The general disturbance which closed the Ordo-vician period appears to have greatly increased the extent of the continent. A relatively narrow strip of coast lands had been added to the northern pre-Cambrian area, converting much of Minnesota, Wisconsin, and the province of Ontario, northern New York and New Jersey, and western New England into land. Southern Ohio and central Kentucky and Tennessee had been raised into the Cincinnati anticline, but it is doubtful whether they remained as islands in the Silurian sea. Much of the Interior Sea had withdrawn, but the emergence was not long, geologically speaking, and the sea was soon reestablished, but with entirely different boundaries and connections. What changes affected the land masses of* the West and Southwest cannot yet be definitely determined, but the absence of the Silurian from extensive areas where the Ordovician is found indicates that these masses were greatly enlarged. How much of this enlargement came at a later date and how far the absence of the Silurian is the result of denudation, there is no present means of finding out.
The Silurian rocks are far thicker in the East, especially along the Appalachian range, than in the interior or western regions, where they thin out and are wanting over large areas.
An important feature in the Silurian geography of eastern North America was the establishment of the Cumberland Basin, or "Appalachian Mediterranean," as it has been called. This large sea lay to the eastward of the Interior Sea, from which it would seem to have been either completely separated, or so nearly so that the species of marine animals inhabiting the two bodies of water were very different. The Cumberland Basin was east of the Catskill-Helderberg line in New York, and its western shore crossed New Jersey and curved westward beyond the centre of Pennsylvania, whence it ran southwest more or less parallel with the Appalachian line, toward which it curved eastward in southern West Virginia. This basin began apparently in western Maryland and adjoining areas very early in Silurian times, and continued to grow larger and deeper until the Devonian was well advanced. The Interior Sea underwent a succession of oscillations much like those which had affected it during the Ordovician; it was apparently closed at the south, but extended northwestward to the Arctic Sea, while its east-west diameter had been greatly reduced from that of the Ordovician.
Fig. 261. - Generalized map of North America in the Silurian. Black areas = known exposures. Lined areas = sea. White areas = land, or unknown.
The submergence which inaugurated the Silurian period (i.e. on the assumption that the Medina is properly referable to the Ordovician) brought the Interior Sea up to the narrow barrier which separated it from the Cumberland Basin, and in it were laid down the sediments of the Clinton stage, shales in the east passing westward into limestones, which extend through New York to Indiana, and perhaps also through Illinois to Missouri. In the Cumberland Basin the Clinton shales followed the trend of the Appalachians to Alabama. It must be remembered, however, that the Appalachian Mountains were not then in existence, as such, but they were foreshadowed by structural lines of depression and low folding which exerted a definite control of the coast-lines and basins through most of the Palaeozoic era. Northeastward, the Clinton recurs in Nova Scotia and at other points in eastern Canada, but is not always readily identifiable. In many places interstratified concretionary haematites are found in the Clinton, especially along the Appalachian line, but also in Wisconsin, New York, and Nova Scotia.
A time of limestone-making on a great scale (the Lockport and Gnelph stages), preceded in New York by the Rochester shales, followed the Clinton. In the East this great limestone has but a limited extension southward, but southwestward it stretches for nearly 1000 miles, to Wisconsin and thence across Illinois, Iowa, Missouri, and western Tennessee. Many scattered outliers in Manitoba and the region west of Hudson's Bay indicate the probable former extension of the limestone unbrokenly to the Arctic shores and islands. . Rocks of corresponding date, laid down in the Cumberland Basin, but with marked faunal differences in the fossils from those of the Interior Sea, are found in western Maryland and Virginia, New England, New Brunswick, and Nova Scotia. East Tennessee, on the other hand, was elevated at the end of the Clinton stage and remained as a land-surface till the middle of the Devonian.
Little is known of the Silurian of the West, for, as already pointed out, there is reason to believe that nearly all of that region was then land. However, the Nevada trough continued to be submerged, presumably forming a gulf from the Pacific, and here the Niagara series is represented by the upper part of a thick mass of limestone which extends upward unbrokenly from the Trenton, the great limestone of the Ordovician.
The limestone of the Niagara epoch (Lockport) is very largely made up of corals, and in some places, as in eastern Wisconsin and areas to the south, distinct coral reefs may be observed, the most ancient which have as yet been found. As we have seen, corals flourished abundantly in the Ordovician, but, so far, no definite reefs have been noted in the rocks of that period.
The next change (Salina stage) was the separation, along the northern part of the northeastern arm of the Interior Sea, of a series of salt lagoons, in which were deposited red marls and shales, interstratified with gypsum and rock-salt, from which are obtained the brines of New York, Ontario, and Ohio. In part contemporaneous with these is a hydraulic limestone, called the Water-lime, which has much the same distribution as the salt-bearing beds, but is thickest where they are thin. The Water-lime indicates the freshening of the Salina lagoons and has preserved a remarkable assemblage of Crustacean fossils, belonging to the Eurypterida. The rocks of the Salina stage are thickest in New York and Pennsylvania, thinning to the westward. In the Shawangunk Mountains of eastern New York the Salina is represented by thick conglomerates of quartz pebbles, which were formerly referred to the Oneida, but are now known to contain, in interstratified shales, the remarkable Eurypterida of the Waterlime. This formation extends along the Appalachian line to Tennessee as a very thick mass of sandstones and conglcmerates.
The beds of salt and gypsum are strong evidence that the climate of Salina times, at least in the northeastern part of the contu nent, was arid, but how far this aridity was local, cannot be determined.
Throughout the Salina age the Interior Sea had been growing shallower, and shortly after the close of that stage the whole interior of the continent became land and remained so for a considerable period, but the Cumberland Basin persisted, and in it the limestones of the closing part of the Cayugan epoch and Silurian period were accumulated, the Cobleskill, Rondout, and Manlius, which are variously distributed in different parts of the basin, due probably to differential movements of the sea-floor, but all occur in succession in New York and in western Maryland, and for some distance north and south of the latter.
The Silurian rocks of North America have yielded no sign of volcanic material; in a few places they are traversed by igneous intrusions, most of which may, however, be of much later date. In Maine are some igneous rocks which decidedly appear to be Silurian, and the same may be true of certain areas in New Brunswick and Nova Scotia.
The division of Europe into northern and southern areas which we found in the Ordovician was maintained in Silurian times, and the southern sea was as peculiar in its animal life as it had been before, the northern being the typical Silurian which is found in the other continents. In the west of Ireland, Wales, northern England, and Scotland, Silurian beds accompany and overlie the Ordovician, but the much greater development of limestone points to a deepening of the water in those seas or a lowering of the surrounding lands. The volcanic materials, so conspicuous in the Ordovician, are no longer found. The Wenlock limestone of Great Britain, which corresponds to the American Niagara (Lockport-Guelph) is, like the latter, largely coralline. In Scandinavia also there is a great development of Silurian limestones, which extend far into Russia. In the latter country the sea had retreated much from its extension in the Ordovician, except toward the southeast, where it was carried into Bessarabia. Most of the Russian Silurian was formed in an interior sea, connected with that of southern Europe. In the southern European countries, which display the Bohemian fades, the Silurian rocks have nearly the same general distribution as the Ordovician. The two systems are also associated in the Arctic islands, in China, north Africa, South America, and Australia. In South Africa the Silurian is probably represented in some of the barren Table Mountain sandstones.
In all of these areas, as also in North America, the fossils resemble those of the northern European region, rather than those of the southern. In general the Silurian rocks are less extensively exposed at the surface than the Ordovician.
The Silurian climate seems to have been like that of the Ordovician, very uniform over the earth and with no indication of climatic zones. The aridity of the New York region in the Salina age, which has already been mentioned, corresponds to the similar conditions in the Ordovician of Siberia. Both were probably local.
In parts of North America the Silurian passed so gradually and gently into the Devonian, that it is difficult to draw the line between the two systems. Some disturbances, however, took place in Ireland, Wales, and the north of England, for in these localities the Devonian lies unconformably upon the Silurian. In other parts of Europe the transition was gradual.