The object of this paper is to show that the Shawangunk grit and conglomerate holds the position of the Medina sandstone and equivalent formations which together throughout the Appalachian region occur at the base of the Silurian system of strata. The Shawangunk-Medina formations have overlying them, in unbroken sequence, as one gets about 30 miles from the main eastern outcrops, the Clinton shales. Then everywhere follows a great break in deposition, for all of the higher Niagaran is absent—that is, the equivalent of at least all of the Lockport and Guelph. Overlying the Clinton or the Shawangunk-Medina, in disconformable contact, is a thick shale series that below is usually of a red or pink color, and above passes into calcareous shales or ribbon limestones, the natural cement rocks of the northeastern Appalachian area. The latter, or Cayugan series, appear to go unbroken into the Lower Devonian, and yet where . . .