We shall now turn our attention to a very brief consideration of the salient points in the physical history of North America during the next great period (Silurian) of the Paleozoic era. As a result of the physical disturbance late in the Ordovician the great interior sea was largely or wholly expelled from the continent, and this was essentially the condition of the continent at the beginning of the Silurian. But this condition was of short duration, for early in the Silurian the sea again began to spread, gradually increasing in extent to a climax in about the middle of the period. At this time the famous and extensive Niagara limestone, so named from the rock at the crest of Niagara Falls, was deposited. Except for the newly formed Taconic Range, standing out as a bold topographic feature along the middle Atlantic Coast, and a somewhat wider extent of land, the condition of the continent during middle Silurian time was very similar to that of middle Ordovician time. [Illustration: Fig. 38.--Map showing the general relations of land and water in North America during middle Mississippian time. (After Willis, courtesy of the Journal of Geology.)] Soon after mid-Silurian time the seas became greatly restricted almost to disappearance as such. In the eastern United States and southeastern Canada strata of that particular age are found only in parts of Ontario, New York, Ohio, Michigan, and from Pennsylvania southward to West Virginia, where they are characterized by red shales and sandstones, and salt and gypsum deposits. Such materials containing few fossils very clearly indicate deposition in either extensive lagoons or more or less cut-off arms of the sea under arid climate conditions rather than in ordinary marine water. Still later in the Silurian the interior seas were partially restored, as shown by the fact that true marine strata corresponding to that age not only cover the salt and gypsum deposits, but are notably more extensive than they. About the close of the Silurian period almost all of the continent was dry land. Unlike the Ordovician period, the Silurian closed without any mountain-making disturbance or great uplift of land. The Silurian period, like the preceding Ordovician and Cambrian, seems to have been free from any more than slight igneous activity as, for example, in Maine and New Brunswick. The total thickness of Silurian strata in North America is seldom more than a few thousand feet.