GEOLOGY AND SOILS. 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 moun tain 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 Archtean and Algon kian. They are represented to the northward lv au 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 metamor phic 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 Hud son 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 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 geo logical time was very long, and includes an ex tended succession of periods, with their sub divisions or epochs. The rocks of the Cambrian, or earliest of these periods. are mainly sand stones. conglomerates, and shales, and are found in limited outcrops about the borders of the Arch can and Algonkian. The Ordovician, Silu rian, 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 Ar chrean axis of the Appalachians, beyond the Mis sissippi 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 terri tory of the United States was a semi-continent on the past, sending lobate areas southward, with the Mississippi emhayment 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 Paleozoie era closed, in North America, with what is known as the Appalachian revolu tion, 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 alt 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 orig inated the sandstones, shales, and limestones of the Cambrian and succeeding periods, to which reference has been made. Along the old Appa• lachinn border these formations acquired a thick ness 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 Penn sylvania. 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.