FROM-THE-LIBRARY- OF 
 WILLIAM -A H1LLEBRAND 
 
 FNir:iNFFRtN6 LIBRARY 
 
BOCKS 
 
 AND 
 
 EOOK MINEEALS 
 
 MANUAL OF THE ELEMENTS OF PETROLOGY 
 WITHOUT THE USE OF THE MICROSCOPE 
 
 FOR THE GEOLOGIST, ENGINEER, MINER, ARCHITECT, 
 
 ETC., AND FOR INSTRUCTION IN COLLEGES 
 
 AND SCHOOLS 
 
 BY 
 
 LOUIS V. PIRSSON 
 
 LATE PROFESSOR OF PHYSICAL GEOLOGY IN THE SHEFFIELD 
 SCIENTIFIC SCHOOL OF YALE UNIVERSITY 
 
 TOTAL ISSUE TWELVE THOUSAND 
 
 NEW YORK 
 
 JOHN WILEY & SONS, INC. 
 LONDON: CHAPMAN & HALL, LIMITED 
 
V\ ' 
 
 COPYRIGHT, 1906, 
 
 BY 
 
 LOUIS V. PIRSSON 
 
 Entered at Stationers" Ha 
 
 ENGINEERING LIBRARY 
 
 Stanhope ipress 
 
 H. GILSON COMPANY 1~ 25 
 
 BOSTON. U.S.A 
 
PREFACE. 
 
 DURING the last fifteen years it has been one of the 
 writer's duties to teach the elements of Petrology to 
 students in various branches of Engineering, Mining, 
 Chemistry, Forestry, etc. The amount of time which 
 these students, in their undergraduate course, can devote 
 to the subject is limited and precludes any attempt to 
 give them such instruction in optical-microscopical 
 methods of research as would be worth while. The 
 subject has to be treated from the purely megascopic 
 standpoint, as indeed the vast majority of those who 
 have to deal with rocks in a practical or technical way 
 are also obliged to consider them. 
 
 In giving this instruction the author has long felt the 
 need of a small, concise and practical treatise in which 
 the rocks and rock-minerals are handled entirely from 
 this megascopic standpoint. In such works as exist 
 either the subject matter has not been brought down to 
 date to express our present knowledge of rocks, or it is 
 treated largely from the microscopical or chemical stand- 
 point, or the classifications used are based on microscopical 
 research and are thus not available for ordinary use, or 
 the rocks are treated incidentally with respect to some 
 other main purpose as in works on soils, ore deposits, etc. 
 The present work is an attempt to fill this need which the 
 writer believes is also felt in many other institutions in 
 which similar courses in Petrology are given. In addition 
 to this purpose its scope has also been somewhat enlarged 
 to meet the wants of many who have to consider rocks 
 from the scientific or practical point of view and who are 
 not in a position to use the microscopical method. It is 
 hoped that it may thus prove of service to field geologists, 
 
 iii 
 
 995702 
 
iv PREFACE 
 
 engineers, chemists, architects, miners, etc., as a handy 
 work of reference. Much of the theoretical side of 
 Petrology which has been developed during the last few 
 years, especially in the line of petrogenesis, does not 
 demand a knowledge of microscopical petrography for 
 its understanding, and the endeavor has been made to 
 present the elements of this in a simple manner. Although 
 the author has incorporated considerable original material 
 it goes without saying that a work of this character must 
 of necessity be mainly one of compilation. It would be 
 nearly impossible and in any case out of place in an 
 elementary treatise to give by reference the thousand 
 and one sources from which the material has been taken. 
 It should be mentioned, however, that in the description 
 of the minerals the writer has drawn freely upon the 
 mineralogies of Dana, Iddings and Rosenbusch and the 
 determinative mineralogy of Brush-Penfield. In the same 
 way in the treatment of the rocks the petrographies 
 of Rosenbusch and Zirkel and the geological text-book 
 of Geikie have been freely used. 
 
 Most of the illustrations have been prepared for this 
 work, but the wealth of material in the published reports, 
 bulletins, etc., of the United States Geological Survey 
 has also been freely used. 
 
 SHEFFIELD SCIENTIFIC SCHOOL OF YALE UNIVERSITY. 
 New Haven, Conn., Jan., 1908. 
 
TABLE OF CONTENTS. 
 
 PART I. 
 
 Introductory and General Considerations. 
 
 Chapter Page 
 
 I. SCOPE OF PETROLOGY; HISTORICAL; METHODS OF STUDY 1 
 II. CHEMICAL CHARACTER OF THE EARTH'S CRUST AND ITS 
 
 COMPONENT MINERALS . 14 
 
 PART II. 
 
 Rock Minerals. 
 
 III. IMPORTANT PROPERTIES OF MINERALS 21 
 
 IV. DESCRIPTION OF THE ROCK-MAKING MINERALS ... 33 
 V. DETERMINATION OF THE ROCK-MAKING MINERALS . . 114 
 
 PART III. 
 
 The Rocks. 
 
 VI. GENERAL PETROLOGY OF IGNEOUS ROCKS 132 
 
 VII. DESCRIPTION OF IGNEOUS ROCKS 205 
 
 VIII. ORIGIN AND CLASSIFICATION OF STRATIFIED ROCKS . 275 
 
 IX. DESCRIPTION OF STRATIFIED ROCKS 293 
 
 X. ORIGIN, GENERAL CHARACTERS AND CLASSIFICATION OF 
 
 THE METAMORPHIC ROCKS 333 
 
 XI. DESCRIPTION OF METAMORPHIC ROCKS 351 
 
 XII. THE DETERMINATION OF ROCKS 398 
 
 INDEX . 409 
 
ROCKS AND ROCK MINERALS 
 
PART I. 
 
 INTRODUCTORY AND GENERAL. 
 
 CHAPTER I. 
 
 SCOPE OF PETROLOGY; HISTORICAL; METHODS 
 OF STUDY. 
 
 EVERYWHERE beneath the mantle of soil and vegetation 
 that covers the surface of the land lies rock, the solid 
 platform upon which the superficial debris of earth rests. 
 Here and there in mountain tops, in cliffs and ledges, we 
 see this underlying rock projecting from the soil and 
 exposed: we know that it must underlie the sea in the 
 same way. The outer shell of the earth then is made of 
 rock, which forms the foundation upon which rest all the 
 surface things with which we are acquainted. How thick 
 this zone of rock is we do not know, but upon it we live 
 and exert our activities; into it we penetrate for coal, oil, 
 gas, metals and other things upon which the material 
 features of our modern civilization depend. It is there- 
 fore of the highest importance to us, and the information 
 which we have acquired concerning it, by examination 
 and study, forms a valuable branch of human knowledge. 
 
 Petrology the Science of Rocks. Our knowledge of 
 all the various things which together make up that part 
 of the earth which it is permitted us to examine and study 
 and which has been comprehended under the heading of 
 Geology has now increased to such a degree that this 
 science has split up into a number of well-defined, sub- 
 ordinate branches or geological sciences. Thus Meteor- 
 ology is the science of the atmosphere, the summation of 
 
 1 
 
2 ROCKS AND ROCK MINERALS 
 
 our knowledge of the causes and movements of winds, 
 storms, rain, the distribution of heat and cold, and in 
 general the study of the various factors that affect the 
 air and its movements and of the laws that govern them. 
 Physiography takes account of the surface features of 
 the earth, of the distribution of land and water and of the 
 agencies which are modifying them, the effects of climates 
 and the various 'causes which together produce the topo- 
 graphy which the earth's surface now exhibits. Paleon- 
 tology is ihe science resulting from the study of the remains 
 of past life upon the earth, as shown by the fossils inclosed 
 in the rocks, and teaches not only the different forms 
 which have existed but also seeks to discover the trans- 
 formation of one form into another and the various move- 
 ments or migrations of life upon the earth in past ages. 
 
 Petrology, in the same way, has now become a separate 
 branch, one of the geologic sciences. It comprises our 
 knowledge of the rocks forming the crust of the earth, the 
 results of our studies of the various component materials 
 which form them, of the different factors and the laws 
 governing them which have led to their formation, and 
 of their behavior under the action of the agencies to which 
 they have been subjected, and endeavors to classify the 
 kinds into orderly arrangement. 
 
 The terms Petrology and Petrography are not abso- 
 lute synonyms though often so used in a general way. 
 The former has been denned above; the latter more 
 particularly refers to the description of rocks and especially 
 with respect to their study by means of the microscope 
 as explained later thus microscopical petrography. 
 Petrology is used for the science in its broader aspects as 
 well and covers the geological and chemical relations of 
 rocks: thus strictly denned petrography may be said to 
 be a branch of petrology. The synonym Lithology has 
 become nearly obsolete. Petrology means the science of 
 rocks; lithology, the science of stones, and the word stone 
 is now used in a popular way for architectural and com- 
 
METHODS OF STUDY 3 
 
 mercial purposes or to designate any loose piece of rock 
 of unknown origin. 
 
 Definition of a Rock. By the term " rock," geologically 
 speaking, is meant the material composing one of the 
 individual parts of the earth's solid crust, which, if not 
 exposed, everywhere underlies the superficial covering 
 of soil, vegetation or water which lies upon it. The 
 popular understanding of this term, that it denotes a 
 hard or firm substance, is not, geologically, a necessary 
 one, for a soft bed of clay or of volcanic ash is as truly a 
 rock as a mass of the hardest granite. Moreover it 
 implies within limits, which will be explained elsewhere, 
 a certain constancy of chemical and mineral composition 
 of the mass recognized as forming a particular kind of 
 rock. Thus the chance filling of a mineral vein by variable 
 amounts of quartz, calcite and ores is not accepted by 
 petrographers as forming a definite kind of rock. The 
 term is also used with different meanings; it may be 
 denotive of the substance forming parts of the earth's 
 crust, as quartz and feldspar arranged in a particular 
 manner are said to form a rock granite or it may 
 refer to the masses themselves and thus possess a larger, 
 geological significance. In a general way the former 
 may be said to be a petrographic, the latter a geologic 
 usage. When used in this broader geologic sense the 
 mass recognized as an individual kind of rock must 
 possess definite boundaries and show by its relations to 
 other rock masses that it owes its existence to a definite 
 geological process. The absolute size of the mass is not 
 involved in this, for a seam or dike of granite cutting rocks 
 of other kinds may be as thin as cardboard or a mile in 
 thickness. 
 
 Composition of Rocks. Rocks are sometimes defined 
 as aggregates of one or more minerals, but this is not a 
 broad enough or wholly correct definition. Rocks may 
 be composed entirely of minerals or entirely of glass or 
 of a mixture of both. Minerals are substances having 
 
4 ROCKS AND ROCK MINERALS 
 
 definite chemical compositions and usually of crystalline 
 structure; glasses are molten masses chilled and solidified 
 without definite composition and structure. Rocks 
 composed wholly of minerals may be simple or compound, 
 that is, the rock may be formed of one kind of mineral 
 alone, as for example, some of the purest marbles which 
 consist of calcite only or of a mixture of two or more like 
 ordinary granite which is made of grains of quartz, feld- 
 spar and mica. These subjects are treated more fully in 
 later chapters. 
 
 History of Petrology. The science of geology may be 
 said to have commenced when rocks as objects of inves- 
 tigation began to be studied. As the individual minerals 
 composing rocks, or contained in their cavities, were 
 investigated by chemical means and by the goniometer, 
 the science of Mineralogy and its related subject, Crys- 
 tallography, began. At first the difference between 
 rocks and minerals was not very clearly perceived; very 
 dense rocks composed of mineral grains so fine that they 
 could not be distinguished by the eye or magnifying lens 
 were thought to be homogeneous substances, and similar 
 in their nature to minerals. This continued in many 
 cases even down to the middle of the last century. 
 
 As the knowledge of the composition and properties of 
 minerals grew it was seen in the case of the coarser 
 grained rocks that they were composed of aggregates of 
 these mineral grains, and according to the kinds of the 
 component minerals many common rocks had already 
 received names early in the last century. As investiga- 
 tion proceeded and geological science grew many new 
 combinations were discovered and the list of named 
 rocks increased, and it may be remarked here that these 
 early geologists, armed only with a simple lens, were 
 exceedingly keen observers and made many surprisingly 
 correct observations on the mineral composition of quite 
 fine grained rocks. Various schemes of classification were 
 proposed, some of them containing admirable features, 
 
METHODS OF STUDY 5 
 
 but the dense varieties defied the means of investigation 
 then at command, and in great part their composition, 
 properties and relations to other rocks remained unknown. 
 About the middle of the last century Sorby, an English 
 geologist, showed that, by a suitable method of operation, 
 very thin slices of rocks could be prepared, and by 
 the study of such thin sections under the microscope the 
 kinds of component mineral grains could be made out, 
 their properties and relations to one another, the order in 
 which they had been formed, the processes to which they 
 had been subjected, and many other interesting and 
 important features discovered, and that it was possible 
 to do this even in the case of the densest and most com- 
 pact rocks. This method of investigation was imme- 
 diately taken up, especially in Germany by Zirkel and 
 others, and with its advent a new era in the study of rocks 
 and the science of Petrography may be said to have 
 begun. A flood of knowledge regarding rocks and 
 especially of the minerals composing them began to rise 
 and has kept on increasing to the present day. The 
 study of the properties of transparent minerals under the 
 action of polarized light received a great impulse, and 
 the facts discovered have in turn been of immeasurable 
 service in the investigation of rocks by this method. 
 Thus at first attention was directed chiefly to the mineral- 
 ogical side of petrology; the kinds of minerals of which 
 rocks are composed and their properties were considered 
 of first importance, and this is reflected in the schemes of 
 classification devised at this period. Later, the chemical 
 composition of rocks, both in mass and as shown in their 
 component minerals, their origin and the relations of the 
 different varieties to each other began to attract more 
 attention and have been regarded as of increasing impor- 
 tance down to the present day. This increasing impor- 
 tance of these aspects of the subject is also seen in the 
 weight placed upon them in the more recent schemes of 
 classification proposed for the igneous rocks, those formed 
 
6 ROCKS AND ROCK MINERALS 
 
 by the solidification of the molten masses coming from 
 the earth's interior. 
 
 Classification of Rocks. According to their mode of 
 origin and the position of the masses with respect to the 
 earth's crust and to each other, rocks naturally divide 
 themselves into three main groups, divisions which are 
 recognized by practically all geologists. These are the 
 igneous rocks made by the solidification of molten material; 
 the sedimentary or bedded rocks formed by the precipita- 
 tion of sediments in water, to which may be added the 
 small group of seolian or wind formed deposits, and the 
 metamorphic rocks, those produced by the secondary 
 action of certain processes upon either igneous or sedi- 
 mentary ones by which their original characters are 
 wholly or partly obscured and replaced by new ones and 
 which are therefore most conveniently considered in a 
 separate group. This grouping will be used in this work, 
 and each group with its further subdivisions, their char- 
 acters, relations, etc., will be treated by itself. 
 
 Summarizing then what has just been stated, we have: 
 
 I. Igneous Rocks, solidified molten masses. 
 II. Sedimentary Rocks, precipitated sediments. 
 III. Metamorphic Rocks, secondary formed from 
 I and II. 
 
 Field and Petrographic Classifications. The sedimentary 
 rocks are classified in two ways, in one they are sub- 
 divided according to the kinds and fineness of the mineral 
 particles which compose them, in the other according to 
 the geological age, as shown by their position and fossils, 
 in which the sediments were laid down. The first is the 
 petrographic, the second the geological, or more strictly 
 the historical classification, and in this work these rocks 
 are treated only according to the former method. In 
 classifying them they have, so far, been simply divided 
 according to the properties mentioned above, and as they 
 have not yet been the subject of the detailed research 
 
METHODS OF STUDY 7 
 
 which their importance demands, the simple classifica- 
 tion adopted by geologists in the field has been followed 
 by the petrographers. 
 
 With respect to the igneous rocks and to a lesser degree 
 the metamorphic ones the case is different. The use of 
 the microscope in the study of thin sections has shown 
 that rocks which may appear absolutely identical, either 
 in the field or as one simply compares hand-specimens, 
 may be composed of entirely different minerals, or their 
 chemical analysis may prove them to be fundamentally 
 different in chemical composition. They may thus be 
 quite different rocks deserving separate names and places 
 in any classification in which all of the essential char- 
 acters of rocks are considered, and yet outwardly to the 
 eye there may be no hint of this. There have arisen two 
 useful terms, megascopic (from the Greek /xcyas great) 
 and microscopic, the first descriptive of those characters 
 of rocks which can be perceived by the eye alone or aided 
 by a simple pocket-lens, and the second referring to those 
 which require the use of the microscope on thin sections. 
 It is obvious that a classification which is based upon 
 microscopic characters as much as upon megascopic ones 
 cannot be used in determining rocks in the field. It may 
 be more correct and scientific, but in the nature of things 
 it cannot be of general application and use. This subject 
 will be treated of more fully in the section devoted to the 
 igneous rocks, and it is sufficient to say here that the 
 object of this work is to supply a field classification 
 based upon the megascopic characters of rocks to be 
 determined by the eye or pocket-lens, aided by a few 
 simple means for the determination of minerals. In 
 addition many important facts regarding rocks and 
 especially igneous ones have been discovered in these 
 later years which are not dependent upon their classifica- 
 tion or microscopic study, and it is intended to give some 
 account of these in a simple general way. 
 
 Microscopical Petrography. Although this volume is 
 
8 
 
 ROCKS AND ROCK MINERALS 
 
 not based upon the microscopical method of research it 
 will be of interest to indicate briefly how this is conducted 
 and the sort of results obtained by it. To prepare the 
 thin sections or slices of rock for study, a chip of the 
 material as thin as can be obtained is taken. It should 
 be for ordinary purposes about an inch in diameter and 
 of firm unaltered material. It is first ground flat on a 
 metal plate with coarse emery powder and water and then 
 very smooth on another plate with very fine powder. It 
 is then cemented by the aid of heat to a piece of glass 
 with Canada balsam and the other side ground down with 
 the coarse emery until it is as thin as cardboard, or as 
 far as it is possible to carry the operation safely with the 
 coarse powder. It is then in a similar way ground down 
 with the finer powder and finally finished on a glass plate 
 with the finest flour of emery until, in the case of dark, 
 dense rocks, it becomes so thin that ordinary print may 
 be read through it. It is then transferred, after melting 
 the cementing balsam, to a microscopic object glass slide, 
 enveloped in balsam, a thin cover glass placed over it, and 
 it is then ready for use. The professionals who make a 
 business of preparing such sections save much time in the 
 coarser work by the use of sawing disks with diamond 
 dust embedded in them or by using car- 
 borundum powder on disks or endless 
 revolving wires. They become very expert 
 in the final grinding and prepare sections 
 whose thickness is quite uniformly about 
 TTfW f an inch, experience having shown 
 that this is best for general work. The 
 appearance of one of these finished sections 
 is shown in Figure 1. By this process the 
 minute mineral grains composing the 
 "** sec T t?oS r ck densest and blackest of basaltic lavas 
 become transparent and may be deter- 
 mined under the microscope. 
 The microscope used in petrographic work differs from 
 
PLATE 1. 
 
METHODS OF STUDY 9 
 
 that ordinarily employed in being furnished with a 
 variety of apparatus arranged for studying the mineral 
 sections in polarized light. Underneath the table or 
 stage which carries the section is a nicol prism of calcite 
 which polarizes the light coming upward from the reflect- 
 ing mirror below before it passes through the section, 
 that is, the vibrations of the ether which produce the 
 phenomenon of light instead of occurring in all directions, 
 as in common light, are reduced to a definite direction in 
 one plane. 
 
 Another nicol prism called the analyzer is fitted in the 
 tube above the object lens so that the effects produced by 
 the mineral particles on the polarized light, as it passes 
 through them, can be tested and studied. The nicols 
 can be also removed so that the effects of ordinary light can 
 be seen. Other arrangements are provided for strongly 
 converging the light as it passes through the minerals 
 and for testing the results produced in a variety of ways. 
 
 Subjected to such processes the transparent minerals 
 of the rocks exhibit a great variety of phenomena by 
 means of which the different species may be definitely 
 determined. Crystals or fragments of crystals of an 
 almost incredible degree of minuteness may be studied 
 with high powers, their properties examined and the par- 
 ticular variety of mineral to which they belong made out. 
 In order to use this means of studying rocks a good 
 knowledge of general Mineralogy, of Crystallography, of 
 Optics, and in particular of the optical properties of the 
 rock-making minerals is essential. Owing to the cost of 
 the apparatus, the technical knowledge required in its 
 use and the difficulty of making thin sections, it is obvious 
 that this method of studying rocks can never become a 
 popular or general one, but many of the results which 
 have been attained by it are easily understandable and 
 may be mentioned. 
 
 Results of Microscopic Research. By the method 
 described above the kind of mineral or mineral grains 
 
10 ROCKS AND ROCK MINERALS 
 
 making up the most compact and dense rocks may be 
 determined; whether the rock is of sedimentary or igneous 
 origin can be told and, if the latter, the general order in 
 which the mineral varieties have crystallized from the 
 molten fluid. It can be seen whether the crystal particles 
 are clear and homogeneous or if they contain inclusions 
 of various kinds, facts which often throw light on their 
 origin and history; whether they are fresh and unchanged 
 or have been decayed by the action of the elements and 
 altered wholly or partly into other substances; whether 
 they have been subjected to the enormous pressures of 
 mountain building in the crust and are strained and 
 crushed or not. It is possible to tell at once if a rock 
 contains more or less glass associated with the mineral 
 grains, and if it does, to thus learn with certainty its 
 igneous origin and the fact that in all probability it is a 
 surface lava, glass, in the nature of things, being almost 
 entirely confined to such rocks. 
 
 Furthermore, if the grains are not too microscopically 
 fine it may be possible, not only to determine the kinds of 
 minerals they are, but to measure their areas or diameters 
 in a given section, ascertain from this the relative pro- 
 portions of the different kinds of grains present, and then, 
 when the chemical composition of the component minerals 
 is known, compute the chemical composition of the rock 
 mass as a whole, a factor, especially in the case of igneous 
 rocks, often of great importance in scientific classification 
 and in other ways. 
 
 These are some of the more important features of 
 rocks which may be discovered by their microscopic 
 study, and they are sufficient to illustrate the value of the 
 method in aiding geological research.* 
 
 * The following works in which rock-making minerals and rocks 
 are treated and classified according to the results of microscopical 
 research in a more or less extensive and detailed way may be men- 
 tioned: Rock Minerals, by J. P. Iddings (Wiley and Sons, New York). 
 Quantitative Classification of Igneous Rocks, by Messrs.Cross, Iddings, 
 
METHODS OF STUDY 11 
 
 Megascopic Study of Rocks. Although the microscope 
 is necessary for the complete investigation of rocks many 
 of their important features may be observed without its 
 use. In the case of the coarser grained ones, those 
 where the size of the grains is one-sixteenth of an inch in 
 diameter or more, the component minerals can usually 
 be identified by the aid of the lens or by simple means. 
 Even when much finer grained than this, some minerals 
 may be distinguished by certain characters they possess 
 as explained in the chapter on the rock-making minerals. 
 Even when they are so dense that the component grains 
 can no longer be discriminated from each other, the color, 
 the hardness, the style of fracture under the hammer, the 
 specific gravity and the behavior of fragments or of the 
 powdered rock under the action of acids, are all impor- 
 tant characters which serve to distinguish different kinds 
 of rocks. 
 
 Implements and Apparatus. The first requisite is a 
 suitable hammer for obtaining material. It should be a 
 square-faced geological hammer of the 
 type shown in the adjoining figure. It is 
 convenient to have one end wedge shaped. 
 The steel should be tempered as hard as 
 possible without making it too brittle, 
 otherwise the edges wear off very rapidly. 
 If made to order it is a great convenience 
 to have the hole as large as possible, 
 consistent with strength, and tapered some- 
 what; the handle may then be somewhat 
 larger at the hammer end and thrust Fig. 2 . Geological 
 through the hole until brought up in the 
 taper by the enlarged end. This device, which is the 
 familiar one used in securing the handles of picks, is a great 
 convenience as it prevents the head coming off when the 
 
 Pirsson and Washington (Chicago University Press). Elemente der 
 Gesteinslehre, by H. Rosenbusch (Stuttgart). Petrology for Students, 
 by A. Harker (Cambridge University Press). Igneous Rocks, by 
 J. P. Iddings (Wiley & Sons, New York). 
 
12 ROCKS AND ROCK MINERALS 
 
 handle shrinks. The hammer should be of good weight, 
 about two and one-half pounds for the head, to enable 
 good-sized pieces of rock to be readily broken up and 
 fresh material within to be secured. Of course anything 
 in the way of a hammer or sledge may be used on occa- 
 sion, but this implement will give the best service for 
 general use. 
 
 If, in addition to procuring material for examination, 
 it is desired to trim and shape it into specimens for the 
 collection a small trimming hammer will be found 
 convenient. It should be double-headed, 
 square-faced, and of very hard steel, and 
 the head may weigh about six ounces. Hand 
 specimens for collections are usually about 
 4X3X1 inches in size and are made by 
 selecting a suitable large flake or spall obtain- 
 ed by the large hammer and knocking small 
 chips from it along the edges first on one side 
 Fig.3. Trimming and then on the other until trimmed to the 
 required shape and size. A well-made 
 specimen should show hammer marks only on the edges 
 and never on the faces. 
 
 A pocket-lens is also essential; one of the apochromatic 
 triplets now made by several makers of optical instru- 
 ments is best, but much cheaper ones will serve the 
 purpose. One with a focal distance of -one inch is most 
 convenient for general use. 
 
 In addition to the above, which are for use in the field, 
 a small amount of the apparatus used in the laboratory 
 for the determination of minerals will often prove of 
 great service. This would include a blowpipe and plati- 
 num tipped forceps for testing fusibility, pieces of quartz, 
 calcite and ordinary window glass for testing hardness, a" 
 simple apparatus for determining specific gravity, a 
 magnet, a few test tubes, dilute acids and a Bunsen gas 
 burner or alcohol lamp for testing solubility, and a glass 
 funnel, filter paper and a few reagents, such as solutions 
 
METHODS OF STUDY 13 
 
 of ammonia, silver nitrate and barium chloride, for 
 making tests by chemical reactions. A small agate or 
 steel mortar is needed for grinding a fragment of the rock 
 or mineral to powder for making chemical tests. This 
 list might be increased almost indefinitely into the full 
 equipment of a mineralogical laboratory, but most 
 chemical laboratories contain all the apparatus and 
 reagents necessary for the determination of minerals and 
 rocks mentioned in this book and, where such a laboratory 
 is not available, the material which has been named above 
 will cover nearly all necessary demands and may be used 
 almost anywhere. 
 
CHAPTER II. 
 
 CHEMICAL CHARACTER OF THE EARTH'S CRUST AND 
 ITS COMPONENT MINERALS. 
 
 Composition of the Earth's Interior. The origin and 
 history of the rocks composing the solid crust of the earth 
 are of necessity bound up with the history and origin of 
 the globe itself. Beyond that history, however, which is 
 revealed in the sedimentary rocks, our ideas on this sub- 
 ject, as regards the earth, must, with our present knowl- 
 edge, be largely of the nature of pure speculation. Below 
 a relatively very shallow depth the same is true with 
 respect to the character and condition of its interior. We 
 do not know what it is like, and it is of course possible that 
 we never shall. The view which is most generally held 
 is that the earth was once a molten mass, the outer shell 
 of which solidified through cooling to, a solid crust, while 
 the interior, though excessively hot, also solidified through 
 the enormous pressures exerted upon it by the overlying 
 portions; between the two is either a zone of liquid because 
 the pressure is not there sufficient to solidify it, or of 
 heated material which will become liquid if for any cause 
 the pressure in the crust above is diminished; this zone is 
 regarded as the seat of volcanic and other important 
 geological activities. 
 
 While this view has been long and is still widely held 
 and has been of great service in explaining many geological 
 phenomena, certain objections to it have been advanced, 
 and recently Chamberlain has propounded another. 
 
 According to this the earth is regarded as never having 
 been liquid but always a solid which has been gradually 
 built up by the infall and accretion of relatively small 
 solid bodies termed planetesimals. Through the enor- 
 
 14 
 
 . 
 
CHARACTER OF THE EARTH'S CRUST 15 
 
 mous pressures exerted under the influence of gravity, 
 contraction has ensued and gaseous matters have been 
 expelled, giving rise to the atmosphere and water on the 
 surface. This contraction is held to be the source of the 
 interior heat, and to the issuance of the heated gases is 
 attributed the origin of volcanic activity. 
 
 Still another view has been advanced in recent years by 
 Arrhenius according to which the interior is neither in a 
 solid or liquid but in a gaseous condition. It is assumed 
 that all substances if sufficiently heated must be in the 
 state of a gas; experiment teaches us that if any gas is 
 heated to or above a certain degree called its " critical 
 temperature " it cannot be reduced to a liquid or solid by 
 pressure alone, and it is held that this will be true even 
 though the pressure be enormous enough to contract the 
 gases to a density far beyond that which the substances 
 would have if in the solid condition. It is assumed that 
 the temperatures reigning in the earth's interior are so 
 great that all substances must be in a gaseous condition 
 and above their critical temperatures, but that the 
 pressures are also so enormous that they are reduced to a 
 state of density far greater than that of solids at the 
 surface, and that on account of this condensation their 
 internal viscosity or resistance to flowage is so great that 
 they possess also a greater rigidity, one sufficient to meet 
 the demand which astronomical investigations have shown 
 that the earth as a whole must possess. 
 
 Following this view then there is, first, a solid outer 
 crust, then a zone of molten liquid or of solid material so 
 greatly heated that it is capable of becoming liquid if the 
 pressure above is in any way lightened, and then finally 
 the' great interior mass consisting of heated gases in a 
 condition of enormous condensation. 
 
 The three hypotheses presented above will serve to 
 show how widely divergent are the views in regard to this 
 subject among scientific men at the present time and how 
 purely speculative our ideas must be. 
 
16 ROCKS AND ROCK MINERALS 
 
 Facts which are known. On the other hand it must 
 not be assumed that nothing is known of the earth beyond 
 that which we can see at the surface. We know, for 
 instance, that there is a considerable increase in heat as 
 we go downward in the crust. We know also that there 
 are bodies of molten material, which, though they may 
 be relatively small as compared with the size of the earth, 
 are yet absolutely large, and we see the upward prolonga- 
 tion to the surface of these masses in active volcanoes. 
 We know that such bodies not only exist in the earth's 
 interior now but have also in past geological ages, as 
 shown by the way in which they have been forced upward 
 into its crust or poured out upon its surface. We know 
 that upon the land surfaces wherever the deepest seated 
 rocks, which underlie all the stratified and metamorphic 
 ones which have accumulated upon them, are exposed by 
 erosion they present the general characters of igneous 
 rocks, and thus lead us to infer that they were at one time 
 in a state of fusion. As the sedimentary and metamorphic 
 rocks are secondary or derived from previously existent 
 ones, this leads to the natural assumption that they came 
 from material originally similar to these deep-seated ones 
 and that their substance had at some previous stage 
 passed through a state of fusion. 
 
 Rock material then having been wholly or at least very 
 largely in a molten condition, it is evidently a matter of 
 importance that we should know something of the nature 
 and properties of the molten fluids which have formed 
 it. We can do this to some extent in active volcanoes 
 where we see some of the properties of these fluids exhib- 
 ited, but those which are most important in rock forma- 
 tion we can best learn by study of the igneous rocks which 
 are the result of the direct solidification of these molten 
 masses, and this subject is, therefore, treated in the 
 chapter upon them. There are, however, certain aspects 
 of it which can well be considered here, and one of these 
 is the general chemical composition of the earth's crust. 
 
CHARACTER OF THE EARTH'S CRUST 17 
 
 Chemical Composition of the Earth's Crust. During 
 
 recent years several thousand chemical analyses have 
 been made of rock specimens from visible parts of the 
 earth's crust. The great majority of them are from 
 Europe and the United States, but enough have been 
 made from other parts of the world to show, in con- 
 junction with the microscopical studies of other specimens, 
 that the essential facts which these analyses teach are 
 almost beyond question of general application. One of the 
 most important general truths learned by these investiga- 
 tions may be thus broadly stated the general chemical 
 composition of the earth's crust is everywhere similar. 
 The statement thus broadly made demands explanation. 
 It does not mean that one portion of the rock crust is 
 composed of exactly the same chemical elements in 
 exactly the same proportions as any other portion. It 
 means that it is composed of the same elements and that, 
 although these may vary greatly in proportions from 
 place to place or from one kind of rock mass to another, 
 if we take large areas involving many kinds of rocks the 
 average of such areas will be very nearly alike. Thus 
 the composition of the average rock computed from all 
 the analyses made of specimens from the United States 
 is essentially the same as the average computed from the 
 analyses of the rocks of Europe. The average rock of 
 New England is essentially that of the Rocky Mountains 
 region. On the other hand a large part of Quebec Province 
 is composed of one kind of rock which extends with 
 monotonous sameness over a vast area; the composition 
 of this has not the same proportions as the average rock, 
 and if we were considering this particular part of the 
 continent we should have to increase greatly our area to 
 obtain an average. Some parts of the continental areas 
 are covered with limestone which is essentially carbonate 
 of lime alone, but is a relatively thin, concentrated coating 
 of a special substance we should have to balance it with 
 large masses of other rocks. 
 
18 
 
 ROCKS AND ROCK MINERALS 
 
 The average rock has been computed from the analyses 
 by Clarke and by Washington and the results are shown 
 in the table below in Column A. 
 
 
 A 
 
 93 Per Cent 
 Lithosphere. 
 
 B 
 
 7 Per Cent 
 The Ocean. 
 
 C 
 
 Average with 
 Atmosphere. 
 
 Oxygen 
 
 47 07 
 
 85 79 
 
 49 77 
 
 Silicon 
 
 28 06 
 
 
 26 08 
 
 Aluminum 
 
 7 90 
 
 
 7 34 
 
 Iron 
 
 4 43 
 
 
 4 11 
 
 Calcium 
 
 3 44 
 
 05 
 
 3 19 
 
 Magnesium 
 
 2.40 
 
 14 
 
 2 24 
 
 Sodium 
 
 2.43 
 
 1 14 
 
 2 33 
 
 Potassium 
 
 2.45 
 
 04 
 
 2 28 
 
 Hydrogen 
 
 0.22 
 
 10 67 
 
 95 
 
 Titanium 
 
 40 
 
 
 39 
 
 Carbon 
 
 20 
 
 002 
 
 18 
 
 Chlorine . . 
 
 07 
 
 2 07 
 
 21 
 
 Phosphorus . . . 
 
 11 
 
 
 10 
 
 Sulphur 
 
 11 
 
 09 
 
 10 
 
 All others 
 
 71 
 
 008 
 
 73 
 
 
 
 
 
 
 100.00 
 
 100.000 
 
 100.00 
 
 Clarke has also calculated that if we assume that the 
 crust has this composition to a depth of ten miles and 
 add in the water of the oceans, the atmosphere and an 
 amount of sedimentary rocks in proper proportions, the 
 general average of the whole will be that shown in Column 
 C, of the above table. These assumptions are reasonable, 
 and correspond with the facts so far as known. Even if 
 these results are not very accurate they must be approxi- 
 mately so and they are of value in showing the relative 
 proportions of the elements in the outer part of the earth. 
 From them important deductions can be drawn. 
 
 The Elements of Geological Importance. From the 
 table just given we see that the first eight elements are 
 present in quantity, and are, therefore, of geologic impor- 
 tance. Oxygen forms about one half of the outer part of 
 
CHARACTER OF THE EARTH'S CRUST 19 
 
 the earth, and the quantity in the atmosphere and in the 
 ocean is small, compared with that locked up in the under- 
 lying rock. Silicon comes next and forms about one 
 fourth and after it are aluminium and five other metals, of 
 which iron is the most important, the others being calcium, 
 magnesium, sodium and potassium, in the order of their 
 importance. After these comes a small group of four 
 elements, which, although of secondary rank in quantity, 
 demand mention: they are, hydrogen, titanium, carbon 
 and chlorine. Of these, titanium is a rather inert element 
 from the geological standpoint, plays no important part 
 in geological processes or results, and may, therefore, be 
 dismissed. The hydrogen and carbon, on the contrary, 
 are of great importance, they are of great activity in 
 geological processes, produce results of petrologic interest, 
 and must therefore be considered with the primary group 
 first mentioned. All the other elements, however impor- 
 tant in special cases, or for organic life or civilized activities, 
 are from the standpoint of general geology and petrology 
 of relatively little interest. 
 
 Combinations of Chemical Elements. Except oxygen, 
 carbon, and possibly to an unimportant extent iron, the 
 elements mentioned above do not occur alone, or native; 
 they are always combined in some form producing com- 
 pounds known as minerals. We may state this chemically 
 by saying that they are either in combination with oxygen 
 as oxides, or these oxides are in combination as salts. 
 Two, carbon and silicon, are negative elements their 
 oxides C0 2 and Si0 2 are anhydrides of acids; the others, 
 leaving hydrogen aside, are metals, or positive elements 
 whose oxides act as bases. The oxide of aluminium, 
 A1 2 O 3 , acts sometimes as a base and sometimes as a weak 
 acid, especially in combination with strong positive bases, 
 such as soda, Na 2 O, and potassa, K 2 O, and in combination 
 with silica, SiO 2 . Fe 2 O 3 acts as an acid in spinels. 
 
 From what has been said we have to deal with these 
 oxides which are of chief petrologic importance. 
 
20 ROCKS AND HOCK MINERALS 
 
 ACIDIC OXIDES. 
 
 SiO 2 , silica > in combination and free as a solid. 
 CO 2 , carbon dioxide, in combination and free as gas. 
 
 BASIC OXIDES. 
 
 A1 2 O 3 , alumina, in combination and free solid, sometimes acidic, 
 
 Fe 2 O 3 , ferric oxide, in combination and free solid. 
 
 FeO, ferrous oxide only in combination in solids- 
 
 MgO, magnesia, in combination and free, solid.* 
 
 CaO, lime only in combination in solids. 
 
 Na 2 O, soda only in combination in solids. 
 
 K 2 O, potassa only in combination in solids. 
 
 The above table is given in order of decreasing acidity 
 and increasing basicity from top to bottom. To this list 
 we should also add water, H 2 O, which occurs free in the 
 gaseous, solid and liquid states, and in combination. 
 
 Since we are considering rocks it is evident that of 
 these oxides and their combinations we need to regard 
 only those which form solids. They are then the com- 
 pounds of silicic and carbonic acid or silicates and car- 
 bonates and the oxides of silica, alumina and iron. 
 
 Ice, the solid form of water, may also be regarded as a rock, 
 but as such, needs no further consideration in this work. Com- 
 binations of the oxides of aluminium, iron and magnesium and 
 of the silicates with water also occur. Combinations with sulphur 
 as sulphides and sulphates and of phosphorus as phosphates and 
 of chlorine as chlorides are at times of local importance though 
 never having the general interest of those mentioned above. 
 They receive attention in their appropriate places. 
 
 * The solid MgO is the mineral periclase, excessively rare, and of no petrc 
 logic importance. 
 
PART II. 
 
 / 
 
 ROCK MINERALS. 
 
 CHAPTER III. 
 IMPORTANT PROPERTIES OF MINERALS. 
 
 SINCE all rocks, with the exception of a few glassy ones 
 of igneous origin, are composed of minerals it is of first 
 importance in their study and determination that a good 
 knowledge of the important rock-making minerals, of 
 their obvious characters and properties, should be had. 
 This is so indispensable, that before taking up the rocks 
 themselves, the following part of this work is devoted: 
 first, to a general account of those properties of minerals 
 which are of value in megascopic determination; second, 
 in a succeeding chapter, to a description of the minerals 
 individually; and third, to methods for their determination. 
 
 Minerals defined. A mineral is defined as any inorganic 
 substance occurring in nature which possesses a definite 
 chemical composition. To this, for petrographic purposes 
 we should add, that it is also a solid and usually it has a 
 definite crystalline structure. The word is also used in two 
 ways with different meanings: in one, which may be termed 
 the abstract chemical way, we refer to a compound having 
 a certain composition, as in speaking of calcite we mean 
 the compound CaCOs, carbonate of lime; in the other 
 when we speak of the minerals of a rock we refer to the 
 actual crystal grains, the minerals as distinct entities or 
 bodies which compose that rock. 
 
 Crystals defined. Most chemical compounds when 
 their molecules are free to arrange themselves in space and 
 the conditions are proper for them to assume solid form, 
 
 21 
 
22 ROCKS AND ROCK MINERALS 
 
 as for example when they solidify from solutions, appear 
 in crystals. That is, the molecules arrange themselves in 
 a definite geometric system, characteristic of that com- 
 pound, and governed by mathematical laws, which give 
 the solid a definite internal structure and an outward 
 form bounded by planes which are always placed at 
 certain angles to one another. Thus minerals crystallize 
 in cubes, octahedrons, prisms, etc. The conditions in 
 rock formation are sometimes such that a mineral can 
 assume outward crystal form, and it is then bounded by- 
 distinct planes; more commonly, however, the growing 
 crystals interfere with one another and have no distinct 
 form, or, as in the sedimentary rocks, they are fragments 
 only of former crystals, or their plane surfaces have been 
 worn off by attrition. Since, however, they possess the 
 inward characteristic structure we still call such bodies 
 crystals, though lacking the outward form. Thus, when 
 we speak of the crystals or crystal grains composing a 
 rock, w r e do not necessarily imply that these have plane 
 surfaces which give them geometric forms. Such grains 
 are sometimes called anhedrons (from the Greek, meaning, 
 without planes). 
 
 List of Properties. The chief properties of the rock- 
 making minerals by which they may be known are their 
 crystal form, color, cleavage and associations; these are 
 perceived by the eye, and in addition we have their hard- 
 ness, specific gravity, and their behavior before the blow- 
 pipe and with chemical reagents, properties which demand 
 some form of testing with apparatus. 
 
 Crystal form. The mineral grains which compose 
 rocks do not, as a rule, possess good outward crystal form 
 as mentioned in a preceding paragraph. The reason for 
 this is that in the igneous and metamorphic rocks, the 
 growing minerals interfere with one another's develop- 
 ment, and thus, while they may roughly approximate to 
 a certain general shape the mineral has endeavored to 
 assume, the outer surface is not composed of smooth 
 
IMPORTANT PROPERTIES OF MINERALS 23 
 
 definite planes; while in the sedimentary rocks the grains 
 are either broken fragments or rounded by rolling and 
 grinding. It may happen, however, that in a liquid 
 molten mass when crystallization begins, one or more 
 kinds of minerals may commence growing in crystals 
 scattered through it and complete their period of growth 
 before the others which will compose. the general mass of 
 the rock have commenced. In this case they have not 
 been interfered with, and they may exhibit good outward 
 crystal forms bounded by distinct planes. This is well 
 shown in those kinds of igneous rocks which are described 
 elsewhere as porphyries. Likewise in the metamorphic 
 rocks, certain minerals often appear in such well-bounded, 
 distinct crystals, as to indicate that they are of later origin, 
 and although formed by molecular rearrangement of 
 materials in a solid or somewhat plastic mass the con- 
 ditions were such that they were not interfered with 
 during their period of growth. This is illustrated by the 
 excellent crystals of garnet, often seen in the rock known 
 as mica schist. 
 
 Thus in general the outward crystal form or shape of 
 minerals in rocks is wanting and cannot be used as a 
 means for determining them, but in many special cases it 
 may be well developed in rock-making minerals and it 
 can then be very useful. The shapes in which each 
 mineral is most apt to occur are described under the 
 heading of that mineral in the descriptive part. 
 
 Color. The color of minerals, when used with proper 
 precautions, is also a very useful property for helping to 
 distinguish them. The color of minerals is dependent 
 upon their chemical composition, in which case it may be 
 said to be inherent, or it may be due to some foreign sub- 
 stance distributed through them and acting as a pigment, 
 and their color may then be termed exotic. It is because 
 the color of the mineral grains of rocks is frequently exotic 
 that precaution must be used in employing it as a means 
 of discrimination. 
 
24 ROCKS AND ROCK MINERALS 
 
 In regard to inherent colors, neither silica nor carbonic 
 acid in combination as silicates and carbonates has any 
 capability for producing color, and so far as they are con- 
 cerned, such compounds would be colorless, or, as will be 
 presently explained, white. So, silica alone, as quartz, is 
 naturally colorless. The same is also true of the metallic 
 oxides alumina, lime, magnesia, and the alkalies soda and 
 potash. Thus, carbonate of lime or calcite, carbonate of 
 lime and magnesia or dolomite, oxide of alumina or corun- 
 dum, silicates of lime and magnesia, silicates of alumina 
 and the alkalies or feldspars are all inherently colorless 
 minerals. The metallic oxides which chiefly influence 
 the color of rock minerals are those of iron, chromium 
 and manganese, and the only one of these which is of 
 wide petrographic importance is iron, especially iron as 
 ferric oxide. The minerals containing iron as a prominent 
 component are dark green, dark brown or black, and 
 these colors may ordinarily be regarded as indicative of 
 this metal. 
 
 With respect to the exotic colors which minerals fre- 
 quently exhibit, this may be due to one of two causes. 
 It may happen that a minute amount of some compound 
 of an intensely colorative character is present in chemical 
 combination. Thus a minute amount of manganese 
 oxide in quartz is supposed to produce the amethyst 
 color, traces of chromic oxide sometimes color silicates 
 green, and probably copper does also. Or the color may 
 be due to a vast number of minute bodies dispersed 
 through the crystal as inclusions. These minute specks 
 may have a distinct color of their own and thus act as a 
 pigment, as when, for example, quartz is colored dark red 
 by a reddish dust of ferric oxide particles in it; or the 
 inclusions may be so arranged in regular systems as to 
 act refractively upon light, breaking it up and producing 
 a play of prismatic colors, or opalescence in the substance. 
 Usually in the latter case, one color predominates and 
 gives its character to the mineral. A good example of 
 
IMPORTANT PROPERTIES OF MINERALS 25 
 
 this is seen in the variety of feldspar called labradorite, a 
 constituent of the rocks called anorthosite and gabbro, 
 which often shows a fine play of colors, a rich dark blue 
 being usually predominant. 
 
 The white color which so many rock-making minerals 
 exhibit may be due to minute inclusions, as when feld- 
 spars are sometimes, through alteration, filled with scales 
 of kaolin or white mica, but more commonly it is due to 
 the reflection of light from the surfaces of innumerable 
 microscopic cracks and crevices which everywhere per- 
 meate the mineral substance. In such cases the material 
 is really colorless and transparent. The effect is the same 
 as if a piece of colorless glass should be ground to powder, 
 which would of course be white. Hence white minerals 
 are not regarded as possessing any color, and they are 
 often free from such cracks and are then colorless and 
 transparent. Good examples of this are seen in such 
 common minerals as quartz, calcite and feldspars. As 
 explained under cleavage, these cracks in feldspar are 
 sometimes so regularly arranged as to produce a play of 
 colors, giving the mineral an opalescence or pearly luster 
 with a distinctly predominating color tone like that 
 mentioned above as produced by inclusions. 
 
 Streak. In addition to that color which minerals show 
 in the solid form, there is another way in which this 
 property may be often usefully employed in determining 
 them. This is the color which the substance presents 
 when reduced to a state of powder. The powder may be 
 obtained by grinding a small fragment in a mortar, but 
 it is more easily produced by scratching a sharp point 
 of the mineral across a plate of unglazed porcelain; the 
 color of the resultant streak is of course that of the 
 powdered mineral. While any piece of unglazed porce- 
 lain will answer fairly well, ,mall plates are specially 
 made for this purpose and sold by dealers in chemical 
 apparatus. 
 
26 ROCKS AND ROCK MINERALS 
 
 The color shown by minerals in the powdered state is usually 
 much lighter than that which they exhibit in the mass and sometimes 
 very different. It is most useful in helping to discriminate the 
 dark colored minerals, especially the metallic oxides and sulphides 
 of the heavy metals used as ores, and hence its application with the 
 light colored silicates and carbonates that chiefly form the rock- 
 making minerals is much more limited and of lesser value. In the 
 case of these minerals it is sometimes useful in distinguishing exotic 
 from natural colors; for the color of the streak is generally that of 
 the mineral substance itself, and the pigment or other impurities 
 which produce an exotic color must be present in very large amount 
 to exert a definite influence. Thus calcite is colorless or white, but 
 sometimes yellow, brown or red, but the streak of all these colors 
 is white or barely tinted except in unusual instances. The feld- 
 spars are normally white or colorless, but in some rocks, such as 
 anorthosite, they sometimes are black and at first glance might be 
 mistaken for an iron-bearing mineral; the streak, however, is white 
 and helps to show their true character. 
 
 In the field, the bruised surface of the rock, where struck by the 
 hammer, often shows the powdered minerals, giving in a rough 
 manner the color of the streak; or a bit of the substance may be 
 ground between two hammer surfaces and the powder rubbed on 
 white paper. 
 
 Cleavage. When mineral bodies possess crystalline 
 structure, it frequently happens that the arrangement of 
 the physical molecules composing them is such, that the 
 force of cohesion among them is less in some particular 
 direction or directions than in others. Along such 
 directions, if suitable means be employed, such as placing 
 the edge of a knife upon the mineral and striking with a 
 hammer, the body will tend to split or cleave. The degree 
 of perfection with which minerals possess this property 
 is very variable; some, like mica, which is used for stove 
 windows and lamp chimneys, are capable of being almost 
 indefinitely split into thin leaves ; others, like the feldspars, 
 have a good cleavage; while some, like quartz, have lio 
 apparent cleavage. When the cleavage is very good the 
 new surfaces are smooth and shining like the original ones 
 of a crystal and it is termed perfect. This property, 
 being then so distinctive, is a most useful one in helping 
 
IMPORTANT PROPERTIES OF MINERALS 27 
 
 to determine minerals, especially in rocks where the 
 mineral grains on the surface, broken by the hammer, if 
 they possess it, everywhere show shining cleavage faces. 
 It must not be imagined that the directions of cleavage 
 occur at random in a mineral; on the contrary, they always 
 bear a definite relation to the special crystal form that 
 characterizes a particular mineral. If the latter has two 
 directions in which it may be cleaved, like feldspar, for 
 example, the angle between the two surfaces is, for a 
 feldspar of a certain definite chemical composition, always 
 the same. Some minerals, like mica, have only one 
 direction in which there is good cleavage; others have two 
 directions, and sometimes the two are exactly alike and 
 sometimes unlike, one being better than the other; again 
 there may be three directions in which cleavage can be 
 produced, all alike as in calcite or unlike as in barite 
 (heavy spar, BaSO 4 ), or there may even be four or more. 
 Whether the cleavages are alike or unlike, when there is 
 more than one, depends not only on their direction in the 
 crystal, but also on the geometric form or system of 
 crystallization the latter exhibits. A description of these 
 relations would involve too much of the principles of 
 crystallography for discussion in this place, but the 
 following will be helpful in understanding certain terms 
 frequently used. 
 
 A. Good cleavage in one direction only: the mineral 
 grains in the rock in this case are apt to be developed in 
 tables, folia, or scales, whose surface is parallel to the 
 cleavage. This is well shown in such minerals as the 
 micas and chlorite. 
 
 B. Good cleavage in two directions and both alike: 
 the minerals are apt to be developed in elongated forms 
 parallel to the cleavage, and the latter is spoken of as being 
 prismatic. This is shown by the minerals hornblende 
 and pyroxene. If the two cleavages are not exactly 
 alike, the mineral still is often elongated in the direction 
 of the edge produced by the meeting of the two cleavage 
 
28 ROCKS AND ROCK MINERALS 
 
 planes. It may be sometimes columnar and sometimes 
 tabular parallel to the better cleavage. The feldspars, 
 which form the free developed crystals in porphyry, often 
 show such relations. 
 
 C. Good cleavage in three directions alike: if the three 
 planes are at right angles to each other the mineral will 
 break up into cubes and the cleavage is cubic or apparently 
 so; if they are at some other angle, rhombs will be produced 
 and the cleavage is called rhombohedral. Cubic cleavage 
 is well shown by galena, PbS, the common ore of lead and 
 by rock salt; it is not exhibited by any common rock- 
 making mineral. Rhombohedral cleavage is character- 
 istic of the common rock-making carbonates, calcite, 
 CaCOs, and dolomite (MgCa)CO 3 . Three unlike directions 
 have the same practical effect as two unlike, and four 
 directions are not of importance in megascopic petro- 
 graphy, as no common rock-making mineral exhibits 
 them. 
 
 If a rock with component mineral grains sufficiently coarse so 
 that they can be readily studied by the pocket-lens, the size of peas, 
 for example, be carefully examined, it will be found that almost 
 without exception, where a mineral shows a cleavage face, ft will be 
 full of minute cracks and fissures. These cracks are paraJ^l some- 
 times to one cleavage and sometimes to all the cleavage directions 
 the mineral has. In addition to the cleavage cracks there are also 
 irregular lines of fracture which do not correspond to any definite 
 direction. Commonly, the mineral grains of rocks contain not only 
 these large cleavage cracks and irregular fractures which can be 
 perceived with the eye or with the aid of the lens, but, in addition, 
 they are everywhere rifted by similar ones so minute that they can 
 only be detected in thin sections of rocks under high powers of the 
 microscope. It is the reflection of light from these minute micro- 
 scopic cracks that renders so many minerals opaque and white in 
 color that would otherwise be colorless and transparent. These 
 cracks and fissures have been produced in the rocks by the various 
 forces to which they have been subjected; sometimes they are due 
 to the contraction following a heated stage as in metamorphic and 
 igneous rocks, and sometimes and more generally to the intense 
 pressures and strains to which the rocks of the earth's crust are and 
 have been subjected. Minute as the rifts in the mineral grains are, 
 
IMPORTANT PROPERTIES OF MINERALS 
 
 29 
 
 they are of great importance in geologic processes, for by means of 
 them, and drawn by capillary action with great force, water con- 
 taining CO 2 in solution penetrates not only the rocks but the indi- 
 vidual grains as well, to their very interiors, and alters and changes 
 them into other minerals and the rocks into soil. 
 
 Fracture. The appearance of the breakage of minerals 
 in directions which are not those of cleavage or in cases 
 where the mineral does not 
 possess cleavage is called its 
 fracture. If the mineral is 
 fibrous in structure, the 
 fracture may be fibrous; or 
 it may be rough and un- 
 even or hackly; if the 
 mineral is dense, compact 
 and homogeneous it will be 
 conchoidal, that is, it will 
 present a sort of shelly 
 appearance such as is shown 
 on surfaces of broken glass 
 which recall the inside or 
 outside of a clam shell. 
 Rocks which are extremely dense and homogeneous, like 
 some flints, or glassy lavas or fine-grained compact ones, 
 have also a. conchoidal fracture more or less pronounced. 
 Quartz is the most common mineral which gives a good 
 example of conchoidal fracture. 
 
 Associations of Minerals. The facts that certain kinds 
 of minerals are apt to be found together in the same kind 
 of rock and that the presence of one mineral excludes 
 the presence of some other mineral are of great value in 
 petrography but of much greater use in microscopic work, 
 where the distinguishing characters of minerals are easily 
 made out, than in field determinations. Even in mega- 
 scopic petrography, however, these facts are at times of 
 practical use; thus the fact that the two minerals, quartz 
 and nephelite, cannot occur naturally together as rock- 
 
 Fig. 4. Conchoidal fracture in obsidian 
 volcanic glass. 
 
30 ROCKS AND ROCK MINERALS 
 
 making components is of value in discriminating between 
 certain rocks. The various relations of this kind that are 
 of importance will be mentioned in their appropriate places. 
 Hardness. This property is of great value in helping 
 to make determinations of minerals, and it is likewise very 
 useful in the field in making rough tests of rocks. The 
 hardness of minerals is determined by comparing them 
 with the following scale: 
 
 Scale of Hardness 
 
 1. Talc. 6. Feldspar. 
 
 2. Gypsum. 7. Quartz. 
 
 3. Calcite. . 8. Topaz. 
 
 4. Fluorite. 9. Corundum. 
 
 5. Apatite. 10. Diamond. 
 
 This means that each mineral, using a sharp point, will 
 scratch smooth surfaces of all the minerals in the list above 
 it but of none below it. If, for example, a fragment of an 
 unknown mineral is found to scratch calcite its hardness 
 is greater than 3; if it will not scratch fluorite, but, on the 
 contrary, is scratched by it, its hardness is not so great 
 as 4, but must be between 3 and 4 or approximately 3j. 
 
 The point of a pocket-knife blade as ordinarily tempered 
 with a hardness of a little over 5 and pieces of common 
 window glass with hardness of about 5 are very useful 
 for testing the hardness of common minerals and of the 
 rocks made up of them. A common brass pin point is a 
 little over 3 and will scratch calcite; the finger nail is a 
 little over 2 and will scratch gypsum. 
 
 Specific Gravity. The specific gravity of a substance 
 is its density compared with water or the number of times 
 heavier a given volume of the substance is than an equal 
 volume of water. It is obtained by weighing a piece of 
 the mineral or rock in air and then in water; the difference 
 
IMPORTANT PROPERTIES OF MINERALS 
 
 31 
 
 between the two is equal to the weight of an equal volume 
 of water (the volume displaced) and we have 
 
 wt. in air 
 
 wt. in air wt. in water 
 
 Sp. Gr. 
 
 The operation may be carried out with one of the special forms 
 of apparatus devised for determining specific gravity and described 
 in the manuals of determinative mineralogy, or it may be done with 
 a chemical, an assay or a jeweler's balance. It is first weighed in 
 the pan and then suspended from it by a hair and weighed in water. 
 
 SPECIFIC GRAVITIES OF ROCK MINERALS. 
 
 ARRANGED IN DESCENDING ORDER. 
 
 5.2 
 
 Magnetite. 
 
 2.86 
 
 Muscovite. 
 
 5.2 
 
 Hematite 
 
 2.85 
 
 Dolomite. 
 
 4.9-5.1 
 
 Pyrite. 
 
 2.80 
 
 Talc. 
 
 4.7-5.1 
 
 Ilmenite. 
 
 2.75 
 
 Anorthite. 
 
 3.95-4.1 
 
 Corundum. 
 
 2.73 
 
 Labrador! te. 
 
 3.6-4.0 
 
 Limonite. 
 
 2.72 
 
 Calcite. 
 
 3.5-4.2 
 
 Garnet. 
 
 2.65-2.75 
 
 Chlorite. 
 
 3.75 
 
 Staurolite. 
 
 2.66 
 
 Quartz. 
 
 3.5 
 
 Topaz. 
 
 2.62 
 
 Albite. 
 
 3.56-3.66 
 
 Cyanite. 
 
 2.6 
 
 Kaolin. 
 
 3.4 
 
 Vesuvianite. 
 
 2.5-2.65 
 
 Serpentine. 
 
 3.2-3.5 
 
 Pyroxene. 
 
 2.57 
 
 Orthoclase. 
 
 3.27-3.37 
 
 Olivine. 
 
 2.55-2.65 
 
 Nephelite. 
 
 3.25-3.45 
 
 Epidote. 
 
 2.45-2.50 
 
 Leucite. 
 
 3.0-3.4 
 3.16-3.2 
 
 Amphibole. 
 Andalusite. 
 
 2.32 
 2.27 
 
 Gypeum. 
 Analcite. 
 
 3.1-3.2 
 
 Chondrodite. 
 
 2.25 
 
 Natrolite. 
 
 3.15 
 
 Apatite. 
 
 2.15-2.30 
 
 Sodalite. 
 
 3.0-3.15 
 
 Tourmaline. 
 
 2.15-2.2 
 
 Heulandite. 
 
 2.95 
 2.8-3.1 
 
 Anhydrite. 
 Biotite. 
 
 2.1-2.2 
 2.1-2.2 
 
 Stilbite. 
 Opal. 
 
 A piece about one-half inch in diameter is convenient both for 
 minerals and rocks, but in the case of minerals it is frequently 
 necessary to select a fragment smaller than this to obtain pure 
 homogeneous material, without which it is perhaps needless to say 
 the determination is of little value. Adherent air bubbles and air 
 in cracks are best gotten rid of by boiling the fragment in water and 
 then allowing it to cool before weighing. If the mineral has an 
 
32 ROCKS AND ROCK MINERALS 
 
 invariable chemical composition and crystal form, as for example, 
 quartz (SiO 2 ), calcite (CaCO 3 ), etc., the specific gravity is an invari- 
 able quantity, and departures from it must be due to the presence 
 of impurities. Many minerals, however, while they retain the same 
 crystal form, vary considerably in chemical composition in that one 
 metallic oxide may be more or less replaced by another similar oxide 
 or oxides. Thus we find minerals which at one end of a series con- 
 tain magnesia, MgO, and at the other end ferrous oxide, FeO, and 
 between these extremes all degrees of mixtures of these two oxides. 
 In accordance with such variations the specific gravity of the mineral 
 varies. The pyroxenes, amphiboles, garnets, oli vines, etc., are 
 examples of this, and it accounts for most of the variations in specific 
 gravity which may be observed in the annexed table. 
 
 Blowpipe Reactions. The rock-making minerals, which 
 are chiefly carbonates and silicates, do not as a rule 
 exhibit before the blowpipe very characteristic reactions 
 by which they may be readily determined, as do so many 
 of the ores, the oxides and sulphides of the heavy metals. 
 Still, however, the relative degree of fusibility shown by 
 thin splinters, the coloration of the flame and the characters 
 of the melted bead which may result are properties which 
 may be of great service in helping to determine these 
 minerals, and so far as they have value in this direction 
 they are mentioned in the description of the minerals. 
 If instruction in the use of the blowpipe is desired it should 
 be sought in one of the manuals devoted to that purpose. 
 
 Chemical Reactions with Reagents. Certain qualitative 
 chemical tests which can generally be made with a few 
 reagents and simple apparatus are of great service in 
 mineral determination and in aiding to classify rocks. 
 In Chapter V, in which the methods for the identification 
 of minerals are given, these tests and the proper ways of 
 making them are fully described. 
 
CHAPTER IV. 
 
 THE ROCK-MAKING MINERALS. 
 SEC. 1. Primary Anhydrous Silicates and Oxides. 
 
 THESE minerals from the geological standpoint are the 
 most important in forming rocks. They are the most 
 abundant and the most widely diffused. They are the 
 chief minerals which are formed by the cooling and 
 crystallization of the molten fluids of the earth's interior, 
 and hence they are the main components of the igneous 
 rocks. The greater part of the metamorphic rocks are 
 also made up of them, and in the sedimentary beds they 
 are also important constituents in many cases. 
 
 It is difficult to draw a sharp line between the absolutely 
 anhydrous minerals and those containing considerable 
 quantities of combined water. Thus, most hornblendes, 
 micas and epidotes contain small amounts of hydroxyl 
 and yet are ordinarily considered as anhydrous, compared, 
 for instance, with kaolin, serpentine and chlorite. In 
 the same way feldspar, hornblende and pyroxene are 
 thought of as primary minerals although we know that in 
 some cases they are of secondary origin, that is, they have 
 been formed at the expense of previously existent minerals. 
 The grouping as given is largely a matter of convenience; 
 it includes those which are always anhydrous and always 
 primary and which thus give a certain distinctive charac- 
 ter to the group, which it is well to enforce, but it also 
 includes many which are at times secondary and some 
 which are hydrous, because on account of their mineralogic 
 positions and affinities it is more convenient and natural 
 to consider these minerals in this connection. 
 
 In the following section only such silicates and oxides 
 are treated as are both hydrous and secondary. 
 
 33 
 
34 ROCKS AND ROCK MINERALS 
 
 a. Silicates. 
 
 The silicates are salts of various silicic acids, in which 
 the hydrogen atoms have been replaced by various metals 
 or radicals composed of metals in combination with 
 oxygen, hydroxyl, fluorine, etc. The three important 
 silicic acids which in this group form rock minerals are 
 poly silicic acid, EUSisOg; metasilidc acid, H 2 SiO 3 , and 
 orthosilicic acid, EUSiO^ The list of those treated as of 
 importance on account of the functions which they have 
 as rock-making minerals includes the feldspar, felds- 
 pathoid, mica, pyroxene, amphibole, olivine, garnet, 
 tourmaline and epidote groups, and a few other less 
 common ones. 
 
 FELDSPARS. 
 
 The term feldspar is not the name of a single mineral 
 of a definite chemical composition like quartz, Si0 2 , but 
 is the designation of a group of minerals which have a 
 general similarity in chemical and physical properties. 
 They are indeed so much alike in general characters and 
 appearance that in determining rocks by megascopic 
 features they cannot be told apart except in special cases, 
 and it is, therefore, best to treat them as a group, and at 
 the same time mention those characters by which, when 
 possible, they may be distinguished. 
 
 The rock-making feldspars are composed of three kinds 
 and their mixtures as follows: 
 
 a. Orthoclase, KAlSisOg, silicate of potash and alumina; 
 
 b. Albite, NaAlSisOg, silicate of soda and alumina; 
 
 c. Anorihite, CaAl 2 Si 2 Og, silicate of lime and alumina; 
 Alkalic feldspar, (KNa)AlSi 3 O8, mixtures of a and 6; 
 Plagioclase feldspar, (NaAlSisOg)* + (CaAl 2 Si 2 O 8 ) y , 
 
 mixtures of b and c. 
 
 The simple feldspars are mostly confined to the crystals 
 found in veins, druses, etc.; they sometimes occur as the 
 
ROCK-MAKING MINERALS 
 
 35 
 
 component grains of rocks, but are comparatively rare; 
 in the great majority of cases the feldspars are either 
 mixtures of orthoclase and albite in varying proportions 
 but usually with a considerable excess of the potash 
 compound and are then called alkalic feldspar, or they are 
 mixtures of albite and anorthite and are then known as 
 soda-lime feldspar or plagioclase. All transitions from 
 pure albite to pure anorthite occur, and the series has been 
 divided into groups according to the different proportions 
 of the soda and lime molecules; one of the most important 
 of these is called labradorite in which there are about 
 equal amounts of the two kinds. 
 
 Mixtures of a and c, of the potash and lime feldspars, have been 
 found to occur but are so rare that for practical purposes they may 
 be neglected. 
 
 Form. Orthoclase is monoclinic in symmetry, and when 
 in distinct well-made crystals it commonly takes the 
 
 Fig. e. 
 
 Fig. 7. 
 
 forms shown in the accompanying figures. Sometimes 
 the crystals are stout and thick in their habit or appear- 
 ance as in Fig. 5, sometimes they are thin and tabular 
 parallel to the face b as in Fig. 6, and again they may 
 be rather long and columnar as in Fig. 7. In or- 
 thoclase the face c is always at right angles with the 
 face b. In albite and anorthite, whose crystallization is 
 
36 ROCKS AND ROCK MINERALS 
 
 triclinic, these faces c and b are not at right angles but are 
 slightly oblique; this is also true for all of their mixtures 
 or the plagioclase group in general. Some mixtures of 
 orthoclase and albite, as well as certain varieties of the 
 pure potash compound KAlSisOg called microdine, are 
 also slightly oblique, but in all these cases mentioned the 
 amount of departure from a right angle is only a few 
 degrees which, even under favorable conditions, can 
 scarcely be perceived by the eye and must be measured 
 by a goniometer to be appreciated. It cannot, therefore, 
 under ordinary circumstances, be used as a means of 
 discrimination between the alkalic and plagioclase feld- 
 spars. The forms of the crystals in which the plagioclase 
 feldspars appear in rocks when they have the opportunity 
 to crystallize freely are similar to those mentioned above 
 for orthoclase in Figs. 5-7. 
 
 It is only in the phenocrysts of porphyritic igneous 
 rocks and in the miarolitic druses of the massive igneous 
 ones that these minerals have an opportunity to assume 
 the free crystal forms described; in ordinary cases their 
 crystallization is interfered with by other minerals or by 
 other crystals of feldspar and they are thus seen in shape- 
 less masses or grains. Nevertheless there is a tendency 
 to assume these forms, and in some rocks, such, for instance, 
 as the syenites, which are mainly composed of feldspar, 
 it may be observed that they have more or less perfectly 
 the shape of flat tables or rude laths as they approximate 
 to Figs. 6 or 7. 
 
 Twinning. Crystals frequently appear compound, as 
 if cut through parallel to some prominent plane on them 
 and one of the halves revolved 180 degrees, usually on an 
 axis perpendicular to the plane of division which is called 
 the twinning plane, and the two parts grown together. 
 Such an arrangement is called a twin crystal. Feldspars 
 very commonly occur in twin crystals, one of the most 
 frequent arrangements being that illustrated in Fig. 8 
 and known as the Carlsbad twinning from the town of that 
 
ROCK-MAKING MINERALS 
 
 37 
 
 Fig. 8 
 
 Fig 9 
 
 name in Bohemia where excellent examples have been 
 found. It is as if a crystal like that shown in Fig. 5 
 were cut through parallel to the face 6, one of the parts 
 revolved 180 degrees around a 
 vertical axis parallel to the edge 
 mb and then joined and the two 
 parts pushed together so that 
 they mutually penetrate. In 
 Fig. 9 the same arrangement is 
 seen looking down on the face b 
 of the crystal; acya is the outline 
 of the original crystal; if this is 
 cut out in a piece of paper and then turned over 180 
 degrees or upside down and laid on acya so that the edges 
 aa are brought together, it will give the result seen in 
 the figure. In the twin crystal illustrated in Fig. 8 the 
 face c slopes toward the observer, the face y slopes away 
 behind; in the twinned half this is reversed; as explained 
 under the cleavage of feldspars this fact is of importance 
 in helping to recognize these twins when the outward 
 crystal form is imperfect or wanting. Carlsbad twins 
 of the character described are found of all the different 
 varieties of feldspar; they are most perfectly developed 
 in the phenocrysts of the porphyritic igneous rocks, 
 especially in the large orthoclase phenocrysts of some 
 granite porphyries. 
 
 In the Carlsbad twin the plane of division of the two 
 parts is one parallel to the face 6; the axis 
 on which one part is revolved is the vertical 
 line parallel to the edge ab of Fig. 10 and 
 not one perpendicular to b or parallel to the 
 edge ac which is usually the case in twinning, 
 as already mentioned. The face c in ortho- 
 clase makes a right angle with b ; the outline 
 of the face a is, therefore, a rectangle, and if the crystal 
 were divided along the dotted line by a plane parallel to b 
 and one of the halves revolved 180 degrees on an axis 
 
 Fig. 10 
 
38 
 
 ROCKS AND ROCK MINERALS 
 
 parallel to the edge ac, that is, perpendicular to b, it would 
 appear precisely as before and no twinning would occur. 
 The crystallographic reason for this is that b is a symme- 
 try plane, since the crystal is monoclinic, and a symmetry 
 plane cannot be a twinning plane. 
 
 In the plagioclase group, in albite, anorthite and their 
 admixtures, the face c makes an oblique angle with the 
 face b; the face a is, therefore, a rhomboid and not a 
 rectangle as shown in Fig. 11: if this crystal is divided 
 along the dotted line and one of the halves revolved 180 
 degrees it will present the appearance seen in Fig, 12; 
 the face c and the lower c now brought on top slope toward 
 
 Fig. 
 
 Fig. 12 
 
 Fig. I 3 
 
 each other, forming a re-entrant angle, while below they 
 produce a salient angle. A twin crystal is, therefore, 
 produced, and this kind of twinning is known as the albite 
 method because it is so generally found in that variety 
 of feldspar. A complete crystal of this kind is seen' 
 in Fig. 13. The crystallographic reason that this can 
 occur is because these feldspars are triclinic; they have, 
 therefore, no symmetry plane, and any one of the faces 
 might serve as a twinning plane. 
 
 Multiple Twinning. In nature, in actual practice, we 
 rarely find a single albite twin of the kind described above. 
 In the rock-making plagioclases the crystals are divided 
 again and again into thin slices, and these are alternately 
 twinned upon one another, producing the effect seen in 
 Fig. 14. Indeed, this albite twinning descends to such 
 a remarkable degree of fineness that the twin layers are 
 
ROCK-MAKING MINERALS 
 
 39 
 
 less than the one hundred thousandth of an inch in thick- 
 ness and are scarcely to be perceived in thin sections in 
 polarized light under the highest powers of the micro- 
 scope. It frequently happens, however, especially in 
 those feldspars containing much lime, like labradorite, 
 that it is coarse enough to be readily seen by the naked 
 eye; one cleavage surface of such a feldspar appears as if 
 
 Fig. 14 
 
 Fig. 15 
 
 ruled by fine parallel lines or striations as illustrated in Fig. 
 15. Even when very fine and on a small cleavage surface of 
 a feldspar grain embedded in the rock, by a proper adjust- 
 ment of the light reflected from the surface and the use of 
 a good lens this multiple twinning may be distinctly seen. 
 Sometimes feldspars are twinned both according to 
 the Carlsbad and the albite laws; they 
 may be seen divided into the Carlsbad 
 halves by the reflection of light from 
 the cleavage and each of these ruled 
 by the fine lines of albite twinning. 
 An illustration of the combination of 
 these two, each Carlsbad half divided 
 into albite halves, is seen in Fig. 16. 
 The practical use of the twinning of feld- 
 spars is explained in the paragraph on methods for their 
 determination. Other methods of twinning beside those 
 
 Fig. 16 
 
40 ROCKS AND ROCK MINERALS 
 
 mentioned occur in the feldspars, but in the megascopic- 
 study of rocks they are not of importance. 
 
 Cleavage. All the different varieties of feldspar are 
 alike in possessing a good cleavage in two directions, one 
 parallel to the face c and another parallel to b (see Fig. 
 7). Since in orthoclase these two faces intersect at a 
 right angle, so also do the cleavages, and from this fact its 
 name is derived (Greek, opQos, straight, right + /c\dv, 
 to break) ; in the lime-soda feldspars, albite to anorthite, 
 these faces are slightly oblique, and so are the cleavage 
 planes; hence the name plagioclase (Greek, TrXcr/io?, 
 oblique + K\av } to break) has been given to the group. 
 
 In rocks, if the feldspar grains are of good size, the 
 cleavages are readily seen by reflected light ; they are com- 
 monly interrupted, giving rise to steplike appearances. 
 Even when the grains are small the cleavage can usually be 
 detected with a lens in good light. Sometimes when the 
 feldspars are more or less altered, as described under 
 alteration, they lose more or less completely their capacity 
 for showing good cleavage faces on a broken surface of 
 the rock, and this fact must be taken into account in 
 making determinations. As in the crystals which show 
 distinct faces, so in cleavage pieces, the amount of obliquity 
 of the plagioclases is too small to be used in distinguishing 
 them from right angled orthoclases by the eye or lens. 
 
 On a fractured rock surface if the crystal grains are of 
 sufficient size the cleavages frequently permit one to 
 recognize that they are twinned according to the Carlsbad 
 method. The grain or broken crystal appears divided 
 into two parts by a distinct line; on one side of this, if the 
 line points away from the observer, the cleavage sur- 
 faces slope or step away in one direction ; in the other half 
 they slope towards the observer at an equal angle, like 
 the two c faces in Fig. 8, to which indeed they are 
 parallel. This can usually be readily seen by shifting the 
 position of the surface in a good light until the cleavages 
 reflect it. At the same time if examined with a good 
 
ROCK-MAKING MINERALS 41 
 
 lens they may often be seen to be ruled by the fine parallel 
 striations of the albite twinning, which indicates that the 
 feldspar grain is a plagioclase. 
 
 Fracture. In directions in which they do not cleave 
 the fracture of feldspars is uneven and sometimes some- 
 what conchoidal. They are brittle. 
 
 Color, Luster and Streak. Feldspars do not possess 
 any natural color, hence, as explained under the color of 
 minerals, they should normally be either limpid and color- 
 less or white. Transparent, colorless, glassy feldspars in 
 rocks are confined to fresh and recent lavas in which they 
 may be frequently seen in the phenocrysts; they practi- 
 cally never occur in massive granular rocks like granites, 
 gneisses, etc. In such lavas the luster may be strongly 
 vitreous. More commonly they are semi-translucent or 
 opaque and white, grayish white or yellowish and of a 
 somewhat porcelain-like appearance. Orthoclase and 
 the alkalic group of feldspars in general are very apt to 
 have a tinge of red; this color varies from a pale flesh 
 color to a strong brick-red or brownish red; a distinct 
 flesh color is the shade most common. It is this which 
 gives many granites used for building stones their color. 
 It is most probable that this variety of color is caused by 
 finely disseminated ferric oxide dust which acts as a pig- 
 ment, and it must be considered as exotic and not a natural 
 color. The plagioclases or lime-soda feldspars more rarely 
 show this; they are commonly gray, and the difference 
 between the two classes of feldspars is apparently due to 
 a difference in the chemical behavior of iron towards soda 
 and potash; soda enters readily into combination with 
 iron in silicate minerals, while potash does not. Thus 
 in the potash feldspars the iron would tend to be present 
 as free oxide and color them. Therefore rocks with 
 potassic feldspars often tend to be of reddish color, those 
 with sodic feldspars tend to be gray. This distinction 
 may be used to some extent as an indicator of the kinds 
 of feldspar, but it must never be taken as an absolute 
 
42 ROCKS AND ROCK MINERALS 
 
 rule, because many potassic feldspars are white or gray, 
 and conversely many instances occur where rocks with 
 soda-lime feldspars are red. In general one may say 
 that if the rock contains two feldspars one of which is red 
 while the other is not, it is almost certain that the red 
 feldspar is a potassic one or orthoclase. 
 
 The potassic feldspars, especially the variety called microcline 
 when occurring in distinct crystals in the miarolitic druses of granitic 
 rocks, have sometimes a green color, pale to bright grass-green. 
 This is also an exotic coloration and is supposed to be due to some 
 organic substance acting as a pigment, since it disappears on heating. 
 
 Sometimes the rock feldspars are gray, dark, smoky or bluish- 
 gray or even black. While this may happen with alkalic varieties, 
 it is much more common with the soda- lime ones, especially lab- 
 radorite. It is caused by a fine black dust disseminated through 
 them which acts as a pigment and which may sometimes be mag- 
 netite dust, but is much more often ilmenite, titanic iron ore. 
 Fine examples of these are seen in the labradorite rocks from Canada, 
 the Adirondack region in New York State and from Labrador which 
 have been called anorthosites. Sometimes these inclusions are of 
 sufficient size and so regularly arranged in the feldspar that, by the 
 interference of light, they produce an opalescence or play of colors 
 in the mineral as seen in the beautiful examples from St. Paul's 
 Island on the coast of Labrador and from Kiev in Russia. 
 
 In other cases feldspars have a pearly bluish opalescence from 
 innumerable minute cracks regularly arranged which reflect light 
 with interference colors. 
 
 The luster is vitreous and on cleavages often pearly. 
 Feldspars which are more or less altered often have a 
 waxlike appearance and a waxy, glimmering luster; if 
 completely altered they may look earthy and have no 
 luster. 
 
 The streak is white and not characteristic. 
 
 Hardness. This is 6. Scratched by quartz, scratches 
 glass, but is not scratched by the knife. 
 
 Specific Gravity. Orthoclase = 2.55, albite = 2.62, 
 anorthite 2. 7 6. That of the various mixtures varies 
 
ROCK -MAKING MINERALS 
 
 43 
 
 between these limits; thus the alkalic feldspars which con- 
 sist of a mixture of orthoclase and albite average about 
 2.57, while the plagioclases vary regularly with the relative 
 amounts of soda and lime, that of labradorite being 2.67. 
 If the specific gravity of a fragment of feldspar can be 
 taken with accuracy to the second place of decimals it 
 affords a fairly good rough method of ascertaining its 
 composition. 
 
 Chemical Composition. This is shown in the following 
 table. 
 
 
 SiO 2 
 
 A1 2 O 3 
 
 CaO 
 
 Na 2 O 
 
 K 2 O 
 
 Total. 
 
 I 
 
 64.7 
 
 18.4 
 
 
 
 16.9 
 
 100 
 
 II 
 
 68.7 
 
 19.5 
 
 
 11.8 
 
 
 100 
 
 III 
 
 43.2 
 
 36.7 
 
 20.1 
 
 
 
 100 
 
 IV 
 
 55.6 
 
 28.3 
 
 10.4 
 
 5.7 
 
 
 100 
 
 V 
 
 66.7 
 
 18.9 
 
 
 5.7 
 
 8.7 
 
 100 
 
 I, Orthoclase (and microcline); II, Albite; III, Anorthite; IV, 
 Labradorite (equal mixture of albite and anorthite); V, Alkalic 
 feldspar (equal mixture of orthoclase and albite). 
 
 The mixtures vary naturally with the proportions 
 of the pure products; examples of equal parts are given 
 in IV and V. The other substances, such as iron oxide, 
 etc., shown in. feldspars by chemical analyses, are due 
 to impurities. 
 
 Blowpipe and Chemical Characters. A fine splinter 
 fuses before the blowpipe with difficulty to a globular 
 ending, more easily with anorthite and the varieties rich 
 in lime than with albite and orthoclase. The flame shows 
 the persistent yellow coloration of soda; only occasionally 
 in the rock feldspars does orthoclase occur, which is pure 
 enough to give the violet flame of potash. Orthoclase 
 and albite are not acted upon by ordinary acids to an 
 appreciable extent; as the feldspars increase in lime they 
 
44 ROCKS AND ROCK MINERALS 
 
 become more soluble, thus labradorite is very slowlj 
 dissolved while anorthite is slowly dissolved and affords 
 gelatinous silica. 
 
 Alteration. Under the action of various agencies the 
 feldspars are prone to alter into other substances, which 
 depend in part on the nature of the agents and in part on 
 the composition of the feldspar attacked. Some of these 
 changes and products are quite complex and their nature 
 and significance have not as yet been sufficiently studied 
 for us to understand them, but some of the simpler and 
 more important ones are as follows. 
 
 When the feldspars are acted upon by water carrying 
 carbonic acid gas in solution, which may be the case in 
 surface waters leaching downward or in hot waters rising 
 from depths below, they may be turned into kaolin or 
 muscovite with separation of free silica and alkaline 
 carbonates. These changes may be expressed chemically 
 as follows. 
 
 Orthoclase 4- Water 4- Garb. diox. = Kaolin + Quartz +Potas. Garb. 
 
 2KAlSi 3 O 8 + 2H 2 O + CO 2 = H 4 Al 2 Si 2 O 9 + 4SiO 2 + K 2 CO 3 
 
 Orthoclase + Water + Garb. diox. = Muscovite + Quartz. + Potas. Garb. 
 
 3KAlSi 3 O 8 + H 2 O + CO 2 = H 2 K(AlSiO 4 ) 3 + 6 SiO 2 + K 2 CO 3 
 
 What determines whether the removal of the potash from the 
 feldspar will be complete so that kaolin is formed or only partial 
 so that muscovite is the resultant product is not clearly understood. 
 In a general way one may say that weathering from the action of 
 surface waters generally forms kaolin while the change to muscovite 
 is more apt to be a deep-seated affair and is especially noted in 
 processes of metamorphism. In mines it is often seen that the 
 solutions which deposited the ores have altered the rocks enclosing 
 them, sometimes to kaolin, sometimes to a form of muscovite (sericite) 
 and sometimes to other products. It is due to this in great part 
 that such rocks are so often changed from their original fresh con- 
 dition. 
 
 All feldspars undergo similar changes to those men- 
 tioned, but in those which contain lime they are more 
 complex, as calcite, the carbonate of lime is also formed. 
 Accordingly, as this change to muscovite or kaolin is more 
 
ROCK -MAKING MINERALS 45 
 
 or less complete, the feldspars lose their original bright 
 appearance and become dull and earthy in character; if it 
 is pronounced they are soft and may be cut or scratched 
 with the knife or even with the finger nail. In certain 
 changes in the lime-soda feldspars they have a faint, 
 glimmering luster, are semi-translucent, often of a pale 
 bluish or grayish tone, lose to a great extent their property 
 of cleavage and resemble wax or paraffin as mentioned 
 under cleavage. Often these changes do not take place 
 regularly through the whole mass of the crystal, some- 
 times the border is altered, sometimes the center only is 
 attacked and sometimes, especially in the lime-soda ones, 
 like labradorite, zones between the two are altered. If 
 the feldspars of a rock do not show bright, glistening 
 cleavage surfaces it may be considered practically certain 
 that they are more or less altered. These alterations of 
 the feldspars are of great importance in geologic processes 
 and especially in the formation of soils. 
 
 In addition to these alterations others are also known, thus under 
 some circumstances the feldspars are changed into zeolites and in 
 metamorphic processes those containing lime may take part with 
 other minerals in forming epidote, garnet, etc., changes which are 
 mentioned elsewhere. 
 
 Occurrence. The feldspars are of wide distribution, 
 perhaps more so than any other group of minerals. They 
 are found in all classes of rocks, in most of the igneous 
 ones, such as granites, syenites, porphyries and felsite 
 lavas; in the sedimentary ones in certain kinds of sand- 
 stones and conglomerates and in the metamorphic rocks 
 in gneisses. Since, so far as our knowledge extends,the 
 crust of the earth, underlying all the sedimentary beds of 
 all ages deposited upon it, is composed chiefly of granites, 
 gneisses, etc., in which feldspars are the main minerals, 
 it is not too much, perhaps, to say that there is more 
 feldspar in the world than any ether substance of whose 
 occurrence we have knowledge. 
 
46 ROCKS AND ROCK MINERALS 
 
 Determination. In general, the two cleavages at right 
 angles or nearly so, the vitreous luster, light color and 
 hardness, which resists the point of the knife, enable one 
 in the field to recognize the feldspar grains of rocks and to 
 distinguish them from the other common minerals, 
 especially quartz, with which they are usually associated. 
 Sometimes the crystal form may also be of assistance, 
 especially in porphyries. In addition one or more of the 
 various chemical and physical properties enumerated 
 above may be determined on separated fragments, if the 
 feldspar grains or masses are of sufficient size. 
 
 The determining of the different varieties of feldspar which may be 
 present in a rock is, however, a much more difficult task when only 
 megascopic means are employed. Sometimes the remarks made 
 under the heading of color will be of assistance. If the cleavage 
 surfaces are closely examined with a lens and the fine lines of stria- 
 tion of the albite twinning are found then one knows that a plagio- 
 clase feldspar is present, since orthoclase cannot have this twinning 
 as previously explained. The only practical exception to this rule 
 is that the large, often huge, crystals of potash feldspar found in 
 granite-pegmatite dikes are often not really orthoclase but micro- 
 cline, a tricHnic variety and a good cleavage surface of this ex- 
 amined in a strong light with a powerful lens frequently shows a 
 minute, scarcely perceptible, multiple twinning like the albite 
 twinning. 
 
 If no multiple twinning is seen it would not be, therefore, safe 
 to conclude that the feldspar is necessarily an orthoclase or alkalic 
 variety and not a plagioclase because this twinning, as already 
 stated, is often so fine that it cannot be detected with the lens and is 
 sometimes wanting. As the grain of rocks grows finer it becomes 
 increasingly difficult to detect, but a good training of the eye by 
 studying a series of rocks in which it is present in the feldspars is a 
 great help and eventually enables one to perceive it clearly in cases 
 where at first it could not be seen. The modern tendency on the 
 part of geologists to refer all difficulties in rocks to microscopic 
 examination of thin sections has led to a great neglect in the training 
 of the eye for megascopic determination of minerals in rocks with a 
 corresponding loss of efficiency in the field. 
 
 If the albite twinning is clearly seen in several of the feldspar 
 grains of a rock it may be quite safely concluded that a considerable 
 proportion of plagioclase is present and this may indeed be prac- 
 
ROCK-MAKING MINERALS 47 
 
 tically the only feldspar present. If it cannot be seen plagioclase 
 may or may not be present. 
 
 Other means which may be resorted to are the determination of 
 the specific gravity, the behavior before the blowpipe, and with 
 acids, as previously mentioned, and the chemical tests for soda, 
 potash and lime, which suggest themselves to those experienced in 
 analytical chemistry. Further information in the subject should be 
 sought in .the special manuals devoted to determinative mineralogy. 
 
 THE FELDSPATHOID GROUP. 
 
 The feldspathoid group owes its name 'to the fact, that, 
 like the feldspars, it is composed of minerals which are 
 silicates of alumina with soda, potash and lime and that 
 they are found in the same associations, accompanying 
 or replacing feldspars and playing a similar function in 
 the making of rocks. Unlike feldspars they are com- 
 paratively rare and are restricted entirely to certain kinds 
 of igneous rocks such as nephelite syenite. Thus in 
 treating of the occurrence of common rocks they are, 
 compared with the feldspars, of relatively much less 
 importance, but, in dealing with questions regarding the 
 origin of igneous rocks, they are of great significance. 
 The more important members of the group are nephelite 
 and sodalite, less common ones are noselite and hauynite, 
 cancrinite and leucite. 
 
 Nephelite. This mineral crystallizes in short, thick, 
 hexagonal prisms or tables with a flat base and top but it 
 rarely shows distinct crystal form in rocks. Most com- 
 monly it occurs in shapeless masses and grains like quartz. 
 Its normal color is white, but it is usually gray, varying 
 from light smoky to dark in tone, sometimes it is flesh 
 colored or brick-red. The white color may shade into 
 yellowish, the gray into bluish or greenish. Streak, light 
 not characteristic. Translucent. Its luster, when 
 fresh, is oily or greasy and much like that of quartz and, 
 like this mineral, it has no good cleavage and its fracture 
 is somewhat conchoidal. Brittle. Hardness, nearly that 
 of feldspar = 6- Specific gravity, 2.55-2.61 . Its com- 
 
48 ROCKS AND ROCK MINERALS 
 
 position is chiefly NaAlSiO 4 with a small varying amount 
 of potash replacing soda. Before the blowpipe a fine 
 splinter fuses quite readily to a globule tingeing the flame 
 deep yellow. Readily soluble in dilute acid with forma- 
 tion of gelatinous silica. 
 
 Sodalite. The form of crystallization is the isometric 
 dodecahedron, so often seen in garnet, but this rarely 
 occurs in rocks, the mineral commonly occurring in form- 
 less grains and lumps. It is sometimes white, pink, or 
 greenish gray, but the usual color is a blue of some shade, 
 often a bright sky-blue to dark rich blue. The blue color 
 may be due to a slight admixture of the lapis-lazuli 
 molecule acting as a pigment. Usually translucent. 
 Cleavage dodecahedral but not striking as a megascopic 
 property; fracture uneven to poorly conchoidal. Luster 
 vitreous to greasy. Streak, white. Hardness nearly 
 that of feldspar, 5.5-6. Specific gravity, 2.15-2.30. Its 
 composition is Na 4 (AlCl)Al 2 (Si04)3 and this may also be 
 expressed 3 NaAlSiO4 . NaCl, but it should not be under- 
 stood from this that it consists of a mixture of nephelite 
 and common salt molecules; it is a definite chemical com- 
 pound into which the chlorine enters. Fuses rather 
 easily before the blowpipe with bubbling, coloring the 
 flame yellow. Easily soluble in dilute acids with forma- 
 tion of gelatinous silica; in the nitric acid solution chlorine 
 may be tested for with silver nitrate. 
 
 The other feldspathoids are less common and in their 
 general properties, modes of occurrence and functions as 
 rock minerals are similar to nephelite and sodalite, which 
 they are usually found associated with or in part replacing 
 in those rocks in which they occur. 
 
 Hauynite and Noselite. These show characters like sodalite but 
 they differ from it in containing the radical SO 3 of sulphuric acid in 
 the place of chlorine and the best method of detecting them is by 
 the test for sulphuric acid with barium chloride in their nitric acid 
 solution. They differ from one another only that in hauynite a 
 part of the soda is replaced by lime while noselite is the pure soda 
 compound. Cancrinite is much like nephelite in its genera! prop- 
 
ROCK-MAKING MINERALS 49 
 
 erties, it contains CO 2 in combination, which affords aid in detecting 
 it as explained later in testing minerals and rocks ; its formula might 
 be written 8 NaAlSiO 4 . CaCO 3 . CO 2 . 3 H 2 O, but as in sodalite it 
 is not a mixture of molecules but a definite compound. The color 
 is variable but frequently a bright yellow to orange which may also 
 help in detecting it. It is supposed at times to be caused by the 
 alteration of nephelite, but in most cases, if not always, it is an 
 original mineral crystallizing from a molten magma, like nephelite 
 and feldspar. 
 
 Leucite is a rare feldspathoid crystallizing in isometric trape- 
 zohedrons, a form illustrated in garnet; the crystals when imperfect 
 appear spherical. Its cleavage is imperfect; fracture conchoidal; 
 color white to gray; luster vitreous. Hardness is 5.5-6; specific 
 gravity, 2.5. Before the blowpipe it is infusible and when mixed 
 with powdered gypsum gives the flame the violet color of potassium. 
 It dissolves in acids without gelatinizing. Its composition is 
 KAl(SiO 3 ) 2 . It occurs almost wholly in lavas and is nowhere 
 common except in those of central Italy, where the magmas are 
 characterized by a high content of potash. The most noted occur- 
 rence is in the lavas of Vesuvius, in some of which it is found in good- 
 sized, well-shaped crystals of the form illustrated in Fig. Blunder 
 garnet. Large crystals, altered, however, to other minerals, have 
 been found in certain syenites and related rocks in Arkansas, Mon- 
 tana, Brazil and elsewhere. 
 
 Alteration. The feldspathoids, like the feldspars, are 
 liable to alteration from the processes of weathering when 
 exposed to the atmosphere and to the action of fluids 
 circulating in the rocks at lower levels. They become 
 converted into kaolin or muscovite and also very com- 
 monly into zeolites. The latter case is very general; all 
 that is necessary is a rearrangement of the molecule and 
 the assumption of water and silica; hence when the feld- 
 spathoids are heated in a closed glass tube they are very 
 apt to yield water. Thus 
 
 Nephelite and silica and water yield analcite. 
 NaAlSiO 4 + SiO 2 + H 2 = NaAl(SiO 3 ) 2 . H 2 O. 
 
 The determination of the feldspathoids in rocks is best done by 
 chemical means. With the exception of leucite, which is too rare a 
 mineral to be considered except in very unusual cases, they yield 
 gelatinous silica and may be tested for as described later under 
 
50 ROCKS AND ROCK MINERALS 
 
 mineral tests. Nephelite is easily confused with quartz which it 
 often closely resembles in rocks; its association with other minerals 
 and the appearances of those rocks in which it chiefly occurs and 
 which are described in their appropriate places, helps in arousing 
 suspicion of its presence and this is readily confirmed by its solubility 
 in acids. Fortunately for field determinations nepheiite is a very 
 rare mineral, quartz an exceedingly common one ; thus the assump- 
 tion that the mineral is quartz in the vast majority of cases will be 
 right. 
 
 MICAS. 
 
 The micas form a natural group of rock minerals, which 
 is characterized by great perfection of cleavage in one 
 direction, and by the thinness, toughness and flexibility 
 of the elastic plates or lamina? into which this cleavage 
 permits them to be split. For practical purposes of 
 megascopic rock study and classification they can be 
 divided into two main groups, light colored micas or 
 muscovite and related varieties, and dark colored biotite 
 and related varieties. 
 
 Form. Micas crystallize in six-sided tablets with flat 
 bases; they appear to be short hexagonal prisms, (see 
 Fig. 17); in reality, as maybe shown by optical methods, 
 their crystallization is monoclinic. Their side faces are 
 rough and striated, the flat bases, which are usually cleav- 
 
 Fig. 17 Fig. 18 
 
 age faces, bright and glittering. Sometimes two of the 
 side faces are much elongated, as in Figo 18. While 
 distinct crystal form is often observed in rocks, par- 
 ticularly the igneous ones, the micas are much more 
 commonly seen in shapeless flakes, scales or shreds, with 
 flat, shining, cleavage faces. Sometimes the folise or 
 leaves are curled or bent. 
 
ROCK-MAKING MINERALS 51 
 
 Cleavage, This has been already mentioned. It is 
 perfect parallel to the base and it is this property combined 
 with its flexibility, transparency and toughness that 
 makes the large crystals and sheets of muscovite found in 
 pegmatite veins so useful in making stove windows, lamp 
 chimneys, etc., where ordinary glass is easily broken. 
 Sometimes when the mineral occurs in an aggregate of 
 minute scales, especially muscovite in the sericite form, 
 the cleavage is not so apparent, but can generally be seen 
 by close observation. 
 
 Color, Luster and Hardness. Muscovite is colorless, 
 white to gray or light brown, often with greenish tones. 
 The other light-colored micas are similar, except that 
 lithia mica or lepidolite, found in pegmatite veins, is 
 usually pink or lilac colored. These micas in thin sheets 
 are transparent. 
 
 Biotite and its congeners are black, in thin sheets 
 translucent with strong brown, red-brown or deep green 
 colors. The phlogopite variety is pale brown, sometimes 
 coppery. The luster of micas is splendent, on cleavage 
 faces sometimes pearly and in the sericite variety of 
 muscovite frequently silky. The hardness varies from 
 2-3; all are easily scratched with the knife. 
 
 Chemical Composition. Chemically the micas which 
 take part in rock-making may be divided into two main 
 groups, one containing iron and magnesia, of which the 
 dark-colored biotite is an example, the other devoid of 
 these oxides, of which muscovite is the most prominent 
 member. They are complex in composition, silicates of 
 alumina with alkalies and containing more or less hydroxyl 
 and fluorine. The two main varieties may be represented 
 as follows: 
 
 Muscovite = 
 
 Biotite = (HK) 2 (MgFe) 2 (AlFe)2(SiO 4 )3. 
 
 The other members of the muscovite group are, paragonite* 
 a rare mineral like muscovite, in which soda replaces 
 
52 
 
 ROCKS AND ROCK MINERALS 
 
 potash and lepidolite, in which the potash of muscovite is 
 partly replaced by lithia. In the biotite sub-group, 
 phlogopite is a variety nearly free from iron and thus a 
 magnesia mica; the lack of iron accounts for its lighter 
 color; lepidomelane, on the contrary, is very rich in iron, 
 especially ferric oxide, while another, zinnwaldite, contains 
 some lithia in place of part of the potash. The formulas 
 of these compounds are very complex and in part not 
 absolutely settled. The adjoining table of analyses 
 shows the chemical differences between the varieties. 
 
 
 I 
 
 II 
 
 III 
 
 IV 
 
 V 
 
 VI 
 
 VII 
 
 SiOo 
 
 44 6 
 
 46 8 
 
 48 8 
 
 36 
 
 39 6 
 
 32 1 
 
 45 9 
 
 ALO, 
 
 35 7 
 
 40 1 
 
 28 3 
 
 18 8 
 
 17 
 
 18 5 
 
 22 5 
 
 Fe O, 
 
 1 
 
 
 3 
 
 5 6 
 
 3 
 
 19 5 
 
 6 
 
 FeO 
 
 1 
 
 
 1 
 
 14 7 
 
 2 
 
 14 1 
 
 11 6 
 
 MgO 
 
 
 6 
 
 
 9 8 
 
 26 5 
 
 1.0 
 
 
 CaO 
 
 1 
 
 1.3 
 
 1 
 
 6 
 
 
 
 
 Na 2 O 
 
 2 4 
 
 6 4 
 
 7 
 
 4 
 
 6 
 
 1.5 
 
 
 K O . 
 
 9 8 
 
 
 12 2 
 
 9 3 
 
 10 
 
 8.1 
 
 10 5 
 
 LLO 
 
 
 
 4 5 
 
 
 
 
 3 3 
 
 H..O . 
 
 5.5 
 
 4 8 
 
 1 7 
 
 2 5 
 
 3 
 
 4.6 
 
 9 
 
 F 
 
 0.7 
 
 
 5 
 
 3 
 
 2 2 
 
 
 7.9 
 
 X* . ..... 
 
 
 
 
 1 9 
 
 1 2 
 
 1.4 
 
 1.7 
 
 
 
 
 
 
 
 
 
 Total 
 
 100.8 
 
 100.0 
 
 101.7 
 
 99.9 
 
 100.6 
 
 100.8 
 
 104.9 
 
 
 
 
 
 
 
 
 
 * X represents small quantities of non-essential oxides present. 
 
 I, Muscovite, Auburn, Me.; II, Paragonite, the Alps; III, Lepid- 
 olite, Hebron. Me.; IV, Biotite, from granite, Yosemite, Cal.; V, 
 Phlogopite, Burgess, Ontario; VI, Lepidomelane, from nephelite 
 syenite, Litchfield, Me. ; VII, Zinnwaldite, Zinnwald, Erzgebirge. 
 
 Blowpipe and Chemical Characters. Usually the micas 
 whiten before the blowpipe and fuse on the edges, when 
 in thin scales. Lepidomelane fuses to a black magnetic 
 globule. Heated in the closed glass tube they yield 
 very little water, which helps to distinguish them from 
 
ROCK-MAKING MINERALS 53 
 
 chlorites and other micaceous rock minerals. When thin 
 scales are treated with a little boiling concentrated sul r 
 phuric acid in a test tube, muscovite and the re- 
 lated light-colored kinds are scarcely acted upon, but 
 biotite and its congeners are decomposed, the scales losing 
 their luster and transparency while the acid becomes 
 turbid.* 
 
 Lepidomelane is soluble in hydrochloric acid, depositing 
 silica in scales, an important character serving to dis- 
 tinguish it from the other micas. The lithia micas impart 
 a red color to the blowpipe flame, paragonite the yellow 
 color of sodium. 
 
 Alteration. Biotite under the action of weathering 
 changes to chlorite, loses its elasticity and becomes soft 
 and of a green color. Muscovite being itself often the 
 product of various alterations of other minerals, especially 
 of feldspars, appears well fitted to withstand the process 
 of weathering and its scales often occur in soils made of 
 broken-down rocks whose other constituents may be 
 greatly changed. It eventually changes, loses its trans- 
 parency and elasticity and perhaps becomes ultimately 
 converted into clay. 
 
 Occurrence. The common micas are minerals of wide 
 distribution as rock components. Biotite is a very 
 common and prominent ingredient of many igneous rocks, 
 especially of those rich in feldspar like granites and 
 syenites in ferro-magnesian rocks like gabbro it is less 
 prominent; it is also seen in many felsite lavas and por- 
 phyries. It occurs commonly in some metamorphic 
 rocks such as gneisses and schists and is frequently one of 
 the products of contact metamorphism of igneous rocks. 
 From its liability to alteration it does not figure as a 
 component of sedimentary beds. The phlogopite variety 
 containing little iron has been found in some rare cases in 
 
 * Care should be used in making this test not to bring the hot 
 acid in contact with water, or the mixture will take place with 
 explosive activity. 
 
54 ROCKS AND ROCK MINERALS 
 
 igneous rocks, but it chiefly occurs as a product of meta- 
 morphism in crystalline limestones or impure marbles 
 and dolomites. Lepidomelane and zinnwaldite appear to 
 occur chiefly in granites and syenites, especially in peg- 
 matitic varieties. Muscovite occurs in granites and 
 syenites, especially in pegmatite veins and in miarolitic 
 druses and in places where the igneous rocks have been 
 subjected to later fumarole actions furnishing water and 
 fluorine. It is sometimes seen in intrusive porphyries 
 and lavas of felsitic character. It is especially common 
 in the metamorphic rocks and is widely distributed in 
 gneisses and schists; sometimes, especially in the latter 
 rocks, it is in the form of an aggregate of minute scales 
 which have a silky luster and largely lack in appearance 
 the evident characters of the mineral, such as its cleavage; 
 this variety has been called sericite. When feldspars are 
 altered to muscovite, rather than to kaolin, this sericite 
 variety is the common product. In the sedimentary 
 rocks, such as conglomerates and sandstones, muscovite 
 is sometimes seen, an unchanged remnant of the original 
 rocks from which their material came. Lepidolite is 
 practically restricted to granite-pegmatite veins and is 
 constantly accompanied by tourmaline. Paragonite has 
 been found in only a few cases, in schists, playing the role 
 muscovite would ordinarily have. 
 
 Determination. From the ordinary rock minerals the 
 micas are at once distinguished by their appearance, high 
 luster and eminent cleavage, the latter quality and their 
 hardness being readily tested in the field by the knife 
 point. From the chlorite group and from talc, which 
 resemble them, they are told by the elasticity of their 
 split-off laminae, those of the chlorites and talc being 
 flexible but not elastic. From chloritoid a micaceous 
 appearing mineral of a gray or green color, a hydrated 
 silicate of alumina, magnesia and iron, which is sometimes 
 seen in distinct crystals in certain metamorphic rocks, they 
 are readily distinguished by its superior hardness = 6.5 
 
ROCK-MAKING MINERALS 55 
 
 and brittleness. The different varieties of mica are 
 best discriminated by the chemical and blowpipe tests 
 already mentioned. 
 
 PYROXENES. 
 
 The pyroxene group embraces a number of important 
 minerals which have in common the fact that they are 
 metasilicates, salts of metasilicic acid, H 2 Si0 3 , in which 
 the hydrogen is replaced by various metals as shown 
 later, and although they may differ in the system in which 
 they crystallize, in having closely related crystal form, 
 notably a prismatic cleavage of 87 and 93 degrees. As 
 rock minerals they are of greatest importance in the 
 igneous rocks though they may be prominent at times in 
 some of the metamorphic ones. Some igneous rocks are 
 composed almost entirely of pyroxene. 
 
 It is often difficult to recognize pyroxene in the rocks 
 and distinguish it from several other minerals purely by 
 simple megascopic methods and largely impossible to 
 tell from one another by such means the many varieties 
 recognized by mineralogists and petrographers. The 
 differences between these varieties are chiefly in chemical 
 composition and optical properties and these must be 
 determined by chemical and optical methods. 
 
 For practical megascopic petrography the pyroxenes 
 may be divided into the following sub-groups dependent 
 on their color, behavior before the blowpipe and chemical 
 reaction for lime as described later: hypersthene, diopside, 
 common pyroxene, augite and aegirite. 
 
 Form. Hypersthene crystallizes in the orthorhombic, 
 the others in the monoclinic systems, but this distinction 
 is not a matter of practical importance in megascopic 
 work, since the former is rarely well enough crystallized 
 to determine the system. The common form, in which 
 the monoclinic rock pyroxenes crystallize, is a prism, 
 usually short and thick though sometimes longer and 
 more slender. Such a prism is shown in Fig. 19, the 
 
56 
 
 ROCKS AND ROCK MINERALS 
 
 ends modified by pyramidal faces. Generally, however, 
 the edges of the prism mm are truncated by a front face 
 a and a side face b sometimes these truncations are 
 
 r ,-- ^xSv 
 
 Fig. 19 
 
 Fig. 20 
 
 Fig. 21 
 
 small so that a and b are slender (Fig. 20); often they 
 are very broad and mm narrow. While these faces are 
 commonly well developed and often lustrous the pyra- 
 midal faces are often very imperfect or wanting, the 
 crystal being rounded at the ends; rarely other pyramidal 
 faces are present and the ends much more complex than 
 in the figures. The augites which occur in igneous rocks, 
 especially porphyries and lavas, very often have the 
 appearance and development shown in Fig. 21. The most 
 important thing in the crystallization is that the angle m 
 on m is nearly a right angle, 87 degrees, so that the prism 
 is nearly square in cross section or when truncated by a 
 
 and 6, octagonal as shown in Fig. 22. Besides occurring 
 in prismatic crystals the pyroxenes also are very common 
 in grains, or in more or less shapeless masses; this is 
 
ROCK-MAKING MINERALS 57 
 
 usually the case in certain massive igneous rocks such as 
 gabbros and peridotites. 
 
 Cleavage and Fracture. As previously mentioned the 
 pyroxenes have a cleavage parallel to the faces mm, nearly 
 at right angles as shown in Fig. 23; this is 
 fundamental and serves to distinguish the 
 mineral from hornblendes. This cleavage is 
 usually very good but not perfect. Some 
 varieties often have a good parting in other 
 directions resembling cleavage which causes 
 the mineral to appear lamellar, perhaps even 
 somewhat micaceous, as seen in the pyroxenes of some 
 gabbros. Fracture uneven; the mineral is brittle. 
 
 Color and Luster. The color varies from white through 
 various shades of green to black, according to the amount 
 of iron present. The pure diopsides are white, rarely 
 colorless and transparent, often pale green, and more or 
 less translucent; common pyroxenes are dull green of 
 various shades; augite and aegirite are black; these are 
 opaque. The luster, which is often wanting, is glassy to 
 resinous. Streak varies from white to gray-green. 
 
 Hardness and Specific Gravity. The hardness varies 
 from 5-6. Some varieties can be just scratched by the 
 knife. The specific gravity varies, chiefly with the iron 
 present, from 3.2-3.6. 
 
 Chemical Composition. Pyroxenes are composed of 
 the metasilicate molecules MgSiO 3 , FeSiO 3 , CaMg(SiO 3 ) 2 , 
 CaFe(SiO 3 ) 2 , NaFe(SiO 3 ) 2 and RR 2 SiO 6 in which last 
 R = MgFe and R = Al and Fe. These molecules are 
 isomorphous, that is, capable of crystallizing in various 
 mixtures which produce the same crystal form and many 
 similar physical properties. The hypersthene sub-group 
 contains mixtures of MgSiO 3 and FeSiO 3 without lime; 
 diopside is CaMg(SiO 3 ) 2 with little or none of the iron 
 molecule, common pyroxene contains variable mixtures of 
 CaMg(SiO 3 ) 2 (diopside) and CaFe(SiO 3 ) 2 (hedenbergite) 
 with small portions of the other molecules ; augite contains 
 
58 
 
 ROCKS AND ROCK MINERALS 
 
 the same but in addition a large amount of RR 2 Si0 6 ; 
 aegirite is mostly NaFe(SiO 3 ) 2 and is thus a soda 
 pyroxene. 
 
 Blowpipe and Chemical Characters. Hypersthene varies 
 from almost infusible in the blowpipe flame when contain- 
 ing little iron (variety enstatite) to difficultly so with much 
 iron; in the latter case it becomes black and slightly 
 magnetic. The other pyroxenes are much more fusible 
 = 4 and melt quietly or with little intumescence to glassy 
 globules whose color depends on the amount of iron, diop- 
 side nearly colorless, common pyroxene green or brown, 
 augite and aegirite black; the last two magnetic. Aegirite 
 fuses quietly and colors the flame yellow. They are but 
 slightly acted upon by acids, those with iron more so 
 than those without. 
 
 These differences in the chemical composition are shown 
 in the table of analyses. 
 
 
 SiO a 
 
 A1 2 3 
 
 Fe 2 3 
 
 FeO 
 
 MgO 
 
 CaO 
 
 Na 2 O 
 
 X* 
 
 Total. 
 
 I . 
 
 53.1 
 
 1.0 
 
 
 17.8 
 
 24.8 
 
 2.7 
 
 
 0.4 
 
 99.8 
 
 II . 
 
 58.0 
 
 1.3 
 
 
 3.1 
 
 36.9 
 
 
 
 0.8 
 
 100.1 
 
 Ill 
 
 55.1 
 
 0.4 
 
 
 1.1 
 
 18.1 
 
 25.0 
 
 0.4 
 
 0.2 
 
 100.3 
 
 IV. 
 
 51.1 
 
 2.0 
 
 1.3 
 
 12.3 
 
 10.0 
 
 22.1 
 
 
 0.4 
 
 99.2 
 
 V . 
 
 47.0 
 
 9.8 
 
 4.5 
 
 4.1 
 
 16.0 
 
 19.0 
 
 
 
 100.4 
 
 VI. 
 
 51.4 
 
 1.8 
 
 23.3 
 
 9.4 
 
 0.3 
 
 2.0 
 
 11.9 
 
 0.1 
 
 100.2 
 
 * X = small quantities of other oxides. 
 
 I, Hypersthene, Romsaas, Norway; II, Hypersthene (var. ensta- 
 tite), Bamle, Norway; III, Diopside, DeKalb, N. Y.; IV, Common 
 pyroxene, Edenville, N. Y.; V, Black augite, Vesuvius lava; VI, 
 Aegirite, from syenite, Hot Springs, Ark. 
 
 Alteration. The pyroxenes are prone to alter into 
 other substances whose nature depends partly on the 
 kind of process to which they are subjected and partly 
 on their own composition. Thus under the action of 
 weathering they may be converted, if containing much 
 magnesia, into serpentine, or into chlorite, if containing 
 
ROCK-MAKING MINERALS 59 
 
 iron, or into both and often carbonates are also formed, 
 such as calcite. Those containing much iron may com- 
 pletely break down into hydrated iron oxides, such as 
 limonite, and carbonates. 
 
 Another very important change is one which they 
 suffer under metamorphic processes, especially regional 
 ones. In this they become altered to masses of fibrous, 
 felty or stringy hornblende needles and prisms, usually 
 of distinct but variable green colors. This process is of 
 great geologic importance for by means of it whole masses 
 of pyroxenic rocks, generally of igneous origin, such as 
 gabbros, peridotites, basalts, etc., have been changed 
 into hornblendic ones to which a variety of names, such 
 as greenstone, greenstone schist, hornblende schist, etc., 
 have been applied. The process is further mentioned 
 under metamorphism, and under gabbro, dolerite, green- 
 stone and amphibolite. 
 
 Occurrence. The pyroxenes are chiefly found in 
 igneous rocks, especially those which are formed from 
 magmas rich in lime, iron and magnesia. Therefore, in 
 the dark colored rocks of this class they should always be 
 looked for. They are not often found in igneous rocks 
 which contain much quartz, hence in granites, felsite 
 porphyries and felsite lavas they are rare. Augite is 
 found in basaltic lavas and dark, trap-like intrusives, 
 often in well formed crystals; when it occurs in gabbros 
 and peridotites it is commonly in grains and lumps. 
 Hypersthene is prominent in masses and grains in some 
 varieties of gabbro and peridotite. Aegirite occurs chiefly 
 in nephelite syenites and the phonolite variety of felsite 
 lava. Some normal syenites and related rocks contain 
 diopside-like or common pyroxene. In the metamorphic 
 rocks common pyroxene and diopside, the latter some- 
 times white or pale greenish and transparent, are found 
 in impure recrystallized limestones and dolomites, some- 
 times in well formed scattered crystals, sometimes 
 aggregated into large masses. Common pyroxene also 
 
60 ROCKS AND ROCK MINERALS 
 
 occurs in some gneisses. Being readily decomposed by 
 weathering they play no part in sedimentary beds. 
 
 Determination. If the mineral under examination is 
 in well formed crystals careful observation will usually 
 show if it is a pyroxene by its possession of the forms 
 previously described. The outline of the section presented 
 by the prisms, especially when broken across, should be 
 noted in this connection. The common minerals in rocks 
 with which pyroxenes may be confused are hornblende, 
 epidote and tourmaline. The lack of good cleavage, the 
 superior hardness, the high luster, dense black color and 
 triangular shape of the prism cross section of tourmaline 
 readily distinguish it from pyroxene. Epidote has one 
 perfect cleavage, one poor one; it is much harder, 6-7; 
 while green it commonly has a yellow tone, giving a yel- 
 lowish green; before the blowpipe it intumesces when 
 fusing. The distinction of pyroxene from hornblende is 
 more difficult and is treated under the head of that 
 mineral. 
 
 To distinguish the different varieties of pyroxene from one another 
 the blowpipe tests previously mentioned should be used in conjunc- 
 tion with the natural color of the mineral. The hypersthenes are 
 most certainly told from other pyroxenes by making a chemical test 
 to prove the absence of lime or at least its presence in only minute 
 quantity. This is best done by making a small fusion with soda as 
 described in the chapter treating of mineral tests. 
 
 AMPHIBOLES (HORNBLENDES). 
 
 The amphiboles, or hornblendes, names which are used 
 interchangeably, are a natural group of silicate minerals 
 which like the pyroxenes are salts of metasilicic acid 
 H^SiOs, in which the hydrogen is replaced by various 
 metals or radicals. They have in common a certain 
 crystal form, a prismatic cleavage of about 55 degrees, 
 and are nearly related in many physical properties. As 
 in the pyroxene group, to which the amphiboles are closely 
 allied in several ways, there are many varieties recognized 
 
ROCK-MAKING MINERALS 
 
 61 
 
 by petrographers, dependent upon differences in chemi- 
 cal composition and physical properties, especially optical 
 ones, which are impossible to distinguish by the eye and 
 many of them indeed by ordinary megascopic tests. 
 
 For practical work in megascopic petrography the 
 amphiboles may be divided into the following sub-groups: 
 Tremolite, Actinolite, Common Hornblende, and Arfved- 
 sonite. These may be distinguished by their colors, asso- 
 ciations and behavior before the blowpipe. 
 
 Form. Amphiboles crystallize in the monoclinic sys- 
 tem. The crystals are usually long and bladed, formed 
 by two prisms mm which meet at angles of 55 and 125 
 degrees. Sometimes there are terminal faces rr as in Fig. 
 24, sometimes the crystals are imperfect at the ends and 
 no terminal faces are seen; this latter is common in rocks. 
 Very often the side face b is present truncating the prism 
 
 m 
 
 m 
 
 m 
 
 Fig. 26 
 
 Fig. 37 
 
 edge and the crystal has a nearly hexagonal cross section 
 as in Fig. 25. More rarely the front face a is present as in 
 Fig. 26. The black hornblendes found as phenocrysts in 
 some basaltic rocks have often a not very short prism 
 and appear as in Fig. 27; these are the hornblendes which 
 most often have distinct terminal planes. The prismatic 
 faces mm and the face b, if it is present, are apt to be 
 shining, the ends are frequently dull. It is not common 
 
62 ROCKS AND ROCK MINERALS 
 
 for amphibole to present itself in rocks in crystals whose 
 planes can be distinctly seen; when this occurs it is mostly 
 with the black hornblendes found in lavas as phenocrysts 
 and in those which occur in limestones and dolomites 
 which have been subjected to metamorphism. The 
 common appearance is in long slender blades with irregu- 
 lar, rough ends; this is usual in the hornblende schists 
 where the crystals are aggregated together in more or less 
 parallel position; they may dwindle in size to shining 
 needles, becoming so fine that the minute prisms can 
 hardly be seen with the lens; the aggregate then has a 
 silky appearance. In the felsitic lavas and porphyries 
 the prisms of the hornblende phenocrysts vary from 
 rather short, like those in the figures, to slender needles; 
 in the massive doleritic rocks like diorite the amphibole is 
 apt to occur in irregular grains and small masses. Some- 
 times as in asbestus the mineral has a highly developed 
 columnar, fibrous form. 
 
 Cleavage. Amphiboles have a highly perfect cleavage 
 parallel to the prism faces mm as illustrated in the cross 
 section, Fig. 28. Like the faces mm 
 these cleavages meet at angles of 125 
 and 55 degrees, a fact of great import- 
 | 6 ance in distinguishing the mineral. The 
 glittering prismatic faces seen on the 
 blades and needles of fractured rock 
 Fig. as surfaces are commonly due to this 
 
 good cleavage. The fracture is uneven. 
 Color and Luster. The color varies with the amount of 
 iron from white or gray in tremolite to gray-green or 
 bright green in actinolite to darker greens and black in 
 common hornblende. Arfvedsonite is black. Some var- 
 ieties found in igneous rocks which appear black are 
 really deep brown. The mineral varies from opaque in the 
 deeper colored varieties to translucent in the lighter 
 ones. The luster is bright and vitreous to somewhat 
 pearly on the cleavage surfaces; in very fine needle-like 
 
ROCK-MAKING MINERALS 63 
 
 or fibrous varieties, silky. Streak, white to gray-green or 
 brownish. 
 
 Hardness and Specific Gravity. The hardness varies 
 from 5-6; some specimens can be scratched with the 
 knife. The specific gravity varies, chiefly with the 
 amount of iron, from 2.9-3.5. 
 
 Chemical Composition. Amphiboles like the pyroxenes 
 are metasilicates, salts of H 2 SiO 3 , in which the hydrogen 
 atoms are replaced by calcium, magnesium, iron, soda 
 and also, as shown by Penfield, by radicals in which 
 alumina plays a prominent part and which contain 
 hydroxyl ( OH) and fluorine. Penfield has also shown 
 that when calcium is present it replaces one fourth of the 
 hydrogen atoms. Thus, while the amphiboles resemble 
 the pyroxenes in being metasilicates and composed of the 
 same elements, they differ from them in being much more 
 complex and in containing hydroxyl and fluorine. Their 
 compositions, as a rule, are too complicated to be repre- 
 sented by simple formulas, but in a general way, disregard- 
 ing the hydroxyl and fluorine, one may say that each 
 type of pyroxene has a corresponding amphibole and in 
 this connection the composition of the pyroxenes should 
 be studied. 
 
 Thus tremolite, if simply represented by CaMg 3 (Si0 3 ) 4 
 corresponds to diopside CaMg(SiO 3 ) 2 : while actinolite, 
 
 Ca(MgFe) 3 (Si0 3 ) 4 , 
 
 with variable amounts of ferrous iron replacing magnesium 
 corresponds to common pyroxene, Ca(MgFe) (SiO 3 ) 2 ; 
 common hornblende or hornblende for short which consists 
 of the actinolite molecule with others in which radicals 
 containing alumina or ferric iron and usually both are 
 present and perhaps some alkalies, corresponds in general 
 to augite which is a variable mixture of pyroxene molecules 
 with alumina and ferric iron; arfvedsonite, which con- 
 tains chiefly soda, lime and ferrous iron, plays the part of 
 aegirite, the soda iron pyroxene, though a very rare 
 
64 
 
 ROCKS AND ROCK MINERALS 
 
 variety, riebeckite, more nearly corresponds in com- 
 position. 
 
 Glaucophane is a rare variety, consisting of a mixture of a soda- 
 alumina molecule with a hypersthene molecule, 
 
 NaAl(SiO 3 ) 2 . (FeMg)SiO 3 . 
 
 It is distinguished from other hornblendes by its blue color, often a 
 rich sky-blue or lavender-blue. It occurs only in a rare variety of 
 hornblende-schists, called glaucophane-schists, which are described 
 under amphibolites. 
 
 The chemical composition is illustrated in the following 
 table of analyses. 
 
 
 Si0 2 
 
 A1 2 3 
 
 Fe 2 3 
 
 FeO 
 
 MgO 
 
 CaO 
 
 Na 2 O 
 
 H 2 O 
 
 F 2 
 
 XyO* 
 
 Total 
 
 I 
 
 57.5 
 
 1.3 
 
 0.2 
 
 0.2 
 
 24.9 
 
 12.8 
 
 0.7 
 
 1.3 
 
 0.8 
 
 0.6 
 
 100.3 
 
 II 
 
 56.1 
 
 1.2 
 
 0.8 
 
 5.5 
 
 21.2 
 
 12.1 
 
 0.2 
 
 1.9 
 
 0.1 
 
 0.6 
 
 99.7 
 
 III 
 
 41.9 
 
 11.7 
 
 2.5 
 
 14.3 
 
 11.2 
 
 11.5 
 
 2.7 
 
 0.7 
 
 0.8 
 
 2.6 
 
 99.9 
 
 IV 
 
 43.8 
 
 4.4 
 
 3.8 
 
 33.4 
 
 0.8 
 
 4.6 
 
 8.1 
 
 0.1 
 
 
 1.5 
 
 100.5 
 
 V 
 
 55.6 
 
 15.1 
 
 3.1 
 
 6.8 
 
 7.8 
 
 2.4 
 
 9.3 
 
 
 
 0.5 
 
 100.6 
 
 * XyO = small quantities of minor components. 
 
 I, Tremolite, Richville, Gouverneur, New York; II, Actinolite 
 Greiner, Tyrol; III, Hornblende, Edenville, Orange County, New 
 York: IV, Arfvedsonite, Kangerdluarsuk, Greenland; V, Glauco- 
 phane, Island of Syra, Greece. 
 
 Blowpipe and Chemical Characters. Tremolite, actino- 
 lite and common hornblende melt quietly or with little 
 intumescence before the blowpipe, fusing rather easily at 
 4. The color of the bead depends on the amount of iron, 
 tremolite nearly colorless, actinolite green or brown; 
 common hornblende dark and shining. Common horn- 
 blende sometimes colors the flame yellow, indicating soda. 
 Arfvedsonite fuses easily (3.5), colors the flame strong, 
 persistent yellow, intumesces decidedly (difference from 
 aegirite) and yields a black, shining, magnetic bead. The 
 
ROCK-MAKING MINERALS 65 
 
 amphiboles are only slightly attacked by the ordinary 
 acids, those rich in iron more than those without. 
 
 Alteration. The amphiboles have methods of alteration 
 similar to those of the pyroxenes. Under the action -of 
 various agencies they may be changed into serpentine or 
 into chlorite or into both, accompanied by the formation 
 of carbonates, sometimes of epidote and also quartz. 
 Under the continued action of weathering they may 
 break down further in limonite, carbonates and quartz. 
 Thus on much weathered rock surfaces only rusty-looking 
 holes and spots may be left to show their former presence. 
 
 Occurrence. Amphiboles are common and widely 
 distributed minerals playing an important role in igneous 
 rocks and especially in the metamorphic ones. The 
 presence of water, hydroxyl and fluorine in them shows 
 that they are not formed by simple reactions like the 
 pyroxenes but require the presence of mineralizing 
 vapors; they are in some sense pneumatolytic minerals. 
 Thus they cannot be artificially formed by allowing simple 
 dry fusions containing their constituent silica and metallic 
 oxides to cool and crystallize; pyroxenes are produced 
 instead of them. And if hornblendes are fused and the 
 melt allowed to crystallize we obtain pyroxenes, iron ore, 
 etc., in their place; this is because the necessary water and 
 fluorine have escaped. 
 
 Tremolite is chiefly found in the impure crystalline limestones and 
 dolomites in the older schistose metamorphic rocks and in contact 
 zones. In such occurrences it not infrequently has an extra- 
 ordinarily fine fibrous structure and is capable of being split into 
 long, flexible fibers of great fineness and strength, forming the 
 greater part of what is known as asbestus. Sometimes actinolite 
 and other hornblendes are found in this asbestus form. Some so- 
 called asbestus is really a fibrous variety of serpentine. 
 
 Actinolite has its true home in the crystalline schists; it is the 
 characteristic light green to bright green amphibole of many horn- 
 blende-schists and greenstones: in many of these cases it is second- 
 ary after original pyroxene of former gabbro and trap rocks as 
 described under uralite. 
 
 Common hornblende occurs both in igneous and metamorphic 
 
66 ROCKS AND ROCK MINERALS 
 
 rocks. It is found in granites, common syenites, and in the doleritic 
 types; is in diorite and some varieties of peridotite. It may also be 
 often observed in the phenocrysts of felsitic intrusive porphyries and 
 lavas. In dark traps and basalt lavas it is rare. In the meta- 
 morphic rocks it is found in gneisses and is the prominent mineral of 
 the hornblende schists. 
 
 Arfvedsonite occurs in nephelite syenites and in rare porphyries. 
 
 Uralite is a fibrous or fine needle-like, columnar hornblende, 
 secondary after pyroxene and as mentioned under that mineral 
 produced from it by metamorphic processes. Instances have been 
 found where the outward crystal form of the pyroxene is retained 
 but the substance composing it is this hornblende in parallel bundles 
 of needle-like prisms. Generally it is in aggregates, which may be 
 very fine and felt-like, lying in the plane of schistosity. It is espe- 
 cially apt to occur when basic, pyroxenic, igneous rocks have been 
 subjected to dynamic changes in the earth's crust attended with 
 squeezing and shearing. It varies in composition from actinolite 
 to common hornblende, depending on the kind of pyroxene from 
 which it was derived. It is clear that it cannot be a simple rearrange- 
 ment of the pyroxene molecule since the latter has twice as much 
 lime as the hornblende and is lacking in the necessary water or 
 fluorine. Lime is separated out in the process to form a carbonate 
 (calcite) or some other mineral and the presence of water, containing 
 often other substances in solution, is a necessary aid to the dynamic 
 processes of pressure and shearing which set up chemical activity 
 and the reactions which produce this mineral. 
 
 In this connection the reader should consult what is said under 
 metamorphism and the hornblende-schists. 
 
 Determination. Amphibole may be confused in mega- 
 scopic work with pyroxene, tourmaline and epidote. To 
 distinguish it from the last two use may be made of the 
 various physical properties mentioned under the deter- 
 mination of pyroxene; the good cleavage separates it at 
 once from tourmaline. The distinction from pyroxene 
 is much more difficult, owing to the fact that these two 
 minerals have similar chemical compositions and physical 
 properties. The following points will be found of service 
 in this connection. If the mineral appears in tolerably 
 distinct crystals the form should be carefully studied, 
 especially the outline of the section of the prism which 
 can often be observed on a fractured surface of the rock 
 and comparison made with Figs. 23 and 28. 
 
ROCK-MAKING MINERALS 67 
 
 In case the crystal form is imperfect or wanting, if it is 
 possible, the angle at which the cleavage surfaces meet 
 should be carefully studied, as this is a fundamental 
 character, the cleavage prism as already described being 
 nearly square in pyroxene and much more oblique in 
 amphibole. Further, the perfection of the cleavage in 
 amphibole and the bright glittering surfaces it yields 
 furnish indications not commonly seen in pyroxene whose 
 cleavage is only fairly good. Amphibole also is apt to 
 occur in needles or long bladed prisms; pyroxene is com- 
 monly in short prismoids or grains. Before the blow- 
 pipe amphibole, on account of the combined water 
 (hydroxyl), is more apt to intumesce than pyroxene 
 (arfvedsonite from aegirite) but this cannot be relied on as 
 a general definite test. If fluorine is obtained by a 
 qualitative test this is also indicative of amphibole, but 
 many do not contain this element and it is not a method 
 which is ordinarily in one's power to make. Finally, in 
 many cases, especially in fine grained igneous rocks, it is 
 impossible by purely megascopic means to tell if the dark 
 ferromagnesian mineral present is hornblende or pyroxene 
 or, as often happens, a mixture of both. Only in a thin 
 section under the microscope can this be certainly deter- 
 mined. This is a limitation which the megascopic method 
 for the study and determination of rocks and rock-minerals 
 imposes. 
 
 OLIVINE. 
 
 Form. Olivine crystallizes in the orthorhombic system ; 
 the crystals are rather complex as illustrated by a common 
 form shown in Fig. 29. The form is not, however, a matter 
 of importance, as the mineral very rarely shows well 
 developed crystals in rocks but occurs in grains or small 
 formless masses composed of grains. 
 
 General Properties. There is a cleavage parallel to the 
 face b but it is not a very perfect or noticeable property 
 in rock grains. The fracture, is conchoidal, The color is 
 
68 
 
 ROCKS AND ROCK MINERALS 
 
 green, generally of a medium shade and varying from 
 olive-green to a yellow-green; a bottle-green is very 
 common. It is often transparent varying 
 to translucent but becomes brown to dark 
 red on oxidation of the iron and more or 
 less opaque; this is frequently noticed in 
 lavas which have been exposed to the action 
 of steam. Luster vitreous; streak white 
 to yellowish. Hardness 6.5-7.0. Specific 
 gravity varies with the iron from 3.3-3.5. 
 Chemical Composition. Olivine is magnesium ortho- 
 silicate, Mg2SiO4, and ferrous orthosilicate, Fe2SiO 4 , which 
 mingle isomorphously in all proportions. The nearly 
 pure magnesium compound is called forsterite, the nearly 
 pure iron compound fayalite; these occur in rocks but are 
 rare. Much more common are variable mixtures of the 
 two which make common olivine or chrysolite as it is 
 often called. These variations may be seen in the fol- 
 lowing table of analyses. 
 
 
 Si0 2 
 
 MgO 
 
 FeO 
 
 XyO* 
 
 Total. 
 
 I . 
 
 41 8 
 
 56 2 
 
 1 1 
 
 7 
 
 99 8 
 
 II .... 
 Ill .... 
 
 IV .... 
 V 
 
 39.9 
 37.2 
 41.9 
 33.6 
 
 49.2 
 39.7 
 
 28.5 
 16 7 
 
 10.5 
 22.5 
 29.2 
 44 4 
 
 50 
 
 99.6 
 99.4 
 99.6 
 99.7 
 
 VI .... 
 
 30.1 
 
 
 68.2 
 
 1.5 
 
 99.8 
 
 * XyO = small quantities of other oxides, chiefly MnO. 
 
 I, Forsterite, Monte Somma, Italy; II, Olivine, Mt. Vesuvius, 
 Italy; III, Olivine, Montarville, Canada; IV, Olivine, Hochbohl, 
 Germany; V, Hortonolite, Monroe, Orange Co., N. Y.; VI, Fayalite, 
 Rockport, Mass. 
 
 Blowpipe and Chemical Characters. Before the blow- 
 pipe nearly infusible; varieties very rich in iron fuse and 
 yield magnetic globules these are apt to turn red on 
 heating. The powdered mineral dissolves in hydrochloric 
 
ROCK-MAKING MINERALS 69 
 
 or nitric acid, yielding gelatinous silica on evaporation. 
 The solution may be tested for iron and magnesium as 
 directed under mineral tests. 
 
 Alteration. In one case this takes place through oxida- 
 tion of the iron, the mineral turns reddish or brownish, 
 and eventually a mass of limonite replaces it, accompanied 
 with carbonates and some form of silica. The rusty iron 
 product is the most noticeable feature of the process. 
 
 A most important mode of alteration is that by which 
 the olivine becomes converted into serpentine. This 
 appears to take place through the agents of weathering 
 near the surface and deeper down through the action of 
 heated waters. This is more fully discussed under the 
 head of serpentine. Other substances such as carbonate 
 of magnesia, iron ores, free silica, etc., are also liable to 
 occur as by-products in the process. Other kinds of 
 alteration of olivine are known but are of less importance 
 in this connection. 
 
 Occurrence. Olivine is a quite characteristic mineral of 
 igneous rocks, especially the ferromagnesian ones. It so 
 rarely occurs in those composed chiefly of alkalic feldspars 
 in the granite-syenite rocks, feldspathic porphyries 
 and felsite lavas that for practical purposes it need not 
 be sought in them. Anorthosite is the only feldspathic 
 rock in which it may become of importance. Thus its 
 true home is in the gabbros, peridotites and basaltic 
 lavas. In the later it usually occurs in bottle-green grains; 
 in the former it is sometimes colored dark by inclusions. 
 It also forms masses of igneous rock known as dunite 
 which consist almost wholly of olivine. Fine trans- 
 parent crystals of olivine from basaltic lavas are fre- 
 quently cut for gems, commonly called peri dotes. The 
 mineral is also often found in meteorites. 
 
 Olivine also occurs in metamorphic rocks, in crystalline lime- 
 stones of dolomitic character and in other rocks found in such 
 associations, composed of varying quantities of other magnesian 
 (and lime) silicates, such as amphibole, pyroxene and talc. Its 
 
70 
 
 ROCKS AND ROCK MINERALS 
 
 origin may be ascribed to a reaction between the magnesium car- 
 bonate of the dolomite and quartz sand or silica-bearing solutions. 
 
 2 MgC0 3 + SiO 2 = Mg 2 SiO 4 + 2 C0 2 
 
 But in many such cases of its occurrence in the crystalline schists., 
 mixed more or less with other silicate minerals, its presence is prob- 
 ably due to the fact that the masses containing it were originally of 
 igneous origin, rather than metamorphosed sedimentary beds. 
 
 Determination. The appearance, associations and 
 characters described above are usually sufficient to readily 
 identify the mineral. It may be confused with greenish, 
 more or less transparent grains of pyroxene, but the lack of 
 pronounced cleavage, the superior hardness and easy 
 gelatinization in acid enable one to distinguish it from that 
 mineral. 
 
 GARNET. 
 
 Form. Garnets crystallize in the isometric system in 
 the simple form of the rhombic dodecahedron shown in 
 
 Fig. 30 or in the trapezo- 
 
 hedron shown in Fig. 31. 
 
 Sometimes, they show 
 
 these forms well developed 
 
 and are then excellent 
 
 crystals, which may be 
 
 more complicated by bev- 
 
 ellings or truncations of 
 the edges of the dodecahedron. Very commonly how- 
 ever the faces are not well developed and the mineral 
 then appears as a spherical mass or grain. 
 
 Cleavage and Fracture. The cleavage is generally poor 
 and not a prominent feature; sometimes a parting, in 
 garnets occurring in sheared rocks, may be seen which 
 suggests a lamellar structure. The fracture is uneven. 
 The mineral is very brittle but some rocks composed 
 largely of massive garnet are very tough. 
 
 Hardness and Specific Gravity. The hardness varies 
 from 6.5-7.5; the specific gravity from 3.55 in grossularite 
 to 4.2 in almandite, common garnet being about 4.0. 
 
 Fig. 30 
 
 Fig. 31 
 
ROCK-MAKING MINERALS 
 
 71 
 
 Color, Luster and Streak. The color depends upon the 
 composition; grossularite is sometimes white but usually 
 tinted pale tones of green, pink or yellow, sometimes 
 yellowish or reddish-brown to brown; pyrope is deep red 
 to black; almandite and most common garnet is deep red 
 to brownish-red; melanite is black. Streak, light-colored, 
 not important. The luster is glassy, sometimes rather 
 resinous. The light-colored garnets are transparent to 
 translucent, the darker ones translucent or opaque. 
 
 Chemical Composition. Garnets are orthosilicates of 
 the general formula RaR^CSiO^a, in which the radical R 
 may be calcium, magnesium, ferrous iron and other 
 bivalent metals, while H may be aluminum, ferric iron or 
 chromium, trivalent elements. There is therefore oppor- 
 tunity for a number of combinations which are isomor- 
 phous. The most common ones which are of importance 
 as rock minerals are grossularite , Ca 3 Al 2 (SiO4)3, pyrope, 
 Mg 3 Al 2 (Si04)3, almandite, Fe 3 Al2(SiO 4 )3 and andradite, 
 Ca3Fe2(SiO 4 )3. These compounds, however, rarely, if 
 ever, occur pure, generally there are variable amounts of 
 the other molecules present and the mineral is named 
 from the one predominating. Common garnet is chiefly 
 almandite with more or less of the others present, especially 
 the andradite molecule, and at times this may predom- 
 inate. Melanite, the black garnet found in some rocks, 
 is chiefly andradite. These facts are illustrated in the 
 following analyses of typical specimens. 
 
 
 Si0 2 
 
 A1 2 3 
 
 Fe 2 3 
 
 FeO 
 
 MgO 
 
 CaO 
 
 XyO 
 
 Total. 
 
 I 
 
 39.8 
 
 22.1 
 
 1.1 
 
 
 0.7 
 
 36.3 
 
 
 100.0 
 
 II . . . 
 
 40.4 
 
 19.7 
 
 4.0 
 
 6.9 
 
 20.8 
 
 5.8 
 
 2.6 
 
 100.2 
 
 Ill 
 
 39.3 
 
 21.7 
 
 
 30.8 
 
 5.3 
 
 2.0 
 
 1.5 
 
 100.6 
 
 IV. . . 
 
 35.9 
 
 19.2 
 
 4.9 
 
 29.5 
 
 3.7 
 
 2.4 
 
 4.8 
 
 100.4 
 
 V . . . 
 
 35.7 
 
 0.1 
 
 30.0 
 
 1.2 
 
 0.1 
 
 32.3 
 
 0.9 
 
 100.3 
 
 I, Grossularite, Hull, Ontario. II, Pyrope, Krems, Bohemia, 
 XyO = Cr 2 O 3 . Ill, Almandite, Fort Wrangell, Alaska, XyO = MnO. 
 IV, Common Garnet (mostly almandite), Shimerville, Penn. 
 
 XyO = MnO; V, Andradite, Sisersk, Ural Mts. 
 
72 ROCKS AND ROCK MINERALS 
 
 Blowpipe and Chemical Characters. The garnets fuse 
 readily before the blowpipe and in the reducing flame 
 those containing much iron become magnetic. After 
 fusion and grinding of the bead to powder they dissolve 
 in hydrochloric acid with gelatinization on boiling. They 
 are slightly attacked by acids, andradite quite strongly. 
 Give little or no water by heating in closed glass tube. 
 Decomposed by fusion with sodium carbonate. 
 
 Alteration. Garnets change into other substances, 
 commonly chlorite, serpentine, etc., and those containing 
 iron oxides may alter into rusty spots of limonite and 
 other products of weathering. 
 
 Occurrence. Common garnet is a widely distributed 
 mineral as an accessory component of metamorphic and 
 sometimes igneous rocks. Its most striking occurrence 
 is in schists, especially in many mica-schists though it is 
 also found in other kinds, in many hornblende-schists and 
 in gneisses for example. It is apt to occur in the ferro- 
 magnesian igneous rocks which have been squeezed and 
 sheared. It is sometimes seen in granite-pegmatites, 
 rarely in granite itself, in occasional scattered crystals. It 
 also occurs in the contact zone of igneous rocks where 
 mixed beds containing clay, calcareous matter and 
 limonite have been metamorphosed. Pyrope, which 
 chiefly furnishes the garnet used as a jewel, is an acces- 
 sory component of some peridotites and the serpentines 
 derived from them. Grossularite is especially found in re- 
 crystallized limestone beds both in contact and regional 
 metamorphism. Melanite occurs mostly in certain 
 igneous rocks and is not an important megascopic 
 mineral. 
 
 Determination. The crystal form of garnets, the 
 appearance, color and hardness are generally sufficient 
 to enable one to easily recognize them and in case of 
 doubt the blowpipe tests will furnish sufficient confir- 
 mation. 
 
ROCK-MAKING MINERALS 
 
 73 
 
 EPIDOTE. 
 
 Form. Epidote crystallizes in the monoclinic system, 
 the simplest form being that shown in Fig. 32, the crystals 
 are apt to be more complex with 
 other faces. Well-developed crystals s c 
 usually occur only in druses in seams \ a / 
 and cavities and the form is there- \ r \ 
 fore not generally a character which Fig. 33 
 
 can be of much use in megascopic rock 
 determination. Commonly seen in bladed prisms extended 
 in the direction of the edge ac and sometimes passing 
 into slender, needle-like forms. Often in bundles or 
 aggregates of prisms or needles. Terminations of prisms 
 often rounded. Also occurs in spherical and angular 
 grains and in aggregates of such grains. 
 
 General Properties. The cleavage is perfect parallel to 
 c, parallel to a imperfect. Fracture uneven. Brittle. 
 Hardness is 6-7. Specific gravity is 3.3-3.5. The 
 color in general is green, usually of a peculiar yellowish, 
 oily green; varying from pistache-green to olive, some- 
 times very dark green; rarely brownish. Luster vitreous. 
 Streak whitish. Translucent to opaque. 
 
 Chemical Composition. Epidote is really the name of a 
 group of complex silicates, salts of orthosilicic acid whose 
 hydrogen atoms are replaced by calcium and by a set of 
 isomorphous radicals composed of variable amounts of 
 alumina, ferric iron and sometimes other oxides and of 
 hydroxyl. Of these only common rock-making epidote 
 is described in this section and its formula may be repre- 
 sented as being mixtures of Ca2(AlOH)Al 2 (Si04)3 and 
 Ca2(FeOH)Fe 2 (SiO 4 )3. The composition may be seen 
 in these two specimen analyses. 
 
 
 Si0 2 
 
 A1 2 3 
 
 Fe 2 O 3 
 
 FeO 
 
 CaO 
 
 H 2 
 
 XyO 
 
 Total. 
 
 I 
 II .... 
 
 37.8 
 37.0 
 
 22.6 
 25.8 
 
 14.0 
 10.0 
 
 0.9 
 1.3 
 
 23.3 
 21.9 
 
 2.1 
 3.0 
 
 1.0 
 
 100.7 
 100.0 
 
 I, Untersulzbach, Pinzgau. II, Macon Co., North Carolina, 
 XyO = MnO and MgO = 0.5 each. 
 
74 ROCKS AND ROCK MINERALS 
 
 Blowpipe and Chemical Characters. Before the blow- 
 pipe epidote fuses easily with intumescence to a black 
 slaggy mass. Intense heating in the closed glass tube 
 causes the finely powdered mineral to give off water. 
 Only slightly acted on by hydrochloric acid but after 
 fusion dissolves and gelatinizes. Reacts with fluxes 
 for iron and decomposes on fusion with sodium car- 
 bonate. 
 
 Occurrence. Epidote is characteristic as a product of 
 alteration of other minerals. It appears through the 
 weathering of igneous rocks which contain largely original 
 lime, iron and alumina silicates and is then usually with 
 chlorite. When igneous rocks of this character also 
 suffer regional metamorphism epidote is apt to form. 
 The occurrences in which it appears most notable from the 
 megascopic view point are those in which mixed sedimen- 
 tary beds containing calcareous matter, with sand clay 
 and limonite (impure limestones) are subjected either to 
 general or contact metamorphism. Then epidote is apt 
 to be formed, usually in company with other silicates, 
 but sometimes so extensively as to form masses which 
 consist almost entirely of this mineral. 
 
 Determination. The peculiar yellow-green color, superior 
 hardness, perfect cleavage in one direction only and 
 the blowpipe characters described above generally suf- 
 fice to distinguish epidote from hornblende, pyroxene 
 and possibly tourmaline with which it might be con- 
 fused. The hardness distinguishes it at once from 
 some varieties of serpentine which resemble it in color. 
 This may be confirmed by a chemical test showing the 
 absence of magnesia as described in the section on 
 mineral testing. 
 
 Zoisite. This is a mineral which has the same chemical 
 composition as epidote and is closely related to it. It 
 consists almost wholly of the lime-alumina molecule pre- 
 viously mentioned and contains little or no iron oxides. 
 It is orthorhombic in crystallization but in the crystals 
 
ROCK-MAKING MINERALS 75 
 
 seen in rocks this can generally only be told by optical 
 methods : it occurs in aggregated blades or prisms, parallel 
 or divergent or in grains and masses. Its color is usually 
 gray of varying shades. From epidotes lacking in iron it 
 can only be told by crystallographic investigations. 
 
 VESUVIANITB. 
 
 Vesuvianite is a tetragonal mineral which generally 
 crystallizes in short thick square prisms terminated by a 
 pyramid commonly cut off by a basal plane 
 as illustrated in Fig. 33. It also occurs 
 in lumps or grains. The cleavage is poor, 
 best parallel to the prism faces m; fracture 
 uneven. The color generally varies from 
 green to brown. The luster is vitreous. 
 Hardness, 6.5. Specific gravity about 3.4. 
 Subtransparent to subtranslucent. The 
 chemical composition of vesuvianite is not exactly known; 
 it is a silicate of calcium and aluminum containing hy- 
 droxyl and fluorine, but small amounts of ferric iron and 
 magnesium are generally present. The formula has been 
 written H 4 Ca 12 (AlFe) 6 Si 10 O43 but this is probably not 
 correct. Before the blowpipe it fuses readily with intu- 
 mescence to a greenish or brownish glass, which gelatinizes 
 with hydrochloric acid. The fresh mineral is slightly 
 soluble in hydrochloric acid. 
 
 As a rock forming mineral vesuvianite characteristically 
 occurs in limestones which have become crystalline 
 through the contact action of igneous rocks and its forma- 
 tion is evidently conditioned by the pneumatolytic 
 emanations of water and fluorine from the igneous mag- 
 mas. In these occurrences it is commonly associated 
 with garnet, pyroxene, tourmaline, chondrodite, and other 
 contact minerals. 
 
 Vesuvianite may be confused with garnet, pyroxene, 
 epidote or hornblende, but the study of its crystal form, 
 
76 
 
 ROCKS AND ROCK MINERALS 
 
 its other physical characters and behavior before the blow- 
 pipe will generally serve to distinguish it from them. 
 
 STAUROLITE. 
 
 Form. Staurolite is orthorhombic in crystallization 
 and usually in distinct crystals of the form shown in Fig. 
 34. They are often stout and thick, sometimes long and 
 more slender but not strikingly so. The angle of the 
 
 Fig. 34 
 
 Fig. 35 
 
 Fig. 36 
 
 faces m on m is 50 40'. They are terminated by flat 
 bases c, though it often happens these cannot be seen 
 in the rock. Staurolite is very apt to form compound 
 twinned crystals as shown in Figs. 35, 36. From this 
 fact its name is derived from the Greek, meaning a cross. 
 
 Physical Properties. The mineral has a moderate but 
 distinct cleavage parallel to the face b; the fracture is sub- 
 conchoidal. The color varies from a dark reddish or 
 yellowish brown to almost black, the light transmitted 
 through thin splinters appears almost blood-red. The 
 streak is white to gray. The hardness is 7-7.5, the specific 
 gravity 3.75. 
 
 Chemical Composition. The formula is rather complex, 
 (AlO) 4 (AlOH)Fe(SiO 4 ) 2 , the alumina may be partly 
 replaced by ferric iron and the ferrous iron by magnesia, 
 as seen in the included analysis of a crystal from Franklin, 
 North Carolina. The percentage of silica is very low; 
 
 SiO 2 A1 2 O 3 Fe 2 3 FeO MgO H 2 Total 
 27.91 52.92 6.87 7.80 3.28 1.59 = 100.37 
 
ROCK-MAKING MINERALS 77 
 
 staurolite is one of the rock-forming silicates containing 
 the least silica and this fact with the high alumina is 
 significant of its place and mode of origin in metamor- 
 phosed clay rocks. 
 
 Blowpipe and Chemical Characters. Staurolite is prac- 
 tically infusible before the blowpipe. It is almost in- 
 soluble in acids. It may be fused with carbonate of soda 
 and the resulting fusion after solution in hydrochloric 
 acid may be tested for alumina, iron and magnesia. It is 
 easily recognized by its color, crystal form, hardness, 
 method of twinning and mode of occurrence. 
 
 Occurrence. Staurolite occurs in the metamorphic 
 rocks; it is a highly characteristic mineral of the crystal- 
 line schists. It is found in mica schists, in certain slates 
 and sometimes in gneiss. Frequently it is associated 
 with dark red garnets in these rocks. 
 
 ANDALUSITE. 
 
 Andalusite is orthorhombic in crystallization and is usually seen 
 in rough prisms, nearly square in cross section. Sometimes the 
 prisms are collected in radiated grpups. The cleavage parallel to 
 the prism is good in other directions poor. Fracture uneven to 
 subconchoidal. The normal color is white to pink or red to brown, 
 but the mineral is very apt to contain impurities, especially particles 
 of carbonaceous matter, which may color it dark or even black. 
 Often these are arranged in a symmetrical manner in the crystal so 
 that the cross section, when it is broken or cut, displays a definite 
 pattern, such as a white cross in a black square. This may help to 
 identify the mineral. It is usually subtranslucent in thin splinters. 
 Brittle. Hardness, 7.5. Specific gravity, 3.2. Streak, whitish. 
 The chemical composition is Al 2 SiO 6 = A1 2 O 3 . SiO 2 . It is insoluble 
 in acids but decomposed by fusion with carbonate of soda. Before 
 the blowpipe it is infusible; after moistening with cobalt nitrate 
 solution it turns a blue color upon intense ignition (as do also cyanite 
 and some other alumina minerals). 
 
 Andalusite is a mineral characteristic of metamorphism, and es- 
 pecially of the contact zones of igneous rocks, such as granite. It 
 is produced by the alteration of clay slates and shales as described 
 on a later page. It occurs in mica schists and gneisses; sometimes 
 though rarely it is found in granite. 
 
78 ROCKS AND ROCK MINERALS 
 
 CYANITE. 
 
 Cyanite usually occurs in long bladcd crystals which rarely show 
 distinct end faces, or in coarsely bladed columnar masses. It is 
 triclinic. It has one very perfect cleavage (parallel to the face a) 
 and another less so (parallel to 6); the angle between these is about 
 74 degrees. The color is white to pure blue, sometimes the center 
 of the blade is blue with white margins ; rarely gray, green to black. 
 Streak whitish. Transparent to translucent. Luster vitreous to 
 pearly. Hardness varies in different directions from 5-7; least on 
 face a (best cleavage) greatest on face b (second cleavage). Specific 
 gravity, 3.56-3.67. Chemical composition, Al 2 SiO 5 , and other 
 chemical and blowpipe properties similar to those of andalusite, 
 mentioned above. 
 
 Cyanite is a mineral characteristically developed in regions sub- 
 jected to intense regional metamorphism. It occurs in gneisses and 
 in mica schists. In the latter case the mica is sometimes muscovite 
 and sometimes the soda-bearing variety, paragonite. It is often 
 associated with garnet, sometimes with staurolite or corundum. It 
 alters to talc and steatite. 
 
 Cyanite is easily distinguished from other minerals, especially 
 andalusite which has the same chemical composition by its form, 
 color, variable hardness, specific gravity and other properties. 
 
 Sillimanite. With andalusite and cyanite may be mentioned 
 sillimanite, a mineral which has a chemical composition identical 
 with them, but which is separated mineralogically because it has a 
 different crystal form as shown by its angles. Its chief importance 
 is microscopical, but it may sometimes be seen with the eye or lens, 
 mostly in gneisses or quartzites, as slender white or light-colored, 
 four-sided prisms, or radiated aggregates of them forming brushes. 
 Its blowpipe and other chemical characters are like those described 
 for andalusite and cyanite. 
 
 TOURMALINE. 
 
 Tourmaline is a mineral of which there are a number of 
 varieties based on the color, which in turn depends on the 
 chemical composition. The chief ones are black, green, 
 brown and red, but of these the black variety known also as 
 schorl is the only one which is of importance in mega- 
 scopic petrography. 
 
 Form. Tourmaline crystallizes in the rhombohedral 
 division of the hexagonal system. The faces therefore 
 
ROCK-MAKING MINERALS 
 
 79 
 
 are in threes or multiples of three. A simple form is 
 shown in Fig. 37 and its appearance looking down upon 
 the upper end in Fig. 39. It consists of the 
 three-cornered prism m its edges bevelled 
 by the prism faces a and terminated by 
 the rhombohedron r. The crystals if well 
 developed are apt to be more complicated 
 than this, other faces being present and if 
 both ends are perfect they have unlike faces. 
 Though sometimes short and thick the crys- 
 tals are commonly elongated prisms, often 
 extremely long and thin. Very often also 
 the faces a and m oscillate or repeat so 
 that the prism is striated or channeled as 
 shown in Fig. 38 and the outline and appear- 
 ance from above is that seen in Fig. 40. This 
 spherical triangle cross section is very characteristic of the 
 prisms of rock-making tourmaline. It is rarely in form- 
 
 Fig. 37 
 
 Fig. 39 
 
 Fig. 40 
 
 less grains or large shapeless masses. The slender prisms 
 and needles are apt to be aggregated together into bundles, 
 sheaves and radiate groups. The section of the latter 
 in rocks furnishes the so-called " tourmaline suns." 
 
 General Properties. Tourmaline has no good cleavage 
 and its fracture is rather conchoidal to uneven. It is 
 brittle. The color is black, the luster glassy, sometimes 
 dull, streak uncolored, not characteristic. Opaque. The 
 
80 
 
 ROCKS AND ROCK MINERALS 
 
 hardness is 7-7.5, the specific gravity 3.13.2. It becomes 
 electrified by friction. 
 
 Chemical Composition. Tourmaline is a very complex 
 silicate of boron and aluminium with hydroxyl and some- 
 times fluorine and with magnesium, iron and sometimes 
 alkali metals. It may be said to be a salt of an alumin- 
 ium-borosilicic acid in which the hydrogens are re- 
 placed by iron, magnesium, alkalies and aluminium in 
 varying amounts. This acid has been formulated as 
 H 9 Al 3 (BOH) 2 Si 4 O 13 and in common black tourmaline 
 the hydrogens are replaced mostly by iron or iron and 
 magnesia as shown in the analyses here given. 
 
 
 SiO 2 
 
 A1 2 3 
 
 Fe 2 3 
 
 FeO 
 
 MgO 
 
 Na 2 O 
 
 B 2 3 
 
 H 2 
 
 XyO 
 
 Total. 
 
 I. 
 
 35.0 
 
 34.4 
 
 1.1 
 
 12.1 
 
 1.8 
 
 2.0 
 
 9.0 
 
 3.7 
 
 0.6 
 
 99.7 
 
 II . 
 
 35.6 
 
 25.3 
 
 0.4 
 
 8.2 
 
 11.1 
 
 1.5 
 
 10.1 
 
 3.3 
 
 4.3 
 
 99.8 
 
 I, Paris, Maine. II, Pierrepont, 
 quantities of other oxides, etc. 
 
 New York. XyO = small 
 
 Blowpipe and Chemical Characters. Difficultly fusible 
 before the blowpipe with swelling and bubbling. When 
 mixed with powdered fluor spar and bisulphate of potas- 
 sium momentarily colors the flame a fine green, showing 
 presence of boron. Decomposed on fusion with sodium 
 carbonate. Not acted on by acids but after fusion gelat- 
 inizes in hydrochloric acid. 
 
 Occurrence. Tourmaline is not a common megascopic 
 component of rocks, but it is of interest and importance 
 because it is . perhaps the most common and typical 
 mineral which is produced in the pneumatolytic or 
 fumarole stage of igneous rock formation as described in 
 another place. This is shown by the boron, hydroxyl and 
 fluorine which it contains. Thus it is one of the most 
 common and characteristic accessory minerals found in the 
 pegmatite dikes associated with intrusions of granites; its 
 
ROCK-MAKING MINERALS 81 
 
 presence in granite indicates, as a rule, nearness to the 
 contact and in the rocks which have suffered contact 
 metamorphism it is very liable to appear. In this way 
 it is not infrequently found associated with certain ore 
 deposits. It appears at times also in gneisses, in schists 
 and in crystalline limestones of the metamorphic rocks 
 and its occurrence in these cases indicates that the meta- 
 morphism has been induced in part by the contact action 
 of igneous masses giving off water vapors and other 
 volatile substances. The beautiful red and green trans- 
 parent tourmalines which are valued as gem material 
 occur in pegmatite dikes of granite, often associated with 
 the common black variety. The red is usually found with 
 lepidolite the lithia mica. 
 
 Determination. The black color, crystalline form, and 
 mode of occurrence of common tourmaline are usually 
 sufficient to identify it. From black hornblende it is 
 easily distinguished by its lack of good cleavage, superior 
 hardness and the shape of the cross section of the prism and 
 this can be made certain by the blowpipe test for boron. 
 
 TOPAZ. 
 
 Topaz crystallizes in the orthorhombic system and the 
 form in which it is generally seen is in pointed prisms, as 
 illustrated in Fig. 41. There is a very 
 perfect cleavage parallel to the base c, at 
 right angles to the prism: the fracture is 
 uneven. The mineral is very hard = 8, 
 and brittle. The specific gravity is about 
 3.5. In color it is, while generally trans- 
 parent, often colorless, sometimes yellow 
 to brown - yellow, sometimes white and 
 translucent. The luster is vitreous. The 
 chemical composition as established by 
 Penfield is (AlF 2 )SiO 4 in which the fluorine may be 
 replaced in part by hydroxyl ( OH). Before the 
 blowpipe it is infusible. If fused in a closed tube with 
 
82 ROCKS AND ROCK MINERALS 
 
 previously fused and powdered phosphorus salt hydro- 
 fluoric acid will be given off which etches the glass and 
 deposits a ring of silica on the colder upper walls of the 
 tube. If the pulverized mineral be moistened with 
 cobalt nitrate solution and intensely heated before the 
 blowpipe on charcoal it assumes a fine blue color showing 
 presence of alumina. 
 
 Topaz, while not a common or important rock-forming mineral, 
 is a very interesting one as it is particularly characteristic of the 
 pneumatolytic stage in the formation of igneous rocks. 
 
 Thus it is found in crystals in the miarolitic cavities of granites 
 where the vapors have collected and in the same way in felsite lavas 
 (especially in rhyolite). It is also found in pegmatite dikes and in 
 the cracks and crevices of the surrounding rocks which have served 
 as channel ways for the escape of gases as explained under the 
 description of pegmatite dikes and of contact metamorphism. It 
 is apt to be associated in these occurrences with quartz, mica, tour- 
 maline and sometimes with cassiterite, tin ore. 
 
 The form, color, cleavage and great hardness of topaz, together 
 with its mode of occurrence, serve to readily distinguish it from 
 other minerals and the determination may be confirmed by the 
 chemical tests mentioned above. 
 
 CHONDRODITE. 
 
 Chondrodite is really one of a small group of minerals chon- 
 drodite, humite, clinohumite, etc. which are so closely allied in 
 all of their general properties that for practical megascopic rock 
 work they are indistinguishable and may all be comprised under 
 this heading. While the mineral is monoclinic it rarely shows, as 
 a rock component, any definite crystal form which is of value in 
 determining it, but appears as embedded grains and lumps. The 
 cleavage is not marked but is sometimes distinct in one direction. 
 Brittle; fracture subconchoidal. The color is yellow, honey yellow 
 to reddish yellow, to brown red. Luster vitreous. Hardness 
 6-6.5. Specific gravity 3.1-3.2. In chemical composition the 
 mineral is closely allied to olivine but differs in containing fluorine or 
 hydroxyl or both, as may be seen from the following formulas deduced 
 by Penfield. 
 
 Olivine = Mg 2 SiO 4 . 
 
 Chondrodite = M gg [Mg(F,OH)] 2 (SiO 4 ) 2 . 
 Humite = M g5 [Mg(F,OH)] 2 (Si0 4 ) 3 . 
 Clinohumite = Mg 7 [Mg(F,OH)] 2 (SiO 4 ) 4 . 
 
ROCK-MAKING MINERALS 83 
 
 As in olivine part of the magnesium is usually replaced by some 
 ferrous iron. The powdered mineral is slowly dissolved by hydro- 
 chloric acid, yielding gelatinous silica. The solution evaporated to 
 dryness, then moistened with acid and taken up in water, after the 
 silica is filtered off, yields tests for iron with excess of ammonia and 
 after its separation by filtering, for magnesia with sodium phosphate 
 solution. From olivine it may be distinguished by a test for fluorine 
 as described under topaz. Before the blowpipe it is nearly infusible. 
 
 The characteristic mode of occurrence of chondrodite is in lime- 
 stone, especially dolomite, which has been subjected to contact 
 action of igneous rocks. In them it forms yellowish or reddish 
 embedded grains or lumps usually associated with other contact 
 minerals such as pyroxene, vesuvianite, magnetite, spinel, phlo- 
 gopite, etc. The presence of the hydroxyl and fluorine shows its 
 derivation by pneumatolytic processes. 
 
 The appearance, color, mode of occurrence and associations are 
 usually sufficient to identify the mineral, and these may be con- 
 firmed by the chemical tests mentioned. 
 
 6. Oxides, etc. 
 
 The list of important rock-making oxides includes first, 
 silica, SiO 2 and then corundum, A1 2 O 3 . Then come the 
 oxides of iron which are of importance as nearly constant 
 accessory minerals in rocks and therefore have a wide 
 distribution. For this reason two other minerals not 
 oxides, pyrite, the sulphide of iron, and apatite, the phos- 
 phate of lime, are also here included. Limonite, the 
 hydrated oxide of iron, which is always secondary, is 
 placed with the other iron ores for the sake of convenience. 
 
 QUAKTZ. 
 
 Form. Quartz crystallizes in the hexagonal system, 
 the ordinary form being a hexagonal prism terminated by 
 a six-sided pyramid. This form, which is the common one 
 for the crystals of veins and is illustrated in Fig. 42, is not 
 often seen in the quartzes of rocks, except in igneous rocks 
 which possess miarolitic or drusy cavities; into them the 
 rock-making quartzes project with free ends which show 
 crystal form. The large crystals seen in pegmatite veins 
 and which sometimes attain huge dimensions are only a 
 
84 
 
 ROCKS AND ROCK MINERALS 
 
 Fig. 42 
 
 Fig. 43 
 
 manifestation of the same thing on a larger scale, as 
 explained under the pegmatite formation of igneous rocks. 
 In porphyries where quartz may have crys- 
 tallized free as phenocrysts it tends to take 
 the form shown in Fig. 43; the two pyramids 
 are present and the prism is very 
 short or even wanting. Since the 
 crystals are usually poorly devel- 
 oped, with rough faces, they appear 
 as spherical objects, like shot or 
 peas, embedded in the rock, with 
 round cross sections where broken 
 across on a fracture face. In general, quartz has no 
 definite form in rocks, especially in igneous ones like 
 granite, where, being usually the last substance to crys- 
 tallize, its shape is conditioned by the other minerals 
 which have already formed. In granites, therefore, 
 it commonly appears in small shapeless lumps and 
 masses, but in some of the fine-textured varieties the 
 quartz tends to appear in granules like those composing 
 lump sugar. In pegmatite dikes it appears on fracture 
 surfaces in curious script-like figures intergrown with 
 feldspar, forming the substance known as graphic granite. 
 Cleavage and Fracture. The cleavage of quartz is so 
 poor that for practical petrographic purposes it may be 
 regarded as not possessing any. It has commonly a good 
 conchoidal fracture which is a great help in distinguishing 
 it in granitic rocks but in some massive forms it is uneven 
 and splintery. The mineral is brittle to tough. 
 
 Color and Luster. Rock-making quartz varies in color 
 from white through shades of gray and dark smoky gray 
 or brown to black. The gray and smoky tones are most 
 common in igneous rocks and the white color in the sedi- 
 mentary and metamorphic ones but there is no absolute 
 rule about this. The black color is rare and mostly con- 
 fined to igneous rocks; sometimes in them it has a strong 
 bluish tone. The colorless, limpid quartz, so characteristic 
 
ROCK-MAKING MINERALS 85 
 
 of the crystals found in veins and geodes and deposited by 
 solution, is rare as a rock-making component but some- 
 times occurs as in some very fresh lavas. The mineral 
 may also at times possess an exotic color given it by some 
 substance acting as a pigment; thus it may be red from 
 included ferric oxide dust or green from scales of chlorite, 
 and in the sedimentary and metamorphic rocks, such as 
 quartzite,it may be very dark from included organic matter 
 or charcoal-like substance. 
 
 The luster varies from glassy to oily or greasy. The 
 streak is white or very pale colored and not a prominent 
 character. Hardness, 7. Scratches feldspar and glass 
 but is not touched by the knife. Specific Gravity = 2.66. 
 
 Composition. Pure silica, Si0 2 . This is the composi- 
 tion of the crystallized common rock-making quartz, but 
 certain massive varieties of silica, which are not crystallized 
 or not apparently so, and are of common occurrence and 
 sometimes take part in forming rocks, such as jasper, 
 opal, chert, etc., contain in addition more or less com- 
 bined water, while impurities like clay, oxides of iron, 
 etc., are usually present and give them distinctive colors. 
 
 Blowpipe and Chemical Characters. Quartz is infusible 
 before the blowpipe varieties dark from organic matter 
 whiten but do not fuse. Fused with carbonate of soda, 
 it dissolves with effervescence of CC>2 gas. In the sodium 
 metaphosphate bead a fragment floats without dissolv- 
 ing. It is insoluble in acids except hydrofluoric, HF. 
 
 Occurrence. Quartz is one of the commonest of all 
 minerals, and is universally distributed, occurring in 
 igneous, sedimentary and metamorphic rocks alike. Not 
 only does it form rocks in company with other minerals, 
 chiefly feldspar, but in pure sandstones and quartzites 
 it may be the only one present in the rock-mass. It is 
 indeed so common that, with the exception of the lime- 
 stones and marbles and dark heavy igneous rocks like 
 dolerite and basalt, its presence in rocks should at least 
 always be suspected. 
 
86 ROCKS AND ROCK MINERALS 
 
 Determination. The hardness of quartz, its lack of 
 cleavage, its conchoidal fracture and generally greasy 
 luster are characters which help to distinguish it, especially 
 from the feldspars with which it is so often associated. 
 The gray and smoky color it often has in granites and 
 other igneous rocks helps in the same way. It may be 
 confused with nephelite but this mineral is readily soluble 
 in acids with gelatinization and moreover is very rare. 
 These characters, with the blowpipe and chemical ones 
 mentioned above, will readily confirm its determination. 
 
 Opal, Jasper, Flint, Chalcedony, etc. Silica, in addition to 
 forming the crystallized anhydrous mineral quartz, occurs in non- 
 crystalline, amorphous masses which contain varying amounts of 
 water. Accordingly as the color, structure and other properties 
 vary, a great number of different varieties are produced which have 
 received particular names. For a description of them the larger 
 manuals of mineralogy should be consulted. They seem to have 
 been formed, in large part at least, by the evaporation of liquids 
 containing soluble silica, which on the drying down has been 
 deposited in an amorphous, more or less hydrated condition instead 
 of as crystalline quartz. Sometimes they are a mixture of quartz 
 particles or fibers mixed with amorphous material. This form of 
 silica is illustrated by the gelatinous product obtained when a 
 silicate like nephelite is dissolved in an acid and the resulting solution 
 evaporated. It is also formed in nature as a secretion from water 
 by various living organisms. 
 
 Amorphous silica is not a rock component of any megascopic 
 importance in igneous or metamorphic rocks, but in the sedimentary 
 ones it forms accompanying masses and sometimes beds which, al- 
 though not of wide general importance, may be of considerable local 
 interest and value. These are further noticed in their appropriate 
 places. It may also act as a cementing substance of the grains of 
 some rocks. 
 
 CORUNDUM. 
 
 Form. The crystallization is hexagonal and the form 
 assumed is either a thick six-sided prism often swelling out 
 in the middle into barrel-like shape or in thinner six-sided 
 tables; also commonly in grains or shapeless lumps. 
 The thick and barrel forms are most common when it 
 
ROCK-MAKING MINERALS 87 
 
 occurs in massive rocks like the syenites, and they are 
 associated with the grains and lumps. Sometimes on 
 parting faces a multiple twinning resembling that illus- 
 trated as occurring on feldspars may be observed, pro- 
 duced however by another method. 
 
 Cleavage. Corundum does not have a good cleavage 
 but possesses a parting that appears like perfect cleavage 
 parallel to the base of the prism and also in three other 
 directions at an angle to it (parallel to the unit rhombo- 
 hedron). In large pieces these partings or pseudo- 
 cleavages may appear nearly at right angles and the 
 mineral has a laminated structure. 
 
 Color, Luster, and Hardness. Rock-making corundum 
 is usually dark gray to bluish gray or smoky. It is very 
 rarely blue forming the variety sapphire, while the red 
 variety or ruby is excessively rare. The luster is adaman- 
 tine to vitreous, sometimes dull and greasy in rock grains. 
 Translucent to opaque. It is the hardest of rock minerals 
 = 9. Brittle, though sometimes very tough. Specific 
 Gravity = 4. 
 
 Blowpipe and Chemical Characters. Before the Blow- 
 pipe it is infusible. The powder moistened with cobalt 
 solution and intensely ignited turns bright blue showing 
 alumina. It is insoluble in acids. Its composition is 
 pure alumina, A1 2 O 3 . By the action of weathering and 
 alteration it is apt to change into muscovite. 
 
 Occurrence. In recent years corundum has been 
 recognized as an important primary mineral in the igneous 
 rocks of a number of regions, in syenites in Canada, 
 Montana and India, in peridotites in North and South 
 Carolina and Alabama, and in other igneous rocks in the 
 Urals and in California. An unrecorded occurrence is in 
 syenite in Orange County, New York State. Many more 
 such will doubtless be discovered. The variety sapphire 
 has been found in basaltic rocks in Montana, the Rhine 
 district and elsewhere. Corundum also occurs in the 
 contact zone of igneous rocks. In these cases it is usually 
 
88 ROCKS AND ROCK MINERALS 
 
 in thin tabular crystals. It also occurs in metamorphic 
 rocks, sometimes in thick beds of the variety called 
 emery. Probably in many of these occurrences it ante- 
 dates the period of metamorphism and is of igneous 
 origin. 
 
 Determination. The crystal form, when present, and 
 color indicate the presence of this mineral which is readily 
 confirmed by a test of its hardness since it cannot be 
 scratched by another of the rock minerals. These tests 
 may be confirmed by the other described properties. 
 
 THE IRON ORES. 
 
 The term ore is commonly applied to the oxides, sul- 
 phides and carbonates of the heavy metals as the sources 
 from which they are obtained in commercial quantities. 
 Of these minerals the only ones, which by reason of their 
 wide distribution and common occurrence as components 
 of rocks, may be considered of general importance from 
 the petrological standpoint are the oxides and sulphides 
 of irpn. Even these play only a subordinate role in 
 rock-making and are considered as accessory minerals, 
 except in certain cases where they have been concen- 
 trated by geologic processes into considerable masses. 
 They are considered accessory because, in one form or 
 another, they are found scattered in small quantities 
 through most rocks and in each of the three great classes 
 of rocks and do not therefore have the' same importance 
 and value in classification that those minerals, such as 
 feldspars and pyroxenes have, which occur in large and 
 varying amounts. They are mentioned here because they 
 are the most common of accessory rock-minerals and are 
 of importance in other ways as well. They include 
 magnetite, ilmenite, hematite, limonite and pyrite. Th^re 
 are other oxides and sulphides of iron but they are rela- 
 tively of small petrographic importance. 
 
KOCK-MAKING MINERALS 
 
 89 
 
 MAGNETITE. 
 
 Form. Magnetite crystallizes in the isometric system, 
 most commonly in octohedrons, Fig. 44, sometimes in 
 dodecahedrons, Fig. 45, sometimes in a combination of 
 
 Fig. 44 
 
 Fig. 45 
 
 Fig. 46 
 
 both, Fig. 46. It is sometimes seen in distinct crystals 
 in rocks but usually is in small grains whose form cannot 
 be made out and is sometimes in larger irregular masses. 
 
 General Properties. No distinct cleavage but some- 
 times a parting parallel to the octahedral faces resembling 
 cleavage. Fracture, uneven. Brittle. Color, dark gray 
 to iron-black; opaque; luster, metallic, fine to dull. 
 Resembles often bits of iron or steel in the rocks. Streak, 
 black. Magnetic. Hardness, 5.5-6.5. Specific gravity, 
 5.2. The chemical composition is Fe 3 O 4 = FeO . Fe 2 O 3 , 
 or FeO =31.0 per cent, Fe 2 O 3 = 69.0. Difficultly fusi- 
 ble before the blowpipe and in the oxydizing flame be- 
 comes non-magnetic. Slowly soluble in hydrochloric acid. 
 
 Occurrence. Magnetite is one of the most widely 
 distributed of all minerals. It is found in all kinds of 
 igneous rocks, usually in small grains, but sometimes 
 segregated into considerable masses. It occurs also in 
 rocks produced by contact metamorphism and in the 
 crystalline schists, sometimes in large bodies. It is 
 uncommon in the unmetamorphosed sedimentary rocks. 
 It is one of the most important ores of iron. 
 
 Determination. The appearance of magnetite in small 
 dark metallic-looking particles is usually sufficient to dis- 
 tinguish it in the rocks, and this may be confirmed by a 
 
90 ROCKS AND ROCK MINERALS 
 
 test of its hardness, streak and magnetism, together with 
 the other properties described above. It is not liable to 
 be confused with any other mineral except ilmenite. 
 
 SPINELS. 
 
 Magnetite may be regarded as the type of a group of minerals 
 known as the spinels. They have the general chemical composition 
 RO . R 2 O 3 and crystallize in isometric octahedrons as illustrated in 
 magnetite. In them the RO is either MgO, FeO, MnO or ZnO or 
 mixtures of them; R 2 O 3 is Fe 2 O 3 , A1 2 O 3 or Cr 2 O 3 or mixtures of 
 them. True spinel is MgAl 2 O 4 (MgO . A1 2 O 3 ) and when trans- 
 parent and of good color is sometimes cut as a gem. Hercynite is 
 iron spinel, FeAl 2 O 4 , and chromite is FeCr 2 4 , more or less mixed 
 with other spinel molecules. Depending on their composition the 
 spinels have various colors, black, green, red and gray. They are 
 extremely hard, 7-8, without good cleavage and are of high luster 
 to pitchy. Some of the spinels are constituents of igneous rocks, 
 especially of those low in silica and rich in iron and magnesia like 
 peridotite and dunite; others are found in metamorphic rocks, 
 especially those produced by contact metamorphism. In all cases 
 they form only accessory and not important components of the 
 rocks and, except in some contact rocks, are rarely found in crystals 
 sufficiently large to make them of megascopic importance. 
 
 ILMENITE. 
 
 General Properties. Ilmenite crystallizes in the hex- 
 agonal system like hematite, but it is so rarely seen in 
 good megascopic crystals in rocks that its crystal form 
 is not a matter of importance. It usually occurs in 
 embedded grains and masses, sometimes in plates of 
 irregular to hexagonal outline. No cleavage; fracture, 
 conchoidal; brittle. Color, iron-black, sometimes with 
 faint reddish to brownish tinge; luster, submetallic; streak, 
 black to brownish-red. Opaque. Hardness, 5-6. Spe- 
 cific gravity, 4.5-5. Composition, FeTi0 3 = FeO . Ti0 2 . 
 FeO = 47.3, TiO 2 = 52.7. Is not generally pure, but 
 more or less mixed with hematite, Fe2O 3 , with which it 
 is isomorphous. Before the blowpipe very difficultly 
 fusible; in the reducing flame becomes magnetic. After 
 fusion with carbonate of soda can be dissolved in hydro- 
 
ROCK-MAKING MINERALS 91 
 
 chloric acid and the solution boiled with tin becomes 
 violet showing titanium. Fresh mineral difficultly soluble 
 in acids; decomposed by fusion with bisulphate of potash. 
 The solutions give reaction for iron with potassium 
 ferricyanide. The test for titanium is the safest method 
 to determine the mineral. 
 
 Occurrence. Ilmenite or titanic iron ore, as it is often 
 called, is a widely spread mineral occurring as a common 
 accessory mineral in igneous rocks in the same manner 
 as magnetite which it often accompanies. In the same 
 way it is found in gneisses and schists. Unless the 
 embedded grains are of such size that they can be safely 
 tested it cannot usually be discriminated from that 
 mineral by simple inspection. The most important 
 megascopic occurrences are in the coarser grained gabbros 
 and anorthosites where the mineral is very common and is 
 indeed not infrequently segregated in places into such 
 large beds and masses that it would be a useful ore of iron 
 if some method of profitably smelting it could be dis- 
 covered. 
 
 HEMATITE. 
 
 Form. Hematite crystallizes in the rhombohedral 
 division of the hexagonal system but is so rarely in distinct 
 well-formed crystals of observable size as a rock con- 
 stituent that this is not a matter of practical importance. 
 
 It occurs as a rock-mineral in three different forms: as 
 specular iron ore, micaceous hematite, and as common red 
 hematite. 
 
 In the first case it forms masses and plates, the latter 
 sometimes hexagonal in outline. Its color is black to 
 steel-gray with sometimes a faint reddish tone. It is 
 opaque, has a metallic luster which is sometimes very fine 
 or splendent so that it resembles polished steel or iron, at 
 other times it is rather dull but metallic-looking. Fracture, 
 subconchoidal ; no cleavage. 
 
 As micaceous hematite it is in thin flakes which some- 
 what resemble mica; often they are so thin as to be trans- 
 
92 ROCKS AND ROCK MINERALS 
 
 lucent and then have a deep red color. The luster is 
 submetallic to metallic, sometimes splendent like the 
 specular form. The thin leaves are usually of ragged 
 outlines but sometimes hexagonal. 
 
 Common red hematite does not appear crystallized. 
 The mineral is massive, sometimes columnar or granular, 
 often in stalactitic or mamillary forms and sometimes 
 earthy. It is dull, without metallic luster, opaque and 
 of a dark red color. 
 
 General Properties. The streak of hematite is of a red 
 color, from bright Indian red to brownish red and fur- 
 nishes the most convenient method of distinguishing it 
 from magnetite and limonite. Before the blowpipe it is 
 very difficultly fusible except in very fine splinters. After 
 heating in the reducing flame magnetic. Dissolves slowly 
 in hydrochloric acid and the solution gives reactions for 
 iron. 
 
 The composition is Fe2O 3 , ferric oxide. The hardness 
 varies from 5.56.5; specific gravity of the specular 
 variety is 5.2. 
 
 Occurrence. Hematite is one of the most widely diffused 
 of minerals. The specular variety is a common accessory 
 component of feldspathic igneous rocks, such as granite. 
 It is also found in the crystalline schists, often in thick 
 beds and masses. 
 
 Micaceous hematite occurs in the crystalline schists 
 in megascopic form, as in itabirite, and also in minute 
 microscopic scales it is the red coloring matter found in 
 igneous and metamorphic rocks. The red color of many 
 potash feldspars is due to it and so is that of many 
 
 Common red hematite is found in sedimentary and 
 metamorphic rocks in beds and masses, often of great 
 size and forming one of the most valuable ores of iron. 
 It is the interstitial cement of many stratified rocks, such 
 as red sandstones, and as a red pigment in the form of 
 powder it is everywhere distributed in all classes of rocks 
 
ROCK-MAKING MINERALS 93 
 
 and in soils, though possibly in some cases it may be 
 replaced by turgite (hydrohematite), 2Fe 2 O 3 .H 2 O, which 
 often closely resembles it. Earthy red hematite, usually 
 more or less mixed with clay, is called red ocher. 
 
 LIMONITE. 
 
 Form. Limonite does not crystallize, but occurs in 
 earthy formless masses in the rocks, and when found in 
 considerable deposits very frequently exhibits compact 
 stalactitic or mammillary shapes which have a fibrous or 
 radiating structure and are sometimes concretionary; 
 sometimes in earthy beds or deposits. 
 
 General Properties. No cleavage. Luster of compact 
 varieties often silky to sub-metallic, but generally dull 
 and earthy. Color, various shades of brown from very 
 dark to brownish yellow. The surface of the compact 
 stalactitic or mammillary forms often has a varnish-like 
 skin. Opaque. Streak, yellow-brown. The hardness of 
 the compact mineral varies from 5-5.5 and the specific 
 gravity from 3.6-4.0. The composition is 2 Fe 2 O 3 .3 H 2 O 
 or Fe 2 (OH) 6 .Fe 2 O 3 , partly dehydrated ferric hydroxide. 
 Fe = 59.8; O = 25.7; H 2 O = 14.5 = 100. Difficultly 
 fusible before the blowpipe; becomes magnetic in the 
 reducing flame. Heated in closed glass tube gives off 
 water. Slowly soluble in hydrochloric acid, the solution 
 giving reactions for iron. The yellow streak is the most 
 convenient means of distinction from hematite. 
 
 Occurrence. Limonite occurs in several different ways. 
 In all cases it is strictly a secondary substance formed at 
 the expense of previously existing minerals, by the various 
 agencies of weathering and alteration. In igneous and 
 metamorphic rocks it is frequently seen as small, earthy, 
 yellowish to brownish masses which represent the decay 
 of some previous iron-bearing mineral, such as pyrite, 
 hornblende, etc. Accumulated in beds, as explained under 
 sedimentary rocks, it frequently has the compact form 
 with stalactitic and mamillary or concretionary structure. 
 
94 
 
 ROCKS AND ROCK MINERALS 
 
 As bog iron ore it is loose, porous and earthy. Mixed 
 with more or less clay it forms yellow ocher and is the 
 yellow pigment of many soils and sedimentary rocks. 
 
 PYRITE. 
 
 Form. Pyrite almost invariably occurs in crystals in 
 the rocks, very seldom in grains and masses. It crystal- 
 lizes in the isometric system. It is frequently seen in 
 cubes or in the twelve-sided form seen in Fig. 47 and called 
 the pyritohedron because this mineral so commonly shows 
 it. Combinations of the two are also very common as 
 
 Fig. 47 
 
 Fig. 48 
 
 Fig. 49 
 
 Fig. 50 
 
 shown in Fig. 48. Very often the cubic faces are striated 
 by fine lines as seen in Fig. 50 produced by oscillating or 
 repeating combinations of the pyritohedron on the cube 
 faces. The octahedron is less frequent and is apt to be 
 modified by the pyritohedron combining with it as in 
 Fig. 49. Other more complex forms also occur. 
 
 General Properties. No good cleavage; fracture, con- 
 choidal to uneven. Color, brass yellow; luster, metallic, 
 splendent, duller when tarnished. Opaque. Streak, 
 greenish to brownish black. Hardness, 6-6.5; specific 
 gravity, 5.0. Composition, FeS 2 ; iron = 46.6, sulphur = 
 53.4 = 100. Easily fusible before the blowpipe, burning 
 and giving off sulphur dioxide gas, and leaving a magnetic 
 globule. In the closed glass tube on heating gives a sub- 
 limate of sulphur and leaves a magnetic residue. Insol- 
 uble in hydrochloric but decomposes in boiling nitric 
 acid with separation of sulphur. 
 
 The color and crystallization are usually sufficient to 
 
ROCK-MAKING MINERALS 95 
 
 at once identify pyrite and distinguish from other rock 
 minerals. From pyrrhotite, FenSi2, and chalcopyrite, 
 FeCuS2, other sulphides of iron which occasionally may 
 be seen in rocks, the test of hardness discriminates it 
 from chalcopyrite (3.5) which can be readily scratched 
 with the knife and gives reactions for copper; pyrrhotite 
 has a bronze color, is also scratched by the knife and gives 
 little or no sulphur in the closed tube. 
 
 Occurrence. Pyrite is a mineral which has many 
 different modes of origin and in consequence is found in 
 all kinds of rocks as a scattered accessory component, 
 usually in small distinct crystals, less commonly aggre- 
 gated. The largest masses are found in ore deposits, 
 chiefly formed in contact zones of igneous rocks through 
 the action of mineralizing solutions. In igneous rocks it 
 appears as a primary product of crystallization from the 
 molten magma. In sedimentary rocks it is frequently 
 found replacing fossils, and its occurrence must be due to 
 reactions between the sulphur of albuminous materials 
 of organic life and the iron in the rocks. It is common in 
 coal seams. 
 
 APATITE. 
 
 Apatite crystallizes in hexagonal prisms either rounded at the 
 ends or capped by a six-sided pyramid. It is scratched by the 
 knife, has a vitreous luster and is white to green or brown in color. 
 No good cleavage. Brittle. Transparent in small crystals to 
 opaque in large. Very difficultly fusible. Dissolves in nitric acid 
 and ammonium molybdate solution added to a few drops of the 
 nitric acid solution gives a bright yellow precipitate showing the 
 presence of phosphorus. Composition (CaF)Ca 4 (PO 4 ) 3 ; phosphate 
 of lime with fluorine ; the fluorine is often replaced wholly or in part 
 by chlorine. Apatite is found in large, sometimes huge, crystals in 
 pegmatite dikes and in metamorphosed limestones in the crystalline 
 schists: these may be said to be the chief megascopic modes of 
 occurrence. In these, however, it cannot be said that its function 
 as a rock-mineral is of any wide or general importance. In addition 
 to this it occurs in minute microscopic crystals, which can seldom 
 be detected with the eye or lens, in all kinds of igneous rocks and in 
 many metamorphic ones. Microscopical study of the thin sections 
 of such rocks has shown that in this form the mineral has a nearly 
 
96 ROCKS AND ROCK MINERALS 
 
 universal distribution as a constant accessory component. Although 
 the relative proportion of the mineral is small, rarely rising above 
 two or three per cent of the rock, its presence is a matter of great 
 importance, since by it the phosphorus, so necessary to vegetable 
 and animal life (in bones, etc.), is furnished to the soil which is 
 formed when the rocks decay and break down under the action of 
 the various agents of weathering. 
 
 SEC. 2. Hydrous Silicates. 
 
 The minerals of this group are of purely secondary 
 origin; they are formed from previously existent ones 
 by the agencies of weathering, water containing carbon 
 dioxide or vegetable acids and by heated water or its 
 vapors circulating in already solid, existent rocks. Thus 
 they do not play any important part in fresh unchanged 
 igneous rocks; only as these alter do they become of 
 importance in them; their true home is in the meta- 
 morphic and sedimentary ones, which at times are made 
 up wholly of these minerals. 
 
 The important ones to be considered in this section are 
 kaolin, chlorite, serpentine, talc and zeolites. Some micas 
 would also naturally be considered here and among 
 secondary minerals also limonite, but, for reasons pre- 
 viously stated, these have been treated in the foregoing 
 section. 
 
 KAOLIN CLAY. 
 
 Under the heading of clay are included certain hydrous 
 silicates of alumina having well-known physical properties 
 by which they are distinguished. By far the most com- 
 mon and important of these is kaolin which may be taken 
 as a type of the group, and the only one which need be 
 considered here in detail. 
 
 General Properties. Kaolin crystallizes in the mono- 
 clinic system forming thin plates or scales often with 
 hexagonal outlines which are flexible and recall mica but 
 are inelastic; these are generally so minute and aggre- 
 gated together that the crystal form is not a matter of 
 
ROCK-MAKING MINERALS 97 
 
 importance in megascopic determination of the substance. 
 Usually in masses, either compact, friable or mealy. 
 Color white, often tinted yellow, brown or gray. Neither 
 the hardness (2-2.5) nor the specific gravity (2.6) can be 
 used for practical tests. On rubbing between the fingers 
 kaolin has a smooth, unctuous, greasy feel, which helps 
 to distinguish it from fine aggregates of some other 
 minerals occurring in nature: thus its presence in soils 
 can usually be told by rubbing out the fine, gritty 
 particles of quartz, feldspar, etc., and observing if there 
 is a smooth, unctuous residue of clay. 
 
 It" is infusible before the blowpipe, but moistened with 
 cobalt nitrate and ignited turns blue showing presence of 
 alumina. Heated in the closed glass tube it yields water. 
 Insoluble in hydrochloric acid. In the phosphorus bead 
 before the blowpipe, undissolved silica is left; this test 
 helps to distinguish it from bauxite, a hydrated oxide of 
 aluminum (A1 2 O(OH) 4 ) which very much resembles it 
 and sometimes occurs in considerable deposits. Bauxite 
 dissolves in the phosphorus bead completely. The 
 chemical composition of kaolin is H 4 Al 2 Si 2 O 9 - a combi- 
 nation of A1 2 3 . 2 SiO 2 . 2 H 2 O. 
 
 Occurrence. Kaolin is always a secondary mineral 
 formed by the alteration or weathering of previously 
 existent aluminous silicates and chiefly feldspar. The 
 reaction by which it is formed from alkalic feldspar is one 
 of the most important in nature, for by it soil is chiefly 
 made and the alkali necessary for plant food liberated 
 and converted into soluble form. It is expressed as fol- 
 lows: 
 
 Orthoclase + Water + Garb. diox. = Kaolin + Potas. Carb. + Quartz. 
 2 KAlSi 3 O 8 + 2 H 2 O + CO 2 = H 4 Al 2 Si 2 O 9 + K 2 CO 3 + 4 SiO 2 . 
 
 This process and reaction have been already described 
 under feldspars. The feldspathoids also yield kaolin and 
 the process could be expressed as follows: 
 
 Nephelite + Water + Carb. diox. = Kaolin + Sodium Carb. 
 2 NaAlSiO* + 2 H 2 O + CO 2 = H 4 Al 2 Si 2 O 9 + Na 2 CO 3 . 
 
98 ROCKS AND ROCK MINERALS 
 
 They are more apt however to first change into musco- 
 vite or zeolites and these ultimately to clay. 
 
 From what has been said it is clear that feldspathic 
 rocks furnish kaolin, and every stage of the change may be 
 observed in nature as described more completely in the 
 chapter dealing with the origin of sedimentary rocks and 
 soils. Thus kaolin occurs intimately mixed with the 
 feldspar substance of such rocks as are undergoing this 
 change; it is found occasionally in quite extensive deposits 
 where such rocks have been completely altered in place, 
 and the products of decay yet remain where they have 
 been formed, and lastly it occurs in extensive beds in "the 
 sedimentary formations. Since the particles of kaolin 
 are very minute, light and flat they remain much longer 
 in suspension than the other products of land waste, and 
 thus in erosive and sedimentary processes there is a con- 
 stant tendency to separate them from the other particles. 
 We find beds of clay with every degree of admixture with 
 sand, etc., that pass into sandstones and other rocks but 
 not infrequently they are of a high degree of purity. 
 
 CHLORITES. 
 
 The chlorites are an ill-defined group of hydrous silicates 
 so named on account of their green color (Greek ^Awpos, 
 green), which are always secondary and formed at the 
 expense of previously existing silicate minerals which 
 contain aluminum, iron and magnesia. Outwardly they 
 resemble the micas, but unlike them their folia are soft 
 and inelastic. They are hydrous silicates of aluminium 
 with ferrous iron and magnesium. They have certain 
 common properties by which they may generally be easily 
 recognized as a group, but the identification of the different 
 members is usually a difficult matter and for ordinary 
 purposes of megascopic petrography of little importance. 
 In the description which follows then it is these general 
 group properties which are given, though these are based 
 
ROCK-MAKING MINERALS 99 
 
 largely on the species clinochlore which is perhaps the 
 most common and best known of the group. 
 
 Form. The chlorites are really monoclinic in crystal- 
 lization, but, like the micas, when crystal form can be 
 observed they are generally in six-sided plates and tablets. 
 More commonly they occur in irregular leaves and scales 
 which are aggregated together into fine granular or coarse 
 leafy massive forms or arranged into fan-like or rosette- 
 like groups. The scales are sometimes flat; often bent or 
 curled. 
 
 General Properties. Chlorite, like mica, has a highly 
 perfect cleavage in one direction parallel to the flat base 
 of the plates. The cleavage leaves are flexible and tough 
 but unlike mica they are inelastic. Luster of cleavage 
 face rather pearly. Color green, variable, usually a 
 rather dark green. Usually translucent. Hardness, 2-2.5 
 soft, just scratched by the finger nail. Specific Gravity 
 about 2.7. Streak pale green to white. The chemical 
 composition of the chlorites is not definitely understood 
 and seems to be complex: it may be illustrated by the 
 formula assigned to clinochlore, H 8 (MgFe) 5 Al 2 Si3Oi8, 
 which may be written 4 H 2 O . 5 (MgFe)O . A1 2 O 3 . 3 SiO 2 : 
 Ferrous iron and magnesia are isomorphous. In kam- 
 mererite, a rare violet-red variety, part of the alumina is 
 replaced by chromic oxide, Cr 2 O 3 . Before the blowpipe 
 chlorites are infusible or very difficultly so; with the 
 borax bead they react for iron. Heated in the closed 
 glass tube they yield water. They are insoluble or 
 difficultly so in hydrochloric acid but are decomposed in 
 sulphuric acid. These reactions are those of the common 
 kinds. 
 
 Occurrence. The chlorites are a widely spread group of 
 minerals, and occur wherever previously existent rocks 
 containing silicates composed of alumina, iron and mag- 
 nesia, such as dark micas, amphibole, pyroxene and 
 garnet, are being altered by geologic processes. To 
 chlorite many igneous rocks owe their green color, the 
 
100 ROCKS AND ROCK MINERALS 
 
 original ferro-magnesian silicates having been broken 
 down by decay and changed more or less completely into 
 this substance. They are apt to lose their original bright, 
 clean appearance and hard clear-cut fracture and become 
 dull green and more or less soft and even earthy. This 
 change can also be often observed in the case of single 
 embedded crystals of the above-mentioned minerals, 
 which become soft, dull green masses. 
 
 Chlorite is also of common occurrence in the schistose 
 rocks; in chlorite-schist it is the prominent component 
 accompanied by other minerals; other schists often owe 
 their green color to its presence, as in green slates for 
 example. Thus in finely disseminated particles it is a 
 common coloring matter. 
 
 SERPENTINE, 
 
 General Properties. Serpentine does not crystallize, 
 and therefore has no crystal form of its own, but it is 
 sometimes found in the crystal form of other minerals 
 which have been altered to this substance. It is usually 
 massive, sometimes finely granular or even slaty; some- 
 times fibrous, the fibers fine, flexible and easily separable, 
 like asbestus. Massive varieties have a conchoidal or 
 splintery fracture. Has a smooth, greasy feel. The 
 color of massive varieties is green, bright yellowish green, 
 olive green, to blackish green, or nearly black; the fibrous 
 varieties are apt to be brownish, yellowish brown, pale 
 brown or nearly white. Luster of the massive varieties 
 greasy, wax-like, glimmering and usually feeble to dull; of 
 fibrous varieties pearly to opalescent. Translucent to 
 opaque. Hardness, 2.5-3.0; apparent greater hardness 
 is caused by presence of remains of the original mineral 
 or by infiltrated and deposited silica. Specific gravity 
 somewhat variable, fibrous 2.2-2.4, massive 2.5-2.7.* 
 Composition, H 4 Mg 3 Si 2 O 9 = 2 H 2 O . 3 MgO . 2 SiO 2 . A 
 small part of the MgO is usually replaced by ferrous oxide, 
 FeO. Before the blowpipe difficultly fusible, fine fibers 
 
ROCK-MAKING MINERALS 101 
 
 fuse more readily. In the closed glass tube yields water 
 on ignition. The finely powdered or divided material 
 decomposes in boiling hydrochloric acid with separation , 
 of silica but does not gelatinize. The solution' may be 
 tested for iron and magnesia. Easily told from epidote 
 and other common green silicates which may resemble it 
 by its greasy feel and softness. 
 
 Occurrence. Serpentine is a secondary mineral result- 
 ing from the alteration of previously existing silicates 
 containing magnesium. Thus pyroxene, amphibole and 
 especially olivine may be altered to this substance. In 
 the case of olivine the process can be illustrated by the 
 following equation : 
 
 Olivine + Water + Carb. diox. = Serpentine + Magnesite 
 2 Mg 2 Si0 4 + 2 H 2 + C0 2 '= H 4 Mg 3 Si 2 9 + MgCO 3 
 
 This would explain the frequent association of the mineral 
 magnesite, MgCO 3 , with serpentine; or it might be taken 
 into solution by the carbonated water and removed. 
 
 A still simpler method would be by the action of heated 
 waters containing some soluble silica. 
 
 3 Mg 2 SiO 4 + 4 H 2 O + SiO 2 = 2 I^MgsSiaOg. 
 
 Therefore as a product of alteration of such minerals, 
 especially by the action of heated waters, serpentine is a 
 common and widely diffused mineral and is found both 
 in igneous and metamorphic rocks. It may occur dis- 
 seminated in small scattered masses in the rocks or form 
 large independent bodies of itself, as described further 
 under the chapters dealing with the rocks. Besides the 
 common massive form, many sub-varieties of serpentine 
 are known; the most important of these is the finely 
 fibrous one, often taken for asbestus, which is known as 
 chrysotile. It usually occurs in seams in the massive 
 variety. Bright green massive material is known as 
 precious serpentine and is cut for ornamental purposes. 
 
102 ROCKS AND ROCK MINERALS 
 
 TALC. 
 
 General Properties. The exact crystal form of talc is 
 doubtful, but this is not a matter of importance since it so 
 rarely occurs in distinct crystals. It is usually seen in 
 compact or strongly foliated masses, sometimes in scaly 
 or platy aggregates which may be grouped into globular 
 or rosette-like forms. Like mica it has a perfect cleavage 
 in one direction, but the laminae though flexible are not 
 elastic; it is sectile. It has a soft greasy feel. The 
 cleavage face has a mother of pearl luster. The color is 
 white, often inclining to green; apple-green; sometimes 
 gray to dark gray. Usually translucent. Hardness 
 = 1-1.5, easily scratched with the finger nail. Specific 
 gravity, 2.7-2.8. Streak, light, usually easily seen on 
 dark cloth. Composition, H 2 Mg 3 (SiO3)4, acid metasili- 
 cate of magnesium. Before the blowpipe it whitens, 
 exfoliates and fuses with difficulty on the edges. Only 
 yields water in the closed glass tube on intense ignition. 
 Scarcely acted on by hydrochloric acid. It is easily 
 recognized by the properties mentioned above. 
 
 Occurrence. Talc is a secondary mineral produced by 
 the action of circulating fluids on magnesium silicates, 
 especially those free from alumina, such as olivine, hypers- 
 thene and some pyroxenes and amphiboles. The process 
 could be illustrated by the following equation. 
 
 Enstatite + Water + Carb. diox. = Talc +Magnesite 
 
 4 MgSi0 3 + H 2 + C0 2 = H 2 Mg 3 (SiO 3 ) 4 + MgCO 3 
 
 Thus talc occurs at times in the igneous rocks as an 
 alteration product of such silicates, especially in the 
 peridotite and pyroxenite groups. The place, however, 
 where it plays an important function is in the meta- 
 morphic rocks, where alone it may form independent 
 masses, as in steatite or soapstone, or be an important 
 component of several varieties of schistose rocks as in 
 talcose schists. 
 
ROCK-MAKING MINERALS 103 
 
 ZEOLITES. 
 
 The zeolites are a group of hydrous silicates, composed 
 like the feldspars of aluminum with alkali and alkali- 
 earth metals. They are indeed for the most part second- 
 ary minerals which have been formed at the expense of 
 feldspars and feldspathoids by the action of heated cir- 
 culating waters and. steam and are thus chiefly found in 
 igneous and especially volcanic rocks. They do not 
 form a group so closely related in crystallization and 
 other properties as the feldspars, but still, in many ways, 
 they have certain common properties by which they may 
 be distinguished. These will be first described, and then, 
 out of the many species, the individual characters of a 
 few of the most important will be treated. 
 
 Group Properties. The zeolites are nearly always well crystallized, 
 the crystals presenting the forms characteristic of the different 
 species. They have a vitreous luster, are usually colorless or white, 
 sometimes tinted yellow or red, like feldspar. They are usually of 
 inferior hardness and can be scratched by the knife. Their specific 
 gravity is low, 2.1-2.4. They fuse very readily before the blowpipe, 
 most of them with intumescence (whence the name, friv, Greek, 
 to boil), but some quietly, to white glasses or enamels. They dis- 
 solve in hydrochloric acid, sometimes gelatinizing and sometimes 
 with separation of slimy silica. Some of the more common varieties 
 are, analcite, natrolite, stilbite and heulandite. 
 
 Analcite. This zeolite crystallizes in isometric trapezohedrons 
 like garnet, which easily enables one to recognize it. Generally 
 colorless to white. Before the blowpipe first becomes opaque, then 
 fuses quietly to a clear glass, coloring the flame yellow. Dissolves 
 in hydrochloric acid with separation of silica but does not gelatinize. 
 Its composition is NaAl(SiO 3 ) 2 + H 2 O. 
 
 Natrolite. Crystallizes in orthorhombic prisms which are 
 generally long, slender and even needle-like and arranged in diver- 
 gent bunches or compacted into fibrous, often radiating masses. 
 Before the blowpipe fuses easily and quietly to a clear glass ; fuses in 
 a candle flame. Dissolves in acid with gelatinization. Composition, 
 Na 2 Al(AlO) (SiO 3 ) 3 + 2 H 2 O. 
 
 Stilbite. Crystallizes in complex monoclinic crystals, which are 
 usually so compounded together that the aggregate has the form of 
 a sheaf. There is a perfect cleavage in one direction and this appears 
 
104 ROCKS AND ROCK MINERALS 
 
 on the side of the sheaf with pearly luster. Sometimes in divergent, 
 sometimes in globular groups. White or red in color. Before the 
 blowpipe swells, intumesces and fuses to a white enamel. Dissolves 
 in acid without gelatinization. Composition, 
 
 H 4 (CaNa 2 )Al 2 (Si0 3 ) 6 + 4 H 2 O. 
 
 Heulandite. Crystallizes in flattened monoclinic crystals which 
 aggregate into compound individuals, the crystals being grown 
 side by side with the flattened surfaces together. There is a perfect 
 cleavage parallel to this flattened side which has a pearly luster. 
 The cleavage plates are often curved and have a lozenge-shaped 
 outline. Blowpipe and chemical characters like stilbite. Com- 
 position, H 4 CaAl 2 (SiO 3 ) 6 + 3 H 2 O. 
 
 Occurrence. As stated above, the zeolites are second- 
 ary minerals chiefly found in igneous rocks. They are 
 found in these especially when they have been subjected 
 to the action of circulating waters and steam which have 
 attacked the feldspars and feldspathoids. Thus, for 
 example, a mixture of albite and nephelite with water 
 would yield analcite, as follows : 
 
 Albite + Nephelite + Water = Analcite 
 NaAlSi 3 O 8 + NaAlSiO 4 + 2 H 2 O = 2 NaAl(SiO 3 ) 2 . H 2 O 
 
 Thus where feldspathic rocks have been somewhat altered 
 they are very apt to contain zeolites in small amounts 
 scattered through them; in some rare cases it has been 
 found that a considerable part of the rock mass is com- 
 posed of them, especially of analcite. Ordinarily the 
 presence of these minerals is not to be detected megascopi- 
 cally, though it may be discovered by heating some of the 
 powdered rock in a closed glass tube, when the easy evolu- 
 tion of abundant water would indicate their presence. 
 
 Their especial home, from the megascopic point of view, 
 is in the lavas, particularly basaltic ones. Here they are 
 found coating and lining cavities and the sides of jointing 
 planes, and composing the materials of the amygdaloids 
 in lavas, as described under amygdaloidal structure and 
 under basalt. They may be associated with crystals of 
 
ROCK-MAKING MINERALS 105 
 
 quartz and of calcite in such occurrences, and, in addition 
 to the common kinds mentioned above, many others may 
 occur whose description must be sought for in the larger 
 manuals on minerals. 
 
 Carbonates. 
 
 The carbonates are salts of carbonic acid, H 2 CO3, and 
 are secondary minerals, in that their metals have been 
 derived from previously existent minerals acted upon by 
 water and carbon dioxide either from the supply already 
 in the atmosphere or coming from interior sources deep 
 within the earth. The carbonates thus formed have been 
 either deposited directly where we now find them, or, being 
 soluble in water containing carbon dioxide, they have been 
 carried in solution into lakes and into the sea and rede- 
 posited by the agencies of organic life in the form of 
 chalk, limestone, etc. The shifting about of carbonates 
 on the earth's surface, owing to their solubility in water 
 containing carbon dioxide, is a geologic process of great 
 importance and gives rise to a variety of products which 
 are described in their appropriate places as rock forma- 
 tions. Here they are treated simply as minerals and out 
 of the considerable number of kinds only two are of such 
 importance that they will be considered in detail calcite 
 and dolomite. 
 
 CALCITE. 
 
 Form. Calcite crystallizes in the rhombohedral system. 
 The crystals are often very fine, perfect, and sometimes 
 of very large size. It displays a great variety of crystal 
 forms many of them being often very complex. Some 
 simple ones are shown in the annexed figures. Fig. 51 is 
 a simple flat rhombohedron, three faces above and three 
 below. Fig. 52 is the unit rhombohedron, so called 
 because the faces are parallel to the cleavage. Fig. 53 is 
 a very acute rhombohedron. Fig. 54 is a very short 
 prism with six prism faces m and the flat rhombohedron 
 e above and below; Fig. 55 is similar but the prism faces 
 
106 
 
 ROCKS AND ROCK MINERALS 
 
 m are elongated.- Fig. 56 is an acutely pointed form, the 
 scalenohedron. All of these are common crystal forms 
 
 Fig. 51 
 
 Fig. 52 
 
 Fig. 53 
 
 Fig. 54 
 
 shown by calcite; they occur when it is found lining 
 cavities in rocks, in druses, in amygdaloids, in geodes and 
 on the surfaces of joint planes and fissures and in caves; 
 in short in all those places where it could be deposited by 
 infiltrating water containing it in solution. As a rock- 
 making mineral it is massive; of- 
 ten crystalline-granular and coarse 
 to fine in structure as in marble, or 
 compact as in ordinary limestone, 
 or loose and powdery in texture as 
 in chalk. It is sometimes spongy 
 in structure as in tufa, or rounded, 
 stalactic, mamillary, etc., as in 
 cave deposits and in concretions, 
 and not uncommonly fibrous. 
 Cleavage. Calcite has a very perfect rhombohedral 
 cleavage; in three directions parallel to the faces r of the 
 crystal shown in Fig. 52. While this 
 is, of course, best produced in iso- 
 lated crystals it can be well observed 
 on the fractured surfaces of coarsely 
 crystalline massive forms, as in many 
 marbles and related rocks and in the- 
 massive calcite of veins. The angles 
 of the face of the rhomb produced by cleavage are just 
 about 78 and 102 degrees, as shown in Fig. 57, and small 
 
 Fig. 55 
 
 Fig. 57- 
 
ROCK-MAKING MINERALS 107 
 
 cleavage pieces can be readily tested by applying them 
 to the edges of the figure on the paper. 
 
 General Properties. The hardness is 3 ; readily scratched 
 or cut by the knife; chalky varieties are of course softer. 
 The specific gravity is 2.71 if pure. The natural color 
 of calcite is colorless or white but it frequently displays a 
 great variety of exotic colors owing to the presence of 
 impurities; thus it may be reddish or yellowish from iron 
 oxides or gray to black from organic matter, or green, 
 purple, blue, etc., from other substances. Streak, white 
 to gray. The luster of the crystallized calcite is vitreous; 
 of massive forms glimmering to dull. In the same way 
 it varies from transparent to translucent to opaque. 
 Chemical composition, CaCO 3 ; or CaO . C0 2 . CaO = 56 
 per cent, CO 2 , 44 per cent = 100. 
 
 Before the blowpipe it is infusible, colors the flame 
 reddish yellow and after ignition if placed on moistened 
 yellow turmeric paper colors it brown. Fragments 
 effervesce freely in cold (difference from dolomite) very 
 dilute acids. 
 
 Occurrence. Calcite is one of the most common and 
 widely diffused of all minerals. It is found in the igneous 
 rocks as the result of alteration of the lime-bearing sili- 
 cates by waters containing carbon dioxide in solution, 
 other substances being formed as by products at the same 
 time. Thus, for example, 
 
 Pyroxene 4- Water 4- Carb. diox. = Calcite + Serpentine + Quartz 
 3CaMg(SiO 3 ) 2 + 2 H 2 O + 3 CO 2 3 CaCO 3 4- H 4 Mg 3 Si 2 O 9 + 4 SiO 2 
 
 The calcite thus formed may remain for a time in the 
 rock but eventually, as the latter breaks down into soil, 
 it is to a greater or lesser extent removed in solution and 
 carried away. The mineral has also been found to occur 
 very commonly in minute cavities in unaltered igneous 
 rocks, especially intrusive ones, and its origin is probably 
 due to infiltration and deposition of the material from 
 neighboring rock masses. In these cases just mentioned 
 
108 ROCKS AND ROCK MINERALS 
 
 the mineral is ordinarily not observable megascopically, 
 but its presence is easily ascertained by immersing a 
 fragment of the rock in cold dilute acid and seeing if it 
 effervesces and gives off carbonic acid gas. Calcite also 
 occurs in amygdaloidal cavities in lavas, especially in 
 basalts, and often in good crystals. 
 
 In the sedimentary and metamorphic rocks calcite 
 plays a much more important part. It is very commonly 
 found distributed through them in fine particles or acting 
 as a cement to the other mineral granules. From this 
 role, if we examine a whole series of these rocks, we find 
 it increasing more and more in abundance and importance 
 as a constituent, until finally there are enormous, widely 
 extended rock-masses, such as the chalks, limestones 
 and marbles, composed practically and in some instances 
 wholly of this substance. Such rocks are found described 
 in their appropriate places in this work ; it is sufficient here 
 to mention that in the sedimentary rocks calcite plays an 
 important part in chalk, limestone, calcareous marls, 
 calcareous sandstones, etc.; in chemical deposits in 
 calcareous tufas, sinters, stalagmitic deposits, veins, etc., 
 and in the metamorphic rocks in marbles and in rocks 
 which are mixtures of calcite and various silicates. 
 
 Determination. Calcite when sufficiently coarsely crys- 
 tallized is easily recognized by its inferior hardness and 
 rhombohedral cleavage. This is confirmed chemically 
 by its ready solubility in cold dilute acids with efferves- 
 cence of CO 2 gas and if necessary a test for the presence 
 of lime. For the distinction from dolomite, reference 
 should be made to the description of that mineral. 
 
 DOLOMITE. 
 
 Form. Dolomite crystallizes in the rhombohedral 
 system and, like calcite, it is found in simple rhombo- 
 hedral crystals whose faces are parallel to the cleavage; 
 Fig. 52 of calcite. Unlike that mineral it rarely occurs 
 in complicated crystals and the simple rhombohedron, in 
 
ROCK-MAKING MINERALS 109 
 
 which it is generally seen when showing outward crystal 
 form, usually has its faces curved as represented in Fig. 
 58 instead of flat. Moreover the curved crys- 
 tals are apt to be compound, made up of a 
 number of sub-individuals. This is the way it 
 occurs when lining druses and cavities, but as 
 a rock-making mineral it is nearly always 
 massive, often crystalline-granular and coarse 
 to fine in texture as in some marbles. It is also often 
 compact- massive as in some limestones; more rarely 
 columnar or fibrous. 
 
 Cleavage. The cleavage, like that of calcite, is perfect 
 in three directions parallel to the faces of the simple 
 rhombohedron. The angles of the cleavage rhombs 
 differ only a degree or so (74 and 106 degrees, nearly) 
 from those of calcite and therefore by form alone they 
 cannot be distinguished by the eye. 
 
 General Properties. The natural color is white and 
 while this is often seen the mineral is very apt to be 
 tinted some exotic color by other substances; thus it may 
 be reddish, brown, greenish, gray or even black. Luster, 
 vitreous, sometimes pearly to dull or glimmering in com- 
 pact varieties. Translucent to opaque. The hardness 
 is 3.5-4.0, harder than calcite but easily scratched with a 
 knife. Specific gravity, 2.8-2.9. Chemical composition, 
 CaMg(CO 3 ) 2 ; CaO 30.4, MgO 21.7. C0 2 47.9 = 100. 
 Before the blowpipe infusible, but placed on moist tur- 
 meric paper after ignition colors it brown. Does not 
 dissolve or is very little acted upon in cold dilute acid but 
 on boiling effervesces and goes into solution (difference 
 from calcite). The solution may be tested for lime and 
 magnesia as directed in the chapter on mineral tests. 
 
 Occurrence. Dolomite occurs as a scattered accessory 
 component of certain crystalline schists and in beds of 
 gypsum, etc., but its chief importance as a rock-making 
 mineral lies in the fact that alone it forms immense 
 extended beds both in the sedimentary series and in the 
 
110 ROCKS AND ROCK MINERALS 
 
 metamorphic rocks. It thus exactly parallels calcite, 
 and in limestones and in marbles we have every degree of 
 transition between these two substances, thus marbles 
 composed of calcite alone, others with increasing amounts 
 of dolomite until pure dolomite marble is reached. This 
 is more fully described under the carbonate rocks. 
 
 Determination. The rhombohedral cleavage and infe- 
 rior hardness separate dolomite, like calcite, from other 
 common rock minerals. The frequent curved surfaces 
 help to distinguish it from calcite but the test with hot 
 and cold acid mentioned above, together with the finding 
 of magnesia in the solution, is the only safe way. 
 
 Siderite, Magnesite and Breunerite. As an appendix to calcite 
 and dolomite these carbonates, which are sometimes of local impor- 
 tance, may be mentioned. Siderite or spathic iron ore is FeCO 3 , 
 ferrous carbonate, while magnesite is magnesium carbonate, MgCO b , 
 and breunerite is an isomorphous mixture of the two, (MgFe)CO 3 . 
 In crystallization, cleavage, hardness, etc., they closely resemble 
 the other carbonates described. Siderite is usually light to dark 
 brown in color; magnesite white; breunerite brownish. Siderite 
 chiefly occurs more or less massive and impure in certain sedimentary 
 deposits, in the so-called " clay iron stone " and is a valuable ore of 
 iron. Magnesite occurs chiefly* in certain metamorphic rocks and 
 is apt to be associated with serpentine, talc, etc. It may be acccom- 
 panied or replaced by breunerite. 
 
 Sulphates. 
 
 The sulphates, like the carbonates, are in general 
 minerals of a secondary nature; the metals they contain 
 have been taken from previously existent minerals, the 
 sulphuric acid has been furnished for the most part by 
 the Oxidation of metallic sulphides or by exhalations in 
 regions of igneous activity. With a few exceptions they 
 are readily soluble and the great bulk of them, which has 
 been formed during geologic time, has therefore been 
 transferred to the sea, which, with the salt lakes in the 
 interior of continents, is now the great reservoir of these 
 substances as well as of many other soluble salts such as 
 
ROCK-MAKING MINERALS 
 
 111 
 
 the chlorides. As rock-making minerals only two of the 
 large number of sulphates known are of importance, 
 gypsum and anhydrite. Barite, BaS(>4, which is one of 
 the few insoluble sulphates, is a very common material in 
 veins and is also found in concretions, but it does not 
 form independent rock-masses or play any role as a rock- 
 component as the two first mentioned do. 
 
 GYPSUM SELENITE. 
 
 Form. Gypsum crystallizes in the monoclinic system, 
 and the common form of the crystals is shown in Fig. 59. 
 The same crystal is shown in Fig. 60 revolved so that the 
 side face b is parallel with the plane of the paper; such 
 crystals may be roughly tested by placing them on the 
 
 Fig. 59 
 
 Fig. 60 
 
 Fig. 61 
 
 diagram and seeing if the angles coincide. Twin crystals 
 are common and they are apt to assume arrow-head 
 forms as shown in Fig. 61. More commonly as a rock 
 constituent, gypsum occurs massive, foliated often with 
 curved surfaces, or granular to compact and sometimes 
 fibrous. 
 
 Cleavage. Gypsum has a perfect cleavage parallel to 
 the side face 6; by means of it on good material very thin 
 sheets with perfect luster may be split off, almost as in 
 mica. Such sheets will be found to break in one direction 
 
112 
 
 ROCKS AND ROCK MINERALS 
 
 Fig. 62 
 
 in straight lines with a conchoidal fracture; this is due to 
 another cleavage parallel to the vertical edge between 
 mm. If such sheets be bent cracks will 
 appear in them making angles of 66 and 
 114 degrees with the straight fracture 
 edge mentioned above; if the bending 
 parallel to this direction is continued the 
 plate will break with a fibrous fracture, 
 and a cleavage rhomb like that shown in 
 Fig. 62 may be obtained. In massive 
 coarsely crystalline gypsum, these cleav- 
 ages can usually be readily obtained and 
 furnish one means of helping to identify 
 it; in fibrous material it simply cleaves parallel to the 
 fibers; in the compact massive forms it may happen that 
 no cleavage is seen. 
 
 General Properties. The natural color of gypsum is 
 colorless or white, and crystals are transparent to trans- 
 lucent, but it is frequently tinted reddish or yellowish or 
 in massive varieties may be even red, brown or black 
 through impurities, and translucent to opaque. The 
 luster of the cleavage face b is glassy to pearly, of fibrous 
 varieties satiny, while massive forms are glistening, 
 glimmering to dull. Streak, white. Hardness, 1.5-2.0, 
 easily scratched by the finger nail. Specific gravity of 
 pure crystals, 2.32. Chemical composition, hydrous sul- 
 phate of calcium, CaSO 4 + 2 H 2 0. CaO 32.5, S0 3 46.6, 
 H 2 20.9 = 100. Before the blowpipe fuses easily and 
 after ignition colors moistened turmeric paper brown. 
 Fused with carbonate of soda and charcoal dust on char- 
 coal and transferred to a moistened surface of silver 
 stains it dark. Finely powdered mineral is readily 
 soluble in boiling dilute hydrochloric acid. Heated 
 intensely in a closed glass tube gives off water and becomes 
 opaque. Heated moderately (not above 200 degrees) 
 it loses some water and becomes plaster of Paris and the 
 powder, if moistened, again takes up water and sets or 
 
ROCK-MAKING MINERALS 113 
 
 becomes solid turning back into gypsum. If heated too 
 highly it loses all its water, becomes anhydrite, CaS0 4 , 
 and is then called dead burnt plaster and does not set as 
 described above. 
 
 The occurrence of gypsum is mentioned later in its 
 description as a stratified rock. 
 
 ANHYDRITE. 
 
 General Properties. Anhydrite crystallizes in the ortho- 
 rhombic system but in the rocks in which it occurs it is 
 seen in granular to compact masses, less commonly in 
 fibrous or foliated forms. It has a cleavage in three 
 directions at right angles, and if coarsely crystalline this 
 may be observed to produce cube-like forms. Usually 
 white but sometimes tinted as in gypsum; luster of 
 cleavage faces pearly to glassy; in massive varieties varies 
 to dull. Harder than gypsum = 3-3.5 but easily cut by 
 knife. Specific gravity, 2.95. Chemical composition, 
 CaSO 4 ; CaO 41.2, SO 3 58.8 = 100. Blowpipe and other 
 reactions as with gypsum, but it does not yield water in 
 the closed glass tube on heating which is the best dis- 
 tinction from gypsum; the difference in cleavage also 
 aids in the discrimination. 
 
 Occurrence. Like gypsum, anhydrite forms inter- 
 stratified beds in sedimentary formations especially in 
 limestones and shales. It is also found in masses and in 
 geodes in such rocks and is a common associate of rock 
 salt and gypsum. 
 
 ROCK SALT HALITE. 
 
 Rock salt, sodium chloride, NaCl, is the only chloride which 
 occurs as a rock-forming constituent in such amounts as to be of 
 importance. It is easily recognized by its cubic crystals, perfect 
 cubic cleavage, solubility and saline taste. Colorless and trans- 
 parent to white, translucent; frequently tinted various colors by 
 impurities. Hardness = 2.5. Occurs in beds, sometimes of enor- 
 mous thickness, in the sedimentary formations, usually clays or 
 shales, and is frequently accompanied by gypsum and anhydrite. 
 
CHAPTER V. 
 THE DETERMINATION OF ROCK MINERALS. 
 
 THE more important of the physical properties of the 
 rock minerals have been described in the previous chap- 
 ter, and in most cases the methods by which these are 
 to be determined have been stated under them. In the 
 present chapter it is proposed to present a number of 
 qualitative chemical tests, which can generally be made 
 with a few reagents and simple apparatus, and which are 
 of great service in mineral determination and in thus 
 aiding to classify rocks. This is followed by a set of 
 tables ; one for rough approximations in the field, the other 
 for more complete identifications in the laboratory by 
 means of the properties and methods described. 
 
 Chemical Tests. 
 
 These consist mainly in observing the effect of acids 
 upon the mineral, whether it is dissolved or only partly 
 attacked or is wholly insoluble; if soluble, or partly so, in 
 ascertaining with certain reagents what substances have 
 gone into solution. These serve to detect the acid radicals 
 and metallic oxides which alone or in combination com- 
 pose the rock minerals. A few useful additional tests are 
 given. 
 
 A. Powdering the Sample. The first thing in testing 
 the solubility of minerals is to prepare a finely powdered 
 sample. Small chips, grains or splinters of the sub- 
 stance about the size of wheat grains and as pure as 
 possible are successively crushed and ground to a fine 
 powder, like flour, in a diamond, steel or agate mortar, 
 until a sufficient amount has been produced. It is 
 usually best to crush the fragments in the steel mortar, 
 
 114 
 
THE DETERMINATION OF ROCK MINERALS 115 
 
 and unless iron is to be tested for they may be ground in 
 this as well. The finer the powder is ground the more 
 readily it will go into solution; it is therefore generally 
 best to grind it until it no longer feels gritty when a small 
 pinch is rubbed between the fingers. 
 
 B. Treatment with Acid. A small bulk of the powder 
 prepared as above, about equal to a pea in volume, is put 
 in a test tube, covered with about an inch of distilled 
 water and a few drops of nitric or hydrochloric acid 
 added. Either may be used, but if it is desired to test 
 for phosphorus or chlorine subsequently in the solution 
 the former should be employed. The test tube, if the cold 
 acid has no apparent effect upon the substance, may then 
 be gently heatea over the flame of a bunsen burner or 
 lamp of a suitable kind until the liquid is brought to 
 boiling. If the effect is slight or apparently none has 
 been produced more acid is added and the boiling repeated 
 until the substance is brought into solution or it is appa- 
 rent that it cannot be dissolved. 
 
 C. Carbonic Acids Carbonates. If the substance 
 effervesces freely and readily in cold dilute acid it indicates 
 that it is a carbonate and that CO2 gas is being given off. 
 It is probably calcite. The carbonates of magnesia and 
 iron (dolomite, siderite, etc.) are scarcely acted upon or 
 but very slowly in cold acid but effervesce freely when the 
 liquid is heated. This serves as a convenient means of 
 distinction between calcite and dolomite which may be 
 confirmed by tests for lime and magnesia as given beyond. 
 The rare rock mineral cancrinite, a silicate containing 
 CO 2 also effervesces very slowly in hot dilute acid; the 
 heated solution should be examined in a good light with 
 a lens when a slow persistent evolution of minute bubbles 
 will be seen. In regarding the heated solution care must 
 be taken not to confuse the ebullition of steam bubbles 
 with the effervescence of CO 2 gas; a moment's pause 
 should be given to allow the former to cease. 
 
 D. Soluble Silicates. Gelatinization. If the sub- 
 
116 ROCKS AND ROCK MINERALS 
 
 stance treated according to A wholly or partly dissolves 
 without effervescence it should be tested for soluble 
 silica. If only partly attacked the insoluble portion 
 should be filtered off and the filtrate used. This is con- 
 centrated by boiling it down in the test tube, the latter 
 being continuously and gently shaken to prevent crack- 
 ing, until the solution is greatly concentrated, if necessary 
 to a few drops. It is then allowed to cool and stand, and 
 if it becomes a jelly the presence of soluble silica is indi- 
 cated. If the amount of silicate which has gone into 
 solution is relatively large, a jelly will probably form 
 while the solution is being boiled down and is still hot, 
 otherwise the solution must be concentrated and allowed 
 to stand, as just stated; in the latter case care must be 
 taken not to confuse a thickening of the solution from 
 concentration in it of the salts, especially basic salts of 
 iron, with the true jelly of soluble silica. If the solution 
 is carefully carried to dryness and the residue heated for a 
 few moments the salts on being moistened with strong 
 hydrochloric acid and then warmed will go into solution 
 in water while the silica is left as an insoluble residue and 
 may be filtered off. In the filtrate the various metallic 
 bases, aluminum, iron, calcium, etc., which may be in 
 solution, can be tested for by the methods described 
 beyond. 
 
 The rock-making silicates which will go into solution on 
 boiling with nitric or hydrochloric acid are nephelite, 
 sodalite, analcite, olivine, chondrodite, serpentine, anor- 
 thite, leucite, noselite, stilbite, heulandite and cancrinite. 
 All except serpentine, leucite, analcite, stilbite and 
 heulandite yield gelatinous silica; with these when the 
 liquid is boiled it turns from the milkiness caused by the 
 suspended material to a translucent appearance with 
 slimy silica suspended in it. The bases, however, hare 
 gone into solution. 
 
 E. Insoluble Silicates. Fusion. Most rock-making sil- 
 icates are insoluble in acids or only partially soluble. To 
 
THE DETERMINATION OF ROCK MINERALS 117 
 
 get them into solution so that the bases may be tested for 
 as in the following sections, a preliminary process of 
 fusion with sodium carbonate, Na 2 CO 3j must be under- 
 taken. For this purpose some of the powder obtained 
 in A is mixed with about 4-5 times its weight of dry 
 anhydrous sodium carbonate, placed in a platinum 
 crucible or spoon, and gently heated to redness over a 
 Bunsen lamp flame. If no crucible is at hand a coil of 
 platinum wire can be used instead; the mixed powder is 
 made into a thick paste with a little water and a quantity 
 taken on the coil and carefully fused before the blowpipe. 
 A fused bead or mass the size of a large pea may be 
 obtained in this way. The fusion must be conducted 
 until bubbling has ceased. 
 
 In this fusion the silicates are decomposed, silica is liberated 
 from them and takes the place of the carbonic acid in the sodium 
 carbonate which'is thus converted into sodium silicate. The liberated 
 CO 2 gas is given off with bubbling and frothing of the fusion and 
 this effect is in itself indicative of the presence of silica in the original 
 substance, provided it is known not to come from combined water 
 by previous trial. The reaction might be illustrated in the case of 
 pyroxene as follows: 
 
 MgCaSi 2 O 6 + 2 Na 2 CO 3 = 2 Na 2 SiO 3 + MgO + CaO + 2 CO 2 
 
 The fused mass obtained is broken up in a diamond 
 mortar, placed in a test tube, and then treated with acid 
 until dissolved, evaporated and the silica separated and 
 the metallic bases brought into solution just as directed 
 for soluble silicates in D. 
 
 If the fusion has been made in a platinum crucible the 
 cake can generally be loosened and removed by boiling it 
 with a little water; if not, it is dissolved with water and acid 
 in the crucible, the latter being set in a beaker or dish. 
 
 F. Alumina. The filtrate from the silica obtained in 
 D or ED combined, may be tested for alumina. It is 
 heated to boiling after addition of a few drops of nitric 
 acid, and ammonia is added in slight excess. If a white 
 or light-colored, flocculent, gelatinous precipitate forms, 
 
118 ROCKS AND ROCK MINERALS 
 
 this is aluminium hydroxide. If it is reddish brown, then 
 it is wholly or in part ferric hydroxide, indicating the 
 presence of iron, which is very apt to be present in silicates, 
 especially colored ones, and the alumina may be masked. 
 
 If much magnesia is present in the mineral its hydroxide may 
 also be precipitated at this point by the ammonia, unless the solutions 
 are rather diluted and a considerable quantity of ammonium chloride 
 or nitrate has been formed by the neutralization of the acid by the 
 ammonia. 
 
 Scrape some of the precipitate from the filter paper, 
 transfer it to a clean test tube, or if it is small in amount 
 transfer it paper and all, and cover with about 5 cubic 
 centimeters of water; drop in a piece of pure caustic 
 potash, KOH, about the size of a pea, and boil. The 
 alumina, if present, will go into solution leaving the iron 
 hydroxide undissolved; the latter may be now filtered 
 off. Make the filtrate slightly acid with hydrochloric 
 acid, boil and then add ammonia in slight excess; alumina, 
 if present, will be precipitated as the white flocculent 
 hydroxide. 
 
 Alumina can also be detected before the blowpipe by 
 intensely heating the powdered mineral, moistened with 
 cobalt nitrate, on charcoal, when its presence is indicated 
 by the mass turning blue, as mentioned under topaz, 
 cyanite, etc., in the following tables. 
 
 G. Iron. The detection of this metal has been men- 
 tioned above in F. A more delicate method is to add a 
 few drops of potassium ferrocyanide solution to a few 
 drops in water of the final filtrate obtained in D or ED 
 combined, after boiling with a few drops of nitric acid in 
 case hydrochloric was originally used. The formation of 
 a deep Prussian blue precipitate or coloration, if the 
 solutions are very dilute, indicates the presence of iron. 
 The nitric acid converts the ferrous salt in the solution into 
 a ferric one. Potassium /erro-cyanide produces a Prussian 
 blue with ferric salts, not with ferrous, while conversely 
 
THE DETERMINATION OF ROCK MINERALS 119 
 
 potassium /em-cyanide produces the same effect with 
 jerrous salts. Thus by testing portions of the original 
 solution of the mineral in hydrochloric acid with these two 
 reagents the state or states of oxidation of the iron in the 
 original mineral can be ascertained. 
 
 Iron is also shown when minerals become magnetic 
 after being heated in the reducing flame of the blowpipe. 
 
 H. Calcium. The ammoniacal filtrate from the 
 hydroxides of alumina and iron obtained in F, or the 
 clear liquid in case ammonia failed to precipitate, may 
 contain lime salts in solution. To prove the presence of 
 lime it should be heated to boiling and some ammonium 
 oxalate added, when the formation of a precipitate of 
 oxalate of lime proves the presence of this element. If 
 it should be desired to further test the solution for mag- 
 nesia the lime oxalate must be removed by filtration; it is 
 allowed to stand for some time and then filtered; if the 
 filtrate runs through turbid it should be again passed 
 through the paper until the liquid is clear. To this a 
 little more ammonium oxalate is added to prove the com- 
 plete precipitation of the lime. 
 
 I. Magnesium. Ordinarily this element should not 
 be tested for until the alumina, iron and lime have been 
 removed, as directed in F and H, or their absence ascer- 
 tained. To the solution thus obtained some sodium 
 phosphate and a considerable quantity of strong ammonia 
 are added. The formation of a precipitate, ammonium 
 magnesium phosphate, proves the presence of this element. 
 If a precipitate does not form at once it is not, however, 
 safe to consider magnesia absent, for if the amount is 
 small and the solution warm it may not appear until the 
 liquid has become cold and has stood for some time. It 
 is then apt to appear as a crystalline precipitate attached 
 to the walls of the vessel. 
 
 J. Sodium. A mineral containing sodium when heated 
 before the blowpipe colors the flame bright yellow. 
 The best effect is obtained with silicates when the 
 
120 ROCKS AND ROCK MINERALS 
 
 powdered mineral is previously mixed with an equal 
 volume of powdered gypsum and a little of this taken upon 
 a clean moistened platinum wire which has been previously 
 tested. The reaction is, however, so delicate and pro- 
 duced so strongly by minute quantities of the element or 
 accidental traces that great judgment must be used in 
 employing it. It is only when the coloration is very 
 intense and prolonged that the mineral should be inferred 
 to contain soda as an essential oxide. 
 
 K. Potassium. This element may be detected by the 
 violet color it communicates to the Bunsen or blowpipe 
 flame. In silicates it is best obtained by powdering the 
 mineral and mixing it with gypsum, as mentioned under 
 sodium in J. The flame color is delicate and entirely 
 obscured by any sodium present- in this case it can be 
 seen by viewing it through a piece of thick, dark blue 
 glass which cuts off all but the potash flame. Through 
 this it will appear of a violet or reddish purple. 
 
 Another test is to take a small portion of the final 
 filtrate obtained in D or ED combined, evaporate it to a 
 very small volume, add an equal volume of alcohol and 
 if turbid filter it. Then add a few drops of hydrochlor- 
 platinic acid, H 2 PtCl 6 , and if a heavy yellow or orange 
 colored crystalline precipitate, potassium platinic chloride, 
 K 2 PtCl6, forms it shows the presence of this element. No 
 ammonium salt must be present or it will yield a similar 
 precipitate. 
 
 L. Hydrogen Water. If a little of the powdered 
 mineral be placed in a glass tube, one end of which has 
 been closed by fusion and drawing off, and gently heated 
 below redness, the evolution of water, which collects on the 
 upper walls of the tube, shows that it contains loosely 
 attached water of crystallization. This occurs with 
 zeolites, such as analcite, NaAlSi2O 6 + H 2 O, and with 
 gypsum, CaSO 4 4- 2 H 2 O. On the other hand, some 
 minerals, and many silicates are among them, contain 
 hydrogen and oxygen firmly attached in the form of 
 
THE DETERMINATION OF ROCK MINERALS 121 
 
 hydroxyl OH, and this is only given off as water at a 
 very high heat. Indeed with some, as for instance 
 staurolite and talc, Mg 3 Si 4 O 10 (OH)o, it is necessary to 
 subject the assay to intense ignition by heating it white 
 hot before the blowpipe before the water is given off. 
 This difference in behavior will often serve as a useful 
 test in determining minerals. Many minerals which 
 contain hydroxyl also contain fluorine and in this case it 
 will be often found that the water evolved in the tube 
 gives an acid reaction to test paper and the glass may be 
 etched. Unless the latter occurs the test is not however 
 decisive of the absence or presence of fluorine. 
 
 M. Fluorine. This is best tested for as described under 
 topaz, on page 81, in a bulb tube with sodium meta- 
 phosphate. 
 
 N. Chlorine. This occurs in rock salt, apatite and 
 sodalite. The test is the precipitation of chlorine as 
 silver chloride, AgCl, in the solution by addition of a few 
 drops of silver nitrate. The white precipitate turns 
 bluish gray on exposure to light. The test for chlorine 
 is very delicate and slight impurities may cause a faint 
 opalescence in the liquid on addition of the silver salt. 
 Rock salt is easily told by its solubility in water, taste 
 and associations. Apatite usually contains only a very 
 little chlorine yielding a faint test, or chlorine may be 
 wanting in it. Sodalite dissolves in dilute nitric acid and 
 silver nitrate produces in this a considerable precipitate 
 of the chloride; the nitric acid solution also yields gelatin- 
 ous silica as in D ; these tests suffice to identify the mineral. 
 
 O. Sulphuric Acid. Barium chloride produces in the 
 solution containing a sulphate a heavy white precipitate 
 of barium sulphate, BaSO-j, insoluble in hydrochloric or 
 nitric acid. Gypsum, anhydrite and noselite contain 
 sulphuric acid; they dissolve in hydrochloric acid and 
 it may be tested for as above. Noselite also yields 
 gelatinous silica, as in D, and the two reactions serve to 
 identify it. 
 
122 ROCKS AND ROCK MINERALS 
 
 P. Phosphoric Acid. Dissolve the powdered mineral 
 (see A) in nitric acid and add some solution of ammonium 
 molybdate, a yellow precipitate of ammonium phospho- 
 molybdate shows the presence of phosphorus. This test 
 is very delicate. It should be conducted with cold or 
 nearly cold solutions. The precipitate is soluble in excess 
 of ammonia. If it is desired to make this test in a mixture 
 of minerals, as in a fine-grained rock for instance, and 
 silica may be in the solution, it is best to evaporate the 
 latter and get rid of the silica as directed in D. The 
 phosphoric acid can then be tested for in the filtrate 
 acidified with nitric acid. Apatite is the only common 
 rock-making mineral containing phosphoric acid, and its 
 presence in rocks and soils can usually be shown by this 
 test when it cannot be detected megascopically. 
 
 Tables for the Megascopic Determination of Rock Minerals. 
 
 The two following tables will be found useful in helping 
 to identify the commoner rock-making minerals. Beside 
 those given in the tables there are many less common 
 minerals which enter into the composition of rocks and 
 which may at times become of local importance. This 
 is especially true in metamorphic limestones and schists. 
 Some of the more important of them have been given in 
 the preceding chapter on the characters of minerals, but 
 only about fifty minerals or mineral groups constituting 
 the kinds which are ordinarily met with in megascopic 
 rock study are here included. The tables can only be 
 used to distinguish from one another the minerals which 
 are named in them; they cannot in general be used to 
 distinguish them from all other minerals. If doubt arises 
 and a mineral seems to be other than any of those described 
 here the larger manuals of descriptive and determinative 
 mineralogy must be consulted for its identification. 
 
 Table 1. This is based solely on the most obvious and 
 easily determinable physical properties and includes about 
 thirty common minerals or mineral groups. It may 
 
THE DETERMINATION OF ROCK MINERALS 123 
 
 often be used to advantage in the field. The only appa- 
 ratus required in its use are a lens, pocket knife and frag- 
 ments of quartz and feldspar, in addition to the hammers 
 usually carried. It will be of advantage to have one 
 blade of the knife magnetized. The streak or color of the 
 powdered mineral can be tested by grinding a small piece 
 to powder between two hammer faces, pouring it on a 
 piece of white paper and rubbing the dust with the finger 
 to observe the color produced. A piece may be cracked 
 into smaller grains and these examined with the lens to 
 observe the cleavage if it is not well shown by the mineral 
 on the fractured rock surface. The transparency or 
 translucency, if not obvious in the mineral in the rock, 
 may be tested by holding a fragment or sliver against the 
 light and observing if light is transmitted through its 
 thinnest edges. The hardness is best tested on a smooth 
 lustrous cleavage face with the knife point or a sharp- 
 pointed fragment of quartz or feldspar, substances which 
 are usually readily obtainable. 
 
 Table 2. This includes about fifty of the prominent 
 rock minerals or mineral groups whose characters are 
 treated in the foregoing descriptive portion. It requires 
 for its use some of the simpler apparatus and reagents 
 found in every chemical and mineralogical laboratory 
 and the knowledge of how to use them. They have been 
 already mentioned on page 12. 
 
 The table is based upon those of the Brush-Penfield 
 Determinative Mineralogy which have been modified to 
 meet the demands of this particular place, and if further 
 information is desired that manual may be consulted to 
 advantage. 
 
 This table is much more complete and certain in its 
 identification than Table 1 and should always be used 
 in preference to it when possible. Table 1 is to be 
 considered a more or less rough method of approximation 
 to be used in the field or when no apparatus or reagents 
 are at hand. 
 
124 ROCKS AND ROCK MINERALS 
 
 It should be again repeated that the table cannot be 
 used for the identification of all minerals which occur in 
 rocks but only to distinguish the commoner ones, men- 
 tioned in it, from one another. In most cases the identi- 
 fication of the mineral is complete, but instances may 
 occur where some comparatively rare one will give similar 
 reactions. Thus the rare mineral aragonite would lead to 
 the same place as calcite, but reference to the description 
 of the latter would show at once that it differs markedly 
 in other properties, such as cleavage and crystallization. 
 This will be usually found to be the case, and if further 
 information is desired it must be sought elsewhere. But 
 within the limits imposed the table should serve a useful 
 purpose to the student of rocks. 
 
 TABLE 1. 
 
 The mineral has a fine cleavage in one direction; is sometimes 
 micaceous and may be split into thin leaves by the use of the 
 knife point. Sec. 1 below. 
 
 Has a good cleavage in two directions. Sec. 2. 
 
 Has a good cleavage in three directions, forming cubes or 
 rhombs. Sec. 3. 
 
 Has a fine fibrous structure and cleavage. Sec. 4. 
 
 No apparent good cleavage. Sec. 5. 
 
 SEC. 1. Cleavage in one direction. 
 
 A. Micaceous. Cleavage leaves tough, flexible, elastic. Occurs 
 
 in crystals, shreds, flakes. Black, brown, gray or white. 
 Transparent-translucent. Mica, p. 50. 
 
 B. Micaceous. Cleavage leaves tough, flexible, non-elastic. In 
 
 crystals, shreds, masses. Usually green to dark green. 
 Chlorite, p. 98. 
 
 C. Often micaceous. Leaves flexible but non-elastic. Greasy feel, 
 
 very soft, marks cloth. White, greenish, gray. Usually in 
 foliated masses. Translucent. Talc, p. 102. 
 
 D. Leaves somewhat flexible but showing cross cleavage cracks 
 
 when bent; in one direction fibrous, the other brittle forming 
 rhombs. Soft, scratched by finger nail, but not greasy in feel. 
 Usually colorless, white or reddish; transparent to trans- 
 lucent. In crystals, masses, seams. Gypsum, p. 111. 
 
 E. Leaves have a brilliant metallic luster, like polished steel. 
 
 Hematite (micaceous variety) p. 91. 
 
THE DETERMINATION OF ROCK MINERALS 125 
 
 F. Leaves brittle; lozenge shaped outline. Usually white, trans- 
 
 lucent. Scratched by the knife. Crystals in cavities. 
 Heulandite, p. 104. 
 
 G. Not micaceous, massive, brittle. Very hard, not scratched by 
 
 knife or feldspar. Yellowish green to dark green, translucent. 
 In crystals or masses. Epidote, p. 73. 
 
 SEC. 2. Cleavage in two directions. 
 
 A. Two cleavages at or very nearly at 90 degrees. Brittle, hard, 
 
 not scratched by knife but by quartz. Usually of a light 
 color, white, pink to red or gray, translucent. In crystals, 
 grains, masses. Feldspar, p. 34. 
 
 B. Usually of a dark color, greenish to black; in grains or short 
 
 prisms; sometimes light colored in metamorphic rocks and 
 then often elongated columnar in cleavage direction. Cleav- 
 age good but not eminent; prismatic. Cleavage angles 87 
 and 93 degrees. Usually scratched by feldspar. Pyroxene, 
 p. 55. 
 
 C. Usually of a dark color, greenish to black. Apt to be in crystals 
 
 elongated or bladed in cleavage direction. Sometimes light 
 colored in metamorphic rocks. Cleavage very good with 
 shining surface. Cleavage angles 55 and 125 degrees. 
 Usually scratched by feldspar. Amphibole, p. 60. 
 SEC. 3. Cleavage in three directions. 
 
 A. Cleavages not at right angles, forming rhombs. Easily scratched 
 
 by knife. Usually white, sometimes tinted various shades 
 to black; transparent to translucent. In crystals, masses, 
 veins, etc. Calcite, p. 105, or Dolomite, p. 108. (If rhombic sur- 
 faces of crystals are curved, probably dolomite.) 
 
 B. Cleavages at right angles forming cubes, soluble, strong saline 
 
 taste. Transparent colorless or white, rarely tinted. In 
 crystalline masses. Halite, rock salt, p. 113. 
 
 C. Cleavage apparently as above. No perceptible taste. Easily 
 
 scratched by the knife. White, bluish. In crystalline 
 masses. Anhydrite, p. 113. 
 
 D. Apparent cleavages sometimes forming rhombs, sometimes 
 
 apparently cubic. Very hard, scratches quartz easily. In 
 hexagonal crystals, grains or lumps of a dark, smoky, or 
 bluish gray; more or less translucent. Corundum, p. 86. 
 
 SEC. 4. Has a fine fibrous or columnar structure 
 
 A, In opaque brown to black masses. Streak yellow-brown* 
 
 Limonite, p. 93. 
 
 B. In opaque red-brown to black masses. Streak brownish red. 
 
 Hematite, p. 91. 
 
126 ROCKS AND HOCK MINERALS 
 
 C. White or reddish; translucent. Brittle. Often radially fibrous. 
 
 Sometimes showing slender prismatic crystals. Difficultly 
 scratched by the knife. Occurs in cavities, veins or seams. 
 Natrolite, p. 103. 
 
 D. White or reddish ; translucent or transparent. Brittle. Often radi- 
 
 ally fibrous. Compound crystals often sheaf shaped. Scratched 
 by knife. In cavities, veins or seams. Stilbite, p. 103. 
 
 E. Shreds easily into fine, flexible fibers like cotton or silk. White 
 
 or light gray. a. Hornblende asbestus, page 65. b. White 
 to yellowish brown; silky; generally in veins in or associated 
 with serpentine. Chrysotile (serpentine) asbestus, page 101. 
 
 F. White or pale colors. Translucent. Brittle. Easily scratched 
 
 by knife but not by finger nail. In masses. Calcite, p. 105. 
 
 G. White to pale red. Silky luster, translucent. Brittle, soft, 
 
 scratched by finger nail. In masses and seams. Gypsum, 
 p. 111. 
 
 SEC. 5. Without good or apparent cleavage 
 
 A. Opaque, brass-yellow crystals with metallic luster. Not 
 
 scratched by the knife. Pyrite, p. 94. 
 
 B. Opaque, earthy, brown to brown-black masses. Streak yellow- 
 
 brown. Scratched by the knife. Limonite, p. 93. 
 
 C. Opaque, reddish brown to black masses, or crystals and grains, 
 
 iron black with metallic luster. Streak brownish red. 
 Scratched by the knife. Hematite, p. 91. 
 
 D. Opaque, iron black masses, grains or octahedrons with metallic 
 
 luster. Streak black. Magnetic. Not scratched by the 
 knife. Magnetite, p. 89. 
 
 E. Opaque, black grains or masses often with reddish tone. Luster 
 
 metallic to submetallic. Streak black to reddish black. Not 
 noticeably magnetic. Scarcely or not scratched by the knife. 
 Ilmenite, p. 90. 
 
 F. In garnet-shaped crystals or spherical. Usually dark red to 
 
 black and translucent. Brittle. Not scratched by feldspar. 
 Garnet, p. 70. 
 
 G. In garnet-shaped crystals. Colorless or white to gray white, 
 
 translucent. Not scratched by knife but by feldspar. Leucite, 
 p. 49, or Analcite, p. 103. 
 
 H . In transparent to translucent crystals or grains of a light yellow- 
 ish- or bottle-green color. Not scratched by feldspar. 
 Olivine, p. 67. 
 
 /. In prismatic crystals, generally slender, shiny and black with 
 triangular cross section. Not scratched by quartz. Tour- 
 maline, p. 78. 
 
THE DETERMINATION OF ROCK MINERALS 127 
 
 /. In grains, masses or hexagonal, pyramidal crystals. Con- 
 
 choidal fracture. Greasy to glassy luster. Colorless, white, 
 
 smoky, dark; transparent to translucent. Not scratched by 
 
 feldspar. Quartz, p. 83. 
 K. In grains or masses, rarely in crystals with rectangular or 
 
 hexagonal sections. Conchoidal fracture. Greasy, oily 
 
 luster. White, gray or reddish; translucent. Scratched by 
 
 feldspar. Nephelite, p. 47. 
 L. In grains or masses, generally of a bright blue color. Sodalite, 
 
 p. 48. 
 M . In masses, of a dark or yellowish green, easily scratched or cut 
 
 by knife. Serpentine, p. 100. 
 N. In masses, often somewhat foliated. Greasy feel; very soft, 
 
 marks cloth. White, greenish, gray. Talc, p. 102. 
 0. In hexagonal crystals, grains or lumps. Dark smoky or bluish 
 
 gray; translucent. Very hard, not scratched by quartz, 
 
 garnet or tourmaline. Corundum, p. 86. 
 P. In masses, compact or chalky. Friable, very soft, easily cut by 
 
 finger nail. Rubbed between the fingers has a soft soapy feel. 
 
 Kaolin, p. 96. 
 
 TABLE 2 
 
 A . The mineral has a metallic luster or is opaque and gives a 
 
 dark or strongly colored streak. 2.* 
 
 B. The mineral is without metallic luster or is transparent or 
 
 translucent on very thin edges and its streak is white or 
 light-colored. 6.* 
 
 A. Heated in the blowpipe flame the mineral burns and gives 
 
 off sulphurous fumes. Has a brass yellow color. Pyrite, 
 p. 94. 
 
 B. Heated in the reducing blowpipe flame becomes magnetic 
 
 when cold. Not brassy in appearance. Infusible or 
 very difficultly so. Iron oxides. 3. 
 
 A. Is magnetic without heating. Magnetite (and in part 
 
 Ilmenite}, p. 89. 
 
 B. Is only magnetic after heating. 4. 
 
 Heated in the closed glass tube gives water. Limonite, p. 93. 
 Gives little or no water. 5. 
 
 !A. Reacts for titanium. Ilmenite, p. 90. 
 B. No reaction for titanium. Streak brownish or Indian red. 
 Hematite, p. 91. 
 
 f> \ A . Fusible before the blowpipe (fusibility 1-5). 7. 
 j B. Infusible or very difficultly fusible. 17. 
 
 * The appended number in each case refers to that in front of a succeeding 
 section. 
 
 I A. 
 \ B 
 
128 ROCKS AND ROCK MINERALS 
 
 r 
 
 IB. 
 
 Become magnetic after heating before the blowpipe in 
 
 reducing flame. 8. 
 Do not become magnetic. 11. 
 
 !A. Soluble in hydrochloric acid with separation of silica, some- 
 times gelatinous. 9. 
 B. Insoluble in hydrochloric acid or only slightly acted on. 10. 
 
 A. Micaceous or foliated. Mica (Biotite or Lepidomelane), 
 
 p. 50. 
 
 B. Isometric crystals. Gelatinizes imperfectly. Garnet 
 
 (Andradite), p. 70. 
 
 C. Gelatinizes. Olivine, rich in iron Fayalite, p. 67. 
 
 A. Micaceous difficultly fusible. Biotite, p. 50. 
 
 B. Isometric crystals or spherical in shape. After fusion 
 
 gelatinizes with HC1. Dark red color. Garnet (Alman- 
 dite), p. 70. 
 
 C. Quietly and difficultly fusible. Greenish black or bronze- 
 10 brown. Good cleavage. Pyroxene (Hypersthene), p. 55. 
 
 D. Fuses with intumescence coloring flame strong yellow. 
 
 Black. Prismatic cleavage, angle 55 degrees. Amphi- 
 bole (Arfvedsonite), p. 60. 
 
 E. Fuses quietly, coloring flame yellow. Black, prismatic 
 
 cleavage, 93 degrees. Pyroxene (Aegirite), p. 55. 
 
 A. Readily and completely soluble in water; has a saline taste. 
 
 Halite, rock-salt, p. 113. 
 
 B. Difficultly soluble in water. After intense ignition colors 
 
 moistened turmeric paper brown. 
 
 a. Gives much water in closed glass tube, Gypsum, p. 1 1 1 . 
 
 b. Gives little or no water in closed tube, Anhydrite. 
 
 p. 113. 
 
 C. Soluble in hydrochloric acid without gelatinizing or separa- 
 
 tion of silica on evaporation. A drop of sulphuric acid 
 in the solution precipitates calcium sulphate. Apatite, 
 p. 95. 
 
 D. Soluble in hydrochloric acid with gelatinization. 
 
 a. Heated in closed glass tube gives off water. 12. 
 6. Heated as above yields little or no water. 13. 
 
 E. Soluble in hydrochloric acid, silica separates but no jelly 
 
 forms. 
 
 a. Heated in closed glass tube gives off water. 14. 
 
 b. Heated as above yields little or no water. 15. 
 
 F. Insoluble in hydrochloric acid. 16. 
 
 A. Fuses quietly to a clear transparent glass. White or 
 
 colorless ; in slender crystals or fibrous bunches. Natrolite,* 
 
 see Zeolites, p. 103. 
 5. A fragment in warm dilute hydrochloric acid gives off 
 
 minute bubbles of CO 2 gas. Cancrinite, see Felds- 
 
 pathoids, p. 48. 
 
 11 
 
 12 
 
THE DETERMINATION OF ROCK MINERALS 129 
 
 A. 
 
 Fuse rather easily 
 before the blow- 
 pipe, coloring 
 the flame strong 
 yellow. Dissolve 
 easily in very 
 dilute nitric 
 acid and gela- 
 tinize. 
 
 a. The nitric acid solution gives a pre- 
 
 cipitate with silver nitrate solution 
 121). Color usually blue. 
 lite, p. 48. 
 
 b. The nitric acid solution gives a pre- 
 
 cipitate with barium chloride solu- 
 tion. Hauynite-Noselite, p. 48. 
 
 c. No reaction with silver nitrate or 
 
 barium chloride. Nephelite. p. 47. 
 
 B. 
 
 14 
 
 15 
 
 Difficultly soluble in hydrochloric acid and colors the flame 
 very little. Has a good cleavage in two directions. Anor- 
 thite, see Feldspars, p. 34. 
 
 A. Usually in greenish masses, compact, greasy, sometimes 
 
 fibrous. Difficultly fusible. Serpentine, p. 100. 
 
 B. Fuses quietly to a clear glass coloring flame yellow. Gen- 
 
 erally in colorless or white garnet-like crystals. Analcite, 
 p. 103. 
 
 C. Fuses with swelling and intumescence. Commonly in 
 
 sheaf-like or radiated crystals. Stilbite, p. 103. 
 
 D. Fuses as in C. Crystals have a fine cleavage with pearly 
 
 luster and lozenge-shaped section. Heulandite, p. 104. 
 
 A. Fuses quietly to a glassy globule. Slowly acted on by 
 hydrochloric acid. Good cleavage in two directions; one 
 
 11 i f* 11 1 t ! /~vfj 
 
 generally shows fine parallel twinning lines. Often 
 
 frayish or bluish with a play of colors. Labradorite, see 
 eldspars, p. 34. 
 
 B. Fuses quietly to white globule. Easily soluble in hydro- 
 chloric acid; solution evaporated to dryness, residue 
 moistened with little hydrochloric and dissolved in water 
 and filtered, ammonia produces little or no precipitate. 
 Wollastonite, CaSiO 3 , a variety of Pyroxene, generally of 
 a white color. 
 
 A. Micaceous. Cleave into thin flexible elastic plates in one 
 
 direction. Micas, p. 50. 
 
 B. Micaceous. Cleaves into thin plates, flexible but not 
 
 elastic, micaceous. Very soft and has a greasy feel. 
 Color white, gray or greenish. Talc, p. 102. 
 
 C. Cleavable, micaceous, but cleavage plates not elastic, though 
 
 flexible. Soft, but not so soft as talc. Color green, 
 usually rather dark green. Chlorite, p. 98. 
 
 D. Not micaceous. Solid and brittle. Good cleavage in two 
 
 directions at or about 90 degrees. Generally light 
 colored, red or gray. Hard, cannot be scratched by 
 knife. Difficultly fusible. Feldspar, p. 34. 
 E. Before the blowpipe fuses with swelling and bubbling. 
 Very hard, scratches feldspar. Generally in black 
 columnar crystals, sometimes red or green. No cleavage. 
 Tourmaline, p. 78. 
 
130 ROCKS AND ROCK MINERALS 
 
 F. Fuses quietly. Gelatinizes with hydrochloric acid aftei 
 
 fusion. Crystals as on page 70 or in spherical forms. 
 Very hard. No good cleavage. Garnet, p. 70. 
 
 G. Fuses with swelling and intumescence to a black slaggy 
 
 mass which gelatinizes in hydrochloric acid. Powdered 
 mineral on intense heating in closed glass tube yields a 
 little water. Yellowish to blackish green. Epidote, p. 73. 
 H. Fuses quietly or with little intumescence. Generally 
 scratched by feldspar. 
 
 a. Prismatic cleavage with angle of 87 degrees. Pyroxene 
 
 p. 55. 
 
 b. Prismatic cleavage with angle of 55 degrees. Amphi- 
 
 bole, p. 60. 
 
 7. Fuses with intumescence to a greenish or brownish glass 
 which will gelatinize with hydrochloric acid. Vesuvianite, 
 p. 75. 
 
 A. After intense ignition before the blowpipe gives a brown 
 
 stain when placed on moistened turmeric paper. 18. 
 
 B. Dissolves in hydrochloric acid but without gelatinizing or 
 
 yielding a residue of silica on evaporation. 19. 
 
 C. a. Dissolves in hydrochloric acid and gelatinizes. Olivine, 
 
 17 J P- 67 - 
 
 b. Reacts for fluorine (see topaz 22 F.). Chondrodite, p. 82. 
 
 D. Dissolves in hydrochloric acid, does not gelatinize but 
 
 silica separates. 20. 
 
 E. Insoluble in hydrochloric acid. 
 
 a. Can be scratched by glass or a knife point. 21. 
 
 b. Cannot be scratched by glass or the knife. 22. 
 
 (A. Effervesces freely in cold dilute acid. Calcite, p. 105. 
 B. Effervesces freely in hot but not in cold acid. Dolomite, 
 p. 108. 
 
 A. Heated in the reducing blowpipe flame becomes magnetic. 
 
 a. Little or no water in closed tube; streak brown-red. 
 
 Hematite, p. 91. 
 
 b. Water in closed glass tube; streak yellow-brown. 
 
 Limonite, p. 93. 
 
 B. In hexagonal crystals usually. Gives reactions for phos- 
 
 phorus. A little dilute sulphuric acid gives a precipitate 
 of white CaSO 4 , in the cold concentrated solution of 
 mineral in hydrochloric acid. Readily scratched by the 
 knife. Apatite, p. 95. 
 
 A. Commonly in compact green masses. Sometimes fibrous. 
 
 like asbestus, then white or brownish or yellowish. Greasy 
 20 feeling, easily scratched by knife. Serpentine, p. 100. 
 
 B. In spherical or garnet-shaped crystals. White to gray 
 
 Leucite, p. 49. 
 
21 i 
 
 22 
 
 THE DETERMINATION OF ROCK MINERALS 131 
 
 A. Micaceous. Cleavage leaves tough and elastic. Micas, 
 
 p. 50. 
 
 B. Micaceous. Cleavage leaves tough and flexible but not 
 
 elastic. Intense ignition in closed tube gives water. 
 Color green. Chlorite, p. 98. 
 
 C. Very soft and has a greasy feeling. Talc, p. 102. 
 
 D. Clay-like, compact or mealy. Leaves undissolved silica in 
 
 the phosphorus salt bead. Gives water in the closed 
 glass tube. Kaolin, p. 96. 
 
 A. Extremely hard. Scratches quartz. Generally has a 
 
 parting that looks like cleavage. Corundum, p. 86. 
 
 B. No cleavage; conchoidal fracture. Scratches feldspar. 
 
 Sometimes in hexagonal crystals with pyramid at end. 
 Quartz, p. 83. 
 
 C. Prismatic cleavage. Becomes black before the blowpipe 
 
 and very fine splinters fuse with difficulty. Brown to 
 green or greenish black. Pyroxene (enstatite-hypers- 
 thene), p. 55. 
 
 D. Good cleavage in two directions at 90 degrees or nearly so. 
 
 Generally light in color, red or gray. Scratched by 
 quartz. Fusibility = 5. Feldspars, p. 34. 
 
 E. In prismatic crystals, often twinned ; scratches quartz ; red- 
 
 brown to brownish black; intense ignition in closed tube 
 gives a little water. Staurolite, p. 76. 
 
 F. Reaction for fluorine when heated in tube with soda meta- 
 
 phosphate. With cobalt nitrate reacts for alumina (see G 
 below). One good cleavage. Scratches quartz. Topaz, 
 p. 81. 
 
 G. Powdered mineral moistened with cobalt nitrate and 
 
 intensely heated by the blowpipe on charcoal becomes blue 
 (alumina}', a, in stout rectangular prisms, often full of 
 impurities, not scratched by knife. Andalusite, p. 77; 6, 
 in bladed, generally blue, crystals; scratched by knife 
 parallel to cleavage, but not at right angles to it. Cyanite, 
 p. 78. 
 
 H. No crystal form or structure. Effervesces in Na 2 CO 3 bead. 
 Yields a little water in closed tube on intense ignition. 
 Opal, etc., p. 86. 
 
PART III. 
 
 ROCKS. 
 
 CHAPTER VI. >*~ 
 
 GENERAL PETROLOGY OF IGNEOUS ROCKS. ^ 
 
 IT has been previously stated that all rocks may be 
 divided into three great natural groups, the igneous, the*, 
 sedimentary and the metamorphic. The igneous are 
 those which have been formed by the solidification of 
 molten masses from within the earth. With reference to 
 their origin they have also at times been called the primary 
 rocks because the material which composes the other two 
 classes has been originally derived from igneous rocks 
 which, from time to time, have been formed either in or on 
 the upper part of the earth's crust or from the earth's 
 original crust itself. And if we follow the view that the 
 earth was once molten, the original cooling crust must 
 have been of the nature of igneous rock. Hence in this 
 sense the igneous rocks are the primary ones. 
 
 Distinguishing Characters of Igneous Rocks. The char- 
 acters of the igneous rocks, by which they may be 
 distinguished from the sedimentary and metamorphic 
 ones, are of two kinds field characters and specimen 
 characters. The field characters are those which can only 
 be observed in the field by studying the mass of rock in 
 its relation to surrounding masses, or in other words its 
 mode of occurrence, that is, whether it is a dike, a laccolith* 
 a lava sheet, etc. These modes of occurrence will be 
 presently described. If that of a given rock mass can be 
 
 132 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 133 
 
 clearly determined it indicates, more definitely than any- 
 thing else, if it is igneous in origin or not. 
 
 But very often it happens that the boundaries of a rock 
 mass are so covered or obscured that its relations to the 
 surrounding rocks and its mode of occurrence cannot be 
 told, or it is often necessary to determine the nature of a 
 specimen which has been removed from a parent mass 
 which is not accessible for study. In this case we are 
 compelled to fall back upon those characters of the rock 
 which are inherent, and to be observed by an examination 
 of the material of the outcrop or specimen. Of these 
 there are three principal ones which distinguish the 
 igneous from the sedimentary and metamorphic rocks. 
 They are: 
 
 a. Entire absence of fossils. 
 
 6. The material composition. 
 
 c. The arrangement of the material, texture or structure. 
 
 The first character is an obvious one, but it is largely of 
 negative value since many sedimentary and most meta- 
 morphic rocks do not contain fossils. 
 
 The second refers to whether the rock contains glass 
 or is wholly made up of mineral grains, and if the latter, 
 the kinds and relative amounts of the minerals present. 
 If a rock is made up wholly or in part of glass it is cer- 
 tainly of igneous origin. The presence of certain minerals 
 is also proof of igneous origin, but no general rule by which 
 a rock may be certainly stated to be of igneous origin from 
 its mineral composition can be given. This would have 
 to be done from a knowledge of the different kinds of 
 igneous rocks themselves, as they are described in a fol- 
 lowing chapter. The third character is that the igneous 
 rocks present a homogeneous appearance; that a surface 
 of the rock in one direction is like a surface in any other 
 direction that they do not show the stratified, banded, 
 or foliated structures which are characteristic of the 
 sedimentary and metamorphic rocks. In addition there 
 
134 ROCKS AND ROCK MINERALS 
 
 are certain minor structures which sometimes appear in 
 igneous rocks, such as the amygdaloidal, which are 
 highly characteristic and will be described later. 
 
 There are exceptions to the rules given above in a and c, but at 
 the outset it is better for the student to consider them as if absolute, 
 and the exceptions, which will be discussed in their appropriate 
 places, will take care of themselves. 
 
 Occurrences of Igneous Rocks. 
 
 There are two chief modes of occurrence of igneous 
 rocks recognized by geologists, the extrusive and the 
 intrusive. In the extrusive the molten mass or magma 
 rising from depths below has attained the surface, come 
 out upon it, solidified and formed the rock. They are 
 also called effusive and sometimes volcanic rocks, though 
 they are not always connected with volcanoes. In the 
 case of the intrusive rocks the magma has stopped before 
 attaining the surface and has cooled and solidified, sur- 
 rounded by other rock masses of the earth's upper crust. 
 Each of these cases has a number of recognized sub- 
 divisions; with the extrusive rocks depending on the 
 conditions under which the magma was ejected and with 
 the intrusive rocks on the relation which the mass bears 
 to the rocks which surround it. Following the course of 
 the magma upward we will describe first the intrusive 
 and then the extrusive modes of occurrence. 
 
 Intrusive Modes of Occurrence. These are dikes, 
 sheets, laccoliths, necks, stocks and bathyliths. Various 
 other modes have been recognized and described, but for 
 simplicity's sake they may be regarded as modifications 
 of these which have just been mentioned. The simplest 
 form of intrusion is that of the dike, and this will be 
 described first. 
 
 Dikes. A dike is the result of the simple filling of a 
 fissure in rock masses by molten magma from below* 
 which there solidified. In shape, its extension in length 
 and breadth is great as compared with its thickness. It 
 
PLATE 2. 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 135 
 
 may " cut, " that is, pass through, other igneous rocks or 
 through sedimentary or metamorphic ones, whatever the 
 material was in which the fissure was formed. In passing 
 through sedimentary rocks it always cuts at some angle 
 across the planes of stratification; if parallel to them it 
 becomes an intrusive sheet. A dike may be of all sizes 
 from a fraction of an inch in thickness up to half a mile; 
 from two or three feet up to twenty are the ones most 
 commonly observed; it may be but a yard or two long as 
 exposed on the surface, or it may be many miles ; a great 
 dike in the north of England has been traced over a 
 hundred miles. The plane of extension of a dike in most 
 cases appears to be vertical or nearly so; often it is inclined 
 at varying angles to the vertical plane. This angle of 
 inclination is called the hade of the dike, and the direction 
 which its outcrop takes in intersecting the horizontal 
 plane is called its trend. Dikes may have attained the 
 surface and given rise to lava outflows, or they may not 
 and have been exposed only by subsequent erosion. In 
 the processes of erosion they may have resisted better 
 than the surrounding rock and thus project as walls, a 
 common feature; or they may have resisted less well and 
 have become ditches, which is less common. Dikes very 
 often show pronouncedly the columnar structure described 
 later, the columns lying at right angles to the walls. 
 Where dikes have cut through sedimentary rocks they 
 have often changed and altered them for some distance in 
 the manner described under contact metamorphism. A 
 view of a dike cutting a sheet of igneous rock and stratified 
 beds is seen in Plate 2. 
 
 Intrusive Sheets. It frequently happens that where 
 molten magma is being forced upward through, or into, 
 stratified rocks, that it attains a place where the con- 
 ditions are such that it is easiest for it to spread out in a 
 layer between the sedimentary beds. This frequently 
 happens where the beds are weak and easily penetrated, 
 as in shales, thinly bedded sandstones, etc. The form of 
 
136 ROCKS AND ROCK MINERALS 
 
 such a. mass is like that of a dike, but unlike the latter it 
 lies concordantly along the planes of stratification. Such 
 sheets may be only a foot or less in thickness, and from 
 this up to several hundred feet or even more: they may 
 spread over many miles in extent. Like dikes they often 
 show a columnar structure, the columns being perpen- 
 dicular to the upper and lower surfaces and thus often 
 vertical. Sometimes they break, dike-like, across the 
 strata and are continued along a new horizon. They are 
 usually of the same hard, firm rock at top and bottom, 
 and to some extent have altered and changed the sedi- 
 mentary beds both above and below them: these char- 
 acters distinguish them from surface flows of lavas which 
 have been buried by later deposits of sediment upon 
 them. They are most apt to occur in connection with 
 larger and more important intrusions of magma, such as 
 stocks, laccoliths, etc., as accompanying or dependent 
 features. In regions where thick extensive sheets occur 
 and the strata have been dislocated, faulted, and upturned 
 they often give rise, through erosion, to prominent topo- 
 graphic features as illustrated in the trap ridges of southern 
 New England, northern New Jersey, and in Scotland at 
 Edinburgh and in many other places. In Great Britain 
 and frequently in Canada an intrusive sheet is called a 
 sill. Examples are shown on Plates 2 and 3. 
 
 Laccoliths. These are great lenticular masses of igneous 
 rock lying between stratified beds which infold them. If 
 in the forming of an intrusive sheet the supply of material 
 from below goes on faster than it can spread at a given 
 thickness laterally, the strata above will begin to arch up, 
 and if the process continues a great thick half lens, 
 flat below and rounded above, of liquid rock, will be 
 formed.* A cross section of such a one is shown in Fig. 
 63. They are apt to run out into intrusive sheets or be 
 accompanied by them. Also on the flanks of folding, 
 uplifting mountain ranges where the folding strata are 
 subjected to horizontal pressure they may tend to open, 
 * Increased viscosity of magma also helps in this result. 
 
PLATE 3. 
 
 LACCOLITH, AND INTRUSIVE SHEETS OF BASALT, IN 
 
 SANDSTONES, SHONKIN SAG, MONTANA. 
 
 (U. S. Geological Survey.) 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 137 
 
 and such openings be filled with magma from below in 
 measure as they open, as illustrated in Fig. 64. In 
 general, laccoliths are 
 more or less circular or 
 oval in ground plan, 
 and while sometimes 
 symmetrical as in the 
 diagrams they are apt 
 
 Fig 
 
 CrossSectionof(lLaccolitll 
 
 not to be so but more 
 or less distorted in shape. The floor may be flat or tilted 
 as in the figures. They differ from intrusive sheets only 
 in being extremely thick in comparison with their lateral 
 
 extension, and all gra- 
 dations between the two 
 may be found. They 
 may be a mile or more 
 in thickness and a num- 
 ber of miles in diameter, 
 
 or but a few hundred 
 Fig. 6 4 . Section of an inclined Laccolith 
 
 above are usually stretched, thinned and broken in the 
 process of formation. Like intrusive sheets they alter 
 and change the strata above and below by contact 
 metamorphism. They are most apt to occur in weak 
 beds of shale, etc., the stronger, thicker beds of sand- 
 stone and limestone being up-arched. The best exam- 
 ples that are known are found in the region of the 
 Rocky Mountains, where in many places they are not 
 uncommon. 
 
 Cases have been described where the roof of a laccolith has been 
 ruptured and driven upward by the magma rising like a plug through 
 the strata. It has been suggested that such forms be called bys- 
 maliths (Greek, plug rocks). It has also been suggested that when 
 a body of magma is injected into the stratified rocks like a laccolith, 
 but of indefinite shape and without the relations to the planes of 
 stratification which a laccolith has, such a mass be termed a chonolith 
 (Greek, mold used in casting rock). 
 
138 ROCKS AND ROCK MINERALS 
 
 Necks. When a volcano ceases its activity and becomes 
 extinct the column of magma, occupying the conduit 
 leading to unknown depths below, may solidify and form 
 a mass of igneous rock. Erosion may cut away a great 
 part of the light porous ashes and lavas, leaving this more 
 solid and resistant rock projecting, as shown by the 
 heavy line abc in Fig. 65. Or the level of erosion may 
 continue to descend into the rocks which form the base- 
 ment on which the volcano 
 is built, all traces of the 
 ashes, lavas, etc., being 
 swept away and only this 
 mass being left to mark 
 its former site. Such a 
 
 Fig. 65. Section Through a Volcano .. , . , 
 
 mass of rock is known .as 
 
 a volcanic neck. It is commonly more or less circular 
 in ground plan and may be from a few hundred yards up 
 to a mile or more in diameter. The rocks about them 
 are apt to be fissured and filled with dikes and in many 
 cases, if stratified, with intrusive sheets. 
 
 Stocks. This term has been applied to large bodies of 
 intrusive rock which in the form of magma have ascended 
 into the upper region of the earth's crust and there solid- 
 ified. They have become visible by extended erosion and 
 tend to have a more or less circular or elliptic ground 
 plan. Their plane of contact cuts across the inclosing 
 rocks, is more or less irregular, and the mass may widen in 
 extent as it descends. Their size may be anything from 
 a few hundred yards to many miles in extent. Since 
 they are apt to form protuberant topographic features 
 through erosion they are sometimes, especially in Great 
 Britain, called bosses. The distinction from a volcanic 
 neck is not one of size alone, though necks tend to be 
 smaller than stocks, but lies in the fact that the term 
 neck is employed only when there is evidence that there 
 has been extrusive volcanic activity from it. Some 
 stocks were doubtless necks, but this cannot now be 
 
PLATE 4. 
 
 Bf 1 
 

GENERAL PETROLOGY OF IGNEOUS ROCKS 139 
 
 proved. The granite hills of New England, of Scotland 
 and of other old eroded mountain regions are often stocks 
 or bosses. 
 
 Bathyliths. This term is used in a general way to 
 designate those huge irregular masses of igneous rock, 
 which, underlying the sedimentary and metamorphic 
 ones or sometimes cutting through them, have been 
 exposed by erosion. They are seen in the oldest exposed 
 areas of the crust where they are characteristically accom- 
 panied or surrounded by metamorphic rocks, as in eastern 
 Canada, or in mountainous regions where they form the 
 central cores or masses of the ranges, as in the Alps. 
 They differ chiefly from stocks in their much greater size, 
 as they are not infrequently many thousands of square 
 miles in surface area. 
 
 While some stocks are clearly intrusive and have displaced the 
 rocks whose site they occupy, the mode of formation of others and 
 of bathyliths is still a subject of speculation. Some have held that 
 they have attained their position by melting and assimilating the 
 previous formation and thus replacing it, while others have urged 
 the view that it has been ruptured, uplifted and driven out by the 
 invading mass and then eroded away. Various modifications of 
 these views have been suggested, but geologic science is not yet in a 
 position to pronounce definitely upon their correctness. 
 
 Extrusive Igneous Rocks. For petrographical purposes 
 two chief modes of extrusion may be recognized, the 
 quiet one, giving rise to outwellings of magma in the liquid 
 state which then solidifies to rock, and the explosive, in 
 which the material by the violent action of gases is pro- 
 jected into the air and falls in a solid but fragmental form. 
 
 Quiet Eruption; Lava Flows. When the magma rises 
 to the surfaces and outpours it is then called lava. The 
 solidified material is often called a sheet of lava or extrusive 
 sheet. Such flows often come from volcanoes; the 
 extrusions of some, like those now active in Hawaii, being 
 wholly of this nature, while in others they alternate with 
 or succeed projections of explosive fragmental material. 
 
140 ROCKS AND ROCK MINERALS 
 
 In other cases they are not connected with volcanic 
 eruptions but have taken place as quiet outwellings from 
 numerous fissures. This has sometimes occurred on a 
 huge scale, as in the Columbia River region of the north- 
 western United States, in western India and in the north 
 of the British Isles. In these regions the repeated lava 
 flows are thousands of feet in depth and cover areas of 
 from 100,000 to as much as 200,000 square miles. 
 
 Not infrequently sheets of lava have sunk below sea-level and 
 been covered by deposits, or they have originated on the sea floor and 
 have been covered. Such buried extrusive sheets are distinguished 
 from intrusive ones by the fact that they have not altered or changed 
 the sediments above them by contact metamorphism (qu. vid.), and 
 their upper surfaces usually show the structures common to the 
 surface of lavas, such as the vesicular, amygdaloidal, scoriaceous 
 and ropy ones described later. 
 
 Explosive Eruption; Tuffs and Breccias. When a 
 magma attains the surface in the canal of a volcano it 
 may give rise to quiet flows of lavas as mentioned above, 
 or if its viscosity is sufficient and it is charged with vapors 
 under great tension it will give rise to explosive activity, 
 
 
 Fig. 66. Diagram to Illustrate the Occurrence of Igneous Rocks : b, bathylith; 
 s, stock; , volcanic neck forming v, a volcano with tuff s and breccias; I, I, lacco- 
 liths; i, intrusive sheet; e, extrusive sheet; d,d, dikes. Horizontal distance 
 shown, thirty miles; vertical distance, three miles. 
 
 and the material will be projected into the air to fall in 
 solid fragmental form. Owing to the expansion of the 
 vapors, chiefly steam, the projected pieces usually have 
 a more or less pronounced vesicular structure, and vary 
 in size from those weighing perhaps several hundred 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 141 
 
 pounds to dust so fine that it floats for long periods in the 
 air. According to size these may be roughly classified 
 as follows. Pieces the size of an apple and upward are 
 called volcanic bombs; those the size of nuts are termed 
 lapilli; those the size of small peas or shot volcanic ashes; 
 while the finest is volcanic dust. The coarser material, 
 the bombs, ashes and lapilli, falls around the vent and 
 builds up the cone; the lighter ashes and dust, carried 
 by air currents, tend to fall after these and at greater 
 distances. The beds of coarser material thus produced 
 are termed volcanic conglomerate or more commonly vol- 
 canic breccia, while the finer is known as tuff.* 
 
 General Characters of Igneous Rocks. 
 
 Since igneous rocks are formed by the consolidation of 
 molten magmas it is evident that the nature of a rock 
 produced must in large measure depend upon the chem- 
 ical composition of the magma which forms it. For 
 most rocks are composed of mineral grains, and the kinds 
 and relative amounts of these must depend upon the 
 kinds and relative amounts of the chemical elements 
 which form the molten fluid. It is pertinent therefore 
 to inquire what the general chemical character of the 
 earth's magmas is' like and if there are any general rules 
 which appear to govern their composition. 
 
 Chemical Composition of Magmas. We cannot of 
 course subject a molten magma directly to investigation, 
 but this may be essentially done if an average sample of 
 an igneous rock is subjected to chemical analysis. Several 
 thousand such analyses have been made of rocks from 
 all parts of the world, and these results show, as might be 
 expected from the discussion given on page 17 and 
 following, that the magmas and therefore the rocks are 
 
 * (Volcanic tuff was formerly commonly called volcanic tufa, but 
 at the present time it is customary to restrict the word tufa to 
 deposits from aqueous solution, especially those of a calcareous 
 nature.) 
 
142 
 
 ROCKS AND ROCK MINERALS 
 
 made up of the following oxides : silica, SiO 2 ; alumina, 
 Al 2 Os' } iron oxides, both ferric, Fe 2 O 3 , and ferrous, FeO; 
 magnesia, MgO; lime, CaO; soda, Na 2 O, and potash, K 2 O. 
 Other oxides, including water, are also present but in 
 such relatively small amounts that they do not exercise 
 any controlling influence and may be neglected. 
 
 The variations in chemical composition which are 
 shown in the magmas are in a general way exhibited in 
 the following table of selected analyses. 
 
 
 I. 
 
 II. 
 
 III. 
 
 IV. 
 
 V. 
 
 VI. 
 
 VII. 
 
 VIII. 
 
 Si0 2 . 
 A1 2 3 
 
 56.6 
 22.4 
 
 65.5 
 17.8 
 
 72.5 
 13.1 
 
 65.1 
 16.2 
 
 56.0 
 15.6 
 
 49.2 
 12.0 
 
 40.1 
 7.8 
 
 38.4 
 0.3 
 
 Fe 2 3 
 
 1.8 
 
 0.7 
 
 1.7 
 
 1.1 
 
 1.2 
 
 2.8 
 
 7.3 
 
 3.4 
 
 FeO . 
 
 0.8 
 
 1.2 
 
 1.0 
 
 3.2 
 
 6.3 
 
 8.8 
 
 8.6 
 
 6.7 
 
 MgO. 
 
 1.3 
 
 1.0 
 
 0.6 
 
 2.3 
 
 6.8 
 
 9.3 
 
 23.7 
 
 45.2 
 
 CaO . 
 
 0.3 
 
 1.9 
 
 1.0 
 
 4.0 
 
 7.3 
 
 10.6 
 
 6.5 
 
 0.4 
 
 Na 2 0. 
 K 2 . 
 
 8.5 
 7.3 
 
 5.6 
 5.6 
 
 4.2 
 4.9 
 
 4.0 
 2.5 
 
 - 2.2 
 1.3 
 
 1.9 
 1.7 
 
 1.2 
 0.5 
 
 |o. 
 
 Rest . 
 
 1.4 
 
 0.7 
 
 0.7 
 
 1.6 
 
 3.3 
 
 3.2 
 
 4.5 
 
 5.7 
 
 Total . 
 
 100.4 
 
 100.0 
 
 99.7 
 
 100.0 
 
 100.0 
 
 99.5 
 
 100.2 
 
 100.2 
 
 I, Nephelite Syenite, Serra di Monchique, Portugal; II, Syenite, 
 High wood Mountains, Montana; III, Granite, Castle Mountains, 
 Montana; IV, Quartz Diorite, Electric Peak, Yellowstone Park; 
 V, Diorite, Montgomery County, Maryland; VI, Gabbro, Red 
 Mountains, Montana; VII, Peridotite, Devonshire, England; VIII, 
 Dunite, Tulameen River, British Columbia. 
 
 Variation of Magmas and Mineral Composition. It is 
 
 not to be understood that all the different varieties of 
 magmas are represented by these analyses; they are only 
 selected to show the most prominent and general features 
 of variation. Certain of these can be readily seen by 
 observing the table. Thus in the first three analyses it 
 is evident that silica, alumina and the alkalies, potash 
 and soda, are the chief oxides composing them, while 
 lime, iron and magnesia play a very subordinate part. 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 143 
 
 It is therefore evident that if such magmas should 
 crystallize into minerals they would be mostly composed 
 of alkalic feldspars because these are composed of silica, 
 alumina and alkalies. Again, if we regard the amounts 
 of silica in these three and remember that orthoclase, 
 potash feldspar, contains about 65 per cent of silica and 
 albite, the soda feldspar, about 68, it is clear that in 
 No. Ill there is more silica than needed to form the alka- 
 lies and alumina into feldspars, and there will therefore 
 be extra silica which will crystallize as free quartz. In 
 No. I, on the contrary, there is not enough silica to turn 
 all of the alumina and alkalies into feldspar, and a certain 
 amount of some mineral, such as nephelite, which con- 
 tains these oxides in combination with a smaller amount 
 of silica must be formed to compensate this deficiency. 
 In No. II the per cent of silica is very nearly that required 
 for the pure feldspars, and these will make up the great 
 bulk of the rock with little either of quartz on the one 
 hand, or of nephelite on the other. 
 
 If now we turn our attention to the oxides of lime, 
 iron and magnesia, it is evident that the minerals which 
 they produce, such as biotite, hornblende and pyroxene, 
 will have but a subordinate role in the first three rocks, 
 but in Nos. IV-IX these oxides continually increase 
 while silica alumina and alkalies also decrease, and even- 
 tually the last two vanish and the silica becomes very 
 low. Expressing this in terms of minerals, if the magmas 
 crystallized, it is evident that in these four the ferro- 
 magnesian minerals those containing iron or magnesia 
 or more commonly both such as pyroxene, amphibole 
 or olivine, would play an increasingly important role, and 
 that the last rock would be wholly composed of them, 
 while feldspars correspondingly become less important 
 and ultimately disappear. 
 
 In this connection the variation of lime deserves a sepa- 
 rate word because lime has a dual function : it may form 
 a feldspar with alumina and silica which then commonly 
 
144 
 
 ROCKS AND ROCK MINERALS 
 
 combines with soda feldspar to form plagioclase (soda- 
 lime feldspar), or it may enter into the ferromagnesian 
 minerals, pyroxene and amphibole. It generally does 
 both and thus for a time as we follow the table of analyses 
 from left to right, as lime increases, the quantity of both 
 plagioclase and of ferromagnesian minerals increases also. 
 Coincident with this the alumina also increases somewhat. 
 Variation shown by Diagrams. The facts which have 
 been stated above may be shown in a graphic manner by 
 
 means of a simple dia- 
 gram, Fig. 67. Thus in 
 the place of the analyses 
 of the foregoing table 
 we may draw verti- 
 cal lines, one for each 
 analysis, at equal dis- 
 tances apart and on each 
 
 Fig. 67. Diagram to Illustrate Chemical 
 Variation of Igneous Rocks 
 
 line set off a vertical 
 distance in millimeters 
 equal to the per cents 
 of each oxide in the 
 analysis. Through these 
 points lines are drawn 
 corresponding to each 
 oxide, the iron and 
 magnesia from the simi- 
 larity of function they 
 
 exhibit being united in one line. The equal distances for 
 each analysis at the foot of the diagram thus serve as 
 abscissas and the percentages are ordinates, while the 
 connecting lines approach curves which show the mutual 
 relations of the oxides. In the description of the variation 
 of the oxides it was pointed out how this caused a corre- 
 sponding variation in the minerals produced by the crys- 
 tallization of the magmas composed of these oxides. By 
 considering the relative amounts of the important minerals 
 which each type of analysis would produce we can con- 
 
PLATE 6. 
 
 A. Anorthosite, all Feldspar. B. Syenite, mostly Feldspar. 
 
 C. Diorite, some Feldspar. D. Peridotite, no Feldspar. 
 
 CONTRAST OF FELDSPATHIC AND FERROMAGNESIAN 
 ROCKS. 
 
: . 11. -l 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 145 
 
 struct a diagram, Fig. 68, which will show the variation 
 of the minerals in a general way in the common rocks. 
 It also shows the relative proportions of the minerals in 
 the more common and important kinds of igneous rocks, 
 
 Fig. 68. Diagram to Illustrate the Variations and Relative Proportions of the 
 Minerals Composing the Important Igneous Rocks. 
 
 and it serves as a basis for their classification as will be 
 explained later. The relative proportions of the minerals 
 are given in the vertical direction, the variation and pas- 
 sage of one kind of rock into another in the horizontal 
 direction. 
 
 It should be repeated that these diagrams and the 
 table of analyses are not to be taken in a hard and fast 
 manner as representing the limits of variation and all the 
 possible mineral combinations of igneous rocks. This 
 would be very wide of the truth. Other analyses might 
 be selected which would yield different diagrams, and if 
 of rare and uncommon rocks, they might be very different 
 indeed, but in a general way these may be accepted as 
 showing the more important chemical and mineralogical 
 features which distinguish the common kinds of igneous 
 rocks from one another. 
 
 Minerals of Igneous Rocks. From what has been 
 
146 ROCKS AND ROCK MINERALS 
 
 stated in the foregoing sections it is evident that the more 
 important minerals which compose the igneous rocks are 
 the feldspars, quartz and the ferromagnesian group. For 
 purposes of classification to be explained later it is con- 
 venient to contrast the ferromagnesian on the one hand 
 with the quartz and feldspars on the other. Recalling 
 that silica (/Si'C^) and alumina (A/ 2 3 ) are prominent 
 substances in the composition of these latter minerals, 
 and following American petrographic usage, we may term 
 this group the saZic one. More specifically the prominent 
 minerals of the igneous rocks are given in the following 
 table: 
 
 SALIC GROUP. FERROMAGNESIAN GROUP. 
 Alkalic Feldspar Pyroxenes 
 Plagioclase Feldspar Amphiboles 
 Quartz Biotite 
 Olivine 
 
 Nephelite 
 
 Sodalite Iron Ores 
 
 Corundum 
 
 The last three in the salic group are of much less impor- 
 tance than the first three on account of their restricted 
 occurrence; the iron ores, hematite, ilmenite and mag- 
 netite, though so widely distributed that nearly all igneous 
 rocks contain one or more of them, are of less importance 
 than the other ferromagnesian minerals because they 
 usually form only a very small proportion of all the 
 minerals in the rocks. A mineral, like these, which may 
 be quite evenly distributed through a rock but makes only 
 a small part of its mass is called an accessory component 
 in contradistinction to those which form its main bulk 
 and are termed chief or essential components. 
 
 The chemical and physical characters of the minerals 
 mentioned in the above list have been described under 
 their appropriate headings in Part II, to which reference 
 may be made, for further information concerning them. 
 
 Order of Crystallization. If a polished surface of a 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 147 
 
 coarse-grained rock be attentively studied with a lens, or 
 better if a thin section be observed under the microscope, 
 it will usually be found that there are more or less distinct 
 evidences that all the minerals composing it have not 
 crystallized simultaneously but successively. Thus in 
 Fig. 69 the crystals of biotite mica (M) contain grains and 
 octahedrons of black iron ore, magnetite; they occur also 
 in the other minerals. They are evidently older than the 
 mica because they are inclosed by it. The mica is older 
 than the soda-lime feldspars or plagioclases (P) because 
 it abuts into them with its own crystal faces or is partly 
 
 Fig. 69. Diagram to Illustrate Successive Crystallization 
 
 inclosed by them as they grow around the already formed 
 crystals. In the same way the plagioclase has its own 
 form as regards the alkali feldspar, orthoclase (0) and 
 the quartz (Q), which surround it, and is therefore 
 judged to be older than they are. When the orthoclase 
 and quartz are considered they do not show any crystal 
 boundaries with respect to one another, and their crys- 
 tallization is therefore judged to be more nearly simul- 
 
148 ROCKS AND ROCK MINERALS 
 
 taneous. The order of crystallization as thus worked out 
 in this particular case is: first, magnetite, then biotite 
 mica, then plagioclase, and lastly orthoclase and quartz. 
 
 The studies which have been made of igneous rocks 
 teach that in general the order of crystallization is : first, 
 the oxides or ores of iron, then ferromagnesian minerals, 
 then soda-lime feldspars, then alkalic feldspars (and 
 feldspathoids) and lastly quartz. One observes from 
 
 this, as illustrated in the 
 
 1. Magnetite, Fe 3 4 . adjoining table, that the 
 
 2. Pyroxene, (MgFe)Ca(SiO 3 ) 2 . process begins with metallic 
 3 Plagioclase $ niCaA! 2 Si 2 O 8 . oxides which contain no 
 
 ' lnNaAlSi 3 O 8 . silica, that next come the 
 
 4. Orthoclase KAISi 3 8 . ferromagnesian minerals, 
 
 5. Quartz, SiO 9 . .,. ^ 
 
 ortho and metasilicates, then 
 
 feldspars which contain more 
 
 silica and finally quartz or free silica. Thus there tends 
 to crystallize out successively minerals richer and richer in 
 silica. It is not to be understood however that one mineral 
 necessarily finishes its period of crystallization before 
 another one begins as in A, but rather that they overlap 
 
 Orthoclase. Quartz. 
 
 Quartz. 
 
 as in B, that is, that one may begin before another has 
 finished, and continue after the former has ceased. Expe- 
 rience shows that with orthoclase and quartz the overlap 
 is so great that they crystallize nearly simultaneously, 
 only orthoclase usually begins and quartz finishes. 
 
 Insolubility vs. Infusibility. A molten silicate magma is 
 to be regarded as a complex solution of some compounds in 
 others, like a solution of mixed salts in some solvent such 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 149 
 
 as water. As the heated solution cools, a point is reached 
 where some compound, or mineral, becomes insoluble in 
 the resulting solution and it therefore crystallizes out. 
 The statement that is sometimes made that the minerals 
 crystallize in the order of their fusibility is entirely wrong; 
 thus from the preceding paragraph we see that pyroxene 
 crystallizes before quartz ; now pyroxene is rather readily 
 fusible before the blowpipe, while quartz is infusible. It 
 is not therefore a question of infusibility but of solubility 
 which determines the order of crystallization. 
 
 Influence of Mineralizers. Experience teaches us that 
 those magmas which attain the surface in volcanoes and 
 in lava flows contain large quantities of volatile sub- 
 stances, especially water vapor, which they give off, 
 frequently with explosive violence. It was formerly 
 considered that the magmas imbibed these from the 
 moisture laden rocks with which they came in contact on 
 their way to the surface. At present these volatile sub- 
 stances are generally held to be wholly or in large part of 
 magmatic origin, that is, original constituents of the 
 earth's interior molten masses, contained therein under 
 pressure. Without further regard to the theories of how 
 they came to be there we know that the magmas contain 
 them and that they are of great importance in a number 
 of ways in the formation of igneous rocks. The most 
 important of these is water, but carbon dioxide, fluorine, 
 boric acid, sulphur and chlorine are also prominent and 
 may produce important results. The work of various 
 investigators, especially the French, has shown, that 
 while some minerals such as pyroxene, magnetite, lime 
 feldspar, olivine and nephelite may be artificially pro- 
 duced by fusing their constituents together and allowing 
 the molten mass to cool slowly, other minerals such as 
 hornblende, biotite, orthoclase and quartz do not form 
 in dry fusions in the same way. For their production 
 more or less of the volatile substances mentioned above 
 must be present, especially water vapor. These sub- 
 
150 ROCKS AND ROCK MINERALS 
 
 stances appear to act in two ways: in one in a chemical 
 manner since some minerals, such as biotite and horn- 
 blende, always contain small quantities of water (in the 
 form of hydroxyl, - OH) or fluorine or both, and these 
 are consequently necessary for their production; and 
 second, in a physical manner in that they lower the 
 melting point of the fusion and greatly increase its fluidity. 
 Thus orthoclase, albite and quartz which have extremely 
 high melting points but only crystallize at much lower 
 temperatures, in a dry fusion become so viscous on cooling 
 that they are unable to crystallize and therefore solidify 
 as glasses. The addition of water under pressure lowers 
 the temperature of solidification and increases the fluidity 
 or mobility of the melted mass and permits such move- 
 ment of the molecules that they can arrange themselves 
 in crystal form, and the above minerals are produced. 
 These substances then, such as water, fluorine, etc., which 
 exert so important a function in processes of crystalliza- 
 tion and on the formation of igneous rocks are called 
 mineralizers. As crystallization progresses, the amount 
 of them, beyond what is chemically (and to some extent 
 mechanically) retained in the minerals, is gradually 
 excluded from the solidifying rock mass to play an active 
 role in new and important processes, such as the forma- 
 tion of pegmatites, contact metamorphism and others 
 which will be described later in their appropriate places, 
 
 Texture of Igneous Rocks. 
 
 It has been pointed out that igneous rocks vary in the 
 kinds and proportions of the minerals that compose them 
 and that this variance is mainly due to the chemical 
 composition of the magmas from which they are derived. 
 Another important way in which these rocks vary is in 
 their texture. Thus one rock may be made up of mineral 
 grains so large that the different minerals are easily 
 distinguished, while in another the grains are so small as 
 to defy identification by the eye or simple lens, Again 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 151 
 
 the grains may be approximately all of about one size or 
 they may vary in size, some being relatively large while 
 the rest are minute. Or again the conditions may have 
 been such that the magma had no opportunity to crys- 
 tallize but solidified as a simple glass, or to only partly 
 crystallize and formed a mixture of glass and crystals. 
 Such variations for the most part are independent of 
 chemical composition, they depend upon the physical 
 conditions under which the molten mass has solidified, and 
 thus a magma of a given composition may appear in any 
 one of the states mentioned above if subjected to the 
 proper conditions. 
 
 The characteristic features which a rock exhibits in this 
 respect constitute its texture, and rocks are distinguished 
 and classified in one way according to their textures, 
 just as in another way they are distinguished and classified 
 according to their mineral composition. 
 
 Factors influencing Texture. If a strong hot solution 
 of a salt, such as common alum in water, is allowed to 
 cool very slowly and regularly, comparatively few centers of 
 crystallization will be set up, and the few resulting crystals 
 will have a long period of growth and will be of good size. 
 If, on the contrary, the cooling is very rapid a great number 
 of centers of crystallization will form, the period of growth 
 will be short and a great number of very small crystals 
 will result. The same is true in the molten liquids from 
 which the igneous rocks are formed. If the cooling has 
 progressed with great slowness and regularity then 
 coarse-grained rocks are produced; if the cooling is rapid 
 then they are fine-grained and the cooling may take place 
 so quickly that there is no opportunity for complete 
 crystallization, and rocks wholly or in part composed of 
 glass will result. The rate of cooling then is a prominent, 
 and in fact the most prominent, factor in the production 
 of rock texture. In addition to the temperature there 
 has been a tendency in the past to ascribe also a prominent 
 role to the pressure. The idea involved is that if a magma 
 
152 ROCKS AND ROCK MINERALS 
 
 remained liquid within the earth at a given temperature 
 and if for any reason the pressure increases, a point will 
 be eventually reached where it will be forced to crystallize 
 and become solid, since in so doing its volume would 
 be reduced. Decrease of temperature and increase of 
 pressure would then work together. While this may be 
 true in theory it does not seem probable that the pressures 
 obtaining in the upper region of the crust are a very 
 prominent factor in this direction, since geological obser- 
 vation has shown that a particular variety of texture can 
 be found unchanged through a range of 10,000 feet 
 vertical. Still it cannot be denied that pressure probably 
 has some influence on the process of crystallization and 
 the production of rock texture. 
 
 The presence of mineralizers, especially water, has 
 undoubtedly a strong influence on the texture, particu- 
 larly in the siliceous rocks, for this greatly increases the 
 fluidity of such magmas and, as they cool down and the 
 crystallizing points of the different minerals are reached, 
 they still retain their mobility instead of becoming stiffly 
 viscous. This increases the range of movement of the 
 mineral molecules forming, and enables larger crystals to 
 grow and a coarser texture to be produced. As we shall 
 see later this reaches its maximum in the pegmatite dikes. 
 
 In connection with what has just been stated chemical 
 composition of the magmas has a certain influence in 
 producing texture. This shows itself in two ways. Those 
 magmas which are deficient in silica and especially those 
 which contain much iron and magnesia and which are 
 shown in the right hand side of the diagrams given on a 
 previous page remain liquid, without becoming stiffly 
 viscous, to much lower temperatures than those with 
 high silica which are found expressed in the middle of the 
 diagrams. This liquid condition enables them to crys- 
 tallize more freely and to form in consequence coarse^ 
 textured rocks under circumstances where the siliceous 
 magmas would produce only types fine-grained in texture 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 153 
 
 or even glassy from inability to crystallize completely, 
 owing to increasing viscosity. The other way in which 
 chemical composition influences texture is this. Dif- 
 ferences in composition in the magmas mean of course 
 differences in the kinds of minerals which they produce. 
 Different minerals crystallize in different shapes and 
 although, owing to interference with one another, they 
 may not form in perfect crystals, they tend to take such 
 shapes. Some form tabular shapes, others spherical or 
 cuboidal grains or elongated prisms. Thus, while the 
 general size of such grains may remain the same through- 
 out a mass of rock, such differences in shape will 
 produce corresponding differences in what we may call 
 the pattern or fabric of the rock and thus influence its 
 texture. 
 
 Relation of Texture to Geologic Mode of Occurrence. It 
 is evident that the condition most favorable for the pro- 
 duction of coarse-textured rocks conditions described 
 in the preceding discussion as slow cooling, pressure and 
 the presence of mineralizers will, in general, be best 
 realized when the magma is in large mass and deeply 
 buried in the earth's crust so that it is completely 
 enveloped by surrounding rock masses. The heavy 
 cover retains the heat and the mineralizers and gives in 
 part the pressure. Such igneous rocks, formed in depth, 
 will only become exposed to our observation when con- 
 tinued erosion has carried away the superincumbent 
 material. They are often therefore called plutonic or 
 abyssal and sometimes massive rocks, and referring to 
 what has been described as the modes of occurrence of 
 the igneous rocks it can be seen that bathyliths, stocks 
 and the lower part of volcanic necks may be particularly 
 expected to exhibit such texture and nearly always 
 do so. 
 
 On the other hand, when the magmas attain the surface 
 and are forced out in volcanic eruptions, lava flows, etc., 
 entirely different conditions will prevail; there is no cover 
 
154 ROCKS AND ROCK MINERALS 
 
 to retain the heat and the cooling in consequence is rapid 
 Also the pressure has been relieved, and with loss of cover 
 and pressure the mineralizers quickly depart. As a 
 result fine-grained ; dense, compact textures are formed, 
 or the cooling may be so rapid that crystallization may 
 fail to occur, either wholly or in part, and rocks entirely 
 or partly composed of glass may be produced. When 
 rocks are more or less glassy it is in general very good 
 evidence that they solidified as surface lavas. 
 
 In the smaller intrusive bodies, such as the dikes, 
 sheets and laccoliths, the conditions in general are between 
 the two sets just described. The volume relative to the 
 surrounding rocks is less, the loss of heat and mineralizers 
 more rapid than in the stocks and bathyliths, and, since 
 in general the depth is less, the pressure is diminished. 
 Thus the textures are usually between those of the larger 
 abyssal masses and the effusive lavas. But the conditions 
 in these occurrences are apt to be very variable, and in 
 accord with this we find the textures sometimes dense 
 like the effusives but very rarely glassy and some- 
 times coarse-granular like the larger abyssal masses. In 
 them, too, the function of chemical composition described 
 in the preceding section is often most strongly displayed. 
 Thus highly siliceous dikes and sheets of fine grain will 
 be found associated under the same geological conditions 
 with other ones low in silica and high in iron and magnesia 
 of relatively much coarser grain. 
 
 It is especially in these occurrences and in the surface 
 lavas that the porphyritic texture, to be presently described, 
 is most liable to be found. 
 
 Textures of Igneous Rocks. Based on the principles 
 which have been enunciated in the foregoing sections the 
 textures of igneous rocks for megascopic study may be 
 classified as follows: 
 
 Grained. All sizes of grain large enough to be seen 
 with the unaided eye. Example, ordinary granite. 
 
PLATE 6. 
 
 3. Coarse Grain. 
 EVEN-GRANULAR TEXTURE. 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 155 
 
 Dense (aphanitic). The rock is crystalline, i.e. not 
 glassy, but the grains are too fine to be perceived by 
 the eye. Example, many felsites. 
 
 Glassy. The rock can be distinctly seen to be wholly 
 or in part composed of glass, as in obsidian. 
 
 The distinctions stated above relate in part to its 
 crystallinity or degree of crystallization, for all grades of 
 transition between rocks composed wholly of glass, partly 
 of glass and partly of crystals and wholly of crystals 
 exist, though to be perceived by the unaided eye the glass 
 must form a great or the greater part of the rock. It 
 relates also in part to the absolute sizes of the crystal 
 grains or what we may term the granularity. 
 
 The phanerocrystalline (Greek, <avepos, visible) rocks 
 according to the size of grain can be divided as follows: 
 (See Figs. 1, 2 and 3, Plate 6.) 
 
 Fine-grained, the average size of the particles less than 
 1 millimeter or as fine as fine shot. 
 
 Medium-grained, between 1 and 5 millimeters. 
 
 Coarse-grained, greater than 5 millimeters or as great as 
 or greater than peas. 
 
 But another very important feature of texture is that 
 of the pattern or fabric and this, for megascopic work, is 
 chiefly due to the relative sizes of the crystal grains in a 
 given rock. There are two chief kinds of fabric which 
 may be distinguished: 
 
 Even-granular fabric (or texture), grains of approxi- 
 mately the same general size. 
 
 Porphyritic fabric (or texture), grains of a larger size 
 contrasted with finer ones or with glass. 
 
 Even-granular Texture. While this means that in a 
 given rock the crystal grains have approximately the 
 same general size, as may be seen by referring to Plate 6, 
 it does not mean that they have necessarily the same 
 shape. Careful examination of granites which have this 
 texture will show that the dark mica is in many cases 
 
156 ROCKS AND ROCK MINERALS 
 
 present in well formed hexagonal tablets or crystals, 
 while the feldspars and quartz are in shapeless masses, 
 or the feldspar tends to have rough tabular or brick-like 
 shapes. This depends on the order of crystallization as 
 previously explained. 
 
 Porphyritic Texture. Porphyry. In this texture, when 
 typically developed, there is a sharp contrast between 
 larger crystals with definite crystallographic bounding 
 faces, which are termed phenocrysts (Greek, <aiVeiv, to 
 show), and the material in which they lie embedded, 
 called the groundmass. This groundmass may have the 
 textural characters described on a preceding page, it 
 may be even-granular, coarse or fine, it may be dense 
 or wholly or partly glassy. A rock with this textural 
 fabric is called a porphyry.* Examples are shown in 
 Plate 7. 
 
 Great variations are seen in the phenocrysts ; they may 
 be extremely numerous and the amount of groundmass 
 small or the reverse; they may be an inch or more in 
 diameter or they may be so small as to require close 
 observation to detect them; they may be of light-colored 
 feldspars and quartz or dark-colored ferromagnesian 
 minerals, hornblende, augite and pyroxene, or of both 
 kinds of minerals. Again, they may be extremely well 
 crystallized and afford such striking specimens of perfect 
 crystal development that they find a place in mineral 
 cabinets or they may be very poorly defined in crystal 
 form. And with increase in numbers and poor crystal 
 form, all degrees of transition into the even-granular 
 texture may be found. The porphyritic texture is 
 extremely common in lavas and in intrusives of small 
 mass such as dikes, sheets and laccoliths; it is rarer in 
 
 * The porphyritic texture is not a contrast of colors of mineral 
 grains but of sizes. Care must be taken, therefore, not to confuse, 
 for instance, a white rock consisting of grains of light-colored minerals 
 such as feldspar, in which are embedded a few conspicuous black 
 grains of a ferromagnesian mineral of the same size, such as horn- 
 blende, with a porphyry. 
 
PLATE 7. 
 
 A. With Phenocrysts of Feldspar. 
 
 B. With Phenocrysts of Augite. 
 PORPHYRY TEXTURE. 
 

GENERAL PETROLOGY OF IGNEOUS ROCKS 157 
 
 the abyssal rocks, but is sometimes seen, especially in 
 granites. 
 
 Origin of Porphyritic Texture. In the case of many 
 effusive rocks or lavas it is easy to understand why they 
 have a porphyritic texture. The lavas of many volcanoes, 
 as they issue to the outer air, are full of growing crystals, 
 often of considerable size, suspended in the molten fluid. 
 The latter, however, subjected to new conditions, is 
 forced to cool rapidly and assumes a fine-grained, or 
 dense crystalline, or even a glassy, solid condition with 
 these larger crystals embedded in it, and thus the com- 
 pleted rock has a porphyritic fabric. The same process 
 may serve to explain this texture in some of the smaller 
 intrusives, such as dikes and sheets, but it cannot serve 
 as a general explanation for all cases because in some 
 dikes, laccoliths, etc., there is good evidence that the 
 phenocrysts have not been brought thither but have 
 formed, like the rest of the rock, in the place where we 
 now find them. It also fails to explain the porphyritic 
 border of many granites and the large phenocrysts found 
 in other granites; nor does it explain the origin of the 
 phenocrysts themselves, why a few large crystals have 
 formed while the rest of the magma fails to crystallize. 
 Evidently some more general explanation is needed. 
 
 It has been previously shown that molten magmas 
 must be considered as strong or saturated solutions of 
 some compounds in others. As the mass cools down it 
 may become supersaturated. Now it has been shown 
 that some saturated solutions cannot crystallize spon- 
 taneously but require to be inoculated with a minute 
 fragment of the substance in solution; this is called the 
 metastable state. Other saturated or supersaturated 
 solutions either crystallize spontaneously or can be 
 induced to do so by shaking or stirring with a foreign 
 substance. 
 
 Miers has shown that the same solution may pass from 
 one to the other of these states in accordance with changes 
 
158 ROCKS AND ROCK MINERALS 
 
 of temperature, and suggests that a magma may be in the 
 metastable condition in which a relatively few crystals 
 induced by inoculation from the surrounding rocks are 
 growing as phenocrysts and by cooling pass into the labile 
 condition when spontaneous crystallization of the remain- 
 ing liquid will ensue and form the groundmass. Or it 
 may start in the labile condition when the formation of a 
 crop of phenocrysts will reduce it to the metastable state, 
 in which condition it may be erupted as a lava, or remain- 
 ing and cooling down it may pass into a new labile state, 
 thereupon crystallize and form the groundmass. The 
 recognition of these states in cooling saturated solutions 
 (and we must regard the molten magmas as such) seems 
 quite sufficient to explain the different variations of por- 
 phyritic texture which occur. 
 
 Some Structures of Igneous Rocks. 
 
 The word texture is reserved for those appearances of 
 the rocks which are occasioned by the size, shape, color, 
 etc., of the component crystal grains. Certain larger 
 features exhibited by the rocks may be classed under the 
 term of structure and will now be described.* 
 
 Vesicular Structures. When a molten magma rises to 
 the surface and especially if it issues in the form of lava, 
 the pressure upon it is relieved and the water and other 
 vapors it may contain are given off. This has a tendency, 
 if it is still soft and stiffening, to puff it up into spongy 
 vesicular forms as illustrated in Plate 8. In the case 
 of very siliceous lavas it may be entirely changed into 
 a light glass froth called pumice. Such forms are espe- 
 cially produced in the lava in the throat of a volcano, 
 where the issue of gases is rapid or in the top portion of a 
 flow. Except in a rare and very limited way on the sides 
 
 * An example of the difference between the two usages would be 
 this. A certain lava from flowage might appear in layers; the 
 layers are of rock composed of exceedingly fine particles. We 
 would say then that the lava had a banded structure and a very 
 fine compact texture. 
 
PLATE 8. 
 
 A. VESICULAR LAVA. 
 
 B. AMYGDALOIDAL BASALT. 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 159 
 
 of dikes they never occur in intrusive rocks, and the 
 presence of well-marked vesicular structure may be taken 
 as pretty sure evidence that the rock exhibiting it was 
 originally a surface lava. In the throat of a volcano such 
 spongy forms of lava may, by explosions of steam, be 
 driven in fragments into the air to fall as dust, ashes, 
 lapilli, etc., making volcanic tuffs and breccias as described 
 elsewhere. 
 
 Amygdaloidal Structure. Amygdaloid. When a lava 
 has been rendered spongy (vesicular, as described above), 
 it may be permeated by heated waters carrying material 
 in solution which may be deposited as minerals in the 
 cavities. This happens especially in basaltic lavas, and 
 the dark rock then appears filled with round or ovoid 
 whitish bodies which from a fancied resemblance to the 
 kernel of a nut are termed amygdules, from the Greek word 
 for the almond. The structure is called the amygdaloidal 
 and a rock exhibiting it is often termed an amygdaloid. 
 It is shown in Plate 8. While the smaller cavities are 
 usually filled solid, the larger ones are often hollow, the 
 minerals projecting in crystals from the walls as in geodes, 
 and from such amygdaloidal cavities some of the most 
 beautiful crystallizations are obtained. The minerals 
 most frequently occurring are quartz, which is sometimes 
 of the amethyst variety, calcite and particularly zeolites. 
 The basaltic lavas are an especial home of these latter 
 minerals, some of the more common kinds being analcite, 
 stilbite, natrolite, heulandite and chabazite. The basalts 
 of India, Iceland, Scotland, Nova Scotia and other 
 localities have furnished specimens which are known in 
 all mineral collections. 
 
 This structure is most commonly and typically 
 developed in surface lavas, that is, in effusive rocks, but 
 it is also seen at times in intrusive rocks, such as dikes 
 and sheets, especially at their margins. 
 
 Miarolitic Structure and Porosity. The volume which 
 a magma occupies in the molten condition is considerably 
 
160 ROCKS AND ROCK MINERALS 
 
 greater than that which it has when changed to a solid 
 crystalline rock. It is probably greater in the liquid state 
 than when cooled to a glass but how much we do not know. 
 This contraction in volume, in passing into the crystalline 
 state, is accompanied by a corresponding rise in specific 
 gravity. Thus an obsidian glass, consisting chiefly of 
 high silica with moderate amounts of alkalies and alumina, 
 has an average specific gravity of about 2.2-2.3, but the 
 same material crystallized into a quartz-feldspar rock 
 (granite) has a specific gravity of 2.6-2.7. There would 
 be a corresponding reduction in volume. 
 
 In general this contraction of volume, during the process 
 of crystallizing, produces minute interspaces or pores 
 between the mineral grains, and cracking and jointing of 
 the mass, a process described in the following section. 
 This production of pores accounts for the capacity of the 
 rocks to absorb moisture. It appears to be greatest in 
 the coarse-textured rocks, much less in the finer-grained 
 ones; greater in granites, less in diorites and other ferro- 
 magnesian rocks. In the case of porous vesicular lavas the 
 amount of pore space may be very great, but in ordinary 
 crystalline igneous rocks it is small, usually less than one 
 per cent of the rock volume. 
 
 In some cases, however, there may be distinct cavities 
 produced. These are commonly very small, sometimes 
 an inch or so in diameter and in rare instances as much 
 as several feet. It often happens that the crystal com- 
 ponents of the rock on the boundary walls of the cavity 
 are much larger in size than the average grain, and project 
 into it, bounded by distinct faces and of good crystal form. 
 One notices also, especially in granites, that the quartz 
 and feldspar crystals are often accompanied by those of 
 muscovite, topaz, tourmaline and others which are foreign 
 to the general mass of the rock but are common in peg- 
 matite veins. These are also well crystallized. The, 
 presence of the water vapor, fluorine, boron, etc., necessary 
 for their production, as well as the larger size and 
 
A. MIAROLITIC CAVITY. 
 
 B. MIAROLITIC CAVITIES PASSING INTO PEGMATITE. 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 161 
 
 distinct form of the crystals, shows that such mineralizers, 
 excluded elsewhere from the magma during the process 
 of crystallization, collected in these cavities, possibly 
 helped to enlarge them, and promoted the formation of 
 the unusual minerals and the good crystal forms which 
 they and the ordinary rock minerals exhibit. Such 
 hollow spaces are called miarolitic cavities and a rock 
 which contains them is said to have miarolitic structure, 
 from a local Italian name (miarolo) for the Baveno 
 granite which shows it. Such drusy cavities are distin- 
 guished from geodes and others, in which the minerals 
 have been deposited from solutions, by the fact that they 
 have no distinct wall separating the minerals from the 
 containing rock. They often furnish fine mineral speci- 
 mens. An example of one is seen in Plate 9. 
 
 Jointing of Igneous Bocks. The most important way 
 in which the contraction of a body of magma, after 
 cooling and crystallizing into rock, manifests itself is in 
 the production of joints. These are the cracks or fissures 
 which, running in various directions, divide the mass into 
 blocks, fitted together like masonry and usually according 
 to more or less definite systems. Sometimes this shows 
 itself in the formation of rudely cubic or rhomboidal 
 blocks, as shown in granites and other abyssal rocks, 
 sometimes in a platy parting which may be quite thin and 
 cause the rock mass at first glance to resemble sedimentary 
 beds, and sometimes in concentric or spheroidal forms 
 which develop rounded or ovoid bodies like melons as 
 the weathering and rock decay progresses. Platy and 
 spheroidal partings, and jointing on a small scale by which 
 the rock body is divided into little blocks, are most com- 
 mon in small intrusions in dikes, sheets, etc. and in 
 surface lavas. Such jointing is a matter of great geologic 
 importance in permitting the entrance of air and water 
 to act in the weathering and decay of rocks and in the 
 processes of erosion, especially the splitting and breaking 
 of them by the action of frost. As can be readily 
 
162 ROCKS AND ROCK MINERALS 
 
 inferred it is also of great practical importance in the 
 work of rock excavation, in mining operations and in 
 quarrying. (See Plate 10.) Were it not for such joints 
 almost every igneous rock mass would furnish suitable 
 material for quarrying, whereas on the contrary it is diffi- 
 cult to find a granite jointed on so large a scale that it 
 will furnish solid blocks, for example, like those from 
 which the celebrated Egyptian obelisks were made. 
 
 Columnar Structure. The most remarkable way in 
 which the jointing of a cooling mass of igneous rock, 
 explained above, manifests itself is in the production of 
 columnar structure. This is found both in intrusive and 
 extrusive occurrences and in all kinds of igneous rocks, but 
 is usually best displayed in basalts. The whole mass is 
 made up of columns, regularly fitted together, from a few 
 inches to several feet in diameter and from one foot to two 
 hundred feet or even more in length. An example is 
 shown in Plate 11. The celebrated Giant's Causeway on 
 the north coast of Ireland is one of the best known 
 examples of this. In the most perfect cases, as in the one 
 just mentioned, the cross sections of the columns are 
 regular hexagons and the columns are divided lengthwise 
 at regular intervals by cross joints whose upper surfaces 
 are shallow cup-shaped. The columns are always per- 
 pendicular to the greatest extension or main cooling 
 surface of the igneous mass, hence in a lava flow or intru- 
 sive sheet they are vertical assuming the flow or sheet 
 to be horizontal while in a dike they tend to be hori- 
 zontal. Such a dike when exposed by erosion tends to 
 resemble a stretch of cord- wood regularly piled. 
 
 The cause of this structure seems to be as follows. 
 When a homogeneous mass is cooling slowly and regu- 
 larly, centers of cracking tend to occur on the cooling sur- 
 faces at equally spaced intervals. From each central 
 interspace three cracks radiate outward at angles of 120 
 degrees from each other. These intersecting produce 
 regular hexagons and the cracks penetrating downward 
 
PLATE 10. 
 
 A. High Isle Quarry, Maine. 
 
 B. Allen Quarry, Mount Desert, Maine. 
 
 JOINTING IN GRANITE AND ITS USE IN QUARRYING. 
 
 (U. S. Geological Survey.) 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 163 
 
 make columns. This regular arrangement produces the 
 greatest amount of contraction with the least amount 
 of cracking, provided the centers are equally spaced. 
 But as the contractional centers are not always equally 
 spaced, three, four, five and even seven-sided columns 
 occur. The columns again, contracting lengthwise, break 
 into sections as they form. The same principle is also 
 seen in drying mud-flats which crack into polygonal 
 shapes and in the prisms of drying and contracting starch. 
 Such columns, however regular their appearance, are 
 not crystals but pieces of rock and should not be confused 
 with the hexagonal prisms produced by the crystallization 
 of certain minerals, such as quartz, beryl, etc., which are 
 due to an entirely different process. 
 
 Inclusions in Igneous Rocks. 
 
 Not infrequently there may be noticed in igneous 
 rocks masses which differ in mineral composition, color 
 and texture from the rock which includes them. They 
 may vary in size from a fraction of an inch to several yards 
 across. Study of them shows that sometimes they present 
 all the characters of distinct kinds of rock and by these, 
 and by their angular shapes, they show clearly that they 
 are fragments of pre-existent rocks which the magma on 
 its way upward has torn loose from the walls of its con- 
 duit and brought along, or blocks from the roof or sides of 
 the chamber, in which the magma came to rest, which were 
 loosened and sank into it. They may be composed of 
 other kinds of igneous rocks or of sedimentary ones, such 
 as shales, limestones, etc. In the former case they are 
 not usually much changed, but the fragments of stratified 
 rocks generally exhibit the results of intense metamorphic 
 action, such as described elsewhere, and are much altered. 
 In large intrusive masses inclusions of this character are 
 most apt to occur near the border. An inclusion in 
 granite is shown in Fig. 1, Plate 12. 
 
 In other cases the inclusions are composed of certain 
 
164 ROCKS AND ROCK MINERALS 
 
 minerals, especially the ferromagnesian ones, which occur 
 in the rocks and which by some process have been aggre- 
 gated into lumps, such as the masses of olivine crystals 
 often found in basalts. It is clear that such aggrega- 
 tion or growth of these minerals must have taken place 
 while the remainder of the rock was still in a liquid 
 condition. They have been termed segregations. 
 
 In still another kind the inclusions are indefinite in 
 form and often of boundary; they are apt to be drawn out, 
 lenticular, streaky in character and they may consist of 
 the same minerals as the main mass of the rock but in 
 quite different proportions, or they may contain dif- 
 ferent minerals. Thus one sees streaks in granite which 
 may be much richer in hornblende or biotite than the 
 enclosing rock. Some have held that these are due to 
 inclusions of other rocks which have been melted up and 
 then recrystallized and in some cases they may have had 
 this origin, but for the most part they are regarded by 
 the majority of petrographers as caused by streaks and 
 spots in the original magma of a different chemical com- 
 position from the main portion. The cause of such non- 
 homogeneousness in the magma is ascribed to differentia- 
 tion, as discussed elsewhere in this volume. Such streaky 
 portions are called -by the Germans schlieren and in default 
 of anything better this word is often used for them in 
 English. 
 
 Sometimes lavas show a streaky or even well banded 
 structure, portions differing from one another in com- 
 position or in texture having been drawn out in the 
 flowage. This is known as the eutaxitic structure. 
 
 Origin of Igneous Rocks Differentiation. 
 
 The fact that lavas differing decidedly from each other 
 in mineral and consequently in chemical composition 
 have been erupted by the same volcano at different 
 periods, early attracted the attention of geologists and led 
 to much speculation as to its cause. Thus felsites and 
 
PLATE 11. 
 
 ti 
 
 2 I 
 
 55 
 
 o .2 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 165 
 
 basalts have both been frequently noticed as the products 
 of eruption from a single vent. One explanation, which 
 used to be advanced, was that within the earth there were 
 two layers of magma, an upper one rich in silica, alumina 
 and alkalies, the other and lower, poor in silica but rich 
 in iron and magnesia; accordingly as the eruption came 
 from one or the other of these, felsites or basalts were 
 produced, while their mixtures gave rise to intermediate 
 products. It was soon seen, however, both on chemical 
 and geological grounds, that this view was insufficient to 
 explain the origin of all igneous rocks. 
 
 As the study of rocks progressed, other facts of a similar 
 nature came to light. Thus in the single rock mass com- 
 posing the core or neck of an old volcano,* where the 
 magma cooled under conditions favorable for the pro- 
 duction of the even-granular or granitic texture, it is not 
 infrequent to find that it is composed of two or more 
 distinct kinds of rock. The boundary between these 
 will sometimes show that one was erupted after the other 
 had solidified in its place, since fragments of the latter are 
 enclosed in the former. This is of course merely carrying 
 deeper down into the volcanic conduit the same facts 
 shown by the surface lavas previously mentioned. Other 
 cases, however, are of a different nature and of such 
 geological importance that they demand separate con- 
 sideration. 
 
 Border Zones. In recent years the study of deep-seated 
 intrusive masses, such as stocks of granite, syenite, etc., 
 which have become exposed by long continued erosion, 
 has shown that not uncommonly such masses have an 
 outer border or mantle of rock which differs in mineral 
 composition from the mass which it enfolds. The thick- 
 ness of such a border zone is very variable, even in the 
 same mass, and in places it may be lacking; it may be 
 several thousand feet thick or only a few hundred or even 
 less. While in general it bears some proportion to the 
 
 * See volcanic necks, page 138. 
 
166 ROCKS AND ROCK MINERALS 
 
 general size of the whole mass there is no rule about this 
 which can be stated. 
 
 In most cases this zone or border jades, as it is some- 
 times called, is produced by an enrichment of the rock 
 in the ferromagnesian minerals, such as pyroxene, horn- 
 blende, biotite and iron ore. Generally the enriching 
 minerals are the same as those more sparsely distributed 
 in the main rock body but very often different ones are 
 observed among them. From this it is clear that chemi- 
 cally the border zone is richer in iron and magnesia, and 
 to some extent in lime, than the main mass, with a corre- 
 sponding diminishing of silica, alumina and alkalies. 
 Since they contain less of silica, the acid oxide, they are 
 commonly called basic zones. Not all border zones, 
 however, are basic ones; a number of instances are known 
 where the margin of the intrusion is poorer in lime, 
 iron and magnesia and consequently in ferromagnesian 
 minerals than the interior rock body and therefore con- 
 tains more silica, alumina and alkalies, which expresses 
 itself miner alogic ally in greater abundance of feldspar 
 and sometimes of this and quartz. In this case they are 
 called acid border zones. Thus on the one hand intrusions 
 of syenite have been found which pass into pyroxenite at 
 the border while on the other hand syenite intrusions are 
 known which become granite towards the margin. It 
 must not be imagined that there is anything approaching 
 a contact between the two .kinds of rock. The one kind 
 passes gradually into the other without change in texture 
 and all the facts indicate that this arrangement was not 
 produced by successive intrusions of different magmas 
 but by some process in a single body of magma after it 
 had entered into its chamber. 
 
 Zoned Laccoliths. The zonal arrangement just men- 
 tioned is still more strikingly shown in the case of certain 
 laccoliths which have been found in Montana and else- 
 where. Where these have been laid bare and dissected 
 by erosion the study of them shows that they consist of a 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 167 
 
 body of rock of one kind, generally one consisting mostly 
 of pyroxene, enclosing within a core of rock of a totally 
 different kind, usually a syenite, which is of course chiefly 
 feldspar. A cross section through such a laccolith is 
 shown in the accompanying diagram, Fig. 70. 
 
 Fig. 70. Diagram of a Zoned Laccolith: a, feldspar rock; 6, pyroxene rock; 
 c, shales and sandstone; rf, underlying sheet of intrusive basalt. Figures 
 in feet are heights above sea-level. 
 
 That the pyroxenic rock once had the extension shown 
 by the restoration in the figure is known from other 
 examples in the neighborhood where the erosion has not 
 been so great, and it is still found above, enwrapping the 
 interior syenite. 
 
 Associated Complementary Dikes. Another phenom- 
 enon, of the same category as those just described, is seen 
 in the dikes so commonly found associated with larger 
 intrusive bodies, such as stocks of granite, syenite, diorite, 
 etc., where these have become exposed by dissective 
 erosion. They are in origin subsequent to the main mass 
 which they accompany and are found cutting it and also 
 the surrounding rocks. In the latter, these minor intru- 
 sions may appear, not only in the form of dikes, but also 
 in intrusive sheets, laccoliths, etc. These rocks are 
 divisible into two classes; in the first they are very poor 
 or entirely wanting in ferromagnesian minerals (salic 
 rocks) and have been called aplitic dikes, since the dikes 
 of aplite usually found associated with granites are the 
 most common and best known representatives of this 
 class. They have also been called leucocratic dikes (from 
 the Greek, prevailing white) in allusion to their general 
 light color, due to the fact that they are mostly composed 
 of feldspars or of these with quartz. They are generally 
 
168 ROCKS AND ROCK MINERALS 
 
 fine-grained rocks, sometimes of a sugar granular texture, 
 sometimes dense and to be classed as felsites. In some 
 cases they are porphyritic. They usually occur in narrow 
 dikes, a few feet wide and sometimes only an inch or even 
 less in breadth. 
 
 In the second class the rocks are heavy, dark or even 
 black, of basaltic aspect and composed chiefly of ferro- 
 magnesian minerals, iron ore, pyroxene, hornblende, 
 biotite and olivine, in variable amounts and with very 
 subordinate feldspar. They are very commonly por- 
 phyritic with good-sized phenocrysts of the minerals 
 mentioned above in a dense dark groundmass, though 
 these are often wanting. Such rocks have been called 
 lamprophyres (from the Greek, meaning glistening por- 
 phyry in allusion to the biotite), and are termed melano- 
 cratic rocks (/xcAavos, black). In our field classification 
 they would be named biotite melaphyre (or mica trap), 
 hornblende melaphyre, etc., according to the prevailing 
 phenocrysts. They also usually occur in narrow dikes 
 and are more apt to cut the surrounding rocks than the 
 main intrusive body they accompany, thus reversing the 
 custom of the aplites. 
 
 These two kinds of rocks, the aplitic, light-colored 
 feldspathic, and the lamprophyric, dark-colored, with 
 ferromagnesian minerals, are termed complementary 
 because taken together they represent the composition of 
 the main masses they accompany. If we could mix them 
 in amounts proportional to the bulk of their occurrence 
 we should obtain a rock whose chemical (and largely 
 mineral) composition would be that of these larger masses 
 upon which they appear to depend as satellite bodies. 
 In some cases this has been actually tested and proved. 
 When all the facts concerning their mode of occurrence are 
 taken into account they appear to have been formed by 
 secondary, later intrusions of the same magma producing 
 the larger stocks, which in some way has divided into 
 two unlike sub-magmas. If they should break through 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 169 
 
 to the surface they would give rise to lava flows also 
 unlike, to felsites and basalts, and thus explain in part the 
 phenomena noticed in many volcanoes. 
 
 It is to be understood of course that not all dikes, 
 sheets and laccoliths belong in this category of com- 
 plementary rocks. On the contrary we very often find 
 that the same magma which produces stocks, necks, 
 etc., occurs in intrusions of this character. They then 
 have the same minerals and composition as the larger 
 masses, or if independent bodies they usually contain both 
 ferromagnesian and feldspathic minerals in due amounts. 
 Only, as explained on page 153, they are liable to differ 
 in texture from the stocks and are very apt to be por- 
 phyries. Dikes, etc., of this kind have been called 
 aschistic, which means undivided, while the complementary 
 aplites and lamprophyres have been termed diaschistic, 
 which means divided, in allusion to their dual nature. 
 
 Differentiation. The varied lavas of volcanoes, the 
 marginal zones of stocks and necks, the zoned laccoliths 
 and the associated complementary rocks, which have been 
 described in foregoing sections, as well as other similar 
 features, present to us a body of geological facts that can 
 only be satisfactorily explained by the assumption that 
 in some way magmas, which form igneous rocks, have 
 the capacity of separating into sub-magmas, unlike the 
 original, but which, if mixed in proper proportions to a 
 homogeneous whole, would again reproduce it. Regard- 
 ing the division there seems to be in general two opposite 
 poles toward which the sub-magmas tend; to one con- 
 centrate the iron, magnesia and to a large extent the lime, 
 to the other the alkalies, alumina and to a great extent the 
 silica. The one gives us ferromagnesian rocks such as gab- 
 bro, the other feldspathic rocks such as granite. While 
 this is so in general, we find in detail the process infinitely 
 varied in nature; thus in some places one may observe a 
 division among the alkalies, an enrichment of potash 
 towards one pole as compared with soda or vice versa. 
 
170 ROCKS AND ROCK MINERALS 
 
 If the body of magma has come to rest in its chamber and 
 this process of differentiation takes place and it then, 
 crystallizing, solidifies and forms rock, it is evident that 
 such a rock body will be unlike in its different parts, and 
 marginal zones, zoned laccoliths, etc., will be produced; or 
 if further movements occur, producing new intrusions or 
 these with extrusions, then associated complementary 
 dikes, sheets and lava flows may occur. 
 
 This division into sub-magmas is termed the differen- 
 tiation of igneous magmas and the reality of it as a process 
 seems well established on geological grounds by a large 
 body of facts. That in some manner such a process 
 takes place and on the other hand the understanding of 
 how and why it does take place, are two entirely different 
 affairs, and while every one who is thoroughly conversant 
 with the facts is obliged to admit the former, a wide 
 diversity of views, owing to insufficient knowledge, pre- 
 vails in regard to the latter. Some phases of this subject 
 are discussed in the following paragraphs. 
 
 Formation of Zones and Ore Bodies. One partial 
 explanation that has been offered for the zoned structures 
 previously mentioned is of importance because it affords 
 at the same time an understanding of the origin of a 
 certain class of ore bodies which in some places are of 
 considerable extent and value. On page 148 it was shown 
 that there was a general order of crystallization of rock 
 minerals beginning with the iron ores, then passing into 
 the ferromagnesian silicates and finishing with the feld- 
 spars and quartz. In an enclosed body of magma, 
 crystallization would generally begin when the tempera- 
 ture had fallen to the proper degree. This would natu- 
 rally first occur at the outer walls where the effect of 
 cooling is felt. Against these tho iron ores and ferro- 
 magnesian minerals, the earliest to crystallize, would 
 form and, if the process were extremely gradual, slow 
 convection currents in the magma would bring fresh 
 supplies of material to crystallize there until large 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 171 
 
 amounts of these minerals had formed. This might go 
 on until the temperature had fallen to a point where the 
 main body of magma was compelled to solidify and the 
 rock mass as a whole produced. The outer margin would 
 be much enriched in the earlier formed minerals, giving 
 a zoned arrangement to the whole mass. In such places 
 at the margin the iron ores are sometimes so locally con- 
 centrated as to yield workable deposits of value, though 
 very commonly the ore is titaniferous and therefore can- 
 not be used commercially. The same explanation has 
 been offered for the occurrence of sulphide ores of iron 
 containing copper and nickel, of corundum and of other 
 useful minerals found in similar situations. 
 
 Origin of Salic Border Zones. The explanation given 
 above would show how marginal zones richer in ferro- 
 magnesian minerals might arise but it has been observed 
 that masses of granitic and syenitic rock are sometimes 
 poorer or deprived of these minerals at the margin of the 
 mass while the main part contains them in considerable 
 amounts, thus making salic zones. An explanation which 
 has been offered for this is as follows : If a solution of 
 a salt in a liquid (such as sea-water) be cooled down until 
 it is forced to crystallize (freeze) it is found that the sub- 
 stance in greatest excess, salt or liquid, will solidify first 
 until a certain definite proportion of dissolved salt and 
 liquid are obtained, called the eutectic mixture, when both 
 remaining salt and liquid will crystallize simultaneously 
 and the whole mass become solid. The proportion of salt 
 to fluid, forming the eutectic, varies with the kind of salt 
 and of solvent. Thus when sea water freezes the ice first 
 formed contains no salt, the latter forming in the remain- 
 ing water a brine of increasing strength until the eutectic 
 point is reached, when both solidify together. In the 
 case of granite and syenite the oxides composing the 
 quartz and feldspars are present in great excess and may 
 be considered the solvent for the others. It is possible 
 that under proper conditions these might solidify at the 
 
172 ROCKS AND ROCK MINERALS 
 
 outer margin, the other oxides, those of iron, magnesia, 
 etc., concentrating in the remaining portion and tending to 
 make an eutectic mixture. Thus when the whole solidifies 
 the inner part will contain ferromagnesian minerals, and 
 the outer part will be poor or wanting in them. In the 
 case of many diorites and gabbros, where the oxides of iron 
 and magnesia are in great excess, they would be the sol- 
 vent, and we should expect border zones of ferromagnesian 
 minerals. It is evident this explanation, and the one 
 previously given, which depends on the order of crystal- 
 lization, in the case of highly feldspathic rocks, are 
 opposed to each other; the first tends to make ferromag- 
 nesian zones around granite and syenite, the latter salic 
 ones. In the diorites and gabbros both tend to produce 
 margins richer in ferromagnesian minerals. 
 
 Zones by Absorption. It has also been suggested that 
 such zones are produced by the magma melting its con- 
 taining walls and thus, by absorbing foreign material, 
 becoming in composition, at its border, unlike the main 
 mass. Being thus unlike it would naturally have a 
 different mineral composition on solidification. It is 
 possible that this may have happened in some cases but 
 it cannot serve as a general explanation because in many 
 cases we find the border of an entirely different mineral 
 (and chemical) composition from that which it ought to 
 have if the rocks with which it came in contact had been 
 melted and absorbed. 
 
 General Explanation. It is obvious that the hypotheses 
 discussed above, while they may serve to explain border 
 zones and marginal ore deposits, do not give a general 
 explanation for the differentiation of igneous rocks. For 
 the occurrence of complementary dikes, of different lavas 
 from the same volcano, and the mixtures of different 
 types, which are not marginal, in the same stock, as well 
 as other facts, show clearly, that in general, differentiation 
 is not a division by a process of solidification, but one 
 which occurs in a magma in such a manner as to produce 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 173 
 
 separate bodies of differing liquids which may be inde- 
 pendently ejected or intruded. It must occur before 
 there is any solidification. While we see that this is so, 
 both from geological and chemical facts, no general 
 explanation, which is in all respects satisfactory, has been 
 offered for this process. Different hypotheses, which it 
 would be beyond the limits of this work to state and 
 discuss, have been suggested by various authorities, but 
 our knowledge of the physical chemistry of molten magmas 
 is yet too limited to know their proper value and appli- 
 cability. It is probable the processes of differentiation 
 are quite complex and that they are produced by a 
 variety of factors, the laws governing which must all be 
 taken into account in any general explanation. It is 
 known that molten artificial glasses and molten alloys 
 of metals, under conditions not yet well known, do not 
 remain homogeneous but undergo a kind of differentiation, 
 and it is along this line of experimental research that 
 light must be sought to explain the facts as we find them 
 in Nature. 
 
 Petrographical Provinces. Consanguinity. It has been 
 noticed in the study of rocks, that , those belonging to 
 certain regions have particular features which to a greater 
 or less degree are found to be distinctive of all the members 
 of the group which occur there. This is shown, sometimes 
 in the presence of particular varieties of minerals, some- 
 times in peculiar textures, sometimes in peculiarities of 
 chemical composition and usually in a combination of 
 these things. They may be shown in varying degrees by 
 all the different rocks of the region: thus, for example, by 
 syenites which are chiefly composed of feldspar and by 
 dolerites in which ferromagnesian minerals prevail; in in- 
 trusive stocks of granular rocks with their associated com- 
 plementary dikes and sheets and in lava flows of felsites 
 and basalts. These common characters are sometimes 
 strongly marked and at other times only to be seen by 
 the experienced observer. The fact that such distin- 
 
174 ROCKS AND ROCK MINERALS 
 
 guishing features occur in the different types of a certain 
 region and serve to indicate their relationship to one 
 another and to show a common origin by differentiation is 
 termed the consanguinity of igneous rocks, and that region 
 over which the rocks thus show genetic relations is called 
 a petrographic province, or comagmatic region. Thus the 
 comagmatic region of South Norway is characterized by 
 the extremely high percentage of soda in the magmas, 
 which gives rise to certain minerals and peculiar rock 
 textures; those of Italy and central Montana by very high 
 potash which shows itself in the formation of the mineral 
 leucite, common in such regions but rare or unknown else- 
 where; that of the western Mediterranean islands and 
 eastern Spain by an abnormally high amount of titanic 
 oxide in its rocks. 
 
 Such evidences of consanguinity in rock groups and the 
 proofs which they furnish of comagmatic regions cannot 
 usually be observed in field work and in the megascopic 
 study and determination of rocks. They generally 
 demand careful and complete investigation of thin sections 
 under the microscope, aided by chemical analyses in the 
 laboratory, together, with a broad acquaintance of the 
 literature of this subject, in order to be perceived and 
 appreciated. The matter, however, is one of great interest 
 and although one may not be either a chemist or petro- 
 grapher, he may yet appreciate the significance of its 
 bearing on the solution of problems of the greatest impor- 
 tance in geology. It is evident that before we can safely 
 theorize as to the origin and history of the earth we must 
 first know the nature of its component parts and the laws 
 governing their distribution. 
 
 Post-intrusive Processes, 
 
 When a body of molten magma has come to rest in the 
 chamber it is destined to thenceforth occupy as a solid- 
 ified rock mass, cooling and eventually crystallization 
 begin. From this point on, so far as the magma is con- 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 175 
 
 cerned, those factors are at work which have been 
 described elsewhere, and which in time will produce the 
 completed rock. During this period of crystallization 
 the volatile substances dissolved in the magma and 
 previously contained under pressure, such as fluorine, 
 boric acid, carbon dioxide and especially and chiefly 
 water, which have been already described as mineralizers, 
 are gradually excluded, except in so far as they may take 
 part in the chemical composition of some of the minerals. 
 
 This period in the history of the formation of a rock 
 body, when it is solidifying and giving off, as it crystal- 
 lizes, heat and vapors, is called the pneumatolytic (Greek, 
 gas, and to loosen), and these agents generate important 
 results. At the surface they give rise to hot springs, 
 fumaroles, solfataras, and similar secondary igneous 
 phenomena; in the depths they produce in the rocks 
 surrounding the igneous mass a variety of features known 
 under the term of contact metamorphism, and in the already 
 solidified parts of the igneous mass they bring about the 
 formation of pegmatite dikes, of greisen (described under 
 Granite) and in some cases of ore deposits; things which 
 are treated in the following sections. 
 
 Pegmatite Dikes or Veins. It has been previously 
 stated that when deeply formed stocks or masses of granite, 
 syenite, diorite, etc., have been laid bare by erosion they 
 are very frequently found to be cut by complementary 
 dikes of felsitic and basaltic aspect. In addition it is also 
 frequently observed that they are penetrated by dikes 
 which display certain definite characters, the most marked 
 of which is the very large and sometimes enormous size of 
 the individual minerals composing them. Such dikes 
 have been termed pegmatite dikes, from the name given 
 by Hauy to the intergrown masses of quartz and feldspar 
 found in them when they occur in granites. Dikes of 
 this character not only cut the stocks and batholiths, to 
 whose intrusion they owe their origin, but are also found 
 penetrating, as offshoots, the rock masses enveloping 
 
176 ROCKS AND ROCK MINERALS 
 
 them. There are a number of features which particularly 
 characterize them, as follows: 
 
 a. They consist in large part of the ordinary minerals 
 which compose the rock to which they belong, but these, 
 instead of having their regular order of successive crys- 
 tallization, show by their interpenetration that they 
 have crystallized more nearly if not entirely, simul- 
 taneously.* The size of the individual crystals is a 
 character that has been mentioned. Feldspar and quartz 
 may occur in crystals a foot or even several feet long, 
 apatite in dimensions like the handle of a broom, mica in 
 crystals yielding plates a foot or more in diameter and 
 other minerals in similar proportions. It is not to be 
 understood that these sizes represent the average; they 
 are the extremes which are, however, not infrequently 
 attained. Moreover, the essence of pegmatite structure 
 does not lie in mere size, for many rocks are very coarse- 
 grained which are not pegmatites, but rather in the other 
 qualities enumerated. 
 
 6. Another peculiar feature is that in many pegmatites 
 there is an obvious tendency for the minerals to grow 
 outward from the walls of the dike on either side and 
 project inward toward the center. This may become 
 very marked and there may even be an empty space at the 
 center into which the minerals project showing crystal 
 faces as in miarolitic cavities (page 159) or in the vuggs of 
 mineral veins. The whole effect is to produce in a rough 
 way a zoned, banded or ribbon structure, which is often 
 so perfectly seen in mineral veins. 
 
 c. Another character is the extreme variability in the 
 relative proportions of the component minerals from 
 place to place, a variability not seen in the main rock 
 mass. Thus in granite pegmatites traced along the 
 outcrop of the dike great variations in the relative amount 
 of quartz and feldspar may often be observed; in tracing 
 
 * See in connection with this the description of graphic granite 
 in the granite pegmatite veins, p. 212. 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 177 
 
 them outward from the parent mass into the enclosing 
 rocks they may even pass into pure quartz veins. In 
 other cases they may turn into fine-grained granite 
 (aplite) or felsite, and this change in the character of the 
 dike may occur quite suddenly. 
 
 d. They are very apt to contain accessory minerals 
 which are either not found at all in the main rock mass 
 or which microscopic examination shows are sparingly 
 distributed in very minute crystals. These minerals may 
 be divided into two classes. In the one their chemical 
 composition shows that they consist of the ordinary 
 oxides which compose the magmas, alumina, lime, iron, 
 soda, etc., plus the volatile elements or oxides which form 
 the mineralizing vapors. Common ones are tourmaline, 
 which shows the presence of boric acid; topaz and fluorite, 
 which demonstrate that fluorine was present and many, 
 of which muscovite mica is perhaps the most prominent, 
 which prove the important role played by water vapor. 
 The other class are characterized by their containing in 
 larger or smaller amounts the oxides of rare elements, 
 such as lithium, caesium, beryllium, molybdenum, cerium, 
 lanthanum, niobium, etc., elements which are detectible 
 with difficulty as minute traces or not at all in the main 
 rock mass. In combination with silica, titanic oxide, 
 phosphoric acid, zirconia, carbonic acid, fluorine, etc., 
 they give rise to a whole host of mineral combinations, 
 too numerous to mention, but of which beryl and spodu- 
 mene may be cited as examples. No sharp distinction 
 can be drawn between these two classes; many minerals 
 might be placed in either, but definite types of both, like 
 those mentioned, can readily be found. 
 
 These accessory minerals often occur also in crystals 
 of great size and sometimes aggregated together in places 
 in the dikes in very large amounts. It is due to this great 
 variety of minerals and the frequent size and perfection 
 of the crystals that the pegmatite dikes are mineralogically 
 of great interest and are, therefore, a favorite hunting 
 
178 ROCKS AND ROCK MINERALS 
 
 ground of mineral collectors. It is to be noted also that 
 each variety of magma (or rock) is characterized by 
 special mineral combinations in its pegmatite dikes, and 
 this applies not only to the ordinary minerals which com- 
 pose the rocks and distinguish them from one another 
 but also to the accessory ones as well. 
 
 Thus the mineral groups found in the pegmatites 
 associated with ordinary granites are quite different from 
 those with the nephelite syenites, as these in turn differ 
 from those of the gabbros. 
 
 Origin of the Pegmatites. The simultaneous method of 
 crystallization, the arrangements along the walls of the 
 dike, the variability in the proportions of the component 
 minerals and their frequent huge size all show that con- 
 ditions, different from those which attended the solidifica- 
 tion of the main rock masses, prevailed during the forma- 
 tion of the pegmatites. The presence of hydroxyl, fluorine, 
 boron, etc., also shows that mineralizing agents were 
 abundantly present. Bearing these facts, and those of 
 the geologic mode of occurrence, in mind, we can present 
 to ourselves a view of their origin which would be some- 
 what as follows: 
 
 When a body of igneous magma, such as will form a 
 stock or batholith, comes to rest in place it will commence 
 to cool. This will naturally take place first in the upper 
 and outer portions and here will begin the solidifying of 
 the mass by crystallization. As it becomes solid it breaks 
 up into jointed masses by contraction. The weight of 
 these masses, aided by the rock pressures from above upon 
 the still liquid material below, tends to force the latter 
 upward into the fissures in the solidified part and into 
 those of the surrounding rocks and produce dikes. If 
 differentiation is taking place and there is a concentration 
 of the iron, magnesia and lime towards the outer border, 
 as explained in previous sections, these dikes will be 
 complementary and we will find aplites (and felsites) 
 more commonly in the central mass, and the corresponding 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 179 
 
 basaltic lamprophyres more commonly in the outer portion 
 and in the surrounding rocks. 
 
 But as the process of crystallization goes on, the volatile 
 substances in the magma, and especially water in great 
 quantities, beyond what is retained by such minerals as 
 use them in their chemical composition, will be excluded, 
 and more and more as the gases accumulate they must 
 find their way outward. Thus they will tend to force 
 their way upward along the fissures in the solidified parts 
 above and at the sides. These cracks will therefore be- 
 come channel ways, not only for the still unconsolidated 
 magma as mentioned above (whether differentiated or not), 
 but also for the vapors which will collect in them and in 
 those of the immediately surrounding rock mantle under 
 pressures, which must often be enormous, until event- 
 ually they escape. It is evident from this that the 
 ascending magmas in the fissures will at various places 
 become supercharged with these vapors far beyond what 
 obtains in the normal rock. Now, both on experimental 
 grounds and what is observed in nature, it may be re- 
 garded as almost certain that no sharp line can be drawn 
 between igneous fusions of silicates (molten silicate mag- 
 mas) containing water under pressure and hot water solu- 
 tions. It appears that under pressure water will mix in 
 all proportions with magma so that at one end are molten 
 fusions, at the other hot solutions.* 
 
 At 360 degrees water reaches its critical point, that is, 
 heated to this degree, or above, its vapor cannot be 
 turned back into liquid by mere pressure, however great 
 this may be. At this temperature its expansive force is 
 almost 3000 pounds to the square inch, which would 
 require a vertical height of about 2500 feet of granite rock 
 to contain it. Above this temperature sufficient pressures 
 cause it to contract rapidly and it may even occupy less 
 
 * A good illustration of this is seen in a solution of thallium 
 silver nitrate which boils down, losing water, until a clear molten 
 fluid of the double salt remains which is anhydrous. 
 
180 ROCKS AND ROCK MINERALS 
 
 volume than it would in the liquid state (see p. 15). 
 The temperatures obtaining in molten rock magmas are 
 far above the critical temperature of water and it must 
 therefore be in the gaseous condition, though under the 
 enormous pressures obtaining under thousands of feet 
 or even several miles vertical of overlying rock, it may 
 well be much denser than water at the surface. Although 
 it has not yet been shown, so far as the writer knows, 
 that water in this state holds substances in solution just 
 as though it were a liquid, we can well imagine that at 
 high temperatures aided by the fluorine and other active 
 substances so commonly with it, its solvent action must 
 be enormously increased, especially its ability to dissolve 
 silica. 
 
 Under such conditions it is easy to see that the minerals 
 would crystallize quite differently from those in the 
 normal rock; in some places the magma would be in 
 excess and the results would more nearly approximate 
 those obtaining in the main rock; with diminished water 
 the dike might pass into an ordinary aplite or felsite 
 phase; with increased amounts, in another place, it 
 might pass from the state of a magma into an aqueous 
 solution and here would be favorable conditions for 
 crystallization on a large scale, for growth outward from 
 the walls and for the segregation of the rarer elements. 
 Finally passing onward the solution phase might become 
 more pronounced, only silica would be carried and the 
 dike turn into a quartz vein. Thus, as the degree of 
 differentiation of the magma and the proportion of 
 magma to water vary, we can see how dikes of ordinary 
 rock, of variable pegmatites and quartz veins may be 
 formed, which show in places very clearly their genetic 
 relationships. Also the slow cooling that would occur in 
 great masses of heated rock enclosing the fissure would 
 be favorable for the production of large crystals. 
 
 Contact Metamorphism. This term is applied to the 
 changes which are caused by a body of magma coming in 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 181 
 
 contact with other rocks already formed. The word 
 metamorphism, from the Greek, means a change of form 
 or body and is applied to those results, induced by a 
 variety of factors, by which rocks are recrystallized with 
 the formation of new minerals and textures. General 
 or regional metamorphism by which rocks are changed 
 over wide areas through various geological agencies is 
 considered in a later chapter; here only the results caused 
 by igneous magmas are treated. In several ways the 
 results of the two are alike and they often merge into one 
 another but in contact metamorphism the extent of the 
 masses involved is, in general, so much less than in regional 
 metamorphism, that from the standpoint of general geo- 
 logy, it is of much less importance. In respect to petro- 
 logy and to practical field work, however, it is a matter 
 of individual interest and great consequence and it is 
 therefore given separate treatment in this place. 
 
 The effects of the contact of a body of magma with 
 other rocks is seen in two ways : in one a change from its 
 general normal character is commonly observed in the 
 igneous rock itself and this is termed the endomorphic 
 effect; in the other, changes in the rocks with which it has 
 come in contact are seen and this is called the exomorphic 
 effect. We will consider the former one first. 
 
 Endomorphic Effects. It has been stated in a previous 
 section that a change in the mineralogical composition of 
 an intrusive rock body is not infrequently observed along 
 the contact, producing a border zone or facies. This is 
 due to a change in the chemical composition of the magma, 
 caused by differentiation, and has been fully discussed. 
 But at times also, even when this process has not occurred, 
 more or less of a change in the minerals of the igneous 
 rock may be seen directly at the contact or as one 
 approaches it, In this case it is due to the presence of 
 mineralizing vapors which, as previously described, tend 
 to be excluded as the mass cools and crystallizes and 
 to escape to the margin and into surrounding rocks. 
 
182 ROCKS AND ROCK MINERALS 
 
 Through their influence minerals are formed which do 
 not generally occur in the main part of the mass and which 
 are those which have been described as characteristic of 
 the pegmatite dikes. In granites the most characteristic 
 is perhaps tourmaline, whose presence is indicative of 
 boron, hydroxyl and fluorine. It is apt to take the place 
 of the biotite in the main rock and its occurrence as a 
 regular component of the granite should always lead to a 
 suspicion of approach to the contact, though it is also 
 found in the neighborhood of fissures which have served 
 as the conduit for pneumatolytic exhalations. 
 
 A variety of the granite of the Black Hills from Harney's 
 Peak illustrates this phase; in addition to the usual 
 quartz and feldspar, the rock contains black tourmaline, 
 abundant and well crystallized muscovite, green beryl 
 and red garnets: such minerals recall the associations 
 seen in pegmatites. 
 
 It may even happen that the accumulation of mineral- 
 izing vapors is so great at the outer margin before crystal- 
 lization begins that the conditions are favorable there for 
 the formation of a. true pegmatite zone. The writer has 
 observed a granite stock in the White Mountains 
 enwrapped by a mantle of pegmatite; the large plates of 
 muscovite are set perpendicular to the contact and the 
 mixture is much enriched in quartz. Similar examples 
 are known from other localities, and in Pelham, Mass., a 
 pegmatite mantle partly enfolds a mass of peridotite in 
 the gneiss. Phenomena of the character described above 
 are most noticeable about the larger intrusions, such as 
 batholiths, stocks, etc.; in dikes, sheets and minor 
 intrusions they are not so conspicuous or are entirely 
 wanting. 
 
 A much more common endomorphic contact effect is a 
 change in texture and this is independent of any change in 
 mineral composition, in fact, is largely observed where the 
 mineral composition remains constant. The most usual 
 feature of this kind is a change in the average size of grain 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 183 
 
 in the rock which grows smaller as the contact is 
 approached. The rock indeed may become exceedingly 
 dense at the contact and thus for instance a granite whose 
 average grain is of the size of coarse shot may turn into 
 a compact homogeneous appearing felsite. This is of 
 course due to a more rapid and general crystallization 
 produced by the chilling effect of the contact wall. In- 
 stances are even known where the cooling caused by the 
 cold rocks, with which the magma came in contact, was 
 so rapid that solidification took place at the margin before 
 crystallization could begin, with the production of a thin 
 selvage sheet of glass. Such instances are most liable to 
 occur in narrow dikes, in which the cooling of the con- 
 tiguous rocks is most strongly felt. 
 
 In other cases this denser contact facies may contain 
 larger distinct crystals or phenocrysts and thus be a 
 porphyry while the main mass is of even-granular texture. 
 The phenocrysts may be anterior in origin to the time 
 when the magma came to rest; in the main rock body they 
 may be of the same size as the rest of the later rock grains 
 but at the contact their contrast with the later dense 
 material produces a porphyry. On the other hand, it 
 has been observed that in many intruded masses of 
 porphyry occurring in dikes and sheets the phenocrysts 
 may be entirely absent at the contact margin, and in such 
 rock bodies they have been formed after the period of 
 intrusion, since if they had been brought up in the ascend- 
 ing magma they would be found at the contact as well as 
 in the interior of the mass. 
 
 The cases treated above are sufficient to illustrate the 
 chief endomorphic effects of contact metamorphism in 
 igneous rocks. 
 
 Exomorphic Effects of Contact Metamorphism in General. 
 The effect of the heat and vapors given off by an intruded 
 mass of magma upon the surrounding rocks with which 
 it is in contact varies with a number of factors. For one 
 thing it naturally varies with the size of the intruded 
 
184 ROCKS AND ROCK MINERALS 
 
 mass; it also varies with the nature of the vapors which 
 are given off, as described under pegmatite formation. 
 Another factor is the nature of the rock that is being 
 affected, some kinds being more susceptible than others, 
 and it also depends on the attitude of these rocks, that 
 is, in the case of sedimentary beds, on the position of the 
 planes of stratification toward the igneous mass. All of 
 these are important features and each deserves separate 
 treatment in order that the subject may be fully under- 
 stood. In general it may be said that the most noticeable 
 field evidence of the exterior effect is a baking, hardening 
 or toughening of the surrounding rocks. It not uncom- 
 monly happens as a result of this process that they 
 resist erosion better than the intruded mass or the un- 
 changed enveloping rocks and thus give rise to distinct 
 projecting topographic forms. This is admirably illus- 
 trated in the Crazy Mountains of Montana, where the 
 resistant rocks of the contact zone give rise to a series of 
 high ridges and peaks which encircle a more eroded mass 
 of intruded igneous rock and rise sharply from a sloping 
 plain of soft unchanged shales and sandstones. In the 
 case of a dike it may thus occur that the dike and the 
 surrounding beds are lowered more rapidly by erosion, 
 while the contact walls on either side are left projecting 
 as two parallel ridges. 
 
 The mineralogical effect is that, in general, where the 
 agencies have made themselves most strongly felt there 
 is a recrystallization of the rocks. This is produced by 
 an interchange of the molecules within short distances 
 whereby former chemical combinations are broken up and 
 new ones formed. In mass, that is, in sum total, the 
 chemical composition of the altered rock generally re- 
 mains the same, except that volatile compounds, water, 
 carbon dioxide, organic matter, etc., are driven out, and 
 in some cases, volatile components, fluorine, boron, etc., 
 may be added by the mineralizing vapors from the 
 igneous mass. 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 185 
 
 Modes of Occurrence. The widest and most pronounced 
 contact zones as a rule are noticed about the great intrusive 
 stocks and batholiths. 'This is most natural, since the 
 vast size of the igneous body supplies heat and vapors for 
 a great length of time. Around them contact zones a 
 mile and even more in breadth have been observed in 
 many places. Next to them perhaps the most striking 
 are seen about old volcanic necks. The breadth and 
 intensity here often seem disproportionate to the size of 
 the igneous mass but this is to be explained by the fact 
 that the necks represent conduits through which fresh 
 supplies of highly heated matter have been successively 
 passing. This renewal of matter in the conduit may thus 
 induce a superadded effect. In such cases there may be 
 no endomorphic effect of cooling on texture as described 
 above; the conduit walls are so highly heated that the tex- 
 ture of the igneous rock remains the same up to the very 
 contact wall. 
 
 In the case of dikes considerable variations may be 
 seen; in small dikes the effect may be noticed only a few 
 inches or even less, while in large ones it may extend 
 many yards on either side. Again, some dikes have served 
 as conduits for magma passing up through them into 
 larger intrusions above, feeding sheets or laccoliths or 
 giving rise to extrusive, outflows. About them the meta- 
 morphism will naturally be greater, other things being 
 equal, than where a fissure was filled by a single charge 
 of magma which immediately came to rest. For this 
 reason the metamorphism induced by intrusive sheets 
 and laccoliths is generally inconsiderable, since they also 
 represent a single charge of magma into the rocks about 
 them, which is not renewed. Immediately at the contact 
 and for a few feet or yards beyond, the rocks may be 
 altered but the effect soon dies out except in the cases of 
 very powerful sheets and large laccoliths. With extrusive 
 lava flows a small amount of baking or hardening of the 
 rocks and soils on which they rest is often seen. 
 
186 
 
 ROCKS AND ROCK MINERALS 
 
 Position of the Rocks. It is a common thing to observe 
 that the width of the contact zone varies considerably 
 from place to place about the intrusive mass. This may be 
 due to underground irregularities in the igneous rock body, 
 a wide extension of the zone pointing to a corresponding 
 
 extension of the mass be- 
 low, as illustrated in Figs. 
 71 and 72. In the stratified 
 rocks the position or atti- 
 tude of the planes of strat- 
 ification to the intrusive 
 mass is also important. 
 Thus in Fig. 73 the beds at 
 B dipping into the mass 
 of granite C tend to have 
 their bedding planes and joints opened by the upward 
 movement of the magma, and their position is such as to 
 facilitate the entrance and wide extension of the vapors 
 
 Fig. 71. Ground Plan or Map of an 
 Intruded Stock and its Contact Zone. 
 
 Fig. 72. Vertical Section along 
 Line A B in Fig. 71. 
 
 Fig. 73. Section showing width 
 
 of Contact Zone depending 
 
 on Position of Beds. 
 
 and heat, thus producing a broad contact zone. On the 
 side A on the contrary the conditions are just the reverse 
 of this and a much narrower contact zone is the result. 
 
 Effect on Different Kinds of Rocks. In a general way 
 the most notable effects are produced on sedimentary 
 rocks and these for purposes of consideration may be 
 divided into the sandstones, limestones, clay shales or 
 slates and their various admixtures. On pure quartz 
 sandstones the effect is relatively slight though for short 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 187 
 
 distances and in the near zone of most intensive action 
 they are sometimes found hardened into quartzites. 
 Pure limestones are recrystailized and changed into 
 marble and not infrequently in large masses and extending 
 over considerable distances. The most notable effects 
 are produced when the limestones are impure, containing 
 quartz sand and clay mixed with them. In this case the 
 SiO 2 drives CC>2 out and carbonates are changed to 
 silicates. If the limestone is a dolomite containing 
 magnesia the results are more complex. Some of the 
 simpler of these changes may be readily shown by equa- 
 tions which represent the chemical changes involved. 
 
 Calcite Quartz Wollastonite Garb. diox. 
 CaCO 3 + SiO 2 = CaSiO 3 + CO a 
 
 Dolomite Quartz Pyroxene 
 CaMg(C0 3 ) 2 + 2-S10 2 = CaMg(Si0 3 ) 2 + 2CO 2 
 
 Calcite Clay Quartz Garnet Carb. diox. Water 
 
 3 CaCO 3 + H 4 Al 2 Si 2 O 9 + SiO 2 = Ca 3 Al 2 Si 3 O 12 + 3 CO 2 + 2 H 2 O 
 
 Calcite Clay Anorthite 
 
 CaCO 3 + H 4 Al 2 Si 2 O 9 = CaAl 2 Si 2 O 8 + 2 H 2 O + CO 2 
 
 In some cases the rock is thus entirely changed into 
 silicates or mixtures of them, but usually it consists of 
 impure marble or altered limestone containing the min- 
 erals aggregated into lumps or bunches. Carbonaceous 
 material which may be present is often changed into 
 graphite. In addition to those minerals mentioned, a 
 variety of others, whose origin depends on the mineral- 
 izing vapors given off by the igneous rock, may also be 
 formed, such as mica (phlogopite), chondrodite, horn- 
 blende, vesuvianite, epidote, tourmaline, etc. In these 
 cases the main materials are those already in the rock; 
 the vapors furnish the volatile components, the hydroxyl, 
 boron, fluorine, etc., needed for their composition. Such 
 minerals furnish transitions to the more typical cases of 
 pneumatolytic contacts mentioned below. It should not 
 be forgotten also that many of these minerals contain 
 
188 ROCKS AND ROCK MINERALS 
 
 oxide of iron, ferrous or ferric or both, and this must come 
 from the limonite or other hydrated iron oxides mixed in 
 with the impure marly beds and deposited with the other 
 material at the time of their formation. Perhaps of the 
 minerals mentioned garnets, pyroxenes and vesuvianite 
 may be taken as among the most typical of such occur- 
 rences in altered limestones. Many instances of such 
 contacts are known in various parts of the world and some 
 of them have become famous for the variety and beautiful 
 crystallizations of the minerals which they afford and 
 which are to be found in all mineral cabinets. 
 
 In the case of day shales and slates variable effects are 
 produced, but usually ones that are well marked and 
 characteristic. While such rocks consist mostly of 
 microscopic fragments of quartz, granules of clay, mica, 
 etc., there is considerable variability in their composition 
 and accordingly a difference in the result of the meta- 
 morphism. Sometimes they are baked into a dense, hard 
 rock with conchoidal fracture, of a black or very dark 
 stone color, called hornstone, which closely resembles trap 
 or basalt. Sometimes they are like the hornstone in 
 hardness, texture and fracture but differ in color, being of 
 a light gray to green-gray or greenish and are known as 
 adinole. 
 
 In other cases where the beds are more rich in kaolin, a 
 mineral, andalusite, is apt to develop according to this 
 formula : 
 
 Kaolin Andalusite Quartz Water 
 
 H 4 Al 2 Si 2 O 9 = Al 2 SiO 5 + SiO 2 + 2 H 2 O 
 
 At the contact a rock composed largely of this, often in 
 recognizable grains and crystals, and mixed with a brown 
 biotite in glimmering specks, forms a granular rock, 
 generally dark in color and much resembling an igneous 
 rock in texture. All visible evidence of bedding of 
 sedimentary character is lost. This would be termed an 
 andalusite hornfels. Further from the contact the rock 
 begins to lose its granular texture; it becomes more schist- 
 
PLATE 12. 
 
 A. INCLUSIONS IN GRANITE. 
 
 B. FRUCHTSCHIEFER. 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 189 
 
 like or perhaps slaty and is dotted with the andalusite 
 prisms. These very frequently gather up the dark 
 organic matter of the rock and arrange it within them- 
 selves in the manner characteristic of this mineral. They 
 then appear dark on a lighter background, as seen in 
 Fig. 2, Plate 12, and this variety of rock is known as 
 " Fruchtschiefer " (fruitschist) by the Germans. The 
 rock has much the character of a fine-textured mica- 
 schist. 
 
 Still further from the contact the effects of meta- 
 morphism are less and less marked, the beds show more 
 and more of their original sedimentary nature; in this 
 part the most evident effect is a spotting of the shales or 
 slates from collection of organic matter or minerals into 
 more or less well defined points or knots. Such a develop- 
 ment of knots is one of the most characteristic features 
 of moderate contact metamorphism and, when encoun- 
 tered in the field, should always lead to search for more 
 intensive effects and the possible nearness of intrusive 
 igneous rock bodies. The latter may of course be below 
 and not yet exposed by erosion. 
 
 Just as all kinds of variations between sandstones, 
 limestones and shales are found in nature, so do the 
 different varieties of rocks produced by contact meta- 
 morphism, as described above, vary and grade into one 
 another. 
 
 In the nature of things the already existent igneous 
 rocks are less altered by the contact metamorphism of 
 following intrusions than the sedimentary ones. This is 
 particularly true of the granites and other very feld- 
 spathic rocks. The ferromagnesian ones, those containing 
 feldspars rich in lime, and especially those composed 
 chiefly of pyroxene, show at times considerable effects. 
 The pyroxene is converted into hornblende and the rock 
 becomes an amphibolite and even at times a hornblende 
 schist. 
 
 Pneumatolytic Contacts. It was mentioned above in 
 
190 ROCKS AND ROCK MINERALS 
 
 connection with the changes observed in limestones that 
 minerals appeared whose origin was due to the mineraliz- 
 ing vapors given off from the igneous mass. At times in 
 contact zones the outer rocks may be converted into 
 masses of such minerals, testifying to the abundance and 
 energetic action of the excluded vapors. Such minerals 
 as tourmaline, topaz, fluorspar, vesuvianite, mica (mus- 
 covite), etc., ones containing hydroxyl, fluorine and 
 boron are characteristic of these occurrences. The 
 masses thus formed are not widespread and regular 
 around the contact but appear here and there, espe- 
 cially near fissures, sometimes in isolated areas in the 
 other rocks, sometimes in large, sometimes in smaller 
 lumps and masses, following the irregular escape of 
 the gases. 
 
 Contact Zones and Ore Bodies. In the contact zones of 
 igneous rocks, the passage of the vapors and the move- 
 ment of heated solutions in them, combined often with 
 their own chemical composition, which causes them to 
 react with the solutions, have made them especially 
 favorable places for the deposit of ores. Their loss of 
 volatile substances causes a reduction of volume, they 
 become more porous, if not too deeply buried, and permit 
 more easily the circulation of fluids. As a result of this 
 we find many valuable deposits of the ores of gold, silver, 
 lead, copper, etc., from magmatic waters, in such contact 
 zones. In places in the mining regions of the Rocky 
 Mountains the contact between sedimentary beds and 
 intrusive masses of granite, porphyry, etc., from some 
 elevated point may be followed with the eye for miles by 
 the successive mines, pits, and heaps thrown out from 
 prospects. So well is this known that contacts between 
 porphyry and limestone are eagerly sought by every pros- 
 pector. Any adequate treatment of this subject would 
 carry us far beyond the limits of this work and further 
 information should be sought in those treatises which 
 deal with the origin of ore deposits. 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 191 
 
 Classification of Igneous Rocks. 
 
 Introductory. There is probably no subject in the 
 domain of natural science concerning which there has been 
 and is to-day less agreement than in the classification of 
 igneous rocks. The reason for this is that there are no 
 distinct boundary lines drawn by nature itself. The 
 igneous rock masses of the earth possess certain features 
 which may be used to distinguish and discriminate them, 
 one from another, such as their geologic mode of occur- 
 rence, their mineral composition, their texture, and their 
 chemical composition, which nearly represents that of the 
 original magma. A very brief inspection serves to show, 
 however, that in each of these features gradations exist 
 without hard and fast lines. If we classify them accord- 
 ing to mode of occurrence and divide them into intrusive 
 and extrusive rocks, then, for example, it is clear that every 
 lava flow is (or was) prolonged into depths below by an 
 intrusive continuation in the form of a dike or volcanic 
 neck. We should have to separate the intrusive from 
 the extrusive at some point by an arbitrary plane; above 
 this the rock would receive one name, below it another, 
 though it is clear that the material just above and that 
 just below would be absolutely alike. The same is true 
 when we consider the other features of rocks mentioned; 
 they are found to grade into each other mineralogically, 
 chemically and texturally, and where lines are drawn it 
 must be done arbitrarily. It is due to these facts that 
 so much diversity of opinion has existed regarding their 
 classification, some laying stress on one feature, some on 
 another. By general common consent among petro- 
 graphers, especially since the use of the microscope has 
 served to reveal the composition of dense rocks, a large 
 number of different kinds or types of igneous rocks are 
 recognized, based primarily on the kinds and relative 
 quantities of their component minerals and on their 
 texture, but as to the manner in which these recognized 
 
192 ROCKS AND ROCK MINERALS 
 
 kinds shall be grouped in a classification there is, as stated 
 above, wide diversity of opinion. It would not be proper 
 to go into the discussion of this subject further, but it 
 should be clearly understood at the outset, that what- 
 ever method of classification of igneous rocks is used, the 
 boundary lines must be artificial ones and in many cases 
 just where a rock should belong must be a matter of 
 opinion, which each must decide for himself. 
 
 Older Megascopic Classification. Before the micro- 
 scope came into use in studying rocks, they naturally 
 divided themselves into two groups, those whose com- 
 ponent mineral grains were large enough to be seen and 
 recognized and those which were too compact to permit 
 this. The former group was divided into different kinds 
 according to the mineral varieties composing them, the 
 latter according to the color, texture, luster and other 
 physical properties they presented. In this manner by 
 common usage a megascopic classification, extremely 
 useful for geologic and common purposes, came about, 
 which gave rise to such terms as granite, diorite, porphyry, 
 greenstone, basalt, etc. 
 
 Effect of the Microscope. When the microscope came 
 into use it was discovered that the dense rocks could be 
 studied and their component mineral grains determined, 
 nearly as easily as the coarse-grained ones, and the result 
 of these studies showed that a vastly greater diversity 
 existed among them than had been suspected. And 
 among the coarser-grained ones it was also found that 
 many minerals, until then not known in them, existed, 
 and that great variations among the minerals known to 
 compose them could be seen, as well as differences in tex- 
 ture, etc. To express these differences among the rocks 
 and to connote the ideas regarding them which they 
 engendered, not only have a whole host of new rock names 
 arisen, but the old megascopic terms have been defined 
 and redefined by various authorities, until they have 
 nearly all lost their original significance. 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 193 
 
 This has been an unfortunate phase of the history of 
 the development of petrography as a science, because 
 these former megascopic or field names, as we should 
 term them now, served a very useful and necessary pur- 
 pose, which the more exact and scientific nomenclature 
 of modern petrography cannot replace. The person who 
 desires to deal with rocks and name them from the mega- 
 scopic, field point of view, such as the field geologist, the 
 engineer, the architect, etc., is left without any equipment 
 for doing so. A single illustration will suffice. The 
 old term granite meant any granular igneous rock and 
 then later one composed of quartz and feldspar. Now, as 
 used by modern petrographers, it is only a granite in case 
 the feldspar is chiefly alkalic, while if it is dominantly 
 soda-lime feldspar, the rock is termed a quartz diorite, 
 a distinction which ordinarily cannot be made without 
 microscopic study. 
 
 The redefinition and specializing of these useful general 
 field terms is very much the same as if the botanists had 
 redefined such terms as bush, tree, vine, shrub, etc., and 
 had made them the names of particular species or genera, 
 so that if tree, for instance, were properly used, it would 
 designate only oaks, or even quercus alba. 
 
 In the meantime in the world at large, where rocks are 
 commercially dealt with, as in mining, architecture, etc., 
 the use of rock names in the old way has gone on 
 quite regardless of the petrographers, but the geologist 
 or engineer who has endeavored to keep up with the de- 
 velopment of the science and use its terms megascopically 
 has carried an ever increasing load until finally he has 
 been compelled to become a petrographer or else give up 
 in large part any independent use of rock names. With- 
 out doubt it is largely due to this fact that every advance 
 in the definiteness and completeness of petrographic 
 scientific nomenclature has raised a wave of protest among 
 geologists. 
 
 Present Need in Classification. It is clear that the 
 
194 ROCKS AND ROCK MINERALS 
 
 parting of the ways has long been reached and it ought 
 to be definitely recognized that the further development 
 of petrology and of the classification and nomenclature of 
 rocks from the scientific standpoint must be left largely 
 to petrographers, while those who have occasion to deal 
 with them in the purely megascopic manner must have a 
 method of classification and a set of terms of a totally 
 different scope and usage. They must in large measure 
 revert to that which was in vogue before the microscope 
 came into use. 
 
 It matters little whether such a classification is com- 
 pletely based on all the principles underlying scientific 
 petrology which the study of rocks has revealed or not ; to 
 be useful it must be practical and to be practical it 
 must be based entirely on the evident megascopic char- 
 acters of rocks, such as can be seen by the eye or pocket 
 lens or be determined by simple means at every one's 
 command. 
 
 Classification used in this Work. As it is the object of 
 this work to treat rocks from this point of view the follow- 
 ing method of classification has been adopted.* First, 
 the rocks are considered according to their texture and 
 from this it will be found that they divide naturally into 
 three classes, grained f, dense, and glassy. 
 
 A. Grained Rocks. By this is meant those rocks 
 whose component mineral grains are large and distinct 
 
 * This is essentially that proposed by the author and several 
 other petrographers. "Quantitative Classification of Igneous Rocks," 
 by Cross, Iddings, Pirsson and Washington, University Chicago 
 Press, 1903, p. 180. 
 
 t The term " grained " is here used instead of " granular " for 
 two reasons. First, because granular (from granule a little 
 grain), strictly speaking, means fine-grained, while the rocks included 
 may be coarse, medium or fine-grained. Second, because granular 
 is used by many petrographers in a technical way as an equivalent 
 to " even-granular " and opposed to porphyritic, while grained 
 rocks may be either. Phanerocrystalline, macrogranular, mega- 
 granular, etc., have much the same meaning but it is better to use 
 & simple English word than a compound Latin or Greek one. 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 195 
 
 enough to be seen and recognized by the eye alone or 
 with the lens. No hard or fast line can be here drawn as 
 to the size of grain; it will vary with the kind of mineral, 
 their association together, and on the experience and skill 
 of the observer. In general it may be said that it includes 
 rocks whose average size of grain is as large or larger than 
 that of ordinary loaf sugar. 
 
 B. Dense Rocks. This will include those which are 
 nearly or wholly of stony appearance and texture but 
 whose minerals cannot be determined, because the con- 
 stituent particles are too minute. They may even appear 
 homogeneous but are generally microcrystalline. 
 
 C. Glassy Rocks. Includes those wholly or in part 
 made up of glass, as shown by their vitreous or pitchy 
 luster, conchoidal fracture, and other characters and 
 appearance. 
 
 Treatment of Porphyries. Reference is had in the 
 above to rocks whose average size of grain is uniform or 
 nearly so. But, as explained in the description of the 
 porphyritic texture on page 156, many igneous rocks are 
 porphyries, that is, they contain distinct crystals or 
 phenocrysts much larger in size than that of the average 
 grain of the groundmass in which they lie embedded. It 
 is assumed that in general the size of the phenocrysts is 
 such that they can be distinctly seen and the particular 
 kind of mineral composing them can be recognized or 
 approximately determined. In classifying porphyries 
 they at once naturally fall into two classes; first, (D) one 
 in which not only the phenocrysts, but also the mineral 
 grains of the groundmass are large enough to be determined 
 and second, another (E) in which the groundmass is 
 either too dense to be made out or (F) glassy. In the 
 former case (E), two subdivisions can be made, one (a) in 
 which the phenocrysts are very abundant and a good idea 
 of the mineral composition of the rock as a whole may be 
 had and another (b) in which the amount of groundmass 
 is predominant and this cannot be done. 
 
196 ROCKS AND ROCK MINERALS 
 
 For a clearer understanding these divisions of porphyries 
 may be shown in tabulated form. 
 
 D. Groundmass grained, recognizable. 
 
 E. Groundmass dense, unrecognizable. 
 
 a. Phenocrysts very abundant and recognisable. 
 
 b. Phenocrysts not very abundant or rare, 
 
 groundmass very abundant. 
 
 F. Groundmass glassy. 
 
 It would be logical to make the same subdivisions a and b under 
 class F as are made in E but cases where glassy groundmasses are 
 filled with abundant recognizable phenocrysts dominating in amount 
 over the groundmass though known, are not sufficiently common to 
 make worth while the subdivision for practical purposes. 
 
 No sharp lines can be drawn between these divisions; 
 they pass into one another gradually, except as to whether 
 the rock is glassy or stony in texture. 
 
 It is to be observed that since the rocks belonging in 
 division D have their mineral constituents determinable 
 they belong in the same category as those in A of the 
 evenly granular ones previously mentioned, so far as this 
 particular is concerned. Likewise in division E, sub- 
 division a, if half or more of the rock is composed of 
 recognizable phenocrysts its general mineral character 
 can be determined and it falls in the same category. But 
 in the remaining divisions not enough of the mineral 
 characters can usually be told to safely identify the rocks 
 on this basis and such rocks are evidently to be classed 
 with division B, dense rocks and C, glasses which are 
 not porphyries and which cannot be subdivided according 
 to mineral composition. 
 
 Subdivisions of Class A. The igneous rocks having 
 been divided into classes on the basis of texture it now 
 remains to show on what grounds these classes can be 
 further subdivided and the individual kinds of rocks, 
 from the megascopic standpoint, obtained. This is done, 
 as already suggested, in the even- and porphyritic-grained 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 197 
 
 rocks by considering their mineral composition. First 
 we may broadly divide them into two main groups. 
 
 a. Rocks in which the feldspars or feldspars and 
 quartz predominate. 
 
 6. Rocks in which the ferromagnesian minerals (py- 
 roxene, hornblende, olivine, etc.) predominate. 
 
 As a rule, the rocks of the first group are light-colored, 
 white, red or gray, but this is not an absolute rule since 
 the feldspars are sometimes very dark from an included 
 pigment. In general the rocks of the second group are 
 dark in color to black but this is also not an invariable 
 rule since some, like those composed wholly of olivine, are 
 rather light. 
 
 The first group a may be further subdivided on the 
 basis of the relation of quartz to the feldspars. Those 
 which contain an appreciable amount of quartz with the 
 feldspars fall in one division and are termed granite, when 
 even granular in texture, and granite porphyry, when of 
 porphyritic texture, while those in which quartz is absent 
 or is present in inappreciable quantity are called syenite 
 and syenite porphyry respectively. Further division of 
 these into varieties on the basis of particular mineral 
 characters will be considered in the description of them 
 in the succeeding chapter. 
 
 The second group b is subdivided on the basis of the 
 relation of the feldspars to the ferromagnesian minerals, 
 into those which contain feldspar, subordinate in amount 
 to the ferromagnesian minerals, and those in which it is 
 wanting. Thus we have as follows: 
 
 c. Rocks with predominant ferromagnesian minerals. 
 
 feldspar subordinate. 
 
 d. Rocks consisting wholly of ferromagnesian minerals. 
 
 Group c is subdivided according to the nature of the 
 predominant ferromagnesian mineral present. For 
 practical purposes there need be only two considered 
 
198 ROCKS AND ROCK MINERALS 
 
 here; if it is hornblende the rock is diorite, if it is pyroxene 
 it is gabbro. The means for distinguishing between horn- 
 blende and pyroxene are discussed in the description of 
 these minerals in a preceding part of this work. In many 
 cases, especially in the finer-grained rocks of this group, 
 it may not be possible to distinguish between hornblende 
 and pyroxene and the rock may then be termed dolerite. 
 This name would then mean that the rock consisted chiefly 
 of indeterminable predominant ferromagnesian minerals 
 with subordinate feldspar.* 
 
 Porphyries occur in this group but they are relatively 
 of less importance than in the preceding ones; they are 
 treated in the descriptive part. Rocks in which ferro- 
 magnesian minerals, other than hornblende and pyroxene, 
 predominate over feldspar are known but are of little 
 practical importance in a megascopic scheme of this 
 character and are therefore omitted. They will be 
 mentioned later. 
 
 The last group d, consisting wholly of ferromagnesian 
 minerals, is divided according to the kinds of these minerals 
 present. The most common and prominent mineral in 
 the group is pyroxene but this is usually associated with 
 olivine and the rock is termed peridotite. This is the 
 most common member and may be used as a general term 
 for the group. If olivine is absent and the rock consists 
 wholly of pyroxene it is pyroxenite, if of hornblende, 
 hornblendite. Varieties are described under peridotite. 
 
 * In this usage of dolerite the author adopts and follows that 
 proposed by Chamberlain and Salisbury (Geology, vol. 1, p. 431, 
 1904) which is already obtaining considerable vogue and from such 
 authority is likely to become general. In Germany the term is 
 restricted to certain coarse-grained basaltic rocks; in England it 
 has had a certain use for all coarse-grained basalts and for rocks 
 termed elsewhere diabases; in America it has been little employed 
 and may well be revived as a field name in the sense suggested. 
 With this meaning it is a very useful term. The word is from the 
 Greek, meaning deceptive, with the idea that the pyroxene cannot 
 be distinguished from the hornblende. 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 199 
 
 Porphyries in this group rarely occur and are of no prac- 
 tical importance. 
 
 In summation, in considering the classification of the 
 group of grained rocks whose constituents are determinable, 
 one should consult what has been said regarding the 
 chemical composition of igneous magmas and the 
 variations in mineral composition beginning on page 141. 
 It is not possible, however, to classify them entirely, for 
 megascopic purposes, by the diagram given on page 145, 
 for, in general, we cannot discriminate between the different 
 kinds of feldspars. Thus the rock there shown as quartz 
 diorite must be classified under the head of granite, while, 
 as compared with the diagram, the diorite and gabbro 
 mentioned above broadly overlap. Still, in a general way, 
 bearing these exceptions in mind, the classification, dis- 
 tinguishing between the feldspathic and the ferromagne- 
 sian rocks, brings out the ideas there expressed. The rocks 
 of this class are nearly always intrusive, rarely extrusive. 
 
 Subdivisions of Class B. In considering the second 
 class of rocks, B, whose texture is so fine or dense that the 
 mineral grains cannot be determined, we have little with 
 which to classify them for field purposes except the 
 color. They are thus divided into two groups, the light- 
 colored and the dark-colored. Of course if the rock is either 
 white or black there can be no difficulty in assigning it to 
 one or the other of these two divisions, but all gradations 
 of color exist and it is often a matter of pure choice to 
 which a particular rock should belong. Evidently some 
 closer definition of the terms is needed. We may do this 
 as follows. The term dark includes rocks that are very dark 
 gray, very dark green or black; all other colors, white, red, 
 purple, yellow, brown, light and medium gray, light and 
 medium green are light. The latter are known under the 
 name of felsite, while the former or dark rocks are basalt. 
 The division thus made also expresses in a general way an 
 important fact concerning their composition, for the 
 former are derived from magmas, which, under different 
 
200 ROCKS AND ROCK MINERALS 
 
 physical conditions producing coarser-grained rocks, 
 would crystallize as granites and syenites. On the other 
 hand, the basalts represent the diorites, gabbros and 
 peridotites in dense or fine- textured forms. While many 
 exceptions will be found, this general rule holds true and 
 the light rocks as defined above are chiefly feldspathic, 
 the dark are mainly ferromagnesian. 
 
 While the rocks of this group are often of homogeneous 
 texture and aspect, they are also very often porphyritic. 
 If the amount or bulk of phenocrysts in relation to the 
 fine or dense (aphanitic) groundmass is very large, say 
 half the mass of the rock or more, such porphyries pass 
 back into class A, of grained rocks as previously explained. 
 But if the amount of phenocrysts is less to much less than 
 the groundmass then we have felsite porphyry and basalt 
 porphyry respectively, according to the color of the ground- 
 mass. It has also been suggested that they may be 
 called leucophyre (light-colored porphyry) and melaphyre 
 (dark-colored porphyry), respectively.* Further sub- 
 divisions of these porphyries can be made according to 
 mineral character of the prominent phenocrysts. Thus 
 we might have quartz- felsite-porphyry; feldspar- felsite- 
 porphyry; hornblende- felsite-porphyry or quartz-, feldspar- 
 and hornblende-leucophyre, and similarly we have augite- 
 basalt-porphyry , mica-basalt-porphyry, feldspar-basalt-por- 
 phyry or augite-, mica- and feldspar-melaphyre. Many 
 combinations of this kind can be made but the above will 
 suffice as examples. The rocks of this class are some- 
 times intrusive, sometimes extrusive. 
 
 Subdivisions of Class C. The rocks of the third class, 
 C, those wholly or partly of glass, are distinguished by 
 their glassy or resinous luster and want of stony texture. 
 They may be classified as follows: 
 
 OBSIDIAN, luster strong, bright, glassy; color usually 
 * black, sometimes red, more rarely brown or 
 
 greenish. 
 * Quantitative Classification of Igneous Rocks, p. 184. 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 201 
 
 PITCHSTONE, luster resinous or pitch-like; colors 
 
 various, as above, but black less common. 
 PERLITE, glassy rock with perlitic structure, produced 
 
 by small spheroidal fractures; usually gray in 
 
 color. 
 PUMICE, highly vesicular glass (see page 158), usually 
 
 white or very light-colored. 
 
 Any of these may be porphyritic or not; though cases 
 of porphyritic pumice are much less common than in the 
 other three. When porphyritic a general name for them 
 is vitrophyre (glass porphyry) and different varieties may 
 be distinguished, as in the porphyries of the class above, 
 according to the kind of predominant phenocrysts; thus 
 quartz-vitrophyre, feldspar-vitrophyre, etc. The rocks of this 
 class are practically wholly confined to extrusive lavas. 
 
 Class D. In addition - to the three main classes of 
 igneous rocks described above we may add as an appendix 
 in a fourth class, D, the fragmental material thrown out 
 in volcanic eruptions and already mentioned on page 
 140 as tuffs and breccias. 
 
 Such material serves as a connecting link between the sedimentary 
 and igneous rocks. For, as it falls through the air, it becomes assorted 
 as to size, and successive outbursts thus produce rough but distinct 
 bedding. Or it may fall into water and become perfectly stratified. 
 Falling on the land it may cover vegetation and contain fossil 
 imprints of plants, leaves, etc. ; or if into water, of marine organisms. 
 Thus if we classify volcanic ash beds as igneous rocks we cannot 
 say that a distinguishing feature of igneous rocks is that they never 
 contain fossils. See remarks on page 133. 
 
 Classification Tabulated. The classification which has 
 been adopted and described in the foregoing may now be 
 shown, for convenience of reference, in tabulated form on 
 the following page. 
 
 Classifications based on Microscopic Research. In the 
 classification previously described, the color and texture 
 of rocks play a prominent part, and mineral composition 
 can be used only in an approximate manner. But where 
 
202 
 
 ROCKS AND ROCK MINERALS 
 
 
 c3 
 
 
 w . -^> 
 
 
 
 o 
 
 
 
 
 
 
 
 
 
 j>> 
 
 "3 <3 
 
 G '* G 
 
 
 <i5 
 
 
 
 
 
 
 
 
 
 
 
 "oil 
 
 o ft 
 
 g ^ 
 
 
 >. 
 
 t(-H 
 
 
 
 
 
 
 
 
 
 E 
 
 1 2 
 
 Q -2 
 
 
 'w 
 
 ^ 
 
 
 
 
 
 
 
 
 
 d 
 
 
 S 2 6 
 
 
 B 
 
 'cS 
 
 
 
 
 
 
 
 
 
 o 
 
 
 
 
 
 9 
 
 
 
 
 
 
 
 
 . 
 
 fafi 
 
 
 S ^ w 
 
 
 w 
 
 oa 
 
 
 (V 
 
 
 
 
 
 
 > 
 
 jf 
 
 
 
 
 g 
 
 -v 
 
 
 
 
 
 
 
 
 rt 
 
 
 
 
 
 
 
 "~^ G 
 
 
 ffi 
 
 
 
 , 
 
 
 
 I 
 
 i 
 
 ^0) 
 
 Ferromagnesian roi 
 to black. 
 
 With subordinate 
 Feldspar. 
 
 DIORITE. 
 GABBRO. 
 DOLERITE. 
 
 [ORITE-PORPHYRY. 
 
 jnizable. Intrusive a 
 
 Dark colored to bla< 
 magnesia 
 
 BASALT. 
 
 BASALT-PORP 
 
 rlass. Extrusive. 
 
 ^ 
 
 O) 
 
 oT 
 
 
 
 a 
 
 3 
 
 PH 
 
 oT 
 
 ^tchstone-porphyry) 
 
 xtrusive. 
 
 o 
 fa 
 D 
 
 I 
 
 * 
 
 
 
 Q 
 
 I 
 
 ^ 
 
 
 
 UP 
 
 M-l 
 
 1^ 
 
 ^ 
 
 OH 
 
 M 
 
 O 
 
 ^anerites). Constituent Grains Reco 
 
 ft 
 
 49 
 
 "3 
 
 (B 
 
 1 I 
 
 
 O 
 
 ft 
 
 03 
 
 Without Quartz. 
 
 SYENITE, 
 a. Syenite. 
 b. Nephelite Syen- 
 ite. 
 c. Anorthosite. 
 
 SYENITE-PORPHYRY. 
 
 1 
 
 "o 
 1 
 
 S 
 
 1 
 
 
 
 
 cks Composed Wholly or in Part c 
 
 OBSIDIAN, Pitchstone, Pel 
 
 Vitrophyre (Obsidian-, anc 
 
 D. Fragmental Igneous Material. 
 
 TUFFS, BRECCIAS (Volcanic as 
 
 ht colored, usually feldspathic. 
 
 FELSITE. 
 
 FELSITE-PORPHYRY. 
 
 1 
 
 e'| 
 
 PORPHYRY. 
 
 & 
 
 1 
 
 "3 
 
 | 
 
 1 
 
 
 
 ^.NITE- 
 
 1 
 
 M 
 
 08 
 
 
 
 (2 
 d 
 
 
 
 
 
 *3 
 
 o3 
 
 
 
 
 
 2 
 
 
 
 
 
 
 
 
 
 o 
 
 
 
 
 O 
 
 S 1 
 
 
 
 
 
 
 
 
 
 <i 
 
 
 | 
 
 
 "s 
 
 g 
 
 
 
 o 
 
 
 
 
 
 
 'g 
 
 o 
 
 1 
 
 't 
 
 o 
 
 
 U 
 
 .2 
 
 
 
 
 
 ft 
 
 "2 
 
 
 ft 
 
 ~S 
 
 
 'ft 
 
 "2 
 
 
 
 
 
 El 
 
 O 
 
 ft 
 
 p 
 
 PQ 
 
 8. 
 
 > 
 
 
 O 
 
 ft 
 
 ft 
 
 
 
 
 
 G 
 
 
 
 G 
 
 t^ 
 
 
 c 
 
 
 
 
 
 
 O 
 
 o 
 
 
 O 
 
 Q 
 
 
 o 
 
 o 
 
 
 
 
 
 g 
 
 
 
 g 
 
 (Vi 
 
 
 ^ 
 
 
 
 
GENERAL PETROLOGY OF IGNEOUS ROCKS 203 
 
 rocks are studied in thin section under the microscope 
 texture becomes of much less importance; all of the 
 minerals and their exact characters can be discovered and 
 their relative proportions made out. In this more exact 
 work the kinds of rocks that are recognized by petrog- 
 raphers are based primarily on the kinds and to some 
 extent the relative proportions of the component minerals. 
 This makes a great number of kinds of rocks which have 
 been named. Generally they are grouped first, according 
 to minerals and second, according to texture; some petrog- 
 raphers lay weight also on their mode of occurrence, 
 whether extrusive or intrusive, while others add to this 
 the genetic relations or groupings which they show in 
 nature. Classifications have also been proposed in which 
 their chemical composition plays the most prominent 
 part. 
 
 Quantitative Classification. Recently several petrog- 
 raphers, including the author, have proposed an exact 
 scientific classification of igneous rocks based on their 
 chemical composition, expressed, however, in terms of 
 minerals of definite composition, called standard minerals. 
 For this purpose a chemical analysis of the rock is neces- 
 sary but, where this cannot be obtained, an approximately 
 correct result may be achieved by measurement of the 
 minerals under the microscope, computing from this their 
 relative bulk and weight, and, their composition being 
 known, reckoning from this the chemical composition of 
 the rock as a whole, as if obtained by chemical analysis. 
 
 The chemical composition is then computed, according 
 to a set plan, into the relative amounts of standard 
 minerals. These standard minerals are divided into two 
 main groups; one characterized by the presence of alumina 
 and silica, such as the feldspars, nephelite, corundum and 
 quartz, but without iron or magnesia, the second charac- 
 terized by iron and magnesia but without alumina, such as 
 olivine, diopside, hypersthene, aegirite and iron ores. The 
 complex ferromagnesian minerals which contain alumina, 
 
204 ROCKS AND ROCK MINERALS 
 
 such as hornblendes, biotite, augite, etc., are not treated 
 as standard minerals because it is better to consider them 
 as compounds of simpler molecules of the two preceding 
 groups. The first of these is called the salic (Si and Al) 
 the second the femic (Fe and Mg) groups of standard 
 minerals and the composition of the rock computed in 
 quantities of them is called its norm, which may thus, 
 when hornblende or biotite are really present in it, differ 
 considerably from its actual mineral composition or mode. 
 All igneous rocks may be expressed in salic and femic 
 minerals and according to the relative amount of each 
 group as compared with the other they are divided into 
 five classes, persalane, nearly or entirely composed of salic 
 minerals (sal : fem > 7 : 1 ) ; dosalane, mostly salic 
 
 (sal: fern < 7:1 > 5:3); 
 
 salfemane, equal or nearly equal quantities of each 
 (sal: fem < 5:3 > 3:5); 
 
 dofemane, mostly femic minerals (sal: fem < 3: 5 > 1:7); 
 and lastly perfemane, nearly or entirely femic 
 
 (sal: fem < 1:7). 
 
 Up to this point it is possible to use this classification in a 
 megascopic manner. The classes thus obtained are sub- 
 divided into orders on the relations of the salic minerals, 
 quartz, feldspars and feldspathoids (generally nephelite), 
 to one another in the first three classes and on somewhat 
 similar relations among the femic minerals in the last two. 
 More minute consideration of the mineral oxides divides 
 the orders into rangs and the rangs into grads. The 
 proportions by which they are thus divided are always the 
 same as that by which classes are made. 
 
 Further details regarding this and other systems of 
 classification founded upon results obtained by micro- 
 scopical research are to be found in the list of works 
 mentioned on page 10. 
 
CHAPTER VII. 
 DESCRIPTION OF IGNEOUS ROCKS. 
 
 Grained Igneous Rocks. 
 
 As explained under the section on classification the 
 grained igneous rocks are those whose mineral grains are 
 approximately of equal size and large enough to be 
 identified with eye or lens, aided when necessary by 
 chemical or physical tests. Those rocks whose grain is 
 too fine to permit this will be found under the heading of 
 the dense igneous rocks. The porphyries, the major part 
 of whose constituent minerals can be distinguished, are 
 described in the following section. 
 
 GRANITE. 
 
 Composition. Granites are granular rocks composed 
 of feldspars and quartz. Sometimes they consist of these 
 minerals alone but generally there is more or less mica 
 present and often hornblende. 
 
 The feldspar is the predominant mineral and is readily 
 recognized by its appearance and cleavage. Sometimes 
 only one kind of feldspar is present but generally there 
 are two, orthoclase and soda-lime feldspar. They may 
 sometimes be distinguished by their colors; if one feldspar 
 is flesh-colored to red while the other is white, gray or 
 yellow, it is pretty certain that the first is orthoclase, the 
 second soda-lime feldspar (plagioclase) . Close inspection 
 of a cleavage surface of the latter with a lens may show 
 the twinning striations (see page 38) but the grains are 
 rarely coarse enough to permit this. Rocks in which the 
 amount of plagioclase is greater than the orthoclase are 
 called quartz diorites by petrographers and are placed in a 
 
 20* 
 
206 ROCKS AND ROCK MINERALS 
 
 separate family, but this cannot be done in megascopic 
 determination and they are all here classed as granites. 
 They have also been called granodiorites in the western 
 United States. 
 
 Quartz normally occurs as formless material, filling the 
 interstices between the other minerals and hence without 
 definite shape. The normal color is white to dark, smoky 
 gray; sometimes it is red from included hematite, more 
 rarely a bluish color. In the finer-grained granites the 
 color is usually light or white, especially in those of a 
 sugar granular texture. It is recognized by its oily, 
 greasy luster and conchoidal fracture. In porphyritic 
 granites it sometimes occurs in large dihexahedral crystals 
 or round grains. 
 
 The mica may be either the light or colorless muscovite 
 or black biotite or both kinds may be present. Cases 
 where muscovite is the only mica are rare. Hornblende 
 occurs in black to dark green grains or prisms. It is 
 sometimes the only dark mineral but is more usually 
 accompanied by biotite. These are the chief minerals, 
 but if the rock is fairly coarse-grained, close inspection 
 will commonly show occasional metallic-looking specks 
 or grains of iron ore. Sometimes other minerals may be 
 seen, brassy crystals of pyrite, dark red grains of garnet, 
 etc., but these are occasional and are not of importance in 
 determining the rock. 
 
 Texture. In ordinary normal granite the texture is an 
 even granular one and alike in all directions through the 
 rock. From this type insensible gradations, sometimes 
 in the same mass, may be observed into the texture of 
 gneiss which becomes noticeable through the linear 
 arrangement of the components, especially the micas. 
 Thus the rock passes into granite gneiss. The normal 
 texture is shown on Plate 13. In other cases a tendency 
 may be noted for some of the orthoclase crystals to be 
 larger than the average grain and of more distinct crystal 
 form. In this way the rock becomes porphyritic and 
 
PLATE 13. 
 
 A. COMMON GRANITE. 
 
 B. PORPHYRITIC GRANITE. 
 
DESCRIPTION OF IGNEOUS ROCKS 207 
 
 when this is pronounced it is the porphyritic granite 
 described below. Often the dark minerals tend to group 
 or bunch together in spots. 
 
 Color. The general color of the rock depends largely 
 on that of the feldspar and in the proportion of this to the 
 dark minerals. Thus the color shades from white into 
 gray to dark gray, resulting from the mottling by the 
 biotite, etc. Such types are very common wherever 
 granites are abundant, as in New England. More rarely 
 the quartz and feldspar are themselves gray to dark gray 
 and thus determine the color. An example of this is the 
 granite of Quincy, Mass., largely used as a building stone. 
 Another very common type of coloring is one in which the 
 rock is flesh-colored, pink to red and even deep red. Such 
 red granites are found in Maine, Missouri, Colorado, 
 Scotland and other localities and are largely quarried and 
 used for building. 
 
 Varieties. The varieties of granite depend on the 
 relative proportions of the light and dark minerals, the 
 color and the texture. The relative amount of the biotite 
 (or hornblende) to the quartz and feldspar may vary 
 widely; it may be entirely wanting or it may be present 
 in large amount and make the rock quite dark. Such 
 extreme cases are less common. The grain may become 
 as coarse as large peas or even larger. These variations 
 combined with those in color produce distinct types of 
 granite which have often received local names. Some 
 other varieties are described in following sections. 
 
 Porphyritic Granite. As mentioned above the feldspar 
 may partly occur in large distinct crystals or phenocrysts. 
 Strictly speaking this would cause the rock to become a 
 granite porphyry but where the groundmass in which these 
 lie is as coarse as an average granite it is the custom to 
 speak of it as porphyritic granite, laying stress on the 
 character of the groundmass rather than on the por- 
 phyritic quality. The feldspar phenocrysts are of ortho- 
 clase and have the forms shown under feldspar, page 35. 
 
208 
 
 ROCKS AND ROCK MINERALS 
 
 Reflection of light from the cleavages of these on the rock 
 surface often shows they are in twin halves, due to Carls- 
 bad twinning. The size of these phenocrysts is some- 
 times quite large, an inch long and broad or even more. 
 An illustration of this type of granite is seen on Plate 13. 
 Such rocks occur in New Hampshire and other localities 
 in New England, in Colorado, in the Sierra Nevada 
 Mountains, in England (Dartmoor and elsewhere), in the 
 Black Forest region and other places. 
 
 Chemical Composition. The mass compositions of a 
 few selected granites are shown in the analyses given here 
 to illustrate the kind of magma from which such rocks 
 have crystallized. 
 
 ANALYSES OF GRANITES. 
 
 
 I 
 
 II 
 
 III 
 
 IV 
 
 V 
 
 Si0 2 
 A1 2 3 
 
 77,6 
 12.0 
 
 74.4 
 13.1 
 
 71.2 
 13.7 
 
 68.0 
 17.2 
 
 66.3 
 16.0 
 
 
 0.6 
 
 0.7 
 
 1.7 
 
 3.1 
 
 1.8 
 
 FeO 
 
 0.9 
 
 0.9 
 
 1.0 
 
 0.4 
 
 1.9 
 
 MgO 
 
 trace 
 
 0.4 
 
 0.8 
 
 1.2 
 
 1.1 
 
 CaO 
 
 0.3 
 
 1.3 
 
 2.3 
 
 2.9 
 
 3.7 
 
 Na 2 O 
 
 3.8 
 
 2.6 
 
 3.6 
 
 3.2 
 
 4.1 
 
 K 2 O 
 
 5.0 
 
 6.1 
 
 3.8 
 
 3.9 
 
 3.5 
 
 H 2 O 
 
 0.2 
 
 0.3 
 
 1.7 
 
 . . . 
 
 0.5 
 
 XyO* 
 
 0.2 
 
 0.4 
 
 0.2 
 
 
 
 0.9 
 
 Total . 
 
 100.6 
 
 100.2 
 
 100.0 
 
 99.9 
 
 99.8 
 
 * XyO =* small quantities or traces of other oxides. 
 
 I, Hornblende Granite, Rockport, Mass.; II, Biotite Granite, 
 Crazy Mountains, Montana; III, Granite, Conanicut Island, Rhode 
 Island; IV, Granite, Kirkcudbright, Scotland; V, " Granodiorite or 
 quartz diorite," Mariposa County, California. 
 
 The large percentages of silica, alumina and alkalies 
 explain the predominance of feldspars and quartz. With 
 the increasing lime in the last two, the alkalic feldspars 
 give place in precedence to plagioclase; the increasing 
 
PLATE 14. 
 
 A. EROSION OF GRANITE IN THE HIGH ALPS. 
 (After Duparc.) 
 
 B. EROSION OF GRANITE IN OLD AND LOW MOUNTAIN 
 
 REGIONS, STONE MOUNTAIN, GEORGIA. 
 
 (Georgia State Geological Survey.) 
 

DESCRIPTION OF IGNEOUS ROCKS 209 
 
 iron and magnesia show increasing amounts of the dark 
 minerals; coincidently with this the silica falls; the amount 
 of free quartz is less and such rocks approach the next 
 class, the syenites. 
 
 Physical Properties. The specific gravity of granites 
 varies with the kinds and relative amounts of the com- 
 ponent minerals; from 2.63-2.75 is the ordinary range, 
 those containing more ferromagnesian minerals being the 
 heavier. The average weight of a cubic foot of granite is 
 about 165 pounds. Usually the porosity of such granites 
 as are quarried for building purposes is very small, the 
 percentage of water absorbed, compared with the weight 
 of the dry rock, being about 0.15 of one per cent. Thus a 
 cubic foot of average granite if completely saturated would 
 absorb about 4 ounces of water. The strength of granites 
 in resistance to crushing is very great and probably far 
 greater than any load they would be called upon to bear 
 in architectural work. A series of Wisconsin granites 
 tested by Buckley showed crushing strengths varying from 
 15,000-40,000 pounds per square inch; some of these were 
 very high, and from 15,000 to 20,000 is perhaps the aver- 
 age. As the pressure at the base of the Washington Monu- 
 ment is 342.4 pounds per square inch, it will be seen there 
 is an ample reserve in most cases. 
 
 Uses of Granite. As is well known, on account of its 
 great strength and durability, granite is extensively used 
 for architectural purposes. Its pleasing colors and the 
 high polish it takes cause it to be employed as an orna- 
 mental stone in interior work, in monuments, etc. In 
 one respect, however, many granites have a defect which 
 somewhat impairs their value for use in buildings in 
 large cities. This defect is that they do not resist fire well, 
 but crack, scale and sometimes crumble under great 
 heat. One reason for this is that the quartz grains are 
 very commonly filled with minute bubbles containing 
 water or liquid carbonic acid gas (62) or both.* They 
 are so minute that they are often only to be detected with 
 
 * The different rates of expansion of quartz and feldspar are another cause. 
 
210 ROCKS AND ROCK MINERALS 
 
 high powers of the microscope in thin sections but they 
 may absolutely swarm in the quartz and constitute an 
 appreciable fraction of its bulk. They represent material 
 taken up or included at the time of its crystallization. 
 Under the action of heat the pressure on these sealed 
 crystal flasks becomes enormous; each quartz grain 
 becomes, so to speak, a veritable tiny bomb and eventually 
 it must crack in all directions and crumble and thus injure 
 the strength and resisting capacity of the stone. Feld- 
 spars practically never suffer from liquid inclusions, like 
 quartz, nor do the other ordinary rock minerals, so that 
 rocks like syenite or diorite, in which quartz is absent or 
 only sparingly present, make in respect to resisting fire 
 much better stone than granite. 
 
 Jointing in Granite. Granite tends to a block jointing 
 on a large scale in the great stocks. There generally 
 tends to be three distinct sets of joints, two of which 
 approximate to the perpendicular, the third to the hori- 
 zontal. Sometimes these are nearly at right angles pro- 
 ducing cubes, more often at angles which make rhomboidal 
 blocks. Sometimes the horizontal one is most pronounced 
 and the mass has a sheeted or layer-like character sug- 
 gesting bedding. In dikes the joints are much more 
 numerous and the mass breaks into small blocks, plates, 
 etc. This jointing of granite is a matter of much impor- 
 tance in work of excavation, in mining, tunnelling, 
 quarrying, etc., in facilitating removal of material, but it 
 also explains why every granite mass is not suited to 
 furnish material in blocks of sufficient size for construc- 
 tional purposes. Quarries like those in Finland, in the 
 so-called Rapakiwi granite, from which the base, a 
 cube of 30 feet, and the shaft, 100 feet high by 15 feet in 
 diameter, of the Alexander monument in St. Petersburg 
 were taken and those in Egypt from which the great 
 obelisks were cut are not common. Compare Plate 10. 
 
 Erosion Forms of Granite. The jointing of granite 
 largely conditions the work of erosive agencies on the 
 
PLATE 15. 
 
 
 A. Craftsbury, Vermont. 
 
 B. Kortfors, Sweden. 
 
 C. Stockholm, Sweden. 
 ORBICULAR GRANITES. 
 
DESCRIPTION OF IGNEOUS ROCKS 211 
 
 mass but the topographic forms produced also depend 
 greatly on the severity with which these act. In the 
 high mountain chains and wherever they are very ener- 
 getic, spires, needles and castle-like forms are produced, 
 but in the lower massive and older ranges and where 
 glaciation has been pronounced the granite stocks form 
 more smoothly modeled, rounded or dome-shaped masses 
 with gentle slopes and broad valleys, such as are seen in 
 the hills and mountains of New England and in parts of 
 Great Britain. The views on Plate 14 are illustrative of 
 this. 
 
 Orbicular Granite. It sometimes happens that the 
 component minerals of a granite, instead of being 
 uniformly distributed in grains of about the same size, are 
 collected in some spots in an unusual way and arranged in 
 ovoid or spherical bodies. Thus in a granite from Crafts- 
 bury, Vermont, called " pudding granite," the rock is full 
 of nodules, varying from the size of a pea to that of a nut, 
 composed almost entirely of agglomerated leaves of black 
 mica, as seen on Plate 15. More commonly the bodies 
 are composed of several minerals and consist of a nucleus 
 with a concentric outer shell or shells. The component 
 minerals are the same as those in the main body of the 
 rock but their proportions differ in the nucleus and in the 
 shells, sometimes consisting mostly or entirely of salic 
 minerals, while some shells consist mostly of ferromag- 
 nesian ones. Their appearance is shown on Plate 15. 
 
 The bodies are round, ovoid and often lenticular or 
 spindle shaped, as if drawn out. It was formerly thought 
 that they represented pebbles and were a proof of the 
 metamorphic origin of granite from conglomerates, but 
 the arrangement and regular internal structure of the 
 ovoids precludes such an idea and it is now generally held 
 that they are due to some process of differentiation or 
 aggregation of material in the magma with subsequent 
 crystallization, though in some cases it is thought that 
 they may represent inclusions of other rocks which have 
 
212 ROCKS AND ROCK MINERALS 
 
 been melted up and recrystallized. Granites of this kind 
 are called orbicular and though not common they have 
 been described from Sweden, Finland, Corsica, Canada 
 and Rhode Island. Similar structures have also been 
 found in diorites and gabbros. 
 
 Miarolitic Structure. The older and deeper seated 
 granites and especially those which have been subjected 
 to heavy mountain making pressures show little or nothing 
 of the miarolitic structure described on page 159. The 
 conditions have been unfavorable for the formation of 
 such cavities or under the pressure they have been oblit- 
 erated. In other occurrences and in the younger, higher 
 or unsqueezed granites these cavities may occur, and on 
 their drusy surfaces fine crystallizations of the minerals 
 may be seen. The crystals from such cavities in the 
 granite of the Pike's Peak region in Colorado, from the 
 Mourne Mountains in Ireland, from Baveno on the Lago 
 Maggiore in Northern Italy and other localities are well 
 known in mineral collections. 
 
 Pegmatite Dikes. These are very common in granites, 
 so much so, that when this word is used a granite pegmatite 
 is usually understood unless the rock is otherwise specified. 
 They have the general characters described on page 175 
 and the following ones. The chief minerals are quartz 
 and feldspar, the latter being mostly orthoclase or the 
 variety of it called microcline, though albite also occurs. 
 The quartz and feldspar are apt to be intergrown in such 
 a manner that the interstices of a spongy quartz crystal 
 are filled by an equally spongy feldspar crystal, the two 
 sponges thus mutually filling each other's interstices and 
 interclasping. As the quartz has no cleavage while the 
 feldspar has, the cleavage through the intergrown mass 
 is that of the feldspar and upon such surfaces the quartz 
 appears, repeating its tendency to crystal form again and 
 again and thus producing figures which recall the script 
 used in Arabic writings. This arrangement is called 
 graphic granite and a figure of it is seen on Plate 16. It 
 
PLATE 16. 
 
 GRAPHIC-GRANITE, OR PEGMATITE. 
 
DESCRIPTION OF IGNEOUS ROCKS 213 
 
 shows that these two minerals have crystallized simul- 
 taneously. The minerals occur also separately and often 
 in huge crystals so that such dikes are mined for com- 
 mercial purposes, the quartz and feldspar being used in 
 several technical processes such as the manufacture of 
 china, porcelain ware, etc. The large crystals of mus- 
 covite mica which occur in them are the source of this 
 material as used in stove windows, lamp chimneys, paper 
 making, etc. In addition to these chief minerals a great 
 variety of accessory ones are found, some of the more 
 common of which are tourmaline, garnet, beryl, and 
 spodumene among the silicates, apatite, triphylite, and a 
 series of related phosphates, and a variety of kinds con- 
 taining rare earths. Some of these minerals like the 
 colored tourmalines, topaz, beryl, etc., are valuable for the 
 material suitable for cutting into gems which they afford; 
 others are useful as sources of the rarer elements used in 
 chemical and some technical processes such as the making 
 of Welsbach mantles. A full list of all the minerals known 
 to occur in these pegmatites would cover a large proportion 
 of all the kinds known in mineralogical science. 
 
 Inclusions. Schlieren. It is not uncommon to find 
 in granite the various kinds of inclusions described on 
 page 163 and the following. Sometimes the composition 
 and form of these show that they are fragments of pre- 
 viously existing rock formations broken off and engulfed 
 in the granite magma. These are most common near the 
 border of the mass. They may vary in size from an inch 
 across or less to masses a number of yards long. When 
 they are found in the center of the mass they may be 
 suspected of having sunk into it from the former overlying 
 roof of other rocks. 
 
 In other cases the apparent inclusions are the schlieren 
 described. They may consist wholly or nearly so of 
 quartz and feldspar and thus be very light in color or 
 extremely rich in biotite or hornblende or both, with 
 iron ore, and thus be very, dark in color. Such dark 
 
214 ROCKS AND ROCK MINERALS 
 
 streaks may at times be due to melted up inclusions but 
 in other cases they may be caused by aggregations of the 
 normal dark minerals of the granite and in general are 
 ascribed to processes of differentiation. 
 
 Complementary Dikes. Very frequently it will be found 
 that bodies of granite are cut by complementary dikes 
 as described on page 167. The leucocratic ones are com- 
 monly composed almost solely of quartz and feldspar 
 with which is usually associated a little white mica. 
 The rock has a granular appearance and this variety of 
 granite is called aplite. Sometimes small black specks 
 of biotite or hornblende or of black tourmaline may be 
 seen in them but always the dark minerals play a very 
 subordinate role. The color of these rocks is nearly con- 
 stantly very light, white, flesh-color, pale yellow, gray or 
 brown being common. Sometimes these rocks are so 
 fine of grain that they pass into felsites of the colors 
 mentioned, and sometimes they are porphyritic with 
 phenocrysts of quartz or feldspar or both and are thus 
 granite or felsite porphyries. But most commonly they 
 are even-granular with a grain about like that of loaf 
 sugar and the dike is the characteristic mode of occurrence. 
 They are mostly noticed cutting the granite mass, less 
 commonly the surrounding rocks. They are of all sizes, 
 from a fraction of an inch to a number of yards in breadth. 
 If the larger ones are traced along the outcrop it may 
 sometimes be found that they change into pegmatite 
 dikes. 
 
 The melanocratic dikes, sheets, etc., complementary to 
 the quartzo-feldspathic aplites described above, are dark 
 to black heavy rocks of basaltic aspect. They are com- 
 posed chiefly of biotite-mica, hornblende, pyroxene and 
 iron ore with feldspars, but very commonly the grain is 
 too fine for these minerals to be distinguished and they 
 are to be classed as basalts, or, in allusion to their mode 
 of occurrence, they may be termed lamprophyric basalt. 
 In many cases, however, when biotite is the prominent 
 
DESCRIPTION OF IGNEOUS ROCKS 215 
 
 ingredient they have a characteristic glimmering appear- 
 ance or the plates of biotite may be distinctly seen, and in 
 this case they are known as mica traps. The most char- 
 acteristic color of these rocks is a dark stone gray. Occa- 
 sionally porphyritic crystals of hornblende or of feldspar, 
 as well as of biotite, appear in them and not uncommonly 
 fragments of the granite which they cut and of other 
 rocks. They also at times contain sulphurets of the 
 heavy metals, usually pyrite, and on this account have 
 been prospected or mined as if ore veins, generally without 
 much result. They alter and weather down into soft 
 greenish material full of chlorite or into brown earthy 
 masses. The earlier stages of alteration by the elements 
 are marked by the formation of carbonates and they then 
 effervesce freely when treated with acid. 
 . They occur characteristically in dikes, usually of but a 
 few feet in width, but as previously mentioned, also in 
 intrusive sheets, small laccoliths, etc. While they often 
 cut the granite they are more apt to be found in the outer 
 zone of rocks surrounding it and sporadic occurrences may 
 be discovered a number of miles distant from the parent 
 mass. The origin of these complementary dikes has been 
 already discussed on pages 167 and 178. 
 
 Contact Phenomena. It is especially around great 
 granite intrusions that the contact phenomena described 
 on page 180 are seen in their greatest development and 
 perfection. In the endomorphic form the granite may 
 become a felsite or granite porphyry at the contact, or it 
 may show a differentiated border zone (see page 165) and 
 become so enriched in the dark silicates as to pass into a 
 diorite or dolerite border facies, or, more rarely, on the 
 other hand, be so poor in these as to present a marginal 
 facies of aplite, quite like that seen in the complementary 
 dikes. The first cases mentioned are purely textural 
 modifications ; the second are chemical and mineralogical. 
 More rarely cases are known where granites have a border 
 of pegmatite. With respect to exomorphism the changes 
 
216 ROCKS AND ROCK MINERALS 
 
 described in the previous chapter, on account of the 
 common occurrence of granites, are more frequently seen 
 and have been more extensively studied in connection with 
 them than with any other variety of igneous rock. Around 
 the great granite batholiths these effects are often profound 
 and far reaching, involving tracts of possibly several miles 
 in width. Such areas are often of great interest and im- 
 portance, not only from the geological standpoint, but 
 because they are frequently the site of important ore 
 deposits. If granite comes directly against sedimentary 
 rocks with vertical contact and the latter show no 
 evidence of metamorphism, it may be safely assumed 
 that faulting or dislocation has brought them together. 
 
 Weathering of Granite into Soil. Through the action 
 of the atmosphere, of water, of heat and cold, granite 
 breaks down into soil. In northern and in temperate 
 regions, such as eastern North America, as Merrill has 
 shown, this change is at first largely a mechanical dis- 
 integration and the resultant material differs in its general 
 chemical composition but slightly from the original rock. 
 
 In appearance, however, as it changes granite may alter 
 considerably. The mica tends to bleach and lighten, and 
 ferrous compounds tend to become ferric and the iron 
 oxide to leach out, staining the rock red to dark brown. 
 At the same time its firm texture is lost and it becomes 
 more or less friable and crumbly. Finally it falls into an 
 angular gravel or sand, composed mainly of particles of 
 quartz and feldspar, called gruss. See Plate 17. 
 
 From this stage as the change into soil becomes more 
 complete, the most important process is the conversion 
 of the feldspar into kaolin, according to the following 
 reaction. 
 
 Orthoclase Water Carb.diox. Clay Quartz Potas. Carb. 
 
 2KAlSi 3 O 8 + 2 H 2 O + CO 2 = H 4 Al,Si 2 O 9 + 4 SiO 2 + K 2 CO 3 
 
 This reaction begins as soon as the rock is exposed; it has 
 only partially taken place when the rock crumbles, but 
 
PLATE 17. 
 
 GNEISSOID GRANITE, THROUGH WEATHERING, PASSING 
 FROM FIRM ROCK BELOW INTO ALTERED 
 
 ROCK AND SOIL ABOVE. 
 (Merrill, Bulletin Geological Society of America.) 
 
DESCRIPTION OF IGNEOUS ROCKS 217 
 
 after that goes on slowly but steadily until the feldspar is 
 eventually wholly changed into clay. As a result, a soil 
 consisting of a mixture of clay and quartz sand, stained 
 reddish or yellowish by the iron compounds, is formed. 
 Such a soil is called a loam. Usually the process is not 
 entirely complete and the soil contains more or less 
 small particles of feldspar undergoing alteration. This 
 has an important bearing on the self-renewal of its 
 fertility. 
 
 This reaction is one of the most important that takes 
 place in the great laboratory of Nature, for by means of it, 
 not only is the solid rock converted into soil, but one of the 
 most essential of plant foods, the potash, is converted 
 into soluble form in which it can be assimilated. At the 
 same time the other essentials of plant food, the silica, 
 magnesia, lime, etc., are also unlocked from the rocks and 
 rendered available. Thus by their aid plant life is able 
 to grow and produce from water, carbon, dioxide, etc., 
 those substances upon which all animal life ultimately 
 depends. 
 
 In tropical regions the decay of granite gives rise to a 
 red or yellow-brown ferrugineous earth to which the name 
 of laterite is given. It has been shown to consist of a 
 mixture of quartz sand with hydrargillite, a clay-like sub- 
 stance with the composition A1(OH) 3 , colored by iron 
 oxides. But the name has also been applied in India to 
 soils formed from the basalts of the great Deccan plateaux 
 mentioned later. 
 
 In tropical deserts the surface of granites becomes 
 coated by a brownish or black skin, sometimes with a 
 luster like varnish, due to the alteration of the iron- 
 bearing components and the formation of iron and man- 
 ganese compounds. This also occurs with other kinds of 
 rocks as well. 
 
 Occurrence of Granites. Granite is one of the most 
 common and widely occurring of igneous rocks, and plays 
 a prominent role in the formation of the continental 
 
218 ROCKS AND ROCK MINERALS 
 
 masses. In the form of great stocks and batholiths it 
 forms the central core of many of the great mountain 
 ranges and is revealed by later erosion. In those parts of 
 the earth's surface which have been subjected to repeated 
 disturbances of the crust and profound erosion granites 
 are common rocks. Thus great stocks of different ages 
 of intrusion are found in eastern Canada, in New England 
 and generally along the region of the Piedmont plateau 
 from southern New York into Georgia. They occur 
 again in Missouri, Wisconsin, etc., in isolated areas, but in 
 general, until the Rocky Mountains region is approached, 
 the central states, which compose the Mississippi Valley, 
 being covered with stratified rocks, are devoid of them, 
 though it may be inferred by analogy that they form a 
 large part of the basement on which these later rocks lie. 
 In the Rocky Mountains and in the far western states 
 they are of importance. Likewise in Europe, in western 
 and southern England, in Ireland and Scotland, in various 
 places in France and Germany and in the Alps they are of 
 common occurrence and their exposures form considerable 
 areas. Such a list of occurrences might be almost in- 
 definitely extended but enough has been said to show 
 their importance and wide distribution. 
 
 SYENITE. 
 
 Composition. Syenites are granular rocks composed 
 chiefly of feldspars. They differ from granites in that 
 they contain no quartz or only a negligible quantity. 
 They may consist entirely of feldspar, but usually more or 
 less hornblende, mica or pyroxene is present. These 
 however are subordinate in amount to the feldspar, since 
 if they are equal to or exceed it, the rock becomes a 
 diorite. If the rock is fairly coarse-grained, occasional 
 particles of magnetite and other minerals may be seen, 
 but these are only accessory and not of the importance of 
 the ones mentioned. Occasional minerals which produce 
 varieties will be mentioned presently. 
 
DESCRIPTION OF IGNEOUS ROCKS 219 
 
 In strict petrographic classification founded on microscopic 
 examination a distinction is made in these rocks based upon the 
 kind of feldspar. If the latter is predominantly an alkalic feldspar, 
 without lime, the rock is called a syenite, as above, but if it is a lime- 
 soda feldspar or plagioclase the rock is termed a diorite without 
 reference to the quantity of dark minerals present. This dis- 
 tinction, however, cannot, except in certain exceptional cases, be 
 carried out by megascopic examination and therefore no attempt is 
 made to separate them in this work. 
 
 From what has been said it may be seen that the mineral 
 composition of syenite may vary considerably; there may 
 be a mixture of feldspars present or only one kind, either 
 alkalic or lime-sodic, and there may be variations among 
 the ferromagnesian minerals. According to the pre- 
 dominant kind of the latter, the rock is spoken of as horn- 
 blende syenite, mica syenite or augite syenite. All of 
 these are here treated under the general heading of syenite, 
 but in two cases the rock may have a particular mineral 
 composition which makes it of especial interest and there- 
 fore deserving of separate description. Generally these 
 two varieties may be identified by observing with care the 
 special features which they present and which are described 
 beyond, otherwise they cannot be distinguished and must 
 be classed in the general group of syenites. These two 
 are as follows: (A) the rock contains in addition to the 
 feldspars and other minerals a notable amount of nephelite 
 or this and its congener sodalite; (B) the rock consists 
 entirely, or very nearly so, of soda-lime feldspar (labra- 
 dorite). We may then divide the group of syenites as 
 follows: 
 
 a. Syenite, in general or common syenite consisting 
 
 chiefly of feldspars, without quartz. 
 6. Nephelite Syenite, consisting chiefly of alkalic 
 
 feldspars with nephelite. 
 c. Anorthosite, consisting almost wholly of labradorite. 
 
 Properties of Syenite. The texture of syenites is usually 
 even granular, but sometimes a tendency may be noticed 
 
220 
 
 ROCKS AND ROCK MINERALS 
 
 for the feldspar to assume a flattened tabular form like 
 that of a book, its cross sections on the rock surface are 
 then elongated and often arranged more or less parallel, 
 an arrangement which is thought to be due to movements 
 of the fluid mass during crystallization. This variety of 
 texture occurs practically only when the feldspars are of 
 the alkalic variety. Porphyritic varieties also occur as in 
 granite and these grade into syenite porphyry. The color 
 is variable like that of granites; white to pink or red, or 
 gray or yellow tones are common, gray especially so. The 
 specific gravity varies with the minerals and their propor- 
 tions; it may extend from 2.6-2.8. In a tendency to 
 miarolitic structure, in jointing, in erosion forms, in altera- 
 tion into soil, inclusions, and in contact metamorphism, etc., 
 what has been said in regard to granite, applies also to 
 syenites and need not be repeated. They are also accom- 
 panied by pegmatite dikes, but these are not so common 
 nor so well known as the granite pegmatites. They also 
 often yield a great variety of minerals. 
 
 Chemical Composition. Chemically, the syenites are 
 distinguished from the granites by a lesser amount of 
 silica, which accounts for the absence of the quartz; in 
 other respects they resemble them. These characters 
 may be seen in the following table of analyses. 
 
 ANALYSES OF SYENITES. 
 
 
 SiO 2 
 
 A1 2 3 
 
 Fe 2 3 
 
 FeO 
 
 MgO 
 
 CaO 
 
 Na 2 O 
 
 K 2 
 
 H 2 
 
 XyO 
 
 Total. 
 
 I 
 
 60.7 
 
 19.6 
 
 1.5 
 
 3.0 
 
 0.8 
 
 2.3 
 
 4.9 
 
 5.9 
 
 0.3 
 
 0.6 
 
 99.6 
 
 II 
 
 60.2 
 
 20.4 
 
 1.7 
 
 1.9 
 
 1.0 
 
 2.0 
 
 6.3 
 
 6.1 
 
 0.3 
 
 0.4 
 
 100.3 
 
 III 
 
 61.6 
 
 15.1 
 
 2.0 
 
 2.2 
 
 3.7 
 
 4.6 
 
 4.3 
 
 4.5 
 
 0.7 
 
 1.0 
 
 99.7 
 
 IV 
 
 62.5 
 
 16.5 
 
 2.4 
 
 2.0 
 
 1.9 
 
 4.2 
 
 4.4 
 
 4.6 
 
 0.6 
 
 1.3 
 
 100.4 
 
 I, Belknap Mountains, New Hampshire ; II, Fourche Mountain, 
 Arkansas; III, Little Belt Mountains, Montana; IV, Plauen by 
 Dresden, Germany. Xyo = Small amounts of other oxides. 
 
DESCRIPTION OF IGNEOUS ROCKS 221 
 
 Occurrence of Syenites. Syenites are not very common 
 rocks, and, while they sometimes occur in independent 
 masses, they are very apt to be connected with larger 
 bodies of granite, which by the diminishing of quartz 
 passes into syenite. In the United States they occur in 
 several places in New England, at Mount Ascutney, in the 
 White Mountains and adjacent region, on the coast north 
 of Boston, also in Arkansas, in Montana and in a number 
 of other localities. They are found in several places in 
 Germany and in the Alps. An important area of them 
 exists in South Norway. In comparison with the great 
 batholiths and stocks of granite, distributed so generally 
 in the continental masses, they are, geologically speaking, 
 of relatively small importance. 
 
 Uses of Syenite. For all constructional and other 
 commercial uses syenite has the same value as granite. 
 On account of its relative rarity, compared with the latter 
 rock, it is however little used. Its crushing strength is 
 equal to that of granite and from experiments by J. F. 
 Williams on syenite from Arkansas it may be even greater. 
 Its weight per cubic foot is about the same. The absence 
 of the quartz, which is harder than feldspar, should make 
 it an easier stone to dress and polish, and practically it 
 resists weathering as well, if not better. The absence of 
 the quartz makes it also a better stone in resisting the heat 
 of fires (compare granite, page 209) and it would be in 
 consequence a more advantageous material for building 
 in our large cities. If these advantages over granite 
 were more generally understood it is probable that the 
 accessible occurrences in New England would be more 
 extensively exploited. The beautiful dark gray syenite 
 of South Norway with pearly blue reflections is con- 
 siderably used in northern Europe as an ornamental 
 stone. 
 
 Nephelite Syenite. This variety is distinguished by the 
 fact that in addition to the feldspars, which are almost 
 wholly alkalic in composition, a considerable amount of 
 
222 ROCKS AND ROCK MINERALS 
 
 nephelite is present. This mineral is sometimes flesh- 
 colored but usually it is smoky gray. In its lack of good 
 cleavage and oily, greasy luster it resembles quartz, but 
 can be readily distinguished from it by the gelatinization 
 test (page 115). It is generally present in formless grains 
 mixed with the feldspars, but sometimes shows the outlines 
 of a crystal form. It is apt to be accompanied by sodalite, 
 which is often of a bright blue color, in grains or, if the rock 
 is very coarse-grained, in lumps and masses; if it is thus 
 present it is useful in aiding to distinguish this rock from 
 common syenite. Nosean and cancrinite may also be 
 present. Mica (lepidomelane), hornblende (arfvedsonite) 
 and pyroxene (segirite) are usually present in variable 
 amounts, in plates, grains or prisms, of a black color, and 
 containing considerable soda and iron. The presence of 
 soda in the minerals of this rock is readily understood 
 from a consideration of the chemical analyses, given 
 beyond, which show the composition of the magma from 
 which they crystallized. 
 
 The color of nephelite syenites is variable but commonly 
 gray. The texture is granular, sometimes rather por- 
 phyritic. The book-shape of the feldspars mentioned 
 above is common. The rock is liable to contain many 
 accessory minerals but usually only in microscopic sizes; 
 some of these are of especial interest on account of the 
 rare earths they contain. It is also prone to exhibit in 
 places great variations of the constituent minerals giving 
 rise to different facies. Many of these varieties have 
 received special names. Usually it is cut by comple- 
 mentary dikes and these are of a different character. from 
 those found associated with granites and common syenites; 
 one is a pale brown or pink felsite, another a bright to 
 dark green rock called tinguaite which owes its color to 
 microscopic needles of segirite ; it usually contains nephelite 
 and gelatinizes with acid. The complementary lam- 
 prophyres to these are heavy dark rocks of basaltic aspect 
 often showing distinct to large phenocrysts of biotite,- 
 
DESCRIPTION OF IGNEOUS ROCKS 
 
 223 
 
 augite or hornblende; they are particular varieties of 
 melaphyre (basalt-porphyry). 
 
 The chemical composition is illustrated in the two 
 following analyses of nephelite syenites. 
 
 
 SiO 2 
 
 A1 2 3 
 
 Fe 2 3 
 
 FeO 
 
 MgO 
 
 CaO 
 
 Na 2 O 
 
 K 2 O 
 
 H 2 O 
 
 XyO 
 
 Total. 
 
 I 
 
 58.8 
 
 22.5 
 
 1.5 
 
 1.0 
 
 0.2 
 
 0.7 
 
 9.6 
 
 4.9 
 
 1.0 
 
 0.3 
 
 100.5 
 
 II 
 
 53.1 
 
 21.2 
 
 1.9 
 
 2.0 
 
 0.3 
 
 3.3 
 
 6.9 
 
 8.4 
 
 1.4 
 
 2.0 
 
 100.5 
 
 I, Salem Neck, Mass. ; II, Magnet Cove, Arkansas. XyO, small 
 quantities of various oxides. 
 
 It will be seen that the most striking thing in respect 
 to these magmas are the high amounts of alumina and 
 alkalies, with moderate silica. It is this which causes the 
 formation of nephelite (Na 2 O . A1 2 O3 . 2 SiO 2 ) rather 
 than albite (Na 2 O . A1 2 O 3 . 6 SiO 2 ), there being not 
 enough silica to convert all the alumina and alkalies -into 
 feldspar. From this it may be seen that free quartz and 
 nephelite cannot crystallize from the same magma; the 
 silica would convert the nephelite into albite, and therefore 
 these two minerals are not found in the same rock. Some- 
 times nosean is present; cancrinite may also occur, and of 
 the associated minerals zircon is perhaps the most charac- 
 teristic. In this connection the description of the felds- 
 pathoid group in the part dealing with minerals should 
 be read. 
 
 Pegmatite dikes occur in connection with nephelifce 
 syenites and those of South Norway and Greenland are 
 especially interesting for the great variety of minerals, 
 many of them composed in part of the rarer elements, 
 which they have afforded. 
 
 Nephelite syenites usually occur in rather small stocks 
 or large dikes; relatively large areas of them are known in 
 only a few places, Greenland, South Norway and Lapland. 
 They are of rare occurrence and geologically are of small 
 
224 ROCKS AND ROCK MINERALS 
 
 importance compared with granites, gabbros and diorites. 
 In the United States they are found at Litchfield, Maine; 
 Red Hill, New Hampshire; Salem, Massachusetts; Beem- 
 erville, New Jersey; Magnet Cove, Arkansas; Cripple 
 Creek, Colorado; in western Texas and a few other local- 
 ities. In Canada, at Montreal; Dungannon, Ontario; Ice 
 River, British Columbia. Noted localities for these rocks 
 and their associated minerals in Europe are in South 
 Norway; Alno Island, Sweden, Kola Peninsula and Miask, 
 Ural Mountains, Russia; Foya, Portugal and Ditro, Tran- 
 sylvania. The rock is too uncommon to be of commercial 
 importance but makes an excellent building stone where 
 it occurs. That in the neighborhood of Magnet Cove, 
 Arkansas, has been thus used. 
 
 Anorthosite. This. rock is composed wholly, or nearly 
 so, of a soda-lime feldspar, usually the variety described 
 as labradorite. Sometimes small quantities of a ferro- 
 magnesian mineral, pyroxene, is sprinkled through it in 
 grains and specks, or small masses of magnetite or some 
 other iron ore can be seen. This simple mineral com- 
 position makes it resemble in the hand specimen, especially 
 when the grain is rather fine and the colors light, both 
 marble and quartzite, also rocks consisting of a single 
 mineral. From the former it is easily told by its superior 
 hardness, since the feldspar cannot be scratched by the 
 knife, while marble is easily cut or scratched, and from the 
 latter by the cleavage of the crystal grains which can 
 usually be easily seen with a lens. While these characters 
 help to distinguish the rock, its identification can only be 
 made certain by the determination of the kind of feldspar 
 present ; otherwise it can merely be referred to the general 
 group of syenites. This can only be done in the field 
 when the cleavage surfaces of the feldspars are sufficiently 
 large to permit one to see on them the characteristic 
 twinning striations of plagioclase (see page 38). In 
 the laboratory the feldspar can be identified by blowpipe 
 and chemical tests. 
 
DESCRIPTION OF IGNEOUS ROCKS 225 
 
 The color of the rock is normally white, and this is 
 sometimes seen, but generally it is colored yellowish to 
 brown, or, more commonly, some shade of gray, light 
 gray, blue or smoky to dark gray and almost black. 
 The very dark exotic color is due to an included pig- 
 ment, perhaps ilmenite dust, but it is notable that 
 where these rocks have been subjected to erogenic pres- 
 sure, and especially when they have been sheared and 
 granulated and have assumed gneissoid structure, the 
 dark color tends to disappear and they become lighter. 
 The chemical composition is practically that of a labra- 
 dorite feldspar (Anal. IV, page 43). 
 
 Mineralogically, the anorthosites are related to the 
 gabbros, for they contain the same kind of feldspar and 
 often, as stated above, there is more or less pyroxene; if 
 this latter increases in amount, passages into gabbro may 
 occur; and in gabbros, phases poor in pyroxene, and thus 
 transitional to anorthosite, are found. Geologically, how- 
 ever, they occur quite independently of gabbros. They 
 are not common rocks, so far as the number of occurrences 
 is concerned, but are of importance from the large and 
 sometimes vast masses which they form, notably in 
 Canada and Norway. They are found in Canada in 
 separate areas from the west coast of Newfoundland and 
 the east coast of Labrador down through Quebec into 
 eastern Ontario. One of these areas drained by the 
 Saguenay River covers nearly 6000 square miles while one 
 near Montreal comprises about 1000 square miles. 
 
 Another region is in the Adirondack Mountains in 
 northern New York state, which is in large part com- 
 posed of this rock. Small occurrences of a nearly related 
 type are found also in the White Mountains, New Hamp- 
 shire. It is found again in considerable masses in Minne- 
 sota on the Lake Superior coast. 
 
 In Europe, anorthosite occurs in Norway in large areas 
 on the west coast at Bergen, at Ekersund and on the 
 Sognfiord. It is alo found in Volhynia in Russia. 
 
226 ROCKS AND ROCK MINERALS 
 
 The labradorite of this rock sometimes shows a beautiful opal- 
 escent play of colors, especially a deep blue. Cleavage pieces from 
 the coarse and massive rock of the coast of Labrador have long been 
 known and cut as ornamental stones. Similar material comes from 
 near Zitomir in Volhynia. 
 
 Corundum Syenite. In all the different varieties of syenite 
 described above, instances have been found in which the rock con- 
 tains, in addition to the usual constituents, a notable amount of 
 corundum. The appearance of this mineral is due to the fact that 
 the magma contains more alumina than the alkalies and lime present 
 can turn into feldspars and feldspathoids, and this excess is forced 
 to crystallize out as corundum (A1 2 O 3 ), just as in granites the excess 
 of silica is compelled to form quartz (SiO 2 ). The mineral occurs in 
 crystals, either hexagonal prisms or barrel-shaped, or in grains and 
 lumps, and is usually of a gray color. It is easily identified by its 
 excessive hardness. 
 
 Such occurrences have been found in central Montana in common 
 syenite; in the counties of Renfrew, Hastings and others in Ontario, 
 Canada, in syenite and nephelite syenite; these rocks have been 
 traced in a belt a distance of over a hundred miles; it occurs in a 
 similar manner in the Ural Mountains and in Coimbatore district, 
 India. Anorthosites containing corundum are known from Clay 
 County, North Carolina ; Lanark County, Ontario, and from the Ural 
 Mountains. Some of these occurrences, notably the ones in Canada, 
 are of economic value as a source of this valuable abrasive. Corun- 
 dum also occurs in other kinds of igneous rocks as mentioned under 
 dunite. 
 
 DIORITE. 
 
 Composition. The diorites are granular igneous rocks 
 composed of hornblende and feldspar of any kind, in 
 which the amount of hornblende equals or exceeds the 
 amount of feldspar. Usually more or less iron ore in 
 fine grains can be seen, and very often considerable biotite 
 is present in shining flakes, with sometimes a bronzy 
 luster. The hornblende is usually black, sometimes 
 dark green, and, while often in bladed or prismatic forms, 
 it is also often in short thick crystals or grains and some- 
 times in small masses of them and of biotite separated by 
 the light-colored feldspar. For its recognition and dis- 
 tinction from pyroxene see page 66. While any kind of 
 feldspar may be present, in the great majority of cases, 
 
PLATE 18. 
 
 A. ORBICULAR DIORITE, 
 CORSICA. 
 
 B. DIORITE. 
 
 C. DIORITE, COMMON TYPE. 
 

DESCRIPTION OF IGNEOUS ROCKS 227 
 
 as learned from microscopical studies, it is a soda-lime 
 variety, containing considerable lime. This latter point 
 however can rarely be determined on the hand specimens 
 because the rock is not often coarse grained enough to 
 permit the recognition of twinning striations on their 
 cleavage surfaces. It is not uncommon for some quartz 
 to be present and this can sometimes be identified with 
 the lens. 
 
 While the rocks determined as diorites by this megascopic classi- 
 fication will correspond in a general way with the greater part of the 
 diorites of the more strict classifications founded on microscopic and 
 chemical methods they also include some less common rocks which, 
 for one reason or another, have been given various names by 
 petrographers. 
 
 General Properties. The color of diorites is dark-gray 
 or greenish, running into almost black in some varieties. 
 It results from the color of the hornblende and the pro- 
 portion of this to feldspar. The different varieties are 
 due to the color, coarseness of grain, etc. The texture of 
 the rock is the granular one. The porphyritic texture, 
 while not unknown, is far less common than in granite. 
 Sometimes the black hornblende prisms are distinct 
 enough to produce an impression of porphyritic texture 
 which is dispelled as soon as one compares the average 
 size of the crystal grains. Orbicular structures are known 
 to occur. A rock from Corsica exhibiting it has been used 
 somewhat as an ornamental stone; it is illustrated on 
 Plate 18. Miarolitic cavities occur as in granite; they are 
 often masked by being filled with calcite. Pegmatite 
 dikes also occur and the minerals are somewhat different 
 from those in the granites. Fluidal or somewhat parallel 
 arrangements of the component minerals are not uncom- 
 monly seen, and these produce tendencies to gneissoid 
 structure. Diorites are also frequently cut by com- 
 plementary dikes, of much the same general appearance 
 as those in granites, or these are found in their immediate 
 neighborhood in dikes and sheets. Thus they may be 
 
228 
 
 ROCKS AND ROCK MINERALS 
 
 traversed by light-colored aplites and felsites and by 
 dark, heavy, basaltic traps. 
 
 Their jointing is like that described for granites. 
 
 Chemical Composition. This varies considerably with 
 the relative amounts of feldspar and hornblende, with 
 the particular varieties of these two which are present, and 
 is also somewhat influenced by the accessory minerals 
 which may occur. The following table illustrates this 
 and it shows also how the increase of lime, iron and 
 magnesia over the proportions of these oxides in granites 
 and syenites, causes the increase in the amount of horn- 
 blende. 
 
 ANALYSES OF DIORITES. 
 
 
 SiO 2 
 
 A1 2 3 
 
 Fe 2 3 
 
 FeO 
 
 MgO 
 
 CaO 
 
 Na 2 
 
 K 2 
 
 H 2 
 
 XyO 
 
 Total. 
 
 I 
 
 55.1 
 
 20.2 
 
 1.5 
 
 4.3 
 
 1.8 
 
 7.0 
 
 4.3 
 
 2.8 
 
 1.1 
 
 1.7 
 
 99.8 
 
 II 
 
 58.0 
 
 18.0 
 
 2.5 
 
 4.6 
 
 3.6 
 
 6.2 
 
 3.6 
 
 2.1 
 
 0.9 
 
 1.2 
 
 100.7 
 
 III 
 
 47.1 
 
 18.1 
 
 3.0 
 
 8.5 
 
 7.3 
 
 6.6 
 
 2.4 
 
 2.8 
 
 3.6 
 
 0.5 
 
 99.9 
 
 IV 
 
 43.9 
 
 16.2 
 
 4.0 
 
 10.1 
 
 5.1 
 
 9.6 
 
 2.9 
 
 1.5 
 
 1.6 
 
 4.9 
 
 99.8 
 
 I, Diorite, Little Belt Mountains, Montana; II, Electric Peak, 
 Yellowstone Park; III, Malvern Hills, England; IV, Belknap Moun- 
 tains, New Hampshire. XyO = Small quantities of various oxides, 
 TiO 2 , MnO, etc. 
 
 Occurrence, Uses, Etc. While diorites in many places 
 are found as independent intrusions, they are also very 
 apt to be, on the one hand, connected with granites, on 
 the other with gabbros, and usually these pass into each 
 other. They also do not form such vast batholiths or 
 stocks as the granites or the gabbros, and, especially as 
 independent masses, are more liable to be found as small 
 stocks, large dikes, etc. They have a very wide dis- 
 tribution and are found in all parts of the world where 
 the older deeply-seated igneous rocks are laid bare by 
 continued erosion. In the later formed mountain regions 
 they are also found as stocks and dikes. What has been 
 said of granites in this respect would be largely true of 
 
DESCRIPTION OF IGNEOUS ROCKS 229 
 
 diorite. Owing to its dark color diorite is not so exten- 
 sively used for architectural purposes as granite, though, 
 so far as strength, durability and capacity for receiving a 
 high polish is concerned, it would furnish in many places 
 excellent material. It is somewhat heavier than granite, 
 its specific gravity ranging from 2.8-3.1; at 3.0 a cubic foot 
 of it would weigh about 187 pounds. 
 
 Relation to Other Rocks. As mentioned under its 
 description as a rock mineral, pyroxene through meta- 
 morphic processes changes into hornblende. Generally 
 this is accompanied in the rock by the production of 
 schistosity as described under metamorphism and horn- 
 blende schist. The description of uralite should also be 
 consulted. It sometimes happens that this change takes 
 place in gabbro without causing the rock to lose its 
 massive character or becoming schistose. In this case, 
 if of sufficiently coarse grain to permit the recognition of 
 the hornblende and feldspar, it would be classed as a 
 diorite. If it can be proved that a diorite has been 
 derived from gabbro, it may well be termed a metadiorite 
 to indicate its secondary origin. Usually, however, the 
 grain of such rocks is quite fine, too much so to permit the 
 individual hornblende prisms to be definitely determined, 
 and the rock would be classed under dolerite. They are 
 very apt to have a green color and for this reason have 
 been called greenstones. The green color is partly due 
 to hornblende, partly to chlorite. These rocks are 
 further mentioned under dolerite. 
 
 GABBRO. 
 
 Composition. The gabbros are granular igneous rocks 
 consisting chiefly of pyroxene and feldspar of any kind, in 
 which the amount of pyroxene equals or exceeds that of 
 the feldspar. Usually more or less iron ore in black 
 metallic-looking grains can be seen, and in some varieties 
 considerable olivine may occur. This latter can some- 
 times be detected with the lens as yellowish or green 
 
230 ROCKS AND ROCK MINERALS 
 
 grains. Careful inspection will often show occasional 
 bronzy flakes of biotite. Of the two chief minerals the 
 pyroxene is usually dark greenish when examined with 
 the lens, often black to the eye alone, and sometimes it is 
 of the variety diallage with a pronounced apparent 
 cleavage in one direction, of a gray-green color and often 
 almost micaceous appearance, at times somewhat brassy 
 or semi-metallic in luster. A test with the knife point for 
 cleavage shows at once its non-micaceous character. The 
 feldspar in the great majority of cases is a soda-lime 
 variety, generally labradorite, as may often be seen by 
 the twinning striations on a cleavage surface. It is 
 usually in formless masses or grains like the other minerals, 
 but not unfrequently it has a tabular or book-like form 
 and the sections on the rock face have an elongated shape. 
 In this case the striations run parallel with the elongation. 
 Sometimes the feldspar is fresh and glassy; in this case the 
 two feldspar cleavages are good and the striations if 
 visible are distinct; sometimes the feldspar is waxy in 
 appearance, of a glimmering luster to dull, often with a 
 bluish tone; in this case the cleavage is poor or even 
 apparently wanting and striations cannot be seen. In 
 the latter case the feldspar is more or less affected by 
 alteration to other minerals as described on page 44. 
 
 A distinction is made by petrographers by which gabbros are 
 divided into two groups, depending on the variety of pyroxene 
 present. If this is the monoclinic, lime-bearing augite or diallage, 
 the rock is called gabbro, if it is the orthorhombic hypersthene which 
 is without lime the rock is called norite. This distinction cannot 
 be made in megascopic determinations unless some of the pyroxene 
 is extracted from the rock and tested chemically, hence the norites 
 are here included under gabbro. A rarer type consists of plagioclase 
 and olivine without pyroxene and is called Troctolite. Some rare 
 rocks with alkalic feldspar are also here included under gabbro which 
 are variously classified and named by petrographers. 
 
 General Properties. The color of gabbros is usually 
 dark, dark gray or greenish to black; very rarely reddish. 
 In some varieties in which diallage is the kind of pyroxene 
 
DESCRIPTION OF IGNEOUS ROCKS 
 
 231 
 
 present and the grain is moderately coarse the rock is 
 much lighter in tone and of a medium gray or greenish- 
 gray. The same is true in many cases where the rock is 
 more or less altered; compare with what is said of the 
 feldspars above. The texture is granitoid or granular, 
 sometimes with a porphyritic tendency from the elonga- 
 tion of the feldspars, but true porphyritic texture is very 
 rare. Miarolitic cavities are much less frequent than in 
 granite and syenite. Orbicular gabbro has been found 
 in California. A fluidal or banded structure which is 
 produced by drawn-out layers of varying composition and 
 which simulates a gneissoid structure has been described 
 from several localities, from the Hebrides, California and 
 Minnesota. Pegmatites are also occasionally found in 
 gabbros; they consist of the usual minerals of the rock. 
 In South Norway the pneumatolytic processes attending 
 the intrusion of gabbros have formed much scapolite and 
 other minerals in the gabbro at its border and in dikes in 
 the contact zone; of these minerals apatite is the most 
 prominent and occurs sometimes in large masses. Com- 
 plementary dikes, etc., occur in gabbro masses but are not 
 perhaps so notable a feature as in the foregoing groups. 
 In this connection what is said concerning peridotites may 
 be consulted. 
 
 ANALYSES OF GABBRO. 
 
 
 SiO 2 
 
 A1 2 3 
 
 Fe 2 3 
 
 FeO 
 
 MgO 
 
 CaO 
 
 Na 2 
 
 K 2 
 
 H 2 
 
 XyO 
 
 Total. 
 
 I 
 
 47.9 
 
 18.9 
 
 1.4 
 
 10.5 
 
 7.1 
 
 8.4 
 
 2.7 
 
 0.8 
 
 0.6 
 
 1.7 
 
 100.0 
 
 II 
 
 48.9 
 
 8.8 
 
 1.0 
 
 9.5 
 
 15.2 
 
 14.7 
 
 0.6 
 
 0.1 
 
 0.6 
 
 0.7 
 
 100.1 
 
 III 
 
 49.9 
 
 18.5. 
 
 2.1 
 
 8.4 
 
 5.8 
 
 9.7 
 
 2.6 
 
 0.7 
 
 1.0 
 
 1.4 
 
 100.1 
 
 IV 
 
 52.8 
 
 17.8 
 
 1.2 
 
 4.8 
 
 4.8 
 
 12.9 
 
 3.0 
 
 0.5 
 
 1.2 
 
 0.5 
 
 99.5 
 
 V 
 
 40.2 
 
 9.5 
 
 9.7 
 
 12.2 
 
 8.0 
 
 13.1 
 
 0.8 
 
 0.2 
 
 0.5 
 
 5.5 
 
 99.7 
 
 I, Adirondacks, New York State; II, Orange Grove, Maryland; 
 III, Pigeon Point, Minnesota; IV, Band rich in feldspar, poor in 
 pyroxene, Isle of Skye, Hebrides. V, Band rich in pyroxene, poor 
 in feldspar, Isle of Skye. XyO = small quantities of various oxides, 
 chiefly TiO 2 . 
 
232 ROCKS AND ROCK MINERALS 
 
 Chemical Composition. The gabbros, as a rule, contain 
 larger amounts of lime, iron and magnesia, and less of 
 silica and alkalies than any of the previously described 
 rocks as may be seen from the table annexed. 
 
 Analyses IV and V show how the chemical composition 
 of the banded gabbros varies in the different streaks with 
 corresponding variation in mineral contents. 
 
 Occurrence. Gabbros are widely distributed and com- 
 mon rocks. They are found as large stocks and bath- 
 yliths and in dikes in the older rock complexes, similarly to 
 granite. They are also found as stocks and necks of old 
 volcanoes cutting the stratified beds of the younger moun- 
 tain regions. In these they may also be found as thick 
 intrusive sheets. Gabbros have been held to occur also 
 as forming the central portion of thick extrusive sheets, 
 as in the Hebrides, in Sweden and in the Lake Superior 
 region. If this is the case it is due to the low freezing 
 point of the magma, its liquidity and ready crystallization. 
 
 In the United States, gabbros are found in many places 
 in New England, as in the White Mountains. They are 
 found in the Adirondacks and at Cortlandt on the Hudson 
 River in New York State, in Maryland, etc. 
 
 They occur in the Lake Superior region and elsewhere 
 in Minnesota and in various places in the Rocky Moun- 
 tains and in California. They are extensively distributed 
 in Europe, in southern England, in northern Scotland, 
 especially on the islands of the Hebrides, in Norway and 
 Sweden and in Germany. They are in fact almost as 
 widely known as granites though they do not form, as a 
 rule, such large masses. 
 
 Alteration of Gabbro. It is common to find that where 
 gabbro massives occur in the older rock complexes and in 
 folded mountain ranges that they are surrounded by a 
 mantle of hornblende schist into which the gabbro grad- 
 ually passes, by transitional phases. The origin of this 
 is the pressure, shearing and other metamorphic agencies 
 brought about by the orogenic processes, as mentioned 
 
DESCRIPTION OF IGNEOUS ROCKS 233 
 
 under metamorphism, hornblende, diorite, etc., which 
 have acted upon the pyroxene of the gabbro converting 
 it into hornblende and producing the schistose structure. 
 It may happen through pressure and shearing that a 
 schistose or, perhaps better, a gneissoid structure may be 
 induced in the gabbro without change of the pyroxene 
 to hornblende and we would have in this case a gabbro- 
 gneiss or gabbro-schist produced, but generally the change 
 to hornblende occurs. If olivine is present it also forms 
 amphibole. Very often garnet appears as a new mineral 
 resulting from the process. While the change to horn- 
 blende is usually accompanied by the assumption of a 
 more or less pronounced gneissoid or schistose structure, 
 this is not always the case; the rock sometimes retains a 
 massive granular character and, if its constituent feldspar 
 and hornblende can be recognized, it would be classed as a 
 diorite, as mentioned under that rock. In another mode 
 of alteration of gabbros the feldspar is changed into a 
 substance called saussurite, which was formerly thought 
 to be a distinct mineral, but which the microscope has 
 shown to be a mixture of albite, zoisite and other minerals. 
 The feldspar, or rather that which replaces it, has no 
 cleavage and is waxy looking. The pyroxene is changed 
 to hornblende, which tends to have a bright to grass- 
 green color and is the variety called smaragdite. Other 
 minerals are also formed, but megascopically the waxy- 
 looking saussurite and green hornblende predominate. 
 This may take place without formation of schistose 
 structure and it seems probable that in this case the 
 alteration is due more to the chemical and less to the 
 dynamic agencies of metamorphism. Such rocks have 
 been called saussurite-gabbro. 
 
 In the process of weathering through the agencies of 
 the atmosphere, gabbros give rise to clay soils deeply 
 colored by the oxides of iron and mingled with fragments 
 of still undecomposed minerals. 
 
 Iron and Other Ore Deposits. There are frequently 
 
234 ROCKS AND ROCK MINERALS 
 
 found in large gabbro intrusions masses of iron ore, 
 sometimes consisting of magnetite, but generally of 
 ilmenite or mixtures of the two. Usually these are more 
 or less mingled with the minerals of the gabbro itself, 
 especially pyroxene and olivine. The character of the 
 occurrences, their lack of definite form and the manner in 
 which they gradually shade into the normal gabbro, show 
 that they are only a phase of the rock in which the iron 
 ore, usually scattered through it in small grains, is here 
 locally concentrated in great abundance. Such ore de- 
 posits are sometimes found at the border of the intrusion, 
 though often scattered in masses through it or at the 
 center. They are known in many places, in the Adiron- 
 dacks, in northern Minnesota, in Canada, Norway, Sweden 
 and elsewhere. If titaniferous iron ores could be success- 
 fully smelted, such deposits would undoubtedly be in 
 many cases of great value. 
 
 In other cases sulphide ores are developed in gaboro 
 rocks in a similar manner. This is especially true of the 
 sulphide of iron called pyrrhotite, which is often nickel- 
 bearing and hence of great value as a source of this useful 
 metal. In some places these deposits are accompanied 
 by valuable amounts of copper in the form of chalcopyrite, 
 copper-iron-pyrites, and it has been remarked that as the 
 percentage of copper rises that of nickel falls. Such 
 deposits in gabbros, or in rocks derived from them, are 
 known and have been worked in Norway and Sweden, in 
 Lancaster County, Penn., and at Sudbury, Ontario. 
 
 The origin of this kind of ore deposit in an igneous rock 
 has been described on page 170. 
 
 Use of Gabbro. The gabbros are well suited for con- 
 structional work and architecture, but as a rule have not 
 been extensively used, probably very largely on account 
 of their dark color. In Sweden they have received con- 
 siderable attention for monumental and other uses. In 
 the United States they have been used for building in the 
 Lake Superior region, as at Duluth, and quarries of them ajt 
 
DESCRIPTION OF IGNEOUS ROCKS 235 
 
 Keeseville in the Adirondacks and in Vergennes, Vermont, 
 have been worked. They take a high polish, are suf- 
 ficiently durable and much easier to work than granite. 
 
 DOLERITE. 
 
 Definition and Minerals. The dolerites, as already 
 explained in the section on classification, comprise those 
 forms of diorite and gabbro in which, generally on ac- 
 count of increasing fineness of grain, the hornblende 
 and pyroxene cannot be safely determined or distin- 
 guished from one another, although the eye or lens 
 clearly sees that the rock is composed of feldspar min- 
 gled with an equal or greater amount of a ferromagnesian 
 mineral. 
 
 This term as here used comprises not only the finer-grained 
 diorites and gabbros but much also of what is termed " diabase " 
 by the petrographers, as well as occasional rare rocks which need no 
 mention here. 
 
 The feldspar, which is seen in larger or smaller grains 
 and sometimes in more or less extended lath-shaped 
 sections, is known from microscopic studies to be chiefly 
 a soda-lime variety, though alkalic ones are also present to 
 some extent and in some cases may replace the former; 
 these distinctions cannot be made megascopically, and the 
 plagioclase twinning can very rarely be seen on a cleavage 
 face with the lens. The ferromagnesian minerals are in 
 dark grains, perhaps short columnar; their cleavage sur- 
 faces can usually be seen but it cannot be said whether 
 they are hornblende or pyroxene or a mixture of both. 
 Sometimes olivine is also present and if its yellow-green 
 grains can be detected it is very probable that pyroxene 
 is the chief ferromagnesian mineral and not hornblende. 
 In addition the lens will often show bronzy-looking 
 flakes of biotite and metallic steel-like specks of iron 
 oxide (magnetite or ilmenite) or sometimes brass-like 
 crystals or grains of pyrite. 
 
236 ROCKS AND ROCK MINERALS 
 
 Color. Owing to the equal or predominant amount of 
 ferromagnesian minerals, the color of these rocks is dark, 
 medium or dark gray or greenish to black. As in most 
 rocks the tone of color is best observed in viewing the 
 rock at a little distance, so that the individual grains 
 become indistinguishable and only their mass effect is 
 seen. 
 
 General Properties. The texture of these rocks is granu- 
 lar to fine granular, they are sometimes porphyritic but 
 these cases are described in the following section on the 
 porphyries. Their chemical composition, in the great 
 majority of cases, is similar to that of the diorites and 
 gabbros already given and need not be repeated. They 
 are heavy, the specific gravity being from 3.0-3.3. Their 
 jointing is usually small cuboidal, wedge shaped or platy, 
 often columnar and sometimes on a very large scale, 
 though generally this structure is not so perfect as in the 
 finer-grained basalts. It is most apt to occur in dikes, 
 very thick intrusive sheets and in massive extrusive flows. 
 
 Occurrence. The dolerites do not occur in large stocks 
 and bathyliths like the diorites and gabbros, though not 
 infrequently these latter rocks pass into an endomorphic 
 phase of dolerite at the margin of the intrusion. As in- 
 trusives they belong in the minor class, being found in 
 dikes, small laccoliths and intrusive sheets, the latter often 
 of great thickness, and in thick massive lava flows whose 
 cooling has been slow. 
 
 In the eastern United States the most conspicuous 
 examples are found in the intrusions and flows of " trap " 
 of the Triassic formations stretching from Nova Scotia to 
 Georgia. Through faulting and erosion they now give 
 rise to definite topographic features, such as the ridges in 
 Connecticut and the Palisades opposite New York City. 
 Similar masses of these rocks are found in the Lake Superior 
 region and in the great lava flows of the western United 
 States. In all these occurrences they are associated with, 
 and pass into, the denser forms of basalt. These larger 
 
DESCRIPTION OF IGNEOUS ROCKS 237 
 
 occurrences of dolerite mostly contain pyroxene as the 
 dominant ferromagnesian mineral and are largely the 
 rock called " diabase " by petrographers, while cases where 
 hornblende is dominant are mostly confined to dikes and 
 smaller intrusions, especially in the older rocks. 
 
 Dolerites are also very common rocks in Great Britain 
 in various localities, in dikes and intrusive sheets, and 
 especially in the north of Scotland and Ireland where they 
 are often extrusive and associated with denser basalts. 
 They are in fact very common rocks in all parts of the 
 world. 
 
 Relation to Other Rocks Alteration. From what has 
 been said it is easy to see that the dolerites are a class 
 of rocks based largely on convenience. On the one hand 
 they form a transition group, based on texture, between the 
 diorites and gabbros and the dense basalts, and on the other 
 they cannot depend wholly on texture, because relatively 
 coarse-grained rocks may occur in which one cannot dis- 
 tinguish between hornblende and pyroxene and which 
 must therefore be placed in this class. 
 
 The case might occur in which, instead of hornblende or pyroxene, 
 biotite was the dominant mineral associated with the feldspar. 
 Such rocks are not very common but sometimes occur, especially in 
 dikes and sheets and with quite fine grain. They form the rocks 
 called mica trap or minette, mentioned later under basalt. 
 
 The pyroxenic members of this group, by regional 
 metamorphism, become converted into hornblende rocks, 
 generally into hornblende schists, and both varieties by 
 alteration may produce chlorite and pass into the so-called 
 " greenstones." These alterations are quite similar to 
 what has been described under gabbro. By weathering 
 they become brownish and discolored and ultimately 
 yield brown ferrugineous soils. 
 
 Uses. The rocks of this group are too dark and somber 
 for general use in fine architectural or interior work, 
 except for monumental purposes. The " trap " of the 
 
238 ROCKS AND ROCK MINERALS 
 
 eastern states has been considerably employed in rough 
 masonry, and where good natural joint faces can be used 
 for wall surfaces, the brown weathering color gives pleas- 
 ing effects. The toughness of the material, which the 
 traps afford, has however caused it to be considerably 
 used for block paving and the crushed stone for road 
 making. 
 
 PERIDOTITE PYROXENITE. 
 
 Composition. Under this group are comprised all of 
 those granular igneous rocks composed of ferromagnesian 
 minerals alone, or in which the amount of detectible feld- 
 spar is so small as to be entirely negligible as a component, 
 and in which the mineral grains are sufficiently large to be 
 determined. The chief minerals which form these rocks 
 are olivine, pyroxene of both the augite and hypersthene 
 varieties, and hornblende. These may occur alone or in 
 various mixtures, and according to these the group has 
 been sub-divided into types, some of the more prominent 
 of which are as follows: 
 
 Pyroxenes and Olivine Peridotite. 
 
 Hornblende and Olivine Cortlandtite. 
 
 Olivine alone Dunite. 
 
 Pyroxenes alone Pyroxenite. 
 
 Hornblende alone Hornblendite. 
 
 The first three, which contain olivine, are comprised 
 under the general name of peridotites, from peridot, the 
 French word for olivine. But all the different types, 
 while they sometimes occur independently, also occur 
 together, with transition forms grading into one another, 
 and it is difficult, and sometimes impossible, to distinguish 
 them megascopically and therefore they are best treated 
 together as one general group and not as separate rocks. 
 
 Beside the minerals mentioned, a brown biotite some- 
 times occurs in these rocks, giving rise to the variety called 
 mica peridotite. Additional accessory minerals, some of 
 which are common and some confined to certain occur- 
 
DESCRIPTION OF IGNEOUS ROCKS 
 
 239 
 
 rences, are titanic iron ore, spinels, of which chromite is of 
 importance, and garnet. 
 
 Texture. The texture is granitoid or granular; its 
 appearance depends somewhat on the minerals present 
 and their arrangement. When pyroxene or hornblende 
 is the dominant mineral the grain is often very coarse and 
 may exhibit large cleavage surfaces. Dunite is not apt 
 to be coarse grained; it commonly has a sugar-granular 
 texture like many aplites, sandstones, marbles, etc. 
 Porphyritic texture is rare or wanting. A common 
 texture is one in which the cleavage surfaces of the 
 pyroxenes or hornblendes are seen to be spotted with 
 grains of olivine included in the larger crystal. Such a 
 spotting of the shining cleavage surfaces of one mineral 
 by smaller included crystals of another, which have no 
 crystal orientation, either with respect to one another or to 
 their host, is called luster mottling and is known as the 
 poikilitic texture. It is sometimes well exhibited in these 
 rocks. The included crystals are of course older than 
 their host. 
 
 Chemical Composition. This varies according to the 
 minerals in the rocks but general characters are the very 
 low silica, the small amount or virtual absence of alkalies 
 and alumina, and the large quantities of iron and magnesia. 
 
 ANALYSES OF PERIDOTITES, ETC. 
 
 
 Si0 2 
 
 A1 2 3 
 
 Fe 2 3 
 
 FeO 
 
 MgO 
 
 CaO 
 
 Na 2 
 
 K 2 
 
 H 2 O 
 
 XyO 
 
 Total. 
 
 I 
 II 
 III 
 
 IV 
 
 v 
 
 40.1 
 43.9 
 53.2 
 46.4 
 38.4 
 
 7.8 
 1.6 
 1.9 
 10.8 
 0.3 
 
 7.3 
 8.9 
 1.4 
 5.9 
 3.4 
 
 8.6 
 2.6 
 7.9 
 5.6 
 6.7 
 
 23.7 
 27.3 
 20.8 
 22.2 
 45.2 
 
 6.5 
 6.3 
 13.1 
 3.7 
 4 
 
 1.2 
 0.5 
 0.1 
 0.3 
 0.1 
 
 0.5 
 
 b.i 
 
 1.2 
 
 4.0 
 8.7 
 1.0 
 3.8 
 4.3 
 
 0.6 
 0.3 
 0.5 
 
 1.4 
 
 100.3 
 100.1 
 100.0 
 100.1 
 100.2 
 
 
 
 
 
 
 
 
 
 
 
 
 
 I, Peridotite, Devonshire, England; II, Peridotite, Baltimore 
 County, Maryland; III, Pyroxenite, Oakwood, Maryland; IV, Horn- 
 blendite, Valbonne, Pyrenees; V, Dunite, Tulameen River, British 
 Columbia. XyO = small quantities of other oxides. 
 
240 ROCKS AND ROCK MINERALS 
 
 Color. The color of these rocks ordinarily varies from 
 dull green to black. The dunites, which are practically 
 composed of the one mineral olivine, are at times much 
 lighter. They may show various shades of light green, 
 medium yellow and light brown, passing into one another, 
 and from these through dull yellowish green into dark 
 green. They may thus be exceptions to the general rule 
 that ferromagnesian rocks are dark colored. 
 
 Occurrence. Relation to Gabbro. The peridotites 
 and allied rocks sometimes occur independently as dikes, 
 sheets, laccoliths or small intrusive stocks. In this way, 
 as small isolated occurrences they have been found cutting 
 the Paleozoic rocks, usually in a more or less altered con- 
 dition, at Syracuse and other localities in New York State, 
 in Kentucky, in Arkansas and elsewhere. But generally 
 speaking they are most liable to occur in connection with 
 greater intrusions of gabbros. Sometimes they form phases 
 of the gabbro mass, with transitions between the two; some- 
 times they cut the gabbros in dikes or are found in small 
 intrusions in their neighborhood. This dependence upon 
 the gabbros has led to their being held in such cases as 
 products of differentiation of the gabbro magma in which 
 they represent the lamprophyres of other rock groups. 
 In this way a great number of occurrences are known 
 in all parts of the world where gabbros are common 
 rocks. 
 
 Dunites occur in masses intrusive in the gneisses of 
 western North and South Carolina and Georgia. Asso- 
 ciated with them are smaller amounts of other peridotites 
 and pyroxenite. These occurrences are of importance on 
 account of the deposits of corundum of commercial value 
 associated with them. The mineral is thought to have 
 formed in them in the same manner as described under 
 syenite. Dunite also occurs in considerable masses in New 
 Zealand, especially in the Dun Mountains, from which came 
 the name. Pyroxenite and hornblendite are compara- 
 tively rare and of relatively small geologic importance. 
 
DESCRIPTION OF IGNEOUS ROCKS 241 
 
 Alteration. Serpentine. The peridot! tes are extremely 
 liable to alteration, so much so that unchanged occurrences 
 are not at all usual. The most common form of alteration 
 is that in which the olivine and other magnesian silicates 
 are changed to serpentine. 
 
 This is illustrated by the following reactions: 
 
 Olivine Enstatite Water Serpentine 
 
 Mg 2 SiO 4 + MgSiO 3 + 2 H 2 O = H 4 Mg 3 Si 2 O 9 
 
 2 Mg 2 SiO 4 + CO 2 +2 H 2 O = H 4 Mg 3 Si 2 O 9 + MgCO 3 
 
 3 MgSi0 3 + 2 H 2 = H 4 Mg 3 Si 2 9 + SiO 2 . 
 
 Other magnesian minerals such as talc are also formed 
 by the alteration of these rocks, but that to serpentine 
 is the most important. All stages of transition to pure 
 serpentine occur, and studies which have been made in 
 recent years show that a large part, perhaps the greater 
 part, of the occurrences of this mineral are to be assigned 
 to the alteration of rocks of this group. 
 
 The peridotites ultimately weather down into brown 
 ferrugineous soils which, on account of their lack of potash, 
 do not favor vegetable growth and are therefore barren. 
 
 As a Source of Valuable Minerals. The magmas which 
 form the peridotites usually carry small amounts of 
 chromic oxide which often crystallizes with iron oxide to 
 form the mineral chromite, FeO 2 04, one of the spinel 
 group. It is often seen in dunite and usually forms small, 
 black, pitchy-looking grains. Sometimes this mineral is 
 concentrated in sufficient amount so that it becomes a 
 useful ore, supplying the chromium used in the arts. 
 
 The olivine of these rocks has been found by analysis to 
 contain a minute amount of nickel oxide; when they 
 change to serpentine it sometimes happens that this 
 nickel is concentrated in the form of nickel silicate, some- 
 times in amounts sufficient to form deposits of value as a 
 source of this metal, as in Douglas County, Oregon, and 
 in the Island of New Caledonia. 
 
 The peridotites, and to some extent their allies the 
 
242 ROCKS AND ROCK MINERALS 
 
 gabbros, are also the source of platinum, which occurs in 
 them as the native metal or as sperrylite PtAs 2 ; by the 
 decay of the rock it is washed down and, like gold, concen- 
 trated in alluvial deposits. The precious garnet, pyrope, 
 used as a gem, also comes from a decayed and serpentinized 
 peridotite from Bohemia, South Africa, etc. Lastly, the 
 diamonds of South Africa have their source in decayed 
 and greatly altered peridotite rocks. This altered rock, 
 which was originally a mica peridotite, is known as kim- 
 berlite, by the miners as " blue ground." Some have held 
 that the carbon forming the diamonds was derived from 
 the shales through which the magma passed, others hold 
 that it was original in the magma and that the diamond 
 is a true crystalline constituent of the igneous rock like 
 any other of its accessory minerals. 
 
 PORPHYEIES. 
 
 Definition. As explained in the former section treating 
 of the classification of porphyries, these rocks may be 
 divided into two main groups ; one in which, on account of 
 its coarse texture, not only the phenocrysts but the grains 
 of the groundmass can be determined or the determinable 
 phenocrysts form so large a proportion of the rock that a 
 good idea of its mineral composition can be obtained and 
 the small amount of dense groundmass may be neglected, 
 and a second group in which the amount of dense ground- 
 mass is large and the phenocrysts are not abundant 
 enough to determine safely the mineral character of the 
 rock. It is the first of these two groups which is described 
 in this section, the one which we may call the group of 
 determinable porphyries; the second group will be con- 
 sidered later in connection with the dense igneous rocks 
 the felsites and basalts of which they form a por- 
 phyritic variety. 
 
 In this first group, porphyries are mainly confined to the 
 feldspathic division of the igneous rocks, apparently for 
 the reason that the magmas which furnish the f error 
 
PLATE 19. 
 
 A. GRANITE-PORPHYRY, MONTANA. 
 
 B. SYENITE-PORPHYRY, 
 MONTANA. 
 
 C. SYENITE-PORPHYRY, 
 NORWAY. 
 
DESCRIPTION OF IGNEOUS ROCKS 243 
 
 magnesian rocks have relatively so low a freezing point 
 and crystallize so readily that they are not apt to form 
 porphyries under conditions where the feldspathic rocks 
 often do so readily. Thus granite porphyry is very com- 
 mon, while gabbro and peridotite porphyries are so rare 
 as to be of no practical importance. In the group of 
 dense igneous rocks porphyries of both divisions are com- 
 mon. The rocks to be treated then are granite porphyry, 
 syenite porphyry and diorite or dolerite porphyry. There 
 are so many points in which they are similar that they 
 are best treated as a group. 
 
 Granite Porphyry. This consists of distinct pheno- 
 crysts of quartz and of feldspar in a granular groundmass 
 of the same minerals whose grains can be determined as 
 such, or one in which the abundance of the phenocrysts of 
 quartz and feldspar give a distinct granite-like character 
 to the rock and make the dense groundmass of less impor- 
 tance. Sometimes the rock consists of these minerals 
 alone, or very nearly so, and sometimes biotite and horn- 
 blende are present , perhaps in considerable amount. The 
 biotite and hornblende may be present separately or 
 together, though hornblende alone is rare. They may 
 occur as distinct phenocrysts, usually smaller than the 
 quartz and especially the feldspar, and also in the ground- 
 mass, in which case the tiny specks of biotite are most 
 easily detected. 
 
 When the groundmass is so coarse as to be equivalent to an 
 ordinary granite it is customary to speak of the rock as porphyritic 
 granite, as explained under granite. 
 
 Syenite Porphyry. This rock consists of distinct pheno- 
 crysts of feldspar in a groundmass, which, if determinable, 
 must be made up mainly of grains of feldspar and with 
 very little or no quartz. If the groundmass is not deter- 
 minable the amount of phenocrysts must be large enough 
 to give the rock a distinctly syenitic character. The 
 ferromagnesian minerals, biotite, hornblende and pyroxene, 
 
244 ROCKS AND ROCK MINERALS 
 
 while they may be absent or practically so, are usually 
 present, either as phenocrysts, or in the groundmass, or 
 both. They may occur in considerable amount, but must 
 not equal or exceed the total amount of feldspar, or the 
 rock becomes a diorite porphyry. They occur separately 
 and together, but the combination of all three or of 
 biotite and pyroxene is not so common as biotite and 
 hornblende. 
 
 The rock defined above is that which corresponds to the common 
 one of the three varieties of syenite described on page 219 and 
 following, and represents it in porphyritic development. Anortho- 
 site porphyry is unknown. Nephelite syenite porphyry is known 
 but is a very rare rock. 
 
 In more exact classification based on microscopic research a 
 distinction is made as to whether the feldspars are chiefly alkalic 
 or mainly soda-lime feldspars, both phenocrysts and groundmass 
 being considered together. In the latter case petrographers term 
 the rock diorite porphyry and only apply the term of syenite por- 
 phyry where they are mainly alkalic. So far as the groundmass is 
 concerned this distinction cannot be made by megascopic exam- 
 ination and but rarely, as described later, with the phenocrysts. 
 Hence, just as in the case of syenite, both kinds are classed here 
 together. 
 
 Diorite and Dolerite Porphyry. Diorite porphyry would 
 be composed of phenocrysts of hornblende and feldspar, 
 either separately or together, in a determinable ground- 
 mass of the same minerals, or if the groundmass is not 
 determinable the diorite character must be clearly shown 
 by the great abundance of the hornblende and feldspar 
 phenocrysts. Also the total amount of hornblende must 
 equal or exceed that of the feldspar. Some biotite may 
 also be present, as well as iron ore grains. 
 
 Such rocks occur and it may be possible at times to 
 determine them megascopically, but in the great majority 
 of instances it will be found that, while the hornblende 
 which is present in phenocrysts may be recognized, that 
 which is present in the groundmass cannot. It can often 
 be seen in these cases that the groundmass is composed of 
 
DESCRIPTION OF IGNEOUS ROCKS 245 
 
 feldspar and a ferromagnesian mineral, either hornblende 
 or pyroxene, but megascopically it is impossible to say 
 which. In fact such groundmasses correspond to the 
 definition and description of dolerite previously given and 
 such rocks therefore are most conveniently called dolerite 
 porphyry. The phenocrysts are either feldspar, horn- 
 blende, or pyroxene, or mixtures of them. The feldspar in 
 these rocks is generally a variety of the soda-lime group, 
 usually labradorite. By increase in the amount and 
 density of the groundmass they pass insensibly into the 
 basalt porphyries, or melaphyres, described later. 
 
 Phenocrysts of Porphyries. As the phenocrysts of 
 porphyries have crystallized freely in the fluid magmas 
 they generally show distinct crystal shapes, such as are 
 described in the foregoing part devoted to the rock 
 minerals. A few words in regard to their crystal habits 
 may be added here. Quartz, as a phenocryst, tends to 
 take the form shown in Fig. 43, but is usually spherical; 
 the crystals may be a half inch in diameter but are usually 
 much smaller, the size of coarse shot or peas; it is usually 
 smoky in color. The feldspars tend to assume the forms 
 shown by Figs. 57; they are often twins, Fig. 8; they 
 are white, pink to red, or yellowish and gray; if feldspars 
 of two colors are present and one of them is a reddish 
 tone it is probably orthoclase, the other albite or a soda- 
 lime feldspar. They not infrequently form very large 
 phenocrysts, an inch or even more in length; the model- 
 like feldspars seen in mineral cabinets often are the pheno- 
 crysts obtained from porphyries. Hornblende occurs in 
 dark greenish or black prisms, usually elongated, and some- 
 times quite slender and with glittering cleavage surfaces 
 if fresh; the terminal faces are poor or wanting; some- 
 times it is weathered out and only a rusty mass left in 
 its place. Pyroxene is also dark green to black, in short, 
 stout prisms, and commonly its cleavage and crystal faces 1 
 lack the luster of hornblende. The method of distil^ 
 guishing them has been already explained. Rusty spots 
 
246 ROCKS AND ROCK MINERALS 
 
 also show the former presence of pyroxene, but less com- 
 monly than hornblende. In size both are apt to be small, 
 compared with feldspar. Biotite as a phenocryst, is in six- 
 sided tablets with fine basal cleavage, black to bronze- 
 brown in color. In these rocks its crystals are apt to be 
 small. 
 
 General Properties. The chemical composition of these 
 porphyries is similar to that of the corresponding kinds of 
 granular rocks previously given and need not be repeated. 
 Their specific gravity and modes of alteration and conver- 
 sion into soil are the same. The jointing depends largely 
 on the mode of occurrence ; it is apt to be platy or small 
 cuboidal, or to form small parallelopipedons with acute 
 angles, in the feldspathic porphyries of dikes and sheets, 
 and larger blocks in the greater intrusions; the doleritic 
 porphyries tend to columnar jointing. 
 
 Occurrence. The porphyries of this class are commonly 
 found in the minor intrusions; in dikes, intrusive sheets 
 and laccoliths, sometimes in volcanic necks. They are 
 also not uncommon as marginal phases of intrusive stocks 
 and bathyliths of granite, syenite, etc.; they here represent 
 an endomorphic contact modification and in traversing 
 areas of such rocks, if it is observed that they are becom- 
 ing porphyries with finer grain, approach to the contact 
 should be suspected. They may also occur in extrusive 
 lava flows, especially if these are very thick and massive, 
 but in this mode of occurrence they are generally replaced 
 by the denser felsite and basalt porphyries described 
 beyond. 
 
 These rocks are far too common to give any list of 
 localities; they are everywhere found where erosion has 
 exposed the older crystalline rocks and where igneous 
 activity has displayed itself. Where larger stocks and 
 intrusions have occurred they are especially apt to be 
 present, sometimes cutting them as dikes, sometimes 
 extending from them in apophyses, and sometimes in 
 dikes, sheets, etc., as satellites grouped about them. 
 
DESCRIPTION OF IGNEOUS ROCKS 247 
 
 Perhaps the most notable instances of the occurrence 
 of these rocks are to be found in the great laccoliths of the 
 Rocky Mountains' region, in Colorado, Utah, Wyoming 
 and Montana, which are generally composed of granite or 
 syenite porphyries. Some of these masses are a mile in 
 thickness by several in breadth, though often smaller. In 
 these regions they often form powerful intruded sheets, 
 several hundred feet in thickness. It is in the contact 
 zones of these intrusions, especially with limestone, that 
 a large proportion of the valuable ore deposits, such as the 
 silver-lead ones, which have made these regions famous 
 for their mining industries, are found. Thus to the 
 western miner the word " porphyry " is always of sugges- 
 tive significance. 
 
 Dense Igneous Rocks. 
 
 In the preceding groups of igneous rocks it is assumed 
 that all the component grains of the rock, or those forming 
 the greater part of it, can be determined and the mineral 
 constitution safely established. In the present group it 
 is assumed that the texture of all of the rock, or of the 
 greater part of it, is so dense that this cannot be done. 
 No definite line can be drawn between the two groups; in 
 many cases, whether a given rock should belong to the one 
 or the other, is largely a matter of opinion, dependent 
 upon the experience of the observer, his knowledge of 
 rocks and minerals, his power of observation, keenness of 
 eyesight, and the excellence of his lens. In this respect 
 we are also limited by our size ; if we were ants, instead of 
 men, who were studying rocks, it is probable that few 
 would be placed in this group. 
 
 As has already been explained, under the section treating 
 of the classification of igneous rocks, these dense varieties 
 are divided into two groups, on the basis of color, into the 
 dark to black basalts and the lighter colored felsites; each 
 of these has a porphyritic subdivision. Of these the 
 felsite will be considered first. 
 
248 ROCKS AND ROCK MINERALS 
 
 FELSITE AND FELSITE PORPHYRY. 
 
 The felsites include all those dense igneous rocks which 
 are of stony texture and not evidently glassy, of all colors 
 except dark gray, dark green or black, these latter belong- 
 ing to basalts. They normally and commonly show light 
 shades of color; white, which is not very common, light to 
 medium gray, light pink or red to dark red, pale yellow or 
 brown, purple or light green. With the lens it can be fre- 
 quently seen that they consist of minute mineral grains, too 
 small for determination, and the texture is then very fine 
 granular. In other cases the grains may be entirely too 
 fine to be seen; the rock has then a dense, horn-like or 
 flinty aspect, appearing like a homogeneous substance. 
 In this latter case it is very apt to have a smooth con- 
 choidal fracture. In other cases, especially in surface 
 lavas, the texture is more or less porous and the fracture 
 surface of the rock rough and hackly, with a harsh feeling. 
 A pronounced cellular or vesicular structure, common in 
 basalts and in glassy rocks, and illustrated on Plate 8 is not 
 very common in this group. The surface lavas not infre- 
 quently show fluidal bandings and streakings, more or less 
 flat lenticular, and often curved or curled, due to flowage, 
 and often clearly brought out on weathered surfaces, as 
 illustrated on Plate 22. 
 
 The division of the dense igneous rocks into felsites and basalts 
 is based on color, and not on mineral composition, since the latter 
 cannot be determined. Nevertheless the felsites as classed above 
 are, in general, feldspathic rocks, and they represent in dense form 
 and often as lavas those magmas, which under different geologic and 
 physical conditions, would have produced granites and syenites, 
 while the basalts correspond to diorites, gabbros and dolerites, as 
 already explained under classification. 
 
 In many cases, where these rocks are of medium gray or drab 
 color, it is difficult to know whether to assign them to the felsites or 
 basalts. This happens especially when they are very dense. In 
 this case, if the rock be sharply examined with a good lens, it may be 
 seen that many tiny chips and flakes, only partly formed and yet in 
 the main a part of the mass, lie upon its surface, their thin edges 
 
DESCRIPTION OF IGNEOUS ROCKS 249 
 
 separated from it by a flat underlying crack. It will be observed 
 that their thin edges are very much lighter in color than that of the 
 rock upon which they lie and are translucent to light; indeed in 
 many cases they will appear practically white, even when the rock 
 is a dark gray or stone color. This is a peculiarity of those dense 
 rocks which are chiefly composed of feldspar and are, therefore, to 
 be classed as felsites; it is not shown at all, or only in a very small 
 degree, by the basalts. The reason for this is that, although they 
 may be colored by a pigment, the feldspars are transparent to 
 translucent minerals and a rock composed mostly of them is also 
 megascopically translucent on thin edges, while one composed 
 mostly of ferromagnesian mineral particles is not, since these minerals 
 are either opaque, or practically so, from the megascopic standpoint. 
 The same effect may be observed on the very thin edge of a flat chip 
 broken from the rock. In case the rock is not so dense, but that the 
 individual grains can be seen with the lens, if these are transparent 
 or translucent with light colors, it may be assumed in general that 
 they are mainly feldspar, the ferromagnesian mineral grains being 
 mostly dark, dull, to opaque. Olivine, with its yellow-green color, 
 however, is an exception and must not be confused with feldspar. 
 It will also be noted that under a good lens the mineral grains, or 
 many of them, appear much lighter in color than does the rock in its 
 general effect to the eye. 
 
 These tests, while they cannot be held to be exact, will often prove 
 of service in helping to decide, in doubtful cases, whether a rock 
 belongs to the felsites or basalts, for the division between them, 
 while based primarily on color, is due also to a mineralogical dif- 
 ference as well. 
 
 It is assumed in what has been said regarding these rocks that 
 one is dealing with reasonably fresh, unaltered material, not those 
 rocks which have been long exposed to atmospheric agencies and 
 are weathered into dull ferrugineous material, or green masses of 
 chlorite. 
 
 Varieties of Felsite. From the megascopic standpoint 
 the different varieties of felsite which can be recognized 
 are those which are due to color and texture alone; the 
 petrographer by the use of the microscope on thin sections 
 is, however, able to determine the different kinds of miner- 
 als, which form the minute grains, and to divide and classify 
 these rocks on a mineralogical basis, just as was done with 
 the coarse-grained rocks, whose grains could be seen and 
 determined by the eye. 
 
250 
 
 ROCKS AND ROCK MINERALS 
 
 This is done on the consideration of whether the rock contains 
 quartz or not, whether the predominant feldspar is alkalic or a soda- 
 lime variety, or if it contains a feldspathoid, such as nephelite, in 
 addition to the feldspar. According to this we have the following 
 kinds recognized by petrographers. 
 
 
 
 Equivalent Coarse- 
 
 Equivalent Coarse 
 
 Chief Component Minerals. 
 
 Rock 
 Name. 
 
 Grained Rock in 
 Petrographic 
 
 Rock in Field 
 Class, of this 
 
 
 
 Class. 
 
 Book. 
 
 Alkalic feldspars and quartz. 
 
 Rhyolite 
 
 Granite . . . 
 
 Granite. 
 
 Lime-soda feldspars and 
 
 Dacite 
 
 Quartz diorite . 
 
 Granite. 
 
 quartz. 
 
 
 
 
 Alkalic feldspars, little or 
 
 Trachyte 
 
 Syenite . . . 
 
 Syenite, mostly. 
 
 no quartz. 
 
 
 
 
 Soda-lime feldspars, little 
 
 Andesite 
 
 Diorite .... 
 
 Syenite and Dio- 
 
 or no quartz. 
 
 
 
 rite. 
 
 Alkalic feldspars and neph- 
 
 Phonolite 
 
 Nephelite Syenite 
 
 Nephelite Syenite. 
 
 elite. 
 
 
 
 
 A variety containing lime-soda feldspars and a feldspathoid is 
 known, but is very rare. The ferromagnesian minerals are present 
 in variable amounts and, according to the dominant one of these, we 
 have such terms as mica-trachyte, hornblende-dacite, augite-andesite, 
 etc., etc. 
 
 These are terms which are constantly seen in geological literature 
 and in general it can be understood that the rock so designated has 
 been subjected to microscopic study. They distinguish varieties of 
 the felsites which cannot be accurately made without such study, 
 but on the other hand the following points will serve in a rather 
 vague and general way to indicate megascopically, to which of the 
 above divisions a given felsite probably belongs. If the rock con- 
 tains phenocrysts of free quartz, as mentioned later in the descrip- 
 tion of the porphyritic varieties of felsite, it almost certainly contains 
 a considerable amount of quartz in the dense groundmass and is 
 either a rhyolite or dacite; of these rhyolite is more common than 
 dacite. If the rock on being tested yields gelatinous silica, according 
 to the method recommended on page 115, it is almost certainly a 
 phonolite. The distinction between rhyolite and dacite, and between 
 trachyte and andesite, cannot be made megascopically, since it 
 depends on the determination of the kind of feldspar, and in dense 
 rocks this can only be done by optical means. 
 
DESCRIPTION OF IGNEOUS ROCKS 251 
 
 Felsite Porphyry or Leucophyre. While felsites occur 
 which contain no porphyritical crystals it is much more 
 common for them to contain phenocrysts. These may 
 be very few, scattered and isolated, or they may be abun- 
 dant. They may be quite evenly scattered, or they may 
 be collected in groups. By increasing abundance, when 
 they form half the bulk of the rock or more, they cause 
 transitions into the group of determinable granular por- 
 phyries previously described. The phenocrysts may be 
 salic, in which case they are quartz or feldspar, or they may 
 be ferromagnesian, biotite, hornblende or pyroxene. For 
 the appearance and properties of these phenocrysts 
 reference may be had to the description of them in the 
 former group of porphyries, page 245. 
 
 In the association of these phenocrysts, while all of the 
 above minerals occur at times alone, there are apt to be 
 two or more present. Quartz and feldspar with the 
 others is not uncommon, if a dark mineral is associated 
 with the quartz it is most apt to be biotite ; hornblende is 
 less common and pyroxene very rare. Feldspar and horn- 
 blende, pyroxene and feldspar are very common. In 
 general feldspar is the most common phenocryst. 
 
 In naming these rocks the general term of felsite por- 
 phyry may be given to them or this may be contracted to 
 leucophyre, from the Greek (ACVKO'S white) meaning 
 light-colored porphyry.* If no mineral qualifier is used 
 with this it is understood that the phenocrysts are of 
 feldspar, since this is most general. If they are of quartz, 
 biotite, hornblende, etc., we have as follows: 
 
 Quartz- felsite-porphyry or quartz-leucophyre. 
 
 Hornblende- felsite-porphyry or hornblende-leucophyre. 
 
 Biotite- felsite-porphyry or biotite-leucophyre. 
 
 Augite- felsite-porphyry or augite-leucophyre. 
 
 Hornblende feldspar-leucophyre. 
 
 Quartz- fe Idspar-leucophyre . 
 
 * As suggested in the " Quantitative Classification of Igneous 
 Rocks." Messrs. .Cross, Iddings, Pirsson and Washington, p. 184. 
 
252 ROCKS AND ROCK MINERALS 
 
 These examples are sufficient to show how mineral 
 qualifiers may be used in designating the phenocrysts of 
 these rocks, and how these may be combined, if desired, to 
 give expression to considerable descriptive detail. Other 
 examples will readily suggest themselves. 
 
 The same distinctions are made by petrographers among these 
 rocks, as in the felsites proper, and as described above. Thus we 
 find such terms in use as " rhyolite-porphyiy/' " mica-andesite- 
 porphyry," " augite-trachyte-porphyry," etc., resulting from the 
 study of thin sections and accurate determination of the different 
 kinds of minerals. 
 
 General Properties. The chemical composition of felsites 
 is variable, depending, like the subvarieties enumerated 
 above, on the prevailing minerals. Some correspond with 
 the analyses of granite already given; these contain free 
 quartz; others are like the analyses of syenites and such 
 must contain little or no quartz. If the amount of lime 
 is small, the silica below 60 per cent, and the alkalies high 
 the rock must be mostly composed of alkalic feldspar and 
 probably nephelite is present. The specific gravity ranges 
 from 2.4-2.65 and is usually lower than that of granite 
 and syenite. The jointing is platy, or in small blocks; 
 columnar structure also occurs, but is not so common nor 
 so perfect as in basalts. In normal weathering to soil 
 these rocks become discolored, brownish, reddish, etc. 
 The ferromagnesian mineral generally disappears, leaving 
 a rusty spot or cavity; the rock crumbles into debris, at 
 first largely through mechanical disintegration. Finally 
 the feldspars change to clay, as described under granite, 
 and the change to soil is complete. Where hydrothermal 
 action takes place, as in the vicinity of ore veins, they are 
 often changed to soft clay-like masses consisting some- 
 times of clays, and sometimes of sericite, the fine scaly 
 form of muscovite. 
 
 Occurrence. Felsites occur intrusively as dikes and 
 sheets, and sometimes as the endomorphic contact facies 
 of larger intrusive masses, whose main character is that of 
 granite, granite-porphyry, syenite, etc. They indicate 
 
DESCRIPTION OF IGNEOUS ROCKS 253 
 
 intrusions of magma into cold rocks, and when found in 
 intrusions, these are usually relatively small, or narrow. 
 
 They are much more common extrusively, as lava flows 
 and sheets, and they sometimes cover very large areas, 
 many hundreds and even thousands of square miles in 
 extent. In these cases, and especially in volcanic cones 
 or their eroded remnants, they are usually interbedded 
 with tuffs and breccias. 
 
 While it would be impossible to give any adequate list 
 of actual occurrences it may be mentioned that felsites as 
 intrusives occur extensively in eastern North America, 
 among the older rocks in the Atlantic border states, along 
 the eastern front of the Appalachian uplift; as extrusives 
 they occur in Maine, in the White Mountains, in Penn- 
 sylvania and to the southward. They occur also in 
 Wisconsin. Much more extensive in western America is 
 their effusive occurrence throughout the whole of the 
 Cordilleran tract, where they play an important role in the 
 upbuilding of many of the ranges, and sometimes occupy 
 large areas. Here all the different varieties are found, as 
 for instance rhyolite in the Yellowstone Park, in Colorado 
 and elsewhere; andesite in the lofty volcanoes of the 
 Coast Ranges, and in those in South America, and in the 
 eroded ones of the main chain of the Rocky Mountains; 
 the rare variety phonolite is found at Cripple Creek, 
 Colorado, and in the Black Hills of South Dakota. 
 
 The felsites are just as extensively found in other parts 
 of the world. In Europe they are well distributed as 
 effusive lavas. So in Great Britain they occur in northern 
 Wales, in the Lake district of England, and in northern 
 Ireland. These are the varieties called rhyolite and 
 andesite. They are found in western Germany, in France, 
 Italy, and in Hungary. Wherever volcanic outbreaks 
 have taken place extensively, felsite lavas occur. 
 
 Felsites and felsite porphyries are often found, in the form of 
 narrow dikes and sheets, traversing larger stocks or intrusions of 
 coarse-grained rocks, such as granite, syenite, etc., or the rocks 
 
254 ROCKS AND ROCK MINERALS 
 
 in their immediate vicinity. They are generally complementary in 
 this case to dark basaltic dikes, mentioned later as lamp rophy res, 
 and have received special names from petrographers. Thus we 
 have bostonite (allied to trachyte), tinguaite (allied to phonolite) and 
 many others. Note in this connection paragraphs on complemen- 
 tary rocks, on aplite, and on lamprophyres. 
 
 BASALT AND BASALT-PORPHYRY. 
 
 The basalts include those dense igneous rocks, of very 
 dark color, whose fabric is so fine that the constituent 
 grains either cannot be perceived by the eye or with the 
 lens, or, if seen, are too small to be recognized, and which 
 are of stony but not of glassy texture. The color varies 
 from grayish black or dark stone color, greenish or purplish 
 black, to pure black. In the great majority of cases they 
 do not show translucency on the edges of thin flakes as 
 described under felsite. When not cellular, and very 
 dense, they have a uniform dull, soft, almost velvety 
 appearance, and do not show the horny, flinty, or greasy 
 luster of many dense felsites. 
 
 The study of thin sections of these rocks shows that in general 
 they are composed of minute crystal grains of soda-lime feldspar 
 generally lab rado rite, pyroxene and iron ore, very often with 
 more or less olivine, and sometimes biotite or hornblende. In some 
 cases nepheJite or leucite may accompany the feldspar, or replace it, 
 giving rise to varieties which have received special names. These 
 varieties, although very interesting from the standpoint of theoretical 
 petrography, are comparatively rare and relatively of small impor- 
 tance in a general geological way. 
 
 Being composed of the same minerals, these rocks represent, in 
 dense form and generally as surface lavas, those magmas which, 
 under different physical conditions, would have solidified as gabbros, 
 peridotes, dolerites and (in part) diorites. A large part of the 
 dolerites in fact are transition rocks between them and gabbros, as 
 previously mentioned in the description of that rock, and instances 
 may be found in the same rock mass where the gradation into 
 dolerite may be seen. 
 
 Basalt Porphyry. While porphyritic varieties of basalt 
 are not uncommon rocks it may be said in general, that this 
 type of texture plays a far less important role in this* 
 
PLATE 20. 
 
 A. Labradorite-Porphyry. 
 
 B. Augitophyre. 
 VARIETIES OF BASALT-PORPHYRY. 
 
* - * '> <* 
 
 v ; : , . 
 
 / . f '*,' 
 
DESCRIPTION OF IGNEOUS ROCKS 255 
 
 group than in the felsites previously described, owing 
 probably to the low freezing point and easy crystallization 
 of the magma. One exception to this, however, is in the 
 presence of olivine, which is very apt to occur scattered 
 through the basalt in transparent yellowish or bottle- 
 green porphyritic grains, averaging about the size of 
 moderately coarse shot. The mineral is so common 
 (indeed at one time a rock was not considered a basalt 
 unless it contained olivine) and produces in general so 
 little of a striking porphyritic effect, that it is rather the 
 custom to ignore it in this respect, and term such rocks 
 oli vine-basalt rather than olivine-basalt-porphyry. 
 
 The chief minerals as phenocrysts, when such occur, are feldspar 
 and pyroxene; hornblende and mica are much less common. The 
 feldspar is commonly labradorite; it occurs in elongate tabular forms, 
 either singly, or in twinned groups. The pyroxene is the variety 
 augite ; it is black, sometimes shining, sometimes dull, and is in short 
 thick prisms or prismoids, as illustrated under pyroxene. The 
 hornblende is also black and has its usual shining and good cleavage. 
 Biotite is in six-sided tablets. 
 
 These rocks would be named in accordance with the 
 prevailing phenocryst, so for example augite-basalt- 
 porphyry. Instead of the term basalt-porphyry the name 
 melaphyre, meaning " black porphyry," may be used as 
 more convenient * and we should then have feldspar- 
 melaphyre, augite-melaphyre, biotite-melaphyre, etc. 
 
 General Properties of Basalt. The chemical composition 
 of basalt varies with its mineral composition; in general 
 it is of the same nature as that of gabbro previously given, 
 as may be seen from the following analysis of one from 
 California, which will serve as an example. 
 
 SiO 2 A1 2 O 3 Fe 2 O 3 FeO MgO CaO Na 2 O K 2 O H 2 O XyO Total 
 51.9 15.3 3.1 3.6 8.7 7.4 3.3 2.5 2.5 1.7 = 100.0 
 
 The specific gravity is high, about 3.0 (2.9-3.1). The 
 jointing is platy or columnar; the best examples indeed of 
 
 * Quantitative Classification of Igneous Rocks, p. 185 . 
 
256 ROCKS AND ROCK MINERALS 
 
 this structure are found in basalt and many notable 
 examples of it are found in all parts of the world, the 
 Giants' Causeway on the north coast of Ireland being one 
 of the best known. This structure is seen in Plate 11. 
 Sometimes basalt on weathering develops a singular "pil- 
 low" structure by which there is formed spheroidal masses. 
 
 Varieties. In the dense non-porphyritic basalts there 
 is little opportunity for variation, save that which is based 
 on a change from the compact into the porous or cellular 
 structure. This last is particularly common in surface 
 lavas, especially in their upper portion, and has been 
 illustrated on Plate 8. It is particularly in these basalts 
 that the amygdaloidal structure occurs, also illustrated on 
 the same plate. The minerals filling the cavities in 
 basalt are commonly quartz, calcite or zeolites; among 
 the latter minerals analcite, natrolite, stilbite and heu- 
 landite may be particularly mentioned. Such rocks are 
 termed amygdaloidal basalt. In a number of places, and 
 particularly in western America, basalts have been found 
 as surface lavas which contain visible grains of quartz. 
 One of the most noted of these is the basalt flow from the 
 Cinder Cone, near Lassens Peak in northern California, 
 which is filled with angular pieces of quartz of varying 
 sizes. As many of these correspond in composition to 
 gabbros and dolerites, the presence of the quartz in them 
 appears anomalous, since magmas so low in silica, as may 
 be seen by referring to the analyses of gabbro, would not 
 be expected to develop free quartz on crystallizing. Some 
 petrographers therefore think that these are fragments of 
 quartz rock in the depths, which have been torn loose and 
 olistributed through the magma, while others regard them 
 as a primary crystallization, produced under exceptional 
 conditions of pressure and mineralizers. These rocks 
 have been called quartz-basalts. 
 
 The porphyritic varieties have been described above, 
 but it may be mentioned that a variety containing distinct 
 and sometimes large crystals of labradorite feldspar has' 
 
DESCRIPTION OF IGNEOUS ROCKS 257 
 
 been called labradorite-porphyry. The greenish-black por- 
 phyry from Greece, employed by the ancients (porfido 
 verde antico), is a somewhat altered example of this type. 
 A variety containing rather large and distinct crystals of 
 augite has been termed augitophyre. 
 
 The name trap has been used in a general way as a field 
 term to designate the rocks called here basalts, and also 
 dolerites. As thus employed it would mean any dark- 
 colored, heavy, igneous rock of undetermined mineral 
 composition. Thus the dolerites and basalts of the 
 Newark formation along the Atlantic coast have been 
 termed " Triassic traps;" the great effusives of western 
 India are known as the " Deccan traps." 
 
 Lamprophyres. The ferromagnesian complementary 
 rocks, occurring in dikes and sheets in or around stocks 
 of granite, syenite, etc., and often called " trap " dikes, 
 etc., belong for the most part under this heading of 
 basalt. Their origin and relations have been discussed 
 in the preceding part of this work and they have been 
 mentioned again under granite. They are very apt to 
 contain phenocrysts of the ferromagnesian minerals, 
 olivine, augite, hornblende and biotite, either separately 
 or together, and sometimes these phenocrysts are of very 
 large size. These are embedded in a groundmass that is 
 usually dense and basaltic. According to the variations 
 in the minerals, as shown by the microscope, a large 
 number of different types have been named by petrogra- 
 phers, distinctions which ordinarily cannot be made mega- 
 scopically. For field work they may be treated simply as 
 basalt-porphyries as described above, and termed augite 
 melaphyre, biotite melaphyre, etc. It may be mentioned 
 that biotite melaphyre is a rock which is frequently found 
 in dikes with granite, and has been called mica trap or 
 minette. A hornblende melaphyre occurs in the same way 
 with many syenites and nephelite syenites; it has quite 
 a wide distribution in New England and has been termed 
 camptonite. 
 
258 ROCKS AND ROCK MINERALS 
 
 Olivine Nodules. It frequently happens that basalts, in 
 addition to the ordinary crystals of olivine, contain yellow- 
 ish, or green lumps, or nodules, made up of grains of this 
 mineral. Grains of other minerals, such as pyroxene, 
 spinel, etc., may be present in them. These lumps may 
 vary in size from a pea to masses as large as one's fist, 
 or even larger. They are generally rounded, but often 
 distinctly angular in shape. Their origin is somewhat 
 problematical; some hold that they are merely agglomera- 
 tions of the earlier formed crystals in the liquid magma, 
 while others regard them as fragments of rock (dunite) 
 torn off below and brought up in it. 
 
 Exotic Minerals. Basalts sometimes contain unusual 
 minerals, which do not appear in the ordinary rock, and 
 whose origin in them must be ascribed to unusual condi- 
 tions, or composition of the magma. The quartz basalt 
 mentioned above is one of these. Another case is seen in 
 the iron-bearing basalts of Greenland, which contain 
 small to large masses of native iron, which is much like 
 the iron found in meteorites. By the use of a solution of 
 copper sulphate specks of native iron have been found in 
 basalts from other places. The Greenland basalts also 
 contain graphite. Corundum, in the form of sapphire, has 
 also been found in basalts, and a dike in Montana has 
 furnished a quantity of valuable gems. In this connection 
 also, may be mentioned the occurrence in places of native 
 copper, especially in the Lake Superior district, where the 
 metal occurs in dolerites and basalts and in connection 
 with them, in quantities which have made it one of the 
 most important sources of the world's copper supply. 
 
 Weathering and Alteration. In many volcanic regions, 
 where basalts have been subjected to exhalations of steam 
 or to heated water, the minerals containing ferrous oxide, 
 such as magnetite and olivine, become reddened through 
 change to ferric oxide. Sometimes the olivines alone are 
 reddened; in other cases the whole rock becomes deep red 
 to reddish brown. Such rocks may be difficult to dis- 
 
DESCRIPTION OF IGNEOUS ROCKS 259 
 
 tinguish in the field from red felsites, and may even have 
 to be classed with them. Sometimes, however, the asso- 
 ciation with other rocks, the retained form of pheno- 
 crysts, and the good amygdaloidal structure, rarely seen 
 in the most common felsites, may help one to recognize 
 the original character of the rock. 
 
 The normal weathering of basalt gives rise to chlorite, 
 serpentine, and carbonates, with clay and iron ores; the 
 rock often turns green and becomes soft when much 
 chlorite is developed. In other cases it turns brown 
 through oxidation and eventually falls away into brownish 
 ferrugineous soil, to which various names are given, as 
 laterite in India, wacke in Germany, etc. Sometimes from 
 such deposits all but the hydroxides of iron and alumina 
 are leached, forming one variety of the so-called beauxiie. 
 
 Under processes of metamorphism the basalts act like 
 the gabbros and dolerites previously described, and give 
 rise to " greenstone " and to greenstone schists and 
 amphibolite. 
 
 Occurrence of Basalt. As intrusive rocks, sheets, and 
 especially dikes, of basalt of various types, both plain and 
 porphyritic in texture, are so common in all regions where 
 igneous rocks occur that they need no further mention. 
 As extrusive lavas, in the form of flows and extended 
 sheets, they are of much greater geological interest and 
 importance. There is scarcely any volcanic region in the 
 world which does not exhibit them in greater or lesser 
 amount, and in some regions, as in the lava fields of the 
 Columbia in western America, and in western India, they 
 have been poured out in stupendous masses, so that tracts 
 of country nearly 200,000 square miles in extent have 
 been covered thousands of feet deep. A similar great 
 field existed in northern Great Britain, and its remnants, 
 portions yet saved from the eroding edge of the Atlantic, 
 form in great part the northern British Isles. 
 
 Leucite Rocks. Basaltic rocks in which the feldspathoid minerals, 
 nephelite or leucite, are present, either accompanying the feldspar 
 
260 ROCKS AND ROCK MINERALS 
 
 or replacing it, while not common, have in certain regions a consider- 
 able local development. Ordinarily these minerals are in the 
 groundmass, and only to be detected by the microscope, and such 
 rocks in the field must be classed as regular basalts. In central 
 Italy, however, the leucite rocks have a great development, and in 
 many cases the leucite crystals appear as phenocrysts as large 
 as peas, or larger at times, and are easily recognized. They are 
 leucite-basalt-porphyries or leucite-melaphyre. For the properties 
 of leucite its description under rock minerals should be con- 
 sulted. According to the other minerals present, several different 
 types of these rocks are distinguished and named. Some of them 
 are so light colored they would be classed as varieties of felsites. 
 Outside of Italy these leucite rocks are very rare, occurrences being 
 known in the Rhine district, in central Montana, western Wyoming 
 and a few other localities, but since the well-known lavas of Vesuvius 
 are composed of them, they are mentioned here. 
 
 Glassy Rocks. 
 
 In the felsites and basalts the use of the microscope on 
 thin sections would show in many cases that a certain 
 amount of glass, uncrystallized and solidified magma, is 
 present in them, acting as a cement to hold the mineral 
 grains together. This cannot be detected megascopic ally, 
 and under the term of glassy rocks, as here used, is meant 
 only such as are entirely of glass, or if partly crystalline, 
 those containing it in such amounts and in such circum- 
 stances, that it is visible and evident to the eye. 
 
 The conditions which will cause a magma to solidify as 
 a glass are evidently those which are unfavorable to crys- 
 tallization, extremely quick cooling in the first place, and 
 probably to some extent the rapid loss of mineralizers in 
 the second. This has been already discussed in connection 
 with the texture of igneous rocks. These conditions are 
 best realized when the magmas are poured out on the 
 surface as effusive lavas, and just as we associate a 
 coarse-textured, entirely crystalline, granular rock, such 
 as granite, with an intrusive or deeply seated origin, so 
 conversely we associate glassy rocks with an extrusive 
 one. Indeed while it is true that dikes may sometimes 
 
DESCRIPTION OF IGNEOUS ROCKS 261 
 
 show glassy selvages along the contact, when they have 
 been intruded into cold rocks, or may indeed be wholly 
 of glass when the exposure is near the original surface, as 
 in recently denuded dikes in volcanic regions, this is so 
 uncommon and so inconsiderable an affair, that in general 
 we may regard the fact, that a rock is composed partly or 
 wholly of evident glass, as a proof of its extrusive origin, 
 that it was originally a surface lava, although it may have 
 been buried under later formations. 
 
 Any of the different magmas, varying as to composition, 
 may form glassy rocks if chilled with sufficient rapidity, 
 but petrographical research has shown that, while glassy 
 forms of the felsite lavas are common, those corresponding 
 to basalt are much rarer and relatively of inconsiderable 
 volume. The reason for this appears to be that the mag- 
 mas, which furnish felsites, or the granite and syenite which 
 correspond to them, have a relatively high freezing point, 
 and as the magma cools down and approaches this, it 
 becomes so enormously viscous that the free movement 
 of molecules necessary for crystallization is prevented. 
 This is due to the large amount of silica that such magmas 
 contain, which has a strong effect in promoting viscosity. 
 The presence of water in the magma tends to neutralize 
 this, and to make the magma more fluid and thus to help 
 crystallization, but when it is poured out on the surface 
 the water is rapidly lost with increase in viscosity. On 
 the other hand the basaltic magmas, or those corresponding 
 to gabbro or diorite in part, which contain relatively 
 low silica and high iron and magnesia, have a much lower 
 freezing point and remain liquid as they approach it, 
 thus permitting easy crystallization and the assumption 
 of stony texture and appearance. Consequently those 
 glasses, which have the highest percentage of silica and 
 correspond to granite in composition, are the most common 
 ones. 
 
 Classification of Glassy Bocks. As already stated in 
 the classification of igneous rocks, we may divide the 
 
262 ROCKS AND ROCK MINERALS 
 
 glassy rocks into two groups, one containing distinct 
 crystals or phenocrysts embedded in a glass base, or 
 porphyritic varieties in short, and second, those without 
 distinct phenocrysts, consisting of either pure glass, or 
 glass more or less filled with spherulites or lithophysae, as 
 described later. The second group is again subdivided 
 according to luster and structure. In accordance with 
 this we have as follows: 
 
 {Obsidian, strong bright vitreous luster. 
 Pitchstone, dull pitchy or resinous luster. 
 Perlite, apparently made of small spheroids . 
 Pumice, cellular structure, glass froth. 
 Glass more or less r 
 
 filled with pheno- J Vitrophyre, glass porphyry, 
 crysts L 
 
 Obsidian. This is pure, solid, natural glass, devoid of 
 all apparent crystal grains, or nearly so. It has a bright 
 luster like that of artificial glass. It usually has a jet- 
 black color, but when the edges of thin chips are examined 
 against the light it is generally seen to be transparent or 
 translucent with a more or less smoky color, and it can be 
 often observed with a lens that the coloring matter is 
 more or less collected into fine parallel streaks, bands, or 
 threads, as if drawn out in the flowage. Less commonly 
 the glass is gray, or Indian red, or rich brown, and this is 
 sometimes mixed with the black in bands and strings, 
 which kneaded through it produce a marbled effect. The 
 microscope shows the black glass as colorless and filled 
 with tiny, black, dust-like particles; they are probably 
 specks of magnetite, which represent the beginnings of 
 crystallization, and diffused through the glass, they act as 
 a pigment, coloring it black. In other cases they have 
 been oxidized to hematite dust and the color is then red 
 or brown. 
 
 Obsidian has a remarkable conchoidal fracture, illus- 
 trated in Figure 4, page 29, due to its homogeneity and 
 lack of structure. It was this quality that made the 
 
DESCRIPTION OF IGNEOUS ROCKS 
 
 263 
 
 substance so highly valued by primitive peoples, for it 
 enabled them by chipping to work it into desired forms, 
 knives, spearheads and other implements and weapons, 
 while long, slender flakes possessed, for cutting pur- 
 poses, knife-edges of razor-like keenness. The ancient 
 Mexicans were especially skilful in working it, and were 
 able to spring off blades of bayonet-like cross-section, 
 half an inch in breadth by six inches or more in length. 
 
 While obsidian corresponding to the various kinds of 
 igneous rocks is known, it usually has a composition 
 similar to that of granite, as may be seen from the analysis 
 of a typical specimen from the Yellowstone Park. 
 
 Si0 2 
 
 A1 2 3 
 
 Fe 2 3 
 
 FeO 
 
 MgO 
 
 CaO 
 
 Na 2 O 
 
 K 2 
 
 H 2 
 
 FeS 2 
 
 
 74.7 
 
 13.7 
 
 1.0 
 
 0.6 
 
 0.1 
 
 0.8 
 
 3.9 
 
 4.0 
 
 0.6 
 
 0.4 
 
 =99.8 
 
 It can be readily shown by calculation that had this 
 magma crystallized, it would have produced a rock con- 
 sisting of 35 per cent of quartz, 60 per cent of feldspar, 
 with 5 per cent of other minerals, that is to say, a granite. 
 The specific gravity varies from 2.3-2.7, depending on the 
 composition; of the most common variety, 2.3-2 .4. The 
 hardness is greater than that of ordinary window glass, 
 which it scratches. Before the blowpipe a splinter of 
 black obsidian fuses readily, with bubbling, to a vesicular 
 gray or white enamel, which, after the removal of the 
 water, becomes exceedingly infusible. This experiment 
 is very instructive in showing the effect of water in lower- 
 ing the fusing point of magmas and in increasing their 
 liquidity. The water in the obsidian is not the product 
 of alteration, for it is present in what the microscope 
 reveals as the purest and clearest glass, nor are there 
 cavities to contain it; it appears to be chemically a part of 
 the mixture, like Na2O and 
 
264 ROCKS AND ROCK MINERALS 
 
 Spherulites. In many obsidians may be seen rounded, 
 sometimes perfectly spherical, bodies of white, gray or red 
 color, varying in size from those of microscopic dimen- 
 sions up to those of an egg, or even larger; usually from 
 the size of fine shot to peas. If closely examined with a 
 lens, it can generally be seen that they are composed of 
 fibers radiating from a common center; at uniform dis- 
 tances from the center the fibers are apt to change color, 
 or to be saturated with a differently colored material, and 
 the body appears built of successive concentric shells. 
 These bodies are called spherulites and are composed of 
 fibers of feldspar. They are indicative of sudden cooling 
 and a very rapidly induced crystallization, the fibers 
 shooting outward from some center where crystallization 
 commences, and branching as they grow, until checked by 
 the viscosity of the rapidly cooling magma. They should 
 not be confused with phenocrysts which are single, 
 individual crystals. An example is shown on Plate 21. 
 They are sometimes formed by accident in artificial glass, 
 as seen on Plate 21; in this case the artificial mineral 
 forming them is wollastonite, CaSiO 3 . 
 
 Frequently the spherulites form before the lava has 
 come to rest and are thus drawn out, so that they are 
 dotted along the rock in lines. When in great num- 
 bers, and minute, they may coalesce; some streaks of the 
 rock are then composed of them, while other bands are 
 of dark, solid glass, as shown in Plate 22. 
 
 Lithophysae. Closely connected with the spherulites 
 there occur also in glassy rocks peculiar formations known 
 as lithophysae (stone bubbles). These consist of a series 
 of concentric shells of crystalline material, resembling some- 
 what nested watchglasses, which surround a central cavity, 
 and are more or less separated from each other. They 
 consist of adherent crystals, and are very fragile. When 
 exposed by the breaking of the rock they appear much 
 like flowers with concentric layers of petals. They vary 
 in size from very small to several inches in diameter. The. 
 
PLATE 21. 
 
 A. Spherulites in Obsidian. B. Spherulites in Glass. 
 
 C. Lithophysae. B. Lithophysae. 
 
 STRUCTURES IN GLASSY ROCKS. 
 
DESCRIPTION OF IGNEOUS ROCKS 265 
 
 walls of the cavities are coated with minute but beautiful 
 crystals of quartz, tridymite and feldspar, and sometimes 
 fayalite, topaz, garnet and tourmaline are found in them. 
 Sometimes they are more or less flattened and strung 
 along the flowage planes of the rock. They occur, not 
 only in ths-pure glassy lavas, but also in those which by 
 more or less crystallization have assumed megascopically 
 a stony texture and appearance. They are illustrated on 
 Plate 21. Their origin is ascribed to repeated shells of 
 crystallization, with consequent liberation of water vapor, 
 and expansions of the cavities through its influence under 
 high temperature. The formation of topaz and other 
 minerals points to the presence of fluorine and other 
 accompanying gases. Thus the lithophysae seem to 
 bear a certain analogy to miarolitic cavities in the intrusive 
 rocks, as described elsewhere. 
 
 Pitchstone. This may be regarded as merely a variety 
 of obsidian in which the luster, instead of being bright and 
 glassy, is duller and the rock appears resinous or pitchlike. 
 There is also a chemical difference in that, while the 
 water contained in obsidian is rarely so much as one 
 per cent and may sink to mere traces, pitchstone contains 
 much more, as much as 5 or 6 per cent or even greater. 
 It is this which probably influences the luster. They are 
 also variable in color, black, gray, red, brown, and green, 
 and are translucent to transparent on thin edges. 
 
 Perlite. This is a peculiar variety of glassy rock which 
 is composed of small spheroids, usually varying in size 
 from small shot to peas. It is generally of a gray to blue- 
 gray color, rarely red, has a soft, pearly, or wax-like luster 
 and resembles enamel. The spheroids either lie separated 
 in a sort of cement and are then round, or they may be 
 closely compressed and are then polygonal. They tend 
 to have a concentric, shelly structure and are the result of 
 a contraction phenomenon in the cooling glass, which pro- 
 duces a spherical, spiral cracking, as shown in thin sections. 
 Analyses of perlites prove them to have a rather constant 
 
266 ROCKS AND ROCK MINERALS 
 
 percentage of combined water, between 3 and 4 per cent, 
 and there may be a connection between this amount of 
 water and the peculiar method of cracking. To the casual 
 observer they somewhat resemble oolites and pisolites of 
 the concretionary sedimentary rocks. Perlite is produced 
 only by felsitic magmas, especially by those high in silica; 
 it does not occur in basaltic glasses. 
 
 Pumice, Scoria, etc. Pumice is highly vesicular glass 
 produced by the extravasation of the water vapor at high 
 temperature, through relief of pressure, as the magma comes 
 to the surface. It is best described as glass froth. Its 
 color is white, gray, yellowish, or brownish, rarely red. It 
 sometimes has a somewhat silky luster. Examined with 
 the lens it is seen to be composed of a mass of silky glass 
 fibers of a cottony appearance, full of pores, and separated 
 by larger holes like a sponge. If drawn out by flowage 
 the fibers are parallel, otherwise they are interwound. 
 The chemical composition of typical pumice is like that 
 of the highly siliceous obsidian, or in other words like 
 that of granite. Pumice does not form independent rock- 
 masses, it occurs as the upper crust of flows of felsite lava, 
 or in fragments among the explosive material ejected by 
 volcanoes. On account of its light, porous nature, and its 
 content of sealed glass cells, it floats almost indefinitely 
 on water, and the material ejected by volcanoes near or 
 in the sea is borne by currents all over the world, and 
 drifts ashore everywhere. Its use as an abrasive and 
 polishing agent, and for toilet purposes, is due to the sharp 
 cutting edges of the thin films and fibers of glass; nearly 
 all that is used comes from the Lipari Islands off the coast 
 of Sicily. Other places of occurrence are mentioned 
 later. 
 
 Scoria. While all magmas, whatever their chemical 
 composition, at times and under proper conditions, form 
 pumiceous rocks, typical pumice, as stated above, is most 
 characteristic of the felsitic ones, while basaltic pumices 
 are of local development and of inconsiderable impor- 
 
PLATE 22. 
 
 A. FLOW STRUCTURE IN GLASSY LAVA. 
 
 B- SLAGGY STRUCTURE OF BASALTIC LAVA 
 
DESCRIPTION OF IGNEOUS ROCKS 267 
 
 tance. Nevertheless the basaltic magmas develop through 
 the expansion of gases vesicular forms, as described under 
 basalt. These pass, especially on the upper surface of 
 basalt flows, and in the material thrown out by volcanoes, 
 into mo*e^or less glassy, partly stony, dark or reddish, 
 loosely compacted, spongy, cindery or slag-like modifica- 
 tions known as volcanic scoria. This form is illustrated 
 on Plate 23. 
 
 A peculiar modification of what may be considered basaltic 
 pumice occurs in the crater of Kilauea in Hawaii, where drops of 
 lava flying up from the boiling lava lakes pull out thin, hair-like 
 threads of glass after them. These threads, drifted by the wind, 
 collect in tow-like masses, called by the natives " Pele's hair " after 
 the titulary goddess of the islands. 
 
 Vitrophyre. Either pitchstone or obsidian may contain 
 embedded crystals or phenocrysts which can be recognized. 
 As in felsite porphyries the amount may vary widely from 
 cases where they are rare and widely scattered to those in 
 which the rock is thickly strewn with them. Such por- 
 phyries, consisting of a glass base and phenocrysts, are 
 called vitrophyre. Perlite porphyries are known, but are 
 rare. The glassy base of vitrophyre has the properties 
 of the obsidian or pitchstone previously described; it 
 often contains spherulites in addition to the phenocrysts. 
 Of the latter feldspar is the most common; it is very apt to 
 be limpid with a glassy habit; the cleavage distinguishes 
 it from quartz, which may also occur, sometimes alone and 
 sometimes with the feldspar. If phenocrysts of a ferro- 
 magnesian mineral are present it is usually biotite, less 
 commonly hornblende, while pyroxene, though known, is 
 rare. The varieties are usually named according to the 
 prevailing phenocryst without regard to the character of 
 the groundmass; so we have quartz-vitrophyre, feldspar- 
 vitrophyre, quartz-biotite-vitrophyre, etc. The phenocrysts 
 are generally rather small. 
 
 The chemical composition of the vitrophyres is similar 
 on the one hand to the felsites and on the other to the pure 
 
268 
 
 ROCKS AND ROCK MINERALS 
 
 glasses; they represent an intermediate stage of develop- 
 ment, as may be seen from the following table, which shows 
 the relations of all these varieties of extrusive rocks or 
 lavas to one another. 
 
 Conditions under which Magma Cooled 
 and Solidified. 
 
 No Formation of 
 Crystals in the 
 Depths Before 
 Extrusion, no 
 Phenocrysts. 
 
 Crystals Formed 
 in Depth Before 
 Extrusion and 
 Brought up by 
 Magma; Pheno- 
 crysts. 
 
 No crystallization of magma on 
 surface, on account of rapid 
 cooling. Glassy texture. 
 
 Pitchstone and 
 Obsidian 
 
 Vitrophyre. 
 
 Crystallization of magma on sur- 
 face; slower cooling. 
 Stony texture. 
 
 Felsite 
 
 Felsite-por- 
 phyry. 
 
 Tachylite. As previously stated, basaltic magmas crystallize 
 easily and rarely form glass, or only in relatively small volume. 
 Basaltic glass is, however, seen occasionally as a thin marginal f acies 
 or selvage in dikes, on lava flows, or among the products of basalt- 
 yielding volcanoes, like those in Hawaii. It is known by the name 
 of tachylite. 
 
 Occurrence of Glassy Bocks. The glasses are found in 
 those regions which are, or have been in the past, scenes of 
 volcanic activity. While obsidian and pitchstone occur 
 as independent flows and masses near volcanic vents, the 
 glassy rocks in general form only the upper surface of 
 lava sheets, which become crystalline as they are penetrated 
 downward; they are also found, especially in pumiceous 
 forms, in the fragmental material ejected by volcanoes. 
 To attempt to name all the different occurrences would be 
 impracticable, but it may be mentioned that obsidian in 
 large masses is found in the Yellowstone Park and is 
 known for its beautiful spherulites and lithophysae; at 
 Mono Lake in California; Glass Butte, Oregon; White 
 
PLATE 23. 
 
 A. VOLCANIC BOMB. 
 
 B. SCORIA. 
 
 C. PUMICE. 
 
* 
 
DESCRIPTION OF IGNEOUS ROCKS 269 
 
 Mountains, Utah; Tewan Mountains, New Mexico, and 
 various other places in the United States; in Mexico, 
 Iceland, Lipari Islands, Italy, Hungary, New Zealand, 
 Transcaucasia, etc. Pitchstone occurs in Colorado near 
 Georgetown and at Silver Cliff; well known localities are 
 on the Island of Arran off the west coast of Scotland, in 
 Ireland, and at Meissen and Tharandt near Dresden, 
 Germany. Perlites and pumice are also found in the 
 Yellowstone Park; in Hungary, Italy, Iceland, Japan, etc. 
 Basaltic glasses occur on the west coast of Scotland, in 
 Iceland and especially in the Hawaiian Islands. 
 
 Alteration of Glassy Rocks. It has been found by 
 microscopic and field study that ancient lavas, in a variety 
 of places, were once glassy, though not so at present. It 
 appears that when the natural glasses are exposed to the 
 various agencies which tend to alter rocks, such as pressure, 
 heat, action of water, etc., they undergo a slow change, 
 the glass is converted into an intimate mixture of exces- 
 sively fine particles of quartz and feldspar, and loses 
 entirely its vitreous character. It then assumes the stony 
 texture and becomes a dense felsite. This change is 
 called devitrification. While the former glassy condition 
 of many felsites cannot be proved, even microscopically, 
 it may often be suspected in them, from the presence of 
 chains of spherulites, flow structures and lithophysae, 
 which may be seen megascopically, and give strong hints 
 of their former character. Ancient altered lavas of this 
 kind have been described from the coast of Maine; from 
 South Mountain, Pennsylvania; from Wisconsin; from 
 Sweden and other places. In Sweden they have been 
 called hdlleflinta, though this name is also used to 
 designate somewhat similar rocks of a different origin. 
 
 Fragmental Volcanic Rocks. 
 
 Origin. The fragmental igneous rocks represent the ma- 
 terial thrown out by volcanoes during periods of activity. 
 The explosive action is due to vapors, chiefly that of 
 
270 ROCKS AND ROCK MINERALS 
 
 water, which is contained under pressure in the magma, 
 and as the latter rises to the surface and the pressure is 
 relieved, departs with violence. While the major part is 
 passing off in great volumes, which rush upward and carry 
 the solid or liquid materials to great heights, a minor part 
 is also expanding in the liquid, converting it into cellular 
 vesicular forms. Consequently the solid particles as they 
 fall are commonly found to be of spongy consistency, but 
 mixed with them are often seen compact pieces of lava 
 and other rocks, parts of the solid lava crust formed by 
 cooling after a previous eruption, mingled with fragments 
 torn from the rock walls of the conduit. As the lava con- 
 tinues rising, the greater volume of the gases may pass off, 
 the explosive activity ceases, and the projection of material 
 may be succeeded by quiet outflows of liquid rock. Hence 
 it is very common to find the beds of fragmental material 
 interspersed with layers of compact lava, felsite or basalt. 
 In this connection it should be also mentioned that the 
 chemical composition of the magma plays a considerable 
 part in explosive activity. Those magmas which corre- 
 spond to felsite, and are high in silica are, as has been men- 
 tioned, very viscous at temperatures where those low in 
 silica, such as the basalt magmas, which are rich in iron, 
 magnesia and lime, are still relatively very liquid. From 
 the former the vapors, on account of their thick viscous 
 condition, escape with difficulty, and with explosive 
 violence; from the latter they pass off readily and easily 
 without explosive activity. While there are many ex- 
 ceptions to this, it may be accepted as a general rule, and 
 we therefore find that vents yielding felsite lavas generally 
 build high and steep cones, composed chiefly of frag- 
 mental materials, while basaltic ones are built up largely 
 of liquid outflows and are therefore low and broad. Many 
 volcanoes, like Vesuvius, are of intermediate character in 
 which explosion and projection of material is succeeded 
 by flows of lava, and the cone is consequently of com- 
 posite character. 
 
DESCRIPTION OF IGNEOUS ROCKS 271 
 
 Classification. The particles of magma driven into the 
 atmosphere and solidified and the pieces of rock are of all 
 dimensions from the finest dust, which may float for years, 
 to huge masses weighing several hundred pounds. By 
 general usage, for the sake of convenience, the following 
 sizes are roughly distinguished: pieces the size of an apple 
 or larger are called bombs; those the size of a nut are 
 termed lapilli; those the size of small peas or shot, ashes; 
 the finest is known as volcanic dust. Sometimes the 
 bombs, lapilli, etc., are sharply angular and sometimes 
 smoothly rounded off a form caused by the grinding 
 and attrition of the pieces upon one another in the upward 
 rush from the volcanic throat. They should be dis- 
 tinguished from bombs which have been afterwards 
 rounded by the action of running water. The larger 
 bombs sometimes present a sub-angular appearance, are 
 porous and their surface is penetrated by cracks as shown 
 in Plate 23. Such have been called bread-crust bombs. 
 The ashes, and lapilli which usually make up the greater 
 part of the material are frequently spoken of as volcanic 
 cinders and cones composed of them are called cinder cones. 
 
 In a sense, of course, this loose material may be estimated 
 as a rock formation; so far as the individual pieces are 
 concerned they are to be considered merely as fragments 
 of the various kinds of rocks treated in the foregoing 
 pages, to be named and described as there set forth. 
 
 But in process of time great accumulations of such 
 material may be spread over wide tracts of country, 
 covering up existing rock formations. The heavier and 
 coarser particles fall first, then the finer, giving a grada- 
 tion from top to bottom and, as successive outbursts occur, 
 there is produced in this way a rough bedding. By its 
 own weight as it accumulates, aided by the action of 
 percolating water which may carry and deposit substances 
 in solution, it gradually becomes compacted into a more 
 or less firm mass, having a certain individuality as a kind 
 of rock and deserving of special treatment. When the 
 
272 ROCKS AND ROCK MINERALS 
 
 rock is composed entirely of the finer particles, dust and 
 ash, it is called volcanic tuff; when this is mixed with the 
 coarser bombs and lapilli it is termed volcanic conglomer- 
 ate, or better, volcanic breccia, with reference to the broken 
 angular character of the embedded fragments. 
 
 Volcanic Tuff. This is generally a fine-grained rock, 
 light in weight, and often of a chalky consistency, some- 
 times dense, compact, and breaking into small chips. The 
 color is usually light, white, pink, pale-brown, gray or 
 yellow, sometimes passing into darker shades. The more 
 compact varieties may be easily mistaken for f elsite lavas ; 
 it is possible, indeed, in some cases, that they cannot be 
 distinguished from them megascopically, but generally 
 attentive examination with a good lens will reveal angular 
 particles of quartz, feldspar, and often other minerals in 
 them, and possibly small fragments of other rocks. When 
 breathed upon, they usually exhale a strong argillaceous 
 odor, probably owing to partial or complete alteration 
 of feldspathic particles to clay. When not too compact 
 they have a rough feel and yield a gritty dust, when 
 strongly rubbed between the fingers, unlike the smooth- 
 ness of pure clay or chalk, owing to the hard, angular 
 character of the dust particles. Sometimes such tuffs con- 
 tain fossil remains of vegetation, when they have fallen 
 upon land surfaces covered with it, and carbonaceous 
 remains of stems, twigs, or leaf imprints may be found 
 in them. If the material has fallen into water the tuff 
 may be rich in various kinds of fossils, such as marine 
 organisms, of possibly great perfection of form, and for 
 the same reason it may be well stratified. All of these 
 varied characters, including the mode of occurrence and 
 relation to other rocks, must be taken into account in 
 judging the nature of the deposit. 
 
 Volcanic Breccia. This has a base or cement of tuff, 
 more or less completely filled with lapilli of angular 
 shapes, and these are often mingled with larger bombs 
 and masses which are apt to be rounded. Interspersed 
 
PLATE 24. 
 
DESCRIPTION OF IGNEOUS ROCKS 273 
 
 with these are apt to be fragments of other rocks, pieces 
 of the basement through which the conduit has drilled, 
 of limestones, shales, sandstones, and massive crystalline 
 rocks, granite, gneiss, schist, etc. They have therefore a 
 strongly conglomeratic aspect, like the. specimens seen in 
 Plate 32. Even when these rocks have been greatly 
 indurated by contact metamorphism, or other agencies, 
 they still reveal, by differences of color and texture on a 
 freshly broken face, the angular shapes of the fragments and 
 their composite character. When not too indurated they 
 are apt to erode very unevenly; the finer cement being 
 less resistant washes away first, leaving the contained 
 masses projecting, and in this way along the edges of 
 cliffs strange and weirdly shaped figures of erosion are 
 produced, called " hoodoos " in the Rocky Mountains' 
 region. The colors of these breccias is variable, browns, 
 reds and chocolate being common, along with lighter tones, 
 depending partly on the state of oxidation of the iron- 
 bearing compounds they contain, and partly on the nature 
 of the magma, whether felsitic, which tends to lighter 
 colors, or basaltic which produces darker ones. 
 
 Occurrence of Tuffs and Breccias. These rocks are of 
 wide distribution, being found in all regions where volcanic 
 activity has taken place; their presence indeed is the best 
 confirmation in many regions of such activity in the past. 
 In places where vulcanism is still active, or has only 
 recently ceased, they are represented by the still uncom- 
 pacted material, but no definite line can be drawn be- 
 tween the different conditions of consolidation. 
 
 In the eastern United States, tuffs and breccias have 
 been found in several localities in Maine; near Boston; and 
 at South Mountain, Penn. They occur also, to a limited 
 extent, with the Triassic eruptives of the Connecticut 
 Valley. Further research will probably reveal other 
 localities, but they are neither common nor conspicuous 
 rocks, being limited in volume and so greatly changed in 
 character by various agencies that in many places their 
 
274 ROCKS AND ROCK MINERALS 
 
 true character is difficultly recognizable. They -probably 
 had once a much greater extension, but erosion has mostly 
 carried them away, during the vast period of time which 
 has elapsed since volcanic activity was displayed. 
 
 In western Am.erica, however, the case is very different; 
 in the various ranges of the Rocky Mountains; in the 
 Coast and Cascade Ranges, and in fact over most of western 
 North America, these are common rocks and in many 
 places in Colorado, Wyoming and Montana, they occur 
 in immense deposits, forming often an important factor in 
 building up the bulk of the mountain masses. They are. 
 especially well displayed in western Wyoming, in the region 
 of the Yellowstone Park, where the serried peaks of the 
 Absaroka Range are mostly carved out of tuffs and 
 breccias aggregating thousands of feet in depth, thus 
 testifying to the enormous volcanic energies which this 
 region formerly displayed. A section cut into them 
 by erosion, exhibiting their rough bedding, is shown in 
 Plate 24. In this region they are frequently interbedded 
 with flows of lava. 
 
 In Europe tuffs and breccias have a wide extension. 
 They occur in many places in the British Islands, as in the 
 Lake district in northern England, and associated with 
 the volcanic rocks of the old red sandstone and Car- 
 boniferous of Scotland. They are often interbedded with 
 sedimentary rocks and are frequently so changed by 
 metamorphic processes as to be recognized only by 
 careful petrographic research, having been changed into 
 slates, etc. Such altered tuffs form a part of the so-called 
 " halleflintas " in Sweden or the " porphyroids " of 
 Continental geologists. Tuffs and breccias occur in many 
 places in Germany, France, Italy, etc. The mention of 
 these localities is sufficient to show their wide extension 
 and importance. In this connection the reader is referred 
 to what is said of adobe. 
 
CHAPTER VIII. 
 ORIGIN AND CLASSIFICATION OF STRATIFIED ROCKS. 
 
 THE stratified rocks consist of material which has 
 already formed a part of pre-existent ones, and which has 
 been deposited from some fluid by which it has been 
 moved from its former position. The shifted material 
 may have been moved and deposited by the action of 
 water, the atmosphere, or glacial ice. The first case is 
 by far the most prominent and important, especially with 
 respect to the volume of the masses involved, and the 
 frequency of their occurrence, and thus when stratified 
 rocks are mentioned such water-formed rocks are always 
 understood, unless it is otherwise stated. In contra- 
 distinction to them, the material which has been moved 
 and deposited by the action of the atmosphere, forms the 
 class known as Aeolian rocks, one of far less importance. 
 From what has been said, it is clear that the stratified 
 rocks are secondary ones in the respect that their material 
 in some form or other has been derived from already exis- 
 tent ones. An exception to this would be found in beds 
 of coal, which are truly stratified rocks derived from plant 
 life, or in beds of volcanic ashes which have been deposited 
 from the atmosphere, and which have been described by 
 preference under the igneous rocks. But in general the 
 statement, that the material of the stratified rocks is sec- 
 ondary, holds true, and it has been derived from former 
 rocks of all classes igneous, metamorphic and strati- 
 fied and in the case of the earliest sediments from the 
 earth's original crust, if such ever existed. 
 
 The rocks which have been formed in water may be 
 divided into two main groups, according to the manner 
 
 275 
 
276 ROCKS AND ROCK MINERALS 
 
 in which the material has been deposited; they consist 
 either of substances mechanically held in suspension, and 
 then directly dropped, or of that which has been in solu- 
 tion, and through chemical agencies, either of organic life 
 or otherwise, has been rendered insoluble, and has been 
 therefore deposited. The first we may call mechanical, 
 the second chemical sediments. Yet even between these, 
 as we shall see later, it is difficult to draw a definite line. 
 We have then the following classes to deal with : 
 
 Sedimentary rocks ; water-formed j 
 
 Aeolian rocks; wind formed . . mechanical. 
 
 Decay of Bocks ; Formation of Soil. When firm and even 
 dense rocks are exposed to the action of the atmosphere, 
 they gradually decay and are turned into soil. This is 
 brought about by a variety of agencies. All rock masses 
 are penetrated in various directions by cracks and fissures 
 called joints; these are both great and small, and in addi- 
 tion the individual mineral grains contain cleavage and 
 other cracks. Thus water is able to thoroughly permeate 
 the rock masses, and in cold regions where alternate 
 thawing and freezing goes on, the expansion of the water 
 in turning to ice keeps on splitting and crumbling the 
 rocks until on the surface they are reduced to a mass of 
 debris. The expansion and contraction of rocks in hot 
 countries and in arid regions, under great daily and yearly 
 changes of temperature, accomplishes the same thing more 
 slowly. The expansion of the growing roots of trees and 
 plants tends to the same end. By such processes there is a 
 constant tendency for the rock masses to be broken up, 
 mechanically, into smaller and smaller fragments. In the 
 meantime the substances dissolved in the water, such as 
 air, acids from decaying vegetation, and especially carbonic 
 acid gas, are acting chemically upon the rock minerals, con- 
 verting the silicates, oxides, and sulphides into other 
 forms, into carbonates, hydrated silicates, hydroxides, 
 
PLATE 26. 
 
CLASSIFICATION OF STRATIFIED ROCKS 277 
 
 sulphates, etc. Much material goes into solution, is leached 
 out, and by running water is carried into lakes and the 
 ocean, where it concentrates, and where we must again 
 consider it under the formation of the chemically precipi- 
 tated sediments. Some minerals, such as quartz, are not 
 attacked to any appreciable extent, or but very slowly, 
 under ordinary circumstances, and these remain to form 
 the chief part of the rock debris. It is for this reason 
 that silicates, and especially quartz, play the chief min- 
 eral role in the sedimentary rocks formed by mechani- 
 cal processes. This debris of broken, crumbled and 
 altered rock, which constitutes a detritus, has been 
 called by various names, and the finer upper portion 
 in which vegetation grows is the soil. Under this latter 
 name for convenience we may consider all of it. The 
 gradual change from rock below to soil above is illus- 
 trated in Plate 17. 
 
 Movement of Soil. The surface of the land in general 
 is covered by a mantle of soil resting on the rocky crust of 
 the earth. The latter, which is popularly known as the 
 " country rock," here and there in ledges, precipices, and 
 the craggy tops of hills and mountains projects through 
 this covering. By the action of running water, aided by 
 gravity, this crumbled rock and soil mantle, which is appar- 
 ently at rest, is, geologically considered, actually in motion, 
 and is continually being urged downward into the sea, its 
 ultimate goal, Plate 25. On steep slopes it goes more 
 rapidly, in valleys more slowly; in level plains, like water 
 in a lake, it is temporarily impounded. Its rate of motion 
 varies continually from time to time and from place to 
 place. Its movement in mass is of course very slow; 
 when suspended in running water, that of the water which 
 carries it; when resting on the stream bottom it varies 
 according to circumstances. Thus the land waste is being 
 ever carried away and ever renewed by the destruction of 
 the rocks. The greater part is carried into the sea, but a 
 considerable part is deposited in inland lakes and seas, 
 
278 
 
 ROCKS AND ROCK MINERALS 
 
 and on the lower plains and deltas of great rivers, which 
 from time to time are heavily flooded. It is this material 
 which forms the sedimentary rocks of mechanical 
 deposition. 
 
 Gradation of Material. The detritus of the land con- 
 sists of material of very variable sizes, and in northern 
 countries over which glaciers have passed this is particu- 
 larly apt to be the case, as rock masses showing great 
 extremes in dimensions are moved and mingled by them. 
 When such material is moved by running water it becomes 
 sorted and graded, according to the strength of the cur- 
 rent, into masses consisting approximately of equal sized 
 particles. When they are larger than peas the material 
 is called gravel and the individual pieces are termed 
 pebbles; large, loose pieces of rock from the size of a small 
 melon up are spoken of as boulders. Pieces smaller than 
 peas, which form a non-cohering mass when wet with 
 water, are termed sand, while the finest particles which 
 are readily lifted and transported by movements of the 
 atmosphere are known as dust, and these when wet and 
 then dried generally cohere into solid material. All these 
 grade into one another. The following table shows a 
 more accurate division according to size. 
 
 Name of Material. 
 
 Diameter in Millimeters. 
 
 Fine Gravel . . . 
 Coarse Sand . . . 
 Medium Sand . . 
 Fine Sand .... 
 Very fine Sand . . 
 Silt 
 Fine Silt 
 Clav 
 
 2-1 
 1-0.5 
 0.5-0.25 
 0.25-0.1 
 0.1-0.05 
 0.05-0.01 
 0.01-0.005 
 0.005-0.0001 
 
 
 
 Thus, roughly speaking, the material may be classified 
 into, (a) gravel, (6) sand, (c) mud, clay or silt. Since 
 this division is made the basis of classification of the 
 
CLASSIFICATION OF STRATIFIED ROCKS 279 
 
 mechanically formed sedimentary rocks, each of them may 
 be examined somewhat more in detail. 
 
 Gravel. The pebbles which compose a gravel are 
 pieces of individual rocks and like them are generally 
 made up of grains of different kinds of minerals. In 
 some cases they are composed of only one mineral, and of 
 these, quartz is by far the most common. Such quartz 
 pebbles may be fragments derived from quartzite strata, 
 from a quartz vein, or from large quartz crystals from 
 some granite-pegmatite dike. Such coarse granites or 
 pegmatites may furnish pebbles consisting of other single 
 minerals, especially feldspar. 
 
 The form and appearance of pebbles depends on the conditions 
 to which they have been exposed. Those which have suffered 
 considerable transport in the bed of streams, or have been rolled 
 on the shores of lakes and of the sea, are, as is well known, rounded 
 and become ovoid to spherical. They are apt to have a very smooth 
 surface with a characteristic faintly dimpled, slightly dented, or 
 inverted shagreen appearance, caused by their repeated collisions 
 under movement. This is best seen on a pebble of a hard homo- 
 geneous substance, as in one of quartz. If composite in nature 
 they are often pitted by the decay and removal of softer or more 
 easily altered particles. 
 
 The degree of rounding shown by pebbles depends on the distance 
 and length of time they have been transported and on the hardness of 
 the material. Sedimentary rocks, as will be shown, are sometimes 
 composed of pebble-sized fragments, which have suffered very little 
 movement, and which still retain their original rough, angular 
 character. 
 
 Pebbles and boulders which have been transported by glaciers 
 are sometimes seen in sedimentary rocks. These have character- 
 istic sub-angular forms, with faces ground upon them, which are 
 polished and scratched by parallel and crossing grooves or 
 scratches. Pebbles, partly buried in the sand of the seashore 
 and of deserts, are also often subangular and facetted, the faces 
 being ground by the sand drifting past them, but these lack the 
 scratches. 
 
 Pebbles buried in the soil often show fern or moss-like markings 
 or dendrites upon them, or are sometimes covered with a shiny skin 
 of dark color. This comes from a deposit from water, of manganese 
 or iron oxides. 
 
280 ROCKS AND ROCK MINERALS 
 
 Sand. Strictly speaking, sand means particles of a 
 certain size, as mentioned above, and has no reference to 
 their composition: thus we have quartz sand, coral sand, 
 volcanic sand, etc. It happens, however, that by far 
 the greater part of the sands are composed of particles 
 of quartz, and some are exclusively made up of it. For 
 this reason when sand is spoken of briefly, quartz sand is 
 always understood. 
 
 The composition of ordinary sand is quite variable, depending 
 on the locality. In addition to the quartz grains, those of many 
 other minerals are present, depending on the rocks of the region. 
 Feldspar, garnet and iron ore are very common. Various silicates 
 such as hornblende, pyroxene, tourmaline, etc., are apt to occur. 
 Some grains may be made of pieces of very fine-grained rocks of 
 composite character. Twenty-three different kinds of minerals 
 were found in the dune sands of Holland by Retgers. 
 
 Like pebbles the sand grains are more or less rounded, depending 
 on the amount of transport. In some rather coarse sea sands they 
 are almost all spherical. Below a certain degree of fineness the 
 grains do not become more rounded by attrition in water among 
 themselves; this is due to the fact that the capillary film of water 
 covering them acts as a buffer and prevents them from coming in 
 contact when they collide; in the larger grains it is not able to do 
 this. 
 
 Mud, Silt and Clay. This consists of the finest ma- 
 terial of the land waste. As -sedimentary deposits they 
 are characteristically found off shore, or in sheltered bays 
 and sounds, where the slow movement of the water does 
 not permit the transport of the heavier sand and gravel, 
 and as the material forming the lower flood plains and 
 deltas of great rivers. On account of their minute size 
 the particles are little apt to be rounded, but under the 
 microscope show angular forms. Like the sands they 
 may be composed of a great variety of minerals, kaolin, 
 mica, quartz, feldspar, etc., but just as quartz is the char- 
 acteristic mineral of the sands, so is kaolin that of muds 
 and clays. As shown elsewhere the decay of the feldspars 
 of the rocks produces kaolin or clay, while the quartz 
 
CLASSIFICATION OF STRATIFIED ROCKS 281 
 
 grains are unaltered; the clay particles are excessively 
 fine and light, while the quartz ones are mostly larger and 
 heavier. From this there tends to be a separation of the 
 two by moving water; as the current slackens the quartz 
 is deposited first, forming sand, while the lighter clays are 
 carried beyond and settle in still water. Fine flakes of 
 white mica are apt to accompany them. 
 
 In fresh water a portion of most clays, consisting of the very 
 finest and lightest particles, will remain in suspension almost indefi- 
 nitely. Turbid water of this kind acts much as if it were a solu- 
 tion of clay in water ; if salts be added to it, or if it be mixed with eea 
 water, the clay then curdles into lumps or flocculates and is quickly 
 deposited, leaving the liquid clear. This behavior is analogous to 
 that of salts in solution, and it has an important bearing on the 
 deposition of material carried into the sea, and on the formation of 
 certain kinds of rocks. 
 
 Muds or clays are characterized according to the pre- 
 dominance of certain constituents; thus some are cal- 
 careous, containing more or less carbonate of lime and 
 are often called marls; some contain a good deal of fine 
 quartz and are spoken of as siliceous, others are rich in 
 deposited iron oxides and are ferrugineous clays or 
 ochers, while in many these constituents are present in 
 minimum amount, or are wanting, and these are plain 
 clays or argillaceous deposits. Such mixed forms are 
 transitional to the chemically deposited rocks described 
 later. 
 
 Dissolved Material. The waste of the land includes not 
 only the material mechanically transported by water, but 
 also that which is taken into solution and ultimately 
 carried into the sea. A rough estimate of this for the 
 continents places it at 5,000,000,000 tons per annum. 
 It is an important fraction of the whole amount removed, 
 compared with the mechanical sediments. It varies 
 greatly in different rivers, depending on the composition 
 of the rocks forming their basins. It is inferred that 
 through the concentration of this material in solution 
 
282 ROCKS AND ROCK MINERALS 
 
 during past ages the salts now in the ocean have been 
 produced. From these salts, those sedimentary rocks, 
 whose material through chemical agencies, either of 
 organic life or otherwise, has been redeposited from solu- 
 tion, have been formed. It includes the important class 
 of carbonates, limestones, dolomites, etc., and the less 
 important sulphates and chlorides, such as gypsum and 
 rock-salt. 
 
 It is probable that the carbonates of lime, magnesia, and 
 alkalies were all originally derived from silicates of these 
 oxides. Water containing carbon dioxide has converted 
 them into carbonates, as illustrated under the decom- 
 position of feldspar, and has then dissolved and carried 
 them into the sea. The sulphates have been formed by 
 the oxidation of sulphides in the original rocks and the 
 union of the sulphuric acid with the stronger, more 
 soluble, alkaline bases. The chlorides have in part been 
 derived from minerals of the original rocks and, perhaps, 
 made in part by volcanic emissions from deeply seated 
 magmas within the earth. 
 
 Structure of the Sedimentary Rocks. The sedimentary 
 rocks, as geological masses, differ greatly from the igneous 
 ones in that they form widely extended, relatively thin 
 bodies, making part of a coating or mantle upon the earth's 
 outer surface; they never prolong themselves by extension 
 into the depths, as the latter always do. It is thus their 
 horizontal, as contrasted with their vertical extension, 
 which gives them importance as geological masses. The 
 most characteristic feature about their structure is that 
 they are stratified. This means that they consist of layers, 
 varying in material, texture and color, and in thickness, 
 which, if undisturbed by geological events more recent 
 than their formation, are in general horizontally disposed 
 one upon another. This is illustrated on Plate 26. 
 It is due to the fact that the mechanical sediments have 
 been deposited by moving currents of water in lakes and 
 seas and on the flood plains of rivers, and these currents, 
 
PLATE 26. 
 
 I 
 
 i 
 
 S 
 
 o 
 
 K" 
 
 
 
CLASSIFICATION OF STRATIFIED ROCKS 283 
 
 acting with diverse material and continually varying in 
 strength, have assorted the sediments and deposited them 
 in strata. For further consideration of this subject 
 information should be sought in any of the numerous 
 manuals of Geology, but it may be stated as a general 
 law that sedimentary deposits are always stratified and 
 that, conversely, perfect stratification, resulting from an 
 assortment of particles, is regarded as a proof of deposit 
 in water. 
 
 Volcanic ash deposits are commonly rudely and sometimes well 
 stratified; the heavier of the particles projected upward falling first 
 to be succeeded by smaller lighter ones ; repetitions of the process 
 make individualized beds and thus a rude stratification (see Plate 
 24 ,) while the lighter dust may be deposited by moving air currents 
 much as if in water. 
 
 For stratification variation, both of conditions and in the size of 
 the particles, is necessary ; during a period in which uniformity pre- 
 vails in either it will be wanting. Thus aeolian deposits, which 
 consist of the finest sands and dust driven by the wind, are often so 
 uniform with respect to the size of the particles in any one area that 
 no stratification, or but very little, is produced. In the same way 
 deposits of carbonate of lime, as limestone and chalk, in the open 
 ocean may take place under such uniform conditions and size of 
 particles that beds of these rocks, perhaps a hundred feet in thick- 
 ness or more, are quite structureless and devoid of apparent strati- 
 fication throughout that extent. 
 
 Mere parallelism of layers in a rock is not in itself a mark of 
 stratification and therefore a proof that the rock exhibiting it is of 
 sedimentary origin. Its mineral composition, texture, and relation 
 to the accompanying rock masses, and to the general geology of the 
 region, must also be taken into, consideration. For the spreading 
 action of flowing lava may draw out portions of unlike character 
 within it into thin superimposed sheets as illustrated in Plate 22. 
 Such lavas when fresh are easily recognizable, but buried in the 
 midst of sedimentary deposits and changed in appearance by geologic 
 ages of exposure to various agencies, they may be confused with the 
 accompanying stratified rocks. And again very perfect parallelism 
 of layers and structure may be induced in all kinds of rocks by the 
 shearing and metamorphism accompanying movements of the 
 earth's crust and mountain building. These may superficially 
 simulate stratification quite perfectly, but consideration of the points 
 mentioned above is generally sufficient to show the difference 
 
284 ROCKS AND ROCK MINERALS 
 
 between them. Many serious errors in understanding the real 
 origin of the rocks of different places and their geology have occurred 
 in the past through failure to properly appreciate these facts. 
 
 The individual layers of stratified rocks, which are uni- 
 form in texture, color and composition, may vary from the 
 thinness of paper to a hundred feet or more. Usually, 
 it will be observed that a certain layer, which has a general 
 similarity of character and composition that serve to 
 distinguish it clearly from others above and below, is 
 made up of much smaller subdivisions whose differences 
 from one another are not very marked. The larger division 
 is usually known as a layer or bed, the smaller ones are 
 termed lamince. The main differences between laminae 
 are generally in coloration; as shown in Plate 33, between 
 beds in texture and composition. As explained above, 
 under uniform conditions lamince may be wanting in a 
 particular bed. The general homogeneity of a bed is 
 shown by its particular hardness and appearance, its 
 individual method of cracking or jointing, and the way 
 in which it is affected by erosion, which differs from the 
 beds above and below it. A collection of beds lying con- 
 cordantly one above another, and deposited during a given 
 geological period of time, is called a formation. 
 
 Texture of Sedimentary Bocks. This depends upon the 
 relative size of the particles, which determine the fineness 
 or coarseness of grain, upon their shape, and upon the 
 amount and character of the cement, which determines the 
 firmness or friability of the rock. The size of grain varies 
 within wide bounds, but as explained previously under 
 gradation of material, this in itself determines largely the 
 kind of rock. Thus conglomerates are of necessity 
 coarse-grained rocks; sandstones, medium-grained ones, 
 and shales, fine-grained or compact. Still within the 
 limits of each class there is variation in this respect and 
 we are accustomed to speak of fine, medium and coarse- 
 grained sandstones; a medium grain in this rock is about 
 that of ordinary loaf sugar. 
 
CLASSIFICATION OF STRATIFIED ROCKS 285 
 
 The shape of the component grains, when these are 
 megascopically visible, depends on the amount of trans- 
 port which they have suffered, as explained under gravel. 
 Usually they are more or less rounded or ovoid, but 
 sometimes quite angular. The latter is more apt to be 
 the case as the size of grain increases. Sometimes this 
 broken, angular character of the particles can be dis- 
 tinctly seen in medium-grained sandstones and arkoses 
 by close observation with a good lens. It shows the rock 
 to have a distinctly clastic nature. In the case of coarser 
 rocks and in conglomerates it becomes very striking, and 
 such rocks are called breccias and are said to have a 
 brecciated structure. This is illustrated in Plate 32. 
 Such breccias are not to be confused, however, with 
 volcanic breccias, as described on page 272. 
 
 The cement is that which binds the particles of sedi- 
 mentary rocks together and converts them from loose 
 material into firm rock. Various substances act in this 
 capacity, according to circumstances; sometimes it is 
 carried into the rock from outside sources in solution 
 and deposited in its pores, sometimes part of the sedi- 
 ment itself goes into solution and is redeposited, and some- 
 times it consists of fine material mechanically enclosed 
 with the sediment. In the first and second cases silica 
 and carbonate of lime are common binding materials, in 
 the third, clay or clay-like substances perform this func- 
 tion. Iron oxide, probably according to the second case, 
 is also a not uncommon cement in the form of hematite, 
 or gothite, or limonite. The fine deposits of mud and 
 clay appear to be able to consolidate into firm rocks, 
 under the pressure of superincumbent masses, without 
 the presence of a perceptible cement, though it is some- 
 times present. 
 
 The firmness of the rock depends then, in part on the 
 amount of cement and its quality, and in part on the pres- 
 sure. As a result all degrees of this character are shown 
 by sedimentary rocks; some are very hard, firm and 
 
286 ROCKS AND ROCK MINERALS 
 
 compact, breaking like igneous rocks under the hammer 
 and susceptible of a polish, as in the case of some 
 limestones and sandstones, while others are so loose, 
 incoherent and friable that they may be readily rubbed to 
 powder under the fingers, as with chalks and some sand- 
 stones. And all gradations may be found between these 
 extremes. 
 
 Color of Sedimentary Rocks. This depends partly on 
 the color of the constituent mineral grains or particles, and 
 partly on included substances which act as a pigment. 
 The most common minerals which form the sedimentary 
 rocks are quartz, kaolin, feldspar, calcite and dolomite; 
 these are white or colorless substances naturally, though 
 they sometimes display exotic coloration, and rocks com- 
 posed purely of them, without included pigment, are white, 
 as illustrated by certain sandstones, clays and chalk. 
 Generally more or less pigment is present, and the common 
 ones are the oxides of iron and carbonaceous matter. 
 The iron occurs in the form of ferric oxides, or hydrated 
 oxides, as hematite, or probably hydrohematite (turgite), 
 which gives red to red-brown colors, or as limonite, or 
 gothite, which produce yellow to yellowish brown tones. 
 Carbonaceous matter or finely divided carbon is black, 
 and this is the color of the rock, if it contains an excess of 
 it; as the amount lessens dark grays are formed, and so 
 on into pale grays. If both organic carbonaceous matter 
 and iron oxides are present in the rock, the former exerts 
 a controlling power over the coloring capacity of the latter 
 in this way; in the presence of organic matter, especially 
 when it is decaying, iron is reduced from the ferric to the 
 ferrous condition, it changes from ferric oxide to ferrous 
 carbonate, and as ferrous compounds are colorless or light- 
 colored the rock has the tones of color produced by the 
 carbonaceous pigment. If such rocks are exposed to 
 weathering and the carbonaceous material destroyed, the 
 iron is reoxidized and the red and yellow colors show 
 themselves. This is illustrated in the outcrops and on the 
 
CLASSIFICATION OF STRATIFIED ROCKS 287 
 
 joint faces of many black slates which weather red or 
 yellow. On the other hand if the rocks are devoid of iron, 
 when the organic pigment bleaches out, they become white 
 or very light in color. And again, if solutions containing 
 organic matter leach through the rocks, the iron oxide 
 is not only changed into the ferrous condition, but when 
 reduced to this state, or in it originally, goes into solution 
 also and is carried out, the rocks thus becoming light or 
 colorless. 
 
 The most common colors then for the sedimentary 
 rocks are white to light gray, to dark gray and black, or 
 from white to pink into red, to dark red and red-brown,, or 
 from pale yellow to buff, to yellow-brown. The reds and 
 yellows are often seen commingled in the same rock mass 
 or layer, according to the varying iron hydroxides. In 
 the case of conglomerates and coarse arkose sandstones, 
 these colors may be modified by those of the frag- 
 ments of the unchanged original rocks which they may 
 contain. 
 
 Chemically formed Rocks. These rocks are formed in 
 those cases where material, which has been in solution, has 
 become insoluble by reason of some agency, and is pre- 
 cipitated. The chief agencies involved are concentration 
 of the solutions and organic life. In the latter case ani- 
 mals living in water, chiefly in the sea, secrete inorganic 
 material in the production of their hard parts, either skele- 
 tons to stiffen them, or shells as defensive armor for their 
 soft organisms. As the animals die these collect as depos- 
 its. The chief substances secreted are carbonate of lime, 
 CaCO 3 , and silica, Si02, the former being much the more 
 abundant and important. Examples are seen in the for- 
 mation of reefs and islands by corals, and in the shell- 
 banks made by mollusks. Vegetable organisms also, 
 under certain conditions, secrete silica, and give rise to 
 deposits of that substance. 
 
 The deposits produced by concentration occur when 
 bodies of sea-water are isolated from the ocean by geol- 
 
288 ROCKS AND ROCK MINERALS 
 
 ogic processes and become so concentrated by evapora- 
 tion that they are no longer able to retain the salts 
 in solution. These are then deposited in the order of 
 their solubility. Gypsum and anhydrite, sulphates of 
 lime, and common salt, sodium chloride, are the most 
 important substances deposited in this way. The same 
 result occurs in lakes and inland seas in arid regions, 
 which have no outlet and where there is a steady concen- 
 tration of material in solution, brought into them by in- 
 flowing streams. Carbonates, sulphates and chlorides 
 are the main salts deposited. In a somewhat similar 
 manner, when water passing through the rocks becomes 
 mineralized by taking substances into solution and then 
 attains the outer air, as in springs, these substances are 
 deposited. Such deposits are, with respect to the masses 
 involved, geologically speaking, of minor importance, and 
 are illustrated by the deposits of carbonate of lime around 
 springs, and in caves, and of silica around geysers and hot- 
 springs in volcanic regions. A more important case is 
 where water, in the presence of organic matter, has leached 
 iron oxide from the rocks and soils and carrying it in 
 solution into swamps and shallow waters, has there depos- 
 ited it, either in the form of ferrous carbonate (siderite), 
 if there is excess of organic matter present, or in the reoxi- 
 dized form of ferric hydroxide (limonite) if it is wanting. 
 By this means widely extended beds of iron ore have 
 been formed, which are of great technical value and 
 importance. 
 
 Circulation of Material. Geological science is not yet 
 in a position to state definitely concerning the origin of 
 the material of the earliest formed sediments upon the 
 earth. We have only the fact that, wherever upon the 
 continents the deepest amounts of erosion have occurred 
 and the basement upon which the visibly earliest sedi- 
 ments have been deposited is exposed, this basement is of 
 igneous rock or of apparently igneous rock which has been 
 metamorphosed, and the sediments such as could have 
 
CLASSIFICATION OF STRATIFIED ROCKS 289 
 
 been derived from its erosion and weathering. What- 
 ever the nature of the original sediments was, it is evident 
 that when they had been elevated to form land, since 
 erosive processes continued, any new sediments would be 
 derived from the old ones plus any material that would be 
 added by the continued erosion of such areas of the 
 original surface as the first sediments had not covered and 
 which still remained land, and of any fresh igneous rocks 
 which had come up to occupy a place in either. This con- 
 dition of affairs has continued to the present time; sedi- 
 ments have been laid down, and then elevated to form 
 land, sometimes being greatly metamorphosed in the 
 process and sometimes not, and these by their erosion 
 have in turn yielded fresh sediments, and so on. Thus 
 there has been a circulatory round of material, with 
 changes of conditions to affect the minerals at each stage, 
 and only the most resistant, such as quartz, have been 
 able to undergo it without change. One is a downward 
 course from land to sea; the return journey is the ascen- 
 sion of the land from the sea. The silicate minerals, which 
 chiefly form the mechanical sediments, have performed the 
 downward journey in suspension, the carbonate minerals, 
 on the other hand, have made it mainly in solution. This 
 means that sandstone, for example, on erosion is mostly 
 carried away mechanically, while limestone, which consists 
 mainly or entirely of carbonate of lime, ultimately disap- 
 pears mostly by going into solution, although at the begin- 
 ning of erosive work upon it, it may be largely mechanical 
 processes, which break down the rock. Some cases of 
 mechanical sediments consisting of carbonate of lime occur, 
 though not relatively of great importance, and these are 
 described under limestone, along with some deposits of 
 lime formed on land by evaporation, which may be 
 regarded as temporary stoppages of the material in solu- 
 tion on its way to the sea. This latter case is illus- 
 trated in the formation of travertine around springs and 
 in caves. 
 
290 ROCKS AND ROCK MINERALS 
 
 Minerals of the Sedimentary Rocks. From what has 
 been said in the foregoing pages, it is evident that the 
 minerals of the sedimentary rocks consist of those which 
 compose the igneous ones and which have been able to 
 endure without change the various conditions to which 
 they have been subjected, as well as the new ones formed 
 by weathering and erosion. The finer the material and 
 the longer the time of its transport has been, the more 
 thoroughly it will be changed into new mineral combina- 
 tions. Hence quartz and feldspar are important in the 
 coarser-grained rocks, quartz, 'kaolin and mica in the finer- 
 grained ones; while calcite, dolomite, siderite, limonite 
 and gypsum represent minerals of the chemical deposits. 
 In the fine-grained and dense sedimentary rocks, formed 
 of silts, muds and clays, the particles are so fine, that from 
 the megascopic point of view the mineralogical composi- 
 tion is an element of little value in determining and classi- 
 fying the rock, compared with its color, texture, structure, 
 hardness and other qualities. 
 
 Chemical Relationships. The chemical and mineral- 
 ogical composition of sedimentary rocks is not dependent 
 on definite laws, as that of the igneous rocks evidently is. 
 There are no rules governing the associations of minerals, 
 since these have been brought together by chance, depend- 
 ing mostly on specific gravity, and on size of grain in the 
 assortment. The chemical composition has not in conse- 
 quence the same significance that it has in igneous rocks. 
 Analyses of a few of the more important types are given 
 in the following descriptive portion, since these may be 
 useful in several ways. 
 
 Classification of Sedimentary Rocks. Two modes are 
 used to classify the stratified rocks; one, without reference 
 to composition and character, is based upon the period 
 of their formation in the geological time scale; the other 
 is founded on composition and physical characters. 
 According to the first, strata are classified as Cambrian, 
 Devonian, Jurassic, Tertiary, etc.; according to the 
 
CLASSIFICATION OF STRATIFIED ROCKS 291 
 
 second, as sandstones, limestones, etc. The first has its 
 bearing in historical geology, the second is the petrological 
 method, and is the one that concerns us here. In this 
 work the following classification is adopted. 
 
 Classification of Stratified Rocks. 
 
 1. Material of chemical origin, from solutions. 
 
 a. Deposits from concentration. 
 
 Sulphates; GYPSUM and ANHYDRITE. 
 Chlorides; ROCK-SALT. 
 Silica; GEYSERITE and related rocks. 
 Carbonates; TRAVERTINE and related rocks. 
 IRON ORES of several kinds. 
 
 b. Deposits through organic life.* 
 
 Carbonates; LIMESTONE and DOLOMITE. 
 Silica; FLINT and related rocks. 
 Phosphate rock. 
 COAL, asphalt, etc. 
 
 2. Material of Mechanical Origin. 
 
 a. Water-laid deposits. 
 
 CONGLOMERATES and BRECCIAS. 
 
 SANDSTONES. 
 
 SHALES. 
 
 6. Wind-formed deposits. 
 Loess. 
 
 Dune-sands. 
 Volcanic ashes. 
 
 c. Surface accumulations. 
 
 Laterite and various soils. 
 
 In the nature of things a classification of stratified rocks 
 cannot always draw exact lines between different kinds of 
 rocks. For shales may pass into limestones on the one 
 * Geyserite, Travertine, and Iron Ore may be also partly organic. 
 
292 ROCKS AND ROCK MINERALS 
 
 hand, and into sandstones on the other, and no sharply- 
 defined boundary can be drawn between them. Many 
 such instances could be cited.* And in cases of many 
 rocks of mixed materials and origin it would be difficult 
 to know just where to assign them. The classification 
 must be considered as based upon clear and unmistakable 
 types, which serve as center points around which the 
 rocks group themselves. In the descriptive portion 
 which follows, the exact order of this classification, in 
 respect to some minor rocks, for convenience in refer- 
 ence, may not be always exactly followed. 
 
 * Thus geyserite and travertine are in places and at times partly 
 organic in origin. 
 
CHAPTER IX. 
 DESCRIPTION OF STRATIFIED ROCKS. 
 
 Chemical Deposits by Concentration and Organic Agencies. 
 
 THE more important of the deposits produced from 
 aqueous solutions by the material becoming insoluble 
 through concentration are gypsum, anhydrite, rock-salt 
 and calcium carbonate. Certain deposits of silica from 
 hot waters should also be placed here and iron ores as well, 
 although in the latter case the process of deposition is not 
 usually one of simple concentration. The connection 
 between gypsum, anhydrite and rock-salt, in respect to 
 their origin and occurrence, is very close. They are 
 formed in bodies of sea-water that have been separated 
 from the ocean by the raising of coast-lines, or by accumu- 
 lations of deposits, and under such climatic conditions that 
 the isolated water concentrates by evaporation to such a 
 degree that its salts must crystallize out of solution and 
 deposit. Or in a similar way they may be formed in 
 inland lakes, which have no outlet because they are in 
 arid regions, where the evaporation equals or exceeds 
 the amount of inflow. All natural flowing waters contain 
 more or less of various salts in solution, and in such 
 a lake they must indefinitely increase until the deposit- 
 ing point of concentration is reached. 
 
 GYPSUM. 
 
 As a rock, gypsum is fine-grained to compact; some- 
 times a foliated aggregate showing the excellent cleavage 
 of the mineral; sometimes it has a fine fibrous structure; 
 these forms are less common than the first one. The foil- 
 
294 ROCKS AND ROCK MINERALS 
 
 ated is sometimes cavernous with crystal ends projecting 
 into the cavities, and this may be from recrystallization 
 of the more compact varieties. The fibrous variety is 
 more apt to occur when gypsum forms thin layers or 
 lenses in shales and sandstones. The usual color is white, 
 but it is sometimes yellow or red from iron oxides, or 
 gray to dark gray from mingled clay or organic matter. 
 It is soft and easily scratched with the finger nail. For 
 other properties reference may be had to the description 
 of gypsum as a mineral. 
 
 Gypsum is likely to be accompanied by a great variety 
 of minerals depending on the local occurrence. The most 
 common and intimately related of these are rock-salt and 
 anhydrite, the three having a common origin as previously 
 stated. Clay, marl and bitumen are common impurities. 
 Dolomite, calcite, quartz, sulphur, iron pyrites, are not 
 uncommon accessory constituents. Varieties containing 
 bituminous substances generally yield a disagreeable odor 
 when broken. Gypsum is used in the manufacture of 
 plaster of Paris, and in the raw state as fertilizer. The 
 very compact white or tinted varieties are sometimes 
 called alabaster, and cut into ornamental forms, vases, etc. 
 
 Occurrence. Gypsum is widely distributed in the strat- 
 ified rocks, in the form of extensive beds, often of great 
 thickness, and is especially associated with limestones and 
 shales. It is very commonly found accompanying beds 
 of rock-salt; in such cases it is likely to underlie the salt. 
 It is also found in sedimentary formations, especially in 
 clays and shales, in lenticular masses or scattered through 
 them in isolated crystals, sometimes of great size, as in the 
 Cretaceous beds of the western United States. 
 
 It also occurs in volcanic regions, around fumaroles, 
 where sulphurous vapors are escaping, and especially 
 where limestones have been subjected to such action. In 
 some places where it is found in rocks it may be due to the 
 oxidation of iron pyrites and a chemical reaction of the 
 product with carbonate of lime. 
 
DESCRIPTION OF STRATIFIED ROCKS 295 
 
 ANHYDRITE. 
 
 As a rock, anhydrite is a compact to fine granular sub- 
 stance; sometimes coarse and showing the apparently 
 cubic cleavages of the individual grains. It may be 
 somewhat translucent, and usually has a somewhat 
 splintery fracture with a shimmering or pearly luster. 
 Its color is generally white, though, like gypsum, it is often 
 tinted reddish, yellowish, bluish, gray or dark by oxides 
 of iron, or commingled clay, or organic matter. It is 
 harder than gypsum but easily cut with a knife. For 
 the other properties see description of it as a min- 
 eral. The most commonly associated minerals are rock- 
 salt and gypsum, but locally it may contain many 
 others, as those stated under gypsum. In the anhydrite 
 beds in the strongly folded regions of the Alps, the 
 clay impurity has been converted into cyanite, sillimanite, 
 mica, etc. 
 
 The occurrence of anhydrite is similar to that of gypsum, 
 which it frequently accompanies. It is changed on ex- 
 posure to the air into that substance. The beds do not 
 usually show any distinct stratification. In America, 
 extensive deposits occur in Nova Scotia. 
 
 ROCK-SALT. 
 
 This is an aggregate of grains of common salt, halite 
 or sodium chloride. It is sometimes fine, sometimes 
 medium, and sometimes coarse grained. The color is 
 white but it is often red or yellowish from oxides of iron, 
 gray from intermingled clay or organic matter, and the 
 latter may at times produce bluish or greenish tints. 
 The properties of halite are mentioned in the chapter on 
 rock minerals. 
 
 Associated minerals sometimes found in the salt are 
 quartz, anhydrite and sometimes, though rarely, carbonates 
 or pyrite. 
 
 Rock-salt occurs in geological formations of the sedi- 
 
296 ROCKS AND ROCK MINERALS 
 
 mentary rocks of all ages and in many parts of the world, 
 The beds vary greatly in thickness, from one foot to 4000 or 
 more. Such enormous thicknesses cannot be explained by 
 the simple concentration of an isolated body of sea-water 
 along an arid coast-line. There must have been sub- 
 sidence gradually going on; at first the less soluble gyp- 
 sum, and then the salt, would be deposited, leaving a 
 mother liquor containing the more soluble sulphates and 
 chlorides of magnesium and potassium. If subsidence 
 and the lowering of the barrier should now occur, there 
 would be an influx of the lighter sea-water above, while 
 the heavier mother liquor would flow out below and the 
 basin would be charged anew with sea-water. If the 
 barrier is again closed, for example by waves building it up 
 as seen along the coast of the Carolinas, the conditions 
 would be repeated and fresh deposits of gypsum and salt 
 formed. Thus by repetitions of such a process we can 
 imagine that great thicknesses of salt might be locally 
 deposited. Finally, if no outflows occur the mother 
 liquor is also evaporated and the more soluble salts 
 deposited. 
 
 In the United States rock-salt occurs in beds in New 
 York, Michigan, Louisiana, Kansas and various other 
 states. It is found in Europe, in Germany, Austria and 
 Poland in vast deposits ; in several counties of England and 
 in many other places. Interior drainages are present in 
 all of the continents and in connection with them there 
 are salt lakes and deposits of salt. 
 
 FLINT, GEYSERITE AND OTHER SILICEOUS ROCKS. 
 
 Deposition of silica, Si(>2, from its solution in water 
 occurs both by simple concentration and evaporation 
 and by the action of organic life. It is possible that it 
 may also happen from chemical reactions. The deposits 
 thus formed, while lacking the wide extent and geologic 
 importance of the sedimentary formations produced by 
 the processes of erosion, have yet considerable interest 
 
DESCRIPTION OF STRATIFIED ROCKS 297 
 
 and may be of local significance. On account of their 
 general similarity of composition they are here included 
 under one heading, but the group does not include the 
 mechanically formed siliceous sandstones. The material 
 composing these rocks is, mineralogically, sometimes in the 
 form of quartz pure crystallized, Si02, and sometimes 
 in the form of opal or chalcedonic silica uncrystallized 
 silica containing more or less water in combination as 
 hydroxyl. 
 
 Flint. This is a dark gray or black rock, so extremely 
 compact that it appears as a homogeneous substance. 
 The fracture is conchoidal and the chips have a translucent 
 edge like many felsites, which indeed it may closely 
 resemble. The hardness is 7. It consists of an intimate 
 mixture of quartz and opal, the coloring matter being 
 organic and disappearing when a chip is heated before 
 the blowpipe. 
 
 Flint is not a rock in the sense that it occurs in extended inde- 
 pendent formations. It occurs in irregular nodules or concretions 
 in chalk which vary widely in size, from that of a pea to extensive 
 layers. Similarly an impure flint, occurring chiefly in limestones 
 from the Cambrian up, is called chert. When these substances are 
 studied under the microscope they are found to contain the hard 
 siliceous parts of various organisms, chiefly of sponges and radio- 
 larians. The matter was first derived from sea-water by such 
 organisms, but appears secondarily to have gone into solution and 
 been chemically deposited around certain centers, and in certain 
 places, where favorable conditions obtained. The uses of flint for 
 savage and prehistoric implements and weapons and for striking 
 fire are well known. Other siliceous masses, similar in a general 
 way to flint and chert, sometimes of the same and sometimes of 
 uncertain origin, have received various names such as lydianite, 
 hwnstone, etc. Jasper is a chemically precipitated opaline silica. 
 In places, as in the Lake Superior region, the jaspers are strongly fer- 
 rugineous and interlaminated with bands and streaks of hematite. 
 They constitute rock masses of considerable size, affording valuable 
 deposits of iron ore. They are called jaspilite. The cherty layers 
 are colored bright red by the iron oxide. Another variety of these 
 siliceous flint-like rocks are the novaculites, which occur in con- 
 siderable beds in Arkansas, and are greatly used in the manu- 
 
298 ROCKS AND ROCK MINERALS 
 
 facture of whetstones and hones. They are very dense, conchoidal 
 in fracture, white or pale gray in color, semi-translucent, and com- 
 posed of silica. Their origin is uncertain. 
 
 Geyserite. Siliceous Sinter. In volcanic regions silica 
 is frequently deposited by hot waters, boiling springs and 
 geysers. Sometimes this is produced by simple evapora- 
 tion and drying of the water and sometimes, as shown 
 by Weed, it is due to vegetable organisms, algce, which 
 secrete silica from the heated waters in which they live 
 and become coated with it. The material of the geyser 
 cones and basins produced by drying is hard, compact, 
 and opaline, while that formed by the plants is more or 
 less loose, spongy, and tufaceous. If pure, it is white in 
 color. Its formation is well illustrated in the hot spring 
 and geyser areas of the Yellowstone Park and New Zea- 
 land (see Plate 27). The material thus formed is known 
 as geyserite, or siliceous sinter. 
 
 Diatomaceous Earth. This is a soft, white, chalk-like, 
 very light rock composed of innumerable microscopic 
 shells of diatoms. The latter are excessively minute, 
 unicelled organisms which possess free motion and are 
 covered with a siliceous shell of great delicacy; they are 
 considered forms of vegetable life. In waters of suitable 
 character they may swarm in incredible numbers and their 
 shells, accumulating at the bottom, may give rise to de- 
 posits of considerable magnitude. Some varieties of the 
 rock are pale yellow, brown or gray. It is easily distin- 
 guished from chalk, which it may resemble, by its non- 
 effervescing with acid; from clay by its gritty feeling, 
 when rubbed between the fingers, and its weak argil- 
 laceous odor or the absence of it. A more positive test is 
 the effervescence produced when it is mixed with car- 
 bonate of soda and fused before the blow-pipe. The 
 loose, scarcely coherent material is called infusorial earth; 
 when more compact it is sometimes called tripolite. It 
 is extensively used for polishing purposes. Beds of con- 
 siderable magnitude occur in the United States in Mary- 
 
PLATE 27. 
 
 r 
 
 w 
 
 c 1 
 
 5 1 
 g 1 
 
 K O 
 
 H ^ 
 W a 
 
DESCRIPTION OF STRATIFIED ROCKS 299 
 
 land, Virginia, Georgia and Alabama, where they are 
 worked commercially, also in Missouri, Nevada, California 
 and elsewhere, often as a layer in swamps which repre- 
 sent the fillings of former lakes. They are also found in 
 Germany and other parts of Europe. 
 
 IKON ORE ROCKS. 
 
 The deposits of iron ore which occur as rocks, inter- 
 stratified or associated with sedimentary beds, have origi- 
 nated through complex processes, sometimes wholly, 
 sometimes partly, of a purely chemical nature and usually 
 more or less influenced by the agencies of organic life. 
 The most important set of processes has been previously 
 mentioned but now deserves a more detailed description. 
 
 Iron exists in the original (the igneous) rocks in the 
 form of silicates, such as biotite, olivine, pyroxene and 
 hornblende, and also as oxides, such as magnetite, hema- 
 tite and ilmenite, as disseminated grains. It also occurs 
 in the secondary metamorphic rocks as silicates and oxides. 
 It is also pretty generally diffused through the sedimentary 
 rocks, in part as coloring matter and cement, and mostly 
 in the form of ferric oxide, ferric hydroxides and ferrous 
 carbonate. In the igneous rocks it is largely in the ferrous 
 state and to a considerable degree also in the meta- 
 morphic ones. Also, to understand the concentration of 
 iron and formation of iron ore rocks, it must be borne in 
 mind that the metal forms only one carbonate, ferrous 
 carbonate or siderite, FeCOs which, like carbonate of 
 lime, is soluble in water containing carbon dioxide. 
 
 When the rocks are decomposed and broken down by 
 the agencies of weathering and erosion, the silicates con- 
 taining iron are altered; the ferrous oxide in them com- 
 bines in part with the carbon dioxide in the circulating 
 ground water to form ferrous carbonate which goes into 
 solution, and in part it is oxidized to ferric oxide. The 
 original oxides of iron react in a similar manner. The 
 ferric oxide thus formed or liberated would be insoluble, 
 
300 ROCKS AND ROCK MINERALS 
 
 but in the presence of decaying vegetable matter in the 
 soil and organic acids leached downward into the rocks, 
 deoxidation of the ferric oxide ensues; it is reduced to 
 ferrous oxide and then becomes ferrous carbonate and 
 goes into solution. The reason for this is that decay of 
 organic matter is a process of oxidation, like slow com- 
 bustion; the organic matter takes oxygen from the air, 
 but in the presence of moisture and ferric oxide it will 
 take oxygen from the latter, reducing it to the ferrous 
 oxide which is then fitted to unite with carbon dioxide 
 and become the carbonate. 
 
 The iron of the rocks, which is thus brought into solu- 
 tion, is leached out, and in standing bodies of shallow water, 
 such as swamps, lagoons or estuaries, with small outlets 
 to the sea, it may be concentrated and give rise to exten- 
 sive deposits. Under some conditions these deposits may 
 be of the carbonate directly, but usually the solution of 
 the carbonate is re-oxidized, carbon dioxide escapes, and 
 the iron is precipitated as ferric hydroxide (limonite). 
 This oxidation is largely, if not wholly, performed by cer- 
 tain bacterial organisms which demand iron in their inter- 
 nal economy, and therefore, secrete the iron from the 
 water, and change it in their cells from the ferrous to the 
 ferric condition, thus rendering it insoluble. Living and 
 dying in unimaginable numbers, though excessively 
 minute, they give rise to large deposits. 
 
 The ferric hydroxide which is thus precipitated may 
 accumulate on the bottom as bog iron ore, or limonite, or, 
 as is so often the case in shallow bodies of standing water, 
 like swamps, etc., it may again come in contact with decay- 
 ing vegetable matter, and be changed back into carbonate. 
 Such beds of iron ore may be quite pure, or they may be 
 more or less mingled with clay and sand, brought in at 
 times of high water, and thus impure limonites, clay iron- 
 stones, black-band ore, etc., are formed. This also 
 explains the not infrequent association of stratified iron 
 ore and coal beds in the same series of rocks, and the reason 
 
DESCRIPTION OF STRATIFIED ROCKS 301 
 
 why in this case the iron ore is commonly ferrous 
 carbonate. 
 
 The moving ground waters containing iron in solution, 
 as described above, may also issue as springs and give rise 
 to deposits of iron ore. 
 
 Certain masses of iron ore, chiefly limonite, are supposed 
 to be residual products of weathering and solution. This 
 is illustrated in the view that masses of limestone con- 
 taining ferrous carbonate have been dissolved and carried 
 away, but the iron, oxidized to the ferric condition in the 
 process, has become insoluble and remaining behind has 
 gradually concentrated. The more important iron ore 
 rocks may now be described. 
 
 Bog Iron Ore. Limonite. This is sometimes loose and 
 earthy, sometimes firm and porous. It consists mainly 
 of limonite, mixed more or less with humus, phosphates, 
 silicates of iron, clay, sand, etc. Its character has been 
 sufficiently described under limonite among the minerals. 
 It sometimes occurs in concretions. With increasing 
 amounts of clay it passes over into yellow ocher. It is 
 found in all parts of the world. In the United States it 
 is widely distributed, and along the Appalachian belt, 
 from Vermont to Alabama, deposits of limonite, most of 
 which are probably residual in character, have furnished 
 iron ore since the early settlement of the country, and in 
 great quantity. 
 
 Clay Ironstone. Siderite. When reasonably pure, side- 
 rite, or spathic iron ore, is a coarse to fine crystalline aggre- 
 gate of siderite grains. It is whitish to yellow, or pale 
 brown in color, but on exposed surfaces much darker 
 brown to black, owing to oxidation of the ferrous carbon- 
 ate to limonite, or of the manganous carbonate to manganic 
 oxides. It generally contains, more or less, carbon- 
 ates of lime, magnesia and manganese. Iron pyrites or 
 hematite are commonly associated minerals. For the 
 properties of siderite, reference may be had to its de- 
 scription among the minerals. 
 
302 ROCKS AND ROCK MINERALS 
 
 An impure variety of siderite mixed with clay, sand 
 and limonite in variable proportions, of a compact appear- 
 ance, and generally of dull brown colors, is known as clay 
 ironstone. It is apt to occur in nodules, often as con- 
 cretions around some fossil, and lenticular masses which 
 increase until they become inters tratified beds of consid- 
 erable thickness. Another variety which contains so 
 much organic, coaly matter that it is colored black is 
 known as black-band ore. It is especially associated 
 in the strata with coal beds from the Carboniferous 
 upward. - 
 
 Carbonate ores of iron are of less importance in the 
 United States than the deposits of limonite and hematite. 
 They occur in Pennsylvania, Ohio and Kentucky, of Car- 
 boniferous age, and in the Lake Superior region in Michigan 
 and Minnesota, of Algonkian age. They occur in Europe 
 in England, Germany, France and Spain, in deposits of 
 great technical value. Black-band is found in the coal- 
 bearing strata of Pennsylvania, England, etc. 
 
 Hematite. Red Iron Ore. This occurs in the form of 
 veins, lenticular masses and beds, in various geological 
 formations and especially in those whose strata have been 
 folded. As a rock, it varies from fine grained and compact 
 to earthy or fibrous, is of a red to brown color or, where 
 crystalline, of a dark gray. Its properties as a mineral 
 have been previously given. It occurs pure or nearly so, 
 but with varying mixtures of clay, sand or silica, it passes 
 insensibly into ferrugineous clays, red ochers, or shales, 
 sandstones, cherts, etc. In this connection see jaspilite 
 under flint. While hematite undoubtedly occurs as a 
 normal sedimentary or stratified rock, interbedded with 
 other unchanged strata, as in the beds which have such 
 a wide distribution in the eastern United States in the 
 Clinton group of the Niagara period, it is more generally 
 to be considered a metamorphic rock, and as such, might 
 be included among the metamorphic iron rocks described 
 in Chapter XI, such as itabirite and hematite schists. 
 
DESCRIPTION OF STRATIFIED ROCKS 303 
 
 Extensive deposits of hematite are found in various 
 parts of the United States and Canada. The greatest 
 amounts mined as ore come from Tennessee and the Lake 
 Superior Region, the vast production in the latter leading 
 the world in output. Large beds are also found in England 
 and other parts of Europe. 
 
 Iron Oolite. The iron rocks described above, and especially red 
 hematite, not infrequently assume a concretionary form in which 
 the rock is composed of rounded, sometimes polygonal, grains which 
 vary in size from that of fine sand to peas. An examination of 
 them shows that they have a concentric shelly structure. The color 
 varies from red to brown. Sometimes the rock is composed of 
 them alone and sometimes they are thickly embedded in a marly or 
 clayey cement. The iron ore appears in many cases to have been 
 deposited around grains of sand, fragments of fossils, etc., as neuclei. 
 The Clinton ores mentioned above frequently assume this oolitic 
 character and it is well known from various European localities. 
 Such ores have sometimes been changed into magnetite while still 
 retaining the oolitic structure. 
 
 LIMESTONE AND OTHER CARBONATE ROCKS. 
 
 This group of rocks has the common property of being 
 composed of carbonate of lime, calcite, CaCOs, or of 
 this substance intermingled more or less with dolomite, 
 MgCa(CO 3 ) 2 . It is also a common property that, so far 
 as known, the carbonate of lime has primarily been 
 separated from water, rendered insoluble and accumu- 
 lated by the action of living organisms of one kind or 
 another. Secondarily, the deposits thus made may be 
 mechanically broken up and redeposited, or they may be 
 taken into solution, carried away and precipitated else- 
 where. There may be some possible exceptions to this 
 rule, that the carbonate of lime is primarily precipitated 
 by organisms, in the cases where it is concentrated in 
 alkaline lakes by inflowing waters and finally deposited, 
 or in the evaporation of shut-off portions of the sea, but 
 these are of small account and negligible in comparison 
 with the great formations produced by life agencies. 
 
304 ROCKS AND ROCK MINERALS 
 
 Hence it is generally held that the great masses of car- 
 bonate rocks, even when they do not contain fossils, 
 are a proof of the existence of life at the time of their 
 original deposition. 
 
 This group of rocks is soluble in hydrochloric acid; 
 entirely so when pure carbonates, but generally leaving 
 more or less of a residue, consisting chiefly of sand, clay, 
 silica, etc. In some cases, where dolomite is present, the 
 acid must be heated. Their hardness is less than 4, 
 hence they may be readily scratched or cut with the 
 knife. 
 
 The following are the important members of the group. 
 
 Limestone. This is the most common and important 
 carbonate rock. It is fine grained to very dense in 
 texture and its color varies from whitish, through tones of 
 yellowish to brown, or from various shades of gray, dove- 
 color, bluish-gray, dark-gray to black. It is rarely of 
 reddish colors. The yellow and brown colors are due to 
 iron oxide, the gray and black to organic matter. The 
 gray colors are most common. Compact varieties have 
 an even to somewhat conchoidal fracture. It effervesces 
 freely with any common acid, with vinegar (acetic acid) 
 or lemon juice (citric acid). It is easily scratched with 
 the knife and many of the less compact varieties are 
 friable to the finger nail. The specific gravity varies 
 from 2.G 2.8. On exposed surfaces it is apt to be cavern- 
 ous and often tinted or blotched reddish or yellowish from 
 oxidation of small amounts of ferrous carbonate it may 
 contain. It occurs in individual beds of all thicknesses 
 up to 100 feet or more. 
 
 Some limestones consist of pure grains of calcite, others 
 possess a fine, clay-like cement between them. Acces- 
 sory minerals, which are sometimes seen, are pyrite and 
 quartz, the latter in minute crystals, sometimes lining 
 cavities. 
 
 In following analyses I, II, and III are of very pure 
 limestones; IV is an impure type containing considerable 
 
DESCRIPTION OF STRATIFIED ROCKS 
 
 305 
 
 dolomite and sand and clay. Such transitions through 
 impurities are common; thus V for example shows one 
 toward the clay-ironstone previously described. Transi- 
 tions to dolomite are not common; an examination of a 
 large number of analyses shows that generally either the 
 rock contains very little or no magnesia, or it has much 
 and is a regular dolomite as described later. 
 
 I.. 
 II... 
 III.. 
 
 SiO 2 
 
 A1 2 3 
 
 Fe 2 O 3 
 
 FeO 
 
 MgO 
 
 trace 
 0.4 
 0.6 
 
 CaO 
 
 H 2 O 
 
 XyO 
 
 C0 2 
 
 Total 
 
 0.4 
 0.6 
 1.2 
 
 0.2 
 
 0.4 
 0.4 
 0.2 
 
 0.3 
 
 54.8 
 54.2 
 53.8 
 
 1.1 
 0.9 
 
 0.1 
 
 42.7 
 44.0 
 42.7 
 
 99.4 
 99.6 
 100.0 
 
 IV .. 
 V ... 
 
 7.0 
 3.2 
 
 3 
 0.1 
 
 y ' 
 
 6 
 10.9 
 
 15.2 
 
 9.0 
 11.0 
 
 39.3 
 26.6 
 
 0.2 
 0.5 
 
 1.2 
 1.5 
 
 38.8 
 41.1 
 
 99.1 
 100.1 
 
 I, Trenton Limestone, Lexington, Virginia ; II, Buff Limestone, 
 Hoosier Quarry, Bedford, Indiana; III, Lithographic Limestone, 
 Solenhofen, Bavaria; IV, Impure dolomitic Limestone, Greason, 
 Pennsylvania; V, Sideritic-dolomitic Limestone, Gogebic dist., 
 Michigan. 
 
 XyO represents small quantities of organic matter, manganese 
 oxide, etc. 
 
 The strength of limestone as a rock varies very much 
 with the texture; that of firm compact varieties is very 
 high while loose porous ones are very weak. Thus a 
 dense variety has been shown to have a crushing strength 
 of over 40,000 pounds per square inch, while others 
 scarcely exceed 3000 pounds per square inch. The well 
 known white oolitic limestone of Bedford, Indiana, has an 
 average crushing strength of 4300 pounds. Any good 
 firm and compact limestone has a strength far in excess of 
 any load that it may be called upon to endure in modern 
 structures. The porosity of limestones varies considerably; 
 those containing the most sand are usually the most 
 porous; the ratio of pore space to rock volume may vary 
 
306 ROCKS AND ROCK MINERALS 
 
 from 15 per cent to practically nothing, the ratio of the 
 weight of water it can absorb to the weight of rock is in 
 general much less than this, usually not more than one- 
 half as much. 
 
 There are many varieties of limestone, depending on circumstances, 
 especially the mode of formation. Thus in some there are abun- 
 dant remains of fossils which may give the rock a distinctive char- 
 acter. These comprise a great variety of organisms, among which 
 may be mentioned corals, crinoids, shells of mollusks, brachiopods, 
 gastropods, foraminifera, remains of sponges, etc. The " encrinal 
 limestone " of Silurian age in western New York is an example. 
 Sometimes these fossils occur in such numbers that the entire rock 
 is composed of masses of shells, or the hard part of one particular 
 organism, with just enough fine carbonate of lime between them 
 to act as a cement. Examples of this are seen in the layers com- 
 posed wholly of brachiopod shells found in the Niagara formation 
 of the Silurian in western New York. Such rocks are sometimes 
 called " shell limestones." Certain limestones composed of corals 
 are also examples of the same thing. 
 
 On the other hand, there are varieties which depend on the presence 
 of some impurity which gives a particular character to the rock. 
 Thus it may contain much clay and is termed an argillaceous lime- 
 stone or it may contain much sand of siliceous character and be an 
 arenaceous limestone : such rocks are transitional to shales and 
 sandstones. Others which are dark colored may yield a strong, 
 disagreeable, bituminous odor when struck and broken and are 
 called bituminous limestones; they contain considerable organic 
 matter. In some, which are termed glauconitic limestone, the rock is 
 more or less rilled with green grains of glauconite. Lithographic 
 stone is a fine, compact, somewhat schistose limestone; the flesh- 
 colored rock from Solenhofen, Bavaria, remarkable for the well 
 preserved fossils it occasionally contains, is especially used for this 
 purpose. It is a very pure limestone, as shown by the analysis 
 given above. 
 
 Limestones are very apt to contain concretions and masses of 
 chert, or hornstone, of the character described in a previous section; 
 they often become so abundant as to form definite bands or layers 
 in the rock. 
 
 By the weathering of limestones the lime carbonate is 
 removed in solution, leaving the insoluble impurities 
 behind. These form clays or loams which are colored 
 
DESCRIPTION OF STRATIFIED ROCKS 307 
 
 deeply red or yellow by the oxidation of the iron min- 
 erals originally present, and commonly contain pebbles 
 of chert or quartzose material and masses of limonite. 
 Such residual soils are commonly very fertile and cover 
 large areas in the southern United States, and in other 
 parts of the world. 
 
 Uses of Limestone. The use of limestone for structural 
 purposes of all kinds is well known and needs no further 
 comment. The same is true of its manufacture, by burn- 
 ing, into quicklime for mortar and cements. Large 
 quantities are also used as a flux in smelting operations, as 
 in the making of iron and steel. In recent years the use of 
 certain impure limestones containing 15-40 per cent of 
 clay, or other substances consisting of silica, alumina and 
 iron oxide, in the manufacture of natural hydraulic 
 cements has risen to very large proportions. 
 
 Dolomite. The geological use of this term is not always 
 the same as the chemical one. Chemically, or mineralogic- 
 ally, by dolomite is meant a chemical compound of a 
 definite composition CaMg(CO3)2 with CaO, 30.4 per 
 cent, MgO 21.7, CO 2 47.8, while geologically the term is 
 used for any limestone which consists dominantly of 
 this compound, although it may also contain a large 
 amount of admixed calcite, CaCO 3 , and in some parts of 
 Europe it is employed to designate limestones of a particu- 
 lar geological period, some of which are not dolomites 
 at all. 
 
 The description of the colors, texture, and other physical 
 characters of limestone given above, applies equally well 
 to dolomite. In fact it cannot ordinarily be told in the 
 field, or by mere inspection of a hand specimen of a rock, 
 whether it is a dolomite or a pure limestone. 
 
 Dolomite is somewhat harder than true limestone and 
 if it is a pure dolomite it will not dissolve with efferves- 
 cence in acetic acid (vinegar) and but very slowly in cold 
 hydrochloric; if it contains admixed calcite this reacts 
 very readily. The best test is a chemical one for magnesia 
 
308 
 
 ROCKS AND ROCK MINERALS 
 
 in a solution obtained by boiling the powdered rock in 
 dilute hydrochloric acid. 
 
 The following analyses show the chemical composition 
 of some examples of this rock. 
 
 
 SiO 2 
 
 A1 2 3 
 
 Fe 2 3 
 
 FeO 
 
 MgO 
 
 CaO 
 
 CO 2 
 
 H 2 O 
 
 XyO 
 
 Total 
 
 I.... 
 
 3.2 
 
 0.2 
 
 0.2 
 
 0.1 
 
 20.8 
 
 29.6 
 
 45.5 
 
 0.3 
 
 
 99.9 
 
 II... 
 
 3.1 
 
 
 
 0.1 
 
 0.9 
 
 20.0 
 
 29.7 
 
 45.3 
 
 0.3 
 
 0.2 
 
 99.6 
 
 III.. 
 
 1.1 
 
 0.4 
 
 
 
 
 
 19.9 
 
 31.5 
 
 45.6 
 
 1.3 
 
 0.1 
 
 99.9 
 
 IV.. 
 
 5.0 
 
 1.0 
 
 0.5 
 
 
 
 16.8 
 
 32.2 
 
 43.8 
 
 
 
 
 
 99.3 
 
 I, Knox Dolomite, Morrisville, Alabama ; II, Dolomite, Sunday 
 Lake, Gogebic district, Michigan; III, Dolomite, Tornado Mine, 
 Black Hills, South Dakota; IV, Dolomite (magnesian limestone), 
 Newcastle, England. 
 
 The origin of dolomite is a matter which has been much discussed 
 and many theories have been propounded by geologists and 
 chemists in explanation of it. When all the facts are taken into con- 
 sideration, it is clear that dolomite is not an original rock, but has 
 been formed from pure limestones by the substitution of a part of 
 the lime by magnesia, from waters containing magnesium salts in 
 solution. Dolomite is a denser and more stable compound than 
 calcite; if the latter were subjected to the action of soluble magne- 
 sium salts there would be a constant tendency for dolomite to form 
 and part of the lime to be liberated, as illustrated in the following 
 reaction, 
 
 2 CaCO 3 + MgCl 2 = CaMg (CO 3 ) 2 + CaCl 2 . 
 
 It is evident that if the magnesia solution was strongly concen- 
 trated the exchange, in a given mass of limestone, would be effected 
 more quickly. If the solution were heated it would also act more 
 quickly; if it acted under pressure the result would also be hastened, 
 and, finally, as time is an important element, the longer the limestones 
 have been subjected to the solutions the more completely we may 
 expect them to be changed to dolomite. If we consider in addition, 
 that not only sea-water contains magnesium salts, but also the circu- 
 lating ground waters and thermal waters ascending from the depths, 
 in greater or lesser amount, it is clear that in harmony with the 
 
DESCRIPTION OF STRATIFIED ROCKS 309 
 
 above principles, the change, which we may call dolomitization, must 
 take place in a variety of ways and under various conditions, not 
 only in the sea, but also on the land; that all limestones are not 
 converted completely into dolomite before they emerge from the sea 
 must be due to certain reasons; that the solution is too dilute, that 
 it is not hot enough, that there has not been sufficient time, that the 
 deposits are too compact to permit sufficient penetration and cir- 
 culation of sea-water, etc. But if lime deposits are subjected in 
 an enclosed basin to constantly concentrating sea-water they may 
 become more rapidly converted. This might happen, for instance, 
 if a coral atoll were somewhat elevated and its lagoon wholly or 
 nearly shut off from access to the sea. The formation of dolomite 
 in such enclosed basins of sea-water would also explain its frequent 
 association with gypsum and anhydrite. The application of the 
 principles stated above would also lead us to conclude, that the 
 older and more deeply buried a limestone was, the more apt it would 
 be to become a dolomite; that in disturbed and folded mountain 
 regions, limestones of the same age and formation would be more 
 likely to be dolomitic than those of undisturbed areas, because the 
 rocks are there more fractured and filled with thermal solutions, and 
 in practice the facts are found to confirm these views. The connec- 
 tion with thermal waters also explains the frequent association with 
 lead and zinc ores. 
 
 The mineral dolomite is denser than calcite and in the change 
 above mentioned a considerable reduction of volume, amounting 
 to 12 per cent, must occur in the limestone. This would apparently 
 help to explain why dolomites are so frequently very porous or 
 cavernous rocks, though if deeply buried, all such cavities would be 
 closed by the pressure. 
 
 Limestones and dolomites are rocks of such general 
 distribution in all parts of the world where stratified 
 rocks are found, that their occurrence needs no special 
 mention. 
 
 Oolite. Oolitic Limestone. This is a well-characterized 
 variety of limestone, which consists of minute to small 
 spherical concretions, presenting very much the aspect of 
 a fish roe, whence the name from the Greek, meaning egg- 
 stone. The round grains vary in size from very minute 
 up to those as large as a pea. In the larger ones it may often 
 be observed that they have a concentric shelly structure 
 and thus consist of successive coats. An illustration of a 
 
310 ROCKS AND ROCK MINERALS 
 
 coarse oolite or pisolitic limestone from Bohemia is shown 
 on Plate 28. There is usually more or less limy cement 
 binding the grains together. 
 
 Examination of oolites generally shows that some object, such as 
 a bit of shell, a grain of sand or something similar, has served as a 
 nucleus around which the coatings of lime carbonate have accumu- 
 lated. On the shores of Great Salt Lake at the present time oolitic 
 sands are forming from the waters which are charged with lime and 
 other salts in solution. As the particles are rolled on the beach, or 
 agitated in the water, all parts become equally coated and the 
 spherical form is assumed. By a similar process oolitic grains are 
 forming in springs charged with lime salts, as at Carlsbad in Bohemia. 
 The concretionary structure is best seen under the microscope; 
 it is rarely sufficiently coarse to be observed with the eye alone, but 
 may be sometimes made out with a lens. Oolitic limestones con- 
 stitute large and important formations, often of great thickness and 
 of different geological ages. They are especially important in the 
 Jurassic strata of England and elsewhere in Europe. It is a 
 structure also assumed by some American limestones. 
 
 Chalk. Typical chalk is a soft, white, friable rock, whose 
 use for marking and blackboard crayons is well known. 
 While generally pure white it may sometimes be tinted 
 gray, flesh color, or buff. It consists of a fine calcareous 
 powder, which by examination under the microscope has 
 been found to consist of the tiny shells of foraminifera, 
 mingled with minute fragments of the shells and hard 
 parts of various organisms, as well as the siliceous spicules 
 of sponges, shells of diatoms and radiolarians, together 
 with occasional microscopic fragments of various min- 
 erals. It is the siliceous material of the sponge spicules, 
 diatom shells, etc., that has concentrated into the 
 nodules and concretions of flint, so commonly found in 
 certain beds of chalk, and whose analogue is seen in 
 the layers and masses of chert in limestones. Chalks, 
 in spite of their fine grain, are very porous rocks, absorb- 
 ing as much water as 20 per cent of their weight in some 
 cases. 
 
 Chemically, chalks are quite pure carbonate of lime, as 
 
PLATE 28. 
 
 A. OOLITE, VARIETY PISOLITE, BOHEMIA. 
 
DESCRIPTION OF STRATIFIED ROCKS 
 
 311 
 
 shown in the following analyses of three specimens given 
 by different authorities. 
 
 It has been customary to consider chalk a formation produced on 
 the bottom of the deep sea, from its resemblance to the calcareous 
 oozes or muds found underlying the depths of modern oceans. It 
 has evidently not always been formed in this way, as shown by the 
 fossils indicative of shallow water which some chalks contain, as well 
 as the perfect skeletons of birds, pterosaurs and other vertebrate 'ani- 
 mals. The facts in some cases would point rather to its having been 
 formed in clear, warm and shallow seas, free from the products of 
 land waste. 
 
 Closely related to chalk, but differing in the fact that they do not 
 predominantly consist of foraminiferal shells, are light, chalky, earthy 
 limestones formed in a variety of ways, such as from coral sands and 
 muds; from those materials accumulated by the wind on coral 
 islands; from ground-up shells in clear, shallow seas, etc. A whitish, 
 fragile rock formed on the coasts of Florida, which consists of shells 
 and their fragments of all sizes somewhat lightly compressed and 
 cemented together, is known as Coquina, from the Spanish word for 
 shell. (See Plate 28.) 
 
 
 CaC0 3 
 
 MgCO 3 
 
 SiO 2 
 
 (FeAl) 2 3 
 
 H 2 
 
 Total 
 
 T 
 
 94.2 
 
 1.4 
 
 3.5 
 
 1.4 
 
 0.5 
 
 101.0 
 
 TT.... 
 
 96.4 
 
 1.4 
 
 1.6 
 
 0.4 
 
 0.2 
 
 100.0 
 
 III... 
 
 98.4 
 
 0.1 
 
 1.1 
 
 0.4 
 
 "* 
 
 100.0 
 
 I, White chalk, White Cliffs, Little River, Arkansas; II, Lower 
 Cretaceous chalk, Burnet Co., Texas; III., White chalk, Shore- 
 ham, Sussex Co., England. 
 
 Chalk is found extensively in Europe in England, 
 Germany, France, etc., where its occurrence is the result 
 of a distinct geologic epoch, named on this account 
 the Cretaceous. It also occurs widely distributed in 
 the Cretaceous formations of the southern trans-Missis- 
 sippi States, in Nebraska, Arkansas, and especially in 
 Texas. 
 
312 ROCKS AND ROCK MINERALS 
 
 Travertine, Calcareous Tufa. In the preceding chapter 
 it was shown how material of the land surface is taken into 
 solution and carried into the sea. This is especially 
 important with regard to lime, which goes to the sea as a 
 sulphate and carbonate, the latter being much the more 
 momentous. This lime carbonate comes, not only from 
 pre-existent carbonate rocks, but also from the lime sili- 
 cate minerals of the igneous and metamorphic ones, which 
 under atmospheric agencies are converted into carbonates. 
 The lime carbonate on its way to the sea may be tem- 
 porarily deposited, giving rise to rock-masses of some 
 magnitude and importance. 
 
 Carbonate of lime has little solubility in pure water, but if the latter 
 contains carbon dioxide, the lime carbonate is converted into a soluble 
 bicarbonate and the amount of the latter formed and taken into 
 solution depends on the amount of carbon dioxide present. Thus 
 in regions where limestones or other carbonate rocks abound, the 
 natural waters, under atmospheric pressures, attack such rocks and 
 take the lime carbonate into solution in a relatively slow manner, 
 but in spring waters, and especially thermal ones coming from depths, 
 the pressure may be great, the amount of contained carbon dioxide 
 large and the quantity of dissolved carbonate of lime proportionately 
 so. Such waters on coming to the surface lose the greater part of 
 the dissolved carbonic acid in the form of gas, and the lime in solu- 
 tion is consequently deposited rapidly and in large amount. In the 
 waters under surface atmospheric pressure the lime is deposited 
 by evaporation and therefore much more slowly. In warm waters 
 the deposit of lime may be much increased by the action of low 
 forms of vegetable life, algae, living in them, which secrete lime from 
 the water. 
 
 The rock thus formed by deposit of carbonate of lime 
 from solution is called travertine, from the old Roman 
 name of the town of Tivoli near Rome, where an exten- 
 sive formation of the substance exists. When deposited 
 slowly, as in the stalactites and stalagmites in caves, it is 
 rather hard and compact, fine crystalline, sometimes 
 white but usually tinted yellowish or brownish; it often 
 has a fibrous or concentric structure; it breaks with a 
 
PLATE 29. 
 
 A. CALCAREOUS TUFA, DEPOSITED ON VEGETATION, 
 COLORADO. 
 
 B. CALCAREOUS TUFA, YELLOWSTONE PARK. 
 (U. S. Geological Survey.) 
 
DESCRIPTION OF STRATIFIED ROCKS 313 
 
 splintery fracture. When deposited more rapidly, as by 
 springs, it is softer, not evidently crystalline, and porous to 
 loose or earthy; when formed coating vegetation it may 
 be open, cellular, spongy, bladed or moss-like, as illustrated 
 in Plate 29. 
 
 These looser, less compact, varieties are commonly called 
 calcareous tufa or calcareous sinter. Deposits of traver- 
 tine, or calcareous tufa, are found in nearly all countries 
 and especially in limestone regions. Many caves are 
 celebrated for the number, size and beauty of the stalac- 
 titic and stalagmitic formations they contain. See Plate 
 30. Springs depositing carbonate of lime are very com- 
 mon, but warm carbonated waters are chiefly found in 
 volcanic regions or those which have recently been so, 
 like the celebrated Mammoth Hot Springs of the Yellow- 
 stone Park, and others found in California, Mexico, Italy, 
 New Zealand, etc. See Plate 31. The so-called Mexican 
 " onyx " or " onyx marble," which is extensively used 
 as an ornamental stone, is a travertine with a banded 
 structure, beautifully brought out by its varied tinting 
 through metallic oxides. 
 
 Marl. This name is given to loose, earthy or friable 
 deposits consisting chiefly of intermingled carbonate of 
 lime, or dolomite, with clay, in variable proportions. The 
 color is usually gray, but they are often yellow, green, 
 blue or black, and sometimes with pronounced color tones 
 due to some special substance, as oxide of iron or organic 
 matter. They show all gradations into clays and shales. 
 On exposure to the air or water they crumble quickly 
 into coarse soils. The carbonates in them are readily 
 detected by their effervescence in acid. According to 
 special substances or objects, which they may contain in 
 addition to those mentioned, different varieties are named; 
 thus sandy marl is full of grains of quartz sand and often 
 of other minerals; shell marl is a whitish, earthy deposit 
 formed of fragments of shells of various organisms formed 
 in enclosed basins of water, mingled with clay, etc. In 
 
314 
 
 ROCKS AND ROCK MINERALS 
 
 the Atlantic and Southern states this name is applied to 
 beds which contain abundantly shells of mollusks, gastro- 
 pods and other shell-fish. 
 
 The chemical composition of marls varies very greatly ; 
 the following analysis of a compact one from Colorado will 
 serve as an example. 
 
 CaC0 3 
 
 MgC0 3 
 
 Si0 2 
 
 A1 2 3 
 
 Fe 2 3 
 
 MgO 
 
 K 2 O 
 
 H 2 O 
 
 Org. 
 Sub. 
 
 XyO 
 
 Total 
 
 21.6 
 
 1.7 
 
 45.9 
 
 13.2 
 
 3.9 
 
 1.3 
 
 2.3 
 
 5.4 
 
 3.5 
 
 1.2 
 
 100.0 
 
 XyO = small quantities of TiO 2 ,Na and P 2 O 5 . 
 Greensand Marl is described under sandstones. 
 PHOSPHORITE PHOSPHATE ROCKS. 
 
 Deposits of phosphate of lime, while not of great geo- 
 logical importance in making extensive formations, are 
 yet of considerable interest and, commercially, of great 
 value, from their use as soil fertilizers. When occurring 
 in stratified rocks and unconnected with igneous intrusion 
 they represent material of organic origin. While some 
 invertebrates, such as a few species of brachiopods, secrete 
 phosphate of lime in their shells and hard parts, it is mostly 
 to the bones and excrement of vertebrates that the origin 
 of this material must be ascribed. Sometimes the deposit 
 appears to be the direct and original one, but more com- 
 monly it is secondary in nature, in that the phosphates 
 have been leached out, carried down and redeposited as 
 nodular, concretionary or lenticular masses in clefts and 
 other cavities in the rocks in which they occur. 
 Especially in limestones, which, being soluble, are carried 
 away, the less soluble phosphatic material tends to accu- 
 mulate in such masses. The general name of phosphate 
 rock or phosphorite may be used for all such material. 
 The appearance of these rocks is variable, sometimes com- 
 
PLATE 80. 
 
 STALACTITES OF TRAVERTINE IN LURAY CAVERN, 
 
 VIRGINIA. 
 (U. S. Geological Survey.) 
 
DESCRIPTION OF STRATIFIED ROCKS 
 
 315 
 
 pact semi-crystalline, fibrous or concretionary, often 
 cavernous or spongy; sometimes in rounded mammillary 
 forms; in other cases more or less earthy. The color is 
 usually gray, but sometimes white, buff, reddish, bluish, 
 or even black. The simplest test for these phosphates is 
 to dissolve a powdered sample in nitric acid, and, after 
 filtering off the insoluble matter, to add an excess of 
 ammonium molybdate solution and ascertain by the 
 yellow precipitate if phosphorus is present. The general 
 chemical composition is shown in the following analyses 
 of samples from various localities in North Carolina. 
 
 Sand and insoluble matter 
 
 1.5 
 
 31 2 
 
 22.1 
 
 23.4 
 
 Carbonate of lime 
 
 12.0 
 
 15.9 
 
 42.1 
 
 64.3 
 
 
 71.8 
 
 42.1 
 
 20.5 
 
 11.2 
 
 Water and other constituents 
 
 14.7 
 
 10.8 
 
 15.3 
 
 1.1 
 
 Total 
 
 100.0 
 
 100.0 
 
 100.0 
 
 100.0 
 
 
 
 
 
 
 Phosphorites are widely distributed; in the United 
 States they are found extensively in the Carolinas, in 
 Florida, in Tennessee, and in some of the other states; 
 they occur in England and Wales, in Belgium, northern 
 France, and Russia. 
 
 COAL AND OTHER CARBONACEOUS ROCKS. 
 
 It is well known that, interbedded with other stratified 
 rocks of the different geological periods down to the present, 
 there occur layers of carbonaceous character, which under 
 the names of coal, lignite, etc., represent the remains of 
 former vegetable life, which once flourished where these 
 beds are now found. The formation of peat in modern 
 lakes, swamps, and bogs, and its occurrence in beds inter- 
 stratified in recent delta deposits with those of sands and 
 clays, as in the Mississippi delta, shows us how these beds 
 of coal were formed. For between the growing vegetation 
 of to-day, its change into peat, from this into lignite or 
 brown coal, and so on into bituminous coal, then into 
 
316 ROCKS AND ROCK MINERALS 
 
 anthracite, and eventually into graphite or practically 
 pure carbon, every step of gradual transition can be 
 traced. 
 
 The vegetable matter composing plants consists for the most pan; 
 of carbon, hydrogen, and oxygen. Its decay in the air, like combus- 
 tion, is a process of oxidation; the hydrogen goes off as water, the 
 carbon as carbon dioxide, the oxygen of the air assisting that in the 
 vegetable matter to effect the change. In this process most of the 
 carbon is removed. If the decay takes place under water, however, 
 the access of the oxygen of the air is prevented and the process 
 becomes much like that where wood is burned with a limited amount 
 of air to form charcoal. Some of the carbon unites with some of the 
 oxygen to form carbon dioxide; some of the hydrogen unites with 
 the rest of the oxygen to form water; the rest of the hydrogen unites 
 with some of the carbon to form marsh gas (methane) and the 
 remainder of the carbon is left behind. This can be illustrated 
 by the following equation in which the formula of cellulose, which 
 comprises the most important part of vegetable matter, is used. 
 
 Cellulose Carb. Diox. Water Methane Carbon 
 3H 2 O + CH 4 + 4C. 
 
 It is not intended to imply that this change takes place at once, 
 or is complete, under water; it goes on gradually, and as the CO 2 
 and CH 4 are evolved, the residual matter becomes richer in carbon 
 and poorer in hydrogen and oxygen. Thus vegetable matter is 
 converted into peat and this by compression and further change 
 into brown coal or lignite. The same process goes on in coal beds, 
 furnishing the deadly gases known to the miners as choke-damp 
 (CO 2 ) and fire-damp (CH 4 ), and lignite thus changes to bituminous 
 coal. Folding of the strata, with compression and heat, and the 
 consequent rupturing and Assuring of the overlying beds, which 
 permits the easy escape of the gases, hastens the process, and under 
 such circumstances the coal is changed to anthracite, which is much 
 richer in carbon, or even into graphitic coal which is practically 
 pure carbon. Thus the degree to which lignite has advanced through 
 bituminous coal to anthracite depends in part on its geological age, 
 and in part on the conditions to which it has been subjected. 
 
 Peat. This varies from a brown to yellowish matted 
 mass of interlaced fibrous material, strongly resembling 
 compressed tobacco, in which remains of plant leaves, 
 
PLATE 3L 
 
DESCRIPTION OF STRATIFIED ROCKS 
 
 317 
 
 stems, roots, etc., are still recognizable, in the upper por- 
 tion of the bed, to a dark brown, or black, compact, homo- 
 geneous mass appearing much like dark clay when wet, 
 in the deeper, lower parts. A dried, compact, very pure 
 peat from Germany is stated to have the following 
 composition: 
 
 Carbon. 
 
 Hydrpgen. 
 
 Oxygen. 
 
 Nitrogen. 
 1.0 
 
 Ash. 
 
 Total. 
 
 55.9 
 
 5.8 
 
 36.4 
 
 0.9 
 
 100.0 
 
 Under enormous pressure it has been found that peat may be 
 artificially converted into a hard, black substance like coal. The 
 wide distribution of peat and its use as a fuel are too well known to 
 need further mention. Its purity depends on the amount of clay 
 and sand mingled with it in the process of formation; even the purest 
 peat, like coal, has a small percentage of ash resulting from the 
 mineral constituents in the plants. 
 
 Lignite. Brown Coal. Usually a chocolate brown in 
 color, but varying to yellowish or black; compact and 
 firm to earthy and fragile; luster dull and soft to pitchy; 
 often shows distinctly the texture and grain of wood or 
 intermatting of vegetable fibers. Hardness varies from 
 1-2.5, the specific gravity from 0.7-1.5. It burns readily 
 with a smoky yellow flame, and strong odor. The carbon 
 in it varies from 55 to 75 per cent. A lignite from Ger- 
 many is stated to have the following composition which 
 will serve as an illustration. 
 
 Carbon. 
 
 Hydrogen. 
 
 Oxygen. 
 
 Nitrogen. 
 
 Ash. 
 
 Total. 
 
 57.1 
 
 4.6 
 
 36.0 
 
 0.2 
 
 2.0 
 
 99.9 
 
 Lignite belongs in the Cretaceous and Tertiary formations and 
 often forms considerable beds where these formations occur. It ie 
 
318 
 
 ROCKS AND ROCK MINERALS 
 
 found in small amount in the eastern United States in the Tertiary 
 at Brandon, Vermont, but in the Cretaceous deposits of the Rocky 
 Mountain states, and in the Dakotas it occurs in large and valuable 
 fields. It is found also on the Pacific Coast, and in Germany in 
 Europe; and elsewhere. Where better coal is not to be had it fre- 
 quently furnishes a valuable fuel. 
 
 Bituminous or Soft Coal. This is a compact, brittle 
 rock of a gray-black to velvet-black color. It has a 
 lamellar, conchoidal or splintery fracture; sometimes more 
 or less cubical. The luster varies from dull to pitchy; 
 the specific gravity from 1.2-1.5. It gives a black to 
 brownish-black streak. It burns with a yellow flame and 
 gives a strong bituminous odor. It often shows distinct 
 stratification through the varying luster of the different 
 layers. Generally there are no traces of organic struc- 
 tures visible to the eye. Some varieties fuse or sinter 
 together on heating, leaving a coherent residue or coke, 
 and are thus called coking coals; others fail to do this. and 
 fall to powder. The amount of carbon in a soft coal 
 varies from 75-90 per cent. A coking coal from Northum- 
 berland in England has been found to have the following 
 composition. 
 
 Carbon. 
 
 Hydrogen. 
 
 Oxygen. 
 
 Nitrogen. 
 
 Sulphur. 
 
 Ash. 
 
 Total. 
 
 78.7 
 
 6.0 
 
 10.1 
 
 2.4 
 
 1.5 
 
 1.4 
 
 100.1 
 
 The sulphur in coal comes from pyrite, which is a very common 
 impurity. Bituminous coals vary considerably in the relative 
 proportions of fixed carbon to volatile matter, that is in the pro- 
 portion of the carbon left behind on heating to the gases, tar, etc., 
 driven off; the latter may be as much as 30-40 per cent and such 
 coals are called fat coals and are used for the making of gas, coke, 
 etc. Those with 15-20 per cent volatile matter are largely used for 
 steam engines and are often called steam coals. They are transi- 
 tional to anthracite. 
 
 In addition to ordinary coal there are, depending on the physical 
 
DESCRIPTION OF STRATIFIED ROCKS 319 
 
 characters, a number of varieties which are well recognized. Thus 
 cannel coal is a dense, lusterless, highly bituminous form without 
 structure and generally showing conchoidal fracture. Jet is some- 
 what similar but characterized by its high luster, intense black color, 
 asphaltic appearance, and toughness, which permits of its being 
 readily turned and worked. Its use in mourning jewelry, buttons, 
 ornaments, etc., is well known. It occurs in small, scattered, isolated 
 masses in the later formations in various places, one of the chief 
 localities being at Whitby in Yorkshire, England. It is regarded 
 by some as representing water-logged fragments of originally 
 coniferous wood. Bituminous coal occurs in North America in 
 Nova Scotia; in the Appalachian coal field of western Pennsylvania, 
 Ohio, West Virginia, Kentucky, Tennessee, Alabama and Georgia ; the 
 Central coal field of Illinois, Indiana and Kentucky; in Michigan; the 
 Western field of Iowa, Missouri, Kansas, Arkansas. Oklahoma and 
 Texas. These are of Carboniferous age. In the Rocky Mountain 
 states and on the Pacific coast there are also large deposits of Creta- 
 ceous and Tertiary age. Elsewhere, in England, Belgium, Germany, 
 France and Russia, in South Africa, Australia, India and China, this 
 coal occurs and is mined in quantities. It is the chief coal of the 
 world, and the enormous increase in production in these later years 
 (in the United States from 137,000,000 tons in 1896 to 463,000,000 
 tons in 1922) points in no uncertain way to its exhaustion in a not 
 distant future. 
 
 Anthracite. Hard Coal. This is a compact, dense 
 rock, iron-black to velvet-black in color. It is brittle; 
 has a strong vitreous to sub-metallic luster, a more or less 
 pronounced conchoidal fracture, and a hardness of 2-2.5. 
 Specific gravity 1.4-1.8. Anthracites vary in the amount 
 of carbon they contain from 80-95 per cent; Pennsylvania 
 varieties from 85-93 and of Wales 88-95. The amount of 
 fixed carbon varies from 80-90 per cent; the volatile hydro- 
 carbons generally do not much exceed 5 per cent and the 
 remainder consists of moisture and ash. An analysis of 
 a Welsh anthracite is given as: 
 
 Carbon Hydrogen Oxygen Nitrogen Sulphur Ash Total 
 90.4 3.3 3.0 0.8 0.9 1.6 <= 100.0 
 
 Anthracite requires a strong heat for ignition and with abundant 
 access of air burns with a pale blue flame, giving great heat without 
 smoke or odor. These qualities, with its relative cleanliness, par- 
 
320 ROCKS AND ROCK MINERALS 
 
 ticularly adapt it to household purposes. Some anthracites exhibit 
 on broken surfaces a strong play of spectrum colors produced by 
 iridescent films and are called " peacock " coal. 
 
 Anthracite occurs not only in regions of folded strata as previously 
 stated, but also, though usually in no great quantity, where beds 
 containing ordinary bituminous coal or lignite have been invaded 
 by intrusive masses of igneous rock, as in New Mexico, Colorado, 
 Montana and Scotland. The largest and most important deposits of 
 anthracite are those of eastern Pennsylvania, a considerable part 
 of which has been already mined. It occurs also in Wales, Belgium. 
 France, Russia, and in the province of Shansi in China, as well as in 
 other places. 
 
 In addition to the carbonaceous rocks described above, 
 other carbonaceous and sometimes combustible substances 
 occur, such as graphite, ozokerite or mineral wax, asphalt 
 and various modifications of it, petroleum, etc., but not in 
 such a manner or relation that they may be properly in- 
 cluded in a work treating solely of rocks. Also between 
 the coals and the sandstones and shales, intermediate 
 types exist in great variety, but these are best treated 
 under the description of the latter rocks. 
 
 Sedimentary Deposits of Mechanical Origin. 
 
 These include the products of land waste by various 
 erosive agencies, which have been laid down in stratified 
 form, by moving currents of water in seas, lakes, and on 
 the flood-plains of rivers, and afterwards consolidated 
 into rocks, as described in the foregoing chapter. Accord- 
 ing to the size of the particles they are divided into the 
 gravel rocks or conglomerates and breccias, into the sand 
 rocks or sandstones, and into the mud or clay rocks, or 
 shales, as previously mentioned. 
 
 CONGLOMERATES AND BRECCIAS. 
 
 Conglomerates. These consist of pebbles of various 
 sizes, intermingled with a finer material which acts as a 
 cement. The pebbles may vary from the size of a pea up 
 to large boulders. They are rounded by the action of 
 
PLATE 32 
 
 A. CONGLOMERATE, OF SEDIMENTARY ORIGIN. 
 
 B. BRECCIA, OF SEDIMENTARY ORIGIN. 
 
DESCRIPTION OF STRATIFIED ROCKS 321 
 
 water. They may consist of any kind of rock, though 
 generally of the harder and more resistant varieties, or 
 they may be of a simple mineral, usually quartz or feldspar. 
 The pebbles may be all of one kind or of a mixture of 
 several kinds of rocks or minerals. The cementing mate- 
 rial may also vary greatly; it may be composed chiefly of 
 consolidated sand, either purely siliceous or mixed sub- 
 stances; it may be calcareous in nature, or chiefly com- 
 posed of clay, or of these substances largely mingled with 
 iron oxide. There may be a sharp distinction between 
 the relatively large pebbles, and the very fine matrix in 
 which they are enclosed, and if this contrast is pronounced 
 and the matrix present in considerable amount, such 
 conglomerates are often called pudding stone. On the 
 other hand there may be gradations in size from the peb- 
 bles down into the matrix. There is of course great vari- 
 ation in the color of these rocks; in some cases the pebbles 
 are sharply defined by their colors from the matrix; in 
 other cases the rock may have one general hue, alike for 
 pebbles and matrix this is more apt to be the case 
 where the rock has been somewhat changed or altered 
 from its original character. 
 
 Breccias. In a breccia the fragments which correspond 
 to the pebbles of a conglomerate, instead of being rounded, 
 are sharp and angular in character. (See Plate 32.) 
 This indicates, if the material has been laid down in 
 water, that they have suffered very little transport and 
 are close to their place of origin. In other respects what 
 has been said in regard to conglomerates will also apply 
 to brecciaso 
 
 Conglomerates and breccias, which are composed of a single type 
 of rock, are generally called by its name and we thus have quartz- 
 ite conglomerates and breccias, limestone conglomerates and 
 breccias, etc. Volcanic breccias, produced by the fragmental 
 accumulations of eruptive activity, are really igneous rocks and 
 have been already described (see page 269). The material may, 
 however, fall into water and be rounded, assorted, and stratified, 
 
322 ROCKS AND ROCK MINERALS 
 
 giving rise to volcanic conglomerates; such rocks are very difficult, and 
 sometimes impossible, to distinguish from conglomerates formed 
 by the erosion of such land areas as are formed for the most part ot 
 surface extrusions of lavas. 
 
 Breccias are sometimes produced as the result of the breakage 
 and grinding of the rock masses along some fault plane upon which 
 powerful movement is occurring. The fragments thus formed may 
 be afterwards cemented together into firm rock by deposits from 
 solutions circulating in the zone of crushed and broken rock. Such 
 types are called friction breccias and they naturally show no evi- 
 dence of stratification. 
 
 Conglomerates are normally formed from deposits laid down by 
 swiftly moving currents of water which tend to carry away the 
 lighter and finer material in suspension. Hence they represent the 
 deposits of rapid rivers and estuarine currents. Also, when a sink- 
 ing land surface passes under the sea and the edge of the latter 
 advances, a beach formation sweeps over the land as the initial 
 stage to later deposits. The waves throw the coarser material, 
 the gravel or shingle, toward the upper part of the beach and as the 
 latter sweeps inland a conglomerate is thus the first deposit laid 
 down on the new sea bottom. Thus it is general to find a conglom- 
 erate or coarse sandstone as the first member of a new series of 
 stratified rocks, resting upon an unconformable lower series, and in 
 thus marking divisions of geologic time they may be of great im- 
 portance. They are quite common rocks and are everywhere dis- 
 tributed in the sedimentary formations. 
 
 In the older, and 'especially in strongly folded mountain regions 
 where the strata have suffered great pressures and shearing, the 
 pebbles of conglomerates are generally distorted and flattened into 
 lenses, or drawn out into spindle-shaped forms. The process is 
 generally accompanied by mineralogical changes which may be 
 especially noticeable in the cement. This is the first stage in the 
 conversion of these rocks into gneisses and schists through meta- 
 morphism, as described in the following chapter. On account of 
 their coarse and irregular appearance and unhomogeneous char- 
 acter conglomerates have been little used for structural purposes, 
 except in the roughest stone work, as in foundations, piers, etc. In 
 some cases breccias, which are compact and capable of a good 
 polish, have been cut as ornamental stones, as a reddish conglomerate 
 breccia from South Dakota and a vari-colored limestone breccia 
 from Japan. Since the discovery of the wonderful gold deposits 
 in conglomerates in the Rand district, South Africa, these rocks 
 have received much attention, as representing possible fossil placers 
 in which, if the gold has been concentrated by natural processes, 
 available sources of the precious metal might be expected. 
 
DESCRIPTION OF STRATIFIED ROCKS 323 
 
 SANDSTONE AND RELATED ROCKS. 
 
 Sandstone. Typical sandstones are composed of grains 
 of quartz held together by some substance acting as a 
 cement. The size of grain may vary from that of peas 
 down to that of fine seeds; as they become finer the rocks 
 pass into shales, just as on the other hand they graduate 
 upward into conglomerates, and thus no sharp line can 
 be drawn between the three kinds. While some sand- 
 stones are very pure, consisting of quartz grains alone, 
 others contain intermingled particles of feldspar, garnet, 
 iron ore, tourmaline, flakes of mica and fragments of other 
 minerals. It can generally be observed with a lens that 
 the grains tend to be spheroidal, and that the larger they 
 are, the more perfect the rounding is apt to be. The 
 general appearance of many sandstones, with respect to 
 their granular texture, is much like that of loaf sugar. 
 As described under quartzite, to which reference should be 
 made, the fracture, in breaking sandstone, takes place 
 chiefly in the cement, leaving the grains outstanding, and 
 this gives the rock its sugary appearance and feeling. 
 
 Sandstones differ very much in regard to the cementing 
 material which holds the grains together, and thus different 
 varieties are produced. Sometimes it is deposited silica, 
 sometimes a carbonate commonly calcite, but on occa- 
 sion dolomite or siderite, sometimes extremely fine 
 argillaceous material or clay, and at other times deposited 
 oxides of iron, either reddish (hematite, turgite), or yellow- 
 ish (limonite). 
 
 The colors are very variable, white to gray, buff to 
 dark yellow, and brick-red to reddish brown and brown, are 
 common; green, purple and black are rare. These colors 
 depend largely on the nature of the cement ; in the yellow, 
 red and brown sandstones oxides of iron predominate, with 
 the other, lighter colors, it is apt .to be calcareous or argilla- 
 ceous. In addition, the calcareous sandstones are readily 
 detected by their effervescing when touched with acid, 
 
324 
 
 ROCKS AND ROCK MINERALS 
 
 while the argillaceous ones give the characteristic odor of 
 clay, when breathed upon. The green color is due to 
 glauconite, or in some cases admixed chlorite. Some 
 varieties appear to be almost devoid of a cement. 
 
 Sandstones are usually very porous rocks, and this 
 appears to depend to a large extent upon the amount and 
 character of the interstitial cement. Thus the ratio of 
 the volume of pore space to that of the rock has been 
 found to vary from 5 to almost 30 per cent, the latter 
 being about the greatest amount theoretically possible in 
 deposited sand grains. 
 
 The same characters also condition to a large degree 
 other physical properties and also explain their variations: 
 thus the weight per cubic foot varies from 125-150 pounds, 
 the crushing strength from 1500 to 15,000 pounds per 
 square inch. The specific gravity is about 2.6 (2.5-2.7), 
 with the rock pores filled with water, when weighed in it. 
 
 The chemical composition of sandstone varies con- 
 siderably; the chief element is silica, but the proportions 
 of the other elements depend on the nature of the asso- 
 ciated minerals and cement. Some analyses of promi- 
 nent sandstones used for building purposes are as follows : 
 
 
 Si0 2 
 
 A1 2 3 
 
 Fe 2 3 
 
 FeO 
 
 MgO 
 
 CaO 
 
 K 2 
 
 Na 2 O 
 
 H 2 O 
 
 CO 2 , 
 etc. 
 
 Total 
 
 I... 
 
 99.4 
 
 
 
 .3 
 
 _ 
 
 _ 
 
 _ 
 
 _ 
 
 _ 
 
 0.2 
 
 
 
 99.9 
 
 II.. 
 
 86.6 
 
 8.4 
 
 1.6 
 
 
 
 
 
 trace 
 
 2.4 
 
 0.7 
 
 
 
 
 
 99.7 
 
 III. 
 
 92.9 
 
 3.8 
 
 trace 
 
 0.9 
 
 trace 
 
 0.3 
 
 0.6 
 
 0.3 
 
 1.2 
 
 _ 
 
 100.0 
 
 IV . 
 
 69.9 
 
 13.6 
 
 2.5 
 
 0.7 
 
 trace 
 
 3.1 
 
 3.3 
 
 5.4 
 
 1.0 
 
 _ 
 
 99.5 
 
 V .. 
 
 87.1 
 
 3.9 
 
 1.3 
 
 
 
 1.1 
 
 2.7 
 
 1.3 
 
 0.8 
 
 0.5 
 
 1.4 
 
 100.1 
 
 VI .. 
 
 90.7 
 
 4.6 
 
 0.4 
 
 0.1 
 
 0.1 
 
 0.1 
 
 0.5 
 
 2.8 
 
 0.4 
 
 0.3 
 
 100.0 
 
 I, White, very pure Potsdam Sandstone, Ablemans, Sauk Co., 
 Wisconsin; II, Lake Superior Brownstone, Houghton, Bayfield Co., 
 Wisconsin ; III, Sandstone, light gray, Berea, Ohio ; IV, Brownstone, 
 Triassic, Portland, Conn.; V, Sandstone, Triassic, near Liverpool, 
 England; VI, Bunter sandstone, Heidelberg, Germany. 
 
PLATE 33. 
 
 A. SANDSTONE, OF FINE GRAIN. 
 
 B. LAMINATED SANDSTONE, WITH SLIGHT FAULTS. 
 
ij/'.$ '' " 
 
DESCRIPTION OF STRATIFIED ROCKS 
 
 325 
 
 The presence of the alkalies points to that of feldspar 
 (or mica) in the rock; in IV the amount of feldspar must 
 be large, and such a rock is to be classed as an arkose 
 rather than a sandstone. 
 
 The structure of sandstones is essentially that of the stratified 
 rocks. They are sometimes thinly laminated and fissile, and some- 
 times very thick bedded and within the individual bed may show 
 a very even texture and be practically free from any evidence of 
 stratification. Sandstones such as the latter are valuable for 
 structural purposes on account of their homogeneous character 
 and capability for cleaving or working equally in all directions; 
 they are often called freestones. 
 
 These rocks are frequently distinguished according to the 
 character of the cement or admixed material as described above; 
 thus there are calcareous sandstones, argillaceous sandstones, 
 ferrugineous sandstones and siliceous sandstones. Micaceous sand- 
 stones contain considerable muscovite; the tabular flakes are par- 
 allel to the bedding and induce a more or less ready cleavage in the 
 rock, giving it a fissile character; the cleavage faces are apt to be 
 somewhat silvery in appearance from the mica films coating them. 
 Grit is a term applied to coarse-grained sandstones whose particles 
 are in general more or less sharply angular, and whose cementing 
 material is, as a rule, quite siliceous. They have been considerably 
 used for grindstones and millstones, hence the term " millstone grit." 
 In siliceous sandstones it may happen that the deposited silica is 
 precipitated upon the rounded or angular quartz grains in crys- 
 talline position, thus reconverting them outwardly into crystals; 
 examination with the lens shows the crystal forms and faces of the 
 little regenerated quartzes; these are known as crystal sandstones. 
 
 Green sandstone is a variety full of grains of glauconite which 
 impart a general greenish color to the rock. Sometimes these 
 rocks are very friable, indeed scarcely coherent, as in the Cretaceous 
 formations of the Atlantic border, especially in New Jersey. They 
 are then called greensand or, inappropriately, greensand-marl. They 
 are apt to contain, in addition to the sand and glauconite, iron oxides 
 and fossil shells, either whole or fragmentary. These deposits have 
 been considerably used as fertilizers. Analyses of typical green- 
 sands from New Jersey are as follows: 
 
 SiO 2 
 
 P 2 5 
 
 SO 3 
 
 A1 2 3 
 
 Fe 2 O 3 FeO 
 
 MgO 
 
 CaO 
 
 K 2 
 
 HiO 
 
 Total 
 
 34.5 
 51.2 
 
 1.2 
 0.2 
 
 1.3 
 0.4 
 
 6.0 
 8.2 
 
 31.5 
 23.1 
 
 2.2 
 2.0 
 
 2.5 
 0.5 
 
 1.5 
 
 7.1 
 
 18.8 
 6.7 
 
 99.5 
 99.4 
 
326 ROCKS AND ROCK MINERALS 
 
 Arkose. This is a special variety of sandstone in which a notable 
 quantity of feldspar grains is mingled with those of quartz. Often 
 there is considerable mica present and, if the material is firmly 
 cemented, the rock to a casual glance may bear no small resemblance 
 to a granite. The particles are generally sharply angular, and the 
 feldspar is apt to be soft and more or less changed to kaolin. Under 
 a lens the irregular, clastic, angular shape of the particles readily dis- 
 tinguishes it from a granite. The mineral composition and the shape 
 of the grains show that the material has been derived from quickly 
 disintegrating granite and has suffered but a very short transport 
 before being deposited. Arkoses often grade into conglomerates 
 and breccias by increasing size of some of the particles. They 
 occur in all of the different geological formations. The red-brown 
 Triassic sandstones of New England are in large part arkose and 
 conglomerate or breccia. 
 
 Graywacke. These are sandstone-like rocks of a prevailing gray 
 color, sometimes brown to blackish, which, in addition to the quartz 
 and feldspar of an arkose, contain rounded or angular bits of other 
 rocks, such as fragments of shale, slate, quartzite, granite, felsite, 
 basalt, etc., or of varied minerals, hornblende, garnet, tourmaline, 
 etc. They are in reality fine-grained conglomerates and readily 
 pass into them by increase in size of some of the component particles. 
 The amount of cement, as in sandstones, is usually small and it is 
 generally argillaceous, but sometimes siliceous or calcareous. Such 
 rocks, when fine grained and compact and largely composed of feld- 
 spathic material, may be difficult in the hand specimen to distinguish 
 from some felsites, but close examination with a good lens will 
 generally show their nonhomogeneous character. The name has 
 been rather loosely used and has never had the vogue in America 
 that it has in Europe. 
 
 Uses of Sandstone. As is well known, sandstone is everywhere 
 used for constructional purposes. The ease with which it is worked, 
 and the large size of the blocks which may be quarried, make it 
 particularly valuable for this purpose. Thus in the United States 
 a very considerable portion of the buildings of the eastern cities are 
 wholly or in part of the red-brown sandstone, generally called 
 " brownstone," of the Triassic areas of the Atlantic border, while 
 for instance in Great Britain the city of Edinburgh is largely built 
 of the Carboniferous sandstones of that region. On account of the 
 insoluble nature of the iron oxide forming their cement, the red and 
 brown sandstones in moist climates retain much better the details of 
 fine cutting and carving for architectural effects, than do the 
 lighter colored gray or buff stones. The latter are liable to have a 
 calcareous cement, which dissolves under the action of atmospheric 
 agencies and water, allowing the stone to crumble, and thus in the 
 
DESCRIPTION OF STRATIFIED ROCKS 327 
 
 course of years the fine details of carving are spoiled. Many ex- 
 amples of this may be seen in the older cities where expensive and 
 beautiful buildings have been much injured. If possible, in building, 
 a sandstone should always be laid upon the quarry bed as it is then 
 much less liable to flake or spall. 
 
 Sandstones are of such wide and general distribution 
 in all parts of the world where stratified rocks are found, 
 that it is^unnecessary to give any detailed account of their 
 occurrence. 
 
 SHALE AND RELATED ROCKS. 
 
 Shale is the name given to compacted muds and clays 
 which possess a more or less thinly laminated, or fissile 
 structure. The parting is parallel to the bedding, and is 
 the result of natural stratification. When such rocks 
 have been subjected to folding and pressure, they assume 
 a slaty cleavage which has nothing to do with stratifica- 
 tion; they are then slates or phyllites and are described 
 among the metamorphic rocks. This distinction, that 
 rocks showing slaty cleavage are not shales, should be 
 clearly noted, as the two are often confused. 
 
 Shales are, in general, too fine grained for the component 
 particles to be determined with the eye, or even with the 
 lens. By microscopical and chemical analysis they are 
 known to be formed mostly of kaolin and related sub- 
 stances, with which may be associated much white mica, 
 but these are often accompanied by tiny fragments of 
 quartz and other minerals. As the amount of quartz 
 increases, and also the size of grain, the shales pass over 
 into sandstones, and such intermediate rocks represent 
 deposited silts. There are also all transitions between 
 clays and shales, depending on the relative firmness and 
 fissility of the mass. 
 
 Clay when dry is a fine, earthy, lusterless mass, giving a char- 
 acteristic odor when breathed upon. It clings to the tongue, and 
 when strongly rubbed to a powder between the fingers, it finally 
 produces a soft, greasy, lubricated feeling, usually thus differing 
 from loess, adobe, and similar appearing deposits. It absorbs water 
 
328 
 
 ROCKS AND ROCK MINERALS 
 
 eagerly and becomes plastic. When pure it is white, but it is 
 generally colored red or yellow by iron oxides, forming the red and 
 yellow ochers, or gray, blue or black by organic substance. The 
 colors are sometimes evenly distributed, and sometimes irregularly 
 blotched, through the mass. 
 
 Shales are apt to be soft, cut more or less readily with 
 the knife, and are brittle and crumbly, so that taken in 
 connection with the fissility, it is often difficult to prepare 
 hand specimens of them. Like clays they exhibit a great 
 variety of colors, white to buff or yellow, red to brown, 
 purple, greenish and gray to black, and from the same 
 causes. Different shades of gray are perhaps the most 
 common. They often contain various accessory mineral 
 substances, such as carbonates, gypsum, rock-salt, pyrite, 
 etc. Some of these are frequently seen in the form of 
 concretions, which may attain large size, up to several feet 
 in diameter. 
 
 The chemical composition is somewhat variable, depend- 
 ing on the relative proportions of clay and other minerals. 
 The following analyses will serve to show the general 
 chemical character. 
 
 SiO 2 
 
 A1 2 3 
 
 Fe 2 O 3 
 
 FeO 
 
 MgO 
 
 CaO 
 
 Na 2 O 
 
 K 2 O 
 
 H 2 O 
 
 XyO 
 
 Total 
 
 55.0 
 
 21.0 
 
 5.0 
 
 1.5 
 
 2.3 
 
 1.6 
 
 0.8 
 
 3.2 
 
 8.1 
 
 1.9 
 
 100.4 
 
 60.6 
 
 16.4 
 
 4.9 
 
 _ 
 
 1.4 
 
 1.6 
 
 0.9 
 
 3.0 
 
 9.7 
 
 1.5 
 
 100.0 
 
 61.2 
 
 15.6 
 
 1.4 
 
 3.0 
 
 4.2 
 
 3.4 
 
 0.4 
 
 6.7 
 
 2.7 
 
 1 . 1 
 
 99.7 
 
 53.6 
 
 17.6 
 
 4.1 
 
 3.7 
 
 5.2 
 
 2.3 
 
 2.5 
 
 2.2 
 
 8.5 
 
 0.2 
 
 99.9 
 
 I, Cambrian Shale, Coosa Valley, Cherokee Co., Alabama; 
 II, Cretaceous Shale, near Pueblo, Colorado; III, Devonian Shale, 
 Morenci district, Arizona; IV, Cretaceous Shale, Mount Diablo. 
 California. 
 
 XyO = Carbonaceous matter, CO 2 , and small amounts of other 
 substances. 
 
 There are many varieties of shales, depending chiefly on the 
 presence of accessory materials. Thus there may be a large amount 
 
DESCRIPTION OF STRATIFIED ROCKS 329 
 
 of organic matter, mostly carbon, present, and such are called car- 
 bonaceous shales. They are black in color, and by increase of car- 
 bon, grade into coaly shales, shaly coals and so on into coal. They 
 are a very common type, and are found associated with coal and also 
 independently of it, sometimes covering wide areas and of great 
 thickness. From the nature of the organic matter they are some- 
 times called bituminous shales. It is probable that the total amount 
 of carbon in the shales far exceeds that existing in coal beds. 
 
 In other varieties of shales large amounts of carbonates, especially 
 carbonate of lime, are present, and these are known as calcareous 
 shales. By increase of this substance they pass into shaly lime- 
 stones. They are apt to be associated with limestones and these 
 calcareous varieties are detected by their ready effervescence with 
 acids. Or the carbonate present may be chiefly carbonate of iron 
 and thus produce transition forms between shales and clay iron- 
 stone previously described. The connection between clays, shales 
 and marls has been mentioned on a previous page. Alum shale is 
 a variety full of pyrite, or of sulphates resulting from its alteration ; 
 it has been used for the manufacture of alum. 
 
 Uses of Clay and Shale. The use of clay in the making 
 of bricks, tiles, pottery, etc., is too well known to need 
 further comment. Shale has no value for structural pur- 
 poses, but in recent years, along with clay, it has become 
 of value and is used in many places as a material for the 
 manufacture of Portland cement, when mixed with the 
 proper proportion of limestone and burned. A pure, 
 clean shale or clay of the general composition shown in 
 analysis No. II, given above, is one best adapted for this 
 purpose, when combined with a non-magnesian limestone. 
 
 Clays and shales are such common rocks in all parts of 
 the world, where the unmetamorphosed stratified forma- 
 tions are found, that their occurrence needs no special 
 description. 
 
 Surficial Deposits. 
 
 This small group of geologic materials is of somewhat 
 diverse origin, and they are here included under this head- 
 ing largely as a matter of convenience. They would 
 include seolian deposits, or those made by the wind, and 
 those formed by the disintegration and decay of previously 
 
830 HOCKS AND ROCK MINERALS 
 
 existent rocks. They are appended here to the stratified 
 rocks, because they are in general closely connected with 
 them, and in many cases pass insensibly into them. 
 Many indeed, which might be classed here wholly or in 
 part, have already been described elsewhere because of 
 their close connection with other rocks. Thus volcanic 
 tuffs and breccias have been described under igneous rocks; 
 clays under shale, marl under carbonate rocks, sands in 
 Chapter VIII, etc. Most of these substances, under the 
 ordinary usage of the word, would not be considered rocks 
 at all, and their treatment entails matters of geological 
 interest rather than such as enter into a work of this char- 
 acter. This applies to such things, for instance, as soils, 
 talus heapings, morainal deposits from glaciers, etc. Their 
 description and mode of origin should be sought in the 
 handbooks on geology, or in special manuals. Only a 
 few of them, which from their widespread occurrence and 
 great importance as geological formations are of particulai 
 interest, are here included. 
 
 Loess. This is a deposit of a pale to buff yellow color, 
 running into brown, of an exceedingly fine grain; friable, 
 with scarcely the consistency of ordinary chalk when 
 coherent, and passing into looser forms, and of a rather 
 harsh feeling when rubbed between the fingers. It is of 
 a remarkably homogeneous appearance, and commonly 
 shows no signs of stratification, though this is sometimes 
 clearly seen. It has been found to consist chiefly of 
 angular grains of quartz, mixed with considerable amounts 
 of clay-like substances, tiny specks of other minerals, and 
 a calcareous cement, the amount of carbonate of lime 
 rising in some cases to 30 per cent. This latter produces 
 an effervescence in acid which quickly ends. The analysis 
 of a loess from Kansas City, Missouri, may be quoted to 
 show the general chemical composition. 
 
 SiO, Al 9 O 3 Fe 2 O 3 FeOMgOCaONa 2 OK 2 OH 2 OCO 2 XyO Total 
 74.5 12.3 3.3 0.1 1.1 1.7 1.4 1.8 2.7 0.5 0.4 =99.? 
 XyO = minute quantities of P 2 O 6 , TiO 2 , SO 3 MnO and C. 
 
DESCRIPTION OF STRATIFIED ROCKS 331 
 
 Loess occurs in widespread areas in the valley of the Mississippi, 
 in the states of Ohio, Indiana, Illinois, Iowa, Kansas, Nebraska, 
 Arkansas, Missouri, Tennessee, Kentucky, Alabama, Louisiana, 
 Mississippi, and Oklahoma. It is found also in Europe in various 
 places, especially in the valleys of the Rhine and its tributaries, 
 lying in isolated patches on the upper hill and mountain slopes and 
 in the same way in the Carpathians. It covers an enormous area in 
 northern central China with thicknesses attaining 1500-2000 feet, 
 and the yellow color which it imparts to the Hoang-ho (Yellow River), 
 and eventually to the Yellow Sea, into which the former discharges, 
 gives to these their names. 
 
 It is now generally accepted that the loess is an seolian, that is, 
 a wind blown deposit of dust which has accumulated through long 
 periods of time. This is shown by its lack of stratification, the 
 spread out manner in which it lies upon the surface, filling former 
 inequalities, the remains of land shells which are found in it, and 
 by the small, vertical tubes running through it caused by the roots 
 and stems of former vegetation. In places, however, where it has 
 been washed down into former lakes, ponds and streams, it becomes 
 stratified. In America and Europe, the material of the loess is 
 supposed to represent the finely ground rock powder of the glacial 
 ice sheet. 
 
 A characteristic feature is the common occurrence of concretions 
 of carbonate of lime and of oxide of iron. They often assume the 
 odd shapes seen in the flint nodules of chalk. The perpendicular 
 tubules give to the loess a vertical cleavage, which produces along 
 river banks bold bluffs. 
 
 Adobe. This name is applied to a very fine-grained, 
 coherent, yet friable material which covers wide areas in 
 the semi-arid and arid regions of western North America, 
 especially in the southwestern states and in Mexico. It 
 resembles loess in many ways, has usually the harsh feeling, 
 when rubbed between the fingers, and is of a yellowish, 
 yellow-brown, gray-brown or chocolate-brown color. Its 
 use in the form of sun-dried brick for building is well- 
 known. It is the result of the finer detritus of rock decay 
 on the higher slopes of hills and mountains accumulated 
 on the lower slopes, plains, valleys and basins, in part by 
 rain wash, and in part by the action of the wind in moving 
 it as dust. It forms a valuable soil when irrigated and 
 brought under cultivation. 
 
332 ROCKS AND ROCK MINERALS 
 
 Laterite. This is a red soil or deposit found in tropical 
 regions and is the result of the sub-serial decay of many 
 rocks, especially of granite. In the process the rocks lose 
 their alkalies and alkali-earths more or less completely, 
 and there remains a reddish, cellular mass, consisting of 
 quartz sand mixed with clay-like substance (chiefly 
 hydrargillite, A1(OH) 3 ) with iron oxides which give the 
 color. When dried it may become very hard and rock- 
 like. It frequently contains concretions of the iron oxides. 
 
 Loam. The common arable soils of the greater part of 
 the world are comprised under this heading. Loam con- 
 sists of a mixture of sand and clay, colored yellow, brown, 
 or reddish by iron oxides, or dark to black from organic 
 matter. The sandy particles are chiefly quartz, often 
 mingled with fragments of other minerals. On rubbing 
 between the fingers, it first feels harsh from the gritty sand 
 particles; if the rubbing is continued and these are allowed 
 to drop out, the greasy smooth feeling of the clay is finally 
 perceived. The proportion of organic matter varies very 
 greatly; the black soils of India and Russia are very rich 
 in it. 
 
CHAPTER X. 
 
 THE ORIGIN AND CLASSIFICATION OF META- 
 MORPHIC ROCKS. 
 
 Introductory. The metamorphic rocks are those which, 
 originally sedimentary or igneous, have been changed 
 either in mineral composition or in texture, or in both, so 
 that their primary characters have been altered, or even 
 entirely effaced. Here constantly, as elsewhere in geology, 
 gradations exist, and no definite line can be drawn on the 
 one hand between the sedimentary rocks and their meta- 
 morpnic products, or between the igneous rocks and the 
 metamorphic ones formed from them, on the other. Thus 
 loose chalks pass into limestones, and these into crystalline 
 marbles, just as dolerites merge into greenstones, and so 
 on into hornblende schists, without any sharp line of 
 d^markation. But there comes a point in the change of 
 each original rock, either of composition or of texture and 
 usually of both, where its characters and relations to other 
 rocks have become so individual that, for practical pur- 
 poses, it is best regarded as a distinct kind of rock. Where 
 this line shall be drawn must depend upon the experience 
 and judgment of the observer; in this work only those 
 cases are treated where the change has been so defi- 
 nite and pronounced as to produce typical metamorphic 
 rocks. 
 
 Rocks for the most part are composed of minerals, and 
 minerals for the most part are definite chemical combina- 
 tions, which are only, as a rule, permanent under stable 
 conditions. If the minerals are submitted to new condi- 
 tions, quite different from those under which they were 
 formed, with new chemical and physical factors operating 
 upon them, they will tend to change into other minerals, 
 
 333 
 
334 ROCKS AND ROCK MINERALS 
 
 that is, to turn into new chemical combinations, which will 
 be the most stable under the new conditions. A familiar 
 example is the decay of the feldspar of igneous rocks, and 
 its change into clay and other substances through the 
 action of water and carbon dioxide, as treated under 
 granite. The change in conditions may be so slight that 
 some rock minerals may be able to resist them indefinitely, 
 while others less stable may succumb. Thus igneous 
 rocks, formed by the cooling and crystallization of molten 
 magmas, may remain in the depths for millions of years, 
 and on coming to the surface through erosion and denuda- 
 tion, may be found entirely unchanged, or with only one 
 or two of the constituent minerals altered. At the surface 
 they are at once subjected to new conditions, to the com- 
 bined effects of changes of temperature, to moisture, the 
 various gases of the atmosphere, the products of organic 
 life, etc., and they commence to break up and to form into 
 new compounds. Then their ultimate conversion is only 
 a question of time. The same is true of the sedimentary 
 rocks, only in lesser degree. They are formed of mineral 
 particles, deposited in water and, usually, cemented by 
 pressure and deposits from solution. While they remain 
 deeply buried and under fairly stable conditions, they are 
 unchanged; when they are exposed to the atmosphere 
 they also tend to change and decay, especially in those 
 minerals that are susceptible. 
 
 All these changes which occur upon the surface are 
 strictly to be classed as metamorphic ones, and the prod- 
 ucts, in a geologic sense, are metamorphic rocks. But 
 for practical purposes all these materials formed by the 
 action of weathering and by the decay of rocks on or near 
 the surface, such as the soils, are not here included. They 
 have been previously mentioned under the foregoing rock 
 types, so far as seems desirable for the object of this work, 
 and only those rocks are treated as metamorphic which, 
 while buried at depth below the surface, have suffered, 
 through the action of certain agencies to be presently 
 
ORIGIN OF THE METAMORPHIC ROCKS 335 
 
 described, changes ; which have practically converted 
 them into new kinds of rocks. 
 
 Metamorphic Agencies. The chief metamorphic agen- 
 cies are mechanical movements of the earth's crust and 
 pressure, the chemical action of liquids and gases, and the 
 effect of heat. We may simplify these into the effects of 
 movement, water solutions, and heat, and all three of 
 these are required to produce complete metamorphism 
 in rocks, though not necessarily all to the same extent, 
 since sometimes one factor is more predominant, and 
 sometimes another. Thus in the metamorphism which 
 has been already described as contact metamorphism, 
 induced by the intrusion of a body of magma, the effect 
 of heat is the most important, that of gases and liquids 
 less so, while the effect of movements of the crust, or 
 pressure, is negligible. The rocks produced, however, are 
 actually metamorphic, but for practical reasons they have 
 been given separate consideration, and are not included 
 among these under treatment. We will consider the 
 different agencies separately. 
 
 Movement and Pressure. Pure simple downward pres- 
 sure, to the amount exerted in the upper part of the earth's 
 outer crust, appears to have little metamorphic effect. It 
 tends without doubt to consolidate the material of sedi- 
 ments by bringing the grains closer together, but many 
 instances may be cited of sediments, buried under great 
 thicknesses of deposits for geologic ages, which on being 
 raised and exposed by erosion without disturbance, such 
 as folding, are found to be practically in unchanged 
 condition. 
 
 On the other hand, as commonly supposed, through the 
 gradual contraction of the earth, the outer crust is under 
 compression, and this finds relief from time to time by 
 buckling or wrinkling up of the outer shell into mountain 
 ranges. This compressive force, thus acting with lateral 
 thrust, is therefore spoken of as erogenic, i.e., mountain 
 forming. By it whole masses of strata with possibly 
 
336 ROCKS AND ROCK MINERALS 
 
 included igneous rocks intrusive, extrusive and frag- 
 mental volcanic are folded, crushed, and mashed 
 together in the most involved and intricate manner. Not 
 only are the rocks then subjected to vast pressure, but 
 they are also subjected to enormous shearing stresses, 
 which tend to produce forced differential movements 
 among the rock particles. It is particularly this latter 
 effect which is of great potency in producing meta- 
 morphism. Its effects may often be seen megascopically 
 by the manner in which large crystals, included pebbles, 
 or fossils are flattened and elongated, or broken into frag- 
 ments which are drawn out into thin, lenticular masses 
 in the direction of shear. The microscope chows that 
 even minute crystals are broken, and their optical proper- 
 ties affected, as the result of the strain. It is possible 
 indeed, for this agency working alone to produce rocks 
 having the characteristic out r //ard metamorphic texture, 
 without any change in their riginal mineral composition, 
 but in combination with heat and water, it is of the highest 
 importance in inducing chemical changes, and the produc- 
 tion of new minerals. It is indeed a noticeable fact that 
 so long as the rocks retain their original position, they are 
 unaltered, but as we commence to find them disturbed 
 by erogenic forces, they begin to show signs of meta- 
 morphism, and in proportion to the degree to which they 
 have been folded up, mashed, and sheared, they become 
 more and more metamorphosed. 
 
 Heat. The effect of heat as a metamorphic agent is very 
 powerful, as is so well shown in local or contact meta- 
 morphism. It increases very greatly the solvent action 
 of solutions; it tends in many cases to break up existing 
 chemical compounds which form minerals, and to promote 
 new chemical arrangements. The heat needed for meta- 
 morphism may come from the interior of the earth, which 
 increases greatly with the depth; it may be supplied in 
 part by the transformation of energy resulting from the 
 movements, the folding and crushing of the rock masses. 
 
ORIGIN OF THE METAMORPHIC ROCKS 337 
 
 and in part it may result from intrusions of molten magma, 
 which are very liable to rise and invade the rock masses 
 as they are uplifted and folded. 
 
 Liquids and Gases. The chief of these is of course 
 water, which under heat and pressure becomes a powerful 
 chemical agency. It acts as a solvent, and promotes 
 recrystallization, and taking part in the chemical compo- 
 sition of some of the minerals, such for example as micas 
 and epidote, it is a substance necessary to their formation. 
 It is, without doubt, aided also in its action by substances 
 it may carry in solution, such as alkalies, and by volatile 
 emanations coming from magmatic intrusions, like boric 
 acid, fluorine, etc., as already explained under contact 
 metamorphism. It is this which explains the presence 
 in metamorphic rocks of such minerals as tourmaline, 
 chondrodite, and vesuvianite, which are characteristic of 
 pneumatolytic contacts, and of micas, hornblendes and 
 other minerals which contain fluorine. 
 
 Effect of Depth. The outer crust of the earth has been 
 divided by geologists into different zones, according to 
 the various geological processes at work. In the outer- 
 most one, down to the level afc which ground water stands, 
 the rocks are full of fractures, and are exposed to atmos- 
 pheric agencies moisture, carbon dioxide, oxygen, etc. 
 In this the rocks tend to decay, to be converted into car- 
 bonates and hydroxides, and to form soils. It is called 
 the belt of weathering, and is the one of rock destruction. 
 Below this lies another, in which the rocks are also full of 
 fractures and cavities filled with water. Its upper level 
 is that of ground water; below, it reaches to the point 
 where the pressure of the superincumbent masses and the 
 contraction of the crust becomes so great that all fractures 
 and openings are closed up, since the stress is so much 
 greater than the strength of the rocks, that they crush 
 under it, and are to be regarded as being in a relatively 
 plastic state. In this zone the chemical action of water 
 is most important, aided by the substances it may carry 
 
338 ROCKS AND ROCK MINERALS 
 
 in solution. The tendency is to change the minerals to 
 hydrates, and to a lesser amount to carbonates; thus 
 olivine, an anhydrous silicate, becomes converted into 
 the hydrous silicate, serpentine. Substances are taken 
 into solution and, reinforced by those leached out from 
 the belt above and carried down, are deposited in the 
 pores and fissures of the rocks; hence it is called by Pro- 
 fessor Van Hise the belt of cementation, because the rock- 
 grains are thus cemented together. 
 
 Below this lies the zone where, as stated above, the 
 pressure becomes so great that all openings are closed up, 
 and the rocks may be regarded as in a plastic condition. 
 Its upper level is variable and depends on geological con- 
 ditions; in times of quiet it may be as deep as six miles 
 below the surface; in times of mountain making, it may 
 rise much higher than this. Of what may be its lower 
 level, we know nothing. In this, the chief agencies are 
 the enormous pressure and the increasing heat of the 
 earth; the role played by liquids and volatile substances 
 is of less importance; the tendency is for them to be gotten 
 rid of, to be squeezed out. The chief work done in this 
 zone is molecular rearrangement, in which less stable 
 mineral compounds are broken up, and new ones of higher 
 specific gravity and smaller volume, through condensation, 
 are formed. Carbonates are converted 'nto silicates and 
 the carbon dioxide expelled; hydrated minerals have 
 their water driven out and new minerals, with less or no 
 water, are formed. This zone of rock flowage, in contrast 
 to the zone of fracture above it, has been called the zone of 
 anamorphism by Professor Van Hise. We may term it 
 the zone of constructive metamorphism. 
 
 It is chiefly in the lower part of the belt of cementation 
 (zone of fracture), and the upper part of the zone of rock 
 flowage, that the greater part of the work of metamorphism, 
 in the production of the metamorphic rocks as we see 
 them, is done. In the upper zone, the results are chiefly 
 those produced by dynamic shearing, and the imposing 
 
ORIGIN OF THE METAMORPHIC ROCKS 339 
 
 upon the rocks of characteristic textures. Chemical 
 work may be done and new minerals produced, but it is 
 possible for new textures to be formed without change 
 in mineral composition. In the lower zone, the work done 
 is largely chemical, new and more stable mineral combi- 
 nations being formed; and here also characteristic tex- 
 tures are produced. 
 
 Minerals of Metamorphie Rocks. Just as certain min- 
 erals, of which nephelite and sodalite might be mentioned 
 as examples, are characteristic of igneous rocks, so other 
 minerals are peculiar to the metamorphic ones, such as 
 cyanite, zoisite, staurolite and talc. Other minerals are 
 found in both groups alike, such as quartz, feldspar, horn- 
 blende, pyroxene, garnet and mica. It should be remem- 
 bered, however, that the names just mentioned are really 
 names of families, under which quite a variety of individual 
 mineral species may be grouped, on account of certain 
 common properties, such as crystal form. Thus in the 
 hornblende group, arfvedsonite is found only in igneous 
 rocks; tremolite and uralite occur practically only in meta- 
 morphic ones; common hornblende occurs in both. Of the 
 pyroxenes, the normal home of augite is in igneous rocks, 
 of wollastonite, a pyroxene-like mineral of the composition 
 CaSiOs, in the metamorphic ones; common pyroxene in 
 both. Of the micas, paragonite has been found only in 
 metamorphic schists, biotite and muscovite are present 
 in both groups of rocks, but muscovite is relatively rare in 
 fresh, normal, igneous ones. In the garnet group, pyrope, 
 the magnesia-alumina garnet, is formed only in igneous 
 rocks very rich in magnesia and low in silica, such as the 
 peridotites; 't occurs m them, or in the serpentines formed 
 from them, while grossularite, the lime-alumina garnet, 
 has its characteristic home in metamorphic limestones; 
 almandite and common garnet are found both in igneous 
 and metamorphic rocks. In the following list are given 
 the minerals which may occur in metamorphic rocks; the 
 first column contains those of wide distribution, and of 
 
340 ROCKS AND ROCK MINERALS 
 
 prime importance, as chief components; the second column, 
 those of lesser importance, which occur either as prominent 
 accessory minerals, or locally developed as chief compo- 
 nents; the third, occasional minerals, which may be at 
 times megascopically developed. But this is true only 
 in a general way, and over emphasis must not be laid on 
 these divisions. 
 
 I II III 
 
 Quartz Garnets Graphite 
 
 Feldspars Staurolite Tourmaline 
 
 Biotite Epidote Chrondrodite 
 
 Muscovite Zoisite Vesuvianite 
 
 Hornblendes Cyanite Hematite 
 
 Calcite Pyroxenes 
 
 Dolomite Magnetite 
 
 Chlorite Talc 
 Serpentine 
 
 Of these minerals, chlorite, serpentine, and talc are 
 specially characteristic of the upper zone, while cyanite, 
 staurolite, and some of the others are formed in the lower 
 zone. Some minerals, like quartr, and some members of 
 the groups may be formed in either zone, or be persistent 
 components of the original rocks. 
 
 Texture of Metamorphic Rocks. The metamorphic 
 rocks resemble the greater part of the igneous ones, in 
 that they possess a highly crystalline character, so much 
 so that they are frequently referred to as the crystalline 
 schists. On the other hand, they resemble the stratified 
 ones in possessing a parallel structure which may closely 
 resemble stratification. Thus they show analogies to 
 both of the other great rock groups. This parallel structure 
 expresses itself to a greater or lesser degree by a foliated, 
 laminated, or, as it is frequently termed, a schistose texture, 
 one in virtue of which the rock tends to split or cleave 
 more or less perfectly in the direction of a certain plane 
 passing through it. This direction of cleavage is called 
 the chief fracture, and the break of the rock at right angles 
 
ORIGIN OF THE METAMORPHIC ROCKS 341 
 
 to it is termed the cross fracture. Highly crystalline rocks 
 exhibiting this texture are called gneisses or schists, ac- 
 cording to their mineral composition, as described later. 
 While it is the characteristic texture for the metamorphic 
 rocks, there are a few, such as serpentine, marble 
 and quartzite, that for certain reasons to be explained, 
 may not show any trace of it, and yet are true meta- 
 morphic rocks. 
 
 Observation of the gneisses and schists shows that this texture 
 is due to arrangements of unlike mineral grains in layers, or very 
 flat lenses, or to a parallel arrangement of minerals having prismatic 
 or tabular forms, such as hornblende or mica, or to a mixture of 
 both. It is a result of the orogenic forces, the shearing and pressures 
 to which the original rocks have been subjected, and it makes no 
 difference whether these were igneous or sedimentary, this texture 
 may be imposed upon both alike under proper conditions. The 
 superficial resemblance, which the gneisses and schists bear to strati- 
 fied rocks in their parallel laminated character, for a long time led 
 geologists to think that the former must have been derived wholly 
 from the latter, and the general recognition that they contain former 
 igneous ones as well has come only in the last twenty-five years 
 through petrographic and chemical studies. From the fact that in 
 places stratified rocks could be traced into metamorphic ones and 
 the latter into igneous ones, it was even assumed that the igneous 
 rocks were in part derived from sediments by extreme metamorphism. 
 Such cases merely represent instances where both have been meta- 
 morphosed in common, with a remnant at either end which is not 
 metamorphosed, and whose original characters may therefore be 
 recognized. In the light of our present knowledge we should be 
 no more justified in tracing out such a deduction, than we would in 
 reversing it, and deriving the stratified rocks from the igneous ones 
 by metamorphic processes! 
 
 Varieties of Texture. Three chief varieties of the schis- 
 tose texture may be recognized, (1) the banded, in which 
 unlike mineral layers are in parallel bands, as shown in 
 Fig. A, Plate 34. This resembles stratification, but may 
 be induced in igneous masses as the result of shear. (2) 
 the lenticular, or foliated, in which some of the components 
 are collected into thinner or thicker lenses, around which 
 the other minerals tend to be wrapped or wound, as shown 
 
842 ROCKS AND ROCK MINERALS 
 
 in Fig. B, Plate 34, which shows a view of the cross frac- 
 ture. The surface of chief fracture in this case is apt to 
 be more or less lumpy, and not to show well the minerals 
 of the lenses Both this and the foregoing variety vary 
 greatly from coarse to fine. (3) The slat}/ texture is one 
 in which the mineral grains are extremely small, usually 
 too small to be seen with the eye, and often even with the 
 lens; the rocks appear dense, but they have the capacity 
 of splitting into thin slabs, as seen in roofing slates. 
 The cause of this is discussed under the description of 
 slates. 
 
 Metamorphic rocks frequently contain large and well- 
 developed crystals of minerals, which have formed as a re- 
 sult of the processes to which the rocks have been subjected. 
 These may be very much greater in size than the average 
 grain of the rock, and this contrast, together with the per- 
 fection of their crystal form, produces a strong analogy 
 to the porphyritic texture of igneous rocks. They are not 
 true porphyries, however, not only because the texture 
 is not of igneous origin, but also because these large crystals 
 are not of an older generation, but are actually of later 
 formation than the minerals of the apparent groundmass 
 in which they lie. It is therefore termed the pseudo- 
 porphyritic texture. That these minerals or pseudo-pheno- 
 crysts are of later formation is shown by the fact that they 
 frequently contain as inclusions the other rock minerals, 
 and sometimes the inclusions, such as bits of quartz, graph- 
 ite, etc., pass through the large crystal in the lines of 
 original stratification, and out beyond it. Moreover, it 
 may be frequently noticed that where these pseudo-pheno- 
 crysts are not equidimensional, but elongated, they may lie 
 in the rock pointing in all directions; their longer axes do 
 not necessarily lie in the direction of schistosity, like those 
 older minerals, which have been arranged by the pressure 
 and shearing. Having grown in the zone of pressure, 
 they are not oriented by it, unless subsequent and later 
 movement and shearing should take place after their for- 
 
PLATE 34. 
 
 A. BANDED GNEISS. 
 
 B- LENTICULAR OR FOLIATED GNEISS. 
 (Maryland Geological Survey.) 
 
ORIGIN OF THE METAMORPHIC ROCKS 343 
 
 mation. The space in the rock in which movement of 
 material goes on to produce these larger crystals is clearly 
 shown in Plate 35, which is a photograph of a garnet 
 in gneiss. Around the garnet is a zone of feldspar, 
 from which all the ferromagnesian minerals, visible be- 
 yond it, have disappeared, having been used up in its 
 formation. 
 
 The crystals described above should not be confused 
 with larger crystals or crystal masses in the rock, which 
 may also give it a porphyritic, appearance, but which are 
 really remains of former structures. Such may be former 
 phenocrysts of some porphyritic, igneous rock, or large 
 grains from some former coarse-granular igneous rock, or a 
 pebble from a conglomerate. They are apt to form ovoi'd 
 masses, and they are then really a pronounced case of the 
 lenticular texture, which is sometimes termed, following 
 the German name, augen (eye) texture. 
 
 Relation to Previous Textures, etc. In proportion to the 
 degree of metamorphism which rocks have suffered do we 
 find that the characteristic textures described above have 
 been imposed upon them. But not infrequently, as 
 though looking through the veil which metamorphism 
 has cast over them, we can see back of these features 
 remains of original textures and structures which are 
 characteristic of igneous and sedimentary rocks. Thus, 
 as indicated above, we may see that the original texture 
 was that of a porphyry, or we may find remnants of the 
 spherulites, lithophysae, and flow lines of some felsite 
 lava,or of the amygdules of some basaltic one; on the other 
 hand, ovoid masses of different mineral composition may 
 indicate a former conglomerate, or parallel layers, differing 
 in general mineral and chemical composition, may show 
 former stratified material. Such indications may be very 
 useful in ascertaining the former origin of a metamorphic 
 rock, and in some cases may positively identify it, but 
 deductions from this source should always be made tenta- 
 tively, and used with great caution, for there are many con- 
 
344 
 
 ROCKS AND ROCK MINERALS 
 
 fusing appearances of this kind which may lead to serious 
 error, unless they are checked by microscopic examination 
 and chemical analyses. 
 
 Chemical Composition. The chemical composition of 
 the metamorphic rocks is extremely variable, and it is 
 evident that this must be the case, when one considers the 
 heterogeneous materials from which they may be derived. 
 If we take them together, as a class of rocks, the com- 
 position, therefore, cannot have the significance which it 
 plays in the igneous ones, in showing their mutual rela- 
 tions. It may, however, be of great importance as an aid 
 in helping to determine their origin. Thus, in examining 
 the chemical analysis of a metamorphic rock, we may be 
 able to say that it is similar to those of known igneous 
 rocks and it may therefore have been originally of 
 igneous nature, and on the other hand the analysis may 
 show definitely that it could not have been any igneous 
 rock, and consequently it must have been of sedimentary 
 origin. 
 
 
 Si0 2 
 
 A1 2 3 
 
 Fe 2 O 3 
 
 FeO 
 
 MgO 
 
 CaO 
 
 Na 2 
 
 K 2 O 
 
 H 2 O 
 
 XyO 
 
 Total 
 
 I... 
 
 75.7 
 
 13.2 
 
 
 1.8 
 
 0.4 
 
 0.6 
 
 2.1 
 
 4.7 
 
 1.0 
 
 
 99.5 
 
 II.. 
 
 50.3 
 
 14.1 
 
 7.0 
 
 5.3 
 
 7.2 
 
 8.1 
 
 4.0 
 
 2.3 
 
 1.6 
 
 
 
 99.9 
 
 III. 
 
 74.7 
 
 8.9 
 
 9.6 
 
 
 1.9 
 
 1.1 
 
 0.4 
 
 1.0 
 
 1.1 
 
 0.5 
 
 99.2 
 
 I, Gneiss, near Freiberg, Saxony; II, Hornblende Schist (amphi- 
 bolite) Vestana, Sweden; III, Gneiss, near Rawdon, Quebec. 
 
 Thus, in the analyses given above, that of No. 1 might well be of an 
 ordinary granite, as may be seen by reference to those given under 
 granite; it might also, however, be that of an arkose derived from such 
 a granite. No. II has the composition of a gabbro; it might have 
 been such a rock originally, or a dolerite, or basalt ; it does not suggest 
 any ordinary sedimentary rock. No. Ill on the other hand has no 
 analogy among igneous rocks; the alkalies and alumina are too low 
 for the silica and the ferric oxide too high; it must be of sedimentary 
 origin and suggests an impure, ferruginous sandstone. 
 
ORIGIN OF THE METAMORPHIC ROCKS 345 
 
 It is inferred, of course, that while movement among the 
 molecules within limited distances has occurred, whereby 
 exchanges among the oxides are produced, involving 
 recrystallization and the formation of new mineral com- 
 pounds, the chemical composition of a rock mass as a 
 whole has remained unaltered. That this is so, is shown 
 by the fact, that in innumerable occurrences stratified 
 rocks, although utterly changed in mineral composition 
 from their former state, still retain the spacing and relative 
 volume relations of the strata which they originally had. 
 Thus one band of strata, perhaps only a fraction of an 
 inch in thickness, is sharply marked off by its grain, 
 minerals, and texture from those above and below it. 
 There has been no melting and no formal transfusion of 
 substance, consequently the changes which have occurred 
 are, so to speak, inward, those which lie within the range 
 of molecular attraction. To this general statement that 
 there is no change in mass composition in metamorphism, 
 there is one exception, and that is, that volatile substances, 
 liquids and gases, may be driven out and, conversely, new 
 ones may enter and pass into mineral combinations, as 
 previously explained under the action of liquids and gases 
 as agents. This is most strikingly seen, perhaps, in the 
 metamorphism of impure limestones, as described in the 
 section dealing with marble, and is thoroughly analogous 
 to what has already been stated under contact metamor- 
 phism. 
 
 Injection of Gneisses and Schists. It has been pre- 
 viously mentioned that part of the heat of metamorphism, 
 and of the liquids and gases involved in the production 
 of minerals, is supplied by intrusions of igneous magma, 
 which are particularly liable to rise and invade those 
 areas where crustal movements are starting metamorphic 
 agencies at work. In such areas, of course, the effect of 
 contact merges into that of general metamorphism and no 
 definite line can be drawn between them. Indeed the 
 earlier formed intrusions may themselves become more 
 
346 ROCKS AND ROCK MINERALS 
 
 or less metamorphosed, or have metamorphic textures 
 imposed upon them by repetitions of the processes, 
 and this may happen while they * are in a solid, or 
 yet partly plastic, condition. There is, however, another 
 function which these magmas, rising under great pressure 
 into rocks already schistose and foliated, may perform; 
 they may squeeze themselves in thin veins, sheets, and 
 lenticles into the schists surrounding them, so that these 
 rocks may become partly igneous, partly metamorphic, in 
 composition. And, as previously explained under con- 
 tact metamorphism and pegmatite dikes, these effects 
 may be greatly aided by liquid and gaseous emanations 
 from the magma masses. This process has been termed 
 the injection of schists by magmatic material and, although 
 as it has been doubted by some geologists, just as it has 
 been given entirely too general an application by others, it 
 has undoubtedly occurred in many places. By it we are 
 enabled to understand the veins, stringers, and lenses of 
 granite penetrating the schists in the neighborhood of 
 larger granite intrusions in many places, which would be 
 otherwise incomprehensible. In this connection also, it 
 should be remembered that the intrusive effects in the 
 lower zone of rock flowage may be expected to be quite 
 different from those in the upper zone of rock fracture. 
 
 Occurrence and Age. The metamorphic rocks have a 
 wide distribution over the earth's surface, and in many 
 places they occupy great areas, over which they are the 
 *>nly ones exposed. There is good reason also for be- 
 lieving that they form the basement upon which all the 
 later unmetamorphosed, sedimentary rocks rest. The 
 reason for this is, that wherever these later strata are 
 sufficiently eroded away, this metamorphic basement has 
 come to light. The only exception to this general dis- 
 tribution over the continental areas is in those places 
 where later intrusions of igneous magmas have come up 
 through them and are now exposed as bathyliths, stocks, 
 dikes, etc. But these constitute but a subordinate part 
 
ORIGIN OF THE METAMORPHIC ROCKS 347 
 
 of the total area. It is their extension over such wide 
 areas which has led to the processes, that have produced 
 them, being called regional metamorphism, in contrast to 
 the forming of the small zones around intrusive igneous 
 masses, which is therefore termed local metamorphism. 
 There is no difference in principle, however, between these 
 two, only in the relative intensity with which the varied 
 agents have operated. The metamorphic rocks are found 
 also in folded mountain ranges, of which they form the 
 interior core, and which subsequent erosion brings to light. 
 In proportion to the intricacy of the folding and mashing 
 of the strata, so is the degree of metamorphism increased. 
 This is so well established, that when we find areas where 
 the rocks are intricately folded and completely meta- 
 morphic, but not of any great elevation, we assume that 
 such an elevation formerly existed, but has been eroded 
 away, or in general that metamorphic rocks can only 
 become exposed at the surface through erosive processes. 
 It is these facts that led to the view, formerly held, that 
 metamorphic rocks must, geologically speaking, be of 
 very great age. This is, however, by no means necessarily 
 the case. For, on the one hand, we find unmodified 
 sands of Cambrian age in eastern Russia, and unaltered 
 beds of Ordovician age in the upper Mississippi valley, 
 which have not been changed from their original position, 
 while on the other, strongly folded strata of Tertiary age 
 in the Coast Range, in the Alps, and in other mountains, 
 are in places, profoundly metamorphosed. Rocks that are 
 metamorphic are likely to be old, but not necessarily so, 
 just as a battle-scarred soldier is likely to be a veteran, 
 rather than a recent recruit. It merely depends on 
 whether they have been subjected to metamorphic 
 processes or not, and the older they are, the more likely 
 they are to have suffered from them. Time, however, 
 is one of the great factors in metamorphism, and even in 
 the recent strata which have been changed, the time in- 
 volved, from our standpoint, is very long. 
 
348 
 
 ROCKS AND ROCK MINERALS 
 
 Classification of Metamorphic Rocks It would be 
 natural to classify the metamorphic rocks according to 
 the origin of their material, and to separate those of igneous 
 from those of sedimentary formation. In some cases this 
 may be done. Thus it is clear that marble is not of igneous 
 origin, but when we attempt to carry this principle 
 through, it quickly becomes impracticable, especially 
 if we can use only megascopic means of determination. 
 
 SiO 2 
 
 A1 2 3 
 
 Fe 2 3 
 
 FeO 
 
 MgO 
 
 CaO 
 
 Na 2 O 
 
 K 2 O 
 
 H 2 0, 
 etc. 
 
 Total 
 
 69.9 
 69.9 
 
 13.1 
 14.9 
 
 2.5 
 1.8 
 
 0.7 
 0.6 
 
 trace 
 0.6 
 
 3.1 
 1.5 
 
 5.4 
 5.3 
 
 3.3 
 3.9 
 
 1.0 
 1.3 
 
 99.0 
 99.8 
 
 Thus in the two analyses given above, the upper one is 
 that of the Portland sandstone of Connecticut, a fine- 
 grained arkose full of feldspar, the lower one that of an 
 intrusive granite porphyry from the Crazy Mountains, 
 Montana. It is evident that if these two rocks, one sedi- 
 mentary, the other igneous, should be so thoroughly 
 metamorphosed as to lose all traces of their original 
 textures, it would be impossible to discriminate them from 
 one another, or to say what their original status was. 
 
 Remembering the simple primary classification of the 
 sedimentary rocks previously given, it is possible, in a very 
 general way, to show the relation between the most com- 
 mon ones and their metamorphic derivatives in the fol- 
 lowing table: 
 
 Sediments. 
 
 Compacted Strata. 
 
 Metamorphic Rocks. 
 
 Gravel 
 
 Conglomerate 
 
 Gneiss, and various schists 
 
 Sand 
 
 Sandstone 
 
 Quartzite " " 
 
 Silt and Clay 
 
 Shale 
 
 Slate " " 
 
 Lime deposits 
 
 Limestone 
 
 Marble " " 
 
 
 
 
ORIGIN OF THE METAMORPHIC ROCKS 349 
 
 In the case of the igneous rocks, recalling that they may 
 be roughly divided into two main groups, the one chiefly 
 feldspathic, and the other mainly of ferromagnesian 
 minerals, we can illustrate also, in a very rough and 
 general way, the relation between them and their meta- 
 morphic derivatives in the following table : 
 
 Igneous Rocks. 
 
 Metamorphic Rocks. 
 
 Coarse-grained feldspathic 
 types, such as granite, etc.. . 
 
 Gneiss. 
 
 Fine-grained feldspathic types, 
 such as felsite tuffs etc 
 
 Slate and Schists. 
 
 
 
 Ferromagnesian rocks, such as 
 dolerites and basalt 
 
 Hornblende-, Talc-, 
 (etc.), Schists and 
 
 
 Serpentine. 
 
 A comparison of the two tables will show that gneisses 
 and schists may have diverse origins, and the reason for 
 this has been previously pointed out. 
 
 Another method of classification which has been recently sug- 
 gested is, disregarding the origin of the material entirely, to consider 
 only its chemical composition. According to this the metamorphic 
 rocks are divided into groups. The earth's crust is divided vertically 
 into zones, somewhat as described above, and the effect of the meta- 
 morphism in these zones upon each group is considered. It is found 
 that material of a given composition yields rocks, differing in mineral 
 composition and texture, according to the zone in which the meta- 
 morphism occurred. Thus the first grouping is a chemical one, while 
 the subdivisions are mineral and metamorphic, and in this way the 
 different rocks are produced and classified. 
 
 While this method may be consistent and based on scientific 
 principles, it is not a practical one for field and megascopic use. We 
 cannot make analyses of rocks under ordinary circumstances, nor 
 can we, in most cases, even estimate megascopicahy the chemical 
 composition from the minerals they contain, as can be done with 
 the microscope and thin sections. And the different mineral com- 
 positions and textures pass into one another so gradually, that only 
 
350 ROCKS AND ROCK MINERALS 
 
 in a very general way, or in specific cases, can we say whether the 
 rocks have been metamorphosed in the zone of fracture, or the zone 
 of flowage. 
 
 At present we are obliged, for practical purposes of field 
 work and megascopic determination, to classify quite arbi- 
 trarily the metamorphic rocks according to their evident 
 mineral composition or texture, or a combination of both. 
 Sometimes, as in the gneisses, stress is laid upon the first 
 feature; sometimes, as in the slates, upon the second one, 
 in accordance with whichever one is the most evident and 
 characteristic. 
 
 We have in agreement with this the following main 
 groups of metamorphic rocks. 
 
 Grouping of Metamorphic Rocks. 
 
 1. Gneisses and Feldspar Rocks. 
 
 2. Mica-schist and Quartzite. 
 
 3. Slates and Phyllite. 
 
 4. Talc and Chlorite Schists. 
 
 5. Hornblende Schist. 
 
 6. Marble, Lime carbonate-silicate Rocks. 
 
 7. Dolomite, Magnesian carbonate-silicate Rocks. 
 
 8. Serpentine. 
 
 9. Iron oxides and other rocks. 
 
 By comparison it may be seen that the above is in the 
 main a combination of the two tables previously given. 
 The more important of the members are given in italics. 
 
CHAPTER XI. 
 
 DESCRIPTION OF METAMORPHIC ROCKS. 
 GNEISS. 
 
 THE term gneiss is not only the name of a particular 
 kind of metamorphic rock, but also, in a wider sense, it is 
 used as an expression of a certain texture. Thus when we 
 use gneiss as a name in the limited sense, we mean a rock 
 which has the composition of granite quartz, feldspar, 
 and mica with a certain foliated texture; if we say 
 granite-gneiss, syenite-gneiss, diorite-gneiss, we use it in 
 the wider sense, and denote rocks whose composition is 
 indicated by the first word, and the texture by the second. 
 The only general definition of gneiss which will cover all 
 cases is, that they are metamorphic rocks, composed of 
 feldspar, with other minerals, which have a certain char- 
 acteristic texture. But, as everywhere generally used 
 when no qualifier is prefixed, common gneiss, which is 
 composed of quartz, feldspar and mica, as stated above, 
 is understood, and the term is so employed in this book. 
 If the wider sense is meant the qualifier is given. 
 
 Mineral Composition. Various kinds of feldspar are 
 found in gneisses, both the alkalic and soda-lime varieties, 
 but they can rarely be distinguished by megascopic 
 means. The mineral is white to gray in color, or reddish, 
 as in granite, and is apt to be in more or less round, or 
 elongated, lenticular, formless grains; this lack of definite 
 form makes it more difficult to distinguish from the quartz 
 than in most granites, and the cleavage should be carefully 
 sought. Sometimes large grains, the size of a pea, or even 
 larger, occur, giving the gneiss a porphyritic character; if 
 the cleavages of these are examined against the light, it 
 
 351 
 
352 ROCKS AND ROCK MINERALS 
 
 may be often observed that they are Carlsbad twins. 
 Such large crystals may indeed have been the phenocrysts 
 of a former porphyritic granite, or they may have been 
 feldspar pebbles of a conglomerate or arkose, or they may 
 have been made by injected material. 
 
 The quartz is also in more or less round grains or lentic- 
 ular masses, or in granular aggregates with the feldspar. 
 Its color is white or gray, sometimes reddish, rarely bluish. 
 In the larger grains it is easily recognized by its greasy 
 luster and conchoidal fracture. 
 
 The mica may be either biotite or muscovite, or a mix- 
 ture of both. The biotite is black or dark brown, the 
 muscovite is white or yellowish to light brown, sometimes 
 pale green. The mineral does not have any distinct crystal 
 form, but is in flakes, shreds or irregular leaves, drawn out 
 in bands, or in thin patches. It usually lies stretcher 1 
 out along the structure planes of the rock, and in large 
 part its easy cleavage, thus arranged in one direction, con- 
 ditions the schistosity or cleavage, and gives emphasis to 
 the gneissoid texture. Thus the surface of chief fracture 
 of a flake of the rock may appear to be largely coated 
 with mica, and, judging from this alone, one would be apt 
 to gain an exaggerated idea of the relative amount of it in 
 the rock; the surface of cross fracture should also be 
 examined to gauge correctly its relative amount, as com- 
 pared with the other mineral constituents. This is also 
 especially true, in the mica schists, and in those gneisses, 
 which, by decrease of feldspar and increase in mica, form 
 transitions into these latter rocks. This effect is also 
 more marked in many gneisses, because there is a tendency 
 for the quartz and feldspar to be collected in layers, which 
 alternate with layers of mica. 
 
 Hornblende may occur in gneisses, sometimes associated with the 
 mica, sometimes alone, forming a special variety. It is seen in dark, 
 prismatic crystals without good terminations, as in granite, syenite, 
 etc. Minute crystals may be aggregated into flattened lumps and 
 layers. 
 
PLATE 35. 
 
 A. GARNET IN GNEISS, WITH ZONE 
 OF GROWTH. 
 
 B. GNEISSOID CONTORTED SCHIST. 
 (U. S. Geological Survey.) 
 

DESCRIPTION OF METAMORPHIC ROCKS 353 
 
 Besides these, many other minerals may occur in gneisses, 
 sometimes so prominently as to form special varieties. Of these 
 garnet, of a dark red common variety, is perhaps the most con- 
 spicuous. The crystals are sometimes large, as compared with the 
 size of the other rock constituents. Epidote may also be discovered, 
 as well as graphite, in some varieties. Sillimanite, a mineral with the 
 same composition as andalusite and cyanite, is sometimes seen in 
 gneiss, in bundles and brush-like groups of slender fibers or prisms. 
 Tourmaline occurs also under circumstances similar to those which 
 obtain in granite. In some gneisses the mica may be partly, or 
 wholly, replaced by chlorite, usually from alteration. 
 
 Texture. This has been already described in large part 
 under the general remarks on metamorphic rocks and what 
 has been said above respecting the mica. The essence 
 of the texture consists in the layers of mingled quartz 
 and feldspar, which are separated by drawn out layers of 
 mica. Where the amount of mica is small, the gneissoid 
 texture is less evident, and it increases with the increase 
 of mica. Sometimes these layers are thick and coarse, 
 giving a pronounced gneissoid effect, sometimes the layers 
 are extremely thin. In some cases the layers continue 
 their individual character for considerable distances, in 
 others they are very short, lenticular, and are closely 
 interlaminated. According to these appearances, dif- 
 ferent varieties of gneiss have been named on a textural 
 basis. The gneissoid texture is sometimes scarcely per- 
 ceptible in a hand specimen, but clearly seen on a large, 
 exposed surface of the rock. This is especially the case in 
 rocks which were originally granites, but which, by pres- 
 sure and shearing, have been converted into gneiss. 
 
 The texture described above, the banding or schistosity, 
 may extend for long distances in straight, regular lines, 
 or it may be curved, folded, contorted, or faulted, often in 
 the most complex and remarkable manner, and on any 
 scale, even to a very minute or even microscopic one. Ex- 
 amples of such intricately folded and compressed gneisses 
 are seen on Plates 35 and 36. Such folding testifies in 
 general to repeated dynamic movements, with shearing 
 
354 ROCKS AND ROCK MINERALS 
 
 and folding, the earlier ones producing the gneissoid 
 structure and the later ones crumpling it up, though it is 
 possible that in some cases the two things are simultaneous, 
 In some gneisses, as in some granites, a definite por- 
 phyritic texture may be present, with large and definite- 
 crystals of feldspar, which show more or less distinct 
 crystal form. 
 
 Such gneisses are to be generally regarded as originally porphyritic 
 granites, which have had the gneissoid texture imposed upon them, 
 though in some cases, it may be, that the large crystals have been 
 formed in gneiss of a different origin by growth from injected material. 
 Such gneisses are allied to, and may pass over into, types, which, with 
 a short, thick, lenticular texture, contain ovoid masses of feldspar or 
 quartz. The ovoid bodies are called " eyes " (German, augen\ 
 and the rocks containing them " augen-gneiss '' from the German 
 name, or " eyed-gneiss." As explained on a previous page on the 
 texture of metamorphic rocks, they may be of quite diverse origin. 
 
 In some gneisses are to be seen pebbles of various kinds 
 of previously existent rock masses, of granite, quartzite, 
 etc. They are apt to be drawn out into flattened lenticu- 
 lar masses, but their original character is evident, and it is 
 clear that the gneiss in such a case was originally a con- 
 glomerate, whose finer material has been metamorphosed, 
 leaving the larger pebbles mostly unchanged, save in 
 shape. 
 
 Color. The color of gneisses is too variable a feature to 
 be of any value as a special character. It depends on the 
 color of the quartz and feldspar, and on the relation of these 
 to the amount of biotite, or other dark colored minerals, 
 they may contain. Also, in gneisses of sedimentary origin, 
 carbonaceous material may be present and in the form of 
 graphitic material color the rock very dark. Hence we 
 find them from almost white passing through light shades 
 of red or gray into darker ones, into brown and green, 
 and even black. 
 
 Chemical Composition. As the sources of the material 
 from which gneisses have been made are varied, so do we 
 
DESCRIPTION OF METAMORPHIC ROCKS 
 
 355 
 
 find great variability in their chemical composition, so 
 much so that this character cannot be relied upon as 
 having any special value as one of their definite features. 
 Since they are composed of quartz and feldspar in 
 notable amount they must contain silica, alumina and 
 alkalies, and they usually have also more or less iron and 
 lime, but these oxides may vary within wide bounds, as 
 may be seen from the following table of analyses of a 
 few typical gneisses. 
 
 
 SiO 2 
 
 A1 2 3 
 
 Fe 2 3 
 
 FeO 
 
 MgO 
 
 CaO 
 
 Na 2 O 
 
 K 2 
 
 H 2 O 
 
 XyO 
 
 Total 
 
 I.. 
 
 71.0 
 
 15.0 
 
 1.1 
 
 1.8 
 
 0.7 
 
 0.3 
 
 2.5 
 
 5.8 
 
 1.1 
 
 0.5 
 
 99.8 
 
 II.. 
 
 65.1 
 
 16.4 
 
 0.9 
 
 5.6 
 
 2.4 
 
 2.4 
 
 3.3 
 
 1.9 
 
 0.7 
 
 1.3 
 
 100.0 
 
 III. 
 
 78.3 
 
 10.0 
 
 1.8 
 
 1.8 
 
 1.0 
 
 1.7 
 
 2.7 
 
 1.3 
 
 1.0 
 
 0.9 
 
 100.5 
 
 IV. 
 
 82.4 
 
 11.3 
 
 1.0 
 
 0.3 
 
 0.2 
 
 0.2 
 
 0.6 
 
 1.0 
 
 2.5 
 
 0.2 
 
 99.7 
 
 V .. 
 
 52.2 
 
 18.8 
 
 2.7 
 
 5.3 
 
 5.1 
 
 8.0 
 
 3.3 
 
 1.6 
 
 1.4 
 
 1.7 
 
 100.1 
 
 VI. 
 
 44.5 
 
 17.5 
 
 3.4 
 
 12.6 
 
 5.7 
 
 3.3 
 
 3.6 
 
 3.5 
 
 1.6 
 
 5.2 
 
 100.9 
 
 I, Granite-gneiss, Lincoln, Vermont; II, Garnet-biotite-gneiss, 
 Fort Ann, Washington Co., N. Y. ; III, Gneiss, fine grained, Great 
 Falls of Potomac River, Md.; IV, Schistose gneiss, Marquette 
 region, Michigan. V, Plagioclase-Gneiss, Mokelumne River, Cali- 
 fornia; VI, Gneiss (Kinzigite), Schenkenzell, Black Forest, Baden. 
 
 On the other hand, as stated in the introduction to 
 metamorphic rocks, the analyses may suggest a clue to 
 the origin of the material. Thus analyses III, IV, and VI 
 above are quite unlike those of any igneous rock, and are 
 almost certainly of material of sedimentary origin, while 
 the others may be of igneous derivation. 
 
 Varieties of Gneiss. A very great number of varieties 
 of gneiss have been distinguished by geologists and 
 petrographers. These have been based, partly on dif- 
 ferences in texture, such as " banded-gneiss," " lenticular- 
 gneiss," " augen-gneiss," etc., partly on the presence of 
 some characteristic or unusual mineral, such as " biotite- 
 gneiss," " hornblende-gneiss," " epidote-gneiss," etc., and 
 
356 ROCKS AND ROCK MINERALS 
 
 partly on general composition, such as " granite-gneiss/' 
 " diorite-gneiss," etc. In the latter case the term, as 
 explained in the introductory paragraph, is used in the 
 sense of a general textural modifier. It would not be 
 suitable in a work of this kind to give a description of all 
 these varieties, but a few of the most prominent may be 
 mentioned. The textural modifications have been already 
 sufficiently considered under the heading of texture. 
 
 Of mineralogic varieties, by common gneiss, or " gneiss " for short, 
 mica-gneiss is meant. If further distinction is required, the kind of 
 mica present may be stated, and we thus have biotite-gneiss, mus- 
 covite-gneiss, or biotite-muscovite-gneiss. If the mica is accompanied 
 or replaced by some prominent mineral, as is often the case, other 
 varieties are formed, such as hornblende-gneiss, epidote-gneiss, tour- 
 maline-gneiss, garnet-biotite-gneiss, etc. The different prominent 
 minerals, which may thus take part, have been already described 
 under composition. Of the varieties based on general composition, 
 it may be said that all of the different varieties of coarser-grained, 
 feldspathic, igneous rocks may occur with pronounced gneissoid 
 texture. In accordance with this we have granite-gneiss by far 
 the most common variety syenite-gneiss, diorite-gneiss, and even 
 gabbro- and anorthosite-gneisses. Sometimes this texture has been 
 imposed upon the igneous rocks after they had solidified, by intense 
 pressure and shearing, and sometimes while they were still soft, pasty 
 and crystallizing, by forced differential flowage, due to various causes. 
 
 Inclusions in Gneiss. It is very common to find 
 inclusions, or smaller rock masses, embedded in gneiss, 
 which differ in a marked degree in mineral composition, 
 texture, etc., from the main rock body which encloses 
 them. Thus lenticular masses of quartz frequently occur, 
 and of very variable size. They may be the remains of a 
 quartz pebble of a conglomerate, as explained under 
 texture, or they may have been deposited from solution 
 in some lenticular cavity, opened in the folding of the rock 
 masses. This case may sometimes be detected, in that 
 the quartz mass tends to possess a comb structure, being 
 composed of an aggregate of crystals whose prism direc- 
 tions are set perpendicular to the wall of the cavity. 
 
PLATE 
 
 8 
 
DESCRIPTION OF METAMORPHIC ROCKS 357 
 
 In many gneisses irregular spots, streaks, and lines of 
 pegmatite occur, similar to those in granite. In addition 
 to the quartz, feldspar and mica, they often contain the 
 accessory minerals seen in granite-pegmatite dikes, such 
 as tourmaline, apatite, beryl, garnet, topaz, etc. In the 
 latter case they probably represent the remains of former 
 granite-pegmatite dikes, which have been folded up or 
 squeezed out in dynamic metamorphic processes, but 
 not all of the pegmatitic modifications seen in gneiss are 
 to be certainly ascribed to such an origin, for they may 
 have been produced by secretions from later solutions of 
 heated waters moving through the rock mass. The 
 beautiful crystals of orthoclase, of the varieties called 
 adular and moonstone, occurring in some gneisses, have 
 been probably produced in this way. 
 
 Also there are frequently seen in gneiss, spots, streaks 
 and irregularly curved and winding ribbons of white or 
 pink felsite, or fine-grained granite similar to the aplite of 
 granites. These may be former aplite dikes folded up, 
 or later granitic intrusions or secretions from heated solu- 
 tions. They are sometimes seen in the most complicated 
 systems of network passing through the rock, and 
 they may not have any definite wall against the gneiss, 
 as is the case with regular aplite dikes. By their fold- 
 ings, faultings, and contortions, they often show very 
 clearly the movements which the general rock body has 
 undergone. 
 
 Included masses of other kinds are also frequently met 
 with in gneiss. Thus the streaks and smears, produced by 
 aggregates of the dark-colored or ferromagnesian minerals, 
 such as areseen in granites and are described as "schlieren," 
 are found in gneiss, and may have a similar origin. Also, 
 irregular masses, strips, and lenticular bodies of other 
 schists occur, which, if the gneiss has been derived from a 
 former mass of igneous rock, may have been included or 
 enveloped fragments of the stratified beds, into which it 
 was intruded. 
 
358 ROCKS AND ROCK MINERALS 
 
 While the study of thin sections under the microscope is often of 
 great assistance to the field study of a gneiss, in the endeavor to 
 ascertain its origin and to thus understand better its relation to other 
 rocks, it is by no means always necessary. Very much may be done 
 by careful observation in the field of all the facts ascertainable, and 
 by the thoughtful correlation of these facts with one another. 
 From place to place the rock should be minutely studied with the 
 lens and any change in mineralogical composition or texture noted. 
 The following embody some of the chief points which should be looked 
 for, to distinguish rocks originally igneous from those of sedimentary 
 origin. The igneous ones are more apt to have a uniform composi- 
 tion and texture over large areas. The region of the contact with 
 other rocks should be carefully observed, to see if there are any 
 remains of a former endomorphic contact visible, such as a diminish- 
 ing of grain, or the assumption of a porphyritic texture, as well as 
 the appearance of pneumatolytic minerals, of which tourmaline 
 may be cited as a specially important example. The remains of 
 former aplite dikes and pegmatite veins, as described above, should 
 also be noted in this connection. The enveloping or bordering rocks 
 should be carefully studied to see if, by change in mineral composition, 
 in texture, and in the presence of tourmaline, or other pneumato- 
 lytic minerals, any remains of a former aureole of contact metamor- 
 phism may be discovered. The character of the plane of contact 
 of the gneiss and its neighboring rocks should be examined to see, 
 if possible, whether they are interwoven, as contorted interlaminated 
 beds might be expected to be, or whether the gneiss cuts directly 
 across them. In fact all of the field characters indicative of intru- 
 sion, which have been described under granite, should be looked 
 for, under the veil which metamorphism has cast over the region 
 under study. They may, of course, have been entirely obliterated, 
 but some of them may persist and be valuable indicators. 
 
 In sedimentary gneisses, on the other hand, more rapid changes, 
 from place to place, in composition and texture may be looked for, 
 both on a large and on a minute scale. The remains of former 
 pebbles, or small, lenticular masses of different composition indicative 
 of them, should be sought for. The presence of carbonaceous matter, 
 or graphite, diffused through the rock, or collected in spots or streaks, 
 is also of use in indicating this origin. The absence of any of the 
 signs of intrusion, and the character of the contact, as mentioned 
 above, may also be of value in this connection. Not too much stress 
 must be placed on the mere presence of felsitic and pegmatitic 
 veins or dikes, as these may have been injected into sedimentary 
 rocks, as well as into igneous ones. Their character, number, dis- 
 position, and contact wall must also all be considered in relation to 
 the rock mass they accompany. 
 
DESCRIPTION OF METAMORPHIC ROCKS 359 
 
 If, to the facts observed in the field, a chemical analysis of a well 
 selected specimen, or series of specimens, of the gneiss can be added, 
 this may prove in addition of great value. This has been commented 
 on elsewhere and need not be repeated. 
 
 When all is said and done, however, it must always be remem- 
 bered, as Rosenbusch, the great German petrologist, has well said, 
 " there is no formula by which the derivation of a gneiss may 
 be invariably determined." It must not be done on any one 
 character alone, but all must be taken into account and relatively 
 balanced, and even when this is done, it is impossible in many cases 
 to say if the gneiss has been derived directly from an igneous rock, 
 or whether the material of the latter may not have passed through 
 an intermediate sedimentary stage. 
 
 General Properties and Uses of Gneiss. Those gneisses, 
 which under the action of metamorphic agencies have 
 been thoroughly recrystallized, form solid and massive 
 rocks, whose general properties closely resemble the 
 massive igneous ones. Thus granitic gneiss closely resem- 
 bles granite, and is used in the same manner for building 
 and structural purposes. But often gneiss contains so 
 much mica, that it has too easy a cleavage to be of much 
 value. In general a gneiss should be so placed, that the 
 plane of chief fracture lies in the mortar bed with the cross 
 fracture exposed; otherwise it is liable, like some sedi- 
 mentary rocks, to split and scale badly. Those gneisses 
 which have assumed their texture under conditions of dry 
 crushing and shearing are very tender and friable rocks, 
 which fall to pieces readily under the blow of the hammer, 
 and are of little value. The granite-gneiss of portions of 
 the Alps, and the anorthosite-gneiss of parts of the Adi- 
 rondacks, are examples of this. The jointing, erosion 
 forms, etc., of granite-gneiss are similar, in general, to what 
 is stated under granite. So also is the weathering, and 
 gneisses form fertile sandy soils, which pass into loamy 
 ones, as the decay of the feldspar and its alteration into 
 kaolin becomes more complete. 
 
 Occurrence of Gneiss. Gneiss, especially common or 
 mica-gneiss, is one of the most common and widely dis- 
 
360 ROCKS AND ROCK MINERALS 
 
 tributed of rocks. The occurrence of the metamorphic 
 rocks in general has been already commented on, and it 
 was stated that they are found in mountain regions and 
 in those areas where the sedimentary beds have been 
 eroded, as a basement upon which these later rocks rest. 
 In such places common gneiss is usually the most promi- 
 nent rock. Owing to this, it is spoken of by many geolo- 
 gists as " the basal gneiss," or " fundamental gneiss," and 
 as, in many places, it is clearly the oldest rock of which we 
 have any knowledge, some believe that they see in it the 
 primitive crust of the earth. The Archaean, as it is now 
 used as a division of geologic time, is almost entirely com- 
 posed of gneiss, and to attempt to mention all the localities 
 of the rock, would be practically equivalent to a descrip- 
 tion of the occurrence of the Archaean. Gneisses are not 
 of course restricted to the Archaean; they occur in later 
 formations, into the Mesozoic. Gneisses are found all 
 over New England, and southward along the Piedmont 
 plateau into Georgia; in the Adirondacks; in the Rocky 
 Mountains' region, the Sierra, and other places in the 
 United States; they cover large parts of eastern Canada 
 and are prominent in Scotland, Norway and Sweden, 
 Finland, parts of Germany, and in the Alps. In all of these 
 regions different varieties, such as hornblende-gneiss, 
 occur associated with the common kind. 
 
 Granulite. Associated with gneisses in a number of localities is a 
 schistose, to thin schistose, rock composed almost wholly of quartz 
 and feldspar. It is nearly, or wholly, free from mica, and is usually 
 of fine to dense grain, so that, except for its schistose character and 
 place of occurrence, it is much like an igneous felsite or aplite. It is 
 apt to carry minute red garnets, and sometimes small quantities of 
 other minerals, such as cyanite, tourmaline, or hornblende, can be 
 detected with the lens. Chemically, it is similar in composition to 
 somefelsites orthe aplite variety of granite, and it probably represents 
 in general former igneous rock of this nature which has been in- 
 volved in the metamorphic processes. Such granulites occur in 
 Saxony and other places in Germany, where they were first studied ; 
 in Sweden, Finland, Austria, etc., in Europe; in New England and in 
 the Adirondack region of New York. 
 
DESCRIPTION OF METAMORPHIC ROCKS 361 
 
 MICA-SCHIST. 
 
 Mica-schist is a rock which is closely related on the one 
 hand to gneiss, and on the other to quartzite. It is not 
 only a very common companion of gneisses, in regions of 
 metamorphic rocks, but in many places gneiss grades into 
 mica-schist, so that no definite line can be drawn between 
 them. It has also many other analogies with gneiss, some 
 of which will be presently mentioned. Of that great class 
 of rocks known as schists, it is, excluding gneiss, if the 
 latter be reckoned among them, the most widely dis- 
 tributed and important. 
 
 Composition Minerals and Texture. The essential 
 minerals of mica-schist are quartz and mica, and it is es- 
 pecially the latter which gives the rock its particular char- 
 acter. Different varieties of mica occur; the most common 
 is a silvery white muscovite; biotite of a dark color is com- 
 mon, while the soda-bearing mica paragonite is rare. 
 Muscovite and biotite occur alone, and also in combination, 
 as in gneiss. The micas are in irregular leaves or tablets, 
 without crystal boundaries, or in leafy or foliated aggre- 
 gates; biotite and muscovite are found intergrown, and 
 often so that they have a common cleavage. The micas 
 lie with their cleavage planes in the direction of schistosity, 
 and it is this which produces the extraordinary fissile 
 character of the rock. They are also very often curved, 
 bent, or twisted, as may be easily seen by the reflections 
 from their cleavage surface. The cleavage of the mica is 
 so marked that the surface of chief fracture, or the schistose 
 plane of the rock, appears completely coated by it, and it 
 may produce the impression that it is the only mineral 
 present; to see the quartz, the other essential component, 
 the cross fracture should be examined with the lens. The 
 quartz forms irregular grains, or aggregates of grains, 
 and these are sometimes arranged in small lenses, and 
 sometimes in thin layers, concordant with the layers of 
 mica. 
 
362 ROCKS AND ROCK MINERALS 
 
 Mica-schists, while they are very often composed of 
 these two minerals alone, also very commonly carry crys- 
 tals, often of large size, of other minerals. The most 
 common of these is a dark red garnet, sometimes sparsely, 
 but generally thickly, sprinkled through the rock, and 
 varying in size from that of coarse shot to that of a plum. 
 These garnets are often in the form of simple, rounded 
 nodules, but in most cases they show more or less distinct 
 crystal form, and sometimes they are beautifully crystal- 
 lized in the shapes mentioned in the description of this 
 mineral. This garnetiferous variety of mica-schist is a 
 very common metamorphic rock; in New England it is 
 widely distributed among the bowlders of the glacial drift. 
 
 Other minerals which occur in mica-schist, in a manner 
 similar to garnet, are staurolite, often with garnet, cyanite, 
 epidote, andalusite, and hornblende. These sometimes 
 are in large and well-formed crystals, which, especially 
 staurolite, andalusite, and cyanite, are not infrequently 
 colored dark, by included carbonaceous matter. Graphite 
 occurs in some mica-schists in quantity sufficient to pro- 
 duce a distinct variety. Graphite is such a strong coloring 
 matter, that a relatively small amount will cause the rock 
 to appear as if almost entirely composed of it; in conse- 
 quence unsuccessful attempts have been made in places 
 to exploit such schists for graphite. 
 
 Hornblende, when it occurs, is in dark-colored prisms; 
 by its increase in amount transitions into amphibolite or 
 hornblende schist are formed. 
 
 Cyanite, andalusite, and staurolite occur in prismatic 
 crystals, which may attain a length of several inches. 
 Their formation is contemporaneous with the metamor- 
 phism of the rock, and they produce a pseudo-porphyritic 
 texture as previously explained on page 342. Another 
 variety of mica-schist is one which contains more or less 
 calcite mingled with the quartz; it is readily detected by 
 its effervescence with acids. This variety is especially 
 apt to contain accessory garnet, epidote, hornblende, etc. 
 
DESCRIPTION OF METAMORPHIC ROCKS 363 
 
 The parallel texture of the rock is its especial feature, 
 and its ready fissility is produced by the mica. If the 
 components are in thin, parallel layers, the surface of rock 
 cleavage is smooth and flat; if the lenticular arrangement 
 of the quartz is prominent, the surface is uneven or lumpy. 
 Frequently the surfaces of schistosity are bent, folded 
 and crumpled, showing pressures and shearing secondary 
 to its production. 
 
 Chemical Composition. As in the gneisses, the chemical 
 composition of these rocks is too variable a feature to be 
 of specific value. This comes from the natural variability 
 in the composition of the sediments from which they are 
 formed. In addition, not many of these rocks have been 
 chemically investigated, and some of the older analyses 
 have been very poorly executed. It is clear, however, 
 that they must contain silica, alumina, and potash, to 
 form the quartz and mica, and also magnesia and iron, if 
 biotite is present. The excess of magnesia over lime, 
 taken with the high silica, is a character foreign to igneous 
 rocks, and is clearly indicative of sedimentary origin. 
 They are probably formed mostly by the metamorphism 
 of feldspathic sandstones. Two analyses of typical sam- 
 ples carried out in the laboratory of the United States 
 Geological Survey are here appended. 
 
 Si0 2 
 
 A1 2 3 
 
 Fe 2 3 
 
 FeO 
 
 MgO 
 
 CaO 
 
 NaaO 
 
 K 2 
 
 H 2 O 
 
 XyO 
 
 H 2 
 
 64.7 
 64.8 
 
 16.4 
 14.4 
 
 1.8 
 1.8 
 
 3.8 
 4.5 
 
 3.0 
 2.3 
 
 0.1 
 2.3 
 
 0.1 
 
 1.4 
 
 5.6 
 5.0 
 
 3.1 
 
 2.0 
 
 0.8 
 1.9 
 
 99.4 
 100.4 
 
 General Properties. The color of these rocks varies from 
 very light, through gray, yellow, or brown tones, into very 
 dark, depending on the proportions of light and dark mica, 
 the presence of carbonaceous material, and in part on the 
 amount of alteration of the iron-bearing biotite. Some 
 pure muscovite schists are almost silvery white or light 
 
364 ROCKS AND ROCK MINERALS 
 
 gray. The hardness and firmness of the rock depend on 
 the proportion of mica; the more this is present, the softer 
 and more easily cleavable it is. For this reason they are 
 of little or no value for practical purposes. Inclusions of 
 various kinds occur in mica-schists as in gneiss, thus veins 
 and lenticular masses of quartz, deposited from solution 
 in cracks and cavities opened by movement and foldings 
 of the rocks, are common. They also contain in places 
 lenticular masses of other schists, which may vary from 
 very small to huge dimensions. And sometimes they are 
 penetrated by seams and patches of granite, felsite, and 
 pegmatite as the result of granitic injections. With 
 respect to the alteration of mica-schist, the varieties com- 
 posed of muscovite are chiefly mechanically disintegrated 
 by the action of weathering without much chemical change. 
 The muscovite resists alteration energetically, and the 
 gravelly or sandy soils formed, are in consequence filled 
 with its sparkling flakes. Where much biotite is present it- 
 alters easily; the rocks turn yellow or brown, lose their 
 luster, and eventually much limonite is separated out. 
 
 Varieties and Occurrence. The varieties composed 
 chiefly of muscovite, or with associated garnet, are the 
 most usual kinds, and are found all over the world as 
 common rocks in metamorphic regions, and are generally 
 associated with gneisses. They cover large areas in New 
 England and extend southward to Georgia. Biotitic 
 varieties are also very commonly found with them. 
 Staurolitic mica-schist occurs in many places in New 
 England, and in Maryland, and elsewhere along the Pied- 
 mont plateau; it is found in Scotland and various localities 
 in Europe, in Brazil and elsewhere. Cyanite-mica-schist 
 occurs in various places in New England; a variety in 
 which the mica is paragonite comes from the St. Gothard 
 region in the Alps, and is seen in mineral collections on 
 account of the beautiful crystals of cyanite it contains; 
 the common kind with muscovite is found in many places. 
 Hornblendic mica-schists occur as included lenticular 
 
DESCRIPTION OF METAMORPHIC ROCKS 365 
 
 masses, often of large dimensions, in various places, in the 
 ordinary mica-schists. Graphitic mica-schist is found in 
 Connecticut and other places in New England, various 
 localities in Germany, Norway, etc. Andalusite-mica- 
 schist occurs in the White Mountains in New England, 
 in Scotland, Spain, Germany, etc. 
 
 An interesting variety is the conglomerate-mica-schist, in which the 
 rock contains pebbles of quartz, granite, and other rocks which are very 
 apt to be flattened, lenticular, or drawn out by pressure and shearing. 
 It is closely related to the conglomerate-gneiss previously described 
 and has had a similar origin. Such rocks occur in Massachusetts, in 
 Vermont, Scotland, Sweden, etc. 
 
 Transitions and Relation to other Rocks. The gneisses 
 formed from sediments and the mica-schists have both 
 been made from similar rocks; from feldspathic sand- 
 stones, shales and conglomerates. In the mica-schists the 
 feldspar has been converted into mica; in the gneisses it 
 has mostly persisted or been recrystallized. It is not in- 
 tended in this statement to affirm that this is the only 
 origin for mica-schists, only the most usual one; they may 
 have been formed in some cases from quartzose-feldspathic 
 igneous rocks, though positive evidence on this point is 
 wanting. In this connection what is said elsewhere of 
 phyllites should be consulted. On the whole it would 
 seem most probable that the gneisses have been formed 
 most often from the conglomerates and coarser-grained 
 sandstones, the mica-schists from the finer-grained ones, 
 and from the shales, though many exceptions must occur. 
 
 It is therefore easy to understand that many mica-schists contain 
 more or less of feldspar grains among those of quartz, which are dim- 
 cult to detect without the aid of microscopic investigation. These may 
 increase in amount until the rock passes over into a gneiss, and no 
 hard and fast line can be drawn between them, as previously stated. 
 The decision as to whether a given rock should be classed as a gneiss 
 or mica-schist is often a very difficult thing to make on purely mega- 
 scopic grounds; in general if the amount of mica is large, and little or 
 no feldspar can be detected with the lens, it is best to classify it as a 
 
366 ROCKS AND ROCK MINERALS 
 
 mica-schist; if the amount of mica is small and feldspar can be seen, 
 to define it as a gneiss. 
 
 On the other hand, in proportion as the original sandstones were 
 more and more purely composed of quartz grains there would be less 
 and less of mica made, and in this way formal transitions into quartz 
 schist and quartzite are produced in their metamorphic representa- 
 tives. We thus see that gneiss, mica-schist, and quartzite form a 
 graded series whose divisional lines must be purely arbitrary. 
 
 Again, as the rocks become finer and finer in grain and in texture, 
 the mica-schists pass into micaceous slates and so on into slates, and 
 this becomes more marked if the amount of carbonaceous matter 
 increases, as it tends to mask the mica. The divisional line thus 
 becomes an arbitrary one in this case also. 
 
 QUARTZITE. 
 
 Quartzite is a firm, compact rock, composed of grains 
 of quartz-sand united by a cement consisting of the same 
 material, that is, of deposited quartz. They are in general 
 metamorphosed sandstones, and while no hard and fast 
 line can be drawn between the two rocks, since all degrees 
 of transition can be found between them, the quartzites 
 are much harder and firmer than the sandstones; the 
 latter have a more or less sugar-granular feeling and 
 appearance; the individual grains are distinctly visible to 
 the eye or lens, while in the quartzites the fractured sur- 
 face is uneven, splintery or conchoidal; the luster vitreous 
 or greasy, like that of quartz, and the grains are imper- 
 ceptible or nearly so. This difference arises chiefly from 
 the fact that in breaking the sandstone the fracture takes 
 place in the cement, leaving the grains unaltered and 
 outstanding, while in quartzite the grains are so firmly 
 cemented, that there is nearly a homogeneous substance 
 formed and the fracture takes place through cement and 
 grains alike. This difference will serve as a practical dis- 
 tinction between the two rocks. 
 
 Minerals and General Properties. While some quartz- 
 ites are very pure in mineral composition, others carry in 
 greater or less abundance other minerals, which may be in 
 part remains of original mineral grains, such as feldspar 
 
DESCRIPTION OF METAMORPHIC ROCKS 367 
 
 mixed with those of quartz, or new ones which have 
 resulted from the metamorphism of the clay or lime 
 cement, which formerly filled the interstices between the 
 grains of the sandstone. Such are muscovite, chlorite, 
 cyanite, epidote, etc. Iron hydroxides may be con* 
 verted into magnetite or hematite, and carbonaceous 
 substance into graphite. These resultant minerals are 
 usually of microscopic size, and may give the rock a dis- 
 tinct color green, blue, purple, black, etc.; sometimes 
 they are large enough to be clearly seen with the lens. 
 The most important of them is muscovite, which, as it 
 increases in amount, gives the rock a more schistose char- 
 acter, through which it attains a capacity for cleavage 
 along the planes of the mica. Eventually this produces a 
 transition into mica-schist, as previously explained under 
 that rock. The normal color of quartzite is white, light- 
 gray or yellowish into brown, but these are often modified 
 by included material acting as a pigment, as explained 
 above. The jointing of quartzite is usually platy, but 
 sometimes very massive, and such rocks are in some 
 places quarried and furnish good material for structural 
 purposes. 
 
 The chemical composition of a pure quartzite is nearly 
 that of silica alone, but as more or less clay or calcareous 
 material was mixed with the sand, small amounts of 
 alumina, iron, lime, and alkalies appear. This is illus- 
 trated in the contrast of the two analyses quoted below. 
 
 
 Si0 2 
 
 A1 2 3 
 
 Fe 2 3 
 
 FeO 
 
 MgO 
 
 CaO 
 
 NaO 
 
 K 2 
 
 H 2 O 
 
 Total 
 
 I.. 
 II.. 
 
 97.71 
 74.22 
 
 1.39 
 10.61 
 
 1.25 
 7.45 
 
 0.85 
 
 0.13 
 1.48 
 
 0.18 
 0.56 
 
 2.12 
 
 1.08 
 
 1.79 
 
 100.66 
 100.16 
 
 I, Pure quartzite, Chickies Station, Pa.; II, Impure quartzite, 
 Pigeon Point, Minn. Contains small quantities of feldspar, mica, 
 chlorite and magnetite. 
 
368 ROCKS AND ROCK MINERALS 
 
 Varieties. The different varieties of quartzite are chiefly those 
 which are occasioned by the presence of some included substance. 
 Thus we have epidotic quartzite, graphitic quartzite, sillimanite- 
 quartzite, and many others. Micaceous quartzite is also called 
 quartz-schist. In very strongly folded and compressed mountain 
 regions even pure quartzite may suffer such shearing as to break 
 and crush the original grains and impose a more or less schistose 
 structure. Such rocks are called stretched quartzites. In some 
 places these rocks contain pebbles, of varying sizes, which retain 
 their original shape and are sometimes by pressure and shearing 
 reduced to lenticular, ovoid, or cylindrical bodies. These are called 
 conglomerate-quartzite and were formed from gravels, like conglom- 
 erate-gneisses and conglomerate-mica-schists. 
 
 Oolitic quartzite is a variety consisting of rounded grains, com- 
 posed of chalcedony, a slightly hydrated form of silica, deposited 
 around fragments of quartz which serve as nuclei. It resembles 
 the roe of a fish, and if the globules are sufficiently large, their con- 
 centric structure can be plainly seen with a lens on the broken or 
 polished surface of the rock. Such quartzites have been found at 
 State College, Pa., and in Sumatra. Buhrstone is a name given 
 to a variety of quartzite which is full of long, drawn out hollows 
 or pores. Notwithstanding the porosity, it is quite firm and its 
 hardness and toughness have caused its use as a millstone. It is 
 thought to have been originally more or less of a limestone, filled 
 with fossils, which, by the action of solutions containing silica, has 
 been converted into a quartzite, consisting mostly of chalcedony, 
 whose cavities represent the leached out fossils. It occurs in 
 western Massachusetts, Georgia, South Carolina and in the Paris 
 Basin in France. It is chiefly of Tertiary age. 
 
 Occurrences. Quartzite is a widely distributed rock, 
 mostly among the older metamorphosed strata. Thus 
 it is common in eastern North America, in the Rocky 
 Mountains Cordillera and in various localities in Europe 
 and other parts of the world. The occurrence of some 
 special varieties has been already mentioned. 
 
 Alteration. On account of the insoluble, unyielding 
 nature of its constituent grains and their cement, quartzite 
 resists erosion and the atmospheric agencies well, and, 
 where it is prominent in mountain regions and areas 
 undergoing denudation, it forms prominent features of the 
 landscape, bold ledges, cliffs, castellated crags, spires, etc. 
 
DESCRIPTION OF METAMORPHIC ROCKS 369 
 
 Eventually the rock breaks down into sandy soil of poor 
 quality. 
 
 Distinction from Other Bocks. Quartzites, which are 
 very homogeneous appearing rocks, may be confused, in the 
 outcrop or hand specimen, with some limestones or felsites 
 of a similar color and texture. From the former they are 
 easily told by a test of the hardness, or by lack of effer- 
 vescence with acid; from the latter, in the field by the 
 different mode of geological occurrence, by the cleavage 
 of the feldspar if visible under the lens, or by blowpipe 
 test. It should be remembered that the chief minerals 
 composing these three rocks are quartz, calcite, and feld- 
 spar respectively, and they should be tested accordingly. 
 
 SLATE OR ARGILLITE. 
 
 Slates are dense, homogeneous rocks, of such fine tex- 
 ture that the individual mineral particles composing them 
 cannot be distinguished by the eye or lens, and character- 
 ized by a remarkable cleavage, by means of which they 
 split readily into broad, thin sheets, which, as is well 
 known, may be used for a variety of purposes. 
 
 The slates represent in metamorphic form the finest 
 material of the land waste by erosion, which, among the 
 unmetamorphosed stratified rocks, appears as clay, 
 shales of various kinds, etc., as previously described. 
 With such material more or less volcanic dust and debris, 
 or tuffs, may be mingled. The cause of the slaty cleavage 
 is discussed in a following paragraph. The difference 
 between slate and shale has been discussed in the de- 
 scription of the latter rock. 
 
 Mineral Composition and Other Properties. The mineral 
 particles are so fine in slate that the composition from 
 the megascopic standpoint is not a matter of importance. 
 It may be mentioned, however, that since the clays, silts, 
 etc., from which they are formed come from a great variety 
 of sources, so the microscope detects in them many and 
 varied minerals, the chief of which are quartz, mica, 
 
370 
 
 ROCKS AND ROCK MINERALS 
 
 chlorite, carbonaceous substance, etc. The kaolin and 
 feldspar particles, which one might naturally expect, are 
 rare and appear to have been converted into other min- 
 erals. They not infrequently contain crystals of pyrite, 
 readily seen with the eye or lens, which may attain large 
 size, sometimes as distinct crystals, sometimes as concre- 
 tions, or replacing fossils. Veins, lumps, and lenses of 
 deposited quartz are also common in them, those of cal- 
 cite more rare. The color is chiefly gray, to dark gray, to 
 black, according to the amount of carbonaceous substance, 
 but they are often green from chlorite, or red, purple, 
 yellow, or brown, from the oxides of iron. The surface of 
 the slaty cleavage is apt to have more or less of a silky 
 luster, sometimes scarcely perceptible; the cross fracture 
 has a dull surface. While the rock is firm and never 
 friable, it is also rather soft, so that it may be quite readily 
 cut, a feature of great value for technical purposes. The 
 specific gravity of an average slate is about 2.8. The 
 chemical composition is shown in the following analyses 
 of typical examples, made in the laboratory of the U. S. 
 Geological Survey. 
 
 
 Si0 2 
 
 A1 2 3 
 
 Fe 2 3 
 
 FeO 
 
 MgO 
 
 CaO 
 
 Na 2 -0 
 
 K 2 
 
 H 2 O 
 
 C 
 
 XyO 
 
 Total 
 
 I 
 
 59.7 
 
 17.0 
 
 0.5 
 
 4.9 
 
 3.2 
 
 1.3 
 
 1.4 
 
 3.8 
 
 4.1 
 
 0.5 
 
 3.8 
 
 100.2 
 
 II 
 
 67.6 
 
 13.2 
 
 5.4 
 
 1.2 
 
 3.2 
 
 0.1 
 
 0.7 
 
 4.5 
 
 3.3 
 
 _ 
 
 0.7 
 
 99.9 
 
 III 
 
 59.8 
 
 15.0 
 
 1.2 
 
 4.7 
 
 3.4 
 
 2.2 
 
 1.1 
 
 4.5 
 
 3.8 
 
 _ 
 
 4.3 
 
 100.0 
 
 IV 
 
 56.4 
 
 15.3 
 
 1.7 
 
 3.2 
 
 2.8 
 
 4.2 
 
 1.3 
 
 3.5 
 
 4.8 
 
 0.6 
 
 6.5 
 
 100.3 
 
 V 
 
 60.5 
 
 19.7 
 
 
 
 7.8 
 
 2.2 
 
 1.1 
 
 2.2 
 
 3.2 
 
 3.3 
 
 
 
 
 
 100.0 
 
 I, Black roofing slate, Benson, Vermont. XyO = TiO 2 , P 2 O 5 , CO 2 , 
 FeS 2 , etc.; II, Red roofing slate, Washington Co., New York 
 State; III, Green roofing slate, Pawlet, Vermont, CO 2 , 3.0; IV, 
 Black roofing slate, Slatington, Pennsylvania, CO 2 , 3.7; FeS 2 , 1.7; V, 
 Roofing slate, Wales. 
 
 The general predominance of magnesia over lime in the 
 analyses, as well as the small amount of the latter, shows 
 
DESCRIPTION OF METAMORPHIC ROCKS 371 
 
 that the soluble lime silicates have been mostly dissolved 
 out of the silt in the process of erosion and laying down 
 of the sediments. The presence of carbon in the black 
 varieties, and of ferric iron in the red, is to be noted. 
 
 Varieties. Roofing slates are compact, very fissile varieties which 
 split with a smooth, even cleavage. All the different colors are used, 
 but the most common is a dark gray. Some slates fade when taken 
 from the quarry, on continued exposure, through incipient altera- 
 tion and the possibility of this can only be determined by practical 
 trial. The presence of pyrite in any notable quantity is very 
 prejudical, as this substance on exposure quickly alters and 
 gives rise to rusty stains. The slates used for blackboards and 
 ciphering are the blackest and most compact kinds. Calcareous 
 slates are those which contain a good deal of intermingled calcite, 
 or chalky material, which may rise to 30 per cent of the whole; 
 they represent slates which have been formed from original 
 marls. 
 
 Cleavage of Slates and its Origin. The cause of slaty cleavage 
 has occasioned much speculation and has been the subject of investi- 
 gation, both experimental and mathematical, as well as geological, 
 by a number of scientists. From this work it has become clear that 
 it is the result of great pressure upon the material and that the 
 planes of cleavage are at right angles to the direction of pressure. 
 When the fine-grained sediments are subjected to intense pressure, 
 unevenly shaped particles tend to rotate, so that their longer axes 
 are perpendicular to the direction of pressure; they also tend to 
 become flattened perpendicularly to it. This tends to give the rock 
 a grain, an arrangement of particles, by which it tends to split more 
 readily along such a direction than in any other. Moreover the rock 
 minerals, which naturally tend to be flattened or elongate in the 
 shape of their particles, such as the micas, kaolin, hornblende, 
 chlorite, etc., possess an excellent cleavage parallel to the elongate or 
 flattened directions, and this is a great help in promoting the capacity 
 of the rock cleavage. Slaty cleavage is thus partly molecular, or 
 mineral cleavage, where it passes through a single mineral particle, 
 and partly mechanical where it passes between arranged, unlike 
 mineral particles. Not necessarily all of the minerals whose cleav- 
 age and arrangement induce the slaty cleavage are original; some 
 of them, micas for example, may have been formed by the meta- 
 morphism accompanying the pressure. 
 
 The planes of cleavage do not necessarily bear any definite rela- 
 tion to those of original bedding. The beds were laid down horizon- 
 
372 ROCKS AND ROCK MINERALS 
 
 tally and the direction of pressure is also usually horizontal; the 
 cleavage planes are at right angles to this, and may therefore cut the 
 
 bedding at right, or highly inclined, 
 angles. But, as the beds may be folded 
 before the pressures become intense, 
 the cleavage planes may pass through 
 the bedding at various angles, although 
 they themselves are strictly parallel, as 
 seen in the diagram, Fig. 74. 
 
 Slates, in addition to their cleav- 
 age, are intersected by cross joints 
 which are frequently so numerous as to divide them into small 
 blocks and prevent their technical use. They generally form sys- 
 tems intersecting at definite angles. In the older mountain ranges 
 the slates are frequently crumpled by repeated movement and show 
 this upon their cleavage surfaces. 
 
 Occurrence. Slates are common rocks in metamorphic 
 regions and range geologically from the Algonkian up to 
 recent periods. In eastern North America they are chiefly 
 Paleozoic and have an extensive developement in Maine, 
 in Vermont, in Pennsylvania, and in Georgia. They are 
 also extensively distributed in the Lake Superior region 
 and in the older ranges of the Rocky Mountains Cordillera. 
 They are found in southern England, in Wales and in 
 many other parts of Europe. 
 
 Phyllite. Closely connected with slate by intermediate 
 types are a group of rocks to which the name of phyllite 
 has been given. The name means " leaf stone " and is 
 used on account of the remarkable cleavage of the rocks, 
 by means of which they split into exceedingly thin sheets, 
 in typical examples. The surface is sometimes flat, 
 sometimes curved, folded, or crumpled by crustal move- 
 ments. It differs from ordinary slate in containing a 
 larger amount of mica, or at. all events the mica is in 
 larger flakes, and is more evident, giving the surface of 
 cleavage a shimmering or micaceous appearance, and thus 
 furnishing a transition form between slate and mica- 
 schist. The mica is a fine, scaly, silky variety of mus- 
 covite to which the name of sericite has been given. 
 
DESCRIPTION OF METAMORPHIC ROCKS 373 
 
 Quartz is the other chief mineral and may sometimes be 
 seen on the cross fracture. Rocks, which in this country 
 have been called " hydromica-schists," are in large part 
 such phyllites. Their color is sometimes pure white, 
 more often tinged with reddish, yellowish, or greenish 
 tones, and sometimes dark colored, or black, from pig- 
 ments, like those of slate. They are apt to have a soft 
 talcy or greasy feel, and to be more brittle than slate, and 
 to lack its toughness and firmness. Sometimes they con- 
 tain visible crystals of pyrite, garnet, and other minerals. 
 
 The origin of phyllites, as shown by the researches which have been 
 made upon them, is a varied one; in some cases they represent sedi- 
 mentary material which has been metamorphosed, like the slates, 
 but has attained a more complete degree of recrystallization than 
 they have. On the other hand a considerable part of the phyllites 
 represent original felsites igneous rocks which have been 
 subjected to the energetic operation of metamorphism through 
 dynamic forces, to pressure and great shearing, aided probably by 
 liquids and heat. Their feldspars have been largely, if not entirely, 
 converted into mica, and a thin schistose or slaty cleavage has been 
 imposed upon them. In some extreme cases the rock appears 
 as if wholly composed of this silky mica. The chemical analyses of 
 these rocks show them to have compositions similar to that of many 
 felsites or felsite tuffs. 
 
 Porphyroid Sheared Felsites. In many places where phyllites 
 occur, they may be traced into types which are firmer, with less pro- 
 nounced but yet distinct cleavage, and which contain visible pheno- 
 crysts of quartz and feldspar, similar to those in felsite-porphyries 
 (embedded in the phyllitic ground mass). Such rocks have been 
 termed porphyroid. These again may be further traced into un- 
 doubted felsites which still retain the phenocrysts, flow structures, 
 spherulites, etc., characteristic of lavas, or the broken, angular, frag- 
 mental features of tuffs and breccias, in spite of the slaty cleavage, 
 which to a greater or lesser degree, has been imposed upon them 
 by the dynamic movements and shearing to which they have been 
 subjected. These again may be followed into undoubted, unsheared 
 felsites. Rocks with these characters, in their varied types as 
 described above, occur in various places among the older metamor- 
 phosed Paleozoic areas of eastern North America, in Maine, at South 
 Mountain, Pa., in Virginia and North Carolina, in Wisconsin, the 
 Lake Superior region, etc. They have been found of various ages 
 in Great Britain, Germany, the Alps and other places in Europe. 
 
374 ROCKS AND ROCK MINERALS 
 
 In Sweden such ancient felsites and felsite tuffs, hardened and 
 more or less metamorphosed, have been termed h&Ueflinta. 
 
 It is only in comparatively recent years that such altered igneous 
 rocks, with more or less schistose appearance and cleavage, have been 
 recognized and their significance appreciated. The older geologists, 
 confused by their cleavage, regarded and mapped them as slates and 
 considered them as of sedimentary origin. They are of interest, 
 because, as stated in the introduction to metamorphic rocks, these 
 latter comprise material both of igneous and sedimentary origin. 
 Of the feldspathic, igneous rocks, the coarser-grained ones, like 
 granite, as we have seen, yield gneisses ; the compact felsites and their 
 tuffs under the metamorphic agencies of pressure, shearing, etc., are 
 turned into phyllites, porphyroids, and compact slaty rocks, according 
 to the degree to which these agencies have acted. The igneous 
 ferromagnesian rocks we shall see later among the amphibolites 
 and other schists. 
 
 TALC-SCHIST. 
 
 Talc-schist is a rock of pronounced schistose cleavage and 
 character, in which talc is the predominant mineral. The 
 talc is present in fine scales to coarse foliated aggregates. 
 Other minerals also occur in different varieties of the rock, 
 such as quartz in grains, lenses, and veins ; or magnetite and 
 chromite in black specks and grains; hornblende, usually 
 in white or green prisms, or crystals of enstatite; chlorite 
 mingled with the talc, etc. The color is usually light, 
 white to pale green, or yellowish, or gray; sometimes dark 
 gray or greenish. The rock is soft and the talc gives it 
 a greasy feeling, and often a pearly or tallowy appearance 
 on the cleavage surface. In addition to its micaceous 
 appearance and soft greasy feel, the talc is easily told by 
 its infusibility before the blowpipe, and its insolubility in 
 acids. The rock cleavage is sometimes thinly fissile, 
 sometimes thicker, and sometimes cleavage is nearly 
 wanting, the rock is more nearly massive, is compact, and 
 has a lard-like or wax-like aspect, and approaches soap- 
 stone in character. Chemically, these rocks consist mostly 
 of silica and magnesia with small amounts of water and 
 other oxides. 
 
DESCRIPTION OF METAMORPHIC ROCKS 375 
 
 
 SiO 2 
 
 A1 2 3 
 
 Fe 2 O 3 
 
 FeO 
 
 MgO 
 
 CaO 
 
 Na 2 O + K 2 O 
 
 H 2 O 
 
 Total 
 
 I... 
 
 58.7 
 
 9.3 
 
 4.4 
 
 
 22.8 
 
 0.9 
 
 
 4.1 
 
 100.2 
 
 II.. 
 
 53.3 
 
 4.4 
 
 5.8 
 
 1.0 
 
 29.9 
 
 1.5 
 
 1.5 
 
 2.6 
 
 100.0 
 
 I, Talc-schist, Falun, Sweden; II, Talc-schist, Zobtau, Moravia, 
 Austria. 
 
 The composition is quite similar in its general features 
 to that of the peridotites among igneous rocks, as may be 
 seen by reference to their analyses. 
 
 The talc-schists undoubtedly represent material which was some- 
 times of igneous origin, peridotite, pyroxenite, or dunite, and some- 
 times of sedimentary origin, dolomitic, ferrugineous marls, etc. It 
 may not be possible from field work and an inspection of specimens 
 alone, unless aided by chemical analyses and microscopical study, 
 to decide in any given case which origin the material had, and some- 
 times not even then. The presence of chromium, either in the form 
 of chromite or of secondary minerals derived from it, such as 
 kammererite or fuchsite (a variety of muscovite green from chrom- 
 ium), is indicative of igneous origin, while that of much quartz and 
 dolomite mingled with the talc, which produces the variety of talc- 
 schist called listwanite, would be on the other hand more indicative 
 of a sedimentary one. 
 
 Talc-schists do not form important formations like 
 gneiss, mica-schist, and slates, but are limited in occurrence, 
 being found as interbedded layers or inclusions, chiefly as 
 lenticular masses, in other metamorphic rocks, and are 
 really not very common. They show transitions in places 
 into other rocks, such as chlorite-schist, crystalline dolo- 
 mite, quartzite, etc. Such transitions, or the lack of 
 them, may furnish useful hints in regard to their origin in 
 particular cases. In eastern North America talc-schists 
 occur, associated with other metamorphic rocks, in 
 Canada, in the New England states, in northern New 
 York, and south to Georgia. They are also found in the 
 Rocky Mountains region and in the Pacific states, Call- 
 
376 
 
 ROCKS AND ROCK MINERALS 
 
 fornia, Oregon, etc. In Europe they occur in the Alps, 
 Germany, and various other places, in Sweden, Finland, 
 etc. They also occur in Brazil and other parts of the world. 
 Their occurrence, though not generally of wide geologic 
 interest, is important because they furnish a source of 
 supply for talc, which is used for a variety of purposes. 
 
 CHLORITE-SCHIST. 
 
 These rocks are schists which have the mineral chlorite 
 as their chief determinant mineral. It occurs as fine, 
 scaly aggregates, sometimes too fine for the individual 
 scales to be seen by the eye; more rarely in foliated to 
 coarse foliated aggregates. It is sometimes thinly, some- 
 times thickly, schistose, and in some cases almost massive; 
 and although the rock is very soft and may be readily cut, 
 it is very tough in the more massive varieties. The color 
 varies through different shades of green, yellow-green, to 
 dark green. Different minerals are apt to accompany the 
 chlorite, some of which may be in megascopic sizes; of 
 these may be mentioned magnetite, often in fine crystals ; 
 hornblende in slender needles or prisms; corundum and 
 cyanite in some cases; quartz, which is generally in veins 
 and lenses; epidote in grains and crystals; in some instances 
 graphite, calcite, dolomite, etc. The chemical composi- 
 tion of these rocks is very variable, so far as is known, 
 for not many have been investigated; it indicates that 
 they have resulted from several different sources, as seen 
 in the following analyses. 
 
 SiO 2 
 
 A1 2 3 
 
 Fe 2 3 
 
 FeO 
 
 MgO 
 
 CaO 
 
 Na 2 O 
 
 K 2 O 
 
 H 2 O 
 
 XyO 
 
 Total 
 
 49.2 
 26.2 
 
 15.1 
 23.7 
 
 12.9 
 15.7 
 
 14.5 
 
 5.2 
 8.3 
 
 10.6 
 1.7 
 
 3.6 
 0.5 
 
 1.5 
 0.6 
 
 1.9 
 7.3 
 
 0.8 
 
 100.0 
 99.3 
 
 I, Chlorite-schist, east of Roton, Sweden; II, Chlo rite-schist 
 Benguet, Luzon Island, Philippines. 
 
DESCRIPTION OF METAMORPHIC ROCKS 377 
 
 No. I has a composition very similar to that of the group 
 of igneous rocks known as gabbros, as may be seen by 
 reference to their analyses, and to which also the dolerites 
 and basalts belong, these being merely textural varieties of 
 magmas similar to gabbros. No. II, on the other hand, 
 is very different, and does not correspond to any igneous 
 rock; it suggests rather a very ferrugineous clay. 
 
 The chlorite-schists are of wide distribution, forming 
 subordinate layers or masses in the midst of gneisses, 
 mica-schists and other such rocks, characteristic of meta- 
 morphic areas. Thus they occur in Canada, New England, 
 New York, Pennsylvania, etc. They are also common 
 in Europe, in the Alps, Germany, Sweden and other places. 
 
 Greenstone. Transitions of chlorite-schist into mica- 
 schist, into slates, into schistose serpentine, and into 
 hornblende-schist occur in places. Under the descrip- 
 tion of gabbro and of dolerite it was mentioned that these 
 rocks by alteration, through processes of regional meta- 
 morphism, passed into hornblende-schists and into so- 
 called " greenstone " or " greenstone-schist." In such 
 cases the original ferromagnesian minerals, or the horn- 
 blende produced from them, have been largely changed 
 by alteration into chlorite which gives the rock its green 
 color. Such greenstones (if massive), or greenstone- 
 schists (if schistose), which thus represent altered dolerites, 
 basalts, and gabbros, form transition types to chlorite- 
 schist. The alteration of hornblende in diorites to 
 chlorite also produces greenstones. It is conceivable that 
 a dolerite might pass directly by alteration into a chlorite 
 rock, or greenstone, and thus be of massive character, or it 
 might be first changed into a hornblende-schist and this 
 secondarily alter into a chlorite-schist. But since horn- 
 blende-schists are produced, not only from igneous but 
 also from sedimentary beds, as described in the account 
 of these rocks, the mere fact that transitions from horn- 
 blende-schists into chlorite-schists occur does not alone 
 prove these latter have been derived from igneous rocks 
 
378 
 
 ROCKS AND ROCK MINERALS 
 
 in any given case. Transitions from dolerite into 
 chlorite rocks, or greenstone-schists, have been observed in 
 many regions; in Michigan, Maryland, Connecticut, in the 
 south of England, in the Alps, Germany, etc. A green- 
 stone-schist from the Menominee River, Michigan, which is 
 known to be an altered dolerite, has the following com- 
 position. 
 
 Si0 2 
 
 A1 2 3 
 
 Fe 2 3 
 
 FeO 
 
 MgO 
 
 CaO 
 
 Na 2 
 
 K 2 
 0.5 
 
 H 2 O 
 
 CO 2 
 
 Total 
 
 44.5 
 
 16.4 
 
 5.1 
 
 5.5 
 
 7.5 
 
 7.9 
 
 2.6 
 
 5.0 
 
 5.4 
 
 100.4 
 
 It is composed of chlorite, with some feldspar, quartz 
 and calcite. This should be compared with the analyses 
 of gabbro which represent the gabbro-dolerite group. 
 
 The greenstones vary in color from pale gray-green 
 through yellowish green to dark green. The color depends 
 on the proportion of chlorite to other minerals. They are 
 generally too compact for the megascopic determination of 
 the individual mineral particles. They are generally rather 
 soft rocks. Sometimes, when the original dolerite or 
 basalt was an amygdaloid, this amygdaloid structure is 
 retained and the rock is filled with little balls of calcite 
 or quartz. In other cases, where the rock has been strongly 
 sheared, these have disappeared, but are still represented 
 in the schist by ovoid spots of a different color and mineral 
 composition from the main mass. In some rare cases 
 they have been replaced by ores. The amygdaloidal 
 structure is a good proof of the original igneous character 
 of the rock. 
 
 Soapstone, Steatite. As an appendix to talc and chlorite-schists 
 may be mentioned soapstone or steatite, a massive rock, usually of a 
 gray or green, but sometimes of a dark color; the lighter colors often 
 with a silvery or shimmering fracture surface. It is very soft, 
 easily cut or worked, without cleavage or grain, and resists well 
 heat and the action of acids. For these reasons it has been exten- 
 
DESCRIPTION OF METAMORPHIC ROCKS 379 
 
 sively used in prehistoric times for the manufacture of pots and other 
 vessels, and is employed at present in table tops, sinks, and other inte- 
 rior fittings where its qualities render it valuable. It is usually a 
 variable mixture of interwoven scales of talc and chlorite mixed with 
 various minerals ; in some cases carbonates are present. The better 
 qualities incline more nearly to pure talc. The minerals are in 
 general too fine for megascopic determination. It occurs in con- 
 nection with talc and chlorite rocks, and sometimes with serpentine, 
 in various parts of the world, in areas of metamorphic rocks. 
 
 AMPHIBOLITE OR HORNBLENDE-SCHIST. 
 
 The amphibolites are a large group of metamorphic 
 rocks whose distinguishing characters are, that they con- 
 sist partly or largely of hornblende, and that they possess 
 a more or less pronounced schistose structure. There are 
 a number of varieties in the group, depending on the kind 
 of hornblende present, and on the minerals associated 
 with it, so that it is difficult to give a general description 
 which will cover all cases. It is best therefore to describe 
 the most common kind first, and then give a brief mention 
 of some of the less common varieties. 
 
 Common hornblende-schists or amphibolites are rocks 
 which vary in color from green to black; the green is of 
 varying tones, clear light green, gray-green, yellowish 
 green to dark green, greenish black to black; the darker 
 colors are more common. The color is given by the 
 hornblende, though in a considerable degree, in some 
 cases, it is influenced by admixed chlorite. The grain of 
 the rocks varies from coarse to fine, the latter being more 
 common. When coarse, the hornblende, which is almost 
 always present in slender prisms or blades and rarely in 
 grains, is easily recognized by the eye from its form and 
 bright, good cleavage. In such cases the prisms may be 
 an inch or more in length and have the thickness of a 
 slender match stick; from this, in the finer-grained 
 types, they sink to tiny needle- or hair-like prisms which 
 can only be seen by careful observation with a good 
 lens. The prisms are usually arranged in the direction of 
 
380 ROCKS AND ROCK MINERALS 
 
 schistosity and thus approach parallel positions; it is this 
 which chiefly gives the rock its cleavage. It also gives 
 the rock, especially in the finer-grained types with needle- 
 like prisms, a shimmering or silky luster on cleavage sur- 
 faces, which is rather characteristic. In some cases the 
 grain is so extremely fine that not even with the lens 
 can the individual minerals be seen; such rocks may 
 appear very much like slates, and are indeed difficult to 
 distinguish from them; they are however, not very com- 
 mon types. 
 
 The amphibolites are rather hard rocks, not easily 
 scratched by the .knife. In the more schistose types 
 they are brittle, but as they become more massive in 
 character they are very tough and difficult to break. 
 They are heavy, the specific gravity ranging from 3.0-3.4. 
 
 In addition to the hornblende other minerals are present in 
 varying kinds and quantities; prominent ones are quartz, feldspar, 
 and mica. The quartz and feldspar in grains are best observed with 
 the lens on the cross fracture; often they are too fine and too much 
 masked by the hornblende to be seen; the quartz also at times 
 forms little lenses or masses, or fills fractures in the shape of veins, as 
 in other metamorphic rocks, and has then been secondarily deposited 
 from solutions. The mica can be generally seen on the surface of 
 chief fracture; both biotite and muscovite occur and may increase 
 to such an extent as to produce formal transitions to mica-schist. 
 
 Other minerals which may be detected megascopically are iron 
 ore, pyrite, garnet in small dark red crystals, chlorite, calcite; the 
 latter sometimes in veins, etc., like quartz. Pyroxene, epidote, and 
 other minerals occur, but partly on account of the fineness of grain, 
 and partly on account of their resemblance to hornblende, it is 
 usually impossible to detect and identify them without microscopic 
 study. 
 
 The chemical composition of amphibolites has not yet 
 been as thoroughly investigated as it should be, but 
 what has been done shows, in agreement with facts 
 to be presently mentioned, that the origin of these 
 rocks is various. The following analyses will do for 
 examples. 
 
DESCRIPTION OF METAMORPHIC ROCKS 
 
 381 
 
 
 SiO 2 
 
 A1 2 O 3 
 
 Fe 2 O 3 
 
 FeO 
 
 MgO 
 
 CaO 
 
 NaaO 
 
 K 2 
 
 H 2 
 
 XyO 
 
 Total 
 
 I.. 
 
 49.9 
 
 15.5 
 
 3.0 
 
 8.0 
 
 7.8 
 
 8.9 
 
 3.3 
 
 0.7 
 
 1.5 
 
 1.7 
 
 100.3 
 
 II.. 
 
 50.4 
 
 13.3 
 
 6.3 
 
 9.3 
 
 5.6 
 
 7.9 
 
 2.1 
 
 1.1 
 
 1.7 
 
 2.0 
 
 99.7 
 
 III. 
 
 55.0 
 
 2.9 
 
 0.8 
 
 6.3 
 
 21.0 
 
 11.5 
 
 0.3 
 
 0.2 
 
 1.0 
 
 1.5 
 
 100.5 
 
 IV . 
 
 55.6 
 
 16.3 
 
 1.2 
 
 7.2 
 
 5.6 
 
 9.2 
 
 0.9 
 
 0.2 
 
 3.1 
 
 1.0 
 
 100.3 
 
 V.. 
 
 45.6 
 
 14.2 
 
 1.2 
 
 9.8 
 
 6.8 
 
 2.3 
 
 1.6 
 
 1.2 
 
 5.1 
 
 12.6 
 
 100.4 
 
 VI. 
 
 52.4 
 
 13.6 
 
 2.7 
 
 9.8 
 
 5.5 
 
 10.0 
 
 2.3 
 
 0.4 
 
 1.7 
 
 1.6 
 
 100.0 
 
 I, Thin schistose amphibolite, Whitman's Ferry, Sunderland, Mass.; 
 II, Amphibolite, Crystal Falls district, Michigan; III, Grass-green 
 Amphibolite, Chiavenna. XyO = Cr 2 O 3 ; IV, Amphibolite, Goshen, 
 Massachusetts ; V, Amphibolite, pyritiferous, Conrad Tunnel, Ophir 
 district, California. XyO = Pyrite 7.9, CO 2 3.0, TiO 2 l.l, plus traces ; 
 VI, Olivine-basalt, main lava flow, Pine Hill, South Britain, Con- 
 necticut. 
 
 Of these analyses Nos. I and II have compositions very much 
 like that of the gabbro-basalt magmas, as may be seen by comparison 
 with No. VI ; No. Ill has the general composition of the peridotite- 
 pyroxenite group of igneous rocks and may be compared with 
 Analysis No. Ill of that group. The presence of Cr 2 O 3 in III is also 
 significant of an igneous origin. On the other hand No. IV is 
 thought on geological grounds to be derived from an impure lime- 
 stone, probably full of clay, and this supposition is rendered probable 
 by the fact that the high alumina is accompanied by an almost 
 entire lack of alkalies, a feature not seen in igneous rocks. In No. V, 
 while alkalies are present with the alumina, high magnesia and 
 ferrous iron are not accompanied by high lime and these make a 
 combination not seen in igneous rocks. Compare again No. VI. 
 This rock, No. V, is probably derived from an impure ferrugineous 
 arkose or silt. 
 
 Origin of Amphibolites. As just shown, the composition 
 of amphibolites sometimes corresponds with that of 
 igneous rocks, and sometimes does not, and this agrees 
 with the results of geological investigation in the field. 
 For in some places we find them under conditions which 
 strongly suggest their derivation from igneous rocks, and 
 in other places such evidence is either wanting, or the 
 contrary is indicated. The use of the microscope on thin 
 
382 ROCKS AND ROCK MINERALS 
 
 sections, by which the inward textures and associated 
 minerals may be seen, also leads to the same conclusions. 
 
 Under the description of gabbro and dolerite it was 
 mentioned under alteration, how these rocks by pressure 
 and shearing became converted into hornblende-schist or 
 amphibolite. Gradual transitions, without geological 
 break, from one kind into the other, are found. Thus as 
 the feldspathic igneous rocks give rise to gneisses, phyllites, 
 etc., so the ferromagnesian, especially the pyroxenic, 
 igneous rocks give rise especially to hornblende-schists, 
 and also to talc-schists, chlorite-schists, and to serpentine. 
 
 Sedimentary beds of impure mixed character, such as 
 limestones containing sand, clay, and more or less of the 
 hydroxides of iron, limonite, etc., or marls of a somewhat 
 similar nature, if subjected to metamorphism might, under 
 suitable conditions, be converted largely into hornblende, 
 mixed with other minerals. In this case the volatile con- 
 stituents the water, carbon-dioxide, etc., are mostly 
 driven out; the bases, lime, iron, magnesia and alumina, 
 combine with the acid silica, to form silicates, of which 
 hornblende is the chief and determinant one of the result- 
 ing rock. Thus hornblende-schists result from the meta- 
 morphism of sedimentary strata, and may be one form of 
 the alteration of limestone, as described later under 
 marble. 
 
 Varieties of Amphibolite. In the midst of gneisses and mica- 
 schists amphibolite sometimes assumes a very massive character. 
 The prisms and grains of hornblende, instead of being arranged in 
 parallel position, and thus producing a schistose cleavage, are inter- 
 woven without arrangement and cleavage is wanting. Especially 
 in such massive types is the hornblende liable to be accompanied by 
 feldspar. If th? feldspar should increase and dominate, transitions 
 to hornblende gneiss would be produced. There is a tendency on 
 the part of some to restrict the term amphibolite to such massive 
 varieties and to use hornblende-schist for those with distinct cleavage, 
 but this distinction has not yet come into general use. 
 
 Eclogite is a variety of hornblende-schist of a rather light green 
 color sprinkled full of red garnets. In the typical examples of this 
 rock the hornblende is accompanied, or more or less replaced, by 
 
DESCRIPTION OF METAMORPHIC ROCKS 383 
 
 a green pyroxene. Other minerals also occur in subordinate amount. 
 It has been found in various places in Europe, and has recently 
 been described as occurring in a Californian locality. A closely 
 related hornblende-schist full of garnets is found also in Hanover, 
 New Hampshire. 
 
 Glaucophane-schist is a variety in which ordinary hornblende 
 is replaced by the soda-bearing species glaucophane, and for this 
 reason the rock is colored blue instead of green. Various other 
 minerals may be present, depending on the occurrence, such as quartz, 
 epidote, pyroxene, chlorite, garnet, etc. Sometimes they are coarse 
 grained and these other minerals may be seen, sometimes dense and 
 appear as slaty blue or blue-gray rocks. Studies which have been 
 made of them show that sometimes they have been produced by the 
 metamorphism of sedimentary, sometimes of igneous, material. 
 While comparatively rare they have been found widely distributed, 
 in California, Brittany in France, the Alps, Island of Syra, Greece, 
 Japan, Australia, etc. 
 
 Greenstone-schist in its relation to amphibolites has been already 
 mentioned under chlorite-schist. As the use of the term " green- 
 stone " has been vague, applying rather to color than to a deter- 
 mined mineral composition, many rocks which are hornblende- 
 schists, rather than chlorite-schists, have been included under it. 
 
 Occurrence of Amphibolites. These form layers and 
 masses in other metamorphic rocks, especially in gneisses 
 and mica-schists, rather than extensive independent for- 
 mations. They often occur in gneisses in long bands or 
 veins in such a manner as to suggest that they are meta- 
 morphosed dikes of doleritic rock. In size the masses may 
 vary within the widest bounds, from one foot to thousands 
 in diameter. In some places, what they lack in size, they 
 make up in frequency of occurrence. The manner in 
 which they are interlaminated in places with other meta- 
 morphic rocks suggests that they may have been some- 
 times formed from intrusive sheets of igneous rock, and 
 sometimes from interbedded sediments, but in general 
 this can only be rendered certain by further chemical and 
 microscopical investigation. Their occurrence as mantles 
 around igneous masses has been already mentioned. 
 
 The amphibolites are extremely common rocks, in all 
 metamorphic regions. Thus they are found commonly 
 
384 ROCKS AND ROCK MINERALS 
 
 distributed in New England and New York State, and 
 southward to Georgia; in Canada, the Lake Superior region, 
 the Sierras, in England, Scotland, the Alps, etc. 
 
 Alteration. It has been already pointed out under 
 chlorite-schists that the hornblende of these rocks may be 
 changed into chlorite. In another form of alteration it 
 may be turned into serpentine with other minerals, and 
 thus give rise to serpentine rocks, whose character is 
 described later. These changes take place in the upper 
 belt of metamorphism, that of hydration and cementa- 
 tion, and are secondary to the processes which have pro- 
 duced the amphibolite from something else. They 
 might thus be spoken of as tertiary changes. 
 
 By the ordinary process of weathering on the surface, 
 these rocks change to masses of limonite, clay, calcite, etc., 
 which form ferrugineous soils. 
 
 MARBLE AND CARBONATE-SILICATE ROCKS. 
 
 Marble is the metamorphic condition of sedimentary 
 rocks which are composed of carbonate of lime, CaCOa, 
 and which in their ordinary stratified form are known as 
 limestone, chalk, etc. It is distinguished from them by 
 its crystallization, coarser grain, compactness and purer 
 colors. But just as we have ordinary limestones which 
 contain only carbonate of lime, and dolomitic limestones 
 which contain magnesian carbonate, MgCO 3 , in variable 
 quantity associated with the lime carbonate, so we have 
 lime marbles and dolomite marbles. As this distinction 
 is a purely chemical one which is rarely made, and indeed 
 rarely can be made in ordinary and commercial usage, the 
 rock is, therefore, called marble, without regard to whether 
 it contains magnesia or not. But geologically, especially 
 from the petrographical standpoint, there is an important 
 difference between the two rocks in respect to the asso- 
 ciated minerals they are apt to contain when impurities 
 were present in them originally, and therefore they are 
 
DESCRIPTION OF METAMORPHIC ROCKS 385 
 
 treated separately in this work for reasons which will 
 presently appear. 
 
 General Properties. Marble is a crystalline granular 
 rock composed of grains of calcite; sometimes these are 
 cemented by a fine deposit of calcite between them. 
 The grain varies from very coarse to such fine compact 
 material that individual grains cannot be distinguished; in 
 the coarsest varieties, the cleavage surfaces of individuals 
 may attain a breadth of half an inch or more, but this 
 is unusual. The fracture surface of the finest-grained 
 kinds has a soft, shimmering luster, while the appearance 
 of coarser kinds is like that of loaf sugar. The normal 
 color is white, like that of the best statuary marble, but 
 the rock is usually more or less colored by various sub- 
 stances which act as a pigment, the principal ones being 
 carbonaceous matter and the oxides of iron. It thus 
 becomes gray, yellow, red or black, and while the color is 
 sometimes uniform, it is more generally spotted, blotched, 
 clouded or veined, producing that effect which is known 
 as " marbled." The hardness is that of calcite, 3; the rock 
 is thus readily scratched or cut by the knife, a ready means 
 of distinction from quartzite or sandstone, which may 
 resemble it. It is readily soluble in weak acids. Unless 
 the grain is too fine, the good rhombohedral cleavage of the 
 calcite grains can be easily seen with a lens. The chemi- 
 cal composition of a perfectly pure marble would be that 
 of calcite, CaO = 56, CO 2 = 44 per cent, but there are 
 usually small quantities of magnesia, alumina, iron and 
 silica present, coming from traces of sand, clay, dolomite, 
 etc., mixed with it; these may increase until the impure 
 marbles are produced, which are described in a later 
 paragraph. 
 
 Unlike most metamorphic rocks, marble, if pure, is very 
 massive and shows no sign of schistose cleavage, even 
 where its association with schists is such as to indicate 
 that it must have been subjected to enormous pressure 
 and shearing stresses. If impurities in the form of other 
 
386 ROCKS AND ROCK MINERALS 
 
 minerals are present it may then assume cleavage, caused 
 by their presence. The reason for this want of cleavage 
 has caused much speculation; it is probably due to several 
 causes, to the purity of the rock, to a rolling of the grains 
 among themselves, but chiefly to a curious property which 
 calcite possesses of permitting movement among its mole- 
 cules, whereby new crystal forms are produced without 
 destruction of its substance; this results in a complicated 
 microscopic twinning, somewhat similar to that explained 
 under feldspar. As a result of this the stresses are 
 absorbed molecularly, instead of producing changes in the 
 outward structure, as in most rocks. 
 
 Varieties of Marble. The varieties of marble from the technical 
 point of view are chiefly those which are based on color. Statuary 
 marble is the purest and whitest kind. Architectural marbles are those 
 of the most uniform tones of color, while ornamental marbles are 
 those distinguished by striking effects of varied colors, as mentioned 
 above. In the trade, the term marble is used for any lime carbonate 
 or dolomite rock which can be procured in large, firm blocks, and is 
 susceptible of a high polish ; under this definition many limestones are 
 included. Shell marble is thus a hard, firm limestone in which a 
 certain pattern is given by the presence of certain fossils, shells of 
 brachiopods and remains of crinoid stems being the most common. 
 The different yellow, red, and black marbles, most of them veined 
 and clouded, of Italy, Greece, and the East have long been distin- 
 guished by a host of names. 
 
 Those varieties which depend on the presence of some mineral, 
 additional to the calcite, are treated in the following section on car- 
 bonate-silicate rocks. 
 
 Occurrence of Marble. The great deposits of marble, 
 from which the material used for structural purposes is 
 taken, are the .result of regional metamorphism and it 
 is thus found in regions of metamorphic rocks associated 
 with gneisses, schists, etc., in the form of interbedded 
 masses, layers, or lenses. These vary in size within wide 
 bounds, from a few feet to many miles in length. It 
 forms immense interbedded layers, or masses, in the 
 Laurentian rocks of Canada; it occurs in quantities in 
 
DESCRIPTION OF METAMORPHIC ROCKS 387 
 
 Vermont, Massachusetts, Georgia and Tennessee, where 
 it is extensively exploited, in Colorado and other places 
 in the west. The marbles of Greece and Italy have at- 
 tained celebrity from their use by the ancient Greeks and 
 Romans in statuary and buildings. It is found in the 
 Alps, Germany, and Scandinavia in Europe, and in 
 various other places in the world. Marble is also produced 
 from limestone (and chalk) by the contact action of in- 
 truded igneous rock. Although some very coarse-grained 
 material may be formed in this way, it is usually quite 
 limited in amount. 
 
 Lime Carbonate-Silicate Rocks. As described under 
 the general properties of sedimentary rocks, all transitions 
 occur between limestones and sandstones, between lime- 
 stones and shales, and between the three combined. This 
 means merely, that the original lime deposits may have 
 had sand, clays, silt, and ferrugineous material in variable 
 amounts, mixed with them. Chemically, it means that 
 the carbonate of lime has silica, the oxides of aluminum 
 and iron, and usually small amounts of other things, such 
 as magnesia, potash, and soda mixed with it. Under the 
 conditions of metamorphism the carbon dioxide, CO2, 
 is driven out, to be replaced by an equivalent amount of 
 silica, SiO 2 , and thus silicates of lime, of lime and alumina, 
 of lime and iron, or mixtures of these, or combinations 
 containing other elements as well, are formed. Also 
 volatile substances, liquids and gases, such as water 
 vapor furnishing hydroxyl, fluorine, boron, etc., emana- 
 tions from magmas resting below or being intruded simul- 
 taneously with the crustal movements which give rise to 
 the metamorphosing conditions, may enter the rock mass 
 and thus, in adding new substances, produce additional 
 mineral combinations. The amount of silica present may 
 be sufficient to completely replace the carbon dioxide and 
 the resulting rock is then composed entirely of silicates, 
 or it may not be sufficient to accomplish this, and the 
 mass then consists of a mixture of lime carbonate, calcite, 
 
388 ROCKS AND ROCK MINERALS 
 
 mixed with silicates. Thus all transitions may be found 
 from pure marble, through varieties containing bunches, 
 masses, and individual crystals of some mineral, or miner- 
 als, into rocks completely made up of sometimes one sili- 
 cate, but usually of a mixture of them. The whole affair 
 is quite analogous to what has already been described as 
 the effect of contact metamorphism of igneous rocks on 
 impure limestones in a previous part of this book, and the 
 chemical reactions which take place are the same as those 
 there mentioned. The resulting rocks are also quite simi- 
 lar, with, however, one difference. In contact metamor- 
 phism the chief agency is heat, while pressure and 
 shearing are either wanting, or are relatively of slight 
 importance, but in regional metamorphism these are 
 factors of great intensity. Thus the rocks of contact 
 metamorphism are massive and with little or no schistose 
 cleavage, while those produced by regional metamorphism 
 may strikingly exhibit it; that cleavage is not always 
 present is due to the reason given above under the des- 
 cription of marble. 
 
 Important minerals which thus occur in limestone are 
 pyroxenes (especially wollastonite, CaSiOs, and diopside, 
 CaMgSi 2 6 ) ; garnets (especially grossularite, Ca3Al 2 (SiO 4 ) 3 ) ; 
 hornblendes (especially tremolite, CaMg 3 Si 4 O 12 ) ; jeldspar 
 (especially anorthite, CaAl2Si2O8); vesuvianite; epidote; 
 fluorite, etc. A whole host of minerals occurs, but many 
 of them, such as graphite, magnetite, spinel, titanite, 
 tourmaline, apatite, phlogopite, etc., come chiefly from 
 the impurities in the original rock, which have been 
 recrystallized. 
 
 It is clear from this, that, depending on mineral com- 
 bination, a great variety of these lime carbonate-silicate 
 rocks exist, but only a few of the most important types 
 can be mentioned. 
 
 Wollastonite-rock. Marble not infrequently contains crystals of the 
 pyroxene-like mineral, wollastonite, CaSiO 3 , and this may increase 
 until the rock is practically composed of it. It is apt to be accom- 
 
DESCRIPTION OF METAMORPHIC ROCKS 389 
 
 panied by diopside, hornblende, etc. The rock is white, generally 
 massive, and resembles marble, from which it is easily distinguished 
 by its superior hardness. It occurs in California, the Black Forest, 
 Brittany, etc. 
 
 Garnet-rock. This is a granular aggregate of grains of garnet, 
 generally accompanied by various other minerals in smaller, variable 
 amounts. If some calcite is yet present the garnets may show more 
 or less crystal form ; sometimes the calcite has been leached out and 
 the rock is porous. Apt to be yellowish, to reddish brown, in color. 
 Considerable magnetite is often present. New England, northern 
 New York, Montana, Germany, Alps, etc. 
 
 Epidote-rock, or Epidosite. Composed chiefly of epidote with 
 other minerals, quartz, garnet, etc. Sometimes massive granular, 
 sometimes schistose. Greenish in color, especially of a yellow- 
 green. Often very tough under the hammer. New England, Brazil, 
 Germany, etc. Sometimes the ferromagnesian igneous rocks, basalt 
 and dolerite, under proper metamorphic conditions, are converted 
 into a rock consisting chiefly of epidote, instead of hornblende or 
 chlorite as previously described, and of a yellowish green color. 
 They may resemble the above, but can usually be distinguished by 
 their mode of occurrence, geologic relations, greater uniformity, and 
 often by the remains of special structures, such as the amygdaloidal. 
 Instances occur in Pennsylvania, Virginia, etc. 
 
 Pyroxene-rock. In this case the rock consists chiefly of pyroxene, 
 of which the variety diopside is prominent. Other minerals, quartz 
 or calcite, etc., may occur. White, greenish, to dark green in color, 
 massive or schistose. Is found in Massachusetts, northern New York, 
 Germany, Bohemia, Sweden, etc. Under the head of metamorphic 
 pyroxene rocks there may be mentioned in this connection jade, 
 which, although extremely rare, is of great interest from its eth- 
 nological and artistic importance. Jade is a fine-grained, and 
 usually compact, aggregate of grains and fibers of the soda-pyroxene, 
 jadeite, NaAlSi 2 O 6 . It is sometimes snow-white, resembling marble, 
 but usually greenish (or with a violet shade) to dark green. The 
 greenish colors are also clouded, veined, or specked through the 
 white. When polished it has a soft, somewhat greasy luster. The 
 extraordinary toughness of the rock is one of its most marked 
 characters and on this account it was greatly prized in the early 
 history of mankind, before the discovery of metals, for the manu- 
 facture of weapons and implements, as shown by its distribution in 
 these forms, and in unworked pieces over the world. It has long 
 been greatly valued by the Chinese, who have devoted the most 
 laborious work to fashioning it into objects for personal adornment 
 and use. These objects, such as vases, bowls, etc., are often carved 
 with wonderful skill and taste and are greatly prized for their 
 
390 ROCKS AND ROCK MINERALS 
 
 artistic value. The rock is only known in place in upper Burma 
 and in the Kuen-lun Mountains of Turkestan. Its origin is uncertain, 
 but its chemical composition suggests that it may be a metamor- 
 phosed igneous rock of high soda content, such as nephelite-syenite. 
 A green hornblende rock called nephrite, from Siberia and New 
 Zealand, has similar properties and uses and is frequently mistaken 
 for jade. 
 
 Cipolin is a marble full of mica, which may show transitions to 
 calcareous mica-schist. Usually other minerals, sometimes in con- 
 siderable variety, are also present. 
 
 Dolomite Marble, Magnesia-Silicate Rocks. As men- 
 tioned under dolomite limestones, the rock name does not 
 necessarily mean that the substance composing it is pure 
 dolomite, in the mineralogical sense. There is generally an 
 excess of lime carbonate present, so that the composition 
 is a mixture of dolomite, MgCa(C0 3 ) 2 , and calcite, CaC0 3 . 
 Just as marble is related to ordinary limestone, so is 
 dolomite marble to ordinary dolomite. In a practical 
 way no distinction can be drawn between the two vari- 
 eties of marble, except chemically. See dolomite under 
 the rock minerals. Like ordinary marble, dolomite is 
 one end of a series of metamorphic rocks, which, 
 beginning with a pure carbonate, becomes a mixture of 
 carbonates and silicates, and ends in pure silicate rocks. 
 The causes and processes are identical with those 
 described under marble, only in this case the presence of 
 magnesia causes the formation of silicate minerals, in 
 which this element is either the only metal, or a vary 
 important one. Thus in distinction to the lime carbonate- 
 silicate series, this may be called the magnesian carbonate- 
 silicate series. The magnesian silicates thus produced 
 in the zone of constructive metamorphism may be anhy- 
 drous, or nearly so; on the rocks rising, by erosion or other- 
 wise, into the zone of hydration, they may be secondarily 
 converted in serpentine, H 4 Mg 3 Si 2 9 , or sometimes into 
 talc, H2Mg 3 (Si03) 4 . Thus these rocks are in many cases 
 closely connected with the talc-schists previously described, 
 while their relation to serpentine is mentioned under that 
 
DESCRIPTION OF METAMORPHIC ROCKS 391 
 
 rock. The more important magnesia silicates which take 
 part in the series are olivine, enstatite, chrondrodite, diop- 
 side, tremolite, phlogopite, etc., and secondarily serpen- 
 tine and talc, as stated above. Of the varied rocks formed 
 by these mixtures, only a few of the most important can 
 be mentioned. 
 
 Crystalline dolomites, or dolomitic marbles filled with variable 
 mixtures of minerals, chrondrodite, phlogopite, pyroxenes, etc., with 
 others, such as magnetite, spinel, apatite, graphite, etc., coming from 
 original impurities, are found rather commonly in the metamorphic 
 areas in the eastern United States and Canada, but have received no 
 distinctive names, as rocks. They appear to have been formed some- 
 times by contact, sometimes by regional metamorphism, often by a 
 combination of both. 
 
 Opkicalcite is a mixture of white calcite and green serpentine, the 
 latter often in veins, spots, or clouded through the rock. A part of 
 the " verde antique " marble of the ancients, used for ornamental 
 purposes, appears to have been a variety of ophicalcite. It occurs 
 in Canada, northern New York, and various places in Europe. 
 
 Soapstone and talcschist. Part of the rocks included under these 
 names belong in this series: they have been already described 
 under a previous section. Listwanite, which occurs in the Ural 
 Mountains, and in Spain, is a mixture of magnesia carbonates (mag- 
 nesite, MgCO 3 , and dolomite), with talc, and with more or less quartz 
 Sagvandite, from Norway, is a granular mixture of varieties of mag- 
 nesite and enstatite (MgSiO 3 ) containing ferrous iron. 
 
 Amphibolites. Many of the hornblende-schists or 
 amphibolites, previously described in a separate section, 
 are the result of the transformation of impure limestones 
 and dolomites into metamorphic rocks. This has been 
 already discussed, but it should be again mentioned here, 
 because the amphibolites, made in this way, form one of 
 the most important members of the lime and magnesia 
 series of carbonate-silicate rocks described above. 
 
 Occurrence of Minerals and Ores. The crystalline marbles and 
 dolomites, in addition to the minerals mentioned above, not infre- 
 quently, owing to local causes, contain a great variety of others. 
 Thus at Franklin, New Jersey, owing to the presence of zinc and 
 manganese, a number cf minerals containing these metals have been 
 
392 ROCKS AND ROCK MINERALS 
 
 produced, forming useful ores. Ore-bodies are mostly developed in 
 these rocks, however, by contact metamorphism, but in some cases 
 the minerals developed by regional metamorphism are of such a 
 character, and in such quantity, that they may be usefully exploited. 
 Many of the famous mineral localities, specimens from which are 
 commonly seen in collections, are in these rocks. The minerals 
 thus found embedded as crystals in calcite and dolomite, are apt to 
 have the angles between the faces more or less rounded, and to be 
 veined with calcite in their cracks. 
 
 SERPENTINE. 
 
 General Properties. No close distinction between ser- 
 pentine, as it has been described as a mineral, and serpen- 
 tine as a rock can be made. As a mineral the chemically 
 pure substance was considered, but serpentine as a rock 
 is generally more or less impure from the presence of 
 other minerals which are mixed with it. Serpentine 
 rocks are generally compact, of a dull to waxy luster, and 
 of a smooth to splintery fracture. If tolerably pure they 
 are soft and can be cut by the knife, but they are some- 
 times saturated by deposited silica, which makes them 
 much harder. The general color is green, characteristically 
 a yellowish-green; but sometimes yellow, yellow-brown, 
 reddish-brown, and dark green to black. On smooth sur- 
 faces the rock has a somewhat greasy feel, recalling talc- 
 schists, from which it is, however, readily distinguished 
 by its superior hardness. Talc leaves its mark on cloth, 
 while serpentine does not. The yellow-green color re- 
 sembles also that of epidote rocks, but here again the 
 superior hardness of the epidote serves as a distinction. 
 
 Associated Minerals. Other minerals which may accom- 
 pany the serpentine, and which may at times be seen in it, 
 are remains of the magnesia silicates from which it has 
 been formed, olivine, pyroxene, and hornblende. Metallic- 
 looking specks or crystals of ores are common, magnetite, 
 chromite, etc. In some varieties garnet occurs, chiefly 
 pyrope, and that which is used for gems comes in large 
 part from a serpentine in Bohemia. In the Ural Mountains 
 
DESCRIPTION OF METAMORPHIC ROCKS 393 
 
 serpentine is the source of platinum, and in other places of 
 nickel ores. Serpentine is apt to be accompanied by 
 other secondary minerals, by chlorite (sometimes the pur- 
 ple-red variety kammererite containing chromium), by 
 talc, and by magnesium carbonates, magnesite, MgCO 3 , and 
 breunnerite, MgFeCOs, etc. Serpentine rocks are usually 
 massive but sometimes schistose, serpentine-schist. Not 
 infrequently they are seamed by veins of the finely 
 fibrous variety of the mineral called chrysotile, which 
 has the structure of asbestus and is often so called. 
 
 Chemical Composition. The chemical composition of 
 serpentine rocks approaches that of the pure mineral, but 
 generally differs somewhat on account of the other minerals 
 present. This is seen in the appended analyses: 
 
 
 SiO 2 
 
 A1 2 3 
 
 Cr 2 O 3 
 
 Fe 2 3 
 
 FeO 
 
 NiO 
 
 Mg 
 
 CaO 
 
 H 2 O 
 
 CO 2 
 
 XyO 
 
 Total. 
 
 I.. 
 
 40.4 
 
 1.9 
 
 0.3 
 
 2.8 
 
 4.3 
 
 0.5 
 
 36.0 
 
 0.7 
 
 10.7 
 
 1.4 
 
 0.6 
 
 99.6 
 
 II. 
 
 36.6 
 
 1.0 
 
 0.3 
 
 7.3 
 
 0.4 
 
 0.3 
 
 40.3 
 
 0.1 
 
 13.4 
 
 
 
 0.4 
 
 100.1 
 
 Ill 
 
 38.6 
 
 1.3 
 
 0.5 
 
 5.6 
 
 2.2 
 
 0.1 
 
 39.1 
 
 0.9 
 
 11.3 
 
 0.5 
 
 0.2 
 
 100.3 
 
 IV 
 
 36.9 
 
 1.4 
 
 _ 
 
 6.9 
 
 4.0 
 
 
 
 36.0 
 
 1.4 
 
 13.1 
 
 
 
 
 
 99.7 
 
 V. 
 
 42.3 
 
 0.8 
 
 
 
 _ 
 
 2.6 
 
 
 
 40.3 
 
 1.3 
 
 12.5 
 
 
 
 0.5 
 
 100.3 
 
 VI 
 
 44.1 
 
 ~~ 
 
 
 
 " 
 
 ~ 
 
 ~" " 
 
 43.0 
 
 
 12.9 
 
 ~" 
 
 *"" 
 
 100.0 
 
 I, Serpentine, dark-green, Rowe, Massachusetts; II, Serpentine, 
 from pyroxenite dike, Mount Diablo, California; III, Serpentine, 
 Iron Mountain, Oregon; IV, Serpentine, from hornblende-schist, 
 Vosges Mountains, Germany; V, Serpentine, white, selected pure 
 mineral, Brewsters, New York; VI, Theoretical composition of pure 
 mineral, H 4 Mg 8 Si 2 9 . 
 
 The presence of small quantities of nickel and chrome 
 oxides is a very common feature. 
 
 Origin. Serpentine rocks are secondary in nature, 
 being formed when previously existent rocks, consisting 
 wholly or chiefly of magnesian silicates, are exposed to 
 the processes at work in the zone of hydration. Their 
 origin may thus be twofold: they may be formed from 
 
394 ROCKS AND ROCK MINERALS 
 
 igneous rocks, such as peridotite, dunite, etc.; or when 
 amphibolites or hornblende-schists, which have been 
 made from sediments in the zone of constructive meta- 
 morphism, are brought by erosion into the zone of hydra- 
 tion, they may be converted into serpentines. Thus the 
 origin of the material may be igneous or sedimentary, 
 but, whereas the igneous rocks pass directly into serpen- 
 tine, the sedimentary ones first pass through an interme- 
 diate metamorphic stage (hornblende-schists, etc.), and 
 are then converted. In this connection what has been 
 said elsewhere concerning the alteration of the peridotites 
 and allied rocks should be read This also explains in 
 part at least the origin of the chromium and nickel. No 
 formula can be given for the recognition of which origin 
 a serpentine has had; the geologic mode of occurrence and 
 relation to other rock masses is often a help, while the 
 presence of nickel and chromium, substances to be 
 expected in igneous, but not in sedimentary rocks, if 
 it can be shown, is very significant. 
 
 Occurrence. Serpentine is a common rock, and, while 
 it rarely forms large masses or covers extensive areas, it is 
 widely distributed over the world. In the form of layers, 
 lenticular masses, etc., it is common in metamorphic 
 regions from the alteration of both igneous and meta- 
 morphic rocks, and it thus occurs in eastern Canada, 
 New England, New York, Pennsylvania, Maryland, Cali- 
 fornia, Oregon, and other states; in southern England, 
 Germany, the Alps and various other places. It also 
 occurs in non-metamorphic sedimentary areas due to the 
 conversion of igneous rocks which have penetrated the 
 strata, as in places in Quebec, New Brunswick, New York 
 state, etc. 
 
 Alteration, Uses. Serpentine shows great resistance to 
 the action of the weathering agencies at the surface, but 
 eventually breaks down into a mixture of carbonates and 
 silica, mixed with ferruginous matter. The soils thus 
 formed, on account of the lack of alkalies and lime, are 
 
DESCRIPTION OF METAMORPHIC ROCKS 395 
 
 extremely barren, and often little or no vegetation grows 
 upon them. 
 
 On account of its beautiful coloring, serpentine has 
 been largely quarried for use as an ornamental stone, being 
 used for interior purposes much as highly colored marbles 
 are. It is sometimes employed for the same objects 
 for which soapstone is used; in many cases its softness is 
 an objection to its employment. In some places the 
 seams of fibrous chrysolite which it contains are mined for 
 use as asbestus. Its value as a source of ores of nickel, 
 chromium, etc., has been already commented upon, and is 
 further mentioned under peridotites and allied rocks. 
 
 IRON OXIDE ROCKS. 
 
 Itabirite. This rock is composed chiefly of micaceous 
 hematite and quartz. The micaceous hematite, or " spec- 
 ular iron ore " as it is often called, is in very thin tablets 
 or leaves of irregular outline, while the quartz is in aggre- 
 gates of grains. It much resembles mica-schist, and if one 
 were to imagine the mica of such a schist replaced by a 
 substance of mica-like thinness but with the metallic 
 luster of polished iron, he would have a good idea of the 
 appearance of this rock. Micaceous hematite is indeed 
 of not infrequent occurrence in genuine mica-schists, 
 and by its increase transition forms to itabirite are pro- 
 duced. Also, just as the relative quantities of quartz 
 and mica vary in different layers of mica-schist, so do the 
 micaceous hematite and quartz vary in itabirite; thus there 
 are layers poor in quartz, and others quite rich in it, of very 
 variable thickness. In addition to the mica, magnetite, 
 pyrite, talc, garnet, and others may occur as perceptible 
 accessory minerals. The rock is generally granular to 
 fine granular; very schistose; of a dark color on the cross- 
 fracture, and exhibits on the chief fracture the shining 
 steel-like luster of the specular iron ore. Sometimes the 
 amount of the iron mineral is so great as to practically 
 
396 ROCKS AND ROCK MINERALS 
 
 conceal the quartz. Itabirite forms extensive areas in 
 Brazil and on the Gold Coast of Africa, and in these places 
 carries native gold. It also occurs in North and South 
 Carolina, in Canada, Norway, Germany, etc. It has 
 probably been formed by the metamorphism of sandstones 
 and shales rich in deposited ferruginous matter, limonite, 
 etc. 
 
 Jaspilite is a name given to somewhat similar rocks 
 which consist of layers of red chert and hematite. They 
 occur in the Lake Superior Region. See page 297. 
 
 Magnetite Rock. This is a compact to granular aggre- 
 gate of grains of magnetite; dark-colored to black, and 
 heavy. The properties are those described under the 
 mineral. Hematite is very commonly mixed with it, and a 
 variety of other minerals, such at ilmenite, pyrite, quartz, 
 calcite, garnet, etc., according to the mode of occurrence. 
 The origin of magnetite rock is various; thus it may occur 
 as masses included in, or associated with, igneous rocks, 
 and is then regarded as a differentiated phase of such rocks, 
 as mentioned under them, and in this case the associated 
 minerals vary with the kind of rock, as nephelite and augite, 
 when with nephelite-syenite (Arkansas, Brazil, Sweden); 
 olivine, pyroxene, lime-soda feldspar when with gabbros 
 (Adirondacks, Sweden, Canada, Colorado, etc.). In other 
 cases it occurs as a contact formation where igneous rocks 
 have metamorphosed beds of limonite, siderite, etc. 
 Finally it occurs in regional metamorphosed areas, in the 
 form of layers and lenses, in the midst of gneisses and 
 schists, and often associated with metamorphosed lime- 
 stones and dolomites. It then often contains carbonates 
 of lime and magnesia, as well as the more common of the 
 silicate minerals described as associates of marble, such 
 as garnet, pyroxene, hornblende, etc. It is probably due 
 to the metamorphism of beds of impure limonite, clay- 
 ironstone, etc. Deposits of magnetite rock occur in many 
 places in the United States and Canada, in Scandinavia, 
 Germany, the Ural Mountains, etc., and are of great 
 
DESCRIPTION OF METAMORPHIC ROCKS 397 
 
 importance as sources of iron ore. Those which are 
 situated in genetic connection with igneous rocks are, 
 however, generally useless on account of the presence of 
 ilmenite, titanic iron ore, which prevents their being 
 profitably smelted. 
 
 Emery. This is a granular rock of a dark-gray to black 
 color, consisting mainly of grains of gray or bluish corun- 
 dum, often mixed with magnetite, and associated with 
 other minerals. It is sometimes quite schistose. It is 
 easily told by its weight and excessive hardness (corundum 
 = 9). It occurs, as layers of relatively small volume in 
 the crystalline schists, in Asia Minor, Island of Naxos, 
 Germany, Massachusetts, etc. Its use as an abrasive 
 material, on account of the corundum it contains, is well 
 known. 
 
CHAPTER XII. 
 THE DETERMINATION OF ROCKS. 
 
 THE determination and classification of rocks presents 
 itself as a problem, whose difficulty depends on what is 
 sought to be done, and the means at command for carrying 
 it out. It is obvious that the fine distinctions made by 
 petrographers among rocks, especially the igneous ones, 
 cannot be carried into ordinary practice, unless the same 
 methods for the study of rocks the use of the micro- 
 scope on sections ground thin and chemical analyses are 
 employed which they use. This, of course, cannot be ordi- 
 narily done, and we are thus limited to the means of 
 observation which have been used in this work, and to 
 simple classifications and the limited number of kinds 
 which they afford. This has already been commented 
 upon, in discussing the classification of the igneous rocks, 
 and need not be repeated. 
 
 Rock Characters used in Determination. The characters 
 of rocks which may be used for their megascopic determi- 
 nation are of two kinds, mineral and general. By the 
 mineral characters it is meant, that if the rock is composed 
 wholly or in part of mineral grains, which are large enough 
 to be distinctly seen with the eye or lens, and which may be, 
 if necessary, handled and tested, then the determination 
 may proceed along the line of a study of the minerals, their 
 kinds, relative abundance, and relation to each other 
 (rock-texture) . In this case there is no essential difference 
 between the microscopic and megascopic study of rocks; 
 one can accomplish, in the main, on the fractured surface 
 of a coarse rock, what the microscope does on the thin 
 section of a compact one. The individual minerals may 
 
THE DETERMINATION OF ROCKS 399 
 
 be studied and tested according to the methods given in 
 Chapter V; if in the field, the simple tests of Table No. I 
 may be used; if the conveniences of a laboratory are at 
 hand, the more complete one, Table No. II can be employed. 
 If it has been already determined, perhaps in the field, 
 whether the rock is igneous, sedimentary, or metamorphic, 
 its place can then be usually very quickly settled. Even 
 if all the different kinds of minerals cannot be told, the 
 determination of one or more will generally be of service. 
 
 The general characters are those which are resultant 
 from the combination of mineral grains; they might be 
 termed composite features of rocks. They include color, 
 structure,* texture, fracture, hardness, and specific gravity. 
 Of these the specific gravity is of the least general applica- 
 bility, because it requires a special apparatus to determine 
 it. The reaction of the rock with acids is also at times 
 extremely useful as a test, and may be added to the list. 
 These general characters are so useful that they deserve 
 some separate mention in regard to their employment in 
 rock determination. 
 
 Color. The rock color is the general resultant of those 
 of the combined mineral grains. Certain general conclu- 
 sions may be drawn from the color of a rock; thus if it is 
 pure white or nearly so, it is certain that compounds of 
 iron are either wanting in it, or are only present in traces, 
 and in general the rock is either a sandstone, quartzite, 
 limestone-marble, gypsum, or a nearly feldspathic igneous 
 rock, such as anorthosite, aplite, syenite, or felsite. Red, 
 brown, and green colors indicate the presence of iron com- 
 pounds ; black or stone gray may also, but in a sedimentary 
 rock, these colors may indicate carbonaceous material. 
 
 Structure. If the rock has a pronounced structure of 
 some kind, it may be of great assistance in determining 
 the general class to which it belongs, and this may be of 
 especial assistance if the geological relations of the rock 
 
 * The difference between the use of " structure " and " texture " 
 has been already explained, p. 158. 
 
400 ROCKS AND ROCK MINERALS 
 
 mass cannot be determined. Thus if a rock mass possesses a 
 pronounced columnar, or a highly vesicular, or an amygda- 
 loidal structure, it is almost certainly of igneous origin; 
 if a laminated or banded structure, it is probably sedimen- 
 tary; but this cannot be definitely relied on, because 
 igneous rocks, especially lavas, may assume a banded 
 structure by flowage, while metamorphic rocks may 
 acquire it by shearing movement. The oolitic structure 
 indicates a sedimentary rock. In general, structure must 
 be considered in connection with texture and other prop- 
 erties. 
 
 Texture. Certain textures are of definite assistance in 
 determining the family to which a rock belongs. Thus 
 the glassy texture is definite proof that the rock is of 
 igneous origin; a porphyritic texture shows the same 
 thing, especially if the phenocrysts are well crystallized, 
 and of quartz, or feldspar, or both. Metamorphic rocks also 
 contain at times well crystallized minerals, which are 
 similar to phenocrysts, as, for instance, garnet and stauro- 
 lite, but in general they also possess at the same time a 
 well foliated structure, which helps to distinguish them. 
 Sedimentary rocks do not exhibit this texture. The mere 
 contrast in color of a few dark mineral grains among many 
 lighter ones must not be mistaken for a porphyritic 
 texture. In general, a hard, firm, highly crystalline tex- 
 ture, alike in appearances in all directions through the rock, 
 is indicative of an igneous origin; but there are many 
 exceptions to this rule, as shown in various marbles and 
 quartzites. If a rock has a highly crystalline texture, 
 and at the same time a foliated structure, it is probably 
 metamorphic. 
 
 Hardness. This character, which can readily be tested 
 in a rough way in the field, is very useful in distinguish- 
 ing between certain classes of rocks. Thus very fine- 
 grained compact sandstones (or quartzites), limestones, and 
 dense igneous rocks often look much alike in specimens. 
 A simple test of hardness with the knife-point will at once 
 
THE DETERMINATION OF ROCKS 401 
 
 distinguish between the limestones (carbonate rocks, soft) 
 and the others mentioned (silicate, or silica rocks, hard). 
 If the rock is not very firm, care must be taken not to con- 
 fuse the mere breaking down or crushing of the rock fabric 
 with actual scratching of its component minerals. If the 
 rock itself is used to scratch with, care must also be taken 
 to test a number of corners, or edges, so that some single 
 grain, harder than the average, may not produce a false 
 impression of the average hardness. 
 
 Fracture. This is of less importance than the foregoing 
 characters, but yet in some ways is of value. Most rocks 
 which are firm and solid enough to have a distinct fracture 
 exhibit a more or less rough, hackly one. Those which 
 are fine-grained, or dense and compact, and which contain 
 a large amount of silica, or are wholly composed of it, 
 such as felsites, quartzites, flint, etc., possess a more or less 
 distinct conchoidal fracture, and the surface may be 
 splintery. Some dense limestones also have a splintery 
 fracture, and may even approach the conchoidal. Natural 
 glasses, such as compact obsidian, have a beautiful con- 
 choidal fracture. 
 
 Specific Gravity. This property is of much greater 
 value in the determination of minerals than of rocks. It 
 cannot of course be used in the field, as it requires definite 
 apparatus to determine it, as described in Chapter III, 
 under minerals; nevertheless, even in the field, a rough 
 distinction may be made between light and heavy rocks, 
 by weighing them in the hand. Rocks that are dark- 
 colored and very heavy, in general, are composed largely, 
 or chiefly, of iron-bearing minerals, and are apt to be of 
 igneous, or of metamorphic origin. 
 
 Treatment with Acid. This is particularly useful in 
 distinguishing the carbonate from the silicate rocks. 
 The method of treatment has been fully described in 
 Chapter V. and need not be repeated here. If necessary 
 almost any acid may be used, such as vinegar (acetic 
 acid), or lemon juice. For field use a few crystals of 
 
402 ROCKS AND ROCK MINERALS 
 
 citric acid powdered up may be conveniently carried, and 
 dissolved, when needed, in a little water; the test for 
 effervescence can thus be readily made. The test for 
 gelatinization, as also described in Chapter V, is also very 
 useful in determining the nephelite syenites from other 
 syenites and from granites, and also its effusive represen- 
 tative, the phonolite variety from the other felsites. It 
 should be remembered that olivine, which, however, chiefly 
 occurs in the dark ferromagnesian rocks, gabbros, perido- 
 tites and basalts, also gives this gelatinization test. 
 
 Determination in the Field. The best method of deter- 
 mining the family to which a rock belongs, that is, whether 
 it is sedimentary, igneous, or metamorphic, is to study its 
 characters in the place in which it occurs, and its relation 
 to other rock masses. For these features, and the larger 
 ones of its structure, may be very apparent in the field, 
 while a simple hand-specimen may entirely fail to show 
 them. The structure of a granite-gneiss, for instance, 
 may be very clear on the surface of a field exposure, and 
 be quite inappreciable in a small specimen. It is not 
 necessary to give here the characters and relations by 
 which the class may be determined; this is geological 
 rather than petrographical, and has been sufficiently com- 
 mented upon in Chapters VI, VIII and X. If the family 
 has been determined in the field, and the rock is coarse- 
 textured, so that the mineral grains can be seen, and if 
 necessary handled, Table I (p. 124) of Chapter V may 
 be used for their identification, and by then referring to 
 the classification of the appropriate family, its place in 
 general can be readily determined. 
 
 Table for Rock Determination. Appended to this chap- 
 ter is a table which may be used for the determination of 
 the more important kinds of rocks. It is based essentially 
 on the one given by Geikie, in his Textbook of Geology, 
 which has, however, been considerably modified, and 
 extended to meet the needs of this work. As the tests 
 which it demands are very simple, consisting for the most 
 
THE DETERMINATION OF ROCKS 403 
 
 part of those relating to hardness and effervescence with 
 acid, it may be readily used, even in the field. It must be 
 remembered, however, that a table of this nature can be 
 only quite general in character, and applicable to rocks of 
 well-defined types. Rock kinds grade into one another in 
 so many ways, as has been described in a number of places 
 in this work, that not only the student, but even the 
 experienced geologist, will sometimes be puzzled as to 
 the proper designation a particular type should receive. 
 But if this fact is borne in mind, it is believed the table 
 will prove useful in aiding one to classify the common 
 rocks. 
 
404 ROCKS AND ROCK MINERALS 
 
 TABLE FOR DETERMINING THE COMMON ROCKS. 
 
 The newly fractured surface of the unaltered rock shows 
 one of the following cases: 
 
 a. It is wholly or partly glassy. See A beyond. 
 
 b. Not glassy; of a dull, even appearance or stony; with- 
 
 out particular texture, or so compact that the indi- 
 vidual grains cannot be seen or recognized. See B. 
 
 c. Distinctly grained and crystalline; the grains can be 
 
 seen and determined. See C. 
 
 d. Has a distinctly foliated or gneissoid structure. 
 
 SeeD. 
 
 e. Has a clearly fragmental composition. See E. 
 
 A. Wholly or partly glassy. 
 
 1. Wholly of glass ; solid ; strong vitreous luster. Obsidian, p. 262. 
 
 2. Wholly of glass; solid; resinous or dull pitchy luster. Pitch- 
 stone, p. 265 (Obsidian and pitchstone may contain spherulites.) 
 
 3. Wholly of glass, but cellular or froth-like. Pumice, p. 266. 
 
 4. Of glass, but enamel-like, and composed of small, concentric 
 spheroids. Perlite, p. 265. 
 
 5. Partly of glass and partly of distinct, embedded crystals. 
 Vitrophyre, p. 267. 
 
 (The above forms are generally associated with, or pass into, 
 felsite lavas.) 
 
 6. Glass associated with, or passing into, basalt; rare. Tachylite, 
 p. 268. 
 
 B. Compact close-grained, and dull or stony; not glassy. 
 
 a. Very soft; can be scratched with the finger-nail. 
 
 1. Has a strong earthy or clay odor when breathed upon; rubbed 
 strongly between the fingers has ultimately a smooth, greasy feeling; 
 does not effervesce with acids. Various colors. Clay, p. 327. 
 
 2. Friable; crumbling; soils the fingers; little or no clay odor; 
 lively effervescence with acids; color white or light yellowish, etc. 
 Chalk, p. 310 or perhaps marl, p. 313. (Marl may give a good clay 
 odor.) 
 
THE DETERMINATION OF ROCKS 405 
 
 TABLE FOR DETERMINING THE COMMON ROCKS 
 
 Continued. 
 
 3. General characters as in 2, but does not effervesce with acids. 
 Diatomaceous earth, p. 298. 
 
 4. Harder, more compact than 1, 2, and 3. No clay odor; does 
 not effervesce; composed of a mineral with a good cleavage; some- 
 times fibrous; occurs in beds or veins. Gypsum, p. 293. 
 
 5. White to green, or gray; does not effervesce; no clay odor; 
 mass has a soft, greasy feel; is often foliated or shows a micaceous 
 cleavage; folia inelastic; marks cloth. Talc-rock, p. 374. 
 
 6. Not scratched by the nail, but easily scratched or cut 
 with the knife. 
 
 1. Composed of excessively fine, almost imperceptible particles; 
 dull, even appearance; gives clay odor when breathed on; no effer- 
 vescence or but feeble; has a laminated or stratified structure and 
 usually breaks easily into chippy flakes ; generally gray, but often red, 
 yellow, brown, bluish, or black. Shale, p. 327. 
 
 2. No clay odor, or but feeble ; brisk effervescence with acid; white 
 streak; commonly gray; sometimes white to brown or black. Lime- 
 stone, p. 304. 
 
 3. As in 2, but feeble effervescence in acid, which becomes brisk 
 when the acid is heated; generally white, yellowish or pale brown. 
 Dolomite, p. 307. 
 
 4. Pale to dark green or black, sometimes reddish; soapy or 
 greasy feel; translucent on thin edges; waxy or oily appearing; 
 subconchoidal or splintery fracture; no effervescence. Serpentine, 
 p. 392. 
 
 c. Not scratched or cut with the knife; scratches glass; 
 does not effervesce with acid. 
 
 1. Various colors, white to red or purple, brown to dark gray; 
 often gives a clay odor; frequently shows banded flow structure. 
 Felsite, p. 248. 
 
 2. Very hard; any corner or angle scratches feldspar; no clay 
 odor; scratches steel readily; light colors to brown or black; pro- 
 nounced conchoidal fracture; glimmering horny appearance. A 
 siliceous rock; either flint, p. 297, or perhaps the rhyolite variety of 
 felsite, p. 249. 
 
406 ROCKS AND ROCK MINERALS 
 
 TABLE FOR DETERMINING THE COMMON ROCKS 
 
 Continued. 
 
 3. Not so hard as 1 and 2. Does not scratch feldspar; color 
 black, very dark gray or green; is heavy; sometimes shows a cellular 
 or slaggy structure; sometimes contains amygdules. Basalt, p. 254. 
 
 C. Distinctly grained and crystalline; grains wholly or partly 
 determinable. 
 
 a. Is easily scratched with the knife. 
 
 1. Effervesces briskly with acid. Limestone p. 304, or more prob- 
 ably marble, p. 384. 
 
 2. Effervesces briskly only when the powdered rock is treated 
 with hot acid. Dolomite marble, p. 390. 
 
 3. Does not effervesce; probably granular crystalline. Gypsum, 
 p. 293, or anhydrite, p. 295. 
 
 4. Soluble, with distinct saline taste. Rock-salt, p. 295. 
 
 b. Hard; cannot be scratched with the knife, or scratches 
 with difficulty. Silicate rocks, two cases arise, x and y. 
 
 x. It is composed of grains of approximately equal size; 
 i.e., it is even-granular, like common granite. See X. 
 
 y. It is composed of larger, distinct crystals embedded 
 in a finer-grained groundmass; i.e., it is a porphyry. 
 SeeY. 
 
 X. An even-granular , massive, silicate rock. See p. 155. 
 
 1. Mainly or wholly composed of quartz and feldspar. Granite, 
 p. 205. See also aplite, p. 214. 
 
 2. Mainly or wholly composed of feldspar without quartz. 
 Syenite, p. 218. See also nephelite syenite, p. 221 and anorthosite, 
 p. 224. 
 
 3. Composed of feldspar and a dark ferromagnesian mineral; the 
 latter equals or exceeds the feldspar; a, the dark mineral is mostly or 
 wholly hornblende. Diorite, p. 226; b, the dark mineral is mostly 
 or wholly pyroxene. Gabbro, p. 229 ; c, the dark mineral is indeter- 
 minable. Dolerite, p. 235. 
 
 4. Composed entirely, or almost entirely, of ferromagnesian min- 
 erals; generally heavy and dark green to black (sometimes yellowish, 
 dunite). Peridotite, pyroxenite, etc., see p. 238. 
 
THE DETERMINATION OF ROCKS 407 
 
 TABLE FOR DETERMINING THE COMMON ROCKS 
 
 Continued. 
 
 5. Composed of grains of quartz ; scratches glass or feldspar readiljr 
 Sandstone, p. 323, or quartzite p. 366. 
 
 6. Much less commonly than the above, massive silicate rocks 
 produced by metamorphism may occur in this division. There are a 
 number of different varieties, depending on the particular mineral, or 
 minerals. Epidote-rock, garnet-rock, etc., would be examples. See 
 contact-metamorphism, pp. 180, 186, and carbonate-silicate rocks, 
 p. 384. 
 
 Y. A porphyry (see p. 156), composed of phenocrysts and 
 a groundmass. 
 
 1. Phenocrysts of quartz and feldspar and, perhaps, of a ferro- 
 magnesian mineral in a groundmass of the same. Granite-porphyry, 
 p. 243. 
 
 2. Phenocrysts of feldspar (and often of a ferromagnesian min- 
 eral) in groundmass of predominant feldspar. Syenite-porphyry, 
 p. 243. 
 
 3. Phenocrysts of ferromagnesian minerals, or feldspar, or both, in 
 a groundmass of feldspar and ferromagnesian minerals; feldspar 
 phenocrysts frequently striated, Diorite-porphyry, p. 244. 
 
 4. Phenocrysts of quartz, or feldspar, or both, and sometimes of 
 ferromagnesian minerals, in a predominant groundmass of light color 
 and dense feldspathic aspect. Felsite-porphyry, p. 251. 
 
 5. Phenocrysts of feldspar, or of a ferromagnesian mineral, or 
 both, in a dense, dark to black, and heavy groundmass. BasaU- 
 porphyry, p. 254. 
 
 D. It has a distinctly foliated, gneissoid, or slaty structure. 
 
 1. It contains feldspar, and generally quartz, with mica (some- 
 times hornblende). Gneiss, p. 351. 
 
 2. It consists mainly or largely of mica; often considerable quartz 
 is present, but feldspar is absent, or indeterminable. Frequently 
 contains crystals of dark red to black garnet, more rarely staurolite, 
 cyanite, etc. Mica-schist, p. 361. 
 
 3. Medium green, dark green or black; consists mostly of a felted 
 or matted mass of small, to very fine or microscopic, bladed, or needle- 
 like crystals arranged mostly, in one general direction, which promotes 
 the schistose cleavage. Other minerals, such as garnet, may be 
 present.Hornblende-schist or amphibolite (p. 379). 
 
408 ROCKS AND ROCK MINERALS 
 
 TABLE FOR DETERMINING THE COMMON ROCKS 
 Continued. 
 
 4. Very compact, or dense and fissile, splitting easily into thin, 
 more or less tough, ringing slabs; usually dark gray, or green to black, 
 but sometimes showing other colors. Slate, p. 369. (Sometimes 
 contains large crystals of staurolite, andalusite, etc.) 
 
 5. Very fissile, but soft to the feel ; laminae not tough, but often 
 brittle or crumbling; pronounced silky luster on the cleavage face. 
 Phyllite, p. 372. 
 
 6. Soft, greasy feel; marks cloth; easily scratched with the finger- 
 nail; usually whitish to light gray, or green. Talc-schist, p. 374. 
 
 7. Smooth feel; soft; green to dark green; glimmering luster. 
 Chlorite-schist, p. 376. 
 
 E. Has a clearly fragmental composition; is seen to be com- 
 posed of fragments or pebbles of other rocks, or of smaller 
 angular or rounded mineral fragments; if the latter , 
 frequently shows evidences of stratification. 
 
 1. The pebbles range from the size of a pea up and are rounded; 
 quartz ones are common; they are embedded in more or less of a 
 cement. Conglomerate, p. 320. 
 
 2. The pebbles are angular in shape. Breccia, p. 321. 
 
 3. Composed of various-sized angular fragments of volcanic rocks, 
 such as felsite .and felsite porphyry, of bits of pumice, or cellular lava, 
 or of rounded, vesicular, volcanic bombs, etc., mixed with fine com- 
 pacted material (volcanic ash). Volcanic tuff and breccia, p. 272. 
 
 4. Composed of more or less angular, but sometimes rounded 
 grains, in size from that of a pea down; the grains are mostly, or 
 wholly, composed of quartz, and scratch feldspar. Generally some 
 cement is present, which, if the rock is light colored, is apt to 
 effervesce with acid (lime carbonate); if red or brown does not. 
 Sandstone, p. 323. 
 
 5. As in 4 but more or less feldspar is also (present among the 
 quartz grains. Arkose, p. 326. 
 
INDEX 
 
 Actinolite, 61, 63. 
 Adinole, 188. 
 Adobe, 331. 
 Aegirite, 55, 58. 
 Aeolian rocks, 275. 
 Albite, 34. 
 
 Alkalic feldspar, 34, 35. 
 Alumina, test for, 117. 
 Amphibole, 60. 
 
 " determination of, 66. 
 
 Amphibolite, 379, 391. 
 Amygdaloidal structure, 159 
 Amygdaloid, basalt, 256. 
 Analcite, 103. 
 Andalusite, 76. 
 Andesite, 250. 
 Anhydrite, mineral, 113. 
 Anhydrite, rock, 295. 
 Anorthosite, 224. 
 Anorthite, 34. 
 Anthracite, 319. 
 Apatite, 95. 
 Aplite, 214. 
 Arfvedsonite, 61, 63. 
 Argillite, 369. 
 Arkose, 326. 
 Aschistic rocks, 169. 
 Ashes, volcanic, 141. 
 Associations of minerals, 29. 
 Augite, 55, 57. 
 Augitophyre, 257. 
 Average rock, composition, 18. 
 
 Basalt, basalt-porphyry, 254. 
 
 " amygdaloid, 256. 
 
 " quartz, 256. 
 Bathylith, 139. 
 Bauxite, 97. 
 Biotite, 50, 51. 
 Bituminous coal, 318. 
 Black-band ore, 302. 
 Bog iron ore, 301. 
 Bombs, volcanic, 141. 
 
 Border zones in igneous rocks 
 
 165. 
 
 Border zones, origin of, 170. 
 Boss, 138. 
 Bostonite, 254. 
 Breccia, friction, 322. 
 
 sedimentary, 321. 
 
 " volcanic, 140, 272. 
 
 " origin, 269. 
 
 Brown coal, 317. 
 Brownstone, 326. 
 Breunerite, 110. 
 Buhrstone, 368. 
 
 Calcite, 105. 
 Calcium, test for, 119. 
 Calcareous tufa, 312. 
 Camptonite, 257. 
 Cancrinite, 48. 
 Carbonates, test for, 115. 
 Cementation, zone of, 338. 
 Chalcedony, 86. 
 Chalk, 310. 
 
 Chemical elements, 18. 
 Chert, 297. 
 
 Chlorine, test for, 121. 
 Chlorite, 98. 
 Chlorite-schist, 376. 
 Chloritoid, 54. 
 Chondrodite, 82. 
 Chrysotile, 101. 
 Cipolin, 390. 
 Classification, general, 6. 
 
 " fragmental volcanic 
 
 rocks, 271. 
 Classification, glassy rocks, 261. 
 
 " igneous rocks, 191, 
 
 194, 203. 
 Classification, igneous rocks, table, 
 
 195. 
 Classification, metamorphic rocks, 
 
 348. 
 
 Classification, stratified rocks, 290. 
 409 
 
410 
 
 INDEX 
 
 Clay, 96, 327, 278, 280. 
 Clay ironstone, 301. 
 Cleavage of minerals, 26. 
 
 effect of, 28. 
 Coal, 315. 
 
 " hard, 319. 
 
 " soft, 318. 
 Color of minerals, 23. 
 
 " " rocks, 399. 
 
 " " sedimentary rocks, 286. 
 Columnar structure, 162. 
 Comagmatic regions, 174. 
 Complementary dikes, 167. 
 Conglomerate, sedimentary, 320. 
 
 volcanic, 322. 
 
 Consanguinity of rocks, 173. 
 Contact metamorphism, 180. 
 
 " " effect on 
 
 limestone, 187. 
 Contact metamorphism, effect on 
 
 sandstone, 186. 
 Contact metamorphism, effect on 
 
 shale, slate, 188. 
 Contact metamorphism, endomor- 
 
 phic, 181. 
 Contact metamorphism, exomor- 
 
 phic, 183. 
 Contact metamorphism, modes of 
 
 occurrence, 185. 
 Contact metamorphism, pneumato- 
 
 lytic, 189. 
 Contact metamorphism, ore bodies, 
 
 190. 
 
 Coquina, 311. 
 Cortlandtite, 238. 
 Corundum, 86. 
 Corundum-syenite, 226. 
 Crystals, denned, 21. 
 
 " form in rocks, 22. 
 " twinning of, 36. 
 Cyanite, 78. 
 
 Dacite, 250. 
 
 Decay of rocks, 276. 
 
 Determination of minerals, 114. 
 
 tables, 
 122. 
 Determination of rocks, 396. 
 
 " " table, 
 
 404. 
 Diabase, 235. 
 
 Diaschistic rocks, 169. 
 Diatomaceous earth, 298. 
 Differentiation, 164, 169. 
 Dikes, 134. 
 
 " complementary, 167. 
 Diopside, 55, 57. 
 Diorite, 226. 
 
 " rock relations, 229. 
 Diorite-porphyry, 244. 
 Dolerite, 235. 
 
 " use of word, 198. 
 
 " alteration of, 237. 
 Dolerite-porphyry, 244. 
 Dolomite, mineral, 108. 
 Dolomite, rock, 307. 
 
 " origin, 308. 
 Dolomite-marble, 390. 
 Dunite, 238, 240. 
 
 Earth's crust, composition, 17. 
 
 " interior, state of, 14. 
 Eclogite, 382. 
 Elements, geologically important 
 
 18. 
 
 Emery, 397. 
 Epidosite, 389. 
 Epidote, 73. 
 
 Eutaxitic Structure, 164. 
 Exotic mineral colors, 24. 
 Extrusive igneous rocks, 139. 
 
 Feldspars, 34. 
 
 alteration of, 44. 
 " cleavage of, 40. 
 color of, 41. 
 crystal form of, 35. 
 determination of, 46. 
 twinning of, 36. 
 Feldspathoid minerals, 47. 
 Felsite, felsite-porphyry, 248. 
 " sheared, 373. 
 " varieties of, 249. 
 Ferromagnesian minerals, 146. 
 Field classification, 6. 
 Flint, 86, 296, 297. 
 Fluorine, test for, 121, 81. 
 Fracture of minerals, 29. 
 " conchoidal, 29. 
 " of rocks, 401. 
 Fragmental volcanics, 269. 
 Freestone, 325. 
 
INDEX 
 
 411 
 
 Gabbro, 229. 
 
 " alteration of, 232. 
 
 " iron ores in, 234. 
 Garnets, 70. 
 Garnet-rock, 389. 
 Gelatinization test, 115. 
 Geyserite, 296, 298. 
 Glassy rocks, 260. 
 
 " alteration of, 269. 
 
 " " classification, 261. 
 
 Glaucophane, 64. 
 Glaucophane-schist, 383. 
 Gneiss, 351. 
 
 " field study of, 358. 
 
 " inclusions in, 356. 
 
 " texture of, 353. 
 
 " varieties of, 355. 
 Granite, 205. 
 
 complementary dikes of, 
 contact of, 215. 
 
 " graphic, 212. 
 
 " orbicular, 211. 
 
 " pegmatites in, 212. 
 
 " porphyritic, 207. 
 
 " weathering of, 216. 
 Granite-porphyry, 243. 
 Granulite, 360. 
 Gravel, 278, 279. 
 Graywackc, 326. 
 Greenstone, 229, 377. 
 Greenstone-schist, 377, 383. 
 Greensand-marl, 325. 
 Grit, 325. 
 
 Ground mass defined, 156. 
 Gruss, 216. 
 
 Gypsum, mineral, 111. 
 Gypsum, rock, 293. 
 
 Halite, 113, 295. 
 Hammer, geological, 11. 
 trimming, 12. 
 Hardness of minerals, 30. 
 
 " " rocks, 400. 
 
 Hauynite, 48. 
 Hematite. 91, 302. 
 Heulandite, 104. 
 Hornblendes, 60. 
 Hornblende-schist, 379. 
 Hornblendite, 238. 
 Harnfels, 188. 
 Hornstone, 188, 297. 
 
 Hydromica-schist, 373. 
 Hypersthene, 55, 57. 
 
 Ice, 20. 
 
 Igneous rocks, 132, 205. 
 
 " " consanguinity of, 
 
 173. 
 Igneous rocks, classification of, 
 
 191, 194, 203. 
 Igneous rocks, crystallization in, 
 
 146. 
 Igneous rocks, dense types, 247. 
 
 " general characters, 
 132, 141. 
 
 Igneous rocks, inclusions in, 163, 
 " jointing of, 161. 
 " minerals of, 145. 
 " occurrence of, 134. 
 214. " origin of, 164. 
 
 " petrology of, 132. 
 " post intrusive work 
 of, 174. 
 Igneous rocks, structure of, 158. 
 
 " " textures of, 150, 
 
 154. 
 Igneous rocks, variation of minerals 
 
 in, 142. 
 Ilmenite, 90. 
 
 Inclusions in granite, 213. 
 " gneiss, 356. 
 " igneous rocks, 163. 
 Injection of schists, 345. 
 Intrusive sheets, 135. 
 Infusorial earth, 298. 
 Iron ores, 88, 299, 395. 
 Iron oxide rocks, 395. 
 Iron, test for, 118. 
 Itabirite, 395. 
 
 Jadeite, 389. 
 Jade, 389. 
 Jasper, 86, 297. 
 Jaspilite, 297, 396. 
 Jet, 319. 
 Jointing, 161. 
 
 " in granite, 210. 
 
 Kammererite, 99, 393. 
 Kaolin, 96. 
 
 " from feldspar, 44. 
 Kimberlite, '242. 
 
412 
 
 INDEX 
 
 Labradorite, 35 (rock, 224). 
 Laccoliths, 136. 
 
 " zoned, 166. 
 Lamprophyre, defined, 168. 
 Lamprophyres, 257. 
 Lapilli, volcanic, 141. 
 Laterite, 332. 
 Lava flows, 139. 
 Lepidomelane, 52. 
 Lepidolite, 52. 
 Leucocratic rocks, 167. 
 Leucophyre, 251. 
 Leucite, 49. 
 
 rocks, 259. 
 Lignite, 317. 
 
 Lime-carbonate-silicate rocks, 387. 
 Limestone, 303. 
 
 " oolitic, 309. 
 Lime, test for, 119. 
 Limonite, 93, 301. 
 Listwanite, 375, 391. 
 Lithographic stone, 306. 
 Lithology defined, 2. 
 Lithophysae, 264. 
 Loam, 332. 
 Loess, 330. 
 Lydianite, 297. 
 
 Magmas, 134. 
 
 " composition of, 141. 
 " variations in, 142. 
 Magnesia-silicate rocks, 390. 
 Magnesite, 110. 
 Magnesium, test for, 119. 
 Magnetite, mineral, 89. 
 Magnetite-rock, 396. 
 Marble, 384. 
 Marble, onyx, 313. 
 Marl, 313. 
 
 " greensand, 325. 
 Megascopic, defined, 7. 
 Melanocratic rocks, 168. 
 Melaphyre, 255. 
 Metadiorite, 229. 
 Metamorphic rocks, 333. 
 
 " " age of, 346. 
 
 " " classification 
 
 of, 348. 
 Metamorphic rocks, composition 
 
 of, 344. 
 
 Metamorphic rocks, injection of, 
 345. 
 
 Metamorphic rocks, minerals of, 
 
 339. 
 Metamorphic rocks, occurrence of, 
 
 346. 
 
 Metamorphic rocks, older struc- 
 tures in, 343. 
 
 Metamorphic rocks, origin of, 333. 
 Metamorphic rocks, textures of, 
 
 340. 
 Metamorphism, 333. 
 
 agents of, 335. 
 constructive, 338. 
 effect of depth, 337. 
 " " heat, 336. 
 " " liquids, 337. 
 Miarolitic structure, 159. 
 Micas, 50. 
 Mica-schist, 361. 
 Mica-trap, 257, 215. 
 Microcline, 36. 
 
 Microscopical petrography, 7. 
 Minerals, associations of, 29. 
 " cleavage of, 26. 
 " color of, 23. 
 " exotic color of, 24. 
 " defined, 21. 
 " determination of, 114. 
 " fracture of, 29. 
 " hardness of, 30. 
 " specific gravity of, 31. 
 " streak of, 25. 
 Mineralizers, 149. 
 Minette, 237, 257. 
 Muscovite, 50, 51. 
 
 Natrolite, 103. 
 Necks, volcanic, 138. 
 Nephelite, 47. 
 Nephelite-syenite, 221. 
 Nephrite, 390. 
 Norite, 229. 
 Noselite, 48. 
 Novaculite, 297. 
 
 Obsidian, 262. 
 
 Occurrence of igneous rocks, 134. 
 
 Ocher, 328. 
 
 Olivine, 67. 
 
 " nodules, 258. 
 
 " rock, 238, 240, 390. 
 Onyx marble, 313. 
 
USTDEX 
 
 413 
 
 Oolite, iron, 303. 
 
 " limestone, 309. 
 
 " siliceous, 368. 
 Opal, 86. 
 Ophicalcite, 391. 
 Order of crystallization, 146. 
 Ore bodies, 190, 391. 
 
 " " formation of, 170. 
 Origin of igneous rocks, 164. 
 
 " " metamorphic rocks, 333. 
 Orthoclase, 34. 
 Oxides, important, 20. 
 
 Paragonite, 51. 
 Peat, 316. 
 Pebbles, 279. 
 Pegmatite dikes, 175. 
 
 " origin, 178. 
 Peridotite, 238. 
 
 ores in, 241. 
 relation to gabbro, 240. 
 Perlite, 265. 
 
 Petrographic provinces, 173. 
 Petrography denned, 2. 
 
 microscopical, 7. 
 Petrology denned, 1, 2. 
 
 " history of, 4. 
 
 of igneous rocks, 132. 
 Phenocrysts, 156, 245. 
 
 " pseudo, 342. 
 
 Phlogopite, 52. 
 Phonolite, 250. 
 Phosphate rock, 314. 
 Phosphoric acid, test for, 122. 
 Phosphorite, 314. 
 Phyllite, 372. 
 Pitchstone, 265. 
 Plagioclase feldspar, 34, 35. 
 Poikilitic texture, 239. 
 Porphyroid, 373. 
 Porphyry, 156, 242. 
 
 " basalt, 254. 
 
 " classification of, 195. 
 
 " diorite, 244. 
 
 " dolerite, 244. 
 
 " felsite, 251. 
 
 " granite, 243. 
 
 " labradorite, 257. 
 
 " syenite, 243. 
 Post-intrusive processes, 174. 
 Potash, test for, 120. 
 Pudding-stone, 321. 
 
 Pumice, 266. 
 Pyrite, 94. 
 Pyroxene, 55, 57. 
 
 alteration of, 58. 
 " determination of, 60. 
 Pyroxene-rock, 389. 
 Pyroxenite, 238. 
 
 Quantitative classification, 203. 
 Quartz, 83. 
 Quartzite, 366. 
 
 * oolitic, 368. 
 
 Rhyolite, 250. 
 
 Rock salt, 113, 295. 
 
 Rocks defined, 3. 
 
 " determination of, 398. 
 " general classification of, 6 
 " table to determine, 405. 
 
 Sagvandite, 391. 
 Salic defined, 146. 
 Sand, 278, 280. 
 Sandstone, 323. 
 Scale of hardness, 30. 
 Schist, talc, 374. 
 
 mica, 361. 
 
 hornblende, 379. 
 
 hydro-mica, 373 
 
 greenstone, 377. 
 
 glaucophane, 383. 
 
 chlorite, 376. 
 Schistose texture, 340. 
 Schlieren, 164. 
 Schorl, 78. 
 Scoria, 266. 
 Selenite, 111. 
 Sericite, 54. 
 Serpentine, 100, 
 Serpentine, rock, 392. 
 Shale, 327. 
 
 " alum, 329. 
 Siderite, 110, 301. 
 Silica, test for, 115. 
 Sillimanite, 78. 
 Silt, 278, 280. 
 Sinter, calcareous, 313. 
 
 " siliceous, 298. 
 Slate, 369. 
 
 " cleavage of, 371. 
 Soapstone, 378, 391. 
 Soda, test for, 119. 
 Sodalite. 48. 
 
414 
 
 INDEX 
 
 Soil, formation of, 276. 
 " gradation of, 278. 
 " movement of, 277. 
 Specific gravity of minerals, 30. 
 " " " rocks, 401. 
 
 " " table of, 31. 
 
 Specular iron ore, 91. 
 Spherulites, 264. 
 Spinel, 90. 
 Staurolite, 76. 
 Steatite, 378. 
 Stilbite, 103. 
 Stocks, 138. 
 Stratified rocks, 275, 293. 
 
 " " chemical origin of, 
 
 287. 
 Stratified rocks, classification of, 
 
 290. 
 
 Stratified rocks, color of, 286. 
 " " minerals of, 290. 
 
 " " origin of, 275. 
 
 " " structures of, 282. 
 
 " " .texture of, 284. 
 
 Streak of minerals, 25. 
 Structures of igneous rocks, 158. 
 
 " " metamorphic rocks, 
 
 340. 
 
 Structures of stratified rocks, 282. 
 Sulphuric acid, test for, 121. 
 Syenite, 218. 
 
 " common, 219. 
 " corundum, 226. 
 " nephelite, 221. 
 Syenite-porphyry, 243. 
 
 Table determining rocks, 404. 
 
 " " minerals, 122. 
 
 Talc, 102. 
 
 Talc-schist, 374, 391. 
 Test for alumina, 117. 
 " " calcium, 119. 
 
 " carbonates, 115. 
 
 " chlorine, 121. 
 
 " fluorine, 121, 81. 
 
 " gelatinization, 115. 
 
 " iron, 118. 
 
 " lime, 119. 
 
 " magnesium, 119. 
 
 " phosphoric acid, 122. 
 
 " potash, 120. 
 
 " silicates, 115. 
 
 " sodium, 119. 
 
 Test for sulphuric acid, 121. 
 
 " " water, 120. 
 Texture, augen, 343. 
 
 even-granular, 155. 
 " factors influencing, 151. 
 " foliated, 341. 
 " igneous rock, 150, 151 
 154. 
 Texture, lenticular, 341. 
 
 " metamorphic rock, 340. 
 
 poikilitic, 239. 
 " porphyritic, 156. 
 
 pseudo-porphyritic, 342. 
 " relation to occurrence 
 153. 
 Texture, schistose, 341. 
 
 " slaty, 342. 
 Thin sections, 8. 
 Tinguaite, 222, 254. 
 Topaz, 81. 
 Tourmaline, 78. 
 Trachyte, 250. 
 Trap, 257. 
 Travertine, 312. 
 Tremolite, 61, 63. 
 Tripolite, 298. 
 Tro polite, 230. 
 Tufa, calcareous, 312. 
 Tuff, volcanic, 140, 272. 
 
 " origin of, 269. 
 Twinning of crystals, 36. 
 " " multiple, 38. 
 
 " use of, 46. 
 
 Uralite, 66. 
 
 Variation of minerals, 142. 
 " diagram of, 145. 
 
 Vesicular structure, 158. 
 Vesuvianite, 75. 
 Vitrophyre, 267. 
 Volcanic tuff, 272. 
 " breccia, 272. 
 
 Wacke, 259. 
 Water, test for, 120. 
 Weathering, belt of, 337. 
 
 of rocks, 276. 
 
 Wollastonite-rock, 388. 
 
 Zeolites, 103. 
 Zinnwaldite, 52. 
 Zoisite, 74. 
 

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