The Silurian System Extent and Subdivisions. If we take the world over, the Silurian system is not as widespread or as important as is the Ordovician system. In extensive areas where the latter is well developed, the Silurian is either wholly absent or is but scantily represented, though there are some additional localities where we find Silurian rocks far from any similar outcrop of the Ordovician beds. The name Silurian was given by Murchison, as the result of his study of the rocks in Southwestern Britain, being based on the name Silures, that of an ancient tribe who used to inhabit Wales. The following classification includes some of the chief formations grouped under the Silurian system: Upper Silurian Manlius limestone Rondout water lime or Monroan Cobleskill and Akron limestones Middle Silurian or Salina Lower Silurian or Niagara Camillus shale (in part) Camillus shale and gypsum (in part) Syracuse salt series Upper Niagara (including Guelph dolomite, and Middle Niagara (including Rochester shale, and Lower Niagara or Medina (including Oneida con- In Europe, the Silurian is made to include the Ludlow (also called the Clunian on the Continent) and the Wenlock. Lithic Characters. The Clinton shales, which change into limestones as they extend westward, run from New York State to Indiana, perhaps also through Illinois to Missouri. These beds extend southward to Alabama; while the same beds (doubtless with extensive interruptions between) have been observed in Nova Scotia and Wisconsin. Extensive stratified beds of red iron ore (hematite) occur in the Clinton formation in the two localities last mentioned, and also in the State of New York. The limestones of the Niagara series (Lockport-Guelph) are among the most extensive limestones in America. They FIG. 241. Map showing Silurian outcrops in North America, which are in black. extend (with interruptions) from the Great Lake district to Wisconsin, across Illinois, and thence to Iowa, Missouri, and Western Tennessee, a distance of nearly 1,000 miles. Similar rocks occur in the Maritime Provinces of Eastern Canada, also in Manitoba and south of Hudson Bay, and on many of the islands in the arctic regions. The limestones of the Niagara contain many corals and crinoids, indicating that these beds were formed in clear water, some of them probably in compara FIG. 242. Anticline in Medina sandstone (L. Silurian), on the shore of Lake Ontario, near Eighteen-mile Point, New York. (Gilbert, U. S. G. S.) FIG. 243. Louisville limestone (L. Silurian), in a bluff of the Ohio River, about one mile northeast of Louisville, Kentucky. (Butts, U. S. G. S.) tively deep water. At Niagara Falls, the Silurian limestone is much harder than the underlying shales; and as the latter are gradually worn away by the spray of the falls, the overlying limestone breaks off, causing the falls slowly to move upstream, as has already been explained. Distribution. This Silurian limestone underlies the city of Chicago, and much of it is quite saturated with oil. It is here called "Athens marble" or "Joliet" building stone. Corals are very plentiful and conspicuous in the limestones of Wisconsin, with many crinoids also; while similarly abundant corals are observed at Louisville, Kentucky, though these are classed as Lower Silurian. FIG. 245. Sectional diagram through the Horseshoe Falls, Niagara, On Anticosti Island, in the Gulf of St. Lawrence, is found what is regarded as one of the most complete sets of the Silurian beds in North America. In Europe, there is the same twofold division in the Silurian rocks which we found in the Ordovician of that continent. They occur in Ireland, Wales, Northern England, and in Scotland. They occur also in Scandinavia and in Russia, one of the typical examples of these rocks being found on the island of Gottland, off the coast of Sweden. Silurian coral rocks are plentiful in Siberia, and they contain the same kinds of corals as are found in the arctic regions of North America, probably implying that these rocks extend, in many places, under the Arctic Ocean. A chain coral (Halysites) and a honeycomb coral (Favosites) occur in these rocks on both sides of the Arctic |