Amongst the parallelisms which may be drawn between the Silurian series of Britain and that of North America, none so far has been so certainly established as the equivalency of the “Niagara Formation” to the Wenlock Group. In its most typical development, as in the State of New York, the Niagara formation, consists of an inferior series of argillaceous sediments, the “Niagara Shales,” and of a superior series of calcareous accumulations, the “Niagara Limestone.” At the Falls of Niagara itself, and at the Falls of the Genesee at Rochester, the shales and limestones are about eighty feet in thickness each. In Pennsylvania, the Niagara formation is wholly shaly, and has a thickness of over fifteen hundred feet. In the States west of New York, again, the formation is almost wholly calcareous, many of its members being true dolomites, and its total thickness rarely reaches three hundred feet, and is usually much less. In Western Canada, finally, the Niagara shales can rarely be detected as a distinct group, and the formation consists mainly of limestones, often magnesian, with subordinate courses of shale, the whole usually varying from one hundred to two hundred feet in thickness.