Page:The New International Encyclopædia 1st ed. v. 19.djvu/756

UNITED STATES. rare. The forests are of yet greater extent than in California, and reach down to the coast. The Douglas spruce, Western hemlock, noble fir, and giant cedar reach here their finest development. See.

. The fauna of the United States is an integral part of that of the (q.v.), and is fully representative of that of North America described in the paragraph Fauna under. (See also ; and Fauna under .) With the exception of a few strictly boreal species, such as the (q.v.), almost all the genera and species of North American animals of every kind are represented within the boundaries of the United States and its adjacent waters; and most of the Arctic absentees are found in (q.v.). Reference to the articles cited above and to such articles as , ,, , , , , , , , , , , , , , , , , , and others, will give a conspectus of the fauna of the country.

. North America, like other continents, combines very ancient areas with those that are geologically young. The primitive framework of the continent was drawn somewhat upon the lines of the existing mountain systems of the east and west, forecasting the Appalachian and Cordilleran uplands and the Mississippi lowlands of to-day. The oldest rock systems are known as Archæan and Algonkian. They are represented to the northward by an irregular V-shaped area inclosing Hudson Bay between its arms, and resting its blunted apex upon the north border of the Great Lakes. Extensions of this early nucleus are found along the Appalachians to Alabama, and consist of hard crystalline rocks of igneous and metamorphic origin, granites, gneisses, schists, marbles, and quartzites being among the common kinds. Such areas are the Adirondacks, some tracts in New England, the Highlands of the Lower Hudson and of New Jersey, South Mountain in Pennsylvania, with the Blue Ridge and Unakas of the Southern States. Other primitive masses are found in Wisconsin and Minnesota and form a straggling archipelago in the West, as in the core of the Black Hills, along the axis of the present Rocky Mountain Range, and in the Wasatch and Sierran regions. It must not be thought, however, that the present boundaries of these belts of ancient rock mark the shores of the ancient islands, but the lands now belonging to the United States began thus in narrow strips and patches, east and west.

The next younger but still very old formations belong to the Paleozoic era. This interval of geological time was very long, and includes an extended succession of periods, with their subdivisions or epochs. The rocks of the Cambrian, or earliest of these periods, are mainly sandstones, conglomerates, and shales, and are found in limited outcrops about the borders of the Archæan and Algonkian. The Ordovician, Silurian, Devonian, and Carboniferous formations, on the other hand, cover wide areas, especially in the Eastern United States. They constitute the bed-rocks of a region extending from the Archæan axis of the Appalachians, beyond the Mississippi River, reaching into eastern Nebraska, central Kansas, and far into Texas. From the Great Lake region they are found southward to middle Georgia and Alabama, but do not appear along the Mississippi south of the Ohio River. Thus at the close of the Paleozoic era the territory of the United States was a semi-continent on the east, sending lobate areas southward, with the Mississippi embayment between them. There was also rock accumulation and land growth in the Cordilleran region, but it still held a group of islands rather than a continental area. The largest western lands of Paleozoic age are in the region of the Great Basin, but the regions of the Colorado plateaus and of the coastal Pacific States were still sea, the Rocky Mountain belt was a chain of islands, and an unhindered sea swept from the tropical waters to the Arctic.

The Paleozoic era closed, in North America, with what is known as the Appalachian revolution, that is, with the disturbances which created the great series of folds which now extend from eastern New York to central Alabama. There had long been mountains in the east, as the Adirondacks and Blue Ridge. What their height may have been is not known. During Paleozoic time also the Green and Berkshire ranges of New England were formed. But there were then no mountains west of the Blue Ridge. During all the periods of the Paleozoic era, the waste of the older lands on the east and north was swept into an interior sea that ranged from central New York far to the west and southwest. Thus originated the sandstones, shales, and limestones of the Cambrian and succeeding periods, to which reference has been made. Along the old Appalachian border these formations acquired a thickness of several miles. In the mountain-building which ensued, these thick beds were crumpled and built into a range of high mountains. These mountains, having wasted away during the long periods which have since elapsed, leave to the United States the low ranges found now in Pennsylvania, and west of the Blue Ridge in Virginia and more southern States. With the building of the mountains there was a general uplift in the east, which permanently banished the sea waters from the eastern and central States of the Mississippi basin, except in the south.

During the Mesozoic era and the succeeding Tertiary period, the additions to the land areas of the east were confined to the Atlantic border and the Gulf region. From the enlarged lands of the east, following the Appalachian uplift, materials were available for further extension. In the west the growth was interior as well as on the border, and these later times are marked by the filling in of partially inclosed seas, and by retreat of waters, due to massive continental uplift. Thus gradually the western interior sea of the Great Plains region disappeared (as did that of the Colorado basin), and the Pacific shore line was pushed to its present position.

Some of the important episodes of Mesozoic and Tertiary continental evolution may now be noted. The eastern border region has a series of areas of red and brown shale and sandstone, of Triassic age. These formations underlie the lowlands of the Connecticut Valley in Massachusetts and Connecticut. Another belt extends from the Palisades of the Hudson into Virginia, and there are other and smaller areas.