When one attempts to write an account of the birds of North America, the heavens seem darkened with such a multitude of varied and beautiful forms and the air filled with such a discordant clamour mingled with the sweetest of music that failure to convey an adequate idea of the countless numbers and diversity of the feathered throng within the compass of a few pages must be recognised from the start.
The important place held by the birds of North America in the avifauna of the world, may perhaps be best indicated by noting first of all what orders and families are without representatives among them.
The orders under which the birds of the world are arranged in the scheme of classification adopted by Wallace in his great work on the geographical distribution of animals are as follows:
| Orders. | Examples. |
| 1. Passeres | Includes the greater number of the smaller birds, such as kingbirds, flycatchers, larks, jays, crows, blackbirds, finches, sparrows, warblers, chickadees, swallows, thrushes, etc., numbering in North America some 328 species and many subspecies. |
| 2. Picariæ | Woodpeckers, cuckoos, toucans, kingfishers, swifts, goat-suckers, humming-birds, etc. |
| 3. Psittaci | Parrots only. |
| 4. Columbæ | Pigeons and the dodo. |
| 5. Gallinæ | Grouse, pheasants, quail, jungle-fowl, turkeys, guinea-fowl, etc. |
| 6. Opisthocomi | The hoazin of Guiana and Brazil only. |
| 7. Accipitres | Eagles, owls, vultures, hawks, buzzards, falcons, etc. |
| 8. Grallæ | Rails, snipes, plovers, cranes, herons, storks, flamingoes, etc. |
| 9. Anseres | Ducks, geese, gulls, petrels, pelicans, penguins, loons, auks, etc. |
| 10. Struthiones | Ostrich, rhea, cassowaries, emeus, apteryx. |
Of these ten orders, all but two are abundantly represented in North America. The missing orders include the ostrich-like birds, of which the only species in the New World is the rhea, of the southern portion of South America, and the peculiar hoazin, represented by a single species found in Guiana and Brazil.
The eight orders under which the birds of North America have been arranged (other classifications, however, have been adopted by various naturalists) have again been subdivided into families, genera, species, etc. According to Wallace's summary, the 8 orders referred to contain 124 families, of which 75 are not represented in North America to the north of the lowlands of Mexico, while 47 families are present. Of these 47 families, 25 are essentially of world-wide distribution, and only 1, containing a single species, a small wren-like bird of the genus Chamæa, found in California, is peculiar to the fauna of the continent.
As the North American continent under the arrangement adopted for the series of books of which the one in hand forms a part, is considered as including the West Indies, Mexico, and Central America, the above summary does not represent its entire bird fauna, but presents, perhaps, the best general idea of it that is at present attainable. To include the birds of the tropical portion of the continent would add greatly to the number of species, but I believe not materially to the number of families and orders, as given above.
In reviewing the distribution of the land birds of North America to the north of Mexico, but including lower California, J. A. Allen places the total number of genera, as given in the check-list of the American Ornithologists' Union, at 181. Of these, 55, or 30 per cent, are circumpolar or otherwise wide-ranging Old World forms; 126 genera, or 70 per cent, are American, of which 35, or 28 per cent, are essentially tropical, leaving 91 genera, or about 50 per cent, as distinctly North American.
The number of species in the avifauna of the continent, according to the latest check-list published by the American Ornithologists' Union (1895), is 768, together with a large number of subspecies. If the tropical portion of the continent were included, this number would be greatly increased and possibly doubled.
A further generalization has been advanced by Allen, who states that in the arctic portion of the continent the number of genera of birds present during the breeding season is 65, of which only 5 are exclusively American. In the cold temperate belt 120 genera are represented, of which 98 are circumpolar and 22 American. In the warm temperate belt 95 genera occur which do not range into the cold temperate belt, and of these only 12 are Old World forms, while 83 are exclusively American, and in addition 60 genera are common to both the cold and the warm temperate zone, of which 46 are represented in the fauna of the Old World, while 14 are American. This gives 155 genera for the warm temperate zone, of which 58 are Old World and 97 exclusively American. There are besides 50 essentially tropical genera which extend into the warm temperate zone, of which 43 are American and 7 tropicopolitan. The avifauna of the warm temperate zone thus contains a total of 205 genera.
The above enumeration indicates the rapid increase in the variety of bird life met with as one travels from the arctic to the Gulf coast of the continent, and in this connection it is to be remembered that the land contracts in breadth towards the south. In number of individuals, however, it is doubtful if there are less per square mile at the north than at the south during the breeding season.
There is a decrease in the number of Old World forms inhabiting North America from north to south. A similar decrease in mammalian species common to America and Eurasia has previously been referred to, and the same explanation applies in each case, namely, the near approach of the land areas of the Old and the New World at the north, and the actual union of the two continents in late geological time.
As has been shown by Allen, the species of birds of the temperate and boreal zones of North America were derived in part from the Old World, in part from types almost universally distributed throughout the warmer latitudes, and in part from tropical America, but to a marked extent the species present developed where they are now found. This generalization is in harmony with the geographical relations of the continent to other land areas, and with the fundamental principles of evolution.
Migrations.—Among the many facts of interest to the geographer in connection with the bird life, none present a more fascinating field for study than the annual migrations in which a very large number of the species participate.
As one travels northward from Mexico or the Gulf States, the number of species of birds which remain in essentially the same area throughout the year, or the residents as they are termed, becomes less and less. In New England and about the southern shores of the Great Lakes there are about 30 species which remain all winter and may justly claim to be citizens. Besides these, there are several visitors that come from the north and belong to the vast army of migrants, but which are contented with a comparatively small change of position during the periods of greatest cold or heaviest snow. In the far north the number of residents is still more restricted. On the tundras fringing the arctic coast even the snow-owls, snow-buntings, and the ptarmigans, the hardiest of birds, move southward during the winter to the shelter of the subarctic forest, and bird life on the vast frozen morasses is practically, if not absolutely, wanting.
The millions of birds that journey southward each fall begin their return migrations at the first promise of spring. Even during unusually mild spells of weather in winter, temporary northward movements occur. The migratory birds are actuated by such a strong desire to regain their nesting places and summer homes that they embrace every opportunity to journey towards them, and not infrequently suffer severely for the risks they take. In some instances species which have begun their northward flight too soon are killed by thousands owing to a return of severely cold weather or die for lack of food.
The first definite northward migration in the southern portion of the Mississippi Valley begins during exceptionally favourable years as early as the end of January, but the great movement of the feathered hosts is not usually at its height before the middle of March or the first of April. In New England the current of migration begins between the middle of February and the first of March, and increases in strength until the middle of May, when it is at its height, and then rapidly declines and is practically over by the beginning of June. In the far north, the first arrival from the southward, and that a species which does not make a long annual journey, usually appears early in April. At Point Barrow, the most northern portion of Alaska, as was observed by John Murdock in 1882 and 1883, the first harbinger of spring was a snow-bunting, which arrived the first year on April 9th. The northward-flowing tide of bird life ends early in July in the region of the Yukon, and by the middle of that month the vast flocks have been separated and the many mates have found their nesting places. The time taken for the general movement is thus in the neighbourhood of four months.
The northward flight of the birds is seldom, if ever, one continuous journey, but like many other movements in nature, progresses by pulsations. Well-defined "bird-waves" have been recognised especially in the Mississippi Valley. The direct or immediate cause of the starting of these waves of life is the coming of a wave of heat. Secondary or modifying conditions are furnished by strength and direction of the wind, cloudiness, rain, etc. As the weather in spring-time is fickle, and its variations not the same for any two consecutive years, so the gathering of the birds into flocks and their northward flights vary, although for a term of years the arrival of a given species at a particular station does not depart far from a mean date. With the northward sweep of the waves of bird life over the temperate and boreal portions of the continent comes the awakening of plant life, but the birds, to a marked extent, precede the unfolding of the flowers. This marvellous renewal of the life of the land after the long cold winter makes the budding and nesting spring-time the most joyous portion of the year, and one which exerts a marked effect on human thought and activities. The spring-time awakening in all nature is like a resurrection, and has apparently exerted an influence on the religions of the world.
The bird-waves referred to above are characterized at the start by the presence of great numbers of a single species, but as they progress, scattering occurs, and at the time of the greatest movements in the northern portion of the United States a large number of species frequently arrive at a given locality in a single night. At sunset the groves may be nearly tenantless and silent, while at sunrise they are alive with the flitting of wings of many colours, and the air pulsates with many different songs.
Judging from extended observations on migrations made in the Mississippi Valley, the definite waves of bird life which sweep northward with the spring-tide of temperature are in some instances 100 or 200 miles long and have a breadth of perhaps a score of miles. The distance between the waves varies with variations in the weather and perhaps other causes, and as they progress they apparently become less definite and at the north have yet to be recognised.
The migrations of the birds are performed principally at night. In the northern part of the United States during the hours of darkness in early spring, even when cloudiness prevails or the land is veiled in mist, the voices of geese may frequently be heard overhead, proving that unseen flocks are then winging their way northward. About the lighthouses along the coast and on the shores of the Great Lakes, when migration is in progress, dead birds are frequently found in considerable numbers and of various species. In these cases the birds are evidently attracted by the lights and killed on striking the windows that protect them. This occurs particularly on stormy or cloudy nights, when the birds fly low. Several of the larger species of birds, as the geese, ducks, cranes, etc., which are strong of wing, make long flights without resting. In many instances a single stage in a journey may include 500 or 600 miles. Most of the smaller birds, however, fly comparatively short distances between the pauses made for rest and food.
A species on reaching the northern portion of the route over which it usually migrates scatters, and the individuals mate, nests are built, and young reared. At the approach of cold weather reassemblage occurs, frequently great flocks being formed, and the southern movement begins. The southward migration is less conspicuous in most instances than the movement en masse of the birds in the spring, and so far as now seems to be recognised is not divided into definite waves.
While the winter habitat of most birds in the temperate and boreal portions of the continent is to the south of their summer homes, the annual migration is not in all cases great in amount. Some species move only a few hundred, or possibly a few score, miles. Even the winter residents make short migrations, dependent on weather conditions. The greater part of the migratory birds, however, pass the winter in the Gulf States, Mexico, the West Indies, and Central and South America. In some cases they go well to the south of the equator. The annual flight going and coming measured in a straight line, between the nesting place and the winter home, cannot be less in many instances than from 8,000 to 10,000 or 12,000 miles. An interesting fact in this connection is that certain species follow definite routes. The region moved over annually, if marked on a map, would resemble two open or partially opened fans, with their handles pointing towards each other and connected by a narrow band.
The causes of the annual migrations of birds have received much study and been the subject of much speculation. The general consensus of opinion in this connection seems to be that the birds are controlled largely by what we in our ignorance term instinct. The true beginning of the migration seems to be in the fall, when the birds are driven from their homes by cold or, perhaps more accurately in most cases, by scarcity of food. This, however, is not the whole story, since many species start southward before cold weather approaches and while food is yet abundant. Then, too, crippled individuals have been known to survive the winter in regions from which their summer companions have departed. Instinct, therefore, plays a part in even the fall migration, where at first glance sufficient physical reasons may seemingly be claimed for it. During the spring migration the birds are moved by a strong impulse to regain their breeding-grounds. Each species seems to have adapted itself to certain conditions of temperature, food, etc., through long ages of development, and acquired a subtle faculty of regaining the environment to which it is best adapted, as soon as the adverse conditions that caused it to leave its home are ameliorated. How a particular bird is enabled to return to the nest it built the year previous is not known. The study of the homing instinct of pigeons assists in this direction, however, and suggests that birds are endowed with something answering to a sixth sense—that is, a sense of direction or of orientation.
Spring-time Music.—The northward-flowing tide of life each spring brings to the temperate zone of North America a marvellous change not only in colour and movements, but in sound. This is the season of bird courtship and more than usual happiness among the feathered millions. From shore to shore of the continent a chorus more seductive than sirens' songs pulsates on the breezes.
The winter is characteristically a season of silence. The sounds heard at a distance from human habitations are mainly those produced by inanimate nature. The wind causes varied discords amid the bare branches of the deciduous trees or sings weird melodies in the pines. Strange muffled roars come from the frozen lakes, as the ice contracts and breaks during periods of excessive cold. The frost in tree trunks causes sharp explosions. The ice-covered streams are still except where cataracts interrupt their even flow. In the profound silence of a calm winter night the distant dismal howl of a wolf, the cry of an owl, or the bark of a fox alone reminds one that life still continues, but these animate sounds are far more frequently absent than present. With the coming of the spring there is a marvellous awakening and unfolding. The brooks, swollen to overflowing by the melting of the snow, make music as they run. The northward flight of the birds brings to every grove a chorus of song. A host of batrachians and reptiles bestir themselves after a long winter sleep and vociferously proclaim their presence. The insect world, with its unnumbered legions, takes wing. The air vibrates with millions of voices. The trees put forth their leaves, each a harp-string which responds to the touch of the fingers of the wind. The organ-notes of the thunder again startle the hibernating echoes. As the winter is the silent season, so the spring is the time of music.
One of the most charming of the many phases of nature's concert season is the matin songs of the birds. Ere the eastern sky along the New England coast becomes roseate with the first blush of morn, the twitter of birds may be heard amid the shadowy branches of the trees. Soon a thrush or a warbler awakens in full song, and is followed by a host of other voices, until the air pulsates with music. As the sun rises and his first level rays reveal the varied tints of the tree tops, the many-voiced chorus passes the height of its ecstasy and the music gradually subsides. But the glad tidings of the coming of the day are passed westward from grove to grove and from meadow to meadow, and a wave of song sweeps on ahead of the wave of light, induced by its coming. The song-wave spreads to the north and south and flows steadily westward over the forest-covered mountains, across the great central basin of the continent, breaking on the treeless plateaus into many streams which follow the grove-fringed rivers, passes through the depressions in the Rocky Mountains, and although weakened in the arid valleys beyond, is not checked. The larks there listen for its coming and pass the joyful message westward. The timid dwellers in the great forests of Oregon awaken at the magic sound and the lofty tree tops are made to thrill with the voices of unseen choirs while it is yet night in the silent aisles below. The onward rush of sound is not reflected or turned back by the lofty Cascades, but flows through their passes and only ceases when the sea-birds of the Pacific renew a note that was dropped on the distant Atlantic coast.
One of the most fascinating incidents in the life of the explorer in his lonely camps in the great forests or amid the solemn mountains is the coming of the wave of song in the spring and early summer-time which precedes and accompanies the rising of the sun.
The fascination of the field of study touched upon in this chapter invites an attempt to present an account of some of the more characteristic birds of North America, and to endeavour to convey to the reader some idea of the varied reptilian, fish, and invertebrate life of the continent, but the limitations of space prohibit such a review. Even the great problems dealing with the intimate relation that exists between geographical conditions and the distribution of animals cannot be given more attention than the suggestions already offered.
- Allen, J. A. The Geographical Distribution of Animals, in Bulletin of the United States Geological and Geographical Survey of the Territories, vol. iv, Washington, 1878, pp. 313-377; The Geographical Distribution of North American Mammals, in Bulletin of the American Museum of Natural History, vol. iv, New York, 1892, pp. 199-243.
- Heilprin, Angelo. The Geographical and Geological Distribution of Animals. International Scientific Series, London and New York, 1897.
- Osborn, H. F. The Rise of the Mammalia in North America. New York, 1893.
- Cope, E. D. Geographical Distribution. In the Annual Report of the Smithsonian Institution for the year ending June 30, 1898, Washington, 1900, pp. 1199-1234.
- Merriam, C. Hart. Life-Zones and Crop-Zones of the United States, United States Department of Agriculture, Division of Biological Survey, Bulletin No. 10, Washington, 1898; Results of a Biological Survey of the San Francisco Mountain Region and Desert of the Little Colorado, Arizona, United States Department of Agriculture, Division of Ornithology and Mammalogy [Bulletin] No. 3, Washington, 1890; The Geographical Distribution of Life in North America, in Proceedings of the Biological Society of Washington, vol. vii, 1892, pp. 1-64.
CHAPTER VI
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 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]
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.
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 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.
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.
| 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 Algæ. | 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 Cañon 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 algæ, 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 algæ, mostly, we presume, of small size, in fact microscopic, and soft tissued. The animal life subsisting primarily on the algæ 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 cañons. To this Sierran epoch, as it is termed, is referred the excavation of the larger valleys of the Sierra Nevada, the world-renowned cañons 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.