The rocks, dominantly limestone, of the Ordovician system out crop widely in the United States, but, like the Cambrian, not over very large areas, though in some regions, like the Lexington basin. in Kentucky and the Nashville basin in Tennessee, they are ex tensively exposed. Like the Cambrian, too, the Ordovician ex posures are generally distributed about the peripheries of the old Pre-Cambrian land masses, but naturally farther out from their shores. The Canadian, the earliest period of the Ordovician (if the Ozarkian be included within the Cambrian), has not been even roughly delimited, but the submergence during which the Canadian deposits were laid down was restricted to the eastern United States and the Great Basin of Nevada and Utah, and the Acadian section of Canada. The Champlainian period, following the Canadian, was characterized by a submergence of the North American continent to an extent not since approached, with conse quent deposit of Champlainian sediments rich in marine fossils bryozoans, brachiopods, gastropods, cephalopods, crinoids locally, silicious sponges, hydroids, ostracods and early corals—over wide areas. The Cincinnatian, the closing period of the Ordovician, was also a period of extensive submergence during parts of its continuance, but the epicontinental seas varied greatly in extent and position as a whole. The Cincinnatian closed with such a wide-spread withdrawal of these seas that the configuration of the American continent at the close of the Ordovician approached its present outline in its major characteristics.
Little indication of volcanic activity during the Ordovician is found in North America, but the period was distinguished by important tectonic movements which resulted in the Taconic de formation in the eastern United States, whereby the sedimentary deposits which had been laid down in a trough between the Ap palachians and a land mass lying eastward in New England, from the St. Lawrence to Long Island, were folded sharply and meta morphosed profoundly, to constitute the Taconic complex; and the Cincinnati arch and other central interior anticlines were ini tiated in the middle of the period and at its close were elevated and greatly enlarged.
In the Ordovician, not only of America, but elsewhere, the adaptation of life to environment seems almost as well established as now. The muddy bottoms of the seas into which rivers of medium grade emptied, were characterized by certain forms that were quite distinct from those that developed in seas with sandy and gravelly bottoms; and the clear water areas developed still other groups. The differentiating effect of isolation is clearly brought out in the fossils of the Ordovician wherever a portion of the epicontinental sea was cut off by such a barrier as an anti cline, or wherever ocean currents of different temperature or salinity prevented intermingling of the faunas. Thus the life of the Ordovician was fuller, richer and higher in the evolutionary stage than that of the Cambrian. Trilobites, cephalopods, gastro
pods, pelecypods, cystoids, graptolites and corals of advanced and varied types predominated, and bryozoans, crinoids and fishes appeared for the first time.
The Ordovician formations have yielded much, if not most, of the natural gas and petroleum of Ohio and Indiana and a great deal in other eastern States; lead and zinc have been produced from the sulphide and carbonate ores from the Galena formation of eastern Iowa, south-western Wisconsin and north-western Illi nois which is a Middle Ordovician member; calcium phosphate, a valuable fertilizer is obtained from the Ordovician of central Ten nessee ; and limestone, marble and cement are widely quarried from the members of the Ordovician from Vermont to Tennessee.
The Silurian of North America is nearly everywhere rather readily cut off from the Ordovician by a relatively distinct, angular unconformity in the east and by an equally distinct disconformity in fossil content in the interior. The Oswegan, the lowest of the three distinct major divisions of the Silurian, initiates the period with the epicontinental seas apparently confined to three major extensions upon the area of the present continent : one stretching up the Mississippi valley to northern Illinois; a second extending across Newfoundland and northern New Brunswick; and a third occupying the Appalachian trough, and stretching east and west over central New York and Ontario. Following the Oswegan, the Niagaran continues the period with an expansion of the Silurian seas over the United States east of the Mississippi river and over a large part of Canada to the Arctic ocean, and an extension of two seas on the west, one from California through Idaho to Canada, and another from Mexico into Arizona and New Mexico. The Cayugan closes the period with a withdrawal of the Silurian seas until they covered only the region from Wisconsin and Illinois through New York and over the Appalachian trough.
During the Silurian the Cincinnati uplift or anticline continued a prominent physiographic feature separating distinct marine basins and faunal provinces. The Ozark uplift likewise constituted a conspicuous feature of the continental landscape ; and more land emerged in western United States than ever before.
The Silurian fauna is characterized by a wealth of types with radial symmetry, as was the Ordovician ; the brachiopods became larger and sturdier, the eurypterids or "sea-scorpions" were com mon and corals widely scattered. It was in the Niagaran when the seas were most widely extended that the life was richest and most cosmopolitan. The first recognized land plants—of low form— are from the Cayugan. Fishes probably occupied the fresh waters of the whole Silurian period, but their fossils appear only in the closing part.