GEOLOGY. Although the geological structure of the State is nearly homogeneous over the greater part of its area, there is nevertheless found represented every age from the Archfean to the Pleistocene except the Devonian. There are two Arch:can nuclei, one in the east and one in the west. The former is a broad tongue of Sioux quartzite belonging to the Huronian sys tem and extending westward to the James River from the neighborhood of Sioux Falls. To the north near Bigstone Lake is a smaller area of Laurentian granite. In the centre of the Black Hills is exposed the core Qf schists, slates, and granite. This is surroumTed by narrow bands forming the denuded sections of the upturned Paleozoic strata. successively Potsdam sandstone of the Cambrian, Silurian limestone, and a broader band of Carboniferous limestone. Around this appear the sand and limestones. clays and marls of the Jura-Trias, and the whole is enveloped in the Cretaceous strata which cove• four-fifths of the area of the State. The principal members are
the Colorado marls, clays, and limestones cover ing nearly the whole of the eastern half, the Laramie formation occupying the northwestern quarter. and the Dakota sandstone underlying the valley of the James. The southwestern quarter of the State is covered with Miocene clays and conglomerates. Igneous rocks in the form of dikes of diabase and porphyry occur in both the eastern and western Archa'an areas. The Pleis tocene age is represented by the immense sheet of glacial drift covering the eastern half of the State to a line nearly coinciding with the Mis souri River. and veiling the older formations. West of this line the Pleistocene deposits con sist of aqueous drift.