BERKELEY 
 
 \RY 
 
 UNIVERSITY OF 
 V CALIFORNIA 
 

 
 
 
A TEXTBOOK OF 
 GEOLOGY 
 
 BY 
 
 AMADEUS W. GRABAU 
 
 \\ 
 
 S.B., MASS. INST. OF TECHNOLOGY; S.M., S.D., HARVARD 
 
 PROFESSOR OF PAL/EONTOLOGY IN THE GOVERNMENT UNIVERSITY 
 
 OF PEKING, CHINA, AND PALEONTOLOGIST TO THE 
 
 CHINESE GEOLOGICAL SURVEY 
 
 FORMERLY LECTURER IN MINERALOGY AND IN GEOLOGY IN TUFTS 
 
 COLLEGE, PROFESSOR OF MINERALOGY AND GEOLOGY IN 
 
 THE RENSSELAER POLYTECHNIC INSTITUTE, AND 
 
 PROFESSOR OF PALEONTOLOGY IN 
 
 COLUMBIA UNIVERSITY 
 
 AUTHOR OF "PRINCIPLES OF STRATIGRAPHY," "GEOLOGY OF THE NON- 
 METALLIC MINERAL DEPOSITS OTHER THAN SILICATES," " NORTH 
 AMERICAN INDEX FOSSILS " (WITH H. W. SHIMER), "GUIDES 
 TO THE GEOLOGY AND PALEONTOLOGY OF NIAGARA 
 FALLS, OF EIGHTEEN MILE CREEK, AND OF 
 THE SCHOHARIE REGION," ETC., ETC. 
 
 PART I 
 GENERAL GEOLOGY 
 
 D. C HEATH & CO., PUBLISHERS 
 
 BOSTON NEW YORK CHICAGO 
 
COPYRIGHT, 1920, 
 
 BY D. C. HEATH & Co. 
 
 2iO 
 
DEDICATED 
 
 TO 
 MY FORMER STUDENTS 
 
 WHO 
 
 IN THE NEW WORLD AND THE OLD 
 
 ARE TRANSMITTING, AUGMENTING, AND APPLYING 
 
 THE KNOWLEDGE OF THE 
 
 EARTH SCIENCE 
 
 IN THE ACQUIRING OF WHICH 
 
 IT HAS BEEN MY PRIVILEGE 
 
 TO AID THEM 
 
 438937 
 
PREFACE 
 
 IN the preparation of this book, I have departed somewhat widely 
 from the prevailing order of treatment in current texts. Instead of 
 beginning with the destruction of rocks, it has seemed more logical to 
 give the student some knowledge of the rocks to be destroyed, and of 
 their character and origin. Instead of treating clastic rocks first and 
 igneous and other non-clastic rocks later, it has seemed more desir- 
 able to begin with those rocks from which elastics are largely derived, 
 before dealing with the elastics themselves. Twenty years of experi- 
 ence as a teacher have convinced me that the average student admitted 
 to courses in geology receives too little instruction in minerals, and al- 
 though we generally recommend mineralogy as a desirable prerequisite, 
 few teachers can insist upon a preparation in this subject on the part 
 of the student. Yet without a knowledge of at least some minerals the 
 study of rocks is impossible, and few geological phenomena can be ade- 
 quately understood without at least a general knowledge of the rocks 
 which they affect. Students who are preparing to make geology their 
 life work, will in any case undertake a more extended study of minerals, 
 and they will turn to the excellent textbooks in that science now avail- 
 able, and some of which are listed on page 51. But the great, majority 
 of students of geology come to this subject only with the desire to gain 
 some knowledge of the world they live in, of the material of which it is 
 composed, of the forces which have fashioned it, and of the laws which 
 have governed its development. They may do so from a desire to master 
 the secrets of nature for the material benefits to be derived from such a 
 mastery, or for the power which such a knowledge will confer upon them ; 
 or they may undertake the study of the earth, because they wish to 
 broaden their mental horizon and subject themselves to that stimula- 
 tion of the intellect, that deepening of spiritual perceptions, and that 
 awakening of dormant faculties, which others have found in a sym- 
 pathetic understanding of, and love for, the out-door world, and which, 
 in its fullest measure, is most frequently vouchsafed to the student of 
 geology. From whatever motive the student approaches the subject, 
 he should be made to realize that his desires can best be attained, 
 if he keep in mind the maxim of La Rochefoucauld, "Pour bien savoir 
 une chose il faut en savoir les details." Detail does net appeal to the 
 
vi Preface 
 
 average student, but a knowledge of a certain amount of detail is neces- 
 sary in any subject, if it is to be well understood, and in geology, as in 
 other sciences, no real understanding of principles and of phenomena 
 is possible without some conscientious devotion to detail. The book 
 of nature is unsealed only to him who is willing to learn the language 
 in which it is written. 
 
 I believe that at the very outset the student -of the earth should sub- 
 ject himself to a moderate discipline in the elements of chemistry and 
 mineralogy at least. To those who can not devote the time to sepa- 
 rate courses in these sciences, Chapter IV may serve as guide for a series 
 of laboratory exercises, which may be carried on simultaneously with 
 the study in the classroom of the more general subjects treated in the 
 first three chapters. In all such studies the individual teacher must 
 select and amplify the subject matter; the text of Chapter IV is in- 
 tended primarily as a guide, while to the student, already familiar with 
 minerals, it may serve as a summary for convenient reference. In 
 its preparation, and especially in the selection of the mineral species in- 
 cluded in the tables, I have had the generous advice of the late lamented 
 Professor Alfred Moses of Columbia University, and that of Professor 
 L. Luquer of the same institution, while the experience gained in teach- 
 ing mineralogy for a number of years at the Rensselaer Polytechnic 
 Institute, at Tufts College and at the Museum of the Buffalo Society 
 of Natural Sciences has been drawn upon. For reasons already set 
 forth, I have next taken up the treatment of the igneous rocks. Many 
 years of teaching elementary students both in college and in the summer 
 sessions has satisfied me that while the study of rocks is carried on in 
 the laboratory the broader relations of those rocks are advantageously 
 treated at the same time in the lectures or classroom exercises. There- 
 fore I would recommend that Chapter VI be used entirely as a labora- 
 tory text, while Chapters VII to IX inclusive serve for simultaneous 
 classroom exercises, of course with proper material for illustration. 
 Chapter X is again best treated as a laboratory text on aqueous (chem- 
 ical) deposits, and here again each teacher will make his selection of 
 material as time and equipment permit. Chapter XI serves as the 
 accompanying text for lectures and classroom exercises, amplified and 
 illustrated according to the teacher's predilections and equipment. 
 
 It has been my experience, as no doubt it has been that of many other 
 teachers, that the study of rocks of organic origin requires some under- 
 standing on the part of the student of the types of organisms which 
 are active in their production, and that the study of organic types 
 should not be relegated exclusively to the chapters on historical geology. 
 I therefore venture to hope that the introduction of illustrations of the 
 various types of rock-forming organisms will be welcomed by the teacher, 
 
Preface vii 
 
 and be of value to the student. Wherever possible I have selected 
 illustrations of material easily obtainable, as in the case of the mol- 
 luscan shells and several of the nullipores, which are common on our 
 Atlantic coast, and in our rivers and lakes, and in that of the bryozoans 
 and echinoderms (except the crinoids) which are easily obtained on our 
 coasts. Most of the corals too are the common species found in, or 
 easily added to, any collection. It is recommended that Chapter XII 
 be used both as classroom and laboratory text. The same thing ap- 
 plies to Chapter XIII, which deals chiefly with the plants that enter 
 into the construction of our peat and coal deposits, and specimens of 
 these can readily be obtained by any one. Here, too, laboratory work 
 with specimens and classroom exercises may go hand in hand. 
 
 The study of clastic rocks per se, treated in Chapter XVIII, may 
 again be carried on in the laboratory during the simultaneous treat- 
 ment of the forces which produce them and the phenomena of weather- 
 ing, erosion, transportation, and deposition (Chapters XV-XVII) in the 
 classroom and field. Although I do not suggest that these subjects be 
 entirely ignored during the earlier part of the course, where the teacher 
 may point out phenomena of weathering, stream and wave erosion, 
 etc., especially during field exercises, I believe that systematic study 
 is most satisfactorily deferred until non-clastic rocks, igneous phenom- 
 ena, and other forces which produce material from which clastic rocks 
 may in the first place be derived, have been considered. Other teachers 
 may not agree with me and they may find it desirable to transpose the 
 chapters. I have endeavored so to arrange the chapters that little 
 if anything will be lost in effectiveness by their transposition, and to 
 this end I have given repeated cross references. Therefore if the teacher 
 so desires, he may pass from Chapter III directly to Chapter XV, taking 
 up the intervening chapters in any order he deems fit. 
 
 I believe, however, that most teachers will agree with me that the 
 broader subject of the sculpturing of the lands (Chapters XXII-XXIII, 
 the essentially physiographic chapters) are best treated after, not only 
 the formation and original structural characters of the earth's crust, 
 but also its deformations have been studied. Although metamorphism 
 by igneous contact has been treated in connection with the igneous 
 phenomena, where it belongs, the main subject of metamorphism and 
 metamorphic rocks (Chapter XX) follows the treatment of deforma- 
 tions (Chapter XIX) and may likewise be treated largely as a labo- 
 ratory text. 
 
 Wherever possible, I have adopted the method of treating typical 
 examples of geological phenomena in some detail, with an abundance 
 of illustrations, because in my own experience, I have found that the 
 student grasps the details of isolated examples more readily and more 
 
viii Preface 
 
 completely assimilates them than he does generalizations with illus- 
 trations drawn from many sources, and from examples which are to 
 him little more than names. On this account I have devoted more 
 space than is usually given in textbooks to descriptions of well-known 
 volcanoes, such as Vesuvius, Etna, etc., to a few typical glaciers such 
 as the Aletsch, the Mer de Glace, the Malaspina, etc., to the Florida 
 reefs and the Great Barrier reef, and to such special examples of com- 
 plex rivers as the Niagara, the Genesee, and the Colorado. My choice 
 of the types has been partly influenced by their accessibility, and it 
 is my hope that the study of such accessible examples will awaken in 
 the student the determination to study them in the field, a possibility 
 any one may look forward to in these days of comparative lessening 
 of distances. The same motive of treatment of types rather than facts 
 and phenomena, has prompted me to devote what some may consider 
 an undue amount of space to the details of the great historic earth- 
 quakes. I believe, however, that the human interest which these phe- 
 nomena have, will appeal to the student, and he will duplicate my own 
 experience, that such a narrative will give him a deeper knowledge of 
 seismic phenomena than a categorical treatment of them could inculcate. 
 
 After due consideration and conference with the publishers, I have 
 decided to use the old style of endings for the geological systems and 
 periods, i.e., Cambrian, Ordovician, Silurian, etc., instead of the more 
 precise and uniform ending in ic (Cambric, Ordovicic, Siluric, etc.) 
 which I have always maintained, and still believe, to be superior, not 
 euphonically, but because they give uniformity to the terminology which 
 has the hit and miss characteristics of a language grown up uncon- 
 trolled, and which is, therefore, not scientific. I am confident that in the 
 future we shall adopt the system of uniform endings, and in my scientific 
 writings I shall continue to use it where I can persuade the editor to 
 permit it. In the present case, however, as the majority of teachers 
 cling to the old terminology, I have, though with some reluctance, 
 adopted it, more especially in view of the fact that it would make the 
 use of the book more difficult if it did not conform to the language em- 
 ployed by the teacher. I desire, however, to have it distinctly under- 
 stood that I have not surrendered my belief in the superiority of the 
 more precise terminology, and that hereafter, as in the past, I shall 
 continue to advocate its use. 
 
 For substantial assistance in the preparation of this text I am under 
 obligation to a number of friends and colleagues. I have especially 
 enjoyed the freely given advice of my former colleagues at Columbia 
 University. The aid in the mineralogical chapter has already been re- 
 ferred to. Mr. Frederick K. Morris of Columbia read the proof of 
 Chapters XV to XVII inclusive and made many valuable criticisms 
 
Preface ix 
 
 and suggestions. A similar valuable office was performed by Professor 
 
 C. P. Berkey for Chapters XIX to XXI inclusive, and by Professor 
 
 D. W. Johnson for Chapters XXII and XXIII. To these gentlemen 
 my best thanks are tendered herewith, and the assurance that much 
 that may be valuable in these chapters is due to their advice, while for 
 any divergence from their views, especially in the order and extent of 
 treatment, I take full responsibility. The entire proof was read by Dr. 
 Marjorie O'Connell of the American Museum of Natural History, and 
 again by my friend Mr. Ernest Welleck of the editorial staff of the 
 Popular Science Monthly, for whose gratuitously given and extremely 
 effective service I gladly render full acknowledgment. My former 
 student Miss Mary Welleck, A.M., has been my assistant throughout 
 the arrangement of this text for the press, and has been of the greatest 
 service in securing illustrations. She has a number of block diagrams 
 entirely to her credit as acknowledged in the text, and also a number of 
 other illustrations. Mr. Frederick K. Morris has also contributed sev- 
 eral of his effective drawings, and has made special effort to secure 
 photographs as elsewhere acknowledged. To Miss Florence Holzwasser 
 of Barnard, I am also indebted for careful work in comparing manu- 
 script and copy, and to Messrs. C. J. Connelly and F. K. Morris and 
 to Dr. J. J. Galloway for reading part of the text and making suggestions. 
 
 To Miss Amy Hepburn, Librarian of Natural Science in Columbia, 
 and to the geological, botanical, and zoological libraries of that Uni- 
 versity I am under special obligation for freely furnished material for 
 illustrations. Prof. J. F. Kemp has generously loaned his portrait of 
 Werner. The United States Geological Survey has furnished a large 
 number of original photographs from which illustrations were made for 
 this book, and Dr. D. W. Johnson of Columbia has placed his entire 
 collection of photographs at my disposal for selection of illustrative 
 material, and I hereby record my great indebtedness to Dr. Johnson 
 and to Dr. George Otis Smith, Director of the Federal Survey, for their 
 courtesy. To Dr. John M. Clarke and the State Museum at Albany 
 I am likewise under obligation for several illustrations in this part of 
 the text and for many more in Part II.. The geological department of 
 Harvard University has generously loaned me a number of photographs 
 from the Gardner Collection through the chairman, Prof. R. A. Daly. 
 To Prof. J. B. Woodworth of that University I am also indebted for 
 several original photographs. The Alaska Engineering Commission 
 has also generously furnished a number of photographs of glaciers 
 through Mr. W. A. Ryan. Prof. Elizabeth Fisher of Wellesley College 
 has furnished several very effective photographs taken by herself and 
 others as acknowledged in the text. From Prof. W. H. Sherzer I have 
 received prints of his photographs of sand grains reproduced in Grabau 
 
x Preface 
 
 and Sherzer's Monroe Formation of Michigan. A number of photo- 
 graphs were taken by Dr. Marcus I. Goldman (U. S. G. S.) during a 
 trip with the author in England and Scotland under the guidance of 
 Dr. Benjamin Peach. Several were taken by Mr. G. W. Stose (U. S. G. S.) 
 during an early trip in company with the author, in Nova Scotia. The 
 Philippine Bureau of Science, at Manila, Dr. Elmer D. Merrill, Di- 
 rector, has generously furnished the fine photograph of Mayon Volcano 
 reproduced in the frontispiece. Prof. W. O. Crosby has furnished a 
 number of illustrations used by him in his illustrated Museum Guide 
 (Dynamical and Structural Geology, Boston Society of Natural History, 
 1892), and a number of photographs of geological features in Utah 
 have been received from Prof. F. J. Pack of the University of Utah. 
 To all these contributors my best thanks are given. To The American 
 Museum of Natural History my thanks are gladly given for the fine 
 photograph of the eel-grass in the Annulate group, constructed by 
 Dr. Roy Miner and reproduced in Fig. 274, and for the photographs of 
 the Spine of Pelee (Figs. 106, 107) taken by Dr. E. O. Hovey. The 
 American Geographical Society, through its director Dr. Bowman, also 
 generously loaned a number of cuts as elsewhere acknowledged, and the 
 Popular Science Monthly has furnished a number of photographs for 
 reproduction. Others to whom I am indebted for furnishing original 
 photographs are Dr. C. P. Berkey, Miss A. D. Savage, Dr. M. O'Con- 
 nell, Dr. Elsworth Huntington, the late Prof. C. S. Prosser, Dr. C. C. 
 Mook, and my brother Mr. P. L. Grabau. My former student Dr. 
 Bela Hubbard has prepared a number of photographs of rocks and rock 
 structures from original specimens and thereby put me under great 
 obligation. To Messrs. Dodd Mead and Co., Ginn and Co., Henry Holt 
 and Co., The Macmillan Co., John Wiley and Sons, and to Yale Uni- 
 versity Press, publishers of Military Geology, I am indebted for per- 
 mission to reproduce illustrations from books published by them. These 
 are acknowledged in the text, as are also the sources of other illustra- 
 tions, especially Kayser's Lehrbuch; Lake and Rastall, Textbook 
 of Geology; Le Conte, Elements of Geology; De Martonne, Geographic 
 Physique; Rosenbusch, Elemente der Gesteinslehre ; LyelTs Principles; 
 Ratzel, Die Erde; Gray's Botany; Davis, Erklarende Beschreibung 
 der Landformen; Haug's Traite; Merrill's Contributions to the History of 
 American Geology; Verrill and Smith, Invertebrates of Vineyard Sound; 
 Binney and Gould, Invertebrates of Massachusetts; and books by Wal- 
 ther, Haas, Bowman, Geikie, Zittel, Steinmann, Krummel, Murray, 
 Heim, Suess, J. M. Arms-Sheldon, and others. To the publisher of my 
 Principles of Stratigraphy, and of North American Index Fossils, Mr. 
 A. G. Seiler, I am indebted for permission to reproduce a number of 
 illustrations from these works. Prof. Moses generously permitted the 
 
Preface xi 
 
 reproduction of a number of illustrations, especially of crystal outlines, 
 from his Elements of Mineralogy, published by D. Van Nostrand Co. 
 Finally, my sincere thanks are due to my publishers, Messrs. D. C. 
 Heath and Co., for their generosity in giving me a free hand in the 
 selection of illustrations, in placing no limit upon their number, and in 
 furnishing a considerable proportion of them. 
 
 NEW YORK, June 30, 1920. 
 
CONTENTS 
 
 CHAPTER PAGE 
 
 I. INTRODUCTION .... i 
 
 The Science of Geology. The Earth Viewed as a Whole. 
 
 II. SUBDIVISIONS OF THE SCIENCE OF GEOLOGY . 13 
 
 Subdivisions of the Science in its Comprehensive Sense. 
 Subdivisions of the Science of Geology in its More 
 Limited Sense. Lithology or the Study of the Litho- 
 sphere. 
 
 III. METHODS OF APPROACH IN THE STUDY OF THE 
 
 EARTH 24 
 
 The Rise of Geological Observation and Interpretation. 
 The Field of Geological Observations. The Impor- 
 tance of Geological Literature. 
 
 IV. MATERIAL OF THE EARTH'S CRUST ... 38 
 
 The Chemical Elements and Their Primary Combinations. 
 Minerals. 
 
 V. ROCKS, THEIR CLASSIFICATION AND PRINCIPAL 
 
 TYPES 64 
 
 Definitions. Age Relations of Rocks. Bed-Rock and 
 Mantle-Rock. Classifications of Rocks. The Unaltered 
 or Little Altered Rocks. 
 
 VI. THE PRINCIPAL TYPES OF IGNEOUS OR PYROGENIC 
 
 ROCKS 84 
 
 The Igneous Magma. Formation of Igneous Rocks by 
 Cooling of Magma. Types of Igneous Rocks Based on 
 Composition and Texture. 
 
 A VII. MODERN VOLCANIC PHENOMENA . . . .109 
 
 Distribution, Classification, and Development of Vol- 
 canoes. Characteristic Forms and Activities of Typical 
 Modern Volcanoes. Classification of Volcanoes Ac- 
 cording to Type of Eruption and Form. Geological 
 Age of Volcanoes and Lava Flows, 
 xiii 
 
xiv Contents 
 
 CHAPTER PAGE 
 
 VIII. STRUCTURAL CHARACTERS OF VOLCANOES, AND 
 
 OTHER IGNEOUS PHENOMENA . . . .144 
 
 Extinct Volcanoes. Extinct Calderas and Sinks. Volcanic 
 Funnels and Pipes, Spines, Plugs, and Necks. Sheet La- 
 vas Formed by Fissure Eruption. Minor Phenomena 
 Generally Associated with Closing Stages of Volcanicity. 
 
 IX. FORM AND STRUCTURE OF OLDER IGNEOUS MASSES 188 
 
 Types of Older Igneous Masses. Contact of Igneous 
 Masses with Other Rocks. 
 
 X. THE AQUEOUS OR HYDROGENIC ROCKS . .214 
 
 General Character and Varieties. The Textures of Aqueous 
 Deposits. The Principal Types of Aqueous or Hydro- 
 genie Deposits. 
 
 XI. MODE OF OCCURRENCE AND ORIGIN OF THE 
 
 AQUEOUS OR HYDROGENIC ROCKS . . .227 
 
 Types of Deposits. Sea-Water and the Evaporation Prod- 
 ucts and Chemical Precipitates Formed from It. Spe- 
 cial Conditions Favoring Deposition of Sea-Salts. An 
 Ancient Rock Salt Deposit Formed by Evaporation of 
 Sea- Water. Deposits of Salt by Concentration in La- 
 goons. Bar Theory of Ochsenius. Deposition of Salt 
 in Inland Desert Basins. Carbonate of Lime Deposits. 
 Other Chemical Deposits in the Sea. Chemical Depos- 
 its and Evaporation Products of Lakes. Chemical De- 
 posits and Evaporation Products of Rivers. Deposits 
 by Springs and Underground Waters. Mineral Veins. 
 
 XII. THE ORGANIC OR BIOGENIC ROCKS . . 269 
 
 Bioliths. Types of Organic Rocks or Bioliths. Deposits 
 of Carbonate of Lime by Plants. Foraminifera and 
 Foraminiferal Oozes and Limestones. Corals and 
 Related Reef-Building Animals. Characters and 
 Types of Modern Coral Reefs. Ancient Coral and 
 Coralline Reefs. Other Lime-Depositing Organisms. 
 Organic Deposits of Phosphate of Lime. Organic 
 Deposits of Silica. 
 
 XIII. THE ORGANIC OR BIOGENIC ROCKS: DEPOSITS 
 FORMED FROM THE ORGANIC TISSUES OF 
 PLANTS AND ANIMALS . 3 28 
 
 Deposits Formed from Vascular Plants. Altered De- 
 posits of Older Vegetal Material. Accumulation of 
 
Contents xv 
 
 CHAPTER PAGE 
 
 Decaying Organic Matter from Animal Tissues, and 
 from Non- Vascular Plants. Alteration Products from 
 Organic Slime Produced by Non- Vascular Plants and by 
 Animal Tissues. 
 
 XIV. ATMOSPHERIC PRECIPITATES AND THEIR DERIVA- 
 TIVES . -354 
 Types of Atmospheric Precipitates. Compacting and 
 Modification of Snow. Glaciers. Ice-Caps. Conti- 
 nental Glaciers. Icebergs. Causes of Ice Movement. 
 
 XV. DESTRUCTION OF ROCKS AND THE FORMATION OF 
 
 CLASTIC MATERIAL 390 
 
 Agents Active in the Formation of Fragmental or Clastic 
 Material. Processes of Erosion. Destructive Work 
 of the Atmosphere. Destructive Work of the Hydro- 
 sphere. Destructive Work of the Pyrosphere. De- 
 struction of Rocks by Movements of the Earth's Crust. 
 Rock Destruction by Glaciers. Rock Destruction by 
 Organisms. 
 
 XVI. TRANSPORTATION, SORTING, AND DEPOSITION OF 
 
 CLASTIC ROCK MATERIAL . . . .438 
 
 Agents of Transportation and Sorting and Regions of 
 Deposition. Transporting and Sorting by Winds. 
 Deposition of Wind-Blown Sands. Dust Deposits. 
 Transporting and Sorting by Streams. River Deposits. 
 Glacial Transportation and Deposition. 
 
 XVII. TRANSPORTATION, SORTING, AND DEPOSITION OF 
 
 CLASTIC MATERIAL IN THE SEA . . . 509 
 
 The Geographical Subdivisions of the Sea. Bathymetric 
 Districts and Zones. Waves and Currents of the Sea. 
 Sources of Clastic Sediments Deposited in the Sea. 
 Transportation and Sorting of Clastic Material in the 
 Sea. Types of Clastic Deposits in the Sea. Summary 
 of Structures of Marine Clastics. Lateral Changes in 
 Facies and Overlap Relations of Marine Clastics. 
 
 XVIII. CONSOLIDATION OF CLASTIC MATERIAL; TYPES OF 
 
 CLASTIC ROCKS 563 
 
 Consolidation of Clastics. Classification of Clastic Rocks. 
 Structural and Other Characters Used in Defining 
 Clastic Rock Types. Varieties of Clastic Rocks. 
 
xvi Contents 
 
 CHAPTER PAGE 
 
 XIX. DEFORMATION OF THE ROCKS OF THE EARTH'S 
 
 CRUST 582 
 
 Effects of Deformation. Types of Deformation Struc- 
 tures. Deformation by Folding. Structures Due to 
 Folding, Erosion and Renewal of Deposition. The 
 Causes of Folding. Deformation by Faulting. Topo- 
 graphic Features Due to Faulting. Minor Features 
 Accompanying Faulting. Other Structures Produced 
 by Deformation. 
 
 XX. METAMORPHISM AND METAMORPHIC ROCKS . . 642 
 
 Definition and Classification of Metamorphism. Activi- 
 ties of the Agencies Producing Metamorphism. Meta- 
 morphic Structures and Textures. Occurrence and 
 Age of Metamorphic Rocks. Types of Metamorphic 
 Rocks. 
 
 XXI. MOVEMENTS OF THE EARTH'S SURFACE AND 
 
 THEIR GEOLOGICAL EFFECTS . . . 655 
 
 Sudden Crustal Movements Earthquakes. Great 
 Earthquakes of Modern Times. Summary of Phenom- 
 ena Due to and Accompanying Earthquakes. Slow 
 Changes in Levels, Bradyseisms. 
 
 XXII. THE SCULPTURING OF THE EARTH'S SURFACE . 697 
 
 Initial Character of the Land Surface. The Erosion 
 Cycle in the Sculpturing of the Land Surface. The 
 Erosion-Cycle on a Simple Coastal Plain. The Erosion 
 Cycle on Domes and Basins. The Erosion Cycle on 
 Anticlines and Synclines. 
 
 XXIII. THE SCULPTURING OF THE EARTH'S SURFACE 
 
 (Continued) .... -747 
 
 The Erosion Cycle in a Faulted Region. Some Illustra- 
 tions of Complicated River Erosion. Land Forms 
 Due to Glacial Sculpture. The Sculpturing of the 
 Edge of the Land. Erosion Forms Produced by At- 
 mospheric Agencies. 
 
 INDEX 825 
 
PART I 
 GENERAL GEOLOGY 
 
TABLE OF THE ERAS AND PERIODS OF GEOLOGICAL TIME 
 
 PSYCHOZOIC OR QUATERNARY ERA 
 Recent or Holocene Period 
 Pleistocene Period 
 
 CENOZOIC OR TERTIARY ERA 
 Pliocene Period 
 Miocene Period 
 Oligocene Period 
 Eocene Period 
 Palaeocene Period 
 
 MESOZOIC (SECONDARY) ERA 
 Cretaceous Period 
 Comanchean Period 
 Jurassic Period 
 Triassic Period 
 
 PALAEOZOIC ERA 
 Permian Period 
 
 Carbonic or Pennsylvanian Period 
 Mississippian Period 
 Devonian Period 
 Silurian Period 
 Ordovician Period 
 Cambrian Period 
 
 PROTEROZOIC AND ARCHEOZOIC ERAS 
 Pre-Cambrian Periods 
 (Algonkian and Archaean) 
 
 XV 111 
 
TEXTBOOK OF GEOLOGY 
 
 PART I GENERAL GEOLOGY 
 
 CHAPTER I 
 INTRODUCTION 
 
 THE SCIENCE OF GEOLOGY 
 
 Definition. Geology is the science of the earth in all its aspects, 
 except those which deal with the relationship of the earth to other 
 planets and to the sun of our solar system. That aspect of the 
 earth properly belongs to the science of Astronomy, for our earth 
 is one of the heavenly bodies with which that science is concerned. 
 It is true that this phase of the subject is often treated by the geol- 
 ogist under the name of astronomical geology, but in this book we 
 shall consider it as belonging primarily to the field of astronomy. 
 At the same time we shall recognize the importance to the student 
 of at least a general understanding of these astronomical relations 
 of the earth, and consider such an understanding a desirable pre- 
 liminary preparation. 
 
 Derivation of Terms. The term Geology is derived from the 
 Greek words ge (yrj), earth, and logia (Aoyox), science, or logical 
 discourse. The science of geology was preceded by and in a meas- 
 ure grew out of the subject of geography, which literally is the 
 description of the earth (graphein, ypdfaw, to write) and was, of 
 course, chiefly confined to a description of the earth's surface fea- 
 tures, and their significance in terms of human existence. But 
 since surface features are generally the expression of underlying 
 structure, and of the geological history of the region, it is evident 
 that geography cannot be divorced from geology, and that, to no 
 inconsiderable extent, the student of the geography of a region must 
 take account of its geological structure and history. 
 
 Scope of Geology. In its broadest sense, then, geology is the 
 science of the earth and all that pertains to it. It is the study in 
 
2 rVn ' l Introduction 
 
 detail of one of the planets in one solar system, by the inhabitants 
 of that planet, who are themselves a part of it. If other planets 
 of our solar system or those of other solar systems are inhabited, 
 the study of those planets by their inhabitants would correspond 
 to our geology it would be the science of a particular planet. 
 By us little can be ascertained regarding the structure and develop- 
 ment of other planets, except in an indirect way, and all investi- 
 gations along such lines fall properly into the domain of the astron- 
 omer, though he concerns himself primarily with interrelations of 
 the heavenly bodies. And thus we may consider that the study 
 of the physical universe, i.e., the cosmos, which may be called 
 the science of cosmology, has only the two primary divisions, 
 astronomy and geology. This may be summarized as follows : 
 
 COSMOLOGY 
 
 The science of the 
 
 universe 
 
 1. Astronomy. The science of all the heavenly bodies, 
 including the earth, their character, distribution, interrela- 
 tions, movements, etc., and the laws which govern them. 
 
 2. Geology. The science which deals with the material, 
 structure, history, etc., of one of those bodies, i.e., the earth. 
 
 Such a view of the science of geology gives it an extremely 
 comprehensive scope, and we must recognize that in ordinary 
 parlance the term is used in a much more restricted sense 
 Nevertheless it is desirable to take this comprehensive view at 
 the outset, and to note the several subdivisions into which such a 
 broad science naturally falls, and with all of which the student 
 of any one division should have at least a general acquaintance. 
 We shall best get the proper view-point by first considering the 
 earth in its entirety. 
 
 THE EARTH VIEWED AS A WHOLE 
 
 Could we view the earth in its entirety from some extraterrestrial 
 vantage point, we would recognize it as an oblate spheroid, that is, 
 a body approaching that of a sphere, but with its polar diameter 
 flattened and its equatorial belt somewhat inflated. By measure- 
 ment the polar diameter of the earth is found to be 7899.7 miles, 
 while the equatorial diameter is 7926.5 miles. If the observer in 
 extraterrestrial space is sufficiently removed from the earth's sur- 
 face, that surface would appear essentially smooth, as does the 
 surface of the moon to our unaided eyes ; the irregularities which 
 the dweller on the earth recognizes as mountains and valleys would 
 
The Earth Viewed as a Whole 3 
 
 become of insignificant proportions. Were we to represent the 
 earth by an accurately scaled model 10 feet in polar diameter, 
 the equatorial diameter would exceed the polar by only a trifle 
 over ^ of an inch, while the highest mountains on the earth's 
 surface would form elevations on the model less than -fa of an 
 inch in height. Thus viewed, the prominences which appear to 
 us formidable are after all of minor significance, and bearing this 
 in mind, we can understand that relatively slight bulgings or crum- 
 plings of the earth's surface may produce what to us appear as great 
 elevations. The wrinklings on the skin of a drying apple consti- 
 tute far more prominent elevations in proportion to the size of the 
 apple than do the highest mountain chains on the earth's surface, 
 when compared with the size of the earth, while the scratches and 
 notchings formed upon the surface of a glass marble, after a brief 
 play, form vastly greater depressions, in proportion, than do the 
 largest river canons, such as that of the Colorado, or the deepest 
 ocean depressions. Thus when the geologist finds evidence that 
 the summit of the high peaks of the Himalaya Mountains or the 
 top of the Apennine chain were once a part of the sea-bottom, he 
 no longer concludes that the sea once covered these mountains, as 
 was formerly supposed, but he infers rightly that these mountain 
 chains were raised by a wrinkling of the earth's crust or by an 
 upward warping which occurred at a time subsequent to that in 
 which this region was beneath the sea. The evidence from the 
 material of the mountain which leads the geologist to such a con- 
 clusion will be discussed in subsequent chapters, and the corrobo- 
 rative evidence from the structure of the mountains will also be 
 given. The chief lesson which it is intended to impress upon the 
 student at present is that surface features are relatively unimpor- 
 tant when we consider the earth as a whole, and that elevations and 
 depressions except those which we recognize as continental 
 masses and oceanic depressions are of local significance chiefly, 
 and may change from one to the other not once but many times. 
 
 There rolls the deep where grew the tree. 
 
 O earth, what changes hast thou seen ! 
 
 There where the long street roars hath been 
 The stillness of the central sea. 
 
 This is more than the expression of the poet's fancy ; it is a truth 
 which the observation of the earth and logical deductions from 
 these observations force upon the geologist's attention at every step. 
 
4 Introduction 
 
 The Successive Inorganic Shells or Spheres of the Earth 
 
 The Spheres Open to Direct Study. The observer from his 
 extraterrestrial view-point will, however, note clearly that a large 
 part of the earth's surface to be precise, about 70.8% of it is 
 covered with water. This is the sea which is divided into a number 
 of oceans and surrounds all the lands, and, moreover, forms a 
 continuous body, the surface of which comprises approximately 
 137,070,000 square miles or 361.1 million square kilometers. The 
 land has a surface area of about 59,870,000 square miles or 148.8 
 million square kilometers, giving a total surface area for the earth 
 of 196,940,000 square miles or 509.9 million square kilometers. 
 Certain portions of the land surfaces are also covered by water 
 bodies, the lakes, which are not in direct connection with the sea. 
 Among these the Great Fresh- water Lakes of North America, and 
 the Caspian, a great salt-water lake, are the most prominent ex- 
 amples. These are to be classed as a part of the land, as conti- 
 nental water bodies, distinct from the sea. Moreover, the upper 
 layers of the land in nearly all parts are water-bearing, this water 
 filling the empty spaces of the soil, and occupying the pore-spaces 
 within the solid rock. This is the ground water, which is tapped by 
 wells, mines, or borings, or issues on the surface in springs which 
 feed the brooks or rivers, or form swamps, ponds, and lakes. It is 
 thus possible to speak of a nearly or quite continuous mantle of 
 water which completely covers some parts of the solid surface of 
 the earth and more or less completely saturates the exposed parts. 
 This mantle of water may be viewed as an aqueous sphere envelop- 
 ing the rock sphere of the earth and it is spoken of as the hydro- 
 sphere. It in turn is enveloped by the sphere of gas and vapor, the 
 well-known atmosphere. 
 
 These two spheres or shells constitute the outer layers of the 
 earth thus viewed in its entirety, and surround the more rigid mass 
 of the earth, with which we are most familiar. This consists of 
 solid rock, and of uncpnsolidated soil and other loose material, all 
 of which, except the surface film of organic matter, we may recog- 
 nize, on examination with a magnifier or by other means, as broken- 
 down rock material in a fine state of division. From observation 
 of soundings, dredgings, etc., and by inference, we know that sim- 
 ilar material forms the ocean floor, and thus we recognize that be- 
 neath the hydrosphere is a sphere or a shell of rock material. This 
 
The Earth Viewed as a Whole 
 
 is called the rock sphere, or lithosphere, and in it are included all 
 of the soil and other unconsolidated rock material of the earth's 
 surface. How thick this shell of rock is, we have no means of know- 
 ing from observation. The deepest borings into the earth's crust 
 have penetrated only a little over a mile in depth, while the deeper 
 mines go less than two miles beneath the surface. But from logical 
 deductions of many observed physical facts, we conclude that this 
 shell has a thickness of at least 75 miles, and perhaps much more. 
 It is indeed generally held that the earth is solid rock to the core, 
 but there are those who have held, and some who still hold, that 
 the central portion of the earth consists of fluid or perhaps even of 
 gaseous matter. 
 
 There are thus three inorganic spheres, the atmosphere, the hy- 
 drosphere and the lithosphere, open to partial observation. The re- 
 lationships and relative 
 magnitudes are shown 
 in the following diagrams 
 (Figs, i a and b). 
 
 The Inner Spheres. 
 From the observation 
 of many phenomena, 
 geologists and physicists 
 have reasoned that be- 
 neath the lithosphere, 
 other spheres character- 
 ized by certain peculiari- 
 ties may be recognized, 
 though their boundaries 
 are variable and probably 
 not very definite. One of these is the sphere or zone in which the 
 temperature of the earth is sufficiently high to permit the fusion 
 of rock if the pressure, which ordinarily keeps it solid by raising 
 the fusing point, were removed or lowered by some structural change 
 in the earth's crust. There may be regions in which essentially 
 permanent pools of molten rock exist within the crust, forming 
 feeders of volcanoes, but the majority of such feeding areas are 
 more probably temporary. It is convenient to speak of this more 
 or less concentric zone as a distinct sphere and the name pyrosphere 
 has been applied to it. 
 
 Another equally indefinite sphere or zone, variously regarded as 
 
 FIG. i a. Diagram of the successive in- 
 organic spheres of the earth. The heavy 
 black line represents the lithosphere, including 
 the hydrosphere, taken as 75 miles. The 
 white semicircle above it indicates the atmos- 
 phere. C, center of the earth. The part in- 
 cluded between the two radii, 2 degrees apart, 
 is enlarged in Fig. i b. 
 
Introduction 
 
 lying from 30 to 75 miles beneath the surface (according to our 
 estimate of the thickness of the lithosphere) , is one of relative weak- 
 ness, between a strong external crust and a rigid central mass. Here 
 the rock yields most readily under long- 
 enduring strains of limited magnitude, 
 and earth-movements, resulting in the 
 deformation of the rocks, occur. This 
 zone or sphere, which has been called the 
 tectosphere (Murray) or the astheno sphere 
 (Barrell), may in part include the pyro- 
 sphere; nevertheless, it is convenient to 
 speak of it as distinct. 
 
 Finally there remains the central portion 
 of the earth, the centrosphere, which is by 
 far the largest part of it, and about which 
 we know nothing from observation and but 
 little from inference. From the known 
 rate of temperature increase in borings 
 and mines, it is inferred that the temper- 
 ature near the center of the earth may 
 be between 200,000 and 350,000 degrees 
 
 FIG. i b. Enlargement 
 of a part of the sector 
 shown in Fig. i a, to show 
 the relative thickness of 
 the crust of the earth, the Fahrenheit, a temperature so high that 
 
 mean and greater depths were it not for the enormous pressure, 
 all the rocks there would probably be 
 vaporized. From the fact that the aver- 
 
 inch=no miles. On this age specific gravity of the rocks which 
 scale the two radii meet at cons titute the earth's crust is only about 
 2.6 while that of the earth as a whole is 
 hydrosphere (black) this 5.6, it is further inferred that the material 
 point then representing the o f tne centrosphere is very heavy and it 
 has been calculated that if there were a 
 
 steady increase in the specific gravity of the material from the 
 surface toward the center, this would, at the latter point, become 
 1 1. 2, which suggests the possibility of a central core of iron, if not 
 of gold and platinum or some other heavy metal. Because of 
 this greater weight, the centrosphere is also spoken of as the bary- 
 sphere or heavy sphere. 
 
 of the sea and the mean 
 and great heights of the 
 land. Approximate scale i 
 
 a distance of about 3 feet 
 from the surface of the 
 
The Earth Viewed as a Whole 7 
 
 The Organic Sphere or Biosphere 
 
 To these inorganic or lifeless spheres there is added the organic 
 sphere, or sphere of life, the biosphere (bios, /ftbs, life). It would 
 surprise the average student could he see the universality of this 
 organic shell of the earth. In and out among the mass of the water 
 and of the air and upon and through the upper layers of the rocky 
 surface, the thread of living matter is woven, now constituting an 
 almost solid mass of tissue, as in the bodies of peat or in the dense 
 vegetation of a forest, or the almost continuous tangle of seaweed 
 or layer of floating organisms in the sea ; again forming a network, 
 the meshes of which vary greatly in size, but are on the whole con- 
 tinuous, except where momentarily broken by the hand of man, 
 or by some abrupt, not to say cataclysmic disturbance,- such as an 
 avalanche or outpouring of a mass of lava. But left- for even a 
 short time, the steepest quarry wall, or the most precipitous cliff 
 formed by the sudden dislodgment of a mountain side or by the 
 sinking of a portion of the earth's crust, such as may take place 
 during an earthquake-producing disturbance, will be again taken 
 possession of by some form of plant, if not of animal life. Even 
 the stony lava field will be covered in time by a succession of organic 
 forms of increasing complexity. The apparently barren desert, too, 
 has its wide-meshed net of living beings, except perhaps where the 
 sands are constantly in motion ; and the presence of these organisms 
 is often shown by the countless tracks and trails which appear upon 
 the surface of the sand on a dewy morning, or the sudden springing 
 up of vegetation from hidden seeds when moisture furnishes the 
 condition for expansive growth. And in the sea, life of some form 
 is never wanting long, here covering the bottom with a continu- 
 ous living carpet, there forming an ever changing web of floating 
 animal and plant life, through which the swimming world of animals 
 weaves an intricate pattern with the threads of its never ceasing 
 wanderings. 
 
 Thus it is perfectly in accord with the facts, when we speak of 
 a shell or sphere of living matter, the biosphere, which surrounds 
 the lithosphere and penetrates its upper layers as well as the hydro- 
 sphere and the atmosphere, but is distinct from all. That such a 
 biosphere has existed throughout most of the past ages of the earth's 
 history is clearly shown by the countless remains of the hard parts 
 of animals, such as the shells of mollusks, the bones of vertebrates 
 
8 
 
 Introduction 
 
 and the like, which fill many of the rocky layers of the litho- 
 sphere, and indeed sometimes actually compose these layers. 
 
 For example, the white 
 
 C ^^f^S^^i^^^ chalk of the English and 
 
 French coastal regions is 
 literally made up of the 
 shells, and fragments of 
 shells, of organisms (Fora- 
 minifera, coccoliths, etc.), 
 as shown in the following 
 illustration (Fig. 2), and 
 many of our great lime- 
 stones are consolidated re- 
 mains of shells of animals 
 which formerly inhabited 
 the seas, or fresh-water 
 lakes. In the following 
 
 FIG. 2. Thin sections of chalk, as they 
 appear under the microscope, showing the 
 shells etc., of minute marine organisms 
 (chiefly Foraminifera and coccoliths) of figure (Fig. 3) IS shown an 
 which the rock is composed. A, Chalk enlargement of a group of 
 from Sussex, England, enlarged 60 times. 
 
 B, Chalk from Farafrah, Libyan desert, 
 enlarged 60 times ; the most characteristic 
 shells are Textularia (a) and Rotalia (ft). 
 
 C, Dried residue of milky chalk water with 
 coccoliths enlarged 700 times. (After 
 Zittel.) 
 
 minute needle-like shells, 
 each one of which was built 
 by a separate marine animal 
 and of which a limestone, 
 traceable over wide areas 
 in the state of New York, 
 
 is almost entirely composed. From careful study, it has been de- 
 termined that a cubic inch of this rock, when pure, contains about 
 40,000 individual shells (J. M. Clarke). 
 Whole mountains are sometimes 
 made up of rock, which is largely, if 
 not entirely, the product of accumu- 
 lation of the hard structures of former 
 organisms. An example of this is 
 found in the famous Dolomites of the 
 Tyrol (Fig. 4), the chief rock masses 
 of which are made up of calcareous 
 matter separated from a sea by marine 
 plants (Fig. 5), and the many centuries 
 of time during which the present forms 
 of these peaks were carved from this 
 
 FIG. 3. Styliolina fissu- 
 rella, a minute molluscan. 
 (pteropod) shell. Fragment 
 with numerous individuals 
 enlarged three times; and a 
 specimen much enlarged. 
 (After Hall,) 
 
The Earth Viewed as a Whole 
 
10 
 
 Introduction 
 
 rock have witnessed only a partial obliteration of the organic 
 structures, though no inconsiderable portion of the mass has been 
 removed. 
 
 The remains of animals and plants which 
 compose, or are included in, the rocks of the 
 earth's crust, are known to the geologist as 
 fossils (from the Latin fossilis, something that 
 is dug up) and their study forms an important 
 and integral part of the science of geology. 
 It is indeed the study of the constantly 
 changing elements of the biosphere, of which 
 the existing animals and plants form only the 
 
 most mo dern phase, that has made possible 
 , . < T i r i 
 
 the deciphering of the history of the earth. 
 
 FIG. 5. Diplo- 
 pora porosa Schafh. 
 A calcareous alga or 
 marine plant which 
 
 was chiefly respon- This subject will be more fully considered in 
 
 sible for the making later chapters of this book. 
 
 of the rock out of , .* t 
 
 Beds of coal > too > P omt to the former ex ~ 
 tensive accumulation of vegetable material, 
 anc j t h e o j} o f our o [\ s hales and pools owes its 
 
 . . , , j i i, Al ,1 
 
 on g m ^IS&* and Perhaps altogether, to the 
 
 distillation, during many centuries of time, of 
 
 which the Dolomites 
 are carved. A, Nat- 
 ural size. B, en- 
 larged 3 times, a. 
 canals which appear 
 upon the surface as 
 
 pores, ,p-,r, constrict- buried organic matter which constituted a 
 ing rings character- . r , , . , . , i i 
 
 istic of this genus, portion of the biosphere of former geological 
 
 (After Steinmann.) periods. 
 
 Contributions to the Lithosphere from Other Spheres 
 
 We now see that the biosphere contributes no inconsiderable 
 portion to the growing lithosphere, and this is true to a certain 
 extent also of several of the other spheres. For not only was the 
 carbon of ancient plants, which is now locked up in the earth's, 
 crust as coal and oil, contributed by the atmosphere of the past, 
 but the water vapor of the air, freezing and descending as snow or 
 hail, or crystallizing directly upon cold surfaces, forms a not unim- 
 portant, though in most regions very transitory, addition to the 
 solid rock of the earth. So, too, the conversion of water to ice forms 
 rock at low temperature, for ice is a rock. More permanent con- 
 tributions are made by the crystallizing of various salts, chief among 
 them the common salt (sodium chloride), which, as we shall see 
 later, may become buried by other rock material, and be preserved 
 
The Earth Viewed as a Whole n 
 
 for millions of years as an essential part of the earth's crust. Thus 
 the salt now mined in New York State and in southeastern Mich- 
 igan was derived from the waters of an ancient sea, the deriva- 
 tion being by a roundabout process, which will be detailed later. 
 That this occurred in a period of time many millions of years ago 
 is amply shown by the 1 ascertainable facts. So, too, the great salt 
 deposits of North Germany, which have furnished the world in the 
 past with most of its potash, are the product of the evaporation of 
 cut-off portions of an ancient sea which covered much of Europe, 
 though the period of its production is not quite so remote as that 
 of the best-known American salts. (See Chapters XXXIV and 
 XXXVIII.) 
 
 That the pyrosphere also contributes its quota to the rock mass 
 of the earth is abundantly shown by the vast extent of ancient 
 lavas and other igneous masses which now form a large proportion 
 of the solid crust of the earth and are visible to us as the result of 
 surface cooling in comparatively modern times, as in the case of 
 the great Columbia lava fields of Northwestern America and the 
 preserved fragments of similar flows on the British coast (Staff a, 
 Giants' Causeway, etc.), or which were uncovered by the removal, 
 during long geological epochs, of the overlying portions of the 
 earth's crust beneath which the ancient igneous masses assumed 
 their solid form. 
 
 Throughout the entire history of the earth, the lithosphere has 
 received additions by the cooling of molten rock material injected 
 into it or poured out on its surface, and this addition is in progress 
 even to-day. 
 
 Contributions to the Lithosphere from Outer Space 
 
 Finally it may be added that the earth's crust receives to-day, 
 and undoubtedly has received in the past, additions from the cosmic 
 spaces which surround our atmosphere. When a meteor reaches 
 the earth it forms an integral part of the unconsolidated or discrete 
 portion of the earth's crust, at least until it is gathered in by the 
 discerning collector of these messengers from the extraterrestrial 
 spaces. 
 
 Summary of Spheres 
 
 We may now summarize in tabular form the several spheres or 
 shells into which the earth as a whole is divisible, and add to this 
 summary the derivation of their names. 
 
12 Introduction 
 
 The Inorganic Spheres, (o-^atpa) = sphere. 
 
 1. Atmosphere, Gr. atmos (aT/xd?) = vapor or gas. 
 
 2. Hydrosphere, Gr.hydor (vSup) = water. 
 
 3. Lithosphere, Gr. lithos (\i'0os) = stone. 
 
 4. Pyrosphere, Gr. pyr (irvp) =fire. 
 
 5. Asthenosphere, Gr. asthenos (do-tfev^s) = feeble. 
 
 6. Centrosphere, Gr. centron (KeVrpov) = center, 
 or Barysphere, Gr. &arys (/Sa/ws) = heavy. 
 
 The Organic Sphere. 
 
 7. Biosphere, Gr. 6^5 (/3tos) = life. 
 
CHAPTER II 
 
 SUBDIVISIONS OF THE SCIENCE OF GEOLOGY 
 
 IT is but natural that the mind of man, always eager to enter 
 into details, should have developed distinct branches of inquiry 
 into the character and history of the several parts of the earth 
 which is his home. In the beginning of such inquiry, a single mas- 
 ter mind may have encompassed the entire field ; but with the in- 
 creasing mass of detail, the individual range became limited to 
 special portions of the field, and in the course of time the several 
 lines of inquiry into the nature and history of our earth developed 
 into distinct sciences, each with its host of devoted searchers after 
 truth. 
 
 SUBDIVISIONS OF THE SCIENCE IN ITS COMPREHENSIVE SENSE 
 
 We may, then, at this point of our study, endeavor to ascertain 
 the natural subdivisions of the Science of the Earth, or Geology 
 in its broadest sense, and after that confine ourselves to those 
 branches which by common consent are reserved as the special 
 field of the geologist, as it is understood at the present time. 
 
 Atmology or Meteorology. From an inspection of the table of 
 spheres into which the earth may be divided, we form the natural 
 conclusion that the first line of cleavage would be in accord with 
 these natural subdivisions. The study of the atmosphere has now 
 developed into a separate science, which is familiarly known as 
 Meteorology because the meteors, or extraterrestrial bodies, the 
 familiar " shooting stars," which on entering our atmosphere be- 
 come luminous through friction and frequently reach the earth as 
 meteorites, have from the remotest days formed one of the chief 
 attractions for those whose gaze was turned away from the solid 
 earth. Thus atmospheric phenomena came to be designated as 
 meteoric phenomena and the science of these phenomena became 
 meteorology. But since the sphere with which this science is con- 
 cerned is the atmosphere, a more appropriate name for this science 
 itself is Atmology. 
 
 13 
 
14 Subdivisions of the Science of Geology 
 
 Hydrology. That the hydrosphere, that is, the oceans and 
 other water bodies, early attracted the serious-minded student 
 was but natural, since these water bodies form the natural high- 
 ways of commerce, and since they furnish so large a part of the 
 earth's population with the means of a livelihood. Thus the science 
 of the water, or Hydrology, was developed, which is variously sub- 
 divided again into the sciences of the oceans, or Oceanography, and 
 into the sciences of the lakes, the ponds, and the rivers. That some 
 of these, for example oceanography, have been developed to a 
 remarkable extent, is shown by the great oceanographic institutes 
 of the world, of which that of Berlin under Director Penck, and that 
 at Paris, under the direction of the Prince of Monaco, are the best 
 known. In America, oceanographic studies have chiefly been car- 
 ried on by the federal government and by private individuals, 
 among these the late Alexander Agassiz ; and there is a hydrographic 
 department of our government, the chief business of which is the 
 charting of our sea-coasts, lakes and rivers. 
 
 In England, too, oceanographic studies have been largely fostered 
 by the government, under whose auspices the famous Challenger 
 Expedition, for the exploration of the oceans, under the leadership 
 of Sir John Murray, was carried to successful completion. The 
 governments of other countries, too, have fostered exploring ex- 
 peditions for oceanographic research, these being generally known 
 by the name of the vessel employed in the expedition. 
 
 The student of geology, even in its narrower sense, must keep 
 abreast of these inquiries, at least to a moderate extent, for without 
 them many of his discoveries lack the clarifying light by the aid of 
 which he must interpret them ; and in proportion as the researches 
 of the oceanographers and the other hydrologists become available, 
 the analysis of the structure and composition of the earth's crust, 
 the special field of the geologists in the modern sense, and the 
 interpretation of these facts in terms of earth history, become 
 more precise. 
 
 Lifhology or Geology Proper. As has just been said, the special 
 field of inquiry of the modern geologist is the crust of the earth, or 
 the lithosphere. He is thus primarily a student of Lithology 
 though that term has also been used in the past in a narrower 
 sense, the study of rocks per se. It will be well, however, for the 
 sake of unity of terminology a great desideratum in every science 
 to adopt this term in its wider sense, namely, that which encom- 
 
The Science in its Comprehensive Sense 15 
 
 passes all the fields of inquiry which are concerned with the com- 
 position, structure, and history of the earth's rocky shell, the litho- 
 sphere, generally assigned as the only legitimate field of the 
 modern geologist. Still, to-day more than ever, the geologist can- 
 not limit his inquiries to the facts disclosed by the rocks alone, 
 but must draw his conclusions with the aid of a wide knowledge 
 of the researches of the atmologist, the hydrologist, and even the 
 biologist. 
 
 Biology ; Palaeontology. Biology is the science of living 
 things. It is the study of the biosphere, and has perhaps been more 
 sedulously developed, and has a larger body of votaries, than any 
 other branch of the earth, science. Biology, as currently under- 
 stood, limits itself to the study of modern living beings, the plants 
 and animals of to-day, and naturally falls into the two divisions, or 
 separate sciences, of Botany (Phy tology *) , the study of plant life, 
 and Zoology, 2 the study of animal life. Biologists are, however, 
 aware of the fact that the modern world of plants and animals is 
 only a fragment of the great host of living beings which has in- 
 habited the earth from the remotest ages ; it is the life record of 
 the youngest and shortest chapter in the history of the earth. 
 Throughout all the past ages, living forms existed upon the earth, 
 and their remains, as we have already seen, are embedded in the 
 rocks of the earth's crust as fossils. The study of these was at 
 first left to the geologist, but has now developed into a separate 
 science, that of Paleontology (from the Greek palaios [TraXtuos], 
 ancient, onto, [ovra], living beings, and logos or logia [Aoyia], science). 
 
 Other Divisions. When we come to the consideration of the 
 inner spheres of the earth, we must admit that, owing to our in- 
 ability to make direct observations, no separate sciences worthy 
 of that designation have yet been developed. It is true that a 
 science of earth disturbances of Seismology exists, and that this 
 properly belongs to the study of the asthenosphere or tectosphere. 
 But the observations on which this science is based are chiefly 
 possible upon surface manifestations and the effects which these 
 produce, though inferences can also be drawn from the study 
 of disturbances which have occurred in the past, the effects of 
 which are recorded in the rocks. The same is true of the study of 
 the pyrosphere which can only be approached through observa- 
 
 Greek phylon [0vr6i>], plant. 2 Greek zoon [fv*']> animal. 
 
1 6 Subdivisions of the Science of Geology 
 
 tions of modern volcanoes (Vulcanology) and of the temperatures 
 in deep borings and mines. Of the interior of the earth we may 
 perhaps never know much through observation, and so deduc- 
 tions from physical phenomena alone must guide us. No separate 
 science of the centro- or barysphere has thus been developed. 
 
 From the foregoing, the student is led to the realization of the 
 truth that all true sciences are based on two great fundamentals, 
 observation and deduction. To observe and record facts alone does 
 not constitute the whole of the work of the man of science. Such 
 used to be the sole aim of the older naturalists, so called, who 
 dissented from the attitude of the ancient philosophers, because 
 these based their speculations on mental processes rather than ob- 
 servation of facts. To-day, however, we know that the inter- 
 pretation of facts, in the light of our growing knowledge of causes, 
 is as legitimate, indeed as important, a function of the student of 
 the earth as the observation of facts, and moreover a certain amount 
 of speculation always held in check by the appeal to facts is 
 not only desirable but necessary. The student of nature must be 
 a philosopher in the true sense of the word, and so long as he does 
 not lose sight of his fundamental base the observation of facts 
 his work will gain in value by allowing his reasoning faculties their 
 fullest play. 
 
 SUBDIVISIONS OF THE SCIENCE OF GEOLOGY IN ITS MORE LIM- 
 ITED SENSE. LITHOLOGY OR THE STUDY OF THE LITHOSPHERE 
 
 Restricting our attention now to the more limited field to which 
 the modern geologist applies himself, namely the study of the ma- 
 terial, structure, and history of the earth's crust, or the lithosphere, 
 we may note at the outset that there are two phases or aspects of 
 this field, one or the other of which is cultivated more sedulously 
 by the geologist according to his predilections or circumstances, 
 but neither of which can be wholly neglected by the worker in 
 either field. These two aspects are the purely scientific, and the 
 practical or applied. The geologist who works in the pure science 
 field of his domain does so primarily for the intellectual satisfac- 
 tion derived from the discovery of facts and principles. His aim 
 is chiefly to search for nature's truths irrespective of their bearing 
 on human welfare, and his principal endeavor is directed toward 
 widening the boundaries of human knowledge and pushing forward 
 
The Study of the Lithosphere 17 
 
 the frontiers of discovery. The chief aim of the practical geologist, 
 on the other hand, is directed toward making the forces and ma- 
 terials of the world available to man, to augment the welfare of 
 the human race, and to push forward the boundaries of civiliza- 
 tion. But this work, important and noble as it is, can only be 
 carried on successfully in proportion as the facts and laws of the 
 science are discovered and demonstrated, and the practical geolo- 
 gist must forever depend on the worker in pure science, whose re- 
 ward too often is little more than the satisfaction which is to be 
 derived from a devotion to the search for'truth. We shall for the 
 sake of convenience speak of the pure-science phase of our subject as 
 the scientific aspect of geology, and of the applied phase of geology as 
 geology in its relation to man. Each phase has its special S 4 ubdi- 
 visions, to which attention may now be drawn. 
 
 I. The Scientific Aspect of Geology 
 
 Structural Geology. A logical analysis of this field of investi- 
 gation reveals a threefold aspect and three corresponding methods 
 of approach in study. In the first place the material of the earth's 
 crust, and the composition and the structure of this material, in- 
 vite attention. This portion of the subject is generally treated 
 under the caption Structural Geology. It takes account of the com- 
 position, form, and architecture of the earth's crust, and is primarily 
 an analytic branch of the science. From this point of view the 
 earth's crust may be considered under the following subdivisions, 
 given in the order of their magnitude. 
 
 Subdivisions of Structural Geology. These include the following 
 fields : 
 
 1. Chemical elements and ions. 
 
 2. The combination of these elements and ions into salts and 
 other compounds which either take on crystalline form or remain in 
 an uncrystalline (amorphous) condition. These compounds as they 
 occur in nature are designated minerals. 
 
 3. The combination of crystals or fragments of the same or 
 different minerals into large masses, either bound together into a 
 more or less solid mass, or remaining in an unconsolidated condi- 
 tion. These are the rocks of the earth's crust, and for their study 
 a recognition of at least the principal component minerals is 
 essential. 
 
i8 Subdivisions of the Science of Geology 
 
 4. The association of rock masses into original or primary struc- 
 tural units, which constitutes the primary architecture of the earth's 
 crust ; or the impression upon it of secondary structures, through 
 deformation or other influences of an outside nature. This is struc- 
 tural geology in the narrower sense and has also been called 
 Geotectology. The original structure corresponds to the initial 
 architecture of a building, the stones or bricks of which correspond 
 to the rock masses of the earth's crust. The secondary structures 
 correspond to the changes subsequently made by additions, removal 
 or modification resulting from sagging with age, etc. 
 
 5. The surface forms resulting through the activities of destruc- 
 tive as well as reconstructive forces, and recognizable in the moun- 
 tains and valleys, the plains and plateaus, and other physical 
 features, generally considered under the subject of Physical Geog- 
 raphy. The term Geomorphology or the study of the details 
 of the earth's surface forms has been commonly applied to this 
 branch of inquiry,' when it is not merely descriptive or analytic, 
 but takes account of the history or genesis of the form as such. It 
 corresponds to the study of the form of a building as a whole, either 
 in its completeness or when changed to a ruin by destructive in- 
 fluences. 
 
 The study of most of these divisions of structural geology has 
 been carried to such detail that separate sciences have been devel- 
 oped. Thus the study of the natural substances, or minerals, has 
 become the science of Mineralogy, and that of the rocks the science 
 of Petrology (also sometimes designated as lithology in its narrower 
 sense), while the science of rock structures is Structural Geology in 
 the narrower sense, and that of the surface forms is Physiography, 
 and their relation to man is Geography. 
 
 In all such studies, while emphasis is laid on the analytical and 
 descriptive sides, the causal or dynamic side and the historical or 
 developmental side are given their due attention. This is expressed 
 by the preference of terms ending in ology (mineralogy, petrology, 
 geotectology, geomorphology) over those ending in graphy (petrog- 
 raphy, physical geography, etc.), which emphasize mainly the 
 descriptive and analytical side. 
 
 Dynamical Geology. -A second mode of approach is that which 
 lays the emphasis upon the forces that work upon and within the 
 earth to produce results, which, in this view, take a secondary rank. 
 This is Dynamical Geology and it naturally falls into the two great 
 
The Study of the Lithosphere 19 
 
 divisions, the physical and chemical. The sciences of physics and 
 chemistry, in so far as they lay the stress upon the forces at work, 
 are the specialized development of these aspects. That chemistry, 
 in its analytical as well as its dynamic aspects, is of fundamental 
 importance to the geologist has generally been recognized,- but the 
 importance of physics to the student of earth science has only re- 
 cently received proper recognition by the establishment of geo- 
 physical laboratories. 
 
 Both chemical and physical forces may be viewed as constructional, 
 or those building up rock masses and rock structures, and as de- 
 structional, or those destroying them. A third view of these forces 
 is that which deals with their effects in modifying or deforming 
 the materials and structures, and this may be termed the reconstruc- 
 tional or deformational aspect. 
 
 Historical Geology or Geogenesis. The third method of ap- 
 proach is the historical or evolutionary method, in which emphasis 
 is laid upon development and the causes which underlie this de- 
 velopment. It is apparent that this phase of geology is the latest 
 and most specialized aspect, and that for its proper prosecution a 
 thorough preparation in the other two phases is needed. More- 
 over, since the history of life upon the earth is intimately bound 
 up with the history of the lithosphere, a knowledge of biology, and 
 especially of palaeontology, is indispensable for the prosecution 
 of any but the most general studies in earth history. 
 
 Several special branches have been developed within this field. 
 One of these deals with the origin or genesis of the rocks and their 
 structures, or the origin of the lithosphere. To this branch the 
 name Lithogenesis is commonly applied. Another branch considers 
 the character, arrangement, succession, order of formation, and 
 age relations of the stratified rocks. This is the science of Stratig- 
 raphy, primarily a descriptive one. A third branch deals with the 
 succession and distribution of the organic remains, the petrifactions 
 or fossils in the rocks in so far as they have a bearing on the geolog- 
 ical history of the earth, i.e., the index fossils. This is the special 
 geological phase of Paleontology, which has also been designated 
 by the name Petrifactology (Haeckel). Again, there is the science 
 which deals with the development or genesis of the surface forms 
 of the earth not merely in the descriptive manner, but from the 
 view-point of origin. This, as already noted, is Geomorphology in 
 its proper sense, though it is more commonly known by the name 
 
2o Subdivisions of the Science of Geology 
 
 of Physiography, which formerly had a very different meaning. 
 Finally there is the study of the changes in the earth's surface, or 
 its geography through the successive ages of the earth's history, 
 - together with the changes in climate and the dispersions and 
 migrations of the organisms and the causes which effected these. 
 This is the latest of the several aspects of Historical Geology and 
 is now termed Palao geography or the geography of the past. In 
 this field the palaeogeography of the Pleistocene period has been 
 most extensively studied, and a separate branch, that' of glacial 
 geology, has been developed. Geography, in the usual sense of the 
 term, is the geography of the modern or Holocene period of the 
 earth's history. 
 
 II. Geology in its Relation to the Welfare of Man 
 
 So far we have been considering geology in its pure science aspect, 
 that which appeals to the inquiring mind of man in search after 
 truth and knowledge, without ulterior motives of usefulness. There 
 is, however, another aspect of our science, and one which in recent 
 times has come strongly to the front. This is applied geology, in 
 which geological facts and forces are viewed in their relation to 
 the needs and requirements of man. As has already been intimated, 
 the application of any science for any purpose whatsoever is success- 
 ful in direct proportion to the profundity of knowledge possessed 
 by the applier. No successful exploration of geological products or 
 application of forces is possible without a thorough understanding 
 of the facts and principles of the science, and the student who 
 wishes to follow the applied side of his science should not fail to 
 make his preparation in the pure science side as broad and as pro- 
 found as circumstances will permit. 
 
 Among the earliest problems, to the solution of which geological 
 knowledge has been applied, are those of mining. Indeed, the 
 science of geology, in a measure, developed in response to the needs 
 of the miner for accurate knowledge of the conditions of occurrence, 
 distribution, and mode of origin of the valuable mineral deposits. 
 To such an extent has this been carried, that a separate branch of 
 mining geology has come into existence. Moreover, as our knowledge 
 increased and the possibility of more detailed application of our 
 science became apparent, special subdivisions of mining geology 
 have been developed, and it is found that individuals can profitably 
 
The Study of the Lithosphere 21 
 
 devote themselves to the cultivation of a narrow field, to the prac- 
 tical exclusion of the others. Thus there have been developed the 
 branches of coal geology, of petroleum and gas geology, and of salt 
 geology, including the geology of potash, phosphates, nitrates, borax, 
 and other salts, and of bauxites and other aluminum ores. In 
 these branches the investigator confines himself to the problems 
 involved in the occurrence of these substances, which are chiefly re- 
 stricted to the stratified rocks. It is now well recognized that suc- 
 cessful search for such deposits can only be undertaken by one well 
 versed in the science of stratigraphy (including index fossils) and 
 structural geology, while a thorough understanding of the prin- 
 ciples of physiography and palaeogeography is almost indispensable. 
 The mining geologist who devotes himself primarily to the problems 
 of the metallic deposits must have not only a thorough knowledge 
 of mineralogy, petrology, and structural geology, but of dynamic 
 geology as well, and especially of the chemical and physical principles 
 involved in ore deposition. As many ores are also found in strat- 
 ified deposits, a knowledge of stratigraphy and of index fossils 
 is necessary, while an understanding of the principles of physiog- 
 raphy and palaeogeography will also be found of value in many 
 cases. 
 
 Geology, too, plays an indispensable role in the solution of many 
 important engineering problems. In the construction of the Cats- 
 kill aqueduct for New York City, a force of competent geologists 
 was constantly employed, and specialists were frequently called 
 upon for consultation. This same need was felt after the con- 
 struction of the Panama Canal had been undertaken, and a resident 
 geologist was appointed to supervise the later phases of construc- 
 tion. That many difficulties might have been avoided had such 
 supervision existed from the outset of the undertaking, is now 
 generally conceded. 
 
 Geological advice has always been employed in the construction 
 of great tunnels such as those piercing mountains or passing under 
 rivers and other water bodies. In many cases, too, the selection 
 of sites for bridges, dams, and other great engineering works has 
 been based on geological advice, while in other cases, where such 
 advice has not been sought or has been disregarded, disastrous 
 consequences have resulted. In consequence of the growing recog- 
 nition of the need of geological study in the undertaking of engi- 
 neering problems, the special branch of engineering geology has been 
 
22 Subdivisions of the Science of Geology 
 
 developed. The geological engineer must be primarily a struc- 
 tural geologist and one who has a thorough grasp of the princi- 
 ples of dynamic geology, including hydrology as well, while physi- 
 ography too is of great importance to him. To a lesser degree a 
 knowledge of rock types, of stratigraphic principles, and of index 
 fossils will be needed by him, and not infrequently a knowledge 
 of palaeogeography, especially that phase which deals with the 
 Pleistocene, or the problems of the glacial period, will be found of 
 the greatest value. 
 
 Finally there has been developed in recent years the special 
 branch of geology which deals with the problems involved in mili- 
 tary campaigns, and to this the name war or military geology has 
 been applied. Some of these problems are concerned with the 
 proper location of sites for camps, and trenches, and with water 
 supply and sanitation, and for these a knowledge of structural 
 geology, of rock types, of stratigraphy, and of glacial geology has 
 been found necessary. Other problems are of an engineering type 
 and require the preparation of the geological engineer. Again, the 
 problems involved in military operations need for their solution 
 a well-trained physiographer and a competent meteorologist as 
 well. Problems concerned with naval warfare require the atten- 
 tion of one well versed in hydrology, especially that phase of it 
 .which deals with the oceans, or oceanography. 
 
 There are other ways in which a knowledge of geology has be- 
 come useful to man, and as the science itself is developed new 
 channels of application into which it may be directed will no doubt 
 be discovered. 
 
 III. The History of Geology 
 
 The student should further realize that the development of his 
 science, the history of geologic thought, cannot be neglected by 
 him. We profit by the mistakes of our predecessors as much as 
 we do by their achievements, and the history of the discovery of 
 facts and of the development of geological opinion since the days 
 of the Greek philosophers is fraught with lessons equal in import 
 to those gained from the pursuit of the history of any other depart- 
 ment of human thought and endeavor. At this point it will be 
 desirable for the student to read the masterly sketch of this history 
 from the pen of Sir Archibald Geikie, the book entitled Found- 
 ers of Geology, and if possible follow this by a perusal of the older 
 
The Study of the Lithosphere 23 
 
 historical sketch by Sir Charles Lyell in volume I of his Principles 
 of Geology. For greater details the student is finally referred to 
 the History of Geology and Paleontology by Carl von Zittel, 
 translated into English by Maria Ogilvie Gordon. The history of 
 geology in America is adequately and fully treated by Dr. "George 
 P. Merrill in his book, Contributions to the History of American 
 Geology. 
 
CHAPTER III 
 
 METHODS OF APPROACH IN THE STUDY OF 
 THE EARTH 
 
 THE RISE OF GEOLOGICAL OBSERVATION AND INTERPRETATION 
 
 THE geologist is, above all things, an observer in the great out- 
 of-door world. The man whose horizon is bounded by the walls 
 
 of a city can never be a geologist, 
 though he may gain much scien- 
 tific knowledge from books and 
 from an inspection of collections 
 in museums and laboratories. 
 The true geologist, however, goes 
 directly to tlie earth and there 
 begins his inquiries. Not until 
 observations of natural facts and 
 phenomena were made in extenso 
 was the inquiry of the philoso- 
 phers regarding the earth and 
 its history placed on a scientific 
 basis. Scattered observations 
 and more or less accurate de- 
 ductions were made even in 
 antiquity. Thus Aristotle, in 
 the third century B.C., had a 
 
 very considerable understanding of the work of rivers and reasoned 
 correctly regarding the changes in the land and sea at former times. 
 The painter Leonardo da Vinci (1452-1519) correctly reasoned, from 
 an observation of the fossils found in the foothills of the Apennine 
 Mountains, that they were the shells of once living animals, though 
 they were generally regarded either as freaks of nature (lusus nature) 
 or as modern shells dropped by the pilgrims in their voyages across 
 these mountains.. It is true that the significance of fossil sea shells 
 was recognized by the Greek philosophers but their explanations 
 
 24 
 
 FIG. 6. Georges Leopold Chretien 
 Frederic Dagobert Cuvier. 
 
The Rise of Geological Observation 
 
 FIG. 7. Jean Baptiste Pierre Antoine 
 de Monet de Lamarck. 
 
 were generally ridiculed during the Middle Ages. It was, however, 
 not until the latter part of the eighteenth and the early part of the 
 nineteenth century that sys- 
 tematic investigations of the 
 rocks of the earth and their con- 
 tained fossils began, and from 
 this period we date the birth of 
 geology as a science. Scientific 
 geology arose more or less sim- 
 ultaneously in the different 
 countries of Europe. In France 
 Etienne Guettard (1715-1786) 
 and Nicholas Desmarest (1725- 
 1815) were among the first to 
 bring observation of facts to the 
 fore, while Buff on (1707-1788) 
 indulged in brilliant speculations 
 on the origin of the earth. Later 
 Alexander Brongniart (1770- 
 1847) investigated the rocks 
 
 around Paris, and Georges Cuvier (1769-1832, portrait, Fig. 6) and 
 the Chevalier de Lamarck (1744-1829, portrait, Fig. 7) described 
 
 their fossils. In Germany Abraham 
 Gottlob Werner (1750-1817, portrait, 
 Fig. 8), who is often called the founder 
 of German geology and who was Pro- 
 fessor at the Mining School at Frei- 
 berg i. S., exerted a profound influence 
 on geology especially by his teachings, 
 to which men flocked from all coun- 
 tries. Being mostly an observer of 
 the details of specimens and rarely 
 venturing beyond his immediate sur- 
 roundings for field observations, many 
 of his geological deductions have 
 proved erroneous though his pupils 
 and followers, notably Leopold von 
 Buch, extended their observations 
 
 over wide areas and added much to the store of geological facts 
 as well as to its philosophy. Switzerland had its enthusiastic 
 
 
 FIG. 8. Abraham Gottlob 
 Werner. 
 
26- 
 
 Methods of Approach 
 
 FIG. 9. James Hutton, M.D. 
 
 student of the structure and history of the Alps in the person of 
 H. B. de Saussure (1740-1799), and Russia had in Pierre Simon 
 
 Pallas (1741-1811) its careful 
 student of the Ural Mountains, 
 and the rocks outcropping there 
 and elsewhere in the empire. 
 In Great Britain many men con- 
 tributed to the discovery of facts 
 and the interpretation of these. 
 Among them the first rank is 
 given to James Hutton (1726- 
 1797, Fig. 9), whose work marks 
 a turning point in the history of 
 geology, for he insisted that 
 " the past history of the globe 
 must be explained by what can 
 be seen happening now, or to 
 have happened only recently," 1 
 a dictum which has since become the very cornerstone of geology. 
 Mutton's great work, Theory of the Earth with Proofs and Illustra- 
 tions, is better known through the classic volume, Illustrations of the 
 Huttonian Theory, by his friend John Playfair (1802, Fig. 10), 
 which no student of geology should 
 neglect to read. In this work are 
 contained many of the fundamental 
 principles with which geologists 
 are concerned to-day, and they are 
 illustrated by a wealth of facts 
 gleaned by Hutton from his ram- 
 bles through Scotland and other 
 countries. 
 
 Another of the Scottish founders 
 of geology was Sir James Hall, to 
 whom we owe the origination of 
 experimental geology. The best 
 known, however, among the early 
 English geologists was William 
 
 Smith (1769-1839, portrait, Fig. n), who is generally called the 
 " Father of English Geology." He determined not only the correct 
 
 1 Geikie, Founders of Geology, p. 299. 
 
 FIG. 10. John Playfair. 
 
The Rise of Geological Observation 
 
 27 
 
 FIG. ii. William Smith. 
 
 successions of the English rock formations, and made the first 
 geological map of England, but gave to many of the formations 
 the names which they bear 
 to-day. 
 
 Finally, the student should 
 remember among English geolo- 
 gists the name of Sir Charles 
 Lyell (1797-1875, portrait', Fig. 
 12), as that of a man who has 
 had the most profound influence 
 on geological thought. His 
 great work, the Principles of 
 Geology, has become a classic of 
 geological literature. 
 
 Among the men who exerted a 
 profound influence on American 
 geology in the early days of its 
 development, the names of William McClure (portrait, Fig. 13) 
 and Amos Eaton (portrait, Fig. 14) stand out prominently. Mc- 
 Clure, born in Scotland in 1763, became an American citizen near 
 the close of the century. In 1809 he published the first important 
 
 work on American Geology, in 
 which appeared the first geolog- 
 ical map of the Eastern United 
 States, and one of the first 
 geological maps of the country. 
 Amos Eaton (1776-1842, Fig. 
 14), born in New York state, is 
 known especially for his Index 
 to the Geology of the Northern 
 States (1818), which was the 
 first geological textbook pub- 
 lished in America, and in this 
 and subsequent works he laid 
 the foundation for the New 
 York geological system. Many 
 
 FIG. 12. - Sir Charles Lyell. of the names of American for- 
 
 mations still current were first 
 
 applied by him. He also made the first geological map of New 
 York. The important influence which McClure and Eaton had 
 
28 
 
 Methods of Approach 
 
 FIG. 13. William McClure. 
 
 on the development of American geology has been recognized by 
 the designation of the first two eras in the history of this science 
 
 in America as the Maclurean 
 (1785-1819) and the Eatonian 
 (1820-1829) (Merrill). Other 
 early American geologists will 
 be referred to in later chapters. 
 
 THE FIELD OF GEOLOGICAL 
 
 OBSERVATIONS 
 " Where, then," the student 
 will ask, "can the facts of 
 geology be observed, and how 
 have they become available ? " 
 To get at the facts of structural 
 geology the student must go 
 to the rocks. True, the rocks 
 and the minerals and the fossils are brought to him in museums 
 and laboratories, and he will dp well to begin his studies of selected 
 examples thus brought together and capable of being examined 
 under the most favorable conditions. But the knowledge thus 
 gained must be amplified and correlated by repeated visits to the 
 home of the rocks, where alone their larger relations and their 
 true significance in the history 
 of the earth can be ascertained. 
 
 The Field for the Study of 
 Rocks and Rock Structures 
 
 Rock Exposures in Flat 
 Countries. The dwellers in 
 the interior of our country, or 
 the traveler on the broad plains 
 of northern Germany, of Rus- 
 sia, of Hungary, or of China, 
 will find little opportunity to 
 get a view of the rocks which 
 underlie these regions, for an 
 almost continuous mantle of 
 soil and drift covers the solid rock. Only where rivers have cut 
 channels through the surface layer of loose material, the mantle- 
 
 FIG. 14. Amos Eaton. 
 
The Field of Geological Observations 29 
 
 rock, or where quarries have been opened, or construction opera- 
 tions have necessitated excavation down to and into the solid bed 
 rock, is there an opportunity for observation of the rocks beneath 
 the mantle-rock. River valleys and gorges are therefore the 
 favorite resorts of the geologist of the plains, while quarries, 
 railroad cuts, and other excavations which expose the rock, like- 
 wise receive his attention. Borings and drillings for oil, gas, or 
 water often prove of use to him, though generally, except where 
 the diamond drill is used, and the core preserved, the record of 
 such borings is only of doubtful and minor value. Salt shafts, 
 such as those in central New York and that near Detroit, Mich., 
 furnish excellent sections at the time of excavation. Fortunately 
 for the geologist of the plains, however, few regions of any great 
 extent are wholly covered by mantle-rock ; more commonly low- 
 lying ridges of rock, formed by resistant layers, are exposed above 
 the general surface of the soil and drift mantle. Such " ledges " 
 generally mark the edges of beds of hard rock which have a gentle 
 slope away from the edge of the ledge. When the ledge-forming 
 layer is very thick, a cliff of some height may result, and this 
 generally furnishes abundant opportunity for observation and 
 deduction. 
 
 All natural exposures of the rock are called outcrops, and the 
 outcropping ledges together with the exposures in the stream chan- 
 nels, especially those which cut across the cliffs, furnish to the geolo- 
 gist of the flat countries his most satisfactory data. We may note 
 a few examples. 
 
 Illustration from New York State. The state of New York 
 furnishes an illustration of the types of rock outcrop in ledges re- 
 ferred to in the preceding paragraphs. Over the greater part of 
 the state the rocks are so gently inclined that they appear horizontal 
 to the eye, and it is only when they are seen in the cliffs of Niagara 
 gorge and along Lake Erie that their gentle southward descent 
 becomes noticeable. Such a section, considerably generalized, is 
 shown in the following diagram (Fig. I5). 1 Where the beds end 
 in the air upon the north, the harder ones, such as limestones and 
 sandstones, form a series of low cliffs, while the ends of the softer 
 shale beds are usually marked by broad flat-bottomed valleys. The 
 
 1 The usual method of drawing sections is to place the north end upon the right, 
 but this is here reversed, because the observer along Lake Erie views the cliffs from 
 the west, and therefore south is on his right. 
 
Methods of Approach 
 
 C J 
 o O * 
 
 6 
 
 bol-a.3 p 
 
 Iflil 
 
 S S-oio 
 
 largest and deepest of these valleys is occupied by Lake Ontario, 
 while others are filled by soil which conceals the rock. These cliffs 
 or escarpments generally have an abrupt northern face across the 
 edge of the hard rock, and in these faces 
 & E fi i quarries are generally opened. 
 
 These cliffs can be traced with more 
 or less interruption across New York 
 state from the Niagara River and Lake 
 Erie (Fig. 16) to the Hudson, although 
 they become modified because some of 
 the beds seen in the western part of the 
 state die out, or change in character and 
 new ones appear (Fig. 17). This is 
 shown on the geological map of the state 
 of New York, where a series of broad 
 color bands extends east and west across 
 the state. Each color band in general 
 represents one rock layer or group of 
 layers, and the width of the color band 
 indicates the amount of exposure of each 
 E ^ which would appear were all the cover- 
 tJ* ^ ing soil and vegetation removed ; or in 
 .- other words, the amount by which each 
 | g lower bed projects beyond the next higher 
 S 2 one which covers it. 
 T5 of Wherever streams have cut across this 
 series of beds, gorges are formed, in the 
 walls of which the cut edges of the rocks, 
 the soft layers as well as the hard ones, 
 are shown. The most striking examples 
 of such gorges are shown .along the 
 Genesee River, which crosses the state 
 from south to north. In the section 
 from Rochester northward it cuts the 
 lower beds, the harder strata producing 
 waterfalls. In the section between Portage and Mt. Morris, it 
 cuts the higher strata, and here too several waterfalls are formed by 
 hard layers. Between these two points many smaller tributary 
 streams have cut into the sides of the valley and exposed 
 the rocks. Here, too, are situated several deep shafts which go 
 
 P" * 
 
 3 
 
 4) -*3 
 
 +* 
 
The Field of Geological Observations 31 
 
 down vertically to the Salina salt beds and during the cutting of 
 which the succession of the rocky beds was ascertained. 
 
 A good understanding of the succession of this series of rocky 
 formations is obtained by the traveler who passes from the Adi- 
 rondack Mountains southwestward across the state to Elmira, 
 especially if he take advantage of the various sections exposed 
 in the gorges of the streams and the banks of the Finger Lakes. 
 
 4 ** 
 
 FIG. 1 6. Cliff of Devonian rocks exposed on the shores of Lake Erie, south 
 of Buffalo. Typical of rock exposures shown on the lake shore from Buffalo 
 to Cleveland. (The section shows Hamilton (Wanakah) shales at the base, 
 the projecting Morse Creek limestone, above which lie the Windom shales and 
 higher Devonian shales.) 
 
 It was by the study of these natural outcrops of the state, sup- 
 plemented by those made on the sections exposed during the cutting 
 of the Erie Canal in 1817-1825, that the foundation of American 
 geology was laid by such men as Amos Eaton, and by James Hall 
 and others associated with him on the geological survey of New 
 York state. 
 
 Other Exposure of this Type. The type of outcrop just described 
 is found in most regions of flat-lying rocks in our own country as 
 
32 Methods of Approach 
 
 well as abroad. Thus the geologist who starts from Baraboo, Wis- 
 consin, and proceeds southeastward to Lake Michigan at Mil- 
 waukee, will cross such a series of rocky ridges separated by soil- 
 filled valleys, and a similar experience awaits the traveler across 
 Kansas from southeast to northwest, or the one who proceeds south- 
 ward across Oklahoma, or journeys from central Texas either north- 
 westward or southeastward. The traveler across England from 
 Liverpool to London also crosses such a series of rocky ridges, which 
 
 FIG. 17. View of West Hill, Schoharie, a terraced edge of the Helderberg 
 escarpment or cuesta, in eastern New York. Note the prominent cliffs formed 
 by the limestone members and the intermediate slopes formed by softer beds. 
 (Courtesy of N. Y. State Geological Survey.) 
 
 in general extend in a northeast-southwest direction, and are formed 
 by a succession of nearly horizontal rock-layers with westward 
 facing cliffs, though these are not generally visible from the train. 
 Similar outcrops of nearly horizontal layers of rock surround Paris 
 in constantly widening circles on three sides. The edges of some 
 of these rocks form cliffs or escarpments facing outward. If one 
 were to represent the rock formations which underlie Paris, and 
 which have a shallow basin-like structure, by a nest of plates, the 
 smallest at the top, the successive rims of these plates would repre- 
 sent the encircling cliffs, while the location of Paris would be in 
 the center of the uppermost and smallest plate. Many rivers have 
 cut channels through the edges of these rock plates, while others 
 
The Field of Geological Observations 33 
 
 flow in the depressions between successive rims. Thus numerous 
 and excellent exposures of the rocks are found, even though the 
 marginal portions between the edges of successive rock layers are 
 frequently covered by soil and vegetation. These numerous rock 
 exposures made possible the observations which gave the French 
 founders of our science the data on which to build their deductions, 
 and the cliffs which they form around Paris were of the utmost im- 
 portance in the conduct of the Great War so recently closed. 
 
 Outcrops in Mountainous Countries. It is, however, in the 
 elevated portions of the earth the mountains and chains of up- 
 lands that rock outcrops are most frequent on the surface of the 
 land. Here the soil and rock debris lodges only in the depressions, 
 while between these the ledges protrude and give opportunities 
 for observation. Here, too, deep canons are cut by the rivers and 
 glaciers and thus additional rock- walls are opened for observation. 
 German, Austrian, and Swiss geologists have for the most part 
 been limited to such regions for their observations, and the wonder- 
 ful rock exposures of the Alps and other mountains have made it 
 possible for them to carry their studies along certain lines to great 
 lengths. Werner, the father of German geology, made most of 
 his observations in the subdued * mountain district of Saxony, es- 
 pecially the Erzgebirge, which has long been famous for its old 
 and extensive mining operations. 
 
 Since France and Italy also border on the Alps, and have mountain 
 ranges of their own, the geologists of these countries were enabled 
 to avail themselves of the rocks and rock structures thus revealed. 
 British geologists, too, have been able to some extent to resort to 
 this type of exposure, though in these moisture-enveloped islands, 
 as over parts of Scandinavia as well, the dense though low cover of 
 vegetation and the peat accumulations obstruct much of the un- 
 derlying rock, as all students of Irish geology know only too well. 
 The Highlands of Scotland, however, furnish many good opportu- 
 nities for observation, as do also many of the higher English dis- 
 tricts and especially the mountain region of Wales. Swedish geolo- 
 gists have frequently had to resort to digging through the surface 
 layers to get at the underlying rock, and it is not an uncommon 
 sight to see the Swedish geologist in the southern interior 
 accompanied by a factotum, whose duty it is to wield pick and 
 shovel. 
 
 1 This term implies that the old mountains have been much worn down. 
 
34 Methods of Approach 
 
 In European Russia and adjoining districts there are vast areas 
 of flat-lying rocks covered by soil and drift, so that, except along 
 the coast and in the river channels, outcrops of the bedrock are 
 difficult to find. But where these rocks are uplifted in the Ural 
 Mountains on the east, the Caucasus on the south, and the Carpa- 
 thians on the west outcrops abound, and here the true relation- 
 ships of the rock formations may be ascertained. 
 
 In America, the New England uplands, the White, Green, and 
 Adirondack Mountains, and the Appalachian Chain furnish an 
 
 FIG. 1 8. Marsten Rock : View on the North Sea coast of England (Dur- 
 ham), showing characteristic erosion features in a detached mass of Magnesian 
 Limestone of Permian age, which was formerly united with the cliff on the left. 
 This is typical of the rocky character of much of the British coast, though the 
 kind of rock, the structures, and the erosion forms vary from point to point. 
 
 abundant series of rock outcrops for the eastern geologist. The 
 old and generally much-subdued mountain system, the rocks of 
 which may be traced by frequent outcrops from New York City 
 to the Highlands of the Hudson, and which can be followed south- 
 westward through New Jersey, Pennsylvania, and Maryland, and 
 indeed all the way down to the Carolinas, where it constitutes the 
 older Appalachian Chain, is a typical example of an elevated, though 
 for the most part not very mountainous, country, and here out- 
 crops abound. Many of our western mountains are especially well 
 adapted for geological observation, for here the aridity of the climate 
 prevents the growth of much vegetation, and the rock structures, 
 
The Field of Geological Observations 
 
 35 
 
 frequently developed on a gigantic scale, are visible for many miles. 
 This makes the Rocky Mountains a veritable paradise for the 
 American geologist. 
 
 Outcrops upon the Coast. Of all the natural rock exposures, 
 however, those of the coast-line are the most attractive, and in 
 many respects the most satisfying. Wherever the sea-coast or the 
 shore of a large lake is formed by rocks which rise above the sur- 
 face of the water, the cutting work of the waves keeps the exposure 
 fresh. A tramp along a rocky sea-coast is replete with interest to 
 the geologist, and many of the choicest bits of geological observa- 
 tion have been made on such sea-cliffs. Great Britain, with its 
 wonderful rocky sea-coast, probably leads the world in the variety 
 and significance of rocks and rock structures there exposed (Fig. 
 iS). 1 No student of geology 
 can afford to neglect the won- 
 derful English and Scottish 
 coast, which has furnished the 
 British geologists so many op- 
 portunities for the observation 
 of facts that there, more than 
 elsewhere, geological science 
 has advanced, since the days 
 of William Smith, with phe- 
 nomenal strides. This is 
 probably the reason why Eng- 
 lish geology quickly became 
 the standard of comparison 
 for other nations, in whose 
 home countries observation 
 was a more arduous task, be- 
 cause they did not include 
 such marvelous coast ex- 
 posures. 
 
 FIG. 19. Pulpit Rock, Nahant (Mass.) . 
 One of the most picturesque and in- 
 structive rock sections upon the Atlantic 
 coast of New England. The rocks are 
 metamorphosed Cambrian shales and 
 limestones with a great diabase sheet 
 (sill) intruded between the strata, and 
 the whole eroded by the waves working 
 chiefly upon the softer strata. (Photo 
 by A. W. G.) 
 
 Northern France, too, has a coast-line of great interest to the 
 geologist, and so has Norway. The coast-line of Germany, on the 
 other hand, is mostly sandy, and there is little diversity in the types 
 of the facts which it discloses. 
 
 The Atlantic coast-line of North America is for the most part a 
 sandy one. Only in New England, in the Canadian coastal prov- 
 
 1 See also Figs. 113, 114, 120, 121, 203, 498, 510, 511, 530-532, 719-721, 723 a, b. 
 
36 Methods of Approach 
 
 inces, and in Newfoundland can be seen coastal sections compa- 
 rable to some extent to those of Great Britain. The northern re- 
 gions, however, are accessible with difficulty, and have only recently 
 been investigated. But the New England coast, and especially that 
 of Massachusetts (Fig. 19), is a Mecca for American geologists, 
 and many of the workers in American geology have had their pre- 
 liminary training through a study of that interesting region. 
 
 Regions for the Study of Dynamic Geology 
 
 Although the principles of dynamic geology, the workings of the 
 chemical and physical forces, may be studied to much advantage 
 in the laboratory such study, too, is incomplete without recourse 
 to the outdoor field. It is upon the sea-shore that some of the pro- 
 foundest lessons of erosion by waves, of transportation by currents, 
 and of deposition in quieter waters can be learned. Here, too, 
 the method of entombment of fossils and the formation of many 
 original structures, such as ripple-marks and the like, can be ob- 
 served. Rocky as well as sandy and muddy shores should be visited. 
 Shores of large lakes may serve as a substitute in inland regions, 
 but lakes have in addition many characters of their own. Ponds 
 and temporary pools also teach their lessons. River valleys and 
 gorges, rapids and waterfalls, brooks, and even the roadside gutter, 
 furnish lessons in dynamic geology, as do also the hillside, the 
 mountain slopes, and the elevated peaks, where rocks are shattered 
 by frost, and decay under atmospheric influence. Glaciers present 
 many illustrations of dynamic geology, while caverns and under- 
 ground channels have special lessons to teach. The deserts and all 
 regions where wind is at work furnish illustrations of the mechan- 
 ical activities of the wind, while pools and salt pans in arid regions 
 furnish illustrations of chemical activities and of precipitation of 
 salts through condensation of the water under evaporation. Springs, 
 too, illustrate dynamical activities, both physical and chemical, 
 and artesian wells, oil wells, geysers, and similar phenomena are 
 replete with them. Finally, volcanoes and other such phenomena 
 furnish the means for the study of igneous activities. 
 
 The great English geologist, Sir Charles Lyell, whom we some- 
 times call the " Father of Modern Geology," has said that the 
 geologist must be primarily a traveler he must go to other lands 
 than his own and so widen the scope of his experience. Werner, 
 
Geological Literature 37 
 
 the founder of German geology, confined his observations mainly 
 to his limited Saxon district, and attempted to formulate from 
 these observations laws which should govern the rest of the world. 
 Naturally he fell into many and profound errors, so that to-day 
 scarcely one of his theories is held. Since his day German geologists 
 have, however, become great travelers, not only in their own but 
 in most other lands. As a -result, their observations have become of 
 wide scope, and they have added much to geological knowledge. 
 British and American geologists have only recently begun to 
 follow the advice of Lyell, but already their efforts have been 
 crowned with considerable success. 
 
 Let the student of geology, then, come to realize that the value 
 of his deductions increases in proportion to the range of his obser- 
 vations, and that no single country or region of the world will give 
 him all he needs. The American student, owing to the wide ex- 
 tent and diversity of his country, is perhaps more favored in this 
 respect than is the geologist of any other nationality, but at present 
 only a limited portion of our country has become sufficiently ac- 
 cessible to make extended observations possible. 
 
 THE IMPORTANCE OF GEOLOGICAL LITERATURE 
 
 Finally, it must not be overlooked that the observations of our 
 predecessors are recorded in the literature of the science, and that 
 here we find a mine of information, the value of which cannot be 
 overestimated. No one can repeat all of the observations which 
 have been made in the past, even were such repetition desirable. 
 In addition to the laboratory and field, then, the student of geology 
 must go to the library, and a thorough understanding of the liter- 
 ature on his special field is of fundamental importance to the worker. 
 Besides special books on different aspects of the science, the student 
 should gain familiarity in the use of the official publications issued 
 by the governments of the various countries, the proceedings of 
 scientific societies, and the special journals devoted to geology and 
 kindred sciences. 
 
CHAPTER IV 
 
 MATERIAL OF THE EARTH'S CRUST 
 
 THE material of which the crust of the earth consists is spoken 
 of as rock, a term which we shall presently define more precisely. 
 Rocks are in turn combinations of minerals or large aggregates of a 
 single material, and these are formed by the combination of chemical 
 elements, or by the union of those elementary combinations of 
 elements which are called ions. The study of chemical elements 
 and of their combination into ions and the union of these to form 
 other substances (salts, etc.), belongs in the domain of chemistry. 
 The study of minerals, their properties and occurrence, belongs 
 to the special branch of the earth science called mineralogy. An 
 elementary preparation in chemistry and mineralogy is necessary 
 to the student, and should be obtained by him if possible before 
 undertaking the study of geology. In this book we can give 
 only a brief summary of the more important elements and minerals 
 with which the student should have some acquaintance. The 
 important minerals which enter into the composition of the rocks, 
 or which themselves occur in rock-like masses, will be. dealt with 
 somewhat more fully in the discussion of these rocks. 
 
 THE CHEMICAL ELEMENTS AND THEIR PRIMARY COMBINATIONS 
 
 Of all the chemical elements which enter into the composition 
 of the earth's crust, only a comparatively small number are of 
 importance in combining to form the more common minerals and 
 rocks. The principal ones are given in the following list from 
 F. W. Clarke, in which their relative importance is also indicated. 
 
 Some of these elements occur pure in nature and are then called 
 native elements. Among these are oxygen, nitrogen, sulphur, 
 carbon, and the metals gold, silver, copper, platinum, etc. The 
 majority of elements, however, form combinations among them- 
 selves, with the result that more or less stable compounds are 
 
 produced. 
 
 38 
 
Chemical Elements and Their Combinations 39 
 
 The More Important Elements and Their Distribution 
 
 NAME OF ELEMENT 
 
 SYMBOL 
 
 LlTHOSPHERE 
 
 93 PER CENT 
 or WHOLE 
 
 HYDROSPHERE 
 7 PER CENT 
 OF WHOLE 
 
 AVERAGE 
 INCLUDING 
 ATMOSPHERE 
 
 Oxygen ... ... 
 Silicon 
 
 
 
 Si 
 
 47-33 
 
 27 74. 
 
 85-79 
 
 50.02 
 2 5 80 
 
 Aluminum 
 
 Al 
 
 7 8s 
 
 
 
 7 3O 
 
 Iron 
 
 Fe 
 
 4- SO 
 
 
 
 4. l8 
 
 Calcium 
 
 Ca 
 
 j An 
 
 OS 
 
 322 
 
 Magnesium 
 
 Mg 
 
 2 24. 
 
 14. 
 
 2 08 
 
 Sodium . . 
 
 Na 
 
 2 4.6 
 
 I 14. 
 
 2 "?6 
 
 Potassium 
 
 K 
 
 2 4.6 
 
 O4. 
 
 2 28 
 
 Hydrogen 
 Titanium 
 
 H 
 Ti 
 
 .22 
 4.6 
 
 10.67 
 
 95 
 
 A-l 
 
 Carbon 
 
 c 
 
 10 
 
 OO2 
 
 18 
 
 Chlorine 
 Bromine ... ... 
 
 Cl 
 Br 
 
 .06 
 
 2.07 
 OO8 
 
 .20 
 
 Phosphorus 
 Sulphur 
 
 P 
 
 s 
 
 .12 
 I 2 
 
 OQ 
 
 .11 
 1 1 
 
 Barium 
 Manganese . ... 
 
 Ba 
 Mn 
 
 .08 
 08 
 
 
 .08 
 08 
 
 Strontium ...... 
 Nitrogen 
 
 Sr 
 
 N 
 
 .02 
 
 
 
 .02 
 
 Q-l 
 
 Fluorine 
 
 Fl 
 
 IO 
 
 
 
 IO 
 
 All other Elements including 
 Gold, Silver, Platinum, 
 Arsenic, Copper, Lead, 
 Mercury, Nickel, Tin, 
 Zinc, Radium, etc. . . . 
 
 
 50 
 
 
 47 
 
 Total 
 
 
 TOO OO 
 
 IOO OO 
 
 IOO OO 
 
 
 
 
 
 
 Chemical Combinations 
 
 The following types of chemical combinations exist in nature 
 or are produced in the laboratory. 
 
 Oxides. Combination of an element with oxygen. Examples : 
 Silica (SiO2) ; Carbon dioxide (COg) ; Iron oxide (Fe 2 O3) ; Water 
 (H 2 0). 
 
 In the first of these, two parts of oxygen unite with one of silicon 
 to form silica or quartz ; in the second, two parts of oxygen in like 
 manner unite with one of carbon to form the gas carbon dioxide. 
 In the third example, three parts of oxygen unite with two of iron 
 to form the sesquioxide of iron ; and in the fourth example one part 
 of oxygen unites with two of hydrogen to form water. 
 
40 Material of the Earth's Crust 
 
 Hydroxides. These are combinations of an element (metal) 
 or a group of elements with oxygen and hydrogen, the last two 
 in equal parts. Examples: Sodium hydroxide or caustic soda 
 (NaOH) ; Potassium hydroxide or caustic potash (KOH) ; Alumi- 
 num hydroxide (the mineral Gibbsite, A1(OH) 3 ). In this last 
 example it requires three parts of the (OH) group to satisfy the 
 combining power of one part of aluminum. 
 
 Oxyhydroxides. Like the preceding, but with an additional 
 molecule of oxygen. 1 Examples: Aluminum oxyhydroxide, the 
 mineral diaspore (AIO(OH)) ; Iron oxyhydroxide or goethite 
 (FeO(OH)). Both hydroxides and oxyhydroxides may also be 
 expressed as combinations of oxides and water (H 2 O) ; thus : 
 
 (Gibbsite) 
 
 (Diaspore) 
 (Goethite) 
 
 The hydroxides and oxyhydroxides also form bases with which 
 acids combine to form salts. 
 
 Acids. These are combinations of certain elements such as 
 chlorine, carbon, sulphur, silicon, etc., which are called negative 
 elements, or their oxides (negative ions), with hydrogen or with 
 the oxyhydrogen (OH) combination or radical. Examples: 
 Hydrochloric acid HC1, (Hydrogen chloride + water) ; Carbonic 
 acid, H 2 CO 3 = CO+2(OH) ; Sulphuric acid, H 2 SO 4 = SO 2 +2(OH). 
 
 Salts. A compound formed by the reaction between an acid and 
 a base (hydroxide or oxyhydroxide) with the simultaneous forma- 
 tion of water, is called a salt. 
 
 Thus: 
 Na(OH) ] f HC1 ] [ NaCl ] [ H 2 O 
 
 Sodium + { Hydrochloric } = { Sodium [chloride } + { Water, 
 
 hydroxide J I acid J I or common salt J I 
 
 Ca(OH) 2 | f H 2 S0 4 1 f CaS0 4 ] f 2H 2 O 
 
 Calcium f + { Sulphuric ) = j Calcium | + { Water 
 hydroxide J I acid J ( sulphate J I 
 
 1 More correctly derived from the hydroxide by the abstraction of water, as shown 
 on comparison of the formulas of Diaspore and Gibbsite, the former having two mole- 
 cules of water less. 
 
 f A1(OH) 3 
 
 2 { Aluminum 
 [ hydroxide 
 
 1 i A1 2 O 3 | 
 
 i == i Aluminum ? 
 J 1 oxide J 
 
 f H 2 
 
 +3 \ Water 
 
 j AIO(OH) 
 
 1 f A1 2 3 ] 
 
 i H 2 O 
 
 2 \ Aluminum 
 I oxyhydroxide 
 
 } = { Aluminum } 
 J 1 oxide J 
 
 + i Water 
 
 FeO(OH) 
 
 1 } Fe 2 3 1 
 
 f H 2 
 
 Iron oxy- 
 
 Iron 
 
 + i Water 
 
 hydroxide 
 
 J 1 oxide J 
 
 I 
 
Chemical Elements and Their Combinations 41 
 
 New salts may also be formed by the reaction, in solution, of a 
 strong acid upon a salt with a weak acid, when the weaker acid is 
 set free. 
 Thus: 
 
 SrCl 2 ] f H 2 SO 4 ] f SrSO 4 1 f 2HC1 
 
 Strontium f + { Sulphuric } = { Strontium } + { Hydrochloric 
 chloride j [ acid J [ sulphate J [ acid 
 
 Or they may be formed by the interaction of two salts in solution 
 to form a less soluble salt. 
 Thus : . 
 
 BaCL, | I Na 2 SO 4 1 f BaSO 4 ] f 2NaCl 
 
 Barium f ~{~ Sodium f = \ Barium f ~^| Sodium 
 
 chloride J 1 sulphate J ( sulphate J I chloride 
 
 The barium sulphate is insoluble and will be precipitated out. 
 
 Ions. When certain chemical compounds such as acids, salts 
 and the bases, are dissolved in water, they are believed to be dis- 
 sociated into two or more parts which are either the elements or 
 their simple combinations, and which are called ions. They 
 exhibit a marked behavior towards the passage of an electric 
 current through the solution; some, regarded ?as charged with 
 positive electricity, being attracted by the negative electrode, and 
 others, regarded as negatively charged, being attracted to the 
 positive electrode. Examples are : 
 
 Acids 
 
 =H P ositive an d Cl . . . . negative ions 
 
 + 
 H 2 SO 4 =H 2 positive and SO 4 1 . . . negative ions 
 
 + 
 Base KOH = K positive and (OH) . . . negative ions 
 
 + 
 
 [ NaCl = Na positive and Cl . . . . negative ions 
 Salts + 
 
 I CaSO 4 = Ca positive and SO 4 . . . negative ions 
 
 1 In dibasic acids the dissociation takes place in two stages. In fairly concentrated 
 solutions sulphuric acid dissociates wholly or in part as a. monobasic acid. Thus: 
 
 I _ 
 
 H 2 S04 = H+HSO4. The second stage takes place when the solution is more dilute. 
 Thus: HSO4 = H+SO4. (Jones, H. C., The Nature of Solutions.) 
 
42 Material of the Earth's Crust 
 
 MINERALS 
 
 All native elements, oxides, hydroxides, oxy hydroxides, acids, 
 and salts which occur in nature in a solid state, are called min- 
 erals. They occur either in crystalline or uncrystalline (amor- 
 phous) form or both. Acids are rare as minerals, but native 
 elements and oxides are common, while hydroxides and oxyhy- 
 droxides are not infrequently met with. By far the larger number 
 of minerals, however, belong to the category of salts, among which 
 the dominant ones are the silicates formed by the combinations 
 of metals, etc., with silicic acid. The determination of minerals 
 depends upon the recognition of their physical characters as well 
 as of their chemical composition. There are many physical char- 
 acters of which the more important ones will be briefly summarized. 
 
 Crystalline Form 
 
 Most minerals assume definite forms in which certain planes 
 appear, which are found to have a definite relation to certain 
 imaginary lines or crystallographic axes (coordinate axes) about 
 which the crystal may be supposed to be built up. There are 
 six systems recognized, based on the relative length and relation- 
 ship of the axes. In the systems with three axes, these axes 
 may differ in length, when they are designated by the letters a, &, 
 and c, respectively, the c axis being the vertical one. If a and b 
 are equal both are designated by the letter a ; if all three are equal 
 they are all called a. The various faces of the crystal are read 
 with reference to the points at which they intersect the axis or 
 would do so if both were extended. 
 
 If, in a simple crystal of one set of planes, a plane intersects all 
 three axes (unit length), these axes being unequal, this plane is 
 given the symbol a:b:c (pyramid). If the two horizontal axes 
 are equal, it is designated a:a:c (tetragonal pyramid) ; if all 
 three axes are equal, it is designated a: a: a (octahedron) . If 
 the plane cuts two axes and is parallel to the third, this parallelism 
 is indicated by the infinity sign (<*>) and the formula becomes 
 a : b : <*> c (prism) ; a : a : o c (prism) or a : a : <*> a (dodecahedron) 
 as the case may be ; if it cuts only one axis and is parallel to the 
 other two the designation is a: aob: <&c or <xa:b: we (pina- 
 coids) ; oo a : o b : c (basal pinacoids) ; a : o a : o c (second order 
 
Minerals 43 
 
 prism) ; o a : <*> a : c (basal plane) or a : <*>a: <*> (cube) according 
 to the relative lengths of the axes. Planes cutting all three axes at 
 the unit length are called pyramid planes; those that cut the two 
 horizontal axes at the unit length and are parallel to the vertical 
 one, are called prism planes, while those that cut only one of the 
 horizontal axes, being parallel to the other and to the vertical one, 
 are called pinacoidal planes except in the case where the two horizon- 
 tal axes are of equal length (tetragonal system), when they are 
 called prism planes of the second order. Those which cut the c 
 axis and are parallel to the others are called basal pinacoids. 
 
 Finally, planes parallel to one horizontal axis and cutting the 
 other and the vertical one are called dome planes, except in the case 
 where the two horizontal axes are equal (tetragonal), when they 
 are called pyramid planes of the second order. Other planes may 
 occur which do not cut the axes at the unit length. These are 
 designated' by the coefficient m for the first variation from the 
 unit length and n for the second. Thus with three equal axes we 
 may have planes with the formula a: a: ma (trigonal trisocta- 
 hedron), or a: ma: ma (tetragonal trisoctahedron) ; or finally, 
 a:na: ma (hexoctahedron) . The system with four axes has the 
 three horizontal ones equal and at angles of 60 with one another, 
 while the vertical one is at right angles to the others. 
 
 The Six Systems of Crystallization 
 
 I. Isometric. Three axes of equal length or interchangeable 
 and at right angles to one another. Fundamental forms : cube 
 
 (a: oo a : QO a) ; octahedron (a : a : a) ; etc. (Fig. 20). 
 
 II. Tetragonal. Two horizontal axes equal and interchange- 
 able, the vertical one (c) of different length. All at right angles 
 to one another. Fundamental forms: tetragonal prism 1 
 (a :a: QO c) ; tetragonal pyramid (a : a:c); etc. (Fig. 21). 
 
 III. Hexagonal. Three equal horizontal or interchangeable 
 axes, forming angles of 60 degrees; a vertical axis of different 
 length at right angles to the horizontal ones. Fundamental forms : 
 hexagonal prism (a : a : <*>a: <*> c) ; hexagonal pyramid (a: a: 
 oo a: c) ; etc. (Fig. 22). 
 
 1 All the prisms require, of course, basal planes or pyramids to complete the solid. 
 
44 
 
 Material of the Earth's Crust 
 
 FIG. 20. Isometric System. Principal Forms. The general symbols and the 
 values of the coefficients for the figures given are added. (After Moses and 
 Parsons.) 
 
 Holohedral. (All planes developed.) 
 
 A. Octahedron. a: a: a. 
 
 B. Trigonal Trisoctahedron. a : a : ma 
 
 (-a). 
 
 C. Tetragonal Trisoctahedron or Trape- 
 
 zohedron. a : ma : ma. (m 2.) 
 
 D. Hexoctahedron. a:na: ma. 
 
 E. Dodecahedron. a: a: oo a. 
 
 F. Tetrahexahedron. a :na: <x>a.(n2.} 
 
 G. Cube or Hexahedron. a : oo a : co a. 
 
 HemihedraJ. 
 
 (In this division only every alternate plane 
 is developed, thus giving only half the num- 
 ber of planes found in the corresponding 
 holohedral form. This is indicated by pre- 
 fixing a / 2 to the symbol.) 
 
 H. Tetrahedron. %(a:a: a). 
 I. Deltohedron. \(a:a:ma). 
 J . Tr is tetrahedron . \ (a : ma : ma) . 
 K. Hextetrahedron. |(a : na : ma). 
 L. Pyritohedron. \(a:na: oo a). 
 
Minerals 
 
 45 
 
 .p 
 
 A 
 
 FIG. 21. Tetragonal System. Principal Forms. 
 (After Moses and Parsons.) 
 
 A. Tetragonal Pyramid, ist order. a : a : c. 1 
 
 B. Tetragonal Pyramid, 2d order. a : oo a : c. 1 
 
 C. Ditetragonal Pyramid. a : na:c. 1 
 
 D. Ditetragonal Prism. a:na :<& c. 
 
 E. Tetragonal Prism, id order. a:<x>a: ooc (with pyramid of ist order 
 (p) . na:na: me} . 
 
 F. Tetragonal Prism, ist order. a : a : oo c. 
 D, E, F, show Basal Pinacoids oo a : oo a : c. 
 
 1 When occurring in combination a unit length for c is selected and the formula be- 
 comes a: a: me. 
 
(<-->-. 
 
 : c } 
 
 
 I \ 
 
 V"- 
 
 4,4-- 
 
 
 , t 
 1 ; 
 
 
 ! i 
 
 ~* 
 
 
 *cl^ 
 
 i 
 
 FIG. 22. Hexagonal System. Principal Forms. 
 
 (After Moses and Parsons.) 
 Holohedral : (All faces developed.) 
 
 A. Hexagonal Pyramid, ist order. a : a :oo a : c. 1 
 
 B. Hexagonal Pyramid, zd order. a : na : na : c. 1 
 
 C. Dihexagonal Pyramid. a : na : pa : c. 1 
 
 D. Dihexagonal Prism. a : na : pa : <x>c. 
 
 E. Hexagonal Prism, 2d order. a : na : na : oo c. (n= 2.) 
 
 F. Hexagonal Prism, ist order. a : a : oo a : oo c. 
 D, E, F, show Basal Pinacoids. oo a : oo a : oo a : c. 
 
 Hemihedral. (Half the number of faces developed.) 
 
 G. Rhombohedron, ist order. ^(a : a : oo a : c). 1 
 H. Scalenohedron. ^(a : na: pa: c). 1 
 
 I. Trigonal Prism, ist order. \(a : a : oo a : oo c). 
 
 J. Ditrigonal Prism. ^(a : na : pa : oo c). 
 
 I and J show Basal Pinacoids. oo a : oo a : oo a : c. 
 
 1 me in combination ; p greater than n. 
 46 
 
Minerals 
 
 47 
 
 IV. Orthorhombic. Three axes, all of unequal length, but all 
 forming right angles with one another. Fundamental forms: 
 orthorhombic prism (a : b : oo c) ; orthorhombic pyramid (aibic); 
 etc. (Fig. 23). 
 
 <4s\. / N 
 
 FIG. 23. Orthorhombic System. Principal Forms in combination. 
 (After Moses and Parsons.) 
 
 A. Orthorhombic (unit) Pyramid (p.). a:b:c (or, na:b:mc). 
 Orthorhombic (unit) Prism (m). a:b:co c (or, na:b:<x>c). 
 Brachy-prism (/). na\b:<x> c. (n = 2.) 
 
 Brachy-dome (/). <x>a:b:c (oroo a : b : 2 c). 
 
 B. Same forms as in A with addition of Basal Pinacoid (c). ooa:oo6:c. 
 
 C. Same forms as in B with addition of two other pyramids (i). a : b : I c, 
 and (q). a :b : 2 c; and two macro-domes (h). a :oo b : f c, and (k). a:<xb: 
 2 c, and a macro-pinacoid (a). a :oo b : oo c. 
 
 V. Monoclinic. Three axes, all of unequal length, the hori- 
 zontal ones at right angles to each other, the vertical one (c) 
 
 J 
 
 FIG. 24. Monoclinic System. Principal Forms in combination. 
 (After Moses and Parsons.) 
 
 A. Monoclinic (unit) Prism (m). a:b:v$c (or, na:b:<x>c). 
 Hemi-pyramid (negative p, positive '). a:b:c (or, na : b : me). 
 Ortho Pinacoid (a). a : oo b : oo c. 
 
 Clino Pinacoid (b). oo a : b : oo c. 
 Basal Pinacoid (c). oo a : oo b : c. 
 
 B. Same planes as in A, except positive hemi-pyramid (v) and basal 
 pinacoids (c). 
 
 C. Same planes as in A except positive hemi-pyramid (v). 
 
 D. Unit prism; basal pinacoid; two positive hemi- pyramids v. and w. 
 (a : b : 3 c ) and a dino-dome z = (ooa:6: 2 c). 
 
Material of the Earth's Crust 
 
 inclined with reference to a, but forming a right angle with b. 
 Fundamental forms : monoclinic prism (a : b : o c) ; monoclinic 
 pyramid (a:b:c) (really 2 hemi-pyramids, a positive and a negative 
 one) ; etc. (Fig. 24). 
 
 VI. Triclinic. Three unequal axes all inclined with reference 
 to one another. Fundamental forms: triclinic prism (hemi- 
 prisms) (a :b: oo c) ; triclinic pyramid (a : b : c) (Fig. 25). 
 
 FIG. 25. Triclinic System. Principal Forms. (After Moses and Parsons.) 
 
 A. Triclinic Pyramid. a:b:c, consists of 4 sets of 2 parallel planes each. 
 
 B. Hemi-br achy-dome (e). oo a : b : c. 1 
 Macro-pinacoid (a). a : oo b : oo c. 
 Brachy-pinacoid (b). <x>a:b:oc. 
 Basal- pinacoid (c). oo a : oo b : c. 
 
 C. Hemi-maoro-dome (d). a : <x b : c. 1 
 pinacoids (c). 
 
 D. Triclinic Hemi-prism (m}. a : b : oo c. 
 Basal-pinacoid (c). 
 
 E. Combination of Macro- (a), Brachy- (b), and Basal- pinacoids (c). 
 
 Macro- (a), Brachy- (b), and Basal- 
 Macro- (a), Brachy- -(6), and 
 
 Other Physical Characters 
 
 Cleavage. The ability of a mineral to split along one or more 
 planes parallel to actual or possible crystal planes is called cleavage, 
 and is an important aid in identifying mineral species. 
 
 Fracture. The mode of breaking in directions other than those 
 of cleavage is the type of fracture of the mineral. It is conchoidal 
 (Fig. 41) when it has rounded surfaces suggestive of a shell ; even 
 
 1 me in combination. 
 
Minerals 49 
 
 or uneven, when nearly plain, or rough and irregular; hackly or 
 splintery, when it has ragged sharp points and depressions, or 
 separates in a fiber- or splinter-like manner. 
 
 Tenacity. A mineral is brittle when it breaks into powder ; 
 sectile, when small slices can be shaved off which crumble under a 
 hammer ; malleable when slices from it will flatten under a hammer ; 
 tough, when great resistance to tearing apart under strain or a 
 blow is shown ; ductile, when it can be drawn into wire. 
 
 Hardness. The resistance of a smooth plane, whether crystal, 
 cleavage, or fracture, to abrasion is called the hardness, and is 
 commonly determined by scratching the surface. It is expressed 
 in terms of a scale of ten common minerals (Mohs scale). Each 
 mineral will scratch all those softer than itself. 
 
 Scale of Hardness 
 
 1. Talc 6. Orthoclase 
 
 2. Gypsum (Selenite) 7. Quartz 
 
 3. Calcite 8. Topaz 
 
 4. Fluorite 9. Sapphire 
 
 5. Apatite 10. Diamond 
 
 Minerals below 2.5 in hardness can usually be scratched with a 
 finger nail ; those below 6 by a pocket knife. Any mineral above 
 5.5 will scratch window glass. By these simple tests hardness 
 can be determined approximately. 
 
 Luster. The brilliancy or shine of a mineral is called its luster. 
 It is dependent upon the refractive power, transparency, and 
 structure of the mineral. The following types are recognized : 
 
 a. Metallic : luster of metals, gold, silver, copper, etc. 
 
 b. Non-metallic luster comprising : 
 
 Vitreous the luster of a fractured surface of glass ; example, 
 quartz. 
 
 Adamantine the luster of uncut diamond, zircon, etc., due 
 to hj.gh index of refraction. 
 
 Resinous the luster of resin ; example, sphalerite. 
 
 Greasy the luster of oiled glass ; example, elaeolite. 
 
 Pearly the luster of mother of pearl ; example, foliated talc. 
 
 Silky the luster of silk ; example, satin spar. 
 
 Dull without luster or shine of any kind; examples, chalk, 
 kaolin. 
 
 The prefix sub- is used to express a lesser degree of the partic- 
 ular luster ; e.g. sub-metallic, sub-vitreous, etc. 
 
50 Material of the Earth's Crust 
 
 Color. This depends on chemical composition and is variable ; 
 or on physical constitution, when a variety of color-changes with 
 the changes in the direction of light is produced. These are: 
 Play of color (opal, labradorite) ; Iridescence, bands of prismatic 
 color; Tarnish, surface discoloration; Opalescence, milky or 
 pearly reflection; Asterism, showing a star by reflected or trans- 
 mitted light, as in ruby, etc., or in some micas. 
 
 Streak. The color of the fine powder of a mineral is its streak. 
 It is obtained by scratching the mineral or rubbing it upon a smooth, 
 white, and hard surface. (Arkansas stone ; streak stone.) 
 
 Translucency. The capacity for transmitting light is the 
 translucency of a mineral. A mineral is transparent when objects 
 can be seen through it with clearness ; translucent, when it transmits 
 light, but objects cannot be seen-; opaque when no light passes 
 through even the thin edges. Sub-transparent and sub-translucent 
 are also used. 
 
 Specific gravity. The weight of a substance divided by the 
 weight of an equal volume of distilled water (at 4 C.) is its specific 
 gravity. Exact determinations are made by fine balances, but 
 rough determinations can be made by weighing in the hand and 
 comparing with a mineral of equal size and known specific gravity. 
 
 Taste. Some minerals have a taste, such as astringent (alum) ; 
 salty (common salt) ; bitter (epsom salts) ; alkaline (soda) ; acid 
 (sassolite) ; cooling (niter) ; pungent (sal-ammoniac) , 
 
 Odor. On heating or burning, some minerals give off odors, of 
 which those of garlic (arsenic minerals), horseradish (selenium 
 minerals), or sulphur are examples. Fetid, bituminous, and 
 argillaceous (clay) odors also occur, the latter noticeable on breath- 
 ing upon the substance. 
 
 Feel. The response of a mineral to the sense of touch may 
 be smooth, soapy (talc), harsh, meager (aluminite), or cold, the latter 
 distinguishing gems from glass. 
 
 Other Characters. A few minerals are magnetic, and there is 
 great variation in transmission of heat-rays and of conductivity. 
 Various electric phenomena also exist. 
 
 Classification of Minerals 
 
 Minerals are classified on a chemical basis, and two distinct 
 methods have been employed which may in general be considered 
 as classifications; first, according to the acid radical (including 
 
Minerals 51 
 
 oxides, etc.) and second, according to the basic radical. In the 
 first system the minerals are divided into the following classes : l 
 
 1. Native elements. 
 
 2. Sulphides, Selenides, Tellurides, Arsenides, Antimonides. 
 
 3. Sulpho salts. 
 
 4. Chlorides, Bromides, Iodides, Fluorides. 
 
 5. Oxides (Hydroxides, Oxy hydroxides). 
 
 6. Carbonates. 
 
 7. Silicates. 
 
 8. Titano-Silicates, Titanates. 
 
 Q. Niobates or Columbates, Tantalates. 
 
 10. Phosphates, Arsenates, Vanadates, Antimonates. 
 
 11. Nitrates. 
 
 12. Borates. 
 
 13. Uranates. 
 
 14. Sulphates, Chromates, Tellurates. 
 
 15. Tungstates, Molybdates. 
 
 16. Oxalates, Mellitates. (Salts of organic acids.) 
 
 17. Hydrocarbon compounds. 
 
 Tables of Important Minerals 
 
 In the following tabular list, arranged essentially according to 
 the basic radical, the more important minerals are given, with a 
 brief characterization of their essential features. For more 
 details the student is referred to the textbooks cited below. 
 
 1. A. J. MOSES AND C. L. PARSONS. Elements of Mineralogy, Crystallog- 
 raphy, and Blowpipe Analysis. 5th edition, 1916. N. Y., D. Van Nostrand 
 Company. 
 
 2. DANA-FORD. Manual of Mineralogy, isth edition. John Wiley and 
 Sons, N. Y. 1912. 
 
 3. H. A. MIERS. Mineralogy. An introduction to the scientific study of 
 minerals. Macmillan and Co., London. 1902. 
 
 4. A. H. PHILLIPS. Mineralogy. The Macmillan Co., N. Y. 1912. 
 
 5. A. F. ROGERS. Introduction to the Study of Minerals. McGraw Hill 
 Book Co., N. Y. 1912. 
 
 6. DANA, EDWARD S. A Text Book of Mineralogy, etc. John Wiley and 
 Sons, N. Y. 
 
 7. J. D. DANA. The System of Mineralogy, Descriptive Mineralogy by 
 E. S. Dana. John Wiley and Sons, N. Y. 
 
 8. W. O. CROSBY. Tables for the Determination of Common Minerals, 
 Chiefly by their Physical Properties. Boston. Published by the Author. 
 
 1 Dana, J. D. and E. S. : The System of Mineralogy. 6th ed. 
 
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CHAPTER V 
 ROCKS, THEIR CLASSIFICATION AND PRINCIPAL TYPES 
 
 DEFINITIONS 
 
 A ROCK may be defined as a mineral mass or an association of 
 minerals, which in their natural occurrence form an essential 
 part of the earth's crust. This distinction is not a very precise 
 one, especially when the material of the rock consists of .only one 
 mineral. Thus the calcite in a vein would be considered a mineral, 
 while essentially the same material in a bed of marble would be 
 considered a rock. We shall see, however, as we proceed with the 
 discussion of the rocks, that in practice the distinction between 
 rock and mineral can as a rule be readily made. 
 
 AGE RELATIONS OF ROCKS 
 
 It will be useful at this early stage of our study to recognize the 
 fact that the rocks of the earth's crust are of various ages. Some, 
 like the rocks which make up the Adirondack Mountains, Pikes 
 Peak in Colorado, the Highlands of the Hudson, the Scottish High- 
 lands, the main mass of Finland, and a great part of central France, 
 etc., are very old; others, like those of the " puys " which are 
 scattered over the central French region, the basalts of the Columbia 
 and Snake River plateaus of the northwestern United States, the 
 rocks immediately underlying Paris, London, Vienna, and Berlin, 
 and the rocks of southern Florida are very young, though not all 
 of. the same age. It is possible to divide the history of the earth 
 into a number of periods and eras, just as human history can be 
 divided. But whereas the successive periods of human history are 
 measured by centuries at the most, those of the pre-human earth 
 history are measured by hundreds of thousands if not by millions 
 of years. And as we can refer the monuments and buildings of 
 human origin to their successive periods in human history, often 
 from the character of these monuments and buildings themselves, 
 
 64 
 
Bed-Rock and Mantle-Rock 65 
 
 so we can refer rrfost of the different rocks and rock structures of 
 the earth's crust to their respective geological periods, or to the 
 period of the earth's history when they came into existence as rocks. 
 It is desirable that the student should begin to familiarize himself 
 at this point with the names of the different periods of earth history 
 
 FIG. 26. Ledge of glaciated rock overlain by glacial drift, showing the 
 sharp contact line between the bed-rock and the mantle-rock. New York City. 
 (F. K. Morris, Photo.) 
 
 as given in the table in Chapter XXIV of this book. At a later 
 stage of his studies he will learn by what means it becomes possible 
 to refer rocks and rock structures to their proper period. When in 
 the succeeding pages we refer to the geological age of any rock mass 
 the student should consult the table until he has become familiar 
 with the succession of the periods. (See p. xviii.) 
 
 BED-ROCK AND MANTLE-ROCK 
 
 In general we may distinguish between the solid or bed-rock and 
 the unconsolidated rock or rock material, which latter is commonly 
 called the mantle-rock because it covers or mantles the bed-rock 
 which everywhere underlies it and projects through it as ledges. 
 The mantle-rock is of course much younger than the bed-rock 
 upon which it rests. In the northern United States and Canada 
 and the northwestern part of Europe, the mantle rock generally 
 rests abruptly upon the solid or bed-rock, the contact line between 
 the two being commonly sharp (Fig. 26). In many other por- 
 
66 Classification and Principal Types of Rocks 
 
 tions of the earth, however, there is a gradation between the two, 
 the mantle-rock becoming more stony downwards, and passing into 
 rotten rock and finally into fresh bed-rock. This relationship 
 will be discussed more fully in a subsequent chapter ; the present 
 one will orily take account of the solid rock which forms all except 
 the surface film of the crust of the earth. 
 
 CLASSIFICATIONS OF ROCKS 
 
 General Principles of Classification. Classification of natural 
 objects may be made either on a natural or on an artificial basis. 
 A natural classification is based on the origin and development 
 of the objects classified or on their genesis. Such a classification 
 is therefore called a genetic one, and it alone has a permanent scien- 
 tific value. It is true that a genetic classification can be made 
 only when the genesis or origin of the objects classified has been 
 determined, and that therefore the artificial systems of classifica- 
 tion will always precede the scientific or genetic one. The artificial 
 classification is generally based upon the possession in common, by 
 the objects classified, of a single character or, at most, a few char- 
 acters. Thus whales were formerly classified with fish because, 
 like these, they lived in the water and had a fish-like form. Their 
 true relationship is, however, more closely with elephants, both 
 being mammals. The eel-grass of our coast is not a grass, but 
 belongs to the family of water lilies, although its leaves are grass- 
 like. Rock-salt and sandstone have little in common, though 
 generally classified together as sedimentary rocks ; both may have 
 been deposited in lagoons near the seashore, and therefore in a 
 sense are sediments. 
 
 In general, it may be said that the progress of any science is 
 indicated by the replacement of the original artificial by the 
 natural or genetic classification. Thus as the study of plants 
 developed into the science of botany, the original artificial classi- 
 fication of plants, based on the number of stamens of the flower, 
 and on other superficial characters, became superseded by the 
 natural classification, which is based upon community of origin 
 of the members of the same group ; and in zoology, we find that 
 artificial classifications, based on superficial resemblances or the 
 possession of a common character, are constantly discarded as the 
 true genetic or natural relationships of animals are becoming more 
 fully understood. 
 
Classifications of Rocks 67 
 
 A Convenient Artificial Classification of Rocks 
 
 A convenient classification of rocks, which has come into general 
 use, recognizes three fundamental divisions, as follows : 
 
 1. Igneous Rocks. Rocks which result from the cooling of a 
 molten mass, or magma, either upon the surface of the earth, or 
 within the crust at greater or less depths. An example of the 
 first is basaltic lava ; of the second, granite. 
 
 2. Sedimentary Rocks. Rocks which were formed (a) as me- 
 chanical sediments in water or air, (b) as chemical precipitates and 
 evaporation products from solution in water, and (c) as deposits 
 formed by organisms either in water or air. Examples of these 
 are : (a) sandstone, shales, etc. ; (b) rock-salt, gypsum, cave 
 deposits (stalactites), etc. ; (c) coral and shell limestones, chalk, 
 guano beds, coal. The three groups here indicated are commonly 
 made subdivisions of the sedimentary rocks, and designated re- 
 spectively as follows: (a) mechanical sediments, (b) chemical 
 sediments, and (c) organic sediments. 
 
 3. Metamorphic Rocks.. These rocks were originally members 
 of one or the other of the preceeding divisions, but have become 
 sufficiently altered by natural agencies, such as pressure, heat, 
 and so forth, so that for the most part the original characters are 
 obliterated or lost, and new and special characters are added. 
 Examples of such metamorphic rocks are : (a) gneiss which 
 may be derived from an original granite, but may also have origi- 
 nated from another rock, even from a sediment ; (b) mica schist, 
 which may have been derived from a shale or an impure sandstone 
 or some other rock ; (c) marble, which is derived from some form of 
 limestone that may have been of organic, of chemical, or of me- 
 chanical origin; (d) graphite, which may be derived from coal 
 beds or from carbonaceous shales, etc. 
 
 Such a classification is sufficiently serviceable for all practical 
 purposes, but it cannot be called a scientific classification, because 
 in the second and third group are included types of very diverse 
 origin; for the three groups mentioned under sedimentary rocks 
 have little or nothing in common, except that they are sediments 
 in a very broad and indefinite comprehension of that term. More 
 correctly speaking, they are not igneous rocks, and not pro- 
 nouncedly metamorphic rocks, and this is almost their only claim 
 to a grouping under a separate division. The metamorphic group, 
 
68 Classification and Principal Types of Rocks 
 
 too, includes, as we have seen, rocks of very diverse origin, but 
 since it is not as a rule possible to determine, except perhaps after 
 prolonged and careful study and analysis, if then, what the char- 
 acter of the original rock was, it will probably always be necessary 
 to retain this group as a matter of convenience. 
 
 Principles of a Natural or Genetic Classification of Rocks 
 
 While the practical worker may find the preceding classification 
 sufficient for all needs, and while in the succeeding chapters we 
 may frequently refer to these convenient three types, the student 
 should nevertheless understand the principles of a natural classi- 
 fication, and so far as it is practicable we shall base our subsequent 
 discussions upon such a natural classification. For in geology 
 as in other sciences, logical thinking is of the first importance, 
 and logical thought is best fostered by the most rigid adherence to 
 exact methods of classification, and the only exact classification 
 of natural objects is one based upon genetic relationship, that is, 
 upon community of origin of the members of each group. 
 
 At the outset of our endeavor to understand the natural relations 
 of rocks we must clearly comprehend two fundamental principles. 
 The first of these is, that in this natural world of ours all things 
 are subject to continued change, even though that change may be a 
 very slow one, so slow that the years of a man's life or those of many 
 successive generations are of insufficient length to permit the 
 recognition of such changes. All rocks are constantly undergoing 
 modification, and though sometimes these changes may be rapid, 
 as in the coking of coal in a burning mine, or the change of clay to 
 brick in a kiln, the more usual method of change is a slow and grad- 
 ual one. In a certain sense all rocks may be considered as meta- 
 morphic or altered rocks, a view strongly held by some geologists. 
 Extreme metamor]5hism, such as produced the common types which 
 are usually spoken of as metamorphic rocks, is only a phase of the 
 general alteration or metamorphism which all rocks undergo, and 
 this phase is characterized by the greater or complete obliteration 
 of the characters which the rock possessed during its early history. 
 This more pronounced change may have been brought about by 
 the greater intensity of the activities responsible for it, or by their 
 longer continuance in time, or by both. 
 
 The second principle, a knowledge of which is fundamental to 
 our understanding of rocks as well as all other natural objects, 
 
The Unaltered or Little Altered Rocks 69 
 
 is that related types are not separated as a rule by sharp lines of 
 demarcation, but that always and everywhere in nature gradation 
 is the predominant rule. 
 
 Thus a sandstone may grade into a shale, on the one hand, and into a lime- 
 stone on the other, and shale and limestone may intergrade, though of course 
 there are always types which retain the characters of one or the other with 
 sufficient distinctness to make their classification possible. A granite may 
 grade into a syenite, and this into a diorite ; and again, a granite and a diorite 
 may approach each other so closely in character that classification becomes 
 difficult. In like manner a sandstone may have so many characters of a schist 
 that it becomes a difficult question to which of the two divisions it belongs. 
 A shale may grade into a slate and a granite into a gneiss. With slightly meta- 
 morphosed rocks, classification is often very difficult, though a thoroughly meta- 
 morphosed rock is easily recognized as such. 
 
 Extreme metamorphism is only a later or final phase of the 
 change which all rocks are undergoing, and in a natural classifica- 
 tion such end-products are to be placed with the group from which 
 they are derived. Two gneisses, for example, one of which is 
 derived from a granite and one from some type of sandstone, are 
 not related to each other any more than two human beings are 
 related because they speak the same language, or obey the same 
 laws of civilization. As before said, however, the practical diffi- 
 culty of determining the original character of a metamorphic 
 rock must be reckoned with, just as in the study of thoroughly 
 civilized human beings of different but entirely disguised nation- 
 alities, the practical difficulty of ascertaining the nationality of 
 each (assuming that the individuals refuse or are unable to disclose 
 it) must be taken into account. In this respect thoroughly meta- 
 morphosed rocks hold the same relation to their ancestral type, 
 as thoroughly Americanized individuals descended from different 
 nationalities hold to their ancestors. 
 
 It thus becomes necessary for us to study first the unaltered or 
 but slightly altered rocks, the history of which is ascertainable 
 from the characters which they retain. 
 
 THE UNALTERED OR LITTLE ALTERED ROCKS 
 
 When we study the unaltered or slightly altered rocks, we again 
 note that they fall into two quite readily recognizable divisions. 
 The first of these comprises rocks which have been produced from 
 material not originally rock, for example, by cooling from a lava, 
 
70 Classification and Principal Types of Rocks 
 
 crystallizing from a solution in water, and the like, a group 
 which in a broad sense may be spoken of as produced in a chemical 
 manner. The other division comprises rocks which are made up 
 of fragments or particles of other rocks, such as sandstone made of 
 grains of sand, a conglomerate made of pebbles of various kinds 
 and of sand, and others like these. This is the group of fragmental 
 or clastic rocks (Latin : clastus, broken) which are always made 
 from some preexisting rock that has been broken down into discrete 
 particles, which then are recombined either directly or after more 
 or less assorting. The first group, on the other hand, is that of the 
 non-fragmental or non-clastic type, which is not formed from frag- 
 ments of other rocks, but from non-rock material. In the regular 
 order of formation this type would appear first, though it is per- 
 fectly possible that non-fragmental rocks may originate from 
 material which itself was derived from other rock, by melting, 
 solution, or vaporization of such a rock and subsequent resolidifica- 
 tion. But in this case there is an intermediate non-rock stage, 
 while the material of the fragmental or clastic rocks is always rock, 
 though it may be broken into the finest particles. We shall con- 
 sider the characteristics of each group briefly. 
 
 The Non-Fragmental Rocks 
 
 (Endogenetic Rocks) 
 
 The non-fragmental rocks may be properly regarded as contri- 
 butions to the lithosphere from three of the other spheres ; namely, 
 the hydrosphere, the pyrosphere, and the atmosphere. This 
 contribution may be direct, as in the case of the hardening of a 
 lava from the pyrosphere, the separation out of salt from the water 
 of the hydrosphere, by the concentration of that water or by 
 chemical reactions, or the separation of snow or hail-stones by 
 the solidification of the water vapor from the atmosphere. Each 
 of these spheres may thus in a measure be credited with the genera- 
 tion of these respective rocks, and it is therefore possible to speak 
 of these three types, respectively, as : pyrogenic, generated by the 
 pyrosphere; hydrogenic, generated by the hydrosphere; and 
 atmogenic, or generated by the atmosphere. Instead of these 
 names we may of course speak of these deposits as of igneous, 
 aqueous, and atmospheric origin, respectively, recognizing the fact 
 that the rocks contributed by the atmosphere direct, that is, the 
 snow and hail, are of an evanescent character. 
 
The Unaltered or Little Altered Rocks 71 
 
 In addition to these three types, the direct contributions of 
 three of the inorganic spheres to the lithosphere, there is an 
 indirect contribution of material from two of these spheres by the 
 agency of organisms. Both animals and plants take lime from the 
 sea-water to build their shells and other calcareous structures, and 
 other organisms take silica from sea-water to build their hard parts. 
 Deposits of such organically secreted lime and silica are very 
 important in the construction of the lithosphere, and form, as a 
 rule, readily recognizable types of rocks. 
 
 Again, carbon is taken by plants from the carbon dioxide of the 
 atmosphere, and the plant tissues built up from it may often become 
 compacted into beds of coal which form important members of 
 the rock series of the lithosphere. Here again is an organically 
 formed rock, the material of which is obtained from the atmosphere. 
 
 Such organically formed rocks are the contribution of the bio- 
 sphere to the lithosphere, and in conformity with the method of 
 designation which implies such a relation to that sphere, they may 
 be called bio genie rocks. 
 
 We may then summarize in the following table these four funda- 
 mental types of non-clastic rocks, rocks which are produced essen- 
 tially by chemical (including physiological) reactions from non- 
 rock material. We term these rocks endo genetic because they 
 are formed by forces in a measure inherent in the material of the 
 spheres which produce them. 
 
 THE FOUR TYPES OF ENDOGENETIC (NON- CLASTIC) ROCKS 
 
 1. Pyrogenic or Igneous ; Produced by the pyrosphere. 
 
 2. Hydrogenic or Aqueous : Produced by the hydrosphere. 
 
 3. Atmogenic or Atmospheric : Produced direct by the atmosphere. 
 
 4. Biogenic or Organic : Produced by the biosphere from material taken 
 from the hydrosphere or atmosphere. 
 
 We may consider these rock types from another point of view. 
 Were we to imagine the rock material of the earth deprived of its 
 solid character, that which is essential to the constitution of a rock, 
 we should have to think of it as existing in one or more of three 
 possible forms. 
 
 i. It may be turned into a molten mass like lava, by the applica- 
 tion of heat, and so constitute an igneous magma, in which all the 
 minerals which go to the making of the rock would become as it 
 were dissolved in one another, and the compounds dissociated into 
 
72 Classification and Principal Types of Rocks 
 
 their ions. It would then become a part of the pyrosphere, and 
 in this state most of the rocks of the earth are believed to have 
 existed at an earlier period according to the believers in one hy- 
 pothesis of the earth's origin (see Chapter XXIX) when the earth 
 as a whole was a molten mass. Whether this theory is correct or 
 not, the fact that many of the rocks of the earth's crust were at 
 one time in this state cannot be doubted. By solidification igneous 
 or pyrogenic rocks are formed. 
 
 2. The rocks of the earth may be turned into a condition of 
 vapor by the application in most cases of still greater heat. 
 In such a state they would become a part of the earth's atmosphere. 
 The same hypothesis of earth-origin holds that this was a condition 
 of the entire rock mass of the earth at a still earlier period, and that 
 from this condition of vapor was separated at a later period the 
 molten material of the earth, and later still the water of the hydro- 
 sphere. Again, the truth of this hypothesis is not essential to the 
 recognition of the fact that some at least of the rocks of the earth 
 were formerly in a condition of vapor, and that they were separated 
 from the earth's atmosphere either by direct condensation, as we 
 see to-day in the formation of snow, or by the work of organisms, 
 such as has resulted in the separation of the carbon of the at- 
 mosphere, which now constitutes our coal beds, by the agency of 
 plants. Rocks derived by direct condensation from the atmosphere 
 are atmo genie rocks, while those separated by the activities (physio- 
 logical) of organisms, are biogenic or organic rocks. 
 
 3. Finally, we may think of the rocks of the earth, or at least 
 some of them, as dissolved in the universal fluid envelope of the 
 earth the water and so become a part of the hydrosphere. 
 If rock material, thus held in solution, is separated out from the 
 water by direct condensation, by chemical reactions, or by. elec- 
 trolytic action, aqueous or hydrogenic rocks are produced. If, how- 
 ever, the dissolved material is separated out by the agency of 
 organisms, as in the formation of limestone masses on coral reefs 
 by polyps and lime-secreting seaweeds (algae), it becomes an 
 organic or biogenic rock. 
 
 There are no other primary states than those of fusion, solution 
 in water, or vaporization, into which the rocks of the earth may be 
 changed, nor are there any other known ways in which rocks are 
 formed from the three states of primary dissociation except by 
 direct precipitation or separation or by organic agencies. Hence 
 
The Unaltered or Little Altered Rocks 
 
 73 
 
 the four rock types the igneous or pyrogenic, the aqueous or 
 hydrogenic, the atmogenic, and the organic or biogenic are the 
 
 FIG. 27. Diagram showing the interrelations of the Endogenetic Rocks. 
 
 only primary types recognized. These relationships are shown in 
 the preceding diagram. (Fig. 27.) 
 
 The Fragmented or Clastic Rocks 
 
 (Exogenetic Rocks) 
 
 These are the rocks which are made up of fragments of other 
 rocks, which may range in size from dust particles of microscopic 
 dimensions to boulders many feet in diameter. For their produc- 
 tion it is evident that preexisting rocks should be broken into frag- 
 ments, and that these fragments should become recemented or 
 bound together again in some manner. The methods by which 
 rocks are broken into fragments, and those by which the fragments 
 are recemented will be taken up later, but we may here use as 
 an illustration of this type of rock one of the artificial rock-making 
 processes carried on by man, and which differs primarily from the 
 natural process in the rapidity with which it is carried forward. 
 This is the process of manufacture of rubble concrete for paving 
 
74 Classification and Principal Types of Rocks 
 
 and other construction. Rocks like the trap of the Palisades, an 
 ancient igneous rock, are quarried and passed through the stone- 
 crusher, where they are reduced to rock rubble or particles of small 
 dimensions. They are assorted into different sizes by screening, 
 and bound together by a cement, which is a mixture of lime, alu- 
 mina, and silica, and of sand, which is the finer rock material obtained 
 either from the screening, or more commonly from natural sand 
 banks. The result is a clastic rock, but an artificial one, and this 
 is produced in a few days, whereas a similar rock produced in 
 nature might require as many centuries or millenniums for its pro- 
 duction. But trap rocks or other igneous rocks are not the only 
 ones used in the making of rubbl^ concrete, nor are igneous rocks 
 the only source of clastic material in nature, though they are often 
 the most common one. All rocks, igneous, aqueous, and organic, 
 and clastic rocks as well, are broken into fragments by natural 
 agencies, and from these fragments new clastic rocks are made. 
 For the rocks of the earth's crust are being constantly reworked, 
 all of them, whether clastic or non-clastic, furnishing material for 
 the formation of a younger bed of clastic rock. Let us take an 
 example for illustration from the eastern United States. 
 
 There is in the foothills of the Catskill Mountains a great deposit 
 of bedded clastic rocks known to the arts as Hudson River Blue 
 Stone, and used for the manufacture of flag-stones for sidewalks 
 in New York City and elsewhere, for curbings, and for many other 
 purposes. To the geologist this rock is known as a member of the 
 Middle and Upper Devonian series of rocks, of which we shall learn 
 more in the future. Examination under the microscope shows 
 that this rock is composed of small particles, some of which are 
 themselves small fragments of clastic rocks (these are called clas- 
 toliths) and this shows that this particular blue-stone rock is made 
 up of material derived in large part, if not wholly, from an older 
 clastic rock which was broken into fragments, assorted according 
 to size by natural agencies, and recemented to form new clastic 
 rocks, the " Blue Stone " being made up of the assorted finer par- 
 ticles only. From the character of the small fragments of clastic 
 rock (the clastoliths), and from a study of the structural and age 
 relations of the " Blue Stone " to the rocks of greater age, it has 
 been possible to determine that the material of the " Blue Stone " 
 was derived from the so-called Hudson River formation, which crops 
 out to the east of the Blue Stone area and forms in part the Taconic 
 
The Unaltered or Little Altered Rocks 75 
 
 Mountain range along the New York-Massachusetts boundary 
 line. This rock is of much greater age than the " Blue Stone," 
 belonging to the Ordovician period of the geological series (see 
 table, Chapter XXIV) . Similar examination of this Ordovician clas- 
 tic rock shows that it in turn was derived from a still older clastic 
 rock, this time of Pre-Cambrian age, some of which can be seen in 
 the ledges of Manhattan Island. This clastic rock finally was de- 
 rived from still older igneous and metamorphic rocks (Berkey). 
 
 A second example from the central region of our country may be 
 given. In Ohio, Michigan, and Western Ontario is a great bed 
 of very pure sandstone, a clastic rock made up almost entirely of 
 small, well-rounded grains of quartz sand bound together. This 
 rock is so pure that it is used for glass-making in Toledo, Ohio. It 
 is known by the name of the Sylvania sandstone, and it belongs 
 to the Silurian division of the geological series. Careful study 
 has shown that the grains were originally a part of a still older 
 sandstone, called the Saint Peter sandstone, which belongs to the 
 Ordovician division and covers a large area in the Mississippi 
 Valley region and east of that in Michigan, Canada, etc., though 
 its outcrops are found only in restricted areas. This rock was in 
 turn derived, at least in part, from the destruction of a still older 
 sandstone, the so-called Potsdam sandstone of Cambrian age, 
 which crops out farther to the north, in Wisconsin, Minnesota, etc. 
 This sandstone finally was derived from the still older granites, 
 gneisses, etc., which are seen in the Canadian region to the north. 
 Thus the Sylvania represents the third generation of sandstone, 
 the Saint Peter the second, and the Potsdam the first. 
 
 Finally we may note that the modern sands of the Libyan desert 
 are derived from the breaking up into its component grains of an 
 older sandstone, the Nubian, and of the rock from which the 
 sphinx has been cut (Fig. 28). If these sands become bound 
 together into a sandstone at some future time, this sandstone will 
 be of the second generation. 
 
 We see, therefore, that clastic rocks may be of different genera- 
 tions. The first generation is always derived from some igneous 
 or other non-clastic rock or the metamorphosed product of such 
 a rock. The later generations are derived successively from older 
 elastics, though new contributions from the crystalline source 
 may also be made. This formation of a new and younger clastic 
 rock from the material of an older one may be compared with the 
 
76 Classification and Principal Types of Rocks 
 
 construction, in more recent times, of man-made structures and 
 monuments from the stones obtained by the demolition of older 
 human monuments or structures. 
 
 The cycle of change which includes the successive generations of 
 a clastic series may be brought to an end by the melting of the 
 
 FIG. 28. The Sphinx, cut from a rock ledge and surrounded (and for- 
 merly partly buried) by desert sands derived largely from the destruction of a 
 sandstone in other parts of the Libyan desert. The Great Pyramid in the 
 background is covered by slabs of Nummulitic limestone. 
 
 elastics, and by their incorporation into a new igneous mass, 
 or by their pronounced metamorphism. Then a new cycle of 
 formation of clastic rock will begin. 
 
 The Agents Active in the Breaking Up or " Clastation " of Rocks 
 
 We may now consider the chief agents which are active in the 
 breaking up or clastation of rocks, that is, those responsible for 
 the formation of clastic material from which clastic rocks are 
 formed by consolidation. We may classify the several types of 
 clastic rocks according to the agent which produces the material, 
 or which arranges it in the form in which it becomes consolidated. 
 The methods of clastation will be considered in a later chapter. 
 
The Unaltered or Little Altered Rocks 77 
 
 The Atmosphere as a Rock Breaker. In the first place, rocks 
 are broken up or " clastated " by the atmosphere acting either 
 chemically, by the action of the various gases and vapors of the air 
 upon the rock, or mechanically, as in the case of freezing moisture, 
 or by the wind. Clastic material thus produced may be called 
 atmoclastic material, and rocks formed by the consolidation of such 
 clastic material may be called atmoclastic rocks. As clastic material 
 is often accumulated in certain localities after transportation by 
 the wind, i.e., the atmosphere in motion, and as this material has 
 generally a very definite form and structure, we may further dis- 
 tinguish wind-arranged or wind-deposited clastic material, such as 
 is found in sand-dunes. When this is consolidated into a rock it 
 becomes a wind-formed or eolian rock, a rock which may also be 
 called anemoclastic from the Greek anemos (ai/e/xos), the wind. 1 There 
 are many examples of such wind-formed, eolian, or anemoclastic 
 rocks to which we shall call attention again in a later chapter. 
 We may mention here as examples the recent dune-rock of the 
 Bermuda Islands, and the much older sandstones of the White 
 Cliffs in the Colorado Plateau region. The first of these was 
 formed in the modern or Holocene period, the second in the Jurassic 
 period, and this has retained its structure ever since that time, 
 though much of the original rock, which had a far wider distribution, 
 has been worn away again from large areas. 
 
 The Hydrosphere as a Rock Breaker. The second great 
 agent which accomplishes the destruction of rocks is the hydro- 
 sphere. This destruction, so far as it bears on our present point 
 of view, is mechanical in its nature, although chemical work, the 
 solution of rocks (such as limestone, salt, etc.) is also performed 
 by the hydrosphere. Such solution, however, results in the 
 reincorporation of the rock material into the material of the hydro- 
 sphere from which in turn it may be deposited as an aqueous or 
 hydrogenic rock (stalactites and other cave deposits). Such a 
 rock is, however, not a clastic but a " genie " rock, and the student 
 should learn to make the proper distinction between these two. 
 
 The mechanical work of water is manifested in streams, where 
 the current moves the rock fragments which it has broken from the 
 ledges, and grinds them down as it carries them along. It is also 
 seen on the seashore, where the waves produce an analogous effect. 
 
 1 We have this same root in the word Anemone, the name of the wind-flower, in 
 anemometer, the instrument for measuring the velocity of the wind, etc. 
 
78 Classification and Principal Types of Rocks 
 
 But in addition to the breaking off of the rock fragments and the 
 grinding of them to sand and pebbles, moving water in streams 
 and on the sea-coast assorts clastic material, however produced, 
 into grades of various sizes, and more or less according to the weight, 
 and therefore the nature, of the material. Moreover, clastic 
 material deposited by and in the water will generally be char- 
 
 FIG. 29. Gorge of the Genesee River below the Lower Falls at Portage, 
 N.Y., showing in the opposite banks, the cut edges of the stratified rocks (shales 
 and sandstones) which formerly were continuous across the gorge. 
 
 acterized by a bedded structure; that is, it willjexhibit, if seen in 
 section, a succession of layers or strata one above the other, each 
 one of which was at one time the topmost layer (Fig. 30). Such 
 deposits are called stratified, and of course, at any one time, only the 
 top of the topmost stratum is visible. Where a river, however, has 
 cut a gorge into an older stratified series, or where the waves on the 
 sea-coast or lake shore again partly wear away such a series which 
 has been lifted above the level of the sea by natural disturbances, 
 or rises above the lake-level, the cut edges of such strata can be 
 seen. Thus on the opposite banks of the Genesee River (Fig. 29) 
 we see the cut edges of the successive strata which were formerly 
 continuous across the chasm. On the sea-coast at Atlantic High- 
 lands, N.J., we likewise see the edges of the strata which were 
 exposed because the waves cut laterally into the old deposit which 
 
The Unaltered or Little Altered Rocks 
 
 79 
 
 had been previously uplifted ; and a similar exposure of successive 
 strata is seen on the shore of Lake Erie (Fig. 16, p. 31) and on 
 many other shores (Fig. 30). 
 
 In addition to the bedded structure called stratification, which 
 is characteristic of water-laid deposits of clastic material (and to 
 some extent of hydrogenic deposits also), there are other features 
 
 FIG. 30. Sea cliff of the south coast of Helgoland, showing the edges of 
 the stratified rocks of which the island is composed, exposed by wave cutting. 
 The strata are of Permian age and dip towards the north, but appear to be 
 horizontal in the sea cliff. (After E. Haase, from Walther.) 
 
 by which water-laid elastics may be recognized. These will be 
 more fully discussed in a later chapter. At present it is merely 
 desired that the student should recognize that water-laid deposits 
 have definite characters. Some of these may point to a deposition 
 of the material on a river flood-plain, an alluvial fan, or a delta; 
 some to deposition in lakes, ponds, or playa basins, and some to 
 deposition in the sea. These last may be regarded as the most 
 typical examples of water-laid elastics, and they are generally 
 characterized by the inclusion of the shells and other remains of 
 marine organisms. A sandstone or mud-rock with marine fossils 
 (Fig. 31), such as may be obtained from numerous localities the 
 world over, will serve as a typical example of a water-laid clastic. 
 In accordance with the general method of naming such deposits, 
 those under consideration may be termed hydrodastic. They 
 
8o Classification and Principal Types of Rocks 
 
 may also be called aqueous elastics or sediments, in distinction from 
 aqueous precipitates or concentrates, which belong to the group 
 of hydrogenic deposits. 
 
 The Pyrosphere as a Rock Breaker. As we have seen, the 
 pyrosphere contributes pyrogenic or igneous material, which on 
 
 FIG. 31. Photograph of a slab of sandstone filled with marine fossils. (Oris- 
 kany sandstone, N. Y.) 
 
 cooling and hardening forms a part of the solid crust of the earth. 
 It also breaks up material already rock, and this is generally 
 ^__ brought about by explosive activities such as 
 
 ^A\t^^ j are characteristic of most volcanoes. By these 
 
 fijf&*> 3M1 activities the rock, whether an older lava or a 
 e'fcwt^B^ roc k f ther origin, located in and around the 
 crater,' may be shattered and the material 
 thrown high into the air, to descend as a shower 
 of ashes or larger particles. Such material, 
 which is readily recognized by its character, is 
 called pyroclastic material, and when bound to- 
 gether to form a rock, this becomes a pyro- 
 
 FIG. 32. Fault- clastic rock. Examples: volcanic tuff, volcanic 
 breccia. 
 
 agglomerate, etc. 
 
 Shattering of Rocks within the Lithosphere by Movements. 
 When one part of the lithosphere moves over or against another 
 
The Unaltered or Little Altered Roeks 
 
 81 
 
 part, as happens when the earth's crust suffers disturbances (see 
 Chapter XXI) the rock along the plane of movement is shattered 
 or ground fine, and clastic material with very definite character- 
 istics is produced. As such a movement of adjoining rock masses 
 generally produces a displacement, or fault (see Chapter XIX), the 
 shattered or ground-up material is commonly called a fault-breccia 
 (Fig. 32). A more general term for such material is autoclastic, 
 because it is produced by the self-destruction or breaking up of the 
 rocks of the earth's crust. 
 
 The Biosphere as a Rock Breaker. That growing plants, 
 such as trees, arising from seeds which lodged in a fissure of the 
 rock, can by expansive growth shatter that rock, has been fre- 
 quently observed (Fig. 33). Animals, too, break rocks. Thus 
 where a spring issues 
 upon a level surface in 
 a more or less arid 
 country, vast herds of 
 hoofed animals will con- 
 gregate to drink, and 
 their constant stamping 
 of the rock surface will 
 break it and produce 
 dust and sand which 
 mingles with the water 
 and which on drying 
 may be carried away 
 by the wind. Thus 
 hollows are produced 
 in the surface of the 
 land, and these may 
 be filled with water 
 and so produce ponds. 
 Many such are known to exist in western North America, in 
 north and central Africa, and elsewhere. . Fish and other animals 
 in the sea will break off branches of coral from reefs (biogenic rock) 
 and grind these to powder to obtain for nourishment the animal 
 matter which surrounds these coral masses. Thus much coral 
 sand and mud is produced, and this is clastic rock material of 
 strictly organic origin. 
 
 But by far the greatest destroyer of rock is man. We have seen 
 
 FIG. 33. A huge gravestone, broken and 
 displaced by the growth of the roots of a birch- 
 tree. Hannover. (After Walther.) 
 
82 Classification and Principal Types of Rocks 
 
 how rock is broken down to be used for the making of artificial 
 rock, that is mart-made rock (rubble-concrete, etc.). As man is a 
 part of the organic world or biosphere, his work must be classed 
 with that of other organisms. Clastic material thus produced 
 by organisms, and the rocks, whether natural or artificial (i.e., 
 man-made) , made from these, may therefore be called bioclastic. 
 
 Summary of Clastic Rocks 
 
 We have thus five main groups or classes of clastic rocks, each 
 produced by one of the spheres and named after it. (See the dia- 
 gram, Fig. 34.) In general the rock is regarded as belonging to 
 
 FIG. 34. Diagram of the interrelations of the Exogenetic Rocks. 
 
 that particular class, the agent of which has placed -upon it its 
 characterization stamp. Thus the material of a water-laid rock 
 may have been originally produced in another way than by aqueous 
 erosion it may have been produced by weathering, after which 
 it was sorted by and deposited in the water. Thus water has 
 stamped it as undeniably an aqueous clastic or hydroclastic rock. 
 If it is possible, as it sometimes is, to determine how the material 
 
. The Unaltered or Little Altered Rocks 83 
 
 originated before it was deposited by and in water, this may serve 
 to further characterize the rock as of a special division in the hydro- 
 clastic group. The types of clastic rocks then are : 
 
 1. Atmoclastic rocks. Rocks produced by atmospheric destruc- 
 tion of rocks, and consolidated without rearrangement. Example : 
 consolidated laterites. 
 
 id. Anemodastic rocks. Eolian rocks. Material of variable 
 origin, transported, assorted, and deposited by wind. Example: 
 eolian sandstone. 
 
 2. Hydrodastic rocks. Rocks produced from material eroded 
 or sorted by, and deposited by and in water, whether by rivers 
 (fluviatile) in lakes (lacustrine) or in the sea (marine). Examples: 
 fossiliferous marine sandstones or shales. 
 
 3. Pyrodastic rocks. Rocks formed from material which has 
 resulted from shattering of older rocks by volcanic explosions. 
 Examples : volcanic tuff, volcanic agglomerate. 
 
 4. Autodastic rocks. Rocks formed of material shattered or 
 ground by movements within the earth's crust. Example : fault- 
 breccia. 
 
 5. Biodastic rocks. Rocks produced from material broken or 
 arranged by animals (more rarely by plants) and by man. Includes 
 artificial rocks. Examples : some consolidated muds from coral 
 reefs, rubble-concrete, etc. 
 
 In the succeeding chapters we will consider first the commoner 
 types of rocks, in which the original characters are retained, and 
 their structures, leaving the metamorphic derivatives for a future 
 chapter. 
 
CHAPTER VI 
 
 THE PRINCIPAL TYPES OF IGNEOUS OR PYROGENIC 
 
 ROCKS 
 
 THE IGNEOUS MAGMA 
 
 THE term molten magma is applied to molten rock material or 
 igneous fluid, which is formed or exists within that part of the 
 earth which we have called the pyrosphere, and which, it will be 
 remembered, interpenetrates the lithosphere and the asthenosphere 
 (tectosphere). When the magma reaches the surface of the earth, 
 either in a volcanic eruption, or through large fissures in the earth's 
 crust, as in parts of Iceland to-day, it is called a fluid lava, from 
 which by solidification in cooling a solid lava or lava-rock is pro- 
 duced. Such a rock mass constitutes an extrusive or e/usive 
 igneous rock mass. When the magma does not reach the surface, 
 but cools within the crust of the earth, into which it has risen or 
 become intruded to a greater or less extent, either upon or by the 
 formation of fissures, or otherwise, it forms an intrusive igneous 
 mass. Finally, a magma 'may be conceived as cooling essentially 
 where it was formed, and to produce a deep-seated or abyssal igneous 
 rock mass. 
 
 Outcrops of Igneous Rock Formed by Solidification from Magmas 
 
 It is obvious that only the surface-flows or lavas will be accessible 
 to man immediately after cooling, and of these generally only the 
 superficial portion. All the other igneous rock masses are located 
 within the earth's crust, and buried beneath the surface rock masses, 
 sometimes to very great depths. Some of those which closely 
 approach the surface may be reached by deep borings or by mining 
 operations, but this can occur only in exceptional cases. Some of 
 these may also be conceived of as brought into view on the face 
 of a great dislocation-block of the earth's crust, where one side 
 of the broken crust is lifted and the other depressed. This too, 
 
 84 
 
The Igneous Magma 85 
 
 however, is probably so exceptional that it may be considered, 
 in the absence of other modifications, to be practically negligible. 
 In general, it is only after prolonged erosion, which results in the 
 removal of much of the surface-covering of the rock, that intrusive 
 masses become exposed, while deep-seated masses may require 
 the removal of many thousands of feet of covering rock before they 
 become visible. This removal of great covering rock masses 
 requires, of course, long periods of time, and it thus becomes 
 evident that the igneous rock masses, other than surface lavas, 
 which are now visible in outcrops, are of great age, and that their 
 formation by cooling has taken place at remote periods of time. 
 Igneous intrusions and deep-seated masses which are now being 
 formed are entirely invisible to us, and will not be exposed until 
 many ages have passed by, if ever. Nevertheless there are indirect 
 ways in which we can infer the existence of igneous masses beneath 
 the surface, which are now undergoing the process of solidification, 
 and to some of these we shall refer later. 
 
 Characters and Composition of Igneous Magmas 
 
 It is not possible to gain a complete knowledge of the composition 
 of an igneous magma from the composition of the igneous rock 
 which has resulted by solidification from that magma, because 
 the magma contains in addition to the substances which make 
 up the solid rock formed from it, large quantities of volatile gases 
 which are expelled upon cooling and relief of pressure. The volatile 
 gases which are expelled under these conditions are, water vapor, 
 carbon dioxide, hydrochloric acid, sulphurous vapors, and the like. 
 Such expulsion of volatile matter is shown by all surface lavas 
 from which clouds of steam arise, and such steam on analysis proves 
 to carry with it many of the other gaseous emanations. Others 
 quickly condense upon the surface and may form salts of various 
 kinds, which either encrust the surface of the cooling lava, 
 or are carried away in solution by the condensing waters from the 
 water vapors. Sometimes the expulsion of gas and vapors is so 
 rapid that violent explosions result, and this is indeed the chief 
 cause in the production of pyroclastic material. In practically 
 every volcanic eruption there is produced a cloud of vapor and 
 gases which carries upward vast masses of finely divided rock 
 material the product of the explosive action and often rises 
 
86 Principal Types of Igneous or Pyrogenic Rocks 
 
 to heights bf many miles above the volcano (Fig. 35). When 
 gases and vapors alone issue from a fissure in the earth, the 
 phenomenon is spoken of as a fumarole. Fumaroles are commonly 
 
 associated with de- 
 clining volcanic ac- 
 tivities. The gases 
 and waters of cool- 
 ing magmas within 
 the earth's crust may 
 escape through fis- 
 sures not penetrated 
 by the magma or 
 opened since cooling 
 began, and these 
 
 FIG. 35. Volcano in eruption, showing the cloud 
 of steam and ashes projected high into the air. 
 
 may reach the sur- 
 face as mineral 
 springs, either hot or cold. In their upward passage, such waters 
 and gases may deposit mineral matter which they carried, and this 
 is believed by many to account for vein and other mineral deposits. 
 The non- volatile material of the magma which solidifies to form 
 the lava or other igneous rocks consists predominantly of only a 
 small number of substances, chief among which is silica (SiO^), 
 which, however, is present in variable amounts, according to the 
 nature of the magma. In addition to this there are the oxides of 
 the metals aluminum (Al), iron (Fe), magnesium (Mg), calcium 
 (Ca), sodium (Na), and potassium (K). These, too, vary in 
 amounts in the different magmas. On solidification the silica 
 unites with them to form various silicate minerals, which in the 
 coarser-grained igneous rocks can be readily distinguished. 
 
 In a general way there can be recognized a gradation in the com- 
 position of the magmas from a point where silica is most abundant, 
 sometimes forming 75 per cent of the whole mass, with alumina 
 and potassium next, by an increase in sodium and later in calcium, 
 magnesium, and iron, and a reduction in silica to a point where this 
 constitutes 50 per cent or less (rarely 35 per cent) of the entire mass. 
 With this occurs a reduction in the oxides of potassium, sodium, 
 and aluminum, sometimes to the complete or nearly complete 
 elimination of some of these. The end high in silica, etc., is called 
 the acid end of the series ; the other end is the basic end. Rocks 
 formed from the acid portion of an igneous magma generally have 
 
Formation of Igneous Rocks by Cooling of Magma 87 
 
 light-colored and light-weight minerals predominating; those 
 formed from the basic portion have mainly dark-colored and 
 heavy minerals. Acid magmas (and lavas) are generally very 
 stiff or viscous even at high temperatures (2000 C. or over), and 
 their contained gases escape with difficulty, and often with explo- 
 sive violence, as the magma approaches the surface. Basic mag- 
 mas (and lavas), on the other hand, are more fluid even at much 
 lower temperatures (1300 C.), and on this account the gases 
 escape more readily, and explosions are less common. The lavas 
 of Vesuvius and Mont Pelee are examples of the first, those of 
 Kilauea in Hawaii, of the second. 
 
 FORMATION OF IGNEOUS ROCKS BY COOLING OF MAGMA 
 
 With the cooling of the non- volatile part of the magma, solidifi- 
 cation takes place and igneous rocks are produced. The kind of 
 rock will of course vary with the variation in the composition of 
 the magma, which must be considered to be the character of first 
 importance. Next to this is the rate of cooling, which determines 
 the grain or texture of the rock, and which in turn is influenced 
 by the relative position of the magma while cooling. 
 
 Influence of Rate of Cooling. Texture 
 
 It is a well-ascertained fact that a rapidly coolirig magma will 
 tend to produce a mass of glass, and that in proportion as the rate 
 is slower will there be opportunity for the growth of mineral 
 crystals, the size of which is, in general, proportional to the slow- 
 ness of the cooling. When the 
 entire rock becomes a mass of min- 
 eral crystals, it is said to be holo- 
 crystalline. Between this and the 
 glassy type in which there are no 
 crystals, there are all gradations. 
 The relative size and arrangement 
 of the component crystals of a 
 rock form its texture. FlG ^ _ Poiphyry . typkal por . 
 
 Primary Textures. Two gen- phyritic texture, 
 
 eral types of primary texture may 
 
 be distinguished, the homogeneous or uniform, and the hetero- 
 geneous or porphyritic. In the first all the crystals of each mineral 
 
88 Principal Types of Igneous or Pyrogenic Rocks 
 
 are essentially of uniform size, though the size % varies with the 
 mineral. In the second, or porphyritic type (Fig. 36), on the other 
 hand, the crystals of certain of the essential minerals (generally 
 the feldspars) are of two sizes, one large and more or less fully and 
 perfectly formed, the other small and less perfect. The larger 
 
 crystals, which are scattered through 
 the mass, are called the phenocrysts ; 
 the finer-grained mass, together with 
 other minerals, also in small grains, 
 forms the ground-mass. 
 
 Secondary Textures. The tex- 
 tures of the ground-mass and of the 
 homogeneous textured rocks are 
 called secondary. When the grains 
 or crystals are all nearly uniform, 
 FIG. 37. Granitic texture the texture is said to be granitic or 
 (holocrystalline), slightly granular (Fig. 37), and it may be 
 
 coarsely or finely granular, i.e., the 
 
 texture is coarse-grained or fine-grained so long as the individual 
 crystals are distinguishable. But when the crystals are no longer 
 distinguishable, the texture becomes dense or felsitic, while a step 
 further brings us to the glassy texture in which no crystals are 
 developed. 
 
 Relation of Textures to Kinds of Magmas 
 
 In general, the basic magmas, being more fluid, will tend to form 
 coarser crystals, the resulting rocks therefore being more com- 
 monly coarse-grained. Glassy textures are correspondingly less 
 frequently developed. Acid magmas, on the other hand, tend to 
 develop the finer textures more frequently, and here glassy rocks 
 are common. 
 
 The presence of much gas and water vapor, too, tends to increase 
 the power of crystal forming, and along certain fissures, presumably 
 the pathway of escape of these gases and vapors, the texture is 
 often of exceeding coarseness. The most common examples are 
 the dikes or other masses of pegmatite, a rock composed sometimes 
 of huge crystals of feldspar, quartz, and mica, the latter mineral 
 being obtained in large plates from this rock, which is the chief 
 commercial source of this mineral. 
 
Formation of Igneous Rocks by Cooling of Magma 89 
 
 Relation of Texture to Place of Cooling and Bulk of Magma 
 
 As the coarseness of grain is in proportion to the slowness of 
 cooling, it is evident that, other things being equal, whatever 
 lowers the rate of cooling will cause an increase in the size of the 
 crystals, and of the coarseness of texture. Thus a magma cooling 
 within the earth's crust, especially at great depth, will of necessity 
 cool slowly, and so become a coarsely crystalline rock. Again, 
 the central part of the magma will cool more slowly than its outer 
 part, which is in contact with the cooler 
 enclosing rock. In like manner small masses 
 will cool more rapidly than large ones, and 
 this is especially the case when the igneous 
 mass is a thin sheet transecting other and 
 cooler rocks (dikes (Fig. 38), sills, etc., see 
 beyond). There is, however, one modifica- 
 tion of this general rule, which should be 
 noted, and that is the conditions under which 
 porphyri tic rocks appear. In these the fine FIG. 38. Small 
 
 ground-mass indicates rapid cooling, but the dike of basalt > show - 
 . , , , - mg dense texture, cut- 
 
 presence of the large phenocrysts shows that ting syenite of finely 
 
 these crystals grew to their full size before granular texture, 
 the ground-mass solidified. In other words, 
 the phenocrysts were floating crystals in a still semi-fluid matrix. 
 Typical porphyries (Fig. 36) (with fine-grained ground-mass) are 
 most common in certain lava flows (as in the typical trachyte 
 from Drachenfels, Germany), somewhat less so in the smaller in- 
 trusive masses, and still less common, and indeed rather rare, in 
 the great subterranean masses. All types of igneous rocks may, 
 however, show a porphyritic texture. 
 
 Classification of Igneous Rocks 
 
 From what has been said up to this point, it appears that the 
 principles which underlie the classification of igneous rock are 
 relatively simple. The first general division is made upon a chem- 
 ical basis, which in the rocks is expressed by the mineral species 
 present. Further subdivisions are based upon the texture of the 
 rock, for the same magma may furnish rocks of different textures 
 as the result of cooling at different rates under different conditions. 
 
go Principal Types of Igneous or Pyrogenic Rocks 
 
 Since, as above stated, the character of the magma is most readily 
 ascertained from the minerals which crystallize from it, chief 
 attention is ordinarily given to these minerals. Furthermore, 
 since these are readily recognizable only in the coarse-grained or 
 holocrystalline products of cooling of the magmas, it is customary 
 to designate the rock groups based on chemical and mineralogical 
 characters by the name of the typical holocrystalline member of 
 each group. Before we consider these types, however, we must 
 briefly review the essential minerals which enter into the construc- 
 tion of these rocks. 
 
 The Essential Minerals of Igneous Rocks 
 
 We may in general distinguish three groups of essential minerals 
 which make up the bulk of the igneous rocks. These are the 
 following, arranged in each case in the order from acidic to basic. 
 They are: (i) Quartz, (2) The Feldspars and Feldspathoids, and 
 (3) the Micas and Ferro-Magnesian Silicates. 
 
 Quartz (SiO 2 ). This generally occurs in glassy fragments of 
 irregular and usually of angular form, more rarely in crystals. 
 It is easily recognized by its hardness (see pp. 49 and 61). 
 
 The Feldspars. In composition these are all double silicates 
 of aluminum and an alkali metal (K,Na, etc.) or an alkaline earth 
 (Ca,Mg, etc.) or both. In general the feldspars are divided into 
 orthoclase 1 or the feldspar with right-angled cleavage, and plagiodase 
 or the feldspar with oblique cleavages. The most acid feldspar con- 
 tains only potassium in addition to both alumina and silicic acid 
 (potash feldspar or orthoclase), the most basic only lime in place of 
 the potash (lime feldspar or anorthite). Between the two stands 
 the soda feldspar (albite) where sodium is the metallic base besides 
 aluminum. Between the pure soda and the pure lime feldspars are 
 a number of intermediate types, consisting of different proportions 
 of both soda and lime ; and between the pure potash and soda feld- 
 spars there are also mixtures of the two. 
 
 These relations may be expressed in the following table, where the 
 three main or (theoretically) pure types of feldspars are each 
 expressed by a molecular symbol and the mixed types by formulae 
 which indicate the proportions of each. 
 
 1 Microline is a potash feldspar like orthoclase, but crystallizing in the triclinic 
 system, and therefore of the plagiodase type. 
 
Formation of Igneous Rocks by Cooling of Magma 91 
 
 Table of the Feldspars 
 
 
 
 PRINCIPAL OR 
 PURE TYPES 
 
 MIXED TYPES 
 
 COMPOSITION 
 (Chemical Formula) 
 
 COMPOSITION 
 (Molecular Symbol 
 or Formula) 
 
 si 
 
 
 Orthodase 
 (Potash 
 Feldspar) 
 
 
 K 2 O A1 2 3 6 SiOa 1 
 
 or | 
 KAlSi 3 8 
 
 Or 
 
 
 
 
 Anorthoc'.ase 
 
 
 to 
 OrjAbi.6 
 
 
 .2 
 .H 
 
 Albite 
 (Soda Feldspar) 
 
 
 Na 2 A1 2 3 6S;0 2 
 or 
 NaAlSi 3 8 
 
 Ab 
 
 to 
 
 (AbgAru) 
 
 
 
 
 Oligoclase 
 
 
 AbAni 
 to 
 Ab 2 Ani 
 
 'LAGIOCLASES 
 (Triclinic) 
 
 
 
 (Andesine) 
 
 
 Ab 3 An 2 
 to 
 Ab 4 An 3 
 
 
 
 
 Labradorite 
 
 
 to 
 Ab:An 2 
 
 
 .U 
 
 1 
 
 
 Bytownite 
 
 
 AbiAn 3 
 to 
 AbiAn 
 
 
 
 Anorthite 
 (Lime Feldspar) 
 
 
 CaO A1 2 O 3 2 SiO 2 
 or 
 CaAl 2 Si 2 O 8 
 
 An 
 
 to 
 
 (AnsAbx) 
 
 In ordinary rock determination it is very difficult to recognize 
 the intermediate feldspars, though this may be done with more or 
 less precision when a thin slice of the rock is placed under the micro- 
 scope, and examined by the us"e of polarized light. From chemical 
 analysis, however, which gives the amount of each substance pres- 
 ent, it is possible to calculate the types of feldspar (and other min- 
 erals) present in the rock, a calculation which must, of course, be 
 checked by microscopic examination. The plagioclase feldspars 
 usually show fine parallel (twinning) striae on some faces. 
 
92 Principal Types of Igneous or Pyrogenic Rocks 
 
 In general, it may be said that orthoclase, albite, and oligoclase 
 are characteristic of acidic rocks, or rocks high in silica, with ortho- 
 clase in the most acidic, while labradorite, bytownite, and anorthite 
 characterize the basic rocks, with the last at the most basic end 
 of the series. 
 
 The Feldspathoids. Under this designation are placed double 
 silicates of aluminum and the alkalies or alkaline earths, which 
 have many characters in common with the feldspars besides the 
 general similarity in their composition, but differ in crystal form 
 and some other characters. Theoretically we again have three types, 
 the potash, soda, and lime feldspathoids, but actually the first two 
 are more commonly intermixtures. This is shown in the following 
 table. In the order of their importance as rock constituents, these 
 minerals are nephelite, leucite, and melilite, the last being very 
 rare. 
 
 Table of the Feldspathoids 
 
 MINERAL 
 
 COMPOSITION 
 (pure) 
 
 USUAL MODIFICATION 
 
 Leucite 
 
 K 2 O A1 2 3 4 Si0 2 
 KALSi 2 O 6 
 
 Some Na 2 O replaces 
 part of K 2 O 
 
 Nephelite 
 
 4 Na 2 O 4 A1 2 O 3 9 SiO 2 
 or (almost) 
 NaAlSi0 4 
 
 Some K 2 O and CaO 
 replaces part of Na^O 
 
 Melilite 
 
 i2CaO-2 Al 2 O 3 -gSiO 2 
 or 
 
 Ca 3 Al(Si0 4 ) 3 
 
 
 In general, the occurrence of leucite in igneous rock implies a 
 magma rich in potash ; nephelite, one rich in alumina and soda ; 
 and melilite, one poor in silica and alumina but rich in lime. 
 Melilite occurs mainly in a few rare basalts. 
 
 Common Mica and the Ferromagnesian Minerals. The 
 name ferromagnesian silicates is given to the minerals (usually dark 
 in color) which include in their composition both iron and magne- 
 sium. They comprise the dark mica, biotite, the hornblendes, pyrox- 
 enes, and olivine. The common or white mica (muscovite) has neither 
 iron nor magnesium, but is a silicate of potassium and aluminum 
 
Formation of Igneous Rocks by Cooling of Magma 93 
 
 together with some hydrogen. Its usual formula is K(OH) 
 A1 2 O 3 2SiO 2 , or HKAl 2 Si 2 O8 and it is commonly called the potash 
 mica. As might be inferred, it occurs in rocks where other potash 
 minerals are common, such as granites, pegmatites, etc. It is also 
 common in metamorphic schists. 
 
 In the ferromagnesian silicates we trace a gradation in com- 
 position from biotite, or black mica, in which potash is present, 
 through the amphiboles and .pyroxenes, in which the potash is re- 
 placed by lime, to olivine, where no alkali or alkaline earth is pres- 
 ent. This is shown in the following table where muscovite is also 
 placed at the acidic end and magnetite, the pure iron oxide, at the 
 basic end. 
 
 Table of the Ferromagnesian Minerals and Their Two End Members 
 
 MINERALS 
 
 COMPOSITION 
 
 USUAL MODIFICATION 
 
 Muscovite 
 
 H 2 KAl 3 (SiO 4 )3 
 to 
 
 (HK)Al 2 (Si0 4 ) 2 
 
 Without iron or magne- 
 sium. 
 
 Biotite 
 
 (HK) 2 (MgFe) 2 Al 2 (Si0 4 ) 3 
 (nearly) 
 
 Addition of magnesium 
 and iron; reduction of 
 alumina and silica. 
 
 Amphibole 
 (Hornblende) 
 
 Ca(MgFe) 3 (Si0 3 ) 4 
 (in general) 
 
 Substitution of calcium 
 for hydrogen, potassium, 
 and aluminum, though 
 some of the last may be 
 present. 
 
 Pyroxene 
 
 (Augite) 
 
 Ca(MgFe)(Si0 3 ) 4 
 (in general) 
 
 Olivine 
 
 (MgFe) 2 Si0 4 
 
 Omission of calcium, some 
 reduction of silica. 
 
 Magnetite 
 
 Fe 3 4 
 
 Omission of silica and mag- 
 nesium. 
 
 It must be understood that the amphiboles and pyroxenes are 
 more complex in composition than here stated, and that there are 
 a number of varieties of each, differing in composition. It is, how- 
 ever, not necessary that these be considered here. The common 
 
94 Principal Types of Igneous or Pyrogenic Rocks 
 
 amphiboles and pyroxenes crystallize in the monoclinic system, but 
 there are also orthorhombic members of each group. The chief 
 means of distinction is the (prismatic) cleavage form, that of the 
 pyroxenes being nearly at right angles, and that of the amphiboles 
 forming angles of nearly 120 and 60 degrees, respectively, as shown 
 
 Amphibole 
 
 FIG. 39. Basal sections of crystals of Pyroxene and Amphibole, showing 
 characteristic differences in outline, and cleavage as seen under the microscope. 
 A characteristic interference figure is shown in Pyroxene. (After Moses and 
 Parsons.) 
 
 in the above outlines (Fig. 39). In general, it may be said that 
 whereas muscovite occurs chiefly in the most acidic rocks, biotite 
 and hornblende indicate greater basicity, pyroxenes still greater 
 basicity, while olivine is characteristic only of the very basic igneous 
 rocks. When pyroxene and olivine are present, free quartz is 
 usually absent. 
 
 Secondary or Accessory Minerals. There are many minerals 
 which occur in small quantities in igneous rocks but are not neces- 
 sary constituents of them. They are called accessory minerals, 
 and their presence is most frequently detected by the microscope. 
 Zircon and jtitanite are good examples of these. By alteration 
 many other accessory minerals are formed from the primary or 
 original ones. 
 
 Order of Crystallization 
 
 In general the order of crystallization of the minerals from an 
 igneous magma follows the order of their basicity, the most basic 
 separating out first, the most acid last. Thus olivine will form in 
 perfect crystals from a basaltic magma, the other minerals being 
 less perfectly crystallized. The pyroxenes and the hornblendes 
 separate out before the feldspathoids and the feldspars, while 
 
Types of Igneous Rocks 95 
 
 the quartz, if there is any free silica remaining, separates out last, 
 filling the interstices between the other minerals. For that reason 
 quartz is practically never in perfect crystal form in granites or 
 other igneous rocks which contain it, but forms irregular grains, 
 the shape of which is determined by the form of the cavities left 
 between the other minerals. It has a crystalline structure, how- 
 ever, though not a crystal form. 
 
 TYPES OF IGNEOUS ROCKS BASED ON COMPOSITION 
 AND TEXTURE 
 
 The table on page 96 summarizes the more important types of 
 igneous rocks, beginning with the most acidic on the left, and ex- 
 tending to the basic types on the right. It will be seen that there 
 are certain composition groups based upon the kind of feldspar (or 
 feldspathoid) present, the kind of ferromagnesian mineral, and the 
 presence or absence of free quartz or olivine. Each of these com- 
 position groups includes a series of rocks ranging in texture from 
 coarse (at the bottom) through fine and dense or f elsitic to glassy (at 
 the top). The coarser-grained rocks are generally formed as deep- 
 seated masses, which have become exposed by erosion. The in- 
 termediate types occur mainly as intrusive masses, while the dense 
 and glassy types (including the cellular) are primarily formed from 
 surface flows. As the mineral character of the several groups is 
 most readily ascertained in the case of the coarse-grained varieties, 
 the various groups will be considered under these respective types. 
 Microscopic examination is generally necessary to ascertain the 
 characters of the finer-grained and dense types, though typical 
 examples may generally be recognized without such aid. 
 
 The Granite-Rhyolite Series 
 
 This contains the most acidic of the common igneous rocks. 
 Common types are : granite, rhyolite, quartz-felsite, obsidian, and 
 pumice, and the porphyritic varieties of these. 
 
 Granite. The coarse-grained members consist primarily of 
 orthoclase feldspar and free quartz. When these alone are present, 
 as is sometimes the case, the rock is called a binary granite. Gen- 
 erally, however, black mica (biotite) and often muscovite and some 
 hornblende are present, these being readily recognized by their 
 dark color, while the biotite is distinguished from the hornblende 
 
96 Principal Types of Igneous or Pyrogenic Rocks 
 
 i 
 
 
 
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 I 
 
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 11 
 
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 M 
 
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 61 
 
 PH& 
 
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 K 
 
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 tt 
 
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 ir 
 
 
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 ; 
 
 Andesite pumice 
 Andesite perlite 
 Andesite obsidia 
 
 Chief Feldspar- 
 Acid Plagioclase 
 
 Botite (or) ( 
 Hornblend 
 metimes with 
 
 
 
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 ti-'g 
 
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 lite or Leucite 
 
 Phonolit 
 felsite 
 
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 eplite- 
 syenite 
 
 t 
 
 :lib b 
 
 Granite 
 
 rphyrit 
 granite 
 
 SSVK 
 
 
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 D, 
 
 o.y 
 
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 DIJIS13J 
 HO 3SN3Q 
 
 Dig 
 
 HVIflNVHQ A13NIJ 
 
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 A13SHVOO 
 
Types of Igneous Rocks 
 
 97 
 
 
 by its scaliness and softness. The orthoclase is commonly recog- 
 nized by its pinkish or flesh color and by the smooth cleavage planes 
 which it shows, as well as the more or less marked crystal form. 
 The quartz is always in irregular, glassy masses, breaks with rough 
 fracture and, as a result, looks darker than the feldspar by reflected 
 light. 
 
 Granites vary by the replacement of some of the orthoclase by 
 acid plagioclase while hornblende may increase in amount, forming 
 a hornblendic granite. With this may go a reduction in the 
 quantity of free quartz present, when the rock approaches a 
 syenite in composition, while with the increase in plagioclase the 
 quartz-diorites or diorites are approached. In rare cases, too, 
 augite may occur, showing an approach to gabbro. Among the 
 common accessory minerals are magnetite, zircon, and garnet. A 
 porphyritic texture is sometimes developed by the formation of 
 large and more or less perfect crystals of orthoclase, which fre- 
 quently show a twinned character (Carlsbad twins), recognizable 
 by the fact that one half of the 
 crystal appears darker than the 
 other in reflected light, the shade 
 being reversed with change in the 
 position of the light. 
 
 Pegmatite. This is the name 
 given to a coarse variety of a 
 granite which occurs in vein or 
 dike-like masses, and generally 
 consists of large crystals of ortho- 
 clase (sometimes acid plagioclase) 
 occasionally up to a foot or more 
 in diameter, large masses of quartz, 
 and large plates of mica (musco- 
 vite). The quartz and feldspar 
 are sometimes found intergrown in such a manner that the surface 
 of the feldspar seems to be scattered over with small, irregular, 
 dark masses of quartz, appearing not unlike cuneiform characters. 
 On this account such a mixture of feldspar and quartz is called 
 graphic granite (Fig. 40). 
 
 Rhyolite and Quartz Felsites. These have essentially the same 
 mineralogical composition as granite, though the association may 
 vary more, producing greater variety. In color the rocks are gen- 
 
 FIG. 40. Graphic Granite, 
 slightly reduced. 
 
98 Principal Types of Igneous or Pyrogenic Rocks 
 
 erally light gray, with yellows, and pale reds, and occasionally darker 
 shades. The texture varies considerably, from very finely crystal- 
 line to dense or felsitic, when the rock consists of minute masses 
 of quartz and feldspar often with more or less glass. A cellular 
 structure may also be developed to a slight degree in rhyolites 
 formed from surface flows. In many rhyolites, especially if they 
 are porphyritic, quartz can be recognized (commonly as small 
 double-sided pyramids), giving the surface a rough feel. Pheno- 
 crysts of feldspar also occur in the porphyritic types. When the 
 phenocrysts of quartz and feldspar make up about half the mass 
 of the rock it is called a rhyolite-porphyry ; when it constitutes most 
 of the rock it is called a granite-porphyry and marks an approach 
 to granite. In both the ground mass is generally dense or felsitic. 
 The name rhyolite is given to this rock because" of the flow 
 structure often exhibited (Greek, /W = flow). 
 
 The Acid Glasses. These include obsidian, pitchstone, perlite, 
 and pumice. In these the excess of silica, which on crystallization 
 would produce free quartz, can generally be recognized only on 
 analysis, except when phenocrysts are developed. As the magmas 
 
 which produce syenites on crystalliza- 
 tion also form glasses of similar ap- 
 pearance, though poorer in silica, it 
 is evident that it is not possible to 
 determine, except by analysis, whether 
 a given glass belongs to that or to the 
 granite series. 
 
 Obsidian. This is a homogeneous 
 glass with a low percentage of water. 
 FIG. 41. Obsidian, showing It is black to red in color, with trans- 
 conchoidal fracture. lvcent edgeSj and a concho idal frac- 
 
 ture, so called because the surface of 
 
 the fracture is generally marked by a series of concentric lines, like 
 the growth-lines of a shell (Fig. 41). 
 
 Pitchstone. This is like obsidian, but contains from 5 to 10 
 per cent of water. It has commonly a sheen or luster suggestive 
 of resin, and its colors are commonly reds and greens. 
 
 Perlite, or pearl stone (Fig. 42). This is glass composed of nu- 
 merous rounded masses, with concentric structure like the coats of 
 an onion, formed by contraction in cooling and separated by larger 
 straight cracks. It usually contains from 2 to 4 per cent of water. 
 
Types of Igneous Rocks 
 
 99 
 
 Pumice (Fig. 43). This is a cellular or porous glass, its charac- 
 ter being due to the liberation of gases on cooling. It may be re- 
 garded as the consolidated sur- 
 face froth of the lavas. 
 
 All the glasses form from sur- 
 face flows of lava, and the move- 
 ment or flowing, after partial 
 solidification, is commonly shown 
 by the occurrence in them of 
 layers of denser or more stony 
 material, in which minute crys- 
 tals of feldspar and quartz are de- 
 veloped. Often these are arranged 
 in rosettes of radiating structure 
 to which the name spherulites is 
 given (Fig. 44). Cavities due 
 to the expansion of steam or 
 gases are also formed, which are generally spherical and often 
 contain crystals of various minerals (topaz, quartz feldspar, 
 garnet, etc.). These are called lifhophysa or stone bubbles 
 (Fig. 45), and they vary in size up to an inch in diameter. 
 Typical localities for obsidian are the Lipari Islands and Yellow- 
 
 FIG. 42. Perlite. Thin section 
 under the microscope, enlarged 
 30 diameters. Hlinik, Hungary. 
 (After Rosenbusch.) 
 
 FIG. 43. Pumice. Surface of a 
 hand specimen. 
 
 FIG. 44. Spherulitic Obsidian. 
 
 stone Park; for pitchstone, Meissen near Dresden, Saxony, 
 the Island of Arran, west Scotland, and Silver Cliff, Colorado; 
 while the best known localities for perlites are in Hungary. 
 
ioo Principal Types of Igneous or Pyrogenic Rocks 
 
 Devitrified Old Glasses. Volcanic glasses of very early geologi- 
 cal time have generally undergone a change by the development in 
 them of excessively minute crystals of feldspar and quartz. From 
 
 
 FIG. 45. Lithophysae in Lithoidite of Obsidian Cliff,- Yellowstone National 
 Park. Slightly reduced. U. S. Geol. Sur. Bull. 150. 
 
 this they lose their glassy appearance and resemble felsites, which 
 name is commonly applied to them. They also have been called 
 petrosilex. They are not uncommon in the old lava flows of the 
 New England states and elsewhere. 
 
 The Syenite-Trachyte Series 
 
 This series is primarily characterized by deficiency in silica, so 
 that no free quartz is formed on crystallization. It includes syenite, 
 trachyte, felsites, and glasses. 
 
 Syenite. This rock typically consists of orthoclase and horn- 
 blende, without quartz. When biotite is present, the rock is called 
 mica syenite. In practically all syenites some of the orthoclase 
 is replaced by plagioclase, and this may occur to such a degree that 
 it makes up half of the feldspar, when the rock approaches a diorite, 
 and is called a monzonite. Augite too may replace the hornblende, 
 occurring with orthoclase and forming an augite syenite. Finally, 
 
Types of Igneous Rocks > 101 
 
 quartz syenite, with a small amount of free quartz, shows a transition 
 from granites. With the appearance of nephelite, they pass into 
 nephelite syenites. Porphyritic syenites have large crystals of feld- 
 spar. Common accessory minerals are magnetite, zircon, and 
 apatite. The name syenite is derived from the ancient Syene (now 
 Assuan) in Egypt, where the rock is, however, a hornblende granite 
 which was formerly used for obelisks. As now used the name was 
 first applied to a granite rock almost without quartz near Dresden 
 (Plauen'sche Grund, or Plauen Gorge). 
 
 Trachytes and Felsites. These have essentially the same com- 
 position as the syenites, of which they form the crypto-crystalline 
 representatives with the same relationship that rhyolite holds to 
 granite. Trachytes (Fig. 46) differ from rhyolites in the absence 
 or great rarity of quartz. Biotite is in general the most abundant 
 dark mineral, but hornblende and augite also occur, forming varie- 
 ties. The texture varies from felsitic in the true felsites to strongly 
 and coarsely porphyritic, and not infrequently the rock is somewhat 
 cellular. Felsites, on account of their dense structure, cannot 
 readily be distinguished from rocks of the same type in the granite 
 or even in the diorite group. In typical trachytes, which are gen- 
 erally porphyritic, the ground-mass is made up of fine rods of ortho- 
 clase arranged more or less parallel and in flowing lines. This is 
 the characteristic trachyte texture, which can often be seen with 
 the naked eye in the coarser crystalline varieties. Large pheno- 
 crysts of a clear vitreous variety of orthoclase (called sanidine) are 
 characteristic of the porphyritic trachytes. 
 
 When the phenocrysts are so abundant as to constitute about 
 half the mass of the rock with felsitic ground-mass, it is called a 
 trachyte porphyry. When phenocrysts form the bulk of the rock, 
 while the ground-mass becomes more coarse-textured, the rock is 
 called a syenite porphyry and marks the transition to syenite. 
 
 The name trachyte is derived from its rough or harsh surface 
 feel (Greek, rpa^vs, rough). The most typical trachytes come from 
 the peak of the Drachenfels on the Rhine. (See map, Fig. 98.) 
 
 The Glasses of the Syenite Series. Because the fusing point 
 of these more basic rocks is lower than that of the granite series, 
 being about 2000 F. (1100 C.) for trachytes as compared with 
 about 2200 F. (1200 C.) for the rhyolites, and 2250 F. (1240 C.) 
 for granites, they remain liquid longer, and hence crystallization 
 occurs more generally with less frequent formation of glasses. 
 
102 Principal Types' of Igneous or Pyrogenic Rocks 
 
 When glasses are formed, they are indistinguishable, except by 
 analysis, from those of the granite series. 
 
 The N ephelite-Syenite Phonolite Series 
 
 These differ from syenites chemically in the greater amount of 
 soda, and mineralogically in the partial substitution of nephelite 
 (eleolite) for the feldspar. 
 
 Nephelite-Syenite. This corresponds to syenite except for 
 the presence of nephelite or sodalite, of both or of leucite. In some 
 cases (Litchfield, Maine) the feldspar is wholly plagioclase. Zir- 
 con is a usual secondary mineral. 
 
 Phonolites. (Klingstein, so called because of its ringing sound.) 
 These rocks are generally dense and finely crystalline, seldom 
 
 vesicular or glassy. They are 
 mostly dull green or gray in 
 color, and when light colored 
 they are not readily distin- 
 guished from trachytes except 
 by the microscope, which 
 shows the presence of the 
 nephelite. The chief feldspar 
 is orthoclase, while the com- 
 mon dark mineral is augite, 
 hornblende being rare. A 
 peculiar character is the fact 
 
 that .the rock breaks into thin 
 FIG. 4 6.-Trachyte-phonolite show- glabs which have a musical 
 ing typical tracmtic texture of the . , , , 
 
 ground-mass and a large phenocryst. nng under the hammer. The 
 The Rhon, Germany. Enlarged 24 rock is frequently porphyritic, 
 diameters, seen under crossed nicols. fofa augite and feldspar 
 (After Rosenbusch.) 
 
 phenocrysts appearing in a 
 
 dense ground-mass. When the phenocrysts are very abundant the 
 rock is called phonolite-porphyry, and when in excess, it becomes a 
 nephelite-syenite porphyry. With increase in orthoclase and decrease 
 in nephelite, the phonolites pass into trachytes. (See Fig. 46.) 
 
 Glasses of the Nephelite-Syenite Series. These are still rarer 
 than those of the preceding case, since the fusing point of phonolites 
 (somewhat less than 2000 F. or 1090 C.) is still lower than that 
 of trachytes. Phonolite obsidians are known from the Peak of 
 Teneriffe. 
 
Types of Igneous Rocks 103 
 
 The Quartz- Diorite Dacite Series 
 
 The members of this series differ from those of the granite series, 
 chiefly in the substitution of plagioclase for orthoclase. The plagio- 
 clase can be recognized by the striated character of the cleavage 
 surface. 
 
 Quartz-Diorite. This resembles granite, but is darker and 
 heavier. Acid plagioclase, quartz, hornblende, and (or) biotite 
 are the essential minerals. When biotite predominates the rock 
 is called a quartz-mica-diorite. These rocks form a transition from 
 granites to diorites, the intermediate forms being called grano- 
 diorites. 
 
 A typical locality for quartz-mica-diorite is found in the Cortland 
 Series near Peekskill, N." Y., one for the hornblendic quartz-diorite 
 in the Yellowstone Park. 
 
 Dacite. This rock is difficult to distinguish from rhyolite except 
 by the use of the microscope. It has the same relation to quartz- 
 diorite that rhyolite has to granite. When the texture is finely 
 felsitic and non-porphyritic, it can only be termed felsite. When 
 porphyritic, the dacites are recognized by the striated surfaces of 
 the plagioclase phenocrysts, which predominate. Glasses and cellu- 
 lar texture are not uncommon. 
 
 When phenocrysts constitute up to half the mass of the rock, it 
 becomes a dacite-porphyry. When they are in marked excess over 
 the ground-mass, it becomes a quartz-diorite-porphyry. The name 
 is derived from the old province of Dacia, now Transylvania (Sieben- 
 biirgen), before the war a part of Hungary, which is a typical local- 
 ity. Dacites generally occur with andesites. 
 
 Glasses of this Series. The glasses of this series are more com- 
 mon than those of the syenite series, for the fusing point is only a 
 little less than that of the granite series. They are generally in- 
 cluded with the glasses of the next series. 
 
 . 
 The Diorite- Andesite Series 
 
 This differs from the preceding in the absence of free quartz (in 
 the crystalline members) and from the syenite series in the substi- 
 tution of acid plagioclase for orthoclase. 
 
 Diorites. These are granitoid rocks, the chief feldspar of which 
 is acid plagioclase, and they are rich in hornblende. When biotite 
 largely replaces the hornblende, the rock is called mica-diorite. 
 
104 Principal Types of Igneous or Pyrogenic Rocks 
 
 Again, augite may replace part of the other dark minerals, forming 
 an augite diorite, which is a passage-rock to gabbro. Typically, 
 the feldspar is in excess of the dark minerals, but in other cases 
 these may lead in the mineral constituents. A rock of diorite compo- 
 sition may also arise by metamorphism of gabbros with the change 
 of the augite to hornblende/ Porphyritic diorites, occurring in 
 dikes, have been called camptonites. Characteristic secondary 
 minerals are : magnetite, titanite, and apatite. The name diorite 
 was given to this rock because of the striking contrast between 
 the light and dark minerals of which it is composed. 
 
 Andesites. These are the fine-textured to felsitic members of 
 the diorite series. The acid plagioclase feldspars are the most 
 abundant minerals, but quartz is rare or absent. Biotite, horn- 
 blende, and augite are the dark minerals, the last two predomi- 
 nating over the biotite. The general colors of the rocks are grays or 
 greens. Typical andesite is felsitic, sometimes cellular, and com- 
 monly porphyritic. The felsitic ground-mass consists of micro- 
 scopic rods of feldspar, forming a felt-like aggregate. From trachyte 
 and dacite it can generally be distinguished only by its darker color, 
 owing to the greater abundance of the ferromagnesian silicates. 
 According to the predominant dark mineral we have mica andesite, 
 hornblende andesite, augite andesite, etc. 
 
 In the porphyritic varieties, the phenocrysts are mostly feldspar ; 
 when very abundant (up to half the mass) they form an andesite- 
 porphyry ; when in excess, a diorite- porphyry. These show an 
 increasingly coarser ground-mass, and grade into diorites proper. 
 
 With increase in orthoclase, andesites pass into trachytes, and 
 with increase in the dark and more basic ferromagnesian silicates 
 and a decrease of feldspars, they pass into basalts ; with addition 
 of quartz they pass into dacites. The type localities for these 
 rocks are in the Andes Mountains, and they are abundant and 
 widespread in the Pacific Coast region of North America, especially 
 in the old volcanic cones of Mt. Hood, Mt. Shasta, Mt. Rainier, 
 and others. 
 
 The Glasses of the Diorite-Andesite Series. Andesites fuse 
 at temperatures around 2000 F. (1100 C.), which is about the 
 same as for trachytes. Like these, therefore, they do not readily 
 form glasses, and most of those referred to andesites are probably 
 referable to the dacites. The glasses of this type which are recog- 
 nized are : andesite-obsidian, andesite-perlite, and andesite-pumice. 
 
Types of Igneous Rocks 105 
 
 They are distinguished from the more acidic glasses only by chem- 
 ical analysis, or by their field relations to recognizable types of 
 andesites or dacites. Andesite obsidian has been obtained from 
 Clear Lake, Cal., andesite perlite from Eureka, Nev.. 
 
 The Gabbro-(Pyroxenite, Peridotite)-Basalt Series 
 
 This includes all the basic igneous rocks, the coarser-grained 
 members of which range through gabbro, olivine-gabbro, pyroxenite 
 andperidotites, while the fine-grained ones form diabases and basalt. 
 The surface flows are represented by scoriaceous, ropy, or other 
 lavas, and more rarely by glasses (tachylite, Peele's hair, etc.). 
 
 Gabbro and Olivine Gabbro. These are generally coarsely 
 crystalline dark rocks, composed chiefly of basic plagioclase and 
 monoclinic pyroxenes, the dark silicates typically predominating. 
 There are, however, varieties which are almost wholly composed 
 of coarse crystalline labradorite (Canada, Adirondacks) with little 
 or no pyroxene. These are also called anortho sites ; where the py- 
 roxene is of the orthorhombic varieties (enstatite, bronzite, hyper- 
 sthene), the rock is called norite. Gabbro may contain some horn- 
 blende and biotite. When olivine is present the rock becomes 
 olivine-gabbro, olivine-norite, etc. In rare cases nephelite becomes 
 an important mineral in some basic gabbros, forming the rock 
 theralite, which occurs in the Crazy Mountains, Montana. Gabbros 
 have a wide distribution. 
 
 Pyroxenites and Peridotites. By the decrease of the plagioclase, 
 the gabbros pass insensibly into pyroxenites and peridotites, which 
 are usually found associated with the gabbros, but also occur in- 
 dependently. Pyroxenites generally contain little else than pyrox- 
 ene, which maybe orthorhombic (enstatite, bronzite, hypersthene) or 
 monoclinic (diallage, augite), but these are not readily distinguish- 
 able by the unaided eye, though the orthorhombic pyroxenes show a 
 bronze luster. Frequent accessory minerals are hornblende, magne- 
 tite, and pyrrhotite. When olivine is added the rock becomes a 
 pcridotite, and when olivine predominates it bec6mes a dunite, of 
 which the nearly pure olivine rock of North Carolina is an example. 
 Sometimes magnetite is so abundant as to make the rock almost an 
 iron ore. Porphyritic peridotites have been called picrites, especially 
 when occurring in dikes. Sometimes hornblende is abundant, and 
 it may even form a rock by itself, which is then called amphibolite. 
 
io6 Principal Types of Igneous or Pyrogenic Rocks 
 
 Pyroxenites and peridotites change with age and by metamor- 
 phism into serpentines. 
 
 Diabase. This is a transitional rock between the gabbros and 
 the basalts, being fine-grained but holocrystalline and differing 
 in detail of texture from the gabbro. It consists of basic plagio- 
 clase, augite, and often olivine, the first occurring as elongated rec- 
 tangular rods arranged in an interlacing manner, while the other 
 
 FIG. 47. a, Basalt, showing typical diabasic structure of lath-shaped crys- 
 tals of plagioclase, Burney Falls, Shasta Co., Cal. b, Bronzite diabase, 
 York, Pa., showing lath-shaped plagioclase crystals. Both sections are seen 
 under crossed nicols enlarged 24 times (after Rosenbusch). 
 
 minerals are packed in between them in irregular masses. This is 
 brought about by the fact that the plagioclase crystallized out first, 
 contrary to the usual order of crystallization, and that the ferro- 
 magnesian minerals had to adapt themselves to the remaining 
 spaces. This diabasic texture (Fig. 47) is sometimes modified into 
 an ophitic one, where the rods of plagioclase are included in large, 
 coarsely crystalline masses of pyroxene. Diabase is common as 
 an intrusive rock, both as dikes and sills. 
 
 Basalt. These are the basic fine-grained or dense igneous rocks 
 which form the surface flows of the gabbros, pyroxenites, and peri- 
 dotites. Typically the texture is dense or felsitic, often cellular, 
 and the rock may be characterized by almond-shaped steam holes 
 which are subsequently filled with secondary calcite, giving the 
 rock an amygdaloidal structure (Fig. 48). Sometimes these cav- 
 ities are filled with copper, as in the Lake Superior region. Por- 
 
Types of Igneous Rocks 
 
 107 
 
 FIG. 48. Amygdaloid. 
 
 phyritic basalts show phenocrysts of augite and more commonly 
 olivine. Such porphyritic olivine basalts are also called melaphyres. 
 The felsitic ground mass is composed 
 of microscopic augite, plagioclase and 
 magnetite crystals. Sometimes a little 
 glass is present. With abundant de- 
 velopment of phenocrysts, of augite 
 and olivine, the rock becomes a basalt 
 porphyry and with excessive develop- 
 ment of phenocrysts a gabbro porphyry 
 is produced. 
 
 In rare basalts nephelite or leucite 
 may replace the feldspar, giving a 
 variety of types, distinguishable only 
 under the microscope. 
 
 Augitites and Limburgites. These 
 
 are rare basaltic rocks, with little or no feldspar, which correspond 
 to the pyroxenites and peridotites, respectively. 
 
 The Basic Glasses. These represent surface formations of the 
 basaltic flows, but are on the whole not common. Some of the 
 porous or vesicular scoria is glassy, but for the most part it has a 
 dense stony or felsitic texture. Typical basic glass, corresponding 
 to obsidian, is known by the name of tachylite, while a fibrous vari- 
 ety found in the crater of Kilauea i called Pele's hair. 
 
 Special Field Names 
 
 Several names are commonly applied to the basic igneous rocks 
 when the exact character cannot be determined in the field. 
 One of these is dolerite, used for rocks which may be diorites or 
 gabbros, but in which the dark mineral is undeterminable. An- 
 other is trap, in common use for dark, dense basalts or diabases. 
 It is derived from the Swedish trappar, a stairway, because the 
 sheets of this rock sometimes form successive steps in the land- 
 scape. Old trap or basaltic rocks which have, by alteration, de- 
 veloped sufficient of the mineral chlorite to give them a greenish 
 cast, are called greenstones. 
 
io8 
 
 Modern Volcanic Phenomena 
 
CHAPTER VII 
 MODERN VOLCANIC PHENOMENA 
 
 EXCEPT in the case of comparatively young volcanic eruptions, 
 the structures and relationships of igneous masses can be studied 
 only in regions where these masses have been uncovered by erosion 
 of the rocks which formerly covered them, and their origin can be 
 determined only from the available facts. We shaljl first consider 
 those masses the formation of which are open to observation or 
 which have been formed so recently that there can be no doubt as 
 to their origin. 
 
 DISTRIBUTION, CLASSIFICATION, AND DEVELOPMENT OF 
 VOLCANOES 
 
 Volcanoes are found in many parts of the world at the present 
 time, but their most extensive distribution is around the borders 
 of the Pacific, where they form a " chain of fire " which surrounds 
 that ocean. The map on the opposite page shows this and their 
 occurrence elsewhere. Along the western border of the Pacific 
 more than 150 active volcanoes are distributed through a length 
 of 16,000 kilometers, while about 100 border the eastern margin of 
 that ocean from the Aleutian Islands to Tierra del Fuego. Typical 
 and perfect examples of the former are Fujiyama in Japan (Fig. 50) 
 and Mayon in the Philippines (Frontispiece), while Bogosloff and 
 Popocatepetl may serve as examples of the latter. Within the 
 Pacific and arising from its floor are other active volcanoes, such 
 as those of the Hawaiian Islands, near its very center, and those of 
 the southwest coral reef regions. 
 
 On the Atlantic borders, active volcanoes are not common, being 
 chiefly confined to the Antillean region in the west, the Icelandic in 
 the north, and the Azores, Canaries, and Cape Verde Islands in 
 the east. These are in reality local or isolated groups of volcanoes, 
 except those of Iceland, which are the last manifestations of vol- 
 canic activity in a vast volcanic field which extends from Greenland 
 on the west to Siberia on the east, and which began far back in 
 
 109 
 
no 
 
 Modern Volcanic Phenomena 
 
 FIG. 50. Fujiyama, a perfect volcanic cone in Japan. 
 
 FlG 5I M a p O f the Naples volcanic region, with the three principal 
 volcanic centers, Vesuvius, the Phlegraean volcanic field west' of Naples 
 (Pampi Flegrei), and the Island of Ischia; together with the submarine vol- 
 canoes. (After J. Walther, from Ratzel.) 
 
Distribution, Classification, and Development in 
 
 the geological history of the region. In the Mediterranean are 
 three important fields of volcanic activity known since ancient 
 times : that around Naples, that of Sicily and its islands, and that 
 of the Grecian archipelago. The Naples volcanic field (Fig. 51) 
 comprises Vesuvius with Monte Somma on the east of Naples, 
 the Phlegraean volcanic field with a number of vents including 
 
 FIG. 52. View of Stromboli from the northwest. 
 Kayser's Lehrbuch.) 
 
 (After A. Bergeat, from 
 
 Monte Nuovo, the Solfatara, the Pozzuoli district, etc., on the west, 
 and the volcanic island of Ischia on the southwest, besides many 
 smaller islands, including Capri on the south. The Sicilian region 
 includes Mount Etna in the northeastern part of that island, and 
 the volcanoes of the Lipari or ^Eolian group of islands north of 
 Sicily, among which are Vulcanoand Stromboli (Fig. 52), the latter 
 called the " Lighthouse of the Mediterranean," because of the 
 
 KIRVN6A-CMA-MOTO 
 
 FIG. 53. Volcanoes of the great Rift Valley of East Africa. (After Moore.) 
 
 flashes of its explosions, given off at intervals of from i to 20 min- 
 utes. In the Grecian archipelago the island of Santorin has been 
 a scene of volcanic activity for more than 2000 years. Volcanoes 
 are also active around the Indian Ocean, especially on the islands 
 which form its eastern margin. Active volcanoes exist in the great 
 " Rift Valley " of East Africa, in which lie many of the large lakes 
 of that region (Fig. 53), and recently extinct volcanoes occur along 
 
ii2 Modern Volcanic Phenomena 
 
 the northern margin of the Indian Ocean in Arabia, while more 
 ancient volcanic activities are recorded in India. 
 
 Active, Dormant, and Extinct Volcanoes 
 
 Volcanoes may be classed as active when eruptions are occurring 
 at intervals ; as dormant, when they have been quiescent for cen- 
 turies, though the possibility of a new eruption exists ; and as ex- 
 tinct, when volcanic activity has entirely ceased and the volcano is 
 undergoing destruction by atmospheric agencies. There is of 
 course every gradation between these ; some that would be classed 
 as dormant may never be revived, while others, after a long sleep, 
 burst forth again, as in the case of Vesuvius at the beginning of 
 the Christian Era. It should be clearly understood that volcanism 
 is not a phenomenon restricted to the present, but that volcanic 
 activities, often on a much grander scale than those of to-day, have 
 been going on during all the periods of the earth's history. Many 
 of the older volcanoes and their products, after suffering more or 
 less destruction by erosion, were buried under more recent deposits, 
 and have been reexposed only in part as the result of the latest 
 phases of erosion. 
 
 Formation of New Volcanoes in the Historic Period 
 
 Most of the modern volcanoes came into existence before the 
 time of recorded history, but a few have been formed during the 
 historic period, and their growth and development has been wit- 
 nessed by man. The most noted of these are : Monte Nuovo, in the 
 Bay of Baiae near Naples (1538) ; Jorullo, Mexico (1759) ; Pochutla, 
 Mexico (1870) ; Camiguin, Philippine Islands (1871) ; a new cone 
 of the Ajusco group, Mexico (1881) ; the New Mountain of Japan 
 (1910) ; and the submarine cones of Sabrina and Graham Islands 
 (1811 and 1831). A few of these may be considered in some detail. 
 
 Monte Nuovo. This volcano (Fig. 54) arose in the Phlegraean 
 fields west of Naples on Sunday, September 29, 1538, beginning 
 about one o'clock in the morning. It appeared mainly on the site 
 of the ancient Lake Lucrinus, which itself was regarded as the 
 crater of an older but extinct preexisting volcano, filled with water. 
 Incandescent gases burst open the earth, and within a week a cone 
 made up of ejected ashes, cinders, and large stones, but no lava, 
 was built up to a height of 440 feet above the level of the sea. The 
 
Distribution, Classification, and Development 113 
 
 depth of the crater is 421 feet, reaching within 19 feet of the sea- 
 level, and its basal circumference about 8000 feet. The eruption 
 was preceded on the day and night before by about twenty earth- 
 quake shocks in Pozzuoli and the neighborhood, and after the erup- 
 tion it was found that the sea had receded for some distance, owing 
 
 FIG. 54. Monte Nuovo, formed in the Bay of Baiae, Sept. 29, 1538. 
 (After Lyell.) i. Cone of Monte Nuovo. 2. Rim of crater of same. 
 3. Thermal spring called Bath of Nero, or Stufe di Tritoli. 
 
 to the elevation of the region, and the strand was covered with 
 quantities of dead fish, as well as dead birds. 
 
 The cone of Monte Nuovo is very regular, and is composed of 
 layers of pumaceous fragments and ashes, and of trachytic blocks, 
 the whole partly consolidated and dipping away from the crater 
 at angles of 26 to 30 degrees. Fragments of marine shells and 
 Roman bricks were also included in the beds as a result of the 
 disturbance of the region by the explosions. 
 
 Jorullo. This cone arose on the night of September 28, 1759, 
 at a point 35 miles distant from any then existing volcano, and 
 1 20 miles from the sea. It appeared on a level plain 2000 to 3000 
 feet above the sea in the state of Michoacan, Mexico. A north- 
 easterly fissure opened in the midst of the sugar and indigo fields 
 of this plain, and ashes and rocks were thrown to a great height, 
 and on falling these built up six conical hills on the line of the chasm, 
 the smallest 300 feet high, while Jorullo itself was built up to 1600 
 feet above the plain, or 4265 feet above sea-level. Great streams 
 
Modern Volcanic Phenomena 
 
 of basaltic lava were poured forth from Jorullo, these including 
 fragments of granitic rock. The ejection did not cease until Febru- 
 ary, 1760. Twenty years later the lava was still hot enough, a 
 few inches below the surface, to light a cigar. This eruption, too, 
 was preceded by earthquakes and subterranean rumblings, which 
 began in the June preceding. There has been no eruption in this 
 region since. 
 
 Camiguin. This volcano also started from a fissure in a level 
 plain on one of the small islands north of Luzon in the Philippines. 
 Beginning in 1871, it continued to be active for four years, by which 
 
 time it had reached 
 a height of about 
 1800 feet. 
 
 Sabrina Island. 
 This submarine vol- 
 canic cone appeared 
 above the waters of 
 the Atlantic in the 
 Azores group, off the 
 coast of St. Michaels, 
 on June 13, 1811, 
 and rose to a height 
 of about 300 feet 
 above the sea, gain- 
 FIG. 55. The volcanic eruption which formed j a c i rcum f e rence 
 Sabrina Island in the Azores, June 13, 1811. (After 
 
 DeLaBeche.) of about a mile " 
 
 Being, however, 
 
 largely made of unconsolidated material, it has since been washed 
 away again. As observed from the nearest cliff on St. Michaels, 
 the explosions resembled a mixed discharge of cannon and mus- 
 ketry and were accompanied by a great abundance of lightning. 
 The appearance of the eruption above water is shown in the 
 accompanying figure from a sketch made at that time (Fig. 55). 
 
 Graham Island (Isle Julia). This island, which existed for only 
 about three months, rose as a submarine cinder cone in 1831 in the 
 Mediterranean between the southwest coast of Sicily and that pro- 
 jecting part of the African coast where ancient Carthage stood. A 
 few years before the appearance of the island, soundings at this 
 locality showed a depth of water of 100 fathoms. Premonitory 
 shocks were felt on June 28 over the spot and on the adjoining 
 
Distribution, Classification, and Development 115 
 
 coast of Sicily. About July 10, a column of water 60 feet high and 
 
 800 yards in circumference was seen rising from the sea, followed 
 
 soon after by dense clouds of steam which rose 1800- feet. Eight 
 
 days later the same 
 
 observer noted at 
 
 this spot a small 
 
 island 12 feet high 
 
 and with a crater at 
 
 its center, from 
 
 which volcanic mat- 
 
 FIG. 56. Supposed section of Graham Island. 
 (After C. McLaren, Geology of Fife and the 
 Lothians, pi. 41, Edin., 1839; from LyelFs Prin- 
 ciples.) 
 
 ter and immense 
 clouds of vapor were 
 ejected, while the sea round about was covered with floating 
 cinders and dead fish. By the end of July the island had become 
 from 50 to 90 feet in height and three fourths of a mile in circum- 
 ference, while on August 4 it was reported above 200 feet high and 
 three miles in circumference. After this it began to diminish in 
 size, owing to the erosion by the sea, decreasing to two miles 
 in circumference by August 25, and to three fifths of a mile and a 
 maximum height of 107 feet by September 3, when the crater was 
 about 780 feet in circumference. On September 29 the island was 
 reduced to a circumference of only about 700 yards, and toward 
 
 the close of Oc- 
 tober it had dis- 
 appeared except 
 for a small point 
 of sand and 
 scoriae. By the 
 commencement of 
 1832 . there was 
 only a shoal, with 
 a mass of igneous 
 rock which ap- 
 parently filled the 
 center of the 
 vent. The ap- 
 pearance and sup- 
 
 FIG. 57. Graham Island, as it appeared on Sept. 
 29, 1831. The apparent bedding planes sloping towards 
 the center of the volcano are not such in reality. 
 (From Lyell's Principles.) 
 
 posed structure of this island and of the cone, which rose thus 
 about 800 feet above the sea floor, are shown in the above 
 figures reproduced from Lyell (Figs. 56, 57). The lava core or 
 
n6 
 
 Modern Volcanic Phenomena 
 
 neck, which probably never appeared above sea-level, now forms 
 the highest part of the submerged remnant of the volcano. The 
 fragmental material included, besides volcanic ash and cinders, 
 blocks of limestone, dolomite, and sandstone. 
 
 Several other submarine eruptions of this type have been re- 
 corded from various sections of the Atlantic and from the Medi- 
 terranean. 
 
 CHARACTERISTIC FORMS AND ACTIVITIES OF TYPICAL MODERN 
 
 VOLCANOES 
 
 Of the many modern volcanoes, a number which have been ob- 
 served for long periods of time may be described somewhat in de- 
 tail so that the student will get a grasp of the essentials of the struc- 
 ture and activities of volca- 
 noes. We shall begin with a 
 type- in which liquid lava is 
 the chief product of erup- 
 tion, then consider types in 
 which both lava and frag- 
 mental material are pro- 
 duced, and enter into the 
 building of the cone, and 
 finally consider some ex- 
 amples which are purely 
 explosive, with the produc- 
 tion of only fragmental ma- 
 terial. The nature and com- 
 position of the fragmental 
 material will be considered 
 more at length in a subse- 
 quent chapter. 
 
 Before proceeding, the 
 student should clearly un- 
 derstand that the hill or 
 mountain which constitutes 
 the volcano is built up 
 around an opening or vent 
 
 from the material ejected 
 FIG. c8. Volcanic bomb. Vesuvius, r ,, . j ,- , ., 
 
 eruption of ,872. (After Ratzel, Die from thls vent - and that ll 
 Erde.) does not represent an upris- 
 
Forms and Activities of Typical Modern Volcanoes 117 
 
 ing or an upheaval of a part of the earth's surface, as was at one 
 time supposed to be the case. Three kinds of material are ejected 
 from volcanic vents; of these, two or all may be present. The 
 first type comprises gases and water vapors mingled with fumes of 
 other substances; this is always present. The second is liquid 
 magma or lava, and the third consists of the shattered lava and 
 shattered rocks resulting from the explosive activities, comprising 
 masses of all sizes, from large lava balls or bombs (Fig. 58), more 
 or less spherical or elliptical from their rotary motion through the 
 air, to fine lava particles or lapUli, and fragments or blocks of 
 older igneous or other rocks, and dust produced by the shattering 
 of these. In general we speak of such material as volcanic ashes 
 and cinders. The eruptive activities range from the quiet upwell- 
 ing or bubbling-up of liquid lava to the most violent explosion due 
 to the sudden expansion of the gases and vapors. 
 
 Kilauea (and Mauna Loo) of the Hawaiian Islands 
 
 The entire group of the Hawaiian Islands (Fig. 59) in the mid- 
 Pacific is a series of volcanic cones built up from the sea-bottom in 
 former times. Most of these volcanoes are now extinct, and many 
 
 FIG. 59. Map of the Hawaiian Islands, showing the principal craters. 
 (After Dana.) 
 
 of them are undergoing erosion ; but a large, active crater, that of 
 Kilauea (4000 feet above sea-level) , exists on the east side of the 
 volcanic mountain of Mauna Loa (13,675 ft. high) and about twenty 
 miles from its summit, which is also marked by an active crater. 
 The crater pit of Kilauea (Fig. 60) is rudely oval in form, with a 
 circumference of about nine miles, and its floor is formed by a rough 
 stony crust of solidified lava resting upon a vast column of molten 
 rock which arises from an unknown depth in the crust of the earth. 
 In places, this floor is broken by lakes of liquid lava, red to white 
 
n8 
 
 Modern Volcanic Phenomena 
 
 hot, and set into boiling activity by the ebullition of gases (Figs. 
 61, 62). From cracks in the floor and on its margin jets of lava are 
 
 FIG. 60. View of outline of the crater of Kilauea from Volcano House. 
 (After Button.) 
 
 frequently projected to great heights, and some of this material, 
 blown like spray by the wind, is drawn out into slender, hair-like 
 fibers. This has become known as Pele's hair, so named after the 
 goddess of the mountains. 
 
 FIG. 61. The Lava Lake at one side of the crater of Kilauea. 
 
 The margin of the crater is formed by a precipitous cliff which 
 varies in height from time to time. This is due to the fact that 
 the lava column which supports the floor rises as the pressure in- 
 creases below, until a point is reached where the wall may be rup- 
 tured, and the liquid lava flows out, whereupon the column dimin- 
 
I2O 
 
 Modern Volcanic Phenomena 
 
 ishes, carrying the floor downward, until it may be 700 feet below 
 the edge of the crater rim. The rate of rising may be as much as 
 100 feet in a year, but in modern times the lava has not overflowed 
 the rim, but issues from lateral fissures due to cracking or fusion, 
 the floor approaching only to within 300 feet of the edge of the 
 
 FIG. 63. A lava stream falling in cascades over a cliff into the sea, 
 Hawaiian Islands. 
 
 crater. During the eruption of 1840 the lava first appeared on 
 the side of the mountain five miles from the main crater, after which 
 it issued at successively lower levels. 
 
 Similar conditions exist in the crater of Mauna Loa proper, but 
 here the top of the lava column is nearly 10,000 feet above that of 
 Kilauea. 
 
 The lava of these volcanoes is very liquid, being of an extremely 
 basic character and forming basaltic rocks on cooling. Because 
 of its liquidity, it will continue to flow for a long time ; some of the 
 lava flows of Hawaii are thirty miles in length. When it reaches 
 a cliff, the lava cascades over it like a waterfall (Fig. 63). The 
 surface of such lava streams often shows local wave-like advances, 
 which produce the appearance of a series of crushed pillows piled 
 one against the other. This type of surface form is known in Hawaii 
 
Forms and Activities of Typical Modern Volcanoes 121 
 
 as Pahoehoe (pron. pa-hoi-hoi) (Fig. 64), and it is seen in older lavas 
 of this type in many regions of the world. A ropy surface, having 
 the Appearance of coils of heavy rope, is also commonly produced. 
 Compare also with Fig. 66. 
 
 As the lava stream moves along, it sweeps away forests in its 
 course, and carries away masses of rock and soil covered with vege- 
 
 FIG. 64. Sluggish lava flow forming pillow lava or " pahoehoe " on 
 Mauna Loa. 
 
 tation (Fig. 65). Sometimes a stream will part around a mass of 
 such rock and, reuniting, enclose it as an island. On reaching the 
 sea, the lava plunges into it with loud detonations and becomes 
 shivered into millions of particles of glass, which may be thrown 
 in clouds into the air. The light from such an eruption has been 
 visible for over a hundred miles at sea, and at a distance of forty 
 miles fine print could be read at midnight (Dana). 
 
 When the crust of the lava stream has cooled, the interior mass, 
 still in a molten condition, will flow on, leaving a tunnel behind. 
 Such tunnels are common in Hawaii, and their roofs are frequently 
 
122 
 
 Modern Volcanic Phenomena 
 
 FIG. 65. End of the lava flow of 1881 on Mauna Loa. Note the trees 
 which were killed but not consumed, and those which escaped. 
 
 incrusted with lava pendants or stalactites up to 20 or 30 inches in 
 length, while on the floor corresponding drip mounds or stalag- 
 mites of lava are 
 found. Small " spatter 
 cones" may also arise 
 on its surface (Fig. 66). 
 The form of these 
 basic lava volcanoes is 
 very characteristic, 
 being a very flat cone 
 of vast dimensions. As 
 they arise from the sea- 
 bottom, only a small 
 part is visible, the total 
 height of Mauna Loa 
 being more than 30,000 
 feet, although consider- 
 ably less than half of 
 this height is seen. 
 The summit is nearly 
 
 FIG. 66. Lava tunnel formed by the cool- 
 ing of the outer surface of the flow, after 
 which the lava within flows out, leaving the 
 tunnel. On the surface of the flow a spatter 
 cone was built up. Hawaiian Islands. 
 
 flat for several square miles, and the slopes of the sides do not 
 average more than seven degrees. 
 
Forms and Activities of Typical Modern Volcanoes 123 
 
 Craterless Volcanoes of Viscous Lava 
 
 When the lava is very viscous, such as is characteristic of very 
 silicious (acidic) lavas, it may happen that the lava mass rises as a 
 dome-like swelling from the surface, producing a mound or hill of 
 lava which is not characterized by a summit crater. The volcano 
 of Chimborazo in Ecuador (20,498 feet above the sea) appears to 
 
 Thera 
 
 FIG. 67. Map of the ruin of the cone of Santorin, in the Greek Archipelago. 
 (From Kayser's Lehrbuch.} 
 
 be of this type, and an eruption of this character occurred in 
 1866 which formed the Isle of Asphroessa at Santorin in the Grecian 
 Archipelago (Figs. 67, 68). A gradual rising of the bottom of the 
 bay occurred, until the island appeared, which apparently was due 
 to slow upward and outward pressure by steam, which was escap- 
 ing at every pore through the scoriaceous lava surface. The red-hot 
 lava could be seen through the fissures, and the whole mass was 
 undulating and swaying from side to side, sometimes appearing 
 to swell to nearly double its size, and to throw out ridges like moun- 
 tain spurs. At last a broad chasm appeared across the top of the 
 cone, accompanied by a tremendous roar of steam, while rocks and 
 
I2 4 
 
 Modern Volcanic Phenomena 
 
 FIG. 68. Bird's eye view of the Gulf of Santorin, during the volcanic 
 eruption of February, 1866, looking west. (From LyelPs Principles.} 
 a. Therasia. b. The northern entrance, 1,068 feet deep. c. Thera. 
 d. Mt. St. Elias, rising 1,887 f ee t above the sea, composed of granular lime- 
 stone and clay-slate; the only non- volcanic rocks in Santorin. e. Aspronisi. 
 /. Little Kaimeni (Kaymeni or Kaemenae). g. New Kaimeni (Nea Kaymeni 
 or Kaemenae). h. Old Kaimeni (Palaea Kaymeni or Kaemenae). i. Asphro- 
 essa. k. George. 
 
 ashes mixed with steam were thrown to heights of 50 to 100 feet, 
 masses of this material, 30 cubic feet in bulk, falling at distances of 
 600 yards from the new crater. Then the activity subsided, the 
 cone was lowered, the crater closed in, and after a few minutes of 
 quiet the process recommenced. 
 
 FIG. 69 a. Grand Puy of Sarcoui, 
 composed of trachyte and rising be- 
 tween two breached scoria cones; a 
 typical example of a pustular cone or 
 volcanic blister formed of highly 
 viscous lava. Auvergne, France. 
 
 FIG. 69 b. Experimental illustra- 
 tion of the mode of formation of vol- 
 canic blister cones composed of viscid 
 lavas. 
 
 An example of such a blister cone or volcanic dome, now extinct, is 
 found in the Grand Puy of Sarcoui (Fig. 69 a) in the old volcanic 
 district of Central France. This mountain is a mass of trachyte 
 
Forms and Activities of Typical Modern Volcanoes 125 
 
 lava having the appearance of an inverted cup, without a crater, 
 and was apparently formed from a mass of viscous lava which was 
 forced upward to form a blister upon the surface of the earth. Blis- 
 ters of this kind have been reproduced experimentally, as shown 
 in the preceding figure (Fig. 69 b). 
 
 Vesuvius 
 
 A very different type of volcano is represented by Vesuvius, 
 probably the best and longest known of active volcanoes. The 
 
 FIG. 70. Map of Vesuvius with its lava streams up to 1872. The darker 
 are the later, and lighter the earlier flows. Scale about i : 250,000. After 
 Le Herr and others. (From Kayser's Lehrbuch.) 
 
 present cone, which lies east of Naples, is surrounded on three sides 
 by the rim of the ancient cone, now called Monte Somma (Figs. 
 70, 71), which was perfect up to the first century of the Christian 
 
126 
 
 Modern Volcanic Phenomena 
 
 Era. That volcano had been dormant for so long that its slopes 
 were clothed with vineyards and gardens and dotted over with 
 
 villas, while at 
 the foot of the 
 mountain lay the 
 populous cities of 
 Herculaneum and 
 Pompeii. Even 
 the interior slopes 
 of the crater, of 
 which only a part 
 remains in Monte 
 Somma, were 
 covered with wild 
 vines, so Plutarch 
 tells us, while the 
 floor of the crater 
 was a sterile plain. 
 On this plain, 
 from which there 
 was only a single 
 outlet, a break in 
 the wall of the 
 crater, the gladi- 
 ators of Spartacus 
 encamped in 72 
 B.C., while the 
 praetor Clodius 
 guarded the out- 
 let and attacked 
 Spartacus by low- 
 ering his soldiers 
 into the crater 
 over the precip- 
 itous walls. 
 
 In the year 63 
 A.D. the first evi- 
 dence of the re- 
 awakening of the 
 volcano made 
 
Forms and Activities of Typical Modern Volcanoes 127 
 
 itself felt in an earthquake which damaged the cities in the vi- 
 cinity. From that time to 79 A.D. slight shocks were frequent, 
 and in August of that year they became more numerous and 
 violent, finally terminating in the first great historic eruption. 
 As described by the younger Pliny, a dense column of vapor first 
 arose vertically from the crater, spreading out laterally so that the 
 
 FIG. 72. Ruins of Pompeii. 
 
 upper part resembled the head, and the lower the trunk, of a pine 
 tree. Flashes of light, vivid as lightning, at intervals pierced this 
 cloud, and ashes began to fall even upon the ships at distant Mi- 
 senum, shoaling the sea in places and burying Herculaneum and Pom- 
 peii (Fig. 72). The violent explosions shattered the crater rim, 
 of which only a part remains in Monte Somma, and the later cone 
 of Vesuvius proper was built upon the floor of the old crater, sur- 
 rounded on three sides by its rim. No lava appears to have been 
 ejected at this time, the material being all of the pyroclastic or 
 f ragmen tal type, such as lapilli, sand, and fragments of older lava. 
 The first lava stream recorded from Vesuvius flowed in the erup- 
 tion of 1036 ; which was the seventh since the reawakening of the 
 volcano. Another eruption occurred in 1049, and still another in 
 1138 or 1139 ; after this the volcano rested for 168 years, though 
 two minor vents opened at distant points, one at Solfatara, near 
 Pozzuoli (Bay of Baiae), in 1198, and the other on the island of 
 
128 Modern Volcanic Phenomena 
 
 Ischia in 1302. Then in '1306 a minor eruption of Vesuvius took 
 place, after which this volcano again became dormant for 325 years 
 or until 1 63 1 , with one slight eruption in 1 500. During this interval 
 the Sicilian volcano Etna was, however, in constant eruption, while 
 within the Phlegraean volcanic field west of Naples arose the new 
 volcano Monte Nuovo in 1538 (ante, p. 112). Between 1139 and 
 1631, or for 492 years, there had been no violent eruption, and the 
 
 FIG. 73. Eruption of Vesuvius in 1872. (After photograph from Ratzel.) 
 
 crater, which was five miles in circumference and about a thousand 
 paces deep, had its sides covered with brushwood forests frequented 
 by the wild boar, while cattle grazed on its floor. Three small 
 pools of water remained upon the floor of the crater, one being hot 
 and bitter, another more salty than the sea, and the third hot but 
 tasteless. Suddenly, in 1631, the floor and sides of the crater were 
 blown to fragments which the wind scattered, and in December of 
 that year seven streams of lava poured forth from the crater, over- 
 whelming several villages on the flanks and at the foot of the moun- 
 tain, one of which, Resina, had been partly built over the ancient 
 site of Herculaneum. Great floods of mud, from the condensed 
 vapor and the ashes, also poured down the sides of the volcano and 
 did much damage. 
 
Forms and Activities of Typical Modern Volcanoes 129 
 
 After a brief rest, the eruptions were renewed in 1666, since which 
 time they have occurred almost constantly, with only short inter- 
 mittent periods of quiescence (Fig. 73). The last great eruption 
 occurred in 1906. This began with premonitory explosions in 1904, 
 while during the whole of 1905 a narrow stream of lava flowed from 
 a fissure in the cone (Fig. 74). On April 4, 1906, began the last 
 great eruption, which was inaugurated by the appearance of a 
 cloud of dust, carried aloft by the gases, and assuming the shape of 
 
 FIG. 74. Looking into the crater of Vesuvius ; hot lava sending up clouds 
 
 of steam. 
 
 a cauliflower. At the same time several lava streams broke out 
 at successively lower levels in the side of the cone. Three days 
 later (April 7) occurred a violent explosion, and a dust cloud arose 
 vertically into the air for a height of four miles, this dust falling 
 in such quantities upon the roofs of the houses in the near-by towns 
 as to cause their collapse. Larger streams of lava also issued from 
 various openings, one of them reaching the town of Boscotrecase 
 and destroying it. The main lava stream descended the steeper 
 slopes at a rate of somewhat less then two miles an hour, but 
 flowed at a much lower rate on the gentler slopes. The lava had a 
 temperature of more than 2000 F., but owing to the rapid cooling 
 
Modern Volcanic Phenomena 
 
 on the surface it did not burn up the trees with which it carne in 
 contact, but charred them, and sometimes broke them off by its 
 weight and carried them along on its surface. 
 
 FIG. 75. Inside the crater of Vesuvius. Note the stratified appearance of 
 the wall of ashes and cinders, and the slopes of loose material. 
 
 Both the old cone of Monte Somma, and the later cone of Vesu- 
 vius, which has a height of about 4000 feet, are built up of layers 
 of cinders, ashes, and lava (Fig. 75). These have a steep inclination, 
 dipping away from the rim of the craters in all directions at angles 
 of 26 to 40 or more. These layers are cut vertically by numerous 
 fissures, which are filled with hardened lavas, forming dikes which 
 

 
 I 
 
 
 II 
 
 a. 
 
 
 
 i 3. 
 
 ' * 
 
132 
 
 Modern Volcanic Phenomena 
 
 bind the entire mass together. Much of the consolidation of the 
 fragmental material is also due to the fact that it falls often in a 
 half -fused condition, and the heat from the volcano tends to bind the 
 particles together. Material carried to greater distances, however, 
 remains incoherent. Since the last eruption (1906) much gullying 
 by erosion has occurred on the slopes of the cone (Fig. 76). (See 
 also Fig. 86 a, p. 142.) 
 
 Etna 
 
 This famous volcano, in the eastern part of the island of Sicily, 
 rises almost 11,000 feet above the sea, and has a nearly circular 
 
 FIG. 77. Map of Etna and the Val-del-Bove, or Valle-del-Bue. After 
 map of Italian general staff. (From Ratzel.) The orientation of this map 
 is such that north is on the right. The coast line runs in a direction east of 
 north. 
 
 base with a circumference of 87 miles, while its lavas cover an area 
 almost twice as great (Fig. 77). The lower part of the cone is cul- 
 tivated ; higher up are forests, and the upper part is a barren lava 
 
133 
 
134 
 
 Modern Volcanic Phenomena 
 
 waste, which terminates in a sort of tableland, from which arises 
 the principal cone, noo feet in height. From this cone sulphurous 
 vapors and steam constantly arise, and lavas are emitted at fre- 
 quent intervals. Viewed from north or south the cone is very sym- 
 metrical ; but on the east it is cut by a deep valley, the Val-del-Bove 
 or Valle-del-Bue, which is a vast amphitheater four or five miles in 
 diameter, and is enclosed by precipices between 3000 and 4000 feet 
 high at the upper end. This valley was probably formed in part 
 by explosions and in part by subsidences (Fig. 78). 
 
 During the eruptions of Etna, which have been known to be more 
 or less continuous since the fourth century B.C., the upper cone 
 has repeatedly been blown away or has been partly engulfed by 
 
 subsidences, being 
 renewed again each 
 time by upbuilding 
 from lava and frag- 
 mental material. 
 Where shown in sec- 
 tions, the structure 
 is that of stratified 
 layers dipping away 
 steeply from the 
 crater, but more 
 complicated than in 
 Vesuvius. Numer- 
 ous dikes intersect 
 the layers vertically 
 and bind them to- 
 gether, such dikes 
 of the Val-del-Bove 
 
 FIG. 
 
 79. Dikes at the base of the Serra del 
 Solfizio, Etna. (After Lyell.) 
 
 being especially well shown in the walls 
 (Fig. 79)- 
 
 The formation of such dikes was illustrated by the eruption of 
 1669, when the whole top of the mountain collapsed. At this time 
 a fissure six feet broad and of unknown depth opened in the side 
 of the mountain, extending north and south for a length of 12 miles 
 from the plain of St. Lio to within a mile of the summit. Five 
 other parallel fissures opened one after the other. The incandes- 
 cent glow from these fissures showed that they were filled up to a 
 certain height with lava which on cooling produced transecting dikes. 
 Near the town of Nicolosi, which lay near the base of the wooded 
 
Forms and Activities of Typical Modern Volcanoes 135 
 
 region, about 20 miles from the summit of Etna, and which was de- 
 stroyed by preliminary earthquakes, a gulf opened from which sand 
 and scoria were thrown, which built up a subordinate cone, Monte 
 Rossi, about 450 feet high (see map, Fig. 77). 
 
 The great lava stream of this eruption overflowed fourteen towns 
 and villages, some having a population of between 3000 and 4000 
 souls, and finally reached the walls of Catania by the sea (Fig. 77) 
 15 miles away. It accumulated against the walls of this city, 
 which were 60 feet high, and finally flowed over them, but without 
 
 FIG. 80. View of a part of the Val-del-Bove with parasitic cones and steep 
 lava streams. The main crater, emitting steam, is in the background. 
 (After Sartorius and Lasaulx, from Kayser's Lekrbuch.) 
 
 destroying them, falling on the inside in a series of fiery cascades 
 which overwhelmed part of the city. It covered the first 13 miles 
 of its journey in 20 days, but required 23 days for the last two miles. 
 Sometimes it moved at the rate of 1500 feet an hour, at other times 
 only a few yards in several days. When it finally reached the sea, 
 it was still 600 yards broad and 40 feet deep. Its surface was 
 generally solid rock, but the hot liquid interior broke this surface 
 and flowed on to be in turn chilled with a repetition of the 
 process. The course of this lava stream is shown upon the map 
 (Fig. 77). 
 The formation of lateral cones or monticules from which two 
 
136 
 
 Modern Volcanic Phenomena 
 
 eruptions proceed for every one that issues from the main cone, 
 is a very characteristic feature of Etna, more than 200 such cones 
 being known. One of these (Monte Minardo, east of Bronte, see 
 map, Fig. 77) reached a height of over 750 feet. When new open- 
 ings form, the lava from these may surround and even bury the 
 old monticules, so that the volcano is covered with numerous, more 
 or less buried, extinct, parasitic cones. 
 
 In the eruption of August, 1852, to May, 1853, two new cones 
 opened close together near the head of the Val-del-Bove, rising in 
 1 6 days to a height of about 500 feet (Fig. 80). The lava poured 
 down the Val-del-Bove, in places completely filling it from side to 
 side, so that it became a barren waste, no longer able to support the 
 cattle from which it had derived its name. 
 
 Mont Pelee 
 
 This volcano, on the island of Martinique in the West Indies, 
 became violently active in May, 1902, the volcano Soufriere on St. 
 Vincent going into activity at almost the same time. The erup- 
 tion of Mont Pelee was characterized by violent explosions, pre- 
 
 FIG. 81. View of the volcano Mount Pelee, on Martinique, showing the 
 spine (a) with a larger view of the same (b). (From E. de Martonne in 
 Geographic Physique.) 
 
 ceded by small premonitory symptoms. There was no actual out- 
 pouring of lava, which was completely shattered, and the mass of 
 minute, incandescent rock particles was carried aloft by the highly 
 heated gases, forming dense fiery clouds, which not only rose into 
 the air, but rushed like a stream through a gap in the crater down 
 the slopes of the mountain into the sea, overwhelming and destroy- 
 ing all life, including all but two individuals of the 30,000 inhabit- 
 
Forms and Activities of Typical Modern Volcanoes 137 
 
 ants of the town of St. Pierre. This ejection of incandescent 
 clouds continued for. several months. Another remarkable feature 
 of this eruption was the formation of the great spine, which will 
 be again considered somewhat later (Figs. Si, 106, 107). 
 
 Krakatoa and Bandai-San 
 
 The volcano of Krakatoa forms an island in the straits of Sunda, 
 between Java and Sumatra in the East Indies (Fig. 82). It sud- 
 denly became active on August 26 and 27, 1883. This activity 
 
 FIG. 82. Map of the straits of Sunda in the East Indies, showing the 
 location of the volcano Krakatoa and the rift lines which center in it. (After 
 R. D. H. Verbeck, from Ratzel, Die Erde.) 
 
 began with a series of premonitory convulsions, after which the 
 greater part of the island was blown away by a succession of terrific 
 explosions, the detonations of which were heard more than 150 
 miles away. A mass of material estimated at a bulk of almost one 
 and one-eighth cubic miles was thrown into the air in the form of 
 lapilli, ashes, and the finest dust some of it to the height of seven- 
 teen miles and the air waves generated by the explosion traveled 
 westward carrying the dust with them, and are supposed to have 
 passed three and a quarter times around the earth (82,200 miles) 
 before they died away. For many months after the eruption the 
 dust in the air caused a series of brilliant sunsets all over the earth. 
 
138 
 
 Modern Volcanic Phenomena 
 
 Dust fell in large quantities on the decks of vessels 1600 miles dis- 
 tant, three days after the eruption, and tracts of deep water were 
 made so shallow from this dust as to become unnavigable. Great 
 sea-waves (tsunamis) were generated, one of which was estimated 
 
 Crater Perbuatan 
 
 Verlaten I. 
 
 Lang I. 
 
 f - Crater Danan 
 
 " Rakata 
 
 Ancient Volcano 
 
 lanan /''Rakata 
 
 Vfrlatenl. . 
 
 Sea Level 
 
 FIG. 83 a. Map and section (on line AB) of Krakatoa before the explo- 
 sion of 1883. After Verbeck. i, older andesite; 2, younger andesite; 
 3, basalt; T, Tertiary basement. (From Kayser's Lehrbuch.) 
 
 to have risen 100 feet, and these destroyed 1295 towns and villages 
 along the shores, killing 36,380 people. By their force a large ship 
 was carried inland for a mile and a half and left stranded 30 feet 
 above sea-level. Great blocks of stone, weighing from 30 to 50 
 tons, were also carried inland for two or three miles. Altogether 
 this was the most stupendous manifestation of volcanic activity 
 
Forms and Activities of Typical Modern Volcanoes 139 
 
 known in modern times. The appearances before and after the 
 explosion are shown in the maps and sections here given (Figs. 83 
 a, b) and the main remaining mass in Fig. 84. 
 
 Bandai-San in Japan was a volcanic cone 2000 feet high and had 
 been dormant for a thousand years. Suddenly in 1888 the greater 
 part of the cone was blown away by a terrific series of explosions 
 
 Lang /. 
 
 FIG. 83 b. Map and section (on line CD) of Krakatoa after the explosion 
 of 1883. T to 3, same as in preceding; 4, product of latest eruption. 
 (From Kayser's Lehrbuch.) 
 
 which continued through a period of less than two hours, leaving 
 only a remnant which terminates in a cliff about 1500 feet high 
 (Fig. 85). This explosion is believed to have been due to the per- 
 colation of surface waters into the volcanic interior and the forma- 
 tion of steam, which caused the disruption. No lava flows were 
 observed in this eruption, and the volcano has since been in- 
 active. 
 
140 
 
 Modern Volcanic Phenomena 
 
 FIG. 84. View of the Rakata of Krakatoa, the chief remaining fragment 
 of an older eruption, showing the numerous dikes which bind the mass together. 
 (After Judd from Ratzel, Die Erde.} 
 
 FIG. 85. Section of the Bandai-San. (After Sekiya.) The dotted line 
 shows the part destroyed by the explosion of 1888. 
 
 CLASSIFICATION or VOLCANOES ACCORDING TO TYPE or 
 ERUPTION AND FORM 
 
 We have now seen something of the mode of eruption of several 
 distinct types of volcanoes in various parts of the earth. Ac- 
 cording to their mode of eruption, we may classify them as the 
 quiet type on the one extreme, represented by the welling up and 
 pouring out of liquid lavas, as in Kilauea, and the explosive type, 
 represented by Krakatoa and Bandai-San, where shattering of 
 
Classification of Volcanoes 141 
 
 rock material occurs, but without the outpouring of liquid lava. 
 In the milder examples of this type a cinder cone is built up 
 (Figs. 86 a, b). Between these two stand the types which show both 
 kinds of eruption, with the result that beds of volcanic ashes and 
 lapilli alternate with beds of solidified lava. Here we place Vesu- 
 
 FIG. 86 a. Cinder Cone, Arizona. Young cinder cone on left, late mature 
 cinder cone on right. The young cone and lava flow are but a -few hundred 
 years old and are located on the northern edge of the Flagstaff, Arizona, topo- 
 graphic sheet. (Photo by D. W. Johnson.) 
 
 vius and Etna, explosive eruptions being more marked in the former 
 and lava eruptions in the latter. 
 
 Comparison of Form. Comparing the form of the cinder cone 
 with that of a pure lava cone, we see a striking difference. The 
 former, illustrated by the wonderfully perfect cinder cone of May on 
 in the Philippines (Frontispiece) has steep slopes, the angle being 
 determined by the nature and coarseness of the fragmental ma- 
 terial. The lava cone, on the other hand, especially that composed 
 of basic lava, is broad and relatively low, though the crater may be 
 situated at a great height above the base. The slopes are very 
 gentle, and the top generally a plateau. This is illustrated by 
 Mauna Loa in the Hawaiian group. 
 
 In the diagrams on page 143 is shown a comparison of a number 
 of modern volcanoes and craters drawn approximately to scale 
 (Fig. 87). 
 
Modern Volcanic Phenomena 
 
Geological Age of Volcanoes and Lava Flows 143 
 
 
 5 Km 
 
 FIG. 87. Cone sections of various types of volcanoes, i. Vesuvius. 
 2. Lake Laach. 3. Rocca Monfina. 4. Lago Bracciano. 5. Krakatoa. 
 6. Peak of Teneriffe. 7. Mauna Loa. (From Kayser's Lehrbuch.) 
 
 GEOLOGICAL AGE OF 
 
 VOLCANOES AND 
 
 LAVA FLOWS 
 
 The geological age of 
 a volcano can be deter- 
 mined from the age of the 
 associated formations. It 
 is obvious that a lava 
 flow is always younger 
 than the formation upon 
 which it rests, and older 
 than that which covers it. 
 In Fig. 88 is shown a lava 
 sheet which rests upon 
 river gravels of Pleisto- 
 cene age and is .therefore 
 younger than these. The 
 extensive erosion which 
 it has suffered, indicates 
 
 that it is probably of late FlG 88 _ Uta flow over Pldstocene gravel> 
 Pleistocene age. Utah. (Photo, by F. J. Pack.) 
 
CHAPTER VIII 
 
 STRUCTURAL CHARACTERS OF VOLCANOES, AND 
 OTHER IGNEOUS PHENOMENA 
 
 THE structural character of volcanoes is revealed in cones that 
 have become extinct, for in these the parts are not only more easily 
 accessible, but dissection has often revealed the internal character 
 as well. 
 
 EXTINCT VOLCANOES 
 
 There are many regions where volcanoes have been active in 
 the recent geological past, and in such cases enough of the form 
 and character of the volcanoes, now extinct, is still retained to 
 enable one to recognize them readily. Such recently extinct and 
 partly dissected volcanoes are not only of interest as showing 
 former distribution of volcanic activity, but they have an added 
 value because their erosion has revealed many features which in an 
 active volcano are not open to view. Thus the study of the 
 recently extinct volcanoes supplements that of active ones. 
 
 Extinct Volcanoes of Central France 
 
 One of the most notable fields of former volcanic activity, and 
 one that has played a prominent part in the history of the science 
 of volcanology, lies in the central part of the great area of crystalline 
 and younger rocks which makes up the so-called Massif Central 
 of France, and which is bounded on the north by the Paris basin 
 of Mesozoic and Tertiary rocks, on the southwest by the basin 
 of the Garonne, and on the east by the valleys of the Rhone and 
 Saone. The center of this massif (Fig. 89) is dissected by the 
 river Allier, which flows north into the Loire, and it is along the 
 western border of the Allier valley, which is bounded by a fault, 
 that the main volcanic district is located, while a second one lies 
 
 144 
 
Extinct Volcanoes 
 
 145 
 
 on the southeast, in the region of the headwaters of the Allier and 
 the Loire. 
 
 The younger eruptive rocks belong to several geological epochs, 
 namely the Pleistocene, the Pliocene, and the Miocene. Of these 
 the youngest, of Pleistocene age, form the famous Chaine des Puys, 
 
 FIG. 89. Geological map of Auvergne (after Michel-Levy, M. Boule 
 and Glangeaud). Scale about i : 1,400,000. Light gray, gneiss and granite. 
 Black, Carbonic beds (mostly confined to the Loire valley). Dark gray, 
 Tertiary trachytes, basalts, etc. Fine lines, Quaternary eruptives of the Puys. 
 White, Tertiary and Quaternary. (From Kayser's Lehrbuch.) 
 
 a group of extinct volcanoes which still retain to a remarkable 
 degree their form and general character. South of this lies the vol- 
 canic district of Mont Dore, and still farther south that of Cantal 
 - both of Pliocene age. To the east of the latter and between 
 the Allier and the Loire is the Chaine du Velay, of Pliocene basalts, 
 and east of this a series of Miocene, eruptive hills (Mezenc and 
 Megal). 
 
146 Structural Characters of Volcanoes 
 
 The Chaine des Puys. This is best approached from Clermont- 
 Ferrand, which lies to the east of the highest of these old volcanic 
 hills, the Puy de Dome (Fig. 90), from the summit of which an 
 inspiring panorama of these silent volcanic cones, some sixty in 
 number, may be seen. They extend for a distance of about 90 
 kilometers north from the Mont Dore region. The Puy de Dome 
 itself is not a perfect volcano, but is formed by a central dike or 
 neck of trachytic rock (called by the French domite), but the great 
 majority of the cones show each their cup-shaped crater at the top, 
 
 FIG. 90. Puy de Dome, an extinct volcano in the Chaine des Puys, central 
 France. (Photo, by D. W. Johnson.) 
 
 and produce a volcanic topography rivaling that of the surface of 
 the moon (Fig. 91). The lava of these volcanoes consists of olivine 
 basalts and andesites, with abundant slags, scoriae, and pumaceous 
 material all rich in ferro-magnesian minerals, and containing from 
 50 to 58 per cent of silica. The outpourings belong to the middle 
 and later Pleistocene time. Besides the perfect crater cones there 
 are, however, great intumescences, or dome-like blisters of trachytic 
 rock without craters, and these represent the type of up-swellings 
 of viscous acidic lavas already discussed. Such an one is the 
 Grand Puy of Sarcoui, shown in the distance on the extreme right 
 of our illustration (Fig. 91), and in outline on page 124 (Fig. 69 a). 
 
Extinct Volcanoes 
 
 147 
 
 The Massif of Mont Dore. This mass, south of the Puys, 
 represents the remnant of a great volcano, active during Pliocene 
 time, but since then partly destroyed by erosion. While this makes 
 
 8< 
 
 OQ s- 
 
 P> 
 "I 
 
 possible trie detailed study of the various parts of the volcano, 
 and gives us an opportunity to observe the successive eruptions 
 and their effects, it also makes a more difficult problem for the 
 student to comprehend, for it must be borne in mind that the 
 
148 
 
 Structural Characters of Volcanoes 
 
 srg- 
 
 &, 
 
 
 
 Jiff 
 
 s 
 
 s a 
 
 -f 
 
 1-3 
 
 .11 
 
 8* 
 
 If 
 8 2 
 3. a 
 
 - n 
 
 <u .'Z -^ 
 
 ill 
 
 c/3 ^ 
 
 I T3S 
 
 -^ a 
 
 r5.y 2 
 
 s -a 
 
 many peaks and prominences 
 now seen are not separate vol- 
 canoes as in the Puy district, 
 but erosion remnants of larger 
 masses, and the imagination 
 must be drawn upon to restore 
 again what is missing and so 
 get a picture of the whole vol- 
 cano as it Was during its prime. 
 In the adjoining diagram (Fig. 
 92) is represented such a view 
 of the relationships of the dif- 
 ferent volcanic rocks which 
 make up this volcano as would 
 be obtained were the entire 
 massif cut through its highest 
 part (Sancy) like a round cake 
 cut through the middle, and half 
 of it removed so that the cut 
 side of the other half is visible. 
 Such a cross-section, as it is 
 called, is built up from innu- 
 merable local observations which 
 are then connected by logical 
 inferences. This section shows 
 that the foundation of the vol- 
 cano is ancient granite and 
 gneiss, and that it was built up 
 on an almost level surface (a 
 peneplane) which had pre- 
 viously been cut across the old 
 foundation rocks by natural 
 agencies. Upon this floor lies 
 a mass of andesitic tuff, the 
 product of the first eruptive 
 activity (upper Miocene) . Then 
 (early Pliocene) followed an 
 eruption of trachyte porphyries 
 through the Sancy vents and 
 that of the Capucin, the flows 
 
Extinct Volcanoes 149 
 
 of which extended laterally for some distance, finally thinning 
 away. The next eruptions (middle Pliocene) were those of the 
 Puy de Pailleret and the Puy de Cliergue on each side of Sancy, 
 these lavas on cooling forming hornblendic andesites. Then came 
 the late Pliocene eruption of the Plateau basalts, which cut and 
 rest upon the others, and this was followed by local Pleistocene 
 eruptions of the basalts and the formation of cinder cones which 
 correspond to those of the Puy Chain. The succession of events is 
 clearly indicated by the relationships, especially the superposition 
 of the various lavas and pyroclastic products, and it will be ob- 
 served that the eruptions proceeded from acidic trachytes (to- 
 gether with rhyolites, phonolites, etc.) to basic types, i.e. basalts. 
 The tuffs often carry impressions of the vegetation of their time 
 from which their geological age can be determined. 
 
 The Cantal. This Tertiary volcanic massif lies south of Mont 
 Dore and is connected with it by a basaltic plateau. It forms the 
 most prominent of the Pliocene volcanoes, and it also shows much 
 dissection, so that the structure and succession of eruptive events 
 can be determined. Its diameter is from 60 to 80 kilometers, and 
 its mass about ten times that of Mont Dore. As in the latter case 
 there are many elevated peaks, the highest of which, the Plomb- 
 du-Cantal, rises 1858 meters. These are all erosion peaks of 
 parts of older eruptions, only one the Puy de Griou (1694 meters) 
 representing a late, though not the latest, eruption from the cen- 
 ter of the volcano, but even it does not show the original height 
 of the mountain. In Fig. 93 two cross sections are shown, one 
 from northwest to southeast, the other at right angles to it, and 
 both passing through the center of the old volcano. These are 
 reconstructed in the same manner as that of Mont Dore, namely, 
 from numerous local observations and the combination of these. 
 The succession of eruptive events here is of similar character to 
 that of Mont Dore, the volcano resting upon an old erosion floor 
 of gneiss upon which were locally deposited beds of Oligocene sedi- 
 ments (4, Fig. 93), which are especially well shown on the south- 
 west. Through these came, first, eruptions of older basalt (5) 
 (not positively known, though suspected, at Mont Dore) and this 
 was followed by eruptions of acid trachytes and phonolites with 
 trachytic tuffs, still of Miocene age (6). These locally rest upon, 
 the gneiss, or upon the Oligocene sediments, and still again, in one 
 part of the section, upon the older basalts, showing that they 
 
Structural Characters of Volcanoes 
 
 succeeded these. Then was thrown out from the central orifice 
 a mass of breccias, cinders, etc., of andesitic and basaltic material 
 (7), intruded by dikes and interbedded with sheets of andesite, 
 
 I1*J 
 
 S ex H 3 
 
 this also forming a capping rock (8) and marking the great early- 
 Pliocene eruption from the central orifice. Through these, in 
 middle Pliocene time, was forced the dike of phonolite which forms 
 the Puy de Griou, and finally came the eruption of the last basalt, 
 
Extinct Volcanoes 
 
152 
 
 Structural Characters of Volcanoes 
 
Extinct Volcanoes 
 
 153 
 
 the so-called Plateau basalt, because the great plateaus of the 
 region are capped by it. 
 
 If the student has succeeded in gaining a clear conception of the 
 succession of events in these two dissected volcanoes, he may next 
 attempt an analysis of the two cross sections of the southeastern 
 district given in the next two diagrams (Figs. 94 and 95), where, as 
 a result of eruption through numerous vents, a more complicated 
 structure is produced. After this he will be ready to analyze 
 
 FIG. 96. Map of the volcanic areas and fracture lines of Central France, 
 the former shaded, the latter in broken lines. Fault lines are shown solid. 
 (After Marcellin Boule.) 
 
 sections of older volcanic districts, including those of our own 
 country, as given in the various folios of the Geological Atlas of the 
 United States. 
 
 Arrangement of these Volcanic Centers Along Lines of Fracture 
 in the Earth's Crust. From the study of the volcanic region of 
 Central France it has become apparent that all of these volcanic 
 manifestations are located along lines of fracture in the earth's 
 crust, these fractures making possible the rise of the lavas and 
 gases which have produced the phenomena. The map of the 
 region (Fig. 96) shows the fractures ascertained and their relation 
 
154 
 
 Structural Characters of Volcanoes 
 
 to the volcanic manifestations. This is a very general arrange- 
 ment of volcanoes the world over, and will be referred to again 
 later on. 
 
 The Extinct Volcanoes of the Rhine Region 
 
 Extinct volcanoes and volcanic activity during Tertiary time 
 are shown in a number of localities in the mountainous region 
 through which the river Rhine has cut its famous gorge. The 
 most impressive of these are the Seven Mountains (Siebengebirge) 
 
 FIG. 97. Volcanic landscape of the Siebengebirge after a photograph from 
 the Rodderberg. (From F. Ratzel, Die Erde.) These low mountains are 
 slightly dissected volcanic peaks of Tertiary age. The high peak on the left 
 is the Drachenfels. (See map, Fig. 98.) 
 
 on the .right bank of the river in the Cologne region not far from 
 Bonn (Fig. 97). Like the Tertiary volcanoes of France, these 
 show only in part their former character, erosion having modified 
 them to a considerable degree. Nevertheless, it can be recognized 
 that they represent a series of eruptions, which, like those of France, 
 proceeded from acidic to basic lavas. 
 
 The eruptions began in Miocene time, and the volcanic masses 
 were built up on the old erosion surface of the Devonian shales 
 
Extinct Volcanoes 
 
 155 
 
 and .sandstones, which form the cliff of the Rhine gorge (map, 
 Fig. 98). The first outpouring resulted in the formation of light- 
 colored trachyte of which the typical trachyte of the Drachenfels, 
 already referred to in the discussion of that rock (p. 101), was the 
 product. Others of the hills of this region were also formed by this 
 
 Devonian Clay & Sand Tuff 
 Basaltic cinders Trachyte Andesite 
 Basalt Diluvium Alluvium 
 
 FIG. 98. Map of the volcanic district of the Rhine (Siebengebirge) . 
 (After Laspeyres, from Walther.) 
 
 eruption. The next eruption was of more basic lava, resulting in the 
 formation of andesites, of which rock another group of these hills is 
 composed. Finally, very basic lavas came to the surface, forming 
 basalts, the hills of this rock being scattered among the others, 
 while dikes of the basalt cut the older andesites and trachytes. 
 This last eruption occurred in Pleistocene time. There is thus a 
 
156 Structural Characters of Volcanoes 
 
 succession from acid to basic lavas as in the Auvergne district. 
 One of the last formed of these volcanoes is the Rodderberg, 
 situated on the west bank of the Rhine between Mehlem and 
 Rolandseck. It consists of basaltic scoriae which in places rest 
 upon, and have by their heat altered and partly fused, some of 
 the older river sediments of the Rhine, and its crater, still perfectly 
 recognizable, is filled with a deposit of wind-blown dust or loess 
 and has now become the site of a thriving farm (the Broichhof). 
 
 EXTINCT CALDERAS AND SINKS 
 
 The term caldera has often been applied to large craters, such 
 as those of Kilauea, but it has recently been suggested that these 
 
 FIG. 99. Cinder Cone within the Crater Lake, Oregon. A volcano 
 built within the basin of a sink. (Photo by D. W. Johnson.) 
 
 be spoken of as sinks, because they are formed by subsidences of 
 the lava column, and that the name caldera be restricted to explo- 
 sion craters or hollows such as that formed at Krakatoa (Daly). 
 Both sinks and calderas are known which were formed by past 
 volcanic activities in a region not subject to such disturbances at 
 present. An example of an older sink is Crater Lake, Oregon 
 (Figs. 99, 100 a-d). This occupies the site of a former volcano, 
 which has been named Mount Mazama (Fig. 100 d) and the sum- 
 mit of which has collapsed. From this summit glaciers descended 
 probably during the Pleistocene glacial period, which scoured and 
 polished the sides of the volcano, as is shown by the marks still 
 
Extinct Calderas and Sinks 
 
 157 
 
 lo/ TRAVEL-GUIDE MAP '-' 
 
 CRATER LAKE F NATIONAL PARK 
 
 OREGON 
 
 1 o ' i Scale 2 3 4 Miles 
 
 FIG. ioo a. Map of Crater Lake, National Park, Oregon. (U. S. G. S.) 
 
158 
 
 Structural Characters of Volcanoes 
 
 FIG. ioo b. Map of Crater Lake, Oregon. (U. S. G. S.) Heights and 
 soundings in feet. (Copied from de Martonne.) 
 
 remaining on the outer slopes of the lake rim. Thus the col- 
 lapse of the mountain summit is shown to have been a recent one, 
 and appears to have followed upon an extensive outpouring of 
 
 FIG. ioo c. Profile section of Crater Lake National Park. (U. S. Dept. 
 
 Interior.) 
 
 lava. Many old calderas are found in various parts of the world, 
 those of the Eifel district in western Germany (Fig. 101), where 
 they are known as Maare, being the most typical (Figs. 102, 
 103). These are readily recognized by their circular character, 
 
Extinct Calderas and Sinks 
 
 and by the fact that around their margins are extensive deposits 
 of scoriae and even of small volcanic bombs together with 
 the fragments of shale and sandstone blown from the craters 
 (Fig. 104). Less frequently are lava flows of basalt trachyte or 
 
 FIG. 100 d. Section of Crater Lake and its rim, with the probable outline 
 of Mount Mazama. Structural details generalized. (Vertical and horizontal 
 scale the same.) Smithsonian Institution. 
 
 phonolite associated with them, which, together with the frag- 
 mental material, built up crater cones. One of the most typical and 
 largest of these hollows is now occupied by the beautiful Laacher 
 Lake (Fig. 105). The lapilli from these explosive eruptions form 
 
 FIG. 101. Map of the volcanic district of the Eifel. (After von Deschen.) 
 Be. Bertrich ; Da. Daun ; Dr. Dires ; Ge. Gerolstein ; Gi. Gillenfeld ; H. Hilles- 
 heim; Klb. Kelberg; Ma. Manderscheid ; U. Ulman; Maare in black; vol- 
 canic rocks, shaded. (From Kayser's Lehrbuch.} 
 
i6o 
 
 Structural Characters of Volcanoes 
 
 FIG. 102. Gmiinden Maar, Eifel. 
 An explosion crater converted into a lake. 
 
 FIG. 103. Schalkenmehren Maar, Eifel. 
 A Tertiary explosion crater converted into a lake. 
 
Volcanic Funnels, Pipes, Plugs, and Necks 161 
 
 extensive deposits of " sand " along the left bank of the Rhine, 
 readily visible from the train. 
 
 Tttff- 
 
 Watt 
 
 FIG. 104. Ideal section through a Maar of the Eifel showing the old crater 
 funnel and pipe, with the lakelet in the upper part, and the wall of tuff and 
 scoriae surrounding it. (From Kayser's Lehrbuch.) 
 
 VOLCANIC FUNNELS AND PIPES, SPINES, PLUGS, AND NECKS 
 
 Funnels and Pipes. From the mouth or rim of the crater 
 of the volcano the slope is generally inward, forming a funnel- 
 shaped depression to its bottom, this constituting the normal 
 crater. This differs from the crater of Kilauea, which is a sink 
 with practically perpendicular sides. The funnel is continued 
 
 FIG. 105. The Laachersee (Lake Laach) Northwest Germany, occupying 
 an old explosion crater. (After Walther.) 
 
 downward into the depths of the earth's crust as a more or less 
 cylindrical tube or pipe, which is the main conduit or vent through 
 which the lava reaches the surface. (See Fig. 87, p. 143.) 
 
 The Spine of Mont Pelee (Figs. 106, 107). We have, of 
 course, no direct means of knowing from observation that this tube 
 
l62 
 
 Structural Characters of Volcanoes 
 
 is in reality a cylindrical one, nor that it penetrates vertically 
 through the country rock upon which the volcano is built. That such 
 is the case, however, may be inferred from the remarkable phenome- 
 non which accompanied the eruption of Mont Pelee in Martinique 
 
 FIG. 106. The great "spine" of Mont Pelee, Martinique, from the east. 
 From the old summit plateau, the basin of L'Etang Sec. The spine rises^ ap- 
 proximately 358 meters above the old crater rim in the middle foreground. 
 (Photo by E. O. Hovey, March 25, 1903 ; courtesy of American Museum of 
 Natural History.) 
 
 in 1902, when a columnar mass of extremely viscous or solid lava 
 was pushed up 700 to 1000 feet above the crater, reaching a height 
 of over 5000 feet above the level of the sea. This remarkable 
 
Volcanic Funnels, Pipes, Plugs, and Necks 163 
 
 column appears to have been the plug of lava which filled the pipe 
 of the volcano, and which lacked the proper fluidity to flow over 
 the edge of the crater, but hardened in the pipe and was pushed 
 upward by the pressure of the gases beneath, retaining essentially 
 the form of the tube in which it had solidified. The growth of 
 this spine of Mont Pelee was actually witnessed. It began in 
 October, 1902, and reached its maximum elevation in seven months. 
 After that it slowly crumbled away under the influence of the 
 
 . FIG. 107. The spine and upper part of the new cone of Mont Pelee, Mar- 
 tinique, from the north ; from the crater rim. (Photo by E. O. Hovey, March 
 26, 1903 ; courtesy of American Museum of Natural History.) 
 
 atmosphere and its own weight, and from the explosion of gases 
 beneath it. 
 
 Plugs and Necks. If we speak of the tube which descends from 
 the base of the crater as the volcanic pipe or vent, the solidification 
 of the lava in this pipe forms the volcanic plug. When this plug 
 is pushed upward and becomes visible, as in Mont Pelee, it con- 
 stitutes a volcanic spine. When it becomes visible as the result of 
 the removal of the enclosing mass of the rock material which 
 constituted the volcanic cone, it becomes a volcanic neck. Necks 
 are often left as the only relief feature of a volcano, because of the 
 
164 Structural Characters of Volcanoes 
 
 solid nature of the lava which has hardened in the tube or pipe, 
 and the more readily erodible character of the material which 
 constitutes the rest of the volcano. Frequently, too, a cross- 
 section of a plug still within the pipe is seen when the country 
 has been worn down until both volcano and projecting neck have 
 been removed. Again, in an erosion cliff, a vertical section of a 
 part of such a volcanic plug may sometimes be seen, showing 
 the relation which it assumed to the country rock on cooling. 
 From such sections the form of the plug, and hence that of the 
 original pipe, may be ascertained. Theoretical considerations, 
 too, would lead us to infer, first that the upward path of the 
 heated gases, vapors, and lavas is most likely a direct one, and 
 secondly that the passage-way would become a more or less cy- 
 lindrical one, even though it was part of an irregular fissure in 
 the first place. 
 
 The French experimental geologist, Daubree, was able to show 
 that gases and vapors under high pressure, when forced through 
 fissures in limestone, granite, steel, or other substances, con- 
 verted their passageway into a cylindrical canal. From this we 
 may argue that the pipe or passageway of some, volcanoes was 
 formed by the advancing heated gases and vapors which are 
 liberated from the magma deep down in the earth, and which, 
 under a pressure of thousands of atmospheres, are forced to find 
 their way to the surface through fissures, which they enlarge 
 to cylindrical canals and thus prepare the way for the uprising 
 lava masses. 
 
 Volcanic Necks and Exposed Plugs of Old Volcanoes 
 
 As we have seen, the term volcanic neck is applied to the hardened 
 mass of lava which filled the upper part of the pipe of the volcano 
 and which has been modeled out in relief by the erosion of the 
 material of the cone which formerly surrounded it. This is to 
 be distinguished from a volcanic plug, which is the mass of lava 
 hardened in the pipe which is seen to be still surrounded by the 
 material through which it passed, whether this is the material of 
 the cone, or the country rock beneath, on which the volcano was 
 built up. 
 
 In districts of comparatively recent but extinct volcanic activity, 
 volcanic necks are not uncommon. Some of the finest examples 
 
Volcanic Funnels, Pipes, Plugs, and Necks 165 
 
 are found in the Tertiary volcanic district of Haute-Loire, France, 
 the eastern part of the volcanic region of Auvergne already referred 
 to. Here, near the town of Le Puy, is the famous Rocher St. 
 Michel (Fig. 108), an almost perfect example of a volcanic neck 
 from which all surrounding material of the cone has been removed. 
 This old neck, the position of which is shown in the section on 
 p. 152 (Fig. 95), is not a uniform mass of lava, however, but is rather 
 
 FIG. 108. Rocher St. Michel, in the Bassin du Puy, south central France. 
 The modeled-out neck of an extinct volcano, now crowned by a chapel. For 
 location and relation to other rocks of the region, see the section, Fig. 95, page 
 152. (After Tempest Anderson, Volcanic Studies in Many Lands.} 
 
 made up of fragments of volcanic rock bound together by igneous 
 material, and represents the product of an explosive eruption, the 
 fragments having fallen back again into the throat of the volcano 
 where they were solidified. Thus this neck represents the filling of 
 the upper part of the old volcanic vent, and although it no longer 
 shows the perfect original form of this vent, having been subjected 
 to narrowing by erosion near the top, it may still be regarded as 
 nearly typical of such structures. Its summit is to-day crowned 
 by a chapel. The neighboring and higher Rocher Corneille 
 is, however, an erosion remnant of a brecciated lava mass, 
 
1 66 Structural Characters of Volcanoes 
 
 probably from this same volcano, and rests on horizontal strata 
 (Oligocene) . 
 
 Typical examples of volcanic necks from our own country are 
 the Leucite Hills of Wyoming, and peaks of the Mount Taylor 
 region of New Mexico (Fig. 109) . Another example, from Colorado, 
 is shown in Fig. no. In diameter such necks may vary from several 
 hundred feet to several miles, and the material may be either solid 
 lava or fragments of the same bound together, that is, a breccia 
 of lava fragments. When the necks have been exposed to atmos- 
 pheric influences for some time, especially in a dry climate, they 
 
 FIG. 109. Great Neck, a volcanic neck in the Mount Taylor region, New 
 Mexico. (Photo by D. W. Johnson.) 
 
 are apt to crumble, and a mass of loose debris will accumulate 
 around them, forming a conical hill or tepee butte, from which the 
 summit of the much reduced neck may project. Such a hill must 
 not be mistaken for the original volcanic cone which surrounded the 
 central plug of lava before it was modeled out in relief as a neck. 
 It is merely a conical heap of fragmental material forming talus- 
 slopes around the central core, from the destruction of which the 
 material was derived. 
 
 Not all projecting neck-like masses of volcanic rock, however, 
 can be interpreted as old volcanic necks. We have already seen 
 that the Rocher Corneille at Le Puy, France, is an erosion remnant 
 of a brecciated lava sheet resting on older rocks, although its form 
 is not unlike that of the old neck of the Rocher St. Michel close by 
 (see Fig. 95). The famous Devil's Tower, or Mato Tepee, of 
 
Volcanic Funnels, Pipes, Plugs, and Necks 167 
 
 Wyoming (Fig. in) has long been, and by some is still, regarded 
 as an ancient volcanic neck, but others consider it as a tower-like 
 erosion remnant of an old lava sheet intruded beneath the sur- 
 face (probably a laccolith, see beyond). The peculiar columnar 
 structure of the rock of this tower favors the last interpretation, 
 as will be more fully shown in the next chapter. The determina- 
 tion of the neck character of such a mass depends, of course, on 
 the possibility of showing that the igneous rock continues down- 
 ward through the rocks of the earth's crust, whereas an erosion 
 remnant of a lava sheet or intruded mass would rest upon the 
 
 FIG. no. Conical butte formed by a typical volcanic plug, a pillar of 
 basalt formed by cooling in the vent of an extinct volcano, and modeled out 
 in relief as a neck by erosion of the volcanic material which formerly surrounded 
 it. It is now surrounded by basaltic debris due to weathering, and this forms 
 talus-slopes. One mile north of Adair station, Colorado and Southern R. R. 
 Elmoro quadrangle. Colorado. (G. W. Stose, photo from U. S. G. S.) 
 
 rocks of the crust which are continuous beneath it, and would 
 not necessarily have direct connection with the deeper parts of 
 the earth. 
 
 Volcanic plugs, i.e., the hardened lava which still fills the old 
 pipes, are exposed in many regions in horizontal or in vertical 
 sections as the result of erosion. Practically all of these are found, 
 not within the old volcanic cone, for sections of such cones are 
 seldom if ever exposed, but in the rock beneath, upon which the 
 now vanished volcano had been built. They therefore represent 
 the lower part of the filling of the old volcanic pipe or conduit, 
 and those that are exposed belong to the older volcanoes of the 
 
i68 
 
 Structural Characters of Volcanoes 
 
 world. The presence of the solid lava plug is of course inferred 
 in all uneroded volcanoes, and indeed it is a necessary part of an 
 extinct example. . The part it takes in the determination of a 
 volcano-like hill as an extinct volcano is illustrated by the classical 
 case of the Kammerbiihl, a small hill in northern Bohemia, which 
 played a leading role in the days when geologists still discussed 
 the question of the origin of beds of basalt, one group, the follow- 
 
 FIG. in. Mato Tepee, Devil's Tower, Wyoming, a supposed volcanic neck 
 showing vertical columnar structure. (Photo by N. H. Barton, U. S. G. S.) 
 
 ers of Werner, holding that this rock was precipitated from water, 
 while another group argued for its volcanic origin. 
 
 The hill in question (Fig. 112 a) is composed mainly of cinders and 
 altered sediments, with a small basalt stream on one side. Werner 
 and his followers contended that the material of this hill and others 
 like it was formed by the combustion of beds of coal, which had 
 not only burned the older slates and the younger rock material, 
 but had also in part melted a layer of basalt of aqueous origin, and 
 so produced the cinders. The poet Goethe, however, believed 
 
Volcanic Funnels, Pipes, Plugs, and Necks 169 
 
 this hill to be an extinct volcano, and argued that if a tunnel were 
 driven into it the hardened plug of lava in the old pipe would be 
 encountered near its center. To prove the correctness of the view 
 of the poet-naturalist, 
 his friend Count Cas- 
 par von Sternberg 
 had the tunnel driven 
 in 1837, with the re- 
 sult that the volcanic 
 plug was found and its 
 connection with the 
 basalt layer proved 
 (Fig. 1126). This 
 virtually ended the 
 controversy about the 
 
 FIG. 112 a.- 
 
 The Kammerbiihl, an old volcanic 
 
 hill in Bohemia, 
 origin of basalt. 
 
 Sometimes old lava plugs may be modeled out in relief by the 
 erosion of the older sediments which were penetrated by the vol- 
 canic pipe. In such a case the resulting hill is essentially a neck, 
 not readily distinguishable, except perhaps in the material of 
 which it consists, from necks produced by the removal of the 
 volcanic cone only. An example of such an older neck is found 
 in Edinburgh, Scotland, where it forms the famous peak known 
 as Arthur's Seat, near Holyrood Castle. The material of this 
 peak consists of hardened basaltic lava and fragmental rock in 
 which not infrequently are found pieces of wood from the trees 
 
 which clothed the 
 slopes of the ancient 
 volcano. These 
 wood fragments, 
 now silicified, were 
 buried in the frag- 
 mental material 
 which filled the old 
 throat of the vol- 
 cano, and which 
 probably extends to some depth because of the sinking of the old 
 lava plug on solidifying. 
 
 Sections of volcanic plugs in the older rocks are often exposed 
 by erosion. An illustration of one such from the coast of Ireland 
 
 FIG. 112 b. Section of the Kammerbiihl, show- 
 ing the probable former outline of the volcano and 
 the old volcanic plug, a, metamorphic rocks; 
 b, basaltic scoriae; c, plug of basalt; d, stream of 
 basalt ; e, alluvial beds. 
 
170 
 
 Structural Characters of Volcanoes 
 
 Chatk 
 
 is here given in Fig. 113 a, and a second, still joined to the basaltic 
 lava sheet, in Fig. 113 b. The basaltic lava penetrates the chalk, 
 which was altered to marble by the heat of the basalt. It is prob- 
 able that these are 
 not the plugs filling 
 the pipes of sepa- 
 rate volcanoes, but 
 that they represent 
 fissures through 
 
 FIG. 1 13 a. Section of volcanic plug (basalt) in wn i cn lava reached 
 chalk. Coast of Antrim, Ireland. (After Geikie.) the surface in the 
 
 form of a great 
 
 sheet, as described later. A similar plug may be seen on the 
 Nova Scotia coast at Wasson's Bluff, not far from Parrsborough. 
 Here the volcanic material penetrates old sandstones and gypsum 
 
 A A A A A A A 
 
 A A A / 
 
 Basalt 
 
 FIG. 1136. Marmorization of chalk beds by basalt. Island of Rathlin 
 on coast of Antrim, Ireland. (Leonard.) The marble is dotted. (From 
 Kayser's Lehrbuch.) 
 
 beds of late Palaeozoic age, and belongs to the eruptions of Triassic 
 time which produced the great lava sheet of Cape Blomidon and 
 adjoining regions. 
 
 Sections of old volcanic plugs are 
 abundant in some districts. Thus on 
 the famous shore of County Fife in 
 Scotland, no fewer than eighty are 
 found in a space twelve miles in length 
 by from six to eight in width. These 
 plugs (Fig. 114) penetrate sandstones, 
 shales, limestones, and coal beds, and FIG. 114. Ground-plan 
 
 are consequently of more recent date section of the P lu s of an old 
 
 ' . , , . , . , Al volcano on the shore of St. 
 
 than the periods during which these Monans> Fife> Scotland . 
 
 sediments were deposited (Mississip- (After Geikie.) 
 
Volcanic Funnels, Pipes, Plugs, and Necks 171 
 
 plan, see Chapter XXXV I). The sections 
 of the plugs are roughly circular or ellip- 
 tical, varying in size, and from them dikes 
 and sheets of igneous material not infre- 
 quently penetrate the adjoining rocks, after 
 the manner of the radiating dikes seen 
 in modern volcanoes, as already described. 
 The plugs of Fife are not placed along 
 lines of great fissures, as might be sup- 
 posed, but penetrate the rocks apparently 
 without any relationship to its structure. 
 That necks are, however, ranged along 
 great lines of fissures in the earth's crust 
 is frequently found, and is seen in the 
 alignment of some of the necks of the 
 Auvergne region in France (see ante) and 
 elsewhere, and may be inferred from the 
 arrangement of the volcanoes of Iceland. 
 Here the lava first welled up through a 
 recognizable fissure in the earth's crust, 
 which subsequently was partly closed by 
 the hardening of the lava, so that only the 
 broader portions remained open. These 
 were converted into volcanic pipes, above 
 which individual volcanoes were built up 
 (Fig. 115). The Japanese volcanoes, too, 
 seem to be arranged along great lines of 
 fissures, and this is probably true for the 
 majority of existing volcanoes, as is in- 
 dicated by their linear arrangement (Fig. 
 116). 
 
 The material filling the volcanic vents, 
 and . therefore the material of necks and 
 plugs, ranges from basalt to rhyolite, ac- 
 cording to the acidity of the volcanic 
 mass. When deeper portions of the necks 
 are exposed they show rocks of coarser 
 crystalline type, even gabbro and grano- 
 phyre, which have resulted from slow 
 cooling. In other cases fragmental ma- 
 
 
 /.u'&. 
 
172 
 
 Structural Characters of Volcanoes 
 
 LO S ALTOS 
 
 (deMatagalpa) 
 
 S\j 
 
 F.IG. 1 1 6. Volcanoes in Nicaragua, showing linear arrangement apparently 
 along a fissure line. This line is parallel to the trend of the mountains and 
 ridges, and to the coast line. (After Karl von Seebach, from F. Ratzel, Die 
 Erde.) 
 
 Miocene Volcano 
 ===== 
 
 GrPheuttaL 
 Fault 
 
 FIG. 117. Diagrammatic section through the Freiburg region (i. Br.) 
 showing the former and present topography and the extensive erosion. (After 
 Th. Lorenz, from Kayser's Lehrbuch.) 
 
Volcanic Funnels, Pipes, Plugs, and Necks 173 
 
 terial (agglomerate and tuff) fills the ancient vent, as in the case 
 of the Nova Scotia plug already cited. A mixture of both may 
 occur, and the structure may show evidence of successive erup- 
 tions through the same vent, and the formation of a cylinder of 
 lava within a cylinder. Finally, as already noted, the lava may 
 enclose fragments of the wall rock, generally from a deeper hori- 
 zon. The relationship of a plug to the old volcano in a much- 
 eroded region is shown in the section on the preceding page 
 (Fig. 117). 
 
 FIG. 1 1 8. Diamond mine in old volcanic plug, South Africa. 
 
 In South Africa, old volcanic plugs have become of great eco- 
 nomic importance, for it is in them that the great diamond mines 
 are located. These plugs, which are often of funnel form, broaden- 
 ing upward in diameter to 300 and in exceptional cases to 685 
 meters, are composed of brecciated rock, the so-called " blue 
 ground" which contains the gems (Fig. 118). Whether the dia- 
 monds represent the crystallized carbon of wood included in the 
 plugs as in those of Scotland, or whether the carbon is of sub- 
 terranean origin, is an open question. The character of the dia- 
 mond-bearing material suggests that it may have risen from great 
 depths. ** 
 
174 
 
 Structural Characters of Volcanoes 
 
 SHEET LAVAS FORMED BY FISSURE ERUPTION 
 
 Not all lava reaches the surface through volcanic pipes above 
 which volcanoes are built. Indeed, we have seen that some of 
 
 these pipes, as in the 
 Icelandic volcanoes, are 
 merely the reduced ex- 
 pression of large fissures 
 which opened in the 
 earth and through which 
 the lava at first reached 
 the surface, after which 
 volcanoes were built up 
 on those spots where 
 the fissure became 
 broken up into discon- 
 nected pipes. In some 
 
 L 
 
 UTAH 
 
 FIG. 119 a. Map of the Columbia and Snake 
 River lava fields. (After Bowman.) 
 
 of the grandest erup- 
 tions of molten rock, 
 generally basalt, no volcanoes are built up, or at least not until 
 long after the eruption begins. Instead, great flat surfaces of lava 
 
 FIG. 119 b. "Gordon Craters," Malheur County, S. E. Oregon. Irregu- 
 larities in surface of recent lava, produced by pressure. (Photo by Russell, 
 U. S. G. S. Courtesy of D. W. Johnson.) 
 
Sheet Lavas Formed by Fissure Eruption 175 
 
 lava plains or plateaus are produced. Fissure eruption, ^in 
 which the lava wells up along the entire length of the fissure, is 
 seen to-day in Iceland. Along the great Eld Cleft, lava has 
 welled up in historic times, spreading on both sides of the fissure 
 to form a flat basalt plain, 270 square miles in extent. Along the 
 southern prolongation of the fissure, however, where it was nar- 
 rower, a row of low 
 cones has been 
 formed, through iso- 
 lation of a series of 
 vents. 
 
 One of the largest 
 lava plains of this 
 type is the Columbia 
 and Snake River 
 plain, which covers 
 an area of 200,000 
 to 250,000 square 
 miles in Washing- 
 ton, Oregon, Idaho, 
 and California (Fig. 
 1 19 a). The lava 
 apparently spread 
 over a region of 
 varying topography, 
 filling the valleys 
 
 FIG. IIQC. Canon in the lava of the Snake 
 River plateau. 
 
 and burying the 
 smaller hills, sur- 
 rounding the larger 
 
 ones, and penetrating the valleys in their sides. A succession of 
 outpourings occurred, sometimes at short intervals, sometimes long 
 enough apart for the production, by weathering, of old soil beds 
 upon the preceding sheets, which were occasionally covered with 
 forests before the next lava sheet was poured out. The present 
 surface of this lava plain is in many places barren and desolate 
 beyond description, but in some places low cones rise above it, 
 formed probably during the later stages of volcanic activity, or the 
 surface is broken by pressure (Fig. ngb). Where canons are cut 
 by rivers, the nature of the material is well shown (Fig. 119 c). 
 Another extensive flat lava plain formed by fissure eruption 
 
i 7 6 
 
 Structural Characters of Volcanoes 
 
 covers Central India, and forms what is known as the Dekkan 
 Plateau. This has a present area of some 200,000 square miles, but 
 probably was originally much larger. The thickness of the basalt 
 sheet, generally spoken of as the Dekkan Trap, is in places more 
 than a mile. No cones have been built upon its surface. 
 
 Remnants of an equally extensive lava sheet of this type are found 
 in western Europe, where they occur on the west coast of Ireland 
 (Giant's Causeway, etc.), Scotland, Staffa, the Orkney and Faroe 
 Islands, and perhaps Iceland. The active work of the sea, aided 
 no doubt by the weather, has removed large parts of this plateau, 
 
 FIG. 120. Basaltic columns of the Giant's Causeway, on the coast of Ireland. 
 
 while others have probably subsided beneath the sea-level by 
 recent disturbances of that part of the earth's crust. This lava 
 sheet, too, is composite, and beds with plant remains, from which 
 the age of the eruption can be ascertained, lie between successive 
 flows. 
 
 Remnants of much older (Triassic or Jurassic) lava flows probably 
 of this type are found in New Jersey (Watchung Mountains), in 
 the Connecticut Valley of Connecticut and Massachusetts, and 
 along the shores of the Bay of Fundy in Nova Scotia. In New 
 Jersey and elsewhere the peculiar character of the base of the 
 sheet sometimes indicates that the lava overflowed bodies of 
 
Sheet Lavas Formed by Fissure Eruption 177 
 
 standing water, with the result that steam pipes and a comminuted 
 basal structure were produced, the mud from the bottom of the 
 lake or pond being forced up into the lava. In cavities thus pro- 
 duced many famous mineral deposits (zeolites, etc.) were subse- 
 quently formed. This structure is also found in lava sheets of 
 basic volcanoes. 
 
 A characteristic feature of many of these basaltic sheets is the 
 development of prismatic columns, which generally stand vertical, 
 
 FIG. 121. Near view of a portion of the columnar basalt of the Giant's 
 Causeway, coast of Ireland. The upper surfaces of the transverse joints are 
 seen to be convex in most of the columns, but in a few cases they are concave, 
 holding small pools of water. 
 
 as so finely shown in the Giant's Causeway on the north coast of 
 Ireland (Figs. 120, 121). Sometimes, however, these columns are 
 curved, as is seen in some parts of the island of Stafla off the Scot- 
 tish coast, while in other parts they are vertical as at the entrance 
 to Fingal's Cave (Figs. 122 a-d). This columnar or basaltic joint- 
 ing is also seen in some of the basalt sheets of the Columbia lava 
 plateau and the basalt of the Watchung Mountains, and is in- 
 deed a fairly common structure in basaltic lavas. The name joint 
 is unfortunate, as these structures have little in common with 
 the true joints, which are fractures in the earth's crust, and of 
 
i 7 8 
 
 Structural Characters of Volcanoes 
 
 FIG. 122 a. General view of the FIG. 122 b. The island of Staffa, 
 
 Island of Staffa from the sea. Note showing the basalt columns capped 
 
 the columnar basalt capped by massive by massive basalt on the left and an 
 
 basalt. Entrance to Fingal's Cave eroded surface of basalt columns on 
 
 near the center of the view. (Photo the right. Curved columns in the 
 
 by author.) 
 
 distance. (Photo by author.) 
 
 FIG. 122 c. Fingal's Cave on the island of Staffa, showing the columnar 
 jointing. This is a sea-cave, eroded by the waves which remove the columns. 
 The floor of the cave is covered by sea-water even at low tide, though a nar- 
 row path has been constructed on one side. The- roof of the cave is formed 
 by the non -columnar capping-mass of basalt, as is shown in Fig. 122 d. (See 
 further: Chapter XXIII, and Figs. 720, 721.) (Photo by author.) 
 
Sheet Lavas Formed by Fissure Eruption 179 
 
 secondary origin. The columnar structure, on the other hand, 
 is a primary structure, and is caused by a radial contraction of the 
 
 FIG. 122 d. Wall and f roof of Fingal's Cave, Staffa, showing the cross- 
 jointing of the columnar basalt, and the irregular appearance of the massive 
 basalt which forms the roof of the cave. (Photo by author.) 
 
 cooling lava mass about a series of equally spaced centers (Fig. 123). 
 As a result six-sided prisms are produced. A characteristic feature 
 of these prisms is the curved form of the horizontal or transverse 
 
 FIG. 123. Diagram illustrating the formation of contraction prisms. 
 The centers of attraction are connected by solid lines. The prisms formed are 
 dotted. (From Grabau, Principles of Stratigraphy.) 
 
 planes, which generally separate them into component blocks 
 (Figs. 121, 122 d). Prismatic structure, though best developed 
 in basalt, is not confined to this rock, but occurs even in such acid 
 lavas as obsidian (Fig. 124). The position of the columns is, in 
 
i So 
 
 Structural Characters of Volcanoes 
 
 general, at right angles to the surface of the mass in which they 
 are developed, while diverging and curved columns are often de- 
 veloped just below the surface of the lava sheet. Examples of 
 
 FIG. 124. Columnar jointing in obsidian, Obsidian Cliff, Yellowstone 
 National Park. (U. S. G. S.) 
 
 such curved columns are seen, as before noted, in parts of the 
 island of Staffa and in parts of the basalt sheets of the Watchung 
 Mountains, New Jersey. 
 
Minor Phenomena 
 
 181 
 
 MINOR PHENOMENA GENERALLY ASSOCIATED WITH CLOSING 
 STAGES OF VOLCANICITY 
 
 There are a number of igneous phenomena which are manifested 
 upon the surface of the earth to-day, which are regarded as pri- 
 marily associated with the dying stages of volcanicity. These may 
 be enumerated in the 
 order of their importance 
 as follows : 
 
 a. Solfataric action. 
 
 b. Fumarolic activity. 
 
 c. Mofettes and effer- 
 vescent springs. 
 
 d. Mud-volcanoes. 
 
 e. Geysers. 
 
 /. Hot springs. 
 
 Solfataric action. 
 The volcano of Solfatara 
 (Fig. 125), in the Phle- 
 graean fields near Naples, 
 represents a dying stage 
 in volcanicity, giving off 
 steam and gases only, 
 since its last eruption in 
 1198. The crater of Sol- 
 fatara is very wide, but 
 its walls are only about 
 100 feet high, while its 
 floor is marshy and salt- 
 encrusted, with occa- 
 sional pools of boiling 
 
 water. On one side, at the foot of the crater wall, a jet of steam 
 escapes from an opening and rises to a height of 6 or 7 yards. In 
 Iceland, Java, New Zealand, the Andes, and elsewhere, such 
 solfataric vents, as they are called, are common, indicating the 
 dying condition of those particular volcanoes. Besides the steam 
 many other gases are given off, these including hydrochloric acid 
 gas, sulphur dioxide, sulphureted hydrogen, ammonium chloride, 
 and, in the case of the Italian Saffioni, boracic acid, which forms 
 the source of the important borax industry of Tuscany. 
 
 FIG. 125. The Solfatara at Pozzuoli, Italy. 
 The steam and gases issue in one corner of 
 the nearly extinct volcano. 
 
182 
 
 Structural Characters of Volcanoes 
 
 Fumaroles. These are typically emanations of steam, from 
 fissures, or from cooling lava surfaces, but they also vary, espe- 
 cially with the decrease in temperature of the lava or other source. 
 Above 350 C. only dry fumaroles exist, these giving off chiefly 
 anhydrous chlorides. Between this and a temperature of 100 C. 
 fall the acid fumaroles, which give off hydrochloric acid and sulphur 
 dioxide, with some steam. At about 100 C. occur the alkaline 
 fumaroles, which give off steam and ammonium chloride, and 
 
 FIG. 126. Active mud- volcanoes near Volcano Lake, Cerro Prieto, Delta 
 of the Rio Colorado. The largest of the group, seen in the distance, is now 
 quiescent. (Courtesy of the American Geographical Society, Broadway at 
 157 St., New York. From the Geographical Review.} (For location, see map, 
 Fig. 166.) 
 
 below 100 C. fall the cold fumaroles with nearly pure steam. 
 The famous valley of Ten Thousand Smokes, in the Katmai Penin- 
 sula of Alaska, is a prominent example of fumarolic action on a 
 large scale. 
 
 Mofettes. These give off only carbon dioxide, nitrogen, and 
 oxygen at the temperature of the atmosphere. Caves in which 
 such gases are given off exist in many volcanic districts ; and they 
 are sometimes exhibited to tourists with the lowering into them of 
 dogs or other luckless animals, which quickly become unconscious, 
 
Minor Phenomena 
 
 183 
 
 after which they are drawn up and revived. The carbon dioxide, 
 on account of its weight, remains near the bottom of the cave. 
 Effervescent springs, i.e. water charged with CO2, occur in many old 
 volcanic regions. 
 
 Mud-Volcanoes. Among the subordinate phenomena asso- 
 ciated with or compa^ble to igneous activities, is the formation of 
 mud-volcanoes (Fig. 126). These are cones built up of mud with 
 small craters at the summit, resembling miniature volcanoes. 
 Their height varies from a few feet to a hundred feet or more, 
 
 FIG. 127. The Great Geyser Basin of the Madison River in Yellowstone 
 National Park. (Guyot.) 
 
 and their activity is either constant or intermittent, quiet or ex- 
 plosive. They are formed by highly heated steam or by gases, 
 which rise through a superficial layer of mud and originate from 
 underlying lava beds or from chemical reaction. The mud is built 
 up into cones of the volcanic type. As the material is soft, how- 
 ever, these cones are readily destroyed by rains, etc. 
 
 Examples are known from the Colorado Desert, from lower Cali- 
 fornia, the Yellowstone Park (the Paint Pots) and elsewhere in 
 this country. They also occur at Baku on the Caspian Sea and in 
 the Crimea, where some of them rise to 250 feet in height, and 
 are provided with apical craters. 
 
 Geysers and Hot Springs. Geysers are springs of hot water 
 which erupt violently at intervals, with periods of quiescence be- 
 tween. They have essentially the same relation to ordinary hot 
 
184 
 
 Structural Characters of Volcanoes 
 
 springs that volcanoes of intermittent explosive eruptions have to 
 those of quiet and constant lava flows. Geysers are abundant in 
 Iceland, in New Zealand, and in the Yellowstone Park, where there 
 
 are about 100 active examples 
 and more than 3000 hot 
 springs (Fig. 127). A geyser 
 consist^ typically of a more 
 or less circular basin sur- 
 rounded by a rim of silicious 
 material (geyserite), and this 
 commonly forms the upper 
 part of a cone comparable to a 
 volcanic cone and its crater. 
 From the center of the basin 
 a pipe of circular section and 
 with smooth wall descends, 
 this and the basin being filled 
 with water. In the typical 
 Great Geyser of Iceland, the 
 cone is about 120 feet in di- 
 ameter and 13 feet high, while 
 the crater-like basin at the 
 top is 60 feet in diameter and 
 5 feet deep. The central 
 tube or pipe has a diameter 
 of about 10 feet, with smooth, 
 cylindrical, vertical walls. 
 The temperature of the water 
 which fills the pipe and basin 
 
 ranges from 75 to 90 C., but at a depth of 70 feet in the pipe 
 the temperature is about 130 C. The eruption occurs at almost 
 twenty-four hour intervals, and the water of the basin is thrown 
 to a height of nearly 100 feet. The characters of the other geysers 
 are similar, though differing in detail. The appearance of the 
 Giant Geyser of the Yellowstone is shown in Fig. 128, and of Old 
 Faithful in Fig. 129. Diagrammatic sections of the two main 
 Icelandic examples, " The Geyser " and " Strokr," are given in 
 Fig. 130. 
 
 The eruption is due to the heating, above the boiling point, of the 
 water at a depth, while at the same time the pressure of the column 
 
 FIG. 128. Giant Geyser of the Yellow- 
 stone. (After Hayden.) The eruptions 
 of this geyser occur at intervals of 7 to 12 
 days, and last for full 60 minutes at 
 each eruption. The column is thrown to 
 heights of 200 to 250 feet. 
 
Minor Phenomena 185 
 
 of water keeps it from changing to steam. With increased heating, 
 however, the point is reached where the water will change to steam 
 in spite of the pressure, and the expansion of the steam raises the 
 
 FIG. 129. Old Faithful Geyser in eruption, Yellowstone National Park. 
 (Photo by D. W. Johnson.) The eruptions of this geyser occur at intervals of 
 60 to 75 minutes, each eruption lasting 4 minutes. The height to which the 
 column is thrown is 125 to 150 feet. The eruptions are heralded by loud 
 rumblings, with spasmodic outbursts of jets 10 to 20 feet in height, then the 
 column is suddenly thrown up with a loud roar, maintaining a height varying 
 from 90 to 150 feet for two or three minutes with occasional steeple-shaped 
 jets rising still higher, the jets varying, and giving off great rolling clouds of 
 steam; then the jets gradually decrease in altitude, and in five minutes the 
 eruption is over, the tube apparently empty but emitting occasional puffs of 
 steam for a few minutes longer. During the eruption the water falls in heavy 
 masses about the vent, filling the basins surrounding it and running off in all 
 directions. The estimated discharge is 3000 barrels at each eruption. 
 
 column of water, and so lowers the pressure, with the result that a 
 large amount of steam is suddenly formed from the super-heated 
 water, and the eruption takes place. The source of the water is 
 believed by some to be the ground-water, and by others it is re- 
 garded as new or juvenile water, given off by subterranean igneous 
 masses in the process of cooling. The heat is probably in all cases 
 of volcanic origin. The geysers of Iceland and New Zealand are 
 situated in regions of still active volcanicity, while those of the 
 
i86 
 
 Structural Characters of Volcanoes 
 
 Observed Temp. 
 
 Strokr 
 
 FIG. 130. Semidiagrammatic sections of the Icelandic geysers, "The 
 Geyser" (the type) and "Strokr." Strokr has a funnel-like pit 36 feet deep 
 and 8 feet across, expanding into a saucer-like basin. The tube is generally 
 filled to within 6 feet of the top with clear water, which boils furiously, owing 
 to the escape of great bubbles of steam coming from two openings in opposite 
 sides of the tube. When the eruption took place, the jets rose in a sheaf-like 
 column to a height of 100 or more feet. Eruptions took place at very irregular 
 and long intervals; but they could be produced in a short time by "putting a 
 lid on the great kettle," by dumping in large pieces of turf. "The Geyser" 
 is a pool of limpid green water, whose surface rises and falls in rhythmic pulsa- 
 tions. The usual temperature is 170 F. (76.6 C.) or 200 F. (93.3 C.) but 
 varies, being greater immediately before eruption. The shallow saucer-like 
 basin is about 60 feet across and slopes gently to a cylindrical shaft 10 feet 
 in diameter, forming the pipe of the geyser; this being about 70 feet deep. 
 The tube is very regular. Before an eruption, bubbles of steam entering the 
 tube suddenly collapse with loud but muffled reports and a disturbance of the 
 quiet surface of the water. During this "simmering," the water rises in dome- 
 like mounds over the pipe and overflows the basin, running down the terraced 
 slope and wetting the cauliflower-like forms of sinter that adorn it. Domes of 
 water rise in quick succession, and finally burst into play, followed by a rapid 
 succession of jets increasing in height until the column is 90 to 100 feet high, 
 accompanied by dense clouds of steam. This lasts a few moments and then 
 ceases, and the basin is empty and apparently lined with a smooth coating of 
 white silica. The great geysers of the Yellowstone surpass this in magnitude 
 of eruption, but not in beauty. (Weed.) 
 
Minor Phenomena 
 
 187 
 
 Yellowstone are in a region where volcanoes have recently become 
 extinct. 
 
 The hot waters commonly carry silica is solution, and this is 
 deposited partly by the cooling and partly by the aid of low organ- 
 isms, especially algae, and so the cone of silicious sinter is built up 
 (Fig. 131). The quiet flowing hot springs more commonly deposit 
 carbonate of lime. These deposits will be discussed in a subsequent 
 chapter. 
 
 Geysers are commonly affected by earthquakes which disturb 
 the arrangement of the rocks of the region. The Icelandic geyser 
 
 FIG. 131. Spike Geyser, Witch Creek, Yellowstone Park. Showing an 
 exceptionally fine mass of silicious sinter (geyserite) built up around the basin. 
 The sinter shows botryoidal surfaces. (Photo J. P. Iddings, from U. S. G. S.) 
 
 Strokr is said to have come into existence during the earthquake 
 of 1789, and the earthquake of 1896 put an end to its activity. 
 It has not been in eruption since. A new geyser or hot spring ap- 
 peared after the shocks of the first day (Sept. 15) of the earth- 
 quake of 1896, this spring throwing water and rocks to an estimated 
 height of 600 feet, but in a few hours it subsided to a height of 10 
 or 12 feet. It ceased to flow altogether after ten days. 
 
CHAPTER IX 
 
 FORM AND STRUCTURE OF OLDER IGNEOUS 
 
 MASSES 
 
 TYPES OF OLDER IGNEOUS MASSES 
 
 LEAVING now modern volcanoes and those which have so recently 
 become extinct that their characteristics can still be readily rec- 
 ognized even though modified by erosion, we must next turn our 
 attention to those rock masses and structures of igneous origin 
 which were formed at a depth in the earth's crust, and have become 
 visible only as the result of erosion of the overlying rocks, or through 
 dislocation of the earth's crust. These may be grouped under the 
 following divisions : i. Dikes; 2. Stocks; 3. Sills; 4. Laccoliths 
 and related types ; 5. Bysmaliths; 6. Bosses; 7. Batholiths. 
 
 The first five groups are classed as intrusive or hypabyssal masses, 
 to which group the volcanic plugs may also be referred, especially 
 that part which penetrates the older rocks beneath the volcano. 
 Bosses and batholiths are classed as deep-seated or abyssal igneous 
 rocks, and they may in general be regarded as representing the 
 hardened pools of igneous material from which both the intrusive 
 and the extrusive or volcanic masses are derived. 
 
 Intrusive or Hypabyssal Igneous Masses 
 
 Dikes. In modern volcanoes, as we have seen, the eruption 
 does not always take place through the crater, but a fissure may 
 open in the mountain side through which the lava wells out, and 
 upon it there are generally built up subsidiary or parasitic cones or 
 series of cones, the so-called monticules. This fissuring of the moun- 
 tain side is due to the fact that the material of the mountain, which 
 is often in large part volcanic ash only, is shattered with compara- 
 tive ease by the pressure of the rising lava, while the hardened plug 
 of lava, which fills the throat of the crater, cannot be lifted except 
 by a much greater force. 
 
 188 
 
Types of Older Igneous Masses 189 
 
 When lava hardens in the fissures formed in the sides of the 
 volcano, a dike is produced. This is so called, because erosion 
 often removes the soft tuffs and other material, leaving the hardened 
 lava mass projecting as a wall. Fine examples of such dikes cutting 
 stratified tuffs are seen on the flanks of the Sicilian volcano Etna, 
 especially in the Val-del-Bove, as already noted (Fig. 79, p. 134). 
 
 Such dikes are also found in many districts of recently extinct 
 volcanoes, but they are not confined to the cones of volcanoes. In- 
 
 FIG. 132. West Spanish Peak, Colorado, from the northwest. In the fore- 
 ground, a large dike and several small dikes weathered-out in relief. (After 
 Stose, from U. S. G. S.) (See map, Fig. 138, p. 195.) 
 
 deed, where eruption takes place through fissures in the earth, with- 
 out the building of volcanoes, the hardening of the lava in the fissure 
 produces a dike. As, in such cases, the country rock is often much 
 older, the contrast which the dikes form with it is generally very 
 marked. Of course, such dikes will not become visible until the 
 lava sheet of which they formed the vents is removed, or until a 
 marginal section is cut into the igneous mass by the sea or other 
 agent with the consequent uncovering of the dike. 
 
 Over much of western North America the great lava sheets are 
 still in place, and the dikes which connect with them are not visible. 
 Over much of Great Britain, however, the basalt sheet of similar 
 age, and which probably extended to Iceland, has been removed by 
 
1 90 Form and Structure of Older Igneous Masses 
 
 erosion, in part by the sea, and in part by atmospheric agencies. 
 As a result, the manifold types of rocks which make up the British 
 Isles are found transected by numerous dikes of basaltic material, 
 most of which have no apparent relation to any existing masses of 
 solidified lava. Some of these have been traced for a distance 
 of 60 or even 90 miles. Dikes connecting with remnants of the 
 basaltic sheets are also exposed on the seashore. In some cases 
 the dikes, being more resistant than the enclosing rock, have with- 
 
 FIG. 133. View of the great dike, running north from West Spanish Peak, 
 Colorado. This dike originally cut nearly horizontal strata which have weath- 
 ered away, leaving a continuous wall 100 feet high. The horizontal markings 
 along the side of the wall indicate the original contact with the stratified rocks. 
 (Stose, photo; from U. S. G. S.) 
 
 stood erosion, while the country rock has been worn away. As 
 a result, they stand above the country like stone walls, a fact which 
 first gave rise to the name dike, since they are not unlike the arti- 
 ficial dikes built in some portions of western Europe, especially the 
 Netherlands, to keep out the sea from the low-lying lands (Figs. 
 132-134). 
 
 Along the border of the great trap sheet which forms the Dekkan 
 Plateau of Central India, where the sea or- other agents have eaten 
 away a part of the outer margin of the sheet, dikes are not infre- 
 quently exposed, penetrating the basement rock upon which the 
 trap sheet lies. For the most part, however, the dikes cannot be 
 traced directly to the trap, though the connection is indicated by the 
 
Types of Older Igneous Masses 
 
 191 
 
 similarity of the material. Dikes of probably much older origin 
 are found abundantly along the Massachusetts coast, and to a 
 lesser extent along the Maine coast and elsewhere. Here they gen- 
 erally consist of the type of basalt known as diabase (see p. 106), 
 a lava which has cooled with sufficient slowness to permit the devel- 
 opment of recognizable crystals. We must, therefore, conclude 
 that these dikes represent the deeper portions of fissures filled by 
 lava, where, on account of the depth below the surface at that time, 
 the cooling was slow. Often a columnar structure is found in these 
 
 FIG. 134. Devil's Wall (Teufelsmauer) near Oschitz, Bohemia. A basaltic 
 dike, 25 km. long, 2 meters thick with horizontal prismatic joints. Weath- 
 ered in relief, wall-like. (From Kayser's Lehrbuch.) 
 
 dikes, the columns extending across them from wall to wall (Fig. 
 134). Where the sea has access to them after they have become 
 exposed on the surface from the erosion of large masses of rock, 
 these columns are frequently removed by the waves, leaving a 
 deep, parallel-sided fissure, into which the sea penetrates with great 
 force during storms and at high tide. Sometimes caverns are 
 formed by the removal of the lower columns only, when the com- 
 pression of the air in these caverns under the impact of the wave 
 will result in producing a regular series of water spouts after each 
 inrush of the waters. Such phenomena are common on dike-in- 
 fested coasts and are known as " spouting horns," etc. Chasms 
 left by the removal of dikes are characteristic features of the rock- 
 bound New England coast (Fig. 135). Many other igneous rocks 
 
192 Form and Structure of Older Igneous Masses 
 
 besides basalt and diabase occur in the form of dikes, and it may be 
 inferred that a majority of them never reached the surface as 
 molten lava but cooled in blind fissures of the earth's crust. Their 
 present exposure is therefore brought about by removal, through 
 erosion, of the rock which once concealed them. Among the more 
 frequent occurrences are those of granite (aplite, etc.), and especially 
 of the coarse or giant granite, pegmatite (Fig. 136) . These pegmatite 
 dikes are often of great width, and form the source of many rare 
 mineral deposits, besides yielding feldspar, quartz, and mica in 
 
 sufficiently large masses to be com- 
 mercially valuable. In some cases dikes 
 of this and other kinds have branches, 
 and sometimes they are extremely ir- 
 regular, having perhaps eaten their 
 way into the country rock. Such ir- 
 regular dikes are also spoken of as 
 igneous veins, though they have little 
 in common with true veins (see p. 
 265). 
 
 The essential character of dikes 
 may be summed up by saying that 
 FIG. 135. An eroded dia- they are generally of considerable 
 base dike in granite; west side lineal extent, continuing sometimes 
 Rockport Point, Cape Ann. - ., , r .. . , , 
 
 Mass. In the bottom of the for man y miles > and of uniform width, 
 chasm the dike is seen covered ranging from a fraction of an inch to 
 by boulders which were formed many f eet W h en they cut bedded 
 by the waves from the dike ,. , 
 
 material. At present the chasm strata > the y ma Y do SO at an y an g le > 
 is above high-water mark, show- but in general, dikes have an approxi- 
 ing recent elevation of the coast. ma tely vertical position, though this 
 (After Shaler, U. S. G. S.) 
 
 may be changed by subsequent move- 
 ments of the entire mass. Around old volcanic centers dikes 
 are often radially arranged, as shown on the island of Mull, west 
 coast of Scotland (Fig. 137), and in the ancient volcanic center of 
 the Spanish Peaks in Colorado (Fig. 138), the dikes of which have 
 already been referred to (Figs. 132, 133). Frequently dikes of differ- 
 ent ages intersect one another, in which case the younger can be 
 recognized by the fact that it cuts the older. In the broader dikes 
 the rock texture of the marginal portion is commonly finer than that 
 of the center, because more rapid cooling took place where the 
 igneous rock was in contact with the cool country rock which it 
 
Types of Older Igneous Masses 
 
 193 
 
 penetrated. This is especially well recognized in the case of the 
 dike rocks of coarser grain. Sometimes the marginal texture is 
 even glassy, and the color and composition may also vary from the 
 margin to the center. The wall rock of dikes and the basement 
 rock of lava flows frequently show a certain amount of alteration 
 due to the heat of the lava mass. This contact metamorphism 
 (see p. 207) is seldom very extensive, however, dying out a short 
 distance from the igneous mass, especially when this cooled rapidly 
 near the surface. Lava streams have indeed been known to flow 
 over ice masses without completely melting them. This argues 
 
 FIG. 136. Pegmatite dike in crystalline dolomite, New York City. 
 
 for a comparatively rapid cooling of the lava exposed on the surface 
 of the earth. 
 
 Stocks. These are dike-like intrusions which are of short ex- 
 tent, sometimes more or less regular and cylindrical, at others ir- 
 regular. They are similar to volcanic plugs or cores, such as fill 
 the conduits of old volcanic pipes, but differ from them in not reach- 
 ing the surface, though the name has been indifferently used for 
 the intrusions of plug-like character of moderate size, even those 
 that are in reality the filling of ancient volcanic conduits. It is 
 true that it is not always possible to determine in any particular 
 case whether the intrusion reached the surface and formed a vol- 
 cano, or whether it extended only part way up into the rocks of the 
 
 
194 Form and Structure of Older Igneous Masses 
 
 earth's crusts. In such a case the name stock is best applied to 
 these structures. In character they partake of the deeper portions 
 of volcanic plugs or pipe fillings, and to some extent of those of 
 dikes as well, especially if they are irregular. 
 
 123 456 
 
 FIG. 137. Map and section (on AB) of the Island of Mull, west coast of 
 Scotland. (After Judd.) i, Non- volcanic basement beds; 2, granite; 
 3, basalt flows; 4, gabbroitic dikes ; 5, acid flows; 6, volcanic tuffs and breccias. 
 Note the radiating and branching dikes. 
 
 Intrusive Sheets or Sills. In many regions of the world we 
 find sheets of basalt and other igneous rock, the chemical and min- 
 eralogical characters of which indicate that they were formed by 
 the cooling of a magma, often with considerable slowness, and which 
 are interbedded with rocks of a clastic character, the latter evi- 
 
Types of Older Igneous Masses 
 
 dently of non-volcanic origin. Some of these have been intruded 
 between the layers, but in other cases igneous sheets of this type 
 may be old flows, which were poured out over a surface composed 
 of horizontal strata, and which were subsequently covered by other 
 strata of non-volcanic origin. It is important that the two types 
 be distinguished. In the cases of interbedded lava sheets, we 
 should expect to find evidence of this succession of deposits 
 (stratification) not only in the igneous sheet itself, but also in the 
 enclosing rock. 
 
 FIG. 138. Map of a part of the Spanish Peaks region, Colorado, showing 
 the numerous dikes radiating from the volcanic necks or ancient centers of 
 volcanic action. (See Figs. 131, 132, pp. 189, 190.) 
 
 Lava streams have very definite and recognizable surface charac- 
 ters, as we have seen, and these are different from the structures 
 found in the bottom of the sheet. In most cases, not only is 
 the surface form of a sheet distinct (ropey, pillowy, rough, etc.), 
 but the lava itself is compact, or even glassy near the surface, and 
 commonly layers of steam holes, either empty or filled secondarily 
 by mineral deposits, are found for some distance down from, and 
 parallel to, the surface. The structure of the enclosing rock also 
 is distinctive, for whereas the under layer over which the lava was 
 poured out shows some effect from the heat of the stream (meta- 
 morphism), and may actually have furnished fragments which are 
 included in the lava mass, the overlying layer will show no such 
 contact phenomena, for the lava will, in most cases, have cooled 
 
196 Form and Structure of Older Igneous Masses 
 
 sufficiently, before being covered by sediments, not to produce any 
 such effect. A still more convincing argument of the contem- 
 poraneity of the lava 
 flow is furnished when, 
 as is frequently the case, 
 fragments of the lava are 
 included in the overlying 
 stratum, having been 
 broken from the surface 
 of the sheet before or at 
 the time of the deposi- 
 tion of the covering layer. 
 Weathering of the sur- 
 face of the sheet and the 
 formation of soil layers 
 is also a clear indica- 
 tion of the exposure of 
 the lava sheet for a time 
 before it was buried by 
 sediment. Such old soil 
 surfaces are seen in nearly 
 all of the fine series of 
 ancient lava flows which 
 are to-day exposed on 
 the east coast of Scotland 
 (Fife), and the length of 
 time of the exposure is 
 shown by the fact that 
 roots and stumps of an- 
 cient trees (Calamites) are 
 found in these old soils, 
 and were completely 
 buried by the sediments 
 which followed. A suc- 
 cession of such ancient 
 forest beds is found in 
 
 FIG. 139. Section of Amethyst Moun- 
 tain, Yellowstone National Park. The 
 mountain consists of a base of Archaean 
 granite and Carbonic limestone, overlain dis- 
 conformably by 2700 feet of Tertiary strata, 
 chiefly of volcanic origin. The coarse beds 
 are conglomerates and breccias, and alternat- 
 ing with beds of finer material are sandstones 
 and shales bearing the abundant silicified re- 
 mains of fossil forests. There are at least 
 fifteen successive forests, indicating that num- 
 ber of volcanic outbursts, separated by suf- 
 ficient time to allow the growth of forest 
 trees varying in diameter of trunks from two 
 to ten feet. (After Holmes.) For photo- 
 graph of several of these trees see illustration 
 in Chapter XLV. 
 
 the series of lava flows 
 exposed by erosion in the Yellowstone National Park (Fig. 139). 
 
 Intruded igneous sheets or sills, however, show not only a basal 
 but also an upper igneous contact, and, indeed, may include frag- 
 
Types of Older Igneous Masses 197 
 
 ments of the overlying as well as the underlying strata. More- 
 over, the sheets themselves show a similar fine-grained or dense 
 character on both surfaces, while toward the center they become 
 more coarsely crystalline. Such sheets are evidently of more re- 
 cent origin than the enclosing rocks, and were intruded between 
 and parallel with them, forcing them apart, and cooling thus within 
 the earth's crust without ever reaching the surface. Such intru- 
 sive sheets are also called sills, 1 and they are well represented by 
 
 FIG. 140. Near view of the Palisades of the Hudson, showing jointed 
 trap (simulating columnar jointing) at top and talus slopes below. (Photo by 
 D. W. Johnson.) 
 
 the rocks which now form the Palisades of the Hudson River op- 
 posite New York City, though these constitute only a small part 
 of the former extent of the intruded sheet (Fig. 140). This is sev- 
 eral hundred feet in thickness and has been traced for a distance of 
 about 100 miles. A similarly extensive example, well known to 
 foreign geologists, and indeed the type from which the term sill 
 has been derived, is the " Great Whin Sill " of the north of England. 
 This can be traced for a distance of 80 miles between the enclosing 
 rocks, its resistance to erosion helping to produce the great cliff 
 known as the Pennine escarpment, which bounds the Vale of 
 
 1 The name sill was originally applied by the miners in the north of England to 
 any prominent or hard projecting bed or stratum. The type of the volcanic sills is 
 the great Whin-sill of northern England mentioned below, this being a prominent bed 
 or sill of whin-stone, a name given to any hard, fine-grained rock, such as basalt, 
 quartzite, etc. 
 
198 Form and Structure of Older Igneous Masses 
 
 Eden on the east (Fig. 141). This sill varies from 20 to 150 feet 
 in thickness, the average being from 80 to 100 feet, and it forms 
 a prominent ledge wherever exposed in 
 section (Fig. 142). It covers an area 
 probably not less than 1000 square 
 miles in extent. Other sills, mostly 
 of lesser thickness and extent, are found 
 in widely distant regions of the world. 
 Among these may be mentioned the 
 one forming Salisbury Crags in Edin- 
 burgh, Scotland, which overlooks Holy- 
 rood Castle, and which figured in the 
 disputes over the origin of basalt in 
 Werner's time (Fig. 143 c). 
 
 Through erosion, the sill is exposed in 
 various ways as shown in the following 
 diagrams (Fig. 143). Sills which were 
 originally intruded between horizontal 
 strata may become inclined by the arch- 
 ing of these strata, as in the case of the 
 Palisade sill (Fig. 143 a), or they may 
 even become folded with the strata. 
 
 A characteristic feature of sills, and 
 one which aids greatly in distinguishing 
 them from contemporaneous flows, is 
 seen in the lack of absolute conformity 
 to the enclosing strata. Though in any 
 given locality this conformity may be 
 unquestioned, it will be found that on 
 tracing the sill for some distance, it gen- 
 erally breaks across some of the layers, 
 passing either to a higher or lower level, 
 and there continuing for a time parallel to 
 the enclosing strata (Fig. 144). The rocks 
 
 of the sills are generally massive or not very coarse-grained, though 
 those of the center of large sills, such as that of the Palisades, are 
 sometimes of moderate coarseness. They are generally darker- 
 colored and denser-grained on both upper and lower margins, and 
 the change in grain toward the center is often a very regular one. 
 In the tunnels which have been cut through the Palisade sill, it has 
 
 rt ra 
 C XJ c/3 
 
 olia 
 
 J3 O CXTJ 
 
 +> W OH a 
 <A "rt ' rt 
 
 g* -3.3 
 u ^ . J 
 
 - sse 
 
 + 5 " o 
 
 5 IH 
 
 $ .2 
 
Types of Older Igneous Masses 
 
 199 
 
 been possible to ascertain, from the size of the grain, the distance 
 from the upper or lower margins at any selected locality. Colum- 
 nar structure is not a characteristic feature of sills, the apparent 
 
 FIG. 142. View looking up Hilton Beek, northern England, showing the 
 out-crop of the Whin Sill, which forms the prominent cliff on either side, and 
 rests on Carboniferous limestone. (Photo by the Author.) 
 
 columnar structure of the Palisade sheet being due to a series of 
 subsequently formed intersecting fissures or true joints. 
 
 Laccoliths. The Henry Mountains of Utah represent rem- 
 nants of a peculiar type of intruded mass which, instead of spread- 
 
 FIG. 143. Diagrammatic sections of volcanic sills, a, Palisades of Hudson; 
 ft, Whin Sill, North England ; c, Salisbury Crag in Edinburgh, Scotland. 
 
 ing out between the strata, is localized, each separate intrusion 
 swelling into a semi-lenticular or dome-like mass, space for which 
 is made by the lifting into an arch of the rocks which overlie the 
 
2oo Form and Structure of Older Igneous Masses 
 
 FIG. 144. Base of Palisade diabase, showing lateral ascent of the diabase 
 across the strata of the Newark group. Kings Point, Weehawken, N. J., 
 looking west. (From photographs, U. S. G. S.) 
 
 intruded mass (Figs. 145, 146). These structures were first de- 
 scribed by -an American geologist, the late G. K. Gilbert, and 
 the type was by him designated a laccolith. Other laccoliths 
 
 FIG. 145. Ideal restoration of the laccoliths of Mt. Holmes, Henry Moun- 
 tains, Utah. 
 
 have since been found in many parts of the world, the best known 
 examples among these being in Colorado and Montana. Lac- 
 coliths become visible by the erosion of the covering rock, and 
 
 FIG. 146. Ideal cross-section of the laccoliths of Mt. Holmes, after restoration. 
 
 then they constitute hills of igneous material, thick in the exposed 
 center, but thinning away in all directions where they pass be- 
 neath the remnants of the original covering sheet or interpenetrate 
 
Types of Older Igneous Masses 
 
 201 
 
 it in a series of wedges (Figs. 147, 148). It is important to bear 
 in mind that where the contact of the thin edge with the cover- 
 
 FIG. 147. Hesperus Mountain, showing wedges and sheets of trachyte 
 intruded into the shales, from the laccolith of Mt. Moss. 
 
 ing rock is exposed around the margin of the hill, this contact is 
 seen to be an igneous one, clearly proving the rock to be intrusive. 
 Moreover, it must be shown that the igneous rocks rest upon rock 
 
 E. 
 
 FIG. 148. Ideal section of La Plata Mountain, Colo., showing the sup- 
 posed original form of the laccolith of Mt. Moss. The line a, a is the present 
 profile which cuts Hesperus Mountain and Mt. Moss. 
 
 of a different type, and that they are not deep-seated igneous masses 
 which have eaten their way into the overlying rock (bosses, etc.). 
 When a laccolithic intrusion has been completely isolated by erosion 
 so that no part of the original mass is in contact with the overlying 
 
202 Form and Structure of Older Igneous Masses 
 
 rock, the recognition of the laccolithic origin of the mass becomes 
 very difficult, and the proof of such an origin is sometimes incon- 
 clusive. The Mato Tepee or Devil's Tower of Wyoming (Fig. in, 
 p. 1 68) has also been interpreted by Jaggar as the remnant of a lac- 
 colith rather than a volcanic neck. 
 
 Small laccoliths may be exposed in cross-section in a cliff, when 
 their character is undoubted. Laccoliths range in maximum 
 thickness from less than a hundred to several thousand feet, 
 and their diameter varies from a few hundred yards to several 
 
 N.W. 
 
 B 
 
 FIG. 149 a. Section of Corndon Hill, in Shropshire, England ; the type of 
 the phacolith. (After Harker.) 
 
 miles. As in the case of sills, which may be regarded as the 
 extreme in one direction of laccoliths, the upper and lower margins 
 are generally finer grained and may be darker-colored and 
 lower in silica content, besides being richer in ferro-magnesian 
 minerals, than the center. Columnar structure is sometimes de- 
 veloped, the columns standing vertically. This is shown in 
 the Mato Tepee, already referred to as probably an erosion 
 remnant of a thick laccolith. 
 
 Phacoliths. Corndon Hill of Shropshire, England (Fig. 149 a), 
 appears to represent a peculiar form of lenticular igneous intrusion 
 in which, however, instead of forcing the overlying strata upward 
 
 into a dome, these masses occupy the 
 axes of both upward (anticlinal) and 
 downward (synclinal) folds of the 
 enclosing strata. These cavities, it is 
 inferred, were formed as the result of 
 the spreading of the strata by lateral 
 compression during the folding, and 
 thus the igneous masses merely oc- 
 cupy the spaces made for them by 
 
 other agencies, instead of actively forcing the strata apart (Fig. 
 149 b). Or it may be that the compression of the stratified rocks 
 into folds produced lines of weakness along which the igneous mass 
 
 FIG. 149 b. Diagram illus- 
 trating the formation of phaco- 
 liths. (After Harker.) 
 
Types of Older Igneous Masses 203 
 
 found it easy to enter. Such types of intrusions have been called 
 phacoliths (Harker) (Greek phacos, <aKo's, a lentil). 
 
 Chonoliths. Another type of this intrusion has been described 
 from the Aletschhorn, a mountain in the Aar Massif of the Alps, 
 and from Ascutney Mountain, N. H. Here, during the folding 
 of the strata, there were formed irregular, instead of lenticular, 
 cavities into which the lava from below found its way. To a greater 
 or less degree the igneous mass may also have forced apart the rock 
 in an irregular manner, partaking of the characters of both the 
 laccolith and the phacolith. The most striking feature of such 
 
 , 
 
 ........ 
 
 , ^i' 1 ' 1 ^' 1 .. ............ -'.. 
 
 FIG. 150. Section of the Mt. Holmes bysmalith. (After Iddings.) 
 
 an igneous mass is, however, the great irregularity of the spaces 
 which it occupies. To such masses the name chonolith (choanolitti) 
 has been applied by Daly, because the cavities formed, acted as a 
 mold into which the igneous rock was poured (Greek choanos, 
 xo'avos or x^vos a mold). It is obvious that small intrusions of this 
 type grade into igneous veins. 
 
 Bysmaliths. Still another type of igneous intrusion has been 
 found in the old volcanic center of the southern end of the Gallatin 
 Range in the Yellowstone National Park in northwestern Montana 
 (Fig. 150). Here the mass which forms Mt. Holmes has the nature 
 of a huge core, resembling a giant volcanic neck, but connected 
 either with no surface flow, or with flows of only secondary signifi- 
 cance. It is a laccolith in which the upward force was so great as 
 to rupture the overlying rock mass, and carry it upward for a great 
 
204 Form and Structure of Older Igneous Masses 
 
 distance. This is therefore a large plutonic plug or core which has 
 forced its way upward as a compact mass into the overlying rock, 
 and the contact of this mass with the rocks around its margin shows 
 the evidence of such upward movement. On this account it has 
 been called a bysmalith 1 (Iddings) , a rock rising from the depths 
 (Gr. /Wcros, the deep) . A bysmalith represents the other extreme 
 of laccolith formation, with the exaggeration of the vertical dimen- 
 sions. Bysmaliths are also called plutonic plugs in distinction from 
 volcanic plugs, which are the filling of pipes that reach the surface. 
 They differ from stocks in the manner of origin, having forced their 
 way into the rock by lifting the obstruction in their path, while 
 stocks are intrusive into fissures, which they may widen and alter 
 by pressure and otherwise. 
 
 It is evident that the recognition of the particular type of igneous 
 intrusion which any given mass represents, can only be determined 
 from careful examination of both the mass itself and of the enclos- 
 ing rock, and that many cases may occur where erosion has ren- 
 dered the interpretation at best a doubtful one. If the student 
 keeps in mind the types here given and the essential characters of 
 each, he may, by elimination, in most cases be enabled to reach a 
 conclusion regarding the nature of an igneous mass with which he 
 becomes confronted in the field. Extended examination of all the 
 exposed parts, however, and especially of their contacts with the 
 enclosing rock, will be necessary before such an interpretation is 
 possible. 
 
 It must be emphasized again, that the types so far discussed show, 
 from the nature of the rock, that they have cooled at some depth 
 below the surface, but that they do not belong to the great mass of 
 deep-seated igneous material which formed the reservoir, so to 
 speak, from which these intrusions were fed. This group of deep- 
 seated rocks will be discussed in the next section. 
 
 Deep-Seated or Abyssal Igneous Masses 
 
 In many portions of the world, especially in such regions of an- 
 cient rock as eastern Canada, the Adirondack Mountains, parts of 
 Sweden and Finland, and elsewhere, igneous rocks of coarse grain 
 (holocrystalline) are found, which may be interpreted as a part of 
 the reservoir of igneous material from which ancient volcanoes 
 
 1 More corrrectly byssolith. 
 
Types of Older Igneous Masses 
 
 205 
 
 were fed, and from which the other types of intrusions (the hypa- 
 byssal) emanated. That these are now exposed upon the surface 
 is due to prolonged erosion, which has removed great thicknesses of 
 overlying rock beneath which the magma cooled. At the same 
 time these igneous magmas forced or ate their way to some extent 
 into the overlying rocks, which, therefore, when still preserved, show 
 an igneous contact, that is, a contact of a cool with a molten rock 
 mass. These igneous masses consist of granite, syenite, diorite, 
 gabbro, or of the more 
 basic types of rocks, and 
 they have been divided 
 on the basis of their form 
 and size into bosses and 
 batholiths. 
 
 Bosses. These are 
 deep-seated igneous 
 masses which show a 
 dome-like surface rather 
 than the form of a plug, 
 and their section as re- 
 vealed by erosion is a 
 more or less circular one 
 (Fig. 151). They are sur- 
 rounded by other rocks, often sediments, which show alteration 
 from contact with the heated igneous mass, and such altera- 
 tion appears often in concentric zones around the boss, the most 
 strongly altered zone being next to the igneous mass. 
 
 Batholiths. These are huge bosses of very irregular form, the 
 exposure of which can sometimes be traced over many square 
 miles. They are particularly characteristic of the older parts 
 of the earth's crust, and they have a very variable relation 
 to the rocks into which they are intruded. The granite head- 
 lands of Cape Ann, Mass., of Mount Desert, Maine, and of 
 Halifax, Nova Scotia, are examples of more or less eroded rock 
 masses of this type. 
 
 Since these masses cooled very slowly, they became coarsely 
 crystalline, while the prolonged heat greatly affected the rocks with 
 which they came in contact. In batholiths, as in bosses, we may 
 generally trace a series of zones of alteration in decreasing intensity 
 from the igneous mass outward, those immediately in contact with 
 
 FIG. 151. Ground-plan of a granite boss, 
 with the ring of contact metamorphism. 
 a, sandstones, shales, etc., dipping at high 
 angles in the direction of the arrows ; b, zone 
 or ring within which these rocks are meta- 
 morphosed; c, granite, sending out veins or 
 apophyses into b. 
 
206 Form and Structure of Older Igneous Masses 
 
 the igneous mass showing most profound alteration, while each 
 zone generally has developed minerals peculiar to it. 
 
 Subordinate Igneous Masses 
 
 Apophyses. This name is applied to offshoots from any in- 
 trusive igneous mass whether a dike, sill, laccolith, or deep-seated 
 magma. Apophyses are generally irregular in form and die out 
 in a short distance. Sometimes they may have the character of 
 small dikes for some distance of their extent. They form the surest 
 means of distinguishing an intruded mass from a surface flow. 
 
 Contemporaneous Veins. As all magmas contain more or less 
 water-vapor, this may become locally segregated during the pro- 
 cess of solidification by crystallization of the magma, thus ren- 
 dering portions of the magma very fluid, because of the abundance 
 of the water in it. If the main mass of the magma which has al- 
 ready solidified but is still highly heated, is fissured, as may happen 
 especially near the margin of the mass, this more liquid magma will 
 flow into the fissures, and there solidify. Because the acidic min- 
 eral combinations will be the last to form, these contemporaneous 
 veins, as they are called, will consist of increasingly lighter minerals, 
 toward the outer part of the cooling mass. Thus it will happen that 
 a magma which solidified to form a dark, dioritic rock, for example, 
 will become intersected near its margin with irregular veins of light- 
 colored rock, consisting mainly of orthoclase, feldspar, and quartz, 
 and still farther, near the margin, by more or less pure quartz veins. 
 This is well shown in the rock ledges of the Massachusetts coast 
 some distance north of Boston, where such a dark dioritic rock is 
 interpenetrated in all directions by veins filled with a pinkish, fine- 
 grained, granitic rock (chiefly orthoclase and quartz), forming a 
 very striking contrast, from which that portion of the coast has 
 received its name of " Marble Head." 
 
 Pegmatite Veins. By the local concentration of much water- 
 vapor and the gases from the solidifying of the main mass of the 
 igneous body, exceptionally fluid magmas may be produced which 
 contain much silica and the substances which go to make up the 
 acid minerals, together with the rarer mineral substances of igneous 
 magmas. This very fluid magma will occupy fissures and cavities 
 in the main mass, from which it is differentiated, and will also be 
 injected into fissures in the adjoining rock. Slow solidification 
 
Contact of Igneous Masses with Other Rocks 207 
 
 produces coarse crystals, often many feet or yards in diameter, and 
 these will be largely the more acidic (potash and soda) feldspars, 
 the lighter colored micas, and much free quartz. Such a coarse 
 rock of acidic minerals is known as a pegmatite, and in it the inter- 
 growth of quartz within the feldspar produces the peculiar structure 
 known as graphic or pegmatitic structure (see p. 97, Fig. 40). Rocks 
 of this type are known as pegmatites, but they are not always of 
 such coarse texture. In size too, the pegmatite masses may vary 
 from dike-like intrusions hundreds of meters across, to veins only 
 a few millimeters thick. In such pegmatites there are commonly 
 found many minerals formed of the rarer element?, most common 
 among which are tourmalines (commonly the black variety, but 
 also the red, green, or blue gem types), huge crystals of spodumene, 
 of beryl, etc., and not infrequently many metals as well. 
 
 CONTACT or IGNEOUS MASSES WITH OTHER ROCKS 
 
 Igneous Contact 
 
 By the term igneous contact we mean the junction which has 
 been produced by a mass of igneous magma while still hot with the 
 rock over which it is poured out, if it is a lava, or with the rock of 
 the earth's crust into which it is intruded. In the first case the 
 cold rock over which the lava flow is poured is called the basement 
 rock, in the second case the rock into which the igneous mass is 
 intruded is called the country rock. In either case the older rock 
 may be assumed to have been cold when the hot magma came into 
 contact with it, and the structures which have resulted from such 
 a coming together of highly heated with cold rocks, are contact 
 phenomena, and the alterations produced in either rock are called 
 contact metamorphism. 
 
 In the case of surface flows of lava, the phenomena of contact 
 metamorphism are seen only at the base of the lava sheet, though 
 a certain change is produced where the surface of the sheet is in 
 contact with the atmosphere, or with water in the case of a sub- 
 marine outpouring of lava, and these changes in the lava sheet may 
 also be referred to as metamorphic changes in a very literal inter- 
 pretation of the term. 
 
 Contact metamorphism is very slight in the case of most intru- 
 sive sheets or sills, in laccoliths, and generally in dikes, though in 
 the case of such large sills as the Palisades it may extend for many 
 
208 Form and Structure of Older Igneous Masses 
 
 yards from the contact. In all cases it must be remembered that 
 in intrusive masses the contact phenomena are found on all sides 
 of the intruded mass. Around volcanic plugs and stocks contact 
 phenomena are well marked, especially in the former, where there 
 was continuous or repeated passage upward of igneous material 
 with surface eruption. Again, where dikes are numerous and close 
 together, much contact metamorphism is observable. But such 
 phenomena are most marked in the large, deep-seated igneous 
 masses of coarse grain, which cooled slowly and which, therefore, 
 subjected the adjoining rocks for a long time to the heat of the 
 igneous body and the action of the gases given off from it. 
 
 Kinds of Contact Metamorphism 
 
 We may distinguish two kinds of contact metamorphism, that 
 produced upon the igneous mass itself from contact with a cool 
 wall rock, and that produced upon the wall or country rock. The 
 first is spoken of as an endomorphic, and the second as an exomorpkic, 
 change. 
 
 Endomorphic Effects. The effect produced by the contact of 
 a magma with 'the wall or country rock upon the resulting igneous 
 rock itself is in the first place a change in texture along the con- 
 tact, as we have already seen. This is shown by finer grain or even 
 a glassy character of the igneous rock at the junction with the wall 
 rock. A porphyritic texture with fine ground-mass and coarse 
 phenocrysts may also develop as the result of more rapid cooling. 
 When, however, the enclosing rock is thoroughly heated by the 
 igneous mass, as in a volcanic conduit, no perceptible change may 
 result in the marginal portion of the igneous mass on cooling, while 
 the effects on the wall rock itself (the exomorphic effects) are the 
 more marked. 
 
 In the second place, new minerals, not found in the main mass 
 of the igneous intrusion, may be formed near the contact, from 
 the chemical activities of vapors and gases which tend to be ex- 
 cluded from the main mass as it solidifies and to escape toward the 
 margin of the mass and thence into the surrounding rock. In 
 granitic intrusions tourmaline is not an uncommon mineral thus 
 formed. 
 
 Exomorphic Effects. The effects of the heated- intrusion upon 
 the wall or country rock are, however, the most marked. Among 
 
Contact of Igneous Masses with Other Rocks 209 
 
 these the most notable are the baking or hardening and toughening 
 of the rock near the contact, from the heat, and its frequent change 
 to a more crystalline condition. Next in importance is the develop- 
 ment of new minerals on the contact zone, these being generally 
 formed by the chemical activity of the gases and vapors which 
 enter the rock and constitute the mineralizers. 
 
 The width of the zone subject' to contact metamorphism varies 
 with the size and the heat of the igneous mass, and with the amount 
 of mineralizing gases and vapors given off. It also varies with the 
 character of the enclosing rocks, some being more easily altered than 
 others, while some are more permeable to heat and mineralizing 
 vapors than others. In general, older igneous rocks into which 
 younger ones are intruded, are less altered than are sediments of 
 chemical, organic, or clastic origin, in which the chemical composi- 
 tion is such as to permit ready change, or in which the conduc- 
 tivity, texture, and other characters allow easy entrance of the 
 heat and the vapors. The special effects on a few types of these 
 may be noted. 
 
 Effects on Limestones. Limestone is the general name applied 
 to rocks consisting of carbonate of lime or of carbonate of lime and 
 magnesia. They may be of aqueous (chemical), organic, or of 
 clastic origin, as more fully discussed in later chapters. Lime- 
 stones are seldom pure, there being commonly an admixture of clay 
 or of silica in the form of flint, chert, intimately admixed grains 
 (sand), or other particles. The first and most general effect of the 
 igneous mass upon limestone is the crystallization of the latter, 
 with the result that a marble is produced. Great masses of marble 
 are, however, not produced in this manner, but by more extended 
 (regional) metamorphism, during mountain-making disturbances 
 (see Chapter XX). Changes in the mineral character also occur 
 by recombination of the various substances present. Thus if silica 
 is present, the lime will combine with it, with the separation of 
 carbon dioxide, and a lime silicate (the mineral wollastonite) is pro- 
 duced. The change may be expressed in the following formula : 
 
 CaCO 3 + SiO 2 = CaSiO 3 + CO 2 
 
 Carbonate of Silica Lime silicate Carbon 
 
 Lime (Wollastonite) dioxide 
 
 If the limestone is magnesian, then a double silicate of lime and 
 magnesium is produced, this giving a mineral of the pyroxene 
 group, known as diopside, abundant crystals of which are found in 
 
210 Form and Structure of Older Igneous Masses 
 
 the northern part of the city of New York (Inwood region), where 
 the magnesian limestones are penetrated by numerous pegmatite 
 dikes (see Fig. 136, p. 193). The formula expressing this change 
 may be written as follows : 
 
 Ca Mg(CO 3 ) 2 +2 SiO 2 = Ca Mg(SiO 3 ) 2 +2 CO 2 
 
 Lime magnesium Silica Lime magnesium Carbon 
 carbonate silicate (diopside) dioxide 
 
 When both clay and quartz are present, a double silicate of lime 
 and alumina may result, the aluminum being furnished by the clay. 
 This new compound will then crystallize out in the form of the 
 mineral garnet, while both water and carbon dioxide are given off. 
 The following formula expresses this change : 
 
 3 CaCO 3 +H4Al 2 Si 2 O 9 +SiO 2 = Ca 3 Al 2 Si 3 O 12 +3 CO 2 +2 H 2 O 
 
 Lime carbonate Clay Silica Lime aluminum Carbon Water 
 
 (Calcite) (Quartz) silicate (Garnet l ) dioxide 
 
 Good illustrations of the formation of garnets, sometimes of con- 
 siderable size and' in large numbers, but of no great value, may be 
 seen in the impurer parts of the same limestones (Inwood marble) 
 in the northern part of New York City, near the contacts with 
 dikes. 
 
 Many substances may be carried into the limestones by gases 
 and vapors, and' so produce a variety of minerals. Those men- 
 tioned are, however, the most important. 
 
 Effects on Mud- Rocks. This is a general name applied to sedi- 
 ments composed most commonly of microscopic particles of quartz 
 and of clay, sometimes the one and sometimes the other substance 
 predominating. Lime particles may also be very abundant and 
 intimately disseminated among the clay and quartz, and other 
 substances in a fine state of division may also occur. Such mud- 
 rocks may be massive, or they may have a fine and irregularly 
 bedded or laminated structure, when they are called shales. By 
 baking along the contact, such a mud-rock may be changed into a 
 dense mass comparable to artificially baked clay (porcelain), 
 forming a rock called porcelanite (porcellanite) or a hornfels, which 
 may have the appearance and hardness of a dense basalt or other 
 igneous rock and may be mistaken for such. Another feature pro- 
 duced in such mud-rocks, a short distance from the contact, is a 
 series of spots or knots of mineral matter or even of crystals of 
 
 1 This is the variety Grossularite. There are several others of different composi- 
 tion. See Table, p. 62. 
 
Contact of Igneous Masses with Other Rocks 211 
 
 minerals, among which the commonest is a silicate of alumina known 
 by the mineral name of andalusite (A^SiOs) ; other minerals may 
 also be developed. 
 
 Effect on Quartz Rocks (Sandstones, etc.). When the country 
 rock consists largely of quartz, commonly in fine grains (sandstone) 
 the effect of the intrusion of an igneous mass is not so marked as 
 in the case of other rocks. Close to the contact, the sandstone 
 may be hardened into a quartzite, and if clay, lime, or other mineral 
 substances are present, new minerals may be produced. 
 
 Alteration by Gases and Vapors 
 
 Where the volcanic activity has subsided into the solfataric or 
 fumarolic stage, with the emission only of vapors and gases, the 
 country rock around the vents and along fissures penetrated by 
 these gases and vapors may be profoundly altered. New minerals 
 may be produced in this zone of alteration and deposits of older 
 minerals may be enriched by the addition of new material from 
 the gases and vapors. In this wise, important mineral and ore 
 deposits may be produced, to some of which reference will again 
 be made in a later chapter. 
 
 Ancient Igneous Masses in Sedimentary Contact with the 
 Overlying Rock 
 
 The. granite mass of Pikes Peak, in Colorado, differs from the 
 granite masses previously discussed in the fact that the contact 
 with the overlying rock is not an igneous, but a sedimentary one. 
 To be sure, when the granite was formed by the cooling of a deep- 
 seated igneous magma, it was in igneous contact with the over- 
 lying rocks of that time. But these covering rocks were entirely 
 removed by erosion, and later sediments (sand, then limestone) 
 were spread over the eroded granite surface, and at a much later 
 date still, after the core of the Rocky Mountains was elevated, 
 these younger rocks were again partly removed by erosion, and in 
 the various canons which cut the lower slopes of the mountains 
 these sediments are seen to rest upon the eroded surface of the 
 granite (Figs. 152, 153). 
 
 Evidently the relationship thus seen between the granite and 
 the sediments does not permit our classifying the Pikes Peak granite 
 mass as belonging to any of the igneous types so far discussed, and 
 
212 Form and Structure of Older Igneous Masses 
 
 the name abyssolith has been proposed for it by Grabau. An abysso- 
 lith, then, is a mass of rock, generally granite, or one of the more 
 
 basic rocks of this type, 
 which was originally a 
 boss, or a batholith, or 
 may even have been a 
 bysmalith, laccolith, or 
 stock, which has been 
 exposed by erosion, then 
 covered by sediments 
 which are, of course, un- 
 affected by the igneous 
 mass because it was cool 
 at the time, and after a 
 more or less domelike 
 uplift the sediments were 
 again eroded from the 
 surface of the dome, re- 
 exposing the igneous core 
 around which unaltered 
 
 FIG. 152. Contact of the stratified basal 
 Palaeozoic beds and the granitic basement rock 
 of the Pikes Peak mass, in Williams' Canon, 
 Colorado. (Photo by Author.) 
 
 sediments crop out. In 
 addition to the Pikes 
 
 Peak region we may note as an example of this type the Black 
 Hills Dome, with its center of old igneous rocks. Many small 
 
 ---- v.v.v. -. ................ -./* + \+ T ^\^ +>7+> T ++s+, + +\V+\' 
 
 A * + *&&*>+* *&&-r* v 
 
 ^^*^3^Jv[J?vFj^^^ 
 
 *\-i- * ^<'7.JS f 4.+^ J L+ O.X-+- +X*~+4 
 
 /I- 
 
 f\ 
 
 Vt 
 
 'inch. JOjeet. 
 
 FIG. 153. Details of contact of Palaeozoic sediments and Pre-Palaeozoic 
 granites in Williams' Canon, Colorado. (After Crosby.) 
 
 examples of such resurrected igneous rocks of dome-like form sur- 
 rounded by unaltered sediments are known from this country as 
 well as from Europe. 
 
Contact of Igneous Masses with Other Rocks 213 
 
 Relative Age of Igneous and Enclosing Rock 
 
 If we then recognize the nature of the contact between an ig- 
 neous mass and the sediments which once covered it, we can de- 
 termine the relative ages of the two series. If the contact is 
 igneous, the sediments are older than the intruded igneous mass. 
 If the contact is a sedimentary one, the igneous mass is the older, 
 and there is a' long-time interval lost, between the two an in- 
 terval during which erosion removed the older covering rocks, with 
 which the mass was in igneous contact, and this erosion occurred 
 before the sediments now seen in contact with the igneous mass 
 were deposited. This shows how important it becomes to deter- 
 mine whether a contact is igneous or sedimentary. 
 
CHAPTER X 
 THE AQUEOUS OR HYDROGENIC ROCKS 
 
 GENERAL CHARACTER AND VARIETIES 
 
 Source of the Material. Practically all natural waters contain 
 mineral matter in solution, the common illustration being ocean 
 water, every liter of which contains about 35 grams of mineral 
 matter, more than 27 grams of this being common salt (sodium 
 chloride), which means that every cubic mile of sea- water contains 
 about 131,526,000 short tons of this important substance in solu- 
 tion. Sea-water is therefore spoken of as salty or saline, and this 
 saltiness is readily recognized by taste. The other dissolved sub- 
 stances are also called salts, but their presence is not so readily 
 recognized. Some water bodies, like the Baltic Sea, contain only 
 one fourth or less of this amount of salts in solution, and such waters 
 are called brackish. The waters of the Hudson River some dis- 
 tance above New York City are brackish, because they are formed 
 by a commingling of the fresh water from the upper river with salt 
 water entering from the sea. 
 
 When the water contains so little substance in solution that it 
 can be used for drinking purposes (whether it is contaminated by 
 organic matter or not) it is called fresh water, but fresh water in 
 nature always contains some substances in solution, lime usually 
 predominating, this when present in sufficient quantity forming 
 hard water. When carbonate of soda and similar substances are 
 present in such quantities as to render the water unfit for human 
 consumption, it is called alkaline, and all travelers in the semi-arid 
 regions of western North America are familiar with such water. 
 Finally there are water bodies like Great Salt Lake, the Dead Sea, 
 and others, in which the quantity of common salt in solution ex- 
 ceeds many times that in the ocean water, and such waters are 
 spoken of as super-saline, or as brines. Brines may also be pro- 
 duced by partial evaporation of ocean water, just as brackish waters 
 are produced by the dilution of ocean water. 
 
 214 
 
General Character and Varieties 215 
 
 Separation of Material. When the substances which waters 
 hold in solution are separated out so as to form solid material, this 
 material constitutes rock masses to which in general we apply the 
 name of salts, after the example of the commonest of these, the 
 ordinary salt. If the separation is produced by organisms, the 
 product is called organic salts, and these belong in the division 
 of organic or biogenic rocks, where they will be discussed. If 
 the separation is by inorganic activities, the product is a normal 
 aqueous or hydrogenic rock, to which the present chapter is 
 devoted. 
 
 Separation by inorganic means is accomplished in a variety of 
 ways, of which the following are the most important : 
 
 1. Separation by the condensation or complete evaporation of 
 the water by heat, drying winds, etc., during which process a stage 
 is reached when the water becomes saturated, that is, it holds in 
 solution as much of a given salt as it can hold for that temperature 
 and pressure. If that stage is passed, the excess of salt separates 
 out, often in a more or less crystalline form. The point of satura- 
 tion varies with the nature of the salt, and when two or more salts 
 are present in the solution, they will not only have separate and dis- 
 tinct saturation points, but the presence of each is likely to in- 
 fluence the saturation point of the others, either lowering or raising 
 them. Complete separation of all the salts- will occur upon com- 
 plete evaporation of the water. Such salts are called evaporation 
 products, or briefly, evaporates. 1 
 
 2. Separation of salts from the solution by the force of attrac- 
 tion which crystals or particles of mineral matter exert on material 
 of the same kind in the solution (generally a saturated one) in which 
 those crystals or particles are immersed. 
 
 3. Separation by the abstracting of the solvent or substance 
 which holds the salt in solution. 
 
 4. Separation by chemical reaction between minerals in solution 
 in the water, and other substances introduced from extraneous 
 sources, either as gases or as solutions, and the consequent forma- 
 tion of new and less soluble compounds which are then precipitated. 
 
 5. Separation by electrolysis. 
 
 1 Strictly speaking, this is also a chemical combination of the dissociated ions, which 
 occurs at the moment of solidification. Such salts are therefore also precipitates, but 
 it is well to keep this type distinct from that resulting through reactions with newly 
 introduced substances as given under 4. 
 
2i6 The Aqueous or Hydrogenic Rocks 
 
 As illustrations of the first class we may cite the separation of 
 salt when sea-water is evaporated or when incrustations of salt 
 are formed from a solution of that substance which is allowed to 
 stand for a time in a warm room. The second method is illustrated 
 by the crystallization of the alum of a saturated solution around a 
 crystal of alum introduced into that solution, or the formation 
 of rock candy from a saturated solution of sugar. Precipitation 
 through abstraction of a solvent is shown by the deposits of lime on 
 the inside of boilers and tea-kettles where " hard " water is used, 
 the solvent, which is carbon dioxide, being driven off by the heat. 
 It is also shown by the precipitation of lime around the mouths of 
 springs, in which the water escaping from under pressure loses some 
 of the carbon dioxide which was the solvent of the lime in the water. 
 Examples of precipitation of salts by the addition of substances in 
 solution which by chemical reaction produce a less soluble com- 
 pound, or by the passing of gases through a solution, are familiar to 
 all workers in chemical laboratories, while the electrolytic method 
 is one much practiced in the arts. 
 
 The first three methods cited are most commonly observed in 
 nature. The precipitation by the addition of reagents, whether 
 liquid or gaseous, is chiefly found in the formation of lime deposits 
 under the influence of ammoniacal gases. Since these are, however, 
 in nearly all cases produced by the direct activities or the indirect 
 influence, through decay, of organisms, such deposits are best 
 referred to the organic or biogenic group, and they will be discussed 
 there in this book. Electrolytic processes in nature are still little 
 understood, but that they are going on cannot be doubted. Some 
 of the inclusions of salt in the pore spaces of marine sediments have 
 been explained in this manner. 
 
 Most precipitates from an aqueous solution are called salts, no 
 matter how they are formed or what their composition. There 
 are, however, some simpler substances, such as the oxide of silicon 
 or silica (SiO 2 ) and others, which cannot properly be called salts. 
 (See Chapter IV.) One of the essential characteristics of a salt is 
 its purity of composition, though it is true that under certain con- 
 ditions precipitation of several salts may take place simultaneously, 
 thus producing what is called a eutectic mixture. When pure, the 
 material is in reality a single mineral mass, as for example, the min- 
 eral Halite or rock salt ; but because of its extensive development 
 it is treated as a rock. 
 
The Textures of Aqueous Deposits 217 
 
 Classification of aqueous precipitates according to composition. It 
 is evident that chemical composition is the primary basis on which 
 aqueous precipitates must be classified. Furthermore, most salts look 
 very much alike except when they are crystallized, or when, as in 
 special cases, color, hardness, and weight make distinction possible. 
 In the following pages will be given some of the more important and 
 common salts and oxides, the classification being made, for the sake 
 of convenience, upon the basic element in the composition. 
 
 THE TEXTURES OF AQUEOUS DEPOSITS 
 
 The texture of aqueous precipitates is either crystalline or non- 
 crystalline the latter also being designated amorphous. Crystals 
 of all sizes and degrees of perfection may form, the most perfect 
 
 FIG. 154 a. Oolitic limestone silicified. (Photo by B. Hubbard.) 
 
 being those least interfered with by other crystals. The amor- 
 phous texture may be either in the form of separate or discrete 
 particles, which may or may not be tied together subsequently, as 
 in oolites, or it may be a solid or concrete mass. 
 
 Discrete particles. These comprise the two following textures : 
 
 (a) Oolitic texture (Fig. 154 a, b), characteristic of oolites. The 
 
 particles are small spheres, generally with a nucleus and with radial, 
 
 or with concentric or zonal, structure, the size suggesting the roe 
 
 of fish ; typically lime carbonate. 
 
218 
 
 The Aqueous or Hydrogenic Rocks 
 
 (b) Pisolitic texture (Fig. 
 155), characteristic of piso- 
 lites. The spherules are of the 
 size of a pea or larger. These 
 are also chiefly lime carbonate. 
 
 Concrete Masses. These 
 include the following textures : 
 
 (a) Botryoidal (Fig. 156), 
 with grape-like rounded sur- 
 faces. 
 
 (b) Banded, in layers which 
 in section show a banded 
 structure. 
 
 (c) Laminated, in thin 
 
 FIG. 1546. Thin section of Jurassic 
 oolite showing the characteristic zonal 
 structure. Brown Jura, Schonberg, near layers or laminae. 
 Freiburg I. B., Germany. Enlarged 24 
 diameters. (After Rosenbusch.) 
 
 (d) Scaly, composed of 
 small scale-like masses. 
 (e) Fibrous, in slender hairlike fibers, often elongated crystals. 
 (/) Tufaceous (Figs. 157, 158), porous as in calcareous tufa. 
 (g) Concretionary (Fig. 1 59) , in large more or less spherical masses. 
 
 FIG. 155. Photograph of a specimen of pisolite somewhat reduced. (Photo 
 
 by B. Hubbard.) 
 
The Textures .of Aqueous Deposits 219 
 
 FIG. 156. Botryoidal structure in calcium carbonate deposits. 
 
 FIG. 157. A fragment of calcare- 
 ous tufa. Reduced. (Photo by B. 
 Hubbard.) 
 
 FIG. 158. Bird's nest and eggs 
 "petrified " or covered with a deposit 
 of lime carbonate, by submersion 
 in tufa-depositing spring-water. Re- 
 duced. (Photo by B. Hubbard.) 
 
 FIG. 159. Concretionary limestone ; basin of ancient Lake Lahonton. 
 
 Russell.) 
 
 (After 
 
22O The Aqueous or Hydrogenic Rocks 
 
 THE PRINCIPAL TYPES OF AQUEOUS OR HYDROGENIC 
 DEPOSITS 
 
 Among the many precipitates or other deposits formed directly 
 from aqueous solutions, a certain number is found in sufficiently 
 large quantities to be treated as rock material, while others are im- 
 portant as sources of valuable substances or are themselves of 
 economic value. Their essential mineral characters have already 
 been given in the tables in Chapter IV. 
 
 Non-Metallic Aqueous Deposits 
 
 Rock Salt. Chloride of sodium (NaCl). This is the most important as 
 well as the most abundant mineral substance obtained from ocean water. It 
 commonly occurs in beds, which may have a thickness of a hundred feet or more 
 but generally are only a few feet thick, forming a succession of beds separated 
 by gypsum, anhydrite, limestone, dolomite, or clay, or more rarely by other 
 mineral matter. Such deposits are formed by concentration of sea- water either 
 in basins cut off from the sea by elevation or by the formation of a barrier, or 
 in lagoons behind a bar with continued supply of sea-water through an inlet, 
 as more fully discussed in the next chapter. Salt is also separated from lake 
 basins in arid regions by the concentration, through partial evaporation, of the 
 water. Finally, salt deposits are formed in desert basins from salt disseminated 
 through the rocks in the rims of those basins, as more fully discussed in the next 
 chapter. 
 
 Rock-salt deposits are seldom very continuous, having in most cases a lens- 
 like form. Salt is also extensively manufactured by the evaporation of sea- 
 water in shallow salt gardens or sea salinas, which are located on nearly all shores 
 where the climatic and other conditions are favorable. Natural as well as ar- 
 tificial brines, formed by the solution of old salt beds in the earth's crust, or of 
 disseminated salt, are also extensively used in the manufacture of salt. Salt is 
 mined in central New York, in southern Michigan, in Louisiana, and in a few 
 other localities in the United States. Extensive salt mines exist in Austria, 
 Galicia, Rumania, and elsewhere in the Old World. The salt mines of North 
 Germany are worked chiefly for their potash deposits. Hills or mountains of 
 salt are found in many of the dry regions of the world, in northern Spain 
 (Cardona), Algeria, Persia, and elsewhere. 
 
 The chief uses are for domestic and dairy purposes and for chemical industries. 
 
 Gypsum. Hydrous sulphate of lime (CaSO 4 2 H 2 O). This is a common 
 associate of salt deposits, being found beneath beds of rock-salt of marine origin 
 (sometimes replaced by anhydrite) and always separating from sea- water, which 
 undergoes concentration before the rock-salt separates out. Such rock gypsum 
 is commonly massive and sometimes impure, but pure white gypsum (alabaster) 
 also occurs. It also occurs as crystals (selenite), scattered through mud and 
 sand deposits, especially in desert regions. A fibrous variety (satin spar) occurs 
 in veins. Gypsum is seldom formed in extensive beds in desert regions except 
 
Principal Types of Aqueous Deposits 221 
 
 where sea-water evaporates. It is also formed as an alteration product, gen- 
 erally of limestones, by waters carrying sulphuric acid ; large beds of gypsum 
 result in this way, those of New York state being an example. Anhydrite 
 deposits are also changed to gypsum when surface waters come in contact with 
 them. Extensive deposits of gypsum are found in the " Red Beds" of the west- 
 ern United States, and in Kansas, Oklahoma, and Texas. It is found in Nova 
 Scotia and in the Paris Basin, where it has been and still is extensively quarried 
 and burned into Plaster of Paris ; also in many other parts of the world. 
 
 Raw gypsum is ground and used as a natural fertilizer (land plaster), to re- 
 tard the setting of cement, and for many chemical purposes. When burned 
 or " calcined " at 350 F. it loses most of its water (CaSO 4 | H 2 O), and is 
 ground into the familiar " Plaster of Paris." When this is mixed with water 
 gypsum is again formed. It hardens rapidly and is used extensively in the 
 arts for molding, statuary, etc., and for stucco work. 
 
 Anhydrite. Calcium sulphate (CaSO4) . This is distinguished from gyp- 
 sum by its greater hardness and specific gravity. Its color is often also more 
 grayish. It is formed as a primary deposit from sea-water in cut-off basins, 
 especially when the water contains an excess of chlorides. The largest known 
 deposits thus formed are in northern Germany, where they underlie the salt and 
 potash beds. Anhydrite slowly changes to gypsum by taking on water, with 
 an expansion of the mass. It is of little economic importance. 
 
 Carbonate of Lime. Calcite, Aragonite, Limestones, etc. (CaCO 3 ). The 
 great bulk of the deposits of carbonate of lime, including the limestone beds, is 
 of organic or of clastic origin, but there are a number of lime deposits which be- 
 long to the hydrogenic division. The most extensive of these is probably cal- 
 careous tufa (Fig. 157), which is formed by springs issuing in limestone regions 
 and depositing the excess of lime which they hold in solution. Calcareous tufa 
 is mostly porous and light, incrusting not infrequently mosses and other plants 
 and also other objects. Some massive deposits of th'is 
 type, however, are known, constituting the so-called 
 " Mexican Onyx." Compact carbonate of lime de- 
 posits are also formed in caves as stalactites and 
 stalagmites. These usually have a banded structure, 
 showing the successive addition of layers. That of 
 the stalactites is concentric around the longitudinal 
 axis, which in the early stages is formed by a delicate 
 tube. 
 
 Beds of limestone are sometimes built up from large 
 rounded masses or " concretions," which have re- 
 sulted from the deposition of carbonate of lime, gen- 
 erally around a nucleus. Sometimes these are - FIG. 160. Concre- 
 formed as original masses of limestone, as in the tionary magnesian lime- 
 limestone deposits of Lake Lahonton (Fig. 159), or * tone ( Permian )> Dur ~ 
 an older limestone of this type which occurs in the ha , m ' *f g ^ *"* 
 Permian series (Magnesian limestone) of northeastern photo ) 
 England (Fig. 160). In other cases these concretions 
 
 occur in shale beds, forming distinct layers, which when they become confluent 
 by ^continued growth form bands of limestone. When separate they often 
 
222 
 
 The Aqueous or Hydrogenic Rocks 
 
 FIG. 161 a. Septarium or 
 Turtlestone. A concretion from 
 calcareous shales. The fissures 
 of the interior were filled with 
 calcite veins, which became 
 exposed after erosion and 
 weathering of the surface of 
 the concretion ; about \ natu- 
 ral size. (B. Hubbard, photo.) 
 
 take on disk-shaped or spherical forms, while a series of radial cracks are de- 
 veloped in the interior with the growth of calcite veins. Such concretions are 
 
 called septaria (Figs. 161 a, b). .All car- 
 bonate of lime deposits effervesce readily 
 with dilute hydrochloric acid, this being the 
 most distinctive test. 
 L Dolomite. Carbonate of lime and mag- 
 
 ; ~"**^ nesia ((CaMg)CO 3 ). This differs from the 
 
 m V I pure carbonate of lime deposits in its greater 
 
 A * ^fl hardness and in the fact that it effervesces 
 
 ^^^^ . ^jB only in strong hydrochloric acid. It is some- 
 
 1^ ^^Jl^BII times a primary deposit in basins where 
 
 anhydrite is forming, with which it becomes 
 more or less intimately mixed. It may also 
 be formed as an original precipitate in por- 
 tions of the sea in which the solution has be- 
 come concentrated. Many dolomitic lime- 
 stones are, however, of secondary origin, the 
 original magnesia content, which was derived 
 for the most part from calcareous algae, etc., 
 being proportionately increased through the 
 solution of carbonate of lime by ground water. Secondary deposition of mag- 
 nesium carbonate also takes place from solutions in the circulating ground 
 water. Pure dolomite contains 54.35% CaCO 3 , and 45.65% MgCO 3 . 
 
 Magnesite. Carbonate of Magnesia (MgCO 3 ). This is harder and more 
 compact than dolomite, and dissolves with effervescence only in hot hydro- 
 chloric acid. It occurs as a crystalline mineral, as replacement of dolomite, 
 and also as an amorphous, earthy, hard, compact mineral, probably a colloidal 
 precipitate. It is often concretionary with 
 conchoidal fracture, appearing like un- 
 glazed porcelain, this type being usually 
 derived from the alteration of serpentine 
 and other magnesian rocks. Beds of mag- 
 nesite are found associated with gypsum in 
 fresh- water limestones of France. 
 
 Apatite and Phosphate Rock. Al- 
 though the most extensive deposits of phos- 
 phate of lime are of organic origin, there 
 are some that must be considered as purely 
 aqueous or hydrogenic deposits. Among 
 these are the apatite veins and the marine 
 concretions of phosphate around an or- 
 ganic nucleus. Phosphate of lime occurs 
 also as a secondary replacement of lime- 
 stone. Apatite crystallizes in hexagonal 
 prisms, etc., and is readily recognized by 
 
 its form, hardness, and color (see Table in Chapter IV). Rock phosphates 
 are generally amorphous and compact. The pure mineral (tricalcium phos- 
 
 FIG. 161 b. A weathered sep- 
 tarium, showing the mineral, 
 which filled the fissures, left in 
 relief, thus producing the typical 
 "septarium" structure. (Photo 
 of a specimen in Columbia Uni- 
 versity, by B. Hubbard.) 
 
Principal Types of Aqueous Deposits 223 
 
 phate) contains 45.8% of phosphoric acid (P 2 O 6 ). The principal use of 
 phosphate is as a fertilizer. 
 
 Potash Salts. There are a number of evaporation products which are pri- 
 marily salts of potash and are important sources of this substance. Among the 
 more common of these are Sylvite, the chloride of potassium (KC1), a very 
 soluble, soft, transparent, milky reddish or yellowish mineral, commonly mixed 
 with rock salt; Carnallile, the hydrous double chloride of potassium and 
 magnesium (KC1 MgCl 2 6 H 2 O), a colorless or snow-white or variously 
 colored salt, easily soluble, and an important source of potash ; and Kainite, 
 the compound chloride of potassium and sulphate of magnesium, with water 
 (KC1 MgSO 4 3 H 2 O), colorless to deep blood-red, and chiefly an alteration 
 product. The chief use of potash is for agricultural purposes and in chemical 
 works. 
 
 Trona. Sodium carbonate (Na 2 CO 3 NaHCO 3 2 H 2 O). This is a com- 
 mon evaporation product of alkaline lakes. It is glassy or transparent 
 when crystallized, but usually forms a white salt. Important American 
 localities are Searle's Marsh, Owens and Mono Lakes, Cal., and Soda 
 Lake, Nevada, while some Russian and Hungarian lakes, and the Natron 
 lakes of Egypt west of Cairo, represent foreign localities. It is used for 
 domestic and chemical purposes, but much of the commercial trona is arti- 
 ficially produced. On account of its solubility it is not generally found 
 in the older rocks. 
 
 Mirabilite. (Glauber salt.) Hydrous sulphate of sodium (Na 2 SO 4 
 10 H 2 O). This is a crystalline, granular salt, usually colorless, and readily 
 soluble in water. It is formed, especially in winter, by certain saline bodies, 
 such as Great Salt Lake, Utah, and the Kara Bugas Gulf on the east coast of 
 the Caspian Sea. On the floor of the Kara Bugas a bed of mirabilite, estimated 
 to contain 1000 million metric tons, has formed but it is not yet exploited. In 
 Wyoming and New Mexico occur beds of this salt mixed with epsomite, natron, 
 and common salt (halite). In some cases the deposit is 15 feet thick and covers 
 an area of 100 acres. 
 
 Glauberite. Double sulphate of sodium and calcium (Na 2 SO 4 CaSO 4 ). 
 This is a white, gray, yellow, or red mineral, a little harder than common salt 
 (2.5-3). It occurs in many playa lakes (Borax Lake, Searle's Marsh, Death 
 Valley), and in the salt deposits of Germany, Spain, Austria, Sicily, etc. It also 
 occurs in a number of Tertiary deposits in Spain. 
 
 Soda Niter, Chile Saltpeter. Nitrate of sodium (NaNO 3 ). This is a 
 white to reddish brown, gray, or lemon-yellow salt, commonly impure, when it 
 is called caliche. It is abundant in the desert tracts of western Chile and else- 
 where in South America and other parts of the world. 
 
 Borax Salts. These include Borax or Tinkal, sodium borate (Na 2 B 4 O 7 
 10 H 2 O), a colorless or yellowish or green to gray, soluble mineral, somewhat 
 harder than common salt ; Colemanite, the hydrous borate of calcium (CaBeOu 
 5 H 2 O), generally a massive, glassy or colorless, more or less transparent min- 
 eral, found as a bed from five to 20 feet thick in San Bernardino Co., Cal., and 
 elsewhere ; and Ulexite, the double borate of sodium and calcium with water 
 (NaCaB 5 O 9 8 H 2 O), which occurs as white balls of fibrous material and satiny 
 luster (cotton balls). Borax salts are found in volcanic regions, such as the 
 
224 The Aqueous or Hydrogenic Rocks 
 
 famous " saffioni," i.e., fumaroles, in the volcanic region of Tuscany; in hot 
 spring and lake deposits of volcanic districts, as in Tibet, etc., and the Coast 
 Ranges of California, and in playa deposits, as in Death Valley and elsewhere 
 
 in the western United States. Colemanite 
 is the chief American source of borax. 
 
 Silica. Oxide of silicon (SiO 2 ) frequently 
 with water. The most familiar form of this 
 is quartz in crystalline or amorphous form, 
 which occurs in veins, geodes, etc", colorless 
 to variously colored; also as addition to 
 quartz grains in sandstones, enlarging them 
 to fill all the interstices. It occurs exten- 
 sively as a porous, white, and light spongy 
 FIG. 162. Concretion of deposit of hydrous quartz, around geysers 
 flint from the chalk beds of (gee r Ig } and . hence k 
 
 England. About one fifth nat- '.,. . . , A . .. 
 
 ural size. (Photo by B. geysente or Sllicious smter ' A g^m, it occurs 
 Hubbard ) as concret ions of flwt m chalk beds (Fig. 
 
 162), or as chert nodules or layers in lime- 
 stone. These types are compact and have a conchoidal fracture, with a black 
 or dark brown color on the freshly fractured surface. Flint and chert are 
 secondary concentrations through solution and redeposition of disseminated 
 silica particles of organic origin. (Fig. 163). 
 
 Glauconite. Greensand. This is a complex silicate of potassium and iron 
 (K-FeSi 2 Oe H 2 O). It consists mainly of dark green grains, generally mixed 
 with impurities, found in strata of Cretaceous and other formations, and is form- 
 ing at the present time along the margin of the continental shelf in the Atlantic 
 Ocean and elsewhere. It promises to be an important source of potassium, in 
 which the New Jersey deposits are especially rich. They also contain much 
 phosphorus. 
 
 Metallic Aqueous Deposits 
 
 Manganese Ores. Nodules of oxide of manganese occur in many parts 
 of the deep sea in the region of red clay deposits (Fig. 174). They are 
 more or less rounded or irregular masses of black color. Oolitic pyrolu- 
 site (MnO 2 ), an iron-black mineral of metallic luster, occurs interstrati- 
 fied with other beds in certain localities. A hydrous oxide of manganese 
 (wad) occurs as an amorphous earthy deposit in bogs, generally associated 
 with iron ores. 
 
 Limonite. Bog iron ore, hydrous oxide of iron (2 Fe 2 O 3 3 H 2 O). This 
 occurs in massive, botryoidal, earthy, or porous masses of yellow or ochery color 
 and yellow streak. It forms deposits in bogs and swamps, in shallow water, in 
 depths above 12 feet, from the oxidation of carbonate of iron carried into the 
 bog in solution. The ores are always mixed with sand or earthy impurities. 
 There are several other ferric hydrates, some with less and some with more 
 water; e.g. Gothite (Fe 2 O 3 . 2 H 2 O). Oolitic limonites occur in older strata, 
 where they form an important source of iron, though of low percentage. Such 
 
Principal Types of Aqueous Deposits 225 
 
 are the minette ores of Lorraine and Luxemburg, formerly one of the chief domes- 
 tic sources of iron in Germany. These are believed by many to have resulted 
 from the oxidation of oolitic siderite or glauconite. 
 
 Hematite. Red oxide of iron (Fe 2 O 3 ). This ranges in color from deep 
 red to reddish brown and black with red streak. It occurs interbedded with 
 shales and limestones in the Palaeozoic formations of Germany, France, Bohemia, 
 
 FIG. 163. Quarry-wall of Cummings Cement Mine, Akron, Erie Co., N. Y. 
 The upper bed (a) is a very cherty limestone (Corniferous) the chert nod- 
 ules standing out in relief as the result of weathering; (b) Onondaga lime- 
 stone without chert; (c) Akron dolomite, 7 feet thick, with fossils of upper 
 Silurian Age. Between it and the Onondaga limestone (Middle Devonian) 
 is a hiatus or break in succession involving the whole of the lower Devonian, 
 which is absent. The two formations are disconformable ; (d) Bertie water-lime 
 mined for natural cement. (Courtesy N. Y. State Museum.) 
 
 and the United States, where the (Clinton) iron ores of New York and the Appa- 
 lachian region form a characteristic example. Many of these beds appear to 
 be replacement of limestone by the iron ; in other cases the iron ore seems to 
 be a primary deposit. 
 
 Siderite. Carbonate of iron (FeCO 3 ). This important iron ore occurs 
 in crystallized form in veins cutting limestone and other rocks, and is readily 
 recognized by its perfect rhombohedral cleavage, vitreous to pearly luster, 
 greenish to brownish color, and translucent to subtranslucent character. It is 
 more common, however, as an interbedded rock in the older sedimentary series, 
 
226 
 
 The Aqueous or Hydrogenic Rocks 
 
 being known as clay-iron-stone, spherosiderite, or black band. Clay-iron-stone 
 has a dense or fine-grained structure, forming concretions, which often include 
 
 FIG. 164. Clay-iron-stone concretion, Connecticut valley. (After 
 Gratacap.) 
 
 organic remains (Fig. 164). The "black band" forms continuous layers in 
 the formations which carry coal. 
 
CHAPTER XI 
 
 MODE OF OCCURRENCE AND ORIGIN OF THE 
 AQUEOUS OR HYDROGENIC ROCKS 
 
 TYPES OF DEPOSITS* 
 
 EACH of the several water bodies of the earth may form precipi- 
 tates of mineral matter, and he'nce we may classify these deposits 
 under the following heads : 
 
 A. Marine deposits, or those formed in the sea and its depend- 
 ent water bodies. 
 
 B. Lacustrine deposits, or those formed in lakes, ponds, fresh- 
 water marshes, salinas, playas, etc. 
 
 C. Flumatile deposits, or those formed by rivers in their beds, or 
 on the flood-plain or delta surfaces, except those formed in lakes 
 along the river course, or at its mouth. 
 
 D. Terrestrial deposits, or those formed by springs and by the 
 ground water in fissures, caverns, cavities in the rock, etc. To 
 these belong many important deposits of mineral matter. 
 
 The several types may grade one into the other, but their main 
 characteristics are quite distinct. 
 
 SEA- WATER AND THE EVAPORATION PRODUCTS AND CHEMICAL 
 PRECIPITATES FORMED FROM IT 
 
 Amount of Salt in Sea- Water. As has been noted in the pre- 
 ceding chapter, the oceans, which are the large bodies of sea-water 
 lying between the continents, contain the normal salt water, in 
 which about 35 grams of salt occur in every liter of water. Since 
 a liter of pure water weighs 1000 grams, the quantity of salt is 
 essentially 35 per thousand by weight, which is expressed by the 
 formula 35 per mille (or 35^). This corresponds, of course, to 
 3.5 per hundred or 3.5 per cent (3.5%), but since the difference in 
 salinity between different water bodies Is often very slight, and 
 
 227 
 
228 Aqueous or Hydrogenic Rocks 
 
 because in brackish and fresh waters the actual quantity of mineral 
 matter in solution is very small, it is more satisfactory to express 
 the quantity in permillages than in percentages. 
 
 The quantity of salt in solution determines the salinity of the 
 water. Thus the average salinity of the ocean water is 35 per mille, 
 (3.5 per cent), varying somewhat for the different oceans, for differ- 
 ent parts of the same ocean, and for different depths. On the other 
 hand, the salinity of the Red Sea surface waters is 38.8 per mille, 
 while that of the surface waters of the Black Sea is only 18.3 per 
 mille, which is due to the fact that this water body is almost entirely 
 cut off from the rest of the sea, and that it receives many fresh- 
 water rivers. Finally, the average surface salinity of the Baltic 
 Sea is only 7.8 per mille, whereas if the water of this enclosed basin 
 is taken as a whole, it is somewhat more saline because of the 
 greater salinity of the deeper layers. Even then, however, it is 
 only 10 per mille. The Baltic Sea, moreover, shows a remarkable 
 gradation in the salinity of its waters from west to east. Where it 
 joins the North Sea at the Skager Rack, the salinity is 34 per mille, 
 but in the Kattegat it is only 22 per mille. Thence it gradually 
 decreases eastward and northward until the waters near the heads 
 of the Gulf of Finland and that of Bothnia are essentially fresh. 
 
 Composition of the Sea-Salts 
 
 Although, strictly speaking, the material held in solution by the 
 water of the sea is not in combination as salts, such as are produced 
 on evaporation, but rather in the form of ions, the basic elements 
 and the acid radicals being separated, nevertheless it is customary 
 and convenient to consider them as combined into the form of 
 salts. Among these, common salt or sodium chloride makes up the 
 bulk of the material, being nearly 78 per cent of the total mass of 
 salt, or over 27 per mille of the salinity (which is taken as 35 in round 
 numbers) . In the following table the composition of the sea- water 
 salts is given in the form of such combinations, together with the 
 permillage of each in normal sea-water, and the number of short 
 tons in a cubic mile of sea-water. 
 
 With the calcium carbonate are included the small quantities 
 of other salts present, such as the iodine, lithium, manganese, 
 and phosphorus salts, and the silver, gold, nickel, and other metals 
 which are present in minute quantities in the solution. 
 
Sea- Water and Its Products 
 
 TABLE OF THE COMPOSITION OF SEA-SALTS 
 
 229 
 
 
 
 
 PERMTLLAGE 
 
 
 
 
 PERCENTAGE 
 
 OR ACTUAL 
 
 TONS OF 
 
 SALT 
 
 SYMBOL 
 
 OF TOTAL 
 SALTS TAKEN 
 
 WEIGHT IN 
 GRAMS PER 
 
 2000 LBS. EACH 
 PER CUBIC MILE 
 
 
 
 AS 100 
 
 LITER OF SEA 
 
 OF SEA WATER 
 
 
 
 
 WATER 
 
 
 i. Sodium Chloride 
 
 NaCl 
 
 77-758 
 
 27.213 
 
 131,526,080 
 
 2. Magnesium Chloride 
 
 MgCl 2 
 
 10.878 
 
 3.807 
 
 18,399,360 
 
 3. Magnesium Sulphate 
 
 MgSO* 
 
 4-737 
 
 1.658 
 
 8,012,480 
 
 4. Calcium Sulphate . 
 
 CaS0 4 
 
 3.600 
 
 1.260 
 
 6,089,440 
 
 5. Potassium Sulphate . 
 
 K 2 S0 4 
 
 2.465 
 
 0.863 
 
 4,169,760 
 
 6. Calcium Carbonate . 
 
 CaCO 3 
 
 o-345 
 
 0.123 
 
 583,520 
 
 7. Magnesium Bromide 
 
 MgBr 2 
 
 0.217 
 
 0.076 
 
 367,360 
 
 
 
 IOO.OOO 
 
 35.000 
 
 169,148,000 
 
 Common Salts Produced by Evaporation of Sea-Water 
 
 The two principal salts which are produced by the evaporation 
 of sea- water are the common salt, sodium chloride (NaCl), and 
 gypsum or calcium sulphate, with two molecules of water (CaSC>4 
 + 2H. 2 O). By local oversaturation with lime, calcium carbonate 
 (CaCO 3 ) may also separate out, but this is more commonly pro- 
 duced by chemical reaction. When the evaporation has gone very 
 far, other salts, such as those of magnesium, and finally potash salts, 
 separate out. Common salt and gypsum are obtained on many 
 sea-coasts, either by evaporation of the water by artificial heat, or 
 by conducting the sea-water into large shallow " pans," that is, 
 fields surrounded with dams and having a hard, flat bottom. When 
 the outlet of the pan is closed, evaporation takes place under the 
 influence of the sun and drying winds, and after a while gypsum 
 separates out. When most of this has been deposited, the water 
 is conducted to another pan, where evaporation continues until a 
 large part of the common salt (sodium chloride) has separated out. 
 Then the remaining dense brine, which is called the mother liquor, 
 is drawn off and either returned to the sea or further evaporated 
 for the rarer salts. In this manner a large part of the world is 
 supplied with its domestic requirements of salt, though vast quan- 
 tities of this commodity are also obtained from inland salt deposits, 
 which are the product of evaporation either of sea-water or of in- 
 land salt-bearing waters, in former geological periods. 
 
230 Aqueous or Hydrogenic Rocks 
 
 Experiments in Evaporation of Sea-Water 
 
 In 1849, the Italian chemist, J. Usiglio, published the results of 
 experiments which he had made at Cette on the south coast of 
 France. He had taken 5 liters of the sea-water from the Medi- 
 terranean and evaporated this, keeping an exact record of the point 
 which the evaporation had reached when separation of the several 
 salts took place, and determining the amount of the various salts 
 separated at the successive stages. 
 
 No separation of salts occurred until the water was evaporated to nearly 
 one half its volume, when the iron oxide and a part of the carbonate of lime of 
 the sea- water separated out. Later still, when the original 5 liters of the water had 
 been reduced by evaporation to about one liter, the remainder of the carbonate 
 of lime, together with the hydrous sulphate of lime, or gypsum, was precipitated. 
 More than 84 per cent of the total amount of gypsum contained in the sea- water 
 was deposited before the water became dense enough to allow separation of the 
 common salt (NaCl), the remainder of the gypsum being thrown down with 
 that salt and with various amounts of magnesium salts (MgSC>4 and MgCla), 
 and finally with sodium bromide. A significant fact was that no sodium sepa- 
 rated out until the evaporation had reduced the original 5 liters to less than 
 half a liter, or to less than one tenth the original volume. At that tune the 
 amount of solid matter still held in solution in the half liter of water remaining 
 was about 184.4 grams, which would correspond to 368.8 grams per liter, or a 
 salinity of 368.8 per mille, whereas the salinity of the original sea-water was 
 about 38.5 per mille. 1 From this we must conclude that a water body must 
 reach this high degree of salinity by evaporation before salt can be deposited 
 in nature. Since there are to-day no known large bodies of water with such a 
 salinity, it follows that extensive salt deposition is not going on to-day by simple 
 evaporation of large bodies of sea- water. To be sure, there are many small and 
 shallow marginal lagoons on the sea-coast, and especially on the shores of more 
 or less enclosed salt-water bodies, such as the Black and Caspian Seas, and Great 
 Salt Lake, where evaporation goes far enough,. to precipitate salt but this is, 
 as a rule, only in comparatively small amounts, though commercially important. 
 Complete evaporation of the 5 liters of sea water was not achieved by Usiglio, 
 for his experiments ceased when the volume had been reduced to about 81 cubic 
 centimeters. This remaining dense " mother liquor " retained all of the potash 
 salt of the original sea- water in solution, together with some of the sodium and 
 magnesium salts. The amounts present were as follows : 
 
 NaCl 12.9425 grams 
 
 MgSO 4 9. 2 7 25 grams 
 
 MgCU . 15. 8200 grams 
 
 NaB r i . 6500 grams 
 
 KC1 2.6695 grams 
 
 Total 42.3545 grams 
 
 1 It is probable that actual separation of salt (NaCl) begins at a somewhat lower 
 salinity, for at the stage here noted something over 3 grams of salt had already 
 separated out. 
 
Conditions Favoring Deposition of Sea-Salts 231 
 
 This corresponds to a salinity of 522.9 grams per liter or 522.9 per mille (52.29 
 per cent) . 
 
 The salts of the mother liquor are precipitated only at very high or very 
 low temperatures, and from this we must conclude that potash deposits in nature 
 are formed only under exceptional conditions. 
 
 SPECIAL CONDITIONS FAVORING DEPOSITION OF SEA-SALTS 
 Modern Examples 
 
 From the foregoing it becomes apparent that the first requisite 
 for the deposition of sea-salts in nature is the concentration of the 
 sea-water under the influence of conditions which favor evapora- 
 tion, such as the heat of the sun and, above all, drying winds. That 
 such evaporation cannot go on in the open ocean is apparent, and 
 so we must look to bodies of sea-water cut off from the main oceans. 
 
 The Caspian Sea (Fig. 165). This, the largest isolated salt- 
 water body of the earth, may in many respects be considered typical. 
 It lies within the region of drying winds, and is partly surrounded 
 by deserts. Evaporation has gone so far that its surface is 85 feet 
 below sea-level, yet the salinity of its water is only 12.94 per mille, 
 or a little over one third that of normal ocean water. 1 This is due 
 to the fact that a large amount of fresh water is brought in by the 
 Volga and other rivers tributary to it, so that in spite of the evapo- 
 ration, the salt content is very low. As we shall see later, much of 
 the original salt has been specially concentrated in the Kara Bugas 
 Gulf and other dependent bodies, and some salt has no doubt been 
 deposited on the bottom of the lake and then preserved by a cover- 
 ing layer of impervious material (gypsum, clay, etc.). 
 
 The Black Sea. This nearly, but not quite, isolated mediter- 
 ranean water body has a surface salinity of only 18.3 per mille, but 
 the lower layers are denser, so that the average salinity of the water 
 as a whole is 22.04 per mille. Obviously no salt can be deposited 
 in the open parts of this sea. 
 
 There can be no question, however, that were it not for the supply 
 of fresh water, both the Black and the Caspian seas would have a 
 high salinity, while the Baltic would be nearer to sea- water in that 
 respect. Indeed, should the fresh-water supply be entirely cut off 
 from the Caspian, continued evaporation would result in the sepa- 
 ration of most of its salt upon the bottom of its basin, and eventu- 
 
 1 Average of five analyses made in 1878. 
 
232 
 
 Aqueous or Hydrogenic Rocks 
 
 ally only a layer of mother liquor would remain to cover these de- 
 posits, and under special conditions this mother liquor might also 
 be forced to part with its salts. That such evaporation of large 
 
 FIG. 165. Map of the Caspian Sea and the salt lake region north of it. 
 
 enclosed bodies of sea-water has occurred in the past history of the 
 earth is indicated by the nature of the salt deposits now found en- 
 closed in the rocks of the earth's crust, as will be seen presently. 
 To-day such deposits are formed in local " salt pans " along the 
 
Conditions Favoring Deposition of Sea-Salts 233 
 
 margin of the Black Sea, the Red Sea, and especially in the Ran of 
 Cutch on the west coast of India, deposits which for centuries have 
 supplied the natives with salt. 
 
 FIG. 166. Map of the Colorado Desert with the Salton Sink and Pattie 
 Basin. (After Sykes, from McDougal, Am. Geog. Soc. Bulletin.) 
 
 The Salton Sink (Fig. 166). This is a great depression at the 
 head of the Gulf of California, surrounded on three sides by high 
 mountains which shut out the moisture-bearing winds, especially 
 on the Pacific side. The valley is separated from the Gulf of Cali- 
 
234 Aqueous or Hydrogenic Rocks 
 
 fornia by the delta of the Colorado River. The lowest part of its 
 floor lies 273.5 feet below sea-level, and is occupied by a small lake, 
 which is surrounded by extensive salt deposits. It is generally held 
 that this depression was formerly a part of the Gulf of California 
 and was cut off from it when the Colorado built its delta. Under the 
 influence of the drying winds which descend from the Coast Ranges, 
 the cut-off portion of the sea-water evaporated, and much of the 
 salt was deposited on the floor of the basin, which was converted 
 into a desert. Some of the salt may have been previously removed, 
 when the Colorado drained into this basin and converted it into 
 a fresh-water lake, which stood 40 feet or more above sea-level, as 
 shown by old shore lines. Recently the Colorado has several times 
 reentered this basin and enlarged the central lake. 
 
 Effects of Condensation of Sea-Water on Its Animal Life 
 
 A moment's reflection will show that in the process of evapora- 
 tion and concentration of the cut-off portion of the sea-water, all 
 the animals which lived in that water would be killed, and their 
 remains would sink to the bottom or be cast upon the shores. The 
 shells of the Mollusca, the horny coverings of Crustacea, and the 
 bones and teeth of fish and other vertebrates would be embedded 
 in the layer upon which the salt later comes to lie. Thus a very 
 definite and restricted fossiliferous substratum is produced for 
 salt deposits of this type, and this will furnish a criterion by which 
 ancient salt deposits can be interpreted. If the change in salinity 
 is gradual, because the water body subject to evaporation is 
 large, extensive fossiliferous deposits may be formed, including 
 important beds of limestone, before the water is dense enough to 
 kill the organisms. After that the water will remain essentially 
 lifeless (though there are certain forms of animals which live only 
 in strong brines), and the deposits formed in it will be barren of 
 organic remains. An exception to this may, however, occur if the 
 sea should break into the basin again, flooding it with normal sea- 
 water, and bringing in with it the normal sea-fauna. Then, if the 
 basin is again cut off from the sea, evaporation will set in with the 
 repetition of the series of evaporation deposits. 
 
 Order of the Deposition of Sediments and Salts 
 
 If the basin whose waters become subject to evaporation is 
 large, the waters, as they shrink, will leave a succession of de- 
 
An Ancient Rock Salt Deposit 235 
 
 posits of various kinds around the margin. Along the shores would 
 be found sands and clays which farther out might merge into or- 
 ganic limestone. As the sea-water becomes concentrated, the or- 
 ganically formed limestones would come to an end, and chemically 
 formed limestones and dolomites would take their place. Gypsum 
 or anhydrite deposits follow next, and finally, as the water becomes 
 very .saline and the area much contracted, salt is deposited. Last 
 
 ESSLlSajxUndCUy 
 
 meatone 
 
 FIG. 167. Diagrammatic cross-section of a cut-off basin, and the deposits 
 formed in it after complete evaporation. 
 
 of all, the mother liquor salts are precipitated under favorable con- 
 ditions, but only in the central area of the original basin, where 
 the last of the water lingers. The relationships of these several 
 deposits are shown in the preceding diagram (Fig. 167), which is 
 entirely schematic. It must be emphasized that normal salt de- 
 posits formed from evaporating sea-water must always be under- 
 lain by a layer of gypsum or its anhydrous equivalent, the mineral 
 anhydrite. 
 
 AN ANCIENT ROCK-SALT DEPOSIT FORMED BY EVAPORATION 
 OF SEA-WATER 
 
 Among the many salt deposits within the earth's crust which 
 were formed during earlier geological periods and preserved through 
 burial by later deposits, a certain number can best be explained as 
 formed by the evaporation of cut-off portions of the sea in the 
 manner above outlined. This means, of course, that they have the 
 essential characteristics which we have seen are the normal accom- 
 paniments of salt deposits formed from complete evaporation of a 
 cut-off body of sea-water. The most notable example is found in 
 the great salt deposits of North Germany (Magdeburg-Halber- 
 stadt region) , most widely known as the Stassfurt deposits, though 
 this is only one of the localities where these salts are mined. Their 
 peculiar interest lies in the fact that they have associated with 
 them the rare potash salts which, we have seen, are precipitated 
 
236 Aqueous or Hydrogenic Rocks 
 
 from the mother liquor on complete evaporation of the sea-water. 
 Before the World War these deposits furnished by far the largest 
 amount of potash to the commerce of the world, and their abun- 
 dance is such that they can supply the entire world at the present 
 rate of consumption for perhaps 2000 years to come. 
 
 If we take a section through these deposits from top to bottom, as revealed 
 by the numerous boreholes and by mines in operation, we find the following char- 
 acteristic succession : 
 
 At the base lies a variable thickness of limestones and dolomites, which in 
 some sections contain old reefs formed by organisms which inhabited these 
 waters while they were still connected with the sea, and for some time after. 
 This limestone is known as the Zechstein, and it takes its definite place with the 
 salt deposits and with the underlying red sandstones (Rothliegendes), in the 
 series of successive formations which were made during the later portion of 
 the Palaeozoic era of the earth's history. 
 
 Above the Zechstein limestones and dolomites lies a formation of anhydrite 
 and gypsum 100 meters in thickness, and this is followed by the salt beds. These 
 average 245 meters in thickness and are subdivided by about 3000 layers 
 of anhydrite, the so-called annual rings. Above this follow the mother liquor 
 salts which have given these deposits their great value. They include more 
 than 30 rare minerals, the more abundant, as a rule, in layers or strata, the whole 
 averaging from about 60 to over 90 meters in thickness. The most important 
 of them from a commercial point of view are, of course, the potash salts. 
 Above these mother liquor salts occurs a second series, beginning generally with 
 a salt clay containing remains of marine animals, followed by anhydrite (30-80 
 meters) and rock-salt, with about 400 annual rings of polyhalite, which is a com- 
 plex hydrous sulphate of calcium, magnesium, and potash. The series is closed 
 by a succession of minor layers of red clay, anhydrite, rock-salt (about 40 meters), 
 anhydrite, and red clay, the last forming the top of the deposits. Through- 
 out the entire series, except in the basal Zechstein limestones and the salt clay 
 at the middle, remains of organisms are wanting, except a few plant fragments, 
 which were blown into the water body. It appears then that both series of 
 deposits, the lower as well as the upper, represent the succession which we have 
 seen is characteristic of a progressively drying up cut-off portion of the sea. 
 Moreover, as we shall see later, they differ from the succession of salt deposits 
 formed in other ways, and they may, therefore, unhesitatingly be interpreted as 
 accumulations of the first type. The regular intercalation of the anhydrite 
 layers in the lower salt suggests that they are due to seasonal fluctuations in the 
 density of the water a periodic slight reduction in the salinity during a moister 
 period putting a temporary end to salt deposition and permitting the forma- 
 tion of anhydrite, which is separated from less concentrated waters rich in salt. 
 If these changes were of annual recurrence, as seems likely, it would appear 
 that the formation of this salt mass took three thousand years. Each annual 
 ring is on the average about 7 millimeters thick, while the salt layers with which 
 they alternate are from 8" to 9 millimeters in thickness. 
 
 After the deposition of most of the mother liquor salts, a period of fluctua- 
 
An Ancient Rock Salt Deposit 237 
 
 tion occurred, and salt with polyhalite layers was formed. This alternation, 
 too, is probably seasonal, but shows that the brine was now more highly con- 
 centrated and of changed composition, and it is probable that regular changes 
 in temperature were most influential in producing the succession of deposits. 
 The higher anhydrite and salt series indicates that the sea- water again filled 
 the basin, then it was cut off anew, and again underwent complete evaporation. 
 It is also quite probable that the waters of these basins were enriched by salt 
 brought in solution by intermittent streams from the surrounding hills, as is 
 the case in many modern desert basins, and that, because of this excess of salt, 
 anhydrite, rather than gypsum, was deposited. 
 
 We cannot, however, settle the question of origin from the de- 
 posits of North Germany alone. If they were formed by the dry- 
 ing up of an enclosed sea, there should be neighboring salts and 
 sediments which should show such variations as we would expect 
 nearer the shores of this water body, and these deposits should 
 be of the same age. The method of searching out and determining 
 formations of like geological age in different parts of the world will 
 be dealt with later. Suffice it to say here that such age determina- 
 tion and correlation of formations in different countries is quite 
 possible by the aid of fossils, by the relationship of the beds to one 
 another, and by other criteria. 
 
 Taking, then, such deposits of the same age in other parts of 
 Europe, we find first that the mother liquor salts fail as we proceed 
 away from the North German region, where apparently was the 
 center of concentration. Salt is still found in a number of sections 
 even in eastern England. Here, however, magnesian limestones 
 prevail, some of them showing a peculiar structure suggestive of 
 chemical deposition (Fig. 160, p. 221). Limestone formed of a 
 restricted number of organic remains elsewhere makes up the de- 
 posit of Permian age in England. Finally, in the north and we&t of 
 England, only red sandstone deposits represent the Permian, these 
 being formed near the old shore-line of the Permian basin, and in 
 part above it, by rivers which brought sands from the uplands and 
 dropped them in their lower shallow courses (Fig. 168). 
 
 In the other direction, toward the Ural Mountains of Russia, a 
 change in the character of the Permian formations may also be ob- 
 served. At first limestones predominate, but farther east these 
 are largely replaced by sandy and clayey beds, among which coal 
 seams are found, which indicate a swamp-land condition. 
 
 It seems practically demonstrated, then, that these ancient salt 
 deposits with their valuable beds of potash salts were formed by 
 
238 
 
 Aqueous or Hydrogenic Rocks 
 
 the drying up of a large body of sea-water which had become sepa- 
 rated from the main ocean by the formation of a land barrier. This 
 water body appears to have extended from central England on the 
 west nearly to the Ural Mountains on the east. Its southern 
 
 ' FIG. 1 68. Map of the Zechstein Sea of Permian time in northern Europe, 
 showing the approximate outline of that water body, and the mountains of that 
 period. B=Berlin; Br. = Breslau; ~Dr. = Dresden; Ha,. = Halberstadt; Hb.= 
 Heidelberg; HI = Halle; L = Leipzig; M = Magdeburg; Mb = Marburg; Th = 
 Thorn; Vf = Wesel. 
 
 boundary was north of the Danube, and its northern in the region 
 of the Baltic Sea of to-day. A significant corollary is that the 
 climate of North Europe was very much dryer at that time than 
 it is to-day, for at present no such complete evaporation would take 
 place. This will be referred to again in a later part of this book. 
 
 DEPOSITS OF SALT BY CONCENTRATION IN LAGOONS. 
 THEORY or OCHSENIUS 
 
 BAR 
 
 Complete evaporation of a salt lake is only one way though 
 a most effective way of producing salt deposits. A second 
 method consists in the concentration of the sea-water in a nearly 
 shut-off lagoon in regions of arid climate. As a typical example of 
 this we may select the Kara Bugas Gulf (Fig. 169), a bay which 
 lies on the eastern coast of the Caspian Sea, from which it is sepa- 
 rated only by a sand bar, across which a narrow strait maintains con- 
 nection with the Caspian. On the other sides the gulf is surrounded 
 by deserts, and there are no streams entering it. Since the Caspian 
 Sea is a large salt-water body, though of lower salinity than the 
 ocean, the Kara Bugas Gulf is nearly as satisfactory an illustration 
 as a similar bay on the open sea would be. 
 
Deposits of Salt by Concentration 
 
 239 
 
 Because of the narrow inlet from the Caspian, sufficient water 
 is not supplied to counterbalance the evaporation over the surface 
 of the Kara Bugas Lagoon, and so a slight difference of level is 
 
 
 FIG. 169. Map of the Kara Bugas (Karabugas) Gulf or Adji-darja (salt 
 water), on the eastern border of the Caspian Sea. 
 
 produced, the Kara Bugas surface being sufficiently lower to cause 
 a constant inflowing current of water from the Caspian (Fig. i6ga). 
 Since the salt thus carried in solution is not removed, the waters of 
 
240 
 
 Aqueous or Hydrogenic Rocks 
 
 the lagoon become more and more saline. A determination made 
 some time ago showed a salinity of 285 per mille (28.5%), while 
 that of the Caspian, from which the salt was abstracted, was 
 only 12. 94 per mille (z.294%). 1 This is a sufficient concentration 
 for the deposition of some salts, but not of the common salt, which 
 is not forming at the present time on the bottom of the gulf. An 
 
 20 ZO 50 IOO ZOO ZOO 100 
 
 FIG. 169 a. Cross-section of the Caspian Sea and the Kara Bugas Gulf, 
 from Baku across the inlet to the northeastern shore of the Gulf. Vertical scale 
 greatly enlarged. Depths given in meters. The level of the Kara Bugas is 
 slightly lower than that of the Caspian. 
 
 extensive bed of sodium sulphate or glauber salt has, however, 
 formed on the bottom of this bay, and much gypsum is being de- 
 posited. 
 
 It is evident that as concentration progresses, ordinary salt will 
 be deposited, if, indeed, beds of salt do not actually underlie the layer 
 of glauber salt, having been formed during a period of former greater 
 concentration. It is also evident that so long as the connection 
 with the Caspian is maintained, no mother liquor salts will be pre- 
 cipitated, since that requires nearly complete evaporation. It is 
 not likely that such separation and complete evaporation of the 
 Kara Bugas waters can take place so long as it remains separated 
 from the Caspian by only a sand bar, for the lowering of the water 
 in the Gulf would create a sufficient inward current to keep the 
 inlet open, and even enlarge it. Therefore, while salt deposits of 
 considerable thickness may accumulate on the bottom of such a 
 gulf as the Kara Bugas, no potash salts can be formed in it. 
 
 The Caspian, like the ocean, abounds in animal life. Thousands 
 of fish, many seals, and other animals are carried through the nar- 
 rows across the bar and into the Kara Bugas, where they are killed 
 by the high salinity of the water. Their carcasses float about and 
 later sink to the bottom, or are cast upon the shore, portions of 
 which are literally covefed with dead fish which furnish food for 
 migratory birds. Shells of dead mollusks, especially the cockle 
 (Cardium edule), which lived in these waters before they became 
 
 1 It must be remembered that the salinity of the ocean water is 35.0 per mille or 3.5%. 
 
Deposits of Salt by Concentration 241 
 
 concentrated to their present degree, occur in enormous numbers on 
 the shore of the Kara Bugas, and equally large numbers are buried 
 with the fish remains in the mechanical sediments on the bottom of 
 the gulf. These sediments are, therefore, highly fossiliferous, and 
 should they harden into rock, they would constitute beds of fossil- 
 iferous clays and sandstones in close association with the salt de- 
 posits. Moreover, while salt deposits are forming in the Kara 
 Bugas, normal sediments free from salts are forming in the Caspian 
 in close juxtaposition to the Kara Bugas, and these, too, are fossil- 
 iferous. All of these factors must be kept in mind when we at- 
 tempt to use this example, or others like it, in the interpretation 
 of the history of older salt deposits. 
 
 From its simplicity this example has gained wide currency as an explanation 
 of the origin of rock-salt deposits in all parts of the world. The theory was 
 first developed, though not originated, by the German chemist, Professor Karl 
 Ochsenius, and it is commonly spoken of as the " Bar Theory of Ochsenius." 
 While it explains many an ancient salt deposit, especially the great series of 
 Tertiary salts in the region of the present Carpathian Mountains and elsewhere, 
 it does not satisfy the conditions found in other older salt deposits, especially 
 those of the United States, for which another mode of origin must be postu- 
 lated, as will be shown in a later section. 
 
 An Older Salt Deposit Formed According to the Bar Theory 
 
 Perhaps the best example of an older rock-salt deposit which 
 can be explained by the Bar Theory was found in the salt beds of 
 the Bitter Seas on the peninsula of Suez (Fig. 170). Before the 
 Suez Canal was cut, these lakes were of very high salinity, and on 
 the bottom of at least one of them an immense bed of rock-salt had 
 formed. This has, however, been entirely redissolved by the fresher 
 waters of the canal. A characteristic feature of this salt bed was 
 not only the presence of many layers of gypsum, but also of innu- 
 merable layers of clay in which were embedded the shells of mollusks 
 such as now live in the Red Sea near by, while similar shells were 
 entombed in the sediments which formed on the bottom of the 
 Red Sea during the period of salt deposition, though these sedi- 
 ments are free from salt. 
 
 We have here a deposit which satisfies all the requirements of 
 the lagoon and bar example of the Kara Bugas, namely, a circum- 
 scribed area, presence of numerous organic remains in the salt- 
 bearing series, and the association in a neighboring area (the Red 
 
242 
 
 Aqueous or Hydrogenic Rocks 
 
 Sea) of normal deposits with the same fossils but with no salt. In 
 the present case it is known that these Bitter Lakes formerly con- 
 stituted an extension of the Gulf of Suez and the Red Sea, forming 
 
 the ancient Heroopolitan Gulf. The 
 silting up of the mouth of this gulf, 
 which was not yet complete during the 
 sixth century before Christ, formed 
 the bar which cut off the lagoon from 
 the remainder of the gulf. According 
 to some authorities, this bar appears to 
 have been the site of the crossing of 
 the Red Sea by the Israelites in their. 
 exodus from Egypt. With the forma- 
 tion of this bar, which now constitutes 
 the southern margin of the isthmus of 
 Suez, the conditions favorable to the 
 deposition of the gypsum and salt were 
 produced. Repeated overflow from the 
 Red Sea supplied the waters and the 
 mud in which were buried the or- 
 ganisms which could live here until 
 the water became too salty. Then 
 their shells and other remains sank to 
 the bottom ; a layer of gypsum was 
 formed over them, and then a layer of 
 salt, covered by a more or less imper- 
 vious layer which prevented re-solution 
 
 when next the waters of the Red Sea poured in again. In the 
 waters thus freshened, new organisms developed from the larval 
 stages brought by these waters, and a new cycle of deposition was 
 inaugurated. The mother-liquor, however, never evaporated in 
 this lagoon, but remained behind, forming the bitter waters of the 
 lakes, which, however, have lost much of their character since the 
 letting in of the sea-water by the canal. 
 
 DEPOSITION OF SALT IN INLAND DESERT BASINS 
 
 Salt deposits are forming in many portions of the world to-day, 
 where no direct connection with the sea exists. The inland salt 
 lakes and salinas are in some cases shrunken bodies of fresh water 
 formerly of greater extent. This is the history of Great Salt Lake 
 
 FIG. 170. Map of the 
 Bitter Lake of Suez and the 
 Suez Canal. 
 
Deposition of Salt in Inland Desert Basins 243 
 
 of Utah, which is only a remnant of a much larger fresh-water lake 
 Lake Bonneville. By evaporation and concentration of the 
 water in the deeper part of the basin, the salinity was increased, 
 and finally in some cases reached the concentration necessary for 
 the deposition of salt. Much additional salt is constantly brought 
 into the lake by the streams which feed it. Naturally the question 
 as to the origin of the salt in these waters arises. The answer is that 
 it is leached out of the rocks within the drainage area of the basin. 
 Not all rocks contain sodium chloride, but this is generally present 
 in clastic and other rocks which have been formed on the bottom 
 of the sea in former geological periods. These rocks include an- 
 cient sea-water, often highly concentrated, within their pores, where 
 it is hermetically sealed 
 up and is set free only 
 when the rocks are sub- 
 sequently exposed to 
 atmospheric decay and 
 erosion. Then the con- 
 nate water, as it is 
 called, evaporates, but 
 the salt remains behind 
 to be redissolved by the 
 surface waters and 
 
 carried away. If it is 
 
 j ' i j FIG. 1 71 a. The irregular salt surface of 
 
 earned into an inland tfae Salt ' pl ain of Lop, Eastern Turkestan, 
 drainage basin from (After a photograph by Ellsworth Huntington.) 
 which there is no escape, 
 
 except by evaporation, salinas and even extensive salt deposits 
 are formed. Such areas, covered with glistening white rock- 
 salt or with irregular salt masses, are found in the desert basins of 
 central Asia (Figs. 171 a, b), and indeed are not uncommon in many 
 other desert areas. So long as the supply of salt lasts, such a de- 
 posit will continue to grow by periodic additions, and thus beds of 
 salt of great thickness may form. If silt is brought in during a 
 period of flooding, or if sand or dust is blown across the salt plain, 
 a layer of clastic sediment may form over the salt, and this in turn 
 may again be covered by pure salt deposits. As a rule, however, 
 gypsum is not deposited in such basins, not because the connate 
 waters imprisoned in the rock did not contain it, but because after 
 it is set free it is less readily redissolved by the surface waters than 
 
244 
 
 Aqueous or Hydrogenic Rocks 
 
 is the common salt, and that which is dissolved is likely to separate 
 out again from the waters before they reach the central basin. In 
 conformity with this we often find the sands which surround such 
 
 basins filled with gyp- 
 sum crystals which have 
 grown from the ground 
 water as it passed 
 through the sands, 
 carrying the more 
 soluble sodium chloride 
 to the central basin. 
 
 In like manner the 
 potash salts will not, as 
 a rule, reach the central 
 basin, for though they 
 are very soluble, the 
 
 FIG. 1716. The southern edge of the Salt 
 Plain of Lop, in Turkestan. (From a photo- 
 graph by Ellsworth Huntington.) 
 
 fine particles of the soil 
 of the desert among 
 which the water must find its way to the central basin have a 
 great affinity for the potash and will, by some still little under- 
 stood process, separate this substance from the water during its 
 passage. Hence the water which reaches the central basin will 
 contain little else than pure sodium chloride, and therefore only 
 
 FIG. 172. Silver Peak Marsh, Nevada, a typical playa (Photo F. R. Porter 
 from U. S. G. S.). 
 
 pure rock-salt deposits are formed. Gypsum may, however, result 
 from the alteration of limestones formed of lime-mud and dust 
 which is washed or blown over the salt. In this manner gypsum 
 beds may be formed above the salt beds, whereas in normal 
 marine deposits of salt, or in those due to the evaporation of cut- 
 
Deposition of Salt in Inland Desert Basins 245 
 
 FIG. 1 73. Professor Johannes 
 Walther, widely known for his 
 investigations of desert phe- 
 nomena. 
 
 oils, the gypsum will underlie the salt. The importance of these 
 facts in the determination of the origin of older salt beds is very 
 great. Another fact that must not 
 be overlooked in desert salt de- 
 posits is that the parting and en- 
 closing layers of sediment will con- 
 tain no marine organisms. They 
 will, indeed, be for the most part 
 entirely free from organic remains. 
 A few desert organisms and mi- 
 gratory birds may, however, become 
 entombed in these deposits or even 
 in the salt itself, but their terres- 
 trial character is readily recognized 
 by the expert. Huntington reports 
 finding, in the salt of Lop Nor in 
 eastern Turkestan, a dead plover 
 which had been preserved in the salt 
 for centuries. Around the border 
 of some of the Persian salinas a 
 zone of mud is sometimes found, 
 
 in which are entombed the bones of animals which came to drink 
 of the salty water and perished there. 
 
 Beyond these salinas in all directions we pass into the region of 
 desert sands and dust deposits, and these have very definite char- 
 acteristics which can be recognized even after they have hardened 
 into rock beneath a cover of other deposits. Thus the geologist 
 will generally be able to recognize in the rocks associated with the 
 ancient desert salts the structures which clearly indicate that 
 origin. The appearance of a typical American salt-marsh or 
 playa is shown in Fig. 172. 
 
 Among the geologists who have made extensive investigations 
 into the origin and mode of deposition of desert salts, the foremost 
 rank must be assigned to Professor Johannes Walther of the Uni- 
 versity of Halle. (Portrait, Fig. 173.) 
 
 An Ancient Example of a Desert Salt Deposit 
 
 In the central part of the state of New York, in western Ontario, 
 and in southern Michigan, occur ancient rock salt deposits, all of 
 which were formerly and some of which are still buried under thou- 
 
246 Aqueous or Hydrogenic Rocks 
 
 sands of feet of rock, a considerable portion of the latter being of 
 marine origin. The rock-salts themselves, however, which rest 
 in some places upon marine beds of Lower Silurian (Niagaran) age 
 and in others upon non-marine sediments, are in some places covered 
 by marine beds of Upper Silurian age (Monroan) and are associated 
 laterally with deposits of clastic material which shows all of the 
 features of sediments found in modern deserts. Moreover, no 
 fossils are found in these deposits nor in those which separate the 
 several salt beds of the series from one another, nor are there any 
 Middle Silurian beds of marine origin to be found within hundreds 
 of miles of the salt deposits. Thus it appears that these very an- 
 cient salt beds of Middle Silurian age were formed in a desert which 
 then occupied much of the area now covered by the Great Lakes 
 and adjoining territory (see further, Chapter XXXIV). 
 
 No potash deposits are found associated with these salts, and 
 from what he has learned so far, the student will realize that there 
 is little likelihood of the finding of these salts unless it can be shown 
 that the basal salt beds of some sections are not of desert but of 
 marine origin, resulting from the drying up of the last remnant of 
 the Niagaran sea which preceded the desert period. 
 
 There is, however, one important fact which seems to argue 
 against such a marine interpretation of the basal beds, and that is 
 the absence of gypsum or anhydrite beds beneath the salt. Indeed, 
 the entire series of Salina salt deposits lacks the foundation layers 
 of gypsum or anhydrite, though gypsum overlies the salt in a 
 number of localities. Much of this is, however, known to have 
 been produced by the alteration of former limestone beds which 
 were invaded by sulphur-bearing waters. 
 
 CARBONATE OF LIME DEPOSITS 
 
 Although some carbonate of lime separates out on evaporation 
 of the sea-water, this is of such small amount that it practically 
 disappears in the evaporation series produced along the seacoast 
 and from cut-off bodies. Nevertheless, carbonate of lime deposits 
 are formed in the sea, not so much by evaporation, though 
 such an origin may be ascribed to some deposits, as by the 
 force of attraction of other particles of lime in sea-water saturated 
 with lime carbonate, or by the abstraction of the solvent carbon 
 dioxide through agitation of the water. Chemical precipitation 
 
Carbonate of Lime Deposits 247 
 
 of lime also takes place and is perhaps the most common mode of 
 lime deposition in some places aside from that due to organic 
 action. This precipitation, however, is due largely to the forma- 
 tion of ammonium carbonate by the decay of organic matter, and 
 this ammonium carbonate reacts with the lime sulphate or other 
 lime salts in the sea-water, producing calcium carbonate, which 
 is precipitated, and an ammonium salt which remains in solution. 
 The reactions may be written in the following way : 
 
 CaSO 4 + (NH4) 2 CO 3 = CaCO 3 + (NH4) 2 SO 4 
 
 Calcium Ammonium Calcium Ammonium 
 
 Sulphate Carbonate Carbonate Sulphate 
 
 Or again 
 
 CaCl 2 + (NH 4 ) 2 CO 3 = CaCO 3 + 2 NI^Cl 
 
 Calcium Ammonium Calcium Ammonium 
 
 Chloride Carbonate Carbonate Chloride 
 
 Where lime is precipitated by such reactions, it often forms more or 
 less spherical or irregular masses or nodules, to which the name 
 concretions is applied. Such concretions have been dredged from 
 many portions of the sea, and they appear to be especially common 
 where organic matter which has reached the floor of the ocean 
 undergoes a process of decay. Such areas are, however, not uni- 
 versal because the organic matter on the ocean bottom is generally 
 devoured by bottom-feeding animals before the decay progresses 
 far. It is only where the character of the water, or the tempera- 
 ture, is such that bottom-feeders are scarce or absent, that decay 
 of stray organic matter can take place. 
 
 Deposition of lime due to the attraction of other lime particles 
 is illustrated by the hardening on the ocean bottom of the loose lime- 
 sands and muds worn from the coral and other organic limestone 
 masses in the sea. It is a general fact that wherever lime-mud, or 
 sand, forms upon the sea-floor, this is soon bound together by the 
 filling in, between the particles, of lime derived from the sea-water. 
 This seems to take place most actively in warm regions, where the 
 amount of lime in the sea-water is above the average. As a result, 
 the floor of the ocean in such regions is a hard surface to which 
 various stationary marine animals attach themselves, while others, 
 such as certain worms or sponges, bore into this rock to a certain 
 depth. 
 
 On the surface of ancient limestone beds we often find the 
 marks of animals which had become cemented to it. Such cemen- 
 
248 Aqueous or Hydrogenic Rocks 
 
 tation could of course take place only if the surface were of 
 sufficient firmness, and this indicates that the old deposits of 
 lime-sand, or mud, from which these beds were formed in the sea, 
 hardened by further separation of lime, so that the animals living 
 there could become attached to it. 
 
 Another illustration of such lime deposition is seen in the coating of lime 
 around grains of quartz, basalt, or other sand, or around fragments of shell, 
 etc., which are found on the shores of the Island of Gran Canaria in the Canary 
 Islands. These coated grains are more or less spherical and of the texture 
 called oolitic (see p. 217). By further deposition of lime between the grains 
 they are bound together into a solid rock, an oolite, which is quarried at low tide. 
 The water here is warm, having throughout much of the year a temperature 
 above 20 C., and there is much lime in solution. Still another interesting 
 example of lime deposition is seen in certain Mexican lagoons where insect 
 eggs are coated with lime, producing a series of rounded oolite grains. Most 
 of the grains of oolitic character are, however, produced by the activities of 
 bacteria or minute algae in the sea, and on this account must be classified as of 
 organic origin. They will be more fully discussed in a later chapter. 
 
 OTHER CHEMICAL DEPOSITS IN THE SEA 
 
 Small quantities of other substances are deposited in the sea 
 as the result of certain chemical reactions, or through attraction 
 by material of like composition. The most important of these 
 are phosphatic concretions and concretions of oxide of manganese, 
 both of which have a wide distribution over the sea-bottom. A 
 third group of such deposits forms grains of the green mineral 
 glauconite, of which " green-sands " are made. 
 
 Phosphate of Lime. - This is produced by many marine animals 
 which take the phosphoric acid either directly from the sea-water, 
 or from their food. The phosphate of lime is built into certain 
 hard tissues, as the shells of some brachiopoda (Lingula), the bones 
 and teeth of fish, etc., and it is also present in the excrements of 
 fish and other animals. Such particles accumulating on the sea- 
 bottom have the power of attracting to them more phosphate and 
 precipitating it upon their surfaces, which thus become a nucleus 
 around which phosphate concretions are built. Such phosphate 
 concretions are found on the ocean floor in many localities at mod- 
 erate depths. They are also found in many old limestones, in 
 which they are generally scattered. By the weathering of this 
 limestone the nodules which are left behind are concentrated into 
 beds which are rich enough to be worked. By solution of the 
 
Other Chemical Deposits in the Sea 
 
 249 
 
 phosphate and by redeposition in veins or on the walls of cavities, 
 or by replacing limestones upon which they rest, rich deposits of 
 phosphate are produced. 
 
 Manganese Concretions. Concretions of oxide of manganese 
 with oxide of iron, clay, and other substances are also found on the 
 floor of the deep sea in 
 many localities (Fig. 174). 
 The manganese and iron 
 form concentric layers 
 about some nucleus, which 
 may be the tooth of a 
 shark, or some other sub- 
 stance. It is not fully 
 known whether the man- 
 ganese is derived from the 
 sea-water in which it is 
 present in very small 
 quantities, or whether it is 
 derived from the decom- 
 position of basic volcanic 
 rocks on the floor of the 
 ocean. Manganese con- 
 cretions are also found in 
 
 ancient marine deposits, and in some cases at least may represent 
 concentration of scattered nodules by the weathering of the rock 
 which contained them. 
 
 Glauconite. Still another deposit formed by chemical means 
 on the sea-floor, is the mineral glauconite, which, when abundant, 
 forms beds chiefly composed of small grains of this mineral. On 
 account of their green color such beds of glauconite grains are 
 commonly called " green-sands." Chemically the mineral is an 
 impure hydrous silicate of iron and potassium, and it is commonly 
 formed from fine mud which fills the interior of small shells of 
 Foraminifera (Fig. 196, p. 275), partly by the reaction with the 
 products of decay of the organic matter in these shells, and partly 
 by reaction with the substances in the sea-water. The whole 
 process is too complex to be further discussed here, and should be 
 taken up again in a more advanced course. 1 
 
 FIG. 174. Nodule of oxide of manganese 
 from Red Clay of abyssal ocean bottom. 
 (After J. Murray.) 
 
 1 See A. W. Grabau, Principles of Stratigraphy, pp. 670-673, and the literature there 
 cited. 
 
250 Aqueous or Hydrogenic Rocks 
 
 Beds of green-sands are not uncommon in older marine and other 
 deposits. The most characteristic examples are found in the strata 
 of Cretaceous age which crop out in New Jersey and Maryland. 
 Sometimes by exposure the iron of the green-sand is changed to 
 an oxide, and ochery or red beds will be produced. Such is the 
 vividly red sand bed which is so prominent in the section at Atlantic 
 Highlands, N. J., and which underlies the town of Red Bank, to 
 which it has given the name. Beds of green-sands are also common 
 in the Cretaceous strata of southern England, where they are 
 generally spoken of as the Green Sands. They also occur in France 
 and elsewhere. Some beds of green-sands are, however, found in 
 deposits which accumulated elsewhere than on the sea-floor. 
 
 CHEMICAL DEPOSITS AND EVAPORATION PRODUCTS or LAKES 
 Lacustrine Deposits 
 
 Lakes may be classed as fresh-water, alkaline-water, salt-water, 
 and brine lakes. The salt-water and brine lakes have already been 
 referred to, and it has been shown that the deposits in these are 
 mainly pure salt (sodium chloride), and in some of the larger ones, 
 like Great Salt Lake, also sodium sulphate or mirabilite. The Cas- 
 pian must be differentiated from salt lakes of smaller size, as it is 
 more properly a portion of the sea which has been cut off. Hence 
 the deposits there are generally like those formed on the sea-coast. 
 
 Composition of Lake Water 
 
 The composition of lake water varies of course in an almost endless manner, 
 no two lakes having water of exactly the same composition. Nevertheless, 
 it is possible to select certain types or averages of groups, which represent in a 
 general way the mineral substances present in such waters. Fresh- water 
 lakes are of course the most abundant, and as their composition varies to a less 
 degree than that of the other lakes, it is possible to give a general average. 
 This is shown in the first column of the annexed table, in which the average also 
 includes that of river waters, which are not essentially different from those of 
 lakes. In the other columns the composition of a typical alkaline water (Owen's 
 Lake, California), saline water (Lake Corongamite, Victoria, Australia), and 
 brine (Dead Sea at 200 meters depth) are given, and the composition of ocean 
 water is again given for comparison. In this table the composition is expressed 
 in terms of ions rather than of salts, which is the more accurate way of state- 
 ment. 
 
Evaporation Products of Lakes 
 
 251 
 
 TABLE OF THE COMPOSITION OF LAKE WATERS 
 
 
 
 FRESH 
 
 ALKALINE 
 
 SALINE 
 
 
 OCEAN 
 
 
 
 WATER 
 
 WATER 
 
 WATER 
 
 
 WATER 
 
 NAME OF ION 
 
 SYMBOL 
 
 Average 
 
 Salinity 
 
 0-1 7 - 5 ,, 
 per mule 
 
 Salinity 
 
 2I 3-7 
 per mule 
 
 Salinity 
 46 
 per mille 
 
 Salinity 
 251.1 
 per mille 
 
 Average 
 Salinity 
 
 per mille 
 
 Carbonic oxide .... 
 
 C0 3 
 
 3S- J S 
 
 24-55 
 
 
 
 Trace 
 
 0.207 
 
 Sulphuric oxide . . . 
 
 S0 4 
 
 12.14 
 
 9-93 
 
 I.6 5 
 
 0.22 
 
 7.692 
 
 Phosphoric oxide . . . 
 
 P0 4 
 
 
 
 O.II 
 
 
 
 
 
 Trace 
 
 Boron oxide .... 
 
 B 4 7 
 
 
 
 0.14 
 
 
 
 
 
 Trace 
 
 Chlorine 
 
 Cl 
 
 5.68 
 
 24.82 
 
 59-32 
 
 67.84 
 
 55-292 
 
 Bromine 
 
 Br 
 
 
 
 
 
 0.22 
 
 i-75 
 
 0.188 
 
 Nitrogen oxide .... 
 
 N0 3 
 
 0.90 
 
 0.45 
 
 
 
 
 
 
 Lithium 
 
 Li 
 
 
 
 O O3 
 
 
 
 
 
 Trace 
 
 Calcium 
 
 Ca 
 
 IO OO 
 
 o 02 
 
 O It 
 
 i 68 
 
 1 107 
 
 Magnesium 
 
 Mg 
 
 3-41 
 
 O.OI 
 
 2.77 
 
 16.72 
 
 3-725 
 
 Sodium 
 
 Na 
 
 
 38 oo 
 
 3 c O7 
 
 IO OO 
 
 30 SO3 
 
 
 
 /v 
 
 
 
 
 
 Potassium 
 
 K 
 
 212 
 
 I 62 
 
 o 84. 
 
 I 7O 
 
 1.106 
 
 Iron oxide 
 Aluminum oxide . . . 
 
 Fe 2 3 1 
 A1 2 3 / 
 
 2-75 
 
 0.04 
 
 
 
 Trace 
 
 Silica 
 
 SiO 2 
 
 1 1 67 
 
 O Id 
 
 
 
 Trace 
 
 Trace 
 
 Arsenic oxide .... 
 
 AsA 
 
 
 0.05 
 
 
 
 
 Trace 
 
 
 
 IOO.OO 
 
 IOO.OO 
 
 IOO.OO 
 
 
 IOO.OO 
 
 Deposits of Fresh-Water Lakes 
 
 The three most important types of deposits formed from the 
 waters of fresh-water lakes and ponds are those of carbonate of 
 lime and carbonate and oxide of iron. 
 
 Carbonate of Lime. This is by far the most important con- 
 stituent of lake waters, and for that matter, of fresh-water bodies of 
 all kinds. Nevertheless, the actual quantity is smaller in fresh than 
 in sea-water. As will be seen from the analysis a cubic mile of fresh- 
 water contains on the average only 360,915 short tons of calcium car- 
 bonate (CaCO 3 ), whereas a cubic mile of sea-water contains 583,520 
 short tons of this mineral in solution. It must be remembered, how- 
 ever, that sea-water contains a vastly larger quantity of other sub- 
 stances, of which common salt forms 131,526,000 tons, while in 
 average fresh- water it forms only about 19,656 short tons per cubic 
 mile. Moreover, the total quantity of mineral substances dissolved 
 in the sea is 169,148,000 short tons per cubic mile, in fresh- water only 
 854,100 short tons; that is, the sea contains about 200 times as 
 
2 5 2 
 
 Aqueous or Hydrogenic Rocks 
 
 much mineral matter in solution. The mineral matter of fresh- 
 water bodies is precipitated by three methods: evaporation, 
 chemical reaction, and organic secretion. The last belongs to the 
 topic of organic deposits. 
 
 Evaporation. Lake waters are, as a rule, very far from being 
 saturated with carbonate of lime, and for this reason such a sub- 
 stance will not be de- 
 posited as an evaporation 
 product until much or all 
 of the lake water is 
 evaporated. An excep- 
 tion to this is the depo- 
 sition of carbonate of 
 lime in the form of cal- 
 careous tufa in the mar- 
 ginal pools or on beaches 
 of great fresh-water lakes 
 situated in dry climates, 
 but constantly supplied 
 with water by a large 
 river. Such tufa forms 
 by local complete evapo- 
 ration of the water which 
 lies in shallow marginal 
 pools above the ordinary 
 water-level and is re- 
 plenished at intervals by 
 spray and the waves. 
 The spray which satu- 
 rates the sands of the 
 beaches may also, on 
 rapid evaporation, leave 
 behind carbonate of lime 
 to bind the sand grains 
 
 together. Such deposits of calcareous tufa are found in the old ' 
 lake beaches on the slopes of the Colorado or Salton desert up 
 to 40 feet above sea-level. These were formed when the Colorado 
 River drained into the basin and kept it full of water up to that 
 level, in spite of the rapid evaporation which was taking place in 
 the dry climate of the region, and which has completely dried 
 
 FIG. 175. Map of Lake Bonneville and its 
 present remnant, Great Salt Lake of Utah. 
 
Evaporation Products of Lakes 
 
 253 
 
 FIG. 176. Terraces and shore-lines of Lake Bonneville, near Wellsville, 
 Utah, showing contrast between littoral and subaerial topography. (After 
 Gilbert.) 
 
 FIG. 177. Abandoned shore-lines of Lake Bonneville. North end of Oquirrh 
 Mountains, Utah. (Photo by F. J. Pack.) 
 
254 
 
 Aqueous or Hydrogenic Rocks 
 
 out that basin since the Colorado has become diverted to the 
 Gulf of California. The nearly complete evaporation of lake 
 water is illustrated by the history of lakes Bonneville and Lahonton, 
 which in an earlier period of the earth's history occupied large 
 
 5 iO i5 20 " 
 
 jca,le in Miles 
 
 FIG. 178. Map of ancient Lake Lahonton and some of the present residual 
 
 water bodies. 
 
 areas in the western United States (Figs. 175, 178), and the old 
 shore lines of which are still traceable (Figs. 1 76, 177). The product 
 of evaporation of the water of these lakes was chiefly carbonate 
 of lime in the form of calcareous tufa, of which several types were 
 formed. This covers the older rock surfaces over wide areas 
 (Fig. 176) and forms layers of tufaceous limestone, alternating with 
 
Evaporation Products of Lakes 
 
 255 
 
 deposits of sands and gravels, which it sometimes cements. In 
 places these deposits form a mass over 50 feet in thickness, though 
 elsewhere they are represented only by an average thickness of 
 20 to 25 feet. 
 
 Three types of calcareous tufa are found within the basin of old Lake Lahon- 
 ton, each type belonging to a separate period of formation. The first and oldest 
 is called a lithoidal tufa because it is rock-like, cementing the old gravels of the 
 
 I 
 
 FIG. 179. Thinolithic tufa or Thinolite, from Lake Lahonton Basin. (After 
 
 Russell.) 
 
 lake floor, and it not infrequently contains shells of fresh- water Mollusca. The 
 next type, formed after an interval of exposure, is known as thinolithic tufa 
 (Fig. 179), and consists of a series of large prismatic crystals six to eight inches 
 long and almost half an inch in thickness. These form a layer from six to 
 eight feet thick where best developed. The final type of tufa is called dendritic 
 (Figs. 159, 180), from its branching structure, and this is the most abundant, 
 
256 
 
 Aqueous or Hydrogenic Rocks 
 
 covering the old rocks of the lake with a deposit from twenty to fifty feet thick. 
 Sometimes this formed dome-shaped or mushroom-like masses up to five or 
 six feet in diameter (Fig. 159, p. 2.19), and where these are crowded they often 
 
 assume a polygonal outline 
 resembling paving blocks. 
 Internally these masses 
 have a more or less radiate 
 structure. 
 
 Significance of lime de- 
 posits of this type. Lime- 
 stones like the above, due 
 to evaporation, indicate a 
 dry climate during the 
 period of their formation. 
 If such limestones are cov- 
 ered by other deposits, 
 which later harden to rocks, 
 they will form a member of 
 a series of stratified rocks 
 similar in general appear- 
 ance to many that are found 
 in the older parts of the 
 earth's crust. If in such 
 an older series the limestone 
 member can be determined 
 by its peculiar character- 
 istics to have originated as 
 an evaporation product of 
 an old lake basin, a definite 
 conception of the physical 
 conditions of that region at 
 the time of the formation of 
 the limestone bed is gained. It is therefore important that the particular 
 characteristics of such limestone beds should be understood. At the same 
 time it must be remembered that with the passage of time, such a limestone 
 will undergo more or less change, so that the old characters are to a greater 
 or less degree obliterated or altered. Nevertheless, enough may remain to 
 indicate the origin of the limestone, and from the adjoining formations and 
 those lying beneath and above it additional evidence for or against the evapora- 
 tional origin of such a limestone may be obtained. What may prove to be an 
 example of this type formed in a past geological period (Permian), is the bed of 
 Magnesian Limestone exposed on and near the coast of Durham in England, 
 which shows a structure not unlike the spherical structure of the dendritic tufa 
 of Lake Lahonton (Fig. 160, p. 221). The internal structure is, however, more 
 strongly crystalline, which may be due to its greater age. (Fig. 18, p. 34.) 
 
 Chemical precipitation. This will take place in stagnant lakes 
 and ponds, the waters of which contain much carbonate of lime in 
 
 FIG. 1 80. Tufa-domes, shore of Pyramid 
 Lake, a remnant of Lake Lahonton. Dendritic 
 tufa. (After Russell.) 
 
Evaporation Products in Lakes 
 
 257 
 
 solution, and which moreover contain decaying organic matter. 
 This decay, as in the ocean, will produce ammonium carbonate, 
 which will tend toward precipitating the lime. It may be doubted, 
 however, that much lime is precipitated in this manner from fresh 
 water, since the presence of carbon dioxide, which is also formed 
 as the product of decay, would tend to keep the lime in solution. 
 
 Organic separation. The most common method of abstraction 
 of lime from fresh water is that by the activities of organisms, both 
 animal and plant. But deposits so formed belong to the group 
 of organic rocks, and their discussion will be deferred to another 
 chapter. 
 
 Iron Carbonate and Oxides. Stagnant swamps are often 
 covered by an iridescent film, which is the result of the oxidation, 
 on contact with the atmosphere, of iron salts which were contained 
 in the water. Such iron salts are originally in the form of ferrous 
 carbonate (FeCO 3 ), which in mineral form is the ore siderite. This 
 iron carbonate is derived from iron salts in the soil and has resulted 
 from the decomposition of iron-bearing 
 rocks (the ferro-magnesian silicates of 
 igneous rocks), the leaching being per- 
 formed by rain and ground waters which 
 have taken up carbon dioxide (CO-j) from 
 decaying vegetation. When such a so- 
 lution of iron carbonate is brought into 
 a lake or swamp, one of two things will 
 happen. When there is much decaying 
 organic matter in the swamp, this will 
 appropriate all the available oxygen, 
 and then the iron is deposited in the 
 form of the carbonate. This is gen- 
 erally impure, being mixed with the mud 
 held in suspension and carried down with 
 the iron carbonate. This mixture is most 
 frequently deposited around some object 
 which forms the nucleus of an iron-stone 
 concretion, as such structures are called 
 (Fig. 181). Such iron-stone or siderite 
 concretions when sufficiently pure form ores of iron, and they are 
 commonly found in formations in which coal (the product of the 
 partial decay of the vegetation) is also found. 
 
 
 Hy, 
 I 
 
 FIG. 181. Clay-iron- 
 stone concretion split in 
 two, showing the impression 
 of a fossil fern (Neurop- 
 teris). Mazon Creek, 111. 
 (Photo by B. Hubbard. 
 Specimen in Columbia Uni- 
 versity.) 
 
258 Aqueous or Hydrogenic Rocks 
 
 If, however, vegetation is not abundant, the carbonate will be 
 changed to the oxide, by contact with the air and with the oxygen 
 dissolved in the water, and in such cases an iron 'oxide will 
 be deposited. This is commonly in the form of limonite, the 
 yellow iron ore, in which water is present, and the loose, porous 
 masses of this mineral which form on the bottom of such ponds are 
 called bog-iron-ore. Such bog-ores are generally most abundant 
 near the margins of the ponds and swamps and are often wanting 
 near their centers. Sometimes such bog-ore forms very rapidly, 
 some Swedish lakes having deposited layers several inches thick 
 in twenty-six years. 
 
 The decomposition of the iron carbonate in the solution and its 
 oxidation to insoluble iron oxide is aided by, and in some cases 
 largely due to, the work of minute organisms in the water, the 
 so-called " iron bacteria." Such deposits might be referred to the 
 organic group, were it not for the difficulty of distinguishing them 
 from purely chemical deposits such as those described. 
 
 Alkaline Lakes and Their Deposits 
 
 Under this heading are classed lakes in which carbonate of soda 
 plays a more important part than carbonate of lime, which is 
 generally present in small quantities only. The relatively great 
 preponderance of the CO2 radical, and the much reduced chlorine 
 content, further differentiate typical alkaline lake waters from those 
 of saline lakes, in which sodium chloride is the dominant salt in solu- 
 tion and the carbonates are insignificant in quantity. Sulphates 
 and chlorides, however, are generally present, and one or the other 
 or both may be abundant, making complex alkaline waters. In 
 some cases, too, potassium may be an important element, exceeding 
 even the sodium, as in Albert Lake, Oregon. 
 
 Some of the principal water bodies in which deposits of this and 
 similar types are formed are : 
 
 Sodium Sulphate. Laramie Lakes, Wyoming 
 Soda Lake, Cal. 
 Sevier Lake, Utah 
 Mono Lake, Cal. 
 Owen's Lake, Cal. 
 Searle's Lake or Marsh, Cal. 
 Albert Lake, Oregon 
 Altai and Domoshakovo Lakes, Russia 
 
Evaporation Products of Rivers 259 
 
 Sodium Carbonate. Soda Lake, etc., Nev 
 
 Owen's Lake, Cal. 
 
 Searle's Lake, Cal. 
 
 Natron Lakes, Egypt 
 
 Lakes in Hungary, Armenia, Venezuela, etc. 
 Borax. Searle's Lake, Cal. 
 Death Valley, Cal. 
 Soda Niter (Chile Saltpeter). Desert Lakes of Chile 
 
 Searle's Lake 
 Potassium Nitrate (Saltpeter). Cochabamba, Bolivia 
 
 Different salts may be deposited at different times by these 
 lakes. Thus Searle's Lake or Marsh deposits both carbonate and 
 sulphate of sodium and borax and niter as well. From Owen's 
 Lake, California, both carbonate and sulphate of sodium are ob- 
 tained. Common salt (sodium chloride) is also an accompaniment 
 of the deposits in many of these lakes. 
 
 The greatest deposits of niter known in the world are found in 
 the Atacama and Tarapaca deserts of Chile. The amount has 
 been estimated at 254,760,000 tons and is found at elevations 
 exceeding 2000 feet above the sea and from 50 to 100 miles from 
 the coast. The crude sodium nitrate is known as caliche and is 
 associated with anhydrite, gypsum, epsomite, halite, and other 
 minerals. The origin of these and of the potash nitrates of Bolivia 
 is not fully understood, but they were deposited from solution in 
 drying basins. 1 
 
 Deposits of Saline Lakes and Brines 
 
 These lakes deposit chiefly sodium chloride or common salt, 
 especially those in which the waters are a brine, as is the case* in 
 most inland salt lakes. Great Salt Lake, Utah, the Dead Sea, 
 and numerous Russian and Siberian lakes serve as examples. 
 They supply vast quantities of common salt for domestic and other 
 purposes. 
 
 CHEMICAL DEPOSITS AND EVAPORATION PRODUCTS OF RIVERS 
 (Flumatile Chemical Deposits) 
 
 These are of comparatively rare occurrence and are confined 
 chiefly to regions of arid climate. As carbonate of lime is the 
 
 1 For fuller discussion, see A. W. Grabau, Geology of the Non-metallic Mineral De- 
 posits, etc. Vol. I, Chapter XIII. McGraw-Hill Book Co. 
 
260 Aqueous or Hydrogenic Rocks 
 
 main mineral constituent of the river water and of that of fresh- 
 water lakes, it forms the chief, indeed practically the only, im- 
 portant chemical deposit of fresh water. 
 
 In Bahia, Brazil, the rivers which flow in and over the older 
 limestones are highly charged with lime in solution, and under the 
 influence of the tropical sun, partial evaporation of the water and 
 precipitation of the lime takes place. Thus deposits of lime rang- 
 ing up to 100 feet in thickness have been formed, and they often 
 contain plant remains as well as shells of river and land mollusks 
 of species still living in the region. Angular as well as water-worn 
 fragments of other rock are also included, and at times the mass 
 becomes brecciated through local disruption and cementation of 
 the fragments. Similar deposits are also formed around waterfalls 
 of rivers in limestone regions in various parts of the world. 
 
 On the broad flood-plains of many tropical rivers and on their 
 deltas are formed crusts of limestone, which are precipitated from 
 the over-charged river water from the limestone hills. In Mexico 
 and the southern United States these are known as tepetate, while 
 the nodular limestone masses embedded with the sediments of the 
 Indus and Ganges, in northern India, are known as kankar. Such 
 limestone nodules of chemical origin are very characteristic of 
 river sediments in arid regions, and their occurrence in older sand- 
 stones leads us to ascribe a similar origin to these deposits. 
 
 DEPOSITS BY SPRINGS AND UNDERGROUND WATERS 
 
 Lime Deposits of Springs. In limestone regions, the under- 
 ground water is generally strongly charged with carbonate of lime 
 in solution. Where this water reaches the surface in springs, 
 the relief of pressure and the escape of carbon dioxide combine to 
 cause the precipitation of some of the carbonate of lime as a 
 flocculent material, which later hardens to solid stone. Where 
 leaves, mosses, or other organisms are bathed by this spring water, 
 they are covered by a deposit of lime or are included as fossils in 
 a mass of calcareous tufa or travertine. In ordinary spring water, 
 the deposition of lime goes on rather slowly, but sometimes the 
 growth of the travertine deposit is very rapid. At the Baths of 
 San Vignone, in Tuscany, for example, travertine is deposited at 
 the rate of six inches a year, while at San Filippo, in Sicily, the rate 
 is one foot in four months. Here a hill, a mile and a quarter long 
 
Deposits by Springs and Underground Waters 261 
 
 and a third of a mile broad, has been formed by such deposits, 
 the height being at least 250 feet. 
 
 The water of hot springs generally carries more mineral matter 
 in solution than that of cold springs. The cooling of the water 
 on reaching the surface 
 is also very conducive 
 to lime deposition. 
 Hence we have here ex- 
 tensive travertine de- 
 posits, as shown in the 
 terraces, dams, and 
 basins of the Mammoth 
 Hot Springs of the Yel- 
 lowstone (Fig. 182). 
 The origin of the ter- 
 races is illustrated in 
 the subjoined diagram 
 (Fig. 183). 
 
 A peculiar form of 
 lime deposit from 
 springs is the well-known 
 onyx marble, or Mexican 
 
 onyx, which occurs interbedded with normal tufas in Arizona, 
 Mexico, California, and elsewhere in America, and in TsTorth 
 Africa, Persia, and elsewhere in the Old World. This is a com- 
 pact, highly crystalline, and often beautifully variegated lime- 
 stone of a semi-translucent character, much used for decorative 
 
 purposes, the con- 
 struction of soda- 
 water fountains, etc. 
 It is frequently found 
 resting on crystalline 
 rocks in regions de- 
 void of other lime- 
 stones, and this sug- 
 gests that the lime 
 and the water which 
 brought it to the sur- 
 
 FIG. 182. Portion of the Sinter Terraces 
 of Mammoth Hot Springs, Yellowstone Na- 
 tional Park. 
 
 FIG. 183. Diagrammatic section of Sinter 
 Terraces formed by the water of hot springs. 
 The rim of the terrace is built up most rapidly 
 because as the water overflows it cools quickly at 
 this point and deposits its mineral matter. Series 
 of terrace-basins are thus formed. (From Kay- 
 ser's Lehrbuch.) 
 
 face were of deep-seated or magmatic origin, that is, derived from 
 hot igneous masses within the crust of the earth. 
 
262 Aqueous or Hydrogenic Rocks 
 
 No beds of this deposit have actually been observed in the 
 process of making, but from the fact that it is generally enclosed by 
 beds of normal tufa, we may assume that these deposits were 
 formed in temporary pools or lakes where standing water prevented 
 the rapid escape of CO 2 , the rock thus becoming compact instead 
 of porous, as is the case in ordinary tufa formed on the land. 
 
 Oolites and Pisolites Deposited by Springs. At the famous 
 springs of Carlsbad, in Bohemia, carbonate of lime is deposited 
 in the form of spheroidal, discrete masses of the size of a pea, and 
 hence forming an accumulation of particles which when bound 
 together into a rock would constitute a pisolite. As the water 
 rises in the springs it holds in suspension minute mineral fragments 
 such as quartz or feldspar, which then receive a coating of lime 
 precipitated from the water. As the particles are turned over 
 and over in all directions like a pith-ball in a fountain jet, the 
 coating will be uniform all over, and spheroidal masses are pro- 
 duced. Sometimes gas and air bubbles form the nuclei around 
 which the lime is deposited, thus forming spheroids with a hollow 
 center. The water of these springs is probably of deep-seated 
 volcanic (magma tic) origin, from which source the lime is also 
 derived. This is indicated by the fact that there are no known 
 beds of limestone through which these waters ascend. 
 
 While no doubt both oolites and pisolites have been formed in 
 the past by springs, the great majority of these deposits now found 
 in the rocks of the earth's crust were formed in standing bodies of 
 water through the influence of organic matter either directly, by 
 the physiological activities of living organisms, such as bacteria 
 or minute algae, or by ammonia generated by decaying organic 
 matter. They will be discussed in a subsequent chapter. 
 
 Lime Deposits in Caves. The best-known types of lime 
 deposits from ground-water are found in caverns. Two types of 
 structures are generally recognized, the stalactite (Fig. 184), 
 depending from the roof of the cavern or from some projecting 
 edge, and the stalagmite, which forms on the floor of the cavern, 
 building up a mound or pyramid, or forming a hummocky lime- 
 stone floor (Fig. 185). 
 
 The formation of stalactites may be observed in tunnels and 
 underground chambers, the roofs of which consist of blocks held 
 together by lime-mortar, as well as in limestone caverns. Percolat- 
 ing ground-water will dissolve a part of this lime, and when a drop 
 
Deposits by Springs and Underground Waters 263 
 
 of this lime-charged water reaches the tunnel and is suspended from 
 the roof, rapid evaporation will cause the formation, around the 
 drop, of a thin shell of lime. The pressure of the percolating 
 
 FIG. 184. Compound stalactites and stalactic-sheets in Luray Cave, 
 Virginia. (From U. S. G. S.) 
 
 water will cause the breaking of this film of lime, leaving only a 
 small ring on the roof. The original drop falls, and a new one, 
 equally charged with lime, suspends itself from the bottom of the 
 lime ring. By constant repetition, a slender delicate tube is formed 
 by the successive additions of minute rings of lime, and this is 
 the basis of the stalactite. Other water, running down the outside 
 of this tube, will thicken and strengthen it by the addition of 
 successive layers of lime. In this process the lower end of the 
 
264 Aqueous or Hydrogenic Rocks 
 
 initial tube is soon closed, and after that the stalactite remains 
 a solid icicle of carbonate of lime. Neighboring stalactites may, 
 from close juxtaposition, become confluent and form broad sheets 
 or curtains of lime which are often beautifully banded. Such 
 sheets of lime depending from an edge in the roof or along the 
 line of a crack are shown in Luray Cavern, Virginia, where they 
 form one of the striking features of this beautiful cave (Fig. 184). 
 Stalagmites are built up on the floor of a cavern by the evapora- 
 tion of the water which drops from the roof and generally from the 
 
 FIG. 185. Stalactites and stalagmites in Marengo Cave, Indiana. Note 
 the numerous small stalactites which depend from the roof of the cave, and 
 confluence of the larger stalactites with the stalagmites to form columns. 
 
 end of the stalactite. The continued evaporation of this water 
 leaves a minute quantity of lime, which is gradually built up into 
 a mound, and this becomes steeper and steeper as it increases in 
 height, and finally forms a conical or even columnar mass beneath 
 the stalactite, with which it may eventually become joined into a 
 continuous column. This is well shown in the above, view of 
 the interior of Marengo Cave in Indiana (Fig. 185). Many such 
 columns may result in the cutting off of chambers and galleries 
 from the original caverns. On the margin of the stalagmite sec- 
 ondary dependent stalactites are often formed, wherever a higher 
 portion of the stalagmite projects cap-like beyond the lower. 
 Thus highly complex and picturesque structures are produced. 
 
Mineral Veins 265 
 
 Where the water spreads laterally before evaporating, an extended 
 sheet of stalagmite material is formed, and the lateral confluence 
 of many such sheets may result in the production of a stalagmite 
 floor. In many of the limestone caverns of southern France and 
 other parts of Europe, which during Palaeolithic time were in- 
 habited by the people of the Old Stone Age, implements and even 
 the bones of these prehistoric people are found embedded in the 
 clay of the cavern floor, over which not infrequently a cover of 
 stalagmitic material has been formed. 
 
 Basins are also formed in caverns where water, holding the lime 
 in solution, runs over a ledge. The edges of such a ledge cause the 
 water to break into ripples as it overflows, and this permits the 
 escape of some of the carbon dioxide which holds the lime in solu- 
 tion. In consequence, this lime is deposited at the edges of the 
 ledge, and so a rim is gradually built up, which holds back more 
 and more of the water in a permanent pool. Many such pools are 
 found in limestone caves such as Luray, where the conditions for 
 their formation are favorable. They are analogous to the sinter 
 terraces of the hot springs (Fig. 183). 
 
 MINERAL VEINS 
 
 Of all the deposits formed by waters circulating in or rising 
 through the crust of the earth, the mineral veins are the most 
 important to man. The mineral-bearing waters which form the 
 veins are probably in most cases hot, and they may even be in the 
 form of vapors. These waters or vapors deposit their load of 
 mineral matter either upon the walls of cavities or fissures, or by 
 replacing the rock material 
 alter it along their passage- 
 way, which may be a 
 minute crack or other 
 avenue of escape. The 
 first group is called fissure 
 veins, the second replace- 
 ment veins or deposits. 
 
 Fissure Veins (Fig. FIG. i860.. Section of the Prinzen Lode, 
 186 a). Deposition in Freiber g- <*, blende; &, quartz; c, fluorite; 
 ,. . , ", , d, barite ; e, pyrite ; /, calcite. 
 
 fissures is brought about 
 
 by the cooling effect produced by the walls upon the solution, or 
 by chemical reaction with the material of the wall rock. Thus 
 
266 
 
 Aqueous or Hydrogenic Rocks 
 
 an acid solution coming in contact with a limestone would become 
 neutralized and lime salts inclosing other minerals and metallic 
 substances might be deposited. Fissure veins have the form of 
 
 sheets, or films, of min- 
 eral matter cutting the 
 rock. If there is a suc- 
 cession of deposits, the 
 vein will be banded par- 
 allel to the wall-rock, 
 and the central portion 
 may be filled with crys- 
 tals. Veins of ore min- 
 erals or metallic sub- 
 stances contain, besides 
 this material, quantities 
 of other minerals such 
 as quartz, calcite, barite, 
 etc., and these constitute 
 the gangue material of 
 the miner. Thus native 
 gold is commonly found 
 in a gangue of quartz, 
 but quartz veins without 
 gold or other metal are 
 
 much more common. So, too, are veins of calcite and other min- 
 erals of little commercial value. 
 
 In rocks containing many veins there can often be recognized 
 several distinct series of successive origin. The relative age of 
 veins can be determined from the fact that the younger veins are 
 continuous across the older ones which they intersect. Some- 
 times cavities of irregular shape in the country rock form the site 
 of mineral and ore deposits, these differing from true fissure veins 
 mainly in their form and extent. Deposits of this kind are called 
 cavity-filled ore deposits (Fig. 1866). 
 
 Replacement Deposits (Fig. i86c). Instead of filling a pre- 
 viously existing fissure or cavity with its deposits, the mineral- 
 bearing waters or vapors may deposit their material as they pass 
 through the rock, by dissolving mineral particles of the country 
 rock and filling their places with new mineral matter. Such a 
 replacement of one mineral by another would go on, molecule by 
 
 FIG. 1 86 b. Transverse section of the great 
 ore chamber in the Emma mine, Utah, 
 i inch = 159 feet. 
 
Mineral Veins 
 
 267 
 
 molecule, until an area of the rock is so re- 
 placed by, or impregnated with, valuable min- 
 eral matter as to become an important ore 
 deposit. Frequently, of course, the replace- 
 ment and fissure type of deposit may inter- 
 grade, for the walls of an open fissure may 
 also be partly replaced by mineral matter. 
 Veins in which ores are scattered, i.e. lean 
 veins, may become locally enriched by the sub- 
 sequent solution and local concentration of the 
 valuable mineral matter. The movement of 
 the ores is generally from higher to lower 
 levels. Concentration may also occur by the 
 weathering or solution and removal of the 
 gangue material. 
 
 Placer Deposits. Concentration of val- 
 uable minerals may also occur at points distant 
 from the original vein. Thus quartz veins 
 carrying gold may be broken up by the 
 weather and the fragments washed away by granite impregnated 
 the streams. On account of the greater with tin ore; D, D, 
 weight of the gold, this will be concentrated 
 in favorable areas and so form the well-known placer deposits 
 (Fig. 187). 
 
 FIG. 1 86 c. Plan 
 of a tin lode at East 
 Wheal Lovell Mine, 
 Cornwall, England. 
 A, B, leader; C, C, 
 
 FIG. 187. Hydraulic mining of Placer deposits (gold-bearing gravels), 
 
 Colorado. 
 
268 
 
 Aqueous or Hydrogenic Rocks 
 
 Source of the Vein Minerals. The material in solution in the 
 vein-forming waters or vapors may be derived either by solution 
 from the rocks with which surface waters, descending into the 
 earth, come in contact in their circulation through the deeper 
 portions of the earth's crust, deriving their material chiefly from 
 
 FIG. 187 a. Park City, Utah, a typical mining camp. (Photo by F. J. Pack.) 
 
 rocks which overlie the point of deposition (descending solu- 
 tion), or the waters and vapors circulating through the rocks 
 laterally may dissolve their ore material and deposit it on reaching 
 the fissure (lateral secretion). Again, water derived as emana- 
 tion from deep-seated igneous masses (juvenile or magmatic 
 water) may carry upwards in solution the mineral substances 
 derived from these masses and deposit them in the higher fissures 
 (ascending solutions). This last mode of formation is regarded 
 by many as the most typical origin of mineral veins. 
 
CHAPTER XII 
 THE ORGANIC OR BIOGENIC ROCKS 
 
 BlOLITHS 
 
 BY organic rocks, using that term in its strictly limited sense, we 
 understand those additions to the lithosphere which have resulted 
 from, or are the product of, the direct physiological activities of 
 organisms, both animal and plant. Rocks secondarily derived from 
 organic deposits, such as beds of limestone made of fragments 
 worn from coral reefs, have sometimes been included under this 
 heading, but they do not belong here, being strictly of f ragmen tal 
 or clastic origin. Only those deposits which are formed in place 
 by organisms or which are 'largely built up of such material which 
 has been transported and has accumulated without much wear, 
 can be included here. Of course it must be recognized that a lime- 
 stone of shells or corals, which is strictly an organic limestone, may 
 pass laterally by degrees into one of fragmental origin. Gradation 
 exists everywhere in nature, but we are now concerned with the 
 study of types which may be readily recognized. The true organic 
 rocks are conveniently termed bioliths. 
 
 TYPES OF ORGANIC ROCKS OR BIOLITHS 
 
 At the outset we must distinguish two groups of material of or- 
 ganic origin which enter into the formation of rocks. The first 
 is the stony material, either carbonate of lime (with some- 
 times phosphate of lime) or silica, which animals and plants take 
 chiefly from the water in which they live, and in which these min- 
 erals were dissolved. This they precipitate upon or within their tis- 
 sues to build shells, corals, bones, and other hard structures. Min- 
 eral matter thus formed may be called organic precipitates. The sec- 
 ond group consists of the soft tissues of organisms, such as the flesh 
 of animals and the tissues of plants, the latter made up in large part 
 of the substance called cellulose. Such organic tissues, as they 
 
 269 
 
270 The Organic or Biogenic Rocks 
 
 may be called, are much more perishable than are the organic pre- 
 cipitates, which are generally preserved during long periods of 
 geological time without undergoing much alteration. Organic 
 tissues, on the other hand, undergo decay as soon as death has en- 
 sued, and if not protected, they will quickly disappear by changing 
 into gaseous and other matter. When protected, however, by 
 burial, the change is commonly incomplete, and a product, largely 
 composed of carbon and hydrogen, remains behind. The least 
 altered of such products may form beds of coal; the more completely 
 altered products form various bitumens. Because these substances 
 are all more or less subject to consumption by fire, they have 
 also been called caustoliths or caustobioliths (/axvo-TiKo? = capable of 
 burning). In the present chapter we shall consider the stony de- 
 posits of plants and animals, and in the next one the deposits formed 
 by the organic tissues and the materials resulting from their decay. 
 
 Kinds of Rock Material Produced by Precipitation of Mineral 
 Matter from Solution by Organisms 
 
 Here again we may distinguish two main types according to the 
 composition of the material precipitated, namely, the calcareous and 
 the silicious bioliths. There are others, such as certain iron oxide 
 deposits, which are formed by the agency of organisms, but they 
 are of minor importance. The calcareous group may again be 
 subdivided into those in which the material is carbonate of lime 
 (calcite, aragonite, etc.), with more or less magnesia, and those 
 in which it is largely phosphate of lime. 
 
 DEPOSITS or CARBONATE OF LIME BY PLANTS 
 
 Deposits of carbonate of lime are formed by plants as well as by 
 animals and are among the most abundant precipitates in the sea, 
 though important ones are also formed in fresh water. Only two 
 groups of plants are active in precipitating carbonate of lime, the 
 Bacteria and the Algae, and in each group only a limited number 
 are active in this way. 
 
 Lime Deposited by Bacteria 
 
 Bacteria are microscopic plants of extremely simple organiza- 
 tion, but of almost universal distribution and vast abundance. 
 Certain bacteria (called denitrifying) which live in the warmer 
 
Deposits of Carbonate of Lime by Plants 271 
 
 portions of the sea effect the reduction of nitrates in the water 
 to ammonia, which, with carbon dioxide, produced by other bacteria, 
 forms ammonium carbonate. This reacts with the calcium sul- 
 phate in the sea-water, and the result is the formation of calcium 
 carbonate. The reaction is: 
 
 CaS0 4 + (NH4) 2 C0 3 = CaCO 3 
 
 Calcium Ammonium Calcium Ammonium 
 
 Sulphate Carbonate Carbonate Sulphate 
 
 The calcium carbonate separates out 
 
 in the form of small spherical or 
 
 elongated grains, which accumulate 
 
 as a mass of such discrete particles 
 
 and form a deposit of oolite. Such ^ \ 
 
 deposits are forming to-day in great FIG. 188. Pseudomonas 
 
 abundance off the Florida coast and calds > lime-precipitating bac- 
 
 . _, teria; greatly enlarged. (After 
 
 near the islands of that region. The Kellermann.) 
 
 most common form of the denitrify- 
 
 ing bacteria has been named Pseudomonas calcis, in allusion to 
 the fact that it precipitates lime. It is illustrated in Fig. 188, 
 greatly enlarged. 
 
 Alga and, Algous Limestones 
 
 The term alga (plural alga) is applied to one of the lowest divi- 
 sions of plants. Most algae inhabit the sea, though many also live 
 in fresh water. They range in size from microscopic forms to the 
 giant kelps of the Pacific Ocean, which sometimes grow several 
 hundred feet in length, and are important because they contain 
 both potash and iodine. A number of algae precipitate lime-car- 
 bonate upon and in their tissues, and so form stony structures, 
 either as distinct masses or as incrustations of rocks, shells or 
 other substances. To such structures the general name nullipore 
 is applied, and while they occur in cooler waters as well, they are 
 most common in tropical seas, where they constitute an impor- 
 tant agent in the building of coral reefs. Many reefs consist, to 
 the extent of more than half of their mass, of these organisms. 
 Another important fact to be noted is that many, if not 
 most, of these lime-secreting algae also separate out a considerable 
 amount of magnesium and precipitate it as the carbonate. Con- 
 sequently, the rock resulting from such algous accumulations will be 
 a limestome rich in magnesium carbonate and may approach a 
 
272 
 
 The Organic or Biogenic Rocks 
 
 dolomite in composition, especially when, by subsequent leaching, 
 the proportional amount of calcium carbonate is reduced. An 
 example of such a rock, formed in the ancient Triassic sea, is seen 
 
 in the peaks called the Dolo- 
 mites, from the character of the 
 rock, and which are situated in 
 the Alps of the Tyrol (Fig. 4, p. 9). 
 The rock is known to have been 
 largely built up from lime- 
 secreting algae of the genus Diplo- 
 pora (Fig. 5, p. 10). 
 
 Among the more important 
 types of nullipores, we may men- 
 tion only a few in addition to 
 the Diplopora. 
 
 Lithothamnium (Fig. 189). 
 This forms irregular masses with 
 knobby and sometimes leaf-like 
 surface features. It abounds on 
 modern coral reefs and also formed 
 extensive limestone masses in 
 former periods. 
 
 Halimeda (Fig. 190). This is 
 a form of much more plant-like appearance, having structures 
 resembling a stem and leaves, covered with lime, and brittle when 
 dry. It grows in the pro- 
 tected lagoons of coral reefs 
 and other regions. 
 
 Corallina (Fig. 191). 
 This is a pink, jointed plant, 
 remotely resembling a finely 
 branched coral or hydroid and 
 common below low tide on 
 all our North Atlantic coasts 
 (Corallina zone). When the 
 plant is dead and dry, the 
 color becomes white. 
 
 Chara (Fig. 192 a). Be- 
 sides these and many other 
 
 FIG. 189. Two modern species 
 of Lithothamnium , a lime-secreting 
 alga, which plays an important part 
 in the building of modern coral- 
 reefs. Isle Maurice. (After 
 Zittel.) 
 
 marine nullipores, there are 
 
 FIG. 190. Halimeda tuna, a lime- 
 secreting green alga from the modern sea ; 
 attached to rock. 
 
Deposits of Carbonate of Lime by Plants 273 
 
 some which live in fresh and mineral-spring waters. The common 
 fresh-water form is the stonewort or Chara, found in fresh-water 
 lakes of limestone regions. It is a green alga, but appears gray 
 from the amount of lime precipi- 
 tated upon its surface. When dry, 
 it is white and very brittle. When 
 abundant, it forms deposits of marl 
 on the lake-bottom. Some older 
 limestones, such as some of those of 
 Tertiary age which underlie the city 
 of Paris and crop out some distance 
 from it, are made of the crushed and 
 more or less compacted limy fila- 
 ments of this alga. Their origin 
 
 FIG. 191. Corallina, sp. A modern 
 lime-secreting alga, i, entire plant, natural 
 size; 2, a small branch enlarged. 
 
 FIG. 192 a. Chara vulgaris, 
 Linn. A modern lime-secreting 
 alga, growing in fresh water. An 
 important marl and limestone for- 
 mer. (From Haas, Leitfossilien.) 
 
 from this plant is recognized by the abundance in them of the little 
 ridged globular vessels, about the size of a pin-head, which were the 
 spore-bearing cases of the plant. These are readily recognized by 
 the peculiar spiral bands which surround them (Fig. 192 b). Such 
 
274 
 
 The Organic or Biogenic Rocks 
 
 limestones also commonly inclose the shells of fresh-water snails 
 Lid other mollusca. 
 
 Filamentous algae are also active in hot springs, separating out 
 the lime carbonate, which then builds up the mounds and basins 
 often found around these, as in the Yellowstone region. It is diffi- 
 
 FIG. 192 b. Chara vulgaris, L. A 
 recent calcareous alga (fresh water) ; 
 spore-vessel with corona. Enlarged. 
 This is frequently found in great num- 
 bers in fresh-water limestones, show- 
 ing their mode of origin. (From 
 Haas, Leitfossilien.) 
 
 FIG. 193. A coccolithophora. 
 A mass of coccoliths; a marine 
 pelagic plant of low order covered 
 with calcareous plates. (Greatly en- 
 larged. After Murray.) 
 
 cult to determine in any case what part of the lime of such hot- 
 spring basins is built up by purely hydrogenic means (see<m/e,p. 261), 
 and to what extent algae are responsible. The oolites of Great 
 Salt Lake and of other highly saline waters have also been regarded 
 
 by some authorities as largely 
 due to the growth and lime- 
 secreting habit of microscopic 
 algae (Rothpletz). 
 
 Coccoliths, etc. Finally, 
 we may mention certain float- 
 ing organisms in the sea, gen- 
 erally regarded as extremely 
 low types of plants called 
 Coccolithophores (Fig. 193), 
 which are covered with an 
 armor of plates. These plates, 
 according to their form, are 
 called coccoliths, discoliths, 
 cyatholiths, etc. When covered 
 
 with rods they are called rhabdoliths, and form a rhabdosphere 
 (Fig. 194). These structures are found in calcareous oozes which 
 
 FIG. 194. Rhabdosphere. Much en- 
 larged. (After J. Murray.) 
 
Foraminifera and Foraminiferal Oozes 
 
 275 
 
 FIG. 195. Globigerina bulloides. 
 A modern pelagic foraminiferan, 
 with expanded pseudopodia. (After 
 Wyville Thompson.) 
 
 remain on the floor of the deeper parts of the oceans and which 
 are largely composed of minute shells of Foraminifera (Globigeriq^ 
 ooze), to be described next. 
 
 FORAMINIFERA AND FORAMINIFERAL OOZES AND LIMESTONES 
 
 The name Foraminifera is given to one of the classes of the 
 
 lowest group of animals, the Protozoa, in which each animal 
 
 secretes a small shell of carbonate 
 
 of lime, to which successive cham- 
 bers are added as the animal 
 
 grows, all the chambers being 
 
 occupied by the living animal 
 
 tissue. Many of the shells are 
 
 pierced by holes, as in the modern 
 
 Globigerina (Fig. 195), through 
 
 which delicate threads of living 
 
 matter (pseudopodia) project, 
 
 which serve to collect food. 
 
 There are many varieties of 
 
 these shell-bearing Foraminifera in the modern ocean (Fig. 196). 
 
 Globigerina Ooze. 
 Globigerina is the 
 most common among 
 the floating organisms 
 in the upper layers 
 of the sea-water. Its 
 shell consists of a 
 number of chambers 
 of increasing size, 
 the whole forming a 
 globular mass (Fig. 
 196, 2). Upon the 
 death of the animal 
 these shells slowly 
 sink to the bottom 
 (this requiring from 
 three to six days) , and 
 if they are not dis- 
 solved again in the 
 process, as happens in 
 
 FIG. 196. Modern Foraminiferal types. Much 
 enlarged, i, Orbulina; 2, Globigerina; 3, Rotalia; 
 4, Polystomella; 5, Calcarina. (After Neumayer, 
 Erdgeschichte ; from Ratzel, Die Erde.) 
 
276 
 
 The Organic or Biogenic Rocks 
 
 very deep water, they will accumulate upon the floor of the ocean 
 as a Globigerina ooze (Fig. 197), made up largely of this shell, but of 
 others as well and of coccoliths and other organisms, including non- 
 calcareous types. This ooze is most abundant in depths between 
 2500 and 4500 meters, the percentage of lime carbonate decreasing 
 
 from 70 per cent in the lesser to 
 50 per cent in the greater depths, 
 where more of the shells have been 
 dissolved. Nearly 30 per cent of 
 the area of the sea-floor is covered 
 with this Globigerina ooze, its 
 greatest distribution being in the 
 Atlantic and its least in the Pa- 
 cific, with the Indian Ocean in- 
 termediate (Fig. 198). 
 
 An Older Globigerina Limestone. 
 An example of a limestone now 
 exposed above sea-level, but formed 
 as a Globigerina ooze in deep water, 
 is found on the Island of Malta in 
 the Mediterranean. The age of 
 this rock is older Tertiary (Oligo- 
 cene), but nearly 40 per cent of 
 the species whose shells compose 
 this rock still live in the neighbor- 
 ing waters of the Mediterranean. 
 Most of the minute shells of which 
 the rock is composed are those of Globigerina. Scattered among 
 them are nodules of phosphate of lime similar to those found in 
 the deeper ocean waters of to-day. Altogether, this limestone, 
 now a solid rock, represents admirably a former deep-sea deposit 
 of Globigerina ooze, which, in the course of time, has solidified and 
 been lifted above sea-level by earth-movements of the kind to be 
 discussed in a subsequent chapter. Such old Globigerina limestones 
 occur in other districts as well. 
 
 Shallow Water and Terrestrial Foraminiferal Deposits 
 
 On tropical coasts, especially those of coral islands, shells of dead 
 Foraminifera often accumulate in large quantities, but these are 
 only exceptionally the shells of Globigerina, other forms which live 
 
 ' FIG. 197. Globigerina ooze, 
 from the deep sea, enlarged about 
 thirteen times. (After Murray and 
 Renard.) Besides the foraminif- 
 eran shells there are pteropods, 
 ostracods, and other organic struc- 
 tures. 
 
Foraminifera and Foraminiferal Oozes 
 
 277 
 
 3 8 o . a 3 
 
 '-3 
 
278 
 
 The Organic or Biogenic Rocks 
 
 in shallow water predominating. Owing to the lightness of these 
 shells, they are often carried far inland by the wind, forming dunes 
 
 FIG. 199 a. Foraminiferal shell, 
 Miliola type. (Spiroloculina badensis 
 d'Orbigny. Miocene, Baden.) Lateral 
 and top views. Important limestone 
 builder. (From Haas, Leitfossilien.) 
 
 FIG. 199 b. A foraminiferal shell 
 of the Miliola type. (Biloculina in- 
 ornata d'Orb. Miocene, Baden.) Note 
 that each new chamber covers all pre- 
 ceding ones. Two views and section. 
 Important limestone builder. 
 
 and even extended deposits chiefly composed of them. In the 
 western part of India (Kathiawar Peninsula) such a limestone, 
 called the Junagarh limestone, from the city of that name which is 
 
 built upon it, overlies the 
 * Deccan trap at a distance 
 of thirty miles from the sea. 
 It has a thickness probably 
 exceeding 200 feet, and its 
 cross-bedded structure in- 
 dicates wind transportation 
 (see Chapter XVI). It is 
 almost entirely made up 
 of foraminiferal shells and 
 other lime particles, with 
 only from 6.5 to 12.5 per 
 cent of silicious material. 
 The chief foraminiferan 
 shell of this rock is known 
 as Miliola (Figs. 199 a, b), 
 on which account the rock 
 is called Miliotic limestone. 
 Such limestones are found 
 on the Arabian peninsula 
 and elsewhere, and in Tertiary deposits as well. 
 
 Chalk. This is a white, soft, friable rock, which consists of 
 minute shells and fragments of shells of Foraminifera, of coccoliths 
 
 FIG. 200. Thin sections of chalk as 
 seen under the microscope. A, chalk from 
 Sussex, England, enlarged 60 times; 
 B, chalk from Farafrah, Libyan desert, 
 enlarged 66 times; C, dried residue of 
 milky chalk- water with coccoliths, enlarged 
 700 times; a, Textularia globulosa; b, Ro- 
 talia (Discorbina) marginata. (After 
 Zittel.) 
 
Foraminifera and Foraminiferal Oozes 279 
 
 and of other calcareous structures, all of them exceedingly minute. 
 A properly prepared slide (Fig. 200) of the material, from which 
 the finest dust has been washed out, shows, under the microscope, 
 a number of scattered shells of Foraminifera, of which a form of 
 triangular outline and composed of a double row of constantly 
 increasing chambers is the most abundant. This form, known as 
 Textularia globulosa (Fig. 200 a), lives to-day in the estuary of the 
 Dee River near Chester, England, and, like the other common species 
 
 FIG 201. Wave-cut cliff in chalk beds near Dover, England. From D. W. 
 Johnson's Shore Processes. By permission of John Wiley & Sons. The chalk 
 is in large part composed of microscopic shells ancl other calcareous organic 
 structures as shown in section under the microscope (Fig. 200). See also the 
 view of the Chalk Cliffs on the French coast, Fig. 713. 
 
 of the chalk (Rotalia marginata, Fig. 200 b), is therefore a shallow- 
 water species and not a surface floater (plankton) as is Globigerina. 
 From this and other facts it appears that the chalk is not a deep- 
 water deposit, as is the Globigerina ooze, but was formed in shallow 
 water, and the absence in it of quartz sands and of clays must be 
 accounted for by assuming that the lands which could supply such 
 material were too low to affect the deposits. 
 
 The chalk forms extensive beds over northern France and Bel- 
 gium and the south and east of England. These beds were once 
 continuous across the Channel and over much of the North Sea, 
 
280 
 
 The Organic or Biogenic Rocks 
 
 while at the same time they extended as far to the northwest as 
 northern Ireland. The cliffs which they now present to the sea 
 and inland are the result of subsequent erosion (Fig. 201 ; see also 
 
 Chalk Cliffs of Fecamp on the 
 
 French 
 
 flints 
 
 p. 224) 
 
 in 
 
 FIG. 202. Shell of a Nummulite 
 cut transversely and in part hori- 
 zontally. Enlarged. (Group of 
 Nummulites lucasana Defr. Eocene, 
 Bavaria. From Haas, Leitfossilien.) 
 
 coast, Fig. 713). The 
 the chalk (Fig. 162, 
 are the result of sec- 
 ondary segregation of silica which 
 originally was scattered through 
 it, and which originated from the 
 silicious skeletons or other parts 
 of marine organisms (Radiolaria, 
 sponge spicules, etc.) . At certain 
 
 levels in the chalk, beds of marine shells or other organisms are 
 found, indicating a temporary cessation of the chalk-forming con- 
 ditions and the inauguration for a time of normal beach or shal- 
 low sea deposition. The possibility that some chalk beds may be 
 formed by the drifting inland of shells and fragments of lime by 
 wind, analogous to the Miliolitic limestone of India, has been 
 suggested. 
 
 Nummulitic Limestone. Over large areas of southern Europe 
 and northern Africa, and in parts of Asia as well, occur thick de- 
 posits of limestones which are largely or almost wholly composed 
 of disk-like or button-like bodies, varying in size from that of a 
 pinhead to an inch or more in diameter. From their resemblance 
 to coins, these bodies have long 
 been known as Nummulites. 
 When worn, broken, or cut, they 
 show a characteristic internal 
 structure, with regular division 
 into chambers (Fig. 202), and they 
 are recognized as belonging to the 
 class of Foraminifera, of which 
 they constitute a remarkable, 
 gigantic, but now wholly extinct 
 type. These rocks all belong to 
 the early Tertiary period, and in 
 Egypt they have been quarried since the days of Herodotus and 
 before, and they were extensively used in the facing of the Great 
 Pyramid (Fig. 28, p. 76). A section of such limestone from the 
 
 FIG. 203. A fragment of Num- 
 mulitic limestone^irom the Pyrenees. 
 The nummulites are shown in sec- 
 tion, and of natural size. (After 
 Haas, Leitfossilien.) 
 
Foraminifera and Foraminiferal Oozes 
 
 281 
 
 Pyrenees is shown in Fig. 203. These large Foraminifera probably 
 lived in shallow water, as do their nearest modern relatives. 
 
 Some of the limestones of our 
 Gulf Coast States (Vicksburg 
 limestone), and those of the West 
 Indies and elsewhere, are largely 
 composed of related Foraminifera. 
 One of these, on the island of 
 Cuba, is made up entirely of the 
 shells of such a form (Orbitoides), 
 somewhat larger than a pinhead ; 
 an enlarged photographic view of 
 one of these is reproduced in 
 Fig. 204. Similar limestones of 
 great thickness occur in Jamaica. 
 It is not impossible that these 
 were formed after the manner of 
 the Miliolitic limestone of India (Junagarh Limestone) described 
 above, the shells being blown inland from the coast. This is sug- 
 gested by the almost total absence of other organisms. 
 
 Fusulina Limestone. Another type of limestone, also formed 
 of large foraminiferal shells, is found in the upper Palaeozoic series 
 (Pennsylvanian and Permian) of western North America, Europe, 
 
 FIG. 204. Orbitoides (Lepidocy- 
 clina) kempi, O'Connell. Enlarge- 
 ment of a single shell in section. 
 Cuba. (After M. O'Connell.) 
 
 FIG. 205. A polished piece of Fusulina limestone of the Carbonic. En- 
 larged nine times. On left sections cut the Fusulina transversely, on the right 
 obliquely to the longitudinal axis. (From Haas, Leitfossilien.) 
 
282 
 
 The Organic or Biogenic Rocks 
 
 and Asia. An enlarged section of such a limestone is shown in 
 Fig. 205. These foraminiferal shells frequently resemble a kernel 
 of rice ; they are elongate and spindle-shaped (Fusulina, Fig. 206) 
 
 FIG. 206. Fusidina cylindrica, a typical foraminiferal shell forming rocks 
 in the later Palaeozoic. A group natural size, and a single shell much en- 
 larged and partly sectioned to show interior. (After Kayser.) 
 
 or more or less like a football in form (Schwagerina, Fig. 207), but 
 seldom more than a fraction of an inch in greatest diameter. They 
 are restricted to that part of the geological series, becoming extinct 
 with the close of the Palaeozoic era, though a form of similar appear- 
 
 A B 
 
 FIG. 207. Schwagerina verbecki, Geinitz. A, diagrammatic view; B, plan of 
 structure ; a, natural size. (After Schwager.) 
 
 ance, but very different structure, occurs in Tertiary rocks. In 
 general, the Fusulina is like a Nummulite with the axis of coiling 
 greatly elongated. Since the Fusulinae indicate the horizon of 
 the oldest extensive coal deposit, their recognition is of importance. 
 This will be more fully discussed in a later chapter. 
 
 CORALS AND RELATED REEF-BUILDING ANIMALS 
 
 Corals and Coral Polyps. The name coral is applied to the hard 
 structures (usually of carbonate of lime), built by delicate and 
 
Corals and Related Reef-building Animals 283 
 
 as a rule small animals, which live only in normal sea- water and 
 mostly in regions of tropical or subtropical climates. The animals 
 are called polyps, and they are more or less cylindrical, fleshy, but 
 very delicate organisms, closed at the bottom, but having a central 
 opening, the mouth, at the top, around which there are one or more 
 rings of tentacles. In the 
 simpler forms, which are 
 known as hydroid polyps, the 
 entire interior of the body- 
 xylinder is hollow and con- 
 stitutes the stomach ; but in 
 the coral polyps proper (Fig. 
 208), the interior is variously 
 modified, chief among the 
 modifications being a series 
 of fleshy plates which extend 
 from the bottom to the top 
 of the cylinder and divide 
 the inner cavity into a num- 
 ber of radial chambers. 
 
 Not all polyps secrete a 
 hard structure, but those that 
 
 FIG. 208. Vertical section through a 
 polyp greatly enlarged (Astroides caly- 
 cularis). (After Lacaze-Duthiers.) Mouth 
 surrounded by tentacles and beneath it 
 the "stomatodaeum. " The fleshy mesen- 
 teries are shown, and the calcareous 
 septa which lie between them in position. 
 In the center of the bottom is the 
 columella arising from the calcareous 
 basal plate (Sk). (From Hass, Leit- 
 fossilien.} 
 
 do so precipitate the lime 
 from the sea-water in and 
 upon the outer layers of their 
 body, especially at the base 
 of the cylinder. These hard 
 structures begin as needles or 
 spicules of lime, which in some 
 groups, the gorgonias, seldom 
 
 or never unite into a solid 
 mass, but remain scattered in the fleshy parts of the body and are 
 left as lime needles upon the decay of the flesh. In other groups, 
 however, these needles are so numerous and crowded that they 
 become welded into a solid, more or less porous, stony mass, the 
 true coral. Of this, two types may be recognized, the solid rod 
 and the star coral. The first of these is built by a colony of 
 polyps which are bound together by a solid fleshy substance into a 
 cylinder of living matter, over the surface of which the individual 
 polyps are scattered. The hollow central axis of the mass is 
 
284 The Organic or Biogenic Rocks 
 
 filled by the carbonate of lime which this fleshy mass separates 
 from the sea-water, and when this central calcareous rod is stripped 
 of the surrounding fleshy substance, it appears as a much branch- 
 ing solid structure of carbonate of lime, colored a deep pink or 
 red in the most familiar types. This is the precious coral of com- 
 merce, from which coral beads and ornaments are cut. In the 
 more common, but less familiar gorgonias, the sea-fans and sea- 
 whips, so abundant on most coral reefs, this central axis is horny 
 instead of calcareous, but is otherwise much of the same charac- 
 ter. The lime secreted by the gorgonias, as already stated, is de- 
 
 FIG. 209. A simple cup-coral (Caryophyllia cyathus), attached to the 
 sea-bottom. (After Dana, Corals and Coral Islands, by permission of Dodd, 
 Mead & Co.) 
 
 posited as lime needles or spicules in the fleshy mass which sur- 
 rounds and builds the horny axis. These spicules are sometimes 
 of importance in the formation of modern limestones. . 
 
 Far more abundant than the group just described, and of more 
 importance in the formation of organic limestones, are the star 
 corals. These are so called because they show upon their surfaces 
 one or many, generally more or less depressed, circular, oval, or 
 polygonal areas or cups, each of which contains a radial series of 
 vertical plates, which converge toward the center of the cup and give 
 the appearance of rays from a central star. These rays are called 
 the septa, and they correspond to the radial fleshy plates within the 
 body of the coral polyps by the base of which they are deposited. 
 
Corals and Related Reef -building Animals 285 
 
 We may recognize simple corals with only one septa-bearing cup, 
 which then is either circular or oval (Fig. 209), and compound corals, 
 in which many such cups occur side by side, separated by inter- 
 vening limestone material, when their outline is circular or oval, 
 
 FIG. 210. Compound coral head (Astrcea pallida), with polyps partly ex- 
 panded and partly contracted. The expanded polyps show the tentacles 
 which surround the mouth ; the contracted polyps show the polygonal outline 
 from crowding. (After Dana, Corals and Coral Islands, by permission of Dodd, 
 Mead & Co.) 
 
 (Fig. 210) or closely crowded, when they assume more or less polyg- 
 onal forms (Fig. 211). But always the septa radiate from the 
 center of each cup to its margins. Sometimes the cups are so mi- 
 nute and separated by such broad intervals of spongy lime matter, 
 that the mass has a more or less homogeneous appearance, the cups 
 
 FIG. 211. A compound coral head with crowded prismatic corallites 
 (Acervularia ananas}. The specimen represents a worn pebble, formerly a 
 part of a larger head. (From Kayser.) 
 
 being recognized only on careful examination, as they are small. 
 Often they form closely crowded tubes (Madrepora, Fig. 212); at 
 other times a massive branch is covered with a closely-set series of 
 minute cups (Forties, Fig. 213). In still other coral heads the cups 
 
286 
 
 The Organic or Biogenic Rocks 
 
 or calices are confluent, producing sinuous valleys and ridges 
 (Fig. 214). 
 
 Some of the ancient corals which were important as limestone 
 makers consist of a series of tubes arranged either in a loose, more 
 
 FIG. 212. Reef-coral, Madrepora palmata, natural size, and a and b slightly 
 enlarged calices. (After A. Agassiz ; from Ratzel, Die Erde.) 
 
 or less chain-like series (chain coral or Holy sites, Fig. 215) or closely 
 crowded and taking on a columnar prismatic form from crowding 
 (honeycomb-coral or Favosites, Fig. 216, and Columnaria). In 
 these corals the septa are generally very short, or they may be 
 
Corals and Related Reef -building Animals 287 
 
 represented by vertical rows of spines, or again they may be absent 
 altogether. Instead, the tubes are divided by numerous horizon- 
 
 FIG. 213. A massive branching coral (Porites mordax) with very small calices; 
 an important reef -builder. (After A. Agassiz ; from Ratzel, Die Erde.) 
 
 tal partitions which are often so closely crowded as to give the tube, 
 when broken lengthwise, a finely cellular structure (Fig. 216). 
 
 FIG. 214. Two small heads of a brain-coral. (Maandrina.) a, with 
 soft parts; b, corallum. Slightly reduced. (After Brehm; from Ratzel, 
 Die Erde.) 
 
288 
 
 The Organic or Biogenic Rocks 
 
 FIG. 215. The chain coral. (Haly- 
 sites catemdaria, E. and H.) Silurian ; 
 with enlargement of a few corallites. 
 This is an important index fossil of 
 the Silurian, and an important lime- 
 stone builder as well. (From Haas, 
 Die Leitfossilien.) 
 
 FIG. 216. A characteristic a-sep- 
 tate compound coral. (Fawsites got- 
 landica.) Silurian. The columns are 
 prismatic and their walls pierced by 
 regular pores. Internally the tubes 
 are divided by horizontal plates or 
 tabulae. This is an important lime- 
 stone builder in the Palaeozoic. (From 
 Haas, Leitfossilien.) 
 
 FIG. 217. A modern hydrocoralline (Millepora alcicornis). Important as 
 a reef-builder. (From Dana, Corals and Coral Islands, by permission of Dodd, 
 Mead & Co.) 
 
Corals and Related Reef-building Animals 289 
 
 Hydroid Polyps and Hydrocorallines. Finally the hydroid 
 polyps which, it will be remembered, have no internal fleshy radi- 
 ating plates, sometimes build calcareous coral-like structures in 
 which, however, the cups are merely holes in the moreX)r less solid- 
 appearing limestone mass (Millepora, Fig. 217) which forms the 
 structure secreted by them and to which the name hydrocoralline 
 is applied. 
 
 Such hydrocorallines often form important and extensive 
 portions of coral reefs, the form known as Millepora (referring 
 
 FIG. 218. Fragments of large masses of stromatoporoids, which were im- 
 portant reef-builders in the Palaeozoic. A, Stromatoporella tuberculata, a 
 weathered fragment showing the hummocky surface, and the successive layers 
 (Devonian) ; B, Stromatopora antiqua, a transverse polished section of a 
 fragment snowing the coarse concentric layers (Silurian). (After Nicholson; 
 from Grabau and Shimer, North American Index Fossils.} 
 
 to the thousands of pores on the surface, i.e. the cups) 
 abounding on certain modern coral reefs, while another, the Stroma- 
 topora, sometimes makes up the main portion of ancient (Palaeo- 
 zoic) coral reefs, growing in masses up to ten feet in diameter. It is 
 readily recognized by the concentric arrangement of the succes- 
 sive layers of which this mass is composed (Fig. 218). Each layer 
 will, on microscopic examination, show a very definite structure 
 such as is found only in limestone deposits of organic origin. With- 
 out careful examination of details, however, the student will not 
 be able to distinguish the numerous kinds of Stromatoporas from 
 one another, nor will he be able readily to distinguish them from 
 ancient limestone masses of similar concentric structure built by 
 marine or even by fresh-water plants of low order (Algae, e.g. Cryp- 
 tozoon, etc.). 
 
290 The Organic or Biogenic Rocks 
 
 CHARACTERS AND TYPES OF MODERN CORAL REEFS 
 
 Corals commonly grow associated in regions of the sea where 
 minute food particles are abundant, where the mean temperature 
 of the coldest months does not usually fall below 21 degrees C., 
 or the minimum annual temperature below 18 C. and where a favor- 
 able hard bottom, free from silt, exists for their attachment. For 
 it must be noted that the coral polyps are not free-moving or float- 
 ing organisms (except in their larval stages), but attach themselves 
 to the sea-bottom and lead a sedentary existence. By such asso- 
 ciation in growth of corals, together with other lime-secreting or- 
 ganisms, a reef is built up which rises toward the level of the sea, 
 and may grow so high that it is exposed at very low tides for a short 
 period of time. Coral reefs must be distinguished from coral islands 
 (see Fig. 220), which represent a stage subsequent to the reef when, 
 by wave action, the broken-off dead coral-masses and coral-sands 
 are heaped up to such an extent that they permanently project 
 above the water. Reefs, on the other hand, are always submerged 
 except for the short period of lowest tides already referred to. Mod- 
 ern coral reefs are chiefly confined to the region limited by the 
 28th degree north and south latitudes, the Bermuda Islands, bathed 
 by the warm Gulf stream, being the^ chief exception to this, lying 
 in latitude 32 N. This distribution is partly due to the inability 
 of polyps to separate out much lime from sea- water in colder regions, 
 and also because whenever ice forms in winter the coral polyps, 
 always growing near the surface of the sea, are readily destroyed 
 by such ice. On this account we may confidently assume that 
 ancient coral reefs required similar warm temperatures for their 
 formation, and if we find that rocks now exposed in the Arctic re- 
 gions are formed from ancient coral reefs, as are those on the North 
 Siberian islands, we must assume that at the time of their forma- 
 tion this region had a more tropical climate. 
 
 In the second place, reef-building corals flourish best in shallow 
 water, usually in water not over 20 to 25 fathoms (120 to 150 feet) 
 in depth, 1 though some occur at greater depths. Many of them 
 flourish so near the surface of the sea that they are exposed at low 
 tide, as is so finely shown in the wonderful series of photographs 
 obtained by Saville-Kent from the Great Barrier Reef of Australia 
 
 1 Vaughan considers 45 meters the maximum. 
 
Characters and Types of Modern Coral Reefs 291 
 
 (Fig. 2I9). 1 Frequent agitation of the sea-water is necessary to 
 the active growth of coral polyps, and as a rule, strong light is re- 
 quired. The salinity of the water must not, as a rule, fall much 
 below 27 per mille, nor rise much above 38 per mille, though coral 
 polyps have been found to flourish in very brackish and very 
 saline waters. 
 
 Reefs vary greatly in the types of organisms which built them. 
 In nearly all of them calcareous seaweeds, or nullipores, play an 
 important part, and some reefs are largely composed of them. In 
 
 FIG. 219. Portion of the Great Barrier Reef of Australia at very low tide, 
 showing the living coral masses growing close together, and withstanding the 
 periodic exposure. (After Saville-Kent.) 
 
 others, hydrocorallines (Milleporq, Fig. 217) abound, while in still 
 others the gorgonias form an important, if not dominant element. 
 Star-corals are always present, and in many cases predominate, 
 comprising both the branching types such as the staghorn coral 
 (Madrepora, Fig. 212), the fringe coral (Forties, Fig. 213), etc., and 
 the massive or head types, such as the star corals proper (Astraa, 
 etc., Fig. 210), the brain coral (Maandrina, Fig. 214), and many 
 others. Besides these reef-builders proper, there are many other 
 animals which live in and about the coral branches, and many of 
 them have hard shells or other structures which, on the death of 
 their possessors, add calcareous material to the growing mass. 
 
 1 The student should examine the photographs published in Saville-Kent's book on 
 the Great Barrier Reef. 
 
292 The Organic or Biogenic Rocks 
 
 According to their location, we may distinguish two groups of 
 reefs: i, the oceanic, and 2, the epicontinental. The first have no 
 direct relationship to the continents, but grow around islands in 
 the open oceans or form separate rings of coral islands or atolls. 
 The second group is marginal to the land, being built upon the 
 continental platform or in seas which indent the continents. Be- 
 tween the two there is a gradational series, but on the whole they 
 are quite distinct. 
 
 Oceanic Coral Reefs 
 
 Of these, three types are recognized; namely, fringing reefs, 
 barrier reefs, and atolls. 
 
 The Fringing Reef. This grows close around volcanic or other 
 islands, from the submerged slopes of which it rises, and is 
 separated from the island it fringes by a very narrow channel. 
 
 The Barrier Reef. This grows at a distance of several miles 
 from the shores of the oceanic island, thus leaving a broad strip or 
 channel of water between it and the island. This water may be 
 from one or two to as much as 30 or 40 meters in depth. The barrier 
 reef of New Caledonia in the Pacific is 400 miles long and about 
 ten miles distant from the shore. The outer or seaward margin 
 of such a barrier slopes off into much deeper water than is the case 
 with that of the fringing reef, and the inner channel, besides being 
 broader, is also much deeper than that of the fringing reef. This 
 channel is connected with the outer ocean by cross channels cut 
 through the reef, and these are kept open by the ebb and flow of 
 the tide. 
 
 The Atoll (Fig. 220). This consists merely of a ring of coral- 
 reef islands in the open ocean, with no land in the center of the ring, 
 but instead a shallow lagoon of quiet water. This lagoon is con- 
 nected with the open sea by cross channels, which are located on the 
 leeward side of the atoll, and so generally form a protected entrance 
 to a quiet inner harbor. The water around the atoll is generally 
 very deep, and the outer slopes of the reefs are steep. At the Kokos 
 Keeling atoll, in the eastern part of the Indian Ocean, for example, 
 the ocean has a depth of 1200 fathoms at a distance of only 2200 
 yards from the edge of the reef. The lagoon, on the other hand, 
 is generally less than 50 fathoms in depth, and in some atolls, as 
 in that of the Kokos Keeling group, it is only from 2 to 7 fathoms 
 deep. 
 
Characters and Types of Modern Coral Reefs 293 
 
 Organisms of Oceanic Reefs. Different organisms are gen- 
 erally found to grow in the lagoon and upon the outer slope of 
 the reef. The latter, bathed by the cool, pure sea-water, rich in 
 food particles, is the region of active coral growth, though a fringe 
 of the nullipore, Lithothamnium (Fig. 189), generally occurs here. 
 In the lagoon, lime-secreting sea- weeds (Halimeda, etc., Fig. 190) 
 
 FIG. 2 20. Whitsunday Island, a typical Atoll in the Pacific. (After Guyot.) 
 
 are generally abundant. Corals also grow there, being commonly 
 of the more delicate branching forms, though others more common 
 on the outside of the atoll are also represented. Some lagoons be- 
 come gradually filled up by the growth of these organisms, while 
 others seem to retain their depth and perhaps increase it and widen 
 the lagoon by solution. 
 
 Theories of the Origin of Oceanic Reefs 
 
 No special theory is needed to account for the formation of a 
 fringing reef, for it can be readily understood that coral polyps and 
 other lime-secreting organisms which attach themselves to the sub- 
 merged slopes of an oceanic island will in time build up a reef which 
 fringes the coast of that island and rises nearly to sea-level, followed 
 in many cases by the formation of islands upon it. The case is dif- 
 ferent, however, with the barrier reefs and the atolls, for here the 
 lagoons must be explained, as well as the fact, that the reefs seem to 
 arise from depths too great for normal reef-building corals to grow. 
 
294 The Organic or Biogenic Rocks 
 
 Several theories have been proposed for the explanation of such 
 conditions. 
 
 The Subsidence Theory. This theory was first proposed by 
 Charles Darwin from his study of oceanic coral islands. It was 
 later amplified and elaborated by the American geologist, James D. 
 Dana, and has received its most recent support from the studies of 
 the American physiographer, William Morris Davis. In general, 
 this theory postulates that regions of barrier reef and atoll forma- 
 tion, are regions of subsidence. Beginning as fringing reefs in moder- 
 ate depths upon the submerged slopes of oceanic islands which are 
 
 FIG. 221. Diagram illustrating the conversion of a fringing into a barrier 
 reef by subsidence.' (After Darwin; from Vaughan.) AA, Outer edge of the 
 fringing reef at the level of the sea ; BB, shore line of the island at this stage ; 
 A' A', outer edge of the reef after its upward growth during a period of sub- 
 sidence and the formation of a new sea-level indicated by the dotted line; 
 B'B', the new shore of the encircled island; CC, the lagoon-channel between 
 the reef and the island. 
 
 slowly sinking, the reefs grow upward and to some extent outward, 
 the region of the greatest growth being the seaward margin of the 
 reefs. Subsidence diminishes the diameter of the island, and a re- 
 treat of its shore-line from the original contact with the reef takes 
 place, so that a constantly widening lagoon is produced between 
 the reef and the island shores (Fig. 221). As Davis has pointed 
 out, were it not for comparatively rapid subsidence, the mechan- 
 ical debris washed from the island itself, during its destruction by 
 the weather, would fill up the lagoon, whereas, in the most typical 
 examples of such lagoons, the amount of visible mechanical sediment 
 from the island is far less than would be expected from the 
 extent to which the island has suffered erosion. Most of this sedi- 
 ment is submerged beneath the waters of the constantly widening 
 lagoon. By continued sinking of the island and the corresponding 
 upward building of the reef, the circular lagoon increases in size, 
 until, when the last peak of the island has disappeared, the ring of 
 water becomes closed into a central continuous lagoon, bounded 
 
Characters and Types of Modern Coral Reefs 295 
 
 only by the circular reef upon which islands are built by waves and 
 wind, and the atoll is complete (Fig. 222). 
 
 Such a theory of origin postulates the existence, among the oceanic 
 reef-surrounded islands, of examples of all stages in this process, 
 from those in which the fringing reef has only recently been built, 
 to those in which the reef forms a well-defined barrier at a distance 
 from the coast, with central islands of old rock of all sizes, from 
 those of great extent and height to mere rocky peaks at the center 
 of an almost perfect atoll. For it is evident that all the islands of 
 
 FIG. 222. Diagram illustrating the conversion of a barrier reef into an atoll. 
 (After Darwin ; from Vaughan.) The barrier reef is closely shaded, the letter- 
 ing corresponding to that of Fig. 221. The dotted lines represent the upward 
 growth of the reef as the island subsides, and the new sea-level is represented by 
 the dotted horizontal line. A" A", the outer edge of the reef which forms the 
 atoll; C f , the lagoon of the atoll, the depth of which on this scale is exag- 
 gerated, as is also that of the lagoon channel C. 
 
 an area did not have the same height or slope of surface at the time 
 of the formation of the initial fringing reef. Therefore, even if 
 the subsidence were uniform over wide areas, which need not at 
 all be the case, some islands would disappear earlier than others, 
 and some lagoons would widen more rapidly than others, because 
 of the gentler slope of the surface of the rocky mass. This is illus- 
 trated in the following diagram (Fig. 223). Similar conditions 
 would be produced in the case of islands of the same height and 
 slope, but located in regions which were undergoing subsidence 
 at different rates. Moreover, this theory requires that dead reef- 
 building corals should be found in the position of growth at depths 
 vastly greater than that normal for the growth of corals, for it 
 is evident that as subsidence goes on, the older parts of the reef, 
 formed in waters of 25 or 50 fathoms, will be carried downward 
 with the sinking of the islands which support these reefs. These 
 
296 
 
 The Organic or Biogenic Rocks 
 
 conditions are satisfied, for not only are all gradations from fring- 
 ing reefs to perfect atolls found among the oceanic islands and reefs, 
 but a boring in one of the atolls, that of Funafuti in the Ellice Island 
 group in the western Pacific, has shown the presence of these corals 
 at a depth of over a thousand feet. 
 
 The chief objection urged against this theory of origin is the 
 necessarily widespread subsidence of the ocean bottoms to carry 
 down so many of the islands in such widely distant regions. Fur- 
 
 FIG. 223. Diagram of two islands, showing varying amounts of submer- 
 gence with the same rate of subsidence, due to variation in slope. The reef- 
 deposits formed on the submerging slopes are shown in black. Note the more 
 rapid increase in the .width of the lagoon in the island on the right. 
 
 ther, it is pointed out that in some of these island groups where 
 barrier reefs and atolls would indicate subsidence, there is evidence 
 of actual elevation at some points where the old coral reefs have 
 been raised to a greater or less extent above the sea, so that in some 
 cases the actual foundation upon which the reef was built has be- 
 come exposed. 
 
 The Theory of Stationary Levels and Upbuilding of Reefs. - 
 The English oceanographer, Sir John Murray, following an older 
 suggestion of the poet-naturalist, Adelbert von Chamisso, and of 
 the naturalist, Carl Semper, proposed a different explanation of 
 the phenomena, and this has been further amplified and extended 
 by the American oceanographer, Alexander Agassiz. These inves- 
 tigators started with the fact that corals will begin to grow wherever 
 a submerged ridge rises to within the proper distance of the sea- 
 level (25 to 50 fathoms), whether any portion of this is exposed above 
 the sea as an old island or not. Such a ridge may be of volcanic 
 origin, it may be an old island which has been worn down by the 
 waves until it has largely or entirely disappeared, or it may be a 
 submarine bank built up by the accumulation of shells and other 
 organic structures until it has reached the proper elevation. When 
 the coral masses growing upon such a foundation approach the 
 surface of the sea, the centrally located individuals will die for lack 
 of food and proper water conditions, while on the margin of the 
 
Characters and Types of Modern Coral Reefs 297 
 
 mass, where the open sea-water bathes the corals, growth is vigor- 
 ous. Fragments broken by the waves from this margin will roll 
 down the seaward slope, and in time build the submerged platform 
 outward, after which the coral polyps, etc., will take possession, 
 and the margin of the reef is widened by outward growth. Mean- 
 while the dead coral masses on the inside will undergo solution by 
 the waters rendered acid by the decaying organic matter, and a 
 lagoon is dissolved out, the latter also increasing in diameter as 
 the coral ring spreads on the outside. In this manner an atoll of 
 any size may be produced from an originally solid reef of smaller 
 size, while a fringing reef around an island would gradually move 
 away from it by outward growth and the space between it and the 
 island would widen by solution and scouring out of the lagoon 
 channel. 
 
 Among the objections to this theory may be mentioned the fact 
 that many lagoons are very wide, up to thirty miles or more, and 
 also very deep, some having a depth of 200 feet. This would re- 
 quire an enormous amount of solution and removal of lime carbon- 
 ate from the lagoon, were this produced from originally solid reef 
 masses inside of the growing ring. Moreover, the lagoons of atolls 
 are generally becoming filled by the growth of lime-secreting sea- 
 weeds, by the accumulation of foraminiferal shells and of other or- 
 ganic products, and by precipitation of lime through bacteria and 
 other agents. While this theory may explain certain cases, it does 
 not seem applicable to the problem as a whole. 
 
 Theory of the Rise of the Ocean Surface. The German physi- 
 ographer, Albrecht Penck, has proposed the theory of a rising sea- 
 Igvel which has been more fully amplified by the American geologist, 
 R. A. Daly, though suggestions along this same line were made pre- 
 viously by others (T. Belt, 1874; W. Upham, 1878, etc.). It is 
 known that in a period preceding that in which the modern barrier 
 reefs and atolls were formed, an enormous ice-cap covered the 
 northern portions of America and of Europe and a similar ice-cap 
 covered the Antarctic regions. Such ice masses would exert an 
 attraction upon the waters of the ocean which would, in conse- 
 quence, flow toward them, lowering the sea-level in the equatorial 
 regions throughout the world to a corresponding degree. The nor- 
 mal depth at which coral polyps could flourish at that time in the 
 equatorial regions could, therefore, be much below that possible 
 for them at the present time. As the ice melted the sea-water would 
 
298 The Organic or Biogenic Rocks 
 
 be returned, raising the level, and the additional water from the 
 melting ice would increase the volume. During this continued 
 rising of the sea-level the coral reefs, beginning as fringes around 
 the islands, would grow upwards, while the central islands would 
 become more and more submerged and some would finally disappear. 
 Thus barrier reefs and atolls would be formed, the character and 
 size of which would depend on the area and height of the original 
 island or submerged platform. According to Daly's estimates, the 
 amount of lowering of the sea-level at the beginning was from 200 
 to 230 feet, and this would also be the extent of the subsequent rise 
 of the water-level. Hence the maximum depth of lagoons would be 
 indicated by these figures, while the thickness of the entire reefs 
 built during that time would range from that amount to that plus 
 150 to 300 feet, the original depth at which the corals began to 
 grow. These requirements are borne out by many facts known 
 concerning the reefs, but they do not apparently account for all of 
 them. 
 
 Complex Origin of Reefs. It seems likely that most or all of 
 the factors here indicated are operative in the formation of coral 
 reefs, and that some barrier reefs and atolls may originate in one 
 way and others in another. Each group must be investigated by 
 itself, and probably no single theory accounts for all the phenomena 
 observed in such reefs in different parts of the oceans. 
 
 Epicontinental Reefs 
 
 Reefs which form in shallow water, either on the continental 
 platform of the open ocean, or in the more or less enclosed bodies 
 of water which form indentations into the land, are classed as epj^ 
 continental reefs, or reefs built upon the submerged portions of 
 the continents. The best-known modern examples of reefs of this 
 type are the Great Barrier Reef of Australia and the reefs of the 
 Florida coast. Both are built up on the continental platform and 
 have been extended nearly or quite to the seaward edge of that 
 platform. 
 
 Great Barrier Reef of Australia. This great complex of reefs 
 extends, with a few interruptions, for 1250 miles, from Torres Strait 
 in 9.5 S. latitude to Lady Elliott Island in 24 S. latitude. At 
 Cape York the seaward edge of this reef is nearly 90 miles distant 
 from the coast, and it descends to a depth often exceeding 1800 
 feet. This edge represents a great submarine wall or terrace which 
 
Characters and Types of Modern Coral Reefs 299 
 
 fronts the whole northeast coast of Australia (Fig. 224). It rests 
 at each end in shallow water, but near the center rises from great 
 depths. The surface of this reef-complex forms a great plateau or 
 platform, regarded by some as a submerged land surface, which is 
 covered by from 10 to 30 fathoms of water, and is studded all over 
 with steep-sided, block-like masses, the individual reef mounds, 
 which rise up to low- water level (Jukes-Browne). These individual 
 reef mounds are especially abundant along the outer edge of the 
 bank or platform where they are bathed by the pure ocean water. 
 
 FIG. 224. Diagrammatic section across the Great Barrier Reef of Aus- 
 tralia. (After J. B. Jukes; from Vaughan.) a, Sea outside the Barrier, gen- 
 erally unfathomable ; b, the actual barrier ; c, clear channel inside the barrier, 
 generally about 15 or 20 fathoms deep; d, the inner reef; e, shoal channel 
 between the inner reef and the shore ; F, the great buttress of calcareous rock 
 formed of coral and the detritus of corals and shells ; G, the mainland formed 
 of granite and other similar rocks. 
 
 This linear series of reefs is the true barrier, but the submerged 
 platform in some places extends beyond it. It is breached by nar- 
 row passages, and at rare intervals by navigable ship canals. The 
 main part of the reef consists of coral heads and fragments bound 
 together into a solid, hard mass, with the living coral polyps covering 
 the outer surfaces. Most common among these are great masses 
 of the irregular coral, Forties (Fig. 213), and the brain coral (Fig. 
 214). These, torn from their anchorage by the waves, are rolled 
 about and worn, while at the same time they grind down the living 
 and dead coral masses of the reef into fine coral sand and powder. 
 Rolled and worn fragments of such corals, six to eight feet in diame- 
 ter, are common on the outer slope of the reef, and they furnish an 
 illustration of the manner in which the reef is worn away by the 
 action of the waves. The coarser sand is washed into the channels, 
 while the finer sand is carried seaward and settles on the bottom in 
 deeper water. The material dredged here has the appearance of 
 an impalpable, pale olive-green mud, which is wholly soluble in 
 
300 The Organic or Biogenic Rocks 
 
 diluted hydrochloric acid, thus showing that it is pure carbonate 
 of lime. When dried it has the character and consistency of chalk. 
 
 Inside the barrier is a clear and broad channel, generally from 
 15 to 20 fathoms deep. The bottom is covered with unconsolidated 
 lime-sand, ground from the reef, or with sand largely composed of 
 the shells of Foraminifera (Orbitolites) which in places constitute 
 the entire sand mass around the coral islands and the neighboring 
 shores. Beyond the channel lie the inner reef mounds, which are 
 separated one from the other by narrow water ways through which 
 the tide rushes with great force. Such tidal currents may con- 
 tinue in the same direction, sometimes for two or three days, es- 
 pecially after great storms, and they form important agencies in 
 the distribution of the lime-sand and mud. 
 
 The reefs of the inner series are peculiar in that many of them 
 are composed chiefly of one kind of coral or other lime-secreting 
 organism. Thus the Organ-Pipe reef of Thursday Island consists 
 largely of the organ-pipe coral (Tubipora musica), while another 
 reef is chiefly formed of the blue coral ( Heliopora ccerulea) . There 
 are reefs largely made up of the hydrocoralline Millepora (Fig. 217), 
 and others composed in large part of gorgonias. The inner reefs 
 are separated from the mainland of Australia, which is formed of 
 older rock, by a shallow channel which is mostly free from coral 
 growth. 
 
 The Florida Reefs. These arise from a shallow platform par- 
 allel to the southern margin of the peninsula and at a variable 
 distance from it (Fig. 225). The southern coast of the peninsula 
 rises from 12 to 15 feet above the sea-level, in the form of a curving 
 ridge, and behind it lies the great fresh-water swamp of the Ever- 
 glades, the surface of which is only two or three feet above sea- 
 level (see section, Fig. 226). This rim has been regarded by Agassiz 
 and Le Conte as marking the line of an older series of reefs behind 
 which, on the site of the present Everglades, lay a lagoon which 
 has since been converted into the fresh-water swamp. More recent 
 investigation, however, has shown that this interpretation is prob- 
 ably not correct, though there are Tertiary coral reefs at Bainbridge, 
 Georgia, and Tampa, Florida (Oligocene), and reef corals occur in 
 the Pliocene (Caloosahatchie). From five to fifteen miles out- 
 side of the southern rim of the peninsula lies a line of small islands, 
 the " Keys," which vary from less than four (Key West) to fifteen 
 miles in length in the largest. These Keys have a gentle north- 
 
Characters and Types of Modern Coral Reefs 301 
 
 ward slope and a steep southward or seaward face, and they clearly 
 represent a line of extinct reefs upon which waves and winds have 
 built up the islands. The channel between the Keys and the pres- 
 ent mainland is very shallow; its floor is covered with fine silt, 
 which at low tide forms exposed mud-flats rich in decaying organic 
 matter. Many small, low mangrove islands dot the channel, and 
 the aerial roots of the mangroves form a tangle where they enter the 
 water, and this is very effective in checking the tidal currents and 
 
 FIG. 225. Map of Florida, showing the Keys and reefs. (After Le Conte.) 
 aa, southern coast; a'a', Keys; a"a", living reef; e, Everglades; e' y inner 
 channel ; e", outer or ship-channel ; GSS, Gulf Stream. 
 
 forcing them to deposit their load of silt. In this silt are buried 
 the remains of marine animals which have migrated from the open 
 sea, and terrestrial and fresh-water forms which have come from 
 the land. Also there are forms especially adapted to muddy bot- 
 toms, and their remains are mingled with the other types. 
 
 Outside of the line of Keys, and from three to fifteen miles dis- 
 tant from it, is the line of living reefs which is still, for the most 
 part, submerged, and consists of a chain of reef mounds formed in 
 part by corals (Madrepora, Porites, etc.) and in part by nullipores 
 (Corallina, Lithothamnium, etc.). The channel between the living 
 reef and the dead one (the Keys) is from five to six fathoms deep 
 
302 The Organic or Biogenic Rocks 
 
 and its floor is covered by coral-sand, shells of marine organisms, 
 and oolitic lime precipitated by the agency of bacteria. These 
 deposits are bedded, and when consolidated, will form stratified 
 limestones. In some sections the delicate branching coralline sea- 
 weeds grow in abundance, covering the floor of the channel with a 
 carpet of " country grass " as it is called. The dead portions of 
 these corallines disintegrate into small fragments, and in places 
 these form the main deposit on the channel floor. 
 
 On the outer or seaward side, the reefs slope steeply, and the 
 sea-bottom quickly descends to great depth (2916 feet). The sweep 
 
 s 
 
 FIG. 226. Diagrammatic section of Florida along the line N. S. in Fig. 225, 
 showing the relative position of the south shore, a ; Keys, a' ; and living reef, 
 a" ; with the Everglades, e ; the inner channel, e' ; and the outer or ship- 
 channel, e" \ rin, ancient submarine platform. The dotted lines indicate 
 hypothetical former conditions. (After Le Conte.) 
 
 of the Gulf Stream here prevents any further southward extension 
 of the deposits. These relationships are shown in the diagrammatic 
 section along the line N. S. of the map (Fig. 225) given in the preced- 
 ing figure (Fig. 226) and the details of the northern end of the reefs 
 in the map (Fig. 227). 
 
 It is not difficult to see that we have here a succession of reef lines 
 built seaward one after another, and that the older reefs became 
 extinct and were converted into islands (the Keys) after the new 
 line of reef was built, and so shut off the food-bearing currents from 
 the inner reefs. In the outer channel, between the modern reefs 
 and the Keys, lime deposits only are forming. This was also the 
 case in the inner channel between the present Keys and the main- 
 land, when the Keys were the outermost living reefs. After they 
 became extinct by the building of the new reefs, the lime deposits 
 
Characters and Types of Modern Coral Reefs 303 
 
 of the inner channel were covered by silts and muds, with much 
 organic material. This would happen to the lime deposits of the 
 outer channel if the outer or modern reef became extinct and were 
 
 FIG. 227. Chart of the northern end of the Floridian Barrier-reef. From 
 United States Coast and Geodetic Survey, Chart No. 165. (After Vaughan.) 
 The depths are given in fathoms. Note the sudden drop of the bottom out- 
 side of the lo-fathom line. 
 
 converted into islands. On consolidation of these deposits, the bed- 
 ded limestones beneath would be covered with a mud rock or shale, 
 probably of black color, on account of the abundance of decayed 
 organic material in it. If the southern rim of Florida were an an- 
 
304 The Organic or Biogenic Rocks 
 
 cient line of reefs, it would represent the later stage in development 
 to which the present Keys advance, namely the formation of a con- 
 tinuous belt of land. The Everglades have been regarded as a 
 later stage in the silting up of the channel and its conversion into 
 a fresh-water swamp in which deposits of decaying plants form the 
 initial steps in the formation of a coal bed. Although this inter- 
 pretation of the Florida rim appears not to be correct, it can be re- 
 garded as a possible stage in the gradual modification of such lines 
 of reefs. The black mud which covers the limestone may, in turn, 
 be covered by a coal bed, above which other deposits, such as wind- 
 borne sands, etc., may accumulate. 
 
 Structures Common to All Reefs 
 
 It is important that we should understand the main structural features which 
 distinguish reefs of corals and other lime-secreting organisms from other types 
 of lime deposits, so that we may have definite means by which we can recognize 
 older limestone deposits as due to reef growth, if such be their origin. In the 
 first place, then, it should be noted that the main mass of the reef-mound is 
 
 FIG. 228. Diagrammatic section of a Palaeozoic coral reef; the black 
 masses represent coral heads in the position of growth forming the reef proper ; 
 around the margin are deposits of coral-sand and mud with steep dips near the 
 reef where the edges of the reef and the coral-sand interfinger. (After Grabau, 
 Principles of Stratigraphy.} 
 
 composed of coral or coralline structures in the position of growth. That is, 
 as each new coral head or coral branch developed, it remained attached to the 
 older dead coral mass or to the original rock-floor which served it as a founda- 
 tion. Thus, in general, such a mound represents a mass of undisturbed coral 
 and coralline structures. As the growth is not uniform, however, in all direct- 
 ions, numerous large and small cavities exist among the coral masses, and these 
 cavities are generally occupied by shell-bearing and other animals whose hard 
 parts remain there on the death of the creature. The lime-sand and lime-mud 
 into which the waves grind the exposed corals is washed into these cavities, 
 which may eventually be filled up by such material. On the margin of the reef, 
 especially on the outer one, many coral heads and branching forms are broken 
 from their anchorage and rolled about by the waves, grinding into sand and 
 mud the coral masses over which they are rolled. When finally they themselves 
 
Ancient Coral and Coralline Reefs 305 
 
 become embedded in the coral sand, they are no longer perfect, but are broken 
 and worn, and they may come to lie in all positions, being even completely over- 
 turned. The fine coral-mud resulting from the grinding will be carried out to 
 deeper or quieter water, though it may also be caught in protected cavities 
 within the reef. The coral-sand remains in shallow water to form bedded de- 
 posits. 
 
 Along the margins of the reef-mounds the bedded deposits of coral sand 
 will often lie at a steep angle, which is sometimes as high as 45 or even more. 
 Frequently a layer of small corals will grow upon such a bedded deposit, and 
 this in turn may be covered by other lime-sands. Thus an interfingering of 
 the organic lime structures, the corals, etc., with the clastic lime, the coral-sand, 
 will result, and this is one of the most characteristic features of the margins 
 of the reef-mound (Fig. 228). 
 
 ANCIENT CORAL AND CORALLINE REEFS 
 
 In the older limestone rocks of the earth's crust we often meet 
 with structures which indicate that parts of these limestones 
 were old reefs similar to those described, while the remainder of 
 the limestone forms bedded deposits of coral sand or shell and nulli- 
 pore fragments, etc., similar to those deposited to-day between 
 lines of reefs. Not infrequently the corals of the reefs are distinctly 
 recognizable, but in some cases they have been so altered in the 
 course of time that they form a compact, structureless mass. When 
 such a mass, devoid of bedding planes, and showing, in sections, 
 more or less of a mound-like form, is enclosed by bedded limestones, 
 which near the mound have a steeper inclination away from it in 
 all directions, and when, furthermore, these bedded lime-deposits 
 alternate with projections from the mound, it is generally safe to 
 regard such a mass as an ancient coral or coralline reef. As has 
 been said, however, many of these ancient mounds still show their 
 coral or other organisms in perfectly recognizable condition, and 
 in that case there can be little doubt of the reef origin of the mass 
 if the structural characters above outlined are shown. 
 
 Silurian Reefs of Wisconsin. Such a line of old reefs, composed largely 
 of Stromatopora with some corals, was formed in ancient Silurian time in Wis- 
 consm and adjoining areas. It has been traced for over sixty miles in length, 
 and extended parallel to the old shore-line of an interior sea and at some dis- 
 tance from it. In general, it was formed under conditions not unlike those 
 found to-day in the Great Barrier Reef. Many of the old reef-mounds have 
 been opened in quarries near the city of Milwaukee and elsewhere, and though 
 the mounds are mostly massive, the presence of the Stromatoporas can be recog- 
 nized on the weathered surfaces of the quarry walls. The bedded lime deposits 
 on the flanks of these mounds commonly show steep dips. 
 
306 The Organic or Biogenic Rocks 
 
 Silurian Reefs of Gotland. Another series of such reefs was formed during 
 the same period, parallel to the Swedish coast, in a sea which occupied the area 
 of the present Baltic, and extended far east into Russia. Much of this reef series 
 has been worn away, but a part remains to form the present Island of Gotland, 
 and around the coast of this island, where the modern sea has cut cliffs, many 
 beautiful sections of these reefs are exposed and their structures are well 
 shown. 
 
 Devonian Reefs of the Eastern United States. At a later period (Middle 
 Devonian) a series of reef-lines, similar to those of the Florida coast, formed in 
 the great interior sea which then covered much of North America. These reefs 
 began as a line parallel to the eastern coast, which was formed by a land mass 
 lying between this interior sea and the Atlantic Ocean, approximately along the 
 line of the Older Appalachian Mountains. After the formation of the first 
 line of reefs, a second one came into existence some miles farther to the north- 
 west, and the old channel between the first reef-line and the mainland became 
 silted up with carbonaceous muds. When a third and a fourth line of reefs 
 appeared, each farther to the northwest, where the open sea of that time lay, 
 the older channels were progressively silted up, and as streams brought mud 
 and sand from the mainland of that time (on the southeast), the old extinct 
 reefs themselves were covered by muds and sands. Several of these lines of 
 reefs have been definitely located. One passes through eastern New York and 
 Pennsylvania, another passes under the city of Buffalo, a third under northern 
 Ohio, and a fourth, the last formed of the series, through northern and western 
 Michigan. While this last line of reefs was forming, the older reefs on the south- 
 east were being buried under heavy layers of mud and sand. Many of these 
 reefs show the characteristic structures in a striking manner, as one passes from 
 one quarry-opening to another. The corals which enter into the construction 
 of these reefs are, of course, very different from those found in modern reefs. 
 The chief types were the honeycomb coral (Favosites, Fig. 216) and various 
 star corals (Prismatophylhtm, Craspedophyllum, etc.). Tube corals (Syrin- 
 gopora) and Stromatoporas also abound, the latter often of very large size. 
 Besides these there were many simple horn-shaped corals and other organisms 
 as well. On the margins of the mounds, where the bedded lime-sand deposits 
 dip away from the reef, many broken and worn coral and Stromatopora fragments 
 are found embedded in the sands, lying in appositions. 
 
 Triassic Reefs of the Dolomites. Many other reefs of this kind are found 
 in various parts of the world, and some of these will be again referred to in the 
 later part of this book. We must note here only one other example, which ap- 
 pears to represent an ancient reef of the oceanic type. This is a mass of dolo- 
 mitic limestone, about 3000 feet thick, which now forms the famous peaks of 
 the Dolomites in the Alps of the Tyrol (Fig. 4, p. 9). These limestones were 
 formed in the Triassic period of the earth's history and are largely composed 
 of the remains of nullipores, though other reef-building organisms also occur. 
 The limestone is massive and without structure except around the margins, 
 where the interfingering character, so typical of reefs, is shown. The surround- 
 ing deposits of which this reef-like mass formed a part are of the bedded type 
 of clastic material. 
 
Other Lime-depositing Organisms 
 OTHER LIME -DEPOSITING ORGANISMS 
 
 307 
 
 Bryozoa and Limestones Formed by Them 
 
 The Bryozoa are chiefly marine animals of a higher grade of 
 organization than the coral polyps, but they secrete structures of 
 carbonate of lime, which in many cases are not easily distinguished 
 
 FIG. 229 a, b. A modern Bryozoan (Membranipora pilosa). a, a group of 
 cells or zocecia seen from above (enlarged), and a single cell seen from the 
 side (still further enlarged); b, a single zooid expanded (much enlarged). 
 (After Verrill and Smith.) 
 
 FIG. 229 c, d. Another modern Bryozoan (Crisia eburnea). c, a cluster of 
 branches (enlarged) ; d, a single branch bearing ovicells and zooid cells (zocecia) 
 (much enlarged). (After Verrill and Smith.) 
 
 from corals, except by the trained student. Modern Bryozoa either 
 incrust seaweeds, rocks, or other substances, or form delicate leaf- 
 like but irregular expansions of carbonate of lime (Figs. 229 a-d). 
 
The Organic or Biogenic Rocks 
 
 Many ancient Bryozoa, however, built branching, often cylindrical 
 masses, which were admirably adapted to the making of beds of 
 limestone (Fig. 230). They more often grew in sheet-like associa- 
 
 FIG. 230. Group of rock-forming Bryozoa from the Ordovician (Cincin- 
 nati Group), i, Halloporaramosa; i a, enlargement of surface; 2,H. rugosa; 
 3, H. dalei; 4 a, Dekayia aspcra, enlargement of surface; 5, Hallopora an- 
 drewsi; 5 a, enlargement of surface. (After Nicholson.) 
 
 tion on the sea-floor, but in some cases also formed reef -like mounds. 
 Examples of the latter are shown in the cliffs cut by the Black Sea 
 and the Sea of Azof on the coast of the Peninsula of Kertch (Crimea) . 
 
 Shell-bearing Animals and Shell Limestones 
 
 There are two important groups of shell-bearing animals in the 
 seas, the Brachiopoda and the Mollusca. The latter are represented 
 by several classes, of which three are especially common in the sea to- 
 day, the bivalves or pelecypods, the gastropods, and the pteropods. 
 A fourth group, the cephalopods, was extremely abundant in 
 former times, but is represented by only a few types to-day. 
 
 Brachiopods. In this class the shell is composed of two prin- 
 cipal parts or valves, one generally larger than the other, but each 
 symmetrical about a median line drawn through the apex of the 
 
Other Lime-depositing Organisms 
 
 309 
 
 FIG. 231. Terebratulina 
 septentrionalis. A characteris- 
 tic modern Brachiopod of the 
 northern Atlantic coast. 
 (From Binney and Gould.) 
 
 FIG. 232. A Palaeozoic brachiopod shell 
 partly broken to show the internal spiral 
 arm-supports. (Spirifer striatus, Sowerby.) 
 Mississippian limestones. Note the sym- 
 metrical character of the shell with refer- 
 ence to a median line drawn through the 
 apex or beak. 
 
 valve (Fig. 231). These animals were far more common in the 
 older geological periods, especially those of the Palaeozoic (Fig. 
 232), than they are to-day, and in the past they often formed beds 
 of limestone largely, or almost 
 entirely, composed of their shells. 
 Some brachiopods (Lingula Fig. 
 233, Obolus) carry a high percent- 
 age of phosphoric acid and are an 
 important source of lime phos- 
 phate. 
 
 Bivalves or Pelecypods. In 
 these mollusks (also called lamelli- 
 branchs) the soft body of the ani- 
 mal is enclosed by a shell of two 
 valves which are generally simi- 
 lar, except in the oyster and some 
 other types, forming the right 
 and left valve respectively. Each 
 valve, however, shows an asym- 
 metry of form, a line drawn 
 
 through the apex not dividing it FlG 2 ^_ Lingula p yramidata> 
 into two equal portions. In this natural size. A deep sea brachio- 
 
 respect the pelecypod shell is P od - ( After Brehm; from Ratzel, 
 ,., -,. , . . , j f Die Erde.} The shell of this ani- 
 
 readily distinguished from the mal conta ; ns about 55 per cent of 
 brachiopod shell. Examples are phosphate of lime and magnesia. 
 
3 io 
 
 The Organic or Biogenic Rocks 
 
 FIG. 234. Pecten irradians, the common scallop of the Atlantic coast. 
 (From Binney and Gould.) 
 
 FIG. 235. Area transversa, 
 
 a common plicated shell of FIG. 236. Venus mercenaria, the common 
 
 the Atlantic coast. (From quahaug or salt-water clam, about three fourths 
 
 Binney and Gould.) natural size. (From Binney and Gould.) 
 
 FIG. 237. Modiola plicatula, a characteristic mussel of the tidal flats and 
 salt-meadow streams of the Atlantic coast. (From Binney and Gould.) 
 
Other Lime-depositing Organisms 
 
 the clam, scallop (Fig. 234), area (Fig. 235), quahaug (Fig. 236), 
 mussel (Fig. 237), etc. 
 
 Gastropods. The second class, abundantly represented to-day, 
 is that of the gastropods or snail-like mollusks, in which the shell 
 is coiled in a spiral (Figs. 238-241). These shells are often highly 
 colored and marked by various features such as ridges, nodes, spines, 
 
 FIG. 238. Lunatia heros, the FIG. 239. Neptunea islandica 
 
 common salt-water snail of the At- (curta). A common fusoid gastropod 
 
 lantic coast. (From Binney and of the northern Atlantic ; three fourths 
 
 Gould.) natural size. (From Binney and Gould.) 
 
 and the like. Both pelecypods and gastropods have formed lime- 
 stone beds in the past. On the coast of Florida such shell limestone 
 is forming to-day, where shells and fragments of them are washed 
 into protected areas, remaining long enough so that the percolating 
 waters may deposit lime between them and bind them together. 
 This rock is locally called coquina (Fig. 242). 
 
 Pteropods. A third class of shell-bearing mollusks is that of 
 the pteropods, so-called because a part of their body (the foot) 
 develops into wing-like appendages. These animals float in^vast 
 numbers upon the surface of the ocean, the shell-less species (Fig. 
 243) serving as an important article of food for the whalebone whales. 
 
312 
 
 The Organic or Biogenic Rocks 
 
 FIG. 240. Chrysodomus decemcos- 
 tatus. A characteristic fusoid gas- 
 tropod of the northern Atlantic ; three 
 fourths natural size. (From Binney 
 and Gould.) 
 
 FIG. 241. Fulgur carica, a char- 
 acteristic gastropod of the Atlantic 
 coast from Cape Cod to Florida; 
 one half natural size. (From Binney 
 and Gould.) 
 
 FIG. 242. A piece of modern 
 shell-limestone or Coquina from the 
 Florida coast ; somewhat reduced. 
 (Photo by B. Hubbard.) 
 
 FIG. 243. Clione limacina, a 
 modern shell-less pteropod ; enlarged 
 twice. (From Binney and Gould.) 
 
Other Lime-depositing Organisms 
 
 In correlation with their floating habit, the shell, when present, is 
 thin and light, and often quite transparent (Figs. 244 a, b). Such 
 shells accumulate in vast quantities upon the sea-bottom in regions 
 where the animals abound in the surface waters, and of them is 
 formed a deep-sea pterdpod ooze (Fig. 245). Limestones made 
 entirely of shells of such animals, though not necessarily of deep- 
 sea origin, are found in our older geological series. One of these, 
 
 FIG. 244. Modern shell-bearing 
 pteropods. a, Styliola vitrea, about 
 two and one half times natural 
 size; b, Cavolinia tridentata, approxi- 
 mately natural size. (After Verrill 
 and Smith.) 
 
 FIG. 245. Deep-sea pteropod ooze, 
 enlarged 16 diameters. (After Mur- 
 ray and Renard; from Grabau's 
 Principles of Stratigraphy.) 
 
 found in New York State, carries on the average 40,000 shells to 
 the cubic inch (Fig. 3, p. 8). This limestone may have been formed 
 by the rapid settling of millions of these animals which were killed 
 by being driven into the mouths of estuaries of that period. Where 
 the remains of organisms of this kind abound in rocks which are of 
 shallow-water origin, they may often be the source of important 
 petroleum deposits, as will be more fully shown in the next chapter. 
 Cephalopods. This group is to-day represented chiefly by the 
 Nautilus (Fig. 246) and by a number of shell-less types (squids, 
 Fig. 247; cuttle-fish, Fig. 248; octopus, etc.). During the Meso- 
 zoic era, however, there lived a great group of such shelled cepha- 
 lopods, the Ammonites (Fig. 249), which in some cases were so 
 abundant that they built up beds of limestone, while in other cases 
 
FIG. 246. The modern Pearly 
 Nautilus (N. pompilius} ; the animal 
 occupies the living chamber of the 
 sectioned shell. (After Owen; from 
 Woodworth.) a, mantle; b, dorsal 
 fold; e, nidamental gland; g, shell- 
 muscle; Hi, siphon; k, funnel or hy- 
 ponome; n, hood; ooo, exterior digi- 
 tations; p, tentacles; s, eye; xx, 
 septa ; z, last or living chamber. 
 
 FIG. 247. The Squid (Loligo 
 vulgaris Linn.). A modern decapod 
 cephalopod with remnant of internal 
 shell only. 
 
 FIG. 248. Cuttle fish "bone" 
 (Sepia officinalis Linn.). Internal 
 shell (much reduced). The fine point 
 at the base of the structure represents 
 the guard of the Belemnite, the main 
 mass corresponds with the pro'os- 
 tracum. 
 
 FIG. 249. Ammonite (AmaUheus 
 margaritalus), Middle Lias, Swabia; 
 side view. Where the shell has been 
 partly worn away near the aperture, 
 the complex " suture line" is shown. 
 
Other Lime-depositing Organisms 
 
 they constitute the chief source of the calcareous substance of the 
 rock. In the Palaeozoic era other types similar to the modern 
 Nautilus, and also straight, conical or 
 gently tapering shells, the orthoceran type 
 (Fig. 250), occurred in great abundance, and 
 
 FIG. 250. A simple 
 straight-shelled ceph- 
 alopod. (Orthoceras 
 tumidum, Barr.) Silu- 
 rian of Bohemia; two 
 thirds natural size. 
 Where the shell has been 
 removed, the straight 
 "sutures" are shown. 
 The position of the 
 central tube or 
 "siphuncle" is seen 
 in the bottom view. 
 (After Barrande.) 
 
 FIG. 251. Unio radiatus. A common species of 
 fresh- water clam, of New England ponds and streams. 
 (From Binney and Gould.) 
 
 FIG. 252. Paludina decisa var. Integra. A char- 
 acteristic snail of fresh-water ponds. Female on 
 left, male on right. (From Binney and Gould.) 
 
 were an important source of lime of many rocks, sometimes forming 
 their chief constituent (Orthoceras limestone). 
 
 Fresh-Water and Land Mollusks 
 
 In fresh water, too, deposits of limestone are formed from the 
 shells of bivalve and gastropod Mollusca. Among the former, the 
 great fresh-water clam (Unio, Fig. 251) is the most important, and 
 among the latter, the pond snails (Paludina or Vivipara, Fig. 252) 
 
316 
 
 The Organic or Biogenic Rocks 
 
 FIG. 253. Planorbis trivolvis, a 
 common pond and river snail. Side 
 and bottom views. (After Binney 
 and Gould.} 
 
 FIG. 254. Limncea elodes. A com- 
 mon snail of stagnant ponds. (From 
 Binney and Gould.) 
 
 FIG. 255. Physa heterostropha, FIG. 256. Helix albolabris, the 
 the common species of left-handed common garden snail. (From Bin- 
 snail of brooks and ponds. (From ney and Gould.) 
 Binney and Gould.) 
 
 FIG. 257. Serpula contortuplicata, slightly reduced. Two of the tubes show 
 the expanded fringe of the animal. (From Ratzel, Die Erde.} 
 
Other Lime-depositing Organisms 
 
 and the river snails (Planorbis, Fig. 253 ; Limnaa, Fig. 254 ; Physa, 
 Fig. 255). More commonly, however, these form impure deposits 
 of marly rock mingled with much 
 mechanical sediment. The common 
 snail (Helix, Fig. 256) lives upon 
 moist land, but the shells may be 
 washed into basins and so become 
 an important constituent of lime- 
 stones. 
 
 FIG. 259. The modern ship- 
 or goose-barnacle (Lepas). After 
 removal of the right valve. 
 a, stem; C, Te, and Sc, shell 
 pieces; C, carina; Te, terga; 
 FIG. 258. The Barnacle (Balanus). Sc, scuta; M k, mouth; F, furca; 
 Type of fixed crustacean, one half nat- P, cirrus (or penis) ; M , muscle, 
 ural size. (After Haug.) (From Haas, Leitfossilien.) 
 
 Worms and Crustaceous Animals as Limestone Formers 
 
 Worms. A small number of marine worms build calcareous 
 tubes in which they live. These tubes often form a dense mass 
 which may become at times important as a limestone former ; Ser- 
 pula is an example (Fig. 257). 
 
 Crustacea. This class is best known from its modern represen- 
 tatives, the lobsters, crabs, crayfish, and the very aberrant barnacles 
 (Fig. 258), and the goose-barnacle, Lepas (Fig. 259). None of these 
 ever occurs in such abundance as to form rocks, but a peculiar class, 
 the Ostracods (Fig. 260), in which the animal secretes a bivalve shell, 
 in form often like that of a small bean, may be a rock-former. Some 
 of these abound in the streams and ponds of localities in western 
 
318 
 
 The Organic or Biogenic Rocks 
 
 North America and elsewhere, and the accumulation of the shells 
 gives rise to a calcareous fresh-water ooze which, from the prevalent 
 form, is called Cypris ooze. Members of this class were also abun- 
 dant in the ocean in various geological periods, and limestones 
 largely composed of os traced shells have been formed in the past. 
 
 FIG. 260. A group of fossil Ostracoda. 
 
 Upper row: Leper ditia angulifera (Silurian). 
 
 Middle row; left to right: Cytherideris impressa, lateral and dorsal views 
 X 20 (fresh-wq.ter Cretaceous, Bear River) ; Cythere monticula, lateral and 
 dorsal views X 20 (Bear River Cretaceous) ; jEchmina abnormis, right valve 
 side and (below) dorsal view Xio (Rochester shale, Silurian). 
 
 Lower row : Drepanella crassinoda, right valve, side and (above) dorsal view 
 Xio (Ordovician) ; Primitiopsis punctulifera, left valve, side and (above) 
 dorsal view Xi8 (Hamilton-Devonian) ; Primitia seminulum, left valve Xi8 
 (Hamilton, Devonian). (From Grabau and Shimer, North American Index 
 Fossils.) 
 
 Finally, the remarkable extinct group of the Trilobites (Fig. 261), 
 so-called because the body is divided longitudinally into three parts 
 or lobes, often becomes an important source of lime in the Palaeozoic 
 era, and in Sweden some thin beds of (Cambrian) limestone are 
 entirely composed of the calcareous outer coverings of these 
 animals (Fig. 262). Crustacean structures contain up to 26 per 
 cent of calcium phosphate and may be an important source of this 
 substance. 
 
Other Lime-depositing Organisms 
 
 319 
 
 FIG. 261. A group of Trilobites, the characteristic Palaeozoic crustaceans. 
 
 Upper row : Ptychoparia kingi, Cambrian ; Microdiscus speciosus, pygidium, 
 
 side and surface views; (enlarged). The same, entire specimen, Cambrian; 
 
 Asaphiscus wheeleri, Cambrian. 
 .Lower row : Zacanthoides typicalis, Cambrian, Paradoxides harlani, Cambrian, 
 
 Isoldes gigas, Ordovician. (From Grabau and Shinier, North American 
 
 Index Fossils.) 
 
 . Crinoids and Crinoidal Limestones 
 
 Among the great division of spiny-skinned marine animals, the 
 echinoderms, to which the starfish (Figs. 263, a, b) and sea-urchins 
 (Figs. 263 c, d) belong, there is one group, that of the Crinoids (Fig. 
 264) and their relatives (Cystoids 
 and Blastoids), which were im- 
 portant rock-formers in the past 
 (Palaeozoic and Mesozoic, Fig. 
 265). Crinoids live to-day in 
 water of considerable depth, but 
 in the Palaeozoic era they appear 
 to have been abundant in shallow FIG. 262. Fragment of limestone 
 seas. The animal has a cup- entirely composed of the remains of 
 . i j .^i a small trilobite, Agnostus. Upper 
 
 Shaped body provided with nu- Cambrian, Sweden; one third natu- 
 merous slender arms, and is ral size. (B. Hubbard, photo.) 
 
3 20 
 
 The Organic or Biogenic Rocks 
 
 affixed to the sea-bottom by a stem, often of great length. This 
 stem is composed of a series of calcareous plates or disks, set one 
 upon the other and held together by muscular tissue. When the 
 animal dies, these disks fall apart because of the decay of the 
 
 FIG. 263 a. A common form of 
 star-fish (Asterias arenicola), some- 
 what reduced. (After Verrill and 
 Smith.) 
 
 FIG. 263 c. The common sea- 
 urchin of the North Atlantic (Strongy- 
 locentrotus drobachiensis). Side view, 
 about half natural size. (After Ver- 
 rill and Smith.) 
 
 FIG. 263 b. A common form of 
 brittle star (Ophiopholis aculeata). 
 Dorsal view, about one half natural 
 size. (After Verrill and Smith.) ' 
 
 FIG. 263 d. The common sand- 
 dollar or flat sea-urchin (Echina- 
 rachnius parma). Upper surface with 
 the spines partly removed, about 
 five sevenths natural size ; a, ambu- 
 lacral zone ; i, interambulacral zones. 
 (After Verrill and Smith.) 
 
 muscular tissue, and they will then accumulate to form a bedded 
 deposit of lime which may be many feet in thickness. _The body 
 and arms of the animal are also composed of calcareous plates 
 which are added to the mass, but the stem disks are the most abun- 
 dant and prominent. They form what is called a crinoidal lime- 
 stone, readily recognized from the form of the disks (Fig. 266). 
 
Other Lime-depositing Organisms 
 
 321 
 
 FIG. 265. An extinct lily-crinoid. 
 (Encrinus liliiformis, Lk.) Muschel- 
 kalk (Triassic). Reduced, with basal 
 view of calyx and stem joints. 
 
 FIG. 264. A modern crinoid 
 (Rhizocrinus lojjotensis) showing stem 
 with "root" or hold-fast for attach- 
 ment, and a crown composed of the 
 calyx and branching arms. Slightly 
 enlarged. (After Brehm; from Rat- 
 zel.) 
 
 FIG. 266. A fragment of crinoidal 
 limestone, or rock composed almost 
 entirely of the stem-joints of cri- 
 noids. (Palaeozoic.) About one half 
 natural size. (B. Hubbard, photo.) 
 
322 The Organic or Biogenic Rocks 
 
 Accumulations of Bones of Vertebrates 
 
 The highest division of the animal kingdom, the vertebrates, 
 secretes an internal skeleton which is composed of carbonate and 
 phosphate of lime, and accumulations of such bones, either around 
 salt licks or in ponds and other basins into which they are washed, 
 may form important beds of limestone rich in lime phosphate. 
 Where fish are suddenly killed in the sea, as by an earthquake or the 
 sudden encroachment of cold waters after a storm, vast quantities 
 of fish-bones may accumulate on the sea-bottom; or, where the sea 
 enters a river estuary, the river fish may suddenly be killed by the 
 salt water, and a deposit of their bones formed on the floor of the 
 estuary. Such fish-beds are often found in the geological series, 
 and they may be of local importance as members of the rock forma- 
 tion. Extensive deposits of bones of land animals are also found 
 in caverns, and elsewhere, forming bone-breccias. Some of these 
 will be referred to in later chapters of this book. 
 
 ORGANIC DEPOSITS or PHOSPHATE OF LIME 
 
 Phosphate of lime is probably, in most cases, of organic origin, 
 but in the sea, where the source is chiefly the bones, teeth, and ex- 
 crements of fish, and the shells of some phosphate-secreting animals 
 (such as Lingula (Fig. 233, p. 309) among the brachiopods, and the 
 Crustacea), secondary chemical deposition seems to occur to pro- 
 duce further phosphatization of the original deposits and to form 
 the phosphatic nodules which are subsequently concentrated by 
 weathering of the enclosing rock, until they form important de- 
 posits. Upon the land, accumulations of bones of animals may form 
 a source of phosphate of lime, but by far the most abundant and 
 striking deposits of this character are formed by the droppings of 
 birds on islands and of bats in caves. These deposits are called 
 guano , and they are common on many of the islands of the Pacific ; 
 they also abound in the West Indies and on the islands off the 
 African coast. Indeed, such deposits of bird guano are almost uni- 
 versal around the ocean borders and on their islands. One of the 
 most extensive occurred on the islands off the Peruvian coast, but 
 this has been largely exhausted. In the past, guano formed an 
 important source of commercial phosphate rock, but at present 
 deposits of other origin (generally concentrated marine deposits) 
 
Organic Deposits of Silicia 323 
 
 are more frequently used. Bat guano is much less important and 
 much less common than bird guano. 
 
 ORGANIC DEPOSITS OF SILICA 
 
 Organic silica is far more uncommon and less widespread than 
 is lime of organic origin. Nevertheless, it becomes locally of much 
 importance. Both plants and animals form deposits of organic 
 silica, sometimes in fresh water, but chiefly in salt water. 
 
 Diatoms (Fig. 267). These are plants of low organization be- 
 longing to the division of the algae. They abound in both fresh and 
 ocean water, being mostly of microscopic dimensions, and they 
 are furthermore remarkable in that they commonly possess the 
 power of locomotion. Within the body, which is a single organic 
 
 FIG. 267. Modern diatoms (Diatoma vulgare, Bory). The individuals are 
 joined in a zigzag band; much enlarged. (From Haas, Leitfossilien.) 
 
 cell, they build up a structure of silica, often of very beautiful form 
 and of great variety. This structure, called the frustule, consists 
 of two pieces which fit together like the body and cover of a pill 
 box. In some fresh-water ponds they are so numerous that they 
 form accumulations on the bottom of the pond, largely composed 
 of these minute silicious bodies. A rock is thus formed which re- 
 sembles chalk in consistency and general aspect, but which, unlike 
 chalk, will polish metals or other hard substances. Such rock is 
 called diatomaceous earth when impure and tripolite when pure (from 
 a famous deposit of this material at Tripoli in north Africa). 
 
 A diatom ooze is also formed in the sea, especially in the South- 
 ern and Antarctic oceans, on the floors of which it is estimated to 
 cover an area of ten million square miles, at an average depth of 
 1500 fathoms (see the map, Fig. 198, p. 277). In the northern part 
 of the North Pacific, an area of about forty thousand square miles 
 is also covered with diatom deposits. These deposits are generally 
 not pure, but other silicious organisms (Radiolaria, sponge 
 spicules) and earthy matter occur with them. Diatoms sometimes 
 
The Organic or Biogenic Rocks 
 
 form the chief element of rocks in marine deposits, not always of 
 deep sea origin. If diatoms are carried by currents from the sea, 
 
 into a more or less 
 enclosed basin in 
 which the waters are 
 stagnant, or if they 
 grow in such stag- 
 nant waters, they 
 will accumulate in 
 quantities upon the 
 bottom, but as in 
 such cases there are 
 few animals which 
 feed upon the or- 
 ganic matter of the 
 diatoms, this will 
 also accumulate and 
 by decay form a 
 hydro-carbon which 
 \ saturates the de- 
 
 FIG. 268 a. A modern Radio- 
 larian, showing the skeletal struc- 
 ture of silica and the extended 
 fleshy threads or pseudopodia 
 (Eucystidium cranoides} . Much 
 enlarged. (From Haas, Leitfos- 
 silien. ) 
 
 posits of silicious diatom 
 shells, and may become an im- 
 portant source of petroleum 
 and natural gas. Such de- 
 posits occur in the Tertiary 
 series of rocks in California, 
 and the great oil accumula- 
 tions of these rocks are be- 
 lieved to have been derived from the organic matter of these 
 diatoms which was retained in this deposit because of the peculiar 
 conditions of formation. 
 
 FIG. 268 b. Heliosphara echin aides, a 
 modern Radiolarian. Greatly enlarged. 
 (After J. Murray ; from Haug Traite.) 
 
Organic Deposits of Silica 
 
 325 
 
 Radiolaria (Figs. 268^,6). These are minute, single-celled 
 animals related to the Foraminifera and with them forming the 
 mineral-secreting members of the division of Protozoa. Unlike 
 the shells of the Foraminifera, however, the hard structure secreted 
 
 FIG. 269. A modern sponge with silicious skeletal structure, and basal 
 glass fibers for fixation (Holtenia carpentcri Thomson). 
 
 by the Radiolaria is internal and consists of a network of glass 
 or silica of wonderful variety of form in the various species. These 
 animals live only in the sea, and their shells accumulate in the 
 greatest purity on the deeper portions of the sea-bottom, where 
 forarniniferal shells are not found because they are dissolved be- 
 fore they reach that depth. Commonly there is, however, an 
 
326 The Organic or Biogenic Rocks 
 
 admixture of red clay, a very characteristic deep-sea deposit. 
 When more than 20 per cent of the deposit consists of Radiolaria 
 it is customary to speak of it as a radiolarian ooze. Radiolarian 
 oozes are not known in the Atlantic, but occur in the deeper por- 
 tions of the Pacific Ocean and in small areas of the Indian Ocean 
 
 OsoQlutn 
 
 . . . . ostla 
 
 FIG. 270. A Cretaceous sponge with silicious skeleton retaining its form. 
 (Ventriculites simplex. Mantell.) 
 
 (see map, Fig. 198, p. 277). There is, however, a deposit now 
 exposed on the island of Barbadoes in the Windward group of the 
 Antilles, which is a typical radiolarian ooze with red clay, and 
 appears to be an old sea-bottom deposit now uplifted. 
 
 Radiolaria are also found in shallow water deposits, where they 
 become included in other sediments. They are not uncommon in 
 the lagoons of coral reefs, and they have been found in the chert 
 bands of older limestones formed in association with reefs. The 
 chert bands themselves are often the redeposited silica derived from 
 the solution of scattered Radiolaria and other silicious organisms. 
 
 Sponge Spicules. Many modern marine sponges (Fig. 269) 
 secrete within their soft, horny, and fleshy masses, minute needles 
 of silica. These are set free on the decay of the sponge, and some- 
 
Organic Deposits of Silica 
 
 327 
 
 times form an important source of silica in other deposits. Many 
 of the sponges of former geological periods built solid structures 
 
 FIG. 271 a. Isolated spicular 
 bodies of an extinct silicious sponge. 
 (Eplstomella clivosa, Quenst. White 
 Jura.) 
 
 FIG. 271 b. Part of the skeletal 
 structure of a lithisdid silicious sponge. 
 (Jereiea polystoma, Roem. Mucro- 
 naten-Kreide. Upper Cretaceous, 
 Germany.) 
 
 by a union of such spicules (Figs. 270-271 a, b) and so became im- 
 portant sources of organic silica. The flint of the chalk (Fig. 162, 
 p. 224) is believed to be largely derived from such sponge spicules. 
 
CHAPTER XIII 
 
 THE ORGANIC OR BIOGENIC ROCKS: DEPOSITS 
 FORMED FROM THE ORGANIC TISSUES OF PLANTS 
 AND ANIMALS 
 
 DEPOSITS FORMED FROM VASCULAR PLANTS 
 Conditions and Processes of Decay 
 
 UNDER ordinary conditions, when a plant, whether herb or tree, 
 dies, it soon decays, and practically nothing remains behind except 
 a minute quantity of mineral matter. The plant tissues, which are 
 composed of carbon, hydrogen and oxygen, chiefly in the form of 
 the material called cellulose, unite with the oxygen of the air, 
 partly through the activities of micro-organisms (bacteria), and 
 carbon dioxide and water are formed. The same result is achieved 
 more rapidly by the burning of the dry plant tissues, when, how- 
 ever, some unconsumed carbon may pass off as smoke. Slow 
 decay under the atmosphere is, in effect, a very slow but complete 
 burning of the tissues without the production of a flame or the 
 development of easily perceptible heat. The reaction in either 
 case is as follows : 
 
 C 6 HioO 6 + i2O(=6O 2 ) = 6CO 2 + sH 2 O 
 
 Cellulose, 6 molecules 6 molecules 5 molecules 
 
 etc. of Oxygen of Carbon of Water 
 
 Dioxide 
 
 The carbon dioxide is a gas, and the water passes off as invisible 
 vapor. If plants are burned where oxygen does not have free 
 access, as in a charcoal oven or kiln, or under a covering of earth, 
 the oxidation is incomplete, the hydrogen and oxygen escaping 
 as water, while some of the carbon may also be converted to carbon 
 dioxide ; but the greater part of the carbon remains behind in the 
 form of charcoal. 
 
 In like manner, when plants become submerged in the waters 
 of a swamp or marsh, free access of oxygen is prevented and only 
 partial decay will result. At first only a part of each of the com- 
 
 328 
 
Deposits Formed from Vascular Plants 329 
 
 ponent elements will pass into the air in the form of gaseous com- 
 binations, the chief of these being carbon dioxide (062), water 
 (H 2 O), and marsh gas, a combination of carbon and hydrogen 
 (CHi). As the change progresses, especially under pressure of 
 other material which may be spread over the plant tissues, more 
 of the oxygen and hydrogen will be eliminated, and the relative 
 amount of carbon remaining will be progressively increased, 
 though there is, of course, no actual increase in carbon, but rather 
 some decrease. This forms the several series of coals. Finally, 
 with the application of both pressure and heat, most or even all of 
 the other substances may be driven off and pure carbon alone 
 remains, producing graphite. In special cases crystallization of 
 the carbon may result in the formation of diamonds. 
 
 Types of Vegetal Deposits and Stages in Alteration 
 
 Peat. The first product of partial decay of vegetable matter 
 is peat. According to the degree of decay and the character of 
 the vegetable material, peat ranges in color from brown to black, 
 and is a loose, spongy mass in 
 which the structure of the veg- 
 etable tissue is only partly ob- 
 literated (Fig. 272). Peat forms 
 only in stagnant waters, for here 
 alone complete decay is arrested. 
 This seems to be due to the fact 
 that the bacteria which are active 
 
 in the decay produce certain by- F f- 272. -A fragment of peat 
 .... about one half natural size, 
 
 products which give to the stag- (Photo by H ubbard.) 
 
 nant water an antiseptic char- 
 acter, with the result that the bacteria are destroyed and the 
 process of decay is arrested. Peat bogs are proverbial for their 
 antiseptic qualities, and in them bodies of animals and of men are 
 often preserved for long periods of time. If these by-products 
 are removed, as in running or much agitated waters, the decay 
 continues until it is complete. 
 
 Types of Peat and Conditions of Formation. The areas of 
 peat formation, or the moorlands, may be either low-lying or they 
 may be high moors. The former division comprises the shore 
 moorlands or marine marshes, and the fresh-water swamps and 
 
330 
 
 The Organic or Biogenic Rocks 
 
 fenlands, the latter the upland bogs. These types will now be 
 more fully considered. 
 
 Marine marshes. Where the waves break on a gently sloping 
 sandy coast, they commonly build up an off-shore or outer bar, 
 
 5 B S P EG MF 5 P SM 
 
 HT 
 
 FIG. 273. Diagrammatic section of an off-shore bar and the barachois or 
 lagoon enclosed by it. HT, high tide-level; LT, low tide-level; SB, sand bar 
 formed by waves from sea-bottom sand; Sp, marsh grass or Spartina zone; 
 EG, eel-grass zone; MF, mud-flat, uncovered at low tide; SM, salt meadow, 
 covered only at highest tides. . 
 
 leaving a lagoon or barachois of protected water between it and the 
 land (Fig. 273). This lagoon may be many miles in width, but is 
 never very deep, seldom over 30 feet, and its waters are never 
 
 FIG. 274. Eel-grass growing upon a muddy bottom, Woods Hole, Mass. 
 (Photograph of part of foreground of reproduction of annulate group ; by cour- 
 tesy of American Museum of Natural History.) 
 
 completely cut off from the open sea, joining it either around the 
 'end of the bar or through narrow openings across it. In the quiet 
 waters of the lagoon, where the depth is not over 1 2 feet, eel-grass 
 
Deposits Formed from Vascular Plants 331 
 
 will begin to grow. This is a plant belonging to the pond-lily 
 family, but adapted to live only in salt water (Fig. 274). Mean- 
 while the bar may be converted into a barrier beach by the forma- 
 tion upon it of sand dunes, heaped up by the wind from the sand 
 of the bar, exposed and dried at low tide, and the deeper parts of 
 the lagoon may slowly become silted up, after which, when the 
 proper depth is attained, the eel-grass will spread over these parts. 
 At low water the eel-grass will form a dense tangle which checks the 
 tidal and other currents and forces them to deposit their load of 
 
 FIG. 275. View of the salt meadows at Winthrop, Mass., showing the rank 
 growth of salt thatch (Spartina) and the tidal stream dissecting the meadow. 
 
 silt, and thus further filling of the lagoon takes place, the silt 
 accumulating around the eel-grass blades. When silting-up has 
 progressed so far that at low tide parts of the bottom are exposed, 
 the eel-grass there will die, and a mud-flat results, in which live 
 clams and other marine organisms, and from which a fetid odor 
 of marsh gas is evolved. Finally, the higher portions of the mud- 
 flat are taken possession of by the marsh grasses (Spartina) and 
 as these die down year by year, patches of peat are formed from 
 their roots and decaying stems. As the growth of grasses spreads 
 and the surface rises through the accumulation of the peat, other 
 species of marsh grass, which can stand less submergence, will 
 take possession and these will continue to build up the peat deposit. 
 Finally the entire lagoon, or a large part of it, will be converted into 
 
332 The Organic or Biogenic Rocks 
 
 a salt meadow, which is submerged only at the highest tides, but 
 which is intersected by numerous tidal channels, on the margins of 
 which the peat can be observed at low water (Figs. 275, 276). 
 Meanwhile the sand dunes, traveling inland under the influence of 
 the winds from the sea, will begin to cover parts of the salt 
 meadow, and the waves, cutting back the bar which they had 
 originally formed, will eventually remove it, exposing the peat beds 
 
 FIG. 276. Map of the salt marsh near Newburyport, Mass., with Plum 
 Island Sound and creek representing the remains of the lagoon; the sand bar 
 is known as Plum Island. (After Shaler.) 
 
 upon the outer shore of the barrier beach, which is now formed of 
 the sand dunes resting upon the peat beds of the old lagoon. Unless 
 a change of level occurs, the cutting process of the waves will 
 eventually again destroy the entire series of deposits which have 
 formed in the lagoon. Hence such accumulations are not generally 
 of a permanent character, though special conditions may occur 
 to preserve them. Where, however, a rock barrier separates an 
 old lagoon from the sea, as in parts of the northern New Jersey 
 meadows, the opportunities for preservation of the peat deposits 
 are more favorable. 
 
 Deposits of peat formed in salt meadows are never very thick, unless the 
 coast is slowly sinking as the peat grows, and at a corresponding rate. On the 
 Massachusetts coast, where such subsidence has taken place, peat deposits 
 composed of the remains of high-tide vegetation have been found to have 
 a thickness of 20 feet. Upon the southern Atlantic coast the average thick- 
 ness is probably not much over four feet, though this would vary with the 
 
Deposits Formed from Vascular Plants 333 
 
 magnitude of the tides. Many variations occur, dependent on the original 
 condition of the coast, and it may even happen that salt-water peat, formed 
 only of the taller marsh grasses, will overlie a fresh-water peat which was 
 formed before the sea encroached over that area owing to slow subsidence of 
 the land or other causes. Examples of such complex deposits are found on the 
 Massachusetts coast. 1 Salt peat is almost always rich in silt, which is brought 
 in by the tides or in fine sand and dust which is blown" there by the wind. 
 Moreover, the contact of the salt water with the decaying vegetation favors 
 the activities of certain bacteria which will decompose the sulphates in solu- 
 tion in the sea- water, and cause the formation of sulphureted hydrogen (H 2 S), 
 
 7 e 
 
 FIG. 277. Diagram of plant zones in small lake near Merryman's Lake, 
 Michigan. (After C. A. Davis.) o, Chara; i, floating bladderworts; 2, yellow 
 pond-lily; 3, lake bulrush; 4, SartwelPs sedge; 5, bottle sedge; 6, spike rush; 
 7, cat-tails. 
 
 a characteristic product of the salt meadows, easily recognized by its odor. 
 This will react upon the iron compounds in the silt, and finely divided iron 
 sulphide (the mineral pyrite) will form. If then in time such salt peat is con- 
 verted into coal, that coal will be high in ash, because of the silt, and will be 
 rich in sulphur minerals, especially iron pyrites. When formed in the normal 
 way above outlined, such a coal should rest upon mud and sand beds which 
 contain the remains of marine organisms, and the coal itself may contain 
 such remains, since shell-bearing Mollusca and other animals, such as crabs, 
 are not uncommonly found in peat deposits of this origin. On the Atlantic 
 coast the shells of the plicated mussel (Modiola plicatida, Fig. 237, p. 310) are 
 especially abundant in the peat beds. Some of the plants found in these salt 
 meadows are shown in Fig. 278 a. 
 
 Swamps. Many small lakes are converted into swamps by 
 becoming choked with vegetation, or the margins of larger ones 
 may become swampy through similar causes. In general, we may 
 distinguish two types of plant deposits in lakes, that formed by 
 algae and that formed by the so-called vascular plants, that is, 
 
 1 See further Grabau, Principles of Stratigraphy, p. 492. Johnson, Shore Processes, 
 P- 385- 
 
334 
 
 The Organic or Biogenic Rocks 
 
 those that have a regular structure like or resembling that of 
 wood, and which include all the plants from mosses and ferns up. 
 Algae abound in fresh as in salt water, each habitat having its own 
 species and genera. As the algae may occupy all parts of the lake 
 waters, the deposits formed by 
 their dead tissues 'will cover the 
 lake floor in a more or less con- 
 tinuous layer, provided there 
 are no bottom animals to feed 
 upon them. Mingled with these 
 algae are the pollen grains of 
 trees such as the conifers which 
 
 FIG. 278 a. Salt marsh plants; 
 enlargements of the flowing portions, 
 by which they are chiefly distin- 
 guished, i, a rush, Juncus gerardi, 
 northern salt marshes; 2, bulrush, 
 sedge family, Scirpus americanus, 
 common on borders of salt and fresh 
 ponds and streams ; 3, rush salt grass, 
 Spartina junca, salt marshes. (From 
 drawings by Mary Welleck.) 
 
 FIG. 2786. Characteristic swamp 
 plants taking part in peat-forming, 
 i, a common peat-moss, Hypnum, 
 X i|; 2, common duckweed, Lemna. 
 The entire plant is reduced to a leaf- 
 like expansion which bears a long 
 slender root and small flowers ; forms 
 green floating scum on ponds and 
 stagnant pools, X8 ; 3, a leaf of arrow- 
 head (Sagittaria), a member of the 
 water-plantain family, growing abun- 
 dantly in swamps. One fourth nat- 
 ural size. 
 
 grow in the neighborhood, while fresh water diatoms also accumu- 
 late. This material will form a homogeneous, structureless mass, 
 more or less mingled with fine sediments, with the calcareous par- 
 ticles from the stone-worts (Chara) (Fig. 192, p. 273) which grow 
 under such conditions, or with the silicious frustules of diatoms 
 (Fig. 267, p. 323). Such a mixture of decaying organic material 
 with mineral matter is called a sapropelite, and according to the 
 amount of impurities present, it becomes, on compacting, either 
 an oil shale, or when nearly pure, a cannel coal. The pure 
 
Deposits Formed from Vascular Plants 335 
 
 decaying organic slime is called sapropel. (Sapros, craTrpos = rotten, 
 pelos, TrcAos = mud.) 
 
 The peat of the lake-swamps, on the other hand, is formed from 
 vascular plants (Figs. 277-279). In the deeper waters of many of 
 
 FIG. 278 c. Cotton grass (Eriopho- 
 rum alpinum) , a member of the sedge 
 family, i, small entire plant; 2, spike; 
 3, a single scale ; 4, a flower from the 
 same; 5, a spike in fruit, the bristles 
 forming a cottony tuft; 6, a single 
 one-seeded dry fruit or achenium, 
 with bristles much enlarged. A 
 characteristic peat plant in cold bogs, 
 northern United States and Europe. 
 (After Gray.) 
 
 FIG. 2 78 d. The twig-rush (Cla- 
 dium mariscoides) , a characteristic 
 plant of the bogs of the northeastern 
 United States, i, summit of plant; 
 2, detached spike; 3, the same 
 opened, showing a staminate and a 
 perfect flower; 4, the nut-like fruit 
 or achenium; 5, longitudinal section 
 of the same. (2-5 enlarged; after 
 Gray.) 
 
 our lakes, pond-lilies form the chief vegetation, these rising from 
 depths not exceeding 25 feet. Next come the reeds, such as bul- 
 rushes, cat-tails, flags, water plantain, arrowheads (Fig. 278 6 3 ), some 
 grasses, and the wild Indian rice (Phragmites, Fig. 2787), most of 
 these not growing well in water more than two feet deep. Be- 
 
336 
 
 The Organic or Biogenic Rocks 
 
 sides these, there are floating plants such as the bladderwort and 
 the duck- weed (Lemna, Fig. 278 & 2 ), which may cover large sur- 
 faces as with a green carpet. 
 Near the shore, sedges (Car ex, 
 Fig. 278 e) and peat-mosses (Hyp- 
 num, Fig. 278 bi) appear, and 
 these may form a floating mat of 
 entangled plants which extends 
 from the shore outward often for 
 a considerable distance, and be- 
 cause of the interwoven roots 
 and branches makes a buoyant 
 structure capable, in its thicker 
 portion, of supporting consider- 
 able weight, but dangerous to 
 enter upon in its outer thinner 
 parts (Fig. 280) . The peat formed 
 from such a mat grows in thick- 
 ness year by year, as the under 
 part suffers partial decay and 
 compacting, and new growth 
 takes place on top, and thus the 
 margins of the lake gradually 
 become filled in by a zone of 
 peat which progressively extends 
 toward the center of the lake. 
 Outside of the zone of sedges and 
 peat-mosses appears the zone of 
 water-loving trees, such as the 
 alders and the tamarack in the 
 northern, and the cypress and 
 tupelo in the more southern 
 regions, and in the shadow of 
 these grows a rich assemblage of 
 ferns, sedges, horsetails, and the 
 peat-moss, Sphagnum. As the 
 mat thickens, these trees will ad- 
 vance over it, growing now upon the site of the former lake 
 margin, which is converted into a " quaking bog." If Sphag- 
 num is present, this will often grow to such an extent as to cover 
 
 FIG. 278 e. A typical peat- 
 bog sedge (Car ex pauciflora), 
 characteristic of the peat-bogs 
 of the northeastern United 
 States, i, entire plant; 2, 
 staminate flower with its scale ; 
 3, scale; 4, mature pistillate 
 flower in its perigynium or en- 
 velope; 6, dry fruit or ache- 
 nium on its stalk with style 
 and stigmas. (After Gray.) 
 
Deposits Formed from Vascular Plants 337 
 
 the lower parts of the trunks of the trees, and keeping them 
 moist, will cause their death. As the trunk breaks off above the 
 covering of moss, the stump or '' stool " remains and is covered 
 by the growing moss, as are also the fallen trunks. A mass of 
 
 FIG. 2 78 /and g. Flowers of Reed and Cane: /. the common reed, Phrag- 
 mites (Arundo) communis, of the American and European reed swamps, growing 
 from 5 to 12 feet high with leaves 2 inches wide (see Fig. 279). i, spikelet en- 
 larged; 2, one of the perfect flowers enlarged; 3, lowest flower with stamens 
 only. g. The large cane (Arundinaria macros perma), which forms the cane 
 brakes of the southern states. It grows from 10 to 20 feet in height with 
 leaves i to 2 inches wide, i, a spikelet; 2, a separate flower magnified. (After 
 Gray.) 
 
 FIG. 279. Reed zone (Arundo phragmites zone) on the border of the high 
 moors. The trees in the background, especially Pinus sifaestris, are smaller here 
 than in the intermediate moor. (After Potonie, Die Entstehung der Steinkohle.) 
 
338 
 
 The Organic or Biogenic Rocks 
 
 peat of variable character is thus produced, carrying in its upper 
 part stools and fallen tree trunks, leaves, ferns, etc., and resting 
 near the center of the lake upon a deposit of sapropelite. The 
 
 FIG. 280. Diagram showing how plants fill ponds from the sides and top. 
 i, zone of Chara and floating aquatics; 2, zone of pond weeds or Potamogetons;' 
 3, zone of water-lilies; 4, floating sedge mass; 5, advance plants of conifers 
 and shrubs ; 6, shrub and Sphagnum zone ; 7, zone of tamarack and spruce ; 8, 
 marginal fosse. (After C. A. Davis.) 
 
 thickness of the deposit is determined in part by the depth of the 
 lake and by other causes. If covered by sediments, such a peat 
 deposit may be preserved and subsequently compacted into coals. 
 Such conditions for preservation are best found on broad river 
 
 FIG. 281. Southern margin of Dismal Swamp 12 miles west of Elizabeth 
 City, N. C., showing general aspect of swamp in the month of May. (Photo 
 by Russell, from U. S. G. S.) 
 
 flood-plains or on the great flat alluvial plains near their mouths. 
 Here not only exist all the conditions which favor the formation of 
 swamps, but at intervals the rivers, charged with sand and mud, 
 may spread this over and cover the peat deposit and so preserve it. 
 Borings in river deltas, such as that of the Ganges and others, have 
 
Deposits Formed from Vascular Plants 339 
 
 revealed successive deposits of peat at various depths, covered 
 by and alternating with sands and muds, and in a condition suit- 
 able for conversion into coal. 
 
 FIG. 282. Cypress trees in the eastern part of Lake Drummond, Va. (Photo 
 by Russell, from U. S. G. S.) 
 
 FIG. 283. A swamp in Florida. 
 
 Examples of our largest swamps in which peat is forming to-day 
 are Okefenoke Swamp in Georgia, 50 miles from the sea and 115 
 
340 The Organic or Biogenic Rocks 
 
 f> 
 i 
 
Deposits Formed from Vascular Plants 341 
 
 feet above it, with peat ten feet thick filled with cypress stumps, 
 and Dismal Swamp in Virginia and North Carolina, with an area 
 of 500 square miles, near the sea and only a few feet above it and 
 with a peat deposit at least fifteen feet deep (Fig. 281). In this 
 swamp, the chief plants are canes, wild grape, the bald cypress 
 (Fig. 282) and junipers, with but little sphagnum. Similar swamps 
 occur in Florida (Fig. 283), and extensive cedar swamps with peat 
 up to fifteen feet in thickness exist on the Atlantic coastal plain 
 
 FIG. 285. Section of a Scottish peat-bog. The peat is underlain by clay, 
 which corresponds to the under-clay of a coal seam. (After Geikie.) 
 
 further north. This peat is full of tree trunks and is very pure, 
 containing only 3.35 per cent of ash. In tropical swamps the peat- 
 moss, Sphagnum, is absent, the peat-forming plants being sedges, 
 grasses, myrtle, etc., and woody plants, as in Bermuda, where the 
 peat is at least 50 feet thick in one swamp, and in the Amazon re- 
 gion, and in the interior of Africa. Woody plants are the chief peat- 
 formers in the great tropical swamp of Sumatra, where the peat 
 has been sounded to a depth of nine meters. Here herbaceous 
 plants are rare, and sedges, grasses, and mosses are practically 
 wanting. Thread-like algae are, however, numerous in the water. 
 Where swamps occur along tropical seacoasts, mangrove trees 
 form the chief vegetation (Fig. 284). 
 
 Bogs. These form on uplands, often directly over rocky or 
 sandy bottoms, where no standing water exists, the water for the 
 
342 
 
 The Organic or Biogenic Rocks 
 
 formation of the peat accumulating as the growing vegetation 
 arrests the drainage. Thus actual ponds or lakes may be formed on 
 gentle hillsides or near their tops, the water being held entirely 
 by dams of vegetable material. Such bogs cover the uplands of 
 many northern countries, as for example Great Britain and Ireland, 
 Scandinavia, parts of Canada, etc., being most common in cool 
 and moist regions (Fig. 285). 
 
 In the formation of these bogs the peat moss Sphagnum is most active, this 
 plant growing rapidly upon moist surfaces and building up a spongy mass which 
 collects and holds back the water. In the Scottish and other uplands, heather 
 is an important peat-maker, for it too holds back the moisture and builds up 
 spongy masses from its decaying older branches and roots (Fig. 301 b, p. 364). 
 The common reed (Phragmites, Fig. 279) is another important peat-former in 
 the uplands, and so is the bulrush (Scirpus, Fig. 278 02), the cotton grass 
 
 (Eriophorum, Fig. 278 c), 
 and others. In the Arctic 
 Tundras, which are great 
 peat-deposits covering the 
 frozen ground, mosses and 
 lichens are important peat- 
 formers. 
 
 The peat formed in 
 bogs is known in Great 
 Britain as hill peat and 
 is extensively used for 
 fuel, being cut into 
 cubes, which are dried 
 in the open (Fig. 286). 
 It is usually a brownish 
 or nearly black, fibrous, 
 spongy substance, with 
 the vegetable structure 
 still clearly visible. 
 Tree stumps are com- 
 mon in it, as are also fallen trunks and branches, for whenever the 
 peat begins to grow in forested areas, the elevation of the water- 
 level around the trunks, due to the retention by the peat, causes 
 the death of the trees. The generally monotonous deforested 
 areas of these uplands are the result of such a process. In thick- 
 ness, these upland peat deposits seldom exceed 50 feet, and usually 
 they are much thinner. 
 
 FIG. 286. Gathering peat in an Irish peat- 
 bog. 
 
Altered Deposits of Older Vegetal Material 343 
 
 ALTERED DEPOSITS OF OLDER VEGETAL MATERIAL 
 
 Brown-Coal. This is a compact or earthy coal, of more or less 
 homogeneous character, and pale yellow to deep brown color, burning 
 with a sooty flame and strong odor. It is an altered peat deposit, 
 often still showing evidence of organic origin, and contains from 55 
 to 75 per cent of carbon. Its specific gravity ranges from 0.5 to 
 1.5. Brown-coal is most common in the Tertiary formations, 
 and often reaches a great thickness, this, in some of the north 
 German deposits (Fig. 287), being from 75 to 150 feet, while in 
 Australia beds of 
 brown-coal of much 
 greater thickness are 
 known. Old tree 
 stumps in the po- 
 sition of growth are 
 commonly found in 
 these deposits, as 
 shown in the illus- 
 tration (Fig. 287). 
 
 Lignite. This 
 name should be re- 
 stricted to altered 
 woody tissue such as 
 is found embedded in 
 
 FIG. 287. Brown-coal quarry near Senftenberg, 
 North Germany. Many stumps of large trees are 
 still standing in the position of growth. (After 
 E. Haase, from Walther.) 
 
 brown-coal, though 
 in some cases the bulk of the deposit may be of this origin. In 
 general, the woody structure is still recognizable in lignite. The 
 lignites which occur embedded in the north German brown-coal 
 deposit still retain the form of the fallen tree trunks and standing 
 stumps (Fig. 287), and they are found to be of the same types 
 as those still growing in our Dismal and other modern swamps, 
 but which have become wholly extinct in northern Europe. Lig- 
 nites are also found embedded in sands and clays of Tertiary and 
 Mesozoic age in many regions. 
 
 Bituminous Coal. This is the common soft coal, of black color, 
 bright luster, and usually very brittle character. It contains 
 from 75 to 90 per cent of carbon, and generally some sulphur. Its 
 specific gravity ranges from 1.2 to 1.35, and it burns with a bright, 
 clear flame, though some varieties cake, while others are entirely 
 
344 
 
 The Organic or Biogenic Rocks 
 
 consumed to ashes. Under the microscope, traces of organic origin 
 are recognizable^ and impressions of plants are commonly found in 
 the shales overlying these coals (Fig. 288). Most of the younger 
 coals (Mesozoic) are of this character, as is also 
 a large proportion of the older coals (Pennsyl- 
 vaniafl) of both America and Europe. 
 
 Anthracite Coal. This is hard coal, and is 
 the purest of all, containing over 90 per cent 
 of carbon. It is of black color, submetallic to 
 vitreous luster, and breaks with a conchoidal 
 fracture. Its specific gravity ranges from 1.35 
 to 1.7. It kindles with difficulty, but burns 
 in a strong draught with great heat without 
 smoke, caking, or odor. Many anthracites 
 are found where the rocks have suffered dis- 
 turbance, and they may often be regarded as 
 somewhat metamorphosed bituminous coals. 
 Other anthracites, however, are of primary 
 origin, the vegetable accumulations having lost 
 most of their volatile material before burial. 
 
 Graphite ; Diamond. Graphite is pure 
 carbon, soft, black, and with the characteris- 
 tics of a good lubricant. It occurs chiefly in the ancient rocks 
 which have been subjected td metamorphism and changed from 
 bituminous shales, sandstones, and limestones, to graphite-bearing 
 mica schist, gneisses, and marbles. Some graphite may be due to 
 purely inorganic chemical processes. Diamond is crystallized car- 
 bon, noted for its hardness and brilliancy of luster, 
 
 Natural Coke. This is another alteration product resulting 
 from contact metamorphism of a coal bed through the heat of an 
 intruded dike or sill. It may also be formed by burning of under- 
 ground coal beds. 
 
 Occurrence and General Character of Coal Deposits 
 
 There were three periods in the earth's history when the conditions 
 for the accumulation of extensive deposits of vegetable material 
 were especially favorable, and these deposits have since been 
 converted into brown-coal, bituminous coal, or into anthracite. 
 By far the most extensive accumulations occurred toward the close 
 of the Palaeozoic era, when all of the important coals of eastern 
 
 FIG. 288. Frag- 
 ment of roof shale of 
 an upper Palaeozoic 
 coal seam, showing 
 the impression of a 
 fern. About one 
 third natural size. 
 (B. Hubbard, photo.) 
 
Altered Deposits of Older Vegetal Material 345 
 
 North America (east of the zooth meridian) as well as most of 
 those of Europe, and of China, were formed. It has been esti- 
 mated that seven tenths of the coal deposits of the world belong 
 to this period. The second period was the Cretaceous, in which the 
 Rocky Mountain coals were formed; while the third period, the 
 Tertiary, witnessed the formation of the least valuable of our coals, 
 which occur chiefly west of the i2oth meridian, and also that 
 of the extensive brown-coal deposits of North Europe. Some 
 coal is also found in the Triassic of eastern North America, and 
 in small amounts in other horizons in various parts of the earth. 
 
 In general, much of the coal was formed from vegetation which grew and 
 was buried where the coal is now found (autochthonous deposits), but some of it 
 appears to have been transported vegetation stranded in favorable localities 
 (al loch tho nous deposits). Most coals "appear to have originated from vegeta- 
 tion which grew in swamps, on broad river flood plains, or on coastal plain areas, 
 and most of them were formed in fresh water. Beds of rock carrying marine 
 fossils are as a rule not directly associated with the coal beds, though such 
 may lie between successive -seams. Coals vary greatly in the amount of mineral 
 matter or ash which they contain. Good coals have only from i to 5 per cent 
 of ash. When the coal contains 30 or 40 per cent of ash it is called bony coal 
 and is valueless. Above that amount it becomes a coal shale. 
 
 A typical coal seam (Fig. 289) rests upon a bed of under-day 
 from which much or all of the alkaline material has been removed by 
 the growth of the plants 
 and by solution. This clay 
 is therefore suitable for the 
 formation of " fire bricks " 
 for lining blast furnaces, 
 etc., and the name fire clay 
 is applied to it. Such clays 
 are, however, not always 
 present, and they may occur 
 where no coal seam overlies 
 them. The seam itself may 
 vary in thickness from a 
 mere film to many feet . F IG. 289. - Typical coal beds or seams 
 , .m country rock all steeply inclined by 
 
 *lg. 290), but the contl- subsequent deformation; California, 
 nuity of the thicker seams 
 
 is generally interrupted by coal shale or layers of bony coal. Over- 
 lying it is the roof shale, which is consolidated mud, and generally 
 
346 The Organic or Biogenic Rocks 
 
 contains the well-preserved impressions of the types of plants which 
 grew in the coal swamps. In the majority of cases the rocks 
 between the coal seams are sandstones and shales, but limestone 
 may also occur. 
 
 FIG. 290. Coal-mine in a mammoth vein or bed. 
 
 ACCUMULATION OF DECAYING ORGANIC MATTER FROM ANIMAL 
 TISSUES, AND FROM NON- VASCULAR PLANTS 
 
 As we have previously seen, there is in every pond or lake a 
 region where only the soft tissues of algae will grow, and where by 
 their accumulation a layer of much decomposed, structureless 
 material, more or less mingled with mechanical sediment, with lime 
 precipitated by some of these algae (Chara), or with the silicious 
 frustules of diatoms, is formed, to which the name sapropelite has 
 been applied (Fig. 291). In deposits of this kind the pollen 
 grains of coniferous trees and other plants growing on the border 
 of the swamp usually abound, and spores of the lower (non-flowering) 
 plants of the neighborhood are also common, and may sometimes 
 predominate. In the stagnant portions of the sea-coast, such as 
 the channels behind the dead coral reefs (Keys) of Florida, pre- 
 viously described, and others like them, there is mingled much 
 decaying animal matter with the decaying plant tissues, and the 
 muds are saturated with the product of this decay. It should be 
 clearly understood that such accumulation cannot, as a rule, take 
 place in unenclosed portions of the sea, for the agitation of the 
 waters there will hasten the dissipation of the products of decay, 
 
Accumulation of Decaying Organic Matter 347 
 
 and the universally present bottom animals (worms, mollusks, 
 crustaceans, echinoderms, etc.) will devour the organic material, 
 many of -them passing the sand and mud through their bodies 
 and extracting its organic contents in the process. 
 
 When, however, bottom feeders are scarce or absent, such 
 organic material will accumulate either in pure form, or become 
 
 --'' ".'.:.< ! '-": .'.': "':' >' . "".-'.''*.:-.. 
 
 FIG. 291. Ideal section, showing the approximate relation (i) of the differ- 
 ent types of peat, and (2) the plant societies of Algal Lake, northern Michigan. 
 (After C. A. Davis.) The succession of plant associations from without inward 
 is: (i) Tamarack-spruce-cedar swamp, with young tamarack at the inner 
 border ; (2) open sedge marsh, with islands of tamarack wood ; (3) swamp loose- 
 strife (Decodon verticillatuni) gradually advancing lakeward, and forming " stools" 
 on which grow mosses, ferns, sedges, and shrubs, finally killing the loose-strife ; 
 (4) cat-tail flags ; (5) Potamogeton. The peat formed by these plants thickens 
 away from the lake, and is humus peat. Below this and forming the lake bottom 
 is a mass of sapropelitic peat, composed of green algae, with diatoms, and an 
 abundance of pollen-grains of conifers, forming a structureless mass. 
 
 mingled with muds and other sediments. The purest material is 
 chiefly confined to lake bottoms, where it is largely composed of 
 algae mingled with pollen and spores. In the sea the material is 
 practically always mingled with foreign matter and so in decay 
 forms various grades of sapropelites. The principal regions for 
 the accumulation of such matter are the following : 
 
 Fresh-water Lakes. In these accumulations are chiefly vegetable 
 material (algae, etc.), though some animal matter may also be included. This 
 material may be very pure, forming on decay a sapropel (Fig. 291). 
 
 Channels and Narrow Lagoons between the Land and Fringing Coral Reefs. 
 The material accumulating in these is both vegetable and animal. The plants 
 are partly algae, but the remains of other plants are also included. The animal 
 tissues are those of worms, mollusks, and many other types. There is always a 
 large amount of mechanical sediment in the form of mud, and the product is 
 black bituminous mud- rock or shale (sapropelite), which rests upon the pre- 
 viously deposited limestones derived from the coral reefs, etc. 
 
 Mud-flats Formed in the Process of Filling Lagoons. As we have seen, 
 there is a stage in the formation of marine marshes behind barrier beaches 
 when the eel-grass stage is succeeded by a mud-flat stage, rich in decaying 
 
348 The Organic or Biogenic Rocks 
 
 organic matter, which, with the exception of the eel-grass, is largely of animal 
 origin. Such mud-flats will also form a black bituminous mud-rock, which 
 underlies the coal bed formed by the salt peat if the process of transformation 
 is complete. 
 
 Mud-flats of Estuaries and of River Deltas. In the estuaries of streams, 
 i.e. the broad stream mouths where the river water mingles with the encroaching 
 tide, much fine mud is precipitated, and many sea animals, killed by the fresh 
 water, or river animals, killed by the sea-water, will be embedded in this mud 
 and the decay of their tissues will form sapropelitic material. Such muds are 
 formed on the floor of the Hudson River at New York and in many other 
 estuaries. Sometimes vast numbers of river fish are killed by the advancing 
 sea-water, and these form extensive fish beds, the muds being saturated by the 
 decaying organic matter of these fish. Again, vast masses of floating sea 
 animals, such as Foraminifera, pteropods, etc., may be carried into the estuary 
 and killed by the fresh water, and their remains sink to the bottom, where the 
 decaying organic matter will saturate the bottom sediments. On the mud deltas 
 of great rivers, such as the Mississippi, the Nile, etc., much organic matter, both 
 vegetal and animal, will be buried, part of this coming from the river and part 
 being cast upon the delta from the sea during storms and high water. Chief 
 among this material will be the seaweeds and the animals which live attached 
 to them. In this manner the mud becomes saturated with decaying organic 
 matter, which will be a mixture of coal-producing higher plants and organic 
 slime-producing algae and animal tissues. The mud ot the Nile delta contains 
 only from 5.5 to nearly 8 per cent of organic matter, but that of the Vistula 
 carries as much as 23.3 per cent. This river forms a deposit of black mud, locally 
 called pitch, in the Bay of Danzig. On the Mississippi delta many mud 
 lumps or mud craters are formed, from which large volumes of gas, the product 
 of the decay of the organic matter, escape (see Fig. 126, p. 182). 
 
 Enclosed Stagnant Seas. The Black Sea is an excellent example of a 
 water body which, while still maintaining connection with the sea (Mediter- 
 ranean), is so nearly isolated that its waters are practically stagnant, especially 
 in the lower part. It has been estimated that it takes 1700 years to renew 
 completely the lower waters of this deep basin through the shallow inlets and 
 intermediate salt seas that connect it with the Mediterranean. The upper 
 layers (125 fathoms) are kept fresher by the inflowing drainage from the land, 
 and here many marine organisms live and die. The young of these float in 
 the surface waters, and there are besides many other surface-living animals 
 (plankton) and all of these sooner or later sink to the bottom. There is thus 
 a perpetual rain of organisms descending through the waters of the Black Sea, 
 and these remains accumulate upon the bottom where, because of the stagnant 
 water and lack of oxygen, there are no bottom feeders, only bacteria. These 
 decompose the dead animal matter, and the bottom mud thus becomes richly 
 saturated with these products of decay, in other words, it becomes a rich 
 sapropelite in which, moreover, much sulphur, usually in combination with 
 iron, occurs. 
 
 Highly Saline Lagoons. As we have seen, the waters of lagoons sepa- 
 rated by a bar from the supplying salt water body, in regions of much 
 evaporation, become intensely saline. The currents flowing into such lagoons 
 
Alteration Products from Organic Slime 349 
 
 carry large numbers of animals from the main water body, and these are 
 quickly killed by the brine of the lagoon. In the case of the Kara Bugas Gulf, 
 described in Chapter XI, untold numbers of fish and floating Foraminifera, etc., 
 are carried into the brine from ftie Caspian Sea. As there are no scavengers, 
 i.e. animals devouring dead organic matter, in these brines as is the case in 
 normal sea^ water, the organic matter accumulates and is embedded with the 
 other deposits of the lagoon. This is a prolific source of organic-decay slime 
 and rich deposits of sapropelite are formed. 
 
 ALTERATION PRODUCTS FROM ORGANIC SLIME PRODUCED BY 
 NON-VASCULAR PLANTS AND BY ANIMAL TISSUES 
 
 According to the degree of admixture of mud and other foreign 
 material with the decaying organic matter from non- vascular plants 
 (algae) and from animal tissues, we have a series of deposits which 
 ranges from nearly or quite pure organic slime (sapropel) on the 
 one hand, to mud deposits, impregnated to a greater or less degree 
 with this slime, on the other. This last type we have learned to 
 call sapropelite, and it is by far the most common. Pure deposits 
 of the slime, however, also occur and are known from their alter- 
 ation products, of which cannel coal forms the most important. 
 The sapropelites are generally known as bituminous muds or as 
 bituminous shales when consolidated. The organic matter con- 
 tained in them is, however, often concentrated elsewhere in the 
 form of petroleum and various other hydrocarbons (asphalts) 
 and may separate out as natural gas, etc. We will briefly enu- 
 merate the chief characteristics of each of the important types. 
 
 Cannel Coal. This is generally held to be formed from the fresh- water 
 algae which accumulate in the deeper portions of ponds and swamps, and also 
 often to a large extent from spores, and from the pollen grains of higher 
 plants, blown into these water bodies. Cannel coal has a compact amorphous 
 structure and a dull luster, and commonly a greasy or silky appearance on the 
 fresh surface, a character quite distinct from that of coals formed from the 
 higher plants. This is often well brought out where layers of cannel coal 
 alternate with layers of ordinary bituminous coal, or where the roots of higher 
 plants were embedded in the slime which formed the cannel coal. Such layers 
 of bituminous coal, or such roots of higher plants, when changed to coal, will 
 always have a bright luster in marked contrast with the dull luster of the cannel 
 coal. That animal tissues also add to the slime from which cannel coal is 
 produced is shown by the remains of their hard parts (bones, etc.) in the 
 coal. Thus the cannel coal of Linton, Ohio, has furnished the skeletons of 
 more than fifty species of fishes and amphibians, many of them represented by 
 numerous individuals. Other names given to cannel coal are boghead and 
 
350 The Organic or Biogenic Rocks 
 
 torbanite (Scotland). According to some authorities, the waxy and resinous 
 spores of higher plants are more important in the formation of cannel coal and 
 fresh-water sapropelites than are the algae. They are certainly better pre- 
 served than the algae, and their apparent importance may be more largely due 
 to this fact. 
 
 Jet. This is a deep black, brittle, solid bitumen, segregated in more or 
 less lens-shaped masses in rocks formed of muds which were saturated with 
 organic slime (sapropelites). It appears to be a concentration product of this 
 
 FIG. 292. Cliff of Jurassic (Liassic) sandstones and shales at Whitby on 
 the North Sea coast of England. In these strata the famous Whitby jet is 
 found. 
 
 organic matter and is often found to saturate or replace pieces of fossil wood 
 embedded in the muds or is associated with fish scales and the remains of other 
 organisms. Jet is characterized by its hardness (which is greater than that of 
 asphalt), its conchoidal fracture, and by being less brittle than coal. It is 
 susceptible of a high polish and is much used for ornamental purposes. Im- 
 portant jet-producing rocks are the shales of Whitby, England (Fig. 292), the 
 oily slates of Wiirttemberg, and the similar shales of ancient Lycia in Asia Minor, 
 where the exposure on the Gagas River forms the original locality from which 
 this mineral (often called gagatite or gagates) was obtained. An analysis of 
 jet from Wiirttemberg gave the following result. 
 
 Carbon (C), 71.0% Nitrogen (N), trace 
 
 Hydrogen (H), 7.7% Sulphur (S), trace 
 
 Oxygen (O), 23.3% Ash, 0.9-2.9% 
 
 Asphaltum and Special Varieties of Solid Hydrocarbons. There is a 
 considerable number of solid hydrocarbons found in various parts of the world, 
 and these are probably, for the most part, the products of alteration of organic 
 matter of the non- vascular plants or of animal tissues and therefore members 
 
Alteration Products from Organic Slime 351 
 
 of the sapropel series. A volcanic or other origin, however, has also been sug- 
 gested for some of them. They are generally black and brittle, and distinguished 
 from coals by their fusibility. The general name asphaltum or bitumen is given 
 to these, and their distribution is world-wide. To the pasty, viscid varieties the 
 general name maltha is applied. When sandstones or other rocks are saturated 
 with asphalt, the mixture is called asphalt rock. What is probably the most 
 remarkable occurrence of asphalt is found on the island of Trinidad off the 
 northeast coast of South America, where, in what is regarded as the crater of an 
 old mud volcano or geyser, occurs the famous " Pitch Lake " of Trinidad, the 
 material of which is an emulsion of water, gas, bitumen, and some other organic 
 substances and mineral matter. The water is saline and contains also borates 
 and ammoniacal salts; the gas is principally hydrogen sulphide and carbon 
 dioxide. The bitumen is high in sulphur, the composition when purified being, 
 
 Carbon (C), 82.33% Sulphur (S), 6.16% 
 
 Hydrogen (H), 10.69% Nitrogen (N), 0.81% 
 
 In other hard asphalts the sulphur has been found to range from 3.28 to 9.76 
 per cent, and in soft asphalts or malthas from 0.6 to 2.29 per cent. 
 
 Some of the more important varieties of solid bitumen are (a) Ozokerite, 
 occurring in the Tertiary rocks of the Caucasus, the Carpathians, and in Utah, 
 
 FIG. 293. A typical view of an oil field, showing derricks and storage tanks. 
 
 and forming an important source of paraffin; (b) Albertite and (c) Grahamite, 
 similar hydrocarbons occurring in vein-like fissures in the country rock in New 
 Brunswick and West Virginia, respectively, having probably been injected when 
 in a fluid state, (d) Uintaite or Gilsonite, also a black, brittle and lustrous 
 mixture of hydrocarbons found in the Uinta Mountains of Utah and in many 
 other localities.. 
 
 Petroleum (Fig. 293). This takes first rank among the products of decay or 
 distillation of the organic slime, but may also be formed by the distillation of the 
 coaly deposits from vascular plants. Nevertheless, the organic matter of sapro- 
 pelites (the sapropel) i.e. algae and spores among plants, and animal tissues 
 
35 2 The Organic or Biogenic Rocks 
 
 probably forms the chief source of petroleum. Different petroleum deposits 
 have a different origin, and those of the several geological horizons will be referred 
 to again in their proper place in the section of this book dealing with Historical 
 Geology. The oldest (Palaeozoic) petroleum deposits of North America 
 probably were derived from bituminous shales formed either in mud-flats or in 
 delta deposits (Trenton-Utica oils), in lagoonal deposits, behind coral reefs 
 (Onondaga-Marcellus oils), or in estuaries (later Devonian and younger Palae- 
 ozoic oils), though an origin in enclosed basins of the Black Sea type has also 
 been suggested for some of these. Although the source of the oil is the bitu- 
 minous mud-rock, the accumulation takes place in more porous limestones or 
 sandstones (oil sands) which are associated with them, and abounds only where 
 special structures (anticlines, domes, etc.) furnish the proper conditions. In 
 the following diagrams (Fig. 294) the relationship of the bituminous shales 
 (the oil producers) and the limestones or sandstones (the oil storers) is shown. 
 
 FIG. 294. Diagram showing the relationship of the black sapropelitic shales, 
 the oil producers (fine lines), to the limestone beds on the left and the sands on 
 the right. If the former contain porous members, or areas of dolomitization 
 (dotted and blocked) these will form oil pools, the oil passing laterally into the 
 reservoir rock. The sandstones on the right will also receive oil from the 
 shales, but unless there is a capping-rock, this oil will become dissipated. 
 
 As will be noted, the oil will have to pass laterally into the more porous rock 
 which replaces the bituminous mud-rock, from the organic matter of which the 
 oil is formed. 
 
 Many if not most of the petroleum deposits in the younger (especially 
 Tertiary) rocks of America and Europe appear to be derived from the alteration 
 of the organic material of algae, especially diatoms, and the flesh of animals, 
 ranging from protozoans to fish or even higher types. In nearly all of the 
 great deposits, these organisms appear to have been carried into enclosed 
 bodies of water such as the Black Sea or the Kara Bugas Gulf, where, owing 
 either to the stagnant character of the water or to its high salinity, they perished, 
 their organic tissues accumulating in the sediments because of the absence in 
 these waters of scavengers or animals feeding upon such matter. Thus the 
 California oils are believed to have been derived from the soft tissues of marine 
 diatoms which accumulated in vast quantities under special conditions which 
 insured the non-consumption or incomplete dissipation of the decaying organic 
 matter. Fresh-water diatoms accumulating in lakes also have furnished such 
 materials. The oil of the Caucasus and perhaps of some of the Carpathian 
 regions may be largely derived from the organic matter of fish killed in large 
 quantities in the stagnant or highly saline waters of embayments or basins of 
 the Black Sea type, as were the oil and asphalt of the Alsace-Lorraine region 
 and that forming to-day in the vicinity of the Kara Bugas Gulf, where vast 
 numbers of fish are constantly killed, as already described. 
 
Alteration Products from Organic Slime 353 
 
 Finally, some of the oil of the Carpathian region may have originated 
 from the Foraminifera and other floating (planktonic) organisms carried into 
 similar lagoons, in some of which salts were being deposited. 
 
 It must also be mentioned that petroleum has been held to be, in some cases 
 at least, of inorganic origin, formed by descending waters which came in contact 
 
 with heated metallic carbides in 
 
 the interior of the earth's crust, 
 the reaction there resulting in 
 the formation of metallic oxides 
 and the liberation of hydro- 
 carbon and of carbon dioxide as 
 the final product. This theory, 
 proposed by the Russian chemist, 
 Mendelieff, is, however, very 
 generally discarded in favor of 
 the theory of organic origin, 
 which is fully supported by the 
 facts known in the cases of most 
 large petroleum deposits. 
 
 Natural Gas. This is es- 
 sentially similar in origin to 
 petroleum, representing the 
 volatile hydrocarbons. It is 
 commonly associated with pe- 
 troleum, but may also occur 
 where this is not found. 
 
 Bituminous Shales, Oil 
 Shales. These represent the 
 solidified muds of lagoons, stag- 
 nant basins, mud-flats, etc., 
 which are impregnated with 
 finely disseminated organic mat- 
 ter. Although the names are used somewhat loosely, the term bituminous shale 
 is best applied to those carrying finely divided, partly decomposed remains of 
 vascular plants, while the oil shales are saturated with the product of decay of 
 non-vascular plant and animal tissues, i.e. the true sapropel, and these form 
 the true sapropelites. Bituminous shales thus pass into coal deposits, but 
 petroleum may also be derived from them. The sapropelites, however, appear 
 to form the true and important oil shales. 
 
 FIG. 295. A gushing oil-well. 
 
CHAPTER XIV 
 
 ATMOSPHERIC PRECIPITATES AND THEIR 
 DERIVATIVES 
 
 TYPES or ATMOSPHERIC PRECIPITATES 
 
 THE atmospheric precipitates include rain, hail, and snow; 
 their derivatives are ice and the special form of the latter the 
 glaciers. There are other atmospheric precipitates, such as nitro- 
 gen compounds formed by electrical discharges, etc., but these need 
 not be considered here, though they may at times become of 
 importance. 
 
 Materials in the Atmosphere 
 
 As has been stated in an .earlier chapter, the atmosphere is a 
 mixture of oxygen and nitrogen and contains, besides minute 
 quantities of other substances, a fairly definite amount of carbon 
 dioxide (CO 2 ) and a variable quantity of water vapor (H 2 O). 
 The carbon dioxide is taken from the atmosphere by the higher 
 plants, which, by means of the leaf-green (chlorophyll) in their 
 cells, can, under the influence of the sunlight, decompose the car- 
 bon dioxide, so that the carbon can be utilized to build up the 
 tissues of the plant while the oxygen is given off again through the 
 breathing pores. Carbon dioxide also combines with certain min- 
 erals in the rocks of the earth, but no direct precipitation of this 
 substance takes place, nor are any notable compounds formed in the 
 air that are precipitated. 
 
 If we omit the precipitates formed by gaseous emanations from 
 volcanoes, the only important substance which returns to the 
 surface of the earth from the atmosphere is the water held there as 
 invisible vapor and the various forms which this compound assumes. 
 
 The Water of the Atmosphere 
 
 Sources of Water Vapor. The great source whence the mois- 
 ture of the air is derived is the sea, and the process of translation of 
 this water into vapor form is evaporation. Evaporation, however, 
 
 354 
 
Types of Atmospheric Precipitates 355 
 
 takes place all over the lands as well, where not only lakes and 
 rivers, but also the moisture of the soil is drawn upon to supply 
 the air with water vapor. Vegetation takes the moisture from the 
 ground, and this, circulating through the plants, is in part evapo- 
 rated from the leaves, sometimes at such a rate that the plant 
 droops. 
 
 Absolute Humidity of the Air. The total amount of water 
 vapor in the air marks its absolute humidity. This, however, 
 signifies little, for the same amount of vapor in the air at high 
 temperature will leave that air very dry, while at low temperature 
 it will be moist. 
 
 Saturation of the Air. When the maximum amount of water 
 which the air can hold at any given temperature is reached, the 
 air is said to be saturated. A cubic foot of air can hold half a 
 grain of water at o F., at 60 it can hold 5 grains, and at 80 it 
 can hold 1 1 grains. The air of a room 40 by 40 by 15 feet in dimen- 
 sions can hold nearly 20 pounds of water when the temperature is 
 60 F., or nearly enough to fill a common water pail. At 80 F. 
 it can hold more than twice that amount. When the saturation 
 point of the air is reached, the slightest fall in temperature will 
 cause precipitation. This critical point is called the dew point. 
 
 Relative Humidity. Ordinarily the air holds less water than it 
 is capable of holding at that temperature. The ratio in per cent 
 between the quantity of water which the air actually contains 
 and the maximum amount which it could contain at that tem- 
 perature marks its relative humidity. Thus in the case of the 
 room cited above, if the amount of water which the air can contain 
 is 20 pounds, but the amount which it actually contains is only 
 10 pounds, the relative humidity is 50 per cent. At saturation 
 (20 pounds in the example cited) the relative humidity is 100 per 
 cent. 
 
 The average relative humidity of the air over the land is about 
 60 per cent, while over the oceans it is about 85 per cent. It is, of 
 course, not uniform, especially over the land, where great variation 
 exists. When the relative humidity is below 65 per cent, the air 
 is said to be dry, no matter what its temperature. Thus it appears 
 that regions of dry air are far more extensive than those of moist, 
 to make the average humidity 60 per cent, or the excessive dry ness 
 of some regions greatly lowers the general average, in spite of 
 the fact that some localities have a very high humidity. In semi- 
 
356 Atmospheric Precipitates and their Derivatives 
 
 arid regions the relative humidity ranges around 45 per cent, and 
 in desert regions between 25 and 30 per cent. The lower the 
 relative humidity, the greater is the evaporation from the surface 
 of the earth. 
 
 Precipitation of Atmospheric Moisture 
 
 When the air is saturated, the slightest drop in temperature will 
 inaugurate precipitation, because with it the capacity of the air 
 for holding water is decreased, and the excess above saturation 
 must be eliminated. If the temperature of the air is above freezing 
 point, this precipitation will take the form of rain ; if below, it will 
 be snow or hail. Clouds merely represent the preliminary stage 
 of separation of the water vapor from the atmosphere. 
 
 The rain water, in so far as it is not re-evaporated, either runs 
 off as surface water in the form of rivulets which unite to form larger 
 streams (the run-off), or it sinks into the ground to become a part 
 of the subsurface water of the hydrosphere. The snow, however, 
 generally accumulates where it falls, except for modification by the 
 wind (snow drifts) and becomes a permanent cover of unconsoli- 
 dated material at least for a period of time, if the temperature of the 
 air in contact with it is below the freezing point. If it is above 
 that, snow melts or evaporates, and the water produced by melting 
 becomes a part of the hydrosphere, running off as surface water or 
 sinking in, to become subsurface water. If the air becomes dry, 
 i.e. if its relative humidity falls or a dry wind blows across the snow 
 field, direct evaporation of the snow takes place, that is, it will 
 pass from the solid directly to the vapor stage. As a result of such 
 evaporation the surface of the snow is commonly pitted or marked 
 by shallow hollows or concavities. 
 
 The Snow-line 
 
 The lower limit of the permanent snow fields of the earth con- 
 stitutes the snow-line which, in general, corresponds to the line 
 which connects the points where the mean summer temperature 
 of the air is o C. ( +32 F.). There are, however, many modifica- 
 tions of this relation, due chiefly to the amount of precipitation 
 during the winter months, the amount of sunlight, the course of 
 drying winds, the steepness of the slope, the altitude, and the 
 protection in ravines and shady valleys. The height of the snow- 
 line above sea-level varies in general with the latitude. In the 
 
Glaciers 357 
 
 Bolivian Andes, near the equator, it is 18,500 feet on the western 
 side and 16,000 feet on the eastern. In Lapland (latitude 70 N.) 
 it lies 3000 feet or more above the sea, and in Greenland (lat. 6o- 
 70 N.) about 2200 feet. In higher latitudes it approaches sea-level. 
 The lower limit of snow fall, and the snow-line, or lower limit 
 of permanent snow, do not of course, correspond. In latitude 
 40 N. snow falls at sea-level, but the permanent snow-line is on the 
 average 3000 meters higher. Mountains in this latitude, below 
 that elevation, have permanent snow only in protected ravines. 
 In the tropics the difference in altitude between the lower limit of 
 snow fall and the snow-line is much less. 
 
 COMPACTING AND MODIFICATION or SNOW 
 
 Granular Snow. When precipitated, snow is a loose aggregate 
 of needles and flakes of crystalline form and structure. This 
 crystalline character is, however, soon lost by partial melting and 
 evaporation, and a fine granular powder results, this representing 
 the first stage in alteration. 
 
 Firn or Neve. When the grains of snow become loosely held 
 together, or united by a cement of ice, the neve or firn is produced. 
 The aggregate thus formed is full of air bubbles, and it represents 
 the second stage in modification (Fig. 304, p. 367). 
 
 Snow Ice or Glacier Ice. In this substance the change has 
 progressed so far that the mass has become a granular crystalline 
 ice in which the individual crystalline ice grains range from the size 
 of a pea near the firn border, to that of a hen's egg in the lower 
 portion. The crystals are intimately united, so that in fresh ice they 
 cannot be distinguished. Ice is formed slowly from snow by a 
 process of compacting which eliminates the air spaces of the firn 
 or neve and brings the crystals in close contact. This may be 
 produced by pressure, by percolating waters which by freezing 
 replace the air cavities, or by the growth of the new crystals them- 
 selves from the moisture due to evaporation of the smaller crystals, 
 or that resulting from partial melting. 
 
 GLACIERS 
 
 Glaciers are produced by the spreading or out-flow from the 
 center of accumulation of the ice which has resulted from the 
 compacting of the snow. According to the mode of occurrence 
 
358 Atmospheric Precipitates and their Derivatives 
 
 of such spreading ice masses, we may recognize (i) true glaciers, i.e. 
 valley or mountain glaciers, (2) confluent or piedmont glaciers, 
 (3) ice caps, and (4) glacial sheets or continental glaciers. 
 
 True Glaciers 
 
 True glaciers are confined to more or less definite channels, 
 bounded generally by rock walls and comparable in many respects 
 to streams of water (Fig. 296). Such ice streams may reach a 
 
 FIG. 296. Ideal glacier landscape. A, firnorneve; B, mouth of glacier 
 tunnel; C, glacial stream; a, lateral moraines; b, medial moraine; c, glacial 
 table ; d, terminal moraine. The ridges between the two glaciers are serrated 
 forming aretes or grats. (After F. Simony; from Ratzel, Die Erde.) 
 
 length of fifty miles or more, as in the great Seward glacier of 
 Alaska, the main feeder of the Malaspina glacier (see map, Fig. 320, 
 p. 381) in which the width at the narrowest point is three miles 
 (Fig. 321, p. 382). Most of the alpine glaciers, on the-other hand, 
 are not over a mile long, though the largest, the Aletsch Glacier, 
 is more than ten miles in length. The thickness of the alpine 
 glaciers rises in some cases to 800 and in exceptional ones to 1200 
 feet. 
 
 The most typical glaciers are found upon the mountain sides 
 and in well-defined mountain valleys, the best known examples be- 
 ing in the Alps. They are commonly called valley or mountain 
 glaciers, the latter term applying especially to the short glaciers 
 
Glaciers 359 
 
 which lie in the depressions in mountain sides, these depressions 
 being often due to the erosive work of the glaciers themselves. 
 Typical valley glaciers, also called dendritic glaciers, because they 
 usually have many branches, most commonly occupy large structural 
 troughs in the mountain regions, especially in high latitudes, though 
 they may also fill old erosion valleys. The course of a typical valley 
 glacier may be straight, but is more often a winding one. It may 
 
 FIG. 297. Abrupt front of an Alaskan glacier, with a deglaciated valley in 
 front partly occupied by a glacial stream. (Seward division, Gov't R. R. ; by 
 courtesy of Alaska Engineering Commission.) Compare Fig. 314, p. 376, 
 abrupt glacier front on coast. 
 
 be simple throughout, or two or more streams may unite to form a 
 main trunk, as in the case of the junction of several rivers into a 
 main trunk stream, while, moreover, the main glacier may receive 
 lateral tributaries. According to location, altitude, and other 
 causes, the lower end of the glacier may be more or less abrupt 
 (Fig. 297) or it may spread out upon the flat foreland to form a fan 
 glacier, which by the union with similar fans of neighboring ice 
 streams forms a piedmont glacier. Examples of the several types 
 will be described in some detail. 
 
 The Great Aletsch Glacier of the Alps (Figs. 298, 299). In 
 the Bernese Alps of central Switzerland, where the Jungfrau, the 
 Aletschhorn, and other great peaks dominate the landscape, we 
 find a great center of modern alpine glaciation. From the southern 
 
360 Atmospheric Precipitates and their Derivatives 
 
 FIG. 298. Map of the Aletsch Glacier and the surrounding territory in 
 the Bernese Alps of Switzerland. From the Swiss Government map. 
 
Glaciers 
 
 361 
 
 and eastern sides of these mountain ranges descends the Great 
 Aletsch Glacier toward the valley of the Rhone, without, however, 
 reaching it. This glacier, which in many respects is the finest in 
 
 the Alps, occupies a mountain valley for about ten miles of its length, 
 and is bordered, for the most part, by high ranges and peaks on 
 either side, the largest of which is the Aletschhorn (4182 meters or 
 
362 Atmospheric Precipitates and their Derivatives 
 
 13,768 feet high, 132 feet higher than the Jungfrau) from the base 
 of which tributary glaciers join the main ice stream (Fig. 298). 
 
 Valley of Viesch 
 
 FIG. 300 a. Diagrammatic section, showing the relative position of the 
 Marjelen Lake, the Aletsch Glacier which holds it and the valley of the Viesch 
 into which it drains when full, a, b, col or dividing ridge between the two 
 valleys; c, vertical cliff of ice forming the dam. (After Lyell.) 
 
 On its eastern border a tributary valley, partly free from ice, is 
 dammed by the glacier of the main valley, and behind this ice 
 dam lies the famed and beautiful Marjelen Lake, a typical example 
 
 FIG. 300 b. Section of glacier lake called the Marjelen See. a, b, c, terrace 
 of detrital matter formed on the margin of the lake when full ; d , surface of the 
 lake 40 feet below its usual level ; e, mass of floating ice with included stones 
 detached from the dam; /, boundary hill composed of mica schist. (After 
 Lyell.) 
 
 of a lake produced by ice-damming of a valley (Fig. 299). This 
 lake is periodically drained, partly or completely, every three 
 to five years by the opening of some outlet under the ice, with dis- 
 astrous floods in the valley below. Ordinarily, however, it is held 
 
Glaciers 363 
 
 in by a precipitous wall of ice of the Aletsch glacier. After drainage 
 it fills up in about a year to a level determined not by the height 
 of the glacier dam, which always rises much above the lake, but 
 by the water-shed or col which separates the lake from the valley 
 of the Fiesch (or Viesch) glacier on the east into which it drains 
 when full (Fig. 300 #). Around the margin of this lake, terraces or 
 beaches of sand and gravel are built, which are exposed when the 
 lake is drained. A characteristic section of such a beach (Fig. 300 b) 
 shows a surface shelf, sloping from 5 to 15 toward the lake, and 
 about 1 6 paces wide. At the edge of this slope, which marks the 
 level of the lake when full, there is a sharp change, the beds de- 
 
 FIG. 301 a. The Parallel Roads of Glen Roy. (W. Lamond Howie.) 
 
 scending at an angle of 29. This is the underwater slope of the 
 deposit, and represents the angle at which the beds (fore-sets) 
 are laid down. The sands are bedded, but contain no organic 
 remains, since the temperature of the water is always near freezing. 
 Over the beach and bottom are scattered many large blocks of 
 stone left by icebergs which break from the main glacier wall. 
 
 In one of the glens in the Scottish Highlands, known as Glen 
 Roy, there are several old beach lines at successive levels, along the 
 sides of the glen, which are known under the name of the Parallel 
 Roads of Glen Roy (Fig. 301 a, 6). The natives regard these as 
 roads made by the "gentry" for fishing purposes when the glen was 
 a lake. A study of the glen has shown that it was once blocked 
 by a glacier at the lower end, as is the Marjelen Lake to-day, and 
 that it was filled by a lake, the several levels being due to the 
 
364 Atmospheric Precipitates and their Derivatives 
 
 FIG. 301 b. Photograph of one of the terraces 
 or "parallel roads" in Glen Roy, Scotland, showing 
 the distinct notch which it forms upon the side of 
 
 , , j , j if r 
 
 the glen, and the dense growth of peat-forming 
 vegetation ; largely heather. (Photo by author.) 
 
 progressive opening of cols permitting drainage, at successively 
 lower levels, into other valleys after the manner of the drainage of 
 the Marjelen Lake into the Viesch Valley, the col of which deter- 
 
 mines the height of 
 the lake and the 
 beaches (Fig. 302). 
 
 From the foot of 
 the Aletsch Glacier 
 issues the turbulent 
 Massa River, which 
 joins the Rhone sev- 
 eral miles below, and 
 more than doubles 
 the volume of that 
 stream. It has been 
 sai ^ th at other gla- 
 ciers send forth tor- 
 r^fc e rr .. f i, Q ,v~ 
 
 1CJ.1LS llVJlil LUC ICC 
 
 caverns at their foot ; 
 
 this one alone pours 
 out a river. The Rhone carries down to Naters the drainage of 
 its own glaciers supplemented by a dozen other ice-fed streams; 
 yet these combined waters are far exceeded in volume by those 
 brought by the single stream from the Aletsch Glacier. It has 
 been questioned, indeed, whether the united torrents of any four 
 glaciers in the Alps could equal that single stream. 
 
 A fine view of the glacier is obtained from the summit of the 
 Eggishorn (Fig. 299), a peak just south of Marjelen Lake and 
 opposite the tributary Aren or Middle Aletsch glacier, which heads 
 in an amphitheater in which lies the middle Aletsch neve, at the 
 eastern foot of the Aletschhorn (see map, Fig. 298). Farther down, 
 on the same side, is the little Trist or Upper Aletsch Glacier or 
 glacieret, which at present does not reach the main ice stream but 
 rests in a valley, the floor of which is high above the surface of the 
 great glacier. This little glacier is therefore called a hanging 
 glacier or glacieret, being hung, as it were, upon the valley side of 
 the great one. In former times this did, however, extend to the 
 main glacier. It heads in the upper Aletsch neve south of the 
 Aletschhorn, and another one, the Beichfirn or neve, is also tribu- 
 tary to it. These neves lie in more or less horseshoe-shaped valleys 
 
Glaciers 
 
 365 
 
366 Atmospheric Precipitates and their Derivatives 
 
 or cirques, and between them rise the pyramidal peaks of triangular 
 base to which the designation horn is applied. 
 
 The great Aletsch Glacier itself heads in a group of large neves 
 which extend north of the Dreieckhorn and Aletschhorn and 
 toward the foot of the Jungfrau. On the east lies the Wannehorn, 
 the western face of which is marked by a number of small 
 cirques, the neves of which are tributary to the Great Aletsch 
 Glacier. 
 
 FIG. 303. Typical glacial cirque with large amount of moraine deposited 
 in foreground. -Massif de Pelvoux, Alpes d'Oisaus, France. (Courtesy of 
 Prof. D. W. Johnson.) 
 
 These cirques, when well developed (Fig. 303), are semicircular 
 or amphitheater-shaped, with rough, precipitous walls, and com- 
 paratively smooth floors filled with neve. Their precipitous walls 
 are due to the plucking and sapping action of the ice which freezes 
 to the blocks and in the cracks at the base of the cliff, and under 
 the influence of partial melting and refreezing pushes away from 
 the wall, carrying the plucked or quarried blocks with it. As the 
 ice moves away from the walls of the cirques a crevasse comes 
 into existence between the rock wall and the ice. This crevasse, 
 called the bergschrund, may vary in width from two or three to 
 more than eighty feet, and in depth to 150 feet or more. At the 
 
Glaciers 
 
 367 
 
 bottom, much disruption of rock takes place by the freezing of 
 the water which drips into it. 
 
 In mountains which were formerly covered by glaciers, such 
 cirques are characteristic features, and their presence, readily 
 
 FIG. 304. Glacier of Three Sister peaks, Oregon, between West and North 
 Sister (the latter shown in the view). Showing junction of glacier and neve" 
 and marginal crevasses. (Photo by I. C. Russell, Aug. 18, 1903, U. S. G. S., 
 by courtesy of Popular Science Monthly.} 
 
 recognized by their peculiar form, is in itself a proof that such 
 mountains formerly held glaciers, even though these have entirely 
 disappeared. As cirques are progressively cut backwards into the 
 mountains, their walls increase in height, and if cirques approach 
 each other from opposite directions, the space between them is 
 narrowed, and eventually only a sharp, narrow, serrated ridge 
 
368 Atmospheric Precipitates and their Derivatives 
 
 remains between them, from which arise the sharp horns (Fig. 296). 
 
 Later still, parts of the ridge will be lowered to form a connecting 
 
 col between two cirques (see further, section on glacial erosion in 
 
 Chapter XXIII). 
 
 The neves of the Aletsch really occupy three large and many 
 
 small cirques or embayments in the mountain complex. On the 
 
 west between the Dreieckhorn and the 
 Gletscherhorn, lies the great Aletsch 
 neve which also feeds the Lang Glacier 
 descending to the southwest. On the 
 northwest lies the Jungfrau firn or 
 neve, which is bounded by the Glet- 
 scherhorn, Jungfrau, and the Monch on 
 the west and north, and the Trugberg 
 ridge on the east. The Ewigschnee 
 Feld to the east of the Trugberg ridge 
 completes the trinity, while a fourth 
 smaller one north of the Faulberg de- 
 scends from the Griinhorn Liicke on 
 the east. Crevasses are not uncom- 
 mon on these neves, but the berg- 
 schrund is not everywhere developed. 
 In some cases the neves connect across 
 the cols or passes with others which 
 feed glaciers descending in other direc- 
 tions. 
 
 FIG. 305. Section of an 
 old glacial pot-hole filled with 
 debris, Christiania. (After 
 Brogger.) 
 
 The surface of the Aletsch glacier, which is about two miles 
 wide, is often almost free from stones and rock debris except along 
 its sides, where the material falling from the bounding cliffs forms 
 lateral moraines. This glacier, unlike most other large glaciers of 
 the Alps, has as a rule no medial moraine or only a feeble one. 
 Along the center, the ice is mostly solid, though narrow fissures or 
 crevasses frequently open in it. Such crevasses are much more 
 pronounced, however, on its eastern margin, which has a convex 
 form (see also Fig. 304). The melting of the ice upon the surface 
 produces pools and streams, which occasionally combine into 
 small rivers. These tumble into the crevasses and produce vertical 
 chimneys or moulins, which carry the waters to the bottom of the 
 glacier where, by their whirling motion, they often cut " pot- 
 holes " in the bed of the ice stream. In regions from which glaciers 
 
Glaciers 
 
 369 
 
 have departed, such pot-holes are sometimes seen filled with 
 debris (Fig. 305). 
 
 The Mer de Glace (Figs. 306, 307). From the northern slope 
 of the Mont Blanc glacial field on the Franco-Swiss border descends 
 the glacier Mer de Glace 
 toward the Chamonix Val- 
 ley. At the head of the 
 glacier is a complex of 
 cirques, which radiate out- 
 ward from the stem glacier 
 to which they are tribu- 
 tary, as the veins of a 
 maple leaf radiate from the 
 petiole. From these 
 cirques, short glaciers or 
 glacierets unite to form the 
 trunk glacier, which flows 
 toward the Chamonix 
 Valley. This is the more 
 common type of glacier 
 found in the Alps, and 
 though the branches are 
 given different names, they 
 are, in a manner, only 
 tributary heads of one prin- 
 cipal stream. These smaller 
 glaciers commonly have a 
 much steeper slope than 
 the trunk glacier, and many 
 of them suggest ice cas- 
 cades, being indeed so 
 named, as for example the 
 Cascade du Talefre, which 
 joins the Mer de Glace 
 
 FIG. 306. Map of the Mer de Glace, 
 French Alps, showing four medial moraines 
 and the terminal glacial stream, the Ar- 
 veyon. (After Le Conte.) The feeding 
 glaciers are : on the right, Glacier du Geant, 
 with ice cascades and seracs, and its tribu- 
 tary, the Glacier des Periades; in the 
 center, the Glacier de Leschaux ; and on the 
 left, the Glacier de Talefre also showing 
 ice cascades and seracs. 
 
 from the east, and the 
 Cascade of the Glacier du Geant on the west (see map, Fig. 306). 
 Such steep glaciers, occupying the depressions in the mountain 
 sides, are more properly termed mountain glaciers, and they are 
 the most common among the 2000 or more glaciers of the Alps, 
 most of which are, however, less than a mile in length. 
 
 v 
 
370 Atmospheric Precipitates and their Derivatives 
 
 Each of these tributary glaciers has its lateral moraine of rock 
 debris, partly derived from the lateral wall, but also in part brought 
 up from below (see page 493) by movements within the ice. As 
 two streams unite, the lateral moraines on adjoining sides combine 
 to form a medial moraine, a more or less continuous line of rock- 
 fragments, sand, etc., which stretches along the central portion 
 of the confluent glacier thus formed. In the Mer de Glace there 
 are several such medial moraines forming parallel lines upon the 
 trunk glacier, which, moreover, has its own lateral moraines. 
 
 r -^ 
 
 FIG. 307. The Mer de Glace near Chamonix, French Alps. Note the 
 snubbed lower ends of the mountain ridges indicating the beginning of the U-- 
 shaped glacial valley form. (Photo D. W. Johnson.). 
 
 At the foot of the glacier where the ice undergoes melting, this 
 material comes to rest to form the terminal moraine, which is also 
 built in part of material carried along within or on the bottom of 
 the ice (englacial and subglacial detritus). From the melting of 
 the ice, glacial streams arise, these heading often far back in 
 tunnels under the ice as subglacial streams. 
 
 Other Glaciers of Similar Character. Though the glaciers of 
 the Alps are the most familiar, and in many respects best-studied 
 there are many others in different parts of the world which show 
 the characters so far described on a much larger scale. In the 
 Karakoram Himalayas, several long and narrow valley glaciers are 
 
Glaciers 
 
 found, which, moreover, have numerous lateral tributaries similar 
 to streams of water. These form the so-called dendritic or true 
 valley type, of which the 
 Great Aletsch is a small 
 example with few tribu- 
 taries. Of these larger ex- 
 amples, the Hispar Glacier 
 (Fig. 308) has a length of 
 over 36 miles, and is char- 
 acterized by numerous trib- 
 utaries on both sides, which FlG 30 g _ Hispar Glacier> a typical 
 join it approximately at valley glacier, with numerous tributaries, 
 right angles. The main Karakoram, Himalayas. (After Martin 
 
 Conway.) Ice and streams in black 
 
 glacier here is very straight. dramage territory in dotted line. 
 The Baltoro Glacier of the 
 
 same region and of similar length occupies a curving valley, and 
 its tributaries also join mostly at right angles. They supply 
 
 Miles: 
 
 FIG. 309. Three of the largest ice tongues of the Swiss Alps superposed on 
 the same scale over Hubbard Glacier, Alaska. (Canadian Geol. Survey.) 
 
372 Atmospheric Precipitates and their Derivatives 
 
Glaciers 
 
 373 
 
 morainal material from different sources, and as a result this glacier 
 has 15 moraines of different colors. The Tasman Glacier of New 
 Zealand is another example. The Hubbard Glacier ?f Alaska 
 (location: map, Fig. 320, p. 381) exceeds the combined area of 
 the Aletsch, Rhone, and Mer de Glace, as shown in the diagram 
 on page 37 1 (Fig. 309). 
 
 Characters of the Glacial Valley. When such glaciers shrink 
 with a change in climate, or disappear entirely, their beds become 
 exposed and show characteristic features. The form of the valley 
 
 FIG. 311. Rounded rock surfaces or Roches Moutonnecs, due to erosion by 
 a former glacier, Colorado. (After Hayden.) 
 
 often resembles in section the letter U, with the sides approximately 
 perpendicular and the bottom rounded (Fig. 310). This is due 
 to the erosive action of the ice and is the characteristic form of a 
 young glaciated valley or one deepened by glacial work. Many 
 ancient valleys in regions now entirely free from ice have this 
 form, and point to former glacial occupancy and erosion or deepen- 
 ing by ice (see also Chapter XXIII). 
 
 The eroded bottom of such a valley often shows hummocky 
 surfaces, sloping and smooth on the side from which the glacier 
 moved (stoss-side) and with striated surfaces, but rough and 
 
374 Atmospheric Precipitates and their Derivatives 
 
 cliff ed on the side away from the movement (lee side). Such 
 rock surfaces when seen from above have a fanciful resemblance 
 to a flock of crouching sheep, on which account the French have 
 called them roches moutonnees, a name which has been generally 
 adopted for such erosion surfaces (Fig. 311). They are not con- 
 fined to glaciated valley floors, but occur in regions of continental 
 glaciation as well. 
 
 Where tributary valleys join the main valley they are generally 
 found to do so at a point much above the floor of the latter. 
 The lateral valleys are indeed hanging valleys, there being an 
 
 abrupt descent from 
 the valley bottoms at 
 their mouths to the 
 bottom of the main 
 trough. This is due to 
 the fact that the main 
 ice stream deepens its 
 valley more readily 
 than the smaller lateral 
 glaciers deepen theirs, 
 and although the sur- 
 faces of lateral glaciers 
 ussually accord with 
 that of the main stream 
 to which they are tribu- 
 tary, .their bottoms do 
 
 FIG. 312. Diagram to illustrate the relation- 
 ship of main and tributary glaciers. The sur- 
 faces of the two glaciers are in accord, but the 
 valley of the main glacier is deepened much 
 below that of the tributary glacier. On the 
 melting of the glaciers the valley of the tribu- 
 tary glacier will be a hanging valley, its mouth 
 joining the main valley at some height above 
 its floor. A river flood-plain deposit is formed in 
 the main valley after abandonment by the ice, 
 while an alluvial fan forms at the mouth of the 
 tributary hanging valley. (After Davis.) 
 
 not correspond (Fig. 
 
 312). Along the borders of the great fjords in the " inside passage " 
 to Alaska, there are very many fine examples of such hanging 
 valleys. These fjords themselves represent the old valleys of the 
 main or trunk glaciers, now abandoned by the ice but filled instead 
 by water from the sea. (See further, Chapter XXIII.) 
 
 Movement of Mountain and Valley Glaciers. Experiments 
 have been made on the Aar Glacier, in the north of the Bernese 
 Alps, where Agassiz conducted his pioneer studies, on the glaciers 
 at the head of the Rhone valley and on others, to determine 
 the character and rate of movement of valley and mountain 
 glaciers. A line of stakes driven into the ice across the glacier 
 was accurately located by instruments with reference to the rock 
 walls. Measurements of their advance were made at intervals to 
 
Glaciers 
 
 375 
 
 determine the rate and mode of progress. A significant fact 
 discovered was that the center of the ice stream moved faster 
 
 FIG. 313. Map showing the movement of the ice of the Rhone Glacier in 
 Switzerland, between the years 1874 anc * 1882 at B, C, and D ; the retreat of the 
 ice front by melting during the same period, and the several terminal moraines. 
 (After Heim; from Chamberlin and Salisbury, Geology. By permission of 
 Henry Holt & Co.) (See also Figs. 319 a and b, p. 380.) 
 
 than the sides, for the alignment of the stakes, originally straight, 
 became more and more curved downstream at the center. This 
 is well shown in the preceding map of the Rhone Glacier from 
 
376 Atmospheric Precipitates and their Derivatives 
 
 which the Rhone River arises (Fig. 313)', the movements indicating 
 the advances between the years 1874 and 1882. The fluctuation 
 in the position of the front of the glacier is also shown, and the 
 locations of the terminal moraines at earlier periods are indicated. 
 In general, the rate of movement of Swiss glaciers ranges from 
 one or two inches to four feet or more a day, but glaciers in other 
 parts of the world have shown a greater rate of advance. Thus 
 the Muir Glacier of Alaska (Fig. 314) has moved at the rate of 
 seven feet or more a day, while some Greenland glaciers have 
 
 FIG. 314. Front of Muir Glacier, Glacier Bay, Alaska, from the east. In 
 the distance is Morse Glacier. (U. S. G. S. ; courtesy of Prof. D. W. Johnson.) 
 (See also Fig. 318, p. 379.) 
 
 moved in summer time as much as 50 or 60 feet a day. These 
 glaciers are, however, of a different type, being tongues from an ice 
 mass of great extent. 
 
 The top of the glacier moves, on the whole, faster than the 
 bottom, as was shown by a line of stakes driven into the side wall 
 of a glacier in a favorable spot. Here, after a while, the alignment 
 of the stakes changed from vertical to forward sloping at the top. 
 
 Surface Features Due to Ablation. The surface of a glacier is 
 affected by the heat of the sun and by drying winds, with the 
 result that irregularities are produced by melting and evaporation. 
 
Glaciers 
 
 377 
 
 FIG. 315. '' Glacier -Tables," ice-pillars pro- 
 tected by slabs of rock. Parker Creek Glacier, 
 California. (After Russell.) 
 
 When the ice is much fissured by crevasses, as along the outer 
 margins where stresses occur, or where the ice passes over an 
 abrupt change in 
 the slope of the bot- 
 tom, the melting 
 along these cre- 
 vasses may pro- 
 duce a rough ridge 
 or pinnacle topog- 
 raphy. Where the 
 surface is protected 
 by rocks and other 
 debris of the mo- 
 raines, which, 
 though absorbing 
 
 heat on the surface, do not readily conduct it downwards, the sur- 
 rounding exposed ice will melt more rapidly, leaving the protected 
 portions in relief. A variety of special features may thus be 
 formed, among which ice tables consisting of large blocks of stone 
 supported by ice pillars are perhaps the most striking (Fig. 315). 
 
 Small stones and dust particles, on the 
 other hand, by absorbing the heat, 
 will cause a more rapid melting and 
 will sink into the ice, forming depres- 
 sions (dust wells). Thin, bouldery mo- 
 raines are sometimes sunk below the 
 surface in this manner, whereas a thick 
 moraine comes to rest upon a ridge of 
 ice, and the spreading of the debris 
 down the slope of the ridges may lead 
 to a partial covering of the ice sur- 
 face, especially in the lower part of 
 the glacier. 
 
 The Expanded Foot of Glaciers. In 
 Arctic regions the valley glaciers con- 
 tinue to the flatter lowland or to the 
 sea-margin, where they expand in a 
 broad, flat lobe, a sort of fan-like ice 
 
 "--" ~ 
 
 FIG. 316. Map of Baird 
 Glacier, a typical expanded- 
 foot glacier, and of Miles 
 Glacier, with partly ex- 
 panded foot and marginal 
 lake formed by expansion of 
 Copper River, Alaska. (After 
 Tarr and Martin.) 
 
 delta or ice apron, or better, an ice lake without retaining margins. 
 The Foster Glacier of Alaska expands into such a foot on the coast, 
 
378 Atmospheric Precipitates and their Derivatives 
 
 and the Baird Glacier of Alaska expands in a similar manner in the 
 valley of the Copper River, which it has pushed to the opposite 
 side (Fig. 316). This is indeed the best known example of such 
 an expanded foot of a glacier. The Miles Glacier, from the 
 opposite side of the same valley, expands in a less regular manner 
 and, moreover, forms a lake of the river along its front. Other 
 examples in Alaska are the Davidson Glacier on the Lynn Canal 
 and the Mendenhall Glacier. When several adjoining glaciers 
 from the same upland coalesce in their expanded portions, the 
 
 FIG. 317. Spencer Glacier, Alaska (Oct. 1918). This is melting back and 
 shows a sloping and crevassed front, and a well-developed terminal moraine, 
 especially on the left. (Seward Div. Gov't Railroad; by courtesy Alaska 
 Engineering Commission.) 
 
 piedmont type is produced. Conversely, a shrinking piedmont 
 glacier will become resolved into the several component expanded- 
 foot portions of the corresponding glaciers. 
 
 Retreat of Glacier Front. When the rate of melting is in 
 excess of that of advance, the glacier front will move backward 
 or " retreat." The fronts of many Alaskan glaciers have been 
 retreating in recent times, in spite of the fact that the glacier 
 as a whole advances. The front of' Spencer Glacier (Fig. 317) is 
 melting away, leaving a part of the valley uncovered. The front 
 of the Muir Glacier (Fig. 314, p. 376), which faces the sea, has 
 
Glaciers 
 
 379 
 
 Co 
 
 M 
 
 00 
 
 I 
 
 I 
 
 HO 
 
 p C 
 
 It 
 
 
 
 o 
 
 3 - 
 
 5 5* 
 
 
 c 
 
 5 
 
 Si 
 
 5 3 
 
 
380 Atmospheric Precipitates and their Derivatives 
 
 FIG. 319 a. Front of the Rhone Glacier in 1875. (After Walther.) (See 
 also Fig. 313, p. 375.) 
 
 FIG. 319 6. Front of the Rhone Glacier in 1900. (After Walther.) 
 
382 Atmospheric Precipitates and their Derivatives 
 
 retreated steadily between 1899 and 1911, the retreat between 
 1907 and 1911 being 2000 feet. The positions of the front in 1899, 
 1903, and 1907 are shown in the diagrams (Fig. 318, p. 379). In 
 the next two figures the change in the front of the Rhone 
 Glacier is shown between the years 1875 (Fig. 319 a) and 1900 
 (Fig. 319^). In speaking of the retreat of the glacier we must 
 remember that it is the front of the ice which alone changes its 
 position, not because the glacier as a whole moves backward, 
 
 FIG. 321. Surface of Seward Glacier, Alaska. The summit of Mt. St. 
 Elias is seen in the distance beyond the hills bordering the glacier. Drawn 
 from a photograph. (After Russell, Glaciers of North America. By permission 
 of Ginn & Co.) 
 
 but because it melts away at the front. The water resulting from 
 the melting forms the glacial stream, while the rock debris, which 
 was carried in and upon the ice, remains behind and forms the 
 terminal moraine. (See further, Chapter XVI.) 
 
 The Piedmont Type of Glacier 
 
 The Malaspina Glacier. At the foot of Mount St. Elias and 
 west of Yakutat Bay in Alaska lies the Great Malaspina Glacier 
 or ice fan which, with its moraines and secondary deposits, here 
 forms the border of the North Pacific Ocean (Fig. 320). This ice- 
 mass is formed by the confluence of the basal expanded portions 
 of a number of valley glaciers which descend from the mountainous 
 
Glaciers 
 
 383 
 
 regions, the largest of these being the Seward Glacier (Fig. 321), 
 while others are the Agassiz and the Tyndall glaciers. 
 
 The surface of the Malaspina Glacier has an exceedingly gentle 
 slope, while the mass as a whole is relatively stagnant. Its area 
 is about 1500 square miles (about the size of Rhode Island) and 
 its marginal thickness perhaps 1000 feet. Along parts of the foot 
 of the Malaspina Glacier an extensive terminal moraine is de- 
 
 FIG. 322. Moraine-covered border of Malaspina Glacier from Blossom 
 Island. Drawn from a photograph. (After Russell, Glaciers of North America. 
 By permission of Ginn & Co.) 
 
 veloped, while other parts deploy into the sea and from them 
 fragments are broken off to form icebergs.. In some portions 
 where the partial melting of the ice surface has permitted the 
 accumulation upon the ice of a mantle or covering of debris (Fig. 
 322) a luxuriant vegetation has sprung up, and in places this 
 glacier actually supports a forest growth (see map, Fig. 320) with 
 trees, some of which have reached a trunk diameter of three feet. 
 In many places stagnant pools of water occur in which sediments 
 are deposited, while over the central portion, where debris is 
 absent, there are many crevasses into which fall the streams from 
 the melting ice. Glacial streams issue from tunnels beneath the 
 ice in several places (Fig. 323). 
 
384 Atmospheric Precipitates and their Derivatives 
 
 Other Piedmont Glaciers. Other examples of piedmont 
 glaciers are Bering Glacier, west of the Malaspina, and of about 
 the same size, and the smaller Alsek Glacier to the east. In Chile, 
 
 FIG. 323. Entrance to ice tunnel in Malaspina Glacier. Drawn from 
 a photograph. (After Russell, Glaciers of North America. By permission of 
 Ginn & Co.) 
 
 south of S. lat. 42, lie the San Rafael piedmont glacier and some 
 others. Piedmont glaciers also existed in the Alps and in the 
 Rocky Mountains dufing the Pleistocene period, as is shown by 
 the abandoned moraines and other features. - 
 
 ICE-CAPS 
 
 The Vatna Jokull. In the southeastern part of Iceland lies the 
 plateau of Vatna Jokull, the largest of the many ice-covered pla- 
 teaus or jokulls found in that northern island (Fig. 324). It has an 
 area of 8500 square kilometers, being covered throughout by an 
 ice-cap which forms an arched dome with slopes descending from 
 1900 meters in the center to 800 meters near the margin, where it 
 sends out ice tongues to the lower levels (20 to 100 meters). As 
 
Continental Glaciers 
 
 385 
 
 there are few or no projecting rocky peaks rising through it (except 
 near the margins) this ice-cap is largely free from debris and very 
 white. Iceland has several such ice-caps and they form a transition 
 to the more extensive inland ice mass, such as that of Greenland. 
 Other Examples. Similar caps or plateau glaciers are found 
 in Scandinavia, but these are smaller and often elongated ; they 
 also send out tongues in all directions. The ice mantle of Red 
 
 FIG. 324. The Ice-Cap of Vatna Jokull in Iceland. Ice in black. 
 Th. Thoroddsen, Petermann Mitt. 1906.) 
 
 (After 
 
 Cliff Peninsula, north of Inglefield Gulf in Greenland, is another 
 example, and still others are found in the Kerguelen Islands, and 
 on the summit of Kilimandjaro in Africa. All such ice-caps imply 
 conditions of exceptional precipitation and, on the whole, low 
 temperatures. 
 
 CONTINENTAL GLACIERS 
 
 The Greenland Ice Cover (Fig. 325). Related to the preceding 
 type, but of vastly greater extent and thickness, is the ice mantle 
 which covers Greenland. This is believed to have the character 
 of a very flat dome, the highest portion of which lies somewhat 
 to the east of the median line of the continent, and the thickness 
 of which approximates perhaps 3000 feet. It forms a continuous 
 mantle, except for a narrow marginal portion in which the under- 
 lying rocks are exposed, this portion varying in width from five to 
 twenty miles, though in places the ice reaches the coast. In two 
 
386 Atmospheric Precipitates and their Derivatives 
 
 localities, However, the uncovered margin reaches a width of from 
 60 to 100 miles. 
 
 Only the northern and southern portions of this ice field have 
 been crossed, the great central area being entirely unknown, but 
 there is little reason to think that it is other than a vast field of 
 
 snow and ice. In this central por- 
 tion the surface is probably nearly 
 flat, rising perhaps 10,000 feet above 
 sea-level. Around the margins, how- 
 ever, for a width ranging from 75 to 
 100 miles, the outward slope is rather 
 abrupt, often being so steep as to be 
 difficult of ascent. This slope is broken 
 into a series of broad terraces or steps, 
 though the rock margins of the con- 
 tinent, where uncovered, are generally 
 mountainous, reaching heights be- 
 tween 5000 and 8000 feet on the east 
 coast, but not over 6000 feet on the 
 west. Within the margin of the ice, 
 high rock peaks occasionally project 
 through it, forming nunataks. From 
 the margin of the ice mass many 
 tongues and lobes project outward 
 into the valleys of the coastal region. 
 The edge of the ice mass, where 
 not extended in glaciers, is often very 
 
 precipitous or even overhung by an ice cornice. This is most 
 marked in northern Greenland, but becomes the exception farther 
 south. Crevasses are common in some parts of the surface of the 
 marginal portion, and many dimples and " basins of exudation " 
 occur in some sections above the margin. 
 
 Where the ice moves outward between adjoining nunataks, a 
 series of crescent-shaped moraines of debris is often formed, these 
 moraines extending from one nunatak to another and having 
 their convexity pointing outward. These are held to be formed 
 by material carried up from the lower or basal portions of the ice, 
 the upward movements being produced by the formation of 
 curved shearing planes due to obstructions, and the outward con- 
 vexity to the accelerated motion of the central portion of the ice 
 
 FIG. 325. Map of the 
 Greenland Ice-Cap. (After 
 Stieler; from Chamberlin and 
 Salisbury, Geology. By per- 
 mission of Henry Holt & Co.) 
 
Icebergs 387 
 
 between the nunataks and marginal retardation by them. The 
 nunataks themselves furnish only minor amounts of debris to the 
 surface of the ice, though in some cases morainal ridges extend 
 from them marginward. 
 
 The chief portion of the ice which encloses rock debris lies within 
 the basal 100 feet of the masses exposed in the sections. Here 
 much englacial material is found, and this appears to be largely 
 derived from the bottom of the ice sheet and carried upward by 
 obliquely outward-rising shearing planes. In many portions along 
 the margins heavy terminal moraines accumulate, being chiefly 
 derived from the subglacial and englacial material brought there 
 by the outward moving ice. 
 
 Comparatively little water issues from the margins of the ice- 
 sheet of Greenland, though small streamlets appear beneath the 
 ice border, bringing sand and gravel which they distribute among 
 the coarser morainal material. 
 
 The Antarctic Ice-Sheet. The Antarctic continent, which is 
 larger than Europe, is covered by a great ice-sheet similar to that 
 which covers Greenland, and which like the latter is dome-shaped, 
 with a height of 10,500 feet above the sea at the pole (Amundsen). 
 Many great mountain masses or nunataks project through the ice, 
 rising to heights of 1 5 ,000 feet. Along the margin the ice-sheet sends 
 out valley glaciers which reach the sea, while part of the great sheet 
 itself abuts upon the ocean as in the case of the floating ice shelf 
 or Great Ice Barrier of Victoria Land, with ice-cliffs many miles 
 in length and in places rising to heights of 280 feet, though else- 
 where low enough to permit landing from a ship alongside of it. 
 
 ICEBERGS 
 
 Where glaciers enter the sea or other water bodies they advance 
 on the bottom until a depth equal to their thickness is reached, 
 when portions are detached from their front and float away as ice- 
 bergs (Fig. 326). Such icebergs may become tilted or even over- 
 
 
 FIG. 326. Glacier descending into the sea, where its front is buoyed up by the 
 water and becomes broken up into icebergs. (After A. Helland.) 
 
388 Atmospheric Precipitates and their Derivatives 
 
 FIG. 327 a. A large iceberg. 
 
 turned, and they soon lose their load of debris. In some cases, 
 however, this may be carried far to sea before the melting of the 
 iceberg permits its deposition on the bottom. In certain cases 
 
 where many successive 
 icebergs melt near the 
 same point, submarine 
 
 banks composed of ice- 
 rafted material may be 
 built up. The Grand 
 Banks off Newfoundland 
 have been regarded as in 
 part, at least, formed by 
 such debris. 
 
 Only a small portion 
 of an iceberg (in pure ice 
 only one ninth) appears 
 above water (Figs. 
 3270, b), and while ice- 
 bergs from Greenland 
 seldom exceed 100 feet 
 in exposed height, some 
 in the Antarctic region 
 
 FIG. 327 b. Floating iceberg, showing the 
 proportion of visible to submerged ice. (From 
 Kayser's Lehrbuch.) 
 
 have been found to rise 500 feet or more, with a length of several 
 miles. This would make these bergs blocks of enormous size. 
 
Causes of Ice Movement 389 
 
 CAUSES OF ICE MOVEMENT 
 
 The subject of the causes of glacial movements belongs to physics 
 rather than to geology, but may be briefly considered. By many 
 (following Forbes) ice has been considered a viscous substance 
 which in large masses will flow under the influence of its own 
 weight like pitch or asphalt. Freezing of descending waters and 
 the consequent expansion was regarded by Agassiz and Charpentier 
 as the chief cause of glacier motion. Partial melting and refreezing 
 within the mass, the momentary liquefaction of minute portions of 
 the mass while the ice as a whole remains solid, may produce 
 a condition of flowage of the ice. This melting may be the result 
 of pressure rather than applied heat. Expansion and contraction 
 have also been appealed to and so have repeated fracturing and 
 refreezing (regelation) . Finally, the growth of the ice crystals 
 themselves has been invoked, and the influence of gravity no doubt 
 is effective in glaciers upon a sloping surface. Altogether the 
 subject is too complicated for elementary treatment, and we may 
 merely remark that different combinations of the above men- 
 tioned causes, and perhaps others, are effective in producing the 
 movement of different glaciers and perhaps of the same glacier at 
 different times. 
 
CHAPTER XV 
 
 DESTRUCTION OF ROCKS AND THE FORMATION 
 OF CLASTIC MATERIAL 
 
 ALL rocks are subject to destruction, and the product of such 
 destruction may be visible fragmental or clastic material, or it 
 may be changed by great heat into molten material (magma), or 
 finally it may be invisible as the result of solution in water, or of 
 vaporization. Molten, dissolved, or vaporized rock material will 
 be redeposited under favorable conditions as igneous rock, as 
 aqueous precipitates, or as gaseous sublimates (sulphur, etc.), 
 respectively, or, by the intervention of animals and plants, as 
 organic deposits. Fragmental material, on the other hand, will 
 produce, when reconsolidated, a new type of rock, the clastic rock, 
 and to this attention is now invited. 
 
 AGENTS ACTIVE IN THE FORMATION OF FRAGMENTAL OR 
 CLASTIC MATERIAL 
 
 In general, we may note that there are two methods of breaking 
 rocks into fragments, the chemical and the mechanical. In the 
 first case, the fragmentation is produced by alteration of the ma- 
 terial of the rock, either by abstraction of some of the component 
 material, in solution, etc., or by the addition of material such as 
 oxygen, water, carbon dioxide, etc., or by both. The chemical 
 method of rock-breaking is called decomposition, and the products 
 of decomposition differ from the material of the original rock. In 
 the second method of rock-breaking, the mechanical, the rocks 
 are merely broken into fragments, or are separated into their com- 
 ponent minerals. This is called disintegration. When both modes 
 of change go on together, under the influence of the atmosphere and 
 its contained water vapor and gases, with accompanying tempera- 
 ture changes, etc., the process is called weathering of rocks. 
 
 390 
 
Processes of Erosion 391 
 
 The agents active in the production of fragmental material from 
 the rocks of the earth's crust, are the following : 
 
 1. The Atmosphere. This operates by the action of its con- 
 stituents, its contained gases and vapors, by its transmission of the 
 heat of the sun, and the escape, by radiation, of the heat waves 
 from the earth, and by the mechanical action of wind. 
 
 2. The Hydrosphere. This operates by solution of rock and 
 alteration of its constituents, and by mechanical activities of its 
 movement, as in the case of waves, river-currents, rains, etc. 
 
 3. The Pyrosphere. This agent operates chiefly through 
 igneous explosions, which shatter the rock. 
 
 4. Movements of the Lithosphere. The grinding and frac- 
 turing of rocks in the movement of one rock mass over or against 
 another, as in faulting, represent the chief work of this agent. 
 Here also are properly placed the movements of large ice masses, 
 such as glaciers, over other rock surfaces, and their destructive 
 work. 
 
 5. The Biosphere. This operates by various rock-destroying 
 activities of plants and animals, including man. 
 
 PROCESSES OF EROSION 
 
 Breaking up or fragmentation (clastation) of rock is one form 
 of erosion, and the product of such work in general remains near 
 the scene of operation unless other agencies are active. Erosion 
 is, however, accomplished in another manner, namely by the break- 
 ing off and removal of material by the agent of erosion. This is 
 called ablation, and also becomes the first step in the transporta- 
 tion of material. Ablation may be effected mechanically by the 
 denuding or stripping of the surface of material loosened by pro- 
 cesses of clastation or rock-breaking in place. Such removal is 
 called denudation. A second process is that of corrasion, where ma- 
 terial is ground or filed away by means of " tools " which are carried 
 by the agent. Thus wind carrying sand grains or dust will corrade 
 any surface over which it blows, after the manner of the artificial 
 sand blast used in renovating old stone structures, such as the stone 
 fronts of buildings, etc. The wear which these sand grains them- 
 selves experience is call abrasion. A third method is analogous to 
 quarrying by man, in that, by a process of undermining and loosen- 
 ing, large masses of rock are removed from their original location. 
 Finally, chemical ablation takes place, when the rock mass under- 
 
392 
 
 The Formation of Clastic Material 
 
 goes solution, melting, or other chemical change which at the same 
 time removes the material. This is expressed by the general term 
 corrosion. It is sometimes spoken of as chemical denudation. We 
 may tabulate these processes as follows : 
 
 Erosion 
 
 I. Fragmentation or dastation. 
 Breaking or shattering material in 
 situ not necessarily accompanied 
 by removal. 
 
 II. Ablation or the 
 separation and simul- 
 taneous removal of 
 rock material by the 
 same agents. 
 
 (a) Mechanical 
 
 (6) Chemical 
 
 (a) Physical, or dis- 
 integration. 
 
 (b) Chemical, or de- 
 composition. 
 
 ' i. Denudation. 
 
 2. Corrasion and 
 
 abrasion. 
 
 3. Quarrying. 
 Corrosion. 
 
 DESTRUCTIVE WORK OF THE ATMOSPHERE 
 
 Weathering 
 
 By weathering we understand all the rock-modifying influences 
 of the atmosphere, other than the mechanical activities of the 
 wind. These weathering processes are both chemical and mechan- 
 ical and belong primarily to the division of rock-breaking or clasta- 
 tion, though some corrosion may also occur. 
 
 Daily and Seasonal Changes of Temperature ; Insolation. All 
 rocks are affected to a greater or less degree by the daily and seasonal 
 changes in temperature, and although the source of the heat which 
 affects the rocks is the sun, the presence of the atmosphere modifies 
 this, so that the effects or changes produced are properly classed 
 with the activities of the atmosphere. In common with nearly 
 all substances, rocks expand under the influence of heat and con- 
 tract under that of cold. Illustrations of the expansive effect of 
 the sun's heat on substances are seen in the lengthening on hot 
 summer days of the rails on a car track, which are purposely placed 
 short of contact to allow for such expansion, which would otherwise 
 cause a buckling of the rails. On very cold days the ends of the 
 rails are separated by an appreciable gap. The buckling of con- 
 crete sidewalks from prolonged exposure to the sun is another ex- 
 ample, and the repeated removals of such bucklings and filling of 
 the gaps with new cement is necessary, and noticeable in most 
 older cement walks. The chief effects produced on rock masses 
 by these changes in temperature are exfoliation and granular dis- 
 integration. 
 
Destructive Work of the Atmosphere 393 
 
 Exfoliation. Where changes of temperature between day and 
 night are great, as in desert regions, the effect upon the rocks is very 
 marked. During the day the exposed rock surfaces are intensely 
 heated, especially in the case of the 
 dark-colored igneous rocks, which ab- 
 sorb more heat than those of lighter 
 color. The daily range in tempera- 
 ture on the surfaces of some rocks has 
 been estimated as high as 80 C. (144 
 F.). As rock, on the whole, is a poor 
 conductor of heat, the surface is chiefly 
 affected, though the heat passes gradu- 
 ally inward. Hence the surface layers 
 are subjected to an expansive force, 
 while the deeper layers are not so 
 
 affected. On cooling by radiation in basic igneous rock. Note the 
 
 successive shells which are 
 the clear atmosphere, the temperature peeling off (From spe cimen 
 
 of the outer layers will sink rapidly in Columbia University. 
 and may easily pass below that of 
 the inner part of the rocks. As a re- 
 sult, the surface portion for some distance inward is subjected to 
 a series of strains which will result in the flaking or peeling off of 
 such outer layers. As the angles of rocks are most exposed, being 
 subjected to heating from all sides, they will fall off first, and the 
 
 FIG. 328 a. Concentric 
 exfoliation in a fine-grained 
 
 B. 
 
 Reduced. 
 
 FIG. 328 b. Concentric exfoliation or " spalling " of biotite granite. Ridge 
 south of Morrison Creek, Yosemite Quadrangle, Cal. (Photo by Turner, from 
 U. S. G, S. ; courtesy of Popular Science Monthly.) 
 
394 
 
 The Formation of Clastic Material 
 
 result will be the production of a curved or rounded outline, and 
 the subsequent layers peeled off will also be curved. Thus con- 
 centric exfoliation or the successive peeling off of layers of rock 
 results, a phenomenon very marked in most dark and fine-grained 
 rocks in such regions (Figs. 328 a-c). When the changes in tem- 
 perature and the consequent expansion and contraction are very 
 rapid, large masses of rock may be thrown off with some violence. 
 Over large, nearly flat surfaces, the effect of insolation is such as 
 to produce, a series of planes of separation parallel to the surface 
 
 FIG. 328 c. Concentric exfoliation in a dike of basic igneous rock. (From 
 Ratzel, Die Erde.) 
 
 and at progressively greater distances apart in depth. Such ex- 
 pansion joints or planes are seen in the faces of most granite 
 quarries, and they greatly facilitate the process of quarrying, but 
 limit the size of the blocks obtainable near the surface (Fig. 329). 
 Granular Disintegration. Besides affecting the rock mass as a 
 whole, changes in temperature have a further detailed effect upon 
 the minerals or particles of which the rock is composed, especially 
 if, as is usually the case, these are not all of one kind. A granite, 
 for example, exposed to such influences, will have its several 
 minerals affected in different degrees. The dark minerals (horn- 
 
Destructive Work of the Atmosphere 395 
 
 blende, black mica) will absorb heat more readily and also give 
 it up more quickly than the light minerals. Moreover, the ability 
 
 FIG. 329. Horizontal jointing in a granite ledge, due to expansion and 
 contraction of the surface layers under insolation. (Courtesy of Prof. J. B. 
 Woodworth.) 
 
 to react to heat and cold is not the same in the feldspar as in the 
 quartz, and both differ in this respect from the dark minerals. In 
 other words, each min- 
 eral has its own coefficient 
 of expansion and con- 
 traction. As a result, 
 internal stresses are set 
 up and the minerals will 
 tend to separate one from 
 the other and finally fall 
 apart, producing a sand 
 of loose minerals. This 
 granular disintegration, 
 as it is called, the sep- 
 aration of the rock into 
 
 its component minerals, 
 is a characteristic 
 feature observable in all 
 granite and similar rock 
 
 FIG. 330 a. Slopes of crystalline sand on 
 the sides of Pikes Peak, Colorado. The sand 
 is the product of granular disintegration of the 
 granitic rock, and consists of quartz, feld- 
 spar, and ferro-magnesian mineral fragments. 
 (Photo by the author.) 
 
The Formation of Clastic Material 
 
 masses. The slopes of Pikes Peak, which is a mountain largely 
 composed of coarse granite, are covered with long screes of such 
 
 disintegration products, 
 which have the appear- 
 ance of a coarse sand 
 with a smooth surface 
 which slopes at the angle 
 at which such material 
 will come to rest, that 
 is, the angle of repose 
 (Fig. 330 a). Similar 
 products of granular dis- 
 integration may be seen 
 around large granite 
 boulders on exposed hill- 
 sides, even in fairly moist 
 regions (Fig. 3306). 
 Such boulders are them- 
 selves often the product 
 
 FIG. 330 b. Disintegrating granite boulder, 
 Dogtown Common, Cape Ann, Mass. Half 
 of the boulder has crumbled into crystalline 
 sand. (Photo by the author.) 
 
 of granular disintegra- 
 tion (Fig. 330 c). Granite surfaces often show areas of bare rock 
 separated by fissures, into which the product of disintegration has 
 
 FIG. 330 c. Granite boulders, the result of decomposition and disintegra- 
 tion of granite in situ; resting on granite ledge. Graniteville, Mo. (Gardner col- 
 lection of photographs, No. 7908. Courtesy Geol. Dept., Harvard University.) 
 
Destructive Work of the Atmosphere 397 
 
 been washed by rains, and here, in the moister climates, where 
 this material is further decomposed, lines of 'vegetation will spring 
 up. In moist climates, 
 dark igneous rocks are 
 often reduced to a mass 
 of residual boulders by 
 combined disintegration 
 and decomposition (Figs. 
 
 33 1 , *) 
 
 Finally, it should be 
 noted that the minerals 
 themselves are affected FIG. 331 a. Boulders of disintegration and 
 by these temperature decomposition. Part of a Camptonite dike 
 Changes, because min- weathered until only scattered residual blocks 
 
 of the original rock remain, 
 erals with several distinct 
 
 crystal axes react differently on the several faces. Thus a crystal 
 of feldspar is itself subjected to internal stresses which open 
 
 FIG. 331 b. Boulders of disintegration. Residual masses left by weathering 
 of a diabase dike, Medford, Mass. The dike had been left in relief by quarry- 
 ing operations, and weathered into a mass of spheroidal boulders. As the dike 
 finally crumbled completely, the boulders were thrown together into the heap 
 here shown. (Photo H. W. Dyer ; courtesy of Prof. Elizabeth Fisher, Wellesley 
 College.) 
 
398 
 
 The Formation of Clastic Material 
 
 minute fractures along the cleavage lines. Into these fine fractures 
 air and moisture will penetrate and effect the decomposition of 
 the mineral along the sides of the fracture. 
 
 The Talus Slope (Fig. 344). By the disruption and disintegra- 
 tion of the rock masses, loose material of all sizes is formed, and this 
 will accumulate at the foot of every cliff, forming a talus, the sur- 
 face slope of which varies with the coarseness of the material. In 
 dry climates, the talus is chiefly formed by the processes above 
 described, but in cold climates where the atmosphere is moist, frost 
 action may become an important factor in its production. By the 
 combined action of these agencies it is brought about that most ex- 
 posed mountain peaks are heaps of coarse, loose material, generally 
 burying the ledges, while long slopes of finer material are formed 
 in every favorable locality. 
 
 Land Slides. When a talus slope becomes saturated with rain- 
 water during a moist season, its potential angle of slope is lowered, 
 because the entire mass becomes more mobile than when dry. For 
 
 a time, the angle of slope 
 may be mainiained, but 
 at a critical moment 
 the cohesive force may 
 be overcome and the 
 entire mass, or a large 
 part of it, will slide 
 down the mountain's 
 slope, forming an earth 
 and rock avalanche or 
 rock slide (Fig. 332), 
 which may produce dis- 
 astrous effects in the 
 valley bottoms below, 
 destroying farmlands 
 and buildings, and dam- 
 ming narrow valleys so that the upper portions may be converted 
 into closed basins in which lakes will form. By the final overflow 
 of the lake-water, and the accompanying destruction of the earth 
 dam, disastrous floods may sweep the valley below. In one of the 
 upper branches of the Ganges River in the Himalayas, a slide, 
 bringing down 800,000,000 tons of rock debris in three days, built 
 a dam across the narrow valley nearly a thousand feet deep. Be- 
 
 FIG. 332. Land-slide, Ausable Lake, N. Y. 
 (A. D. Savage, Photo.) 
 
Destructive Work of the Atmosphere 399 
 
 hind this a lake accumulated, 
 reaching a length of four miles 
 in a year. Then the overflow 
 partly destroyed the dam, al- 
 lowing 400,000,000 cubic yards of 
 water to discharge in about four 
 hours and flooding the valley be- 
 low to a depth of 100 to 170 
 feet, destroying every vestige of 
 habitation for a distance of 150 
 miles down the valley. Even on 
 gentle slopes the saturation of 
 the talus by water will produce 
 disastrous slides. 
 
 Frost Work. Water on freez- 
 ing expands one tenth of its 
 volume and so becomes a power- 
 ful agent in disrupting rock. 
 Where the moisture of the air is 
 condensed on the cold surfaces of 
 rocks, long ice crystals may form, 
 and if these develop in crevices, 
 their growing force pushes the 
 
 walls apart and eventually loosens the smaller fragments, which 
 then fall to the bottom of the cliff to aid in building the talus. By 
 the combined action of frost and insolation remarkable erosion 
 
 forms may be pro- 
 duced, an example 
 of which is seen in 
 the " old man of the 
 mountain " in the 
 White Mountains 
 
 (Fig- 333)- 
 
 Boulders on a rock 
 FIG. 334. Diagram showing three stages in or other hard sur f ace 
 the downward progress of a boulder on a sloping . ... 
 
 hillside under the action of frost. The original are otten llted to a 
 position is shown by heavy dotted lines. Frost slight degree by the 
 crystals forming under the boulder raise it at right formation of innu- 
 angles to the surface (fine dotted lines) ; when , . . 
 
 these supporting crystals melt the boulder settles merab l e lce crystals 
 back at right angles to the horizon (solid outline), beneath them. If the 
 
 FIG. 333. The Old Man of the 
 Mountain, Franconia Notch, White 
 Mountains, N. H. Illustrating pe- 
 culiar result of rock weathering. 
 
400 The Formation of Clastic Material 
 
 surface on which the boulder rests has a slope, lifting by the crys- 
 tals will be at right angles to this slope. On the melting of the 
 ice, the boulder will settle back vertically, and thus make a slight 
 advance down hill. In this manner a large boulder may travel for 
 considerable distances down hill in the course of time and may 
 eventually reach the edge of a precipice over which it will finally 
 fall (Fig. 334). 
 
 Frost acts in a similar manner on pebbles and on the finer ma- 
 terial which forms the soil. The effect^ of the frost upon the soil 
 
 FIG. 335. View of the Slumgullion rock-flow from its source to Lake San 
 Cristobal, which was formed when the great slide dammed up the river valley. 
 (Photo by W. Cross, U. S. G. S. Courtesy of D. W. Johnson.) 
 
 is one of expansion, by filling it with ice-crystals, which lift and 
 push apart the particles. This may readily be seen in dirt paths 
 on a frosty morning, in the spring or autumn. When the ice 
 crystals melt, they leave the soil full of small cavities which give 
 the mass a loose spongy character. Under the beat of the rain, 
 the running and soaking in of water, the movement of the 
 groundwater, the trampling of animals, etc., this uplifted, loosened 
 soil is again compacted, and in this process slowly moves down hill. 
 
Destructive Work of the Atmosphere 401 
 
 By the alternate freezing and thawing of the moisture in a talus 
 slope, this may undergo a slow process of creeping down-hill, form- 
 ing a "rock glacier" (Figs. 335, 336). Such creeping talus masses, 
 often of considerable length, have been found in many mountain 
 regions where intense cold prevails for part of the year. Their 
 movement is often indicated by the complete absence of vegetation, 
 even over the lower, gentler slopes, plants being unable to main- 
 tain a foothold on account of the motion. 
 
 FIG. 336. View northeast across Cleveland gulch at landslide mass and 
 conglomerate rock-glacier. Silverton Quadrangle, Colo. (Willis, Photo ; from 
 U. S. G. S.) 
 
 Soil on hillsides also creeps under the influence of freezing mois- 
 ture, as well as gravitative control, and this creep is well seen where 
 the underlying rock is exposed in cuts or otherwise, and where it is 
 composed of vertical or steeply inclined beds, especially shales 
 or schists. The upper ends of these beds below the soil are then 
 frequently found bent forward as the result of the dragging effect 
 of the creeping soil (Fig. 337). 
 
 Chemical Work of the Atmosphere. The atmosphere consists 
 of a mixture of oxygen and nitrogen, the former being somewhat 
 less than 21 per cent of the volume, and the latter something over 
 78 per cent. Besides this, there is a small quantity of the element 
 argon and other rare gases (less than one per cent by volume) and 
 
402 
 
 The Formation of Clastic Material 
 
 a fairly constant admixture of carbon dioxide (CO^ about 0.03 per 
 cent by volume) and water vapor (HkO) in very variable quantity. 
 Minute quantities of other compounds, such as ozone, nitric acid 
 and ammonia, are also found, together with free hydrogen, sulphur 
 compounds and other substances. The agents active in producing 
 chemical changes in the rock are, however, few, comprising chiefly 
 oxygen, carbon dioxide and water vapor. Accordingly the chem- 
 ical- activities of the atmosphere may be grouped under (a) 
 
 FIG. 337. Nearly vertical beds of shale bent over and broken at the top, 
 by superincumbent weight, or creep of the overlying soil. Columbia, Pa. 
 (Walcott, Photo U. S. G. S.) 
 
 oxidation, (b) carbonation and (c) hydration and dehydration. 
 Special combinations of these produce other changes such as kao- 
 linization and laterization. 
 
 Oxidation. The oxygen of the air accomplishes its chief work by uniting 
 with the iron and sulphur of the rock-forming minerals and with the organic 
 matter present. This is called oxidation. The iron of the ferro-magnesian 
 minerals and of other iron compounds in the rocks is transformed into the oxide 
 (Fe 2 O3), or in the presence of moisture into the hydrous oxide of iron. The 
 former has a red color (the mineral hematite, etc.) and the latter a yellow or 
 ocher color (limonite, etc.). In their formation there is generally an in- 
 crease in volume of the material, this being especially great when limonite is 
 formed by simultaneous oxidation and hydration. When magnetite (Fe 3 O 4 ) is 
 
Destructive Work of the Atmosphere 403 
 
 altered to limonite by these processes, the increase in volume is 64 per cent. 
 On the other hand, if iron carbonates are oxidized, the loss of carbon dioxide 
 is not wholly compensated for by the addition of oxygen and water, and the 
 volume decreases. By the oxidation of the sulphur of the rock minerals, such 
 as pyrite, both sulphurous and sulphuric acids may be formed, which unite 
 with the iron or other substances to form new compounds (sulphites and sul- 
 phates). By the oxidation of the organic matter, carbon dioxide (CO2) and 
 water are produced. 
 
 Carbonation. This most commonly affects the silicate minerals, the carbon 
 dioxide combining with the basic elements to form carbonates, while silica is 
 set free. Besides the carbon dioxide of the atmosphere, there is generally an 
 abundant supply of this gas furnished by the oxidation of the organic matter, 
 and this also becomes active in attacking the rock, altering some of the min- 
 erals. Water takes up carbon dioxide, from the air, and from decaying organic 
 matter in the soil and in stagnant ponds ; indeed more CO2 is obtained from 
 organic matter than from the air. Water thus charged with CO 2 is a power- 
 ful solvent, attacking and dissolving many minerals. We shall return to this 
 subject in the study of groundwater. 
 
 Hydration. This is the union of water with the mineral substances of the 
 rocks and is the most important single process of change, but hydration by at- 
 mospheric moisture is only a superficial process often associated with oxidation. 
 More effective, however, is the hydration due to the circulation of ground-water 
 in the pores of the rock. Hydration causes the expansion of the minerals, form- 
 ing stresses in the rock, which lead to its crumbling. As the most important 
 single example of hydration is the changing of feldspars to kaolin or clay, we 
 will consider this and the related process of laterization more in detail. 
 
 Kaolinization. We have seen that feldspars are apt to be traversed by nu- 
 merous fine fissures due to the influence of heat and cold. Moisture entering 
 these in the presence of carbon dioxide changes the feldspar to clay or kaolin, 
 which may be seen in a thin slide as a clouding of the otherwise transparent 
 mineral, along the sides of the cleavage lines. 1 Complete change to kaolin (clay) 
 occurs in time, according to the following chemical reaction : 
 
 2 KAlSi 3 O 8 + 2 H 2 + C0 2 = H 4 Al 2 Si 2 9 + 4 SiO 2 + K 2 CO 3 . 
 Orthoclase Water Carbon Kaolin Silica Potassium 
 
 feldspar dioxide or clay carbonate 
 
 In this change from feldspar to kaolin or pure clay, there is a decrease in volume 
 of 54.44 per cent, owing to the removal of the silica as quartz and of the potas- 
 sium carbonate. Kaolin is the chief substance of which clay is produced, 
 clay being commonly a variable mixture of kaolin and impurities, such as quartz- 
 flour, iron oxides etc. We see here the process by which this common substance 
 originates as the result of the weathering of feldspars. 
 
 Laterization. In moist, tropical regions the feldspars are not as a rule changed 
 to clay, which is a hydrous silicate of alumina, but to hydrous oxides, chief 
 among which is the mineral Hydrargillite (Al 2 Os ' 3 H 2 O), or to a mixture of 
 oxides, forming bauxite. There is usually much iron oxide liberated, especially 
 if the decaying rock is basic, such as basalt, etc. This iron oxide stains the 
 
 1 This clouding may be due to the development of sericite rather than kaolin. 
 
404 
 
 The Formation of Clastic Material 
 
 product a deep red or brown, and may also form iron nodules or concretions. 
 The combined product is called laterite. It is a very widespread product of 
 rock decay in tropical regions and has been known to extend to a depth of 
 300 feet in Brazil. In the process of laterization, the silica is separated out and 
 accumulates in separate areas as quartz (agate or chalcedony). 
 
 A comparison of the more important compounds of a fresh igneous rock 
 (dolerite, see p. 107) and its decomposition product, brings out the change and 
 shows the difference between kaolinization and laterization. 
 
 IMPORTANT COMPOUNDS 
 
 PRESENT IN 
 FRESH ROCK 
 (DOLERITE), 
 ENGLAND 
 
 PRESENT IN 
 KAOLIN- 
 IZATION 
 PRODUCT 
 
 PRESENT IN 
 FRESH ROCK 
 (DOLERITE), 
 INDIA 
 
 PRESENT IN 
 LATERIZATION 
 PRODUCT 
 
 Silica (SiO 2 ) 
 Aluminum oxide (A1 2 O 3 ) . 
 Iron oxide (Fe 2 O 3 ) - - . 
 
 49-3% 
 
 17-4% 
 2-7% 
 
 47-0% 
 
 i8-5% 
 14-6% 
 
 50.4% 
 
 22.2% 
 
 9-9% 
 
 0-7% 
 50-5% 
 23-4% 
 
 Corrosive Work of the Atmosphere 
 
 This is of little importance, as a rule, in rock destruction, though 
 very marked in the case of snow and ice, where the surfaces are 
 often strongly pitted. The corrosive effect of the atmosphere in 
 this case manifests itself largely by a process of direct evaporation. 
 Entire banks of snow and ice may thus be removed by evaporation 
 without passing through a liquid state. The moisture in the air 
 may also have a corrosive effect upon easily soluble deposits, such 
 as salt and gypsum beds, or even limestones, but in general it would 
 be difficult to distinguish this from the work of rain and other waters 
 upon these rocks. Corrosion by hot gases and steam is not an un- 
 common process in volcanic districts. 
 
 Destructive Work of the Wind 
 
 Deflation. Air in motion, i.e. wind, often exerts a powerfully 
 destructive effect upon rocks. This is especially the case where 
 material has previously been loosened by weathering, so that the 
 wind merely picks up the loose material and carries it away. The 
 removal of such material by the wind is called deflation, and to it 
 is probably due the most extensive effect of wind erosion. Of 
 course, material previously deposited in a region, either that trans- 
 ported by the wind from elsewhere or that brought by running 
 water or by ice, may also be removed by the wind, and this too is 
 
Destructive Work of the Atmosphere 405 
 
 deflation. Many exposed uplands are thus kept free from loose 
 material, which is removed as fast as weathering produces it. 
 The great elevated plateau of southern Germany, known as the 
 Swabian Alp, is in this manner kept almost bare of soil, so that 
 agricultural activities are strongly interfered with. So-called stony 
 deserts or Hammadas are produced by the removal by wind of 
 all fine products of disintegration, leaving only the coarser frag- 
 ments, which are worn and polished by the corrasive effects which 
 this finer material has as it is swept along by the wind. 
 
 Corrasion. This is the second important mode of wind erosion, 
 and this method has already been compared to the work of the arti- 
 ficial sand blast. By means of the sand swept along by the wind, 
 rock surfaces are polished and grooved, such grooves often being 
 .very marked and in parallel alignment upon the surfaces of some 
 rocks, such as limestones. 
 
 Wind-worn grooves of this type are found in the limestone plateau of the 
 Libyan desert in northern Africa. They extend in a north-northwest by 
 south-southeast direction, and vary in depth up to a meter. They were cut 
 by the sand-laden winds, which for thousands of years have swept across this 
 surface with little or no variation of direction. 
 
 Such wind-channeled surfaces may become of considerable importance to 
 the student of earth history, when he finds them tfn older rock surfaces which 
 were subsequently covered and buried by material deposited upon them and 
 also consolidated into rock. They become visible, of course, only when the 
 later deposited material is removed again by erosion or by quarrying, and it 
 will then be found that the old wind channels are filled in by this younger rock 
 material, a part of which will commonly remain after the removal of the gen- 
 eral covering mass. The presence of this material will show that the grooves 
 or channels are not of recent origin, but were formed before the younger rock 
 material was deposited. Such conditions clearly show that after the formation 
 of the older rock, a period of land succeeded, with strong wind activities such 
 as are found chiefly in deserts, and that therefore this interval of erosion marks 
 a considerable period between the epochs of deposition of the older and of the 
 younger rock. An ancient example has been found in Michigan, where lime- 
 stone surfaces, of Upper Silurian age, are grooved in this manner and are 
 covered by limestones of Middle Devonian age, which fill these grooves as 
 well. This indicates strong wind activity during the interval between the 
 periods of deposition of these two rocks (the Lower Devonian interval), and 
 some of the old, well-rounded sand grains which were active in the production 
 of these grooves are still found in some of them. 
 
 In central Asia, surfaces of argillaceous rock are similarly sculptured, but 
 here the sides of the grooves are often strongly fluted. Such surfaces are called 
 locally yardangs (Fig. 338), and they too are recognizable when found along the 
 contact of two older rock series, and tell a similar story. 
 
406 
 
 The Formation of Clastic Material 
 
 Good illustrations of the corrasive activities of sand-bearing 
 winds are found in the San Bernardino Pass in southern California, 
 where the telegraph poles along the pass are greatly damaged by 
 
 FIG. 338. Map of a desert area with yardangs, and cross sections of the 
 same on a larger scale. (After Sven Hedin; from Kayser's Lehrbuch.) 
 
 those blasts, so that they have to be protected by piles of rock and 
 supplemental pieces placed on the windward side. In the Trans- 
 Caspian deserts the telegraph wires stretched along the railroad 
 line were so affected by the sand blasts that they became dimin- 
 ished by half their diameter, and had 
 to be replaced after eleven years. 
 Even the fine dust of the city streets, 
 blown across tombstones in old ceme- 
 teries, will in time efface their inscrip- 
 tions. 
 
 Some of the most striking effects 
 of wind corrasion are seen in pebbles 
 exposed to more or less constant 
 sand-bearing winds, such as are found 
 in desert regions, but also on exposed 
 portions of the coast. Upon such 
 pebbles, smooth, flat faces will be 
 cut, several of these intersecting gen- 
 erally in well-marked angles. Such 
 faceted pebbles or dreikanter (so- 
 called because of the usual presence 
 of three edges) are of common oc? 
 
 FIG. 339. Eolian carved 
 pebble ' ' Einkanter. ' ' If the 
 small face on the lower side 
 were enlarged the usual type of 
 "dreikanter" would result. 
 Marthas Vineyard, Mass. 
 Gardner collection of photo- 
 graphs, No. 542. (Courtesy of 
 Geological Department, Har- 
 vard University.) 
 
 currence. Sometimes only two facets are cut, these meeting in an 
 edge; then the pebble is called an einkanter (Fig. 339). 
 
 Abrasion of Sand Grains. The sand grains carried by the 
 wind are themselves affected by the impact against the rock sur- 
 
FIG. 340. Erosion monuments of white sandstone, with harder beds ce- 
 mented by iron oxide. One of these cuts the large monument near the middle, 
 and forms the capstones of the smaller ones. These beds were formerly con- 
 tinuous. Monument Park, Colorado. (Darton, photo ; from U. S. G. S.) 
 
 FIG. 341 a. Erosion forms of a jointed sandstone (Quader Sandstein) 
 Bastei, Saxon Switzerland, on Elbe River. (After Kayser, Lehrbuch.) 
 
408 
 
 The Formation of Clastic Material 
 
 faces as well as against one another. A result of this is, that the 
 grains of softer rocks such as limestones, and those of cleavable 
 minerals, such as feldspar, are often completely ground to dust, 
 which is carried far away, while the grains of the harder minerals, 
 such as quartz, have their angles worn off and their surfaces 
 more or less pitted and given the appearance of ground glass. 
 Grains which have been repeatedly worn in this way may in 
 
 FIG. 341 b. Erosion pillars 
 (Three-finger tower) in sandstone, 
 due to weathering and deflation 
 along joint cracks in the rock. 
 Note the size of the men for com- 
 parison. Bastei, on the River 
 Elbe, Saxony. 
 
 FIG. 342 a. Erosion forms in jointed 
 sandstone, Cedar Breaks, Utah. (Photo 
 by F. J. Pack.) 
 
 time approach a perfectly spherical form (millet-seed sands), by 
 which, as well as by other characters, their origin as wind-blown 
 sands can be recognized. Sand grains are, however, rounded 
 in other ways, though on the whole less perfectly. (See Figs. 361 a, 
 b,p. 440, 370 a, b, p. 452.) 
 
 Combined Deflation and Corrasion. Where materials are 
 loosened by alternate heat and cold, they can be removed from the 
 exposed surface by wind and at the same time act as tools for the 
 
Destructive Work of the Atmosphere 409 
 
 corrasion of the rock. By such means extensive masses of rock are 
 removed in the course of time, and' the material carried elsewhere, 
 where it is redeposited. 
 
 Rock masses are com- 
 monly cut by crevices, 
 which are called joints, 
 and which penetrate 
 them in all directions. 
 Wind erosion is most 
 active in these crevices, 
 widening and enlarging 
 them. In this manner, 
 
 FIG. 342 b. Weird sculptur- 
 ing by the wind in Tertiary 
 rocks of the Uinta Bad Lands, 
 north of White River, Utah. 
 The harder portions have been 
 etched in relief. (American 
 Museum Natural History.) 
 
 pillars of rock may become 
 
 separated from the original 
 
 mass, and these pillars may 
 
 be carved by the wind into 
 
 fantastic shapes; for the 
 
 weaker portions of the rock 
 
 will be cut away more 
 
 rapidly, leaving the harder 
 
 portions in relief (Fig. 
 
 342 b). Exceptionally fine 
 
 examples of such pillars are 
 
 found in Monument Park, 
 
 Colorado (Fig. 340), in the 
 
 famous Bastei region along 
 
 the river Elbe in Saxony 
 
 (Figs. 341 a, b), in the 
 
 semiarid regions of the west 
 
 (Figs. 342 a, b), in Egypt, 
 
 and elsewhere. Sometimes 
 
 only isolated tables or buttes 
 
 FIG. 343. Erosion monument, Utah. 
 The spire rises 2000 feet above the base in 
 the foreground. (Photo by F. J. Pack.) 
 
4io 
 
 The Formation of Clastic Material 
 
 of rock will arise from an otherwise flat surface (Figs. 343, 344) 
 and represent the last remnant of a formerly continuous layer 
 which has been almost completely removed by wind work 
 
 FIG. 344. A butte of horizontal red sandstone capped by white gypsum, 
 and surrounded by talus slopes. The strata once extended widely over the 
 region. Northeast of Cambria, Wyoming. (Photo by \Darten, U. S. G. S. 
 Courtesy of D. W. Johnson.) 
 
 DESTRUCTIVE WORK OF THE HYDROSPHERE 
 Rain-Water 
 
 Erosive Work of Rain. Falling rain forms, in a measure, a 
 transition from the atmosphere to the hydrosphere, originating in 
 the former and becoming a part of the latter as it reaches the earth's 
 surface. Rain erosion is, on the whole, confined to soft rocks, such 
 as salt beds, and to unconsolidated clays and sands, from which it 
 carves pillars, buttresses, and other erosion forms (Figs. 345 a, b). 
 Its effect is partly mechanical, through impact, followed up by 
 the rivulets into which the rain-water unites, and partly chemical, 
 effecting solution or corrosion on salt, gypsum, and limestone beds 
 (Figs. 346 a, b). Cold rain-water falling upon highly heated rocks 
 will greatly aid in shattering them. 
 
Destructive Work of the Hydrosphere 411 
 
 Disposition of Rain-water. The water which falls upon the 
 earth as rain is disposed of in three ways. A part of it again evap- 
 orates, this being most pro- 
 nounced where there is much 
 vegetation to retain it for a 
 time. A second part runs 
 off, following the slope of the 
 land. This is most abundant 
 in regions of little or no vege- 
 tation, on surfaces composed 
 of hard, impervious material, 
 and on steep slopes. This 
 run-off marks the beginning 
 of river formation. A third 
 part finally sinks into the 
 ground, and becomes the 
 ground-water. This is most 
 abundant where the surface 
 is composed of loose, porous 
 
 material, and where its 
 
 _ . FIG. 345 a. Earth pillars protected 
 
 flatness prevents much run- by a cap of roc k and due to rain erosion, 
 
 off. Colorado. (After Hayden.) 
 
 Rivers and the Products of Their Erosion 
 
 Origin of Rivers. As we have seen, the run-off of the surface 
 waters follows the slope of the land. At first, if the slope is a ho- 
 mogeneous one, a sheet-flood may result, i.e. a broad, shallow sheet 
 of water, without definite boundaries, runs down the slope. Soon, 
 however, because the water washes away loose material, some por- 
 tion of the surface will be excavated somewhat more deeply than 
 other's, and more water will be concentrated in this deeper portion. 
 This leads to increased removal of the loose material along that 
 line, and a gully is formed, which is gradually deepened and wid- 
 ened into a stream bed. At first it is a dry gully, carrying only 
 the run-off after rains. But as soon as the gully is deepened suffi- 
 ciently to tap the surface of the ground-water, springs are formed 
 along its sides, and a more or less permanent stream comes into 
 existence. In arid regions where the surface of the ground-water 
 lies deep, large valleys or arroyos (called wadis in Africa) may be 
 
412 
 
 The Formation of Clastic Material 
 
 cut by the run-off without reaching the ground- water level. These 
 arroyos will therefore carry water only in rainy periods, being dry 
 for the remainder of the season. 
 
 , 
 
 FIG. 345 b. Earth pillars at Bozen, in the Tyrol, due to rain erosion of 
 glacial till. The larger rocks often form protecting caps of the pillars. (After 
 Walther.) 
 
 Erosive Work of River-water. The erosive power of river 
 water varies as the square of the velocity of that water. This 
 will be appreciated, when it is realized that, if the speed of the river 
 
Destructive Work of the Hydrosphere 413 
 
 is doubled, it will, in the same time period, hurl twice as many 
 sand-grains as before, against an exposed rock surface in its bed ; 
 but it will also throw each grain with twice the force of its former 
 
 FIG. 346 a. Typical exposure of limestone showing peculiar weathering 
 forms due to solution. (Photo by Collier, U. S. G. S. Courtesy D. W. John- 
 son.) 
 
 speed. Thus the rock-surface will be eroded four times as fast as 
 before the doubling of the speed. The velocity of the water is 
 determined partly by the slope of the river bed, and partly by the 
 volume of the water passing a given point within a given time. 
 
 
 FIG. 346 b. Characteristic surface features on the Sentis in Switzerland 
 produced by solution on exposed limestone surfaces. These are called lapiaz 
 or rasdes by the French and karren by the Germans. (After A. Heim.) 
 
414 The Formation of Clastic Material 
 
 Thus in steeper portions of a river-bed, a greater velocity exists 
 and hence more erosion is accomplished. Again, if the volume of 
 river-water is increased after a rainfall, its velocity and erosive 
 power are increased. 
 
 The erosive work of river- water comprises denudation, or the re- 
 moval of loose material produced by weathering, etc. ; corrasion, 
 or the actual scraping or rubbing-off of material from the rock over 
 which it flows ; abrasion of the pebbles and sand grains which it 
 moves along ; quarrying by undermining of the banks or at water- 
 falls and the subsequent breaking down of the undermined rock, 
 and chemical corrosion or solution. 
 
 Denudation is chiefly confined to the early stages of the river 
 when the water first runs off on the surface of the mantle rock, and 
 by removing part of this, produces its gully. In mature rivers, too, 
 the weathering of the banks provides material: which the river re- 
 moves, and so do lateral branches, which deposit their own debris 
 upon the floor of the main valley, whence the main stream must 
 remove it by denudation. 
 
 Corrasion, or the mechanical wearing away of the rock of the 
 river bottom and sides, is perhaps the most important erosive ac- 
 tivity of streams. This is accomplished by the tools which the 
 river-water carries along, these tools being the sand or pebbles ob- 
 tained by denudation or by corrasion farther up stream. If the 
 sand and fragments carried along by the river are largely quartz 
 or a similar hard mineral, while the bed and banks are composed 
 of softer rock, the erosive effects will be most marked in the latter, 
 though the fragments themselves will be gradually worn to rounded 
 pebbles, and the quartz grains will likewise be rounded. This wear- 
 ing of the pebbles and sand is called abrasion. 
 
 Abrasion is also accomplished by mutual attrition, or the impact of fragment 
 upon fragment, as they are moved along by the water. If the loose material 
 is of soft rock, it will be rapidly destroyed by such abrasive activities, and the 
 average size of the material will become smaller as we proceed downstream. 
 This is well illustrated by observations made on the river Mur in Austria, where 
 at the city of Graz the average size of the rock fragments in the stream was 224 
 cubic centimeters. Farther downstream the average size diminished at the 
 following rates, there being here no new material supplied along the course : 
 At Graz, 224 cc. 56 km. below Graz, 81 cc. 
 
 10 km. below Graz, 184 cc. 71 km. below Graz, 60 cc. 
 
 26 km. below Graz, 132 cc. 83 km. below Graz, 50 cc. 
 
 43 km. below Graz, 1 17 cc. 101 km. below Graz, 33 cc. 
 
 1 20 km. below Graz (at Unter Manthdorf), 21 cc. 
 
Destructive Work of the Hydrosphere 415 
 
 The average distance necessary for rock fragments of different hardness and 
 consistency to be carried by rivers before they become completely destroyed 
 has been determined for some rocks to be as follows : 
 
 Sandstone (Rhaetic formation) av. wt. 40 grams, 15 km. 
 
 Clay-slate, av. wt. 24 gr. 42 km. 
 
 Compact limestone (Ordovician Orthoceras limestone) av. wt. 61 gr. 64 km. 
 
 Granular limestone, av. wt. 40 gr., 8 5 km. 
 
 Granite, av. wt. 36 gr. , 2 78 km. 
 
 The vast superiority of resistance of granite over that of the other rocks is at 
 once apparent. A similar differential destruction of the minerals derived from 
 the disintegration of rocks is effected by the abrasive process. As an example 
 may be cited the sands of some Scottish rivers (Eastern Moray) which are 
 derived from the destruction of the fundamental gneiss. In the river Spey, 
 the percentage of feldspar at one point (Cromdale) was 18 and of mica i, while 
 farther down-stream (at Orton), the feldspar made up only 12 per cent of the 
 sand. In the river Findhorn, near and parallel to the Spey, 42 per cent of 
 feldspar was found in the sand at one point (above Dulsie Bridge), while lower 
 down (between Forres Bridge and the sea) the percentage was reduced to 21. 
 This more rapid destruction of the feldspar is in large part due to the ready 
 cleavability of this mineral. The fine particles resulting from this destruction 
 are carried away by the current. 
 
 Sand grains carried by river- water are rounded by mutual attri- 
 tion and by contact with larger rock masses. The problem is a 
 complex one, but in general it may be said that the rounding is less 
 effective than in the case of wind-blown sand. Moreover, it is 
 generally true that the smaller grains will be less well rounded than, 
 the larger, and very small grains may not be rounded at all (except 
 by solution), while grains of the same size will readily be rounded 
 by abrasion in air. This is one of the means by which river sand 
 may be distinguished from wind-blown sand, but it must be used 
 with caution because of the many modifying factors. 1 
 
 Quarrying, by undermining the banks, is an effective way of 
 widening the river valley and of supplying new material for pebbles 
 and sand. In river gorges the bank on the outer or convex side 
 of the current is commonly kept vertical or overhanging by this un- 
 dermining process, because the principal current of the river always 
 makes more pronounced curves than the river as a whole, and so 
 flows close to the foot of the concave bank on the convex side of the 
 river, as illustrated in the following diagram (Fig. 347 a). On the 
 
 1 For a fuller discussion see Grabau, Principles of Stratigraphy, pp. 253-257, with 
 literature references, and a recent paper by J. J. Galloway, which attacks the problem 
 from the experimental point of view (Am. Journal of Science, vol. xlvii, pp. 270-280). 
 
416 
 
 The Formation of Clastic Material 
 
 concave side of the river (convex bank) a talus slope is generally 
 formed from the accumulation of the products of weathering 
 (Fig. 347 b). 
 
 The manner in which a stream changes from 
 a straight to a winding or meandering course 
 may be briefly set forth. 
 
 In a straight channel of uniform cross-section, 
 occupied by running water, the greatest velocity 
 of current is found in the center and, with still 
 air, just below the surface of the water. This is 
 readily shown by experiments with floats in 
 mill-races, or other artificial canals. The reason 
 for this is the fact that the center of the stream 
 is free from friction which affects the sides and 
 bottom -of the stream, while the surface film of 
 water is affected, though to a much less degree, 
 by friction against the atmosphere. If an ob- 
 P _ jy struction is placed in a channel on one side, the 
 
 showing the "stronger me- main velocity current is at once deflected away 
 
 andering of the current as f rom this obstruction and obliquely towards the 
 
 compared with the river as opposite side, against which it may impinge, and 
 
 a whole. from which it may in turn be again deflected. 
 
 This time the deflection will be in the opposite 
 
 direction, and the current will again cross the channel obliquely in its down- 
 
 stream course. Thus a single obstruction may throw the main current into a 
 
 zigzag or winding course, 
 
 which causes it to impinge 
 
 alternately against the right 
 
 and left banks in its onward 
 
 flow. 1 When the current 
 
 strikes the bank it will remove 
 
 loose material by its impact, 
 
 this being especially marked, 
 
 where the banks consist of 
 
 unconsolidated material. 
 
 When the current rolls and 
 
 sweeps along pebbles and 
 
 sand, these will be hurled 
 
 against the bank at the point 
 
 of impingement, and such 
 river tools will perform their 
 chief corrasive work at this 
 place. Thus the banks be- 
 come indented alternately on 
 
 FlG 34? & . _ Gorge of . the Genesee River 
 below p or tage, showing the vertical bank (260 
 f ee t high) on the outer or convex curve of the 
 river (concave bank) and the talus-covered 
 slope on the opposite side. (Photo by author.) 
 
 1 The right and left bank of a stream are those on our right and left hand, respect- 
 ively, when we face downstream. 
 
Destructive Work of the Hydrosphere 417 
 
 opposite sides, and this causes a further increase in the deviation of the current 
 from the original straight line. Meanwhile a part or the whole of the material 
 removed from the bank at the point of impingement is dropped again in the 
 slacker water below the point of erosion, or in general opposite the point where 
 the current next strikes the bank. Thus erosion and deposition alternate on 
 the same side. As the result of the building of a new sandbank on one side, 
 and the indentation of the old bank on the opposite side, the stream itself will 
 
 FIG. 348. Diagram illustrating the development of meanders in a river. 
 a, a straight channel, the main current has been deflected by an obstruction 
 above, and has assumed a winding course ; b, the entire river has begun to me- 
 ander by cutting into the banks at one place and building sandbanks on the-same 
 side next below; c, extreme meandering course, two of the meanders nearly 
 touch, and the formation of an oxbow on the left is imminent ; d, completion 
 of the oxbow or cut-off. 
 
 assume a winding course, as shown in diagram 6, Fig. 348. The continuation 
 of this process will result in changing the original straight form of the river 
 into a series of curves or meanders, as they are called, from the river Meander 
 (now called Mendere) which flows into the yEgean Sea near Miletus (Palatia) 
 in Asia Minor and in which such curves are very pronounced (Fig. 348 c. See 
 also Fig. 604). The continuation of this process may result in the cutting off 
 of a very pronounced meander which then remains behind as an "oxbow" 
 frequently forming a crescentic lake (Fig. 348, diagram d. See also Fig. 605). 
 
 Undermining is also carried on actively at the foot of a waterfall, 
 where the spray from the falling water attacks the cliff behind it. 
 This is the origin of the Cave of the Winds at Niagara, where a hard 
 limestone forms the top of the fall and a softer shale, or mud-rock, 
 the lower part. This softer rock is worn away by the spray, both 
 by the impact and by freezing, while the harder limestone projects. 
 
4i 8 The Formation of Clastic Material 
 
 At intervals this overhanging mass bre'aks down, and large blocks 
 of limestone accumulate at the foot of the cliff (Fig. 349). The 
 several recorded falls of Table Rock are illustrations of such sudden 
 changes after a long period of undermining. Waterfalls also per- 
 form effective erosion upon the river-bottom at their base, espe- 
 cially if the volume of water is great. This impact of the water and 
 the grinding work of the rock fragments, which it sets in motion, 
 
 FIG. 349. The American Falls at Niagara, as seen from the Canadian 
 side; showing the numerous blocks of limestone which have fallen from the 
 edge of the cliff, because the soft shale beneath them has been eroded by the 
 spray. The force of the water is insufficient to destroy these rocks or to use 
 them in eroding the bottom as is the case at the Horseshoe Falls. (Photo by 
 author. See also Figs. 666-669.) 
 
 result in deep excavation of the bottom. The " swimming hole " 
 at the foot of a small waterfall is a. familiar example. The Horse- 
 shoe Falls at Niagara have excavated the river-bottom to a depth 
 of 150 to 200 feet, or more than the height of the falls above the 
 river-level at their base. As these falls have been slowly receding 
 for a long time, the gorge has been deepened to this extent through- 
 out a distance of over three miles by this process. (See further, 
 Chapter XXIII.) 
 
 There are many special erosion features of rivers, which will be 
 discussed under the sculpturing of the earth's surface. Of these, 
 
Destructive Work of the Hydrosphere 419 
 
 pot-holes may be mentioned in this connection (Fig. 350 a). These 
 are local excavations in the river bed, formed wherever an eddy is 
 
 FIG. 350 a. Pot-holes at Harris Salt Springs, worn in granitic rock by eddies 
 in the bed of a stream, Tuolumne River, Cal. (Photo by Turner, from U. S. 
 G. S.) 
 
 produced which sets up a whirling motion of the sand and pebbles. 
 The holes cut by this whirling motion are round in section and have 
 
 FIG. 350 b. Pot-holes above the present level of the stream that formed 
 them. (After Geikie.) 
 
420 The Formation of Clastic Material 
 
 vertical sides. They may vary up to 20 feet or more in diameter, 
 and may be of equal or greater depth. They are most frequently 
 
 FIG. 351. Watkins Glen, N. Y., a narrow gorge largely formed of confluent 
 pot-holes, cut in shaly sandstones of upper Devonian age. 
 
 found in the beds of mountain torrents and near waterfalls (Fig. 
 3506). A series of confluent pot-holes of large size may produce 
 
Destructive Work of the Hydrosphere 421 
 
 a deep and narrow gorge, as is the case in Watkins Glen in southern 
 New York (Figs. 351, 352). 
 
 FIG. 352. One of the lafger pot-holes in Watkins Glen, N. Y. 
 
 Corrosion. Solution or chemical corrosion is on the whole of 
 little importance in rivers, though it is accomplished wherever 
 limestones or other soluble rocks are encountered. Most of the 
 material held by river-water in solution is, however, furnished to it 
 
422 The Formation of Clastic Material 
 
 by the underground water which issues as springs along the river 
 border. 
 
 Underground Water 
 
 Types of Underground Water. The water which sinks into 
 the ground forms a part of the underground or subsurface water. 
 The quantity of such water depends on the climate, the porosity 
 of the rock or soil, the surface slope of the land, and other factors. 
 As the water sinks into the ground, it reaches a level at which its 
 further progress through the rock and soil becomes so slow that it 
 may be regarded as stationary. This forms the zone of saturation, 
 and the water there forms the true ground-water. The upper level 
 of this zone is called the water-table or ground-water level, and it 
 varies in depth in the same locality with the season, and for longer 
 periods, with the character of the rock, the amount of rainfall, etc., 
 and in different localities with the climate etc. (p. 441). It is 
 not a level surface, but varies with the contour of the land and 
 other factors. It is this lowest average level of the water-table 
 which must be reached by wells to be permanently supplied by 
 unfailing springs. 
 
 The spaces in which this ground- water flows are partly large fissures and 
 partly minute interstices between the grains. These interstices vary with the 
 character of the rock and to some extent with its age. The larger openings 
 also vary with the character of the rock and the disturbances to which it has 
 been subjected. They comprise the planes of bedding between sedimentary 
 rocks, the joints and other fissures which traverse it, solution cavities, etc. In 
 the larger fissures the ground-water is chiefly controlled by gravitation, but in 
 the smaller interstices within the rock the water is subject to the force of ad- 
 hesion, which is the mutual attraction of rock and water, in minute quantities. 
 This force of adhesion is most pronounced in rocks with the smallest interstices. 
 When the interstices are so fine that their walls will attract all the water they 
 can hold, or if interstices are few and far between or are absent altogether, the 
 rock is called impervious to water under ordinary conditions, that is, it will not 
 permit water to pass through it. This of course implies that no cracks or fissures 
 of subsequent origin are present. Other rocks are more or less pervious or per- 
 meable to ground-water under ordinary conditions, the degree of permeability 
 depending on the porosity of the rock and on the amount of fissuring which it 
 has suffered. 
 
 That solid appearing rocks also will take up water can easily be shown by 
 immersing them in water, and noting the difference in weight between the dry 
 and water-soaked rock. The water occupies the spaces between the rock 
 particles. Such spaces may constitute as much as thirty per cent or over 
 (in some cases 60 per cent) of the volume of the rock, or they may form less 
 
Destructive Work of the Hydrosphere 423 
 
 than one per cent of its volume. The greatest porosity is found in rocks com- 
 posed of grains of uniform size and spherical form, especially if the arrangement 
 of the grains is such that they rest vertically one upon the other. Rocks made 
 up of rounded and uniform grains are best produced from wind-blown sands, 
 and hence, where -such rocks are part of the bedded rock series of the earth s 
 crust they form good media for the transmission and storage of underground 
 water provided the changes in character and position which they have under- 
 gone 'are not unfavorable to such processes. Unconsolidated sands and 
 pebble beds are of course among the best of media for the transmission and 
 storage of underground water, but if these are surface deposits, they do not 
 afford good reservoirs for well waters, since they also admit the surface drainage. 
 The rate of flow of underground water varies greatly, being especially in- 
 fluenced by the porosity of the rock. Though locally it may be relatively rapid, 
 the average rate of flow is probably not much over a mile a year. 
 
 As stated above, the name ground-water is applied to the water 
 in the zone of saturation, that is, the water below the water-table. 
 This water is also called phreatic water, a term derived from the 
 Greek word for well, because it is the water which supplies wells 
 and springs. The water in the rock and soil above the water-table, 
 held there temporarily by molecular attraction against gravity, 
 but slowly sinking to the level of the water-table, constitutes a part 
 of the vadose water, which is the name applied to all the subsurface 
 water circulating through the soil or rocks above the water-table. 
 
 The best conditions for the storage of well-water are found when a horizontal 
 or gently inclined pervious bed is covered by an impervious one, for this wi 1 
 keep out the surface drainage. It is of course necessary that all wells sunk 
 through such an impervious bed should be properly protected above the level 
 of that bed, to prevent the entrance of surface waters. The entrance of the 
 water into the pervious bed takes place at its outcrop, which may b 
 miles removed from the region where the water is tapped. 
 
 The surface of the ground-water or the water-table is not a level one. It 
 rises inland from the sea, and conforms somewhat to the irregularities of the 
 country but is much less diversified. The porosity of the rock, the amount 
 rainfall in the region, the presence or absence of much vegetation, especially 
 forests, etc., are among the factors which influence the position of the watei 
 table in a given region. 
 
 Where valleys are cut below the surface of the ground-water level, tl 
 water will issue along their sides in the form oi. hillside springs. Where t 
 ground-water level and that of the surface of the land coincide, swamps are 
 produced ; where the land-surface is depressed below the normal wate /el, 
 
 lakes are formed. 
 
 Where porous or permeable layers enclosed by impervious ones crop c 
 along the mountain sides or at a considerable elevation, the water entem 
 them will gradually find its way to lower levels, but because of the elevation 
 
424 
 
 The Formation of Clastic Material 
 
 . ^3 tn O .t} 
 
 s g - .2 
 
 of the point of entrance, the water thus stored in the porous rock will be under 
 great hydrostatic pressure. If now wells are sunk in the low country to this 
 
 porous rock, the water will rise, because of 
 the pressure it is under, and overflow at the 
 surface, or if the country in which the well is 
 sunk is much lower than the head of water at 
 the intake, a gushing well may result. Such 
 wells are called artesian wells. Their rela- 
 tionship to the intake near the mountains 
 -is shown in the diagram (Fig. 353). If the 
 rock of the low country is intersected by 
 dislocation-planes or faults (see Chapter 
 XIX), the water may rise along such planes 
 and form artesian springs. 
 
 Beside the two types of subsurface 
 waters mentioned, there are two 
 others to which reference has already 
 been made in previous chapters. 
 These are the connate waters, or the 
 waters originally included in the sedi- 
 ments deposited in water, either salt 
 or fresh, and the magmatic or juvenile 
 waters, which are formed as emana- 
 tions from deep-lying igneous mag- 
 mas. These are, however, of little 
 inportance in rock destruction. 
 
 Destructive Work of Underground 
 Water. The only kind of destruc- 
 tive work which underground water 
 performs is chemical, comprising so- 
 lution and hydration, and indirectly, 
 oxidation and carbonation by carry- 
 ing oxygen and carbon dioxide. Oxi- 
 dation, hydration and carbonation 
 have already been described. 
 
 Solution. This is most effective 
 in limestone regions, where caverns 
 are dissolved out by the circulating 
 ground-water above the water-table, 
 i.e. within the vadose belt. Solution 
 
 of limestones is accomplished by the aid of carbon dioxide which 
 the water carries, and in the resulting solution, an extra mole- 
 
 (L pr 
 
 -5 8 
 
 ll*sf.a 
 
 <fi 
 
 iff 
 
 e & = 
 
 .3- 1 
 
 " -a E 
 
 I 8-5 j I 
 
 
Destructive Work of the Hydrosphere 425 
 
 cule of that gas is locked up for each molecule of calcium car- 
 bonate. On the re-deposition of the calcium carbonate, this extra 
 molecule of CO 2 is again liberated. Caverns are most readily dis- 
 solved out along the bedding planes of the limestone and along 
 the cracks or joints which intersect it. After the cavern has been 
 dissolved out it is slowly filled up again by stalactite and stalag- 
 mite growths, though this may go on in one part of -a cavern 
 while solution enlarges another. If the water-table rises so that 
 the ground-water will drown the cave, its walls may become lined 
 with crystals of calcite, often of enormous size, or with other 
 
 FIG. 354. Diagram of a cavern, showing different levels of the under- 
 ground stream. (After G. C. Matson, U. S. G. S.) AB, Connecting funnels 
 between the upper and lower cavern. The latter is undergoing active solu- 
 tion, the ground-water level having been lowered to it, while deposition of 
 stalactites and stalagmites goes on in the upper level, where pillars have also 
 been formed. A sink hole is seen on the left of the diagram, and several others 
 filled with soil are indicated. (See also Figs. 184 and 185, pp. 263, 264.) 
 
 deposits (copper salts, etc.). The crystal caves of Missouri are 
 examples. The characteristic deposits in limestone caves have 
 already been discussed (pp. 262-265). 
 
 On the surface of the country where caves abound there are com- 
 monly found more or less circular depressions called sink-holes or 
 swallow-holes (Fig. 354), which form entrances for the waters to 
 the cave and were dissolved by the waters thus entering the under- 
 ground drainage. Other irregular hollows are formed by the break- 
 ing or incaving of the roof of the cavern. The Natural Bridge of 
 Virginia (Fig. 355) is the last remnant of a cavern roof. 
 
 Other rocks besides limestones are subject to solution by the sub- 
 surface waters. Among these rock salt and gypsum are the most 
 conspicuous. Even quartz is dissolved, especially when the sub- 
 
426 The Formation of Clastic Material 
 
 surface waters contain much humic acid from plant decay, and it 
 is indeed this dissolved silica, which is carried by the streams into 
 
 FIG. 355. Natural Bridge, Virginia. A remnant of a former cave roof. 
 The rock is lower Ordovician limestone. (Courtesy of D. W. Johnson.) 
 
 the sea, that constantly supplies the material from which the radio- 
 larians and the diatoms construct their silicious cases. 
 The rate of solution varies greatly, and is strongly influenced by 
 
Destructive Work of the Hydrosphere 427 
 
 the temperature of the ground-water. When this temperature 
 approaches the freezing point, the solvent action of the water is 
 practically lost. The warm springs of Bath, England, with a mean 
 temperature of 120 F., discharge annually a quantity of dissolved 
 sulphates of calcium and sodium, and chlorides of sodium and mag- 
 nesium, sufficient to make a square column of mineral matter nine 
 feet in diameter and 140 feet high. The great deposits of litho- 
 graphic limestone at Solnhofen in Bavaria are reduced by solu- 
 tion one meter in thickness in every 72,000 years, and the similar 
 extensive limestones of the Nittany Valley in Pennsylvania, one 
 meter in every 30,000 years. The materials removed by solution 
 each year throughout the entire globe have been estimated at 
 96 tons per square mile, of which calcium carbonate constitutes 
 50 tons, calcium sulphate 20 tons, sodium chloride 8 tons, silica 7 
 tons, alkaline carbonate and sulphates 6 tons, and oxide of iron i ton 
 (T. Mellard Reade), leaving 4 tons for other substances. 
 
 Indirect Formation of Clastic Rock Due to Underground Solu- 
 tion. When the roof of a cave breaks down, the rock comprising 
 it is broken into large and small angular fragments, which are 
 piled together in a confused heap. Such breaking down of the sur- 
 face rock is frequently seen in limestone regions, and is even more 
 marked where gypsum and salt have been extracted. Thus in 
 many regions of western England, where the salt (Permian) is mined 
 by the solution process, extensive cavings have occurred, often de- 
 stroying villages and parts of small towns. Because of the angular- 
 ity of the material resulting from such a break, it has a distinctive 
 character, and the name brecciation is applied to the process, and 
 brecciated material to the result. When reconsolidated it becomes 
 a brecciated rock, or, for short, a breccia. Such cavern breccias 
 are not uncommon among the rocks of the earth's crust. Other 
 breccias, such as fault breccias, volcanic breccias, etc., have al- 
 ready been referred to. (See further Chapter XVIII.) 
 
 Destructive Work of the Waves 
 
 The waves of the sea and of lakes exert a powerfully destructive 
 effect upon the margins of the land on which they border. This de- 
 structive effect is due in part to the mere impact and lifting power 
 of the water and its prying power where it enters crevices, and in 
 part it is due to the work of tools which, in the form of sand and 
 
428 The Formation of Clastic Material 
 
 rock fragments, are hurled by the waves against the cliff, or moved 
 along over a rock surface. 
 
 The force of the waves in great storms is often very surprising. Measure- 
 ments on the north Scottish coast have shown the force of impact to be 611 
 pounds per square foot in summer and 2086 pounds per square foot in winter, 
 while exceptional storms may produce an impact of waves nearly three times 
 that amount. The greatest recorded pressure was 6083 pounds per square foot 
 during a heavy westerly gale near the end of March, 1845. Measurements 
 recorded from the French, Italian, and north African coasts show ranges from 
 600 to more than 3000 pounds per square foot. As an illustration of the power 
 of the waves to lift rock material may be cited the breakwater in the harbor of 
 Wick on the northeast coast of Scotland. This breakwater consisted, above 
 the foundation, of three large blocks, weighing 80 to 100 tons each, across which 
 a huge concrete monolith, weighing over 800 tons, was cast in place and .firmly 
 anchored to the blocks by iron chains. The total mass weighed 1350 tons, and 
 yet during the great storm of December, 1872, it was lifted by the waves and 
 hurled into the inner harbor, a distance of 10 to 15 meters, the monolith and 
 three foundation stones remaining anchored together and being moved as a 
 single mass. 
 
 The erosive work of waves is confined to ablation processes, and 
 these are chiefly restricted to quarrying and corrasion and abrasive 
 work. There is, in addition, a certain amount of solution going on, 
 as well as prying by the freezing of the spray and the compression 
 of air in crevices, but these are of minor importance. 
 
 Wave Quarrying. By the constant impact of waves against 
 the cliffs, rock fragments are loosened and lifted from their position. 
 This is especially the case where the rock of the cliff is traversed 
 by cracks or fissures. On the Massachusetts coast, the basaltic 
 dikes commonly have a rude columnar structure, the columns ex- 
 tending across the dike from wall to wall. Wave impact loosens 
 these columns, and they are then lifted out by the waves and car- 
 ried away, and thus the shoreward ends of these dikes are marked 
 by deep parallel-sided chasms (see Fig. 135, p. 192). On the Irish 
 coast, the columns of basalt which form the Giant's Causeway 
 (Fig. 1 20, p. 176) are similarly removed by the waves, and on the 
 island of Staffa on the Scottish coast, such removal has produced 
 the famous FingaPs Cave (see Fig. 122, p. 178). Other examples 
 are found on the coast of the Bay of Fundy and elsewhere, and 
 such removal of columns produces, usually, a steep or vertical cliff. 
 When granite cliffs dip into water of moderate depth, steep cliffs 
 are seldom produced, especially if the granite shows few fissures 
 or structure planes which permit such quarrying operations. The 
 
Destructive Work of the Hydrosphere 429 
 
 waves have comparatively little effect upon granite, which, because 
 
 of its hardness, is not easily corraded. The upper parts of the cliff, 
 
 therefore, being subject to the attack of the weather, will retreat 
 
 more rapidly than the 
 
 lower, and a sloping 
 
 granite headland will 
 
 project into the sea 
 
 (Fig. 356 a). If the 
 
 granite is fine-grained 
 
 and is traversed by 
 
 many joint fissures, the 
 
 quarrying operations of 
 
 the waves are of course 
 
 facilitated. 
 
 Rocks which are char- 
 acterized by bedding 
 planes may be quarried 
 into large blocks by 
 waves. Such effects are 
 seen in the limestone 
 and chalk cliffs of the 
 English and French 
 
 FIG. 356 a. Granite headland, Marble- 
 head, Mass., showing the sloping surface which 
 is due to the more effective attack of the 
 weather on the upper part of the cliff as com- 
 pared with wave erosion which is effected 
 mainly in the joint fissures. (Photo by 
 author.) 
 
 coasts (Fig. 201, p. 279, 
 
 and Fig. 713), especially 
 
 upon the North Sea coast of England. Many of the vertical cliffs 
 
 of softer rocks are, however, produced by corrasion at their base 
 
 and by the falling of the masses of rock thus undermined. 
 
 Corrasion and Abrasion. When the waves hurl sand and rock 
 fragments against the base of a cliff, this becomes subject to wear 
 by corrasion. The sand and rock fragments at the same time suffer 
 erosion by a process of abrasion. Cliffs of soft rock are especially 
 attacked in this manner, and where the wave work is more or less 
 constant at their base, they are apt to present a vertical face (Fig. 
 16, p. 31). The rate of wear varies, of course, with the force and 
 constancy of the waves and the degree of softness or corradability 
 of the rock, as well as with the hardness of the tools employed by 
 the waves. 
 
 The rock fragments employed as tools by the waves, whether 
 broken off by the waves themselves or supplied by the weathering 
 of the cliff, suffer wear. This is most pronounced upon the angles 
 
430 
 
 The Formation of Clastic Material 
 
 and corners of the fragments. In this manner angular blocks are 
 rapidly worn into rounded pebbles and boulders (Figs. 356 b, c). 
 Sand grains suffer in a similar manner, and the softer or more 
 
 cleavable minerals 
 among them are 
 rapidly destroyed, 
 so that the remain- 
 ing sand is largely 
 or wholly composed 
 of quartz grains. 
 
 On shores charac- 
 terized neither by 
 rock cliffs nor by 
 glacial material, 
 
 FIG. 356 b. A sand and cobble-stone beach with 
 rock ledges in the background. 
 
 such as drumlin 
 moraines, etc. (see 
 Fig. 447), pebbles 
 
 and boulders are relatively- uncommon, though some of them may 
 be transported for considerable distances by shore currents. Even 
 where the pebbles are not hurled against a cliff, but are merely 
 rolled back and forth by the waves, mutual impact will effect a 
 
 FIG. 356 c. Pebbles worn from bricks by the waves,- Nahant, Mass. These 
 fragments illustrate three stages in pebble making. The first is rectangular, 
 with only the corners and edges worn off; the second is sub-rounded and the 
 third fully rounded, though still retaining the elongated form of the original. 
 (F. H. Lahee photo, from Gardner collections of photographs, No. 7401. Cour- 
 tesy Geol. Dept., Harvard University.) 
 
 wearing off of angles and corners, and rounded pebbles result. 
 By continued wear these will be reduced to smaller and smaller 
 size and may eventually be reduced to fine dust or rock-flour. 
 Sands, which make up most of the material of shores not bounded 
 
Destructive Work of the Pyrosphere 431 
 
 by cliffs, also have their grains worn round by mutual abrasion. 
 Such rounding, however, affects primarily the larger grains, which 
 from their size and weight come more constantly in contact with 
 one another ; the smaller grains, being more readily held in sus- 
 pension by the agitated waters, suffer less abrasion and so remain 
 more generally in an angular state. Moreover, the wet sand of a 
 beach commonly forms a compact mass, owing to the water be- 
 tween the grains, so that a gently sloping sand beach is often nearly 
 as hard surfaced as a rock floor. Here the waves have little effect, 
 and the rounding of the grains by mutual abrasion is greatly re- 
 tarded (Fig. 357). 
 
 FIG. 357. Daytona Beach, Florida. A hard-packed sand beach, extending 
 for many miles. A favorite beach for automobile races. 
 
 On the whole, the grains of beach sands may be regarded as much 
 less well rounded, than those worn by wind, this being especially 
 true of the smaller grains. Nevertheless, there are many modify- 
 ing influences, and caution must be used in the employment of this 
 criterion for the determination of the origin of a given mass of 
 sand grains. 
 
 DESTRUCTIVE WORK OF THE PYROSPHERE 
 
 In volcanic eruptions of the explosive type, both the semi-fluid 
 and the solid lava and the solid rock of non- volcanic origin may be 
 shattered. The size of the fragments thus produced may range 
 from large blocks to the finest dust, the finer material being as a 
 rule more abundant. When lava, which has not yet become 
 thoroughly hardened, is shattered by an explosion during the 
 eruption, these f ragmen tal lava masses are hurled into the air and 
 may harden in the process. The larger blocks will fall back to the 
 
432 The Formation of Clastic Material 
 
 surface in the neighborhood of the volcano, and form rounded, elon- 
 gated, or otherwise shaped volcanic bombs (Fig. 58, p. 116). Such 
 bombs generally show rounded surfaces from the molding process 
 which the mass of lava underwent in its passage through the air, this 
 being often accompanied by a gyrating or spinning movement. 
 When solidified lavas are shattered, large angular fragments are pro- 
 duced, which may form great accumulations and be subsequently 
 bound together to form volcanic agglomerates. Other rocks shat- 
 tered by these explosions will also produce angular fragments, 
 though of course all such fragments may subsequently be rounded 
 by becoming water-worn. The finer products of shattering of still 
 viscid lavas are known as lapilli, and they also show rounded or 
 smooth surface characters. Fine fragments resulting from the 
 shattering of solid rocks, on the other hand, will be sharply angular. 
 The finest volcanic dust is carried into the upper atmosphere and 
 may be transported by the air-currents over wide areas before it 
 settles again. Such material is generally recognized only by 
 microscopic examination, which reveals the fact that it is largely 
 or wholly of volcanic origin. (See Fig. 488.) 
 
 DESTRUCTION OF ROCKS BY MOVEMENTS OF THE EARTH'S CRUST 
 
 When through some disturbance in the earth's crust, either vol- 
 canic or under the influences of stresses and strains, a fracture arises, 
 movement of the rock masses along such a fracture will result in the 
 grinding and fragmentation of the contiguous rock walls. Such 
 movements may be back and forward, or they may be mainly in 
 one direction, the moving mass producing a displacement of the 
 crust or & fault (Figs. 542 , 543 , Chapter XIX) . Such disturbances are 
 commonly manifest on the surface as earthquakes, and may be shown 
 by actual displacements, either horizontally or vertically, of the 
 adjoining masses of rock and of the soil and other structures upon it. 
 (See further, Chapter XXI.) Within the fissure the shattered ma- 
 terial from the walls will accumulate, commonly in the form of 
 coarse and fine fragments thoroughly intermingled. The fragments 
 will be angular and will consist of the material of the adjoining 
 walls of the fracture. Such a mixture of angular fragments and 
 fine rock, sand, and rock-flour is called a fault rubble, and when 
 consolidated forms & fault breccia (Fig. 32, p. 80). Fault breccias 
 may occur in vertical or inclined fissures, or they may lie in hori- 
 
Rock Destruction by Glaciers 
 
 433 
 
 zontal beds between two masses of rock, one of which has been 
 pushed over or under the other, or both of which may have 
 moved past each other. Its angular character, and the fact that 
 the material consists only of the broken fragments derived from the 
 enclosing rocks, distinguish such a fault breccia from other rubble 
 rocks. 
 
 ROCK DESTRUCTION BY GLACIERS 
 
 Ice is a rock, though only a temporary one in most climates, and 
 the movement over other rock of an ice mass, such as that of a gla- 
 cier, is analogous to the movement described in the preceding sec- 
 tion. One may of course consider a glacier as a frozen river, but 
 the erosive effects of moving ice are much more like those of moving 
 rock masses than they are like those of running water. 
 
 Glacial erosion is wholly mechanical, and is accomplished by 
 plucking and sapping or quarrying and by grinding or corrasion and 
 
 FIG. 358. Limestone polished, furrowed, and scratched by the modern 
 glacier of Rosenlaui, Switzerland. (After Agassiz, from Lyell.)^ aa, white 
 streaks or scratches caused by small grains of flint frozen into the ice ; bb, fur- 
 
 rows. 
 
 abrasion (see Chapter XIV). In addition to this a newly formed 
 glacier will perform denudation or the removal of the material which 
 had accumulated by weathering and otherwise on the surface over 
 which it moves, and prior to its formation or advent. This is the 
 most important erosive work of a glacier in the early stages, while 
 later, quarrying (plucking and sapping) and corrasion become the 
 chief erosive work of the ice. In general, the loose material en- 
 
434 The Formation of Clastic Material 
 
 countered is removed by becoming incorporated in the basal portion 
 of the ice through the freezing of the moisture which saturates this 
 material. In quarrying, the ice mass freezes to projecting rocks or 
 in fissures, surrounding such a projection, and in its movement tears 
 or plucks the rock away. In the performance of corrasion the rock 
 material, frozen firmly into the base of the ice, scratches and pol- 
 ishes the rock floor over which it moves. If the motion is uni- 
 form, series of parallel scratches are formed, which indicate the 
 direction of ice movement (Figs. 358, 359, 360 a). 
 
 FIG. 359. A glaciated surface of gneiss, Bronx Park, N. Y. The bands 
 extending across the surface from the lower left-hand side are the gneissic 
 bands ; the glacial striae run nearly at right angles to these. (Photo by Willis ; 
 from U. S. G. S.) 
 
 The fragments which the ice uses as tools are themselves affected 
 by this corrasive process. The sands are ground and crushed to 
 fine rock flour, which is one of the most characteristic types of ma- 
 terial produced by glacial erosion. This must be distinguished 
 from clay or the similarly fine material due to atmospheric decom- 
 position. Under the ice there is little or no opportunity for chem- 
 ical decomposition, and even after this rock flour is carried away 
 by glacial streams, which it renders turbid or milky, and is de- 
 posited elsewhere, it may not undergo decomposition except 
 near the surface. Rock flour, therefore, though resembling clay 
 
Rock Destruction by Glaciers 
 
 435 
 
 in fineness, is distinct from it. Its minute grains may consist of 
 a variety of minerals, according to the rock from which it was 
 derived. 
 
 The larger rock fragments are ground and polished upon their 
 surfaces, partly by being dragged over the rock-bottom and partly 
 by the movement of the sand-laden ice over them. If the rock 
 fragments were originally flat, they will be polished and striated only 
 on the two larger faces, the others remaining more or less angular 
 (Fig. 360 b). If the fragments are of approximately equal dimen- 
 sions in all directions, they may have turned over frequently, and 
 
 FIG. 360 a. Photograph of a 
 pebble from Permian glacial deposits 
 near Jaquariakyva, Parana, South 
 Brazil. This is a fragment of the 
 old pre-Permian rock floor, showing 
 its glacial striae. (Courtesy of Prof. 
 J. B. Woodworth.) 
 
 FIG. 360 b. A small glaciated 
 boulder from the Pleistocene till. 
 About one half natural size. (Photo 
 by B. Hubbard.) 
 
 all sides may be polished and striated. Unless subsequently worn 
 by water, such striated rock fragments constitute a characteristic 
 feature of the deposits .formed by glaciers and will lead to the 
 identification of such deposits even after consolidation (Fig. 419). 
 It should, however, be remarked that in talus slides and under 
 certain other conditions, rock fragments may be polished and stri- 
 ated, but they will never be so perfect nor so abundant as those 
 produced by glacial activities. 
 
 Glacially produced debris never rests where formed, but always 
 suffers more or less transportation, partly by the ice itself and partly 
 by the streams formed from the melting of the ice. This will be 
 more fully discussed in the next chapter. 
 
436 The Formation of Clastic Material 
 
 ROCK DESTRUCTION BY ORGANISMS 
 
 If we except man, who is by far the most powerful agent active 
 in the destruction of the rocks of the earth's crust, we must confess 
 that organisms play but a minor role in the destruction of rocks. 
 True, there are certain bacteria which appear to be very active 
 agents in bringing about the weathering of rock surfaces, and the 
 decay of plants furnishes both carbon dioxide and humic acids, 
 which are powerful agents in rock-solution, but aside from this 
 the destructive work of organisms is slight. Plants, whose roots 
 grow in fissures in the rock, will by continued growth force the walls 
 of the fissures apart and so perform a certain amount of superficial 
 destruction (Fig. 33, p. 81). Great herds of animals, too, perform 
 a certain amount of surface destruction by pounding the rock to 
 powder under their hoofs, along the paths of their migration and 
 especially around drinking pools. Such destructive work performed 
 by herds .of cattle in Hereroland, former German Southwest Africa, 
 has been described as follows by Pechuel Loesche : 
 
 " In extensive manner these animals aid in the leveling of many land areas. 
 As the dryness increases, the herds of grazing cattle become more numerous 
 around the last of the sparsely distributed water bodies. Thousands and tens 
 of thousands of the large and the small animals overrun for miles the surround- 
 ing country for days, weeks, and months. Through countless hoof-beats the 
 ground is loosened and so furnishes enormous masses of dust, while at the same 
 time all inequalities are trampled down and destroyed. The inclined surfaces 
 would be furrowed by numerous rain water gullies if these were not constantly 
 destroyed by the hoofs of the roaming animals and .if it were not for the fact 
 that rain water is constantly guided along the paths formed by the animals 
 going to and from the water in long lines, ranged one behind the other. Fur- 
 thermore, the cover of dust prevents to an astonishing degree the penetration 
 of the short heavy downpour of rain into the deeper strata." 
 
 This loosening and pounding of rock to dust becomes an effect- 
 ive agent in the lowering of the surface, because the finer particles 
 will be constantly removed by the winds, and it is not too much 
 to attribute to the destructive work of great herds of animals in 
 semi-arid regions the principal role in the lowering of the land over 
 large areas and the production of gently inclined planes free from 
 furrows, above which project remnants of the older surface in the 
 form of butte-like hills or ridges. Such " island hills," as they are 
 called, abound in the great Kalahari desert region of Central Africa. 
 The work of burrowing land animals, such as the moles, prairie 
 
Rock Destruction by Organisms 437 
 
 dogs, etc., also aids to a certain extent in the further destruction 
 of unconsolidated material. 
 
 When we come to the smaller land animals we must note espe- 
 cially the worms and the ants and termites which produce a super- 
 ficial loosening and rearrangement of the unconsolidated material 
 'of the earth's crust, but perform relatively little destructive work 
 in the solid rocks. The many galleries produced by ants and ter- 
 mites admit the air to the soil and so favor decomposition, while 
 organic matter carried down by these animals furnishes acids for 
 the solution of rock particles. 
 
 In the sea, too, animals effect a certain amount of rock destruc- 
 tion. Thus many fish, feeding upon growing coral polyps, crop off 
 the coral ends with the polyps and grind the coral to powder, which 
 eventually passes from them as lime flour. Calcareous algag or 
 nullipores are also ground up in this manner. Crabs, and other 
 crustaceans also, break up shells of mollusks and echinoderms on 
 which they feed, and so produce a not inconsiderable quantity 
 of shell fragments which will enter into the construction of new 
 limestones. Other marine animals active in this way are the echi- 
 noids, which destroy rocks to a certain extent, by drilling holes in 
 them ; but their work as rock breakers is of moderate significance. 
 
CHAPTER XVI 
 
 TRANSPORTATION, SORTING, AND DEPOSITION OF 
 CLASTIC ROCK MATERIAL 
 
 ONLY the clastic material due to weathering may remain in the 
 place where it is produced or suffer only slight shifting of locality, 
 before becoming reconsolidated into a rock, which then will be un- 
 assorted in respect to material, and essentially without structure. 
 Such old soils, subsequently changed to rock, are found in some 
 localities, but for the most part the product of weathering is trans- 
 ported and redeposited elsewhere, and in this process undergoes 
 more or less sorting, while new structures incident to these 
 modifications are impressed upon it. 
 
 AGENTS or TRANSPORTATION AND SORTING AND REGIONS 
 OF DEPOSITION 
 
 Transportation. The chief agents of transportation of clastic 
 material are wind, and water currents, both those of rivers and those 
 of the sea. Glaciers, however, and floating icebergs also transport 
 clastic material, though much of that transported at first by 
 glaciers is subsequently carried farther sometimes by water, and 
 sometimes by wind. There are of course minor agents of trans- 
 portation, among which may be mentioned other floating structures, 
 such as vegetation, etc., to which various kinds of rock material 
 are attached ; snow and landslides, and animals which carry 
 rock material either attached externally or in their stomachs. 
 Man is of course the most effective agent of transportation, calling 
 to his aid the resources of wind, water currents, steam, electricity, 
 and animal energy ; but he works, as yet, upon a far smaller scale 
 than do the less efficient rivers, winds, waves, and ice. 
 
 Sorting. Both wind and running water accomplish sorting 
 of material, and the waves of the sea and lakes are also active in 
 this respect. Glaciers and icebergs, on the other hand, transport 
 material without sorting it in any way. 
 
 438 
 
Transporting and Sorting by Winds 439 
 
 Deposition. Deposition of clastic material takes place in the 
 sea (marine deposits), in lakes and ponds (lacustrine deposits), and 
 on dry land (terrestrial deposits). The last two types are often 
 designated continental as distinct from marine deposits. Inter- 
 mediate between the lake deposits, on the one hand, and those of 
 dry dust and sand, on the other, are the deposits formed on river 
 flood-plains and on alluvial fans, while between the terrestrial and 
 marine deposits, the sea-border delta sediments form a transition 
 type. 
 
 TRANSPORTING AND SORTING BY WINDS 
 
 Whatever the origin of clastic material, if it is light enough and 
 small enough in grain, it can be transported by wind. As we have 
 seen, the removal, by wind, of loose material from a surface where it 
 has been produced is called deflation, and this is the beginning 
 of eolian transport. Such transport is accomplished either by 
 carrying the material in suspension as in sand and dust storms, or by 
 rolling and pushing it along over the surface without lifting. Only 
 fine material such as dust can be retained in suspension for any 
 length of time, and thus be carried far. The volcanic dust pro- 
 duced by the explosion of Krakatoa, in 1883, referred to in Chap- 
 ter VII (p. 137), was carried by the upper air currents repeatedly 
 around the earth before settling. But even pebbles, 8 mm. in 
 diameter or larger, may be picked up by a strong wind and carried 
 a short distance. Much larger sand grains and pebbles may of 
 course be rolled along the surface than could be lifted by wind 
 of the same strength. 
 
 Both the material which is carried in suspension and that which 
 is rolled along by the wind become assorted according to size and 
 weight of grain, the latter varying, of course, for particles of the 
 same size according to their mineral character. The smaller 
 grains are picked out and carried farthest ; the larger ones settle 
 out earlier. The lighter minerals too are carried far, leaving the 
 heavier behind. Thus after repeated transportation by wind, the 
 material will become well sorted, only grains of about the same 
 size and of the same mineral character accumulating at any given 
 locality. In this process of transportation the grains will also be 
 more or less worn and rounded by mutual abrasion, as already de- 
 scribed (p. 406). 
 
440 Deposition of Clastic Rock Material 
 
 When an older sand deposit consists of well-rounded grains of 
 uniform size and of only one mineral species, the supposition is very 
 strong that such a deposit represents the work of the wind at a 
 former time. A rock formed of such material, and known as the 
 Sylvania sandstone of Silurian age, occurs in Ohio and Michigan. 
 The grains are wonderfully well rounded (millet seed type) ; they 
 are of essentially uniform size in each locality, though those of one 
 locality may differ from those of another, as is shown in the two 
 photographs (Fig. 361 A, B) which represent grains from different 
 localities, enlarged to the same degree. They consist wholly of 
 
 FIG. 361. Microphotographs of Sylvania sand grains, enlarged about n 
 times. A y from National Silica Co., Monroe county, Mich., near top; B, 
 from Rockwood pits, four feet down, American Silica Co. The grains are well 
 rounded and of nearly uniform size, but differ in size in the different localities. 
 (W. H. Sherzer, photo; from Grabau and Sherzer, Monroe Formation of 
 Michigan.) 
 
 quartz without any foreign admixture. This fact and the ready 
 separation of the grains makes the rock valuable for glass manu- 
 facture. This rock has been interpreted as formed of wind- trans- 
 ported, wind-sorted, and wind-worn sand grains, and the larger 
 structural characters of the deposit bear out this hypothesis. (See 
 Fig. 374, p. 455.) 
 
 Dust and Sandstorms (Fig. 362). All deserts are subject to 
 violent sand and dust storms, when the strong wind picks up the 
 dry material from the surface and whirls it along in a cloud, which 
 may appear like a solid moving wall. After such a storm the air 
 will be hazy with dust for a long time, and indeed in some desert 
 regions the air is seldom free from suspended dust. As the wind 
 dies down, the coarsest material settles first, the finest being long 
 
Transporting and Sorting by Winds 441 
 
 held aloft and carried far. Sandstorms may be very destructive to 
 life, and as a result of their occurrence great changes in the aspect 
 of the country are produced. To this constant motion of the sand 
 and dust, as much as to the absence of moisture, is due the general 
 lack of vegetation in the sandy desert (Figs. 363 a, b). 
 
 FIG. 362. Sandstorm in the Sahara; from nature. (After Ratzel.) 
 
 Distances to which Sand and Dust are Transported. The dis- 
 tance over which material is transported depends not only upon the 
 size and weight of the particles and the strength and continuity 
 of the wind, but also upon the initial position of the material. 
 When dust is projected high into the air, as by volcanic explosions, 
 the force of the wind is wholly employed in carrying it, and 
 hence such substances will be transported farther than those picked 
 up by the wind directly. As already noted, the fine dust of Kra- 
 katoa was repeatedly carried around the world, and its suspension in 
 the upper air caused everywhere the brilliant sunsets for which the 
 period following the eruption was noted. The coarser ashes fell 
 inches deep at a distance of a thousand miles from the volcano, and 
 some even fell in Holland. Volcanic dust from Iceland has fallen 
 repeatedly in Scandinavia, Holland, and northern Britain. Dust 
 
442 Deposition of Clastic Rock Material 
 
 from the Sumatran volcano, Tomboro, has fallen a thousand miles 
 
 away, and dust from the Mexican volcano, Colima, fell in February 
 
 and March, 1903, at 
 points more than 200 
 miles northeast of the 
 volcano. 
 
 The ash from the 
 eruption in 1835 of the 
 Nicaraguan volcano, 
 Cosequina, covered an 
 area of 1,500,000 square 
 miles, and even reached 
 Jamaica, more than 750 
 miles away. Dust from 
 the eruption of Mont 
 Pelee in 1812 is said to 
 have reached the Azores 
 on the other side of the 
 Atlantic, and dust from 
 
 Vesuvius has been observed to fall in Greece, in France, and in 
 
 Austria. 
 Dust of non-volcanic origin is also carried to great distances. 
 
 Some known to have been derived from the Sahara, was repeatedly 
 
 FIG. 363 a. View of a portion of the Desert 
 of Sahara. 
 
 FIG. 363 b. General view in the arid region, western North America. 
 
Deposition of Wind-blown Sands 443 
 
 carried across the Alps and fell in such distant regions as England 
 and North Germany, 2000 miles away. Dust from Australia some- 
 times reaches New Zealand, 1500 miles away, and yellow dust 
 from the interior of China has fallen on the decks of vessels south- 
 west of Japan, having been carried at least a thousand miles from 
 its source. 
 
 Volume of Dust Falls. The quantity of dust transported during 
 a single storm may be very great. In 1863 a rain of dust fell on the 
 Canary Islands which was estimated to have a weight of 6,500,000 
 tons. The great dust storm of March 9-12, 1901, brought 1,960,000 
 tons of dust to Europe and 1,650,000 tons to North Africa, cover- 
 ing a total area of 300,000 square miles of land surface and 170,000 
 square miles of ocean. It was estimated that this dust traveled, 
 in part at least, a distance of 2500 miles. It is thus clear that wind- 
 transported material plays an important role in nature. 
 
 DEPOSITION OF WIND-BLOWN SANDS 
 
 Wind-blown material may be deposited in water or upon dry 
 land. Only the latter has distinctive characters, for the former 
 is incorporated with other water-laid (lacustrine or marine) elastics. 
 The most characteristic types of wind-borne sand deposits are the 
 dunes. These are found on most coasts, but also abound inland, 
 especially in desert regions, where they often assume considerable 
 proportions and cover wide areas. 
 
 Types of Sand Dunes 
 
 Three main types of sand dunes are recognized : (a) the conical 
 hill, (b) the linear, and (c) the curved or crescent-shaped type, 
 known as the barchane (Fig. 368). Between the last two are often 
 combination types which have some of the characters of both. The 
 linear type is most common on the coast, where a uniform and 
 constant supply of sand is furnished ; while the crescent-shaped or 
 barchane type is most characteristic of the interior, the river flood- 
 plains of arid regions, and the deserts. According to location, 
 dunes may be classed as (a) coastal, (b) river-bottom and flood- 
 plain, and (c) inland or desert dunes. 
 
 Coastal Dunes. These are chiefly of the linear type and ex- 
 tend in general at right angles to the prevailing wind. They al- 
 ways have an asymmetrical cross-section, the opposite sides sloping 
 
444 Deposition of Clastic Rock Material 
 
 EAST 5EA 
 
 at decidedly different angles. The side toward the coast, exposed 
 to the direct force of the wind, generally has a slope of only about 
 10 ; the angle of the opposite or leeward slope is often as high 
 as 30. Sand grains are rolled up the windward side, and passing 
 the crest of the dune, roll down upon the lee slope. Thus the 
 
 dune slowly advances in the 
 
 direction in which the pre- 
 vailing wind blows, which, 
 being from the sea, causes 
 the dunes to move inland 
 at a very variable rate. 
 
 The advancing sand dunes 
 bury whatever lies in front 
 of them. The dunes formed 
 on the barrier beaches of 
 our coasts advance over the 
 salt meadows formed in the 
 lagoon behind the bars (see 
 p. 330). In long-inhabited 
 coastal regions, the dunes 
 may cover buildings and 
 
 FIG. 364 a. Map of the region between 
 Danzig and Memel on the East Sea or 
 Baltic, Eastern Germany, showing the 
 Kurische Nehrung (or bar) and Kurische 
 Haff (or lagoon) north of Kdnigsberg, 
 and the Frische Nehrung and Frische 
 Haff south of Konigsberg. These are 
 typical examples of dune covered sand 
 bars enclosing lagoons. 
 
 even villages. The dunes 
 of the North Sea coast of 
 Denmark and Schleswig- 
 Holstein, which cover an 
 area of 165,557 acres, and 
 rise in places to a height of 
 nearly a hundred feet, fur- 
 nish a good illustration of 
 
 this phenomenon, especially as the coast here is also cut back 
 rapidly by the sea, and the region is believed to be subsiding. In 
 1757, it was found necessary to tear down the church of the village 
 of Rantum on the island of Sylt, on the west coast of Schleswig- 
 Holstein, because it was reached by the advancing dunes. In 
 1791 or 1792, the entire dune chain had passed over the church 
 ruins, these then lying on the shore, which had advanced to this 
 point. Sixty years later, the site of the church was 700 feet from 
 shore, the depth of water at that point being 12 feet. The second 
 church has since been buried by the advancing dunes. 
 
 On the east shore of the Baltic (East Prussia) are two long sand 
 
Deposition of Wind-blown Sands 
 
 445 
 
 bars, the Kurische and the Frische Nehrung, which separate large 
 lagoons, the Kurische and the Frische Haff, from the Baltic (Fig. 
 364 a). These bars are covered with extensive sand dunes, some 
 
 Kircht von Kunzen. 
 
 
 Kurisches Haff 
 
 Stelle ier vertandeten Kirche, 
 
 Ruine dtr Kirche von Kunzen. 
 
 FIG. 364 b. Diagrams showing the advance of the coastal dunes over 
 the village and church of Kunzen on the Kurische Nehrung (bar), from the 
 east shore of the Baltic Sea to the Kurische Haff (barachois). (i) At the 
 beginning of the nineteenth cerltury, when the dunes had just reached the 
 church of Kunzen. (2) Conditions in 1839, when the church was buried by sand. 
 (3) Conditions in 1869, when the dunes had passed beyond the village site, 
 uncovering again the ruins of the church. (After Behrend ; from Ratzel.) 
 
 of which reach a height of sixty meters or more, being among the 
 most imposing dunes of the European coast. Between the years 
 1809 and 1869 the dunes of the Kurische Nehrung (bar) in their 
 
 FIG. 365 a. Tower of the buried church of Eccles, Norfolk, England, 1839. 
 The inland slope of the hill of blown sand is shown in this view, with the light- 
 house of Hasborough northwest of the tower in the distance. (See Fig. 365 6.) 
 (From Lyell's Principles.) 
 
446 Deposition of Clastic Rock Material 
 
 advance buried the church of the village of Kunzen by passing 
 over it and later resurrected it by migrating further inland (Fig. 
 364 b). An analogous phenomenon has been observed on the 
 coast of Norfolk, England, as shown in the next two figures 
 (Figs. 365 a, b). 
 
 The largest dune area of the European coast, and one of the 
 largest coastal dune areas known anywhere, is on the western shores 
 of France, along the Bay of Biscay. The dunes extend almost 
 
 FIG. 365 b. Eccles Tower as it appeared after the storm of November, 
 1862. From a drawing by Rev. S. W. King, taken from nearly the same 
 position as Fig. 365 a. (From LyelPs Principles.) 
 
 without interruption for a length of 240 km. and lie in parallel 
 rows up to ten in number, over a coastal strip from four to eight, 
 and in some cases 10 km. wide, covering a total area of 120,000 
 hectares (296,520 acres) . In height the dunes range up to 90 meters, 
 making them the highest known coastal dunes, though some in- 
 land or desert dunes which have advanced toward the coast in North 
 Africa reach twice this height. 
 
 Behind the great Biscayan dune belt lies the low, swampy tract 
 known as the Landes. The dunes travel inland at a rate ranging 
 from 15 to 105 feet per year, burying not only the swamp lands, 
 but forests, farms, vineyards, and even villages. The church at 
 Lege was taken down at the end of the seventeenth century and 
 rebuilt two and a half miles farther inland. One hundred and sixty 
 
Deposition of Wind-blown Sands 447 
 
 years later it had to be removed again, because the sands had once 
 more reached it. This gave an average rate of advance for the 
 dunes of 81 feet per year. The sea, too, advances, cutting, back 
 the coast in places at the rate of not less than 2 meters per year. 
 
 The dunes on the Atlantic coast of North America are mostly 
 of inferior size and formed in connection with off-shore sand bars. 
 At the extreme end of Cape Cod, however, where the waves have 
 built the series of bars which form the foundation of the Province- 
 town dune-lands (see Fig. 714), great sand dunes, some of them 
 
 FIG. 366 a. Dune of barchane form overwhelming trees on the Province- 
 lands of Cape Cod. The top branches of a tree protrude from the crest of the 
 dune. (D. W. Johnson, Shore Processes and Shoreline Development; John 
 Wiley & Sons.) 
 
 exceeding 3oor 40 meters in height, have been formed by the wind. 
 These dunes slowly advance southward and westward, burying 
 forests and even buildings in their advance (Fig. 366 a). Their 
 movement has, however, been checked to some extent by the 
 planting of grasses and shrubs on the windward slopes. Between 
 the years 1826 and 1838, some $28,000 were spent in this work of 
 arresting the dunes. The dunes of the Virginia and Carolina coasts 
 are also of considerable magnitude, and they, too, bury forests and 
 whatever else lies in the path of their advance (Fig. 366 b). 
 
 In the mastery of sand dunes by means of skillfully planted 
 vegetation France, Belgium, Holland, Germany, and other countries 
 
448 Deposition of Clastic Rock Material 
 
 have far outstripped America, because the lands in the old world 
 coastal district are far more valuable on account of the denser 
 populations. 
 
 On the shores of the Great Lakes, dunes are likewise well de- 
 veloped. The most extensive dune area lies on the eastern and 
 southern shores of Lake Michigan, and some of the dunes there rise 
 to the height of a hundred feet, especially in the Dune Park area 
 of Indiana. A dune near Muskegon, Michigan, has a height of 
 several hundred feet, and from its rate of advance has become 
 known as " Creeping Joe." Many of these dunes are grassed over 
 
 FIG. 366 b. Shore dunes near Cape Henry, Virginia, migrating inland over 
 the forest. (D. W. Johnson, Share Processes and Shoreline Development; John 
 Wiley & Sons.) 
 
 and even wooded, but they are still recognizable by their form. 
 Others are in active motion, burying and killing forests and again 
 uncovering them in their advance. In many cases only the very 
 tops of the trees now project above the surface of the dune. The 
 sand of these dunes is chiefly of glacial origin. 
 
 River-bottom and Flood-plain Dunes. In semi-arid regions 
 the rivers partly dry away during the summer season, leaving broad 
 flood-plains and river-bottoms covered with the sands brought 
 down by the rivers. These sands are then heaped into dunes by 
 the winds and travel across the country, sometimes from one river- 
 bottom to another. Such dunes are extensively developed along 
 
Deposition of Wind-blown Sands 
 
 449 
 
 the river borders in the semi-arid regions of southern Russia ; as, 
 for example, along the Dnieper, the Don, and the Volga, where 
 they occur in belts sometimes as wide as 30 kilometers, and have 
 a length of 150 kilometers or more. The dunes are seldom more 
 than five to seven meters in height, though some reach a height of 
 12 meters or more. 
 
 FIG. 367. Outline map of the drainage region of the Oxus and Jaxartes 
 rivers (Amu-darya and Syr-darya) and the deserts of Kyzyl-Kum and Kara- 
 Kum. 
 
 As typical examples of river-bottom dunes may be cited those of 
 the rivers Oxus and Jaxartes in Turkestan, Central Asia, both of 
 which flow into the Aral Sea. " These streams bring vast quan- 
 tities of sand and mud from the Tian Shan, Great Pamir, and Hindu- 
 Kush mountains, in which they arise, and spread them over the 
 low ground of their flood-plains, which range in width up to ten 
 kilometers. The thickness of the deposit made by the Oxus was 
 found to be 23 meters at Tschard-schui. The rivers rise three 
 meters from March to July, and overflow the flood-plains, deposit- 
 ing sandy sediment. As the water of the Jaxartes falls, the hot 
 
450 Deposition of Clastic Rock Material 
 
 northern winds soon dry the deposit and carry away all the finer 
 dust particles, leaving only the pure quartz sand, which is heaped 
 into dunes. These wander southward across the Kyzyl-Kum 
 desert [Fig. 367], sometimes at a rate of 20 meters during a stormy 
 day, but generally the sand masses move at an average rate of six 
 miles per year. Reaching the Oxus, these sands are incorporated 
 in its sediment, and the operations of sorting the deposits on the 
 flood-plain of this river are repeated, and the sands are again heaped 
 into dunes which wander southward across the Trans-Caspian or 
 Kara-Kum desert until they reach the borders of the Caspian Sea. 
 The activities of the streams are unceasing, and the supply of 
 material in the mountains in which they rise is practically inex- 
 haustible. Thus there is a constant succession of sand dunes 
 wandering southward across these deserts, and layer upon layer 
 of sand accumulates, each showing the characteristic eolian struc- 
 tures and helping to build up a deposit of pure, unf ossilif erous sand 
 of almost unlimited thickness." 1 Large sand dunes are also found 
 on western American river-bottoms, such as those of the Columbia 
 (Fig. 368) and Snake rivers in Oregon and Washington. 
 
 The dunes of these river bottoms and intervening deserts are* both 
 of the linear and the crescent-shaped type. The linear dunes are 
 generally symmetrical in section and extend parallel to the direc- 
 tion of the wind, where this is strong and the supply of sand 
 large. They may, indeed, be regarded in many cases as the 
 extended horns of the crescent or barchane type. 
 
 Desert Dunes. In deserts the barchane or crescent type of dune 
 is most typical. When best developed, this presents a crescent 
 form with the convex side to the wind and its surface a gentle slope 
 (Fig. 368). The lee side is steep and abrupt, with a' concave out- 
 line, from the ends of which the horns extend in the direction of 
 advance. By an elongation of these horns, when much sand is 
 supplied and the winds are strong, long, parallel ridges, united at 
 intervals by cross ridges, may be produced, their direction running 
 parallel to that of the wind. 
 
 The great desert dune districts are found to-day in Asia, Africa, 
 and Australia. Dunes cover only one ninth of the total area of 
 the Sahara, but even this aggregates a total of 18,000 geographical 
 square miles. Arabia is par excellence the land of desert dunes, 
 nearly one third of the entire surface, or not less than 15,000 
 
 1 A. W. Grabau, Principles of Stratigraphy, p. 561^ 
 
Deposition of Wind-blown Sand 
 
 geographical square miles, being covered with sand. Almost the 
 whole southern area is occupied by the terrible Desert of Roba-el- 
 Khali, or the desert Dehna, which is wholly covered by eolian 
 sands, and is without the relief of oases. This desert is 150 geo- 
 graphical miles in length and 80 in width. In the northern part of 
 the peninsula lies the Nefud Desert, where the sands are red and 
 water practically absent. 
 
 FIG. 368. Sand dunes of the barchane type in the valley of the Columbia 
 River, near Biggs, Oregon. River terraces shown in the background. (Photo 
 by Gilbert, U. S. G. S. Courtesy of D. W. Johnson.) 
 
 Sandy deserts exist in many parts of Asia, and the whole in- 
 terior of Australia is a desert, many parts of which are covered with 
 drifting sands. Sandy deserts also abound in southwestern North 
 America, especially in southern California and Arizona. Almost 
 one fourth of the state of Nebraska, or about 18,000 square miles, is 
 covered with drifted sands, forming the " Sand Hills " region 
 with its many lakes, from some of which potash salts are ex- 
 tracted. These lakes are the result of a change in climate from 
 dry to moist, in consequence of which the old dunes are mostly 
 covered with vegetation, though many fresh hollows or " blow- 
 outs " occur. In the central plains 'of Hungary, too, are many 
 old sand dunes now covered with vegetation, but readily recogniz- 
 able by their form. 
 
452 
 
 Deposition of Clastic Rock Material 
 
 The sands of many deserts are derived from the disintegration pf 
 older sandstones ; as, for example, the sands of the Libyan Desert, 
 which are derived from the Nubian sandstone, and from a younger 
 
 69. The edge of the Libyan Desert. 
 
 rock from which the Sphinx is carved. This sand (Fig. 369) 
 has been carried in places for a hundred miles from its source, and 
 often rests upon old limestone surfaces, the weathered-out fossils 
 of which it encloses in its basal portion. From long transport 
 
 FIG. 370. Microphotographs of sand grains from the Libyan Desert, 
 enlarged about n times. A, finer sand only partly rounded, due to constant 
 accession of new material ; B, coarser grains (mechanically separated), showing 
 pronounced rounding and relative uniformity of size. (W. H. Sherzer, photo ; 
 from Grabau and Sherzer, Monroe Formation of Michigan.) 
 
Deposition of Wind-blown Sands 
 
 453 
 
 the sand is well sorted according to size of grain and purity of ma- 
 terial, and from wear the grains are well rounded (Fig. 370, B). 
 Where much fresh material is furnished by local disintegration of 
 rock, a certain amount of angularity of the smaller grains is still 
 seen (Fig. 370, A). The more perfectly rounded grains are similar 
 to those of the Sylvania sandstone of Silurian age (Fig. 361, p. 
 440). 
 
 Structure of Sand Dunes 
 
 Ripple Marks. We have seen that the wind carries the sand up 
 the gentler side of the dune, and that it rolls down the steeper side. 
 The progress of the sand on the windward side is commonly shown 
 
 FIG. 371. Typical sand d 
 California, Colorado Desert. 
 D. W. Johnson.) 
 
 nes with ripple marks in desert region of southern 
 (Photo by. Mendenhall, U. S. G. S. Courtesy 
 
 by ripple marks of an asymmetric character (Fig. 371), these being 
 in reality secondary dunes of minute size upon the surface of the 
 larger ones. The following diagram shows types of ripple marks 
 observed in desert sands (Fig. 372 a). 
 
 Cross-bedding of Dunes. With different velocities of the wind, 
 particles of different size will be deposited upon the slopes, and 
 thus layers of different texture will be formed, which may appear 
 distinctly in a section of the dune. These layers have a gentle 
 
454 Deposition of Clastic Rock Material 
 
 slope on one side and a steeper one on the other, and at the base of 
 the dune the gentler sloping layers will gradually pass into a 
 
 FIG. 372 a. Various types of ripple marks in Lop-Nor Desert, Turkestan. 
 (After Sven Hedin, Scientific Results of a Journey in Central- Asia.) 
 
 FIG. 372 b. Three cross-sec- 
 tions of dunes near Ostend, 
 Belgium, showing Eolian cross- 
 bedding. (From Kayser's Lehr- 
 buch.} 
 
 horizontal position. If now by a change in the direction or force 
 of the wind a part of the dune is cut away again, the remaining 
 
Deposition of Wind-blown Sands 
 
 455 
 
 basal portion will present layers which slope in two directions 
 and will be abruptly truncated above by a horizontal or oblique 
 erosion plane. A second dune deposited above this remnant may 
 become so placed that its 
 layers slope in the opposite 
 direction, or at a different 
 angle from those beneath it. 
 Thus will be formed a type 
 of cross-bedding, as it is 
 called, in which there are 
 successive divisions of the 
 mass, separated by erosion 
 lines, and with the layers of 
 each division dipping in one 
 or more directions without 
 reference to the dip of the 
 layers in other sections. The F f- 373- - Thin section of quartz 
 J , sandstone shown under the microscope, 
 
 preceding illustration repre- The original rounded quartz grains are 
 sents actual cross-sections of surrounded by new quartz (secondary en- 
 dunes near Ostend, and shows ^gement) forming a quartrite, Haupt- 
 
 buntsandstem, near Heidelberg; en- 
 
 the result of such repeated larged 12 diameters. (After Rosenbusch.) 
 partial destruction of older, 
 
 and deposition of newer, dunes, and the wind or eolian cross- 
 bedding thus formed (Fig. 372 b). 
 
 Ancient Deposits with Eolian Cross-bedding. Many ancient 
 sandstones show a cross-bedding of this type, from which it may be 
 inferred that they represent older dune deposits now consolidated. 
 As such dunes are at present most characteristic of and most ex- 
 tensive in desert areas, we are led to conclude that the deposits in 
 question, if at all extensive, were formed under similar circumstances. 
 
 For corroborative 
 evidence, the char- 
 acter of the grains 
 must be examined, 
 since in typical 
 eolian deposits 
 these should be 
 
 FIG. 374. Cross-bedding in Sylvania sandstone. 
 
 more or less well rounded, assorted according to size, and essentially 
 of the same mineral substance. The original roundness of the 
 grains may be obscured toy the subsequent deposition around them 
 
Deposition of Clastic Rock Material 
 
 of mineral matter of the same character, so that they appear angu- 
 lar. Such secondary enlargement of the grains, as it is called, can 
 generally be detected under the microscope (Figs. 373, 472). 
 
 FIG. 375. Eolian cross-bedding in Mesozoic sandstone, Little Meadow 
 district, Utah. (Gardner Collection of Photographs. Courtesy 'of the Geo- 
 logical Department of Harvard University.) 
 
 Eolian cross-bedding is well shown in the Sylvania sandstone, 
 above referred to, as having the characteristic grains of eolian 
 deposits (Fig. 374). Eolian cross-bedding is strikingly shown in 
 
 the Jurassic sand- 
 stone which forms 
 the White Cliff of 
 the Colorado Plateau 
 and in other sand- 
 stones of western as 
 well as eastern North 
 America. This 
 seems to point to the 
 eolian origin of these 
 rocks (Figs. 375, 
 
 FIG. 376. Large boulder of sandstone show- 37 / 
 ing Eolian cross-bedding, Mt. Pisgah, Greene Possible Sources of 
 County, N. Y. Error. It should, 
 
 however, be borne in mind that only when such cross-bedding is 
 developed on a large scale and over a wide extent can it be regarded 
 as indicative of eolian deposition. When restricted to a narrow 
 
Dust Deposits 
 
 457 
 
 area, it may be due to the cross-bedding produced in sand bars 
 along the coast. As will be shown later, this is of limited extent 
 only. Cross-bedding of this type, but with the successive divisions 
 
 FIG. 377. Cross-bedding of the Eolian type in the Orange sand or La 
 Fayette Formation, Mississippi Central Railroad, Oxford, Miss. (After 
 Hilgard.) 
 
 measuring inches rather than feet in thickness, can also be pro- 
 duced by successive current ripples such as may be formed upon 
 the bottom of a shallow water body. Cross-bedding on a large 
 
 FIG. 378. Eolian type of cross-bedding in ancient limestones (Mississip- 
 pian), formed of uniform lime-sand grains (calcarenyte) south of St. Louis, 
 Mo. Scale, i inch = 3^ feet. (From Principles of Stratigraphy.} 
 
 scale cannot, however, be produced in this manner. The preceding 
 illustrations show such cross-bedding on a large scale, in uncon- 
 solidated older sands (Fig. 377), and in ancient limestones 
 
 (Fig. 378). 
 
 DUST DEPOSITS 
 
 Mode of Deposition. Dust-laden wind sweeping across a 
 steppe land, where herbaceous vegetation grows in a scattered 
 manner, will have its velocity checked when it comes in contact 
 with this vegetation, and a part of its dust load will be dropped. 
 This will accumulate around the plants, which thus become buried 
 in the dust deposits essentially in the position of growth. Succes- 
 sive accumulations of dust will raise the surface of the plain until 
 the plants die and new ones take their place. In this manner, 
 thick deposits of dust may be built up. 
 
458 Deposition of Clastic Rock Material 
 
 When dust-laden winds reach a rainy district they are washed 
 clean by the rains, and similar deposits of dust accumulate, but 
 these are likely to be more or less modified by running water or 
 become incorporated in pond or lake sediments. 
 
 Character of Deposits. Unmodified dust deposits, owing to their 
 uniformity and fineness of grain, lack as a rule all evidence of 
 bedding or deposition in layers i.e., stratification. They will 
 form a more or less homogeneous mass of uniform structure 
 throughout. 
 
 The Loess an Example of a Dust Deposit 
 
 In the eastern part of China, extensive deposits of fine dust-like 
 material have been formed, to which the name loess is applied. 
 This dust is believed to have been carried by the wind from the 
 great deserts of the interior, especially the desert of Gobi, and it 
 has accumulated to such an extent that in some sections its thick- 
 ness is one or even two thousand feet. This loess has a very high 
 fertility, and although its surface has been cultivated for thousands 
 of years without the application of artificial fertilizers, it is still 
 productive, owing perhaps largely to the constant addition of new 
 material. Its extent appears to coincide with the limits of Chinese 
 agriculture. Loess has the property of maintaining a vertical face 
 whenever a section is cut, due largely to the presence in it of verti- 
 cal tubes which are more or less filled with mineral matter, and 
 which cause the vertical scaling off of layers ; thus walls of loess 
 200 feet in height are produced. Where roads have been worn into 
 it by constant travel and where the material loosened thereby 
 has been removed by wind, these roads lie in canyon-like depres- 
 sions with vertical walls often of considerable height. In some 
 places in China, caverns are dug into the base of such a wall and 
 occupied as dwelling places by the humbler natives. 
 
 Loess is also found in the United States, the material having 
 been supplied by glacial streams during the ice age of a former 
 period. During the drier periods between ice advances (inter- 
 glacial periods), this dust has been taken up by the winds and de- 
 posited in favorable localities. Such deposits frequently include 
 the shells of land mollusks, such as snails (Helix, Fig. 256, p. 316), 
 and occasionally those of river border types. (See Figs. 252-255, 
 pp. 315-316.) In it are also found the peculiar little concretionary 
 masses which characterize the Chinese and many European loess 
 
Dust Deposits 459 
 
 deposits and which have become known as loess dolls (Loess- 
 pilpchen or Loessmannchen), and which were apparently formed at 
 subsequent periods by the concentrative work of ground-water 
 
 (Fig. 477 d, p. 573)- 
 
 Loess contains a small percentage (4 or 5 per cent) of iron oxide 
 generally in the hydrous form. Being uniformly disseminated 
 through the mass, it gives the loess a yellowish color, and where 
 streams cut into such deposits, their waters are colored by the fine 
 sediment carried along. The Yellow River of China (Hoang-Ho) 
 owes its color and name to such loess sediment carried by it, and 
 the color of the Yellow Sea is likewise derived in this manner. 
 
 We can picture to ourselves what would happen if such loess 
 deposits became buried by other sediments laid down upon them, 
 and consolidated into a rock. Such a rock would be of uniform 
 grain and show no bedding structure (stratification), except per- 
 haps at considerable intervals. Moreover, from the gradual 
 effect of aging and of heat, the water of the iron hydrate would be 
 driven off, and the color of the deposit would change from yellow 
 to a uniform red. Thus a bright-red or brick-red rock of uniform 
 fine grain, without bedding planes, would be produced. Ancient 
 rocks of this character are not unknown, and they have sometimes 
 been interpreted as altered loess deposits of a former time. Such 
 rocks are of course as a rule free from the remains of organisms, 
 except that those of terrestrial animals and the seeds and other 
 wind- transported parts of plants may become included in them. 
 The plant remains will tend, by decomposition, to furnish sub- 
 stances which locally change or remove the iron so that white or 
 greenish spots and streaks may mark the otherwise red rock. 
 
 The Black Earth of Russia. Over considerable areas in Russia, 
 the surface of the earth is covered with a loess-like deposit, the 
 upper part of which, for 20 feet or more, is colored black by a 
 thorough admixture of organic matter (Fig. 379). This Black 
 Earth or Tchernozom or Tschernosem, as it is called, appears to 
 represent the accumulation of fine dust among actively growing 
 herbaceous vegetation, the decay of which furnished the organic 
 matter of the black earth, of which it sometimes forms ten or 
 more per cent. This organic matter renders the soil exceedingly 
 fertile and capable of growing large crops continually without 
 manure. A similar black soil, called the Regur or black cotton soil, 
 occurs in India. Such soils when buried under other deposits and 
 
460 
 
 Deposition of Clastic Rock Material 
 
 compacted, become black carbonaceous shales, such as are found 
 throughout the southern states in the late Palaeozoic series (Chat- 
 tanooga Shale), though other interpretations have also been pro- 
 posed for them. 
 
 FIG. 379. Map showing the distribution of the Black Earth or Tscher- 
 nosem in Russia. (After Glinka.) 
 
 TRANSPORTING AND SORTING BY STREAMS 
 Transportation of Sediments 
 
 Streams are among the most effective agents of transportation. 
 Besides carrying substances in solution, they carry fine particles 
 in suspension, and push and roll larger ones along their bottoms. 
 In this manner material may be transported by streams for hun- 
 dreds of miles before it finally comes to rest. 
 
 Conditions of Transportation. The materials which streams 
 carry are the products of rock weathering and of mechanical wear, 
 partly produced by the stream itself, and partly supplied by rain 
 water, by rills, and by wind. The power of the stream to carry, roll, 
 or push along material varies with the velocity, and in any given 
 section of the stream this velocity varies as the cube-root of the 
 volume. Thus a stream swollen to eight times its original size will 
 
Transporting and Sorting by Streams 461 
 
 have its velocity doubled, and therefore its carrying power is in- 
 creased. This in general varies as the sixth power of the velocity, 
 and therefore, a stream the velocity of which has been doubled will 
 be able to carry sixty-four times the quantity of material which it 
 could transport before. 
 
 This may be demonstrated in the following manner (Fig. 380). Given, a 
 current which can just move along a cube of rock (a) of a certain size. If 
 the velocity were doubled, twice as much water as before would strike the cube 
 in a given time, and with twice the force. Therefore, such a current would 
 move along four such cubes placed end to 
 end. If 1 6 such prisms, each of four cubes 
 placed end to end, were piled together, a cube 
 would be produced, and as each of the six- 
 teen prisms would be subjected to the same 
 impact of water which would move it sepa- 
 rately, it is apparent that when united into 
 a cube (b), the entire mass will be moved as a 
 body, for the area against which the water 
 impinges has been enlarged 16 times. It is of 
 course evident that this holds only for the case 
 here outlined, namely a cube 64 times as large 
 as the original cube. A rock of any other 
 shape would not be moved in the same de- 
 gree, and as few rock fragments approach a 
 cubical form, the formula can be applied only 
 in a general way. Nevertheless, it is evident 
 that with increase in velocity an enormous in- 
 crease in carrying power results, and this explains why streams after rains 
 become heavily laden with debris and can perform destructive work of almost 
 unthought-of magnitude, besides carrying away huge amounts of material in 
 a short time and moving along very large blocks of rock. 
 
 In general, a river current flowing at the rate of one fifth of a mile 
 an hour can carry along fine clay ; one running at the rate of half 
 a mile an hour transports sand ; at the rate of a mile per hour, the 
 current can roll along medium sized gravel ; while at ten miles an 
 hour it can roll along pebbles of the size of an egg. There is, how- 
 ever, much variation according to the nature of the bottom, whether 
 covered with sediment or free from it, and whether the finer ma- 
 terial is stirred up by eddies, etc., or has to be picked up by the 
 current itself. 1 
 
 FIG. 380. Diagrams illus- 
 trating the increased carrying 
 power of streams with in- 
 creased velocity. (After Le 
 Conte.) 
 
 1 For citation of detailed measurements see A. W. Grabau, Principles of Stratigraphy, 
 pp. 248-251. 
 
462 Deposition of Clastic Rock Material 
 
 Volume of Material Transported by Rivers. The total amount 
 of material transported by rivers is surprisingly great, and also 
 varies much for different rivers. Taking some of the large rivers 
 of the earth, we find that the Mississippi, with a flow of 17,500 
 cubic meters of water per second, carries 211,300,000 cubic meters 
 of material per year; while the La Plata, with a flow of 19,820 
 cubic meters of water per second, carries only 44,000,000 cubic 
 meters of material per year. Again, the Hoang-Ho or Yellow River 
 of China, with a flow of only 3,285 cubic meters of water per second, 
 carries the enormous quantity of 472,500,000 cubic meters per 
 year, its waters being turbid with sediment. The Mississippi 
 River actually carries more than 400,000,000 tons of sediment into 
 the Gulf of Mexico each year, or more than a million tons a day. 
 The exact volume of material, according to the measurement of 
 Humphreys and Abbot, is 7,471,411,200 cubic feet (211,273,000 
 cubic meters), a mass sufficient to cover an area of one square mile 
 to a depth of 268 feet. The amount carried to the sea by all the 
 rivers of the earth in one year has been estimated to be about 
 forty times this quantity. 
 
 While the coarser material is, as a rule, pushed or rolled along 
 the river-bottom, the sands generally assume the form of low banks 
 alternating in position on the two sides of the stream. They are 
 generally of triangular form, their bases lying against the river 
 bank, and between them and the opposite bank lies the deeper 
 channel with the main current. (See map, Fig. 381, at Delta, and 
 Fig. 605, at Georgetown Bend.) Sand is removed on the 
 upstream and deposited on the downstream side of the sand bank, 
 which thus slowly wanders down stream. This may go on at the 
 rate of 200 to 400 meters per year in some cases (Rhine), and in 
 others (Loire) from less than two meters per day in summer to more 
 than 1 8 meters per day in winter, which is the season of floods. In 
 all cases the movement of the sands is much slower than that of 
 the water, and the amount of water passing a given point may be 
 a thousand times more than the amount of waste shifted past the 
 same point. 
 
 Sorting of Sediments by Rivers 
 
 River- transported material is subject to assortment by the 
 destruction of the softer materials in the transport, so that after a 
 while only the most resistant material will remain. A much 
 
FIG. 381. Flood-plain of the Mississippi River, near Vicksburg, Miss. The 
 highland to the east rises 150 to 250 feet above it. Note, (i) the meandering 
 course; (2) its change since the interstate boundary was fixed; (3) recently 
 abandoned channels : Paw Paw Chute and Old Channel ; (4) Wilton Bayou ; 
 (5) ox-bow cut-off at De Soto Island; (6) crescent lake: Long Lake; 
 (7) artificial channels : Yazoo River ; Diversion Canal and Grant's attempted 
 diversion near Delta ; (8) cut banks near Vicksburg ; (9) artificial banks or 
 levees near Delta; (10) deposition on inner side of curve south of Delta; 
 (n) "made land" or islands of deposition; (12) concentric ridges and silted 
 hollows near Wilton Bayou, representing lateral migration of the channel; 
 (13) swampy lowland recently abandoned and imperfectly silted. (Military 
 Geology.} 
 
 463 
 
464 Deposition of Clastic Rock Material 
 
 worked-over series of river deposits may, indeed, have been sub- 
 jected so thoroughly to this searching out of destructible material 
 that nothing but quartz pebbles and sand will remain. Observa- 
 tion on Scottish rivers has shown that the percentage of feldspar 
 in the sand derived from the crystalline rocks of the Highlands 
 slowly decreases downstream owing to its progressive destruction. 
 (See p. 415.) 
 
 In general, great masses of pure quartz-sand and pebbles, whether uncon- 
 solidated or bound together to form a solid rock mass, may be regarded as the 
 product of prolonged and repeated working over by water. While in most cases 
 that work is probably performed by rivers which carry the material a long way, 
 it may also be accomplished by the waves and shore currents of the sea-coast, 
 and in many cases it may be a combination of both. 
 
 RIVER DEPOSITS 
 
 The deposits of clastic material formed by rivers may be located 
 upon land, in standing bodies of water (lakes, ponds), or in the sea. 
 The last will always be subject to more or less reworking by the 
 ocean waves and currents, which will impress characteristic fea- 
 tures upon them. They are, therefore, properly classed as marine 
 sediments. Deposits formed in large lakes also have certain dis- 
 tinctive characters, which require separate treatment. Those 
 deposits, which were built upon the margins of lakes and of the sea, 
 that is, the deltas, are properly considered as special types of 
 river deposits. According to the place and manner of deposition, 
 we may recognize the following types : (i) Alluvial fans and plains, 
 (2) flood plain deposits, (3) playas and (4) deltas. 
 
 Alluvial Fans and Plains 
 
 General Forms and Character. When a river issues from the 
 mountains, where the slope of its bed is steep and hence its velocity 
 and carrying power great, and descends to the plains or low ground 
 at the foot of the mountains, where as a result of the decrease of 
 slope its velocity and carrying power become suddenly diminished, 
 it is forced to drop a part of the load which it has carried in its 
 mountain course, and in this manner an alluvial fan is built up 
 (Figs. 382 a and b). This will generally have the form of a low 
 half-cone resting against the high ground in the back, and sloping 
 outward at a low angle in all directions from the point where the 
 river issues (Fig. 382 b). The angle of surface slope depends upon 
 
River Deposits 
 
 465 
 
 the velocity of the issuing stream, the amount and size of the 
 material which it carries, the size of the fan at any given time, 
 and on other factors. It may be as high as 20 or even 30 in small 
 fans, but in general, the angle is much lower, especially in large 
 fans, where the beds appear nearly horizontal. As the fan grows, 
 the stream breaks up into a number of diverging terminal fringes 
 or distributaries, each of which may build a separate lobe of the 
 main fan, whose outer margin will thus become lobate or scalloped. 
 The paths of these distributaries are generally marked by channels 
 cut during seasons when the amount of material brought by the 
 
 FIG. 382 a. Diagram of alluvial cones, showing surface and underground 
 structures. (Drawn by F. K. Morris.) In this case the original plain, F, on 
 which the alluvial cone is built, and the mountainous mass, C, are separated 
 by a fault. Note the progressive overlapping of the successive beds of the fan 
 as shown in the section. 
 
 stream is smaller and when, therefore, some of the energy of the 
 flowing water-currents is expended in erosion. When several 
 streams issue near together from a mountain region, their separate 
 fans may in time become confluent, forming a more or less continu- 
 ous deposit along the mountain front. When this is broad and flat 
 it is called an alluvial plain. 
 
 The area covered by an alluvial fan may vary in size from a 
 few square feet to thousands of square miles, while confluent 
 plains may cover hundreds of thousands of square miles. 
 
 Alluvial Fans as Sources of Water. The steeper alluvial fans 
 at the mountain front commonly furnish a ready supply of water 
 in their deeper layers. This water sinks into the sands near the 
 
4 66 
 
 Deposition of Clastic Rock Material 
 
 head of the fan and slowly makes its way through the sediments. 
 The sloping character of the layers also tends to supply the water 
 with a sufficient " head " to make it available. 
 
 FIG. 382 b. Small alluvial fan, showing distributaries, Utah. (Photo by 
 
 F. J. Pack.) 
 
 Overlaps of the Successive Beds. As the alluvial fan grows, the 
 upper layers will extend farther than the lower ones, and overlap 
 them around the radius of the fan. Thus in the outer zone of the 
 fan only the highest layers will be present, resting directly upon the 
 old surface upon which the fan is built or upon deposits formed in 
 front of it. As we approach the head of the fan, borings in it would 
 reveal the presence of successively earlier and earlier layers at the 
 bottom. This is illustrated in the following diagram (Fig 382 c), 
 
 FIG. 382 c Diagram showing the progressive overlapping of the successive 
 divisions of an alluvial fan, away from the source of supply. 
 
 which represents a radial section of such a fan, the oldest layer 
 being lettered a, the youngest , /. 
 
River Deposits 467 
 
 Modern Examples of Alluvial Fans and Plains 
 
 Among the many modern examples of great alluvial fans those of the Merced 
 River of California, the Cooper River of South Australia, the Yellow River 
 (Hoang-Ho) of China, and the Indo-Gangetic Plain may be given. 
 
 The Merced River Fan. The Merced River rises in the Sierra Nevadas and 
 carries a large amount of waste down their western slope. As the river reaches 
 the great open California Valley, which lies between the Sierra Nevadas and the 
 Coast Ranges, it drops its material and builds an alluvial fan, which has at pres- 
 ent reached a radius of about 40 miles. The material of this fan consists of 
 coarse gravel near the mountains, and of fine silt in its outer portion. The 
 slope of the fan, on the whole, is a very gentle one, on account of which the river 
 is easily diverted at the point of issuance, and may follow different directions 
 across the fan at different times. 
 
 The Merced fan is only one of a number of such fans built by the rivers which 
 flow from the Sierra Nevadas into the Great Valley. These fans are so large, 
 their slopes so low, and their confluence so complete, that it is difficult to recog- 
 nize the individual convexity of each without the aid of surveying instruments. 
 Similar, though smaller, fans descend from the eastern slopes of the Coast Range 
 which bounds the valley on the west, and these two sets meet Jn the center of 
 the valley to form a broad, flat-floored trough. 
 
 The Delta of the Cooper River, Australia. The Cooper River is one of the 
 many intermittent streams which rise in the mountains of Queensland and flow 
 westward to the lake district of South Australia during seasons of flood. It 
 enters Lake Eyre, a saline body of water 1 2 meters below sea-level. This river 
 has built a delta or alluvial plain of large size, but as it is not built in a perma- 
 nent body of standing water, its character lies midway between that of an 
 alluvial fan and a true delta. The area of this delta-plain is more than twice 
 that of the Nile Delta, its length being nearly 185 miles and its width over 170 
 miles. The surface of the delta-plain is dissected by numerous dry channels 
 through which the water flows after periods of rain. 
 
 Alluvial Plain of the Hoang-Ho. The Hoang-Ho or Yellow River of China 
 leaves the mountains at a point about 300 miles from the present sea-shore, 
 and over this distance it has built a very gently sloping alluvial plain, which 
 spreads out in the form of a triangle, the base of which, along the present coast, 
 extends for about 400 miles south from Pekin to the great plain of the Yangtze- 
 Kiang, including and surrounding the rocky headland of Shantung. The head 
 of the plain, where the river leaves the mountains, is only 400 feet above sea- 
 level, hence there is only an average fall of i^ feet per mile, making a surface of 
 such gentle slope that it appears in all respects horizontal (Fig. 383). This 
 very gentle slope is due to the fact that the material of which this plain is built 
 is mostly fine silt derived from the loess of the interior. It is the yellowish color 
 of this material, due to the hydrated iron in it, which has given the Yellow River 
 its name (see ante, p. 459). This fine material is deposited in a series of nearly 
 horizontal layers, one above the other, forming thus a regular succession of beds 
 or strata, and giving the deposits a regular stratified character. 
 
 Because of the gentle slope, the river is easily diverted from its course when 
 swollen, and a slight change at the head may produce a marked alteration of 
 
468 Deposition of Clastic Rock Material 
 
 the distributing streams over the surface. The main mouth of the stream has 
 been repeatedly shifted, the extent being as much as 200 miles. The plain is 
 intensively cultivated, and there are many lakes, ponds, and swamps in which 
 vegetable deposits accumulate, forming peat beds, which in time may be con- 
 verted into coal. When the river breaks its banks, inundations of vast extent 
 result, and such deposits of peat become buried by the silt. In 1887 such a 
 flood covered an estimated area of 50,000 square miles of immensely fertile 
 
 FIG. 383. View of the flat alluvial plain of the Hoang-Ho in eastern China. 
 
 and densely inhabited land. At least a million people were drowned, and an 
 even greater number succumbed to the famine and diseases which resulted be- 
 cause of the flood. 
 
 The Indo-Gangetic Plain. The streams descending from the southern 
 slopes of the Himalaya Mountains in northern India, and the ranges extending 
 westward, carry large quantities of clastic material produced by the disintegra- 
 tion of the rocks during the dry season of the year and their decomposition during 
 the moist season (Fig. 384). Along the foot of the mountains numerous alluvial 
 fans of coarse material and steep grade are built, while the finer material, carried 
 forward into the lowland of northern India, becomes incorporated in the great 
 alluvial plain which is traversed by the two major streams of north India, the 
 Ganges, with its tributary the Brahmaputra on the east, and the Indus on the 
 west. This Indo-Gangetic Plain, as it is called, has an area of about 300,000 
 square miles, varying in width from 90 to nearly 300 miles. It entirely separates 
 the lower peninsula of India from the Himalayas on the north, and forms the 
 richest and most densely populated district of India. The highest portion 
 of this plain rises only 924 feet above the sea-level, the average slope of the sur- 
 face being of similar degree to that of the Hoang-Ho plain (Fig. 385). Borings 
 into this plain, which have penetrated to nearly 1000 feet below sea-level, 
 have shown that the material is essentially the same throughout. This means, 
 of course, that the region is sinking as the deposits are being formed, otherwise 
 the beds now a thousand feet below the sea could not have been formed above 
 that level as are the modern beds, which these older ones resemble in all respects. 
 This belt of country parallel to the Himalayas thus constitutes a modem ex- 
 ample of a geosyndine of deposition, a structural feature of the earth's crust to 
 which we shall refer again in subsequent pages. 
 
River Deposits 
 
 469 
 
 The material of which this great deposit is composed varies greatly in char- 
 acter. Along the sloping borders, especially in the north, gravels are com- 
 mon, but away from these, fine material prevails, pebbles being scarce at a 
 distance of more than 20 or 30 miles from the hills. This finer material consists 
 of sands more or less well assorted, of clays, and of other substances. Beds 
 of wind-blown or eolian sand of great thickness are found in some regions. In 
 some sections the shells of river and pond mollusks are common in the clays, 
 and in other sections, as along the banks of the Jumna River, the bones of many 
 land and river animals are embedded in the sediments, among these being the 
 remains of elephant, hippopotamus, ox, horse, antelope, crocodile, and various 
 
 FIG. 384. Khyber Pass in the Himalayas, showing the extensive forma- 
 tion of rock- waste. 
 
 fish. Peat beds are also forming in many sections, and older peat beds, buried 
 by later sediments, have been found at a depth of 20 to 30 feet below the sur- 
 face in the borings. Bones of terrestrial mammals and crocodiles, etc., have 
 been found at considerable depths, but nowhere are there any traces of marine 
 organisms, showing that throughout the period of deposition of these sediments 
 the region stood sufficiently above sea-level to prevent marine waters from 
 entering. 
 
 At various levels beds of earthy limestones or layers of calcareous concre- 
 tions, called kankar, are found, these representing the lime which was separated 
 out from the river- water under the semi-tropical heat of the sun (see ante, p. 260). 
 Such lime, derived from the solution of older limestones in the upper river 
 courses, is not uncommonly deposited in regions of semi-aridity in various 
 parts of the world. 
 
 On the eastern side of the plain, where the Ganges enters the sea, it builds a 
 normal delta, and here some of the beds enclose the remains of marine organisms. 
 Similar conditions exist upon the western side, where, moreover, extensive sea- 
 
470 Deposition of Clastic Rock Material 
 
 border marshes and salt-pans exist, and where salt deposits are included in the 
 series of stratified sediments which are forming there (ante, p. 233). Salt lakes 
 and playas also exist inland, where climatic conditions are favorable for the 
 evaporation of salt-bearing waters. 
 
 The thickness of the deposits of the Indo-Gangetic Plain is not known, but 
 it probably exceeds several thousand feet. It rests in part upon an older de- 
 posit of precisely the same character, a portion of which, near the mountains, has 
 been bent or tilted by a comparatively recent uplift or rising of the Himalayas. 
 These uplifted ends have been more or less dissected by the modern streams, 
 so that the character of the deposits is ascertainable. Thus it is seen that 
 the material of this older alluvial plain is similar to that which forms the 
 
 FIG. 385. - Flood-plain of the Ganges; from Calcutta. 
 
 modern plain, and like it includes the remains of land and fresh- water animals. 
 The remains of marine organisms are found only in its lowest portion, 
 where it grades into the underlying marine formation. The most surprising 
 thing about this older alluvial fan deposit is its enormous thickness, which is in 
 the neighborhood of 15,000 feet. As the material of this fan was laid down 
 at a comparatively slight elevation above the sea, judging from what is seen 
 in the deposits now forming, it is apparent that during its formation there must 
 have been a slow but constant sinking of the area over which it was deposited, 
 and that when the topmost layer was spread out, the bottom layers must have 
 been more than ten thousand feet below sea-level, from which position they 
 were lifted by the subsequent disturbance which affected this region. 
 
 Another striking feature of this older deposit is the presence in it of beds of 
 red clay, often of great thickness. These red beds were originally deposits of 
 yellow sandy clays such as now form upon the modern plain. The yellow color 
 is due to the oxidation and hydration of finely disseminated iron in the sedi- 
 ments, the thorough oxidation of which is accomplished whenever, during the dry 
 seasons, the ground- water level sinks so low that air can penetrate and to some 
 extent circulate through the sediments. The red color of the older deposit is 
 
River Deposits 471 
 
 merely a later stage in change, the iron losing its water with age, and so changing 
 to the red oxide (hematite), just as the burning of bricks from yellow clay drives 
 off the combined water of the iron and changes the color from yellow to red. 
 
 Deposits in Mountain-enclosed Basins 
 
 Where a basin lies within the mountains with only a single outlet 
 or with none at all, clastic material formed by weathering on the 
 mountain sides will be washed by the rivers into the center of the 
 basin where it accumulates, often to a very great thickness. If 
 the climate is moist, the basin will be filled with water to the point 
 of overflow, forming a lake or series of lakes, while the amount of 
 clastic material washed into this basin will be comparatively small, 
 because weathering will be interfered with by the cover of vegeta- 
 tion which forms upon the mountain sides. Thus the clastic de- 
 posits formed on the floors of the glens and other valleys in the 
 Scottish Highlands are never very extensive, though peat deposits 
 form on all the mountain sides and on the valley bottoms as well. 
 In regions of dry climate, however, where vegetation is scant or 
 absent at least for parts of the year, much weathering results, and 
 the product of such weathering accumulates upon the floor of the 
 intermontane basin. A typical example of such a waste-filled basin 
 is the Vale of Kashmir, which lies within the northwestern ranges 
 of the Himalaya Mountains. Its area equals that of the Connecti- 
 cut Valley of the eastern United States, being of elliptical form, 
 100 miles long from southeast to northwest, and 40 or 50 miles 
 broad. The floor of the valley is more than 5000 feet above 
 sea-level, and it is deeply filled with waste material from the 
 mountain sides, this material being coarse around the margins, 
 but fine toward the center, where it is free from pebbles. Its 
 thickness is probably several thousand feet, though no borings 
 have been made to determine this. The waters of the various 
 streams which bring the material from the mountains and deposit 
 it upon the valley floor are gathered to form the Jehlam River, 
 which meanders across the plain and escapes from the basin by a 
 deep and rocky gorge. 
 
 A similar waste-filled basin exists in the Rocky Mountains in the upper valley 
 of the Arkansas River, wnich escapes from the basin by the Royal Gorge. The 
 deposits within this basin slope from the mountains toward the river borders. 
 
 Deposits formed at an earlier period (Tertiary) in a similar basin in south- 
 western Wyoming, which is enclosed by the Wasatch, Uinta, and Wind River 
 
472 Deposition of Clastic Rock Material 
 
 ranges of mountains, have now been thoroughly dissected, apparently as the 
 result of a change in climate, which brought with it increased precipitation of 
 moisture. This dissection is accomplished by the Green River and its tribu- 
 
 FIG. 386. Flood-plain of the river escaping from the front of the Bartlett 
 Glacier, Alaska. (Courtesy of Alaska Engineering Commission.) 
 
 FIG. 387 a. Morainal and river terraces on east side of Columbia Gorge, 
 Chelan Ferry, Wash. (Photo by Baily Willis, from U. S. G. S.) 
 
 taries, the waters of which escape by a deep gorge through the Uinta Mountains. 
 As a result of this dissection the older parts of the deposit have been laid open 
 to view, in cliffs sometimes a thousand feet high, and the details of structure 
 
River Deposits 
 
 473 
 
 and composition of such deposits can thus be studied to great advantage, whereas 
 those of the undissected deposits can be ascertained only by borings. 
 
 River Flood-plain Deposits 
 
 When a river is heavily laden with sediment, it is bound to de- 
 posit this wherever, by an expansion of its bed or a lessening of its 
 grade, the velocity is de- 
 creased. A river escaping 
 from the foot of a glacier 
 usually carries a large 
 quantity of fragmental 
 material, both coarse and 
 fine, and the former is de- 
 posited soon after the 
 river leaves the ice front 
 (Fig. 386). During the 
 last glacial period, when 
 
 vast ice masses covered FlG ^ b _ old terraces of the West 
 much of northern North Brattleboro, Vt. (Photo by Prof. E. Fisher.) 
 America and northwest- 
 ern Europe, the rivers escaping from the front of this ice, during 
 
 periods of prolonged 
 
 melting, carried large 
 quantities of coarse 
 debris and deposited this 
 in their valleys, often 
 filling them from side to 
 side to a depth of even 
 a hundred feet or more. 
 Subsequently when, 
 owing to the melting 
 away of the ice, less 
 debris was furnished, the 
 energy of the river was 
 expended in again cut- 
 ting away the old de- 
 posit. Commonly, how- 
 ever, the deposit was not 
 
 FIG. 388 a. Flood-plain of the Saco River 
 at Intervale, N. H., showing the coarse ma- 
 terial derived from an erosion of the terraces. 
 (Photo by the author.) 
 
 completely removed, but 
 terraces of the old sedi- 
 
474 Deposition of Clastic Rock Material 
 
 ment were left on both sides of the river valley. Such terraces 
 may be seen along most of the larger streams in the northern 
 
 FIG. 388 b. Flood-plain of Connecticut River, in Vermont. 
 
 states, often occurring in several successive series, each lower than 
 the preceding, and each set marking, by its summit level, which is 
 in accord on opposite sides oi the valley, a temporary surface of 
 
 FIG. 389. The Ohio River in flood, showing inundation of flood-plain. 
 
 the flood-plain of the river before it resumed cutting toward a 
 lower level (Figs. 387 a, b). Close to the front of the old ice sheet 
 the material of which these terraces are composed is very coarse, 
 boulders and cobblestones of the size of a man's fist often pre- 
 
River Deposits 
 
 475 
 
 A MILES 
 
 FIG. 390 a. A well-developed river flat, Mississippi Valley, near Prairie 
 du Chien, Wis. Note the steep confining bluffs and numerous lagoons, cres- 
 cent lakes, ox-bows, cut-offs, and abandoned channels, showing various stages 
 of silting. (From Military Geology.) 
 
 FLOOD-PLAIN DEPOSITS NATURAL LEVEES 
 
 FIG. 390 b. Block diagram showing the flood-plain of a river, with ox-bows 
 and marginal streams ; and, in section, the flood-plain deposits and natural 
 levees. (Drawn by F. K. Morris.) 
 
476 Deposition of Clastic Rock Material 
 
 dominating (Fig. 388 a). Farther down stream the material be- 
 comes largely sand, increasing in fineness with its distance from 
 
 the original source. 
 
 Where rivers flow at a 
 gentle grade, as in the 
 case of the lower reaches 
 of most large rivers (Fig. 
 388 b), the flood-plain, 
 which is that part of the 
 valley floor inundated 
 only at high water (Fig. 
 FIG. 391- T Break in levee of the Mississippi g ) ig commonly 
 
 River opposite New Orleans, submerging a ' 7 f 
 
 plantation. (Photo by Howell.) made U P ot la Y ers o: 
 
 mud, with many ponds 
 
 and swamps scattered over its surface (Fig. 390 a). The mud is 
 derived from the river, which, on rising, overflows its banks. If 
 
 FIG. 392. Mud cracks, Nile flood-plain ; sufficiently wide and deep to admit 
 a man's arm to the elbow. Abu Simbel, near second Nile Cataract. (After 
 Hobbs. Courtesy of the American Geographical Society, Broadway, at i52d 
 St. From the Geographical Review.} 
 
River Deposits 
 
 477 
 
 FIG. 393. The flood-plain of the Nile. Plowing with the aid of camels. 
 
 Evarts VsO ' 
 
 4- MILES 
 
 FIG. 394 a. A braided stream, Platte River, in the broad alluvial valley 
 near Kearney, Neb. A mile-wide, sandy chamiel filled with water only at 
 flood time. Over the bottom during most of the year a little water, not diverted 
 for irrigation, percolates through the sand, or finds its way in a tortuous 
 course through a series of interlacing channels whose pattern changes with 
 every flood. Northwest winds here lift the sand from the channel, sweep it 
 across a grassy plain, and pile it in dunes nearly two miles south of the river. 
 (Kearney, Neb., topographic sheet, U. S. G. S. From Military Geology.) 
 
TT X! 
 
 ||3M 
 
 OOJ3T.6M xi 
 
 ||3M 
 
 |00qo6 ueipui 
 
 liaM uacio 
 
 HOM X 4 djn^ Pt : X: 
 
 MM 
 
 K>J?CN ll>Jujuny 
 
 118/VN pJB8H 
 
 c J 
 
 a E 
 
 478 
 
River Deposits 
 
 479 
 
 UNCONSOLIDATED SEDIMENTS 
 
 Impervious clay 
 
 Porous sand and 
 gravel above ground- 
 water table 
 
 Porous sand and 
 gravel below ground- 
 water table 
 
 FIG. 394 c. Diagrammatic section showing artesian conditions in Sulphur 
 Spring Valley, a, dry hole, which if sunk deeper would strike rock without 
 finding water ; b, dry hole which if sunk deeper would find water ; c, shallow 
 pump well ; d and e, flowing wells. (Military Geology.} 
 
 vegetation grows along the bank, this will commonly retard the 
 current of the spreading river, and as a result, a considerable amount 
 of the river silt will "settle out along its banks, building up natural 
 levees. Such natural levees 
 are very crAacteristic of the 
 Mississippl^fc^pther large 
 rivers (Figs. v ^o /;, 391). 
 Beyond them the*^ 
 generally lower and swampy, 
 with many lakes, as in the 
 "Back Swamps "of the lower 
 Mississippi (see map, Fjg. 
 381, p. 463). Because the 
 mud settles out in relatively 
 small quantities at each 
 flooding, the layers compos- 
 ing the flood-plain will, as a 
 rule, be thin, and a finely 
 stratified structure results. 
 As the mud dries after a 
 flooding, it breaks into 
 polygonal blocks separated 
 by cracks, the depth of 
 which depends on the length 
 of exposure and on other 
 causes. Such sun-cracked 
 
 FIG. 395. Mud-cracked surface on 
 bank of Little Colorado River, Texas, 
 formed during drought of 1918. Some 
 of the large cracks are from 4 to 10 inches 
 deep, and a secondary set of finer ones has 
 formed upon the larger blocks. (Photo- 
 graphed and contributed by Prof. Eliza- 
 beth Fisher, Wellesley College.) 
 
480 
 
 Deposition of Clastic Rock Material 
 
 or " mud-crack" surfaces are very characteristic of river flood- 
 plains (Fig. 392). Illustrations of mud-cracks from the flood 
 and delta-plains of American rivers are given in figures 395 and 
 
 396. 
 
 The flood-plain of the Nile (Fig. 393) extends for a length of 
 500 miles and has a width ranging from five to fifteen miles, which 
 on the delta increases to 100 miles. On both sides it is bordered 
 by rocks or by desert sands, while the banks at low-water are from 
 20 to 30 feet in height. The river overflows its banks every year, 
 the flood beginning in June and usually rising 25 feet or more 
 
 FIG. 396. Mud-cracks on the delta of the Colorado River. (Photo by 
 G. K. Gilbert, from U. S. G. S.) 
 
 at Cairo in the late summer or early autumn. The annual addition 
 of the river silt causes a slow rising of the flood-plain at a rate esti- 
 mated to equal 4^- inches in a century. 
 
 Flood-plains of rivers of variable flow, where a large amount of 
 material is brought from the mountains during flood time, while 
 little or no water occupies the channel during the dry season, have 
 special surface characters and structure. Trie excessive amount of 
 silt causes the river to break up into a series of interlacing threads, 
 forming a " braided " structure (Fig. 394 a). The deposits them- 
 selves consist of variable and discontinuous layers of interfingering 
 coarse and fine material (Fig. 394 b, c). 
 
River Deposits 
 
 481 
 
 Playa Deposits 
 
 When rivers end in desert basins from which there is no outlet, 
 they form saline or more rarely fresh lakes (Fig. 397) or they 
 spread out after a period of flood into flat and very shallow playa 
 lakes which disappear again by evaporation. Such playa lakes 
 come into existence very suddenly on account of the usual flat 
 
 FIG. 397. Lake Sorkul, without outlet, in Great Pamir Desert. (After Reclus.) 
 
 bottom of the basin, and they are often of considerable size. In the 
 Black Rock desert of Nevada a large playa lake forms every winter, 
 covering an area of 450 to 500 square miles, but seldom reaching a 
 depth exceeding a few inches. An Old World playa lake has been 
 known to become full grown in twenty minutes, reaching a width of 
 10 to 15 km. and an unascertained length, with a depth ranging 
 only from one to six incjies. Often such a playa lake is only a body 
 of very liquid mud, and as the water dries away this mud settles 
 down as a continuous but thin sheet over the entire bottom. In a 
 few hours or a few days the water has evaporated, leaving a hard, 
 dry, and absolutely barren playa surface, cracked in all directions as 
 the mud contracts in drying. When the river water carries salts 
 
482 
 
 Deposition of Clastic Rock Material 
 
 in solution, these remain behind on evaporation of the lake water, 
 either impregnating the muds or forming distinct layers. Besides 
 the mud cracks and occasional impressions of raindrops left by a 
 passing sharp shower, the footprints of many animals which come 
 to these waters to drink may be impressed upon the mud surface 
 
 and retained in it on 
 drying. If, subse- 
 quently, sands are 
 blown across such a 
 surface or washed 
 there by a later inun- 
 dation, they will not 
 only fill the cracks 
 between the polygonal 
 blocks, into which the 
 su'rface has been 
 broken while drying, 
 but will also cover, and 
 preserve a relief im- 
 pression of, the rain- 
 drops and foot-prints. 
 On solidifying, the 
 resulting layer of 
 sandstone will have 
 such relief features 
 on its under side, 
 while the hardened 
 mud retains the actual 
 
 impressions. In the sandstones of Triassic age found in eastern 
 North America and in Europe, many such relief impressions 
 of footprints of now extinct gigantic land reptiles (Dinosaurs) and 
 of Stegocephalians (Fig. 398) are found (see Chapter XLIV). The 
 actual impressions are also present in the mud-rock beneath, but 
 these are not easily obtained on account of the readiness with which 
 the mud layers shatter on quarrying. Trails of insects and other 
 organisms are also formed upon the play a surfaces, and sometimes 
 these are preserved ; but the actual remains of animals are seldom 
 found, since those that die are rarely buried by the sands and muds 
 before their bones have completely disintegrated. 
 
 Playa lakes which endure for some months may become stocked 
 
 FIG. 398. Tracks (in relief) of Chirotherium, 
 Buntsandstein, Hessberg, Germany. (From 
 Haas' Leitfossilien.)- This slab represents the 
 consolidated sand which was spread over the 
 original surface on which the foot-print impres- 
 sions were formed. The relief structures here 
 shown are, therefore, the natural "casts" of the 
 original impressions and represent the form of 
 the animal 's feet. The side with these relief 
 structures is the under side of the slab. 
 
River Deposits 483 
 
 with animals whose eggs can withstand prolonged periodic drying, 
 and develop only when the lake comes into existence. Originally 
 such eggs may be brought by the wind or by birds or otherwise, 
 but few long-existing lakes are without them unless their water is 
 very salty. Chief among such forms are the small fresh-water 
 crustaceans of the genera Estheria, Cypris, etc. (Fig. 399), the 
 shells of which may cover the playa surface in considerable number 
 
 F" 
 
 F' 
 
 FIG. 399. Cypris, a modern ostracod. Female before sexual maturity, 
 right valve of shell removed to show internal anatomy. A' A", first and second 
 pair of antennae ; Ob, upper lip ; Md, mandible with leg-like feelers ; G, cerebral 
 ganglion with impaired eye ; Sm, shell muscle ; MX', MX", first and second pair 
 of maxillae; F', foot for. crawling; F", foot for cleaning; Fu, furka; M, 
 stomach; D, intestine; L, liver; Ge, genitals. Much enlarged. (From Haas.) 
 
 after drying, and may even form thin deposits mostly composed 
 of such shells. Estheria is known to live in playa ponds which 
 are dry for. eleven months in the year. Forms of this type are also 
 found in some of the mud layers in the Triassic series of sediments 
 referred to above. River fish, too, may be swept in large numbers 
 into such temporary lakes, and their remains may become buried 
 in the accumulating deposits. 
 
 On the whole, playa deposits have many characters in common 
 with the mud deposits formed on river flood-plains, and ancient 
 deposits of one type may easily be mistaken for those of the other. 
 
 Deltas 
 
 Where rivers laden with sediments enter a lake or the sea, they 
 will build a normal delta, provided the shore currents are of insuffi- 
 cient strength to remove all the material brought by the rivers. 
 Because of the absence of tides in lakes, the deltas built into- such 
 water bodies will be more perfect and less variable than those built 
 
484 Deposition of Clastic Rock Material 
 
 into the sea. The name delta is derived from the resemblance of 
 one of the best-known examples, the delta of the Nile, to the Greek 
 letter delta (A), though few deltas have such a regular triangular 
 
 flosett 
 
 Port Said 
 
 FIG. 400. Map of the Nile Delta. (After Kayser.) 
 
 form (Fig. 406). Small deltas, especially those built in lakes, 
 show two sets of beds, one sloping lakewards at a considerable 
 angle (up to 20 degrees or more) and generally composed of the finer 
 material, and one set which is nearly horizontal or but slightly 
 
 FIG. 401. Section of a small delta in a lake, showing the foresets of pebbles 
 and sand, the bottom-sets of clay, and the topsets of coarse sand and pebbles. 
 (After Kayser.) 
 
 inclined, and is generally composed of coarser material and rests 
 upon the steeper beds with an abrupt change of angle. The first 
 series is called the " foresets," the second the " topsets " (Figs. 401, 
 402). Deltas built in the sea are of more complicated structure, 
 though in general the two sets of beds may also be recognized. 
 
River Deposits 485 
 
 On the surfaces of large deltas, such as those of the Nile and the 
 Mississippi, many ponds and more or less permanent lakes may 
 exist. Most of these will be fresh- water lakes, but along the sea- 
 coast salt-water lagoons and shallow lakes will often be found. If 
 the climate is dry, some of these shallow lagoons may become 
 natural salt pans, as is the case with some of them along the edge 
 of the Nile delta. The main stream, too, commonly divides into 
 many ramifying branches or distributaries, which intersect the sur- 
 
 FIG. 402. Section of a delta-plain built into Lake Bonneville, Utah. The 
 finer-grained inclined foreset beds are covered by horizontal and coarser topset 
 beds. (Photo by F. J. Pack.) 
 
 face of the delta, and some of these streams may build independent 
 lobes or even long arms into the sea, as is the case in the " bird-foot 
 delta " of the Mississippi (Fig. 403). 
 
 The muds of the delta are very apt to bury and preserve, the re- 
 mains of fish and other animals which live in the rivers or in the 
 ponds upon the delta surface, and if the river comes from wooded 
 regions, tree trunks and branches may also be buried. The vege- 
 tation, too, which grows upon the surface of the delta and in the 
 ponds will leave its remains in the deposits. Along the sea margin, 
 marine animals, especially floating types and seaweeds, may be 
 cast upon the delta surface during storms, or may become stranded 
 after a temporary flooding of the delta by the sea-water. Lakes or 
 ponds near the coast may also receive such organisms by overwash 
 
4 86 
 
 Deposition of Clastic Rock Material 
 
 from the sea. Thus a commingling of the remains of river, pond, 
 and marine organisms is to be expected along the borders of the 
 delta. (For illustrations of the common forms see ante, pp. 310 
 to 316.) 
 
 - 6mEO 20 - 50 m 
 6 lOmEZS 50 - 100m 
 I .IHlO-20iaMI 100- 200m 
 
 FIG. 403. Bird-foot delta of the Mississippi. (After coast survey chart.) 
 The various shades. show the depths of water in meters as indicated in the 
 legend. (From Ratzel, Die Erde.) 
 
 Structural and Other Physical Characters of River Deposits 
 
 Among the important structural characters found in most river deposits, 
 the following may be mentioned : (a) stratification, (b) cross-bedding, (c) 
 cut-and-fill structures, (d) ripple marks, (e) rill-marks, (/) mud-cracks, (g) 
 raindrop impressions, (ti) footprints, (i) clay galls, etc. It must be clearly 
 understood, however, that these structures are not confined to river deposits, 
 though special phases of them may be so restricted. 
 
 Stratification (Pig. 404). This is the arrangement of rocks in layers, each 
 of which was at one time the topmost one. The individual layers or beds are 
 called the strata (singular, stratum}, especially if they differ markedly in char- 
 acter, as, for example, a stratum of clay which is followed by a stratum of sand 
 or by one of peat. Within each stratum there may be minor layers or lamina 
 
River Deposits 
 
 487 
 
 which vary slightly in texture or color or other characteristics. The strata of 
 river-laid deposits vary with the coarseness and character of the material (Fig. 
 394 b, p. 478) . Coarse deposits, such as pebbles, are often very irregular, varying 
 from thick to thin, and often passing laterally into sands. Even the strata of 
 sand are not always regular, but vary in thickness, and often thin away or pinch 
 out. Such variation is readily seen when two sections, a short distance apart, 
 are compared (Fig. 405). The fine deposits, on the other hand, such as charac- 
 terize the flood-plains and playas, are commonly well stratified, regular, and 
 occur in layers of uniform thickness over wide areas and generally show a finely 
 
 FIG. 404. Cliff cut on horizontal well-stratified rocks. Note the heavy 
 stratum near the center. Weathering of weaker strata has left the harder 
 ones projecting and this has emphasized the stratified appearance. 
 
 laminated structure. In this respect, they are as well stratified as are marine 
 deposits. (Compare Fig. 16, p. 81, and Fig. 487, p. 579, the former an ancient 
 marine, the latter a flood-plain deposit of the same age.) 
 
 Cross-bedding. This structure is well marked in river deposits which 
 are formed by more or less torrential currents that carry forward a large 
 amount of material at a time. Consequently it is best developed in the coarser- 
 grained deposits, such as sands and the finer pebble beds. Typically it is 
 characterized by a succession of sloping layers separated by horizontal beds. 
 The parts characterized by the sloping layers may be as much as four or five 
 feet in thickness, the angle of slope of the layers being from 20 to 30 degrees. 
 At the bottom these layers tend to change toward horizontality, but at the 
 top they are generally abruptly truncated, and a set of horizontal beds up to 
 several feet in thickness rests upon their edges. Above these horizontal layers 
 is a second series of sloping beds, similar to the first and with the inclination 
 in the same direction. There may be many such successions of sloping and 
 horizontal beds, but in all cases the inclined beds dip in the same direction, 
 which is that of the flow of the current. Such a type of cross-bedding is readily 
 
488 Deposition of Clastic Rock Material 
 
 distinguished from the eolian type, in which the slopes are in varying directions 
 and the successive divisions are separated by erosion planes instead of horizontal 
 layers. The following diagram (Fig. 406) illustrates this type of cross-bedding, 
 
 which may be taken as in- 
 
 m 
 
 fireclay 
 
 Sand and 
 /Sandstone 
 
 Fireclay 
 
 Sand and \ 
 Sandstone] 
 
 Lignite with seams 
 
 of Fireclay, Pebbles 
 
 <& Gypsum. 
 
 Clay 
 
 Soft, 
 Sandstone 
 
 Lignite with seams 
 
 of Sand, Clay 
 
 <& Pebbles 
 
 i 
 
 dicating deposition by tor- 
 rential rivers whenever 
 found. An irregular form of 
 cross-bedding is also pro- 
 duced by rivers where there 
 is a confluence of opposing 
 currents. Lyell described 
 the structure of a sand bank 
 formed in the spring of 1828, 
 where the opposing currents 
 
 soft white 
 Sandstone 
 
 Lignite 
 
 ' Lignite\ 
 
 Clay 
 Sand 
 
 FIG. 406. Diagram 
 showing the type of cross- 
 bedding which is charac- 
 teristic of torrential river 
 deposits. (Compare with 
 Eolian cross-bedding, Figs. 
 372 b, and 374~377.) 
 
 of the Rhone and the Arve 
 met and neutralized each 
 other, causing a retarda- 
 tion of their motion. Into 
 this sand bank a section was 
 subsequently cut, which is 
 reproduced in the following 
 figure (Fig. 407). This sec- 
 tion was about twelve feet 
 long and five feet high. 
 The stratum A A consists 
 of irregular alternations of 
 pebbles and sand in undu- 
 lating beds ; below thtse are 
 
 seams of very fine sand, BB, some as thin as paper, others about a quarter of 
 an inch thick. The stratum CC is composed of layers of fine, greenish gray 
 sand as thin as paper. Some of the inclined beds will be seen to be thicker at 
 their upper, others at their lower, extremity, the inclination of some being 
 very considerable. 
 
 ::---._-.-";:::" 
 
 Clau 
 
 Lignite 
 
 ^-fc-i-aSSsS&SSfia 
 
 
 
 Clay 
 
 
 
 
 
 
 20 feet - 
 
 FIG. 405. Parallel vertical or columnar 
 sections on the face of Pulpit Rock, near Colo- 
 rado Springs, through identical strata, and 
 only 20 feet apart; illustrating rapid lateral 
 changes in the character of the strata. 
 (From Crosby.) 
 
River Deposits 
 
 489 
 
 Cut-and-fill Structure. This is especially characteristic of the coarser river 
 deposits, such as those formed in alluvial fans and waste-filled basins. It may, 
 however, also occur in the finer sediments. It is characterized by abrupt chan- 
 
 FIG. 407. Section of a sandbank in the bed of the Arve at its confluence 
 with the Rhone, showing the stratification of deposits where currents meet. 
 (From LyelPs Principles.} 
 
 nels which cut off, or are excavated into, the older layers, and are filled by a part 
 of the next higher layer. They indicate the channelling so characteristic of 
 the surface of alluvial fans, and the subsequent filling in of these channels 
 where a change in direction of the current has occurred and new deposits 
 are added. 
 
 Ripple Marks. These structures are formed under varying conditions. In 
 river deposits, they generally represent a series of low, parallel ridges, one side 
 
 FIG. 408. Diagram showing : a, the formation of current ripples (after 
 Darwin) ; and b, plan showing arrangement of ripples in parallel lines with 
 transverse connections. (After Walther.) 
 
 of which is steeper than the other (Fig. 408 a). These are the current ripples 
 which form -at right angles to a gently flowing current in the shallow pools, as 
 sand dunes form at right angles to the direction of the wind. Such ripples are, 
 however, by no means confined to river deposits, but may occur in all strata 
 laid by and in shallow water, and occur in wind deposits as well. (See Fig. 371, 
 p. 453, and Fig. 453, p. 537.) 
 
 Rill-Marks. These are formed upon the mud surfaces of flood-plains and 
 playa surfaces, where a small stream of water trickles from a bank or bubbles up 
 from beneath and runs away, spreading into numerous distributaries, which 
 become finer and finer outwards until they fade away. Such spreading rill- 
 marks are the reverse of those most common on the shore in which the channels 
 
490 Deposition of Clastic Rock Material 
 
 converge like the branches of a stream system. When covered by sand, these 
 
 rill-marks may be preserved in relief, and they often have a strong resemblance 
 
 to some form of plant, for which fossil, 
 rill marks have often been mistaken 
 (Fig. 409). 
 
 Mud- cracks. These are most 
 characteristic of the playa and flood- 
 plain surfaces and are formed by con- 
 traction of the surface layer of mud 
 in drying. As a result, such layers 
 split up into polygonal blocks, which 
 generally curve gently upward at the 
 margin, producing a saucer-shaped 
 surface. (See Figs. 392, p. 476, and 
 395-396, pp. 479, 480.) The cracks in- 
 crease in width and depth with pro- 
 gressive drying, and this may go on 
 to such an extent that their depth 
 is measured by several feet. Ordi- 
 narily, however, the depth is not over 
 one, or, at most, a few inches. A 
 covering of sand preserves these mud- 
 cracks, as it fills the fissures between 
 the blocks (Fig. 410). On the under 
 side of the sand layers when hard- 
 ened, the polygonal surfaces will be 
 gently convex and bounded by raised 
 
 ridges of hardened sand. Mud-cracks may be formed on tidal flats which 
 
 are exposed for a long period 
 
 of time. Ordinarily, however, 
 
 such flats do not dry suffi- 
 ciently between tides to permit 
 
 the formation of mud-cracks, 
 
 or if they are formed, the mud 
 
 does not harden sufficiently to 
 
 withstand the softening effect 
 
 of the returning tide. Figure 
 
 411 shows a photograph of an 
 
 ancient clay-rock with very 
 
 narrow mud-cracks. 
 
 Raindrop Impressions. 
 
 These are also most typical 
 
 on the mud surfaces of flood- 
 plains and playas (Fig. 412). 
 
 When raindrops strike the mud 
 
 sharply, they leave a concave FIG. 410. Fossil ripple marks cut by 
 
 impression, around the margin mud-cracks which have been filled in by 
 
 of which the pressed-out mud other material. (Reverse.) (U. S. G. S.) 
 
 FIG. 409. Fragment of a relief 
 mold of rill-marks on the under side 
 of the stratum which covered the 
 layer in which the rill-channels were 
 originally made. Triassic sandstone, 
 Portland, Conn. (After Newberry.) 
 
River Deposits 
 
 491 
 
 forms a low rim. If the raindrop strikes obliquely, as when driven by wind, 
 the impression will be asymmetrical, being deeper on the side from which 
 the rain struck and the wind blew, while 
 an asymmetrical bordering rim is also 
 produced. Raindrops also may be pre- 
 served in relief on the under side of a 
 stratum subsequently deposited upon the 
 pitted surface. 
 
 Footprints. The footprints of land 
 animals are most commonly preserved in 
 the flood-plain and playa deposits when 
 these are subject to hardening by ex- 
 posure. Footprints of the camels of a 
 caravan in the Sahara were still distinctly 
 recognizable fifteen years later in the 
 hardened mud. Such footprints may also 
 be preserved in relief on the under side 
 of a covering stratum. (See Fig. 398, 
 p. 482.) 
 
 Clay Galls. When only a thin layer 
 of mud is deposited on the flood-plain or 
 playa, this may curl up into fine spirals 
 resembling wood shavings, and these may 
 be transported by the wind and come to 
 rest in sands either of eolian or river- 
 laid origin. On being wetted, these clay shavings suffer compression and 
 eventually form only a thin oval plate or film of clay on the sand mass. Such 
 " clay galls " are a characteristic feature of many hardened sand deposits of 
 subaerial origin. 
 
 Red Color of Ancient River-laid Deposits. Many ancient deposits which 
 by their general character suggest their origin as river deposits now have a red 
 
 color, such as is seldom if ever seen in modern 
 river sediments, where the prevailing tones are 
 dark or light grays or blues or else yellows. 
 The latter is most commonly met with in al- 
 luvial fans and plains formed in regions of little 
 vegetation, because of protracted dryness of 
 climate for at least part of the year. This 
 yellow color, as already described in an earlier 
 section (page 459), is due to the oxidation and 
 hydration of the disseminated iron in the sedi- 
 ment, and is especially noticeable where such 
 sediments are fine grained with much clay or 
 rock flour. Where much vegetation is present, 
 such oxidation is not readily effected, nor can 
 subsequent oxidations of the sediments take 
 
 place easily when the deposit is saturated with ground water, which prevents 
 the free access of the oxygen except in so far as the water carries it. In dry 
 
 FIG. 411. Photograph of a 
 specimen of clay rock from the 
 Silurian formation of Pennsyl- 
 vania (Longwood shale), showing 
 mud-cracks formed upon an an- 
 cient river flood-plain or playa. 
 The fissures in this case are very 
 narrow, although the structure 
 affects the rock mass for a depth 
 of many inches. About one 
 eighth natural size. (Columbia 
 University collection; photo by 
 B. Hubbard.) 
 
 FIG. 412. Raindrop im- 
 pressions hi a layer of 
 hardened clay from a 
 modern roadside surface. 
 About one half natural 
 size. (B. Hubbard, photo.) 
 
492 Deposition of Clastic Rock Material 
 
 climates, however, where the level of the ground-water sinks low, the upper 
 layers of the deposit are sufficiently dry to permit the entrance of the air, and 
 here oxidation is accomplished. If this can take place for each successive 
 layer added to the surface of the alluvial fan or plain, the entire deposit will 
 come to have its iron content oxidized throughout. In the course of time, 
 this hydrous iron oxide will lose its water, and the color will change from yellow 
 to red. In this manner may be explained many of the extensive red-bed for- 
 mations found in several geological series, though at the time of deposition 
 such beds were yellow rather than red. 
 
 GLACIAL TRANSPORTATION AND DEPOSITION 
 
 Transportation 
 
 Modern glaciers transport clastic material partly upon their 
 . surfaces (superglacial), and partly within their mass (englacial), 
 and partly frozen into their bottoms (subglacial). All the material 
 transported by ice, whether modern or ancient, is called glacial 
 drift, while the large transported boulders are termed erratics. 
 The subglacial material is derived in part from the erosion of 
 the rock floor by the glaciers and in part from material which 
 is loosened and pried off on the margins of the cirque, and especially 
 in the bergschrund (p. 366). Englacial drift originates partly from 
 the material carried up from the bottom wherever an oblique shear- 
 ing plane permits faulting in the ice, the mass from behind over- 
 riding that in front of it and carrying its debris upward. Part of 
 the englacial material is, however, of superglacial origin and repre- 
 sents the fragments which have fallen into the crevasses or other- 
 wise become inclosed in the ice mass. Superglacial material is 
 derived in various ways. In the valley glaciers, the rock frag- 
 ments falling upon the ice from the valley sides, or sliding down 
 in avalanches, accumulate upon the side of the ice, and as the 
 glacier moves along, such accumulations are strung out in lines of 
 drift along the margin. This constitutes the lateral moraine. (See 
 ante, p. 368.) Where two glacial streams unite, their adjoining 
 lateral moraines become confluent and are continued as a medial 
 moraine (p. 370). 
 
 These moraines are, however, also fed from below ; for it is known that in 
 a moving glacier there are diverging currents, by virtue of which the lower ice 
 layers move outward and upward along the sides of the glacier as well as toward 
 its front (Fig. 413 a). Such currents carry material from the bottom upward, 
 and as this rises and the inclosing ice melts, the material becomes incorporated 
 in the lateral moraines. Furthermore, when two ice streams meet they never 
 
Glacial Transportation and Deposition 493 
 
 commingle as two water streams will, but flow along side by side with more or 
 less independence. The upward currents which formed part of the lateral moraine 
 above the point of junction will continue and the material brought up along 
 the sides of the glaciers in contact with one another will thereafter contribute 
 to the medial moraine (Fig. 413 b). 
 
 FIG. 413 a. Diagram showing ice currents carrying sub-glacial material 
 upwards to form lateral moraines. (After Chamberlin and Salisbury, Geology; 
 by permission of Henry Holt & Co.) 
 
 This change from subglacial to englacial and finally to a superglacial position 
 of the material makes possible the renewal of moraines on the surface of the ice, 
 when great floods of water from prolonged melting have washed the surface 
 clear of debris. Such renewal of moraines after the removal of the older ones 
 by a flood has actually been observed in glaciers of the Alps. 
 
 Large ice masses like the ice-caps of Iceland and Scandinavia and the con- 
 tinental ice-sheet of Greenland have little or no superglacial material, since they 
 are not confined in valleys. It is true that near the margin of the Greenland 
 ice-cap, rocky peaks or nunataks project through the ice, and these furnish a 
 
 FIG. 413 b. Diagram to illustrate ice currents carrying sub-glacial material 
 upwards to form a medial moraine. (From Chamberlin and Salisbury, 
 Geology; by permission of Henry Holt & Co.) 
 
 certain amount of debris which accumulates on its surface. Where upward 
 currents are developed in the ice which is forced to pass between two adjoining 
 nunataks, a semicircular morainic band may be produced, extending from one 
 nunatak to the other. (See page 386.) But on the whole, ice-sheets transport 
 their debris primarily as subglacial material, and as englacial, where local 
 upward currents or shearing planes cause the subglacial material to rise. Such 
 material is commonly characterized by the polishing and scratching of all the 
 coarser fragments, a feature not characteristic of superglacial accumulations 
 of rock falls. (See Fig. 360 b, p. 435.) 
 
 Deposits Formed by Modern Glaciers 
 
 Terminal Moraine. Modern valley or mountain glaciers de- 
 posit their load of debris chiefly at their lower ends, where melting 
 takes place (Figs. 414, 415). This constitutes the terminal moraine, 
 which is composed of superglacial, englacial, and subglacial material 
 more or less intimately commingled and showing no regularity 
 
494 
 
 Deposition of Clastic Rock Material 
 
 of structure. Many ice blocks are buried in such a terminal 
 moraine, which, on subsequent melting, will cause a caving-in of 
 
 FIG. 414. Terminal moraine topography at foot of Kotsina Glacier, Alaska. 
 (Photo by Schraeder, U. S. G. S. Courtesy D. W. Johnson.) 
 
 FIG. 415. Terminal moraine of a former glacier, Oregon. 
 
 this covering of debris, and the production of a cup or kettle-shaped 
 hollow, a feature to which the name kettle-hole is applied (Fig. 416). 
 
Glacial Transportation and Deposition 495 
 
 These may be dry or contain water forming a glacial pond (Fig. 
 417). Such terminal moraines are best developed where the ice 
 front rests for a considerable period of time at the same point, 
 which implies that the rate of melting at the front and the rate of 
 
 A B 
 
 FIG. 416. Diagrams to illustrate the mode of formation of kettle-holes in 
 glacial deposits. A , a portion of a glacial sand-plain burying a boulder of ice ; 
 B, the same after the ice-boulder has melted, and the sand caved in to form 
 the nearly circular kettle-hole. 
 
 ice advance are equal and balance each other. When there is no 
 such equality, the material left on melting will be scattered over 
 a belt of greater or less width and form no pronounced terminal 
 moraine. If the stream issuing from the front of the glacier is 
 very strong, a large part of the glacially transported material may 
 
 FIG. 417. A kettle pond in a glacial landscape. 
 
 be carried away by it, and the terminal moraine will suffer accord- 
 ingly. When well formed, such a moraine may reach a height of 
 more than two hundred feet, but heights of 100 to 200 feet are 
 more frequent. 
 
 Submarginal Moraines. In some valley glaciers the material 
 carried toward the sides of the glacier by the diverging ice currents 
 
496 Deposition of Clastic Rock Material 
 
 may accumulate there as a submarginal moraine, and become ex- 
 posed on the melting away of the glacier. Such moraines accumu- 
 late when the supply of bottom debris is large as compared with the 
 carrying power of the glacier. Submarginal moraines are dis- 
 tinguishable from lateral moraines, which may be let down upon 
 the valley sides on the melting of the ice, by the fact that the sub- 
 marginal accumulations are very compact from the pressure of the 
 ice and that the stones and boulders in them are scratched and 
 polished, features not found in the lateral superglacial moraines. 
 
 Ground Moraines. When the supply of drift in the bottom of 
 the ice is larger than can be moved along by the glacier, part of it 
 will remain upon the bottom as ground moraine and will be over- 
 ridden by the ice as it moves onward. Such material will be 
 thoroughly compacted and will consist of an intimate and irregular 
 mixture of rock-flour, sand, and striated pebbles and boulders, a 
 mixture to which the name till is applied. In modern glaciers, 
 this accumulation takes place chiefly near the front, where the 
 ice is thinner, or behind an obstruction over which the ice moves 
 but behind which it leaves part of its debris. 
 
 Deposits Formed by Former Ice Sheets 
 
 From the wide extent of surface deposits which have all the 
 characteristics of glacially transported material, as well as from 
 scratched and polished rock surfaces (Fig. 359, p. 434) and 
 the effects of erosion upon the landscape such as are known to be 
 produced by moving ice, it has become apparent that a considerable 
 portion of northern North America and western Europe was 
 covered by continental ice-sheets during a period preceding that 
 in which we live, and from the association of these deposits 
 with the remains of early man in Europe, we recognize that this ice- 
 covering still existed after man had made his appearance upon the 
 earth, though he had not yet spread widely over its surface. The 
 deposits formed by these ice-sheets' consist mainly of ground 
 moraines and terminal moraines and the various modifications of 
 these, together with the deposits formed by the streams from the 
 melting ice, both upon the land and in standing bodies of water, 
 either permanent or temporary. 
 
 Ground Moraines ; Drumlins. Vast areas of northern North 
 America and western Europe are covered by the ground moraine 
 
Glacial Transportation and Deposition 497 
 
 of the ice of the last glacial period. This is a till or boulder clay 
 which consists of a heterogeneous mixture of fine and coarse 
 material, with polished and striated boulders and without strati- 
 
 FIG. 418. Typical section of glacial till exposed by cutting a road through 
 the southern end of a drumlin, Whitewater Quadrangle, Wis. Note the unstrati- 
 fied and unassorted character of the material. The appearance of the pebbles 
 in this till is shown in Fig. 419. (Photo by Alden ; from U. S. G. S.) 
 
 fication or assortment. It is often so thoroughly compacted that 
 it requires almost as much labor for its removal as does a mass of 
 solid rock, on which account it is commonly known as hardpan 
 (Figs. 418, 419). This deposit of till often fills in irregularities in 
 
 FIG. 419. Glaciated pebbles and boulders from drift of North America, show- 
 ing characteristic form and scratches. (Photo by Alden ; from U. S. G. S.) 
 
498 
 
 Deposition of Clastic Rock Material 
 
 the rock topography beneath it, and so produces a more or less 
 monotonous surface. At other times it rises in regular rounded 
 or elliptical hills of elongate form, with their major axis in the 
 
 FIG. 420 a. Drumlin near Groton, Mass. (After Frye.) 
 
 direction of the ice movement. Such hills are known as drumlins, 
 and they form a characteristic feature of the topography of eastern 
 Massachusetts, parts of New York, Wisconsin, and other areas 
 in the glaciated regions (Figs. 420 a, b). 
 
 The thickness of the till varies greatly, rising to several hundred 
 feet in some drumlins, and falling to a few inches elsewhere. The 
 rock surface below the till is generally smooth, showing much 
 erosion and polishing by the ice, and generally exhibiting a series of 
 
 parallel scratches which 
 
 indicate the direction of 
 the ice movement (Fig. 
 359). In some cases,- 
 however, deep grooves 
 and flutings were formed, 
 where large boulders 
 were moved along upon 
 soft rocks into which 
 they gouged deeply. 
 Such glacial groovings 
 have been found wonder- 
 fully developed on the 
 limestone surfaces of 
 
 FIG. 420 &. Section of part of a drumlin, 
 near Boston, Mass., showing the hetero- 
 geneous mixture of fine and coarse material 
 including boulders. (Photo by the author.) 
 (See also Figs. 447, p. 531, and 726.) 
 
 Kelley's Island in Lake 
 Erie, and in many other 
 regions (Fig. 421). 
 
 When the ice first ac- 
 cumulated, many parts of the country were covered by a mantle 
 of residual soil due to the weathering of the rock. This material, 
 which included much clay, was the first to be carried away, and 
 
Glacial Transportation and Deposition 499 
 
 where it was deposited the resulting till consists partly of clay. 
 Subsequent erosion by the moving ice, however, produced only 
 the finest rock-flour, or mechanically ground-up undecomposed 
 
 FIG. 421. Glacial fluting on limestone, Kelley's Island, Lake Erie. 
 
 rock, besides the larger fragments, and in consequence such ma- 
 terial predominates over the clay, which may be largely or wholly 
 wanting. 
 
 Terminal Moraines. Whenever the ice halted for a period of 
 time along a given line, a terminal moraine was built up from the 
 subglacial and englacial material, the size of the moraine de- 
 
 FIG. 422. Bird's eye view of about 2 square miles of terminal moraine. 
 Lakes shown by horizontal shading ; swamps are dotted. (Drawn by Fred K. 
 Morris.) 
 
 pending upon the length of time during which the ice front was 
 stationary. Such moraines exhibit the characteristic kettle topog- 
 raphy (Fig. 422) due to the inclosure of ice blocks which subse- 
 quently melted, while in many cases the removal of the finer 
 surface material by the waters from the melting ice has left 
 them covered over with great boulders. Whenever the sea or 
 
500 Deposition of Clastic Rock Material 
 
 other agent has cut into such a moraine, its structure is seen to con- 
 sist of irregular accumulations of sand and finer material with 
 many boulders, often of large size and of heterogeneous char- 
 
 FIG. 423. Crest of northern frontal moraine, looking northwest. Dog- 
 town Common. (After Shaler, U. S. G. S.) 
 
 acter. The so-called " backbones " of Long Island and Cape 
 
 Cod are formed by the terminal moraines, and the Narrows 
 
 in New York harbor represent a cleft in the great terminal mo- 
 raine, the cut ends 
 of which can be 
 seen on both sides, 
 modified of course 
 by man's activity. 
 This moraine and 
 many smaller ones 
 to the north of it 
 can be traced with 
 more or less con- 
 tinuity for long dis- 
 tances across the 
 continent, being 
 FIG. 424. Boulder moraine, Dogtown Common, recognizable by 
 
 Cape Ann. These bpulders are chiefly granite, and tneir topography 
 
 they are piled up in great heaps with little or no fine 
 
 material between. (Photo by the author.) and internal con- 
 
 stitution. 
 Boulder Trains and Moraines. In some cases the moraines 
 
 appear to consist almost wholly of large and small boulders, forming 
 
Glacial Transportation and Deposition 501 
 
 a topography of striking wildness and complexity. Such a moraine 
 covers a part of the surface of Cape Ann in eastern Massachusetts, 
 
 FIG. 425. Section of frontal moraine on side of Warner Street, Gloucester, 
 Mass. (After Shaler, U. S. G. S.) 
 
 its most picturesque portion near Gloucester being known as " Dog- 
 town Common " (Figs. 423 to 425). This moraine is believed to be 
 the result of a disturbance, by readvance of the ice, which by shaking 
 up an older moraine 
 caused the finer material 
 to settle down between 
 the coarse blocks, which 
 then alone appeared upon 
 the surface. Part of the 
 fine material may also 
 have been washed away 
 by glacial waters. Some- 
 times trains or long lines 
 of boulders alone mark 
 the moraine, or they may 
 form a line in front of some 
 projecting rock mass 
 from which they were 
 derived. Some of these 
 glacial boulders are of as- 
 tonishing size (Fig. 426). 
 
 Interlobate Moraines. As the ice which covered much of the 
 northern lands consisted of several distinct lobes which abutted 
 
 FIG. 426. House Rock, a large glacial 
 boulder or erratic near Hingham, Mass. Its 
 size is indicated by a comparison with men 
 standing near it. (Photo by -the author.) 
 
502 
 
 Deposition of Clastic Rock Material 
 
 one against the other, moraines were formed between the abutting 
 lobes by the drift brought to the margin from each lobe. Such 
 moraines therefore often extend more or less at right angles 
 to the main terminal moraine, and their opposite sides may be com- 
 posed of very different materials. The great north-south moraine 
 which forms the hills of Plymouth and extends southward to Woods 
 Hole in Massachusetts is such an interlobate moraine. 
 
 Modified Deposits of the Great Ice Age 
 
 The waters resulting from the melting of the ice-sheet, on issuing 
 at its front, formed many special types of deposits, recognizable 
 by their form and structure. These, as a rule, are more or less 
 stratified, often pronouncedly so, and on this account, the modified 
 drift is also spoken of as stratified drift in contradistinction to the 
 unstratified drift or till. The most important of such deposits are 
 the kames, apron-plains, sand-plains or glacial deltas, and the 
 eskers. 
 
 Kames. These are more or less conical mounds of sand depos- 
 ited at the ice-front by temporary glacial streams. They are not 
 uncommon in the moraine belt, of which they may constitute a part. 
 
 The Apron-plain. This is formed of the outwash along the 
 ice front, and generally lies in front of the terminal moraine. 
 
 FIG. 427. Diagrams illustrating the mode of formation of the frontal 
 apron-plain and its relation to the terminal moraine. (Adapted from J. B. 
 Woodworth.) The ice-front partly covers the terminal moraine which is sub- 
 marginal. In front of the ice the stratified sands washed outward by the 
 glacial waters build a gently sloping apron-plain. When the ice melts away, a 
 depression or "fosse" appears between the moraine and the apron-plain, the 
 upper end of which, formerly banked against the ice, now slumps, and forms a 
 steep back- or ice-contact slope. 
 
Glacial Transportation and Deposition 503 
 
 Near the moraine border there is generally a depression or fosse 
 which may be occupied by a chain of lakes, and from it the apron- 
 or outwash plain rises, often with steep slopes, where the sand 
 rested against the face of an ice mass which occupied the place 
 now marked by the fosse (Fig. 427). From the summit of this 
 slope the plain descends with a very gentle inclination and gener- 
 ally a nearly smooth surface which may dip into the sea, as in the 
 case of the outwash plain which forms the southern half or more of 
 
 FIG. 428 a. Section of a glacial delta or sand-plain showing fine-grained 
 fore-set beds dipping to the left, covered by coarser top-sets. College Hall 
 Hill, Wellesley, Mass. (Photo by W. P. Haynes ; courtesy of Prof. E. Fisher.) 
 
 Long Island and that which forms the main southern part of Cape 
 Cod. Commonly a series of transverse channels, the paths of the 
 main streams from the ice, transects the outwash plain, and these 
 channels may be dry or rilled with water (as on the Nan tucket 
 plain). Kettle-holes may also occur on this plain, and in regions 
 of high ground-water level, as near the coast, these may be con- 
 verted into ponds. Boulders are rare on such a plain, the material 
 being mostly sand, generally well stratified. 
 
 The Sand-plain or Glacial Delta. Where a body of water was 
 held up by the ice front in a valley which sloped toward it, a sta- 
 tionary lake was produced, the level of which depended on the 
 
504 Deposition of Clastic Rock Material 
 
 FIG. 428 b. Section of a glacial delta or sand-plain, showing the level top, 
 the steeply dipping fore-set beds, and the horizontal top-set beds. Brockton, 
 Mass. (Photo by the author.) 
 
 FIG. 429. Ice contact slope of a glacial sand-plain or delta. Note the steep 
 character of the slope. (Photo by the author.) 
 
Glacial Transportation and Deposition 505 
 
 elevation of the lowest exposed point in the rim of the valley, as does 
 the level of the Marjelen Lake by the side of the Aletsch glacier 
 in Switzerland, on the level of the divide which separates its 
 valley from another on the east. (See p. 362.) Into such a 
 body of water deltas may be built by the sands washed down from 
 the ice, and such deltas will have all the characters of normal stream 
 
 7/y 
 
 FIG: 430 a. Diagram illustrating a successive series of sand-plains or 
 glacial deltas built into a temporary lake held up by an ice-dam in a northward 
 sloping valley. The southern plains were built when the lake stood at its 
 highest level, this being indicated by the edge between the surface and frontal 
 slope. As the ice front melted backward lower outlets were uncovered and the 
 level of the lake sank. Then the lower deltas on the north were built. 
 
 deltas built into permanent lakes, including lobate front and a 
 series of sloping fore-set and nearly horizontal top-set beds (Figs. 
 4280, b). As ice boulders may be buried in such a delta, kettle- 
 holes may develop subsequently by their melting, and these are not 
 uncommon in the larger deltas of this type. The most character- 
 istic feature of these deltas, however, is due to the fact that on the 
 
 ICL 
 
 
 A B 
 
 FIG. 430 b. Diagrams showing the origin of eskers. A, A tunnel under the 
 ice, nearly filled with sand and gravel by the subglacial stream which built up 
 its bed to the level of the outlet, or the level of the lake held in front of the ice. 
 B, The same deposit after the melting of the supporting ice-walls. Slumping 
 has produced the characteristic steep side-slopes. 
 
 melting of the retaining ice wall of the lake, this will be drained, and 
 the delta will then form a plateau-like elevation in the midst of the 
 valley. The side of the delta which rested against the ice will, by 
 slumping, become a steep slope often as steep as 30 degrees 
 and one easily distinguished by its angle and general outline from 
 the frontal slopes of such deltas (Fig. 429). 
 
 When successively lower passes are opened in the valley side by 
 the melting back of the retaining ice front, the level of the lake 
 
506 Deposition of Clastic Rock Material 
 
 FIG. 431 a. An esker, near East Weymouth, Ma?s. Seen from the side. 
 (Photo by the author.) 
 
 FiG."43i b. Top of the esker shown in Fig. 431 a, East Weymouth. (Photo 
 
 by the author.) 
 
Glacial Transportation and Deposition 507 
 
 will sink in accordance therewith, and a succession of plains or deltas 
 at lower levels may be built up (Fig. 430 a) . There are many such 
 plains in the valleys of Massachusetts which show the progressive 
 lowering of the old ice-dammed lakes, the shores of a number of 
 which have been traced and the successive outlets located, in con- 
 formity with the levels indicated by the heights of the abandoned 
 deltas. 
 
 Eskers. Streams flowing in ice gorges near the front of the 
 great ice sheet or in tunnels beneath it, built up their beds by de- 
 positing sand and gravel r 
 
 on their floors, until a 
 height was reached which 
 corresponded to that of 
 the standing water level 
 in front of the ice or the 
 elevation of the rocky col 
 across which the stream 
 had to discharge. Dur- 
 ing their formation these 
 deposits rested against 
 the side of the ice gorge 
 or tunnel, but on the 
 melting of the ice this 
 support was removed, 
 
 FIG. 431 c. A small esker, winter scene 
 among the Waverly Oaks, Waverly, Mass. 
 (Photo by the author.) 
 
 was 
 
 and the sides of the sand 
 and gravel mass began to 
 slump and come to rest 
 at the steepest angle con- 
 sistent with the nature of the material (Fig. 430 b). In this manner, 
 more or less winding ridges of triangular section were formed, often 
 extending across country for many miles like a huge railroad em- 
 bankment, but of somewhat variable height and generally winding 
 course, this course being that of the stream which made the de- 
 posit (figs. 431 a-c). Such ridges are known as esker s or osars, and 
 they abound in New England, in Scandinavia, and in other regions 
 formerly covered by the ice and of a topography favorable to their 
 formation. They often constitute striking features of the land- 
 scape. Not all eskers were formed in this manner, some probably 
 representing accumulations of sand and gravel in gorges cut into 
 the ice, or in crevasses open to the sky. 
 
508 Deposition of Clastic Rock Material 
 
 Ancient Glacial Deposits 
 
 In very ancient geological formations in India, South Africa, Aus- 
 tralia, and South America great beds of rock are known, which have 
 all the characteristics of a glacial till, including the polished and 
 striated boulders and bed-rock (Fig. 3600, p. 435). These rocks, 
 known as tillites or consolidated tills, are interpreted as the deposits 
 of ice-sheets which covered these regions in former geological periods. 
 Similar, even more ancient, glacial deposits have been reported from 
 Canada and elsewhere, and others from northern Norway. They 
 will be considered more fully in the section on historical geology 
 under their respective periods. 
 
CHAPTER XVII 
 
 TRANSPORTATION, SORTING, AND DEPOSITION OF 
 CLASTIC MATERIAL IN THE SEA 
 
 THE GEOGRAPHICAL SUBDIVISIONS OF THE SEA 
 
 WHEN we speak of the sea we refer to the entire extent of the 
 connected salt waters of the earth, those that lie between the con- 
 tinents and those that lie within them. The completely enclosed 
 salt water lakes such as Great Salt Lake, the Caspian Sea (lake), 
 and the Dead Sea (lake) are not included here, though it is a gen- 
 eral European custom to speak of such water bodies as seas (Ger- 
 man, Seen). The term seas is most commonly applied to the more 
 or less, but never completely, enclosed portions of the sea, such as 
 the Mediterranean Sea, the Black Sea, the Yellow Sea. Com- 
 pletely enclosed bodies of water are properly called lakes, whether 
 they are fresh or salt, though it will probably be difficult to correct 
 such popular misnomers as " The Dead Sea," " The Caspian Sea," 
 etc., many of which received this appellation when it was believed 
 that they were a part of the' great salt sea. 
 
 Other loosely used geographic terms are gulf and bay, which are 
 indiscriminately applied, sometimes to open and sometimes to 
 partly enclosed marginal bodies of water; sometimes to deep bodies 
 of this type, sometimes to shallow. It is therefore important that 
 certain more precise terms should be used, terms which designate 
 the various types of water bodies, divided not only according to 
 form and location, but also according to depth and relation to other 
 water bodies. 
 
 The Oceans or Intercontinental Seas 
 
 The Three Continental Masses. From our present point of 
 view, we may consider that there are only three continental masses, 
 to one or the other of which all the so-called continents of the 
 geographer belong. These continental masses are the three great 
 
510 Deposition of Clastic Material in the Sea 
 
 land blocks of the earth's crust, and although several of them seem 
 to be nearly or quite separated into distinct continents, they are in 
 reality units, and in former periods of time they were in some cases 
 more intimately united than they are to-day. Named in the order 
 of their magnitude, these three continental masses are: i. The 
 Old World Mass, comprising the geographical continents of Europe, 
 Asia, Africa, and Australia; 2.. The New World Mass, comprising 
 the geographical continents of North and South America; and 
 3. The Antarctic Mass, forming a single Antarctic continent. 
 
 The Four Oceans. Between these continental masses lie the 
 oceans, which are thus truly intercontinental in position, and repre- 
 sent the oceanic blocks of the earth's crust which probably, because 
 of their greater density, have a sunken position with reference to 
 the continental blocks. In the opinion of many students of the 
 earth, these differences of relative position have been constant 
 since the earliest recorded geological time, though there are others 
 who hold that at one period or another some of the oceanic blocks 
 were also elevated to such an extent that their surfaces appeared 
 as land above the sea-level. Named in the order of their magni- 
 tude, the four oceans of the earth are: i. The Pacific Ocean, 
 lying between the Old World block on the west and the New World 
 block on the east, the convergence of which bounds this ocean on 
 the north, while the southern boundary is formed by the Antarctic 
 continent; 2. The Atlantic Ocean, lying between the same conti- 
 nental masses except that the Old World block is on the east and 
 the New World block on the west, while the Antarctic block also 
 bounds it on the south, and the convergence of the other two blocks 
 on the north ; 3. The Indian Ocean, lying between the Old World 
 block on the north, and the Antarctic block on the south, while on 
 the east and the west it becomes more or less confluent with the 
 Pacific and Atlantic oceans, respectively; 4. The Arctic Ocean, 
 the smallest of all, which lies between the northern ends of the Olo^ 
 and New World blocks, and is more or less completely bounded 
 by them (Fig. 432). 
 
 The Continental Shelf. At present the oceans are overfull 
 of water, and so they spread out over the margins of the continental 
 blocks, in some places for a distance of only a few miles, in others 
 for a hundred miles or more. This margin has in general the form of 
 a shelf or gently seaward sloping platform, partly due to erosion of 
 the land margin and partly to deposition. From this shelf arise the 
 
Geographical Subdivisions of the Sea 511 
 
 continental islands which are either residual masses left by the 
 erosion of the edge of the land, or have been built up on the shelf, 
 by clastic sediments from the land, by organisms (coral reefs, etc.),' 
 
 FIG. 432. Map of the Arctic. Ocean and a part of the North Atlantic Ocean 
 
 showing the depths. The continental shelf (0-200 meters) is left in white like 
 
 continents. Note its great width off the Eur-Asian coast (long. 40 K. 
 
 i i , Als n te the many epicontinental seas or shallow indentations 
 
 the land. The Baltic Sea (between long. o and 40 E. is an epeiric sea. The 
 
 bathyal zone is shown by the lighter shading (horizontal lines, 200 to 1000 
 
 s) and the deeper bathyal zone by the darker shading (cross-lines 1000- 
 
 > meters). These two form the continental slope. The abyssal district 
 
 is represented in solid black. Note the double character of the Arctic Ocean 
 
 and the submerged ridges which divide it from the North Atlantic. These 
 
 are the Wyville-Thompson ridge between Scotland and the Faroe Islands, the 
 
 Faroe-Iceland ridge between these islands, and the Denmark straits ridge be- 
 
 t*een Iceland and Greenland. (After Nansen; from Grabau's Principles of 
 
512 Deposition of Clastic Material in the Sea 
 
 or by volcanic eruptions. The depth of water over this shelf 
 ranges from zero to about 100 fathoms (600 feet), at which point, 
 approximately, the main margin of the continental mass is located. 
 This margin is also called the edge of the continental shelf (Fig. 
 
 FIG. 433. Map of the peninsula of Florida, showing the bottom contours 
 of the surrounding sea. The line of keys and old reefs is indicated by crosses. 
 (After Vaughan.) The loo-f athom contour line marks the edge of the continental 
 shelf. Note the width of this shelf on the west, and its narrowness on the south- 
 east where it is swept by the Gulf Stream. Note that on the west the descent 
 from 100 to 1000 fathoms takes place in a shorter distance than the descent from 
 20 to 100 fathoms. The Keys are continental islands of organic origin built 
 upon the shelf. The part of the sea-bottom bounded by the looo-fa thorn line 
 represents the abyssal district. (Compare Fig. 438, p. 519.) 
 
 433). It is upon this shelf that the epicontinental seas are located, 
 while deeper seas transect it. (See Fig. 434.) The continental 
 shelf is also the chief region of clastic deposition in the open sea, 
 though of no greater importance than some of the seas next to be 
 described. (See Fig. 438, page 519.) 
 
Geographical Subdivisions of the Sea 513 
 
 The Epicontinental and Intracontinental Seas 
 
 Upon the edge of the land, that is, upon the continental shelf, 
 there are a number of distinct water bodies more or less outlined 
 by land extensions, 
 submerged ridges, or 
 islands rising from 
 these. These are 
 called epicontinental 
 seas (epi, upon) be- 
 cause they lie upon 
 the continent. Some 
 of these extend far 
 into the land masses 
 (epeiric seas), but are 
 still shallow films of 
 sea-water resting 
 upon the land. Ex- 
 amples of these are 
 seen in the Baltic 
 Sea, and in Hudson 
 Bay. Again, the 
 edge of the conti- 
 nental shelf may be 
 deeply intrenched, 
 excavated, or hol- 
 lowed by down- 
 
 FIG. 434. Map of the Gulf of California, a typi- 
 cal funnel-sea of the narrow type, transecting the 
 
 sinkings, and such ex- continental shelf, which is represented by the hori- 
 cavations mav also zontal lining (o ~ 6o ft) ' Note the regular and 
 
 extend far into the 
 
 land. These are the 
 
 . .. . j 
 
 true intracontinental 
 
 progressive deepening of this funnel-sea and corn- 
 pare with the Red Sea, a typical mediterranean 
 sea ( Fig - 43 7) On the weste J" n border of Lower 
 California is a partly enclosed epicontinental sea, 
 the Bay of Sa P n Sebastian vizc amo. The con^ 
 seas, because they lie tinental slope is shown by diagonal lines (600 to 
 within the continen- 6oo ^- or I0 to I00 fathoms), though the next 
 
 ZOIie might als be included here (OO to IO,OOO 
 
 ft } Below that is the deep sea (io>ooo feet and 
 lower). (See section, Fig. 438.) 
 
 tnl msec TV,o, 
 
 iney are 
 
 illustrated by the 
 Gulf of California 
 
 (Fig. 434), and by the Red Sea (Fig. 437). The principal types 
 may be briefly noted. 
 
514 Deposition of Clastic Material in the Sea 
 
 Intracontinental Seas 
 
 ex- 
 
 Funnel Seas. Where the continental margin is deeply 
 cavated or incised, so as to form a trough which extends into the 
 land with regularly narrowing sides and a regularly rising bottom, 
 a funnel sea is produced, so called because its form may be compared 
 with that of one half of a longitudinally divided funnel. The Gulf 
 of California is a typical example, for not only do its sides converge 
 toward the head, but its floor rises progressively (Fig. 434). If the 
 sea-level should sink or the land block rise, this funnel would be 
 gradually emptied by the withdrawal of the water and no residual 
 
 o-iooo ft 
 
 *IOOO-5OOO ft 
 
 sooo-io.ooofc. 
 
 Over lo.oooft 
 
 FIG. 435. Broad or Biscayan type of funnel-seas, a, Bay of Biscay be- 
 tween France and Spain, a closed-head funnel-sea ; b, Gulf of Cadiz, an open- 
 head funnel-sea of this type, receiving the waters from the Mediterranean 
 through the Straits of Gibraltar. 
 
 lake would remain. The California funnel sea represents the nar- 
 row type ; a broader type is represented by the Bay of Biscay on 
 the west coast of Europe, which differs from the California type 
 only in the more rapid convergence of its sides and corresponding 
 rise of its bottom (Fig. 435 a). A modification of this type is seen 
 in the Gulf of Cadiz, which receives the outlet of the Mediterranean 
 Sea (Fig. 435 b). This may be referred to as having an open head, 
 as compared with the closed-head type shown by the Bay of Biscay. 
 It is evident that not only the character of the water, its tempera- 
 ture, currents, etc. differ in the two types, but also the quantity 
 and character of the sediments, the closed-head type receiving 
 the drainage of the land -direct, together with the sediments 
 brought in, while the open type receives only the finer sediments 
 
Geographical Subdivisions of the Sea 515 
 
 which escape from the adjoining water body, besides the sediments 
 brought in by lateral streams. 
 
 An example of the narrow type of deep funnel sea with an open 
 head is seen in the Gulf of Aden (Fig. 436) which receives the out- 
 let of the Red Sea, another mediterranean. Finally, there are 
 shallow or epicontinental funnel seas, the most conspicuous ex- 
 ample being the Bay of Fundy. This is also a narrow type, and 
 this and its shallowness are responsible for the great range of the 
 tides for which this water body is famous. (Compare with Estuary.) 
 
 FIG. 436. The Gulf of Aden, which exchanges its waters with the Red Sea. 
 A narrow type of funnel-sea with open head. The continental shelf (littoral 
 zone) is lined horizontally. The next two zones belong to the continental 
 slope the higher (600-3000 ft.) is the typical bathyal zone (diagonal lines) ; 
 the next (3000-6000 ft., cross-lines) the deeper bathyal zone. Beyond this (below 
 6000 feet or 1000 fathoms, dark shading) is the abyssal district. (Compare with 
 section, Fig. 438.) 
 
 Mediterraneans. When the continental shelf or an inland por- 
 tion of the continent is deeply excavated, but in such a manner 
 that the deeper portion is everywhere surrounded by a higher rim, 
 a mediterranean sea is produced. Such seas may also be formed by 
 the dropping down of small blocks of the continental mass, or by 
 very pronounced slow warping of a part of the continental shelf. 
 A mediterranean sea, while truly intra-continental or within the 
 land block, as is the case with the deep funnel sea, differs from the 
 latter in having its bottom rise in all directions, while its outline 
 
516 Deposition of Clastic Material in the Sea 
 
 may vary from nearly circular to elongated oval or irregular. In 
 all cases, however, there is a shallow outlet across the rim or there 
 may be several such, these outlets leading to the open ocean (Gulf 
 of Mexico) to a funnel sea (Roman Mediterranean or Red Sea) or to 
 another mediterranean (Black Sea). Should the land rise, carrying 
 the mediterranean with it, or should the sea-level sink, such a medi- 
 terranean sea will be converted into a lake, either without outlet, 
 when the water remains salt, as in the case of the Caspian, or 
 drained by a river (Lake Baikal, Lake Tanganyika), in which case 
 the water will generally become fresh from excess precipitation. 
 
 The type of the mediterraneans is the one generally so called (the Roman 
 Mediterranean), which lies between Europe, Asia, and Africa. It is really a 
 compound body, with several deep centers. The Red Sea is another example, 
 being land-locked like the Roman, and opening into a funnel sea (Fig. 437). The 
 Black Sea, on the other hand, is a very nearly enclosed water body, almost a 
 lake. The Gulf of Mexico is a mediterranean with several openings across a 
 ridge from which islands arise, and the Caribbean Sea is a mediterranean with 
 a still more extensively submerged outer rim, as are Behring Sea and the Sea 
 of Okhotsk in the northern Pacific. These are generally spoken of as marginal 
 mediterraneans lying within the continental shelf. 
 
 The importance of mediterraneans lies in the fact that they have their own 
 ranges of temperature and salinity, currents, and other characteristics, which are 
 on the whole distinct from those of the neighboring oceans. The differences 
 are, of course, more pronounced as the enclosure becomes more complete, until 
 such types as the Black Sea, with practically stagnant deeper waters, are 
 produced. 
 
 Most mediterraneans date their origin from long past geological 
 time. They are not as old as the oceans, but many of them have 
 existed during successive geological periods. 
 
 Epicontinental Seas 
 
 Epeiric Seas. The shallow seas of an epicontinental character 
 which lie within the land masses, such as the Baltic Sea and Hudson 
 Bay, have come to be called epeiric seas (e7rei/>os, epeiros, a conti- 
 nent). Their depth seldom exceeds 50 or 100 fathoms (300-600 
 feet), and they are formed chiefly by a gentle down-warping of 
 the land surface until it has passed below sea-level. Such seas 
 are always connected with the oceans by shallow passages, the 
 narrowness of which often prevents much change in level of the 
 water body during tidal fluctuation of the open ocean. The 
 salinity of the water, its temperature, and its currents are also dis- 
 
Geographical Subdivisions of the Sea 517 
 
 tinct in such a water body. In the past many such epeiric seas 
 existed where now is the dry land ; indeed, such seas have often 
 been replaced repeatedly by dry land. They are of extreme im- 
 
 MER ROUGE 
 
 d'apres les sondages 
 
 du Navire de guerre autrichien 
 
 "POL A". 1895 -1898. 
 
 FIG. 437. Map of the Red Sea, a mediterranean, showing the depths ac- 
 cording to the soundings of the Austrian warship Pola, 1895-1898. (From 
 Suess and de Margerie.) The continental shelf (littoral zone) is shown in white 
 (0-200 meters or roughly, o-ioo fathoms). The zones to a depth of 2000 
 meters (roughly 1000 fathoms) represent the continental slopes, while the deeper 
 parts (in black) are the abyssal depths. Note that the Gulf of Suez is an epi- 
 continental water-body, resting entirely upon the continental shelf, while the 
 Gulf of Akaba is a subsidiary mediterranean. 
 
518 Deposition of Clastic Material in the Sea 
 
 portance in the history of older sediments. Marginal seas of this 
 type situated on the continental shelf are the North Sea, Irish Sea, 
 Tasmanian Sea, and others. 
 
 Geosynclines. Along the base of high or mountainous land 
 masses, there generally extends a belt of subsidence in which the 
 bulk of the sediment brought from the mountains is deposited. 
 This is the geosyncline. The rate of subsidence is usually con- 
 stant, so that great thicknesses of material of uniform character 
 may accumulate. Sometimes this material accumulates wholly 
 above sea-level, as i,n the case of the north Indian geosyncline at the 
 southern foot of the Himalayas already referred to (page 468). In 
 other cases the sea has access to this trough, when great thicknesses 
 of limestones or of other marine deposits may form. Not infre- 
 quently there is an alternation of continental and marine deposits, 
 showing a variable rate of subsidence or a variable volume of sedi- 
 ment. In general the rate of subsidence and that of supply of 
 material is evenly balanced, so that for a long time similar deposits 
 may be formed at essentially the same elevation above, or depres- 
 sion beneath the sea-level. Such geosynclines of the past will be 
 referred to at greater length under their proper periods in the section 
 on historical geology. 
 
 BATHYMETRIC DISTRICTS AND ZONES 
 
 The bathymetric districts and zones of the sea are those deter- 
 mined by the depth of water. As we have seen, this depth over the 
 continental shelf does not exceed, as a rule, 100 fathoms (600 feet). 
 This district and the shallow epeiric sea is illumined throughout 
 by sunlight and agitated by waves, and constitutes the littoral 
 district of the sea, though this term is restricted by some to the shore 
 zone. This district naturally falls into two zones, (a) that of the 
 shore between high and low tide (shore zone) and (b) that perma- 
 nently submerged even at low water (neritic zone) (Fig. 438). 
 Beyond the edge of the continental shelf the ocean floor slopes 
 more rapidly to depths of about 1000 fathoms (6000 feet), and this 
 constitutes the bathyal district, though some authorities limit this 
 zone to depths of 500 fathoms. Beyond this lies the abyssal 
 district, which comprises the larger area, between 1000 and 3000 
 fathoms (2400 and 5500 meters) and the smaller oceanic depressions 
 or deeps, which descend to greater depths (30,000 feet or more). 
 
Waves and Currents of the Sea 
 
 519 
 
 The characteristic deposits of these several districts and zones will 
 be considered in a later section of this chapter. 
 
 Littoral District- 
 
 Strdnd ShoreZone 
 
 Df strict 
 
 ( A tyjso- Pelagic District 
 Abyssal District 
 
 FIG. 438. Diagrammatic cross-section of a 1 part of the sea to show the 
 several life-districts and the bathymetric zones. The edge of the continental 
 shelf is marked by the 2OO-meter or ico-fathom line (approximately). That 
 part of the sea which lies above the continental shelf and extends to "high- 
 water mark is called the Littoral District, this being divided into the shore-zone 
 between high and low water and the neritic zone beyond that. From the edge 
 of the continental shelf the "continental slope" descends to the deep sea (200 
 to 2400 meters, 100 to 1000 fathoms, roughly). The upper part of this, to a 
 depth of 500 fathoms, is called the bathyal zone proper, the lower part the deeper 
 bathyal zone. Above the floor of the deep sea lies the abyssal district of the ocean. 
 Long, narrow depressions of the ocean floor parallel to the continents form the 
 fore-deeps. The pelagic district is the upper part of the ocean to the depth to 
 which sunlight penetrates approximately 100 fathoms. 
 
 WAVES AND CURRENTS or THE SEA 
 
 Waves. There are many kinds of water waves, produced in 
 a variety of ways ; of these the waves produced by winds are the 
 most important, because most common, though those produced by 
 earthquakes and explosions, the so-called tsunamis, are often more 
 destructive. Some of these will be noted in connection with our 
 studies of earthquakes. The great waves known as the tides, 
 two of which sweep around the earth in somewhat more than 
 twenty-four hours, are of great importance as producers of oceanic 
 currents, while the periodic rise and fall of the waters becomes 
 significant along the shores, influencing not only the forces which 
 modify such shores, but the life of the shallow sea as well. 
 
 The waves of the open ocean are due to a rotary movement of 
 the particles of water (Fig. 439). Originally these are set in motion 
 by the wind, but they are propagated beyond the zone of wind 
 activity, so that even on calm days great waves or swells will be 
 found in the ocean, far from their source of origin. As a result of 
 
520 Deposition of Clastic Material in the Sea 
 
 the rotary motion of the water particles the mass of the water 
 rises and falls rhythmically, forming alternately the crest and the 
 trough of the wave. The water particles move forward on the 
 crest of the wave, downward on the back, backward in the trough, 
 
 FIG. 439. Diagram illustrating the formation of waves in the open ocean 
 by a*rotary motion of the water particles. As the particles revolve in the 
 direction indicated by the small arrows, the wave form advances as shown by 
 the heavy arrows. (From Grabau, Principles of Stratigraphy.} 
 
 and upward on the front of the wave. In water of limited depth, 
 however, the path of the moving particles is an ellipse, with the 
 major axis horizontal. 
 
 The height of the wave is measured by the vertical distance between the 
 trough and the crest, and corresponds to the diameter of the orbit in which 
 the water particles move. In the open ocean, wave heights from 5 to 15 feet 
 are most common, but 20 or 40 feet is not an unusual height in storms, while 
 heights of 45 to 50 feet have been recorded. The length of the wave is the 
 horizontal distance from crest to crest. During stormy weather, in the open 
 ocean, the wave length may be from 200 to 500 feet or over. When the wave 
 reaches a length of a thousand feet or more, it is called a swell or ground swell 
 and such waves have been found in exceptional cases to have a length of more 
 than 2700 feet from crest to crest. The height and length of the wave depend 
 upon the strength and continuity of the wind, and to some extent on other 
 factors. The height of the wave decreases downward at a rate proportional 
 to the wave length. If the diameter of the orbit of motion of the water particles 
 is 20 feet, giving surface waves 20 feet high, and if the wave length is 400 feet, 
 the diameter of the orbit of the water particles, at a depth of 400 feet, is T 4 ^ of 
 an inch. 
 
 Because of the regular rise and fall of the water in wave formation, the crest 
 of the wave advances, although the water itself only rises and falls. The 
 rapidity with which the wave crest travels is called the velocity of the wave, 
 and this varies from 15 miles or less to 35 miles or more an hour and may be 
 as high as 60 miles. For ocean waves of large size, the wave velocity is apt 
 to be six or seven times as great as the orbital velocity of the water particles. 
 For example, a wave 400 feet long and 15 feet high will have a velocity of about 
 
Waves and Currents of the Sea 
 
 45 feet per second (about 30 miles an hour), while the surface water particles 
 will move round in their orbits at a speed of only 5^ feet per second. The 
 time taken by a crest to travel a wave length is called the period of the wave, 
 and this, in storm waves, varies from 6 to 10 seconds or over. It corresponds 
 to the time required for a particle to move around its orbit. 
 
 When great waves, by propagation, reach shallow water the 
 free orbital movement of the water particles is interfered with, 
 partly, as some hold, by friction upon the bottom, which retards 
 the backward motion, but more especially because of the insuffi- 
 cient quantity of water to keep up the normal rotary motion. 
 The wave becomes higher and shorter, its front steepens, while 
 the crest arches forward (Fig. 440, D, ), and being unsupported 
 
 FIG. 440. Diagram showing the development of the breaking wave (A-G), 
 and the "swash" after breaking (//). (After Davis.) In general the wave 
 breaks when the depth of water reckoned from the undisturbed sea-level is 
 equal to the height of the crest above the trough. 
 
 by sufficient water in front of it, dashes downward with a roar, 
 producing a breaking or combing wave (Fig. 440, F, G, Fig. 441). 
 After the wave has broken, the water rushes up on the beach, 
 forming the " swash," and returns seaward, carrying the sand and 
 pebbles with it, the impact of the latter against one another often 
 producing a loud rattling noise. In this manner rock fragments 
 are rapidly worn into round pebbles (see p. 430).* 
 
 Waves of Translation. The waves so far described are the ordinary ones, 
 and are called waves of oscillation. There is, however, another kind, illustrated 
 by the movement produced when still water is pushed into a mound by the 
 shoving action of a boat. In that case a single prominent wave rolls forward 
 over the surface of the water, there being no trough as in the oscillation wave. 
 Such waves are called waves of translation, because the water particles rise and 
 move forward to a new position as the wave form advances. When large waves 
 
 1 For further details see A. W. Grabau, Principles of Stratigraphy, A. G. Seller and 
 Co., Chapter V, and D. W. Johnson, Shore Processes and Shoreline Development, John 
 Wiley and Sons, 1919, Chapters I and II. 
 
522 Deposition of Clastic Material in the Sea 
 
 of oscillation break on a gently sloping shore far from land they give rise to a 
 series of waves of translation which will advance up the beach, carrying the 
 wrack of the sea landwards. 
 
 FIG. 441. Combing wave,, showing water completing orbital movement 
 although insufficient in quantity to fill the wave form. Note the steep wave 
 front in the foreground, and the decreasing steepness away from the comber 
 in the distance. (Compare with Fig. 440.) (After D. W. Johnson, Shore 
 Processes, etc. John Wiley and Sons.) 
 
 Wave Currents. In shallow water, where the wave front is 
 steepest, it will rush forward with a short, quick movement, while 
 the return, or backward, motion is slower and of longer duration. 
 The forward movement may be strong enough to carry large 
 pebbles and cobblestones which the backward current is unable 
 to move. Hence this coarser material will be carried landwards, 
 while the finer material moves seawards. Coarse material dropped 
 at a distance of 7 to 10 miles from land, where the water was from 
 10 to 20 fathoms deep, has been thrown on shore by storm waves, 
 and even pig lead, from a vessel wrecked more than a mile from 
 shore, has been cast upon the beach. Stones with large seaweeds 
 or kelp attached to them are often cast ashore from the deeper 
 water in which these seaweeds grow. Here a certain buoyancy 
 is given to the stone by the seaweed. On steep coasts, however, 
 the backward current or undertow may be very strong. 
 
 Beach Drift. This term has been applied by D. W. Johnson 
 to the transportation of material on the beach-slope, and parallel 
 
Waves and Currents of the Sea 523 
 
 to the shore, by current action due to waves which break obliquely 
 upon the shore. In this case the swash advances obliquely up 
 the slope of the beach, but the back-wash, moving under the control 
 of gravity, tends to return directly down the steepest slope. Thus 
 such material will zigzag along the shore, but the path of the 
 particles will in reality be a series of parabolic curves, as shown 
 in Fig. 442. These paths 
 are larger for the finer than 
 for the coarser material, the 
 finer particle therefore trav- 
 eling faster along the shore. 
 The direction of the drift FlG . ^ _ Diagram to "niustrate the 
 Will depend on the direction "zigzagging" of the particles in beach 
 of the prevailing winds, on drift under the influence of oblique waves. 
 the position Of the greatest The direction of movement is indicated 
 
 by the arrows. The paths are parabolic 
 stretch of open water, since curves. (After Johnson.) 
 
 this largely determines the 
 
 direction from which the larger waves will come, and on other 
 factors. In some cases drift movement is in opposite directions 
 from a given point. A similar movement of particles goes on in 
 the off-shore deeper water region, where the material is moved to 
 and fro on the bottom by the waves which advance parallel 
 to the shore, but moves in a series of parabolic curves, if- the 
 wave advance is oblique. This is called long-shore drifting. 
 
 Undertow and Long-shore Currents. When, by the breaking 
 of waves off shore, a succession of forward-moving water waves 
 (waves of translation) is formed, the water will be piled up on the 
 shore, above the normal level of the sea, and has to escape either 
 seaward along the bottom or parallel to the shore. In the first 
 case it forms the undertow, .in the second the long- shore current. 
 The undertow is especially marked in regions of steep off-shore 
 bottom, and deep water close to shore, and further, where broad 
 zones of waves strike the shore at right angles. The long-shore 
 current is best developed where the waves strike the shores 
 obliquely, and where the bottom shallows gradually. 
 
 Tidal Currents. Under the influence of the attraction of the 
 moon and, to a lesser degree, that of the sun, the water of the sea 
 rises into two elevations on opposite sides of the earth, and these 
 would sweep around the earth as two great continuous tidal-waves, 
 were it not for the presence of the continents. Twice each month, 
 
524 Deposition of Clastic Material in the Sea 
 
 at new moon and at full moon, the tides are exceptionally high, 
 owing to the relative position of the sun and moon at such times, 
 when they exert a combined influence of the same character upon 
 the waters. Such tides are called Spring Tides. Twice a month 
 also, at the period of the first quarter and last quarter of the moon, 
 the interval between high and low water is at its lowest, since at 
 such times the moon and sun act in contrary direction upon the 
 waters, each tending to neutralize the force of attraction of the 
 other. This constitutes the Neap Tides. 
 
 FIG. 443. Reversible Falls, St. John, N. B. High tide, 
 westward into the harbor. 
 
 Water pouring 
 
 At a given point, the crest of the tidal wave arrives at intervals of 1 2 hours 
 and 26 minutes. This produces the flood-tide. Six hours and thirteen minutes 
 later the trough of the tidal wave replaces the crest, and low water or ebb-tide 
 results. Because of the interference of the lands, the tidal wave is retarded 
 and the time of high or low tide may differ widely from that of neighboring 
 regions, though the interval remains the same. It may even happen that a 
 low tide from one direction meets a high tide from another, as at Hellgate, 
 where the high tide, entering from New York Harbor, arrives simultaneously 
 with a low tide approaching through Long Island Sound, while six hours later 
 the reverse is true. The strong tidal disturbances of the Straits of Dover, 
 experienced by all who have made the crossing, are due to the interferences 
 of tides arriving, the one from the North Sea, the other from the Atlantic 
 
Waves and Currents of the Sea 
 
 525 
 
 through the Channel. The famous " Scylla " and " Charybdis " in the Straits 
 of Messina are whirlpools, produced by the meeting of tides from opposite 
 directions, and a similar phenomenon is noted on the coast of Massachusetts at 
 Woods Hole. 
 
 Where the tides pass through narrow channels, powerful and 
 dangerous currents are generated, and these may affect the bottom, 
 even at considerable depth, by sweeping away all loose material 
 and depositing it elsewhere. When the tides are compressed in 
 
 FIG. 444. Reversible Falls, St. John, N. B. Low tide. Water pouring 
 from the harbor, flowing eastward. 
 
 narrow inlets, such as the Bay of Fundy, they may pile up near 
 the head of the bay to a height of 30 or 40 feet and in exceptional 
 cases to a height of 70 feet. The rapidly rising tide of this bay 
 pours into St. John Harbor through a narrow inlet, the surface of 
 this basin rising more slowly than that of the ocean without, 
 on account of the narrowness of the passage. Conversely, the 
 tide in the Bay of Fundy falls more rapidly than the water can 
 pour out from St. John Harbor. Hence a reversible fall is pro- 
 duced, facing inward when the ocean is highest and outward 
 when it is lowest. The maximum difference of elevation of the 
 water surfaces is nearly 10 feet (Figs. 443, 444). Related phe- 
 
526 Deposition of Clastic Material in the Sea 
 
 nomena exist upon the coast of Massachusetts. The influence of 
 the tidal currents is very marked in estuaries and will be referred 
 to later. 
 
 Planetary Currents. These represent the great oceanic circu- 
 lation set in motion primarily by the planetary winds and modified 
 
 by the rotation of the earth, the con- 
 figuration of the lands, and by other 
 factors. In an ideal ocean of sym- 
 metrical form, reaching from pole to 
 pole, and covering a meridional dis- 
 tance of 90, the circulation would be 
 essentially like that shown in Fig. 445. 
 
 FIG. 445. Diagram show- 
 ing the circulation in an ideal 
 ocean extending from pole to 
 pole and covering one fourth 
 the circumference of the earth 
 (90). (After Krummel.) 
 
 On both sides of the equator, at 10 north 
 and south latitudes, the equatorial stream 
 would flow westward under the influence of 
 the easterly winds. On approaching the 
 western shore it would bend northward and 
 southward, respectively, crossing eastward 
 again in latitudes 50 north and south, and 
 returning to the equatorial region on the 
 eastern side. This constitutes the principal 
 circulation. Arctic and antarctic currents 
 would also exist, flowing as indicated in .the 
 diagram. Between the two westward-flowing 
 equatorial currents is an eastward-flowing 
 
 equatorial counter-current, which is, however, much weaker. The equatorial 
 currents are warm and they impart their heat to the westerly shores where 
 they turn, respectively, north and south, while the return currents are cooled 
 in their northerly and southerly passages. 
 
 The outline of the continents is responsible for marked deflections 
 of the currents, while the position of the equatorial currents is also 
 modified by the shifting of the heat equator. The south-equatorial 
 current crosses the Atlantic with an average velocity in June, 
 July, and August of from 1.6 to 1.8 km. per hour. Striking the 
 projecting point of South America, at Cape St. Roque, it divides, 
 one arm passing southward to become the Brazil current, while 
 the other (northern) unites with the north-equatorial current to 
 produce the Guiana current, which later becomes the Gulf Stream. 
 This makes the circuit of the Gulf of Mexico and escapes by the 
 narrow passage between Cuba and Florida (Fig. 225, p. 301). It 
 turns northward between Florida and the Bahama banks, crossing 
 
Sources of Clastic Sediments in the Sea 527 
 
 the Atlantic approximately in latitude 40 N. Its further course 
 may be noted by reference to a map of these currents in any good 
 atlas. At the Florida Straits, the average velocity is 5.55 km. per 
 hour, but rises to 9 km. per hour in the warmer season. The 
 velocity rapidly decreases northward, and where the Gulf Stream 
 crosses the Atlantic, it averages not much over i km. per hour. In 
 the center of the great North Atlantic eddy, the ocean is relatively 
 quiet, and is filled with the floating sea-weed Sargassum. This 
 region has, therefore, become known as the Sargasso Sea. 1 
 
 It should be noted, however, that these great oceanic currents seldom come 
 near the coast, or if they do, tKeir effect is overcome by other local currents. 
 This is true even of the Gulf Stream in the Florida Straits, which is separated 
 from the land by another and slower current moving in the opposite direction. 
 
 Other Currents. Besides those described there are other 
 currents in the sea and in great lakes, most notable among which 
 are those produced by winds which blow steadily in a given direc- 
 tion. These currents are often of great importance, especially 
 in enclosed or nearly enclosed water bodies. Other currents are 
 due to difference in salinity of the waters, to convection, to rivers 
 entering the sea or lakes, and to other causes. 2 
 
 SOURCES OF CLASTIC SEDIMENTS DEPOSITED IN THE SEA 
 
 The clastic material which finds its final resting place in the sea 
 is derived from a number of sources, and brought to it in a variety 
 of ways. The principal of these are as follows : 
 
 Land-derived or Terrigenous Material 
 
 Clastic material derived from the land is called terrigenous 
 (earth-born) ; and this may be the product of weathering or of 
 mechanical erosion as already outlined in an earlier chapter (p. 424). 
 This material is supplied to the sea: (a) by rivers, glaciers, 
 icebergs, etc. which bring it from the land, (b) by the wind, often 
 from great distances, (c) by erosion of the seashore by the waves, 
 and (d) by the scouring action of the oceanic currents. 
 
 1 For further details, and for description of currents in the other oceans, see the 
 author's Principles of Stratigraphy, pp. 231-244. 
 
 2 For a discussion of these see Johnson, D. W., Shore Processes, etc., chapter iii. 
 
528 Deposition of Clastic Material in the Sea 
 
 Clastic Material Derived from Organic or Chemical Deposits in the Sea 
 
 Under this heading belongs the clastic material which is torn from 
 structures built in the sea from material formerly in solution; 
 namely, coral reefs, shell deposits, and the like, as well as from 
 chemical deposits in the sea. Of these the coral reefs are by far 
 the most important sources of clastic material, this being of course 
 wholly composed of carbonate of lime with some magnesia. The 
 destructive work is performed by the waves and by animals which 
 feed upon the reef-building organisms and crush their calcareous 
 structures (see ante, p. 437). 
 
 Clastic Material of Subcrustal Origin 
 
 Explosive eruptions of submarine volcanoes furnish clastic 
 material to the waves and currents, this being of ten of considerable 
 amount. Lava streams poured out upon the sea bottom will, if 
 they come within the reach of the waves, be subject to their attack 
 and so furnish clastic material for other sediments. 
 
 Material of Meteoric Origin 
 
 Meteoric dust and stones from spaces outside of our solar system 
 frequently reach the earth, and probably fall into the sea in no in- 
 considerable quantities. These are incorporated in all kinds of 
 marine sediments. 
 
 TRANSPORTATION AND SORTING OF CLASTIC MATERIAL 
 IN THE SEA 
 
 The material furnished to the waves and currents of the sea by 
 the various agencies above enumerated may be transported by them 
 over wide areas before it finally comes to rest. The work of the 
 waves consists mainly in stirring up the material so as to place it 
 in the best position, that of suspension, for the currents to trans- 
 port it. In general, such transportation is most effective in the 
 shore-zone of the littoral district and in the shallow part of the sub- 
 merged zone, but it is also effective in the deeper part of that zone. 
 On the shore the chief currents are the long-shore current, which 
 carries the material more or less parallel to the coast, and the under- 
 tow, which is the return current on the bottom, inaugurated upon 
 the breaking of a wave against the shore. This current tends to 
 
Transportation and Sorting 529 
 
 carry the material farther out to sea. There are, however, also 
 other bottom currents in the deeper parts of the submerged zone 
 which may do no inconsiderable amount of transportation. Chief 
 among these are probably the tidal currents, though others, set in 
 motion by the normal circulation of the ocean (the great oceanic 
 or planetary currents), are also active. 
 
 Both waves and currents perform a certain amount of sorting 
 of the material supplied ; the finer-grained the material the longer 
 will it be kept in suspension by the waves and the farther away 
 will it be carried by the currents. Sorting of clastic material is 
 best accomplished when there is vigorous wave action accompanied 
 by strong undertow, and when the volume of sediment supplied is 
 moderate. On the other hand, if large quantities of sediment are 
 furnished to the waves, only a moderate amount of sorting is ef- 
 fected. Where strong currents support material in suspension for a 
 long period of time, picking it up again repeatedly after dropping it, 
 a considerable amount of sorting may be produced. Such sorting 
 will be according to both size of grain and character of material, 
 and as a result pure quartz sands will be found in some localities, 
 sands largely composed of -heavy minerals, such as garnet and 
 magnetite, in others, and muds in still other localities. By mutual 
 attrition of the grains, a considerable amount of rounding is pro- 
 duced, but it must be recognized that such rounding is seldom or 
 never as perfect, nor the sorting according to size of grain as com- 
 plete, as is the case with wind-transported material. As a result, 
 the sands of the sea-beach generally consist of commingled coarser 
 and finer grains, most of them subangular, this angularity, and 
 the presence of water held by capillary attraction between the 
 grains, serving to bind them into a compact mass. This, in some 
 sections, results in the production of so firm a floor at low tide that 
 it forms a favorite region for the trials of speed of high-power 
 racing machines. (See Fig. 357, p. 431.) 
 
 Fine sediments may be carried out to sea for hundreds of miles, 
 and this accounts for the fact that the entire surface of the conti- 
 nental shelf is covered with sand, while the mud-line begins, as a rule, 
 at the edge of the shelf and continues over the bathyal district. 
 Terrigenous muds are probably seldom carried to the deeper sea, 
 where volcanic or meteoric matter forms the chief clastic sediment, 
 together with the inorganic matter derived from the structures of 
 organisms. 
 
530 Deposition of Clastic Material in the Sea 
 
 In such regions, however, rafted material may form important deposits. 
 We have already referred to the ice-rafted material which forms some subma- 
 rine banks and which has been brought there by floating icebergs. Materials 
 attached to or held by the roots of floating trees and other vegetation from the 
 land may also be distributed widely. Thus leaves of land plants are frequently 
 dredged in deep water, and tree trunks from the tropical rivers of America have 
 been carried by the Gulf Stream to the Arctic regions where they have been cast 
 ashore on far-distant northern lands and islands. 
 
 TYPES OF CLASTIC DEPOSITS IN THE SEA 
 
 Deposits in the Shore Zone 
 
 The Beach. The sea beaches of the modern oceans consist 
 very largely of sand. Even where the sea is faced by a rocky cliff 
 
 FIG. 446. Rock fragment beach at the foot of a rocky cliff, a portion of 
 which remains in the foreground as a marine "stack" or chimney, coast of 
 France. The beach here forms a bench and the coast topography is young. 
 (From Johnson's Shore Processes ; John Wiley and Sons.) 
 
 which is actively undergoing erosion by the waves at the base and by 
 weathering at the top, coarse materials such as large blocks, boul- 
 ders, and pebbles are restricted to a relatively narrow zone, beyond 
 which the material of the sea-bottom is sand (see Fig. 356 b, p. 430). 
 Where cliffs are undermined by the waves, huge blocks often cover 
 
Types of Clastic Deposits in the Sea 531 
 
 the beach at the foot of the cliff, but these will undergo gradual 
 destruction in time (Fig. 446). Sometimes, however, the condi- 
 tions are favorable for their preservation, the spaces between them 
 being filled by finer material and the whole becoming bound to- 
 gether into rock. Such rock formed during the Jurassic period of 
 the earth's history, and carrying blocks of stone twenty feet in 
 length, is found exposed on the eastern coast of Scotland, where 
 the blocks had accumu- 
 lated much as they ac- 
 cumulate in parts of 
 that region to-day. 
 Mingled with the ma- 
 terial of the coarse frag- 
 ments of rock are heads 
 of corals and other or- 
 ganic remains, the larger 
 ones generally water- 
 worn. 
 
 Boulders are also 
 characteristic of the 
 coast where, old glacial 
 moraines, either termi- 
 nal or ground moraines, 
 are exposed to the at- 
 tack of the waves (Fig. 
 447). At "Woods Hole 
 and along the shores of 
 the Elizabeth Islands 
 group on the south- 
 eastern coast of Massa- 
 chusetts, many good 
 
 sections of the terminal moraines are exposed, and huge boulders 
 are strewn over 'the beach for some distance out from the shore. 
 At several places in Boston Bay drumlins have been cut into by 
 the sea and great accumulations of boulders form the shore zone 
 (Fig. 447). These boulders are often overgrown with seaweeds, 
 which sometimes act as a buoying agent, making possible their 
 movement by the waves even on the gently sloping coast. This 
 buoyant power of seaweeds together with that of shore ice in 
 winter has enabled the waves to arrange the boulders in close juxta- 
 
 FIG. 447. Section of Grover's Cliff, near 
 Winthrop, Mass., showing erosion of drumlin 
 by waves and the formation of a boulder beach 
 from the larger stones of the drumlin material. 
 These boulders are partly overgrown with 
 seaweeds which give them a certain buoyancy 
 at high tide. This and the shore-ice serves to 
 arrange these rocks into a solidly packed 
 pavement of boulders. (Photo by the author.) 
 
532 Deposition of Clastic Material in the Sea 
 
 position so as to produce at low tide the appearance of a boulder 
 pavement. On the gently sloping portion of the North Sea coast 
 
 of Scotland, such 
 closely arranged 
 boulders are not only 
 thickly overgrown 
 with seaweeds, but 
 also serve the limpets 
 and other shell-bear- 
 ing mollusks, and 
 even the soft-bodied 
 sea anemones, as a 
 place of attachment. 
 Where the coast 
 slopes more steeply, 
 the boulders are 
 moved by the strong 
 waves, and here all 
 organisms are ground up and the boulders appear barren. If 
 strong waves break upon the shore, they often effect the forma- 
 tion of terraces with 
 gently sloping upper sur- 
 faces and steeper sea- 
 ward faces. These ter- 
 races are more common 
 where the rock fragments 
 are relatively small, as 
 of the size of a man's 
 fist, and in that case the 
 fragments are mostly 
 worn into round, smooth 
 cobble-stones. Such ter- 
 races of cobble-stones are 
 shown along some por- 
 
 FIG. 448 a. Cobble-stone terrace on the coast of 
 Marblehead, Mass. Note the flat summit and the 
 steep frontal slope of the terrace. (Photo by the 
 author.) 
 
 tions of the Massachu- 
 setts coast as at Marble- 
 head (Fig. 4480), and 
 also along the borders of 
 some of our Great Lakes. 
 
 FIG. 448 b. Barrier beach, or cobble ter- 
 race, North Sea Coast of Scotland. (Photo 
 by M. I. Goldman.) 
 
 They are also characteristic of many 
 
 parts of the Scottish and other coasts (Fig. 4486). 
 
Types of Clastic Deposits in the Sea 533 
 
 Where cobble and pebble beaches have been raised by recent 
 earth movements, as in many places along the Scottish and other 
 coasts, these beach deposits are often full of the shells of shore mol- 
 lusks, such as the limpets, as well as the hard parts of other marine 
 organisms. When consolidated they will form a fossiliferous con- 
 glomerate. Examples of 
 such rock are known 
 from many older geo- 
 logical formations. 
 
 Sections of boulder 
 beaches and terraces are 
 rarely seen, but it is ap- 
 parent that their struc- 
 ture must be an irregular 
 one, with perhaps the 
 development of coarse 
 cross-bedding. Where 
 rivers descend from high 
 mountains near the coast, 
 great beds of " shingle" 
 or pebble beaches are 
 formed along the coast, 
 as is shown along the 
 base of the- Maritime 
 Alps between Toulon and 
 Genoa, and especially 
 near Nice, on the coast 
 of the Mediterranean. 
 A great part of the 
 pebbles is, however, 
 swept into the Mediterranean, which in some sections, as at Nice, 
 drops off rapidly to a depth of 2000 feet at a few hundred yards 
 from the beach. North of Messina on Sicily, a torrential river 
 carries annually vast masses of granite pebbles into the sea. 
 
 Sand beaches, on the other hand, are commonly broad and flat, 
 sloping gently toward the water and under it. Here and there 
 the surface will be characterized by ripple marks and the peculiar 
 lines which are left on flat shores by each little wavelet, the extent 
 of which they outline. Shells of mollusks and often those of forami- 
 niferans, and the remains of crustaceans, fish, and other animals 
 
 FIG. 449. Low tide near the head of the 
 Bay of Fundy. The rocks and boulders are 
 overgrown with seaweeds (Fucus, etc.), and 
 these are exposed between tides. The stacks 
 in the view are the result of marine erosion 
 at high tide. (Photo by M. O'Connell.) 
 
534 Deposition of Clastic Material in the Sea 
 
 are buried in the sand, sometimes in large numbers, sometimes as 
 scattered individuals. Frequently the currents and waves collect 
 the shells in protected embayments, and there they accumulate in 
 large quantities and may become embedded as shell layers in the 
 sands. On the coast of Florida such shell layers of lenticular form 
 are inclosed in the beach sand, and because of their consolidation 
 into a shell rock, or coquina, they form limestone shelves at the 
 water's edge. A number of these have been described by J . F. Kemp. 
 One of the most remarkable beaches in existence, is found in the 
 Bay of Fundy, where the rise of the tide is. unusual, being commonly 
 from 30 to 40 feet or over, as noted previously (Fig. 449) . Where 
 
 l^^^^^^^^H^B^^^^^^^^^^^^BMBBBMBB^^BBBBBBBH 
 
 FIG. 450- The tidal-bore at^t. John, N. B. The water rushes up the river 
 valley, presenting a steep front and producing a roaring sound as of violent 
 rapids. 
 
 the beach is gently sloping, as in the Basin of Minas, the water 
 recedes for miles, laying bare huge flats of red mud and sand, 
 which were derived from the erosion of the older red beds of the 
 shore. Across these flats,, the streams and retreating tidal waters 
 cut channels of varying complexity, with rill marks upon their 
 borders, while raindrop impressions are found wherever a shower 
 strikes the drying mud. Worms crawl over the surface or move 
 just beneath it, forming characteristic tracks ; shells of dead 
 mollusks are left stranded upon the red mud surface, and the re- 
 mains of many other organisms are strewn about. When the tide 
 returns, it sweeps across the flat surface with surprising speed and 
 enters the narrow river channels in the form of a steep frontal wave 
 called the " bore," which rushes up the river with the noise and the 
 rapidity of a breaking wave upon the sea-shore (Fig. 450). The 
 
Types of Clastic Deposits in the Sea 535 
 
 mud of the bottom is stirred up, a.nd the water is thick with sedi- 
 ment which later, as the agitation of the water subsides, settles 
 upon the bottom, covering any structure not destroyed by its mad 
 shoreward rush. 
 
 Here we have a good example of the reworking of red sands and muds de- 
 rived by erosion of older red beds, which themselves were a continental deposit- 
 These reworked beds will possess the characters of a seashore deposit, with its 
 peculiar features impressed upon it. Among the older rock series of the earth's 
 crust, there are several such red stratified formations with marine features and 
 with characters which appear in all respects to conform to the sediments now 
 
 FIG. 451. Beach cusps on shore lines of Carmel Bay, Monterey County, 
 California. Photo by W. S. Cooper. (Courtesy of D. W. Johnson.) 
 
 forming in parts of the Bay of Fundy. These can confidently be regarded as 
 ancient products of activities similar to those visible in Nova Scotia to-day. One 
 of the best known of these is the so-called Medina sandstone, which was formerly 
 extensively quarried in western New York, and has been used widely for flagstone 
 sidewalks. On this rock, not only solid ripple marks are seen (often recogniz- 
 able in the older sidewalks of Buffalo, Rochester, and other cities where this rock 
 is used for flagstones), but beautifully preserved wave marks, shell-protected 
 sand ridges, and other characteristic beach features are found. Here, too, 
 are preserved, in some cases, the beach cusps similar to those formed on modern 
 beaches and next to be described. The beach here fossilized was a very ancient 
 one, belonging to the interior or epeiric seas which covered North America in 
 Silurian time. 
 
 Beach Cusps and Ripple Marks. When the waves strike the 
 shore directly, the front of the beach will commonly become exca- 
 vated into a series of scallops or concavities with sharp ridges or 
 
536 Deposition of Clastic Material in the Sea 
 
 cusps between them (Fig. 451). These beach cusps project at right 
 angles from the front of the beach and form a series of sand or 
 pebble ridges of triangular outline with the base of the triangle on 
 the shoreward, and the apex on the water side. They vary 
 greatly in their distance apart, according to the size of the waves 
 which produce them, but in any given series they have a relatively 
 uniform spacing. " Where the waves are about an inch in height, 
 the cusps are from 3 to 9 inches apart; where the waves are 
 from one and a half to two and a half feet high, they are 30 to 60 
 
 FIG. 452. Remnant of the concavity between two beach-cusps, preserved 
 because of the consolidation of the old beach sand into a solid rock. This rock 
 forms a part of the Medina series (Whirlpool sandstone) of the Silurian, and the 
 position of the cusps indicates that the old Medina shore-line was approxi- 
 mately parallel to the railroad track of to-day. Niagara gorge. (Photo by 
 G. K. Gilbert; from U. S. G. S.) 
 
 feet apart, while large storm waves build cusps 100 feet or more 
 apart " (Johnson) . What appear to be fossil beach cusps have been 
 found in ancient beach-formed sandstones, in one of which (Medina 
 sandstone of Silurian age, already referred to), exposed in the gorge 
 of Niagara below the whirlpool, they are wonderfully well preserved 
 (Fig. 452). Ripple marks are not common on open beaches, but in 
 protected areas they are often well developed (Fig. 453). They 
 are of the asymmetrical or current ripple type. (See further, p. 550.) 
 It is difficult to get at the internal structure of modern beaches, 
 but in a few cases, where sections are exposed by wave cutting, an 
 irregular but strongly-marked stratification is seen. The layers 
 
Types of Clastic Deposits in the Sea 537 
 
 are essentially horizontal and in some cases very perfect, while 
 in others a certain amount of pinching out and overlapping of the 
 ends of layers is seen. Cross-bedding on a large scale is rarely in- 
 dicated, though that due 
 to ripple marks, on a 
 scale of inches, is not 
 uncommon. 
 
 The Bar. -- On a 
 gently sloping coast the 
 large waves commonly 
 break at a distance from 
 shore, often several miles 
 from it, where the depth 
 is such that the waters 
 in their rotary motion 
 just strike the bottom. 
 
 The breaking wave 
 (Fig. 441) digs up the 
 sand of the sea-bottom 
 and hurls it forward, de- 
 
 . 453. Beach near St. Monans, east 
 coast of Scotland, showing current ripple 
 marks in protected embayment of shore with 
 numerous worn castings in the troughs. Dr. 
 Benjamin Peach of the Scottish Geological 
 Survey on the right. (M. I. Goldman 
 photo.) 
 
 positing it in front of the 
 
 line of 'breakers, and by 
 
 repeated work of this 
 
 kind builds up a sand 
 
 ridge or bar parallel to 
 
 the line of the breaking wave and therefore in general parallel to 
 
 the shore. When this bar has been built so high that at low tide 
 
 it is exposed, sand dunes are likely to be formed upon it, and it 
 
 becomes a barrier beach with a lagoon between it and the land as 
 
 already outlined (page 330). 
 
 When the emerged bar or barrier beach is attached at one end 
 to a projecting headland of sand or gravel, a large amount of 
 material is furnished by the erosion of this headland. If this 
 material is carried by the long-shore current parallel to and along 
 the shore of the barrier beach, it tends to fill up the tidal inlets 
 or breaks in the continuity of this beach. The result is that 
 barrier beaches are always most continuous in those portions 
 which spring from the sand-supplying headlands, becoming less 
 so away from this. On the south shore of Long Island the great 
 bar known as Fire Island Beach extends unbroken for more than 
 
538 Deposition of Clastic Material in the Sea 
 
 40 miles soumwestward, while the three main barrier beaches 
 farther west, which are not joined to headlands, are each less than 
 10 miles in length. Rockaway Beach, still farther west, also 
 springs from a sand-supplying headland, and is rapidly growing 
 in length, as shown in the maps (Fig. 457 b). When the beaches 
 are shorter, that is, when the tidal inlets across the bar are more 
 numerous, the normal condition for the growth of salt-water 
 vegetation is best developed, and the lagoon is converted into a 
 marsh, as previously described (p. 331). The protected lagoon 
 behind the continuous bar, however, favors such growth to a 
 lesser degree, chiefly because there is less rapid filling of the lagoon 
 to the required depth for the growth of such plants, and also 
 because this portion is often freshened by the inpouring of stream- 
 waters. Hence the protected lagoon will remain open for a longer 
 time. Conditions of this type also exist on the New Jersey and 
 other coasts characterized by barrier beaches. 
 
 From the way in which the sands are piled up to form the bar, we may infer 
 that its internal structure is very irregular, and that if a section were cut across 
 it, an irregular cross-bedded structure would appear. Such a bar may, under 
 favorable conditions, harden into a rock by cementation of its sand grains by 
 lime or other substance, as happens on the coast of Brazil, where wave-formed 
 bars form lines of solid " stone reefs " as they are called. Again, near the shore 
 such a bar may be preserved by changes in sea-level, which leave it submerged 
 and allow the formation of other sediments over it. When such a bar is changed 
 to rock, this rock will exhibit in its structure the characteristic oblique bedding 
 which is probably not very different in general appearance from the cross-bedding 
 of wind-laid sediments. 
 
 As has been briefly outlined in a previous chapter (p. 332), the 
 off-shore bar is not a stationary structure, but is subject to con- 
 tinued wave attack, for now that the ocean bottom has been 
 sufficiently deepened in front of the bar by the excavation of the 
 sand from which that bar was built (see Fig. 273, p. 330), the 
 waves will break close to the shore of the bar and will therefore 
 be able to erode it. Such erosion is more active of course where 
 little material is supplied by the long-shore currents, and there- 
 fore in those portions of the bar which are farthest away from the 
 sand-supplying headlands. If the lagoon has been transformed 
 into a peat marsh (see p. 332), the shore dunes will advance over 
 it, as the bar is being eroded, the sand compressing the peat by 
 its weight. The barrier beach thus migrates shoreward in the 
 
Types of Clastic Deposits in the Sea 539 
 
 course of time. As the erosion on the seaward side continues, the 
 peat-beds of the lagoon will be reached, and become exposed 
 upon the shore at low tide. Such peat exposures are seen in a 
 number of places upon the Atlantic coast, those most readily 
 accessible from New York being just east of Fort Hamilton on 
 the Long Island coast. Eventually, with continued erosion the 
 
 FIG. 454. - Diagram illustrating the development, migration, and complete 
 destruction of an off-shore bar and the lagoon deposits behind it. (After 
 Davis.) i. The bar just beginning to emerge, the lagoon open, and the shore 
 of the mainland marked by a small erosion scarp or "nip"; 2. Completion 
 of the barrier-beach and formation of salt marsh, with lagoon in the center; 
 3. Advance of the beach and dunes on the marsh with progressive erosion 
 of the bar on the seaward side. The lagoon is now converted into a salt marsh 
 intersected by a tidal stream N; 4. Complete destruction by the waves and 
 currents of the original bar advance of the beach and dunes over the 
 marsh which has now become exposed upon the shore; 5. Final stage, show- 
 ing complete destruction of the lagoon and its deposits, and the advance of 
 the deeper water to the original shore, which is now being cliffed by the 
 waves. The dotted line shows the original slope of the sea-bottom. 
 
 entire lagoonal deposit is cut away, and the sea is able to attack 
 the mainland with the full force of its waves, which it was unable 
 to do before the building of the bar because of the shallowness 
 of the water near shore. The various stages in the building and 
 migration of the bar and barrier beach are shown in the preceding 
 diagram (Fig. 454). 
 
 Bars of this type are found to form a more or less continuous 
 series along the Atlantic coast of North America from Long Island 
 southward (Fig. 455, p. 540). They also occur on the Massachusetts 
 coast, and are characteristic of many other coasts as well. On 
 their seaward side they exhibit all the normal features of the 
 beach. Because of the exposed character of the beaches, the winter 
 
540 Deposition of Clastic Material in the Sea 
 
 storms often effect a considerable amount of cutting, sometimes 
 doing much damage to buildings and other structures (Fig. 456). 
 
 The bars of the Baltic, and the 
 movements of the sand dunes 
 upon them have been de- 
 scribed in a previous chapter 
 
 (P- 445 > F ig s - 364 a, fy- 
 
 The Sand-spit. The usual 
 on-shore movement of the 
 waves is seldom direct, but 
 nearly always more or less 
 oblique. As a result, the sands 
 (and pebbles), moved up the 
 beach by the swash or the 
 breaking wave itself, proceed 
 in an oblique direction, but the 
 undertow carries this material 
 back at right angles to the 
 shore line. In consequence, 
 the material moves in a zigzag 
 fashion along the beach, here 
 in one direction, there in 
 another, according to the con- 
 formation of the shore. (See 
 p. 523.) If the shore recedes 
 abruptly into the land as at an 
 inlet, the moving sands reach 
 water too deep for their further 
 progress, and they come to a 
 temporary rest. By constant 
 piling up of such sands, etc., a 
 tongue begins to project into 
 the water, and this continues 
 to grow in length until a sand- 
 spit of considerable extent is 
 formed. Where strong tidal 
 or wind-driven shore currents 
 exist, they greatly aid in the 
 
 FIG. 455. Map of the coast of 
 North Carolina, showing a coast with 
 drowned river valleys, modified by the 
 subsequent formation of off-shore bars 
 and barrier beaches, built at a great 
 distance from shore because of the 
 gently sloping character of the sub- 
 coastal plain. CK, Corrituck bar and 
 lagoon; N, New inlet; H, Cape Hat- 
 teras; 0, Ocracoke Inlet; L, Cape 
 Lookout. The sharp angle at which 
 the bars meet to form Cape Hatteras 
 has been explained by some as due to 
 deposition of debris derived from both 
 north and south, in a triangle of quiet 
 water between two adjacent circling 
 currents, while others have suggested 
 the influence of an initial projecting 
 shore line or shoal. 
 
 transportation of the material along the shore. Sand-spits thus 
 represent a prolongation of a beach into the deeper water where 
 
Types of Clastic Deposits in the Sea 541 
 
 FIG. 456. Ruins of summer residence at North Long Branch, New Jersey. 
 (Photo by D. W. Johnson.) 
 
 FIG. 457 a. A sand-spit, Traverse Bay, Lake Michigan. (Shaler.) 
 
542 Deposition of Clastic Material in the Sea 
 
 Charts from George R. Putnam 
 
 FIG 457 b. Westward growth of the compound sand-spit of Rockaway 
 Beach between the years 1835 an d 1908 (National Geographic Magazine). In 
 1889 the curved spit near meridian 73 54' was the westernmost, although at 
 that time it was broader and extended across the meridian. It has suffered 
 erosion during the formation of the later spits. The outermost spit has been 
 built since 1905. Between 1908 and 1912 the change has been very slight. 
 The influences of the tidal current through Rockaway inlet in producing the 
 curvature of the spits is well shown. 
 
 the shore has receded. According to circumstances and the direc- 
 tion and interference of currents, sand-spits may be straight or 
 curved, or pass from one form to the other (Figs. 457 a, b). Sand- 
 spits of great extent are shown at the end of the northward pro- 
 jecting forearm of Cape Cod (see Fig. 714), where they have 
 
Types of Clastic Deposits in the Sea 543 
 
 combined to form the 
 Provincetown headland, 
 now crowned with sand 
 dunes, and in the simi- 
 lar northward project- 
 ing strip of land on 
 the New Jersey coast 
 known as Sandy Hook. 
 Smaller sand-spits are 
 built within the waters 
 partly enclosed by both 
 of these larger dune- 
 covered spits. Sand- 
 spits are found on many 
 other parts of our coast, 
 and in our Great Lakes 
 as well (Fig. 457 a). FlG . 45? Ct _ A Bay-bar, with narrow inlet 
 They are equally com- which leaves the water of the bay salty, 
 mon on foreign coasts. South Shore of Mart kas Vineyard. (U. S. G. S.) 
 When the sand-spit is built completely across the mouth of a bay, 
 so that only a narrow inlet, or none at all, remains, it is called a 
 
 bay-bar. If the bay re- 
 mains connected with the 
 sea, its waters will differ 
 little in salinity from 
 that of the ocean; but 
 if the bar is complete 
 (Fig. 457 d)j the waters 
 of the cut-off lagoon will 
 become fresh if the cli- 
 mate is pluvial, or saltier 
 than the sea if the cut- 
 off is in an arid region. 
 The increase in salinity 
 may also take place if 
 the inlet is so shal- 
 low and narrow that 
 FIG. 457 d. A Bay-bar completely cut- there is no free inter- 
 tmg off a bay and converting it into a closed i, r 
 shore-pond. South Shore of Marthas Vine- ChangC f Waters > a con - 
 yard. (U. S. G. S.) dition characteristic of 
 
544 Deposition of Clastic Material in the Sea 
 
 tideless waters like the Caspian, where the Kara Bugas Gulf, 
 described on p. 239, forms a typical example. In many such bar- 
 enclosed bays, salt deposits are forming, while in others, situated 
 
 in pluvial climates, fresh- 
 water vegetation and 
 fresh-water animals re- 
 place the marine types 
 which formerly inhabited 
 the bay. In this way may 
 be readily explained the 
 association of fresh-water 
 with marine deposits, 
 which is sometimes found 
 in the older rock series. 
 The bay-bar may be 
 formed by a single sand- 
 spit growing in one di- 
 rection (Fig. 457 d) or by 
 two spits growing from 
 opposite sides. The con- 
 ditions necessary for the 
 formation of such bay- 
 bars are: sufficient sup- 
 ply of detritus or suffi- 
 
 FIG. 457 e. Nahant, a rocky headland 
 tied to the shore by a sand beach or tombolo. 
 U. S. G. S. 
 
 cient strength of the long-shore current, or both, to overcome the 
 effects of the currents flowing into and out of the bay, so that the 
 equilibrium is not established until the bay is nearly or entirely closed. 
 
 Sand-spits may grow between islands, eventually tying them 
 together, or between an island and the mainland, tying the former 
 to the latter. Such a connecting bar has been called a tombolo. 
 It is well illustrated by the bar which unites Great and Little 
 Nahant, and by that which ties this group to the mainland at 
 Lynn, Mass. (Fig. 457 e). It is also shown in the beach which 
 unites Marblehead Neck and the mainland (Fig. 716) and in the 
 Nantasket beaches (Figs. 727-730). These examples will be more 
 fully described in a later chapter. A complex shore-form produced 
 by sand-spits is shown in Fig. 458. 
 
 Cape Canaveral, on the eastern coast of Florida, illustrates a 
 striking series of modifications produced in the coast line by beach, 
 bar, and spit building by waves and currents (Fig. 459). The 
 
Types of Clastic Deposits in the Sea 545 
 
 original coastline, shown on the left of the figure, was nearly 
 straight, being the shore of a coastal plain. In front of this was 
 built a cuspate foreland, i.e., a triangular projecting land-mass of 
 low relief, with the apex of the triangle pointing seaward. A 
 body of water, known as Indian River, lies between the mainland 
 and the cuspate foreland, which terminates in what is now called 
 the False Cape. An irregular winding stream also bisects this 
 foreland, extending nearly to the apex. This cuspate foreland 
 consists of a series of beaches or bars, built one in front of the 
 
 FIG. 458. Tidal lagoon formed by sand-spit at the mouth of San Luis 
 Obispo Creek, California. Oxbow in the foreground. (G. W. Stose, photo ; 
 from U. S. G. S.) 
 
 other because of an excessive supply of sand, both from the north 
 and from the south, the relative uniformity in the amount supplied 
 from both directions having determined the symmetry of outline 
 of the old foreland of the False Cape. Such a broad plain, formed 
 of a succession of beach-ridges with shallow depressions or swales 
 between, is also called a beach-plain. The swales are sometimes 
 marshy, or may be occupied by narrow lagoons; for the most 
 part they are dry. The ridges are commonly dune-covered. 
 
 Such beach-ridges are formed by direct building of new bars 
 in front of the older ones by successive storm waves ; by addition 
 
546 Deposition of Clastic Material in the Sea 
 
 FIG. 459. Map of Cape Canaveral, Florida. (U. S. 
 Coast Survey chart.) Showing formation of bars, 
 beach plains,- spits, etc. For location see map, Fig. 
 433> P- 512. (Copied from de Martonne.) 
 
 of material along 
 the whole front, 
 because of diver- 
 gence of shore- 
 currents, which 
 then assume a 
 greater cross- 
 section and a di- 
 minished velocity, 
 and in conse- 
 quence permit de- 
 position of trans- 
 ported debris; by 
 the extension of 
 su c ce s sively 
 formed embank- 
 ments ; or by the 
 interaction of all 
 three processes. 
 Again they may be 
 produced by nor- 
 mal bar-building, 
 one in front of the 
 other, because an 
 excess of debris is 
 supplied by the 
 shore-currents, 
 which become de- 
 flected as the re- 
 sult of this build- 
 ing of successive 
 bars. 
 
 A pronounced 
 migration of the 
 site of bar-build- 
 ing has produced 
 the later addi- 
 tions, which form 
 the present Cape 
 Canaveral. This 
 
Types of Clastic Deposits in the Sea 547 
 
 is separated from the older cape by a broad lagoon called the 
 Bananc River, while a similar lagoon lies between the outer bar 
 and the old cape on the north. In front of the present cape is 
 built a new series of sand-spits and bars which are, for the most 
 part, still submerged. 
 
 Estuarine Deposits. Estuaries are indentations in the sea- 
 coast where large rivers enter the sea. They may indeed be re- 
 garded as the broadened mouths of rivers, and are often formed by 
 the sinking of the land which is traversed by the lower river valley, 
 
 52' 
 
 CHANNEL 
 
 FIG. 460 a. Map of the Estuary of the Severn River, England. 
 
 so that the sea has access to this valley. The lower Hudson River 
 is an example of such a submerged valley cut in a rocky land sur- 
 face, and because of that it has nearly parallel sides. Most estuaries, 
 however, have their sides diverging seawards so that their form is 
 more or less funnel-shaped. The estuaries of the River Severn 
 on the west coast of England (Fig. 460 a), and of the La Plata 
 (Figs. 460 b, c] in South America are of this type. The latter is 125 
 miles long and receives the waters of the Parana and the Uruguay 
 rivers, and the currents of these rivers come into periodic conflict 
 with those of the entering tides from the Atlantic. Where the two 
 
548 Deposition of Clastic Material in the Sea 
 
 currents neutralize each other, sedimentation is most pronounced, 
 the maximum being between 10 and 20 miles above the mouth of 
 the estuary, where slack water conditions may continue for hours. 
 From the sediment dropped here, chiefly the mud brought by the 
 rivers, submerged banks are built up which may eventually rise 
 into islands. At Buenos Aires, constant dredging is necessary, 
 to keep the channels open for navigation. In the narrower 
 channels between the islands, the tidal current, aided by the 
 river currents, will effect scouring and little or no sedimentation 
 
 Bdcwofr. 
 
 FIG. 460 b. Map of the La Plata Estuary. 
 
 takes place. The original length of the La Plata estuary was 325 
 miles, but about two-thirds of this has been filled up by sediments. 
 In these sediments fine muds prevail, and with -them are buried 
 organic remains brought by the rivers. 
 
 Marine organisms of floating or swimming habit may also enter 
 with the tide, and being killed by the freshening of the water, 
 their remains become embedded in the sediments. Thus such 
 sediments contain a mixture of marine and fresh-water organisms, 
 though on the whole the organic remains are few in kind as well as 
 number. 
 
 The floor of the Hudson, too, has been built up in places by mud 
 deposits to a considerable extent. In these muds have been found 
 
Types of Clastic Deposits in the Sea 549 
 
 the remains of marine animals of a dwarfed character, this dwarfing 
 being due, according to Shimer, to the freshening of the water. 
 
 The waters of the estuary of the Severn, southwestern England, at high tide 
 are thick and opaque with tawny-colored sediments, while at ebbing tide a 
 broad expanse of shining mud-flat is revealed along the coast. Owing to the 
 heavy load of silt in the water, the boundary line between the water and the 
 mud-flat is not easily distinguishable, especially from a distance. Part of the 
 mud is brought down by the river, part of it is derived by wave and current 
 erosion along the banks and is carried into the estuary by the tide. With the 
 muds and fine sands many floating marine organisms, especially Foraminifera, 
 
 FIG. 460 c. View near the mouth of the Estuary Rio de La Plata (at 
 Montevideo). A mud bank is shown in the foreground. 
 
 and the fragments of larger organisms are brought into the estuary and buried 
 in the muds, while farther up the remains of fresh-water sponges and of diatoms 
 are found. Much vegetable matter also occurs in the older muds of the estuary. 
 A special case of river-borne muds is seen in the Bay of Danzig, on the Baltic 
 Sea, where the river Vistula brings black carbonaceous muds from the Russian 
 plains (the tschernosem, see p. 459). This bay is of low salinity and so corre- 
 sponds in character to the water of an estuary. Extensive deposits of deep black 
 mud, inclosing dwarfed marine organisms, are formed upon the floor of this bay, 
 and they illustrate one way in which black shales with dwarfed marine organisms 
 may originate. 
 
 Deposits in the Of -Shore Shallow Waters 
 
 The deposits found between the shore and the edge of the con- 
 tinental shelf, estimated by Murray as covering at present some 
 10,000,000 square miles, consist mainly of the finer sands, and more 
 rarely of the muds which are washed out from the shores, together 
 with organic and chemical deposits of the type outlined in previous 
 chapters. As we have seen, however, on coasts which fall off 
 
550 Deposition of Clastic Material in the Sea 
 
 rapidly, and which are bordered by high mountain areas, coarse 
 stream-transported material may be deposited beyond the shore 
 
 zone. 
 
 Since this shallow portion of the sea is the great theater of organic activity,- 
 the regions where most of the shell-bearing and other hard-structure secreting 
 organisms live, it is evident that all deposits formed here are likely to inclose 
 the remains of these animals and will therefore be highly fossiliferous. Again, 
 large portions of the floor may be covered by sands consisting of the fragments 
 of shells and other calcareous organic structures, as is the case on the floor of 
 large parts of the shallow sea which surrounds the British Isles. In warmer 
 waters coral sands and muds worn from the reefs there building will form an 
 important, if not the whole of the bottom deposit. 
 
 In general, deposits formed in these shallower waters are well 
 stratified, but cross-bedding and irregularity of structure are 
 absent. Around coral reefs, however, the slope of the layers is 
 often steep, being sometimes as high as 60 degrees, and inter- 
 fingering of the clastic and organic deposits produce an irregular 
 
 structure. (See 
 p. 304.) Ripple 
 marks are com- 
 mon in the shallow 
 parts and may 
 even be developed 
 at considerable 
 depths. These 
 
 FIG. 461. Diagram illustrating the forms of ripple- rippl es differ in 
 marks and of the reverse impressions of the same, character from 
 a, reverse of current ripple, original position inverted ; 
 6, form of same; c, form of oscillation ripple; d, re- 
 verse of same, original position inverted. Note con- 
 trasting form of two ripples in center (6, c). Also 
 note general resemblance of a and c, and b and d, ex- 
 cept for asymmetry of a and b. 
 
 those formed on 
 the shore (Fig. 
 
 4S3 P- 537) and 
 in terrestrial de- 
 
 posits (Fig. 371, 
 P- 453) > consisting of sharp, symmetrical ridges between regularly 
 concave troughs (Fig. 461 c). They represent, indeed, the form 
 of the wave on a small scale, and are due to the oscillatory move- 
 ment of the water caused by wave motion. Such oscillation 
 ripples, as they are called, are readily distinguished from the 
 current ripples by their form, but when a relief impression of 
 them is formed, as in the bottom of a layer of sediment deposited 
 over them and perserved by hardening, this relief is not so 
 
Types of Clastic Deposits in the Sea 551 
 
 readily distinguished from the normal current ripples, as the 
 difference in angle of slope of the two sides of the latter is not 
 always very great (Fig. 461 b, d}. The significance of this will ap- 
 pear in the discussion of the structure of ancient sediments. 
 
 Deposits in the Bathyal Zone 
 
 Beyond the edge of the continental shelf we meet the zone of 
 mud deposits, though where cpnditions are favorable this may 
 extend into shallower water as well. The principal clastic de- 
 posits of terrigenous origin in the bathyal zone are blue muds, red 
 jnuds, green muds, and green-sands. Volcanic sands and muds, 
 either derived from volcanoes on the land or from submarine vol- 
 canoes, also cover wide areas, and besides these there are frequently 
 deposits of coral sands and coral muds or elastics of oceanic origin. 
 
 The Blue Muds. These are more often slate-colored, and they are the 
 most widely distributed of the mud deposits, covering an area estimated at 
 14,500,000 square miles in the several oceans of the world, including the Arctic 
 Ocean. (See map, Fig. 198, p. 277.) In the Gulf of Naples the mud begins 
 at a depth of 15 meters, but its greatest distribution is below the 2oo-meter (or 
 loo-fathom) line, while the greatest depth from which it has been obtained is 
 5120 meters. Its upper part is commonly stained red or brown by iron oxide 
 or. hydrate, but the lower layers are bluish. In composition it varies greatly ; 
 sometimes as much as 97 per cent is clay, while at other times this may fall to 
 1 6 per cent. In like manner the content of carbonate of lime varies greatly, 
 ranging from almost nothing to 35 per cent. Quartz in fine grains or as rock 
 flour is a characteristic constituent, and other rock-forming minerals are also 
 present. Some parts of this clay are rich in shells and other hard structures of 
 marine organisms, especially Foraminifera, while in others these are compara- 
 tively rare. The fresh mud generally smells strongly of sulphureted hydrogen, 
 due apparently to the decomposition of organic matter. By combination with 
 the iron in the mud, iron sulphide is formed, which may separate out as crystals 
 or specks of the mineral pyrite. 
 
 The Red Muds. Opposite the mouths of tropical rivers such as the Ama- 
 zon and the Yang-tse-Kiang, the floor of the sea is covered by a red mud which 
 is derived from the lateritic soil of the tropical regions. (See p. 403.) Its red 
 color is due to the high proportion of iron oxide present, a constituent charac-^ 
 teristic of the laterite. As in the case of the blue mud, the composition of the 
 red mud varies, pure clay forming from 28 to 68 per cent of its mass, while 
 the content of carbonate of lime ranges from 6 to 60 per cent. Quartz is also 
 a characteristic constituent. The area covered by this mud in the present oceans 
 is about 100,000 square miles. As in the case of the blue mud, the red mud also 
 contains the remains of marine organisms, and with these may readily be mingled 
 organic bodies that were brought from the land by the rivers. 
 
55 2 Deposition of Clastic Material in the Sea 
 
 Green Mud and Green-sand. In the upper slope of the bathyal zone, from 
 the edge of the continental shelf to a depth of 1000 fathoms or over (180-2300 
 meters), is found a deposit of mud of a character similar to that of the two pre- 
 ceding types, except that it is colored green by the mineral glauconite, which 
 is a hydrous silicate of potassium and iron, often with some aluminum. This 
 mineral forms in the sea under the influence of the decaying organic matter 
 from the elements of the terrigenous muds brought there. Glauconite forms 
 abundantly in the shells of dead Foraminifera, which it fills completely, and on 
 solution of the shell it remains behind as grains of glauconite sand. This con- 
 stitutes the green-sand ; between this and the green muds the difference is mainly 
 one of fineness of grain. The green muds contain from 24 to 48 per cent of clay, 
 and their content of carbonate of lime may be as high as 56 per cent. 
 
 Green-sands sometimes constitute extensive formations in the older rock 
 series of the earth, especially those of the Cretaceous period. They are 
 probably not always formed in the same way nor always near the edge of 
 the continental shelf. Some of them contain an abundance of coarse clastic 
 quartz and feldspar grains and have the appearance of having been formed in 
 shallow water. 
 
 Volcanic Sands and Muds. These are especially abundant in the neigh- 
 borhood of volcanic islands and off those coasts which are characterized by 
 strong volcanic activities. They grade into other types of deposits and may be 
 mingled with calcareous sands and muds to such an extent that on hardening 
 they form impure limestone deposits. 
 
 Coral Sands and Muds. As these are derived by the erosion of coral reefs, 
 their distribution is chiefly in the neighborhood of such structures. They 
 merely represent the finer products of reef erosion carried into the deeper water, 
 where they mostly form beds of lime-mud, which has much the characteristics 
 of the other muds except for the difference in composition. Such muds and 
 fine sands are especially abundant on the floor of the tropical portions of the 
 Pacific Ocean. 
 
 Deposits of the Deep Sea 
 
 Prevailing Types. We have already seen that the deeper parts 
 of the sea, the abyssal regions, are chiefly characterized by the 
 accumulation of organic structures (foraminifers, pteropods, cocco- 
 liths, diatoms, radiolarians, etc.), derived from organisms which 
 spend their lives in a floating manner in the upper or pelagic dis- 
 trict of the sea. With these may be mingled the bones and teeth of 
 aquatic animals (whales, sharks, etc.) and the volcanic dust and 
 pumice fragments which are carried out over such portions of the 
 ocean and eventually reach the bottom. Clastic material is, how- 
 ever, not common in the deep sea, though some of the sands or 
 muds deposited in the bathyal zone may be carried to the deeper 
 waters. 
 
Summary of Structures of Marine Clastics 553 
 
 Red Clay. In some of the deeper parts of the oceans, however, below depths 
 of 2400 to 2600 fathoms, there is a peculiar deep-sea red clay which covers an 
 area aggregating 51,500,000 square miles. (See map, Fig. 198, p. 277.) In color 
 this material ranges from brick-red to chocolate tints, though some samples 
 have a bluish color. When fresh it is soft, plastic, and greasy, but it becomes 
 very hard on drying. It is chiefly derived from the decomposition of volcanic 
 dust which has slowly settled to these depths, along with minute quantities of 
 earthy matter left on solution of the foraminiferan shells, which never reach this 
 depth but are dissolved before they sink so far.' Volcanic glass in minute frag- 
 ments is an abundant constituent, and the silicious structures of the radiolarians 
 constitute an important element of its mass. Manganese nodules and those 
 of other minerals are also present. 
 
 This clay accumulates with exceeding slowness upon the floor of the deep sea, 
 so that the teeth of sharks which have lain upon the ocean floor since Tertiary 
 time have in many cases not been so deeply covered by it, but that by dredging 
 they may be obtained alongside of those of species still living in these oceans. 
 
 SUMMARY or STRUCTURES OF MARINE CLASTICS 
 
 All marine elastics are well stratified, but cross-bedding structure 
 is found only in the deposits of steep beaches, bars, and spits. 
 There is, however, an inclined stratification in the calcareous de- 
 posits around coral reefs (Fig. 228, p. 304). Ripple marks are 
 characteristic of all the shallower deposits. ' They are commonly 
 of the oscillation type with symmetrical troughs separated by sharp 
 ridges (Fig. 461 c). The current type of ripple occurs, how- 
 ever, on beaches and in very shallow water where the influence 
 of currents is manifested (Fig. 453, p. 537) . On the beaches we also 
 find the cusps, which are sharp ridges projecting at right angles 
 to the shore and separated by concavities (Fig. 451, p. 535). 
 
 Rill marks formed by the running off of waters are common upon 
 beaches and tidal flats, but their character is generally the reverse 
 of those formed on inland mud surfaces. On the coast, rill marks 
 are formed by the union of many minor rills into larger ones, 
 being thus more of the nature of miniature river systems, while 
 the rill marks on inland flats are comparable to the distributaries 
 of rivers on the delta. This type, however, also occurs upon the 
 beach (Fig. 462). Other structures restricted to the beaches are 
 wave marks, sand-ridges behind shells and pebbles, and shallow 
 gougings of the sands and muds around various objects. Certain 
 markings due to shore ice and ice crystal growth, and channels 
 due to dragging of seaweeds and other objects over the floor of 
 shallow water may also be mentioned. Mud-cracks, raindrop and 
 
554 Deposition of Clastic Material in the Sea 
 
 FIG. 462. Rill marks upon the coast of 
 Rockaway Beach. (M. O'Connell photo.) 
 
 footprint impressions, on the other hand, so characteristic of 
 river flood-plains and playa surfaces, are rarely preserved in sea- 
 coast deposits, though 
 this may be the case in 
 special regions where 
 muddy surfaces are ex- 
 posed for long intervals 
 between highest tides. 
 
 Above all, however, 
 marine elastics are char- 
 acterized by the presence 
 of organic remains (shells, 
 etc.) which are seldom 
 absent over wide areas 
 or through successive 
 layers. Indeed, such or- 
 ganic remains may be re- 
 garded as the surest in- 
 dication of the marine origin of older sediments and very often 
 the only reliable one. If the remains of marine organisms are 
 absent from a formation of wide extent and great thickness, it 
 may reasonably be concluded that that formation was not de- 
 posited under normal marine conditions. 
 
 LATERAL CHANGES IN FACIES AND OVERLAP RELATIONS 
 OF MARINE CLASTICS 
 
 Change in Fades 
 
 When we consider the deposits formed in the sea at a given time, 
 it is evident that they cannot be of uniform lithological character 
 throughout. Close to the shore, pebbles may accumulate, while 
 at the same time sands are deposited farther out to sea, and at a 
 still greater distance muds are forming. At another place the shore 
 deposit may be wholly quartz sand, but as we pass seaward, es- 
 pecially in tropical regions, the sands may become more calcareous, 
 for the material worn from a coral reef at some distance from shore 
 may become mingled with the shore-derived terrigenous quartz 
 sands. Finally, at greater distances from shore, the quartz grains 
 may disappear and lime-sands and lime-muds will alone be de- 
 posited. Thus when we consider the deposits formed in a given 
 
Lateral Changes in Fades 
 
 555 
 
 short period of time as a whole, we find a lateral gradation in facies 
 from the shore outward. Either the change is from coarse terrig- 
 enous deposits at the shore to fine ones at a distance from shore, 
 
 FIG. 463 a. Diagram Illustrating change in facies of marine elastics, from 
 sands and muds near the shore to calcareous beds near coral reefs, etc. Note 
 also the overlap of the successive formations. (From Principles of Stratig- 
 raphy.) 
 
 or it is from terrigenous sands to ocean-derived (coral) sand and 
 muds (Fig. 463 a). Such a lateral gradation in character of 
 material or in facies is practically universal for marine deposits. 
 
 Overlaps and Off-Laps 
 
 Progressive Overlap. When the sea slowly advances upon a 
 sinking land, it is obvious that from period to period of deposition 
 there is a shifting of the facies in the direction of transgression. 
 In the case of varying terrigenous deposits, the pebble zone of the 
 second stage of deposition comes to rest farther up on land than 
 
 FIG. 463 b. Section to show normal progressive overlap of the formations 
 a-c and their shoreward change from muds to sands and pebbles. (From 
 Principles of Stratigraphy.) 
 
 that of the first, while the sand zone of the second stage is shifted 
 so as to rest more or less upon the pebble zone of the first, and the 
 mud zone of the second in part upon the sand zone of the first. 
 With further advance of the sea, a continued shifting of the zones 
 in the same direction takes place, until the mud zone of the last 
 stage may rest vertically above the pebble zone of the first, being 
 separated from it, however, by a sandy zone (Fig. 463 b). Similar 
 
556 Deposition of Clastic Material in the Sea 
 
 conditions occur when the lateral change in facies is from quartz 
 sand to coral sands or muds (Fig. 463 c). 
 
 It is further evident that if we separate the deposit formed simul- 
 taneously at the different periods or stages of deposition we shall 
 find not only a lateral change in facies in each, and essentially the 
 same type of lateral change, but we shall also find that the deposits 
 of each successive period or stage extend farther up on the old land 
 than did those of the preceding period or stage. In other words, 
 the deposits of successive stages progressively overlap one another 
 in the direction of sea advance. This is illustrated in the diagrams 
 (Figs. 463 b, c), in which the periods of deposition are lettered from 
 below upward, or in the order of formation. 
 
 It must be recognized, however, that such overlaps are of signif- 
 icance only when they extend over wide areas and long time inter- 
 vals, for though now and then such structures can actually be seen 
 
 FIG. 463 c. Section to show normal progressive overlap of marine forma- 
 tions a-c, and their shoreward change from limestones to sandstones. 
 
 in small sections, it usually requires the comparison of sections at 
 a distance from one another to bring out this relationship. More- 
 over, it is necessary to have some means by which the different 
 divisions of the series can be separated from one another, and by 
 which the same division can be traced from point to point. In a 
 series, the formation of which occupied a considerable period of time, 
 the organic remains or fossils serve such a purpose of identification, 
 for the character of the organisms undergoes a progressive change 
 from period to period, and at each period the organisms peculiar 
 to it are distributed over much if not the whole of the area in which 
 the sediments of such an overlapping series are formed. Thus, in 
 the following illustration (Fig. 464) , we may assume that each one 
 of the divisions from a to c had its own peculiar organic remains 
 which form its " index fossils," and that the index fossils of each are 
 distributed horizontally over the entire extent of the division. 
 If we now examine the deposits of such an overlapping series, 
 
Overlaps of Marine Clastics 
 
 357 
 
 which we will assume have been consolidated into rock and have 
 been subjected to considerable erosion, we may find the following 
 conditions along a line at three points, each separated from the other, 
 we will assume, by an interval of fifty miles. The only exposure at 
 locality A is a cliff of sandstone resting with a basal pebble bed 
 upon the crystalline rocks (granite or gneiss). At B, fifty miles 
 distant, we find a river valley cut into horizontal beds down to the 
 underlying crystalline rocks. Here we find again a sandstone and 
 basal pebble bed resting upon the crystallines, and we might as- 
 sume that this was the same sandstone which we found resting upon 
 the crystallines in section A. This assumption would, however, 
 be erroneous, as we shall see. Overlying this lowest series are 
 sandy muds and finally beds of clay rock. Fifty miles farther in 
 
 FIG. 464. Natural and columnar sections at each end and at the center of 
 a line 100 miles long and at right angles to the original trend of the shore. The 
 progressive overlap and change in facies is shown. Note that in each section 
 the clastic formation next above the crystallines is a sandstone, but that it 
 belongs to different formations in each. 
 
 the same direction we meet with the outcrops at C, where the beds 
 have been disturbed (by faulting and tilting; see Chapter XIX), 
 and here we find a still more extensive series of stratified beds 
 resting upon the crystallines, but beginning again with a sand and 
 pebble rock* followed by a succession of beds very similar to those 
 seen at B, but with an additional formation at the top which con- 
 tains more or less lime. 
 
 If we now study the index fossils of the successive divisions a 
 to c at C and then compare them with the index fossils found in the 
 two divisions shown in the river gorge at B, we shall recognize that 
 the two divisions at B correspond only to the upper two (b and c) 
 at C, although in their physical characters, that is, their petrology, 
 they resemble the lower two. In other words, the lowest division 
 at B has the same fossils as the second division at C, although 
 
558 Deposition of Clastic Material in the Sea 
 
 in its general character it resembles the first division at C. In 
 like manner the second division at B corresponds to the third 
 division at C in its index fossils, though in its physical characters 
 it corresponds more closely to the second division at C. Now ex- 
 tensive studies have shown that index fossils are more reliable 
 guides than are the physical (lithologic) characters of the forma- 
 tions, and from this we conclude that the formations exposed at 
 B represent divisions b and c as shown at C, formation a being ab- 
 sent at B through overlap, while the physical characters or facies 
 of b and c have changed laterally. From the character of the index 
 fossils we further conclude that the only bed shown at locality A 
 corresponds to bed c at C and B, though in its physical characters 
 it resembles bed b at B and bed a at C. Beds a and b are thus en- 
 tirely absent at A, having been overlapped by bed c. In the 
 chapters on historical geology, in the later part of this book, we 
 shall give several examples of such overlaps. 
 
 Progressive Offlapping. In the foregoing section we have 
 dealt with the arrangement of strata produced in a transgressing 
 
 FIG. 465. Section to illustrate the arrangement and change in facies in an 
 offlapping series formed during a retreatal movement of the sea. Note that 
 at A, only the lowest bed (a) is present as a sand-facies, this being limestone 
 farther out at sea. At B all the formations are present, the highest (d) being 
 sandy, the lowest calcareous. (From Principles of Stratigraphy.) 
 
 sea. The reverse condition obtains in a retreating sea, forming a 
 regressive series. Here each succeeding division will cover a lesser 
 area than the preceding one, and a portion of the older one is 
 always exposed beyond the younger. This is illustrated in the pre- 
 ceding diagram (Fig. 465) in which the sea is assumed to have 
 successively retreated from A to B. As the shore line retreats, the 
 types of deposits will change with it, and the zone of sands will 
 migrate seaward with the retreat, and come to rest upon the finer 
 muds, etc., which were previously deposited here while the shore 
 stood at A. 
 
 If now we have natural exposures of such a series (Fig. 466) we 
 shall find that the sandstone bed at A has the fossils of division a, 
 
Overlaps of Marine Clastics 
 
 559 
 
 while the beds set B are known by their fossils to be, respectively, 
 beds a and b instead of b and c, as in the transgressive series (Fig. 
 464). At C all three beds are exposed. Moreover, it will be seen 
 that in general the change in the physical characters of the suc- 
 cessive beds is the reverse of that seen in the previous series, the 
 
 FIG. 466. Natural and columnar sections at each end and in the center of 
 a line 100 miles long and extending at right-angles to the original shore-line. 
 
 finer ones (muds and calcareous beds) being at the bottom and the 
 sands at the top. The top sandstone is one of emergence, whereas 
 in the previous series of sections the bottom sandstone was one of 
 submergence. Conditions of this kind, though rarely as simple as 
 here outlined, are found in nature and some of them will be discussed 
 in later chapters. 
 
 Compound Regressive and Transgressive Series 
 
 Offlaps Followed by Overlap. It is obvious that if, after a 
 retreatal movement of the sea and the formation of an offlapping 
 series as outlined in the preceeding section, a transgressive move- 
 
 
 
 FIG. 467. Section to illustrate the relationships of strata formed by a 
 compound, transgressing, retreatal and transgressive movement of the sea with 
 the resulting overlapping, offlapping and overlapping series separated by a 
 compound sandstone of emergence and submergence. (From Principles of 
 Stratigraphy.} 
 
 ment with overlapping of formations should follow, the sandstone 
 bed of emergence would be in part reworked by the advancing 
 waters and transformed into a sandstone of submergence, at least 
 
560 Deposition of Clastic Material" in the Sea 
 
 in its upper part. Thus a complex series would be produced, sepa- 
 rated by this compound sandstone. The resulting relationships 
 of the strata are shown in the diagram (Fig. 467, p. 559), which 
 represents an actual case of a transgressive, followed by a regres- 
 sive, and again by a transgressive series. The appearance in the 
 exposures of such a series is shown in the next figure (Fig. 468). 
 
 FIG. 468. Natural and columnar sections to illustrate compound off- 
 lapping and overlapping series. 
 
 At locality A the sandstone of emergence, a, is followed by the 
 sandstone of submergence, c r . At locality B these two sandstones 
 are represented by formations b and b f , respectively, and at locality 
 C by c and a'. In both localities B and C, this compound sand- 
 stone will occupy the middle of the series and the other beds will 
 become finer grained both downward and upward. 
 
 Replacing Overlap. When a series of delta or other clastic 
 beds is built outward from the land, while a series of marine beds 
 
 
 FIG. 469 a. Diagrammatic section to illustrate replacing overlap of shore 
 or continental sands on the right and marine shales, sandstones and calcareous 
 beds on the left. (From Principles of Stratigraphy.) 
 
 such as limestones is forming farther out to sea, the advancing 
 deposits from the land will progressively overlap the purer marine 
 deposits, successively replacing them. This relationship is shown 
 in the preceding diagram (Fig. 469 a). Instead of delta or other 
 clastic seashore deposits (of terrigenous origin), deposits of purely 
 continental character may progressively replace those of marine 
 
Overlaps of Marine Clastics 
 
 561 
 
 character, the resulting relationships being essentially similar (Fig. 
 469 b). The replacement may be abrupt or gradual, or on account 
 of oscillations the two series may interfinger in various degrees 
 along the line of contact. 
 
 FIG. 469 b. Diagrammatic section to illustrate the overlap of continental 
 beds over a retreatal marine series. (From Principles of Stratigraphy.) 
 
 In the following two diagrams such replacing series of various 
 types are shown (Figs. 469 c, 469 d). These represent interpreta- 
 tions of actual cases, as are also the simpler ones shown in the 
 
 FIG. 469 c. East-west section across the state of New York, showing the 
 various types of overlap characteristic of the formations. The depression, at 
 Utica is due to subsequent erosion. The strata were formerly continuous. 
 (From Principles of Stratigraphy.) 
 
 Interior Region 
 
 FIG. 469 d. Restored section from the Appalachians to the Cincinnati 
 region, to show the overlaps of continental and marine strata. (From Prin- 
 ciples of Stratigraphy.) 
 
562 Deposition of Clastic Material in the Sea 
 
 preceding figures. In the next figure (Fig. 470) non-marine, a, 
 and marine, b, overlaps are compared, the former overlapping 
 
 FIG. 470. Sections to illustrate progressive overlap ; a, in a continental 
 series, away from the source of supply ; b, in a marine series towards the source 
 of supply. (From Principles of Stratigraphy.) 
 
 primarily away from the source of supply of terrigenous material, 
 the latter towards it, the source being indicated by the arrows. 
 
CHAPTER XVIII 
 
 CONSOLIDATION OF CLASTIC MATERIAL; TYPES 
 OF CLASTIC ROCKS 
 
 CONSOLIDATION OF CLASTICS 
 
 WITH the passage of time clastic material is in most cases bound 
 
 together into solid rock, but the degree of such solidification does 
 
 not always correspond to the 
 
 length of time during which these 
 
 deposits have existed. Thus in 
 
 the Baltic provinces of Russia 
 
 there are sands and muds which 
 
 belong to the oldest deposits of 
 
 the Palaeozoic era, deposits which 
 
 everywhere else have been com- 
 pletely consolidated into rock, or, 
 
 as it is technically expressed, have 
 
 become thoroughly indurated. 
 
 The Russian deposits, on the 
 
 other hand, are for the most part 
 
 still unconsolidated, though in 
 
 some cases this may be the result FIG. 471 a. Roadway, cut along 
 
 of later removal of the binding the side of a cliff of consolidated 
 
 glacial gravels (Monchsberg). The 
 rock, which is called Nagelfluh, be- 
 cause of the resemblance of the de- 
 
 substance which once had in- 
 durated them. On the Atlantic 
 
 coast of New Jersey and Mary- Passions left by the pebbles when 
 
 . , , j * removed, to that formed by a large 
 
 land are soft clays and sands of nail _ head> forms part i y overhanging 
 
 the same age as the consolidated cliffs and is in places tunneled by 
 chalk beds of England and France roadways. Salzburg, Austria, 
 and of the hard limestones and 
 
 other rocks of the Alps. Finally, some comparatively young de- 
 posits, formed during the ice age, are already solidified in certain 
 regions, the most noted example being the great pebble beds 
 called " Nagelfluh " which form some of the remarkable hills in 
 the city of Salzburg, Austria (Fig. 471 a). These are honey- 
 
 563 
 
564 Consolidation of Clastic Material 
 
 combed in places by catacombs which date back to Roman times 
 (Fig. 471 b). Deposits of lime-sands and muds are frequently 
 
 FIG. 471 b. Maximus Chapel, in the Catacombs of Salzburg (Roman 
 Juvavum). These catacombs are hewnjout of the Nagelfluh, or consolidated 
 glacial gravels. They date back to the third century. 
 
 bound together very shortly after their formation, especially in the 
 sea, where cementing lime is deposited by precipitation from solu- 
 tion in the sea-water. 
 
 Causes and Agents of Induration 
 
 Clastic material may become consolidated or indurated in a vari- 
 ety of ways, the chief of which are through pressure, by infiltration 
 of a cementing medium into the pores, or by recrystallization and 
 the formation of new minerals which bind the grains together. The 
 last method belongs to the changes due to metamorphism, and 
 will be more fully discussed in Chapter XX. The others may be 
 briefly considered here. 
 
 Induration by Pressure. When an older is buried beneath 
 younger sediments, the pressure of the latter will tend to bring the 
 grains of the older more closely together, forcing out the air and 
 water from the pores. This will result in a certain amount of inter- 
 locking of the grains and cause them to adhere to one another more 
 or less firmly. When sand grains are thus pressed together a soft 
 
Consolidation of Clastics 565 
 
 sandstone is produced, which because of its ready yielding to the 
 stone-cutter's tools is called a freestone. Finer-grained clastic de- 
 posits are more strongly solidified in this manner than those of 
 coarser grain, and thus strata of clay and rock-flour may be changed 
 into a rock called shale (Fig. 473, p. 570). There is probably in all 
 cases a certain amount of cementation by mineral matter, and in 
 the older deposits there is generally some recrystallization as well. 
 Induration by Cementation. The common cements which bind 
 clastic fragments together are carbonate of lime, silica, and iron 
 oxide. Carbonate of lime is of course the chief cementing agent of 
 all calcareous deposits, but it also binds quartz grains and pebbles 
 into a calcareous sand or conglomerate, and it may be an impor- 
 tant cementing agent of 
 muds of various kinds. 
 When sand and pebble beds 
 are penetrated by waters 
 which carry lime in solution, 
 either rising from below or 
 descending through a soil 
 
 rich in lime or through an 
 
 FIG. 472. Well-rounded quartz grains 
 overlying calcareous de- from ancient eolian rock (shaded), en- 
 
 posit, the grains and pebbles larged by secondary addition of silica in 
 are quickly cemented into JP^ continuity with the old quartz 
 
 Sandstone Dike, Buffalo, N. Y. (Much 
 a solid rock, just as sand enlarged.) 
 
 grains, pebbles, and large 
 
 blocks are cemented into an artificial rock mass by mortar, which 
 is mainly carbonate of lime. Silica brought in solution by water 
 will cement many kinds of fragmental material, but is especially 
 effective when the clastic material consists of pure quartz grains. 
 In such cases, the silica will be deposited around the grains in 
 such a way that the physical (especially crystallographic) charac- 
 ters of the new quartz will be continuous with the quartz grain of 
 the clastic sands. This is called secondary enlargement of the 
 clastic particles and is of common occurrence (Fig. 472). When 
 the process has gone so far that the grains are no longer dis- 
 tinguishable a bed of quartzite is produced. 
 
 Iron oxide is a common cementing agent especially of impure 
 sands. Such sands are then colored yellow or brown, forming 
 the familiar brownstone so much used in the past for dignified 
 buildings. 
 
566 Types of Clastic Rocks 
 
 CLASSIFICATION OF CLASTIC ROCKS 
 
 Important Characters. When we remember that the clastic 
 material from which the clastic rocks are produced is the product 
 of rock destruction or fragmentation, it is apparent that in any 
 natural classification the agent causing this fragmentation demands 
 first consideration, just as the medium from which the various 
 chemical (endogenetic) rocks were separated out, and the agent 
 active in the separation, demanded our first attention among the 
 non-clastic rocks. In those non-elastics, the formation of the rock 
 from the magma or the state of solution and vapor, was primarily 
 a chemical process, and therefore the chemical composition of the 
 medium and of the resulting rock was "of next importance. In the 
 case of the clastic rocks, however, the chemical composition of the 
 material from which the clastic matter is derived (that is, the older 
 rocks) is of less significance, since it plays generally only a subor- 
 dinate part in the process of clastation. Of much greater signifi- 
 cance as indicative of the kind and amount of destructive work 
 performed, is the coarseness of the fragments produced, and it is 
 this degree of coarseness which in the consolidated rock produces 
 its grain or texture. Hence, in the clastic rocks, texture may be con- 
 sidered of more importance than chemical composition, whereas 
 in the non-elastics the reverse was true. Chemical composition, it 
 is true, may play a very important part in clastic rocks as well, as 
 for example in limestones and in clay rocks, but this composition 
 is never constant over more than very small areas, because 
 clastic rocks, more than any other, are subject to the mechanical 
 mixture of materials of varying composition. We shall then, in 
 the classification of clastic rocks, consider the basis of subdivision 
 in the following order : (i) agents of clastation, transportation, and 
 deposition ; (2) size of grain or texture ; (3) chemical composi- 
 tion ; (4) other characters. 
 
 Classification according to Agents of Clastation, Transportation, 
 and Deposition 
 
 It is obvious that these activities may not always be due to the 
 same agent, though there are examples where this is undoubtedly the 
 case. Thus sand and larger-grained fragments worn by the waves 
 of the sea from a cliff of older rock, transported by waves and cur- 
 rents, and deposited in quieter water in the ocean, are throughout 
 
Classification of Clastic Rocks 567 
 
 the product of sea erosion, transportation, and deposition. Frag- 
 ments broken from rocks by a river current, transported by it, and 
 deposited upon the flood-plain or alluvial fan, form, throughout, a 
 river-made deposit. So too, a rock is, throughout, a wind-formed 
 or eolian rock if the fragments are removed from the older rock by 
 the erosive action of the wind, and if they are transported and de- 
 posited by it. While such pure types, as they may be called, are 
 perhaps not at all uncommon, actual purity of origin will be very 
 difficult to determine in any but the simplest cases. Thus the 
 sands transported by and deposited in the sea may have been 
 furnished to it by rivers, a very common occurrence. The sands 
 washed away by the rivers or blown away by the wind may origi- 
 nally have been the product of disintegration and decomposition 
 under the atmosphere or they may be the product of glacial erosion. 
 Indeed, it is probably true that clastic deposits and the clastic rocks 
 formed from them have, as a rule, a varied history and are the 
 product of the interaction of various agencies. Only in the con- 
 solidated residual soils, the product of atmospheric decay or 
 weathering without transport, can we be reasonably sure that 
 but one agent, the atmosphere, was active in their production. 
 To state it in another way, most clastic rocks are of multiple par- 
 entage they are polygenetic though some are of single parent- 
 age, or monogenetic. 
 
 If it is not possible to determine all the agents active in the pro- 
 duction of a given rock, we are constrained to give prominence to 
 the agent which has impressed its most characteristic features upon 
 the deposit, and this is commonly the last of the agents, namely, 
 that effecting deposition, or the medium by and in which the material 
 was deposited. Thus an eolian rock is one deposited by wind upon 
 dry land, whatever the origin of the material. If the origin can 
 be determined, this adds to the precision of the classification. Thus 
 certain eolian beds may consist wholly of volcanic fragments, i.e. 
 pyroclastic material. Others again, like the loess, are believed to 
 be in part of glacial origin. 
 
 In general, it may be said that the origin of rocks according to 
 agent is a matter primarily for field determination, and one only 
 rarely indicated in small fragments. True, a marine rock may al- 
 ways be recognized from its fossils, and so may a lake or pond- 
 formed rock. But when fossils are absent, as they may be even 
 iii rocks thus formed, there is no positive indication of origin. In- 
 
568 Types of Clastic Rocks 
 
 deed, some rocks may be wholly composed of minute shells of marine 
 organisms and yet be wind-laid deposits, as in the case of the Milio- 
 litic limestone of India, already referred to (p. 278). Again, a rock 
 may show all the purity, roundness of grains, and uniformity of 
 their size, characteristic of wind deposits, and yet may be reworked 
 by marine waters and so be a typical marine rock. 
 
 Taking first, then, the agent or medium which has given the clas- 
 tic rock its most distinctive character, we may distinguish the fol- 
 lowing types : 
 
 1. Residual Rocks. Those formed in place by disintegration 
 and decomposition with little or no transport (Atmoclastics) . 
 
 2. Eolian Rocks. Those transported and deposited by wind 
 upon dry land (Anemoclastics) . 
 
 3. Water-laid Clastic Rocks. Those deposited by and in water 
 (Hydroclastics). Among these we may distinguish: 
 
 (a) Marine Clastics. Those deposited in the open sea. 
 
 (b) Estuarine Clastics. Those deposited in estuaries. 
 
 (c) Lacustrine Clastics. Those deposited in lakes, ponds, and 
 playas. 
 
 (d) Fluviatile Clastics. Those deposited on river flood-plains, 
 alluvial fans, and deltas. 
 
 4. Volcanic Clastics. Those produced by volcanic explosions, 
 and primarily settling down without much reworking by wind or 
 water (Pyroclastics) . 
 
 5. Fault-Crush and Glacial Clastics. Those produced by the 
 movement of one rock mass past or over another, including those 
 produced by ice erosion (Autoclastics) . 
 
 6. Organically Produced Clastics. Those produced by the 
 rock-breaking activities of organisms, a small class except for arti- 
 ficial rock-masses produced by man (Bioclastics). 
 
 The Texture of Clastic Rocks 
 
 The texture of clastic rocks is their most obvious character and 
 the one most readily determined in hand specimens or other frag- 
 ments which may give no indication concerning the agent active in 
 their formation. Hence clastic rocks are more often classified by 
 their textures than by any other character. 
 
 Three main types of texture may be recognized, the coarse, inter- 
 mediate, and fine, according as the material consists primarily of 
 
Classification of Clastic Rocks 569 
 
 coarse fragments (rubbly material), of sand grains, or of impal- 
 pable rock-flour or clay. Three terms will here be used to express 
 these textures. 
 
 1. Rudaceous, for the coarse rubbly texture (Latin, nidus , 
 rubble). 
 
 2. Arenaceous, for the sandy texture (Latin, arena, sand). 
 
 3. Lutaceous, for the rock-flour and clay-size texture (Latin 
 lutum, mud). 
 
 Accordingly three textural types of rock may be recognized: 
 
 (1) the rubble-rock or rubble-stone, or rudyte, which when the frag- 
 ments are rounded is a conglomerate and when angular a breccia; 
 
 (2) the sand-rock or sandstone or arenyte ; and (3) the mud-rock 
 or mud-stone, or lutyte. 
 
 Several distinct types can be recognized under each division, but 
 as these are further modified by the composition, this will be con- 
 sidered first. 
 
 Composition of Clastic Rocks 
 
 All clastic rocks which have been subject to prolonged transport 
 and sorting of material before consolidation will be relatively pure, 
 or of more or less uniform composition. With material derived 
 from crystalline rocks (igneous or metamorphic) quartz will pre- 
 dominate, and in exceptional cases the rock may consist wholly of 
 quartz, as in that of the Sylvania sandstone of Ohio and Michigan 
 already referred to (p. 440), and of the Shawangunk and Olean 
 conglomerates of New York state. In other cases the rock may 
 contain large quantities of feldspar because there has been little 
 transport and sorting. Such are the arkose sandstones of New 
 Jersey (Triassic) and the arkose Torridon breccia or fragment rock 
 of Loch Marie and other regions of West Scotland (pre-Cambrian), 
 which looks superficially like a coarse granite, having been produced 
 by the disintegration of such rock and the recementation of the 
 material. 
 
 When the materials of a clastic rock are derived from the erosion 
 of a coral reef, or shell accumulations, or older limestones, the 
 grains and larger fragments are apt to be pure calcium carbonate 
 with little or no admixture of other material. This produces a 
 lime sandstone (calcarenyte} such as that found on Bermuda, or 
 a limestone breccia (calcirudyte) , of which the Point of Rocks " Mar- 
 ble " of Maryland is an example. 
 
570 
 
 Types of Clastic Rocks 
 
 The mineral glauconite is often a characteristic constituent of 
 sands, as we have seen (p. 552), and thus glauconitic sandstones may 
 be produced. When iron oxide abounds as a cementing agent, 
 ferruginous clastic rocks (sandstones, conglomerates) are produced. 
 When the material of the rock is largely clay, its texture is that of 
 a mud-rock, or lutaceous, and the rock becomes a claystone or argillite 
 (Latin, argillum, clay). When clay is one, but not the only constit- 
 uent, the rock is called argillaceous. Rocks of all textures may be 
 
 FIG. 473. A bank of typical shale (Hamilton formation, Devonian) on 
 the shore of Lake Erie. The shale splits into small chips on exposure, and 
 rapidly weathers to clay. It is full of marine fossils. (Photo by the author.) 
 
 argillaceous, those of rubbly (rudaceous) texture and those of 
 arenaceous texture generally carrying the clay as an admixture or 
 as part of the cement. When carbon is present, the rock is called 
 carbonaceous. 
 
 While there are other composition types, the silicious, calcareous, 
 and argillaceous are the most common, and from a purely chemical 
 point of view these are called quartz-stones, limestones, and clay- 
 stones. Other substances are present chiefly in smaller quantities 
 and form modifications of these primary types. It must, however, 
 be remembered that there are quartz rocks and limestones of other 
 than clastic origin. 
 
Structural and Other Characters 
 
 STRUCTURAL AND OTHER CHARACTERS USED IN DEFINING 
 CLASTIC ROCK TYPES 
 
 Shaly and Slaty Structures. Certain structures are so commonly 
 associated with special composition and textural types of clastic 
 rocks, that they have been made the chief basis of classification. 
 Mud-rocks (lutytes) in which much clay occurs (argillaceous) gen- 
 erally show a peculiar 
 mode of breaking in 
 small plates parallel to 
 the bedding, these plates 
 having curved surfaces 
 (gently convex on one 
 side and concave on the 
 other) which give the 
 fragment a superficial 
 resemblance to a piece 
 of a shell. On this ac- 
 count the name shale 
 has been given to such 
 rocks and the structure 
 is spoken of as shaly 
 (shelly). When exposed, 
 typical shales quickly 
 split into thin chips, and 
 weather into clay (Fig. 
 473) . The term shale is, 
 
 FIG. 474. A bank of fissile shale (Genesee 
 Shale, Upper Devonian) exposed in the gorge 
 of Eighteen Mile Creek in western New York. 
 Note the perfect jointing. The rock splits 
 into thin slate-like sheets parallel to the bed- 
 ding. (Photo by the author.) 
 
 however, loosely used 
 for all kinds of mud-rocks which split into thin layers, whether 
 these are shaly, i.e. have curved surfaces, or are smooth-surfaced. 
 To the latter type the term fissile shale is sometimes applied, and 
 the name slate is also popularly used in some cases where this 
 smooth splitting is parallel to the bedding surfaces (Genesee 
 Shale, Fig. 474). Such rock is generally well jointed or separated 
 by vertical planes with smooth faces. True slate, however, is a 
 metamorphic mud-rock, the splitting into thin plates having, as 
 a rule, no reference to the original bedding, but being secondarily 
 produced. (See Fig. 562.) 
 
 Platey Structure. When rocks split into layers a few milli- 
 meters or centimeters in thickness, and with smooth surfaces which 
 
572 
 
 Types of Clastic Rocks 
 
 are generally the bedding planes, they are called platey rocks. The 
 most common platey rocks are calcareous and fine-textured or lime- 
 mud-rocks (calcilutytes). 
 Such are the famous 
 platey layers of the 
 Solnhofen region in Ba- 
 varia (Plattenkalke) 
 (Fig. 475), which are 
 used for roofing purposes 
 in that country. Many 
 other limestones in our 
 own and other countries 
 split into thin layers, 
 producing suitable ma- 
 terials for " tablets of 
 stone " on which inscrip- 
 tions can easily be en- 
 graved on account of 
 their softness. When 
 seen in sections, such 
 thin-bedded limestones 
 
 often present a banded aspect, and they are then called ribbon 
 limestones (Fig. 476). They abound among the older formations. 
 
 Concretionary Structure and Con- 
 cretions. The name concretion is ap- 
 plied to a stony mass included in 
 stratified rocks and which has been 
 formed as the result of segregation of 
 material either during the deposition 
 of the rock or afterwards under the 
 influence of circulating waters or other- 
 wise. Concretions are very common 
 in clay rocks or unconsolidated clay 
 beds, and generally consist of clay 
 bound together by carbonate of lime or 
 carbonate of iron. In form they vary 
 greatly from spherical to disk-shaped 
 (Fig. 477 a), often compound (Figs. 
 477 b and c), cylindrical, tubular, or 
 irregular (Loess piippchen), etc., fre- 
 
 FIG. 475. Deep quarry in the lithographic 
 and platey limestone deposits (Jurassic) of 
 Solnhofen in Bavaria. Most of the rock con- 
 sists of thin layers (platey strata), but be- 
 tween these are the thicker, purer layers 
 which furnish the lithographic stone. (Photo 
 by author.) 
 
 FIG. 476. A piece of 
 "ribbon limestone" or thin- 
 bedded, fine-grained lime- 
 mud-rock (calcilutyte) seen 
 in section at right angles 
 to the bedding. Cambro- 
 Ordovician beds of Penn- 
 sylvania, slightly reduced. 
 (Photo by Bela Hubbard.) 
 
Structural and Other Characters 
 
 573 
 
 quently simulating organic forms (Fig. 47 7 d). Sometimes the 
 entire rock may be a mass of such concretions, as are certain beds of 
 the Magnesian Limestone on the coast of Durham, England, already 
 
 FIG. 477. Clay-stone concretions from the glacial clays of the Connecticut 
 valley. (After J. M. Arms Sheldon.) a, single disk-shaped form, about three 
 fifths natural size ; b, confluence of two disk-shaped forms, one half natural size ; 
 c, three confluent disks, about one half natural size ; d, forms imitative of 
 animal shapes (Loesspuppchen) less than half natural size. 
 
 referred to (p. 221). Thin beds of limestone in shales may be 
 formed from confluent concretions. Sometimes concretions are 
 full of radiating veins which, when the outer layer of the concretion 
 
574 
 
 Types of Clastic Rocks 
 
 FIG. 478. Large concretion of the Sep- 
 tarium or "Turtle Stone " type, weathered 
 out of the shale banks (Upper Devonian) on 
 the Genesee River. Others are seen forming 
 bands in the shale bank. (See also Figs. 
 161 a, 6, p. 222.) 
 
 p. 224), and in sandstones they often 
 sand (Fig. 479). 
 
 is worn away, may 
 weather out in relief. 
 Such concretions are 
 called septaria (Fig. 478). 
 The oolites and pisolites, 
 previously discussed, 
 form aggregates of small, 
 regular concretions oc- 
 curring as separate de- 
 posits. Not infrequently 
 concretions are formed 
 around a nucleus, which 
 may be a plant or a 
 shell or other organic 
 structure (Fig. 181, 
 p. 257). In chalk and 
 limestones the concre- 
 tions are mostly of silica, 
 flint, and chert (Fig. 162, 
 consist of iron-cemented 
 
 FIG. 479. Concretionary masses of sandstone (Laramie formation), south- 
 west of New Castle, Wyo. (Darton, photo ; from U. S. G. S.) 
 
Varieties of Clastic Rocks 
 
 575 
 
 VARIETIES OF CLASTIC ROCKS 
 
 As we have seen, the texture of clastic rocks is their most evident 
 characteristic, as seen in fragments or hand specimens. Hence it 
 is usual to divide clastic rocks first 
 upon a textural basis, and this is 
 threefold as shown above. The more 
 common types of each of these tex- 
 tural groups may now be reviewed. 
 
 Rubble Rocks or Rudytes 
 (Texture Rudaceous) 
 
 These are the clastic rocks in which 
 the prevailing size of the fragments 
 is larger than that of a sand grain, or 
 in general larger than 2 or 2.5 
 millimeters. Two main varieties are recognized, as follows: 
 
 FIG. 480. Conglomerate (re- 
 duced). (B. Hubbard, photo.) 
 
 Conglomerates (Fig. 480). These are composed predominantly of rounded 
 fragments, generally well water-worn, and most commonly of river or seashore 
 
 origin. In size the fragments 
 may range from pebbles of 
 the dimensions of peas to 
 large boulders. Some parts 
 of the Old Red Sandstone of 
 Scotland are composed almost 
 wholly of well-rounded bould- 
 ers, mostly of the size of a 
 man's head. Such rocks are 
 generally spoken of as boulder 
 conglomerates (Fig. 481). 
 When the pebbles are few and 
 scattered among sand grains, 
 the rock is called a pebbly 
 sandstone. When the pebbles 
 are very small the rock is 
 called a grit. In the decom- 
 position of certain basic 
 igneous rocks, such as ba- 
 salts, etc., a mass of rounded 
 residual boulders is left (see 
 p. 397), and these may remain 
 FIG. 481. Boulder conglomerate. Old piled together in their original 
 Red Sandstone (Devonian) at Oban, Scotland. position separated by de- 
 (Photo by author.) composition sand. When 
 
FIG. 482. Roxbury Pudding-stone, Franklin Park, Boston, Mass. (P. L. 
 
 Grabau, photo".). 
 
 
 FIG. 483. A breccia. Devonian limestone which has been broken into 
 angular fragments which have then become recemented into a solid rock a 
 typical breccia. Iowa. (After Diller, U. S. G. S.) 
 
 576 
 
Varieties of Clastic Rocks 
 
 577 
 
 close-pfled, their structure is spoken of as a cannon-ball structure, and the rock 
 is called a residual boulder conglomerate (atmoclastic) (Fig. 33 1 a) . Conglomerates 
 of small pebbles may be well stratified, and may show cross-bedding and other 
 structures. Boulder conglomerates generally show little structure. 
 
 According to the composition of the pebbles, we may have quartz conglomer- 
 ates, arkose conglomerates, limestone conglomerates, etc. Examples of these have 
 already been cited.- Many conglomerates are, however, heterogeneous ; that is, 
 the pebbles are composed of a variety of material granite, gneiss, sandstone, 
 etc. Sometimes. conglomerates are formed mostly of one kind of such material. 
 Thus we may have granite con- 
 glomerate, volcanic (lava) con- 
 glomerate, sandstone conglomer- 
 ate, etc. According to the nature 
 of the cementing material, we 
 may have quartz-sand conglomer- 
 ates, calcareous conglomerates, 
 argillaceous conglomerates, fer- 
 ruginous conglomerates, etc. 
 When the pebbles (generally 
 large) and the cementing mixture 
 between them show little grada- 
 tion and a marked contrast, the 
 name pudding-stone is often ap- 
 plied (Fig. 482). If the pebbles 
 are formed of worn organic struc- 
 tures, we may have coral-con- 
 glomerates, shell-conglomerates, 
 etc. Finally, there is the artificial 
 conglomerate or rubble concrete, 
 so extensively produced by man. 
 
 Breccias. This term is ap- 
 plied when the fragments are 
 
 angular, showing little or no evidence of water wear (Fig. 483). The principal 
 types of breccia are the fault breccia (an autoclastic rock) (Fig. 32, p. 80), the 
 talus breccia (an atmoclastic rock), and the volcanic breccia or volcanic ag- 
 glomerate (Fig. 484), formed around and in volcanic vents (pyroclastic). A 
 glacial breccia may also be produced, but generally the fragments of a glacial 
 till are scratched and polished. Such a rock when consolidated forms a tillite 
 (p. 508). In deposits formed from glacial moraines and outwash material, the 
 pebbles and boulders are commonly well worn, forming a glacial conglomerate. 
 
 FIG. 484. Projecting cliff of volcanic 
 
 agglomerate, tunneled by waves along a 
 
 joint fissure. North shore, Bay of Fundy, 
 
 Nova Scotia. (G. W. Stose, photo.) 
 
 Sandstones or Arenytes 
 (Texture Arenaceous) 
 
 When the prevailing size of grain of a clastic rock falls between 
 2.5 and 0.05 millimeters, the rock is said to have an arenaceous tex- 
 ture, and is called a sandstone or arenyte. Generally the term sand- 
 
578 
 
 Types of Clastic Rocks 
 
 stone is restricted to 
 such rocks in which the 
 grains are chiefly quartz. 
 For this reason the term 
 arenyte is preferable, 
 since it implies nothing 
 but textural character. 
 According to origin, 
 composition, and struc- 
 ture a number of com- 
 mon types are recog- 
 nized. 
 
 Quartz Sandstone. 
 
 This is the common type in 
 which quartz grains prevail. 
 Many varieties occur deter- 
 mined by the nature of the 
 cementing material. When 
 this is oxide or hydrate of 
 iron, & ferruginous sandstone 
 is produced ; when carbonate 
 of lime, a calcareous sand- 
 stone is formed ; when much clay is present, the rock is an argillaceous sand- 
 stone. When pure it is valued for 
 glass-making, as are the Sylvania and 
 the Oriskany sandstones (Fig. 485). 
 
 Quartzite. This is a quartz sand- 
 stone in which the grains have become 
 enlarged by the deposition around 
 them of new quartz in such a way as 
 to form a crystallographic continuation 
 of the older quartz. The pores are 
 completely filled and the rock becomes 
 a hard, very resistant mass. (Ex- 
 ample : Potsdam quartzite of Ausable 
 Chasm, N. Y.) Quartzites are fre- 
 quently placed among metamorphic 
 rocks. (See Fig. 373, p. 455.) 
 
 Freestone. This name is given to 
 a quartz sandstone in which the grains 
 are loosely held together, so that they 
 will readily yield to the graver's tools. 
 This is commonly due to the fact that 
 the grains are bound together only by 
 pressure. (Example: Ohio freestone.) 
 
 FIG. 485. Ledge of Oriskany sandstone, 
 Pennsylvania. The rock is a pure quartz sand- 
 stone covered by talus from higher sandstone 
 masses. It is quarried here for glass-making 
 under the name Juniata glass sand. 
 
 FIG. 486. Micro-photograph of a 
 thin section of Hudson River Blue- 
 stone. The clear grains are quartz, 
 the rest of the field is made up chiefly 
 of secondary derivatives from the 
 original feldspars and ferromagnesian 
 minerals, and of clastoliths. (Photo 
 made and contributed by C. P. 
 Berkey.) 
 
Varieties of Clastic Rocks 
 
 579 
 
 Brownstone. This is a highly ferruginous quartz sandstone in which the 
 grains are generally coated with iron oxide. It has been extensively used for 
 building purposes. (Example : Connecticut Valley brownstone.) 
 
 Bluestone, Flagstone. This is a highly argillaceous sandstone of even tex- 
 ture and bedding. Commonly the argillaceous material of the cement has been 
 altered to the mineral sericite (Fig. 486). It is probably in most cases the prod- 
 uct of ill-assorted sediments in lagoons and lakes near the mouth of a river. 
 Such rock is much used for flags and sidewalk curbings. (Example : Hudson 
 River Bluestone.) " (Fig. 487.) 
 
 FIG. 487. Ledge of Hudson River Bluestone, showing solid layers of blue- 
 stone alternating with shaly beds. Near Kingston, N. Y. (Photo by C. P. 
 Berkey.) 
 
 Graywacke. This name is applied to an impure, highly argillaceous sand- 
 stone of variable composition, texture, and structure. Generally fragments 
 of other clastic rocks are present. The name has been widely used in the past 
 for all the harder argillaceous sandstones of the older geological systems. (Ex- 
 ample : Silurian graywackes of England.) 
 
 Arkosic Sandstone. A sandstone in which much feldspar is present. This 
 may range from unassorted products of granular disintegration of fine or medium 
 grained granite (recomposed granites or pure arkoses), to a partly sorted river- 
 laid or even marine arkosic sandstone. (Example: Triassic arkose of New 
 Jersey, some beds of the Potsdam sandstone of New York, etc.) 
 
 Glauconitic Sandstone. (Green-sand.) This is a quartz sandstone or an 
 arkosic sandstone rich in glauconite grains. (Example : Green-sand of England, 
 and of New Jersey and Maryland.) 
 
580 Types of Clastic Rocks 
 
 Lime Sandstone (Calcarenyte). Arenytes in which the grains are largely 
 or wholly composed of carbonate of lime are called calcarenytes. They are 
 common around coral reefs, and many of the common " limestones " of the 
 various geological formations are really calcarenytes. (Example : Coral sand- 
 stone of Bermuda.) 
 
 Eolian Sandstone. Sandstones in which the eolian origin can be recognized 
 in small specimens, either by structure or by character of grains, are referred to as 
 eolian sandstones. They may be pure quartz (Example : Sylvania sandstone) 
 or pure lime. (Example : eolian coral sandstone of Bermuda.) 
 
 Volcanic Sandstone or Ash Rock (Volcanic Tuff). This consists of the 
 material of arenaceous texture produced by volcanic explosions. It is recog- 
 nizable both by its structure, which is often porous, and by the composition and 
 form of the grains. The rock is generally called a volcanic tuff, but this term 
 is also applied to the finer-grained varieties, which in reality are lutytes (Fig. 488). 
 
 FIG. 488. Fragments of volcanic glass in tuff as seen under the microscope 
 X5o. a, minute particle of pumice; b, curved wall of a broken bubble; 
 c, small vesicle still complete and surrounded by tubular glass ; d, interstitial 
 glass between bubbles not tubular. (After Diller, U. S. G. S.) 
 
 Mud-stones or Lutytes 
 (Texture Lutaceous) 
 
 When the dominant material of a clastic rock consists of grains 
 less than 0.05 mm. in diameter, the rock is a mud-stone or lutyte. 
 Clay is a common constituent of such a rock, but may be largely 
 or wholly wanting, and the rock may consist of quartz-flour or in 
 part or entirely of limestone flour, or of lime mud. It may also 
 consist of very small wind-transported or water- worn shells of 
 Foraminifera, etc., and of fragments of these. Some of the common 
 types are : 
 
 Claystones (Argillutytes). Rocks in which much clay is present or which 
 are largely composed of clay. Sometimes these are bound together by iron 
 carbonate, forming clay iron-stones, generally of a concretionary character. 
 In large typical masses clay rocks show no regularity of splitting. 
 
 Shale. Mud-rocks containing much clay and splitting into thin layers with 
 curved surfaces parallel to the bedding are called shales. Many shales or similar 
 thin-bedded mud-rocks contain much carbonaceous material, in the form of 
 disseminated coaly matter, or they are saturated with oil. Such shales are 
 called carbonaceous shales, coal shales, oil shales, or black shales. When extremely 
 rich in carbonaceous material they are called pyroschists and form a transition 
 to coal. Carbonaceous shales generally split with smooth surfaces. Shales 
 may also DC highly fossiliferous, when they are generally calcareous. (See Fig. 
 
Varieties of Clastic Rocks 
 
 581 
 
 473, p. 570). Pyritiferous shales contain much scattered iron pyrite, which 
 weathers to iron hydrate on exposure, and causes the splitting of the shale. 
 Silicious shales contain much quartz-flour and are apt to split into irregular 
 pencil-like and other forms of fragments on weathering. Red shales are colored 
 by finely disseminated iron oxide. 
 
 Slates. These are more or less metamorphosed mud-rocks, the metamor- 
 phism being due to compression. As a result the slates split into thin layers on 
 weathering or under properly 
 applied force, but this split- 
 ting is seldom parallel to the 
 bedding. (Example : roofing 
 slate.) 
 
 Lime Mud-rocks (Cal- 
 cilutytes). These are com- 
 mon around coral reefs, and 
 many old limestones are of 
 this character, having been 
 formed from lime-flour gen- 
 erally deposited in the sea or 
 near its border. Sometimes 
 such rocks show mud-bracks, 
 ripple-marks, and other 
 structures. When alumina 
 and silica are present a water- 
 lime or natural cement rock 
 is produced (Fig. 489). Pure 
 lime mud-rocks form the 
 famous lithographic stone of 
 
 Solnhofen, Bavaria. Rocks of this type often preserve organic remains in won- 
 derful perfection, as is seen in the ancient birds with their feathers preserved 
 and in the dragon flies and other organisms for which the lithographic stone 
 has become famous. 
 
 Quartz-flour Rock (Silicilutyte). Rocks composed apparently largely or 
 wholly of quartz-flour are known. They have a very fine and uniform grain, 
 and furnish smooth and hard surfaces. They are excellent for hone-stones and 
 for polishing, etc. The Arkansas Novaculite appears to be such a deposit, but 
 may be a chemical precipitate. 
 
 FIG. 489. An abandoned quarry in natural 
 cement rock or water-lime. The pillars which 
 support the roof are remnants of the water- 
 lime. Rondout, N. Y. 
 
CHAPTER XIX 
 
 DEFORMATION OF THE ROCKS OF THE 
 EARTH'S CRUST 
 
 ALL rocks are subject to deformation after they have come into 
 existence. These deformations disturb the original arrangement 
 of the material of the rock, and modify the original structure which 
 the rock assumed in formation. Such changes are of a superinduced 
 nature, and they are produced by forces other than those operative 
 in the formation of the rock, or by these forces acting upon it in 
 a new way. Deformation structures are therefore secondary, as 
 compared with the primary or original structures. 
 
 EFFECTS OF DEFORMATION 
 
 The effects of deformation are of several kinds, the most impor- 
 tant being: (i) production of new structures in the rock masses; 
 (2) change in the character of the material (metamorphism) ; (3) 
 superficial disturbances of the earth's surface (earthquakes) ; (4) 
 changes in topography (formation of mountains, etc.). In the pres- 
 ent chapter we will consider only the changes in structure and 
 incidentally their topographic expression. Metamorphism and 
 earthquakes will be discussed in subsequent chapters. 
 
 TYPES OF DEFORMATION STRUCTURES 
 
 In general we may consider four types of deformations; viz. 
 (i) Foldings and warpings of the rocks; (2) fracturing with dis- 
 placement of rocks, or faults; (3) fracturing without displacement, 
 or joints; (4) slaty cleavage. Although all rocks are affected by 
 deformation the effects are most readily recognized in the stratified 
 series, and for that reason the illustrations will be taken from 
 these. 
 
 582 
 
Deformation by Folding 
 
 DEFORMATION BY FOLDING 
 Inclined Strata 
 
 583 
 
 In the mountain regions of the earth it is generally seen that the 
 stratified rocks lie in positions deviating markedly from the hori- 
 zontal (Fig. 490), the latter, as we have learned, being the approxi- 
 mate position in which they are deposited, barring minor local 
 variations as in deltas, around coral reefs, etc. It is true that in 
 some mountains, as in the Catskills, parts of the Rocky Mountains, 
 
 FIG. 490. Folded strata south of Heaven's Peak, Livingston Range, 
 Montana. (Limestones and argillites of Algonkian age.) Vertical range 2000 
 feet. (U. S. G. S. ; courtesy of D. W. Johnson.) 
 
 and elsewhere, the strata may still appear horizontal, but this is 
 rather the exception than the rule. Again, there are regions of the 
 earth, such as eastern New York and New England, numerous areas 
 in Great Britain, Ireland, and parts of the northwestern continent 
 of Europe (Belgium, Northwest Germany, etc.) (Fig. 491), where 
 the strata are not horizontal, although the regions are not now 
 mountainous. Such regions may generally be considered as the 
 sites of former mountain ranges, which have long since been worn 
 away, leaving only their stumps exposed in what is now level or un- 
 dulating country. In such regions the most general aspect of the 
 strata is that of rising at an angle or vertically from the surface and 
 ending abruptly in the air, and their appearance may be compared 
 
584 Deformation of Rocks of the Earth's Crust* 
 
 with that of the leaves of a book standing on end or resting in an 
 inclined position. This comparison is, however, apt to mislead, 
 because the leaves of the book are unrelated to the base on which 
 
 FIG. 491. The Lorelei rock on the Rhine, formed by inclined Devonian 
 shales and sandstones. The summit of the cliff is a part of the peneplane which 
 has beveled all the strata of this region. (See Fig. 6oO 
 
 the book rests. A better illustration of the simplest form of in- 
 clined strata is furnished by the metal sheeting of a nearly flat roof, 
 when this sheeting is bent up at one end to form a leader or gutter 
 for the raia water. The inclined strata represent the bent-up por- 
 tion of this sheeting, the continuation in the horizontal or slightly 
 sloping part being covered. For it must be clearly understood that 
 inclined strata do not continue downwards into the earth with the 
 ,--.. same inclination for 
 
 an indefinite dis- 
 tance, but that be- 
 low the surface they 
 either bend upward 
 again, or in the 
 simplest case as- 
 
 FIG. 492. Diagram to illustrate the underground 
 extension of inclined strata which crop out on the sur- 
 face, and the former relationship of the two inclined 
 beds A and B ; S, soil. 
 
 sume a nearly hori- 
 zontal position. 
 This is illustrated 
 
 in diagrammatic 
 
 manner in Figure 492 at A , where the inclined bed, rising above the 
 surface of soil (s), is seen to be merely the bent-up portion of the 
 bed which beneath the surface changes again to the horizontal. 
 
Deformation by Folding 
 
 585 
 
 Moreover, it is obvious that the upper edge of the inclined bed, 
 which now ends in the air, was not its original end but that it is 
 the cut or eroded edge of a formerly much more extensive bed which 
 continued beyond the present point of outcrop (in our diagram to 
 the right) for an unknown distance, as suggested by the dotted lines. 
 Indeed, it often happens that the continuation of the interrupted 
 bed, recognized by its character, its fossils, or otherwise, is found 
 again at another point (in our diagram farther to the right), where 
 it may be represented by another inclined layer descending into the 
 earth in the opposite direction. In that case it is evident that the 
 
 FIG. 493. Diagram illustrating the relationships of dip and strike of the 
 inclined strata of an outcropping series of ledges. The strike is represented 
 by the line parallel to the horizon (horizontal line) ; the dip by the line at right 
 angles to the strike. 
 
 two outcrops of inclined layers are parts of an arch, the top of which 
 has been cut away by erosion. Inclined layers ending in the air 
 may be produced in other ways (faulting, etc.), but in practically 
 all such cases the inclined part visible is only a portion of a much 
 more extensive bed which continues horizontally or otherwise in 
 one direction and the cut-off edge of which was formerly continuous 
 in the other direction. 
 
 Dips. In the study of the deformational structures of rocks it 
 becomes necessary to measure the amount of inclination of the 
 strata and the changes from place to place. The angle of inclina- 
 tion from the horizontal is called the angle of dip (Fig. 493), and 
 it is measured by an instrument -called the clinometer. 
 
 A simple form of clinometer can easily be made by any one. A piece of card- 
 board or wood with parallel sides, or the inside of a firm note-book cover, forms 
 the foundation. A graduated semicircle of paper or metal is then fastened to 
 this, so that the ends of the semicircle are on a line parallel with the upper and 
 
586 Deformation of Rocks of the Earth's Crust 
 
 lower e.dges of the supporting board. In the center of this line the end of a 
 thread is fastened (by a pin or through a hole in the board and a knot in the 
 thread), and a small weight is suspended at the other end in such a way that 
 
 FIG. 494 a. A home-made clinometer. (For description see text.) 
 
 it hangs below the arc but still above the lower edge of the supporting board 
 (Fig. 494 a). If the work is properly done, the thread should cut the middle of 
 the graduated semicircle when the lower edge of the board rests tipon a hori- 
 zontal surface. This point is marked zero. When the lower edge of the 
 
 board (or the upper, which 
 is parallel to it) rests against 
 a vertical plane, the string 
 should cut either end of the 
 arc, and this is marked 90. 
 If the surface on which the 
 clinometer is placed slopes 
 halfway between these two 
 extremes, the string cuts the 
 semi-circle at a corresponding 
 point, and this is marked 
 45. The other divisions are 
 marked accordingly. Those 
 
 FIG 494 b. Diagram illustrating gently who desire may obtain dabo _ 
 inclined strata, with the clinometer in proper j constructed clinom- 
 
 position to measure the dip at right angles to 
 
 the strike. (Drawn by Mary Welleck.) eters > but the Beater ac- 
 
 curacy of observation with 
 
 them will scarcely be of much value because of the almost constant variation 
 in dip from point to point. 
 
 Strike. The intersection of an inclined bed with a horizontal 
 surface is called the strike of the outcrop (Fig. 493), and its direc- 
 
Deformation by Folding 587 
 
 tion is measured by a compass, the' dial of which is graduated to 
 degrees. Thus if this line of intersection extends in a direction 
 30 degrees east of north, the strike is recorded as N. 30 E. (or in 
 exceptional cases S. 30 W.). The dip is then recorded as either 
 east or west, though in the case cited it would really be north of 
 west or south of east, because it is at right angles to the line of strike. 
 Thus a record of strike N. 30 E. and dip 25 W. means that the 
 angle of dip is 25 to the westward, but that the actual direction 
 of the dip (which is at right angles to the strike) is W. 30 N. (or 
 N. 60 W.). Upon a map these facts are recorded by the symbol 
 T, the cross-arm of which is placed in the direction of the strike and 
 the stem in the direction of the dip. As the position of the cross- 
 arm upon the map indicates the direction of strike, it is not neces- 
 sary to record this in figures (except for greater accuracy), but the 
 angle of dip is recorded opposite the stem of the T- Thus if the 
 strike is due north and south, a direction represented by the edge 
 of this page, and the dip is 25 to the west, the symbol would have 
 this position: 25 f 
 
 Deflection of Strike. If the surface on which an inclined stra- 
 tum crops out is not horizontal, but inclined, it becomes evident, 
 on reflection, that the direction which the outcrop takes upon 
 such a surface is not that of the true strike, but a deflection of the 
 same, the amount of deflection depending on the angle of dip of the 
 stratum and the angle of slope of the surface. Only if the dip of 
 the bed is 90, or vertical, will the line of outcrops be the same, 
 whatever the slope of the surface. The mode of deflection is illus- 
 trated in the following diagram (Fig. 495). A BCD represents 
 
 FIG. 495. Diagram illustrating the deflection of outcrop of an inclined 
 stratum, a, b, c, d, from the true strike be, on a horizontal surface (EFGH) to 
 a false strike (cV) on an inclined surface, A BCD. , 
 
588 Deformation of Rocks of the Earth's Crust 
 
 the sloping surface of the ground, EFGH a horizontal plane ; abed 
 is the inclined bed, the direction of true dip of which is indicated 
 by the sides of this bed. It is evident that the true strike is the 
 line be, the intersection with the horizontal plane, whereas, the 
 actual outcrop of this stratum on the sloping surface is represented 
 by the line b'c. 
 
 The degree of deflection of the outcrop from the true strike can be calcu- 
 lated from the following formula: 
 
 tan ^ = cot 6 tan <, or ^ = tan- 1 (cot tan <). 
 
 when ^ is the angle of deflection ; 6 the angle of dip of the stratum, and < 
 the angle of inclination of the sloping surface from the horizontal. 1 When it is 
 possible to get the true strike on a horizontal surface and its deflection on a 
 sloping surface, the angle of dip ( 6} may be calculated by the following formula : 
 tan 0=tan< cot ^, or 6 = tan- 1 (tan < cot^). 
 
 The illustrations in Fig. 496, a, b show the manner in which 
 the outcrops of inclined beds are deflected along the sides of 
 valleys with different slopes. 
 
 
 FIG. 496. Models showing deflection of outcrop of dipping strata on sloping 
 surfaces, a, slope of valley 40, dip of strata 20 ; b, slope of valley 20, dip 
 of strata 50. (After Lyell.) 
 
 Relation between Dip and Width of Outcrop. When a stratum 
 of rock is tilted to the vertical and ends in a horizontal erosion sur- 
 face, the width of the outcrop on the surface is equal to the thickness 
 of the stratum. If the tilting is less than vertical, the width of the 
 outcrop on the horizontal surface will be greater than the thickness 
 of the bed, and will increase in proportion as the dip of the stratum 
 decreases. From the width of the outcrop and the angle of dip, 
 the thickness of the bed may be ascertained by simple mathematical 
 calculation. 
 
 1 For fuller discussion, see A. W. Grabau, Principles of Stratigraphy, pp. 800-806. 
 
Deformation by Folding 
 
 589 
 
 Thus in the following diagram (Fig. 497) the width of the outcrop is shown 
 oy AB, and the thickness of the bed to be ascertained by AC, which is a line 
 at right angles to the upper and 
 lower surfaces. The dip is the 
 angle ABC, which is one angle 
 of a right-angled triangle of which 
 the length of the hypotenuse, 
 AB, is known and the side AC 
 to be ascertained. This is found 
 by the following formula : 
 
 or 
 
 FIG. 497. Diagram illustrating width 
 of outcrop AB of the bed, the thick- 
 ness of which is AC; its depth at X is 
 measured by the length of XY. 
 
 AC=sin ABCXAB. 
 If the angle ABC, that is, the 
 dip, is 30 degrees, and the width 
 
 of the outcrop AB 100 feet, we have AC = sin 30 X 100 = 0.5X100 = 50 feet. 
 In like manner, if the thickness of a bed and its dip are known while the outcrop 
 is mostly covered by soil, its width for purposes of mapping can be determined 
 
 by the formula : AB = 
 
 AC 
 
 sin^BC 
 
 , if the thickness, AC, is 50 feet, and the 
 
 dip 30, gives A B = ^- = ioo feet. If the bed in question is a coal seam or a 
 
 o-5 
 
 water-bearing stratum, its depth below the surface at any point within the 
 belt of outcrop may be determined by noting, the distance from the outcrop and 
 the dip. Thus at X the depth X Y to the bed is to be ascertained. The dis- 
 tance A X is measured, and the dip XA Y determined. Then we apply the 
 
 formula : tan XA Y = or X Y = A X tan XA Y. With the dip 30 and the 
 
 yl J\. 
 
 distance AX 300 feet, we have XF = 3ooXtan 3o = 3ooXo.5774=i73.22 
 feet. If the dip of a water-bearing stratum is i and the distance from the 
 point of outcrop 50 miles, while the elevation of the outcrop is 1000 feet above 
 the point at which the well is to be sunk, the depth beneath the surface will be 
 (tan i X 50 X S 280) 1000 = (.01 75 X 5oX 5 280) 1000 = 4620 1000 = 3620 feet. 
 This assumes that the dip is constant, which for so low an angle in a region of 
 little disturbance is likely to be the case. 
 
 Types of Folds 
 
 Three principal types of simple folds are recognized : (a) anti- 
 clines, (b) synclines, and (c) monoclines. Each of these has various 
 modifications. 
 
 The Anticline. Arched folds are called anticlines (Fig. 498), 
 and their sides, which are called the limbs, dip away from the crest- 
 line or axis of folding. Such folds may vary from broad and gentle 
 to sharply ridged arches. If both limbs dip at the same angle, 
 
590 Deformation of Rocks of the Earth's Crust 
 
 the anticline is called symmetrical, and this type is most character- 
 istic of the Jura Mountains of Switzerland (Fig. 499, a), though ex- 
 
 amples are also found 
 in the Appalachians 
 (Fig. 500) and else- 
 where. If one limb 
 has a steeper dip than 
 the other, the anti- 
 cline is asymmetrical 
 (Figs. 499, b, 501). 
 In the Appalachian 
 Mountains, the folds 
 of which have their 
 axes extending in a 
 northeasterly direc- 
 
 FIG. 498. A symmetrical anticlinal fold, the top 
 of which has been eroded. Near St. Abb's Head, 
 Scotland. (After Geikie.) 
 
 tion, the western limb 
 of the anticline is 
 generally steep, often 
 vertical or even overturned, while the eastern limb is more gentle. 
 Overturned anticlines (Fig. 499, c) are characteristic of some strongly 
 folded regions, and in these one limb comes to lie under the other, 
 and the beds of this limb appear in the reversed order. If the 
 overturning is so extreme that the limbs lie nearly or quite hori- 
 
 FIG. 499. Types of folds, a, symmetrical anticline ; b, asymmetrical anti- 
 cline ; c, overturned anticline ; d, recumbent fold ; e, fan- fold. 
 
 zontally, the folds are said to be recumbent (Fig. 499, d). Over- 
 turned strata, can often be recognized as such by the position on 
 their surfaces of ripple-marks, rain-prints, foot-prints, rill marks, 
 mud-cracks, and other structures. These will be seen on the under 
 side of overturned strata, while their reverse impressions will ap- 
 pear on the upper side, both being the reverse of the normal posi- 
 tion. 
 
 Fan-shaped folds (Fig. 499, e) are produced when the lower parts 
 of the limbs of the anticline are so^ compressed that the upper part 
 bulges beyond them on both sides. Such folds are found in the 
 Alps (Fig. 523, p. 606). 
 
Deformation by Folding 
 
 When the limbs of the fold are parallel or nearly so, an isoclinal 
 folding is produced (Fig. 502). It will sometimes be difficult to 
 
 FIG. 500. Classic arch or symmetric anticline of Upper Silurian red sand- 
 stone (in Wills Creek formation), Roundtop, Md. Banks of the Chesapeake 
 and Ohio Canal. The center of the arch has crumbled away, leaving a cavern. 
 (Photo by Stose, from U. S. G. S.) 
 
 FIG. 501. A small asymmetric anticline in Upper Silurian shales and sand- 
 stones, High Falls, N. Y. 
 
 determine the structure of an isoclinal series of folds if the top has 
 been eroded away. In such a case all the beds may appear to be- 
 long to one series (Fig. 502, a), unless it can be shown by the charac- 
 
592 Deformation of Rocks of the Earth's Crust 
 
 
 
 ter of the beds and by their fossils and otherwise that there is a 
 repetition of the same strata in the series. In the diagram (Fig. 
 502) such a regular repetition of strata is shown, but even here it 
 
 FIG. 502. Isoclinal folds, a, outcrop of strata in which there is a certain 
 repetition of similar beds, which by their fossils or otherwise are recognized 
 to be repetitions of the same strata; b, restoration as two anticlines and a 
 syncline ; in this case, 4, 4 is the oldest bed ; c, restoration as two synclines 
 separated by an anticline ; in this case, 4, 4 is the youngest bed. (From Prin- 
 ciples of Stratigraphy.) 
 
 is possible to reconstruct the series in two ways, as anticlines with 
 a narrow syncline between (Fig. 502, ), or as synclines separated 
 by an anticline (Fig. 502, c). The correct reconstruction depends 
 upon the recognition of the relative ages of the beds, the older being 
 at the centers of the anticlines, and the younger at the centers of 
 the synclines. If this cannot be determined, recourse may be 
 
 FIG. 503. Eroded folds, a, Symmetrical anticline, from the axis of which 
 the higher beds have been eroded down to a harder stratum; b, breached asym- 
 metrical anticline, the hard stratum forming two opposing uniclines. 
 
 had to structures such as ripple-marks, mud-cracks, etc., which 
 will indicate the upper and under sides of the strata. It is apparent 
 that the upper sides of the strata will be on the inside of the syn- 
 cline but on the outside of the anticlines. 
 
Deformation by Folding 
 
 593 
 
 In general, when folding is pronounced, the beds in the axial 
 part of the anticline as in that of the syncline (see beyond) are 
 thickened, while those of the limbs become thinner. This reaches 
 its extreme in fan-folds, but is also observable in others. It im- 
 
 FIG. 504. Block-diagram of the uniclinal ridges of the Appalachian type; 
 the original anticline from which the uniclines are cut was asymmetric, with 
 the steeper limb on the west. 
 
 plies a certain amount of transference of the material of the beds 
 from the limbs, to the axes. 
 
 Small anticlines may be complete, but the larger structures 
 of this type have commonly suffered erosion along their tops. 
 This is comparatively slight in the Jura Mountains, where large 
 anticlines are still nearly complete with only their upper layers 
 partly removed from the axis (Fig. 503, a). In the Appalachian 
 Mountains, on the other hand, the anticlines have been for the most 
 part deeply dissected so that a depression or valley lies at their axes 
 and the cut ends of the limbs end in the air (Figs. 503, b, 504). If 
 
 FIG. 505. Uniclinal ridges formed by the erosion of an anticline. Utah. 
 (F. J. Pack, Photo.) 
 
 the folds of the original anticlines consist of alternating hard and 
 soft strata, a series of parallel ridges formed by the hard layers, and 
 valleys formed on the soft layers may be produced as the result of 
 erosion. The beds of corresponding ridges will dip in opposite 
 directions. This is of common occurrence in the Appalachians, 
 
594 Deformation of Rocks of the Earth's Crust 
 
 and to such erosion remnants of anticlinal folds, the name unicline 
 is applied (Fig. 505). (See further, Chapter XXII.) 
 
 Pitch of Axis of Anticline. The axis of the anticline may con- 
 tinue in a horizontal position for a long distance (sometimes for 
 
 FIG. 506. Diagram showing pitching folds and the topography formed by 
 their erosion. (Drawn by F. K. Morris, Military Geology.) 
 
 more than a hundred miles), but eventually it descends or pitches 
 into the .ground and the fold dies away (Fig. 506). When the 
 pitching axis of an anticline is planed across horizontally by erosion, 
 
 FIG. 507. Eroded anticlines with horizontal and with pitching axes. In 
 the first case the strata crop out in parallel series, the oldest at the center; in 
 the second case they converge in the direction of pitch. (From Principles of 
 
 Stratigraphy.) 
 
 the upper ends of the several beds will be seen to converge in 
 the direction of the pitching and eventually to meet (Fig. 507). 
 When some of these beds form ridges, these too meet and enclose 
 a semicircular canoe-shaped valley (Fig. 508). 
 
Deformation by Folding 
 
 595 
 
 Domes. Anticlines with very short axes are called domes. The 
 length of the axis may be several times the transverse diameter of the 
 dome, or it may nearly equal it, but domes are seldom quite circular 
 in basal outline. Such domes may be pronounced, with steeply 
 dipping sides, as in the case of the Black Hills of South Dakota, or 
 
 FIG. 508. Development of concentric ridges in anticlinal structures. A . An 
 anticlinal or cigar-shaped mountain results from the position of the hard rock. 
 B. The anticlinal mountain A is replaced by an anticlinal valley because of 
 the position of the strata. (After A. K. Lobeck.) Note that the cut faces of the 
 ridge-forming strata face inwards. These ridges are "monoclines of erosion" 
 or "uniclines." 
 
 they may be so gentle that the dip of the strata is scarcely perceived. 
 The latter are by far the more common, and of them the Cincinnati 
 dome may serve as an example. On account of their large size 
 and gently dipping strata they are recognized only when plotted 
 upon a geological map. 
 
 Anticlinoria. A mountain mass composed of a number of anti- 
 clines may have these so arranged that they form a part of a larger 
 arch. This compound anticlinal series is called an anticlinorium. 
 
 FIG. 509. Anticlinorium. Generalized section in the Alps. (After Heim.) 
 
 Such an anticlinorium may constitute a part of a mountain mass, as 
 in the Alps (Fig. 509) ; or the central beds may be eroded because 
 these are soft, and so a valley is produced. The Wealden district 
 of southeastern England is such an eroded anticlinorium, the minor 
 foldings being seen in the variable dips of the strata in the center, 
 which, because of the softness of the strata, has generally been 
 transformed by erosion into a valley. 
 
596 Deformation of Rocks of the Earth's Crust 
 
 Synclines. When two or more anticlines occur in a folded sys- 
 tem such as the Jura or Appalachian Mountains, they are separated 
 by trough folds or synclines (Figs. 510,511). These are symmetrical 
 
 FIG. 510. Natural section, showing two anticlines and a syncline. The 
 surface is a peneplane cutting across the folds. Scotland. (After Geikie.) 
 
 FIG. 511. Synclinal fold near Banff, Scotland. (Geikie.) 
 
 in the Jura but asymmetrical in the Appalachians, corresponding in 
 character to the anticlines. In the Appalachians the eastern limb 
 (or arm) of the syncline is steep, vertical, or overturned, being in 
 
Deformation by Folding 
 
 597 
 
 FIG. 512 a. Rapid change in thickness of beds, due to strong folding, w 1 , 
 Neocomian ; n 2 , Schrattenkalk ; g, Gault ; Kr, Seewer limestone (Upper Cre- 
 taceous). After Alb. Heim. Santis. (From Kayser's Lehrbuch.) 
 
 FIG. 512 b. Synclinal fold of Knox dolomite, showing thinning on the 
 limbs of the fold and thickening in the axis. One half mile north of Embree- 
 ville, Tenn. (Photo by Keith, from U. S. G. S.) 
 
598 Deformation of Rocks of the Earth's Crust 
 
 fact the western limb of the anticline next east. The western limb 
 (or arm) of the syncline, which is also the eastern limb of the anti- 
 cline next west, has a moderate inclination. In the- center of the 
 syncline the beds are usually thickened, as is well shown in the sec- 
 tion of Alpine folds in Fig. 512 a. It is also seen in the Appala- 
 chian Mountains (Fig. 51 26). This portion of the fold may also 
 be characterized by numerous smaller folds, this complex when 
 
 FIG. 513. Synclinorium. Section of Mt. Greylock, Mass. (After Dale.) 
 
 occurring on a sufficiently large scale being called a syndinorium 
 (Fig. 513). Synclines may also be overturned and recumbent, and 
 their limbs may be parallel, forming a part of an isoclinal series of 
 folds as in Fig. 502, p. 592. 
 
 Topographically, the beds of a much eroded syncline may also 
 form parallel ridges, but the cut edges of these face outward, instead 
 
 FIG. 514. Development of concentric ridges in synclines. A, a region 
 with synclinal or canoe-shaped folds. In B the synclinal valley shown in A 
 is replaced by a mountain because of the occurrence of a hard stratum in the 
 center. (A. K. Lobeck.) Compare with Fig. 508, p. 595. Note that the cut 
 edges of the hard ridge-forming strata face outward. The ridges, too, are 
 "monoclines of erosion" or uniclines. 
 
 of inward as in the case of the dissected anticline (Fig. 514). For 
 such ridges the term unicline is used. They are also called 
 " monoclinal ridges of erosion." 
 
 As the syncline dies out, the axis rises, and in eroded synclines 
 the cut edges of the strata and the ridges which they form converge 
 and finally unite. Thus another series of rimming ridges and canoe- 
 shaped valleys is produced. 
 
Deformation by Folding 
 
 599 
 
 Chief Min 
 
 Lewis Range 
 
 By deep erosion of the centers of adjoining anticlines, the center 
 of the syncline may be left standing in relief and a synclinal 
 mountain is produced (Fig. 
 515, see also Figs. 513 and 
 514, B). Such mountains are 
 not uncommon in the Appala- 
 chian and other deeply eroded 
 mountain systems. 
 
 Basins. A syncline with 
 much shortened axis produces a 
 basin, which is the reverse of 
 the dome. The majority of 
 basins have their strata so 
 gently inclined that they appear 
 to be practically horizontal. 
 They can be recognized, how- 
 ever, in eroded regions by the 
 fact that the edges of the older 
 or lower strata project progres- 
 sively beyond those of the 
 younger or upper, while their 
 outcrops form a more or less 
 completely encircling series of 
 bands. Where hard strata are 
 involved, these form rimming 
 ridges with their cut ends facing 
 outward. Such a series of rim- 
 ming cliffs partly encircles the 
 Paris basin, in the center of 
 which lies the city which gives 
 it its name. The basin in the 
 center of which London lies is 
 another, though less perfect ex- 
 ample. In our own country, 
 the Michigan basin, which com- 
 prises the whole of the Lower 
 Peninsula, is a typical example. 
 
 On the west, north, and east of this basin the lowlands cut on the 
 softer strata are partly occupied by the waters of some of the Great 
 Lakes and their embayments (Figs. 516, 517). 
 
 Livingston 
 
6oo Deformation of Rocks of the Earth's Crust 
 
 The Monocline. A simple flexure in the rocks in which the beds 
 bend up or downward and then continue in the same direction, is 
 called a monocline. Such monoclines are found on the margins of 
 
 folded areas and in 
 
 ' VJ /5^^r\ regions of minor dis- 
 
 turbance as in the 
 Colorado Plateaus 
 (Figs. 690 a, b), where 
 they form important 
 structural features. 
 A monocline may 
 pass into a fault as 
 shown in the diagram 
 (Fig. 518 a). The 
 monoclines of the 
 Colorado Plateau 
 region (Grand Canon 
 district) commonly 
 pass into what may 
 be termed an in- 
 verted fault, where 
 the upflexed side 
 becomes the down- 
 throw side of the 
 fault, and the down- 
 flexed, the upthrow 
 side of the fault 
 
 (Fig. 5186). In these faults the beds of the upthrow block bend 
 upwards as they approach the fault plane and those of the down- 
 throw block bend downward. This probably indicates two move- 
 ments, a flexing to the breaking point, and a settling back with 
 faulting, of the elevated block. 
 
 Relation of Folded Mountain Series to Geosyndines of Deposition 
 
 All the larger folded mountain systems appear to be located along 
 regions which were formerly geosynclines of deposition (p. 518). 
 This is shown by the fact that in such folded areas the formations 
 are all of much greater thickness than elsewhere. Several of these 
 may be noted. 
 
 FIG. 516. Geological map of the Michigan 
 Basin, showing the successive rims formed by the 
 outcrops of the strata around the central coal 
 basin. (Mich. Geol. Survey.) 
 
Deformation by Folding 
 
 60 1 
 
 The Appalachian Mountains. These were 'formed by the fold- 
 ing of the strata which accumulated in the Appalachian geosyncline 
 to a thickness of perhaps 40,000 feet. This geosyncline was bounded 
 
 on the east by a high more or less mountainous land mass, the an- 
 cient Appalachia, from which the clastic material which makes up 
 a large part of the strata was chiefly derived. The series comprises 
 alternating marine and continental beds, but on the whole is 
 rather uniform, and constant subsidence is indicated for the old 
 
602 Deformation of Rocks of the Earth's Crust 
 
 FIG. 518 a. Monocline replaced in 
 the lower strata by a fault. 
 
 the steep or overturned limb of 
 would seem that the movement 
 which produced the folds came 
 from the east, or from the Ap- 
 palachian old land. The most 
 intense folding is on the east, the 
 next upon the western border 
 (Fig. 519). There are also pro- 
 nounced overthrusts (see p. 626). 
 Beyond the strongly folded west- 
 ern part minor folds occur, but 
 soon die out, and the strata be- 
 come approximately horizontal. 
 The force, however, appears to 
 have been transmitted over a great 
 part of eastern North America, 
 with the development of an ex- 
 tensive series of low domes and 
 shallow basins, such as the Cin- 
 cinnati dome and Michigan basin. 
 Some, or perhaps most, of these 
 had begun to form at an earlier 
 time, probably during one or more 
 of the earlier periods of folding 
 but their most pronounced struc- 
 ture was given to them during 
 the Appalachian folding. It must 
 be remembered that these domes 
 and basins are so large and so 
 gentle that the strata appear hori- 
 zontal and their dome- and basin- 
 character can be seen only on the 
 
 geosynclinal trough. Folding 
 occurred locally at several 
 periods, but the great folding 
 took place toward the end of 
 the Palaeozoic era. This pro- 
 duced the asymmetrical anti- 
 clines and synclines character- 
 istic of this system. Because 
 the anticlines is on the west, it 
 
 FIG. 5186. Diagram showing 
 early stages in the development of 
 the Hurricane fault in the Grand 
 Canon region, a monoclinal 
 fold develops into an "inverted" 
 fault, the upflexed side becomes 
 the downthrow side, the down- 
 flexed the upthrow. Note that 
 the strata at the border of the 
 fault plane bend in the reverse" 
 direction of that due to drag along 
 a simple fault. The upper dia- 
 gram represents the monoclinal 
 flexure; the middle, the faulting; 
 and the lower, the appearance after 
 erosion (peneplanation). This was 
 followed by further movements 
 in the same direction along the 
 fault plane. East on the left. 
 (After D. W. Johnson.) 
 
Deformation by Folding 
 
 603 
 
 study of extended areas. Wherever two basins adjoin each other, 
 there is a sharp but small anticline or series of anticlines formed 
 between them, as shown in the lower sections on this page (Fig. 
 
 FIG. 519. Restoration of the Appalachian folds of Pennsylvania between 
 Harrisburg on the east (right) and Tyrone on the west (left), showing the differ- 
 ent intensities of folding, and the portion of the earth's crust (ABC) affected 
 by the compressive movement. (After R. D. Chamberlin.) 
 
 520). In some cases these anticlines have proved favorable for the 
 accumulation of oil and gas. In like manner, a syncline or group 
 of synclines lies between adjoining domes. 
 
 The trend of the Appalachian folds conforms more or less to the 
 position of the domes and basins adjoining them, being bowed out- 
 ward around the basins and inward between them (Fig. 521). 
 
 a 
 
 FIG. 520. Cross-sections of shallow basins, separated (a) by a simple anti- 
 cline, (6) by two anticlines and a syncline. 
 
 The Carpathians. As a second example, the Carpathian 
 Mountains of Europe may be cited. These form a semicircular 
 arc, enclosing the basins of Hungary and Transylvania on the 
 north, east, and south, and they are a part of the Alpine system of 
 folds (map, Fig. 522). They are of much more recent origin than 
 the Appalachians, having been formed in Tertiary time. The out- 
 line of this arc appears to have been determined by the form of 
 
604 Deformation of Rocks of the Earth's Crust 
 
 the geosyncline in which the strata, which became involved in the 
 folds, were deposited. The old land from which the clastic material 
 was derived lay to the north and the east of the present mountain 
 chain, and the sediments were carried toward the Hungarian basin, 
 most of them lodging in the geosyncline, around its outer (eastern 
 and northern) margin. 1 The first folding took place at the begin- 
 ning of Mid-Tertiary (Miocene) time, and resulted in the formation 
 of mountain chains from the deposits in the geosyncline. The 
 movement was probably toward the Hungarian basin. During 
 
 FIG. 521. Map of the domes and basins of eastern North America, and of 
 the outline of the Appalachian chains. 1-6, land lobes ; i a-6 a, sea lobes. 
 
 this folding, or probably as a part, if not a cause of it, a new geo- 
 syncline came into existence by the sinking of a part of the old land 
 which had supplied the sediments. This sinking occurred along a 
 belt parallel to and outside of (nqrth and east of) the newly formed 
 semicircle of mountains. In this new geosyncline, later sediments, 
 to the extent of several thousand feet, were deposited, these being 
 in large part derived from the newly formed Carpathians. In this 
 geosyncline were also formed many of the important salt deposits 
 of that region. Then came a second folding, this time with a move- 
 ment toward the new geosyncline (eastward and northward) so that 
 the strata of this geosyncline were thrown into asymmetric anti- 
 
 1 A. W. Grabau, Geology of the Non-Metallic Mineral Deposits, Vol. II, Chapter 
 XXXII, McGraw-Hill Book Co. 
 
Deformation by Folding 
 
 605 
 
 > -r '- ' 
 
 +J <-i I I -M t/1 <U 0"^ V 
 
606 Deformation of Rocks of the Earth's Crust 
 
 clines, with steep limbs on the east and north or away from the 
 mountains. At the same time, the already folded strata of the 
 
 earlier Carpathians were folded 
 more intensely, and portions were 
 thrust over toward the Russian 
 regions on the east. Thus the 
 horizontal rocks of Russia and the 
 coal fields of Silesia on the north 
 pass under the folded rocks of the 
 Carpathians, which have over- 
 ridden them. 
 
 The movement toward the Hun- 
 garian plain during the first fold- 
 ing appears to have affected the 
 rocks of this plain also, by break- 
 ing them into a series of blocks in- 
 stead of developing domes and 
 basins as in North America. 
 
 The Alps. These are among 
 the most complex mountain sys- 
 tems known. The main line forms 
 an arc extending around the plains 
 of North Italy, in general north 
 and south along the Franco-Italian 
 border, and turning eastward in 
 Switzerland and the Tyrol (see 
 map, Fig. 522, p. 605). As in the 
 case of -the Carpathians, this arc 
 was outlined by the old geosyncline 
 in early Tertiary time and the form 
 of the old land on the west and 
 north. After deposition of great 
 thicknesses of clastic material, de- 
 rived from this old land, had taken 
 place in the geosynclines, the first 
 folding occurred at the beginning 
 of Mid-Miocene time. The move- 
 ment there was apparently toward 
 the Italian region, which was 
 broken into blocks, while the 
 
Deformation by Folding 
 
 607 
 
 strata were overturned toward that region, 
 thians, another new geosyncline came into 
 land, north and west 
 of the newly formed 
 chain, and in this 
 geosyncline great 
 masses of clastic ma- 
 terial, derived from 
 .the newly formed 
 Alps, were deposited. 
 Then came the sec- 
 ond folding toward 
 the new geosyncline 
 (west and north) so 
 that the strata were 
 folded over in the 
 other direction, while 
 at the same time 
 great blocks were 
 thrust over in this 
 direction. In this 
 manner was probably 
 formed the peculiar 
 fan structure of the 
 Alpine folds (Fig. 
 523)- 
 
 Structures Due to Re- 
 peated Folding 
 
 In the Alps, Car- 
 pathians, and other 
 great mountain sys- 
 tems of Europe and 
 elsewhere, at least 
 two periods of folding 
 and perhaps more are 
 recognized, following 
 each other with com- 
 paratively short in- 
 tervals. These two 
 
 As in the Carpa- 
 existence in the old 
 
 fgJie?ij 
 
 R fe 
 
 O T3 
 
 G ^gl *..* *s a 
 
 ' % $ s a * 
 
 3 H s'5 g .o ^ ^ 
 
608 Deformation of Rocks of the Earth's Crust 
 
 .a ^ 
 
 ^ -2 
 So 
 
 3 
 
 "<Q O 
 13 ^ 
 
 1 I 
 
 * & 
 
 < 
 
 r* CJ 
 
 5^ 5 W 
 
 s ^ 
 
 <^i 
 ^s 
 
 ^H ^3 
 
 1- 
 
 M 
 
 a 
 
 I! 
 
 S 
 
 u 
 
 t \ 
 
 'S vd 
 h N 
 ^ " 
 
 2 
 
 
Deformation by Folding 
 
 609 
 
 foldings appear to have been in opposite directions, the last move- 
 ment being toward the west and northwest in the French Alps 
 (Fig. 524), and toward the north in the Swiss-Italian and Austrian 
 Alps (Fig. 525) ; while in the Carpathians it was northward in the 
 northern, eastward in the eastern, and southward in the southern 
 portion of the arc. The results of this compound folding have 
 produced extremely complex mountain systems, the complexity 
 
 FIG. 527. Photograph of the unconformity between the Silurian and 
 Ordovician formations on the Vlightberg above Rondout (Kingston), N. Y. On 
 the right are the Hudson River sandstones and shales with a strike of N. 30 W. 
 and a dip of 51 to the northeast. Against the eroded surface of these rest 
 the Upper Silurian limestones (Cobleskill) which strike N. 56 E. and dip 72 
 to the northwest. The large inclined masses of limestone in the left half of 
 the view are the highest beds of the Silurian (Manlius) which originally had 
 the same inclination as the Cobleskill, the space between these two series being 
 occupied by the Rondout water lime which has been quarried out, after which 
 the Manlius fell over into its present position. (Photo by M. O'Connell.) 
 (See Fig. 528 a.) 
 
 being increased by great overthrusts. In the Caucasus the folding 
 was very intense, so that all the beds are either vertical or steeply 
 inclined (Fig. 526). 
 
 In the Appalachians, where more than one period of folding is 
 recognized, the foldings appear to have been in the same general 
 direction, though in part, at least, with a slight change in strike. 
 Moreover, the movements of at least two of these foldings of which 
 we have positive evidence occurred at long intervals, one near the 
 close of the Ordovician period, the other toward the close of the 
 
6io Deformation of Rocks of the Earth's Crust 
 
 Permian, although there were, in some localities at least, others 
 between these. In the intervals between the foldings, extensive 
 erosion of the older folded series took place, after which more strata 
 were deposited upon these eroded surfaces, and the whole complex 
 folded again. Thus a complicated structure was produced, but the 
 complication is less than in the Alps. The illustration on page 
 609 (Fig. 527) presents the appearance of an outcrop in a hillside 
 above Rondout, N. Y., which shows strikingly the complexity 
 resulting from such double folding. (See also Fig. 528 a.) 
 
 A bed of limestone, S, of Upper Silurian (Monroan) age is seen to 
 have a nearly vertical position. It is part of a thick series which has 
 
 the same position 
 and represents the 
 western vertical 
 limb of an asym- 
 metric anticline as 
 shown in other sec- 
 tions. This belongs 
 to the last period of 
 folding, which was 
 in general toward 
 the northwest as the 
 strike of this bed is 
 to the northeast. 
 The exposed side of 
 this bed is its upper surface. Resting against it on the right is 
 a series of sandy beds (O), which apparently dip at an angle of 
 about 50 to the northeast, or in the direction of strike of the lime- 
 stone bed. These belong to the Middle Ordovician series. From 
 observation of ripple marks, mud cracks, and raindrop impressions, 
 on the overhanging portions of the cliff and other exposures near 
 by, it appears, however, that these beds are completely overturned ; 
 that, in fact, the overhanging surface is the top of the beds (see p. 
 550). This apparent complication is simplified when we attempt to 
 restore the conditions which existed here before the last folding. 
 Turning the bed (S) back to the horizontal, we find that the ap- 
 parent dip of the Ordovician beds (O) is really their strike as it ex- 
 isted after the first folding, and that their dip was vertical, this 
 being transformed into the present strike of these beds by the second 
 folding (Fig. 5286). By this return to the conditions before the 
 
 FIG. 528 a. Diagram showing the relationships 
 of the Ordovician sandstones (0) to the Silurian 
 limestones (S) on the Vlightberg at Rondout, N. Y. 
 Shown in Fig. 527. . 
 
Folding, Erosion, and Deposition 
 
 6n 
 
 second folding, it is seen that the strike of the first set of folds was 
 more nearly north and south instead of northeast, as is that of the 
 later folding in this region. 
 
 If the beds are returned to a vertical position, it becomes apparent 
 that the present under side was the western side of this series 
 after the first folding. As 
 we have seen, this represents 
 the upper sides of the strata, 
 and it thus becomes evident 
 that we are dealing here 
 with the western limb of an 
 anticline of the first period 
 of folding. Since the west- 
 ern limb was vertical, the 
 eastern one must have had 
 
 FIG. 528 b. Relation of the Ordovician 
 sandstones and Silurian limestones of the 
 Vlightberg region at Kingston, N. Y., 
 before the last folding. 
 
 a gentler dip unless the folds 
 
 were isoclinal. Observation of other sections in this region con- 
 firms the supposition of an asymmetric anticline, which thus ap- 
 pears to represent folding in the same general direction, i.e. toward 
 the west, as was that of the later foldings. These conditions be- 
 fore the second folding are restored in the section (Fig. 528 b), 
 which covers a larger field than is shown in the preceding figure. 
 It is by such analysis of the facts, and reasonings therefrom, that 
 the geologist is enabled to reconstruct the conditions which existed 
 at a given period in the earth's history at the locality under ob- 
 servation, and from many such observations over wide areas he is 
 enabled, by combinations, to write a moderately complete chapter 
 of the history of his country at a given period in the far distant past. 
 
 STRUCTURES DUE TO FOLDING, EROSION, AND RENEWAL 
 OF DEPOSITION 
 
 Unconformities and Disconformities 
 
 Unconformities. In the example of complex structure de- 
 scribed in the last section (Fig. 528 b), we have seen that horizontal 
 beds (S) are deposited upon the eroded edges of an older folded 
 series. The two series of beds thus have a discordant relation, their 
 dips being in different directions and of different degrees, in one 
 part at right angles to each other. Such a relationship is termed 
 an unconformity, a term sometimes qualified by prefixing the word 
 
6i2 Deformation of Rocks of the Earth's Crust 
 
 angular. When such an unconformity exists, it always points 
 to the folding and erosion of the older series before the deposition 
 of the younger, which rests unconformably upon the older. Such 
 
 FIG. 529. Typical unconformity between inclined sandstones ("Laramie") 
 below (dipping to the left) and horizontal conglomerates (Basal Wasatch) above. 
 Near Meeteetsie, Wyo. (Photo : C. A. Fisher, from U. S. G. S.) 
 
 a relation implies the existence of a time-interval between 
 the deposition of the two series, during which the folding of 
 the older series and its partial erosion took place. In the example 
 cited, where the older beds are of Middle Ordovician age and the 
 
 younger of Upper 
 Silurian, the in- 
 terval comprises 
 the Upper Or- 
 N x dovician and the 
 
 Lower and Mid- 
 dle Silurian eras. 
 It is probable that 
 the folding of the 
 older series oc- 
 curred in Upper Ordovician time, which would leave the Lower 
 and Middle Silurian periods for the accomplishment of the 
 erosion. 
 
 Fir.. 530. Unconformity at Siccar Point, Berwick, 
 Scotland, a, folded and truncated Siluro-Ordovician 
 beds; d, Old Red Sandstone beds (Devonian). (After 
 Lyell.) See Figs. 531 a, b. 
 
Folding, Erosion, and Deposition 
 
 613 
 
 Unconformities are met with in all geological formations in regions 
 where folding of strata has occurred. Sometimes the later series has 
 not been folded and then it represents horizontal, or nearly hori- 
 
 FIG. 531 a. Irregular, unconformable contact between the vertical (on 
 right) or steeply inclined (on left) Siluro-Ordovician shales, and the gently dip- 
 ping Old Red Sandstone beds in the center and foreground. Note that the older 
 beds often project into the basal beds of the Old Red series, which were deposited 
 in the hollows between. The higher Old Red beds have been removed by sub- 
 sequent erosion. (Photo, M. I. Goldman.) 
 
 zontal, strata resting abruptly upon inclined beds (Fig. 529). This 
 is well shown in the unconformity between the Siluro-Ordovician 
 beds and the Old Red Sandstone at Siccar Point, near St. Abb's 
 Head (Berwick), Scotland (Figs. 530-5316), and at Banff on the 
 
 FIG. 531 b. Near view of the unconformity at Siccar Point, Scotland, 
 showing horizontal basal Old Red conglomerates resting on vertical Siluro- 
 Ordovician shales and sandstones. (Photo, M. I. Goldman.) 
 
614 Deformation of Rocks of the Earth's Crust 
 
 south shore of the Moray Firth (Fig. 532). Other illustrations of 
 unconformities will be cited in the chapters on Historical Geology. 
 
 FIG. 532. Unconformity and disconformity exposed on the coast of Banff- 
 shire, south shore of Moray Firth, Scotland, q, quartzite (Siluro-Ordovician) ; 
 s, Old Red Sandstone ; d, drift resting disconformably upon the Old Red beds, 
 and unconformably against the older series. (After Geikie.) 
 
 FIG. 533. Diagram showing the relationship of the strata in five successive 
 sections, in a compound regressive-transgressive series. The intercalated sand- 
 stone xy encloses the hiatus. (From Principles of Stratigraphy.} 
 
 FIG. 534. Diagram showing the relative magnitude of the hiatus in the 
 various sections shown in Fig. 533. (From Principles of Stratigraphy.) 
 
Folding, Erosion, and Deposition . 
 
 615 
 
 Disconformities. Where the surface of a low dome, such as 
 that of the Cincinnati dome, or the margins of a shallow basin, such 
 
 FIG. 535. Diagram showing the relationships of a transgressive-regres- 
 sive-transgressive series. A hiatus is found at the base of the transgressive 
 series (xz) and between the regressive and upper transgressive series (xy). Ow- 
 ing to the vertical exaggeration of the scale the essential parallelism of the strata 
 is not expressed. (From Principles of Stratigraphy.) 
 
 as the Michigan basin, are eroded and later strata deposited upon 
 the eroded surfaces, the difference in the dip of these two series 
 will be so slight that it cannot be measured, much less seen by the 
 
 FIG. 536. Disconformable contact between the Bedford shales below and 
 the Berea sandstone above, the latter resting in erosion hollows in the former. 
 Near Cleveland, O. (C. S. Prosser, photo.) 
 
 eye. Nevertheless, there may be a pronounced break in continuity 
 between the two apparently concordant series, and this break may 
 represent a long time interval. A similar break in continuity may 
 
616 Deformation of Rocks of the Earth's Crust 
 
 be produced when a series of strata left exposed on the retreat of the 
 sea, after a long time interval is again covered by the advancing 
 sea in which strata of much younger age are deposited in the manner 
 illustrated in the preceding diagrams (Figs. 533-535). When such 
 a break in continuity is established between two successive forma- 
 tions which have a parallel or concordant position, it is designated 
 
 a disconformity. The break 
 and disconformity may be 
 indicated in many ways. The 
 older series may show evi- 
 dence of erosion on its upper 
 surface (Fig. 536) ; pebbles 
 of the older series may be in- 
 cluded in the base of the 
 upper one; an old soil bed, 
 or a bed of eolian sands, 
 may separate two marine 
 formations (Fig. 535), this 
 bed indicating an emergence 
 followed by a submergence. 
 Sometimes, however, no such 
 detailed characters are shown, 
 and then the" disconformity 
 and break are indicated only 
 by the great difference in age 
 FIG. 537. Disconformable contact be- of the two formations and the 
 tween the* Cobleskill limestone (Upper absence of an intermediate 
 Silurian) above, and the Brayman shales . , , , , . 
 
 (Middle Ordovician) below. Near Howe's senes tnat should intervene 
 Cave, N. Y. (C. C. Mook, photo.) (Fig. 537). This difference 
 
 in age is shown primarily by 
 
 the fossils which the rocks contain and by the known succession 
 of the series elsewhere. In some cases the line of disconformity 
 is marked by springs (Fig. 538). In Fig. 532, a section is given 
 showing both unconformity and disconformity. 
 
 Diastems. This term has been applied to minor breaks in the continuity 
 of a series of strata due to a temporary cessation in sedimentation followed by 
 its resumption at a later period, but as a rule without erosion or even emergence 
 of the older strata (if they are marine), as is always the case in a disconform- 
 ity (Barrell). 
 
The Causes of Folding 
 
 617 
 
 FIG. 538. Disconformable contact between the Akron dolomite (Upper 
 Silurian) and the Onondaga limestone (Middle Devonian), at Williamsville, 
 N. Y. The contact is marked by the line of springs. (Photo by author.) 
 
 THE CAUSES or FOLDING 
 
 Folding of the rocks of the earth's crust always implies compres- 
 sion, and in a folded region the crust has become correspondingly 
 shortened. The monoclinal flexure must, however, be excepted, 
 
 FIG. 539. Machine for producing folded strata. Baily Willis, 
 tonne, Geographic Physique.) 
 
 (From Mar- 
 
 for this is not necessarily due to compression. That folds are pro- 
 duced by lateral compression of horizontal layers of material can 
 easily be shown by experiment. It is only necessary to press the 
 
618 Deformation of Rocks of the Earth's Crust 
 
 leaves of a book against the baqk of the cover to produce folds in 
 the paper. More elaborate experiments with layers of Wax and 
 other substances to represent the strata have been made in a ma- 
 chine permitting slow and regular compression. In such an experi- 
 ment the strata to be folded are generally weighted by a load of 
 
 shot, on the 
 supposition 
 that folding 
 is best ac- 
 complished in 
 nature beneath a 
 load of surface 
 rock material, and 
 that in the absence 
 of such a load the 
 layers will fracture and 
 become dislocated. A ma- 
 chine of this type is illus- 
 trated in figure 539, and 
 by its use many of the charac- 
 teristic types of foldings found 
 in the strata of mountain re- 
 gions have been reproduced 
 (Fig. 54o). 
 
 It should be noted, however, 
 that under lateral pressure not 
 all strata undergo folding. Some 
 of them become merely com- 
 pacted or thickened, and their 
 material flows into any spaces 
 released under the pressure. 
 Such strata, among which clay 
 rocks are found, have been called 
 " incompetent beds " in contradistinction to the " competent beds," 
 which are thrown into folds by the compression. In many cases 
 thrust faulting accompanies folding, the one passing into the other 
 (Fig- 54i). 
 
 In the folding of the Appalachian Mountains, the crust of the 
 earth has been shortened to an amount estimated at from 40 to 50 
 miles and in some portions probably more. In the Swiss Alps 
 
 FIG. 540. Folds made by com : 
 pression of layers of wax and other 
 substances in the machine illustrated 
 in Fig. 539. (U. S. G. S.) 
 
Deformation by Faulting 
 
 619 
 
 the foreshortening was originally estimated at 74 miles, but more 
 recent studies have led to the conclusion that the original Alpine 
 geosyncline was from 400 to 750 miles broad, and that it has 
 been reduced by folding and mashing to 100 miles. In the 
 
 FIG. 541. Diagram showing passage of thrust fault into fold. 
 (After de Martonne.) 
 
 Rocky Mountains of British Columbia an area originally 50 
 miles wide has been reduced to a width of 25 miles by such 
 compression. 
 
 There are a number of theories which aim to explain the causes of the com- 
 pressive movement. Among these the one held longest, appeals to the gradual 
 shrinking of the earth's interior through loss of heat, and the inward pressure 
 of the cool crust, with the result that tangential strains are set up within this 
 crust. Other causes appealed to are : the transference of subcrustal molten 
 material to the surface ; isostatic readjustment ; escape of volatile substances ; 
 changes in the oblateness of the spheroid toward a more spherical form ; migra- 
 tion of the equatorial bulge with change in polar position, etc. The subject is 
 too complex for elementary treatment, nor are all of the elements of the problem 
 as yet fully understood. 
 
 DEFORMATION BY FAULTING 
 
 Definition of Terms 
 
 A fault is a displacement of the strata of the earth's crust on 
 opposite sides of a fracture plane or surface. Faults affect all 
 kinds of rocks, but are most readily recognized in the stratified 
 deposits, and the illustrations will therefore be taken from them. 
 The surface along which the movement has taken place is called 
 the fault-plane, or better, fault surface, as it is not a perfect plane 
 except for short distances, but curved, warped, irregularly broken, 
 and even with offsets. Sometimes there are a number of more 
 or less parallel surfaces along each of which a small amount of 
 slipping has taken place. In such cases the entire series is re- 
 
620 Deformation of Rocks of the Earth's Crust 
 
 ferred to as the fault zone, and the displacement itself is called a 
 
 step-fault (Fig. 542). 
 
 Owing to the friction along the fault surfaces by the movement 
 
 against each other of the rock masses, these surfaces are often pol- 
 ished and striated in the direction 
 of movement. Such surfaces are 
 said to be slickensided. The line 
 of intersection of the fault sur- 
 face with a horizontal surface is 
 the fault-line (Fig. 543, F. L.\ 
 and its direction is called its 
 trend. (Strike is also used, but 
 this is properly restricted to the 
 intersection of the strata with 
 FIG. 542. Transverse section of the sur f ace .) The normal posi- 
 
 a piece of banded sandstone, showing 
 
 series of small faults. (Photo, B. tlon of a fault P lane or sulface 
 
 Hubbard.) is assumed to be vertical, and this 
 
 is called its zero (Fig. 543, F^). 
 
 Any departure from this (F 2 ) is measured with reference to a verti- 
 cal plane passed through the fault line and is called the hade (Fig. 
 543, F 2 ti). It will be seen that this is the complement of the dip, 
 which is the departure from the horizontal, but the term dip should 
 be restricted to the inclination of strata. The dip of the fault 
 plane may be measured with the clinometer and the amount sub- 
 
 FIG. 543. Diagram illustrating various types of fault. F. L., fault-line; 
 FI, F/, normal fault with vertical fault plane, the fault scarp or cliff has been 
 worn back from the fault-line ; F 2 , normal fault with inclined fault plane, with 
 hade h ; F.W., foot wall ; H.W., hanging wall ; dk, dike showing offset because 
 of inclination ; F. S., fault scarp, uneroded; F 3 , thrust fault; F. L. S., fault- 
 line scarp formed by erosion ; F 4 , fault without vertical but with horizontal 
 displacement or shove as shown by offset in dike (dk) at yz. 
 
Deformation by Faulting 621 
 
 tracted from 90, the difference being the angle of hade. With an 
 inclined fault-plane or surface the side projecting below is called 
 the foot wall (Fig. 543, F. W.), while the overhanging side is called 
 the hanging wall (Fig. 543, H. W.). In a vertical fault plane 
 neither is present. Most faults are best shown in vertical sections. 
 
 FIG. 544. Section of the Island of Helgoland in the North Sea, showing 
 numerous faults in the inclined strata, and the decrease in the size of the island, 
 and formation of a submarine platform, by wave-erosion. (After Walther.) 
 
 The vertical cliff section r of the island of Helgoland cut by the 
 waves of the North Sea shows a number of faults not otherwise 
 noticeable (Fig. 544). 
 
 Types of Faults 
 
 Nature of Movement in Faulting. The movement of the two 
 sides of a broken mass in faulting is relative. Both may undergo 
 a change in position of the same or of different extent, or either may 
 remain stationary while the other moves. In any case a displace- 
 ment results, which is all that is visible of the fault. This displace- 
 ment is best measured by the change in position of the two parts of 
 a definite, recognizable stratum, such as a coal bed, a limestone layer, 
 or some other bed (x, Fig. 543), which can be recognized by its lithic 
 character and thickness. The movement may be vertical, hori- 
 zontal, or oblique, or it may be a rotary one, the displacement 
 being of a corresponding character. Only the simpler types will 
 be considered. 
 
 Normal Faults. If the displacement or slipping along the fault 
 plane is in a direction at right angles to the fault line, two distinct 
 cases may be recognized, the normal and the reverse fault. With 
 reference to an assumed stationary footwall, the hanging wall may 
 slip down, thus producing a normal fault. This is best recognized 
 upon a vertical face cut at right angles to the fault line (abed, Fig. 
 543), where it will be seen that the guide or index stratum has been 
 vertically displaced (Fig. 543, FI, F Zj ). The extent of vertical dis- 
 
622 Deformation of Rocks of the Earth's Crust 
 
 placement is called the throw, this, inF 2 , Fig. 543, being measured 
 by the vertical distance s-t. There is, however, also a horizontal 
 displacement at right angles to the fault line, and this is called the 
 heave. In the present case this is a separation or positive displace- 
 ment of the beds, and its amount is measured by the distance r-s. 
 It is evident that in this case there has been a lengthening of the 
 earth's crust by the amount of the heave, and this implies a stretch- 
 ing movement such as may exist at the top of an anticlinal arch. 
 If the fault plane is vertical the heave is zero. (Fig. 543, F/.) 
 Normal faults may also develop upon a monoclinal flexure, where 
 the upflexed side becomes the downthrow side, and the down- 
 flexed, the upthrow side. (See p. 602.) 
 
 Reverse Faults. If the hanging wall moves upward with ref- 
 erence to an assumed stationary footwall, the movement again 
 being only in a direction at right angles to the fault line, a reverse 
 fault is produced (Fig. 543, F 3 ). Here, too, we meet with a throw, 
 but in the opposite direction, and if the fault plane is inclined, with 
 a heave which is also in the opposite direction, the result being an 
 overlapping or negative displacement. Here again the amount of 
 throw is measured by the vertical line s-t, and the amount of 
 heave by the horizontal line t-r, as seen in a vertical section at 
 right angles to the fault plane (bcfe). It is clear that in this case 
 there has been a shortening of the earth's crust by the amount of 
 the heave, which would imply lateral compression of the type 
 which produces folded structures. 
 
 Surface Expression of Normal and Reverse Faults. Both nor- 
 mal and reverse faults would be characterized by a cliff or initial 
 fault scarp upon the surface immediately after faulting has taken 
 place, if this were more rapid than the erosion which would tend to 
 level the inequalities (Fig. 543, F. 5.). Fault scarps of this type are 
 sometimes seen in the modern topography where faulting has been 
 so rapid as to be accompanied by an earthquake (Fig. 582, p. 677). 
 Such scarps and the modifications which they undergo by erosion 
 will be considered fully in the chapter on surface sculpture (Chap- 
 ter XXIII). Two examples, an eroded fault scarp (of F t ) and the 
 revived fault-line scarp F. L. S. , are indicated in the diagram (Fig. 
 543). In normal faulting the initial fault scarp is formed by the 
 footwall, in reverse faulting by the hanging wall. 
 
 Horizontal, Oblique and Rotary Faults. Instead of vertical 
 movements up or down the fault plane, the movement may be con- 
 
Deformation by Faulting 623 
 
 ceived as merely a horizontal one along the fault plane, though this 
 is probably rare. Such a fault would produce no initial fault scarp, 
 though a secondary one might be developed .later by erosion. The 
 extent of horizontal displacement along the fault plane is called 
 the shove, and if the fault plane cuts across strata or across a dike, 
 vein, pebble, or other guiding structure, the amount of the shove 
 along the fault plane can be measured (yz in F 4 , Fig. 543). An ap- 
 parent horizontal displacement of the dike is produced by a simple 
 vertical fault, because of the inclination of the dike (Fig. 543, dk 
 at F,). 
 
 Probably most faultings involve a more or less oblique movement down or 
 up the fault plane, comprising at the same time lateral displacement along that 
 plane, or the movement may be a double one, partly up or down the plane and 
 partly parallel to it, or it may be rotary, one block being twisted with reference 
 to the other. In any case the total amount of displacement between any two 
 points, originally in contact, can be measured by a direct but oblique line, and 
 this is called the slip of the fault. It may be resolved into the three components, 
 the throw, the heave, and the shove, from which measurable units the amount 
 of slip may be calculated. If the fault plane is vertical there is no heave, but 
 the line of slip forms the hypotenuse of a right-angled triangle, the other sides 
 of which represent the throw and shove, respectively. Rotary faults may some- 
 times be recognized by the difference in dip or strike of the strata on opposite 
 sides of the fault plane. 
 
 Strike Faults. Faults in tilted stratified rocks may have their 
 trend essentially parallel to the strike of these rocks. Such a fault 
 is called a strike fault, and it may be either normal or reverse, and 
 the inclination of the fault plane may coincide with the dip of the 
 strata, or may have any angle with, reference to it. This is the 
 common type of fault found in folded beds where, moreover, fault- 
 ing is commonly reverse. Whenever the angle of inclination of 
 the fault plane and the dip of the strata differ in a strike fault, there 
 will be either a concealment or a duplication of the beds in the sur- 
 face exposures as the result of faulting. The following diagrams 
 (Fig. 545) illustrate these results with varying angles of inclination 
 of the fault plane and constant dip of strata, both in normal faults 
 (A to G) and in reverse faults (H to J) . It will be seen that in figures 
 A, F, I, and / certain beds are concealed at the surface, while in 
 figures B, C, , G, and H certain beds are repeated. 1 
 
 Dip Faults, Oblique Faults. When the trend of the fault line 
 is at right angles to the strike of the strata or nearly so, the fault is 
 
 1 See further, A. W. Grabau, Principles of Stratigraphy, pp. 817-818. 
 
624 Deformation of Rocks of the Earth's Crust 
 
 spoken of as a dip fault, while faults whose trend makes an angle 
 of approximately 45 with the strike are called oblique faults. Such 
 faults are commonly indicated by an offset or shifting of the strata 
 on either side of the fault line (Fig. 546). 
 
 21 23456.1 21 2 3456 
 
 FIG. 545. Strike faults. A-F, Normal or gravity faults, the arrows 
 indicating the downthrow side ; H-J, thrust faults, the arrows indicating 
 the upthrust side. The diagrams represent the conditions after erosion of 
 the surface. In A, F, 7, and / certain strata are eliminated, in B, C, E, G, 
 and H certain strata are repeated. (From Principles of Stratigraphy.') 
 
 Gravity Faults. While the terms heretofore used have refer,- 
 ence to the nature of the movement (normal, reverse, horizontal, 
 rotary, etc.), or to the relation of the fault and the strata (strike, 
 dip, and oblique faults), there are two other terms in common use 
 which refer to the agent or cause of the movement. These are 
 gravity and thrust faults. The term gravity fault implies that one 
 
Deformation by Faulting 625 
 
 side slipped down as the result of its own weight, and normal faults 
 are generally assumed to be of such character, the terms being some- 
 times used interchangeably. A normal fault may, however, be 
 produced by the up-pushing of the footwall, rather than the down- 
 slipping of the hanging wall, as is illustrated by the fault formed 
 during the New Zealand earthquake of 1855 (pp. 675-677, Figs. 
 581-582). 
 
 Thrust Faults. These, on the other hand, are of common occur- 
 rence, and they are generally of the reverse type, but are also much 
 more complicated. They occur on the margins of folded areas and 
 
 FIG. 546. Diagram illustrating the offset of strata produced by vertical 
 slipping along an inclined plane, when the fault is oblique with reference 
 to the strata. The dotted outline restores the fault scarp and gives the 
 appearance immediately after faulting, before erosion has removed this por- 
 tion of the block. 
 
 sometime^ within them. Indeed, within the same area an over- 
 turned fold at one locality may be seen to pass into an overthrust 
 fault in another (Fig. 541, p. 618). 
 
 Magnitude of Thrusts. Thrust faults may range in magni- 
 tude from minute slippings of a few feet or less to cases in 
 which the movement has resulted in carrying one rock mass over 
 another for a distance of as much as 70 miles. The most stu- 
 pendous examples of such thrustings are seen in the Alps, where 
 successive sections have overridden one another, producing a 
 structure of extreme complexity. In the great thrust along the 
 Front Range of the Rocky Mountains in Montana, the observed 
 distance of movement is fifteen miles, while actually it was probably 
 much greater. 
 
 Duplication and Inversion of Order of Strata by Thrusting 
 
 As the result of great overthrusts, the strata of a given region 
 may be repeated, and because of the nearly horizontal nature of 
 
626 Deformation of Rocks of the Earth's Crust 
 
 the thrust the repetition may appear to be perfectly conformable. 
 Unless the thrust is recognized, such a series may be conceived 
 as representing one of unbroken succession. By such thrusting, 
 formations of older may come to rest upon those of younger age, 
 and thus an inversion of the strata may seem to have taken place. 
 Several examples may be cited. 
 
 The Helderberg Overthrusts. In the Helderberg range of 
 mountains, from Catskill southward, a great overthrusting of the 
 strata of Silurian and Devonian age has occurred along the western 
 border of the Appalachian folds, probably for the most part as a 
 
 FIG. 547. Section through North Hill, Kingston, N. Y., showing the 
 overthrust which led to a repetition of the strata. E, Esopus shale ; 0, Ori- 
 kany sandstone; P, Port Ewen; B, Becraft; N, New Scotland; C, Coey- 
 mans with Manlius below ; W, Cobleskill ; Z, Hudson River strata. (After 
 Van Ingen ; N. Y. State Museum Report.) 
 
 single overthrust (with perhaps locally a number of parallel slips). 
 By erosion the overthrust mass has been divided into a series of 
 blocks of which the above is an illustration (Fig. 547). In all 
 cases the thrusting is from the southeast, and the entire series of 
 strata from the Upper Silurian to the Middle Devonian is repeated. 
 Before the thrust was recognized it was thought that the beds ex- 
 posed in those ridges represented a single conformable series in 
 which there occurred a remarkable repetition 6f similar sediments 
 enclosing similar organic remains. 
 
 The Hudson Highlands Thrust (Fig. 548). The Highlands of 
 the Hudson likewise present a thrust of considerable magnitude, 
 with the result that the older crystalline rocks of the Highlands rest 
 
Deformation by Faulting 
 
 627 
 
 upon the younger ones, which are exposed farther to the north. 
 This thrusting appears to have been of a compound nature, the 
 
 FIG. 548. Diagrammatic section (generalized), showing the overthrust 
 at the northern end of the Hudson Highlands. Irregular dashes, granite, 
 gneiss, and other crystalline rocks of the Highland region; dotted, Lower 
 Cambrian (Poughquag) quartzite; blocked, Cambro-Ordovician limestones 
 (Wappinger, etc.) ; lined, Hudson River shale Ordovician. The main over- 
 thrust on the right brings the crystalline rocks against the Cambrian quartzite, 
 while the subsidiary thrust brings the crystallines above the Hudson River 
 slates. Thrust movement towards the northwest 
 
 FIG. 549 a. Chief Mountain, Montana. An erosion remnant of a mass of 
 Algonkian limestone carried by thrust faulting over soft sandstones and shales 
 of Mesozoic age, for a distance of at least fifteen miles. The monolith has a 
 height of 1500 feet above the thrust plane, which is indicated in the photograph 
 by the upper line of the forest. (After Campbell, U. S. G. S.) (See Fig. 515, 
 page 599, and Fig. 549 6.) 
 
 movement occurring along several parallel planes of low inclination. 
 As a result the old crystalline rocks have been repeatedly brought up 
 
628 Deformation of Rocks of the Earth's Crust 
 
 to the same level, where subsequent erosion has uncovered them. 
 At several places the old Highland gneisses of Archaean age may 
 be seen to rest upon the upturned Hudson River shales and sand- 
 stones of Ordovician age. The movement in this case also was 
 from the southeast to the northwest. 
 
 The Chief Mountain Thrust of Montana (Figs. 549 a, b). In 
 northern Montana along the Front Range of the Rocky Mountains, 
 
 
 FIG. 549 b. A series of diagrams showing the development of the Chief 
 Mountain thrust. The older rocks from which the mountains are cut are repre- 
 sented by cross-lines, the younger rocks of the plains (Mesozoic) in white. 
 (After Campbell, U. S. G. S.) 
 
 strata of Algonkian age are seen to rest with apparent conformity 
 upon nearly horizontal beds of Cretaceous age. These Algonkian 
 masses are erosion remnants of a formerly continuous series, Chief 
 Mountain being the most conspicuous of these remnants. This 
 position of pre-Cambrian upon late Mesozoic rocks at once 
 suggests overthrusting, and an examination of the region to the 
 west shows that this has actually occurred. From the data avail- 
 able the extent of the movement is recognized to have been at 
 least fifteen miles, the direction of movement in this case being 
 eastward. The actual movement was probably much greater, and 
 a considerable portion of the overthrust mass has apparently been 
 
Deformation by Faulting 
 
 629 
 
 removed by erosion. The development of this thrust 
 shown in the diagram (Fig. 549 &), and the position 
 Mountain with reference to the other 
 rocks is shown in Fig. 515 on p. 599. 
 
 Thrusts in the Northwest High- 
 lands of Scotland. The northwest 
 region of the Highlands of Scotland- 
 shows a wonderful series of thrust- 
 ings by which the older rocks are 
 repeatedly made to override the 
 younger ones, the ancient gneiss in 
 many places resting upon Cambrian 
 or Ordovician strata. The adjoin- 
 ing illustration shows some of the 
 overthrusts in this region (Fig. 550). 
 
 The Salt Range District of India. 
 - In northwestern India a series of 
 mountains forming what is known 
 as the Salt Range, because of the 
 abundance of rock-salt in it, illus- 
 trates an interesting problem which 
 may arise from great thrustings. 
 The rocks of this region appear to 
 represent a conformable series, with 
 red sandstones, shales, and extensive 
 salt beds at the base, overlain by 
 marine strata with organic remains 
 of Cambrian age. From this re- 
 lationship it was originally assumed 
 that the great salt beds of the Salt 
 Range were of early Cambrian or of 
 Pre-Cambrian age, constituting thus 
 the oldest salt deposits of the earth. 
 More recent studies, however, have 
 shown that we are dealing here, not 
 with a conformable series, but with 
 an enormous overthrust, which has 
 carried the fossiliferous Cambrian 
 sediments over the much younger 
 salt beds, upon which they have 
 
 plane is 
 of Chief 
 
630 Deformation of Rocks of the Earth's Crust 
 
 come to rest in apparently conformable position. The salt beds 
 themselves are probably of Tertiary age. 
 
 The Alpine Overthrusts. By far the most complicated series 
 of overthrusts which have yet been worked out are found in the 
 
 Sentis 
 
 FIG. 551. Block diagram [A] and section [B] illustrating overthrust in the 
 Alps. (After Lugeon.) 
 
 Alps, where the magnitude of the thrust movement seems at times 
 to be almost incredible. In the preceding illustration some of these 
 great overthrusts are shown (Fig. 551, see also Fig. 524, p. 607). 
 
 TOPOGRAPHIC FEATURES DUE TO FAULTING 
 
 Fault Scarps. We have already seen that both normal and re- 
 verse faults are expressed upon the surface of the earth by fault 
 scarps, if the dislocation is more rapid than the erosion which tends 
 to cut away inequalities (Fig. 543, p. 620). In all but the most re- 
 cent faults, the scarp is modified by erosion, which may remove 
 it completely or to a position distant from the fault line. Scarps 
 along fault lines may also be resurrected by erosion so that the 
 "cliff is not the original fault scarp, but an erosion cliff due to the 
 wearing away of the rocks on one or the other side of the fault line. 
 (See further, Chapter XXIII.) 
 
 Rift Valley or Graben. By the downfaulting of a long series 
 of narrow blocks of the earth's crust or a series of parallel blocks a 
 rift valley or graben is produced. This is well illustrated by the 
 Graben of the Rhine in the Basel-Strassburg region (Fig. 552), 
 where ancient rocks form the Vosges Mountains and Black Forest on 
 opposite sides, while the center represents a series of much younger 
 strata which originally were, in part at least, continuous with 
 
Topographic Features Due to Faulting 631 
 
 \I 
 
 those now lying on the outer flanks of these mountains, and 
 probably formed an arch with them, the center of which was 
 broken down, as 
 illustrated in the 
 following diagram 
 
 (Fig. 553)- 
 
 Other rift valleys 
 are found in eastern 
 Africa (Fig. 554), 
 and of one of these 
 the Dead Sea of 
 Palestine is appar- 
 ently the northern 
 continuation or a 
 part of a parallel 
 rift which also in- 
 volves the Vale of 
 Araba and the Gulf 
 of Akaba (see map 
 Fig. 649). On the 
 African Continent 
 these rift valleys 
 have been divided 
 by the building up 
 of volcanic cones at 
 
 FIG. 552. Map of rift valley of the Rhine, 
 showing the main faults on either side, and the 
 dying-out folds of the Jura Mountain system on the 
 south where they are crowded against the Vosges 
 and Black Forest massifs. ' (After Lake and Rastall.) 
 
 various points, and the resulting divisions of the valleys are partly 
 occupied by lakes, of which Lake Tanganyika is one of the largest. 
 
 VOSGES 
 
 etACKfOREST 
 
 FIG. 553. Section of the rift valley of the Rhine, i, granite; 2-7, Meso- 
 zoic rocks ; 8, 9, Tertiary and recent. (After Lake and Rastall.) For the 
 development of this graben see Fig. 650. 
 
 The rift valley in which this lake lies is more complicated in 
 structure than that of the Rhine, consisting of a number of tilted 
 
o So 100 200 3oo too SooKilom 
 
 Fault block depressions 
 
 FIG. 554. Rift valleys of East Africa. After Suess etc. from de Martonne. 
 
 AN, Lake Albert; AE, L. Albert Edward; K, L. Kivu; RK, L. Rikwa or 
 Rukwa; , L. Eyassi; H, L. Hohenlohe; M, L. Manyara; GN, L. Guano 
 Nyiro; KJ, Mt. Kilimanjaro; X^, Mt. Kenia; N, L. Naivasha; EG, Mt. 
 Elgon; St. L. Stefanie; AB, L. Abaya; 47, L. Afdjada. 
 
 632 
 
Topographic Features Due to Faulting 633 
 
 blocks instead of a normal downfaulted mass, as in the latter case. 
 This is shown by the following cross section (Fig. 555). 
 
 FIG. 555. Cross-section of Lake Tanganyika and Lake Rukwa, Africa, 
 showing the block and graben faulting. Vertical scale exaggerated five times. 
 (After Moore.) 
 
 T ^t;-- : "--._ ^x?"V-____ A : :Cp '~'^^~^f^!^^:'-- 
 
 FIG. 556. Sketch of Lake Albert, Oregon, a fault basin lake. (After Russell.) 
 
 A similar valley, with 
 occupied by Albert Lake 
 other examples of such 
 asymmetric rift valleys, 
 all of which in reality 
 represent block fault- 
 ing. The rift valley oc- 
 cupied by the Dead 
 Sea of Palestine is of 
 comparatively recent 
 origin, for on opposite 
 sides are still found 
 remnants of beds which 
 tended across this region 
 
 a fault scarp on one side only, is in part 
 in Oregon (Fig. 556), and there are many 
 
 FIG. 557 a. Diagram illustrating block 
 faulting, and the initial stage in the formation 
 of block mountains. See further, Figs. 643, 
 pp. 647 and 648. (After W. M. Davis.) 
 
 were deposited in a lake which once ex- 
 before the valley was formed. 
 
634 Deformation of Rocks of the Earth's Crust 
 
 Block Faulting and Mountains. When by faulting large blocks 
 of the earth's crust become tilted, they will present a steep fault- 
 scarp on one side and a slope, that of the original surface, on the 
 other. When of sufficient magnitude, such blocks will form moun- 
 
 FIG. 557 b. Block faulting with the formation of elevated horsts and 
 depressed rifts. Wasatch Mountains, Utah. Men are standing on the ends of 
 the fault blocks. (Photo F. J. Pack.) 
 
 tains. Of this type the Great Basin Ranges of the western United 
 States are the most conspicuous examples (Figs. 557 o, 645). An 
 uplifted block, bounded by faults, is called a horst. Faulting of 
 this type, but without surface expression, is illustrated in the 
 above view from the Wasatch Mountains (Fig. 557 b). 
 
 MINOR FEATURES ACCOMPANYING FAULTING 
 
 Slickensides. We have already seen that along the sides of 
 the fault plane, the rocks are often polished and grooved by the 
 movement, producing the characteristic slickensided surfaces. 
 Such surfaces are often the best indications of the position of the 
 fault planes, but they are also developed along minor planes of 
 movement parallel to the main fault. 
 
 Fault Breccia. Another feature often produced between fault 
 surfaces, is the complete fracturing and partial powdering of the 
 rocks which form either side of the fault plane. This broken ma- 
 
Minor Features Accompanying Faulting 635 
 
 terial may accumulate in a fissure in the fault and there produce a 
 fault breccia (Fig. 32, p. 80). Fault breccias are most conspicu- 
 ous in normal faults, but they are 
 also found along thrust planes, where 
 they may be so much modified by 
 the movement that the larger frag- 
 ments lose their angularity and come 
 to resemble a bed of pebbles. The 
 material produced by great ice 
 masses moving over a rock surface 
 is analogous to this, as already out- 
 lined in a previous chapter. When 
 a single large mass of rock is caught between two faults it is spoken 
 of as a " horse " (Fig. 558). 
 
 Collapse Breccias. Masses of angular fragments produced by 
 the collapse of the roof of a cave or other hollow, may have a very 
 similar appearance to a fault breccia, but would occupy a more 
 circumscribed area, while at the same time they are apt to be of 
 greater thickness. 
 
 Crumplings and Drags. Minor crumplings of the rocks along 
 the fault plane may also result from the movement, and the angle 
 of the beds on either side of the plane may be modified by a dragging 
 
 FIG. 558. A "horse." 
 
 FIG. 559. Diagrams illustrating drag of strata along a fault plane a, 
 normal fault; b, reverse fault. (The reverse bending of the strata near the 
 fault plane is shown in inverted faults formed upon a monoclinal flexure as 
 shown in Fig. 518 b, p. 602.) 
 
 movement. Thus in a normal fault in horizontal strata, the beds 
 of the hanging wall may be sharply dragged upward along the 
 fault plane while in a reverse fault they may be dragged downward 
 (Fig. 559). The vertical strata from which the Gateway to the 
 Garden of the Gods and the " Cathedral Spires " are cut have been 
 considered as having been dragged to a vertical position along a 
 
636 Deformation of Rocks of the Earth's Surface 
 
 fault plane (Figs. 560, 561). Where " inverted " faults are formed 
 upon a monoclinal flexure, the strata near the fault plane bend in 
 
 FIG. 560. Gateway to the Garden of the Gods, Colorado, with Pikes Peak 
 in the distance. The strata which form the "Gateway" are red sandstones 
 apparently dragged to a vertical position by faulting. 
 
 the opposite direction, i.e. down on the downthrust and up on the 
 upthrust side. (See p. 600, and Fig. 518, p. 602.) 
 
 OTHER STRUCTURES PRODUCED BY DEFORMATION 
 
 In the deformation of the strata of the earth's crust, structural 
 features of a less conspicuous character than the folds and faults 
 are produced. The metamorphism of the strata due to pressure 
 and the development of heat thereby, and the special structural 
 features of these, will be discussed in the next chapter. Two types 
 of structure, however, may be considered here ; namely, slaty cleav- 
 age and fracture planes or joints. 
 
 Slaty Cleavage 
 
 When the finer-grained rocks are subject to intense squeezing, 
 the particles of such a rock may become flattened and expand at 
 right angles to the compression. This results in a squeezing to- 
 gether of the mass in the direction of compression and a proportional 
 swelling at right angles thereto. If mica scales or other fragments 
 which already have a scaly form are present, they will become so 
 
Other Structures Produced by Deformation 637 
 
 arranged that their longer axis is at right angles to the direction of 
 compression. In this manner, a secondary parallel structure is 
 produced in the rock thus compressed, this parallel structure being at 
 right angles to the direction of compression, and having no reference 
 to the original structures, such as bedding planes, etc. On weather- 
 
 'f ' 
 
 i 
 
 
 , 
 
 
 FIG. 561. The Cathedral Spires, Garden of the Gods, Colorado. These 
 spires are parts of vertical beds of red sandstones, which are modeled out by 
 erosions of the softer beds. (Photo by Darton, U. S. G. S.) 
 
 ing, the rock thus affected will split into thin plates along these 
 newly-produced structure planes, or the rock may be artificially 
 split along them. This is called slaty cleavage, and it is most com- 
 monly developed on clay mud-rocks, such rocks when thus af- 
 fected producing the common slates used for roofing purposes, etc. 
 (Fig. 562). Slates of this kind may appear banded transversely to 
 
638 Deformation of Rocks of the Earth's Surface 
 
 their surfaces, such bands commonly representing the original bed- 
 ding-planes of the mass before it was affected by the compression. 
 
 When the original rock contains 
 fossils, these will be compressed 
 and variously distorted, but such 
 rocks are not as a rule capable 
 of furnishing good roofing or 
 other slates. 
 
 Joints 
 
 The term joints and jointing 
 as applied to rocks covers a num- 
 ber of structures, such as the 
 columnar jointing of basalts, the 
 horizontal or arched jointing of 
 granites, etc., and the vertical 
 fissures which are found in nearly 
 all clastic rocks. It is this last 
 type alone which is of defor- 
 mational origin, though it is 
 not necessarily confined to the 
 clastic rocks, where, however, it is most perfectly developed. 
 As seen in beds of shale, limestones, or fine-grained sandstones, 
 such joints are com- 
 monly found arranged 
 in systems, the joints of 
 each system being par- 
 allel, while the different 
 systems form angles 
 with one another. In 
 general, there are two 
 such systems present, 
 crossing each other at a 
 high angle and dividing 
 the rock mass perpen- 
 dicularly to the bedding 
 planes into a series of 
 
 quadrangular blocks or 
 
 , . i ,, FIG. 563. Master joints in soft shale (Upper 
 
 prisms, which greatly Marcellus) on the shore of Lake Erie. (Photo 
 
 facilitate quarrying by author.) 
 
 FIG. 562. A piece of rock in 
 which slaty cleavage has been de- 
 veloped, as shown by the fine lines 
 (p) ; the coarser bands represent the 
 original bedding planes, now com- 
 pressed and contorted. (After Le 
 Conte.) 
 
Other Structures Produced by Deformation 639 
 
 operations and permit 
 weathering and mechanical 
 erosion to produce a va- 
 riety of forms. In figures 
 563 to 566 some illustra- 
 tions of such jointing are 
 given ; in soft shales on the 
 shore of Lake Erie (Fig. 
 563) ; in sandy shales and 
 sandstones on Cayuga Lake 
 (Fig. 564), and in limestone 
 or chalk where they control 
 the erosion on the coast of 
 France (Figs. 565, 566). 
 (See also Figs. 341 a, p. 407, 
 and 474, p. 571.) 
 
 Some of these joints ex- 
 tend through the entire 
 mass of the formation, 
 others are limited to a 
 single stratum. The first 
 are called master joints and 
 often exert an important influence on the topography (Figs. 564, 
 565). The others, the minor joints, are of small significance. 
 
 FIG. 564. Joint planes in Sherburne 
 sandstone, shore of Cayuga Lake, N. Y. Two 
 sets of joints cut the horizontally bedded 
 fine-grained sandstones, and erosion has re- 
 moved the portion on the right, leaving the 
 huge square prisms of rock facing the lake. 
 (E. M. Kindle, photo; from U. S. G. S.) 
 
 FIG. 565. Sea cliff 100 meters high at Treport, France, showing the effect 
 on the coast topography of joints which traverse the beds of chalk in two 
 principal directions. (Copied from Crosby.) 
 
640 Deformation of Rocks of the Earth's Surface 
 
 Such joints appear to be the result of the fracturing of strata 
 under a torsional strain. If a great horizontally lying plate of glass 
 were raised by one corner, the weight of the plate would tend to 
 set up sagging or torsional strains and these might be so powerful 
 
 FIG. 566. Plan of the joints in the chalk cliffs of Trport, showing how 
 they influence the process of erosion. (Copied from Crosby.) 
 
 as to overcome the cohesion of the mass and produce fracture. 
 This can be illustrated by gently twisting a strip of glass held 
 firmly by one end, as illustrated in the annexed figures (Figs. 
 567 a, b). When the strain becomes too great, the glass will- break, 
 
 FIG. 567 a. Apparatus for breaking a plate of glass by torsion, with an 
 example of results produced. (After Daubree.) 
 
 with the formation of two or more regular systems of parallel 
 cracks, the cracks of one system crossing those of the other 
 at a high angle. Such cracks reproduce in all essentials the joint 
 cracks formed in stratified rocks. 1 If the twisting is not carried 
 far enough to produce actual shattering of the glass, this can be 
 
 1 In this experiment the glass should be reenforced by a sheet of paper glued to one 
 side which will keep the fragments in place after cracking. 
 
Other Structures Produced by Deformation 641 
 
 produced by striking a blow with a hammer on the table on which 
 the experiment is made. The shock thus produced will com- 
 plete the shattering not accomplished by the torsion alone. This 
 illustrates how great sheets of stratified rocks placed under a 
 
 FIG. 567 b. Arrangement of fractures in a large plate of glass which was 
 broken by torsion. (After Daubree.) 
 
 torsional strain by unequal elevation or warping may be shattered 
 by the passage through them of an earthquake shock, and this 
 may be the usual way in which such jointing is produced, as sug- 
 gested by W. O. Crosby. Joint-like fissures, but of lesser regularity 
 of arrangement, may also be produced in a variety of other ways. 
 
CHAPTER XX 
 METAMORPHISM AND METAMORPHIC ROCKS 
 
 DEFINITION AND CLASSIFICATION OF METAMORPHISM 
 
 Definition. All rocks are subject to alteration in nature. 
 This alteration may be slight or intense ; it may be accompanied by 
 disturbances in the earth's crust, or it may be the direct result of 
 such disturbances. Changes in the character of the rock are gen- 
 erally recognized by changes in mineral constitution, in texture, or in 
 both, as well as in other characteristics. Such changes are termed 
 metamorphic, and the process is one of metamorphism (Greek 
 /xera, dftioting interchange -f- /xop</>rj, form). Some geologists con- 
 sider all changes as metamorphic changes, but in practice the term 
 metamorphic rocks is generally restricted to those that have been 
 strongly altered, and as a result have taken on a crystalline 
 character. 
 
 Classification of Metamorphism according to Forces. The 
 natural forces producing rock metamorphism are (a) chemical 
 energy, (b) heat, and (c) pressure. All three are commonly active, 
 but one or the other may predominate to such an extent as to 
 give the process its distinctive character. Accordingly we may 
 in a general way classify metamorphism as (i) chemical or dia- 
 genetic, (2) thermal and (3) dynamic, but it must be clearly under- 
 stood that these divisions refer only to the dominant force, and 
 that there can be no complete dissociation of any one of them from 
 the others. Chemical energy is indeed active in all processes 
 of metamorphism and may be regarded as the chief of the forces 
 producing changes .in rock, but this may be set in motion, or ac- 
 celerated, by the special intervention of heat, as in the case of a 
 contact with igneous masses or by the influence of great pressure 
 in mountain-making disturbances. It may also act, though 
 slowly, under the ordinary conditions of aging of rocks with time. 
 
 Classification of Metamorphism according to Extent. Metamor- 
 
 642 
 
Activities of Agencies Producing Metarnorphism 643 
 
 phism may be local, when only a limited area is affected, as in the 
 case of the contact of a rock with a hot igneous mass or with heated 
 waters or gases, or it may be regional, when extensive areas are 
 affected, as in the case of great deformation of the earth's crust. 
 Because of the association of regional metamorphism with ex- 
 tensive dynamic disturbances, the two terms, regional and dynamic, 
 are often used interchangeably. It will be well, however, to keep 
 in mind the fact that one term refers to the areal extent of the 
 change and the other to the process producing it. And it must 
 be further understood that in extensive regional metamorphism, 
 deformational changes are not the only causes of alteration, for 
 all other causes, such as invading heat from igneous masses and 
 the passage of gases and liquids which produce chemical changes, 
 may be equally active. 
 
 Local metamorphism is most readily seen where heated igneous 
 masses come in contact with other rocks, which they alter, and by 
 which they are altered to a certain extent. This is therefore 
 called contact metamorphism, and its characteristics, so far as 
 igneous contacts are concerned, have already been described 
 (p. 207). It should, however, be noted that contact metamorphism 
 is also produced by the passage of heated gases and waters, which 
 generally carry solutions of various substances, and by these the 
 rocks in the neighborhood are affected. Since the chief cause of 
 contact metamorphism is the heat of the igneous body, and since 
 such bodies in contact with other rocks are the chief, though not 
 the only source of heat, the terms contact and thermal metamor- 
 phism are often used synonymously. In the following table some 
 of these relations are shown : 
 
 Force Condition Area 
 
 Chemical or diagenetic. Static. Local or regional. 
 
 Thermal. Contactic. Local. 
 
 Dynamic. . Tectonic. Regional, more rarely local. 
 
 ACTIVITIES OF THE AGENCIES PRODUCING METAMORPHISM 
 
 Pressure. Simple pressure is exerted upon rocks by the weight 
 of other rock masses which overlie them. Such pressure tends 
 to bring closely together the particles of which the rock is composed 
 and may weld them into a more or less solid mass. Certain more 
 plastic layers may also give way at some points and undergo a 
 
644 Metamorphism and Metamorphic Rocks 
 
 flowing movement, with the result that crowding and crumpling 
 of that layer (enterolithic structure] (Fig. 568) is produced else- 
 
 FIG. 568. Enterolithic structure in fine-grained or compact limestone ; 
 Muschelkalk, Neckar Valley, Wiirttemberg. (After Koken.) 
 
 where. Pressure and unequal solution along irregular bedding 
 surfaces in massive limestone produces the remarkable structure 
 known as stylolite (Figs. 569, 570). Similar deformations are pro- 
 duced in the formation of the great domes of salt which character- 
 ize certain districts 
 of the southern 
 United States and 
 of the Old World 
 as well. In these 
 contortion of the 
 salt mass takes 
 place often from the 
 
 F,G. 569. - Appearance on the transverse face of P ressure of g rowin g 
 a limestone layer, showing the formation of stylolitic salt crystals, aided 
 structure. The interlocking masses have been pro- m some cases by 
 duced by solution along a minute fracture plane, 
 producing hollows on opposite sides into which the 
 projecting masses fit. The residual clay left on 
 solution is found in both the under and upper hol- 
 lows and indicated in black. The amount of solu- 
 tion is approximately indicated by the depths of 
 the hollows. (After Wagner.) Greatly reduced. 
 
 tectonic forces (Fig. 
 
 571)- 
 
 It is probably also 
 true that rocks un- 
 der enormous pres- 
 
 sure of overlying 
 
 beds are subject to intense thermal ai\d chemical activities, 
 but of this there is as yet no positive evidence. It has some- 
 times been assumed that the older rocks of the earth's crust 
 
Activities of Agencies Producing Metamorphism 645 
 
 have become metamorphosed in part because of the great weight 
 of overlying sediments, of which those of the Palaeozoic alone, in 
 the Appalachian region, 
 aggregate some 40,000 
 feet. But it appears 
 that the older meta- 
 morphic rocks, wherever 
 they have been exposed 
 by erosion, are overlain 
 by sediments altered 
 but slightly or not at 
 all, and that those rocks 
 were already meta- FIG. 570. Two of the interlocking prisms or 
 
 morphosed before these stylolites, showing the striated sides produced 
 , . by friction during the process of interpenetra- 
 
 sediments were de- tion About one half natural size . (After 
 
 posited upon them. Wagner.) 
 
 Thus the mere pressure 
 
 of superincumbent rocks seems a minor cause of metamorphism. 
 Movement. The case is otherwise where movement of 
 
 rock masses has taken place, especially where such movement has 
 
 been intense and resulted in the production of greatly disturbed 
 
 structures. Simply 
 folded strata, such 
 as those of the Jura 
 and Appalachian 
 mountains, have 
 been but little af- 
 fected by the dis- 
 turbances, and as a 
 rule the amount of 
 metamorphism in 
 them is compara- 
 tively slight, though 
 locally it may be 
 
 of considerable in- 
 FIG. 571. Contorted salt layers in the roof of . w , 
 
 a chamber in Myles Salt Mine, Weeks Island, La. 1 W' W n 
 
 (Photo by Veatch; from U. S. G. S.) These def- rocks are intensely 
 ormations are believed by many to be wholly due 
 to the pressure of growing salt crystals, while others 
 hold that tectonic forces from without aid in their 
 formation, or may even be the chief cause. 
 
 deformed, metamor- 
 phism is generally 
 pronounced. 
 
646 Metamorphism and Metamorphic Rocks 
 
 Intense deformation is accompanied not only by a change in 
 the form and position of the strata, as in the change from a hori- 
 zontal to a folded structure, but also by interior readjustment 
 of rock substances, a rearrangement of the rock particles com- 
 monly in a parallel manner, together with chemical and mineral- 
 ogical changes. This is called rock flowage, as distinct from rock 
 fracture, and it is believed to be a process which in most rocks 
 can take place only at a considerable depth, whereas rock 
 fracture is essentially a surface phenomenon. Consequently, 
 the earth's crust is believed to be divisible beneath the belt of 
 weathering into an upper zone of rock fracture (katamorphism), 
 and a lower one of rock flowage (anamorphism). 1 The limit be- 
 tween the two is at the level where pressure is so great as to close 
 all fractures which may result from movements. It must, how- 
 ever, be emphasized that the depth at which rocks flow rather 
 than fracture depends upon many things, chief among them being 
 perhaps, the nature of the rock. Some rocks, like shales, undergo 
 a flowage comparatively close to the surface, while rigid rocks, 
 like quartzites, may still fracture at a considerable depth beneath 
 it. Therefore if we speak of zones of flowage and fracture, we 
 refer to conditions rather than to actual depth. Nevertheless, 
 at great depths probably no fracturing takes place, all rocks under- 
 going flowage. Therefore it may be better to speak of the upper 
 portion of the earth's crust as the zone of combined fracture and 
 flowage (according to the nature of the rock), and the lower part 
 as the zone of flowage only. 
 
 Heat. Heat not only affects the mineral particles of the rocks 
 which it invades, but also aids greatly in making the rock undergo 
 flowage rather than fracture. Thus where a rock under pressure 
 is invaded by heat from an igneous mass, flowage may occur much 
 nearer the surface than is the case for the same rock unaffected 
 by the heat. Heat also greatly increases the activity of 
 liquids and gases which enter the rock masses. While heat 
 may be produced in various ways, as by pressure, movements, 
 etc., its most frequent source is probably found in intruded 
 igneous masses. 
 
 Liquids and Gases. These are important agents in producing 
 rock metamorphism. Water is, of course, the. most important of the 
 
 1 Leith and Mead (" Metamorphic Geology "), use katamorphic for destructive altera- 
 tions, including weathering, and anamorphic for constructive alterations. 
 
Metamorphic Structures and Textures 647 
 
 liquids. Combined with heat and pressure it becomes a powerful 
 solvent agent and aids in the crystallization of the minerals. Gases 
 arising from an intruded igneous magma are also very active in 
 altering the minerals of the affected rock and in introducing new 
 ones. These agencies appear to be most effective in contact 
 metamorphism. 
 
 METAMORPHIC STRUCTURES AND TEXTURES 
 
 The more important structures produced by metamorphism 
 in rocks are: (i) cleavage, (2) schistosity, (3) gneissic structure and 
 banding. The chief texture of metamorphic rocks is crystalline. 
 
 Slaty Cleavage. This has already been discussed in connection 
 with structures produced by deformation (p. 636). 
 
 Schistosity. This structure represents a further development 
 of cleavage in rocks which are subject to intense deformation, with 
 the development of a crystalline texture. The crystals in this 
 case are elongated and have a more or less parallel arrangement 
 not only of form but of mineral cleavage as well. As a result, 
 schistose rocks split readily along the planes of schistosity, some- 
 times separating between the mineral partings, but more generally 
 along the cleavage planes of these minerals. This is especially 
 the case where mica and hornblende abound, whereas if they are 
 less common than quartz and feld- 
 sparj the splitting is mostly between 
 these mineral particles. As a rule the 
 mica and hornblende are new to the 
 schist, having been developed there 
 during the process of metamorphism 
 by recrystallization . of substances 
 already in the rock mass. 
 
 Gneissic Structure. "Gneissic 
 structure means a banding of con- 
 stituents of which feldspar is impor- 
 tant, with or without the parallel 
 dimensional arrangement necessary FIG. 572 <z. Banded and 
 r , , ,, /T .,,v /T ^. contorted gneiss, Fordham, 
 
 for rock cleavage" (Leith) (Figs. Ny (B . Hubbard, photo.) 
 
 572 a, b). Cleavage is not necessarily, 
 
 nor commonly, very well developed in gneissic structure, but the 
 parallel banding is always marked. " The essential mineralogical 
 difference between gneisses and schist is the possession by the 
 
648 Metamorphism and Metamorphic Rocks 
 
 gneisses of a relatively small amount of the platy and columnar 
 minerals so necessary for a good rock cleavage, and correspond- 
 ingly more feldspar and 
 quartz " (Leith). 
 
 Crystalline Texture. 
 This may or may not 
 be accompanied by the 
 formation of schistose or 
 gneissic structure in rock 
 flowage. Marble is one 
 of the best examples in 
 which recrystallization of 
 the carbonate of lime may 
 take place without the 
 formation of a flow struc- 
 ture or cleavage. Re- 
 crystallization tends 
 toward an enlargement of 
 the rock particles, and as 
 a result, the original tex- 
 ture of the rock is de- 
 stroyed (Fig. 573). If the original limestone was of homogeneous 
 character, the bedding planes also are obliterated, but if there 
 were layers differing in character and composition, these are 
 affected in a separate manner, and the bedding structure of the 
 marble or other highly crystalline rock will be retained. 
 
 FIG. 572 b. Banded and folded gneiss, Utah. 
 (Photo, by F. J. Pack.) 
 
 OCCURRENCE AND AGE OF METAMORPHIC ROCKS 
 
 Metamorphic rocks are widespread over the earth's surface. 
 Vast areas, such as those of the Canadian Shield, and the greater 
 part of Scandinavia and Finland, show chiefly metamorphic rocks. 
 These are among the oldest rocks of the earth's crust, and it will 
 generally be found that the later rocks (Palaeozoic and younger) 
 rest upon them with a very pronounced unconformity, indicating 
 a long time interval between the formation of the two series and 
 a pronounced erosion of the metamorphic rocks before the de- 
 position of the overlying formations. 
 
 Highly metamorphosed rocks also occur in the axes of many 
 mountain ranges, where they have become exposed as the result 
 
Types of Metamorphic Rocks 649 
 
 of erosion of the younger rocks. They in some cases also belong to 
 the older rock series of the earth, but in others are metamorphosed 
 sediments of quite recent date. Thus in the Alps and in the Coast 
 Ranges of Western America, rocks of Mesozoic age have been 
 
 FIG. 573. Granular crystallized limestone (marble) seen under crossed 
 nicols; enlarged 24 diameters. Fichtelgebirge. (After Rosenbusch.) 
 
 highly metamorphosed, and rocks of Tertiary age have suffered 
 a similar change in some parts of the world. In general, however, 
 we may say that the bulk of metamorphic rocks belongs to the oldest 
 geological divisions, being for the most part of pre-Palaeozoic age. 
 
 TYPES OF METAMORPHIC ROCKS 
 
 For the sake of convenience, we shall group the more important 
 metamorphic rocks under the following divisions, which in a general 
 way lead from the less metamorphosed, i.e., those whose original 
 character can still be determined, to the strongly metamorphosed 
 types. 
 
 1. Quartzites, slates and phyllites. 
 
 2. Crystalline schists. 
 
 3. Gneisses. 
 
 4. Marbles, crystalline dolomites, ophicalcites, etc. 
 
 5. Serpentines, magnetitic and graphitic rocks, etc. 
 
 In general, it is easier to predict the types of metamorphic rocks 
 derivable from the common unaltered rocks than to state positively 
 what was the original condition of a given metamorphic rock. 
 In the following table, the more usual metamorphic derivatives 
 of the common rocks are given. 
 
650 Metamorphism and Metamorphic Rocks 
 
 METAMORPHIC DERIVATIVES OF COMMON ROCKS 
 
 ORIGINAL ROCK 
 
 METAMORPHIC DERIVATIVES 
 
 1. Igneous or Pyrogenic Rocks 
 
 a. Coarse-grained acid feldspathic types, 
 
 such as granites, syenites, etc. 
 
 b. Fine-grained acid feldspathic types, 
 
 felsites, etc. 
 
 c. Coarse-grained basic types 
 
 d. Finer-grained basic types rich in ferro- 
 
 magnesian minerals (basalts, dolerites, 
 etc.) 
 
 2. Aqueous (Hydro genie) and Organic (Bio- 
 
 genie) Rocks 
 
 e. Precipitated and organic limestones 
 
 3. Clastic Rocks 
 
 f. Conglomerates and breccias (rudytes) 
 
 g. Sandstones (arenytes) 
 
 h. Clay and quartz flour, i.e., mud-rocks 
 (lutytes) 
 
 i. Clastic limestones 
 
 j. Pyroclastic rocks (tuffs, etc.) 
 
 a. Gneiss 
 
 b. Phyllites and schists 
 
 c. Basic gneiss ; serpentine 
 
 d. Hornblende schists and 
 
 other basic schists 
 
 e. Marbles 
 
 /. Gneisses; schists 
 
 g. Quartzite; quartz mica 
 schist, etc. 
 
 h. Porcelanites, argillites and 
 hornfels ; slates ; phyllites ; 
 mica schists 
 
 i. Marbles ; calcareous schists 
 
 j. Slates; mica schists; horn- 
 blende schists 
 
 Characters of the More Important Metamorphic Rocks 
 
 Quartzites. These range in composition from nearly pure silica to a 
 mixture with 1 5 per cent or more of aluminum oxide, iron oxide, etc. Quartzites 
 are derived from quartz-sandstones and are characterized by the presence of 
 silica as the cementing agent of the grains, this silica being commonly deposited 
 in crystallographic continuity with the quartz of the grains. Quartzites thus 
 have a crystalline character, and they are harder than ordinary sandstones 
 into which they grade. When much clay was present in the original sandstone, 
 mica, especially muscovite, is developed, producing a micaceous quartzite, 
 which passes by degrees into a quartz-mica schist. A flexible form of mi- 
 caceous quartzite, in which the grains have a slight power of movement on one 
 another, is called itacolumite. Quartz conglomerate, too, may be changed to 
 quartzite and the pebbles may be flattened by dynamic movements. 
 
 Quartzites are most abundant in the older (pre-Cambrian) strata, but are 
 not confined to them. They may indeed be of any age. 
 
 Slates. These are mud-rocks or lutytes in which a high degree of slaty 
 cleavage has been developed as the result of compression. This cleavage, as 
 already outlined, has commonly no relationship to the bedding planes, though 
 the name slate is also applied to carbonaceous mud-rocks which split smoothly 
 along the stratification. Mica, especially sericite, and hornblende scales may 
 be developed, but these are not visible except under the microscope. When 
 
Types of Metamorphic Rocks 651 
 
 they become large and dominant, the slate passes into phyllites and mica 
 schists. (See Fig. 562, p. 638.) 
 
 Slates may also be developed from pyroclastic rocks (tuffs) which have been 
 subject to compression without extensive recrystallization. In color, slates 
 generally range from drab to black, but green, red, and purple tints may occur. 
 
 The chief uses are for roofing purposes, blackboards, and (formerly) for 
 school slates. The most important American slates are altered Cambrian 
 and Ordovician rocks, but pre-Cambrian and also younger slates occur, the latter 
 chiefly foreign. 
 
 Porcelanite, Baked Clay, Hornfels. Fused clays and shales in the roofs 
 and floors of burned coal seams are changed into a hard, homogeneous rock 
 resembling porcelain and designated Porcelanite. Similar results are produced 
 in contact metamorphism, where the shales are baked into a hard, flinty rock 
 to which the name hornfels is applied. It breaks in irregular, angular masses, 
 and closely resembles dense trap-rock, for which it may be mistaken. Biotite 
 is commonly an important constituent of this rock, though visible only under 
 the microscope. Various minerals, such as andalusite, garnet, cyanite, stauro- 
 lite, tourmaline, ottrelite, rutile, hornblende, feldspar, etc., may be developed, 
 sometimes in crystals of considerable size. When the minerals are rather 
 evenly scattered, a knotty slate or hornfels is produced. 
 
 Phyllites. These rocks are intermediate between slates and mica schists, 
 partaking of the structure of the former, but with the development of much 
 fine mica. They may be derived from silicious clay rocks like ordinary slates, 
 but they may also represent altered tuffs or even felsitic rocks. The fine mica 
 scales are commonly sericite, etc., which has also been called hydromica, on 
 which account the name hydromica schist is frequently used. 
 
 Mica Schists. These are schistose rocks in which mica, chiefly muscovite, 
 but also biotite appears in prominent scales, while quartz is the other important 
 mineral. When quartz predominates, the rock passes into a micaceous quartzite. 
 Sometimes the rock is high in lime, when it passes into a micaceous marble or 
 calc schist. Feldspar in greater or less quantity is also present, especially if 
 the schist has been derived from an igneous rock. As accessory minerals, 
 garnet, staurolite, cyanite, sillimanite, tourmaline, apatite, pyrite, and magnetite 
 may be mentioned. 
 
 Mica schists are perhaps most commonly derived from the alteration of 
 argillaceous sandstones and silicious shales, etc., but they may also originate 
 from igneous rocks (rhyolites, trachites, etc.). When derived from elastics, 
 they are generally lower in alkalies, and the magnesia is in excess of the lime. 
 
 Schists with graphite disseminated through them are not infrequent. These 
 are probably in most cases developed by the metamorphism of carbonaceous 
 shales. 
 
 Hornblende Schists. These are derived from the basic igneous rocks high 
 in ferromagnesian silicates, by the development in them of schistose structure. 
 More rarely, sediments have yielded such rocks by metamorphism, when 
 analysis generally shows a low alumina content and a great excess of magnesia 
 over lime. 
 
 While hornblende forms the chief mineral of this rock, biotite, augite, and 
 plagioclase may also be present in varying proportion. Besides these, there 
 
652 Metamorphism and Metamorphic Rocks 
 
 are accessory minerals, such as garnet, magnetite, pyrite, and pyrrhotite, but 
 quartz is normally absent, or at least very rare. By alteration, the hornblende 
 passes into chlorite, with the production of a chlorite schist. The plagioclase 
 may be replaced by secondary products, such as epidote, calcite, scapolite, etc. 
 The schistose structure is due to the parallel arrangement of the hornblende 
 crystals. When this structure is indistinct, the name amphibolite has been 
 used for the rock. Hornblende schists may form extensive areas by them- 
 selves or they may represent the altered basic dikes intruded in more acid rocks 
 with which they have become thoroughly metamorphosed by dynamic dis- 
 turbances. Such dikes, changed to hornblende schist, abound in the Man- 
 hattan Island rocks and are also found elsewhere in the older metamorphic 
 series of eastern North America. 
 
 Chlorite Schists. These are further alteration products of hornblende 
 schists, but may also be derived from other rocks rich in anhydrous iron-alumina 
 silicates. The schistosity is well developed in these rocks and the chief mineral 
 is chlorite, a green, micaceous, and rather soft mineral. More or less quartz is 
 also present, and besides this, plagioclase, talc, epidote, and magnetite may 
 form common accessories. Owing to the pronounced green color, certain of 
 these rocks are sometimes called " green schists." They are not uncommonly 
 members of the metamorphic series of the Appalachian, New England, and 
 Lake Superior regions. 
 
 Talc Schists. These are the metamorphic products of rocks high in an- 
 hydrous magnesian silicates but low in iron. The chief mineral is talc, but 
 quartz is also often quite abundant. Feldspars and some micaceous minerals 
 also occur in the latter, and are often difficult to distinguish from the talc scales. 
 The MgO content of these rocks may be 30 per cent or more. It is possible that 
 silicious dolomites in a sedimentary series may in some cases be the original 
 rock from which talc schists are derived. The distribution of talc schists is 
 similar to that of chlorite schists. 
 
 Epidote Schists. These are rarer schists, characterized by the predomi- 
 nance of the mineral epidote. They have a light greenish appearance from the 
 color of the ferromagnesian silicate, epidote, and they are generally the product 
 of metamorphism of pyroxenic and hornblendic rocks. Their CaO and MgO 
 content is often very similar (between 7 and 8 per cent). These schists have a 
 distribution similar to the preceding but are less common. 
 
 Graphite Schists. These are the common product of the metamorphism 
 of carbonaceous sedimentary (clastic) rocks, and they are not uncommon in the 
 metamorphosed Palaeozoic series of eastern North America, but also occur in 
 the older rocks. Graphite is present as a rule in only moderate amounts, 
 occurring in scaly flakes along the planes of schistosity. Mica, quartz and 
 feldspar are the common associates in these schists, and when the graphite is 
 present only in small quantities, the rock may be a graphitic mica schist. 
 
 Gneiss. The rocks classed under this term (pronounced nice) include a 
 variety of types in which the characteristic gneissic structure is developed. 
 The structure is more coarsely laminated than that of schists, and the rocks 
 split less readily along these planes of lamination. Feldspar is always a char- 
 acteristic constituent of typical gneisses and, besides this, there occur, as a 
 rule, quartz, mica and hornblende. Many accessory minerals may also occur. 
 
Types of Metamorphic Rocks 653 
 
 There are, however, gneisses produced from the alteration of basic igneous rocks 
 in which quartz is absent, while the ferromagnesian minerals abound. 
 
 Gneisses may be produced from the alteration of igneous as well as clastic 
 rocks. In many cases it is possible to refer the rock back to its original type, 
 but in other cases this cannot be done. According to the rocks from which 
 gneisses are derived, the following types have been recognized : 
 
 Corresponding Gneisses 
 Original Rock , , , , , . . , . 
 
 Produced by Metamorphtsm 
 
 Granite Granite-gneiss 
 
 Syenite Syenite-gneiss 
 
 Diorite Diorite-gneiss 
 
 Gabbro Gabbro-gneiss 
 
 Pyroxenite Pyroxenite-gneiss 
 
 Peridotite Peridotitic-gneiss 
 
 Conglomerate, etc. Conglomerate-gneiss 
 
 Sandstone Quartzite-gneiss 
 
 The conglomerate gneiss of Munson, Mass., a well-known building stone, 
 is so thoroughly crystallized that it is commercially referred to as a granite. 
 The conglomerate was probably originally derived from granites and diorites, 
 and its age appears to have been basal Palaeozoic. 
 
 Gneiss forms the dominant rock of the old crystalline areas around and 
 upon which the younger sediments were deposited. In America the largest 
 area of such ancient gneisses forms the so-called Canadian Shield. Gneisses also 
 occur in the older Appalachians and in the Cordilleran region. In Europe a 
 similar area is found in the Scottish Highlands, Scandinavia, and Finland. 
 Younger gneisses are also abundant, especially in New England, in parts of 
 Scotland, and in the Alps, where even Mesozoic rocks have been metamor- 
 phosed into gneisses by the great dynamic disturbances. 
 
 Marbles. These comprise both crystalline limestones and dolomites, 
 but not all of these have the qualities demanded of commercial marble. 
 Some very slightly altered limestones occasionally furnish this product. All 
 kinds of limestones, of whatever origin, may be changed to marbles, and in 
 this process the original bedding-structure and the fossils are commonly ob- 
 literated. When the original limestone was impure, as is usually the case in 
 clastic limestones, silicate minerals, such as tremolite, light colored pyroxenes, 
 various micas, especially phlogopite, etc., are developed as the result of meta- 
 morphism. When the original limestone was carbonaceous, the product of meta- 
 morphism is a graphitic marble. Sometimes the rock becomes very coarsely 
 crystalline, as in the case of the Westchester dolomite marble, used in the con- 
 struction of St. Patrick's Cathedral in New York City. (See also Fig. 573, 
 p. 649.) 
 
 Marbles abound in the metamorphic districts of the Appalachian belt, es- 
 pecially in Pennsylvania, Vermont, Massachusetts, New York, and Georgia. 
 They are also extensively developed in western Colorado and in the Sierras 
 of California, etc. In the Pyrenees, Alps, Carpathians, and Himalayas many 
 marble quarries exist. The famous Carrara marble is a metamorphosed upper 
 Triassic limestone of the Apennines in North Italy. It is associated with sericite, 
 
654 Metamorphism and Metamorphic Rocks 
 
 chlorite, ottrelite, and mica schists, and is underlain by conglomerate gneisses 
 believed to be of Permian age. The main zone of this marble has a thickness 
 ranging up to 1000 meters. Both white and dark common marble (ordinario) 
 make up the main deposit. " This, however, incloses locally the famous fine- 
 grained snow-white statuary marble (statuario), which comprises about 5 per 
 cent of the entire deposit. The " antique marble " of eastern Greece is in- 
 cluded in crystalline schists and is partly metamorphosed Cretaceous limestone 
 and in part belongs to the Archaean. Among the several varieties are the snow- 
 white to yellowish Pentelic marble found northeast of Athens, the translucent 
 Parian marble from the Island of Paros in the ^Egean, of snow-white color 
 with frequently a bluish tinge, uniformly grained, and prized as the finest of 
 all marbles, and others. Great marble deposits also occur in the metamor- 
 phic regions of Norway, and good statuary marble has been obtained here. 
 Excellent statuary marble also occurs in the Tyrol (Laas region). Fine 
 marbles for decorative purposes are obtained from various parts of Africa. 
 
 Ophicalcites. These are crystalline magnesian limestones or dolomites, 
 mottled with inclusions of the mineral serpentine in varying amounts, and 
 they mark a transition from marbles to serpentines. In color, the stone is a 
 beautiful mottled green and white. The serpentine is probably derived from 
 crystals of pyroxene which originally were included in the rock, and this may 
 have been a silicious magnesian limestone of sedimentary (clastic) origin, 
 altered by regional metamorphism. The rock is also called Verd-antique. 
 
 Serpentine. This is a rock composed of green or red scales, fibers, and 
 massive aggregates of the mineral serpentine, a hydrous silicate of magnesium 
 and iron and generally formed by the static alteration of basic igneous rocks, 
 such as pyroxenites and peridotites. Serpentines vary greatly in texture, and 
 they may contain small quantities of secondary minerals such as chromite, 
 magnetite, garnet, etc. Veins of calcite or magnesium carbonates often intersect 
 the serpentine in all directions and produce a striking appearance. The rock 
 is much used as a building and ornamental stone. 
 
 Soapstone or Steatite. This is a massive talc rock, differing from talc 
 schist mainly in the absence of schistosity. Quartz veins and scattered quartz 
 grains are not uncommon, and magnesium carbonate is also present. The rock 
 often occurs in association with crystalline limestones, as in the Adirondacks, 
 and may be the alteration product of a silicious dolomite or a non-ferruginous 
 basic intrusive igneous rock. Soapstones are of great, economic importance, 
 and their distribution is generally similar to that of the crystalline limestones and 
 the serpentines. 
 
 Magnetite Rock. This is a metamorphic product of iron ores, and where 
 found in abundance constitutes a valuable source of iron. 
 
 Anthracite, Graphite Rock. Anthracite is sometimes considered the 
 product of slightly metamorphosed coal beds, but may also originate as an 
 original deposit. The extreme metamorphism of carbonaceous deposits produces 
 graphite, which is pure carbon. 
 
CHAPTER XXI 
 
 MOVEMENTS OF THE EARTH'S SURFACE AND THEIR 
 GEOLOGICAL EFFECTS 
 
 THE crust of our earth is subject to a variety of movements, 
 some of which are so pronounced and sudden that they produce the 
 tremblings known as earthquakes, while others are slow and gradual, 
 and are only recognized by careful measurements, or by a com- 
 parison of the characters of a region at widely separated time 
 intervals, or by special features which indicate them to the trained 
 observer. Among these are the great earth-movements by which 
 areas once land became submerged beneath the sea, while sea- 
 bottoms are upraised into land areas or even into mountain masses 
 of great height and extent. 
 
 From a purely geological point of view earthquakes are of less 
 significance than the other movements referred to, but they have a 
 very real interest for us, because they occur practically everywhere 
 and at all times and because their immediate effect upon human 
 welfare is more pronounced than that of any other geological phe- 
 nomenon. On this account we shall devote more space to them 
 than their geological importance would demand. 
 
 SUDDEN CRUSTAL MOVEMENTS EARTHQUAKES 
 General Consideration 
 
 Earthquakes are the tremors or vibrations set in motion in the 
 outer layers of the earth's surface by sudden disturbances in the 
 crust. These disturbances may be due to volcanic or other ex- 
 plosions, or to local deformation of the crust. They are not in- 
 frequently indicated upon the surface by changes in topography, 
 while the tremors resulting from and accompanying the disturb- 
 ances are often the cause of enormous destruction of life and 
 property, as well as modification of the surface features of the 
 earth. In how far the great disturbances in the earth's crust, of 
 
 655 
 
656 Movements of the Earth's Surface 
 
 which we have a record in the rock structures, affected the surface 
 in the past, can only be conjectured, but it is safe to say that most, 
 if not all, of them were accompanied by earthquake tremors, 
 and many of these may have been more violent than any recorded 
 in historic time. It is, however, conceivable that great disturbances 
 may go on slowly, deep down in the earth's crust, and that they 
 are manifested upon the surface only by occasional or intermittent 
 earthquakes, which are due to secondary disturbances, set in 
 motion near the surface of the earth during the progress of readjust- 
 ment of the rocks at greater depths. 
 
 Types of Seismic Disturbances 
 
 The name seismic disturbances is applied to shocks produced by 
 sudden changes in the earth's crust, and they may be manifested 
 in earthquakes, seaquakes and atmospheric disturbances (air- 
 quakes). Seaquakes and great air- waves may be originated by 
 violent explosions, such as those of the volcano Krakatoa in Java 
 in 1883, where the air-waves passed around the earth several times, 
 while the great sea-disturbances, the so-called tsunamis, 1 were 
 noticeable more than five hundred miles away. 
 
 The quaking of the lands may also be brought about by vol- 
 canic explosions, but the larger earthquakes are probably all pro- 
 duced by dislocations within the earth's crust, and the readjust- 
 ment of sections under strain. Such disturbances, if near the 
 coast or upon the sea-bottom, are communicated to the water, 
 and thus sea-disturbances or tsunamis are also induced. At- 
 mospheric disturbances are, however, as a rule, of slight pr negli- 
 gible importance as effects due to readjustment in the crust. 
 
 According to their mode of origin, seismic disturbances, of which 
 earthquakes are the typical expressions, have been divided into 
 the two following groups : 
 
 Volcanic or Explosive Earthquakes. These affect the earth, 
 sea and air. As a subordinate type, the disturbances due to ex- 
 plosions of gases in coal mines, gas reservoirs, etc., and the con- 
 cussions due to the detonations of artificial explosives either on 
 land or in the sea or air may be included. Such explosions affect 
 chiefly the air and the water, though minor tremors may be induced 
 by them in the rocky crust of the earth. 
 
 1 A word of Japanese origin for the great seismic waves often wrongly called "tidal 
 waves." 
 
Sudden Crustal Movements Earthquakes 657 
 
 Dislocation, Fault or Fracture Earthquakes. These produce 
 the larger seismic disturbances of the land and are communicated 
 to the sea as well, but affect the air only to a minor degree. The 
 actual dislocation is probably the accompaniment rather than 
 the cause of the shock, this being produced chiefly by the sudden 
 fracturing of the rock along a line where strains have accumulated 
 until they exceed what the strength of the rock can withstand. As 
 a subordinate type, the tremors due to the collapse of the roofs of 
 caves., such as frequently characterize the " Karst " region of 
 the former Austrian Coast-land, may be cited. They affect chiefly 
 the land, but only to a minor degree. Tremors due to the collapse 
 of buildings form another subordinate type. 
 
 While even these subordinate tremors may become of great 
 human or economic importance, they are insignificant in their 
 geological effects, and we may confine our attention primarily 
 to the disturbances due to volcanic explosions and those due to 
 dislocations of the earth's crust, and chiefly to the latter. 
 
 Centers and Areal Extent of Disturbances 
 
 In the case of disturbances due to volcanic explosions, we may 
 consider that they center about a circumscribed spot or point, 
 which is termed the focus or the hypocenter of the earthquake 
 (also called seismic center, centrum or origin). In the case of a 
 dislocation, the disturbance extends along a line, though it may be 
 most violent at one or several points along that line. In general, 
 the shock appears to originate comparatively near the surface, so 
 that the focus or center is probably never deeper than 30 geo- 
 graphical miles, and probably does not, as a rule, exceed 5 to 15 
 miles. The point or locus upon the earth's surface immediately 
 above this focal point or line is called the epicenter or the epicentral 
 or epifocal point or line. 
 
 Earthquake Waves. The disturbance at the focus starts a 
 series of vibrations or earthquake waves which rapidly spread from 
 this point through the earth in ever widening spheres (Fig. 574). 
 
 The rate at which such vibrations travel is enormous ; they will pass through 
 the 8000 miles of the diameter of the earth in 20 to 22 minutes, or at the rate 
 of about 375 miles per minute. The rate of transmission, however, varies with 
 the depth. From data afforded by the California earthquake of 1906, Reid 
 has calculated that while the transmission at the surface was 4.5 miles per 
 second, at 272 miles below the surface it was 6 miles per second; at 612 miles 
 
6 5 8 
 
 Movements of the Earth's Surface 
 
 depth, 6.9 miles per second; at 1225 miles, 7.8 miles per second; and at 1968 
 miles depth, it was 7.9 miles per second. This relatively lessening rate of in- 
 
 FIG. 574. Diagrammatic representation of the propagation of the earth- 
 quake waves and their record by the seismograph in different parts of the 
 earth. (After Sieberg, from Keilhack's Praktische Geologic.) 
 
 Explanation and translation of terms : H, hypocentrum ; Ortsbeben, local 
 vibration (at epicentrurn) ; Nahbeben, vibrations not far distant ; Fernbeben, 
 distant vibrations; Stoss-strahl, the "wave normal/' represented by the heavy 
 lines which extend outwards from the hypocentrum (H) in all directions at 
 right angles to the successive wave lines (shown by the fine lines in the figure) 
 and along which a vertical movement takes place. FI, V^ first and second 
 preliminary vibrations (Vorlaufer) ; TFi, surface waves sent from the epicen- 
 trum; W^ surface waves returned from the antiepicentrum ; Wa, third set of 
 waves (rarely recorded) sent out again from epicentrurn ; B, principal or great 
 vibrations ; N, af tervibrations (Nachlaufer) ; Oberflachenwellen, surface waves ; 
 Erd-Rinde, earth's crust; Erd-Kern, earth's center. 
 
 crease with depth indicates that the density and elasticity of the earth's crust 
 increases with depth, down to a certain point. 
 
 Earthquake waves may be resolved into their longitudinal or compressive 
 components which vibrate in the direction of the propagation of the shock, 
 
Sudden Crustal Movements Earthquakes 659 
 
 and into the transverse components which vibrate at right angles to the di- 
 rection of propagation. To record these waves, special instruments called 
 seismographs are devised (Fig. 575 a), in which three heavy pendulums are sus- 
 pended in such a way that they will record the three movements at right angles, 
 N. and S., E. and W., and up and down, though in many seismographs the latter 
 is omitted (Fig. 576). The weight of the pendulums keeps them stationary for a 
 
 FIG. 575 a. The Ewing seismograph, constructed to record all three 
 movements (University of California, after Le Conte). Three pendulums are 
 arranged to swing in the manner of a bracket or a gate, in three planes, at right 
 angles to one another. Two of them are placed vertically and oscillate in a 
 horizontal manner, a, position north and south records east- west movement; 
 b, position east and west records north-south movement ; the third, c, oscillates 
 in a vertical manner, being placed in a horizontal position and being retained 
 by sensitive spiral springs. Styles from these pendulums make the record on 
 a circular smoked-glass plate rotating in a horizontal plane. (Fig. 575 &.) 
 d, driving clock ; e, time-recording clock. 
 
 > 
 
 long tune, while the earth vibrates beneath them. The point of a pencil inserted 
 in the pendulum makes the record upon sheets of paper which are moved by 
 clockwork at a regular rate. These records are called seismograms, and two or 
 three are formed for each earthquake, according to the construction. They 
 consist of a series of wave-like or zigzag lines crossing the central line of the 
 paper, and their amplitude records the varying magnitude of the vibration. 
 
66o 
 
 Movements of the Earth's Surface 
 
 Thus a record made on the side of the earth opposite that of the epicentrum 
 (Fig. 577 b) will record first a series of minor or preliminary tremblings (V\, V 2 ), 
 
 FIG. 575 b. Part of record 
 made by Ewing seismograph. 
 a, east and west motion ; b, north 
 and south motion; c, up and 
 down motion. (After Sek'iya 
 from Le Conte.) 
 
 FIG. 576 a. Seismograph recording 
 two horizontal movements (Spindler 
 
 and Hoyer, Manuf. Gottingen). A cylindrical mass or pendulum weighing 
 from 80 to 200 kilograms is supported on a vertical pillar about i meter long, 
 which is hinged at the base to one side of the framework so as to permit motion 
 in two directions at right angles. From the center of gravity of the pen- 
 dulum mass extend two horizontal shanks, oriented respectively in a north- 
 south and east-west position, and which through the medium of two levers 
 operate two styles or markers. These rest upon a cylinder, covered with smoked 
 paper (see Fig. 576 6), which revolves on its horizontal axis under the influence of 
 a weight. During an earthquake the suspended mass, owing to its inertia, 
 remains immobile with reference to the vertical, and with it the shanks which 
 bear the levers and styles, while the framework with the recording cylinder 
 moves with the oscillations of the earth. The levers amplify the movements, 
 which are recorded upon the cylinder in two sets of lines, one representing the 
 east-west and the other the north-south movement. From a practical view- 
 point the framework and recording cylinder remain stationary, while the ar- 
 ticulated pendulum with the shanks, levers, and styles sways with the earth 
 movements. (After Stanislas Meunier. Courtesy of Popular Scie nee Monthly.) 
 
 which appear at the antiepicentrum 20 to 22 minutes after the shock. These 
 are followed by the strong vibrations of the principal shock (B), and these, in 
 
Sudden Crustal Movements -Earthquakes 661 
 
 turn, are followed by the feeble vibrations of the dying shock (N). The pre- 
 liminary tremors are recorded only at points more than 1000 kilometers from 
 the seat of disturbance, and 
 they are believed to have 
 come by the shortest path 
 through the earth, or in gen- 
 eral along the direction of the 
 chord between the seat of 
 disturbance and the recording 
 station (Sloss-strahl, Fig. 
 574), while the larger waves 
 travel by a longer route over 
 the surface (Oberflachenwellen, 
 Fig. 574)- 
 
 These earthquake 
 waves represent actual 
 vibratory movements of 
 the rock particles, such 
 movements being ex- 
 
 FIG. 576 b. Diagrammatic view, from 
 above, of the seismograph shown in Fig. 5 76 a. 
 (After Stanislas Meunier. By courtesy of 
 
 tremely complex and in Popular Science Monthly.} 
 
 all directions (Fig. 578). 
 
 The distance to which the particles vibrate from their original 
 
 point of rest, or, as it is called, the amplitude of the vibration, 
 
 FIG. 577 a. One of the seismograms of the Messina earthquake of De- 
 cember 28, 1908, as registered at Gottingen, Germany, by the seismograph 
 illustrated in Figs. 576 a, b. (After Stanislas Meunier. By courtesy of Popular 
 Science Monthly.) 
 
662 
 
 Movements of the Earth's Surface 
 
 tt 
 
 o <U o 
 
 I" 831- 
 
Great Earthquakes of Modern Times 663 
 
 is seldom very great, though the surface waves produced in water 
 or unconsolidated rock material or soil are often pronounced. 
 Water waves 40 feet in height and piling up on the coast to 60 
 feet were produced by the Lisbon earthquake of 1755, and the 
 loose alluvial material of the Mississippi Valley bottom was thrown 
 into such wave-like commotions during the New Madrid earth- 
 
 FIG. 578. Model of a part of the path traveled by a particle on the earth's 
 surface during the earthquake of Tokyo, January 15, 1887. (After Seikei 
 Sekiya.) This model is constructed by the combination of the three movements 
 recorded by the Ewing type of seismograph. 
 
 quake of 1811 that the trees bent over and interlocked with their 
 branches. Here the actual movement of the particles of the solid 
 underlying rock, or the amplitude of vibration, was probably not 
 over a few centimeters. It has been ascertained that the ampli- 
 tude of the vibration of rock particles in earthquakes of sufficient 
 violence to destroy an entire city is often not greater than 20 mil- 
 limeters, or about three fourths of an inch, while amplitudes of 
 half or even one quarter that amount are productive of destruc- 
 tive effects. The pronounced effects of earthquakes are produced 
 by the suddenness of the shock rather than by the amount of 
 motion of the rock particles. 
 
 GREAT EARTHQUAKES OF MODERN TIMES 
 
 In order that the student may gain some concrete understanding 
 of earthquakes and the phenomena accompanying them, we will 
 briefly review some of the more violent earthquakes of modern 
 times, after which the main characteristics may be summarized. 
 
664 Movements of the Earth's Surface 
 
 
 
 The Lisbon Earthquake of 1755 
 
 One of the most destructive earthquakes which has visited 
 Europe in historic times was that which, on November i, 1755, 
 devastated the city of Lisbon, Portugal. Without previous warn- 
 ing, except for a thunderous underground noise immediately 
 before the shock, the greater part of the city was laid in ruins, and 
 60,000 persons perished within the next six minutes. The sea 
 retired abruptly, laying bare the bar off the coast, and then rolled 
 back over the land as a huge series of waves, rising 50 feet or more 
 above its ordinary level. The shock was marked in the mountains 
 of Portugal, which seemed to rock, while fissures opened at the 
 summits of some of them, rending them in a most intricate manner, 
 huge masses being precipitated into the valleys below. Flames 
 appeared to issue from these clefts, apparently the play of electrical 
 phenomena, and clouds of dust gave the appearance of smoke. 
 
 A new quay .built entirely of marble, upon which many persons 
 had congregated for safety, sank suddenly, carrying the hapless 
 mortals with it, and of their bodies not one is said to have floated 
 again to the surface. Many boats and small vessels anchored 
 near by, and filled with people, were swallowed by the waters, the 
 depth of which in the region of the quay became 100 fathoms or 
 more. The lower part of the city and the quay were built upon 
 the blue clay and other Tertiary and younger strata at the mouth 
 of the Tagus River, and it was the structures thus located, which 
 suffered chiefly, not a building upon the Mesozoic limestones or 
 the basalts being injured. 
 
 The shock of this earthquake affected a portion of the earth's 
 surface estimated to have been four times greater than the extent 
 of Europe. It was felt in the- Alps, on the coast of Sweden and 
 elsewhere on the Baltic, over much of north Germany, in Thuringia 
 and in Great Britain, where the water of Loch Lomond in Scotland 
 rose suddenly over two feet against the banks and then subsided 
 below its usual level. At Kinsale in Ireland a body of water 
 rushed into the harbor, whirling around vessels there stationed, 
 and pouring into the market place. In the West Indies, where the 
 tide is usually only two feet high, the water rose suddenly by more 
 than 20 feet, appearing discolored and of an inky blackness. Even 
 in the region of the Great North American Lakes the shock was 
 felt. It was, of course, most violent in the Mediterranean region, 
 
Great Earthquakes of Modern Times 665 
 
 the agitation being as marked in Algiers, north Africa, and in 
 Morocco as in Portugal and Spain. Many persons (8000 to 10,000 
 in one district) were said to have been swallowed up by fissures 
 which opened and closed again. Even at a distance out at sea, 
 the shock was felt on the decks of vessels, being so violent at a 
 point 40 leagues west of St. Vincent that the men were said to 
 have been thrown a foot and a half perpendicularly up from the 
 deck. A great sea wave or tsunami, originating on the ocean floor 
 50 or more miles off the coast of Lisbon, and estimated to have 
 been 60 feet high at Cadiz, swept the coast of Spain. It was 
 followed by others of decreasing heights. At Tangier, Africa, the 
 water rose and fell 18 times ; on the Madeira coast (at Funchal) 
 it rose fully 15 feet above high-water mark, the tide at the time 
 being at half ebb. Several of the coast cities were flooded. It 
 took the sea-wave 2-| hours to reach the Madeira coast, although 
 the shock was transmitted through the earth in 25 minutes. This 
 shock caused no retreat of the sea there on account of the steep- 
 ness of the coast. The effects of those waves were felt even on 
 the shores of the West Indian Islands across the whole expanse 
 of the Atlantic. 
 
 The Calabrian Earthquake of 1783-86, and the Messina Earthquake 
 
 of 1908 
 
 The Calabrian peninsula of Italy (Calabria Ultra) (Fig. 579 a) 
 was the scene of a violent series of earthquake shocks, which began 
 in February, 1783, and lasted for nearly four years to the end 
 of 1786. This is the first of the great earthquakes the effects of 
 which were carefully noted by men of scientific training. The 
 convulsions extended not only over the whole of Calabria Ultra, 
 but also over the southeastern part of Calabria Citra and across 
 the sea to Messina, affecting an area of about 500 square miles. 
 The concussions were noted over a great part of Sicily and as far 
 north as Naples. 
 
 The formations covering the area chiefly affected consist of 
 thick argillaceous beds with marine fossils and some sands and 
 limestone, all of Tertiary or younger age and abutting against the 
 central Apennine chain of granite and other rock. The Calabrian 
 plain formed by these Tertiary and younger rocks is flat and level 
 except where dissected by streams, which have cut gorges, in places 
 
666 
 
 Movements of the Earth's Surface 
 
 600 feet deep, and with steep, sometimes almost perpendicular 
 sides, due to the binding together of the upper beds by roots of 
 trees, etc. 
 
 The greatest destruction occurred within a radius of 22 miles of 
 the city of Oppido in Calabria Ultra. The first shock, which oc- 
 curred on February 5, 1783, " . . . threw down in two minutes 
 
 the greater part of the 
 houses in all the cities, 
 towns, and villages, from 
 the western flanks of the 
 Apennines in Calabria 
 Ultra to ~ Messina in 
 Sicily, and convulsed the 
 whole surface of the 
 country " (Lyell). A 
 second shock of almost 
 equal violence occurred 
 on March 28, and this 
 rudely shook the granite 
 chain which passes 
 through Calabria from 
 north to south, and which 
 
 FIG. 579 a. Map of Calabria, showing the 
 regions chiefly affected by the earthquakes. 
 (After Lyell.) 
 
 was only slightly shaken 
 by the first shock. Along 
 the flanks of this chain 
 
 the soil slid downwards, producing a chasm] from 9 to 10 miles 
 in length between the solid granite and the sandy soil. These 
 landslides were sometimes carried for considerable distances over 
 the lower ground, thus causing the properties of different indi- 
 viduals to become superimposed and leading to subsequent dis- 
 putes of ownership. The chasm along the mountain base has 
 been explained as caused by the reflection and refraction of the 
 earthquake wave in passing from a body of low elasticity, such 
 as the clay and gravel, to one of high elasticity like the granite, a 
 shock being thus produced in the opposite direction. 
 
 The surface of the plain was thrown into undulating movements 
 by each shock to such an extent that rooted trees are said to have 
 touched the ground with their branches, a fact attested by com- 
 petent authority. A vorticose movement was indicated by the 
 behavior of the large stones of two obelisks at the convent of 
 
Great Earthquakes of Modern Times 667 
 
 St. Bruno, the pedestals of which remained in position while the 
 separate stones were partly rotated horizontally (Fig. 579 b). 
 Pavement stones in 
 many towns were 
 thrown up and over- 
 turned, and a round 
 tower at Terranuova 
 was faulted through 
 the center (Fig. 
 579 c). Along this 
 fault-line houses on 
 one side were lifted 
 above those of the 
 
 t> r VrTi cant ^ IG< 579^' Shifts in the two obelisks in the 
 
 er > convent of St. Bruno, Calabrian earthquake of 
 
 with the ground, and x 7 s 3 . (After Lyell.) 
 
 walls crossing it were 
 
 faulted, yet with sides so firmly adhering that the fault was only 
 "marked by the displacement of the tiers of stone on opposite 
 sides. In this town, too, a stone well was apparently driven from 
 
 FIG. 579 c. Fault in the round tower of Terranuova in Calabria occasioned 
 by the earthquake of 1783. (After Lyell.) 
 
 the ground, resembling a small tower eight or nine feet in height, 
 and a little inclined. This was probably effected by the settling 
 of the soil around the stone well. In Monteleone, some streets 
 
668 Movements of the Earth's Surface 
 
 had all their houses but one thrown down, others all but two, 
 these excepted buildings being often scarcely injured. In many 
 Calabrian cities all the more solid buildings were destroyed, the 
 lighter ones escaping, but the reverse was true at Messina and 
 elsewhere. 
 
 Rents and chasms were opened and closed again along the path 
 of the earthquake, engulfing houses, cattle, and human beings, 
 but in a few cases, it was stated, individuals thus engulfed were 
 thrown out again, sometimes still alive, by an immediately follow- 
 ing shock. Radiating fissures were formed in many places and 
 the surface of the country was broken by cracks resembling those of 
 a shattered pane of glass. 
 
 In the central district around Oppido, many houses were com- 
 pletely engulfed in the opening and closing fissures, while at Canna- 
 maria ". . . four farm houses, several oil-stores, and some spacious 
 dwelling houses were so completely engulfed in one chasm, that 
 not a vestige of them was afterwards discernible " (Lyell). Similar 
 phenemona occurred elsewhere, and later excavations showed that 
 detached parts of the buildings and their contents were so firmly 
 jammed together by the closing of the fissure that they formed 
 one compact mass. Many fissures closed more gradually, several 
 near Mileto, which had engulfed an ox and nearly one hundred goats, 
 being, when later visited, still nearly a foot in width. One fissure, 
 however, on a hillside near Oppido, which had swallowed part 
 of a vineyard and a considerable number of olive trees with much 
 soil, remained open for a length of 500 feet and a depth of 200 feet. 
 Many fissures formed by the shock of February 5 were greatly 
 widened, lengthened, and deepened by the shock of March 28, 
 some of them becoming nearly a mile in length by from 150 to 
 200 feet .in depth. They were usually straight, but sometimes 
 crescent form. 
 
 But the most remarkable features formed were numerous small 
 craterlets or funnel-shaped sinks, of about the size of a carriage 
 wheel or larger, which covered parts of the plains (Fig. 579 d). These 
 were in some cases filled witji dry sand, in others with water which 
 arose through a neck or tube at the base. Innumerable cones of 
 sand were thrown up in marshy places by the spouting upward 
 of the water in jets. Rivers were dried up by the violent shocks, 
 but immediately afterward overflowed their banks. River courses 
 were deranged by extensive landslides, and 215 lakes and small 
 
Great Earthquakes of Modern Times 669 
 
 ponds were formed. In one case, a river valley was transformed 
 by a landslide into a lake two miles long and one mile broad/ Oaks, 
 olive trees, vineyards, and corn slid with the land into the river- 
 valley at Terranuova, where they continued to grow as did those of 
 the portion from which they were detached at least 500 feet higher 
 and about three quarters of a mile distant. Near Seminara an 
 extensive olive orchard was hurled to a distance of 200 feet into a 
 valley 60 feet in depth, the region from which it was detached 
 opening in a deep chasm which was appropriated by the river, 
 
 FIG. 579 d. Craterlets formed during the Calabrian earthquake of 1783. 
 
 (From C. Vogt.) 
 
 leaving its former channel completely dry. On this mass of earth 
 stood a small house, which was carried down with it entire and 
 without injury to its inhabitants. The olive trees thus transported 
 bore an abundant crop of fruit the same year. The greater part 
 of the town of Polistena, " consisting of some hundreds of houses, 
 travelled into a contiguous ravine and nearly across it, about half 
 a mile from their original site, " and several of the inhabitants 
 were dug out of the ruins alive and unhurt. Near Mileto, two 
 tenements, " occupying an extent of ground about one mile long 
 and half a mile broad, were carried for a mile down a valley." 
 Here the ground had been long undermined by rivulets. A 
 thatched cottage and large olive and mulberry trees were carried 
 uninjured for this same distance, most of the trees remaining 
 erect. 
 
 In many places mud-streams were formed which buried houses 
 and trees in their paths. In one case two such mud-streams, 
 rolling forward like streams of lava, united in a valley, forming a 
 
670 Movements of the Earth's Surface 
 
 stream 225 feet wide and 15 feet deep, and before it ceased to move 
 it covered a surface one Italian mile in length. " In its progress 
 it overwhelmed a flock of 30 goats and tore up by the roots many 
 olive and mulberry trees, which floated like ships upon its surface" 
 (Lyell). This mud was highly calcareous, and it gradually dried 
 and hardened, the mass decreasing 7^ feet in thickness. 
 
 Along the straits of Messina, near the famous rock of Scylla, 
 huge masses of rock were detached from the lofty cliff and over- 
 whelmed many gardens and villas. " At Gian Greco, a continuous 
 line of cliffs, for a mile in length, was thrown down. The sea was 
 violently agitated, and rising more than 20 feet rushed back and 
 forth over the low coast, causing great destruction of life. The 
 aged prince of Scylla, and 1430 of his people, who had taken to the 
 fishing vessels for safety, perished, all the boats being destroyed." 
 
 The total number of persons that perished by this earthquake 
 in the two Calabrias and Sicily was estimated at 40,000, and about 
 20,000 more died from epidemics caused by insufficient nourishment 
 and exposure, and by malaria arising from the new stagnant pools. 
 Many people were burned to death, as numerous fires resulted, 
 and many were swallowed alive by the fissures which opened and 
 closed and might have been saved if help had been at hand. But 
 by far the greater number perished in the ruins of their houses, 
 while on the coast drowning was the chief cause of death. 
 
 This same region, along the straits which separates Sicily from 
 the mainland of Italy, was visited by the most destructive earth- 
 quake of modern times on December 28, 1908. The cities of Mes- 
 sina and Reggio were completely destroyed, and so were many 
 smaller towns and villages. The whole of Calabria and of eastern 
 Sicily was affected by the shock. The catastrophe has been called 
 the most appalling of its kind that has visited any country. The 
 number of persons killed was approximately 78,000, while the 
 number of injured was beyond calculation. 
 
 The New Madrid (Missouri) Earthquake of 1811-12 
 
 On March 26, 1812, violent earthquake shocks destroyed the 
 city of Caracas, Venezuela, and previously to this and continuing 
 for some time after, earthquake shocks were felt in South Carolina 
 and in the valley of the Mississippi from New Madrid to the 
 mouth of the Ohio in one direction, and to St. Francis, Arkansas, 
 
Great Earthquakes of Modern Times 671 
 
 in the other. At New Madrid, subterranean rumblings had 
 been heard frequently for many years before and up to within a 
 year of the earthquake. The first great shock came about 2 
 o'clock in the night of December 16, 1811, accompanied by a noise 
 like thunder, and in a few minutes the air was saturated with sul- 
 phurous vapors. Between December 16, 1811, and March 16, 
 1812, a total of 1874 shocks was recorded, eight of them being of 
 the first order of intensity. The most violent one occurred on 
 February 7 and was accompanied by sulphurous vapors and 
 unusual darkness. 
 
 The region affected is a part of the flood plain of the Mississippi 
 and is underlain by unconsolidated sands and muds. The quaking 
 of the ground continued for several successive months, and great 
 changes in the surface were produced. Large lakes were formed, 
 sometimes in the course of an hour ; others already in existence 
 were drained. The most noted of these newly formed water bodies 
 is Reelfoot Lake, in Obion County, northwest Tennessee, which 
 has a length of more than 20 miles and a width of 7 miles, while 
 the water in places covers ihe tops of submerged cypress trees 
 which grew on the ground before it settled. Near Little Prairie, 
 a lake many miles in length but only from 3 to 4 feet in depth came 
 into existence. Later it disappeared, leaving behind a stratum of 
 sand. Lake Eulalie, near New Madrid, 300 yards long and 100 
 yards wide, was suddenly drained through parallel fissures which 
 opened in the bottom and which were not yet closed when Lyell 
 visited the region 34 years later. The ancient bed of the lake is 
 now largely overgrown with forest trees. 
 
 During the first shock, the current of the Mississippi River 
 was reversed in direction north of New Madrid, continuing so for 
 several minutes. During later shocks, mountainous waves were 
 generated in the river, which receded from its banks, leaving boats 
 high upon the sand, and then moving forward as a wall of water 
 15 to 20 feet high, tore them from their moorings and swept them 
 into a creek as a close-packed mass, a quarter of a mile long. The 
 graveyard at New Madrid was precipitated into the bed of the 
 Mississippi ; and the ground on which the town is built, and the 
 river bank for 15 miles above it, are said to have sunk eight feet 
 below their former level. The trees of the region suffered break- 
 ing, and many were inclined in all directions, continuing to grow 
 thus. Many fissures were opened, usually in a northeast-southwest 
 
672 Movements of the Earth's Surface 
 
 direction, and much water, sand, and lignite was discharged often 
 with great force. Hundreds of these chasms were still visfble in the 
 aUuvial so, seven years after the event, and Lyell noted many 
 of them only partly filled 35 years after their formation 
 
 Numerous circular craterlets or sinkholes, from 10 to 30 yards 
 mde and 20 feet in depth, appeared. These were located on the 
 border of the Sunk Country," west of New Madrid, which extends 
 along the course of the White Water and its tributaries for a dis- 
 ance of between 70 and 80 miles north and south and 30 miles 
 east and west. In this area innumerable dead trees are sub- 
 merged, some erect, but many prostrate. The borders of the 
 submerged area are gradually filling up by growth of swamp vege- 
 tation and sediments washed into it. East of this area a low 
 ae, 20 miles in diameter and rising 20 or 25 feet above the 
 alluvial plain, was formed. 
 
 In this case also the trees are said to have bent down during the 
 shocks many of them becoming interlocked with the branches of 
 trees similarly bent, and being thus prevented from righting 
 themselves again. One result of the disturbance due to this earth- 
 quake was the great confusion into which boundary lines were 
 rown, so that the government found it necessary to resurvey 
 an area of 1,000,000 acres. 
 
 The Chilean Earthquakes of 1822 and Later Periods 
 Chile has been visited by many earthquakes, of which those 
 22 1835, and 1906 are especially noteworthy. The coastal 
 ion (Fig 580) was visited by a particularly destructive earth- 
 quake on November 19, l8 , which was felt simultaneously for 
 Distance of I2 oo miles north and south. Much damage was 
 done m Valparaiso, Santiago, and other places, and the coast near 
 Valparaiso was raised from three to four feet above its former level 
 exposing beds of oysters, while mussels and other Mollusca and 
 saweeds adhering to the rocks were left high and dry Vast 
 quantities of fish were killed and their bodies left to decay over - 
 the raised ground. The slopes of streams back of the coast were 
 increased, one of them gaining a fall of 14 inches in little more than 
 This indicates a greater inland rise, which was esti- 
 ated to be from five to six or even seven feet at a distance of a 
 mile from the coast. Parallel fissures opened in the granitic rocks 
 thus elevated, some of these being traceable for a mile and a half 
 
Great Earthquakes of Modern Times 673 
 
 inland. As in the Calabrian earthquake, cones of earth up to four 
 
 feet in height were built up on alluvial soil by water, mixed 
 
 with sand, being forced 
 
 through funnel-shaped 
 
 openings. The houses 
 
 built on alluvial soil were 
 
 more damaged than 
 
 those built upon the 
 
 granite. 
 
 The total area elevated 
 during this earthquake 
 is estimated at 100,000 
 square miles, an area 
 equal to half that of 
 France. Accepting this 
 estimate, Lyell figured 
 that the entire mass of 
 land raised above sea- 
 level was 57 cubic miles 
 in bulk, and of a weight 
 100,000 times that of 
 the great Pyramid ; and 
 if a moderate estimate 
 of the weight of the entire 
 mass displaced is made, 
 assuming the depth af- 
 fected to be two miles, 
 it would be 3630 times 
 that much. Lyell has 
 further estimated that 
 the amount of solid mat- 
 ter thus raised above 
 the sea-level by this 
 single earthquake is equal to that which the Ganges River would 
 carry into the sea during a period of four centuries. 
 
 The shocks continued at intervals of 24 to 48 hours for nearly 
 a year, or until the end of September, 1823. Twelve years later, 
 on February 20, 1835, another great earthquake visited this coast, 
 being felt for nearly a thousand miles from north to south between 
 Copiapo, 400 miles north of Valparaiso, and Chiloe, and for about 
 
 FIG. 580. Map of a part of Chile, showing 
 the regions principally affected by earth- 
 quakes. (After Lyell.) 
 
674 Movements of the Earth's Surface 
 
 500 miles east and west from Mendoza to the island of Juan 
 Fernandez, 365 miles from Chile. Its effects were particularly 
 noted in Concepcion Bay, where the sea retreated, stranding vessels 
 which lay in seven fathoms of water, and then rushed in again with 
 several repetitions, the waves being thrown 1 6 to 20 feet in height 
 and rushing far up on the shelving beach. Large numbers of 
 cattle were washed into the sea by these waves, and others stand- 
 ing on a steep slope near the shore were rolled into the sea by the 
 shock. 
 
 Upward of a hundred villages were destroyed on the coast and 
 the islands of the bay, and rocks broken off beneath the sea were 
 cast high upon the shore. Darwin found one of them to have a 
 length of six feet, a breadth of three, and a thickness of two feet. 
 On Quiriquina Island, Darwin found many fissures extending in 
 a north-south direction, some of them a yard wide. The hard, 
 slaty rock of the island was shivered superficially as if blasted by 
 gunpowder, and huge blocks were precipitated to the beach, 
 others being loosened and left in a position where the heavy rains 
 would tend to their further displacement. Darwin believed that 
 this single convulsion " has been more effectual in lessening the 
 size of the island of Quiriquina than the ordinary wear and tear of 
 the sea and weather during the course of a whole century." 
 
 The island of Santa Maria, about 25 miles southwest of Con- 
 cepcion, and about seven miles long by two broad, was raised ten 
 feet at the northern and eight feet at the southern end, as shown by 
 the mussels which were found clinging to the steep faces of the rock. 
 A large flat at the northern end of the island, formerly submerged, 
 became permanently exposed, causing the extermination of great 
 beds of shellfish, while the water all around the island was dimin- 
 ished in depth by a fathom and a half. 
 
 The great sea waves or tsunamis which originated from this 
 earthquake " traversed the ocean to the Society and Navigator 
 Islands, 3,000 and 4,000 miles distant, and to the Hawaiian Islands, 
 6,000 miles away " (Dana). The velocity of such waves is very 
 great, one originating in the earthquake of 1868 running to the 
 Hawaiian Islands at a rate of 465 miles per hour. 
 
 During the earthquake of 1835 and for some time preceding and 
 following it, the whole volcanic chain of the Chilean Andes, 1300 
 miles in length, was in a state of unusual activity. Lava flowed 
 from the crater of Osorno at the southern end, and a submarine 
 
Great Earthquakes of Modern Times 675 
 
 volcano broke out about a mile from the island of Juan Fernandez 
 (365 geographical miles from Chile), this island being violently 
 shaken and devastated by a great wave. 
 
 Another earthquake shock occurred at Concepcion in November 
 of the same year, and at the same time the volcano Osorno, 400 
 miles distant, renewed its activity. 
 
 Two years later, on November 7, 1837, Valdivia, situated near 
 the coast about 300 miles south of Concepcion, was destroyed 
 by a violent earthquake, and the sea-bottom near the island of 
 Lemus in the Chonos Archipelago was raised more than eight feet. 
 
 In August, 1906, the coast of Chile was subjected to another 
 severe earthquake which did much damage in Valparaiso and 
 other places, killing several thousand persons. After-shocks 
 continued for a long time, while the readjustment along the fault 
 lines took place. 
 
 The New Zealand Earthquake of 1855 
 
 New Zealand has been visited by many earthquakes, which 
 have caused profound alteration of its surface and coast line. One 
 of the most marked of them occurred on the night of January 23, 
 1855, being most violent in the narrowest parts of Cook Strait, 
 between the two main islands (Fig. 581), and affecting an area of 
 land and water estimated at 860,000 square miles three times the 
 size of the British Islands. Near Wellington, in the North Island, 
 a tract of land comprising 4600 square miles was permanently 
 upraised from one to nine feet. The uplift was especially marked 
 along the eastern flank of the Rimutaka Mountains, a range running 
 northeast from Cook Strait, and rising to heights of 4000 feet above 
 the sea. A fault scarp came into existence along the eastern face 
 of the range, and where shown at the Muka M.uka cliff, on the 
 coast, 12 miles southeast of Wellington, it was found that the 
 older rocks of the mountains had experienced an elevation of nine 
 feet, while the Tertiary rocks on the east remained undisturbed 
 (Fig. 582). The elevation of the older rock on the west of the 
 fault was clearly marked by a line of nullipores or calcareous 
 seaweeds, originally at sea-level, but immediately after the shock 
 nine feet above it. A beach 100 feet wide also came into existence 
 at the foot of this cliff, where formerly the water had washed it 
 closely at high tide. 
 
Great Earthquakes of Modern Times 677 
 
 The fault scarp extends continuously into the interior of the 
 country along the base of the Rimutaka Mountains, being marked 
 by nearly perpendicular fresh cliffs nine feet in height, and traceable 
 for a distance of about 90 miles. In many places the fault line 
 was marked by an open 
 fissure from six to nine feet 
 broad and filled locally with 
 soft mud and loose earth. 
 
 The effects of the eleva- 
 tion were also seen at Port 
 Nicholson, about 12 miles FIG. 582. Section of Muka Muka 
 
 west of Muka Muka cliff, the cliff > Cook Strait > New Zealand, showing 
 
 ,, r r c recent fault. A, argillite: B. Tertiary 
 
 elevation there being five feet beds . c ^ Hne of fissure and fauk (Affa 
 
 on the eastern and four feet Lyell.) 
 on the western side of the 
 
 harbor. We have here an interesting example of block faulting 
 or the tilting of a great block of the earth's crust by differential 
 elevation. It is also significant that although the fault plane 
 was apparently vertical, the strata on the east remained undis- 
 turbed and without change in position, while the block was 
 actually raised along the fault plane. Thus the fault is really 
 an upward thrust although it has the appearance of a gravity 
 fault. 
 
 Owen's Valley, California, Earthquake of 1872 
 
 Owen's Valley lies in eastern California near the eastern base of 
 the Sierra Nevada Mountains. The earthquake occurred on March 
 26, 1872, and the ground sank in strips producing several fault 
 scarps, the principal one of which followed the base of the 
 mountains, and in places rose to 20 feet and extended for about 
 40 miles. Opposite the highest point a second scarp appeared 10 
 feet high and facing in the opposite direction. Other parallel 
 faults were formed and an area of several thousand acres of land 
 was not only lowered bodily, but also shifted northward for about 
 15 feet. At Big Pine, many extensive fissures were opened, trace- 
 able for several miles, while an area of ground 200 to 300 feet wide 
 sank in places to a depth of 20 feet or more, leaving vertical fault 
 scarps on opposite sides. This depression was filled with water, 
 forming a pond one third of a mile in length. A road crossed by 
 
678 
 
 Movements of the Earth's Surface 
 
 a north-south fissure had the part to the west of the fissure shifted 
 1 8 feet to the south. 
 
 The earthquake consisted of only one violent shock, and all 
 the changes were produced within a few seconds. After-shocks 
 and slight tremors, however, continued for two months longer. 
 
 The Charleston Earthquake of 1886 
 
 On August 31, 1886, a violent shock visited the city of Charleston, 
 S. C., at 9:51 P.M., there having been two light premonitory 
 shocks on the 28th and 2;th preceding. About 14,000 chimneys 
 
 were thrown down, and 
 in some streets walls and 
 roofs of buildings col- 
 lapsed. The shocks 
 lasted a little more 
 than half a minute, the 
 greatest destruction be- 
 ing accomplished in the 
 first twenty seconds. 
 
 Numerous craterlets 
 of the type formed in 
 Calabria and the New 
 Madrid region were 
 opened on the flat 
 country, some of them, 
 
 FIG. 583. A small craterlet or funnel- 
 shaped depression formed during the Charleston 
 earthquake. The umbrella indicates the size. 
 
 which were aligned along 
 a fissure, being twenty 
 
 feet in diameter. Water, mud, and sand gushed from these, in 
 some cases to a height of twenty feet (Fig. 583). 
 
 Three railroads entering Charleston from different directions 
 had their rails twisted, bent, and wrenched, especially where the 
 fissures crossed the tracks. The number of killed and wounded 
 was slight compared with other earthquakes. The shock of this 
 disturbance was distinctly felt in Chicago, 800 miles away 
 (Fig. 584). 
 
 The Sonora Earthquake of 1887 
 
 On May 3, 1887, a violent earthquake affected more than one 
 half of Mexico and two thirds of New Mexico and Arizona, but 
 
Great Earthquakes of Modern Times 679 
 
 owing to the sparsely settled condition of the district, little damage 
 was done to life and property. The region most affected was the 
 old province of Sonora on the northern border of old Mexico. 
 " Here a range of mountains, the Sierra Teras, was uplifted between 
 faults which opened on 
 either side" (Hobbs). 
 The displacement varied 
 from zero to 20 feet. In 
 places along the western 
 fault, the displacement 
 was in the opposite di- 
 rection. 
 
 " Millions of cubic feet 
 of rock were thrown down 
 from the slopes into the 
 canyons and water 
 courses, and cliffs of com- 
 pact rock were shattered 
 and split as though by a 
 charge of giant powder " 
 (Hobbs). Hundreds of 
 small fissures, accom- 
 panied frequently by ver- 
 tical displacements of 
 one or two feet, opened 
 in the flat country (Fig. 
 585), some of these dis- 
 charging water, and nu- 
 merous craterlets two 
 feet or more in diameter 
 opened along the fissure 
 lines and gushed forth 
 water and sand. All the 
 
 water courses of the San Bernardino Valley experienced a change 
 in level of from six inches to two feet. While much water 
 gushed up and the streams became swollen during the shocks, 
 the water dried away immediately after, the springs also going 
 dry. 
 
 Numerous forest fires were started, probably by friction or by 
 sparks struck from flint rocks during the land-slips. 
 
 FIG. 584. Map of the Charleston earth- 
 quake, showing the isoseismic curves formed 
 by connecting the points in which the shocks 
 were of equal magnitude. The order of 
 shocks is: i, microseismic ; 2, extremely 
 feeble; 3, very feeble; 4, feeble; 5, of 
 moderate intensity ; 6, fairly strong ; 7, strong; 
 8, very strong ; 9, extremely strong ; 10, shock 
 of extreme intensity. 
 
68o 
 
 Movements of the Earth's Surface 
 
 Japanese Earthquakes of 1891 and 1896 
 
 Japan is probably one of the most unstable countries of the 
 earth. During a period of nearly 1500 years, 225 destructive 
 earthquakes have been recorded, and the careful records kept 
 since the beginning of the seventeenth century show that a destruc- 
 tive earthquake 
 has occurred some- 
 where in Japan 
 about once in every 
 two and a half 
 years. 
 
 On October 28, 
 1891, occurred the 
 greatest shock so 
 far recorded in 
 Japan. It affected 
 an area of 243,000 
 square miles, or 
 more than three 
 fifths of the whole 
 
 FIG. 585. Small fissure and fault in the Arizona 
 desert, formed during the Sonora earthquake of 
 1887. (After Branner.) 
 
 of Japan. The most violent manifestations were, however, largely 
 confined to the provinces of Mino and Owari, which form a kettle- 
 shaped basin covered by broad rice fields and surrounded by high 
 mountains except on the south. 
 
 This region had been undisturbed for a long time, and the great 
 shock came with unforeseen suddenness. Within a few moments 
 20,000 buildings were destroyed, 7000 persons were killed, and 1 7,000 
 more injured, many perishing by the fires which blazed up every- 
 where. The center of the district was fissured in an extraordinary 
 way, and an open fissure a mile in length appeared on the banks of 
 the Shonai River, where a bamboo grove " with pines and thatched 
 houses was shifted en bloc 60 feet back from the river embankment, 
 the trees remaining upright and the thatched roofs falling to the 
 ground without fracture " (Hobbs). Small mud- volcanoes and 
 sand craters arose in numbers all over the plain. 
 
 This earthquake was the accompaniment of a great fault which 
 transected the valley in a general north-northwest and south- 
 southeast direction. The part on the east of this line was lowered 
 with reference to the other side and also was shifted along the 
 
Great Earthquakes of Modern Times 68 1 
 
 fault to the northwestward from three to six feet. In general, the 
 vertical displacement was less than two feet, and there was no 
 fault scarp, but a ridging of the ground resembling a gigantic mole 
 track. In some places, however, as at Midori, there was a vertical 
 displacement of not less than 18 feet, but in the opposite direction, 
 with the formation of a great fault scarp. The lateral displace- 
 ment in this case was in the same direction as elsewhere, and 
 amounted to about 12 feet (Fig. 586). 
 
 FIG. 586. View of the great fault produced by the earthquake of 1891 at 
 Midori, in the Neo Valley, Japan. (After B. Koto.) 
 
 Many smaller displacements occurred along the rifts opened 
 around Omori, some blocks rising, others sinking, as in the cases 
 of the Owen's Valley earthquake of 1872. At one place a reservoir 
 was cut in two by the fault, the northern half being depressed and 
 shifted and the southern half drained. Two village sites were 
 transformed by depression into a deep swamp two square kilo- 
 meters in area. 
 
 During the five months succeeding this shock, no less than 
 2588 after-shocks were recorded, as many as 318 occurring on the 
 day after (102 on the first day), gradually diminishing in number 
 and intensity thereafter. 
 
682 
 
 Movements of the Earth's Surface 
 
 On August 31, 1896, a heavy earthquake affected the province 
 of Northern Honshiu, killing, however, only about 1000 persons, 
 since preliminary shocks from six to eight hours before had given 
 
 the warning. A great mag- 
 **" 3 * netic disturbance occurred 
 
 33 hours before the great 
 shock. A mountain range 
 forms the backbone of Japan 
 in this province (Fig. 587), 
 and on opposite sides of this 
 two great clefts, the Senya 
 and Kawafune clefts, opened 
 with the great shock. The 
 latter cleft was accompanied 
 by a vertical displacement 
 of six feet. Where it crossed 
 a crooked road near the vil- 
 lage of Kawafune, it cut out, 
 raised, and laterally displaced 
 a section of it. A small 
 house on the fault line was 
 
 stood upon its head without further injury. The fault was traced 
 for 15 kilometers, though not always showing displacement. 
 
 The Senya cleft or fault on the opposite side of the mountain 
 range was traced with some interruption for 60 kilometers, and it 
 had a maximum vertical displacement of 10 feet. It appears to 
 be the continuation of an older cleft farther to the southwest at 
 Sakata, formed by the earthquake of 1894. 
 
 Icelandic Earthquake of 1896 
 
 A series of destructive earthquake shocks affected southwestern 
 Iceland on August 26 and 27 and September 5, 6, and 10, 1896. 
 The region affected is a triangular plateau bordered by lofty moun- 
 tains, which include Mount Hecla and other volcanoes. These, 
 however, were quiescent before, during, and after the earthquake. 
 
 Each of the five great shocks affected a different area or block 
 of the region, though these were all contiguous. The blocks thus 
 affected were outlined by great fissures upon the surface, and the 
 succession of movements passed, in general, from the east west- 
 ward. 
 
 FIG. 587. Map of part of the prov- 
 ince of Northern Honshiu, Japan, show- 
 ing the area affected by the earthquake 
 of August, 1896. (After Yamasaki.) 
 
Great Earthquakes of Modern Times 683 
 
 The surfaces of the blocks were agitated so violently that neither 
 men nor cattle could stand. Persons lying near a cliff were thrown 
 over it, and in one village a heavy stove, six feet in height, was 
 thrown a distance of twenty-five feet. Many fissures were opened, 
 one of them extending for seven and another for nine miles in length. 
 The mountains around the plain were fissured, and land-slips 
 occurred on the steep bordering slopes. Many funnel-shaped 
 openings were formed, and swamps and ponds in some cases were 
 drained by them. A new hot spring arose after the shock of Sep- 
 tember 5, throwing water streams and volcanic rock to an esti- 
 mated height of 600 feet. After a few hours, however, it rose only 
 10 or 12 feet in the air, and ten days later had entirely ceased to 
 flow. The geysers of the region were also affected, one of them, 
 the Strokr, which had come into existence during an earlier earth- 
 quake (1789), becoming suddenly extinct and remaining inactive 
 since. (See p. 186.) 
 
 Assam (India) Earthquake of 1897 
 
 On June 12, 1897, a violent earthquake shook the district about 
 Assam, India, laying in ruins a region 150,000 square miles in area 
 within the period of fifteen seconds. The heavy shocks were all 
 over in less than two and one half minutes, having shaken an area 
 of 1,750,000 square miles. 
 
 A rumbling sound like thunder preceded the shocks by a second 
 of time, increasing to intensity so as to drown the noise of falling 
 heavy masonry near by. The ground rocked in waves " as though 
 composed of soft jelly." Many monuments were twisted, and 
 posts and houses were driven into the sandy ground, only the roofs 
 of the houses remaining visible. 
 
 Numerous fissures, generally parallel to the mountain ranges, 
 were opened, and some closed again as if under great pressure. 
 Craterlets six or more feet in diameter appeared, and sand and 
 water were thrown from these to heights of seven or eight feet or 
 more. With the sand were fragments of peat, lignite, resin, half 
 petrified pieces of timber, and a black earth not previously known 
 from that district. Similar material issued from the cracks and 
 was spread by the water over the surrounding country. The 
 streams were temporarily swollen, the Brahmaputra advancing as 
 a wall of water 10 feet in height. 
 
 The largest of the many displacements formed at this time was 
 
684 Movements of the Earth's Surface 
 
 the great Chedrang fault, which crossed and recrossed a meandering 
 stream, closely following its general course for a distance of about 
 12 miles. The maximum displacement was 33 feet, but generally 
 less. In some cases the fault was distributed over a series of parallel 
 rifts, with shiftings of as much as several inches along these. In 
 cutting the stream channel, the displacement caused the produc- 
 tion of many ponds, lakes, and waterfalls. Readjustment in the 
 hills resulted in many local changes in level, in amounts up to 
 twelve feet, and changes in location of similar amounts as well. 
 
 After-shocks of lesser intensity followed the initial destructive 
 shocks and continued for more than a week, but gradually faded 
 away. 
 
 The Yakutat Bay (Alaska) Earthquake of 1899 
 
 A heavy earthquake occurred in southern Alaska in September, 
 1899, but little was known of it until the region was explored in 
 detail in 1905. The country was broken into blocks, some being 
 elevated, others depressed, the extent of such changes being fre- 
 quently measurable on the fjords and other parts of the coast. 
 In general, the changes of level ranged from 5 to 12 feet, but ex- 
 treme changes up to 30 and even 47 feet were noted. Large 
 blocks were broken into smaller ones, and these underwent indi- 
 vidual adjustments. 
 
 New reefs and islands appeared as the result of this block adjust- 
 ment, one of these being 450 feet long, about 75 feet broad, and 
 apparently rising from deep water. In some places the sea has 
 encroached on sunken forest lands, killing the trees; in others, 
 raised beaches indicate elevation, several periods of elevation 
 sometimes being shown. 
 
 The great wave or tsunami which accompanied this earthquake 
 devastated a forest 40 feet above the level of the bay, the twisted 
 and fallen trunks now lying in utter confusion. Great changes 
 were also produced in the glaciers of the region. 
 
 California Earthquake of 1906 
 
 The great earthquake which partly destroyed San Francisco 
 on April 18, 1906, is still fresh in the minds of the present genera- 
 tion. The heavy shocks came without warning at 5.12 A.M. 
 They continued for about a minute and then graded off into lighter 
 shocks, which were repeated during a period of several days. The 
 
Great Earthquakes of Modern Times 685 
 
 FIG. 588. Map of the fault trace of the San Francisco earthquake of April 
 18, 1906. Broken lines indicate alternative hypothesis as to its extension 
 north of Point Arena. (U. S. G. S.) 
 
686 
 
 Movements of the Earth's Surface 
 
 shocks accompanied movement along the Great Fault Line which 
 runs, in general, parallel to the coast and is traced from Punta 
 Arena on the north to the vicinity of Mount Pirnos in Ventura 
 county on the south, a distance of about 400 miles, and with a 
 general direction of N. 35 W. (Fig. 588). This is an old fault line, 
 and repeated movements have occurred along it, all probably 
 accompanied by earthquakes. In the latest earthquake, move- 
 
 ments took place along this 
 
 line for at least 185 miles, 
 these movements being partly 
 vertical and partly lateral 
 displacements. On the south- 
 west side of the fault, the 
 shifting was generally to the 
 northward, ranging from a 
 few inches to 20 feet. A 
 reverse displacement, how- 
 ever, occurred at Tomales 
 Bay, north of San Francisco, 
 where the offset in the op- 
 posite direction was about 
 20 feet. The southwestern 
 side was usually also the up- 
 lift side, the amount being 
 not over four feet. In some 
 cases, however, an uplift of 
 as much as two feet occurred 
 on the eastern side. 
 
 As no large cities are lo- 
 cated upon the fault line, the 
 amount of destruction along it was limited. The chief destruction 
 was done along a line nearly parallel to the fault and northeast of it, 
 where it is marked by the straight eastern shore of San Francisco 
 Bay. On it are located the towns of Ukiah, Cloverdale, Healdsburg, 
 and Santa Rosa in the north, and San Jose, south of Oakland, 
 through which it passes. All of these towns suffered severely, 
 especially the parts built on unconsolidated material, which was 
 thrown into vibrations of greater amplitude than was the case with 
 the solid rock. San Francisco lies between these two lines of 
 faulting and over a subordinate fault line. The main destruction 
 
 FIG. 589. Earthquake fissure, as- 
 sociated with faulting. California earth- 
 quake. The main fault between Point 
 Keys Station and Olima looking south- 
 east. The ground at the right of the 
 fault has moved toward the observer ; at 
 the left, from the observer. (U. S. G. S.) 
 
Great Earthquakes of Modern Times 687 
 
 wrought there by the earthquake itself extended roughly in a north- 
 east-southwest line (along the direction of Market Street). When 
 this line is extended, it roughly forms the straight northwestern 
 coast of Suisun Bay, and there the railroad track sank in the marsh 
 at the time of the earthquake. 
 
 This earthquake was accompanied by the usual phenomona of 
 fissuring of the ground (Fig. 589), of lateral displacement of 
 
 FIG. 590. Fence displaced during the California earthquake of 1906. (Photo 
 by G. K. Gilbert, from U. S. G. S.) 
 
 fences, roads, etc., to the extent of 10 feet (Fig. 590), the lateral 
 shifting of the foundations underneath the buildings, the twisting 
 of statues on their pedestals or their overthrow, etc. 1 
 
 The loss of life was comparatively small, though perhaps 1000 
 persons perished. Much of the destruction in San Francisco was 
 due to the- fires, which could not be extinguished because the 
 water pipes laid across the fault line were cut in two. 
 
 1 For details see Bulletin 324, U. S. G. S., 1907 ; 170 pp., 57 plates. 
 
688 Movements of the Earth's Surface 
 
 The Jamaica Earthquake of 1907 
 
 The island of Jamaica in the Greater Antilles lies along lines 
 of deformation which go back at least to Tertiary time, and has 
 been repeatedly subjected to seismic disturbances. The city of 
 Kingston seems to be a focal point from which the lines of chief 
 disturbance radiate. The old town of Port Royal, situated 
 across the harbor, at the end of a seven mile sand-spit, was 
 destroyed by an earthquake in 1692, the ground settling and 
 causing the submergence of a large part of the old city. This 
 settling has recurred lately. A heavy shock, preceded by subterra- 
 nean rumblings, occurred on January 14, 1907, about 3.30 P.M., 
 and was followed about 20 seconds later by a second one. Within 
 a period of 35 seconds the main destruction was completed, although 
 eight after-shocks followed, between February 5 and June 14. 
 The east-west walls of the buildings within the city suffered most 
 damage. Destruction almost equal to that at Kingston was 
 wrought at Bluff Bay on the north coast, and it appears that a 
 line of disturbance (seismotectonic line) crosses eastern Jamaica 
 between these points. Extended northward, this line passes through 
 Santiago de Cuba, where the shocks were strongly felt. Other 
 lines of this type intersect Jamaica westward from Kingston, where 
 they converge. Along one of these lines the cable to the city of 
 Colon was fractured, four miles out from Bull Bay. 
 
 The faulting produced at this time seems to have been confined 
 to a zone around the river harbor. A series of parallel step 
 faults was produced on the inner side of the sand-spit, descending 
 progressively toward the shore, and making an aggregate displace- 
 ment of not less than 24 feet. At the intersection of one of these 
 faults with a railroad track the latter was deformed by a short, 
 sharp kink. The fissures repeatedly opened and closed, ejecting 
 water and sand, and a series of craterlets was formed on the 
 bottom of some of them. 
 
 In one instance the harbor was increased in depth to the extent 
 of 27 feet, while the western end of the peninsula on which Port 
 Royal stands was submerged from 8 to 25 feet, so that only the 
 tops of the palm trees and roofs of buildings project above the water 
 level. The usual twisting of statues and other phenemona were 
 manifested during this earthquake. 
 
Phenomena Accompanying Earthquakes 689 
 
 SUMMARY or PHENOMENA DUE TO AND ACCOMPANYING 
 EARTHQUAKES 
 
 Having now reviewed some of the more important earthquakes 
 of historic time, we may summarize their characteristics and the 
 accompanying phenomena. In the first place, it must be empha- 
 sized that although volcanic eruptions are commonly accompanied 
 by earth tremors, the great earthquake disturbances are independ- 
 ent of such volcanisms and are due to the giving way of the earth's 
 crust in places under heavy strains. Such giving way produces 
 
 fissures, or opens pre- _ 
 
 viously formed fissures, '~^~ ] j | ' y ' / ^^ 
 and along these both ver- 
 tical and horizontal dis- 
 placements suddenly 
 take place. Vertical 
 movements are both up 
 and downward, some- 
 times only one occurring, 
 as in the case of the up- 
 ward tilting of the block 
 mountain in New Zea- 
 land; in others both 
 movements occur. Along 
 
 the same fault line dif- 
 ferential vertical move- 
 ments may be in opposite 
 directions, so that in dif- 
 ferent places the same 
 side of the fault may 
 form the relatively raised 
 or depressed portions. 
 Again, block faulting, the 
 dropping down or raising 
 of circumscribed blocks, 
 is not uncommon. Upon 
 the surface, the topo- 
 
 FIG. 591. Diagram of an ancient fissure in 
 fine Upper Silurian limestone, etc., filled with 
 rounded grains of sand, secondarily enlarged, 
 and including fragments of the wall-rock. The 
 character of the fissure shows that it was 
 formed suddenly and that the sand, which 
 originally covered the surface, was violently 
 injected, and sometimes driven into horizontal 
 fissures between the strata. It is interpreted 
 as an earthquake fissure, and the date of the 
 earthquake is seen to have been sometime in 
 Lower Devonian time, as the fissure is in 
 Upper Silurian rocks, and the covering rock, 
 which is unaffected, is of Middle Devonian 
 age. Cement quarry, Buffalo, N. Y. For detail 
 of included fragment, see Fig. 592, and for en- 
 larged sand grains from the injected mass, see 
 Fig. 472, p. 565. 
 
 graphical effect may be 
 
 a single, or a series of step-like fault scarps, or a ridge resembling 
 
 a gigantic mole track. In general, the height of the scarp is only 
 
690 Movements of the Earth's Surface 
 
 a few feet, though displacements of as much as 47 feet have been 
 recorded. Beneath the sea, however, much greater fault scarps 
 have come into existence. Thus the earthquake of October 26, 
 1873, caused the cable to break seven miles from the cable office 
 at Zante, Greece, by the formation of a submarine fault scarp 600 
 feet in height, the change in depth being from 1400 to 2000 feet. 
 Other fault scarps 1000 feet in height have been formed in this 
 region, and in one case a difference in depth of 2000 feet has been 
 
 found between the bow and 
 stern of the cable repair ship. 
 
 By the repeated opening and 
 closing of fissures, sands from 
 the surface, together with other 
 objects, are injected into them, 
 and thus sandstone dikes are 
 FIG. 592. - Fragment of the wall formed, penetrating the rocks of 
 rock (limestone) included in an an- the region. There are many 
 cient sandstone dike; the rock is anc i ent examples of such sand- 
 shattered and the sand (shaded) in- ... /T ,. >. 
 jected into the fissures. Somewhat stone dlkes ( Fl f- 591, 592). 
 reduced. Sandstone dike. Buffalo, Another superficial effect pro- 
 N.Y. (For character of sand grains, duced upon the roc k s j s t h e ir 
 Fig. 472, P. 565-) shattering and the dislodgment 
 of huge masses which are often precipitated to some distance. 
 
 When the surface consists of unconsolidated material, great 
 earth waves are produced, which cause a rocking of structures, 
 a bending back and forth of trees, and a rifting of the surface, 
 with the spouting forth of sand and water and the formation of 
 craterlets and mud-volcanoes. Huge landslides are also produced, 
 and great masses of soil with trees and buildings may be shifted 
 bodily. Subsidences also occur by a settling of the soil, and thus 
 lakes, ponds, and submerged areas along the coast result. The 
 damage to buildings on such unconsolidated bottoms is often much 
 greater than to those built on rock. 
 
 Earthquakes originating near or under the sea produce great 
 water-waves or tsunamis. These may rise 40 feet or more, and 
 travel with enormous velocities and over distances of many thou- 
 sands of miles. Recorded velocities range from 370 to 465 miles 
 per hour, or from six to seven and three fourths miles per 
 minute. 
 
Slow Changes in Levels, Bradyseisms 691 
 
 SLOW CHANGES IN LEVELS, BRADYSEISMS 
 
 In addition to the rapid modification of the earth's surface, de- 
 scribed as accompanied by earthquakes, there are gradual changes 
 in elevation which are not associated with detectable earthquake 
 movements. These are often of great geological importance, 
 especially when they produce depressions into which the sea may 
 enter. We have already discussed the formation of geosynclines 
 of deposition, and have seen that these imply slow sinking with 
 simultaneous deposition of sediment, the aggregate of the slow 
 subsidence amounting in the end to many thousands of feet. The 
 actuality of such subsidences can be deduced only indirectly from 
 the nature and extent of the sediment deposited in the subsiding 
 area, and indeed actual observation of all slow earth movements, 
 or bradyseisms , as they are called, is out of the question. There 
 are, however, other evidences than the sediments to indicate slow 
 changes in level, both upward and downward movements, and these 
 may be found not only upon the sea-coast, but in certain cases in- 
 land as well. We will take three illustrations of such changes, one 
 from the Temple of Jupiter Serapis at Pozzuoli, Italy, another from 
 the raised beaches of the Atlantic coast, and the third from the 
 changes of level observed in the region of the Great North Amer- 
 ican Lakes. 
 
 The Temple of Jupiter Serapis at Pozzuoli 
 
 The coastal district of Naples is of interest, not only because of 
 the varied manifestations of volcanic activities, but also because 
 of the unequivocal evidence of elevation and subsidence since the 
 beginning of the Christian Era which is afforded by such monu- 
 ments of antiquity as the ruined temple of Jupiter Serapis at Poz- 
 zuoli. (See map, Fig. 51, p. no.) It is true that there is other 
 evidence of change of level in this region, but none that is quite 
 so convincing as that furnished by these ruins. 
 
 This temple (Fig. 593), built some centuries before the opening 
 of the Christian Era, was of circular form, 70 feet in diameter, 
 surrounded by a quadrangular court, and the roof was supported 
 by 46 noble columns, of which 24 were of granite and the rest of 
 marble. Only three of the marble pillars remain erect, each carved 
 out of a single block, their height being 40 feet 3^ inches. One 
 is nearly bisected by a horizontal fissure, the others are entire, 
 
692 
 
 Movements of the Earth's Surface 
 
 and they are all out of the perpendicular, inclining slightly to the 
 sea. For a height of about 12 feet above their pedestals, the sur- 
 face of the pillars is smooth and uninjured. Above this is a zone 
 about nine feet in height, where the marble has been pierced by the 
 
 boring mollusk, Lithodo- 
 mus, many shells of which 
 still exist in the pear- 
 shaped inward-enlarging 
 " hollows. In some of these 
 borings, shells of another 
 marine mollusk (Area) 
 also occur. These holes 
 clearly indicate that at the 
 time of their formation the 
 parts of the columns thus 
 affected were beneath sea- 
 level, and their depth and 
 size is such as to suggest 
 that this state of sub- 
 mergence lasted for a con- 
 siderable time. The fact 
 that the lower part of the 
 pillars is unaffected, indi- 
 cates that at that time 
 this part was protected 
 by sediments or otherwise. 
 The amount of submer- 
 gence is marked by the 
 height of the bored zone, 
 the upper parts of the 
 pillars projecting above water-level. Marble columns lying on the 
 pavement of the temple were also attacked by borers, while 
 worm-tubes (Serpulce) became attached to them. 
 
 At the time of Lyell's visit in 1828, the platform was about a 
 foot under water, the sea, only 100 feet distant; soaking through 
 the intervening soil. The top of the bored zone was, therefore, 
 at least 23 ieet above high- water mark, the tides in the Bay of 
 Naples being small. An older, costly mosaic pavement was found 
 by excavation about 5 feet below the upper one, indicating sub- 
 sidence before the building of the later temple floor. That the 
 
 FIG. 593. Ruins of the temple of Jupiter 
 Serapis at Pozzuoli as they appeared in 1840, 
 with the floor partly submerged. (After 
 Lyell.) 
 
Slow Changes in Levels, Bradyseisms 693 
 
 temple was intact, and stood above water-level between the years 
 222 and 235 A.D. is clearly attested by historical evidence, and, 
 therefore, the great subsidence occurred much more recently. That 
 it antedated the close of the fifteenth century is shown by a 
 description of an Italian writer dated 1580, in which he said that 
 50 years before, or in 1530, the sea washed the base of the hills which 
 rise behind the flat land on which the remains of the temple stand, 
 and " a person might then have fished from the site of those ruins 
 which are now called the stadium," and which stand upon these 
 hills. 
 
 A series of deposits around the lower part of the pillars and the 
 outer walls of the temple, record the changes in history and show 
 that the subsidence was gradual. " The sea first entered the 
 court or atrium and mingled its waters partially with those of the 
 hot springs [which still exist]. From this brackish medium a dark 
 calcareous precipitate was thrown down, which became, in the 
 course of time, more than two feet thick, including some serpulae 
 in it. The presence of these annelids teaches us that the water was 
 salt or brackish. After this period the temple was filled up with 
 an irregular mass of volcanic tuff, probably derived from an erup- 
 tion of the neighboring crater of the Solfatara, to the height of from 
 five to nine feet above the pavement. Over this again a purely 
 fresh-water deposit of carbonate of lime accumulated with an irreg- 
 ular bottom (because of the uneven surface of the tuff)." The sur- 
 face of this fresh-water deposit was level, and upon it followed 
 another deposit of volcanic ashes and rubbish. Then came the 
 subsidence, which permitted the boring of the submerged part 
 of the pillars not protected by these deposits. Later showers of 
 ashes buried the pillars in places to a height of 35 feet above the 
 pavement, from which they were exhumed in recent times. 
 
 Certain historic evidence shows that the upward movement 
 had begun before 1530, but the main elevation appears to have 
 occurred at the time of the great eruption of Monte Nuovo in 1538, 
 when, as we have seen (p. 112), the region thereabout was elevated. 
 
 Since the rediscovery of the temple in 1 749, when it stood higher 
 than now, it has undergone a renewed subsidence. At the begin- 
 ning of the nineteenth century the floor was above sea-level, but in 
 1838 fish were caught every day over parts of the pavement, the 
 conditions being as shown in the illustration published in 1840 
 (Fig. 593). Between 1822 and 1838 the rate of subsidence was 
 
694 Movements of the Earth's Surface 
 
 about one inch in four years, in 1847 it was about one inch a year, 
 and in 1852 it had practically ceased'. In 1857-58 the floor of 
 the temple was covered by about two feet of water at- high tide, on 
 calm days. At the present time the surface of the pavement is 
 slightly below sea-level. 
 
 Evidences are not wanting that these changes of level affected 
 the whole of the Bay of Naples, and represent warpings of con- 
 siderable areal extent instead of being a local phenomenon con- 
 nected with volcanism, as has been supposed by some. 
 
 Raised Beaches 
 
 Scandinavia. On the coast of Norway raised sea-beaches 
 are found, which are held to indicate a movement in recent times 
 to a height of 600 feet or more, although this interpretation has 
 been questioned. It is a noteworthy fact, however, that these 
 terraces, or beaches, are not only found in fjords, where they might 
 have been formed on lakes dammed by ice, but that they also face 
 the open sea, and occur even upon some of the islands. These 
 raised beaches are both of the wave-built type of shingle beach 
 and the wave-cut terrace type. On the Swedish coast, measure- 
 ments of the rate of elevation have been made on monuments 
 especially planted for this purpose. In one place, the rate of 
 elevation was found to be about two feet per century, but it is not 
 everywhere uniform, and varies from period to period. 
 
 Scotland. Around the Scottish coast, such beaches occur at 
 levels of 25, 50, and 100 feet above the sea, indicating step-like 
 elevation. The higher beaches date back to the glacial period, 
 as is shown by their organic contents and their association with 
 boulder clay, but the lowest one is very recent, containing in many 
 cases the shells of the Mollusca still living upon that coast (espe- 
 cially limpets) and incorporated in the modern beach. The lower 
 two beaches can be traced nearly around Scotland, but the upper 
 one occurs only in certain places. 
 
 Eastern North America. In the coastal region of Maine, New 
 Brunswick, and other districts, beds of clay are associated with 
 beaches and sand-spits, and cliffs are cut into the glacial deposits, 
 these cliffs being at present found at an elevation of about 300 to 
 600 feet above the sea-level. In these sands and clays are found 
 the shells of cold-water mollusks (Saxicava arctica, Fig. 594, 
 Leda, etc.), which are still living upon the coast. As the beaches 
 
Slow Changes in Levels, Bradyseisms 695 
 
 are partly cut into the glacial deposits at those elevations, it is clear 
 that the region has been relatively raised some time after the forma- 
 tion of these deposits or, in other words, in comparatively modern 
 times. 
 
 There are many other examples known which demonstrate re- 
 cent changes in level along the coast, but those cited are so clear 
 that they admit of no other explanation. Evidences of subsidence, 
 such as the drowned Valley of the Hudson and other rivers, the 
 
 FIG. 594. Shells of a pelecypod (Saxlcava arctka) found in the elevated sand 
 beds of northeastern New England, New Brunswick, etc. This species still 
 lives in the modern ocean. 
 
 submergence of fresh- water peat deposits, of former forests, etc., 
 also occur, but they are in some cases explicable in other ways. 
 Enough has been said, however, to show that changes of level have 
 occurred in recent times. Some of these, no doubt, were due to a 
 change in the position of the sea-level, caused in part by the attrac- 
 tion of the great ice mass which covered the northern Atlantic 
 regions, as already outlined (p. 297). Others, however, can only 
 be regarded as representative of a change in the altitude of the 
 land itself. 1 
 
 Change of Level in the Great Lakes Region 
 
 The evidence of change of level in the region of the Great Lakes 
 of North America is of an interesting and peculiar type. On the 
 northeastern side of the Lakes, the old beaches are raised above 
 the present water-level, in some cases to a height of several hun- 
 dred feet. Followed southward and westward, these beaches 
 descend toward the lake level and finally pass under it. This in- 
 dicates a tilting of the lake-basins toward the southwest, and in 
 support of this hypothesis, it is found that the water of Lake Michi- 
 gan at Chicago rises at the rate of about 9 inches a century. At 
 
 1 For further discussion of this problem see D. W. Johnson, " Shore Processes and 
 Shore Line Development." John Wiley and Sons. 
 
696 Movements of the Earth's Surface 
 
 this rate it is estimated that the permanent discharge of the Great 
 Lakes will be by this route to the Mississippi 'in 3000 years from 
 now. 
 
 The tilting of the lake basin is explained by the rising of the old 
 Ontario dome on the northeast, a dome which has repeatedly risen 
 since Palaeozoic time, with intervening periods of rest. The present 
 rate of rise is estimated to be several inches in a hundred miles per 
 century. 
 
CHAPTER XXII 
 THE SCULPTURING OF THE EARTH'S SURFACE 
 
 INITIAL CHARACTER OF THE LAND SURFACE 
 
 Plains. The simplest form of land surface upon which the 
 agents of erosion perform their sculpturing process is the plain. 
 By this name is designated the surface of a country which over 
 extensive areas shows very little relief. Indeed, some plains ap- 
 pear to the eye almost as level as the surface of the ocean. The 
 rivers of the plain have as a rule shallow valleys and are for the 
 most part sluggish, frequently with swampy or boggy borders. 
 
 Young Plains. The most typical plains are plains of construc- 
 tion and are formed of horizontal or nearly horizontal strata upon 
 which little erosion has been performed. Such plains are called 
 young plains, and they may be divided into coastal plains and in- 
 land plains. A typical coastal plain borders the sea-coast, and rep- 
 resents portions of the sea-bottom which have been elevated but 
 recently, so that erosion has made little progress, or elevated to 
 such a slight extent that the work of the rivers and other agents 
 of erosion has been unable to accomplish much dissection. Such 
 a coastal plain borders the sea in southern New Jersey, and ex- 
 tends inland for many miles. It has a deep, sandy soil, and is 
 generally overgrown with pine forests; its roads and -railroads 
 run in straight lines for long distances, and its population is 
 scanty because of the infertility of the soil and the sluggishness of 
 the streams and the consequent lack of water-power. 
 
 Inland plains may be formed by the deposition of strata of sand 
 and clay either in old lake basins or by the confluence of river flood 
 plains, as in the case of the Indo-Gangetic plain of northern India, 
 (p. 468) ; the great alluvial plain of the Hoang-Ho of China (p. 467), 
 and others of this type ; or they may represent the flat till-covered 
 surfaces left during former glaciation by the continental ice-sheet 
 which once occupied that region. 
 
 697 
 
698 The Sculpturing of the Earth's Surface 
 
 Extensive plains may also be formed by wide-spreading lava 
 flows, but these are seldom absolutely level. Plains always stand 
 
 FIG. 595. Winter scene on the plains of North Asia, showing the broad and 
 level expanse of a young plain. 
 
 at a moderate altitude above the sea or other level which controls 
 erosion, so that the rivers do not cut down deeply beneath the 
 surface. The great plain of western Siberia, in latitude 50 to 
 
 FIG. 596. Relief map of Asia. 
 
Initial Character of the Land Surface 699 
 
 60 N., is not greatly elevated above sea-level and preserves an 
 even surface over hundreds of miles. " Vast areas, stretching 
 further than the eye can reach, are monotonous in the extreme, 
 almost as uniform in soil as in surface. The flat areas between 
 the streams, having no distinct lines of water parting and no dis- 
 tinct channels of water discharge, are as yet practically undivided 
 among the rivers. Marshes, alternately wet and dry in winter and 
 summer, and many shallow lakes lie in faint depressions, as if slight 
 
 FIG. 597. Badlands of the Great Plains, showing horizontal plains structure. 
 South side of White River, three miles below Porcupine Creek. (U. S. G. S. ; 
 courtesy of D. W. Johnson.) 
 
 inequalities in the original surface of the plain had not yet been 
 drained by river action. The valleys are few and far between ; 
 they can never be cut deep while the region stands low. They are 
 narrow ; hence the rivers have as yet worked only for a compara- 
 tively short time in the earth's history. The plains are still young." 
 (Davis.) (Fig. 595. See also relief map of Asia, Fig. 596.) 
 
 Mature Plains. When a plain has been thoroughly dissected 
 by streams and by the work of the atmosphere, especially the 
 wind, so that its topography presents the maximum of ruggedness 
 which it can acquire under the existing conditions, it is said to be 
 mature. Such thoroughly dissected plains are also spoken of as 
 
700 The Sculpturing of the Earth's Surface 
 
 "Bad Lands," because of the difficulty of traversing them 
 (Fig. 597). The same plane may be maturely dissected in one 
 part, while in another the original evenness of surface may show 
 but little modification. 
 
 Old Plains (Erosion plains). A plain which has been produced 
 as the result of prolonged erosion of a region, until that has been 
 worn down again to a level surface, that is, a plain of denuda- 
 tion, is spoken of as an old plain. The great plain of central 
 Russia is one of the largest examples of such a plain of erosion, and 
 
 ATLANTIC 
 . OCEAN 
 
 \ 
 
 EUROPE 
 
 5&.I5 BLACK SEA 
 
 FIG. 598. Relief map of Europe, 
 
 it might at first sight be mistaken for a young plain, it is so level 
 and monotonous over vast areas. The rocks underlying it do not, 
 however, correspond with the surface, although they are nearly 
 horizontal. They are mostly firm and consolidated, instead of 
 loose sands, etc., as in young plains, and the surface is not formed 
 by the same stratum as is the case in young plains, but bevels 
 across the layers at a very gentle angle. Thus from place to 
 place different layers are exposed, with the result that slight 
 changes in the form of the surface and the soil, resulting from the 
 decomposition of the strata, are produced. The rivers flow in 
 
Initial Character of the Land Surface 
 
 701 
 
 narrow channels of moderate depth, indicating that little erosion 
 has been accomplished since the formation of the plain. It is 
 customary to designate such an erosion surface by the term plane 
 
 (Fig. 598). 
 
 The Peneplane. 1 An old plane of erosion is also designated a 
 peneplane, because it is seldom a perfect plane, but generally " al- 
 most a plane," as a peninsula is almost an island. The great 
 Russian peneplane above referred to is an example of such a sur- 
 
 FIG. 599. Jail Rock, Cheyenne County, Nebraska. A butte of sandstone 
 capping clay, rising as a monadnock above the peneplaned adjacent portions of 
 the Great Plains. (Photo by Barton, U. S. G. S. ; courtesy of D. W. Johnson.) 
 
 face of moderate elevation; but most peneplanes have been up- 
 lifted again, and are undergoing renewed dissection. The Great 
 Plains of western North America represent such an erosion surface 
 on nearly horizontal strata, and they appear to the eye as absolutely 
 level or gently rolling surfaces, as shown in the preceding illus- 
 tration, where only a small remnant of erosion, " the Jail Rock, " 
 rises above the very level plane, representing one of a number of 
 remnants of the former higher surface (Fig. 599). This surface is, 
 however, undergoing renewed dissection in other regions. The 
 Prairie Plains of the central United States represent a broadly 
 
 1 It has become customary to use the terms plane and peneplane for surfaces of 
 erosion, restricting the term plain to surfaces of construction of low relief and hori- 
 zontal strata. 
 
702 The Sculpturing of the Earth's Surface 
 
 peneplaned surface which still stands at a moderate altitude above 
 the sea (from 500 to 1000 ft.) and is therefore subject only to 
 slight dissection. Much of this area is covered by the till or 
 ground moraine left by the great ice sheets which buried this 
 region in Pleistocene time, while other parts are covered by the 
 river and loess deposits, derived from the ice-borne material. 
 These newest sediments mantle over the peneplane surface, and 
 thus in reality convert it into a constructional or true plain. It 
 
 FIG. 600. The untamed prairie. The broad monotonous expanse of these 
 plains is due to the peneplanation of the underlying rock and the veneering of 
 the surface with glacial, alluvial, eolian and loess deposits. 
 
 is this mantling by glacial debris which gives the surface of the 
 prairie its level, monotonous character (Fig. 600). 
 
 While many peneplanes are cut on nearly horizontal strata, there 
 are others which have been cut on complexly folded rocks. The 
 New England peneplane is of this type, and so is that which cuts 
 the old, much-folded slate mountains of western Germany, in which 
 the Rhine has sunk its beautiful gorge (Fig. 601). It is evident 
 that such a peneplane, cutting across strata of varying hardness, 
 must represent the lowest level to which streams could erode at 
 the time this erosion surface was formed, this" in the examples 
 cited being approximately sea-level. From the fact that the 
 
Initial Character of the Land Surface 
 
 703 
 
 modern streams have again cut gorges and valleys below this 
 level we must conclude that the peneplane has been elevated, with 
 reference to the sea-level, since its formation. 
 
 Plateaus. A typical plateau, like a typical plain, is composed 
 of horizontal strata, but its surface stands at a much greater eleva- 
 tion above the floors of the valleys which transect it, than does 
 that of the plain. A young plateau presents a wide, level surface, 
 with few but deep and narrow canons cut in it, which form its 
 most pronounced feature, since the upland surface itself, because 
 of its even, monotonous character, attracts little attention. 
 
 I 
 
 FIG. 601 . Gorge of the Rhine at St. Goar, Germany. The gorge is seen to 
 be incised in an older valley, remnants of the floor of which are seen somewhat 
 below the remarkably even skyline of the peneplane. (Photo by D. W. John- 
 son.) 
 
 Many so-called plateaus are really uplifted peneplanes cut on 
 horizontal strata, as in the case of the Alleghany Plateau of the 
 eastern United States. Some of the little dissected plateaus of 
 the western states are, however, regarded as young plateaus in 
 which erosion has progressed only to a slight 'extent. In the rivers 
 dissecting young plateaus, as well as in those which dissect uplifted 
 peneplanes, waterfalls often form a characteristic feature. An- 
 other marked feature of many recently uplifted plateaus is the 
 broken and flexed character of their strata, so that recent fault 
 scarps and monoclinal flexures diversify the surface. Such modi- 
 fications of the surface character are clearly shown in the Colorado 
 
704 The Sculpturing of the Earth's Surface 
 
 Plateau of Arizona and southern Utah represented in the diagram 
 (Fig. 6196). 
 
 Deformed Surfaces. Many initial surfaces are far from being 
 level' but are more or less strongly deformed. Such deformed sur- 
 faces are found on fault blocks, dome mountains and anticlines. 
 Besides these there are minor constructional surfaces, such as 
 volcanic cones, drumlins, etc., which present inclined surfaces and 
 offer special opportunities for the sculpturing processes. 
 
 THE EROSION CYCLE IN THE SCULPTURING OF THE LAND 
 SURFACE 
 
 The surface features of the earth are largely the result of the 
 sculpturing processes which are constantly and everywhere at work, 
 and the details of operation of which we have studied in previous 
 chapters. But it must be clearly understood that the nature of 
 the rock and its attitude, that is, the structural features of the 
 earth's crust, exercise an important controlling influence upon 
 the sculpturing processes, and that it is only by taking those fea- 
 tures into account that we can understand the bewildering series 
 of land forms which have been produced by the agents of erosion 
 or sculpture upon a surface of diverse structure and history. Hence 
 this subject has been reserved until the discussion of such features 
 had been completed. 
 
 The Cycle. We shall first discuss the general progress of sur- 
 face sculpture from the beginning to the end, that is, to the oblit- 
 eration of the surface features produced during the process. For, 
 unlike a human sculptor, nature is not content to produce finished 
 land forms from the earth-block she is working on, but must needs 
 continue, after the sculpture is finished, to reduce its salient fea- 
 tures, and keep up the process until the originally produced com- 
 plex form has been again destroyed and the block brought back 
 nearly to the relative simplicity of form which it had at the be- 
 ginning of the process. It is as if the sculptor, having produced 
 his bas-relief upon a stone front, were to continue his carving and 
 cutting, gradually removing the salient features until all resem- 
 blance to the work produced originally is lost, and the surface of 
 the stone again becomes plain and without relief features. 
 
 This process of earth sculpture from formless surfaces to com- 
 plex bas-relief, and on to formless surface again, constitutes the 
 
The Erosion Cycle 705 
 
 cycle of erosion, and the agents which accomplish this cycle are the 
 familiar ones of the weather, that is, the gases and vapors of the 
 atmosphere, the wind, rain, frost, and diurnal and seasonal tem- 
 perature changes ; and also the streams and the glaciers, and the 
 waves and currents upon the coast. 
 
 Stages of Relief. In the beginning, when the diversity of re- 
 lief is slight, the surface is said to be in the stage of youth. As dis- 
 section progresses, the bas-relief reaches its maximum complexity 
 and diversity of form, when the condition of maturity is approached, 
 beyond which it passes on to old age, which is reached when the 
 surface is again reduced to a condition of little or no relief. The 
 last stage, in which the surface would be a perfect plane, is seldom 
 if ever reached, for as it is approached the agents of erosion work 
 more and more slowly, and before its final accomplishment the 
 unstable nature of the earth's crust may effect a change of condi- 
 tions and a new cycle of sculpture or erosion become inaugurated. 
 Such an unfinished surface, the end stage of an incomplete cycle, 
 is presented by the peneplane. 
 
 Monadnocks. The unreduced relief features which rise above 
 the peneplane, and which are the residual remnants of the former 
 diverse relief, are called monadnocks, from the mountain of that 
 name in New Hampshire, which was first recognized as representing 
 such a residual relief feature upon a peneplane to which we shall 
 refer again later on (Fig. 635, p. 742). Jail Rock, shown in Fig. 
 599, is such a monadnock, rising above the peneplane surface of 
 the Great Plains. 
 
 Relation of Peneplane to Base-level of Erosion. Since the 
 materials removed by the natural agencies of land-sculpture are 
 carried away by agents acting under the influence of gravitation, 
 it is evident that the sculpturing processes may continue until the 
 surface of the country subjected to them is lowered to the level, 
 which for the time being, limits the force of gravitational control. 
 This downward limiting surface is called the base-level of erosion, 
 and in most regions this is the level of the sea, for beneath this 
 level, few of the agents of erosion may cut to any great extent. 
 Therefore, we may assume that sea-level is the ultimate base- 
 level of erosion, and that in a prolonged cycle the surface of the 
 land as it approaches the peneplane condition also approaches the 
 level of the sea. This level will of course not be reached by the 
 peneplane the rivers of which are tributary to the sea, except at its 
 
706 The Sculpturing of the Earth's Surface 
 
 margin, for the surface which is undergoing subaerial erosion must 
 always have a certain amount of slope, in order that the material 
 removed from it may be carried away. A peneplane which faces 
 
 the sea may, however, 
 be planed down by 
 the waves from the 
 margins inward, thus 
 producing a plane of 
 marine erosion, or ma- 
 rine peneplane. 
 
 Local Base-levels. 
 -There are, how- 
 ever, local base-levels 
 of erosion which are 
 independent of the 
 level of the sea. The 
 floor of the Sahara 
 Desert lies in part at 
 least beneath the level 
 of the sea, and so 
 does the surface of the 
 Caspian Sea (Lake). 
 Both of these deeper 
 levels control the ex- 
 tent to which erosion 
 of the bordering lands 
 can be carried, and 
 erosion surfaces 
 formed about their 
 margins will be low- 
 ered until they ap- 
 proach these deeper 
 levels. In the great 
 interior deserts, such 
 as those of Tibet and 
 other Asiatic regions, 
 the base-level of 
 erosion, which is the 
 desert floor, lies thou- 
 sands of feet above 
 
The Erosion Cycle 707 
 
 the sea-level. Nevertheless, for the time being it controls the level 
 to which erosion of the surrounding country may progress. Where 
 the local base-level of erosion in an arid region is an enclosed dry 
 basin which receives the sediment from the surrounding regions, 
 this base-level will gradually rise as the basin is filled with the 
 wastage of the surrounding hills, and so the ruggedness of the dis- 
 trict will be partly reduced by filljnj^or aggrading, and partly by 
 down-cuttingor^i^^g^g. In this manner a complex high-level 
 plane, partly of erosioo? and partly of local filling by sediments, 
 will come into existence. Sucha plain is, of course, produced only 
 where the hollows formed by wind erosion are filled by sediments 
 washed into them, and this can take place only in a region which 
 is not absolutely rainless. 
 
 The Inauguration of the New Cycle. When the peneplane is 
 raised with reference to the sea-level, or where by a change in con- 
 ditions the base-level of erosion controlling an interior peneplane is 
 lowered, a second cycle of erosion begins. The streams once more 
 will incise their courses into the surface, until they again approach 
 a condition of grade, that is, have such a relation to the new base- 
 level of erosion that their cutting power has practically ceased. 
 As the streams upon a peneplane generally assume a meandering 
 course, winding about over the surface to follow the easiest path, 
 they will upon elevation of the peneplane maintain this course, and 
 so large rivers of a region in the second or later cycles of erosion 
 will present the phenomenon of intrenched meanders (Fig. 602). 
 Such intrenched meanders may also present the phenomenon of 
 a cut-off, leaving an intrenched oxbow, as at Lauffen on the 
 Neckar River in Wiirttemburg (Fig. 603). Lateral tributary 
 streams will gradually dissect the elevated upland. Finally, as 
 the second cycle approaches completion, the region will again 
 assume the character of a peneplane. 
 
 Relative Ages of the Rivers and of the Land 
 
 There is no relation between the topographic age of the land sur- 
 face and that of the river dissecting it. It is true that in newly 
 emerged coastal plains, young rivers are a characteristic feature, 
 but previously existing rivers will also flow across such a coastal 
 plain from the old land bordering it, and while the part entering 
 the coastal plain will at first have the characters of a youthful 
 stream, its greater volume of water will soon produce a valley that 
 
708 The Sculpturing of the Earth's Surface 
 
 has all the characteristics of maturity. In like manner, young 
 rivers may come into existence on an old land surface, such as a 
 peneplane, by uplift of this surface, while the older streams upon 
 such a surface will become rejuvenated and assume the charac- 
 teristics of a youthful stream. 
 
 FIG. 603. Intrenched meander cut-off of the Neckar River at Lauffen and 
 at Kirchheim. The cut-off portions are occupied by small streams. The cut- 
 off at Lauffen is the most recent, the form of the old valley still being clearly 
 marked. At Kirchheim the river has cut below the level of the old cut-off. 
 (From de Martonne.) 
 
 A young stream is characterized by a narrow valley or gorge 
 with steep sides, and with little or no flood-plain surface on the 
 valley floor, which is occupied for the most part by the stream. 
 The gorge of Niagara and that of the Genesee at Portage present 
 such youthful conditions, and these are also seen in the Colorado 
 River, and in fact in all rivers which run in narrow canons which 
 they are still actively eroding; such rivers are frequently char- 
 acterized by waterfalls. A mature river, on the other hand, has 
 
The Erosion-Cycle on a Coastal Plain 709 
 
 widened its valley by lateral cutting, and built a flood-plain upon 
 its floor, which it no longer completely fills. On this flood-plain 
 it will meander, cutting away portions at one place (the concave 
 bank), and building at another (the convex bank) (Fig. 604). 
 Such continued erosion results frequently in the cutting off of the 
 larger meanders, leaving them as " oxbows " still occupied by 
 water, as in the case of Lakes Chicot and Lee and others in the 
 Mississippi Valley (Fig. 605), or as dry oxbow valleys. Such 
 
 FIG. 604. Crooked Creek, near Long Valley dam site, California. Meanders 
 of stream on flat floor of a mature valley whose sides are unusually steep in cer- 
 tain localities where hard rock ledges project into the valley. (U. S. G. S.) 
 
 cutting-off shortens the river, often by many miles, and increases 
 its velocity along the new course. The manner of their forma- 
 tion has been discussed in a previous chapter (p. 417). 
 
 THE EROSION-CYCLE ON A SIMPLE COASTAL PLAIN 
 
 Characters of the Coastal Plain. A coastal plain such as that 
 which faces the Atlantic from New Jersey to the Gulf, with a 
 width varying from a few miles to over a hundred miles, represents 
 an emerged strip 'of former sea-bottom. The sediments of which 
 it is composed represent the deposit formed in the littoral district 
 of the sea when it covered this region, and the shore-line of which 
 
710 The Sculpturing of the Earth's Surface 
 
 
 FIG. 605. The meanders and oxbows of the Mississippi. (Miss. River 
 Comm., 1881-82 and 1894 ; from Ratzel.) Lakes Chicot and Lee represent 
 cut-off portions of the river, now forming oxbow-lakes. The portion shaded 
 by waterlining represents the river bed according to the surveys of 1881-82, 
 the sandbanks of that period being dotted. The black line represents the bed 
 of the river according to the survey of 1894, and the sandbanks of that period 
 are outlined by dotted lines. The new landing-places erected since 1882 are 
 underlined. 
 
The Erosion-Cycle on a Coastal Plain 711 
 
 lay approximately along the inner margin of the present coastal 
 plain. Not all coastal plains are composed of simple marine 
 deposits; indeed, the example cited contains delta and alluvial 
 deposits which were formed above sea-level, these being especially 
 characteristic of the lower part of the series. Nor is the marine 
 series always continuous, but, as in the example cited, it is more 
 often the product of successive encroachments and withdrawals 
 of the sea. Nevertheless, this series may be treated as a unit, and 
 in our discussion of erosion we may assume simplicity of structure 
 as typical of such a coastal plain. 
 
 Although appearing horizontal, the strata of the coastal plain 
 are in reality gently inclined seaward, finally passing beneath the 
 
 FIG. 606. A. Diagrammatic section of coastal plain strata in a trans- 
 gressive or overlapping series. B. A similar series formed in a retreating sea 
 (off -lapping series), a and c are soft strata, b and d are hard strata. The strata 
 are represented as of the same character throughout, though in reality they 
 change in lithic character toward the shore. 
 
 level of the sea. This may be in conformity with the slope of the 
 surface, or it may be somewhat in excess of that, depending upon 
 whether we are dealing with a transgressive series of strata sud- 
 denly emerged, or with a retreatal one, the product of slow emer- 
 gence. The two types are shown here in section (Fig. 606, A, B). 
 
 For the. sake of simplicity we shall consider the transgressive 
 series only, and we shall further assume that this series consists of 
 alternating deposits of hard and soft strata. The former may con- 
 sist of sands bound together by calcareous material (b,d\ the latter 
 of muds, etc. (a, c) (Fig. 606, A). 
 
 Drainage Systems of the Simple Coastal Plain. Upon a sim- 
 ple coastal plain, where the surface is comparatively uniform and 
 regular, sloping gently to the sea, a definite type of drainage will 
 develop. The rain-water run-off (see p. 411) will flow down the 
 slope to the sea, soon forming for itself definite lines of drainage, 
 which, following the shortest course, will in general be at right 
 angles to the coast. Such streams, the first to be formed, are 
 called consequent streams. Streams which were in existence on 
 
yi2 The Sculpturing of the Earth's Surface 
 
 the oldland when the coastal plain had not yet emerged will, if 
 persisting, become extended across the coastal plain to the sea. 
 These are the extended consequent streams, and since they origi- 
 nate in the mountains or higher oldland, they will generally 
 have more water than the newly formed consequents upon the 
 coastal plain, and so be able to cut deep and wide channels more 
 quickly than these. Such larger streams are spoken of as the 
 master consequents. 
 
 Along the margin of the consequent streams others will come 
 into existence from the near-by run-off, which finds its shortest 
 course into the consequent. These streams are not consequent 
 on either the initial slope of the surface, or difference of rock 
 resistance sufficiently pronounced to be recognizable. Hence 
 they are spoken of as inconsequent, or, more briefly, insequent 
 streams. They cut back their channels from the edge of the 
 consequent valley and prolong their own valleys by headward 
 cutting. 
 
 If the coastal plain structure is of the simple type above out- 
 lined, with its strata progressively overlapping upon the oldland, 
 and if the capping stratum is of uniform character throughout, 
 the drainage pattern developed will consist of the two types of 
 streams so far described, namely, the consequents and the inse- 
 quents. The former cut downwards, the latter headwards, in- 
 creasing in length and complexity until a complicated drainage 
 system of the multiple-branching type is produced. 
 
 It is, however, conceivable that near the inner end of the coastal 
 plain the conditions may be such that the insequent streams 
 developed there may have an advantage over those formed farther 
 down on the coastal plain. If the oldland is abrupt and high, 
 the moisture-bearing winds, on striking them, may be forced to 
 rise and part with some of their burden of water, and many of the 
 mountain streams thus formed may find their way into, or be 
 gathered up by, the growing insequents, as these prolong their 
 valleys by headward cutting. In this way a portion of the inner 
 edge of the coastal plain may be removed, the oldland being 
 stripped of its cover of coastal plain strata. 
 
 Coastal plains are, however, seldom, if ever, as simple as here 
 outlined. As we have seen, they are often composed of strata 
 due to repeated advances and retreats of the sea, with the result 
 that the inner edge of the coastal plain differs considerably from 
 
The Erosion-Cycle on a Coastal Plain 713 
 
 that of the simple type first described. Moreover, the strata of 
 a coastal plain are never uniform throughout, but, as has been 
 shown in a previous chapter, their lithic character changes as we 
 approach the shore of the sea in which they were formed. All of 
 these characters combined may render the inner edge of the coastal 
 plain less resistant to erosion, and as a result, a new type of stream, 
 the subsequent, will develop, which differs from the insequent in 
 that it develops by headward erosion along this belt of weaker 
 rocks. If the coastal plain consists primarily of a series of strata 
 deposited in a retreating sea, the ends of the weaker strata will 
 be exposed, as shown in diagram B, Fig. 606, and along the weaker 
 strata subsequent streams will develop. Again, if weaker strata 
 are exposed because of the peneplanation of a coastal plain series, 
 subsequent streams will develop along these exposures; but these 
 belong to a later cycle of erosion. 
 
 By the erosion of the inner edge of the coastal plain, the valley 
 of the subsequent stream is produced, and this, in a general way, 
 will be at right angles to the valley of the consequent. Erosion 
 in this region progresses with relative rapidity because of the 
 weaker character of the strata upon which it takes place, and 
 thus, partly by the wearing activities of the stream, and partly 
 by that of rain 
 water and the rapid 
 weathering of the 
 softer rock, the val- 
 ley of the subse- 
 quent will be broad- 
 ened until it be- FIG. 607 a. Diagram illustrating the coastal plain 
 of overlapping strata, with consequent and insequent 
 comes wide enough drainage. 
 
 to be termed a low- 
 land. As this lowland lies next to the oldland, or on the inside 
 of the coastal plain, it is called an inner lowland. It consists 
 in part of a stripped belt, where the coastal plain strata have 
 been removed down to the rock of the oldland, and in part it is 
 cut from the coastal plain strata. (Compare Figs. 607 a and b.} 
 
 The inner lowland will be bordered on one side by the slop- 
 ing surface of the oldland, and on the other, the seaward side, 
 by the eroded edge of the coastal plain. If the top stratum 
 of this coastal plain is of resistant rock, a cliff will crown this 
 cut edge, especially if the cutting has been deep enough to expose 
 
714 The Sculpturing of the Earth's Surface 
 
 the softer bed just beneath. In this manner the coastal plain will 
 end landwards in a steeper slope or in/ace, and between this in- 
 face and the oldland lies the inner lowland (see Fig. 607 b). 
 The remnant of the coastal plain thus separated from the oldland is 
 
 called a cuesta (pro- 
 nounced kwest.a), 
 and this topographic 
 feature is character- 
 istic of the mature 
 stage of dissection 
 
 FIG. 607 b. - The same region shown in Fig. 607 a, f the C astal P lam ' 
 
 after the development of subsequent streams and Two types of 
 
 the formation of an inner lowland. The remnant streams thus are 
 of the coastal plain now forms a cuesta ending 
 landward in an inface, and transected by the conse- isar y 
 
 quent stream valley. duce the cuesta : the 
 
 consequent which 
 
 flows down the dip of the strata, and the subsequent which flows 
 along the strike of the strata. 
 
 The inface of the cuesta may also become the site of develop- 
 ment of a fourth type of stream which flows down it and into the 
 subsequent stream which occupies the inner lowland. These 
 streams likewise increase in length by head ward erosion, and as 
 they flow in the opposite direction from that of the original con- 
 sequent, they have been called ob sequent streams. They form the 
 third side of a rectangular drainage pattern thus established. By 
 their activities, portions of the front of the cuesta may become 
 separated from the main part and constitute plateau- or butte-like 
 outliers or table mountains, if high. These are characteristic 
 features of many cuesta fronts. (See Fig. 633, p. 740.) 
 
 It must always be borne in mind that the depth of the inner 
 lowland, which is occupied by the subsequent streams, cannot be 
 greater than the depth of the consequent stream valley to which 
 these subsequents are tributary, and that it is the master stream 
 of the region which determines the maximum depth to which ero- 
 sion of all the valleys may proceed. The present Atlantic coastal 
 plain of eastern North America appears to be in essentially this 
 state of dissection, although parts of it have suffered great modifi- 
 cations. In the southern part of the plain, the normal cuesta to- 
 pography is easily recognizable. Thus in Alabama we find next to 
 the oldland which is formed by the subdued mountains of the Appa- 
 
The Erosion-Cycle on a Coastal Plain 715 
 
 lachians, the Black Prairie Valley, which forms the inner lowland, 
 and the name of which is derived from the dark color of the rich 
 soil, weathered from the underlying weak limestone. This flat 
 inner lowland includes the best cotton district of the state, and in 
 it lie the cities of Montgomery and Selma. Proceeding seaward 
 we note the rising ground of the inface of the cuesta, locally 
 called the Chunnemugga Ridge, which is rather abrupt, though not 
 characterized by steep cliffs, and upheld by a more resistant lime- 
 stone stratum. Numerous small obsequent streams run down this 
 inface of the cuesta and dissect it. At the top of this inface, which 
 rises 200 feet above the inner lowland, the Hill Prairies and Pinelands 
 
 Copyright by Detroit Furnishing Co. 
 
 FIG. 608. The "Pine-Barrens" on the southern part of the Atlantic 
 Coastal Plain of the United States. . 
 
 begin, and the cuesta surface falls with an imperceptible slope to 
 the sea (Fig. 608). The Alabama, Tombigbee and Chattahoochee 
 rivers are the chief consequent streams rising in the oldland and 
 traversing the cuesta to the sea. 
 
 Farther north, in Maryland, Delaware, and New Jersey, the 
 coastal plain has suffered depression after the erosion topography 
 was produced. As a result the sea has entered the river valleys 
 and produced shallow inlets or bays, of which Chesapeake and 
 Delaware bays are conspicuous examples, though marine erosion 
 has greatly modified and widened these original river valleys 
 (Fig. 629, p. 738). Still farther north, the drowning of the 
 coastal plain has been so complete that it allowed the sea to 
 enter the inner lowland. In this manner Long Island Sound was 
 
716 The Sculpturing of the Earth's Surface 
 
 produced between the inface of the cuesta which forms the north- 
 ern shore of Long Island (here modified by the superposition upon 
 it of the terminal moraine of the ice age) and the Connecticut 
 shore, which is a part of the oldland. Much of the seaward side 
 of the old coastal plain has also been submerged, this accounting 
 in part for the narrowness of the remaining portion which forms 
 Long Island. 
 
 Completion of Cycle of Erosion ; Formation of the Peneplane. 
 As erosion continues, the inface of the cuesta is pushed farther sea- 
 ward and the inner lowland is widened. If a second hard layer is 
 discovered during the progress of erosion, the inface will assume a 
 terraced condition, each hard layer forming a cliff, and the softer 
 
 FIG. 609 a. A coastal plain in the first cycle of erosion. The presence of 
 two hard layers produces a terraced cuesta front. In exceptional cases the 
 upper terrace may weather back with sufficient rapidity to constitute a second 
 cuesta with a lowland of some width between it and the lower cuesta, if the 
 weak stratum between the harder is of sufficient thickness. (Drawn by Mary 
 Welleck.) 
 
 one between, a slope. Continued erosion will eventually result in 
 the complete destruction of the cuesta and the formation of a pene- 
 plane. As the surface of this peneplane will have a gentler slope 
 than that of the strata composing it, it is evident that this surface 
 and the strata will intersect at a moderate angle, with the result 
 that the outcropping ends of the strata are beveled across by the 
 erosion plane. This is shown in the following diagrams (Figs. 609 
 a, b), the first of which (a) shows the terraced cuesta of two hard 
 layers, while the second (b) shows the completion of the peneplane 
 and the beveling across of the strata. If the coastal plain con- 
 sists of a retreatal series of strata (Fig. 606 B) each weak stratum 
 will form a lowland and each hard one a cuesta. 
 
 It will be observed that, as the result of this beveling, the out- 
 crops of the strata are in the form of broad belts, and that, in con- 
 
The Erosion-Cycle on a Coastal Plain 717 
 
 sequence, the outcrops of the two hard layers b and d are separated 
 by a broad belt of soft strata. 
 
 Across such a peneplane the original consequent streams will 
 continue to flow, but because of the disappearance of the original 
 bounding walls of these streams, they are no longer confined to a 
 straight course, but may begin to wind or meander, partly because 
 
 FIG. 609 b. The same region shown in Fig. 609 a, after peneplanation. 
 The strata are now beveled across by the peneplain, and the rivers are no longer 
 controlled by the topography. (Drawn by Mary Welleck.) 
 
 of obstructions left by the diminished current in dropping its load 
 of sediment, and partly for other reasons ; and as the outcrops of 
 the harder beds are more and more reduced to the general level 
 of the peneplane, they will come to have less and less influence 
 upon the course of the streams. 
 
 Beginning of the Second Cycle of Erosion. If, after the region 
 has been reduced to the condition of a peneplane, it suffers a re- 
 
 FIG. 609 c. The same district shown in the two preceding diagrams, after 
 elevation and renewed dissection. Two cuestas and two lowlands are now 
 produced from the hard and weak strata, respectively. (Drawn by Mary 
 Welleck.) 
 
 newed bodily elevation, the streams will be rejuvenated because 
 their slope is increased, and they will once more begin to cut down- 
 wards. Here again the consequent streams, especially those which 
 flow from the oldland to the sea, will determine the rate and extent 
 of downward cutting, for all other streams are tributary to these 
 consequents, and cannot cut below the level attained by them. 
 
7iS The Sculpturing of the Earth's Surface 
 
 If the consequent streams have begun to meander upon the sur- 
 face of the peneplane, they are apt to retain that meandering course, 
 and the new channel or gorge which they cut during the second 
 cycle will be characterized by incised or intrenched meanders, the 
 existence of which is generally suggestive of a second or later cycle 
 of erosion (Fig. 610). (See also Figs. 602, p. 706, and 603, p. 708.) 
 
 FIG. 6 10. Intrenched meanders of the Seine in the old peneplane cut upon 
 the strata of the Paris Basin ; near Rouen. (After de Martonne.) 
 
 As the consequent streams cut their transverse channels across 
 the outcropping belts of the soft and the hard layers alike, the lat- 
 eral streams will also begin cutting, but they will be largely con- 
 fined to the softer belts which are more easily eroded. As a result, 
 subsequent valleys will be formed upon these softer belts, and the 
 harder ones will gradually be modeled in relief as new ridges, and 
 eventually each will assume the appearance of a cuesta inface, 
 fronting a lowland cut upon the soft belt. But there will now be 
 two such cuesta infaces and two broad lowland valleys, one the 
 inner lowland between the oldland and the inface formed by the 
 
The Erosion-Cycle on a Coastal Plain 719 
 
 lower hard stratum, the other between this latter and the inface 
 formed by the upper hard stratum. The diagram (Fig. 609 c) 
 illustrates the result thus produced. 
 
 These two cuesta ridges may now be many miles apart, but their 
 summit elevation will be in the same plane, as they formed part 
 of the original peneplane surface. They may, indeed, retain a 
 part of the beveling which was produced during the peneplanation, 
 as is indicated in the diagram. 
 
 Central England as an Illustration. (Fig. 611.) What is gen- 
 erally regarded as a good illustration of such a belted structure 
 with two cuestas and lowlands, is seen in the topography of central 
 England. If we consider the rugged hills and mountains of Wales 
 
 FIG. 611. Block diagram of the cuestas and lowlands of Central England. 
 A, Oldland of Wales; B, Inner lowland (Triassic) ; C, Oolite cuesta, Cots wold 
 Hills ; D, Outer lowland (Green-sands and Gault) ; E, Chalk cuesta, Chiltern 
 Hills; F, London coastal lowland. (After Davis.) 
 
 as a part of the oldland (^4) we find to the east and southeast of 
 this a broad inner lowland cut upon .soft red sandstones and 
 marls (Triassic). In this lowland (B) are situated the cities of 
 Bristol, Nottingham, and York. Rising from this lowland belt 
 upon the east is the first ridge or cuesta (C) , which extends in a 
 curving line across central England from southwest to northeast, 
 and is formed by the edges of the lower hard limestone strata 
 (Oolites) which overlie the red sandstones. Near Bristol this up- 
 land belt forms the Cotswold Hills, and it is characterized by a 
 relatively steep westward and very gentle eastward slope. Next 
 east of this lies the second lowland (D), cut upon the second belt 
 of soft strata (Gault and Green-sands) and in this are situated the 
 cities of Cambridge and Oxford. A second upland or cuesta (E), 
 bounds the eastern and northeastern border of this lowland, this 
 being formed by the resistant chalk. It constitutes the Chiltern 
 
720 The Sculpturing of the Earth's Surface 
 
 Hills on the south, and from the summit of the cuesta there is a 
 gentle slope toward the London coastal lowland (F). 
 
 While this illustrates in a general way the double cuesta type of 
 topography produced in a second erosion cycle, there are many 
 local modifications, owing to the disturbances of the strata by folds 
 and faults, and the history of the region is on the whole more com- 
 plex than here outlined. 
 
 River Capture in Coastal Plain Dissection. It has been shown 
 that in every coastal plain undergoing dissection there is apt to 
 be a master consequent stream which, because of its greater water 
 supply, cuts deeper and more rapidly than the other consequent 
 streams. The tributaries to such a stream have the advantage 
 over the tributaries to another consequent, in that they are enabled 
 
 j i 
 
 FIG. 612. Diagrams showing the progress of river capture and beheading. 
 At first the divide is halfway between the two consequent streams at A ; but 
 the subsequent tributary of the stronger consequent encroaches upon the 
 territory of the weaker one, pushing the divide to B ; continuing, it beheads the 
 weaker stream, capturing and diverting its headwaters (C). The beheaded 
 stream now follows a wriggling course in its old valley, which becomes much 
 obstructed by the debris from the sides which this stream is no longer able to 
 carry away. 
 
 to cut deeper and tend more rapidly to extend their territory by 
 headward cutting. Of two subsequent streams flowing in opposite 
 directions into separate consequents, the one tributary to the 
 master will have the steeper slope, and so be enabled to extend its 
 drainage territory at the expense of the other subsequent, which is 
 progressively robbed by the more powerful stream. Thus while 
 the divide between the two tributaries (subsequents) may at first 
 have been at A (Fig. 612), it has gradually been pushed to B, and 
 finally, at C, tapped the headwater portion of the smaller con- 
 sequent, making it a captured tributary to the master stream. 
 The beheaded portion of the smaller consequent will remain as a 
 much diminished stream in its original valley. Since the lateral 
 insequent streams continue to bring in at least the same amount 
 of sediment as before, this must now be partly dropped upon 
 
The Erosion-Cycle on a Coastal Plain 721 
 
 the floor of the valley occupied by the beheaded consequent, be- 
 cause this river is no longer able, on account of its diminished 
 water supply, to carry this material away. Because of the ob- 
 structions thus produced, the diminished stream of the valley must 
 frequently change its course. To the extreme irregular windings 
 thus assumed, the name " staggering course " is applied, and such 
 a course may in general be said to be characteristic of recently 
 beheaded streams. 
 
 Meanwhile the area appropriated by the master stream will be 
 deepened in conformity with the depth of that stream, and so the 
 inner lowland floor may be cut far below the level at which the 
 beheaded consequent continues to flow, the valley of this stream 
 ending abruptly against the much deeper subsequent valley. This 
 is illustrated in the following diagram (Fig. 613), and may be re- 
 
 FIG. 613. Overdeepened subsequent. (Drawn by Mary Welleck.) 
 
 garded as a characteristic feature of the valleys of beheaded streams 
 on dissected coastal plains. Such conditions may develop during 
 the first, and during any later cycle of erosion, and as a result the 
 form of the drainage system of the master stream, after it has 
 captured a number of streams, will resemble that of a well-trained 
 grape vine. 
 
 Waterfalls in the Coastal Plain Drainage System. Large 
 waterfalls are not a characteristic feature of the coastal plain 
 proper, though small ones are formed in all the streams which 
 flow over hard strata of the coastal plain deposits. Along the 
 head of the coastal plain, where the extended consequents and 
 other streams pass from the oldland of hard rocks to the low- 
 land cut on the soft belt of the coastal plain, a series of waterfalls 
 is likely to come into existence, for the hard rocks of the oldland 
 permit less rapid erosion than do the softer rocks of the valley 
 adjoining. The inner border of the coastal plain thus becomes a 
 
722 The Sculpturing of the Earth's Surface 
 
 " fall line," and it is here that water power is developed, which 
 favors the location of large cities and towns, Trenton, Phila- 
 delphia, Washington, Richmond, Raleigh, Camden, Augusta, and 
 Columbus are all located upon the fall line of the Atlantic coastal 
 plain where it is crossed by large rivers. 
 
 THE EROSION CYCLE ON DOMES AND BASINS 
 The Black Hills Type 
 
 The Black Hills of South Dakota represent an uplift of the 
 strata in the form of a very regular dome of oval outline 100 miles 
 long north and south and 50 miles broad east and west. The 
 strata on the flanks are raised to angles of 45 degrees or more, and 
 the original height of the dome was about 5000 feet above the 
 surrounding plain. The normal erosion cycle upon such a dome 
 proceeds somewhat as follows. 
 
 Radial Consequents. The first type of stream to come into 
 existence on a newly raised dome of this kind is a series of radial 
 consequent streams which flow down the sides of the dome in all 
 directions. At the base of the dome they become tributary to 
 one or more streams which carry away the drainage, but these 
 have no especial significance in the development of the drainage 
 topography of the dome. As the strata are removed from the 
 summit of the dome by erosion they gradually begin to form a 
 series of rimming outcrop belts around the center, the highest or 
 youngest stratum forming the outermost, and the oldest the inner- 
 most belt. Wherever a hard bed of sandstone or limestone alter- 
 nates with softer shaley or marly beds, a ridge will be formed from 
 the hard bed, which, because of the steep inclination of the strata, 
 will have a triangular section, the outer slope being formed by the 
 surface of the hard stratum and the inner by the cut edges of the 
 series. Such a uniclinal ridge is called a " hog-back" (Fig. 631, 
 p. 739), and the Black Hills are encircled by a number of such 
 hog-backs concentrically placed around the central region. Be- 
 tween the hog-backs are circling valleys cut upon the softer 
 strata, and one of these, cut on red shales and soft sandstones, has 
 such a regular form that the Indians called it the " race course." 
 Because of the red color of the rock it is also known as the Red 
 Valley. The drainage of these valleys is carried through a series 
 
The Erosion Cycle on Domes and Basins 723 
 
 of water gaps in the hog-backs or uniclines, and these gaps form 
 the avenues of approach to the central mountainous area. 
 
 This central area is formed by the old crystalline rocks, here 
 raised high above the surrounding country and largely stripped of 
 their former covering of sediments. They have been dissected 
 into a series of peaks and ridges, which to-day rise to 2000 or 3000 
 feet above the plain and which by forcing the winds to rise com- 
 pel precipitation. They are therefore clothed with dark forests 
 (hence the name Black Hills), and they form a marked contrast to 
 the more arid rolling treeless plains which surround the hills. 
 The presence of precious metals in these ancient rocks has made 
 this a center of active mining operations. The following cross- 
 section shows the position of the hog-back-forming strata and 
 their former extent across the dome (Fig. 614). 
 
 FIG. 614. Cross-section of the Black Hills dome, showing the present 
 structure and topography and the former continuation of the beds, i, Archaean 
 slates and schists; 2, granite; 3, basal Cambrian sandstone; 4, limestone 
 (Ordovician at base, Carbonic for the most part) ; 5, Red beds shales and 
 sandstones (Triassic) forming the Red Valley ; 6, Jurassic shales, etc. ; 7, Dakota 
 sandstones; 8, Cretaceous shales; 9, Cretaceous limestone; 10, Tertiary 
 strata ; DH, Dakota hog-back. 
 
 Flat Domes 
 
 Ozark Plateau. The Ozark Plateau of Missouri represents a 
 low, flat, but somewhat asymmetrical dome of great extent, 
 steeper in the south, and which has been dissected so that the 
 strata which once covered it are now found chiefly as rimming 
 belts around the flanks. Only a small area of the underlying 
 crystalline rocks is exposed in the St. Francis Mountains in the 
 northeastern part of the dome. Over the greater part of the 
 Missouri area, the Cambro-Ordovician strata, which immediately 
 overlie the crystallines, form the surface rock of the so-called Salem 
 upland. These are dolomitic limestones and sandstones, dipping 
 at such low angles that they appear horizontal. They are dis- 
 
724 The Sculpturing of the Earth's Surface 
 
 sected by streams which in general have a radial arrangement, 
 partly with reference to the St. Francis Mountain area, but chiefly 
 with reference to the geographic center of the dome. Some of the 
 valleys cut by these streams are 250 feet or more in depth (Fig. 615). 
 The northern slope of the dome is characterized by a succes- 
 sion of broad, flat plateaus, separated by escarpments of irregular 
 
 outline formed by 
 the .harder strata, 
 which here dip 
 northward at a very 
 low angle, so that 
 the escarpments 
 have the character- 
 istic appearance of 
 cuesta fronts. They 
 are partly formed by 
 Silurian and partly 
 by Mississippian 
 limestones. Many 
 outliers are found in 
 front of the escarp- 
 ments and mark the 
 progress of differ- 
 ential retreat. The 
 western and south- 
 
 Carbonic Mississippi&n 
 
 Pre- Cambrian 
 Crystallines 
 
 Silur.&nd 
 Upper Ord 
 
 Tertiary 
 Cover 
 
 Carnbrp- 
 
 FIG. 615. Geological map of the Ozark dome, 
 showing the concentric arrangement of the out- 
 cropping formations and the radial drainage. 
 
 western margins of 
 the dome are formed 
 by the Springfield 
 
 Plain, which faces the center of the dome in the Burlington es- 
 carpment and is another cuesta-like structural feature. It is pro- 
 duced by the harder Mississippian limestones which dip gently 
 to the west in Missouri and to the southwest in Arkansas, the 
 slope of the surface of the plain corresponding to the dip of 
 the strata. Outliers also characterize the front of this escarpment. 
 On the south, the margin of the dome is formed by the continua- 
 tion of the Burlington escarpment, and farther away by the Boston 
 Mountains, the rocks of which dip southward at a somewhat 
 greater angle than do those of other portions of the dome. The 
 front of this ridge is a bold escarpment capped by a resistant layer 
 of sandstone, and from it many finger-like prolongations extend 
 
The Erosion Cycle on Domes and Basins 725 
 
 northward, while dissociated remnants form outliers to the north 
 of the main escarpment. The topography becomes rougher toward 
 the east, where the plateau and escarpment characters are ob- 
 scured. On the southeast the dome passes beneath the alluvial 
 lands of the Mississippi River. 
 
 This dome differs from the Black Hills Dome in that its rimming 
 elements have a cuesta-like character, owing to the nearly hori- 
 zontal attitude of the strata, whereas the steeply inclined strata 
 of the Black Hills Dome form rimming hog-backs. Like the 
 Black Hills case, however, the strata which now end in escarp- 
 ments on the north, west, and south, formerly extended across the 
 Ozark dome, and have since been removed from the central part 
 by erosion. 
 
 Ontario Dome. A less perfectly preserved low dome of the 
 Ozark type is seen in the Ontario region north and east of the 
 Great Lakes. Over the greater part of this dome only the old 
 crystalline rocks are now exposed, but around its eastern, south- 
 ern, and southwestern border, and in part on its northwestern 
 as well, the sedimentary strata which once covered it to a large 
 extent, if not completely, are still visible, forming a series of more 
 or less rimming escarpments which face the center of this dome. 
 As is the case with all the domes of eastern North America, this one 
 is not in its first cycle of dissection, but has been at least once 
 reduced to the condition of a peneplane since the end of the 
 Palaeozoic, when its main elevation took place. As a result, the 
 strata which dip from 5 to 15 feet per mile are beveled across 
 at a low angle and their outcrops form broad belts upon the 
 surface. After the peneplanation a renewed upward movement 
 of the dome took place in early Tertiary times, and this revived 
 the radial consequent drainage. Many of the valleys of these 
 revived radial consequents are still recognizable, though some 
 have been deepened by glacial erosion and transformed into lakes, 
 while others are filled by drift, or are partly occupied by streams 
 which flow in the opposite direction. Of the over-deepened 
 valleys, those occupied by the " Finger Lakes " of New York are 
 typical examples. Their striking radial arrangement is the direct 
 result of their origin as radial consequents, and a series of lines 
 passed through them and marking the former courses of the rivers 
 would- converge somewhere near the center of the dome north of 
 Lake Ontario (Fig. 616). The waters of these rivers, then flowing 
 
726 The Sculpturing of the Earth's Surface 
 
 southward, were gathered by the various branches of the Susque- 
 hanna and carried to the Atlantic. The continuity of these old 
 valleys to their junction with the old valley of the Susquehanna 
 can still be traced, and so can others, like that of the Genesee, now 
 occupied by northward flowing rivers. Where not modified by 
 ice erosion, these valleys are often from one to several miles in 
 
 width, with gently slop- 
 ing sides and flat bot- 
 toms, mostly covered 
 to some depth, by sands 
 and clays of later origin. 
 The master stream 
 valley of the region is 
 now entirely filled in 
 by glacial drift, but has 
 been traced by borings. 
 It passed through the 
 western end of Lake 
 Ontario, where, near the 
 city of Hamilton, it was 
 about three miles wide 
 and of great depth. 
 Its general direction of flow was to the southwest (Figs. 617, 6 1 8). 
 Another of the radial consequent valleys can be partly traced 
 through the gap in the ridge, which separates Georgian Bay from 
 Lake Huron, and its southwestern continuation was apparently 
 in the submerged valley which now forms Saginaw Bay. The 
 waters of these streams were gathered and carried to the Mississippi 
 by paths as yet only partially known. 
 
 In the later cycle of erosion the outcrops of the softer beds of 
 this peneplaned dome were eroded into lowlands, and the harder 
 beds formed new rimming cuestas. One of these lowlands is 
 occupied by Lake Ontario on the south and by Georgian Bay 
 on the west of the dome, the connecting portion being filled by 
 glacial drift. Along the southern border of Lake Ontario ex- 
 tends the Niagara escarpment, which is the inface of the revived 
 cuesta. The upper part of this cuesta rises as a ridge 200 
 feet high above the surface of the lake, but the greater part is 
 submerged. The escarpment can be traced to and around the 
 western end of Lake Ontario (where it is breached by the valley 
 
 FIG. 616. Map of a portion of central New 
 York, showing the radial arrangement of the 
 Finger Lakes. 
 
FIG. 617. Map of the ancient drainage system of the southern and western 
 part of the Ontario dome ; early stage. The present hydrography is dotted ; 
 the principal cuesta-fronts are indicated. 
 
 FIG. 6 1 8. Map of the ancient drainage system of the southern and western 
 part of the Ontario dome, showing a later stage when by capture the head- 
 waters of the other consequents have become tributary to the Dundas River. 
 
 727 
 
728 The Sculpturing of the Earth's Surface 
 
 of the drift-buried consequent above mentioned the Dundas 
 River) and throughout the peninsula which divides Georgian Bay 
 from Lake Huron, continuing beyond the. gap, in Manitoulin 
 Island. A second escarpment, formed by a higher limestone bed 
 (the Onondaga), can be traced east and west through the city of 
 Buffalo, across western Ontario in a northwest direction, where 
 it is chiefly buried by glacial drift, and diagonally across Lake 
 Huron from near Goderich, Ontario, to Mackinac Island. Even 
 where submerged, it is clearly shown by soundings in Lake Huron 
 to have a. height of about 400 feet (Fig. 619). A third escarp- 
 ment forms the front of the Alleghany Plateau in southwestern 
 New York, and extends along the western margin of Lake 
 Huron. The lowland in front of it is partly occupied by the east- 
 
 FIG. 619. Cross-section of Lake Huron from Point au Sable, (a) Michigan, 
 across nine fathom ledge^i) to Cape Hurd, (c) Canada, showing the submerged 
 Onondaga cuesta and the lowland in front of it. 
 
 ern end of Lake Erie. The general relationship of the radial 
 streams and revived cuestas is shown in the diagram on page 760 
 (Fig. 653). 
 
 These three escarpments can be traced eastward through New 
 York state, gradually approaching one another as the Mohawk 
 River is reached, because the softer beds, which form the lowlands 
 on the west, die out eastward. They finally unite to form the 
 ridge known as the Helderberg Mountains in eastern New York, 
 culminating in the Catskill Mountains, which project as a group 
 of monadnocks above the surface of the peneplane. 
 
 These various cuestas and lowlands have the relation to the 
 center of the Ontario Dome that normal cuestas and lowlands have 
 to their oldland, and they are sometimes referred to as normal 
 cuestas in the second cycle of erosion. They are, however, the 
 remnants of the rimming strata of a peneplaned dome, which, be- 
 cause of further elevation of that dome, have undergone renewed 
 dissection with the revival of an incomplete rimming cuesta 
 topography. 
 
 Nashville Dome. As a final example of a dissected dome of 
 gently dipping strata, the Nashville Dome of central Tennessee 
 
The Erosion Cycle on Domes and Basins 729 
 
 may be noted. Although a structural dome, it now forms a 
 topographic basin, the Central Basin of Tennessee. This is due 
 to the fact that after the peneplanation of this dome, the central 
 area exposed only soft strata, and these were largely cut away 
 down to a lower series of hard strata by the tributaries of the 
 radial streams which dissected the dome on renewal of uplift. The 
 chief of these streams is the Tennessee River, which has cut a 
 narrow, gorge-like valley in the surrounding rim. The rim is con- 
 tinuous except for the breaches by the radial streams, which lie 
 chiefly upon the western side. It is formed by the edges of the 
 harder strata which overlie the softer ones not far from their 
 position of outcrop after the peneplanation. The central de- 
 pression (the topographic basin) is about 70 miles across, and 
 
 FIG. 620. Section of the Nashville dome, showing the center eroded into a 
 topographic basin or encircled lowland, the so-called Central Basin of Tennessee. 
 C, Cambrian ; Ot, Ordovician (Trenton) limestone ; On, Ordovician (Nashville) 
 shale; Sn, Silurian (Niagaran) limestone; Db, Devono-Mississippian shale; 
 Ms, Mississippian sandstone; Mlm, Mississippian (Mountain) limestone; 
 C, coal measures; Length of section about 120 miles. (After Safford.) 
 
 its bottom, which is about 600 feet above sea-level, is formed by 
 the lower series of hard rocks and has a gently undulating surface 
 (Fig. 620). Outliers from the surrounding plateau characterize it 
 in many places as we approach the rimming margin. The Cin- 
 cinnati Dome is a larger but less perfect example of similar 
 character. 
 
 Shallow Basins 
 
 Shallow structural basins are the complements of the flat 
 domes with which they are commonly associated. In America 
 the best example is found in the lower peninsula of Michigan 
 (Michigan Basin), while in Europe the Paris Basin is the best 
 known example. Both of these represent basins not in the first 
 cycle of erosion, for both have suffered peneplanation and the 
 present topography is a revival in the second or later cycle. As 
 a result of this, the strata now form a succession resembling a 
 nest of plates, the largest at the bottom and the smallest at the 
 top, the successive edges of the plates forming the outer rims of 
 the several formations. 
 
730 The Sculpturing of the Earth's Surface 
 
 By the excavation, during the second cycle, of subsequent valleys 
 upon the outcrops of the softer beds there are produced con- 
 
 centric annular lowlands. The outcropping edges of the hard 
 strata form cuesta ridges, the escarpments of which face out- 
 ward in all directions. This is especially well seen in the eastern 
 
The Erosion Cycle on Domes and Basins 731 
 
 part of the Paris Basin, where a succession of such outward- 
 facing escarpments, separated by broad flat valleys, marks the 
 approach from Germany towards Paris, which lies in the center 
 of the basin. These escarpments form, indeed, a series of huge 
 steps up which, or through the gaps in which, invading armies 
 from the east must fight their way; while the gentle slope from the 
 top of each step toward the foot of the next inner one, and finally 
 toward Paris, makes the approach from the west an easy one 
 (Fig. 621). The heights of Nancy and Metz form the eastern- 
 most of these escarpments (Middle Jurassic). Beyond them, 
 nearer Paris, lies the Woevre lowland, west of which rises the second 
 escarpment (Upper Jurassic), which has to be ascended to reach 
 Verdun. Still farther west lies the Wet Champagne lowland sepa- 
 rated by the chalk escarpment from the Dry Champagne lowland, 
 
 FIG. 622. Cross-section of the Michigan Basin showing the rimming 
 cuestas which are largely submerged. Or, Ordovician; N, Niagaran; 5 & 
 M, Salman and Monroan; On, Onondaga; Tr, Traverse; A, Antrim black 
 shale ; M , Mississippian ; C, Coal Measures. 
 
 which in turn is bounded on the west by the Rheims-Epernay- 
 Sezanne escarpment (Tertiary), the last to be ascended before reach- 
 ing the center of the basin and Paris. These escarpments are also, 
 though less perfectly, developed on the south and west, but on the 
 northwest they are obliterated by the English Channel. Between 
 the Ardennes Mountains and Calais on the coast they are not 
 developed because, owing to a tilt in the basin, erosion has not 
 gone deep enough. Hence the approach to Paris through Belgium 
 is the only one which avoids these great step-like natural 
 defenses. 
 
 The center of the Michigan Basin, in which Alma lies, is sur- 
 rounded by a similar series of outward-facing escarpments, but 
 owing to the heavy glaciation of the region the lowland valleys 
 are generally filled with drift and obliterated, while others are 
 occupied by the waters of Lakes Huron and Michigan. If these 
 lakes were drained, central Michigan would be well defended by 
 cliffs from Canadian and Wisconsin approaches, though , easily 
 accessible from the south (Fig. 622). (See also Fig. 516, p. 600.) 
 
732 The Sculpturing of the Earth's Surface 
 
 THE EROSION CYCLE ON ANTICLINES AND SYNCLINES 
 
 The First Cycle. The erosion of anticlines proceeds much 
 after the manner of that on steep domes, except that the initial 
 consequent drainage is not radial, but consists of a series of parallel 
 streams on either side of the anticline and flowing away from 
 its axis. These lateral consequents carry the drainage into the 
 synclines, where the initial longitudinal consequent streams, i.e., 
 those flowing along the axis of the fold, are situated (Fig. 623). 
 Only at the ends of the anticlines, where these flatten out or pitch, 
 is the radial form of initial drainage partly realized. 
 
 FIG. 623. A longitudinal consequent valley in a syncline, with hog-backs or 
 uniclines facing outward on both sides. 
 
 As the consequent streams cut into the flanks of the anticlines, 
 they will develop gorges, along the sides of which insequent streams 
 arise, especially where the strata become flat on the axis. In the 
 course of time there will be opened a longitudinal breach in the 
 axial portion of the anticline (Fig. 624, I). If a hard stratum 
 forms the capping rock, with a softer one beneath it, this breach 
 will be enclosed by cliffs broken only by the narrow gaps through 
 which the consequents flow. Continued erosion will not only 
 lengthen this breach on the axis (Fig. 624, II), but will push the 
 sides farther and farther apart, that is, down the flanks of the 
 anticline. If a second hard stratum is discovered on the axis of 
 the anticline, this will form a central ridge, flanked on either side 
 by longitudinal subsequent valleys. 
 
The Erosion Cycle on Anticlines and Synclines 733 
 
 Further erosion may open up a new longitudinal valley in the 
 top of this axis, at the same time pushing the sides of the older 
 
 FIG. 624. Diagrams showing the development of river systems in anticlinal 
 folds with anticlinal valleys as the end product (IV). As a result of this erosion 
 the streams flowing in the synclinal valleys are diverted by capture and the 
 main streams become anticlinal. (After de Martonne.) 
 
 valleys farther down the flanks. Thus a series of uniclinal ridges 
 is produced, facing the center of the original anticline. The 
 drainage system produced will at first comprise longitudinal subse- 
 
734 The Sculpturing of the Earth's Surface 
 
 quent streams which occupy the valleys on the softer strata parallel 
 to the ridges, and transverse consequents passing through gaps 
 in the ridges to the longitudinal consequent streams which flow 
 down the axis of the syncline. Some of the larger transverse 
 consequent streams may cut so deep that their tributary sub- 
 sequents will encroach upon the territory of adjoining weaker 
 consequents, which they may finally behead, carrying the captured 
 drainage to the master stream. If this happens, the original 
 water-gap through the ridge formed by the weaker consequents, 
 will be abandoned below the elbow of capture and become a dry 
 notch in the ridge. It is then known as a wind-gap. 
 
 In the following diagram (Fig. 625) a stream (Q) flowing towards 
 
 FIG. 625. Diagram illustrating the imminence of river capture. (After 
 
 Davis.) 
 
 the anticlinal valley on the right across two hard strata G, J, is 
 encroached upon by another flowing upon soft strata H, the valley 
 of which, as shown by the figures, is much lower. In the next 
 stage, shown in diagram Fig. 626, the capture of the upper part 
 of the stream (Q) has been completed. 
 
 When the breaching of the anticline has proceeded as far as 
 the altitude of the land with reference to the base-level of erosion 
 permits, a series of parallel uniclines or hog-back ridges and 
 longitudinal strike- valleys, with generally a central axial or 
 anticlinal valley, has been produced, the cut edges of the ridges 
 successively facing the center of the anticline. Further erosion 
 will result in the lowering of these ridges, and when they have 
 
The Erosion Cycle on Anticlines and Synclines 735 
 
 nearly disappeared, a peneplane will be formed, the surface of 
 which will be marked by a series of parallel belts of alternating 
 hard and soft strata ; but none of these produce marked relief 
 features. Where the ends of the anticline pitch beneath the erosion 
 surface, the corresponding strata will curve around from opposite 
 sides, joining along the axial line, or better, a continuous curve 
 will unite the corresponding strata on opposite sides, forming canoe- 
 shaped valleys (see Figs. 508, p. 595, and 514, p. 598). As the hard 
 strata which confined the longitudinal subsequents to their re- 
 spective valleys are reduced to the level of the peneplane, these 
 streams may shift their location, meandering aimlessly across hard 
 
 FIG. 626. Diagram showing the same region with the capture completed. 
 
 (After Davis.) 
 
 and soft strata alike. Thus the initial character of the drainage 
 is destroyed. 
 
 The Second Cycle. If a peneplaned region of anticlinal and 
 synclinal structure is elevated, rejuvenation of the streams will 
 take place, and these will begin to cut downwards where they were 
 located at the time of the uplift (Fig. 627). If, as generally 
 happens, the surface of the peneplane becomes warped or slightly 
 tilted on uplift, the direction of the new drainage will be deter- 
 mined by the slope thus produced, and the streams may begin to 
 cut downward across hard and soft strata alike. So soon as a 
 master stream has begun to incise its bed beneath the surface of 
 the peneplane, lateral streams will begin to open out valleys 
 along the softer strata, leaving the harder in relief, and thus the 
 
736 The Sculpturing of the Earth's Surface 
 
 original topography of longitudinal valleys and parallel uniclinal 
 ridges will be revived (Fig. 628). By capture of the weaker 
 streams which flow across the strata, wind gaps are produced, 
 and the drainage all becomes tributary to the master stream or 
 streams of the region. 
 
 Recognition of the Second Cycle. How can we recognize 
 that a dissected anticlinal region is in the second cycle, when the 
 ridges all stand out in bold relief and the valleys are occupied by 
 longitudinal streams, tributary to a transverse consequent which 
 
 FIG. 627. A stream incised upon the axis of a broad anticline, where it 
 happened to be located at the beginning of a new cycle of erosion. 
 
 has cut gaps for its passage across the successive ridges? If we 
 were dealing with a single anticline, it would probably be difficult 
 to determine the cycle, though here, too, certain guiding principles 
 might help to solve the problem. In the first cycle, the drainage 
 from the corresponding longitudinal valleys on opposite sides of 
 the axis is carried outward in opposite directions. It is, however, 
 conceivable that by capture the entire drainage system of one side 
 might be reversed and carried out in the opposite direction. While 
 this might happen in the case of one anticline, it is extremely 
 doubtful that by capture the entire drainage can become tributary 
 to a stream which, as the result of such capture, crosses two anti- 
 clines and all the uniclinal ridges derived from them. Still more 
 
The Erosion Cycle on Anticlines and Synclines 737 
 
 doubtful does it become when the number of parallel anticlines 
 is more than two. If a master stream has cut gap after gap across 
 all the uniclinal ridges of several parallel anticlines, carrying the 
 drainage of all the longitudinal valleys and of the original syncline 
 as well, it is practically certain that this stream was not built up 
 from progressively captured portions of other streams, but that 
 it represents a new stream formed upon the surface of a sloping 
 peneplane. This stream cuts its channel regardless of hard and 
 soft strata; for there is no chance for readjustment during the 
 cutting of the channel, because such a stream controls the drain- 
 
 FIG. 628. Diagram showing the development of a peneplane from anti- 
 clinal ridges and the reappearance of the uniclinal topography after elevation 
 and renewed erosion. This is the type of structure found in the Appalachians. 
 (After Davis.) 
 
 age of the region, and no valley can be cut by any of its tributaries 
 which is lower than the floor of the valley of the master stream. 
 
 It should, however, be noted that many if not most of the 
 phenomena here cited may be' explained by the hypothesis of an 
 antecedent stream in the first cycle; but there are other criteria 
 which will serve to determine the applicability of that hypothesis. 
 Antecedent streams are discussed in a later section. 
 
 It is because of the existence of a number of such transverse 
 streams, which cut across all the ridges of the region, and more- 
 over, have a meandering course, that the Appalachian Mountains 
 are believed to be in at least a second cycle of erosion (Fig. 629). 
 The Susquehanna is the most prominent of these transverse streams, 
 and its tributaries have opened up again a large part of the valley 
 system of the present Appalachians of Pennsylvania, etc. This 
 opening of the Appalachian valleys was essentially coincident with 
 the opening of the cuesta-like topography of the Ontario and other 
 
738 The Sculpturing of the Earth's Surface 
 
 domes to the north, and the drainage accomplishing the latter 
 was in part carried out by the Susquehanna. 
 
 As a result of this erosion in the second cycle, the axes of the 
 original synclines, now high above the new base-level of erosion, 
 have often been left in relief by the cutting of the valleys on either 
 side to a greater depth. Thus synclinal mountains are actually 
 produced, these being a characteristic feature of some parts of the 
 Appalachian system. 
 
 When the center of the anticline consists of hard rocks, erosion 
 progressing in the manner outlined will leave a central ridge in- 
 
 FIG. 629. Map of Northern Appalachians. (U. S. G. S.) 
 
 stead of a valley. This central ridge seen in parts of the Appala- 
 chians will be bordered on either side by uniclinal ridges formed 
 by the hard strata of the sedimentary series (Fig. 628). Such 
 uniclinal ridges or hog-backs, as they are called (Fig. 630, I, II), 
 which have been developed in the first cycle of erosion, may be- 
 come obliterated again when the region approaches a condition 
 of peneplanation. Renewed elevation will steepen the inclination 
 of the layers (Fig. 630, III), after which erosion will once more 
 model out the hog-back, or uniclinal topography (Fig. 630, IV). 
 
 This has been essentially the later history of the Rocky Moun- 
 tain Front Range, though by the further development of thrust 
 faults this history has been rendered more complex. The central 
 
The Erosion Cycle on Anticlines and Synclines 739 
 
 axis of the range consists of crystalline rocks (granites, etc.), and 
 is margined on either side by pronounced hog-backs, formed by 
 the harder strata of the 
 Mesozoic series which 
 once extended across 
 the crystallines. Typical 
 examples of such hog- 
 backs are shown in Figs. 
 63 1 , 63 2 . The upturned 
 strata along the moun- 
 tain front, will, if they 
 are sandstones or other 
 porous rocks, take in 
 surface waters and so 
 form a head for artesian 
 conditions farther out 
 in the plains, as shown 
 in the section of the 
 Dakota artesian system 
 
 FIG. 630. Diagrammatic sections represent- 
 ing the development of hog-backs. I, a simple 
 anticlinal ridge consisting of hard and soft .sedi- 
 ments overlying crystalline rocks ; II, the same 
 after erosion and development of hog-backs 
 (uniclines) on opposite sides; III, the same 
 region after peneplanation and further arching ; 
 IV, Development of the hog-backs in the second 
 cycle of erosion. 
 
 on p. 424 (Fig. 353). 
 
 As the strata are 
 flexed only near the mountains, and change toward horizontality 
 away from these, it is evident that as the hog-back is pushed 
 
 FIG. 631. Hog-back of Dakota sandstone, near Canyon City, Colorado. 
 The ridge-making hard stratum dips steeply to the right. A second, minor 
 ridge is formed by another hard sandstone on the right. Valley of the 
 Graneros River. (C. D. Walcott, photo, U. S. G. S.) 
 
740 The Sculpturing of the Earth's Surface 
 
 farther away from the mountain by erosion, the steepness of the 
 dip of its strata will decrease, until it becomes so low that the 
 
 FIG. 632. Hog-back near Colorado City, Colorado. (I. C. Russell, photo.) 
 
 beds have the appearance of gently dipping coastal plain strata, 
 and the hog-back changes to a cuesta. Still farther away, where 
 the strata are absolutely horizontal, a step topography is produced 
 
 FIG. 633. Hog-backs RR, changing into cuestas (CC) and these into 
 steps (HH) by progressive flattening of strata ; UU, oldland ; 7T, inner low- 
 land; SS, second lowland; P, plateau between steps; V, nose; Z, outlier. 
 (After Davis.) 
 
 by the edges of the hard formations. In some cases all of these 
 stages may be observed along a line of outcrop, as shown in the 
 preceding diagram (Fig. 633) where the hog-backs (RR) pass into 
 
The Erosion Cycle on Anticlines and Synclines 741 
 
 cuestas (CC) and these into steps (HH). Such a change is also 
 seen along the present eastern front of the Appalachians, where 
 the Helderberg front, of the cuesta type, passes southward into 
 the uniclinal ridge through which the Delaware River has cut its 
 water-gap (Fig. 634). 
 
 Complexly Folded Strata. When erosion attacks complexly 
 folded strata, the process, though following the general laws ob- 
 served in the case of simple folds, is correspondingly more com- 
 plicated, and the erosion forms produced are more diverse. In- 
 
 FIG. 634. View of Delaware Water-Gap from the Great Valley, showing 
 the even sky line of the mountains and the abrupt cut of the water gap. (U. S. 
 G.S.) 
 
 stead of long, regular ridges, such as those of the Appalachians, 
 individual peaks and irregular ridges, such as those of the White 
 Mountains, the Alps, the Caucasus, and others, result. In these, 
 moreover, glacial erosion has aided in the sculpturing process, as 
 will be set forth later. Peneplanation of such a region is apt to 
 leave peaks of harder rock standing above the peneplane level as 
 monadnocks. Such is Mt. Monadnock in southern New Hamp- 
 shire, the type of residual erosion peaks, which from some points 
 of view has almost the regularity of outline of a volcanic cone 
 (Fig. 635). 
 
742 The Sculpturing of the Earth's Surface 
 
 FIG. 635. The New England peneplane with Mt. Monadnock (New 
 Hampshire) rising above it. From Be^ch Hill, Keene, N. H. Tertiary valleys 
 in the foreground. (Gardner Collection of Photographs, 2634. Courtesy 
 Geological Department, Harvard University.) 
 
 FIG. 636. Glen Coe, a glaciated stream valley in the Scottish upland. 
 The glens of Scotland are splendid glacial troughs, of which Glen Coe is a typical 
 example. (Courtesy of D. W. Johnson.) 
 
The Erosion Cycle on Anticlines and Synclines 743 
 
 The New England peneplane was uplifted with a tilt, so 
 that there is a rise of the surface inland. Large portions of 
 the peneplane are still intact, forming broad uplands beneath 
 which the streams have intrenched their courses. The Scottish 
 Highlands region, on the other hand, represents a much more 
 thoroughly dissected peneplane, so that the upland has been 
 reduced to narrow ridges between the glens, which are the 
 product of a later cycle of erosion by streams and glaciers 
 (Fig. 636). 
 
 Antecedent Streams. Streams cutting across anticlinal or 
 complex mountain systems are not necessarily inherited from a 
 former cycle or devel- 
 oped upon a peneplane 
 surface. They may 
 represent streams 
 across the path of 
 which a series of folds 
 or fault blocks have 
 arisen at such a slow 
 rate that the down- 
 ward cutting of the 
 stream kept pace with 
 the rising of the folds 
 or other structures 
 which would otherwise 
 have diverted the 
 stream. Such streams 
 are called antecedent, 
 because they existed 
 in the region before the structure in question made its appearance. 
 The Columbia River is believed to have such an antecedent re- 
 lation to the Cascade Mountains which rise across its course and 
 are cut by it. The Meuse River of France is also believed to be 
 antecedent with reference to the Ardennes highland, through which 
 it cuts a deep, narrow gorge or canon. Although the uprising 
 of this highland of ancient strata across the river path was so 
 slow that the Meuse was able to keep pace with it by cutting 
 downward, and so maintain its course, it lost its tributaries which 
 were diverted by capture to other streams not so handicapped 
 (Figs. 637, 638). 
 
 FIG. 637. Map of the ancient Meuse when 
 the Toul, now diverted, formed a part of it. The 
 Moselle of that time included the Meurthe (a 
 tributary to the modern Moselle) and the Pom- 
 pey, a tributary of that time, but now with the 
 Toul forming a part of the modern Moselle (see 
 Fig. 638). (After Davis.) 
 
744 The Sculpturing of the Earth's Surface 
 
 Superimposed Streams. There is still another way in which 
 streams can be forced to cut across rock structures, hard and weak 
 alike, without being able to adjust themselves, by selective erosion, 
 to the weaker structures. This is effected where a country of 
 diverse structure is covered by a deposit of coastal plain strata 
 which present a new surface upon which rivers may develop irre- 
 spective of the character of the underlying rock. Having ac- 
 quired a certain course, determined by the slope of the coastal 
 
 plain strata, the river 
 will be forced to con- 
 tinue downward cut- 
 ting into the underly- 
 ing rocks, no matter 
 how diverse their struc- 
 ture. Eventually the 
 coastal plain strata 
 may be largely or 
 wholly removed by 
 eiosion, revealing the 
 underlying rock and the 
 topographic features 
 which marked its sur- 
 face before the coastal 
 plain strata were de- 
 posited upon it, and these may be entirely out of harmony with 
 the course of the river superimposed upon them. The lower course 
 of the Connecticut through the crystalline rocks of the New 
 England upland is believed to be due to superimposition of the 
 course acquired upon a former covering of coastal plain strata, 
 now wholly removed from that region by erosion. 
 
 Special cases of superimposed streams are occasionally met with. 
 One of the most interesting is that of the famous Lake District of 
 Northwestern England. This is a roughly oval area, formed of 
 complexly folded older Palaeozoic shales, sandstones, and igneous 
 rocks, with many high mountain peaks formed by resistant strata, 
 between which the river valleys expand in a series of beautiful 
 long and narrow lakes which have made the region famous. The 
 area is almost completely surrounded by belts of late Palaeozoic 
 and younger strata, including the Carboniferous limestone of the 
 English geologists (Mississippian age), the Coal Measures which 
 
 /W E, U 
 
 FIG. 638. Map of the Meuse and Moselle, 
 showing the formation of the modern Moselle 
 by capture of former tributaries to the Meuse. 
 (After Davis.) 
 
The Erosion Cycle on Anticlines and Synclines 745 
 
 overlie these, the next higher Permian sandstones, and finally, 
 on the west, the New Red sandstone beds (Triassic). Wherever 
 
 FIG. 639. Map of the Lake District, showing radial drainage. The dotted 
 area represents Carboniferous and younger rocks, gently dipping away from 
 the center of the dome. The plain area is formed by Silurian and Ordovician 
 strata much deformed and with a general N. E. and S. W. strike. (From Lake 
 and Rastall. Textbook of Geology.) 
 
 these rocks rest upon the older Palaeozoics, they are found to have 
 an unconformable relationship, these younger rocks being but 
 slightly disturbed, while the older ones are strongly so. More- 
 over, with negligible exceptions, these younger rocks dip away in 
 
746 The Sculpturing of the Earth's Surface 
 
 all directions from the center of the area which they surround, and 
 this indicates that the region was a dome once covered by these 
 younger strata which have since been eroded from the central 
 area. The radial drainage developed upon this dome has become 
 superimposed upon the underlying rocks of complex structure as 
 they were uncovered, and hence, despite this complex structure 
 and the general northeast and southwest strike of the folds of 
 the older strata, the drainage is a radial one, wholly out of har- 
 mony with the rock structure of the region, except locally where 
 adjustment has taken place. There can be no doubt that the 
 general direction of these streams is inherited from their courses on 
 the domed strata which formerly covered this region, and along the 
 margins of the area, where they still lie within these younger 
 strata, they have the normal arrangement of radial consequents 
 (Marr). The lakes in these river valleys owe their origin in part to 
 glacial erosion and obstruction by drift. 
 
 The map on the preceding page (Fig. 639), copied from Lake 
 and Rastall, shows the radial arrangement of the lakes of this 
 district and the streams which determined their position. 
 
CHAPTER XXIII 
 
 THE SCULPTURING OF THE EARTH'S SURFACE 
 
 (Continued) 
 
 THE EROSION CYCLE IN A FAULTED REGION 
 
 A REGION that has -been subjected to extensive faulting, presents 
 some interesting conditions which exercise a controlling influence 
 upon the topography produced by erosion. We can consider only 
 the simpler types of features produced by faulting, but they will 
 serve to point the general principles of which cognizance must 
 be taken in the interpretation of the land forms produced in a 
 faulted region by the agencies of erosion. 
 
 The Fault Block or Block Mountain 
 
 Characteristics of Block Mountains. In certain regions the 
 crust of the earth appears to have been broken* into a series of 
 parallel blocks, and by what appears to be a tilting movement of 
 these blocks one side has been raised and the other depressed, 
 resulting in the formation of parallel ridges 'and valleys of tri- 
 angular cross-section, each characterized by a long, gentle slope 
 on one side and a short abrupt slope on the other. This type of 
 block faulting, as it is called; may be illustrated by placing upon 
 the table a row of books or blocks of the same size and thickness, 
 and then tilting the whole series in one direction, when the upper 
 edges of the books or blocks will produce a series of ridges and val- 
 leys of the type described (Fig. 640 a, b), although it must be clearly 
 borne in mind that such regularity does not exist in nature. When 
 these tilted fault blocks are large enough, they constitute block 
 mountains. In a region of horizontal strata such block faulting 
 will produce a succession of hog-back-like ridges (Fig. 640 c d.) 
 but these differ from the hog-backs of erosion, the true uniclines 
 produced upon the side of an anticline, in the fact that each of the 
 tilted fault blocks shows the same succession of strata, whereas 
 
 747 
 
748 The Sculpturing of the Earth's Surface 
 
 in a series of hog-backs of erosion, the stratum capping each suc- 
 cessive ridge will be a different one, either higher or lower, accord- 
 ing to the order in which the successive hog-backs are examined. 
 The capping bed of the inner hog-back (i.e., that nearest the center 
 
 FIG. 640. Diagrams illustrating block faulting, a, b, ridges produced by 
 tilting of blocks ; c, region of horizontal strata intersected by vertical fissures ; 
 d, the same after tilting, note the repetition of the strata in the successive 
 blocks ; e, the same series with the fault faces of the blocks eroded ; /, a series 
 of normal uniclines showing a similar appearance, but each ridge is capped by 
 a different bed. 
 
 of the original anticline) will pass under that capping the next 
 outer one and the capping bed of this one again under that of the 
 one next outward from it. This is shown in diagram / in the 
 illustration (Fig. 640) which should be compared with the diagram 
 e of the same figure. A more complex and irregular type of block 
 faulting has already been illustrated in Fig. 557 a, p. 633. An actual 
 
The Erosion Cycle in a Faulted Region 749 
 
 example of complex block faulting on a small scale, from Nevada, 
 is illustrated in the following map and section (Figs. 641, 642). 
 
 Typical Examples. Typical examples of mountains formed 
 by the tilting of large blocks which themselves are broken into 
 minor blocks, are found in the Sandia and Magdalena mountains 
 
 FIG. 641. Plan of the principal faults in the Bullfrog district, Nevada. 
 (Emmons, U. S. G. S.) 
 
 of New Mexico. The displacement occurs in originally hori- 
 zontar strata underlain by crystalline rocks (Fig. 643, Johnson). 
 Block faulting in regions of more complex structure is seen in the 
 Basin Ranges of Utah and Nevada. On the eastern edge of the 
 Great Basin stands the block which forms the Wasatch Mountains. 
 
 Horizontal Scale 
 
 FIG. 642. Diagrammatic section illustrating fault block displacements in 
 the Bullfrog district, Nevada. (Emmons, U. S. G. S.) 
 
 Its surface is tilted so as to slope eastward and its fault surface 
 faces westward. On the opposite side of the Great Basin stands 
 the block which forms the Sierra Nevada Range. Its fault face 
 is on the east, and its long back slope drops off gently to the Valley 
 of California on the west. Between the two are many narrow ridges 
 from i o to 50 miles in length which represent modified fault blocks 
 of this type. The fault faces of some are turned east, of others west. 
 
750 The Sculpturing of the Earth's Surface 
 
 Complex block faulting has also occurred in a number of cases. 
 The general relationship is shown in the lower diagram on this 
 page, after Le Conte (Fig. 644), the faulting being interpreted as 
 the breaking down of a great arch. 
 
 FIG. 643. Block faulting, Sandia Mountains. The mass consists of crys- 
 talline rocks, shown by dots, covered by limestones and other sediments, 
 which originally were horizontal. A single great fault forms the left-hand face 
 of the mountain, which consists of a tilted block, broken into minor blocks 
 by lesser faults. (After D. W. Johnson.) 
 
 The block mountains of the Great Basin ranges are not generally 
 formed from horizontal strata, but are complex. The Wasatch 
 block contains many crystalline and metamorphic rocks; the 
 Sierra Nevada block is in large part igneous. In the ranges within 
 the Basin, the strata are part of an old folded mountain system 
 similar to that of the Appalachians and, like that, peneplaned. 
 Instead of simple elevation at the beginning of the second cycle, 
 the Great Basin region then suffered block faulting. The direction 
 
 of the faults does not conform 
 to that of the old folds, i.e., to 
 the strike of the strata, but is 
 often obliquely across them. 
 This relationship is shown in 
 the following diagram (Fig. 
 645). In some of the Basin 
 
 FIG. 644. Generalized cross-section 
 of the Great Basin from the Sierra 
 Nevada to the Wasatch, showing the 
 origin of the block mountains of the 
 Basin Ranges by collapse of a former 
 arched surface. (After Le Conte.) 
 
 Ranges slight faulting or up- 
 lift of the fault block, has 
 occurred in comparatively 
 
 recent times, and some are probably still undergoing move- 
 ment (Fig. 646). 
 
 Erosion -of Block Mountains. The streams which come into 
 existence upon a faulted block mountain are of two types, one 
 flowing down the back slope of the tilted block, that is, the original 
 
The Erosion Cycle in a Faulted Region 751 
 
 FIG. 645. Diagram illustrating the development of the block mountain 
 topography of the Basin Ranges. The region is one of folded Palaeozoic strata, 
 their folds forming the ancient Palaeocordilleran mountains. Peneplanation 
 ensued, followed by block-faulting, after which erosion carved the present 
 mountain topography, while the depressions between the blocks were partly 
 filled by. the waste from the mountains. (After Davis.) 
 
 FIG. 646. Recent fault crossing moraine. Wasatch Mountains, Utah. 
 (F. J. Pack, photo.) 
 
752 The Sculpturing of the Earth's Surface 
 \ 
 
 surface of that part of the land, whether it was of horizontal strata 
 or a peneplaned surface ; the other flowing down the much steeper 
 fault scarp. The first type may be compared to the consequent 
 streams of an anticline, or if the tilting is slight, to the normal 
 consequent of the coastal plain. The second type, that flowing 
 down the fault scarp, may be compared to the obsequent stream 
 of the cuesta, or the similar obsequent stream on the inner side 
 of the hog-back. In- a measure, of course, both stream types de- 
 veloped on the fault block are consequent, one being the dip-slope 
 consequent, the other the fault-scarp consequent, and they are 
 so classed by many physiographers. 
 
 Because of the greater steepness of the fault scarp of the tilted 
 block, the streams upon it will have greater erosive power than 
 
 FIG. 647. Diagram showing mountains and valleys due to block faulting 
 in the background, and the dissection of the blocks and filling of the valleys, 
 in the foreground. A stage of maturity has been reached in the development 
 of these mountains. (After Davis. Erklarende Beschreibung der Landformen.) 
 
 those on the back slope. In consequence, the divide between 
 the two types of streams flowing in opposite directions, which in 
 the beginning was at the crest of the tilted block, is pushed away 
 from the edge of the fault scarp, and the drainage basin of the 
 fault-scarp stream will encroach more and more upon that of the 
 outer-slope stream. This is true of course, primarily, where the 
 two stream systems receive an approximately equal supply of 
 water. Equilibrium will be established when the gradients of the 
 two streams are essentially alike, though of course differences in rock 
 character and structure on opposite sides of the block, as well as 
 differences in the amount of water supplied and other factors, enter 
 to complicate the process. The material worn from the blocks 
 may be deposited in the valleys, which thus take on a level surface 
 (Fig. 647). 
 
The Erosion Cycle in a Faulted Region 753 
 
 The Wasatch Mountains as an Example. The Wasatch Moun- 
 tains, which, as already stated, form the eastern fault block that 
 bounds the Great Basin, have a gentle slope toward the east for a 
 distance of 15 to 20 miles. The western slope, the fault scarp, 
 is an extremely abrupt one, elevations of 10,000 feet being attained 
 within one or two miles of the western base, where the mountain 
 rises abruptly from the broad flat plains of the Utah Basin. These 
 plains are formed by an unknown thickness of alluvial deposits 
 which cover the valley floor (Fig. 648). In spite of this difference 
 of slope, the main crest of the mountains produced by erosion lies 
 
 FIG. 648. Diagram illustrating several stages in the development of the 
 Wasatch Mountains from the original fault block (A) through rugged mountain 
 topography (B) ; to subdued mature topography (C) ; and final obliteration 
 (in the future) and formation of a peneplane (Z>). (After Davis. Erklarende 
 Beschreibimg der Landformen). 
 
 near the eastern border, and the westward flowing streams are 
 generally from two to three times the length of the eastward flow- 
 ing ones. Thus the divide has been pushed by erosion two thirds 
 or more of the way across the original block. The various stages 
 in dissection of the fault block are illustrated in the diagram (Fig. 
 648). 
 
 On the crystalline and metamorphic rocks of this block, peaks from 11,000 
 to 12,000 feet in height have been developed, and they have rugged pinnacle- 
 like forms. On the slightly tilted sedimentary rocks, peaks of pyramidal out- 
 line with cliffs and slopes are developed. These erosion forms are not wholly 
 due to river wear and weathering, for ice erosion has also played an important 
 part in the higher regions. Below the level of glaciation, however, that is, in 
 summits less than 9000 feet in height, the outlines are rounded and softened by 
 heavy slopes of land waste. 
 
754 The Sculpturing of the Earth's Surface 
 
 The Sinai Type of Fault-Bounded Cuesta Block 
 
 A striking example of a block isolated by graben-faulting, but 
 having otherwise the character of a normal cuesta, is seen in the 
 Sinai Peninsula. This is of triangular outline, the apex being 
 on the south, where two rift valleys converge, that of the Gulf of 
 Suez on the west and that of the Gulf of Akaba, continued in the 
 Vale of Araba and the Dead Sea, on the east. The apex of the 
 peninsula is formed of a series of much dissected peaks of crystalline 
 rocks (granite, porphyry, diorite, gneiss, etc.). Some of the peaks 
 rise to considerable heights, notably the Jebel Musa group, which 
 culminates in Mt. Catherine (8540 ft.). This, or the more iso- 
 lated Mt. Serbal (6750 ft.), is to be identified as Mt. Sinai or the 
 " Mountain of the Law." North of this oldland mountain group 
 lies a series of great valleys which represent the inner lowland, 
 and from which arises on the north the great cuesta front of Jebel 
 El-Tih, which attains a height of 3000 ft., and is much dissected 
 into fantastic forms. (See map, Fig. 649.) This great cliff is formed 
 of the cut edges of nearly horizontal strata, comprising chiefly 
 the Nubian Sandstone, Cretaceous limestones, and, farther north, 
 Tertiary Nummulitic limestones, the strata dipping at a gentle 
 angle northward to the Mediterranean. From the summit of 
 this cuesta-inface the surface descends northward in conformity 
 with the dip of the strata for 250 kilometers (about 160 miles) 
 passing beneath the level of the Mediterranean like a normal coastal 
 plain. The surface thus has an average slope of less than 20 ft. 
 per mile, and though under the prevailing climatic conditions it is 
 partly a desert region (the desert El-Tih), it is dissected by wadis 
 which form a consequent drainage system descending from near 
 the south edge of the cuesta to the sea. On the east the edge of 
 the cliff is dissected by numerous insequent valleys tributary to 
 the Araba Graben. (See map, Fig. 649.) 
 
 The Rift Valley or Graben 
 
 The name Graben (ditch) has been given in Germany to valleys 
 formed by the down-faulting of a long, narrow block of the earth's 
 crust, and the typical example of such a Graben is the Rhine 
 trough north of Mayence (Mainz) already described. The region 
 was formerly a peneplane eroded on crystalline rocks, and upon 
 this were deposited red sandstones and shales (Triassic with some 
 
The Erosion Cycle in a Faulted Region 755 
 
 Permian), followed by shales (Lias) and limestone (Jurassic), 
 which extended uninterruptedly across this region, uniting the 
 remnants of these beds now seen on the one side in the cuestas of 
 eastern France and on the other in the Swabian Alp, etc., of Ger- 
 many. Later this region arose as an anticline or arch, the south- 
 ward continuation of which was formed by a group of anticlines 
 and synclines which constitute the Jura Mountains. (See Fig. 
 
 FIG. 649. Topographic map of the Sinai Peninsula and the Nile region. 
 
 (From Ratzel.) 
 
 552, p. 631.) At that time, or somewhat later, the center of the 
 arch on the north collapsed, sinking down as a long, narrow block, 
 bounded by a series of parallel fault planes on both sides (Fig. 
 650 A). Peneplanation of the region followed, and in this pro- 
 cess the crystalline rock was uncovered in several places on each 
 side of the fault block. This peneplanation appears to have 
 extended widely over this entire region, being the extension of the 
 peneplane which truncated the strata of the Paris Basin. We 
 know that the faulting took place before the peneplanation, be- 
 
756 The Sculpturing of the Earth's Surface 
 
 cause upon the floor of the Rhine Graben are found the strata 
 (Triassic and Jurassic) which this peneplanation removed on both 
 sides, cutting, as we have seen, in places, even to the crystallines. 
 
 Conditions at the time of the completion of the peneplane were 
 somewhat like those shown in the diagram Fig. 650 B. After the 
 
 FIG. 650. Diagrammatic sections to represent the development of the 
 Rhine Graben and its present structural and topographic features. A (upper), 
 conditions after the first faulting, which formed the original Graben which is 
 occupied by the sea in which the Tertiary strata (7) were deposited ; B (middle), 
 the same region after peneplanation; C (lower), the same region after renewed 
 faulting and dissection; V, Vosges Mountains; R.G., Rhine Graben; B.F., 
 Black Forest (Schwarzwald) ; N, Neckar valley ; o, ancient crystallines ; c, 
 folded Carbonic beds; i. Buntsandstein (Triassic, including some Permian); 
 2. Muschelkalk; 3. Keuper; 4. Lias; 5. Dogger shales and sandstones; 
 6. Malm, forming the Swabian Alp ; 7. Tertiary (chiefly Oligocene) ; 8. Quater- 
 nary and modern alluvium. 
 
 peneplanation, renewed faulting set in, the movements continuing 
 in some places even to the present time. The northward continu- 
 ation of the Graben was blocked by the outbreak of a series of vol- 
 canoes, especially between Darmstadt and Kassel, and in conse- 
 quence the Rhine, which had begun to occupy this trough, had 
 to cut its path across the slate mountains and so form its famed 
 gorge (Fig. 601, p. 703). In the south, the uplifted sides of the 
 
The Erosion Cycle in a Faulted Region 757 
 
 Graben carried the crystallines to a high altitude, and these consti- 
 tute to-day the Vosges Mountains on the west and the Black Forest 
 on the east. Because of the hard character of these rocks, they 
 have resisted erosion and now constitute prominent barriers on 
 both sides of the Rhine Graben. A steep fault scarp, somewhat 
 modified by later erosion, faces the Rhine Valley on each side, 
 while from the top of each range a gentle slope extends backward 
 to the foot of the first cuesta formed by the erosion, in the second 
 cycle, of the softer beds beneath the harder cliff-making limestones 
 (Fig. 650 C). 
 
 Recent rift valleys of this type exist, as we have seen (Fig. 
 554, p. 632), in east Africa, where volcanic eruptions likewise 
 have blocked the continuity of the valley in many places, with the 
 formation of lakes, of which Tanganyika is one (Fig. 555, p. 633). 
 The Palestine rift valley, occupied partly by the Dead Sea, the 
 River Jordan, and the Sea of Galilee, and continued southward 
 in the Gulf of Akabah, is also a very recent one. In this, erosion 
 is apparently still in the first cycle, having modified only the sides 
 of the trough to a certain extent (Fig. 649, p. 755). Among the 
 minor erosion phenomena along the Dead Sea are the salt pillars 
 carved from ancient salt beds exposed by the faulting, and one or 
 another of which has been identified since time immemorial as the 
 statue of Lot's wife. 
 
 Checker-board Fault Structure 
 
 Where a country is traversed by many parallel master joints 
 arranged in two intersecting series, dislocation of the blocks thus 
 produced, results in the elevation of some and the depression of 
 others, the amounts being variable in the different blocks. Such 
 a checker-board dislocation of a country produces a series of more 
 or less rectangular elevated and depressed fields, and on these 
 erosion will produce a most complex series of river systems. If 
 such a region is finally peneplaned, a very complicated rock surface 
 will result, some squares consisting of one kind, others of different 
 material, and all abruptly bounded by fault lines. In the second 
 cycle of erosion, the softer beds will be attacked and a complicated 
 topography is produced. 
 
 Southern Sweden is one of the best known examples of this 
 structure. Here one passes abruptly from rounded hills of crystal- 
 
The Sculpturing of the Earth's Surface 
 
 line rock to buttes or table mountains cut on horizontal strata, 
 and from these to regions covered with more or less inclined strata, 
 and on again to hills of crystalline rock, there being absolutely no 
 regularity of structure. The geologist working in such a field 
 passes abruptly from rocks of Archaean age to those of Mesozoic. 
 These may in the next block be replaced by Cambrian and Ordovi- 
 cian strata, beyond which a block exposes Tertiary beds at the 
 
 FIG. 651. Diagram showing block faulting of the checkerboard type ; pene- 
 planation of the region, and the development of a complicated topography by 
 erosion in the second cycle. Illustration of the geology of central Sweden. 
 (After Davis.) 
 
 surface, and then follows perhaps one with Silurian strata. Along 
 the fault lines, valleys are often carved, and some of these may be 
 occupied by lakes. Constant diversity appears to be the keynote, 
 not only of the rocks and rock structures, but of the topography as 
 well (Fig. 651). 
 
 Fault-Line Valleys 
 
 When a country traversed by faults of some extent has become 
 peneplaned during the first cycle of erosion, a valley may be cut 
 by streams along the fault line during the succeeding cycle. For 
 not only is the fault line one of weakness in the earth's crust, but 
 along it are apt to be found many remnants of weaker strata 
 which, being faulted below the level of peneplanation, escaped 
 removal during the first cycle but are readily attacked in the 
 second cycle. As a result, valleys will be formed which have no 
 marked adjustment to the kinds of rocks, and which, moreover, 
 may extend in a continuous manner for great distances. Such 
 
The Erosion Cycle in a Faulted Region 759 
 
 a valley is the Great Glen which traverses the Scottish Highland 
 region from Loch Linnhe on the southwest to the Moray Firth on 
 the northeast, and which, on account of the succession of beautiful 
 lochs situated along it, 
 makes this not only the 
 most picturesque but 
 also the easiest line of 
 travel across Scotland 
 from Oban to Inverness. 
 Such fault-line "valleys 
 
 must be distinguished 
 
 FIG. 652. A fault-line valley. (From Pnn- 
 from fault valleys or dpks of Stratigraphy.) 
 
 troughs, the latter rep- 
 resenting purely a structural feature, the valley always being 
 in the first cycle. The fault-line valley, on the other hand, is an 
 erosion feature, often in a later cycle after peneplanation has de- 
 stroyed the original fault topography (Fig. 652). 
 
 Renewal of Fault-Scarp Topography 
 
 If in a faulted region the original fault topography has been 
 destroyed by peneplanation, erosion in the second cycle may renew 
 the fault-scarp topography by removing the softer rock on one side 
 of the fault and leaving the harder rock on the other side in relief. 
 The scarp or cliff thus produced along the fault-line is called a 
 fault-line scarp, in distinction from the fault scarp which is the orig- 
 inal cliff due to dislocation. According to the nature of the strata 
 uncovered on either side of the fault-line by peneplanation, the 
 fault-line scarp may face in the same direction as the original fault 
 scarp, that is, face the down- throw block, in which case it is called 
 resequent, since it retakes a consequent position [re (con) sequent]. 
 On the other hand, it may face in the opposite direction, i.e., face 
 the original upthrow block, if the strata exposed by peneplanation 
 upon the surface of this block, prove to be the softer. In such a 
 case, the fault-line scarp is said to be obsequent. Neither fault 
 scarp nor fault-line scarp will long maintain its position along 
 the fault-line, but erosion will progressively push them back until 
 the cliff may be many miles from the fault-line, though essentially 
 parallel to it. Such cliffs are characteristic topographical features 
 of the plateau country west of the Rocky Mountains, though in most 
 cases they have not weathered back very far from the fault plane. 
 
760 The Sculpturing of the Earth's Surface 
 
 SOME ILLUSTRATIONS OF COMPLICATED RIVER EROSION 
 
 We may here introduce a few typical examples of complexly 
 adjusted rivers and the topographical features which accompany 
 them, in order that the student may appreciate the interrelations 
 of the various forces operative in producing a complex series of 
 land forms. We will select the Niagara, the Genesee, and the 
 Colorado rivers. 
 
 Niagara River and Falls 
 
 The Niagara River differs from the normal form of river, which 
 we have been considering, in so many respects that it may be taken 
 as the type of a special class, that of the spillway of one water 
 body into another. 
 
 FIG. 653. Block diagram illustrating the formation of three cuestas and 
 lowlands by normal drainage on the peneplaned strata which surround the 
 old Ontario dome on the south and west. Three principal streams are indi- 
 cated, of which the middle one is the master stream (Dundas river). (Drawn 
 by Mary Welleck.) See also maps, Figs. 617, 618, p. 727. 
 
 Pre-Niagara Topography. Before Niagara River came into 
 existence, the region was the site of normal dissection of part of 
 a low dome that had been peneplaned, as has previously been out- 
 lined (p. 725). As a result, three lowlands and cuestas had come 
 into existence, of which the northernmost was the most profound 
 and the middle one the shallowest (Fig. 653). These three cuestas 
 are formed by the three resistant formations of the region, and are, 
 respectively, the Niagara cuesta (Fig. 654, a), the Onondaga cuesta 
 (b), and the Portage-Chemung cuesta or front of the Allegheny 
 
Illustrations of Complicated River Erosion 761 
 
 plateau (c), while the lowlands in front of these are the Ontario 
 lowland (A), the Salina lowland (5), and the Erie lowland (C). 
 The peneplane is indicated by the beveled surfaces of the Niagara 
 and Onondaga limestones at the edges of the respective cuestas, and 
 the summit elevation of the Alleghany front south of Buffalo. 
 When . these points are connected by a line which restores the 
 peneplane surface, it is seen that this old surface now rises to the 
 south, and this indicates that the peneplane has been tilted to 
 the north with possibly some warping which carried the southern 
 region to greater altitudes. Such tilting occurred, however, only 
 after the cuesta and lowland topography had been etched out of 
 the hard and soft strata, respectively,- by a revival of the conse- 
 quent and subsequent drainage from the old Ontario dome toward 
 the southwest (p. 726). Indeed, this tilting appears to have been 
 due to the depression of the land on the north during, and prob- 
 
 FIG. 654. North-south section across western New York, showing the 
 three cuestas and lowlands and the tilting of the peneplane. a, Niagara 
 cuesta; b, Onondaga cuesta; c, Portage-Chemung cuesta; A, Ontario low- 
 land; B, Salina lowland; C, Erie lowland; LO, Lake Ontario; NF, Niagara 
 Falls; LE, Lake Erie. 
 
 ably because of, the accumulation of the continental ice sheet of 
 Pleistocene time. As a further result of this northward depression, 
 the floor on the Ontario lowland was carried several hundred feet 
 below the present sea-level, from its position some distance above 
 that at the time of its formation by river erosion (Fig. 654). 
 
 As these various lowlands extend in a direction at right angles 
 to that of the movement of the great ice sheet in Pleistocene time, 
 they suffered no appreciable deepening, though the tops of the 
 cuestas may, in some cases, have been planed down to a slight 
 extent. 
 
 In addition to the depression of the country on the north, the 
 valley of the master consequent stream had been rilled by glacial 
 drift, and the same thing happened to the shallow Salina lowland. 
 Thus, with the outlet of the inner lowland choked by drift, and 
 the actual slope of the country reversed by depression, the Ontario 
 lowland became a circumscribed basin and was filled with water 
 
762 The Sculpturing of the Earth's Surface 
 
 up to the level of the lowest point in the rim, which is at present 
 at the Thousand Islands, a region that was, in the days of former 
 great elevation, the head of the St. Lawrence River and the divide 
 between it and a westward flowing stream which joined the subse- 
 
 FIG. 655. Map of Lake Iroquois, with the modern outlines of Lake Ontario, 
 etc., shown in dotted lines. (After Gilbert.) 
 
 quent by which the Ontario lowland was carved (see maps, Figs. 
 617 and 618, p. 727). Before the present St. Lawrence outlet came 
 into action, however, the waters of the lake in the Ontario lowland 
 overflowed along the line of the old Ontario River and across the 
 divide at Little Falls into the Mohawk and Hudson. This was 
 
 L CM, 
 
 FIG. 656. The edge of the Niagara escarpment. NS, showing the posi- 
 tion of the ancient Iroquois Beach (IB) and the relation of the Iroquois level 
 to that of Lake Ontario. 
 
 necessitated by the fact that a part of the great ice sheet still 
 lingered in the region of the Thousand Islands and so blocked 
 that outlet (Fig. 655). As a result, the level of the lake, called 
 Lake Iroquois, stood higher than the present Ontario level and 
 extended farther over the flat terrace which lies between the Niag- 
 ara escarpment and the submerged part of the cuesta. The 
 
Illustrations of Complicated River Erosion 763 
 
 ancient shore-line is distinctly marked by one or more abandoned 
 beaches, which can be traced along the southern border of Lake 
 Ontario but distant from it some miles (Fig. 656). The principal 
 one is followed by the so- 
 called Ridge Road. 
 
 Because of the northward 
 tilting of the land and the 
 choking of the old outlets, 
 the Erie lowland also was 
 transformed into a lake 
 basin, its level being likewise 
 determined by the lowest 
 place of outlet in its rim, 
 which happened to be across 
 the Onondaga cuesta where 
 Buffalo now stands. Spilling 
 over at this point, the waters 
 took their course across the 
 country in a northward di- 
 rection, this being the only 
 avenue of escape open to 
 them, until they reached the 
 edge of the Niagara escarp- 
 ment where Lewiston stands 
 to-day, and there fell over it 
 as the newborn Niagara Falls. 
 That no fall or only a slight 
 one came into existence over 
 the Onondaga cuesta was due 
 to the filling of the lowland 
 in front of it by glacial drift, 
 and also because at the point 
 of overflow there appears to 
 have been an old notch cut in 
 the escarpment by some ob- 
 sequent stream during the 
 period of normal valley 
 erosion. 
 
 Peculiar Course of Niagara River. If we examine the course 
 of Niagara River, we find certain peculiarities, especially repeated 
 
 FIG. 657. Map of the Niagara Gorge, 
 showing variable course and physical 
 features. (After Gilbert.) Old river 
 banks are shown by dotted lines; shell 
 localities, by crosses. 
 
764 The Sculpturing of the Earth's Surface 
 
 changes in direction, unlike what we might expect in a normal 
 spill-over from one water body to another across a comparatively 
 level plain (Fig. 657). From its head at Buffalo, northward, the 
 river is a broad placid stream dividing into two arms which reunite 
 and enclose a large flat island called Grand Island. Beyond this 
 it turns almost due westward and soon becomes a turbulent, though 
 still very broad, stream in which no boat can make headway. 
 Then it plunges over a succession of low limestone ledges as a 
 magnificent series of rapids until it has descended about 50 feet 
 vertically, when it reaches the present falls, of which there are two, 
 the American, parallel to the line of rapids, and the Horseshoe 
 
 FIG. 658. Niagara Falls. 
 
 Falls, opening in a curve almost due north. Between the two lies 
 Goat Island, a drift-covered rock mass which rises nearly to the 
 level of the river above the upper rapids (Fig. 658). 
 
 From the foot of the Horseshoe Falls the gorge extends in a 
 direction somewhat east of north for about three miles, and it is 
 virtually over the side of this gorge that the American cataract 
 falls. The width of this part of the chasm varies from 1250 to 
 1700 feet at the top, its banks being for the most part nearly verti- 
 cal. The depth to water-level is nearly 200 feet, and the water 
 itself is from 150 to 200 feet deep. 
 
 Just before it passes under the railroad bridge at Clifton, the 
 gorge suddenly turns to the northwest, making almost a right 
 angle. It also contracts to a width of about 700 or 750 feet at the 
 top and 550 feet at the water-line, and the water becomes only 
 
Illustrations of Complicated River Erosion 765 
 
 about 35 feet deep. This is the beginning of the Whirlpool Rapids, 
 the waters here tumbling over each other with indescribable fury 
 and descending 50 feet in the space of less than a mile (Fig. 659). 
 This portion of the gorge ends at the Whirlpool, a deep circular 
 basin a thousand feet or more in diameter which has a depth of water 
 ranging from 150 to 200 
 feet (Fig. 660). At the 
 great swollen elbow of 
 the Whirlpool, the gorge 
 makes another right- 
 angled turn, extending 
 northeast for nearly two 
 miles. This part is again 
 a wide gorge, the width at 
 the top varying from 1300 
 to 1700 feet, though it is 
 less at the water-line. 
 The depth of water is, 
 however, not great, being 
 at the Whirlpool outlet 
 50 feet and ranging from 
 35 to 70 feet in other 
 parts. In this portion of FIG. 659. The Whirlpool Rapids gorge 
 
 the gorge on the Canadian * Nia s ara - (^m Grabau, Geology of 
 
 Niagara Falls, N. Y. State Educational 
 side is an interesting table Department.) 
 
 rock, known as Foster's 
 
 Flats, to which reference will be made again. At the end of this 
 part of its course, the river turns due north, retaining about the 
 same width to the edge of the escarpment at Lewiston, and thence 
 continuing in the same direction as a broad navigable stream with 
 relatively low banks for seven miles more to Lake Ontario. 
 
 The three bends in the gorge, together with the changing width 
 and depth (Fig. 66i)> are its most striking feature, while the sudden 
 bend in the river at the Horseshoe Falls is another feature of sig- 
 nificance. Tracing the lower part of the gorge, which extends due 
 north, southward to the bend, we find it aligned with a shallow 
 trench in the upland extending for a mile or more due southward 
 and occupied by a small stream, the Bloody Run, famous as the 
 scene of an Indian massacre during the wars of 1763, and which 
 forms the chasm of the Devil's Hole where it enters the present 
 
766 The Sculpturing of the Earth's Surface 
 
 gorge. The valley of this stream is readily recognized as a pre- 
 glacial one, and it appears that it is the upper part of the valley 
 of an obsequent stream which flowed over the edge of the escarp- 
 
 FIG. 660. The Whirlpool in Niagara gorge. (From Grabau, Geology of 
 Niagara Falls. Courtesy N. Y. State Educational Department.) 
 
 ment during the earlier period of normal erosion and long before 
 Niagara came into existence (Fig. 662). Thus the course of the 
 lower gorge and that of the river beyond to Lake Ontario appears 
 to have been predetermined by the cutting of a shallow obsequent 
 
 FIG. 66 1. Longitudinal section of Niagara gorge from the Falls (F) to 
 Queenston Heights () showing the west bank and the depth of water (in 
 black) at the several points, W, Whirlpool ; R, railroad bridges. (After Gilbert.) 
 
 valley in the surface of the cuesta, though it probably was not cut 
 back very far from the edge of the escarpment at Lewiston. 
 
 The gorge of the Whirlpool Rapids south of the Whirlpool 
 presents another such case. As noted, this extends almost due 
 
Illustrations of Complicated River Erosion 767 
 
 northwest to the Whirlpool. An examination of the northwestern 
 border of the Whirlpool discloses the fact that it is formed by glacial 
 drift instead of rock, and a series of borings has shown that an old 
 drift-filled, gorge extends from the Whirlpool northwestward to 
 the edge of the Niagara escarpment at St. Davids, gradually widen- 
 ing in this direction (Fig. 662). Beyond St. Davids it has not 
 been traced, but without doubt it extends to Lake Ontario. This 
 ancient St. Davids Gorge appears to represent another obsequent 
 stream which had cut back from the edge of the cuesta, probably, 
 to the head of the Whirlpool Rapids gorge at the Clifton railroad 
 
 FIG. 662. Bird's-eye view of Niagara gorge, showing the course of the river, 
 the falls, the railroad bridges, Whirlpool, location of Foster's Flats, escarp- 
 ments at Queenston, pre-glacial valley of Bloody Run, and flaring mouth of 
 pre-glacial St. Davids Gorge. (Modified after Gilbert.) 
 
 bridge. At the inlet of the Whirlpool, where the water is less than 
 50 feet deep, the gorge is crossed by a heavy bed of white sandstone 
 called the Whirlpool sandstone, and just beyond this the depth 
 of the water becomes 150 to 200 feet. Evidently there was here a 
 waterfall of that height in the ancient obsequent river determined 
 by the presence of this bed of sandstone. 
 
 These facts adequately account for the direction of the gorge 
 of the Whirlpool Rapids and for its narrowness and shallowness 
 as compared with other parts. Evidently it was largely drift- 
 filled to its head when the overflow from Lake Erie was directed 
 into it, and followed it for some distance before the drift-filling 
 became so high that the waters were forced to spill over the side of 
 
768 The Sculpturing of the Earth's Surface 
 
 the shallow channel and find their way across the country to another 
 ancient and partly drift-filled obsequent valley. The following 
 section (AB, Fig. 663) gives the approximate profile of this old 
 St. Davids valley. When the falls had cut back nearly to the site 
 of the Whirlpool (section CD, Fig. 663) only a narrow rock wall 
 upheld the drift-filling of the Whirlpool and Whirlpool Rapids 
 Gorge, and when this barrier broke the drift was quickly cleared 
 out and the falls became suddenly transferred to near the head of 
 the Whirlpool Rapids Gorge. 
 
 FIG. 663. Sections of the Whirlpool at Niagara before the former drift- 
 filling was cleared out by the river, i, Queenston shales; 2, Whirlpool sand- 
 stone; 3, Medina shales and sandstones; 4, Thorold quartzite; 5, Clinton 
 shale and limestone ; 6, Rochester shale ; 7, Lockport dolomite ; x, the narrow 
 rock barrier northeast of the St. Davids Gorge, the destruction of which caused 
 the sudden clearing of drift from the Whirlpool and the transference of the falls 
 for some distance upstream. 
 
 The upper portion of the gorge, for three miles down stream 
 from the falls, was also directed by a shallow valley in the upland 
 across which the Erie waters spilled. This appears to have been 
 the valley of an old consequent stream originally flowing in a south- 
 westerly direction, but with the present slope reversed owing to tilt- 
 ing and to partial drift-filling. It was cut into the upland to a 
 depth not much over 50 feet, but this was sufficient to guide the 
 waters as they spilled into it. The western side of this valley is 
 seen in the cliff opposite the Horseshoe Falls on the Canadian side, 
 
Illustrations of Complicated River Erosion 769 
 
 and the eastern in the rapids above the falls, which here extend 
 continuously across the river. 
 
 We thus see that the course of the Niagara River is due to the 
 spilling over of the waters from one ancient channel to another, 
 and that the falls cut back along these channels, sometimes as a 
 single fall, at others probably as a succession of falls, with a sudden 
 transference upstream for about a mile when the barrier which 
 held in the drift filling of the upper end of the St. Davids Gorge 
 (Whirlpool Rapids Gorge) was broken and the drift cleared from 
 this part of the old channel. 
 
 FIG. 664. Map of glacial Lake Algonquin and the discharge by the Trent 
 river. (After Taylor.) 
 
 1 There are some other larger geographic factors which must 
 be taken into account in the history of Niagara. Thus it has 
 been clearly shown that, for a time at least, the waters of the 
 upper Great Lakes discharged, not as they do now into Lake Erie 
 by way of the Detroit River and thence over Niagara Falls, but by 
 a more direct route across Ontario along a valley now 'occupied by 
 the Trent River (Fig. 664). This meant, of course, that the water 
 flowing over Niagara was only the spill-over from the much smaller 
 Lake Erie of that time, and therefore the Falls were not able to 
 cut so deep and broad a gorge as they do to-day, nor retreat at so 
 rapid a rate. It has been held that during the existence of these 
 
770 The Sculpturing of the Earth's Surface 
 
 conditions the shallower part of the gorge north of the Whirlpool 
 was cut. Still later, when the ice had melted away from the St. 
 Lawrence outlet, which was then even lower than at present, owing 
 to the greater depression of the land, and was, moreover, because 
 of this depression, probably flooded by water entering from the 
 sea (Fig. 665), the upper Great Lakes discharged directly to the 
 St. Lawrence or the sea which occupied its valley, along a course 
 now partly occupied by the Ottawa River. This still left the 
 Niagara only the drainage from Lake Erie, and it is held by some 
 that during this period the Whirlpool Rapids Gorge was cut, or 
 
 FIG. 665. Map of the Nipissing Great Lakes, and the marine submer- 
 gence of the Champlain and Ontario valleys. The outlet of the Great Lakes 
 is by the Ottawa River. (After Taylor.) 
 
 at least deepened, and that because of the small quantity of water 
 carried over the falls, this deepening and widening was much less 
 than it would have been with the present volume of water. 
 
 After this period, the land in the north began to recover from its 
 depression, and as we have seen (p. 695) may still be slowly rising, 
 as indicated by the abandoned beaches in the north and the en- 
 croachment of the waters of the lakes on the south shores. When 
 the tilting back toward the original position had progressed so far 
 that the upper Great Lakes began to spill over by way of the 
 Detroit River into Lake Erie, Niagara was forced to carry out the 
 combined drainage of these lakes and assumed its present volume 
 
Illustrations of Complicated River Erosion 771 
 
 of 22,440,000 cubic feet or more than 7,000,000 tons per minute. 
 Since this time it has cut its broad deep gorge from the railroad 
 bridge to the present Falls. This cutting, judging by the measured 
 rate of retreat during the last three quarters of a century, must 
 have taken about 3000 years. Thus the beginning of the great 
 cataract about three miles north of its present site, dates back to 
 about the eleventh century B.C., or to about " 300 years before the 
 time of Romulus, or to the reign of King David at Jerusalem " 
 (Hitchcock). How long it 
 took to cut the other parts 
 of the gorge is more difficult 
 to determine, because of the 
 variable factors above re- 
 ferred to. It is probably safe 
 to say that it was at least 
 50,000 years ago when the 
 falls began near Lewiston, 
 and it may have been several 
 hundred thousand years ago. 
 Mode of Cutting of Ni- 
 agara Falls. Niagara River 
 carries no sediment. Where 
 it leaves Lake Erie the water FIG. 666. Section of the Horseshoe 
 is so pure that the intake Falls, Niagara, to show the depth of water 
 
 of the water supply of the ^ elow f * e faU ! ^ d ,. the arra *gement f 
 
 f the strata. 7 b, Medina sandstones and 
 
 City 01 Buffalo is located at shales with Whirlpool sandstone at base 
 
 that point. What sediment (resting on Queenston shales) and Thorold 
 
 is brought in by lateral ? uart f ite ab 8 v " ; * a > C^ton stoile and 
 
 limestones; 8b, Rochester shale; &c, 
 
 streams or picked off the bank Lockport dolomite. (After Gilbert.) 
 is dropped in the quieter 
 
 water long before the falls are readied, over which the water 
 passes as a pure stream. The erosion accomplished by the 
 cataract is entirely due to undermining by spray and to the force 
 of the falling water. The greatest amount of water passes over 
 the Horseshoe Falls, and the section here is shown in the above 
 diagram (Fig. 666). It will be observed that a heavy bed of lime- 
 stone (about 80 feet thick) (8c) caps the cliff and is underlain by 
 softer shales with some limestone and sandstone layers. It is these 
 softer beds which are slowly removed by the water and spray 
 driven against their edges and by the quarrying activity of the 
 
 
772 The Sculpturing of the Earth's Surface 
 
 frost in winter. Thus the limestone ledge comes to overhang more 
 and more, and eventually it breaks down from its own weight and 
 that of the water above it. Thus the gorge is lengthened by 
 periodic rock falls. The fallen blocks are broken and ground up 
 by the enormous force of the descending water, and in this process 
 they dig into the soft red shales and sandstones of the river bot- 
 tom (yft), thus producing a depth of water approaching 200 feet. 
 In other words, the force of the plunging water is so great that the 
 gorge-cutting goes on for a depth of 200 feet below the level of 
 the river in front of the falls, producing a " plunge basin " of un- 
 usual magnitude and depth. 
 
 FIG. 667. Section of the American Falls at Niagara ; the rapids above the 
 Falls are formed by the thin-bedded Guelph dolomites (7) ; the edge of the 
 falls is formed by the Lockport dolomite (6) ; the Cave of the Winds is cut by 
 the spray from the Rochester shale (5) ; and its floor is formed by the Clinton 
 limestone (4) ; beneath this lie the Clinton shale (3), the Thorold quartzite (2), 
 and the Medina sandstones and shales (i). In front of the falls lie the larger 
 blocks of Lockport dolomite, broken from the edge of the falls. 
 
 The American Falls, on the other hand, having so much less 
 water, are not able to cut such a plunge basin. Indeed, the water 
 is unable to move or destroy the larger fallen blocks of limestone, 
 and so the foot of these falls, as seen from the Canadian side, is 
 lined by rows of such large blocks (Fig. 349, p. 418). The section 
 of these falls is shown in the preceding diagram (Fig. 667). Here 
 too, the soft shale (Rochester) immediately below the capping lime- 
 stone is removed by spray and frost, so that the limestone overhangs. 
 Another limestone series (Clinton), about 40 feet thick, lies beneath 
 this shale, and this is not destroyed by the spray. Thus the 
 " Cave of the Winds " is produced, the floor of which is formed 
 by the lower (Clinton) limestone and the roof by the upper (Lock- 
 
Illustrations of Complicated River Erosion 773 
 
 port) limestones, the cave itself being due to the retreating face of 
 the shale (Rochester), which is about 80 feet thick. 
 
 Unequal Retreat of the Falls. It is readily seen that because of 
 the greater amount of water which flows over the Horseshoe Falls, 
 these retreat more rapidly than do the American Falls. Between 
 the years 1842 and 1890, the measured retreat, according to sur- 
 
 FIG. 668. Successive crest lines of the Horseshoe Falls, Niagara, from 1842 
 to 1890. i, Terrapin rocks; 2, Former Table Rock. (After Grabau, Geology 
 of Niagara Falls.) 
 
 veys (Fig. 668), when distributed over the entire falls, gave for the 
 American Falls a mean total recession of 30.75 feet, or a mean annual 
 recession of 0.64 feet, and for the Horseshoe. Falls a mean total 
 recession of 104.51 feet, or a mean annual recession of 2.18 feet. 
 Figured in area, this gives a removal of rock surface from the 
 edge of the falls between these years of 0.755 acres for the 
 American and 6.32 acres for the Horseshoe Falls. The greater 
 volume of water is due, of course, to the fact that the Horseshoe 
 
774 The Sculpturing of the Earth's Surface 
 
 Falls lie in the path of the main current of the river which, strik- 
 ing the left bank above the falls, is deflected so as to carry its full 
 measure over the Horseshoe Falls. The outline of the crest of the 
 American Falls, as it appeared at the beginning of this century, is 
 shown in the photograph (Fig. 669) from Goat Island. 
 
 FIG. 669. Crest of American Falls at Niagara, as seen from Goat Island 
 in 1911, showing irregular recession by the fall of large blocks which have be- 
 come undermined. (Photo by author.) (See also Ing. 349, p. 418.) 
 
 It is easy to see that when the Horseshoe Falls have retreated to 
 beyond the head of Goat Island, -the American Falls will become 
 entirely dry. Such an event has happened once before in that 
 part of the gorge which lies below the Whirlpool. Only here, 
 because the bend at the Whirlpool was a sharper one, the main 
 current was thrown over to the right side, and this side was there- 
 fore deepened. An abandoned platform, once the site of a fall 
 similar to the American Falls, is now seen on the Canadian side, 
 forming Foster's Flats (Ongiara Park) (Fig. 670). At the foot of 
 
Illustrations of Complicated River Erosion 775 
 
 the cliff over which the cataract once fell are found huge blocks of 
 limestone similar to those seen at the foot of the present American 
 Falls, and in some of these blocks the falling waters have bored 
 
 FIG. 670. View of Niagara Glen or Foster's Flats looking south. Forests 
 omitted. (After Gilbert.) This is the site of .a former fall. 
 
 large pot-holes. This platform and its cliffs present precisely 
 what will be seen in the American Falls when its waters are drawn 
 off by the retreating Horseshoe several thousand years from now, 
 or by man in the less distant future. 
 
 The Genesee River 
 
 As a second example of a complicated river history we will select 
 the Genesee River, which traverses New York state from south 
 to north, rising in the uplands of the Alleghany Plateau in Pennsyl- 
 vania and ending in Lake Ontario north of Rochester, New York. 
 
 Preglacial Character. The Genesee River lies in a region char- 
 acterized by ancient valleys formed during the period of normal 
 river erosion on the peneplaned strata which flank the Ontario Dome 
 on the south. The erosion, which occurred in Tertiary time, resulted 
 in the formation of numerous consequent valleys with subsequent 
 branches. Many of the consequents were beheaded in the deep- 
 ening of the Ontario lowland by the subsequent tributary to the 
 master stream of the region (see map, Fig. 618, p. 727). The 
 
776 The Sculpturing of the Earth's Surface 
 
 map (Fig. 671) shows the location of several of these ancient 
 valleys which are concerned in the history of this river. The prin- 
 cipal ones are the Dansville Valley on the east with several tribu- 
 taries, the upper Genesee Valley, and the Oatka Valley with the Dale 
 Valley as a tributary. The largest of these is the Dansville Valley, 
 which near Dansville is ahout a thousand feet deep and from two to 
 
 , . Rochester 
 I I 
 I I 
 
 Waisa: 
 
 FIG. 671. Map of the Genesee River country; showing old valleys and 
 modern drainage. Modern lakes in solid black. 
 
 three miles wide. Traced southward, it becomes merged with other 
 ancient valleys, and it is possible to trace the entire drainage sys- 
 tem to the Susquehanna. But the most significant feature of the 
 system is that the Dansville Valley north of that town is much 
 deeper than the valley which continues the drainage system south- 
 ward. Indeed, south of Dansville, the rock floor of the valley 
 rises very rapidly for several hundred feet until it reaches the ele- 
 
Illustrations of Complicated River Erosion 777 
 
 vation of the rock floor of the more southerly valleys. This is 
 illustrated in the following diagram (Fig. 672), which shows the 
 appearance of the valleys. The contrast would be greater if all 
 the drift and other unconsolidated deposits were removed. This 
 relationship is most readily explained by regarding the Dansville 
 Valley as overdeepened by ice-erosion during the period when the 
 great Pleistocene ice sheet was still in active motion in this 
 region. Corroboration of this hypothesis is furnished by the form 
 of the Dansville Valley, the sides of which above Dansville are 
 much steeper than is characteristic of a mature valley due to normal 
 river erosion and weathering in rocks of such soft nature. Rocky 
 
 FIG. 672. Diagram illustrating the relation of the Dansville to the Way- 
 land Valleys. The floor of the Wayland Valley at Wayland has an elevation 
 of 1372 feet. Its continuation on the extreme left hand of the diagram is 1300 
 feet. The elevation of the Dansville Valley at Dansville is 700 feet. Both 
 valleys are partly filled by drift. The streams which dissect the Wayland 
 Valley have cut narrow gorges in the rock bottom. (Drawn by Mary Welleck.) 
 
 spurs on the valley side have been truncated, a very characteristic 
 feature of glacial erosion (see p. 799), and finally, a vast amount 
 of morainal material dug up from this valley has been deposited 
 in the valley south of Dansville and in the others which continue 
 southward from this, such deposition being largely due to the waters 
 from the melting ice. This is shown by the fact that the drift- 
 covering of the valley-floor presents the stratification and surface 
 characters of river-laid deposits. 
 
 The other north-south valleys have also been deepened by 
 glacial erosion, but not to the same extent. This is well illustrated 
 by the Oatka Valley, the floor of which lies more than 100 feet 
 below that of the undeepened Dale Valley which joins it at 
 Warsaw. Formerly the floor of the Dale Valley joined the 
 
7 78 The Sculpturing of the Earth's Surface 
 
 Oatka at grade, that is, the two levels were essentially in ac- 
 cord. To-day the point of junction is marked by a cliff of rock 
 and a descent to the level of the Oatka Valley, which is here 
 
 FIG. 673. Diagram showing the junction of the Oatka and Dale Valleys 
 near Warsaw. The Dale is a hanging valley on the side of the over-deepened 
 Oatka. (Drawn by Mary Welleck.) 
 
 filled by more than 100 feet of alluvial material deposited 
 upon it after the overdeepening. Thus the Dale Valley is a hang- 
 ing valley on the side of the Oatka Valley, a type characteristic 
 
 of glaciated regions 
 (see p. 801). This 
 is illustrated in the 
 diagram, Fig. 673. 
 The sides of the 
 Oatka Valley are, 
 moreover, steepened 
 and its spurs are 
 truncated. The 
 overdeepening con- 
 tinues for some dis- 
 tance south of the 
 town of Warsaw, and 
 beyond that are 
 heavy morainal de- 
 posits composed in 
 part, at least, of the 
 material scraped by 
 
 the ice from the Oatka Valley. This moraine crosses the line of 
 junction of the Oatka with the ancient Genesee Valley and com- 
 
 ^-^ 
 
 FIG. 674. Map of the junction of the ancient 
 valleys, now partly drift-filled, in the Portageville 
 region, showing the approximate character of the 
 valleys before they became modified by glacial 
 erosion; the course of the modern Genesee with 
 its gorges and falls is represented. 
 
Illustrations of Complicated River Erosion 779 
 
 pletely divides this valley into two 
 parts by a huge dam of drift at Port- 
 ageville (Fig. 674). 
 
 Post-Glacial Development. The 
 subsequent history of the region was 
 somewhat as follows. The land, it 
 will be remembered, had been de- 
 pressed on the north, so that the 
 slopes of the valleys, which formerly 
 were southward, now descended to 
 the north. For a time, the Ontario 
 Valley was occupied by ice, blocking 
 the outlets to the east. This resulted 
 in the damming of all the north-south 
 valleys and their conversion into lakes 
 up to the lowest point of outlet on the 
 south, since drainage to the north was 
 prevented by the ice. The outlets 
 and levels of practically all of these 
 lakes have been traced with consid- 
 erable detail. When the outlet of 
 the Ontario waters into the Mohawk 
 (Rome outlet) was uncovered by the 
 melting of the ice, the waters were 
 lowered to the level of Lake Iroquois 
 (p. 762), and, as will be recalled, the 
 birth of Niagara took place. This re- 
 sulted also in the draining of the 
 Dansville and Oatka Valleys, but the 
 upper Genesee Valley, being dammed 
 by heavy moraines at Portageville, re- 
 mained a lake. Instead of south- 
 ward drainage, however, the overflow 
 of this lake took place northward at 
 a lower level, and thus was born the 
 modern Genesee (Fig. 675). The over- 
 flow of the waters of the lake occurred 
 on the west, then they turned north 
 through a depression in the moraine, 
 crossed the buried valley of the Oatka 
 
 FIG. 675. Map of the 
 Genesee River from south of 
 Portageville to about 9 miles 
 south of Mt. Morris. The 
 1 1 oo and 1200 foot contours 
 represent the rock topography. 
 The broken contour lines rep- 
 resent the same elevations on 
 the drift filling. Note the 
 broad and regular character of 
 the old valley north and south 
 of St. Helena. This has been 
 glaciated. The valley south 
 of Portageville shows rocky 
 spurs and has been less gla- 
 ciated. The modern stream 
 has cut a narrow gorge in the 
 bottom of the St. Helena valley 
 as shown in the cross section 
 near St. Helena, given in Fig. 
 683, p. 783. Drifts-fillings of 
 old valleys dotted. A , Lower 
 Falls, 70 ft. ; B, Middle Falls, 
 107 ft. ; C, Upper Falls, 71 ft. 
 
 above its junction with the 
 
780 The Sculpturing of the Earth's Surface 
 
 old Genesee Valley, and finally spilled into the northern part of 
 the old Genesee Valley which Had been separated from the southern 
 
 part by the heavy Port- 
 ageville moraine. The 
 new river was prevented 
 from following this val- 
 ley to Lake Ontario 
 (Iroquois) by other 
 heavy morainal ma- 
 terial, and so the waters 
 spilled over the side 
 and flowed across the 
 plateau surface to the 
 Dansville Valley, plung- 
 ing into it over the 
 
 FIG. 676. The lower falls of the Genesee 
 at Rochester over the upper hard Medina 
 (Thorold) sandstone. (Photo by author.) 
 
 rocky side of that valley 
 at the site of the modern 
 town of Mt. Morris, and 
 thence continuing for 60 
 
 miles more to the Lake, before reaching which, however, they had 
 
 to descend over the exposed part of the inf ace of the Niagara cuesta, 
 
 i.e., the Niagara escarpment (Fig. 679, A). 
 
 Gorge cutting by retreating waterfalls began at two points and 
 
 occurred simultaneously with the cutting of the Niagara gorge 
 
 farther west. One gorge was cut by the new Genesee from the 
 
 Niagara escarpment 
 
 southward, and the 
 
 other from Mt. Morris 
 
 southwestward. In the 
 
 cutting of the gorge 
 
 through the Niagara 
 
 escarpment three hard 
 
 layers were discovered, 
 
 the lowest a white 
 
 quartzite (Thorold) 
 
 overlying softer red 
 
 sandstones (Medina), 
 
 the next a heavy lime- 
 stone (Clinton) overlying shales, and the third another limestone 
 
 (Lockport) overlying a still heavier shale bed (Rochester). These, 
 
 FIG. 677. The lower and middle falls of 
 the Genesee River at Rochester, the latter over 
 Clinton limestones. (Photo by the author.) 
 
Illustrations of Complicated River Erosion 781 
 
 it will be observed, are the same beds as those the Niagara River 
 had to cut through, but because that river had so much more 
 water, it has long since accomplished this cutting process and is 
 now at work upon the 
 single upper hard lime- 
 stone (Lockport) at the 
 present cataract. The 
 Genesee, on the other 
 hand, is still in an early 
 stage, because, although 
 it has been at work just 
 as long as Niagara has, 
 and on the same rocks, 
 the smaller amount of 
 water determined a 
 slower rate of cutting. 
 Hence there are still 
 three falls in existence 
 in the Rochester Gorge, one about 100 feet high, over the Thorold 
 quartzite (Fig. 676), a second 40 or 50 feet high, over the Clinton 
 limestone (Fig. 677), and a third about 100 feet high, over the 
 
 FIG. 678. The upper falls of the Genesee 
 River at Rochester over the lower beds of 
 the Lockport limestone. (Photo by the author.) 
 
 FIG. 679. A. Section of the lower Genesee River from Lake Ontario to 
 Rochester, N. Y. B. Section of Niagara river from Lake Ontario to the Falls. 
 
 Lockport limestone (Fig. 678). In the Niagara gorge the lower 
 two of these falls have long since disappeared, because the beds 
 which produced them have passed below water-level on account 
 
782 The Sculpturing of the Earth's Surface 
 
 of the southward dip. The relative position of the waterfalls of 
 
 the two rivers is shown in the diagram (Fig. 679). 
 The waterfall which began at the same time at Mt. Morris, 
 
 cut more rapidly because it 
 had chiefly soft shales and 
 some sandstones to work 
 upon. Hence, in the same 
 period of time, it cut back 
 not only across the plateau, 
 where it formed a magnificent 
 gorge 500 or more feet in 
 depth and of a meandering 
 character, but all the way 
 back to Portageville, a dis- 
 tance of 1 8 miles, completely 
 draining the Portageville 
 lake. This cutting was not 
 uniform, but wherever hard 
 layers -existed a separate fall 
 
 ' 
 
 FIG. 680. The Upper 
 falls of the Genesee River at 
 Portage. (Photo by author.) ' 
 
 came into being. To- 
 day there are three of 
 these, located at A, B, 
 and C, respectively (Fig. 
 675), the upper one at C 
 (Fig. 680) carrying the 
 water down to the floor 
 of the Oatka Valley, but 
 the other two cutting 
 below that because of the 
 greater depth of the val- 
 ley at Mt. Morris (Figs. 
 681, 684). 
 
 Contrasting Features 
 of the Modern Stream. 
 -It is thus seen that FlG 581. The middle falls of the Genesee 
 the Genesee River is not River at Portage. (Photo by the author.) 
 
Illustrations of Complicated River Erosion 783 
 
 a normal stream, but 
 one which has come into 
 existence as the result 
 of a complicated series 
 of changes in the land 
 surface, by reversal of 
 the direction of drain- 
 age, and by the appro- 
 priation of parts of val- 
 leys of several older 
 streams and the cutting 
 of connecting gorges. 
 Where the stream flows 
 
 FIG. 682. The open valley of the Genesee 
 at Portageville above the upper falls. (Photo 
 by the author.) 
 
 in the ancient valleys, the floors of which are always covered to a 
 
 considerable depth with 
 deposits 
 
 FIG. 683. Cross-section of the old valley of 
 the Genesee River, north of Portageville, N. Y., 
 showing the mature broad valley, now partly 
 drift obstructed, in the center of which the modern 
 stream (reversed in flow) has cut a narrow gorge. 
 
 ley with sloping soil-covered sides (Fig. 
 the stream has cut a connect- 
 ing gorge, it is confined in 
 a narrow chasm with steep 
 rock walls. These are ver- 
 tical wherever the current 
 strikes the bank but form a 
 talus-covered slope on the 
 opposite side. A succession 
 of such cliffs and slopes alter- 
 nates on each side of the 
 gorge. (Seep. 416.) 
 
 Where the stream has be- 
 come incised below the level 
 of the old valley-floor, a steep- 
 sided narrow gorge occupies 
 the center of a broad, open, 
 shallow valley, as shown in 
 the above diagram (Fig. 683). 
 
 made when 
 these valleys were lakes, 
 the stream itself has 
 the aspect of maturity, 
 winding across the flat 
 bottom of a broad val- 
 682). Where, however, 
 
 >. 684. The lower falls of the 
 Genesee River at Portage as it appeared 
 in 1892. (Photo by the author.) 
 
784 The Sculpturing of the Earth's Surface 
 
 FIG. 685. The lower falls of the Genesee at 
 Portage in 1900. 
 
 If the old valley was much encumbered by drift, some of this is 
 
 also found in the gorge, whither it has been carried by landslides. 
 
 Finally, where the stream 
 is still actively cutting, 
 a series of waterfalls 
 (A-C) exist in narrow, 
 steep-sided gorges, each 
 waterfall being caused by 
 a hard stratum of rock. 
 Of the two portions flow- 
 ing in mature valleys, 
 the one above Portage- 
 ville lies at a level which 
 is about 500 feet above 
 that of the mature por- 
 tion below Mt. Morris. 
 This difference of depth 
 is taken up by the falls 
 
 and rapids in the intervening gorge portions. 
 
 In the lower of the three falls in the Portage gorge (Fig. 684) 
 
 the phenomenon of the table rock or abandoned waterfall seen at 
 
 Foster's Flats, Niagara, is 
 
 repeated. Owing to a 
 
 bend in the river, the 
 
 current was deflected to 
 
 the right side and cut 
 
 there a deeper gorge (Fig. 
 
 685), leaving a triangular 
 
 table rock, the site of the 
 
 former fall, upon the left 
 
 (Fig. 686). The follow- 
 ing woodcut (Fig. 687), 
 
 represents the conditions 
 
 as they existed in 1840, 
 
 while at present the falls 
 the narrower gorge 
 
 in 
 
 FIG. 686. The table rock and narrow lateral 
 gorge at the lower falls of the Genesee River, 
 Portage, N. Y. 1892. (Photo by the author.) 
 
 have moved many hun- 
 dred feet upstream. 
 
 Owing to the jointing of this rock, the peculiar " Sugar Loaf " 
 rock, shown in that illustration (and shown from below in Fig. 
 
Illustrations of Complicated River Erosion 785 
 
 FIG. 687. View of the lower falls of the Genesee at Portage as it appeared 
 in 1840. (After Hall.) 
 
 688), was formed. The steps by which these features were devel- 
 oped are shown in the following series of diagrams (Fig. 689). 
 
 FIG. 688. The "Sugar 
 Loaf" rock seen from be- 
 low (1892). 
 
 FIG. 689. Diagram showing the de- 
 velopment of the characters at the lower 
 falls of the Genesee River at Portage. 
 I-III, hypothetic; IV, conditions in 1840 
 (after Hall) ; V, conditions in 1892 (from 
 a survey by the author). 
 
786 The Sculpturing of the Earth's Surface 
 
 The Grand Canon of the Colorado River 
 
 General Characters of the Grand Canon. The Grand Canon 
 of the Colorado River is the most stupendous example of a river 
 
 FIG. 690 a. Sketch map of the Grand Canon district, showing the suc- 
 cession of plateaus. (See fig. 6906.) EC, Echo Cliff. EK, East Kaibab 
 double monocline. WK, West Kaibab fault. S, Sevier fault; T, Toroweap 
 fault, and scarp. H, Hurricane fault and ledge. G W, Grand Wash Cliffs and 
 fault. (After Johnson.) 
 
 gorge to be found anywhere on earth. It is more than a mile in 
 depth in its principal division, and its average width is eight miles, 
 though some portions are wider. " Its sides are a succession of 
 
Illustrations of Complicated River Erosion 787 
 
 rocky slopes and sinuous cliffs, some of which are huge steps with 
 from 300 to 500 feet sheer descent" (Darton). The main section, 
 the Grand Canon proper, has a length of 125 miles, and it is divided 
 into the Kaibab or eastern and the Kanab or western portions. 
 The Kaibab is the deepest and most complex portion of the canon, 
 and in this the river has cut an inner gorge into the crystalline 
 
 FIG. 690 b. Block diagram and section of a part of the Colorado Plateau 
 (after Powell). The section is drawn north of the Colorado and shows the 
 principal formations. These are, at the base, the pre-Cambrian crystallines 
 and ancient sediments (in -white) ; the Tonto group of sandstones and shales 
 (Cambrian) (in circles) ; the Carboniferous beds (limestones, shales, sand- 
 stones and limestones) (blocked) ; and the Permo-Triassic sandstones, etc. (fine 
 dots). 
 
 On the right is Echo Cliff, an erosion ridge of Permo-Triassic sandstones in a 
 monocline, the flexure of which is downward on the east. Along the section-line 
 this forms a triangular ridge with a westward slope in conformity with the dip, 
 almost as steep as the bold erosion scarp on the west. Next,' west of this cliff 
 lies the Marble Plateau formed "by the nearly horizontal Carbonic (Kaibab) 
 limestone and transected by the Marble Canon. This was formerly also 
 covered by the Permo-Triassic sandstone, a remnant of which forms the high 
 Paria Plateau which rises above it in the background denned by the Vermilion 
 Cliff. West of the Marble Plateau lies the Kaibab Plateau, which rises 2500 
 to 4000 feet higher and is formed of the same Carboniferous limestone (Kaibab). 
 This plateau is bounded on the east by the East Kaibab monocline, which is 
 generally a double flexure. On the west is the West Kaibab fault, here shown 
 as a flexure. Southward this splits into three secondary faults with associated 
 scarps, and they extend almost to the brink of the canon. This fault lowers 
 the formation and the surface to the Kanab Plateau still of the same limestone 
 series. This is transected by the Kanab Canon. In the distance is seen the 
 Vermilion Cliff of Permo-Triassic sandstones and shales. The Kanab is sepa- 
 rated from the Uinkaret Plateau by the Toroweap fault with downthrow on 
 the west amounting to 600 or 700 feet near the Canon. The Uinkaret is 
 separated from the Shivwitz Plateau, still farther west, by the Hurricane fault 
 ledge, with a recent displacement up to 1400 feet in addition to an older displace- 
 ment. (See map on opposite page, Fig. 690 a.) 
 
 basement-rocks. In the Kanab portion, which is much less diversi- 
 fied and picturesque, the inner gorge is cut into the lower member 
 of the late Palaeozoic series (the Red Wall limestone), the surface 
 of which forms a broad platform about two miles wide on either 
 side and a thousand feet below the summit of the plateau, which 
 stands nearly 7000 feet above sea-level. Above the Kaibab portion 
 of the Grand Canon the river has cut the Marble Canon, which is 
 
788 The Sculpturing of the Earth's Surface 
 
 65 miles in length and extends nearly north and south, being joined 
 nearer its lower end by the canon of the Little Colorado. (See 
 map, Fig. 690 a, and the more detailed map, Fig. 691.) The 
 Marble Canon is cut through the Carbono-Mississippian lime- 
 stones, shales, and sandstones down to and into the Cambrian 
 (Tonto beds), but to the west of it a broader open valley exposes 
 the older pre-Cambrian rocks, here raised to greater altitude. The 
 reason for these differences in the rock sections will become apparent 
 when the general structure of the country is understood. 
 
 FIG. 691. Map of the Grand Canon and part of the Marble Canon on 
 the right. (U. S. G. S.) 
 
 Rocks Exposed in the Grand Canon. In the immediate vicinity of 
 the canon the strata which compose the plateaus are of Palaeozoic and older 
 age. At the base of the series lie the granite and other crystalline base- 
 ment rocks, these being exposed in the bottom of the canon from about the 
 Grand View Point westward for 35 miles or more (map, Fig. 691). The 
 river has cut an inner gorge in this resistant rock, to a depth of 800 to 
 1000 feet, as is shown in the photograph taken from El Tovar and repro- 
 duced in Fig. 693, and in the cross-section of the canon from the same 
 point (Fig. 694). These rocks, which form dark rugged ledges in the 
 inner gorge, terminate upwards in a very level surface of erosion (a pene- 
 plane) and upon them, over wide areas, rest the horizontal Tonto sand- 
 stones, about 150 feet thick, followed by the Tonto shales of greenish color 
 and about 800 feet in thickness. These formations belong to the Cambrian 
 series of rocks. -The surface of the Tonto sandstone forms a plateau or 
 terrace on either side of the inner gorge, averaging a mile in width, while 
 the shales which overlie it form long slopes interrupted by subordinate 
 limestone and sandstone ledges. The platform and inner gorge are char- 
 
Illustrations of Complicated River Erosion 789 
 
 acteristic features and are well shown in the topographic map of the canon 
 (Fig. 691) and in the photograph (Fig. 693). 
 
 In the broad part of the canon which lies northeast of Grand View 
 (west of the Marble Canon) and in some other sections (Shinumo basin, 
 parts of Bright Angel canon, etc.) another series of old rocks appears 
 between the Tonto sandstone and the basal granites and other crystallines. 
 This pre-Cambrian series is known as the Grand Canon Series, and is divis- 
 ible into two parts. The lower division (Unkar group} consists of basal 
 conglomerates, limestones in thick dark beds, bright red shades and upper 
 heavy quartzites and brown sandstones. The higher division (Chuar 
 group] is seen only in the region west and northwest of the mouth of the 
 Little Colorado, and consists of sandstones, shales, and limestones. The 
 
 FIG. 692. General view of the Grand Canon. 
 
 beds of the Grand Canon series, which have a total aggregate thickness 
 of 12,000 feet, dip rather steeply to the north and northeast, and are in 
 marked contrast with the nearly horizontal Tonto beds which overlie them 
 unconformably. The relation is shown in the section (Fig. 694). The 
 surface of the inclined Grand Canon beds was beveled across by erosion and 
 worn into an undulating plain before the Tonto beds were deposited upon it. 
 Next above the Tonto shales, and like these nearly horizontal, is a 
 massive hard compact limestone about 500 feet thick and of pale-blue 
 gray color when freshly broken. It forms great cliffs the faces of which 
 are stained red from the wash of the red beds which overlie it, and on this 
 account the rock has been named the Red-wall limestone. Its cliffs form 
 prominent features in the scenery of the canon, and they are well indicated 
 by the heavy shading above the Tonto platform in the map (Fig. 691) 
 
790 The Sculpturing of the Earth's Surface 
 
 (see also the section Fig. 694). In the photograph (Fig. 693) the prominent 
 dark cliff above the middle of the view is formed by this limestone. This 
 limestone also forms the caps of many flat-topped spurs and buttresses, and 
 of outlying buttes and towers such as Cheops Temple, Newberry Butte, 
 Sheba Temple, Solomon's Temple, and many others. 
 
 Next above the Red-wall limestone lies a series of red shales interbedded 
 with thick layers of red and red-brown sandstone, the whole forming the 
 
 FIG. 693. Telescopic view of the Grand Canon of the Colorado, from El 
 Tovar Hotel, showing the crystalline rocks (granite and gneiss) at the base 
 unconformably overlain by the horizontal Palaeozoic strata. Resting directly 
 upon the granite, the Tonto sandstone forms a platform in the foreground ; 
 the first cliff beyond the gorge is formed by Unkar quartzites covered by Tonto 
 shale. The prominent cliff three fourths the way up is the Red-wall limestone, 
 and it is covered by the Supai Red Beds. The Red-wall limestone forms the 
 prominent butte, Cheops Pyramid, near the center of the picture, while the higher 
 butte is Buddha Temple, a detached part of the Kaibab Plateau which forms 
 the background. A section across this part of the canon is given in Fig. 694. 
 
 Supai formation, which has a total thickness of noo feet. Because of 
 their hardness, the sandstones cause a succession of steps in the middle of 
 the slope formed by the red shales. These red beds constitute a large 
 part of many fine promontories and ridges which project far out into the 
 canon. Many of the buttes and towers are also formed oi them. The 
 next higher division is the heavy Coconino sandstone, 300 feet thick, and 
 
Illustrations of Complicated River Erosion 791 
 
 this forms a vertical gray cliff in the canon wall, 800 feet below the top. 
 It is massive and strongly cross-bedded. On top of it lies the Kaibab lime- 
 stone, a slabby light gray limestone nearly 800 feet thick, which everywhere 
 causes the terminal cliff and the summit of which forms great forest- 
 covered plateaus. This is the highest rock exposed in the canon walls, and 
 it and the underlying formations to the top of the Tonto group belong 
 to the " Carboniferous " series of rocks. Between them and the under- 
 lying Tonto group is a great break in succession, i.e., a hiatus, which leaves 
 the great middle portion of the Palaeozoic series unrepresented. 
 
 This rock series was once covered by higher beds, but these have all 
 been eroded from the immediate vicinity of the canon. They comprise 
 vari-colored shales and red sandstones of Permian and Triassic age, por- 
 tions of which are now seen in the Vermilion Cliffs and in Echo Cliff (see 
 Fig. 690 a and the stereogram, Fig. 690 b}. A still higher series of white 
 sandstones (Jurassic) is seen in the White Cliffs, which form the edges of 
 more elevated plateaus, far to the north of the canon. 
 
 FIG. 694. Section across the Grand Canon west of Bright Angel Creek. 
 From El ToVar to Kaibab Plateau, showing the unconformity between the basal 
 crystallines and the Unkar Group, and that between the latter and the Tonto 
 formation of the Palaeozoic. (U. S. G. S.) A, Kaibab- limestone; B, Coconino 
 sandstone; C, Supai (Red Sandstones and shales); D, Red-wall limestone; 
 E, Tonto shales and Sandstones (at base) ; F, Granite, etc. 
 
 General Character and Development of the Region. Taking a 
 broad view of the country into which the Grand Canon is cut, we 
 recognize that it consists of a series of great plateaus composed of 
 nearly horizontal strata. These plateaus, known collectively as the 
 Colorado Plateaus, are separated from one another by monoclinal 
 flexures or by cliffs located along lines of faulting. The general 
 trend of the axes of the flexures, and that of the fault lines, is north 
 and south, or approximately at right angles to the Grand Canon. 
 (See map, Fig. 690 a, and diagram, Fig. 690 b.) Originally the 
 deformation was entirely by monoclinal flexures which ascended 
 westward, so that the region was composed of a series of huge 
 steps rising westward, each separated from the others by flexures, 
 and elevated from one to several thousand feet above the one next 
 below. During or shortly after the flexing, the monoclines of the 
 western half of the series broke and the elevated blocks settled 
 
792 The Sculpturing of the Earth's Surface 
 
 down again, the depression in general being progressively more 
 pronounced westward, so that a descending series of steps was 
 produced. Thus originated the peculiar type of faulting of this 
 region previously referred to (p. 600), in which the up-flexed portion 
 became the downthrow block, the strata of which bend down near 
 the fault-plane, while the strata of the upthrow block bend up near 
 that plane, this being the reverse of the bending produced by drag 
 in ordinary faulting. (See Fig. 518 b, p. 602.) In the eastern half 
 of the region, the original monoclinal flexures remained unbroken 
 so that the steps descended both eastward and westward from the 
 high or 'central area: eastward by a succession of monoclinal 
 flexures ; westward by a succession of faults. 
 
 This period of disturbance was followed by extensive erosion, 
 during which the country was reduced to a peneplane, both the 
 fault and the flexure-steps being obliterated. It was into this 
 peneplaned country, the surface of which was now formed of dif- 
 ferent rocks in the various blocks, that the westward-flowing 
 Colorado began to cut its canon. Over parts of this peneplaned 
 surface basaltic lavas were poured out, covering adjoining portions 
 of contiguous fault-blocks, in which the surface was often cut on 
 different formations. A second period of faulting, in the same 
 direction, ensued, but this affected chiefly the western faults (Grand 
 Wash and Hurricane). Where these were crossed by basalt flows, 
 the part resting upon the western block was lowered, relatively, 
 with that block, during the faulting. In the ensuing second cycle 
 of erosion, the softer strata were stripped from off the various 
 blocks, except where protected by the basalt layers, and thus 
 the step-like topography was reproduced, but this time the surfaces 
 of the several plateaus in the vicinity of the canon were formed 
 by the Kaibab limestone, the resistant upper member of the 
 Carbonic series. Portions of the higher formations, however, 
 remained to form the more elevated plateaus north of the canon, 
 as seen in the stereogram (Fig. 690 b) and in the map (Fig. 690 a). 
 The principal westward-facing scarps of the region thus represent 
 resequent fault-scarps, though parts of the Hurricane and the Grand 
 Wash cliffs are new fault-scarps, produced by a third period of 
 faulting, after the cutting of the Grand Canon had progressed to a 
 considerable extent. 
 
 As the Colorado had learned to flow westward upon the old pene- 
 planed surface, formed during the first cycle of erosion, it continued 
 
Land Forms Due to Glacial Sculpture 793 
 
 in this direction upon the renewed great uplift of the country. 
 This latest period of erosion witnessed the cutting of the great 
 canon, which, though of such magnitude, is only the latest erosion 
 feature in a region which has under- 
 gone a succession of uplifts and 
 down cutting of vastly more stupen- 
 dous proportions and all within the 
 space of time which has elapsed since 
 the close of the early Tertiary period. 
 Because of the differential elevation 
 of the several blocks into which the 
 canon is cut, the bottom of the canon 
 in the several sections is formed by 
 different members of the rock suc- 
 
 FIG. 694 a. Major John 
 Wesley Powell. 
 
 cession. 
 
 The characters of the Grand Canon 
 were practically unknown to the 
 world, until a heroic exploring party 
 traversed it from end to end in 1869, 
 
 under the leadership of that intrepid geologist, Major John 
 Wesley Powell (portrait, Fig. 694 a), one of the organizers and 
 early directors of "the United States Geological Survey. Since that 
 day it has become a favorite region for study and inspirational con- 
 templation, having in recent times been made readily accessible 
 to the general tourist. To the student of geology it serves as the 
 most wonderful object lesson in canon cutting which our country 
 affords. 
 
 LAND FORMS DUE TO GLACIAL SCULPTURE 
 
 Glaciers are very important agents in the sculpturing of the 
 landscape, their mode of cutting being that already set forth in 
 an earlier chapter (pp. 433-435). We shall here summarize only 
 the characteristics of the main types of land-forms due to glacial 
 sculpturing. 
 
 The Cirque, Arete, and Horn. One of the most characteristic 
 erosion features of glaciated mountain districts is the cirque or 
 glacial amphitheater at the head of the glacier (Fig. 303, p. 366; 
 Figs. 695 a, b). Where glaciers have entirely disappeared, these 
 steep-walled semicircular indentations remain as eloquent witnesses 
 
794 The Sculpturing of the Earth's Surface 
 
 of former glacial occupancy, for no agent but the glacier is known 
 to be capable of producing such a feature. Where several cirques 
 occur side by side, they may be separated by narrow ridges, which, 
 with the growth of the cirque by continued cutting, will become 
 sharper and sharper, with knife-edge crests, and this eventually 
 results in the production of a strongly serrated ridge or arete 
 (Fig. 6q6 a). 
 
 FIG. 695 a. South slope of Mt. Rainier, with Paradise Park in the fore- 
 ground. Note the glacial cirques which are being cut back into this maturely 
 dissected volcano. (Photo by A. H. Barnes, courtesy of D. W. Johnson.) 
 
 As we have seen in an earlier chapter, the cirque is produced 
 by the plucking action of the lower portion of the glacier mass which 
 occupies the valley in the mountain side. The bergschrund or 
 crevasse at the head of the glacier formed by its forward movement 
 permits the thawing and refreezing of the ice at the bottom, and 
 this brings about a loosening of rock masses from the base of the 
 cirque. Its walls, which originally were those of a sloping valley, 
 become more and more steepened. The floor of the cirque also may 
 be deepened into one or more basins by the scouring action of the 
 moving ice, and such basins may remain filled with water after 
 the ice has melted away entirely, forming rock basin lakes or tarns. 
 Lakes may also be held in an ice-abandoned cirque by morainal 
 matter deposited across its mouth (Fig. 696 b). 
 
Land Forms Due to Glacial Sculpture 795 
 
 Where cirques have been cut into an old table-land from various 
 directions, the region after the disappearance of the ice presents 
 a remarkably scalloped character which has been aptly compared 
 with a heavy piece of 'dough from which biscuits have been cut, 
 the hollows left by the biscuit cutter representing the cirques, and 
 the remnant of the dough, the irregular ridges of upland remaining. 
 
 FIG. 695 b. Glacier at Cascade Pass, Glacier Peak quadrangle, Wash- 
 ington ; showing steep walls of cirques separated by narrow aretes ; marginal 
 crevasses on the ice and abundant morainal material are shown in the fore- 
 ground. (Photo by B. Willis, for U. S. G. S., courtesy of Popular Science 
 
 Monthly.) 
 
 At first, the cirque is scarcely wider than the valley below in which 
 the glacier lies, but by continual sapping it becomes enlarged in 
 all directions, so that its form, with reference to that of the valley 
 extending from it, has been aptly compared to that of the large 
 rounded head of a nail. Further growth of the cirques may pro- 
 duce compound or scalloped forms with subordinate cirques around 
 the margin of the principal one. In this manner a complex 
 
796 The Sculpturing of the Earth's Surface 
 
 " grooved " upland is produced, which later, by the sharpening 
 and partial destruction of the dividing ridges, produces a sharply 
 
 FIG. 696 a. Diagram of a mountain region from which the former glaciers 
 have melted away, leaving the cirques to testify to their former presence. Some 
 of the cirques diverge, leaving a broad rounded ridge between them, others con- 
 verge, being separated only by a serrated knife-edge or arete (grat). (After 
 Davis.) 
 
 " fretted " upland surface, such as is characteristic of the summit 
 of Mont Blanc and other regions of the Alps. Sharp-toothed 
 comb-like ridges, called aretes or grats, dividing the cirques, are 
 
 FIG. 696 b. A glacial lake or tarn in an old cirque. Utah. (Photo by 
 
 F. J. Pack.) 
 
Land Forms Due to Glacial Sculpture 797 
 
 characteristic of this 
 stage, and at the junc- 
 tion points of such comb- 
 ridges sharp pyramidal 
 peaks or " horns " arise 
 as the erosion remnants 
 of the highest portion of 
 the original mountain 
 mass. Of these, the 
 Matterhorn (Fig. 697), 
 the Aletschhorn, and 
 many others are typical 
 Alpine examples. Mount 
 Sir Donald in the Selkirk 
 Mountains of Canada is 
 a typical American ex- 
 ample. 
 
 The U-shaped Valley. 
 The normal section of 
 a young river valley is more or less V-shaped, the greatest cutting 
 taking place at the bottom and downward, while weathering pushes 
 
 FIG. 697. The Matterhorn, a typical Al- 
 pine peak or "horn" in Switzerland, as seen 
 from the Gornergrat. (Photo by D. W. 
 Johnson.) 
 
 FIG. 698. The Yosemite Valley, a typical U-shaped young glaciated val- 
 ley, much over-deepened so that the side streams now flow in Hanging Valleys 
 and enter the main valley by Waterfalls. 
 
798 The Sculpturing of the Earth's Surface 
 
 FIG. 699. The Lauterbrunnen Valley ; a young glacial trough, similar to the 
 Yosemite Valley. It is especially noted for the splendid cataracts which enter 
 it from hanging valleys. (Courtesy D. W. Johnson.) (See Fig. 310, p. 372.) 
 
 FIG. 700. Upper part of Kern Canon, California. A splendid mature 
 glacial trough showing the typical catenary curve. (Photo by Gilbert, U. S. 
 G. S. Courtesy D. W. Johnson.) 
 
Land Forms Due to Glacial Sculpture 799 
 
 back the sides of the valley most rapidly in the upper portion. 
 With increasing age the floor of the valley is widened to a flat sur- 
 face, but the sides retain their slopes. When such valleys are oc- 
 cupied by moving glaciers, cutting takes place not only at the 
 bottom, but on the sides as well, and the section of the valley 
 changes to a U-form, with precipitous cliffs often of great height. 
 This constitutes the stage 
 of youthful deepening of 
 river valleys by ice and 
 presents the most pic- 
 turesque aspect of such 
 erosion. It is well shown 
 in the great Yosemite 
 Valley of California (Fig. 
 698), which has been 
 scoured out by ice to a 
 depth of more than two 
 thousand feet, and in the 
 beautiful Lauterbrunnen 
 Valley of Switzerland 
 (Fig. 699), which has a 
 very similar character. 
 (See also Fig. 310^.372.) 
 Where a valley has long 
 been occupied by a large 
 ice stream, the walls of 
 the resulting trough are 
 smooth and somewhat 
 flaring, and the bottom 
 beautifully rounded (Figs. 700, 701). It no longer has the parallel 
 sides of a U-shaped valley and is better called a glacial trough. 
 Most glaciated valleys belong to this category. 
 
 If the original river valley is an irregular one, with rocky spurs 
 projecting from alternate sides, these spurs will gradually be worn 
 off by the ice, and their truncated faces will form a characteristic 
 feature of such a valley that has been glaciated. Hence the 
 steepened sides and the truncated spurs form characteristic 
 topographic features by which young glaciated valleys may be 
 recognized, while the flaring sides and general smoothness of form 
 indicate a valley long occupied by ice (Fig. 702 a, 6). 
 
 FIG. 701. Catenary curves of glacial 
 trough in Norway. View from Moldestadt. 
 (Courtesy D. W. Johnson.) 
 

 
 51^ 
 
 FIG. 702 a. River valley with nu- FIG. 702 b. The same valley as 
 
 merous spurs and a V-shaped form. that shown in Fig. 702 a after glacia- 
 
 (U. S. G. S.) 
 
 tion. The spurs have been truncated 
 and the valley is broadly U-shaped. 
 (U. S. G. S.) 
 
 AL 
 
 C C Irque 
 
 GT Glacial Trou 
 
 HV Hanging 
 
 MG Main Glacier 
 
 MP Matterhorn Peak 
 
 R Rock Step 
 
 T Tarn 
 
 TG Tributary Gl elder 
 
 us Unylaeiafect Summii 
 
 uv UnglaciateJ Va/ley 
 
 FIG. 703. Land forms due to ice sculpture. A . A mountain mass occupied 
 by a glacier system. The surfaces of the main and tributary valleys are in 
 accord. B. The same mountain mass after melting of the glaciers. The 
 floor of the main valley lies far below that of the tributary valleys, which now 
 form hanging valleys upon its side. (After Davis.) C. Sketch of a glaciated 
 valley showing the U-shaped form. (Military Geology.) 
 
 800 
 
Land Forms Due to Glacial Sculpture 801 
 
 Overdeepening ; Hanging Valleys. A normal river valley 
 will, in general, be joined by its tributary valleys at grade, that is, 
 the floors of the two valleys will be essentially in accordance at 
 the point of junction. In glacially eroded valleys, however, this 
 
 FIG. 704. Diagrams illustrating the transformation of a lateral stream 
 valley into a hanging valley by glacial erosion of the main valley: A, by 
 widening of the main valley without deepening; and B, by over-deepening of 
 the same without essential widening. 
 
 is frequently not the case, for the main valley which is occupied 
 by the larger glacier may be greatly deepened by it, while the 
 lateral one, occupied by smaller glaciers or by none, may be deep- 
 ened slightly or not at all. Tributary glaciers have their surfaces 
 
 FIG. 705. The Norwegian fjords are partially submerged glacial troughs 
 into which side streams fall in cataracts from hanging valleys. The Naero 
 Fjord, from Gudvangen. (Courtesy D. W. Johnson.) 
 
 in accord with the surface of the main glacier at the point of 
 junction, but their floors may be at very different levels (Fig. 703). 
 This might be due to the widening of the main valley at the bot- 
 tom (A) (Fig. 704), but is more commonly produced by overdeep- 
 ening (B) or by both. When the ice finally melts away from the 
 
802 The Sculpturing of the Earth's Surface 
 
 valleys, the point of junction of the lateral with the main valley 
 is found to be high up on the side of the main valley, the lateral 
 
 valley then being spoken of 
 as a hanging valley. Such 
 hanging valleys are very char- 
 acteristic of glaciated dis- 
 tricts, as is so well shown in 
 the falls of the Yosemite and 
 Lauterbrunnen valleys, which 
 drop from hanging valleys 
 high up on the side of the 
 main trough (Figs. 698, 699), 
 and in many of the beautiful 
 falls of the Norwegian fjords 
 (Fig. 705). Hanging valleys 
 may also be produced in 
 other ways. Thus, where a 
 waterfall in the main valley 
 cuts beyond the mouth of the 
 small tributary valley which 
 formerly joined it at grade 
 above the waterfall, this val- 
 ley will, for a time after the 
 
 FIG. 706. Falls of a hanging valley 
 on the side of the Genesee Gorge at 
 Portage, N. Y. The difference in eleva- 
 tion between the floors of the two valleys 
 is about 200 feet, and is due to the re- 
 cession of the cataracts of the main val- 
 
 ley. (Photo by author.) recession of such a waterfall, 
 
 remain as a hanging valley on 
 
 the sides of the main gorge. The valley of the Bloody Run on 
 the side of the Niagara Gorge is such a hanging valley, and there 
 are many examples of this type 
 in the gorges of young streams, 
 as for example in the Genesee 
 (Fig. 706). In broad, open 
 valleys, however, such a re- 
 lationship generally points to 
 glacial overdeepening of the 
 main valley. 
 
 FIG. 707. Longitudinal profile of 
 a river valley (a) ; and of the same 
 valley after it has been deepened by a 
 glacier (6). Note that the valley in 
 the second case is deeper above the 
 mouth, so that it would hold a lake 
 after the melting of the glacier. 
 
 Glacial Lakes, Tarns, and 
 Fjords. Glaciers do not 
 deepen their channels in- a uni- 
 form manner any more than rivers do. Near the head of the 
 glacier, as well as near its front, the deepening is, as a rule, to a 
 
Land Forms Due to Glacial Sculpture 803 
 
 lesser degree than between these two points. In consequence, 
 the profile of a glacial trough will be more strongly concave than ' 
 that of a normal river valley (Fig. 707). When such a valley 
 is uncovered by the melting of the ice, a rock tarn or lake will 
 remain behind in the deepened part. This lake may attain addi- 
 tional depth by the building < 
 
 of a morainic dam across the 
 mouth of the valley. Many 
 of the beautiful Scottish lochs 
 are examples of such lake 
 basins of glacial origin, and 
 a similar origin is a,scribable 
 to the Finger Lakes of New 
 York state. 
 
 When the deepening by ice 
 has proceeded to a level be- 
 low that of the sea, or if an 
 over-deepened valley near the 
 coast is carried down by a 
 subsidence of the land, the 
 sea will occupy a part of 
 such a valley, and a fjord is 
 produced. Such fjords are 
 very characteristic of the bold 
 coast of Norway (Figs. 705, 
 708), and they are found in FIG. 708. A typical Norwegian fjord, 
 Scotland and elsewhere as showing U-shaped glacial trough and the 
 11 rrii TT j T>- relatively flat upland of the Norwegian 
 
 well. The Hudson River penep]an ^ (From D. W. Johnson's sL* 
 (Fig. 709) has the character Processes, etc. John Wiley and Sons.) 
 of a fjord, the depth being 
 
 over 600 feet from water level to rock bottom in the Highlands, 
 whereas opposite New York City, it is only somewhat over 300 feet. 
 That this is not due to differential subsidence or downward bowing 
 of the land is shown by the regular rise of the crest-line of the 
 gorge (the surface of the old peneplane), for any unequal sub- 
 sidence affecting the river bottom would also affect this crest-line. 
 The relationship is shown in the diagram on page 804 (Fig. 710). 
 
 There are many drowned river valleys along the Maine coast, 
 but it is not known whether these have the characteristics of true 
 fjords, that is, whether they are deepened by glacial erosion, less at 
 
804 The Sculpturing of the Earth's Surface 
 
 the mouth than some distance back of it (Fig. 711). A map of a 
 typical fjorded coast, that of New Zealand, is shown in Fig. 712. 
 
 The Tinds and Nunataks. Around the margins of ice caps 
 which cover elevated tracts of country, a special form of erosion 
 takes place. Where the ice spills through notches in the rock 
 
 FIG. 709. The gorge of the Hudson in the Highlands. North of West Point. 
 
 margin, active erosion goes on at those points, and the result is 
 often the production of a conical rock hill by the reduction of a 
 mass of rock enclosed by the spill-overs of such an ice cap. So 
 long as such a mass is enclosed by ice, above which it projects, it 
 is called a nunafak, as are all rock-masses projecting above the 
 
 FIG. 710. Profile of the Hudson River from New York City through the 
 Highlands, showing the fjord-like excavation above the mouth and the gradual 
 northward rise of the peneplane. 
 
 marginal portion of the ice-field. When freed by the melting of 
 the ice, such a conical hill due to glacial erosion is called a tind. 
 Unlike the conical hills or horns produced at the head of the 
 mountain glaciers by sapping, the tind is produced at the lower 
 end of the glaciers by lateral erosion. Such tinds are fre- 
 quently met with in higher latitudes where ice-caps are still in 
 existence. 
 
The Sculpturing of the Edge of the Land 805 
 
 Erosive Work of Valley and Mountain Glaciers Contrasted with That 
 of Ice-Caps and Continental Ice-Sheets 
 
 Valley and mountain glaciers tend to accentuate the relief of 
 the country, especially in their middle and upper courses. Ice- 
 caps and continental glaciers, on the other hand, tend to plane 
 down the relief of the country which they cover and produce an 
 accentuation of the topography only at their margins. 
 
 % 
 
 THE SCULPTURING OF THE EDGE OF THE LAND 
 
 The edge of the land, that is, the sea-coast and the coasts of large 
 lakes, are subjected to a sculpturing process by the waves and 
 
 FIG. 711. Map of a section of the coast of Maine, showing the topography 
 produced by drowning of river valleys. 
 
806 The Sculpturing of the Earth's Surface 
 
 currents, with the production of special erosion forms, the more 
 important of which we may consider. 
 
 Young and Mature Coasts. A young coast-line is one but 
 recently established by the relative change in the level of land 
 and sea. Such a coast-line is a regular one if it is formed upon a 
 young surface of a recently emerged sea or lake-bottom, for in such 
 a case the coast marks merely the intersection of two planes, that 
 
 FIG. 712. Fjord coast of Dusky Sound, New Zealand. A young coast- 
 line upon a strongly dissected land surface. (British Admiralty Chart, from 
 Ratzel, Die Erde.) 
 
 of the sea and that of the young coastal plain. A young coast- 
 line upon an old land surface, i.e., a peneplane, will also be on 
 the whole fairly regular, though such regularity will be less pro- 
 nounced than in the case of a young land. But a young coast-line 
 formed upon a maturely dissected land surface will be one of ex- 
 treme irregularity, such as is shown, for example, on the Maine 
 coast of New England (Fig. 711), or the coasts of Norway, Sweden, 
 and elsewhere (Fig. 712). A mature coast-line on any land surface 
 tends toward regularity of outline, for the projecting headlands will 
 
The Sculpturing of the Edge of the Land 807 
 
 be cut back and the reentrants will be bridged by the formation 
 of bars and beaches. 
 
 Erosion on the Coast of a Young Land. A newly emerged coastal 
 plain of gently seaward-dipping strata will not immediately be 
 effectively attacked by the waves, because the water near the shore 
 is as a rule too shallow for vigorous wave erosion. In such a 
 case, an off-shore bar will be built first, at the line where the shoal- 
 ing determines the breaking of the prevailing storm waves for that 
 
 FIG. 713. Wave-cut chalk cliffs near Fecamp, France. (From D. W. 
 Johnson, Shore Processes, John Wiley and Son.) Note the marine bench 
 in front of the cliffs, only partially covered by the lighter beach sand. 
 
 coast. This bar is built by the eroding of the bottom by the break- 
 ing wave, the hurling forward of the material removed, and its de- 
 position in shallower waters, from which it eventually emerges as 
 an off-shore bar, as already described (p. 330, p. 539). In front of 
 the bar the water has been sufficiently deepened for wave attack, 
 behind the bar a shoaling of the lagoon takes place, with the forma- 
 tion of peat deposits, and eventually, perhaps, a strip of dry new- 
 made coastal land. 
 
 Meanwhile the waves continue to attack the bar, cutting it 
 away, while the sand dunes travel inland over the lagoon deposits. 
 
8o8 The Sculpturing of the Earth's Surface 
 
 FIG. 714. Map of Cape Cod, showing the long outer erosion cliff of the 
 " Forearm," and the sand spits built from the sands thus eroded, both southward 
 and northward, where they form the foundation of the dune headland of Prov- 
 incetown. (After Coast Survey Chart, from Ratzel, Die Erde.) 
 
The Sculpturing of the Edge of the Land 809 
 
 These deposits of peat, etc., now begin to appear upon the shore, 
 and as erosion continues, the entire new-formed series will be 
 removed again. With the advancing sea, the water retains suf- 
 ficient depth for efficient wave work, and when the lagoon deposits 
 have been wholly removed, the original shore will be vigorously 
 attacked, the water now being deep enough for this process. As the 
 sea cuts inland like a horizontal saw, a sea-cliff is produced, which, 
 because of the regularity of the coastal plain, will, other factors re- 
 maining inactive, be uniform and continuous. This cutting into 
 the land may continue indefinitely if the tidal and other currents 
 are strong enough to carry away the product of erosion. If not, 
 a beach is formed at the foot of the cliff, and erosion eventually 
 
 FIG. 715. Work of the waves in cutting away rocky coasts. A. Suc- 
 cessive stages in the destruction of the island of Helgoland. B. Cliffs of Nor- 
 mandy ; the coast has been cut back so far that the tributaries of the old river 
 systems now enter the sea independently. (After Lobeck, in Military Geology.) 
 
 becomes checked by the accumulation of a protecting belt of 
 material. (See Fig. 446, p. 530, and Fig. 447, p. 531.). The bold 
 Yorkshire coast of England, the Channel Coast of France (Fig. 
 713), and the long sand cliffs of the outr arm of Cape Cod on 
 the Massachusetts coast (Fig. 714) illustrate the continued cutting 
 of the sea into the land in regions which, in many respects, have 
 the characteristics of coastal plain strata. One striking effect of 
 such inward cutting on the coast is the betrunking of the streams 
 which formerly ran down the slope of the coastal plain into the 
 sea. Eventually, the main stream may be cut back so far that 
 its former branches enter the sea as independent streams. Good 
 illustrations of this are furnished by the Channel Coast of France. 
 (See Fig. 715, B.) 
 
8io The Sculpturing of the Earth's Surface 
 
 Erosion of Coasts on Maturely Dissected Lands. The coast 
 of Maine and that of Sweden present typical examples of a relatively 
 young coastline upon a maturely dissected drowned land. The 
 numerous parallel river valleys have been converted into inden- 
 tations, and the ridges between the valleys project into the sea 
 as long spurs or rock tongues, and by the submergence of low places 
 across them, numerous islands, aligned with the ridges, are pro- 
 duced. Travel along such a coast is possible only by boats ; all 
 roads and railroads must be placed far inland. Erosion (Fig. 711, 
 p. 805) is restricted to the clifnng of the headlands and islands, 
 
 FIG. 716. Marblehead Neck, Mass. A rocky island tied to the mainland 
 by a narrow beach (or tombolo) of cobblestones and sand. (From D. W. John- 
 son's Shore Processes, John Wiley and Sons.) 
 
 but at the same time, the narrower and shallower indentations 
 are bridged across by ^nd and gravel bars built by the waves, 
 and rocky islands are tied to the main land by sand and pebble 
 beaches (Fig. 716). Thus the coast is straightened, but the process 
 is necessarily a very slow one, and a change of level may occur be- 
 fore it has proceeded far. If elevation takes place, there will 
 appear abandoned sea-scarps inland, as well as abandoned beaches. 
 Where a normally dissected coastal plain is drowned, as on the 
 coast of Maryland and New Jersey, the sea will enter the con- 
 sequent river channels and wave erosion will widen the bays thus 
 formed, as in the case of Chesapeake and other bays on the 
 
The Sculpturing of the Edge of the Land 811 
 
 Atlantic coastal plain.. Where drowning has gone so far that the 
 inner lowland is submerged, the cuesta of the coastal plain may be 
 broken into islands, Long Island, New York, being an example of 
 this. Here wave attack produces cliffs along the northern coast, 
 which is the drowned inface of the cuesta, but on the southern 
 coast, where the submergence has produced a new intersection of 
 the sloping coastal plain and glacial outwash strata with the sea, 
 erosion is preceded by bar-building and lagoon-filling. 
 
 FIG. 717. Sea-caves on the coast of California. An upper cave is 
 shown which was cut when the land stood ten feet lower with reference to sea- 
 level. (Photo by G. W. Stose, from U. S. G. S.) 
 
 Special Erosion Features 
 
 Stratified and Jointed Rocks. Vigorous wave erosion on hori- 
 zontally stratified rocks is apt to produce vertical cliffs, especially 
 if the beds at the base are weak. By the attack of the waves sea- 
 caves may be produced on softer strata (Figs. 717, 718), or by 
 extensive undermining harder beds may form overhanging ledges 
 until they break down from overweight. The formation^of sea- 
 caves is especially favored by joints which traverse the rocks 
 vertically and permit the waves to cut along them. On the north 
 shore of Lake Superior, an extensive series of such wave-cut caves 
 
812 The Sculpturing of the Earth's Surface 
 
 was formed in the soft sandstone of the Pictured Rocks. Many of 
 these caves have since collapsed, but new ones are constantly form- 
 
 FIG. 718. Wave-cut arch known as the Natural Bridge one half mile north 
 of mouth of Medder Creek, Santa Cruz County, California. (U. S. G. S.) 
 
 / 
 
 ing. Caves of this kind are not uncommon on the British and 
 French coasts, and in the past some of these served as the haunts 
 of smugglers (Figs. 719 a, 6). They are cut in other jointed rocks 
 
 FIG 719 a Tilly Whim Caves. Elevated sea-caves cut by waves in hori- 
 zontal ' (Jurassic) strata. Coast of English Channel, Durlston Head, south of 
 Swanage, England. 
 
The Sculpturing of the Edge of the Land 813 
 
 as well, good examples being found on the basaltic coast of Nova 
 Scotia, while Fingal's Cave on the Island of Staffa, west coast of 
 
 FIG. 719 b. Tilly Whim Caves. Caves due to marine erosion in horizontal 
 (Jurassic) rocks, coast south of Swanage, England. 
 
 Scotland, is a classic example of a wave-cut cavern of consider- 
 able depth in a jointed basaltic rock (Figs. 720, 721, see also Figs. 
 122 a-d, pages 178 to 179). 
 Caverns of this type are 
 often found at some eleva- 
 tion above sea-level, as on 
 
 FIG. 720. Fingal's Cave. 
 Isle of Staffa, off west coast 
 of Scotland. Looking seaward 
 from within the cave. (Photo by 
 author.) 
 
 FIG. 721. Interior view of Fingal's 
 Cave, a sea-cave in columnar lava on the 
 Island of Staffa, Scotland. (From D. W. 
 Johnson's Shore Processes, John Wiley and 
 Sons.) 
 
814 The Sculpturing of the Earth's Surface 
 
 the coast of the Island of Gotland in the Baltic, on the coast of 
 Mackinac Island in Lake Huron (Fig. 722), and elsewhere. They 
 indicate an elevation of the land or a subsidence of the water-level 
 since their formation. (See also Fig. 717, p. 811.) 
 
 When joints are numerous and traverse the rock mass through- 
 out, a series of ramparts may be produced, such a character being 
 
 FIG. 722. The Arch or Natural Bridge cut from brecciated limestone 
 (Upper Silurian) by the waves of Lake Huron during a former higher level. 
 The arch is the remnant of the roof of a wave-cut cavern. 
 
 especially well shown in the jointed sandstones along Cayuga 
 Lake, New York (Fig. 564, p. 639). Isolated masses or sea-stacks 
 may also be severed from the mainland in this manner. Such 
 structures are especially marked on the coast of Scotland and the 
 Orkney Islands, where the jointed and nearly horizontal Old Red 
 Sandstone cliffs are rapidly succumbing to wave attack. Similar 
 sea-stacks are formed (Figs. 723 a, b) from the chalk on the English 
 coast and on the French coast (see Fig. 446, p. 530), and from the 
 
The Sculpturing of the Edge of the Land 815 
 
 basalt on the coast of Nova Scotia. Several ancient stacks are 
 found on the northern shore of Lake Michigan, Lake Huron, and 
 on Mackinac Island, marking a former higher level of the water. 
 
 FIG. 723 a. "Old Harry" and "Old Harry's wife." Sea-stacks cut by 
 the waves of the English Channel from jointed chalk cliffs. The Foreland, 
 north of Swanage, England. 
 
 They also abound in places on the coast of the island of Gotland at 
 a level coinciding with that of the abandoned sea-caves. 
 
 Where stratified rocks are steeply inclined, their erosion on the 
 
 FlG. 723^6. "Old Harry Rocks" and chalk cliff. Attacked by storm waves. 
 Coast north of Swanage, England. 
 
816 The Sculpturing of the Earth's Surface 
 
 coast produces a series of " skerries " or reef-like ridges formed by 
 the harder beds ; when submerged they form dangerous shoals. 
 
 Igneous Rocks. Except when fine-grained and much jointed, 
 igneous rocks present a less favorable medium for cliff-cutting 
 than do the stratified rocks. Wherever a .fairly coarse granite 
 
 forms the coast, wave 
 
 erosion does not keep 
 pace with the weathering 
 of exposed parts of the 
 mass, and the surface will 
 thus often be rounded 
 rather than abrupt. A 
 part of this rounding 
 may also be due to pre- 
 vious glaciation, as on 
 our northern Atlantic 
 coast, where the topog- 
 raphy produced by the 
 ice has not yet been de- 
 stroyed by wave erosion. 
 Such masses, if large, 
 form a subdued rocky 
 coast, even though ex- 
 posed to very violent 
 wave attack (Fig. 356 a, 
 p. 429). Basaltic rocks, 
 on the other hand, be- 
 cause of their excellent 
 jointing, produce frown- 
 
 Bras. 
 
 FIG. 724. Purgatory Chasm, Newport, 
 R. I. A cleft in the conglomerate rocks due 
 to erosion by the waves of a weathered zone 
 between joint cracks. It simulates a chasm 
 left by the erosion of a dike, and was formerly 
 regarded as such. 
 
 ing cliffs, those of Nova 
 Scotia and farther north, 
 and those of the west 
 Scottish coast, being 
 typical examples. 
 
 Erosion of Dikes. Along many of the cliffed portions of the 
 New England and the British coasts, basaltic or diabase dikes 
 which cut other strata and are exposed to wave attack have been 
 removed by the loosening of the successive joint blocks into which 
 such dikes are generally divided. As a result, a deep, narrow 
 chasm is formed, with vertical sides if the dike was vertical (see 
 
The Sculpturing of the Edge of the Land 817 
 
 
 FIG. 725. Winthrop Great Head, a drumlin near Boston, whose eastern 
 end has been removed by wave erosion. In the foreground is a beach built 
 from the eroded material. (Photo by D. W. Johnson.) 
 
 Fig. 135, p. 192), and at the end of such a chasm is often found a 
 cavern-like hollow into which the waves dash, compressing the air, 
 whereupon the water is forced put again with great force, producing 
 various forms of waterspouts. When the jointing of the dike is not 
 well developed, it is not so 
 readily eroded, especially 
 when the surface is gently 
 sloping, in which case the 
 surface of the dike will be 
 even with that of the en- 
 closing rock. Similar fis- 
 sures may, however, be 
 eroded in massive much- 
 jointed rocks, along a 
 weak zone, as is well il- 
 lustrated by the chasm 
 called Purgatory on the 
 coast at Newport, R. I., 
 which is worn along a 
 
 weak zone in massive FlG ?26 . _ Grover>s cliff) winthrop . An 
 conglomerate (Fig. 724). eroded drumlin on the Massachusetts coast. 
 
8i8 The Sculpturing of the Earth's Surface 
 
 Erosion of Glacial Deposits. When unconsolidated glacial 
 deposits are exposed along the shore, erosion by waves progresses 
 
 on the whole very rapidly 
 until it becomes checked by 
 the accumulation of boul- 
 ders which are left behind 
 during the removal of the 
 finer material. This is es- 
 pecially well shown where 
 the terminal moraine of the 
 last ice age is exposed upon 
 the shore, as is the case 
 along the south coast of 
 Massachusetts at Woods 
 Hole and the Elizabeth 
 Islands and on parts of the 
 shore of Long Island. In 
 these regions the beach at 
 low tide presents an accu- 
 mulation of large boulders 
 which are not readily moved 
 
 FIG. 727. Development of Nantas- by the waves. Drumlins 
 
 ket Beach. (After Johnson and Reed) , h t fe ^ 
 First stage, original drumlms restored. 
 
 Restoration in dotted lines. are apt to be fronted by a 
 
 AL. Allerton Lost Drumlin boulder pavement (Fig. 447, 
 
 AtL. Atlantic Lost Drumlin p ^ T \ wn i c h greatly re- 
 
 BL. Bayside Lost Drumlin ^ , ,, 
 
 BI. Bumkin Island tards further erosion - 
 
 G. Great Hill Drumlins cliffed by the 
 
 H. Hampton Hill sea present a very charac- 
 
 LHI. Little Hog Island teristic P rofile > which varies 
 
 N. Nantasket Hill with the distance to which 
 
 Q. Quarter Ledge the drumlin has been cut 
 
 Sa. Sagamore Head , , ,. ,, TT , 
 
 Sk. Skull Head back - Wmthrop Head on 
 
 SL. Strawberry Lost Drumlin the Massachusetts coast is 
 
 St. Strawberry Hill a drumlin half of which has 
 
 T. Thornbush Hill , ' , 
 
 W. White Head been removed by the sea. 
 
 WL. White Head Lost Drumlin The profile there formed is 
 
 WP. Windmill Point Sand Spit shown in the illustration 
 (Fig. 725). A near view of the profile of an eroded drumlin cliff 
 from the same coast is shown in Fig. 726. The sands worn from 
 
The Sculpturing of the Edge of the Land 819 
 
 the drumlins are partly used in the building of the beach shown 
 in the foreground of Fig. 725. Because of the abundant supply of 
 such sands, extensive bars and spits are produced by the long-shore 
 currents and the waves, and these frequently tie rocky islands to 
 the main shore, as in the case of Nahant, which is tied by a long 
 sand beach to the Massachusetts coast at Lynn. Formerly isolated 
 drumlins, more or less eroded by the sea, may thus be tied together 
 
 FIG. 7 28 a. Development of 
 Nantasket Beach. (Johnson and 
 Reed.) 2d stage. Early erosion and 
 connection of some of the drumlins 
 by bars or tombolos. (For notation 
 see Fig. 727.) 
 
 FIG. 728 b. Development of Nan- 
 tasket Beach. (Johnson and Reed.) 
 3d stage. Further connection of 
 eroded drumlins by beaches (tom- 
 bolos). (For notation see Fig. 727.) 
 
 by sand beaches, as has been the case on the Massachusetts coast, 
 in the formation of Nantasket beach. Here beaches have in some 
 cases been built so rapidly that the old cliffs have been protected 
 from further erosion, and thus drumlins in various stages of dis- 
 section, but now some distance inland from the shore, enter into 
 the construction of this remarkable land-mass. The successive 
 stages in the formation of this beach are shown in the maps here 
 reproduced from the studies of D. W. Johnson and W. G. Reed, Jr. 
 
820 The Sculpturing of the Earth's Surface 
 
 (Figs. 727 to 730). Extensive sand spits and bars are also built 
 along the outer coast of Cape Cod (Fig. 714), where the waves 
 cut away the unconsolidated stratified drift of fluvio-glacial origin 
 and carry it both southward and northward. On the north 
 they have built a succession of bars on which the sands have 
 been piled up into dunes by the wind to form the headland of 
 
 FIG. 729 a. Development of Nan- 
 tasket Beach. (Johnson and Reed.) 
 4th stage. Development of beach- 
 ridges and beach-plain. (For nota- 
 tion see Fig. 727.) 
 
 FIG. 729 b. Development of Nan- 
 tasket Beach. (Johnson and Reed.) 
 5th stage. The modern beach, con- 
 sisting of old cliffed drumlins with 
 broad beach-plain in front of it. 
 (Notation as in Fig. 727.) 
 
 Provincetown. The seaward face of the forearm, subject to con- 
 stant wave erosion, presents a continuous and regularly curving 
 line of cliffs with scarcely any break for many miles. Where clays 
 are present in the sands, as at Highland Light, the cliffs present 
 picturesque erosion features due chiefly to rain- wash (Fig. 731). 
 
 EROSION FORMS PRODUCED BY ATMOSPHERIC AGENCIES 
 
 The atmospheric agencies, under which are classed the diurnal 
 and seasonal changes of temperature; the moisture and gases of 
 the air and the activities of frost, rain, and wind, produce erosion 
 
Erosion Forms Produced by Atmospheric Agencies 821 
 
 forms essentially peculiar to themselves. Some of these have 
 already been mentioned, but they may again be briefly referred to, 
 while others not yet noted may be added. Several of these agents 
 generally act in conjunc- 
 tion, and their indi- 
 vidual activities cannot 
 always be dissociated. 
 
 The Cliff and Talus. 
 These are primarily 
 the products of tempera- 
 ture changes and frost 
 work. The fragments 
 broken by these agents 
 from the face of the cliff 
 will accumulate at its 
 foot to form the talus, 
 the slope of which is in 
 general harmonious with 
 the size and other char- 
 acters of the fragments. 
 When not removed by 
 the agents of denuda- 
 tion such a talus may 
 accumulate until it 
 mantles much if not the 
 whole of the cliff-face. 
 Old talus surfaces are 
 generally overgrown with forests if the climate is sufficiently 
 moist. Mountains exposed to much frost action, but not covered 
 by glaciers, are generally characterized by extensive accumula- 
 tions of broken fragments, which may entirely mask the under- 
 lying ledges. This is the case to a considerable extent in the 
 White Mountains of New Hampshire, in Pikes Peak, in Ben Nevis 
 in west Scotland, and in other mountains like these. In some 
 cases these fragments give to the summit a remarkably perfect 
 conical form. 
 
 Sculpturing by Rain and Atmospheric Moisture. The earth pil- 
 lars of Colorado and of Bozen in the Tyrol (Figs. 345 a, b, pp. 41 1 , 41 2) 
 have already been referred to as products of rain erosion. Where 
 cliffs are composed of soft clays or shales, or of soluble material, 
 
 FIG. 730. Development of Nantasket 
 Beach. (Johnson and Reed.) A hypothetical 
 future stage. The outer drumlins are all worn 
 away and the beach extends from the rocky 
 ridges at Nantasket (Atlantic) to World's End 
 drumlin (WE} ; thence to Bumkin Island 
 (BI) and thence to Nantasket Hill (JV) in Hull. 
 (For notation see Fig. 727.) 
 
822 The Sculpturing of the Earth's Surface 
 
 fantastic shapes will be carved by the rain (Fig. 731) or produced 
 by a process of solution, this being especially marked in certain 
 elevated limestone peaks in which, if the strata are inclined, a strik- 
 ingly fretted upland surface is produced (Fig. 346 b, p. 413). Re- 
 markable solution forms are also produced upon the surfaces of 
 salt mountains, such as those of Cardona, Spain, and of southern 
 Persia. The salt pillars of the Dead Sea, which have given rise to the 
 
 legend of the transfor- 
 mation of Lot's wife into 
 salt, are likewise the 
 products of subaerial 
 sculpture upon an an- 
 cient cliff of salt. 
 
 Sculpturing Process of 
 
 
 
 
 FIG. 731. Clay cliffs at Highland Light, 
 Truro, on Cape Cod, showing characteristic 
 erosion forms due to rain and wind. 
 
 Wind. The sculptur- 
 ing process of wind com- 
 prises both deflation of 
 material loosened under 
 the influence of the 
 weather, and active cor- 
 rasion of the rock surface 
 1 by the sand-blast. By 
 
 the combined action of these processes, wind-carved structures, 
 often of remarkable form, are produced. We have already referred 
 to the faceted pebbles or " dreikanter " which represent faces 
 carved by the -sand-blast upon partly exposed pebbles in regions 
 of much sand-drifting (p. 406). But structures of a much larger 
 type are also produced by wind erosion. Such are the natural 
 monuments carved from soft sandstone of uniform grain in the 
 plains about Pikes Peak, Colorado, and which are generally capped 
 by a projecting mass of more resistant sandstone due to cementa- 
 tion of the grains by iron oxide (Fig. 340, p. 1 407). The great 
 sandstone buttes which dominate the Plains country in various 
 regions in western North America (Fig. 344, p. 410, and Fig. 599, 
 p. 701) are much larger examples of such erosion remnants left after 
 much of the rock formerly continuous with them had been removed 
 by stream and by eolian erosion. 
 
 But by far the most striking products of stream and eolian 
 erosion, supplemented in part by other agencies of denudation, 
 are seen in the wonderful sandstone arches or natural bridges 
 
Erosion Forms Produced by Atmospheric Agencies 823 
 
 found in several localities within the semi-arid district of southern 
 Utah and other regions (Fig. 732). One of these, the Rainbow or 
 
 FIG. 732. Natural Bridge, or sandstone arch formed by stream and wind 
 erosion. Utah. (F. J. Pack, photo.) 
 
 " Barohoini " Natural bridge of southern Utah, has a height of 
 398 feet, a width between abutments of 278 feet, and is 33 feet 
 
 FIG. 733. Looking-Glass Rock; a mass of sandstone carved and hollowed 
 by erosion. The size is indicated by th'at of the white horse in the left fore- 
 ground, and that of the two men in the rear opening. Near La Sal Mountains, 
 Utah. (Whitman Cross, photo; from U. S. G. S.) 
 
 wide at the top. Another rock-form due chiefly to wind erosion 
 is seen in Looking-Glass Rock near La Sal Mountains, Utah 
 (Fig- 733)- 
 
824 The Sculpturing of the Earth's Surface 
 
 In arid regions these products of subaerial erosion are numerous 
 and present such a bewildering series of fantastic shapes that even 
 the most prosaic are beguiled into comparisons with organic forms, 
 while the more imaginative here find the prototypes of all the fanci- 
 ful* creations of fairy-tale and folk-lore. 
 
 The sculpturing processes create the landscape as we see it; 
 they supplement the constructional and deformational processes, 
 and it is they that are primarily responsible for the diversified 
 nature of the earth's surface, and for the great variety of habitats 
 in which the multifarious .types of plant and animal life find a 
 momentary place of occupancy, during the ceaseless procession 
 of organic forms upon the earth. Finally, it is the sculpturing 
 processes which bring about the never-ceasing changes in the 
 contours of the face of the earth, which like the changing human 
 countenance is never the same from day to day, though we, with 
 our limited power of vision, are able to perceive only the more 
 pronounced and abrupt of these modifications. 
 
 The hills are shadows, and they flow 
 
 From form to form, and nothing stands ; 
 They melt like mist, the solid lands, 
 
 Like clouds they shape themselves and go. 
 
 FIG. 734. Alexander von Humboldt. The first great leader in the study 
 of land forms, their origin, and the adaptation of organisms to them. (From the 
 portrait by Schrader. Guyot's Physical Geography.) 
 
INDEX 
 
 Figures in italics refer to pages where descriptions or definitions are given ; figures 
 followed by an asterisk (*) refer to pages on which illustrations are given. Generic 
 and specific names are in italics. 
 
 Aar glacier, experiments on, 374 
 
 Aar massif, 203 
 
 Abaya Lake, Africa, on map, 622* 
 
 Ablation, 391, 392 ; features of glaciers, 376 
 
 Abrasion, 391; by rivers, 414; by waves, 
 
 429 ; of glacial material, 433 ; of sand 
 
 grains, 406 
 
 Absolute humidity of air, 355 
 Abyssal, district, 515, 518; igneous masses, 
 
 204 ; igneous rock, 84 
 Abyssolith, 212 
 
 Accessory minerals of granite, 97 ; of ig- 
 neous rocks, 94 
 Acervularia, 285* 
 Acid feldspars, 91 ; glasses, 98 ; lavas, 122 ; 
 
 magmas, 86, 87, 88; taste, 50 
 Acidic rocks, minerals of, 94 
 Acids, 40; ionization of, 41 
 Adamantine luster, 49 
 Aden, Gulf of, 515 
 Adirondack Mts., 64, 204; anorthosites 
 
 from, 105 ; rock exposures in, 34 ; 
 
 sections from, 31 
 Adji-darja, see Kara Bugas 
 Mchmina,, 318* 
 ^olian Islands, in 
 Afdjada Lake, Africa, on map, 632* 
 Africa, dunes of, 450; marbles of, 654; 
 
 rift valleys of, 1 1 1 *, 63 1 
 African swamps, 341 
 After- vibrations, in earthquakes, 658* 
 Agassiz, Alexander, 14 ; on reef origin, 296 
 Agassiz Glacier, 383 
 Agassiz, L., cited on reefs, 300; experiments 
 
 by, on motion of glaciers, 374 
 Agassiz, L., and Charpentier, on movement 
 
 of glaciers, 389 
 Agate, 404 
 
 Age, relative, of river and land, 707 
 Agents of clastation, 76 
 
 Agglomerate, volcanic, 432, 577* 
 
 Aggrading of basins in arid regions, 707 
 
 Agnostus limestone, 319* 
 
 Ajusco group, 112 
 
 Akaba, Gulf of, 517*, 631, 754, 755*, 757 
 
 Akron dolomite, disconformity above, 616* 
 
 Alabama, coastal plane of, 714; River, 715 
 
 Alabaster, 220 
 
 Alaskan glaciers, 359*, 371*, 376*, 377*, 
 
 378*, 379*, 38i*-384* 
 Albert Edward Lake, Africa, on map, 632* 
 Albert Lake, Africa, on map, 632* 
 Albert Lake, Oregon, 358, 633* 
 Albite, 91 
 Albertite, 351 
 Alders, in swamps, 336 
 Aletsch Glacier, 358, 359; map, 360*, 361*, 
 
 364 
 Aletschhorn, 358, 361, 366, 797 ; chonolith 
 
 of, 203 
 
 Aletsch neve, 364, 368 
 Aleutian Islands, 109 
 Algae, 72, 272*, 273*, 274*, 323 ; and algous 
 
 limestone, 271 ; as source of petroleum, 
 
 352; deposits by, 333; in lakes, 334 
 Algal Lake, section of, 347* 
 Algonkian strata, folded, 583* 
 Algonquin Lake, map of, 769* 
 Alkaline, lakes, 258; taste, 50; water, 214 
 Alleghany Plateau, 703, 728, 775 
 Allier River, 144 
 Allochthonous deposits, 345 
 Alluvial cone, 465* 
 Alluvial fan, deposits of, 79, 800*; and 
 
 plains, 464 ; modern examples of, 467 
 Alma, Michigan, 731 
 Alpine, folds, origin of structure of, 607 ; 
 
 folds, thickness of, 598; geosynclines, 
 
 619; glaciers, 358; overthrusts, 630*; 
 
 system of folds, 603 
 
 825 
 
826 
 
 Index 
 
 Alps, 590, 595, 607, 609, 610, 741 ; de- 
 scribed, 606 ; gneisses in, 653 ; on map, 
 605 * ; marbles of, 653 ; metamorphic 
 rocks of, 649 ; rock exposures of, 33 ; 
 shortening by folds in, 619 
 
 Alsace-Lorraine, petroleum of, 352 
 
 Alsek Glacier, 384 
 
 Altai Lake, 258 
 
 Alteration products of organic slime, 349 
 
 Alteration stages of vegetal deposits, 329 
 
 Altitude, criterion of dissection, 702 
 
 Aluminum minerals, 57, $8 
 
 Alunite, 58 
 
 Amaltheus, 314* 
 
 Amazon River, red muds from, 551 ; valley 
 swamps of, 341 
 
 Amber, 61 
 
 America, rock exposures in, 34 
 
 American Falls, Niagara, 418*, 764, 773, 
 774, 775; crest line, 774* ; map, 763*; 
 section of, 772* 
 
 Amethyst Mountain, 196* 
 
 Amiens, on map, 730* 
 
 Ammonites, 3 13,. 3 14* 
 
 Amorphous texture, 217 
 
 Amphibole, 62, 93 ; distinctive form and 
 cleavage of, 94* 
 
 Amphibolite, 105; described, 652 
 
 Amplitude of vibrations of earthquake 
 waves, 66 1 
 
 Amu-darya, on map, 449* 
 
 Amundsen, cited, 387 
 
 Amygdaloid, 107* 
 
 Amygdaloidal structure, 106 
 
 Anamorphism, 646 
 
 Ancient reefs, 305 
 
 Andalusite, 62, 211 
 
 Andes Mts., 674 ; altitude of snow line in, 
 357; andesite from, 104; solfataric 
 vents of, 181 
 
 Andesine, 91 
 
 Andesite, 96, 104 ; obsidian, 96, 104, 105 ; 
 perlite, 96, 104; porphyry, 96, 104; 
 pumice, 96, 104 
 
 Anemoclastic rock, 77, 83 
 
 Anemoclastics, $68 
 
 Angle of repose, 396 
 
 Anglesite, 54 
 
 Anhydrite, 59, 220, 221; in caliche, 259; 
 order of deposition of, 235 
 
 Animal life, effects of condensation of sea 
 water-on, 234 
 
 Animal tissues, deposits of decaying, 346 
 
 Animals as rock-breakers, 81 
 
 Anorthoclase, 91 
 
 Anorthosite, 105 
 
 Anorthite, 90, 91 
 
 Antarctic, ice-sheet, 387; mass, 510; 
 Ocean, diatom ooze of", 323 
 
 Antecedent stream, 743 
 
 Anthracite coal, 344, 654 
 
 Anticlinal valleys, 736* ; development of, 
 733* 
 
 Anticline, 589, 59o*~596*; between 
 basins, 603*; erosion cycle on, 732 
 
 Anticlinoria, 595* 
 
 Anti-epicenter, 658* 
 
 Antillean volcanoes, 104 
 
 Antilles, radiolarian ooze of, 326 
 
 Antimony minerals, 54 
 
 Antique marble, 654 
 
 Anti-Taurus range, on map, 605* 
 
 Antrim, Ireland, sections on coast of, 170* 
 
 Ants, work of, 437 
 
 Apatite, 61, 222 
 
 Apennine Mountains, 24; on map, 605* 
 
 Aphroena, 122 
 
 Aplite dikes, 192 
 
 Apophyllite, 63 
 
 Apophyses, 206 
 
 Appalachia, 60 1 
 
 Appalachian, belt, marbles of, 653; fold- 
 ing, 602 ; folds, diagrams of, 603 ; folds, 
 relations of to domes and basins, 603; 
 map, 604* 
 
 Appalachian Mts., 590, 593, 596, 645, 698 ; 
 described, 60 i ; rock exposures in, 34 ; 
 shortening by folds in, 618 ; topography, 
 development of, 737* 
 
 Appalachians, 609 ; gneisses in, 653 ; 
 northern, map of, 738* 
 
 Applied geology, 20 
 
 Apron-plain, 502 
 
 Aqueous elastics, 80; precipitates, classi- 
 fication of, 217 
 
 Aqueous rocks, 70-73, 214; mode of oc- 
 currence of, 227 ; principal types of, 220 ; 
 textures of, 217 
 
 Araba, graben, 754; vale of, 631, 754; 
 map, 755* 
 
 Arabia, dunes of, 450 
 
 Arabian peninsula, Miliolitic limestone of, 
 278 
 
 Aragonite, 50, 221 
 
 Area, 310*, 311 ; at Pozzuoli, 692 
 
 Architecture of earth's crust, 18 
 
Index 
 
 827 
 
 Arctic Ocean, 510; map of, 511* 
 Arctic regions, coral reefs in, 290 
 Ardennes Mts., 731 ; effect of uplift of, on 
 
 Meuse, 743 ; on map, 730 
 Aren Glacier, 364 
 Arenaceous texture, 560 
 Arenyte, 569, 577, 578 
 Arete, 793, 794, 795*, 796, 800* 
 Argentite, 57 
 Argillaceous, 570 
 
 Argillaceous odor, 50 ; sandstone, 578 
 Argillite, 570, 650 
 Argillutytes, described, 580 
 Argonne. Forest, on map, 730* 
 Arid region, 442* 
 Aristotle, 29 
 Arizona, dunes in, 451; earthquake jeffects 
 
 in, 678 
 
 Arkansas River, 471 
 Arkose conglomerate, 577 ; sandstone, 569, 
 
 579 
 
 Armenia, soda lakes of, 259 
 Arran Island, pitchstone of, 99 
 Arrowhead, 334*, 335 
 Arroyos, 411 
 Arsenite minerals, 54 
 Arsenopyrite, 54 
 Artesian, conditions in river valley, 479 ; 
 
 springs, 424; system, Dakota, 424*; 
 
 wells, 424 
 Arthur's Seat, 169 
 
 Artificial classification, 66 ; of rocks, 67 
 Arundinaria, 337* 
 Arundo, 337* 
 
 Arve, irregular deposits by, 488, 489* 
 Asaphiscus, 319* 
 Ascending solution, 268 
 Ascutney Mountain, 203 
 Ash rock, described, 580 
 Asia, dunes of, 450 ; salt deserts of, 243 
 Asiatic plains, 698* 
 Asphalt rock, 351 
 Asphaltum, 350, 351 
 Asphalts, 349 
 
 Assam, India, earthquake, described, 683 
 Asterias, 320* 
 Asterism, 50 
 Asthenosphere, 6, 84 ; derivation of term, 
 
 12 
 
 Astraea, 285* 
 Astringent taste, 50 
 A sir aides, 283* 
 Astronomical geology, i 
 
 Astronomy, i, 2 
 
 Asymmetrical anticline, 590*, 591* 
 
 Atacama Desert, 259 
 
 Atlantic coast, raised beaches of, 691 
 
 Atlantic coastal plain, 714; fall line on, 
 
 722 
 Atlantic Highlands, N. J., 78; oxidized 
 
 green sands of, 250 
 Atlantic ocean, 510 
 Atlas ranges, on map, 605* 
 Atmoclastic, 568 
 
 Atmoclastic material, origin of, 77 
 Atmoclastic rocks, 77 ; defined, 83 
 Atmogenic rocks, 70-73 
 Atmology, science of, 13 
 Atmosphere, 4, 5, 70 ; as rock-breaker, 77 ; 
 
 chemical work of, 401 ; contribution of, 
 
 to lithosphere, 10; corrosive work of, 
 
 404 ; derivation of, 12 ; materials of, 354 ; 
 
 operation of, in rock-breaking, 391 ; 
 
 science of, 13 
 
 Atmospheric agents, erosion by, 820 
 Atmospheric moisture, precipitation of, 
 
 356; sculpturing by, 821 
 Atmospheric rocks, 70, 71 
 Atoll, 292, 293*; origin of, 295* 
 Augusta, Ga., 722 
 Australia, dunes of, 450 
 Australian brown-coal, thickness of, 343 
 Autochthonous deposits, 345 
 Autoclastic material, 81 ; rocks, 83 
 Autoclastics, 568 
 Augite, 93, 105; andesite, 96; andesite 
 
 porphyry, 96 ; diorite, 104 ; syenite, 100 
 Augitite, 96, 107 ; porphyry, 76 
 Auvergne volcanic district, map of, 145*, 
 
 165 
 
 Axis of folding, 589 
 Azores, 114; volcanoes of, 109 
 Azov Sea, Bryozoa reefs on border of, 308 
 Azurite, 56 
 
 Backbones, of Long Island and Cape Cod, 
 500 
 
 Back swamps, 479 
 
 Back- wash, 523 
 
 Bacteria, aiding in decay of plant tissues, 
 328; denitrifying, 270; lime-depositing, 
 270, 271; iron-depositing, 258 
 
 Badlands, 699*, 700 
 
 Bahia, Brazil, 260 
 
 Baird Glacier, Alaska, 378, map, 377* 
 
 Bakoni Forest range, on map, 605* 
 
828 
 
 Index 
 
 Baku, mud volcanoes of, 182 
 
 Balanus, 317* 
 
 Bald cypress, 341 
 
 Balkan Mts., on map, 605* 
 
 Baltic provinces of Russia, 563 
 
 Baltic Sea, 513, 516; bars of, 540; dunes 
 
 on, 444, 445 ; salinity of, 228 ; salts of, 
 
 214 
 
 Baltoro Glacier, 371 
 Bandai-San, as type of volcano, 140* ; 
 
 explosion of, 139 
 Banded texture, 218 
 Banff, synclinal fold near, 596* 
 Banff shire, unconformity in, 614* 
 Bar, coastal, 537 
 Bar and barachois, 330* 
 Baraboo, Wis., section beginning at, 32 
 Barachois, 330* 
 Barbadoes, 326 
 Barchanes, 443, 451* 
 Barite, 5Q 
 
 Barium minerals, 59 
 Barnacle, 317* 
 
 Barohoini Natural Bridge, Utah, 823 
 Barrell, J., cited, 6, 616 
 Barrier beach, 331, 537; in Scotland, 532* 
 Barrier reefs, 292 ; origin of, 294* 
 Bars, 807 ; wave built, 810 ; on Cape Cod, 
 
 820 ; produced by longshore currents 
 
 and waves, 819 
 Bar theory of Ochsenius, 241 
 Bartlett Glacier, Alaska, 472* 
 Bary sphere, 16; 166, derivation of term, 
 
 12 
 Basal pinacoids, 45*, 46, 47; monoclinic, 
 
 47*; triclinic, 48*. 
 Basal planes, 42, 43 
 Basalt, 96, 106 ; controversy about nature 
 
 of, 168 
 
 Basalt porphyry, 96, 107 
 Basaltic magma, minerals in, 94 
 Basaltic obsidian, 96 
 Basaltic rocks, erosion features on, 816 ; of 
 
 Kilauea, 119 
 
 Base, 40 ; ionization of, 41 
 Basel, Switzerland, 630 
 Base-level, relation of peneplane to, 705 ; 
 
 local, 706 
 
 Basement rock, 207 
 Basic feldspars, 91 ; glasses, 107 ; magmas 
 
 defined, 86, 87 ; magma, texture of, 88 ; 
 
 rocks, minerals of, 94 
 Basin of Minas, 534 
 
 Basin Ranges, 749, 750, 751 
 
 Basins, 599; erosion cycle in, 722 ; in arid 
 
 regions, relation to base-level, 707; of 
 
 exudation, 386 ; shallow, 602, 729 
 Bastei, Saxony, 409; erosion pillars in, 
 
 407*, 408* 
 Bat guano, 323 
 
 Bath, England, warm springs of, 427 
 Batholiths, 205 
 Bathyal district, 515, 518, 529; zone, 511 ; 
 
 deposits in, 551 
 
 Bathy metric districts, 518, 519* 
 Bauxite, 58, 403 
 Bay, 509 
 Bay-bar, 543* 
 Bay of Fundy, deposits of, 534 ; erosion in, 
 
 428 ; tides of, 525 ; see also Fundy, bay of 
 Beach, 530 ; cusp, 535, 553 ; drift, 522 
 Beaches, 807 ; abandoned, 810 ; hard 
 
 packed, formation of, 431; raised, 
 
 fossiliferous, 533 
 
 Beach-plain, 545 ; of Nantasket, 820 
 Beach-ridges, 545 ; Nantasket, 820 
 Bedford shale, disconformity above, 615* 
 Bed rock covered by mantle -rock, 65* 
 Beheaded streams, 721 
 Beheading in river capture, 720* 
 Beichfirn, 364 
 Belgium, 731 ; chalk of, 279; deformed 
 
 strata of, 583 
 
 Belt, T., on reef origin, 297 
 Ben Nevis, erosion on, 821 
 Bench, marine, 807* 
 Berea sandstone, disconformity beneath, 
 
 615* 
 
 Bergschrund, 366, 368, 492, 794 
 Bering Glacier, 384 
 Berkey, C. P., cited, 75 
 Berlin, oceanographic institute of, 14; 
 
 age of formation beneath, 64 
 Bermuda Islands, 290 ; calcarenyte of, 
 
 569 ; eolian rock of, 77 ; swamp of, 341 
 Bernese Alps, 359 
 Beryl, 62 in pegmatite, 207 
 Betrunking of streams, 809 ; map, 809* 
 Beveling of strata in peneplane, 716, 717* 
 Big Pine, Cal., earthquake effects at, 677 
 Biloculina, 678* 
 Binary granite, 95 
 Bioclastic rock, 82, 83, 568 
 Biogenic rocks, 71, 72, 73, 81, 269 
 Bioliths, 269 
 Biology, 15, 19 
 
Index 
 
 829 
 
 Biosphere, 7, 71 ; as rock-breaker, 81 ; con- 
 tribution of to lithosphere, 10; deriva- 
 tion of term, 12; of former periods, 10; 
 operation of, in rock-breaking, 391 
 
 Biotite, 63, 93; distinction from horn- 
 blende, 95 
 
 Birdfoot delta of Mississippi River, 485, 
 486* 
 
 Biscay, Bay of, map, 514* 
 
 Biscayan dune belt, 446 
 
 Bismite, 54 
 
 Bismuth, 54 
 
 Bismuth minerals, 54 
 
 Bitter Seas, of Suez, 241 ; map of, 242* 
 
 Bitter taste, 50 
 
 Bitumens, origin of, 270 
 
 Bituminous coal, 343, 344 ; odor, 50 
 
 Bivalves, 309, 310* 
 
 Black earth of Russia, 459; map, 460* 
 
 Black Forest, 630, 757 ; on diagram, 756* 
 
 Black Hills, 595, 722, 723*; dome, 725; 
 origin of name, 723 
 
 Black Prairie Valley, 715 
 
 Black Rock Desert, Nev., 481 
 
 Black Sea, 230, 348, 352, 509, 516; de- 
 scribed, 231; salinity of, 228; salt 
 pans of, 232 
 
 Black shales, 580 
 
 Bladderwort, 336 
 
 Blastoids, 319* 
 
 Blister-cone, artificial, 129* 
 
 Block conglomerate, 531 
 
 Block faulting, diagram, 633, 634, 747, 748*, 
 752*; dissection of , 252*; in earthquakes, 
 689 ; in Nevada, map and section, 749*; 
 in Wasatch, 634* 
 
 Block Mts., 634; character of, 747; ero- 
 sion of, 750* 
 
 Bloody Run, 765, 802; on map, 763* 
 
 Blue coral, 300 
 
 Blue ground, of South African diamond 
 fields, 173 
 
 Blue muds, 551 
 
 Bluestone, 74, 75, 578*, 579* 
 
 Boghead, 349 
 
 Bog iron ore, 224, 258 
 
 Bogosloff, 109 
 
 Bogs, 329, 341 
 
 Bohemia, old volcano of, 168 
 
 Botany, 15- 
 
 Bolivia, potash niter of, 259 
 
 Bombs, volcanic, 431 
 
 Bone-breccias, 322 
 
 Bones, deep sea accummulations of, 552 ; 
 of vertebrates, accumulations of, 322 
 
 Bonneville Lake, 243, 254; delta in, 485*; 
 map of, 252*; shore lines of, 253* 
 
 Bony coal, 345 
 
 Borax Lakes, 259 
 
 Borax of Tuscany, 181 
 
 Borax salts, 223 
 
 Bornite, 560 
 
 Boron minerals, 60 
 
 Boscotrecase, destruction of, 129 
 
 Bosses, 205 
 
 Boston Mts., 724 
 
 Botryoidal texture, 218, 219* 
 
 Bottom feeders in the sea, 347 ; work of, 
 247 
 
 Bottom-sets, 484 
 
 Boulder, giant, 501*; movement on hill- 
 side, 399* 
 
 Boulder clay, 497 
 
 Boulder conglomerate, 575* ; residual, 577 ; 
 moraine, 500 
 
 Boulder pavement, 531, 532 
 
 Boulders, glacial, 492 ; formation of, on 
 beaches, 430; glaciated, 497*; of dis- 
 integration, 396*, 397*; of Scottish 
 coast, 532; on coast, 531; on moraine, 
 499 
 
 Bozen, Tyrol, earth pillars of, 412*, 821,, 
 
 Bracciano, Lago, 143* 
 
 Brachiopods, 308, 309* 
 
 Brachy-dome (orthorhombic), 47* 
 
 Brachy-pinacoid (triclinic), 48* 
 
 Brachy -prisms (orthorhombic), 47* 
 
 Brackish water, 214 
 
 Bradyseisms, 691 
 
 Brahmaputra River, 468 ; effects of earth- 
 quake on, 683 
 
 Braided river, map, 477*, 480 
 
 Brain coral-, 287*, 299; on reefs, 291 
 
 Brayman shale, disconformity above, 616* 
 
 Breaking wave, 521*, 522* 
 
 Breccia, def., 569, 576*, 577; fault, 634; 
 collapse, 635 
 
 Breccia ted material, 427 
 
 Brecciation, 427 
 
 Bright Angel Canon, 789 
 
 Brines, 214; deposits of, 259 
 
 British coast, erosion of dikes on, 816; 
 sea-caves on, 812*, 813* 
 
 British Isles, organisms in shallow sea 
 around, 550 
 
 Brittle, denned, 49 
 
8 3 o 
 
 Index 
 
 Brittle-star, 320* 
 
 Bristol, England, 719 
 
 Broichhof, on map, 155* 
 
 Brongniart, A., 25 
 
 Bronx Park, glaciated surface in, 434* 
 
 Bronzite, 105; diabase, 106* 
 
 Brown-coal, 343; quarry, 343* 
 
 Brownstone, 565, 579 
 
 Bryozoa and Bryozoa limestone, 307 
 
 Bryozoa from Ordovician, 308* 
 
 Buch, Leopold von, 25 
 
 Buddha temple, 790 
 
 Buenos Aires, 548 ; on map, 548* 
 
 Buffalo, N. Y., 761, 763, 764, 771 ; ancient 
 earthquake fissure in, 689*; old 
 coral reefs near, 306*; sandstone dike 
 in. 585; section near, 318* 
 
 Buffalo River (ancient), on map, 727* 
 
 Buffon, 25 
 
 Bulk of magma, relation of texture to, 89 
 
 Bulrushes, 334*, 335; in upland bogs, 
 
 243 
 
 Buntsandstein, tracks in, 482* 
 Buoyancy, due to seaweeds, 522 
 Burlington escarpment, 724 
 Butte of sandstone, 410* 
 Bytownite, 91 
 Bysmalith, 203*, 204 
 
 Cadiz, Gulf of, map, 514* .. 
 
 Cairo, rise of Nile at, 480 
 
 Calabrian earthquake, described, 665 ; 
 map of, 666* 
 
 Calais, 731 
 
 Calamine, 53 
 
 Calcarenyte, 569, 580 
 
 Calcareous bioliths, 270 
 
 Calcareous sandstone, 578 
 
 Calcareous tufa, 219*, 252 ; rate of deposi- 
 tion of, 260 
 
 Calcarina, 275* 
 
 Calcilutyte, 572*, 581 
 
 Calcirudyte, 569 
 
 Calcite, 59, 221 
 
 Calcium minerals, 59 
 
 Calderas, extinct, 157, 158 
 
 Calices, 286 
 
 Caliche, 259 
 
 California, dunes in, 451 ; marbles of, 653 
 
 California earthquake, described, 684; 
 displacements in, 687* 
 
 California, Gulf of, 232, 514; map of, 513* 
 
 California Valley, 467, 749 
 
 Cambrian, age of Potsdam sandstone, 75 ; 
 
 trilobite limestone, 318, 319*; of Na- 
 
 hant, 35 
 
 ambridge, Eng., 719 
 Camden, S. C., 722 
 
 amel, foot-prints of, in Sahara, 491 
 Caminguin, 112; described, 114 
 Camptonite, 104 
 Canada, anorthosites from, 105 ; coastal 
 
 sections of, 35 ; granites and gneisses 
 
 of, 75 ; mantle rock of, 65 ; St. Peter 
 
 sandstone of, 75 ; upland bogs of, 342 
 Canadian shield, metamorphic rocks of, 
 
 648 
 
 Canaries, volcanoes of, 109 
 Canaveral, Cape, 544; map of, 546* 
 Cane, 337* 
 Cannel coal, 334, 349 
 Cannon-ball structure, 577 
 Cantabrian Mts., on map, 605* 
 Cantal, 145*, 149; section of, 150* 
 Cape Ann, batholith of, 205 ; moraine of, 
 
 5i 
 
 Cape Cod, apron plain of, 503 ; dunes of, 
 447*; erosion of, 809; map of, 808*; 
 sandspits of. 542 
 
 Cape Henry, Va., dunes of, 448* 
 
 Cape Verde Islands, volcanoes of, 109 
 
 Capri, in 
 
 Capucin, 148 
 
 Caracas, Venezuela, destroyed by earth- 
 quake, 670 
 
 Carbon minerals, 61 
 
 Carbonaceous, 570 
 
 Carbonaceous shales, 580 
 
 Carbonation, 403 
 
 Carbonate of lime, 221 ; deposited by 
 plants, 270; deposited from sea water, 
 246; from fresh-water lakes, 251 
 
 Cardium shells in Kara Bugas, 240 
 
 Cardona, Spain, salt mountain of, 220, 822 
 
 Carex, 336* 
 
 Caribbean Sea, 516 
 
 Carlsbad twins, 97 
 
 Carnellite, 58, 223 
 
 Carnotite, 55 
 
 Carolina, coast dunes of, 447 ; rock ex- 
 posures of mountains of, 34 
 
 Carpathian Mountains, 607, 609; de- 
 scribed, 603; folding of, 606; marbles 
 of, 653; on map, 605*; rock exposures 
 in, 34 
 
 Carpathian oils, 352 
 
Index 
 
 Carrara marble, 653 
 
 Caryophyllia, 284* 
 
 Cascade Mts., 743 
 
 Cascade Pass, glacier of, 795* 
 
 Cascades, Swiss, glacial, 369 
 
 Caspian Sea, 4, 230, 238, 349, 509, 516; 
 
 characters of, 231; map of, 232*; 
 
 section of, 240; sub-sea level of, 706 
 Cassel, 756 
 Cassiterite, 53 
 Cat-tails, 335 
 
 Catacombs of Salzburg, 564* 
 Catania, lava of ^Etna at, 135 
 Catenary curves in glacial trough, 799* 
 Cathedral spires, 635, 637* 
 Catskill aqueduct, geological advice in 
 
 construction of, 21 
 Catskill Mountains, 74, 583, 728 
 Caucasus Mts., 741 ; cross section of, 
 
 608; oils of, 352; rock exposures in, 
 
 34 
 
 Caustoliths, 270 
 Cave deposits, 77 
 Cave-diagram, 425* 
 
 Cave of the Winds, 772* ; origin of, 417 
 Caves, lime deposits in, 262 
 Cavern breccias, 427 
 Caverns, bone deposits in, 322 
 Cavity-filled ore deposits, 266 
 Cawlinia, 313* 
 Cayuga Lake, N. Y., erosion of jointed 
 
 rocks on, 639*, 814 
 Cedar Breaks,' Utah, erosion monuments 
 
 in, 408* 
 
 Cedar swamps, 341 
 Celestite, 59 
 Cellulose, 269, 328 
 Cementation, induration by, 565 
 Centrosphere, 6, 16; derivation of term, 
 
 12 
 
 Cephalopods, 313, 314*, 315* 
 Cerargyrite, 57 
 Cerium minerals, 53 
 Cerussite, 54 
 Cette, France, evaporation experiments at, 
 
 230 
 
 Chain coral, 286 
 Chaine du Deves section, 152 
 Chaine des Puys, described, 146 
 Chaine du Velay, 145 ; section, 152 
 Chalcedony, 61, 404 
 Chalcocite, 56 
 Chalcopyrite, 56 
 
 Chalk, concretions in, 574; section, 278*; 
 
 thin section of, 7* 
 Chalk cliffs, 279*; of France, 807*; 
 
 sea-stacks from, 815* 
 Chalk cuesta, England, 719* 
 Challenger Expedition, 14 
 Chamisso, A. von, on reef origin, 296 
 Chamonix valley, 369 
 Champagne, 730*; lowlands, 731 
 Champlain submergence, map, 770* 
 Channel coast, France, erosion of, 809 
 Chara, 272, 273*, 274*, 334, 346 
 Charcoal, 328 
 Charlestown earthquake, described, 678, 
 
 679* 
 
 Charpentier, see Agassiz and Charpentier 
 Chary bdis, 525 
 Chattahoochee River, 715 
 Checker-board fault structure, 757, 758* 
 Chedrang fault, formation of, by Indian 
 
 earthquake, 684 
 Chemical ablation, 391 
 Chemical changes, agents producing, 402 
 Chemical combinations, 39 ; elements, 38 ; 
 
 precipitation of lime in lakes, 256 ; rock 
 
 destruction, 390; sediments, 67; work 
 
 of atmosphere, 401 
 Chemically formed rocks, 70 
 Chemistry, 38 
 
 Cheops' pyramid, 790*; temple, 790 
 Chert concretions, 574 
 Cherty limestone, 225* 
 Chesapeake Bay, 715, 810 
 Chicago, 111., earthquake tremors at, 678; 
 
 rise of Lake Michigan at 695 
 Chi cot Lake, 709; map, 710 
 Chief Mt., Mont., 599*, 627*, 629; thrust, 
 
 628* 
 Chile, desert lakes of , 259 ; earthquakes of, 
 
 672, 673*; saltpeter of, 223 
 Chiltern Hills, England, 719* 
 Chimneys, glacial, 368 
 China, age of coal in, 345 
 Chirotherium tracks, 482* 
 Chrysocolla, 56 
 Chimborazo, 132 
 Chlorite, 63 ; schists, 652 
 Chlorophyll, 354 
 Chonolith, 203 
 Chonos Archipelago, earthquake effects in, 
 
 675 
 
 Chromite, 55 
 Chromium minerals, 55 
 
Index 
 
 Chrysodomus, 312* 
 
 Chrysolite, 62 
 
 Chuar group, 789 
 
 Chunnemugga Ridge, 715 
 
 Cincinnati Dome, 595, 602, 615, 729 
 
 Cinder cone, Ariz., 141 
 
 Cinnabar, 56 
 
 Cirques, 366*, 492, 793, 794*, 795, 800*; 
 cutting of, 367 ; enlargement of, 795, 
 796* 
 
 Cladium, 335* 
 
 Clarke, F. W., cited, 38 
 
 Clarke, J. M., cited, 8 
 
 Classification of minerals, 50 ; principles 
 of, 66 
 
 Clastation, 391, 392 ; agents of, 76 
 
 Clastic deposits in sea, 530 ; sources of, 527 
 
 Clastic material, 78; formation of, 390 
 
 Clastic rocks, 70, 73, 74, 390; classifica- 
 tion of, 566; composition of, 569 ; inter- 
 pretations of, 82*; summary of, 82; 
 texture of, 568; varieties of, 575 _ 
 
 Clastics, consolidation of, 563 ; structures 
 of marine, 553 
 
 Clay, formation of, 403 
 
 Clay -galls, 491 
 
 Clay iron-stone concretion, 226*, 257* 
 
 Clay, percentage in blue muds, 551 ; 
 percentage in green muds, 552 ; per- 
 centage in red muds, 551 
 
 Clay stones, 570, 580 
 
 Clear Lake, Cal., andesite obsidian from, 105 
 
 Cleavage, denned, 48 
 
 Cleveland gulch rock flow, 401* 
 
 Cliergue, Puyde, 149 
 
 Cliffs, as sources of geological information, 
 29; formation of, 821 
 
 Clifton, Ontario, bend of Niagara River at, 
 764 
 
 Clinometer, 585 ; directions for making, 
 585 ; fig. of home made, 586* ; use of, 
 586* 
 
 Clino-pinacoid (monoclinic), 47* 
 
 Clinton iron ore, 225 
 
 Clione, 312* 
 
 Clouds, 356 
 
 Coal, as evidence of former life, 10; de- 
 posits, 344; geology, 21 ; mine, 346*; 
 origin of, 270; seams, 345*; shales, 
 345, 580 
 
 Coastal dunes, 443 
 
 Coastal plain, Atlantic, 714; Alabama, 
 714; denned, 697; drainage system of, 
 
 711, 713*; drowned, 810; erosion cycle 
 
 on, 709; first cycle of erosion on, 716*; 
 
 regressive strata of, 711*; river capture 
 
 in, 720*; transgressive strata of, 711*; 
 
 waterfalls in drainage system of, 721 
 Coast ranges, 467 ; metamorphic rocks of, 
 
 649 
 Coasts, straightening of, 810; young and 
 
 mature, 806 
 Cobaltite, 52 
 Cobalt minerals, 52 
 Cobblestone terrace, 532* 
 Cobleskill limestone, disconformity be- 
 neath, 616* 
 Coccolithophore, 274* 
 Coccoliths, 274 ; deep-sea accumulations 
 
 of, 552; in chalk, 7*, 278* 
 Cochabomba, Bolivia, 250 
 Cockle shells in Kara Bugas, 240 
 Coconino sandstone, 790 
 Col, 364 
 
 Cold feel of gems, 50 
 Colemanite, 60, 223, 224 
 Collapse breccia, 635 
 Color, of minerals, 50 ; play of, 50 
 Cologne region, volcanoes of, 155 
 Colorado Canon, relative size of, 3 ; youth- 
 ful condition of, 708; see also Grand 
 
 Canon 
 
 Colorado delta, mud-cracks on, 480* 
 Colorado Desert, 252; map of, 232* 
 Colorado, earth pillars in, 411*, 821; 
 
 marbles of, 653 ; volcanic neck, 166 
 Colorado plateaus, 600, 703, 704, 791 ; 
 
 diagram of, 787*; eolian rock of, 77; 
 
 map of, 787* 
 Columbia lava plateau, n, 64; basaltic 
 
 structure of, 177; map, 174*, 175* 
 Columbia River, 743; dunes of, 450, 451* 
 Columbus, Ga., 722 
 Columnar structure, development of, 179; 
 
 in basalt, 177, i79*; in obsidian, 179, 
 
 1 80; of dikes, 191* 
 Columnaria, 286 
 Combing wave, 522* 
 Competent beds, 618 
 Complexly folded strata, 741 
 Compound corals, 285 
 Compressive movements, theories of causes 
 
 of, 619 
 
 Concentric exfoliation, 393*, 394* 
 Concepcion Bay, earthquake effects in, 
 
 674, 675 
 
Index 
 
 833 
 
 Conchoidal fracture, 48 
 
 Concrete masses, 217, 218 
 
 Concretionary texture, 218 
 
 Concretions, 572, 573*; of lime, 247 
 
 Conductivity of minerals, 50 
 
 Cone, alluvial, 465* 
 
 Cones, parasitic, Etna, 135*, 136 
 
 Conglomerate, 569, 575*; gneiss, 653 
 
 Connate water, 243, 424 
 
 Connecticut River, course of, 744; flood 
 
 plain of, 474* 
 
 Connecticut Valley, brownstone of, 579 
 Consequent streams, 717 ; downward rate 
 
 of cutting determined by, 717 ; examples 
 
 of, 715 ; radial, 722 
 Constructional forces, 19 
 Contact metamorphism, 193, 206, 643 ; 
 
 kinds of, 208 
 
 Contemporaneous veins, 206 
 Continental deposits, 439 ; glaciers, 385 ; 
 
 ice-sheets, erosion by, 805 ; masses, 3, 
 
 509 
 Continental shelf, 510, 512, 529; depth of 
 
 water, 512 ; etige of, 512 ; map of, 511* 
 Continental water bodies, 4 
 Continents, 509 
 Convection currents, 527 
 Cook Strait, 675, 676* 
 Cooling taste, 50 
 Cooper River delta, 467 
 Copper, 55 ; minerals, 55, 56 
 Copper River, Alaska, 377*, 378 
 Coquina, 311, 312,* 534 
 Coral conglomerate, 577 
 Corallina, 272, 273*, 301 
 Coral polyps, 283* 
 Coral reefs, 83 ; characters and types of, 
 
 290; nullipores on, 271 
 Corals and coral polyps, 282 
 Coral sands and muds, 552; in deep sea, 
 
 277* 
 
 Coral sandstone of Bermuda, 580 
 Corals, destroyed by fish, 437 
 Cordilleran region, gneisses in, 653 
 Corndon Hill, section of, 202 
 Cornwall, Eng., vein of, 267* 
 Corongamite Lake, Australia, 250 
 Corrasion, 391, 392, 405, 822; by rivers, 
 
 414, 421 ; by waves, 429 ; glacial, 433 
 Corrosion, 302 ; by rivers, 414 
 Corrosive work of atmosphere, 409 
 Cortland series, quartz mica diorite in, 103 
 Corundum, 57 
 
 Cosmic space, contribution from, to litho- 
 
 sphere, n 
 Cosmology, i 
 
 Cotswold Hills, England, 719* 
 Cottongrass, 335* ; in upland bogs, 342 
 Country grass (see Corallina), 302 
 Country rock, 27 
 Crabs, 317 
 
 Craspedophyllum, of old reefs, 306 
 Crater Lake, Oregon, origin, 15; view, 
 
 157*; map, 157*, 158*; section, 158*, 
 
 159 
 Craterlets, earthquake, 668, 669*, 672, 
 
 678*, 679, 690 
 Crayfish, 317 
 
 Crazy Mts., Mont., theralite from, 105 
 Creep of soil on hillsides, 401, 402* 
 Cretaceous, coal of, 345 ; green sands of, 
 
 250, 552 
 Crevasses, marginal on glacier, 795; on 
 
 surface, 368, 377 
 
 Crimea, Bryozoa reefs in, 308 ; mud vol- 
 canoes of, 183 
 
 Crimo-Caucasian chain, 605* 
 Crisia, 307* 
 
 Crinoidal limestone, 319, 320, 321* 
 Crinoids, 319, 321* 
 
 Crooked Creek, Cal., meanders of, 709* 
 Crosby, W. O.,. cited, 641; referred to, 51 
 Cross-bedding, 455; of dunes, 453, 454*; 
 
 of river deposits, 487, 488* ; of shallow 
 
 water deposits, 550; river type, 488* 
 Cross section of volcanoes, construction of, 
 
 148 
 Crustacea, 317*, 319*; as bottom feeders, 
 
 347 
 
 Cryolite, 57 
 Cryptozoon, 289 
 Crystal caves of Missouri, 425 
 Crystalline form, 42 
 Crystalline texture, in metamorphic rocks, 
 
 648 
 
 Crystallization, order of, 94 
 Crystallographic axis, 42 
 Cube, 44* 
 Cuesta, 714*, 740*, 741 ; second cycle of 
 
 erosion in, 717*; terraced, 716*, 
 Cuestas of Ontario dome region, block 
 
 diagram, 760* 
 Cup coral, modern, 284* 
 Cuprite, 56 
 
 Current, course of, in rivers, 416* 
 Current ripples, 550* 
 
834 
 
 Index 
 
 Currents of the sea, 519 
 
 Cuspate forelands, 545 
 
 Cut and fill structures, 489 
 
 Cuttle fish, 313, 314* 
 
 Cuvier, L., portrait, 24*, 25 
 
 Cyanite, 62 
 
 Cyatholiths, 274 
 
 Cycle, inauguration of new, 707 
 
 Cycle of erosion, beginning of second, 717 ; 
 
 completion of, 716; second, 707 
 Cycle of formation of clastic rocks, 76 
 Cypress, in swamps, 336 ; stumps, 341 ; 
 
 trees in Lake Drummond, 339* 
 Cypris, 585*, in playa lakes, 483; zone, 
 
 3i8 
 
 Cystoids, 319 
 Cy there, 318* 
 Cytherideris, 318* 
 
 Dacite, 96, 103 ; origin of name, 103 
 Dacite porphyry, 96, 103 
 Dakota artesian system, 424*, 739 
 Dakota sandstone, hog-back of, 723*, 
 
 739*, 740 
 Dale Valley, N. Y., 776, 777; on map, 
 
 776* ; relation of, to Qatka, 778* 
 Daly, R. A., estimate of change in sea- 
 level, 298 ; on reef origin, 297 
 Dana, E. S., ref., 51, 120 
 Dansville, N. Y., 777, 779 ; on map, 776* 
 Dansville Valley, 776, 777*, .780 
 Danzig, Bay of, 549 ; black mud deposits 
 
 in Bay of, 348 
 Darmstadt, 756 
 Darton, N. H., quoted, 787 
 Darwin, C., cited on coral reefs, 294 ; on 
 
 Chilean earthquakes, 674 
 Datolite, 63 
 Daubree, experiments by, on origin of 
 
 volcanic pipes, 164 
 Davidson Glacier, 378 
 Davis, W. M., cited on reefs, 294 ; quoted, 
 
 299 
 
 Daytona Beach, Fla., 431* 
 Dead Sea, 259, 509, 633, 754, 757 ; brine of, 
 
 214; graben, 631; map, 755*; salt 
 
 pillars of, 757, 822 
 Death Valley, Cal., 259 
 Decay of plants, 328 
 Decomposition, 390, 392 
 Dee River, Textularia in, 279 
 Deep sea, deposits of, 552; map, 277*; 
 
 rocks, exposure of, 84 
 
 Deflation, 404, 439, 822; and corrosion, 
 408 
 
 Deformation, by faulting, 619; effects of, 
 582 ; of rocks, 582 
 
 Deformation of rocks, 582 
 
 Deformation structures, types of, 582 
 
 Degrading of basins in arid regions, 707 
 
 Dehna Desert, 451 
 
 Dekkan Plateau, 176; dikes of, 190 
 
 DekkanTrap, 176, 278 
 
 Delaware, coastal plain in, 715 ; Bay, 715 ; 
 river, 741; Water-gap, 741* 
 
 Delta, deposits in, 79 ; glacial, 503 ; ice 
 contact slope of, 504*; origin of, 505*; 
 plain, 485 ; section of, 484 ; mud flats 
 of, 348, 483 ; origin of name, 484 
 
 Deltohedron, 44* 
 
 Dendritic glaciers, 359 
 
 Dendritic tufa, 255*, 256* 
 
 Denmark Straits, ridge, 511* 
 
 Denudation, 391, 392 ; by waves, 414 
 
 Deposition, agents of, 439 
 
 Descending solutions, 268 
 
 Deserts, salts in, 242 ; dunes of, 450 ; 
 El-Tih, 754, 755*; illustration of dy- 
 namic geology in, 36 ; salt deposits, 
 ancient example of, 245" 
 
 Desmarest, N., 25 
 
 Destructional forces, 19 
 
 Detroit River, 769 ; discharge by, 770 
 
 Devil's Hole, 765; Tower, 166, 168*, 
 202 ; Wall, Oschitz, 191 
 
 Devitrified old glasses, 100 
 
 Devonian, flagstone rocks, of, 74; reefs 
 of U. S., 396; rocks of Lake Erie, 31* 
 
 Dew point, 355 
 
 Diabase, 96, 106, 191 ; dikes of, erosion of 
 on New England coast, 816 : of Nahant, 
 35; structure, 106*; texture, 106 
 
 Diagenetic metamorphism, 642 
 
 Diallage, 105 
 
 Diameter of earth, polar, 2 ; equatorial, 2 
 
 Diamond, 61, 344 ; mine, South Africa, 173* 
 
 Diamonds, 329; in old volcanic plug, 173 
 
 Diaspore, 40 
 
 Diastems, 616 
 
 Diatomaceous earth, 323 
 
 Diatomaceous ooze, map of distribution of, 
 277* 
 
 Diatoms, 323*, 346; deep-sea accumula- 
 tions of, 552; fresh water, 334; in 
 Severn estuary, 549 ; silica obtained by, 
 426 
 
Index 
 
 835 
 
 Dihexagonal pyramid, 46* 
 
 Dike chasms, 191; on Cape Ann, 192*; 
 formations of, 428 
 
 Dikes, 89*, 1 88, 189*, 191*; erosion of, 
 816; essential characters of, 192; for- 
 mation of, 134 ; in Vesuvius, 130 ; of 
 British Isles, 190; on Etna, 134 
 
 Dinarian Alps, on map, 605* 
 
 Dinosaurs, tracks of, 482 
 
 Diopside, 209 
 
 Diorite, 96, 97, 103 
 
 Diorite-andesite series, 103; glasses of, 
 104 
 
 Diorite-gneiss, 653 
 
 Diorite-porphyry, 96, 104 
 
 Dip, 585 ; diagram of, 585*; measurement 
 of, 586*; of fault, 620; relation to 
 width of outcrop, 588 
 
 Dip faults, 623, 624 
 
 Diplopora porosa of Dolomites, 10*, 272 
 
 Disconformities, 614, 615*, 616*, 617; 
 marked by springs, 616* 
 
 Discrete particles, 217 
 
 Disintegration, 390, 392 
 
 Dislocation fault or fracture, 657 
 
 Dismal Swamp, 338*, 34*> 343 
 
 Displacement by faulting, 621 ; in Cal- 
 ifornia earthquake, 687 
 
 Distributaries, 465 ; of deltas, 485 
 
 Ditetragonal prism, 45*; pyramid, 45* 
 
 Ditrigonal prism, 46* 
 
 Diurnal temperature changes, 820 
 
 Dodecahedron, 44* 
 
 Dogtown Common, disintegration of 
 boulders on, 396* ; moraine of, 500*, 
 Soi* 
 
 Dolerite, 107 ; decomposition of, 404 ; 
 products of decomposition of, 404 
 
 Dolomite, 59, 146, 222 ; algal origin of, 272 
 
 Dolomites, 272, 649; of Tyrol, 8, 9*; 
 organisms which formed the rock, 10*; 
 Triassic reefs of, 306 
 
 Domes, 595 ; erosion cycle on, 722 ; flat, 
 723 ; low, 602 
 
 Domoshakovo Lake, 258 
 
 Dover Cliffs, 279* 
 
 Dover Straits, tides of, 524 
 
 Drachenfels, 155*; trachyte of, 89, 101, 
 156 
 
 Drag with faulting, 635* 
 
 Drainage system, form of,' 721 
 
 Dreieckhorn, 366, 368 
 
 Dreikanter, 406, 822 
 
 Drepanella, 318* 
 
 Drift, beach, direction of, 523; types of, 
 502 
 
 Drowned river valleys, 803 
 
 Drumlins, 496, 498*; sea cliff of, 531; 
 wave-cut, 817 ; erosion on, 818 
 
 Drummond, Lake, 339* 
 
 Dry Champagne lowland, 731 
 
 Duckweed, 334*, 336 
 
 Ductile, 49 
 
 Dull, 49 
 
 Dundas River (ancient), 728; on block 
 diagram, 760*; on map, 727* 
 
 Dunes, 443, 445*~453*; arresting of move- 
 ment of, 447 ; rate of motion of, 444, 
 450; structure of, 453* 
 
 Dunite, 105 
 
 Durham, Eng., concretionary limestone 
 of, 221*, 256; Magnesian Limestone of, 
 573 ; rock section on coast of, 34* 
 
 Dusky Sound, New Zealand, 806* 
 
 Dust deposits, 457 ; character of, 458 
 
 Dust falls, volumes of, 443 ; storms, 440 ; 
 volcanic, 431 ; wells on glaciers, 377 
 
 Dwarfed organisms, in black mud, 549 
 
 Dynamical geology, 18; regions far study 
 of, 36 
 
 Dynamic metamorphism, 642, 643 
 
 Earth, area of surface, 4; method of ap- 
 proach in study of, 21 ; movements, 655 ; 
 pillars, 411*, 412*, 821 
 
 Earthquake, diagram, 658; fissure, Ariz., 
 680* ; Cal., 686* ; center of disturbance, 
 657; fossil, 689*; path of particle in, 
 663*; phenomena summarized, 689; 
 waves, 657 ; waves, due to, 690 ; with 
 faulting, 622 
 
 Earthquakes, 655; volcanic or explosive, 
 656 
 
 Earth's crust, material of, 38 
 
 East Africa, rift valleys of, 632* 
 
 East Kaibab monocline, 786*, 787* 
 
 Eaton, Amos, 27; portrait, 28*, 31 
 
 Eatonian era in American geology, 28 
 
 Ebb-tide, 524 
 
 Eccles tower, burial of, by dunes, 445*; 
 resurrection of, 446* 
 
 Echinarachnius, 320* 
 
 Echinoderms, 319, 320*, 321*; as bottom 
 feeders, 347 
 
 Echinoids, as rock-destroyers, 437 
 
 Echo Cliff, 786*, 787*, 7Qi 
 
8 3 6 
 
 Index 
 
 Edge of land, sculpturing of, 805 
 
 Eel-grass, 330* 
 
 Effervescent springs, 181, 183 
 
 Eggishorn, 364 
 
 Egypt, Nummulitic limestone of, 280 
 
 Eifel district, Maare of, 158; map, 159* 
 
 Eighteen Mile Creek, 571* 
 
 Einkanter, 406* 
 
 Eld Cleft, Iceland, 175 
 
 Elements, chemical, 17, 38; distribution of 
 
 important, 39 
 
 Elevation of beaches, rate of, 694 
 Elgon Mt., Africa, 632* 
 Elizabeth Islands, 531 ; erosion on, 818 
 Ellice Island reefs, 295 
 Elmira, section from Adirondacks to, 31 
 El Tovar, Colorado Canon, 788 
 Emergence, sandstone of, 559 
 Emma mine, Utah, 266* 
 Encrinus, 321* 
 Endogenetic agents, 566 
 Endogenetic rocks, 70; defined, 71 ; 
 
 interrelations of, 73*; types of, 71 
 Endomorphic effects, 208 
 Engineering geology, 21 
 Englacial detritus, 370; drift, 492 
 England, central, cuesta topography of, 
 
 719*; sections in, 32 
 English coast, sections of, 35; sea-stacks 
 
 on, 814, 815* 
 Enstatite, 105 
 Enterolithic structure, 644* 
 Eolian cross-bedding, ancient deposits with, 
 
 455 ; in Mesozoic sandstone, 456* ; 
 
 in Mississippi limestone, 457*; Orange 
 
 sand, 457*; Palaeozoic sandstone, 456* 
 Eolian rock, 83, 567, 568, 580 ; origin of, 
 
 77 
 
 Epeiric sea, 511, 513, 516 
 Epernay, 730* 
 Epicenter, 657, 658* 
 Epicontinental reefs, 298, 513, 516 
 Epidote, 63 schist, described, 652 
 Epsomite, 60 ; in caliche, 259 
 Epistomella, 327* 
 Erie Canal, sections on, 31 
 Erie lowland, 761 ; 763, section of, 761 
 Eriophorum, 335* ; in upland bogs, 342 
 Erosion, 392 
 Erosion cycle, 704, 705 ; agents in, 705 ; 
 
 on coastal plain, 709 
 Erosion features, special, 811 
 Erosion monuments, 407* 
 
 Erosion plains, 700 
 
 Erosion processes, 391 
 
 Erratics, 492 
 
 Erzgebirge, 33 
 
 Esker, 506*, 507*; origin of, 505 
 
 Estheria, in playa lakes, 483 
 
 Estuaries, mud flats of, 348 
 
 Estuarine elastics, 568 ; deposits, 547 
 
 Etna, in, 189; as type, 141; described, 
 
 132 ; map of, 132, 133 ; eruption of, 128 
 Eucystidium, 324* 
 Eulalie Lake, 671 
 
 Eureka, Nev., andesite perlite from, 105 
 Europe, age of coals of, 345 ; mantle rock 
 
 of, 65 ; relief map of, 700 
 Eutectic mixture, 216 
 Evaporate, 215 
 Evaporation, 354 356 ; of lake water, 252 ; 
 
 products from sea water, 227 
 Even fracture, 48 
 
 Everglades, 300 ; supposed origin of, 304 
 Ewigschneefeld, 368 
 Excrements, source of phosphate, 322 
 Exfoliation, 303 
 
 Exogenetic rocks, 73 ; interrelations of, 82 
 Exomorphic effects, 208 
 Expanded foot of glaciers, 377 
 Explosive type volcanoes, 140 
 Extended consequent streams, 712 
 Extinct volcanoes, 144 
 Extrusive igneous rock, 84 
 Eyassi Lake, Africa, on map, 632* 
 Eyre Lake, Australia, 467 
 
 Faceted' pebbles, 406*, 822 
 
 Facies, change of, in marine elastics, 554 ; 
 
 diagram, 555* 
 Fall-line, 722 
 
 False Cape (Canaveral), 545* 
 Fan-alluvial, 465, 466* 
 Fan glacier, 359 
 Fan- shaped fold, 590* 
 Farafrah, chalk from, 278* 
 Faroe-Iceland ridge, 511* 
 Faulberg, 368 
 Fault, 81, 432, 619; recent, at Muka, 
 
 Muka cliff, New Zealand, 675, 677*; 
 
 formed during earthquake, 681, 681*; 
 
 in Wasatch Mts., 751* 
 Fault-block, 747 
 Fault-breccia, 80*, 81, 427, 432, 577, 634, 
 
 635 
 Fault-crush elastics, 568 
 
Index 
 
 837 
 
 Faulted region, erosion cycle in, 747 
 
 Faulting, nature of movement of, 621 
 
 Fault-line, 620* 
 
 Fault-line scarp, 620* 622, 759 
 
 Fault-line valleys, 758, 759* 
 
 Fault plane, 619; scarp, 622, 630, 759; 
 
 9f Rhine Valley, 757 ; on diagram, 620 ; 
 
 submarine formed by earthquakes, 690 ; 
 
 topography, renewal of, 759 
 Fault-rubble, 432 
 Faults, developed on monocline, 622*; 
 
 horizontal, 622 ; oblique, 622 ; rotary, 
 
 622 ; types of, 621 
 Fault surface, 619 
 Fault zones, 620 
 
 Fawsites, 286, 288* ; of old reefs, 306 
 Fecamp, France, 807* 
 Feel of minerals, 50 
 Feldspar, clouding of grains, 403; effect 
 
 of -temperature on, 395, 397 
 Feldspars, 61, 90; separation of, 94; 
 
 table of, 91 
 Feldspathoids, 61, 90; described, 92; 
 
 separation of, 94 ; table of, 92 
 Felsite, 96, 100, 101, 103 ; porphyry, 96 
 Felsitic texture, 88 
 Fenlands, 330 
 Ferns in swamps, 336 
 Ferromagnesian minerals, as source of iron, 
 
 257 ; table of, 93 ; silicates, 90 
 Ferruginous sandstone, 578 
 Fetid odor, 50 
 Fibrous texture, 218 
 Field names of rocks, 107 
 Fiesch Glacier, see Viesch Glacier 
 Fife, Scotland, lava flows of coast of, 196 ; 
 
 volcanic plugs of coast of, 170 
 Findhorn River, destruction of feldspar 
 
 in, 4*5 . 
 Fingal's Cave, 178, 179*, 180*, 813*; 
 
 origin of, 428 
 Finger Lakes, N. Y., 725, 803; radial 
 
 character of, map, 726*; sections on, 31 
 Finland, 64; gneisses in, 653; metamor- 
 
 phic rocks of, 648 
 Fire bricks, 345 
 Fire clay, 345 
 Fire Island beach, 537 
 Firn, 357 
 Fish, as rock-breakers, 81 ;- as source of 
 
 petroleum, 352; destruction by, of 
 
 corals, etc., 437; sudden killing of, 322 
 Fish-bones, beds of, 322, 348 
 
 Fissile shale, 571* 
 
 Fissure eruption, 174; veins, 265* 
 
 Fjorded coast, 804 
 
 Fjords, 801-803* 
 
 Flags, 335 
 
 Flagstone, described, 579 
 
 Flattop Mt., 599* 
 
 Flint, concretions of, 224*, 574 ; origin of, 
 327 
 
 Flood-plain deposits, 79 
 
 Flood plain, dunes of, 448 
 
 Flood plains of rirer, 472*, 474*, 479* 
 
 Flood tide, 524 
 
 Florida, oolites of, 271 ; map of coasf of, 
 301*, 312; map of end of barrier reef 
 of, 303*; reefs of, 298, 300; rocks of, 
 64*; swamp, 339*; section of Keys of, 
 302* 
 
 Fluid lava, 84 
 
 Fluorite, $Q 
 
 Fluvialite, chemical deposits, 259; elas- 
 tics, 568; deposits, 227 ; rocks, 83 
 
 Focus (earthquake), 657 
 
 Folding, 583; causes of, 617; deforma- 
 tion by, 583 ; machine reproducing, 617 ; 
 structures due to, 607 
 
 Folds, artificially produced, 618* ; eroded, 
 591, 592*; reconstruction of, 592; 
 thickening of axes of, 593; types of, 
 589, -590* 
 
 Foot-prints, fossil, 482 * ; in river deposits, 
 491 
 
 Foot- wall, 621 ; on diagram, 620* 
 
 Foraminifera, 275*, 325 ; deep-sea accumu- 
 lations of, 552; glauconite in shells of, 
 552; in blue muds, 551; in Severn 
 estuary, 549; killed in estuaries, 348; 
 of chalk, 7 ; lutytes of, 580 
 
 Foraminiferal oozes, 275 
 
 Foresets, 484*, 485*, 503*, 504* 
 
 Fort Hamilton, N. Y., peat beds on coast 
 near, 539 
 
 Fosse, glacial, 503 
 
 Fossiliferous character of marine deposits, 
 550 
 
 Fossiliferous sandstone, 80* 
 
 Fossils, 10, 15 ; formation of, illustrated 
 on sea coast, 36 
 
 Foster Glacier, Alaska, 377 
 
 Foster's Flats, Niagara, 765, 774, 775*, 784 ; 
 on map, 763* 
 
 Fracture, denned, 48 
 
 Fragmental rocks, 70, 73 
 
8 3 8 
 
 Index 
 
 Fragmentation, 302 
 
 France, central, 64 ; chalk of, 279 ; coastal 
 section of, 35 ; joints in rock on coast of, 
 639*, 640*; sea caves on coast of, 812 ; 
 sea- stack on coast of, 530* , 814 ; sec- 
 tions in mountains, of, 33 ; volcanic area 
 of central, 153 ; volcanoes of central, 144 
 
 Franklinite, 53 
 
 Freestone, 565, 578 
 
 Freiburg, Mining School, 25 
 
 Freiburg region, section of old volcano, 172 
 
 Fresh water, 214 
 
 Fresh-water lakes, 347 ; deposits of, 251 
 
 Fresh- water mollusks, 315-316* 
 
 Fretted upland, 796 
 
 Fringing reefs^292 
 
 Frische Nehrung and Haff, 444, 445 
 
 Front Range, Rocky Mts., 738 
 
 Frost as agent of erosion, 820 
 
 Frost work, 399, 821 ; on pebbles and soil, 
 400 
 
 Frustule, 323 
 
 Fujiyama, 109, no* 
 
 Fulgur, 312* 
 
 Fumarole, 86, 182 
 
 Fumarolic action, 181 
 
 Funafuti reefs, 295 
 
 Fundy, Bay of, 515; low tide in, 533*; 
 see also Bay of Fundy 
 
 Funnel seas, 514, maps, 513*, 515* 
 
 Funnels, volcanic, 161 
 
 Fusulina, 282* 
 
 Fusulina limestone, 281* 
 
 Gabbro, 96, 97, 104, 705 
 
 Gabbro, basalt series, 105 
 
 Gabbro-gneiss, 653 ; porphyry, 96, 107 
 
 Gagates, 350 
 
 Gagatite, 350 
 
 Galenite, 54 
 
 Galilee, Sea of, 757 
 
 Gallatin Range, 263 
 
 Galloway, J. J., ref., 415 
 
 Ganges Delta, peat of, 338 
 
 Ganges River, 468 ; kankar of, 260 ; flood 
 
 plain of, 470*; land-slide on branch of, 
 
 398 
 
 Gangue material, 266 
 Garden of the Gods, 635, 636* 
 Garlic odor, 50 
 
 Garnet, 62; development of, 210 
 Garnierite, 52 
 Gas geology, 21 
 
 Gases, metamorphism by, 646 ; and vapors, 
 
 metamorphic action of, 211 
 Gastropods, 311*, 312* 
 Geant, Glacier du, 369 
 Genesee Falls, at Portage, Lower, 784*, 
 
 785*; Middle, 782*; Upper, 783*; 
 
 at Rochester, Lower, 780*; Middle, 
 
 780* ; Upper, 780* 
 Genesee Gorge, hanging valley in, 802 ; 
 
 youthful condition of, 708 
 Genesee region, post-glacial development 
 
 of, 779 
 Genesee River, 726 ; (ancient), map, 727*; 
 
 banks of, 416 ; contrasting features of, 
 
 782 ; section of old valley of, 783 ; 
 
 geological history of, 775 ; gorge of, 77* ; 
 
 Lower Falls at Portage, development of, 
 
 diagrams, 785*; map of part of, 779*; 
 
 on map, 776*; sections on, 30 
 Genesee River region, map of, 776*; 
 
 post-glacial development of, 779; pre- 
 
 glacial character of, 775 
 Genesee shale, 571* 
 
 Genesee Valley, 776, 778, 779, 780; ma- 
 ture stage at Portage, 783* 
 Genesis, as basis of natural classification, 
 
 66 
 
 Genetic classification, 66; of rocks, 68 
 Genie rocks, 77 
 Geogenesis, 19 
 Geography, i, 18, 20 
 
 Geological, engineer, 22 ; literature, im- 
 portance of, 37 ; observation, field of, 28; 
 
 and interpretation, rise of, 24 
 Geology, analytic and descriptive, 18; 
 
 applied, 20; causal or dynamic, 18; 
 
 defined, i, 2 ; history of, 22 ; historical 
 
 or developmental, 18; in narrower 
 
 sense, 14 ; relation of, to human welfare, 
 
 20 ; scientific aspect of, 17; scope of, i ; 
 
 subdivisions of science, 13 
 Geomorphology, 18, 19 
 Geophysical laboratories, 19 
 Georgia, swamps in, 339 
 Georgian Bay, 726, 728; map, 727* 
 Geosyncline, Indian, 518; of deposition, 
 
 468 ; relation of mountains to, 6bo 
 Geotectology, 18 
 German brown-coal, thickness of, 343 ; see 
 
 also brown-coal 
 Germany, coast of, 35 ; deformed strata 
 
 of, 583 ; peneplane of western, 702 ; 
 
 salt deposits of northern, n 
 
Index 
 
 839 
 
 Geyser (Iceland), 184, 186* 
 
 Geyserite, 184, 224 
 
 Geysers, 181, 183; distribution of, 184; 
 effect of earthquakes on, 683 
 
 Giant's Causeway, n, 176*, i77*J erosion 
 at, 428 
 
 Giant Geyser, 184* 
 
 Gibbsite, 40, 57 
 
 Gibsonite, 351 
 
 Gilbert, G. K., cited, 200 
 
 Glacial, elastics, 568; deposits, ancient, 
 508; erosion of, 818; Pleistocene, 496; 
 drift, 492 ; erosion, 433 ; geology defined, 
 20; pond, 494, 495*; sculpture, land 
 forms due to, 793 ; streams, 383 ; ma- 
 terial, transported, 495; tarn, 796; 
 till, 497*; transportation, 492 ; trough, 
 mature, 798 *, 799 ; of Norway, 799*, 
 800*; valley, character of, 373 
 
 Glaciated boulder, 435 ; pebbles and 
 boulders, 497*; rock ledge, 434*; 
 valley, old, 799*; valley, young, 799. 
 
 Glacier, currents, work of, 492, 493 ;_ ice, 
 357 ; lakes, 802 ; landscape, ideal, 350* ; 
 tables, 377*; tributary, 800* 
 
 Glaciers, 357 ; deposits of modern, 493 ; 
 several types contrasted, 805 ; illustra- 
 tion of dynamic geology by, 36 ; move- 
 ment of, 374; relation of parts, 375, 376 ; 
 rock destruction by, 433; types, 358, 
 359 
 
 Glasses, old de vitrified, 100 
 
 Glassy texture, 87, 88 
 
 Glauberite, 223 
 
 Glauber salt, 223 
 
 Glauconite, 224, 249, 552 
 
 Glauconitic sandstones, character of, 570 ; 
 described, 579 
 
 Glencoe, 742 
 
 Glen Roy, parallel roads of, 363*-365* 
 
 Gletscherhorn, 368 
 
 Globigerina, 275*; limestone, 276; ooze, 
 275, 276*; map of distribution of, 277* 
 
 Gloucester, Mass., moraine near, 501 
 
 Gmiinden Maar, 160* 
 
 Gneiss, 67; described, 652, 647*, 648* 
 
 Gneisses, 649, 650 ; of Canadian region, 75 
 
 Gneissic structure, 647 
 
 Goat Island, 763, 764, 774 
 
 Gobi, Desert of, 458 
 
 Goderich, Ont., 728 
 
 Goethe, on volcanic structure of Kammer- 
 btihl, 168 
 
 Goethite, 40, 52, 224 
 
 Gold, 57 ; minerals, 57 
 
 Goose barnacle, 317* 
 
 Gordon Craters, 174* 
 
 Gorgonias, 283, 284 ; on reefs, 291 
 
 Gotland, caves on coast of, 814 ; old reefs 
 of, 306 
 
 Grabau, A. W., cited, 212 ; ref., 46, 249, 
 259, 333, 415, 52i, 527, 588, 604, 623 
 
 Graben, 630, 754 
 
 Graben faulting, 754 
 
 Graham Island, 112, 115*; described, 
 114; section, 115* 
 
 Grahamite, 35 
 
 Gran' Canaria, oolites of, 388 
 
 Grand Banks, Newfoundland, 388 
 
 Grand Canon, geological history of, 786; 
 map of, 786*, 788, 789*, 790; section 
 across, 791 ; region, character and de- 
 velopment of, 791 ; rocks of, 788 
 
 Grand Canon district, 600 
 
 Grand Canon series, 789 
 
 Grand Island, 764 
 
 Grand Viewpoint, Colorado Canon, 788 
 
 Grand Wash, cliffs, map, 786*; fault, 
 792 
 
 Granite, 95, 96 ; cliffs, erosion of, 428, 429* ; 
 boss, 205*; boulder, disintegration of, 
 396* ; boulders, formed by disintegra- 
 tion, 396 ; dikes, 192 ; disintegration of, 
 394 ; gneiss, 653 ; horizontal joints in, 
 395*; porphyry, 96*, 98 
 
 Granite-rhyolite series, 95 
 
 Granites, 93 ; of Canadian region, 75 
 
 Granitic texture, 88* 
 
 Granodiorite, 103 
 
 Granular, disintegration, 394, 395 ; snow, 
 357 ; texture, 88 
 
 Graphic granite, 97*, 207 
 
 Graphite, 61, 67, 329, 344; rock, 654; 
 schist, described, 652 
 
 Graphitic rocks, 649 
 
 Grasses, in swamps, 335 
 
 Grat, 796* 
 
 Gravity fault, 624 
 
 Graywacke, described, 579 
 
 Graz, Austria, destruction of rock frag- 
 ments in river below, 414 
 
 Greasy luster, defined, 49 
 
 Great Aletsch Glacier, see Aletsch Glacier 
 
 Great Balkan Mts., on map, 605* 
 
 Great Barrier Reef, Australia, 290, 291*, 
 305; described, 298 ; section of, 299* 
 
840 
 
 Index 
 
 Great Basin, 749, 753; cross section of, 
 
 750* ; ranges of, 634 
 Great Britain, deformed strata of, 583 ; 
 
 sea-coast section of, 35 ; upland bogs 
 
 of, 342 
 
 Great Fault Line, Cal., 686* 
 Great Geyser Basin, 83* 
 Great Glen, Scotland, 759 
 Great Ice Barrier, 387 
 Great Lakes, 4; cobble terraces of, 532; 
 
 discharge of, 769; dunes of, 448; 
 
 future permanent discharge of, 696 
 Great Lakes region, change of level in, 695 
 Great Neck, Mt. Taylor, N. M., 166* 
 Great plains, 701, 705; Badlands of, 699* 
 Great Pyramid, 76*; facing by Nummu- 
 
 litic limestone, 280 
 Great Salt Lake, 230, 242, 259, 509 ; 
 
 brine of, 214; map of, 252*; mirabilite 
 
 in, 223 
 
 Great Whin Sill, 197 
 
 Grecian Archipelago, volcanic field of, in 
 Greenland, altitude of snow-line in, 357 ; 
 
 ice-cap of, 493 ; ice cover of, 385, 386* 
 Green Mts., rock exposures in, 34 
 Green muds, 551, 552 
 Green River, 472 
 
 Green-sand, 224, 249, 551, 552, 579 
 Greenstone, 107 
 Griou, Puy de, 149, 150 
 Grit, 575 
 
 Grooved upland, 796 
 Grossularite, 210 
 Ground-mass, denned, 88 
 Ground moraines, 496 
 Ground water, 4, 411 
 Grover's Cliff, Mass., 817*; section of, 
 
 53i* 
 
 Grunhorn, Liicke, 368 
 Guano, 322 
 
 Guano Nyiro Lake, Africa, on map, 632* 
 Guettard, .,25 
 Gulf, 509 
 Gulf Coast, Orbitolitic and Nummulitic 
 
 limestone of, 281 
 Gulf Stream, 302 ; influence on coral reef 
 
 growth of, 290; origin of, 526; velocity 
 
 of, 527 
 Gypsum, 59, 220; crystals of, in desert 
 
 deposits, 244 ; from alterations of lime 
 
 stones, 244; from sea-water, 229; in 
 
 caliche, 259 ; order of deposition of, 
 
 235 ; solution of, 404 
 
 Hackly fracture, 49 
 
 Hade of fault, 620*, 621 
 
 Haeckel, E., cited, 19 
 
 Hail, 70, 356 
 
 Halifax, N. S., batholith of, 205 
 
 Halimeda, 272*; of reefs, 293 
 
 Halite, 58 ; in caliche, 259 
 
 Hall, James, 31 
 
 Hall, Sir James, 26 
 
 Halysites, 286, 288* 
 
 Hamilton, Ont., 726 
 
 Hamilton shales, 31*, 570* 
 
 Hammadas, 405 
 
 Hanging valleys, 374*, 802; Genesee 
 gorge, 797*, 800*, 802*; formation of, 
 diagram, 801*; on block diagram, 778* 
 
 Hanging wall, 621 ; on diagram, 620* 
 
 Hardness, defined, 49 
 
 Hardpan, 497 
 
 Hard water, 214 
 
 Harsh feel, 50 
 
 Haute Loire, volcanic district of, 165 
 
 Hawaiian Islands, 109 ; lava cascades in, 
 119; map of, 117; tsunamis at, 674 
 
 Heat, metamorphism by, 646 
 
 Heather, as peat former, 342 
 
 Heave, 622 
 
 Heaven's Peak, Montana, 583* 
 
 Height of waves, 520 
 
 Helderberg front, 741 ; Mts., 728 ; over- 
 thrusts, 626* 
 
 Helgoland, faults of, 621*; map, showing 
 erosion of, 809* 
 
 Heliopora, 30x3 
 
 Helios phara, 324* 
 
 Helix, 316*, 317*, 458 
 
 Hellgate, 524 
 
 Hematite, 52, 225, 402 
 
 Hemi-brachy-dome (triclinic), 48* 
 
 Hemi-macrodome (triclinic), 48* 
 
 Hemi-prism (triclinic), 48* 
 
 Hemi-pyramid (monoclinic), 47* 
 
 Herculaneum, 126, 128; burial of, 127 
 
 Herds of animals, rock destruction by, 436 
 
 Hereroland, 436 
 
 Herodotus, Nummulitic limestone known 
 to, 280 
 
 Hesperus Mountain, section of, 201* 
 
 Heterogeneous conglomerate, 577 ; tex- 
 tures, 87 
 
 Hexagonal prism, 46*; pyramid, 46*; 
 system, 43 ; principal types of, 46 
 
 Hexahedron, 44* 
 
Index 
 
 841 
 
 Hexoctahedron, 44* 
 
 Hextetrahedron, 44* 
 
 High Falls, N. Y., asymmetrical anticline 
 at, 591* 
 
 Highland Light, Cape Cod, 820 ; erosion 
 of cliffs at, 822* 
 
 Highlands of Hudson, 64 
 
 High moors, 329 
 
 Hill peat, 342 
 
 Hill prairies, 715 
 
 Hillside springs, 423 
 
 Himalaya Mts., 3, 471 ; marbles of, 653 
 
 Hingham, Mass., boulder near, 501* 
 
 Hispar Glacier, 371 
 
 Historical Geology, defined, IQ 
 
 History of geology, 22 
 
 Hitchcock, C. H., quoted, 771 
 
 Hoang-Ho, 459, 697 ; material transported 
 by, 462 ; plain of, 467, 468* 
 
 Hobbs, W. H., quoted, 679, 680 
 
 Hog-back, of Dakota sandstone, 739*, 
 740* 
 
 Hog-backs, 722, 734, 738; development of, 
 diagram, 739* 
 
 Hohenlohe Lake, Africa, on map, 632* 
 
 Holocene period, 20, 77 
 
 Holocrystalline, 90 ; texture, 87, 88* 
 
 Holtenia, 325* 
 
 Homogeneous texture, 87 
 
 Hone-stones, 581 
 
 Honey-comb coral, 286, 288* 
 
 Honshiu, northern earthquake in, 682 ; 
 map, 682* 
 
 Horn (mountain), 366, 368, 793, 797, 804 
 
 Hornblende, 93 ; andesite, 104 ; recogni- 
 tion of, in granite, 95 ; schists, described, 
 651 ; separation of, from magma, 94 
 
 Hornblendic granite, 97 
 
 Hornfels, 210, 650; described, 651 
 
 Horse (structural), 635* 
 
 Horseradish odor, 50 
 
 Horseshoe Falls, 764, 765, 768, 774, 775; 
 map of, 763* ; section of, 771* ; surveys 
 of crest line, 773* 
 
 Horse-tails, in swamps, 336 
 
 Horst, 634 
 
 Hot springs, 181, 183 
 
 Hubbard Glacier, 373; map, of, 371* 
 
 Hudson Gorge, 804* ; Bay, 516 ; Highlands, 
 rock exposures in, 34 ; thrust in High- 
 lands, 626, 627* 
 
 Hudson River, brackish water of, 214; 
 drowned valley of, 695; estuarine 
 
 character of, 547 ; fjord character of, 
 803 ; variable depth of, 803 ; subsidence 
 of, 803 ; formation, 74 ; mud deposits 
 on, 548 ; profiles of, 804 * 
 
 Hudson River Bluestone, 74, 578*, 579 
 
 Humboldt, Alexander von, portrait, 824* 
 
 Humic acid, action of in solution of quartz, 
 426 
 
 Humidity, absolute and relative, 355 
 
 Hungarian soda lake, 250 
 
 Hungary, 603 ; dunes of, 451 ; perlite of, 99 
 
 Huntington, E., cited, 245 
 
 Hurricane Fault, 792; diagram, 602*, 
 787*; map, 786* 
 
 Hutton, James, portrait, 26* 
 
 Huttonian theory, 26 
 
 Hydrargillite, 403 
 
 Hydration, 403 
 
 Hydraulic mining, 267* 
 
 Hydrocarbon minerals, 61 
 
 Hydrocarbons from diatoms, 324 
 
 Hydrochloric acid, from magmas, 85 
 
 Hydroclastic, 79, 568; rocks, defined, 83 
 
 Hydrocoralline, 289 
 
 Hydrogenic deposits, 79 ; rocks, 70-73, 77, 
 214 
 
 Hydrogen minerals, 60 
 
 Hydroid polyps, 283, 289 
 
 Hydrology, science of, 19 
 
 Hydrosphere, 4, 5, 14, 70, 77 ; as rock- 
 breaker, 77 ; contribution of, to litho- 
 sphere, 10; derivation of term, 12; 
 destructive work of, 410; operation of, 
 in rock-breaking, 391 
 
 Hydroxides, defined, 40 
 
 Hydrozincite, 53 
 
 Hypabyssal, 18, 83, 205 
 
 Hypersthene, 105 
 
 Hypnum, 334*, 336 
 
 Hypocenter, 657, 658* 
 
 Ice, 60, 357, 433 
 
 Icebergs, 383, 387*; origin of, 387*, 388*; 
 size of, 388 ; submerged part of, 388* 
 
 Ice-caps, 384, 493 ; erosion by, 805 
 
 Ice contact slope, 502 
 
 Iceland, earthquake of, described, 682 ; 
 ice-caps of, 384, 493 ; fissure eruption in, 
 84; lava plains of, 175; solfataric 
 vents of, 181 ; volcanoes of, 109, 174; 
 volcanic fissure in, 171* 
 
 Ice movement, causes of, 387 ; direction of, 
 indicated by scratches, 434 
 
842 
 
 Index 
 
 Ice rafted material of submarine banks, 388 
 Ice sculpture, 800; land forms, due to, 
 
 800* 
 
 Ice-tables, 377 
 Iddings, J. P., cited, 204 
 Igneous contact, 205, 207 ; significance of, 
 
 213 
 Igneous magma, 71, 84; character and 
 
 composition of, 85 ; order of minerals 
 
 crystallizing from, 94 
 Igneous masses, form and structure of, 188 ; 
 
 relative ages of, 213 
 Igneous material, 80 
 Igneous rocks, 47, 72, 73 ; classification of, 
 
 89 ; erosion features on, 816 ; es- 
 sential minerals of, 90; formation of, 
 
 87 ; outcrop of, 84 ; types of, 95 
 Impervious beds, 422 
 Impregnation vein, 267 
 Inclined strata, 583 ; continuation of, 584 ; 
 
 diagram of, 584* 
 Incompetent beds, 618 
 Index fossils, 556 
 India, lava plain of central, 176 ; Miliolitic 
 
 limestone of, 568 
 Indian, geosyncline, 518; Ocean, 510; 
 
 Ocean, atolls of, 292 ; radiolarian ooze 
 
 in, 328; rice, 335 
 Indo-Gangetic plain, 468, 697 
 Indurated elastics, 563 
 Induration, causes and agents of, 564 
 Indus River, 468 ; kankar of, 260 
 Inface, defined, 714 
 Inglefield Gulf, Greenland, 385 
 Inland, desert basins, salts deposited in, 
 
 242 ; plain defined, 697 
 Inner lowland, 713, 714*, 715, 716*, 717*; 
 
 control of depth of, 714; of English 
 
 cuestas, 719* 
 Inorganic spheres, 4, 12 
 Insect eggs, coated by lime, 248 
 Insects, trails of fossil, 482 
 Inselberge (island hills), 436 
 Insequent streams, defined, 712 
 Insolation, 392 
 
 Intercontinental seas, 509, 510 
 Interlobate moraines, 501 
 Intracontinental seas, 513, 514 
 Intrenched meander, 706*, 707, 718* 
 Intrenched oxbow, 707, 708* 
 Intrusive, rocks, igneous masses, 188 ; 
 
 rocks, 84 ; exposure of, 84 ; sheets, 194 
 Inverness, Scotland, 759 
 
 Inverted fault, 600; development of, dia- 
 grams, 602* 
 
 Inwood,"N. Y., 210 
 
 Iodine from kelp, 271 
 
 Ions, 17 ; defined, 41 
 
 Ireland, deformed strata of, 583; upland 
 bogs of, 342 ; volcanic plugs of, 169, 
 170* 
 
 Iridescence, 50 
 
 Iridosmine, 57 
 
 Irish, peat bog, 342*; Sea, 518 
 
 Iron, bacteria, 258 ; carbonate and oxide, 
 257 ; minerals, 52 ; nodules, of laterite, 
 404 
 
 Iron-stone nodule, 257 
 
 Irondequoit Bay, on map, 776* 
 
 Iroquois beach on section, 762* 
 
 Ischia, eruption in, 128; Island of, map, 
 no* 
 
 Island hills, 436 
 
 Isoclinal folds, 591, 592, 595 
 
 Isometric system, 43 ; principal types, 
 
 44* 
 
 Isoldes, 319* 
 Israelites, exodus of, 242 
 Italian marble, 6 r 3 
 Italy, sections in mountains of, 33 
 
 Jaggar, T. A., cited, 202 
 
 Jail Rock, Nev., 701 *, 705 
 
 Jamaica, earthquake, described, 688; 
 
 Island, foraminiferal limestone of, 281 
 Japanese earthquake, described, 680 
 Java, solfataric vents of, 181 
 Jaxartes River, dunes of, 449; map of, 
 
 449* 
 
 Jebel el-Tih, 754 
 Jebel Musa, 754 
 Jehlam River, 471 
 Jereica, 327* 
 Jet, 350 
 Johnson, D. W., cited, 749; quoted on 
 
 beach cusps, 536; ref., 333, 521, 527, 
 
 695 
 
 Jointed rocks, erosion features on, 8n 
 Joints, 409, 638, 63 8 "-640* 
 Jones, H. C., ref., 41 
 Jordan River, 757 
 Jorullo, 112; described, 113 
 Juan Fernandez, 674 ; submarine volcano 
 
 at, 675 
 
 Jukes-Browne, cited, 299 
 Jumna River, 469 
 
Index 
 
 843 
 
 Junagarh limestone, 278 
 
 Juncus, 334* 
 
 Jungfrau, 359, 366, 368 
 
 Juniata glass sand, 578* 
 
 Junipers in swamp, 341 
 
 Jupiter Serapis, temple of, 691, 692* 
 
 Jura Mts., 590, 593, 596, 645, 755 ; on map, 
 
 605* 
 
 Jurassic period, 77 
 
 Jurassic sandstone, cross-bedding of, 456 
 Juvenile waters, 185, 268, 424 
 
 Kaibab, Canon, 787 ; limestone, 791 ; 
 
 Plateau, 786*, 787*, 790* 
 Kainite, 58, 223 
 
 Kalahari Desert, rock destruction in, 436 
 Kames, 502 
 
 Kammerbtihl, 168, 169* 
 Kanab Canon, 787 
 Kankar, 260, 469 
 Kansas, sections across, 32 
 Kaolin, production of, 403 
 Kaolinite, 63 
 Kaolinization, 403 
 Kara Bugas Gulf, 231, 238, 349, 352 ; 
 
 asphalts of, 352; map of, 239*; mirab- 
 
 ilite in, 223 ; section of, 240* 
 Karakoram Himalayas, glaciers of, 370 
 Kara Kum Desert, 232*, 450; map of, 
 
 449* 
 
 Karren, 413* 
 Kashmir, Vale of, 471 
 Katamorphism, 646 
 
 Kathiawar Peninsula, India, 278 \ 
 
 Kawafune fault-cleft, Japan, 682* ' 
 
 Kelley's Island, glacial flutings on, 498, 499* 
 Kelps of Pacific, 271 
 Kemp, J. F., cited on shell layers of Florida, 
 
 534 ; table of rocks modified after, 96 
 Kenia, Mt., Africa, on map, 632* 
 Kerguelen Islands, 385 
 Kern Canon, Cal., 798* 
 Kertch, Bryozoa of, 308 
 Kettle-hole, 494, 503 ; origin of, 495* 
 Kettle-ponds, 503 
 Kettle topography, 499 
 Keys, 300, 301, 302 ; of Florida, 346 
 Key West, 300 
 
 Khyber Pass, Himalayas, 469* 
 Kieserite, 60 
 Kilauea, 87, 117, 157; as type, 140; 
 
 basic glasses of, 107; crater of, 118*; 
 
 lava lake of, 118*, 119* 
 
 Kilimandjaro, Mt. (Kilimanjaro), 385 ; 
 
 on map, 632* 
 
 Kingston, N. Y., thrust fault near, 626* 
 Kirchheim, Wiirttemberg, intrenched ox- 
 bow at, 708* 
 
 Kivu Lake, Africa, on map, 632* 
 Klingstein, 102 
 
 Knife-edge crests (glacial), 794, 796* 
 Knox dolomite, 597 
 Kokos Keeling atoll, 292 
 Kotsina Glacier, Alaska, terminal moraine 
 
 of, 494* 
 Krakatoa, 137 ; air waves from eruption 
 
 of, 656 ; as type, 140 ; dust of, 439 ; 
 
 maps of, 137*, 138*, 139*; Rakata of, 
 
 140*; section of, 143* 
 Kunzen, burial of church of, by dunes, 
 
 445 
 
 Kurische Nehring and Haff, 444*, 445* 
 Kyzyl Kum desert, dunes of, 450; map 
 
 of, 449* 
 
 Laacher Lake, 159, 161*; section of, 143* 
 Labradorite, 91 
 
 Laccolith, 167, 199 ; section of, 200* 
 Lacustrine, elastics, 568 ; deposits, 83, 227, 
 
 25, 439 
 Lagoons, of atoll, 292; salts formed in, 
 
 238 ; sapropelite in, 347 
 Lahonton Lake, 221 ; concretionary lime- 
 stone of, 2 19*; map of, 254*; thinolite 
 
 from, 255*; tufa domes in, 256* 
 Lake Albert, on map, 632* 
 Lake Baikal, 516 
 
 Lake Bonneville, see Bonneville, Lake 
 Lake district, England, dome structure of, 
 
 744; map of, 745* 
 Lake Erie, 767 ; joints in rocks on, 638* ; 
 
 outcrops on, 29; section along, 30*, 31*; 
 
 strata on, 79 
 Lake Huron, 726, 731 ; cross-section of, 
 
 728*; map of, 727* 
 Lake Iroquois, 779, 780; map of, 762*; 
 
 section of, 762* 
 
 Lake Lahonton, see Lahonton Lake 
 Lake Michigan, 731; change of level in, 
 
 695 ; dunes of, 448; section from Bara- 
 
 boo to, 32 
 
 Lake Ontario, 30, 725, 767, 775, 780 
 Lake, P., and Rastall, R. H., cited, 746 
 Lake Superior, caves on shore of, 811 
 Lake waters, composition of, 250 ; table of 
 
 composition of, 251 
 
8 4 4 
 
 \ak 
 
 Index 
 
 4; evaporation .products of, 250; 
 
 origin of, 423 ; temporary, formed by 
 
 land slides, 398, 399 
 Lakes of Africa, map of, 632 ; see under 
 
 names 
 
 Lamarck, J. B., portrait, 25* 
 Lamellibranch, 309, 310* . 
 Lamina, 486 
 Laminated texture, 218 
 Land, area of, 4; mollusks, 315, 316*; 
 
 plaster, 221; surface, original character 
 
 of, 697 
 Land slides, 398*; due to earthquakes, 
 
 690 
 
 Landes, 446 
 Lang Glacier, 368 
 Laon, on map, 730* 
 Lapiaz. 413* 
 Lapilli, 117, 432 
 
 Lapland, altitude of snow-line in, 357 
 La Plata estuary, 547, 549* ; map of, 548*; 
 
 material transported in, 462 
 La Plata Mountain, 201* 
 Laramie Lakes, Wyoming, 258 
 La Sal Mts., Utah, erosion features near, 
 
 823 
 Lateral moraines, 368, 370, 492; origin 
 
 of, 403* 
 
 Lateral secretion, 268 
 Laterite, 404 
 Lateritic soil, 551 
 Laterization, 493 
 Laufen, Wurttemberg, intrenched oxbow 
 
 at, 707, 708* 
 Lauterbrunnen canon, 198*, 372*, 799, 
 
 802 
 Lava, age of flows of, 143 ; plains, Oregon, 
 
 Idaho, and California, 175; rock, 84; 
 
 surface of sheets, 1 74 ; tunnels, 1 20, 1 2 1 * ; 
 
 viscous lava of craterless volcanoes, 122 
 Lead minerals, 54 
 Le Conte, J., cited on reefs, 300 
 Leda, in raised beaches, 694 
 Ledges, as sources of geological information, 
 
 29 
 
 Lee Lake, 709; map, 710* 
 Leith, C. K., quoted, 647, 648 
 Leith, C. K., and Mead, W. J., ref., 646 
 Lemna, 334*, 336 
 
 Lemus Island, earthquake effects near, 675 
 Length of waves, 520 
 Leonardo da Vinci, 24 
 Lepas, 317* 
 
 Leperditia, 318* 
 
 Lepidocydina, 281* 
 
 Lepidolite, 58 
 
 Le Puy, France, 166 
 
 Leschaux, Glacier de, 369 
 
 Leu cite Hills, Wyoming, 166 
 
 Levees, natural, 479 
 
 Lewiston, N. Y., 763, 766, 771 
 
 Libyan Desert, 452*; chalk from, 278; 
 origin of sands of, 75, 452 ; wind grooves 
 in, 405 
 
 Lignite, 343 
 
 Ligurian Alps, on map, 605* 
 
 Limbs of anticline, 589 
 
 Limburgite, 96, 107 
 
 Limburgite-porphyry, 96 
 
 Lime, organically secreted, 71 
 
 Lime carbonate, deposits of, 246 ; percent- 
 age in blue muds, 551; percentage in 
 green muds, 552; percentage in red 
 muds, 551 
 
 Lime mud-rocks, 581 
 
 Lime sandstones, described, 580 
 
 Limestone, 221 ; conglomerate, 577 ; 
 formed by Bryozoa, 307 ; peaks, solu- 
 tion in; 822; solution forms on, 413* 
 
 Limestones, 570; contact metamorphic 
 effects on, 209 
 
 Limncea, 315, 317* 
 
 Limonite, 52, 224, 402 
 
 Limpets, in raised beaches of Scotland, 533 
 
 Lingula, 309 ; phosphate of lime in shells 
 of, 322 
 
 Linton, Ohio, cannel coal of, 349 
 
 Lipari Islands, 1 1 1 ; obsidian of, 99 
 
 Lisbon earthquake, 664 
 
 Litchfield, Maine, nephelite syenite from, 
 102 
 
 Lithium minerals, 58 
 
 Lithodomus, 692 
 
 Lithogenesis, IQ 
 
 Lithographic stone, 58 
 
 Lithoidite with lithophysae, 100* 
 
 Lithology, science of, 14; subdivisions of, 
 16 
 
 Lithophysae, 99, 100* 
 
 Lithosphere, 5, 14, 15, 19, 71, 84; contribu- 
 tion to, from other spheres, 10; deri- 
 vation of term, 12 ; movements of, in 
 rock breaking, 391 ; organic remains 
 in, 7 ; thickness of, 5 
 
 Lithothamnium, 272*, 301 ; of reef, 293 
 
 Little altered rocks, 69 
 
Index 
 
 845 
 
 Little Colorado River, 788, 789; mud- 
 cracks along, 479 
 Littoral district, 515, 518 
 Liverpool section, beginning at, 32 
 Livingston and Lewis ranges, section, 599* 
 Livingston Range, Montana, 583* 
 Lobsters, 317 
 Local metamorphism, 643 
 Loch Linnhe, Scotland, 759 
 Loch Maree, 569 
 
 Loess, 458, 567 ; consolidation of, 459 
 Loessmannchen (Loesspiippcheri), 459, 572, 
 
 573* 
 
 Loire River, 144 ; sandbanks of, 462 
 Loligo, 314* 
 London, rocks under, 64; section from 
 
 Liverpool to, 32 
 London Basin, 599 
 London coastal lowland, 719, 720* 
 Long Branch, destruction by storm waves 
 
 on, 541* 
 Long Island, N. Y., apron plain of, 503 ; 
 
 development of, 811; erosion on, 818; 
 
 origin of, 716 
 Long Island Sound, 715 
 Long-shore currents, 523, 528; bar sand 
 
 spits formed by, 819 
 Long-shore drifting, 523 
 Longwood shale, mud-cracks in, 491* 
 Looking Glass Rock, Utah, 823* 
 Lop Nor, salt plain of, 243*, 249*; types 
 
 of sand ripples in, 454 
 Lorelei Rock, 584* 
 Lorraine, minette ores of, 225 
 Lot's wife, 757, 822 
 Lowland, annular, 730; inner, see Inner 
 
 lowland 
 
 Lucrinus, Lake, 112 
 Lunatia, 311* 
 Luray Cave, 263*, 264 
 Luster, 49 
 
 Lutaceous texture, 569, 570 
 Lutyte, 56 Q, 580 
 Luxemburg, minette ores of, 225 
 Luzon, 114 
 Lycra, jet from, 350 
 Lyell, C., cited, 23, 36, 37, 671 ; quoted, 
 
 668, 670 
 Lyell, Sir Charles, portrait, 27 
 
 Maare, of Eifel, 158, i59*-i6i* 
 Maar, Schalkenmehren, 160*; Gmunden, 
 1 60; ideal section of, 161* 
 
 Mackinac Island, Mich., 728; erosion on 
 
 coast of, 814 ; natural bridge on, 814* 
 Maciurean era in American geology, 28 
 Macro-domes (orthorhombic), 47* 
 Macro-pinacoids (orthorhombic), 47*; 
 
 (triclinic), 48* 
 Madrepora, 285, 286*, 287*, 306; on reefs, 
 
 291 
 
 Maandrina, on reefs, 291 
 Magdalena Mt., N. Mex., 749 
 Magdeburg-Halberstadt region, 235 
 Magma, 390 ; igneous, 84 
 Magmatic origin of tufa-depositing waters, 
 
 261 ; waters, 268, 424 
 Magnesite, 60, 222 
 Magnesian Limestone, concretions of,. 221* ; 
 
 of Durham, 573 
 Magnesium carbonate, precipitated by 
 
 algae, 271 ; minerals, 59, 60 
 Magnetic character of minerals, 50 
 Magnetite, 52, 93, 402 
 Magnetite rock, described, 654 
 Magnetitic rocks, 649 
 Maine coast, 806, 810 ; raised beaches of, 
 
 694 ; dikes of, 191 ; map of portion of, 
 
 805* 
 
 Mainz, see Mayence 
 Malachite, 56 
 Malaspina Glacier, 358; map of, 381*; 
 
 described, 382; moraine on surface of, 
 
 383* 
 
 Malleable, denned, 49 
 Malta, Globigerina limestone of, 276 
 Maltha, 351 
 
 Mammoth hot springs, terraces of, 261 
 Man, as rock-breaker, 82 ; associated with 
 
 glacial deposits in Europe, 496 ; destruc- 
 tion of rocks by, 436 
 Manganese-concretions, 249*; ores, 224; 
 
 minerals, 52 ; nodules in red clay, 553 
 Mangrove, Islands, 301 ; swamp, 340* ; 
 
 trees, 341 
 
 Manitoulin Island, 728 
 Mantle rock, denned, 65; interfering of, 
 
 with geological study, 29 ; on bed rock, 
 
 65* 
 
 Manyara Lake, Africa, on map, 632* 
 Marble, 67, 209, 569 ; described, 649, 650, 
 
 653 ; microsection of, 649* 
 Marble Canon, 788, 789; on map, 786*; 
 
 on diagram, 787* 
 Marble Plateau, on map, 786*; diagram, 
 
 787* 
 
8 4 6 
 
 Index 
 
 Marblehead, coast, terrace of, 532 ; Neck, 
 
 Mass., 810*; rocks of, 206; tombolo, 544 
 Marcasite, 52 
 Marengo Cave, 264* 
 Marine bench, 530*, 807* 
 Marine elastics, defined, 568; structures 
 
 of, 553 
 Marine, deposits, 227, 439; marshes, 330; 
 
 overlap, diagram, 562*; rocks 83, 567 
 Maritime Alps, pebble beaches on coast 
 
 at foot of, 533 
 Marjelen See, 363, 364; deltas of, 362; 
 
 section of, 362*; view of, 361* 
 Marl, origin of, 273 
 Marne River, on map, 730* 
 Marr, J. E., cited, 746 
 Marsh gas, 329, 331 
 Marsh grasses, 331* 
 Marshes, marine, 329, 330, 331* 
 Martha's Vineyard, coast of, 543 
 Maryland, coast of, 810 ; coastal plain of, 
 
 7i5 
 
 Massa River, 364 
 
 Massachusetts coast, dikes of, 191 ; drum- 
 lins of, 498; morainal sections of, 531; 
 peat of, 332, 333; pebbles of, 430*; 
 tidal falls of, 526 ; wave erosion on, 428, 
 429*, 807*, 808*, 810*, 817*, 8i8*-822* 
 
 Massif, Central France, 144 
 
 Master consequent streams, defined, 712 
 
 Master stream, drainage system of, 721 
 
 Mato Tepee, 166, 168* 
 
 Matterhorn , 797* 
 
 Matterhorn peak, 800* 
 
 Mature lands, erosion of coast on, 810 
 
 Mature river, characters of, 708 
 
 Maturity of land, 705 
 
 Mauna Loa, 117, 119 ; end of lava flow of, 
 121*; form of, '141; height of, 121; 
 section of, 143* 
 
 Mayence, 754 
 
 Mayon volcano, 109 ; form of, 141 ; view 
 of, Frontispiece* 
 
 Mazama Mt., 156; restoration of, 159* 
 
 McClure, W., 27 ; portrait, 28* 
 
 Meager feel, 50 
 
 Meandering course of streams, 709 ; on 
 peneplane, 707 ; of river current, 416* 
 
 Meander River, 417 
 
 Meanders, development of, 417*; of Mis- 
 sissippi River, map, 710* 
 
 Mechanical, rock destruction, 390; sedi- 
 ments, 67 
 
 Medford dike, disintegration boulders of, 
 
 397* 
 
 Medial moraine, 370, 492 ; origin of, 493* 
 Medina sandstone, origin of, 535 ; fossil 
 
 beach cusps in, 536* 
 Mediterranean, 509 
 
 Mediterraneans, 515 ; importance of, 516 
 Meeteesie, Wyoming, unconformity near, 
 
 612 
 
 Megal, 145 
 Mehlem, 157 
 
 Meissen, pitchstone of, 99 
 Melaphyres, 107 
 Melilite, 92 
 Melito, Italy, earthquake manifestations 
 
 in, 668, 669 
 Membranipora, 307* 
 Mendelieff, on origin of petroleum, 353 
 Mendenhall Glacier, 378 
 Mender, see Meander 
 Mendoza, Chile, 674 
 Mer de Glace, 370*; described, 369; 
 
 map of, 369* 
 Merced River fan, 467 
 Mercury minerals, 56 
 Merrill, G. P., cited, 23 
 Merryman's Lake, section of, 333* 
 Mesozoic, coals, 344 ; lignites, 343 
 Messina, effects of earthquake on, 670; 
 
 earthquake manifestations in, 668 ; 
 
 earthquake described, 670; seismo- 
 
 gram of, 66 1 ; straits, tides of, 525 ; 
 
 torrential deposits in sea near, 533 
 Metallic, aqueous deposits, 224; carbides, 
 
 as source of petroleum, 352 ; luster 
 
 defined, 49 
 Metamorphic rocks, 67, 68 ; age of, 648 ; 
 
 types of, 649 
 
 Metamorphic structures and textures, 647 
 Metamorphism, 68, 69, 642 ; agencies 
 
 producing, 643 ; classification of, 642 ; 
 
 contact, 193 
 
 Meteoric, dust, 528; phenomena, 13 
 Meteorites, 13 
 Meteorology, science of, 13 
 Meteors, 13 
 Metz, 731 
 
 Meuse River, 743*, 744* 
 Mexican onyx, 221, 261 
 Mexico, earthquake effects in, 678 ; Gulf 
 
 of, 516 
 Mezenc, 145*; Massif of, section, 151*; 
 
 on map, 153* 
 
Index 
 
 847 
 
 Mica, 63, 90; andesite, 104; commercial 
 
 source of, 88; diorite, 103; syenite, 
 
 100 
 
 Mica schist, 67, 651 
 Michigan, 1 1 ; old coral reefs of, 306 ; 
 
 St. Peter sandstone of, 75 ; salt deposits 
 
 of, n, 245; Sylvania sandstone of, 75; 
 
 wind grooves in Silurian rocks of, 405 
 Michigan Basin, 599, 600*, 602, 615, 729 ; 
 
 cross-section of, 601, 731 
 Microcline, 90 
 Microdiscus, 319* 
 Middle Aletsch Glacier, 364 
 Midori, Japan, earthquake fault-scarp near, 
 
 681* 
 
 Miers, H. A., ref., 51 
 Migration of geosynclines in Carpathians, 
 
 607 
 
 Miles Glacier, 378; map of, 377* 
 Miletus, 417 
 Miliola, 278* 
 
 Miliolitic limestone, 278, 281, 568 
 Military geology, 22 
 Millepora, 288, 289; on reefs, 291; reef, 
 
 300 
 
 Millerite, 52 
 
 Millet seed type of sand grain, 440 
 Milwaukee, Wis., 32 ; reefs near, 305 
 Minas, basin of, 534 
 Mineral, coal, 61 ; deposits, 86 ; springs, 86 ; 
 
 veins, 265 
 Mineralizers, 209 
 Mineralogy, 18, 38 
 Minerals, 17, 38; classification of, 50; 
 
 defined, 42 tables of, 51, 52-62 
 Mines, 4 
 
 Minette ores, 225 
 Mining camp, 268* 
 Mining geology, 20 
 Minnesota, Potsdam sandstone of, 75 
 Mino province, Japan, earthquake in, 680 
 Mirabilite, 58, 223; in Great Salt Lake, 
 
 250 
 
 Misenium, 127 
 Mississippi River, amount of sediment 
 
 carried annually by, 462 ; delta of, 485 ; 
 
 flood -plain of, map, 463*, 475*; levees 
 
 of, 475, 476* ; material transported 
 
 by, 462 ; mud-flats of, 348 ; mud-lumps 
 
 and craters on delta of, "348 ; oxbows 
 
 and meanders of, map, 710* 
 Mississippi Valley, St. Peter sandstone 
 
 of, 75 
 
 Modified drift, 502 
 
 Modiola, 310*; in marshes, 333 
 
 Moffettes, 181, 182* 
 
 Mohawk River, 728, 779 
 
 Mon's scale of hardness, 49 
 
 Mollusks, 3io*~3i5*; as bottom feeders, 
 
 347 
 
 Molybdenum minerals, 55 
 Monaco, Prince of, 14 
 Monadnock, Mt., 741, 742* 
 Monadnocks, 741 ; described, 705 ; on 
 
 Great Plains, 701 * 
 Monazite, 53 
 Monch, the, 368 
 Mono Lake, California, 258; trona in, 
 
 223 
 
 Monocline, 600; diagram of, 602* 
 Monoclinic, prism, 47*; pyramid, 48*; 
 
 system, 47 ; principal types in, 47 
 Monogenetic elastics, 567 
 Mont Blanc, 369 
 Mont Dore, 145, 146; massif of, 147*; 
 
 section of, 148* 
 Mont Pelee, 87 ; described, 136* ; spine of, 
 
 136*, 162*, 163* 
 Monte Minardo, 135 
 Monte Nuovo, in, 112, 113*; described, 
 
 112 ; eruption of, 128, 693 
 Monte Rossi, 135 
 
 Monte Somma, 127*; map of, 125* 
 Monteleone, Italy, earthquake manifesta- 
 tions in, 667 
 Montgomery, 715 
 Monticules, 135*, 136, 188 
 Monument Park, erosion pillars in, 407*, 
 
 409 
 Monuments of erosion, 407*; wind carved, 
 
 822 
 
 Monzonite, 100 
 Moon, 2 
 Moorlands, 329 
 Moraines, ground, 496 ; interlobate, 501 ; 
 
 lateral, 368, 370/492, 493* ; medial, 370, 
 
 492, 493*; terminal, 370, 493, 494*; 
 
 relation of terminal and interlobate, 502 ; 
 
 sub-marginal, 493 
 Morainic dam, 803 
 Moray Firth, 614, 759 
 Morse Creek limestone, 31* 
 Morse Glacier, 376* 
 
 Moselle River, development of, 743, 744* 
 Moses, J. A., and Parsons, C. L., ref., 
 
8 4 8 
 
 Index 
 
 Moss Mountain, section and restoration, 
 
 201* 
 Mother liquor, composition of, 220; 
 
 denned, 229 
 Moulins, glacial, 368 
 Mount Taylor, New Mexico, volcanic 
 
 necks of, 166* 
 
 Mountain chains of Europe, map of, 605* 
 Mountain-enclosed basins, deposits in, 471 
 Mountain of the Law, 754 
 Mountainous countries, outcrops in, 33 
 Mountains, illustrations of dynamical 
 
 geology in, 36 ; relative size of, 2, 3 
 Movements, metamorphism in, 645 
 Movements of lithosphere, with rock 
 
 shattering, 80 
 
 Mt. Catherine, Sinai, 754; map, 755* 
 Mt. Desert, batholith of, 205 
 Mt. Grey lock, Mass., 598* 
 Mt. Holmes, Mont., 203* 
 Mt. Monadnock, N. H., 741, 742* 
 Mt. Morris, N. Y., 780, 782, 784 ; on map, 
 
 776*; sections at, 30 
 Mt. Rainier, 794*; andesite from, 104 
 Mt. Serbal, 754 
 Mt. Shasta, andesite from, 104 
 Mt. Sinai, 754 
 Mt. Sir Donald, 797 
 Mt. St. Elias, 382 
 Mud-cracks, 480; fossil, 482*, 490*, 
 
 491; in river deposits, 490; of Little 
 
 Colorado River, 479* 
 Mud-flats, 347, 348 
 Mud-line, 529 
 
 Mud-rock, denned, $6g ; contact metamor- 
 phism of, 210 
 
 Mud-stones, described, 580 
 Mud streams of Calabrian earthquake, 669 
 Mud volcanoes, 181, 182*, 183, 680; due 
 
 to earthquakes, 690 
 Muir Glacier, 376*, 378; map of retreat 
 
 of, 379* 
 Muka Muka cliffs, elevation at, 675 ; on 
 
 map, 676* ; section of, 677* 
 Mull, Island of, map and section, 193* ; 
 
 radial dikes of, 192, 194* 
 Munson, Mass., conglomerate gneiss of, 
 
 653 
 Mur River, Austria, destruction of rock 
 
 fragments in, 414 
 
 Murray, J., 143 ; cited, 6 ; on reef origin, 296 
 Muscovite, 63, 93 
 Mussel, 310*, 311 
 
 Naero Fjord, 801* 
 
 Nagelfluh, 563*, 564* 
 
 Nahant, Mass., 819 ; map of, 544* ; sec- 
 tion, on coast of, 35* 
 
 Naivasha Lake, Africa, on map, 632* 
 
 Nancy, 731 
 
 Nantasket Beach, 544 ; development of, 
 diagrams, 818*, 821* 
 
 Nantucket, apron-plain, 503; Sound, on 
 map, 808* 
 
 Naples, Italy, 691 
 
 Naples, N. Y., on map, 776* 
 
 Naples Bay, change of level, 694 
 
 Naples volcanic field, in ; map of, no* 
 
 Nashville dome, 728, 729* 
 
 Naters, 364 
 
 Natrolite, 63 
 
 Natron Lakes, Egypt, 259 
 
 Natural bridge on California coast, 812*; 
 Utah, 823*; of Virginia, 425, 426* 
 
 Natural bridges, 822 
 
 Natural classification of rocks, 68 
 
 Natural gas, 353 
 
 Nautilus, 313, 314*, 315 
 
 Navigator Islands, tsunamis at, 674 
 
 Neap tide, 5, 24 
 
 Nebraska, dunes of, 451 
 
 Neckar Valley, on diagram, 756* 
 
 Necks, volcanic, 163, 165*, 166* 
 
 Nefud Desert, 451 
 
 Nephelite, 61, 92 
 
 Nephelite syenite, 96, 101, 102 ; . series, 
 glasses of, 102 
 
 Nephelite-syenite-phonolite, series, 102 
 
 Nephelite-syenite-porphyry, 96, 102 
 
 Neptunea, 311* 
 
 Neritic zone, 518; on diagram, 519* 
 
 Nero, Bath of, 113 
 
 Neve, 357, 358*, 367*; on map, 360* 
 
 Newberry Butte, 790 
 
 New Brunswick, raised beaches of, 694 
 
 New England, coastal section of, 35, 36 ; 
 deformed strata of, 583 ; eskers of, 
 506*, 507* ; old lavas of, 100 ; rock ex- 
 posures in, 34 ; sand plains of, 503*, 504* 
 
 New England coast, 806 ; dike chasms of, 
 191*; erosion of dikes on, 816 
 
 New England peneplane, 702, 742*, 743 
 
 New England upland, 744 
 
 Newfoundland, coast sections of, 36 
 
 New Jersey, coast of, 810 ; coastal plain in, 
 715 ; meadows of, 332 ; rock exposures 
 of mountains of, 34 
 
Index 
 
 849 
 
 New Madrid earthquake, 670 
 
 New Mexico, earthquake effects in, 678; 
 volcanic neck in, 166 
 
 New Mountain, Japan, 112 
 
 Newport, R. I., wave-cut chasm near, 
 816* 
 
 New world mass, 510 
 
 New York City, flagstones of, 74 ; rock 
 exposure near, 34 
 
 New York State, deformed strata of, 583 ; 
 drumlins of, 498; geological map of, 
 30; geological system, 27; Mass. 
 boundary line, rocks of, 306 ; old coral- 
 reefs of, 306; salt deposits of, n, 245; 
 illustrations of rock outcrops from, 29 
 
 New Zealand, earthquake of, 675; fjords 
 of, 804, 806* ; map of, 676* ; solfataric 
 vents of, 181 
 
 Niagara, 779; section of falls at, 781* 
 
 Niagara cuestas, map, 727*; on block dia- 
 gram, 760*; section of, 761* 
 
 Niagara escarpment, 763, 767, 780 
 
 Niagara Falls, 418*, 764* ; geological his- 
 tory of, 760; mode of cutting of, 771; 
 on map, 763*; unequal retreat of, 773; 
 on map, 773* 
 
 Niagara Glen, 775* 
 
 Niagara Gorge, 802 ; bird's eye view of, 
 767*; map of, 763*; outcrops in, 29; 
 section of, 776*; youthful condition of, 
 708 
 
 Niagara River, geological history of, 760 ; 
 peculiar course of, 763, 769 ; rate of re- 
 treat of, 771 ; section of, 30* ; volume of, 
 771 
 
 Niagara sea, 246 
 
 Nicaragua, alignment of volcanoes in, map, 
 172* 
 
 Niccolite, 52 
 
 Nickel minerals, 5, 52 
 
 Nicolosi, destruction of, 135 
 
 Nile, flood plain of, 477*, 480 ; mud-cracks 
 
 on flood plain of, 476* ; mud flats of, 348 ; 
 
 delta of, 484*, 485 ; mud, organic matter 
 
 of, 348; region, map of, 755 
 
 Nipissing, Great Lakes, map, 770* 
 
 Niter soda, 58 
 
 Nittany Valley, limestone solution of, 427 
 
 Nodules of oxide of manganese, 249* 
 
 Non-clastic rocks, 70 
 
 Non-fragmental rocks, 70 
 
 Non-marine overlap, 562* 
 
 Non-metallic luster, 49 
 
 Non-vascular plants, deposits of decaying, 
 
 346 
 Norfolk, England, sand dunes on coast of, 
 
 445, 446 
 ^orite, 105 
 
 Normal faults, 621 ; on diagram, 620* 
 Normandy cliffs, map showing erosion of, 
 
 509* 
 
 North Africa, dunes of, 448 
 North America, Atlantic coast of, 35; 
 
 raised beaches of, 694 
 North Carolina, sand bars on coast of, 
 
 540* ; diorite from, 105 
 North Island, New Zealand, 675 ; on map, 
 
 676* 
 
 North Pacific, diatom ooze of, 323 
 North Sea, 518 
 
 North Sea coast, England, erosion on, 429 
 Norway, coast of, 806 ; marbles in, 654 ; 
 
 raised beaches in, 694 
 Norwegian fjords, falls in, 802 
 Nose (topographic), 740 
 Nottingham, England, 719 
 Novaculite, 581 
 Nova Scotia, erosion of basaltic rocks in, 
 
 813 ; erosion features on coast of, 816 ; 
 
 sea-stacks on coast of, 815; volcanic 
 
 agglomerate in, 577*; volcanic plugs in, 
 
 173 
 Nubian sandstone, 75 ; as source of sands of 
 
 Libyan Desert, 452 
 Nullipores, 271; of Triassic reefs, 306; 
 
 on coral reefs, 291 
 Nummulites, 280* 
 Nummulitic limestone, 76, 280* 
 Nunataks, 386, 493, 804 
 
 Oatka creek, on map, 776* 
 Oatka valley, 776, 779 
 Oban, Scotland, 759 
 Oblique faults, 623, 624 
 Obolus, 309 
 Obsidian, 95, 96, 98 
 
 Ocean, salinity of water of, 228 ; tides of 
 ideal, 526; velocity of currents of, 526 
 Oceanic coral reefs, 292 
 Oceanic currents, 529 
 Oceanic reefs, organisms of, 293 
 Oceanography, 14 
 Oceanographic institutes, 14 
 Oceans, 4, 509; the four, 510 
 Oceanic depressions, 3 
 Ochsenius, Bar theory of, 241 
 
850 
 
 Index 
 
 Octahedron, 44* 
 Octopus, 313 
 Odor, 50 
 
 Offlap and overlap, 559, 560* 
 Offlapping, progressive, 558, 559* 
 Offset, 620*; in faulting, 625* 
 Off-shore shallow water, deposits in, 549 
 Ohio, old coral reefs of, 306 ; Sylvania 
 
 sandstone of, 75 ; freestone, 578; 
 
 River, in flood, 474* 
 Oil, field, 351*; producers, 352; sands, 
 
 352; shales, 334, 353, 580; storers, 352 
 Okefenoke Swamp, 339 
 Oklahoma, section in, 32 
 Obsequent streams, 714 
 Old age of land, 705 
 Old Faithful geyser, 184, 185* 
 Old Harry, sea-stack, 815 
 Old Man of the Mountain, 339* 
 Old plains, 699, 744 
 Old Red Sandstone, 575* 
 Old Stone Age, 265 
 Old world man, 510 
 Older igneous masses, 188 
 Older vegetal deposits, 343 
 Oldland of Wales, 719* 
 Olean conglomerate, 569 
 Oligoclase, 9 
 Olivine, 93 ; gabbro, 96. 105 ; porphyry, 
 
 96 ; norite, 105 
 
 Omori, Japan, earthquake rifts near, 68 1 
 Onondaga, cuesta, 760, 761*, 763; lime- 
 stone, disconformity beneath, 616* 
 Onondaga-Marcellus oil, 352 
 Ontario, Lake, see Lake Ontario 
 Ontario, salt deposits of, 245 ; western, 
 
 Sylvania sandston ^, 75 
 Ontario dome, 725, 728, 761, 775; block, 
 
 diagram of, 760*; map of, 727; rate of 
 
 rising of, 696 
 
 Ontario lowland, 761, 775; section of, 761* 
 Ontario Valley, 779 
 Onyx, -Mexican, 221, 261 
 Oolites, 217*, 574; of bacterial origin, 
 
 271*; deposited by springs, 262; of 
 
 great Salt Lake, origin of, 274 
 Oolite cuesta, England, 719* 
 Oolitic limestone, 217*; precipitated by 
 
 bacteria, 302 
 Oolitic texture, 217* 
 Opal, 61 
 Opalescence, 50 
 Opaque, 50 
 
 Opheopholis, 320* 
 
 Ophicalcites, 649, 654 
 
 Ophitic textures, 106 
 
 Oppido, Italy, earthquake manifestations 
 in, 666, 668 
 
 Orbitoides, from Cuba, 281* 
 
 Orbitolites, 300 
 
 Orbulina, 275* 
 
 Ordinario (marble), 654 
 
 Ordovician, Bryozoa, 308* ; elastics, 75 
 
 Ore chamber, 266* 
 
 Organ pipe reef, 300 
 
 Organic, elastics, 568; matter from Nile 
 mud, 348; precipitates, 269*; remains, 
 in marine elastics, 554; rocks, i, 72, 
 73, 269; rocks, as source of clastic 
 material, 528; salts, 215; sediments, 67 ; 
 separation of lime in lakes, 257 ; silica, 
 322; spheres, 12; tissues, 269 
 
 Organisms, as rock producers, 71; com- 
 mingling of remains, in delta, 486; 
 rock destruction by, 436 
 
 Origin of rivers, 411 
 
 Oriskany sandstone, 80*, 578* 
 
 Orkney Islands, sea-stacks on, 814 
 
 Orpiment, 54 
 
 Orthoceran type, 315 
 
 Orthoceras, 315*' limestone, 315 
 
 Orthoclase, 61, ^ j, 91 ; recognition of, in 
 granite, 95 
 
 Orthopinacoid (Monoclinic), 47* 
 
 Orthorhombic, prism, 47*; pyramid, 47*; 
 system, 47 ; principal types in, 47 
 
 Osar, see Esker 
 
 Oscillation waves, 521 
 
 Osorno, eruption of, 674, 675 
 
 Ostend, dunes at, sections, 459* 
 
 Ostracods, 317, 318* 
 
 Ottawa outlet, on map, 770* 
 
 Outcrop, 2Q; deflection of, diagram, 587*, 
 588*; relation between width of, and 
 dip, 588 ; width of, diagram, 589* 
 
 Outcrops, in mountainous countries, 33; 
 of igneous rocks, 84 
 
 Outer lowland, England, 71.9* 
 
 Outliers, 740*; origin of, 714 
 
 Outwash plain, 503 
 
 Overdeepened subsequent, 721* 
 
 Overdeepening, 801 
 
 Overlap, marine, 562*; non-marine, 562*; 
 of beds of alluvial fan, 466 ; progressive, 
 555*, 556*, 557*; replacing, 560*, 
 561* 
 
Index 
 
 851 
 
 Overturned anticline, 590* 
 
 Owari province, Japan, earthquake in, 
 
 680 
 Owen's Lake, Cal., 250, 258, 259 ; trona in, 
 
 223 
 
 Owen's Valley earthquake, 677 
 Oxbow, 709; intrenched, 707, 708*; 
 
 formation of, 417* ; of Mississippi River, 
 
 map, 710* 
 
 Oxford, England, 719 
 Oxidation, 402 
 Oxides, 39 
 
 Oxus River, map, 449* 
 Oxyhydroxides, 40 
 
 Ozark dome, map, 724*; plateau, 723 
 Ozokerite, 61, 351 
 
 Pacific Ocean, 510 ; coral sands and muds 
 of, 552 ; radiolarian ooze in, 326 
 
 Pahoehoe, 120 
 
 Pailleret, Puy de, 149 
 
 Paint Pots, Yellowstone, 183 
 
 Palaeogeography, 20 
 
 Palaeolithic time, 265 
 
 Palaeontology, 19 ; science of, 15 
 
 Palaeozoic, coal, 345 ; crinoidal limestone, 
 321*; petroleum, 352 
 
 Palestine rift valley, 631, 633, 757 
 
 Palisades, 197*, 198; columnar structure 
 of, 199 ; contact metamorphism in, 
 207 ; diabase, base of, 200 ; section of, 
 199*; trap rock of, 73 
 
 Pallas, P. S., 26 
 
 Paludina. 315* 
 
 Panama Canal, geological advice in con- 
 struction of, 21 
 
 Paradoxides, 319* 
 
 Paraffin, 351 
 
 Parallel Roads of Glen Roy, 363*, 364*; 
 map, 365* 
 
 Parana, 547 
 
 Parasitic cones, 188 
 
 Parian marble, 654 
 
 Paris, 731; Oceanographic Institute of, 
 14; on map, 730; rocks of, 64; sec- 
 tions near, 32, 33 
 
 Park City, Utah, 268* 
 
 Pattie Basin, map of, 232* 
 
 Pearlite, 98 
 
 Pearly nautilus, 314* 
 
 Pearly luster, 49 
 
 Peat, 3^9*; beds in alluvial plain, 469; 
 bog, Scottish, 341* 
 
 Peat deposits, formation of, 329; on coast, 
 
 807 ; on shore, 809 ; thickness of, 332 
 Peat marsh, 538 
 Peat-moss, 334*, 336 
 Pebble beaches, 532 
 Pebbles, form of, 430* ; formation by wave 
 
 erosion, 430; glaciated, 497 
 Pechuel-Loesche, quoted, 436 
 Pecten, 310* 
 Peekskill, N. Y., quartz-mica diorite from, 
 
 103 
 Pegmatite, 93, 97, 207; dikes, 88, 192, 
 
 193*; veins, 206 
 Pegmatitic structure, 207 
 Pekin, see Peking 
 Peking, 467 
 Pelagic district, 519 
 Pelecypods, 309, 310* 
 Pelee, spine of, 161 ; view, 162*, 163* 
 Pele's hair, 96, 107, 118 
 Peloponnesia Mts., on map, 605* 
 Pelvoux, Massif de, 366* 
 Penck, A., 74 ; on reef origin, 297 
 Peneplane, 148, 701, 705, 806; formation 
 
 of, 716; New England, 702; relation 
 
 to base-level, 705 
 Pennine chain, section, 198* 
 Pennine escarpment, 197 
 Pennsylvania, old coral reefs of, 306; 
 
 rock exposures of mountains of, 34 
 Pennsylvanian coals, 344 
 Pentelic marble, 654 
 Pentlandite, 52 
 Periades, Glacier des, 369 
 Peridotite, 96, 105 ; change to serpentine, 
 
 106 
 
 Peridotitic-gneiss, 653 
 Period of waves, 521 
 Perlite, 96 ; thin section of, 99 
 Permeability of rock, 422 
 Permian age of Helgoland rocks, 79 
 Permian glaciated rock fragment, 435* 
 Permillage, 228 
 Permille, 227 
 
 Persia, salt mountain of, 822 
 Persian salinas, 245 
 Peruvian coast, guano of, 322 
 Pervious beds, 422 
 Petrifactions, 19 
 Petrifactology, 19 
 Petrified bird's nest, 219 
 Petrography, 18 
 Petroleum, 351; chemical origin of, 353; 
 
Index 
 
 formation diagram, 352* ; from diatoms, 
 324; geology, 21 ; origin, 313; source, 
 352 
 
 Petrology, 18 
 
 Petrosilex,-ioo 
 
 Phacolith, 202, 203 
 
 Phenocrysts, 88, 89 
 
 Philadelphia, Pa., 722 
 
 Phlegraean field, map of, no*; solfataric 
 action in, 181 
 
 Phlogopite, 63 
 
 Phonolite, 96, 102 ; felsite, 96 ; obsidian, 
 102 ; porphyry, 06, 102 
 
 Phosphate of lime, in sea, 248; organic, 
 322 ; secreted by brachiopods, 309 
 
 Phosphate rock, 222 
 
 Phosphorus minerals, 61 
 
 Phragmites, 335, 337*; in upland bogs, 342 
 
 Phyllites, 649, 650, 651 
 
 Physa, 315, 317* 
 
 Physical geography, 18 
 
 Physiography, 18, 20 
 
 Phytology, 15 
 
 Picrites, 105 
 
 Pictured Rocks, Lake Superior, 812 
 
 Piedmont glaciers, 359, 378, 382, 384 
 
 Pikes Peak, 64, 396, 636*; contact at, 
 21 1 ; disintegration of granite on, 395*; 
 erosion on, 821 ; wind-carved monu- 
 ments near, 822 
 
 Phillips, A. H., ref., 51 
 
 Pillow lava of Hawaii, 119 
 
 Pillowy lava, 195 
 
 Pinacoidal planes, 42, 43 
 
 Pindus Mts., on map, 605* 
 
 Pine Barrens, 715* 
 
 Pinelands, 715 
 
 Pinus, 337* 
 
 Pipes, volcanic, 161, 163 
 
 Pisolite, 218*, 574; deposited by springs, 
 262 
 
 Pisolitic texture, 218 
 
 Pitch Lake of Trinidad, 351 
 
 Pitch of axis of anticline, 594 
 
 Pitching folds, 594*; eroded, 594*, 595* 
 
 Pitchstone, 96, 98 
 
 Place of cooling, relation of texture to, 89 
 
 Placer deposits, 267 
 
 Plagioclase, 61, go, 91 
 
 Plain, 607 ; alluvial, 465 ; definition lim- 
 ited, /or 
 
 Plains, mature, 699 ; of construction, 697 ; 
 young, 697 
 
 Plains country, sandstone buttes of, 822 
 Plane, 707 ; high-level, 709 ; of marine 
 
 erosion, 706 
 Planetary currents, 529 
 Plankton, 279, 348 
 Planktonic organisms, 352 
 Planorbis, 315*, 317 
 Plant types in lake, 347* 
 Plant zones in lake, 333 * 
 Plants, carbonate of lime deposited by, 
 
 270; as rock-breakers, 81 ; of salt 
 
 marshes, 334* 
 Plaster of Paris, 221 
 Plateau basalts, 149 
 Plateaus, 703 
 
 Platinum, 57; minerals, 57 
 Platte River, braided character of, map, 
 
 477* 
 
 Plattenkalke, 572 
 Platy rocks, 572 ; structure, 572 
 Plauen'sche Grund, syenite from, 101 
 Play fair, John, portrait, 26* 
 Playa deposits, 79, 480; characters of, 
 
 483 ; lakes, 244*, 480 
 Pleistocene, 20; glacial deposits of, 496; 
 
 gravel covered by lava, 143* 
 Pliny, on eruption of Vesuvius in 79 A. D., 
 
 127 
 
 Plomb-du-Cantal, 149 
 Plucking, glacial, 433 
 Plug, lava, of Mt. Pelee, 163 
 Plugs, volcanic, 163 
 Plutarch, on early condition of Monte 
 
 Somma, 127 
 Plutonic plug, 204 
 Pochutla, 112 
 Point of Rocks, Md., 569 
 Polarized light, examination of rocks by, 
 
 9i 
 
 Polistena, Italy, earthquake manifesta- 
 tions in, 669 
 
 Pollen grains in lake deposits, 334 
 Polygenetic elastics, 567 
 Poly halite, 236 
 Polyps, 283 
 Polystomella, 275* 
 Pompeii, 126; burial of, 127; ruins of, 
 
 127* 
 
 Pond, plant-filled, section, 338* 
 Pond-lilies, 335 
 Ponds, 4 
 
 Popocatepetl, 109 
 Porcelanite, 210, 650, 651 
 
Index 
 
 853 
 
 Pore spaces of rocks, 4 
 
 Porites, 285, 287*, 299, 301; on reefs, 291 
 
 Porphyries, 89 
 
 Porphyritic, andesite, 96 ; augite andesite, 
 96 ; augitite, 96 ; basalt, 96 ; dacite, 
 96; diorite, 96; gabbro, 96; granite, 
 96; limburgite, 96; olivine gabbro, 
 96 ; peridotite, 96 ; phonolite, 96 ; pyrox- 
 enite, 96 ; quartz diorite, 96 ; rhyolite, 
 96; syenite, 96; texture, 87*, 88, 89; 
 trachyte, 96 
 
 Porphyry, 87* 
 
 Port Nicholson, New Zealand, earthquake 
 effects at, 677 
 
 Port Royal, Jamaica, earthquake effects, 
 at, 688 
 
 Portage-Chemung cuesta, 760; on block 
 diagram, 760*; section of, 761* 
 
 Portage, gorge at, 784, 78* ; Lower Falls of 
 Genesee at, 783*, 784, 785; Middle 
 Falls of Genesee at, 782*; sections at, 
 30; Upper Falls of Genesee at, 783* 
 
 Portageville, N. Y., 782, 784 ; moraines, 
 780; on map, 776*, 779*; region, map 
 of ancient river valleys in, 778* 
 
 Potamogeton, 347 
 
 Potash from kelps, 271 
 
 Potash salts, 223 ; .in desert deposits, 244 ; 
 of Germany, 235 
 
 Potassium minerals, 58 
 
 Pot-holes, 4i9*~42i*; elevated, 419; 
 glacial, 368 ; in boulders of ancient 
 Niagara, 775 
 
 Potsdam quartzite, 578; sandstore, 75, 
 579 
 
 Poughquag quartzite, 627 
 
 Pozzuoli, in, 113, 691 
 
 Prairie, origin of, 702* 
 
 Prairie Plains, 701 
 
 Pre-Cambrian of Manhattan Island, 75 
 
 Precious coral, 284 
 
 Precipitate, 215 
 
 Precipitation, methods of, 216; of car- 
 bonate of lime, methods of, 252 
 
 Prehnite, 63 
 
 Pressure, induration by, 564 ; metamor- 
 phism by, 643 
 
 Primary textures, 87 
 
 Primitia, 318* 
 
 Primitiopsis, 318* 
 
 Prismatic structure in basalts, obsidian, 
 etc., 179 
 
 Prisms, 43 
 
 Prochlorite, 63 
 
 Progressive offlapping, 558*, 559* 
 Progressive overlap, 555*, 556*, 557*, 
 Protozoa, 275, 325 ; as source of petroleum, 
 
 352 
 
 Proustite, 57 
 Provincetown, 820; dune lands of, 447; 
 
 headland, 543 ; on map, 808* 
 Pseudomonas, 271* 
 Pseudopodia, 275 
 Psilomilane, 52 
 Pteropod ooze, 313* 
 Pteropods, 311, 312*, 313*; deep-sea 
 
 accumulations of, 552 ; killed in estua- 
 ries, 348 
 
 Ptychoparia, 319* 
 Pudding-stone, 576*, 577 
 Pulpit Rock, Nahant, 35* 
 Pumice, 95, 96, 99* 
 Pungent taste, 50 
 
 Purgatory chasm, Newport, R. I., 816*, 817 
 Puy, Bassin de, section, 152*; de Cliergue, 
 
 149; de Dome, 146*; de Grioce, 149; 
 
 de Pailleret, 149 
 Puys, chaine des, 145, 146, 147* ; of France, 
 
 64 
 
 Pyramid Lake, tufa dams of, 256* 
 Pyramids, 42 
 Pyrenees Mts., marbles of, 653 ; num- 
 
 mulitic limestone of, 280; on map, 605* 
 Pyrite, 52 ; in marsh deposits, "3 3 3 ; oxida- 
 tion of, 403 
 
 Pyritiferous shales, 581 
 Pyritohedron, 44* 
 
 Pyroclastic material, 80, 85 ; rocks, 80, 83 
 Pyroclastics, 568 
 Pyrogenic material, 80; rocks, 70, 71, 72, 
 
 73 ; principal types, 84 
 Pyrolusite, 52, 224 
 Pyrosphere, 5, 70, 84; as rock-breaker, 
 
 80 ; contribution to lithosphere, 1 1 ; 
 
 derivation of term, 12 ; destructive 
 
 work of, 431 ; operation in rock-breaking, 
 
 391 
 Pyroxene, 62, 93 ; distinctive form and 
 
 cleavage of, 94* 
 
 Pyroxenes, 93 ; separation of, 94 
 Pyroxenite, 96, 105, 106 ; gneiss, 653 ;' 
 
 porphyry, 96 
 Pyrrhotite, 52 
 
 Quahaug, 310*, 311 
 Quaking bog, 336 
 
854 
 
 Index 
 
 Quarries as sources of geological informa- 
 tion, 29 
 
 Quarrying, 391, 392 ; by rivers, 414, 415 ; 
 glacial, 433 
 
 Quartz, 39, 61, go ; cause of imperfect 
 crystallization of in igneous rocks, 
 95 ; conglomerate, 577 ; dacite series, 
 103 ; diorite, 96, 97, 107 ; diorite 
 porphyry, 96, 103 ; diorite series, glasses 
 of, 103 ; felsite, 95, 96, 97 ; felsite por- 
 phyry, 96 ; flour rock, 581 ; mica diorite, 
 103 ; rocks, contact metamorphism of, 
 211 ; sandstone, 570, 578; solution of, 
 425 ; syenite, 101 
 
 Quartzite, 211, 565, 578, 649, 650; gneiss, 
 653 
 
 Queensland, 467 
 
 Quiet type of volcano, 140 
 
 Quiriquina Island, earthquake effects on, 
 674 
 
 Radial consequents, 722 
 Radial dikes, 192, 194*, 195* 
 Radiolaria, 323, 324, 325 ; deep-sea ac- 
 cumulations of, 552; flint from, 280; 
 
 in red clay, 553 ; silica obtained by, 426 
 Radiolarian ooze, 326 ; map of distribution 
 
 of, 277 
 
 Radium minerals, 55 
 Rafted material in sea, 530 
 Rain, as agent of erosion, 820 ; sculpturing 
 
 by, 821 
 
 Rainbow Natural Bridge, 823 
 Raindrop impressions, 490, 491* 
 Raindrops, fossil, 482 
 Rain-water, disposition of, 411; erosive 
 
 work of, 410 
 Raised beaches, 694; in Great Lakes 
 
 .region, 695 
 
 Rakata of Krakatoa, 140* 
 Raleigh, N. C., 722 
 Ran of Cutch, 232 
 Rantum, Church of, overwhelmed by 
 
 dunes, 444 
 
 Rare earth minerals, 53 
 Rascles, 413* 
 Rate of cooling of igneous rock, influence 
 
 on texture, 87 
 Reade, T. Mellard, cited on solution of 
 
 rock, 427 
 Realgar, 54 
 
 Reconstructional forces, 19 
 Recumbent anticline, 590* 
 
 Red Bank, N. J., oxidized greensands of, 
 250 
 
 Red beds, gypsum of, 221 ; of loessic origin, 
 459 ; origin of, 492 ; origin of, in Bay of 
 Fundy, 535 
 
 Red clay, 553 
 
 Red Cliff Peninsula, ice mantle of, 385 
 
 Red muds, 551 
 
 Red Sea, 513, 515; map of, 517*; salinity 
 of, 228 ; salt pans of, 232 
 
 Red shales, 581 
 
 Red Valley, 722, 723* 
 
 Red Wall limestone, 787, 789, 790 
 
 Reeds, 335, 337* ; in upland bogs, 342 
 
 Reef, 290 
 
 Reef-building animals, 282 
 
 Reef-section of Palaeozoic, 304* 
 
 Reefs, common structures of, 304 
 
 Reelfoot Lake, 671 
 
 Regelation theory of glacier movement, 
 389 
 
 Reggio, Italy, effects of earthquake in, 
 670 
 
 Regional metamorphism, 643 
 
 Regur of India, 459 
 
 Reid, H. F., cited, 657 
 
 Relative humidity of air, 355 
 
 Relief, stages of, 705 ; impression of tracks, 
 etc., origin of, 482 
 
 Replacement deposits, 266 
 
 Replacing overlap, 560*, 561* 
 
 Resequent fault-line scarp, 759; fault 
 scarps, 792 
 
 Residual, boulder conglomerate, 577 ; rocks, 
 568; soil, consolidated, 567 
 
 Resina, destruction of, 128 
 
 Resinous luster, 40 
 
 Retreat of glacier front, 378, 382 
 
 Reverse faults, 622 
 
 Reversible Fall, St. John, 524*, 525* 
 
 Rhabdoliths, 274 
 
 Rhabdosphere, 274* 
 
 Rheims, on map, 730* 
 
 Rheims-Epernay-Sezanne escarpment, 731 
 
 Rhine, gorge and peneplane of, 702, 703*; 
 graben, 630, 756, 757 ; diagrams of 
 development of, 756 ; region, extinct 
 volcanoes of, 154; rift valley of, 631*; 
 sandbanks of, 462 ;" trough, 754; vol- 
 canic district of, 156* 
 
 Rhizocrinus, 321* 
 
 Rhodochrosite, 52 
 
 Rhodonite, 52 
 
Index 
 
 855 
 
 Rhombohedron, 46* 
 
 Rhone Glacier, map, 375*; views of 
 
 change in past, 380 
 Rhone River, 364, 376 ; irregular deposits 
 
 by, 488, 489* 
 Rhone Valley, 361 
 Rhyolite, 95, 96, 97 ; origin of name, 98 ; 
 
 porphyry, 96, 98 
 Ribbon limestone, 572* 
 Richmond, Va., 722 
 Ridge Road, 763 
 
 Rift valley, 630, 754; of Rhine, map, 631* 
 Rift valleys of Africa, 631 ; map of, 632* 
 Rikwa Lake, Africa, on map, 632* 
 Rill-marks, 489, 553, 554* ', fossil, 490* 
 Rimutaka Mts., earthquake effects in, 
 
 675 ; fault cliff in, 677 ; on map, 676* 
 Ripple marks, 553; current, 489*; dia- 
 grams of types of, 550; fossil, 490*; 
 
 in shallow water deposits, 550 ; on dunes, 
 
 453*; on sea shore, 535 
 Ripples, types of, in desert, 454 
 Rise of ocean surface, theory of coral reef 
 
 origin, 297 
 
 River-bottom dunes, 448 
 River capture, 734*, 735* ; in coastal plain, 
 
 720* 
 \ River deposits. 464 ; characters of, 486 ; 
 
 red color of ancient, 491; variation of, 
 
 section, 488* 
 
 * River flood-plain deposits, 472 
 
 - River sediments, sorting of, 462 ; varia- 
 
 tion in, sections, 478* 
 
 River terraces, Columbia River, 472*; 
 West River, 473* 
 
 River valleys as sources of geological in- 
 formation, 29 ; illustrations of dynamical 
 .^ geology in, 36 
 
 River water, erosive work of, 412 
 ^ Rivers, 4 ; chemical deposits of, 259 ; 
 erosion products of, 411 
 
 Roba-el-Khali, 451 
 
 'Rocca Morifina, section, 143* 
 
 Rocher Corneille, 165, 166; St. Michel, 
 165*, 1 66 
 
 Roches moutonnees, 373*, 374 
 
 Rochester, N. Y., 775; Lower Falls of 
 Genesee at, 780* ; Middle Falls of Gene- 
 see at, 780* ; section near, 30 ; sections 
 of falls at, 781* ; Upper Falls of Genesee 
 at, 780* 
 
 Rock, 38, 64 ; avalanche, 398 ; destruction, 
 390; destruction by rivers, variation 
 
 of, 415; exposures in flat countries, 
 28; flour, of glaciers, 499; flour, of 
 ground moraine, 496 ; origin of, 434 ; 
 flowage, 646 ; fracture, 646 ; glaciers, 401 ; 
 salt, 220; shattered by plant growth, 
 81*; step, 800*; structures, field for 
 study of, 28 
 
 Rockaway Beach, 538 ; map of change in, 
 542 ; rill-marks on, 554* 
 
 Rocks, 17 ; age relations of, 64 ; classifica- 
 tion of, 64, 66 ; principal types of, 64 
 shattering of heated rock by cold rail 
 water, 410 
 
 Rocky Mts., 583, 759 ; Front Range of, 
 738 ; rock exposures of, 35 ; reduction in 
 width by folding of, 619 
 
 Rodderberg, 157* 
 
 Rogers, A. T., ref., 51 
 
 Rolandseck, 157 
 
 Roman Mediterranean, 516 
 
 Rome outlet, 779 
 
 Rondout, unconformity near, 609*; dia- 
 grams, 610*, 6n* 
 
 Roofing slate, 581, 638 
 
 Roofshale, 345 ; fern in, 344* 
 
 Ropy lava, 100, 195 
 
 Rosenlaue Glacier, polished rocks of, 433* 
 
 Rotalia, 7*, 275*; in chalk, 278* 
 
 Rothliegendes, 236 
 
 Rothpletz, A., cited, 274 
 
 Rouen, in trenched, meanders near, 718* 
 
 Round Top, Md., arch at, 591* 
 
 Roxbury Pudding-stone, 576* 
 
 Royal Gorge of Arkansas, 471 
 
 Rubble concrete, 73, 74, 82, 83, 577 ; rock, 
 569, 575 
 
 Rubbly material, 569 
 
 Rudaceous texture, 569 
 
 Rudyte, 560, 575 
 
 Rukwa Lake, Africa, on map, 632* 
 
 Run-off, 356, 411 
 
 Rush salt grass, 334* 
 
 Rushes, 334* 
 
 Russia, great plains of central, 700 ; rock 
 exposures of, 34 
 
 Russian peneplane, 701 
 
 Rutile, 53 
 
 Sabrina Island, 112; described, 114* 
 Saco River, flood-plain of, 473* 
 Safnoni, 181, 224 
 
 Saginaw Bay, 726; map, 727*; river 
 (ancient), map, 727* 
 
856 
 
 Index 
 
 Sagittaria, 334* 
 
 Sahara Desert, 442*; foot-prints in, 491 ; 
 
 sub-sea level of floor of, 706 
 Sahara dust, transportation of, 443 
 St. Abb's Head, Scotland, unconformity 
 
 near, 613* 
 St. David's gorge (ancient), 767, 768, 769 ; 
 
 diagram, 767* 
 St. Francis Mts., 723, 724 
 St. Gotthard massif, section of, 606* 
 St. Helena, N. Y., on map, 776*, 779* 
 St. John, N. B., reversible fall at, 524*, 
 
 525* 
 St. Monans, Scotland, ripples on coast of, 
 
 537* 
 St. Patrick's Cathedral, N. Y., building 
 
 stone of, 653 
 Saint Peter sandstone, 75 
 St. Pierre, destruction of, 137 
 Salem upland, 723 
 
 Salina lowland, 761 ; section of, 761* 
 Salina salt beds, shafts sunk to, 31 
 Saline lagoons, 348; water, 214 
 Saline lakes, 480 ; deposits of, 250 
 Salinity currents, 527 ; defined, 228 ; 
 
 of water favoring coral reef growth, 291 
 Salisbury Crags, 198; section of, 199 
 Salt, 17, 38, 40; amount in sea water, 227 ; 
 
 formation of, 40, 41 ; from sea water, 
 
 229; in ocean water, 24; solution of, 
 
 404 ; use of term, 215 
 Salt, deposits, origin of, 1 1 ; gardens, 
 
 220; geology, 21 ; lick, 322; marsh, 
 
 map, 332*; meadow, 331, 332; mines, 
 
 220 ; mines, contortion of strata in, 645* ; 
 
 mountain, solution forms of, 822 ; 
 
 pans, 229 ; pans, of Black Sea, 232 ; peat, 
 
 silt in, 333 ; Range, India, overthrust in, 
 
 629 ; River, Arizona, section, 478* 
 Salton Desert, 252; Sink, 232; map of, 
 
 232* 
 
 Saltpeter, 259 
 Salts, ionization of, 41 ; order of deposition 
 
 of, 235 
 
 Salty water, 214; taste, 50 
 Salzburg, 563 
 San Bernardino, Pass, wind erosion in, 
 
 406 ; Valley, change of water courses in, 
 
 679 
 
 Sancy vent, 148, 149 
 
 Sand beaches, 533 ; firm character of, 529 
 Sand blast, 822 ; work of, 391, 405 
 Sand-craters, 680 
 
 Sand-dollar, 320* 
 
 Sand dunes, 77, 44S*~453*; types of, 443 
 
 Sand, garnet and magnetite, 529; grains 
 of Libyan Desert, 452 ; grains, rounding 
 of, in rivers, 415 ; grains, rounding of, on 
 beach, 43 1 ; grains, secondarily enlarged, 
 455*, 456; plain, glacial, 503, 504*; 
 ridges, 553 ; spit, 540 ; spits on Cape 
 Cod, 820; storms, 440; storms, in 
 Sahara Desert, 441 
 
 Sand Hills, Neb., 451 
 
 Sandia Mt., N. Mex., 749, 750* 
 
 Sandstone, 569, 577 ; buttes, 822 ; dike as 
 earthquake record, 689*, 690; of emer- 
 gence and of submergence, 559 
 
 San Filippo, Sicily, tufa deposits at, 260 
 
 San Francisco earthquake, 684; map of 
 fault trace, 685*; seismogram of, 662 
 
 Sanidine, 101 
 
 San Juan Canon, 706* 
 
 San Rafael piedmont glacier, 384 
 
 San Sebastian Vizcaino, 513 
 
 Santa Maria Island, Chile, earthquake 
 effects on, 674 
 
 Santiago, Chile, damaged by earthquake, 
 672 
 
 Santorin, in ; map, 122*; view of, 123* 
 
 San Vignone, Baths of, in Tuscany, rate of 
 tufa deposits in, 260 
 
 Sapping, 795 ; glacial, 433 
 
 Sapropel, 335, 351 
 
 Sapropelite, 334, 338, 346, 347, 348, 349, 
 351 ; in lakes and lagoons, 347 
 
 Sarcoui, Grand Puy of, 124*, 146 
 
 Sargasso Sea, 527 
 
 Sargassum, 527 
 
 Sassolite, 60 
 
 Satin spar, 220 
 
 Saturated solution, 215 
 
 Saturation, of air, 355 ; point, 215 
 
 Saussure, H. B. de, 26 
 
 Saville Kent, cited, 290 
 
 Saxicava arctica, in raised beaches, 694, 
 
 695* 
 
 Scale of hardness, 49 
 
 Scalenohedron, 46* 
 
 Scallop, 310*, 311 
 
 Scaly texture, 218 
 
 Scandinavia, eskers of, 507 ; gneisses in, 
 653; ice-caps of, 384, 493 ; metamor- 
 phic rocks of, 648 ; raised beaches in, 
 694 ; rock exposures of, 33 ', upland bogs 
 of, 342 
 
Index 
 
 857 
 
 Schalkenmehren Maar, 160* 
 Scheelite, 55 
 
 Schist, 649, 650 ; hornblende, 650 ; mica, 650 
 Schistosity, 647 
 Schwagerina, 282* 
 Schwarzwald, see Black Forest, 756 
 Sceria, 107 
 
 Scirpus, 334* ; in upland bogs, 342 
 Scotland, block conglomerate of, S3i ', 
 raised beaches of, 694; rock exposures 
 of Highlands of, 33 
 
 Scottish coast, erosion features on, 816 ; 
 
 sea stacks on, 814; sections of, 35 J 
 
 glens, deposits in, 71; lochs, 803; 
 
 rivers, sorting of sediment by, 464 
 
 Scottish Highlands, 64, 743 ; gneisses 
 
 of, 653 ; thrust in, 629* 
 Sculpturing processes of wind, 822 
 Scylla rock, 525 ; effects of earthquake on, 
 
 670 
 
 Sea, area of, 4 ; subdivisions of, 509 
 Sea-anemones on shore boulders, 532 
 Sea-caves, California coast, 811*; forma- 
 tion of, 811; on British coast, 812; 
 on French coast, 812 
 Sea-cliffs, 35 
 
 Sea coast, rock exposures on, 35 
 Sea fans, 284 
 Sea salinas, 220 
 Sea salts, composition of, 228, 229; 
 
 conditions favoring deposition of, 231 
 Sea scarps, abandoned, 810 
 Sea shore, illustrations of dynamical 
 
 geology on, 36 
 Sea stacks, 530*, 814 ; ancient, on Macki- 
 
 nac Island, 815 ; on Gotland, 815 
 Sea urchins, 319, 320* 
 Sea water, amount of salt in, 227 ; experi- 
 ments in evaporation of, 230; salinity 
 of, 228 
 
 Seaweeds, buoyancy given by, 522 ; illus- 
 tration of buoyant power of, 531 ; on 
 deltas, 348 
 
 Seasonal temperature changes, 820 
 Searle's Lake, Cal., 259; trona in, 223 
 Searle's Marsh, see Searle's Lake 
 Secondary enlargement of sand grains, 
 
 565* 
 
 Secondary minerals of igneous rocks, 94 
 Secondary structures, 582 
 Secondary textures, 88 
 Sectile, 49 
 Sedan, on map, 730* 
 
 Sedges, 336*; in swamps, 336 
 Sedimentary contact, 211; significance of, 
 
 213 
 
 Sedimentary rocks, 47 
 Seine, intrenched meanders of, 718* 
 Seismic center, 657 
 Seismic disturbances, types of, 656 
 Seismogram, 659, 660*, 661*, 662* 
 Seismograph, 659*, 660*, 661* 
 Seismology, 15 
 Selkirk Mts., 797 
 Selma, 715 
 
 Seminara, Italy, earthquake manifesta- 
 tions in, 669 
 
 Semper, C., on reef origin, 296 
 Sentis, erosion forms in, 413 
 Senya fault cleft, Japan, 682* 
 Separation of mineral matter from water, 
 
 215 
 
 Sepia, 314* 
 Septaria, 222*, 574* 
 Sericite, 403 ; in bluestone, 579 
 Serpentine, 63, 106, 649, 650, 654 
 Serpula, 316*, 317; at Pozzuoli, 692 
 Seven Mts., 154 
 
 Severn estuary, 549 ; map of, 547* 
 Sevier fault, map of, 786*; diagram of, 
 
 787* 
 
 Sevier Lake, Utah, 258 
 Seward Glacier, 358, 382*, 383 
 Shafts (salt, etc.), as sources of geological 
 
 information, 29 
 Shale, 565, 570*, 571, 580 
 Shales, 210; bituminous and oil, 353 
 Shallow water deposits, characters of, 
 
 550 ; foraminiferal deposits, 276 
 Shaly structure, 571 
 Shantung, 467 
 
 Shawangunk conglomerate, 569 
 Sheba Temple, 790 
 Sheet lava, 174 
 Shell conglomerate, 577; layers on coast 
 
 of Florida, 534 
 Shimer, H. W., cited, 549 
 Shinumo basin, 789 
 Ship barnacle, 317* 
 Shivwits Plateau, map, 786*; diagram, 
 
 787* 
 Shonai River, Japan, earthquake fissure 
 
 on, 680 
 
 Shore zone, 518; deposits in, 530 
 Shortening by folding, in Appalachians, 
 
 618 ; in Swiss Alps, 619 
 
858 
 
 Index 
 
 Shove in faulting, 623 ; on diagram, 620* 
 Siberia, great plains of western, 698 
 Siberian islands, coral reefs of, 290 
 Siccar Point, Scotland, unconformity at, 
 
 612*, 613* 
 
 Sicily, volcanic field of, 1 1 1 
 Siderite, 52, 225 
 
 Siebengebirge, map of, 155*; view, 154* 
 Sierra Nevada Mts., Cal., 467, 677, 749, 
 
 750 
 
 Sierra Nevada Mts., Spain, on map, 605* 
 Sierra Teras, uplift of, 679 
 Silesian coal fields, strata of, overridden 
 
 by Carpathians, 606 
 Silica, 224; organic, 323; organically 
 
 secreted, 71 ; minerals, 61 
 Silicate minerals, 61-63 ; from magmas, 86 
 Silicilutyte, 581 
 Silicious, bioliths, 270; shales, 581 ; sinter, 
 
 224; sponge, 325* 
 Silky luster, 49 
 Sill at Nahant, 35 
 Sillimanite, 62 
 
 Sills, 89, 194 ; origin of term, 197 
 Silurian, division, 75 ; reefs, of Gotland, 
 
 306 ; of Wisconsin, 305 ; salt beds, 246 
 Silver, 56; minerals, 56, 57 
 Silver Cliff, Col., pitchstone of, 99 
 Silver Peak Marsh, Nev., 244* 
 Simple corals, 285 
 Sinter terraces, formation of, 261* 
 Simplon massif, 608* 
 Sinai fault-block, 754; map, 755* 
 Sink- holes, 425 
 Sinks, extinct, 157 
 Skerries, 816 
 Slates, 581, 637, 649, 6 so 
 Slaty cleavage, 636, 637, 638*, 647 
 Slaty structure, 571 
 Slickensides, 620, 634 
 Slime, organic, alteration products of, 349 
 Slip (in fault), 623 
 Slumgullion rock flow, 400* 
 Smaltite, 52 
 
 Smith, William, 26, 35; portrait, 27* 
 Smithsonite, 53 
 Smooth feel, 50 
 Snake River lava plateau, 64 ; canon in, 
 
 175*; map of, 174*, 175* 
 Snow, 70, 356; compacting of, 357; 
 
 drifts, 356 ; evaporation of, 404 ; ice, 
 
 357 ; line, 356 
 Soapstone, 654 
 
 Soapy feel, 50 
 
 Society Islands, tsunamis at, 674 
 
 Soda Lake, Cal., 258 
 
 Soda Lake, Nev., 259 ; trona in, 223 
 
 Sodalite, 102 
 
 Soda niter, 58, 223, 259 
 
 Sodium carbonate lakes, 259 
 
 Sodium minerals, 58 
 
 Sodium sulphate lakes, 258 
 
 Solfatara, in, 181*; eruption of, 127; 
 
 material due to eruption of, 693 
 Solfataric, action, 181 ; vents, 181 
 Solid red coral, 283 
 Solnhofen, Bavaria, 572, 581 ; limestone, 
 
 solution of, 427 ; quarry, 572* 
 Solomon's Temple, 790 
 Solution, by underground water, 424; 
 
 of rocks, 77 ; annual amounts, 427 ; 
 
 of salt, cavings due to, 427 ; . surface 
 
 forms due to, 822 
 Sonora earthquake, 678 
 Sonora province, Mex., earthquake effects 
 
 in, 678 
 
 Sorkul Lake, 481* 
 Sorting, agents of, 438 ; conditions of 
 
 sorting by waves, 529 
 South Africa, diamond mines of, 173* 
 Spalling of granite, 393 * 
 Spanish Peaks, radial dikes of, 192, 195* 
 Spartacus, gladiators of, encamped on 
 
 floor of Monte Somma, 127 
 Spartina, 331, 334* 
 Spatter cones, volcanic, 121* 
 Specific gravity, 50; average of rocks of 
 
 earth's crust, 6 ; of earth, 6 
 Spencer Glacier, 378* 
 Spey River, destruction of feldspar in, 
 
 4i5 
 
 Sphagnum, 336, 341, 342 
 Sphalerite, 53 
 Spheres of the earth, relationships of, 5*, 
 
 6*; summary of, n 
 Spherosiderite, 226 
 Spherulites, 99 
 Spherulitic obsidian, 99* 
 Sphinx, 76*, 452 ; rock of, 75 
 Spicules of lime, 283 
 Spike Geyser, 187* 
 Spillway, 760 
 Spine, volcanic, 163 
 Spinel, 5Q 
 Spirifer, 309* 
 Spiroloculina, 278* 
 
Index 
 
 859 
 
 Spits, produced by long-shore currents and 
 
 waves, 819 
 Splintery fracture, 49 
 Spodumene, 58 ; in pegmatite, 207 
 Sponge, Cretaceous, 326*; spicules, 323, 
 
 326, 327*; flint from, 280* 
 Sponges, in Severn estuary, 549 
 Springfield Plain, 724 
 Spring tides, 524 
 Springs, 4; artesian, 424; as illustrations 
 
 of dynamical geology, 36; deposits 
 
 by, 260 ; hill side, 423 
 Squid, 313, 314* 
 Staffa, n, 176, 178*, 179, 180; erosion at, 
 
 428; Fingal's Cave on, 813* 
 Staggering course of streams, 721 
 Stagnant seas, 348 
 
 Stalactites, 77, 263, 425; of lava, 121 
 Stalagmites, 264*, 425; of lava, 121 
 Star coral, 283, 284 
 Starfish, 319, 320* 
 Stassfurt deposits, 235 
 Static metamorphism, 643 
 Stationary level theory of reefs, 296 
 Statuario (marble), 654 
 Statuary marble, 654 
 Staurolite, 62 
 Steam holes, 195 
 Stegocephalians, tracks of, 482 
 Stefanie Lake, Africa, on map, 632* 
 Step (topographic), 740*, 741 ; fault, 620 
 Stephanite, 57 
 Sternberg, Count Caspar von, on Kammer- 
 
 biihl, 169 
 Stibnite, 54 
 Stillbite, 63 
 Stocks, 193 
 Stomatodaeum, 283 
 Stonewort, 273, 334 
 Stone reefs, 538 
 Stools, in swamps, 337 
 Stoss-strahl, 658*, 661 
 Strassburg, 630 
 Strata, 78 
 
 Stratification, 79, 486, 487* 
 Stratified, character of beds of alluvial 
 
 plain, 467; deposits in shallow water, 
 
 550 ; drift, 502 ; rocks, 78 ; erosion 
 
 features on, 811 
 Stratigraphy, IQ 
 Stratum, 486; relation to surface, of 
 
 beveled, 700 
 Streak, 50 
 
 Streams, rejuvenated, 717 
 
 Strike, 585*, 586* ; deflection of, 587 ; dia- 
 gram of, 587*, 588*; faults, 623, 624* 
 
 Stripped belt, 713 
 
 Strokr Geyser, 186*; extinction of, 187, 
 683 ; origin of, 187 
 
 Stromatopora, 289*; of Devonian reefs, 
 306 ; of Wisconsin reefs, 305 
 
 Stromboli, in* 
 
 Strongylocentrotus, 320* 
 
 Strontianite, 59 
 
 Strontium minerals, 59 
 
 Structural geology, 18 ; field for study of, 
 28; in broader sense, 17; in narrower 
 sense, 18 
 
 Structures, secondary and primary, com- 
 pared, 582 ; types of deformation, 582 
 
 Styliola, 313* 
 
 Styliolina fissuretta, 7* 
 
 Stylolite, 644*, 645* 
 
 Subcrustal source of clastic material, 528 
 
 Subglacial, detritus, 370; drift, 492; 
 streams, 370 
 
 Sub-marginal moraines, 493 
 
 Submarine banks, 388 
 
 Submergence, sandstone of, 559 
 
 Submergence of islands, variation of, 296* 
 
 Sub-metallic, 40 
 
 Subsequent streams, 713; overdeepened, 
 721* 
 
 Subsidence theory of coral reefs, 294 
 
 Subsidences due to earthquakes, 690 
 
 Subsurface waters, solution by, 425 
 
 Sub-translucent, 50 
 
 Sub-transparent, 50 
 
 Sub-vitreous, 49 
 
 Suez, Gulf of, 754; map of, 517*, 755* 
 
 Suez Canal, 241 ; map of, 242* 
 
 Sugar Loaf, Portage, 784, 785* 
 
 Sulphur, 60 ; minerals, 60; odor, 50 
 
 Sulphur Springs Valley, artesian condi- 
 tions in, 479 
 
 Sulphureted hydrogen in marshes, 333 
 
 Sulphurous vapors from magmas, 85 
 
 Sumatra, tropical swamp of, 341 
 
 Sunda straits, map of, 137* 
 
 Sunsets, brilliant, due to dust of Krakatoa, 
 441 
 
 Supai formation, 790 
 
 Superglacial drift, 492 
 
 Superimposed streams, 744 
 
 Super-saline water, 214 
 
 Susquehanna River, 726, 776 
 
86o 
 
 Index 
 
 Swabian Alp, 755, 756*; wind work on, 
 
 405 
 
 Swallow-holes, 425 
 Swamp plants, 334*, 337* 
 Swamps, 4, 333 ; fresh water, 3 29 ; origin 
 
 of, 423 
 Swanage, Eng., sea-stacks on coast of, 
 
 815 
 
 Swash, 521, 523 
 Sweden, coast of, 806, 810 ; fault structure 
 
 of, 757, 758*; trilobite limestone of, 
 
 318, 319* 
 Swells, 519, 520 
 Switzerland, 606 
 Syene, syenite from, 101 
 Syenite, 96, 100 ; origin of name, 101 ; 
 
 porphyry, 96, 101 ; series, glasses of, 
 
 101 ; trachyte series, 100 ; gneiss, 653 
 Sylt, Island of, coast destruction at, 444 
 Sylvania sandstone, 75, 440*, 453, 569, 578, 
 
 580 ; cross-bedding of, 455* 
 Sylvanite, 57 
 Sylvite, 58, 223 
 
 Symmetrical anticline, 590*, 591* 
 Synclinal, mountain, 599*; ridges, 598; 
 
 valley, 732* 
 Synclines, 596*; between domes, 603; 
 
 erosion cycle on, 732 ; limb or arm of, 
 
 596 ; thickening of axis of, 597* 
 Synclinorium, 598* 
 Syr-darya, map of, 449* 
 Syringopora of old reefs, 306 
 
 Table mountains, origin of, 714 
 
 Table Rock in Genesee River at Portage, 
 
 784* 
 Table Rock, Niagara, on map, 773*; 
 
 rock falls of, 418 
 Tachylites, 96, 107 
 Taconic mountains, 75 
 Tagus River, 664 
 Talc, 63; schist, 652 
 Talefre, Cascade du, 369 
 Talus, breccia, 577; formation, 821; 
 
 slope, 398 
 
 Tamarack in swamps, 336 
 Tampa, Fla., reefs, 300 
 Tanganyika Lake, 516, 631, 633*, 757 
 Tarapaca Desert, 259 
 Tarn, 796, 800*, 802, 803 
 Tarnish, 50 
 Tasman Glacier, 373 
 Tasmanian Sea, 518 
 
 Taste, 50 
 
 Tchernozom, see Tschernosem 
 
 Tectonic metamorphism, 643 
 
 Tectosphere, 6, 84 
 
 Teeth, deep-sea accumulations of, 552 
 
 Temperature change, effect of, in rock 
 
 destruction, 392 
 
 Temperature of center of earth, 6 
 Tenacity, 49 
 
 Teneriffe, Peak of, 143*; phonolite ob- 
 sidian from, 102 
 Tennessee, central basin of, 729*; River, 
 
 729 
 
 Tepetate, 260 
 Terebratulina, 309* 
 Terminal moraines, 370, 382, 493, 494*, 
 
 499* 
 
 Termites, work of, 437 
 Terra Nuova, Italy, earthquake manifesta- 
 tions in, 667 
 
 Terrapin Rocks, Niagara, on map, 773* 
 Terrestrial deposits, 227, 439 ; of Foraminif- 
 
 era, 276 
 
 Terrigenous deposits in deep sea, map, 277* 
 Terrigenous material, 527 
 Tertiary, brown-coal, 343, 345; lignites, 
 
 343; oils, 352; rocks formed by Char a, 
 
 273 
 Tetragonal, prism, 45*; pyramid, 45*; 
 
 system, 43 ; principal forms, 45 ; tris- 
 
 octahedron, 44* 
 Tetrahedrite, 56 
 Tetrahedron, 44* 
 Tetrahexahedron, 44* 
 Teufelsmauer, 191* 
 Texas, section in, 32 
 Textularia, 7*; in chalk, 278*, 279 
 Textures, of aqueous rocks, 217 ; of elastics, 
 
 566, 568 ; of igneous rocks, 87 ; relation 
 
 to magma, 88 
 Theories of reef origin, 293 
 Theralite, 105 
 
 Thermal metamorphism, 642, 643 
 Thinolitic tufa, 255 
 Thousand Islands, 762 
 Three Sister Peaks, Oregon, 367* 
 Throw, 622 
 Thrust fault, 625 ; magnitude of, 625 ; 
 
 on diagram, 620*; passing into fold, 
 
 619* 
 Thrusting, duplicating and inversion of 
 
 strata by, 625 
 Tibet, borax of, 224; deserts of, 706 
 
Index 
 
 861 
 
 Tidal, bore, 534*; currents, 523, 529; 
 
 lagoon, 545* 
 Tierra del Fuego, 109 
 Tillite, 508, 577 
 
 Tilly Whim caves, British coast, 812*, 813* 
 Tind, 804 
 Tinkal, 223 
 Tin minerals, 53 
 Titanite, 63 
 Titanium minerals, 53 
 Toledo, O., glass-making from Sylvania 
 
 Sand at, 75 
 
 Tombigbee River, 715 
 Tombolo, 810*, 819*; Nahant, 544 
 Tonto beds, 788 
 Topaz, 62 
 
 Topographic features due to fault, 630 
 Topsets, 484 
 Torbanite, 350 
 Toroweep fault, map, 786*; diagram, 
 
 787* 
 Torrential currents, cross-bedding due to, 
 
 487* 
 
 Torridon breccia, 569 
 
 Torsion joints, 640*, 641* 
 
 Tough, 4Q 
 
 Tourmaline, 62 ; in pegmatite, 207 
 
 Trachyte, 89, 96, 100, 101 ; of Sarcoui, 
 124; origin of name, 101 
 
 Trachyte-phonolite, 102*; -porphyry, 96, 
 101 
 
 Trachytic texture, 102* 
 
 Transportation, agents of, 438; and sort- 
 ing of clastic material in the sea, 528; 
 by streams, 460; of sand and dust, dis- 
 tances of, 441 
 
 Transporting power of streams, variation 
 of, 460, 461 
 
 Trans-Caspian deserts, wind erosion in, 
 406 
 
 Translation, waves of, 521 
 
 Translucency, 50 
 
 Translucent, 50 
 
 Transparent, 50 
 
 Transylvania, 603 
 
 Transylvanian Alps, on map, 605* 
 
 Trapezohedron, 44* 
 
 Trap rock, 73, 96, 107 
 
 Traverse Bay, sand spit in, 541* 
 
 Travertine, rate of deposition of, 260 
 
 Trend of fault, 620 
 
 Trenton, N. J., 722 
 
 Trenton-Utica oils, 352 
 
 Trent River outlet on map, 769* 
 
 Triassic, coal, 345 ; reefs of Dolomites, 306 
 
 Triclinic, hemi-prism, 48*; pyramid, 48; 
 system, 48; system, fundamental forms 
 of, 48* 
 
 Trigonal, prism, 46*; trisoctahedron, 44* 
 
 Trilobites, 318, 319* 
 
 Trinidad asphalt lake, 351 
 
 Tripoli, Africa, 323 
 
 Tripolite, 323 
 
 Trisoctahedron, 44 
 
 Tristetrahedron, 44* 
 
 Trist Glacier, 364 
 
 Trona, 223 
 
 True glaciers, 358 
 
 Trugberg, 368 
 
 Tschard-schui, thickness of dune deposits 
 at, 449 
 
 Tschernosem, 459, 549; map of distribu- 
 tion, 460* 
 
 Tsunamis, 519, 684; defined, 656; due to 
 earthquakes, 690; during eruption of 
 Krakatoa, 138 
 
 Tubipora, 300 
 
 Tufa dams and terraces, 256*, 261 
 
 Tufaceous texture, 218 
 
 Tundras, arctic, 342 
 
 Tungsten mineral, 5$ 
 
 Tuolumne River, pot-holes on, 419* < 
 
 Turquois, 5# 
 
 Turtle stone, 222*, 574* 
 
 Tuscany, borax of, 181, 224 
 
 Twig-rush, 335* 
 
 Twinning of plagioclase, 91 
 
 Tyndall Glacier, 383 
 
 Types of peat, 329 
 
 Tyrol, 606 ; marbles in, 654 ; reefs in, 306 
 
 Uinkaret Plateau, map, 786*; diagram, 
 
 787* 
 
 Uinta Mountains, 472 
 Uintaite, 351 
 Ulexite, 60, 223 
 Unaltered rocks, 69 
 Unconformities, 611, 612*, 613*, 614* 
 Under-clay, 345 
 Underground water, deposits by, 260; 
 
 destructive work of, 424 ; rate of flow of, 
 
 423 ; types of, 422 
 Undermining by waterfalls, 417 
 Undertow, 522, 523, 528 
 Uneven fracture, 40 
 Unglaciated summit, 800*; valley, 800* 
 
862 
 
 Index 
 
 Uniclinal ridges, diagram, 593*, 722, 733, 
 
 738 
 
 Unicline, 594, 595*, 598, 732*, 734 
 Unio, 315* 
 United States, mantle rock of the northern, 
 
 65 
 
 Unkar group, 789 
 Unstratified drift, 502 
 Upham, W., on reef origin, 297 
 Upland bogs, plants of, 342 
 Upper Aletsch Glacier, 364 
 Ural Mountains, 237 ; rock exposures in, 34 
 Uraninite, 55 
 Uranium minerals, 55 
 Uruguay, 547 
 U-shaped trough, 803* 
 U-shaped valley, 797, 799, 800* 
 Usiglio, J., experiments by, 230 
 Utah Basin, 753 
 Utah erosion monument, 409* 
 
 Valdivia, Chile, destroyed by earthquake, 
 675 
 
 Vale, of Eden, 197; of Kashmir, 471 
 
 Val-del-Bove, 134, 189; map of, 133* 
 
 Valle-del-Bue, see Val-del-Bove 
 
 Valley profiles, river and glacier, 802* 
 
 Valleys, circling, 722 ; relative size of, 2 
 
 Valparaiso, Chile, damaged by earthquake, 
 672 ; destruction by earthquake, 675 
 
 Vanadinite, 55 
 
 Vanadium minerals, 55 
 
 Vascular plants, 333, 335; deposits from, 
 328 
 
 Vatna Jokull, 384; map, 385* 
 
 Vaughan, T. W., cited, 290 
 
 Vegetal deposits, types of, 329 
 
 Vein deposits, 86 
 
 Vein minerals, sources of, 268 
 
 Veins, 265; contemporaneous, 206; rela- 
 tive age of, 266 
 
 Velocity of waves, 520 
 
 Venezuela, soda lakes of, 259 
 
 Vent, volcanic, 116, 163 
 
 Ventriculites, 326* 
 
 Venus, 310* 
 
 Verdun, 731 ; -on map, 730* 
 
 Vermilion Cliffs, 791 
 
 Versailles, on map, 730* 
 
 Vertical movements in earthquakes, 689 
 
 Vesuvianite, 62 
 
 Vesuvius, 87, 112; as type, 141; cinder 
 cone of, 142*; described, 125; 1872 
 
 eruption of, 128; furrowed cone, 131*; 
 map of, 125*; map of vicinity of, no*; 
 section of, 143*; view in crater, 129*, 
 130*; view of, 127* 
 
 Vicksburg limestone, 281 
 
 Victoria Land, ice barrier of, 387 
 
 Vienna, rocks of, 64 
 
 Viesch Glacier, 363, 364; section of, 362* 
 
 Virginia, coast dunes of, 447 
 
 Vistula River, black mud of the, 549; 
 organic matter of mud in, 348 
 
 Vitreous luster, 49 
 
 Vivipara, 315 
 
 Vlightberg, unconformity on, 609*; dia- 
 grams, 610*, 611* 
 
 Volcanic, agglomerate, 80, 431, 577*; 
 area, central France, map, 153*; ashes 
 and cinders, 117; bombs, 116*, 117, 
 431; breccia, 427, 577; conduit, 167; 
 elastics, denned, 568; centers on frac- 
 ture lines, 153; district of Rhine, map 
 of, 156*; dome, 124; dust, 431; dust, 
 transportation of, 443 ; eruption, 84 ; fun- 
 nels, pipes, spines and necks, 161 ; glass 
 in red clay, 553 ; lapilli, 431 ; material of 
 old vents, 171; neck, 166*, 167*; 
 phenomena, modern, 109; pipe, 167; 
 plug, 164, 167, 204 ; plug of Fife, 170, 171 ; 
 sands and muds, 551, 552; sandstone 
 described, 580; tuff, 80, 580; vents, 
 materials ejected from, 117 
 
 Volcano, section of ancient, in Freiburg 
 region, 172* 
 
 Volcano in eruption, 86* 
 
 Volcanoes, active, dormant and extinct, 
 112; age of, 143; alignment of, 171; 
 characteristic form and activity of 
 modern, 116; classification of, 140; 
 comparison of form of, 141 ; craterless, 
 122; distribution of, 109, map of, 108; 
 extinct, of Rhine region, 154*, 155*; 
 formation of new, in historic period, 112 ; 
 illustrations of dynamic geology by, 36; 
 old necks and plugs of, 164; sections 
 of various types of, 143*; structural 
 characters of, 144; types of, 140-141 
 
 Volcanology, 16, 144 
 
 Volga River, 231 
 
 Volume of river transported material, 462 
 
 Vosges Mountains, 630, 757 ; on diagram, 
 756* 
 
 V-shaped valley, 797, 800* 
 
 Vulcano, in 
 
Index 
 
 863 
 
 Wad, 224 
 
 Wadis, 411 
 
 Wales, oldland of, 719*; rock exposures of, 
 
 33 
 
 Walther, Johannes, portrait, 245* 
 
 Wanakah shales, 31* 
 
 Wannehorn, 366 
 
 Wappinger limestone, 627 
 
 Warsaw, N. Y., 777; on map, 776* 
 
 Wasatch Mts., 749, 750, 753 ; recent fault 
 in, 75i* 
 
 Washington, D. C., 722 
 
 Watchung Mts., 180; columnar structure 
 of basalt of, 177 
 
 Water, 60; amount of, in air, 355 ; mechan- 
 ical work of, 77 ; types of, 214 
 
 Waterfalls, characteristic of young streams, 
 708; in coastal plain drainage system, 
 721 
 
 Water-gap, 723, 734, 741 
 
 Water laid elastics, 79 ; denned, 568 
 
 Waterlime, 581* 
 
 Water plantain, 335 
 
 Water-table, 422 
 
 Water vapor, from magma, 85 ; sources of, 
 
 354 
 
 Watkins Glen gorge, 420*, 421* 
 
 Wave-built beaches, raised, 694 
 
 Wave currents, 522 
 
 Wave-cut, cliff, 807 ; terraces, raised, 694 
 
 Wave-marks, 553 
 
 Wave normal, 658* 
 
 Wave quarrying, 428 
 
 Waverly Oaks, esker of, 507 * 
 
 Waves, 519; diagram, 520*, 521*, 522*; 
 bars and spits formed by, 819 ; destruc- 
 tive work of, 427 ; force of, 428 
 
 Wave-worn bricks, 430* 
 
 Wayland, N. Y., on map, 776* 
 
 Way land Valley on block diagram, 777* 
 
 Wealden district, 595 
 
 Weathering, 390, 392 
 
 Wellington, New Zealand, earthquake 
 effects near, 675; on map, 676* 
 
 Wells, 4 ; oil, gas, etc., as sources of geo- 
 logical information, 29 
 
 Werner, A. G., 25, 33, 36; on basalt, 168; 
 portrait, 25* 
 
 Wernerite, 62 
 
 West Alps, cross section of, 607* 
 
 Westchester dolomite, 653 
 
 West Indies, fora'miniferal limestones of, 
 281 
 
 West Kaibab fault, map, 786* ; diagram, 
 
 787* 
 
 West River, terraces of, 473* 
 West Spanish Peaks, 189*, 190* 
 Wet Champagne lowland, 731 
 Whin Sill, 197 ; section of, 199* 
 Whirlpool, 766*, 767, 768, 770, 774; dia- 
 gram of development of, 768*; on map, 
 
 763* 
 Whirlpool Rapids, 765*, 766, 768; on map, 
 
 763* 
 
 Whirlpool sandstone, 767 
 Whitby, jet-bearing cliffs of, 350 
 White Cliffs, Colorado Plateau, 791; 
 
 eolian rock of, 77 
 White Mts., 741; erosion in, 821; rock 
 
 exposures in, 34 
 Whitsunday Island, 293* 
 Wick, Scotland, wave destruction at, 428 
 Willemite, 53 
 
 Williams Canon, contacts in, 212* 
 Willis, Bailey, machine for producing folds, 
 
 617*; folds produced by, 618* 
 Wind, as agent of erosion, 820 ; destructive 
 
 work of, 404; gaps, 734, 736; grooves 
 
 in Libyan Desert, 405 ; sculpturing, 
 
 409*; sculpturing processes of, 822; 
 
 transportion by, 439 
 Windom shales, 31* 
 
 Winthrop Great Head, Mass., 817*, 818 
 Wisconsin, drumlins of, 498 ; old coral reefs 
 
 of, 305 ; Potsdam sandstone of, 75 
 Witherite, 59 
 Woevre lowland, 731 
 Wolframite, 55 
 Wollastonite, 62, 209 
 Wood in old lavas, 169 
 Woods Hole, 531; erosion at, 818; tides 
 
 at, 525 
 Worms, 316*, 317; as bottom feeders, 
 
 347 ; work of, 437 
 Wrack, of sea, 522 
 Wulfenite, 55 
 
 Wyoming, waste basin in, 471 
 Wyville-Thompson ridge, 511* 
 
 Yakutat Bay, Alaska, . 382 ; earthquake 
 
 of, 684; map of, 381 
 Yangtse-Kiang, 467; red muds from, 
 
 55i 
 
 Yardangs, 405, 406* 
 Yellow River, China, 459, 467 
 Yellow Sea, 509 ; origin of color of, 459 
 
86 4 
 
 Index 
 
 Yellowstone National Park, 203 ; algae of 
 hot springs of, 274; geysers of, 184 ; lava 
 of, 196; lithophysae in lithoidite of, 100; 
 obsidian of, 99 ; quartz-diorite from, 103 
 
 York, Eng., 719 
 
 Yorkshire coast, erosion of, 809 
 
 Yosemite Valley, 797 *, 799, 802 
 
 Young land, erosion on coast of, 807 
 
 Youth of land, 705 
 
 Zacanthoides, 319* 
 
 Zechstein, 236 ; Sea, map of, 238* 
 
 Zeolites, 63 
 
 Zinc minerals, 53 
 
 Zircon, 53 
 
 Zirconium minerals, 53 
 
 Zittel, C. von, cited, 23 
 
 Zones, bathymetric diagram, 519* 
 
 Zoology, 15 
 

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