Chapter 6 GEOLOGY

Introduction

In the preceding chapters an attempt has been made to present outline sketches of the geography, fauna, and flora of North America as they exist now. Yesterday, we may say for the sake of emphasis, there were differences from what exists to-day in each of these great groups of facts. That is, changes are everywhere in progress. With the recognition of this idea comes logically the conclusion that similar changes must have taken place in the past, and that the geography of the earth's surface, and its flora and fauna, at no very distant time must have been markedly different from what they are to-day. To test this hypothesis the geologist studies the records preserved in the rocks in much the same manner that the historian searches the papyri or the monuments of Egypt to discover what changes in the affairs of men have occurred since the days of the Pharaohs. The changes referred to are not essentially different from those now in progress, but in reality the two are parts of a single series. For a very long time there have been continents and oceans, lakes and rivers, and the land has been diversified by mountains and hills, plains and valleys, in the same general way as at the present time. When once the idea is grasped that we are living in a geological age, and that there is no break between the present and the past, it is evident that the history of the past can be interpreted by means of the results produced by known causes. Familiar formulas which express this idea are: "The present is the key to the past"; "Geography is the geology of to-day," etc. The forces or agencies which are now modifying the earth's surface, such as the rending of rocks by changes of temperature and the action of frost, erosion and deposition by streams, the dash of ocean waves against the land, volcanic eruption, the chemical action of organic acids, movements producing upheaval and subsidence, etc., have been in action for geological eras, but their intensity has varied from time to time and from place to place.

THE GROWTH OF THE CONTINENT

The geological history of North America is, in general, the same as that of other continents, but claims attention in certain particulars, largely for the reason that with the exception of Europe it has been studied more thoroughly than any other comparable land area. In Europe, throughout much of geological time, there have been numerous islands, and as a large portion of the records of past changes which have been presented were formed in the ocean, the results are complex. But in North America there has been a comparatively steady growth from one main continental centre or nucleus, and the records of the principal changes that have occurred are, to a greater degree, simple. Not only in the major features of the relief of the continent, as already described, but in its growth and geological history, it is, so far as can be judged from the present state of our knowledge of the various land areas, the most typical of all the continents.

Changes in the outlines and area of a continent are brought about principally by movements of elevation or depression in the earth's crust. Of less importance is the erosion of the margin of the land by waves and currents and the deposition of material brought from the land by streams, together with the spits, bars, and embankments made by waves and currents. By these and other and less conspicuous processes the shape of North America has undergone numerous changes in outline and is still being modified.

General maps have been prepared by J. D. Dana and others, showing the outlines of North America at various stages in the course of its development, and from a series of such maps recently compiled by D. C. Schaffner those here reproduced (Fig. 33) have been selected to illustrate the growth of the continent. As has been shown by various geologists, the outlines of the present continents and ocean-basins had their major features determined at a very early stage in the history of the earth, and at a time preceding the existence of the oldest known sedimentary rocks. At the close of the Archean, the earliest geological era now recognised, and, so far as has been determined, before life existed on the earth, the principal nucleus of North America was a land mass some 2,000,000 square miles in area, situated mainly in what is now the eastern half of Canada, from which there was a southward prolongation represented by the Adirondack hills of New York (Fig. 33, A).

The rocks forming this earliest known land in the Western Hemisphere consist of crystalline schists, gneisses, and granite, which are considered by some geologists at least as having resulted from the metamorphism of sedimentary beds. Penetrating and intimately intermingled with these greatly altered rocks, some of them perhaps metamorphosed lavas and allied terranes, are many rocks that were forced upward from deep in the earth into fissures in a molten condition and have since cooled and crystallized. More than one epoch of metamorphism has perhaps occurred, and the entire record now accessible is exceedingly complicated.

The physical conditions at the earth's surface at the close of the Archean period, as may reasonably be inferred, were not essentially different from what they are now. The land areas were eroded by streams, and the débris carried to the sea and deposited, the coarser near shore and the finer farther seaward. Upward movements in the earth's crust in various places subsequently laid bare a portion of the sea-floor adjacent to the former land, and the continent was enlarged. The outline of the land as it existed previous to the upheaval which exposed this portion of the ocean's bottom would be defined by the landward margin of the material deposited. The exposed sediments would be coarsest near the former coast-line and become finer and finer seaward from it, and the fossils contained in the consolidated sands and clays would also supply evidence bearing on the origin of the rocks. It is by such interpretation of the ancient records in the light of what is now taking place that the geologist is enabled to map approximately the outline of North America at several stages in its growth in the manner shown on the series of maps here presented. Information in this connection, however, concerning both the northern and southern portions of the continent is too meagre at present to be largely utilized in these outline sketches.[5]

[5]The relations of the eras referred to on these maps and the positions they occupy on the geological time-scale are shown a few pages later on a chart of the geological history of North America.

The next system thus far recognised, following the Archean, is the Algonkian, at the close of whose deposition some additions had been made to the Archean or pre-Algonkian land. Succeeding the Algonkian system come, in succession, the Cambrian, Ordovician, and Silurian systems. At the close of the Silurian there was a decided increase in the size of the main nucleus of the continent. Owing principally to an excess of elevation over subsidence in the portion of the earth's crust beneath the northeastern part of the region now occupied by the United States, portions of the sediments deposited previous to the close of the Silurian were upraised and important additions made to the extent of the land southward from the Archean area of Canada. This "Appalachian peninsula" would be conspicuous in a map representing the outline of the continent at the close of the Silurian. The eastern margin of the growing continent was then well to the eastward of its present position, but how far beyond the present coast we have no means of determining. Although at the close of the Silurian the continent had greatly increased in area over that of the nucleus at the close of the Archean, it bore but little resemblance to its present form. It is worthy of note, however, that with the exception of the eastward extension of the land at the time referred to, the growth had been within the present continental outline.

A later stage in the growth of the continent is shown in Fig. 33, B, when its eastern margin had much of its present general outline and the Appalachian Mountains were in their prime. The time indicated is at the close of the Paleozoic era, and after the great coal-fields extending from Pennsylvania southward to Alabama and westward to beyond the Mississippi were formed. The eastern half of the continent was approximately completed at the time just referred to, and is older than the western half.

During the Cretaceous period great changes took place in the geography of the still growing continent, as may be seen by the map illustrating that period. The conspicuous features in the geography are the submerged Atlantic and Gulf borders, and the presence of a broad belt of ocean water in the continental basin which reached from the then much expanded Gulf of Mexico to the Arctic Ocean, and divided the land into an eastern and a western continental island.

Following the Cretaceous period came the Tertiary period, during which the continent assumed very nearly its present outline. During this period, however, as is indicated in Fig. 33, D, the Atlantic border of the United States from New England southward and a wide area about the Gulf of Mexico, were submerged and had deep layers of sediment deposited on them. During the Tertiary, bodies of fresh water became for the first time a conspicuous feature on the land, and large lakes and broad silt-depositing rivers existed particularly in the Pacific mountain region of the United States, and at its close the continent was practically completed as we now know it, but several important oscillations, particularly at the north, have since occurred.

Fig. 33a.-Maps showing the growth of the North American continent.

Fig. 33b.

Fig. 33c.

Fig. 33d.

With the growth of the continent, briefly outlined above, came greater and greater diversity in its relief, due principally to the upraising of various mountains in a somewhat orderly succession from east to west.

The oldest mountains on the continent are the Laurentian Highlands of eastern Canada. Although the region referred to-the one mentioned above as being composed of Archean crystalline rocks-is not now of sufficient elevation or ruggedness to be termed mountainous, it shows in the nature and structure of its rocks that deep erosion has taken place. The inference is that truly great mountains have been removed, but the evidence may also sustain the interpretation that slow upheaval has been accompanied by erosion, and that at no time was the land conspicuously elevated.

Next in age after the Laurentian Highlands come the mountains of New England and the maritime province of Canada, which were upraised at the close of the Silurian period. The next great step was the crumpling into folds and upheaval of the rocks in the Appalachian region at the close of the Paleozoic era. The Park and Stony Mountains were upraised at the close of the Mesozoic era, and later came the Sierra Nevada and Cascades, followed by the Coast Ranges. Youngest of all, and in part for that reason the boldest and most lofty, are the magnificent mountains of southern Alaska, with a host of sublime peaks, like Mounts Fairweather, Logan, St. Elias, and perhaps McKinley. The last-named and highest peak of all, however, may be of volcanic origin.

In the above list showing the progressive westward movement of the birth of mountain systems, account is taken only of the elevations produced by upheaval. The mountains due to volcanic eruptions, which are still conspicuous, are all young, in comparison with the mountains situated to the eastward of the Sierra Nevada. The majestic cones of the northwestern portion of the United States, of which Mounts Shasta, Hood, Adams, Rainier, Baker, etc., are the most glorious, are of Tertiary or later age. The same is true, so far as known, of the still more lofty volcanoes in Mexico. The "pine-tree" forms of steam rising from the volcanoes of the Caribbees, Central America, southern Mexico, and southwestern Alaska, proclaim the recency of the birth of the frequently magnificent craters built of rocks that were once molten, from which they emerge.

THE ROCKS OF WHICH THE CONTINENT IS COMPOSED

The rocks of which North America is built belong to three classes, which are world-wide in their distribution. These are: First, rocks produced by the cooling and crystallizing of formerly molten magmas; second, those deposited by water; and third, those which previously belonged to either of the two classes just referred to, but have been recrystallized and so greatly changed that their preceding condition is no longer clearly recognisable.

These three classes or subkingdoms, as perhaps they might be termed from analogy with systems of biological classifications, are in technical language:

1. Igneous rocks, such as the lava of Vesuvius.

2. Sedimentary rocks, such as sandstone, shale, limestone, coal, etc.

3. Metamorphic rocks, such as gneiss, schist, some granites, etc.

These major divisions are based principally on mode of origin, but do not indicate relative age. While theoretically at least, and in a general way, the rocks of these three great classes came into existence on the earth in the order named, it is convenient to consider first those of sedimentary origin.

Plate IV.-Leading geological features.

Click image to enlarge.

The Sedimentary Rocks (Plate IV).-Whenever land exists or the waves and currents of the ocean come in contact with the rocks denudation occurs. That is, the rocks are broken through the action of mechanical or chemical agencies, such as the friction of the gravel and sand swept along by streams, the solvent power of water, etc., and the fragments thus produced are removed principally through the action of flowing water and deposited. Resulting from this general process of rock decay and disintegration, combined with transportation and deposition, there result mechanically formed sedimentary beds, such as shale, sandstone, conglomerate, etc.; chemically formed sedimentary beds, such as the deposits of springs, the saline precipitates from inclosed lakes, etc.; and organically formed sedimentary beds, as, for example, peat, coal, and limestone.

Since the first appearance of land in the region now occupied by North America, sedimentary rocks have been in process of formation, and in this way the growth of the continent, with the aid of movements in the earth's crust, has been produced.

The superficial extent of the sedimentary beds in North America is very great, as is indicated on the map referred to above. By far the larger portion of the surface of the continent is underlain by them. Their thickness varies from place to place, but probably reaches a maximum in the Appalachian region, where a depth of some 40,000 feet has been measured. Throughout the continental basin their depth is in general from 3,000 to 4,000 feet. In the Pacific mountains their thickness embraces tens of thousands of feet, and the same is true in Mexico, Cuba, and Jamaica. These sedimentary rocks contain fossils which, with comparatively few exceptions, show that they were deposited in the ocean; thus sustaining in an important manner the conclusion already presented in reference to the growth of the continent.

Great as is the area of the sedimentary beds at the present time, it does not show the entire extent to which what is now land has at some time been submerged beneath the sea. In certain broad regions, sedimentary beds which formerly existed have been removed by erosion; in other extensive areas they are covered by volcanic rocks, and in still other portions of the continent, embracing thousands of square miles, they have been metamorphosed and their original characteristics obliterated.

The system of classification of the sedimentary beds that has been adopted, as is well known, is based on the relative age of the formations, determined primarily by the occurrence of one formation above another, in regions where but moderate disturbances in position have occurred. Many of the stratified rocks contain fossils-that is, records of the life of the time they were deposited, and after the order of succession of a large number of formations has been ascertained, the life records they contain may be used as a means of determining the age of a newly discovered terrane.

By grouping the information obtained from the study of the vertical sequence of the formations in many regions, and also the records of life contained in them, a composite geological column has been constructed which shows the relative age of all known formations. The larger divisions of such a scheme of classification are world-wide in their application, but the smaller divisions are usually of restricted geographical extent.

The scheme of classification of general application in North America is shown in the chart on page 308. The arrangement is in order of age, the oldest formation being at the bottom. There is some lack of uniformity among American geologists as to certain of the terms used, more especially in the lower portion of the column, and in part the scheme is provisional, but in general it may be taken as expressing the progress made in the study of the geology of North America up to the present time.

The names of the larger divisions in this scheme of classification, or those designating the groups and systems and the eras and periods, have for the most part been adopted from European geologists. Two important ones, however-namely, Archean and Algonkian-are of American birth.

Outline Chart of the Geological History of North America

Rock-Scale. ? Group. System.

Time-Scale. ? Era. Period.

Zoic time: embracing the history of the earth since the appearance of life. Time of Mammals. Time of Palms and Angiosperms. Psychozoic. Human.

Cenozoic. Pleistocene.

Time of Reptiles. Tertiary.

Time of Cycads.

Mesozoic. Cretaceous.

Time of Amphibians. Time of Acrogens (Ferns, club-mosses, etc.). Jura-Trias.

Paleozoic. Carboniferous.

Time of Fishes. Devonian.

Silurian.

Time of Molluscs and Crustaceans.

Time of Alg?. Ordovician.

Cambrian.

Eozoic. Algonkian.

Time of Protozoa? (As yet unknown pre-Algonkian sediments.)

Azoic time: preceding the dawn of life. Azoic. Archean or Basement Complex.

Prehistoric Solid Earth.

Molten Earth.

Gaseous Earth.

While this scheme of classification is based on the succession of sedimentary beds, igneous and metamorphic rocks have a place in it, providing their age can be determined.

The Archean period includes the time previous to the deposition of the oldest known sedimentary beds, and its lower limit is as yet undefined. The Archean system, or the rocks formed during the Archean period, are without known fossils, and consist largely of gneisses and foliated schists, which are metamorphosed sedimentary or igneous terranes, together with various eruptives. The typical area where these rocks are exposed at the surface is in the Laurentian Highlands of eastern Canada, the main Archean nucleus of the continent, but rocks of the same age and same general character occur in several of the mountain systems of both the Atlantic and Pacific cordilleras, and underlie the sedimentary beds throughout a large part of the Continental basin. The Archean system was named by J. D. Dana, and divided into two portions, namely, the Laurentian below and the Huronian above. More recent studies, especially by C. R. Van Hise, have shown the necessity of removing from the system many of the terranes formerly referred to it, and of placing them in the Algonkian. The Archean as it remains after this adjustment is termed by Van Hise the Basement Complex. This term, although thus far not generally adopted, has much to commend it, since the terranes designated by it are highly complex, and may perhaps be ultimately subdivided into two or more systems, and besides occupy a basal position lower than any known sedimentary formation that has escaped metamorphism.

The Algonkian series embraces a great thickness of sedimentary beds, in part metamorphosed, which in certain localities rest unconformably on the eroded surface of the Basement Complex and in places are overlain unconformably by Cambrian rocks. Both the upper and lower contacts, however, in certain localities, have been rendered obscure by metamorphism. The system derives its name from a tribe of Indians that inhabited the region about the shores of Lake Superior, where it is well developed. The Algonkian terranes are exposed in the Grand Ca?on of the Colorado, in the Wasatch and Uintah Mountains, the Black Hills of Dakota, about the southern shore of Lake Superior, and in many parts of eastern Canada, as well as in several other localities. The oldest known fossils occur in these rocks, and consist of a small number of brachiopods, molluscs, crustaceans, etc. These scanty records are suggestive, and at least stimulate the hope that an extensive pre-Cambrian fauna will ultimately be discovered. The few forms found seem to be not far different from the similar life records of the Cambrian.

The Cambrian system, although first studied in Europe, has an important development in North America, and occurs at the surface at a large number of localities ranging from Newfoundland to California. The known distribution of the system and the nature of the rocks composing it indicate that it occurs widely in the Continental basin beneath subsequent deposits. The most interesting results derived from the study of the Cambrian, carried on especially by C. D. Walcott, pertain to its life records. With the exception of a few obscure alg?, all the fossils thus far discovered are marine invertebrates. As regards rank in the zoological scale, certain molluscan remains are the highest, but outclassing them in size, abundance, and degree of specialization are the trilobites, the nearest living representatives of which are certain crustaceans. Of the trilobites about 100 species have been discovered in the Cambrian rocks of North America, the largest individual being about 20 inches in length.

The picture of the continent which the facts just referred to enables one to sketch in fancy includes land areas destitute of animal life, and probably without vegetation, except perhaps the lichens, the lowest of the cryptogams. The sea, especially in its shallower portions near land and over its surface, contains alg?, mostly, we presume, of small size, in fact microscopic, and soft tissued. The animal life subsisting primarily on the alg? are all invertebrates, and nearly all of them, excepting the crustaceans, simple in organization. None of the animals the remains of which have thus far been discovered had strong shells or other well-developed protective or supporting tissues, thus indicating that they were not subject to the attacks of formidable enemies.

As compared with later faunas, the animals of the Cambrian were primitive, but their diversity-every subkingdom of invertebrates being represented-is positive evidence that they were not the first inhabitants of the waters. Considered from the point of view of development, this fauna stands at least half-way, and some students of the ancient history of the earth place it as far as nine-tenths of the way, up the life column-that is, the time from the first appearance of life on the earth to the beginning of the Cambrian was at least as long and possibly nine times as long as the time that has since elapsed. This is a sufficient promise that many records of life, and it seems safe to predict as varied an assemblage of organisms as the at present known Cambrian fauna, will ultimately be discovered in the Algonkian or lower rocks.

The Paleozoic era witnessed the first appearance of vertebrate life. The earliest known forms were fish-like in character and were succeeded in sequence by batrachians and reptiles. In this connection the most important contribution to the world's knowledge, from the study of the American records, include the discovery of a large number of fishes, or fish-like forms, some of them of gigantic size, in the Devonian and Carboniferous rocks of the Ohio region, by J. S. Newberry; numerous batrachians in the Coal Measures of Ohio, by E. D. Cope; of batrachians and probably reptiles in rocks of similar age in Nova Scotia, by J. W. Dawson and O. C. Marsh.

During the Paleozoic era land plants appeared, and before its close the continent was densely clothed with forests consisting of flowerless plants such as ferns and club-mosses, together with a less abundance of trees related to the existing conifers.

Great additions to the world's knowledge of the varied and beautiful floras of the swamps in which the coal-beds of Pennsylvania, Ohio, Nova Scotia, etc., were accumulated have been made by H. D. Rogers, J. S. Newberry, Leo Lesquereux, J. W. Dawson, I. C. White, David White, and others.

The Mesozoic era is characterized among other events by the first appearance and rapid development of flowering plants, the cycads being especially numerous, and of our ordinary broad-leaved trees, such as the oak, willow, sassafras, etc., and by the coming in of palms; and in the animal kingdom by the culmination of reptilian life and the advent of birds and mammals.

The American Mesozoic rocks have yielded a rich store of fossil plants, as is well known from the painstaking studies of J. S. Newberry, Leo Lesquereux, W. M. Fontaine, L. F. Ward, F. H. Knowlton, and others. These same students of the progress of plant life on the continent have also made extensive and critical studies of the Cenozoic floras.

The relics of reptilian life brought to light from the Mesozoic rocks of New Jersey, Kansas, Wyoming, etc., by Joseph Leidy, O. C. Marsh, E. D. Cope, and others, have astonished the world, even though marvellous results in a similar direction had previously been made known in Europe. The reptilian age was marked in America by the presence of such huge reptiles, and by the strange development and adaptations in various directions that they surpass the wildest dreams of fable. Lizard-like reptiles walked the earth that were 40 to 60 feet in length and stood 10 to 14 feet high where the massive hind limbs joined the body. Their thigh-bones in certain instances measured over 6 feet in length. Some of these monsters, it is estimated, weighed at least 10 tons. These, the hugest of all land animals, were vegetable feeders. Others, of less size, although still gigantic and more active, were carnivorous. Some of the old lizard-like forms which left their footprints in great abundance in the sands now hardened into sandstone in the Connecticut Valley and New Jersey walked on their hind feet, after the manner of birds, and left three-toed footmarks, some of them 20 inches in length, which are strikingly bird-like in appearance. Other great reptiles, whale-like in appearance, inhabited the ocean. Yet more marvellous forms were provided with wings, resembling those of bats, and in the case of the great Pteranodons found in the rocks of Kansas had a "stretch of wing" of fully 20 feet. But the strange menagerie that has been resurrected contains such a marvellous array of grotesque shapes that not even a catalogue of the genera can be presented here.

While the Mesozoic era was emphatically the age of reptiles, the coming of a more highly developed fauna was foreshadowed. Bird life was represented, and the skeletons of reptilian birds, or birds with teeth like those of reptiles, have been discovered in the Mesozoic rocks of Kansas. Important additions to our knowledge of these strange creatures, which furnish much instructive data in reference to the development of the higher from the lower forms of life, have been made by O. C. Marsh. The humble beginning of mammalian life is shown by insectivorous marsupials, the jaws of which were discovered in the Newark system (Lower Mesozoic) of North Carolina.

The Cenozoic era is the age of mammals, so called because during that time brute mammals succeeded reptiles as the rulers of the earth. From the rocks deposited in North America during this era, principally the sediments of fresh-water lakes and the gravel-beds laid down by streams in the Pacific mountain region, a great number of skeletons of truly remarkable mammals, differing widely from anything now living, have been discovered by Joseph Leidy, O. C. Marsh, E. D. Cope, H. F. Osborn, and others. The profound interest attached to this fauna, and the bearings it has on the study of the geographical distribution of animals, climatic changes, etc., is indicated by the fact that it includes forms related to the rhinoceros, elephant, camel, etc., which are not represented among the animals now living on the continent, although having relatives in other and principally tropical countries.

During the Psychozoic era mind gained ascendency over brute force, and man became the leader. The mammals continued to dominate the earth throughout the Pleistocene period and were then probably more numerous and of even larger size than during the preceding Tertiary period. During the Pleistocene great climatic changes occurred, and large glaciers existed in several regions which now enjoy a temperate climate and are densely populated.

The presence of man in North America during the Pleistocene has not been proved, but important contributions to knowledge concerning the brute mammals, and in reference also to the climatic and physiographic changes, have been made.

In stream-deposited gravels, caverns, peat swamps, etc., over the surface of practically the entire continent, the bones of many species of large mammals have been obtained. These include the mastodon and elephant, megatherium, megalonyx, mylodon, a large horse, a great bison, an elk much exceeding the living species in size, a giant beaver, and many others remarkable for their large dimensions as compared with their living representatives. Several of these large animals survived the vicissitudes of climate characteristic of the Glacial epoch, but have since become extinct.

The chief contributions to Pleistocene history, however, made by American geologists, are in connection with the records of climatic changes. During the earlier portion of the period, and beginning perhaps in late Tertiary time, the continent in large part at least was more elevated than now and the energetic streams of the mountainous portions eroded deep ca?ons. To this Sierran epoch, as it is termed, is referred the excavation of the larger valleys of the Sierra Nevada, the world-renowned ca?ons of the Colorado and Snake Rivers, and probably the deep Valley of the St. Lawrence and the Hudson.

A climatic change perhaps initiated by the greater elevation of the land, but not as yet wholly explained, caused glaciers to form about the higher portions of a number of the ranges in the Pacific mountains, and continental glaciers of the type of the ice-sheet now covering Greenland to expand from at least three centres, termed the Labradorean, Keewatin, and Cordilleran, in what is now Canada. During this time of great ice accumulation and of glacial advance and retreat, or the Glacial epoch, as it is termed, fully one-half of North America was buried beneath ice-sheets of the continental type. A composite map showing the portions of the continent which were covered with ice at one time or another during the Glacial epoch is reproduced in Plate V.

Plate V.-Pleistocene glacial deposits.

Note: This map presents what may be termed a composite picture of the extent of glacial ice during Pleistocene and Recent time; Greenland, much of the Arctic archipelago, and many areas in the Pacific mountains are still occupied by ice. The broken blue lines on the Atlantic and Pacific coasts show approximately the seaward extension of the Pleistocene ice-sheets. The detached areas of glaciation in the western portion of the United States are here assigned to the Wisconsin stage, but in the Rocky Mountains and Sierra Nevada there are records of two ice advances. The drift in western Canada here colored as Wisconsin is perhaps in part of later date.

Click image to enlarge.

During the maximum advance of the ice from the Labradorean centre into the Continental basin it nearly reached the mouth of the Ohio River (near Cincinnati). An earlier advance from the Keewatin centre extended to the Missouri River in Missouri. There is evidence of a succession of advances and retreats of the ice forming a very complex history. With its final retreat the Great Lakes came into existence and the continent reached the stage in its development when man became prominent.

The study of glacial geology in North America was initiated, or at least given a fresh start and in the proper direction, by Louis Agassiz, and within recent years energetically carried forward by a large number of earnest workers. The stage of advance reached in this branch of geology which serves so admirably to link the present with the past is well presented in the numerous publications of T. C. Chamberlin and his associates.

The instructive history of the growth of North America and the successive appearance of higher and higher forms of life, the records of which have been discovered in the sedimentary rocks, has been made known by the combined studies of a large number of investigators, but the great task has been carried on mainly under the auspices of various national and State surveys. Chief among these is the present United States Geological Survey, which has published what may be justly termed a library of valuable literature and of topographic and geologic maps.

The Igneous Rocks (Plate IV).-Under the at present popular explanation of the origin of the earth, namely, the nebular hypothesis, and also the modification of it termed the meteoric hypothesis, the planet itself is considered to have been at one time in a molten condition. The starting-point of the study of the rocks composing the earth should be, therefore, the primitive crust cooled from fusion. In addition to this there have been throughout history geologic migrations of molten matter from deep within the earth towards the surface, and a part of the material thus forced outward, principally through fissures, has cooled in the rocks it penetrated, forming intrusions of various kinds, and a part has reached the surface and been extruded, as during volcanic eruptions.

Probably every known phase of vulcanism is illustrated by the igneous rocks of North America, and in certain branches of the subject, as the nature of intrusions and the changes which occur in the cooling of igneous magmas, marked advances in the world's knowledge have been made by American geologists.

Examples of volcanic phenomena on a grand scale are furnished by the still active volcanoes of the Caribbees, Central America, Mexico, and Alaska. Between southern Alaska and south-central Mexico there are no active craters, but a large number of volcanic mountains in various stages of erosion which form an instructive series illustrating the internal structure of the mode of accumulation of ejected fragment material and of lava-flows. In this series of mountains built by igneous agencies belongs the great volcanic piles of the Cascade region, of which Mounts Baker, Rainier, Adams, Hood, Jefferson, Mazama, Shasta, etc., are among the leading examples. Many other illustrations in the same connection, some of them in an advanced stage of erosion and now revealing only the dikes and necks of resistant rock that cooled and hardened well below the surface, occur widely throughout the southwest portion of the United States. The still recognisable volcanic mountains of the continent, with the exception of those of the Caribbees, are confined to its western half, and with the exception of certain almost perfect craters in eastern New Mexico are all within the Pacific mountains. A great belt of volcanoes, including a large number of both active and extinct examples, extends from Panama to the Aleutian Islands, a distance of some 7,000 miles, and is a part of the so-called "circle of fire" surrounding the Pacific Ocean. This belt is about 1,000 miles broad in its central part, where only extinct volcanoes exist, and narrows towards both its northern and southern extremities, which are defined by still steaming craters. The narrow northern portion, inclusive of the active volcanoes of the Alaskan Peninsula and the Aleutian Islands, is prolonged westward, and forms a curve concave to the southward, while the equally narrow southern portion marked by the energetic craters of Central America forms a curve concave to the northward. The entire belt has something the shape of a sigmoid curve, with a wide central portion.

In the preceding sketch of the growth of the continent it was shown that the Pacific mountain region is younger than the Atlantic mountain region. In this same connection certain interesting general conclusions have been reached in reference to igneous activity. In each of the great cordilleras referred to there have been extensive breaks in the earth's crust through which molten rocks have been forced upward. Volcanoes and various intrusions have been formed in each region, but in the eastern half of the continent the time since the last eruptions has been so great that all evidence in the relief of the land of the former presence of volcanic mountains has been obliterated. Erosion has cut deeply into the rocks on which the ancient volcanoes stood, and revealed in some instances the dikes occupying the fissures which supplied them. A large number of dikes of igneous rock occur in the Atlantic coast region from Prince Edward Island southward to Alabama and Georgia, and vast lava-flows of ancient date are still preserved about the south shore of Lake Superior. Volcanic eruptions in the older half of the continent have long since ceased and the breaks which gave them existence have been healed. The later movements in the western half of the continent have caused fresh fractures to form, through which molten matter has been forced to the surface. Many facts have been observed in each region which show an intimate connection between movements in the earth's crust which have produced fractures and the distribution of volcanoes.

The lavas poured out by the more recent volcanoes of North America are mainly dark basic rocks, among which basalt predominates. An exception occurs in the case of the Mono craters near Mono Lake, California, which in recent time extruded a thick, viscous, highly siliceous, rhyolitic lava, much of which cooled quickly and formed volcanic glass or obsidian.

In addition to streams and sheets of lava, many volcanoes, and especially those in a state of explosive eruption, blew into the air quantities of fragmental material, such as scoria, bombs, volcanic gravel (lapilli), dust, etc., which was scattered far and wide over the land. More or less extensive sheets of this material, in many instances interstratified with sedimentary beds, and especially with the strata laid down in Tertiary lakes, or separating lava-flows, occur widely throughout the Pacific mountains. Dust showers of the nature just referred to have occurred at a recent date, and the fine white material that fell is now found at the surface in a large number of localities, ranging from Central America to the Yukon Valley and from Kansas and Nebraska to Oregon and Washington.

The most remarkable instance of the addition of volcanic rocks to the surface of North America is in the case of the Columbia River lava, which covers some 200,000 or more square miles of country in Washington, Oregon, and neighbouring States. In that region outwellings of highly liquid rock came from fissures and spread widely over the surface as veritable inundations, which on cooling became black, basaltic rock, but without forming mountains or craters. Where the Snake River has excavated its magnificent ca?on in these still horizontal layers of basalt, a thickness of 4,000 feet is revealed, although the stream has not as yet cut through the formation, and in Stein Mountain, Oregon, a similar series of lava-sheets over 5,000 feet thick has been measured. The Columbia River lava was spread over the surface of a deeply eroded land in a series of vast overflows of molten material. The liquid rock covered the broad plains and extended into the valleys in the adjacent mountains, giving them level floors of basalt. Mountain spurs became capes and headlands and outstanding buttes were transformed into islands in the molten sea. The lava since cooled and crystallized has in places been folded and tilted; streams like the Columbia, Snake, Spokane Rivers, etc., have carved great ca?ons in it, and the surface, especially where it is still nearly horizontal, has decayed and yielded a wonderfully rich soil. It is the fine, rich residual material of these lava plains, redistributed in part by the wind, which furnishes the basis for the immense wheat industry of the northwestern portion of the United States.

The extrusion of molten rock from deep within the earth so as to form volcanoes or fissure eruptions at the surface is only a part of a widely extended and highly varied process. As geologists have discovered, particularly in deeply eroded regions, by no means all of the fissures which permit of the forcing upward of molten material in them reach the surface. Many of them died out before coming to the light and favoured the production of various forms of intrusion.

A fissure originating deep in the earth's crust and extending upward, perhaps with many branches and irregularities, if injected with molten rock from below gives origin to dikes. That is, a dike is a more or less vertical sheet of igneous rock which has cooled and crystallized in a fissure. Such sheets of intruded material cutting across the bedding of stratified rocks, or traversing older igneous or metamorphic terranes, are of common occurrence and are frequently abundant in deeply eroded regions. They occur particularly in mountains of upheaval, thus demonstrating the fact that to a large extent the fissure which became injected with molten magmas and perhaps gave origin to volcanoes, are due to movements in the rocks composing the earth's crust. The force which causes molten rock to rise in such fissures also tends to prolong and enlarge them. The heat of an intruded magma affects the rocks it traverses and produces what is termed contact metamorphism. Examples of dikes in the Newark system have already been referred to, and others are common throughout the Pacific mountains. Where the Columbia River lava in central Washington has been removed by erosion, hundreds, and in fact thousands, of dikes are exposed in the terranes on which it formerly rested.

When a dike ends above in horizontally bedded rocks it sometimes happens that the injected magma, especially if highly fluid, is forced in between the strata and spreads widely between the layers, forming an intruded sheet, which lifts a broad cover to a height equal to its own thickness. An example of an intrusion of this nature is furnished by the palisade trap-sheet in New Jersey and New York, which has a maximum thickness of about 1,000 feet, and is fully 100 miles in length from north to south. The portion which remains is but a remnant and is seldom over 2 or 3 miles wide. This sheet in common with its associated sandstones and shales has been tilted so as to dip westward at an angle of about 15 degrees, and its eastern border eroded so as to form the picturesque Palisades on the west bank of the Hudson opposite New York city. Many other similar intruded sheets are known in Nova Scotia, the Connecticut Valley, among the Pacific mountains, etc.

A marked variation in the process just outlined occurs when, as the controlling condition, the intruded magma is highly viscous instead of highly fluid, and the friction of contact and of flow is greatly increased. Under such circumstances the intruded magma expands less widely than is the case when an intruded sheet is formed, and a thick intrusion results, which lifts a small cover perhaps to a great height. Intrusions of this nature are sometimes expanded in their upper portions into a more or less mushroom shape, and from their fancied resemblance to cisterns of once molten rock within older terranes have been termed laccoliths. The typical examples are furnished by the Henry Mountains in southern Utah, described by G. K. Gilbert. Other similar intrusions in Colorado have been studied by Whitman Cross, and yet other examples have been discovered in various parts of the Pacific mountains. In the case of certain of the laccoliths in the Henry Mountains, now laid bare by erosion, the cistern-like mass of intruded material is 12,000 feet or more in diameter, some 5,000 feet thick in the central part, and lifted a cover of stratified rocks fully 7,000 feet thick.

Where a dike ends above in older rocks, and particularly in horizontally stratified sedimentary beds, in a pipe-like form, similar to the conduit of a volcano, but without reaching the surface, the unexpanded or but slightly enlarged summit portion lifts a comparatively small cover into a dome, and what has been termed a plutonic plug results.

All the various phases of intrusions thus far referred to, it will be readily seen, are variations of one process. The wide range in the results produced are dependent on local conditions, either in respect to terranes invaded, as, for example, whether or not they are undisturbed sedimentary beds, and on the physical condition of the intruded material, in reference especially to its degree of viscosity. There is an intimate and even a genetic connection between intrusions on the one hand and volcanic and fissure eruptions on the other. If fissures lead from portions of the earth's crust sufficiently deep to permit the rocks to become plastic or fused on account of the relief of pressure due to the opening of the fissure, the magma may be forced to the surface, becoming more and more plastic or more perfectly fluid as the weight upon it decreased, and volcanic phenomena result; or if the fissure fails to reach the surface intrusions of various forms may be produced. The simplest form of intrusion, the dike, results under whatever condition the summit portion of the magma comes to rest. A magma forced upward in fissures in the earth's crust may meet moist rocks or even reservoirs of water, and in such instances steam or gases are produced and a new force is added, which may produce explosions.

In addition to the intrusions of the various classes just referred to there are others on a far larger scale, examples of which occur in North America, but as yet their mode of origin has been but little studied. I refer to vast upwellings of molten or plastic material beneath the more rigid portions of the earth's crust, which elevate domes, perhaps 200 or 300 miles or more in their various horizontal diameters. The great areas occupied by intrusive granite, as the one from which the Bitter Root Mountains in Idaho have been sculptured, are of this nature. These "regional intrusions," as they may be termed, elevate mountains in the same general manner as in the case of laccoliths, but of far greater size. To the elevations produced in this manner I have ventured to apply the name subtuberant mountains, in expression of the idea that they have resulted from vertical uplifts, due to the upswelling of molten material beneath.

The Metamorphic Rocks (Plate IV).-At the contact of either sedimentary or igneous rocks with intrusive rocks of whatever form, such as dikes, sheets, laccoliths, etc., there has been in many well-known instances an alteration of the terranes penetrated or uplifted which is most intense along the contact and diminishes at a distance. This change or metamorphism, as it is termed, consists of an alteration in the colour, texture, hardness, mineral and chemical composition, etc., of the rocks affected, and may be manifest throughout a thickness of but a few feet, or perhaps only a few inches, but near large intrusions is apt to be traceable for scores or hundreds of feet. In the case of intense contact metamorphism, the altered rock assumes a new form, and may exhibit a crystalline and foliated or schistose structure. The changes referred to are most marked when water is present, and are thought to be due largely to the influence of heated water percolating through the rocks and producing changes by solution and deposition. The principal agencies which take part in contact metamorphism are heat, heated waters, pressure, and perhaps movements within the rocks.

There are extensive regions throughout which the rocks have been changed in a manner similar to the alterations commonly found adjacent to igneous intrusions which, in general, have been brought about in some other way. This regional metamorphism, as it is termed, has affected the rocks in certain instances throughout districts measuring many hundreds of square miles in surface extent, and with a vertical range of many thousands of feet. The rocks referred to have been changed without fusion from a previous condition, during which they were either sedimentary beds or cooled and crystallized igneous magma. This conclusion has been verified in numerous instances by tracing the thoroughly altered rocks to regions where the change has been less intense and finally to where they pass by insensible gradations into easily recognisable sedimentary or igneous terranes. Common examples of metamorphic rocks are mica, schist, gneiss, statuary marble, certain granites, etc. These rocks frequently have a foliated or fissile structure, such as it is presumed would result from a flowing movement within the mass while under great pressure. Characteristically also the rocks are composed of interlocking crystals or portions of crystals, which are not contained in a glassy base, as is the case with most rocks that have crystallized from fusion. That is, the metamorphic rocks are characteristically holocrystalline, while igneous rocks are porphyritic, or cryptocrystalline.

The analogy between rocks altered by contact metamorphism and those affected by regional metamorphism had led to the conclusion that the latter, like the former, have been changed by heat and the passage through them of heated water bearing mineral matter, and especially silica, in solution. More than this, the foliation frequently so characteristic of metamorphic rocks is considered as evidence of a flowing movement or shearing of the material while under pressure. In short, rocks are altered by heat, especially if water is present in them, by motion, and by chemical changes produced by percolating waters, and perhaps in still other ways. The degree of heat required is not definitely known, and probably varies according to the nature of the rocks, the presence or absence of water, etc., but is certainly less than that necessary to produce fusion, and is thought, in general, to be in the neighbourhood of 750° F. While heat alone is considered as sufficient to produce metamorphism, it is probable that in most instances two or more of the agencies just referred to have been in operation at the same time. In the case of the foliated rocks motion within the mass seems to have been the predominating factor, and dynamical metamorphism is considered as important as heat metamorphism.

In North America, as is indicated roughly on the map forming Plate IV, metamorphic rocks occur at the surface over a great region in eastern and northeastern Canada, in Labrador and Newfoundland, in the New England States, and thence southward along the eastern side of the Appalachians. Other extensive regions occupied by similar rocks occur in many of the ranges of the Pacific mountains, from Alaska to Panama, and are known in the West Indies.

Not only do the metamorphosed rocks outcrop at the surface over large areas, but, as may be inferred from such outcrops, as well as from the records of numerous borings, underlies nearly the entire extent of the sedimentary formations. The basal portion of the continent, with the exception of certain areas where igneous rocks occur, is formed of metamorphosed terranes. So generally is this true, that it is safe to say that if a boring is begun at any locality on the continent where sedimentary beds occur, and is continued downward until the sedimentary rocks are passed through, metamorphic terranes will be found beneath. The same is true also where the surface is composed of lava-sheets. The exceptions, where metamorphosed rocks do not occur beneath sedimentary or volcanic beds, are when igneous intrusions or ancient lava-flows are present at a depth.

In the brief description given of the Archean system on a preceding page, it was stated that the rocks composing it are largely metamorphic. But rocks of practically any age may be altered in the several ways mentioned above, and the resulting gneisses, schists, etc., be indistinguishable from those of the Archean. In fact, some of the metamorphosed rocks of North America, as certain gneisses, schists, etc., of the Sierra Nevada and Cascade Mountains, are known to be of Mesozoic and even Cenozoic age.

In speaking of the growth of North America, and again in connection with the distribution of volcanic mountains, it was shown that there has been a progressive migration of the field of action of the forces which upheave the rocks so as to form land areas, and also of the movements in the rocks which produce fractures and lead to the origin of volcanoes. In a similar way the sphere of influence of metamorphism as indicated by the age of the transformed rocks in various regions has in a general way migrated from east to west across the continent.

In the Laurentian Highlands the metamorphosed rocks are of pre-Cambrian age; in New England and the Appalachian region they are, in part at least, of Paleozoic age; and in the Sierra Nevada and Cascade Mountains metamorphosed Mesozoic and Cenozoic rocks occur. As movements in the outer portion of the earth's crust may produce fractures in any class of rocks, and as such fractures favour the intrusion of igneous material, the metamorphic rocks may contain igneous intrusions similar to those noted above in connection with sedimentary rocks. As the stratification so marked in sedimentary beds is lacking in metamorphosed rocks, it is not to be expected that intrusions will take the form of sheets, laccoliths, etc., but rather appear as dikes with perhaps irregular branches. As the same region may experience two or more periods of metamorphism, it is evident that great complexities may arise, as, for example, when a metamorphosed terrane is penetrated by dikes and irregular intrusions and again subjected to metamorphosing conditions. These considerations lead to the suggestion that rocks metamorphosed in pre-Cambrian time, for example, would be apt to be more complex than those of Mesozoic date. In general, this has been found to be true, as is suggested by the fact that to the pre-Cambrian metamorphosed terranes, as previously stated, the name Basement Complex has been applied.

Summary.-The relation of the three great divisions into which the rocks composing North America, in common with all other portions of the known lithosphere, are divided, may perhaps be better understood when it is remembered that the igneous rocks came from below in a molten condition; that the sedimentary rocks have been formed at the surface from the débris of either igneous, metamorphic, or previously formed sedimentary beds; and that metamorphic rocks have been produced within the earth's outer crust by the alteration of either igneous or sedimentary rocks. When the heat which produced certain phases of metamorphism is sufficiently increased, greater freedom of molecular and chemical changes occur and the material acted on passes to the condition of an igneous magma. The three great classes of rocks considered above are thus seen to be but stages in a cycle which the material of the lithosphere passes through.

The conditions which bring about these changes are still in action and are intimately associated with movements in the rocks of the earth's crust. When elevation raises a portion of the earth's crust above sea-level, erosion and redeposition ensue and sedimentary rocks are formed; the greater the elevation the more energetically the forces act which bring about denudation, transportation, and sedimentation. When depression occurs of sufficient amount to carry rocks previously at or near the surface into the zone of metamorphism, alterations follow, and in general the deeper the depression the greater the changes until metamorphism culminates in fusion, providing pressure does not counteract the influence of heat. Dynamical and chemical metamorphism may occur at less depth than purely heat metamorphism, and it may be presumed takes place in the axes of mountain ranges, even above sea-level. Such a broad view of the relations and genesis of the three great lithologic divisions of the material forming the earth's outer crust is necessary to the understanding of the conditions observed in the basal portion of the geological column, as it is termed, in which the age and order of succession of the sedimentary rocks is indicated. In certain localities, for example, the Cambrian rocks rest unconformably on a surface of metamorphic and igneous rocks-that is, the Basement Complex was raised above sea-level, eroded and subsequently depressed before the Cambrian sediments were laid upon it. In other localities the Cambrian rocks pass indefinitely into metamorphosed terranes beneath, which means that metamorphism invaded the series after the deposition of the Cambrian, and the characteristics of its junction with older rocks was obliterated. Similar relations may evidently be discovered at any horizon in the geological column. Obviously the chances of a system of stratified rocks becoming metamorphosed or of being removed by erosion, are greater the nearer their position to the base of the sedimentary series; in a similar way the chances of a sedimentary terrane becoming invaded by igneous intrusions is greater the greater its age; again, the older a sedimentary terrane the greater the chances of its becoming buried by subsequent deposition and the less the likelihood of its being exposed for study. The only position in which a sedimentary formation can maintain its integrity and be safe from destruction by erosion or transformation by metamorphism is below sea-level and above the zone of heat metamorphism; but even in this position it may have its distinctive features, including its fossils, obliterated by dynamical and chemical alterations. These suggestions are offered for the sake of indicating, as stated on a previous page, that the Cambrian and Algonkian rocks should not be considered as the first formed sediments, and that there is hope of the discovery of a rich fauna of older date than any at present known. In the search for the earliest evidence of animal life on the earth, North America holds out favourable conditions.

THE CONCENTRATION OF MINERAL SUBSTANCES

The most important branch of geology treats of the substances in the earth's crust that are of direct service to man, as, for example, building stones, coal, iron, petroleum, gold, etc. Only a glance can here be given at the conditions which have led to the origin of the materials of commercial value and to their geographical distribution.

From the mode of origin of the principal classes of rocks it may be reasonably inferred that certain minerals and ores will be developed or concentrated in one class of rocks and not in the others. To a great extent the facts observed during the development of mines, etc., sustain this prediction.

In the cooling and crystallizing of igneous rocks from a state of fusion many minerals are formed, the most common being silicates of the alkaline earths, which are usually inclosed in a glassy or cryptocrystalline base. The igneous rocks have characteristically a highly complex chemical composition, and although frequently containing the metallic element, etc., which are of economic importance, these are widely disseminated, and in nearly all cases in chemical combinations, as the minor ingredients of siliceous minerals. Although the igneous rocks sometimes contain valuable ores, they are in many, if not all instances, due to secondary enrichment and are not a result of primary crystallization from fusion. As all the material of the earth's crust was at one stage in the series of changes it has experienced consolidated from fusion, it follows that the ores and minerals now of economic value did not then exist, or were widely diffused and have since been formed or concentrated.

The processes of concentration referred to are carried on in various ways through the agency of mechanical, chemical, vital, molecular, and electrical forces, acting singly or in association. For example, concentration through the action of mechanical agencies is illustrated by the manner in which rocks are reduced to fragments in the every-day process of denudation and the resulting débris removed by streams and redeposited. In this process an assorting in reference to size, specific gravity, etc., takes place, and certain substances, as sand, for instance, is accumulated in one locality, and certain other substances, as clay, deposited in another locality. During this process gold, platinum, etc., owing to their high specific gravity, may be concentrated in stream channels. The accumulation of mineral matter through the action mainly of chemical agencies, occurs when the waters percolating through rocks dissolves certain substances, as calcium carbonate, for instance, and on coming to the surface as springs, or dripping from the roofs of caverns, deposit calcareous tufa, stalactites, etc. Silica, iron, manganese, and other substances are frequently concentrated in a similar manner.

Concentration of previously widely disseminated substances principally through the agency of vital forces, is illustrated by the manner in which molluscs and polyps obtain calcium carbonate from water and deposit it in their shells or skeletons. The part played by plants in this same connection is shown by the way in which they eliminate carbon dioxide from the air or from water, and concentrate the carbon in their tissues. From the carbon accumulated in this manner, under certain conditions, deposits of peat, lignite, coal, graphite, etc., have resulted.

What may provisionally at least be termed molecular concentration occurs when similar molecules are brought together largely by water and crystallized to form mineral species. In order to simplify this brief discussion as much as practicable, this phase of concentration will be included under the chemical processes referred to above.

The three principal methods by which mineral substances are concentrated, namely, the mechanical, chemical, and vital, have in the main different fields of action. The mechanical and vital agencies operate at the surface of the lithosphere, although organic products, principally certain acids, descend into the earth in solution in water and play an important part in deep-seated chemical changes, as in the formation of mineral veins. The chemical agencies bring about the concentration of mineral substances both at or near the surface and at a depth.

The intensity with which the several agencies just referred to operate varies according to conditions. The mechanical agencies, for example, acting mainly through the aid of flowing water, are in general most potent in humid regions and where the land is high above sea-level. Vital agencies depend largely on climate and are most active in warm humid regions. The chemical agencies are influenced largely by heat, the presence of water, and by pressure.

It is interesting to note that a high degree of heat leads to the dissipation and wide distribution of substances previously concentrated; fusion, for example, permitting of the intimate mingling or recombination of substances, previously segregated, although during the dying stages of volcanic activity minerals like sulphur, cinnabar, etc., may be directly condensed and thus concentrated from a vaporous condition.

During the formation of the three main classes of rocks composing the earth's crust, the agencies leading to the concentration of various substances now of economic importance have to a great extent been different, and hence in a marked way the stones, ores, fuels, gems, etc., to be expected in each of the three classes of rocks, respectively, are distinct. Certain exceptions to this broad conclusion, however, arise from the fact that rocks belonging to each of the classes referred to may have been brought within the influence of the same or similar concentrating agencies and like results produced in each class.

Economic Importance of the Igneous Terranes.-The igneous rocks, as previously noted, are such as have cooled from fusion. On the cooling of magmas various minerals are formed, most commonly silicates, and except in a minor way in connection with the weaker stages of volcanic activity and the slow cooling of the rocks, there does not seem to be any marked tendency towards the concentration or segregation of metallic minerals or ores. Although igneous rocks do contain gold, silver, copper, etc., and a large variety of the rarer metals, they are widely disseminated. As is well known, however, igneous rocks are in some instances of value for the metallic mineral, gems, and ores associated with them, but in the great majority of instances at least, and as a rule, these minerals and ores are the result of subsequent changes and owe their origin mainly to deposition from heated, percolating water. Rich ore bodies frequently occur on the borders of igneous dikes, and in fissures and cavities in igneous rocks, but the process by which they have been formed is similar to that leading to the concentration of mineral matter in metamorphic rocks, and will be referred to later.

The igneous rocks themselves furnish desirable building stones, such as granite, diorite, porphyry, diabase, etc. With the exception of granite and the nearly related diorite, these have not as yet been extensively utilized in North America. Certain of the igneous rocks have been altered to serpentine, which on account of its pleasing green colour and the ease with which it can be cut and polished furnishes a stone valuable for interior uses. It is also employed, usually with a rough surface, in the construction of exterior walls of dwellings, gateways, etc. Large bodies of serpentine occur at a number of localities in the Atlantic mountains from Pennsylvania and Maryland northward, including eastern Canada, and also over extensive areas in the Pacific mountains, particularly in California, Washington, and Alaska.

The principal ores and minerals of commercial importance in the igneous rocks are native copper, as in northern Michigan; copper pyrites, as at Butte, Montana; gold, at many localities, including the Treadwell mine, Alaska; opal, which is mined on a small scale in Idaho and Washington. In practically all these instances, and numerous others that might be enumerated, the substances referred to have been deposited from solution in cavities in the rocks or have replaced other substances, and are due to what is termed above chemical concentration.

Economic Importance of the Sedimentary Terranes.-The sedimentary rocks are composed principally of fragmental material derived from the disintegration of older rocks transported and deposited mechanically, and resulting in the formation of sandstone, shale, etc., and of organically concentrated material, such as shells and corals, which form limestones. The deposits originating in these ways furnish excellent building stones, the principal classes being sandstones and limestones. These occur widely throughout North America, and in formations of all ages subsequent to the Archean. The sandstones were deposited near the shores of the seas, or in lakes, and the limestones principally in moderately deep oceans.

Sandstones occur largely in the Cambrian formation on the south shore of Lake Superior and about the borders of the Adirondack hills of New York. They are usually red or reddish-brown rocks, and their pleasing colours, durability, even grain, and the readiness with which they may be broken in any direction make them desirable building stones.

The Newark system, extending in detached areas from Nova Scotia to South Carolina, contains immense quantities of brown and gray sandstone, which have been extensively quarried, particularly in the Connecticut Valley, New Jersey, Pennsylvania, and Maryland, and largely used in Atlantic coast cities. The Carboniferous and Devonian sandstones, usually of a gray colour, of Pennsylvania, Ohio, and neighbouring States, are largely used in the cities of the interior portions of the United States. Extensive deposits of Mesozoic and Cenozoic sandstones occur throughout the Pacific mountains, and afford a practically unlimited supply of good building material, which as yet has been but little utilized. The colours of sandstones vary from bright red through brown-yellow to gray, and in some cases are nearly white, depending largely on the condition of the iron present. The red rocks are dyed with ferric oxide; the brownstones contain iron, frequently in the cementing material that unites the grains, in various stages of oxidation and hydration; the gray stones may also contain iron, but if present it is in union with organic matter, as the ferric carbonate, for example. The Cambrian and Newark sandstones are prevailingly of some shade of red, for the reason that not enough organic matter is present to change the iron to a carbonate.

The sandstones when of an even fine grain and not too hard, are suitable for sharpening tools, and large quantities of grindstones, whetstones, etc., are made from them, as on the Lake Huron shore of Michigan, in Ohio, etc. Other sandstones, practically free from iron, are used in the manufacture of glass. The best example of "glass sand" is the Sylvania sandstone of southeastern Michigan. Unconsolidated sand is largely used in mixing mortars and cements, for smoothing stones used for architectural and monumental purposes, as foundry sand in making moulds for casting, and many other ways. Seaward from where sand is being deposited we find in the present oceans that as a rule fine bluish or greenish mud occurs, and still farther seaward, except where coral-polyps thrive, usually at a distance of 100 miles or more from land, the bottom is composed of calcareous mud or ooze. The sand and mud are derived from the land, and if consolidated form sandstone and shale. The calcareous ooze is derived from the life of the sea, largely minute lime-secreting foraminifera, together with shells of molluscs, and in the vicinity of coral islands or reefs the hard parts of coral growth are added. That is, the calcareous oozes are formed by the concentration of calcium carbonate through the vital action of animals and to a less extent of plants. Such material, if consolidated, would form ordinary limestone.

In North America there are terranes scores of hundreds of miles across in various directions and hundreds and even thousands of feet thick that have been formed in the manner just indicated. From this mode of origin it may be truthfully inferred that limestone may have been formed during any age since organisms having the power of secreting calcium carbonate existed on the earth. The limestones of North America range in age from the Algonkian period to the present time, and are still being formed in the ocean and in a minor way in lakes.

Impure limestones, frequently coloured or clouded with red, due to ferric oxide, are quarried on an extensive scale in eastern Tennessee, and are used for decorative purposes. The Tennessee limestones referred to are of Paleozoic age; in Florida porous rocks, known as coquina, composed of imperfectly consolidated shells of living species of molluscs, are used in the construction of buildings. Gray limestones susceptible of a good polish occur in Ohio and neighbouring States and are utilized to some extent for columns and interior finish of buildings, but in the main the stones of this nature when employed for architectural purposes are rough-faced. Vast amounts of limestone suitable for masonry occur widely throughout the Mississippi Valley in many of the ranges of the Pacific mountains, especially in the United States and Mexico, and are also of immense thickness in the West Indies.

In many instances limestone has been metamorphosed, as will be described below, and converted into crystalline marble. Commercially, however, all limestone, whether crystalline or not, which is susceptible of a polish, is termed marble.

Under certain conditions calcium carbonate is concentrated at or near the earth's surface by chemical agencies, as about springs where calcareous tufa, travertine, etc., are precipitated, and in caverns where stalactites and stalagmites are formed. Stalagmite sheets are sometimes composed of variegated, laminated layers, and when polished produce a beautiful decorative stone which passes under the name of onyx marble. Deposits of this character of commercial importance occur in Arizona and Mexico.

Calcium carbonate concentrated in lakes through the combined action of chemical and vital agencies produces the so-called marl, now extensively utilized in the manufacture of Portland cement. In this mode of accumulation the calcium carbonate is dissolved by percolating waters from the rocks and soils and carried to lakes in solution; it is there precipitated largely through the vital action of certain alg? and deposited as a fine white ooze. Thousands of deposits of this nature, varying in extent up to several hundred acres, and having a depth of from a few feet to 40 and even 60 or more feet, occur in the portion of the continent covered with glacial drift, and especially in the States from New England to Minnesota. The reasons for the greater abundance of marl in this region than elsewhere are that the glacial drift is there highly calcareous, numerous lakes are present, and the climatic conditions are such as to favour the growth of certain aquatic plants, and especially the Charace? or stoneworts, which have the property of eliminating calcium carbonate from ordinary lake waters.

The importance of the vital agencies in concentrating substances of economic value is illustrated by the manner in which coal, petroleum, and natural or rock-gas, etc., have been formed.

Land plants have the power, under the influence of light, of decomposing the carbon dioxide (carbonic-acid gas) of the air and fixing the carbon in their tissues, the oxygen being liberated and rendered available for animal respiration. Carbon is thus concentrated, and when plant remains accumulate and are preserved beneath water in swamps, a slow change takes place and peat is formed. The essential conditions for the accumulation of vegetable matter have been present on the earth ever since a land flora existed, and coal-beds occur at many different horizons. The earliest date at which land plants seem to have been sufficiently abundant to furnish material for coal-beds was the Carboniferous period. Although a similar flora existed during the preceding period, the Devonian, no coal-beds of workable thickness are known in the rocks of that age. Since the Carboniferous period coal has been found at many horizons in the sedimentary rocks, and peat is being accumulated at the present day.

Fig. 34.-Map showing the distribution of coal in North America.

The coal-fields of North America are more extensive than those of any other continent, excepting, perhaps, the at present but little known coal-bearing formations of Asia, and are distributed in temperate latitudes, from tide-water on the Atlantic to tide-water on the Pacific coasts, where the greatest commercial and intellectual development has been reached.

Coal of Carboniferous age occurs in large and valuable deposits in Nova Scotia and New Brunswick; there is a small area of graphitic anthracite, not now utilized, in Rhode Island; but the great fields are in Pennsylvania and the States southward to central Alabama, and westward to beyond the Mississippi. A detached coal-basin containing some 6,700 square miles, but a small part of which is productive, however, occurs in the central part of southern Michigan. Small coal-fields in Virginia and North Carolina, the first to be worked in America, are of Jura-Trias age and form part of the Newark system. Extensive fields of valuable coal of Mesozoic age, principally in the Laramie system, occur in New Mexico, Colorado, Wyoming, Montana, and still farther north along the same great belt in Canada.

Another highly valuable field of Mesozoic coal is now being extensively worked on Vancouver Island. The coals of the west side of the Pacific mountains, largely lignites, but in many instances of high grade and serviceable for steam coal, are mostly of Cenozoic age (Tertiary) and occur in California, Oregon, Washington, and Alaska. The distribution of the various coal-fields is indicated on the above map, and space will not be taken in describing their geographical relations.

Peat is present in innumerable swamps throughout the humid, temperate portion of the continent, especially from Louisiana and Florida northward, to the region about the Great Lakes and widely throughout Canada, but is at present of small commercial importance, although steps are being taken for its extensive utilization.

The most valuable of the coal deposits are of Carboniferous age, and lie to the east of the Rocky Mountains. The most of the coal is bituminous, or soft coal, used principally in generating steam and for manufacturing gas and coke. The exceptions occur in eastern Pennsylvania and in Rhode Island. These are considered as metamorphosed coals, although in the Pennsylvania region there is no evidence of the action of a high degree of heat. In the Rhode Island field the rocks associated with the coal are plainly metamorphic in character, and the coal has, in large part, been changed to graphitic anthracite.

That anthracite may be of any age, however, is indicated by the local changes that have occurred in Mesozoic and Cenozoic coals, where they have been penetrated by dikes and other varieties of intrusions, or have been altered by surface lava-flows. In such situations the coal has lost nearly all its volatile matter, and in composition and in certain instances, as in western Colorado, in physical character as well, is essentially an anthracite.

In addition to the various coal deposits referred to above there is a second series of organic compounds found stored in sedimentary rocks which consists of hydrocarbon. This series of substances includes natural or rock-gas, petroleum, maltha or semifluid hydrocarbon, and solid hydrocarbons, such as asphaltum, albertite, grahamite, ozokerite, etc. These substances are usually considered as being of organic origin and to have resulted from changes which take place in vegetable and animal tissues when buried and in most cases subjected to heavy pressure. A large part of the hydrocarbons referred to is thought to have been derived from animal organisms, an opinion which is sustained in an important manner by the fact that large stores of both petroleum and rock-gas have been discovered in rocks which were laid down before land vegetation is known to have existed. Marine alg? were present, however, so that it cannot be affirmed that the hydrocarbon of the earlier Paleozoic rocks came entirely from animal organisms. It is highly probable, however, that a large portion of the hydrocarbons stored in Paleozoic and later strata was derived from the animals whose hard parts occur so abundantly as fossils in the same or adjacent beds.

Fig. 35.-Ideal section showing favourable conditions for the storage of petroleum and gas.

Besides the concentration of carbon in plant and animal tissues and its change to hydrocarbons, there is a still further concentration necessary in order that stores of petroleum, gas, etc., shall be accumulated so as to be of economic value. This accumulation is dependent largely on physical conditions. The production of hydrocarbons from organic matter contained in sedimentary rocks, and particularly in shale, is going on in many regions, and probably nearly everywhere, especially when the soft parts of animals are buried in the rocks, but the petroleum, gas, etc., generated escape at the surface and pass into the air and are again widely disseminated, unless conditions are present which lead to their accumulation. The conditions favouring the natural storage of the substances referred to are cavities, or more usually porous beds, such as sandstone, beneath impervious beds, such as clay or shale. The conditions are still more favourable when lateral as well as vertical escape is cut off, as, for example, when arches or domes occur. The most favourable conditions result when a bed of shale or other rock, as a, Fig. 35, from which hydrocarbons are being evolved occur beneath a sheet of porous sandstone or fissured rock of any kind, b, above which there is a close-textured, unfractured stratum, such as shale, c, and the series is bent along certain axes into upward folds or anticlinals. Under these conditions, as extended experience has shown, a well drilled at d should yield in succession gas, petroleum, and water.

The conditions for the production of petroleum, gas, etc., have been present on the earth since the first appearance of life, and reservoirs may have originated at any subsequent time. The oldest known reservoirs still charged with these substances that have been discovered occur in the earlier Paleozoic rocks, just above the formations containing the oldest known fauna. Important petroleum and gas fields in rocks of the Trenton period occur in New York, Ontario, Ohio, and Indiana. The Devonian rocks of Pennsylvania, New York, Ontario, etc., also yield large supplies of both oil and gas. Mesozoic rocks of Colorado, Wyoming, etc., are also rich in the concentrated hydrocarbon referred to, and on the Pacific coast, particularly in California, rocks of Cenozoic age are highly productive. Petroleum and gas may occur also in rocks more recent than the Cenozoic, but owing to the absence of reservoirs, and possibly the lack of sufficient time, no important accumulations are known in beds more recent than the Tertiary, unless they come from a deeper source in older rocks. The vast quantity of petroleum stored in the rocks of various ages in North America is indicated by the fact that in 1900 the yield from the wells of the United States was 63,362,704 barrels, and from Canadian wells about 280,000 barrels, making a total of nearly 64,000,000 barrels.

The stores of rock-gas are also enormous, as is indicated by the fact that a single well at Bairdstown, Ohio, yielded over 17,000,000 cubic feet per day. In 1890 the average daily flow of the Indiana gas-wells was 779,525,000 cubic feet. The value of the natural gas consumed in the United States in 1900 was $23,606,463.

In the sedimentary rocks of North America there occur also extensive and valuable deposits of semifluid and solid hydrocarbons, such as maltha, asphaltum, albertite, grahamite, uintahite, etc., which have arisen, under the most plausible explanation thus far offered, from the concentration by evaporation of fluid hydrocarbons such as petroleum. The evaporation, particularly of heavy petroleum, leads to the formation of a solid residue, similar to asphaltum. In fact, there is no definite boundary between the lightest naphtha and the most coal-like asphaltum. They form a connected hydrocarbon series, analogous to the coal series.

Albertite, a bright, coal-like substance, exceedingly rich in volatile hydrocarbon, occupies fissures in Carboniferous rocks in Nova Scotia, and a similar but less lustrous mineral, termed grahamite, occurs in fissures in rock of the same age, near a rich oil-pool in West Virginia. Other similar deposits, but usually wax-like and dull, are found in Utah and neighbouring States. Asphaltum occurs in vast quantities in southern California, and also in Cuba; these deposits resemble the celebrated asphaltum of Trinidad and give promise of being fully as extensive and valuable.

In brief, gaseous, fluid, semifluid, and solid hydrocarbons in great variety are widely distributed throughout the portions of North America where the surface is composed of sedimentary beds, and in a few instances occur in cavities in igneous rocks as well.

The influence of life in leading to the concentration of substances of commercial value is still further illustrated by the beds of diatomaceous earth which are found in various portions of North America and elsewhere, particularly in Cenozoic and more recent terranes. Beds of diatomaceous earth reported to be 40 feet thick and of wide extent have been found near Richmond, Virginia, and similar deposits occur at several localities in Oregon, California, etc. The uses of this fine, white, flour-like powder, each minute grain of which is a beautiful siliceous organism, are for polishing powder, as an ingredient in friction soap, as an absorbent for nitroglycerine in the manufacture of high explosives, etc.

A class of substances of economic importance which owe their accumulation to chemical agencies acting at the surface of the earth is well illustrated by deposits of rock salt and gypsum.

In the Silurian system in New York, Ontario, Michigan, etc., several beds of rock salt and gypsum occur, indicating that there were formerly a number of separate evaporating basins in that region. The beds of salt vary in thickness from a few inches to over 300 feet, as at Tulley, New York. At Goodrich, Ontario, 6 beds of salt from 6 to 35 feet thick have been penetrated in a single well. With the salt in this the Salina formation there are many beds of gypsum. In rocks of Carboniferous age in Michigan, other extensive beds of salt and gypsum have been discovered. In Louisiana, Texas, Utah, and other States, salt and gypsum occur in Mesozoic and Cenozoic rocks. One of the most remarkable of these deposits is beneath small islands in the Gulf of Mexico off the Louisiana coast. On Jefferson Island, for example, rock salt was reached recently at a depth of 260 feet beneath Cenozoic rock, and was penetrated for over 1,800 feet without reaching the base of the deposit. The supply of salt stored in the rocks, and the natural brines of the arid region, such as the waters of Great Salt Lake, afford an inexhaustible supply upon which comparatively small demands have thus far been made.

In addition to salt and gypsum there are other substances that have been accumulated in a similar manner, such, for example, as sodium sulphate, of which large beds occur in the desiccated lake basins of the arid region, sodium bromide, which is obtained from some of the ancient brines pumped from deep wells in Michigan.

Next to the fossil fuels, the most important products of the rocks in North America are the iron ores. Although certain igneous rocks are rich in iron, and in some instances contain it even in a pure or metallic state, none of the rocks that have cooled from fusion carry iron in any form in sufficient quantities to be of commercial importance. Most of the iron in igneous rocks is contained in mineral, usually silicates, and would be difficult to separate. When exposed to the air and to percolating water, the iron-bearing minerals of the igneous or other rocks decay and the iron enters into various new combinations. When organic acids are present, and especially carbon dioxide, ferrous carbonate is formed, which is quite soluble, and is taken into solution by percolating water, some of which emerges as springs, and joins the surface run-off, which may also take up ferrous carbonate in solution. One of the most common methods by which iron ore is accumulated is when water carrying ferrous carbonate in solution forms swamps and lakes, and in many instances as the water is exposed to the air and aided by evaporation it parts with a portion of its carbon dioxide, and the hydrated sesquioxide of iron or limonite results. When, under similar conditions, an excess of organic matter is present, beds of ferrous carbonate are formed. In other instances iron oxide is precipitated in swamps and lakes through the action of low forms of plant life. The ores of iron concentrated in these ways are in many instances in well-defined layers, or lenticular bodies, which are thickest in the central portion and thin out in all directions. Their forms are determined mainly by the shapes of the depressions they occupy. Both ferrous carbonate and limonite, however, occur in irregular surface deposits.

In North America, bog-iron ores occur at the surface in many regions, in existing swamps and about springs, but are seldom of economic importance, owing in part to the great abundance of better ores. Limonite occurs at the surface also, having been deposited in cavities and as a cement for loose fragments, particularly on the weathered outcrops of formations rich in iron. When rocks contain but a fraction of 1 per cent of iron, the soil on their weathered outcrops, owing to the removal of the more soluble ingredients and the leaving of the less soluble oxidized iron, have a yellow, brown, or red colour, and in some instances this process of concentration has produced workable iron ore. Limonite and earthy hematites occur widely throughout the Appalachian region, in central New York, and westward to the Mississippi Valley. One of the most productive formations is the Clinton, a division of the Silurian, the outcrop of which extends in a nearly continuous band from Alabama, where at Birmingham, etc., it is extensively worked, northward along the west side of the Appalachians to central New York, and thence westward to Ohio, and appears again in Wisconsin. At many localities throughout this belt, some 1,300 miles in length, iron furnaces have been built, although now mostly abandoned, the ore supply being the weathered outcrop of the Clinton limestone.

In the Carboniferous rocks of Pennsylvania and neighbouring States to the south and west, layers of ferrous carbonate, formed when there was an excess of organic matter present, termed black-band ore and kidney ore, occur. The former is present as regular strata and the latter in oval concretionary masses. These ores, although not as rich in iron and less pure than certain other and more abundant and more accessible deposits, have been extensively utilized, largely for the reason that they occur in the same formation which furnishes coal available for their reduction.

Deposits of iron ore accumulated in the several ways referred to above may be metamorphosed and changed to hematite and magnetite. The richest iron ores of North America are of this nature, and will be referred to below in connection with other substances of economic importance contained in the metamorphic rocks.

There are various other substances in the stratified rocks of North America of economic importance which owe their value to some process of concentration. Certain rocks, as the so-called greensands or marls of eastern New Jersey, contain from 3 to 10 per cent of potash, which makes them valuable fertilizers. In this instance the concentration took place on the floor of the sea, through the action of decomposing organic matter, and the potash-bearing mineral of the greensand, namely, glauconite, was deposited in the interiors of the minute tests of foraminifera. The importance of this material is indicated by the fact that the greensands of New Jersey have been actively worked for more than half a century, the annual products during many years being upward of 100,000 tons.

Extensive areas in the Carolinas, Florida, etc., underlaid by rocks of Cenozoic age, are rich in phosphatic nodules, which have been derived from organic matter. The guano deposits of the low arid islands in the West Indies illustrate another mode of accumulation of organic material useful as a fertilizer.

The assorting of surface débris by streams and currents has led to the formation of extensive deposits of clay which occur widely throughout the portions of North America where the surface is composed of stratified rock, which is extensively used in the manufacture of earthenware, bricks, tiles, terra-cotta, Portland cement, etc.

When rocks containing gold in nuggets, grains, scales, etc., are disintegrated, and the resulting débris removed by streams, mechanical separation of the heavier from the lighter material takes place and all but the very finest of the gold is concentrated on the stream beds. In this manner the rich placers of the Pacific mountain region from California to Alaska have originated.

The general nature of the ore bodies formed through the action of chemical agencies in sedimentary rocks, by solution and redeposition, is illustrated by the lead and zinc ores of Wisconsin, Missouri, the silver-bearing lodes of the Pacific mountains, etc. In the case of the lead and zinc deposits the ores occupy the interspaces between broken sedimentary beds or line caverns. Under the best explanation of the origin of these deposits that has been offered, although certain modifications of the general hypothesis have been suggested which it is not necessary to consider at length at this time, the lead and zinc are considered to have been at one time widely distributed in the adjacent sedimentary rocks, mainly limestone, and to have been taken in solution by percolating waters and carried to cavities where they were precipitated, together with various other mineral substances, such as calcium carbonate or calcite, barium sulphate or barite, carbonate of calcium and magnesium or dolomite, etc. The minerals containing lead are principally galenite or lead sulphate, cerussite or lead carbonate; while the zinc is contained in the minerals, sphalerite or zinc sulphide, calamine or zinc silicate, smithsonite or zinc carbonate, etc. These minerals, including both those containing lead and zinc, and those intimately associated with them which are at present of no commercial value, are such as are known to crystallize from solution without the aid of high temperatures. In the Missouri lead and zinc districts the ore deposits occur near the surface, the depth of the present working seldom exceeding 150 or 200 feet, and, as nearly as can be judged, have been formed by the downward transfer of mineral matter through the process of solution and recrystallization, as the surface of the land has been lowered by chemical and mechanical denudation.

Many of the rich silver-mines of the Pacific mountains occur in fissures and cavities in sedimentary rocks, mainly limestone. Instances of this nature are furnished by certain mines in northeastern Mexico, where the ore is found in cavities in Cretaceous limestone; at Leadville and Aspen, Colorado; Big and Little Cottonwood ca?ons, and the Horn silver-mine, Utah, where the principal country rock is Carboniferous limestone; the Eureka district, Nevada, where the ore occurs in cavities in Cambrian limestone. In the case of several of these mines, igneous rock is near at hand, and the ores are believed to owe their concentration largely to the action of heated waters.

In other regions deep fissures, occupied in part by dikes of igneous rock, have permitted of the ascent of water charged with mineral matter from far below the surface; such waters are heated, in part by the general heat of the earth's interior, or, if in association with dikes, by the heat of the once molten intruded rock. The ascending hot water is an active solvent, and as it rises becomes cooled, and for this and other reasons precipitates many mineral substances. Veins are thus formed, which are many times banded-that is, result from the filling of fissures by the successive deposition of minerals of various kinds on their walls, each different layer of minerals indicating a change in conditions. Fissures filled in this manner from below, as denudation progresses, become exposed at the surface and reconcentration through the influence of disintegration and decay, and of solution and redeposition by descending water takes place. Ore bodies of this character carrying gold, silver, mercury, etc., are of wide occurrence, especially in the Pacific mountains, but the process of concentration is independent of the nature of the country rock. Segregated and fissure veins occur in either igneous, sedimentary, or metamorphic terranes, but are more commonly of economic importance in the metamorphic rocks than elsewhere, and will be referred to again in that connection.

Economic Importance of the Metamorphic Terranes.-The great laboratory in which rocks undergo important changes in their physical condition and in mineralogical and chemical composition, is what has been termed on a previous page the zone of metamorphism. The depth of the upper limit of this zone is variable, dependent in part on the nature of the rocks and on movements within them, as is the case of mountain building. In fact, there is probably no well-defined limit to the zone either above or below, as in the former direction metamorphism merges by gradations into alteration produced by the descent of surface water, and in the latter direction as heat increases passes again, as we imagine, by insensible and irregular gradations into a region where the rocks are so highly heated that diffusion rather than concentration results. Whether the rocks below the zone of metamorphism are fused or not depends on pressure. They are probably solid, but in a potentially plastic condition, and become fused and may be forced upward through fissures in the condition of igneous magmas when pressure is relieved. The zone of metamorphism lies between a superior zone where alteration by descending water is dominant, and a lower region where alteration due mainly to heat is in control. In the zone of metamorphism the influence of heated percolating waters, combined with movements in the rocks, are the principal factors which lead to the concentration of mineral substances.

Under the influence of percolating, heated waters, new minerals are formed in sedimentary or igneous rocks, and rocks once metamorphosed may undergo additional changes. Mineral matter previously widely disseminated through rocks is, under the action of percolating, heated water, brought together and the regeneration and crystallization of a large variety of ores and minerals result. The birthplace of a large variety of ores and minerals is in the zone of metamorphism. It is in metamorphic rocks that the geologist looks for gems, the precious metals, crystalline marble, magnetic iron, etc.

For the most part, however, the native metals and ores of the precious and many of the common metals are too widely disseminated in the metamorphic rocks to be of commercial importance, and a still further concentration, principally in fissures and other cavities, is necessary before they can be of value to man. This secondary concentration is much the same as in the case of the deposition of lead and zinc ores in cavities in sedimentary rocks, and results largely from the solution and redeposition, sometimes by replacement, of mineral matter by heated waters.

Certain ores and rocks contained in metamorphic terranes owe their concentration to previously acting processes of concentration, but have undergone chemical changes in place. Illustrations of this class of ores, etc., are furnished by the magnetite and hematite contained in the metamorphic rocks on the eastern border of the Appalachians, in New England, eastern Canada, and the Lake Superior region. These ore bodies, frequently of great size, in some instances furnish evidence of having been originally lenticular masses of bog-iron ore, or ferric carbonate, associated with sedimentary beds, and originally concentrated, as already mentioned, at the surface through the action of water charged with carbon dioxide, but principally on account of the influence of heat have been changed to a higher degree of oxidation and now appear as hematite, as, for example, in the iron districts of the northern portions of Michigan, Minnesota, Wisconsin, and the Ozark Hills, or still further altered as in the richest of all iron ores, magnetite, so abundant in the metamorphic rocks of the Appalachian region, about the Adirondack hills, widely and in extensive bodies in eastern Canada, about the south shore of Lake Superior, in Texas, etc.

In certain instances, as has been shown by C. R. Van Hise and others, hematite ore, like that of the Lake Superior region, has resulted from the alteration of ferrous carbonate which had replaced limestone by a chemical process of solution and double decomposition.

As bodies of iron ore in the form of the carbonate, or limonite, may occur in rocks of any age, and as rocks of any age may be metamorphosed, it follows that hematite and magnetite may be present in any formation which has been subjected to metamorphosing conditions.

Limestone when metamorphosed is changed to a crystalline marble, frequently white in colour owing to the dissipation of its previously contained organic matter. The white marbles so extensively utilized in Georgia, Vermont, etc., are of this nature. Other similar metamorphosed layers occur in several of the ranges of the Pacific mountains from Mexico to Alaska.

The influence of metamorphism on deposits of coal when the heat has been of moderate intensity serves to drive off a large part of the volatile matter present and converts the coal into a substance resembling coke, as has happened adjacent to dikes or intruded sheets of igneous rock in the Richmond coal-field, Virginia, in New Mexico, Washington, etc. When the heat is somewhat more intense, the coal is changed to what is termed graphitic anthracite, as in the Rhode Island coal-fields, and when still greater or long-continued, results in the production of graphite, as in the Algonkian rocks about the Adirondack hills and over a wide region in eastern Canada.

An important result of metamorphism is the production of new minerals in the rocks acted on. Many of the metamorphic terranes consist essentially of quartz, feldspar, and mica, which have been formed by the rearrangement of the mineral matter contained in the rocks during their previous state. Besides these constituent minerals there are frequently others present, such as the garnets, tourmaline, emerald, sapphire, corundum, etc., which are of economic importance. In a large number of instances the minerals of metamorphic rocks are contained in veins of one class or another, in part resulting from segregation in the rocks themselves while yet in a heated condition, and in part deposited in fissures or other openings as a result of secondary concentration through the action of heated waters. The principal difference between the minerals concentrated in the metamorphic rocks and those deposited in cavities in unaltered sedimentary beds seems to be that in the former instance the percolating water which carried the material in solution had a higher temperature than in the latter case.

Among the numerous mineral substances of value in the arts, occurring in the metamorphic terranes of North America, other than building stones and the previously concentrated deposits, such as iron ore, graphite, etc., mention can only be made at present of the following:

Mica, which is used in thin sheets for the windows of stoves and furnaces, and when ground and mingled with other substances furnishes a good insulating material for electric wires, fireproofing, and also used as a lubricant, etc., occurs in large quantities in the metamorphic rocks of New Hampshire and Ottawa, and less abundantly in North Carolina, South Dakota, Wyoming, Idaho, etc. It is widely distributed, but to find transparent colourless sheets of large size is difficult.

Talc and soapstone, consisting of the hydrated silicate of magnesia, and useful for hearths, mantels, fire-brick, linings for stoves, laundry-, bath-, and acid-tubs, etc., and when ground, employed as an adulterant of soap, paper, rubber, and as a lubricant, etc., occurs widely in the metamorphic terranes on the eastern side of the Appalachians, in Canada, and at numerous localities in the Pacific mountains. The chief centres of production at present are in Pennsylvania, New Jersey, New Hampshire, and Vermont.

Asbestos, valuable on account of its fibrous structure and non-conductivity of heat, which make it an excellent insulator, and largely used in the manufacture of fireproof paper, cloth, etc., occurs in connection with serpentine, in metamorphic terranes, and is extensively mined in the Thetford district, Quebec.

Corundum, consisting of aluminum oxide, and having essentially the same composition as the sapphire and ruby, and a less pure variety of similar composition termed emery, is largely used as an abrasive in polishing metal, sharpening tools, etc., and also as "sand-paper" in working wood, occurs in commercial quantities, largely in crystalline limestone, at Chester, Mass., in Georgia, North Carolina, and several other localities. Although corundum is next to the diamond in hardness, and therefore highly favourable, when reduced to a powder, for polishing various substances, the demand for it has in recent years been diminished owing to the manufacture of an equally if not superior material termed commercially carborundum.

Among the crystals used as gems, which occur in the metamorphic rocks of North America but thus far in minor quantities, and as a rule of inferior quality, may be enumerated sapphires, rubies, tourmalines, garnets, quartz, etc.

Apatite, a mineral rich in phosphoric acid, and largely used in the manufacture of fertilizers, occurs associated with limestone in the metamorphic rocks of Quebec and Ontario in the form of veins, beds, and irregular pockets, and a few years since was extensively mined, but now, owing to foreign competition, is held in reserve.

By far the most valuable of the minerals and native metals that occur in the metamorphosed terranes is gold. Although this metal has been found in paying quantities in association with nearly every kind of country rocks and in terranes of all ages, the place of its original concentration from a previously widely disseminated condition is to a great extent in the zone of metamorphism. It occurs principally as native gold, although usually alloyed with silver, but is frequently contained in iron pyrites. In the crystalline rocks, such as gneiss, schist, slates, granite, etc., it occurs in flakes and grains, but so far as its occurrence in commercial quantities is concerned its deposition has for the most part been secondary, and the metal, usually in association with quartz, is found in veins, lodes, contact deposits, etc., and owes its concentration to chemical agencies not well understood, acting in connection with percolating water. That this general statement is correct is clearly shown by the fact that gold occurs in crystals, flakes, grains, etc., most frequently in quartz and iron pyrites, which, as can be shown in a number of ways, have crystallized from solution. The gold and its commonly associated mineral in countless instances occupy fissures and must have been carried to such localities after the surrounding rock had been fractured. So intimate is the association of gold with metamorphic rocks that this is one of the main guides in searching for it, although, as already stated, it is frequently present in other rocks as well. With the disintegration of the metamorphic terranes the gold is set free, and may be still further concentrated by streams so as to form the well-known placers.

A very large proportion of both the quartz and placer mining of North America is in regions occupied by metamorphic rock. This is true of all gold-mines, previously quite largely exploited, of the Atlantic mountain region from Georgia to eastern Canada. The mines of California are also largely in schistose rocks, as are also those to the northward, throughout the Pacific mountains, to British Columbia and Alaska, including the recently established mining district at Cape Nome.

With placer gold, and probably derived largely, if not entirely, from metamorphic rocks, there are frequently found grains of platinum. The annual production of this metal in the United States and Canada has a value of about $5,000.

The study of the distribution of native metals and ores in the metamorphic rocks of North America indicates that in general the older rocks, as the Archean, for example, are less rich than the younger terranes, such as the schist, etc., of the Sierra Nevada and Cascade Mountains. This seems to indicate that the older rocks were once deeply buried and their more soluble substances removed by ascending waters, and in part redeposited in higher terranes. Erosion has since carried off the rocks which were mineral-charged and laid bare the depleted terranes beneath. This hypothetical explanation of the general poverty of the Archean rocks is coupled with another consideration, namely, that the younger metamorphic terranes, where they have been elevated, as in the Pacific mountains, are more broken than the Archean rocks, and afford more cavities in which minerals may be deposited. Whether this is a complete explanation or not remains to be demonstrated, but observation shows that the Archean terranes-all of which as yet discovered are composed of either metamorphosed or igneous rocks-are, in comparison with younger metamorphosed rocks, relatively poor in minerals and ores of commercial importance.

Among the economic products of the rocks are included mineral waters. The direct commercial value of such waters, not including their use for baths, etc., in the United States, is about $7,000,000 annually. The demand for these waters depends largely on the mineral substances they hold in solution, and which in many instances is in process of transference from one locality to another. Much might be written in this connection in illustration of the fact that the processes by which minerals, ores, etc., have been concentrated are still in progress.

LITERATURE

An extensive literature is available concerning the geology, minerals, ores, etc., of North America, but only a few of the more important publications can here be referred to. The numerous publications of the United States Geological Survey, the Geological Survey of Canada, and the Geological Survey of Mexico contain vast amounts of valuable information. Several of the States of the United States have independent surveys and have published numerous reports. Of journals containing articles of American geology, the more important are: The Journal of Geology, published at the University of Chicago; The American Geologist, published at Minneapolis, Minn.; The American Journal of Science, published at New Haven, Conn. The publications of a large number of learned societies in Canada and the United States should also be consulted.

The most useful bibliographies of North American geology are:

Darton, N. H. Catalogue and Index of Contributions to North American Geology, 1732-1891. Published as Bulletin No. 127 of the United States Geological Survey, Washington, D. C., 1896.

Dowling, D. B. General Index to the Reports of Progress [of the Geological Survey of Canada], 1863-1884. Published by the Canadian Geological Survey, Ottawa, Canada, 1900.

Warman, P. C. Catalogue and Index of the Publications of the United States Geological Survey, 1880 to 1901. Published as Bulletin No. 177 of the United States Geological Survey, Washington, D. C., 1901.

Weeks, F. B. Bibliography of North American Geology, Paleontology, Petrology, and Mineralogy for the Years 1892-1900, Inclusive. United States Geological Survey, Bulletins No. 188 and 189, Washington, D. C., 1902.

Of the numerous general treatises on the geology, the following will be found especially helpful to the student:

Dana, J. D. Manual of Geology. Fourth edition. American Book Company, New York, 1895.

Kemp, J. F. The Ore Deposits of the United States and Canada. Scientific Publishing Company, New York, 1900.

Le Conte, J. Elements of Geology. Fifth edition, revised and partly rewritten by Prof. H. L. Fairchild. D. Appleton and Company, New York, 1903.

Merrill, G. P. A Treatise on Rocks, Rock-Weathering, and Soils. The Macmillan Company, New York, 1897.

Shaler, N. S. Outlines of the Earth's History. D. Appleton and Company, New York, 1898.

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