Economic geology of the United States

Preface

The object in preparing this treatise is to supply the pressing need of a text-book to accompany a series of lectures given by the author to a class in Economic Geology at Cornell University. At first it was the intention to prepare merely a set of lecture notes ; but since this is at best a temporary and unsatisfactory expedient, and since the literature is so barren of works on this important subject, the author has decided to issue a text-book, with the hope that it may find a wider field than that for which it is primarily intended. It is a surprising fact that there is no text-book on Economic Geology which is sufficiently recent to be of value, and that nearly all of the books which have been issued on the subject treat exclusively of that part of economic geology which is in reality of least importance, and give no consideration to the large and important group of non-metallic minerals and rocks.

In the preparation of the book especial attention has been given to the mineral products of the United States ; and foreign localities are referred to only where they are of marked importance, or where they throw some light upon the origin of the materials. Even in this country only the most important and best known localities are described, types being chosen rather than large numbers of variable occurrences ; and, throughout the various chapters, the prime object is to point out the geological aspect of the subject, and secondarily the economic importance

VI PREFACE.

and relation of the several products. The scope of the work would need to be considerably enlarged to include much more than has been included here, and the expansion of the subject beyond this elementary outline must be left to the instructor. At times the treatment of the subject is somewhat dogmatic, since it is not possible, in such a small space, to always explain in detail the arguments upon which conclusions are based ; but so far as possible this has been done. There is also some repetition ; but this is intentional, and is introduced in order to illustrate the same principle from different standpoints.

Whenever possible, the localities chosen for description are those with which the author is personally familiar ; but, in a field so broad, and covering so many diverse industries, it is scarcely possible for one person to possess information concerning all, or even a large proportion, of the typical occurrences, or even of all the industries. The work is, therefore, in many parts a compilation, and free use has been made of all available sources. This has sometimes amounted to an abstract of descriptions or conclusions, but rarely to actual quotations, since a more condensed statement than is usually found in such sources is needed here. The statistics are almost entirely compiled from the standard sources, and, where more detailed information is desired upon these subjects, reference may be made to these original sources.

Aside from the geological reports of the state and national geological surveys, and from special articles in the scientific journals, particularly the Engineering and Mining Journal, geological descriptions have been obtained from Phillips's Ore Deposits and the Census Reports. The purely statistical, and a considerable part of the economic portions of the treatise have been obtained from the annual reports of the Director of the

PREFACE. VI 1

Mint ; the reports upon the Mineral Resources of the United States, edited by Dr. D. T. Day under the auspices of the United States Geological Survey ; the reports of the Census Bureau, particularly the volume on Mineral Industries in the reports of the Eleventh Census ; and the Mineral Industry, etc., for 1892, edited by Mr. R. P. Eothwell, editor of the Engineering and Mining Journal. Particular acknowledgment is due the last-named work, since it contains a more valuable body of statistics and economic material, written by experts upon these subjects, than any other source ; and, moreover, by the remarkable energy of its editor, the statistics are brought down to the close of 1892, and published a few months after its close. One will find there almost every variety of statistical and economic information upon the subjects treated. Prom these sources the statistics have been compiled, in some cases from one, in some from all combined, according as the needs of the work were best served. It has not seemed desirable to make direct acknowledgments in the following pages, excepting where some especial service can be rendered the student by direct reference to particularly valuable essays and monographs.

R. S. TARR.

Ithaca, N.Y., Sept. 5, 1893.

Part I. — Introductory Chapters, chapter I. Elements and Minerals 1-4

Common Rock-forming Minerals 5-10

Quartz o-Q

Feldspar Group 6-7

Amphibole Group . 7-8

Pyroxene Group . . .8

Mica Group 8-9

Calcite 9

Dolomite 10

Useful Minerals .11-15

Carbon Group . . . 11-12

Phosphate of Lime 12-13

Gypsum 13

Salt 13

Cobalt Minerals 13-14

Chromite 14

Magnesite 14

Common Vein-forming Minerals and Ores 15-27

Character of Ores 15-17

Common Veinstones 17

Fluorite 17

Barite 17

Common Ores 17-27

Gold ' .20

ix

CONTENTS.

Platinum

Lead

Zinc

Mercury

Manganese

Aluminum

Tin .

Nickel

Antimony

21-22

22-23

23-24

24-25

CHAPTER II. Rocks or the Earth's Crust 28-51

Sedimentary Rocks . . . 28-34

Chemically precipitated Rocks 30-31

Organic Rocks 31-32

Conditions of Accumulation 32-34

Igneous Rocks 34-38

Metamorphic Rocks . . . 38-41

Geological Age of Rocks 41-48

Age of Metamorphic Rocks ....... 41-42

Age of Eruptive Rocks 42-43

Age of Sedimentary Rocks ' . 43-48

Disturbance of the Rocks 48-51

Chapter III

Physical Geography and Geology of the United States Coastal Plains .

Quaternary Coastal Plains

Florida Plains

Tertiary Coastal Plains .

Cretaceous Coastal Plains

Triassic Coastal Area .

Mountains of the Eastern States

The Eastern Archean Mountains

Appalachian Mountain Region Central Plains ....

52-71

52-58

52-54

54-55

55-56

56-58

58-61

58-59

59-61

61-62

CONTENTS.

XI

Lake Superior Region 63

Cordilleran Region 63-65

Summarized Geological History . . . • . . . 65-71

CHAPTER IV Origin of Ore Deposits Original Condition

Removal of Original Ores .

Origin of Cavities Joint Planes Fault Planes Solution Cavities Minor Cavities .

Classification of Ore Deposits .

I. Eruptive Deposits II. Mechanical Deposits .

III. Chemical Deposits

(a) Precipitated Deposits (6) True Veins .

i. Chamber Deposits ii. Gash Veins iii. Fissure Veins iv. Ore Channels .

(c) Replacement Deposits

(d) Impregnation Deposits

(e) Concretionary Deposits (/) Segregated Deposits (g) Contact Deposits .

Distribution of Ore Deposits

72-95

72-73

74-75

75-78

75-76

77-78

78-94

80-81

81-82

82-94

82-83

83-87

84-86

86-87

87-88

88-89

89-90

90-92

92-94

94-95

CHAPTER V. Mining Terms 96-101

Variation in Veins 101-103

Mining Methods 103-109

Concentration of Ores 110-113

Xll CONTENTS.

Part II. — Metalliferous Deposits, chapter VI. Brown Hematite Ores 120-122

Red Hematite Ores 122-127

Magnetite Ores 127-132

Carbonate of Iron Ores 132-133

Foreign Occurrences . . . . 133-135

General Mode of Occurrence 135-136

Uses of Iron 136-137

Distribution of Iron Ores in the United States .... 137-144

Production of Iron 144-146

CHAPTER VII. Appalachian Gold Fields 147-148

California Quartz Mines 148-153

California Auriferous Gravels 153-158

Origin of Nuggets . 157-158

Other Western Gold Fields 158-160

Alaskan Gold Mines 160-161

Foreign Gold Regions 161-168

Russia and Siberia . . . . . . . . 163-164

South African Fields 164-165

Other European and Asiatic Countries . . . 165-166

South American Countries 166-167

Mexico and Canada ■ 167-168

Origin of Gold Deposits 168-170

Uses of Gold 170-172

Production of Gold ....... 173-175

Platinum Group ■ 176-179

Occurrence of Platinum 176-177

Uses of Platinum 177-178

Production of Platinum ....... 178

Other Metals of Platinum Group . . . . . 179

CONTENTS. Xlll

CHAPTER VIII. Silver Mines in the United States 181-193

Comstock Lode 181-187

Eureka District 187-189

Other Silver Mines of the United States .... 189-193

Mexico, Central America, and Canada 193-195

South American Silver Mines 195-197

European Silver Mines 197-199

Origin of Silver 199-201

Uses of Silver 201-204

Production of Silver . . 204-207

CHAPTER IX. Appalachian States 209-210

Lake Superior District 210-213

Montana Mines 214-215

Arizona and Other Western Mines . . . . . . 215-216

Foreign Copper Occurrences 216-220

Occurrence and Origin of Copper 220-222

Uses of Copper 222-224

Production of Copper . . . . . . . . 224-227

CHAPTER X. Appalachian District 229

Missouri Lead District 229-233

Colorado Lead Mines 233-235

Other Western Lead Mines . . . . . . 235-236

Foreign Lead Occurrences 236-238

Origin of Lead Ores 238

XIV

CONTENTS.

Zinc in the United States . . 244-245

Foreign Zinc Mines 245-247

Origin of Zinc Deposits 247-248

Production of Zinc 249-252

CHAPTER XI Mercury and Manganese Mercury

California Mines

Foreign Mercury Mines

Origin of Mercury

Uses of Mercury

Production of Mercury Manganese

Manganese in the United States . Foreign Manganese Mines

Origin of Manganese

Uses of Manganese .

Production of Manganese .

253-273

253-262

253-255

255-258

258-260

261-262

262-273

262-264

264-266

266-268

268-270

270-271

271-273

CHAPTER XII. Tin Mines of the United States 275-277

Foreign Tin Mines . . . . . . . 277-281

Origin of Tin Ore 281-282

Uses of Tin . . 282-283

Production of Tin 283

Occurrence of Aluminum ....... 283-286

History and Metallurgy of Aluminum .... 286-287

Uses of Aluminum 288-290

Production of Aluminum . . . . , . . 290-291

CONTENTS. XV

CHAPTER XIII. Miscellaneous Ores and General Review of Metals . . 292-307

Nickel and Cobalt 292-296

Mines in the United States 292-293

Foreign Mines 293-294

Origin of Nickel and Cobalt 294

Uses of Nickel and Cobalt 294-295

Production of Nickel and Cobalt 295-296

Antimony 297-299

Chromium 299-300

Iron Pijrite 300-301

General Review of Metals 302-307

CHAPTER XIV. New England Coal Basin . . . . . . . 314-315

Appalachian Coal District 315-318

Central, Western, and Northern Coal Areas .... 318-319

Rocky Mountain, Pacific Coast, and Alaskan Coal Areas . 319-321

Origin of Coal 321-326

Conditions Existing in Carboniferous Times .... 326-331

Uses of Coal , 331-333

Production of Coal 333-336

CHAPTER XV. Distribution of Petroleum . . . . . . 338-340

Origin of Petroleum 340-346

Uses of Petroleum 346-347

Production of Petroleum ....... 347-349

Uses of Natural Gas 353-354

Production of Natural Gas . . . ... . 354-355

Asphaltum 355-357

Ozokerite . 357-358

XVI CONTENTS.

CHAPTER XVI. Building- Stones 359-386

Granite 360-368

Sandstone 369-372

Bluestone 372-373

Summary of Building-Stone Production .... 383-386

Natural and Artificial Cements 387-390

CHAPTER XVII. Soils, Clays, Fertilizers, Artesian Wells, and Mineral

Limestone and Marl . . 402-403

Phosphatic Fertilizers 405-412

Mineral Phosphates 405-406

Rock Phosphates 407-412

Artesian Wells 412-418

Chapter XVIII

Precious Stones, Abrasive Materials, Salt, Miscellaneous Minerals, and General Summary of Mineral Production . 421-456

Precious Stones 421-425

Abrasive Materials 425-429

Infusorial Earth . . . . . . . 426

Corundum and Emery ....... 427

CONTENTS. XV11

Grindstones and Buhrstones 427-428

Oilstones and Whetstones 428-429

Statistics 429

Bromine 434

Natural Soda 437

Magnesite 437-438

Fluorite 440

Graphite 441

Lithographic Stone 442

Tale and Soapstone 445-447

Asbestos 447-448

Mineral Paints 449-450

General Summary of Mineral Production .... 450-456

Appendix

Literature of Economic Geology 457-465

Text-Books of Mineralogy and Geology 459

General Treatises upon Economic Geology .... 459-460

Special Articles 460

Works upon Special Subjects 460-464

Gold, Silver, and Lead 461

Zinc, Copper, Mercury, Manganese, Tin, and Aluminum . 461-462

Coal 462

Building- Stones, etc . . . 463

Miscellaneous Minerals 464

Mineral Statistics . 464

Mining Methods 464-465

Alloys and Metallurgy . . 465

List op Authors and Works of Reference referred to in

the Text 467

LIST OF ILLUSTRATIONS.

Plate Page

I. Hydraulic Mining • . Frontispiece

II. Rockport Granite Quarry . • 363

FlGTJKE

1. Unconformity 46

2. Normal and Reverse Fault 50

3. Anticlines and Synclines 51

4. Formation of Cavities by Faulting 76

4a. Formation of Cavities by Faulting 77

5. Bedded Ore Deposit 83

6. Fissure Vein 84

7. Replacement Ore Deposits 87

8. Impregnation Ore Deposits 88

9. Concretionary Ore Deposits 89

10. Segregation Veins 90

11. Contact Ore Deposits 93

12. Section of Mineral Vein 98

13. Section of Mineral Vein showing Horses 99

14. Section showing Shafts, Tunnels, etc 106

15. Section showing Mode of Occurrence of Iron in the Penokee

Region 125

16. Cross- section of Table Mountain, California 154

17. Section showing Position of Gold-bearing Gravels in New South

18. Cross-section of Comstock Lode, Nevada 185

xix

XX LIST OF ILLUSTRATIONS.

Figure Page

19. Cross-section of Eureka Vein, Nevada 188

20. Ideal Section showing Mode of Occurrence of Lead in Wisconsin . 229

21. "Openings " and Ore Deposits in Galena Limestone, Wisconsin . 230

22. " Flats," etc., in Galena Limestone, Wisconsin .... 231

23. Gash Veins and Cave Openings, Galena Limestone, Wisconsin . 232

24. Cross-section of Lode at Leadville, Colorado 234

25. Mode of Occurrence of Manganese in Arkansas .... 265

26. Artesian Wells . 415

27. Artesian Wells . . .416

Part I. INTRODUCTORY CHAPTERS

ECONOMIC OEOLOQY

OF THE

UNITED STATES.

Chapter I. Common Rock And Vein-forming Minerals

Elements and Minerals. — Chemical investigations have shown that about -f^ of the earth's crust is composed of the following sixteen elements : oxygen, silicon, aluminum, calcium, carbon, magnesium, potassium, sodium, iron, sulphur, hydrogen, chlorine, phosphorus, fluorine, manganese, and barium. About ninety-seven per cent of the earth's crust is composed of the first nine of these elements. Besides these there are over fifty other elements of greater or less rarity. Of the elements, oxygen is by far the most important. Nearly every element is found united with it, and in these various compounds it forms about one-half, by weight, of all the rocks. Silicon, next in abundance, is always found united with oxygen.

Some of the elements occur in the earth's crust free from union with other elements. Graphite and diamond are pure carbon, and gold, silver, copper, and even iron, are also found

l

A ECONOMIC GEOLOGY OE THE UNITED STATES.

free or native. These native elements are, however, extremely rare when compared with the great mass of minerals. The crust of the earth is oxidized, and whatever may have been its original condition or what may be the condition of the interior, upon the surface nearly all of the elements are in chemical combinations of greater or less definiteness, and these are called minerals.

Minerals occur in the crust in several conditions. Typically they are crystalline, or with definite molecular structure, as in the igneous and metamorphic rocks and most mineral veins. Their structure may be glassy when formed by the rapid cooling of a lava, or amorphous (without form), as in flint, when precipitated, without crystallization, from solution. Any of these minerals, partly by decay, partly by mechanical destruction, may be removed from the parent rock and deposited as sedimentary or stratified rocks. The minerals are then fragmented, as in sandstones or shales. Some minerals are dissolved from the rock and later precipitated, as in the case of gypsum, or formed into a sedimentary rock by the intervention of calcareous secreting animals. In such cases the minerals are usually amorphous.

Theoretically, minerals have not only a definite chemical composition, but also a definite geometric form. The component molecules build themselves together according to definite laws ; and when their growth is not interfered with, crystals are formed according to these laws, and the resulting crystal form is capable of mathematical calculation. Conditions which permit this perfect development are comparatively rare in nature, and consequently perfectly formed crystals are not common ; but in most cases the tendency is shown by definite molecular structure with well-defined optical characters. In mineralogy the ciystallographic study is of prime importance, 1 but in an economic study of minerals it is less important than the chemical composition and the ever-present physical characters.

A few predominant chemical compounds make up the greater part of the earth's crust. Of these, silica (Si0 2 ), a combination of silicon and oxygen, is the most important. This forms quartz and its numerous varieties, amethyst, agate, flint, etc. ; and, combined with other elements, often with an extremely complicated chemical composition, silica makes the great group of silicates, which includes the larger number of the common rock-forming minerals. Oxygen combined singly with an element forms another great group, the oxides to which many ores, such as those of iron, belong. Combined with aluminum oxygen forms alumina (A1 2 3 ), a common mineral; and this combined with silica is the base of our clays and an important rock constituent. Oxygen with carbon and some other element forms the carbonates to which limestones belong; with sulphur and some other element it forms the sulphates (gypsum, etc.) ; and with phosphorus and another element the phosphates. Sulphur, without oxygen, combined with an element forms a sulphide, fluorine a fluoride, chlorine a chloride, etc.

The elements are divided into two groups, the metals and the metalloids. This division was made at a time when the known elements were fewer than at present, and was based upon the possession or absence of metallic characters ; but it is now known that this division of elements is not as sharp as was at first supposed, though for the types of the two groups it still

1 In Dana's works on mineralogy the subject of crystallography is treated, but the best work upon the subject is Williams' Elements of Crystallography .

holds. Of the sixteen elements above mentioned, oxygen, silicon, carbon, sulphur, hydrogen, chlorine, phosphorus, and fluorine are metalloids ; the remaining eight are metals. Many of the metals are of economic value, because of certain properties which they possess ; but the metalloids, excepting carbon and sulphur, are of little importance in the free state. Nearly all common minerals, excepting quartz, contain metals in their composition, but the great majority have the metals either in such small quantities or in such refractory combinations that they are not separated. From the economic standpoint minerals are of importance in three distinct conditions. As rock-forming minerals, making a structure which can be utilized in the arts, some of the more common silicates are important. Secondly, certain minerals are of importance as minerals; the phosphates for fertilizers, the hydrocarbons for fuels and light, gypsum for plaster, and certain minerals which, because of rarity or beauty, are used as gems. Thirdly, many minerals are of value for the metal which they hold in chemical combination, and which man finds it profitable to extract. These three groups will be considered separately. In the first group are included quartz, feldspar, hornblende, augite, mica, calcite, and dolomite ; in the second, carbon, sulphur, phosphate of lime, gypsum, salt, corundum, cobalt minerals, chromite, magnesite, talc, soapstone, and asbestos ; in the third group, the ores and veinstones. 1

1 This consideration of minerals is general, and should be supplemented by a study of the minerals themselves, preferably by a thorough course in mineralogy. The object in treating the minerals here is to give prominence to an aspect of the subject not usually found in text-books of mineralogy. The relations and characters of the rock-forming minerals are found in Rosenbusch's Microscopical Physiography, Vol. I. Translated by Iddings.

Common Rock-forming Minerals. — Quartz (Si0 2 ) occurs in many forms in the earth, being very variable in colour, but commonly either glassy, or clouded whitish or black, though frequently coloured, as in amethyst. It has a glassy lustre, is hard, scratching glass readily, and is not scratched by the knife. There is no cleavage, but the mineral breaks with a rounded conchoidal fracture like glass. The specific gravity is low. Being both hard and not easily destroyed chemically, it is a very durable mineral, and remains long after many of the associated minerals have been decayed. Ordinary acids do not attack it, but it is soluble in small quantities in water, particularly when the water is charged with organic acids or alkaline carbonates. Hence quartz is transported by the water which creeps through the earth and is deposited in veins, and as a cement to many siliceous rocks, sometimes transforming sandstones to compact quartz or quartzite. That it is present both in fresh and salt water is shown by the fact that certain plants (Diatoms) and animals (Infusoria) build their shells or tests of silica which they extract from the water. There is no mineral more common than quartz. Sandstones are made up almost entirely of quartz grains, the fragmental remnants of previously existing rocks which have disintegrated and given up their less durable minerals to form clay. Gneiss and schist, rocks derived by metamorphism from other rocks, are in part made up of quartz, which, in some cases, forms the bulk of the rock. The original condition of quartz, as in the case of a great majority of minerals, is in eruptive rocks. Certain of these rocks, notably the granites, rhyolites, and quartz porphyries, contain this mineral as an essential constituent, it being one of the two predominant minerals. Aside from these common occurrences quartz is found in less abundance in many other rocks, sometimes in large pieces, sometimes in minute and even microscopic grains.

Feldspar Group. — The group of feldspars is divided into many species, but it is necessary to recognize here only the two main subdivisions, — the orthoclase, or potassium group (K 2 OAl 2 3 Si0 2 ), and the plagio.clase, or lime soda group (Na 2 OCaOAl 2 3 Si0 2 ). Aside from the chemical basis for this division there is also a crystallographic difference, each group belonging to a different system. Both groups have certain characteristics in common. There are well-defined cleavages, 1 the specific gravity is low, and the hardness less than in quartz, but just beyond the touch of the ordinary knife. The colour is usually dull and whitish, from decay, though green and reddish colours are not uncommon, and some fresh feldspars are colourless and glassy. On the cleavage faces the lustre is pearly, but on the rough fracture it is glassy. They differ somewhat in their ability to decay, but all are destructible ; and the different species, being of different chemical composition, decay at different rates. The chief difference, and the only one that can be made use of in the absence of a microscope, or a chemical analysis, provided the crystal form is not present, is the presence or absence of fine striations on one of the cleavage faces. These are very noticeable on some plagioclases, but never on orthoclase ; but, as it is not universally present, it is not of much value, and hence in this statement but little stress can be placed upon the division of the feldspars. An acquaintance with the rocks usually serves to indicate this difference in the feld-

1 A cleavage is a smooth fracture plane in a definite direction with reference to the crystal faces or the crystallographic axes.

spars, but this can only come after long experience in examining them. Feldspar, even in small grains, can usually be told from quartz by the presence of the smooth cleavage faces, and usually by the clouded, somewhat decayed appearance, contrasted with the fresh glassy appearance of the quartz.

The feldspars are extremely abundant and their distribution very widespread. In the igneous rocks they are more abundant than quartz, since in one form or another they appear in nearly all the common varieties. The feldspars are almost equally abundant in the metamorphic gneisses and schists. When attacked by weathering and percolating water, being chemically complex, they begin to decay and form simpler compounds. The salts of potassium, sodium, and calcium are transformed chiefly to soluble minerals which are carried away in solution, while the silica and alumina form quartz and kaolin, a hydrous silicate of alumina (H 2 Al 2 Si 2 8 + H 2 0), which is the chief constituent of many clays.

Amphibole Group. — To this group belong hornblende, which is the most common, tremolite, actinolite, etc. Hornblende is a silicate of magnesia with lime, iron, usually alumina, and a small percentage of other compounds. The colour varies in the amphiboles from black to white, but common hornblende is usually either black or dark green, the intensity of the colour varying with the per cent of iron and other metallic compounds. It is hard, heavier than either quartz or feldspar, has a glassy lustre, a conchoidal fracture, and a fairly perfect cleavage in two directions, with a pearly lustre on these faces. In granites, diorites, gneisses, and schists, it is common, and in many other rocks it occurs in less abundance. Owing to the readiness with which it decays, it is rarely found in the sedimentary rocks, although the products of its destruction enter frequently into the clays and shales.

Pyroxene Group . — The only common mineral of this group is augite, which resembles hornblende very closely both in chemical composition and physical structure. Indeed, unless the crystal form or the cleavage can be seen, the two cannot be told apart in the rocks. Generally the augite is more greenish in colour than hornblende, but unless the two are seen side by side, this hardly serves to distinguish them. The cleavage of augite is nearly rectangular, while that of hornblende forms a much greater angle. Augite is much less abundant than hornblende, but in some of the lavas, such as basalt and diabase, as well as in some of the gneisses, it is the predominating mineral.

Mica Group. — Mica is a family name applied to a wide variety of species which, however, can be considered here under two general groups, the white or muscovite mica and the black or biotite mica. Both are silicates of alumina with iron, potassium, magnesium, sodium, and other elements in smaller quantities. The white mica contains little iron, while the black mica sometimes carries as much as twentyeight per cent, the intensity of the blackness being usually proportional to the amount of iron. The true muscovite is very nearly transparent, while the extremely black biotite barely allows light to pass through thin flakes. In specific gravity they are about as heavy as hornblende, but there is some difference among them, the very ferruginous micas being a little heavier than the muscovite. They resemble each other in hardness (being easily scratched with the knife), in their

COMMON ROCK AND VELN-FORMING MINERALS. 9

pearly lustre, but primarily in their remarkable cleavage, which allows them to be split readily into extremely thin, elastic plates. When occurring in fine grains, as in granites, it is sometimes difficult to distinguish the black mica from hornblende ; but the remarkable cleavage is easily recognized, and this serves as a distinguishing feature.

In the rocks mica is perhaps most abundant in the schists and gneisses, though it is present also in many granites as an essential constituent, and also in other igneous rocks as an essential or accessory mineral. The white mica does not decay readily, and is consequently present in many sandstones. Biotite is also present in some of the sedimentary rocks, but it usually decays by the loss of iron and some of the soluble salts, and the absorption of water. Chlorite is then formed, and this mineral is found in clays and other sedimentary rocks often in great abundance. It is present in some schists, giving to them, as also to the clays, a greenish colour. It can be distinguished from anhydrous mica by its green colour and the marked cleavage, forming flakes which are no longer elastic.

Calcite (CaC0 3 ) is commonly either colourless or white, as in white marbles, though various colours are sometimes assumed. It is soft, of low specific gravity, and can be told from other common minerals by its action upon the application of dilute hydrochloric acid, when a brisk effervescence is caused. In the rocks it occurs, as a very widespread constituent, in minute quantities, in many igneous, metamorphic, and sedimentary rocks, but as the main constituent of limestone and its metamorphosed product, marble. In these strata it is usually impure by an admixture of clay or some foreign minerals which give various colours to the rock.

Dolomite. — Not uncommonly the carbonate of lime is combined chemically with magnesium, and the resulting mineral is dolomite ([Ca, Mg]C0 3 ). This resembles calcite very closely, but does not effervesce with weak, cold acids, though it will effervesce when the acid is heated. There are many dolomitic limestones, but in other rocks the mineral is uncommon. Both dolomite and limestone are used not only for building-stones, but also for a source of lime, and as a fertilizer.

The above minerals constitute the greater part of the rocks of the earth's crust. The only other really abundant minerals are the ores of iron which are scattered through all rocks as well as gathered together into veins. Sandstone is practically all quartz, or quartz and kaolin ; limestone and marble are either dolomite or calcite, and shales and clays are practically all composed of the decayed products, or finely ground fragments of some of the above minerals. Granite and the metamorphic gneisses and schists are composed chiefly of quartz and feldspar with either hornblende, mica, or augite, or a combination of these. The other igneous rocks are chiefly composed of feldspar, with hornblende, mica, or augite, or a combination of two or all three of these minerals. They are the essential minerals of the earth's crust, and other minerals are accessories, although in some rocks a rarer mineral may be in such quantities as to be an essential constituent of these particular rocks. For instance, a basalt may have enough olivine to become an olivine basalt, or a schist may be a tourmaline, epidote, or talc schist when one of these minerals is abundant ; but for our purposes the rarer kinds of rock-forming minerals may be neglected.

Useful Minerals. — This group, which includes those individual minerals used without chemical change, is a very large one, but only a few will be described. There are many rare minerals, such as the gems belonging to this class, but it is hardly necessary to describe these here.

Carbon Group. — The minerals belonging to this group are of three distinct kinds, — (1) the organic material of indefinite composition stored in the earth and in places transformed to coal or mineral oils, (2) graphite, and (3) diamond. Coal is not properly a mineral ; at least, it is not strictly the element carbon, but rather a hydrocarbon consisting of a great variety of chemical and mechanical mixtures. Yet from the stage of pure wood there is every gradation to graphite, through peat, lignite, bituminous coal, anthracite, and graphitic anthracite. Allied to this group is asphaltum, which in turn grades to mineral oils, and these to various indefinite carbonaceous compounds stored in the rocks. Carbon in these forms exists not only in beds and other accumulations, but also disseminated through many of the stratified rocks, particularly shales and limestones, where the black colour is often due to these hydrocarbons derived from the remains of animals or plants fossilized in the rock. With the hydrocarbon there is frequently some pure carbon.