/MRKEIEY 
 
 LIBRARY 
 
 UNIVERSITY OF 
 ^CALIFORNIA 
 
 EAKTH 
 
 SCIENCES 
 
 LIBRARY 
 
 
 THE LIBRARY 
 
 OF 
 
 THE UNIVERSITY 
 OF CALIFORNIA 
 
 GEOLOGY LIBRARY 
 IN MEMORY OF 
 
 PROFESSOR 
 GEORGE D. LOUDERBACK 
 
 1874-1957 
 
KOCKS, KOCK-WEATHEKING, AND SOILS 
 
A TREATISE 
 
 ON 
 
 ROCKS, ROCK-WEATHERING 
 AND SOILS 
 
 BY 
 
 GEORGE P. MERRILL 
 
 CURATOR OF GEOLOGY IN THE UNITED STATES NATIONAL MUSEUM, AND PROFESSOR 
 OF GEOLOGY IN THE CORCORAN SCIENTIFIC SCHOOL AND GRADUATE 
 
 SCHOOL OF COLUMBIAN UNIVERSITY, WASHINGTON, D.C. 
 AUTHOR OF "STONES FOR BUILDING AND DECORATION," ETC. 
 
 gork 
 THE MACMILLAN COMPANY 
 
 LONDON: MACMILLAN & CO., LTD. 
 
 1897 
 
 All rights reserved 
 
SCIENCES Lit, 
 
 COPYEIGHT, 1897, 
 
 BY THE MACMILLAN COMPANY. 
 
 Norbjooti 
 
 J. S. Cushing & Co. Berwick & Smith 
 Norwood Mass. U.S.A. 
 
' ' THE rmns of an older vtovld are visible in the present structure 
 of our planet ; and the strata which now compose our continents 
 have been once beneath the sea, and were formed out of the waste 
 of pre-existing continents. The same forces are still destroying, 
 by chemical decomposition or mechanical violence, even the hard- 
 est rocks, and transporting the materials to the sea, where they 
 are spread out, and form strata analogous to those of more ancient 
 date." HUTTON. 
 
 978 
 
PREFATORY NOTE 
 
 IN the work here presented the writer has endeavored to 
 bring together in systematic form the results of several years' 
 study of the phenomena attendant upon rock degeneration 
 and soil formation. Although beginning with a discussion 
 of rocks and rock-forming minerals, the work must be con- 
 sidered in no sense a petrology as this word is commonly 
 used. What is here given relative to the origin, structure, 
 and composition of rock masses is regarded as an essential 
 introduction to the chapters on rock-weathering. The por- 
 tion dealing with the structure and composition of the result- 
 ant materials is an essential corollary to these same chapters. 
 
 It is believed that no apology is necessary even in this day 
 of many books for bringing out the present work. The origin, 
 structure, and mineral composition of rocks, particularly the 
 eruptive varieties, are matters which have of late received much 
 attention. In fact, it is to these rocks that the petrologists 
 have devoted their best efforts. Since the introduction of the 
 microscope into petrographic work, there has, however, been 
 very little time devoted to the study of rocks in a weathered 
 condition. The chemists have made analyses, but have disre- 
 garded the physical and mineralogical nature of the material 
 analyzed. Other workers have studied the physical properties 
 of rocks decayed, in the form of soils, but have in their 
 turn disregarded their mineral and chemical nature. The 
 writer has aimed to bring together here such results obtained 
 by these workers in divers fields as it is believed will be for 
 the mutual benefit of all concerned. The state of comminu- 
 tion reached by rocks during the processes of long-continued, 
 
 vii 
 
yiii PREFATORY NOTE 
 
 secular decay, and the amount of leaching such have under- 
 gone, are certainly of as much practical interest to the agri- 
 culturist as of theoretical interest to the geologist. 
 
 To the one, these residues are essential to the life and well- 
 being of man through furnishing the soils from whence is 
 derived directly and indirectly the food for life's sustenance ; 
 to the other they are but transitory phases in the earth's his- 
 tory, ijepresenting the materials from which, through a process 
 of fractional separation by running waters, have been made 
 up the thousands of feet of secondary rocks which to-day 
 occupy so large a portion of its surface. 
 
 The very general scheme of classification adopted in the 
 treatment of the unconsolidated clastic materials may at first 
 seem disappointing. It has, however, been the writer's special 
 aim to introduce into this preliminary volume as few new terms 
 as possible, using only those which through years of service 
 have become a part of our language. It is of course possible 
 that in his desire to avoid any possible confusion such as might 
 arise through putting forward a purely tentative classification 
 he has been overcautious. 
 
 It is possible, further, that in numerous instances it may 
 appear that too much reliance is placed upon single analyses, 
 particularly in the discussions relating to the character of 
 decomposed material. Regarding this it can only be said that 
 in those instances upon which most reliance is placed, the 
 materials were not merely collected by the author himself, but 
 that he made his own chemical analyses and microscopic deter- 
 minations as well. It is believed that the fresh and residual 
 materials examined are in each instance as truly representative 
 of the same rock mass, as would be samples of fresh rock col- 
 lected equal distances apart. In all cases special effort was 
 made to obtain material concerning the lithological identity 
 of which there could be no doubt, and in the majority of cases 
 the residuary matter was collected from positions immediately 
 overlying the still unaltered rock. Where such a procedure 
 
PREFATORY NOTE i x 
 
 was impossible, especial care was exercised to obtain only such 
 as was originally of the same lithological nature as the fresh 
 rock, and which had suffered no contamination from extrane- 
 ous sources. The fact that stratified rocks are likely to vary 
 so greatly within short distances, and hence that a residual clay 
 cannot be relied upon to represent the residue from rocks of 
 the same nature immediately underlying, will serve to explain 
 in part the author's limiting himself so largely to a discussion 
 of massive eruptive materials. That so little use has been 
 made of other analyses, made in greater detail or by those more 
 skilled in analytical methods, is due to a lack of satisfactory 
 information relative to the mutual association of the fresh and 
 decomposed materials and the mineralogical and physical nature 
 of the residual product. 
 
 As will be readily perceived by those at all acquainted with 
 the general literature, the publications of the U. S. Geological 
 Survey, the U. S. National Museum, and the Bulletins of the 
 Geological Society of America have been drawn upon to furnish 
 materials for illustration. The writer is under special obliga- 
 tion to Dr. Milton Whitney of the U. S. Department of Agri- 
 culture for many of the mechanical analyses given, and to Mr. 
 L. H. Merrill of the Maine Experiment Station for numerous 
 criticisms and suggestions. 
 
 To the late Dr. G. Brown Goode he is indebted for permis- 
 sion to utilize photographs and specimens forming a part of 
 the collections of the National Museum and also for electro- 
 types of sundry plates and figures in its publications. 
 
 GEORGE P. MERRILL. 
 
 U. S. NATIONAL MUSEUM, January, 1897. 
 
CONTENTS 
 
 PART I 
 
 THE CONSTITUENTS, PHYSICAL AND CHEMICAL 
 
 PROPERTIES, AND MODE OF OCCURRENCE 
 
 OF ROCKS 
 
 PAGE 
 
 I. INTRODUCTORY : ROCKS DEFINED 1 
 
 II. THE CHEMICAL ELEMENTS CONSTITUTING ROCKS ... 4 
 
 III. THE MINERALS CONSTITUTING ROCKS 9 
 
 IV. THE PHYSICAL AND CHEMICAL PROPERTIES OF ROCKS . . * 33 
 
 1. The Structure of Rocks, macroscopic and microscopic . 33 
 
 2. The Specific Gravity of Rocks 43 
 
 3. The Chemical Composition of Rocks ..... 44 
 
 4. The Color of Rocks 45 
 
 V. THE MODE OF OCCURRENCE OF ROCKS 49 
 
 PART II 
 
 THE KINDS OF ROCKS 
 
 GENERALITIES, AND CLASSIFICATION 56 
 
 I. IGNEOUS ROCKS : ORIGIN OF, AND CLASSIFICATION ; RELATION- 
 SHIP EXISTING BETWEEN PLUTONIC AND EFFUSIVE ROCKS 59 
 
 1. The Granite-Liparite Group 65 
 
 2. The Syenite-Trachyte Group 73 
 
 3. The Foyaite-Phonolite Group 77 
 
 4. The Diorite-Andesite Group 81 
 
 5. The Gabbro-Basalt Group 85 
 
 6. The Theralite-Basanite Group 93 
 
xii CONTENTS 
 
 PAGE 
 
 7. The Peridotite-Limburgite Group 95 
 
 8. The Pyroxenite-Augitite Group 99 
 
 9. The Leucite-Nepheline Rocks 102 
 
 II. AQUEOUS ROCKS 105 
 
 1. Rocks formed through Chemical Agencies .... 105 
 
 (1) Oxides 106 
 
 (2) Carbonates Ill 
 
 (3) Silicates 114 
 
 (4) Sulphates 117 
 
 (5) Phosphates 119 
 
 (6) Chlorides . 119 
 
 (7) Hydrocarbon Compounds 120 
 
 2. Rocks formed as Sedimentary Deposits .... 129 
 
 (1) Rocks composed mainly of Inorganic Material . . 131 
 
 (1) The Arenaceous Group : Psammites . . 131 
 
 (2) The Argillaceous Group : Pelites . . . 135 
 . (3) The Calcareous Group : Calcareous Conglom- 
 erate and Breccia 139 
 
 (4) The Volcanic Group : Tuffs . . . .139 
 
 (2) Rocks composed mainly of debris from Plant and 
 
 Animal Life. Organagenous .... 141 
 
 (1) The Siliceous Group : Infusorial Earth . 141 
 
 (2) The Calcareous Group : Limestone, Marl, etc. 143 
 
 (3) The Carbonaceous Group : Peat, Lignite, 
 
 and Coal 148 
 
 (4) The Phosphatic Group 151 
 
 III. JEOLIAN ROCKS 153 
 
 Volcanic Dust ; Dune Sands, etc 153 
 
 IV. METAMORPHIC ROCKS 155 
 
 Agencies and Results of Metamorphism and Metasomatosis . 155 
 
 1. Stratified or Bedded 162 
 
 (1) The Crystalline Limestones and Dolomites . 162 
 
 2. Foliated or Schistose 164 
 
 (1) The Gneisses . . . . . . .164 
 
 (2) The Crystalline Schists 168 
 
CONTENTS xiii 
 
 PART III 
 THE WEATHERING OF ROCKS 
 
 PAGE 
 
 I. THE PRINCIPLES INVOLVED IN ROCK-WEATHERING : Statement 
 of General Problem ; Weathering defined ; Reference to Au- 
 thorities and Opinions held 173 
 
 1. Action of the Atmosphere 176 
 
 (1) Nitrogen, Nitric Acid, and Ammonia of the 
 
 Atmosphere 176 
 
 (2) Carbonic Acid of the Atmosphere . . . 178 
 
 (3) Oxygen of the Atmosphere . . . .180 
 
 (4) Effects of Heat and Cold . . . .180 
 
 (5) Effects of Wind 184 
 
 2. Chemical Action of Water 186 
 
 (1) Oxidation 187 
 
 (2) Deoxidation 187 
 
 (3) Hydration 187 
 
 (4) Solution 189 
 
 3. Mechanical Action of Water and of Ice ; Erosion by 
 
 Water ; Daubree's Experiments ; Action of Freez- 
 ing Water and of Ice ...... 195 
 
 4. Action of Plants and Animals ; Effect of Lichens, 
 
 Mosses, Root Action, Organic Acids, etc. ; Solvent 
 Power of Citric Acid ; Action of Bacteria ; Action 
 of Ants and Termites ; Action of Marine Inver- 
 tebrates ; Production of Carbonates . . . 201 
 
 II. CONSIDERATION OF SPECIAL CASES 206 
 
 (1-) Weathering of Granite, District of Columbia . . .206 
 
 (2) Weathering of Gneiss, Albemarle County, Virginia . . 214 
 
 (3) Weathering of Elseolite Syenite, Little Rock, Arkansas . 216 
 
 (4) Weathering of Phonolites, Marienfels, Bohemia . . 217 
 
 (5) Weathering of Diabase, Medford, Massachusetts . . 218 
 
 (6) Weathering of Diabase, Venezuela 222 
 
 (7) Weathering of Basalt, Kammar Bull, Bohemia . . . 223 
 
 (8) Weathering of Basalt, Haute Loire, France . . . 223 
 
xiv CONTENTS 
 
 PAGE 
 
 (9) Weathering of Diorite, Alberaarle County, Virginia . . 224 
 
 (10) Weathering of Peridotites and Pyroxenites . . . 225 
 
 (a) Serpentine of Harford County, Maryland . . . 226 
 (&) Soapstones of Albernarle and Fairfax Counties, Vir- 
 ginia 226 
 
 (11) Weathering of Clastic Rocks . . . . . .228 
 
 (a) Argillites of Harford County, Maryland . . .229 
 
 (6) Cherts of Missouri and Arkansas .... 230 
 
 (12) Weathering of Limestones, Arkansas .... 232 
 
 (13) Resume: Importance of Hydration ; Loss of Constituents ; 
 
 Relative Durability of Various Minerals ; Discussion of 
 
 Processes involved in Feldspathic Decomposition . . 234 
 
 HI. THE PHYSICAL MANIFESTATIONS OF WEATHERING . . . 241 
 
 (1) Disintegration without Decomposition .... 241 
 
 (2) Weathering influenced by Crystalline Structure . . 243 
 
 (3) Weathering influenced by Structure of Rock Masses . . 244 
 
 (4) Weathering influenced by Mineral Composition . . 248 
 
 (5) Results due to Position 252 
 
 (6) Induration on Exposure 254 
 
 (7) Changes in Color incidental to Weathering . . . 257 
 
 (8) Relative Amount of Material removed in Solution . . 258 
 
 (9) Incidental Surface Contours 259 
 
 (10) Effacement of Original Characteristics .... 262 
 
 (11) Simplification of Chemical Compounds incidental to 
 
 Weathering 265 
 
 (12) Other Results incidental to Decomposition and Erosion . 266 
 
 IV. TIME CONSIDERATIONS 268 
 
 (1) Rate of Weathering influenced by Texture . , . 268 
 
 (2) Rate of Weathering influenced by Composition . . 269 
 
 (3) Rate of Weathering influenced by Humidity . , . 270 
 
 (4) Rate of Weathering influenced by Position .' . . . .270 
 
 (5) Relative Rapidity of Weathering among Eruptive and Sedi- 
 
 mentary Rocks . . . , . . . . 271 
 
 (6) Time Limit of Decay: Post-Cretaceous Weathering of 
 
 Granite; Weathered Implements of Human Workman- 
 ship ; Post-Glacial Weathering of Diabase ; Post-Jurassic 
 
CONTENTS XV 
 
 PAGE 
 
 and Post-Pliocene Decay of Rocks of the Sierras; Pre- 
 Palaeozoic Weathering of Archaean Rocks . . . 272 
 
 (7) Extent of Weathering: In the District of Columbia, 
 
 Georgia, Missouri, Nicaragua, Brazil, and South Africa . 276 
 
 (8) Relative Rapidity of Weathering in Warm and Cold Cli- 
 
 mates : Opinions hitherto held ; Supposed Protective 
 Action of Frost Effects of Forests 278 
 
 (9) Difference in Kind of Weathering in Cold and Warm Cli- 
 
 mates 283 
 
 (10) Relative Amounts of Materials lost through Weathering 
 
 in Hilly and Plains Regions ...... 284 
 
 PART IV 
 
 TRANSPORTATION AND REDEPOSITION OF ROCK 
 DEBRIS 
 
 1. ACTION OF GRAVITY 286 
 
 2. ACTION OF WATER AND ICE 287 
 
 3. ACTION OF WIND 292 
 
 PART V 
 
 THE REGOLITH 
 
 I. CLASSIFICATION AND GENERAL DESCRIPTION .... 299 
 
 1. Sedentary Materials 300 
 
 (1) Residuary Deposits : Residual Sands and Clays ; 
 
 Terra Rossa ; Laterite, etc 301 
 
 (2) Cumulose Deposits : Peat ; Muck and Swamp Soils 
 
 in part ; Infusorial Earths 313 
 
 2. Transported Materials 318 
 
 (1) Colluvial Deposits : Talus, Cliff Debris and Material 
 
 of Avalanches . . . . . . .319 
 
 (2) Alluvial Deposits : Modern Alluvium ; Sea-coast 
 
 Swamps ; Loess ; Adobe in part ; Champlain Clays ; 
 Beach Sands and Gravel 320 
 
XVi CONTENTS 
 
 PAGE 
 
 (3 ) .ZEolian Deposits : Wind-blown Sand ; Sand Dunes ; 
 
 Volcanic Dust .344 
 
 (4) Glacial Deposits : Moraine Material ; Eskers ; Drum- 
 
 lins, etc . 350 
 
 3. The Soil .358 
 
 (1) The Chemical Nature of Soils 358 
 
 (2) The Mineral Composition of Soils . . . .374 
 
 (3) The Physical Condition of Soils . . . .379 
 
 (4) The Weight of Soils 382 
 
 (5) The Kinds and Classification of Soils . . .382 
 
 (6) The Color of Soils 385 
 
 (7) The Age of Soils 387 
 
 (8) Soils as Affected by Plant and Animal Life . . 390 
 
ILLUSTRATIONS 
 
 FULL-PAGE PLATES 
 
 FACING PAGE 
 
 PLATE 1 Frontispiece 
 
 Stone Mountain, Georgia. A Residual Boss of Granite. From 
 a photograph by J. K. Killers. 
 
 PLATE 2 . . .33 
 
 Porphyritic and Flow Structures. 
 
 PLATE 3 . . 35 
 
 Slaggy and Vesicular Structures. 
 
 PLATE 4 38 
 
 Brecciated Structures. 
 
 PLATE 5 41 
 
 Microscopic Structures of Rocks. 
 
 PLATE 6 65 
 
 Fig. 1. Lithophysse in Liparite. 
 
 Fig. 2. Cross-section of Stalagmite. 
 
 Fig. 3. Concretionary Aragonite. 
 
 Fig. 4. Pegmatite. 
 
 PLATE 7 . . 70 
 
 Fig. 1. Liparite, Nevadite Form. 
 
 Fig. 2. Liparite, Rhyolite Form. 
 
 Fig. 3. Liparite, Obsidian Form. 
 
 Fig. 4. Liparite, Pumiceous Form. 
 
 PLATE 8 82 
 
 Fig. 1. Orbicular Diorite. 
 Fig. 2. Granite Spheroid. 
 
 PLATE 9 . 107 
 
 Fig. 1. Botryoidal Hematite. 
 Fig. 2. Septarian Nodule. 
 
 PLATE 10 113 
 
 View in Limestone Cavern. 
 
 xvii 
 
XViii ILLUSTRATIONS 
 
 FACING PAGE 
 PLATE 11 . 130 
 
 Fig. 1. Shell Limestone. 
 
 Fig. 2. Coquina. 
 
 Fig. 3. Crinoidal Limestone. 
 
 PLATE 12 143 
 
 Fig. 1. Pisolitic Limestone. 
 Fig. 2. Oolitic Limestone. 
 
 PLATE 13 164 
 
 Banded and Foliated Gneisses. 
 
 PLATE 14 172 
 
 Weathered Granite, District of Columbia. From a photograph 
 by George P. Merrill. 
 
 PLATE 15 193 
 
 Corroded Limestones. 
 
 PLATE 16 199 
 
 Fig. 1. Diorite Boulder split along Joint Planes by Frost. 
 
 Fig. 2. Corroded Surface of Pyroxenic Limestone. 
 
 Fig. 3. Corroded Limestone. 
 
 PLATE 17 219 
 
 Weathered Diabase Dike, Medf'ord, Mass. From a photograph by 
 G. H. Barton. 
 
 PLATE 18 241 
 
 Fig. 1. Exfoliated Granite in the Sierras. From a photograph 
 
 by H. W. Turner. 
 Fig. 2. Talus Slopes on Pike's Peak. From a photograph by W. 
 
 H. Jackson. 
 Fig. 3. Disintegrated Granite, Ute Pass, Colorado. From a 
 
 photograph by W. H. Jackson. 
 
 PLATE 19 .248 
 
 Fig. 1. Weathered Schists, Coast of Cape Elizabeth, Maine. 
 
 Fig. 2. Sandstone bored by Bees. 
 
 Fig. 3. Slab of Glaciated Limestone. 
 
 PLATE 20 258 
 
 Fig. 1. Weathered Boulder of Oriskany Sandstone. 
 Fig. 2. Concentric Weathering in Diabase. 
 Fig. 3. Zonal Structure in Weathered Argillite. 
 Fig. 4. Weathered Sandstone showing Induration along Joint 
 Planes. 
 
ILLUSTRATIONS xix 
 
 FACING PAGK 
 
 PLATE 21 . . ... 267 
 
 Fig. 1. Sink-hole near Knoxville, Tennessee. From a photograph 
 
 by George P. Merrill. 
 Fig. 2. Beds of Marble corroded by Meteoric Waters, Pickens 
 
 County, Georgia. 
 
 PLATE 22 285 
 
 Fig. 1. Forest destroyed by Wind-blown Sand. From a photo- 
 graph by I. C. Russell. 
 
 Fig. 2. Calcareous Conglomerate carved and polished by Wind- 
 blown Sand. 
 
 Fig. 3. Rock being undermined by Wind-blown Sand. After G. 
 K. Gilbert. 
 
 PLATE 23 319 
 
 Rock Disintegration and Formation of Talus, Mount Sneffels, 
 Colorado. From a photograph by Whitman Cross. 
 
 PLATE 24 345 
 
 Fig. 1. Section of Beds of Leda Clay, Lewiston, Maine. From a 
 
 photograph by L. H. Merrill. 
 Fig. 2. Beds of Volcanic Dust, Reese Creek, Gallatin County, 
 
 Montana. From a photograph by George P. Merrill. 
 
 PLATE 25 357 
 
 Fig. 1. Section of Glacial Till. From a photograph by G. F. 
 
 Wright. 
 Fig. 2. Glaciated Landscape. From a photograph by L. H. 
 
 Merrill. 
 
 Plates 2, 3, 4, 5, 6, 7, 8, 9, 11, 12, 13, 15, 19, and 20, and Fig. 3 Plate 16, 
 and Fig. 2 Plate 22, from specimens in the Geological Department of 
 the United States National Museum. 
 
 FIGURES IN TEXT 
 
 'IG. PAGE 
 
 1. Augite partially altered into Hornblende 40 
 
 2. Mounted Thin Section of Rock 43 
 
 3. Microscopic Structure of Muscovite-Biotite Granite, Hallowell, 
 
 Maine 67 
 
 4. Microscopic Structure of Diabase, Weehawken, New Jersey . . 88 
 
 5. Microscopic Structure of Peridotite (Porphyritic Lherzolite) . 96 
 
 6. Microscopic Structure of Pyroxenite 100 
 
XX ILLUSTRATIONS 
 
 FIG. PAGE 
 
 7. Microscopic Structure of Oolitic Limestone 112 
 
 8. Pyroxene partially altered into Serpentine 115 
 
 9. Microstructure of Sandstone 131 
 
 10. Section through Lake Basin, showing Bed of Infusorial Earth . 142 
 
 11. Microstructure of Oolitic Limestone ...... 144 
 
 12. Microstructure of Fossiliferous Limestone ..... 145 
 
 13. Microstructure of Quartzite ........ 158 
 
 14. Microstructure of Crystalline Limestone ..... 163 
 
 15. Microstructure of Gneiss 165 
 
 16. Microstructure of Quartzite 169 
 
 17. Influence of Joints in the Production of Boulders .... 244 
 
 18. Exfoliation of Granite, Stone Mountain, Georgia .... 245 
 
 19. Concentric Exfoliation of Granite, Canada 246 
 
 20. Microstructure of Sandstone, with Large Absorptive Power . . 269 
 
 21. Microstructure of Diabase, with relatively Little Absorptive Power 269 
 
 22. Flint Implement showing Weathered Surface .... 274 
 
 23. Sketch showing Pre-Palseozoic Decay of Rocks .... 276 
 
 24. Diagram showing Direction and Rate of Motion of Soil . . 287 
 
 25. Diagram showing Flood Plain of River 289 
 
 26. Angular Outlines of Particles in Residual Soil from Gneiss . . 301 
 
 27. Section across Central Kentucky, showing Inherited Characteris- 
 
 tics of Soils 303 
 
 28. Angular Quartz Particles from Decomposed Gneiss . . . 304 
 
 29. Outlines of Kaolinite Crystals and Kaolin Particles . . . 309 
 
 30. Section across Small Lake 314 
 
 31. Talus Slopes 319 
 
 32. Alluvial Plains 323 
 
 33. Outlines of Particles in Chinese Loess ,329 
 
 34. Particles washed from Leda Clays 335 
 
 35. Cross-section of Marine Marsh 338 
 
 36. Quartz Granules in Beach Sand ....... 343 
 
 37. Outlines of Particles of Glass in Volcanic Dust . . . 349 
 
 38. Section through Carboniferous Soil 386 
 
 39. Section showing Varying Character of Residual Soil . . . 387 
 
 40. Section through Ant Nest 390 
 
 41 and 42. Sections showing the Effect of Tree Roots in Soil . .395 
 
 Fig. 1, after G. W. Hawes ; 5 and 6, after G. H. Williams ; 18 and 22, 
 
 after Robert Bell ; 10, 23, 24, 26, 29, 30, 31, 34, 37, 38, 39, 40, and 41, after 
 Shaler, Twelfth Annual Report United States Geological Survey, 1890-1891. 
 
ROCKS, ROCK-WEATHERING, 
 AND SOILS 
 
 PART I 
 
 THE CONSTITUENTS, PHYSICAL AND CHEMICAL 
 PROPERTIES, AND MODE OP OCCURRENCE OP 
 ROCKS 
 
 I. INTRODUCTORY 
 
 A ROCK is a mineral aggregate ; more than this, it is an 
 essential portion of the earth's crust, a geological body occu- 
 pying a more or less well-defined position in the structure of 
 the earth, either in the form of stratified beds, eruptive masses, 
 sheets or dikes, or in that of veins and other chemical deposits 
 of comparatively little importance as regards size and extent. 
 In giving this definition, origin, chemical composition, and state 
 of aggregation of the individual particles are for the time 
 ignored. From a strictly geological standpoint, the beds of 
 loose sand, and even the water of the ocean itself, may be 
 considered as rocks, and either, under favorable circumstances, 
 may undergo a process of induration such as shall be produc- 
 tive of the condition of solidity commonly ascribed to rocks 
 by the popular mind. 
 
 In ever-varying conditions as regards compactness, color, 
 texture, and structure, rocks form the entire mass of the globe 
 so far as it is as yet made known to us, with the exception of a 
 scarcely appreciable proportion of organic matter. It is rock 
 which forms the substance of mountain ranges and the vast 
 stretches of valley and plain. It is from the rocks that we 
 gain our food, our fuel, and the supplies of metal which are 
 seemingly so essential to our well-being; we cannot ignore 
 
 B 1 
 
2 INTRODUCTORY 
 
 them, even if we would. We borrow from the rocks that 
 which is essential to our life to-day, but when that brief day 
 is ended return it once more, with neither loss nor gain, to its 
 original source. 
 
 Those portions of the earth's crust which are available for 
 study comprise at best but a few thousand vertical feet, though 
 from the fact that the stratified rocks have been so extensively 
 thrown out of their original, horizontal position, and again 
 eroded, we are enabled to measure their thickness, and may 
 hence claim to know with a reasonable degree of accuracy the 
 character of the material forming this crust down to a depth 
 of perhaps twenty miles. 1 Throughout all this vast thickness, 
 comprising millions upon millions of cubic feet, in weight far 
 beyond all comprehension, we find a constant recurrence of 
 materials alike in composition and similarity in origin to those 
 upon the immediate surface. There is at times, as noted later, 
 a difference in structure due to metamorphism, between the 
 older, deeper lying portions and those more recent, but the 
 ultimate composition is essentially the same, and all the know- 
 ledge thus far gained points to a wonderful unity in nature's 
 methods, and shows with seeming conclusiveness that the geo- 
 logical agencies of the past, the methods by which rocks were 
 made and again destroyed, differed in no essential particular 
 from those in progress to-day. What these processes were, 
 how they operated, and with what results, it shall be our aim 
 to here set forth. 
 
 Among the many interesting,, and at first thought seemingly 
 unaccountable, things we shall encounter in the progress of 
 our work, not the least is the fact that so large a proportion 
 of natural objects are more or less out of harmony with their 
 surroundings. Throughout life every organic being is in a 
 constant struggle with the elements to preserve that life, fulfil 
 all its functions, and gratify its natural desires. No sooner 
 does life depart than decomposition and disintegration ensue. 
 As with organic beings, so with inorganic substances. Every 
 mass of rock pushed up by the faulting and folding of the 
 earth's crust, exposed by denudation, or erupted as molten 
 matter from the earth's interior, finds almost at once that its 
 various elements, in their existing combinations, are not in har- 
 
 1 The total mean depth of the fossiliferous formations of Europe as stated by 
 Geikie (Text-book of Geology, p. 675) has been set down as 75,000 feet. 
 
INTRODUCTORY 3 
 
 mony with their environment. The summer's heat and winter's 
 cold, the chemical action of atmospheres and acidulated rains, 
 combine their forces ; a breaking up ensues, to be succeeded 
 by new combinations and perhaps reconsolidations more in 
 keeping with the then existing circumstances. An intermedi- 
 ate product in all this endless cycle of change, of disintegration 
 and recombination, is a comparatively thin, superficial mantle 
 of loose debris, which, mixed with more or less organic matter, 
 nearly everywhere covers the land, and by its combined chemi- 
 cal and mechanical properties furnishes food and foothold for 
 myriads of plants, and hence, indirectly, sustenance for man and 
 beast as well. In brief, what is commonly known as soil is but 
 disintegrated and more or less decomposed rock material, inter- 
 mingled, perhaps, with organic matter from plant decay. Such 
 being the case, a study of the processes of rock weathering and 
 the transportation, deposition, and physical properties of the 
 resultant debris, is but a study of the origin of soils on the 
 broadest and most comprehensive basis, and soils themselves 
 may justly be regarded as secondary rocks in a state of in- 
 complete consolidation. Their study belongs, therefore, as 
 legitimate^ to the realm of geology as does that of any sub- 
 ject relating to rock formation or other phases of the earth's 
 history. 
 
 Accepting the above, we will begin our studies by a consid- 
 eration of (1) the elements which in their single or combined 
 state make up the minerals ; (2) the minerals which make up 
 the rocks ; (3) the rocks themselves, with particular reference 
 to their mineralogical and chemical natures ; (4) the breaking 
 down or degeneration of rocks through processes in part chemi- 
 cal and in part mechanical ; and (5) the result of this clasmatic 
 process as manifested in the production of clay, sand, gravel, 
 and incidental soil. There are other points which will be 
 touched upon more briefly, in order to make our work system- 
 atic, as the action of wind and water in assorting and redeposit- 
 ing rock debris and tending to reduce the land surface to one 
 general level. 
 
II. THE CHEMICAL ELEMENTS CONSTITUTING 
 
 ROCKS 
 
 Although there are 69 elements now known, but 16 occur in 
 any abundance or form more than an extremely small proportion 
 of the material of the earth's crust. Indeed, of this number 
 probably fully one-half, taken collectively, will not constitute 
 more than 4 or 5% of the earth's crust so far as known. These 
 16, arranged according to their chemical properties and order 
 of their abundance, are as follows : oxygen, silicon, carbon, 
 sulphur, hydrogen, chlorine, phosphorus, fluorine, aluminum, 
 calcium, magnesium, potassium, sodium, iron, manganese, and 
 barium. The eight more important, with their approximate 
 percentage amounts as given by Roscoe and Schorlemmer, 1 are 
 
 as below : 
 
 Oxygen 44.0 to 48.7% 
 
 Silicon 22.8 to 36.2 
 
 Aluminum 9.9 to 6.1 
 
 Iron 9.9 to 2.4 
 
 Calcium 6.6 to 0.9 
 
 Magnesium 2.7 to 0.1 
 
 Sodium 2.4 to 2.5 
 
 Potassium 1.7 to 3.1 
 
 It must not for a moment be imagined, however, that these 
 elements exist for the most part in a free or uncombined state : 
 on the contrary, in the majority of cases so great is their affinity 
 for one another that it is only momentarily, or under abnormal 
 conditions, that they are met with at all in this elementary 
 form. Those elements which are most common in the free 
 state, though even these occur more commonly combined with 
 others, are, (1) the gas oxygen, and (2) the solids, carbon, sul- 
 phur, and, more rarely, iron. Still more rarely, and under such 
 abnormal conditions, as exist during volcanic eruptions, are found 
 the free gases, hydrogen, chlorine, and fluorine. The gas nitro- 
 gen, although so abundant a constituent of the atmosphere, 
 
 1 Treatise on Chemistry, Vol. I, p. 55, 1878. 
 4 
 
OXYGEN 5 
 
 is, as a primary constituent of the earth's crust, almost wholly 
 unknown, and needs no consideration at this stage of our 
 work. 
 
 Oxygen, as is well known, is the active, even the aggressive, 
 principle of the atmosphere, of which it constitutes about one- 
 fifth by bulk. Combined with other elements, it is, however, 
 of vastly greater geological importance, being estimated, as 
 noted above, to constitute from 44 to 48.7% of the entire mass 
 of the earth's crust ; that is to say, could the earth's crust be 
 once more resolved into its original elements, the oxygen thus 
 liberated would be found very nearly equal to all the other 
 elements taken together. The simpler forms of oxygen com- 
 pounds are known as oxides, and of these the oxide of hydrogen, 
 water (H 2 O), is by far the most common, and, anomalous as it 
 may at first seem, is a true mineral and to be classed as an 
 anhydrous oxide at that. Aside from being so essential to 
 human life, oxygen, as will be noted later, is a very potent 
 factor in the manifold changes which are constantly taking 
 place in the more superficial portions of the earth's crust. 
 
 Silicon. Next to oxygen silicon is the most abundant of 
 the earth's constituents, though it exists only in combination, 
 either as an oxide (SiO 2 ), or with other elements to form 
 silicates. In these two forms it is the predominating con- 
 stituent in all but the calcareous rocks. As silica (SiO 2 ), or 
 quartz, it forms one of the most indestructible of natural com- 
 pounds, and hence is to be found as the prevailing constituent 
 in nearly all sands and soils. 
 
 Aluminum is next to oxygen and silicon probably the most 
 important element when regarded from our present standpoint. 
 It occurs mainly in combination with silicon and oxygen, form- 
 ing an important series of minerals known as aluminous sili- 
 cates. As a sesquioxide it is well known in the minerals 
 corundum and beauxite. 
 
 Iron, although less abundant than either oxygen or silica, 
 occupies a very important place as a rock constituent, owing to 
 the variety of compounds of which it forms a part, as well as 
 to the decided colors which are characteristic of its oxides and 
 of the iron-bearing silicates. The most conspicuous forms of iron 
 on the immediate surface of the earth are the oxides, but which 
 at greater depths, or where the atmosphere has as yet exercised 
 less influence, give way to carbonates, sulphides, and silicates. 
 
6 CHEMICAL ELEMENTS CONSTITUTING THE ROCKS 
 
 Iron, although so common in combination with other ele- 
 ments, occurs but rarely free, owing to its affinity for oxygen. 
 It is possible that far below the surface, beyond the reach 
 of meteoric waters and atmospheric air, it is to be found in 
 a metallic state much more abundantly, but of- this we have 
 no other proof than that the specific gravity of the globe, in 
 its entirety, is much greater than that of the most dense minerals 
 which constitute its outer portion. The inference seems un- 
 avoidable that at great depths some of these elements exist 
 uncombined, and in a state of greater molecular density than 
 at the surface. 
 
 Calcium is a very important element of the earth's crust, 
 although, as we have seen, it has been estimated to compose 
 only about one-sixteenth of its mass. Its most conspicuous 
 form of occurrence is in combination with carbon dioxide, 
 forming the mineral calcite (CaCO 3 ), or the rock limestone. 
 In this form it is slightly soluble in water containing carbonic 
 acid, and hence has become an almost universal ingredient of 
 all natural waters, whence it furnishes the lime necessary for 
 the formation of shells and skeletons of the various tribes of 
 mollusca and corals. In combination with sulphuric acid, 
 calcium forms the rock gypsum. It is also an important con- 
 stituent of many silicates. 
 
 Magnesium is found in combination with carbonic acid as 
 carbonate, forming thus an essential part of the rock dolomite. 
 The bitter taste of sea-water and some mineral waters is due to 
 the presence of salts of magnesia. In combination with silica 
 as a silicate it forms an essential part of such rocks as serpen- 
 tine, soapstone, and talc. 
 
 Potassium combined with silica is also an important element 
 in many mineral silicates, as orthoclase, leucite, and nepheline. 
 In smaller amounts it is found in silicates of the mica, amphi- 
 bole, and pyroxene groups. The following table will serve to 
 show the varying amounts of potash (K 2 O) in rocks of various 
 kinds : 
 
 Granite 2.6 to 6.50% 
 
 Diorite 0.1 to 2.42% 
 
 Basalt 0.058 to 0.50% 
 
 Gabbro 0.00 to 0.93% 
 
 Limestone , 0.19 to 1.22% 
 
 Sandstone 0.00 to 3.30% 
 
 Slate (fissile argillite) 0.00 to 3.83% 
 
SODIUM 7 
 
 As a chloride, potassium is invariably present in sea-water, 
 and as a nitrate it forms the rare, but valuable mineral nitre, or 
 saltpetre. 
 
 Sodium. The most common and wide-spread form of the 
 element sodium is the compound with chlorine known as 
 sodium chloride (NaCl) or common salt. In this form it is 
 the most abundant of the salts occurring in sea-water, and 
 constitutes also rock masses of no inconsiderable dimensions 
 interstratified Avith other rocks of the earth's crust. Combined 
 with silica, lime, and alumina, sodium is an important constitu- 
 ent of the soda-lime feldspars, and of numerous other silicate 
 minerals. In the form of carbonate and sulphate it occurs as 
 an incrustation on the surface, or disseminated throughout the 
 soils in poorly drained portions of arid countries, giving rise to 
 the so-called "alkali soils," for which such regions are frequently 
 noted. As a nitrate, sodium occurs in the desert regions of 
 Chili, forming the soda nitre so valuable for fertilizing purposes. 
 
 Manganese is, next to iron, the most abundant of the heavy 
 metals, occurring as oxide, carbonate, or in combination with 
 two or more other elements as a silicate. 
 
 Barium is found mainly combined with sulphuric acid, to 
 form the mineral barite or heavy spar. It sometimes occurs 
 as a carbonate, and more rarely as a silicate. 
 
 Phosphorus, although existing in comparatively insignificant 
 proportions, is nevertheless an important element, though in 
 nature it occurs only in combination with various bases, prin- 
 cipally lime, to form phosphates. In this form it is found in 
 the bones of animals, the seeds of plants, and constitutes the 
 essential portions of the minerals apatite and phosphorite. 
 Though small in proportion, phosphorus is a very important 
 constituent of any fertile soils. Its chief source, in the older, 
 crystalline rocks, is the mineral apatite, as noted later. As 
 found in the secondary rocks, as limestones and marls, it is 
 evidently derived from animal remains. (Seep. 151.) Analy- 
 ses have shown that the amount of phosphorus, in the form of 
 phosphoric anhydride (P 2 O 5 ), in rocks rarely exceeds 1%, and 
 usually falls much lower, being most abundant in the basic 
 eruptive rocks, as diorites and gabbros, and most lacking in 
 the siliceous fragmentals, as sandstones and slates. The fol- 
 lowing table will serve to show the small percentages of this 
 constituent in rocks of various kinds: 
 
8 CHEMICAL ELEMENTS CONSTITUTING THE ROCKS 
 
 Granite 0.07 to 0.25% 
 
 Diorite 0.18 to 1.06% 
 
 Basalt 0.03 to 1.18% 
 
 Limestone 0.06 to 10.00% 
 
 Shale 0.02 to 0.25% 
 
 Sandstone 0.00 to 0.1 % 
 
 Of the solid elements occurring free, or uncombined, carbon 
 is by far the more abundant, being found in the forms known 
 as diamond and graphite, or when quite impure as coal. In 
 combination as a dioxide (CO 2 ), it forms the well-known car- 
 bonic acid gas, which, like oxygen, is a powerful agent in 
 bringing about important changes in the rocks with which it 
 comes in contact. Free sulphur occurs more rarely, being as a 
 rule a product of volcanic activity, or due to the reduction of 
 the sulphides and sulphates of the metal with which it more 
 commonly exists in combination. 
 
III. THE MINERALS CONSTITUTING ROCKS 
 
 A rock, as previously stated, is a mineral aggregate. As a 
 rule, the number of mineral species constituting any essential 
 portion of a rock is very small, seldom exceeding three or four. 
 In common crystalline limestones, the only essential constitu- 
 ent is the mineral calcite ; granite, on the other hand, is, 
 as a rule, composed of minerals of three or four independent 
 species. As has been elsewhere stated, the mineral composition 
 of rocks in general is greatly simplified by the wide range of 
 conditions, under which the commonest minerals can be formed, 
 thus allowing their presence in rocks of all classes and of what- 
 ever origin. Thus quartz, feldspar, mica, the minerals of the 
 hornblende or pyroxene group, can be formed in a mass cooling 
 from a state of fusion ; they may be crystallized from solution, 
 or be formed from volatilized products. They are therefore 
 the commonest of minerals and rarely excluded from rocks of 
 any class, since there is no process of rock formation which 
 determines their absence. Moreover, most of the common 
 minerals, like the feldspars, micas, hornblendes, pyroxenes, and 
 the alkaline carbonates, possess the capacity of adapting them- 
 selves to a very considerable range of compositions. In the 
 feldspars, for example, the alkalies, lime, soda, or potash may 
 replace each other almost indefinitely, and it is now commonly 
 assumed that true species do not exist, all being but isomorphous 
 admixtures passing into one another by all gradations, and the 
 names albite, oligloclase, anorthite, etc., are to be used only as 
 indicating convenient stopping and starting points in the series. 
 Hornblende or pyroxene, further, may be pure silicates of lime 
 and magnesia, or iron and manganese may partially replace these 
 substances. Lime carbonate may be pure, or magnesia may 
 replace the lime in any proportion. These illustrations are 
 sufficient to indicate the reason of the great simplicity of rock 
 masses as regards their chief constituents, and that whatever 
 may be the composition of a mass within nature's limits, and 
 
 9 
 
10 THE MINERALS CONSTITUTING ROCKS 
 
 whatever may be the conditions of its origin, the probabilities 
 are that it will be formed essentially of one or more' of a half 
 a dozen minerals in some of their varieties. 
 
 But however great the adaptability of these few minerals may 
 be, they are, nevertheless, subject to very definite laws of chemi- 
 cal equivalence. There are elements which they cannot take 
 into their composition, and there are circumstances which retard 
 their formation while other minerals may be crystallizing. In 
 a mass of more or less accidental composition it may, there- 
 fore, be expected that other minerals will form in consider- 
 able numbers, but minute quantities. It is customary to speak 
 of those minerals which form the chief ingredients of any 
 rock, and which may be regarded as characteristic of any 
 particular variety, as the essential constituents, while those 
 which occur in but small quantities, and whose presence or 
 absence does not fundamentally affect its character, are called 
 accessory constituents. The accessory mineral which predomi- 
 nates, and which is, as a rule, present in such quantities as to 
 be recognizable by the unaided eye, is the characterizing acces- 
 sory. Thus a biotite granite is a stone composed of the essential 
 minerals quartz and potash feldspar, but in which the accessory 
 mineral biotite occurs in such quantities as to give a definite 
 character to the rock. The minerals of rocks may also be con- 
 veniently divided into two groups, according as they are prod- 
 ucts of the first consolidation of the mass or of subsequent 
 changes. This is the system here adopted. We thus have : 
 
 (1) The original or primary constituents, those which formed 
 upon its first consolidation. All the essential constituents are 
 original, but, on the other hand, all the original constituents 
 are not essential. Thus, in granite, quartz and orthoclase are 
 both original and essential, while beryl and zircon or apatite, 
 though original, are not essential. 
 
 (2) The secondary constituents are .those which result from 
 changes in a rock subsequent to its first consolidation, changes 
 which are due in great part to the chemical action of percolat- 
 ing water. Such are the calcite, chalcedony, quartz, and zeo- 
 lite deposits which form in the druses and amygdaloidal cavities 
 of traps and other rocks. 
 
 Below is given a list of the more important rock-forming 
 minerals, arranged as above indicated. Although these are 
 sufficiently described as regards their chemical and crystallo- 
 
ROCK-FORMING MINERALS 
 
 11 
 
 graphic properties in any of the mineralogies, it has seemed 
 advisable to devote some space here to a reconsideration of 
 those most prominent as rock constituents, in order that the 
 individual characteristics of the rocks of which they form a 
 part may be better understood. In passing them in review 
 we will also note briefly the characteristic alteration and de- 
 composition products to which they give rise, though the cause 
 of such changes must be left for another chapter. 
 
 A. ORIGINAL MINERALS. 
 
 1. Quartz. 
 
 2. The Feldspars. 
 2 a. Orthoclase. 
 2 b. Microcline. 
 2 c. Albite. 
 
 2 d. Oligoclase. 
 2 e. Andesite. 
 2/. Labradorite. 
 2 g. Bytownite. 
 
 2 h. Anorthite. 
 
 3. The Amphiboles. 
 
 3 a. Hornblende. 
 3 b. Tremolite. 
 
 3 c. Actinolite. 
 3 d. Arvedsonite. 
 
 3 e. Glaucophane. 
 3/. Smaragdite. 
 
 4. The Monoclinic Pyroxenes. 
 
 4 a. Malacolite. 
 4 b. Diallage. 
 4c. Augite. 
 
 4 d. Acmite. 
 
 4 e. JSgerite. 
 
 .">. The Rhombic Pyroxenes. 
 
 5 a. Enstatite (Bronzite). 
 5 b. Hypersthene. 
 
 6. The Micas. 
 
 6 a. Muscovite. 
 66. Biotite. 
 6 c. Phlogopite. 
 
 7. Calcite (and Aragonite). 
 
 8. Dolomite. 
 
 9. Gypsum. 
 
 10. Olivine. 
 
 11. Garnet. 
 
 12. Epidote. 
 
 13. Zoisite. 
 
 14. Andalusite. 
 
 15. Staurolite. 
 
 16. Scapolite. 
 
 17. Elseolite and Nephelirie. 
 
 18. Leucite. 
 
 19. Sodalite. 
 
 20. Hauyn (nosean). 
 
 21. Apatite. 
 
 22. Menaccanite. 
 
 23. Magnetite. 
 
 24. Hematite. 
 
 25. Chromite. 
 
 26. Halite (common salt). 
 
 27. Muorite. 
 
 28. Graphite. 
 
 29. Carbon. 
 
 30. Pyrite. 
 
 B. SECONDARY MINERALS. 
 
 1. Quartz. 
 
 1 a. Chalcedony. 
 
 16. 
 Ic. 
 
 Opal. 
 Tridymite. 
 
12 
 
 THE MINERALS CONSTITUTING KOCKS 
 
 2. Albite. 
 
 3. The Amphlboles. 
 3 a. Hornblende. 
 3 6. Tremolite. 
 
 3 c. Actinolite. 
 3d. Uralite. 
 
 4. Muscovite (Sericite). 
 
 5. The Chlorites. 
 5 a. Jeff erisite. 
 5 6. Ripidolite. 
 5 c. Penninite. 
 5 d. Prochlorite. 
 
 6. Calcite (and aragonite). 
 
 7. Wollastonite. 
 
 8. Scapolite. 
 
 9. Garnet. 
 
 10. Epidote. 
 
 11. Zoisite. 
 
 12. Serpentine. 
 
 13. Talc. 
 
 14. Glaueonite. 
 
 15. Kaolin. 
 
 16. The Zeolites. 
 16 a. Pectolite. 
 16 6. Laumontite 
 16 c. I^renite. 
 16 d. Ttiortisonite 
 16 e. Natrolite. 
 16/. Analcite. 
 16 g. Datolite. 
 16 h. Chabazite. 
 16 i. Stilbite. 
 
 16 k. Heulandite. 
 16 1 Phillipsite. 
 16m. Ptilolite. 
 16 n. Mordenite. 
 16 o. Harmotome 
 
 17. Hematite. 
 
 18. Limonite. 
 
 19. Gothite. 
 
 20. Turgite. 
 
 21. Pyrite. 
 
 22. Marcasite. 
 
 Quartz. Composition : Pure silica, SiO 2 ; specific gravity 2.6; 
 hardness, 7. 1 
 
 This is one of the commonest and most widely distributed 
 minerals of the earth's crust, and forms an essential constituent 
 in a variety of eruptive and sedimentary rocks, such as granite, 
 
 1 For convenience in determining minerals, the "scale of hardness" given 
 below has been adopted by mineralogists. By means of it one is enabled to 
 designate the comparative hardness of minerals with ease and definiteness. 
 Thus, in saying that serpentine has a hardness equal to 4, is meant that it is of 
 the same hardness as the mineral fluorite, and can therefore be cut with a knife, 
 but less readily than calcite or marble. 
 
 1. Talc : Easily scratched with the thumbnail. 
 
 2. Gypsum : Can be scratched by the thumbnail. 
 
 3. Calcite : Not scratched by the thumbnail, but easily cut with a knife. 
 
 4. Fluorite : Can be cut with a knife, but less easily than calcite. 
 
 5. Apatite : Can be cut with a knife, but only with difficulty. 
 
 6. Orthoclase feldspar : Can be cut with a knife only with great difficulty and 
 
 on thin edges. 
 
 7. Quartz : Cannot be cut with a knife ; scratches glass. 
 
 8. Topaz : Will scratch quartz. 
 
 9. Corundum : Will scratch topaz. 
 10. Diamond : Will scratch corundum. 
 
QUARTZ 13 
 
 quartz porphyry, liparite, gneiss, mica schist, quartzite, and 
 sandstones. In the granites, gneisses, and schists it occurs in 
 the form of irregular granules destitute of crystal outlines. 
 In the quartz porphyries and liparites it is found as a porphy- 
 ritic constituent, usually with well-defined crystal outlines, 
 which may however have become more or less obliterated 
 through the corrosive action of a molten magma. (See Fig. 3, 
 PI. 5.) In the secondary rocks, quartzite and sandstone, the 
 quartz occurs as more or less rounded or irregularly angular 
 grains without crystal outlines, except it may be through a 
 secondary deposition of silica, as explained on p. 158. Quartz 
 is the hardest and most indestructible of the common constitu- 
 ents, and hence when rocks containing it decompose and their 
 debris becomes exposed to combined chemical and mechanical 
 agencies, it remains unaltered to the very last, forming the 
 chief constituent of beds of sand and gravel, which in turn 
 may become transformed into sandstones, quartzites, or con- 
 glomerates. 
 
 Quartz is usually easily recognized, either under the micro- 
 scope or by the unaided eye, by its clear, colorless appear- 
 ance, irregular, glass-like fracture, having no true cleavage, 
 hardness, and insolubility in any acids but hydrofluoric. 
 Under the microscope it appears in clear, pellucid grains, often 
 highly charged with minute cavities filled with liquid and 
 gaseous carbonic acid, the latter like the bubble in a spirit 
 level, dancing about from side to side of its minute chamber as 
 though endowed with life. In other cases the cavity may be 
 filled with a saline solution from which has separated out a 
 minute cube of common salt. 
 
 As a secondary constituent quartz occurs, filling veins and 
 cracks in other rocks, and in the impure crypto-crystalline and 
 amorphous forms known as chalcedony, chert, flint, opal, hya- 
 lite, and agate is found as an infiltration product in the cavities 
 of many trappean rocks, in lenticular and oval concretionary 
 masses in limestones, and replacing the organic matter of wood 
 and other organisms. The name tridymite is given to a quartz 
 occurring in minute, usually microscopic, tablets in cavities in 
 volcanic rocks, particularly the more acid varieties. (See fur- 
 ther on p. 71.) 
 
 The Feldspars. Hardness, 5 to 7; specific gravity, 2.5 to 
 2.8. The feldspars are essentially anhydrous silicates of alu- 
 
14 THE MINERALS CONSTITUTING KOCKS 
 
 minum, with varying amounts of lime, potash, or soda, and 
 rarely barium. They have in common the characteristics of 
 two easy cleavages inclined to one another at an angle of 
 90, or nearly 90; close relationship in optical properties; 
 similarity in colors, which vary from clear and transparent 
 through white, yellowish pink, and red; more rarely greenish, 
 and often opaque through impurities or decomposition; and 
 lastly, a constant intergradation in composition, as already 
 noted on p. 9. 
 
 Nine varieties of feldspar are commonly recognized, which 
 on crystallographic grounds are divided into two groups: the 
 first, crystallizing in the monoclinic system, including ortho- 
 clase and hyalophane; and the second, crystallizing in the 
 triclinic system, including microcline, anorthoclase, and the 
 albite-anorthite series albite, oligoclase, andesine, labradorite, 
 and anorthite. 
 
 The Monoclinic Feldspars : Orthoclase (Sanidin)^ Potash Feld- 
 spars. Composition : K 2 Al 2 Si 6 O 16 = silicia, 64.7 % ; alumina, 
 18.4%; potash, 16.9%. 
 
 This is one of the commonest and most abundant of feldspars, 
 and forms an essential constituent of the acid rocks, such as gran- 
 ite, gneiss, syenite, and the orthoclase and quartzose porphyries; 
 more rarely it occurs as an accessory in the more basic erup- 
 tives. Under the name sanidin is included the clear glassy 
 variety of orthoclase occurring in tertiary and modern lavas, 
 such as trachyte, phonolite, and the liparites. 
 
 Among the older rocks orthoclase not infrequently occurs in 
 very coarse pegmatitic crystallizations with quartz and mica, 
 and is quarried for utilization in pottery manufacture. As a 
 rock constituent the potash feldspars are of primary impor- 
 tance, imparting by their preponderance, not merely color 
 and important structural features, but on their decomposition 
 yielding up the alkali potash, valuable for plant food, and the 
 mineral kaolin so essential for porcelain ware, or in its impure 
 state, as clay for pottery and brick making. In the thin sec- 
 tions, under the microscope, the orthoclase of the older rocks 
 is, as a rule, found to be quite opaque, or at least muddy, 
 through impurities or incipient kaoliiiization. In many erup- 
 tives it has been one of the first minerals to separate out from 
 the molten magma, and shows, therefore, more or less well- 
 defined crystallographic boundaries is idiomorphic, to use a 
 
THE TRICLINIC FELDSPARS 
 
 15 
 
 more technical term. A well-defined zonal structure is fre- 
 quently observed, which is due to interrupted periods of 
 growth, and not infrequently to a gradual change in the char- 
 acter of the magma, whereby the outer zones are more or less 
 translucent or opaque from impurities. Twin structure is very 
 common after what is known as the Carlsbad law, and when 
 the crystals are of sufficient size is easily recognized by the 
 unequal reflection of the light from the two sides of a crystal 
 on a cleavage surface. 
 
 The Triclinic Feldspars. The chemical relationship exist- 
 ing between the triclinic feldspars is shown in the following 
 table: 
 
 
 SiO 2 
 
 A1 2 3 
 
 K 2 
 
 Na 2 
 
 CaO 
 
 Microcline 
 
 65.00% 
 
 18.00% 
 
 17.00% 
 
 
 
 Albite 
 
 68 00 
 
 20 00 
 
 
 12 00 / 
 
 ^^^^\^^^ 
 
 Olio-oclase 
 
 62.00 
 
 24.00 
 
 
 9 00 
 
 aJIUL) h 
 
 5 00 
 
 Labradorite 
 
 53.00 
 
 30.00 
 
 
 4 00 
 
 13 00 
 
 Anorthite . ... 
 
 43.00 
 
 37.00 
 
 
 
 20 00 
 
 
 
 
 
 
 
 Considering only the last four of these, as arranged, it will 
 be noted that they become gradually poorer in the acid element 
 silicia, and richer in alumina and other bases; that is, they 
 become more basic. Also that albite carries some 12 % of soda 
 and no lime; that oligoclase carries 9 % of soda and 5 % of lime; 
 labradorite but 4 % of soda and 13 % of lime, while anorthite, 
 the most basic of all, has no soda, and carries 20 % of lime. 
 They have hence come to be known, respectively, as soda feld- 
 spar, soda-lime feldspar, lime-soda feldspar, and lime feldspar. 
 As a matter of fact, however, these varieties all grade into one 
 another, through the replacing power of the various elements, 
 and are regarded, not as true species, but rather as isomorphous 
 admixtures, forming what is known as the albite-anorthite 
 series. 
 
 Their distinction, either in hand specimens by the unaided 
 eye, or in thin sections by the microscope, is a matter of con- 
 siderable difficulty, and as in addition to other characteris- 
 tics they have in common two eminent cleavages occurring at 
 oblique angles, it has become customary to group all under 
 the general term of plagioclase, a name derived from two 
 
16 THE MINERALS CONSTITUTING ROCKS 
 
 Greek words signifying oblique and fracture. We can then 
 treat of the subject under the heads of (1) microline and 
 (2) plagioclase. 
 
 (1) Microcline (Triclinic Potash Feldspar). As a rock con- 
 stituent, this feldspar is in every way nearly, if not quite, 
 identical with orthoclase, from which it can be distinguished 
 only in thin sections under the microscope. Its composition, 
 manner of occurrence, and associations are those of orthoclase, 
 and need not be repeated here. Anorthoclase is a triclinic 
 soda-potash feldspar of a form closely resembling that of ortho- 
 clase and which for all present purposes may be regarded as 
 orthoclase in which soda replaces a considerable proportion of 
 the potash. 
 
 (2) The Plagioclases. With the exception of albite the 
 plagioclases are all prominent and essential constituents of 
 the basic eruptives. As a rule they are recognizable only as 
 fSldsJTars by the unaided eye, and recourse must be had to 
 the microscope or to chemical tests for their final determina- 
 tion. Examined in thin sections and by polarized light, they 
 almost invariably show a beautiful parallel banding in light 
 and dark colors, which is due to multiple twinning, the alter- 
 nate bands becoming light and dark in turn as the stage of the 
 microscope is revolved. When the crystals are of sufficient 
 size, this twinning is sometimes evident in the form of fine 
 straight, parallel bands, or strise, but in rock masses, as already 
 noted, recourse must be made to microscopic methods. In form 
 the plagioclase of effusive rocks is most frequently slender and 
 elongated, lath-shaped, as commonly described, and often with 
 very perfect crystal outlines. In the norites and gabbros. they 
 are often short and stout, imparting a granular character to the 
 rock. They occur frequently in crystals of two or more gen- 
 erations, of which the earlier formed are usually the largest 
 and best developed. The common forms are described in de- 
 tail below : 
 
 (1) Albite, or soda feldspar, occurs as an original constituent 
 in many granites in company with orthoclase; it is also found 
 in gneiss, the crystalline schists, and not infrequently in diorite, 
 phonolite, trachyte, and other eruptives. (2) Oligoclase, a soda- 
 lime feldspar, occurs like albite in the acid eruptives like gran- 
 ite and quartz porphyry, but is also a common constituent of 
 diorite, and the younger eruptives such as trachyte, the ande- 
 
 
THE TRICLINIC FELDSPARS 17 
 
 sites, and more rarely of the diabases. It is also a constituent 
 of many gneisses. (3) Labradorite, or lime-soda feldspar, is a 
 prominent constituent of the basic eruptives of all geologi- 
 cal ages, such as the norites, diabases, diorites, and basalts. 
 Andesine and bytownite are closely allied varieties of similar 
 'habit, the first being a trifle more acid, and the second more 
 basic than labradorite. (4) Anorthite, or lime feldspar, is 
 also a prominent and important constituent of the basic 
 eruptives, and has been found in meteorites and terrestrial 
 peridotites. 
 
 On account of their abundance and wide distribution, as well 
 as on account of the character of their decomposition products, 
 the feldspars are to be considered as among the most important 
 of rock constituents. As it is from the debris of the older 
 feldspathic rocks that have been made up a large proportion 
 of all the sedimentaries of more recent date, so too it may be 
 claimed that from the decomposition of this feldspathic con- 
 stituent has been derived a large share of the salts of potash, 
 lime, and soda, as well as aluminous silicates which form so 
 essential a portion of the soils. The method of feldspathic 
 decomposition as commonly understood is given on p. 237. 
 
 This decomposition usually manifests itself by a whitening 
 of the mass, accompanied by opacity and a general softening, 
 whereby it falls away to loose powder unless confined. As seen 
 in thin sections under the microscope, the decomposition goes 
 on most rapidly along lines of cleavage, naturally attacking the 
 outer portions first, so that the crystals show fresh unaltered 
 cores surrounded by opaque and " muddy " borders. In cases 
 where the feldspars carry iron this usually makes its presence 
 known by a reddening or browning of the mass, due to oxida- 
 tion. In presence of abundant carbonic acid, the liberated iron 
 may enter into combination as a carbonate and the color remain 
 unchanged. 
 
 Daubree, who submitted feldspathic fragments to trituration 
 in revolving cylinders of stone and iron, found that in all such 
 cases not merely were the particles worn down to the condi- 
 tion of fine silt, but that there was an actual decomposition, 
 whereby a certain proportion of the alkalies in the form of 
 soluble silicates were formed in the water with which the cyl- 
 inders were partially filled. When the trituration was carried 
 on in iron cylinders, a certain amount of iron oxides were 
 
18 THE MINERALS CONSTITUTING ROCKS 
 
 formed which combined with the silica of the alkaline silicate, 
 leaving the alkali itself free. As in nearly all decomposing 
 rocks there exists more or less of iron oxides from decomposing 
 ferruginous minerals, it is not impossible that a similar reaction 
 is there going on. 
 
 The production of kaolin through feldspathic decomposition 
 has become so well recognized that it is customary to speak 
 of this form of decomposition as kaolinization, a term which we 
 shall have frequent cause to use as we proceed. 
 
 It should be noted that orthoclase, though so frequently 
 found muddied and impure, apparently in an advanced stage 
 of decomposition, does not in reality decompose so readily as 
 the plagioclase (soda-lime) varieties. This fact has been noted 
 by Lemberg, 1 who states that the apparent decomposition may 
 be due to physical causes, as disintegration, inclusions of some 
 easily decomposable silicate, or to originally water-filled cavities 
 whose contents have been absorbed through the formation of 
 secondary hydrous silicates. 
 
 Leucite. Composition: Silica, 55.0 % ; alumina, 23.5 % ; pot- 
 ash, 21.5%. 
 
 Leucite occurs as an original and essential constituent of 
 many volcanic rocks, such as leucitophyre, leucotephrite, and 
 leucitite. More rarely it occurs in trachyte. It is a common 
 associate of nepheline in recent lavas, and has been found asso- 
 ciated with elseolite in the elseolite syenites of Hot Springs, 
 Arkansas. When well developed it shows polyhedral, garnet- 
 like outlines. 
 
 Leucite as a rock constituent is not an abundant mineral 
 except in rare instances. Its chief interest, from our present 
 standpoint, lies in its high percentage of potash which must 
 become available as plant food on decomposition. Leucite is 
 a common constituent of certain Vesuvian lavas, and it is not 
 improbable that this fact may account in part for the well- 
 known fertility of the soils of this region, though naturally 
 climatic influence has much to do. 
 
 Nepheline (Elaeolite). These names are given to what are 
 varietal forms of one and the same mineral. In composition 
 they are silicates of alumina, soda, and potash of the formula 
 (NaK) 2 Al 2 Si 2 O 8 = silica, 41.24; alumina, 35.26; potash, 6.46; 
 soda, 17.04. 
 
 i Zeit. Deut. Geol. Gesellschaft, 35, 1883. 
 
THE AMPHIBOLES 19 
 
 Nepheline occurs in Tertiary and post-Tertiary eruptive rocks, 
 and is an essential constituent of phonolite, tephrite, and nephe- 
 linite. Secondary nepheline has been found in the ejected vol- 
 canic blocks found in the lava of Mount Somma. The variety 
 elseolite occurs only in older rocks, and is an essential constitu- 
 ent of elseolite syenite. Cancrinite is a yellowish granular 
 mineral, in some cases apparently resulting from the alteration 
 of ekeolite, with which it occurs. 
 
 Both nepheline and elseolite gelatinize readily with hydro- 
 chloric acid, and the powdered rock when treated on a glass slide 
 with this acid yields abundant microscopic cubes of sodium 
 chloride. This is one of the easiest of microchemical tests for 
 the determination of the mineral. Nepheline occurs as a rule 
 in well-defined short and stout hexagonal prisms, which in 
 longitudinal sections show up as short, colorless rectangular 
 areas extinguishing parallel with the sides of the prism. Elseo- 
 lite differs in being more opaque and occurring in less well- 
 defined, more granular forms. When occurring in sufficient 
 abundance in a rock mass it is readily recognized by its char- 
 acteristic greasy appearance. The mineral undergoes a ready 
 alteration, giving rise to zeolitic minerals and on ultimate 
 decomposition through weathering, yielding a rich and fertile 
 soil. 
 
 The Amphiboles. Composition : Two principal varieties are 
 recognized. (1) Non-aluminous, consisting mainly of the 
 meta-silicates of magnesium and calcium, with 55 to 59 % of 
 silica, 21 to 27 % of magnesia, 11 to 15 % of lime, and small pro- 
 portions of protoxides of iron and manganese. Under this head 
 are included the white, gray, and pale green, often fibrous forms, 
 as tremolite, actinolite, and asbestos. (2) Aluminous, contain- 
 ing silica, 40 to 51 % ; magnesia, 10 to 23 % ; alumina, 6 to 14 % ; 
 lime, 10 to 13%; ferrous and ferric oxides, 12 to 20%. Here 
 are included the dark green, brown, and black varieties. 
 
 The aluminous variety, common hornblende, is an original 
 and essential constituent of diorite, and of many varieties of 
 granite, gneiss, syenite, schist, andesite, and trachyte, and is 
 also present as a secondary constituent in many rocks, result- 
 ing from the molecular alteration of the augite. The non- 
 aluminous varieties occur in gneiss, crystalline limestone, and 
 other metamorphic rocks. 
 
 By the unaided eye, or by means of blowpipe tests, it is often 
 
20 
 
 THE MINERALS CONSTITUTING ROCKS 
 
 impossible to distinguish the minerals of this group from 
 the pyroxenes. In the thin sections this distinction is, however, 
 a matter of comparative ease, basal sections showing not merely 
 a greater development of prismatic faces, but also cleavages 
 cutting at angles of 66 and 124 instead of nearly at right 
 angles, as in the latter. Green fibrous hornblendes frequently 
 result from the molecular alteration of augite, and all varieties 
 are susceptible of alteration into chloritic and ferruginous 
 products with the separation of calcite. In the recent lavas it 
 is a common occurrence to find the hornblendes surrounded by 
 a black border, or wholly changed by corrosion of the molten 
 magma into an aggregate of small black opaque granules, which 
 in certain instances have been proven to be augites. 
 
 On decomposing, the amphiboles give rise to ferruginous and 
 aluminous or magnesian products, as do the pyroxenes, next to 
 be described. With the darker colored varieties, the decompo- 
 sition begins with hydration and the peroxidation of the iron 
 along lines of cleavage and fracture, whereby the crystal 
 becomes riddled with corroded areas filled with the liberated 
 iron in the form of hydrated sesquioxide. 
 
 When the disintegration is complete, the whole mass is con- 
 verted into an ochre-brown, earthy substance. These chemical 
 changes are indicated in the following analysis of I. fresh, and 
 II. decomposed hornblende from Haavi on Fillef jeld, Norway : 1 
 
 
 I 
 
 
 II 
 
 Silica . 
 
 4537 
 
 
 4032 
 
 Alumina 
 
 1481 
 
 
 1749 
 
 Iron protoxide .... 
 Manganese 
 
 8.74 
 1 50 
 
 Iron peroxide . . . 
 
 18.26 
 2 14 
 
 Lime 
 
 1491 
 
 
 537 
 
 Magnesia 
 
 1433 
 
 
 923 
 
 Water 
 
 
 
 800 
 
 
 
 
 
 
 99.66 
 
 
 100.81 
 
 The most striking features of the above analyses are (1) 
 the complete conversion of the protoxides into sesquioxides, 
 (2) the loss in lime and magnesia which have presumably 
 
 1 Bischof's Chemical Geology, Vol. II, p. 354. 
 
THE MONOCLINIC PYROXINES 21 
 
 been carried away in the form of carbonates, and (3) the 
 assumption of 8 % of water. As the dark aluminous and 
 ferruginous hornblendes are among the commonest and most 
 wide-spread of minerals, it is apparent from the above that 
 they may have an important bearing upon the color and physi- 
 cal qualities of the residual clays ; to which they thus give 
 rise. The peroxidation of the iron gives yellow, brown, or red 
 colors, while the hydrated aluminous silicate (clay) imparts 
 tenacity. The final product of such decomposition is, then, a 
 ferruginous clay. 
 
 The Pyroxenes. The rock-forming pyroxenes are divided 
 upon crystallographic grounds into two groups, the one ortho- 
 rhombic in crystallization, and the other monoclinic. All varie- 
 ties, when in good crystalline form, show in basal sections an 
 octagonal outline bounded by prismatic and pinacoidal faces 
 and with a well-defined cleavage parallel with the prism faces. 
 Chemically they are silicates of magnesia and iron with lime 
 and alumina in varying proportions. They are hard, tough 
 minerals and have an important bearing upon the physical 
 properties of the rocks of which they form a part. Their dis- 
 tribution, in some of their varieties, is almost universal, being 
 found in metamorphic and eruptive rocks of almost every class 
 and every age. 
 
 The Monoclinic Pyroxenes. Two principal varieties are recog- 
 nized. (1) Pyroxenes containing little or no alumina, and com- 
 posed of silica, 45.95 to 55.6 % ; lime, 21.06 to 25.9 % ; magnesia, 
 13.08 to 18.5 %, with sometimes varying quantities of iron oxides 
 and water. Under this head are included the lighter colored 
 varieties, malacolite, sahlite, and diallage. (2) Pyroxenes con- 
 taining alumina, and composed of silica, 49.40 to 51.50 %; alu- 
 mina, 6.15 to 6.70%; magnesia, 13.06 to 17.69%; lime, 21.88 
 to 23.80%; iron oxides, 0.35 to 7.83%, with sometimes small 
 quantities of soda and water. Under this head are included 
 the darker varieties, augite and leucaugite. 
 
 The lighter colored, non-aluminous varieties, malacolite and 
 sahlite, are common in mica and hornblendic schists, gneiss, 
 and granite, though not always in sufficient abundance to be 
 noticeable to the naked eye. The foliated variety, diallage, 
 is an essential constituent of the rock gabbro, and is also 
 common in peridotites. The darker colored, aluminous vari- 
 ety, augite, is an essential constituent of diabase and basalt, 
 
22 THE MINERALS CONSTITUTING ROCKS 
 
 and also occurs in many syenites, andesites, and other eruptive 
 rocks. 
 
 In the thin sections the monoclinic pyroxenes are usually 
 readily recognized by their nearly rectangular cleavages on 
 basal sections (see Fig. 1), lack of pleochroism, and high 
 extinction angles on sections parallel to the clinopinacoids. 
 The aluminous varieties undergo alteration into chloritic and 
 ferruginous products, while the non-aluminous give rise to ser- 
 pentine, either process being attended by the separation of 
 free calcite. 
 
 JEgerine and acmite are soda-bearing pyroxenes corre- 
 sponding to the formula Na 2 OFe 2 O 3 4SiO 2 . They are less 
 abundant than the above-mentioned varieties, and so far as 
 yet described seem to be confined mainly to the elseolite 
 syenites. 
 
 The OrthorJiombic Pyroxenes. These are essentially silicates 
 of magnesia and iron, the latter replacing the former in varying 
 proportions up to as high as 25 %. Two principal varieties are 
 recognized, the distinction being founded mainly upon their 
 optical properties which seem to be affected very largely by the 
 percentages of iron. Enstatite is the theoretically pure mag- 
 nesian silicate of the formula MgSiO 3 , but which, as a matter 
 of fact, usually contains from 2 to 10 % or more of iron. The 
 highly ferruginous varieties are known as bronzite, from their 
 bronze-like lustre. Hypersthene differs from enstatite in being 
 strongly pleochroic in thin sections, and it contains from 10 to 
 25 % of ferruginous oxide. 
 
 Both enstatite and hypersthene are common constituents of 
 basic igneous rocks, such as the gabbros, norites, and perido- 
 tites. Enstatite is a common constituent of meteorites, occur- 
 ring not infrequently in peculiar fan-shaped, radiating masses 
 not greatly unlike certain organic forms for which they were 
 once mistaken. Both forms are liable to alteration, giving rise 
 to serpentinous pseudomorphs to which the name bastite has 
 been given, and to talcose and chloritic products. The general 
 character of the decomposition products of the pyroxenes, as 
 well as the methods by which the decomposition progresses, 
 are in every way similar to those of the amphiboles, and need 
 not be further dwelt upon here. 
 
 The Micas. There are several species of mica which are 
 prominent as rock constituents, the more important being the 
 
THE MICAS 23 
 
 white variety, muscovite, and the dark variety, biotite. Both 
 occur, as a rule, in thin, platy forms, splitting readily into thin, 
 elastic folia, which in crystalline form are hexagonal in outline. 
 The folia are often bent and distorted, and the mineral not 
 infrequently undergoes alteration into a chloritic or sericitic 
 product. The micas exercise an important influence upon the 
 rocks containing them, on both color and structural grounds. 
 Other things being equal, the muscovite-bearing rocks are 
 lighter in color than those carrying biotite. If the mica plates 
 are arranged in definite planes, the rock assumes a schistose 
 structure and splits more or less readily into sheets an impor- 
 tant feature from an economic standpoint. Muscovite, or 
 potash mica, a silicate of alumina and potash, is a constituent 
 of many granites, gneisses, and schists, but is rarely met with 
 in other rocks, and is wholly wanting in the basic eruptives. 
 Another white or nearly colorless mica is sericite, a silvery 
 white, or greenish, hydrous, secondary constituent of metamor- 
 phic schists, or occurring as an alteration product from feldspar : 
 paragonite and margarite are other hydrous micas, confined 
 mainly to the schists and to veins. Lepidolite, a lithia mica of 
 a white or faint pink color, is frequently found in pegmatitic 
 veins in the older rocks. 
 
 Biotite, the black iron mica, is a silicate of alumina, iron, and 
 magnesia, and is much more general in its distribution than is 
 muscovite, occurring in both eruptive and metamorphic rocks 
 of all kinds and of all ages. It undergoes alteration into 
 chloritic and ferruginous products and is often an impor- 
 tant feature in hastening rock disintegration. Other black 
 micas, sometimes distinguishable from biotite only by chemi- 
 cal means, are lepidomelaiie and houghtonite. A pearl gray 
 potash mica phlogopite is an important constituent of many 
 limestones, as in northern New York and adjacent portions of 
 Canada. 
 
 All micas, owing to their eminently fissile structure, allow the 
 ready percolation of moisture, and hence, though in themselves 
 of difficult solubility, are elements of weakness in any stone 
 of which they may form a part. The characteristic form of 
 decomposition begins as in other silicate minerals, with hydra- 
 tion. This in the dark varieties is accompanied by a higher 
 oxidation of the iron. The foliie gradually lose their elasticity 
 and crumble away, the bases being removed in solution as 
 
24 
 
 THE MINERALS CONSTITUTING ROCKS 
 
 before. The complete decomposition of the micas is, however, 
 brought about very slowly, and almost any granitic soil, how- 
 ever thoroughly decomposed, will, on washing, show small flakes 
 of the mineral still remaining. However rusty, too, these may 
 appear, a little hydrochloric acid cleans them up, showing rem- 
 nant shreds still fresh and readily recognizable. For some 
 unexplained reason those granitic rocks containing a consider- 
 able proportion of white mica are almost invariably more friable 
 and easily disintegrated than those containing biotite. 
 
 Olivine (Chrysolite, Peridote). Composition: Silicate of iron 
 and magnesia, (MgFe) 2 SiO 4 . 
 
 This is an essential constituent of basalt, dunite, limburgite, 
 Iherzolite, and pikrite, and a prominent ingredient of many 
 lavas, diabases, gabbros, and other igneous rocks. It also occurs 
 occasionally in metamorphic rocks and is a constituent of most 
 meteorites. Olivine is subject to extensive alteration, becom- 
 ing changed by hydration into serpentine or talcose and chloritic 
 products, with the separation of free iron oxides. Under the 
 microscope olivine is as a rule easily recognized by its lack of 
 cleavage and brilliant polarization colors. It occurs in well- 
 defined crystals and also in irregular grains, either singly or 
 grouped in peculiar clusters to which the name poly somatic has 
 been applied by Tschermak. The serpentinous alteration takes 
 place along the irregular curvilinear lines of fracture, and under 
 favorable conditions continues until the transformation is com- 
 plete. The following analyses by Holland, as quoted in Teall's 
 British Petrography, illustrate the simplicity of the chemical 
 changes which here take place : 
 
 
 I 
 
 II 
 
 III 
 
 Si0 2 
 
 41 32 / 
 
 42 72 / 
 
 43 48 / 
 
 Alo0 3 
 
 XL.VtJ /Q 
 
 028 
 
 ***! /O 
 
 006 
 
 
 Fe 2 3 ...... 
 
 239 
 
 225 
 
 
 CrO 
 
 005 
 
 Trace 
 
 
 M^O 
 
 5469 
 
 42 52 
 
 43 48 
 
 H 2 O . . . 
 
 020 
 
 1339 
 
 13 04 
 
 
 
 
 
 
 98.93 % 
 
 100.94 % 
 
 100.00 % 
 
 I. Olivine, Snarum, Norway. II. Serpentine derived from the same. 
 III. The theoretical composition of serpentine. 
 
EPIDOTE AND ALLANITE 25 
 
 Aside from the assumption of some 13 % of water, the princi- 
 pal change, as will be noted, is a loss in magnesia which as a 
 rule separates out as a carbonate. The iron, which existed as 
 protoxide, is further oxidized and crystallizes out along lines of 
 fracture as magnetite or hematite, or in the hydrous sesquioxide 
 form known as limonite. Through decomposition, a portion or 
 all of the silica may be set free as opal or. chalcedony, the mag- 
 nesia going 'over to the condition of carbonate, and the iron 
 passing into various hydrated oxide forms such as are most 
 stable under the existing circumstances. 
 
 Epidote. Composition: Silica, 37.83%; alumina, 22.63%; 
 iron oxides, 15.98 % ; lime 23.27 % ; water 2.05 %. 
 
 This mineral is a common constituent of many granites, 
 gneisses, and schists, especially the hornblendic varieties. It 
 is particularly abundant, however, as a secondary constituent 
 in basic eruptives, where it results from the alteration of the 
 original ferromagnesian constituents such as the augites, horn- 
 blendes, or micas. It is the presence of this mineral or a sec- 
 ondary chlorite that gives the characteristic color to many of 
 the so-called greenstones (altered basalts, diabases, diorites, etc.). 
 
 The name piedmontite is given to a red manganese epidote, 
 which has been found in certain Japanese schists and has also, 
 in sparing amounts, been observed by Professor Ha worth, 1 in the 
 quartz porphyries of Missouri, and a few foreign porphyrites. 
 Zoisite is a closely related mineral crystallizing in the ortho- 
 rhombic system and relatively poorer in iron and richer in 
 alumina than is epidote. It is chiefly characteristic of the crys- 
 talline schists, though sometimes found in granitic rocks, inter- 
 grown with common epidote as has been noted in Maryland, by 
 Keyes. 2 
 
 Allanite, or orthite, as it is sometimes called, is closely allied 
 to common epidote, but contains cerium and other of the more 
 rare alkaline earths. In the form of brown acicular crystals it 
 is a common constituent of New England granites and has 
 recently been described in a granite porphyry near Ilchester, 
 Maryland, where it occurs enclosed as a nucleus in the ordinary 
 epidote. 
 
 Calcite (Calcium Carbonate). Composition: CaCO 3 = Car- 
 bon dioxide, 44 % ; lime, 56 %. Hardness, 3. 
 
 1 American Geologist, Vol. I, p. 365. 
 
 2 15th Ann. Rep. U. S. Geol. Survey, 1890-94. 
 
26 THE MINERALS CONSTITUTING ROCKS 
 
 This is an original constituent of many secondary rocks, 
 such as limestone, ophiolite, and calcareous shales. It is the 
 essential constituent of most marbles, of stalactites, travertine, 
 and calc-sinter. The shells of foraminifera, brachiopods, crus- 
 taceans, and many lamellibranchs and gasteropods are also of 
 this material. As a secondary constituent, resulting from the 
 decomposition of other minerals, it occurs almost universally, 
 filling wholly or in part cavities in rocks of all ages, such as 
 granite, gneiss, syenite, diabase, diorite, liparite, trachyte, 
 andesite, and basalt. 
 
 The effervescence of the mineral when treated with a dilute 
 acid furnishes the most ready means for its detection. Under 
 the microscope it appears as colorless grains with faint irides- 
 cent polarization, and is best recognized by its cleavage and 
 characteristic twinning lines as shown in the figure on p. 163. 
 Being soluble in carbonated waters, it is liable to complete 
 removal, or leaves only its impurities behind as a mark of its 
 decay. 
 
 Aragonite. Composition : CaCO 3 = Carbon dioxide, 44 % ; 
 lime, 56 % . 
 
 This mineral has the same chemical composition as calcite, 
 but differs in its crystalline form and specific gravity. It 
 occurs with beds of gypsum and veins of ore, and also in 
 stalactitic and stalagmitic forms. In small quantities it occurs 
 as a secondary product in many trap rocks and basalts, and is 
 the substance of which the shells of many gasteropod and 
 lamellibranch molluscs are composed. 
 
 The mineral occurs nearly always in clustered aggregates of 
 radiating, divergent needles, and is distinguished from calcite 
 by its crystallization and cleavage. As a rock constituent it is 
 comparatively unimportant, but frequently occurs as a decom- 
 position product in basic eruptives. This form of calcium 
 carbonate, as long ago pointed out by Sorby, is less stable than 
 calcite, and in many instances where the substance has first 
 crystallized in the orthorhombic form aragonite, it is found to 
 have undergone a molecular alteration into calcite. 
 
 Dolomite. Composition: (CaMg)CaO 3 = Calcium carbonate, 
 54.35%; magnesium carbonate, 45.65%. Hardness, 3.5-4. 
 
 This mineral, like calcite, is a wide-spread constituent of 
 rocks, and not infrequently forms extensive masses which are 
 of value as sources of building material. It is distinguishable 
 
APATITE AND THE IRON ORES 27 
 
 from calcite by its greater hardness, higher specific gravity, 
 and in being but slightly acted on by acetic or dilute hydro- 
 chloric acid. In itself the mineral is less susceptible to atmos- 
 pheric influence than calcite, yielding much less readily to 
 decomposing agencies of a purely chemical nature. Never- 
 theless, Roth 1 has shown that in the weathering of dolomitic 
 limestones the magnesia is sometimes removed by leaching, in 
 greater proportional quantities than the more soluble lime 
 carbonate. 
 
 Apatite. Composition: Phosphate of lime. Hardness, 5. 
 
 Apatite is an almost universal constituent of eruptive rocks, 
 both acid and basic, though as a rule present only in micro- 
 scopic proportions. In the granular limestones, schists, and 
 other metamorphic and vein rocks it sometimes occurs in large 
 crystals or massive forms in such abundance as to be of value 
 as a source of mineral phosphate for fertilizing purposes. In 
 the thin sections the apatites of eruptive rocks are as a rule 
 colorless, and without evident cleavage, though presenting 
 good crystallographic forms. Rarely the mineral is pleochroic 
 in red or brown or bluish colors. If a drop of an acid solution 
 of ammonium molybdate be placed upon an apatite crystal in 
 an uncovered slide, the mineral will be slowly dissolved and 
 minute crystals of phosphomolybdate of ammonium be contem- 
 poraneously deposited. The process is an easy one, readily 
 performed while the slide is still 011 the stage, and forms one of 
 the most interesting and accurate of the many microchemical 
 tests. Though present in but small amounts, apatite is an 
 important constituent, since it is the only common rock con- 
 stituent containing the valuable element phosphorus. 
 
 THE IRON ORES 
 
 Under this head we may conveniently treat the several 
 forms of iron oxides which commonly occur as rock constitu- 
 ents, and which from their opacity in even the thinnest sec- 
 tions, and similarly in crystallographic outline, are separable 
 with difficulty by optical tests alone. 
 
 Magnetite. Composition: FeO -}- Fe 2 O 3 = iron sesquioxide, 
 68.97%; iron protoxide, 31.03%. 
 
 This is a wide-spread and almost universal constituent of 
 
 1 Chemische u. Allgemeine Geologie. 
 
28 THE MINERALS CONSTITUTING ROCKS 
 
 eruptive rocks, occurring as a rule in the form of scattering, 
 small, and rather inconspicuous granules, which under the 
 microscope are characterized by a complete opacity and bluish 
 lustre. When of sufficient size to be distinguished by the 
 unaided eye, magnetite is easily recognized by its brilliant 
 lustre, weight, and its property of being readily attracted by 
 the magnet. It is as a rule one of the first minerals to sepa- 
 rate out from the molten magma, and hence presents good 
 crystal outlines in which octahedral forms prevail. Skeleton 
 forms of great beauty are not infrequent. Magnetite also 
 occurs as a constituent of metamorphic rocks and is some- 
 times found in large beds, constituting a valuable ore of iron. 
 Under continual alternations of heat and cold, moisture and 
 dryness, it slowly decomposes, giving rise to hydrated sesqui- 
 oxides which impart color, but no valuable qualities, to the 
 resultant sands and clays. 
 
 Menaccanite (Ilmenite or Titanic Iron). Composition : 
 (TiFe) 2 O 3 , a mixture in varying proportions of the oxides 
 of iron and titanium. 
 
 This, like magnetite, occurs in scattering granules as an 
 original constituent of many eruptive rocks, and also in mica- 
 ceous lamellar and vein-like masses in other rocks. Under the 
 microscope it shows, by incident light, a brownish rather than 
 bluish lustre, but is best recognized by its characteristic altera- 
 tion products, which are whitish, gummy, and opaque. The 
 name leucoxene was given by Gumbel to the final product of 
 this alteration. This form of iron ore is extremely refractory 
 to atmospheric agencies and is to be found scarcely, if any, 
 changed in the residuary materials resulting from the breaking 
 down of the rocks in which it originated. 
 
 Hematite (Specular Iron Ore.) Composition: Anhydrous ses- 
 quioxide of iron, Fe 2 O 3 = iron, 70.9 % ; oxygen, 30.20 %. H = 
 5.5-6.5. 
 
 This mineral occurs in varying proportions and under vary- 
 ing conditions in rocks of all ages. In the form of minute 
 scales of a blood-red color, it is found not infrequently in 
 granitic and other eruptive rocks. It occurs, also, in large 
 beds, forming a valuable ore of iron. In the amorphous 
 condition, it may form the cementing constituent of sand- 
 stones, and is the cause of the red color of many rocks, both 
 clastic and metamorphic, and of soils as well. The usual color- 
 
LIMONITE AND PYRITE 29 
 
 ing constituent is, however, limonite or turgite, as noted below. 
 The specular and massive forms are best recognized by opacity, 
 brilliant, black, metallic lustre, and red streak. 
 
 Limonite (Brown Hematite). Composition: Hydrous ses- 
 quioxide of iron, H 6 Fe 2 O 6 + Fe 2 O 3 =iron sesquioxide, 85.6%; 
 water, 14.4 %. H = 5-5.5. 
 
 This is a common constituent of rocks of all ages, but is as 
 a rule wholly secondary, resulting from the decomposition of 
 ferruginous silicates, sulphides, and anhydrous oxides. As a 
 coloring constituent it is even more abundant than hematite, 
 and like it forms a valuable ore of iron. (See p. 10T.) Turgite 
 (Fe 4 H 2 O 7 ) in the form of a brilliant red ochreous material is 
 also a common constituent of soils and clays resulting from the 
 decomposition of siliceous rocks, and is presumably, like limo- 
 nite, a product of the spontaneous hydration of the iron salts 
 thus set free. (See further under Color of Soils, p. 385.) 
 
 Pyrite (Iron Pyrites). Composition: Iron disulphide, FeS 2 
 = iron, 46.7 % ; sulphur, 58.3 %. H = 6-6.5. 
 
 Two principal forms of iron disulphide occur in nature, alike 
 in chemical composition, but differing in forms of crystalliza- 
 tion and in density. The one is common pyrites which crys- 
 tallizes in the isometric system, and is easily recognized by its 
 strong brassy yellow color and hardness. Its usual form of 
 occurrence is that of cubes, the corners and edges of which may 
 be more or less modified by secondary planes, and in concre- 
 tionary masses. The second form marcasite, also called gray, 
 white, or cockscomb pyrites, is of lighter color, inferior hard- 
 ness and density, and crystallizes in the orthorhombic system. 
 Its most common form of occurrence is that of irregular con- 
 cretionary masses. 
 
 Both forms of pyrite are susceptible to oxidation when 
 exposed to atmospheric agencies, though of the two the pyrite 
 proper is much the more refractory. 
 
 Mr. A. P. Brown has shown 1 that in this form of the com- 
 pound a large proportion of the iron exists in a ferric condition 
 while in marcasite it is ferrous. In other words, marcasite is 
 an unsaturated compound, and hence unstable. This readily 
 explains the relatively more rapid decomposition of the latter 
 mineral. There is also a difference in the character of the 
 products arising from the decomposition of the two compounds, 
 
 i Proc. American Philos. Soc., Vol. XXXIII, 1894, p. 225. 
 
30 THE MINERALS CONSTITUTING ROCKS 
 
 pyrite yielding, as a rule, limonite and free sulphur, while mar- 
 casite, under the same conditions, yields ferrous sulphate, though 
 when decomposing under water, it may also yield much limonite. 
 The sulphate of iron, resulting from pyritiferous decomposition, 
 is, if present in quantity, injurious to plant growth. This fact 
 was well illustrated some years ago on the west front of the 
 National Museum at Washington. Several large masses of iron 
 sulphide, too large for exhibition within the building, were 
 placed here upon a floor of cement bordered by a narrow strip 
 of lawn. Under the oxidizing influence of rain and air the 
 sulphide became slowly converted into sulphate which was 
 washed down upon the cement and thence into the soil, which 
 it so poisoned as to kill the roots and necessitate an entire 
 resodding. 
 
 The experiments of Prichard, 1 however, showed that the 
 presence of a small amount of sulphate of iron in a soil may, 
 under certain conditions, be beneficial, in that it serves to pre- 
 vent the loss of ammonia in rapidly decomposing materials. 
 In processes involving slow decomposition, its antiseptic quali- 
 ties render it of doubtful value. 
 
 Chlorite (Viridite). Under the general name chlorite are 
 included several minerals occurring in fibres and folia, closely 
 resembling the micas, from which they differ in their large per- 
 centage of water, and in their folia being inelastic. The three 
 principal varieties recognized are, ripidolite, penninite, and pro- 
 chlorite, any one of which may occur as the essential constitu- 
 ent of a chlorite schist. Chlorite as a secondary product often 
 results from and entirely replaces the pyroxene, hornblende, or 
 mica in rocks of various kinds, and also occurs filling wholly or 
 in part the amygdaloidal cavities of trap rocks. In this form 
 it is frequently visible only with the microscope, and owing to 
 the difficulties in the way of an exact determination of its 
 mineral species is sometimes called viridite. It is this mineral 
 which gives the green color to a large share of the more or 
 less altered eruptives, like the diabases and diorites, the 
 "greenstones" of the older geologists. 
 
 Serpentine. Composition : A hydrous silicate of magnesium 
 corresponding to the formula H 4 Mg 3 Si 2 O 9 = silica, 44.1 % ; mag- 
 nesia, 43.0 % ; and water, 12.9 %. 
 
 The prevailing color is green, though often spotted and 
 1 Ann. de Chernie et Physique, 1892. 
 
GLAUCONITE AND THE ZEOLITES 31 
 
 streaked ; hence the name from the Latin serpentinus, a ser- 
 pent. It has a somewhat greasy lustre and may be cut with a 
 knife, having a hardness of about 4 of the scale. The mineral 
 is always secondary, resulting mainly from the hydration of 
 pure magnesian or lime magnesian silicates. (See further on 
 p. 115.) 
 
 Glauconite. This name is given to a somewhat variable 
 compound consisting essentially of silica, iron, alumina, and 
 water, with smaller amounts of potash, and incidentally lime, 
 magnesia, and soda. The prevailing color is green, and as it 
 occurs in single granules or granular aggregates, it is com- 
 monly known as greensand. It is always a secondary mineral, 
 and has been formed and is still forming on many shallow sea- 
 bottoms which receive fine sediments derived from the breaking 
 down of siliceous crystalline rocks. (See under Greensand 
 Marl, p. 133.) 
 
 The Zeolites. Under this head are grouped a number of 
 minerals alike in being hydrous silicates of alumina with vary- 
 ing percentages of lime, potash, and soda. They are altogether 
 secondary minerals, resulting from chemical changes taking 
 place in pre-existing rocks, and indicate not infrequently the 
 first or deep-seated stages of rock decay. In a more or less 
 perfect condition they have been assumed to occur in soils, 
 having been derived from the rocks, or, as is contended by some 
 authorities, having formed during the process of rock decompo- 
 sition or in the soil itself. It is possible that those constituents 
 of a soil which on analysis are found to be " soluble " as the 
 term is ordinarily used, may, in part at least, have existed as 
 zeolites. Hence their consideration in this connection is of 
 importance. 
 
 Out of the 22 species of minerals classified as zeolites by 
 Dana in this " System of Mineralogy " there are but 11 which, 
 on account of their abundance or chemical composition, need 
 consideration here. The theoretical composition of these, as 
 indicated from a comparison of several to many analyses, is 
 shown in the accompanying table. In addition to the true 
 zeolites are included several other hydrous silicates closely 
 related, both as regards chemical composition and mode of 
 occurrence, and which, in our present discussion, cannot well 
 be excluded. 
 
32 
 
 THE MINERALS CONSTITUTING ROCKS 
 
 
 SILICA 
 
 (Si0 2 ) 
 
 ALUMINA 
 (A1 2 3 ) 
 
 LIME 
 (CaO) 
 
 BARIUM 
 (BaO) 
 
 POTASH 
 (K 2 0) 
 
 SODA 
 (Na 2 0) 
 
 WATER 
 (H 2 0) 
 
 Ptilolite . . . 
 Mordenite . 
 HsulanditG 
 
 70.0 
 67.2 
 
 59 2 
 
 11.9 
 11.4 
 
 16.8 
 
 4.4 
 
 2.1 
 9.2 
 
 .... 
 
 2.4 
 3.5 
 
 0.8 
 2.3 
 
 10.5 
 
 13.5 
 
 14 8 
 
 Phillipsite . . 
 Harmotome . 
 Stilbite . . . 
 LauinontitG . 
 
 48.8 
 47.1 
 57.4 
 51 1 
 
 20.7 
 16.0 
 16.3 
 21.7 
 
 7.6 
 
 7.7 
 11 9 
 
 20.6 
 
 6.4 
 2.1 
 
 1.4 
 
 16.5 
 14.1 
 47.2 
 15 3 
 
 Chabazite . . 
 Analcite . 
 
 47.2 
 54.5 
 
 20.0 
 23.2 
 
 5.5 
 
 .... 
 
 .... 
 
 6.1 
 14.1 
 
 21.2 
 8 2 
 
 Natrolite 
 
 47.4 
 
 26.8 
 
 
 
 
 16.3 
 
 9.5 
 
 Thomsonite . 
 Prehnite 
 
 36.9 
 43 7 
 
 31.4 
 24 8 
 
 11.5 
 
 27 1 
 
 .... 
 
 
 
 6.4 
 
 13.8 
 4 4 
 
 Apophyllite . . 
 
 53.7 
 
 
 25.0 
 
 .... 
 
 5.2 
 
 .... 
 
 16.1 
 
 
PLATE 2 
 
 FIG. 1. Quartz porphyry showing porphyritic structure. 
 FIG. 2. Quartz porphyry showing flow structure. 
 
IV. THE PHYSICAL AND CHEMICAL PROPER- 
 TIES OF ROCKS 
 
 1. STRUCTURE 
 
 In considering the structure of rocks it will facilitate mat- 
 ters to do so under two heads : (1) the macroscopic (or mega- 
 scopic) structures, or structures visible to the unaided eye 
 (macros, from Greek word fta%/9o?, signifying large); and 
 
 (2) microscope structures, or those visible only with the aid 
 of the microscope. 
 
 1. Macroscopic Structures. From a structural standpoint all 
 rocks may be classified sufficiently close for present purposes, 
 under the heads of : (1) Crystalline, (2) vitreous or glassy, 
 
 (3) colloidal, and (4) clastic or fragmental. Of the first of 
 these, ordinary granite or crystalline marbles are good types, 
 being made up wholly of crystal aggregates, without interstitial 
 amorphous or fragmental material. The term crystalline gran- 
 ular, or granular crystalline, is often applied to such as have a 
 distinctly granular structure, as do many of the granitic rocks. 
 Vitreous or glassy structures are found only among igneous 
 rocks, and are due always to a cooling of the molten magma 
 too rapidly for the production of crystals. Obviously, as the 
 rate of cooling in rock masses must be extremely variable, so 
 we find all intermediate stages between the completely glassy 
 and the crystalline forms. To these intermediate stages such 
 names as felsitic and microlitic are given, names the precise 
 meaning of which will be stated under the head of microscopic 
 structures. Rocks originating as chemical deposits, and which 
 have since undergone no structural changes, often present a 
 jelly or glue like structure known as colloidal. Such are exem- 
 plified in the flints from the English chalk cliffs, the siliceous 
 sinters from the Yellowstone National Park, and by various 
 other forms of silica, as opal, agate, etc., and occasionally by 
 serpentines. 
 
 n 33 
 
31 PHYSICAL AND CHEMICAL PROPERTIES OF ROCKS 
 
 A clastic or fragmental structure is found only in secondary 
 rocks, and is the result of a breaking down or disintegration of 
 pre-existing rocks, and a reconsolidation of their particles with- 
 out crystallization. There are many minor points of structure, 
 some of which are common to all of the primary groups above 
 mentioned, while others are limited to one or more. Rocks 
 which are made up of distinct grains, whether crystalline or 
 fragmental, are spoken of as granular ; when the structure be- 
 comes too tine and dense for macroscopic determination it is 
 spoken of as compact, though there is no reason why the term 
 should not equally well be applied to the coarser grained rocks 
 in which the individual grains are closely cohering without 
 interstices. The term massive is applied to such igneous rocks 
 as show no signs of bedding or stratification, while limestones, 
 sandstones, and such other rocks as are arranged in more or 
 less parallel layers are described as stratified. (See Fig. 1, 
 PI. 13.) The name foliated or schistose is given to a rock in 
 which the arrangement of the constituent minerals in parallel 
 planes is sufficiently marked to cause it to split in this direction 
 more readily than in any other. Not infrequently the quartzes 
 or feldspars occur in lens-shaped forms about which curve the 
 hornblende or mica folia as shown in Fig. 2, PI. 13. As ex- 
 plained elsewhere, this structure may be due to original deposi- 
 tion or may be secondary. In eruptive rocks a fluidal or fluxion 
 structure is not uncommon, as shown in Fig. 2, PI. 2, and is due 
 to the onward flowing of the mass while gradually cooling and 
 passing into a solid state. Eruptive magmas at the time of 
 their extrusion contain more or less moisture, which, being 
 highly heated, expands whenever sufficient force is developed 
 to overcome the pressure of the overlying mass. In this way 
 are formed innumerable cavities or bubbles, comparable to the 
 cavities caused by carbonic acid from the yeast in well-raised 
 bread. Such cavities are called vesicles, and the rocks contain- 
 ing them are vesicular (Fig. 2, PI. 3). By the subsequent 
 action of percolating waters these cavities may become filled 
 with a variety of secondary minerals, among which chalcedony, 
 epidote, calcite, and various zeolites are not uncommon. Such 
 refilled cavities are called amygdules, from the Greek word 
 a^v^a\ov, an almond, in allusion to their shape, and the rocks 
 containing them are therefore described as amygdaloidal. The 
 upper part of a lava flow not infrequently cools in peculiar ropy 
 
PLATE 3 
 
 FIG. 1. Basalt showing slaggy structure. FIG. 2. Basalt showing vesicular structure. 
 
MACROSCOPIC STRUCTURE 35 
 
 forms like the slag from a smelting furnace. Such forms are 
 known as slaggy. (See Fig. 1, PL 3.) 
 
 When a rock consists of a compact, glassy, or fine and evenly 
 crystalline ground-mass, throughout which are scattered larger 
 crystals, usually of feldspar, the structure is said to be porphy- 
 ritic (Fig. 1, PI. 2). This structure is quite common in granite, 
 but is not particularly noticeable, owing to the slight contrast in 
 color between the larger crystals and the finer ground-mass. It 
 is most noticeable in such effusive eruptives as the quartz por- 
 phyries, in which, as is the case with some of those of eastern 
 Massachusetts, the ground-mass is exceedingly dense and com- 
 pact and of a black or red color, while the large feldspar 
 crystals are white and stand out in very marked contrasts. 
 This structure is so striking in appearance that rocks possess- 
 ing it in any marked degree are popularly called porphyries, 
 whatever may be their mineral composition. The term por- 
 phyry is said to have been originally applied to certain kinds 
 of igneous rocks of a reddish or purple color, such as the 
 celebrated red porphyry or " roseo antico " of Egypt. The 
 word is now used by the best authorities almost wholly in its 
 adjective sense, since any rock may possess this structure 
 whatever its origin or composition may be. 
 
 Glassy rocks on cooling sometimes have developed in them 
 a series of concentric cracks whereby the rock on a broken sur- 
 face shows numerous rounded or globular bodies with an onion- 
 like shell. This structure, which may be visible only with a 
 microscope, is known as perlitic. It is not uncommon in glassy 
 forms of Hungarian trachytes. 
 
 Glassy and felsitic eruptives, particularly of the liparite and 
 quartz porphyry groups, frequently show spherulitic masses of 
 all sizes, from microscopic to several inches or even feet in 
 diameter, usually with a well-defined radiating structure and 
 which are due to incipient crystallization. Such are known as 
 spherulites, and hence rocks in which they occur are described 
 as spherulitic. 1 
 
 A concretionary structure is not infrequently developed in 
 rocks either as a primary structure or as due to segregating 
 processes acting subsequent to the formation of the rocks in 
 
 1 The structure and origin of these forms has been worked out in detail by 
 Whitman Cross. Bull. Philosophical Society of Washington, Vol. XI, 1891, 
 pp. 411-462. 
 
36 PHYSICAL AND CHEMICAL PROPERTIES OF ROCKS 
 
 which they are found. Many of the forms thus developed are 
 peculiarly deceptive, and it may not be out of place to enter 
 into a discussion of their nature and origin with some detail. 
 
 On genetic grounds we may divide such forms, as intimated 
 above, into two groups: (.A) Primary concretions, formed con- 
 temporaneously with the rocks in which they are found, and 
 (^) secondary concretions, or those which are due to segregat- 
 ing influences acting subsequent to the formation of the rocks 
 of which they now form a part. All are due to that peculiar 
 and little understood tendency which atoms or molecules of 
 like nature so often manifest in concreting or gathering in 
 amorphous masses or concentric layers about some foreign body 
 which serves as a primary point of attachment. The extreme 
 development of this tendency is seen in crystallization, of which 
 we may perhaps regard this first form of concretionary structure 
 as incipient stages. Under primary concretions may be included 
 the flint and chalcedonic nodules found in chalk and the older 
 limestones, the material of which was in part without doubt 
 derived from the siliceous remains of diatoms and sponges. 
 Such sometimes occur in the form of lenticular nodules with 
 or without an appreciable concentric structure and lying in 
 parallel layers or beds, sometimes continuous for long distances. 
 Clay iron stone, an impure carbonate of iron, occurs character- 
 istically in this form. These latter often crack on drying 
 and consequent shrinkage, the cracks extending from within 
 outward. In these cracks calcite is subsequently deposited, 
 whereby the nodule is divided up into septa of a white or 
 yellowish color. On being cut and polished, these often form 
 beautiful and unique objects. To such the name septarian 
 nodule is commonly given. (See Fig. 2, PI. 9.) The car- 
 bonate of lime in inland lakes and seas may not infrequently 
 become deposited in the form of thin pellicles about a minute, 
 perhaps microscopic nucleus, forming small, spherical bodies 
 which, when ultimately consolidated into beds, give rise to the 
 oolitic and pisolitic limestones. (See p. 143.) All primary 
 concretions are not, however, chemical deposits ; but, rather, 
 aggregates of mineral particles in a finely fragmental condition. 
 
 Such are the clay concretions which are found in the beds 
 of streams and lakes, and which may not so closely simu- 
 late animal forms as to be very misleading. The manner in 
 which concretions of this nature are formed was shown in a 
 
MACROSCOPIC STRUCTURE 37 
 
 very interesting manner a few years ago during the process of 
 the work of filling in the so-called Potomac flats, on the river 
 front at Washington, District of Columbia. For the double 
 purpose of raising the flats and deepening the channel, gigantic 
 pumps were employed which raised the sediment from the river 
 bottom in the form of a very thin mud and forced it through 
 iron pipes to the flats, where it flowed out, spreading quietly 
 over the surface. The material of this mud was mainly fine 
 siliceous sand and clay intermingled with occasional fresh- 
 water shells and plant debris. As this mud flowed quietly 
 from the mouth of the pipe and spread out over the surface, 
 the clayey particles began immediately to separate from the 
 siliceous sand in the form of concretionary balls, and in the 
 course of a very short time these would grow to be several 
 inches in diameter. Such, owing to the rapidity of their 
 formation, contained a large amount of sand and shells, though 
 clayey matter predominated. 
 
 In crystalline rocks concretionary structure is rarely devel- 
 oped. Cases such as shown on Plate 8 are quite unique, and 
 in the case of the orbicular diorite of the greatest interest on 
 account of the beauty of the stone and its adaptability for 
 small ornamentation. 
 
 Concretionary structure of a secondary nature may be de- 
 veloped through the process of weathering. Thus, by the 
 oxidizing action of meteoric waters percolating through a 
 porous sand or sandstone, included nodules of iron disulphide 
 (pyrite) may be converted into an oxide which gradually 
 segregates in zones about the original nodule. This oxide, 
 by its cementing action, binds the grains together in the form 
 of a hard crust, leaving the central portion, formerly filled by 
 pyrite, either empty or occupied by loose sand. 1 A zonal 
 banding or shelly structure closely simulating concretionary 
 structure is common in rocks more or less weathered and 
 decomposed, but which is due not to original deposition or 
 crystallization of mineral matter about a centre, but rather to 
 the weathering of jointed blocks, the various chemical agencies 
 acting from without inward. 
 
 A botryoidal structure is not infrequent among rocks and 
 minerals of chemical origin. It is, as a rule, confined to such 
 
 1 See On the Formation of Sandstone Concretions, Proceedings U. S. National 
 Museum, Vol. XVII, pp. 87, 88. 
 
38 PHYSICAL AND CHEMICAL PROPERTIES OF ROCKS 
 
 as are amorphous or radiating crystalline aggregates of a single 
 mineral, such as chalcedony or the hematite iron ores. (See 
 Fig. 1, PL 9.) 
 
 A brecciated structure, produced by the presence of angular 
 fragments in a finer ground, is of common occurrence among 
 fragmental rocks (the breccias), but is more rare among the 
 crystallines. It is sometimes produced in volcanic rocks by the 
 imbedding in the still pasty magma of angular fragments of 
 previously consolidated material, as shown in Fig. 2, PI. 4. 
 Columnar structure, though comparatively common as the 
 structure of a geological body, is rarely developed among the 
 constituents of the rock itself. The columnar structure of 
 many lavas and dike rocks has already been alluded to : oc- 
 casionally the mineral constituents of some secondary rocks 
 are arranged after this manner. A cavernous or cellular struct- 
 ure is not infrequently developed through the removal by 
 solution of some constituent or the weathering out of a fossil. 
 As an original structure it occurs in many rocks of chemical 
 origin as the stalagmitic deposits in caves, travertines, etc. 
 
 A laminated or banded structure, due to the arrangement of 
 the constituents in parallel layers or bands, is common in rocks 
 of sedimentary origin, particularly in sandstones and shales. 
 
 2. Microscopic Structures. Many, if not indeed the majority, 
 of rocks are so fine grained and compact that little of their 
 mineral nature or structural features can be learned from exami- 
 nation by the unaided eye. This difficulty made itself apparent 
 very early in the history of geological science, and to it is per- 
 haps due, more than to any other single cause, the apparent 
 crudities and fallacies of the early workers. As long ago as 
 1663, the microscope had been to some extent utilized for the 
 examination of minerals ; but its application to the study of 
 rocks remained long unrecognized, though early in the present 
 century Cordier and others utilized it in the study of rocks in 
 a pulverized condition. It was not until about 1850, when the 
 subject was taken up by H. Clifton Sorby of England, that the 
 possibility of studying rocks in thin sections under the micro- 
 scope began to be appreciated. Even then the idea failed to 
 bear its legitimate fruits until transplanted to German soils, 
 where, under the fostering care of Professor Zirkel of Leipzig, 
 it soon began to yield an abundant harvest ; and to-day the 
 branch of the science of geology known as microscopical pe- 
 
PLATE 4 
 
MICROSCOPIC STRUCTURE 39 
 
 trography holds a prominent place in all the leading universi- 
 ties, both domestic and foreign. The efficiency of the method 
 is based upon the fact that every crystallized mineral has cer- 
 tain definite optical properties ; i.e. when cut in such a way as 
 to allow the light to pass through it, will act upon this light in 
 a manner sufficiently characteristic to enable one working with 
 an instrument combining the properties of a microscope and 
 stauroscope to ascertain at least to what crystalline, system it 
 belongs, and in most cases by studying also the crystal outlines 
 and lines of cleavage the mineral species as well. To enter 
 upon a detailed description of the method by which this is done 
 would be out of place here, since it involves the polarization of 
 light and other subjects which must be studied elsewhere. The 
 reader is referred to any authoritative work on the subject of 
 light, and to Professor J. P. Idding's translation of Professor 
 Rosenbusch's work on optical mineralogy. 1 
 
 This method of study is of value, not merely as an aid in 
 determining the mineralogical composition of a rock, but also, 
 and what is often of more importance, its structure and the 
 various changes which have taken place in it since its first 
 consolidation. Rocks are not the definite and unchangeable 
 mineral compounds they were once considered, but are rather 
 ever- varying aggregates of minerals, which, even in themselves 
 undergo structural and chemical changes almost without num- 
 ber. It is a common matter to find rock masses which may 
 have had originally the mineral composition and structure of 
 diabase, but which now are mere aggregates of secondary prod- 
 ucts, such as chlorite, epidote, iron oxides, and kaolin, with 
 perhaps scarcely a trace of the unaltered original constituents ; 
 yet the rock mass retains its geological identity, and to the 
 naked eye shows little, if any, sign of the changes that have 
 gone on. These and other changes are in part chemical and in 
 part structural or molecular. A very common mineral trans- 
 formation in basic rocks is that from augite to hornblende. 
 This takes place merely through a molecular readjustment of 
 the particles, whereby the augite, with its gray or brown colors 
 and rectangular cleavages, passes by uralitic stages over into a 
 green hornblende, a mineral of the same chemical composition, 
 but of different crystallographic form. This transformation in 
 
 1 Microscopic Physiography of Rock-making Minerals, Wiley & Son, New 
 York. See also Professor A. Barkers' Petrology for Students. 
 
40 PHYSICAL AND CHEMICAL PROPERTIES OF ROCKS 
 
 its incompleted state is shown in the accompanying figure, in 
 which the central, nearly colorless portion with rectangular 
 cleavage represents the original augite, while the outer dotted 
 
 portion with cleavage lines cutting at 
 sharp and obtuse angles is the second- 
 ary hornblende. This change is due 
 to slow and gradual pressure exerted 
 through unknown periods of time upon 
 the rock masses, and the final result is 
 the production of a rock of entirely 
 different type and structure from that 
 which originally cooled from the molt- 
 en magma. The change such as above 
 FIG. i. Augite partially described is further alluded to in the 
 
 chapter on metamorphism. 
 
 This science of microscopic petrography, as it is technically 
 called, has also been productive of equally important results in 
 other lines. As an instance of this may be mentioned the dis- 
 covery that the structural features of a rock are dependent, not 
 upon its chemical composition or geological age, but upon the 
 conditions under which it cooled from a molten magma, portions 
 of the same rock varying all the way from holocrystalline 
 granular through porphyritic to glassy forms. To this fact 
 allusion has already been made. 
 
 The general subject of the microscopic structure of rocks of 
 various kinds, will be discussed more fully in describing the 
 rocks themselves. Nevertheless, as in describing these struct- 
 ures it has become necessary to use sundry technical terms, it 
 will be well to refer to them briefly here. 
 
 When a rock is made up wholly of crystalline matter, it is 
 spoken of as holocrystalline ; when, however, it shows interstitial 
 glassy or felsitic matter, it is hypocrystalline. Rocks wholly 
 without crystalline secretions are amorphous. The glassy, or 
 felsitic matter occupying the interstices of the other constitu- 
 ents is spoken of as the base. This base, together with the 
 microlites and smaller crystallizations of the second generation, 
 is called the ground-mass; such may be made up of microlites 
 small needle-like crystals imperfectly developed when it is 
 called microlitic, or of a dense aggregate of quartzose, felds- 
 pathic and other materials, when it is known as felsitic. The 
 larger crystals developed in a glassy, felsitic, microlitic, or finely 
 
PLATE 5 
 
 FIG. 1. Microstructure of granite. 
 
 FIG. 2. Microstructure of micropegmatite. 
 
 FIG. 3. Microstructure of quartz porphyry. 
 
 FIG. 4. Microstructure of porphyritic obsidian. 
 FIG. 5. Microstructure of trachyte. 
 FIG. 6. Microstructure of serpentine. 
 
MICROSCOPIC STRUCTURE 41 
 
 granular micro crystalline ground-mass are called phenocrysts. 
 When a mineral in a rock shows good crystal outlines, having 
 been uninfluenced in its growth by the proximity of other 
 minerals, it is called idiomorphic : when, however, its outline is 
 due not to cryst/allographic forces, but to interference to the 
 action of external forces it is allotriomorphic. Many rocks 
 show indications of two or more periods of crystallization, 
 whereby minerals of the same species may be developed. Thus 
 in a molten magma the augites may begin to form under such 
 conditions that for some time their growth is unimpeded and 
 they take on large and well-developed forms. After a time, 
 owing to changed conditions, their growth is stopped, and the 
 rock solidifies with a new crop of smaller and less perfectly 
 developed forms. It is customary to speak of such a mineral 
 as occurring in crystals of two generations. In the case above 
 described, the first developed form the porphyritic constitu- 
 ents, the phenocrysts, while the latter formed are a part of 
 the ground-mass. Vitreous or glassy rocks not infrequently 
 show, under the microscope, minute, hair-like or rod-shaped 
 forms, representing the first stages of crystallization, but in 
 which the process was arrested before they were sufficiently 
 developed to render possible an accurate determination of 
 their mineral nature. Such are termed crystallites; those in 
 drop-shaped or globular forms being called globulites, the 
 rod-shaped ones belonites, and the twisted, hair-like forms 
 trie kite s. 
 
 Tile wide variation in microstructure in rocks of essentially 
 the same chemical composition, but which have cooled under 
 the varying conditions indicated above, is shown in Figs. 1 to 
 4 of PI. 5, Fig. 1 being a holocrystalline type, and Fig. 4 one 
 almost completely glassy, the first being a deep-seated rock, and 
 the last a surface lava flow. Intermediate structures are often 
 produced through a beginning of crystallization at certain 
 depths below the surface, after which, and while a portion of 
 the magma was still fluid, it was pushed upward toward the 
 surface, or brought under such other conditions as resulted in 
 a more rapid cooling, the final result being a glassy, or micro- 
 crystalline rock with scattering porphyritic crystals, or pheno- 
 crysts. It has not infrequently happened that, subsequent to 
 the formation of these earliest products of crystallization, a 
 second elevation of temperatures has taken place whereby the 
 
42 PHYSICAL AND CHEMICAL PROPERTIES OF ROCKS 
 
 magma has eaten into or corroded them, as is the case with 
 the quartz crystal shown in the centre of Fig. 3 of PI. 5. 
 
 Inasmuch as this study by the microscope involves the prepa- 
 ration of thin sections, a brief description of the methods pur- 
 sued may well be given here. The fact that a chip of rock, 
 however dense, can, without breaking, be ground so thin as 
 to be transparent, may at first seem strange, but in reality it 
 is readily accomplished. The work requires only patience and 
 the skill which comes from practice. A small chip of the rock, 
 about the size of a nickel five-cent piece, is broken off with a 
 hammer, care being taken to get it as thin as possible without 
 fracturing. One side of this is then ground flat and smooth by 
 rubbing it in water and emery on a smooth, cast-iron plate. 
 Toward the close of the process fine flour of emery should be 
 used, as the final surface must be very smooth and free from 
 scratches. This chip is then cemented smooth side down on 
 a piece of ordinary double-thick window glass, a convenient 
 size being about 2x1 inches, the cementing material being 
 Canada balsam which has been evaporated to the extent that, 
 when cold, it is sufficiently hard to hold firmly, is not at all 
 sticky, but yet is not so hard as to be brittle. The exact degree 
 can only be learned by experience ; a hardness such as to be 
 barely indented by the thumb nail will be found about right. 
 This operation of cementing will be best done by means of a 
 thin iron plate laid horizontally on a support and heated not too 
 hot by a lamp beneath. The glass with the balsam upon it is 
 heated to the right temperature, the balsam being fluid and free 
 from bubbles. The rock chip, heated sufficiently to expel all 
 moisture, is then pressed firmly into the balsam, in such a way 
 as to exclude air bubbles, and brought within as close contact 
 with the glass as possible. It is then removed from the iron, 
 plate and allowed to cool, when the grinding process is resumed, 
 the glass plate serving merely as support for the film of stone 
 and something for the fingers to hold by. Being transparent, 
 the worker can see just how the grinding is progressing without 
 continually stopping to examine. When sufficiently thin, 
 usually from -^-^ to g^- of an inch, the film is remounted as 
 follows : While on the thick glass on which it was ground, it 
 is thoroughly washed with a brush an ordinary tooth-brush 
 serves well to get rid of all particles of emery and other dirt 
 that may adhere. It is then washed in alcohol to get rid of the 
 
THE SPECIFIC GRAVITY OF ROCKS 
 
 43 
 
 old hard balsam, which is usually quite dirty from mud pro- 
 duced in grinding. Fresh mounting slips and clean cover 
 glasses being ready, the first is laid upon the warm iron plate 
 with a couple of drops of fresh balsam in the centre, and allowed 
 to heat until it just begins to smoke. Care must here be exer- 
 cised, as, if heated too much, the balsam becomes hard and 
 brittle, and if too little, the mount is sticky from the balsam 
 which constantly oozes from under the cover. The thick glass, 
 with its film of stone still adhering, is likewise laid upon the 
 warm iron plate, and a drop of fresh balsam placed upon the film. 
 This is then gently heated, and the cover-glass, first warmed, 
 gently laid upon it one edge placed in position and loAvered 
 gradually in such a manner as to force out any accidental air 
 bubbles, being finally pressed flat down against the stone film. 
 The film itself, if sufficiently warmed, no longer adheres to the 
 thick glass, and may be removed to the ^ 
 clean slip for its final mounting. This is 
 best accomplished by taking up the thick 
 glass by means of a pair of forceps and 
 pushing cover-glass and film together, with 
 a needle point set in a handle, off into the 
 balsam on the new slide. The cover-glass 
 here serves merely as a support for the thin 
 film during the process of transferring. 
 Without it there is danger of breakage. 
 When fairly transferred, the new slide is 
 removed from the hot plate, the cover 
 pressed close down against the film, ad- 
 justed in proper position and allowed to cool. FlG - 2 - Mounted thin 
 
 rrn nil ^ ii Section of rock. 
 
 Ine superfluous balsam may be then re- 
 moved with a hot knife and the section finally washed in alcohol. 
 Thus completed, it forms the " thin section " of the petrologist. 
 
 2. THE SPECIFIC GRAVITY OF BOCKS 
 
 The term specific gravity is used to designate the weight of 
 any substance when compared with an equal volume of distilled 
 water at a temperature of 4 C. This property is therefore 
 dependent upon the specific gravity of its various constituents 
 and their relative proportions. The exact or true specific 
 gravity of a rock may be obscured by its structure. Thus an 
 
44 PHYSICAL AND CHEMICAL PROPERTIES OF ROCKS 
 
 obsidian pumice will float upon water, buoyed up by the air 
 contained in its innumerable vesicles, while a compact obsidian 
 of precisely the same chemical composition will sink almost 
 instantly. This property of any subject is spoken of as its 
 apparent specific gravity in distinction from the actual com- 
 parative weight, bulk for bulk, of its constituent parts, which 
 could in the case of a pumice be obtained only by finely pul- 
 verizing so as to admit the water into all its pores. Inasmuch 
 as the structural peculiarities of any igneous rock as will be 
 noted later are dependent upon the condition under which it 
 cooled, it is instructive to notice that a crystalline aggregate 
 has a higher specific gravity, i.e. a greater weight, bulk for 
 bulk, than does a glassy, non-crystalline rock of the same chemi- 
 cal composition. The property is therefore dependent upon 
 chemical (and consequently mineral) composition and struct- 
 ure, and as a very general rule it may be said that among the 
 siliceous rocks those which contain the largest amount of silica 
 are the lightest, while those with a comparatively small amount, 
 but which are correspondingly rich in iron, lime, and magnesian 
 constituents, are proportionately heavy. 
 
 3. THE CHEMICAL COMPOSITION OF ROCKS 
 
 This varies naturally with their mineral composition. It is 
 customary to speak of sedimentary rocks as calcareous, sili- 
 ceous, ferruginous, or argillaceous, accordingly as lime, silica, 
 iron oxides, or clayey matter are prominent constituents. 
 Among eruptive rocks it is customary to speak of those show- 
 ing, on analysis, upwards of 60 % silica as acidic, and those 
 showing less than 50 %, but rich in iron, lime, and magnesian 
 constituents, as basic. The extremes, as will be noted, are rep- 
 resented by the rocks of the granite and peridotite groups. 
 
 A series illustrating the above-mentioned properties may be 
 arranged as below. With the eruptive rocks only the silica 
 percentages are here given. The results of the complete chemi- 
 cal analysis of each variety are given further on, in the pages 
 devoted to their description. 
 
THE CHEMICAL COMPOSITION OF ROCKS 
 
 45 
 
 (1) STRATIFIED ROCKS 
 
 KIND 
 
 SPECIFIC GRAVITY 
 
 COMPOSITION 
 
 Calcareous : 
 Compact limestone . . . 
 Crystalline limestone . . 
 Compact dolomite . . . 
 Crystalline dolomite . . 
 Siliceous : 
 Gneiss 
 
 1 2.6 to 2.8 
 1 2.8 to 2.95 
 
 2 6 to 2 7 
 
 Carbonate of lime. 
 Carbonate of lime and magnesia. 
 
 Same as granite 
 
 Siliceous sandstone . . . 
 Schist 
 
 2.6 
 2 6 to 2 8 
 
 Mainly silica. 
 60 to 80 per cent silica 
 
 Argillaceous : 
 Clay slate (argillite) . . 
 
 2.5 
 
 Mainly silicate of aluminum. 
 
 (2) ERUPTIVE ROCKS 
 
 KIND 
 
 SPECIFIC GRAVITY 
 
 PER CENT SILICA 
 
 Acidic group : 
 Granite 
 
 2 58 to 2 73 
 
 77 65 to 62 90 
 
 Liparite 
 
 2 53 to 2 70 
 
 76 06 to 67 61 
 
 Obsidian 
 
 2.26 to 2 41 
 
 82 80 to 71 19 
 
 Obsidian pumice 
 
 Floats on water. 
 
 82.80 to 71.19 
 
 Intermediate group : 
 Syenite 
 
 2.73 to 2.86 
 
 72.20 to 54.65 
 
 Trachyte 
 
 2.70 to 2.80 
 
 64.00 to 60.00 
 
 Hyalotrachyte 
 Andesite 
 
 2.40 to 2.50 
 2 54 to 2 79 
 
 64.00 to 60.00 
 66.75 to 54.73 
 
 Basic group : 
 Diabase 
 
 2 66 to 2.88 
 
 50.00 to 48.00 
 
 Basalt . . 
 
 2.90 to 3.10 
 
 50.59 to 40.74 
 
 Peridotite 
 
 3.22 to 3.29 
 
 42.65 to 33.73 
 
 Peridotite (iron rich) 
 Peridotite (meteorite) 
 
 3.86 
 3 51 
 
 23.00 
 37.70 
 
 
 
 
 4. THE COLOR OF ROCKS 
 
 The color of a rock is dependent upon a variety of circum- 
 stances, but which may all be generalized under the heads of 
 mineral and chemical composition and physical condition. Iron 
 and carbon, in some of their forms, are the common coloring 
 
46 PHYSICAL AND CHEMICAL PROPERTIES OF ROCKS 
 
 substances and the only ones that need be considered here. 
 The yellow, brown, and red colors, common to fragmental rocks, 
 are due almost wholly to free oxides of iron. The gray, green, 
 dull brown, and even black colors of crystalline rocks are due 
 to the presence of free iron oxides or to the prevalence of sili- 
 cate minerals rich in iron, as augite, hornblende, or black mica. 
 Rarely copper, manganese, and other metallic oxides than those 
 of iron are present in sufficient abundance to impart their char- 
 acteristic hues. As a rule, a white or light gray color denotes 
 an absence of an appreciable amount of iron in any of its forms. 
 The amber, bluish and black colors of many rocks, particularly 
 the limestones and slates, are due to the prevalence of carbona- 
 ceous matter. 
 
 Among siliceous crystalline rocks the more basic, like those 
 of the diabase, diorite, or basalt groups, are as a rule of a darker 
 color than the acid varieties, the color being due to the fine 
 grain and predominance of dark iron-magnesian silicates, such 
 as hornblende, augite, or black mica, or their chloritic alteration 
 products. The red or pink color sometimes occurring in gran- 
 itic rocks is due to the predominance of red or pink feldspars, 
 which in their turn owe their color to the presence of iron. 
 
 Among feldspar-bearing rocks the color is not infrequently 
 due to the physical condition of this important constituent. 
 Thus in many rocks like the norite of Keeseville (New York), 
 and the Quincy (Massachusetts) granite, the dark color is 
 largely due to the fact that the feldspar is clear and glassy, 
 allowing the light rays to penetrate and become absorbed. The 
 beautiful chatoyant play of colors sometimes shown by labra- 
 dorite-bearing rocks like those of northern New York and of 
 Norway is apparently due to a separation of the individual 
 crystals along cleavage lines, into thin, transparent plates which 
 reflect and partially polarize the light which would otherwise 
 penetrate and become absorbed. Through weathering, such 
 feldspars undergo a further physical change, becoming soft 
 and porous, and no longer allowing the light to penetrate, but 
 wholly reflecting it arid causing the stone to appear white. 
 These white feldspars, as has been very neatly expressed by the 
 late Dr. Hawes, bear the same relation to the glassy forms 
 as does the foam of the sea to the water itself, the difference 
 in color being in both cases due to the changed physical con- 
 dition. Indeed, the color of rocks, as may be imagined, is 
 
THE COLOR OF ROCKS 47 
 
 not constant, but liable to change under varying conditions, 
 particularly those of exposure. Rocks black with carbonaceous 
 matter will fade to almost whiteness on prolonged exposure, 
 owing to the bleaching out of the coloring materials. Rocks 
 rich in magnetite or free iron oxides, protoxide carbonates, or 
 sulphides, or in highly ferruginous silicate minerals, are like- 
 wise liable to a change of color, becoming yellowish, red, or 
 brown, through oxidation of the ferruginous constituents. (See 
 p. 257.) Translucent, nearly colorless rocks or minerals, as 
 those made up of crystals of calcite or selenite, will on exposure 
 become nearly opaque and snow-white, owing to purely physi- 
 cal causes, as already noted in the case of the feldspars. (See 
 further in chapter on weathering.) 
 
 The cause of the color variations in certain rocks and min- 
 erals is, however, a matter concerning which it will not do, as 
 yet, to speak too decidedly. Analysis of a mineral may show 
 the presence of metallic oxides, but it does not necessarily fol- 
 low that whatever color the mineral may have is due or in any 
 way related to these oxides. Thus the writer has shown 1 that 
 the onyx marbles (travertines) of Arizona and Mexico may 
 vary from pure white to green, and from yellow through brown 
 to red, without appreciable change in the actual amounts of 
 iron, though there may be a change in the form of combination. 
 In the white and green varieties the iron exists as a carbonate ; 
 in the yellow, red, and brown varieties as a more or less hydrated 
 sesquioxide. Certain dark amber and bright rose-colored va- 
 rieties from California, and the Californian Peninsula, show, 
 however, 110 iron or other of the usual metallic coloring con- 
 stituents, but burn perfectly white when submitted to high 
 temperatures and yield volatile organic compounds. The fact 
 that serpentines so frequently contain small traces of chromium, 
 early gave rise to the opinion that it was to this element that 
 was due the characteristic green color of the mineral. The 
 writer has elsewhere 2 described serpentines of a beautiful oil 
 yellow and deep green color which, however, contain not a 
 trace of chromium or manganese, but only iron, which in this 
 case is in combination as a silicate. (See p. 114.) 
 
 These color characteristics are of greater importance than 
 
 1 Annual Report U. S. National Museum, 1893, p. 558. 
 
 2 On the Serpentine of Montville, New Jersey, Proc. U. S. National Museum, 
 1888, p. 105. 
 
48 PHYSICAL AND CHEMICAL PROPERTIES OF ROCKS 
 
 may at first appear, particularly from an economic standpoint. 
 One of the first essentials in a rock designed for architectural 
 use should be permanency of color. Deleterious changes are 
 particularly liable to occur in stone taken from below the water 
 level, where, protected from oxidation, or from variations in 
 temperature. Certain of the Ohio sandstones are of a blue- 
 gray color below the water level, but buff above, where the 
 included iron sulphides and protoxide carbonates have been 
 acted upon by oxidation. The student should early make 
 himself acquainted with these characteristics, as in the field it 
 is as a rule only the more or less weathered surfaces that pre- 
 sent themselves for inspection. This subject is again referred 
 to in the chapter on rock weathering. 
 
 Lustre as a property of rocks does not, owing to their com- 
 plex nature, possess the same value as a determinative charac- 
 teristic as among minerals. Certain of the more compact and 
 homogeneous varieties possess lustres which may be described 
 as vitreous, greasy, pearly, metallic, or iridescent. 
 
 The meaning of such terms is sufficiently evident, and the 
 subject need not be further dwelt upon here. The fracture, 
 or manner of breaking of any rock, is dependent more upon 
 structure than upon chemical or mineralogical composition. 
 Many fine and evenly grained crystalline or fragmental rocks 
 break with smooth, even surfaces, and are described as having 
 a straight or even fracture. Others break with shell-like con- 
 cave and convex surfaces, and are said to have a conchoidal 
 fracture. Still others are splintery, hackly, or shaly, words the 
 meaning of which is sufficiently evident without their being 
 described in detail. 
 
V. THE MODE OF OCCURRENCE OF ROCKS 
 
 It is ordinarily assumed that the earth owes its present form 
 to its having originated from a mass of incandescent vapor, and 
 to have passed, by gradual cooling and consequent condensa- 
 tion, from gaseous through pasty or fluidal, and all intermediate 
 stages, to its present condition. This, in brief, is the hypothesis 
 of Kant, and which seems most readily to account for the facts 
 as we now know them. As to the character of the rock masses 
 resulting from this primary cooling, we know but little. Rea- 
 soning from analogy, it seems safe to assume that they resem- 
 bled the slags from a smelting furnace, or some form of modern 
 lavas, more nearly than any other rock masses of which we 
 have knowledge. Whatever may have been their nature, they 
 have long since been obscured by rocks of secondary origin, 
 or become so altered through dynamic and incidental chemical 
 agencies as to be no longer recognizable. 
 
 The oldest rocks of which we now have knowledge belong 
 to the group of gneisses and crystalline schists. They are as 
 a rule highly siliceous rocks, though not infrequently includ- 
 ing considerable thicknesses of crystalline limestone. They 
 contain no traces of what can be referred beyond doubt to an 
 organic origin, though from their banded or foliated structure, 
 so closely simulating bedding, they have in the past, as a rule, 
 been considered metamorphic rocks ; that is to say, rocks laid 
 down as sediments and crystallized by the complex processes 
 comprehended under the term metamorphism. Rocks of this 
 type, according to Dana, first appeared in North America in 
 the wide V-shaped area extending from Labrador southwesterly 
 to the Great Lake, and thence northwesterly, to the Arctic 
 regions. This area has since been added to by the folding and 
 crumpling processes incident to the formation of the Appa- 
 lachian and Rocky Mountain systems. Concerning the geo- 
 graphical distribution of these rocks, as they now appear 
 exposed, we have little to say here. They seem to form, as 
 E 49 
 
50 THE MODE OF OCCURRENCE OF ROCKS 
 
 has been stated, the actual floor of the continents upon which 
 all later deposits have been laid down, and through which and 
 into which have been extruded and intruded the great variety 
 of igneous rocks which form so conspicuous a feature in many 
 a mountainous region. In order to properly understand that 
 which is to follow, we may well devote a little space here to a 
 consideration of the manner in which these rock masses occur, 
 so far as exposed to investigation. 
 
 Several varieties of igneous rocks, and particularly the gra- 
 nitic types, occur not infrequently in the form of immense oval 
 or rounded masses, protruded into overlying materials which 
 dip away on all sides ; such forms are ordinarily designated 
 as bosses. (PL 1.) It is a form common to granite, gab- 
 bros, norites, etc. A laccolite 1 is a somewhat similar form 
 due to the welling up of a magma through a comparatively 
 small vent, but which, instead of coming to the surface, spread 
 out laterally into dome-shaped masses between the sheets of the 
 overlying strata. When the intruded matter has been so 
 forced into or between overlying bedded rocks as to appear 
 like more or less regularly defined beds, they are known as 
 sheets or sills. Such, as a rule, may be distinguished from 
 superficial lava flows by their like condition of compactness 
 along both upper and lower contacts, surface streams being 
 more or less vesicular along the upper portions, owing to the 
 expansion of their included moisture. The name dike is given 
 to an eruptive mass of varying width included between well- 
 defined walls, and occupying a fissure or fault in previously 
 consolidated rocks. Such are inclined at all angles with the 
 horizon, and are usually of very moderate width, but may ex- 
 tend for miles. The dikes in any one region will frequently 
 be found to belong to one or more well-defined systems, each 
 system occupying fissures essentially parallel with one another. 
 Any one dike may remain comparatively uniform in width for 
 long distances, excepting when split up into smaller dikes. 
 At times, dikes may be traced to the parent mass a boss or 
 laccolite from which they radiate with more or less regu- 
 larity, being in such cases widest at the start, and gradually 
 
 1 It is to be regretted that this name in its present form has been so generally 
 adopted by geologists, since its termination, ite, should indicate a kind of rock, 
 whereas, in fact, it but denotes a form of occurrence. Laccolith would be 
 preferable. 
 
IGNEOUS ROCKS 51 
 
 thinning out to, it may be, mere knife-like edges. The name 
 volcanic neck or plug is given to the cylindrical mass which 
 results from the congealing of that portion of the lava which 
 remains in the volcanic vent when eruption ceases. Through 
 the erosion of the matter composing the cone of a volcano, 
 such are sometimes left, owing to their superior hardness, form- 
 ing thus a very striking feature of the landscape. The gen- 
 eral name lava is applied to any igneous rock, regardless of 
 geological age or mineral composition, which has been poured 
 out on the surface of the earth in a molten condition. Such 
 are characterized, as a rule, by less perfect crystallization and a 
 more slaggy and vesicular structure than the deep-seated rocks. 
 A columnar jointing, due to cooling, is by no means uncom- 
 mon, particularly among basaltic lavas, although it is by no 
 means confined to them. 
 
 But a comparatively small proportion of the rocks composing 
 the superficial portions of the earth's crust the portions with 
 which we are more or less familiar are eruptive. They are 
 rather what are known as secondary rocks ; that is to say, they 
 are rocks made over from these so-called primary rocks, which 
 we have been just discussing, by processes which will be described 
 later. 
 
 Any rock mass, be it eruptive or otherwise, lying exposed at 
 or near the surface of the ground finds itself subjected to a 
 multitude of disintegrating and decomposing agencies, such as 
 will be described more in detail under the head of rock weather- 
 ing. Leached and decomposed by meteoric waters, disintegrated 
 by heat and frost, or the mechanical action of waves and cur- 
 rents, the rock masses slowly succumb, their materials being 
 gradually removed in solution, or as debris mechanically trans- 
 ported by every wind, rain, or running stream, down the slopes 
 into the valleys, and from the valleys into the seas. This 
 debris, in various stages of coarseness and fineness, and to 
 which we give the name of bowlders, gravel, sand, and silt, 
 undergoes by these transporting agencies a system of assorting 
 more or less complete, and is carried to distances dependent 
 upon its weight and the force of the transporting agent. It 
 requires no geological or other special training to enable one to 
 understand that the force being the same, the finer and lighter 
 materials will be carried the farthest, and that all must be de- 
 posited when the force shall be expended. Consider, then, for 
 
52 THE MODE OF OCCURRENCE OF ROCKS 
 
 purpose of illustration, a stream flowing out from a mountainous 
 region and emptying itself into a lake. Materials falling by 
 gravity from the mountain slopes, or washed by spasmodic rains 
 into the stream, are transported certain distances, according to 
 the strength of the current. For our present purposes, it is 
 sufficient to consider only those portions which are transported 
 quite to the mouth of the stream and dumped into the lake. 
 But as the water leaves its narrow channel and spreads out into 
 the lake, there is an almost instant diminution of the force of 
 its current, and consequent carrying power. As a result, it 
 begins to deposit its load, the coarsest and heaviest first, and 
 the finer materials further out from the shore, the very finest, 
 an impalpable silt it may be, remaining suspended until the 
 very last. There will thus be formed on the bottom of the lake 
 or sea, whichever it may be, a bed, or series of beds of varying 
 thickness, of gravel, sand, and clay, the coarsest at the bottom 
 and nearest the shore, and the finest and last the most remote. 
 But the streams emptying into the lake vary from time to 
 time in their carrying capacity, and the action of the waves in 
 the sea itself, together with the salts dissolved therein, exert a 
 modifying action, whereby this process of sedimentation, as it is 
 called, may not be quite so simple as it first appears. 1 Enough 
 has, however, been said to show that beds of detritus laid down 
 in this manner must occur in approximately horizontal layers, 
 and that the layers may vary greatly in the coarseness and 
 fineness of their materials, as well as in their mineral character. 
 But there are still other processes of sedimentation than the 
 purely mechanical methods described above. All natural waters 
 contain more or less mineral matter, of which lime is the more 
 abundant. Through the secreting power of marine animals, this 
 lime is taken up in the form of a carbonate to form shells and 
 calcareous skeletons of molluscs, corals, and other forms of 
 marine life. On the death of the secreting animal, the calca- 
 reous material is left to accumulate in a more or less fragmen- 
 tal condition, forming thus the material of the coral islands, 
 and to a considerable extent the beds of limestone the world 
 over. I have said to a considerable extent, for the reason that 
 it is doubtful if many of our limestones are of purely animal 
 origin ; in many a true chemical precipitation plays a not unim- 
 
 1 See Conditions of Sedimentary Deposition, by Bailey Willis, Journal of 
 Geology, 1893, p. 476. 
 
BEDDED OR STRATIFIED ROCKS 53 
 
 portant part. This is especially true of the oolitic varieties, 
 and the fact is readily apparent when we come to study such 
 in detail. Consider a shallow sea-bottom on which are gradu- 
 ally accumulating in a finely divided condition the fragmental 
 remains of calcareous organisms of any kind. By the undu- 
 latory action of the waves these are kept in almost constant 
 motion, though it may be but gently rolling from 'side to side. 
 Owing to evaporation, or a too rapid accumulation of the lime 
 for it to be abstracted by the lime-secreting animals, the water 
 becomes supercharged with this constituent, which is then pre- 
 cipitated in the form of a thin pellicle around the most availa- 
 ble nucleus, in this case the grains of calcareous sand upon the 
 bottom. Thus are gradually built up beds of no inconsiderable 
 thickness, such as the well-known Carboniferous oolitic lime- 
 stones of Indiana and Kentucky. The microscopic structure of 
 stones of this class is shown in Fig. 7 on p. 112. Rocks which 
 are laid down in the manner we have just described, whether 
 composed of inorganic particles or fragmental materials from 
 marine and fresh water organisms, are designated as sediment- 
 ary. They occur in more or less well-defined beds or strata, 
 and hence are spoken of as bedded or stratified. Owing to the 
 fact that they have in most cases been deposited in compara- 
 tively shallow water, they retain not infrequently the superficial 
 markings made upon them by waves and other agencies prior to 
 their final consolidation. 
 
 Deposits laid down as above described naturally lie approxi- 
 mately horizontally where not subsequently disturbed by earth 
 movements. The earth's crust, however, is by no means in a 
 state of stable equilibrium, but, being subjected to continuous 
 stress or compressive force, is often broken, crushed, or folded, 
 and crumpled to an extraordinary degree. The name fault is 
 applied to the profound fractures made by these movements, 
 and which, inclined at various angles to the horizon, may extend 
 for miles. Usually the rocks on one side of a fault will be 
 found to have sunk down, while those of the other remain sta- 
 tionary or are raised, producing thus an inequality of surface 
 that may assume mountainous proportions. Most mountain 
 ranges, in fact, are due to a combination of faulting and fold- 
 ing processes. It not infrequently happens that the masses 
 of rock, sliding over one another along a line of fault, produce 
 smooth or striated and often highly polished surfaces, to which 
 
54 THE MODE OF OCCURRENCE OF ROCKS 
 
 the name slickensides is given. Such are particularly noticeable 
 among serpeiitinous rocks, being apparently due to motion gen- 
 erated in the mass by increase in bulk incident to its conver- 
 sion into serpentine. 1 The name vein is given to rock masses 
 of chemical origin, deposited along previously existing fractures 
 which may or may not be true faults. By some authorities 
 the name is also made to include the smaller injections of igne- 
 ous rocks. Such are here classed under the head of dikes, 
 though it must be understood that it is not in all cases pos- 
 sible to state to which of the two classes an occurrence is 
 to be referred. It is customary to divide the veins into two 
 classes : (1) the mineral veins, in which the materials have 
 been deposited from aqueous solution or sublimation between 
 the walls of a fissure ; and (2) segregation veins, in which the 
 component materials have crystallized or segregated out of the 
 still unconsolidated, pasty, or colloidal rock. It is not in all 
 cases possible to decide to which of the two classes a vein may 
 belong, but as a rule the mineral (or fissure) veins are separated 
 by sharp and well-defined walls from the country rock, and 
 show a comb or banded structure. The segregation type is less 
 distinctly marked, the vein material being welded to the enclos- 
 ing rock, or seemingly passing into it by gentle gradations. 
 
 The unconsolidated materials, as sands and gravels, occur 
 not only in regularly bedded or stratified forms, but also in 
 hillocks and ridges to which special terms are applied. The 
 loose material washed down the mountain slopes by ephemeral 
 streams, and deposited at the mouth of gorges, not infrequently 
 assumes the form of " a conical mass of low slope descending 
 equally in all directions from the point of issue." To such 
 forms Gilbert has given the name of alluvial cones. The mate- 
 rial of these cones, as described, varies in size from the finest 
 powder to angular rocks weighing many tons. It exhibits no 
 regular bedding or stratification, but coarse and fine debris are 
 mingled in endless variety. There is a well-marked gradation, 
 however, to be seen as one travels from the apex of a cone 
 toward its periphery. At the apex it is composed mostly of 
 coarse, angular material, with fine silt-like clays filling the 
 interspaces, while toward the periphery the fine material pre- 
 dominates. The name talus is given to the accumulations of 
 
 1 See On the Serpentine of Montville, New Jersey, Proc. U. S. National 
 Museum, 1888, p. 105. 
 
CLASTIC MATERIALS 55 
 
 debris at the foot of rocky cliffs, and which are composed of 
 angular fragments, large and small, which have fallen from the 
 cliffs above. The name dune is given to the rounded hills of 
 wind-blown sand common in arid regions and on windy shores. 
 Such are naturally of moderately fine and quite uniformly 
 assorted materials. In form and position they are ever chang- 
 ing, like drifts of snow, but are usually much steeper on the 
 leeward than on the windward sides. The character of the 
 material of which they are composed is most commonly sili- 
 ceous sand. 
 
 The names kame, esker, osar, or horseback are given to ridges 
 and mounds of sand and gravel deposited by the melting ice of 
 the glacial epoch. The materials are as a rule well rounded, 
 and as deposited usually show rude lines of stratification. 
 Such, as described, vary greatly in breadth and height, some 
 being 400 to 500 feet broad at the base and from 25 to 60 feet 
 in height. Drumlin is the name given to the peculiar low, 
 gently and smoothly sloping lenticular hills composed of un- 
 assorted glacial debris, and which are common in eastern Massa- 
 chusetts and other glacial regions. The general name moraine 
 includes the heterogeneous materials brought down by glaciers 
 and ultimately deposited in undulating hills and ridges on 
 their final disappearance. (See further under The Regolith, 
 p. 299.) 
 
PART II 
 
 THE KINDS OP BOCKS 
 
 " Some rin up hill and down dale knapping the chucky stones to pieces wi 
 hammers like sae many road-makers run daft. They say it is to see how the 
 warld was made." St. Ronan's Well. 
 
 REFERENCE has already been made to the fact that but 
 sixteen out of the sixty-nine known elements enter into the 
 composition of the earth's crust in other than comparatively 
 minute quantities. Also to the equally important fact that the 
 combination of these elements as represented in not above a 
 score of well-known mineral species go to make up the essential 
 portion of nearly all rock masses. Nevertheless, owing to the 
 variety of forms under which these rock masses occur, the vary- 
 ing forces or conditions under which they originated, or the 
 proportional quantities of the various minerals which they may 
 contain, we find numerous and widely varying types of rocks, 
 a satisfactory consideration of which necessitates first some 
 attempt at systematic classification. We may say at the outset, 
 however, that rock species, in the sense in which the word is 
 o used in mineralogy and zoology, scarcely exist. It is true we 
 may have, and particularly among igneous rocks, certain forms 
 which on casual inspection, or indeed on close inspection, with 
 regard only to limited geographical areas, seem to possess an 
 individuality of their own sufficient to entitle them to being 
 considered as true species. Yet, when we come to compare 
 these with others, to take into account their physical and chem- 
 ical composition, their structure and mode of occurrence, and 
 above all to consider how any rock varies within its own mass, 
 and the still greater variation which may have been produced 
 through alteration, we shall see that one form grades into an- 
 other almost without limit, that, indeed, no two are exactly 
 alike, and that, were we to attempt any hard and sharp lines of 
 discrimination, our species-making would practically resolve it- 
 
 56 
 
THE KINDS OF ROCKS 57 
 
 self into an enumeration of individual occurrences, or specimens. 
 This fact will become apparent as we proceed, and further 
 remarks on the subject may well be deferred until we come to 
 a discussion of individual groups. Indeed, in the present, tran- 
 sitional state of knowledge regarding the chemical and minera- 
 logical composition of rocks, their structural features, and 
 methods of origin, no scheme of classification can be advanced 
 that will prove satisfactory in all its details. The older sys- 
 tems, which were made to answer before the introduction of the 
 microscope into geological science, are now known to be founded 
 upon what were in part false, and what have proven to be 
 wholly inadequate, data. This is especially true in regard to 
 eruptive rocks. The time that has elapsed since this intro- 
 duction has been too short for the evolution of a perfectly satis- 
 factory system ; many have been proposed, but all have been 
 found lacking in some essential particulars. To enter upon a 
 discussion of the merits and demerits of the various schemes 
 would obviously be out of place here, and the student is re- 
 ferred to the published writings of Naumann, Senft, Von Cotta, 
 Richtofen, Vogelsang, Zirkel, Rosenbusch, Michel-Levy, Cred- 
 ner, Jukes Brown, and Geikie, as well as those of the American 
 geologists, Dana, 1 Wadsworth, 2 and Iddings. 3 In the scheme 
 here presented the writer has aimed to simplify matters so far 
 as is consistent With observed facts, and has not hesitated to 
 adopt or reject any such portions of systems proposed by others 
 as have seemed desirable. 
 
 All the rocks forming any essential part of the earth's crust 
 are here grouped under four main heads, the distinctions being 
 based upon their origin and structure. Each of the main 
 divisions is again divided into groups or families, the distinc- 
 tions being based mainly upon mineral and chemical composi- 
 tion, structure, and mode of occurrence. We thus have : 
 
 I. Igneous Rocks : Eruptive. Rocks which have been 
 brought up from below in a molten condition, and which 
 owe their present structural peculiarities to variations in con- 
 ditions of solidification and composition. Having as a rule two 
 
 1 On Some Points in Lithology, Am. Jour, of Science, Vol. XVI, 1878, pp. 335 
 and 431. 
 
 2 On the Classification of Rocks, Bull. Mus. Comp. Zool. Howard College, 
 No. 13, Vol. V ; also Lithological Studies. 
 
 3 The Origin of Igneous Rocks, Bull. Philosophical Society of Washington, 
 1892. 
 
58 THE KINDS OF ROCKS 
 
 or more essential constituents. In structure massive, crystal- 
 line, or glassy, or in certain altered forms, colloidal. 
 
 II. Aqueous Rocks. Rocks formed mainly through the 
 agency of water, as (J.) chemical precipitates or as (#) sedi- 
 mentary beds. Having one or many essential constituents. In 
 structure laminated or bedded ; crystalline, colloidal, or f rag- 
 mental ; never glassy. 
 
 III. JEolia.n Rocks. Rocks formed from wind-drifted ma- 
 terials. In structure irregularly bedded ; fragmental. 
 
 IV. Metamorphic Rocks. Rocks changed from their orig- 
 inal condition through dynamic or chemical agencies and 
 which may have been in part of aqueous, seolian, or of igne- 
 ous origin. Having one or many essential constituents. In 
 structure bedded, schistose or foliated, and crystalline. 
 
I. ROCKS FORMED THROUGH IGNEOUS 
 AGENCIES. ERUPTIVE 
 
 This group includes all those rocks which having once been 
 in a state of igneous fusion have been forced upward and in- 
 truded into the overlying rocks in the form of bosses, laccolites, 
 dikes, and sheets, or poured out upon the surface as lavas. 
 
 Concerning the source of eruptive rocks we are yet in igno- 
 rance. By many they have been supposed to represent portions 
 of the still unconsolidated interior of the earth. The great 
 variety of igneous rocks, the wide variation in chemical compo- 
 sition as well as the apparent independence of closely adjacent 
 volcanoes, both in the matters of time of eruption and character 
 of erupted material, seem, however, to show that they come not 
 from a common reservoir, but from isolated and comparatively 
 small areas where, for reasons not now well understood, pre- 
 viously solidified rock masses have been so highly heated as to 
 become pasty or liquid ; and then, through their own expan- 
 sion, or that of included vapors, or by compressive forces 
 generated in the earth's crust, forced upward into the positions 
 they now occupy. The origin of igneous rocks belongs as yet 
 largely to the realm of speculation. We must here confine 
 ourselves more to their mineral and chemical nature, general 
 physical properties, and the conditions under which they occur. 
 
 Consider, then, a mass of molten rock material, to which 
 the term magma may be conveniently applied, and which by 
 the processes of eruption is forced upward toward the surface, 
 and let us first dwell briefly upon the forms assumed by this 
 magma on cooling under the various conditions in which it 
 finds itself. It is obvious at the start that we can have actu- 
 ally to do with but a comparatively limited portion of the 
 products of any eruption. If the molten material is poured 
 out upon the surface and there remains for our inspection 
 to-day, it is a necessary consequence that the deeper-lying 
 portions are obscured. If, on the other hand, the superficial 
 
 59 
 
60 ROCKS FORMED THROUGH IGNEOUS AGENCIES 
 
 portions have been removed by erosion so as to expose the 
 deeply lying parts, we have only these for study and observa- 
 tion. It is rare indeed that erosion has so acted on any one 
 rock mass as to expose superficial and deep-seated portions 
 alike. In the older regions, those of greatest geological an- 
 tiquity, erosion, either glacial or otherwise, has not infre- 
 quently removed more or less completely the superficial parts 
 and left for our inspection those portions of a magma that at 
 the time of eruption never reached the surface, but cooled, it 
 may be, under thousands of feet of superincumbent matter. 
 Such rocks are as a rule more highly crystalline than those 
 which in the newer, less eroded portions, flowed out upon the 
 surface like our modern lavas. Hence it is that from a very 
 early period it has been found convenient, for purposes of dis- 
 cussion, to divide the eruptive rocks into two general groups : 
 first, the intrusive or plutonic rocks ; and second, the effusive, 
 or volcanic rocks. 
 
 Although this classification has not been strictly adhered to 
 in the present work, a few words descriptive of the essential 
 distinctions between plutonic and effusive rocks will not be out 
 of place, since such distinctions, particularly in eroded regions, 
 afford the only criteria for discrimination as to the original 
 conditions under which a rock mass has been formed, and hence 
 are of value in the field. 
 
 As a general rule, it may be said that the structural features 
 of an eruptive rock depend upon the conditions under which 
 a magma has cooled, although undoubtedly the amount of 
 included vapor of water may exert a powerful influence. As 
 Professor J. P. Iddings has well expressed it, u the chemical 
 differences of igneous rocks are the result of a chemical differ-- 
 entiation of a general magma, and the structure of a rock is 
 dependent upon the physical conditions attending its eruption 
 and solidification." Now it is at once apparent that the greater 
 the depth below the surface at which a magma 'undergoes 
 solidification, or the greater its mass, the slower, more gradual, 
 will be that solidification, and hence the more complete and 
 coarser will be the crystallization. Hence the strictly plutonic 
 rocks are always holocrystalline. And, inasmuch as the weight 
 of the superincumbent matter has been such as to prevent the 
 expansion of included vapors to form steam cavities, so these 
 rocks are never vesicular or pumiceous, but compact and gran- 
 
STRUCTURAL FEATURES OF IGNEOUS ROCKS 61 
 
 ular throughout. In cases where a plutonic rock has been 
 voided upward to fill a pre-existing rift in the form of a dike, 
 those portions of the magma coming in contact with the cold 
 walls on either hand will cool most quickly. Hence a dike is 
 as a rule most coarsely crystalline near the centre, becoming 
 finer grained and perhaps microcrystalline or even glassy at 
 the immediate contact. These two phenomena often afford 
 the only means of determining whether a rock mass occurring 
 in the form of a sheet parallel with the stratification, between 
 sedimentary beds, is an intrusive or a contemporaneous lava 
 flow ; whether it was injected as we now find it between 
 two previously existing beds ; or whether, as a lava flow, it 
 was poured out over the lower, first formed, after which the 
 second was laid down upon its surface. If formed as an intru- 
 sive sheet, we may expect to find the rock more dense along 
 both contacts, in addition to which there may, very probably, 
 be more or less contact metamorphism on the sedimentary beds 
 from the action of the hot intruded material. If poured out 
 as a lava, on the other hand, contact metamorphism and the 
 dense, fine-grained portions will be limited to the lower con- 
 tacts, while, provided there had been 110 great amount of erosion 
 between the time of the pouring out of the molten mass as a 
 surface flow and the deposition of the newer sediments, the 
 upper portions will be less dense, perhaps even vesicular, sco- 
 riaceous, and glassy, while the sediments themselves, having 
 been laid down on cold consolidated material, remain wholly 
 unchanged. Such means of discrimination have been of the 
 greatest value in ascertaining the relative ages of portions 
 of the Triassic sandstones and associated traps in the eastern 
 United States. 
 
 The lava flows, cooling so much more rapidly than the plu- 
 tonic rocks, owing to their exposed position and relief from 
 pressure, often show but incipient forms of crystallization, or 
 are quite glasslike, as is the case with the obsidians of the 
 Yellowstone Park and elsewhere. Chemically these are iden- 
 tical with granite, but they have cooled too quickly for the 
 forces of crystallization to act. Owing, further, to the expan- 
 sive force of the included vapor of water, a constituent of 
 all lavas, these surface flows are not infrequently so filled 
 with cavities as to be quite pumiceous. The pumice pur- 
 chased at the drug-stores is but the froth from a lava which, 
 
62 ROCKS FORMED THROUGH IGNEOUS AGENCIES 
 
 had it cooled under greater pressure, might have given us a 
 granite. 
 
 A common feature of the effusive or volcanic rocks is a flow 
 structure, sometimes visible only with the microscope, and which 
 is due to a flowing movement of the magma while undergoing 
 consolidation. (See Fig. 2, PI. 2.) The characteristic structure 
 of effusive rocks is porphyritic, instead of granular, and repre- 
 sents two distinct phases of cooling and crystallization : (1) an 
 intratellurial period, marked by the crystallization of certain 
 constituents while the magma, still buried in the depths of the 
 earth, was cooling very gradually, and (2) an effusive period, 
 marked by the final consolidation of the material on or near 
 the surface. As this final cooling was much the more rapid, 
 the ultimate product is a glassy, felsitic, or sometimes holo- 
 crystalline ground-mass, enclosing the porphyritic minerals, or 
 phenocrysts, formed during the first or intratellurial stage. 1 
 Naturally the deeper-lying portions of an effusive mass, those 
 forming the under or lower portions of deep lava streams, will 
 be under conditions essentially similar to plutonic magmas, and 
 may cool so slowly as to become holocrystalline. It is, more- 
 over, obvious that, could we trace any superficial mass of 
 erupted material back to its original deep-seated source, we 
 would pass gradually from the volcanic to the plutonic type 
 without at any one point being able to indicate the line of 
 separation. Hence it is that in the laboratory it is not always 
 possible, from the examination of the hand specimen or thin 
 section only, to determine to which of the two classes it may 
 belong. We can easily discriminate between the extremes, 
 but there is a wide intermediate zone where any such attempts 
 are impracticable, as indeed they are unnecessary. 2 
 
 1 Whitman Cross has shown that there are exceptions to this rule. See The 
 Laccolitic Mountain Groups of Colorado, 14th Ann. Rep. U. S. Geol. Survey, 
 pp. 231-235. 
 
 2 Intermediate between these plutonic and effusive types is still a third phase 
 of prevailing holocrystalline porphyritic structure, and which, owing to the fact 
 that such have thus far been found only in dikes, it has been proposed to group 
 under the head of dike rocks (gangesteine). Since such are but local phases of 
 plutonic magmas, which have been left to cool and crystallize between narrow 
 walls, instead of poured out upon the surface, such a subdivision seems scarcely 
 called for and as tending to still further confuse that which is already sadly 
 confounded. The same may be said with reference to the now prevailing ten- 
 dency to give varietal names to every phase of magmatic differentiation, and 
 which has resulted already in such monstrosities of nomenclature as ouachitite, 
 monchiquite, yogoite, and absarokite. 
 
RELATIONSHIP OF PLUTONIC AND IGNEOUS ROCKS 63 
 
 Owing to a false impression which formerly prevailed relative 
 to the nature of the Palaeozoic effusives and those of Mesozoic, 
 Tertiary, and more recent times, dissimilar names have, in very 
 many instances, been applied to rocks which in other respects 
 than that of geological age are essentially one and the same. 
 Thus the name andesite is given to a rock in every respect 
 similar to porphyrite, with the possible exception of a slight 
 amount of devitrification the latter may have undergone owing 
 to its greater geological antiquity. 
 
 The name rhyolite likewise includes rocks with the structure 
 and composition of the older quartz porphyries, and though 
 intended by Richthofen to include only certain comparatively 
 modern acid lavas, has been shown by the late Dr. Williams l 
 to be equally applicable to the pre-Cambrian lavas of the South 
 Mountain region of Pennsylvania. These and other names have, 
 however, become too firmly engrafted upon the literature to be 
 too hastily set aside, and may well be retained here. 
 
 The following table will serve to show the relationship, so 
 
 INTRUSIVE OB PLUTONIC 
 
 EFFUSIVE OR VOLCANIC 
 
 Palseovolcanic 
 
 Neovolcanic 
 
 Acid 
 
 
 
 
 65% -75% } Granites .... 
 
 Quartz porphyries . . 
 
 Liparites (rhyolites) 
 
 Si0 2 J 
 
 
 
 r Syenites .... 
 Intermediate N heline syenites j 
 
 00% to 65% P Foyai 4 
 
 Quartz-free porphyries 
 Phonolites 
 
 Trachytes 
 Phonolites 
 
 
 bl 2 iDiorites . . . . 
 
 Porphy rites .... 
 
 Andesites 
 
 f Gabbros, norites, ( 
 
 Melaphyrs and augitej 
 
 "Rooqlfo 
 
 
 and diabases \ 
 
 porphyrites \ 
 
 
 Basic 
 
 Theralites . . . 
 
 (Not known) .... 
 
 (Thephrites and 
 | basanites 
 
 40% to 55% H 
 
 Peridotites . . 
 
 Picrite porphyrites 
 
 Limburgites 
 
 Si0 2 
 
 Pyroxenites . 
 
 (Not known) . . . 
 
 Augitites 
 
 
 (Not known) . . 
 
 (Not known) . . . 
 
 Leucite rocks 
 
 
 (Not known) 
 
 (Not known) . . . 
 
 Nepheline rocks 
 
 
 (Not known) . . 
 
 (Not known) . . . 
 
 Melilite rocks 
 
 far as known, which exists between the plutoiiic rocks and 
 their effusive equivalents of whatever age. Thus the palseo- 
 
 Am. Jour, of Science, Vol. XLIV, p. 482, 1892. 
 
64 ROCKS FORMED THROUGH IGNEOUS AGENCIES 
 
 volcanic equivalents of the syenites are the quartz-free por- 
 phyries, and the neovolcanic equivalents, the trachytes. The 
 terms acid, intermediate, and basic, as used, have reference to 
 the percentage amounts of silica, both free and combined, 
 contained by the representatives of the several groups. Rocks 
 which, like some of the peridotites, carry even less than 40 % 
 of silica are sometimes spoken of as ultra basic. 
 
 The researches of the past few years have made it apparently 
 evident that* eruptive rocks are to be satisfactorily studied only 
 when considered, in their geographical as well as geological 
 relationships ; that is to say, the eruptives of any particular 
 region must be considered with reference to their genetic rela- 
 tion to others of the same region ; such a relationship as is 
 suggested by regarding them all as but varying phases of a 
 process of differentiation from a common magma. 
 
 That such a relationship in many cases exists has apparently 
 been conclusively demonstrated by the work of Iddings 1 in the 
 Yellowstone Park, J. F. Williams 2 in Arkansas, Pirsson 3 in 
 Montana, and Brogger 4 in Norway. The attempt at correla- 
 tion of local types with those of a somewhat similar nature at 
 a distance is interesting and instructive, as showing on the 
 whole a remarkable unity in nature's methods ; but we must 
 never lose sight of the fact that each eruptive centre, through- 
 out periods of activity interrupted it muy be by thousands of 
 years, works out its own results according to local conditions 
 which may or may not harmonize with those at distant points. 
 It is possible to conceive that, could all the rocks of any suc- 
 cessive periods of eruption from a single centre be once more 
 relegated to a common magma, such might, in its entirety, be 
 an exact equivalent of others in remote portions of the globe. 
 The consolidated results from the cooling of extruded portions 
 of this magma may, however, show ever-varying differences 
 due to local conditions. In short, eruptive rocks must be 
 considered by geographic groups and with reference to magmas. 
 
 Attempts at a satisfactory classification on other grounds 
 must prove invariably futile and tend only to retard, rather 
 than to promote, the science. 
 
 1 Bull. Philos. Soc. of Washington, XII, 1892. 
 
 2 Ann. Rep. Geol. Survey of Arkansas, Vol. II, 1890. 
 
 3 Bull. Geol. Soc. of America, Vol. VI, 1895. 
 
 4 Die Eruptivgesteine des Kristianiagebiete, Christiana, Norway, 1894. 
 
PLATE 6 
 
 *ttf!. 
 
 'S 
 
 / V * *.'>/ r ' - , ^' > *^ * " 
 
 -*- 
 
 FIG. 1. Lithophysse in liparite. 
 FIG. 2. Cross-section of stalagmite. 
 
 FIG. 3. Concretionary aragonite. 
 FIG. 4. Pegmatite. 
 
THE GRANITE-LIPARITE GROUP 
 
 65 
 
 In the following pages the rocks are discussed in groups, 
 each group comprising all those rocks having essentially the 
 same chemical composition, but differing (1) in degree of 
 crystallization, (2) in mode of occurrence, and (3) in geological 
 age. In all, there is, within certain limits, a considerable range 
 in mineral composition, or at least in the relative proportion of 
 the various essential constituents. 
 
 1. THE GRANITE-LIPARITE GROUP 
 
 This group includes the most acid of all eruptive rocks ; that 
 is, those which on analysis are found to yield the highest per- 
 centages of silica. Their chief essential constituents are quartz 
 and potash feldspars, while the more basic ferruginous minerals 
 are in quantities proportionately small. The group includes a 
 deep-seated or plutonic type, granite, and two effusive or vol- 
 canic types, quartz porphyry, and liparite or rhyolite. They 
 may be described in detail as below : 
 
 (1) THE GRANITES 
 
 Granite, from the Latin " granum," a grain, in allusion to the 
 granular structure. 
 
 Mineral Composition. The essential constituents of granite 
 are quartz and a potash feldspar (either orthoclase or micro- 
 cline), and plagioclase. Nearly always one or more minerals of 
 the mica, hornblende, or pyroxene group are present, and in 
 small, usually microscopic forms, the accessories magnetite, 
 apatite, and zircon ; more rarely occur sphene, beryl, topaz, 
 tourmaline, garnet, epidote, allanite, fluorite, and pyrite. De- 
 lesse 1 has made the following determination of the relative 
 proportion of the various constituents in two well-known gran- 
 ites : 
 
 EGYPTIAN RED GRANITE 
 
 PARTS 
 
 PORPIIYRITIC GRANITE, VOSGES 
 
 PARTS 
 
 R6d orthoclase 
 
 43 
 
 White ortlioclase 
 
 28 
 
 White albite .... 
 
 9 
 
 Reddish oligoclase .... 
 
 7 
 
 Gray quartz 
 
 44 
 
 Gray quartz 
 
 59 
 
 Black mica . 
 
 4 
 
 Mica 
 
 6 
 
 
 
 
 
 Total 
 
 100 
 
 Total 
 
 100 
 
 
 
 
 
 1 Prestwich, Chemical and Physical Geology, Vol. I, p. 42. 
 
66 
 
 ROCKS FORMED THROUGH IGNEOUS AGENCIES 
 
 Chemical Composition. A general idea of the varying char- 
 acter of these rocks may be gained from the following analy- 
 ses : 
 
 KINDS AND LOCALITIES 
 
 Si0 2 
 
 A1 S 0, 
 
 FeO 
 Fe 2 3 
 
 CaO 
 
 MgO 
 
 K 2 
 
 Na a O 
 
 Biotite granite, near Dublin, 
 Ireland 
 
 73.0 
 
 13.64 
 
 2.44 
 
 1.84 
 
 2.11 
 
 4.21 
 
 3.53 
 
 Biotite granite, Silesia . . 
 Biotite granite, Raleigh, 
 North Carolina .... 
 Hornblende granite, Salt 
 Lake Utah 
 
 73.13 
 69.28 
 71 78 
 
 12.49 
 17.44 
 14 75 
 
 2.58 
 2.30 
 1 94 * 
 
 2.40 
 2.30 
 2 36 
 
 0.27 
 0.27 
 71 
 
 4.13 
 
 2.76 
 4.89 
 
 2.61 
 3.64 
 3 12 
 
 Hornblende granite, Sauk 
 Rapids, Minnesota . . . 
 Gneissoid biotite granite, 
 District of Columbia . . 
 Hornblende mica granite, 
 Svene EffVDt 
 
 64.13 
 69.33 
 
 68.18 
 
 21 
 14.33 
 
 16.20 
 
 01 
 3.60 
 4.10 
 
 6.90 
 3.21 
 1.75 
 
 1.26 
 2.44 
 0.48 
 
 1.22 
 2.67 
 6.48 
 
 3.31 
 2.70 
 
 288 
 
 
 
 
 
 
 
 
 
 Although the mineral apatite is so universally a constituent 
 of granitic rocks, yet it occurs in such small quantities as to 
 be quite overlooked in the ordinary methods of analysis. Such 
 tests as have been made show that the amount of phosphoric 
 acid (P 2 O 5 ) contained by rocks of this class rarely exceeds 
 0.2 % and may fall as low as 0.05 %. Small as is the amount, 
 it is nevertheless probable that it was from just such minute 
 quantities in granites and the more basic eruptives, that was 
 derived the main supply of phosphates existing in soils. 
 
 Structure. The granites are holocrystalline granular rocks. 
 As a rule none of the essential constituents show perfect crystal 
 outlines, though the f eldspathic minerals are often quite perfectly 
 formed. The quartz has always been the last mineral to so- 
 lidify, and hence occurs only as irregular granules occupying the 
 interspaces. It is remarkable from its carrying innumerable 
 cavities filled with liquid and gaseous carbonic acid or with 
 saline matter. So minute are these cavities that it has been esti- 
 mated by Sorby that from one to ten thousand millions could 
 be contained in a single cubic inch of space. The microscopic 
 structure of a mica granite from Maine is shown in Fig. 3 and 
 in Fig. 1, PI. 5. 
 
 1 Yielded also 1.09% manganese oxide. 
 
THE GRANITE-LIP ARITE GROUP 
 
 67 
 
 FIG. 3. Microstructure of muscovite-biotite 
 granite, Hallowell, Maiue. 
 
 The rocks vary in texture almost indefinitely, presenting all 
 gradations from fine evenly granular rocks to coarsely porphy- 
 ritic forms in which the 
 feldspars, which are the 
 only constituents porphy- 
 ritically developed, are 
 several inches or feet in 
 length. 
 
 Concretionary forms 
 are rare. A variety from 
 Craftsburg, Vermont, is 
 unique on account of the 
 numerous concretionary 
 masses of black mica it 
 carries. 
 
 Colors. The prevail- 
 ing color is some shade 
 of gray, though greenish, 
 yellowish, pink, to deep 
 red, are not uncommon. 
 The various hues are due to the color of the prevailing feldspar 
 and the abundance and kind of the accessory minerals. Granites 
 in which muscovite is the prevailing mica, are nearly always very 
 light gray in color. The dark gray varieties are due largely to 
 abundant black mica or hornblende, the greenish and pink or 
 red colors to the prevailing greenish, pink, or red feldspars. 
 
 Classification and Nomenclature. Several varieties are com- 
 monly recognized and designated by names dependent upon the 
 predominating accessory mineral. We thus have (1) musco- 
 vite granite, (2) biotite granite or granitite, (3) biotite-muscovite 
 granite, (4) hornblende granite, (5) Jiornblende-biotite granite, and 
 more rarely (6) pyroxene (7) tourmaline and (8) epidote granite. 
 The name protogine has been given to a granite in which the 
 mica is in part or wholly replaced by talc. The name is not 
 very generally used. 
 
 G-raphic granite, or pegmatite, is a granitic rock consisting 
 essentially of quartz and orthoclase so crystallized together in 
 long parallel columns or shells that a cross-section bears a 
 crude resemblance to Hebrew writing. (See Fig. 4, PI. 6.) 
 Aplit is a name used by the Germans for a granite very poor in 
 mica and consisting essentially of quartz and feldspar only. 
 
68 HOCKS FORMED THROUGH IGNEOUS AGENCIES 
 
 The names granitell and Unary granite have also been used 
 to designate rocks of this class. Ghreisen is a name applied to 
 a quartz-mica rock, with accessory topaz, occurring associated 
 with the tin ores of Saxony and regarded as a granite meta- 
 morphosed by exhalations of fluoric acid. Luxullianite and 
 Trowlesworthite are local names given to tourmaline or tour- 
 maline-fluorite granitic rocks occurring at Luxullian and 
 Trowlesworth, in Cornwall, England. The name Unakite has 
 been given to an epidotic granite with pink feldspars occurring 
 in the Unaka Mountains in western North Carolina and eastern 
 Tennessee. 
 
 The name granite porphyry is made to include a class of rocks 
 placed by Professor Rosenbusch under the head of "gaiige- 
 steine," or dike rocks, and differing from the true granites 
 mainly in structural features. They consist in their typical 
 forms of orthoclase feldspars and quartzes porphyritically de- 
 veloped in a finer holocrystalline aggregate of the minerals 
 common to the granite group. 
 
 The granites are among the most wide-spread and commonest 
 of rocks, and are of great economic importance for structural 
 and monumental work. In the United States they are to be 
 found mainly in the Appalachian region and from the front 
 range of the Rocky Mountains westward to the Pacific coast. 
 
 Geological Age and Mode of Occurrence. The granites are 
 massive rocks, occurring most frequently associated with the 
 older and lower rocks of the earth's crust, sometimes inter- 
 stratified with metamorphic rocks or forming the central por- 
 tions of mountain chains. They are not, as once supposed, the 
 oldest of rocks, but occur frequently in eruptive masses or 
 bosses invading rocks of all ages up to late Mesozoic or Ter- 
 tiary times. Thus Professor Whitney considered the eruptive 
 granites of the Sierra Nevada to be Jurassic. Zirkel divides 
 the granites described in the reports of the 40th Parallel Sur- 
 vey into three groups : (1) Those of Jurassic age ; (2) those of 
 Palaeozoic age, and (3) those of Archaean age. The granites 
 of the eastern United States, on the other hand, have, in times 
 past, been regarded as mainly Archaean, though Dr. Wadsworth 
 has shown that the Quincy, Massachusetts, stone is an eruptive 
 rock of late Primordial or more recent age, while Professor 
 Hitchcock regards the eruptive granites of Vermont as having 
 been protruded during Silurian or perhaps Devonian times. 
 
THE QUARTZ PORPHYRIES 69 
 
 (2) THE QUARTZ PORPHYRIES 
 
 Composition. The mineral and chemical composition of the 
 quartz porphyries is essentially the same as that of the gran- 
 ites, from which they differ mainly in structure. Their essen- 
 tial constituents are quartz and feldspar, with accessory black 
 mica or hornblende in very small quantities ; other acces- 
 sories present, as a rule only in microscopic quantities, are 
 magnetite, pyrite, hematite, and epidote. 
 
 Structure. The prevailing structure is porphyritic. (Fig. 1, 
 PI. 2.) To the unaided eye they present a very dense and com- 
 pact ground-mass of uniform reddish, brown, black, gray, or yel- 
 lowish color, through which are scattered clear glassy crystals 
 of quartz alone, or of quartz and feldspar together. The quartz 
 differs from the quartz of granites in that here it was the first 
 mineral to separate out on cooling, and hence has taken on a 
 more perfect crystalline form ; the crystal outlines of the feld- 
 spar are also well defined. Under the microscope the ground- 
 mass in the typical porphyry is found to consist of a dense 
 felsitic, almost irresolvable substance, which chemical analysis 
 shows to be also a mixture of quartzose and feldspathic ma- 
 terial. The porphyritic quartzes show frequently a marked 
 corrosive action from the molten magma, the mineral having 
 again been partially dissolved after its first crystallization. 
 (Fig: 3, PI. 5.) This difference in structure in rocks of the 
 same chemical composition is believed to be due wholly to the 
 different circumstances under which the two rocks have solidi- 
 fied from a molten magma. The structure of the ground-mass 
 is not always felsitic, but may vary from a glass, as in the 
 pitchstones of Meissen, Isle of Arran, and the Lake Lugano 
 region, through spherulitic, micropegmatitic, and porphyritic 
 to perfectly microcrystalline forms as in the microgranites. 
 This difference in structure may be best understood by refer- 
 ence to Plate 5, which shows the microscopic structure of (1) 
 granite from Sullivan, Hancock County, Maine, (2) micropeg- 
 matite from Mount Desert, Maine, and (3) a quartz porphyry 
 from Fairfield, Pennsylvania. Marked fluidal structure is 
 common. (See PI. 2, Fig. 2.) 
 
 Colors. The colors of the ground-mass, as above noted, vary 
 through reddish, brownish gray to black and sometimes yellowish 
 or green. The porphyritic feldspars vary from red, pink, and 
 
70 ROCKS FORMED THROUGH IGNEOUS AGENCIES 
 
 yellow to snow-white, and often present a beautiful contrast with 
 the ground-mass, forming a desirable stone for ornamental pur- 
 poses. 
 
 Classification and Nomenclature. Owing to the very slight 
 development of the accessory minerals, mica, hornblende, etc., 
 it has been found impossible to adopt the system of classifica- 
 tion and nomenclature used with the granites and other rocks. 
 Vogelsang's classification as modified by Rosenbusch is based 
 upon the structure of the ground-mass as revealed by the micro- 
 scope. It is as follows: 
 
 Ground-mass holocrystalline granular Micro-granite. 
 
 Ground-mass holocrystalline, but formed of quartz and feld- 
 spar aggregates, rather than district crystals Granophyr. 
 
 Ground-mass felsitic Felsophyr. 
 
 Ground-mass glassy Vitrophyr. 
 
 Intermediate forms are designated by a combination of the 
 names, as granofelsophyr, felsovitrophyr, etc. The name felsite 
 is often given to rocks of this group in which the porphyritic 
 constituents are wholly lacking. The names felstone arid 
 petrosilex are also common, though gradually going out of use. 
 Elvanite is a Cornish miner's term and too indefinite to be of 
 great value. Eurite, now little used, applies to felsitic forms. 
 The n&mQ felsite pitchstone or retinite has been given to a glassy 
 form with pitch-like lustre, such as occurs in dikes cutting the 
 old red sandstone on the Isle of Arran. Kugel porphyry is a 
 name given by German writers to varieties showing spheroids 
 with a radiating or concentric structure. Micropegmatite is the 
 term not infrequently applied to such as show under the micro- 
 scope a pegmatitic structure. (Fig. 2, PI. 5.) Various popular 
 names, as leopardite and toadstone, are sometimes applied to such 
 as show a spotted or spherulitic structure. 
 
 (3) THE LIPARITES 
 
 Mineral Composition. These rocks may be regarded as the 
 younger equivalents of the quartz porphyries, or the volcanic 
 equivalents of the granites, having essentially the same mineral 
 and chemical composition. The prevailing feldspar is the clear 
 glassy variety of orthoclase known as sanidin ; quartz occurs in 
 quite perfect crystal forms often more or less corroded by the 
 molten magmas, as in the porphyries, and in the minute, six- 
 
PLATE 7 
 
 
 FIG. 1. Liparite, nevadite form. 
 FIG. 2. Liparite, rhyolite form. 
 
 FIG. 3. Liparite, obsidian form. 
 FIG. 4. Liparite, pumiceous form. 
 
THE LIPARITES 
 
 71 
 
 sided, thin platy forms known as tridymite. The accessory 
 minerals are the same as those of the granites and quartz 
 porphyries. 
 
 Chemical Composition. Below is given the composition of : 
 (I) Nevadite, from the northeastern part of Chalk Mountain, 
 Colorado, as given by Cross. 1 (II) That of a rhyolitic form, 
 from the Montezuma Range, Nevada, as given by King, 2 and 
 (III) that of a black obsidian from the Yellowstone National 
 Park, Wyoming, as given by Iddings. 3 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 III 
 
 Silica (Si0 2 ) ... 
 
 74.50% 
 
 74.62% 
 
 74 70 L 
 
 Alumina (AlgOs) 
 
 14.72 
 
 11.96 
 
 13.72 
 
 Ferric oxide (Fe^Og) 
 
 None 
 
 1.20 
 
 1.01 
 
 Ferrous oxide (FeO) 
 Ferric sulphide (FeS2) 
 
 0.56 
 
 0.10 
 
 0.62 
 0.40 
 
 Manganese (MnO) 
 
 0.28 
 
 
 Trace 
 
 Lime (CaO) ... 
 
 0.83 
 
 0.36 
 
 0.78 
 
 Magnesia (MgO) . . . 
 
 0.37 
 
 
 0.14 
 
 Soda (Na 2 O) 
 
 3.97 
 
 2.26 
 
 3.90 
 
 Potash (K 2 0) 
 
 4.53 
 
 7.76 
 
 4.02 
 
 Phosphoric anhydride (P 2 05) 
 
 0.01 
 
 
 
 Ignition. 
 
 0.66 
 
 1.02 
 
 0.62 
 
 
 
 
 
 
 100.38 % 
 
 99.28 % 
 2 2 
 
 99.91 % 
 2 3447 
 
 
 
 
 
 Colors. These are fully as variable as in the quartz por- 
 phyries ; white, though all shades of gray, green, brown, yel- 
 low, pink, and red are common. Black is the more common 
 color for the glassy varieties of obsidian, though they are often 
 beautifully spotted and streaked with red or reddish-brown. 
 
 Structure. The liparites present a great variety of structural 
 features, varying from holocrystalline, through porphyritic and 
 felsitic, to clear, glassy forms. These varieties can be best 
 understood by reference to Plates 5 and 7, prepared from 
 photographs. Fig. 1, PI. 7, is that of the coarsely crystalline 
 variety nevadite from Chalk Mountain, Colorado ; Fig. 2 is 
 
 1 Geology and Mining Industry of Leadville, Monograph XII, U. S. Geol. 
 Survey, p. 349. 
 
 2 Geological Exploration 40th Parallel, Vol. I, p. 652. 
 
 3 Ann. Rep. U. S. Geol. Survey, 1885-86, p. 282. 
 
72 KOCKS FORMED THROUGH IGNEOUS AGENCIES 
 
 that of a common felsitic and porphyritic type ; Fig. 3 is that 
 of the clear, glassy form, obsidian ; Fig. 4 shows also an obsid- 
 ian, but with a pumiceous structure ; Fig. 1 on PI. 6 shows the 
 hollow spherulites or lithopkyzce, which have been studied and 
 described by Professor J. P. Iddings, of the United States Geo- 
 logical Survey. 1 Such forms are regarded by Mr. Iddings as 
 .resulting u from the action of absorbed vapors upon the molten 
 glass from which they were liberated during the process of 
 crystallization consequent upon cooling." A pronounced flow 
 structure is quite characteristic of the rocks of this group, as 
 indicated by the name rhyolite. The microscopic structure of 
 an obsidian is shown in Fig. 4, PI. 5. Transitions from com- 
 pact obsidian into pumiceous forms, due to expansion of included 
 moisture, are common. 
 
 Classification and Nomenclature. The following varieties are 
 now generally recognized, the distinctions being based mainly 
 on structural features, as with the quartz porphyries. We thus 
 have the granitic-appearing variety nevadite, the less markedly 
 granular and porphyritic variety rhyolite, and the glassy forms 
 hyaloliparite, hyaline rhyolite, or obsidian as it is variously called. 
 Hydrous varieties of the glassy rock with a dull pitch-like lustre 
 are sometimes called rhyolite pitchstone. 
 
 The name rhyolite, from the Greek word pew, to flow, it may 
 be stated, was applied by Bichtofen as early as 1860 to this 
 class of rocks as occurring on the southern slopes of the Carpa- 
 thians. Subsequently Roth applied the name Liparite to similar 
 rocks occurring on the Lipari Islands. The first name, owing 
 to its priority, is the more generally used for the group, though 
 Professor Rosenbusch in his latest work has adopted the latter. 
 The name Nevadite is from the state of Nevada, and was also 
 proposed by Richtofen. The name Obsidian as applied to the 
 glassy variety is stated to have been given in honor of Obsid- 
 ius, its discoverer, who brought fragments of the rock from 
 Ethiopia to Rome. The name pantellerite has been given by 
 Rosenbusch to a liparite in which the porphyritic constituent 
 is an orthoclase. 
 
 Rocks of these types occur, in the United States, only in 
 the regions west of the front range of the Rocky Mountains. 
 Apo-rhyolite is the name proposed by Dr. Williams for the 
 
 i Obsidian Cliff, Yellowstone National Park, Ann. Rep. U. S. Geol. Survey, 
 1885-86. 
 
THE SYENITE-TRACHYTE GROUP 73 
 
 devitrified and otherwise altered pre-Cambrian rhyolite found 
 at South Mountain in Pennsylvania. 
 
 2. THE SYENITE-TRACHYTE GROUP 
 
 This group stands next to that of the granites in point of 
 acidity, from which it differs mainly in the lack of free silica 
 (quartz) as an essential constituent. On chemical grounds this 
 and the next group to be described belong to the intermediate 
 series, standing midway between the acid granites and the basic 
 basalts. As with the last, we have plu tonic and effusive forms. 
 These may be described as below : 
 
 (1) THE SYENITES 
 
 The name Syenite, from Syene, a town of Egypt. The word 
 was first used by Pliny to designate the coarse red granite from 
 quarries at Syene, and used by the Egyptians in their obelisks 
 and pyramids. Afterwards (in 1787) Werner introduced the 
 word into geological nomenclature to designate a class of gran- 
 ular rocks consisting of feldspar and hornblende, either with or 
 without quartz. Later, when a more precise classification 
 became necessary, the German geologists reserved the name 
 syenite to designate only the quartzless varieties of these 
 rocks, while the quartz-bearing varieties were referred to the 
 hornblendic granites. This is the classification now followed 
 by all tlie leading petrologists and is therefore adopted here. 
 Much confusion has arisen from the fact that the French geolo- 
 gist Roziere insisted upon designating the quartz-bearing rock 
 as syenite, a practice which has been followed to a considerable 
 extent both in this country and England. 
 
 Mineral Composition. The syenites differ from the granites 
 only in the absence of the mineral quartz, consisting essentially 
 of orthoclase feldspar in company with biotite, or one or more 
 minerals of the amphibole or pyroxene group. A soda-lime 
 feldspar is nearly always present and frequently microcline ; 
 other common accessories are apatite, zircon, and the iron ores : 
 more rarely sodalite. 
 
 Chemical Composition. In column I below is given the com- 
 position of a hornblende syenite from near Dresden, Saxony, 
 in II that of a mica syenite (minette) from the Odenwald, and 
 in III and IV that of augite-sodalite syenites from Montana. 
 
KOCKS FORMED THROUGH IGNEOUS AGENCIES 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 ill 
 
 IV 
 
 Silica (Si02) . . .... 
 
 60.02% 
 
 57.37% 
 
 54.15% 
 
 56.45% 
 
 Alumina (AloOa) . 
 
 16.66 
 
 13.84 
 
 18.92 
 
 20.08 
 
 Ferric iron (F620g) . . . 
 
 1 
 
 f 2.44 
 
 ) 
 
 f 1.31 
 
 Ferrous iron (FeO) 
 
 1 7.21 
 
 t 3.44 
 
 | 6.79 
 
 I 4.39 
 
 Magnesia (MgO) 
 
 2.51 
 
 6.05 
 
 1.90 
 
 0.63 
 
 Lime (CaO) 
 
 3.59 
 
 5.53 
 
 3.72 
 
 2.14 
 
 Soda (Na 2 0) . . 
 
 2.41 
 
 1.53 
 
 5.47 
 
 5.61 
 
 Potash (K 2 0) . .... 
 
 6.50 
 
 4.47 
 
 8.44 
 
 7.13 
 
 Ignition (H 2 O) 
 
 1.10 
 
 3.17 
 
 
 1.77 
 
 Chlorine (Cl ) 
 
 
 
 0.42 
 
 0.43 
 
 Phosphoric acid (PsO*-) . 
 
 
 
 
 0.13 
 
 
 
 
 
 
 
 100.00 % 
 
 97.84% 
 
 99.81 
 
 100.07 % 
 
 Structure. The structure of the syenites is wholly analo- 
 gous to that of the granites, and need not be further described 
 here. In process of crystallization the apatite, zircon, and iron 
 ores were the first to separate out from the molten magma, and 
 hence are found in more or less perfect forms enclosed by the 
 feldspars and later-formed minerals. These were followed in 
 order by the mica, hornblende, or augite, and lastly the feld- 
 spars, the soda-lime feldspars, when such occur, forming subse- 
 quent to the orthoclase. 
 
 Color. The prevailing colors are various shades of gray, 
 through pink to reddish. 
 
 Classification and Nomenclature. According as one or the 
 other of the accessory minerals of the bisilicate group predomi- 
 nates we have (1) hornblende syenite, (2) mica syenite, or minette, 
 and (3) augite syenite. 
 
 Other varietal names have from time to time been given 
 by various authors. The name minette, first introduced into 
 geological nomenclature by Voltz in 1828 (Teall), is applied 
 to a fine-grained mica orthoclase rock, occurring only in the 
 form of dikes and further differing from the typical syenites in 
 having a porphyritic rather than granitic structure. Vogesite 
 is the name applied to a similar rock in which hornblende or 
 augite prevails in place of mica. These rocks are placed by 
 Professor Rosenbusch in his latest work in the group of 
 syenitic lamprophyrs. Monzonite is a varietal name for the 
 augite syenite of Monzoni in the Tyrol. 
 
THE ORTHOCLASE OR QUARTZ-FREE PORPHYRIES 75 
 
 The mode of occurrence of the syenites is similar to that 
 of the granites, though they are much more limited in their 
 distribution. In the United States they have thus far been 
 described but sparingly. Marbleheacl Neck, Massachusetts ; 
 Jackson, New Hampshire, are well-known localities ; a beauti- 
 ful hornblende syenite is found among the glacial drift boulders 
 about Portland, Maine, but its exact source is not known. The 
 hornblende syenite described by Hawes as occurring at Red 
 Hill, Moultonborough, New Hampshire, has been shown by 
 Professor W. S. Bayley 1 to carry elseolite, and to belong to the 
 group of elseolite syenites. Hornblende syenites occur in the 
 Vosgea Mountains of Germany and in Saxony ; mica syenites 
 or minettes in the Odenwald, Germany, Baden, Saxony, and in 
 the Fichtelgebirge. A mica-augite syenite carrying sodalite 
 occurs as a Cretaceous eruptive in Jefferson County, Montana, 2 
 and a similar rock has been described by Lindgreii from the 
 Highwood Mountains in the saine state. 3 
 
 (2) THE ORTHOCLASE OR QUARTZ-FREE PORPHYRIES 
 
 Mineral Composition. The essential constituents are the 
 same as those of syenite. They consist therefore of a compact 
 porphyry ground-mass with porphyritic feldspar (orthoclase) 
 and accessory plagioclase, quartz, mica, hornblende, or minerals 
 of the pyroxene group. More rarely occur zircon, apatite, 
 magnetite, etc., as in the syenites. 
 
 Chemical Composition. Being poor in quartz, these rocks are 
 a trifle more basic than the quartz porphyries which they other- 
 wise resemble. The following is the composition of an ortho- 
 clase porphyry from Predazzo as given by Kalkowski : 4 Silica, 
 64.45% ; alumina, 16.31% ; ferrous oxide, 6.49% ; magnesia, 
 0.30%; lime, 1.10%; soda, 5.00%; potash, 5.45%; water, 
 0.85 %. 
 
 Structure. Excepting that orthoclase is the porphyritic con- 
 stituent, they are structurally identical with the quartz porphy- 
 ries, and need not be further described here. 
 
 Colors. --These are the same as the quartz porphyries already 
 described. 
 
 1 Bull. Geol. Soc. of America, Vol. IK, 1802. 
 
 2 Proc. U. S. Nat. Museum, Vol. XVII, 1894. 
 
 3 Proc. Cali. Acad. of Sciences, Vol. Ill, 2d series, p. 47. 
 
 4 Elemente der Lithologie, p. 80. 
 
76 ROCKS FORMED THROUGH IGNEOUS AGENCIES 
 
 Classification and Nomenclature. The orthoclase or quartz- 
 free porphyries bear the same relation to the syenites as do the 
 quartz porphyries to granite, and the rocks are frequently 
 designated as syenite porphyries. Like the quartz porphyries, 
 they occur in intrusive sheets, dikes, and lava flows associated 
 with the Palaeozoic formations. Owing to the frequent absence 
 of accessory minerals of the ferro-magnesia group, the rocks can- 
 not in all cases be classified as are the syenites, and distinctive 
 names based upon other features are often applied. The term 
 orthopTiyr is applied to the normal orthoclase porphyries, and 
 these are subdivided when possible into biotite, hornblende, or 
 augite orthophyr according as either one of these minerals is the 
 predominating accessory. The term rhombporphyry has been 
 used to designate an orthoclase porphyry found in southern 
 Norway, and in which the porphyritic constituent appears in 
 characteristic rhombic outlines, and which is further distin- 
 guished by a complete absencre of quartz and rarity of horn- 
 blende. The name keratophyr has been given by Gumbel to a 
 quartzose or quartz-free porphyry containing a sodium-rich alka- 
 line feldspar. So far as can be at present judged, rocks of this 
 type are much more restricted in their occurrence than are the 
 quartz porphyries already described. 
 
 (3) THE TRACHYTES 
 
 Trachyte, from the Greek word rpa^v^ rough, in allusion to 
 the characteristic roughness of the rock. The term was first 
 used by Hauy to designate the well-known volcanic rocks of the 
 Drachenfels on the Rhine. 
 
 Mineral Composition. Under the name of trachyte are com- 
 prehended those massive Tertiary and post-Tertiary lavas, con- 
 sisting essentially of sanidin with hornblende augite or black 
 mica, and which may be regarded as the younger equivalents of 
 of the quartz-free porphyries. The common accessory minerals 
 are plagioclase, tridymite, apatite, spheiie, and magnetite, more 
 rarely olivine, sodalite, humite, hauyne, and melilite. 
 
 Chemical Composition. The following analyses show the 
 range in chemical composition of these rocks, I being that of 
 the trachyte of Game Ridge, Colorado, and II that of a La 
 Guardia stone. 
 
THE FOYAITE-PHONOLITE GROUP 
 
 77 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 Silica (SiOo) 
 
 66 03 / 
 
 fy 00 <V 
 
 Alumina (A1 2 O 3 ) 
 
 18 49 
 
 oo.ujy 
 
 96 OQ 
 
 Ferric oxide (Fe 2 3 ) 
 
 2 18 
 
 
 Manganese oxide (MnO) 
 
 Trace 
 
 Tnrp 
 
 Lime (CaO) 
 
 96 
 
 q 41 
 
 Magnesia (MgO) 
 
 39 
 
 9 70 
 
 Potash (K 2 0) 
 
 5 86 
 
 6 4Q 
 
 Soda (Na 2 0) 
 
 5 22 
 
 Q QQ 
 
 Ignition (H 2 0) 
 
 85 
 
 1 0^ 
 
 Phosphoric acid (PoOs) 
 
 04 
 
 
 
 
 
 Total 
 
 100 24 / 
 
 100.74 <y 
 
 
 
 
 Structure. In structure the trachytes are rarely granular, 
 but possess a fine, scaly or microfelsitic ground-mass, rendered 
 porphyritic through the development of scattering crystals of 
 sanidin, hornblende, augite of black mica. The texture is 
 porous, and the rock possesses a characteristic roughness to 
 the touch; hence the derivation of the name as given above. 
 Perlitic structure is common in the glassy forms. The micro- 
 scopic structure of the trachyte of Monte Vetta is shown in 
 Fig. 5, PI. 5. 
 
 Colors. The prevailing colors are grayish, yellowish, or 
 reddish. 
 
 Classification and Nomenclature. They are divided into horn- 
 blende, auyite, or mica trachytes, according as any one of these 
 minerals predominates. The name sanidin-oligoclase trachyte is 
 sometimes given to trachytes in which both these feldspars ap- 
 pear as prominent constituents. The presence of quartz gives 
 rise to the variety quartz trachytes. (See .under rhyolite.) The 
 glassy form of trachyte is commonly known under the name of 
 trachyte pitchstone, or if with a perlitic structure simply as per- 
 lite. In his most recent work Professor Rosenbusch has included 
 the glassy forms under the name of hyalotrachyte. 
 
 3. THE FOYAITE-PHONOLITE GROUP 
 
 This group differs from the last mainly in the partial replace- 
 ment of the potash feldspars by the closely related mineral 
 ekeolite or nepheline. It includes therefore those plutonic and 
 effusive rocks commonly known under the name of elceolite or 
 
78 
 
 ROCKS FORMED THROUGH IGNEOUS AGENCIES 
 
 nepheline syenites and the phonolites. In their silica and potash 
 percentages it will be observed they differ not greatly from the 
 syenites proper, but are much more rich in soda and corre- 
 spondingly poor in lime. They may be described in detail as 
 follows : 
 
 (1) THE NEPHELINE (ELJEOLITE) SYENITES: FOYAITS 
 
 Nepheline from the Greek i/e^eX?;, a cloud, since the mineral 
 becomes cloudy on immersion in acid. Elaeolite from e\alov, oil, 
 in allusion to the greasy lustre. Syenite from Syene in Egypt. 
 
 Mineral Composition. The essential constituents of this 
 group are nepheline (elyeolite) and orthoclase, with nearly 
 always a pyroxenic or amphibolic mineral and a plagioclase 
 feldspar. The common accessory minerals are sphene, sodalite, 
 cancrinite, zircon, apatite, black mica, and the iron ores ilme- 
 nite and magnetite, with occasional leucite, eucolite, melino- 
 phane, and also tourmalines, perowskite, and olivine. Calcite, 
 epidote, chlorite, analcite, and sundry minerals of the zeolite 
 group occur as secondary products. 
 
 Professor W. S. Bayley has computed 1 the relative propor- 
 tions of the various constituents in the elseolite syenite of Litch- 
 field, Maine, as follows : Elaeolite, 17 % ; potash feldspar, 27 % ; 
 albite, 47% ; cancrinite, 2 % ; and black mica (lepidomelane), 7%. 
 
 Chemical Composition. The composition of the elseolite sye- 
 nite from several well-known localities is given below : 
 
 CONSTITUENTS 
 
 ALGKAVE, 
 PORTUGAL 
 
 HOT SPRINGS, 
 ARKANSAS 
 
 LlTCHFIELD, 
 
 MAINE 
 
 BEEMERVILLE, 
 NEW JERSEY 
 
 Silica (Si0 2 ) 
 
 54 61 % 
 
 59.70% 
 
 60.39% 
 
 50.36% 
 
 Alumina (A1 2 3 ) . . 
 Ferric oxide (Fe 2 0a) . . . 
 Ferrous oxide (FeO) . . . 
 Magnesia (MgO) 
 
 22.07 
 2.33 
 2.50 
 88 
 
 18.85 
 4.85 
 
 68 
 
 22.51 
 .42 
 2.26 
 0.13 
 
 19.84 
 J6.94 
 
 Manganese oxide (MnO) . . 
 Lime (CaO) 
 
 2 51 
 
 1 34 
 
 0.08 
 32 
 
 0.411 
 343 
 
 Soda (Na 2 O) 
 
 7 58 
 
 6 29 
 
 8 44 
 
 7.64 
 
 Potash (K 2 O) 
 
 5 46 
 
 5 97 
 
 4.77 
 
 7.17 
 
 Titanium oxide (Ti0 2 ) . . 
 Phosphoric anhydride (P 2 5 ) 
 Water (H 2 O) 
 
 0.09 
 0.15 
 1 13 
 
 1 88 
 
 .57 
 
 3.512 (loss) 
 
 
 
 
 
 
 1 Bull. Geol. Soc. of America, Vol. Ill, 1892, p. 231. 
 
THE NEPHELINE SYENITES 79 
 
 The essential points to be noted are the larger percentages 
 of the alkalies over those yielded by syenites of the ordinary 
 type, or the granites. 
 
 Color. The colors are light to dark gray, and sometimes 
 reddish. 
 
 Structure. The syenites, like the granites, are massive holo- 
 crystalline granular rocks, and as a rule sufficiently coarse in 
 texture to allow a partial determination of the constituent 
 parts by the unaided eye. In the Litchfield (Maine) syenite 
 the elgeolite often occurs in crystals upwards of 5 centimetres 
 in length, and zircons 2 centimetres in length are not rare. 
 Neither of the essential constituents occur in the form of per- 
 fect crystals, while the apatite, zircon, black mica, and pyrox- 
 enic constituents often present very perfect forms. The can- 
 crinite occurs both as secondary after the elieolite and as a 
 primary constituent in the form of long needle-like yellow 
 crystals with a hexagonal outline. This last form is especially 
 characteristic of the Litchfield rock. The sodalite occurs both 
 as crystals and in irregular massive forms, coating the walls of 
 crevices. 
 
 Classification and Nomenclature. Several varietal names have 
 been given to the rocks of this group as described by various 
 authors. Miascite was the name given by G. Rose to the sye- 
 nite occurring at Miask in the Urals ; Ditroite to that occurring 
 at Ditro in Transylvania, and Foyaite, by Blum, to that from 
 Mount Foya, in the province of Algrave in Portugal. The 
 name zircon syenite, or Laurvikite, has been given to the vari- 
 ety from Laurvig in southern Norway, which is rich in zircons. 
 Tinguaite is the name proposed for a varietal form from Serra 
 de Tingua, province of Rio Janeiro, Brazil. 
 
 American petrographers have not been at all delinquent in 
 the matter of names, and have added to an already over- 
 burdened nomenclature such terms as Litchfieldite, Ouachitite, 
 Pulaskite, and Fourchite to varieties from Litchfield (Maine) 
 and the Hot Springs region of Arkansas. Liebnerite is the 
 name given to an eleeolite syenite porphyry occurring in the 
 Tyrol. 
 
 Rocks of this group, although wide-spread in their distribu- 
 tion, are nevertheless not abundant. The more important 
 localities thus far described have already been noted ; there 
 remains to be mentioned only the locality at Red Hill, Moul- 
 
80 ROCKS FORMED THROUGH IGNEOUS AGENCIES 
 
 tonborough, New Hampshire, the rock of which was first de- 
 scribed as an ordinary syenite, and that of Hastings County, 
 Ontario. 
 
 (2) THE PHONOLITES 
 
 Phonolite, from the Greek word <f>a>vri, sound, and \l0os, stone, 
 in allusion to the clear ringing or clinking sound which slabs 
 of the stone emit when struck with a hammer ; formerly called 
 clinkstone for the same reason. 
 
 Mineral Composition. The phonolites consist essentially of 
 sanidin and nepheline or leucite, together with one or more 
 minerals of the augite-hornblende group, and generally hauyne 
 or nosean. The common accessories are plagioclase, apatite, 
 sphene, mica, and magnetite ; more rarely occur tridymite, 
 melanite, zircon, and olivine. The rock undergoes ready alter- 
 ation, and calcite, chlorite, limonite, and various minerals of 
 the zeolite group occur as secondary products. 
 
 Chemical Composition. The average of six analyses given 
 by Zirkel 1 is as follows: Silica, 58.02%; alumina, 20.03%; 
 iron oxides, 6.18 %; manganese oxide, 0.58%; lime, 1.89%; 
 magnesia, 0.80 % ; potash, 6.18 % ; soda, 6.35 % ; water, 1.88 % ; 
 specific gravity, 2.58. 
 
 Structure. The phonolites present but little variety in 
 structure, being usually porphyritic, seldom evenly granular. 
 The porphyritic structure is due to the development of large 
 crystals of sanidin, nepheline, leucite, or hauyne, and more rarely 
 hornblende, augite, or sphene, in the fine-grained and compact 
 ground-mass, which is usually microcrystalline, rarely glassy or 
 amorphous. 
 
 Colors. The prevailing colors are dark gray or greenish. 
 
 Classification and Nomenclature. Three varieties are recog- 
 nized by Professor Rosenbusch, the distinction being founded 
 upon the variation in proportional amounts of the three miner- 
 als, sanidin, nepheline, or leucite. We thus have (1) nepheline 
 phonolite, consisting essentially of nepheline and sanidin, and 
 which may therefore be regarded as the volcanic equivalent 
 of the nepheline syenite ; (2) leucite phonolite, consisting 
 essentially of leucite and sanidin ; and (3) leucitophyr, which 
 consists essentially of both nepheline and leucite in connection 
 with sanidin, and nearly always melanite. 
 
 1 Lehrbuch der Petrographie, II, p. 193. 
 
THE DIORITE-ANDESITE GROUP 81 
 
 So far as now known, these rocks are of comparatively rare 
 occurrence in the United States, having been described as 
 occurring only in the Black Hills of South Dakota and the 
 Cripple Creek district of Colorado. 
 
 4. THE DIORITE-ANDESITE GROUP 
 
 We come now to groups of rocks which show a still greater 
 falling off in their total amount of silica, as indicated by analy- 
 ses, and a like diminution in the amount of potash. The cause 
 of this falling off is due to the absence as an essential constituent 
 of quartz and potash feldspars, the latter being replaced by soda- 
 lime varieties, and which in their turn cause a corresponding in- 
 crease in the elements sodium and calcium. The group includes 
 the plutonic type diorite, and the effusive types hornblende por- 
 phyrite, and andesite. These may be described as below: 
 
 (1) THE DIORITES (GREENSTONES IN PART) 
 
 Diorite, from the Greek word Siopi^elv, to distinguish. Term 
 
 first used by the mineralogist Haiiy. 
 
 Mineral Composition. The essential constituents of diorite 
 are plagioclase feldspar, either labradorite or oligoclase, and 
 hornblende or black mica. The common accessories are mag- 
 netite, titanic iron, orthoclase, apatite, epidote, quartz, augite, 
 black mica, and pyrite, more rarely garnets. Calcite and 
 chlorite occur as alteration products. 
 
 Structure. Dioritesare holocrystalline granular rocks, and as 
 a rule, massive, though schistose forms occur. The individual 
 crystals composing the rock are sometimes grouped in globular 
 aggregates, thus forming the so-called orbicular diorite, kuyel 
 diorite, or napoleonite from Corsica. (Fig. 1, PI. 8.) The 
 texture is, as a rule, fine, compact, and homogeneous, and its 
 true nature discernible only with the aid of a microscope ; more 
 rarely porphyritic forms occur as in the camptonites. 
 
 Colors. T The colors vary from green and dark gray to 
 almost black. 
 
 Chemical Composition. The following table shows the wide 
 range in chemical composition found in rocks commonly grouped 
 under this head. 
 
 Classification. Accordingly as they vary in mineral comno- 
 sition the diorites are classified as (1) diorite, in which horn- 
 
82 
 
 ROCKS FORMED THROUGH IGNEOUS AGENCIES 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 III 
 
 IV 
 
 V 
 
 VI 
 
 Silica (Si0 2 ) 
 
 67 54 % 
 
 61.75% 
 
 56.71 % 
 
 50.47 % 
 
 43.50% 
 
 39 32 % 
 
 Titanic oxide (TiO ) . 
 
 
 
 
 
 
 1.70 
 
 Alumina (A1 2 3 ) . . . 
 Ferric iron (Fe 2 3 ) . . 
 Ferrous iron (FeO) . . 
 Manganese oxide (MnO) 
 
 17.02 
 2.97 
 0.04 
 
 18.88 
 0.52 
 3.52 
 
 18.36 
 6.45 
 
 18.73 
 4.19 
 4.92 
 
 17.02 
 13.68 
 
 14.48 
 2.01 
 
 8.73 
 0.71 
 
 Lime (CaO) 
 
 2 94 
 
 3 54 
 
 6.11 
 
 8 82 
 
 8 15 
 
 8.30 
 
 Magnesia (MgO) . . . 
 Potash (K 2 O) .... 
 Soda (Na 2 0) 
 
 1.51 
 
 2.28 
 4.62 
 
 1.90 
 1.24 
 3.67 
 
 3.92 
 2.38 
 3.52 
 
 3.48 
 3.56 
 4.62 
 
 6.84 
 2.84 
 2.84 
 
 11.11 
 0.87 
 3.76 
 
 Phosphoric acid (P 2 O 5 ) . 
 Carbonic acid (CO 2 ) . . 
 Water (H 2 0) .... 
 
 1 0.55 
 
 4.46 
 
 
 0.58 
 
 4.35 
 
 0.61 
 5.25 
 
 2.57 
 
 I. Quartz-mica diorite: Electric Peak, Yellowstone Park (J. P. Iddings). 
 II. Diorite: Pemnaen-Mawr, Wales (J.A.Phillips). III. Diorite: Comstock 
 Lode, Nevada (40th Parallel Survey). IV. Augite diorite : Custer County, 
 Colorado (Whitman Cross). V. Porphyritic diorite (cainptonite) : Fairhaven, 
 Vermont (J. F. Kemp). VI. Porphyritic diorite: Lewiston, Maine (G. P. 
 Merrill). 
 
 blende alone is the predominating accessory; (2) mica diorite, 
 in which black mica replaces the hornblende, and (3) augite 
 diorite, in which the hornblende is partially replaced by augite. 
 The presence of quartz gives rise to the varieties, quartz, quartz 
 augiie, and quartz-mica diorites. The name tonalite has been 
 given by Vom Rath to a quartz diorite containing the feldspar 
 andesine and very rich in black mica. Kersantite is a dioritic 
 rock occurring, so far as known, only in dikes, and consisting 
 essentially of black mica and plagioclase, with accessory apatite 
 and augite, or more rarely hornblende, quartz, and orthoclase. 
 It differs from the true mica diorite in being, as a rule, of a 
 porphyritic rather than granitic structure. Professor Rosen- 
 busch, in his latest work, has placed the kersantites, together 
 with the porphyritic diorites (camptonites), under the head 
 of dioritic lamprophyrs in the class of dike rocks or "gange- 
 steine." The name, it should be stated, is from Kersanton, a 
 small hamlet in the Brest Roads, department of Finistere, 
 France. 
 
 The diorites were formerly, before their exact mineralogical 
 nature was well understood, included with the diabases and 
 melaphyrs under the general name greenstone (Ger. G-riinsteiii). 
 
PLATE 8 
 
 FIG. 1. Orbicular diorite. 
 
 Fia. 2. Granite spheroid. 
 
THE PORPHYRITES AND ANDESITES 83 
 
 They are rocks of wide geographic distribution, but apparently 
 less abundant in the United States than are the diabases. The 
 lamprophyr varieties are still less abundant, so far as now 
 known. 
 
 (2) THE PORPHYRITES 
 
 Mineral and Chemical Composition. The essential constitu- 
 ents of the porphyrites are the same as of the diorites, from 
 which they differ mainly in structure. 
 
 Structure. The porphyrites, as a rule, show a felsitic or 
 glassy ground-mass, as do the quartz porphyries, in which are 
 embedded quite perfectly developed porphyritic plagioclases, 
 with or without hornblende or black mica. At times, as in 
 the well-known " porfido rosso antico," or antique porphyries 
 of Egypt, the ground-mass is microcrystalline, forming thus 
 connecting links between the true diorites and diorite porphy- 
 rites. Indeed, the rocks of the group may be said to bear the 
 same relation to the diorites in the plagioclase series as do 
 the quartz porphyries to the granites in the orthoclase series, 
 or better yet, they may be compared with the hornblende an- 
 desites, of which they are apparently the Palaeozoic equivalents. 
 
 Colors. The prevailing colors are dark brown, gray, or 
 greenish. 
 
 Classification. According to the character of prevailing 
 accessory mineral, we have hornblende porphyrite, or diorite 
 porphyrite, as it is sometimes called, and mica porphyrite. 
 When, as is frequently the case, neither of the above minerals 
 are developed in recognizable quantities, the rock is designated 
 as simply porphyrite. The porphyrites are wide-spread rocks, 
 very characteristic of the later Paleozoic formations, occurring 
 as contemporaneous lava flows, intrusive sheets, dikes, and 
 bosses. 
 
 (3) THE ANDESITES 
 
 The name Andesite was first used by L. V. Buch in 1835, to 
 designate a type of volcanic rocks found in the Andes Moun- 
 tains, South America. 
 
 Mineral Composition. The essential constituents are soda- 
 lime feldspar, together with black mica, hornblende, augite, or 
 a rhombic pyroxene, and in smaller, usually microscopic pro- 
 portions, magnetite, ilmenite, hematite, and apatite. Common 
 
84 
 
 ROCKS FORMED THROUGH IGNEOUS AGENCIES 
 
 accessories are olivine, sphene, garnets, quartz, tridymite, anor- 
 thite, sanidin, and pyrite. 
 
 Chemical Composition. The composition of the andesites 
 varies very considerably, the quartz-bearing members naturally 
 showing much the higher percentage of silica. The following 
 table shows the composition of a few typical forms : 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 III 
 
 IV 
 
 V 
 
 VI 
 
 Silica (Si0 2 ) 
 
 66.32 % 
 14.33 
 5.53 
 0.25 
 2.45 
 4.64 
 3.90 
 1.61 
 1.13 
 
 69.51 % 
 15.75 
 3.34 
 
 2.09 
 1.71 
 3.89 
 3.34 
 
 61.12% 
 11.61 
 11.64 
 
 0.61 
 4.33 
 3.85 
 3.52 
 4.35 
 
 56.07 % 
 19.06 
 5.39 
 0.92 
 2.12 
 7.70 
 4.52 
 1.24 
 0.99 
 
 56.19% 
 16.21 
 4.92 
 4.43 
 4.60 
 7.00 
 2.96 
 2.37 
 1.03 
 
 58.33% 
 18.17 
 
 6.03 
 2.40 
 6.19 
 3.20 
 3.02 
 0.76 
 
 Alumina (A1 2 3 ) . . . 
 Ferric oxide (Fe 2 0s) . . 
 Ferrous oxide (FeO) . . 
 Magnesia (MgO) . . . 
 Lime (CaO) 
 
 Soda (Na>0) 
 
 Potash (K 2 0) .... 
 Water (H 2 0) .... 
 
 100.16% 
 
 99.63% 
 
 101.03% 
 
 98.01 % 
 
 99.62 
 
 98.10% 
 
 I. Dacite from Kis Sebes, Transylvania. II. Dacite from Lassens Peak, 
 California. III. Hornblende andesite from hill north of Gold Peak, Nevada. 
 IV. Hornblende andesite from Bogoslof Island, Alaska. V. Hypersthene ande- 
 site, Buffalo Peaks, Colorado. VI. Augite andesite from north of American 
 Flat, Washoe, Nevada. 
 
 Structure. To the unaided eye the andesites present as a 
 rule a compact, often rough and porous ground-mass carrying 
 porphyritic feldspars and small scales of mica, hornblende, or 
 whatever may be the prevailing accessory; pumiceous forms 
 are not uncommon. Under the microscope the ground-mass is 
 found to vary from clear glassy through microlitic forms to 
 almost holocrystalline. The minerals of the ground-mass are 
 feldspars in elongated microlites, specks of iron ore, apatite in 
 very perfect forms, and one or more of the accessory ferro-mag- 
 nesian minerals.- 
 
 Colors. The prevailing colors are some shade of gray, green- 
 ish or reddish. 
 
 Classification and Nomenclature. Specific names are given 
 dependent upon the character of the prevailing accessory. We 
 thus have : 
 
 Andesites with quartz = Quartz andesites or dacites. 
 
 Andesites in which hornblende prevails = Hornblende andesites. 
 
THE GABBRO-BASALT GROUP 85 
 
 Andesites in which augite prevails = Augite andesites. 
 Andesites in which hypersthene prevails = Hypersthene andesites. 
 Andesites in which mica prevails = Mica andesites. 
 
 The glassy varieties are often known as hyaline andesites. 
 The name propylite was given by Richthofen to a group of 
 andesitic rocks prevalent in Hungary, Transylvania, and the 
 western United States, but these rocks have since been shown 
 by Dr. Wadsworth J and others to be but altered andesites, and 
 the name has fallen largely into disuse. 
 
 5. THE GABBRO-BASALT GROUP 
 
 We have here a large and variable group of rocks which on 
 structural and mineralogical grounds might well be subdivided. 
 Thus the gabbros, norites, and hypersthene andesites might 
 well be considered as a group by themselves, while the diabases, 
 augite porphyrites, melaphyrs, and basalts could form a second. 
 Owing, however, to the similarity of the magmas from which 
 they have been derived, it is believed the wants of the student 
 will be best subserved by grouping them all together as above. 
 They may be described in detail as below : 
 
 (1) THE GABBROS 
 
 Gabbro, an old Italian name originally applied to serpen- 
 tinous rocks containing diallage. 
 
 Mineral Composition. The gabbros consist essentially of a 
 basic soda-lime feldspar, either labradorite, bytownite, or an- 
 orthite, and diallage or a closely related monoclinic pyroxene, 
 a rhombic pyroxene (enstatite or hypersthene), and more rarely 
 olivine. Apatite and. the iron ores are almost universally pres- 
 ent, and often picotite, chromite, pyrrhotite, more rarely com- 
 mon pyrites, and a green spinel. Secondary brown mica and 
 hornblende are common. Quartz occurs but rarely. 
 
 Chemical Composition. As with other groups, the percent- 
 age amounts of the various constituents obtained by analyses 
 is dependent upon the relative proportion of the constituent 
 minerals. In the tables given below, analyses like I and III, 
 showing very little iron and magnesia, but rich in lime and 
 soda and alumina, are of rocks in which the pyroxenic con- 
 
 1 Proc. Boston Society of Natural History, Vol. XXI, 1881, p. 260. 
 
86 
 
 KOCKS FORMED THROUGH IGNEOUS AGENCIES 
 
 stituents are almost wholly lacking, and which consist essen- 
 tially of lime feldspars only. 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 Ill 
 
 IV 
 
 V 
 
 VI 
 
 Silica (Si0 2 ) 
 
 59.55 % 
 
 54.72 % 
 
 53.43 % 
 
 49.15% 
 
 46.85% 
 
 45.66% 
 
 Alumina (A1 2 O 3 ) . . . 
 
 25.62 
 
 17.79 
 
 28.01 
 
 21.90 
 
 19.72 
 
 16.44 
 
 Ferric iron (Fe 2 3 ) . . 
 
 0.75 
 
 2.08 
 
 0.75 
 
 6.60 
 
 3.22 
 
 0.66 
 
 Ferrous iron (FeO) . 
 
 .... 
 
 6.03 
 
 .... 
 
 4.54 
 
 7.99 
 
 13.90 
 
 Lime (CaO) 
 
 7.73 
 
 6.84 
 
 11.24 
 
 8.22 
 
 13.10 
 
 7.23 
 
 Magnesia (MgO) . 
 
 Trace 
 
 5.85 
 
 0.63 
 
 3.03 
 
 7.75 
 
 11.57 
 
 Potash (K 2 0) .... 
 
 0.96 
 
 3.01 
 
 0.96 
 
 1.61 
 
 0.09 
 
 0.41 
 
 Soda (Na 2 O) 
 
 5.09 
 
 3.02 
 
 4.85 
 
 3.83 
 
 1.56 
 
 2.13 
 
 Ignition and loss . . . 
 
 0.45 
 
 .... 
 
 .... 
 
 1.92 
 
 0.56 
 
 0.07 
 
 I. Anorthosite : Chateau Richer, Canada (T. S. Hunt). II. Gabbro : near 
 Cornell Dam, Croton River, New York (J. F. Kemp). III. Anorthosite: 
 Labrador (A. Wickman). IV. Gabbro : near Duluth, Minnesota (Streng). 
 V. Gabbro: near Baltimore, Maryland (G. H. Williams). VI. Gabbro: North- 
 west Minnesota (W. S. Bay ley). 
 
 Structure. The gabbro structure is quite variable. Like 
 the other plutonic rocks mentioned, they are crystalline granu- 
 lar, the essential constituents rarely showing perfect crystal 
 outlines. As a rule the pyroxenic constituent occurs in broad 
 and very irregularly outlined plates, filling the interstices of 
 the feldspars, which are themselves in short and stout forms 
 quite at variance with the elongated, lath-shaped forms seen in 
 diabases. This rule is, however, in some cases reversed, and 
 the feldspars occur in broad, irregular forms surrounding the 
 more perfectly formed pyroxenes. Transitions into diabase 
 structure are not uncommon. In rare instances the pyroxenic 
 constituents occur in concretionary aggregates as in the peculiar 
 kugel gabbro or potato rock from Smaalanene, in Norway. 
 Through a molecular change of the pyroxenic constituent, the 
 gabbros pass into diorites, as do also the diabases. 
 
 Colors. The prevailing colors are gray to nearly black ; 
 sometimes greenish through decomposition. 
 
 Classification. The rocks of this group are divided into 
 
 (1) the true gabbros that is, plagioclase-diallage rocks and 
 
 (2) norites, or plagioclase-bronzite and hypersthene rocks. 
 Both varieties are further subdivided according to the presence 
 or absence of olivine. We then have : 
 
THE DIABASES 
 
 87 
 
 True gabbro = Plagioclase -f diallage. 
 Olivine gabbro = Plagioclase -f- diallage and olivine. 
 Norite = Plagioclase -f hypersthene or bronzite. 
 
 Olivine norite = Plagioclase -f hypersthene and olivine. 
 
 Nearly all gabbros contain more or less rhombic pyroxene, and 
 hence pass by gradual transitions into the norites. Through 
 a diminution in the proportion of feldspar they pass into the 
 peroditites, and a like diminution in the proportion of pyroxene 
 gives rise to the so-called forellenstein. Hyperite is the name 
 given, by Tornebohm, to a rock intermediate between normal 
 gabbro and norite. Anorthosite, as above indicated, is the name 
 given to the granular varieties poor or quite lacking in pyrox- 
 enes. 
 
 (2) THE DIABASES 
 
 Diabase, from the Greek word &a/3acrt9, a passing over ; so 
 called by Brongniart because the rock passes by insensible 
 gradations into diorite. 
 
 Chemical Composition. The table below shows the average 
 range in composition of (I and II) the plutonic diabase and 
 (III, IV, V, and VI) the effusive forms melaphyr and basalt. 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 III 
 
 IV 
 
 V 
 
 VI 
 
 Silica (Si0 2 ) .... 
 
 53.13% 
 
 45.46 % 
 
 56.52 % 
 
 51.02 % 
 
 57.25% 
 
 46.90 % 
 
 Alumina (Al 2 Oa) 
 
 13.74 
 
 19.94 
 
 13.53 
 
 18.86 
 
 16.45 
 
 10.17 
 
 Ferric iron (Fe 2 3 ) . . 
 Ferrous iron (FeO) . 
 
 1.08 
 9.10 
 
 j 15.36 
 
 12.56 
 
 J6.57 
 
 "1 4. 68 
 
 1.67, 
 1.77 
 
 1.22 
 5.17 
 
 Lime (CaO) .... 
 
 9.47 
 
 8.32 
 
 5.31 
 
 7.36 
 
 7.65 
 
 6.20 
 
 Magnesia (MgQ) . . . 
 
 8.58 
 
 2.95 
 
 2.79 
 
 5.57 
 
 6.74 
 
 20.98 
 
 Potash (K 2 O) .... 
 
 1.03 
 
 3.21 
 
 3.59 
 
 2.10 
 
 1.57 
 
 2.04 
 
 Soda (Na 2 0) .... 
 
 2.30 
 
 2.12 
 
 3.71 
 
 2.54 
 
 3.00 
 
 1.16 
 
 Ignition 
 
 0.90 
 
 0.30 
 
 0.81 
 
 2.86 
 
 0.45 
 
 5.42 
 
 Specific gravity 
 
 2 96 
 
 2 945 
 
 
 
 
 2.86 
 
 
 
 
 
 
 
 
 I. Diabase: Jersey City, New Jersey (G. W. Hawes). II. Diabase: Palmer 
 Hill, Au Sable Forks, New York (J. F. Kemp). III. Melaphyr: Hockenberg, 
 Silesia. IV. Melaphyr, Falgendorf, Bohemia (quoted from Zirkel's Lehrbuch 
 der Petrographie). V. Quartz basalt: Snag Lake, California (J. S. Diller). 
 VI. Basalt (absarokite) : near Bozeman, Montana (G. P. Merrill). 
 
 Mineral Composition. The essential constituents of diabase 
 are plagioclase feldspar and augite, with nearly always mag- 
 
88 
 
 HOCKS FORMED THROUGH IGNEOUS AGENCIES 
 
 iietite and apatite in microscopic proportions. The common 
 accessories are hornblende, black mica, olivine, enstatite, hyper- 
 sthene, orthoclase, quartz, and titanic iron. Calcite, chlorite, 
 hornblende, and serpentine are common as products of altera- 
 tion. Through a molecular change known as uralitization the 
 augite not infrequently becomes converted into hornblende, 
 as already described (p. 40), and the rock thus passes over 
 into diorite. The plagioclase may be labradorite, oligoclase, 
 or anorthite. 
 
 Structure. In structure the diabases are holocrystalline. 
 Rarely do the constituents possess perfect crystal outlines, but 
 
 are more or less imper- 
 fect and distorted, owing 
 to mutual interference in 
 process of formation, the 
 granular hypidiomorphic 
 structure of Professor 
 Rosenbusch. The augite 
 in the typical forms oc- 
 
 FIG. 4. Microstructure of diabase. 
 
 curs in broad and sharply 
 angular plates enclosing 
 the elongated or lath- 
 shaped crystal of plagio- 
 clase, giving rise to a 
 structure known as ophi- 
 tic. (See Fig. 4.) The 
 rocks are, as a rule, com- 
 pact, fine, and homoge- 
 neous, though sometimes porphyritic and rarely amygdaloidal. 
 Colors. The colors are sombre, varying from greenish through 
 dark gray to nearly black, the green color being due to a dissemi- 
 nated chloritic or serpentinous product resulting from the alter- 
 ation of the augite or olivine. 
 
 Classification. Two principal varieties are recognized, the 
 distinction being based upon the presence or absence of the 
 mineral olivine. We thus have: (1) diabase proper and (2) oli- 
 vine diabase. 
 
 Many varietal names have been given from time to time by 
 different authors. Gumbel gave the name of leucophyr to a 
 very chloritic, diabase-like rock consisting of pale green augite 
 and a saussurite-like plagioclase. The same authority gave 
 
THE DIABASES 89 
 
 the name epidiorite to an altered diabase rock occurring in 
 small dikes between the Cambrian and Silurian formations 
 in the Fichtelgebirge, and in which the augite had become 
 changed to hornblende. He also designated by the term pro- 
 terobase a Silurian diabase consisting of a green or brown, 
 somewhat fibrous, hornblende, reddish augite, two varieties of 
 plagioclase, chlorite, ilmenite, a little magnetite, and usually a 
 magnesian mica. The name ophite has been used by Pallarson 
 to designate an augite plagioclase eruptive rock, rich in horn- 
 blende and epidote, and occurring in the Pyrenees. The 
 researches of M. Levy Kuhn 1 and others have, however, shown 
 that both hornblende and epidote are secondary, resulting from 
 the augitic alteration, and that the rock must be regarded as 
 belonging to the diabase. 
 
 The Swedish geologist, Tornebohm, gave the name sahlite 
 diabase to a class of diabasic rocks containing the pyroxene 
 sahlite, and which occurred in dikes cutting the granite, gneiss, 
 and Cambrian sandstones in the province of Smaaland, and in 
 other localities. The name teschenite was for many years ap- 
 plied to a class of rocks occurring in Moravia, and which, until 
 the recent researches of Rohrbach, were supposed to contain 
 nepheline, but which are now regarded as merely varietal forms 
 of diabase. Variolite is a compact, often spherulitic, variety 
 occurring in some instances as marginal facies of ordinary 
 diabase. The name eukrite or eucrite was first used by G. Rose 
 to designate a rock consisting of white anorthite and grayish 
 green augite occurring in the form of a dike cutting the Car- 
 boniferous limestone of Carlingford district, Ireland. These 
 rocks were included by Professor Zirkel under the head of 
 "anorthitgesteine." The name is now little used, and rocks 
 of this type are here included with the diabases. 
 
 The diabases are among the most abundant and wide-spread 
 of our so-called trap rocks, occurring in the form of dikes, 
 intrusive sheets, and bosses. They are especially characteristic 
 of the Triassic formations of the eastern United States. It 
 should be noted, however, that many of these Triassic traps 
 have been shown to be true lava flows, and that on both litho- 
 logical and geological grounds such might with propriety be 
 classed with the basalts. 
 
 1 Untersuchungen liber pyrenaeische Ophite, Inaugural Dissertation Univer- 
 sitat, Leipzig, 1881. 
 
90 KOCKS FORMED THROUGH IGNEOUS AGENCIES 
 
 (3) THE MELAPHYRS AND AUGITE PORPHYRITES 
 
 The term melaphyr is used to designate a volcanic rock 
 occurring in the form of intrusive sheets and lava flows, and 
 consisting essentially of a plagioclase feldspar, augite, and 
 olivine, with free iron oxides and an amorphous of porphyry 
 base. The augite porphyrites differ in containing no olivine. 
 The rocks of this group are therefore the porphyritic, effusive, 
 forms of the olivine-bearing and divine-free diabases and 
 gabbros. 
 
 Structure. As above noted, they are porphyritic rocks with, 
 in their typical forms, an amorphous base, are often amygda- 
 loidal, and with a marked flow structure. 
 
 Colors. In colors they vary through gray or brown to nearly 
 black ; often greenish through chloritic and epidotic decompo- 
 sition. 
 
 Classification and Nomenclature. According as olivine is 
 present or absent, they are divided primarily into melaphyrs 
 and augite porphyrites, the first bearing the same relation to 
 the olivine diabases as do the quartz porphyries to the granites, 
 or the hornblende porphyrites to the diorites, and the second 
 a similar relation to the olivine-free diabases. The augite 
 porphyrites are further divided upon structural grounds into 
 (1) diabase porphyrite, which includes the varieties with holo- 
 crystalline diabase granular ground-mass of augite, iron ores, 
 and feldspars, in which are embedded porphyritic lime-soda 
 feldspars, mainly labradorite, idiomorphic augites, and at 
 times accessory hornblende and black mica ; (2) spilite, which 
 includes the non-porphyritic compact, sometimes amygdaloidal 
 and decomposed forms such as are known to German petrog- 
 raphers as dichte diabase, diabase mandelstein (amygdaloid), 
 kalk-diabase, variolite, etc.; (3) the true augite porphyrite, in- 
 cluding the normal porphyritic forms with the amorphous base, 
 and (4) the glassy variety augite vitrophyrite. 
 
 (4) THE BASALTS 
 
 Basalt, a very old term used by Pliny and Strabo to designate 
 certain blacks rocks from Egypt, and which were employed in 
 the arts in early times. 1 
 
 1 Teall, British Petrography, p. 136. 
 
THE BASALTS 91 
 
 Mineral Composition. The essential minerals are augite and 
 plagioclase feldspar with olivine in the normal forms; accessory 
 iron ores (magnetite and ilmenite), together with apatite, are 
 always present, and more rarely a rhombic pyroxene, horn- 
 blende, black mica, quartz, perowskite, hauyne and nepheline, 
 and minerals of the spinel group. Metallic iron has been 
 found as a constituent of certain basaltic rocks on Disco Island, 
 Greenland. 
 
 Chemical Composition. The composition is quite variable, 
 as shown by analyses in columns V and VI on p. 87. The fol- 
 lowing shows the common extremes of variation : Silica, 45 % 
 to 55 %; alumina, 10 % to 18 %; lime, 7 % to 14 %; magnesia, 
 3 % to 10 % ; oxide of iron and manganese, 9 % to 16 % ; 
 potash, 0.058 % ; soda, 2 % to 5 % ; loss by ignition, 1% to 5 % ; 
 specific gravity, 2.85 to 3.10. 
 
 Structure. Basalts vary all the way from clear glassy to 
 holocrystalline forms. The common type is a compact and, 
 to the unaided eye, homogeneous rock, with a splintery or 
 conchoidal fracture, and showing only porphyritic olivines in 
 such size as to be recognizable. Under the microscope they 
 show a ground-mass of small feldspar and augite microlites, 
 with perhaps a sprinkling of porphyritic forms of feldspar, 
 augite, and olivine, and a varying amount of interstitial brown- 
 ish glass; the glass may be wholly or in part replaced by devit- 
 rification products, as minute hairs, needles, and granules. A 
 marked flow structure is often developed, the feldspars of the 
 ground-mass having flowed around the olivine belonging to the 
 earlier period of consolidation, giving rise to an appearance 
 that may be compared to logs in a mill stream, the olivines 
 representing small islands. Pumiceous and amygdaloidal 
 forms are common. 
 
 Colors. The prevailing colors are dark, some shade of gray 
 to perfectly black. Red and brown colors are also common. 
 Mineralogically it will be observed the basalts resemble the 
 olivine diabases and melaphyrs, of which they may be regarded 
 as the younger equivalents. Indeed, in very many cases it has 
 been found impossible to ascertain from the study of the speci- 
 men alone to which of the three groups it should be referred, so 
 closely at times do they resemble one another. 
 
 Classification and Nomenclature. In classifying, the varia- 
 tions in crystalline structure are the controlling factors. As, 
 
92 ROCKS FORMED THROUGH IGNEOUS AGENCIES 
 
 however, these characteristics are such as may vary almost 
 indefinitely in different portions of the same flow, the rule has 
 not been rigidly adhered to here. We thus have : 
 
 (1) Dolerite, including the coarse-grained almost holocrys- 
 talline variety ; (2) anamesite, including the very compact 
 tine-grained variety, the various constituents of which are not 
 distinguishable by the unaided eye ; (3) basalt proper, which 
 includes the compact homogeneous, often porphyritic, variety, 
 carrying a larger proportion of interstitial glass or devitrifica- 
 tion products than either of the above varieties, and (4) tacky - 
 lite, hyalomelan, or hyalobasalt, which includes the vitreous or 
 glassy varieties, the mass having cooled too rapidly to allow it to 
 assume a crystalline structure. These varieties, therefore, bear 
 the same relation to normal basalt as do the obsidians to the 
 liparites. Other varieties, though less common, are recogniz- 
 able and characterized by the presence or absence of some 
 predominating accessory mineral. We have thus quartz, horn- 
 blende, and hypersthene basalt, etc. An olivine-free variety is 
 also recognized. 
 
 The basalts are among the most abundant and wide-spread of 
 the younger eruptive rocks. Jn the United States the} r are 
 found mainly in the regions west of the Mississippi River. 
 They are eminently volcanic rocks, and occur in the form of lava 
 streams and sheets, often of great extent, and sometimes show- 
 ing a characteristic columnar structure. According to Rich- 
 thofen, the basalts are the latest products of volcanic activity. 
 A quartz-bearing basalt has been described by Mr. J. S. Diller 
 as occurring at Snag Lake, near Lassens Peak, California, and 
 which is regarded by him as a product of the latest volcanic 
 eruption within the limits of the state. This lava field covers 
 an area of only some three square miles, and trunks of trees 
 killed at the time of the eruption are still standing. 1 
 
 Under the name of melilite basalt is included a group of rocks 
 in which the mineral melilite is the characterizing constituent, 
 with accessory augite, olivine, nepheline, biotite, magnetite, 
 perowskite, and spinel. The normal structure is holocrystal- 
 line porphyritic, in which the olivine, augite, mica, or occasion- 
 ally the melilite, appear as porphyritic constituents. These are 
 rocks of very limited distribution, and at present known in 
 North America only near Montreal, Canada. Professor Rosen- 
 1 Bull. No. 79, U. S. Geol. Survey, 1891. 
 
THE THERALITE-BASANITE GROUP 93 
 
 busch, in his latest work, separates this entirely from the basalts, 
 and considers it in a group by itself under the name of Melilite 
 Rocks. 
 
 6. THE THERALITE-BASANITE GROUP 
 
 This is a small, and so far as now known, comparatively in- 
 significant group of rocks, representatives of which are confined 
 to limited and widely separated areas. They are described as 
 below : 
 
 (1) THE THERALITES 
 
 This name, derived from the Greek word Orjpav, to seek 
 eagerly, is given by Professor Rosenbusch to a class of intru- 
 sive rocks consisting essentially of plagioclase feldspar and 
 nepheline, and which are apparently the plutonic equivalents of 
 the tephrites and basanites. 
 
 The group is founded by Professor Rosenbusch upon certain 
 rocks occurring in dikes and laccolites in the Cretaceous sand- 
 stones of the Crazy Mountains of Montana, and described by 
 Professor J. E. Wolff, 1 of Harvard University. 
 
 Mineral Composition. The essential constituents as above 
 noted are nepheline and plagioclase with accessory augite, 
 olivine, sodalite, biotite, magnetite, apatite and secondary horn- 
 blende, and zeolitic minerals. 
 
 Chemical Composition. The chemical composition of a sam- 
 ple from near Martinsdale, as given by Professor Wolff, is as 
 follows: Silica, 43.175%; alumina, 15.236%; ferrous oxide, 
 7.607 % ; ferric oxide, 2.668 % ; lime, 10.633 % ; magnesia, 
 5.810%; potash, 4.070%; soda, 5.68%; water, 3.571%; 
 sulphuric anhydride, 0.94 %. 
 
 Structure. The rocks are holocrystalline granular through- 
 out. 
 
 Colors. These are dark gray to nearly black. 
 
 The theralites, so far as known, have an extremely limited 
 distribution, and in the United States have thus far been re- 
 ported only from Gordon's Butte and Upper Shields River basin 
 in the Crazy Mountains of Montana. 
 
 1 Notes on the Petrography of the Crazy Mountains and other localities in 
 Montana, by J. E. Wolff. Neues Jahrb. fur. Min., etc., 1885, I, p. 69. 
 
94 
 
 ROCKS FORMED THROUGH IGNEOUS AGENCIES 
 
 (2) THE TEPHRITES AND BASANITES 
 
 Mineral Composition. The essential constituent of the rocks 
 of this group as given by Rosenbusch are a lime-soda feldspar 
 and nepheline or leucite, either alone or accompanied by augite. 
 Olivine is, essential in basanite. Apatite, the iron ores, and 
 rarely zircon occur in both varieties. Common accessories are 
 sanidin, hornblende, biotite, hauyne, melanite, perowskite, and 
 a mineral of the spinel group. 
 
 Chemical Composition. The following is the composition of 
 
 (I) a nepheline tephrite from Antao, Pico da Cruz, Azores, and 
 
 (II) a nepheline basanite from San Antonio, Cape Verde 
 Islands, as given by Roth. 1 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 Silica (SK>2) . . . . 
 
 47 44 % 
 
 43 09 L 
 
 
 2371 
 
 17.45 
 
 
 6.83 
 
 18.99 
 
 Iron protoxide (FeO) 
 
 353 
 
 
 Magnesia (MgO) . . . 
 
 1 95 
 
 463 
 
 Lime (CaO) 
 
 647 
 
 976 
 
 Soda (Na 2 0) 
 
 6.40 
 
 502 
 
 Potash (K 2 O) 
 
 334 
 
 1 81 
 
 Water (H 2 0) 
 
 1 73 
 
 033 
 
 
 
 
 Structure. The rocks of this group are as a rule porphyritic 
 with a holocrystalline ground-mass, though sometimes there is 
 present a small amount of amorphous interstitial matter or 
 base; at times amygdaloidal. 
 
 Colors. The colors are dark, some shade of gray or brownish. 
 
 Classification and Nomenclature. According to their vary- 
 ing mineral composition Rosenbusch divides them into : 
 
 Leucite tephrite = Leucite, augite, plagioclase rocks. 
 
 Leucite basanite = Leucite, augite, plagioclase and olivine rocks. 
 
 Nepheline tephrite = Nepheline, plagioclase rocks. 
 
 Nepheline basanite = Nepheline, plagioclase and olivine rocks. 
 
 The group, it will be observed, stands intermediate between 
 the true basalts and the nephelinites to be noted later. Their 
 distribution, so far as now known, is quite limited. 
 
 1 Abhandlungen der Konig. Akad. der Wissenschaften zu Berlin, 1884, p. 64. 
 
THE PERIDOTITE-LIMBURGITE GROUP 
 
 95 
 
 7. THE PERIDOTITE-LIMBURGITE GROUP 
 
 This and the following groups include eruptive rocks in 
 which neither quartz nor feldspars of any kind longer appear 
 as essential constituents, and which are therefore very low in 
 silica, causing them to be classed as ultrabasic. Although in 
 most cases comparatively insignificant as rock masses, they are 
 peculiarly interesting as mineral aggregates, and even more on 
 account of the character of their alteration products. The 
 peridotites are further of interest in presenting the nearest 
 homologues to meteorites of any of our terrestrial rocks. The 
 group includes the plutonic peridotites '(serpentine in part), and 
 effusive picrite porphyrites and limburgites. In detail these are 
 as below : 
 
 (1) THE PERIDOTITES 
 
 Peridotite, so called because the mineral peridot (olivine) is 
 the chief constituent. 
 
 Mineral Composition. The essential constituent is olivine 
 associated nearly always with chromite or picotite and the iron 
 ores. The common accessories are one or more of the ferro- 
 magnesian silicate minerals augite, hornblende, enstatite, and 
 black mica ; feldspar is also present in certain varieties and 
 more rarely apatite, garnet, sillimanite, perowskite, and pyrite. 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 III 
 
 IV 
 
 V 
 
 VI 
 
 Silica (Si0 2 ) .... 
 
 41.58o/ 
 
 43.84 % 
 
 39.103% 
 
 42.94% 
 
 38.01 % 
 
 45.68 % 
 
 Alumina (AloOs.) . . . 
 
 0.14 
 
 1.14 
 
 4.94 
 
 10.87 
 
 5.32 
 
 6.28 
 
 Magnesia (MgO) . . . 
 
 49.28 
 
 44.33 
 
 29.176 
 
 16.32 
 
 23.29 
 
 34.76 
 
 Lime (CaO) .... 
 
 0.11 
 
 1.71 
 
 3.951 
 
 9.07 
 
 4.11 
 
 2.15 
 
 Iron sesquioxide(Fe 2 3 ) 
 
 
 8.76 
 
 4.315 
 
 3.47 
 
 6.70 
 
 9.12 
 
 Iron protoxide (FeO) . 
 
 7.49 
 
 .... 
 
 11.441 
 
 10.14 
 
 4.92 
 
 
 
 Chrome oxide (Cr 2 3 ) . 
 
 
 
 0.42 
 
 0.436 
 
 .... 
 
 
 
 0.26 
 
 Manganese (MnO) . . 
 
 .... 
 
 0.12 
 
 0.276 
 
 Trace 
 
 .... 
 
 
 Potash (K 2 O) .... 
 
 .... 
 
 .... 
 
 Trace 
 
 0.15 
 
 0.22 
 
 
 Soda (Na 2 0) 
 
 
 
 
 0.90 
 
 4.15 
 
 
 Nickel oxide (NiO) 
 
 0.34 
 
 
 
 
 
 
 Water and ignition . . 
 
 1.72 
 
 1.06 
 
 5.669 
 
 6.09 
 
 10.60 
 
 1.21 
 
 Specific gravity . . . 
 
 
 
 3.287 
 
 2.93 
 
 2.88 
 
 2.83 
 
 3.269 
 
 I. Dunite : Macon County, North Carolina. II. Saxonite : St. Paul's Rocks, 
 Atlantic Ocean. III. Picrite : Nassau, Germany. IV. Hornblende picrite : 
 Ty Cross, Anglesia. V. Picrite : Little Deer Isle, Maine. VI. Lherzolite : 
 Monte Rossi, Piedmont. 
 
96 
 
 ROCKS FORMED THROUGH IGNEOUS AGENCIES 
 
 Chemical Composition. The chemical composition varies 
 somewhat with the character and abundance of the prevailing 
 accessory. The preceding table shows the composition of 
 several typical varieties. 
 
 Structure. The structure as displayed in the different 
 varieties is somewhat variable. In the dunite it is as a rule 
 even crystalline granular, none of the olivines showing perfect 
 crystal outlines. In the picrites the augite or hornblende often 
 
 occurs in the form of 
 broad plates occupying 
 the interstices of the oli- 
 vines and wholly or par- 
 tially enclosing them, as 
 in the hornblende pic- 
 rite of Stony Point, New 
 York. The saxonites and 
 Iherzolites often show 
 a marked porphyritic 
 structure produced by 
 the development of large 
 pyroxene crystals in the 
 fine and evenly granular 
 ground-mass of olivines. 
 (See Fig. 5, as drawn by 
 Dr. G. H. Williams.) 
 The rocks belong to the 
 class designated as hypidiomorphic granular by Professor 
 Rosenbusch; that is, rocks composed only in part of minerals 
 showing crystal faces peculiar to their species. 
 
 Colors. The prevailing colors are green, greenish gray, yel- 
 lowish green, dark green to black. 
 
 Nomenclature and Classification. Mineralogically and geo- 
 logically it will be observed the peridotites bear a close resem- 
 blance to the olivine diabases and gabbros, from which they 
 differ only in the absence of feldspars. Indeed, Professor Judd 
 has shown that the gabbros and diabase both, in places, pass by 
 insensible gradations into peridotites through a gradual dimi- 
 nution in the amount of their feldspathic constituents. Dr. 
 Wadsworth would extend the term peridotite to include rocks 
 of the same composition, but of meteoric as well as terrestrial 
 origin, the condition of the included iron, whether metallic or 
 
 FIG. 5. Microstructure of porphyritic Iherzo- 
 lite, partly altered into serpentine. 
 
THE PERIDOTITES 97 
 
 as an oxide, being considered by him as non-essential, since 
 native iron is also found occasionally in terrestrial rocks, as 
 the Greenland basalts and some diabases. 
 
 In classifying the peridotites the varietal distinctions are 
 based upon the prevailing accessory mineral. We thus have : 
 
 Dunite, consisting essentially of olivine only. 
 
 Saxonite, consisting essentially of olivine and enstatite. 
 
 Picrite, consisting essentially of olivine and augite. 
 
 Hornblende picrite, consisting essentially of olivine and hornblende. 
 
 Wehrlite (or eulysite), consisting essentially of olivine and diallage. 
 
 Lherzolite, consisting essentially of olivine, enstatite, and augite. 
 
 The name Dunite was first used by Hochstetter and applied 
 to the olivine rock of Mount Dun, New Zealand. Saxonite 
 was given by Wadsworth, rocks of this type being prevalent in 
 Saxony. The same rock has since been named Harzburgite by 
 Rosenbusch. The name Lherzolite is from Lake Lherz in the 
 Pyrenees. 
 
 The peridotites are, as a rule, highly altered rocks, the older 
 forms showing a more or less complete transformation of their 
 original constituents into a variety of secondary minerals, the 
 olivine going over into serpentine or talc and the augite or 
 hornblende into chlorite. The most common result of this 
 alteration is the rock serpentine, the transformation taking 
 place through the hydration of the olivine and the liberation of 
 free iron oxides and chalcedony. (See Fig. 5.) Recent inves- 
 tigations have shown that a large share of the serpentinous 
 rocks were thus originated. The chemistry of the process has 
 been already discussed under the head of olivine, p. 24. ' 
 
 Since in this process of hydration the combined iron becomes 
 converted into the sesquioxide form, and the calcium of the 
 lime-magnesian silicates separates out in large part as free cal- 
 cite, or as mixed carbonates of lime and magnesia, so these ser- 
 pentinous rocks are rarely uniform in color or composition. 
 The prevailing color is some shade of green, though not infre- 
 quently brown, yellow, red, or nearly black. Through the 
 presence of still unaltered grains of pyroxene, many varieties 
 are porphyritic. The rock is almost universally badly jointed, 
 an evident necessary accompaniment to the alteration, and into 
 these joints have filtered the lime or magnesia carbonate solu- 
 tions, where, depositing their load, they have formed the numer- 
 
98 ROCKS FORMED THROUGH IGNEOUS AGENCIES 
 
 ous white, yellow, and greenish veins with which the stone is 
 traversed. Many varieties indeed, like the rosso de Levante, 
 verde di Pegli, and verde di Crenora of Italy, are but breccias of 
 serpeiitinous fragments cemented by calcareous and ferruginous 
 cements. 1 
 
 It is, perhaps, as yet too early to state definitely that all peri- 
 dotites are eruptive. In many instances their eruptive nature 
 is beyond dispute. Others are found in connection with the 
 crystalline schists, so situated as to suggest that they may them- 
 selves be metamorphic. 
 
 (2) THE PICRITE PORPHYRITES 
 
 Under this head is placed a small group of rocks so far as 
 now known very limited in their distribution, and which are 
 regarded as the effusive forms of the plutonic picrites, as bear- 
 ing the same relation to these rocks as do the melaphyrs to the 
 olivine diabases. The essential constituents are therefore oli- 
 vine and augite with accessory apatite, iron ores, and other 
 minerals mentioned as occurring in the true picrites. Struct- 
 urally they differ from these rocks in presenting an amorphous 
 base rather than being crystalline throughout. Rocks of this 
 type are supposed to have had an important bearing on the 
 origin of the diamond, the diamond-bearing rocks of South 
 Africa being picrite porphyrite (kimberlite) cutting highly 
 carbonaceous shales. An examination of the Kentucky peri- 
 dotite locality, where the same rock occurs under quite similar 
 conditions, failed to show that similar results have been there 
 produced, a fact which is supposed to be due in part to the 
 small amount of carbonaceous matter in the surrounding shales. 
 
 The group is very limited, and is represented in the United 
 States only in Elliott County, Kentucky ; Pike County, Arkan- 
 sas ; Syracuse, Onondaga County, New York. 
 
 (3) THE LIMBURGITES 
 
 This is a small group of lavas described by Rosenbusch in 
 1872 as occurring at Limburg, or the Kaiserstuhl in the Rhine. 
 The essential constituents are augite and olivine with the usual 
 iron ores. Structurally the rock is so far as known never holo- 
 crystalline, but glassy and porphyritic. The composition of the 
 
 1 See the Stones for Building and Decoration, Wiley & Sons, New York. 
 
THE PYROXENITE-AUGITITE GROUP 99 
 
 Prussian limburgite is given as below. So far as known, the 
 group has no representatives in the United States. 
 
 CONSTITUENTS 
 
 PER CENT 
 
 Silica (Si0 2 ) 
 
 42 24 
 
 
 18 66 
 
 Iron sesquioxide (Fe 2 03) 
 
 7 45 
 
 Magnesia (M'0) 
 
 12 27 
 
 Lime (CaO) 
 
 11 76 
 
 Soda (Na0) 
 
 4 02 
 
 Potash (K 2 O) 
 
 1 08 
 
 Water (H 2 0) 
 
 3 71 
 
 
 
 
 99.19 
 
 8. THE PYROXENITE-AUGITITE GROUP 
 
 Here are included a small group of eruptive rocks differing 
 from the last mainly in the absence of olivine as an essential 
 constituent. They are represented, so far as now known, only 
 by the plutonic pyroxenites and effusive augitites. 
 
 (1) THE PYROXENITES 
 
 Pyroxenite, a term applied by Dr. Hunt to certain rocks con- 
 sisting essentially of minerals of the pyroxene group, and which 
 occurred both as intrusive and as beds or nests intercalated with 
 stratified rocks. The author here follows the nomenclature and 
 classification adopted by Dr. G. H. Williams. 1 
 
 Mineral Composition. The essential constituents are one or 
 more minerals of the pyroxene group, either orthorhombic or 
 monoclinic. Accessory minerals are not abundant and limited 
 mainly to the iron ores and minerals of the hornblende or mica 
 groups. 
 
 Chemical Composition. The following analyses serve to show 
 the variations which are due mainly to the varying character 
 of the pyroxenic constituents : 
 
 1 American Geologist, Vol. VI, July, 1890, pp. 35-49. 
 
100 
 
 ROCKS FORMED THROUGH IGNEOUS AGENCIES 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 in 
 
 Silica (SiO 2 ) 
 
 50.80 % 
 
 53.98 % 
 
 55 14 % 
 
 Alumina (AlgOs) 
 
 340 
 
 132 
 
 066 
 
 Chrome oxide (Cr 2 8 ) 
 Ferric oxide (Fe 2 03) 
 
 0.32 
 1.39 
 
 0.53 
 1.41 
 
 0.25 
 348 
 
 Ferrous oxide (FeO) 
 
 8 11 
 
 390 
 
 4 73 
 
 Manganese (MnO) 
 
 017 
 
 021 
 
 003 
 
 Lime (CaO) 
 
 12.31 
 
 1547 
 
 839 
 
 
 22.77 
 
 22 59 
 
 2666 
 
 Soda (Na 2 0) 
 
 Trace 
 
 
 030 
 
 Potash (K 2 0) 
 
 Trace 
 
 
 
 Water (H 2 0) 
 
 052 
 
 083 
 
 038 
 
 Chlorine (Cl) 
 
 024 
 
 
 023 
 
 
 
 .... 
 
 
 
 100.03% 
 
 100.24 % 
 
 100.25% 
 
 I. Hypersthene-diallage rock : Johnny Cake Road, Baltimore County, Mary- 
 land. II. Hypersthene-diallage rock : Hebbville post-office, Baltimore County, 
 Maryland. III. Bronzite-diopside rock from near Webster, North Carolina. 
 
 Structure. The pyroxenites are holocrystalline granular 
 rocks, at times evenly granular and saccharoidal, or again 
 
 porphyritic, as in the 
 websterite 
 Carolina. 
 
 North 
 
 from 
 
 The micro- 
 scopic structure of this 
 rock is shown in Fig. 6 
 from the original draw- 
 ing by Dr. Williams. 
 
 Colors. The colors 
 are, as a rule, greenish or 
 bronze. 
 
 Classification and No- 
 menclature. The pyrox- 
 enites, it will be observed, 
 differ from the peridotites 
 only in the lack of olivine. 
 Following Dr. Williams's 
 
 .bio. 6. Microstructure of websterite, Webster. , 
 
 North Carolina. nomenclature, we have 
 
 the varieties diallagite, 
 
 bronzitite, and hypersthenite, according as the mineral diallage, 
 brohzite, or hypersthene forms the essential constituent. Web- 
 sterite is the name given to the enstatite-diopside variety, such 
 
AUGITITE 
 
 101 
 
 as occurs near Webster, North Carolina, and hornblendite to 
 the hornblende-augite variety. The pyroxenites rank, in geo- 
 logical importance, next to the peridotites. Through processes 
 of hydration and other chemical changes, these rocks pass into 
 amphibolic and steatitic masses to which the name soapstone 
 or potstone is not infrequently applied. These are dark gray 
 or greenish rocks, soft enough to be readily cut with a knife 
 and with a pronounced soapy or greasy feeling ; hence the 
 name soapstone. The name potstone was given on account of 
 their having been utilized for making rude pots, for which their 
 softness and fireproof properties render them well qualified. 
 Although it is commonly stated in the text-books that soap- 
 stone is a compact form of steatite or talc, few are even ap- 
 proximately pure forms of this mineral, but all contain varying 
 proportions of chlorite, mica, and tremolite, together with per- 
 haps unaltered residuals of pyroxene, granules of iron ore, iron 
 pyrites, quartz, and, in seams and veins, calcite and magnesian 
 carbonates. The variation in chemical composition is shown in 
 the following analyses, I being that of a compact, homogeneous- 
 appearing, quite massive variety from Alberene, in Albemarle 
 County, Virginia, and II one from Francestown, New Hamp- 
 shire. 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 Silica (Si0 2 ) 
 
 39.06% 
 
 42.43% 
 
 Alumina (AloOs) 
 
 12.84 
 
 6.08 
 
 Ferric and ferrous iron (Fe203) and (FeO) .... 
 Lime (CaO) 
 
 12.90 
 5.98 
 
 13.07 
 3.27 
 
 Magnesia (IVIgO) 
 
 22.76 
 
 25.71 
 
 Potash (KoO) . . . 
 
 0.19 
 
 0.32 
 
 Soda (Na 2 0) . . 
 
 0.11 
 
 0.16 
 
 Ignition 
 
 6.56 
 
 8.45 
 
 
 
 
 
 100.40% 
 
 99.49% 
 
 (2) AUGITITE 
 
 The effusive form, augitite, differs from the pyroxenite proper 
 mainly on structural grounds. In common with many lavas it 
 has a glassy base, in which are embedded the crystals of augite 
 and iron ores. The composition of an augitite from the Cape 
 Verde Islands, as given by Roth, is as below : 
 
102 ROCKS FORMED THROUGH IGNEOUS AGENCIES 
 
 CONSTITUENTS 
 
 PER CENT 
 
 Silica (SK>2) . ... 
 
 41 83 
 
 Alumina (A^Oa) . . . 
 
 18 60 
 
 
 16.11 
 
 Magnesia (MsjO) . 
 
 4 98 
 
 Lime (CaO) . ... 
 
 11 83 
 
 Soda (Na 2 0) 
 
 4 70 
 
 Potash (K 2 O) 
 
 2.47 
 
 Water (H 2 0) 
 
 91 
 
 
 
 
 101.43 
 
 9. THE LEUCITE-NEPHELINE ROCKS 
 
 Under this head are grouped two small but interesting groups 
 of effusive rocks, having, so far as known, no exact equivalent 
 among the plutonics, and characterized by the presence of leu- 
 cite or nepheline as essential constituents and which here seem 
 to play the role of feldspars. In detail they are as below: 
 
 (1) THE LEUCITE ROCKS 
 
 Mineral Composition. The essential constituent is leucite 
 and a basic augite. A variety of accessories occur, including 
 biotite, hornblende, iron ores, apatite, olivine, plagioclase, nephe- 
 line, melilite, and more rarely garnets, hauyne, sphene, chromite, 
 and perowskite. Feldspar as an essential fails entirely. 
 
 Chemical Composition. The average chemical composition as 
 given by Blaas 1 is as follows : Silica, 48.9 % ; alumina, 19.5 % ; 
 iron oxides, 9.2%; lime, 8.9%; magnesia, 1.9%; potash, 
 6.5%; soda, 4.4%. 
 
 Structure. The rocks of this group are, as a rule, fine 
 grained and only slightly vesicular, presenting to the unaided 
 eye little to distinguish them from the finer-grained varieties 
 of ordinary basalt. 
 
 Colors. The prevailing colors are some shades of gray, 
 though sometimes yellowish or brownish. 
 
 Classification and Nomenclature. The varietal distinctions 
 are based upon the presence or absence of the mineral olivine 
 
 1 Katechismus der Petrographie, p. 117. 
 
THE NEFHELINE ROCKS 
 
 103 
 
 and upon structural grounds and various minor characteristics. 
 We have the olivine-free variety leucitite and the olivine-holding 
 variety leucite basalt. 
 
 These rocks have also a very limited distribution, and, so far 
 as known, are found within the limits of the United States only 
 at the Leucite Hills, Wyoming. 
 
 (2) THE NEPHELINE ROCKS 
 
 Mineral Composition. These rocks consist essentially of 
 nepheline with a basaltic augite and accessory sanidin, pla- 
 gioclase, mica, olivine, leucite, minerals of the sodalite group, 
 magnetite, apatite, perowskite, and melanite. 
 
 Chemical Composition. Below is given the composition of 
 (I) a nephelinite from the Cape Verde Islands, and (II) a 
 nepheline basalt from the Vogelsberg, Prussia. 1 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 Silica (Si0 2 ) 
 
 46 95 % 
 
 42 37 / 
 
 Alumina (Al 2 0s) 
 
 21.59 
 
 8 88 
 
 Iron sesquioxide (I^Og) 
 Iron protoxide (FeO) 
 
 8.09 
 
 11.26 
 
 7 80 
 
 Magnesia (MsjO) . . . 
 
 2 49 
 
 13 01 
 
 Lime (CaO) ... 
 
 7 97 
 
 10 93 
 
 Soda (Na 2 0) .... 
 
 8.93 
 
 4 51 
 
 Potash (K 2 0) ... . . 
 
 2.04 
 
 1.21 
 
 Water (H 2 0) 
 
 2.09 
 
 0.34 
 
 
 
 3.103 
 
 
 
 
 Colors. The prevailing colors are various shades of gray 
 to nearly black. 
 
 Structure. Structurally they are porphyritic, with a holo- 
 crystalline or in part amorphous base, usually fine grained and 
 compact, at times amygdaloidal. 
 
 Classification and Nomenclature. These rocks differ from the 
 basalts, which they otherwise greatly resemble, in that they bear 
 the mineral nepheline in place of feldspar. Based upon the 
 presence or absence of olivine, we have, first, nepheline basalt, 
 
 1 Roth, Abhandl. der Konig. Preus. Akad. der Wiss. zu Berlin, 1884. 
 
104 ROCKS FORMED THROUGH IGNEOUS AGENCIES 
 
 and second, nephelinite. The name nepheline dolerite has been 
 given in some cases to the coarser, holocrystalline, olivine- 
 bearing varieties. 
 
 Like the leucite rocks, the members of this group are some- 
 what limited in their distribution. 
 
II. AQUEOUS ROCKS 
 1. ROCKS FORMED THROUGH CHEMICAL AGENCIES 
 
 This comparatively small, though by no means unimportant, 
 group of rocks comprises those substances which, having once 
 been in a condition of aqueous solution, have been deposited as 
 rock masses either by cooling, evaporation, by a diminution of 
 pressure, or by direct chemical precipitation. It also includes 
 the simpler forms of those produced by chemical changes in 
 pre-existing rocks. Water, when pure or charged with more 
 or less acid or alkaline material, and particularly when acting 
 under great pressure, is an almost universal solvent. Thus, 
 heated alkaline waters, permeating the rocks of the earth's 
 crust at great depths below the surface, are enabled to dis- 
 solve from them various mineral matters with which they come 
 in contact. On coming to the surface or flowing into crevices, 
 the pressure is diminished, or evaporation takes place, and the 
 water, no longer able to carry its load, deposits it wholly or in 
 part as vein material or a surface coating. In other cases alka- 
 line or acid water, bearing mineral matters, may, in course of 
 their percolations, be brought in contact with neutralizing solu- 
 tions, and these dissolved materials be thus deposited by direct 
 precipitation. In these various ways were formed the rocks 
 here described. It will be observed that the various members 
 of the group are composed mainly of minerals of a single species 
 only. 
 
 This group cannot, however, be separated by any sharp lines 
 from that which is to follow, inasmuch as many rocks are not 
 the product of a single agency, acting alone, but are rather the 
 result of two or more combined processes. This is especially the 
 case with the limestones. It is safe to assume that few of these 
 are due wholly to accumulations of calcareous, organic remains, 
 but are, in part at least, chemical precipitates, as is well illus- 
 trated by the oolitic varieties. 
 
 105 
 
106 AQUEOUS ROCKS 
 
 According to their chemical nature, the group is divided 
 into (1) Oxides, (2) Carbonates, (3) Silicates, (4) Sulphates, 
 (5) Phosphates, (6) Chlorides, and (7) the Hydrocarbon Com- 
 pounds. 
 
 (l) OXIDES 
 
 Here are included those rocks consisting essentially of oxygen 
 combined with a base, though usually other constituents are 
 present as impurities. 
 
 Hematite. Anhydrous sesquioxide of iron. Fe 2 O 3 = oxy- 
 gen, 30 % ; iron, 70 %. In nature nearly always more or less im- 
 pure through the mechanical admixture of argillaceous silicates 
 or calcareous matter, manganese oxides, sulphur, phosphates, 
 etc. Several forms are recognized, the distinction being based 
 mainly upon physical properties. Specular hematite is a mica- 
 ceous or foliated variety with a black, metallic, often splendent 
 lustre ; this variety is mainly a metamorphic form, and prop- 
 erly should be classed with the metamorphic rocks. Compact, 
 columnar, fibrous, and earthy forms also occur, the latter often 
 known as ochre, as are similar forms of limonite. Although 
 classified here under the head of aqueous rocks, it does not 
 follow that the hematites have all originated in precisely the 
 same manner. To a limited extent the specular variety is found 
 about volcanic craters and fumaroles, where it was originally 
 deposited by a process of sublimation. Through a process of 
 oxidation, beds of magnetic iron become locally altered into 
 hematite, giving rise to pseudomorphous granular, octahedral, 
 and dodecahedral forms, to which the name martite is given. 
 Many extensive beds undoubtedly arise from the dehydration 
 by dynamic agencies the folding and metamorphosing of the 
 enclosing rocks of beds of limonite. Others, like the fossil 
 and oolitic ores of the Clinton formations, arise in part from a 
 process of chemical precipitation and subsequent segregation, 
 the ore being originally disseminated throughout a ferruginous 
 limestone, and having accumulated as an insoluble residue as 
 the lime carbonate was carried away through the action of car- 
 bonated waters. The extensive hematite deposits of the Lake 
 Superior region of Michigan are regarded as oxidation prod- 
 ucts from pre-existing carbonates (siderite), the oxide having 
 been precipitated from solution in synclinal troughs, and subse- 
 quently crystallized by metamorphism. 1 The ores of the Mesabi 
 
 1 Van Hise Monograph XIX, U. S. Geol. Survey, 1892. 
 
FIG. 1. Botryoidal hematite. 
 
 FIG. 2. Clay-iron stone septarian nodule. 
 
OXIDES 107 
 
 range, on the other hand, are regarded by at least one writer 
 as having originated through a somewhat complicated process 
 of oxidation and metasomatosis, whereby a pre-existing glauco- 
 nitic rock (a ferruginous silicate) became converted into an 
 admixture of free iron oxide and silica, the one or the other, 
 according to the intermittent character of the permeating solu- 
 tions, being leached out and redeposited at no great distance in 
 a fair condition of purity. 1 A discussion of this subject belongs 
 more properly to economic geology, and need not be dwelt 
 upon further here. 
 
 Limonite (Brown Hematite). Iron sesquioxide plus water. 
 H 6 Fe 2 O 6 + Fe 2 O 3 . An earthy or compact dark brown, black, 
 or ochreous-yellow rock, containing, when pure, about two- 
 thirds its weight of pure iron. It occurs in beds, veins, and 
 concretionary forms, associated with rocks of all ages, and 
 forms a valuable ore of iron. (See Fig. 1, PL 9.) On the bot- 
 toms of lakes, bogs, and marshes it often forms in extensive 
 deposits, where it is known as bog-iron ore. The formation of 
 these deposits is described as follows : Iron is widely diffused 
 in rocks of all ages, chiefly in the form of (1) the protoxide, 
 which is readily soluble in waters impregnated with carbonic 
 or other feeble acids, or (2) the peroxide, which is insoluble in 
 the same liquids. Water percolating through the soils becomes 
 impregnated with these acids from the decomposing organic 
 matter, and then dissolves the iron protoxide with which it 
 comes in contact. On coming to the surface and being exposed 
 to the air, as in a stagnant lake or marsh, this dissolved oxide 
 absorbs more oxygen, becoming converted into the insoluble 
 sesquioxide, which floats temporarily on the surface as an oil- 
 like, iridescent scum. Finally this sinks to the bottom, where 
 it gradually becomes aggregated as a massive iron ore. This 
 same ore may also form through the oxidation of pyrite, or 
 beds of ferrous carbonate. At the Ktaadn Iron Works, in 
 Piscataquis County, Maine, the ferrous salt as it oxidizes is 
 brought to the surface by water and deposited as a coating 
 over the leaves and twigs scattered about, forming thus beauti- 
 fully perfect casts, or fossils. 
 
 Pyrolusite, Psilomelane, and Wad. These are names given 
 to the anhydrous and more or less hydrated forms of manganese 
 
 1 J. E. Spurr, Bull. No. 10, Geol. and Nat. Hist. Survey of Minnesota, 1894. 
 
108 AQUEOUS ROCKS 
 
 oxides, and which, though wide in their distribution, are found 
 in such abundance as to constitute rock masses in comparative 
 rarity. The origin of such deposits is at times somewhat ob- 
 scure. In all cases they are doubtless secondary. The original 
 source of the material appears to have been the manganiferous 
 silicates of Archsean and more recent eruptive rocks, whence it 
 was derived by leaching, being transported in the form of 
 soluble salts and finally precipitated as oxide or carbonate, the 
 latter being subsequently converted into oxide. The deposits 
 which are of sufficient extent to be of commercial value occur 
 as a rule in residual clays, as interbedded strata in shales and 
 sandstones, or as occupying superficial seams and joints, and 
 in the form of pockets and nests. True fissure veins of man- 
 ganese oxide are not known. It is often associated with the 
 form of limonite known as bog-iron ore, and, apparently, has 
 been deposited contemporaneously. 
 
 Beauxite (so called from Beaux, near Aries, France) is the 
 name given to a somewhat indefinite mixture of alumina and 
 iron oxides, and occurring in the form of compact concretion- 
 ary grains of a dull red, brown, or nearly white color, and 
 also in compact and earthy forms. The mode of occurrence of 
 the mineral is somewhat variable. At Beaux and several other 
 localities it occurs in pockets in limestone, and also in beds 
 alternating with limestones, sandstones, and clays belonging 
 to the Cretaceous period. In the Puy-de-D6me the beds rest 
 directly upon gneiss, and are overlaid by basalt. At Oberhes- 
 sen, Germany, the mineral occurs in rounded masses embedded 
 in clay, as is also the case at Vogelsberg. In America, beaux- 
 ite has been found in Alabama, Georgia, and Arkansas. In 
 Alabama and Georgia it occurs in beds of irregular extent, 
 associated with limestones of Upper Cambrian age (the Knox 
 dolomite); in Arkansas the deposits are Tertiary. 
 
 The origin of the beauxite is somewhat obscure. It has been 
 argued that the beds at Beaux, and those of Var, are deposits 
 from mineral springs. Those of the Puy-de-D6me, the West- 
 erwald, Vogelsberg, and of Ireland, on the other hand, are 
 regarded as derived from basalt by a metasomatic process. 
 The Alabama and Georgia deposits, like those of Beaux, are 
 regarded as of chemical origin. 1 
 
 1 See resume of the subject, by R. L. Packard, in Mineral Resources of the 
 United States for 1891. 
 
OXIDES 
 
 109 
 
 According to C. Willard Hayes, 1 the prevailing rocks of this 
 region are dolomites underlaid by aluminous shales. It is 
 assumed that heated waters, in their passage upward from 
 greater depths, have oxidized the iron sulphides of the shale, 
 giving rise to sulphates of iron, of alumina, and the double 
 sulphates of alumina and potash. As the ascending water, 
 carrying these salts in solution, passes through the dolomite, it 
 becomes charged with calcium carbonate, which causes the pre- 
 cipitation of the aluminum salts in the concretionary, pisolitic 
 form so characteristic. 
 
 Beauxite has, of late, come to be of considerable economic 
 value as an ore of aluminum, and as a source of alum, in place 
 of clay. 
 
 The material from various sources varies greatly in chemical 
 composition, as shown by the following analyses : 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 III 
 
 IV 
 
 V 
 
 Silica (Si0 2 ) 
 
 2.8% 
 
 1 10 % 
 
 21 08 % 
 
 2 80 % 
 
 10 38 / 
 
 Alumina (A1 2 3 ) . . . 
 Iron sesquioxide (Fe 2 C>3) . 
 Water (H 2 0) 
 
 57.6 
 25.3 
 
 10.08 
 
 50.92 
 15.70 
 27.75 
 
 48.92 
 2.14 
 23.41 
 
 52.21 
 13.50 
 
 27.72 
 
 55.64 
 1.95 
 27.62 
 
 Titanium oxide (TiO 2 ) 
 
 3.1 
 
 3.20 
 
 2.52 
 
 3.52 
 
 3.50 
 
 I. Beaux, France. II. Vogelsberg, Germany. III. Jacksonville, Alabama. 
 IV. Floyd County, Georgia. V. Pulaski County, Arkansas. 
 
 Silica. Silica, as has been already noted under the head of 
 rock-forming minerals, is one of the most abundant constituents 
 of the earth's crust. In its various forms, which are sufficiently 
 extensive to constitute rock masses, it is always of chemical 
 origin, that is, results by deposition from solution, by precipi- 
 tation, or evaporation, as noted above. Varietal names are 
 given to the deposits, dependent upon their structure, method 
 of formation, color, and degree of purity. Siliceous sinter, 
 geyserite, or fiorite is the name given to the nearly white, 
 often soft and friable, hydrated varieties formed on the evapo- 
 ration of the siliceous waters of hot springs and geysers, or 
 through the eliminating action of algous vegetation, as de- 
 scribed by W. H. Weed in the reports of the United States 
 
 1 Trans. Am. Inst. of Mining Engineers, February, 1894. 
 
110 AQUEOUS ROCKS 
 
 Geological Survey. 1 The material is, in reality, an impure 
 form of opal. Throughout the geyser regions of the Yellow- 
 stone Park, Iceland, and New Zealand, the sinter has been 
 deposited as a comparatively thin crust over the surface, or in 
 the form of cones about the throats of the geysers. The vari- 
 eties of silica known as opal are hydrous forms occurring in 
 veins and pockets, in a variety of rocks. Not infrequently it 
 forms the replacing material in silicified or " petrified " woods. 
 In the old lake beds of the Madison valley, Montana, may not 
 infrequently be found large logs composed wholly of this mate- 
 rial, no sign of organic matter remaining, but yet with the 
 woody structure beautifully preserved. 
 
 The origin of these silicified logs, so far as it has been traced, 
 appears to have been somewhat as follows : The water which 
 permeated the lake beds in which these logs lay, was more or 
 less alkaline, and carried small amounts of silica in solution. 
 As the logs slowly decayed, there were given off minute quan- 
 tities of organic acids which, neutralizing the alkaline water, 
 caused a gradual precipitation of the silica, building up thus an 
 exact cast of the decaying structure. Chalcedony is the trans- 
 lucent, massive, c^ptocrystalline variety of silica occurring 
 mainly in cavities in older rocks, where it has been deposited 
 by infiltration. It is a common secondary product formed 
 during the decomposition of many rocks, and, like opal, not 
 infrequently forms the petrifying medium of fossil woods and 
 other organisms. Not infrequently, also, it occurs in continu- 
 ous layers of several inches in thickness, interstratified with 
 limestone, as may be seen in the walls of the Wyandotte caves 
 in southern Indiana, or, more rarely, in beds from 2 to 8 feet 
 thick, interstratified with coal and fire-clay, as at the well- 
 known " Flint Ridge " of Licking County, Ohio. Such depos- 
 its are considered to be due to accumulations of the siliceous 
 tests of diatoms. Flint is a variety of chalcedony formed by 
 segregation in chalky limestone, and is composed, in part, of 
 the broken and partially dissolved spicules of sponges, and 
 the siliceous casts of infusoria. The source of the silica is, 
 doubtless, the sponge spicules above noted and diatomaceous 
 remains. Chert is an impure flint containing not infrequently 
 fossil nummulitic remains, and with sometimes a pronounced 
 
 1 9th Ann. Rep. U. S. Geol. Survey, 1887-88. See also Bischof s Chemical 
 and Physical Geology, Vol. I, pp. 184-200. 
 
CARBONATES 111 
 
 oolitic structure. It occurs in rounded, nodular, concretionary 
 masses interbedded with limestones, particularly Palaeozoic vari- 
 eties, and doubtless originated as did the flints in the chalky 
 limestones. Jasper is a dull or bright red, or yellow variety 
 of chalcedony containing alumina, and owing its color to iron 
 oxides. It is sometimes used in jewellery. 
 
 The name novaculite is frequently given to very fine-grained 
 and compact quartz rocks, such as are suitable for hones. As 
 commonly used, the name is made to include rocks of widely 
 different origin, some of which are evidently chemical precipi- 
 tates, while others are indurated clastic or schistose rocks. The 
 well-known novaculites of Arkansas are clear white masses of 
 chalcedonic silica, containing scattering quartz granules, minute 
 grains of garnet, and numerous small rhomboidal cavities which 
 seemingly were once occupied by crystals of calcite or dolomite. 
 Opinions differ as to the origin of this rock. Owen. 1 regarded 
 it as a sandstone metamorphosed by percolating hot water. 
 Branner 2 looked upon it as a metamorphosed chert ; Griswold, 3 
 as a chemical deposit in the form of a siliceous slime on a sea- 
 bottom, while Rutley 4 argues that it is but a siliceous replace- 
 ment of beds of dolomite or dolomitic limestone. It seems 
 probable that the views of Branner or Rutley are the most 
 nearly correct. 
 
 Quartz is a massive form of crystalline silica occurring in 
 veins, disseminated granules, and pockets in rocks of all kinds 
 and all ages. It is one of the most wide-spread and commonest 
 of minerals, and is frequently quarried and crushed for abrasive 
 purposes or use in pottery manufacture. It is not infrequently 
 of a pink or rose color from metallic oxides. It is a common 
 gangue of ores of the precious metals, particularly of gold. 
 Lydian stone is an exceedingly hard impure quartz rock, of a 
 black color and splintery fracture. It was formerly much 
 used in testing the purity of precious metals. 
 
 (2) CARBONATES 
 
 Water carrying small amounts of carbonic acid readily dis- 
 solves the calcium carbonate of rocks with which it comes in 
 
 1 2d Rep. Geological Reconnaissance of Arkansas, 1860. 
 
 2 Ann. Rep. Geol. Survey of Arkansas, Vol. I, 1886, p. 49. 
 
 3 Ann. Rep. Geol. Survey of Arkansas, Vol. Ill, 1890. 
 
 4 Quarterly Journal Geological Society of London, August, 1894. 
 
112 
 
 AQUEOUS ROCKS 
 
 contact ; on evaporation and through loss of a portion of the 
 carbonic acid, this is again deposited. In this way are formed 
 numerous and at times extensive deposits, to which are given 
 varietal names dependent upon their structure and the special 
 conditions under which they originated. Gale sinter or tufa is 
 a loose friable deposit made by springs and streams either by 
 evaporation or through intervention of algous vegetation. Such 
 are often beautifully arborescent and of a snow-white color, as 
 seen at the Mammoth Hot Springs of the Yellowstone National 
 Park. Somewhat similar deposits are formed by springs in 
 Virginia, California, Mexico, New Zealand. Others, like those 
 from Niagara Falls, New York, and Soda Springs, Idaho, were 
 formed by the deposition of the lime on leaves and twigs, form- 
 ing beautifully perfect casts of these objects. 
 
 Tufa deposits of peculiar imitative shapes have been described 
 by Mr. I. C. Russell of the United States Geological Survey, 
 as formed by the evaporation of the waters of Pyramid Lake, 
 
 Nevada. Oolitic and pi- 
 solitic limestones are so 
 called on account of their 
 rounded, fish - egg - like 
 structure, the word oolite 
 being from the Greek 
 word (oov, an egg. (See 
 PI. 12.) These are in 
 part chemical and in part 
 mechanical deposits. The 
 water in the lakes and 
 seas in which they were 
 formed became so satu- 
 rated that the lime was 
 deposited in concentric 
 coatings about the grains 
 of calcareous sand on the 
 bottom, and finally the little granules thus formed became 
 cemented into firm rock by the further deposition of lime in 
 the interstices. This structure will be best understood by 
 reference to Fig. 7. Rocks of this nature are now forming 
 along the beaches of Pyramid Lake. Concerning the occur- 
 rence of these Mr. Russell writes : 
 
 " Among The Needles the rocky capes are connected by cres- 
 
 FIG. 7. Microstructure of oolitic limestone. 
 
CARBONATES 113 
 
 cent-shaped beaches of clean, creamy sands, over which the 
 summer surf breaks with soft murmurs. These sands are oolitic 
 in structure, and are formed of concentric layers of carbonate 
 of lime which is being deposited near where the warm springs 
 rise in the shallow margin of the lake. In places these grains 
 have increased by continual accretion until they are a quarter 
 of an inch or more in diameter, and form gravel, or pisolite, as 
 it would be termed by mineralogists. In a few localities this 
 material has been cemented into a solid rock, and forms an 
 oolitic limestone sufficiently compact to receive a polish. No 
 more attractive place can be found for the bather than these 
 secluded coves, with their beaches of pearl-like pebbles, or the 
 rocky capes, washed by pellucid waters, that offer tempting 
 leaps to the bold diver." 
 
 Such forms as these may or may not show a nucleus. It 
 seems safe to assume that such a nucleus, at first, in all cases 
 existed, though it may be in microscopic dimensions only. 
 
 Travertine is a compact and usually crystalline deposit formed, 
 like the tufas, by waters of springs and streams. The traver- 
 tines are often beautifully veined and colored by metallic oxides 
 and form some of the finest marbles. Such are the so-called 
 "onyx marbles" of Mexico and Arizona. 1 
 
 Stalactite and stalagmite are the names given to the deposits 
 formed from the roofs and on the floors of caves ; water, perco- 
 lating through the limestone roof, by virtue of the carbonic acid 
 it contains, dissolves out a small amount of the lime, which, on 
 evaporation, is again deposited either as pendent cones from 
 the ceiling, or as massive and pillar-like forms upon the floor. 
 The pendants are known as stalactites ; the corresponding 
 growths upon the floor as stalagmites. Stalactite and stalag- 
 mite sometimes meet, forming thus continuous pillars, or col- 
 umns extending from floor to ceiling. The lime of these 
 deposits, it may be said, is as a rule in the form of calcite, 
 though sometimes, as in the old portions of the Wyandotte 
 caves in Indiana, it is aragonite. The so-called " oriental ala- 
 baster " of the ancients is a stalagmitic deposit derived in part 
 from crevices and pockets in the Eocene limestones of the Nile 
 valley. 
 
 Magnesite, a carbonate of magnesia, occurs frequently as a 
 
 1 The Onyx Marbles, Ann. Rep. U. S. National Museum for 1893. Also 
 Stones for Building and Decoration, Wiley Sons, New York, 2d ed., p. 120. 
 
114 AQUEOUS ROCKS 
 
 secondary mineral in the form of veins in serpentinous rocks, 
 but rarely itself forms rock masses of any importance. Rhodo- 
 chrosite, a carbonate of manganese, sometimes occurs in rock 
 masses, but is found most commonly in the form of veins asso- 
 ciated with ores of silver, lead, or copper. 
 
 Another carbonate, less common than that of lime, but which 
 sometimes occurs in such quantities as to constitute true rock 
 masses, is siderite, or carbonate of iron. A common form of 
 this is dull brownish or nearly black in color, very compact and 
 impure, containing varying amounts of calcareous, clayey, and 
 organic matter. In this condition it is found in stratified beds 
 and in the shape of rounded and oval nodules, or concretions, 
 which are called clay -ironstone nodules, septaria, and sphcero- 
 siderite. (See Fig. 2, PI. 9.) These septarian nodules are 
 often beautifully veined with calcite, and when cut and polished 
 form not undesirable objects of ornamentation. Other forms of 
 siderite are massive, coarsely crystalline, and of a nearly white 
 or yellowish color, becoming brownish on exposure. Pure sider- 
 ite yields about 48 % metallic iron, and is of value as an ore. 
 
 (3) SILICATES 
 
 Silica, combined with magnesia and water, gives rise to an 
 interesting group of serpentinous and talcose substances, which 
 are often sufficiently abundant to constitute rock masses. Pure 
 serpentine consists of about equal parts of silica and magnesia, 
 with from 12 to 13 % of water. It is a compact, amorphous, or 
 colloidal rock, soft enough to be cut with a knife, with a slight 
 greasy feeling and lustre, and of a color varying from dull 
 greenish and almost black, through all shades of yellow, brown- 
 ish, and red. It also occurs in fibrous and silky forms, filling 
 narrow veins in the massive rocks, and is known as amianthus, 
 or chrysotile. These fibres, when sufficiently long, are used for 
 the manufacture of fireproof material, and the mineral is com- 
 mercially confounded with asbestos, a fibrous variety of amphi- 
 bole. It is very doubtful if serpentine is ever an original 
 rock ; it is rather an alteration product of other and less stable 
 magnesian minerals. Here will be considered only those which 
 have originated by a series of chemical changes known as meta- 
 somatosis, a process of indefinite substitution and replacement, 
 in simple mineral aggregates occurring associated with the 
 
SILICATES 
 
 115 
 
 older metamorphic rocks. Such are the serpentines derived 
 from non-aluminous pyroxenes, like those of Montville, New 
 Jersey, and Moriah, New York, and those from Easton, Penn- 
 sylvania, derived from a massive tremolite rock. The analyses 
 given below will serve to illustrate the chemical changes which 
 occur in this process of metasomatosis, I being that of a nearly 
 white pyroxene, and II that of the serpentine derived therefrom. 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 Silica (8162) 
 
 54.215 L 
 
 42 38 % 
 
 Magnesia (MgO) . 
 
 19.82 
 
 42.14 
 
 Lime (CaO) 
 
 24 71 
 
 00 
 
 Alumina (AloOa^ 
 
 59 
 
 07 
 
 Ferric oxide (Fe20s^ 
 
 0.20 
 
 97 
 
 Ferrous oride (FeO) 
 
 0.27 
 
 17 
 
 Ignition (H 2 0) . 
 
 0.14 
 
 14.20 
 
 
 
 
 
 99.945% 
 
 99.85 % 
 
 The pyroxene, it should be observed, occurs in nodular 
 masses in a crystalline granular dolomite. Various stages of 
 the process are shown in Fig. 8, in 
 which the white and gray central por- 
 tions are nucleal masses of unchanged 
 pyroxenes, surrounded by the darker 
 crusts of secondary serpentine. 1 Ser- 
 pentine as an alteration product of the 
 mineral chondrodite is also known to 
 occur, though this form is less common. 
 At Brewster, New York, are extensive 
 deposits of this nature. (See further 
 on, p. 158.) 
 
 Several varieties of serpentine are 
 
 popularly recognized. Precious or noble Fm g _ Pyroxene partiall y 
 serpentine is simply a very pure com- altered to serpentine, 
 pact variety of a deep oil-yellow or 
 
 green color. Amianthus, or chrysotile, as noted above, is the 
 name given to the fibrous variety. Williamsite is a deep bright 
 green, translucent, and somewhat scaly variety, occurring asso- 
 
 1 See On the Serpentine of Montville, New Jersey, Proc. U. S. National 
 Museum, Vol. XI, 1888, p. 105. 
 
116 AQUEOUS ROCKS 
 
 elated with the chrome iron deposits in Fulton township, Lan- 
 caster County, Pennsylvania. Deiveylite is a hard, translucent 
 variety occurring in veins in altered dunite beds. Bowenite is 
 a pale green variety forming veins in limestone at Smithfield, 
 Rhode Island. Picrolite, marmolite, and retinolite are varieties of 
 minor importance. Serpentine alone, or associated with calcite 
 and dolomite, forms a beautiful marble, to which the names verd 
 antique, ophite, and ophiolite are given. The so-called Eozoon 
 Canadense, a supposed fossil rhizopod, is a mixture of serpen- 
 tine and calcite or dolomite. The name serpentine is from the 
 Latin serpentinus, a serpent, in allusion to its green color and 
 often mottled appearance. 
 
 Those serpentines which were derived from basic eruptives, 
 or complex metamorphic rocks are described with those rocks 
 with which, in their unaltered state, they would naturally be 
 grouped. 
 
 The mineral steatite, or talc, when pure, differs from ser- 
 pentine in containing 63.5 % of silica, 31.7% of magnesia, and 
 4.8 % of water. Its common form is that of white or greenish 
 inelastic scales, forming an essential constituent of the talcose 
 schists. As is the case with serpentine, it sometimes results 
 from the alteration of eruptive magnesian rocks, such as the 
 pyroxenites, and rarely occurs as a direct result of precipita- 
 tion. It will be described more fully under the head of schists 
 and pyroxenites. Rensselaerite is a closely related rock of a 
 white or gray color, found in St. Lawrence County, New York. 
 Its composition is essentially that of talc. 
 
 Pyrophyllite, or agalmatolite, is a hydrous silicate of alumina, 
 somewhat harder than talc, which it otherwise resembles, and 
 which is used in making slate pencils and small images. It 
 occurs in a schistose form in the Deep River region of North 
 Carolina. 
 
 Kaolin, also a hydrous silicate of alumina, is a chemical 
 product in that it is a residue left by the chemical decomposi- 
 tion of the feldspars. These minerals, as explained elsewhere, 
 consist of silicates of alumina and lime, with more or less of 
 the alkalies potash and soda, and iron oxides. In the process 
 of decomposition new compounds are formed, the more soluble 
 of which are leached out, leaving the less soluble silicates, 
 including kaolin, behind in a condition of more or less purity. 
 The mineral is of great value for fictile purposes, and is de- 
 
SULPHATES 117 
 
 scribed more fully under the head of argillaceous fragmental 
 rocks. 
 
 (4) SULPHATES 
 
 Gypsum. The rock gypsum is chemically a hydrous sul- 
 phate of lime, that is to say, consists of sulphur, lime, and 
 water, and in the proportion of 32.6 parts of lime and 20.9 
 parts of water, combined with 46.5 parts of sulphur trioxide. 
 When crystallized, the mineral is nearly colorless and trans- 
 parent, and splits readily into thin, inelastic sheets. The com- 
 pact massive varieties are white, gray to black, and sometimes 
 pink from various impurities. The most characteristic feature 
 is its softness, which is such that it can be readily cut with a 
 knife or even by the thumbnail. 
 
 Four varieties of gypsum are recognized : (1) The common 
 massive form, dull in color and often more or less impure; 
 (2) the pure white, fine-grained variety, alabaster; (3) the 
 fibrous variety, satin spar ; and (4) the broadly foliated, trans- 
 parent variety, selenite, so called from the Greek word o-eXe^e, 
 the moon, in allusion to its soft and pleasing lustre. 
 
 The following is an analysis of a commercial gypsum from 
 Ottawa County, Ohio, as given by Professor Orton : l 
 
 Lime (CaO) 32.52% 
 
 Sulphuric acid (S0 3 ) 45.56 
 
 Water (H 2 0) 20.14 
 
 Magnesia (MgO) 0.56 
 
 Alumina (A1 2 3 ) 0.16 
 
 k Insoluble residue 0.68 
 
 99.62 % 
 
 Gypsum occurs mainly associated with stratified rocks, and 
 is regarded as a chemical deposit resulting from the evapora- 
 tion of waters of inland seas and lakes ; it may also originate 
 through the decomposition of sulphides and the action of the 
 resultant sulphuric acid upon limestone ; through the mutual 
 decomposition of the carbonate of lime (limestone) and the sul- 
 phates of iron, copper, and other metals ; through the hydration 
 of anhydrite ; and through the action of sulphurous vapors and 
 solutions from volcanoes upon the rocks with which they come 
 in contact. According to Dana, 2 the gypsum deposits in western 
 
 1 Geology of Ohio, 1888, Vol. VI, p. 700. 
 
 2 Manual of Geology, p. 234. 
 
118 AQUEOUS ROCKS 
 
 New York do not form continuous layers in the strata, but lie 
 in embedded, sometimes nodular, masses in limestones. In all 
 such cases this authority says the gypsum is the product of the 
 action of sulphuric acid from springs upon the limestone. " The 
 sulphuric acid, acting on the carbonate of lime, drives off its 
 carbonic acid and makes sulphate of lime or gypsum ; and this 
 is the true theory of its formation in New York." W. C. Clarke, 
 however, regards it as a product of deposition from solution in 
 sea- water. 1 
 
 The gypsum deposits of northern Ohio form apparently con- 
 tinuous beds over thousands of square miles, and are regarded 
 by Professors Newberry and Orton as deposits from the evapo- 
 ration of landlocked seas at the same time as was the rock-salt 
 which overlies it. 
 
 Geological Age and Mode of Occurrence. As may be readily 
 inferred from what has gone before, beds of gypsum have 
 formed at many periods of the earth's history, and are still 
 forming wherever proper conditions exist. 
 
 In New York there are extensive deposits belonging to the 
 Salina period of the Upper Silurian. In Ohio, gypsum asso- 
 ciated with limestones and shales of Lower Helderbergage occur 
 over areas comprising thousands of square miles. The follow- 
 ing section of beds in Ottawa County, this state, will serve to 
 show the conditions under which the rock may occur : 
 
 Drift clays 12 to 14 feet 
 
 Gray rock carrying impure gypsum - . 5 to 14 feet 
 
 Blue shale % to 14 feet 
 
 Boulder bed carrying gypsum embedded in shaly limestone . . . o to 14 feet 
 
 Blue limestone 1 to 14 feet 
 
 Main gypsum bed 7 to 14 feet 
 
 Gray limestone 1 to 14 feet 
 
 Gypsum . 3 to 5 feet 
 
 Anhydrite is an anhydrous variety of calcium sulphate some- 
 what less common than gypsum. Barite, or heavy spar, the 
 sulphate of barium, also occurs in nature, but less abundantly 
 than the calcium sulphates. It is found commonly in con- 
 nection with metallic ores (silver, lead, and zinc), or as a 
 secondary mineral associated with limestone, sometimes in 
 distinct veins, or, as in southwest Virginia, filling irregular 
 fractures in certain beds of the Cambrian limestones, or in 
 
 i Bull. New York State Museum, Vol. Ill, No. 1, 1893. 
 
PHOSPHATES 119 
 
 part replacing the limestone itself. It is easily distinguished 
 from coarsely crystalline calcite, for which it might possibly 
 be mistaken, by its weight, the specific gravity being about 
 4.5 as against 2.7 for the latter. 
 
 (5) PHOSPHATES 
 
 The mineral apatite, a phosphate of lime, as already noted, is 
 a common accessory, in the form of small crystals, in crystal- 
 line rocks of all ages, both metamorphic and eruptive. In 
 rare instances, as among certain Laurentian rocks of Canada, it 
 occurs in coarsely granular aggregates of a green or pinkish 
 color and of such dimensions as to constitute true rock masses. 
 Here we have to do, however, more with the amorphous, fibrous, 
 or concretionary forms to which the name phosphorite is com- 
 monly applied. These occur nearly if not quite altogether as 
 secondary products, due to the leaching out of phosphatic mate- 
 rial from older rocks, and its redeposition in clefts and cavities 
 at lower levels. It is thus that the phosphorites of Estre- 
 madura, Spain, are accounted for. From these very pure, 
 semi-crystalline masses, to the amorphous nodular and earthy 
 forms, such as are found in the eastern Carolinas and in Flor- 
 ida, there are no well-defined lines of demarcation. All have 
 resulted apparently either from the leaching out of the phos- 
 phate as above, or from the dissolving and carrying away of the 
 lime carbonate in a phosphatic limestone, leaving the phosphatic 
 material to accumulate as a residual product. Some of the latter 
 products, like the phosphatic sandstones of the Carolinas, might 
 with equal propriety be classed with the fragmental rocks, as 
 are the residual clays. (See p. 151.) 
 
 (6) CHLORIDES 
 
 Sodium chloride, or common salt, is one of the most wide- 
 spread constituents of the earth's crust, and from the standpoint 
 of human comfort a most important constituent as well. The 
 theoretically pure mineral consists of 66.6 parts of sodium and 
 39.4 parts of chlorine, though in nature it is almost univer- 
 sally contaminated with chlorides, sulphates, and carbonates 
 of potassium, calcium, and magnesium, together with oxides of 
 iron and aluminum. A large number of analyses of rock-salts 
 
120 AQUEOUS BOOKS 
 
 from world-wide sources show them to range from 94 to 99 % 
 sodium chloride. The pure mineral is white in color, but 
 shows often yellow, red, or purplish hues due to iron oxides or 
 organic matter. When crystallizing freely from solution, it 
 ordinarily assumes the form of a cube, the faces being frequently 
 cavernous or hopper-shaped ; rarely it occurs in octahedrons, 
 and occasionally in fibrous forms. Sodium chloride in solution 
 is an almost universal constituent of carbonated waters, though 
 often in but the merest traces. Its prevailing solid form is that 
 of coarsely granular aggregates constituting the so-called rock- 
 salt, the beds of which are often of such thickness and extent 
 as to constitute true rock masses and entitle them to considera- 
 tion here. These rock masses are invariably products of depo- 
 sition from solution, a deposition brought about through the 
 evaporation of saline waters in enclosed lakes or seas. They 
 are not limited to any particular geological period, but are to be 
 found wherever suitable conditions have existed for their for- 
 mation and preservation. Some of the more important beds 
 now known belong to either the Upper Silurian, Carboniferous, 
 Triassic, or Tertiary ages, and vary in thickness from a mere 
 film to upwards of 1200 feet. In the United States, beds of 
 rock-salt are known to occur in the states of New York, Penn- 
 sylvania, Ohio, Virginia, West Virginia, Michigan, Kansas, 
 Kentucky, Texas, Wyoming, California, and Nevada. Canada, 
 England, the Carpathian Mountains, the Austrian and Bavarian 
 Alps, West Germany, the Vosges, the Jura, Spain, the Pyrenees 
 and Celtiberian mountains, all contain important beds. With 
 the rock-salt are not infrequently associated other salts, as above 
 noted. In the celebrated Stassfurth deposits, sixteen different 
 compounds in the shape of chlorides and sulphates of sodium, 
 potassium, magnesium, calcium, and iron have been determined, 
 many of them in sufficient quantity to be of commercial value. 
 
 (7) THE HYDROCARBON COMPOUNDS 
 
 Under this head are included a series of hydrocarbon com- 
 pounds varying in physical properties from solid to gaseous, 
 and in color from coal-black through brown, greenish, red, and 
 yellow to colorless. Unlike the other members of the hydro- 
 carbon series yet to be described, they are not the residual 
 products of plant decomposition in situ, but are rather distilla- 
 
THE HYDROCARBON COMPOUNDS 121 
 
 tion products from deeply buried organic matter of both animal 
 and vegetable origin. The different members of the series 
 differ so widely in their properties and uses that each must be 
 discussed independently. The grouping of the various com- 
 pounds as given below is open to many objections from a strictly 
 scientific standpoint, but, all things considered, it seems best 
 suited for our present purposes. 1 
 
 Gaseous ..... Marsh gas (natural gas) 
 Fluidal ..... Petroleum (naphtha) 
 
 .. , f Pittasphalt (maltha) 
 Viscous and semi-solid { ^^ ^ 
 
 c Asphalt (bitumen) 
 Elastic ... . . ( Elaterite 
 
 BltummOUS ..... < Iwurtzilite 
 
 f Albertite 
 Solid ...... -I Grahamite 
 
 I Uintaite 
 f Succinite 
 Resinous ............... J Copalite 
 
 I Ambrite 
 
 Marsh G-as (Natural Cras). This is a colorless and odor- 
 less gas arising from the decomposition of organic matter 
 protected from the oxidizing influence of atmospheric air. By 
 itself it burns quietly with a slightly luminous flame, but when 
 mixed with air forms a dangerous explosive. It is this gas 
 which forms the dreaded fire-damp of the miners. 
 
 Under this head may properly be considered the so-called 
 natural gas, which has of late years become of so much impor- 
 tance from an economic standpoint. This is, however, by no 
 means a simple compound, but an admixture of several gases, 
 samples from different wells showing considerable variation in 
 composition, as well as those from the same well collected at 
 different periods. This last is shown by the six analyses fol- 
 lowing, and which may serve well to illustrate the average 
 composition, though in some instances the percentage of marsh 
 gas has been found greater. 
 
 1 W. P. Blake, Trans. Am. Inst. of Mining Engineers, Vol. XVIII, 1890, 
 p. 582. 
 
122 
 
 AQUEOUS EOCKS 
 
 CONSTITUENTS 
 
 . 
 
 II 
 
 III 
 
 IV 
 
 v 
 
 VI 
 
 Mash gas 
 
 57.85% 
 
 75.16 % 
 
 72.18% 
 
 65.25 % 
 
 60.70% 
 
 49.58% 
 
 Hydrogen 
 
 9.64 
 
 14.45 
 
 20.02 
 
 26.16 
 
 29.03 
 
 35.92 
 
 Ethylic hydride .... 
 Olifiant gas .... 
 
 5.20 
 0.80 
 
 4.80 
 0.60 
 
 3.60 
 0.70 
 
 5.50 
 
 0.80 
 
 7.92 
 0.98 
 
 12.30 
 0.60 
 
 Oxygen 
 
 2.10 
 
 1.20 
 
 1.10 
 
 0.80 
 
 0.78 
 
 0.80 
 
 Carbonic oxide .... 
 Carbonic acid .... 
 Nitrogen . .... 
 
 1.00 
 0.00 
 23.41 
 
 0.30 
 0.30 
 
 2.89 
 
 1.00 
 
 0.80 
 0.00 
 
 0.80 
 0.60 
 0.00 
 
 0.58 
 0.00 
 0.00 
 
 0.40 
 0.40 
 0.00 
 
 
 
 
 
 
 
 
 
 100.00 % 
 
 99.70% 
 
 99.40% 
 
 99.91 % 
 
 99.99 % 
 
 100.00 % 
 
 Natural gas in quantities sufficient to be of economic impor- 
 tance is necessarily limited to rocks of no particular horizon. 
 The tendency of recent studies seems to be to show that it 
 results, as above stated, from the deeply buried organic matter, 
 of both plant and animal origin. It is not, however, indige- 
 nous to the rocks in which it is now found, but occurs in an 
 overlying, more or less porous, sand or lime rock into which 
 it has been forced by hydrostatic pressure. The first necessary 
 condition for the presence of gas in any locality may, indeed, 
 be said to depend upon the existence of such a porous rock as 
 will serve as a reservoir to hold it, and also the presence of an 
 impervious overlying stratum to prevent its escape. In Penn- 
 sylvania the reservoir rock is a sandstone of Carboniferous or 
 Devonian age ; in Ohio and Indiana, a cavernous dolomitic 
 limestone of Silurian (Trenton) age. 
 
 Natural gas, as may readily be understood, is still in process 
 of formation, though at a rate vastly slower than it is being 
 utilized, or wasted, in many regions. It is a necessary conse- 
 quence that the available supply must sooner or later become 
 exhausted. Indeed this contingency has already made itself 
 apparent in many fields, necessitating continuous activity in 
 prospecting, and in more than one instance all known sources 
 of supply are already exhausted. Few more marked illustra- 
 tions of man's unreasonable squandering of nature's resources 
 have ever been offered than that relating to the utilization of 
 natural gas. 
 
 Petroleum. This is the name given to a complex hydro- 
 carbon compound, liquid at ordinary temperatures, though 
 varying greatly in viscosity, of a black, brown, greenish, or 
 
THE HYDROCARBON COMPOUNDS 
 
 123 
 
 more rarely, red or yellow color, and of extremely disagreeable 
 odor. Its specific gravity varies from 0.6 to 0.9. Through 
 becoming more and more viscous, the material passes into the 
 solid and semi-solid forms, asphalt and maltha. Chemically it 
 is considered as a mixture of the various hydrocarbons included 
 in the marsh gas, ethyline, and paraffin series. 
 
 An ultimate analysis of several samples, as given by the 
 reports of the 10th Census of the United States (1880), showed 
 the following percentages of the three essential constituents: 
 
 LOCALITIES 
 
 HYDROGEN 
 
 CARBON 
 
 NITROGEN 
 
 West Virginia 
 
 13.359 % 
 
 85.200 % 
 
 0.540 % 
 
 Mecca Ohio 
 
 13.071 
 
 86 316 
 
 0.230 
 
 California 
 
 11.819 
 
 86.934 
 
 1.109 
 
 As with marsh gas, petroleum is considered as a product of 
 organic decomposition, which has been for the most part forced 
 up from the rocks in which it originated into overlying strata. 
 It is therefore limited to no particular geological horizon, but 
 is found in rocks of all ages, from the Cambrian to the most 
 recent, its existence in quantities sufficient for economic pur- 
 poses being dependent upon local conditions for its generation 
 and subsequent preservation. Inasmuch as its accumulation 
 in large quantities necessitates a rock of porous nature to act as 
 a reservoir, the petroleum-bearing rocks are mostly sandstones, 
 though not uniformly so. Petroleums are found in California 
 and Texas, in Tertiary sands ; in Colorado, in the Cretaceous ; 
 in West Virginia, both above and below the Crinoidal (Car- 
 boniferous) limestones ; in Pennsylvania, in the Mountain sands 
 (Lower Carboniferous) and the Venango sands (Devonian); in 
 Canada, in the Corniferous (Lower Devonian) limestone ; in 
 Kentucky, in the Hudson River shales (Lower Silurian); and 
 in Ohio, in the Trenton limestone, also of Lower Silurian age. 
 
 In some instances petroleum oozes naturally from the ground, 
 forming at times a thin layer on the surface of pools of water, 
 whence in times past it has been gathered and used for chemical 
 and medicinal purposes. The so-called " Seneca oil " thus used 
 some fifty or sixty years ago was obtained from a spring in Cuba, 
 Alleghany County, in New York. The immense supply now 
 
124 AQUEOUS ROCKS 
 
 demanded for commercial purposes is, however, obtained alto- 
 gether from artificial wells of varying depths, and which are 
 in some cases self-flowing, while .in others the oil is raised by 
 means of pumps. Wells of from 500 to 1500 feet in depth are 
 of common occurrence, while those upwards of 2000 feet are not 
 rare. The principal sources of petroleum, in the United States, 
 are in New York, Pennsylvania, and Ohio, with smaller fields 
 in West Virginia, Kentucky, Tennessee, Indiana, Texas, Colo- 
 rado, and California. The chief foreign source is the Baku 
 region, on the Caspian Sea, and Galicia, in Austria. 
 
 The quantity of petroleum and semi-solid bituminous com- 
 pounds contained in the rocks of certain areas is sometimes 
 enormous. Dr. Hunt estimated that the dolomite underlying 
 the city of Chicago and vicinity contains for each square mile 
 over 7,000,000 barrels. A like computation by Professor 
 Orton 1 led to the conclusions given in the following quotation 
 relative to the water-lime stratum of Ohio, which is almost 
 universally petroliferous: 
 
 "Estimating its petroleum contents at one-tenth of one per 
 cent, and the thickness of the stratum at 500 feet, both of 
 which estimates are probably within the limits, we find the 
 petroleum contained in it to be more than 2,500,000 barrels to 
 the square mile. The total production of the great oil field 
 of Pennsylvania and New York to January, 1885, is 261,000,000 
 barrels. It would require only three ordinary townships, or a 
 little more than 100 square miles, to duplicate this enormous 
 stock from the water-lime alone. But if the rate of one-tenth 
 of one per cent should be maintained through a descent of 
 1500 feet at any point in the state, each square mile would, in 
 that case, yield 75,000,000 barrels, or nearly one-third of the 
 total product of the entire Pennsylvania and New York oil 
 fields. These figures pass at once beyond clear comprehension, 
 but they serve to give some idea of the vast stock of petroleum 
 contained in the earth's crust. If petroleum is generally dis- 
 tributed through a considerable series of rocks in any appre- 
 ciable percentage, it is easy to see that the aggregate amount 
 must be immense. Even one-thousandth of one per cent would 
 yield 750,000 barrels to the square mile in a series of rocks 1500 
 feet deep, but this amount is nearly equal to the greatest actual 
 production per square mile of any part of the leading Pennsyl- 
 
 1 Ann. Rep. U. S. Geol. Survey, 1886-87, Part II, p. 507. 
 
THE HYDROCARBON COMPOUNDS 125 
 
 vania fields. It is obvious that the total amount of petroleum 
 in the rocks underlying the surface of Ohio is large beyond 
 computation, but in its diffused and distributed state it is 
 entirely without value. It must be accumulated in rocks that 
 serve as reservoirs before it becomes of economic interest. In 
 respect to the importance of concentration, it agrees with most 
 other forms of mineral wealth." 
 
 Asphaltum (JBitumen, or Mineral Pitch). These are names 
 given to what are rather indefinite admixtures of various 
 hydrocarbons, in part oxygenated, and which, for the most part 
 solid or at least highly viscous at ordinary temperatures, pass 
 by insensible gradations into pittasphalts or mineral tar, and 
 these in turn into the petroleums. They are characterized by 
 a black or brownish black color, pitchy lustre, and bituminous 
 odor. The solid forms melt ordinarily at a temperature of 
 from 90 to 100 F., and burn readily with a bright flame, 
 giving off dense fumes of a tarry odor. The fluidal varieties 
 become solid on exposure to the atmosphere, owing to evapora- 
 tion of the more volatile portions. 
 
 The crude asphalt of Trinidad has the following composition 
 and physical characteristics : 1 
 
 Specific gravity, 1.28 ; hardness at 70 F., 2.5 to 3, Dana's 
 scale ; color, chocolate-brown. Composition : 
 
 Bitumen 39.83 % 
 
 Earthy matter 33.99 
 
 Vegetable matter .... 9.31 
 
 Water 16.87 
 
 100.00% 
 
 The mode of occurrence of asphalt deposits varies greatly, 
 owing to the fact that, as with petroleum and natural gas, it 
 has come up through fissures and cracks in the earth's surface, 
 and as a rule no longer occupies its place of origin. On the 
 island of Trinidad is an immense superficial deposit having an 
 area of about 114 acres and a depth varying from 18 to 78 feet. 
 The surface is sufficiently solid over nearly every part for the 
 passage of teams, is of a brownish black color, and nearly level. 
 The deposit has in numerous publications been compared to a 
 lake, and stated to be fluidal and at a high temperature in the 
 centre. This statement is quite erroneous and misleading. 
 
 1 Trans. Am. Inst. Mining Engineers, Vol. XVII, 1889, p. 363. 
 
126 AQUEOUS ROCKS 
 
 In Ventura County, California, the material occurs in a fissure 
 vein in siliceous clay of Miocene age, the vein being from 7 to 
 15 inches thick on the surface, but widening rapidly in descent 
 to a thickness of 5 feet at a depth of 65 feet below the surface. 
 The material of the vein is, however, far from pure asphalt ; 
 but rather an asphaltic sand. In western Kentucky the as- 
 phalt exudes from the ground in the form of u tar springs," and 
 occurs also disseminated through sandstones and limestones of 
 sub-Carboniferous age. Frequently, as in the dolomite under- 
 lying Chicago, Illinois, the bituminous matter is so diffused 
 throughout the rock as to give it, on exposure, a brownish 
 black appearance, and cause it to exhale an odor of petroleum 
 appreciable for some distance. In the Dead Sea, bituminous 
 masses of considerable size have in times past risen like islands 
 to the surface of the water, and furnished thus the material 
 used by the ancients in pitching the walls of buildings and 
 rendering vessels water-tight. The ancient name of this body 
 of water was Lake Asphaltites, and from it our word asphalt 
 is derived. 
 
 The above illustrations are sufficient to indicate the numerous 
 conditions under which the substance occurs. The material is 
 world-wide in its geographic distribution and equally cosmo- 
 politan in its geological range, being found in gneissic rocks of 
 presumably Archaean age in Sweden, and in rocks of all inter- 
 mediate horizons down to late Tertiary. 
 
 Elaterite (Mineral Caoutchouc). This is the name given to a 
 soft and elastic variety of asphalt much resembling pure india- 
 rubber. It is easily compressible in the fingers, to which it 
 adheres slightly, of a brownish color, and of a specific gravity 
 varying from 0.905 to 1.00. It has been described from mines 
 in Derbyshire and elsewhere in England, but, so far as the 
 writer is aware, is of no commercial value. Its composition so 
 far as determined is, carbon, 85.47 %; hydrogen, 13.28 %. 
 
 The name wurtzilite has been given by Professor W. P. 
 Blake to a hydrocarbon very similar in appearance to the 
 uintaite (described below), but differing in physical and chem-- 
 ical properties. It is described as a firm black solid, amorphous 
 in structure, brittle when cold, breaking with a conchoidal 
 fracture, but when warm, tough and elastic, its elasticity being 
 best compared with that of mica. If bent too quickly, it snaps 
 like glass. It cuts like horn, has a hardness between 2 and 3, 
 
THE HYDROCARBON COMPOUNDS 127 
 
 a specific gravity of 1.03, gives a brown streak, and in very 
 thin flakes shows a garnet-red color. It does not fuse or 
 melt in boiling water, bnt becomes softer and more elastic ; in 
 the flame of a candle it melts and takes fire, burning with a 
 bright, luminous flame, giving off gas and a strong bitumi- 
 nous odor. It is not soluble in alcohol, but sparingly so 
 in ether, in both of which respects it differs from elaterite 
 proper. 
 
 Albertite. This is a brilliant jet-black compound, breaking 
 with a lustrous, conchoidal fracture, having a hardness of 
 between 1 and 2 of Dana's scale, a specific gravity of 1.097, 
 a black streak, and showing a brown color on very thin edges. 
 In the flame of a lamp it shows signs of incipient fusion, intu- 
 mesces somewhat, and emits jets of gas, giving off a bituminous 
 odor ; when rubbed it becomes electric. According to Dana, 
 it softens slightly in boiling water, is scarcely at all soluble in 
 alcohol, and only slightly so in ether and in turpentine. The 
 following is the composition as given by Witherill : Carbon, 
 86.04%; hydrogen, 8.96%; oxygen, 1.97%; nitrogen, 2.93%; 
 ash, 0.10%. The mineral occurs in fissures in rocks of sub- 
 Carboiiiferous age, at the Albert Mines, in Hillsborough County, 
 Nova Scotia ; hence the name. 
 
 Formerly it was used for the distillation of oils for illumi- 
 nating purposes. Since the discovery of petroleum its use has 
 been discontinued. 
 
 G-rahamite. This variety resembles the last in its general 
 appearance and its conduct toward solvents, and it is a question 
 if it is not identical therewith. It was described by Dr. Wurtz 
 from Ritchie County, in West Virginia, where it occurred in a 
 vein some four feet in width in Carboniferous sandstones. 
 
 Uintaite (Grilsonite). This is a black, brilliant, and lustrous 
 variety giving a dark-brown streak, breaking with a beautiful 
 conchoidal fracture, and having a hardness of 2 to 2.5 and a 
 specific gravity of 1.065 to 1.07. It fuses readily in the flame 
 of a candle, is plastic but not sticky while warm, and unless 
 highly heated will not adhere to cold paper. Its deportment 
 is stated to be much like that of sealing wax or shellac. Like 
 albertite and grahamite, it dissolves slightly in turpentine and 
 is not soluble in alcohol. It is a good non-conductor of elec- 
 tricity, but like albertite becomes electric by friction. Its 
 composition as given is, carbon, 80.88%; hydrogen, 9.76%-, 
 
128 AQUEOUS ROCKS 
 
 nitrogen, 3.30 %; oxygen, 6.05 % ; and ash, 0.01 %. The min- 
 eral as first described occurred in a vertical vein from 3 to 5 
 feet in thickness, cutting through nearly horizontal sandstones 
 some 3 miles east of Fort Duchesne, on the reservation of the 
 Uinta Indians. 
 
 /Succinite (Am~ber). The mineral commonly known as amber 
 is a fossil resin, consisting of some 78.94 parts of carbon, 10.53 
 parts of oxygen, and 10.53 parts of hydrogen, together with 
 usually from two to four-tenths of a per cent of sulphur. It 
 is not a simple resin, but a compound of four or more hydro- 
 carbons. According to Berzelius, as quoted by Dana, it " con- 
 sists mainly of (85% to '90%) two other resins in soluble 
 alcohol and ether, and an oil, and 2J- % to 6 % of succinic 
 acid." 
 
 The mineral, as found, is of a yellow, brownish, or reddish 
 color, frequently clouded, translucent, or even transparent, 
 tasteless, becomes negatively electrified by friction, has a hard- 
 ness of 2 to 2.5, a specific gravity, when free from enclosures, of 
 1.096, a conchoidal fracture, and melts at 250 to 300 Fahr. 
 without previous swelling, but boils quietly, giving off dense 
 white fumes with an aromatic odor and very irritating effect 
 on the respiratory organs. 
 
 Amber, or closely related compounds, has been found in 
 varying amounts at numerous widely separated localities, but 
 always under conditions closely resembling one another. The 
 better known localities are the Prussian coast of the Baltic ; on 
 the coast of Norfolk, Essex, and Suffolk, England ; the coasts 
 of Sweden, Denmark, and the Russian Baltic provinces; in 
 Galicia, Westphalia ; Poland ; Moravia ; in Norway ; Switzer- 
 land ; France ; Upper Burma ; Sicily ; Mexico ; the United 
 States at Martha's Vineyard, and near Trenton and Camden, 
 New Jersey, and at Cedar Lake in Northwest Canada. 
 
 The amber of commerce comes now, as for the past 2000 
 years, mainly from the Baltic, where it occurs in a stratum of 
 blue earth of from 4 to 20 feet in thickness underlying the 
 brown coal formation. 
 
 Ozokerite (Mineral Wax; Native Paraffin). This is a wax- 
 like hydrocarbon, usually with a foliated structure, soft and 
 easily indented with the thumb nail ; of a yellow, yellow 
 brown, or sometimes greenish color, translucent when pure, 
 with a greasy feeling, and fusing at 56 to 63 F. ; specific 
 
ROCKS FORMED AS SEDIMENTARY DEPOSITS 129 
 
 gravity, 0.955. It is essentially a natural paraffin. The name 
 is derived from two Greek words, signifying to smell, and ivax. 
 Below is given the composition of (I) samples from Utah, and 
 (II) from Boryslaw, in Galicia. 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 Carbon 
 
 85.47% 
 
 85 78 % 
 
 Hydrogen 
 
 14 57 
 
 14 ">9 
 
 
 
 
 
 100.04% 
 
 100.07 % 
 
 The substance is completely soluble in boiling ether, carbon 
 disulphide, or benzine, and partially so in alcohol. 
 
 Ozokerite occurs in the United States, in Emery and Uinta 
 counties, Utah, where in the form of small veins in Tertiary 
 rocks it extends over a wide area. It is also found in Galicia, 
 Austria, in Miocene deposits, in Roumania, Hungary, Russia, 
 and other Asiatic and European sources. As a rule the de- 
 posits are in beds of Tertiary or Cretaceous age. The Galician 
 deposits are the most noted of the above. According to 
 Boverton Redwood, 1 the material occurs here in the form of 
 veins from the thickness of a few millimetres to some feet, and 
 is accompanied by petroleum and gaseous hydrocarbons. 
 
 The names scheerite, hatchettite, ficlitellite, and konlite are 
 applied to simple hydrocarbons allied to ozokerite found in 
 beds of peat and coal, but so far as the writer is aware never in 
 such abundance as to be of commercial value. 
 
 The name retinite includes a considerable series of fossil 
 resins allied to amber, differing mainly in containing no suc- 
 cinic acid. They occur in beds of brown coal of Tertiary and 
 Cretaceous age. The so-called copalite, a hard brittle, clear 
 yellow, or brownish variety used in making varnishes, belongs 
 here. 
 
 2. ROCKS FORMED AS SEDIMENTARY DEPOSITS AND FRAG- 
 MENTAL IN STRUCTURE: CLASTIC 
 
 The rocks of this group differ from those just described in 
 that they are composed mainly of fragmental materials derived 
 from the breaking down of older rocks, or are but the more or 
 
 1 Jour. Soc. of Chem. Industry, February, 1892. 
 
130 AQUEOUS ROCKS 
 
 less consolidated accumulations of organic and inorganic debris 
 from plant and animal life. The group shows transitional 
 forms into the last, as will be illustrated by certain of the lime- 
 stones and the quarzites. They are water deposits, and, as a 
 rule, are eminently stratified or bedded, although this structure 
 is not always apparent in the hand specimen. 
 
 As will be readily comprehended when one considers from 
 what a multitude of materials the fragmental rocks have been 
 derived, the amount of assorting, admixture with other sub- 
 stances, solution, and transportation by streams these materials 
 have undergone, they cannot be classified by any hard and fast 
 lines, but one variety may grade into another, both in texture 
 and structure as well as in chemical composition, almost indefi- 
 nitely. Indeed, many of them can scarcely be considered as 
 more than indurated muds, and only very general names can 
 be given them. 
 
 Accordingly as these rocks consist of mechanically formed 
 inorganic particles of varying composition and texture, or of 
 the more or less fragmental debris from plant and animal life, 
 they are here divided into two main groups, each of which is 
 subdivided as below : 
 
 I. Rocks formed by mechanical agencies, and mainly of in- 
 organic materials. 
 
 (1) The Arenaceous group Psammites: Sand, gravel, sand- 
 stone, conglomerate, and breccia. 
 
 (2) The Argillaceous group Pelites : Kaolin, clay, wacke, 
 shale, clayey marl, argillite. 
 
 (3) The Calcareous group : Arenaceous and brecciated 
 limestones. The rocks of this group are often in part organic, 
 and in part chemical deposits. Only those are considered 
 here in which the fragmental nature is the most pronounced 
 characteristic. 
 
 (4) The Volcanic group : Fragmental rocks composed 
 mainly of ejected volcanic material : Tuffs, lapilli, sand and 
 ashes, pumice-dust, trass, peperino, pozzuolano, etc. 
 
 II. Rocks formed largely or only in part by mechanical 
 agencies and composed mainly of the debris from plant and 
 animal life Organagenous. 
 
 (1) The Siliceous group Infusorial earth. 
 
 (2) The Calcareous group Fossiliferous and oolitic lime- 
 stone, marl, shell-sand, shell-rock. 
 
PLATE 11 
 
 
 FIGS. 1 and 2. Shell limestones. FIG. 3. Crinoldal limestone. 
 
ARENACEOUS ROCKS: PSAMMITES 
 
 131 
 
 (3) The Carbonaceous group Peat, lignite, coals, oil shale, 
 etc. 
 
 (4) The Phosphatic group Phosphatic sandstone, guano, 
 coprolite nodules. 
 
 (1) ROCKS COMPOSED MAINLY OF INORGANIC MATERIAL 
 
 (1) The Arenaceous Group: Psammites. Arenaceous, from 
 the Latin arenaceous, sandy or sand-like ; psammite from the 
 Greek -v/ra/LtyLuVr;?, sandy. 
 
 These rocks are composed mainly of the siliceous materials 
 derived from the disintegration of older crystalline rocks and 
 which have been rearranged in beds of varying thickness 
 through the mechanical agency of water. They are, in short, 
 more or less consolidated beds of sand and gravel. In composi- 
 tion and texture, they vary almost indefinitely. Many of them 
 having suffered little during the process of disintegration and 
 transportation, are com- 
 posed of essentially the 
 same materials as the 
 rocks from which they 
 were derived. Others, 
 in which the fragmeiital 
 materials suffered more 
 prior to their final con- 
 solidation, have had the 
 softer and more soluble 
 minerals removed, leav- 
 ing the sand composed 
 mainly of the hard, al- 
 most indestructible min- 
 eral quartz. 
 
 In structure, the sand- 
 stones also vary greatly, 
 in some the grains being rounded, while in others they are 
 sharply angular. Figure 9 shows the microscopic structure of 
 a brown Triassic sandstone from Portland, Connecticut. 
 
 The material by which the individual grains of a sandstone 
 are bound together is as a rule of a calcareous, ferruginous, or 
 siliceous nature ; sometimes argillaceous. The substance has 
 been deposited between the granules by percolating water or 
 
 FIG. 9. Microstructure of sandstone, 
 Portland, Connecticut. 
 
132 
 
 AQUEOUS ROCKS 
 
 during the process of sedimentation, and forms a natural 
 cement. It sometimes happens that the siliceous cement is 
 deposited about the rounded grains of quartz in the form of a 
 new crystalline growth, converting the stone into quartzite ; 
 such are in this work classed with the crystalline rocks. 
 
 Upon the character of this cementing material and the close- 
 ness with which the grains are bound together, is very largely 
 dependent the power of the stone to resist disintegration under 
 the trying action of percolating carbonated waters and the 
 mechanical action of heat and frost. The calcareous, and to a 
 less extent the ferruginous cements are liable to removal in 
 solution, allowing the rock to fall away to sand, or at least 
 allowing it to absorb water, which, on freezing, brings about 
 the disintegration. The argillaceous cementing material, while 
 in itself inert, also permits a high degree of absorption, with 
 like results. Those sandstones cemented by silica, and which 
 therefore partake of the nature of quartzite (see p. 169), are 
 by far the more refractory. 
 
 The following analyses will serve to indicate the consid- 
 erable range in composition of rocks of this class : 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 III 
 
 IV 
 
 Silica (Si0 2 ) 
 
 69 94 % 
 
 84.40 % 
 
 95.24% 
 
 90.86% 
 
 Alumina (Al 2 0s) 
 
 13.15 
 
 7.49 
 
 0.56 
 
 4.76 
 
 Iron oxides (Fe 2 3 ) and (FeO) . . 
 Manganese (MnO) 
 
 2.48 
 70 
 
 3.87 
 
 1.28 
 
 1.58 
 
 Lime (CaO) . 
 
 3.09 
 
 0.74 
 
 1.40 
 
 0.15 
 
 Magnesia (MgO) 
 
 Trace 
 
 2.11 
 
 1.23 
 
 0.59 
 
 Potash (K 2 0) 
 
 3.30 
 
 0.24 
 
 
 1.06 
 
 Soda (Na 2 0) 
 Loss 
 
 5.43 
 1 01 
 
 0.56 
 
 0.56 
 
 0.45 
 
 
 
 
 
 
 Totals 
 
 99. 10 % 
 
 99 41 % 
 
 99.27 % 
 
 99.45% 
 
 
 
 
 
 
 I. Brown Triassic sandstone: Portland, Connecticut. II. Gray sub-Carbo- 
 niferous sandstone: Berea, Ohio. III. Red Carboniferous sandstone: Anan, 
 Scotland. IV. Cambrian sandstone : Siskowit Bay, Wisconsin. 
 
 The table given on p. 166 will serve to show the close chemi- 
 cal relationship existing between many rocks of this group, 
 and their metamorphic equivalents. 
 
 The colors of sandstone are dependent upon a variety of 
 circumstances. The red, brown, and yellowish colors are due 
 
ARENACEOUS ROCKS: PSAMMITES 133 
 
 to iron oxides in the cementing constituent. Some of the dark 
 colors are due to carbonaceous matter. 
 
 Many varieties of sandstone are popularly recognized. Cal- 
 careous, ferruginous, siliceous, or argillaceous sandstones are those 
 in which the cementing materials are of a calcareous, ferrugi- 
 nous, siliceous, or argillaceous nature. The name arkose is given 
 to a coarse feldspathic sandstone derived from granitic rocks, 
 with a minimum amount of loss of original material. Conglomer- 
 ate or puddingstone is merely a coarse sandstone ; it differs from 
 ordinary sandstone only as gravel differs from sand. Breccia 
 is a fragmental rock differing from conglomerate in that the 
 individual particles are sharply angular instead of rounded. 
 The term is made to include also certain volcanic rocks with a 
 brecciated structure. (See PI. 4.) 
 
 Crreywacke or Grramvacke is an old German name for brecci- 
 ated fragmental rocks made up of argillaceous particles. The 
 name is now little used. Other names, &s flagstone, freestone, and 
 brownstone, are applied to such as are used for flagging or other 
 structural purposes. Itacolumite is a feldspathic sandstone, or 
 perhaps more properly quartzite, in which the feldspathic mate- 
 rial plays the role of a binding constituent to the quartz gran- 
 ules. The so-called flexible sandstone is an itacolumite from 
 which the feldspathic portions have been removed by decompo- 
 sition leaving the interlocking quartz grains with a small amount 
 of play between them. The rock is in no sense elastic, but 
 merely loose jointed. 
 
 The name greensand, greensand marl, and glauconitic sand are 
 given to a prevailing dull green, loosely coherent, clayey or 
 arenaceous deposit which owes its peculiarities to the presence 
 of the hydrous silicate of iron and potassium glauconite, but 
 which is variously contaminated with minute particles of quartz 
 and siliceous minerals such as feldspar, hornblende, augite, 
 garnet, epidote, tourmaline, zircon, and the iron ores, clay, rock 
 fragments, and particles of shells. 
 
 Beds of glauconitic sand are most abundant among terranes 
 of Cretaceous age, but are by no means limited to them, as has 
 been already intimated on p. 31. They are aqueous deposits, 
 formed during processes of slow sedimentation along coasts 
 receiving debris from the continental slopes and of a nature 
 such as is derived from the breaking down of granitic and other 
 feldspathic rocks. The depth at which such deposits form is 
 
134 
 
 AQUEOUS ROCKS 
 
 naturally quite variable, but conditions most favorable to their 
 accumulation seem to lie just beyond the reach of wave agita- 
 tion and under a depth of 900 fathoms. 
 
 The following table of analyses of glauconitic marls is from 
 the Report of the Geological Survey of New Jersey, for 1893. 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 III 
 
 IV 
 
 VI 
 
 X 
 
 XI 
 
 XII 
 
 XIII 
 
 Phosphoric acid . . 
 
 l.15 
 
 0.58 
 
 1.51 
 
 1.14 
 
 0.84 
 
 0.19 
 
 0.50 
 
 6.87 
 
 3.73 
 
 Sulphuric acid . . . 
 
 1.28 
 
 .... 
 
 2.40 
 
 0.14 
 
 0.12 
 
 0.41 
 
 0.34 
 
 3.12 
 
 2.44 
 
 Silica and sand . . . 
 
 34.50 
 
 45.50 
 
 55.69 
 
 38.70 
 
 52.07 
 
 51.15 
 
 47.50 
 
 44.68 
 
 49.68 
 
 Potash . . . 
 
 1.54 
 
 3.79 
 
 5.27 
 
 3.65 
 
 6.46 
 
 7.08 
 
 5.29 
 
 397 
 
 4.98 
 
 Lime 
 
 2.52 
 
 1.51 
 
 0.65 
 
 9.07 
 
 1.01 
 
 0.49 
 
 0.56 
 
 4.97 
 
 4.14 
 
 Magnesia 
 
 2.15 
 
 2.20 
 
 0.79 
 
 1.50 
 
 1.53 
 
 2.02 
 
 2.70 
 
 2.97 
 
 0.47 
 
 Alumina 
 
 6.00 
 
 5.80 
 
 6.61 
 
 10.20 
 
 6.96 
 
 8 23 
 
 860 
 
 604 
 
 f> 
 
 Oxide of iron .... 
 
 31.50 
 
 24.50 
 
 21.63 
 
 18.63 
 
 21.55 
 
 23.13 
 
 20.52 
 
 18.97 
 
 28.71 
 
 Water . ... 
 
 18.80 
 
 15.40 
 
 8.85 
 
 10.00 
 
 9.31 
 
 6.67 
 
 1357 
 
 863 
 
 554 
 
 Carbonic acid . . . 
 
 
 
 
 6.14 
 
 
 
 
 
 
 Carbonate of lime 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 99.43 
 
 99.18 
 
 102.40 
 
 99.16 
 
 99.85 
 
 99.37 
 
 99.58 
 
 99.32 
 
 99.69 
 
 I. Clay marl, from near Mattawan. II. Clay marl, from Matchaponix 
 Creek, three miles south of Spottswood. III. Lower marl, from Navesink 
 Highlands. IV. Lower marl, from north shore of Navesink River, at Red Bank. 
 VI. Lower marl, from northwest slope of Mount Pleasant Hills. X. Middle 
 marl, from near Eatontown. XI. Middle marl, from southeast of Freehold. 
 XII. Upper marl, from Poplar. XIII. Upper marl, from Shark River. 
 
 The most extensive and best known deposits in the United 
 States are included in what are known as the Upper, Middle, 
 and Lower marl beds of the Cretaceous formations in south- 
 eastern New Jersey, and which has been very thoroughly 
 described in the various reports of the State Survey. 1 The 
 marl is somewhat variable in different localities, but may in a 
 general way be described as a dull green, arenaceous deposit of 
 such consistency as to be easily removed by the shovel alone, 
 or pick and shovel. The beds vary from 30 to 60 feet in thick- 
 ness, but the glauconitic layers are not uniformly distributed 
 through it. Through weathering, the ferruginous constituents 
 become more highly oxidized, and the color changed from dull 
 green to red and yellow. 
 
 1 The reader is especially referred to Professor W. B. Clarke's paper on " The 
 Cretaceous and Tertiary Formations of New Jersey," in the Ann. Rep. State 
 Geologist of New Jersey for 1892. 
 
ARGILLACEOUS ROCKS: PELITES 135 
 
 Rocks belonging to the arenaceous group are world wide 
 in their distribution, covering not infrequently thousands of 
 square miles of territory to depths, it may be, of thousands of 
 feet. They are, in some of their varieties, among the most 
 common and wide-spread of materials. Being themselves the 
 products of disintegration and decomposition of pre-existing 
 rocks, and having become consolidated under conditions not 
 greatly different from those now existing at or near the surface 
 of the earth, the rocks of this group are as a whole in a state of 
 comparatively stable chemical equilibrium. Unless including 
 calcareous matter or readily oxidizing ferruginous compounds, 
 such are subject to disintegration more through physical than 
 chemical agencies, as will be noted later. 
 
 (2) The Argillaceous Group: Pelites. The rocks of this 
 group are composed of more or less hydrated aluminous sili- 
 cates admixed in almost indefinite proportions with siliceous 
 sand, various silicate minerals in a more or less fragmental and 
 decomposed condition, and calcareous and carbonaceous matter. 
 In their least consolidated form they are best represented by 
 the common plastic clays used for brick and pottery manufac- 
 ture. Such, although alike in their general physical or even 
 ultimate chemical nature, have widely diverse origins. In fact, 
 the term clay, like silt, indicates physical condition rather than 
 chemical or miiieralogical composition, and it may perhaps be 
 defined as an indefinite admixture of more or less hydrated 
 aluminous silicates, free silica, iron oxides, carbonates of lime, 
 and various silicate minerals which in a more or less decom- 
 posed and fragmental condition have survived the destructive 
 agencies to which they have been subjected. About the only 
 feature characteristic of all clays, is that of plasticity, when 
 wet, and this is dependent, apparently, wholly upon texture 
 and structure, i.e. upon the size and shape of the individual 
 particles. Pure quartz, chalcedony, flint, feldspar, or other 
 silicates, will, when reduced to an impalpable powder, possess 
 the plasticity and even odor usually ascribed to clay, and in the 
 pages following, the term is used only with reference to degree 
 of comminution, regardless of mineral nature or chemical com- 
 position. It includes residual products of any or all forms of 
 rock degeneration, and which may or may not have been re- 
 assorted through the agency of water. (See further under The 
 Regolith, Part V.) The oft-repeated statement that kaolin 
 
136 
 
 AQUEOUS ROCKS 
 
 forms the basis of clays, or that clay is impure kaolin, is there- 
 fore to a certain extent misleading, and if accepted at all it 
 must be with the reservations made by Johnson and Blake, 1 
 who limit the term kaolin itself to the impure material, quite 
 distinct from true kaolinite, which is a definite chemical com- 
 pound corresponding to the formula H 4 Al 2 Si 2 O 9 . 
 
 Throughout the glaciated region of the northeastern United 
 States the clays are mostly glacial or water deposits from the 
 floods of the Champlain epoch. The latter are often beauti- 
 fully and evenly stratified, as shown in the illustration on PI. 24. 
 The plastic clays and siliceous sands about Woodbridge, New 
 Jersey, are regarded as derived from the Azoic rocks and 
 deposited by sea- water in enclosed basins. The exact source of 
 the material is not always apparent ; the porcelain clays of Law- 
 rence County, Indiana, on the other hand, are residual deposits 
 resulting from the decomposition of impure Carboniferous 
 (Archimides) limestones, the lime carbonate being removed in 
 solution, while the less soluble clay remains. Kaolin, as already 
 noted, is a residual deposit from the decay of feldspathic and 
 other aluminous rocks, while the ordinary brick and tile clays 
 of the Southern states, as well as the clayey soils, are residual 
 aluminous deposits resulting from the decay and leaching out 
 of soluble constituents from a variety of rocks, both sedimentary 
 and eruptive. (See chapter on rock weathering.) 
 
 As showing the comparative compositions of kaolins and 
 clays, the following table is given : 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 III 
 
 IV 
 
 V 
 
 VI 
 
 SI02 (combined) . . . 
 Si0 2 (free) 
 
 46.4 % 
 
 39.00% 
 
 34.70% 
 12.20 
 
 28.30% 
 
 27.80 
 
 42.71 % 
 0.70 
 
 J60.97% 
 
 A1 2 3 
 
 39.7 
 
 36.00 
 
 31.34 
 
 27.42 
 
 39.24 
 
 26.38 
 
 H 2 (combined) . . . 
 H 2 at 212 ... 
 
 13.9 
 
 14.00 
 950 
 
 12.00 
 
 8 00 
 
 6.60 
 
 2 90 
 
 13.32 
 1 58 
 
 } 8.93 
 
 CaOandMgO .... 
 Alkalies . 
 
 .... 
 
 0.63 
 54 
 
 0.10 
 95 
 
 0.18 
 2 71 
 
 0.20 
 89 
 
 } 1.90 
 
 Fe 2 3 .... 
 
 
 
 16 
 
 2 68 
 
 46 
 
 146 
 
 
 * 
 
 
 
 
 
 
 
 99.00 % 
 
 99.67% 
 
 99.45% 
 
 98.59% 
 
 99.10% 
 
 99.64% 
 
 I. Kaolin. II. Indianite, a white clay residual from St. Lawrence County, 
 Indiana. III. Potter's clay, from Pope County, Illinois. IV. Brick clay from 
 New Jersey. V. Fire clay from New Jersey. VI. Fire clay from Illinois. 
 1 Am. Jour, of Science, 1867, p. 351. 
 
ARGILLITES AND SHALES 
 
 137 
 
 Amongst the older formations the clays have undergone 
 induration, giving rise to what are known as argillites, or if 
 fissile, slates or day slates, such as are used for roofing and 
 similar purposes, the fissile property having been imparted by 
 pressure or shearing. Such forms pass by imperceptible gra- 
 dations into argillaceous schists which are classed with the met- 
 amorphic rocks. (See p. 170.) The argillites are, as a rule, 
 among the most indestructible of rocks, since they are them- 
 selves composed of the least destructible debris of pre-existing 
 rocks. Their ultimate chemical composition is much like that 
 of the clays, and scarce any two samples will show similar 
 results when submitted to analysis. The table given below 
 shows the composition of some schistose argillites used for 
 roofing purposes from (I) Harford County, Maryland, (II) 
 Lancaster County, Pennsylvania, and (III) Llangynog, North 
 Wales. 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 III 
 
 Silica (SiOo) 
 
 58 37 % 
 
 60 32 % 
 
 60 150 % 
 
 Sulphuric acid (HoS04) 
 
 22 
 
 
 
 Alumina (A^Os) 
 
 21.985 
 
 23.10 
 
 24.20 
 
 Iron oxides (FeO) and (FesOg) . . 
 Lime (CaO) 
 Magnesia (MgO) 
 Soda (Na 2 0) . 
 
 10.661 
 0.30 
 1.203 
 1 933 
 
 7.05 
 
 0.87 
 49 
 
 7.65 
 I 4 278 
 
 Potash (K 2 O) 
 Water (H 2 0) 
 
 4.03 
 
 3.83 
 
 4.08 
 
 3.72 
 
 
 98.699 % 
 
 99.74% 
 
 99.998% 
 
 Shale is a somewhat loosely defined term, indicating struc- 
 tural rather than chemical or mineralogical composition. The 
 word is perhaps best used in its adjective sense, as a shall/ 
 sandstone, or shaly limestone. By many authors it is used 
 with reference more particularly to thinly stratified or lami- 
 nated, clayey rocks. Many shales are but the finer, more fissile 
 portions of sandstone beds; such may represent the off-shore 
 or deep-water portions of arenaceous sediments, which, begin- 
 ning with gravels near the shore-line, become gradually finer 
 as the distance from the shore increases, passing through coarse 
 to finer sands and finally to sandy clays and silts as the water, 
 
138 
 
 AQUEOUS ROCKS 
 
 through the lessening of its carrying power, lays down its load. 
 Or they may represent later stages in the cycle of sedimenta- 
 tion ; the finer silts brought down after erosion have so far 
 reduced the level of the land as to greatly diminish the currents 
 and consequent carrying power of the seaward-flowing streams. 
 Such beds, on consolidation, yield then what are commonly 
 known, in the order of their formation, as conglomerates, sand- 
 stones, shales and argillites, or clay slates, the shales occu- 
 pying, both in texture and composition, a position intermediate 
 between the argillites and sandstones. 
 
 The following table will serve to show the varying character 
 of the rocks included under this name. Those such as given 
 in columns I and II carry their sulphur in combination with 
 iron, as iron pyrites (FeS 2 ). This, on decomposing, through 
 the action of meteoric waters, yields iron sesquioxides and sul- 
 phuric acid, the latter combining with a portion of the alumina 
 in the rock to form sulphate of aluminum, or common alum. 
 Hence they have been called alum shales. 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 Ill 
 
 Silica (Si0 2 ) 
 
 50.13% 
 
 72.40% 
 
 66.96 % 
 
 Alumina (Al 2 0s) 
 
 10.73 
 
 16.45 
 
 15.626 
 
 Iron sesquioxide (Fe 2 0s) 
 
 5.78 
 
 1.05 
 
 8.38 
 
 Lime (CaO) 
 
 0.40 
 
 0.17 
 
 0.493 
 
 Magnesia (MgO) 
 
 1.00 
 
 1.48 
 
 0.677 
 
 Potash (K 2 0) 
 Soda (Na 2 0) 
 
 
 5.08 
 0.53 
 
 3.295 
 0.628 
 
 Sulphur (S) 
 
 4.02 
 
 1.21 
 
 
 Carbon (C) 
 
 22.83 
 
 Undet. 
 
 3.787 
 
 Water (H 2 O) 1 
 
 2.21 
 
 Undet. 
 
 
 Phosphoric acid (P 2 O 5 ) 
 
 
 
 0.154 
 
 I. An alum shale from Garnsdorf, near Saalst'eld. II. An alum shale from 
 Bornholm. III. A "marly shale " from Breckenridge County, Kentucky. 
 
 The name till or boulder clay is given to a sandy clay of 
 glacial origin and consisting of the usual indefinite mixture. 
 Professor W. O. Crosby, who has studied the composition of 
 the normal till of the Boston Basin, reports it as composed, 
 exclusive of the larger pebbles, of "about 25%, or one-fourth, 
 of coarse material which may be classed as gravel ; about 20 %, 
 
 1 Ignition. 
 
CALCAREOUS FRAGMENTAL ROCKS 139 
 
 or one-fifth, of sand; 40 to 45 % of extremely fine sand, or rock 
 flour, and less than 12 % of clay." l 
 
 Laterite is a red, ferruginous residual clay found in tropic 
 and semitropic regions. (See p. 310.) Catlinite, or Indian 
 pipe-stone, is an. indurated clay rock formerly used by the Da- 
 kota Indians for pipe material. The name porcellainite has 
 been given to a compact porcelain-like rock consisting of clay 
 indurated by igneous agencies. The name wacke is sometimes 
 used to designate an earthy or compact, dark-colored clayey 
 material resulting from the decomposition in situ of basaltic 
 rocks. Adobe is the name given to a calcareous clay of a 
 general gray-brown or yellowish color, very fine grained and 
 porous, and which is widely distributed throughout the more 
 arid regions of the West. It is described in greater detail 
 under the head of soils (p. 333). Loess is a somewhat similar 
 material forming the surface soil over wide areas in the Missis- 
 sippi valley, and at times sufficiently plastic for brick making. 
 (See also p. 327.) 
 
 (3) The Calcareous Group. Here are brought together a 
 small series of fragmental rocks composed mainly of calcareous 
 material, but of which the organic nature, if such it had, is not 
 apparent. These rocks form at times beautifully brecciated 
 marbles. Their structure may be best comprehended by remem- 
 bering that the original beds, whether crystalline or amorphous, 
 whether f ossilif erous or originating as chemical precipitates, 
 have by geological agencies been crushed and shattered into a 
 million fragments, and then, by infiltration of lime and iron- 
 bearing solutions, been slowly cemented once more into solid 
 rock. The composition is essentially the same as the ordinary 
 sedimentary limestones and need not be further dwelt upon 
 here. It may be stated, however, that owing to the softness 
 and ready solubility of their materials limestones do not, on 
 breaking down, except under rare instances, give rise to exten- 
 sive beds of arenaceous rocks, as do the siliceous varieties. 
 One of the best known rocks of this group is the breccia marble 
 near Point of Rocks in Maryland, which has been used in the 
 United States Capitol building at Washington. 
 
 (4) The Volcanic Group : Tuffs. Under this head are in- 
 cluded a great variety of fragmental rocks, composed of the 
 more or less finely comminuted materials ejected from vol- 
 
 i Proc. Boston Society of Natural History, Vol. XXV, 1800, p. 123. 
 
140 AQUEOUS ROCKS 
 
 canoes as ashes, dust, sand, and lapilli. Some of them are made 
 up of minute shreds of pumiceous glass. These occur, in many 
 instances, interbedded with lava flows of the same lithological 
 nature, and which are a product of the same periods of vol- 
 canic activity, the eruption of molten lava being accompanied 
 by intervals of explosive action, during which only fragmental 
 material was ejected. To such materials the name ^/roclastic 
 (Greek Trf/oo?, fire) is appropriately given. 
 
 The lithological character of the materials varies almost 
 indefinitely, and only very general names are given them in 
 the majority of cases. The name tuff or tuff a is given to the 
 entire group of volcanic materials formed as above, and also 
 by some authorities to fragmental rocks resulting from the 
 breaking down and reconsolidation of older volcanic lavas. 
 It would seem advisable to designate these last, as has F. 
 Lowinson-Lessing, 1 as pseudotuffs or tuffoids. 
 
 The names volcanic ashes, sand, and dust are applied to 
 the finer of these volcanic materials, and lapilli or rapilli to the 
 coarser fragments. 
 
 The dusts and sands are not infrequently composed of 
 minute shreds of volcanic glass, which were blown from the 
 volcanic vents and carried unknown distances, to be ultimately 
 deposited as stratified beds in comparatively shallow water. 
 Such are described more in detail under the head of ^Eolian 
 rocks (p. 153). The term trass is used to designate a compact 
 or earthy fragmental rock composed of pumice dust, in which 
 are embedded fragments of trachytic and basaltic rocks, car- 
 bonized wood, etc., and which occupies some of the valleys of 
 the Eifel. Peperino is a tufaceous rock composed of fragments 
 of basalt, leucite, lava, and limestone, with abundant crystals 
 of augite, mica, leucite, and magnetite. It occurs among the 
 Alban Hills, near Rome, Italy. Palagonite tuff is composed of 
 dust and fragments of basaltic lava, with pieces of a pale yellow, 
 green, reddish, or brownish glass called palagonite. The general 
 name of volcanic mud is given to the finely comminuted volcanic 
 material which in a more or less pasty or liquid condition is thrown 
 from volcanic vents during the incipient stages of eruption. 
 
 The tuffs are as a rule more or less distinctly stratified, of 
 very uneven texture, and with rarely a pisolitic structure. 
 They are found associated with volcanic rocks of all ages, and 
 1 Tschermaks Min. u. Petrog. Mittheilungen, Vol. IX, 1889, p, 530. 
 
VOLCANIC TUFFS 
 
 141 
 
 at times so highly metamorphosed as to render the original 
 nature of some doubt. Certain English authorities have con- 
 tended that a part of the so-called argillites and lire clays were 
 of finely comminuted volcanic materials. 
 
 The composition of the tuffs naturally varies with that of the 
 character of the lava from which they were derived. Being 
 in a more or less finely comminuted condition, often porous 
 and readily permeated by water or rootlets, they undergo de- 
 composition, forming soils the character of which is dependent 
 to some extent upon their lithological nature. The following 
 table shows the varying composition of rocks of this class : 
 
 
 o" 
 
 
 "S ,*- - 
 
 s 
 
 
 
 
 o 
 
 
 
 
 
 KINDS AND LOCALITIES 
 
 33 
 
 15 
 
 
 & 
 
 ! 
 
 S 
 
 C3 
 
 r 
 
 ,| 
 
 X 
 
 
 1 
 
 IS 
 
 |S 
 
 
 
 s 
 
 te 
 
 1 
 
 ^0 
 
 42 
 
 o 
 
 | 
 
 JS * 
 
 3 
 
 O 
 
 
 02 
 
 *< 
 
 &, 
 
 3 
 
 s 
 
 
 Oj 
 
 ^ 
 
 "2. 
 
 H 
 
 Pozzuolana, Naples, 
 
 % 
 
 % 
 
 % 
 
 
 % 
 
 % 
 
 % 
 
 % 
 
 % 
 
 % 
 
 Italy .... 
 
 59.144 
 
 21.28 
 
 4.76 
 
 1.90 
 
 
 4.37 
 
 6.2*3 
 
 
 
 100.24 
 
 Tuff, Crater of 
 
 
 
 
 
 
 
 
 
 
 
 Monte Nuova, 
 
 
 
 
 
 
 
 
 
 Chlorine 
 
 
 Italy .... 
 
 56.31 
 
 15.23 
 
 7.11 
 
 1.74 
 
 1.36 
 
 6.54 
 
 4.84 
 
 6.12 
 
 0.27 
 
 100.22 
 
 Trass, Andernach, 
 
 
 
 
 
 
 v , ' 
 
 
 
 
 Prussia. . . . 
 
 54.00 
 
 16.50 
 
 6.10 
 
 4.00 
 
 0.70 
 
 10.00 
 
 7.00 
 
 
 
 99.00 
 
 Tuff, Lacher See, 
 
 
 
 
 
 
 
 
 
 
 Prussia . . . 
 
 60.49 
 
 19.95 
 
 9.37 
 
 3.12 
 
 1.43 
 
 3.40 
 
 1.33 
 
 
 99.09 
 
 (2) ROCKS COMPOSED MAINLY OF DEBRIS FROM PLANT AND 
 
 ANIMAL LIFE 
 
 (1) The Siliceous Group : Infusorial or Diatomaceous Earth. 
 This is a fine white or pulverulent rock, composed mainly of 
 the minute shells, or tests, of diatoms, and often so soft and 
 friable as to crumble readily between the thumb and fingers. 
 It occurs in beds which, when compared with other rocks of 
 the earth's crust, are of comparatively insignificant proportions, 
 but which are nevertheless of considerable geological impor- 
 tance. Though deposits of this material are still forming, and 
 have been formed in times past at various periods of the earth's 
 history, they appear most abundantly associated with rocks 
 belonging to the Tertiary formations. 
 
 The beds are wide-spread, and some of them of economic 
 importance as a source of tripoli, absorbents for nitro-glycerine 
 
142 
 
 AQUEOUS ROCKS 
 
 compounds, non-conducting materials, etc. A deposit in Biln, 
 Bohemia, is some 14 feet in thickness, and is estimated by 
 Ehrenberg to contain 40,000,000 shells to every cubic inch. 
 Beds occur in the United States at South Beddington, Maine ; 
 
 FIG. 10. Section through lake basin showing the formation of infusorial earth, 
 a, bed rock ; bb, floating peat ; cc, decayed peat ; d, infusorial earth. 
 
 Lake Umbagog, New Hampshire ; in Morris County, New Jer- 
 sey ; near Richmond, Virginia ; in Calvert and Charles coun- 
 ties, Maryland; in New Mexico; Graham County, Arizona; near 
 Reno, Nevada, and in various parts of California and Oregon. 
 
 The New Jersey deposit covers about 3 acres, and varies 
 from 1 to 3 feet in thickness ; the Richmond bed extends from 
 Herring Bay, on the Chesapeake, to Petersburgh, Virginia, and 
 is in some places 30 feet in thickness ; the New Mexico deposit 
 is some 6 feet in thickness and has been traced some 1500 feet. 
 Professor Leconte states that near Monterey, in California, is 
 a bed some 50 feet in thickness, while the geologists of the 
 Fortieth Parallel Survey report beds not less than 300 feet in 
 thickness, of a pure white, pale buff, or canary-yellow color, 
 as occurring near Hunter's Station, west of Reno, Nevada. 
 
 The earth is used mainly as a polishing powder, and is some- 
 times designated as tripolite. It has also been used to some 
 extent to mix with nitro-glycerine in the manufacture of dyna- 
 mite. Chemically the rock is impure opal, as will be seen from 
 the following analyses made on samples from (I) Lake Umba- 
 gog, New Hampshire, (II) Morris County, New Jersey, and 
 (III) Paper Creek, Maryland: 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 ill 
 
 Silica (SiO 2 ) . . 
 
 80 53 % 
 
 80 60 % 
 
 81.53% 
 
 Iron oxides (Fe 2 3 and FeO) . . . 
 Alumina (Al 2 0s) 
 
 1.03 
 
 5.89 
 
 3.84 
 
 3.33 
 3.43 
 
 Lime (CaO) .... 
 
 35 
 
 58 
 
 2 61 
 
 Water (H 2 0) 
 
 11 05 
 
 14 00 
 
 6.04 
 
 Organic matter 
 
 98 
 
 
 
 
 
 
 
 
 99.38% 
 
 99.02% 
 
 96.94% 
 
 Number III showed also small amounts of potash and soda. 
 
PLATE 12 
 
 ;*-v ^ 
 
 ,.^^^-^d^ ? v^*V^-' : y^^''^'- ' ^ ; ^^ : ^t 
 
 tl^*^^-^,*; ^ ;i ! -^^^w'^V u t-V 4 ^.^.'^ .^v^J^ 
 ^3fc5S- : .-. "'^Pt^C^^i. ! ^, w:^^--^-i^v ;> ^^<^ -^ 
 
 ^ j i^M^^i^ 
 
 ,^-^i ^ ;^SM^:&^ 
 
 
 PIG. 1. Pisolitic limestone. 
 
 FIG. 2. Oolitic limestone. 
 
LIMESTONES 143 
 
 (2) The Calcareous Group. These rocks are made up of the 
 more or less fragmental remains of molluscs, corals, and other 
 marine and fresh-water animals. Many of them are but con- 
 solidated beds of calcareous mud, full of more or less fragment- 
 ary shells or casts of shells, as shown in Fig. 1, PI. 11. The 
 name coquina (Spanish for shell) is given to such as that 
 shown in Fig. 2, PI. 11, from St. Augustine, Florida. The 
 rock, it will be observed, is composed almost wholly of very 
 perfect shells of a bivalve mollusc, loosely cemented by calcare- 
 ous materials in a finely divided condition. From such forms 
 as these we have all possible gradations to compact crystalline 
 limestone. Special names are often given these calcareous 
 rocks, designating the character of materials from which they 
 are derived. Coral and shell limestones, as the names denote, 
 are composed mainly of the debris from these organisms. In 
 like manner such names as crinoidal, fusulina, etc., are applied. 
 
 Lumachelle is the name given to a shell limestone from the 
 Tyrol, in which the shells still retain their pearly lining and 
 original beauty. Nummulitic limestone carries fossil nummulites. 
 Rocks of this type were used in the construction of the pyramids 
 of Cheops. Chalk is a fine-grained, white, pulverulent rock, 
 composed of finely broken shells of marine molluscs, among 
 which minute foraminifera are abundant. Shell sand is a 
 loose aggregate of shell fragments, formed on sea-beaches by 
 the action of the winds and waves. On certain Hawaiian 
 beaches, such sands give out a distinct note, or peculiar crunch- 
 ing sound when walked over, or even when shaken in a closed 
 vessel, and are popularly known as sounding, or singing, sands. 
 The property is manifested only when the sand is dry and is 
 assumed to be due to the minute air cavities enclosed by the 
 shells. Oolitic and pisolitic limestones, as previously noted, are 
 made up of rounded concretionary masses of calcium carbonate, 
 and are in part of mechanical origin, and in part chemical de- 
 posits (PL 12). 
 
 The microscopic structure of an oolitic limestone from Prince- 
 ton, in Caldwell County, Kentucky, is shown in the accompany- 
 ing figure (p. 144). It will be noticed that the first step in the 
 formation of this stone was the deposition of concentric coat- 
 ings of lime about a nucleus which is sometimes nearly round, 
 but more frequently quite angular and irregular. After the 
 concretions were completed there were formed in all cases about 
 
144 
 
 AQUEOUS HOCKS 
 
 each one, narrow zones of minute radiating crystals of clear, 
 colorless calcite; then the larger crystals formed in the inter- 
 stices. The nuclei are composed in some cases of single frag- 
 ments or, again, of a group of fragments. Certain of the oolites 
 present no distinct concentric structure, but appear as mere 
 rounded masses merging gradually into the crystalline inter- 
 stitial portions. Recent microscopic studies have tended to 
 show that many of the oolitic limestones owe their structure to 
 the lime-secreting power of microscopic algie. 1 
 
 Limestones vary almost indefinitely in structure and color. 
 From the soft tufaceous or highly fossiliferous varieties there 
 
 is a constant gradation 
 to dense compact rocks 
 breaking with a conchoi- 
 dal or splintery fracture 
 and the true nature of 
 which is sometimes to be 
 ascertained only by chem- 
 ical tests. There is a like 
 variation in color. White 
 through all shades of gray 
 to black is common, and 
 more rarely occur yellow, 
 brown, pink, or red vari- 
 eties, the colors depend- 
 ing on organic matter and 
 metallic oxides, mainly 
 ferruginous. 
 
 Owing to the readiness with which calcium carbonate 
 undergoes crystallization, even at ordinary temperatures, few 
 limestones are wholly amorphous, but grade insensibly into 
 holocrystalline varieties such as are classed with the metamor- 
 phic rocks. The name marble is given to such limestones as 
 are of sufficiently close texture to take a polish and of such 
 colors as to make them desirable for ornamental work. A large 
 proportion of the marbles belong, however, to the metamor- 
 phic group. (See p. 162.) Figure 12 shows the microscopic 
 structure of a dark gray, variegated, highly fossiliferous lime- 
 stone belonging to the Cincinnati group, near Hamilton, Ohio. 
 It is a natural result of their method of formation that few 
 1 American Geologist, Vol. X, No. 5, 1892. 
 
 FIG. 11. Microstructure of oolitic limestone. 
 
LIMESTONES 
 
 145 
 
 limestones are of pure calcium carbonate. A portion of the 
 calcium is not infrequently replaced by magnesium, giving rise 
 to magnesian limestones, or when the proportion of magnesia 
 rises to 45. G5 % to dolomite. This last can as a rule be distin- 
 guished from limestone only by its increased hardness (3.5-4.5) 
 and specific gravity (2.8-2.95). Frequently chemical tests are 
 necessary, limestone effervescing readily when treated with 
 dilute hydrochloric acid, while dolomite is unacted upon. 
 
 Mechanically included materials, as sand and clay, are com- 
 mon, giving rise to siliceous and argillaceous varieties. The so- 
 called hydraulic limestone 
 is one containing 10 % 
 and upwards of these 
 impurities, and which, 
 when burnt and ground, 
 forms a cement charac- 
 terized by its property 
 of setting under water. 
 Many limestones, like the 
 dolomitic varieties in 
 Cook County, Illinois, 
 contain so large a pro- 
 portion of bituminous 
 matter as to give off a 
 distinct odor of petro- 
 leum when struck with a 
 hammer, or even to be- 
 come blackened on the 
 surface by its exudation when exposed to the weather. Others 
 contain phosphatic matter, and pass by insensible gradations 
 through what are known as phosphatic limestones to true phos- 
 phates (phosphorites, etc.). 
 
 In chemical composition the limestones vary, like other sedi- 
 mentary rocks, almost indefinitely, as will naturally be inferred 
 from what is said above. As a general rule, those varieties, 
 which have been formed in deep waters and at a distance from 
 the shores, will be of greatest purity, since less likely to have 
 become contaminated through detrital materials washed in from 
 the land. Even these may, however, be intermingled to a very 
 considerable extent with the fine siliceous and ferruginous mat- 
 ter, such as deep-sea dredgings have shown to be common to 
 
 FIG. 12. Microstructure of fossiliferous 
 limestone. 
 
146 
 
 AQUEOUS ROCKS 
 
 our modern sea-bottoms, and which are assumed to be in part at 
 least of volcanic origin. (See under JEolian Rocks, p. 153.) The 
 following table will give some idea of the wide range in chemi- 
 cal composition to be found in rocks of this class : 
 
 
 2 
 
 2 
 
 O 
 
 c 
 
 s 
 
 
 "S 5 S 
 
 s 
 
 ;- 2 S 
 
 5 ~r a 
 
 c s 
 
 
 l> 
 
 "S .2 j 
 
 c c 
 
 "So 
 
 s 
 
 CONSTITUENTS 
 
 n 
 
 &".- 1 
 
 |Ss 
 
 .2 ^ > 
 '5^">. 
 
 a 
 
 
 |||| 
 
 *i _o "S ^ 
 
 |="ci 
 
 Igl 
 
 >., fl 
 
 
 
 S^S 
 
 fSS^fc 
 
 S^5-i 
 
 Pt^^ 
 
 
 Carbonate of lime (CaC0 3 ) . . . 
 
 98.00 % 
 
 54.62% 
 
 41.48% 
 
 72.95-% 
 
 96.60 % 
 
 Carbonate of magnesium (MgCOs) 
 
 .... 
 
 45.04 
 
 24.55 
 
 3.84 
 
 0.13 
 
 Oxides of iron (FeO and Fe 2 O 3 ) . 
 Oxide of aluminum (A1 2 3 ) . . 
 
 
 J0.23 
 
 J4.03 
 
 1.34 
 4.50 
 
 0.98 
 
 Silica (Si0 2 ) and insol. silicates . 
 
 0.57 
 
 .... 
 
 29.93 
 
 14.79 
 
 0.50 
 
 Potash (K 2 0) 
 
 
 
 1 22 
 
 
 31 
 
 Soda (Na 2 0) .... 
 
 
 
 1 12 
 
 
 0.40 
 
 Water (H 2 0) 
 
 
 
 
 
 0.96 
 
 Sulphate of lime (CaS0 3 ) . . . 
 
 
 
 
 1.75 
 
 
 Organic matter 
 
 
 
 
 1.46 
 
 
 
 
 
 
 
 
 Totals 
 
 98 57 / 
 
 99 89 / 
 
 98 33 % 
 
 100 64 % 
 
 99 88 % 
 
 
 
 
 
 
 
 Researches by the Kentucky Geological Survey have shown 
 that the older limestones are, as a general rule, richer in soda, 
 phosphoric acid, and, when non-magnesian, in lime carbonate, 
 than are the younger more recently formed, and correspondingly 
 poorer in silica and insoluble silicates. This inverse ratio is 
 shown in the table on the opposite page, in which the rocks are 
 arranged by geological horizons, the oldest at the bottom. 
 
 The name shell marl, or merely marl, is given to an illy denned, 
 often arenaceous, soft and earthy rock consisting essentially of 
 shell material in a more or less fragmental condition, and usu- 
 ally intermixed with more or less clayey matter or siliceous 
 sand and silt. Geikie 1 would limit the term to fresh-water 
 accumulations of remains of mollusca, entomostra , and fresh- 
 water algse, but unfortunately the word has not been so used 
 in much of the literature extant. These marls, being easily 
 decomposed, and on account of their occasional richness in 
 phosphoric acid, or, perhaps, merely on account of the lime 
 they contain, are of value as fertilizers. The following analy- 
 ses of North Carolina marls, consisting largely of comminuted 
 
 1 Text-book of Geology. 3d ed. 
 
COMPOSITION OF LIMESTONES 
 
 147 
 
 83XVO 
 
 CS CS 
 
 ^5 o t- cs o 
 
 t- O 
 co cs o o -t t~- 
 
 O O rH T* O O O 
 
 3 K$% 
 
 t O GO CO 
 
 
 cs m ooj 
 
 O 'N 00 (M T-H -tO 
 
 (M T-H r-i GO 
 
 CO O T-H ^ 
 
 
 O <M ' I 
 
 cs co -to 
 
 .O CO (M CS O CO 
 
 O GO O . 
 
 S 2! W 
 
 vaog 
 
 
 
 
 
 
 O T-H -t O 
 
 CO 00 CS O 
 ^5 TtH rH CO O 
 
 .O t- Tt CO CO O 
 rvj O O rH T-H T 1- 
 
 1 lla 
 
 
 
 
 * 
 
 ^ 
 
 ( 8 OS) 
 
 CO T-H CO CS 
 
 .O O "t CO O (M 
 rv -N -tO CS O CO 
 ".CO O CO O (M CO 
 
 J ?, 
 
 rt 01 -". 
 
 
 T-H T-H 
 
 ^H rH 
 
 ^ 
 
 [OH I} 
 
 C\j CO T 1 O 
 
 O CM rH CO 
 
 ^ 3 ^ ^ ^ 
 
 O O CS O O CO 
 O O T-H O O 00 
 I-H (MO 00 -^ CM 
 
 tO ' 
 
 
 
 1 =- 
 
 -NVJ^ a_s;v NOHJ 
 
 CO <M T-H co 
 
 oo cs o t 
 
 00 (M 00 
 
 00 
 
 O rH ^t t- O CO 
 
 CO (NO CO rH O 
 CS CO CS CM <N t- 
 
 o o o o 
 
 O H O O 
 
 OMV vKiremy 
 
 ^ rH CO 
 
 ^0 CO CS rH 
 
 <M CO T-H 
 
 axvNOs 
 
 CO t- rH CO 
 
 o co -to 
 
 O CO O CM 
 
 co cs o co ^t I-H co 
 co cs o co co o o 
 
 CO T-H c-J O 
 
 
 CS CO -* O 
 <M CM 
 
 t- O CS Tt (M rH rH 
 
 rH rH (M 
 
 rH 0? CM O 
 CM (M 
 
 axvs 
 
 CO T)H O t~ 
 
 o T-H o cs 
 
 CO O (M t O O (M 
 
 -t T- i CS O O CO T-H 
 
 O CO (M rH CM rH CO 
 
 o co o t- 
 
 I~- Ol rH O 
 
 
 (M O CO rH 
 
 r- oo o o 
 
 (MO 'M CS O rH O 
 
 ^t cs o ** co cs 
 
 O -t O GO 
 
 CS CS 
 
 
 r>l O GO 
 
 
 CO rH 
 
 AXT 
 -AVHf) Ol^JIOajg 
 
 I <M CM CM" 
 
 m m % m 
 
 ; ; 
 
 
 
 
 
 
 00 ^ 
 
 'Ji V 2 ^ OS 
 
 G W) 
 
 LIMESTONES 
 
 nposition of G Coal-measure limestone 
 nposition of 10 upper sub-Carbonifer( 
 
 3 
 
 nposition of 3 lower sub-Carbonifen 
 5 (hydraulic) 
 nposition of Ulack Slate limestones . 
 
 nposuum 01 -i uaruoniieruus iimesLon 
 h 2 are magnesian (hydraulic ?) . . 
 re non-magnesian 
 nposition of 14 Niagara group (or Up 
 limestones 
 nposition of 3 Clinton group limeston* 
 nposition of 3 upper Hudson group lii 
 
 mposition of 7 middle Hudson grc 
 us mudstone") 
 nposition of 9 lower Hudson limeston< 
 
 nposition of 7 Trenton limestones (n 
 n) 
 nposition of 11 Trenton limestones (m 
 
 nposition of 2 Bird's-eye limestones 
 nposition of 3 Chazy limestones 
 
 
 ||| || | 
 
 |-sl||||||f| 
 
 fcC JH t S tJO tO 
 
 gyflgg 
 
148 
 
 AQUEOUS ROCKS 
 
 shells and sometimes coprolite nodules, will serve to show the 
 widely varying character of the materials grouped under this 
 
 name. 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 III 
 
 IV 
 
 V 
 
 VI 
 
 VII 
 
 Silica . . 
 
 % 
 6.97 
 
 % 
 61 61 
 
 % 
 18 84 
 
 % 
 
 58 25 
 
 % 
 25 28 
 
 39 36 
 
 % 
 5 65 
 
 Oxide of iron and alumina . 
 Lime 
 
 0.86 
 47.62 
 
 2.80 
 19.60 
 
 2.72 
 41.48 
 
 11.28 
 13.49 
 
 3.02 
 37 52 
 
 3.47 
 
 28 96 
 
 3.30 
 
 48 51 
 
 Magnesia 
 Potash 
 
 1.03 
 037 
 
 
 56 
 
 
 0.12 
 22 
 
 0.16 
 75 
 
 1.96 
 23 
 
 Soda 
 
 0.15 
 
 
 0.09 
 
 
 25 
 
 17 
 
 30 
 
 Phosphoric acid 
 Sulphuric acid 
 
 0.19 
 0.41 
 
 0.06 
 
 0.18 
 0.64 
 
 .... 
 
 0.40 
 40 
 
 0.11 
 
 18 
 
 trace 
 31 
 
 Carbonic acid 
 Organic matter and water . 
 
 38.15 
 4.25 
 
 15.37 
 
 32.07 
 3.42 
 
 10.59 
 
 29.02 
 2.98 
 
 22.73 
 4.11 
 
 39.80 
 0.60 
 
 (3) The Carbonaceous Group: Peat, Lignite, and Coal. Here 
 are included a variety of more or less oxygenated hydrocarbons 
 varying widely in physical and chemical properties, but alike 
 in originating from decomposing plant growth protected from 
 the oxidizing influences of the air. Plants, when decomposing 
 upon the surface of the ground, give off their carbon to the 
 atmosphere in the shape of carbonic acid gas (CO 2 ), leaving 
 only the strictly inorganic or mineral matter behind. When, 
 however, protected from this oxidizing influence by water, or 
 other plant growth, decomposition is greatly retarded, varying 
 portions of the carbonaceous and volatile matters are retained, 
 and the material becomes slowly converted into coal. Accord- 
 ing to the amount of change that has taken place in the original 
 plant material, the amount of volatile matter still retained by it, 
 its hardness and burning qualities, several varieties are recog- 
 nized, which, however, pass into each other by insensible gra- 
 dations. 
 
 Peat is the plant matter in its least changed condition. 
 It results from the gradual accumulation in bogs and marshes 
 of growths consisting mainly of sphagnous mosses, a low order 
 of plants having the faculty of continuing in growth upwards 
 as they die off below. In this way the deposits often assume 
 a very considerable thickness. Where sufficiently thick, the 
 1 Geology of North Carolina, Vol. I, 1875, p. 195. 
 
PEAT AND LIGNITE 
 
 149 
 
 lower portions have sometimes been converted into a dense 
 brownish black mass somewhat resembling true coal. The 
 deposits of peat are all comparatively recent and occur only 
 in humid climates. They are developed to an enormous 
 extent in Ireland about one-seventh of the entire country 
 being covered by them and average in some cases 25 feet 
 in thickness. They are also abundant in Europe and various 
 parts of North America. In Europe, and especially in Ireland, 
 the material is extensively utilized for fuel, and there would 
 seem no good reason for not so utilizing it in America. An 
 impure variety containing a considerable quantity of siliceous 
 sand, and locally known as " muck," is used as a fertilizer and 
 for " multching " throughout New England. Below are given 
 the results of analyses of (I) peat from the bog of Allan, 
 Ireland, (II) Maine (United States), and (III) Commander 
 Islands in Bering Sea. 
 
 CONSTITUENTS 
 
 I 
 
 11 
 
 III 
 
 Carbon 
 
 61.04% 
 
 21.00% 
 
 60.48% 
 
 Volatile matter 
 
 37.53 
 
 72.00 
 
 39.53 
 
 Ash 
 
 1.83 
 
 7.00 
 
 3.30 
 
 
 
 
 
 Lignite, or brown coal, is the name given to a brownish black 
 variety characterized by a brilliant lustre, conchoidal fracture, 
 and brown streak. Such contain from 55 % to 65 % of carbon, 
 and burn easily, with a smoky flame, but are inferior to the 
 true coals for heating purposes. They are also objectionable 
 on account of the soot they create, and their rapid disintegra- 
 tion and general deterioration when exposed to the air. They 
 occur in beds under conditions similar to the true coals, but 
 are of more recent origin. The lignitic coals of the regions of 
 the United States, west of the Mississippi River, are mainly of 
 Laramie (Upper Cretaceous) age, and, as a rule, show easily 
 recognizable traces of their organic origin, such as compressed 
 and flattened stem and trunks of trees with traces of woody 
 fibre. 
 
 Bituminous Coal. Under this name are included a series of 
 compact and brittle products in which no traces of organic 
 remains are to be seen on casual inspection, but which, under 
 
150 
 
 AQUEOUS KOCKS 
 
 the microscope, often show traces of woody fibre, spores of 
 lycopods, etc. These coals are usually of a brown to black 
 color, with a brown or gray brown streak, breaking with a 
 cubical or conchoidal fracture, and burning readily with a 
 yellow, smoky flame. They contain from 35 % to 70 % of fixed 
 carbon, 18 % to 60 % of volatile matter, and from 2 % to 20 % 
 of water, and only too frequently show traces of sulphur due 
 to included iron pyrites. Several varieties of bituminous coals 
 are recognized, the distinctions being based upon their manner 
 of burning. Coking coals are so called from the facility with 
 which they may be made to yield coke ; such give a yellow 
 flame in burning, and make a hot fire. They are soft, and 
 break with a cubical fracture. Other varieties of apparently 
 the same composition and general physical properties, cannot, 
 for some unexplained reason, be made to yield coke, and are 
 known as non-coking coals. Cannel coal has a very compact 
 structure, breaks with a conchoidal fracture, has a dull lustre, 
 ignites easily, and burns with a yellow flame. It does not coke. 
 Its chief characteristic is the large amount of volatile matter 
 given off when heated, whereby it is rendered of particular 
 value for making gas. Before the discovery of petroleum it 
 was used for the distillation of oils. Below is given the com- 
 position of (I) a coking coal from the Connelsville Basin of 
 Pennsylvania, and (II) a cannel coal from Kanawha County, 
 West Virginia. 1 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 Water 
 
 1.05% 
 
 
 Volatile matter 
 
 29.885 
 
 
 Fixed carbon 
 
 57.754 
 
 58.00% 
 
 Ash 
 
 9 895 
 
 23 50 
 
 Sulphur 
 
 1 339 
 
 18.50 
 
 
 
 
 
 100.00 % 
 
 100.00 % 
 
 Anthracite Coal. This is a deep black, lustrous, hard and 
 brittle variety, and represents the most highly metamorphosed 
 variety of the coal series. Such have been generally regarded 
 as bituminous coals from which a very large proportion of the 
 
 1 F. P. Dewey, Bull. 42, U. S. National Museum, 1891. 
 
THE PHOSPHATES 151 
 
 volatile constituents have been driven off by the agencies in- 
 volved in the production of mountain systems, or by the heat 
 incident to the injection of igneous rocks. Traces of organic 
 nature are almost entirely lacking in the matter of the anthra- 
 cite itself, though impressions of ferns, lycopods, sigillaria, and 
 other coal-forming plants are frequently associated with the 
 beds in such a manner as to leave little doubt as to their origin. 
 Anthracite is ignited with difficulty and burns with little flame, 
 but makes a hot fire. Below is given the average composition 
 of anthracite from the Kohinoor Colliery, Shenandoah, Penn- 
 sylvania. 1 
 
 Water 3.163% 
 
 Volatile matter 3.717 
 
 Fixed carbon 81.143 
 
 Sulphur 0.899 
 
 Ash 11.078 
 
 100.000 % 
 
 Like the other coals, anthracite occurs in true beds, but is 
 confined mostly to rocks of the Carboniferous age. Thin seams 
 of anthracite sometimes occur in Devonian and Silurian rocks, 
 but which are too small to be of economic value. Rarely 
 coals of more recent geological horizon have been found locally 
 altered into anthracite by the heat of igneous rocks. Through 
 a still further metamorphism, whereby it loses all its volatile 
 constituents, coal passes over into graphite. 
 
 The principal anthracite coal regions of the United States are 
 in eastern Pennsylvania. From here westward throughout the 
 interior states to the front range of the Rocky Mountains the 
 coals are all soft, or bituminous coals. Those of the Rocky 
 Mountain regions proper are largely lignitic, passing into the 
 bituminous varieties. 
 
 (4) Phosphatic Group: Phosphatic Sandstone; Bone Breccia; 
 Guano ; Coprolite Nodules. This is a group of rocks limited in 
 extent, but nevertheless of considerable economic importance, 
 owing to the high values of certain varieties for fertilizing 
 purposes. 
 
 G-uano consists mainly of the excrements of sea fowls, and is 
 
 to be found in beds of any importance only in rainless regions 
 
 like those of the western coast of South America and southern 
 
 Africa. The most noted deposits are on small islands off the 
 
 1 Bull. 42, U. S. National Museum, 1891. 
 
152 AQUEOUS ROCKS 
 
 coast of Peru. Immense flocks of sea fowls have, in the course 
 of centuries, covered the. ground with an accumulation of their 
 droppings to a depth of sometimes 30 to 80 feet, or even more. 
 
 An analysis of American guano gave : Combustible organic 
 matter and acids, 11.3 % ; ammonia (carbonate, etc.), 31.7 % ; 
 fixed alkaline salts, sulphates, phosphates, chlorides, etc., 
 8.1%; phosphates of lime and magnesia, 22.5%; oxalate of 
 lime, 2.6 % ; sand and earthy matter, 1.6 % ; water, 22.2 % 
 (Geikie). 
 
 Coprolite nodules are likewise the excrements of vertebrate 
 animals ; those among the Carboniferous shales of the basin of 
 the Firth of Forth are regarded as accumulated excretions of 
 ganoid fishes. 
 
 Phosphatic sandstones, as the name denotes, are arenaceous 
 rocks containing more or less phosphatic matter. Inasmuch 
 as the phosphatic material is derived largely by leaching and 
 segregation, these rocks have been already described under the 
 head of chemical deposits (p. 119). In the river beds of the 
 Carolinas are found rounded and nodular masses of this nature, 
 consisting of siliceous and calcareous sand, with embedded 
 bones, teeth of sharks, and other animal remains. Bone breccia, 
 consists of fragmental bones of mammals cemented by argil- 
 laceous, earthy, or calcareous matter. 
 
III. AEOLIAN ROCKS 
 
 This group comprises a small and comparatively insignificant 
 class of rocks formed from materials drifted by the winds, and 
 more or less compacted into rock masses. They are, as a rule, 
 of a loose and friable texture and of a fragmental nature. 
 Many of the volcanic fragmental rocks (tuffs) are grouped 
 here, their materials having been thrown from the volcanic 
 vents in small fragments and drifted long distances by wind 
 prior to falling upon the surface of the ground or into the 
 water for their final consolidation. 
 
 One of the most common results of wind action on the land 
 is the production of sand-dunes billowy masses of loose sand 
 which, like drifts of snow, though more slowly, gradually 
 change their outlines and creep onward under the restless goad- 
 ing of the Avind. 
 
 Such, owing to their superficial nature, recent origin, and 
 loose state of consolidation, are considered more in detail in 
 the chapter on The Regolith, p. 299. On undergoing consoli- 
 dation, these dune sands may give rise to sandstones in many 
 instances indistinguishable from those of aqueous origin, though 
 less regularly bedded. The finely disintegrated shell and coral 
 material thrown up by the waves on the beaches of Bermuda 
 is caught up by the winds and drifted inland, forming hills 
 which, in some instances, are 250 feet in height. Being soluble 
 in the water from rainfalls, these become shortly reconsolidated 
 through the deposition of lime carbonate in the interstices of 
 the fragments, and form thus the drift rock which comprises a 
 large portion of the mass of the islands above tide level. 
 
 The finely comminuted materials ejected from volcanic vents 
 may be likewise transported by atmospheric currents and, far 
 from their source, again deposited in beds of no insignificant 
 proportions. These, on induration, give rise to fine-grained 
 tuffs, and, where the final deposition has taken place in water, 
 to distinctly laminated, fine white rocks the lithological nature 
 
 153 
 
154 
 
 KOCKS 
 
 of which can be made out only by means of the microscope. 
 Such are many of the Pliocene sandstones of Idaho and Mon- 
 tana. 1 The following analyses of samples of tuffs from (I) 
 Marsh Creek Valley, Idaho ; and (II) Little Sage Creek, Mon- 
 tana, will serve to show their composition. 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 Ignition (H 2 0) 
 
 7.60 % 
 
 7 62 % 
 
 Oxide of iron and aluminium (Fe 2 3 and A1 2 O 3 ) . 
 Silica (Si0 2 ) 
 
 16.22 
 68.92 
 
 18.24 
 65.56 
 
 Lime (CaO) 
 
 1.62 
 
 2.58 
 
 Magnesia (MgO) 
 
 Trace 
 
 0.72 
 
 Soda (Na 2 O) 
 
 1.56 
 
 2.08 
 
 Potash (K 2 0) 
 
 4.00 
 
 3.94 
 
 
 
 
 
 99.92 % 
 
 100.74% 
 
 1 On the Composition of Certain Pliocene Sandstones from Montana and 
 Idaho, Am. Jour, of Science, Vol. XXVII, 1886, p. 199. 
 
IV. METAMORPHIC ROCKS 
 
 Before proceeding to describe in detail the metamorphic 
 rocks, it will be well to devote a brief space to a discussion 
 of the processes by which this metamorphism has been brought 
 about. 
 
 The word metamorphism as used in geology includes changes 
 in the structure of rocks induced through agencies in part 
 physical, and in part chemical, in their nature. It is, in fact, 
 a very general term, and indicates any transformation taking 
 place in the composition and structural features of rocks 
 of any kind, whether sedimentary or igneous, and from any 
 cause whatever. Rocks laid down in the form of sediments 
 may become so deeply buried as to be subject to intense heat 
 from the earth's interior, as well as to pressure from weight of 
 the overlying material. In this way, a partial or complete 
 fusion of the constituents takes place, which is followed by a 
 crystallization whereby the original fragmental nature may be 
 wholly or in part obscured. This form of change is included 
 under the general name of regional metamorphism. In this 
 manner, it was once assumed, were formed the gneisses, a part 
 of the granites, and the vast series of crystalline schists and 
 calcareous rocks (marbles, etc.). It has, however, been shown 
 that the banded and foliated structure shown by gneisses and 
 schists is not in all cases necessarily an indication of an original 
 bedded structure, but may be due to pressure acting through- 
 out long periods of time, and accompanied by the heat thereby 
 generated. A common and readily understood illustration of 
 this principle of metamorphism by pressure is offered by the 
 roofing slates. These, first laid down as fine silts, rarely show 
 their eminent cleavages whereby they are rendered so useful to 
 man, parallel to their original bedding, but inclined at any and 
 all angles thereto. In such cases the bedding is not infre- 
 quently indicated by the dark bands or "ribbons" which are 
 so evident on a split surface. 
 
 155 
 
156 METAMORPHIC ROCKS 
 
 But it is not alone the fragmental rocks which thus become 
 schistose under pressure. Originally massive, igneous rocks, 
 in regions of profound disturbance have been found converted 
 into schistose aggregates, indistinguishable from rocks ordina- 
 rily assumed to be sedimentary. Thus the greenstone schists 
 of the Menominee and Marquette regions of Michigan have 
 been shown by Williams l to be highly altered eruptive rocks, 
 mainly gabbros, diabases, and diorites, originally massive, but 
 now foliated, schistose, and variously crumpled through the 
 squeezing and shearing to which they have been subjected since 
 the period of their first extrusion. The changes in these and 
 similar cases, is rarely purely physical, though at times the 
 chemical alterations may be quite inconspicuous. The ulti- 
 mate composition of the rock may remain essentially the same, 
 while the method of combination of its various elements may 
 have undergone extensive alteration. Quartzes and feldspars 
 may be crushed and distorted, drawn out into lens-shaped and 
 variously elongated forms, while secondary minerals like feld- 
 spars, quartz, zoisite, garnet, hornblende, epidote, and the micas 
 may be abundantly generated. 
 
 One of the commonest results of pressure effects upon 
 igneous rocks is the conversion of augite or other minerals of 
 the pyroxene group into hornblendes. The coarse hypersthene 
 gabbro occurring about Baltimore is fpund locally altered into 
 a rock consisting essentially of a schistose aggregate of horn- 
 blende and plagioclase feldspars, or what, on mineralogical 
 grounds, might be classed as a diorite. 2 The chemical compo- 
 sition in this case has undergone no appreciable change ; there 
 has been simply a molecular rearrangement of the particles. 
 In such cases proof of the character of the change that has 
 taken place is usually found in the fractured and otherwise 
 distorted condition of many of the constituent minerals, as well 
 as intermediate stages of alteration, whereby a residual augite 
 crystal is found enclosed in an envelope of secondary horn- 
 blende, as shown in Fig. 1, on p. 40. To the secondary 
 minerals formed in this way the technical name paramorpJiic 
 is applied. To such changes as are above described the name 
 dynamic metamorphism is given. 
 
 The protrusion of a mass of molten matter into the over- 
 
 1 Bull. 62, U. S. Geol. Survey, 1890. 
 
 2 Bull. 28, U. S. Geol. Survey, 1886. 
 
CONTACT METAMORPHISM 157 
 
 lying strata may give rise to a series of changes differing from 
 the last in that they are due mainly to heat and to the chemical 
 action of accompanying vapors and solutions. Since these 
 changes are confined to limited areas along the line of the 
 contacts between the two bodies, they are defined as contact 
 metamorphisms. As illustrative of such changes, a few cases 
 may be described. 
 
 Near Gefrees, in Bavaria, an eruptive biotite granite has 
 been protruded into clay slates and phyllites. At the line 
 of contact both phyllites and slates are converted into a 
 hard, compact blue-black " hornfels " consisting of a crystal- 
 line granular aggregate of quartz, deep reddish brown mica 
 (biotite), a little muscovite and andalusite. This zone, some 
 120 paces in width, is succeeded by a second some 380 paces in 
 width in which the rocks are converted into andalusite mica 
 schists, and this by a third zone some 500 paces wide in which 
 the gradually failing energy was sufficient only to give rise to 
 a spotted mica schist (knoten schiefer), and lastly, a zone some 
 400 paces wide in which the clay slates has become converted 
 into a chiastolite schist, and the phyllites to a biotite-bearing 
 variety. In all these cases the chemical character of the rock 
 remains essentially the same. Through the metamorphosing 
 action of intruded basic rocks crystalline schists near Peekskill, 
 New York, have near the line of contact become puckered and 
 filled with lens-shaped eyes of quartz containing garnets and 
 other minerals, while crystals of staurolite, sillimanite, cyanite, 
 and garnet appear, the amount of change being directly propor- 
 tional to the nearness of the line of contact. At the contact 
 the schistose structure is almost completely obliterated and the 
 schists become hard and massive, appearing more or less fused 
 with the eruptive, and consist of a large number of minerals. 
 Briefly expressed, the progressive change, approaching the line 
 of contact, consists in a gradual decrease in the proportional 
 amount of silica and alkalies, with a corresponding increase in 
 iron and alumina, this being accompanied by a disappearance 
 of the quartz and muscovite and the development of biotite, 
 sillimanite, staurolite, cyanite, and garnet, as above mentioned. 
 Where limestones abound, they have become bleached and 
 rendered more closely crystalline, while a variety of meta- 
 morphic minerals, as lime-bearing pyroxenes, hornblendes, 
 zoisite, sphene, and scapolite have been developed. 
 
158 
 
 METAMORPHIC ROCKS 
 
 A common form of metamorpliism is manifested in the pro- 
 duction of a quartzite from siliceous sandstone. This, in its 
 simplest form, is brought about by a secondary deposit of silica 
 about the original rounded granules of sand, whereby the entire 
 mass is converted into an aggregate of quartz crystals, the out- 
 lines of which are more or less imperfect through mutual in- 
 terference in process of 
 growth. The microscopic 
 structure of a quartzite 
 of this nature is shown in 
 Fig. 13. In this case the 
 original rounded gran- 
 ules are readily recog- 
 nized from the fact that 
 not merely did they fre- 
 quently contain small 
 cavities and needle-like 
 enclosures, but exteriorly 
 they were covered with a 
 thin pellicle of iron ox- 
 ide, while the secondary 
 deposit, which now fills 
 all the interspaces, is free 
 from enclosures of all 
 kinds and quite pellucid. 
 
 In many quartzites a shearing force has acted a prominent 
 part, whereby the granules have become elongated and more or 
 less pulverized along their margins by the friction of rubbing 
 one over the other. In such cases mica and other secondary 
 minerals are often developed, and the rock passes over into a 
 mica schist. 
 
 Still another form of change, or metamorpliism, is that 
 known by the name of metasomatosis, a process of indefinite 
 substitution and replacement. Through the chemical action 
 of percolating solutions certain constituents of a rock may be 
 leached out and replaced by others in indefinite proportions. 
 It is by such processes that have originated a large share of 
 the serpentinous rocks, dolomites, etc. The mineral olivine, 
 an anhydrous ferruginous silicate of magnesia, passes over into 
 serpentine by a simple process of hydration, and a more or less 
 complete change of its combined iron from the ferrous to the 
 
 FIG. 13. Microstructure of quartzite, showing 
 secondary deposit of silica about the original 
 quartz grains. 
 
METASOMATOSIS 159 
 
 ferric state ; this constituent not infrequently separating out 
 during the process of change, and crystallizing as magnetite, 
 or remaining as an amorphous hematite or limonite. Provided 
 there be no loss in silica, this change in the olivine, according 
 to T. Sterry Hunt, must be accompanied by an increase of 
 volume amounting to some 33%. Through the hydration of 
 eruptive olivine-bearing rocks, or rocks rich in other magnesian 
 silicate minerals, have originated a large proportion of the so- 
 called serpentines and verd-antique marbles. Many serpentines 
 and serpentinous limestones are derived from metamorphic 
 rocks rich in lime-magnesian pyroxenes or amphiboles, as mala- 
 colite and tremolite. To such an origin are to be referred 
 such serpentinous limestones as those of Essex County, New 
 York ; Easton, Pennsylvania, and Montville, New Jersey. In 
 the last-named instance the original rock was a coarsely crys- 
 talline dolomitic limestone containing numerous nodular masses 
 of white pyroxene (malacolite). Under this metasomatic pro- 
 cess they yielded up their calcium, which recrystallized as 
 calcium carbonate or calcite, while the silica and magnesia, 
 combined with some 13 % of water, remained as a beautiful 
 green and yellow serpentine. The transformation was accom- 
 panied by a considerable increase in bulk, whereby the exterior 
 of the nodules, pressed against the rough walls of the enclos- 
 ing rock, became scratched and polished like boulders from the 
 glacial drift, or the entire mass even took on a platy, schistose 
 structure. Figure 8, from a specimen in the National Museum, 
 illustrates a transitional phase of this change, the interior 
 rounded mass of a gray color being of still unaltered pyroxene, 
 while the dark material forming the exterior shell, or travers- 
 ing the gray in fine thread-like veins, is the secondary ser- 
 pentine. In a like manner in all probability originated the 
 peculiar structure imitative of animal organisms known as 
 Eozoon Canadense. 1 
 
 The conversion of a limestone into a dolomite is believed to 
 have been brought about by a somewhat similar process. Indeed 
 it is doubtful if this last-named rock is ever a product of direct 
 sedimentation or precipitation. Although sea-water contains 
 
 1 See On the Serpentine of Montville, New Jersey, Proc. U. S. National 
 Museum, Vol. XI, 1888 ; Notes on the Serpentinous Rocks of Essex County, 
 New York, etc., ibid., Vol. XII, 1889; and On the Ophiolite of Thurman, 
 Warren County, New York, Am. Jour, of Science, Vol. XXXVII, 1889. 
 
160 METAMORPHIC ROCKS 
 
 from three to four times as much magnesia as lime, evidence is 
 wanting to show that the material is ever secreted in appre- 
 ciable quantities by marine animals, and hence the sedimentary 
 deposits, resulting from the accumulation of the remains of these 
 animals, must be correspondingly lacking in this constituent. It 
 has been argued by Beaumont and others that through a process 
 of partial molecular replacement (metasomatosis) pre-existing 
 limestones were converted into dolomites, the process consisting 
 in the replacement of every other molecule of calcium carbonate 
 by one of the magnesium carbonate. As the dolomite molecule 
 is the more dense of the two, such replacement, in any given 
 limestone bed, must result in a contraction amounting to some 
 12 J / . Assuming that a dolomitic mass resulting in this way 
 is of the same bulk as the original limestone, this shrinkage 
 must manifest itself in the production of interstitial rifts and 
 cavities, such as do actually occur in many dolomitic lime- 
 stones, as those of the Ohio Trenton formations. The principal 
 objection to this theory lies in the difficulty of accounting for 
 the large amount of magnesia in solution ; whence its source, 
 etc. The same objections apparently apply to the explanation 
 given by M. C. Klement. 1 This writer describes a series of 
 experiments in which solutions of sodium chloride and magne- 
 sium sulphate were made to act upon pulverized calcite and 
 aragonite. From the results obtained, he concludes that dolo- 
 mite is formed by the action of sea-water, concentrated in en- 
 closed basins and heated by the sun, on the aragonite deposited 
 by marine organisms, in such a way that a mixture of carbon- 
 ates of calcium and of magnesium is first produced, and which 
 is subsequently converted into dolomite. 
 
 Still another theory is that which regards the dolomite as a 
 residuary product formed by the leaching out of the lime car- 
 bonate from beds of impure, slightly magnesian limestone, 
 leaving behind the less soluble magnesian carbonate. The 
 amount of material lost, and the consequent contraction of the 
 original beds, must necessarily vary with their purity ; but in 
 any case where the residual mass has reached the condition of 
 a true dolomite, the proportional loss must have been enormous, 
 since in no cases are unaltered sediments known to contain 
 more than 4 or 5 % of magnesian carbonate. Although on 
 first thought this theory seems the more plausible of the two, 
 1 Bull, de la Societe Geologique de Beige, Tome IX, 1895. 
 
HYDRO-METAMORPHISM 161 
 
 it is apparently rendered invalid by the presence in these dolo- 
 mites of very perfect casts of fossils which have undergone no 
 crushing or distortion whatever, and which tend to show that 
 the beds as a whole, so far from having undergone a shrink- 
 age of 95 % and upwards, are of essentially the same bulk as 
 when laid down. The subject is too large for complete dis- 
 cussion here, and the reader is referred to standard works on 
 chemical geology, as well as the current literature. 1 
 
 Still another form of change in the structure and mineral 
 composition of a rock is that brought about through the action 
 of water below the zone of oxidation and of true weathering. 
 It may be best described as a process of hydro-metamorphism, 
 since the influence of water is paramount. It is to this form 
 of metamorphism that is due the production, in part, of secondary 
 epidote, chlorite, sericite, leucoxene, kaolin (?) pyrite, and vari- 
 ous zeolitic compounds from pre-existing minerals, but without 
 in any way changing the character as a geological body of the 
 rock mass in which they occur. Such changes are in part meta- 
 somatic, and in many instances are rendered more intense by 
 dynamic causes. This form of change has, unfortunately, been 
 too frequently confounded with weathering and decomposition. 2 
 
 Under the head of metamorphic, then, is grouped a large 
 series of rocks which have been changed from their original 
 condition through the dynamical and chemical agencies above 
 described, and which may have been in part of. aqueous and in 
 part of eruptive origin. Were it possible, it might have been 
 better to describe each' class of these rocks together with the 
 corresponding igneous or aqueous form from which it was de- 
 rived by this process of change, or metamorphism. In only 
 too many cases, however, the change has been so complete as 
 to quite obliterate all such traces of the original character as 
 would lead to safe and satisfactory conclusions, and consistency 
 demands that all be grouped together. 
 
 1 See The Magnesian Series of the Northwestern States, by C. W. Hall and 
 F. W. Sardeson. Bull. Geol. Soc. of America, Vol. VI, 1895, p. 167. 
 
 2 While it is true that no new compound can be formed without first a break- 
 ing up, or decomposition, of those already existing, still, as this decomposition 
 affects only the individual minerals, and not the integrity of the rock mass as a 
 whole, it would seem preferable to include such changes under the name of 
 alteration and metamorphism. Weathering it certainly is not, though it is 
 essentially the form of change which Roth (Allegemeine u. Chemische Geologie, 
 Vol. I, -pp. 159-412) has designated as complex weathering (Complicirte Vtr- 
 witterung}. 
 
162 METAMORPHIC ROCKS 
 
 Accordingly as they vary in structure, we may divide these 
 metamorphic rocks into two general groups as below: 1. Strati- 
 fied or bedded; 2. foliated or schistose. 
 
 1. STRATIFIED OR BEDDED 
 (1) THE CRYSTALLINE LIMESTONES AND DOLOMITES 
 
 . Here are included the metamorphosed form of the sedimen- 
 tary rocks described on p. 143. 
 
 Mineral Composition. The essential constituent of the crys- 
 talline limestones is the mineral calcite. The common acces- 
 sories are minerals of the mica, amphibole, or proxene group, 
 and frequently sphene, tourmaline, garnets, vesuvianite, apatite, 
 pyrite, graphite, etc. 
 
 Chemical Composition. As may be inferred from the mineral 
 composition, these rocks, when pure, consist only of calcium 
 carbonate. They are, however, rarely if ever found in a state 
 of absolute purity, but show more or less magnesia, alumina, 
 and other constituents of the accessory minerals. The analyses 
 given on pp. 14647 will serve equally well here, and need not 
 be repeated. 
 
 Structure. The limestones are eminently stratified rocks, 
 though this peculiarity is not always sufficiently marked to be 
 seen in the hand specimen. The purest and finest crystalline 
 varieties often show a granular texture like that of loaf sugar, 
 and hence are spoken of as saccharoidal limestones. Statuary 
 marble is a good illustration of this type. Under the micro- 
 scope the stone is shown to be made up of small grains, which, 
 having mutually interfered in process of growth, do not possess 
 perfect crystal outlines, but are rounded and irregular in out- 
 line, as shown in Fig. 14. All grades of textures are common, 
 the coarser forms sometimes showing individual crystals an inch 
 in length. Though in their unchanged conditions highly fossi- 
 liferous or tufaceous, these structural features may be wholly 
 or in part obliterated by crystallization. 
 
 Colors. The color of pure limestone is snow-white, as seen 
 in statuary marble. Other common colors are pink or reddish, 
 greenish, blue-gray through all shades of gray to black. The 
 pink and red colors are due to iron oxides, the greenish as a 
 
STRATIFIED OR BEDDED 
 
 163 
 
 rule to micaceous minerals, the blue-gray and black to carbo- 
 naceous matter. 
 
 Geological Age and Mode of Occurrence. The crystalline 
 limestone and dolomites are but the metamorphosed sedimentary 
 deposits such as have al- 
 ready been described on 
 p. 143. They occur asso- 
 ciated with rocks of all 
 ages, but only in regions 
 that have been subjected 
 to disturbances such as the 
 folding and faulting inci- 
 dent to mountain-making, 
 or the heat from intruded 
 igneous rocks. From an 
 economic standpoint, the 
 rocks of this group are 
 not infrequently of great 
 economic value for struct- 
 ural and decorative pur- 
 poses. 
 
 Classification and Nomenclature. It is common to speak of 
 this entire group of rocks as simply limestones, though many 
 varietal names are often rather indefinitely applied. The name 
 marble is given to any calcareous or magnesian rock suffi- 
 ciently beautiful to be utilized in decorative work. Argilla- 
 ceous and siliceous limestones carry clayey matter and sand. 
 Dolomite (so named after the French geologist Dolomieu) 
 consists of 45.50% carbonate of magnesia and 54.40% car- 
 bonate of lime, as already noted. The names ophiolite and 
 opliicalcite are popularly applied to stones consisting of a 
 granular aggregate of calcite and serpentine, such as occur 
 in Essex County, New York, and are used as marbles. The 
 so-called Eozoon Canadenses, a supposed fossil rhizopod, belongs 
 here. The serpentinous matter in such cases originates from a 
 non-aluminous pyroxene by a process of hydration, as already 
 explained. 
 
 FIG. 14. Microstructure of crystalline lime- 
 stone (marble). 
 
164 METAMORPHIC ROCKS 
 
 2. FOLIATED OR SCHISTOSE 
 
 (1) THE GNEISSES 
 
 Gneiss, from the German Gf-neis, a term used by the miners 
 of Saxony to designate the country rock in which occur the 
 ore deposits of the Erzgebirge. The word is pronounced as 
 though spelled nice. 
 
 Mineral and Chemical Composition. The composition of the 
 gneisses is essentially the same as that of the granites, from 
 which they differ only in structure and origin. They, how- 
 ever, present a greater variety and abundance of accessory 
 minerals, chief among which may be mentioned (besides those 
 of the mica, hornblende, or pyroxene group) garnet, tour- 
 maline, beryl, sphene, apatite, zircon, cordierite, pyrite, and 
 graphite. 
 
 Structure. Structurally the gneisses are holocrystalline 
 granular rocks, as are the granites, but differ in that the 
 various constituents are arranged in approximately parallel 
 bands or layers, as shown in PI. 13. 
 
 In width and texture these bands vary indefinitely. It is 
 common to find bands of coarsely crystalline quartz several 
 inches in width, alternating with others of feldspar, or feld- 
 spar, quartz, and mica, or hornblende. A lenticular structure 
 is common, produced by lens-shaped aggregates of quartz or 
 feldspar, about and around which are bent the hornblendes or 
 mica laminae. The rocks vary from finely and evenly fissile 
 through all grades of coarseness, and become at times so mas- 
 sive as to be indistinguishable in the hand specimens from 
 granites. The causes of the foliated structure are mentioned 
 below. 
 
 Colors. Like the granites, they are all shades of gray, 
 greenish, pink, or red. 
 
 Geological Age and Mode of Occurrence. The true gneisses are 
 among the oldest crystalline rocks, and have been considered by 
 many geologists as representing "portions of the primeval crust 
 of the globe, traces of the surface that first congealed upon the 
 molten nucleus." By others they are regarded as metamor- 
 phosed sedimentary deposits resulting from the breaking down 
 of still older rocks, and may not in themselves, therefore, be con- 
 fined to any particular geological horizon. They are in large 
 
PLATE 13 
 
 i mm 
 
 FIG. 1. Banded gneiss. 
 
 FIG. 2. Foliated gneiss. 
 
THE GNEISSES 165 
 
 part, however, indisputably the oldest known rocks, lying be- 
 neath or being cut by all rocks of later formation or injection. 
 The origin of the gneisses, as above suggested, is in many 
 cases somewhat obscure, the banded or foliated structure being 
 considered by some as representing the original bedding of the 
 sediments, the different bands representing layers of varying 
 composition. This structure is now, however, considered to be 
 due to mechanical causes, and in no way dependent upon origi- 
 nal stratification. The name, as commonly used, is made to in- 
 clude rocks of widely different structure, and which are beyond 
 doubt in part sedimentary and in part eruptive, but in all cases 
 altered from their original conditions. 
 
 FIG. 15. Microstructure of gneiss, showing at the points a broken feldspars. 
 
 This alteration, it should be stated, has been brought about 
 not by heat and crystallization alone, but in many cases by 
 processes of squeezing, crumpling, and folding so complex as 
 almost to warrant the application of the term kneading thereto. 
 It is even possible to conceive that some of them may be origi- 
 nal massive or foliated rocks into which eruptive materials have 
 since been injected along lines of foliation or of weakness due to 
 shearing, and the entire mass again submitted to such a knead- 
 ing as to render it practically impossible to now decide what 
 are portions of the original rock and what of the subsequently 
 injected. 
 
166 
 
 METAMORPHIC ROCKS 
 
 The close chemical relationship which may exist between 
 clastic, metamorphic, and eruptive rocks is shown in the selected 
 series of analyses here given. 
 
 CONSTITUENTS 
 
 GRANITE 
 
 GNEISS 
 
 8 
 
 N 
 Jz, 
 
 o 
 
 SANDSTONE 
 
 M 
 
 3 
 
 j 
 
 OQ 
 
 I 
 
 OQ 
 
 DISINTE- 
 GRATED 
 GRANITE 
 
 
 I 
 
 II 
 
 III 
 
 IV 
 
 v 
 
 VI 
 
 VII 
 
 Silica (Si0 2 ) 
 
 68 18 
 
 % 
 61.96 
 
 69.24 
 
 % 
 69 94 
 
 % 
 
 61.91 
 
 % 
 
 60.32 
 
 65.69 
 
 Titanium oxide (Ti02) . 
 
 
 1.66 
 
 
 Not det 
 
 
 
 0.31 
 
 Alumina (A1 2 03) .... 
 Ferric oxide (Fe 2 Os) . . . 
 Ferrous oxide (FeO) 
 
 16.20 
 4.10 
 
 19.73 
 4.60 
 
 14.85 
 2.62 
 
 13.15 
 
 2.48 
 
 21.73 
 4.73 
 
 23.10 
 7.05 
 
 15.23 
 4.39 
 
 Ferrous sulphide (FeS2) 
 
 
 4.33 
 
 
 
 
 
 
 Manganese oxide (MnO) . . 
 Lime (CaO) 
 
 1 75 
 
 Trace 
 35 
 
 0.45 
 2 10 
 
 0.70 
 
 3.08 
 
 09 
 
 .... 
 
 Not det. 
 2 63 
 
 Magnesia (MgO) . . 
 
 0.48 
 
 1.81 
 
 096 
 
 Trace 
 
 0.59 
 
 0.87 
 
 2.64 
 
 Soda (Na 2 0) 
 
 2.88 
 
 0.79 
 
 4.30 
 
 5.43 
 
 0.25 
 
 0.49 
 
 2.12 
 
 Potash (K 2 0) 
 
 6 48 
 
 2 50 
 
 4 33 
 
 3 30 
 
 3 16 
 
 3 83 
 
 2 00 
 
 Ignition 
 
 
 1 82 
 
 70 
 
 1 01 
 
 7 43 
 
 4 08 
 
 4.70 
 
 
 
 
 
 
 
 
 
 
 100.07 
 
 99.55 
 
 99.56 
 
 99.10 
 
 99.89 
 
 99.74 
 
 99.71 
 
 I. Granite : Syene, Egypt. II. Gneiss : St. Jean de Matha, Province of Que- 
 bec, Canada. III. Gneiss : Trembling Mountain, Province of Quebec, Canada. 
 IV. Sandstone : Portland, Connecticut. Y. Shale : England. VI. Slate : Lan- 
 caster County, Pennsylvania. VII. Disintegrated granite : District of Columbia. 
 
 Figures 1 and 2 on PI. 13 shows two rather extreme types of 
 these gneissoid rocks. Figure 1 is that of a banded gneiss from 
 Madison County, Montana, and which, so far as we know, may 
 be an altered sedimentary rock. In Fig. 2 of the same plate 
 is shown a foliation rather than a banded rock, and whatever 
 may have been its origin, it undoubtedly owes its foliated 
 structure to dynamic agencies. The effect of the shearing 
 force whereby the foliation was produced is evident in the 
 figure, even to the unaided eye, to the left and just above the 
 centre, where an elongated feldspar is seen broken transversely 
 in four pieces. The same features are brought out even more 
 plainly in Fig. 15, which shows the structure of this same gneiss 
 as seen under the microscope. 
 
 As in the present state of our knowledge it is in most cases 
 impossible to separate what may be true metamorphosed sedi- 
 
GNEISS 
 
 167 
 
 mentary rocks from those in which the foliated or banded 
 structure is in no way connected with bedding and which may 
 or may not be altered eruptives, all are grouped together here. 
 
 Classification and Nomenclature. The varietal distinctions 
 are based upon the character of the prevailing accessory min- 
 eral, as in the granites, forming a parallel series. We thus 
 have biotite gneiss, muscovite gneiss, biotite-muscovite gneiss, horn- 
 blende gneiss, etc. Rarely the mineral cordierite occurs in suffi- 
 cient abundance to become a characterizing accessory. Such 
 forms occur in Gilford County, Connecticut, and in Saxony. 
 
 The name granulite or leptynite is applied to a banded quartz- 
 feldspar rock, the constituents of which occur in the form of 
 small grains and show under the microscope a mosaic structure. 
 The Saxon granulites are regarded by Lehman as eruptive 
 rocks altered by pressure. Halleflinta is a Swedish name for 
 a rock resembling in most respects the eruptive felsites or 
 quartz porphyries already described. Such, however, show a 
 banded structure and are, as a rule, regarded as metamorphic 
 rocks. PorpJiyroid is also a felsitic rock with a more or less 
 schistose structure, and with porphyritic feldspar or quartzes. 
 Such have been described from the Ardennes, France. 
 
 GNEISS 
 
 ANALOGOUS MASSIVE TYPE 
 
 OF IGNEOUS ORIGIN 
 
 ORIGIN UNKNOWN 
 
 Granite : 
 Biotite granite . . . 
 Hornblende granite . . 
 
 Syenite : 
 Hornblende syenite . . 
 
 Mica syenite .... 
 Pyroxene syenite . . 
 Diorite : 
 Mica diorite .... 
 Gabbro . 
 
 Granite gneiss : 
 Biotite granite gneiss . 
 Hornblende granite \ 
 gneiss . . . . / 
 Syenite gneiss : 
 Hornblende syenite 1 
 gneiss . . . . / 
 Mica syenite gneiss. . 
 Pyroxene syenite gneiss 
 Diorite gneiss : 
 Mica diorite gneiss . . 
 Gabbro gneiss .... 
 
 Granitic gneiss : 
 Biotite granitic gneiss. 
 Hornblende granitic 
 gneiss. 
 Syenitic gneiss : 
 Hornblende syenitic 
 gneiss. 
 Mica syenitic gneiss. 
 Augite syenitic gneiss. 
 Dioritic gneiss : 
 Mica dioritic gneiss. 
 Gabbroic gneiss, or gab- 
 
 Pyroxenite 
 
 Pyroxenite gneiss . . . 
 
 brie gneiss. 
 Pyroxenitic gneiss. 
 
 Inasmuch as the structure characteristic of gneisses is found 
 developed in rocks of diverse types, many petrologists now use 
 
168 METAMORPHIC KOCKS 
 
 the term in an almost wholly structural sense, as in itself non- 
 committal as to composition or origin, but merely designating a 
 rock of foliated or schistose structure. C. H. Gordon has pro- 
 posed 1 a scheme of classification of gneissoid rocks as above, 
 and which has much in its favor. 
 
 (2) THE CRYSTALLINE SCHISTS 
 
 Under this head are grouped a large and extremely variable 
 series of rocks, differing from the gneisses mainly in the lack of 
 feldspar as an essential constituent. They consist, therefore, 
 essentially of granular quartz, with one or more minerals of the 
 mica, chlorite, talc, amphibole, or pyroxene group. In acces- 
 sory minerals the schists are particularly rich. The more 
 common of these are feldspar, garnet, cyanite, staurolite, 
 tourmaline, epidote, rutile, magnetite, menaccanite, and pyrite. 
 Through an increase in the proportional amount of feldspar the 
 schists pass into the gneisses, and through a decrease in mica, 
 hornblende, or whatever may be the characterizing mineral, 
 into the quartz schists, in which quartz alone is the essential 
 constituent. Occasional forms are met with quite lacking 
 in quartz and other accessory minerals and consisting only of 
 schistose aggregates of minerals of a single species, as is the 
 case with the pyrophyllite schists (or, more properly, schistose 
 pyrophyllites) from North Carolina, talcose schists, and with 
 the more massive "soapstones." 
 
 The rocks of this group are characterized as a whole by a 
 pronounced schistose structure, due to the parallel arrangement 
 of the various constituents, this structure being most pro- 
 nounced in those varieties in which mica is the predominating 
 accessory mineral. They are ordinarily considered as having 
 originated from the crystallization of sediments, and in many 
 cases 'the microscope still reveals existing " traces of the origi- 
 nal grains of quartz sand and other sedimentary particles of 
 which the rocks at first consisted." Like the gneisses, they 
 are in part, however, mechanically deformed massive rocks and 
 their schistosity in no way relates to true bedding, as has been 
 already noted (p. 156). 
 
 The varietal names given are dependent mainly upon the 
 character of the prevailing ferro-magnesian silicate. We thus 
 
 1 Bull. Geol. Soc. of America, Vol. VII, p. 122. 
 
THE CRYSTALLINE SCHISTS 
 
 169 
 
 have mica schists, chlorite schists, talc schists, hornblende, actinol- 
 ite, glaucophane schists, etc. The term slate was originally 
 applied to these and other types of rocks of schistose or fis- 
 sile character. In the arrangement here adopted this term is 
 restricted to the argillaceous fragmental or semi-crystalline 
 and foliated rocks next to be described. 
 
 Of the above-mentioned varieties the mica schists are the 
 most common and widely distributed, the mica being in some 
 cases biotite, in others muscovite, or perhaps a mixture of 
 the two. The principal accessories sufficiently developed to 
 be conspicuous are staurolites, chiastolites, garnets, and tour- 
 malines. In the sericite schists the hydrous mica sericite 
 prevails; paragonite schist carries the hydrous sodium-mica par- 
 agonite ; ottr elite schist carries the accessory mineral ottrelite. 
 
 The name phyllite is used by German petrographers to desig- 
 nate a micaceous semi-crystalline rock standing intermediate 
 between the true schists 
 and clay slates. Quart- 
 zite is a more or less 
 schistose or banded rock 
 consisting essentially of 
 crystalline granules of 
 quartz. Such originate 
 from the induration of si- 
 liceous sandstones. This 
 induration is brought 
 about through a deposi- 
 tion of crystalline silica 
 in the form of a bind- 
 ing material or cement 
 around each of the sand 
 particles of which the 
 stone is composed. Each 
 of these granules then forms the nucleus of a more or less per- 
 fectly outlined quartz crystal. This structure is shown in Fig. 
 16, drawn from a thin section of a Potsdam quartzite from 
 St. Lawrence County, New York. The rounded, more or less 
 shaded, portions represent the original grains of quartz sand, 
 and the clear, colorless, interstitial portions the secondary silica. 
 
 The quartzites consist, as a rule, only of silica, or silica 
 colored brown and red by iron oxides. At times a greenish 
 
 FIG. 16. Microstructure of quartzite. 
 
170 METAMORPHIC ROCKS 
 
 tinge is imparted through the development of chloritic minerals; 
 accessory minerals are not, as a rule, abundant. 
 
 Among the hornblende schists there are but few needing 
 especial attention. These are, as a rule, less finely schistose 
 than are the mica-bearing schists, owing to the fact that the 
 mineral hornblende itself has not a platy structure. The glau- 
 cophane schists are perhaps the least abundant of the hornblendic 
 varieties. Such have been described from the Isle of Syra, in 
 the Mediterranean Sea, Switzerland, Wales, and Italy ; a more 
 massive form, probably an altered eruptive, is found near the 
 mouth of Sulphur Creek, Sonoma County, California. Am- 
 phibolite is the name given to an extremely tough and often 
 massive rock of obscure origin, and consisting essentially of 
 the mineral amphibole or hornblende. In some instances the 
 varieties of amphibole, actinolite, and tremolite take the place 
 of the common hornblende. The tremolite rock may undergo 
 alteration into serpentine under proper conditions. Edogite is 
 a tough, massive, or slightly schistose rock, consisting of the 
 grass-green variety of pyroxene, omphacite, and small red gar- 
 nets, with which are frequently associated bluish kyanite, green 
 hornblende (smaragdite), and white mica. Grarnet rock, or 
 garnetite, is a crystalline granular aggregate of garnets with 
 black mica, hornblende, quartz, and magnetite. Kinzigkite is 
 a somewhat similar, though fine-grained and compact, rock 
 consisting of garnets, plagioclase feldspar, and black mica, 
 and which is found in Kinzig and the Odenwald. 
 
 Many of the rocks of this group are but products of dynamic 
 or contact metamorphism, as is the case with many of the 
 chiastolite and argillaceous schists or roofing slates. Rocks of 
 the latter group pass by insensible gradations into clastic ar- 
 gillites. They owe their cleavable property to shearing, as 
 already explained. Under the microscope these rocks are 
 found to be quite variable. Hawes describes clay slate from 
 Littleton, New Hampshire, as consisting of a mixture of quartz 
 and feldspar, in particles as fine as dust. They contained also 
 amorphous carbonaceous matter and little needles of a mineral 
 assumed to be mica. A slate from Hanover, in the same state, 
 contained garnets and staurolites. Wichman found slates from 
 Lake Superior to consist of a colorless, isotropic ground-mass 
 carrying quartz and feldspar particles, scales of iron oxide, car- 
 bonaceous matter, minute tourmalines, and mica fragments. 
 
THE CRYSTALLINE SCHISTS 
 
 171 
 
 The red slates of New York state are composed of an impal- 
 pable red, dust-like ground-mass, carrying grains of quartz 
 and feldspar, all arranged with their longer axes parallel to 
 the plane of schistosity. These can scarcely be considered as 
 other than clastic rocks, the dynamic action not having been 
 sufficient to produce crystallization in more than incipient 
 stages. In this case the plane of schistosity is very nearly 
 parallel with that of bedding, but in many cases, as in the 
 roofing slates of Pennsylvania, the schistose structure is devel- 
 oped at a very considerable, though ever-varying, angle with 
 the bedding. In such cases the true bedding plane is often 
 determinable only by the dark bands, or ribbons, by which the 
 split slates are traversed. 
 
 Chemical Composition. As may be readily imagined, the 
 schists vary almost indefinitely in composition, approximating 
 pure quartzite on the one hand and the gneisses on. the other. 
 The table given below is intended to show the composition of 
 a few characteristic types only. All gradations, from the most 
 acid of quartzites to the most basic of the amphibolites, may 
 readily be found. 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 in 
 
 IV 
 
 v 
 
 VI 
 
 Silica (Si0 2 ) . . 
 
 82 38 / n 
 
 40.00 % 
 
 52 39 % 
 
 49.18% 
 
 50 81 % 
 
 97 1 % 
 
 Alumina (A1 2 3 ) . . . 
 
 11.84 
 
 23.65 
 
 16.33 
 
 15.09 
 
 4.53 
 
 1.39 
 
 Ferric oxide (Fe 2 03) . . 
 
 
 
 8.07 
 
 1.64 
 
 12.90 
 
 3.52 
 
 1.25 
 
 Ferrous oxide (FeO) . . 
 
 2.28 
 
 .... 
 
 1.44 
 
 
 4.26 
 
 .... 
 
 Lime (CaO) 
 
 .... 
 
 0.63 
 
 8.76 
 
 10.59 
 
 
 0.18 
 
 Magnesia (MgO) . . . 
 
 1.00 
 
 0.94 
 
 4.70 
 
 5.22 
 
 31.55 
 
 0.13 
 
 Potash (K 2 0) .... 
 
 0.83 
 
 9.11 
 
 1.42 
 
 1.51 
 
 
 .... 
 
 Soda (Na 2 0) 
 
 38 
 
 1 75 
 
 2 59 
 
 3.64 
 
 
 
 Ignition . 
 
 77 
 
 3 41 
 
 17 
 
 1.87 
 
 4.42 
 
 
 
 
 
 
 
 
 
 I. Mica schist : Monte Rosa, Switzerland. II. Sericite schist : Wisconsin. 
 III. Hornblende schist: Grand Rapids, Wisconsin. IV. Chlorite schist : Klippe, 
 Sweden. V. Talc schist: Gastein, Austria. VI. Quartzite: Chickies Station, 
 Pennsylvania. All analyses quoted from J. F. Kemp's Lecture Notes on Rocks. 
 
PART III 
 
 THE WEATHERING OP ROCKS 
 
 " In the economy of the world, I can find no traces of a beginning, no prospect of 
 an end." HUTTON. 
 
 THE stability of chemical compounds is governed by prevail- 
 ing conditions. A form of combination stable under conditions 
 existing to-day may, under those of to-morrow, become impos- 
 sible. As was suggested in the introductory chapter, the con- 
 ditions under which the more superficial portions of the earth's 
 crust exist are ever changing, and as a result old compounds 
 are broken up and new continually formed. All over the earth 
 rocks laid down as sediments on oceanic floors have been 
 folded, faulted, and pushed out of place until brought under 
 influences as different from those under which they were formed 
 as it is possible to conceive. Molten magmas cooling suddenly 
 on the immediate surface formed compounds in which mere loss 
 of heat was the controlling factor, but which time proves to be 
 unstable. Slow cooling, deep-seated magmas have been, and 
 are being, continually exposed by denudation, and thus brought 
 under new influences and environments. Hence a constant 
 readjustment is everywhere going on, which, as we shall see, is 
 manifold in its physical manifestations. As where an entire 
 building is razed to the ground, and another of quite different 
 architectural features constructed from the old materials ; or 
 again, where, without change of general plan, old timbers are 
 here and there replaced by new, so here we have at work a 
 series of processes in part seemingly destructive and in part 
 constructive, but all tending toward one end. 
 
 The firm and everlasting hills we must learn to regard as 
 neither firm nor everlasting. Whole mountain chains of the 
 geological yesterday have disappeared from view, and as with 
 
 172 
 
PLATE 14 
 
 
 Weathered granite, District of Columbia. 
 
THE WEATHERING OF ROCKS 173 
 
 the ancient cities of the East, we read their histories only in 
 their ruins. Yet, in all this seemingly destructive process of 
 breaking down, decomposition, and erosion, there is traceable 
 the one underlying principle of transformation from the un- 
 stable toward that which is to-day more stable. Nothing is 
 lost or wasted: It is a change which began with the beginning 
 of matter ; which will end only with the blotting out of matter 
 itself. There are no traces of a beginning, there is no prospect 
 of an end. 
 
 I. THE PRINCIPLES INVOLVED IN ROCK- 
 WEATHERING 
 
 The processes involved in this readjustment from unstable 
 to stable compounds, as above outlined, and of incidental 
 soil formation, are in part physical and in part chemical in 
 their nature ; they operate under ever- varying conditions, and 
 through processes at times simple, or again complex. What 
 these processes are, and how they operate, it must be our 
 purpose to now consider. 
 
 It may be said at the outset, that whatever the forces en- 
 gaged, they are, with a few isolated exceptions, superficial, 
 they work from without downwards. However much they may 
 have accomplished since the first rock masses appeared above 
 the primeval ocean, in no case can the actual amount of debris 
 in situ have formed at one time more than a scarcely appreciable 
 film over the underlying and unchanged material. The decom- 
 posing forces early lose their active principles and become quite 
 inert at depths comparatively insignificant. It is only where 
 through erosion the results of the disintegration are gradually 
 removed, that the processes have gone on to such an extent as 
 to pexhaps quite obliterate thousands of feet of strata or of 
 massive rock, and furnished the necessary debris for the vast 
 thicknesses of sandstone, slate, and shale which characterize the 
 more modern horizons. In certain isolated cases, it is true, 
 ascending steam and heated waters, arising from depths un- 
 known, have been instrumental in promoting decomposition, as 
 is well illustrated in the areas of decomposed rhyolites in the 
 Yellowstone National Park. Nevertheless, it is to the almost 
 incalculably slow process of superficial weathering that we owe 
 
174 THE PRINCIPLES INVOLVED IN ROCK- WEATHERING 
 
 a very large share of the apparent rock decomposition and inci- 
 dental soil formation. 1 
 
 This transformation, as already noted, takes place through 
 processes that may be simple, or again complex. It is but 
 rarely that one, alone, prevails for any length of time, and as a 
 rule several or many go merrily on together. Were it possi- 
 ble, it might be well to consider briefly each of these in its turn 
 and by itself. From the fact, however, as above stated, that 
 any one, either physical or chemical, rarely goes on alone, it is 
 thought best to treat the subject as below, and describe in more 
 or less detail the action, first, of the atmosphere, second, of water, 
 in both the solid and liquid form, and third, that of plant and 
 animal life, finally considering the combined action of all these 
 forces, as manifested on the various types of rock which go to 
 make up the earth's crust. 
 
 So striking a phenomenon as the breaking down, or degenera- 
 tion as we may call it, of a mass of firm rock, naturally did not 
 escape the observation of the earlier workers in this and allied 
 branches of science, and the older literature from the time of 
 Hutton contains numerous references to it, though the full 
 significance of atmospheric agencies in bringing about the 
 results, seems not at first to have been fully realized. 
 
 The exciting cause of the degeneration, particularly in warm 
 latitudes, where phenomena of this nature are, as a rule, more 
 apparent, has been a matter of some speculation, and at the out- 
 set it may be well to indicate in brief their tendencies. 
 
 1 The term weathering, as here used, is applied only to those superficial 
 changes in a rock mass brought about through atmospheric agencies, and result- 
 ing in a more or less complete destruction of the rock as a geological body, as 
 where granitic rocks are resolved into sand, and kaolinic material, with liberation 
 of carbonates of the alkalies and of lime, and oxides of iron. It does not include 
 those deeper-seated changes changes taking place below the zone of oxidation 
 arid which result mainly in hydration and the production, it may be, of new 
 mineral species, as chlorite, sericite, zeolites, etc., but during which the rock 
 mass as a whole retains its individuality and geological identity. The distinction 
 is not one that has been sharply insisted upon, and indeed geologists and petrolo- 
 gists as a rule have been extremely careless in their use of such terms as altera- 
 tion, decomposition, and weathering. The distinction drawn here is essentially 
 that made by Roth (Allegemeine u. Chemische Geologie), between Verwitterung 
 and Complicirte Verwitterung. For reasons above stated and others given on 
 p. 161, it seems best to limit the terms weathering and decomposition to processes 
 involving the destruction of the rock mass as a geological body, and to designate 
 the purely mineralogical deeper-seated changes as alteration, which may or may 
 not be due wholly to hydrometamorphism. 
 
OPINIONS OF EARLY WORKERS 175 
 
 Fournet, as quoted elsewhere, writing as early as 1833, in- 
 sisted upon the efficacy of water containing carbonic acid in 
 promoting the decomposition of igneous rocks, while Brogniart, 
 writing with particular reference to feldspathic decomposition 
 and the origin of kaolin, laid great stress on the acceleration 
 of the ordinary process of decay through the electric currents 
 resulting from the contact of heterogeneous rock masses. Dar- 
 win 1 believed the extensive decomposition observed by him in 
 Brazil, to have taken place under the sea, and before the present 
 valleys were excavated. Hartt 2 gave it as his opinion that the 
 decomposition was due to the action of warm rain water soaking 
 through the rock, and carrying with it carbonic acid derived 
 not only from the air, but from the vegetation decaying in the 
 soil as well, together with organic acids, nitrate of ammonium, 
 etc. Further, that the decomposition had gone on only in re- 
 gions once covered by forests. Heusser and Claraz 3 suggested 
 that the decomposition was brought about through the influence 
 of nitric acid. They say " it is without doubt determined by 
 the violence and frequency of the tropical rains, and by the dis- 
 solving action of water, which increases with the temperature. 
 It is necessary to observe, moreover, that this water contains 
 some nitric acid, on account of the thunder storms which follow 
 each other with great regularity during many months of the 
 year." 
 
 Belt, 4 in discussing the extensive decomposition observed by 
 him in Nicaragua, says : " This decomposition of the rocks 
 near the surface prevails in many parts of tropical America, 
 and is principally, if not always, confined to the forest regions. 
 It has been ascribed, and probably with reason, to the percola- 
 tion through the rocks of rain water charged with a little acid 
 from the decomposing vegetation." 
 
 The elder Agassiz laid much stress on the decomposing effects 
 of the hot water from rainfall, 5 while Mills and Branner, 6 in 
 addition, attributed no insignificant amount of the decomposi- 
 tion to the action of decomposing organic matter carried into 
 
 1 Geological Observations, p. 417. 
 
 2 Phys. Geog. and Geol. of Brazil. 
 
 Ann. des Mines, 5th series, 17, 1860, p. 291. 
 4 The Naturalist in Nicaragua, 1874. 
 6 Journey in Brazil, p. 89. 
 
 3 Bull. Geol. Soc. of America, Vol. VII, 1896. 
 
176 THE PRINCIPLES INVOLVED IN ROCK-WEATHERING 
 
 the ground by ants, and also to the acid secretions of the ants 
 themselves. 
 
 The chemical changes involved in the process of decompo- 
 sition received attention from several of the earlier workers, 
 among whom the names of Berthier, J. G. Forschammer, 
 Brogniart, Gustav Bischof, and Ebelmen stand out in greater 
 prominence. More recently the name of S terry Hunt becomes 
 conspicuous, while the purely geological side of the question 
 has been ably set forth in numerous papers by L. Agassiz, R. 
 Pumpelly, N. S. Shaler, O. A. Derby, R. Irving, J. C. Branner, 
 and others, to whom reference is frequently made in these pages. 
 
 1. ACTION OF THE ATMOSPHERE 
 
 Atmospheric air, as is well known, consists in its normal state 
 of a mechanical admixture of free nitrogen and oxygen in the 
 proportion of four volumes of the former to one of the latter. 
 In addition are small and comparatively insignificant amounts 
 of various combined gases and salts, of which carbonic acid is 
 by far the most abundant, constant, and, from our standpoint, 
 important. Still smaller quantities of ammoniacal vapors exist, 
 and in volcanic regions there have been detected appreciable but 
 variable quantities of sulphuric and hydrochloric and nitric acids 
 as well. With rare exceptions these last exist in combination 
 as sulphates, chlorides, and nitrates and with the exception of 
 the last-named need little consideration. 
 
 (1) Nitrogen, Nitric Acid, and Ammonia. Nitrogen, by it- 
 self, is believed to be wholly inoperative in promoting rock 
 decomposition. In works on agricultural chemistry, much has, 
 however, been written concerning the presence in the atmos- 
 phere of the compounds of nitrogen, nitric acid, and ammonia, 
 and it will be well to devote a little space to a consideration of 
 the facts as known, and their possible application to the subject 
 under consideration. 
 
 The well-known experiments of Cloez, Boussingault, De Luca, 
 Kletzinsky, and Way, as well as the recent ones of G. H. 
 Failyer, 1 prove conclusively the existence of ammonia and nitric 
 acid in the air, from whence it is brought to the surface of the 
 earth in the water of rainfalls. 
 
 1 Ammonia and Nitric Acid in Atmospheric Waters, 2d Ann. Rep. Kansas 
 Experiment Station, 1889. 
 
ACTION OF THE ATMOSPHEHE 
 
 17T 
 
 In nearly every case, however, the percentage of ammonia, as 
 determined, equalled or exceeded the amount necessary to com- 
 bine with the acid, forming thus the salt ammonium nitrate. 
 Failyer's experiments in Kansas, carried on for a period of four 
 years, during which time water was collected from 266 rain- 
 falls, showed in but seven instances nitric acid equalling or 
 exceeding the ammonia. In all other cases the amount is less, 
 with the possible exception of the reported occurrence (at 
 Nismes, in 1845) of a fall of hail sufficiently acid to be sour 
 to the taste. As direct promoters of rock decomposition, 
 neither atmospheric nitrogen nor free nitric acid need, then, 
 serious attention. The following tables are, however, of inter- 
 est, the first being abridged from Johnson's How Crops Feed, 
 and the second from Professor Failyer's paper above quoted. 
 
 AMOUNTS or RAIN AND OF AMMONIA, NITRIC ACID, AND TOTAL NITROGEN THEREIN, 
 
 COLLECTED AT ROTHAMSTEDD, ENGLAND, IN THE YEARS 1855-56, CALCULATED 
 PER ACRE, ACCORDING TO MESSRS. LAWES, GILBERT, AND WAY. 
 
 Total . . 
 
 Quantity of rain in 
 Imperial gallons. 
 1 gal. = 10 Ib. water 
 
 Ammonia 
 (in pounds) 
 
 Nitric Acid 
 (in pounds) 
 
 Total Nitrogen 
 (in pounds) 
 
 1855 
 
 663.332 
 
 1856 
 
 616.051 
 
 1855 
 
 7.11 
 
 1856 
 9.53 
 
 1855 
 
 2.98 
 
 1856 
 
 2.80 
 
 1855 
 6.63 
 
 1856 
 
 8.31 
 
 AMOUNTS or RAIN AND OF AMMONIA, NITRIC ACID, AND NITROGEN THEREIN, COL- 
 LECTED AT MANHATTAN, KANSAS, 1887-90, ACCORDING TO G. H. FAILYER. 
 
 
 Total Nitrogen. 
 Means for 4 
 
 Nitrogen in 
 ammonia. 
 Means for 3 
 
 Nitrogen in 
 nitric acid. 
 Means for 3 
 
 
 
 
 
 
 
 years 
 
 years 
 
 Parts per million of water .... 
 
 0.522 
 
 .388 
 
 0.156 
 
 Grammes per acre . ... 
 
 1563.0 
 
 1196.0 
 
 480.0 
 
 Pounds per acre 
 
 3.44 
 
 2.63 
 
 1.06 
 
 
 
 
 
 It has been demonstrated, however, that nitrogen compounds 
 and nitrogenous matter in the soil may become subject to nitri- 
 fication through the action of bacteria, whereby ammonia, 
 nitrous or nitric acid, carbon dioxide, and water are formed, 
 
178 THE PRINCIPLES INVOLVED IN ROCK-WEATHERING 
 
 though, as Wiley says, " The ammonia and nitrous acid may 
 not appear in the soils, as the nitric organism attacks the latter 
 at once and converts it into nitric acid." 1 (See further under 
 influence of plant and animal life, p. 203.) 
 
 In considering the possible efficacy of these compounds, one 
 must not lose sight of the fact that the amount of nitrogen in 
 the soils is as a rule far too small to supply the demands of 
 growing plants, and it is probable that a very large proportion 
 of that which finds its way there is quickly taken up again by 
 these organisms. It is possible that other salts of ammonium 
 than the nitrate may be locally efficacious. Thus M. Beyer, 
 as quoted by Van Den Broeck, 2 has shown that the feldspars 
 decompose very rapidly under the influence of water contain- 
 ing ammonium sulphate or even sodium chloride, either of 
 which substance may be found in vegetable soil. Daubree, 
 who experimented by means of revolving iron cylinders (see 
 p. 197), found, however, that the presence of sodium chloride 
 retarded decomposition. 
 
 (2) Carbonic Acid. The amount of carbonic acid in the air 
 under natural conditions is not a widely variable quantity, ex- 
 cepting near volcanoes and the immediate vicinity of gaseous 
 springs. In the vicinity of large cities and manufactories 
 consuming great quantities of coal, the amount is naturally 
 increased. Although carbonic acid is the most abundant gas 
 given off by decomposing vegetable matter, it has apparently 
 been definitely ascertained that the amount of this gas in regions 
 of abundant vegetation is no greater than elsewhere. This has 
 been accounted for on the assumption that, as fast as liberated, it 
 is taken up by growing organisms or carried by rains into the soil. 3 
 
 1 Wiley, Principles and Practice of Agricultural Analysis. 
 
 2 Mem. sur les phenomenes d' Alteration des Depots Superficial, p. 16. 
 
 3 The researches of Boussingault and Lewey (Mem. de Chemie Agricole, 
 etc.), as quoted by Johnson (How Crops Feed, p. 139), showed the following 
 proportions existing between the C0 2 of the air of the atmosphere and of various 
 soils : 
 
 C0 2 IN 10,000 PARTS 
 BY WEIGHT 
 
 Ordinary atmosphere 6 parts 
 
 Air from sandy subsoil of forest 38 parts 
 
 Air from loamy subsoil of forest 124 parts 
 
 Air from surface soil of forest 130 parts 
 
 Air from surface soil of vineyard 146 parts 
 
 Air from pasture soil 270 parts 
 
 Air from soil rich in humus 543 parts 
 
ACTION OF THE ATMOSPHERE 179 
 
 Twenty-one tests of the air in various parts of Boston, during 
 the spring, 1870, showed the presence of 385 parts of carbonic 
 acid in 1,000,000. Eleven tests of the winter air in Cambridge 
 yielded 337 parts in 1,000,000.! Dr. J. H. Kidder found the 
 out-door air of Washington to contain 387 to 448 parts in 
 1,000,000, while Dr. Angus Smith, after an elaborate series of 
 experiments, reported the atmosphere of Manchester (Eng- 
 land) as containing 442 parts in 1,000,000. 2 
 
 These amounts are considerably in excess of those reported 
 by Miintz and Aubin, 3 who give the following figures relative 
 to the proportional amounts in 10,000 by volume, as determined 
 at the various widely separated stations. The amount, it will 
 be perceived, is slightly greater during the night than during 
 the day. 
 
 DAY NIGHT 
 
 Hayti 2.704 2.920 
 
 Florida 2.897 2.947 
 
 Martinique 2.735 2.850 
 
 Mexico 2.665 2.860 
 
 Santa Cruz, Patagonia 2.664 2.670 
 
 Chubut, Patagonia 2.790 3.120 
 
 Chili 2.665 2.820 
 
 The general mean is then 2.78 parts in 10,000, that for the 
 night alone being 2.82. For the north of France the mean is 
 given as 2.962, for the plain of Vincennes 2.84, and for the 
 summit of the Pic du Midi 2.86. 
 
 Fischer, as quoted by Branner, 4 has shown that in rain and 
 snow water the amount of carbonic acid varies between 0.22% 
 and 0.45 % by volume of water. Assuming that the mean of 
 these figures fairly represents the general average, it is easy, 
 knowing the rainfall of any region, to calculate the amount of 
 the gas thus annually brought to the surface. Professor Bran- 
 ner has thus calculated that from 3.21 to 11.80 millimetres of 
 carbonic acid (CO 2 ) are annually brought to the surface in cer- 
 tain parts of Brazil. The same method of calculation applied 
 to the various parts of the United States, would give us for the 
 Atlantic coast states 3.75 mm.; for the upper Mississippi val- 
 ley, 2.50 mm.; for the lower Mississippi valley, 4.50 mm. ; and 
 
 1 2d Ann. Rep. Mass. State Board of Health, 1871. 
 
 2 Air and Rain, p. 52. 
 
 3 Comptes Rendus, Vol. XCIII, 1881, p. 797 ; also XCVI, 1883, pp. 1793-97. 
 
 4 Op. cit. 
 
180 THE PRINCIPLES INVOLVED IN ROCK- WEATHERING 
 
 for the northern Pacific states, 6.25 mm. As it is mainly when 
 this carbonic acid is thus brought to the surface by the rain 
 and snows that its effects become of direct significance in our 
 present work, the matter may be dropped here, to be taken up 
 again when considering the chemical action of water. 
 
 (3) Oxygen. Under ordinary conditions oxygen is the most 
 active principle in atmospheric air, and it is to this agent that 
 is due the process of oxidation which almost invariably char- 
 acterizes the decomposition of silicates and other minerals con- 
 taining iron in the protoxide state. Such oxidation is, however, 
 almost inactive unless aided by moisture, and a further discus- 
 sion of the subject may well be deferred, to be taken up again 
 when discussing the action of water. 
 
 (4) Heat and Cold. The ordinarily feeble action of the air 
 is greatly augmented through natural temperature variations. 
 That heat expands and cold contracts is a fact too well known 
 to need elaboration. That, however, the constant expansion 
 and contraction due to diurnal temperature variations may be 
 productive of weakness and ultimate disintegration in so inert 
 a body as stone, seems not so generally understood, or is, at 
 least, less well appreciated, and hence a little space is devoted 
 to the subject here. Rocks, it must be remembered, as the 
 writer has noted elsewhere, 1 are complex mineral aggregates of 
 low conducting power, each individual constituent of which 
 possesses its own ratio of expansion, or contraction, as the case 
 may be. In crystalline rocks these various constituents are 
 practically in contact. In clastic rocks they are, on the other 
 hand, frequently separated from one another by the interposi- 
 tion of a thin layer of calcareous, ferruginous, or siliceous matter 
 which serves as a cement. As temperatures rise, each and every 
 constituent expands and crowds with almost resistless force 
 against its neighbor ; as temperatures fall, a corresponding con- 
 traction takes place. Since in but few regions are surface tem- 
 peratures constant for any great period of time, it will be readily 
 perceived that almost the world over there must be continuous 
 movement within the superficial portions of the mass of a rock. 
 
 The actual amount of expansion and contraction of stone 
 under ordinary temperatures has been a matter of experiment. 
 W. H. Bartlett 2 has shown that the average rate of expansion 
 
 1 Stones for Building and Decoration, Wiley & Sons, New York. 
 
 2 Am. Jour, of Science, Vol. XXII, 1832, p. 136. 
 
ACTION OF THE ATMOSPHERE 181 
 
 for granite amounts to .000004825 inch per foot for each de- 
 gree Fahrenheit ; for marble .000005668 inch, and for sandstone 
 .000009532 inch. Adie, in a series of similar experiments, 
 found the rate of expansion for granite to be .00000438 inch, 
 and for white marble .00000613 inch. 1 Slight as these move- 
 ments may seem, they are sufficient to in time produce a decided 
 weakening and afford a starting-point for other physical and 
 chemical agencies, such as are ever lying in wait for an oppor- 
 tunity to get in their work. The writer well remembers the 
 peculiar impressions produced during one of his earlier trips 
 into the comparatively arid regions of Montana, at finding, at a 
 certain place, the slopes and valley bottoms strewn with small, 
 beautifully fresh, concave and convex chips of a dense, coal- 
 black, andesitic rock that occupied the crest of one of the higher 
 hills. So fresh were the fractures, so free were they from oxi- 
 dation or other signs of decomposition, it was at first felt that 
 they must be of human origin, that they were chips flaked off 
 by aboriginal workmen in making stone implements, and some 
 time was wasted in seeking for the more complete results of 
 their handiwork. It, however, did not take long to convince 
 him that the flakes were far too abundant and too widely spread 
 to have originated in any such way, while the finding, on the 
 top of the hill, of the coal-black rock, broken into larger colum- 
 nar blocks, each with its angles rendered more obtuse or even 
 fluted by the springing off of just such flakes, this, coupled 
 with the knowledge that during the day, exposed under a 
 cloudless sky, the rocks became so highly heated as to be un- 
 comfortable to the touch, whilst at night the temperature sank 
 nearly to the freezing-point, sufficed to teach, as it must have 
 taught the most obtuse, that the ordinary daily temperature 
 variations were amply sufficient to account for the phenom- 
 enon. 
 
 Shaler states 2 that rock surfaces in the eastern United States 
 may be subjected to temperatures varying from 150 F. at 
 midday in summer to and below in winter. This change of 
 150 in a sheet of granite 100 feet in diameter would produce a 
 lateral expansion of about one inch of surface. That this ex- 
 pansion must tend to lessen the cohesion and tear the upper 
 from the deeper lying layers, is self-evident. As exemplifying 
 
 1 Trans. Royal Soc. of Edinburgh, Vol. XIII, p. 366. 
 
 2 Proc. Boston Soc. of Nat. History, XII, 1809, p. 292. 
 
182 THE PRINCIPLES INVOLVED IN ROCK-WEATHERING 
 
 this, Professor Shaler states that there are on Cape Ann (Massa- 
 chusetts) hundreds of acres of bare rock surface completely 
 covered with blocks of stone, which have been separated from 
 the mass beneath by just this process. 1 
 
 The size of such flakes may vary from those of microscopic 
 proportions to masses of several tons' weight. The higher 
 slopes of Lone Mountain, east of the Madison, in Montana, 
 are covered above timber line with thousands upon thousands 
 of these loose flakes of all sizes up to ten or more feet in diam- 
 eter. Such, here, as in general, are characterized by a roughly 
 lenticular outline in cross-section, possessing a large superficial 
 area in proportion to their thickness, and are further distin- 
 guished from boulders of decomposition by the entire freshness 
 of their materials even to the very surface. In close-grained, 
 black andesitic and basaltic rocks the chip or flake not infre- 
 quently shows a beautiful concave and convex form and is 
 greatly elongated in proportion to its breadth, resembling the 
 long and slender chips of obsidian or flint found on the sites 
 of aboriginal workshops. The surface left by the springing off 
 of these flakes is of course fluted as though the work were 
 done with a carpenter's gouge. 
 
 It is natural that this form of disintegration should be most 
 pronounced in massive, close-grained rocks. In regions of great 
 extremes of daily temperature the rupturing of these masses 
 from the parent ledge is frequently attended by gun-like reports 
 sufficiently loud to be heard at a considerable distance. H. von 
 Streeruwitz states 2 that the rocks of the Trans Pecos (Texas) 
 region undergo a very rapid disintegration from diurnal tem- 
 perature variations, which here amount to from 60 to 75 F. 
 He says: "I frequently observed in summer, as well as in winter 
 time, on the heights of the Quitman Mountains a peculiar crack- 
 ling noise and occasionally loud reports, . . . and careful 
 
 1 The rifting action of heat upon granitic masses is said to have been made a 
 matter of quarry utility in India. It is stated (Nature, January 17, 1895) that a 
 wood fire built upon the surface of the granite ledge and pushed slowly forward 
 causes the stone to rift out in sheets six inches or so in thickness, and of almost 
 any desired superficial area. Slabs 60 x 40 feet in area, varying not more than 
 half an inch from a uniform thickness throughout, have been thus obtained. In 
 one instance mentioned, the surface passed over by the line of fire was 460 feet, 
 setting free an area of stone of 740 square feet of an average thickness of five 
 inches. This stone is undoubtedly one of remarkably easy rift, but the case 
 will, nevertheless, serve our present purposes of illustration. 
 
 2 4th Ann. Rep. Geol. Survey of Texas, 1892, p. 144. 
 
ACTION OF THE ATMOSPHERE 183 
 
 research revealed the fact that the crackling was caused by the 
 gradual disintegration and separation of scales from the surface 
 of the rock, and the loud reports by crackling and splitting of 
 huge boulders." The scales thus split off, he says, vary in 
 thickness from one-half to four inches, and their superficial 
 area from a few square inches to many feet. This form of 
 disintegration is necessarily confined to slopes unprotected by 
 vegetation, and is the more pronounced the greater the diurnal 
 variations. 
 
 In Arabia Petrea, according to Marsh, 1 " when a wind pow- 
 erful enough to scour down below the ordinary surface of the 
 desert and lay bare a fresh bed of stones is followed by a sudden 
 burst of sunshine, the dark agate pebbles are often cracked and 
 broken by the heat." According to Livingstone, the rock tem- 
 peratures in certain parts of Africa, on the immediate surface, 
 rise during the day as high as 137 F. and at night fall so rap- 
 idly as to throw off by their contraction sharp, angular masses 
 in sizes up to 200 pounds' weight. Stanley, in his reports, 
 is inclined to lay considerable stress on the effects of cold rains 
 upon the heated rock surfaces, though it is doubtful if this. is 
 as powerful an agent as his descriptions would give us to under- 
 stand. (See further under action of water.) Throughout the 
 desert regions of lower California, as observed by the writer, 
 the granitic and basic eruptive rocks subject to very little 
 rainfall, and hence almost completely bare of vegetation, under 
 the blistering heat of the desert sun have weathered down into 
 dome-shaped masses, their debris in the form of angular bits of 
 gravel being strewn over the plain. Particles of this gravel, 
 when compared with those which* are a product of chemical 
 agencies, are found to differ in that each, however friable, is a 
 complex molecule of quartz, feldspar and mica or other mineral 
 that may have composed the rock from which it was derived. 
 Aside from a whitening of the feldspathic constituent, due to 
 the reflection of the light from its parted cleavage planes, 
 scarcely any change has taken place, and indeed it more resem- 
 bles the finely comminuted material from a rock-crusher than 
 a product of natural agencies. 
 
 Owing, however, to the low conducting power of rocks, dis- 
 integration from this cause alone can go on to any extent only 
 at the immediate surface, and on flat and level plains, where 
 1 The Earth .as modified by Human Action, p. 552. 
 
184 THE PRINCIPLES INVOLVED IN KOCK-WEATHERING 
 
 the debris is allowed to accumulate, must in time completely 
 cease. 1 It is only on hillsides and slopes, or where by the 
 erosive action of running water, or by wind, the debris is 
 removed as fast as formed, that such can have any geological 
 significance, although the rate of such disintegration is suffi- 
 ciently rapid in exposed places to be of serious consequence in 
 stone used for architectural application. (See further on p. 198, 
 Action of Ice.) 
 
 (5) Wind. But it is to the action of the air when in motion 
 to the wind that is due a very considerable part of atmos- 
 pheric work. Particles of sand drifting along before the wind 
 become themselves agents of abrasion, filing away on every hard 
 object with which they come in contact. As a matter of course, 
 this phenomenon is most strikingly active in the arid regions, 
 though the results, when looked for, are by no means wanting 
 in the humid east. It is thought by Professor Egleston that 
 many of the tombstones in the older churchyards of New York 
 City have become illegible by the wearing action of the dust 
 and sand blown against them from the street. There is among 
 
 1 Observations on soil temperatures made at the Orono, Maine, Experimental 
 Station showed that the mean daily range of temperatures from April to Octo- 
 ber, at a depth below the surface of 1 inch, was 5.62 ; at a depth of 3 inches, 
 5.26 ; at 6 inches, 1.9 ; and at 9 inches, 1.18 ; and at 12 inches very slight. At 
 the depth of 1 inch the temperature was lower than that of the air by 2.4 ; at 
 3 inches by 2.11 ; at 6 inches by 3.16 ; at 9 inches by 3.94 ; at 12 inches by 
 4.18 ; at 24 inches by 5.78 ; and at 36 inches by 7.10. 
 
 The following table, compiled by Forbes (Trans. Royal Society of Edinburgh, 
 Vol. XVI, 1849), from observations made near Edinburgh, Scotland, during 
 1841-42, shows the range of earth temperatures at varying depths in soil, sand- 
 stone, and trap rock. 
 
 
 TRAP EOCK 
 
 SAND OF GARDEN 
 
 CRAIGLEITII SANDSTONE 
 
 DEPTH 
 
 
 
 
 
 Max. 
 
 Min. 
 
 Range 
 
 Max. 
 
 Min. 
 
 Range 
 
 Max. 
 
 Min. 
 
 Range 
 
 3 feet . . . 
 
 52.85 
 
 38.88 
 
 13.97 
 
 54.50 
 
 37.85 
 
 17.65 
 
 53.15 
 
 38.25 
 
 14.90 
 
 6 feet . . . 
 
 51.07 
 
 40.78 
 
 10.29 
 
 52.95 
 
 39.55 
 
 13.40 
 
 51.90 
 
 38.95 
 
 12.95 
 
 12 feet . . 
 
 49.00 
 
 44.20 
 
 4.80 
 
 50.40 
 
 43.50 
 
 6.90 
 
 50.30 
 
 41.60 
 
 8.70 
 
 24 feet . . 
 
 47.50 
 
 46.12 
 
 1.38 
 
 48.10 
 
 46.10 
 
 2.00 
 
 48.25 
 
 44.35 
 
 3.90 
 
 It has been shown that the thermal conductivity of rocks varies in direction 
 according to their structure, being greatest in the direction of their schistosity, 
 where such exists. In massive, homogeneous rocks the conductivity is the same 
 in all directions. In finely fissile rocks, on the other hand, it may be four times 
 as great in the direction of their fissility as at right angles thereto. 
 
ACTION OF THE ATMOSPHERE 185 
 
 the heterogeneous collections of the National Museum at Wash- 
 ington a large sheet of plate glass, once a window in a light- 
 house on Cape Cod. During a severe storm, of not above forty- 
 eight hours' duration, this became on its exposed surface so 
 ground from the impact of grains of sand blown against it as to 
 be no longer transparent, and to necessitate its removal. Win- 
 dow panes in the dwelling-houses of the vicinity are, it is stated, 
 not infrequently drilled quite through by the same means. 
 
 Apply now this agency to a geological field in a dry region. 
 The wind, sweeping across a country bare of verdure and 
 parched by drought, catches up the loose particles of dust and 
 sand and drives them violently into the air in clouds, or sweeps 
 them along more quietly close to the surface, where they are 
 at first scarce noticeable. The impact of a single one of these 
 moving grains on any object with which it may come in con- 
 tact is far too small to be appreciable ; but the impact of 
 millions, acting through days, weeks, and years, produces re- 
 sults not merely noticeable, but strikingly conspicuous. We 
 have here, in fact, a natural sand blast, an illustration on a 
 grand scale of a principle in common use in glass-cutting, and 
 to a small extent in stone-cutting also. Constantly filing away 
 on every object with which they come in contact, the grains 
 go sweeping on, undermining cliffs, scouring down mountain 
 passes, wearing away the loose boulders, and smoothing out all 
 inequalities. Naturally the abrading action on exposed blocks 
 of stone is most rapid near the ground, as here the flying sand 
 grains are thickest. First the sharp angles and corners are 
 worn away, and the masses gradually become pear-shaped, 
 standing on their smaller ends. Finally the base becomes 
 too small for support, the stone topples over, and the process 
 begins anew without a moment's intercession, and continues 
 until the entire mass disappears, becomes itself converted 
 into loose sand drifted by the wind and an agent for destruc- 
 tion. Professor W. P. Blake was the first, I believe, to call pub- 
 lic attention to this phenomenon, having observed it while in the 
 Pass of San Bernardino (California) in 1853. G. K. Gilbert 
 has also published some interesting facts as noted by himself 
 while geologist of the Wheeler Expedition west of the 100th 
 meridian, in 1878. 1 In acting on the hard rocks, the sand cuts 
 
 1 It should be noted that the "sand-blast carving" described by Gilbert in 
 this report is not due wholly to the action of wind-blown sand. The rock is fine 
 
186 THE PRINCIPLES INVOLVED IN ROCK-WEATHERING 
 
 so slowly as at times to produce only grooved or fantastically 
 carved surfaces, often with a very high, polish. The geologists 
 of the 40th Parallel Survey in 1878 described like interesting 
 phenomena as observed on the western faces of conglomerate 
 boulders exposed to the sand blast of the desert regions 'of Ne- 
 vada. The surface of the otherwise light-colored rock was 
 found to have assumed a dark lead-gray hue and a polish equal 
 to that of glass, while the sand had drilled irregular holes and 
 grooves, often three-fourths of an inch deep and not more than 
 an eighth of an inch in diameter, through pebbles and matrix 
 alike. Professor W. M. Davis, 1 G. H. Stone, 2 and J. B. Wood- 
 ward 3 have described pebbles occurring in the glacial deposits 
 of Cape Cod and of Maine, carved and facetted by the same 
 agencies. 
 
 2. CHEMICAL ACTION OF WATER 
 
 Pure water, although an almost universal solvent, neverthe- 
 less acts with such slowness upon the ordinary materials of the 
 earth's crust, that its results are scarcely appreciable to the 
 ordinary observer. But it by no means follows that its effects 
 are not worthy of our consideration here. This is particularly 
 true when we reflect that the results being discussed are not 
 merely those of days and weeks, but of years even when counted 
 by the tens of thousands and millions. Moreover, absolutely 
 pure water, as a constituent of our sphere, presumably does not 
 exist. We have to consider its action as well when contami- 
 nated with sundry salts and acids which it almost universally 
 holds, having taken them up in passing through the atmosphere, 
 and in filtering through the overlying layer of organic matter 
 and decomposition products which cover so large a portion of 
 the surface of the land. It is when thus contaminated that are 
 manifested the wonderful solvent and other chemical reactions 
 which have been instrumental in promoting rock destruction, 
 and it is here, then, that will be considered the complex chemical 
 
 calcareous shale. Through the solvent action of meteoric water the calcareous 
 cement is removed, the fine, argillaceous interstitial material mechanically 
 eroded, while the more resisting granules of quartz sand stand in relief, giving 
 rise to elevated points and ridges. 
 
 1 Proc. Boston Soc. of Natural History, Vol. XXVI, 1893, p. 166. 
 
 2 Am. Jour. Science, Vol. XXXI, 1886, p. 133. 
 
 3 Ibid., Jan., 1894, p. 63. 
 
CHEMICAL ACTION OF WATER 187 
 
 processes commonly grouped under the head of oxidation, deoxi- 
 dation, hydration, and solution. 
 
 (1) Oxidation. Oxidation is perceptibly manifested only in 
 rocks carrying iron either as sulphide, protoxide carbonate, or 
 silicate. The sulphides, in presence of water and when not 
 fully protected from atmospheric influences, readily succumb, 
 producing sulphates which, being soluble, are removed in solu- 
 tion, or hydrated oxides, sulphuretted hydrogen, and perhaps 
 free sulphur, as already noted (p. 29). Such an oxidation is 
 attended by an increase in bulk, so that if nothing escapes by 
 solution, there may be brought to bear a physical agency to aid 
 in disintegration. Weathered rocks, containing iron sulphides, 
 may not infrequently be found with cubical cavities quite empty 
 or partially filled with the brownish, yellow, or red product of 
 its oxidation in a more or less powdery condition. Pyrites, 
 though a wide-spread constituent, is, nevertheless, a less con- 
 spicuous agent in promoting rock decomposition than the pro- 
 toxide carbonates and silicates. In these the iron passes also 
 over to the hydrated sesquioxide state, as is indicated by the 
 general discoloration, whereby the rock becomes first streaked 
 and stained, and finally uniformly ochreous. The more com- 
 mon minerals thus attacked are the ferruginous carbonates of 
 lime and magnesia, and silicates of the mica, amphibole, and 
 pyroxene groups. As the oxidation progresses, the minerals 
 become gradually decomposed and fall away into unrecogniz- 
 able forms. The red and yellow colors of soils are due invari- 
 ably to the iron oxides contained by them. In many cases, the 
 mineral magnetite, a mixture of proto- and sesqui-oxides, under- 
 goes further oxidation and also loses its individuality. 
 
 (2) Deoxidation is a less common feature than oxidation. 
 Water, carrying small quantities of organic acids, may take 
 away a portion of the combined oxygen of a sesquioxide, 
 converting it once more into the protoxide state. The local 
 bleaching of certain ferruginous sands and sandstones is due 
 to this action and a partial removal of the ferriferous salt 
 in solution. Through a similar process of deoxidation, ferrous 
 sulphates may be converted into sulphides, a process which 
 undoubtedly takes place in marine muds protected from atmos- 
 pheric action. 
 
 (3) Hydration the assumption of water more commonly 
 accompanies oxidation, and, indeed, is an almost constant accom- 
 
188 THE PRINCIPLES INVOLVED IN ROCK-WEATHERING 
 
 paninient of rock decomposition, as may be observed in com- 
 paring the total percentages of water in fresh and decomposed 
 minerals and rocks, as given in the analyses. 
 
 This assumption, provided it be not accompanied by a loss of 
 constituents, either by solution or erosion, must be attended by 
 an increase in bulk, such as may be quite appreciable. The 
 Comte de la Hure, as quoted by Branner, 1 has expressed the 
 opinion that some of the hills of Brazil have actually increased 
 in height through this means. The present writer has calcu- 
 lated that the transition of a granitic rock into arable soil, pro- 
 vided the same took place without loss of material, must be 
 attended by an increase in bulk amounting to 88 % . 
 
 Hydration as a factor in rock disintegration is, in the writer's 
 opinion, of more importance than is ordinarily supposed. Granitic 
 rocks in the District of Columbia have been shown 2 to have be- 
 come disintegrated for a depth of many feet with loss of but 
 comparatively small quantities of their chemical constituents 
 and with apparently but little change in their form of combina- 
 tion. Aside from its state of disintegration, the newly formed 
 soil differs from the massive rock, mainly in that a part of its 
 feldspathic and other silicate constituents have undergone a cer- 
 tain amount of hydration. Natural joint blocks of the rock 
 brought up from shafts were, on casual inspection, sound and 
 fresh. It was noted, however, that on exposure to the atmos- 
 phere such shortly fell away to the condition of sand. Closer 
 inspection revealed the fact that the blocks when brought to the 
 surface were in a hydrated condition, giving forth only a dull, 
 instead of clear, ringing sound, when struck with a hammer, and 
 showing a lustreless fracture, though otherwise unchanged. 
 That such had not previously fallen away to the condition of sand 
 was evidently due to the vice-like grasp of the surrounding rock 
 masses. These observations seem to have since received confir- 
 mation from Professor Derby, 3 who states that the sedimentary 
 rocks of Sao Paulo, Brazil, as seen in the deep railway cuttings, 
 " are almost invariably soft even when they show no signs of 
 decay, and go to pieces by a kind of slaking process when 
 broken up and exposed to the air, though they may have 
 required blasting in the original opening of the cuttings." 
 
 1 Op. cit., p. 284. 
 
 2 Bull. Geol. Soc. of America, Vol. VI, p. 321. 
 
 8 Decomposition of Rocks in Brazil, Jour, of Geol., Vol. IV, 1896, p. 205. 
 
CHEMICAL ACTION OF WATER 189 
 
 Professor W. O. Crosby l gives it as his opinion that the dis- 
 integration of the Pike's Peak (Colorado) granite is due mainly 
 to hydration, the mica particularly being affected. 
 
 Professor Alexander Johnstone showed 2 by experimentation 
 that normal muscovites, when submitted to the action of pure 
 and carbonated waters for the space of a year, underwent very 
 little change other than hydration, and a diminution in lustre, 
 hardness, and elasticity. They appeared, in fact, to be converted 
 merely into hydromuscovites, the hydration in pure water hav- 
 ing gone on nearly as rapidly as in that which was carbonated. 
 Biotite, when similarly treated, showed a slight discoloration 
 or bleaching on the edges, accompanied also by hydration, and, 
 in the case of that in carbonated water, a distinct loss of iron 
 and magnesia through solution. Lepidolite, voigtite, vermicu- 
 lite, and pyrosclerite were similarly acted upon, the iron and 
 magnesia being removed in the form of carbonates. The fact 
 was noted " that whenever anhydrous micas, or lower hydrated 
 micas, become hydrated, they always at the same time increase 
 in bulk." This fact he regarded as accounting for the rapid 
 weathering of micaceous sandstones. 
 
 (4) Solution. The solvent action of water is perhaps the 
 most important of its immediate effects, though there are many 
 incidental chemical changes set in operation which, in the end, 
 are of equal or even greater significance. It is the solvent 
 action only that concerns us here. 
 
 Rain and nearly all superficial waters contain small quantities 
 of carbonic, humic, ulmic, crenic, and apocrenic acids, which 
 greatly increase their solvent capacities. The last-named forms 
 are complex, unstable, and little understood products of plant 
 decomposition, 3 and might logically be considered under effects 
 
 1 Personal Memoranda to the Writer. 
 
 2 Quar. Jour. Geol. Soc. of London, Vol. XLV, 1889. 
 
 3 The following are the chemical formulas of these acids, as commonly 
 given: 
 
 ULMIN AND ULMIC ACID 
 
 Carbon* C7.1/U 
 
 Hydrogen 4.2 I Corresponding to C 4 oH 28 Oi2 + H 2 
 
 Oxygen 8.7 J 
 
 HUMIN AND HUMIC ACID 
 
 Carbon C4.4%\ 
 
 Hydrogen 4.3 L Corresponding to C 2 iH 24 Oi 2 -f 3 H 2 O 
 
 Oxygen 31.3 J 
 
190 THE PRINCIPLES INVOLVEP IN ROCK-WEATHERING 
 
 of plant and animal life, but that they act only in presence of 
 moisture. 
 
 " There is reason to believe that in the decomposition effected 
 by meteoric waters and usually attributed mainly to carbonic 
 acid, the initial stages of attack are due to the powerful solvent 
 capacities of the humus acids. Owing, however, to the facility 
 with which these acids pass into higher stages of oxidation, it is 
 chiefly as carbonates that the results of their action are carried 
 down into deeper parts of the crust or brought up to the sur- 
 face. Although CO 2 is ho doubt the final condition into which 
 these unstable organic acids pass, yet during their existence 
 they attack not merely alkalies and alkaline earth, but even 
 dissolve silica." 1 P. Thernard found that the solvent power 
 of these acids was largely controlled by the amount of nitrogen 
 they contained. 2 
 
 CRENIC ACID 
 Carbon . 44.0 
 
 Oxygen 46.6 
 
 APOCRENIC ACID 
 
 Carbon 34.4 %-, 
 
 Hydrogen 3.5 I 
 
 Nitrogen 3.0 f Corresponding to C M 
 
 Oxygen 39.1 J 
 
 Berthelot and Andre (Comptes Rendus Academie de Paris, 114, 1892, pp. 41- 
 43) have shown that the brown substance of humus and analogous compounds 
 undergo direct oxidation under the influence of the air and sunlight, forming 
 carbonic acid. These reactions are purely chemical, taking place without the 
 intervention of microbes, and are accompanied by a change in color of the orig- 
 inal humus. The oxidation is rendered more active through the division and 
 mellowing of the humus by cultivation. Through chemical union of the carbonic 
 acid with certain bases, as lime soda and potash, there are found soluble car- 
 bonates which may be leached out by meteoric waters. 
 
 1 Geikie, Text-book of Geology, 3d ed., p. 472. 
 
 The writer was shown not long since, by Professor Charles E. Munroe, a 
 very practical illustration of the remarkable corrosive power of organic acids. 
 A highly ornate Erench clock, with case of black marble, was packed for storage 
 in excelsior which was a trifle damp. The clock remained in storage from the 
 last of May until about the first of October of the same year. When the pack- 
 ing material was removed, the marble was found to be so corroded as to need 
 rehoning and polishing. The roughness could be easily felt by passing the 
 finger over the surface, and long lustreless lines indicating the contact of excel- 
 sior fibres traversed the surface in every direction. 
 
 2 Julien, The Geological Action of Humus Acids, Proc. Am. Assoc. Adv. of 
 Science, 1879, p. 324. 
 
CHEMICAL ACTION OF WATER 191 
 
 It is stated by Storer 1 that "on the tops of the higher hills 
 of New Hampshire, and on the coast of Maine also, a cold, sour 
 black earth will often be noticed at the surface of the ground, 
 immediately beneath which is sometimes a layer of remarkably 
 white earth. The whiteness is due to the solvent action of 
 acids that soak out from the black humus, and which leach out 
 from the underlying clay and sand the oxides of iron that for- 
 merly colored them." 
 
 As long ago as 1848 the Rogers brothers showed 2 that pure 
 water partially decomposed nearly all the ordinary silicate 
 minerals which form any appreciable part of our rocks. The 
 action of carbonated water was recognizable in less than ten 
 minutes, but pure water required a much longer time before 
 its effect was sufficient for a qualitative determination. So pro- 
 nounced was the action of carbonated water that the presence of 
 the alkalies of lime and magnesia could be recognized in a single 
 drop of the nitrate from the liquid in which the powdered min- 
 erals were digested. By digestion for forty-eight hours they 
 obtained from hornblende, actinolite, epidote, chlorite, serpen- 
 tine, feldspar, etc., a quantity of lime, magnesia, oxide of iron, 
 alumina, silica, and alkalies amounting to from 0.4 % to 1 % of 
 the whole mass. The lime, magnesia, and alkalies were ob- 
 tained in the form of carbonates ; the iron, in the case of horn- 
 blende, epidote, etc., passing from the state of carbonate to that 
 of peroxide during the evaporation of the solutions. Forty 
 grains of finely pulverized hornblende, digested for forty- 
 eight hours in carbonated water at a temperature of 60, with 
 repeated agitation, yielded silica, 0.08%; oxide of iron, 
 0.095%; lime, 0.13%, and magnesia, 0.095%, with traces of 
 manganese. Commenting on these results, Bischof remarks 3 
 that "by repeating this treatment 112 times with fresh carbon- 
 ated water, a perfect solution might be effected in 224 days. 
 If now," he says, "40 grains of hornblende, unpowdered, in 
 which, according to the above assumption, the surface is only 
 one millionth of the powdered, were treated in the same way, 
 and the water renewed every two days, the time required for 
 perfect solution would be somewhat more than six million 
 years." In considering these figures and their practical bear- 
 
 1 Chemistry as applied to Agriculture. 
 
 2 Am. Jour, of Science, Vol. V, 1848. 
 
 3 Chemical and Physical Geology, Vol. I, p. 61. 
 
192 THE PRINCIPLES INVOLVED IN ROCK- WEATHERING 
 
 ing, it must be remembered that while in nature the quantity 
 of water coming in contact with a crystal embedded in a rock 
 during a given time is much less than that assumed above, the 
 mineral is undergoing a gradual splitting up, becoming more 
 and more porous, so that the process is gradually accelerated. 
 
 To quote Bischof again, it is probably admissible to assume 
 that the time in which water produces similar effects of decom- 
 position or solution on minerals, is inversely as the magnitude 
 of the surface of contact. If, therefore, a mineral were so far 
 subdivided that the surface was increased ten million-fold, the 
 quantity then dissolved during a certain time would be the same 
 as that dissolved during a period ten million times as long. 
 
 Richard Miiller has also shown 1 that carbonic acid waters 
 will act even during so brief a period as seven weeks upon the 
 silicate mineral with such energy as to permit a quantitative 
 determination of the dissolved materials. The accompanying 
 table shows (1st) the percentages of the various constituents 
 thus taken out by the carbonated water, and (2d) the total per- 
 centages of the materials dissolved. That is to say, the figures 
 0.1552 given for adular under SiO 2 , indicate that 0.1552% of 
 the total 65.24% of the silica contained by the mineral have 
 been removed, and so on. The last column gives the total per 
 cent of all the constituents extracted. 
 
 MINERAL 
 
 Si0 2 
 
 A1 2 3 
 
 K 2 
 
 Na 2 O 
 
 MgO 
 
 CaO 
 
 P0, 
 
 FeO 
 
 Total 
 
 Adular . 
 
 1552 
 
 01368 
 
 
 % 
 
 % 
 
 
 % 
 
 trace 
 
 0328 
 
 Oligoclase . . 
 Hornblende . 
 
 0.237 
 0.419 
 
 9.1713 
 trace 
 
 .... 
 
 2.367 
 
 .... 
 
 3.213 
 
 8528 
 
 .... 
 
 trace 
 4.829 
 
 0.533 
 1.536 
 
 Magnetite . . 
 
 trace 
 
 
 
 
 
 
 
 0.942 
 
 0.307 
 
 Apatite . 
 
 
 
 
 
 
 2 168 
 
 1 822 
 
 
 2018 
 
 Olivine . . . 
 Serpentine 
 
 0.873 
 0.354 
 
 trace 
 
 .... 
 
 .... 
 
 1.291 
 2649 
 
 trace 
 
 
 8.733 
 1.527 
 
 2.111 
 1.211 
 
 
 
 
 
 
 
 
 
 
 
 The summary of his investigations he gives as below : 
 
 (1) All the minerals tested were acted upon by the carbonated 
 
 water. 
 
 (2) In this process there were formed carbonates of lime, iron, 
 
 manganese, cobalt, nickel, potash, and soda. 
 
 1 Untersuchen liber die Einwirkung des kohlensaurehaltigen Wassers auf 
 einige Mineralien und Gesteine, Tschermaks Min. Mittheilungen, 1877, p. 25. 
 
PLATE 15 
 
 v^v^iSSl 
 
 > , * J 
 , -. ^ -^--r. 
 
 Corroded limestones. 
 
CHEMICAL ACTION OF WATER 193 
 
 (3) In the action of the carbonated waters upon the alkaline 
 
 silicates, like the feldspars, a small amount of silica went 
 always into solution, presumably in the form of hydrate. 
 
 (4) Even alumina was dissolved in appreciable quantities. 
 
 (5) Adular proved more resisting to the action of the acid than 
 
 did the oligoclase. 
 
 (6) The first stage of decomposition in the feldspars is a redden- 
 
 ing process ; the second, kaolinization. 
 
 (7) Hornblende was more easily decomposed than feldspar. 
 
 (8) Increase of pressure on the solution was productive of more 
 
 energetic action than prolonging the time. 
 
 (9) Of all the minerals tested, the magnetic iron was least 
 
 affected. 
 
 (10) Apatite was readily acted upon, as could be detected by its 
 
 appearance under the microscope. 
 
 (11) Olivine was the most readily attacked of all the silicates 
 
 tested, probably twice as easily decomposed as the ser- 
 pentine. 
 
 (12) Magnesian silicates were attacked by the carbonated waters. 
 
 Hence serpentine cannot be considered a final product of 
 decomposition. 1 
 
 Of all the materials forming any essential part of the earth's 
 crust the limestones are most affected by the solvent power of 
 water. It is stated that pure water will dissolve lime carbon- 
 ate in the proportions of one part in 10800 when cold and one 
 part in 8875 when boiling. 
 
 Since rock-weathering is, as already stated, a superficial 
 phenomenon, we have to do only with waters of ordinary tem- 
 peratures and under ordinary conditions of pressure, though 
 this expression must not be taken as necessarily meaning cold 
 waters, since, if we accept the statements of Caldcleugh, 2 rain 
 waters falling upon the heated rocks may have their tempera- 
 tures raised as high as 140 F. The enormously destructive 
 effect of carbonated waters on limestone is scarcely apparent 
 011 casual inspection, owing to the fact that the material is 
 carried away in solution, leaving only the insoluble impurities 
 behind. In such cases it is possible to estimate the amount of 
 corrosion through a comparison of the proportional amounts of 
 various constituents in this residue with those in the fresh rock 
 
 1 Serpentine, however, cannot be properly considered a decomposition prod- 
 uct. It is rather a product of alteration. 
 
 2 Trans. Geol. Soc. of London, 1829. 
 
194 THE PRINCIPLES INVOLVED IN ROCK-WEATHERING 
 
 (see p. 209 et seq.), and the time limit of corrosion through 
 determining the percentage amounts of the constituents in the 
 water which annually drains from any given area. By such 
 methods it has been estimated 1 that some 275 tons of calcium 
 carbonate are annually removed from each square mile of Cal- 
 ciferous limestone exposed in the Appalachian region alone ; 
 while a well-known English authority 2 has calculated that with 
 an annual rainfall of 32 inches, percolating only to a depth of 
 18.3 inches, there are annually removed by solution from the 
 superficial portions of England and Wales an average of all 
 constituents amounting to 143.5 tons per square mile of area. 
 He further calculates that the average amount of carbonate of 
 lime annually removed from each square mile of the entire 
 globe amounts to 50 tons. 3 It is to this corrosive action of 
 meteoric waters that still another authority 4 would attribute 
 the slight thickness and nodular condition of many beds of 
 Palseozoic limestone. He argues that originally thick-bedded 
 limestones have, during the ages subsequent to their formation 
 and uplifting, become so impoverished through the dissolving 
 out and carrying away in solution of the lime carbonate, as to 
 have been quite obliterated, or reduced to mere nodular bands, 
 and given rise to important palseontological breaks in the geo- 
 logical record. Other than organic acids may locally exert a 
 potent influence. Thus Robert Bell has described the dolomitic 
 limestones underlying the waters along Grand Manitou Island, 
 the Indian peninsula, and adjacent portions of Lake Huron and 
 the Georgian Bay, as pitted and honeycombed in a very pecu- 
 liar and striking manner. This corrosion, it is believed, is 
 produced through the solvent action of sulphuric acid in the 
 water, the acid itself arising from the decomposition of the sul- 
 phides of iron, pyrites and pyrrhotite, which exist in great 
 quantities in the Huronian rocks to the northward. 5 
 
 1 A. L. Ewing, Am. Jour, of Science, 1885, p. 29. 
 
 2 T. Mellard Reade, Chemical Denudation in Relation to Geological Time. 
 
 3 The total dissolved constituents thus removed are divided up as follows : 
 Carbonate of lime, 50 tons ; sulphate of lime, 20 tons ; silica, 7 tons ; carbonate 
 of magnesia, 4 tons ; peroxide of iron, 1 ton ; chloride of sodium, 8 tons ; alka- 
 line carbonates and sulphates, 6 tons. 
 
 4 F. Rutley, The Dwindling and Disappearance of Limestones, Quar. Jour. 
 Geol. Soc. of London, August, 1893. 
 
 5 Bull. Geol. Soc. of America, Vol. VI, pp. 47-304. 
 
 Messrs. C. \V. Hayes and M. R. Campbell, of the United States Geological 
 
MECHANICAL ACTION OF WATER^AND OF ICE 195 
 
 3. MECHANICAL ACTION OF WATER AND OF ICE 
 
 Aside from its solvent capacity, water acts as a powerful ero- 
 sive agent, as well as an agent for the transportation of the 
 eroded materials. It is only its erosive power that need con- 
 cern us here, though, as will be seen, this is to a considerable 
 extent dependent upon its power of transportation. Every 
 raindrop beating down upon a surface already sorely tried by 
 heat and frost serves to detach the partially loosened granules, 
 and, catching them up in the temporary rivulets, carries them 
 to the more permanent rills, to be spread out over the valley 
 bottoms, or perhaps if the slopes be steep and the current ac- 
 
 Survey, have recently reported some remarkable examples of corroded quartz 
 pebbles which should be mentioned here, although a satisfactory explanation for 
 the phenomenon has not yet been given. 
 
 Dr. Hayes, in a personal memorandum to the writer, describes the occur- 
 rence as follows: 
 
 "At three rather widely separated points in the South, conglomerates have 
 been observed in which the projecting portions of the pebbles have been etched 
 or partly dissolved. 
 
 "The first, observed by Mr. Campbell, is at Nuttall, West Virginia. The 
 conglomerate in question, which belongs to the coal measures, is composed of 
 rather coarse quartz sand with slightly yellowish cement, in which are embedded 
 well-worn pebbles of white vein quartz. The latter vary in size up to three- 
 quarters of an inch in diameter, and are somewhat irregularly distributed. 
 Ordinarily the pebbles, wholly unaltered, weather out by the chemical or 
 mechanical disintegration of the sandy matrix. In the case observed, however, 
 where the conglomerate received the drip from an overhanging cliff, the project- 
 ing portions of the pebbles are deeply pitted, evidently by solution. Mechanical 
 wear is precluded by the form of the resulting surface, which is not smooth like 
 the portions of the pebble still protected by the matrix, but is rough and irregu- 
 lar. The outer portion of the pebbles is evidently less easily affected by the 
 solvent than the interior, and forms a sharp rim about the irregular cavities 
 hollowed out within. In some cases a third of the pebble has thus been re- 
 moved. The surface of the sandstone matrix in which the pebbles are embedded 
 is also pitted, possibly by the same process of solution as that which has affected 
 the pebbles, but such a surface might also be produced by mechanical means in 
 case the cement were less indurated in some places than in others. 
 
 "The second case is on Clifty Creek, White County, Tennessee. The con- 
 glomerate, also a member of the coal measures, forms the bottom of a small 
 canon, and is covered by the creek at high water, but uncovered throughout 
 the greater part of the year. The matrix is a coarse white sandstone which 
 weathers yellow by the oxidation of the slightly ferruginous cement. Embedded 
 in this are rather abundant pebbles, varying in size up to two inches in diameter, 
 and composed chiefly of quartz, with a few of chert and possibly of quartzite. 
 The projecting portions of these pebbles have been in part removed, though they 
 still project somewhat above the enclosing matrix. As in case of the Nuttall 
 conglomerate, the exterior portions of the pebbles are less easily affected than 
 
196 THE PRINCIPLES INVOLVED IN ROCK- WEATHERING 
 
 cordingly strong, to the rivers and thence to the sea. The 
 amount of detrital matter thus mechanically removed from 
 the hills and spread out over valley and sea-bottoms quite ex- 
 ceeds our comprehension, but it is estimated that at the rate 
 the Mississippi River is now doing its work, the entire Ameri- 
 can continent might be reduced to sea-level within a period of 
 four and one-half million years. The Appalachian Mountain 
 system, whose uplifting began in early Cambrian times and 
 terminated at the close of the Carboniferous, has already 
 through this cause lost more material than the entire mass of 
 that which now remains. But the rivers, like the winds arid 
 glaciers, in virtue of this load they bear, become themselves 
 converted into agents of erosion, filing away upon their rocky 
 beds, undermining their banks, and continually wearing away 
 the land by their ceaseless activity. The pot-holes in the bed 
 of a stream, formed by the constant swirl of sand and gravel 
 in an eddy, furnish on a small scale striking illustrations of this 
 cutting power, while the rocky canons o'f the Colorado of the 
 West, where thousands of feet of horizontal strata have been cut 
 through as with a file, show the same thing on a scale so gigan- 
 tic as to be at first scarce comprehensible. 1 An item of no 
 insignificant importance to be considered here is the possibility, 
 
 the interiors, and when the pebble has been a third or half removed the outer 
 shell forms a rim within which is a depression with a slight elevation in the 
 centre. The chert pebbles show less evidence of corrosion by a solvent than 
 those composed of quartz. Their upper surfaces are somewhat worn down and 
 even slightly hollowed, but this might easily have been produced by mechanical 
 means, which is not the case with the quartz. 
 
 " The third case is a block of conglomerate from Starrs Mountain, Tennessee, 
 collected by Mr. Bailey Willis. This is of Lower Cambrian age. The matrix is 
 a coarse feldspathic sandstone containing layers of well-rounded pebbles, mostly 
 quartz, with a few probably of some feldspar. The former are between one-half 
 and one inch in diameter and the latter somewhat larger. The projecting por- 
 tions of the quartz pebbles on one side of the block are almost entirely removed, 
 and as in the other cases evidently by solution. A slight rim projects above the 
 matrix in which the pebbles are embedded ; within this is a depression, while a 
 slight elevation occupies the centre. 
 
 "The projecting portions of the feldspathic pebbles also are partly removed, 
 but this may be due to corrasion instead of corrosion, that is, to the action of 
 mechanical rather than chemical agents. The pebbles on the lower side of the 
 block have their original water- worn surfaces without any trace of etching." 
 
 1 Captain C. E. Button has estimated (Tertiary History of the Grand Canon 
 of the Colorado) that from over an area of 13,000 to 15,000 square miles drained 
 by the Colorado River, an average thickness of 10,000 feet of strata have been 
 removed. 
 
MECHANICAL ACTION OF WATER AND OF ICE 197 
 
 indeed probability, of an incidental chemical decomposition 
 taking place during this abrasive action. Daubree showed l 
 that when feldspathic fragments were submitted to artificial 
 trituration in a revolving cylinder containing water, a decompo- 
 sition was effected whereby the alkalies were liberated in very 
 appreciable amounts. He found further that the principal 
 product of mutual attrition of feldspar fragments in water was 
 not sand, but an impalpable mud (limon). This mud was of 
 such tenuity as to remain for many days in suspension, and 
 on desiccation became so hard as to be broken only with 
 the aid of a hammer, resembling in many respects the argillites 
 of the coal measures, but differing in that it carried a high 
 percentage of alkalies. Granitic rocks thus treated yielded 
 angular fragments of quartz and very minute shreds of mica, 
 while the feldspars ultimately quite disappeared in the form 
 of the impalpable mud above mentioned. It was noted that 
 after the quartzose particles had reached a certain degree of 
 fineness further diminution in the size ceased, owing to the 
 buoyant action of the water, which in the form of a thin film 
 between adjacent particles acted as a cushion and prevented 
 actual contact to the extent necessary for mutual abrasion. It 
 is to a similar action on the part of sea-water that Shaler 2 would 
 attribute the lasting qualities of the sand grains upon our sea 
 beaches. Indeed the conditions of Daubree's experiments as 
 a whole were not so different from those existing in nature that 
 we need hesitate, as it seems to the writer, to conclude similar 
 action, both chemical and physical, may be going on wherever 
 abrasion takes place in the presence of continual moisture, as in 
 the bed of a river or glacier. 
 
 1 It will be remembered that this authority placed rock fragments in stone and 
 iron cylinders containing water and made to revolve horizontally at a measured 
 rate of speed, so that the actual distance travelled by any of the particles dur- 
 ing a given time could be readily calculated. The product of this disintegration, 
 even when carried to the condition of fine silt, was always sharply angular. His 
 experiments further showed that when feldspathic fragments were thus treated, 
 there was always a certain amount of decomposition, whereby salts of potash were 
 liberated ; in one instance, when 3 kilogrammes of feldspar were revolved for 192 
 hours in iron cylinders containing 5 litres of water, 2.72 kilogrammes of finely 
 comminuted mud were obtained, and in solution in the water, 12.6 grammes of 
 potash, or 2.52 grammes per litre. The presence of carbonic acid in the water 
 increased the amount of potash. When the feldspar was triturated dry and then 
 treated with water, no such solvent action could be detected. Geologic Experi- 
 mental, p. 268. 
 
 2 Bull. Geol. Soc. of America, Vol. V, p. 208. 
 
198 THE PRINCIPLES INVOLVED IN ROCK- WEATHERING 
 
 The hammering action of waves upon the sea-coast exerts a 
 powerful erosive action, particularly upon particles of rock of 
 such size as to be lifted or moved by wave action, but too heavy 
 to be protected from attrition by the thin film of water above 
 alluded to. Shaler's observations 1 at Cape Ann were to the 
 effect that ordinary granitic paving blocks (weighing perhaps 
 twenty pounds) were, when exposed to surf action, worn in 
 the course of a year into spheroidal forms such as to indicate 
 an average loss of more than an inch from their peripheries. 
 Experiments made with fragments of hard burned brick showed 
 that in the course of a year they would be reduced fully one-half 
 their bulk. Even the crystallization of the salt thrown up by 
 wave action and absorbed into the pores of rocks 2 serves in its 
 way the purposes of disintegration. 
 
 The Action of Freezing Water and of Ice. The action of 
 dry heat and cold in disintegrating rocks has already been 
 described. The effects of such temperature changes upon 
 stone of ordinary dryness are, however, slight in comparison 
 with the destructive agencies of freezing temperatures upon 
 stones saturated with moisture. The expansive force of water 
 passing from the liquid to the solid state has been graphically 
 described as equal to the weight of a column of ice a mile high 
 (about 150 tons to the square foot). Otherwise expressed, 100 
 volumes of water expand, on freezing, to form 109 volumes of 
 ice. Provided, then, sufficient water be contained within the 
 pores of a stone, it is easy to understand that the results of 
 freezing must be disastrous. That stones as they lie in the 
 ground do contain moisture, often in no inconsiderable amounts, 
 is a well-known and well-recognized fact by all those engaged 
 in quarrying operations, and indeed no mineral substance is 
 absolutely impervious to it. The amount contained, naturally 
 varies with the nature of the mineral constituents and their 
 state of aggregation. According to various authorities, granite 
 may contain some 0.37% by weight; chalk, 20%; ordinary 
 compact limestone, 0.5% to 5% ; marble, about 0.30% ; and 
 sandstones, amounts varying up to 10% or 12%, while clay 
 
 1 Bull. Geol. Soc. of America, Vol. V, p. 208. 
 
 2 According to Dana (Wilkes' Exploring Expedition, Geology, p. 529), the 
 sandstones along the coast of Sydney, Australia, are subjected to a mechanical 
 disintegration through the crystallization of salt which is absorbed from the 
 saline spray of the ocean waves. 
 
% \ 
 
 3 2 
 
 
 
 O g> CD 
 g | 1 
 
 < 'E-S 
 SS 
 
 I i 
 
 ii 
 
 5-g 
 
MECHANICAL ACTION OF WATER AND OF ICE 199 
 
 may contain nearly one-fourth its weight. This water is largely 
 interstitial the quarry water, as it is sometimes called. In 
 addition to this, the quartz, particularly of granitic rocks, 
 almost universally contains innumerable minute cavities par- 
 tially filled with water, and which are, in extreme cases, so 
 abundant as to make up, according to Sorby, at least 5 % of the 
 whole volume of the mineral. 
 
 That the passage of this included moisture from the liquid 
 to the solid state, must be attended with results disastrous to 
 the stone is self-evident, though the rate of disintegration may 
 be so slow under favorable circumstances as to be scarce notice- 
 able. Freezing of the absorbed water is one of the most fruit- 
 ful sources of disintegration in stones confined in the walls of 
 a building, and even in the quarry bed it is by no means uncom- 
 mon to have stone so injured as to render it worthless. How- 
 ever slight may be the effects of a single freezing, constant 
 repetition of the process cannot fail to open up new rifts, and 
 still further widen those already in existence, allowing further 
 penetration of water to freeze in its turn and to exert a chemical 
 action as well., So year in and year out, through winter's cold 
 and summer's heat, the work goes on until the massive rock 
 becomes loose sand to be caught up by winds or temporary 
 rivulets and spread broadcast over the land. In some instances, 
 it may be, the rock is of sufficiently uniform texture to be af- 
 fected in all its mass alike. More commonly, however, it is 
 traversed by veins, joints, or other lines of weakness along 
 which the rifting power is first made manifest, as in our illus- 
 tration. Naturally disintegration of this kind is confined to 
 frigid and temperate latitudes. As bearing upon the extreme 
 rapidity with which such disintegration may take place, the 
 following is quoted from a letter of Dr. L. Stejneger, of the 
 United States National Museum, who passed several months 
 among the islands of Bering Sea. 
 
 u ln September, 1882, I visited Tolstoi Mys, a precipitous 
 cliff near the southeastern extremity of Bering Island. At the 
 foot of it I found large masses of rock and stone which had 
 evidently fallen down during the year. Most of them were 
 considerably more than six feet in diameter, and showed no 
 trace of disintegration. The following spring, April, 1883, 
 when I revisited the place, I found that the rocks had split up 
 into innumerable fragments, cube-shaped, sharp-edged, and of 
 
200 THE PRINCIPLES INVOLVED IN ROCK-WEATHERING 
 
 a very uniform size, about two inches. They had not yet 
 fallen to pieces, the rocks still retaining their original shape. 
 I may remark, however, that the weather was still freezing 
 when I was there. The winter was not one of great severity, 
 and several thawing spells broke its continuity. These cubic 
 fragments did not seem to split up any further, for everywhere 
 on the islands where the rock consisted of the coarse sandstone, 
 as in this place, the talus consisted of these sharp-edged stones." 
 
 Ice acts as a disintegrating agent in still other ways than 
 that mentioned. The phenomenon of the glacier is now so 
 well known that we need dwell upon it but briefly here. Long- 
 continued precipitation of snow upon regions of such elevation, 
 or in such latitudes as to preclude anything like an equally 
 rapid melting, gives rise to deep fields of snow, compacted in 
 the lower portions into the condition of ice. These, in virtue 
 of the weight of the overlying mass, and perhaps the steepness 
 of the slopes, aided by a certain amount of plasticity possessed 
 in some degree by even the most rigid of so-called solids, creep 
 slowly down the slopes in the form of glaciers or rivers of ice. 
 Advancing, it may be, but an inch or several feet a day, now scarce 
 moving at all, or even retreating temporarily through a diminu- 
 tion in the amount of their supplies, or an increase in the sun's 
 heat, these bring, either upon their surfaces as moraines, or 
 frozen into their mass, large quantities of fragmental rock mate- 
 rial fallen upon them from above, or picked up from the surfaces 
 over which they flow. Those fragments which remain upon the 
 upper surface, or frozen into the upper portions, are but trans- 
 ported to the lower levels where, the temperature being suffi- 
 cient, the ice is melted and the load deposited in the form of a 
 moraine. 
 
 Beneath, and frozen into the lower portion of the ice sheet, 
 there is, however, a variable amount of rock material, which, as 
 the glacier moves along, is crowded with all the weight of the 
 overlying mass, and all the resistless energy of the ice behind, 
 over the surface of the underlying rock. In virtue of this 
 material, this sand, gravel, and boulder aggregate, the glaciers 
 become converted into what we may compare to extremely 
 coarse files, to tear away the rocks over which they pass, and 
 grind and crush them into detritus of varying degrees of 
 fineness. The small streams which originate from the melt- 
 ing of these glaciers become, hence, not infrequently charged 
 
ACTION OF PLANTS AND ANIMALS 201 
 
 to the point of turbidity with the fine silt-like detritus ground 
 from the ledges and in part from the boulders themselves. 
 Figure 3 of plate 19 shows a slab of limestone still bear- 
 ing upon its surface the evidences of the severity of the 
 onslaught. A consideration of the amount of detritus thus 
 brought down either merely as transported or as abraded 
 material belongs properly to the chapter on transportation, 
 but a few illustrations are not without interest here. The 
 Aar in Switzerland is stated by Geikie to discharge every day 
 in August some 440,000,000 gallons of water, carrying some 
 280 tons of sand. A portion of this is in a state of such 
 minute subdivision as to remain a long time in suspension, 
 and give the water a milky appearance for several miles. 
 I. C. Russell has described 1 the Tuolumne River, issuing from 
 the foot of the Lyell Glacier in the Sierras of California, as 
 turbid with silt which has been ground by the moving ice. 
 
 At the foot of the Dana Glacier there is a small lakelet 
 whose waters are of a peculiar greenish yellow color from 
 the silt held in suspension, and which, when submitted to 
 microscopic examination, is found to be made up of fresh 
 angular fragments of various silicate minerals of all sizes from 
 0.35 mm. in diameter down to impalpable silt. 
 
 4. ACTION OF PLANTS AND ANIMALS 
 
 Both plants and animals aid to some extent in the work of 
 rock disintegration. Plants are also not infrequently an im- 
 portant factor in promoting sedimentation, while burrowing 
 insects and animals may exert an important influence upon 
 the texture of soils and in bringing about a more general 
 admixture by transferring to the surface that which is below. 
 
 The lowest forms of plant life, the lichens and mosses, - 
 growing upon the hard, bare face of rocky ledges send their 
 minute rootlets into every crack and crevice, seeking not 
 merely foot-hold, but food as well. 
 
 Slight as is the action, it aids in disintegration. The plants 
 die, and others grow upon their ruins. There accumulates 
 thus, it may be with extreme slowness, a thin film of humus, 
 which serves not merely to retain the moisture of rains and 
 thus bring the rock under the influence of chemical action, 
 
 1 5th Ann. Rep. U. S. Geol. Survey, 1883-84. 
 
202 THE PRINCIPLES INVOLVED IN ROCK- WEATHERING 
 
 but supplies at the same time small quantities of the humic 
 and other organic acids to which reference has already been 
 made. 1 These act both as solvents and deoxidizing agents. 
 As time goes on, sufficient soil gathers for other, larger and 
 higher types of life, which exert still more potent influences. 
 It may be the rock is in a jointed condition. Into these joints 
 each herb, shrub, or sapling pushes down its roots, which, in 
 simple virtue of their gain in bulk, day by day, serve to enlarge 
 the rifts and furnish thereby more ready access for water, and 
 the wash of rains, to still further augment disintegration. 
 
 This phase of root action is often well shown in walls of 
 ancient masonry, either of brick or stone, whereby the usual 
 rate of destruction is greatly accelerated. The depth to which 
 such roots may penetrate has often been noted, varying, as is 
 to be expected, with the nature of the soil. 2 In the limestone 
 caverns of the Southern states, the writer has often been im- 
 pressed by the number of long thread-like rootlets, so fine as 
 to be almost imperceptible, which have found their way through 
 rifts in the rocky roof. 
 
 H. Carrington Bolton has shown that very many minerals 
 are decomposed by the action of cold citric acid for a more or 
 less prolonged period, the zeolites and other hydrous silicates 
 being especially susceptible. Such tests have a peculiar sig- 
 nificance when we consider that the roots of growing plants 
 secrete an acid sap, which, by actual experiment, has been found 
 capable of etching marble. The exact nature of this acid is 
 not accurately known, but it is considered probable that in the 
 rootlets of each species of plant there exists a considerable 
 variety of organic acids. 3 
 
 But the effects of plant growth are not necessarily always 
 destructive ; such may be conservative or even protective. In 
 glaciated regions, it is often the case that the striated and pol- 
 ished surfaces of the rocks have been preserved only where pro- 
 tected from the disintegrating action of the sun and atmosphere 
 
 1 It is stated by Storer (Chemistry as applied to Agriculture) that some 
 lichens have been found to contain half their weight of oxalate of lime. 
 
 2 Aughey has found roots of the buffalo berry (Sherperdia argophylla') pene- 
 trating the loess soils of Nebraska to the depth of 50 feet. 
 
 3 See Application of Organic Acids to the Examination of Minerals, H. Car- 
 rington Bolton, Proc. Am. Assoc. for the Advancement of Science, XXXI, 1883, 
 and Available Mineral Plant Food in Soils, B. Dyer, Jour. Chem. Society, March, 
 1894. 
 
ACTION OF PLANTS AND ANIMALS 203 
 
 by a thin layer of turf or moss. As a general rule, however, 
 the manifest action of plant growth is to accelerate chemical 
 decomposition, through keeping the surfaces continually moist, 
 and to retard erosion. (See further on p. 280.) 
 
 Action of Bacteria. The researches of A. Miintz, 1 Wido 
 gradsky, Schlosing, and others tend to show that bacteria may 
 exercise a very important influence in promoting rock disinte- 
 gration and decomposition. Their influence in promoting nitri- 
 fication has been already alluded to. It would appear that 
 while these organisms secrete and utilize for their sustenance 
 the carbon from the carbonic acid of the atmosphere, as do 
 plants of a higher order, they may also assimilate carbonate 
 of ammonium, forming from it organic matter and setting free 
 nitric acid. Being of microscopic proportions, the organisms 
 penetrate into every little cleft or crevice produced by atmos- 
 pheric agencies, and throughout long periods of time produce 
 results of no inconsiderable geological significance. The depth 
 below the surface at which such may thrive is presumably but 
 slight, and their period of activity limited to the summer months. 
 They have been found on rocks of widely different character 
 granites, gneisses, schists, limestones, sandstones, and volcanic 
 rocks and on high mountain peaks as well as on lower levels. 
 The Pic Pourri, or Rotten Peak, in the Lower Pyrenees of south- 
 western France, is composed of friable and superficially decom- 
 posed calcareous schists, throughout the whole mass of which 
 are found the nitrifying bacteria, which are believed to have 
 been instrumental in promoting its characteristic decomposition. 
 The organism acts even upon the most minute fragments, reduc- 
 ing them continually to smaller and smaller sizes. Each frag- 
 ment loosened from the parent mass is found coated with a film 
 of organic matter thus produced, and the accumulation begun 
 by these apparently insignificant forces is added to by residues 
 of plants of a higher order, which come in as soon as food and 
 foothold are provided. 2 
 
 Mr. J. E. Mills, 3 and after him J. C. Branner, 4 lay con- 
 siderable stress on the decomposing effect of vegetable matter 
 
 1 Comptes Kendus de 1' Academic des Sciences, CX, 1890, p. 1370. 
 
 2 It is, perhaps, as yet, too early to say to what extent the presence of bacteria 
 may be incidental to decomposition, rather than causative. 
 
 3 American Geologist, June, 1889, p. 357. 
 
 4 Bull. Am. Geol. Soc. of America, Vol. VII. 
 
204 THE PRINCIPLES INVOLVED IN ROCK- WEATHERING 
 
 carried into the ground by ants in certain parts of Brazil, Mills 
 going so far as to describe the ants as continually pouring car- 
 bonic acid into the ground. Be this as it may, the evacuations 
 of the ants themselves are undoubtedly of such a nature as to 
 further the processes of decomposition. Certain species of ants, 
 locally known as saubas, or sauvas, live, according to Branner, 
 in enormous colonies, burrowing in the earth, where they exca- 
 vate chambers with galleries that radiate and anastomose in 
 every direction, and into which they carry great quantities of 
 leaves. Certain species of termites, the white ants of Brazil, are 
 also active promoters in bringing about changes in the structure 
 of the soil, and incidentally accelerating decomposition. The 
 organic matter carried by these creatures into the ground, there 
 to decompose, furnishes organic acids to promote further decay 
 in the material close at hand, and by its downward percolation 
 to attack the still firm rocks at greater depths. Indeed, these 
 numerous channels, through affording easy access of air and 
 surface waters with all their absorbed gases or alkaline salts, 
 may serve indirectly a geological purpose scarcely inferior to 
 that of the joints in massive rocks. (See further under soil 
 modified by plant and animal life.) 
 
 The mechanical agency which has already been referred 
 to as instrumental in bringing about a certain amount of de- 
 composition in silicate minerals, is greatly augmented when 
 such trituration takes place in connection with organic matter. 
 J. Y. Buchanan has shown, 1 that the mud of sea-bottoms is being 
 continually passed and repassed through the alimentary canals 
 of marine animals, and that in so doing the mineral matter not 
 merely undergoes a slight amount of comminution and conse- 
 quent decomposition, but a chemical reduction takes place 
 whereby existing sulphates are converted into sulphides. Such 
 sulphides and the metallic constituents of the silicates and other 
 compounds, particularly those of iron and manganese, would 
 on exposure to sea-water become converted into oxides. It is 
 through such agencies that he would account for the presence 
 of sulphur in marine muds, and the variations in color, from 
 shades of red or brown to blue and gray, in the former the iron 
 occurring as oxides, while in the latter it exists as a sulphide. 
 Of course either form may be more or less permanent according 
 
 1 On the Occurrence of Sulphur in Marine Muds, Proc. Royal Soc. of Edin- 
 burgh, 1890-91. 
 
THE PRODUCTION OF CARBONATES 205 
 
 as the mud may be devoid of animal life, or protected from 
 oxidizing influences. These reactions, being subaqueous, are 
 somewhat beyond the scope of the present work, but are never- 
 theless not without interest in this connection. 
 
 One of the most conspicuous results of rock-weathering 
 through the agencies of water and organic acids, as above enu- 
 merated, is manifested in the production of carbonates of lime 
 and more rarely of magnesia, iron, and the alkalies. Thus in 
 the decomposition of lime-bearing silicates, as the feldspars, 
 pyroxenes, and amphiboles, the lime almost invariably separates 
 out as calcite or aragonite, and often may be found filling cracks 
 and crevices, as veins of " spar " in the very rock masses from 
 which it was derived. The celebrated verde di Genova and 
 vercle di Prato marbles are but secondary rocks derived by 
 hydration from pre-existing pyroxenic masses and in which the 
 lime and magnesia have separated out as carbonates forming 
 the white veins by which the stone is traversed. The almost 
 universality of carbonate formation incident to rock-weathering 
 manifests itself in the ready effervescence of freshly decomposed 
 material when treated with an acid. It is indeed difficult to 
 find weathered rocks of any kind that will not show at least 
 traces of secondary carbonates, of which those of calcium are by 
 far the more abundant. 
 
 It is further to be noted that the solvent and general chemical 
 activity of water is often greatly augmented by the salts and 
 acids it acquires through the decomposition of various minerals 
 with which it comes in contact. Thus through the decomposi- 
 tion of iron pyrites there may be formed free sulphuric acid, 
 or through the decomposition of a feldspar, carbonates of the 
 alkalies, any of which, when in solution, are more energetic 
 factors in promoting decomposition than water alone. Hence 
 under certain conditions the process of decomposition once set 
 in operation augments itself, and goes on with increasing vigor 
 until such a depth is reached that the percolating solutions 
 become neutralized and further action, aside from hydration, 
 practically ceases. 
 
THE WEATHERING OF BOCKS (Continued} 
 II. CONSIDERATION OF SPECIAL CASES 
 
 Let us now enter into a consideration of the composition of 
 a few prominent rock types, and note the changes they have 
 undergone in this process of weathering, assuming, as we must 
 for the time being, that they have been all subjected to essen- 
 tially the same conditions. Inasmuch, as has been noted already, 
 there are divers types of rocks, differing not merely in chemical 
 and mineral composition, but in structure as well, it is an easy 
 assumption that the results of prolonged weathering may be 
 widely divergent. Yet, as will become apparent, the ultimate 
 products from all but the purely quartzose rocks, present strik- 
 ing similarities. 
 
 In the tables following are given the results of chemical and 
 mechanical analyses of rocks of various kinds and in varying 
 stages of degeneration. We will begin with a consideration 
 of the granitic rocks of the District of Columbia. 1 
 
 The rock (see PL 14) in its fresh condition is a strongly 
 foliated gray micaceous granite showing to the unaided eye 
 a finely granular aggregate of quartz and feldspars arranged in 
 imperfect lenticular masses from 2 to 5 mm. in diameter, about 
 and through which are distributed abundant folia of black 
 mica. In the thin section the structure is seen to be cataclastic. 
 Quartz and black mica are the most prominent constituents, 
 though there are abundant feldspars of both potash and soda- 
 lime varieties, "which, owing to their limpidity, can by the 
 unaided eye scarcely be distinguished from the quartz. The 
 potash feldspar has in part a microcline structure. Aside from 
 these minerals, a primary epidote, in small granules and at times 
 quite perfectly outlined crystals, is a strikingly abundant con- 
 stituent. Small apatites, a few flakes of white mica (sericite), 
 
 1 Disintegration of the Granitic Rocks of the District of Columbia, Bull. Geol. 
 Soc. of America, Vol. VI, 1895, pp. 321, 332. 
 
 206 
 
WEATHERING OF GRANITE 
 
 207 
 
 and widely scattering black tourmalines and iron ores complete 
 the list of recognizable minerals. 
 
 The outcrops from which the samples for the analyses to 
 which attention is first called were selected are shown in the 
 plate. At the very bottom, the rock is hard, fresh, and com- 
 pact, without trace of the decomposition products other than 
 as indicated by minute infiltrations of calcite from above. Just 
 above the level of the small creek which flows at the foot of 
 the bluff, at the point indicated by the first series of right-and- 
 left joints near the centre of the view, the character of the rock 
 changes quite suddenly,, becoming brown and friable, though 
 still retaining its form and easily recognizable granitic appear- 
 ance. A few feet above a third zone begins, in which the rock 
 is converted into sand and gravel and which becomes more and 
 more soil-like to the top of the bank, where it becomes admixed 
 with organic matter from the growing plants. The amount 
 of organic matter is quite small, however, and in making the 
 analyses care was taken to remove such as was recognizable in 
 the form of rootlets, leaves, and twigs. 
 
 Bulk analyses of these three types, (I) fresh gray granite, 
 (II) brown but still moderately firm and intact rock, and (III) 
 the residual sand, yielded the results given in the columns cor- 
 respondingly numbered below: 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 in 
 
 Ignition .... 
 
 1.22% 
 
 3.27 % 
 
 4.70% 
 
 Silica (Si0 2 ) 
 
 09.33 
 
 66.82 
 
 65.69 
 
 Titanium (Ti0 2 ) 
 Alumina (A1 2 3 ) 
 Iron protoxide (FeO) 
 
 not det. 
 14.33 
 3.G0 1 
 
 not det. 
 15.62 
 1.69 
 
 0.31 
 15.23 
 
 Iron sesquioxide (Fe203) 
 Lime (CaO) 
 
 3.21 
 
 1.88 
 3.13 
 
 4.39 
 2.63 
 
 Magnesia (MgO) 
 
 2.44 
 
 2.76 
 
 2.64 
 
 Soda (Na 2 0) 
 
 2 70 
 
 2.58 
 
 2.12 
 
 Potash (K 2 0) 
 
 2.67 
 
 2.04 
 
 2.00 
 
 Phosphoric acid (P 2 C>5) 
 
 0.10 
 
 not det. 
 
 0.06 
 
 
 99.60 % 
 
 99.79% 
 
 99.77 % 
 
 In glancing over these figures it is at once apparent that 
 there is a surprisingly small difference in ultimate composition 
 
 1 4.00% when calculated as Fe 2 3 . 
 
208 ROCK DISINTEGRATION AND DECOMPOSITION 
 
 between the sound rock and the residual sand, the more marked 
 differences being a slightly smaller amount of silica, more alu- 
 mina, and slightly diminished amounts of lime, magnesia, pot- 
 ash, and soda, with a considerable increase in the amount of 
 water. The ferrous salts have moreover been converted into 
 ferric forms. It does not necessarily follow, however, that no 
 more actual gain or loss of material or change in manner of 
 combination than is here indicated may not have taken place, 
 and at the very outset it may be well to enter into a discussion 
 of the manner in which the results of such analyses are to be 
 considered. 
 
 We must first of all remember that any indicated loss or 
 gain of a constituent may be only apparent, and that the true 
 relative proportions can be learned only by calculating results 
 of analyses of both fresh and decomposed materials on a com- 
 mon basis. Thus the first glance at analysis III, as given, 
 might lead one to surmise that the decomposed rock had actually 
 lost only some 3.3% of silica. This, however, is not strictly 
 the case, since this analysis shows 4.7% volatile constituents 
 against 1.22% in analysis I of the fresh material. Could we 
 assume that this difference of 3.48 % was due wholly to a 
 uniform absorption of moisture, as by a clay, the problem would 
 resolve itself into simply recalculating all analyses upon a 
 water-free basis. 
 
 The results obtained thus are not quite satisfactory, however, 
 and it is thought a more correct view of the changes taking 
 place may be obtained by assuming for one of the constituents 
 a fairly constant value and using this as a basis for comparison. 
 
 Of all the essential constituents occurring in appreciable 
 quantities in siliceous crystalline rocks the alumina and the iron 
 oxides are the most refractory and the least liable to be removed 
 by a leaching process, although they may undergo manifold 
 changes in mode of combination. Although not absolutely 
 correct, therefore, we will for our present purposes assume the 
 one or the other of these (in this case the iron as Fe 2 O 3 ) as a 
 constant factor, and in order to show the proportional or actual 
 amount of loss of any constituent will recalculate the analyses 
 upon this basis, a proceeding for which, so far as alumina is 
 concerned, we have already good authority. 1 This method will 
 be adopted, however, only with the siliceous crystalline rocks, 
 1 G. Roth, Allegemeine u. Chemische Geologic, 3d ed. 
 
WEATHERING OF GRANITE 
 
 209 
 
 in which, for reasons noted later, the process of decomposition, 
 we have reason to suppose, is more complex than in calcareous 
 and magnesian rocks poor or lacking in the alkalies. The 
 entire discussion is one beset with great difficulties, since we 
 lack definite knowledge as to the exact processes which have 
 been going on and need constantly to guard against assump- 
 tions too hastily drawn or based upon insufficient data. Indeed, 
 any assumption based upon the results of chemical analyses 
 alone is likely to lead to grave error. 
 
 If, then, we consider the iron in the form of Fe 2 O 3 as a constant 
 factor, we may, by proper calculation, obtain the results given 
 in column (IV) below, which represent the proportional gain 
 and loss of the various constituents of the rock in passing from 
 the condition indicated in column (I) above, to that indicated 
 in column (III). Such a comparison is instructive as showing 
 not merely the relative loss and gain, but also the total loss of 
 material, in this case 13.47 %, accompanied by a gain of 2.16%, 
 in volatile matter. 
 
 DISINTEGRATED AND DECOMPOSED GRANITE, DISTRICT OF COLUMBIA, SHOWING 
 PROPORTIONAL Loss OF CONSTITUENTS 
 
 
 
 IV 
 
 V 
 
 VI 
 
 CONSTITUENTS 
 
 PERCENTAGE 
 Loss FOR EN- 
 TIKE ROCK 
 
 PERCENTAGE 
 OF EACH CON- 
 STITUENT SAVED 
 
 PERCENTAGE 
 OF EACH CON- 
 STITUENT LOST 
 
 Silica (Si0 2 ) 
 Alumina (Al 2 0s) 
 
 10.50% 
 0.46 
 
 85.11% 
 96.77 
 
 14.89% 
 3.23 
 
 Iron sesquioxide (Fe 2 3 ) 
 Iron protoxide (FeO) 
 
 | 0.00 
 
 100.00 
 
 0.00 
 
 Lime (CaO) 
 
 0.81 
 
 74.79 
 
 25.21 
 
 Magnesia (MgO) 
 Soda (Na 2 0) 
 
 0.36 
 0.77 
 
 98.51 
 71.38 
 
 1.49 
 
 28.62 
 
 Potash (K 2 0) 
 
 0.85 
 
 68.02 
 
 31.98 
 
 Phosphoric anhydride (P 2 5 ) . . . 
 Ignition . .... 
 
 0.04 
 2.16 1 
 
 60.00 
 100.00 
 
 40.00 
 0.00 
 
 
 
 
 
 Total loss 
 
 13.47 % 
 
 
 
 
 
 
 
 Such results are still far from satisfactory, and it is believed 
 the tables will be more useful and instructive can we show the 
 
 Gain. 
 
210 ROCK DISINTEGRATION AND DECOMPOSITION 
 
 percentage loss and gain of each constituent as compared with 
 the same constituent in the original rock. This can also readily 
 be accomplished by a process the formula for which is given 
 below, 1 and by which are obtained the results given in columns 
 V and VI. 
 
 From a perusal of these figures, it appears that the residual 
 sand retains 85.11% of the original silica; 96.77% of the alumina; 
 all the ferric oxide; 74.79% of its lime; 98.51% of its mag- 
 nesia, together with 71.38 % of its soda and 68.02 of the potash, 
 while there has been an actual gain, as was to be expected, in 
 volatile matter. 
 
 Let us not, however, too hastily assume that we have ex- 
 hausted the subject. 
 
 We must remember, further, that while an analysis shows the 
 actual composition of a rock so far as the various elements are 
 concerned, it quite fails to show the manner in which those 
 elements are combined. While the ultimate composition of 
 the fresh and decomposed samples may be closely similar, it is 
 possible, indeed probable, that in some cases at least the manner 
 of combination of these elements is quite different. This is 
 well illustrated in the case of the figures showing the percent- 
 ages of alumina in analyses I and III and which differ only 
 nine-tenths of one per cent in total amount; yet in the first the 
 alumina exists mainly in the form of anhydrous silicates of 
 alumina, potash, iron, and magnesia (as in the feldspars and 
 mica), while in the last a very considerable proportion, or indeed 
 all in extreme cases of weathering, may exist as a hydrous sili- 
 cate of alumina only (kaolin). It is in instances of this kind 
 that the microscope may render efficient service, and much may 
 be learned by means of such mechanical analyses as can be made 
 by sifting and washing. Such separations made on this disin- 
 tegrated rock showed it to consist of particles as given in the 
 following table, the 4.25% silt being obtained by washing the 
 
 1 The formula employed in these calculations is as follows : - = x : and 
 
 Jj X O 
 
 100 x y, in which A = the percentage of any constituent in the residual 
 material ; B = the percentage of the same constituent in the fresh rock, and 
 C = the quotient obtained by dividing the percentage amount of alumina (or 
 iron sesquioxide, whichever is taken as a constant factor) of the residual mate- 
 rial by that in the fresh rock, the final quotient being multiplied by 100. x then 
 equals the percentage of the original constituent saved, in the residue, and y the 
 percentage of the same constituent lost. 
 
WEATHERING OF GRANITE 211 
 
 10.75% of material which passed through fine bolting-cloth of 
 120 meshes to the lineal inch, and which represents the impal- 
 pable mud remaining in suspension while the 6.5 % of fine sand 
 sank quickly to the bottom of the beaker in which the washing 
 was made. The residual sand yielded then: - 
 
 Silt 4.25% Largest grains 0. 1 mm. in diameter 
 
 Very fine sand 6.50 " " 0.18 
 
 Fine sand 11.25 " " 0.25 
 
 Medium sand 3.80 " 0.65 
 
 Sand) 11.00 " " 1.00 
 
 Sand/ 23.50 " " 1.50 
 
 Coarse sand 29.50 " " 2.00 
 
 Gravel . 10.20 " 8.00 
 
 Total 100.00% 
 
 The coarser of these particles, like the gravel and coarse 
 sand, are of a compound nature, being aggregates of quartz 
 and feldspar, with small amounts of mica and other minerals. 
 In the finer material, on the other hand, each particle repre- 
 sents but a single mineral, the process of disaggregation having 
 quite freed it from its associates, excepting, of course, the 
 microscopic inclusions which could be liberated only by a 
 complete disintegration of the host itself. These particles, 
 as seen under the microscope, are all sharply angular, and in 
 many cases surprisingly fresh, though the analyses, as given 
 above, had suggested only a slight change in chemical composi- 
 tion. The mica shows the greatest amount of alteration, the 
 change consisting mainly in an oxidation of its ferruginous 
 constituent, whereby the folia becomes stained and reduced 
 to yellowish brown shreds. The feldspars are, in some cases, 
 opaque through kaolinization, but in others are still fresh and 
 unchanged even in the smallest particles. The finest silt, 
 when treated with a diluted acid to remove the iron stains, 
 shows the remaining granules of quartz, feldspar, and epidote 
 beautifully fresh, and with sharp, angular borders, the mica 
 being, however, almost completely decolorized. 
 
 An analysis of the silt, which was found to constitute 4.25% 
 of the entire mass of disintegrated material, as noted above, 
 is given below, and also a partial separation and analysis of 
 the 39.7% soluble, and 60.3% insoluble portions. 1 
 
 1 In all analyses made by or under the direction of the author, the matter 
 tabulated as soluble is that extracted by boiling for three hours in hydrochloric 
 
212 ROCK DISINTEGRATION AND DECOMPOSITION 
 
 ANALYSES OP SILT PROM DISINTEGRATED GRANITE 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 III 
 
 BULK ANALYSIS 
 
 OF SILT 
 
 ANALYSIS OF 
 SOLUBLE PORTION 
 (39.7%) SILT 
 
 ANALYSIS OF 
 INSOLUBLE PORTION 
 (60.3 %) SILT 
 
 Ignition 
 
 8.12% 
 49.39 { 
 
 23.84 
 3.69 
 4.41 } 
 4.60 -1 
 3.36 [ 
 2.49 J 
 
 8.12% 
 InHCl 1.123 
 InNa 2 C0 3 11.147 
 9.-21 
 4.47 
 
 Not det. 
 
 0.97% 
 
 } 37.30 
 
 13.40 
 0.82 
 
 r 2.90 
 
 J Trace 
 j 2.75 
 1.07 
 
 Silica (Si0 2 ) 
 
 Alumina (A1 2 3 ) .... 
 Iron sesquioxide (Fe20s) . 
 Lime (CaO) . . . . 
 
 Magnesia (MgO) .... 
 Soda (Na 2 0) 
 
 Potash (K 2 0) .... 
 
 
 99.90% 
 
 34.07 
 
 59.21 
 
 93.28 % 
 
 From these analyses it would appear that of the 17 grammes 
 of silt, representing 4% of the total disintegrated material, 
 only 39.7% is soluble ; and, further, that a very considerable 
 proportion of the insoluble residue, as indicated by the high 
 percentages of alkalies and lime, still consist of unaltered soda- 
 lime and potash feldspars, the iron and magnesia alone having 
 been largely removed. 
 
 These results are not quite what one would be led to expect 
 from a perusal of the literature bearing upon the subject of 
 rock decomposition. As long since noted by J. G. Forch- 
 hammer, G. Bischof, T. Sterry Hunt, and others, the ordinary 
 processes of decay in siliceous rocks containing ferruginous 
 protoxides and alkalies consists in the higher oxidation and 
 separation of the protoxides in the form of hydrous sesqui- 
 oxides and a general hydration of the alkaline silicates, accom- 
 panied by the formation of alkaline carbonates, which, being 
 readily soluble, are taken away nearly as fast as formed. More 
 or less silica is also removed, according to the amount of car- 
 bonic acid present, a portion of the alkalies forming soluble 
 
 acid of one-half normal strength, to which is added the silica set free in a gelati- 
 nous form by the acid and subsequently extracted by sodium carbonate solu- 
 tion. All analyses made on material first dried at 100 C. 
 
WEATHERING OF GRANITE 213 
 
 alkaline silicates when the supply of the acid is insufficient to 
 take them all up in the form of carbonates. The apparent 
 anomaly here shown is partially explained by examination of 
 the various separations with the microscope. Thus the low 
 percentage of silica is found to be in large part due to the fact 
 that the residual quartz granules are, in many cases, too large 
 to pass the 120-mesh sieve, or, if passing, have been largely 
 separated in the process of washing. Further, it is found that 
 the sifting has served to concentrate the small epidotes in the 
 fine sand, and a portion of them have even come over with 
 the silt. The presence of this epidote also explains in part the 
 high percentage of lime shown, since the mineral itself carries 
 some 20 to 24 % of this material. The large percentages of 
 magnesia, soda, and potash cannot, however, be thus accounted 
 for, and we are led to infer that either these elements are there 
 combined in minute amorphous zeolitic compounds, unrecog- 
 nizable as such under the microscope, or, as seems more prob- 
 able, the feldspathic constituents, to which the alkalies are to be 
 originally referred, have undergone a mechanical splitting up 
 rather than a chemical decomposition. This view is, to a 
 certain extent, borne out by microscopic studies, but it is diffi- 
 cult to measure by the eye the relative abundance of these 
 constituents with sufficient accuracy to enable one to form any 
 satisfactory conclusion. The magnesia must come from the 
 shreds of mica, many of which, from their small size and almost 
 flocculent nature when decomposed, would naturally be found 
 in the silt obtained as stated. 
 
 It is to be noted that the magnesia, together with the iron, 
 exists almost wholly in a soluble form. 
 
 It is evident at once that we have had to do here with 
 but the preliminary stages of granitic weathering, that the 
 process is more one of disintegration than decomposition, and 
 it will be well to consider now a case in which the decom- 
 position has gone on to the condition of a residual clay, as 
 found in many of the Southern states. For this purpose a 
 biotite gneiss or gneissoid granite found near North Garden, 
 in Albemarle County, Virginia, is selected. The rock is a 
 coarse gray feldspar-rich variety with abundant folia of black 
 mica. Under the microscope it shows the presence of both 
 potash and soda-lime feldspars, a sprinkling of apatite and 
 iron ores, sporadic occurrences of an undetermined zeolite, and 
 
214 ROCK DISINTEGRATION AND DECOMPOSITION 
 
 an extraordinary number of minute zircons which are mostly 
 enclosed in the feldspars. There are also present occasional 
 small garnets and aggregates of decomposition products the 
 exact nature of which was not made out. The residual soil 
 resulting from the decomposition of this rock is highly plastic, 
 of a deep red-brown color, and has a distinct gritty feeling in 
 the hand, owing to the presence of quartz and undecomposed 
 silicate minerals. In columns I and III below are given the 
 results of analyses of fresh rock and residual soil, and in II, IV, 
 and V the analyses of the soluble and insoluble portions. In 
 columns VI, VII, and VIII are given the calculated percentage 
 amounts of the various constituents saved and lost, as before. 
 
 The particular features to which attention need here be 
 called, are (1) that 30.47 % of the fresh rock and 69.18 % of 
 the decomposed are soluble in hydrochloric acid and sodium 
 carbonate solutions, and that more than half the potash and 
 nearly the same proportion of the soda in the fresh rock is 
 found in the acid extract. (2) That the insoluble portion of 
 the residuary material is mainly in the form of free quartz. 
 (3) That 44.67 % of the original matter has been leached away, 
 and that (4) of the original silica 52.45 % is lost, while 85.61 % 
 of the iron and all the alumina remain. All the lime has dis- 
 appeared, 83.52 % of the potash, 95.03 % of the soda, and 74.70 % 
 of the magnesia. The total amount of water, as indicated by 
 the ignition, has increased very greatly, as was to be expected. 
 The small original amount of phosphoric acid prohibits our 
 placing too much reliance upon the indicated gain in this con- 
 stituent, since it may be due to errors in manipulation. 
 
 Passing from the acid group of granular crystalline rocks, 
 we will consider next a closely allied form differing mainly in 
 the absence of quartz as an essential constituent, and in the 
 presence of eleeolite, the elseolite syenites of the Fourche Moun- 
 tain region of Arkansas. These are somewhat coarsely crystal- 
 line granitic-appearing rocks, in which an orthoclase feldspar 
 in broadly tabular forms is the prevailing constituent, though 
 always accompanied by nepheline, biotite, pyroxene, titanite, 
 and apatite, while fluorite, analcite, and thomsonite, together 
 with calcite, occur as secondary products. The rock weathers 
 away to a coarse gray gravel which ultimately becomes a clay, 
 from which, by washing, may be obtained kaolin in a fail- 
 degree of purity. 
 
WEATHERING OF GNEISS 
 
 215 
 
 
 a 
 
 a 
 w 
 
 i 
 
 s 
 
 irH 
 
 r^OI^COO O0 
 
 j 
 
 
 
 z 
 
 ,_, 
 
 ^^.n 
 
 o ooooaot^oo 
 
 OOOCOTPC100 
 
 
 
 4 
 
 HH 
 
 g 
 
 Q 
 
 ' 
 
 JOa8 " raDJ9J 
 
 l- >0 O Tt< 
 
 -* O CO CN i-t O C 
 
 
 
 s 
 
 > 
 
 <! 
 D 
 
 
 SSOT 
 
 s gg^sssii 
 
 
 
 
 K>1 
 
 H 
 
 s 
 
 o 
 
 
 
 rH Or-^OCO(MOO 
 
 ^ 
 
 
 fc 
 
 P 
 
 O 
 O 
 
 a 
 
 s 
 
 s 
 
 - 
 
 -iq jo sisXjBuy 
 
 o Jg^g^^oo^ 
 c<i H H 
 
 CO ^ 
 
 
 PS 
 
 <3 
 
 ^ 
 
 o 
 
 a 
 
 
 E 00 Z M pan IOH 
 
 S3S8SSSS :5 
 
 o 
 
 0' 
 
 co ' 
 
 
 w 
 
 P3 
 t_3 
 
 1 
 
 
 -aoj jo siSA[Biiy 
 
 T 1 (M r-H rH 
 
 J 
 
 
 <5 
 c/T 
 
 P 
 
 
 
 SS<5|SSS55S 
 
 co 
 Oi 
 
 
 co 
 
 3 
 2; 
 
 
 
 
 uo" d c<i .H d I-H d d co' 
 
 
 
 
 o 
 
 Q 
 
 
 
 WKPUMOH 
 
 -<tfico o oao^fr-i<Mo 
 
 r- 
 
 'S 
 
 a 
 
 W 
 
 1 
 
 2 
 O 
 
 
 -joj jo sisX[t3uy 
 
 rHCO CO r-lOGOr-100 
 
 
 
 
 
 O 
 
 o 
 
 | 
 
 
 
 g s3sssg 
 
 s 
 
 
 H 
 P 
 
 
 
 
 CDCi^rHTtl^OO 
 
 1 
 
 
 P 
 
 ^ 
 
 <<< 
 
 
 
 
 
 
 
 X 
 
 
 
 
 
 
 
 
 w 
 
 p^ 
 
 
 
 
 
 
 
 PH 
 
 
 
 
 
 
 
 PH 
 O 
 
 
 
 s 
 
 
 
 
 
 ANALYSES 
 
 
 
 w 
 
 C^ c3 Q> ^^ 
 
 f fill fill 
 
 
 
216 
 
 KOCK DISINTEGRATION AND DECOMPOSITION 
 
 The following analyses from the work of Dr. J. F. Williams 1 
 will serve to show the changes which have here taken place in 
 the transformation from (I) fresh syenite through (II and III) 
 intermediate stages of decomposition to (IV) a kaolin-like 
 residue. 
 
 ANALYSES OF FRESH AND DECOMPOSED SYENITE, ARKANSAS 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 III 
 
 IV 
 
 Silica (Si0 2 ) .... 
 
 59.70% 
 
 58.50% 
 
 50.65% 
 
 46.27 % 
 
 Alumina (Al 2 0s) . . 
 
 18.85 
 
 25.71 
 
 26.71 
 
 38.57 
 
 Ferric oxide (Fe 2 3 ) . 
 
 4.85 
 
 3.74 
 
 4.87 
 
 1.36 
 
 Lime (CaO) .... 
 
 1.34 
 
 0.44 
 
 0.62 
 
 0.34 
 
 Magnesia (MgO) . . . 
 
 0.68 
 
 Trace 
 
 0.21 
 
 0.25 
 
 Potash (K 2 0) .... 
 
 5.97 
 
 1.96 
 
 1.91 
 
 0.23 
 
 Soda (Na 2 0) .... 
 
 6.29 
 
 1.37 
 
 0.62 
 
 0.37 
 
 Ignition (H 2 0) . . . 
 
 1.88 
 
 5.85 
 
 8.68 
 
 13.61 
 
 
 99.56 % 
 
 97.57 % 
 
 94.27 % 
 
 101.00% 
 
 Recalculating the numbers given in columns I and IV upon 
 the basis of 100, we may obtain by further calculation, as already 
 described, the figures given in columns V and VI and VII below, 
 which represent the proportional loss of each constituent, as 
 before. 
 
 CALCULATED Loss OF MATERIAL 
 
 
 v 
 
 VI 
 
 VII 
 
 CONSTITUENTS 
 
 PERCENTAGE 
 Loss FOR ENTIRE 
 KOCK 
 
 PERCENTAGE 
 OF EACH CON- 
 STITUENT SAVED 
 
 PERCENTAGE 
 OF EACH CON- 
 STITUENT LOST 
 
 Silica (Si0 2 ) . 
 
 37. 28% loss 
 
 37.82% 
 
 62.18% 
 
 Alumina (Al 2 0a) 
 
 0.00 
 
 100.00 
 
 0.00 
 
 Ferric oxide (Fe 2 3 ) 
 
 4.19 
 
 13.83 
 
 86.17 
 
 Lime (CaO) 
 
 1 19 
 
 12.10 
 
 87.90 
 
 Magnesia (MgO) 
 
 0.57 
 
 17.90 
 
 82.10 
 
 Potash (K 2 0) 
 
 5.90 
 
 18.15 
 
 81.85 
 
 Soda (Na 2 0) 
 
 6.15 
 
 2.89 
 
 97.11 
 
 Water (H 2 0) .... 
 
 00 
 
 100.00 
 
 0.00 
 
 
 
 
 
 Total loss of original material, 56.28%. 
 1 Ann. Eep., Vol. II, 1890, Arkansas Geol. Survey. 
 
WEATHERING OF SYENITE AND PHONOLITE 
 
 217 
 
 Here, as with the granitic rocks, it will be noted we have a 
 gradual increase in the percentage of water as the decomposi- 
 tion advances, and a decrease in the amount of silica even more 
 pronounced. This last, as may be readily imagined, is due to 
 the absence of free quartz in the Fourche Mountain rocks. 
 
 The phonolites of Marienfels, near Assig, in Bohemia, have 
 been described by Lemberg l as weathering into a bright- 
 colored, porous, friable mass, the composition of which, as 
 compared with the fresh rock, is shown below. Each column, 
 it should be stated, represents an average of three analyses, 
 I being the fresh and II the weathered material, while in III, 
 IV, and V are given the percentage calculations of gain and 
 loss, as before. 
 
 ANALYSES OF FRESH AND DECOMPOSED PHONOLITE, BOHEMIA 
 
 
 I 
 
 II 
 
 ill 
 
 IV 
 
 V 
 
 CONSTITUENTS 
 
 w 
 
 H 
 
 c 
 
 - H 
 * 
 
 3 
 
 12 
 
 K 
 
 w 
 
 t 
 
 IJ 
 
 s 
 
 LI 
 
 | a 
 
 - 
 
 si 
 
 fe PH 
 
 li 
 
 2 
 
 Q 
 
 H 
 
 % % 
 O o 
 
 ^6 
 
 BhiS 
 
 * oo 2 
 
 a W g |; 
 ^63 
 
 Silica (8162) 
 
 55.67 % 
 
 55. 72 % 
 
 4.83 % 
 
 91 46 % 
 
 8 54 % 
 
 Alumina (A^Os) ... . 
 
 20.64 
 
 22.19 
 
 0.37 
 
 98.40 
 
 1.60 
 
 Ferric oxide (Fe 2 3 ) 
 Lime (CaO) 
 Magnesia fMgO) 
 
 3.14 
 1.40 
 42 
 
 3.44 
 1.28 
 44 
 
 0.00 
 0.25 
 02 
 
 0.00 
 83.66 
 95 65 
 
 100.00 
 16.34 
 4 35 
 
 Potash (K 2 O) 
 Soda (Na 2 0) 
 
 5.56 
 7.12 
 
 6.26 
 2.65 
 
 O.OO 1 
 4.79 
 
 100.00 
 34.01 
 
 0.00 
 65.99 
 
 Ignition 
 
 4.33 
 
 7.79 
 
 O.OO 1 
 
 100.00 
 
 0.00 
 
 
 98.28% 
 
 99.77% 
 
 10.26 % 
 
 .... 
 
 .... 
 
 This phonolite, it should be remarked, consisted essentially 
 of sanidin feldspars and a soda zeolite, together with accessory 
 augite, black mica, magnetic and titanic iron, and possibly 
 hauyne. The zeolite is assumed to have originated from the al- 
 teration of the nepheline. The process of decomposition would 
 seem to consist, then, in the breaking down of this zeolite, and 
 the conversion of the rock into an earthy mass, with little other 
 
 1 Zeit. der Deutschen Geol. Gesellschaft, Vol. 35, 1883, p. 559. 
 
 2 Gain. The calculations for potash in column IV gives: 107.79% and for 
 ignition 164.77%. 
 
218 HOCK DISINTEGRATION AND DECOMPOSITION 
 
 change, so far as ultimate composition is concerned, than a loss 
 of a considerable proportion of its soda, and an assumption of 
 nearly 3.5 % of water. The decomposed rock yielded 55.44 % 
 of material insoluble in hydrochloric acid, and with essentially 
 the composition of sanidin, showing that this mineral underwent 
 only a physical disintegration, the decomposition proper being 
 limited to the other constituents. 1 
 
 Turning to still more basic rocks, we will next consider 
 a disintegrated diabase occurring in the form of a large dike 
 extending from Granite Street in Somerville, Massachusetts, to 
 Spot Pond in Stoneham, and beyond. 2 The rock at the point 
 selected for study (Medford) is a coarsely granular admixture 
 of lath-shaped feldspar, black mica, augite, and brown basaltic 
 hornblende, with the usual sprinkling of apatite, magnetite, and 
 ilmenite. Secondary uralite, chlorite, biotite, leucoxene, kaolin, 
 calcite, pyrite, and quartz are common. 3 
 
 The rock has undergone extensive disintegration, giving rise 
 to loose sand and gravel of a deep brown color, in which lie 
 rounded boulders of all sizes of the still undecomposed material. 
 These boulders, as is usually the case, show a more or less con- 
 centric structure, from without inward, until a solid core of 
 unaltered diabase is met with. (See PI. 17, and Fig. 2, PI. 20.) 
 
 A mechanical separation of the disintegrated material yielded 
 results as below : 
 
 1. 
 
 2. 
 3. 
 4. 
 
 5. 
 6. 
 
 7. 
 8. 
 9. 
 10. 
 
 Coarse gravel ab 
 Fine gravel 
 Coarse sand 
 Medium sand 
 Fine sand 
 Very fine sand 
 Silt 
 Fine silt 
 Clay 
 Loss at 110 C. 
 
 Dve 2 mm. 
 2-1 mm. 
 1-5 mm. 
 .5-.25 mm. 
 .25-. 1 mm. 
 .1-.05 mm. 
 .05-. 01 mm. 
 .01-.005 mm. 
 .005-.0001 mm. 
 
 in diameter 
 
 42 300 / 
 
 in diameter . 
 
 . . 20 355 
 
 in diameter . 
 
 12 723 
 
 in diameter . . 
 
 . . . . 9 567 
 
 in diameter . 
 
 4 907 
 
 in diameter . 
 
 . . . . 4.181 
 
 in diameter . 
 
 1 128 
 
 in diameter . 
 in diameter . 
 
 .... 0.370 
 . . . . 1 670 
 
 
 0.660 
 
 11. Loss on ignition 1.730 
 
 99.691 % 
 
 1 In calculating these analyses, it was found that the loss of alumina had 
 exceeded that of iron oxide, necessitating the assumption of the last-named as 
 a constant for comparison. The apparent gain in potash is presumably due to 
 errors in analysis, since, as will be noted, the analysis of the fresh material, given 
 in column I, foots up only 98.28%. 
 
 2 See Disintegration and Decomposition of Diabase at Medford, Massachu- 
 setts, by G. P. Merrill, Bull. Geol. Soc. of America, Vol. VII, 1896, pp. 349-362. 
 
 3 On the Petrographic Characters of a Dike of Diabase in the Boston Basin, 
 by W. H. Hobbs, Bull. Mus. Comp. Zoology, Vol. XVI, No. 1, 1888. 
 
I 
 
WEATHERING OF DIABASE 219 
 
 Of the above, the first three sizes could be easily recognized 
 by the unaided eyes, as composed of particles of a compound 
 nature. In number 4 the separation had gone a trifle farther, 
 though even here inspection with a pocket lens revealed the 
 compound nature of many of the granules, somewhat obscured 
 by the prevailing discoloration from the oxides of iron. It 
 forms a gray-brown sand composed of feldspathic particles, 
 dirty brown augites, and lustrous scales of brown mica. Num- 
 bers 5 and 6 seemed composed almost wholly of beautifully 
 lustrous, dark mahogany-brown mica scales, while 7 would pass 
 for a finely micaceous umber. Numbers 8 and 9 were uni- 
 formly ochreous, the last being several shades lighter than 
 number 8, and without appreciable grit. 
 
 The chemical nature of the fresh and decomposed rock is 
 shown in the accompanying table, the results being in nearly 
 every case averages obtained from two or more analyses. The 
 "fresh" material, obtained from the interior of one of the boul- 
 ders, is firm in texture, has a bright clean fracture, and shows to 
 the unaided eye no signs of decomposition. When pulverized 
 and treated with acid, however, it effervesces distinctly, indi- 
 cating the presence of free carbonates, which are also observ- 
 able as secondary calcite when thin sections are examined under 
 the microscope. Some of this calcite is evidently a deposit from 
 infiltrated waters, being derived from the surrounding decom- 
 posed material, while a portion results from the decomposition 
 of the silicate minerals in place. Aside from a slight kaolini- 
 zation of the feldspars and development of chlorite from the 
 ferruginous silicates, there are no other observable signs of de- 
 composition, though the presence of a soda-bearing zeolite is indi- 
 cated by cubes of chloride of sodium, which separate out when 
 an uncovered slide is treated with a drop of hydrochloric acid. 
 
 A glance at this table is sufficient to show that the disinte- 
 gration is accompanied by decomposition and a leaching action 
 which has resulted in the removal of a portion of the more 
 soluble constituents. The fact that the fresh rock yields the 
 larger percentages of its constituents to the solvent action of 
 acid and alkaline solutions is readily explained on this ground, 
 though it may be doubted if the full significance of the fact, so 
 far as it relates to siliceous crystallines, is as yet appreciated. 
 It will be observed that 36.23 % of the fresh rock and 32.28% 
 of the decomposed is thus extracted. 
 
220 ROCK DISINTEGRATION AND DECOMPOSITION 
 
 ANALYSES OF FRESH AND DISINTEGRATED DIABASE FROM MEDFORD 
 
 
 
 
 SILT FROM DISINTEGRATED 
 
 
 FRESH DIABASE 
 
 DISINTEGRATED 
 DIABASE 
 
 DIABASE, Nos. VII, VIII, 
 AND IX OF TABLE, ON 
 
 
 
 
 P. 218 
 
 
 I 
 
 II 
 
 III 
 
 IV 
 
 V 
 
 VI 
 
 VII 
 
 CONSTITUENTS 
 
 .2 
 
 ig 
 
 
 1-2 * 
 
 s 
 
 .2 ^ 
 
 
 
 5 
 
 a 
 
 a 
 
 2 ' 
 
 "S3 
 | 
 
 it 
 
 III 
 
 'o " 
 a 
 
 2 -u C3 
 
 
 
 
 t<81 
 
 
 *> ci 
 
 "33 a a 
 
 >> c3 
 
 S c 
 t^ g O 
 
 
 
 M 
 PQ 
 
 111 
 
 W 
 
 111 
 
 111 
 
 Us 
 
 I 
 
 Silica (Si0 2 ) 
 
 % 
 
 % 
 
 
 % 
 
 % 
 
 % 
 
 % 
 
 fin HC1 ) 
 tin Na 2 Co 3 ) 
 
 47.28 
 
 r 1.19 
 
 I 9.66 
 
 1 44.44 
 
 f 0.85 
 \ 8.65 
 
 0.47 
 22.63 
 
 J13.51 
 
 36.61 
 
 Alumina (Al 2 0s) 
 
 20.22 
 
 4.74 
 
 23.19 
 
 4.86 
 
 21.98 
 
 
 
 Ferric oxide 
 
 
 
 
 
 
 
 
 (Fe 2 3 ). . . 
 
 3.66 
 
 ) 
 
 
 
 
 5.88 
 
 40.68 
 
 Ferrous oxide 
 
 
 1 10.91 
 
 12.70 
 
 10.00 
 
 12.83 
 
 
 
 (FeO) . . . 
 
 8.89 
 
 J 
 
 
 
 
 
 
 Lime (CaO) . . 
 
 7.09 
 
 3.09 
 
 6.03 
 
 1.50 
 
 3.32 
 
 0.12 
 
 3.44 
 
 Magnesia (MgO) 
 
 3.17 
 
 2.20 
 
 2.82 
 
 1.84 
 
 3.23 
 
 0.79 
 
 4.02 
 
 Manganese oxide 
 
 
 
 
 
 
 
 
 MnO. . . . 
 
 0.77 
 
 Not det. 
 
 0.52 
 
 Not det. 
 
 Not det. 
 
 Not det. 
 
 Not det. 
 
 Potash (K 2 0) . 
 
 2.16 
 
 1.21 
 
 1.75 
 
 0.68 
 
 1.30 
 
 0.52 
 
 1.82 
 
 Soda (Na 2 0) . . 
 
 3.94 
 
 0.50 
 
 3.93 
 
 0.17 
 
 0.90 
 
 1.24 
 
 2.14 
 
 Phosphoric acid 
 
 
 
 
 
 
 
 
 (P 2 6 ) . . . 
 
 0.68 
 
 Not det. 
 
 0.70 
 
 Not det. 
 
 Not det. 
 
 Not det. 
 
 .... 
 
 Ignition . . . 
 
 2.73 
 
 2.73 
 
 3.73 
 
 3.73 
 
 10.86 
 
 0.11 
 
 10.97 
 
 
 100.59 
 
 36.23 
 
 99.81 
 
 32.28 
 
 77.52 
 
 22.17 
 
 99.68 
 
 Of the material classed as silt in columns V, VI, and VII, or 
 as silt and clay, on p. 218, and which constitutes only some 
 3.17 % of the entire residual debris, 77.87% is soluble in dilute 
 hydrochloric and sodium carbonate solutions. The insoluble 
 portion, constituting 22.13% of the silt, consists of unaltered 
 feldspar and iron, lime and magnesian silicates, which are easily 
 recognized under the microscope, in the form of minute, sharply 
 angular particles. Recalculating, as before, the matter in col- 
 umns I and II on the basis of 100 and considering the alumina 
 as a constant factor, we get the results given in columns VIII to 
 XII inclusive, representing, so far as it can be obtained by this 
 
WEATHERING OF DIABASE 
 
 221 
 
 method, the actual percentage loss of materials attending the 
 breaking down. 
 
 CALCULATED Loss or MATERIAL 
 
 
 VIII 
 
 IX 
 
 X 
 
 XI 
 
 XII 
 
 
 RECALCULATED ON 
 
 J 8 
 
 o *-* 
 
 3 
 
 CONSTITUENTS 
 
 BASIS OF 100 
 
 
 
 73 
 
 
 
 ti 
 
 til 
 
 fj- 
 
 
 
 
 Fresh 
 
 Decomposed 
 
 8 w 
 
 8 ,a M 
 
 fu-2 
 
 
 Diabase 
 
 Diabase 
 
 q; fr-i 
 
 <n c* ^ 
 
 } * + J 
 
 
 
 
 P-i 2 
 
 cupa 
 
 PH W 
 
 Silica (Si0 2 ) 
 
 47.01% 
 
 44.51% 
 
 8.48 
 
 81.97% 
 
 18.03% 
 
 Alumina (A1 2 3 ) . . . 
 
 20.11 
 
 23.24 
 
 0.00 
 
 100.00 
 
 0.00 
 
 Ferric oxide (Fe 2 O 3 ) . . 
 Ferrous oxide (FeO) . . 
 
 3.63 
 
 8.83 
 
 1 12.71 
 
 2.42 
 
 81.90 
 
 18.10 
 
 Lime (CaO) 
 
 7.06 
 
 6.04 
 
 1.83 
 
 74.11 
 
 25.89 
 
 Magnesia (MgO) 
 
 3.15 
 
 2.85 
 
 0.68 
 
 78.30 
 
 21.70 
 
 Manganese (MnO) . 
 
 0.77 
 
 0.52 
 
 0.32 
 
 58.43 
 
 41.57 
 
 Potash (K 2 0) .... 
 
 2.14 
 
 1.75 
 
 0.62 
 
 70.85 
 
 29.15 
 
 Soda (Na 2 0) 
 
 3.91 
 
 3.94 
 
 0.50 
 
 87.17 
 
 12.83 
 
 Phosphoric acid (P 2 6 ) . 
 
 0.68 
 
 0.70 
 
 0.08 
 
 88.61 
 
 11.39 
 
 Ignition 
 
 2.71 
 
 3.74 
 
 0.00 
 
 100.00 
 
 0.00 
 
 
 
 
 
 
 
 100.00 % 
 
 100.00% 
 
 14.93% 
 
 .... 
 
 .... 
 
 From the figures in column X it appears that there has 
 been a loss of some 14.93% of all constituents. The increase 
 in water, as indicated by the ignition, is a natural consequence 
 of hydration and the presence of a small amount of organic 
 matter. This increase, it should be stated, is greater than may 
 be at first apparent, for the reason that the fresh rock contains 
 a considerable amount of secondary calcite, which is quite lack- 
 ing in the residual sand. A large part of the ignition in col- 
 umns I and VIII is therefore to be accredited to carbonic acid, 
 and not to water of hydration. 
 
 From columns XI and XII it appears that of all the essential 
 constituents, the lime and potash salts have suffered the most, 
 though the iron oxides have been carried away to the amount 
 of 18.10 %. Magnesia has also proven very susceptible to the 
 solvent action, disappearing to the amount of 21.70%; and 
 lastly, silica, to the amount of 18.03%. The small original 
 amounts of manganese and phosphoric acid render the results 
 
222 
 
 KOCK DISINTEGRATION AND DECOMPOSITION 
 
 obtained by these calculations of doubtful value, since it is pos- 
 sible they may be due to errors of analysis. 
 
 In this case, as in that of the granite from the District 
 of Columbia, we have to do with only the earlier stages of de- 
 generation, with conditions which are as much in the nature 
 of mechanical disintegration as of chemical decomposition. As 
 before, then, it will be instructive to consider cases in which, in 
 rocks of similar nature, the decomposition has proceeded much 
 farther. For this purpose we will select a diabase from Spanish 
 Guiana, 1 and basalts from Bohemia and the Haute Loire as 
 described by Ebelmen; 2 in each instance the actual analysis 
 being recalculated to the basis of 100. 
 
 ANALYSES OF FRESH AND DECOMPOSED DIABASE FROM SPANISH GUIANA, 
 
 VENEZUELA 
 
 
 I 
 
 II 
 
 III 
 
 IV 
 
 v 
 
 CONSTITUENTS 
 
 1 
 
 DECOMPOSED 
 
 PERCENTAGE 
 Loss FOR EN- 
 TIRE EOCK 
 
 PERCENTAGE OF 
 EACH CONSTIT- 
 UENT SAVED 
 
 PERCENTAGE OF 
 EACH CONSTIT- 
 UENT LOST 
 
 Silica (Si0 2 ) 
 
 49 35 / 
 
 43 38 / 
 
 20 92 / 
 
 57 60 / 
 
 42 40 / 
 
 Alumina (A1 2 3 ) . . . 
 Ferric iron (Fe 2 0s) . . 
 Ferrous iron (FeO) . . 
 Lime (CaO) 
 Magnesia (MgO) . . . 
 Potash (K 2 0) .... 
 Soda (Na 2 0) . 
 
 15.30 
 
 12.28 
 9.60 
 7.38 
 0.85 
 1 98 
 
 18.36 
 20.39 
 
 2.37 
 3.45 
 0.59 
 14 
 
 3.27 
 0.00 
 
 8.05 
 5.12 
 0.33 
 1 82 
 
 78.62 
 100.00 
 
 16.17 
 30.63 
 54.12 
 4 63 
 
 21.38 
 0.00 
 
 83.23 
 61.37 
 45.88 
 95 37 
 
 Ignition 
 
 3 25 
 
 11 34 
 
 00 
 
 OO 3 
 
 00 
 
 
 
 
 
 
 
 
 100.00 % 
 
 100.00 % 
 
 39.51 % 
 
 .... 
 
 .... 
 
 In the case of the diabase, it appears, from a comparison 
 of the figures in columns I and III, that the total loss of 
 material equals 39.51 %, there being the usual gain in volatile 
 matter. 
 
 1 Quar. Jour. Geol. Soc. of London, Vol. XXXV, 1879, p. 586. 
 
 2 Ann. des Mines, Vol. VII, 1845. 
 
 3 Gain. 
 
WEATHERING OF DIABASE AND BASALT 
 
 223 
 
 ANALYSES or FRESH AND DECOMPOSED BASALT FROM KAMMAR BULL, 
 
 BOHEMIA 
 
 
 I 
 
 II 
 
 III 
 
 IV 
 
 V 
 
 VI 
 
 
 
 
 
 K ^ 
 
 1 
 
 fe 
 
 h 
 
 
 
 *~" 5 
 
 fi O 
 
 
 H 
 
 c , 
 
 CONSTITUENTS 
 
 w 
 
 
 H 
 
 c w 
 
 EH 
 X 
 
 H 
 W H 
 
 
 o 
 
 ^ 
 
 ^ a 
 
 o 
 
 g I 
 
 - 
 
 << | x 
 
 
 
 < 5 
 
 ^ O? 
 
 
 - ^ & 
 
 > r X "; 
 
 
 M 
 
 ^ P 
 
 )-3 ? 
 
 K ^ 
 
 N- 
 
 M - 
 
 
 
 
 I I 
 
 J 
 
 5 
 
 III 
 
 K < 
 
 ||| 
 
 Silica (Si0 2 ) .... 
 
 43.61 
 
 43.00 
 
 43.27 
 
 15.04 loss 
 
 67.01 
 
 32.99 
 
 Alumina (A1 2 3 ) 
 
 12.26 
 
 13.90 
 
 18.13 
 
 0.00 " 
 
 100.00 
 
 0.00 
 
 Ferric iron (Fe 2 O 3 ) . 
 Ferrous iron (FeO) . 
 
 3.51 
 12.16 
 
 5.40 \ 
 8.30 / 
 
 11.70 
 
 9.10 " 
 
 49.83 
 
 50.17 
 
 Lime (CaO) .... 
 
 11.87 
 
 12.10 
 
 2.60 
 
 9.60 " 
 
 54.47 
 
 84.53 
 
 Magnesia (MgO) . 
 
 9.14 
 
 7.30 
 
 3.40 
 
 6.83 " 
 
 25.90 
 
 74.10 
 
 Soda (Na 2 O) .... 
 Potash (K 2 0) .... 
 
 2.721 
 0.81 J 
 
 0.50 
 
 0.20 
 
 3.39 " 
 
 38.31 
 
 61.69 
 
 Water (H 2 O) .... 
 
 4.42 
 
 9.50 
 
 20.70 
 
 0.00 
 
 100.00 
 
 
 
 
 100.00 % 
 
 100.00 % 
 
 100.00 % 
 
 43.96 loss 
 
 
 
 ANALYSES OF FRESH AND DECOMPOSED BASALT FROM CROUZET, IN THE 
 HAUTE LOIRE, FRANCE 
 
 
 I 
 
 II 
 
 III 
 
 IV 
 
 V 
 
 
 
 
 o 
 
 j. 
 
 o 
 
 c , 
 
 
 
 
 
 H 
 
 H 
 
 CONSTITUENTS 
 
 g 
 
 3 
 
 K y 
 C BS 
 
 
 2 * u 
 
 
 
 1 
 
 V. 
 
 H ^ 
 
 ||f 
 
 III 
 
 
 i 
 
 8 o 
 
 K 
 
 Es3 ^ O 
 
 W < 
 
 K < 
 
 
 fa 
 
 P M 
 
 CH s: w 
 
 ft- a uj 
 
 fc, W a 
 
 Silica (Si0 2 ) .... 
 
 48.29% 
 
 37.09 % 
 
 30. 34% loss 
 
 34.44% 
 
 65.56% 
 
 Alumina (A1 2 3 ) . . . 
 
 13.25 
 
 30.75 
 
 0.00 " 
 
 100.00 
 
 0.00 
 
 Ferric iron (Fe 2 3 ) . 
 Ferrous iron (FeO) . 
 
 0.00 
 16.66 
 
 4.31 } 
 0.00 J 
 
 16.64 " 
 
 11.16 
 
 88.84 
 
 Lime (CaO) .... 
 
 7.33 
 
 8.97 
 
 3.46 " 
 
 52.76 
 
 47.24 
 
 Magnesia (MgO) . . . 
 
 7.03 
 
 0.61 
 
 6.77 " 
 
 3.62 
 
 96.38 
 
 Potash (K 2 0) .... 
 
 1.81 
 
 0.71 
 
 1.51 " 
 
 16.66 
 
 83.34 
 
 Soda (Na 2 O) .... 
 
 2.71 
 
 1.01 
 
 1.40 " 
 
 25.59 
 
 74.41 
 
 Ignition 
 
 4.92 
 
 16.55 
 
 0.00 
 
 100.00 
 
 0.00 
 
 
 100.00 % 
 
 100.00% 
 
 60. 12% loss 
 
 .... 
 
 .... 
 
224 ROCK DISINTEGRATION AND DECOMPOSITION 
 
 Of the individual constituents, 83.23% of the original lime, 
 61.37 % of the magnesia, 45.88% of the potash, 95.37 % of the 
 soda, 42.40 % of the silica, and 21.38 % of the alumina have dis- 
 appeared, the calculations being made on a Fe 2 O 3 constant 
 basis. 
 
 In the case of the Bohemian basalt, the decomposition com- 
 menced with the formation of boulders, which, when the 
 process had not gone too far, still showed fresh, unchanged 
 basalt interiorly, but became more and more altered toward 
 their peripheries. The first stage of decomposition (column II), 
 it will be noted, consists, aside from hydration, in a slight appar- 
 ent loss of silica, a considerable oxidation of the iron magnesia 
 minerals, accompanied by a slight loss of both constituents, and 
 an almost complete loss of alkalies. In the second stage (column 
 III) lime and magnesia are both lost in considerable amounts, 
 the iron passing over wholly to the condition of sesquioxide, and 
 there is a further slight diminution in the proportional amount 
 of silica. It is evident that here the feldspars were the first of 
 the constituents to yield to the decomposing forces, the augite 
 and olivine proving most refractory. The total loss of material, 
 it will be noted, amounts to 43.96 %, the lime, magnesia, alka- 
 lies, iron oxides, and silica disappearing in the order here 
 mentioned. 
 
 In the case of the basalt from Crouzet, the analyses show a 
 total of 60.12% loss, or over one-half of the original material. 
 This loss includes nearly two-thirds of the original silica, 
 88.84% of the iron, and 96.38% of the magnesia. The loss 
 of both iron and magnesia in such proportionally large quan- 
 tities is quite unusual, and indicates, so far as the iron is con- 
 cerned, that the decomposition took place under conditions 
 excluding a sufficient supply of oxygen to convert the same 
 into the insoluble sesquioxide, or where subjected to the 'de- 
 oxidizing and solvent action of organic acids. The removal of 
 the magnesia, which must have existed mainly in the mineral 
 olivine, indicates that the decomposition has gone on even to 
 the production of carbonate of magnesia and the separation 
 of free silica and iron oxides. 
 
 An analysis by the present writer of a closely related rock, 
 a diorite, and its residual soil, from North Garden, Albemarle 
 County, Virginia, yielded the results given in columns I and 
 II below. The rock here was fine-grained, of an almost coal- 
 
WEATHERING OF DIORITE 
 
 225 
 
 black color finely speckled with whitish flecks due to the 
 presence of feldspars. The microscope showed it to be com- 
 posed mainly of hornblende with interstitial soda-lime feldspars 
 and scattering areas of titanic iron. The clay, or soil, to which 
 it gave rise was deep brownish red in color and highly plastic, 
 though distinctly gritty from the presence of undecomposed 
 minerals. In columns III, IV. and V are given the loss and 
 gain of the various constituents calculated on an alumina 
 constant basis, as before. 
 
 ANALYSES OF FRESH AND DECOMPOSED DIORITE FROM ALBEMARLE COUNTY, 
 
 VIRGINIA 
 
 
 I 
 
 II 
 
 III 
 
 IV 
 
 V 
 
 CONSTITUENTS 
 
 FRESH 
 
 DECOM- 
 POSED 
 
 CALCULATED 
 Loss FOR EN- 
 
 PER CENT 
 OF EACH 
 CONSTITU- 
 
 PER CENT 
 OF EACH 
 
 CONSTITU- 
 
 
 
 
 
 
 
 
 
 
 
 ENT SAVED 
 
 ENT LOST 
 
 Silica (Si0 2 ) 
 
 46.75% 
 
 42.44 % 
 
 17. 43% loss 
 
 62.69% 
 
 37.31 % 
 
 Alumina (A1 2 3 ) . . . 
 
 17.61 
 
 25.51 
 
 0.00 
 
 100.00 
 
 0.00 
 
 Iron sesquioxide (Fe 2 3 ) l 
 
 16.79 
 
 19.20 
 
 3.53 
 
 78.97 
 
 21.03 
 
 Lime (CaO) 
 
 9.46 
 
 0.37 
 
 9.20 
 
 2.70 
 
 97.30 
 
 Magnesia (MgO) . . 
 
 5.12 
 
 0.21 
 
 4.97 
 
 2.83 
 
 97.17 
 
 Potash (K 2 0) .... 
 
 0.55 
 
 0.49 
 
 0.21 
 
 61.25 
 
 38.75 
 
 Soda (Na 2 0) . 
 
 2 56 
 
 0.56 
 
 2.17 
 
 15.13 
 
 84.87 
 
 Phosphoric acid (P 2 0) 5 . 
 
 0.25 
 
 0.29 
 
 0.00 
 
 80.11 
 
 19.87 
 
 Ignition 
 
 0.92 
 
 10.92 
 
 0.00 
 
 100.00 
 
 0.00 
 
 
 
 
 
 
 
 
 100.01 % 
 
 99.99 % 
 
 37. 51% loss 
 
 .... 
 
 .... 
 
 The ultra basic rocks, peridotites and pyroxenites, from 
 the very nature of their composition, must yield on decompo- 
 sition residues poor in the presence of alkalies and rich in 
 iron or aluminum and magnesian compounds. Owing, further, 
 to their poverty in alkali-bearing silicates, the process of decom- 
 position must be less complex, consisting essentially in hydra- 
 tion, oxidation, and a production of iron, lime, and magnesian 
 carbonates and a liberation of chalcedonic silica. 
 
 During the process these rocks as a rule become brownish, 
 and, on the surface, often irregularly checked with a fine net- 
 work of rifts which become filled with secondary calcite, mag- 
 nesite, and chalcedony. If the original rock is an olivme-rich 
 
 1 All iron calculated as Fe 2 O 3 . 
 
226 
 
 ROCK DISINTEGRATION AND DECOMPOSITION 
 
 peridotite, these clefts may become filled with the silicates of 
 nickel, noumceite and garncerite, which may be of sufficient 
 abundance to form valuable ores. This, in brief, is the history 
 of the nickel ores of Riddles, Oregon, and of New Caledonia, 
 though the process is more properly a form of hydrometamor- 
 phism than weathering. 
 
 The deep green serpentines of Harford County, Maryland, 
 weather slowly down into a gray-brown soil, which consists of 
 60.17% silica, 10.40% of iron oxides, 14.81% of alumina, and 
 only 7.23% magnesia. The fresh rock, on the other hand, car- 
 ries nearly 40 % of magnesia, 8.50% iron and other metallic 
 oxides, and less than one-half of one per cent of alumina. 
 
 Natural joint blocks occur in which the preliminary stages 
 of weathering are manifested -by a brown, ferruginous, though 
 tough and hard, vesicular crust of from a millimetre to two or 
 more centimetres' thickness, enclosing the slightly hydrated but 
 otherwise unchanged material. 
 
 ANALYSES OF FRESH AND DECOMPOSED SOAPSTONE (ALTERED PYROXENITE) 
 
 
 I 
 
 II 
 
 III 
 
 IV 
 
 V 
 
 CONSTITUENTS 
 
 M 
 o 
 o 
 H 
 
 1 
 1 
 
 KESIDUAL SOIL 
 
 PERCENTAGE OF 
 Loss FOB ENTIRE 
 EOCK 
 
 PERCENTAGE OF 
 EACH CONSTIT- 
 UENT SAVED 
 
 PERCENTAGE OF 
 EACH CONSTIT- 
 UENT LOST 
 
 Silica (Si0 2 ) 
 
 38 85 / 
 
 38 82 / 
 
 16 92 / 
 
 56 42 / 
 
 43 58 / 
 
 Alumina (A1 2 S ) . . . 
 Iron sesquioxide (Fe20s) l 
 Lime (CaO) 
 Magnesia (MgO) . . . 
 Potash (K 2 O) .... 
 Soda (Na a O) 
 Ignition 
 
 12.77 
 12.86 
 6.12 
 22.58 
 0.19 
 0.11 
 6 52 
 
 22.61 
 13.33 
 6.13 
 9.52 
 0.18 
 0.20 
 9 21 
 
 0.00 
 5.33 
 2.66 
 17.20 
 9.03 
 0.00 
 1 32 
 
 ciu.-*^ /O 
 
 100.00 
 58.52 
 55.55 
 23.81 
 52.94 
 100.00 
 79 74 
 
 0.00 
 41.48 
 44.45 
 76.19 
 47.05 
 0.00 
 20 26 
 
 
 
 
 
 
 
 
 100.00 % 
 
 100.00 % 
 
 52.46 % 
 
 
 
 In columns I and II above are given (I) the composition of 
 an altered pyroxenite (soapstone) from Albemarle County, 
 Virginia, and (II) a residual soil derived from the same, the 
 
 1 All iron calculated as Fe 2 3 . 
 
WEATHERING OF PYROXENITES 
 
 227 
 
 latter being of a dull, oclireous, brown-red color, somewhat 
 lumpy, but with no appreciable grit when rubbed between the 
 thumb and fingers. 
 
 The fresh rock is of a blue-gray color, close texture, and 
 consists, as shown by the microscope, of elongated crystals of 
 colorless tremolite, with folia of talc and chlorite, and occasional 
 opaque granules of chromic iron. The general petrologic feat- 
 ures are those of an altered pyroxenite. 
 
 Recalculated as before, the analyses give the results shown 
 in columns III, IV, and V. 
 
 Total loss of material 52.46%, including water of hydration. 
 The most striking feature brought out is the fact that the mag- 
 nesia has been carried away in greater proportional quantity 
 than has the lime. A like result was noted by Ebelmen in his 
 analyses of the decomposed basalts of Crouzet, which are given 
 on p. 223. 
 
 ANALYSES OF FRESH AND DECOMPOSED SOAPSTONE, FAIRFAX COUNTY, VIRGINIA 
 
 
 I 
 
 II 
 
 III 
 
 IV 
 
 V 
 
 CONSTITUENTS 
 
 a 
 1 
 
 ej 
 PM 
 
 RESIDUAL SOIL 
 
 PERCENTAGE OF 
 Loss FOB ENTIRE 
 KOCK 
 
 PERCENTAGE OF 
 EACH CONSTIT- 
 UENT SAVED 
 
 PERCENTAGE OF 
 EACH CONSTIT- 
 UENT LOST 
 
 Silica (SiOo) 
 
 58 40 / 
 
 54 84 / 
 
 46 31 / 
 
 20 70 / 
 
 79 30 / 
 
 Alumina (A1 2 3 ) . . . 
 Iron oxides(FeO and Fe 2 3 ) 
 Lime (CaO) . . . 
 
 }7. 
 
 00 
 
 <jt.a- / 
 
 33.75 
 00 
 
 I.UL / 
 0.00 
 
 100.00 
 
 0.00 
 
 Magnesia (MgO) 
 Alkalies (K 2 O and Na 2 0) 
 Ignition (H 2 0) .... 
 
 29.19 
 0.00 
 4.97 
 
 4.36 
 0.00 
 7.05 
 
 28.23 
 3.41 
 
 3.29 
 
 31.28 
 
 96.71 
 68.72 
 
 
 100.00 % 
 
 100.00% 
 
 77.95% 
 
 
 
 A varietal form of this same rock occurring near Fostoria in 
 Fairfax County, this state, is thoroughly decomposed throughout 
 nearly the entire area to a depth of twenty or more feet. The 
 fresh rock is composed mainly of a light greenish, almost white 
 talc, with sporadic patches of chlorite some five or more millime- 
 tres in diameter, and scattering granules of iron ores. The 
 
228 ROCK DISINTEGRATION AND DECOMPOSITION 
 
 decomposed material is dull brownish or gray, and when washed 
 and submitted to microscopic examinations is found to consist 
 almost wholly of brown and yellow-brown scales of talcose 
 material, intermingled with an impalpable silt, composed so far 
 as determinable of talcose and chloritic shreds. It is wholly 
 without grit, and with a decided soapy or greasy feeling. 
 Analyses of fresh and decomposed material, and calculations as 
 already given, yielded results as shown in table on p. 227. 
 
 The principles involved in the decomposition of fragmen- 
 tal and crystalline stratified rocks are not so different from 
 those we have been discussing as to call for detailed considera- 
 tion. It is well to note, however, that the materials composing 
 rocks of this type are themselves a product of these very dis- 
 integrating and decomposing agencies, but which have become 
 consolidated into rock masses and now, once more in the infinite 
 cycle of change, are undergoing a breaking up. It follows from 
 the very nature of the case that such rocks, with the exception 
 of the purely calcareous varieties, will undergo less chemical 
 change than do those we have been discussing. Their feld- 
 spathic and easily decomposable silicate constituents long ago 
 yielded to the decomposing processes, and were largely removed 
 before consolidation took place. Thus, most sandstones are 
 composed largely of quartzose sand, the least soluble and least 
 changeable product, it may be, of many a previous disintegra- 
 tion. Hence, the processes involved in the degeneration of the 
 sandstones, shales, and argillites are largely mechanical, with 
 the exception of those which carry a feldspathic or calcareous 
 cement. In these last-named, the cementing material is gradu- 
 ally leached away, and the rock becomes susceptible to the action 
 of frost, or falls away to loose sand simply through loss of cohe- 
 sion. Heusser and Claraz l described the itacolumites of Brazil 
 as subject to this mechanical degeneration, the process being 
 characterized by fissuration, succeeded by complete disintegra- 
 tion. Among siliceous sandstone it is the binding constituent 
 that yields first, as is naturally to be expected, and as has been 
 shown by the experiments conducted by R. Schutze. 2 
 
 The rocks grouped under the name of argillites, though com- 
 posed of detrital materials from pre-existing rocks, and of parti- 
 
 1 Ann. des Mines, 5th, Vol. XVII, 1860. 
 
 2 Ueber Verwitterungsvorgange bei Krystallinisclien u. Sedimentargesteinen, 
 Inaug. Dissertation der Friedrich-Alexanders Universitat, Berlin, 1886. 
 
WEATHERING OF ARGILLITE 
 
 229 
 
 cles reduced to an extreme degree of fineness, are, nevertheless, 
 quite variable in composition, as already noted. As a rule, they 
 are among the most indestructible of rocks, and on breaking 
 down yield only clays which differ from the original argillites 
 mainly in degree of hydratioii and condition of oxidation of the 
 iron and other metallic constituents. Those argillites which 
 carry appreciable quantities of still undecomposed silicates, par- 
 ticularly alkali-bearing varieties, are, of course, more susceptible, 
 other things being equal, as texture, fissility, etc. 
 
 The deep blue-black argillites of Harford County, Mary- 
 land, as shown in the analyses given below, do contain very 
 
 ANALYSES OF FRESH AND DECOMPOSED ARGILLITE, HARFORD COUNTY, 
 
 MARYLAND 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 III 
 
 IV 
 
 V 
 
 FRESH ARGILLITE 
 
 KESIDUAL CLAY 
 
 PERCENTAGE OF 
 Loss FOR EN- 
 TIRE EOCK 
 
 PERCENTAGE OF 
 EACH CONSTIT- 
 UENT 6AVED 
 
 PERCENTAGE OF 
 
 EACH CONSTIT- 
 UENT LOST 
 
 Silica 1 (Si0 2 ) 
 
 44.15% 
 30.84 
 14.87 
 0.48 
 0.27 
 4.36 
 0.51 
 4.49 
 
 24.17% 
 39.90 
 17.61 
 None 
 0.25 
 1.24 
 0.23 
 16.62 
 
 25.34 % 
 0.00 
 1.23 
 0.48 
 0.08 
 3.39 
 0.33 
 0.00 
 
 42.43 % 
 100.00 
 91.22 
 0.00 
 71.84 
 22.04 
 0.36 
 287.37 
 
 57.57% 
 0.00 
 8.78 
 100.00 
 28.16 
 77.95 
 99.64 
 None 
 
 Alumina fA! 2 03) 
 
 Iron oxide (FeO and Fe 2 3 ) . . 
 Lime (CaO) 
 
 Magnesia (MgO) 
 
 Potash (K 2 0) 
 
 Soda (Na 2 O) 
 Ignition (C and H 2 0) .... 
 
 
 99.97 % 
 
 100.02 % 
 
 40.83% 
 
 .... 
 
 .... 
 
 considerable quantities of these undecomposed silicates, and 
 though extremely tough and enduring from a human stand- 
 point, in time decompose in a very interesting manner. In the 
 field these rocks are found standing nearly, if not quite, verti- 
 cally, that is, with their evident cleavage vertical, and form- 
 ing steep, high ridges flanked by valleys carved from the softer 
 rocks on either hand. In the fresh cuts made during the work 
 
 1 With traces of Ti0 2 . 
 phur ; hence no pyrite. 
 
 Manganese in traces, but not determined. No sul- 
 
230 ROCK DISINTEGRATION AND DECOMPOSITION 
 
 of stripping, to open new quarries, the sound rock is found over- 
 lain by a ^variable thickness of ferruginous residual clay. Joint 
 blocks and splinters of the slate scattered through this clay, in 
 all stages of decomposition leave no doubt as to its origin. 
 Blocks, deep velvety black on the interior, are surrounded by 
 a crust of ochreous brown-red decomposition product, the decay 
 penetrating irregularly like the processes of oxidation into a 
 piece of metal. The first physical indication of decay is shown 
 by a softening of the slate, so that it may be readily scratched 
 by the thumb nail, and an assumption of a soapy or greasy feel- 
 ing, the entire mass finally passing over to the deep red-brown 
 unctuous clay, sufficiently rich in iron to serve as a low-grade 
 ochre, for paints. The incidental chemical changes are surpris- 
 ingly large, as shown by the analyses given on p. 229, column I 
 being an average of two analyses of the black, little altered 
 material from the interior of one of these blocks, and II that of 
 the residual clay. In III, IV, and V are given the calculated 
 losses of constituents, as before. 
 
 This residual clay, when boiled with hydrochloric acid and 
 sodium carbonate solutions, yielded up nearly 70% of its matter 
 to these solvents, leaving a residue which, when examined under 
 the microscope, shows only faint yellow-brown scale-like par- 
 ticles, rarely over a tenth of a millimetre in diameter, acting 
 very faintly, if at all, on polarized light, and with borders often 
 serrate, through corrosion, though this latter feature may be 
 due, in part, to the action of the solvents used. 
 
 Among siliceous rocks poor in alkalies or iron-bearing 
 silicates the degeneration is mainly disintegration, though a 
 small amount of silica, existing in either crystalline or chalce- 
 donic forms, is usually lost through solution. Thus the cherts 
 of southwest Missouri break down into porous friable forms, 
 sometimes passing into the condition of loose powder, or again 
 retaining sufficient tenacity to be utilized for filter discs and 
 tubes, as at Seneca, in Newton County. 
 
 Analyses of fresh and altered forms of this material, as given 
 by Dr. E. O. Hovey, 1 show no differences that are of sufficient 
 importance to warrant us in assuming any of them as the direct 
 cause of disintegration. The change is evidently mainly physi- 
 cal, though it is more than probable that a certain amount of 
 interstitial silica has been removed. It is, of course, possible 
 1 Appendix A, Vol. VII, Missouri Geological Survey, 1894, pp. 727-739. 
 
WEATHERING OF CHERTS 231 
 
 that here, as in other forms of decomposition, extensive solution 
 may have taken place, leaving a residue which, so far as compo- 
 sition is concerned, gives no clew to the changes which have 
 occurred. Dr. Penrose, however, describes l a process of chert 
 decay, or more properly disintegration, as manifested in the 
 Batesville region of Arkansas, in which the cause of the break- 
 ing down is more apparent. There are two stages in the proc- 
 ess, as described : (1) A transition into a light, porous, opaque, 
 buff-colored rock of the consistency of ordinary pressed brick, 
 and (2) into an impalpable white or brown powder, locally 
 known as a polishing powder. This second stage is not so con- 
 spicuous a feature as the first, since the finer materials thus 
 formed are carried off by surface waters. The white residual 
 powder often contains masses of the porous, semi-decomposed 
 rock, the latter in turn encircling kernels of hard, unaltered 
 chert. Throughout this region, the cherts (of Carboniferous 
 age) are generally decomposed into the condition of a more 
 or less porous mass to all depths up to ten or more feet. 
 In all cases the disintegration may be traced to the removal, 
 by leaching, of a small amount of interstitial carbonate of 
 lime. 
 
 When we come to a consideration of the Calcareous rocks, 
 we find, almost invariably, the chemical agencies of degenera- 
 tion preponderating over those that are purely physical. In 
 arid regions, and with granular crystalline types, physical 
 agencies may for a time prevail, but as a rule the process 
 is largely chemical, and notable for its simplicity. The de- 
 composition is due mainly to the action of meteoric waters 
 trickling over the surface, or filtering through cracks and crev- 
 ices, under ordinary conditions of atmospheric pressure and 
 atmospheric temperature. Hence the process is one of super- 
 ficial solution, and the incidental chemical processes set in 
 motion, as in the feldspar-bearing rocks, are almost entirely 
 lacking. It follows that, only the lime carbonate is removed 
 in appreciable quantities, while the less soluble impurities are 
 left to accumulate in the form of ferruginous clays, admixed 
 with quartzose particles, chert nodules, etc. Since in many 
 limestones the amount of these constituents is reduced to a 
 minimum, even perhaps to the fraction of one per cent, so it 
 happens that hundreds, or even thousands of feet of strata may 
 1 Ann. Rep. Geol. Survey of Arkansas, Vol. I, 1890. 
 
232 
 
 KOCK DISINTEGRATION AND DECOMPOSITION 
 
 disappear without leaving more than a very thin coating of soil 
 in their place. 
 
 An interesting illustration of the changes taking place in the 
 decomposition of an impure Carboniferous limestone is described 
 by Penrose in his treatise on the genesis of manganese deposits. 1 
 The stone in its least changed condition is of a granular crys- 
 talline structure and dark chocolate-brown color. The residual 
 clay from its decomposition is a trifle darker, highly plastic, 
 and quite impervious. Below are given the analyses of (I) the 
 fresh rock and (II) the clay, both being taken from the same 
 pit, the latter being of about fifteen feet in thickness and over- 
 lain by a capping of chert, which reduced to a minimum the 
 possibility of any admixture of foreign matter. The materials 
 were dried at a temperature of 110 to 115 C. before analyzing. 
 
 ANALYSES OF FRESH LIMESTONE AND ITS RESIDUAL CLAY 
 
 
 I 
 
 II 
 
 III 
 
 IV 
 
 V 
 
 
 
 5 
 
 H 
 
 o g 
 
 &, 
 S 
 
 H 
 
 CONSTITUENTS 
 
 fc 
 
 o 
 
 Si 
 
 sig 
 
 W H 
 111 
 
 
 = 1 
 
 to 
 
 fc 
 
 Q ^ W 
 
 | 3 
 
 
 
 oO W 
 
 w 
 
 
 1 1 * 
 
 III 
 
 
 PH ^ 
 
 M 
 
 PH (-1 M 
 
 fS H to 
 
 Pi fd to 
 
 Silica (Si0 2 ) 
 
 4.13 % 
 
 33.69% 
 
 0.00% 
 
 100.00 % 
 
 00 / 
 
 
 I.J-U JQ 
 
 /l/ /O 
 
 Vf.WW ^ 
 
 
 V.UU IQ 
 
 Alumina (A1 2 3 ) . . 
 
 4.19 
 
 30.30 
 
 0.35 
 
 88.65 
 
 11.35 
 
 Ferric iron (Fe 2 O 3 ) . . 
 
 2.35 
 
 1.99 
 
 2.13 
 
 10.44 
 
 89.56 
 
 Manganic oxide (MnO) . 
 
 4.33 
 
 14.98 
 
 2.49 
 
 42.41 
 
 57.59 
 
 Lime (CaO) 
 
 44.79 
 
 3.91 
 
 44.32 
 
 1.07 
 
 98.93 
 
 Magnesia (MgO) . . . 
 
 0.30 
 
 0.26 
 
 6.25 
 
 10.62 
 
 89.38 
 
 Potash (K 2 0) .... 
 
 0.35 
 
 0.96 
 
 0.23 
 
 33.63 
 
 66.37 
 
 Soda (Na 2 0) 
 
 0.16 
 
 0.61 
 
 0.085 
 
 46.74 
 
 53.26 
 
 Water (H 2 0). .... 
 
 2.26 
 
 10.76 
 
 0.95 
 
 58.37 
 
 41.63 
 
 Carbonic acid (C0 2 ) . . 
 
 34.10 
 
 0.00 
 
 34.10 
 
 0.00 
 
 100.00 
 
 Phosphoric acid (P 2 05) 
 
 3.04 
 
 2.54 
 
 2.73 
 
 10.24 
 
 89.76 
 
 
 100.00 % 
 
 100.00% 
 
 97.635% 
 
 
 
 These analyses have been recalculated in the same manner as 
 before, excepting that silica, instead of alumina, is taken as the 
 constant factor. This for the reason above suggested. It is 
 believed that one is safe in assuming little or no silica is lost 
 
 1 Ann. Rep. Geol. Survey of Arkansas, 1890, p. 179. 
 
WEATHERING OF CALCAREOUS ROCKS 233 
 
 here through the action of alkaline carbonates, since the alka- 
 lies are almost wholly lacking in the fresh rock, and a large 
 portion of the silica doubtless exists as free quartz. Recalcu- 
 lating, then, in the same manner as before, but on a silica con- 
 stant basis, we obtain the matter in columns III, IV, and V. 
 
 These columns bring to light some unexpected features, not 
 the least interesting of which is the fact that the residual clay, 
 in spite of its highly hydrated condition, in reality contains 
 scarcely half the amount of water it would, had the small amount 
 (2.26%) in the original limestone been allowed to accumulate 
 without loss. A more important, though perhaps more to be ex- 
 pected, feature is the entire removal of that portion of the lime 
 which existed as carbonate, as indicated by the absence of car- 
 bonic acid in the clay. It will be noted that 97.635 % of the 
 entire rock mass has disappeared through leaching, leaving only 
 2.365 % to accumulate as an insoluble residue in the form of soil. 
 
 This leaching out of the lime carbonates and the accumula- 
 tion of insoluble residues is a strikingly conspicuous feature in 
 regions abounding in limestone caverns, and to it is due the 
 tenaceous ferruginous clays which cover their floors. So rich 
 indeed are some of these residual deposits in iron oxide that 
 in some instances they are locally used for pigments, under the 
 name of ochre or mineral paint, or again, where occurring in 
 large quantities, as ores of iron. (See p. 267.) 
 
 It is possible that loosely consolidated beds of shell limestone 
 may undergo a process of change, perhaps more nearly akin to 
 alteration than decomposition, through agencies quite different 
 from those we have been considering. 
 
 Darwin, it will be remembered, found the shells in the raised 
 sea-beaches of San Lorenzo, South America, altered to the con- 
 dition of a white powder without trace of organic structure, and 
 consisting of carbonate, sulphate and chloride of lime with sul- 
 phate and chloride of sodium. This alteration he believed to 
 be due to a mutual reaction taking place between the original 
 sodium chloride derived from the sea-water and the lime car- 
 bonate of the shells, and he speaks of it as an interesting illus- 
 tration of the fact that the dry climate of the west coast of 
 South America is much less favorable to the preservation of 
 shell structures than would be a moist one where the salt would 
 be removed too rapidly for the double decomposition to be 
 brought about. 
 
234 BOCK DISINTEGRATION AND DECOMPOSITION 
 
 Resume. Making all due allowance for possible sources of 
 error in our methods, there are certain general deductions that 
 may be safely drawn. Not, it may be, from our own analyses 
 alone, but from numerous others as found in existing literature. 1 
 
 Let us briefly review the subject and make the deductions 
 accordingly. 
 
 In glancing over the columns of our analyses, it is at once 
 apparent that hydration is an important factor, the amount of 
 water increasing rapidly as decomposition advances. In the 
 earlier stages of degeneration it is doubtless the most important 
 factor. There is, moreover, among the siliceous crystalline 
 rocks, in every case a loss in silica, a greater proportional loss in 
 lime, magnesia, and the alkalies, and a proportional increase in 
 the amounts of alumina and sometimes of iron oxides, though 
 the apparent gain may in some cases be due to the change in 
 condition from ferrous to ferric oxide. As a whole, however, 
 there is a very decided loss of materials. Among siliceous 
 crystalline rocks, this loss, so far as shown by available analyses 
 and calculations, rarely amounts to more than 50 % of the entire 
 rock mass. Among calcareous rocks, on the other hand, it may, 
 in extreme cases, amount to even 99 % . 
 
 Of all the ordinary essential mineral constituents the free 
 quartz is the most refractory toward purely chemical agen- 
 cies, and the amount of silica lost from this source must be 
 small, though Sorby 2 thinks to have distinguished chemically 
 corroded quartz granules in some of the sands examined by him. 
 It is, however, safe to say that the mineral suffers chiefly from 
 mechanical disruption, that silica in any rock which is re- 
 moved during the process of decomposition comes mainly from 
 the silicates, and not from the free quartz. According to Bis- 
 chof, and as shown by our own work, the silicates in any rock 
 that are most readily decomposed are, as a rule, those contain- 
 ing protoxides of iron and manganese, or lime, and the first 
 indication of decomposition is signalled by a ferruginous dis- 
 coloration and the appearance of calcite. The evidence bearing 
 upon the relative durability of the various minerals consti- 
 tuting rocks is, however, quite conflicting and unsatisfactory. 
 Doubtless much depends on local conditions. 
 
 1 See especially Roth's Allegemeine u. Chemische Geologic, Vol. Ill, and Ebel- 
 men's papers in Ann. des Mines, Vols. VII, 1845, and XII, 1847. 
 
 2 Proc. Geol. Soc. of London, 1879. 
 
 
GENERAL DEDUCTIONS 235 
 
 Dana observed 1 that in the decomposition of the granitic 
 rocks of the Chilean coast the feldspars yielded first, becoming 
 white and opaque and of a friable earthy appearance. But it 
 should be noted that there is nothing in Professor Dana's de- 
 scription to show that this change may not have been a purely 
 physical one, and due to the splitting up of the feldspars along 
 cleavage lines. Fournet, from a study of the processes of kao- 
 linization, was led to state 2 that hornblende yields less readily 
 to decomposing forces than does feldspar, when the two are 
 associated in the same rock. Becker, however, in studying 
 deep-seated decomposition in the Comstock Lode of Nevada, 
 arrived at a precisely opposite conclusion, the feldspars as a 
 whole offering more resistance than the augite, hornblende, or 
 mica. 
 
 The present writer has described 3 thick sheets of augite por- 
 phyrite in Gallatin County, Montana, in which the feldspathic 
 disintegration has gone on so far that the mass falls away to a 
 coarse sand, from which still perfectly outlined crystals of coal- 
 black augites may be gleaned in profusion. This last is, 
 however, a semi-arid region, and the process thus far one of 
 disintegration more than decomposition. In a moist, or perhaps 
 in any- climate, minerals consisting essentially of silicates of 
 alumina and magnesia are less liable to decomposition than 
 those containing considerable proportions of iron protoxides or 
 of lime. This for the reason that the first-named are scarcely 
 at all affected by the ordinary atmospheric agents of solution. 
 Bischof goes so far as to say that the silicate of alumina is not 
 at all affected by carbonic acid, but the researches of Miiller, to 
 which reference has been made, and our own calculations tend 
 to disprove this. Dana states 4 that in the decomposition of 
 basalt, on the island of Tahiti, the olivine is the earliest to give 
 way, becoming first iridescent and finally falling away to a soft, 
 pulverulent, ochreous yellow or brown powder. The compact 
 base of the rock yielded next, the augites holding out until the 
 last. Those silicates which are least liable to atmospheric de- 
 composition are, as is to be expected, those which have resulted 
 from the alteration of less stable silicates, as serpentine from 
 
 1 Report, Wilkes's Exploring Expedition, Geology, p. 578. 
 
 2 Ann. de Chimie et de Physique, Vol. LV, 1833, p. 240. 
 8 Bull, U. S. Geol. Survey, No. 110, 1894. 
 
 4 Op. cit., p. 298. 
 
236 ROCK DISINTEGRATION AND DECOMPOSITION 
 
 olivine, epidote from hornblende, or kaolin from feldspar, etc. 
 A few silicates like tourmaline and zircon, or garnet, or oxides 
 like rutile and magnetite, or the salts of rarer earths like mona- 
 zite, etc., are scarcely at all affected by any of the ordinary 
 agents of decomposition, but remain in the form of residual 
 sands in the beds of streams, from whence the lighter, more 
 decomposed material is removed by erosion. 
 
 In the weathering of potash-feldspar rocks carrying black 
 mica, the latter mineral is as a rule the first to give way, and at 
 times almost wholly disappears. With basic rocks, on the 
 other hand, the dark mica is one of the most enduring of the 
 constituents, and in the residual sands may be found in surpris- 
 ingly large proportions. 
 
 In the kaolinized gneisses of northern Delaware, the biotite, 
 as a rule, is in an advanced stage of decomposition, while the 
 small amount of primary muscovite is still fresh and intact, 
 retaining all its original lustre and elasticity. 
 
 Among the feldspars the potash varieties are, as a rule, far 
 more refractory than the soda-lime, or plagioclase varieties. 
 This is shown not merely by our own investigations, but by 
 those of others as well. Roth shows 1 from analyses of fresh 
 and weathered phonolites, nepheline basalts, and dolorites, that 
 the loss of soda is almost invariably greater than that of 
 potash. 
 
 In the coarse, pegmatitic dikes of Delaware County, Penn- 
 sylvania, the microcline masses, as mined for pottery purposes, 
 are beautifully fresh and translucent, while the associated oligo- 
 clase is snow-white through a splitting up along cleavage lines 
 and partial decomposition. Where thrown out upon the dumps, 
 this whitened mineral shortly falls away to fine sand, 'resembling, 
 .at first glance, kaolin, but is distinctly gritty. 
 
 Max Geldmacher noted 2 that in the weathering of quartz 
 porphyry oligoclase always gave way before the oligoclase. 
 
 Indeed, as shown in our analyses, in certain phases of rock 
 degeneration, the potash feldspars may lose very little by 
 decomposition, but be converted into the condition of fine 
 silt merely through a mechanical splitting up. This fact will 
 in part explain the relative scarcity of free potassium salts 
 
 1 Op. cit., 3d ed., 2d Heft. 
 
 2 Beitrage zur Verwitterung der Porphyre, Inaug. Dissertation, Konigl. 
 Freidrich Alexander Universitat, Leipzig, 1889. 
 
GENERAL DEDUCTIONS 
 
 237 
 
 (carbonates, sulphates, and nitrates) as compared with those of 
 soda. 1 
 
 The chemical processes involved in this feldspathic decompo- 
 sition are of sufficient importance to warrant further discussion, 
 even though it may involve a certain amount of repetition of 
 what has gone before. 
 
 Berthier, Forschammer, Brogniart, 2 Fournet, 3 and others ex- 
 plained more than fifty years ago the process of feldspathic dis- 
 integration through the breaking up of its complex molecule 
 into alkaline silicates soluble in water, and aluminous silicates 
 which are insoluble. The loss in silica, as noted above, was 
 supposed to be due to the removal, by solution, of these alka- 
 line silicates. Ebelmen, 4 however, subsequently showed that 
 silicate minerals poor or quite lacking in alkalies lost a portion 
 of their silica with equal facility, as is also shown in our analy- 
 ses of pyroxenites on pp. 226 and 227. He accounted for this on 
 the supposition that the silica set free in a nascent state was 
 soluble either in pure water, or water containing carbonic acid. 
 Bischof states that when meteoric waters containing carbonic 
 acid filter through rocks containing alkaline silicates, the first 
 
 1 An oligoclase occurring in a tourmaline granite on the southern slope of 
 Monte Mulatto, near Predazzo, undergoes, according to Lemberg (Zeit, der Deut. 
 Geol. Gesellschaft, 28, 1876), a much more rapid decomposition than the ortho- 
 clase with which it is associated, and gives rise to a green, lustreless, serpentine- 
 like product. The chemical changes incidental to the alteration are as shown in 
 the following tables, I being the fresh oligoclase, and II the decomposition 
 product. 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 Silica (Si0 2 ) 
 
 59.51% 
 
 45.29 % 
 
 Alumina (A^Os) 
 
 25.10 
 
 25.68 
 
 Iron sesquioxide (Fe 2 03) 
 
 1.08 
 
 12.49 
 
 Lime (CaO) 
 
 4.03 
 
 0.52 
 
 Magnesia (M^O) 
 
 Trace 
 
 2.88 
 
 Potash (K^O) .... 
 
 2.10 
 
 3.00 
 
 Soda (Na 2 0) .... 
 
 7.26 
 
 2.14 
 
 Water (H0) 
 
 0.92 
 
 8.00 
 
 
 
 
 
 100.00% 
 
 100.00 % 
 
 2 Arch du Museum, Vol. I, 1839 (cited by Ebelmen). 
 8 Ann. de Chimie et de Physique, Vol. LV, 1833. 
 4 Ann. des Mines, Vol. VII, 1845. 
 
238 KOCK DISINTEGRATION AND DECOMPOSITION 
 
 action consists in the partial decomposition of these substances 
 by the carbonic acid and the formation of alkaline carbonates, 
 which are dissolved. If the water thus impregnated, on pene- 
 trating further below the surface, comes in contact with cal- 
 careous silicates, another change will take place consisting of a 
 decomposition and replacement of these calcareous silicates by 
 the alkaline silicates, and a removal of the lime set free, as a 
 carbonate, provided the water still contains a sufficient amount 
 of carbonic acid. This replacing process and the retention of 
 the alkaline silicates is accounted for on the supposition that, 
 in their nascent state, they form new combinations with the 
 other silicates present, while the lime remains as a carbonate to 
 be removed or not, as the case may be. He further states that 
 the alkaline carbonates originating in the manner described 
 are among the most soluble substances known ; the carbonate 
 of soda requires for solution only six times its weight of water 
 at ordinary temperatures. Silica, on the other hand, even in 
 its most soluble form, requires ten thousand times its weight of 
 water for solution. If, therefore, the decomposition of feld- 
 spar by such carbonated water were ever so energetic, there 
 would be sufficient water for the solution of the carbonate of 
 soda formed. But if the silica separated meanwhile amounted 
 to more than Yolhro ^ ^ ne wa ^ er present, the excess could not 
 be dissolved, but would remain mixed with the kaolin. 
 
 The case is very different when the decomposition of feldspar 
 is affected by fresh water containing only the minute quantity 
 of carbonic acid derived from the atmosphere. By the action 
 of such water, only very small quantities of alkaline carbonates 
 are formed ; consequently it is possible that the silica separated 
 at the same time, also small' in quantity, may find enough water 
 for solution. In such cases the whole of this silica would be 
 removed with the alkaline carbonates, and pure kaolin would 
 be left. Such an action as this does not, however, appear to 
 take place ; for the purest of kaolin nearly always contains an 
 admixture of quartz sand, or of free silica in some of its forms. 
 
 K. V. Murakozy has shown 1 that in the decomposition of 
 rhyolite from Nagy-Mihaly, the sanidin passes into kaolin and 
 opal, the latter separating out as hyalite in veins or impure 
 concretionary forms. 
 
 It follows from this consideration that in the decomposition 
 1 Abstract by F. Becke, Neues Jahrbuch, 1894, 1 Band, 2 Heft, p. 291. 
 
GENERAL DEDUCTIONS 239 
 
 of feldspar into kaolin more of the silica separated remains 
 mixed with the kaolin formed, the greater the quantity of 
 carbonic acid in water, and that, perhaps, the amount of car- 
 bonic acid is never so small that the whole of the silica sep- 
 arated in the decomposition of feldspar can be removed. 1 The 
 above, however, overlooks the possible presence of nitrates, such 
 as we now know from the researches noted on p. 203 may in 
 many cases exist, even though in extremely small proportions. 
 It is probable that the small amount of nitric acid formed 
 by the bacteria would, if not taken up by plant growth, com- 
 bine immediately with the alkalies, forming nitrates which, 
 owing to their ready solubility, would be carried away. The 
 larger the proportion of nitric acid, therefore, the greater 
 would be the amount of silica intermingled with the kaolin, 
 since whatever proportion of the alkalies failed to be carried 
 away as nitrates would pretty certainly disappear 'as carbo- 
 nate. There is also the possibility, especially in the rocks 
 rich in iron protoxides, that a portion of the silica may com- 
 bine with the iron, as already noted. 
 
 In cases where the decomposition takes place under the 
 influence of a sufficient supply of oxygen, all iron, and presum- 
 ably the manganese as well, would be converted into the in- 
 soluble hydrous sesquioxide form and remain with the residue. 
 Where, however, the supply of oxygen is insufficient, a por- 
 tion or all of these v constituents may be removed in the form 
 of protoxide carbonates, or, in the case of iron, of a ferrous 
 sulphate. These facts well account for the variation in sta- 
 bility of the iron, as indicated in the preceding analyses. 
 
 Reference has already been made to the fact that the mag- 
 nesia from the decomposition of magnesian silicates was some- 
 times removed in greater relative portions than was the lime. 
 This seeming anomaly is also sometimes met with in cal- 
 careous stratified rocks. Roth 2 showed that in the weather- 
 ing of dolomitic limestones, the magnesia is often removed in 
 greater proportional quantities than the more soluble lime 
 carbonate. 
 
 The researches of Hitterman 3 showed, however, that carbonic 
 
 1 Chemical and Physical Geology, by Gustav Bischof, Vol. II, pp. 182, 183. 
 *Op. cit., Vol. III. 
 
 3 Die Verwitterungeproducte von Gesteinen der Triasformation Frankers, 
 Inaug. Dissertation, Freidrich-Alexanders Universitat, Munich, 1889. 
 
240 ROCK DISINTEGRATION AND DECOMPOSITION 
 
 acid solutions may exert a scarcely appreciable effect upon mag- 
 nesian carbonate, which therefore accumulates in the residual 
 soils. 
 
 It is safe to say that while the general process of rock- 
 weathering may be quite simple, as outlined, there are many 
 minor reactions which it is not possible to describe in detail. 
 
 It has been shown that even in firm rocks a mutual chemi- 
 cal reaction is not uncommon among minerals lying in close 
 juxtaposition, giving rise to what are known as reaction rims 
 or zones composed of secondary minerals. This is a par- 
 ticularly conspicuous feature in many gabbros, where olivine 
 and feldspar are closely adjacent. In these cases, a mutual 
 interchange of elements may take place, giving rise to garnets, 
 free quartz, or other minerals as the case may be. This is, 
 to be sure, a deep-seated change, to be classed as alteration 
 rather than decomposition, and taking place presumably under 
 conditions of temperature and solution quite at variance with 
 those existing on the immediate surface. It is, nevertheless, 
 self-evident that when elements are set free through any 
 process, they must almost immediately recombine, taking those 
 forms which existing circumstances may dictate and that close 
 contact of particles would be favorable to the more rapid for- 
 mation of new compounds. In a mass of decomposing rock, 
 circumstances are almost continually changing, and the infer- 
 ence is fair that new combinations are continually being made 
 and unmade, the intricacies of which we are unable to follow. 
 
PLATE 18 
 
 FIG. 1. Exfoliated granite in the Sierras. 
 
 FIG. 2. Talus slopes on Pike's Peak. 
 
 FIG. 3. Disintegrated granite, Ute Pass, Colorado. 
 
THE WEATHERING- OP BOCKS (Continued} 
 III. THE PHYSICAL MANIFESTATIONS 
 
 Rock-weathering manifests itself in a great variety of ways, 
 much depending upon climate, though naturally the controlling 
 factor is that of mineral composition. The manner of weather- 
 ing is often sufficiently characteristic to be of great importance 
 in determining surface contours, as well as incidentally afford- 
 ing a means for the identification of rock masses when the 
 outcrops themselves are obscured by decomposition products. 
 Such a means is of only local importance, however, since under 
 varying conditions the resultant forms assumed, even by similar 
 rocks, are themselves quite variable. It is, nevertheless, not 
 without interest to note the varying phases of weathering in 
 different kinds of rocks, the incidental contours assumed, the 
 character of the resultant debris, and, at the same time, the 
 controlling forces that have -been instrumental in bringing 
 about the final result. 
 
 (1) Disintegration without Decomposition. That in weather- 
 ing, physical and chemical agencies may go on either singly or 
 conjointly has been noted in previous pages. In the case of 
 single minerals, the preliminary disintegration is beautifully 
 illustrated in the large oligoclase masses associated with micro- 
 cline in the feldspar mines of Delaware County, Pennsylvania. 
 In the dumps of waste about the mines these are found, in all 
 stages of disintegration, the mineral splitting up along cleavage 
 lines, becoming snow-white, and ultimately falling away to a 
 kaolin-like product, but which, when submitted to microscopic 
 examination, is found to be made up of sharply angular cleavage 
 particles, showing no sign of decomposition other than that in- 
 dicated by occasional opacity. In the analyses given below are 
 shown (I) the composition of a fresh oligoclase (as given by 
 Dana) from near Wilmington, Delaware, (II) the snow-white 
 R 241 
 
242 THE PHYSICAL MANIFESTATIONS OF WEATHERING 
 
 cleaved, but still moderately firm mineral mentioned above, and 
 (III) the flour-like or kaolin-like product. 
 
 
 I 
 
 II 
 
 III 
 
 CONSTITUENTS 
 
 FEESH OLIGOCLASE 
 
 OPAQUE WHITE, 
 
 BUT STILL FlEM 
 
 OLIGOCLASE 
 
 FINE DUST FROM 
 DISINTEGRATED 
 
 OLIGOOLASE 
 
 Si0 2 
 
 64.75% 
 
 61.23 % 
 
 56.73% 
 
 A1 2 3 
 
 23.56 
 
 25.65 
 
 28.44 
 
 CaO. 
 
 2 84 
 
 2 37 
 
 2 95 
 
 K 2 O 
 
 1 11 
 
 0.72 
 
 1 12 
 
 Na 2 
 
 9.04 
 
 7.66 
 
 5 81 
 
 Ignition 
 
 
 1 00 
 
 5 67 
 
 
 
 
 
 
 101.33% 
 
 99.63% 
 
 100.72% 
 
 The fact that granitic and gneissic rocks may undergo ex- 
 tensive disintegration with slight decomposition, even in a 
 moist climate, was noted by Nordenskiold 1 in Ceylon. He 
 says : ' ' The boundary between the un weathered granite and 
 that which has been converted into sand is often so sharp that 
 a stroke of the hammer separates the crust of granitic sand 
 from the granite blocks. They have an almost fresh surface, 
 and a couple of millimetres within the boundary the rock is quite 
 unaltered. No formation of clay takes place and the alteration 
 to which the rocks are subjected, therefore, consists in a crum- 
 bling or formation of sand, and not, or at least only to a very 
 small extent, in a chemical change. At every road section 
 between Galle, Colombo, and Ratnapoora the granite and gneiss 
 crumbled down to a coarse sand, which was again bound to- 
 gether by newly formed hydrated peroxide of iron to a peculiar 
 porous sandstone, called by the natives cabook. 2 This sandstone 
 forms the layer lying next the rock in nearly all the hills on that 
 part of the island which we visited. It evidently belongs to 
 an earlier geological period than the Quaternary, for it is older 
 than the recent formation of valleys and rivers. The cabook 
 often contains large, rounded, unweathered granite blocks, quite 
 resembling the rolled stone blocks in Sweden. In this way 
 
 1 Voyage of the Vega, Vol. II, 1881, p. 420. 
 
 2 Laterite ? It seems so regarded by H. F. Alexander, Trans. Edinburgh 
 Geol. Society, Vol. II, 1869-74, p. 113. 
 
WEATHERING INFLUENCED BY STRUCTURE 243 
 
 there arises at places where the cabook stratum has again 
 been broken up and washed away by currents of water, forma- 
 tions which are so bewildering, like the ridges (osars) and hills 
 with erratic blocks in Sweden and Finland, that I was aston- 
 ished when I saw them." 
 
 The same features are brought out in the previous descrip- 
 tions relative to the weathering of the granite of the District 
 of Columbia, the diabase of Medford, Massachusetts, and other 
 localities mentioned in these pages. (See pp. 206 and 218.) 
 This tendency toward disintegration without decomposition is 
 exaggerated among coarsely crystalline rocks, as is abundantly 
 exemplified in the rocks of the Pike's Peak (Colorado) area. (See 
 PL 18.) Among those of finer grain, particularly the quartz- 
 free varieties, as the Fourche Mountain (Arkansas) syenites, 
 decomposition may follow so closely on disintegration that little 
 or no sand is formed, sound fresh rock passing within the space 
 of a few millimetres into the condition of residual clay. 1 
 
 (2) Weathering influenced by Crystalline Structure. It is else- 
 where observed that, other things being equal, a coarsely gran- 
 ular rock will disintegrate more rapidly than one of finer grain. 
 
 Lone Mountain, one of the high eruptive peaks on the Avest 
 side of the Madison valley in Montana, presents in its upper 
 portions all the features of a volcanic crater broken down on 
 one side by the lava flow. The facts of the case are, however, 
 that the coarser grained central portion has been disintegrated, 
 and swept by wind and rain into the valleys, while the fine- 
 grained, more compact outer portions, those which solidified near 
 the line of contact with adjacent rocks, remain intact. Pro- 
 fessor Bell 2 describes an interesting case of this kind where the 
 coarsely crystalline central portion of a " greenstone " dike has 
 yielded more readily to erosion than at the sides and afforded 
 channel-way for the Mattagami River, north of Lake Huron, in 
 Canada. The gneiss adjoining the dike having been shattered, 
 yielded also to decomposing agencies and forms now a second 
 parallel channel on each side of the central one. " Between 
 them the finer grained, hard, and undecayed 4 greenstone ' con- 
 
 1 Dr. Max Fesca has noted that the granitic rocks of Kai province, Japan, 
 yield on decomposing gravel, sand, and clayey loams, while those rocks poor in 
 quartz, such as the syenites, give rise only to clays (Abhandlungen und Erlau- 
 terungen zur Agronomischen Karte de Prov. Kai, Kaiserlich Japanischen Geo- 
 logischen Reichsanstalt, 1887). 
 
 2 Bull. Geol. Soc. of America, Vol. V, 1894, p. 364. 
 
244 THE PHYSICAL MANIFESTATIONS OF WEATHERING 
 
 stituting the outer portions of the dike rises up in the shape of 
 ridges and chains of islands, so that the river flows as a main, 
 central channel, more or less separated from the smaller lateral 
 ones." The same writer describes several instances in which 
 long straight valleys in the Archaean regions of Canada, now 
 occupied by straight river stretches, long narrow lakes or inlets 
 of the larger lakes, are due to the decay and removal of the 
 wide " greenstone " dikes, or of parallel dikes with narrow belts 
 of rock between. Long Lake, north of Lake Superior, some 52 
 miles in length, is mentioned as typical of lakes of this class. 
 
 (3) Weathering influenced by Structure of Rock Masses. In 
 any rock mass weathering is greatly augmented by lines of 
 weakness, such as joint and bedding planes, since these furnish 
 so many additional points of attack. In homogeneous massive 
 rocks the rate of disintegration is retarded by a lack of vulner- 
 able points, and the resultant form is that of rounded bosses 
 such as are shown in plate 1. 
 
 As a rule, however, the most massive of rocks are traversed 
 by one or more series of joints (see PI. 14) whereby they are 
 
 FIG. 17. Showing the influence of joints in the production of boulders. 
 
 divided up into rhomboidal blocks of varying sizes. Even 
 when not sufficiently developed to be conspicuous, such joints 
 not infrequently exist as lines of weakness along which moisture 
 and the accompanying agents of disintegration make their way, 
 gradually rounding the corners until there is left an oval mass 
 of which the so-called " niggerheads " of the gabbro area about 
 Baltimore are typical examples. In nearly all such rocks the 
 exfoliation and decomposition take place in the form of con- 
 centric layers, like the coatings on an onion. This holds true 
 with the huge granitic bosses, as well as with the smaller joint 
 blocks, and has been argued by some of the earlier geologists 
 as indicative of an original concretionary structure. Such an 
 
WEATHERING INFLUENCED BY STRUCTURE 
 
 245 
 
 assumption seems, however, wholly uncalled for. If the block or 
 mass is reasonably homogeneous, the agencies of decomposition 
 will penetrate nearly uniformly from all exposed surfaces, pro- 
 ducing an exfoliation nearly parallel to that surface, and the 
 concentric structure is inevitable, as was long ago pointed out 
 by Werner. 
 
 In some cases the tendency to assume the boss-like form is 
 accentuated through the presence of joints running approxi- 
 mately parallel to the exposed surface, such joints as give rise 
 to the step-like arrangement of the stone so frequently seen in 
 granite quarries. Stone Mountain, Georgia, an immense boss 
 of light gray granite some 2 miles long by 1| wide and 650 feet 
 
 9SSSS&.'. 
 
 FIG. 18. Exfoliation of granite. 
 
 high, owes its form, apparently, wholly to exfoliation parallel 
 to pre-existing lines of weakness. The entire mass, so far as 
 exposed by quarrying operations, is made up of imbricated sheets 
 of granite, which, of unknown thickness beneath the surface, 
 thin out to mere knife edges above, like shingles on a roof. 
 Through prolonged exposure the superficial layers have become 
 detached from the parent mass, and doubtless hundreds of feet 
 in vertical thickness completely disintegrated and swept away. 
 With many geologists these joints, in themselves, would be 
 accepted as due to atmospheric action. In the writer's present 
 opinion they are, however, the result of torsional strains and 
 once existing are lines of weakness which become more and 
 more pronounced as weathering progresses. The boss-like form 
 is therefore incidental and consequent. The process of exfo- 
 liation has, in the case mentioned, been productive of some 
 peculiar results which may be described in detail. 
 
246 THE PHYSICAL MANIFESTATIONS OF WEATHERING 
 
 As above mentioned, the sheets of granite, varying from a few 
 inches to several feet in thickness, conform in a general way to 
 the present surface of the hill. Constant expansion and con- 
 traction from temperature changes have, in the manner already 
 described, so expanded these sheets that, bound at the sides, 
 they have found relief in an upward direction where resistance 
 was least, and risen in dome or roof shaped forms, as shown in 
 the sketch. The weight of the sheets higher up the slopes, im- 
 pinging upon the edges of those below, has in some cases 
 undoubtedly aided in the work, but the larger part is due to 
 simple expansion, such as was referred to on p. 180. 
 
 These ruptured sheets are rarely more than 10 inches thick, 
 but 10 or 20 feet in diameter. The material, though quite 
 fresh appearing, is loosely granular and friable, easily reduced 
 to sand. In a few instances small avalanches have been caused 
 by the giving way of the sheets below and the consequent slid- 
 ing down of those above through lack of support. (See Fig. 18.) 
 This same mass of granite sometimes shows upon its surface 
 peculiar circular depressions, one within another, separated by 
 
 intervening ridges of low 
 relief, such as have been 
 described in a much more 
 perfect stage of develop- 
 ment by Dr. Robert Bell l 
 in the Huronian rocks of 
 Canada. These, as shown 
 in Fig. 19 from Bell's 
 paper, are some 3 or 4 
 feet in diameter and 3 or 
 4 inches high. The cause 
 of this form of weather- 
 ing at Stone Mountain 
 is not apparent, though 
 
 Bel1 ' Y 1 *" case /^ r l d ' 
 
 regards it as induced by 
 FlG 19 an original concretionary 
 
 structure. 
 
 The spheroidal structure so frequently seen in basaltic rocks, 
 and as typified in the sphaeroidische absonderung of German 
 writers, may perhaps be due to an original spheroidal ten- 
 Geol. Soc. of America, Vol. V, 1894, p. 362. 
 
 
t ' WEATHERING INFLUENCED BY STRUCTURE 
 
 dency caused by cooling, 1 but a very large proportion of the 
 spheroidal masses so typical of the decomposition of massive 
 rock is, as already suggested, due wholly to external causes. 
 W. P. Blake in 1855 called attention to this form of disintegra- 
 tion in the massive sandstones near San Francisco (California) 
 and pointed out the true explanation. 2 
 
 This sandstone is described as occurring in the form of layers 
 from a few inches to 6 and 8 feet in thickness, alternating with 
 beds of slate and shale. Down to a depth of 10 or 20 feet, or 
 to the limits of atmospheric action, all the beds have turned from 
 gray to rusty brown or drab. " There are, however, parts of the 
 upper beds that have not yet been reached and changed by de- 
 composition; these parts are found in the condition of spherical 
 or ellipsoidal masses, from which the weathered parts scale off 
 in successive crusts. These nuclei have the appearance of great 
 rounded boulders, and have accumulated in great numbers at 
 the base of the cliff." In this case the sandstone is composed 
 mainly of grains of quartz and a little feldspar cemented by 
 calcite, the disintegration being due mainly to the removal of 
 this cement by percolating water, while the change in color is 
 doubtless due to oxidizing pyrite or ferrous carbonate. 
 
 The effect of percolating waters is not, however, always im- 
 mediately destructive. Though in themselves carrying cement- 
 ing materials, or causing an oxidation of the iron carbonates or 
 sulphides, a local induration may be induced along the joint 
 lines such as becomes conspicuous only through the weathering 
 away of the non-indurated portions. Resultant forms may be 
 extremely regular or again irregular, according to the character 
 of the lines along which percolation takes place, and that of the 
 rock itself. An interesting illustration of this form of weather- 
 ing is that given by Wyville Thompson 3 as occurring on the 
 islands of Bermuda. 
 
 " This dissolving and hardening process," he writes, " takes 
 place irregularly, the water apparently following certain courses 
 in its percolations, which it keeps open, and the walls of which 
 it hardens ; and in consequence of this, the rock weathers most 
 unequally, leaving extraordinary rugged fissures and pinnacles, 
 
 1 T. G. Bonney, Quar. Jour. Geol. Soc. of London, Vol. XXXII, 1876, p. 153. 
 
 2 Expl. and Survey for a Railroad from the Mississippi to the Pacific Ocean; 
 Report on the Geology of the Route, near the 32d Parallel, by W. P. Blake. 
 
 3 See The Atlantic, Vol. I. Also Bull. 25, U. S. National Museum. 
 
248 THE PHYSICAL MANIFESTATIONS OF WEATHERING 
 
 and piled up boulders, the cores of masses which have been 
 eaten away, more like slags or cinders than blocks of limestone. 
 The ridges between Harrington Sound and Castle Harbor are a 
 good example of this. It is like a rockery of the most irregular 
 and fantastic style, and there seems to be something specially 
 productive in the soil; for every crack and crevice is filled with 
 the most luxuriant vegetation, mossing over the stones and train- 
 ing up as tier upon tier of climbers, clinging to the trees and 
 rocks. Frequently the percolation of hardening matter, from 
 some cause or other, only affects certain parts of a mass of rock, 
 leaving spaces occupied by free sand. There seems to be little 
 doubt that it is by the clearing out of the sand from such 
 spaces, either by the action of running fresh water or by that 
 of the sea, that those remarkable caves are formed which add 
 so much to the interest of the Bermudas." 
 
 A form of weathering due to similar causes, but productive 
 of results much more regular in arrangement, is shown in 
 Fig. 4, PI. 20, from a block of weathered sandstone in the 
 National Museum. The original joints through which the 
 waters filtered are easily recognized in the sharp straight lines 
 running diagonally across the specimen. Blocks of fine shale 
 and argillite, in their incipient stages of weathering, often show 
 concentric bands of varying color, due to the oxidizing effect 
 of water percolating inward from all sides of the natural joints 
 as shown in Fig. 3, PI. 20. 
 
 In stratified rocks there is, as a rule, a lack of homogeneity, 
 certain layers being more porous than others, or containing 
 mineral constituents more susceptible to the attacking forces. 
 Such rocks, therefore, weather unevenly, and give rise to ex- 
 ceedingly ragged contours. The finely fissile schists standing 
 nearly on edge along the coast of Casco Bay, in Maine, under 
 the combined influence of wave and atmospheric action, weather 
 into peculiarly fantastic forms resembling nothing more than 
 piles of old lumber in which the multitudinous channels formed 
 by boring coleopterous larvse have become irregularly enlarged 
 by decay. (See Fig. 1, PI. 19.) The numerous quartz veins by 
 which these schists are traversed stand out in bold relief until 
 no longer supported by the matrix, when they fall to the beach, 
 where, together with fragments of the schist, they are gradually 
 reduced to pebbles and fine sand. 
 
 (4) Weathering influenced by Mineral Composition. Although 
 
PLATE 19 
 
 FIG. 1. Weathered schist, coast of Cape Elizabeth, Maine. 
 FIG. 2. Sandstone bored by bees. FIG. 3. Slab of glaciated limestone. 
 
WEATHERING INFLUENCED BY COMPOSITION 249 
 
 the soda-lime feldspars yield to the decomposing agencies more 
 readily than the potash varieties, basic eruptives do not in all 
 cases decompose more rapidly than the granitic rocks into which 
 they -are intruded, as is well illustrated in some of the glaciated 
 areas about Boston, where small, compact dikes form low ridges 
 a few inches above the surface of the enclosing granite. Much 
 seems to depend upon the character of the secondary minerals 
 which have been generated in a rock at some period prior to 
 its decomposition proper. Thus those dikes containing so large 
 a proportion of secondary epidote as to be of a dull greenish 
 hue are almost invariably more enduring than the granites, 
 while those, on the other hand, in which the secondary minerals 
 are largely chlorite, calcite, and zeolitic compounds, yield to the 
 decomposing agencies more readily. Even when the dike as 
 a whole gives way, the presence of epiclotic aggregates fre- 
 quently manifests itself in protruding knots and bunches above 
 the corroded surface. Knots caused by segregations of black 
 tourmalines stand out in the same way from the surface of the 
 granite boss called Stone Mountain, near Altanta, Georgia. 
 Garnets, staurolites, quartz veins, and other of the less easily 
 decomposed minerals may stand out in like manner from the 
 surface of the rocks of which they form a part. 
 
 Granitic and other complex crystalline granular rocks will, on 
 exposure, sometimes take on a pitted surface, owing to the re- 
 moval of the more easily decomposed materials. The boulders 
 of nepheline syenite in the glacial drift about Portland, Maine, 
 are thus corroded to trie depth of several millimetres through 
 the removal of the granular nepheline, while the feldspars and 
 hornblendes project irregularly. 
 
 Calcareous rocks containing silicates, like the amphiboles or 
 pyroxenes, show like roughened surfaces due to the dissolving 
 away of the calcareous matter, leaving the silicates projecting, 
 or, as is the case with some of the tremolite-bearing dolomites 
 used for building, may become pitted by the dropping out of 
 the tremolite as the calcareous cement gives way. 1 
 
 Many sandstones become likewise roughened through the 
 removal of a portion of the cementing constituent, leaving the 
 siliceous granules projecting. In the coarsely crystalline lime- 
 stones and dolomites the solution and weathering effects are 
 often first manifested along cleavage lines and the contacts of 
 1 As in the U. S. Capitol Building at Washington. 
 
250 THE PHYSICAL MANIFESTATIONS OF WEATHERING 
 
 the individual granules, as may be observed in many an old 
 tombstone or polished column. 
 
 Even where the decomposition is almost purely chemical, the 
 corroded surfaces are . peculiarly irregular, as shown in PI. 17. 
 This feature is doubtless due to some imperceptible difference in 
 the texture of the stone, or to the presence of joints and flaws 
 which give direction to the solvent fluids. Calcareous rocks 
 consisting of an admixture of calcite and dolomite crystals may 
 undergo disintegration through a complete or partial removal of 
 the calcite granules by solution, the dolomite remaining almost 
 untouched. Certain dolomitic limestones near Stockton, Min- 
 nesota, have been described 1 as peculiarly subject to this form 
 of disintegration. The mass of the rock consists of dolomitic 
 crystals and granule's, but often interlaminated with narrow 
 bands of calcite. Through the removal of the latter, the 
 stone becomes porous and its degeneration so complete that 
 " shovelfuls of loose sand consisting of dolomitic rhombohedra 
 can be taken up." 
 
 Fine-grained, compact, and, seemingly homogeneous rocks 
 may, on account of imperceptible differences in composition 
 and structure, weather out in strikingly irregular and peculiar 
 forms. Figure 2 on PI. 15 is that of a limestone fragment from 
 Harrisonburg, Virginia. The resemblance to cuneiform charac- 
 ters Is so close that it is not surprising that such were at first 
 supposed to be of human origin. 
 
 Massive granitic rocks seemingly of quite uniform composi- 
 tion will sometimes weather very irregularly, giving rise to 
 erven-like cavities, in general shape resembling the pot-holes in 
 the beds of streams. Reusch has described 2 such in exposed 
 faces of granite ledges on the island of Corsica, the holes 
 extending inward horizontally, or sometimes with a slight up- 
 ward tendency. The cause of this is not apparent from the 
 description given, but it is presumably due to slight textural 
 differences such as are not readily discernible in the decom- 
 posed rock. 
 
 In any rock consisting of a variety of minerals, disintegration 
 is likely to constitute a more prominent feature of weathering 
 than in one of less complexity of composition, owing to the 
 unequally refractory properties of its constituents. Thus a 
 
 1 Hall and Sardeson, Bull. Geol. Soc. of America, Vol. VI, 1895, p. 184. 
 
 2 Eorhandlinger i Videnskabs-Selskabet i Christiania, 1878, No. 7, pp. 24-27. 
 
WEATHERING INFLUENCED BY COMPOSITION 251 
 
 granite must yield a sand, while a purely feldspathic, pyrox- 
 enic, or calcareous rock may yield only clays. 
 
 Beds of feldspathic quartzite, through the decomposition of 
 the feldspar, undergo disintegration, giving rise to beds of 
 friable siliceous sand interlaminated with kaolin, as described by 
 Dana. 1 The same author also describes an interesting pseudo- 
 breccia formed by a quartzite divided up by a succession of 
 cracks into which limonite from decomposing pyrite has fil- 
 tered and acted as a colored cement. He says : " Many of the 
 pieces lie in place barely separated from one another, and ap- 
 pear to be undergoing new divisions. But in the lower part, 
 large pieces look^as if there had been wide displacements; yet 
 the hardly disturbed condition of the upper half proves that 
 the apparent displacement is due to the extension of the color- 
 ing and penetrating limonite. The cracks are made in part 
 by the extremely slow, wedge-like action of the depositing 
 limonite." 
 
 Heusser and Claraz 2 describe somewhat similar breccias 
 formed in Brazil through the weathering of crystalline schists 
 rich in iron. These breccias consist of. angular fragments of 
 schist, more or less decomposed, firmly cemented by limonite. 
 
 The boulders of Oriskany quartzite in the Cretaceous gravel 
 about Washington, District of Columbia, are composed of 
 rounded and angular quartz fragments tightly bound together 
 by a fine granular crystalline aggregate of quartz and feldspar. 
 Disintegration first manifests itself on the exterior of the 
 boulders in the form of an irregular network of grooves or 
 channels, which gradually become more and' more conspicuous 
 until the boulder falls into bluntly pyramidal fragments and 
 finally into sand. The microscope shows that the disintegra- 
 tion is due wholly to the disaggregation and partial kaoliniza- 
 tion of the binding constituents whereby all cohesion is lost> 
 and disintegration follows from necessity. (Fig. 1, PI- 20.) 
 
 This form of disintegration seems to take place only in 
 boulders exposed at or near the surface, and is believed to be 
 due primarily to expansion and contraction from alternations 
 of temperature. 
 
 Many rocks, owing to a lack of homogeneity, weather with 
 extreme irregularity and give rise to odd and sometimes fan- 
 
 1 Am. Jour, of Science, Vol. XXVIII, 1884. 
 
 2 Ann. des Mines, 5th Series, Vol. XVII, 18GO, p. 290. 
 
252 THE PHYSICAL MANIFESTATIONS OF WEATHERING 
 
 tastic forms. In the case of a friable sand or limestone, sub- 
 ject to wind or rain erosion or to solution, certain portions may 
 be protected by a capping of other rock while the intervening 
 material is carried away. There thus arise spindle-shaped 
 forms of varying proportions, each capped by the roof or hat- 
 like block to which it owes its origin. Such have been noted 
 in many regions, and have been described by Hayden as occur- 
 ring on a colossal scale in Colorado. In* the case of strata 
 lying nearly horizontal, it rarely happens that all possess the 
 same power of resistance, the more friable weathering away 
 with the greatest rapidity, leaving the harder layers for a 
 time projecting in rib-like masses, to ultimately break down 
 in large angular blocks as the support below is gradually 
 removed. Friable beds of sedimentary rock are thus not infre- 
 quently protected by a capping of impervious lava. Continual 
 percolation of water through existing joints and fractures in 
 time, however, erode away, in part, the underlying material, 
 causing the landscape to assume the Table Mountain appear- 
 ance, where each flat-topped hill represents residual masses of 
 a once continuous plateau, now isolated in the manner described. 
 
 (5) Results due to Position. In very many instances loose 
 blocks of stone lying exposed upon the ground, will undergo 
 a more rapid disintegration from the lower surface, a feature 
 evidently due to the fact that this portion of the rock is kept 
 in a state of continual moisture. This form of disintegration 
 results in the production of oval, flattened, scale-like masses, 
 quite independent of the original jointing. In other cases 
 decomposition going on from all exposed sides of a joint block 
 may in time produce the so-called rocking-stones or " logans " 
 and "tors" of English writers, though some of these are un- 
 doubtedly nicely balanced boulders from the glacial drift. 
 
 A mass of rock may be prevented from undergoing disinte- 
 gration, even though partially decomposed, by its surroundings. 
 Thus, in driving the tunnel for the waterworks extension, in 
 Washington, natural joint blocks of hard and apparently firm 
 rock brought to the surface would fall away to loose sand in 
 course of a few days, or months, as the case "might be, much 
 depending on the conditions of the weather and the state of 
 decay. This characteristic was sufficiently pronounced to 
 attract even the attention of the workmen, who described the 
 rock as " slaking " and believed it to contain quicklime. 
 
RESULTS DUE TO POSITION 253 
 
 The fact was that percolating waters had brought about a 
 partial kaolinization of the feldspar, and hydration, without 
 great oxidation of the iron-magnesian constituent. The origi- 
 nal pressure, coupled with that incidental to expansion from 
 hydration, had, however, been sufficient to hold the mass intact 
 until exposed briefly to atmospheric influences. 
 
 The protective action of water, as sometimes shown in the 
 beds of streams and in deep ravines, may be only apparent, and 
 due to the fact that erosion exceeds decomposition, the stream 
 having cut its way down to fresh bed-rock. Professor Dana, 
 to be sure, writing more than half a century ago, 1 described the 
 basaltic rocks of Kiama, Australia, as in a condition of advanced 
 decomposition except where protected by sea-water. He says : 
 " It is a general and important fact that a rock which alters 
 rapidly when exposed to the united action of air and water, is 
 wholly unchanged when immersed in water, or exposed to a 
 constant wetting by the surf." While no exception can be 
 taken to the conclusion regarding those rocks wholly immersed, 
 the question naturally arises in one's mind, if the absence of 
 decomposition products in those rocks constantly wetted by 
 the surf and in many stream beds may not be due, in part at 
 least, to erosion. That rocks so situated are in a condition far 
 from fresh, is well known to any petrologist who has attempted 
 to gather specimens. 
 
 It is obvious that where a large series of sedimentary rocks 
 composed, it may be, of interbedded limestones, sandstones, and 
 argillites are turned up on edge and exposed alike to atmos- 
 pheric agencies, they will become eroded very unequally. If 
 chemical agencies alone prevail, the limestone will dwindle 
 away and perhaps give rise to long valleys or depressions 
 walled in by the more enduring sands and shales, and carry- 
 ing upon its bottom a fertile clayey soil representing not 
 merely the insoluble impurities contained by the original lime- 
 stone, but also the mechanically disintegrated particles washed 
 in from the hills on either hand. This indeed may be consid- 
 ered the history of the fertile Shenandoah valley of Virginia, 
 famous alike for soft contours, beautiful scenery, and the exu- 
 berant fertility of its soils. 
 
 When stratified rocks lie nearly or quite horizontally, much 
 must depend upon the character as regards permeability, etc., 
 1 Reports of Wilkes's Exploring Expedition, Geology, p. 614. 
 
254 THE PHYSICAL MANIFESTATIONS OF WEATHERING 
 
 of the upper layers, since these may so protect the lower lying 
 as to retard or quite stop further disintegration. Further than 
 this, an easy and rapidly disintegrating superficial layer may 
 yield a residual clay so impervious as to protect the underlying 
 rocks as securely as a mass of rock itself, or so hard and tough 
 as to put a stop to purely mechanical erosion, as in the case of 
 the laterite beds of central India. 
 
 In cases where thinly bedded rocks lie sharply inclined, it 
 nearly always happens that certain layers decompose more 
 readily than others. There may thus arise strikingly ragged 
 saw-tooth contours, the more enduring layers standing out in 
 sharply serrate or wall-like masses, while the softer give way 
 and become obscured by their own debris. 
 
 (6) Induration on Exposure. Many rocks, instead of becom- 
 ing disintegrated on exposure, undergo a kind of induration 
 upon the exposed surfaces. This is particularly the case with 
 some siliceous sandstones. The water with which the stone is 
 permeated holds in solution certain constituents, as silica, car- 
 bonate of lime, or iron oxides. When the rock is so situated 
 that this " quarry water," as it is popularly called, is brought 
 to the surface and evaporated, it binds together the granules 
 composing the stone, forming thus a more or less superficial 
 coating of a more enduring nature. The induration sometimes 
 takes place so rapidly that even an exposure of but a few months 
 is sufficient to produce very marked results on freshly broken 
 surfaces. This peculiarity of certain classes of rocks has long 
 been known to quarrymen and stone workers, who recognize 
 the fact that a well-seasoned stone yields much less readily under 
 the chisel than one that is newly quarried. 1 
 
 A somewhat similar induration, due to purely superficial 
 causes, has been described by Dr. M. E. Wadsworth as taking 
 place on the surface of exposed blocks of siliceous sandstone in 
 Wisconsin. " The St. Peters Sandstone " he writes, 2 " is com- 
 posed almost wholly of a pure quartz sand, and in the outliers 
 of it found on the hilltops south of the town, the parts covered 
 by the soil were more or less friable, and the grains distinct; 
 while the exposed portions of the same blocks and slabs were 
 greatly indurated, the grains almost obliterated, and the rock 
 possessed the conchoidal fracture and other characteristics of a 
 
 1 See Stones for Building and Decoration, p. 415. 
 
 2 Proc. Boston Soc. of Natural History, Vol. XXII, 1883, p. 202. 
 
INDURATION ON EXPOSURE 255 
 
 quartzite." In this and other cases cited by Dr. Wadsworth, 
 the cementing matter is silica. 
 
 The explanation given (in letter to the present writer) is to 
 the effect that all water, including that of rains, as well as ter- 
 restrial, dissolves silica, which is again deposited under suitable 
 conditions. Part of the silica apparently comes from the solu- 
 tion of the quartz, chalcedony, and opal, and a part from the 
 alteration and destruction of the silicates. Both solution and 
 deposition seem at times to take place on the immediate surface, 
 the interior waters in such cases playing no part. 
 
 P. Choffat regards it as possible that silica set free through 
 feldspathic decomposition in granitic rocks may, on evaporation, 
 be redeposited in an insoluble form in the interstices of the fresh 
 rock in the immediate vicinity, thus retarding if not wholly 
 preventing further decay in that direction. 1 
 
 Professor W. O. Crosby, in a personal memorandum to the 
 writer, calls attention to the fact that in the disintegrated 
 granites of the Pike's Peak, Colorado, area, the rock is almost 
 invariably exceptionally firm and impervious along the joints, 
 indicating a local induration due perhaps to infiltration of iron 
 oxides or silica. Where a joint face bounds a ledge of rock, it 
 often maintains its integrity, weathering out in relief like a 
 quartz vein, while the granite is in a condition of advanced 
 degeneration all around. A slight break in the face of a joint 
 plane, in such cases, may lead to extensive disintegration behind 
 it, until it finally falls away from the disintegrating mass, a slab 
 of relatively sound rock. 
 
 Andesitic rocks in regions of limited rainfall have been 
 noted by Professor G. Vom Rath as having become covered 
 on the upper surface with a thin layer of brown iron oxide, 
 which protected them from further disintegration. Such 
 crumbled away only from the under surfaces, where they ab- 
 sorbed moisture from the ground, and gave rise thus to peculiar 
 tent-like and mushroom-shaped forms. 
 
 The present writer has noted in the Madison valley, north 
 of the Yellowstone Park, rounded masses of a vesicular rhyolite 
 which have, through the same causes, been reduced to the con- 
 dition of mere shells with openings on the under sides and that 
 
 1 Sur quelques cas d' erosion atmospherique dans les garnites du Minho, Com- 
 munica95es da Direc^ao Dos Trabalhos Geologicos de Portugal, Tome 3, Fasc. 1, 
 1895-96, p. 17. 
 
256 THE PHYSICAL MANIFESTATIONS OF WEATHERING 
 
 facing the direction of the prevailing winds. In these cases, 
 however, the wind seemed to have aided their formation, not 
 merely through transporting the disintegrated material, but by 
 catching up and whirling about the loosened granules within 
 the gradually enlarging cavity, where, by force of impact, as 
 already described, they became themselves agents of abrasion. 
 Some of the cavities observed were of sufficient size to afford 
 shelter for a human being and had in some instances served as 
 temporary dens for wild animals. 
 
 Roth mentions l an induration evidently somewhat similar to 
 that described by Vom Rath above, as having taken place, on 
 the surface of a reddish yellow sandstone in Fezzan, North 
 Africa. The crust thus formed was so dense and hard as to 
 break with a shell-like fracture resembling basalt. A similar 
 incrustation on sandstone from the Lydian desert was found to 
 consist of: manganese oxide, 30.57% ; iron oxide, 36.86% ; 
 alumina, 8.91% ; silica, 8.44% ; barium oxide, 4.89% ; sul- 
 phuric acid, 4.06 % ; phosphoric acid, 0.25 % ; and water, 5.90 %. 
 
 W. P. Blake has described boulders from the Colorado 
 desert colored exteriorly by what he regarded as organic matter 
 received from water during a period of submergence. Similarly 
 discolored quartzitic boulders brought by G. K. Gilbert from 
 the Sevier desert in Utah, and examined by the present writer, 
 show a thin dark varnish-like coating, not inaptly named by Mr. 
 Gilbert " desert varnish," and which consists largely of oxides 
 of iron and manganese, though a slight amount of organic 
 matter is present. In this case the rock is composed not wholly 
 of quartz granules, but carries interstitial calcite and feldspathic 
 granules. Near the discolored surface of the boulders these in- 
 terstitial calcites are found quite dissolved away, leaving cavities 
 stained by a dark deposit which reacts for iron and manganese. 
 Inasmuch as acid solutions obtained from fresh and uncolored 
 portions of the boulders give faint reactions of the same nature, 
 it seems very probable that the crust is due to a concentration 
 of these metals in a condition of higher oxidation on the surface, 
 whither they have been brought by capillarity, while the more 
 soluble lime carbonate was removed. 2 
 
 1 Allegemeine u. Chemische Geologie, 2d ed., Vol. Ill, p. 215. 
 
 2 Although such discolorations seem to have been noted principally in desert 
 regions, they are by no means limited thereto. The quartzitic boulders in the 
 superficial deposits of the District of Columbia show at times a like discoloration, 
 due to a very thin coating of iron and manganese oxide. 
 
INCIDENTAL COLOR CHANGES 257 
 
 The Potsdam quartzites of Minnesota have had, in many in- 
 stances, an almost glass-like polish imparted to their exposed 
 surfaces through no other apparent agency than that of wind- 
 blown sand. Unlike a polish produced by artificial methods, 
 this wind polish extends to the bottoms of every little groove 
 and cavity, or over every protruding knob alike. In softer 
 rocks, or rocks of less homogeneous structure, the same agencies 
 carve out the softer portions, leaving the more resisting pro- 
 truding, as already described on p. 186. This polish is so per- 
 fect, even on rough surfaces, as to suggest a partial solution of 
 the granules, and a redeposition of the dissolved matter in the 
 form of a glaze, but the microscope proves to the contrary. 
 The gloss is due wholly to superficial smoothing and has no 
 thickness whatever, nor has any new matter been deposited 
 either on the surface or between the granules. 
 
 (7) Changes in Color incidental to Weathering. That in 
 nearly every rock a change in color, the assumption of a 
 brownish or reddish hue, is an early indication of decomposition 
 has been made sufficiently apparent in the chapter devoted to a 
 discussion of the chemical changes involved. This discolor- 
 ation is, however, merely incidental, and not essential, and is 
 found to diminish, if not wholly disappear, as the distance from 
 the surface increases, as was noted in the case of the granites of 
 the District of Columbia (p. 207) and the diorites of the Sierra 
 Nevadas (p. 274. See further under Color of Soils, p. 385). 
 
 Granitic and other highly feldspathic rocks carrying pro- 
 portionately small amounts of iron become almost invariably 
 bleached or whitened on the immediate surface, owing in part to 
 kaolinization and in part to the splitting up of the feldspars 
 along cleavage lines. 
 
 In extreme cases rocks consisting of an admixture of feldspars 
 and iron-bearing silicates, but in which the first-named, owing 
 to its glassy nature, is in the fresh rock quite inconspicuous, 
 become almost snow-white in the earlier stages of weathering. 
 This, as in the case above mentioned, is due to the change in 
 the feldspars and the consequent obscuring of the darker sili- 
 cates by the white product of kaolinization. Continued decom- 
 position must, however, attack the ferruginous constituent and 
 the usual staining ensue, unless, as in some cases possible, suffi- 
 cient carbonic acid may exist to convert the iron immediately 
 into carbonate and permit of its removal in solution. 
 
258 THE PHYSICAL MANIFESTATIONS OF WEATHERING 
 
 Allusion has been already made to the fact that oxidation 
 or other chemical action, with the possible exception of hydra- 
 tion, practically ceases below the permanent water level. Hunt 
 and Le Conte have both called attention to the fact that the 
 hornblendic and feldspathic rock fragments occurring in the 
 Pliocene auriferous gravels of California are firm and intact in 
 those portions below the drainage level (the blue gravel layer), 
 but more or less completely oxidized, kaolinized, and otherwise 
 altered in the red or upper gravel. 
 
 Van den Broeck has called attention l to the possibility that 
 the so-called red and gray diluvium of the Quaternary deposits 
 near Paris may be but portions of one and the same geological 
 body, the " diluvium rouge " being but an upper member of 
 the " diluvium gres" oxidized and impoverished in lime by the 
 action of meteoric waters. 
 
 The same feature is noticeable in many of our quarries for 
 building stone, as those in the Berea sandstones of Ohio. 
 These below the drainage level, are of a gray or blue-gray 
 color, while above, where they have been subjected to the 
 oxidizing influence of meteoric waters, they are buff. The 
 Jurassic oolites of England, are blue-gray at some depths below 
 the surface, but white above. 
 
 In cases where natural joint blocks are exposed to the perco- 
 lation of meteoric waters, the weathering may for a time mani- 
 fest itself only in differential oxidation and zonal segregation 
 of the iron whereby are produced concentric bands of varying 
 hues. Figure 3, PI. 20, is a slab from a natural joint block of 
 argillite in the collections of the National Museum, in which 
 the bands, due to this cause, vary from yellow-brown, drab, to 
 ochreous yellow and red, while the rock as a whole still retains 
 its compact structure and susceptibility to polish, forming an 
 ornamental stone of no mean order. 2 
 
 (8) Relative Amount of Material removed in Solution. 
 Among siliceous rocks, chemical action proceeds but slowly, 
 and the amount of material actually removed in solution is 
 rarely over 50 %, and may be so small that, as the writer has 
 shown, 3 the residue in extreme cases occupies some 80 % more 
 space than the rock from whence it was derived. Carbonate 
 
 1 Bull. Soc. Geologique de France, 5, 1876-77, p. 298. 
 
 2 Stones for Building and Decoration, p. 169. 
 
 8 Bull. Geol. Soc. of America, Vol. VI, 1895, pp. 321-332. 
 
PLATE 20 
 
 FIG. 1. Weathered boulder of Oriskany sandstone. 
 
 FIG. 2. Concentric weathering in diabase. 
 
 FIG. 3. Zonal structure in weathered argillite. 
 
 FIG. 4. Weathered sandstone, showing- induration along joint planes. 
 
INCIDENTAL SURFACE CONTOURS - 259 
 
 of lime, the essential constituent of ordinary limestone, is, 
 however, as has been observed, soluble in the carbonated water 
 of rainfalls, and, in time, may undergo complete removal, 
 leaving but the insoluble impurities behind. This is, indeed, 
 the almost universal history of limestone soils. They are not 
 infrequently so siliceous or ferruginous as to be quite barren 
 and of a nature to be benefited by the application of lime as a 
 manure. 
 
 Throughout the areas occupied by the Trenton limestones, in 
 Maryland, nearly every farm has, in years past, had its quarry 
 and lime-kiln where the stone was fitted for supplying lime 
 once more to soils from which it had been so thoroughly leached 
 as to render them lean and poor. It is almost wholly to this 
 solvent action that is due the formation of the multitudinous 
 caverns, large and small, of the limestone regions. Even where 
 caverns are not apparent, the corrosive action is evident to the 
 practised eye. In the quarry regions of Tennessee surface 
 blocks of limestone are often grooved to a depth of an inch or 
 more with wonderful sharpness, simply from the water of rain- 
 falls with its acids absorbed from the atmosphere and surface 
 soils, while in the quarry bed the stone is found no longer in 
 continuous layers, but in disconnected boulder-like masses. 
 (Figs. 3 and 2, Pis. 16 and 21.) In such cases casual 
 examinations give very little clew to the rapidity of the de- 
 struction going steadily on, since all is removed in solution 
 excepting the comparatively small amount of insoluble matter 
 (usually clay or silica) existing as an impurity. 
 
 (9) Incidental Surface Contours. In limestone regions the 
 solvent action of water has frequently gone on so extensively 
 as to leave its imprint upon the topographic features of the 
 landscape. The drainage is no longer wholly superficial, but 
 by subterranean streams sinking entirely into the ground to 
 reappear again at lower levels, it may be miles away, having 
 traversed the intervening distance in some of the numerous 
 passages (fissures enlarged by solution) with which the rocks 
 abound. Entire landscapes are undulating through the abun- 
 dance of sink-holes shallow depressions down through which 
 the water has percolated and escaped into the underground 
 passages. 
 
 The writer recalls a beautiful illustration of this nature seen 
 in the limestone regions of southern Indiana, some years ago. 
 
260 ' THE PHYSICAL MANIFESTATIONS OF WEATHERING 
 
 The season was that of the wheat harvest. On every side, far as 
 the eye could reach, were undulating fields of waving grain, of 
 that charming golden hue of which poets sing, with intervening 
 patches of woodland. From every farm was heard the click of 
 the reaper, and from every fence the whistle of 'the " Bob 
 White." Owing to the fact that the ridges between these de- 
 pressions were drier than the bottoms, the wheat here ripened 
 earlier, and field after field showed long reaches of saucer- 
 shaped depressions green in the centre, with intervening ridges 
 of golden brown, making, with that charming hazy atmosphere, 
 a picture long to be remembered. Through accident or design, 
 the opening in the bottom of these sink-holes sometimes becomes 
 closed, giving rise thus to temporary pools, or ponds, as shown 
 in the accompanying plate. It is this same action that has 
 given rise to the so-called " saiidpipes " of the English geolo- 
 gists. These are slender funnel- or tube-shaped cavities found 
 in chalk and calcareous sandstone, sometimes filled with drift 
 gravels, sands, brick-earths, or again with fragmental materials 
 fallen into them from the overlying beds as the support beneath 
 was gradually removed. In all these cases it is assumed that 
 direction was given the percolating water by pre-existing fissures 
 or lines of weakness. 1 (Fig. 1, PI. 21.) 
 
 In regions underlaid by massive siliceous crystalline rocks, 
 and where mechanical erosion is reduced to a minimum, land- 
 scapes are softly undulating, with few abrupt escarpments or 
 precipitous ledges, owing to the uniform rotting away of the 
 materials, and the gradual accumulation of the debris. It is to 
 this form of weathering that is due the beautiful rolling hills 
 of southwestern Maryland. The prevailing rock is granite or 
 gneiss. Decomposition follows out each line of weakness. 
 Streams erode through the softened material down to hard 
 bed-rock, while the relatively large proportion of insoluble 
 debris is left to accumulate on the gentle slopes which form 
 such an enchanting feature of these landscapes. 
 
 In regions of gneissic or granitoid rocks traversed by large 
 veins of quartz, as in the northwestern part of the District of 
 Columbia, the superior resisting power of the quartz causes it 
 to stand out in relief from the gradually dwindling rock masses 
 on either hand, giving rise thus to prominent knolls, or ridges, 
 
 1 See Prestwich's paper, Quarterly Journal Geological Society of London, 
 1855, p. 62. 
 
INCIDENTAL SURFACE CONTOURS 261 
 
 the occasion for which is a mystery until we come to examine 
 their foundation materials. Belt, in describing the auriferous 
 quartz lodes at San Domingo, 1 states that the prevailing trend 
 of the main ranges is nearly east and west, and is probably due 
 to the direction of the outcrops of the lodes which have resisted 
 the action of the elements better than the soft dolerites. 
 
 So striking a feature of the landscape as the Devil's Tower 
 or Bear Lodge on Little Sun Dance River, Wyoming, is due to 
 the weathering away and erosion of sedimentary beds from 
 around a dense crystalline core or plug of eruptive rock in- 
 truded into them in some past period of volcanic activity. 
 Through its greater powers of resistance, this still stands, 
 towering over 1000 feet above the level of the river, though in 
 time this, too, must go. Quite similar forms have resulted, 
 within a comparatively brief geological period through the 
 erosion of tufaceous cones from around the compact, crystalline 
 plug of lava which solidified within the crater when volcanic 
 activity ceased. Beautiful examples of these are to be seen in 
 Arizona and New Mexico, where they are known as " volcanic 
 necks." The formation of bosses through the influence of 
 joint planes has been described elsewhere (p. 244). 
 
 In regions abounding in intrusive olivine or pyroxene rocks 
 which have undergone alteration into serpentine and talc or 
 "soapstone," one frequently finds these materials forming the 
 main mass of the hills, while the valleys are carved out of the 
 softer, more readily decomposed granite, or whatever the country 
 rocks may be. The same feature is prominently developed in 
 the slate regions of Harford County, Maryland, where the slate 
 is the more enduring rock, and forms steep ridges, flanked by 
 valleys, carved out from less resisting materials. Regions of 
 trappean dikes in siliceous schists or gneisses, particularly 
 along sea-shores where swept by incoming tides, are often 
 characterized by narrow, straight-walled chasms, or canons due 
 to the weathering out of the basic rocks, while the more refrac- 
 tory schists on either hand remain. 
 
 In cases where trappean dikes have cut through friable sand- 
 stones, they have in some instances so indurated these rocks 
 along either contact as to cause them to be more durable than 
 the original rock or than even the trappean rock itself. There 
 may thus arise long parallel ridges of indurated sandstone sepa- 
 1 The Naturalist in Nicaragua. 
 
262 THE PHYSICAL MANIFESTATIONS OF WEATHERING 
 
 rated by an intervening depression due to the weathering out 
 of the dike material. 
 
 In regions where climatic conditions or the nature of the 
 rock are more favorable to mechanical disintegration than 
 chemical decomposition, contours may be ragged in the ex- 
 treme. Entire crests may be but successions of jagged peaks 
 and intervening narrow valleys which are gradually becoming 
 choked up by the debris fallen from the cliffs above. 
 
 (10) Effacement of Original Characteristics through Weather- 
 ing. In cases of extreme decomposition, in place, the residual 
 products may so slightly resemble the parent rock as to give 
 rise to very conflicting opinions concerning their origin. This 
 was for a long time the case with the laterite of India, already 
 described, and the terra rossa of Europe. 
 
 Dana describes l an interesting case of basaltic decomposition 
 which, on account of the peculiar nature of the residual product, 
 is worthy of mention here. He writes: " The process of decom- 
 position is finely exhibited on the second cliff north of Kiama 
 (Australia) towards the north end. At first sight, a distinct 
 argillaceous deposit was supposed to overlie the columnar basalt; 
 for it was twenty feet thick, and of a whitish color, resembling 
 a soft crumbling marl, thus wholly unlike the basalt, and the 
 common results of basaltic decomposition. Still it had pro- 
 ceeded from the alteration of a regular columnar variety, having 
 a dull grayish blue color. The original rock is exceedingly 
 compact, showing no trace of crystallization, excepting an 
 occasional minute crystal of feldspar ; and within the reach 
 of the swell, it was still compact and solid. 
 
 " The rock has a concentric structure, and to this it owes in 
 part its rapid decomposition. The alteration commences be- 
 tween the concentric layers, rendering them apparent, although 
 not so before. At first a thin ochreous line appears, arising 
 from iron ; either magnetic iron disseminated in the rock, or 
 from that of the constituent mineral augite. This ochreous 
 color afterwards mostly disappears, and the concentric coats 
 become separated by thin clayey layers of a white color, more 
 or less striped with ochreous lines. In a more advanced stage 
 of the process large ovoidal masses of basalt (but little changed 
 in appearance excepting the development of a slaty concentric 
 structure) lie in the cliff separated by a considerable thickness 
 
 1 Reports Wilkes's Exploring Expedition, Geology. 
 
EFFACEMENT OF ORIGINAL CHARACTERISTICS 263 
 
 of the whitish clayey layers, which are stained by irregular 
 ochreous lines. At last the centres of the spheroidal masses 
 yield, and finally the change is so complete that the concentric 
 arrangement is entirely lost, and a soft whitish or yellowish- 
 white argillaceous deposit, with few ochreous spots or lines, 
 takes the place of the compact basalt. 
 
 " In basalts of more compact structure these changes take 
 place more slowly. The grayish blue basalt in the Illawarra 
 range, near Broughton's Head, when long exposed, is discolored 
 exteriorly to a depth of an inch and a half. The colors, begin- 
 ning within, are dirt-brown, grayish yellow, ochre-yellow, 
 brownish red ; and they are evidently dependent mostly on 
 changes in the condition of the iron which the rock or its 
 minerals contain. 
 
 " When the rock includes much chrysolite, the results of 
 decomposition in some instances give a fissile or micaceous 
 appearance to the rock. At Prospect Hill, five miles west of 
 Paramatta, this change is in progress. The rock is a black 
 ferruginous basalt of homogeneous aspect, breaking with a 
 smooth fracture and no appearance of crystallization. It con- 
 tains chrysolite ; but the grains are small and not apparent 
 except on very close examination. . . . 
 
 " Were we unable to trace the transitions, and distinguish 
 the columnar structure through the whole, we should scarcely 
 suspect its basaltic origin. Indeed, it was pointed out to me 
 as an instance of mica slate overlying basalt. Particles of 
 rusted mica, as they seemed, were distinct, and it much re- 
 sembled a decomposing variety of that rock. On close inspec- 
 tion and an examination of the rock in different stages of 
 change, it became evident that the pseudo-mica was nothing 
 but altered chrysolite, which had rusted from partial decompo- 
 sition, and split into thin cleavage scales. 
 
 " The crystals of chrysolite have evidently a parallel position 
 in the rock, and hence the plane of easiest cleavage lies in the 
 same direction, or, as the cleavage shows, parallel with the 
 upper surface, that is, at right angles with the vertical axis of 
 the columns. The passage from the compact to the decomposed 
 rock is, in this case, unusually abrupt. Alteration takes place 
 (through the elimination of oxide of iron as before suggested) 
 slowly at the surface, which therefore chips off as soon as de- 
 composed and exposes a new portion. This sudden transition 
 
264 THE PHYSICAL MANIFESTATIONS OF WEATHERING 
 
 may, in part, proceed from the absence of any natural planes of 
 fracture (which are brought out when there is a concentric 
 structure), and perhaps in part also from the presence of 
 chrysolite. The layer of pseudo-mica schist is in some places 
 five feet thick and has a rusty brownish color. Above it passes 
 into three feet of earth of the same origin, having a brownish 
 black color, and this is covered again by four feet of brownish 
 red soil." 
 
 Such an effacement is not, however, an invariable accom- 
 paniment of decomposition, since where the amount of residuary 
 material is relatively large, and allowed to accumulate in place, 
 the mass may for a long period retain its original structural char- 
 acteristics. Indeed, the original features are sometimes so per- 
 fectly preserved that casual inspection alone quite fails to reveal 
 the havoc that has gone on. Every detail of bedding, jointing, 
 or foliation, or even of internal structure, as brought about by 
 the arrangement or size of the individual particles, may be re- 
 tained with perhaps only a slight change of color due to oxida- 
 tion. This feature is often strikingly conspicuous in the newer 
 railway cuts of the southern Appalachian regions, particularly 
 where the country rock is of the nature of gneisses or schists. 
 In the work of grading the streets, in the extensions of the city 
 of Washington, masses of strongly foliated granites, so soft as 
 to be readily removed with pick and shovel, would be cut 
 through, and which yet showed every vein or other structural 
 detail as plainly marked as in the original rock, and it was 
 only when by thrusting one's cane or other implement into it 
 that its thoroughly decomposed condition became apparent. 
 Russell describes 1 a similar condition of affairs prevailing in 
 the coarse Triassic conglomerate near Wadesborough, North 
 Carolina. This conglomerate is here composed of rounded and 
 angular pebbles of talcose schist and other crystalline rocks. 
 In the fresh cuts along the line of the North Carolina railroad, 
 every detail of the original rock is brought out almost as sharply 
 as in the so-called " Potomac marble " phase of the same forma- 
 tions as used in the Capitol building at Washington. "On 
 examining more closely, however, one is surprised to find that 
 it is completely decomposed, and that when moist it can be cut 
 with a pocket knife through pebbles and matrix alike, as easily 
 as so much potter's clay. The full depth of the alteration in this 
 
 1 Bull. 52, U. S. Geol. Survey, 1889. 
 
INCIDENTAL SIMPLIFICATION OF COMPOUNDS 265 
 
 instance is not revealed, but it extends more than 30 feet below 
 the surface without change in character." 
 
 W. B. Potter described l the feldspar porphyry of Iron Moun- 
 tain, Missouri, as decomposed to the extent that it can be easily 
 whittled away with a penknife or scratched with the thumb nail. 
 " At the same time," he writes, " the original porphyritic 
 structure of the individual crystals scattered through the mass 
 is beautifully preserved, and is even frequently more distinctly 
 visible than in the original rock, owing to stronger contrasts of 
 color in the kaolinized material." In many dense massive rocks, 
 indeed, such features as flow structure and inequalities of text- 
 ure are frequently rendered evident only on weathered surfaces. 
 The same is often true of fossiliferous limestones, a weathered 
 surface revealing the presence of organic forms wholly imper- 
 ceptible on one freshly broken. 
 
 The crude kaolin as removed from the pits near Brandywine 
 Summit, Pennsylvania, and at Hockessin, Delaware, still retains 
 .more or less distinctly the structure of the original gneiss or con- 
 glomerate from whence it was derived. The quartz granules 
 of the gneiss are, in these cases, almost invariably shattered, 
 as though crushed by dynamic agencies, and show distinctly 
 corroded surfaces, presumably caused by the alkaline carbo- 
 nates formed during the kaolinizing of the feldspars. The 
 black mica makes its former presence known by rust-colored 
 spots which, in those cases where the mineral was sufficiently 
 abundant, have ruined the material for the purposes of the 
 potter. 
 
 (11) Simplification of Chemical Compounds, incidental to 
 Weathering. It has been noted on p. 172 that the process of 
 weathering is but an attempt on the part of the elements in 
 their various combinations to adjust themselves to existing con- 
 ditions. This adjustment consists in the formation of new com- 
 pounds which are characterized by a less complex structure than 
 those first formed. 
 
 Indeed, one of the most striking features of chemical geology 
 is the tendency toward simplification in composition as mani- 
 fested all over the superficial portions of the earth. During 
 the process of decomposition there is almost invariably a con- 
 stant breaking down of complex molecules of mixed silicates of 
 alumina, iron, lime, magnesia, and the alkalies, and a recombi- 
 1 Jour. U. S. Assoc. Charcoal Iron Workers, Vol. VI, p. 25. 
 
266 THE PHYSICAL MANIFESTATIONS OF WEATHERING 
 
 nation of their various elements as simpler silicates, carbonates, 
 sulphates, and oxides. 
 
 (12) Other Results incidental to Decomposition and Erosion. 
 That all the minerals of a rock mass are not equally acted upon 
 by atmospheric agencies has been sufficiently noted in previous 
 pages. The more refractory, freed by the breaking down of 
 their host, remain to gradually accumulate in vastly greater 
 proportions than they existed in the original rock. If, in 
 addition to their refractory qualities, such possess, as is usually 
 the case, greater density, decomposition and erosion may act but 
 as agents of concentration, and in such residues minerals like 
 xenotime and monazite have been found in abundance, although 
 occurring so sparingly in the fresh rock that their existence was 
 scarcely suspected. 
 
 It is in this manner that has originated the gem sand of 
 Ceylon. Precious stones have been found disseminated in lim- 
 ited numbers in the granite converted into the cabook described 
 on p. 242. In weathering, the difficultly decomposable precious 
 stones have not been attacked, or attacked only to a limited ex- 
 tent. They have therefore retained their original form and hard- 
 ness. When in the course of thousands of years streams of water 
 have flowed over the layers of cabook, their soft, already half- 
 weathered constituents have been for the most part changed into 
 a fine mud, and as such washed away, while the hard gems have 
 only been inconsiderably rounded and little diminished in size. 
 The current of water therefore has not been able to wash them 
 far away from the place where they were originally embedded 
 in the rock, and we now find them collected in the gravel bed, 
 resting for the most part on the fundamental rock which the 
 stream has left behind, and which afterwards, when the water 
 has changed its course, has been again covered by new layers of 
 mud, clay, and sand. It is this gravel bed which the natives 
 call nellan, and from which they chiefly get their treasures of 
 precious stones. 1 The same process in states bordering along 
 the Appalachian Mountain system in North America has given 
 rise to auriferous sands, as well as to sands bearing monazite, 
 zircons, and other valuable minerals, which become segregated 
 merely through their greater density and power to resist decom- 
 position. The stream tin ores of the Malayan Peninsula, the 
 
 1 Nordenskiold, Voyage of the Vega. See also Judd, On the Rubies of Burma, 
 etc., Fhilos. Trans. Royal Soc. of London, Vol. CLXXXVII, 1896, p. 151. 
 
PLATE 21 
 
 FIG. 1. Sink-hole near Knoxville, Tennessee. 
 
 FIG. 2. Beds of marble corroded by meteoric waters, Pickens County, Georgia 
 
RESULTS INCIDENTAL TO DECOMPOSITION 
 
 267 
 
 diamond-bearing gravels of Brazil, and indeed placer deposits in 
 general are illustrative of this same principle. The very soil 
 itself, although so indispensable to human existence, is but an 
 incidental and transitory phase of rock-weathering, as has been 
 made sufficiently apparent in previous pages. The deposits of 
 kaolin in western Pennsylvania and northern Delaware, as else- 
 where noted, are but decomposed highly feldspathic gneisses 
 and conglomerates, while the phosphate deposits of middle Ten- 
 nessee are insoluble residue left by the leaching out of the cal- 
 cium carbonate from phosphatic limestones. 1 
 
 According to Russell, 2 the Clinton iron ore of Alabama is 
 but the insoluble residue from ferruginous Silurian limestones. 
 On the immediate surface the ore is quite pure, containing, it 
 may be, but a trace of lime. When followed downward, the 
 amount of lime is found to gradually increase, until the ores 
 may become so poor in iron as to be valueless. The following 
 figures show this gradual increase in lime carbonate, and neces- 
 sary decrease in iron, from the surface downward. 3 
 
 PERCENTAGE OF CALCIUM CARBONATE IN CLINTON IRON ORE 
 
 DEPTH 
 
 PER CENT 
 
 DEPTH 
 
 PER CENT 
 
 Surface 
 
 Trace 
 
 70 feet below surface . . 
 
 25.61 
 
 10 feet below surface . . . 
 
 Trace 
 
 80 feet below surface . . 
 
 29.92 
 
 20 feet below surface . . . 
 
 Trace 
 
 90 feet below surface . . 
 
 29.89 
 
 30 feet below surface . . . 
 
 Trace 
 
 100 feet below surface . . 
 
 23.37 
 
 40 feet below surface . . . 
 
 21.06 
 
 110 feet below surface . . 
 
 28.82 
 
 50 feet below surface . . . 
 
 23.90 
 
 120 feet below surface . . 
 
 21.32 
 
 60 feet below surface . . . 
 
 27.01 
 
 130 feet below surface . . 
 
 30.55 
 
 William Whitaker in 1864 4 noted the decomposition of the 
 English chalk beds, in Middlesex, and the gradual accumulation 
 of a stiff brown-red residual clay interspersed with many flint 
 nodules. It is by this same leaching action on aluminous lime- 
 stones that is formed the so-called " rottenstone" so commonly 
 used in polishing brasses and other metals. 
 
 1 J. M. Safford, American Geologist, October, 1896, p. 261. 
 
 2 Op. cit., p. 22. 
 
 8 Trans. Am. Ins. of Mining Engineers, Vol. XV, 1886, p. 189. 
 4 Mem. Geological Society of Great Britain, 1864, p. 64. 
 
THE WEATHERING OP ROCKS (Continued} 
 IV. TIME CONSIDERATIONS 
 
 Concerning the rate of decomposition of rocks of various 
 kinds, only very general rules can be laid down, since much 
 depends upon climatic conditions and the position of rock 
 masses relative to the action of frost, moisture, and the various 
 growing organisms. 
 
 (1) Rate of Weathering influenced by Texture. From the 
 study of building materials it has become apparent that a 
 coarsely crystalline rock will, all other conditions being the 
 same, disintegrate more rapidly than one of finer grain. This 
 is doubtless owing in part to expansion and contraction from 
 ordinary temperature variations, which act the more energeti- 
 cally the larger the crystalline particles. 1 
 
 It has already been remarked (ante. p. 44) that crystalline 
 rocks have a greater density than do glassy forms of the same 
 chemical composition. This indicates a contraction during the 
 processes of crystallization, which manifests itself, according to 
 at least one authority, in the development of minute interspaces 
 between the individual crystals. The coarser the crystalliza- 
 tion, then, the greater the amount of interstitial space, and 
 hence the greater the absorptive power. 
 
 These coarser rocks, owing to their tendency to undergo a 
 mechanical disintegration, or disaggregation, may also yield to 
 
 1 The coefficient of cubical expansion for several of the more common rock- 
 forming minerals has been determined as follows : 
 
 Quartz. . . 0.0000360 Tourmaline 0.000022 
 
 Orthoclase . . . . . . 0.0000170 Garnet 0.000025 
 
 Hornblende 0.0000284 Calcite 0.000020 
 
 Beryl 0.0000010 Dolomite 0.000035 
 
 The strain brought to bear upon a mass of rock through the unequal rate of 
 expansion of its various constituents is further complicated through the unequal 
 expansion of the individual minerals along the direction of their various axes. 
 Thus quartz gives a coefficient of 0.00000769 parallel to the major axis, and of 
 0.000001385 at right angles thereto. Adularia gives 0.0000156, 0.000000659, and 
 0.00000294 for its three axes, and hornblende 0.0000081, 0.00000084, and 0.0000095 
 (Stones for Building and Decoration, p. 419). 
 
 268 
 
RATE OF WEATHERING 
 
 269 
 
 the decomposing agencies more readily than those of finer 
 grain, though from the fact that they first fall away to coarse 
 sand, whereby the rock- 
 like character is lost, one 
 might, on casual inspec- 
 tion, be led to the oppo- 
 site conclusion. It need 
 scarcely be said that, 
 among rocks having the 
 same composition, wheth- 
 er fragmental or crystal- 
 line, those of a granular 
 structure will undergo 
 disintegration more 
 quickly than will those 
 in which the individual 
 minerals are closely com- 
 pacted or interknit, as in 
 many quartzites or dia- 
 bases. 
 
 (2) Rate of Weather- 
 ing influenced by Compo- 
 sition. Among rocks of 
 the same structure as re- 
 gards crystallization and 
 size of particles, the basic 
 varieties, such as the dia- 
 bases and gabbros, as a 
 rule succumb more read- 
 ily than do the more acid 
 varieties like the gran- 
 ites. This for the reason 
 that the iron-magnesian 
 as well as the soda-lime 
 minerals are more sus- 
 
 FIG. 20. 
 
 FIG. 21. 
 
 Microstructure of sandstone (Fig. 20), showing 
 relatively large amount of interstitial space 
 and absorptive power, and (Fig. 21) of dia- 
 base, with relatively little. 
 
 ceptible than are the pot- 
 ash silicates and other 
 essential constituents of 
 the rocks of the granitic group. It is possible also that these 
 dark colors cause them to become more highly heated, where 
 exposed to direct sunlight, and hence subject to mechanical dis- 
 
270 TIME CONSIDERATIONS 
 
 integration. The fact that many of our trappean rocks, as seen 
 in dikes cutting other rocks, do not in all cases succumb with 
 greater comparative rapidity is due to their very compact struct- 
 ure, whereby percolating waters are so largely excluded. 
 
 (3) Rate of Weathering influenced by Humidity. The ra- 
 pidity of rock weathering and soil formation is, even among 
 rocks of the same nature, widely variable, being dependent 
 upon climatic conditions of any particular locality. In the arid 
 regions north of Flagstaff, Arizona, are wide areas of country 
 covered with coal-black lapilli ejected from volcanoes whose 
 craters are now occupied by growing pines upwards of two 
 feet in diameter. Yet these fields are, with the exception of the 
 pines, as bare of vegetation as though but yesterday scorched 
 by fire. The fine lapilli, resembling nothing more than crushed 
 coke, cover everywhere the undulating plains, greedily absorb- 
 ing the moisture from melting snows and scanty rainfalls, but 
 undergoing no appreciable decomposition and affording foot- 
 hold for only a few desert shrubs and grasses. Yet in a 
 moister clime, and one more adapted for luxuriant vegetation, 
 we might expect that these lapilli should long ago have suc- 
 cumbed and given fairly fertile soils. 
 
 (4) Rate of Weathering influenced by Position. Among the 
 siliceous crystalline rocks superficial disintegration is undoubt- 
 edly greatly aided by temperature variations, which, by render- 
 ing the rocks porous, facilitate chemical decomposition. Such 
 action must, however, be merely superficial, and at considerable 
 depths below the surface the change must be purely chemical. 
 The chief conditions favoring chemical action are those of con- 
 tinual percolation by waters carrying the organic acids already 
 described. It naturally follows, therefore, that a purely chemi- 
 cal decay will progress more rapidly where the rock mass is 
 covered by such a layer of vegetable soil as shall give rise to 
 the decomposing solutions. Hence, that such an accumulation 
 having begun, decomposition will keep on at an ever-increasing 
 rate to a depth concerning which we have at present no data 
 for calculation. It must not be too hastily assumed from this 
 that rocks thus protected do in reality break down more rapidly 
 than those exposed on bare hillsides, since here, where physical 
 causes predominate, the loosened particles are removed as fast 
 as formed, and, besides leaving no measure of the destruction 
 going steadily on, new surfaces for attack are being continually 
 
RELATIVE RAPIDITY OF WEATHERING 271 
 
 exposed. Moreover, in assuming that rocks decay rapidly where 
 covered by vegetation, we must not overlook the fact that the 
 character of the overlying soil may be such as to be protective 
 rather than otherwise. Thus in glaciated regions it is a well- 
 known fact that the striae on rock surfaces are found best pre- 
 served where they have been protected from heat and frost 
 by a mantle of drift, or the compact turf so characteristic of 
 the Northern states. (See further under Influence of Forests, 
 p. 280.) 
 
 (5) Relative Rapidity of Weathering among Eruptive and 
 Sedimentary Rocks. As to the relative rapidity of chemical 
 decomposition among eruptive and sedimentary rocks, there 
 can with the exception of the calcareous varieties be no 
 question, the eruptives being far the more susceptible. This 
 for reasons which will be at once apparent when we consider 
 their origin. The eruptive rocks result from the comparatively 
 sudden cooling of magmas originating far below the action of 
 atmospheric agencies, and which are pushed up and allowed to 
 solidify under conditions which are not at all conducive to chemi- 
 cal equilibrium. They are compounds of elements which have 
 combined according to the conditions under which they tempo- 
 rarily existed, but which undergo continual changes as they 
 become exposed by erosion and other causes. They become, in 
 short, out of harmony with their surroundings, and there are at 
 once set up a series of physical and chemical changes such as 
 shall result in products more in harmony with existing condi- 
 tions, and hence more stable. These changes, briefly put, are 
 those involved in the weathering processes we have described. 
 Indeed, we may well say that rock weathering and all the seem- 
 ingly endless processes of rock decay and rock consolidation 
 are but stages in the continual efforts being made by these inor- 
 ganic particles to adjust themselves to existing conditions. But 
 the sedimentary rocks (exclusive of the calcareous varieties) are 
 themselves the actual products of these adjustments. The con- 
 glomerates, sandstones, shales, and argillites are but the detri- 
 tal remains of eruptive rocks which under the various weathering 
 influences have become disintegrated and decomposed, their more 
 soluble constituents quite or in part removed, and the residues 
 laid down and consolidated under conditions such as to-day 
 exist upon or near the surface of the earth. They have, it is 
 true, been laid down under water; they are subaqueous, but 
 
272 TIME CONSIDERATIONS 
 
 their decomposition and disintegration was subaerial. Hence, 
 when elevated above the ocean's level to become a part of the 
 dry land, they are for the most part comparatively stable, sub- 
 ject to only such chemical changes as oxidation, and it may be 
 dehydration. All other things being equal, then, those siliceous 
 rocks which are the product of mechanical sedimentation will be 
 found far less susceptible to the chemical action of the atmos- 
 phere and meteoric waters than are the eruptives. While they 
 may undergo a transformation into soils, it is mainly through 
 the disintegrating effects of heat and frost. Sedentary soils 
 resulting from such disintegration resemble, therefore, their 
 parent rock more than those of any other class. 
 
 Turning now to calcareous rocks, we shall find a quite differ- 
 ent state of affairs prevailing, owing to the different chemical 
 nature of the material and its ready solubility. These rocks 
 represent, in fact, the soluble portions of the eruptive rocks 
 which have been leached out during the process of decomposi- 
 tion. They are themselves solution products, although their 
 immediate deposition may have been brought about through 
 mechanical agencies, as in the laying down of beds of shell 
 marl upon a sea-bottom. The lime leached out of terrestrial 
 rocks is carried in solution into the sea, where, taken up by 
 molluscs and corals as a carbonate, it becomes precipitated to 
 the bottom on their death, and may reappear as a limestone, or, 
 if mixed with sufficient quantities of other constituents, as a 
 marl, calcareous sandstone, or shale. Such on their re-elevation 
 are still subject to chemical change, owing to the ready solu- 
 bility of lime carbonate in terrestrial waters, and so the endless 
 round begins once more. Reference has already been made to 
 the amounts of lime carbonate that may thus be annually re- 
 moved from the earth's surface, but one may add here, that, 
 according to J. G. Goodchild, certain English limestones waste 
 away, superficially, at the rate of one inch in 300 years. 1 
 
 (6) Time Limit of Decay. We are sometimes enabled to 
 put a time limit on the beginnings of decomposition such as 
 shall enable us to gain at least a geological measure of the 
 rapidity of the process. This is the case with the disintegrated 
 granite of the District of Columbia described on p. 206. The 
 residual material is here now overlaid by clastic deposits of such 
 a nature as to force the conclusion that they were laid down by 
 1 Geological Magazine, 1890, p. 463. 
 
TIME LIMIT OF DECAY 273 
 
 water under such conditions as would have thoroughly eroded 
 away all underlying pre-existing decomposed material. It is 
 therefore inferred that this decomposition has taken place since 
 the clastic material was deposited, or, since these are of Creta- 
 ceous age, that it has taken place since the close of Cretaceous 
 times. In the same way, since glaciation must have carried 
 away the pre-existing disintegrated matter from the dike of 
 diabase at Medford, leaving the surface smooth and hard, so 
 here it is inferred that the decomposition is post-glacial. It is 
 but rarely that the rate of decomposition of any rock has been 
 sufficiently rapid since the beginning of human history, to be 
 of geological significance, though weathered surfaces in old 
 quarries, or the walls of old buildings, not infrequently offer 
 abundant illustration of what we might expect, could observa- 
 tion be extended over whole geological periods instead of at 
 most a few years. We must not forget, however, that, in the 
 latter case, the conditions are quite different from those exist- 
 ing in nature, and the rate of weathering may be accelerated or 
 retarded, as the case may be. 
 
 Stone implements, made by prehistoric man, as now found 
 in graves, or dug from the soil, sometimes show incipient signs 
 of decomposition, as indicated, when broken across, by a change 
 in color and texture from without inward. Flint arrow and 
 spear-heads from prehistoric caves or mounds in Europe, 
 England, or America, often present on the outer surface a thin 
 crust or patine of a gray or white color extending inward, it 
 may be, for the distance of two or more millimeters. A grooved 
 stone axe of diorite found in eastern Massachusetts and now in 
 the collections of the National Museum at Washington, 1 shows 
 concentric exfoliation in every way comparable to that on the 
 diabase boulder figured on PL 20, extending inward to a depth 
 of from three to six millimetres. It is of course possible that 
 the axe was made from a boulder, itself not quite fresh, but this 
 seems scarcely probable, and the inference is fair that both the 
 patine and the exfoliation are due wholly to weathering sub- 
 sequent to the manufacture of the implements on which they 
 occur. 
 
 Mills 2 regards the extreme condition of decomposition exist- 
 ing in the Archrean rocks of Brazil as having taken place prior 
 
 1 Specimen No. 172,794, Archaeological Series. 
 
 2 American Geologist, June, 1889, p. 345. 
 
274 
 
 TIME CONSIDERATIONS 
 
 to the deposition of the loess, that is, in the long interval between 
 the elevation of the Archaean rocks and the beginning of Qua- 
 ternary times. Inasmuch, however, as the Quaternary gravels 
 and loess are all readily permeable by water and not of a nature 
 to be themselves readily affected, it would seem possible that 
 at least a portion of the decomposition might have been brought 
 about since their deposition and, indeed, to be still in progress. 
 The writer is informed by Mr. W. Lindgren that the granitic 
 diorites of the Sierra Nevadas of California, and which are of 
 
 FIG. 22. Flint implement showing weathered surface. 
 
 late Jurassic or early Cretaceous age, are often decomposed and 
 disintegrated to a maximum depth of 200 feet, the extreme 
 upper, more superficial portions being reduced to the condition 
 of a red clay, while the lower are merely rendered soft and 
 friable, with little if any change in color. This disintegration 
 has gone on to such an extent that where the rock is traversed, 
 as is sometimes the case, by numerous gold-bearing quartz veins, 
 the entire mass of material is washed down by water hydrau- 
 licked as in the ordinary process of placer mining. The 
 Pliocene andesites are also in places, decomposed to a depth of 
 
TIME LIMIT OF DECAY 275 
 
 20 feet. The region is one of heavy annual precipitation, but 
 the rainfall is limited almost wholly to the winter season. 
 
 Rock disintegration and decomposition, after the manner 
 already described, has been by no means limited to the present 
 era, but has been going on since the first land appeared above 
 the surface of the primeval ocean. The results of the recent 
 decomposition are more apparent, since the derived materials are 
 still recognizable as rock debris, while that formed in past ages 
 may have been so changed by the solvent and assorting power 
 of water, the chemical action of the atmosphere, and the general 
 agents of metamorphism, as to have quite lost its identity. 
 
 Dr. R. Bell, of the Canadian Geological Survey, has described * 
 an interesting illustration of pre-Palseozoic decay in the crystal- 
 line rocks north of Lake Huron. The red granite, where it has 
 been protected from glacial action, is found to be eaten into 
 hollows in the form of round and sack-like pits and small 
 caverns, the last-named generally occurring on steep slopes or 
 perpendicular faces of the rock. These pits are, in places, of 
 sufficient size to allow two men to crouch within. The sack- 
 like ovens, such as are shown in Fig. 23, are most usually on 
 sloping surfaces. The granite around these pits shows no in- 
 dications of decay. That they are of pre-Palseozoic origin is 
 demonstrated by the presence in them of residual patches, in 
 situ, of the fossiliferous Black River limestone and which Pro- 
 fessor Bell regards as veritable inliers of the Black River forma- 
 tion, which once filled all the inequalities and still overlies the 
 granite at lower levels, though elsewhere almost wholly removed 
 by erosion. Figure 23, after Bell, shows diagrammatically the 
 old granitic corroded floor up on which the calcareous sediments 
 were laid down, with pits still containing residual masses of the 
 limestone, and the intact beds passing under the waters of Lake 
 Huron at the lower right. 
 
 Pumpelly, too, has shown 2 that the diabase dike at Stamford, 
 Massachusetts, had undergone extensive decomposition prior 
 to the deposition of the Cambrian conglomerates. Of equal 
 interest and still greater economic importance was the sugges- 
 tion by this same authority, subsequently abundantly confirmed 
 by W. B. Potter, 3 that beds of iron ore lying on the western 
 
 1 Bull. Geol. Soc. of America, Vol. V, 1894, pp. 35-37. 
 
 2 Ibid., Vol. II, 1891, p. 209. 
 
 3 Jour. U. S. Assoc. Charcoal Iron Workers, Vol. VI, p. 23. 
 
276 TIME CONSIDEKATIONS 
 
 flank of Iron Mountain, Missouri, and covered by Silurian lime- 
 stones, were true detrital deposits resulting from the pre-Silurian 
 breaking down of the ore-bearing porphyry forming the mass 
 of the mountain. These and other 1 illustrations that might be 
 given point unmistakably to the identity of geological processes 
 and correspondence in results since the earliest times, even did 
 not analogy and the thousands of feet of secondary rocks furnish 
 us safe criteria upon which to base our inferences. 
 
 FIG. 23. 
 
 (7) Extent of Weathering. The depth to which weather- 
 ing has penetrated necessarily varies greatly. In cases where 
 the detrital material is removed nearly or quite as rapidly as 
 formed, it may go on indefinitely, until, it may be, thousands 
 of feet of material have melted away ; where, however, remain- 
 ing in place, decomposition must be gradually retarded until a 
 time comes when it practically ceases. In the region about 
 Washington, District of Columbia, the writer has observed the 
 granitic rock so disintegrated at a depth of 80 feet from the 
 present surface as to be readily removed by pick and shovel. 
 Even greater depths have been noted by writers on the geology 
 of our own Southern states and Central and South America. 
 
 !See also T. Sterry Hunt, The Decay of Rocks Geologically Considered, 
 Am. Jour, of Science, Vol. XXVI, 1883, p. 190. 
 
EXTENT OF WEATHERING 277 
 
 Spencer states 1 that in the region about Atlanta, Georgia, the 
 rocks are " completely rotted" to a depth of 95 feet, while 
 ''incipient decay" may reach to a depth of 300 feet. W. B. 
 Potter describes 2 the feldspar porphyry of Iron Mountain in 
 Missouri as decomposed to a visible extent as far into the hill 
 as mining operations had been carried, while to depths varying 
 from 10 to 80 feet the kaolinization is complete. 
 
 The coarse granite of Pike's Peak, Colorado, is reported as 
 disintegrated to a depth of from 20 to 30 feet. Belt 3 describes 
 dolerites in Nicaragua as decomposed, as shown by deep cut- 
 tings in mines, to a depth of 200 feet. " Next the surface," he 
 says, " they were often as soft as alluvial clay, and might be 
 cut with a spade." 
 
 Derby describes 4 certain shales in Rio Grande do Sul, Brazil, 
 as decomposed into the condition of reddish, drab, greenish, 
 black, and umber-colored clays to the depth of 120 metres 
 (394 feet). 
 
 W. H. Fuiionge has described 5 the granite of the Dekaap 
 gold fields, in the Transvaal, South Africa, as decomposed to a 
 depth of 200 feet. Rain erosion has carved out from this 
 decomposed mass deep "dongas," as they are locally called, 
 and which sometimes present most striking and picturesque 
 appearances. 
 
 The apparent depth to which weathering has gone on is 
 often greater among siliceous than calcareous rocks. This is, 
 however, due merely to the facts that (1) the siliceous rocks 
 are composed largely of insoluble materials, and hence leave a 
 proportionately large amount of debris, and (2) that among 
 calcareous rocks the change is mainly chemical and takes place 
 only from the immediate surface. As a result of this, residuary 
 nodules of limestone may be found perfectly fresh and unal- 
 tered at a depth of but a few millimetres below the surface, 
 while granites and allied rocks may show signs of disintegra- 
 tion and incipient decay for many inches, or even feet. 
 
 Pumpelly states 6 that in the Ozark Mountains of Missouri 
 the secular dissolving away of limestones containing from 2 to 
 
 1 Geol. Survey of Georgia, 1893. 
 
 2 Jour. U. S. Assoc. Charcoal Iron Workers, Vol. VI, p. 25. 
 
 3 The Naturalist in Nicaragua, p. 86. 
 
 4 Am. Jour, of Science, February, 1884, p. 138. 
 
 5 Trans. Am. Inst. of Mining Engineers, Vol. XVIII, 1890, p. 337. 
 
 6 Am. Jour, of Science, 1879, p. 136. 
 
278 TIME CONSIDERATIONS 
 
 9 % of insoluble matter has left residual clays from 20 to 120 
 feet in thickness, indicating a removal of not less than 1200 
 vertical feet by solution. According to Whitney, the dark, 
 reddish brown, residual clays of .southern Wisconsin, of an 
 average depth of perhaps 10 feet over the entire area, repre- 
 sent the insoluble accumulations from the decomposition of 
 from 350 to 400 vertical feet of dolomite, limestone and cal- 
 careous shale. 
 
 (8) Relative Rapidity of Weathering in Warm and Cold Cli- 
 mates. For many years an impression has prevailed to the 
 effect that rocks decomposed more rapidly in warm and moist 
 than in cold climates. While, owing to abundance of vegeta- 
 tion and other supposed favorable conditions, a more rapid 
 decomposition might be expected, such has not as yet been 
 proven to actually take place, and indeed many facts tend to 
 prove the impression quite erroneous. Lack of decomposition 
 products in high latitudes is not infrequently due to glaciation 
 or erosion by other means. Whitney, 1 Irving, 2 Chamberlain, 
 and Salisbury 3 have shown the presence of residual clays of 
 all thicknesses up to 25 feet in the driftless area of Wiscon- 
 sin, and Chamberlain has described 4 limited areas of strongly 
 decomposed gneiss in the non-glacial areas of Greenland. 
 
 Moreover, we have no actual proof that the action of frost 
 is, on the whole, protective, as is stated by Branner. 5 It must 
 be remembered that frost, excepting in the extreme north, 
 penetrates to but a slight depth, and while it undoubtedly puts 
 a temporary stop to chemical action on the immediate surface, 
 it remains yet to be shown that the mechanical disruption that 
 ensues, and as described in previous pages, is not as efficacious 
 as would have been the chemical agencies alone, had they been 
 permitted to continue their work. Through bringing about a 
 finely fissile or pulverulent structure, whereby a vastly greater 
 amount of surface becomes exposed, frost prepares the way for 
 chemical action at a thousand-fold more rapid rate than could 
 otherwise have been possible. If, further, as the writer has 
 elsewhere at least suggested, 6 hydration is the most potent 
 
 1 Rep. Geol. Survey of Wisconsin, 1861. 
 
 2 Trans. Wisconsin Acad. of Science, Vol. Ill, 1875. 
 
 3 Ann. Rep. U. S. Geol. Survey, 1884-85, p. 254. 
 
 4 Bull. Geol. Soc. of America, Vol. VI, 1895, p. 218. 
 
 6 Bull. Geol. Soc. of America, Vol. VII, 1896, p. 282. 
 6 Bull. Geol. Soc. of America, Vol. VI, p. 331. 
 
RELATIVE RAPIDITY OF WEATHERING 279 
 
 factor in rock disintegration, the process can go on uninter- 
 ruptedly below the level of freezing. 
 
 Professor H. P. Gushing has described 1 the argillites in the 
 vicinity of Glacial Bay, Alaska, as in a condition of great dis- 
 integration, wholly through the action of frost. " Disintegra- 
 tion," he says, "takes place with amazing rapidity, as shown 
 by the enormous piles of morainic matter furnished to the tribu- 
 taries of Muir Glacier, whose valleys are adjoined by mountains 
 of argillite, and by the massive talus heaps that are rapidly 
 accumulating at the bases of other mountains made up of the 
 same material." In a private communication to the present 
 writer, he further states that the diabases of the region are 
 fully as much decomposed as are those in the Adirondacks of 
 New York, and that the blocks of eruptive rocks occurring in 
 the moraines of Muir Glacier are far gone in decomposition. 
 
 Mr. C. W. Purrington has made similar observations, and 
 states 2 that on the south side of Silver Bow Basin, some three 
 miles west of Juneau, at an elevation of 2000 feet above sea- 
 level, he found schistose diorites disintegrated over a consider- 
 able area to a depth of 20 feet. The particular locality cited 
 was on a mountain slope, where landslides were frequent, and 
 other conditions prevailed such as would prevent the accumula- 
 tion of the debris throughout a prolonged geological period or 
 to a very great depth. There could be, however, no doubt as 
 to the residuary character of the material observed, and the 
 inference drawn was to the effect that the disintegration had 
 taken place within a comparatively brief space of time. G. E. 
 Culver has also described 3 a diabase dike in Minnehaha County, 
 South Dakota, an arid region lying within the 'glacial area, as 
 decomposed throughout the whole exposures from its upper 
 surface down to a depth of 20 or 25 feet, the limit of disinte- 
 gration being the drainage level of the region as marked by 
 the bed of a stream cutting through it. 
 
 On the other hand, Professor I. C. Russell, who has devoted 
 much attention to the subject of rock-weathering in both high 
 and low latitudes, is of the opinion that rock decay is a direct 
 result of existing climatic conditions. He states further that 
 decay goes on most rapidly in warm regions where there is an 
 
 1 Trans. N. Y. Academy of Science, Vol. XV, 1895. 
 
 2 Personal Memoranda to the writer. 
 
 3 Wisconsin Academy of Sciences, Art, and Literature, 1886-91, p. 206. 
 
280 TIME CONSIDERATIONS 
 
 abundant rainfall, and is scarcely at all manifest in arid and 
 frigid regions. 1 Professor Russell's observations are of more 
 than ordinary value, since he has discriminated between decay 
 and disintegration, which most writers have failed to do. 
 
 Relative to the subject of rock degeneration in temperate 
 regions, we have further to consider the possible increased 
 amounts of atmospheric gases brought down by snowfalls, over 
 those brought by rain. The siiowflakes, in falling, so com- 
 pletely fill the air as to rob it of a larger proportion of its 
 impurities than would a corresponding amount of precipitation 
 in the form of rain. Further, the snow in melting slowly away 
 affords the water better facilities for soaking into the ground 
 than though it was poured down during the comparatively brief 
 period of a shower. How far these agencies may go toward 
 counterbalancing the effects of the continued higher tempera- 
 tures of the tropics, we have no means of judging. 2 
 
 It is even questionable if decomposition has actually gone on 
 to greater depths in regions covered by forests, as contended 
 by Hartt 3 and Belt 4 than elsewhere. The accumulation of a 
 large amount of organic matter is undoubtedly favorable to 
 decomposition, but the growing vegetation constantly robs the 
 soil beneath of moisture and other elements necessary for its 
 growth, storing it away in the form of woody fibre or sending 
 it off into the atmosphere once more. The amount of moisture 
 that a full-grown tree evaporates daily through its leaves is 
 simply enormous, and is often made conspicuously apparent 
 by the dry knolls that may be seen surrounding isolated trees 
 or groups of trees in swampy areas. Indeed, Mr. R. L. Fulton, 
 in discussing 5 the influence of forests in the mountain regions 
 of the West, states it as his belief that the local springs and 
 streams are " more diminished by the water used by the tree& 
 than by evaporation in their absence." 
 
 It has been shown 6 that the total amount of moisture returned 
 
 1 Surface Geology of Alaska, Bull. Geol. Soc. of America, Vol. I, 1890. 
 
 2 There is an old saying among Eastern farmers to the effect that a late 
 spring snowstorm is as good as a dressing of manure. It undoubtedly arose 
 from an appreciation by the farmers of the fact that the snow was more benefi- 
 cial than rain for the reasons above mentioned. 
 
 3 Physical Geography and Geology of Brazil, 
 
 4 The Naturalist in Nicaragua, p. 86. 
 
 5 Science, April 10, 1896. 
 
 6 See Bull. No. 7, Forestry Division, U. S. Dept. of Agriculture, 1893. 
 
INFLUENCE OF FORESTS 
 
 281 
 
 into the atmosphere from a forest by transpiration and evapora- 
 tion from the trees and underlying soil, is about 75% of the 
 total precipitation. For other forms of vegetation it varies 
 between 70 % and 90 %, the forest as a rule being surpassed by 
 the cereals, while the evaporation from a bare soil is but 30 % 
 of the precipitation. To this should be added the fact that 
 the activity of evaporation from forested areas is continued 
 throughout a longer period of each year, as a rule, than in 
 non-forested, for the simple reason that the grasses and cereals 
 early ripen, and practically cease to exhale altogether. This 
 is particularly the case in cultivated areas and prairie regions. 
 Hence, while the daily evaporation from given areas might for 
 a time be nearly equal, the annual amount is likely to be great- 
 est for that which is forested. 
 
 Further, it has been shown that only 70 % as much rainfall 
 reaches the soil in the woods as in the open fields, the rest 
 being caught in the leaves, branches, and trunks, whence it is 
 returned directly to the atmosphere by evaporation. These 
 percentages naturally vary with the character of the forest 
 growth. In this connection the following table, showing the 
 measured amounts of water at varying depths in a loamy soil 
 under forests of spruce, twenty-five, sixty, and one hundred 
 and twenty years old, and one base of all vegetation, is instruc- 
 tive. It will be observed that the average amount is apprecia- 
 bly greater in the bare soil, and that the least amount is found 
 under forests 60 years old, when we may assume the trees are 
 in their prime. 
 
 WATEU CONTENTS OP A LOAMY SAND; RESULTS BY SEASONS EXPRESSED ix 
 PERCENTAGES OF THE WEIGHT OF THE SOIL 
 
 SEASON 
 
 SPRUCE 
 
 25 YEAKS OLD 
 
 60 YEARS OLD 
 
 16 inch 
 
 32 inch 
 
 Average 
 
 16 inch 
 
 82 inch 
 
 Average 
 
 Winter (January and February) . 
 Spring (March to May) .... 
 Summer (June to August) . . . 
 Fall (September to November) . . 
 
 20.23 
 18.62 
 15.10 
 16.57 
 
 17.00 
 18.02 
 16.22 
 17.57 
 
 18.61 
 18.32 
 15.96 
 17.07 
 
 18.06 
 15.29 
 14.42 
 13.49 
 
 17.76 
 16.28 
 17.03 
 16.52 
 
 17.91 
 15.78 
 15.72 
 15.00 
 
282 
 
 TIME CONSIDERATIONS 
 
 SEASON 
 
 SPRUCE 
 
 NAKED SOIL 
 
 120 YEARS OLD 
 
 16 inch 
 
 32 inch 
 
 Average 
 
 16 inch 
 
 32 inch 
 
 Average 
 
 Winter (January and February) . 
 Spring (March to May) .... 
 Summer (June to August) . . . 
 Fall (September to November) . . 
 
 19.75 
 17.47 
 
 17.78 
 14.88 
 
 22.44 
 20.83 
 20.90 
 19.46 
 
 21.09 
 19.15 
 19.97 
 17.17 
 
 19.96 
 20.66 
 19.77 
 20.04 
 
 24.73 
 20.51 
 19.98 
 20.20 
 
 22.35 
 20.58 
 19.97 
 20.12 
 
 Other experiments have shown a marked difference in the 
 distribution of the water in the forest-covered and naked soils, 
 in the first-named a much larger proportion being held in the 
 extreme upper portion than in that which was unprotected. 
 This is a natural consequence of the absorptive properties of 
 the accumulated humus. The following table, as compiled by 
 Fernow l from the work of Ebermayer, illustrates this point. 
 
 AVERAGE OF WATER CAPACITY, EXPRESSED IN PERCENTAGES OF THE WEIGHT 
 
 OF THE SOIL 
 
 
 
 SPRUCB 
 
 
 
 DEPTH 
 
 25 Years 
 Old 
 
 60 Years 
 Old 
 
 120 Years 
 Old 
 
 SOIL 
 
 to 2 inches 
 
 30 93 % 
 
 29 48 L 
 
 40 32 % 
 
 22 33 / 
 
 6 to 8 inches 
 
 19.19 
 
 18.99 
 
 19.30 
 
 20 62 
 
 12 to 14 inches 
 
 19.10 
 
 16.07 
 
 18 28 
 
 20 54 
 
 19 to 20 inches 
 
 18 40 
 
 16 26 
 
 20 16 
 
 20 14 
 
 30 to 32 inches . 
 
 17 91 
 
 17 88 
 
 21 11 
 
 20 54 
 
 
 
 
 
 
 It is obvious that it is only that portion of the water which 
 passes through this superficial blanket of mould that can be 
 instrumental in promoting rock decomposition. Hence the 
 presence of such a blanket may exert a protective, or at least 
 conservative, rather than destructive action. Further than this, 
 we have to remember that plant growth tends to reduce the 
 extremes of temperature and, even more, to diminish evapora- 
 
 1 Bull. No. 7, Forestry Division, U. S. Dept. of Agriculture, 1893. 
 
WEATHERING IN COLD AND WARM CLIMATES 283 
 
 tioii from the immediate surface. The constant action of grav- 
 ity and capillarity in pumping the water down and up through 
 the soil is therefore largely diminished. Since it is by temper- 
 ature changes and water action that decomposition is so largely 
 brought about, it is apparent that we must not be too hasty in 
 assuming that forest action is actually destructive ; it may be 
 largely conservative. It is possible that the apparent amount 
 of decomposition in wooded areas is due to protection from ero- 
 sion, and the consequent accumulation of the residuary material. 
 More facts are necessary before this question can be decided. 
 
 (9) Difference in Kind of Weathering in Cold and Warm. 
 Climates. That, however, there may be a difference in kind 
 in the degeneration in warm and cold climates, or at least in 
 moist and dry climates, is possible and even probable. 1 In cold 
 and in dry climates subject to extremes of temperature, as in 
 the arctic regions or in the arid regions of lower latitudes, the 
 weathering is at first almost wholly in the nature of disintegra- 
 tion, a process of disaggregation whereby the rock is resolved 
 into, first, a gravel and ultimately a sand composed of the 
 isolated mineral particles which have suffered scarcely at all 
 from decomposition. The writer has elsewhere referred to this 
 form of degeneration as manifested in the desert regions of the 
 Lower California!! peninsula. 2 In a warm, moist climate chem- 
 ical decomposition may or may not keep pace with the disin- 
 tegration, according to local conditions, so that the resultant 
 material may be in the form of an arkose sand, as in the District 
 of Columbia, or a residual clay, as in the more superficial portions 
 of the residual deposits to the southward. In certain cases, or 
 among certain classes of rocks, the decomposition proceeds at so 
 rapid a rate that there is scarcely any apparent preliminary dis- 
 integration. Local circumstances and character of rock masses 
 being the same, we are, however, apparently safe in assuming 
 that in warm and moist climates decomposition follows so closely 
 upon disintegration as to form the more conspicuous feature of 
 the phenomenon, while in dry regions, or those subject to ener- 
 getic frost action, mechanical processes prevail and disintegra- 
 tion exceeds decomposition. 
 
 1 The majority of writers have failed to discriminate between decomposition 
 and disintegration. That there may be a very marked difference, due mainly to 
 climatic conditions, is the point I wish to emphasize here. 
 
 2 Bull. Geol. Soc. of America, Vol. V, 1894, p. 499. 
 
284 TIME CONSIDERATIONS 
 
 Accepting these facts, there is at once suggested the idea 
 that the lithe-logical nature of sedimentary rocks, as well as 
 their fossil contents, may be regarded as indicative of prevalent 
 climatic conditions. 
 
 The possibility of estimating these conditions by the char- 
 acter of the debris resulting from the degeneration of feld- 
 spathic rocks was first suggested by the geologists of the Indian 
 Survey, 1 the undecomposed feldspars in the Panchet (Mesozoic) 
 sandstones being regarded as indicating a recurrence of a cold 
 period during which mechanical forces preponderated over those 
 purely chemical. The same idea was subsequently put forth, 
 quite independently, by the present writer. 2 That rocks in arid 
 regions do actually undergo less decomposition during the 
 weathering processes is shown not only by the fresh character 
 of the residuary material. Judd has shown 3 that rivers like 
 the Nile, draining regions of great aridity, though in a con- 
 dition of high concentration from prolonged evaporation, carry, 
 in solution, smaller proportional amounts of derived salts than 
 do those of humid regions. 
 
 Russell has noted that in the Yukon River region of Alaska 
 disintegration so far exceeds decomposition that the talus from 
 the mountains, composed of loose, angular masses of rock quite 
 free from vegetation, forms what he calls debris streams, which 
 actually creep slowly down the slopes, the movement taking 
 place principally in the winter time and being due apparently 
 to the slow settling, or creep, of deep snows. He states it as 
 his opinion that the mountains of the region have suffered more 
 through this form of disintegration than have those of Colorado 
 or the southern Appalachians, but less than those of the Great 
 Basin area. The range of limestone mountains along the Yukon 
 is pictured as presenting a crest of sharp, blade-like crags, flanked 
 by vast slopes of loose, angular stones on either side, the rock 
 being everywhere fresh and undecomposed, but badly shattered 
 and fissured. 
 
 (10) Relative Amount of Material lost. Other things being 
 equal, it is also safe to infer that more material has actually 
 been lost through disintegration and decomposition in moun- 
 
 1 Geol. of India, 2d ed., Vol. I, p. 201. 
 
 2 Bull. Geol. Soc. of America, Vol. VII, p. 362. 
 
 3 Report on Deposits of the Nile Delta, Proc. Royal Society of London, Vol. 
 XXXIX, 1885. 
 
PLATE 22 
 
 FIG. 1. Forest destroyed by wind-blown sand. 
 
 FIG. 2. Calcareous conglomerate carved and polished by wind-blown sand. 
 
 FIG. 3. Rock being undermined by wind-blown sand. 
 
RELATIVE AMOUNT OF MATERIAL LOST 285 
 
 tainous and hilly countries than from the level plains. This 
 for the reasons that (1) through the upturning of the beds there 
 were exposed, it may be, friable and soluble strata that might 
 otherwise have been protected, and (2) that through the shat- 
 tering incident to this upturning the rocks were rendered more 
 susceptible to the weathering forces. Further, (3) the steeper 
 slopes in mountain regions promote more rapid removal of the 
 resultant debris, whereby fresh surfaces are continually exposed, 
 such as might otherwise shortly become protected through its 
 accumulation, as above noted. 
 
PART IV 
 
 THE TRANSPORTATION AND RBDBPOSITION OP 
 ROCK DEBRIS 
 
 IT rarely happens that more than a comparatively small pro- 
 portion of the products of disintegration and decomposition are 
 left to accumulate on the site of the parent rock. In most 
 instances a very considerable proportion, in some instances all, 
 the debris is removed immediately, or soon after its formation, 
 and deposited elsewhere. A portion of this material is removed 
 in solution, as has already been described (ante, p. 194). A 
 still larger portion is transported mechanically, and it is to a 
 discussion of the method of this transportation that a few pages 
 may now be devoted with profit. 
 
 The chief agencies involved in this transportation are grav- 
 ity, water, in either a solid or liquid form, and the wind. Un- 
 doubtedly the major part of the work is done by water, but as 
 the wind's action is so frequently overlooked, and as, moreover, 
 the results thus produced are of more than ordinary interest 
 from our present standpoint, it may perhaps be well to dwell 
 upon this branch of the subject with considerable detail. 
 
 (1) Action of Gravity. Gravity, especially when aided by 
 the lifting power of frost, may locally exert no insignificant 
 influence. The tremendous power of landslides, or avalanches, 
 have, owing to their devastating effects, been impressed upon 
 us from the beginnings of written history. There are, how- 
 ever, other results, due to similar causes, but which, going on 
 on an almost microscopic scale, are wholly overlooked by the 
 ordinary observer, and the full meaning of which can be dis- 
 covered only when the results of years are taken into account. 
 Professor W. C. Kerr, in 1881, described 1 the manner in which 
 the superficial cap of soil from the decomposition of micaceous 
 
 1 Am. Jour, of Science, 3d Series, Vol. XXI, p. 345. 
 286 
 
ACTION OF WATER AND ICE 287 
 
 and hornblendic gneisses near Philadelphia had crept down 
 the inclined surface on which it rested, and the gradual attenu- 
 ation of the bands of variously colored debris of Avhich it was 
 composed. This creeping process he ascribed wholly to the 
 expansive action of included water passing into the condition 
 of ice, the expansion taking place laterally and the material 
 being pushed down the slope along the line of least resistance. 
 Mr. C. Davidson has since taken up the subject experimentally 
 
 FIG. 2-L Showing direction and rate of motion of soil ; the arrows showing, by 
 their relative lengths, the rate of movement at various points, a, soil; b, bed- 
 rock. 
 
 and shown that the amount of the creeping could be accounted 
 for by the ordinary laws of gravity, the frost, by its expansion, 
 raising the individual particles a slight distance, and, on thaw- 
 ing, allowing them to drop back again a greater or less distance 
 down the slope, according to the angle of inclination. Dr. 
 Milton Whitney has, however, shown 1 that there is an almost 
 continual movement among soil particles, dependent upon 
 meteorological conditions quite aside from those involved in 
 freezing and thawing. The creeping appears therefore to be 
 but the manifestation, in mass, of the inclination of each indi- 
 vidual particle to slide down the slope. 
 
 The accumulations of talus at the foot of every cliff and on 
 the slopes of hills and mountains are matters of such every -day 
 observation as to need no mention in detail. 
 
 (2) The Action of Water and Ice. 2 The power of a stream 
 to transport rock debris depends naturally upon its volume 
 and the rapidity of its current. This, on the supposition that 
 the character of the sediment to be transported remains the 
 
 1 Some Physical Properties of Soils, Bull. No. 4, U. S. Weather Bureau, 1892. 
 
 2 Students are referred to Professor R. D. Salisbury's article on Agencies 
 which transport Material on the Earth's Surface, Journal of Geology, Vol. Ill, 
 1895, p. 70. 
 
288 TRANSPORTATION AND REDEPOSITION OF ROCK DEBRIS 
 
 same. According to the calculations of Hopkins, as quoted by 
 Geikie, 1 the capacity of transport increases as the sixth power 
 of the velocity of the current ; that is to say, the motor power 
 is increased sixty-four times, by doubling the velocity. The 
 following table is taken from the work quoted as showing the 
 power of transport of river currents of varying velocities: 
 
 INCHES MILES 
 
 PER SECOND PER HOUR 
 
 3 0.170 : will just move fine clay. 
 
 6 0.240 : will lift fine sand. 
 
 8 0.4545: will lift sand as coarse as linseed. 
 
 12 0.6819 : will sweep along fine gravel. 
 
 24 1.3638 : will roll along rounded pebbles 1 inch in diameter. 
 
 36 2.045 : will sweep along slippery, angular stones of the size of an 
 egg. 
 
 There are, of course, other factors that should be taken into 
 consideration, such as the character of a river bed, the density 
 of the water, etc., but which lack of space prevents our touch- 
 ing upon here, and which are, moreover, sufficiently enlarged 
 upon in other works. 
 
 The writer has stood at the head waters of the Missouri, and 
 seen the Jefferson, Madison, and Gallatin rivers uniting their 
 floods to form one grand rushing stream of clear green water, 
 full of trout and grayling. He has seen it again at Mahdan, 
 Dakota, a sluggish stream actually yellow with suspended silt. 
 At St. Louis, one beholds it a mighty torrent, whirling along 
 trunks and stumps of trees, twigs, and all manner of organic 
 debris and inorganic detritus picked up from its banks, or 
 washed in by rains and tributary streams, till, one vast sea of 
 liquid mud, it pours every year into the Gulf of Mexico a mass 
 of sediment equal to 812,500,000,000,000 pounds (7,468,694,400 
 cubic feet), or enough to cover a square mile of territory to 
 a depth of 268 feet. But only a portion of the detritus car- 
 ried by running streams reaches the ocean ; otherwise we need 
 devote little time here to its consideration. Nearly all streams, 
 in some part of their courses, flow through level plains with low 
 banks which are subject to inundation during seasons of high 
 water. Picture a muddy stream such as is shown in cross-sec- 
 tion in Fig. 25, and which at ordinary periods is confined 
 within the narrow channel near the centre. In time of freshet, 
 however, the volume of water is so greatly augmented as to 
 
 1 Text-book of Geology, 3d ed. 
 
ACTION OF WATER AND ICE 289 
 
 cause it to overflow its banks and spread out over the plains on 
 either hand. But 110 sooner does the water leave the channel 
 than the force of its currents becomes checked, its carrying 
 power lessened, and it therefore begins to deposit its load of 
 silt upon this flood plain, as it is called, where it remains to 
 permanently enrich the land when the waters subside. It is to 
 such processes of formation that we owe some of the most fer- 
 tile lands in existence, as the valley of the Mississippi, that of 
 the Red River of the North, the Nile, and scores of others that 
 might be mentioned readily attest. 1 
 
 FIG. 25. 
 
 To the same process, coupled with the accumulation of 
 organic matter, we owe the filling in and gradual extinction of 
 thousands of glacial lakes throughout New England and the 
 North, and the formation of rich, flat-bottomed valleys known 
 locally as meadows, swales, and bogs. 
 
 Ice in the form of glaciers is an efficient agent for transpor- 
 tation as well as for erosion, as already noted. While the work 
 being done by existing glaciers may seem comparatively insig- 
 nificant, that done by the ice sheet of the glacial epoch was by 
 no means so, and deserves a more than passing notice. The 
 manner in which the ice carries and deposits its load has already 
 received attention in speaking of its erosive power, and but 
 little more need be said on the subject. That material which 
 existed in a loose, unconsolidated condition, on the surfaces on 
 
 1 The Arkansas River is stated by Owen (Geol. of Arkansas, 2d Rep., 1860, 
 p. 52) to be at certain seasons of the year almost blood-red from the quantity of 
 suspended fine ferruginous clay and saliferous silt brought down from the regions 
 of ferruginous shales, which prevail in the Cherokee County, through which the 
 river flows. This material, deposited along the banks and in the eddies of still 
 water, produces the celebrated red buckshot land. Material washed from the 
 bluffs of argillaceous shell marl, near the confines of Jefferson and Pulaski 
 counties, is deposited again farther down the stream as a fine silt, imparting, 
 like the red silt, extraordinary fertilizing properties to the soil. 
 
290 TRANSPORTATION AND REDEPOSITION OF ROCK DEBRIS 
 
 which the glacier formed, was pushed and dragged along by 
 the onward movement of the ice, which in extreme cases may 
 have exerted a pressure of 200,000 pounds to the square foot. 
 On the final retreat of the glacier, this was left in the form of a 
 compact structureless mass of almost stony hardness, commonly 
 known as till or ground moraine, Materials falling upon the 
 surface from greater heights were likewise transported, so long 
 as the ice sheet continued to advance, and finally deposited in 
 the form of terminal or frontal moraines. 
 
 Inasmuch as the ice sheet was almost continually melting 
 upon its surface, it is practically impossible to consider its 
 action wholly independent of that of water also. Thus, 
 streams resulting from such melting would gradually wear 
 channels in the ice, as on the land. In these channels would 
 accumulate sand and boulders of such size and weight as to 
 resist the current, and such accumulations would, on the final 
 melting of the sheet, be deposited on the surface of the ground 
 in the form of ridges known as eskers, or osars. Other forms 
 of water action on the materials of the ice sheet, are hillocks 
 of stratified sand and gravel deposited near the terminal mo- 
 raines, and known as kames. Since during the advancing of 
 this ice sheet existing rivers flowing eastward must have been 
 dammed, we can safely imagine the formation of large tempo- 
 rary lakes, on the bottom of which would be deposited the 
 glacial silt, like the so-called loess of the Mississippi valley. 
 Lake Agassiz, a glacial lake of this type, is supposed to have 
 occupied an area of more than 100,000 square miles in north- 
 western Minnesota, northeastern Dakota, and a considerable 
 portion of Manitoba. On the bottom of this lake there was 
 deposited during the comparatively brief time of its existence, 
 silt to a depth as yet undetermined, but known to be at least 
 100 feet. 1 
 
 Waters issuing from the melting ice sheet tend to reassert the 
 material of the terminal moraine, redepositing it in approxi- 
 mately concentric zones beyond its margin. These deposits 
 are naturally thicker and coarser near the moraine and thinner 
 and finer at increasing distances. Their form and mode of 
 occurrence is such as to have suggested for them the name of 
 glacio-fluvial aprons, or frontal aprons. Their materials are 
 nearly always loose sands and gravels, the lithological nature 
 1 Ice Age in North America, by G. F. Wright, p. 355. 
 
ACTION OF WATER AND ICE 291 
 
 of the individual particles being of course dependent upon that 
 of the moraines from which they are derived. 
 
 The effects upon the landscapes of this ice sheet have been 
 lasting and peculiar. We may safely imagine that, before the 
 ice invasion, the surface was covered with decayed and softened 
 materials like the residual soils of our Southern states, and 
 which had been cut up into valleys and intervening ridges by 
 the stream of that time. The ice sheet stripped from these 
 surfaces their mantle of decomposed materials, and in addi- 
 tion cut, in many cases, into the fresh rock, actually planing 
 the entire country so deeply that in most cases the preglacial 
 surface is no longer recognizable. The hills were thus lowered 
 and the valleys in some cases deepened or again filled by sand 
 and gravel. Since a protruding rock mass would, from neces- 
 sity, be most eroded on the side from whence the ice sheet 
 approached, and since, moreover, such would serve to catch 
 and hold back a part of the loose earth and stony matter 
 brought from the north, a peculiar feature in the topography 
 of glaciated hills has been brought about as shown in Fig. 2, 
 PI. 25. 
 
 The direction taken by this drift material was quite variable. 
 It was, as a rule, from the north toward the south, with many 
 minor deflections. Boulders of Laurentian rocks north of Lake 
 Huron are abundant in the drift about Oberlin, Ohio, and even 
 further south. Boulders of native copper from the Lake Supe- 
 rior region are found even as far south as Kankakee, Illinois, 
 and a large boulder of a peculiar conglomerate known in place 
 only near Ontario, has been found a few miles south of the 
 Ohio River in Kentucky. Dawson states " that boulders from 
 the Laurentian axis of the continent, which stretches from 
 Lake Superior northward to the west of Hudson Bay, have 
 been transported westward a distance of 700 miles, and left 
 upon the flanks of the Rocky Mountains at an elevation of 
 something over 4000 feet." 1 
 
 All over the states once occupied by this ice sheet the ma- 
 terial originating from the decomposition of rocks in situ, or 
 deposited on alluvial plains, was, with a few minor exceptions, 
 carried away to the southward and in part dumped into the 
 Atlantic, while its place was supplied by mongrel hordes from 
 the north. In process of digging for the foundations of the 
 1 Ice Age in North America, p. 171. 
 
292 TRANSPORTATION AND REDEPOSITION OF ROCK DEBRIS 
 
 Maine Experiment Station at Orono, the fresh and highly 
 polished slaty rock was found but a few feet below the sur- 
 face, proving incontestably that, with the exception of the 
 small amount of organic matter that had since been added, 
 not an ounce of the soil was truly native, but all of foreign 
 birth, and a mongrel creature of compulsory migration. We 
 shall dwell more fully upon the character and distribution of 
 these soils later. The single illustration above given will 
 answer present purposes. 
 
 In a less degree the ice along the shores of lakes and rivers 
 may exert a transporting influence. Thus the ice first formed 
 along the shores encloses sundry pebbles, boulders, and sand. 
 Through the expansion force of the freezing water as the entire 
 surface becomes frozen over, this shore ice, together with its 
 enclosures, may be pushed up some distance beyond the water 
 line, where the included debris is deposited on melting. Or, 
 on the breaking up of the ice in the spring, the shore ice may 
 be drifted to other parts of the lake, or down the stream, per- 
 haps for miles before melting sufficiently to cause it to deposit 
 its load. 
 
 (3) Action of Wind. 1 While abrasion by the wind is im- 
 possible without transportation, the converse is by no means 
 true ; indeed it is as an agent of transportation for rock detri- 
 tus, without appreciable abrasion, that the wind accomplishes 
 its greatest work, though in like manner this phase is most 
 manifest in arid regions. 
 
 It is stated by Darwin that for several months of the year 
 large quantities of dust are blown from the northwestern shores 
 of Africa into the Atlantic over a space some 1600 miles in 
 width and for a distance of from 300 to 600 and even 1000 
 miles from the coast. During a stay of three weeks at St. Jago 
 in the Cape Verde Archipelago, this authority found the atmos- 
 phere almost always hazy from the extremely fine dust coming 
 from Africa and falling upon the land and water for miles 
 around. So abundant was this dust that a distance of between 
 300 and 400 miles from the coast the water was distinctly 
 colored by it. In the arid lands of Central Asia the air is also 
 reported as often laden with fine detritus which drifts like snow 
 around conspicuous objects and tends to bury them in a dust 
 
 1 See article on Erosion performed by the Wind, by Professor J. A. Udden, 
 Journal of Geology, Vol. II, 1894, p. 318. 
 
ACTION OF WIND 293 
 
 drift. Even when there is no apparent wind, the air is described 
 as often thick with fine dust, and a yellow sediment covers 
 everything. In Khotan this dust sometimes so obscures the 
 sun that even at midday one cannot see to read fine print with- 
 out the aid of a lamp. The tales of the overwhelming of trav- 
 ellers and entire caravans by sand storms in the Great Desert 
 of Sahara are familiar to every schoolboy. Greatly exagger- 
 ated though these may be, the accounts of Layard and of 
 Loftus show us that the sand storms which are of frequent 
 occurrence during the early part of summer throughout Meso- 
 potamia, Babylonia, and Susiana are by no means of insignifi- 
 cant proportions. Layard states that during the progress of 
 the excavations at Nimrud, whirlwinds of short duration but 
 almost inconceivable violence would suddenly arise and sweep 
 across the face of the country, carrying along with them clouds 
 of dust and sand. Almost utter darkness prevailed during 
 their passage, and nothing could resist their force ; the Arabs 
 would cease their work and crouch in the trenches almost suf- 
 focated and blinded by the dense cloud of fine dust and sand 
 which nothing could exclude. 
 
 The accounts of Loftus are equally impressive. Describing 
 their departure from Warka to Sinkara, he says: "A furious 
 squall arose from the southeast and completely enveloped us 
 in a tornado of sand, rendering it impossible to see within a 
 few paces. Tellig and his camels were as invisible as though 
 they were miles distant. A continuous stream of the finest sand 
 drove directly in our faces, filling the eyes, ears, nose, and mouth 
 with its penetrating particles, drying up the moisture of the 
 tongue, and choking the action of the lungs." With such 
 descriptions before one it is not difficult to believe that these 
 ruined cities have in the course of centuries been completely 
 hidden and their sites obscured by mounds of wind-drifted 
 sand and dust. 
 
 We need not, however, confine ourselves wholly to the Old 
 World for illustrations. Not longer ago than May of 1889 a 
 dry southwesterly wind which for several days had prevailed 
 in various parts of the Northwest, particularly in Dakota, cul- 
 minated in a storm peculiarly suggestive from a geological 
 standpoint. It is stated 1 that during the prevalence of this 
 wind, on the 6th and 7th of the month mentioned, the air be- 
 1 American Geologist, June, 1889, p. 398. 
 
294 TRANSPORTATION AND REDEFOSITION OF ROCK DEBRIS 
 
 came filled with flying particles caught up from the ploughed 
 fields, fire-blackened prairies, public roads, and sandy plains. 
 The particles formed dense clouds and rendered it as impos- 
 sible to withstand the blast as it is to resist the blizzard 
 which carries snow in winter over the same region. The soil 
 to a depth of 4 or 5 inches in some places was torn up and 
 scattered in all directions. Drifts of sand were formed in 
 favorable places, several feet deep, packed precisely as snow- 
 drifts are packed by a blizzard. It seemed as if there were 
 great sheets of dust and dirt blown recklessly in mid air, and 
 when the wind died down for a few moments, the dirt, fine 
 and white, appeared to lie in layers in the atmosphere, clouding 
 the sun and hiding it entirely from sight for an hour or more 
 at a time. (See also on p. 184.) 
 
 Over the wide, dry, and bare flat-topped terraces of the upper 
 Madison valley the wind sweeps in a strong steady current 
 for days together, or during the heated portion of the year, 
 when the sun pours from a cloudless sky its hottest rays upon 
 the parched soil, starts up spasmodically here and there in the 
 form of small whirlwinds made visible by the dust they carry, 
 and which wander spectre-like across the plain to noiselessly 
 disappear in the distant mid air. 
 
 Dust columns of this nature are common in all arid regions, 
 and doubtless have been observed by the many who have 
 crossed the Humboldt desert in Nevada. Seated comfortably 
 in a Pullman car on the Union Pacific, one may not infrequently 
 see at a single view not less than a half dozen of these geologi- 
 cal spectres, each in the distance doing its apportioned task 
 and silently disappearing, laying down its load of sand as its 
 strength gives out and leaving it for its successor. 1 
 
 Under proper conditions such of these wind-blown sands as 
 are too heavy to be carried into the air as dust accumulate 
 upon the surface in the form of drifts, or dunes, all lying with 
 their longer axes approximately at right angles with the pre- 
 vailing currents. Excepting during periods of calm, such are 
 in a state of almost constant, though it may be imperceptible, 
 motion, ever changing their shapes and moving onward like 
 long parallel drifts of snow. The rate of motion of a dune 
 
 1 Professor J. A. Udden estimates that the dust in a cubic mile of lower air 
 during a dry storm weighs not less than 225 tons, while in severe storms it may 
 reach 126,000 tons (Popular Science Monthly, September, 1886) . 
 
ACTION OF WIND 295 
 
 from necessity is governed by the strength and constancy of 
 the winds, and the fineness and dryness of the sand. Urged 
 into temporary activity, each little grain goes scurrying up the 
 slope, across the crest, and tumbles to rest in the steeper 
 declivity upon the leeward side, to be slowly buried by those 
 which follow. This is the sum total of the movement taking 
 place in the march of a dune, whatever its pace and however 
 great its bulk. Yet in this very faculty of moving itself for- 
 ward by but a ten billionth part of its bulk at a time lies the 
 whole secret of its power. Silently, imperceptibly it may be 
 except when measured by months and perhaps years of time, 
 retarded by no walls nor ordinary declivities, it relentlessly 
 performs its task. 1 
 
 A writer in one of the recent popular magazines estimates 
 the dunes of Hatteras and Henlopen as in some cases upwards 
 of 70 feet in height and moving at least 50 feet a year. Swamps 
 have thus been filled, forests and houses buried, and it is stated 
 that but a few years can elapse before the entire island lying 
 north of Cape Hatteras will be rendered uninhabitable. The 
 sand dunes on the coast of Prussia commenced not over a 
 century ago, and already fields and villages have been buried 
 and valuable forests laid waste by them. In one instance a 
 tall pine forest covering many hundred acres was destroyed 
 during the brief period intervening between 1804 and 1827. 
 Loftus, writing of Niliyga, an old Arab town a few miles east 
 of the ruins of Babylon, says that in 1848 the sand began to 
 accumulate about it, and in six years the desert within a radius 
 of six miles was covered with little undulating domes, while 
 the ruins of the city were so buried that it is now impossible 
 to trace their original form and extent. A still more striking 
 illustration of the rapidity of these sand accumulations is offered 
 by the same authority in describing the burial customs of some 
 of these ancient people, it being stated that the earthen coffins 
 were merely stacked in layers one on top of another, and left 
 thus to be covered by the finer sand sifted over them by the 
 winds from the desert. Even Nineveh, founded some twenty 
 centuries before Christ and destroyed 1400 years later, became 
 so covered by drifted sands that at the time of the Greek 
 Xenophon (about 400 B.C.) the very site of the once famous 
 
 i The Wind as a Factor in Geology, Engineering Magazine, 1892, p. 596. 
 
296 TRANSPORTATION AND REDEPOSITION OF ROCK DEBRIS 
 
 city was unknown. Marsh 1 gives the rate of movement of dunes 
 along the western coast of Jutland and Schleswig-Holstein as 
 averaging 13 J feet a year, while Anderson estimates the aver- 
 age depth of the sand over the entire area as about 30 feet, 
 equalling therefore about 1| cubic miles for the total quantity. 
 
 It is not in all cases possible to trace the drifted sands to 
 their various sources. Dunes along the sea-coasts are in nearly 
 all cases composed of materials thrown up by the waves on 
 the beaches in the immediate vicinity. This is the case with 
 those of Hatteras, Cape Cod, Gascony, Algeria, and Schleswig- 
 Holstein. But the origin of the large inland dunes, like those 
 of Nevada, is not always so clear. It has been suggested that 
 these last are formed of beach sand driven in by the prevail- 
 ing westerly winds from the Pacific coast. This is, however, 
 a matter of very grave doubt, and it seems more probable, as 
 stated by geologist Russell, 2 that they were derived from the 
 disintegrating granites of the Sierras. They certainly have 
 travelled far, and are not a product of disintegration of rocks 
 in the immediate vicinity. 3 
 
 By wind action, accompanied by the carrying power of spas- 
 modic or perennial streams, were formed the wide stretches 
 of adobe in the western United States, and according to many 
 authorities the deposits of loess which cover, as in Europe and 
 Asia, areas aggregating many square miles and which have a 
 depth, in extreme cases, of 2000 feet. 4 
 
 The tendency of the wind is not, however, in all cases toward 
 
 1 The Earth as modified by Human Action, p. 562. 
 
 2 Quaternary History of Lake Lahonton, Nevada, Monograph, U. S. Geol. 
 Survey, 1885. 
 
 3 The sands covering the Egyptian Sphinx and Pyramids are stated to have 
 come mainly from the sea on the north, and not from the desert, as is popularly 
 supposed. Sand showers having their origin in the desert of Sahara extend 
 across the Mediterranean, and as far as northern Italy (Nature, July 18, 1889, 
 p. 286). 
 
 4 The wind plays an important part in the transportation of soils in Wyoming, 
 owing to the incoherent state of the soils, due to the lack of clay. The arid 
 regions of this state, which are chiefly Tertiary and Cretaceous plains and table- 
 lands, receive very little rain. Consequently the soils become loosened by great 
 earth cracks, and during the dry and windy winter weather are transported in 
 dense clouds, which almost suffocate travellers, to the broken country and dis- 
 tant hills and mountains. In a single season it is not an uncommon sight to see 
 banks of earth, like huge banks of snow, behind a reef of rock, or in the lee of 
 large bunches of sage brushes (U. S. Dept. of Agriculture, Office of Experiment 
 Stations, Vol. V, No. 6, 1894, p. 567). 
 
ACTION OF WIND 297 
 
 forming drifts and ridges, but at times rather to reduce the land 
 to one general level. Thus J. Flinders Petrie l states that near 
 the ancient cemetery of Tell Nebesheh, on the Isthmus of Suez, 
 the surface of the country has been cut down at the rate of 4 
 inches a century until some 8 feet have been removed from 
 the dry areas and deposited in the intervening depressions, 
 slowly converting the existing lakes into marshes, and the 
 marshes into dry land. An even more rapid change of con- 
 tours is that described by Dwight 2 as having taken place on 
 Cape Cod, Massachusetts. The entire country here is com- 
 posed of sand so susceptible to the drifting action of the wind 
 that it has for years been the custom of the people to sow pines 
 and coarse beach grass to hold it in place. In the instance 
 described by Dwight, however, reckless pasturage had so far 
 destroyed the grass as to lessen its protecting power, and 
 under the strong breezes from the open Atlantic it began to 
 drift rapidly. Over an area of about 1000 acres the sand was 
 blown away to a depth, in many places, of 10 feet. " Nothing," 
 says Dwight, " could exceed the dreariness of this scene. Not a 
 living creature was visible; not a house, nor even a green thing 
 except the whortleberries which tufted a few lonely hillocks 
 rising to the height of the original surface, and prevented by 
 this defence from being blown away also. The impression made 
 by this landscape cannot be realized without experience. It 
 was a compound of wildness, gloom, and solitude. I felt 
 myself transported to the borders of Nubia, and was well 
 prepared to meet the sand columns so forcibly described by 
 Bruce, and after him by Darwin. A troup of Bedouins would 
 have finished the picture, banished every thought of my own 
 country, and set us down in an African waste." 
 
 One more instance of contour changes of this sort must suffice.. 
 It is stated 3 that in Pipestone and Rock counties in Minnesota, 
 the bluffs facing to the westward are, as a rule, more precipi- 
 tous and more rocky than those facing in the opposite direc- 
 tion. This fact is regarded by Professor Winchell as due to 
 the action of the prevailing westerly winds, combined with 
 the drying effects of the southwestern sun in summer. Such 
 winds would uncover and keep bare the coarser materials of 
 
 1 Proc. Royal Geographic Soc., November, 1889, p. 648. 
 
 2 Travels in New England and New York, Vol. Ill, p. 101. 
 
 3 Geol. of Minnesota, Vol. I, p. 575. 
 
298 TRANSPORTATION AND REDEPOSITION OF ROCK DEBRIS 
 
 the western surface by blowing away the sand and clay, while 
 the bluffs on the east are not only protected from the winds, 
 but collect upon their slopes all the flying particles from the 
 prairies above. 
 
 The finely comminuted rock dust blown from volcanic vents 
 is often drifted for long distances by atmospheric currents, and 
 ultimately deposited in beds of no insignificant proportions. 
 Dense clouds of such dust were blown from Icelandic volcanoes 
 to the coast of Norway in 1875, and subsequent to the eruption 
 of Krakatoa (in 1883) the ship Beacomfield of Philadelphia, 
 while at a distance of 831 miles from the source, sailed for three 
 days through clouds of dust which fell upon her decks at the 
 rate of an inch an hour. That such are not or have not in 
 the past been unusual instances is shown by results obtained 
 by the Challenger Expedition, volcanic ashes and sand being 
 repeatedly dredged up from almost abysmal depths at points 
 in the central Pacific far remote from land areas. The day 
 following the explosive eruption of St. Vincent, in 1812, the 
 Barbadoes Island, 80 miles to the windward, was completely 
 shrouded in darkness for many hours, the light of the sun being 
 almost wholly obscured by the cloud of impalpable dust which 
 in the form of a slow, silent rain fell over the whole island. 
 u The trade wind had fallen dead; the everlasting roar of the 
 surf was gone; and the only noise was the crushing of the 
 branches snapped by the weight of the clammy dust. About 
 one o'clock the veil began to lift, a lurid sunlight stared in 
 from the horizon, but all was black overhead. Gradually the 
 dust cloud drifted away; the island saw the sun once more, 
 and saw itself inches deep in black, and in this case fertiliz- 
 ing, dust." J 
 
 1 Kingsley, as quoted by Belt, in The Naturalist in Nicaragua, p. 354. 
 
PART V 
 
 THE RBGOLITH 
 
 THROUGHOUT the millions of years which have elapsed since 
 the earth assumed its present form and essentially solid con- 
 dition, the rocks composing its more superficial portions have 
 been constantly undergoing degeneration in the manner de- 
 scribed, and, in so doing, have given rise to the immense masses 
 of materials which constitute the thousands of feet of secon- 
 dary rocks, and the still unconsolidated sands, gravels, and other 
 products which will be considered in detail later. With those 
 products which have undergone lithification, which are now in 
 the state of consolidation commonly ascribed to rocks by the 
 popular mind, we shall have little more to say. These have 
 already been sufficiently described as rocks in Part II of this 
 work. It is to the most superficial and unconsolidated portion 
 of the earth's crust that we will now devote our attention. 
 
 Let the reader for a moment picture to himself the present 
 condition of this crust, with particular reference to the land 
 areas. Everywhere, with the exception of the comparatively 
 limited portions laid bare by ice or stream erosion, or on the 
 steepest mountain slopes, the underlying rocks are covered by 
 an incoherent mass of varying thickness composed of materials 
 essentially the same as those which make up the rocks them- 
 selves, but in greatly varying conditions of mechanical aggrega- 
 tion and chemical combination. 
 
 In places this covering is made up of material originating 
 through rock-weathering or plant growth in situ. In other 
 instances it is of fragmental and more or less decomposed mat- 
 ter drifted by wind, water, or ice from other sources. This 
 entire mantle of unconsolidated material, whatever its nature 
 or origin, it is proposed to call the regolith, from the Greek 
 words /07?709, meaning a blanket, and Xt#o9, a stone. Within 
 
 299 
 
300 
 
 THE REGOLITH 
 
 certain limits it varies widely in composition and structure, and 
 many names have, on one ground and another, been applied to 
 its local phases, the more important of which are given in tabu- 
 lar form below, and described in detail in the pages following. 
 According to its origin, whether the product of transporting 
 agencies as noted above, or derived from the degeneration of 
 rocks in situ, the regolith is found lying upon a rocky floor of 
 little changed material, or becomes less and less decomposed 
 from the surface downward until it passes by imperceptible 
 gradations into solid rock. 
 
 The 
 regolith 
 
 Sedentary 
 
 f Residual deposits 
 [Cumulose deposits 
 
 Transported 
 
 f Residuary gravels, sands and clays, 
 I wacke, laterite, terra rossa, etc. 
 Peat, muck, and swamp soils, in 
 part. 
 
 f Talus and cliff debris, material of 
 Colluvial deposits { avalanches . 
 
 Alluvial deposits ( Modern alluvium, marsh and swamp 
 (including aqueo- -I (paludal) deposits, the Champlain 
 glacial) ( clays, loess, and adobe, in part. 
 
 f Wind-blown material, sand dunes, 
 JEokan deposits { adobe and loesg) in part> 
 
 f Morainal material, drumlins, es- 
 Glacial deposits { kers , osars , e tc. 
 
 The extreme upper, most superficial portion of this regolith, 
 that which affords food and foothold for plant life, is commonly 
 designated as soil; that immediately underlying the soil, and 
 passing into it by insensible gradations, is known as the sub-soil. 
 This last differs from the soil proper only in degree of compact- 
 ness and in such chemical changes as may have been induced 
 in the soil through growing organisms and more extensive 
 weathering. Indeed, the soil is but derived from the sub-soil, 
 and were it entirely removed, would shortly be replaced through 
 the same agencies as first gave it birth. 
 
 The characteristics of individual soils can be best discussed 
 when speaking of those local phases of the regolith of which 
 they form a part, and with this understanding we will proceed. 
 
 1. SEDENTARY MATERIALS 
 
 Here are to be considered those deposits which, resulting 
 from chemical decomposition or disintegration, from any or all 
 of the processes involved in rock-weathering, or from organic 
 
SEDENTARY MATERIALS 
 
 301 
 
 accumulation, are found to-day occupying their original sites. 
 They are, in fact, the primeval types of nearly all soils and sec- 
 ondary rocks, since those of drift origin are but derived from 
 sedentary materials through the transporting agencies of air 
 and water. They may be 
 conveniently divided into 
 two classes, (1) residual 1 
 and (2) cumulose. 
 
 (1) Residuary Deposits. 
 Under this name, then, are 
 included all those prod- 
 ucts of rock degeneration 
 which are to-day found oc- 
 cupying the sites of the 
 rock masses from which 
 they were derived, and im- 
 mediately overlying such 
 portions as have as yet 
 escaped destruction. The 
 
 name is peculiarly appro- FIG. 26. Showing angular outlines of residuary 
 priate, since thev are actu- Particles from decomposed gneiss. 1, mica ; 
 * . , , J c . , , . , 2, feldspar; 3, quartz. 
 
 ally residues, left behind 
 
 while the more soluble portions have been leached away by 
 
 meteoric waters. 
 
 The residual deposits of North America reach their maximum 
 development in the portion of the United States east of the 
 Mississippi and south of the southern margin of the ice sheet 
 of the Glacial epoch. Their mode of accumulation and gen- 
 eral characteristics have been very thoroughly discussed by 
 Professors I. C. Russell, Chamberlain, and Salisbury, 2 on whose 
 papers we shall draw for some of the facts given here. 
 
 1 Various names have from time to time been proposed for deposits of this 
 nature, but obviously it is impossible to include under a single lithological term 
 materials so widely variable. The term saprolite (from the Greek crairpos, rotten, 
 recently suggested by G. F. Becker, 16th Ann. Rep. II. S. Geol. Survey, Part III, 
 p. 289) is objectionable as conveying the idea of putridity. The old provincial 
 term geest adopted by De Luc, and recently endorsed by McGee (llth Ann. 
 Rep. U. S. Geol. Survey, 1889-90, p. 279), has lost whatever precise meaning it 
 may have had, being defined in both the Standard and Century dictionaries as 
 (1) a bed derived from rock decay in situ, (2) high gravelly land, and (3) gravel 
 or drift. The term gruss, although advocated by some American authorities, is 
 of old German origin and open to the same objection. 
 
 2 Bull. 52, U. S. Geol. Survey and Ann. Rep. U. S. Geol. Survey, 1884-85. 
 
302 THE REGOLITH 
 
 The prevailing characteristic of an old residual deposit, from 
 whatever rock it may be derived, is a ferruginous clay. Exam- 
 ined by a microscope, its mineral particles, when not too thor- 
 oughly decomposed, are found to be sharply angular in outline. 
 With the exception of the quartz, the various mineral constitu- 
 ents are often in an advanced stage of decay, and the more 
 soluble constituents are wholly or partially lacking, having been 
 leached out, in the manner already described. 
 
 Owing to the prevalence of the aluminous constituents, these 
 deposits, when thoroughly decomposed, as on the immediate sur- 
 face, are very tenacious, and may well be termed clays. Their 
 colors are dull, or some shade of brown or red, owing to the 
 higher oxidation and perhaps dehydration of the ferruginous 
 matter set free by the decomposition of the iron-bearing sili- 
 cate constituents. Such in general are the residual soils of the 
 southern Appalachian regions of the United States and which 
 are apparently in every way comparable with the terra rossa of 
 Europe, but only in a slight degree with the laterite of India, 
 to which they have often unfortunately been referred. 1 From 
 a chemical standpoint the soils forming the upper portion of the 
 residuary deposits, though of a prevailing aluminous character, 
 vary widely from the rock masses from whence they were de- 
 rived, much depending upon their age and the amount of actual 
 decomposition and leaching that has taken place. On p. 306 
 are given a few typical but widely varying analyses which will 
 serve to illustrate this point. 
 
 Deposits of this nature are never truly stratified, excepting 
 where, through having remained wholly undisturbed, they re- 
 tain the original structure of the parent rock. (See under 
 Effacement of Original Characteristics, p. 262.) 
 
 The residuary differ from the drift deposits in that they con- 
 tain no materials foreign to their vicinity, but only such more 
 enduring matter as has been handed down to them from the 
 
 1 The term terra rossa, according to Neumayer (Erdgeschichte, Vol. I, p. 
 405) was first applied to the red residual deposits in the Karst maritime lands 
 of the Adriatic Sea. The material is described as a highly ferruginous clay 
 resulting from the leaching out, by meteoric waters, of the soluble portions of 
 the prevailing limestones. Its distribution is by no means limited to the mari- 
 time provinces of the Karst, but it is found also on the Grecian coasts and in 
 the Schwabia-Frankonia Jura Plateaus of Bavaria. In fact it is to be found any- 
 where in these regions where the prevailing country rock is a marine limestone 
 and erosion not sufficiently active to remove the residuary material. 
 
RESIDUARY DEPOSITS 303 
 
 parent rock. In the case of limestones such matter consists 
 mainly of aluminous and ferruginous matter, grains of sand, 
 and nodular masses of chert which existed as mechanically 
 admixed impurities. 
 
 The inherited characteristics of deposits of this nature may 
 be illustrated by the accompanying exaggerated section across 
 central Kentucky where, it is easy to see, the regolithic mate- 
 rial overlying the Lower Silurian and Cambrian limestones may 
 
 FIG. 27. 
 
 contain a portion of all the insoluble residues from the hundreds 
 of feet of Upper Silurian, Devonian, Lower and Upper Carbonif- 
 erous beds which formerly stretched above them. Upon the 
 nature of this inheritance must depend the adaptability of the 
 regolith to soil purposes and its consequent fertility. 
 
 The transition from a regolith of this type to fresh rock is 
 usually quite sharp, owing to the fact that limestones decompose 
 mainly through solution from the immediate surface. Never- 
 theless there is a gradual change in the character of such a 
 deposit from above downwards, owing to the oxidizing influence 
 of the air and percolating waters. (See p. 307.) 
 
 As above noted, the mineral particles in the older residuary 
 deposits are, with the exception of the quartz, found to be as a 
 rule in a state of advanced decomposition. Nevertheless the 
 ultimate individual constituents of even the darkest clays of the 
 driftless regions of Wisconsin, as examined by Messrs. Chamber- 
 lain and Salisbury, are transparent, although stained by iron 
 oxides. 
 
 Concerning the physical properties of limestone residues as 
 occurring in this driftless area, the following statements are 
 made by Messrs. Chamberlain and Salisbury. " Above, the 
 clay graduates into soil which, outside the valleys, is uniformly 
 shallow. Beneath the soil, the clay loses the dark color of the 
 latter, due to the presence of organic matter, but is for a certain 
 distance downward not unlike the superior portion in texture. 
 The deeper lying clay, where limestone is the subjacent rock, 
 is the most characteristic member of the residuary earth series. 
 
304 
 
 THE REGOLITH 
 
 It is not like that above, structureless, although, like that, it is 
 without trace of stratification. It generally shows a tendency to 
 cleave, breaking up into little pieces which are roughly cubical. 
 This is often conspicuous, and especially so on the faces of 
 
 sections which are thor- 
 oughly dry. In such sit- 
 uations large quantities 
 of the clay in small angu- 
 lar blocks may be removed 
 by slight friction. The 
 size of the cuboids varies, 
 within somewhat narrow 
 limits, from a small frac- 
 tion of an inch to one or 
 two inches in diameter. 
 This cleavage is probably 
 a phenomenon of shrink- 
 age due to drying, as it 
 partially disappears when 
 
 ^ ne clay becomes wet. 
 ^^ structure hag giyen 
 
 joint ' clay, an appellation not alto- 
 
 FIG. 28. Showing angular character of quartz 
 particles in decomposed gneiss. 
 
 rise to the local name of 
 gether inappropriate. 
 
 " Upon drying, this variety becomes very hard and rock-like. 
 It only becomes adapted to serve as soil by surface amelioration, 
 as is shown by the fact that, from the thousands of mineral holes 
 scattered over the southern part of the mining district, the 
 material ejected still lies beside the excavations as heaps of clay, 
 without covering of vegetation, although it has been exposed in 
 most cases for many years. Notwithstanding this fact, the 
 clay, even in its deepest parts, wherever examined, is found to 
 abound in minute perforations. These, in many cases at least, 
 indicate the penetration of rootlets, for the rootlets themselves 
 may sometimes be found. In some cases, too, the perforations 
 have been seen to undergo a gradual variation in size, and to 
 branch now and then, much as rootlets do. On the other hand, 
 it is probable that some of the perforations have had a different 
 origin, for in one case a small insect was found in one of the 
 little canal-ways. The clay is exceedingly tenacious, and hence 
 the perforations, once formed, would endure for long periods of 
 time. 
 
RESIDUARY DEPOSITS 305 
 
 " Another characteristic of certain portions of the clay is its 
 power of retaining moisture. It can rarely be found, even in 
 the driest season, unless exposed to the direct rays of the sun, 
 without visible moisture a few inches from the surface. The 
 regions where it is present are conspicuously less affected by 
 drought than adjacent localities where it is wanting. For this 
 reason it is a valuable sub-soil. 
 
 u Fragments of residuary rock are not uncommon in the deeper 
 portions of this earth. Of these, chert fragments are most 
 abundant, and occur scattered sparingly throughout the clay or 
 sometimes arranged in more or less distinct layers in it. Even 
 where they appear to be entirely wanting, the microscope often 
 reveals minute flakes scattered sparsely throughout the clay. 
 The larger pieces are more numerous near the basal portion of 
 the clay than higher up. 
 
 " It is natural to suppose that the residuary earths derived 
 from the decomposition of limestone would differ very notably 
 from those which take their origin from sandstones or from 
 shales or mixed crystalline rocks. Yet the difference is far 
 less than might be anticipated. There usually overlies the 
 sandstone strata a loamy earth not very far removed in char- 
 acter from that which mantles limestones. It is somewhat 
 more sandy, and consequently less cohesive, and presents the 
 opposite variations in vertical sections, becoming less cohesive 
 below, instead of more so. In the limestone region the tough- 
 est clay lies next to the rock. In the sandstone regions the 
 soil graduates below into sand. The difference is most con- 
 spicuous where the mantle has been washed and redeposited 
 and mingled with mechanically derived sand and secondary 
 products, as occurs in some of the valleys." 1 
 
 The following analyses, in part from this same report, will 
 answer, in connection with those already given, to show the 
 prevailing type of the residuary deposits throughout widely 
 separated areas. It will be noted that silica exceeds as a rule 
 all other constituents, while alumina, iron oxides, and moisture 
 make up the main bulk of the residue. This generalization 
 holds good of nearly all sedentary soils, whatever the character 
 of the rocks from which they were derived, and is the more 
 pronounced the more advanced the decomposition. 
 
 1 6th Ann. Rep. U. S. Geol. Survey, 1884-85, pp. 240-242. 
 x 
 
306 
 
 THE REGOLITH 
 
 I-H t- OS CO CO 
 
 O CM l>- O CO 
 O I-H O -^ t-^ 
 
 t- . O 
 
 O O O O CO 
 
 CO 00 
 
 1 a 
 
 r*- 1 CO 
 
 t* lO . CO CO Oi CO QO ^H i-H 
 
 dco" CM "o" 'coooo't^cM 
 
 ^ .-H rH CM 
 
 CM !> O O iO tr- CM CO 
 
 I-H -^ rH CO OO 00 
 
 OCM CO I I I O rH O C4 OS 
 iO C^ A 
 
 .s 
 
 COCO'^COCO'^tlCOCMCMCOOCMOSOS CX) 
 
 I-H O O OS rH ^ O CM OS CO CO t^- CO O CO 
 
 >-<OOOOl-Hl-Hl-Hi-Hl-HOrH O 
 
 COCOCOCOCOCOOCOOCOOSOSCM OS 
 
 i-^OOOST^I^cOIr^cMCM O 
 
 COi-HCOOOOOOrHrHO'o'od 
 vo CM I-H 
 
 '5* 
 
 g^OSt^COCOi-HCOCOOCOCOOTfl 
 > COi-H(MCMOOOSI>-OOOS'H^COCO 
 
 oscdt^oddo'ddoo'dod 
 
 ^ 
 
 ^ss^^sss^ssg^s 
 
 rHCMOoddoddcM'i-H-Hido 
 
 e 
 
 Q ^ 
 
 ^IIIs f :;:::?: 
 
 O^^aj^.2 O Cx 
 
 3 1 V-/ !J ! 3 S ^ ' s-^ ' rrt 
 
 / T^ <s CB "^ 'S ^ ^^-^ x N S / ~ x o i^ -2 
 
 0-2 g So 8 P * 5 S 2- I ^ 
 
 a I 1 1 1 a 1 1 r & -I r 
 
 ill ii tig Infill 
 
 g^SSHgsas.gSfeSl 
 
 8 
 
 ll 
 
 S t3 
 
 33 
 
 G fl 
 O O 
 
 O O 
 
RESIDUARY DEPOSITS 307 
 
 Columns I, II, III, and IV of this table (see opposite page) 
 are limestone residuals from southern Wisconsin. Columns I 
 and II are from the same vertical section, I being 4J feet from 
 the surface, and II 8J, and in contact with the underlying lime- 
 stone. Columns III and IV are similarly related, III being 3 
 feet from the surface, and IV 4J feet, the lower sample lying on 
 the unchanged rock. The larger percentages of silica in the 
 samples from nearest the surface indicate a higher state of 
 decomposition, the soluble portions having been more largely 
 removed. The presence of larger percentages of alkalies in 
 these same samples indicates that these salts existed in the form 
 of silicates which have resisted the decomposing influences, and 
 remain mechanically included in the residues. Column V is a 
 clay from the decomposition of the Knox dolomite at Morris- 
 ville, Alabama ; VI the characteristic red earth from the decom- 
 position of coralline limestone on the islands of Bermuda; VII 
 a product of the decay of a diabase dike at Wadesboro, North 
 Carolina; VIII a gabbro sub-soil from Maryland; IX a sub-soil 
 from the decomposition of Trenton limestone near Hagerstown, 
 Maryland ; and X a residual soil from the decomposition of a 
 Triassic sandstone, Maryland. 
 
 A microscopic examination of the material represented by 
 analyses I and IV, as given by the authorities quoted, showed 
 it to consist of particles in an extreme condition of comminu- 
 tion. An actual measurement of over 700,000 of these particles 
 yielded results as below : 
 
 Particles less than .0025 mm. in diameter 721.866 % 
 
 Particles between .0025 mm. and .005 mm. in diameter 9.812 
 
 Particles over .005 mm. in diameter . 0.634 
 
 732.312 % 
 
 Of those over .005 millimetre in diameter, particles reaching 
 0.06 millimetre were not rare. Nearly all those above 0.1 milli- 
 metre were found to be of flints and cherts which graded up 
 into chips and flakes of notable sizes. Particles much coarser 
 than those above enumerated do indeed occur, but their actual 
 number is comparatively small, though their comparative bulk 
 may be considerable. 
 
 Work of a like nature, but done under somewhat different 
 conditions, by Dr. Milton Whitney, showed the residues from 
 the Trenton limestones near Hagerstown, Maryland, to contain 
 
308 
 
 THE REGOLITH 
 
 on an average some 45% of finely comminuted material, the 
 individual particles of which vary in size between .005 and 
 .0001 millimetre in diameter, and which may appropriately 
 be termed clay. As Dr. Whitney has calculated, there are 
 approximately 22,000,000,000 grains of sand and clay in each 
 gramme of such a sub-soil, presenting in every cubic foot not 
 less than 158,000 square feet of surface to the action of water 
 and air, as well as to the roots of growing plants. 
 
 The results of mechanical analyses of (I and II) resi- 
 dues from the Trenton limestone, (III) Triassic sandstone, 
 (IV) gabbro, and (V) gneiss are presented in tabular form 
 below. 1 
 
 DIAMETER 
 
 OF 
 
 PARTICLES 
 
 MM. 
 
 CONVENTIONAL NAMES 
 
 I 
 
 II 
 
 III 
 
 IV 
 
 V 
 
 2-1 
 
 
 0.54 
 
 0.17 
 
 0.00 
 
 00 
 
 0.19 
 
 1-.5 
 
 Coarse sand 
 
 0.32 
 
 0.00 
 
 0.23 
 
 0.26 
 
 1.80 
 
 .5-.25 
 
 Medium sand . 
 
 72 
 
 15 
 
 1 29 
 
 18 
 
 3 12 
 
 .25-.! 
 
 Fine sand .... 
 
 62 
 
 25 
 
 4 03 
 
 66 
 
 6 96 
 
 .1-.05 
 
 Very fine sand .... 
 
 4.03 
 
 2.34 
 
 11 57 
 
 6 73 
 
 8 76 
 
 .05- 01 
 
 Silt 
 
 36 02 
 
 19 04 
 
 38 97 
 
 47 32 
 
 34 92 
 
 .01-.005 
 
 Fine silt 
 
 14.99 
 
 20 88 
 
 8 84 
 
 10 04 
 
 12 14 
 
 .005-.0001 
 
 Clay 
 
 41 24 
 
 51 77 
 
 32 70 
 
 34 90 
 
 28 82 
 
 
 
 
 
 
 
 
 
 Total mineral matter .... 
 Organic matter, water, and loss 
 
 88.48 
 1.52 
 
 94.60 
 5.40 
 
 97.63 
 2.37 
 
 94.44 
 5.56 
 
 96.71 
 3.29 
 
 
 
 100.00 
 
 100.00 
 
 100.00 
 
 100.00 
 
 100.00 
 
 Many of the products of weathering of siliceous crystalline 
 and calcareous rocks are of economic importance as soils, clays, 
 and iron ores, as elsewhere noted. The kaolin beds of northern 
 Delaware and southwestern Pennsylvania are mainly decom- 
 posed, highly feldspathic, gneissic rocks, and which as dug 
 from the pits still retain their gneissic structure, but which 
 are now plastic clays full of angular quartz fragments, mica 
 scales and feldspar particles in various stages of decomposi- 
 tion. The change that has taken place consists in a kaoliniza- 
 tion of the feldspars, whereby the alkalies are largely removed, 
 and a residue consisting essentially of a hydrous silicate of 
 
 1 Bull. No. 21, Maryland Agricultural Exp. Station, by Milton Whitney, 1893. 
 
RESIDUARY DEPOSITS 
 
 309 
 
 alumina left in their place. The quartz granules are disaggre- 
 gated, and their surfaces sometimes slightly etched by the 
 action of the alkaline car- 
 bonates ; the black mica, 
 where such existed, de- 
 composed, giving rise to 
 rust-colored spots. The 
 material is dug from the 
 pits and washed with wa- 
 ter to separate the im- 
 purities, the " kaolin " or 
 clay remaining in sus- 
 pension, and being ulti- 
 mately saved by nitration 
 through canvas. This 
 finest material, as seen 
 under the microscope, still 
 contains particles of un- FIG. 29. Showing, on 
 decomposed feldspars and 
 shreds of white mica, to- 
 gether with other extremely irregularly outlined, sometimes 
 almost amoeba-shaped forms, as shown in Fig. 29. An average 
 of two mechanical analyses of this clay, made under Dr. Whit- 
 ney's direction, yielded the results given below : 
 
 the left, the mineral 
 kaolinite as seen under the microscope, and 
 on the right, washed kaolin. 
 
 MOISTURE IN AIR- 
 DRY MATERIAL AT 
 100 C. 
 
 MOISTURE ON 
 IGNITION 
 
 SILT 
 
 .05-.01 MM. 
 
 FINE SILT 
 .01-.005 MM. 
 
 CLAY 
 .005-.0001 MM. 
 
 0.41% 
 
 11.41 % 
 
 31.79% 
 
 7.31 % 
 
 47.78% 
 
 Chemical analyses of the same material, made in the labora- 
 tories of the United States Geological Survey, yielded : - 
 
 Silica (Si0 2 ) 48.73% 
 
 Titanic oxide (Ti0 2 ) 0.17 
 
 Alumina (A1 2 3 ) 37.02 
 
 Ferric iron (Fe 2 3 ) 0.79 
 
 Lime (CaO) 0.16 
 
 Magnesia (MgO) 0.11 
 
 Potash (K 2 0) 0.41 
 
 Soda (Na 2 0) 0.04 
 
 Water at 100 0.52 
 
 Ignition 12.83 
 
 Phosphoric acid (P 2 5 ) 0.03 
 
 100.81 % 
 
310 THE REGOLITH 
 
 Among the special names that have from time to time been 
 given to local phases of residuary accumulations, there remain 
 two, the laterite and wacke, which are sufficiently common to 
 merit some attention. The first mentioned of these, laterite, 
 like loess and several other terms that might be mentioned, 
 has to a considerable extent lost its true lithological signifi- 
 cance through careless usage. Originally the name was applied 
 to a vesicular highly ferruginous clay, soft in the mass, but hard- 
 ening on exposure to the weather, and which has a wide distribu- 
 tion throughout India and Ceylon. Two forms are commonly 
 recognized, the one capping the summits of hills and plateaux 
 on the highlands of central and western India, and underlain 
 by the Deccan traps; and the second occurring on the lowlands, 
 in part overlying gneisses and granites. The prevailing colors 
 of the laterite, when freshly broken, are various tints of brown, 
 red and yellow mottled, or whitish ; after exposure it is usually 
 covered with a brown or blackish brown coating of limonite. 
 When first dug out, the material is sufficiently soft to be cut 
 with a pick or shovel, but becomes greatly indurated on expos- 
 ure. In some instances the material is of so compact a texture 
 and so hard as to resemble jasper. In many forms of laterite 
 the material is traversed by "small irregular tortuous tubes 
 from a quarter of an inch to upwards of an inch in diameter." 
 These penetrate the mass in all directions, though most com- 
 monly nearly vertical, and are often lined with a coating of 
 limonite. On weathering, these give rise to extremely irregu- 
 larly pitted or scoriaceous surfaces, which, together with the 
 dense, often botryoidal structure, cause it to resemble certain 
 types of igneous rocks, for which it has more than once been 
 mistaken. The more massive forms show usually a horizontal 
 banding. Some forms of laterite show a brecciated structure, 
 due to its detrital fragments becoming recemented into masses 
 closely resembling the original rock. The high level form, that 
 which occurs capping the hills and plateaux on the highlands 
 of central and western India, is fine grained and compact and 
 of a fairly homogeneous structure, although the iron oxide may 
 be somewhat irregularly distributed and sometimes segregated 
 in pisolitic nodules sufficiently abundant to form an ore. The 
 lower level form, that which covers large areas of both east and 
 west coasts, frequently contains grains of sand and pebbles 
 embedded in a ferruginous matrix. It is, as a rule, less homo- 
 
LATERITE AND WACKE 
 
 311 
 
 geneous than the high level form, but nevertheless passes into it 
 by insensible gradations. 
 
 The origin of both high and low level forms of the laterite 
 has been the subject of much speculation. It is probable that 
 all of it is of a residual nature, i.e. represents the less soluble 
 portions of pre-existing rock masses. That which is found on 
 the high levels occurs overlying the Deccan trap sheets, into 
 which it can in many instances be traced, proving conclusively 
 its origin from this rock by the ordinary processes of weather- 
 ing. The low-lying variety can, in many instances, in like man- 
 ner be traced back to its origin from more siliceous, gneissic, and 
 granitic rocks. A part of the material, however, has the ap- 
 pearance and structure of a clastic rock of sedimentary origin, 
 and so it is considered by the best authorities to be. 
 
 The chemical composition of a very ferruginous laterite from 
 Rangoon is as below : 
 
 CONSTITUENTS 
 
 INSOLUBLE 
 
 SOLUBLE 
 
 BULK 
 
 Silica (SiC>2) 
 
 30.728% 
 [ 2.728 j 
 
 I 6.802 
 
 6.848% 
 5.783 
 46.279 
 0.742 
 
 0.090 
 
 37.576% 
 I 52.802 
 
 6.892 
 
 Alumina (AloOs) 
 
 Iron sesquioxide (I^Os) 
 
 Lime (CaO) 
 
 Magnesia (MgO) 
 
 Alkalies 
 
 \Vater and loss 
 
 
 40.258 
 
 59.742 
 
 100.00 % 
 
 100.00% 
 
 " The surface of the country composed of the more solid 
 forms of laterite is usually very barren, the trees and shrubs 
 growing upon it being thinly scattered and of small size. This 
 infertility is due, in great part, to the rock being so porous that 
 all the water sinks into it, and sufficient moisture is not retained 
 to support vegetation. The result is that laterite plateaux are 
 usually bare of soil, and frequently almost bare of vegetation." 1 
 
 Wacke is an old German name now but little used, designat- 
 ing the gray, brown to black earthy residue or clay resulting 
 from the decomposition in place of basic eruptive rocks, as 
 
 1 Manual of the Geology of India, by R. D. Oldham, 2d ed., 1893, pp. 369-390. 
 
312 THE REGOLITH 
 
 basalt, melaphyr, etc. In composition the material naturally 
 varies with the character of the rock from which it was derived, 
 and the amount of decomposition and leaching it may have 
 undergone. 
 
 It seems advisable to call attention here, a little more emphati- 
 cally, to the fact that the same processes which in ages past 
 have been instrumental in the formation of sandstones, shales, 
 slates, or marls are to-day, and have in late Tertiary and in 
 Quaternary times, given us soils; in other words, many of our 
 soils are but secondary rocks in a state of loose consolidation, 
 and many of the accumulations classed as residual were de- 
 rived by disintegration, in situ, of alluvial materials ; materials 
 brought down years ago and deposited in shallow seas. The 
 amount of consolidation undergone by the more recent of these 
 sediments has in many instances been so slight that on elevation 
 above the water level they are ready almost at once to assume 
 the role of soil with little if any preparatory disintegration. 
 Nevertheless consistency demands that .such be here grouped 
 as residuary. 
 
 Over what is known as the coastal plain of the middle Atlan- 
 tic slope, a narrow belt bordering on the Atlantic and extending 
 from the Hudson River on the north to the Roanoke on the 
 south, have been deposited in late Mesozoic and Tertiary times 
 a series of gravels, sands, and clays which constitute the well- 
 known Potomac, Appomattox, and Columbian formations of 
 Darton, McGee, and others. These are all detrital deposits 
 from the eastern Appalachian regions, brought down by streams 
 and deposited in the shallow estuaries and deltas of these 
 periods, but which have remained in a condition of slight con- 
 solidation, and through subsequent elevation and weathering 
 form the soils. Such vary widely and abruptly. In the region 
 northeast of Washington, the Potomac formation consists of 
 feldspathic sands, gravels, and clays irregularly bedded and 
 often enclosing notable accumulations of rounded pebbles of 
 quartzite brought down from the Appalachian and Piedmont 
 regions. The Appomattox formation, from which was derived 
 surface soil in the vicinity of the Rappahannock and Appomat- 
 tox in Virginia, is a yellowish or orange-colored clay and sand 
 with sometimes interbedded gravel. The Columbian formation, 
 which yields the surface soil of the main portion of Washington 
 City and the immediate valley of the Potomac and contributary 
 
CUMULOSE DEPOSITS 
 
 313 
 
 streams southward, is a delta and littoral deposit made up of 
 materials worked over from the older Potomac and Lafayette 
 formations and also of granitic sands and clays from the decom- 
 posed rocks of the Piedmont plateau. 
 
 The clays of the Potomac formation above mentioned are not 
 infrequently sufficiently homogeneous and plastic to be utilized 
 in the manufacture of brick, tiles, and pottery. The following 
 table shows the finely comminuted condition of the materials 
 which go to make up these clays in Maryland, as determined 
 by Whitney. 1 
 
 DIAMETER 
 
 MM. 
 
 CONVENTIONAL NAMES 
 
 EED CLAY, 
 TILE 
 
 EED CLAY, 
 PUDDLING 
 
 BLUE CLAY, 
 STONEWARE 
 
 2-1 
 
 Fine gravel ... 
 
 0.00 % 
 
 0.31 % 
 
 0.00 % 
 
 1-5 
 
 Coarse sand 
 
 0.00 
 
 0.82 
 
 0.00 
 
 .5-.25 
 
 Medium sajnd 
 
 0.50 
 
 2.69 
 
 0.29 
 
 .25-.! 
 
 Fine sand 
 
 2.63 
 
 3.23 
 
 1.27 
 
 1-05 
 
 Very fine sand . 
 
 962 
 
 889 
 
 8.93 
 
 .05-.01 
 
 Silt 
 
 25.13 
 
 26.17 
 
 20.16 
 
 01- 005 
 
 Fine silt 
 
 13.44 
 
 11.18 
 
 16.72 
 
 .005-.0001 
 
 Clay 
 
 42.34 
 
 42.36 
 
 50.02 
 
 
 Total 
 
 93.76 % 
 
 95.65 % 
 
 97.39 % 
 
 
 Organic matter, water loss . . 
 
 6.24 
 
 4.35 
 
 2.61 
 
 (2) Cumulose Deposits. To be classed with the sedentary 
 deposits, in that they result from the gradual accumulation of 
 material in situ, but differing radically in both composition and 
 origin from those just described, are those portions of the rego- 
 lith which result from the gradual accumulation of organic 
 matter with only small amounts of foreign detritus ; which are 
 made up almost wholly of the combined accumulations, organic 
 and inorganic, of growing plants. Such may not infrequently 
 be found in all stages of formation, in enclosed ponds or lakes, 
 without appreciable inlet or outlet, being merely due to stand- 
 ing water in low places. " Such pools, when not exposed to 
 periodical drying up, are invaded by a peculiar vegetation, first 
 mostly composed of confervse, simple thread-like plants of vari- 
 ous color and of prodigious activity of growth, mixed with a 
 mass of infusoria, animalcules, and microscopic plants, which, 
 
 . 4, U. S. Dept. of Agriculture, 1892. 
 
314 THE KEGOLITH 
 
 partly decomposed, partly containing the floating vegetation, 
 soon fill the basins and cover the bottom with a coating of 
 clay-like mould. So rapid is the work of these minute beings, 
 that in some cases from 6 to 10 inches of this mud is deposited 
 in one year. Some artificial basins in the large ornamental 
 parks of Europe have to be cleaned of such muddy deposits 
 of floating plants, mixed with small shells, every three or four 
 years. 
 
 " When left undisturbed, this mud becomes gradually thick 
 and solid ; in some cases, of great thickness ; affording a kind of 
 soil for marsh plants, which root at the bottom of the basins or 
 swamps and send off their stems and leaves to the surface of 
 the water or above it ; where their substance becomes in the 
 sunshine hard and woody. 
 
 " As these plants periodically decay, their remains of course 
 drop to the bottom of the water ; and each year the process is 
 repeated, with a more or less marked variation in the species 
 of the plants. After a time the basins become filled by these 
 successive accumulations of years or even centuries, and the 
 top surface of the decayed matter, being exposed to atmospheric 
 action, is transformed into humus and is gradually covered by 
 other kinds of plants, making meadows and forests. In other 
 cases when basins of stagnant water are too deep for vegetation 
 
 FIG. 30. Section across a small lake, a, bed rock ; 66, drift ; cc, growing peat ; 
 dd, decaying peat ; ee, climbing bog. 
 
 of aquatic plants, nature attains the same result by a different 
 special process ; namely, by the prolonged vegetation of certain 
 kinds of floating mosses, especially the species known as sphagna. 
 These grow with prodigious speed, and expanding their branches 
 in every direction over the surface of ponds or small lakes, soon 
 cover it entirely. They thus form a thin floating carpet, which 
 as it gradually increases in thickness serves as a solid soil for 
 another kind of vegetation, that of the rushes, the sedges, and 
 some kinds of grasses, which grow abundantly mixed with the 
 mosses, and which by their water-absorbing structure furnish 
 
CUMULOSE DEPOSITS 
 
 315 
 
 a persistent humidity sufficient for the preservation of their 
 remains against aerial decay. The floating carpet of moss be- 
 comes still more solid, and is then overspread by many species 
 of larger swamp plants, and small arborescent shrubs, especially 
 those of the heath family ; and so, in the lapse of years, by the 
 continual vegetation of the mosses, which is never interrupted, 
 and by the yearly deposits of plant remains, the carpet at last 
 becomes strong enough to support trees, and is changed into a 
 floating forest, until, becoming too heavy, it either breaks and 
 sinks suddenly to the bottom of the basin, or is slowly and grad- 
 ually lowered into it and covered with water." 1 
 
 It is to such processes that are due, in large part, the inland 
 swamp soils of many localities. Beginning at and near the 
 shore and upon a soil of wet sand, the organic matter has accu- 
 mulated year by year till now several feet in thickness and in 
 some cases covering miles of territory. The proportion of or- 
 ganic matter in such a deposit naturally increases from the shore 
 outward until in the upper and central layers it may comprise 
 90 % of the total weight. 
 
 This feature is well brought out in the following analyses of 
 material from an open ground prairie swamp in Carteret County, 
 North Carolina. 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 Silica (insoluble) (Si02) 
 
 80.84% 
 
 1.62% 
 
 Silica (soluble) (SiO ) 
 
 3.70 
 
 0.00 
 
 Alumina (A^Os) . . ... 
 
 2.69 
 
 0.39 
 
 Oxide of iron (Fe20s) . 
 
 1.18 
 
 0.15 
 
 Lime (CaO) 
 
 0.44 
 
 0.36 
 
 Magnesia (MgO) 
 
 0.22 
 
 0.14 
 
 Potash (K 2 0) 
 
 0.07 
 
 0.06 
 
 Soda CNa.O) . 
 
 0.02 
 
 0.13 
 
 Phosphoric acid (P^Og) 
 
 0.08 
 
 0.06 
 
 Sulphuric acid (SOg) . ... 
 
 0.06 
 
 0.00 
 
 Chlorine (Cl) 
 
 Trace 
 
 0.02 
 
 Organic matter (C) . 
 
 7.70 
 
 87.25 
 
 Water (H 2 O) 
 
 2.50 
 
 9.60 
 
 
 
 
 Column I of the above is from the margin the oak fringe 
 of this great swamp, near North River, about 8 miles north of 
 
 1 Geol. Survey of Pennsylvania, 1885, p. 106. 
 
316 THE REGOLITH 
 
 Beaufort ; it is light gray to ash-colored with a growth of white 
 oak, gum, maple, pine, and palmetto trees ; the situation is low 
 and flat. " This margin belt of semi-swamp is from a half mile 
 or less in width to above a mile. The surface rises towards the 
 interior and is covered by a soil, if it may be called such, repre- 
 sented by column II, which is 2 to 3 feet deep and upwards, and 
 lies on a bed of white sea-sand. It consists of a loose open mass 
 of half-decayed woody matter, of a brown color, and is in fact 
 a superficial, uncompressed lignite ; for it will be observed that 
 the analysis includes nearly 10 % of water, so that the dry sub- 
 stance would give but 3|- % of inorganic matter, not more than 
 would be accounted for by the ash of the woody matter. The 
 growth is a dense thicket of spindling shrubs with small scat- 
 tered maples and bays." 1 
 
 Wiley has described 2 deposits of a somewhat similar nature 
 as covering 1,000,000 acres in the Kissimmee valley of Florida. 
 These, which are of a dark brown to deep black color, contain 
 in some cases as much as 96.16% of volatile matter, a and vary 
 from 3 to 20 feet in depth. Such, when properly drained, may 
 be made extremely fertile, though in periods of drought endan- 
 gered by fire which, once started, may burn for months, doing 
 immense damage. The partially reclaimed areas of the Great 
 Dismal Swamp of Virginia are fair representative types of 
 swamp soils. 
 
 The formation of cumulose deposits is not, however, limited 
 to lakes, stagnant ponds, or even to swamps as the word is ordi- 
 narily used, excepting as the swamp itself may be incidental 
 and consequent. Regions of poor drainage, particularly in 
 moist and cool climates, may give rise to growths of sphagnous 
 mosses and subsequently to plants of a higher type, which in 
 course of years assume no insignificant proportions. 
 
 In accounting for such accumulations, we have but to remem- 
 ber that ordinarily when a plant dies, its organic constituents 
 are returned to the atmosphere once more in a comparatively 
 brief period of time through the usual processes of decay. It 
 needs only such conditions of moisture as shall prevent the 
 complete decay and hence favor the accumulation of the organic 
 matter, to give us beds of peat and ultimately of coal. Plants 
 of the type of sphagnous mosses, growing continuously above 
 
 1 Geology of North Carolina, Vol. I, 1875. 
 
 2 Agricultural Science, Vol. VII, No. 3, 1893, pp. 106-120. 
 
SWAMP DEPOSITS 317 
 
 and dying beneath, hold in their mass sufficient moisture to 
 exclude atmospheric air, and thus themselves bring about the 
 proper conditions for bog making. In virtue of this property 
 such may gradually rise above the level of the surrounding 
 country, as is the case with the Great Dismal Swamp of Vir- 
 ginia and numerous others that need not be mentioned here. 
 Instances are on record where bogs of this nature have grown 
 so far above the natural level, that during seasons of unusual 
 rainfall they have burst, and flooded adjacent regions, with dis- 
 astrous results. The rate of growth of such accumulations is 
 naturally quite variable. H. S. Gesner, as quoted by T. Rupert 
 Jones, 1 states that in Bavarian moors the observed increase in 
 peat, in forty-five years, amounted to from 2 to 3 feet in thick- 
 ness ; in Oldenberg, in one hundred years, to 4 feet ; in Ham- 
 melsmoor, Denmark, to 2J feet ; and in Alpine districts to 4 and 
 5 feet in from thirty to fifty years. 
 
 The peat bogs, so characteristic of Ireland, Scotland, and 
 other northern latitudes, are of this type. A section of the 
 well-known Bog of Allen, made in county Kildare, is given 
 below. 2 
 
 THICKNESS 
 
 (1) Dark reddish brown ; mass compact ; no fibres of moss visible ; 
 
 surface decomposed by atmosphere 2 feet 
 
 (2) Light reddish brown ; fibres of moss very perfect 3 " 
 
 (3) Pale yellowish brown ; fibres of moss very perceptible 5 " 
 
 (4) Deep reddish brown ; fibres of moss perceptible 8| " 
 
 (5) Blackish brown ; fibres of moss scarcely perceptible, contains 
 
 numerous twigs and small branches of birch, elder, and fir . 3 " 
 
 (6) Dull yellow-brown ; fibres not visible ; contains much empyreu- 
 
 matic oil ; mass compact 3 " 
 
 (7) Blackish brown ; mass compact ; fibres not visible ; contains much 
 
 empyreumatic oil 10 " 
 
 (8) Black mass, very compact ; has a strong resemblance to pitch or 
 
 coal ; fracture conchoidal in all directions; lustre shining ... 4 " 
 
 Total depth of bog 38 feet 
 
 Underlaid by 3 feet of marl containing 64 % carbonate of lime, 4 feet of blue 
 clay, and this in its turn by clay mixed with limestone gravel of an unknown 
 thickness. 
 
 1 Proc. Geologists' Association, Vol. VI, No. 5, January, 1880. 
 
 2 T. Rupert Jones, Proc. of the Geologists' Association, London, Vol. VI, No. 
 5, January, 1880. This authority classifies the peat bogs, swamps, and marshes, 
 as follows : 
 
 I. Peat bogs and turf moors on such plateaux as flat mountain tops and wide 
 hill moors. 
 
318 THE REGOLITH 
 
 Deposits of the cumulose type pass by all gradations into 
 the paludal, swamp, or marsh type and these in turn into ordi- 
 nary alluvium. Or it would perhaps be better to reverse this 
 order, since, as in the gradual silting up of an enclosed lake, 
 we may have, in the first stages, stratified alluvium, then when 
 the waters become sufficiently shallowed, swamp and muck 
 deposits, and lastly the deposits of pure organic, or cumulose 
 material. 
 
 2. TRANSPORTED MATERIALS 
 
 Because of the constant action of gravity, the well-known 
 transporting power of water, the wind or moving ice, few re- 
 sidual products retain for any length of time their virgin purity, 
 but become more or less contaminated with materials from near 
 or distant sources. The avalanches of mountain regions afford 
 an illustration of the bodily transfer of, it may be, millions of 
 tons of matter from the mountain slopes to be debouched into 
 the valley below ; the slow-creeping glacier brings down its 
 load and deposits its moraine when, succumbing to the blan- 
 dishments of warmer climes, it is no longer able to bear it fur- 
 ther : spasmodic winds catch up the smaller particles as clouds 
 of dust to be transported, assorted, and redeposited as their 
 
 II. Peat bogs of valleys : (1) At the heads of valleys ; (2) at the salient angles 
 within river curves ; (3) in deserted beds of rivers ; (4) in plains and lakes of 
 expanded valleys ; (5) special peat bogs of Denmark and the black earth of Rus- 
 sia ; (6) river deltas ; (7) maritime peat marshes, where certain valleys and plains 
 open to the sea. 
 
 Regarding the black earth of Russia, it should be stated that this is now 
 regarded by at least one authority (Hume, Geol. Mag., Vol. I, No. 2, 1894) as 
 being but a local phase of the loess, the color being due to the prevalence of 
 organic matter. 
 
 Shaler (Ann. Rep. U. S. Geol. Survey, 1888-89), on a basis of physical char- 
 acters, classifies the inundated lands of the United States as below : 
 
 c . . ( Grass marshes. 
 
 Marine marshes /Above ^ tide . . - { Mangrove marshe , 
 
 IT,, . , f Mud banks. 
 
 (Below mean tide . . - 1 Eel _ grass areas . 
 
 /. (Terrace. 
 
 River swamps . . . .{ Estuarine 
 
 T , f Lake margins. 
 
 Fresh-water swamps. . . >ake swamps . . . j Quaking b & ogs . 
 
 f Wet woods. 
 Upland swamps . . j Climbing bogs . 
 
 vAblation swamps. 
 
COLLUVIAL DEPOSITS 
 
 319 
 
 force is spent. It is, however, through the continual transpor- 
 tation of running streams, both in the past and present, and 
 through the action of moving ice in ages gone, that have been 
 brought about the great amount of transportation and admixt- 
 ure characteristic of that part of the regolith comprised under 
 the general name of drift. According to which of the agencies 
 enumerated prevailed, we may subdivide our subject as folloAvs : 
 (1) Colluvial deposits, (2) alluvial deposits, (3) seolian de- 
 posits, and (4) glacial deposits, though as we proceed we shall 
 find that the lines of separation are not in all cases sharply 
 drawn, and in many an area the regolith bears impress of com- 
 pounded agencies. 
 
 (1) Colluvial Deposits. 1 Under this head it is proposed to 
 include those heterogeneous aggregates of rock detritus com- 
 monly designated as talus and cliff debris. The material of 
 avalanches may also be classed here. Such result 
 wholly from the transporting action of gravity. The 
 deposits in themselves are comparatively limited 
 in extent, ever varying in composition, and are 
 composed of an indiscriminate admixture of 
 N^ particles of all sizes, from those as fine as 
 dust to blocks it may be of hundreds of 
 tons' weight. Such are necessarily limited 
 to the immediate vicinity of the cliffs or 
 mountains from which they are derived. 
 As loosened by heat or frost from the 
 
 FIG. 31. Diagram 
 showing the history 
 of a talus, a, bed 
 rock; bb, talus; c, de- 
 stroyed portion of a cliff, 
 the material being now in 
 the talus. 
 
 So&tearlng ponton, 
 "talus 
 
 parent masses, the fragments 
 
 tumble down the slopes, gradually 
 
 accumulating in beds the slope of which is limited only by the 
 
 laws of gravity and the character of the debris. (See PI. 23.) 
 
 Inclinations of 30 are common ; less commonly of 40. From 
 
 1 From the Latin "colluvies," a mixture. The term as here used is more 
 restricted in its meaning than as used by Professor Ililgard. 
 
320 
 
 THE REGOLITH 
 
 their mode of origin it is natural that the individual particles 
 should be mainly angular and comparatively fresh. In fact, 
 they represent rock-weathering through disintegration, and 
 not decomposition, which will come later. Below, i.e. further 
 down the slopes and in the edges of the valleys, these coarse, 
 illy assorted deposits pass gradually into soils ; above, they 
 consist simply of masses of loose rock wholly unfitted for the 
 support of vegetable life. (Fig. 31.) Through becoming sat- 
 urated with water, ice, or snow, such at times become loosened 
 from the steep slopes on which they lie and slide down in the 
 form of avalanches into the valleys. Although comparatively 
 limited in their extent, these latter, owing to the resistless 
 energy and suddenness of their advance, are sometimes appall- 
 ingly destructive, as has been repeatedly illustrated in the Swiss 
 Alps, and other mountain regions. The geographic distribution 
 of talus deposits as controlled by climatic conditions has been 
 already noted (p. 283). 
 
 (2) Alluvial Deposits. The deposits included under this 
 head differ structurally from those thus far described in that 
 they are always more or less distinctly stratified, or bedded. 
 In writing of the formation of sedimentary rocks, and again 
 when treating of the action of running water, a few figures 
 were given relative to the amount of transported debris de- 
 posited yearly in the Gulf of Mexico. In a similar way the 
 amount of debris carried annually to the ocean by some of the 
 chief rivers of the world has been estimated as below : 
 
 CUBIC FEET 
 
 Mississippi 7,468,694,400 
 
 Upper Ganges . . . 6,368,077,440 
 Hoang-Ho 17,520,000,000 
 
 CUBIC FEET 
 
 Rhone 600,000,800 
 
 Danube 1,253,738,600 
 
 Po 1,510,147,000 
 
 The muddy condition of the water, caused by this sus- 
 pended matter, is so conspicuous a feature of certain rivers that 
 they have received special names on this account. Hoang-Ho 
 means simply yellow river ; Missouri is the Indian name 
 for Big Muddy ; while the famous Red River of the North is 
 so called merely because of the red mud it carries. Such silt- 
 bearing streams, .flowing into lakes and tideless seas, begin 
 depositing their loads so soon as their currents are checked, 
 building up thus the so-called delta deposits for which the 
 Mississippi, the Po, Ganges, and the Nile are noted. 
 
 The character of the material in the delta deposits is vari- 
 
ALLUVIAL DEPOSITS 
 
 321 
 
 able only within certain limits, consisting always of siliceous 
 sand and mud intermingled with organic matter. 
 
 Professor Judd, who examined samples from borings in the 
 alluvial deposits of the Nile delta, found the materials to vary 
 abruptly in texture from the surface downward, the variations 
 following no recognizable law. The percentage amounts of 
 constituents classed as sand and mud, as obtained from (I) 
 borings at Kasr-el-Nil, Cairo, (II) Kafr-ez-Zayat, and (III) 
 Tantah, are given in the table below. 
 
 in 
 
 DEPTH 
 
 SAND 
 
 MUD 
 
 SAND 
 
 MUD 
 
 SAND 
 
 MUD 
 
 
 QI 
 
 10 
 
 % 
 
 % 
 
 
 % 
 
 % 
 
 3'0" . . . 
 
 .... 
 
 .... 
 
 2.35 
 
 97.65 
 
 
 
 4'0" . . . 
 
 
 .... 
 
 30.42 
 
 69.58 
 
 1.71 
 
 98.29 
 
 6' 0" ... 
 
 5.77 
 
 94.33 
 
 
 
 .... 
 
 
 8'C" . . . 
 
 
 
 
 
 7.27 
 
 92.73 
 
 11' 0" . . . 
 
 .... 
 
 
 50.99 
 
 49.01 
 
 .... 
 
 
 10' 0" ... 
 
 86.27 
 
 13.73 
 
 .... 
 
 .... 
 
 
 .... 
 
 17' 6" . . . 
 
 79.65 
 
 20.35 
 
 .... 
 
 .... 
 
 
 .... 
 
 18' 0" ... 
 
 .... 
 
 .... 
 
 .... 
 
 .... 
 
 8.78 
 
 91.22 
 
 19' 0" . . . 
 
 .... 
 
 .... 
 
 87.41 
 
 12.59 
 
 .... 
 
 
 22' 6" . . . 
 
 
 .... 
 
 .... 
 
 .... 
 
 31.16 
 
 68.44 
 
 20' 0" 
 
 
 
 90.19 
 
 9.81 
 
 
 
 31' 0" ... 
 
 .... 
 
 ..,. 
 
 
 
 39.43 
 
 60.57 
 
 35' 0" ... 
 
 .... 
 
 
 86.42 
 
 13.58 
 
 .... 
 
 
 38' 0" ... 
 
 65.05 
 
 34.95 
 
 
 
 
 
 .... 
 
 
 40' 0' . . . 
 
 .... 
 
 .... 
 
 81.94 
 
 18.06 
 
 80.70 
 
 19.30 
 
 40' 6' ... 
 
 80.83 
 
 19.17 
 
 .... 
 
 .... 
 
 .... 
 
 
 45' 0' ... 
 
 68.72 
 
 31.28 
 
 .... 
 
 
 .... 
 
 .... 
 
 40' 0' . . . 
 
 .... 
 
 .... 
 
 .... 
 
 
 95.90 
 
 4.10 
 
 48' 0' ... 
 
 
 .... 
 
 87.23 
 
 12.77 
 
 .... 
 
 .... 
 
 55' 0' ... 
 
 .... 
 
 
 
 0.25 
 
 99.75 
 
 97.71 
 
 .... 
 
 50' 0' . . . 
 
 .... 
 
 
 
 .... 
 
 99.53 
 
 2.29 
 
 58' 0' ... 
 
 .... 
 
 .... 
 
 
 
 .... 
 
 59.09 
 
 0.47 
 
 60' 0' ... 
 
 .... 
 
 
 
 12.60 
 
 87.40 
 
 .... 
 
 40.91 
 
 66' 0' . . . 
 
 
 .... 
 
 62.07 
 
 37.93 
 
 .... 
 
 
 68' 0" ... 
 
 
 .... 
 
 
 
 7.76 
 
 .... 
 
 73' 0" ... 
 
 
 
 
 
 
 .... 
 
 59.95 
 
 92.24 
 
 75' 0" . . . 
 
 .... 
 
 
 66.38 
 
 36.62 
 
 .... 
 
 40.05 
 
 The material described as sand consists of rounded, angular, 
 and sub-angular grains. The well-rounded granules are mainly 
 of quartz and feldspar ; the angular and sub-angular of quartz, 
 feldspars, hornblende, and augite, with smaller quantities of 
 mica, tourmaline, sphene, iolite, zircon, fluor-spar, and magnetite 
 
322 THE REGOLITH 
 
 all in a nearly unaltered condition. The feldspars are mainly 
 orthoclase and microcline rarely a soda-lime variety and 
 in a state of surprising freshness. The quartz is in part the 
 quartz of granitic rocks and the larger grains well rounded, 
 best described as microscopic pebbles. He says : " It is evi- 
 dent that these sand grains have been formed by the breaking 
 up of granitic and metamorphic rocks, or of older sandstones 
 derived directly from such rocks. The larger grains exhibit 
 the perfect rounding and polishing now recognized as charac- 
 teristic of yeolian action ; the smaller ones from their larger 
 surfaces in proportion to their weight, have undergone far less 
 attrition in their passage through the air ; but it is fair to con- 
 clude that they are really desert sand, derived from the vast 
 tracts which lie on either side of the Nile valley, and swept 
 into it by the action of the wind." The material described 
 as mud is composed of essentially the same materials as the 
 sands, but in a more finely divided state. There is an entire 
 absence of anything like kaolin, though there are present 
 particles of organic matter and frustules of diatoms. The 
 surprising freshness of the materials and lack of kaolin is 
 regarded as indicative of an origin through the action of heat 
 and frost ; i.e. through mechanical agencies rather than through 
 the processes of rock decomposition. 1 
 
 But, as has been already noted, only a part of the sediment 
 carried by any stream reaches its mouth. A comparatively 
 small, but, from our present standpoint, very important portion 
 is carried during seasons of high water beyond the usual chan- 
 nels and spread out over the flood plains, as described on p. 287. 
 Such deposits are, as a rule, plainly stratified, and consist of 
 mineral matter in a finely comminuted condition derived, it may 
 be, from the breaking down of a great variety of rocks. Their 
 physical and chemical properties, as well as the periodic char- 
 acter of their deposition, are favorable to the formation of soils 
 possessing great strength and fertility. Both fertility and rate 
 of deposition in such cases are augmented through plant growth, 
 which takes place with great rapidity wherever climatic condi- 
 tions are favorable. So soon as the water leaves the flood plain, 
 a host of moisture-loving plants, as reeds and rushes, spring up 
 in countless numbers to die down again in the fall, and yield 
 the carbon and nitrogeneous constituents to serve as fertilizers, 
 1 Proc. Royal Soc. of London, Vol. XXXIX, 1885, p. 213. 
 
ALLUVIAL DEPOSITS 
 
 323 
 
 and augment the crop of the following year. Moreover, the 
 remaining stems and fallen leaves of the plants serve to retard 
 the running waters of each succeeding flood, catching in their 
 meshes the floating sediments which might otherwise be carried 
 seaward. The Anacostia, which empties into the Potomac River 
 east of Washington, serves as a good illustration of the working 
 of these agencies. A century ago the stream was navigable by 
 coasting crafts as far as Bladensburg. Now, owing to shallow 
 waters, nothing but rowboats can navigate beyond the Navy 
 Yard at Washington. Each season the stream, murky with 
 suspended silt from cultivated fields along its shores, comes 
 down, till, ponded back by tides, it begins to deposit its load. 
 As year by year its bed was thus raised, water plants, encroach- 
 ing more and more from shallow shores, still further dammed 
 
 FIG. 32. 
 
 its sluggish current till now, during summer months, it is little 
 more than a stagnant pond full of rank vegetation, and a source 
 of odors foul and atmospheres enervating. The so-called 
 " Potomac Flats " south of the city of Washington owed their 
 origin and unhealthy conditions to similar processes. 
 
 The method of alluvial deposition in the flood plain, or 
 delta, of the lower Mississippi has been worked out by McGee, 1 
 from whom we cannot do better than quote in considerable 
 detail. 
 
 In length this flood plain reaches from the mouth of the Ohio 
 1100 miles measured along the river, or half as far measured 
 in an air line, to the Gulf, and is bounded on the east by the 
 bluff rampart separating it from the contiguous district ; it is 
 bounded on the west by a less continuous and less conspicuous 
 rampart crossing the Arkansas River at Little Rock and grad- 
 ually failing southward until this district and its more westerly 
 
 1 The Lafayette Formation, Ann. Rep. U. S. Geol. Survey, 1890-91. 
 
324 THE REGOLITH 
 
 neighbor nearly blend. The surface of this otherwise monoto- 
 nous district is relieved by a few small tracts of higher land. 
 Most conspicuous of these is Crowley Ridge in eastern Arkansas, 
 a long belt of upland stretching from the southeastern Missouri 
 southward between the White and St. Francis rivers to the 
 Mississippi at Helena. This belt of upland rises 100 or 200 feet 
 above the insulating flood plain, and in its steepness of slope 
 and rugosity of outline fairly simulates the eastern rampart 
 overlooking the " delta " in corresponding latitudes. 
 
 The vast lowland tract comprised in and constituting most of 
 this district is at once the most extensive and most complete 
 example of a land surface lying at base-level or a trifle below 
 that the continent affords. 
 
 It is trenched longitudinally by the Mississippi, and trans- 
 versely by the White, Arkansas, Red, and other large rivers ; 
 between these greater waterways it is cut into a labyrinth of 
 peninsulas and islands by a network of lesser tributaries and 
 distributaries, the former gathering the waters from its own 
 surface and from adjacent country, and the latter aiding the 
 main river to discharge its vast volume of water and its immense 
 load of detritus into the Gulf. The whole surface lies so low 
 that it is flooded by periodic overflows of the Mississippi and 
 its larger tributaries, and with each flood receives a fresh coat- 
 ing of river sediment ; and much of the flood plain, fertilized by 
 freshet deposits, is clothed with luxuriant forests and dense 
 tangles of undergrowth, or with brakes of cane, or with sub- 
 tropical shrubbery, only a few of the broader inter-stream tracts 
 being grassed. Partly by reason of this mantle of vegetation, 
 the current of each overflow is checked as the river rises above 
 its banks, and most of the sediment is dropped near by; and so 
 the Mississippi, the White, the Arkansas, and the Red, as well 
 as each lesser tributary and each distributary from the great 
 Atchafalaya down, are flanked by natural levees of height and 
 breadth proportionate to the depth and breadth of the stream. 
 The network of waterways is thus a network of double ridges 
 with channels between; and each inter-stream area is virtually 
 a shallow, dish-like pond in which the waters of the floods lie 
 long, to be drained finally, perhaps, through fresh-made breaks 
 in the natural dikes, weeks after the stream flood subsides. In 
 the southern part of the district the inter-stream basins approach 
 tide level and drain still more slowly ; in the sub-coastal zone 
 
ALLUVIAL DEPOSITS 
 
 325 
 
 many of the basins are permanent tidal marshes. In the western 
 part of the district is an area in which the inter-stream basins 
 lie so high that they are invaded only by the highest floods and 
 veneered with only the finest sediments ; in some cases these 
 sediments are so fine and so compactly aggregated and the 
 surface is so ill drained and watered that trees may hardly 
 take root, and these are either drowned by the floods or with- 
 ered by the sun. in the drought. Such portions of the sur- 
 face are but scantily covered with coarse grass and form 
 the "black prairies" of southern Arkansas and northwestern 
 Louisiana. 
 
 It is to just such processes as those described that the Nile 
 valley owes its remarkable fertility. The sediments depos- 
 ited over these plains during the season of freshets consist of 
 fine sand brought down by the Blue Nile and the Atbara from 
 the decomposing siliceous rocks of mountainous Abyssinia. The 
 gneisses and granites yield their detritus to the lixiviating in- 
 fluence of the mountain torrents and majestic Nile, the clayey 
 particles being borne seaward, while the fresh quartzose, feld- 
 spathic and other siliceous particles, and smaller traces of apa- 
 tite and alkaline carbonates remain in just the right stage of 
 subdivision to yield a soil, which has brought forth for a period 
 of over 4000 years crop after crop without artificial fertilization. 
 
 The following table will serve to show the physical character- 
 istics of alluvial deposits, a portion of which are but reasserted 
 materials from the glacial drift. 
 
 APPROXIMATE NUMBER OF GRAINS OF SAND, SILT, AND CLAY IN ONE GRAMME 
 OF ALLUVIAL SUB-SOIL FROM ILLINOIS 
 
 DIAMETER 
 
 CONVENTIONAL 
 
 (a) 
 
 (6) 
 
 (c) 
 
 MM. 
 
 NAMES 
 
 CHILLICOTHE 
 
 EOCKFOBD 
 
 AMERICAN BOTTOMS 
 
 2-1 
 
 Fine gravel 
 
 
 
 1 
 
 
 
 1-.5 
 
 Coarse sand . 
 
 83 
 
 48 
 
 
 
 .5-.25 
 
 Medium . . . 
 
 6,755 
 
 3,428 
 
 5 
 
 .25-.! 
 
 Fine sand . . 
 
 18,660 
 
 29,300 
 
 194 
 
 .1-.05 
 
 Very fine sand 
 
 53,470 
 
 212,400 
 
 151,400 
 
 .05-.01 
 
 Silt .... 
 
 4,670,000 
 
 5,888,000 
 
 12,230,000 
 
 .01-.005 
 
 Fine silt . . 
 
 86,860,000 
 
 115,100,000 
 
 195,600,000 
 
 .005-.0001 
 
 Clay .... 
 
 2,537,000,000 
 
 3,842,000,000 
 
 14,680,000,000 
 
 
 Total. . . . 
 
 2,628,608,968 
 
 3,963,232,177 
 
 14,887,981,599 
 
 (a) Terrace of Glacial age. (6) Flood deposits, 
 (bottom land of Mississippi) . 
 
 (c) Post-glacial terrace 
 
326 THE REGOLITH 
 
 The same processes active in delta formation are manifested 
 on a smaller scale in the gradual silting up of many inland 
 lakes, particularly those of glacial origin, the rapidity of the 
 filling being augmented by aquatic plants. 
 
 These lakes lie not infrequently between high hills, being 
 fed by one or more streams flowing through narrow valleys, 
 and having outlets at the opposite extremity. Soon after the 
 close of the Glacial epoch, we may imagine one of these to 
 have existed as a lake of clear blue water of varying depths, 
 filled with abundant fish and wild fowl. But the little streams 
 which fed it brought down continually sand and silt to be de- 
 posited at varying distances so soon as the currents fall to sleep 
 within the bosom of the lake. Hence each year it shallows, 
 and the pure white water-lily, reeds, and the rotting trunks of 
 trees and shrubs encroach upon its shores until in course of 
 time there remains but a flat plain, for a time subject to annual 
 inundations, but ultimately permanently above the level of but 
 the most severe floods, and through which flow in a meander- 
 ing course the sluggish streams that first gave it birth and then 
 wrought its extinction. This is the story of thousands of the 
 so-called meadows, swales, swamps, and intervals throughout 
 the northern portion of the United States, and the process in 
 some easily recognizable stage may be found in almost any lake 
 or pond now remaining. 
 
 It is a striking thought that all our lakes are but transient 
 enlargements of pre-existing streams, and will in time, perhaps 
 even before our own species is extinct, become converted into 
 broad expanses of meadow lands ; and that our children's chil- 
 dren may yet sow and reap from rich and fertile areas which 
 now echo only to the cry of water-fowls, and whose blue ex- 
 panse is broken but by wind-born waves and leaping fish. 
 
 The lithological character of the deposits thus formed vary 
 within certain limits almost indefinitely, since everything de- 
 pends on the character and quantity of the silt brought down 
 by the streams. Rarely, if ever, are they clayey, since the finer 
 particles are carried beyond. In nearly all instances they are 
 found to consist of very fine sand, largely siliceous, permeated, 
 often quite blackened, through the presence of organic matter. 
 Such are the mucks or mucky soils of New England. 
 
 So abundant is this organic matter that, when dried, such are 
 not infrequently used locally for mulching purposes, though 
 
ALLUVIAL DEPOSITS 327 
 
 in their fresh condition they are sour and almost worthless 
 except for growing sedges and the ranker kinds of forage 
 grass. During the later stages of the process of filling up, 
 deposition of sediments may almost entirely cease, since the 
 water no longer rises above the level of past accumulations. 
 In such cases the final stages consist simply in the accumu- 
 lation of organic matter and the deposits come to closely 
 resemble, or are even superficially identical with, the cumulose 
 deposits already described. This same statement holds good 
 also for the closely related salt-water marsh or paludal deposits, 
 to be noted later. 
 
 Loess and Adobe. Under the head of transported deposits, 
 we must also consider the so-called loess of the Mississippi val- 
 ley in our own country ; of the Rhine valley, and other parts 
 of Europe ; of northern China and the Russian steppes, though, 
 as we shall see, the name includes deposits which, while having 
 many physical properties in common, may vary widely in com- 
 position as well as in method of deposition. It is more than 
 doubtful, indeed, if the name, through misapprehension, has not 
 been so loosely applied as to rob it of its proper geological 
 significance. 
 
 The loess of China, made famous through the researches of 
 Richtofen, is now regarded by some authorities 1 as of the 
 same nature as our adobe. Richtofen himself, it will be re- 
 membered, regarded the Chinese loess as largely an yeolian 
 deposit, as due to the action of wind in transporting for long 
 distances the fine detritus swept by rain and wind from moun- 
 tain slopes into enclosed basins, to ultimately become entangled 
 and deposited among the growing vegetation. This foreign 
 material, intermingled with the collective residue of herba- 
 ceous plants, with the inorganic residuum from the decay of 
 prairie vegetation for countless generations, makes up its mass 
 over many hundreds of square miles of territory, and to depths 
 in places of thousands of feet. The characteristics of the loess, 
 as found in China, are those of a fine calcareous silt or clay, of 
 a yellowish or buff color, so slightly coherent that it may be 
 readily reduced to powder between the thumb and fingers, and 
 yet possessing such tenacity as to resist the ordinary weather- 
 ing action of the atmosphere, and, wherever cut by stream 
 erosion or other means, to stand with vertical walls, even 
 1 See I. C. Russell, Subaerial Deposits of North America, Geol. Mag., August, 1889. 
 
328 THE REGOLITH 
 
 though they may be hundreds of feet in height. The loess 
 country is described as thus cut up by an almost impassable 
 system of gorges, so that to cross it in any fixed direction is 
 almost an impossibility. "Wide chasms are surrounded by 
 castles, towers, peaks, and needles, all made up of yellow earth, 
 between which gorges and chasms radiate labyrinthically up- 
 wards into the walls of solid ground around. High upon a 
 rock of earth steeper than any rock of stone stands the 
 temple of the village, or a small fortress which affords the 
 villagers a safe retreat in times of danger. The only access 
 to such a place is by a spiral stairway dug out within the mass 
 of the bluff itself. In this yellow defile there are innumerable 
 nooks and recesses, often enlivened by thousands of people, 
 who dwell in caves dug in the loess." 1 
 
 One of the striking features of the loess, both in China and 
 elsewhere, is the abundance of minute tubes or canals lined 
 with carbonate of lime which traverse it from above down- 
 ward, and which are assumed by some to be due to root fibres. 
 It is the presence of these presumably that causes the vertical 
 cleavage, and at the same time the remarkable absorptive quali- 
 ties for which the loess is noted. Such is the material which 
 for more than three thousand years has brought forth crops 
 continuously, and without exhaustion, over many square miles 
 of the Chinese Empire. Its distribution in Europe is given as 
 extending from the French coast at Sangatte, eastward across 
 the north of France and Belgium, filling up the depressions of 
 the Ardennes, passing far up the valleys of the Khine and its 
 tributaries, the Neckar, Main, and Lahr ; likewise those of the 
 Elke above Meissen, the Weser, Mulde, and Saale, the upper 
 Oder and Vistula. Spreading across upper Silesia, it sweeps 
 eastward over the plains of Poland and southern Russia, where 
 it forms the substratum of the tschernoseun, or black earth. 
 It extends into Bohemia, Moravia, Hungaria, Galicia, Transyl- 
 vania, and Roumania far up into the Carpathians, where it 
 reaches heights of from 2000 to 5000 feet above sea-level. In 
 northern China it spreads over a large portion of the region 
 drained by the Hoang-Ho. For nearly a thousand miles 
 from the borders of the great alluvial plain of Pechele, through 
 the provinces of Shan si, Shensi, and Kansu, everywhere to the 
 
 J The Chinese Loess Puzzle, by J. D. Whitney, American Naturalist, 
 December, 1877. 
 
LOESS AND ADOBE 
 
 329 
 
 northern base of the range of the Tsing-ling-shan, the loess 
 may be followed to the very divide which separates the basin 
 of the Hoang-Ho from the region destitute of drainage, into 
 the sea. Toward the north it reaches almost to the edge of 
 the Mongolian plateau. The entire area covered continuously 
 is stated to be as large as the whole of Germany, while it is 
 found in more or less detached portions over an area in addi- 
 tion, nearly half as large. In the United States the loess 
 covers thousands of square miles throughout the drainage 
 basin of the Mississippi River. It is found in Ohio, Indiana, 
 Michigan, Iowa, Kansas, Nebraska, Illinois, Tennessee, Ala- 
 bama, Mississippi, Louisiana, Arkansas, Missouri, Kentucky, 
 and the Indian Territory. According to Professor Aughey it 
 prevails over at least three-fourths of Nebraska, to a depth 
 ranging from 5 to 150 feet, and furnishes a soil of extraor- 
 dinary strength and fertility. As here found, however, the 
 aeolian hypothesis fails to satisfactorily explain all the exist- 
 ing conditions, arid there is little doubt but that it represents 
 in large part the fine silt, the glacial flour brought down by 
 the ice of the Glacial epoch, borne southward by streams, and 
 deposited in water just sufficiently in motion to carry the fine 
 clay farther away. The 
 loess, in fact, illustrates 
 in a remarkable manner 
 the wonderful assorting 
 power of water. 
 
 Microscopic and chemi- 
 cal examinations of loess 
 sustain this hypothesis. 
 The particles are as a rule 
 quite fresh and sharply 
 angular. Out of 150,000 
 particles examined under 
 the microscope only about 
 3% measures above .0025 
 of a millimetre and 1 % 
 over .005 of a millimetre. 
 
 f ^ . FIG. 33. Showing outlines of particles in 
 
 (Quartz IS the prepon- Chinese loess. 
 
 derating material, with 
 
 lesser amounts of orthoclase and plagioclase feldspars, white 
 
 and dark micas, hornblende, augite, magnetite, dolomite, and cal- 
 
330 
 
 THE KEGOLITH 
 
 BAD 
 RLA 
 
 O t^ 
 
 ^ 
 
 11 . i i co o <r> 
 
 11 .CCgtNCOO 
 
 g 
 
 i-H rH C<c 
 
 C1OCM ' ' ' ' CO O CO H CO O '. ', CO 
 
 g 
 
 9 So Ot-CNOOOOODTtir-iT-ir-iOOO 
 
 'e 
 
 w 
 a 
 
 a" 
 
 COCOCOCO(MCOOOi-iCOCOOOi-HOi 
 CO O O O O i i f"i i^ C^ G^ O O O 
 
 : ; g ^ ; : : : ; i" 
 
 ^ ^ o C /-^ <D 
 
 o o :s o 3 
 
 S E ^ ^ ' ^ B -S 
 
 ac^ *^ GJ 
 
 ^ --H O r^ 
 
 Ii!l|!SltlP!i! 
 
 II If Iff IN ft 111 
 
LOESS AND ADOBE 
 
 331 
 
 cite. The loess of the Rhine valley and of China offers no differ- 
 ences that can be readily described, though, as will be noticed 
 by reference to the analyses, there may be a wide difference in 
 chemical composition. Indeed, the essential characteristic of 
 the loess is a physical rather than a chemical one, and it is 
 doubtless to this that is due its uniform fertility. On p. 330 
 are given analyses of loess from the United States, the Rhine 
 valley, and from Switzerland. 
 
 The following table will serve to show the fine state of sub- 
 division in which the particles exist in loess as well as in the 
 dust brought down by snow, which will be described on p. 344. 
 
 
 I 
 
 n 
 
 III 
 
 IV 
 
 CONSTITUENTS 
 
 it 
 
 H?0 
 g|l 
 
 IP 
 
 KIVER LOESS : 
 VIRGINIA CITY, 
 ILLINOIS 
 
 LOESS : 
 NEBRASKA 
 
 DUST FROM SNOW : 
 ROCKVILLE, 
 INDIANA 
 
 Moisture 
 
 
 
 5.40% 
 
 3.17% 
 
 Organic matter 
 
 
 
 4.96 
 
 11.98 
 
 
 0.00 % 
 
 0.00 % 
 
 0.00 
 
 0.00 
 
 
 0.00 
 
 0.00 
 
 0.00 
 
 0.00 
 
 
 0.00 
 
 0.01 
 
 0.00 
 
 0.00 
 
 Fine sand. 
 
 0.01 
 
 0.10 
 
 0.00 
 
 0.00 
 
 Very fine sand 
 
 7.68 
 
 24.84 
 
 23.14 
 
 0.00 
 
 Silt 
 
 61.85 
 
 60.98 
 
 54.81 
 
 69.37 
 
 Fine silt .... 
 
 9.60 
 
 2.80 
 
 2.46 
 
 5.80 
 
 Clay 
 
 15.15 
 
 6.15 
 
 9.45 
 
 9.68 
 
 
 
 
 
 
 Aughey l gives the following section of the loess and soil in 
 
 Nebraska. 
 
 (1) Loess 4 feet 
 
 (2) Black soil 2 
 
 (3) Loess 4 
 
 (4) Black soil 1 
 
 (5) Loess 6 
 
 (6) Black soil 
 
 (7) Stratified loess 15 
 
 This alternation is accounted for on the assumption of fre- 
 quent changes of level during the loess-forming period. It 
 would seem that the loess was deposited in shallow water and 
 that as the lake became filled plant life came in as in modern 
 
 1 Physical Geology and Geography of Nebraska, p. 276. 
 
332 THE REGOLITH 
 
 bogs and marshes, and throve until sufficient organic matter. was 
 formed to make the black soil layer. A period of subsidence 
 followed, more loess was deposited and the previous condition 
 repeated, this process going on till all the layers were formed. 
 The material of the loess, in this case, would seem most likely 
 to have been of seolian origin. 
 
 The name adobe is given to a calcareous clay of a gray-brown 
 or yellowish color, very fine-grained and porous, which is suffi- 
 ciently friable to crumble readily in the fingers, and yet, like 
 loess, has sufficient coherency to stand for many years in the 
 form of vertical escarpments, without appreciable talus slopes. 
 The material of the adobe is derived from the waste of the 
 surrounding mountain slopes, the disintegration being largely 
 mechanical. According to Prof. I. C. Russell, 1 from whose de- 
 scriptions is drawn a portion of what is given here, it is assorted 
 and spread out over the valley bottom by the action of ephemeral 
 streams, where it becomes mixed with dust blown by the winds 
 from the neighboring mountains, and rendered more or less 
 coherent by the cementing action of interstitial carbonate of lime. 
 
 Hilgard 2 limits the name adobe to the distinctly clayey soils of 
 the arid regions, and divides them into two classes, the upland 
 and the valley adobes, the first being derived mainly from the 
 disintegration, in place, of clay shales, while the second are 
 mostly paludal or swamp formations, and represent either the 
 finest materials that remain suspended in slack water, from any 
 source, or sometimes the direct washings of the clayey soils of 
 the hills. Whichever authority we follow, it is evident the 
 name includes materials alike not in mode of origin or com- 
 position, but only in physical characteristics. 
 
 Adobe forms the soil of a large portion of the rainless region 
 of the United States. It is found therefore in Colorado, Utah, 
 Nevada, southern California, Arizona, New Mexico, and west- 
 ern Texas, as well as in the southern portion of Idaho, Wyoming, 
 and Oregon. It has also a wide distribution in Mexico. In 
 the United States it occurs from near the sea-level in Arizona, 
 and even below it in southern California up to an elevation of 
 at least 6000 or 8000 feet along the eastern border of the Rocky 
 Mountains, and in the elevated valleys of New Mexico, Colorado, 
 and Wyoming. 
 
 1 Subaerial Deposits of North America, Geol. Mag., August, 1889. 
 
 2 Bull. 3, U. S. Weather Bureau, Dept. of Agriculture, 1892. 
 
LOESS AND ADOBE 333 
 
 The maximum thickness of the various deposits grouped 
 under this name is not in all cases readily determined, for the 
 reason that it is still accumulating and has not been sufficiently 
 dissected by erosion to expose sections to any considerable 
 depth. Many of the valleys of the arid region have been filled 
 by it to a depth of 2000 or 3000 feet. In the larger valleys 
 there are rocky crests, called " lost mountains," which project 
 above the broad level desert surface, and which are in reality 
 the summits of precipitous mountains that have been almost 
 completely buried beneath these recent accumulations. The 
 prevailing color of adobe is light buff to gray, excepting when 
 contaminated with organic matter. In its typical form it is so 
 fine as to be quite without grit when rubbed between the fingers. 
 When examined under the microscope, it is seen to be com- 
 posed of irregular unassorted flakes and grains, principally 
 quartz, but fragments of other minerals are also present. The 
 adobe of Salt Lake shows flocculent masses of amorphous matter, 
 which, when thoroughly disintegrated, are found to consist of 
 minute sharply angular fragments of quartz and feldspar with 
 much calcareous matter, and only rarely a shred of micaceous 
 or hornblendic material. In size the particles vary from those 
 too small for measurement up to .08 millimetre in diameter. 
 
 The valuable characteristics of the adobe are its extreme 
 fineness, great depth, and wonderful fertility. 
 
 Although comprising the soil of almost the entire region that 
 was but recently known as the Great American Desert, it needs 
 but water to make it laugh with harvests. While its physical 
 properties undoubtedly have much to do with its fertility, this 
 quality must also be in part due to the fresh arid undecomposed 
 condition of its constituent parts. Originating doubtless by 
 purely mechanical agencies, it has been swept by winds and 
 spasmodic rains into closely adjacent basins occupied by but 
 temporary lakes, where, spread out over a floor sometimes almost 
 absolutely level, it has been subjected to a minimum amount of 
 leaching and retains until to-day its youthful strength and 
 powers of recuperation. 1 The analyses given on p. 334 will 
 serve to show the varying character of the deposits included 
 under this name. Especial attention need only be called to 
 the relatively high percentages of lime and the alkalies. 
 
 Under the head of alluvial deposits we may also consider 
 i See further on p. 369. 
 
334 
 
 THE KEGOLITH 
 
 those clay accumulations which result from the deposition of 
 fine aluminous sediments sorted by running streams from gla- 
 cial debris and like the loess laid down in quiet water, though 
 usually estuarian rather than lacustrine. These are the well- 
 known Leda clays 1 of glacial regions, and which on genetic 
 grounds might well be classed as aqueo-glacial deposits. 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 Silica (Si0 2 ) 
 Alumina (A^Os) 
 
 66.69 % 
 14 16 
 
 44.64 % 
 13 19 
 
 Ferric oxide (Fe 2 0s) 
 
 4.38 
 
 5.12 
 
 Manganese oxide (MnO) 
 
 0.09 
 
 13 
 
 Lime (CaO) 
 
 2.49 
 
 13.91 
 
 Magnesia (MgO) 
 
 1 28 
 
 2 96 
 
 Potash (K 2 0) 
 
 1 21 
 
 1 71 
 
 Soda (Na 2 0) 
 
 0.57 
 
 59 
 
 Carbonic acid (C0 2 ) 
 
 77 
 
 8 55 
 
 Phosphoric acid (P 2 05) 
 
 0.29 
 
 94 
 
 Sulphuric anhydride (S0 3 ) 
 Chlorine (Cl) 
 
 0.41 . 
 34 
 
 0.64 
 14 
 
 Water (H 2 0) 
 
 4 94 
 
 3 84 
 
 Organic matter 
 
 2.00 
 
 3 43 
 
 
 
 
 
 99.72% 
 
 99.84 % 
 
 I. Adobe from Santa F, New Mexico. II. Adobe from Fort Wingate, New 
 Mexico. 
 
 Such are very abundant along all the lower valleys of the 
 principal rivers of New England, sometimes coming to the im- 
 mediate surface or overlaid with a thin layer of sandy material 
 which, together with a little organic matter, forms the true soil. 
 They form, according to Dawson, 2 the sub-soils over a large part 
 of the great plains of Lower Canada, varying in thickness up 
 to 50 or even 100 feet, usually resting upon the boulder clay. 
 They are, as a rule, of almost impalpable fineness, unctious, and 
 extremely plastic. Excepting where superficially oxidized to 
 buff or brown, they are of a blue-gray color and may show on 
 analysis considerable quantities of lime carbonate and alkalies, 
 features whereby they are readily distinguished from the resid- 
 ual clays, and which are regarded as indicative of an origin by 
 mechanical rather than chemical means. When dried, they be- 
 come greatly indurated, and when unmixed with other mate- 
 rials, bake so hard during seasons of drought, or are so plastic 
 1 So called from their most characteristic fossil, Leda. 2 The Canadian Ice Age. 
 
THE CHAMPLAIN CLAYS 
 
 335 
 
 during seasons of rainfall, as to be quite uiisuited for cultivation. 
 Mixed with varing proportions of siliceous sand to counteract 
 shrinkage, they form the common brick-making materials of the 
 Northeastern states, burning red and brown. 
 
 The materials of the Leda clays naturally vary in different 
 localities, being dependent on the characteristics of the rocks 
 from which they were de- 
 rived. Those of Canada, 
 according to Dawson, 
 were derived from the 
 waste of the Utica and 
 Quebec groups. This 
 authority believes that 
 when the clay was in sus- 
 pension, it was probably 
 of a reddish or brown 
 color from the iron per- 
 oxide it contained, but 
 that, like the bottom mud 
 now forming in the deeper 
 parts of the St. Lawrence, 
 
 the coloring matter be- FIG. 34. -Showing 7article7 from Leda clays. 
 Came deoxidized by or- 1, quartz ; 2, orthoclase ; 3, plagioclase ; 4, mica ; 
 ganic matter SO soon as Mourmaline ; 6, pyroxene; 7, chlorite; 8, Horn- 
 
 deposited, the iron being 
 
 converted into a sulphide or protoxide carbonate. Inasmuch, 
 however, as the materials were so largely derived by the grind- 
 ing action of the glaciers on fresh rocks, it is not impossible 
 that they may have been again deposited as clay without hav- 
 ing ever undergone the oxidizing process. 
 
 Unlike the till or boulder clays, these Leda clays are dis- 
 tinctly stratified, as shown in the accompanying illustration. 
 (PL 24.) An analysis of a sample from this locality yielded 
 the author results as given in column I on p. 336. In column II 
 is given that of the portion (33.26 %) soluble in hydrochloric 
 acid and sodium carbonate solutions, while in column III is given 
 the composition of a "semi-assorted glacio-lacustrine " clay 
 bordering on Lake Michigan near Milwaukee, Wisconsin, and 
 in IV a glacial pebbly clay underlying II at the same locality. 1 
 
 1 Analyses II and III from Chamberlain and Salisbury's paper, 6th Ann. Rep. 
 U. S. Geol. Survey, 1884-85. 
 
336 
 
 THE REGOLITH 
 ANALYSES OF STRATIFIED CLAYS 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 III 
 
 IV 
 
 Silica (Si0 2 ) .... 
 
 56.17 L 
 
 10.98% 
 
 40.22 % 
 
 48.81 % 
 
 Alumina/ (A1 2 03) 
 
 24.25 
 
 8.66 
 
 8.47 
 
 7.54 
 
 Phosphoric acid (P 2 5 ) 
 Titanic oxide (Ti0 2 ) 
 Ferric iron (Fe 2 O3) 
 
 Not det. 
 Not det. 
 Not det. 
 
 Not det. 
 Not det. 
 Not det. 
 
 0.05 
 0.35 
 
 2.83 
 
 0.13 
 0.45 
 2.53 
 
 Ferrous iron (FeO) 
 
 3.54 
 
 5.19 1 
 
 ' 0.48 
 
 0.65 
 
 Manganese oxide (MnO) 
 Lime (CaO) 
 
 Not det. 
 2.09 
 
 Not det. 
 1.02 
 
 Trace 
 15.65 
 
 0.03 
 11.83 
 
 Magnesia (JVI '0) 
 
 2 57 
 
 2.19 
 
 7 80 
 
 7.05 
 
 Potash (K 2 0) 
 
 4.06 
 
 1 12 
 
 2.36 
 
 2.60 
 
 Soda (Na 2 0) 
 Water (H 2 O) 
 
 2.25 
 4.69 
 
 0.75 
 3.65 
 
 0.84 
 1.95 2 
 
 0.92 
 2. 02 2 
 
 Carbonic acid (C0 2 ) 
 Organic carbon (C) 
 
 None 
 None 
 
 None 
 None 
 
 18.76 
 32 
 
 15.47 
 0.38 
 
 Sulphuric anhydride (S0 3 ) .... 
 
 None 
 
 None 
 
 0.13 
 
 0.05 
 
 
 99.56% 
 
 33.26 % 
 
 100.21 % 
 
 100.46% 
 
 Related to the delta deposits already described, but differing 
 in that their inorganic materials are in large part derived im- 
 mediately from the sea, are due to the transporting and assort- 
 ing poAver of tide and wave action, are the salt-water marsh, or 
 paludal deposits so common along the Atlantic border of North 
 America. In discussing the formation of these and their grad- 
 ual transitions into arable lands, we cannot do better than 
 follow Professor N. S. Shaler. 3 
 
 The formation of a sea-coast swamp is due mainly to wave 
 action and plant growth. It is dependent upon the configura- 
 tion of the coast. Wave action upon an irregular coast such 
 as that of New England nearly always results in a breaking or 
 wearing away of the exposed headlands and the transportation of 
 the debris from these into intervening inlets, and thrown upon, 
 or at least in a direction toward, the beaches. On these beaches, 
 as one may any day observe, the rock fragments are ever being 
 ground smaller and smaller, and must in time be reduced to 
 the condition of the finest sand and mud. Each incoming 
 
 1 All iron determined as FeO. 
 
 2 Contains H of organic matter dried at 100 C. 
 
 3 Ann. Rep. Director of the U. S. Geol. Survey, 1884-85. 
 
SEA-COAST SWAMP DEPOSITS 337 
 
 wave hurls more or less of this fragmental material upon the 
 beach, whence a considerable portion of it may be again carried 
 seaward by the bottom current or undertow as the wave re- 
 cedes. One who has stood upon a high rock on the sea-shore 
 and watched the waves come tumbling at his feet and then go 
 creeping silently oceanward once more cannot have failed to 
 notice the continual seething sound due to the constant drag 
 of the rock fragments one over the other as they are impelled 
 inward and outward by the alternating currents. A consider- 
 able part of this mud is taken out to sea by the undertow, or 
 bottom current, which always sets from a storm-beaten beach 
 along the bottom, but another part is urged by the movement 
 of the water caused by the waves and of the tidal flow into the 
 fjords, where it falls to the bottom. In this process of carriage 
 the mud is generally conveyed along the shores and is most 
 commonly deposited in the parts of the inlets near the shore 
 line. Wherever there is a bay within which the tidal current 
 is deadened and where the waves have little play, this sediment 
 is most rapidly laid down. If the process of deposition begins 
 on a pebbly bottom, it is at first aided by the irregularities be- 
 tween the stones and the friction of the water among the sea- 
 weeds, which frequently attach themselves to the stones. As 
 soon as a sheet of mud is established, it commonly becomes 
 occupied by a dense growth of eel-grass. This plant, by its 
 habit of growth, greatly favors the deposition of sediment. The 
 separate stems are set very closely together, the interspaces not 
 generally exceeding 1 or 2 inches. A tidal current of 2 miles 
 an hour, swift enough to carry much sediment, is almost en- 
 tirely deadened in this tangle of plants. 
 
 At half tide on the New England coast these eel-grass fields 
 are generally covered with water to the depth of several feet ; 
 at this stage the tidal currents are commonly strongest. The 
 water above the level of the grass has its usual freedom of 
 motion and brings much sedimentary matter above the level of 
 the foliage. As the tide falls, a part of this waste is entangled 
 and held until it gradually sinks to the bottom, so that each 
 run of the tide gives a certain contribution of sedimentary 
 matter, which goes to shallow the water. This process is easily 
 observed from a boat floating over a field of these plants. The 
 deadening of the current when the lowered tide brings the tops 
 of the plants near the surface is very noticeable. The mass of 
 
338 THE KEGOL1TH 
 
 floating matter mud, fronds of sea-weed (often with shells or 
 small pebbles attached to their bases), dead fish, and a mass of 
 other refuse, is seen to collect in the mesh of foliage and sink 
 to the bottom. The dead stems of the eel-grass and the bodies 
 of many small crustaceans and mollusca which live on its stalks 
 or on the bottom contribute to the deposit, so that it thickens 
 with considerable rapidity. 
 
 When the bed formed on the sea-bottom by the action of the 
 eel-grass and its associated plants has risen to the point where 
 it is dry at low water of the ordinary run of tides, the eel-grass 
 can no longer maintain itself, but gives place to other groups 
 of sea-weeds and grasses. 
 
 These species of plants find their place first near the shore 
 line, where the eel-grass platform is naturally the highest. At 
 first their vegetation is quite sparse, owing to the difficulty 
 with which they endure the depth of water at high tide. There 
 is often, indeed, a considerable difficulty in establishing the 
 growth of the second group of plants, and for a while the de- 
 posit takes the shape of bare mud-flats, dependent in the main 
 for their accumulation of detrital matter on the growth of 
 certain mollusca, especially of the genera Mytilus and Modiola. 
 
 Ide 
 
 FIG. 35. Cross-section of marine marsh, a, original surface of shore line ; 6, grassy 
 marsh; c, mud-flats; d, eel-grass; e, mud accumulated in eel-grass. 
 
 When, as is usually the case, the more highly organized plants 
 have difficulty in establishing themselves over the broad sur- 
 face of the mud-flat, they win their way to it in the following 
 manner. 
 
 From the vantage ground of the shore line, where these plants 
 easily find the conditions of submergence which suit their needs, 
 the plants slowly extend the front of their bench out over the 
 mud-flats. (See Fig. 35.) 
 
 This process of growth can be more easily studied than that 
 of the earlier or eel-grass stage of the marshes, for it is visible 
 along miles of our sea-shore. The higher grasses have even 
 
SEA-COAST SWAMP DEPOSITS 339 
 
 more thick-set stems than those of the eel-grass flats ; they en- 
 tangle sediment even more effectively. At first their stems are 
 covered for a few hours at each ordinary tide ; they gather 
 waste rapidly, and soon lift the plain which s they are construct- 
 ing up to the point where only at the highest tides are the tops 
 covered by water. At this stage the growth of the deposit is 
 practically arrested, there being no means of increase save from 
 the decay of the grasses themselves. 
 
 " On the central parts of the New England shore, as about 
 Boston, the mud-flat occupies at most 2 or 3 feet in the alti- 
 tude above mean low tide and the annual addition to its mass 
 in a year is very small," perhaps not so much as the tenth of an 
 inch in a year. " On the other hand, in the Basin of Minas, 
 one of the principal inlets leading from the Bay of Fundy, the 
 contribution of sediment is so great that vast areas have been 
 easily reclaimed from the sea by building a rude enclosure 
 around an area of the higher parts of the mud-flat, so that the 
 speed of the sediment-laden waters is checked and they are 
 made to lay down their burdens. In a few years, often in a 
 few months, this enclosed area is raised to near the level of 
 high tides. It is then only necessary to erect a barrier suffi- 
 cient to exclude the tide, with gates for the rain water, in order 
 to have the land completely reclaimed from the sea. In this 
 simple way there has been an area of many thousand acres of 
 excellent arable land created along these shores." l 
 
 The lithological and chemical character of deposits of this 
 nature have been but little studied, and we are here able to give 
 only two analyses, as below, in which, however, it is probable 
 that the matter tabulated as insoluble silica includes as well all 
 silicates insoluble in acid. 
 
 Column I of the table is mud from the marshes of Newport 
 River, a few miles above Beaufort, in Carteret County, North 
 
 1 As the total reclaimable area between New York and Portland (Maine) 
 probably exceeds 200,000 acres, their money value in their best state will 
 amount to at least $40,000,000,000. The cost of reclaiming these areas and re- 
 ducing them to cultivation should not exceed the fifth part of that sum. It may 
 be noted that from the chemical composition of these soils, they are practically 
 inexhaustible, and that from their position they are often well placed for irriga- 
 tion. South of the New England shore the marsh area is much more extensive 
 than in that region. It is probable that the improvable marshes of the Atlantic 
 coast amount to at least 3,000,000 acres and they may exceed double this amount, 
 (Shaler, p. 380.) 
 
340 
 
 THE KEGOLITH 
 
 Carolina. This marsh, formed by the filling up of the old river 
 channel, several miles wide, is continually enlarging at the ex- 
 pense of the water surface ; and similar formations, to the extent 
 of hundreds of square miles, are accumulating in very many 
 shallow bays and sounds and rivers near the sea. 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 Silica insoluble 
 
 54 42 % 
 
 72 70 / 
 
 Silica soluble 
 
 
 '"" /o 
 
 1 92 
 
 Oxide of iron and alumina 
 
 16.45 
 
 5 69 
 
 Lime . . . 
 
 1.18 
 
 1.39 
 
 Magnesia 
 
 0.07 
 
 0.05 
 
 Potash 
 
 1.18 
 
 1.82 
 
 Soda 
 
 79 
 
 35 
 
 Phosphoric acid . 
 
 0.25 
 
 0.13 
 
 Sulphuric acid 
 
 1.46 
 
 0.33 
 
 Organic matter 
 
 
 10 35 
 
 Water . 
 
 20 92 \ 
 
 
 Oxide of manganese . 
 
 54 J 
 
 3.65 
 
 Sulphide of iron 
 
 1.09 
 
 11 
 
 Common salt 
 
 1 63 
 
 1 71 
 
 
 
 
 
 99.98 % 
 
 100.10% 
 
 Column II is the sea mud or slime which is deposited in the 
 shoal waters of Beaufort Harbor and along the sounds and estu- 
 aries of the North Carolina coast. It is a fine, dark-colored 
 salt mud, formed of the silt brought down by the rivers, mixed 
 with decaying vegetable matter (mostly sea-weed and marsh 
 grass), and animal remains, of fish, molluscs, and all sorts of 
 marine organisms. 1 
 
 What is described by Whitney 2 as a typical swamp bog or 
 peat soil, from a rice field near Georgetown, South Carolina, 
 yielded the results as below, in columns I, II, III, and IV, the 
 last two being simply recalculated from columns I and II on an 
 organic and water-free basis. These are the so-called sob-field 
 soils, in themselves poor, but responding readily to fertilizers. 
 When exhausted by cultivation, they recuperate quickly through 
 the aid of silt deposits from the rivers, brought about by the 
 continual ebb and flow of the tides. 
 
 1 Geology of North Carolina, Vol. I, 1875, p. 214. 
 
 2 Rice, Its Cultivation, Production, and Distribution, Rep. No. 6, Misc. Series, 
 U. S. Dept. of Agriculture, 1893. 
 
BEACH SANDS 
 
 341 
 
 DIAMETER 
 
 
 I 
 
 II 
 
 III 
 
 IV 
 
 PARTICLES 
 
 CONVENTIONAL NAMES 
 
 SOIL 
 
 SUB-SOIL 
 
 SOIL 
 
 SUB-SOIL 
 
 mm. 
 
 
 0-6 inches 
 
 6-9 inches 
 
 0-6 inches 
 
 6-9 inches 
 
 2-1 
 
 Fine gravel 
 
 0.00% 
 
 00 % 
 
 00 / 
 
 00 / 
 
 
 
 
 
 
 
 1-.5 
 
 Coarse sand 
 
 0.71 
 
 0.08 
 
 1.86 
 
 0.14 
 
 .5-.25 
 
 Medium sand . 
 
 2.70 
 
 0.25 
 
 5 18 
 
 43 
 
 .25-.! 
 
 Fine sand 
 
 0.83 
 
 0.13 
 
 1.59 
 
 0.23 
 
 .1-.05 
 
 Very fine sand .... 
 
 0.37 
 
 0.15 
 
 0.71 
 
 0.26 
 
 .05-.01 
 
 Silt 
 
 10.32 
 
 13 97 
 
 19 79 
 
 24 30 
 
 .01-.005 
 
 Fine silt 
 
 5.32 
 
 7.10 
 
 10.20 
 
 14.09 
 
 005- 0001 
 
 Clay 
 
 31.90 
 
 34 85 
 
 61 17 
 
 60 65 
 
 
 
 
 
 
 
 
 Total mineral matter . . 
 
 52.15% 
 
 57.53% 
 
 100.00 % 
 
 100.00 % 
 
 
 Organic matter, water loss 
 
 47.85 
 
 42.47 
 
 
 
 
 
 100.00 % 
 
 100.00 % 
 
 
 
 
 Loss by direct ignition . 
 
 47.36 
 
 39.65 
 
 
 
 Beach Sands. Although differing radically in composition 
 from the sea-coast swamp deposits already described, we must, on 
 account of their intimate geological relationship, include here a 
 brief description of those fragmental deposits formed by wave 
 action along beaches and in many instances almost absolutely 
 free from organic matter of any kind. Such are the clean 
 white beach sands, the delight of the summer visitor at the sea- 
 sides. These are found here and there in isolated stretches 
 along the Atlantic slopes, particularly where, as at Old Orchard, 
 Maine, they receive the full sweep of wave and tide from the 
 open sea. In many instances the material forming these beaches 
 is siliceous sand, from glacial deposits which the ocean has 
 reasserted according to its own liking. In other cases it is 
 sand brought down by rivers, and which has undergone frac- 
 tional separation through the varying strength of transporting 
 agencies. In still others it is material derived immediately 
 from the shore rocks through the weathering action of atmos- 
 pheres and the hammering of the waves. In other cases yet, 
 as along the coasts of Florida, the source is problematical. We 
 can only say, knowing the character of rocks forming the main- 
 land, that they could not have here originated, but must have 
 been transported and probably down the coast, from the areas 
 of crystalline rocks to the northward. It is sometimes, though 
 
342 THE REGOLITH 
 
 not always, possible to gain an idea of the probable source of 
 these sands through a study of their mineralogical nature and 
 the physical condition of the individual particles. 
 
 Sorby, who devoted careful attention to the microscopic ap- 
 pearance of granules of quartz sand belonging to various geo- 
 logical periods, divided them into five types, "which though 
 characteristically distinct, gradually pass into one another." 1 
 These types are : - 
 
 1. Normal, angular, fresh-formed sand, as derived almost 
 directly from granitic or schistose rocks. 
 
 2. Well-worn sand in rounded grains, the original angles 
 being completely lost, and the surface looking like fine ground 
 glass. 
 
 3. Sand mechanically broken into sharp angular chips, show- 
 ing a glassy fracture. 
 
 4. Sand having the grains chemically corroded, so as to pro- 
 duce a peculiar texture of the surface, differing from that of 
 worn grains or crystals. 
 
 5. Sand in which the grains have a perfect crystalline out- 
 line, in some cases undoubtedly due to the deposition of quartz 
 over rounded or angular nuclei of ordinary non-crystalline 
 sand. 
 
 The material of most beach sands is largely quartz, though 
 this is not invariably so. Those of the Bermudas are, as a 
 matter of necessity, calcareous. Those of isolated deep-sea 
 islands like the Hawaiians, are derived in part from the vol- 
 canic rocks of the islands, and in some instances are composed 
 almost wholly of minute shells of the size of a pin's head. 
 These last from their faculty of emitting a crunching sound 
 when disturbed, are known as " sounding " or " singing sands." 
 
 The beach sand at Diamond Head, Oahu, is mainly of olivine 
 and magnetite granules, with smaller amounts of calcareous 
 matter. As usual, the grains in samples from the same level 
 are of fairly uniform dimension, varying from 0.51.0 millime- 
 tre, the larger forms being often fairly well rounded, while the 
 smaller may still show crystal outlines. The granules, even 
 in the same sample, however, vary greatly in the amount of 
 rounding they have undergone. Like the quartz granules from 
 the Florida beach, these show conchoidal drippings due to the 
 shock of impact as one granule strikes against another. 
 
 1 Proc. Geol. Soc. of London, Anniversary Address, Session, 1879-80, p. 58. 
 
BEACH SANDS 
 
 343 
 
 FIG. 36. Quartz granules in sand from beach, 
 Santa Rosa island. 
 
 The beach of Santa Rosa island, south of Pensacola, Florida, 
 is composed of clear white quartz sand of almost ideal purity. 
 The grains, though water-worn and with the lesser angles 
 rounded, are still in many 
 cases angular, and of very 
 uniform size (about .5- 
 1.0 millimetre), as shown 
 in Fig. 36.. These gran- 
 ules offer a very beauti- 
 ful illustration of Sorby's 
 type No. 2, the surface of 
 each one, through abra- 
 sion, being reduced to the 
 condition of ground glass. 
 Examination with a high 
 power brings out minute 
 fractures and conchoidal 
 chippings, at once sug- 
 gestive of the prelimi- 
 nary stages of manufact- 
 ure of the quartz spheres 
 for which the Japanese are so noted. It is as though each 
 granule had been held in the hand of some pigmy aboriginal, 
 and its surface reduced by hammering with another pebble, 
 after the manner known among archaeologists as "pecking." 
 
 The shape assumed by a rock or mineral fragment subjected 
 to wave action varies somewhat with the nature of the material, 
 schistose rocks and easily cleavable minerals naturally giving 
 rise to pebbles or granules of quite unequal dimensions in three 
 directions. The schist on the coast of Cape Elizabeth, Maine, 
 for instance, gives rise to pebbles in the form of a greatly flat- 
 tened oval, while the more homogeneous quartz, with which it 
 is associated, yields nearly spherical forms. " But of whatever 
 character the material, the normal shape of a beach-formed 
 boulder or pebble is oval, and this for the reason that the wave 
 action is a dragging rather than a carrying one ; the stone is 
 not lifted bodily and hurled toward the shore to roll back with 
 the receding wave, but is rather shoved and dragged along. 
 Gravity tends to hold the fragments in one position so that 
 the wear is greatest on the side which is down, and this in 
 itself would cause them to assume an oval or flattened form 
 
344 THE EEGOLITH 
 
 even were they spherical and of homogeneous material at the 
 start." 1 
 
 (3) JEolian Deposits. That no sharp lines can in all cases be 
 drawn between alluvial and ceolian deposits has been made evi- 
 dent in our discussion of the loess and adobe. We will now con- 
 sider those deposits which owe their origin and present structural 
 features almost altogether, if not entirely, to wind action. 
 
 The efficacy of the wind as an agent of transportation was 
 dwelt upon in considerable detail on pp. 184 and 292. The 
 material thus carried into the air, often to great heights, is 
 brought to the surface again by gravity, though the normal rate 
 of descent is not infrequently greatly accelerated by rain or 
 snow. Indeed, the clearness, limpidity, of the atmosphere after 
 a rainfall is due simply to the fact that it has been washed, is 
 cleansed of its suspended impurities. 
 
 The very fogs which infest our cities, particularly those of 
 the soft coal regions, are but indices of the dust particles in the 
 atmosphere, each globule of fog being condensed about a nucleus 
 of floating matter. 
 
 The amount of this dust brought down even from moderately 
 clear atmospheres is often sufficiently abundant to attract the 
 attention of the most casual observer. Professor H. L. Bruner 
 of Irvington, Indiana, has stated 2 that during a snowstorm in 
 February, 1895, a layer of snow about one-fourth of an inch in 
 thickness was colored distinctly brown by the dust it contained. 
 One sample of snow collected yielded .37% of dust, by weight, 
 and it was calculated that dust was thus deposited at the rate of 
 30.7 pounds avoirdupois for each acre. Another observer calcu- 
 lated the fall as taking place at the rate of 12.77 pounds per acre. 
 
 From a gallon of water melted from a snowfall of but 4 
 inches, which fell in London in January, 1895, there was obtained 
 10.65 grains of solid matter, 5.75 grains being inorganic and 
 4.90 grains carbonaceous. Water from a snow collected near 
 the centre of the city, January 30 of this same year, gave 6.25 
 grains of mineral and 11.07 grains of carbonaceous matter. It 
 was also found that 75 % of these impurities were brought 
 down with the first 2 inches of the snowfall. 
 
 Dr. Whitney, who examined samples of the black earth 
 
 1 Merrill, Preliminary Handbook, Dept. of Geology, U. S. National Museum, 
 1889, p. 23. 
 
 2 Monthly Weather Review, U. S. Dept. of Agriculture, January, 1895. 
 
PLATE 24 
 
 FIG. 1. Section of beds of Leda clay, Lewiston, Maine. 
 
 FIG. 2. Beds of volcanic dust, Reese Creek, Gallatin County, Montana. 
 
DEPOSITS 
 
 345 
 
 brought down near Rockville, Indiana, during a snowfall of the 
 winter of 1895, reported 1 it as consisting of material almost 
 identical with the prevailing loess of that region, from whence 
 it was doubtless derived. The individual particles varied in size 
 between .10 and .05 millimetre. The results of a mechanical 
 analysis of the dust are tabulated with those of loess on p. 331. 
 Samples of the same dust submitted to microscopic examination 
 were found to consist of fully 96 % silt and 4 % organic matter, 
 the latter consisting mainly of fresh-water algte, diatoms, fungi, 
 cells from decayed grasses, and shreds of woody tissue. 
 
 Hilgard, who has examined the so-called " dust soils " of 
 Oregon, California, and Washington, and which during the dry 
 seasons are so loose and fine as to rise in clouds at the merest 
 puff of wind, gives the following tables to show their 
 chemical and physical natures, and which he regards as fairly 
 typical for soils of the arid regions of the United States. 2 
 
 CHEMICAL ANALYSES OF DUST SOILS 
 
 
 I 
 
 II 
 
 III 
 
 CONSTITUENTS 
 
 ATATHNAM 
 PRAIRIE, 
 YAKIMA 
 COUNTY, 
 WASHINGTON 
 
 RATTLESNAKE 
 CREEK, KITTI- 
 TAS COUNTY, 
 WASHINGTON 
 
 PLATEAU ON 
 WILLOW CREEK, 
 MORROW 
 COUNTY, 
 OREGON 
 
 Insoluble matter 
 
 % 
 71 67 ) 
 
 % 
 78 33 1 
 
 % 
 7921 1 
 
 Soluble silica 
 
 sni 76 - 78 
 
 2 20 I 80 ' 53 
 
 2 SO I 81 ' 51 
 
 Potash (K 2 O) . 
 
 1 07 
 
 70 
 
 89 
 
 Soda (Na 2 O) 
 
 035 
 
 024 
 
 005 
 
 Lime (CaO) . 
 
 200 
 
 208 
 
 1 37 
 
 Magnesia (MgO) 
 
 1 34 
 
 1 47 
 
 1 08 
 
 Brown oxide of manganese (Mn 3 4 ) . 
 Peroxide of iron (Fe 2 3 ) 
 Alumina (A^Oa) 
 
 0.04 
 6.88 
 791 
 
 0.07 
 6.13 
 6 12 
 
 0.06 
 5.63 
 602 
 
 Phosphoric acid (P 2 5 ) 
 Sulphuric acid (S0 3 ) 
 
 0.13 
 0.02 
 
 0.18 
 0.02 
 
 0.18 
 0.03 
 
 Water and organic matter .... 
 
 2.82 
 
 2.35 
 
 2.55 
 
 Total .... 
 
 99 33 % 
 
 99 90 % 
 
 99 35 % 
 
 Humus 
 
 4.10 
 
 
 044 
 
 Hygroscopic moisture . . 
 
 4.98 
 
 320 
 
 4.92 
 
 
 
 
 
 1 Monthly Weather Review, U. S. Dept. of Agriculture, January, 1895. 
 
 2 Bull. No. 3, Weather Bureau, U. S. Dept. of Agriculture, 1892. 
 
346 THE REGOLITH 
 
 MECHANICAL ANALYSES OP DUST SOILS 
 
 CONVENTIONAL NAME 
 
 DIAMETER OF 
 PARTICLES 
 
 I 
 
 II 
 
 III 
 
 Clay 
 
 0023 mm. 
 
 0.93% 
 
 3.59% 
 
 1 27 % 
 
 Fine silt 
 Silt 
 Very fine sand 
 
 .005-. Oil 
 .013-. 027 
 027 05 
 
 30.93 
 3.20 
 
 7 18 
 
 13.06 
 5.82 
 27.37 
 
 32.29 
 12.75 
 37 51 
 
 Fine sand. 
 
 05-. 122 
 
 21 88 
 
 43.78 
 
 10 92 
 
 Medium sand 
 
 .122-. 5 
 
 32.39 
 
 4.57 
 
 3.97 
 
 
 
 96.57% 
 
 98.18% 
 
 98.72% 
 
 Sand Dunes. The influence of the wind in the formation 
 of sand hills or dunes, as they are commonly called, has received 
 attention on p. 184. A few words more regarding their physi- 
 cal qualities and lithological nature are here essential. 
 
 The effect of the single whirlwind or it may be that of the 
 more constant air current for days, weeks, or even months, 
 may be from a geological standpoint comparatively insignifi- 
 cant ; but they are, nevertheless, interesting, and at times 
 important. In certain regions of the West, and notably in 
 parts of the Colorado desert, as described by W. P. Blake, in 
 1853, all the fine loose sand on the surface of the ground is 
 blown away, leaving every pebble and boulder standing out 
 in strong relief from the hard sun-baked soil, or ledge of 
 bed-rock. 
 
 Under favorable conditions the material thus blown along 
 may gather in the form of dunes, which themselves travel 
 slowly across the country, ever changing their outlines like 
 drifts of snow. A few miles north of Winnemucca Lake, in 
 western Nevada, is a belt of these dunes described by geologist 
 Russell 1 as fully 75 feet in thickness and about 40 miles in 
 length by 8 miles in breadth. These, under the restless 
 goading of the winds, are constantly varying in shape, and 
 though moving in mass probably but a few feet a year have 
 already, in more than one instance, made necessary the splicing 
 of telegraph poles to prevent the burial of the wires. Another 
 range of sand dunes, at least 20 miles in length, and forming 
 
 1 Geological History of Lake Lahontan, Monograph XI, U. S. Geol. Survey, 
 
JEOLIAN DEPOSITS 347 
 
 hills 200 to 300 feet high, occurs on the eastern end of Alkali 
 Lake in the same state. On the eastern shore of Lake Michi- 
 gan are also dunes of sand sometimes 200 feet in height, and 
 which at Grand Haven and Sleeping Bear have drifted over 
 the adjacent woodlands, leaving only the dead tops of trees 
 exposed. Similar dunes occur frequently on the Atlantic 
 coast, as at Hatteras, Long Island, and Cape Cod. The island 
 of Bermuda is made up almost altogether of coral and shell 
 fragments. These are washed by the waves upon the beach, 
 dried by the winds, and blown gradually inland, thus forming 
 hills in some cases, as stated by Professor Kice, 1 not less than 
 250 feet in height. In other instances, as at Elbow Bay, on 
 the south shore of the main island, the sand, like a huge 
 glacier, has quite filled a valley, and still progressing in a 
 mass some 25 feet in thickness, is covering houses, gardens, and 
 even woodlands, leaving, as at Lake Michigan, only the trunks 
 of dead trees standing partially exposed in the midst of sandy 
 plains. 
 
 One of the most interesting and remarkable of the many 
 regions for the observation of sand dunes, lies between Bor- 
 deaux and Bayonne in Gascony, and which has been admirably 
 described by Reclus. 2 The sea here throws every year upon 
 the beach along a line 100 miles in length some 5,000,000 
 cubic yards of sand. The prevailing westerly winds, contin- 
 ually picking up the surface particles from the seaward side, 
 whirl them over to the inland or leeward slope, where they 
 are again deposited, and the entire ridge by this means alone 
 moves gradually inland. In the course of years there have 
 thus been formed a complex series of dunes all approximately 
 parallel with the coast and with one another, and of all alti- 
 tudes up to 250 feet. These are still marching steadily inward, 
 though at the rate of but 3 to 6 feet annually, and whole vil- 
 lages have more than once been torn down to prevent burial, 
 and rebuilt at a distance, to be again removed within 200 
 years. 3 
 
 The lithological nature of the dunes is widely variable, 
 
 1 Geology of Bermuda, Bull. 25, U. S. National Museum. 
 
 2 The Earth, Atmosphere, and Life. 
 
 3 The church of Lege, owing to the encroachment of the sand dunes, was torn 
 down in 1690, and rebuilt at a distance of "2\ miles from its first site. By 1850 
 the dunes had traversed the intervening space, and again necessitated its removal. 
 
348 THE KEGOLITH 
 
 though naturally siliceous sand is the prevailing constituent 
 in the majority of cases. J. W. Retgers describes l the dune 
 sands of Holland as consisting principally of quartz granules, 
 together with those of garnets, augite, hornblende, tourmaline, 
 epidote, staurolite, rutile, zircon, magnetite, ilmenite, ortho- 
 clase, calcite, and apatite ; and, more rarely, microcline, cor- 
 dierite, titanite, sillimanite, olivine, kyanite, corundum, and 
 spinel. The majority of these minerals occur in the form of 
 well-rounded granules, though many of the garnets, zircons, 
 and magnetites show quite well-preserved crystal outlines. It 
 is noticeable that these sands contain no mica, although the 
 mineral occurs in the sea-sand, from whence the dunes are 
 derived. Retgers accounts for this on the supposition that 
 during the transportation of the material the mica folia be- 
 come so finely shredded as to be sifted out from the heavier 
 particles of sand, and quite dissipated. It is well to note that 
 the abrasive power of wind-blown particles is greater than 
 among those carried by water, since, as noted by Daubree, a 
 thin intervening film of water may serve to buoy up the gran- 
 ules, and keep them apart. To this fact is ascribed the angular 
 nature of many of the wind-blown grains, they having become 
 shattered through the shock of impact. This same authority 
 seems to think that with wind-blown sand, as with water- worn 
 material, there is a minimum limit, beyond which reduction 
 in size of particles rarely goes. This minimum he places at 
 about .25 millimetre in diameter. It seems, however, more 
 probable that attrition may go on to an almost indefinite limit, 
 but that the finer and lighter materials are driven farther 
 away perhaps not collecting in the form of dunes at all 
 leaving, as one would naturally expect, the sands of any one 
 series of dunes of nearly uniform size. 
 
 It was noted by Blake during the surveys of the railway 
 routes to the Pacific that the wind-blown sands of the Colorado 
 desert were sometimes in the form of almost perfect spheres, all 
 their sharp edges and asperities having been worn away by 
 mutual attrition. The grains were composed mainly of quartz, 
 agate, garnet, and dark granules derived from the debris of vol- 
 canic rocks. In places there is a black iron sand, and usually 
 a considerable proportion of lime carbonate, as indicated by its 
 brisk effervescence when treated with acid. The sand dunes of 
 1 Neues Jahrbuch fur Mineralogie u. Geologie, etc., 1895, 1 B. 1st Heft, p. 22. 
 
AEOLIAN DEPOSITS 
 
 349 
 
 the Bermudas, as elsewhere noted, are composed wholly of cal- 
 careous material from finely comminuted shells and corals, while 
 those of the Sevier desert region of Utah, as described by Gilbert, 1 
 are of fine gypseous sand formed by the evaporation of the water 
 in the neighboring play a lakes. 
 
 Volcanic Dust. The finely comminuted materials ejected 
 from volcanoes and caught up by atmospheric currents, as de- 
 scribed on p. 153, are sometimes carried long distances to be 
 again deposited either on land or in the water, forming loose, 
 often flour-like deposits of varying thickness. At various points 
 in Colorado, Kansas, Nebraska, Montana, and other of the West- 
 ern states, are remnant beds of fine volcanic dust such as must 
 originally have covered many square miles of territory, and the 
 materials of which were 
 derived from sources now 
 wholly obscured. 2 The 
 illustration given on PL 
 24 is from a photograph, 
 taken by the writer, of 
 one of these beds in 
 the lower Gallatin val- 
 ley, Montana. From the 
 height of the man's shoul- 
 der to his feet the bed is 
 of pure glassy dust, very 
 light gray in color, and 
 so fine and light that 
 when thrown into the 
 air it floats away at the 
 slightest breath. The fig- 
 ure given shows the ap- 
 pearance of this glass as seen under the microscope. Beds of 
 this nature upwards of 4 feet in thickness occur underlying the 
 loess or surface soil along the Republican River in Nebraska 
 and Kansas and even as far east as Omaha in the first-named 
 state. The source of their materials is problematical. 
 
 Deposits of this nature thus far described are of very recent 
 origin, and the beds loosely coherent. There are, however, good 
 
 1 Monograph I, U. S. Geol. Survey, 1890. 
 
 2 See On Deposits of Volcanic Dust and Sand in Southwestern Nebraska, 
 Proc. U. S. National Museum, Vol. VIII, 1885, p. 99. 
 
 FIG. 37. Showing outlines of shreds of volcanic 
 dust, as seen under the microscope. 
 
350 
 
 THE REGOLITH 
 
 reasons for supposing that similar processes were carried on in 
 the earlier stages of the earth's history, but that the peculiar^ 
 susceptible deposits have since undergone such extensive altera- 
 tion as to be no longer recognizable as wind-drifted materials. 
 Where the material still exists as a surface deposit, it undergoes 
 ready decomposition on account of its porosity and easy perme- 
 ability. The character of the resultant soil is dependent some- 
 what upon the character of the material, which varies indefinitely. 
 The volcanic dusts are as a rule siliceous, more nearly allied to 
 the acid potash rocks than to the basalts. 
 
 The analyses given below show the chemical nature of (I) a 
 fine, white, almost flour-like pumice dust from Harlan County, 
 Nebraska, and (II) of dune sands from the Pamlico Peninsula, 
 North Carolina. This last is described 1 as a tolerably fine, 
 nearly white sand consisting of smooth, well-rounded grains, 
 mainly quartz, but containing also occasional shell fragments 
 and black granules of iron ore. 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 Silica (SiOo) 
 
 69 12 V 
 
 92 12 V 
 
 Alumina (ALOs) | 
 
 1 7 AA I 
 
 K oq 
 
 Iron oxide (Fe 2 3 ) j 
 Lime (CaO) 
 
 0.86 
 
 1.13 
 
 Magnesia (MgO) 
 
 24 
 
 03 
 
 Potash (KoO) . 
 
 6 64 
 
 64 
 
 Soda (Na 2 0) .... 
 
 1 69 
 
 35 
 
 Sulphuric acid (SOg) . . . 
 
 
 33 
 
 Ignition 
 
 4 05 
 
 60 
 
 
 
 
 
 100.23% 
 
 100.49% 
 
 (4) Glacial Deposits. Under this name are included those 
 drift deposits which are the product mainly of glacial action, 
 though their immediate deposition may have been brought about 
 in part through the instrumentality of water. The strictly 
 aqueo-glacial materials have been noted under the head of 
 alluvial deposits. 
 
 Allusion has been already made to the manner in which gla- 
 ciers erode and transport. During a comparatively recent 
 period in geologic history, there appears to have come over a 
 
 1 Geology of North Carolina, Vol. I, 1875, pp. 182-183. 
 
GLACIAL DEPOSITS 351 
 
 portion of North America a gradual loAvering of the normal 
 temperature or increase in the annual precipitation, or perhaps 
 both, until the condition of affairs now existing in northern 
 Greenland prevailed as far south as the 39th parallel of north 
 latitude. Now whether the ice sheet extended at any one 
 time over the area outlined below or whether there were 
 periods of advancement and retreat ; whether the glaciation 
 was produced by floating ice and local glaciers as argued by 
 certain Canadian geologists, or by a truly continental ice sheet 
 thousands of feet in thickness, are for our present purposes mat- 
 ters of slight concern. We have more to do with results than 
 methods. Suffice it for the moment, that over the entire north- 
 eastern part of the United States and eastern Canada, all the ex- 
 isting loose materials from rock decay that had been gathering 
 for untold ages was carried bodily northward, westward, or south- 
 ward, as the case might be. From over a considerable part of 
 southern New England the original residual soils were stripped 
 and dumped into the Atlantic, portions of the transported mate- 
 rial still protruding above sea-level in the forms known now by 
 the names of Nantucket, No Man's Land, and Block Island. In 
 process of this transfer the rocks were planed down to hard 
 fresh surfaces, over and upon which were deposited new mate- 
 rials from the north. It follows that over this entire glaciated 
 area, estimated by Upham 1 as some 4,000,000 square miles, with 
 the exception of a few comparatively insignificant patches here 
 and there, scarcely a foot of clastic matter is to be found that 
 is truly native. Wherever road cuts or stream erosion favors, 
 the regolith in various conditions of compactness may be found 
 lying directly upon the hard, smooth, and striated rock with 
 which it has perhaps no affinity in composition or structure. 
 The rotten and mechanically triturated detritus of many rocks 
 from many sources more or less admixed by the moving glacier 
 or commingled by resultant streams, is spread out to form the 
 soils on lands to which it is as truly foreign as are the emigrants 
 who land to-day upon our shores. The stone wall, built of 
 boulders found loose in the field, may consist of granites, dia- 
 bases, schists, or shales even though the underlying rock may be 
 a limestone ; or the wall may be of limestone though the coun- 
 try rock be a gneiss, or slate. A similar distinction exists in 
 the soil itself, which, while it may in part consist of the material 
 1 Ice Age in North America, p. 579. 
 
352 THE KEGOLITH 
 
 of these boulders in a finely divided state, is more likely to con- 
 sist of detritus of softer rocks which yielded more readily to the 
 abrasive force. Sand and gravel or clay, dust or mud, black 
 with organic matter or red-brown from iron oxides, the ad- 
 mixture is ever varying, dependent only on the nature of the 
 materials to the north. But the material of the glacial drift 
 is spread out over the land in a manner far from uniform and 
 under conditions widely variable. Following Professor Salis- 
 bury 1 and others, we may, according to its physical charac- 
 ters and method of deposition, separate the deposits into 
 two general groups: (l)*the stratified or assorted drift, 2 and 
 (2) the unstratified or unassorted, the first having been laid 
 down under the influence of water and hence showing a more 
 or less stratified condition, while the second, deposited directly 
 from the ice, consists of a heterogeneous aggregate of coarse and 
 fine materials without evident marks of stratification. The two 
 forms are not always readily separable nor is their relative posi- 
 tion always the same, either one not infrequently occurring up- 
 permost, and "not rarely they alternate with each other several 
 times between the surface and the bottom of the drift." 
 
 A large part of the drift is composed of this unstratified and 
 unassorted material, consisting of clay, sand, gravel, and boulders 
 in ever-varying proportions, and to which the name till or 
 boulder clay is commonly applied, or from its mode of deposi- 
 tion, that of ground moraine. As already noted, it is the 
 material carried along bodily beneath the ice sheet and left 
 in the position it now occupies on its final retreat. This, 
 entirely unmodified except upon the immediate surface where 
 it has become converted into soil through the agencies else- 
 where described, forms the regolith over large areas of the 
 northeastern portion- of America and of northern Europe as 
 well. Where as yet unaffected by oxidation, it is of a gray 
 or blue-gray color, and often so intensely tough and hard as to 
 necessitate, in process of excavation, recourse to blasting. The 
 upper portion, through percolation of meteoric waters, is as a 
 rule of a buff or brownish color, owing to oxidation of the 
 ferruginous constituents. Through the combined agencies of 
 this oxidation, of plant and animal life and of cultivation, 
 considerable contrasts in both physical and chemical properties 
 
 1 Ann. Rep. State Geologists of New Jersey, 1891. 
 
 2 Here included in large part with the aqueo-glacial deposits. 
 
GLACIAL DEPOSITS 353 
 
 are brought about between the superficial and deeper-lying 
 portion, which are commonly recognized by the terms soil and 
 sub-soil respectively applied to them, though originally they 
 may have been one and the same thing. The composition of 
 this till naturally varies with the character of the rocks from 
 whence it was derived. It may have, and indeed probably has, 
 in most cases.travelled but a short distance, and its constituent 
 particles may be the same as that of the rocks which it overlies, 
 though in a finely divided condition, only the harder and 
 tougher rocks retaining their lithological identity, while the 
 more friable, like the shales and sandstones, have been ground 
 to the condition of clay and sand. To attempt to give then 
 the composition of the till would necessitate its study and analy- 
 sis in innumerable localities, an endless and profitless task. 
 It will be sufficient to here describe a few representative occur- 
 rences. In nearly all till the boulders, consisting of the harder 
 and more resistant of the materials, are in a more or less rounded 
 or rhomboidal form, with their surfaces scarred and with other 
 marks of the rough treatment to which they have been subjected. 
 They are in fact the tools with which the glacier has done its 
 work, and these scars are but the signs of wear. Intermingled 
 with these is an ever-variable amount of finer detritus largely 
 a result of mechanical abrasion. Professor W. O. Crosby, who 
 has studied in great detail the physical properties of the till 
 about Boston, states J that, excluding the larger stones, it con- 
 sists of 25 % of coarse material which may be classed as gravel ; 
 20 % of sand ; 40 to 45 % of extremely fine sand, or rock flour, 
 and less than 12 % of clay. The gravel in these cases consists 
 mainly of pebbles of the harder and more massive rocks of the 
 region, such as granite, diorite, diabase, quartzite, and sandstone. 
 In passing from sand to gravel, there is noted an increase in the 
 proportional amount of quartz, in clear and angular or sub- 
 angular forms, due mainly to the disintegration of the granite, 
 quartzite, and sandstone pebbles. The " rock flour " also con- 
 sists essentially of quartz. The most striking feature here 
 brought out is the very small proportion of actual clay material, 
 which varies from one-tenth to one-eighth of the total bulk. 
 
 The following table, as given by F. Leverett, shows the 
 approximate physical condition of the till as represented by 
 the sub-soil in various parts of Illinois. 
 
 1 Proc. Boston Soc. of Natural History, 1890, p. 123. 
 2 A 
 
354 
 
 THE REGOLITH 
 
 55 25 
 
 pq 
 
 O 
 
 
 (N CO O CO O 
 CM O O O 
 
 i 
 
 o 
 
 
 00 
 CO 
 
 
 r- rH 
 
 o 
 
 o 
 
 
 oo^ 
 
 fc 
 
 CO Ttl 
 
 1 - 
 
 , 
 
 
 < 
 
 o 
 
 
 
 
 
 
 ^ 
 
 rH 
 
 
 II 
 
 
 CO 
 
 s 
 
 
 ^ 
 >> 
 
 o" 
 
 S 
 
 j 
 
 2 
 
 
 
 f^Vj 
 
 o 
 
 CO 
 
 5 
 
 
 
 
 r^ 
 
 rH 
 
 
 co cc 
 
 1 
 
 
 
 O 
 
 o 
 o 
 
 O 
 
 CO 
 
 o 
 
 S 
 
 g" 
 
 s 
 
 8" 
 
 o 
 
 ^ 
 
 rH 
 
 C/L^ 
 
 ' ^ 
 
 C3 
 
 ^^ 
 
 
 
 ^M 
 
 
 
 
 
 *tM 
 
 02 
 
 
 rH 
 
 s 
 
 s 
 
 I 
 
 H 
 
 O 
 
 CO rH O O O 
 CO O O O 
 05 
 
 
 
 i 
 
 1 
 
 o 
 
 1 
 
 o 
 
 co cs 
 
 K5 
 
 
 
 o" 
 
 3 
 
 <N 
 
 X) 
 
 
 
 0^ 
 
 j 
 
 
 o" 
 
 ^D" 
 
 cT 
 
 co" 
 
 1 
 
 
 
 S 
 
 rH 
 
 ^ 
 
 ^ 
 
 rH CO (M O 
 rH t- O O 
 
 Tfl t- OS 
 
 
 
 
 
 | 
 
 1 
 
 
 
 lO CM 
 
 o 
 
 
 
 oT 
 
 
 CO 
 
 
 
 
 OS 
 
 hJ 
 
 
 CO 
 
 X) 
 
 
 t- 
 
 S3 
 
 
 rH 
 
 ^f 
 
 'O^ 
 
 "* 
 
 CC 
 
 
 rH 
 
 ^H- 
 
 CO 
 
 CO 
 
 
 
 
 
 ou 
 
 rH 
 
 
 
 "" 00 O 00 
 <M O -<ti 
 
 
 
 I 
 
 I 
 
 
 
 CO 
 
 H 
 
 <M CC 
 ! t 
 
 I 
 
 O 
 
 O 
 
 
 
 8 
 
 1 
 
 a 
 
 
 rH 
 
 Q0~ 
 
 8~ 
 
 i 
 
 o 
 
 
 
 iA 
 
 CO 
 
 
 
 
 
 
 cT 
 
 rH 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 K 
 
 
 
 
 
 
 jj 
 
 
 
 
 
 
 O 
 
 
 
 
 
 
 
 
 H 
 
 ' ' -j ' ^ 
 
 
 
 
 
 i 
 
 -3 'S el ' i 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 a 
 
 c3 02 H C ^ 
 &) eg | S 
 
 . 
 
 '33 
 
 . 
 
 "eS 
 
 
 
 *, 
 
 0) 
 
 .S 
 
 t 
 
 1 
 
 
 S Q S pin r> 
 
 X 
 
 s 
 
 
 
 
 
 d ^ 
 
 
 
 
 
 M 
 
 S 
 
 
 
 rH 
 
 
 H 
 
 g 
 
 ^0 
 
 ^ O CM rH O 
 
 
 
 8 
 
 i 
 
 
 fi 
 
 i r r r r 
 
 (M T 1 lO >O rH 
 
 ^ 
 
 r 
 
 
 
 iO 
 
 
GLACIAL DEPOSITS 355 
 
 The till is not, however, always spread out evenly over the 
 land, but though partaking in a general way of the topography 
 of the slopes which it covered, lies much deeper in certain 
 places than others. Indeed, it thickens and thins out very 
 irregularly and in many places fails entirely either through 
 having never been deposited, as over many a rocky hillside in 
 New England, or through having been removed by running 
 water. Moreover, there are found in certain parts of the drift- 
 covered areas rounded hills of very symmetrical form, composed 
 of material identical with the till, but which must have been 
 deposited under slightly different conditions. These range in 
 height up to 200 or 300 feet, though rarely more than half that 
 amount. Such forms are known as drumlins. 
 
 The moraines, as already noted, represent those portions of 
 the ice drift which gathered near the edge of the ice sheet in 
 the form of submarginal accumulations, to be left as broad belts 
 or ridges of sand and gravel on its retreat. Such with refer- 
 ence to their position to the margin of the ice are known as 
 terminal, marginal, and frontal moraines. The materials of 
 which they are composed represent (1) that which accumulated 
 beneath the edge of the ice while it was practically stationary 
 for a considerable length of time ; (2) that dumped from the 
 surface at its margin; and (3) that pushed up by the ice sheet, 
 in front of itself during its forward movement. Such ridges 
 are not sharp as a rule, but broad and low, it may be from a 
 fraction of one to several miles in width. Unlike the subgla- 
 cial drift, the till, the materials are but loosely consolidated, 
 and but a small part, if any, of the boulders show the scarred 
 and abraded surfaces so characteristic of those of the till proper. 
 
 This frontal moraine, occupying the southern and western 
 margin of the glaciated area, forms one of the most striking 
 and unique of geological bodies. Composed of materials of a 
 most heterogeneous nature, ever varying, and limited in range 
 of variation only by the lithological character of the rocks to 
 the northward and eastward ; in all degrees of coarseness and 
 fineness, from boulders of many tons' weight to particles too 
 small to be visible to the unaided eye, only obscurely and some- 
 times scarcely at all stratified excepting where subsequently 
 modified by running water ; in the form of broad low hillocks, 
 domes, and ridges, the moraine sweeps in an interrupted, sin- 
 uous belt from eastern Massachusetts to North Dakota and over 
 
356 THE KEGOLITH 
 
 400 miles into British America, having a length, in all its wind- 
 ings and turnings, of not less than 3000 miles. 
 
 The water arising from the melting ice sheet flowed off, in 
 part, over the surface, forming superglacial streams, or in part 
 upon the surface of the ground beneath as subglacial streams, 
 of which last the river Rhone of to-day is a good example. 
 Presumably also a portion of the water became concentrated 
 and flowed for short distances in the mass of the ice itself, 
 forming thus englacial streams. In all cases the running water 
 would collect, reassort, and variously modify the rock debris 
 found either in immediate connection with the ice itself or at 
 its extremity, in the terminal moraines. There were thus 
 formed hillocks and ridges or low fan-shaped masses of " modi- 
 fied drift." The sand, gravel, and boulders which collected in 
 the troughs of superglacial streams would, on the final melting 
 of the ice, be deposited as ridges running essentially parallel 
 with that of the movement of the ice on which they formed. 
 Such are known as eskers, or osars. Other deposits closely 
 resembling these and sometimes confounded with them, but 
 formed, it is believed, only by swift and changeable currents 
 near the frontal margin of the ice, present often a rude and 
 disturbed and distorted stratification, and are known as kames. 
 They differ from the eskers in their outlines as well as positions 
 with reference to the glacier from 'whence their materials were 
 derived, being as a rule in the form of hills, rather than ridges, 
 and with their longer axes at right angles with that of the ice 
 motion. 
 
 Beyond the margin of the ice and its terminal moraines are 
 found still other loosely aggregated deposits of a similar hetero- 
 geneous nature wliich are likewise due to swiftly running water 
 caused by the melting ice. Such, according to their position 
 and form, are known as valley drift, morainic or frontal aprons, 
 and overwash plains. 
 
 The thickness of these glacial deposits varies greatly, as has 
 been already indicated. Variations of upwards of a hundred 
 feet may occur within the limits of even less than one square 
 mile. Professor Newberry estimated that the area south and 
 west of the Canadian highlands covered with glacial drift was 
 not less than 1,000,000 square miles, and that its average 
 depth would not be less than 30 feet. Other estimates on 
 deposits in Ohio, Indiana, and Illinois give an average thickness 
 
PLATE 25 
 
 
 i 
 
 FIG. 1. Section of glacial till. FIG. 2. Glaciated landscape. 
 
THE SOIL 357 
 
 in these states of 62 feet. In extreme cases the deposit 
 has been found to extend to a depth of 300 to 500 feet. Bell 
 has stated l that glaciation of the surface of British America has 
 been almost universal in the regions east of the Rocky Moun- 
 tains, and all over the Palaeozoic districts west and south of 
 Hudson and James Bay the average depth of the till is 100 feet, 
 and perhaps 200 feet in Manitoba and the northwest territories. 
 The following section is given by James Geikie 2 as showing 
 the varying character of the glacial drift and its interstratified 
 interglacial lacustrine deposits: 
 
 FEET INCHES 
 
 Sandy clay 5 
 
 Brown clay and stones (till) 17 
 
 Mud 15 
 
 Sandy mud .31 
 
 Sand and gravel 28 
 
 Sandy clay and gravel 17 
 
 Sand 5 
 
 Mud 6 
 
 Sand 14 
 
 Gravel 30 
 
 Brown sandy clay and stones (till) .... 30 
 
 Hard red gravel 4 6 
 
 Light mud and sand 1 8 
 
 Light clay and stones 6 6 
 
 Light clay and whin block 26 
 
 Fine sandy mud 36 
 
 Brown clay, gravel, and stones 14 4 
 
 Dark clay and stones (till) ... .68 
 
 355 
 3. THE SOIL 
 
 There remains now to be summarized a few of the character- 
 istics of those superficial portions of the regolith to which the 
 name soil is commonly applied, and these, too, only in direct 
 relation to their properties as soils, since as integral portions 
 of the regolith they have already been sufficiently touched 
 upon. 
 
 (1) The Chemical Nature of Soils. --The prevailing con- 
 stituent of any soil, whatever its source, is nearly always silica, 
 with varying amounts of alumina, oxides of iron, lime, magne- 
 sia, and the alkalies. 3 A small amount of organic matter, from 
 
 1 Bull. Geol. Soc. of America, Vol. I, 1890, p. 289. 
 
 2 The Great Ice Age, 3d ed., 1894, p. 120. 
 
 s The peat deposits furnish almost the only exception to this rule. 
 
358 THE REGOLITH 
 
 extraneous source, is usually present. This prevalence of 
 silica, as may be readily understood, is an essential conse- 
 quence of soil formation through the breaking down of rocks 
 by the processes of weathering, whereby all but the most in- 
 destructible portions are lost. 
 
 The predominantly inorganic nature of any soil may easily 
 be shown by fractional separations, made either by washing, 
 or by sieves of varying degrees of fineness, whereby it is 
 brought into portions of like size and weight such as can con- 
 veniently be submitted to microscopical and chemical analyses. 
 All portions, from the finest dust to particles of such size as to 
 be classed as pebbles, will thus be found to be but mineral 
 matter; particles of quartz, feldspar, shreds of mica, and other 
 silicates in ever-varying proportions and stages of alteration 
 or decomposition. 
 
 Owing to the destructive nature of their formation, it is but 
 natural that a soil, particularly one of considerable antiquity, 
 should but slightly resemble the parent rock. This fact was 
 more than suggested in the chapter on rock-weathering. In 
 order that its significance may be fully comprehended, the 
 analyses of fresh rock and corresponding residual material from 
 various sources are given in the table on p. 359. 
 
 The most striking of the dissimilarities shown by this table 
 are, as is to be expected, those of the limestone soils, as in 
 columns I and II, where the proportional amounts of silica, 
 iron, and alumina are increased, roughly speaking, nearly one 
 hundred fold, while the amount of lime carbonate is corre- 
 spondingly diminished. This condition of affairs is still further 
 exaggerated in the case of the red soil of Bermuda (columns 
 III and IV) and which offers particularly favorable opportuni- 
 ties for study, owing to the isolated condition of the islands 
 and the consequent freedom from danger of contamination by 
 other than local drift. 
 
 The shells and corals which in a more or less consolidated 
 condition form the entire mass of these islands, although es- 
 sentially of carbonate of lime, are nevertheless not entirely so, 
 carrying, aside from the magnesia, about 1 % of inorganic im- 
 purities, chiefly oxides of iron and alumina and earthy phos- 
 phates, which are practically insoluble in the water of rainfalls, 
 with which alone we have to do here. As time goes on, the 
 lime is slowly leached out and carried away into the ocean, the 
 
CHEMICAL NATURE OF SOILS 
 
 359 
 
 M 
 
 3 3 
 
 
 
 CO 
 
 II 
 <M 
 
 OS 
 
 -r 
 
 
 
 
 
 
 
 g 
 
 ^ 
 
 <>) O 
 ^ <M 
 
 CS 
 
 
 
 o 
 
 
 
 
 
 
 
 o 
 
 
 
 i i 
 
 * 
 
 ^-i 
 t- 
 
 t^ 
 
 CS 
 
 i 
 
 - 
 
 d 
 
 s 
 
 
 
 d 
 
 d 
 
 d 
 
 
 OO tC 
 
 CO 
 
 o 
 
 o 
 
 
 
 
 
 ^ 
 
 8 
 
 -? 
 
 
 
 
 O CD 
 ^ <M 
 
 ! 
 
 1 
 
 o 
 
 "" 
 
 
 
 
 
 o 
 
 CO 
 
 
 CS CS 
 
 CD CO 
 
 
 
 i i 
 
 5 
 
 $ 
 
 
 
 " : f 
 
 . 
 
 
 
 CD 
 
 
 CO i-H 
 
 OS 
 
 * 
 
 rH 
 
 ^i 
 
 CM 
 
 
 
 
 
 
 
 ^ 
 
 O CO 
 O t- 
 
 1 
 
 CO 
 f 
 OS 
 
 C: 
 
 cc 
 
 CC 
 CC 
 
 I 
 
 CO 
 
 S3 
 
 
 i 
 
 
 O iO 
 
 Tfl 1 1 
 
 w- 
 
 CO 
 1 1 
 
 v ' 
 
 CC 
 
 O 
 
 o 
 
 
 
 CM 
 
 
 CO 
 
 3 
 
 1 
 
 o 
 
 O 
 
 | 
 
 -t-' 
 
 
 
 lO 
 
 
 
 o 
 
 
 CM 
 
 -H 
 
 ; 
 
 CO 
 
 CO 
 
 d 
 
 
 r i~- 
 
 o o 
 
 A s 
 
 CO 
 
 so 
 
 CC 
 
 
 
 o 
 
 (M 
 
 O 
 
 . 
 
 CO 
 CS 
 
 
 co o 
 
 V 
 
 v ' 
 
 
 
 
 
 
 rH 
 
 ^ 
 
 
 r-l 
 
 M 
 
 o 
 
 / 
 
 -^ , 
 
 y 
 
 H 
 
 1 
 
 <o 
 
 I 
 
 CM 
 
 
 CO 
 
 o 
 
 I-H 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 O3 
 
 
 
 
 
 
 
 
 
 
 H 
 gj 
 
 
 
 
 
 
 
 
 
 .2 
 
 t> 
 H 
 
 
 
 
 
 
 
 
 c 
 
 ID 
 
 1 
 
 Silica (Si0 2 ) . . . 
 Alumina (Al 2 Os). 
 
 | 
 
 - 
 | 
 
 1 
 
 1 
 
 J 
 
 Magnesia (MgO). 
 
 \ 
 
 ,~, 
 
 Carbonic acid (CO^ 
 
 Phosphoric acid (P 2 
 
 1 
 
 z 
 1 
 
 1 
 
 OJ <D 
 
 aa 
 
 ii 
 
 M -d 
 ^ 2^ 
 
 s 'S'g 
 12 1 
 
 o? S* 
 
 2 
 
 13 
 
 . 2 
 
 e8 03 
 
 I'll 
 
 *>- g 
 
 3*8 
 
 l|< 
 
 T3 o . 
 
 Hi 
 
 ^^ s 
 
 M .. s 
 
 5^2 
 IIS 
 SO > 
 
 *>:| 
 
 1 
 
 *" o 2 
 
 1 5 > 
 
 Ijj 
 
 Ii! 
 
 $R '^ > 
 
 OJ <x> *^ 
 
 !?i? 
 
 o C 
 rt w d 
 o^o 
 
 B g^ 
 
 s-^ 
 **'g S 
 
 J 9 
 
360 THE REGOLITH 
 
 insoluble parts remaining. Throughout the centuries of decay, 
 this 1 % of insoluble impurities, representing but one ton of 
 earth to every 99 tons removed, slowly accumulates until it 
 forms the common red earth of the islands. Though usually 
 fertile, in numerous instances where the leaching has been ex- 
 cessive the resultant soil is so rich in iron and other deleterious 
 constituents as to be quite barren. 
 
 There are few more impressive facts in agricultural geology, 
 than that each foot in depth of such soil, as it now lies at our 
 feet, may indicate the removal of at least 100 feet in actual thick- 
 ness of limestone. In other words, assuming that nothing has 
 been lost by mechanical erosion, the surface of the ground has 
 been lowered this much in bringing about the present conditions. 
 
 From what has gone before, it is obvious that soils derived 
 by purely mechanical agencies will, if unmixed with other ma- 
 terials, show a composition closely resembling the mother rock, 
 as in the case of that derived from granite as described on p. 207 
 or those derived from argillites and siliceous sandstones ; others 
 in which chemical agencies prevailed may by solution and other 
 changes have so far lost important constituents as to be scarce 
 recognizable as rock derivatives at all. Obviously a rock mass 
 containing in itself none of the elements of plant food cannot, 
 merely through its decay, furnish soil of appreciable fertility. 
 This fact is well illustrated in the region known as the Bare 
 Hills north of Baltimore, Maryland, or the Chester County 
 Barrens in southern Pennsylvania. Both regions are under- 
 laid by peridotites rocks rich in iron-magnesian silicates, but 
 almost wholly lacking in lime, potash, or other desirable con- 
 stituents. Such rocks not merely decompose very slowly, but 
 the stingy product of such decomposition consists only of hya- 
 line forms of silica, magnesian carbonates, or silicates and fer- 
 ruginous products quite devoid of nutrient matter, affording 
 food and foothold to scanty growths of grass and stunted 
 shrubs. That, however, a rock contains all the desired mate- 
 rials, is no certain indication as to character of its decomposition 
 product, since in this process of decomposition much desirable 
 matter may have become lost. Nevertheless most soils retain 
 what we may call inherited characteristics, and a direct com- 
 parison whenever possible is by no means uninteresting, as will 
 be noted later. 
 
 It need scarcely be remarked that the value of any soil de- 
 
CHEMICAL NATURE OF SOILS 361 
 
 pends wholly upon its capacity for plant growth. Hence a 
 satisfactory treatise on the subject should be written with a 
 view to showing to what this capacity is due, and what are 
 the laws governing its fertility and its rejuvenation when that 
 fertility becomes exhausted. Such a method of treatment is, 
 however, far beyond the limits of the present work, and we must 
 content ourselves with merely touching upon a few of the most 
 salient points, leaving the at present little understood subject 
 of fertility for other and abler writers. It may be well to re- 
 mark, however, that a soil left to itself and nature's processes 
 rarely becomes barren or exhausted except it may be under 
 changed geological conditions. A growing organism takes 
 temporarily from the soil that which is essential, but restores 
 it again with accrued interest in the form of carbonaceous and 
 nitrogeneous matter derived from the atmosphere, when it dies. 
 Thus, under normal conditions, the soil grows yearly richer 
 and richer and capable of supporting larger and more luxuriant 
 crops. It is only when the husbandman comes in, and by his 
 improvident harvesting robs the soil not merely of its interest 
 due, but of a part of the principal as well, that bankruptcy 
 results. 
 
 For a long period the fertility of a soil was felt to be dependent 
 very largely upon its chemical composition, and older treatises 
 and reports of geological surveys are filled with tables of analy- 
 ses which the acquired knowledge of years now shows us to be 
 almost as worthless as can be, either for the purposes for which 
 they were first intended, or as indicative of the mineral nature 
 of the soil itself. 1 A soil which, under certain conditions of 
 climate or moisture, is utterly barren may, under changed con- 
 ditions, be fruitful in the extreme, as has been repeatedly de- 
 monstrated in the case of the so-called American deserts, dreary 
 stretches of aridity given over to sage brush and a few degraded 
 forms of animal life, but which need only moisture to cause 
 them to laugh with harvests. 
 
 1 The common practice of making soil analyses, whereby the results are tabu- 
 lated as soluble and insoluble (meaning by soluble the portion extracted by boil- 
 ing hydrochloric acid) and putting down the latter as silica (or sand) and 
 insoluble silicates, cannot be too strongly condemned. It means nothing. A 
 growing plant is capable of extracting only a small, and as yet unknown, portion 
 of that taken out by the acid, and as to what silica and insoluble silicates may 
 be, we are left in ignorance. Such analyses can be of use to neither the student 
 of soils or of geology. 
 
362 THE REGOLITH. 
 
 Naturally, a soil containing in itself nothing in the way of 
 available plant food can be made to produce crops only when 
 the needed constituents are supplied. Investigations have, 
 however, shown that, though varying in different species, the 
 proportional amount of food demanded by plants which can be 
 supplied by the atmosphere and meteoric waters is very large. 
 
 It seems to be now pretty well conceded that of all the con- 
 stituents found in soil aside from moisture, only potash, lime, 
 magnesia, phosphoric and sulphuric acids, can be considered 
 absolutely essential as plant food. The ash of all plants, to be 
 sure, contains silica, soda, and it may be iron and other min- 
 eral ingredients, but such are to be regarded as accidental 
 rather than otherwise. Of the constituents enumerated as 
 essential, magnesia and sulphuric acid are almost invariably 
 present in sufficient quantities, while potash, lime, and phos- 
 phoric acid, even though sufficiently abundant in a virgin soil, 
 are liable to exhaustion under the ordinary methods of culti- 
 vation. The source of these materials has been shown in the 
 previous pages and need here be only touched upon. The 
 potash and the lime must have come originally from the de- 
 composition of potash-lime-bearing silicates, as the feldspars and 
 micas, amphiboles and pyroxenes. The original source of the 
 phosphoric acid was undoubtedly the apatite of the eruptive 
 rocks, though now to be found in bones and skeletons of ani- 
 mals, whose remains become entombed in sedimentary rocks 
 of all ages. How small and proportionally insignificant are 
 the percentages of these constituents in any soil, fertile or 
 barren, is shown in the table below, 1 in which are given the 
 general average composition of a large number of soils, seden- 
 tary and transported. The sulphuric acid, which is not given 
 in this table, rarely amounts to more than from 0.05 % to 0.5 % 
 when calculated as sulphuric anhydride (SO 3 ). 
 
 So small, comparatively, are these percentages, that it is rare, 
 indeed, to find a soil which on complete analysis will not be 
 shown to contain them in sufficient proportion. The varying 
 degrees of fertility in such cases are due then, not to differ- 
 ences in ultimate composition, but to difference in combination 
 of these elements whereby they are or are not available for 
 plant food, aiid'to physical and climatic differences as well. 
 
 iFrom Part A, Vol. II, Part II, Chemical Analyses, Geological Survey of 
 Kentucky, p. 113. 
 
CHEMICAL NATURE OF SOILS 
 
 363 
 
 X 
 
 SllOg 3NOi9 
 
 -areii ( amg ) 
 
 ^5 co 
 
 co' 
 
 i i 
 
 ?, 
 
 d 
 
 -f 
 
 CO 
 
 d 
 
 CO 
 
 d 
 
 1 
 
 d 
 
 j 
 
 <N 
 
 CO 
 
 X 
 
 siiog Nviamis 
 
 o 
 
 CJ 
 
 *^p 07 
 
 I 
 
 CO 
 
 "T 
 
 s 
 
 01 
 01 
 
 H 
 
 i 
 
 
 aaaan 91 
 
 00 
 
 
 rH 
 
 o 
 
 c 
 
 o 
 
 
 
 125 
 
 
 
 M 
 
 raofluonn 
 
 r>- 
 
 1 
 
 
 
 1 
 
 os 
 
 C7 
 
 SB 
 
 
 
 i 
 
 ^ 
 
 
 CO 
 
 oo 
 
 Oi 
 
 o 
 
 o 
 
 
 
 
 
 521 
 
 o 
 
 B 
 
 eiiog 
 
 ^ 
 
 1 - 
 
 s 
 
 10 
 
 1 - 
 rH 
 
 O_| 
 
 s 
 
 rH 
 
 H 
 
 os 
 
 3 
 
 
 aivig Hovng g 
 
 & 
 
 s 
 
 O 
 
 O 
 
 o 
 
 
 
 521 
 
 o 
 
 
 enog 
 
 CO 
 
 s 
 
 r^ 
 
 01 
 
 ~ 
 
 CO 
 
 -r 
 
 H 
 
 
 
 
 [AV^i 01 
 
 g 
 
 -t 
 
 d 
 
 d 
 
 d 
 
 d 
 
 * 
 
 "* 
 
 
 87IOg 
 
 ^s 
 
 X 
 
 o 
 
 TO 
 
 ^ 
 
 kfl 
 
 CO 
 01 
 
 
 
 OJ 
 
 | 
 
 i 
 
 
 ^agnuunw 
 
 GO 
 
 X 
 
 0' 
 
 
 
 o 
 
 d 
 
 ^ 
 
 CO 
 
 
 snog eaansvarc 
 
 | 
 
 1 
 
 1 
 
 1 
 
 -* 
 CO 
 
 S 
 
 CO 
 
 
 
 
 -IVOQ 09 
 
 oo 
 
 ~ 
 
 d 
 
 o 
 
 o 
 
 o 
 
 r 1 
 
 TJ; 
 
 M 
 
 e-iiog (eeaoi) 
 
 00 
 OS 
 
 OS 
 
 
 
 i - 
 
 CO 
 
 01 
 
 OS 
 01 
 
 QO 
 rH 
 
 
 
 01 
 
 5 
 
 t - 
 
 
 
 
 AavKaaxvn^) tg 
 
 00 
 
 oo 
 
 S3 
 
 
 
 d 
 
 o 
 
 
 
 1-1 
 
 (M 
 
 
 81IOg 1VIA 
 
 
 
 **! 
 
 g 
 
 I 
 
 rH 
 
 2 
 
 1 
 
 01 
 
 rH 
 
 OS 
 CO 
 QO 
 
 O 
 
 
 IddI8SIS8IK3 
 
 s 
 
 o 
 
 11 
 
 
 
 
 
 o 
 
 T ^ 
 
 OS 
 
 
 9^IOg iviAmiy 
 
 
 
 rH 
 
 CO 
 
 QO 
 
 01 
 
 
 
 | 
 
 X 
 
 I 
 
 1 
 
 g 
 
 
 AanvA OIHQ 8 
 
 S 
 
 os 
 
 o 
 
 o 
 
 
 
 o 
 
 fH 
 
 co- 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 02 
 
 
 
 
 
 
 
 
 1 
 
 Sand and insoluble silicates . . 
 
 ; 
 
 V, 
 o 
 
 1 
 
 ^ 
 
 i 
 
 i 
 
 < 
 
 Carbonate of lime (CaC0 3 ) . . 
 
 Magnesia (MgO) 
 
 1 
 
 1 
 
 f 
 
 rH 
 
 Potash in acid extract .... 
 
 Potash in the insoluble silicates 
 
 Organic and volatile matters . . 
 
364 THE REGOLITH 
 
 Naturally a growing plant can take up only that which is 
 soluble by the means at its command. A high percentage of 
 any of the above constituents counts for little when they are 
 combined in the form of difficultly soluble silicates. A granitic 
 rock, as has already been noted, contains locked up in its mass 
 all the mineral elements necessary for a fertile soil, but remains 
 barren simply because these are in a condition of slight solu- 
 bility and its physical structure is such that even the soluble 
 portions are unavailable. Pulverize this rock sufficiently, and 
 it will become immediately available for soil, though naturally 
 its fertility is slight, and rendered enduring only by gradual 
 decomposition. It is of course possible, that by nature's 
 methods, decomposition and incident leaching may have gone 
 so far that a soil on the immediate surface, though derived 
 from rocks rich in essential constituents, has become quite 
 impoverished and barren. This is especially true with lime- 
 stone residuals, as has been already noted. It is doubtless to 
 this fact that is due the enduring qualities of the glacial till 
 as a soil, though its immediate fertility may not be as great 
 as one of sedentary origin. The undecomposed feldspathic 
 and other mineral particles contained by the till, due to its 
 mechanical origin, yield up slowly but continually their sup- 
 ply of plant food, and such a soil may long outlast the residual 
 clays of non-glaciated regions. 
 
 The soils derived from deposits of modified glacial drift are 
 almost invariably sandy or gravelly in their nature. Such, on 
 account of their easy working qualities, great porosity, and 
 ready permeability, are commonly known as light soils, even 
 though their actual specific gravities may be greater than the 
 so-called heavy soils of the ground moraine. 1 
 
 1 Mechanical analysis of a glacial soil from an old pasture, Cape Elizabeth, 
 Maine, yielded results as below. The portion selected was of just the thickness 
 turned up by the plough, about 7 inches. In color it was dark gray, at the 
 immediate surface almost black from organic matter, and penetrated throughout 
 by grass roots. Fine angular grains of white quartz were the most conspicuous 
 feature on macroscopic examination. Eight hundred and thirty grammes of this 
 soil on sifting yielded : (1) 2.5 grammes gravel, which failed to pass a sieve con- 
 taining 8 meshes to the lineal inch. This consisted mainly of angular quartz and 
 cleavage bits of feldspar with occasional rounded lumps of impure limonite, and 
 not completely disintegrated particles of granitic rock. (2) 40 grammes coarse 
 sand retained by 20-mesh sieve and consisting of clear glassy and white opaque 
 quartz in angular and sub-angular fragments, the largest forms being some 3 
 millimetres in greatest diameter ; cleavage bits of white and pink feldspar, rarely 
 
CHEMICAL NATURE OF SOILS 365 
 
 There is many an humble homestead throughout the glaciated 
 areas of North America whose lack of worldly prosperity is due 
 to the dry and barren soil supplied by these deposits of modi- 
 fied drift. On the other hand, there are numerous regions, like 
 those of northern Ohio, where a light, barren, residual soil de- 
 rived from sandstone has become enriched by an admixture of 
 glacial clays from the north, and thus brought prosperity to 
 thousands of happy homes. Nature works out her own com- 
 pensations, impoverishing, it may be, here but correspondingly 
 enriching there. 
 
 R. H. Loughbridge has shown 1 that the percentage of soluble 
 
 folia of white mica, a few bits of mica schist, and lastly hard, rounded pellets of 
 indurated silt and organic matter. (3) 170 grammes retained by 40-mesh sieve 
 and consisting of a clean sand composed of some two-thirds its bulk white quartz 
 particles and one-third opaque, partially kaolinized feldspathic particles ; rarely 
 any mica or free iron oxides. (4) 180 grammes retained by 60-mesh sieve and 
 consisting, like the last, of clean quartz and feldspar sand, the quartz particles in 
 excess of the feldspar, and rarely a little mica. (5) 82 grammes retained by the 
 80-rnesh sieve. This, very clean sand of quartz and feldspar, in the proportion 
 of about | quartz and f feldspar. (6) 150 grammes retained by a sieve of silk 
 bolting cloth of 120 meshes to the lineal inch. Like the last, composed almost 
 wholly of bright quartzes and somewhat kaolinized feldspars with scarcely a 
 trace of other silicates. (7) 185 grammes which passed the silk bolting cloth. 
 This was submitted to washing, the lighter finer material being poured off as silt. 
 By this means were obtained 118 grammes very fine sand and 67 grammes silt. 
 The fine sand, as before, showed under the microscope only quartz and feldspars, 
 the quartzes still in excess. The silt to the naked eye consisted of a light brown, 
 almost impalpable material, which the microscope revolved into quartz and 
 feldspar particles with shreds of ferruginous products evidently derived from 
 tlje decomposition of iron-magnesian silicates, such as micas or amphiboles. 
 (8) Organic matter, 19.5 grammes. 
 
 A bulk analysis of the air dry-soil, excluding all grass and roots, yielded 
 results as below : 
 
 Ignition (water and organic matter) 2.72% 
 
 Silica 76.80 
 
 Alumina and iron oxides 14.04 
 
 Lime 0.78 
 
 Magnesia Traces 
 
 Potash 2.87 
 
 Soda . . 1.18 
 
 98.39 % 
 
 Such a soil is plainly little more than a highly quartzose granite or gneiss in a 
 pulverulent condition and in which the agencies of decomposition have scarcely 
 begun their work. Its composition could have been almost foretold by the 
 microscopic examination. 
 
 *On the Distribution of Soil Ingredients among the Sediments obtained in 
 Silt Analysis, Am. Jour, of Science, Vol. VII, 1874, p. 17. 
 
366 
 
 THE REGOLITH 
 
 material in a soil rapidly increases with the degree of commi- 
 nution; i.e. the finer the material the larger the proportional 
 amount of soluble matter, and hence of matter available as 
 plant food. This is well brought out in the following table 
 abridged from the one given in Mr. Loughbridge's original 
 paper, the figures in the upper spacerof each column indicating 
 the size of the particles, and the percentage amount of each as 
 determined by fractional separations. 
 
 CONVENTIONAL NAME : 
 
 CLAY 
 
 FINEST SILT 
 
 FINE SILT 
 
 MEDIUM 
 SILT 
 
 COARSE 
 
 SILT 
 
 DIAMETER OF PARTICLES : 
 
 21.64% 
 
 ? 
 
 23.56 % 
 ram. 
 .005-.011 
 
 12.54% 
 mm. 
 .013-.016 
 
 13.67% 
 mm. 
 
 .022^.027 
 
 13.11% 
 mm. 
 .033-.03S 
 
 CONSTITUENTS 
 
 % 
 
 % 
 
 % 
 
 % 
 
 % 
 
 Insoluble residue .... 
 Soluble silica 
 Potash (KgO) . . 
 
 15.96 
 33.10 
 1.47 
 
 73.17 
 9.95 
 53 
 
 87.96 
 4.27 
 29 
 
 94.13 
 2.35 
 0.12 
 
 96.52 
 
 Soda (Na 2 0) . . . 
 
 (1.70) 1 
 
 24 
 
 0.28 
 
 0.21 
 
 
 Lime (CaO) 
 
 0.09 
 
 0.13 
 
 0.18 
 
 0.09 
 
 
 Magnesia (MgO) .... 
 Manganese (Mn02) . . ' . 
 Iron sesquioxide (Fe 2 3 ) . 
 Alumina (A1 2 3 ) .... 
 Phosphoric acid (P 2 0s) . 
 Sulphuric acid (SOs) . . . 
 Volatile matter .... 
 
 1.33 
 0.30 
 18.76 
 18.19 
 0.18 
 0.06 
 9.00 
 
 0.46 
 0.00 
 4.76 
 4.32 
 0.11 
 0.02 
 5.61 
 
 0.26 
 0.00 
 2.34 
 2.64 
 0.03 
 0.03 
 1.72 
 
 0.10 
 0.00 
 1.03 
 1.21 
 0.02 
 0.03 
 0.92 
 
 .... 
 
 Totals 
 Total soluble constituents . 
 
 100.14 
 
 75.18 
 
 99.30 
 20.52 
 
 100.00 
 10.32 
 
 100.21 
 5.16 
 
 .... 
 
 According to Hilgard, 2 the substance which assumes com- 
 manding importance as controlling the fertility of a soil, aside 
 from physical conditions, is lime, in the presence of which, in 
 adequate proportions, smaller percentages of the other plant 
 foods will suffice for high and lasting productiveness, than 
 would otherwise be the case. Since lime is the essential con- 
 stituent of the rock limestone, it follows that, other things 
 being equal, a "limestone country is a rich country." As else- 
 where noted, however, a limestone soil may have become so 
 
 1 An excess of original amount, due to the addition of sodium chloride to 
 produce flocculation of clay in suspension. 
 
 2 The Relation of Soil to Climate, Bull. No. 3, U. S. Weather Bureau, 1892. 
 
CHEMICAL NATURE OF SOILS 367 
 
 leached of its lime, through prolonged decay, as to be benefited 
 by artificial applications of this same constituent. Lime is, 
 moreover, so generally distributed throughout the great major- 
 ity of rocks that few soils would be lacking in this constituent, 
 were even a small proportion of the original amount left in the 
 residue from rock decay, instead of being so largely removed 
 in solution. 
 
 It would follow from this that the composition and fertility 
 of a soil is dependent not more upon the character of the rock 
 mass from which it is derived, than upon the prevalent climatic 
 conditions under which it originated, the general average tem- 
 perature and the amount and distribution of the rainfall being 
 particularly important factors. This branch of the subject has 
 also been considered in some detail by Hilgard, to whom we are 
 indebted for the only satisfactory resume. Concerning condi- 
 tions of temperature, this author says : - 
 
 " Within the ordinary limits of atmospheric temperatures all 
 the chemical processes active in soil formation are intensified 
 by high and retarded by low temperatures, all other conditions 
 being equal. This being true, we would expect that the soils 
 of tropical regions should, broadly speaking, be more highly 
 decomposed than those of the temperate and frigid zones. 
 While this fact has not been actually verified by the direct 
 comparative chemical examination of corresponding soils from 
 the several regions, yet the incomparable luxuriance of the 
 natural as well as the artificial vegetation in the tropics, and 
 the long duration of productiveness, offer at least presumptive 
 evidence of the practical correctness of this deduction. In 
 other words, the fallowing action, which in temperate regions 
 takes place with comparative slowness, necessitating the early 
 use of fertilizers on an extensive scale, has been much more 
 rapid and effective in the hot climates of the equatorial belts, 
 thus rendering available so large a proportion of the soil's in- 
 trinsic stores of plant food that the need of artificial fertilization 
 is there restricted to those soils of which the parent rocks were 
 exceptionally deficient in the mineral ingredients of special 
 importance to plants that ordinarily form the essential material 
 of fertilizers." 1 
 
 1 While the action of frost in bringing rock masses into the condition of soil 
 is, in temperate climates, of very great importance, there seems to be a limit 
 beyond which it accomplishes little in the way of directly promoting decomposi- 
 
68 THE REGOLITH 
 
 Concerning the concentration and leaching out of certain con- 
 stituents by the action of meteoric waters, the same authority 
 says : 
 
 " When, however, the rainfall is either in total quantity or in 
 its distribution insufficient to effect this leaching, the sub- 
 stances which otherwise would have passed into the sea are 
 wholly or partially retained in the soil stratum, and when in 
 sufficient amount may become apparent on the surface in the 
 form of efflorescences of 4 alkali ' salts. One of the most im- 
 portant modifications produced by scantiness of rainfall on soil 
 formation is the great retardation of formation of clay from 
 feldspathic rocks (kaolinization) and the sediments derived 
 therefrom. As a result, it is observed that the soils of the 
 Atlantic slope are prevalently loams, containing considerable 
 clay, and even in the case of alluvial lands, oftentimes very 
 heavy, while the character of the soils of arid regions is pre- 
 dominantly sandy or silty with but a small proportion of clay, 
 unless derived directly or indirectly from clay or clay shales. 
 In the former case, the clay, becoming partially diffused in 
 the rain water when a somewhat heavy fall occurs, percolates 
 through the soil in that condition and tends to accumulate in 
 the sub-soil, the result being that almost without exception 
 the sub-soils of the humid regions are very decidedly more 
 clayey than the corresponding surface soils. Not only does 
 this clay water tend to make the sub-soil more compact and 
 heavy, making it less pervious to water and air, but it is as- 
 sisted materially in this by the action which tends to leach the 
 lime carbonate out of the surface soil into the sub-soil. The 
 accumulated clay is thus frequently more or less cemented into 
 a 'hardpan' by lime partly in the form of carbonate and partly 
 in that of zeolitic (hydrous silicate) compounds, adding to the 
 compactness of the sub-soil, and therefore to the usual specific 
 difference between the soil and sub-soil ; viz. the deficiency or 
 absence of humus and the difficulty of penetration by an aera- 
 tion of the roots of plants." 
 
 For these reasons the soils of arid regions, even though con- 
 taining the same materials, are often of uniform physical and 
 chemical character to great depths. The soluble salts, as car- 
 
 tion, and presumably disintegration as well. Collier's (8th Ann. Kep. New 
 York Exp. Station, 1889) experiments showed that 47 successive freezings and 
 taawings of a soil did not perceptibly increase the percentage of soluble potash. 
 
CHEMICAL NATURE OF SOILS 
 
 369 
 
 bonate of lime and salts of potash and soda^ which are leached 
 away in regions of great average humidity, remain in those 
 where the annual precipitation is less, or where, on account of 
 its uneven distribution throughout the warmer months of the 
 year, its permeability and consequent leaching action is less. 
 Hilgard brings out this fact prominently in tables from which 
 that below is condensed, the original being compiled from sev- 
 eral hundred analyses of soils from the humid regions of North 
 and South Carolina, Georgia, Florida, Alabama, Mississippi, 
 Arkansas, Kentucky, and the arid regions of California, Wash- 
 ington, Montana, Utah, Colorado, Wyoming, and New Mexico. 
 
 SHOWING THE PROPORTIONAL AMOUNTS OF SOLUBLE SALTS IN SOILS OF ARID 
 AND HUMID REGIONS 
 
 CONSTITUENTS 
 
 ARID EEGION 
 
 HUMID REGION 
 
 Insoluble residue 
 
 69 681 % 
 
 84 472 / 
 
 Soluble silica ..... 
 
 6 289 
 
 3 873 
 
 Potash 
 
 825 
 
 187 
 
 Soda 
 
 251 
 
 071 
 
 Lime 
 
 1 645 
 
 112 
 
 Magnesia 
 
 1 384 
 
 209 
 
 Brown manganese oxide 
 
 056 
 
 126 
 
 Iron peroxide 
 
 5 431 
 
 3 455 
 
 Alumina 
 
 7 309 
 
 4.008 
 
 Phosphoric acid 
 Sulphuric acid 
 Water and organic matter 
 
 0.144 
 0.035 
 5 585 
 
 0.114 
 0.065 
 3 557 
 
 
 
 
 Total . . 
 
 99 978 % 
 
 100 093 % 
 
 
 
 
 Discussing these figures, Professor Hilgard says : " Concern- 
 ing this table with reference to the lime, a glance at the col- 
 umns for the two regions shows a surprising and evidently 
 intrinsic and material difference approximating to the propor- 
 tion of 1 to 14J. This difference is so great that no accidental 
 errors in the selection of analysis of the soils can to any mate- 
 rial degree weaken the overwhelming proof of the correctness 
 of the inference drawn upon theoretical grounds ; viz. that the 
 soils of the arid regions must be richer in lime than those of 
 the humid countries." These remarks hold good also for the 
 percentages of magnesia and the alkalies. From the fact that 
 
 2 B 
 
370 THE REGOLITH 
 
 in humid regions the more soluble constituents are leached 
 out, we may safely infer a corresponding proportional increase 
 in the insoluble constituents. This is also made manifest by 
 the tables, there being a difference of nearly 15% in favor of the 
 humid regions. The table shows, further, a probably greater 
 proportion of zeolitic material in the soil of arid regions, the 
 assumption being based upon the percentages of soluble silica. 
 Concerning this difference, the author says : 
 
 4 ' Nor should this be a matter of surprise when we consider 
 the agencies which are brought to bear upon the soils of the 
 arid regions with so much greater intensity than can be the 
 case where the solutions resulting from the weathering process 
 are continually removed as fast as formed by the continuous 
 leaching effect of atmospheric waters. In the soils of regions 
 where summer rains are insignificant or wanting, these solu- 
 tions not only remain, but are concentrated by evaporation to 
 a point that in the nature of the case can never be reached in 
 humid climates. Prominent among these soluble ingredients 
 are the silicates and carbonates of the two alkalies, potash and 
 soda. The former, when filtered through a soil containing the 
 carbonates of lime and magnesia, will soon be transformed into 
 complex silicates in which potash takes the precedence of soda, 
 and which, existing in a very finely divided (at the outset in a 
 gelatinous) condition, serve as an ever-ready reservoir to catch 
 and store the lingering alkalies as they are set free from the 
 rocks, whether in the form of soluble silicates or carbonates. 1 
 The latter have still another important effect. In the concen- 
 trated form, at least, they themselves are effective in decom- 
 posing silicate minerals refractory to milder agencies, such as 
 calcic carbonate solutions, and thus the more decomposed state 
 in which we find the soil minerals of the arid regions is intel- 
 ligible on that ground alone. But it must not be forgotten 
 that lime carbonate, though less effective than the correspond- 
 ing alkali solutions, nevertheless is known to produce, by long- 
 continued action, chemical effects similar to those that are more 
 quickly and energetically brought about by the action of 
 caustic lime. In the analysis of silicates we employ caustic 
 lime for the setting free of the alkalies and the formation of 
 easily decomposable silicates by igniting the mixture ; but the 
 carbonate will slowly produce a similar change, both in the 
 1 See author's remarks on page 374. 
 
CHEMICAL NATURE OF SOILS 371 
 
 laboratory and in the soils, in which it is constantly present. 
 This is strikingly seen when we contrast the analyses of calca- 
 reous clay soils of the humid region with the corresponding 
 non-calcareous ones of the same. In the former the propor- 
 tions of dissolved silica and alumina are almost invariably 
 much greater than in the latter, so far as such comparisons are 
 practicable without assured absolute identity of materials." 
 
 It is evident from the above that, provided the amount of de- 
 composition be the same, the soil of an arid region may contain 
 a larger proportion of desirable constituents than one in a region 
 of considerable annual precipitation. It may, also, and for the 
 same reasons, contain a larger proportion of constituents that 
 are positively deleterious. This is particularly true of arid and 
 semi-arid regions of poor drainage, like the Great Basin regions 
 of the United States, where salts of sodium not infrequently 
 accumulate to such an extent as to render the land sterile and 
 barren in the extreme. 
 
 The primary origin of the sodium in these salts lies in the 
 soda-bearing silicate minerals forming the rocks of the region 
 and from which they have been set free through their decom- 
 position. 
 
 It should be stated, however, that the so-called " alkali " is 
 not composed wholly of sodium compounds, but contains also 
 salts of magnesia, lime, iron, and potash. Nor is the form under 
 which the salts exist at all constant. As a rule, the larger por- 
 tion of the alkali is in the form of sulphate of soda, though a 
 considerable portion may exist as carbonate or chloride, and 
 smaller proportions in the form of nitrates. Concerning the 
 formation of these carbonates, Hilgard says : 1 
 
 " There seems to be a consensus of opinion that the carbona- 
 tion of the soda is connected in some way with the presence 
 of limestone or carbonate of lime, and that an exchange has 
 occurred in which either common salt or Glauber salt have 
 transferred their acidic components to lime and have become 
 carbonates instead. . . . Yet the simple explanation of the 
 contrary reaction was given and published as early as 1826 by 
 Schweigger. In 1859 it was again observed by Alex Muller, 
 in a different form, but neither of these chemists, nor any of 
 their readers, appear to have perceived the important bearing of 
 this reaction, not only upon the formation of the natural clepos- 
 1 Bull. No. 3, Weather Bureau, U. S. Dept. of Agriculture, 1892. 
 
372 THE REGOLITH 
 
 its of carbonate of soda, but also upon a multitude of processes 
 in chemical geology. Without going into details ... it may 
 be broadly stated that the formation of carbonated alkalies oc- 
 curs whenever the neutral alkaline salts (chlorides or sulphates) 
 are placed in presence of lime or magnesia carbonates and car- 
 bonic acid, or of alkali ' supercarbonates ' (hydrocarbonates) 
 containing even a slight excess of carbonic acid above the nor- 
 mal carbonates, the latter being the actual condition of all 
 natural sodas." 
 
 We have thus far considered, only those elements of the soil 
 that are derived directly from the rocks from which they are 
 formed. 
 
 To this list we should add the element nitrogen, not so much 
 on account of its quantity, as its value as plant food and of the 
 great economic value of some of its compounds. The common 
 forms under which this element exists, are (1) atmospheric 
 nitrogen, a colorless, tasteless, and innocuous gas and which 
 forms some three-fourth by bulk of the air we breathe, and 
 (2) the nitrogen of the soil, where it exists in at least three 
 distinct forms, (1) organic nitrogen, (2) as ammonia or ammonia 
 salts, and (3) as nitric acid. 
 
 The average amount of nitrogen present in agriculture soils 
 is given by authorities as varying from 0.1% to 0.3 %, though 
 occasionally, as in certain soils rich in organic matter, 4 or 5 % . 
 Of these forms only the ammonia salts and nitric acid are of 
 direct value for plant food. Nitrogen, in the form of nitrate 
 of soda, forms an important mineral fertilizer, as noted on p. 71. 
 
 The extraordinary richness in nitrates of the soils in tropical 
 countries, and particularly in South America, has often been 
 remarked since the subject was first broached by Humboldt 
 and Boussingault. According to Miintz and Maracano, nitrates 
 occur in the soils of Venezuela, the valley of the Orinoco, and 
 other localities sometimes to the amount of 30 % of their mass. 
 These nitrates they show to be due to the oxidation of organic 
 nitrogen through the agency of bacteria. They state that in 
 the caverns of the regions, a guano composed mainly of the 
 excreta of birds and bats, but admixed also with the dead bodies 
 of these and other animals, has accumulated to the amount of 
 millions of cubic metres. Through the gradual nitrification of 
 this guano, and a combination of the nitrogen with the lime 
 of bones, or existing as a carbonate in the soil, a gradual tran- 
 
MINERAL NATURE OF SOILS 
 
 373 
 
 sition is brought about wherever there is free access of air or 
 the temperature is sufficiently high to stimulate the nitrifying 
 organisms to their fullest activity. There is thus a gradual 
 change in the character of the nitrogeneous combination from 
 the interior to the exterior portions of the cave, as shown in the 
 following analyses : 
 
 CONSTITUENTS 
 
 GUANO FROM 
 INTERIOR OF 
 CAVE 
 
 EARTH FROM 
 THE ENTRANCE 
 
 EARTH FROM 
 SOME DISTANCE 
 FROM ENTRANCE 
 
 Organic nitrogen 
 
 11-74% 
 
 2.41 % 
 
 0.80% 
 
 Nitrate of lime 
 
 0.00 
 
 3.03 
 
 10.36 
 
 These authorities would account for the presence of extensive 
 deposits of nitrates as in Chili, on the assumption that the solu- 
 ble nitrate, originally derived from decomposing organic matter, 
 as noted above, had been leached out from its place of origin 
 by percolating water and redeposited elsewhere on evaporation. 
 The invocation of atmospheric electricity to account for any 
 part of the nitrates of the soils, they regard as quite unneces- 
 sary, the same being of indirect influence only, furnishing first 
 nitrogen for growing plants which in their turn serve as food 
 for animals. These same authorities give the following figures 
 relative to the amounts of nitrates and nitrogen in South 
 American soils : 
 
 CONSTITUENTS 
 
 SAN JUAN 
 
 Los MORROS 
 DE PARAPARA 
 
 EL ENCANTADO 
 
 Nitrate of lime . 
 
 2.85% 
 
 3.50% 
 
 0.62 % 
 
 Organic nitrogen 
 
 0.15 
 
 0.27 
 
 0.21 
 
 (2) The Mineral Composition of Soils. This is essentially 
 the same as that of the regolith of which the soil forms a part. 
 Fragmental quartzes and feldspars form the larger proportion 
 of most soils. These are intermingled with shreds of mica, 
 amphibole, pyroxene, calcite or aragonite, iron and manganese 
 oxides, and in variable proportions, kaolin and other silicates, 
 carbonates and oxides. The presence of these constituents is 
 
374 THE REGOLITH 
 
 usually somewhat obscured by iron oxides and carbonaceous 
 matter ; but when these are removed by acids or by ignition, 
 and the residue submitted to microscopic analyses, the true 
 mineral nature can be, as a rule, made out with approximate 
 accuracy. 1 
 
 From what has gone before, it must be evident that the con- 
 stituents of any soil are almost universally in a finely fragmen- 
 tal condition, a few of the smaller more resisting minerals, like 
 the rutiles, tourmalines, zircons, etc., having perhaps escaped 
 the comminuting processes. Of the silicate minerals we may 
 be sure that many are in an advanced stage of hydration and 
 the ferruginous constituents in a state of peroxidation. It is 
 possible that under favorable conditions new minerals of fairly 
 perfect crystalline development may be temporarily formed. 
 Since the work of Lemberg was made public, 2 it has been very 
 commonly assumed that various minerals of the zeolite group 
 were present and exercised an important function in the con- 
 servation of soil fertility. Notwithstanding the somewhat 
 enthusiastic endorsement by Hilgard, of this idea, as set forth 
 in the previous pages, the writer can but feel that too much has 
 been assumed, both regarding their actual presence and their 
 possible utility. 
 
 We must not lose sight of the fact that the actual occurrence 
 of zeolites in soils, where they have been formed, is as yet not 
 proven. Their presence is inferred from the fact that weak 
 acids, such as are known to be capable of decomposing zeolitic 
 minerals, will extract from the soil, among other constituents, 
 certain ones which are characteristic of minerals of the zeolitic 
 group; and it is assumed, purely for lack of a better reason, that 
 these elements are those thus combined. Even if this be true, 
 their efficacy as potash holders may well be questioned, since 
 potash is not as a rule an element of great importance in zeo- 
 litic minerals. Out of the 23 known species of zeolites (includ- 
 ing apophyllite), in but five is potash considered an essential 
 constituent. These five, as already noted on p. 32, are apo- 
 phyllite, ptilolite, mordenite, phillipsite, and harmotome, of 
 which phillipsite alone carries upwards of 6 % (theoretically), 
 
 1 See Anleitung zur Mineralogischen Bodenanalyse, etc., by Franz Steinriede, 
 Inaug. Dis. Friedrichs-Universitat Halle- Wittenberg. Halle, 1889. 
 
 2 Zur Kenntniss der Bildung und Umbildung von Silicaten, Zeitschrift der 
 Deutschen Geolischen Gesellschaft, Vols. XXXVII and XXXVIII, 1885 and 1887. 
 
MINERAL NATURE OF SOILS 375 
 
 the other smaller amounts, the average for the five being about 
 4 %. Now assuming that all the zeolites in the soils belonged 
 to these five groups and none to the 18 potash-free varieties, 
 and that 10 % of any soil consisted of zeolitic material, even 
 then we have thus combined only 0.4 % of K 2 O. 
 
 We must remember, further, that the zeolites are invariably 
 secondary minerals, as already noted, and as such are com- 
 monly regarded as decomposition products. This does not 
 necessarily mean, however, that they are products of superficial 
 weathering. Indeed, in the majority of cases the evidence is 
 all to the contrary, they being plainly a result of deep-seated 
 processes going on in the rock masses long before atmospheric 
 action began to manifest itself. (See under Hydrometamor- 
 phism, p. 161.) It is even questionable if the conditions preva- 
 lent in soil are not unfavorable rather than otherwise to the 
 formation of zeolitic compounds, and if such traces as there 
 exist are not rather residuals from the breaking down of rock 
 masses in which they had been previously formed. 
 
 In this connection it is well to remember that zeolites as a 
 whole are characteristic of basic eruptive rocks, such as have 
 yielded but a proportionately small amount of our soils. Also 
 that the mutual chemical reactions that may go on in a rock 
 mass due to close juxtaposition of the various minerals may 
 largely, if not entirely, cease in a soil where the amount of in- 
 terspace is so enormously exaggerated. 
 
 The researches made during the Challenger Expedition 1 
 show, it is true, that even at so low temperatures as from 2 to 
 3 C. phillipsite is being formed in the deep-sea muds of the 
 Central Pacific and Indian oceans. But in these cases the 
 mud is the finely comminuted debris from basic eruptive rocks, 
 itself peculiarly liable to decay, and containing all the materials 
 necessary for zeolitic formation. It is, moreover, in a condition 
 of continual moisture and under the weight of the thousands 
 of fathoms of overlying water which is here in a state of ex- 
 treme quiescence, being beyond the influence of superficial move- 
 ments, as waves, tides, and currents. These conditions are so 
 widely different from those which exist in the superficial parts 
 of land areas,' that they can be regarded as merely suggestive. 
 The same may be said relative to the zeolite (phillipsite and 
 apophyllite) formations at Plombieres as described by Dau- 
 1 Rep. on the Scientific Results, 1873-76, Deep-sea Deposits, 1891, pp. 400-411. 
 
376 THE EEGOLITH 
 
 bree. 1 Another fact which mitigates against the theory of 
 zeolitic formation in soils, is the almost universal absence of 
 these minerals in such secondary, unmetamorphosed rocks as 
 are the product of the reconsolidation of the same class of ma- 
 terials as in their unconsolidated condition form soils. If they 
 once existed, it would seem strange they have not in some cases 
 at least survived. If formed in soils, why should they not be 
 formed in secondary rocks where the conditions are apparently 
 so much more favorable? 
 
 It would, to the writer at least, seem more probable that the 
 soluble potash of soils exists, not in zeolitic combination, but 
 in some of the numerous decomposition products of feldspar, 
 nepheline, scapolite, etc., to which the name pinite is commonly 
 applied. Such at least is the case in the potash-rich soils of 
 Maryland, examined by R. L. Packard. 2 It is possible also 
 that it may exist in compounds allied to glauconite. 
 
 The writer has elsewhere 3 pointed out that, particularly 
 among basic rocks, there may be actually a larger percentage 
 of matter soluble in hydrochloric acid and sodium carbonate 
 solution in rocks ordinarily designated as fresh, than in the 
 debris resulting from their decomposition. This fact he has 
 since emphasized in a paper read at the December (1896) meet- 
 ing of the Geological Society of America, and from which the 
 following statements are drawn. Rock -weathering, it must be 
 remembered, is in the majority of instances accompanied by a 
 leaching process, whereby original soluble compounds, or new 
 soluble compounds formed during the process of decomposition, 
 are gradually removed. The final result is therefore, as already 
 many times noted, a residue consisting of the least soluble con- 
 stituents, and which forms the ordinary surface soil. Even in 
 cases where the actual amount of soluble matter is greatest in 
 a soil, the apparent excess may be due to water of hydration 
 and to the large amount of sesquioxide of iron, the latter being 
 practically insoluble in meteoric waters so long as there is a 
 free supply of oxygen, though readily soluble in hydrochloric 
 acid. These conclusions are based upon the following table, in 
 which the total percentage loss on ignition, minus the ignition 
 in the insoluble residue, is tabulated with the soluble matter. 
 
 1 Geologie Experimental, pp. 180 et seq. 
 
 2 Bull. 21, Maryland Agricultural Experiment Station, 1893. 
 
 3 Bull. Geol. Soc. of America, Vol. VII, 1895, p. 355. 
 
 
SOLUBLE CONSTITUENTS OF ROCKS 
 
 377 
 
 O 
 
 
 
 
 
 <: 
 
 S 
 
 - 
 
 W 
 
 a 
 
 O 
 
 t. 
 
 ^o 
 
 0^~ 
 
 t^ 
 
 CO 
 O 
 
 (N 
 
 ss 
 t^ 
 
 CO 
 i-O 
 <N 
 
 w 
 g 
 
 s 
 
 55 &. 
 ^ 
 g 
 
 1 
 
 p 
 
 a 
 
 o 
 
 & 
 
 S5 
 
 o co o 1-1 co co cc 
 
 ^ CO CO I O O (M 
 
 O 00 r-i O r-H 
 
 S^ 
 o 
 t^ 
 
 c<i 
 
 <M 
 
 
 
 
 
 w 
 to % 
 
 M 55 
 
 H 
 gig 
 
 Decomposed 
 
 = 
 
 CiO O O t^ O (M O 
 OOD GO O CO t- <N **< 
 t-I r}5 -i O O O O co' 
 
 rH (M T-H I 
 
 55 
 
 00 
 
 ci 
 
 CO 
 
 >ED ROC1 
 SOLUTIO 
 
 S i 
 o ^ 
 
 K 
 K 
 3 
 ^ 
 
 1 
 
 o S S 8 3 2 
 
 S S rH CN rH 
 
 5S 
 
 GO 
 <M 
 
 i 
 
 ECOMPOS 
 BONATE 
 
 f I 
 l| 
 
 s a . 
 
 Decomposed 
 
 5^ 
 
 8O i-H O 00 CO CN ^ 
 00 00 CO <M_ CO 
 
 ci o t^ T-4 o o d c<i 
 
 55 
 s 
 s 
 
 og 
 
 p5 
 
 ^ 
 
 ^5 
 
 <! 3 
 
 s^ 
 2 s 
 
 "<! 
 
 M PR 
 
 1 
 
 g ^ Ci o s. | ! ! 
 
 d co "^ <^ i i o . j 1 -^ 
 
 <M Q ^00 
 ^5 15 
 h 
 
 0^ 
 
 CO 
 
 ^ - 
 O 
 
 s p 
 
 w o 
 
 PH ^ 
 
 *o 
 
 f^ fc 
 
 i 
 3 
 
 I 
 
 < ji 
 
 Decomposed 
 
 ^O ^. v ' ' 
 
 oo o o^ooi^t 
 
 OOO O O CO O - i O 
 
 ci ** d r-J - i d o co 
 
 s5 
 
 
 
 CO 
 
 ^ 
 
 sa 
 
 fc o 
 
 p ^ 
 2 
 
 3 
 3 ft 
 
 I 
 3 
 
 I 
 PR 
 
 55: 
 
 i/D -rH T-H Ci O i"^ ^^ CO 
 CO ? CN <N t- 
 
 d ^* d co' c<J I-H d o<i 
 
 T-H 1 1 
 
 0^ 
 
 CO 
 5^ 
 
 
 
 y 
 
 * 
 
 ^ ^ 
 
 < 
 | 
 
 1 
 
 &c 
 
 S5 
 
 000 (N ! ! !i 2 92 
 
 C1C<I t O^T^Or- 
 
 55 
 
 S 
 
 < w 
 
 fc 
 O O 
 
 
 
 S5 
 
 8 
 n 
 
 ^ ^ <M' ^H d d d od 
 
 r- ( i i 
 
 (N 
 
 * 
 
 s* 
 
 SB 
 
 o p-t 
 
 i ^ 
 1 
 
 PH 
 
 & 
 
 8 
 
 S5 
 
 rtlt-- CO t OOO^OO 
 
 Sco 5 o^c^^'-i 
 
 55 
 
 S 
 
 r 
 
 H 
 -0 
 S 
 
 
 to 
 
 ^0 (Nr-ldOO'* 
 (M ^ 
 
 O 
 <3< 
 
 PH 
 
 O 
 
 
 
 
 
 55 
 
 
 
 
 
 fc 
 
 1 
 
 
 
 
 g 
 
 CO 
 
 I 
 
 
 . - 
 
 
 TABLE 
 
 1 
 
 
 <NO o cO^^o 
 
 o^ ^ 5 S J^ S 
 
 i2<- f^oSW^S 
 
 
378 THE REGOLITH 
 
 (3) Physical Condition of the Soil. Chemically, as previ- 
 ously noted, a soil differs from the parent rock in the amount 
 of leaching it has undergone, and in the finely comminuted and 
 more or less decomposed condition of its particles. There are 
 other distinctions, not the least important of which are its state 
 of loose coherency and porous condition due to interstitial air 
 spaces. It has been estimated by Whitney l that the approxi- 
 mate number of grains in one gramme of soil varies between 
 2,000,000 and 15,000,000, the lowest estimate being that for a 
 sandy soil containing only some 4.77% of material in such an 
 extremely fine state of comminution as properly to be classed 
 as clay, while the highest number is that in a sub-soil contain- 
 ing some 32.45 %. Our interest in these remarkable figures is 
 still further heightened when we are called upon to remember 
 that these grains are not in actual contact, but each separated 
 from the other by thin films of moisture, or, in a dry soil, by 
 actual air spaces. That such spaces exist is easily proven by the 
 fact that any soil may be greatly diminished in bulk by pressure. 
 The amount of this empty space is naturally quite variable, but 
 it is estimated to constitute on an average some 50 %, by volume, 
 of the soil. That is to say, a cubic foot of soil, in its natural 
 condition, contains an amount of space between its grains, filled 
 by air or water, equal to one-half the entire mass. 
 
 These extraordinary figures are given, not merely to illustrate 
 the wonderful degree of comminution reached in rock-weather- 
 ing, but also, and what is of more importance from the stand- 
 point of an agriculturist, the amount of surface exposed to the 
 solvent action of roots and percolating waters. Indeed, it has 
 been estimated that the total surface areas of the grains in a 
 cubic foot of soil amounts, on the average, to 50,000 square feet. 
 The amount is of course greater in a fine than a coarse soil, but 
 in any case sufficiently large to enable us to understand how, 
 under the ordinary conditions of cultivation, all the materials 
 essential to plant growth may in a brief time be removed, unless 
 renewed by artificial fertilizers. 
 
 Further than this, the amount of space between the grains 
 is of very great importance in determining the circulation of 
 water in the soil, and its capacity for retaining the right propor- 
 tion essential to plant growth as noted later. 
 
 The experimental work of late years goes to show that fertil- 
 1 Bull. No. 4, U. S. Dept. of Agriculture, Weather Bureau. 
 
PHYSICAL CONDITION OF SOILS 379 
 
 ity is dependent upon these physical properties perhaps even 
 more than upon chemical composition. If the structure, i.e. 
 the manner of arrangement of the soil particles, is such as to be 
 most favorable to root action and conservation of moisture, there 
 are few soils but may be made fertile by proper treatment, even 
 cannot the desired physical properties be imparted by artificial 
 means. A soil which contains too large a proportion of fine 
 clay matter may hold so large a proportion of moisture as to 
 be quite unsuited for cultivation when saturated, and become 
 equally unfitted by induration when dry. A light, porous, 
 sandy soil on the other hand, though fertile during seasons of 
 abundant precipitation, parts with its moisture so readily as to 
 be quite barren in seasons of drought. Porosity and capillarity, 
 two properties dependent wholly on the size and shape of the 
 soil particles, are therefore very essential items in this consid- 
 eration. Moisture precipitated in the form of rain soaks into 
 the ground or flows off upon the surface in varying proportions, 
 according to local conditions, an open porous soil naturally 
 absorbing more rapidly than one that is close and compact. 
 
 When, after the rain ceases, evaporation sets in from the 
 surface, the water which has soaked in is brought back again 
 in part by capillarity, though a part escapes through leaching 
 downward beyond the reach of capillarity, ultimately coming 
 to the surface again, at lower levels, in the form of springs. 
 The capacity of a soil to care for the water it receives from 
 rains is, perhaps, the most important of any one property. 
 It has been demonstrated that the soils of the semi-arid regions 
 of the West will produce abundant crops of wheat and corn, 
 though receiving but about half the amount of water from rain- 
 fall that would be requisite in the East. This is accounted 
 for wholly on physical grounds, and is explained as follows : : 
 Water falling upon a perfectly dry soil descends very slowly, 
 and indeed, in extreme cases, may continue to fall for hours 
 without wetting the mass for more than a few inches below the 
 surface, while it will be absorbed very rapidly by a soil already 
 wet, but not saturated. This is due to the fact, as explained 
 by Whitney, that in a dry soil the tension or contracting power 
 of the surface of the water is greater than the attraction of the 
 soil grains. If, on the other hand, there is any appreciable 
 
 1 Conditions in Soils of the Arid Region, by Milton Whitney, Yearbook 
 U. S. Dept. of Agriculture, 1894. 
 
380 THE REGOLITH 
 
 amount of moisture in the soil, the tension of the water sur- 
 face will cause it to contract and pull the water from above 
 into the sub-soil. It follows, then, that the water of rains fall- 
 ing in semi-arid regions would not penetrate into the dry sub- 
 soil, until the overlying portions had become 'successively so 
 far saturated that they could no longer hold the water back, 
 and it would pass downward, therefore, very gradually into the 
 lower depths, saturating, or nearly saturating, each successive 
 depth as it progressed. Unless, then, as rarely happens in this 
 region, the rainfall was so great and so continuous as to saturate 
 the soil to a considerable depth, the whole supply of moisture 
 absorbed would remain within a short distance of the surface, 
 either immediately within reach of plant roots, or where it can 
 be brought upwards once more by capillarity when evaporation 
 from the surface begins. With a continuously wet sub-soil, 
 however, as in the East, a very considerable portion of the 
 water passes at once to depths beyond the reach of roots or 
 capillary attraction, and is, so far as our present considerations 
 are concerned, completely lost until, in the course of nature's 
 endless cycle, it shall once more be returned as rain. Within 
 certain limits, a small rainfall, equitably distributed, is more 
 advantageous to agriculture than are the heavier precipita- 
 tions which characterize the Atlantic slopes of the American 
 continent. 
 
 The capacity of soils for moisture has been the subject of 
 experiment, and is found to vary widely, being naturally largely 
 dependent upon the size of the individual particles and the con- 
 sequent amount of interspace. Whitney states 1 that sub-soils 
 of Maryland truckland having 45 % of interspace will hold but 
 22.41 % by weight of water, when this space is completely filled. 
 The sub-soil of the Helderberg limestone, having 65 % of space, 
 will hold 41.22%. King 2 gives the following table to show 
 the actual amount of water held by field soils when their sur- 
 faces are only 11 inches above standing water, this water having 
 been lifted into them by capillarity : 
 
 1 Some Physical Properties of Soils, Bull. No. 4, U. S. Dept. of Agriculture, 
 Weather Bureau, 1892. 
 
 2 The Soil, p. 159. 
 
KINDS OF SOILS 
 
 381 
 
 SOIL 
 
 PER CENT 
 OF WATER 
 
 POUNDS OF 
 WATER 
 
 INCHES OK 
 WATER 
 
 Surface foot of clay loam contained . . . 
 
 32.2 
 
 23.9 
 
 4.59 
 
 Second foot of reddish clay contained . 
 
 23.8 
 
 22.2 
 
 4.26 
 
 Third foot of reddish clay contained 
 
 24.5 
 
 22.7 
 
 4.37 
 
 Fourth foot of clay and sand contained 
 
 22.6 
 
 22.1 
 
 4.25 
 
 Fifth foot of fine sand contained .... 
 
 17.5 
 
 19.6 
 
 3.?7 
 
 Total 
 
 
 110.5 
 
 21.24 
 
 According to Meister, different soils show water-holding 
 capacities as follows : 1 - 
 
 
 PER CENT 
 
 
 PER CENT 
 
 KIND OF SOIL 
 
 OF WATER 
 
 KIND OF SOIL 
 
 OF WATER 
 
 
 IMBIBED 
 
 
 IMBIBED 
 
 Clay 
 
 50.0 
 
 Chalk 
 
 49.5 
 
 Loam 
 
 60.1 
 
 Gyseous 
 
 52.4 
 
 Humus 
 
 70.3 
 
 Sandy (82 % sand) . . . 
 
 45.4 
 
 Peat 
 
 63.7 
 
 Sandy (64% sand) . . . 
 
 65.2 
 
 Garden 
 
 69.0 
 
 Pure quartz sand. 
 
 46.4 
 
 Lime 
 
 59.9 
 
 
 
 (4) The Weight of Soils. This is dependent upon (1) the 
 character of the particles composing the soil and (2) their 
 degrees of compactness. The figures given below are those of 
 Schubler. 2 
 
 WEIGHT PER CUBIC FOOT IN POUNDS, OF VARIOUS SOILS 
 
 Dry siliceous or calcareous sand 
 
 Half sand and half clay 
 
 Common arable soil 
 
 Heavy clay 
 
 Garden mould, rich in vegetable matter . . 
 
 Peat soil 
 
 110 
 96 
 
 80-90 
 75 
 70 
 30-50 
 
 (5) Kinds and Classification of Soils. Being derived from 
 rocks of all kinds and under greatly varying conditions; in 
 
 i Handbook of Experiment Station Work, U. S. Dept. of Agriculture, 1893, 
 p. 317. 
 
 * Handbook of Experiment Station Work, U. S. Dept. of Agriculture, 1893, 
 
 p. 315. 
 
382 THE REGOLITH 
 
 almost infinitely variable conditions of comminution, decay, and 
 proportional amounts of their various constituents, no hard and 
 fast lines for soil classification can be laid down. All things 
 considered, they are best classed with the regolith of which they 
 form a part, the general divisions of which are given in tabular 
 form 011 p. 299. We thus have the primary divisions of seden- 
 tary.and transported soils, accordingly as they have been formed 
 in place, or transported. Each of these is again subdivided 
 according to the agencies involved in its transportation or 
 original formation. 
 
 Many varietal names have been applied to soils, but as a rule 
 in so loose and ill-defined a manner as to give them only a very 
 general significance. A common practice is to name one of 
 sedentary origin according to the rock from which it was de- 
 rived, as granite soil, limestone soil, etc. Transported soils, on 
 the other hand, are often designated either by the agencies in- 
 volved in transportation, as glacial, or ceolian soils, their position, 
 as terrace soils, or their physical or chemical characteristics, as 
 sandy or clayey soils. A loam is usually defined as an admixture 
 of sand and clay with more or less organic matter, a clayey 
 loam being one in which clay predominates and a sandy loam 
 one in which sand prevails. The terms peat, muck, loess, marl, 
 etc., have been already sufficiently defined. Local names indi- 
 cative of suitability for particular crops, or sometimes of doubt- 
 ful or obscure meaning, are frequently met with. The bluegrass 
 soils of central Kentucky are limestone residuals celebrated for 
 the luxuriant growths of Poa pratensis which they bear. The 
 red " buckshot " soils of the Yazoo bottoms, Louisiana, are stiff 
 clayey alluvial soils mottled with ferruginous spots. 
 
 Many names indicative of mode of formation have already 
 received attention, but a few others may be here noted. The 
 names Endogeneous and Exogeneous have been proposed for 
 soils formed in place (sedentary) or derived from other sources 
 (transported). It is presumably scarcely necessary to remark 
 that such terms are quite inapplicable and inappropriate. 
 
 The name regur is locally applied to a fine dark argillaceous 
 soil particularly suited for cotton growing and which has a wide 
 areal distribution throughout southern India. Its origin ap- 
 pears to be mainly subaerial, though a part of the material so 
 called is undoubtedly alluvial. The material is highly plastic 
 when wet, and expands and contracts to a remarkable degree 
 
KINDS OF SOILS 
 
 383 
 
 under varying conditions of moisture and dryness. This soil 
 is very retentive of moisture and rarely requires to be irrigated 
 artificially. It is, as a rule, of great fertility and of wonderful 
 lasting powers, it being stated that in some localities it has 
 borne crops for 2000 consecutive years, without the aid of ma- 
 nures. In depth this soil is rarely over 6 to 8 feet. The follow- 
 ing analyses show the chemical character of the regur (from 
 near Seoni) on the surface and at depths of (An) 5 feet and 
 (Bn) 3 feet below. The analyses A, like those given on 
 p. 306, are instructive as showing the large increase in the 
 amount of lime from the surface downward. Although not so 
 stated, the slight differences in Bi and Bn are probably due to 
 the lesser depth below the surface from which Bn was taken. 
 
 
 l 
 
 L 
 
 I 
 
 I 
 
 CONSTITUENTS 
 
 I 
 
 II 
 
 I 
 
 II 
 
 Insoluble matter 
 Organic matter 
 Water 
 Oxide of iron . ... 
 
 62.7% 
 9.2 
 8.4 
 11.0 
 
 47.61 % 
 8.4 
 7.6 
 15.9 
 
 62. 8% 
 9.0 
 8.2 
 10.9 
 
 63.7% 
 8.7 
 6.5 
 11.8 
 
 Alumina . 
 
 7.5 
 
 8.6 
 
 7.6 
 
 8.4 
 
 Carbonate of lime 
 
 1.2 
 
 11.89 
 
 1.5 
 
 1.3 
 
 
 100.00% 
 
 100.00% 
 
 100.00% 
 
 100.00% 
 
 In many cases this regur is derived directly from basaltic 
 rocks, by surface decomposition in situ, whilst other varieties 
 were derived from other aluminous rocks, or are alluvial depos- 
 its in river valle}^s, lakes, lagoons, and marshes. The dark 
 color, as is usual, is due to the presence of organic matter. 1 
 
 The term sub-soil is applied to that portion of the regolith 
 which immediately underlies the soil proper, and from which 
 it differs mainly in compactness, and the lesser amount of oxi- 
 dation and decomposition it lias undergone. In a soil which 
 has never been cultivated, the sub-soil may pass gradually up- 
 ward into the soil without distinct lines of demarcation. Pro- 
 longed cultivation may, however, have so thoroughly oxidized 
 and physically altered the superficial portions down to the limit 
 of plough and root action, as to bring about a very marked cliff er- 
 
 i Manual of the Geology of India, 2d ed., by R. D. Oldham, 1893, p. 411. 
 
384 THE REGOLITH 
 
 ence, both in color and texture, as well as in actual composition. 
 At times the sub-soil becomes so thoroughly compacted as to be 
 almost impervious, forming a so-called hardpan. 
 
 (6) The Color of Soils. The color of soils is due mainly to 
 carbonaceous matter and iron oxides. To the first are due the 
 dark gray to black colors characteristic of prairie and swamp 
 soils. To iron oxides are due the buff, yellow, ochreous-brown, 
 and red hues, the source of the oxides being mainly the silicate 
 minerals from whence the soils were derived. It not infre- 
 quently happens, as abundantly demonstrated in the southern 
 Appalachian states, that it is possible in passing over any sec- 
 tion of the country to designate with a fair degree of accuracy 
 the lithological nature of the underlying rocks from the color 
 of the residual soils, even though the rocks themselves may 
 be wholly ob'scured by decomposition products. In such cases 
 rocks rich in iron silicates, like hornblende and augite, give 
 rise to bright red soils, while those poor in these constituents 
 yield soils of a gray or slightly yellowish hue. Much, however, 
 apparently depends on extent of decomposition and on climatic 
 conditions, as noted below. 
 
 One of the most striking features of the landscape observed 
 in travelling southward along the Appalachian belt is the 
 abrupt transition in color of the soil, as the limit of glacial 
 action is past. Within the glaciated area, except where de- 
 rived directly from rocks themselves highly colored, like the 
 Triassic sandstones, the soils are everywhere dull in color, 
 some shade of gray, drab, or brown. South of this limit ochre- 
 ous-red and yellowish prevail. Along the line of the Virginia 
 Midland railway, south of Washington, these colors prevail in 
 hues of astonishing brilliancy. Although the soils throughout 
 the region are residual, their colors seem in many cases quite 
 independent of the kind of rock to which they owe their origin. 
 Granite, gneiss, schist, or basic trappean rocks alike give rise to 
 red and yellow highly tenacious soils of such depth and brill- 
 iancy of color that every gully, ravine, and roadway stands out 
 against the green background of the landscape, as though 
 painted by some Titanic hand with brushes dipped only in 
 yellow, red, and vermilion ochres. These contrasts are par- 
 ticularly striking in the early summer and directly after a 
 rain. But he who wishes to admire had best do so from his 
 window, and without too much attention to detail. 
 
THE COLOR OF SOILS 385 
 
 The soil is plastic and adherent to an intolerable degree. 
 The grass forms no compact sod, as in the North, and as a re- 
 sult the walls of houses, fences, feet, legs and clothes of pedes- 
 trians become uniformly stained a dirty ochreous color equally 
 trying to the housewife and to ploughman. 
 
 The cause of this color variation has been the subject of 
 speculation by Professors Crosby, 1 Dana, 2 Russell, 3 and others. 
 So far as our knowledge now extends, it is apparent, as first 
 stated by Crosby, that the difference is due to a spontaneous 
 dehydration which takes place in the warmer regions, whereby 
 the hydrous sesquioxides of the type of limonite and gothite 
 are converted into the less hydrated or anhydrous forms tur- 
 gite and hematite with corresponding changes in color from 
 yellow or brown to red. 
 
 This view is rendered the more plausible from the fact that 
 the most brilliant hues are entirely superficial, and below the 
 surface gradually fade out into brown and yellow or even gray 
 hues. Such a transition may be observed in any fresh road 
 cut, but quickly becomes obscured by the washing down of the 
 deeply colored material from the higher levels. Sometimes 
 the brilliant red will be found a mere wash, but a fraction of 
 an inch in thickness, or again it penetrates to the depth of 
 a foot or more before giving way to more modest hues. In 
 such cases the brilliant colors will be found to have penetrated 
 deepest along joint lines, or the more porous portions, leaving 
 the intervening compact masses of more sombre hue. 
 
 In discussing this matter, there is, however, one point that we 
 should not overlook, although its importance seems not to have 
 been fully realized by the authorities quoted, and that is, a 
 change in color not due alone to a change in the conditions of 
 the iron, but to the relative greater abundance of this constitu- 
 ent in the upper portions. The iron oxides, as already noted, 
 owing to their less soluble nature accumulate in the residues, 
 and as a rule, the more thorough the decomposition the greater 
 the proportional amount of iron. A small percentage of free 
 oxide disseminated throughout a relatively large amount of 
 detritus imparts but little color ; the more iron, the more color. 
 
 1 Proc. Boston Society of Natural History, 1885, p. 219, and Technological 
 Quarterly, Vol. IV, 1891, p. 30. 
 
 2 Am. Jour, of Science, Vol. XXXIX, 1890, pp. 317-319. 
 8 Bull. No. 52, U. S. Geol. Survey, 1889. 
 
 2c 
 
386 THE REGOLITH 
 
 The residue from the Medford diabase described on p. 220 is 
 of a deep brown color, as a whole, but the finest silt washed 
 from it is several shades brighter, of a dull ochreous red. Had 
 the entire mass decomposed to the condition of this silt, we 
 might expect it to have the same color. This change in color, 
 due to increased proportional amounts of iron oxides, is particu- 
 larly marked in limestone residuals where the original rock may 
 contain merely traces of free oxides, or ferruginous silicates. 
 Neumayer has shown 1 that the snow-white Karst limestones 
 contain only some 0.044 % of ferruginous silicates which them- 
 selves carry 20 % of iron oxides. Yet the residual soil left by 
 the decomposition of this limestone is of so pronounced a color, 
 as to have long ago received the name terra rossa or red earth. 
 Other things being equal, brilliancy of color may then be 
 regarded as (1) indicative of advanced decomposition, and 
 (2) of geological antiquity. 
 
 (7) The Age of Soils. No sooner were the first rocks pushed 
 above sea-level than the various agencies described under the 
 
 head of weathering began the 
 work of disintegration, de- 
 composition, and transporta- 
 tion. Of this we have ample 
 proof in the entire series of 
 sedimentary rocks extending 
 from the Archaean down to 
 the most recent and which 
 are but the reconsolidated 
 FIG. 38. Trunk of tree still standing in residues of preexisting masses. 
 
 soil of Carboniferous age. a, bed-rock ; ^hat guc ] 1 a breaking down 
 b, under clay or ancient soil; c, coal; 11-1 i , * 
 
 d, bedded rock; e , fossil tree. resulted in the production of 
 
 soils is a fair inference, though 
 
 we have no absolute evidence of land plants and hence, a 
 priori, of soils, before the beginning of the Upper Silurian 
 period, when plants of the lycopod type appeared. Such soils, 
 as soils, have, however, long since disappeared in the never- 
 ending cycle of change and it is not until we reach the Car- 
 boniferous period that we meet with soils which have been 
 preserved in place and in recognizable form even to the present 
 day. Even here induration and partial metamorphism has 
 rendered them no longer fitted for the support of plant life, 
 
 1 Verhandl. k. k. Geol. Reichsandstalt, 1875-76, p. 50. 
 
THE AGE OF SOILS 387 
 
 but that they once did so serve is amply proven by the occa- 
 sional finding of still erect, though fossil, trunks with roots 
 buried in their native soil, as they grew in the marshes and 
 woodlands of the coal period. But as to the time of the begin- 
 nings of the formation of such soils as still retain their soil 
 characteristics, we have not in all cases reliable data. Those 
 which are but the unconsolidated sediments of recent geological 
 time, like those of the eastern shore of Maryland, the loess and 
 alluvium of the Mississippi valley, or the swamp and glacial 
 soils of the north and east may, of course, be located with a 
 reasonable amount of accuracy. But as for the residual soils, 
 those which result from the breaking down in place of rock 
 masses, we can only say that they must be younger than the 
 rocks from which they were derived. The writer has shown 
 that the granite soils of the District of Columbia are post- 
 Cretaceous ; in other parts of the Piedmont plateau of Mary- 
 land, they may be post-Tertiary. In but few instances, as at 
 
 Poor Soil 
 
 FIG. 39. 
 
 Medford in Massachusetts, have we evidence of any consider- 
 able amount of soil formation by decomposition and disintegra- 
 tion since the close of the glacial period. Obviously the older 
 a residual soil, the greater the amount of decomposition and 
 leaching it will have undergone and the less will it resemble 
 the parent rock. Where horizontally lying strata of varying 
 character have successively undergone decomposition and a loss 
 of their soluble constituents, the resultant soil must periodically 
 vary according to the nature of the rock undergoing decompo- 
 sition and the inherited characteristics handed down from the 
 strata earlier decomposed. In such a case as that here figured, 
 we have a residual soil containing the least soluble constituents 
 of the hundreds of feet of dissolved and disintegrated rock which 
 
388 THE KEGOLITH 
 
 once extended across the entire country becoming commingled 
 with that now undergoing, in its turn, the soil-making process. 
 Such a soil may, therefore, in extreme cases, contain materials 
 of all ages from the first product of disintegration of the upper- 
 most strata, which may have been Carboniferous, to that which 
 formed to-day, and may be Cambrian. 
 
 It is, of course, true that through the erosive action of water 
 these soils are continually losing their finer silt and clay-like 
 particles, it may be almost as fast as formed, especially in hilly 
 regions, and that as the soil drops lower and lower in the geo- 
 logical horizons indicated, it becomes more and more impover- 
 ished in those constituents derived from the upper beds. But 
 as to what proportion of the material of one horizon is handed 
 down to become admixed with that from the rocks below, we 
 have no means of judging, and in fact it must be ever-varying. 
 
 The matter of the geological age of any soil, or the age of 
 the rocks from which it was derived, is therefore of only very 
 general interest, and may w^ell be dismissed here. The attempt 
 which has been made by another writer 1 to discriminate or 
 classify soils according to the geological horizons of the rocks 
 from which they were derived, is believed by the present writer 
 to be futile and wholly inexpedient. 
 
 No attempt should be made, as has been done by at least one 
 writer, to state the character of soil that may arise from the 
 weathering of any particular class of rocks, since much depends 
 upon the extent to which weathering has been carried. The 
 ultimate product of weathering of rocks of any but the purely 
 siliceous type is a more or less ferruginous clay, which may 
 be contaminated or admixed with coarser, foreign particles. 
 It is the extent of decomposition, more than its lithological 
 derivation, that determines both the chemical composition and 
 physical characteristics of any soil. 
 
 Rocks of essentially the same type so far as composition is 
 concerned, regardless of structural modifications induced by 
 metamorphism, have been formed and re-formed throughout 
 every period of the earth's history, and the attempt made to 
 classify those of igneous origin from the standpoint of geologi- 
 cal age has invariably resulted in failure. As has been already 
 indicated, the greater portion of the granitic, gneissic, or highly 
 metamorphosed crystalline schists and calcareous rocks belong 
 1 See Stockbridge's Rocks and Soils, p, 12, 
 
EFFECT OF PLANT AND ANIMAL LIFE 389 
 
 either to the Archaean or older Palseozoic formations, but this 
 merely because they, being older, have been longer subjected to 
 metamorphosing agencies, and not because in themselves they 
 possess essential differences. It is true that some authorities 
 lay stress on the supposed abundance of animal remains in cer- 
 tain Palseozoic formations, but no one but the veriest amateur 
 would now dream of attempting to discriminate between either 
 igneous or aqueous rocks of the same nature, but of different 
 geological ages, on purely chemical grounds. 
 
 It is a fact, however, that within certain climatic limits, the 
 rocks of any one horizon may impart such characteristics to a 
 residual soil as shall render it adapted to plant growth of a 
 particular kind. Thus, 1 throughout the central portion of 
 Kentucky, where, within the distance of a few miles, rocks 
 occur of several distinct geological horizons, each bearing its 
 mantle of residual soil, each horizon may be traced for long 
 distances, though the rocks themselves are wholly obscured, 
 merely by the character of its forest growth. 
 
 (8) Soils as affected by Plant and Animal Life. There are 
 various forms of animal and plant life the action of which is 
 worthy of note in connection with the subject of decomposition ; 
 but since it is probable that they are of greater efficiency in 
 promoting changes in soils once formed than in bringing about 
 the preliminary rock disintegration, their consideration has 
 been left to form a portion of the present chapter. 
 
 Ants, by means of their numerous borings, penetrating at 
 times to depths of many feet, bring about not merely a rear- 
 rangement of soil particles through a transfer of materials from 
 lower to higher levels, but also a condition of porosity whereby 
 air and water gain access to the deeper lying portions, there 
 to promote further chemical and physical changes. 
 
 Naturally these insects limit their work to dry and light 
 soils, where their operations may be compared with that of 
 earthworms whose operations are confined to moist ones. 
 Shaler has calculated 2 that over a certain field in Cambridge 
 (Massachusetts) the ants have made an average transfer of soil 
 matter from the depths to the surface sufficient to form a layer 
 each year of at least a fifth of an inch over the entire four acres 
 under observation. He further mentions a curious effect aris- 
 
 1 As the writer is informed by Mr. J. R. Proctor. 
 
 2 12th Ann. Rep. U. S. Geol. Survey, 1890-91, p. 278. 
 
390 THE REGOLITH 
 
 ing from the interference of the ants with the original condi- 
 tions, in the separation of the finer from the coarser particles. 
 In certain parts of New England where sandy soils had laid 
 for a long time uncultivated, fields were covered to a depth of 
 some inches with a layer of fine sand without pebbles larger 
 than the head of a pin, while below the level of perhaps a foot 
 the soil was mainly pebbles, with very little finer material. 
 This condition, it is argued, was brought about by the tens of 
 thousands of ants which each year, over every acre, in the 
 process of building their dwelling brought up the finer material 
 and deposited it in the form of a mound about the surface 
 opening, leaving behind the coarser particles, too heavy for 
 them to move. The common black and brown ants of the 
 United States (Formica exsectoides^) build upon the surface 
 mounds not infrequently from 1 to 2 feet in height and 3 to 
 5 feet in diameter and which are composed of materials brought 
 up from below intermingled with twigs and shreds of bark and 
 leaves from the surface. Shaler calculates the mass of some 
 
 of these mounds as equal 
 to 2 cubic yards. Being 
 of unconsolidated, loosely 
 coherent material, such 
 are constantly being de- 
 
 FIG. 40. -Effects of ant-hills on soils. a,sand S raded ^ wind and rail! 
 accumulated in hill; bb, material washed and their particles dis- 
 
 Sl PeS> mingled with vegetable tributed over the sur- 
 rounding surface. "Where 
 these structures are numerous, as they are in certain districts 
 in the United States, by their constant deposits of matter on 
 the surface of the ground, they bury a good deal of vegeta- 
 ble waste in the soil ; at the same time the animals are con- 
 stantly conveying into the earth large quantities of organic 
 matter which serves them as food, and the waste of this, 
 including the excreta of the animals themselves, is of con- 
 siderable importance in the refreshment of the soil." The 
 geological efficacy of insects of this and other types is un- 
 doubtedly greater in warmer climes, where not only are they 
 found in greater abundance, but their period of activity ex- 
 tends over a larger portion of the year. Messrs. Mills and 
 Branner, as already noted, are inclined to lay considerable 
 stress on the work of ants and termites in bringing about soil 
 
EFFECT OF PLANT AND ANIMAL LIFE 391 
 
 changes and rocks decomposition in Brazil. Branner states 
 that in some parts of the Amazon valley, of Minas Goyaz and 
 Matto Grosso, the soil " looks as if it had been literally turned 
 inside out by the burrowing of ants and termites." The species 
 popularly known as saubas excavate chambers and build gal- 
 leries which are frequently from 50 to 100 feet long, from 10 
 to 20 feet across, and from 1 to 4 feet high, and contain tons of 
 earth. The white ants or termites, like the true ants, burrow 
 extensive channels in the ground, and build up huge nests 
 upon the surface from the size of which one may gain some 
 idea of the extent of the underground galleries. In the region 
 extending from the state of Parana to north of the Amazon 
 and along the upper Paraguay in Matto Grosso may be seen 
 places where the nests are so close together that one can al- 
 most walk upon them for several hundred yards at a time, 
 while no one of the nests is more than 10 feet from another 
 over many acres of ground. Such nests vary in size from 1 
 to 12 feet in height and 1 to 10 feet in diameter, and do not 
 seem to be confined to any particular kind of country, though 
 especially noticeable in the interior and timberless regions. The 
 constant transference of such quantities of soil from below to 
 the surface, and of organic matter from the surface downward, 
 cannot fail to bring about marked changes in its physical as 
 well as chemical condition, while at the same time affording 
 passageways for air and meteoric waters, as already noted. 
 
 Certain animals, like the crayfish, have likewise a habit of 
 burrowing in the ground, though as they are wholly subterra- 
 nean or aquatic in their nature, the results are less conspicuous 
 to the casual observer. In searching for their food, these ani- 
 mals bore numerous horizontal channels or galleries some- 
 times an inch or so in diameter and extending for many feet, 
 usually ending in an upward shaft extending to the surface, 
 or at the margin of a pond or stream. These form natural 
 drainage channels and allow a more ready access of air, con- 
 verting what might under other conditions be a heavy, clayey 
 or even marshy soil, unfit for cultivation, into one light and 
 fertile. 
 
 By burrowing through dams and embankments, they have, 
 however, in some instances so weakened these structures as to 
 cause them to give way, whereby large districts have become 
 inundated and for a time rendered unfit for cultivation. 
 
392 THE REGOLITH 
 
 Probably none of the forms of animal life thus far mentioned 
 produce such wide-spread and beneficial results as have been 
 ascribed by Darwin 1 to the common earthworm, the angleworm 
 of the New* England disciples of Izaak Walton. These insig- 
 nificant creatures, as is well known, burrow in the moist rich 
 soil, and derive their nourishment from the organic matter it 
 may contain. In order, however, to obtain this comparatively 
 small amount of nutritive matter, they devour the earth with- 
 out any selective power, and pass it through their alimentary 
 canals, rejecting the remainder, which nearly equals in bulk 
 that first taken in. The nufnerous holes made, while in part 
 perhaps to afford passage to the surface, are mainly excavated 
 in this process of soil eating and actually represent the amount 
 of material which the worms have passed through their diges- 
 tive systems. 
 
 Darwin states that in certain parts of England these worms 
 bring to the surface every year, in the form of excreta, more than 
 10 tons per acre of fine dry mould, " so that the whole superficial 
 bed of vegetable mould passes through their bodies in the course 
 of every few years." By actually collecting and weighing the 
 excretions deposited on a small area during a given time, he 
 found that the rate of accumulation was at the rate of two- 
 tenths of an inch a year, or an inch in every five years. The 
 importance of these worms, then, both as mellowers of the soil 
 and as levellers of inequalities by burying stones and filling 
 hollows is therefore very great, and we cannot afford to 
 overlook it here. 
 
 While the main influence of the worm is manifested in a 
 mellowing by burrowing and a transfer of material from a 
 lower to a higher level, they bring about a slight admixture 
 of organic matter through a habit of coming to the surface at 
 night time, and dragging down into their burrows small shreds 
 of leaves and grass, which, taken into account in connection with 
 the excrementitious matter of the worms themselves, must tend, 
 though it may be ever so slightly, to enrich the soil. The sub- 
 ject should not be dropped without referring to the abundance 
 of these worms, which in England has been estimated as at the 
 rate of 53,767 to each acre of garden land, and about one half 
 that number for pasture land. It is scarcely necessary to re- 
 mark that their distribution is very unequal throughout the 
 1 The Formation of Vegetable Mould. 
 
EFFECT OF PLANT AND ANIMAL LIFE 393 
 
 world, and that in dry sandy regions they are almost, if not 
 wholly, unknown. 
 
 In northern temperate climates, such as that of New England, 
 and particularly where the soil is of a clayey nature like the 
 ground moraine, the burying action of the earthworm, as de- 
 scribed above, may be wholly overcome through the heaving 
 action of frost. Every farmer boy who has been condemned 
 to pick the drift boulders from a field knows through bitter 
 experience that, however well he may do his work in the fall, 
 however clean the surface may be when winter sets in, the fol- 
 lowing spring, after the frost is out of the ground, will find a 
 new crop in no way distinguishable from the old, and which, 
 for all that he can see, may have rained down during the win- 
 ter's storms. The fact is, however, that they have been actually 
 thrown up, "heaved out," the farmers will say, from below the 
 surface by the frost which here penetrates not infrequently to 
 a depth of two or more feet. As the water-soaked clay under- 
 lying one of these buried boulders freezes, it expands upwards, 
 since this is the direction of least resistance. The stone is 
 carried up bodily for a distance dependent on the amount of 
 expansion. When the frost leaves the ground, the soil sinks 
 back nearly to its first position; but the boulder never quite 
 regains its former place, being prevented by particles of soil, 
 or clay or pebbles which fall into the cavity as the soil shrinks 
 away from it. The amount of actual lifting for each season 
 may be but slight, but as the process goes on unceasingly there 
 is always an abundance of new material at the surface each 
 succeeding spring. This heaving action of the frost is abun- 
 dantly exemplified in these clay regions by the throwing out 
 of fence posts and clover roots ; sometimes, when the winter is 
 one of frequent freezing and thawing, causing the destruction 
 of a crop as completely as though it had been pulled up by the 
 roots. In wet boggy lands this heaving action of frost, as 
 exerted on partially buried boulders of small size, is sometimes 
 exemplified in a peculiarly striking manner. The surface of 
 the ground will be dotted here and there with small hummocks, 
 each with a comparatively large crater-like opening at the top. 
 Investigation reveals the fact that at a distance of but a few 
 inches at most below the surface of this crater-like opening is 
 a rounded boulder. The heaving action of the frost forces the 
 boulder gradually upward, causing the turf to first rise with 
 
394 THE REGOLITH 
 
 smooth rounded outline, till, through continual pressure from 
 the boulder, it bursts at the top. When the frost leaves the 
 ground, the boulder drops back a short distance, but enough 
 to be quite out of sight, leaving the cavity at the top filled 
 with mud, and looking in outline like a small mud volcano. 
 So far as the writer's observations go, the heaving action rarely 
 progresses, in these areas, to the point of actually throwing 
 the boulder out upon the surface. Each summer the growing 
 turf makes an attempt at healing the wound, but each winter's 
 frost opens it once more, the alternating forces so nearly bal- 
 ancing that little is accomplished after this pseudo-volcanic 
 stage is reached. 
 
 Insects like the boring bee, the burying beetle, or larger bur- 
 rowing animals, like the " woodchuck " of the Eastern states, 
 the prairie dogs, badgers, and spermophiles of the West, in the 
 same way exert powerful though local influences in admixing 
 the lower with the upper portions of the soil, and through 
 allowing perhaps a more ready passage of water facilitating 
 oxidation and decomposition at greater depths. (Fig. 2, 
 PI. 19.) 
 
 While the effect of these animals may be comparatively in- 
 conspicuous in the regions east of the Mississippi, in the drier 
 regions of the West the surface is not infrequently so under- 
 mined by burrows as to make travelling on horseback at more 
 than a very moderate pace a matter of grave difficulty. W. P. 
 Blake, in the early reports of the Pacific Railroad Survey, 
 states that the fine, silty soil of the Tulare valley in California 
 is so undermined that it is almost impossible to travel over it. 
 " Mules often break through the thin crust and sink to their 
 shoulders in these holes." 
 
 The action of plant life in the accumulation of vegetable 
 mould has been fully discussed under the head of cumulose 
 and alluvial deposits. There is, however, one phase of action 
 which may well be mentioned here. A growing tree, as already 
 noted, sends its roots deep down into the earth in search of food 
 and foothold. So long as the tree remains alive and standing, 
 in firm soil the amount of change in the soil itself, except in the 
 way of abstraction of certain.constituents taken up by the grow- 
 ing plant, is presumably very small. When, however, the tree 
 dies, the roots slowly decay, and besides yielding up their con- 
 tents to form new soil, afford passageway for percolating water 
 
EFFECT OF PLANT AND ANIMAL LIFE 
 
 395 
 
 Forest Mould. 
 
 FIG. 41. 
 
 with all its attendant results. Moreover, cases are by no means 
 infrequent in which trees are upturned by the winds, bringing 
 entangled in their roots it may be tons of soil and boulders 
 which in part gradually fall back into the hole and in part re- 
 main to form a mound 
 which marks the spot long 
 after the tree has de- 
 cayed. Into the cavity 
 thus formed, dead leaves 
 and other organic debris 
 accumulate, which in time 
 form -deep rich loam to be 
 commingled with the stony 
 matter of the soil. In sec- 
 tions of the country where 
 heavy winds and hurri- 
 canes are of frequent oc- 
 currence, the efficacy of 
 trees in thus burying or- 
 ganic matter, and produc- 
 ing a more complete inter- 
 mingling of the soils, is by 
 no means inconsiderable. 1 
 The influence of plants in 
 adding carbon and inci- 
 dentally carbonic and other FlG> 42 
 organic acids to the soils 
 
 has been described in previous pages. When plants die and 
 decay upon the immediate surface, there is left only the inor- 
 ganic matter or ash behind, the carbonic acid escaping into the 
 air or being carried by rains into the soil. Hence it would 
 seem to naturally follow that the soil where supporting an 
 abundant vegetation should contain a larger percentage of 
 carbonic acid than the atmosphere itself. That it does not 
 contain, in all cases, a greater amount of free carbonic acid is 
 apparently brought out in the table . from the works of Bous- 
 singault and Lewy, as quoted on p. 178. 
 
 1 Some of our archaeologists go so far as to assert that the stone implements 
 found buried several feet below the surface in glacial deposits, and brought for- 
 ward as proving the existence of pre-glacial man, have been brought into that 
 position by just such agencies. See Holmes, Early Man in Minnesota, American 
 Geologist, April, 1893, p. 228. 
 
396 THE REGOLITH 
 
 Bacteria as agents of nitrification are undoubtedly efficacious 
 in preparing nitrogeneous matter in the soils for assimilation 
 by growing plants. Their influence as decomposers of rock 
 masses was noted on p. 203. According to Wiley, 1 it is highly 
 probable that organic nitrogen in the soil, in passing into the 
 form of nitric acid, exists at some period of the process in the 
 form of ammonia. The products of nitrification, he says, are 
 ammonia, nitrous or nitric acid, carbon dioxide, and water. 
 The ammonia and nitrous acid may not appear in the soils as 
 the final products of nitrification, as the organism attacks the 
 nitrous acid at once, converting it into the nitric form. 
 
 It may at first seem strange that man, who prides himself on 
 being the highest type in the animal kingdom, as well as the 
 only animal endowed with reasoning powers, should prove the 
 most destructive ; yet such is the case. Through prodigality, 
 due in part to thoughtlessness and in part to a wilful disregard 
 for any but immediate interests, man has, apparently from the 
 very beginning of his existence, so conducted himself with re- 
 lation to natural resources as to leave little less than ruin in 
 his path. This is true not merely with reference to his treat- 
 ment of the soil, but of the deeper lying rocks and their min- 
 eral contents. In the name of development he has squandered ; 
 through careless husbandry he has not merely impoverished 
 the soil, but in many cases allowed it to run waste and be lost 
 beyond recovery. So long ago as 1846, when Lyell made his 
 second visit to America, he was struck by the rapid denuda- 
 tion of the land in our Southern states due to the reckless cut- 
 ting away of the forests. He describes near Milledgeville, in 
 Georgia, a washout in a lately deforested area. " Twenty years 
 ago," he writes, " before the land was cleared, it [the washout] 
 had no existence ; but when the trees of the forest were cut 
 down, cracks 3 feet deep were caused by the sun's heat in the 
 clay ; and during the rains, a sudden rush of water through the 
 principal crack deepened it at its lower extremity, from whence 
 the excavating power worked backwards, till in the course of 
 20 years, a chasm measuring no less than 55 feet in depth, 300 
 yards in length, and varying in width from 20 to 180 feet was 
 the result. The high road has been several times turned to 
 avoid this cavity, the enlargement of which is still proceeding, 
 and the old line of road may be seen to have held its course, 
 1 Principles and Practice of Agricultural Analysis, p. 464. 
 
EFFECT OF PLANT AND ANIMAL LIFE 397 
 
 directly over what is now the widest part of the ravine. In 
 the perpendicular walls of this great chasm appear beds of clay 
 and sand, red, white, yellow, and green, produced by the de- 
 composition in situ of hornblendic gneiss, with layers of veins 
 of quartz, which remain entire, to prove that the whole mass 
 was once solid and crystalline." 1 
 
 The same lack of foresight or wanton disregard for coming 
 generations is still manifested, and every muddy stream bears 
 downward to the sea an increased load of silt from lands im- 
 properly cultivated and from which every rain removes a por- 
 tion of the finest and richest of the soil, leaving behind but the 
 barren gravel, channelled it may be beyond the possibility of 
 cultivation. McGee 2 has more recently made observations of 
 a similar nature in southern Mississippi, where the softer loam 
 of the Columbia formation, which here forms the soil, has 
 been allowed to become eroded down to the barren sandy loam 
 of the Lafayette. " Old fields are denuded by the acre, leaving 
 mazes of pinnacles divided by a complex network of runnels 
 glaring red toward the sun and sky in strong contrast to the 
 rich verdure of the hillsides never deforested ; the plantations, 
 mansions, and ' quarters ' are undermined, and whole villages, 
 once the home of wealth and luxury, are being swept away at 
 the rate of acres for each year/' 
 
 " The ravages committed by man," writes Marsh, 3 " subvert 
 the relations and destroy the balance which nature had estab- 
 lished between her organized and her inorganic creations, and 
 she avenges herself upon the intruder by letting loose upon her 
 defaced provinces destructive energies hitherto kept in check 
 by organic forces destined to be his best auxiliaries, but which 
 he has unwisely dispersed and driven from the field of action. 
 When the forest is gone, the great reservoir of moisture stored 
 up in its vegetable mould is evaporated, and returns only in 
 deluges of rain to wash awa} r the parched dust into which that 
 mould has been converted. The well-wooded and humid hills 
 are turned to ridges of dry rock, which encumbers the low 
 grounds and chokes the watercourses with its debris, and 
 except in countries favored with an equable distribution of rain 
 
 1 Lyell, Principles of Geology, 9th ed., 1846, p. 204. 
 
 2 12th Ann. Rep. U. S. Geol. Survey, 1890-91. 
 
 3 The Earth as modified by Human Action, by Geo. P. Marsh, a new edition 
 of Man and Nature, pp. 43, 44. 
 
398 THE REGOLITH 
 
 through the seasons, and a moderate and regular inclination of 
 surface the whole earth, unless rescued by human art from the 
 physical degradation to which it tends, becomes an assemblage 
 of bald mountains, of barren, turfless hills, and of swampy and 
 malarious plains. There are parts of Asia Minor, of northern 
 Africa, of Greece, and even of Alpine Europe, where the opera- 
 tion of causes set in action by man has brought the face of the 
 earth to a desolation almost as complete as that of the moon ; 
 and though, within that brief space of time which we call ' the 
 historical period,' they are known to have been covered with 
 luxuriant woods, verdant pastures, and fertile meadows, they 
 are now too far deteriorated to be reclaimable by man, nor can 
 they become again fitted for human use, except through great 
 geological changes, or other mysterious influences or agencies 
 of which we have no present knowledge, and over which we 
 have no prospective control. The earth is fast becoming an 
 unfit home for its noblest inhabitant, and another era of equal 
 human crime and human improvidence, and of like duration 
 with that through which traces of that crime and that improvi- 
 dence extend, would reduce it to such a condition of impover- 
 ished productiveness, of shattered surface, of climatic excess, 
 as to threaten the depravation, barbarism, and perhaps even 
 extinction of the species." 
 
LIST OF AUTHORS CITED OR REFERRED TO 
 
 Agassiz, L., 175. 
 
 Alexander, H. F., 242. 
 
 Aughey, S., 331. 
 
 Bartlett, W. H., 180. 
 
 Bayley, W. S., 7f>, 78, 80. 
 
 Beaumont, Elie de, 160. 
 
 Becker, G. F., 235, 301. 
 
 Bell, Robert, 194, 243, 246, 275. 
 
 Belt, T., 175, 261, 277, 280, 298. 
 
 Berthier, P., 237. 
 
 Beyer, M., 178. 
 
 Bischof, G., 20, 27, 191, 192. 
 
 Bkias, J., 102. 
 
 Blake, W. P., 121, 126, 185, 247, 250, 34(5, 
 
 348, 394. 
 
 Bolton, H. Carrington, 202. 
 Bonney, T. G., 246. 
 Boussingault, J. B., 176. 
 Branner, J. C., Ill, 175, 179, 188, 203, 278. 
 Brogger, W. C., 64. 
 Brougniart, A., 87, 175, 237. 
 Brown, A. P., 29. 
 Bruner, H. L., 344. 
 Buchanan, J. V., 204. 
 Caldcleugh, Alexander, 193. 
 Chamberlain, T. C., 278, 301, 303. 
 Cholfat, P., 255. 
 Clark, W. B., 134. 
 Clark, W. C., 118. 
 Cloez, 176. 
 Collier, P., 369. 
 Cointe de la Hure, 188. 
 Crosby, W. O., 138, 189, 255, 353, 385. 
 Cross, C.W., 35, 62, 71,81. 
 Culver, G. E., 279. 
 Gushing, H. P., 279. 
 Dana, E. S., 31, 127. 
 Dana, Professor J. D., 49, 57, 117, 198, 
 
 235, 251, 253, 262. 
 Darton, N. L., 312. 
 Darwin, E., 175, 233, 292, 392. 
 Daubree, A., 16, 197, 376. 
 Davis, W. M., 18(5. 
 Davidson, C., 287. 
 Dawson, J. W., 291, "334. 
 
 De Luca, 176. 
 
 Derby, O. A., 188, 277. 
 
 Diller, J. S., 87, 92. 
 
 Dutton, C. E., 196. 
 
 Dwight, 297. 
 
 Dyer, B., 202. 
 
 Ebelmen, M., 237. 
 
 Egleston, Thomas, 184. 
 
 Ewing, A. L., 194. 
 
 Failyer, G. H., 176. 
 
 Fernow, B. E., 282. 
 
 Fesca, Dr. Max, 243. 
 
 Forbes, 184. 
 
 Forschammer, J. G., 237. 
 
 Fournet, 175, 237. 
 
 Fulton, R. L., 280. 
 
 Furlouge, W. H., 277. 
 
 Geikie, A., 2, 146, 201, 288. 
 
 Geikie, James, 357. 
 
 Geldmacher, Max, 236. 
 
 Gesner, H. S., 317. 
 
 Gilbert, G. K., 50, 185, 256, 349. 
 
 Gordon, C. H., 108. 
 
 Griswold, L. S., 111. , 
 
 Gumbel, C. W., 28, 88. 
 
 Hall, C. W., and Sardeson, F. W., 161, 250. 
 
 Harker, A., 39. 
 
 Hartt, C. F., 175, 280. 
 
 Hawes, G. W., 46, 75, 87, 170. 
 
 Ha worth, E., 25. 
 
 Hayden, F. V., 252. 
 
 Hayes, C. W., 109, 194. 
 
 Heusser and Claraz, 175, 228, 251. 
 
 Hilgard, E. W., 333, 346, 300, 367, 369, 371, 
 
 374. 
 
 Hitchcock, C. H., 68. 
 Hitterman, 239. 
 Hobbs, W. H., 218. 
 Holmes, W. H., 395. 
 Hovey, E. O., 230. 
 Hunt, T. S., 86, 99, 124, 159, 258. 
 Iddings, J. P., 22, 39, 57, 60, 64, 71, 72, 81. 
 Irving, R. D., 278. 
 Johnson, S. W., 177, 178. 
 Johnson and Blake, 136. 
 
400 
 
 LIST OF AUTHORS CITED OR REFERRED TO 
 
 Johnstone, Alexander, 189. 
 
 Jones, T. Rupert, 317. 
 
 Judd, J. W., 284, 321. 
 
 Julien, A. A., 190. 
 
 Kalkowski, E., 75. 
 
 Kemp, J. F., 81, 86, 87, 171. 
 
 Kerr, W. C., 286. 
 
 Keyes, C. R., 25. 
 
 Kidder, J. H., 179. 
 
 King, F. H., 381. 
 
 King, Clarence, 71. 
 
 Klement, M. C., 160. 
 
 Kletzinsky, W., 176. 
 
 Kuhn, M. Levy, 89. 
 
 Layard, A. H., 293. 
 
 Le Conte, J., 258. 
 
 Lemberg, J., 18, 217, 374. 
 
 Lindgren, W., 75, 274. 
 
 Livingstone, David, 183. 
 
 Loftus, 293. 
 
 Loughbridge, R. H., 365, 366. 
 
 Lyell, Sir Charles, 396. 
 
 Marsh, George P., 183, 297. 
 
 McGee, W. J., 301, 312, 323, 397. 
 
 Meister, 381. 
 
 Merrill, G. P., 37, 47, 54, 81, 87, 
 
 115, 154, 159, 206, 218, 349. 
 Mills, J. E., 175, 203, 273. 
 Muller, Alex, 371. 
 Miiller, R., 192. 
 Munroe, C. E., 190. 
 Miintz, A., 203. 
 Miintz and Aubin, 179. 
 Miintz and Maracano, 373. 
 Murakozky, K. V., 238. 
 Neumayer, M., 302. 
 Newberry, J. S., 118, 356. 
 Nordenskiold, A. E., 242. 
 Oldham, R. D., 311. 
 Orton, Edward, 117, 118, 124. 
 Owen, D. D., 111. 
 Packard, R. L., 108, 376. 
 Pallarsen, 89. 
 Penrose, R. A. F., 231, 232. 
 Pirsson, L. V., 64. 
 Pliny, 73, 90. 
 Porter, J. B., 266. 
 Potter, W. B., 265, 275, 276. 
 Prestwich, Joseph, 65, 260. 
 Prichard, 30. 
 Pumpelly, R., 275, 277. 
 Purrington, C. W., 279. 
 Read, T. Mellard, 194. 
 Redwood, Boverton, 129. 
 Retgers, J. W., 349. 
 Reusch, H., 250. 
 
 113, 
 
 Richthofen, F. von, 63, 85. 
 
 Rogers Brothers, 191. 
 
 Rohrbach, C. E. M., 89. 
 
 Roscoe and Schorlemmer, 4. 
 
 Rose, G., 79, 89. 
 
 Roseubusch, H., 57, 62, 70, 72, 74, 82, 93, 
 
 97, 98. 
 Rosiere, 73. 
 Roth, Justus, 72, 94, 101, 103, 208, 239, 
 
 256. 
 Russell, I. C., 112, 201, 266, 279, 280, 284, 
 
 296, 301, 333, 385. 
 Rutley, F., Ill, 194. 
 Safford, J. M., 267. 
 
 Salisbury, R. D., 278, 287, 301, 303, 352. 
 Schlosing, 203. 
 Schutze, R., 228. 
 
 Shaler, N. S., 181, 197, 318, 336, 389, 498. 
 Smith, Angus, 179. 
 Sorby, H. C., 26, 38, 199, 243, 342. 
 Spurr, J. E., 107. 
 Stanley, H. M., 183. 
 Stejneger, L., 199. 
 Stone, G. H., 186. 
 Storer, F. H., 191, 202. 
 Strabo, 90. 
 
 Streeruwitz, H. von, 182. 
 Streng, A., 86. 
 Teall, J. J. H., 24, 74, 90. 
 Theiiard, P., 190. 
 Thompson, Wyville, 247. 
 Tornebohm, A. E., 87, 89. 
 Tschermak, G., 24. 
 Van Den Broeck, E., 178, 258. 
 VanHise.C. R., 106. . 
 Vom Rath, G., 255. 
 Von Buch, L., 83. 
 
 Wadsworth, Dr. M. E., 57, 68, 85, 97, 254. 
 Weed, W. H., 109. 
 Werner, A. G., 73. 
 Whitaker, W., 267. 
 Whitney, J. D., 68, 127, 278, 328. 
 Whitney, Milton, 287, 307-309, 313, 340, 
 
 344, 353, 379. 
 Wichman, A., 170. 
 Widogradsky, 203. 
 Wiley, H. W., 178, 316. 
 Williams, G. H., 63, 72, 86, 96, 99, 100, 
 
 156, 216. 
 
 Williams, J. F., 64. 
 Willis, Bailey, 52. 
 Winchell, N. H., 297. 
 Wolff, Professor J. E., 93. 
 Woodward, J. B., 186. 
 Wurtz, H., 127. 
 Zirkel, F., 38, 57, 68, 80, 87, 89. 
 
INDEX 
 
 Abrasive action of wind-blown sand, 185. 
 
 Acid rocks, meaning of term, 64. 
 
 Acmite, 22. 
 
 Adobe, 139, 332. 
 
 JEgerine, 22. 
 
 ./Eolian deposits, 344. 
 
 ^Eolian rocks, 153 ; defined, 58. 
 
 Agalmatolite, 116. 
 
 Age of soils, 386. 
 
 Air in motion, effects of, 189. 
 
 Alabaster, 117. 
 
 Alaska, rock-weathering in, 279, 284. 
 
 Albertite described, 127. 
 
 Albite as a rock constituent, 16. 
 
 Alkalies in soils, 371. 
 
 Alkaline carbonates, when formed, 372; 
 in soils, 371 ; formed during weather- 
 ing, 205. 
 
 Alkaline silicates in soils, 370. 
 
 Allanite, 25. 
 
 Allotriomorphic minei-als defined, 41. 
 
 Alluvial cones defined, 54. 
 
 Alluvial deposits, 320. 
 
 Alteration defined, 174. 
 
 Alum shale, 138. 
 
 Aluminum as a constituent of the earth's 
 crust, 5. 
 
 Amber, 128. 
 
 Amianthus, 115. 
 
 Ammonia in atmosphere, 177. 
 
 Ammonium sulphate, influence in decom- 
 posing feldspars, 178. 
 
 Amorphous, definition of, 40. 
 
 Ainphiboles as rock constituents, 19. 
 
 Amygdaloidal structure, 34. 
 
 Anacostia, deposits of the, 323. 
 
 Analyses, calculations of, 210; discus- 
 sion of, 212. 
 
 Anamesite, 92. 
 
 Andesites, 83. 
 
 Andesitic rocks, induration of surface, 
 2.55. 
 
 Anhydrite described, 118. 
 
 Animal life, effect on soils, 389. 
 
 Anorthite as a rock constituent, 17. 
 
 Anorthit-gesteine, 89. 
 
 Anthracite coal, 150. 
 
 Antique porphyry, 83. 
 
 Ants, effect on soils, 389 ; as promoters of 
 rock decomposition, 204. 
 
 Apatite as a rock constituent, 27. 
 
 Apo-rhyolite, 72. 
 
 Appalachian Mountain system, material 
 eroded from, 196. 
 
 Appomattox formation, 312. 
 
 Aqueo-glacial clays, 334. 
 
 Aqueous rocks, 105 ; defined, 58. 
 
 Aragonite as a rock constituent, 26. 
 
 Arenaceous group, the, 131. 
 
 Argillaceous rocks described, 135. 
 
 Argillites, 137; fissile, 170; Harford 
 County, Maryland, weathering of, 229. 
 
 Arkansas River referred to, 289. 
 
 Asphaltum described, 125. 
 
 Atmosphere, action of, 176. 
 
 Augite, molecular alteration of, 39; rela- 
 tive durability of, 235. 
 
 Augite porphyrite, 90; Montana, disin- 
 tegration of, 235. 
 
 Augite vitrophyrite, 90. 
 
 Augitite described, 101. 
 
 Auriferous sands, origin of, 266. 
 
 Bacteria, as agents of nitrification, 396; 
 decomposing action of, 203. 
 
 Banding in gneisses, origin of, 165. 
 
 Barbadoes Island, volcanic dust on, 298. 
 
 Barite described, 118. 
 
 Barium as a constituent of the earth's 
 crust, 7. 
 
 Basalt, described, 90; Bohemia, weather- 
 ing of, 223; Haute Loire, France, 
 weathering of, 223. 
 
 Basalts, geographical distribution in 
 United States, 92 ; weathering of, 262. 
 
 Basanite described, 94. 
 
 Base, definition of, 40. 
 
 Basic rocks, meaning of term, 64. 
 
 Beach sands, 341. 
 
 Beauxite described, 108. 
 
 401 
 
402 
 
 INDEX 
 
 Bedded rocks defined, 53. 
 
 Bedded structure, 34. 
 
 Bermuda, weathering of limestones in, 
 
 247. 
 
 Binary granite, 68. 
 Biotite as a rock constituent, 23. 
 Bitumen, 125. 
 Bituminous coals, 149. 
 Bituminous dolomite of Chicago, 145. 
 Black earth, Russian, 318. 
 Bleaching of rocks on exposure, 257. 
 Bluegrass soil, 382. 
 Bog of Allen, 317. 
 Bogs, classification of, 317. 
 Boss, defined, 50. 
 
 Boss-like form accentuated by joints, 245. 
 Botryoidal structure, 37. 
 Boulder clay, 138. 
 Boulder clays, 353. 
 Boulders, of decomposition resembling 
 
 those of the drift, 242; formed by 
 
 weathering, 244. 
 Bowenite, 116. 
 Breccia, 133. 
 
 Brecciated limestones, 139. 
 Brecciated structure, 38. 
 Bronzitite, 100. 
 Brown hematite, 29, 107. 
 Brownstone, 133. 
 
 Cabook, formation of, 242. 
 
 Calc sinter, 112. 
 
 Calcareous group of rocks, 137. 
 
 Calcareous rocks, 143; rate of weather- 
 ing, 272. 
 
 Calcite as a rock constituent, 25. 
 
 Calcium as a constituent of the earth's 
 crust, 6. 
 
 Calcium carbonate, amount annually re- 
 moved in solution, 194. 
 
 Camptonite, 8. 
 
 Cannel coal, 150. 
 
 Cape Cod, wind action on, 297. 
 
 Carbonates of alkalies, influence of, 238. 
 
 Carbonates of the alkalies formed during 
 weathering, 205. 
 
 Carbonates, production of, during weath- 
 ering, 205. 
 
 Carbonaceous rocks, 148. 
 
 Carbonic acid, influence of, in feldspathic 
 decomposition, 237, 239; amount annu- 
 ally brought to the surface, 179 ; in air 
 of soils, 178; in the atmosphere, 178. 
 
 Carboniferous soils, 386. 
 
 Catlinite, 139. 
 
 Cavernous structure, 38. 
 
 Cellular structure, 38. 
 
 Ceylon, rock disintegration in, 242. 
 
 Chalcedony, 110. 
 
 Chalk, 143 ; decomposition of, 267. 
 
 Chemical composition of rocks, 44. 
 
 Chemical elements constituting rocks, 4. 
 
 Chert, 110 ; of Arkansas, weathering of, 
 231 ; of Missouri, weathering of, 230. 
 
 Chilian nitrates, origin of, 373. 
 
 Chlorides, 119. 
 
 Chlorite as a rock constituent, 30. 
 
 Chrysotile, 115. 
 
 Citric acid, solvent property of, 202. 
 
 Classification of soils, 381. 
 
 Clastic rocks, 129 ; classification of, 130. 
 
 Clastic structure, 34. 
 
 Clay concretions, formation of, 37. 
 
 Clay, defined, 135; effect on soils, 368; 
 protective influence of, 254. 
 
 Clay ironstone, 114. 
 
 Clay slates, 137. 
 
 Clays, aqueo-glacial, 334. 
 
 Climate, influence of, on weathering, 278. 
 
 Clinton iron ores, origin of, 266. 
 
 Coefficient of cubical expansion of min- 
 erals, 268. 
 
 Coking coals, 150. 
 
 Cold, effect on rocks, 180. 
 
 Colloidal structure, 33. 
 
 Colluvial deposits, 319. 
 
 Color ; changes incidental to weathering, 
 257 ; of rocks, 45 ; of soils, 384 ; of soils, 
 cause of, 385 ; variation, cause of, 47. 
 
 Colorado Kiver, erosion by, 196. 
 
 Columbian formation, 312. 
 
 Columnar structure, 38. 
 
 Complexity of structure favoring disinte- 
 gration, 250. 
 
 Concentric exfoliation, 244; not indica- 
 tive of an original concretionary struct- 
 ure, 245. 
 
 Concentric structure inevitable to joint- 
 ing, 245. 
 
 Concretionary structure, 35; in granite, 
 246; in crystalline rocks, 37. 
 
 Conductivity of rocks, unequal, 184. 
 
 Conglomerate, 133. 
 
 Conservative action of plants, 202. 
 
 Contact metamorphism, 157. 
 
 Contours incidental to weathering, 259. 
 
 Coprolite nodules, 152. 
 
 Coquina, 143. 
 
 Coral limestone, 143. 
 
 Corroded surfaces, irregularity of, 250. 
 
 Corsica, weathering of granite on, 250. 
 
 Crayfish, effects on soils, 391. 
 
 Creeping of soil cap, 287. 
 
 Crenic acid, 190. 
 
 Crystalline limestones and dolomkes, 
 162. 
 
 Crystalline schists, the, 168. 
 
 Crystalline structure, 33. 
 
INDEX 
 
 403 
 
 Crystallites defined, 41. 
 Cumulose deposits, 313. 
 
 Dacite, 84. 
 
 Daubree's experiments in rock tritura- 
 tion, 17, 197. 
 
 Decay, time limjt of, 272; of rocks, how 
 characterized, 212. 
 
 Decomposition and disintegration, dis- 
 crimination between, 283. 
 
 Decomposition, depth of, 278 ; following 
 disintegration, 243; incident to ero- 
 sion, 197; of fragmental rocks, 228; of 
 greenstone dikes, effects of, 244 ; of 
 rocks, chemical processes involved in, 
 238 ; of shells through the aid of salt, 
 233 ; natural acceleration of, 205. 
 
 Degeneration of rocks, 174. 
 
 Degradation of North America, rate of ,196. 
 
 Delta deposits, 320. 
 
 Delta of the Nile, section of, 321. 
 
 Deoxidation, 187 ; by marine animals, 204. 
 
 Desert varnish, 256. 
 
 Devil's Tower, origin of, 261. 
 
 Deweylite, 116. 
 
 Diabase, described, 87; mandelstein, 90; 
 Medford, Massachusetts, weathering 
 of, 218 ; porphyrite, 90 ; Venezuela, 
 weathering of, 222. 
 
 Diallogite, 100. 
 
 Diamonds, origin of, 98. 
 
 Diatom aceous earth, 141. 
 
 Dichte diabase, 90. 
 
 Dike denned, 50. 
 
 Diluvium, rouge et gres, 258. 
 
 Diorite, Albemarle County, Virginia, 
 weathering of, 224. 
 
 Diorite-andesite group, 81. 
 
 Diorites, 87. 
 
 Discoloration, above drainage level, 258; 
 incidental to weathering, 257. 
 
 Discussion of analyses, 234. 
 
 Disintegration of rocks in Lower Califor- 
 nia, 183 ; prevented by surroundings, 
 252; without decomposition, 241. 
 
 District of Columbia, rock-weathering 
 in, 283. 
 
 Ditroite, 79. 
 
 Dolerite, 92. 
 
 Dolomite, as a rock constituent, 26 ; de- 
 scribed, 145 ; origin of name, 163 ; ori- 
 gin of, by metasomatosis, 159. 
 
 Dolomites, 162. 
 
 Dolomitic limestones, disintegration of, 
 250 ; weathering of, 239. 
 
 Drift, extent of, 291. 
 
 Drumlins, 355; defined, 55. 
 
 Dune defined, 55. 
 
 Dune sand, chemical composition, 350. 
 
 Dunite, 97. 
 
 Dust, in rain and snow falls, 344; vol- 
 canic, 298, 349. 
 
 Dust soils, 345. 
 
 Dust storms, 292 : in Dakota, 293 ; in Mon- 
 tana and Nevada, 294. 
 
 Dynamic metamorphism, 156. 
 
 Earth's crust, thickness of, 2. 
 
 Earthworms, effects on soils, 392. 
 
 Eclogite, 170. 
 
 Effacement of characteristics by weath- 
 ering, 262. 
 
 Effusive .rocks, characteristics of, 61 ; 
 defined, 60. 
 
 Elaeolite as a rock constituent, 18. 
 
 Elreolite syenites, 78. 
 
 Elaeolite syenite porphyry, 79. 
 
 Elaterite described, 126. 
 
 Elvanite, 70. 
 
 Eozoon Canadense, 159, 163; origin of, 
 116. 
 
 Epidiorite, 89. 
 
 Epidote, as a rock constituent, 25 ; altera- 
 tion of, 25. 
 
 Erosion by rivers, 196. 
 
 Eruptive rocks, 59. 
 
 Eskers, 290, 356. 
 
 Eucrite, 89. 
 
 Eulysite, 97. 
 
 Eurite, 70. 
 
 Exfoliated rocks, shape and size of flakes, 
 182. 
 
 Exfoliation, attended by gun-like reports, 
 182 : due to heat and cold, 181 ; of rocks 
 on Cape Cod, 182. 
 
 Expansion through hydration, 188. 
 
 Extent of weathering, 276 : in Brazil, 
 Colorado, District of Columbia, Mis- 
 souri, Nicaragua, South Africa, South 
 America, 277. 
 
 Fault defined, 53. 
 
 Feldspars, as rock constituents, 13; de- 
 composition of, 17. 
 
 Feldspathic decomposition, process of, 
 237; in Comstock Lode, 235; by fresh 
 water, 238 ; influenced by ammonium 
 sulphate and sodium chloride, 178. 
 
 Feldspar porphyry of Iron Mountain, 
 weathering of, 265. 
 
 Feldspars, relative durability of, 235. 
 
 Felsitic structure, 33. 
 
 Felsite pitchstone, 70. 
 
 Felsophyr, 70 
 
 Felstone, 70. 
 
 Ferrous carbonate, solubility of, 239. 
 
 Fertility of soil dependent on physical 
 condition, 379. 
 
404 
 
 INDEX 
 
 Fichtellite, 129. 
 
 Fiorite, 109. 
 
 Fissile argillites, roofing slates, 170. 
 
 Flagstone, 133. 
 
 Flexible sandstone, 134. 
 
 Flint, 110. 
 
 Flood plain of the Mississippi, 323. 
 
 Fluidal or fluxion structure, 34. 
 
 Fogs, indices of dust in atmosphere, 
 344. 
 
 Foliated or schistose rocks, 164. 
 
 Foliated structure, 34. 
 
 Forellenstein, 87. 
 
 Forests, buried by sand, 295; influence 
 of, 280 ; protective action of, 282. 
 
 Fourchite, 79. 
 
 Foyaite, 79. 
 
 Foyaite-phonolite group, 77. 
 
 Fracture of rocks, 48. 
 
 Fragmental structure, 34. 
 
 Freestone, 133. 
 
 Freezing water, disintegrating action of, 
 198. 
 
 Frontal aprons, 356. 
 
 Frontal moraines, 355. 
 
 Frost, action in accelerating decomposi- 
 tion, 278 ; action on soil, 367 ; disin- 
 tegrating action, 199; heaving effects 
 on boulders, 393; supposed protective 
 action of, 278. 
 
 Gabbro described, 85. 
 
 Gabbro-basalt group, 85. 
 
 Garnet rock, 170. 
 
 Garnserite, formation of, 226. 
 
 Garnetite, 170. 
 
 Geest, 301. 
 
 Gem sands of Ceylon, origin of, 266. 
 
 Genetic relationship of rocks, 64. 
 
 Geological age, of soils, 389 ; a basis for 
 classification, 63. 
 
 Geyserite, 109. 
 
 Gilsonite, 127. 
 
 Glacial deposits, 351. 
 
 Glacial detritus, amount of, 201. 
 
 Glacial drift, extent of, 291. 
 
 Glacial lakes, extinction of, 289; filling 
 of, 326. 
 
 Glacial landscape, 291. 
 
 Glacial moraine, 290. 
 
 Glacial soil of Cape Elizabeth, composi- 
 tion of, 364. 
 
 Glacier, the, as an erosive agent, 200. 
 
 Glaciers, as agents of transportation, 200 
 289. 
 
 Glass abraded by wind-blown sand, 185. 
 
 Glauconite, 31, 134. 
 
 Glauconitic marl, 134. 
 
 Globulitos defined, 41. 
 
 Gneiss, Albemarle County, Virginia, de- 
 generation of, 213. 
 
 Gneisses, the, 164. 
 
 Grahamite described, 127. 
 
 Granite, described, 65 ; extent of weather- 
 ing in District of Columbia, 276. 
 
 Granitell, 68. 
 
 Granite-liparite group, 65. 
 
 Granite porphyry, 68. 
 
 Granite soil defined, 383. 
 
 Grauitite, 67. 
 
 Granofelsophyr, 70. 
 
 Granophyr, 70. 
 
 Granular structure, 34. 
 
 Granulite, 167. 
 
 Grauwacke, 133. 
 
 Graphic granite, 67. 
 
 Gravels superficially oxidized, 258. 
 
 Gravity, action of, in transporting debris, 
 286. 
 
 Greenland, rock-weathering in, 278. 
 
 Greensand marl, 134. 
 
 Greenstone, 81. 
 
 Greisen, 68. 
 
 Greywacke, 133. 
 
 Ground-mass defined, 40. 
 
 Ground moraine, 352. 
 
 Gruss, 301. 
 
 Guano, 151. 
 
 Gypsum described, 117. 
 
 Halleflinta, 167. 
 
 Hardpan, 368. 
 
 Harzburgite, 97. 
 
 Hatchettite, 129. 
 
 Hatteras and Henlopen, sand dunes of, 
 295. 
 
 Heat, action on pebbles in Arabia Petrea, 
 183 ; expansive action on rocks, 180. 
 
 Heat and cold, as agents of decomposi- 
 tion, 180 ; effects of, in Africa, 183 ; 
 effects limited to surface, 183; most 
 effective on slopes, 184. 
 
 Heavy spar, 118. 
 
 Hematite, 106 ; as a rock constituent, 28. 
 
 Holocrystalline, definition of, 40. 
 
 Hornblende, as a rock constituent, 19; 
 decomposition of, 20; relative durabil- 
 ity of, 235. 
 
 Hornblende picrite, 97. 
 
 Hornblendite, 100. 
 
 Humic acid, 189. 
 
 Humidity, weathering influenced by, 270. 
 
 Hyaline andesite, 85. 
 
 Hyalite formed during feldspathic de- 
 composition, 238. 
 
 Hyalobasalt, 92. 
 
 Hyaloliparite, 72. 
 
 Hyalomelan, 92. 
 
INDEX 
 
 405 
 
 Hyalotrachyte, 77. 
 
 Hydration, 187 ; importance of, 188, 234, 
 
 253, 278 ; of micas, 189. 
 Hydraulic limestone, 145. 
 Hydrocarbou compounds, description of, 
 
 121. 
 
 Hydro-metamorphism, 101. 
 Hyperite, 87. 
 Hypersthenite, 100. 
 Hypocrenic acid, 190. 
 Hypocrystalliiie, definition of, 40, 
 
 Ice, disintegrating action of, 198 ; influ- 
 ence in transporting rock debris, 287 ; 
 mechanical action of, 195. 
 
 Idiomorphic minerals defined, 41. 
 
 Igneous rocks, 59; defined, 57. 
 
 Ilmenite as a rock constituent, 28. 
 
 Induration, cause of, 255; of rocks on 
 exposure, 254 ; of sandstone by igneous 
 contacts, 261. 
 
 Infusorial earth, 141. 
 
 Insects, effects on soils, 394. 
 
 Intrusive rocks defined, GO. 
 
 Inundated lands, classification of, 318. 
 
 Iron, as a constituent of the earth's crust, 
 5 ; removed in form of ferrous sul- 
 phate, 239; removed in form of pro- 
 toxide carbonate, 239; variation in 
 solubility, 239. 
 
 Iron Mountain, Missouri, pre-Silurian 
 weathering of, 276. 
 
 Iron ores as rock constituents, 27. 
 
 Iron pyrites as a rock constituent, 29. 
 
 Itacolumite, 133. 
 
 Itacolumites, Brazilian, weathering of, 
 228. 
 
 Jasper, 110. 
 
 Joints, as aids to weathering, 244 ; cause 
 of, 245 ; influence of, in producing 
 boulders, 244; influence in producing 
 boss-like forms, 245. 
 
 Kalk diabase, 90. 
 
 Kames, 290. 
 
 Kaolin, 116, 136, 205; composition of, 
 
 309; origin of, 308. 
 Kaolinite distinct from kaolin, 309. 
 Kaolinization defined, 18. 
 Keratophyr, 76. 
 Kersantite, 82. 
 Kimberlite, 98. 
 Kinds of rocks, 50. 
 Kinzigkite, 170. 
 Konlite, 129. 
 
 Krakatoa, dust from, 298. 
 Ktaadn Iron AVorks referred to, 107. 
 Kugel porphyry, 70. 
 
 Labradorite as a rock constituent, 17. 
 
 Laccolite defined, 50. 
 
 Lake Agassiz, deposits in, 290. 
 
 Lake Asphaltites, 120. 
 
 Lakes, filling of, 314 ; transient charac- 
 ter of, 326. 
 
 Laminated or banded structure, 38. 
 
 Landscape, glacial, 291. 
 
 Lapilli, 140. 
 
 Laterite, 139, 310. 
 
 Laurvikite, 79. 
 
 Lava defined, 51. 
 
 Leda clays, 334. 
 
 Leopardite, 70. 
 
 Leptinite, 107. 
 
 Leucite as a rock constituent, 18. 
 
 Leucite basalt, 103. 
 
 Leucite-nepheline rocks, 102. 
 
 Leucite rocks described, 102. 
 
 Leucitite, 103. 
 
 Leucitophyr, 80. 
 
 Leucophyr, 88. 
 
 Leucoxene, 28. 
 
 Lherzolite, 97. 
 
 Lichens, action of, 201. 
 
 Liebuerite, 79. 
 
 Lignite, 149. 
 
 Limburgite described, 98. 
 
 Lime carbonate, decomposing action of, 
 370. 
 
 Lime in soils, 366. 
 
 Limestone, unequal weathering of, 250; 
 weathering of, 232. 
 
 Limestone residuals, character of, 303. 
 
 Limestone soils poor in lime, 259. 
 
 Limestones, 143; and dolomites, 162; 
 corroded by acids, 194; corroded by 
 meteoric waters, 259; unequal indura- 
 tion of, 247; variation in composition, 
 147. 
 
 Limit of diminution in size of particles 
 by erosion, 197. 
 
 Limonite, 107 ; as a rock constituent, 29. 
 
 Liparite described, 70. 
 
 Litchfieldite, 79. 
 
 Lithophysfu, 72. 
 
 Loess, 139, 290, 327. 
 
 Logans, or tors, 252. 
 
 Lower California, rock-weathering in, 
 283. 
 
 Lumachelle, 143. 
 
 Lustre, 48. 
 
 Luxullianite, 70. 
 
 Lydian stone, 111. 
 
 Magma, definition of, 59. 
 Magnesian limestones, 145. 
 Magnesia removed in excess of lime, 
 239. 
 
406 
 
 INDEX 
 
 Magnesium as a constituent of the earth's 
 crust, 6. 
 
 Magnesite, 113. 
 
 Magnetite as a rock constituent, 27. 
 
 Man, has squandered in the name of 
 development, 397; ravages committed 
 by, 397. 
 
 Marbles, 163. 
 
 Marcasite as a rock constituent, 29. 
 
 Marginal moraines, 355. 
 
 Marine animals, influence of, on marine 
 muds, 204. 
 
 Marl, 146. 
 
 Marmolite, 116. 
 
 Marsh gas, 121. 
 
 Marsh lands, reclaimable areas, 340. 
 
 Martite, 106. 
 
 Massive structure, 34. 
 
 Material lost through weathering, 208. 
 
 Materials lost during decomposition, pro- 
 portional amounts, 234. 
 
 Mechanical action of water and ice, 195. 
 
 Mechanical disintegration most active in 
 regions of extreme temperatures, 182. 
 
 Melaphyr described, 90. 
 
 Melaphyrs and augite porphyrites, 90. 
 
 Melilite basalt, 92. 
 
 Menaccanite as a rock constituent, 28. 
 
 Metamorphic rocks, 155; defined, 58. 
 
 Metamorphism defined, 155. 
 
 Metasomatosis defined, 158. 
 
 Miascite, 79. 
 
 Mica, relative durability of, 236. 
 
 Micaceous sandstone, cause of weather- 
 ing, 189. 
 
 Micas, alteration and decomposition of, 
 23 ; as rock constituents, 22. 
 
 Microcline as a rock constituent, 16. 
 
 Microcrystalline structure, variation in, 
 41. 
 
 Micro-granite, 70. 
 
 Microlites defined, 40. 
 
 Microlitic structure, 33. 
 
 Micropegmatite, 70. 
 
 Microscope used in geology, 38. 
 
 Microscopic structure, 38 ; of rocks, 33. 
 
 Microscopic study of rocks, efficiency of, 
 39. 
 
 Mineral caouchouc, 126. 
 
 Mineral composition of soils, 373. 
 
 Mineral matter, dissolved by water, 191 ; 
 in solution, removed annually from 
 England and Wales, 194. 
 
 Mineral pitch, 125. 
 
 Minerals constituting rocks, 9 ; list of, 11. 
 
 Mineral variation of rocks, cause of, 9. 
 
 Mineral wax, 128. 
 
 Minette, 74. 
 
 Minnesota, wind action in, 297. 
 
 Mississippi, flood plain of, 323. 
 
 Mississippi River, amount of material 
 transported by, 288. 
 
 Missouri River, muddy character of, 
 288. 
 
 Mode of occurrence of rocks, 49. 
 
 Monazite sands, origin of, 266. 
 
 Monoclinic feldspars, 14. 
 
 Monoclinic pyroxenes as rock constitu- 
 ents, 21. 
 
 Monzonite, 74. 
 
 Moraine defined, 55. 
 
 Moraines, classified, 355 ; glacial, 290. 
 
 Mosses, action, 201. 
 
 Muck, 149. 
 
 Muscovite, as a rock constituent, 23 ; rel- 
 ative durability of, 236. 
 
 Natural gas, 121. 
 
 Nepheline as a rock constituent, 18. 
 
 Nepheline basalt, 107. 
 
 Nepheline dolerite, 104. 
 
 Nepheline rocks described, 103. 
 
 Nepheline syenites, 78; weathering of, 
 249. 
 
 Nephelinite, 104. 
 
 Nevadite, 72. 
 
 Niggerheads, how formed, 244. 
 
 Nile delta, section of, 321. 
 
 Nineveh, site obscured by sand dunes, 
 295. 
 
 Nitrates, influence of, in feldspathic de- 
 composition; 239; in soils, 372; source 
 of, 372. 
 
 Nitric acid, in atmosphere, 177 ; influence 
 of, in feldspathic decomposition, 239. 
 
 Nitrogen, in atmosphere, 176 ; in soils, 372. 
 
 Non-coking coal, 150. 
 
 Norites, 86. 
 
 Noumseite, formation of, 226. 
 
 Novaculite, 111. 
 
 Nummulitic limestone, 143. 
 
 Obsidian, 72. 
 
 Oldest known rocks, 49. 
 
 Oligoclase, as a rock constituent, 16 ; 
 disintegration of, 241 ; decomposition 
 of, 237. 
 
 Olivine, as a rock constituent, 24 ; altera- 
 tion into serpentine, 24 ; relative dura- 
 bility of, 235. 
 
 Onyx marbles, 113. 
 
 Oolites, English, coloration of, 258. 
 
 Oolitic limestone, 143 ; origin of, 53, 112. 
 
 Ophicalcite, 163. 
 
 Ophiolite, 89, 116, 163. 
 
 Organic acids, action of, 189; corrosive 
 power on marble, 190; solvent power 
 augmented by nitrogen, 190. 
 
INDEX 
 
 407 
 
 Oriental alabaster, 113. 
 
 Original constituents of rocks, 10. 
 
 Original structures preserved during de- 
 composition, 264. 
 
 Orthoclase, relative durability of, 236. 
 
 Orthoclase porphyries, 75. 
 
 Orthoclase as a rock constituent, 14. 
 
 Orthophyr, 76. 
 
 Orthorhombic pyroxenes as rock constit- 
 uents, 22. 
 
 Osars, 290, 356. 
 
 Ouachitite, 79. 
 
 Overwash plains, 356. 
 
 Oxidation, how manifested, 187; inci- 
 dental to decomposition, 234. 
 
 Oxides, silica, 10!) . 
 
 Oxygen, as a constituent of the earth's 
 crust, 5 ; influence in preventing loss 
 of iron during rock decomposition, 239 ; 
 of the atmosphere as an agent of de- 
 composition, 180. 
 
 Ozokerite, 128. 
 
 Palagonite tuff, 140. 
 
 Paludal deposits, 336. 
 
 Pantellerite, 72. 
 
 Paraffin, native, 128. 
 
 Paraniorphic minerals, 156. 
 
 Peat, 148. 
 
 Peat bogs, 317. 
 
 Pebble, normal shape of, 348. 
 
 Pegmatite, 67. 
 
 Pelites, 135. 
 
 Peperino, 140. 
 
 Peridotite, described, 95; weathering of , 
 
 225. 
 
 Peridotite-limburgite group, 95. 
 Perlite, 77. 
 Perlitic structure, 35. 
 Petroleum described, 122. 
 Petrosilex, 70. 
 Phenocrysts defined, 41. 
 Phlogopite, 23. 
 
 Phonolite, weathering of, 217. 
 Phosphates, 119. 
 
 Phosphates of Tennessee, origin of, 267. 
 Phosphatic sandstone, 152. 
 Phosphorite, 119. 
 Phosphorus, as a constituent of the 
 
 earth's crust, 7 ; relative proportion of, 
 
 in rocks, 8. 
 Phyllite, 169. 
 Physical and chemical properties of 
 
 rocks, 33. 
 
 Physical condition of soils, 378. 
 Physical manifestations of weathering, 
 
 241. 
 
 Picrite, 97. 
 Picrite porphyrites described, 98. 
 
 Picrolite, 116. 
 
 Pic Pourri, decomposition of, by bacteria, 
 
 203. 
 
 Piedmontite, 25. 
 Pike's Peak, Colorado, weathering of 
 
 granite, 243, 255. 
 Pisolitic limestone, 143. 
 Pitchstone, 77. 
 
 Placer deposits, origin of, 267. 
 Plagioclase feldspars, relative durability 
 
 of, 236. 
 
 Plagioclases as rock constituents, 16. 
 Plant and animal life, effect on soils, 
 
 389. 
 
 Plant life, effect on soils, 394. 
 Plants and animals, agents of disintegra- 
 tion, 201. 
 Plutonic rocks, characteristics of, 60; 
 
 defined, 60. 
 
 P or ft do rosso antico, 83. 
 Porphyrites, 83. 
 Porphyritic structure, 35. 
 Porphyroid, 167. 
 
 Post-Cretaceous decay of granite, 272. 
 Post-Glacial decay of diabase, 273. 
 Post-Jurassic weathering of grano- 
 
 diorites, 274. 
 Post-Pliocene weathering of andesites, 
 
 274. 
 Potash, in soils, replacing power of, 370; 
 
 soluble in soils, 376. 
 Potassium, as a constituent of the earth's 
 
 crust, 6; relative proportion of, in 
 
 rocks, 6. 
 
 Pot-holes, formation of, 196. 
 Potomac flats, 323. 
 Potomac formation, 313. 
 Potstone, 101. 
 Precious serpentine, 115. 
 Pre-Paloeozoic decay of rocks, 275. 
 Primary rocks, 51. 
 Primary constituents of rocks, 10. 
 Principles involved in rock-weathering, 
 
 173. 
 
 Propyllite, 85. 
 Protective action, of plants, 202 ; of soil, 
 
 271. 
 
 Protogine, 67. 
 Psammites, the, 131. 
 Pseudotuffs, 140. 
 Psilomelane, 107. 
 Puddingstone, 133. 
 Pulaskite, 79. 
 
 Pyrite, as a rock constituent, 29 ; decom- 
 position of, 29. 
 Pyroclastic rocks, 140. 
 Pyrolusite, 107. 
 Pyrophyllite, 116. 
 Pyrophyllite schist, 168. 
 
408 
 
 INDEX 
 
 Pyroxenes, alteration and decomposition 
 of, 22 ; as rock constituents, 21. 
 
 Pyroxenite-augitite group, 99. 
 
 Pyroxenites, described, 99; weathering 
 of, 225. 
 
 Quarrying by aid of fire, in India, 182. 
 
 Quarry water, 199, 254. 
 
 Quartz, 110; as a rock constituent, 12; 
 
 the most refractory mineral, 234. 
 Quartz basalt, 92. 
 Quartz-free porphyries, 75. 
 Quartz porphyry described, 69. 
 Quartz veins, influence of contours, 260. 
 Quartzite, origin of, 158 ; feldspathic, 
 
 disintegration of, 251; polished by 
 
 wind-blown sand, 257. 
 Quartzites, weathering of, in the District 
 
 of Columbia, 251. 
 
 Quaternary deposits, weathering of, 258. 
 Quitman Mountains, exfoliation of rocks, 
 
 182. 
 
 Rainfall, amount reaching the soil, 281. 
 
 Rain waters, temperatures of, 193. 
 
 Rapilli, 140. 
 
 Rate of weathering influenced by texture, 
 268 ; by composition, 269 ; by humidity, 
 270; by climate, 278; by position, 270. 
 
 Reaction rims, 240. 
 
 Regional metamorphism, 155. 
 
 Regolith, classification of, 300 ; origin of 
 name, 299. 
 
 Regur defined, 382. 
 
 Relationship between plutonic and effu- 
 sive rocks, 63. 
 
 Relative amount of material lost through 
 weathering, 284. 
 
 Relative durability of minerals, 234. 
 
 Relative rapidity of weathering among 
 eruptive and sedimentary rocks, 271. 
 
 Rensselaerite, 116. 
 
 Residual clays, 302; in caves, 233. 
 
 Residuary deposits, 301 ; analysis of, 306 ; 
 names proposed for, 301. 
 
 Results, incidental to weathering, 266; 
 of weathering due to position, 252. 
 
 Retinite, 70, 129. 
 
 Retinolite, 116. 
 
 Rhodochrosite, 114. 
 
 Rhombporphyry, 76. 
 
 Rhyolite, 72 ; weathering of, 255. 
 
 Ribbons in slates, 155. 
 
 River channels formed by rock-weather- 
 ing, 243. 
 
 River erosion, 196. 
 
 Rivers, flood plains of, 289. 
 
 Rock, definition of, 1 ; disintegration of 
 on Bering Island, 199. 
 
 Rock-forming minerals, classification, 10 ; 
 list of, 11. 
 
 Rocking stones, 252. 
 
 Rock temperatures, in Africa, 183; at 
 Edinburgh, Scotland, 184. 
 
 Rock - weathering, 206 ; a superficial 
 phenomenon, 193; complexity of pro- 
 cess, 240; early references to, 175; on 
 Lone Mountain, Montana, 243. 
 
 Rocks, absorptive power of, 198 ; chemi- 
 cal composition of, 44; classification 
 of, 57 ; color of, 45 ; composed mainly 
 of inorganic material, 131; composed 
 of debris from plants and animals, 141 ; 
 expansion and contraction under natu- 
 ral temperatures, 181 ; formed through 
 chemical agencies, 105 ; formed as sedi- 
 mentary deposits, 129; kinds of, 56; 
 mode of occurrence, 49; physical and 
 chemical properties of, 33 ; specific 
 gravity of, 43. 
 
 Roofing slate, microstructure of, 170. 
 
 Root action, how manifested, 202. 
 
 Roots, depth of penetration, in caves 
 and soils, 202. 
 
 Rosso de Levante, 98. 
 
 Rottenstone, origin of, 267. 
 
 Salt, common, 119 ; disintegrating effects 
 of, 198. 
 
 Sand, seolian, 346; Sorby's classification 
 of, 342 ; of dunes, sources of, 296. 
 
 Sand blast carving, 186 ; natural, 185. 
 
 Sand dunes, 346 ; formation of, 295 ; rate 
 of movement, 296. 
 
 Sand grains, lasting power of, 197. 
 
 Sandpipes, formation of, 260. 
 
 Sandstone, cause of disintegration, 247; 
 cementing matter of, 132; induration 
 of, 256; siliceous, weathering of, 228; 
 spheroidal, weathering in, 247; un- 
 equal weathering of, 248. 
 
 Sandstone concretions, formation of, 37. 
 
 Sandstones, weathering of, 249. 
 
 Sanidin, kaolinization of, 238. 
 
 Sanidin-oligoclase trachyte, 77. 
 
 Saprolite, 301. 
 
 Satin spar, 117. 
 
 Saxonite, 97. 
 
 Scheerite, 129. 
 
 Schistose structure, 34. 
 
 Schists, the, 168; crystalline, weathering 
 of, in Brazil, 251; of Cape Elizabeth, 
 weathering of, 248 ; origin of, 156. 
 
 Seacoast swamps, 336. 
 
 Secondary constituents of rocks, 10. 
 
 Secondary minerals, influence of, 249. 
 
 Sedentary materials, classification of, 
 300. 
 
INDEX 
 
 409 
 
 Sedimentary rocks, origin of, 52. 
 
 Seleuite, 117. 
 
 Seneca oil, 123. 
 
 Septarian nodules, 36, 114. 
 
 Sericite, 23. 
 
 Serpentine, composition, 30; after peri- 
 dotite, 97 ; origin of, 115, 159; origin of 
 name, 31 ; Harford County, Maryland, 
 weathering of, 226. 
 
 Shale, 137. 
 
 Sheet defined, 50. 
 
 Shell limestone, 143. 
 
 Shell marl, 146. 
 
 Shell sand, 143. 
 
 Shore ice, transportation by, 292. 
 
 Siderite, 114. 
 
 Silica, loss of, how accounted for, 237; 
 lost during decomposition, 234; possi- 
 bility of combination with iron during 
 rock decomposition, 239; solubility of, 
 238. 
 
 Silicates, 114 ; most refractory, 235. 
 
 Siliceous sinter, 109. 
 
 Silicified wood, 110. 
 
 Silicon as a constituent of the earth's 
 crust, 5. 
 
 Sill defined, 50. 
 
 Simplification of compounds incidental 
 to weathering, 265. 
 
 Singing sands, 143. 
 
 Sink-holes, formation of, 259. 
 
 Slaggy structure, 34. 
 
 Slates, 137. 
 
 Slaty cleavage, origin of, 155. 
 
 Slickensides defined, 54. 
 
 Snow, effect in promoting decomposition, 
 280. 
 
 Snowfall, influence compared with rain- 
 fall, 280. 
 
 Soapstone, Amherst County, Virginia, 
 weathering of, 226; Fairfax County, 
 Virginia, weathering of, 227 ; origin of, 
 101. 
 
 Sodium as a constituent of the earth's 
 crust, 7. 
 
 Sodium chloride, influence in decompos- 
 ing feldspars, 178. 
 
 Sodium salts in soils, 371. 
 
 Soil, chemical nature of, 358 ; capacity 
 for water, 379; definition, 3: mineral 
 nature of, 373 ; nitrates in, 322 ; nitro- 
 gen in, 372; soluble matter of, 365; 
 water content of, 281. 
 
 Soil cap, creeping of, 287. 
 
 Soil particles, movements of, 287. 
 
 Soil temperatures at Orono, Maine, 184. 
 
 Soils, age of, 386 ; affected by plant and 
 animal life, 389; affected by winds, 
 2%; as affected by man, 396; classifi- 
 
 cation, 381; color of, 384; destructive 
 process of formation, 360; essential 
 constituents of, 362; fertility of, 361; 
 fertility dependent on physical condi- 
 tion, 379; how affected by climates, 
 367 ; how affected by leaching, 368 ; in- 
 herited characteristics, 303, 360, 387: 
 mineral composition of, 373; of arid 
 regions, character of, 368 ; of arid re- 
 gions, composition of, 369; of humid 
 regions, composition of, 379; of Nile 
 valley, cause of fertility of, 325 ; phys- 
 ical condition of, 378 ; resemblance to 
 parent rock, 360; soluble salts in, 369; 
 the, 357 ; weight of, 381. 
 
 Soluble matter in fresh and decomposed 
 rocks, 377. 
 
 Soluble salts in soils, 369. 
 
 Solution, 189 ; rate increased by commi- 
 nution, 192; relative amount of mate- 
 rial removed in, 258. 
 
 Sounding sand, 143. 
 
 South Dakota, rock-weathering in, 279. 
 
 Specific gravity of rocks, 43. 
 
 Specular iron ore, 28. 
 
 Sphserosiderite, 114. 
 
 Sphagnous mosses, rate of growth, 317. 
 
 Spheroidal structure, 247. 
 
 Spheroidal weathering of sandstone, 247. 
 
 Spherulitic structure, 35. 
 
 Spilite, 90. 
 
 Stalactite, 113. 
 
 Stalagmite, 113. 
 
 Stamford dike, pre-Pala3ozoic decay of, 
 275. 
 
 Steatite, 116. 
 
 Stone implements, weathered, 273. 
 
 Stone Mountain, Georgia, weathering of, 
 245. 
 
 Stratification defined, 53. 
 
 Stratified rocks, weathering of, 248. 
 
 Stratified structure, 34. 
 
 Structure, as affecting weathering, 249 ; 
 of rocks, 33. 
 
 Sub-soil defined, 383. 
 
 Succinite described, 128. 
 
 Sulphates, 117. 
 
 Sulphuric acid formed during rock- 
 weathering, 205. 
 
 Swamp deposits, section of, 317. 
 
 Swamp soils, 315. 
 
 Swam ps.causeof, 316 Classification of, 317. 
 
 Syenite, Little Rock, Arkansas, weather- 
 ing of, 214. 
 
 Syenite-trachyte group, 73. 
 
 Syenites described, 73. 
 
 Table Mountain structure, how produced, 
 252. 
 
410 
 
 INDEX 
 
 Tachylite, 92. 
 
 Talus, defined, 54; slopes, 319. 
 
 Temperatures, effect on soils, 367. 
 
 Tephrite and basauite described, 94. 
 
 Terminal moraines, 355. 
 
 Termites, effects on soils, 391. 
 
 Termites, or white ants, as promoters of 
 
 decomposition, 204. 
 Terra rossa, 302. 
 Teschenite, 89. 
 Theralite-basanite group, 93. 
 Thin sections, preparation of, 42. 
 Till, 138. 
 
 Time considerations, 268. 
 Time limit of decay, 272. 
 Titanic iron as a rock constituent, 28. 
 loadstone, 70. 
 Tonalite, 82. 
 Trachytes described, 76. 
 Transportation and deposition of debris, 
 
 286. 
 Transported materials, classification of, 
 
 318. 
 
 Trap rocks, 89. 
 Trass, 140. 
 Travertine, 113. 
 Trees, effect on soils, 394. 
 Triassic conglomerate, weathering of, 
 
 264. 
 
 Trichites defined, 41. 
 Triclinic feldspars, 15. 
 Tri polite, 142. 
 Trowlesworthite, 68. 
 Tufa, 112. 
 Tuffoids, 140. 
 Tuffs, 139. 
 
 Uintaite described, 127. 
 Ulmic acid, 189. 
 Unakite, 68. 
 
 Valley drift, 356. 
 
 Valleys, formed by decomposition of 
 
 greenstone dikes, 244. 
 Valleys of solution, 253. 
 Variolite, 89, 90. 
 Vegetable matter, decomposing action 
 
 of, 203. 
 
 Veins defined, 54. 
 Verd antique, 116. 
 Verde di Genora, 98, 205. 
 Verde di Pegli, 98. 
 Verde di Prato, 205. 
 Vesicular structure, 34. 
 Viridite, 30. 
 
 Vitreous or glassy structure, 33. 
 Vitrophyr, 70. 
 Vogesite, 74. 
 Volcanic ashes, 140. 
 
 Volcanic dust, 140, 298, 349. 
 
 Volcanic group of fragmeutal rocks, 
 
 139. 
 
 Volcanic mud, 140. 
 Volcanic neck defined, 51. 
 Volcanic necks, origin of, 261. 
 
 Wacke, 139, 311. 
 
 Wad, 107. 
 
 Water, action of, in dry soil, 379 ; amount 
 absorbed by rocks, 198; apparent pro- 
 tective action of, 253 ; chemical action 
 of, 186; contents of soil, 281; effects 
 of freezing, 199; expansive force of 
 freezing, 198; in cavities of quartz, 
 199 ; mechanical action of, 195 ; solvent 
 power augmented, 186 ; solvent power 
 tested, 191. 
 
 Water and ice, influence in transporting 
 rock debris, 287. 
 
 Wave erosion, rapidity of, 198. 
 
 Waves, erosive action of, 198. 
 
 Weathering, character of, indicative of 
 climate, 284; defined, 174; difference 
 in kind in cold and warm climates, 283 ; 
 effacement of characteristics of, 262; 
 incidental results, 266; influenced by 
 crystalline structure, 243; influenced 
 by mineral composition, 248 ; influ- 
 enced by position, 270; influenced by 
 structure of rock masses, 244 ; irregu- 
 lar, due -to lack of homogeneity, 251; 
 of andesites, 274; of argillite, Har- 
 ford County, Maryland, 229 ; of basalt, 
 Bohemia, 223; of basalt, France, 223; 
 of calcareous rocks containing silicate 
 minerals, 249; of chert, 230; of clastic 
 rocks, 228; of crystalline schists, 251; 
 of diabase, Medford, Massachusetts, 
 218; of diabase, Venezuela, 222; of 
 diabase, Stamford, Connecticut, 275; 
 of diorite, Albemarle County, Virginia, 
 224; of dolomitic limestones, 250; of 
 eruptive and sedimentary rocks, rela- 
 tive rapidity of, 271 ; of feldspathic 
 quartzite, 251 ; of fine-grained homo- 
 geneous rocks, 250; of gneiss, Albe- 
 marle County, Virginia, 213 ; of granite 
 of the District of Columbia, 206; of 
 granite, Lake Huron, 275; of granite, 
 Pike's Peak, 243; of grano-diorites, 
 274; of limestone, 232, 250; of lime- 
 stones, process one of solution, 231 ; of 
 peridotite, 225; of phonolite, 217; of 
 pyroxenite, 225; of quartzite boulders 
 on deserts, 256 ; of quartzite in the Dis- 
 trict of Columbia, 251; of rhyolite, 
 255; of soapstone, Albemarle County, 
 Virginia, 226; of soapstone, Fairfax 
 
INDEX 
 
 411 
 
 County, Virginia, 227 ; of syenite, Lit- 
 tle Rock, Arkansas, 214; rate of, 2(58; 
 rate of, influenced by climate, 278 ; rela- 
 tive amount of material lost through, 
 284 ; surface contours due to, 257 ; ulti- 
 mate product of, 388; unequal, of 
 bedded rocks, 253. 
 
 Weathered stone implements, 273. 
 
 Websterite, 100. 
 
 Wehrlite, 97. 
 
 Weight of soils, 381. 
 
 Whirlwinds, effects of, 346. 
 
 White ants, effects on soils, 391. 
 
 Williamsite, 115. 
 
 Wind action, 153, 184, 292. 
 
 Wind action on Cape Cod, 297. 
 
 Wind action on Wyoming soils, 296. 
 Wind-blown sand polish, 257. 
 Wisconsin, rock-weathering in, 278. 
 Wurtzilite described, 126. 
 
 Zeolites, as conservators of potash, 374; 
 as rock constituents, 31 ; at Plombieres, 
 375 ; composition of, 32 ; formed in deep- 
 sea bottoms, 375; in soils, 374; origin 
 of, 31 ; products of hydro-metamor- 
 phism, 375. 
 
 Zeolitic matter in soils, 370. 
 
 Zircon syenite, 79. 
 
 Zonal structure, 37. 
 
 Zonal structure incident to weathering, 
 258. 
 
ELEMENTARY 
 PHYSICAL GEOGRAPHY, 
 
 By RALPH S. TARR, B.S., F.G.S.A., 
 
 Assistant Professor of Dynamic Geology and Physical Geography at 
 
 Cornell University : 
 Author of "Economic Geology of the United States." 
 
 8vo. Cloth. 488 pp. Price $1.40, net. 
 
 Comments. 
 
 " I regard Professor Tarr's book as one of the first publications in this 
 country to embody the new principles and advanced methods in the 
 study of physical geography. ... It seems to me eminently adapted 
 as regards its style and the nature of the illustrations for the grade of 
 students for whom it is intended ; to wit, those of high schools. Most 
 of the book is, indeed, written in a style so simple and plain that par- 
 ticularly the part of the work relating to physical geography might well 
 find a place in the upper class of many of the grammar schools." J. B. 
 WOODWORTH, Instructor in Geology, Harvard University, Cambridge, 
 Mass. 
 
 " I have recommended the study of Professor Tarr's admirable book 
 to be required of students entering the Engineering Department of the 
 University of Michigan." Professor ISRAEL C. RUSSELL, Department 
 of Science, University of Michigan, Ann Arbor, Mich. 
 
 " It is beyond question the most thoroughly scientific elementary 
 text-book on this important subject which has yet appeared." Boston 
 Daily Advertiser. 
 
 " The subject is treated with scientific breadth, accuracy, and fulness, 
 and is presented in an exceedingly attractive manner. The style is 
 clear, forcible, and instructive. In fact, the entire arrangement of divi- 
 sions and subdivisions of the subject, with abundant illustrations, most 
 aptly and beautifully executed, explanatory of, and giving increased 
 interest to, the text, altogether makes the work a valuable contribution 
 to science and well adapted to the use of schools and colleges. "- 
 F. B. WATSON, Superintendent of Schools, Chatham, Va. 
 
 THE MACMILLAN COMPANY, 
 
 66 FIFTH AVENUE, NEW YORK. 
 
PHYSICAL GEOGRAPHY. 
 
 Comments. 
 
 " I have received Professor Tarr's Physical Geography, and have read 
 it with very great pleasure. It gives an excellent and accurate presen- 
 tation of the important facts relative to the surface of the earth, and the 
 forces acting upon it." DAVID S. JORDAN, President Stanford Univer- 
 sity, Cat. 
 
 "After a careful reading, I do not hesitate to pronounce it a most 
 excellent book. Professor Tarr has given us a book that has long been 
 needed in the preparatory schools, not of merely one phase of the sub- 
 ject, but covering, and well too, the entire subject of physical geog- 
 raphy." JAMES PERRIN SMITH, Associate Professor of Geology, Stanford 
 University, Cal. 
 
 " I have reviewed the book very carefully, and it is excellent. The 
 chapter on storms is especially worthy of commendation. I have no 
 hesitation in recommending it as in every way well adapted for use in 
 the class-room. The mechanical execution of the book is beautiful. 
 The list of reference books at the end of each chapter makes it espe- 
 cially valuable to teachers and students." EDWARD H. MCLACHLIN, 
 Superintendent of Schools, South Hadley, Mass. 
 
 " I like the book very much. It is fresh and modern in style, and 
 presents the subject in an attractive manner. I shall recommend its 
 use here next year." EDWARD M. SHEPARD, Department of Biology 
 and Geology, Drury College, Springfield, Mo. 
 
 " I have found it exceedingly valuable and helpful. In clear, orderly 
 treatment, in the selection, character, and number of illustrations, in the 
 prominence given to the physical features as illustrated in our own coun- 
 try, in the references to the bibliography of the various subjects, it is 
 certainly very much the best book accessible to the American teacher." 
 CHARLES B. SCOTT, State Normal School, Oswego, N. Y. 
 
 " Its simplicity of statement, very full treatment of all points worthy 
 of consideration, and lavish use of most excellent illustrations, call forth 
 my hearty approval and admiration." CHARLES F. KING, Master Dear- 
 born School, Boston Highlands, Mass. 
 
 THE MACMILLAN COMPANY, 
 
 66 FIFTH AVENUE, NEW YORK. 
 
ECONOMIC GEOLOGY 
 
 OF THE 
 
 UNITED STATES, 
 
 With Briefer Mention of Foreign Mineral Products. 
 By RALPH S. TARR, B.S., F.G.S.A., 
 
 Assistant Professor of Geology at Cornell University. 
 
 Second Edition, Revised. $3.50. 
 
 Comments. 
 
 " I am more than pleased with your new ' Economic Geology of the United States.' 
 An introduction to this subject, fully abreast of its recent progress, and especially 
 adapted to American students and readers, has been a desideratum. The book is 
 admirably suited for class use, and I shall adopt it as the text-book for instruction in 
 Economic Geology in Colorado College. It is essentially accurate, while written in a 
 pleasant and popular style, and is one of the few books on practical geology that the 
 general public is sure to pronounce readable. The large share of attention given to 
 non-metallic resources is an especially valuable feature." FRANCIS W. CRAGIN, 
 Professor of Geology, Mineralogy, and Paleontology at Colorado College. 
 
 " I have examined Professor R. S. Tarr's ' Economic Geology ' with much pleasure. 
 It fills a felt want. It will be found not only very helpful to students and teachers by 
 furnishing the fundamental facts of the science, but it places within easy reach of the 
 business man, the capitalist, and the statesman, fresh, reliable, and complete statistics 
 of our national resources. The numerous tables bringing out in an analytic way the 
 comparative resources and productiveness of our country and of different states, are 
 a specially convenient and admirable feature. The work is an interesting demonstra- 
 tion of the great public importance of the science of geology." JAMES E. TODD, 
 State Geologist, South Dakota. 
 
 " It is one of those books that are valuable for what it omits, and for the concise 
 method of presenting its data. The American engineer has now the ability to acquire 
 the latest knowledge of the theories, locations, and statistics of the leading American 
 ore bodies at a glance. Were my course one of text-books, I should certainly use it, 
 and I have already called the attention of my students to its value as a book of refer- 
 ence." EDWARD H.WILLIAMS, Professor of Mining, Engineering, and Geology at 
 Lctiigh University. 
 
 " I have taken time for a careful examination of the work, and it gives me pleas- 
 ure to say that it is very satisfactory. Regarded simply as a general treatise on 
 Economic Geology, it is a distinct advance on anything that we had before; while 
 in its relations to the Economic deposits of this country, it is almost a new creation, 
 and certainly supplies a want long and keenly felt by both teachers and general 
 students. Its appearance was most timely in my case, and my class in Economic 
 Geology are already using it as a text-book." WILLIAM O. CROSBY, Assistant 
 Professor of Structural and Economic Geology at the Massachusetts Institute of 
 Technology, 
 
 THE MACMILLAN COMPANY, 
 
 66 FIFTH AVENUE, NEW YORK. 
 
By SIR ARCHIBALD GEIKIE. 
 
 A TEXT-BOOK OF GEOLOGY. 
 
 Third Edition. $7.50. 
 
 " Mr. Geikie is not a mere man of learning, but a very successful original thinker; he is 
 also endowed with excellent judgment in the fields in which his special work has not lain ; 
 and he possesses in a high degree the literary faculty. This happy union of qualifications 
 has enabled him to write a text-book which marks an era in British science. . . . This 
 text-book, while stating conclusions as definitely as the condition of the science warrants, 
 points out the uncertainty of many of them, and is full of suggestion as to what is yet to be 
 learned, and how it may be approached. In short, it is a candid and suggestive book as 
 well as a sound one." The Nation. 
 
 OUTLINES OF FIELD GEOLOGY. 
 
 16mo. $1.00. 
 
 " The work is already favorably known in this country, especially as a hand-book for 
 teachers, but in revising it the author has so far extended its scope and changed its order 
 as to make it practically a new book, far more perfectly adapted to its purpose than the 
 original pamphlet was." New York Evening Post. 
 
 GEOLOGICAL SKETCHES, 
 
 AT HOME AND ABROAD. 
 With Illustrations. 12mo. $1.50. 
 
 " Most of these sketches have found their way into different periodicals, but the lover 
 of geology will be glad to see them in this form. Dr. Geikie adds to his great acquire- 
 *ments in this branch of science a remarkable faculty of expressing his ideas in clear and 
 attractive language. The book does not pretend to give an exhaustive geological treat- 
 ment of any country, but describes prominent features of various lands. . . . After read- 
 ing the book one has a keener interest in the earth, and the study of its structure and the 
 changes that have taken place." The School Journal. 
 
 CLASS-BOOK OF GEOLOGY. 
 
 Third Edition. Illustrated. 12mo. $1.10. 
 
 " We have no hesitation in declaring the book an excellent one, containing exactly . ^^a 
 material as renders it especially fitted for instruction. More than that, to the per? jn with 
 no geological turn of mind the whole matter is so well combined, and the explanations so 
 simple, that by reading the volume, nature's actions in the past, as in the present, can be 
 better understood." New York Times. 
 
 "Professor Geikie is recognized in England as one of the leading authouties in this 
 science, and his interesting work is unusually well adapted to create and fostr.- a taste for 
 this subject. While witten in a remarkably plain and simple style, it contains an immense 
 amount of solid and useful information, and is profusely illustrated with engravings of the 
 various fossils and geological formations." Popular Science News. 
 
 ELEMENTARY LESSONS IN PHYSICAL 
 GEOGRAPHY. 
 
 Illustrated. 18mo. $1.10. 
 
 " The language is always simple and clear, and the descriptions of the various ph >nom- 
 ena are no less vivid than interesting ; the lessons are never dull, never wearisome and 
 they can scarcely fail to make the study of Physical Geography popular wherever they jre 
 used." Academy. 
 
 Questions on the Same for Use in Schools. 18mo. 40 cents. 
 
 THE MAGMILLAN COMPANY, - - 66 Fifth Avenue, New York. 
 

 
RETURN EARTH SCIENCES LIBRARY 
 
 230 McCone Hall 642-2997 
 
 LOAN PERIOD 1 
 1 MONTH 
 
 2 
 
 3 
 
 4 
 
 5 
 
 6 
 
 ALL BOOKS MAY BE RECALLED AFTER 7 DAYS 
 
 Books needed for class reserve are subject to immediate recal 
 
 DUE AS STAMPED BELOW 
 
 FORM NO. DD8 
 
 UNIVERSITY OF CALIFORNIA, BERKELEY 
 BERKELEY, CA 94720 
 
U.C. BERKELEY LIBRARIES 
 
 Sfcraqe 
 
 \*J