Economic geology of the United States : with briefer mention of foreign mineral products

Front matter

Google to make the world's books discoverable online.

It has survived long enough for the copyright to expire and the book to enter the public domain. A public domain book is one that was never subject

to copyright or whose legal copyright term has expired. Whether a book is in the public domain may vary country to country. Public domain books

Preface

The object iu 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 Greology 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 unsatis- ^tory 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 Eiigineering 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. Vll

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, f(yr 1892, edited by Mr. R. P. Rothwell, 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. From 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, 1803.

Preface To Second Edition

Since a second edition of the "Economic Geology of the United States " has been called for in so short a time since the publication of the first, the author has not deemed it wise to attempt a thorough revision. Other duties have prevented him from giving the attention to the matter which would be de-

VUl PREFACE TO SECOND EDITION.

manded if he would caxry out the plan of revision which he had made with the expectation that several years would elapse before the first edition was exhausted. Since the first edition was published but little more than a year ago, the statistical part of the book is still fairly recent. Therefore, with the exception of a few corrections, no changes are made in the body of the book. In an appendix the statistics are brought as nearly down to date as is possible ; and some notes upon important recent developments are added. In the text these are referred to by italic letters.

Since in the minds of some of the reviewers there appears to be a misconception as to the object sought by the author in preparing the book, a word or two on that point seems in place here. The book does not claim to be an exhaustive treatise on Economic Geology, — with its present size it could not be. It is intended as a text-book for use in those colleges where in a short course the students are given a general survey of the mineral resources of the country, accompanied by a few specific and typical illustrations of the more important classes of deposits, and a statement of the economic bearing, including the statistical aspect, of the subject. Throughout the work an attempt has been made to treat the subject philosophically, and wherever possible, to show the relations between the various parts of the subject. In writing the book the author has also attempted to prepare a statement which an intelligent person could read, and thereby gain a knowledge of the wonderful mineral resources of the country, as well as to learn something of the principles of Economic Geology. It is for these two classes only that the book was written. For this pui-pose there have been introduced certain elementary, and, from one standpoint, outside subjects, such as the chapter on minerals of economic importance. It is for this reason, also, that technical terms have been avoided, wherever possible, and technical statements put into as simple form as was feasible.

Part I. — Introductory Chapters. CHAPTER L

PAOBB

Common Rock and Vein-formino Minerals 1-27

Elements and Minerals 1-4

Common Rock-forming Minerals 5-10

QuarU 5-6

Feldspar Group 0-7

Amphibole Group 7-8

Pyroxene Group 8

Mica Group 8-0

Calcite 9

Dolomite 10

Useful Minerals 11-15

Carbon Group 11-12

Phosphate of Lime 12-18

Gypsum 13

Salt 13

Corundum 13

Cobalt Minerals 13-14

Chromite 14

Magnesite 14

Asbestos 15

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

X CONTENTS.

PAon

Platinum 21

Manganese * 24-25

Aluminum 25

Tin 25

Nickel 26

Antimony 26

CHAPTER n.

RocKB OF THE Earth's Crust 28-61

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 . . 52-71

Coastal Plains 52-^8

Quaternary Coastal Plains 52-54

Florida Plains 54

Tertiary Coastal Plains 54-55

Cretaceous Coastal Plains 55-60

Triassic Coastal Area 66-58

Mountains of the Eastern States 68-61

The Eastern Archean Mountains 68-59

Appalachian Mountain Region 59-61

Central Plains 61-62

COKTSNTS. XI

Lake Superior Begion 6S

Cordillenn R^on t;:)-^

CHAPTER IV. Origi!? op Orb Dbpo^its 7S-0o

Original Condition 72-73

Removal of Original Ores 74-75

Origin of Cairities 75-78

Joint Planes 75-76

Fault Planes 77

Solution Cavities 77-78

Minor Cavities 78

Classification of Ore Deposits 78-M

I. Eruptive Deposits 80-81

II. Mechanical Deposits 81-82

ni. Chemical Deposits 82-04

(a) Precipitated Deposits 82-83

(6) True Veins 83-87

i. Chamber Deposits 84

IL Gash Veins 84

UL Fissure Veins 84-86

iv. Ore Channels mS7

(c) Replacement Deposits 87-88

(d) Impregnation Deposits 88-80

(c) Concretionary Deposits 80-00

(/) Segregated Deposits 00-02

(flr) Contact Deposits 02-04

Distribution of Ore Deposits 04-05

CHAPTER V. Mining Tbbms and Methods 00-115

Mining Terms 06-101

Variation in Veins 101-103

Mining Methods 103-100

Concentration of Ores 110-113

Reduction of Ores . . 113-115

XU CONTENTS.

Paet II. — Metalliferous Deposits.

CHAPTER VI. PAon

Brown Hematite Ores 120-122

Red Hematite Ores 122-127

Magnetite Ores 127-132

Carbonate of Iron Ores 132-133

Foreign Occurrences . 133-136

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 VIL

Appalachian Gold Fields 147-148

California Quartz Mines 14&-153

California Auriferous Gravels 153-168

Origin of Nuggets 157-168

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 . . . 166-166

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. Xm

CHAPTER Vm.

FAOn

Silver Mines in the United States 181-103

Comstock Lode 181-187

Eureka District 187-189

Other Silver Mines of the United States .... 180-103

Mexico, Central America, and Canada 103-106

South American Silver Bifines 106-107

European Silver Mines 107-100

Origin of SUver 100-201

Uses of Silver 201-204

Production of Silver 204-207

CHAPTEB IX.

Appalachian States 200-210

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

CHAPTEB X.

Lead akd Zikc 228-252

Appalachian District 220

Missouri Lead District 220-233

Colorado Lead Mines 233-235

Other Western Lead Mines 235-236

Foreign Lead Occurrences 236-238

Origin of Lead Ores 238

Uses'of Lead 238-230

Production of Lead 230-243

XIV CONTENTS.

PAOFit

Zinc in the United States 244-245

Foreign Zinc Mines 245-247

Origin of Zinc Deposits 247-248

Uses of Zinc . • 248-249

Production of Zinc 249-252

CHAPTER XI.

Mercury and Mai^oanesb 253-273

California Mines 253-255

Foreign Mercury Mines ....... 255-258

Origin of Mercury 258-260

Uses of Mercury 260

Production of Mercury 261-262

Manganese 262-273

Manganese in the United States 264-266

Foreign Manganese Mines 266-268

Origin of Manganese 268-270

Uses of Manganese 270-271

Production of Manganese - . 271-273

CHAPTER Xn.

Tin Mines of the United States 275-277

Foreign Tin Mines 277-281

Origin of Tin Ore 281-282

Production of Tin 283

Aluminum 283-291

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 Xin.

PAOF.»

Nickel and Cobalt 202-206

Mines in the United States 202-203

Foreign Mines 203-204

Origin of Nickel and Cobalt 204

Uses of Nickel and Cobalt 204-205

Production of Nickel and Cobalt 205-206

Antimony 207-200

Chromium 200-300

Iron PyrUe 300-301

General Beoiew of Metals 302-307

Part III. — Non-Metallic Mineral Products. CHAPTER XrV

New England Coal Basin 314-315

Appalachian Coal District 315-318

Central, Western, and Northern Coal Areas .... 318-310

Rocky Mountain, Pacific Coast, and Alaskan Coal Areas . 310-321

Origin of Coal 321-326

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

Uses of Coal 331-333

Production of Coal 333-336

CHAPTER XV.

Petrolbum, Natural Gab, and Asphaltum .... 337-358

Distribution of Petroleum 338-340

Origin of Petroleum 340-346

Uses of Petroleum 346-347

Production of Petroleum 347-340

Uses of Natural Gas 353-354

Production of Natural Gas 354-355

Asphaltum 355-357

Ozokerite 357-368

Xvi CONTENTS.

CHAPTER XVL

Building-Stones 350-386

Granite 360*368

Sandstone 360-372

Blaestone 372-373

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

Natural and Artificial Cements 387-300

CHAPTER XVII.

FertilUers 402-412

Limestone and Marl 402-403

Phosphatic Fertilizers 405-412

Mineral Phosphates 405-406

Rock Phosphates 407-412

CHAPTER XVra.

Precious Stones 421-425

Abrasive Materials 425-420

Infusorial Earth 426

Corundum and Emery 427

CONTENTS. XVil

9Aam

Grindstones and Buhrstones 427-428

Oilstones and Whetstones 428-420

StatisUcs 429

Bromine 434

Natural Soda 437

MagnesUe 437-438

FluorUe 440

Graphite 441

Lithographic Stone 442

Talc and Soap9tone 445-447

Aabestoe 447-448

Mineral Paints 449-450

General Summary of Mineral Prodtiction .... 460-450

Appendix

LiTSRATuss OF ECONOMIC Gboloot 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

• • •

XVIH CONTENTS.

PAon Table of Minbral Pbodccts of thb United States, 1884-

1803 (Folder)

List of Authors and Works of Reference referred to in

THE Text 483

Index . . 487

LIST OF ILLUSTRATIONS.

Plate Paor

I. Hydraulic Mining Frontispiece

II. Rockport Granite Quany 3G3

FlOVBB

1. Unconformity 46

2. Normal and Reverse Fault 60

3. Anticlines and Syndines 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

0. Concretionary Ore Deposits 80

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 164

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

Wales 163

18. Crosa-section of Comstock Lode, Nevada 186

xix

XX LIST OF ILLUSTBATIONS.

FiGUBX Paox

19. Cro68-6ection of Eureka Vein, Nevada 188

20. Ideal Section showing Mode of Occurrence of Lead in Wisconsin . 220

21. *' Openings ** and Ore Deposits in Galena Limestone, Wisconsin . 280

22. ** Flats,*' etc., in Galena Limestone, Wisconsin .... 231 28. Gash Veins and Cave Openings, Galena Limestone, Wisconsin . 282

24. Cross-section of Lode at Leadville, Colorado 284

26. Mode of Occurrence of Manganese in Arkansas .... 206

Part I. INTRODUCTORY CHAPTERS

n

ECOI^OMIC GEOLOGY

OF THE

UNITED STATES.

Chapter I. Common Bock And Vein-forming Minerals

Elements and Minerals. — Chemical investigations have shown that about ^^ 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 ninetynseven 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 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 wliatever 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 fragmental^ 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

COMMON ROCK AND YBIN-FORMING MINERALS. 3

optical characters. In mineralogy the crystallographic study is of prime importance,^ 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 (SiOj), 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 (AljOg), 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 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-fonning minerals are found in Rosenbusch's Microscopical Physiography y Vol. I. Translated by Iddings.

COMMON BOCK AND VfilN-FOB^lBIIKG MINERALS. 5

Common Book-forming Minerals. — Qvuirtz (SiO,) 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 Grroup. — 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 (K2OAI2O8 6 SiOg), and the plagioclase^ or lime soda group (NajOCaO 2 Al^Og 8 SiOg) . 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,^ 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-

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

COMMON ROCK ASD VEIN-FORMING MINERALS. 7

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 (HjAljSijOg + HjO), which is the chief constituent of many clays.

AmpMbole Group. — ^ To this group belong hornblende, which is the most common, tremolite, actinolite, etc. Homblende 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 augitCj 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 BOCK AND VEIN-FOBMINO 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 (CaCOg) 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 minemls 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.