Page:The Journal of geology (IA journalofgeology21894univers).pdf/461

the rocks have been disturbed very little from the horizontal, the average dip of the strata being in a southeasterly direction at an angle probably not exceeding 5 degrees. Intrusive rocks are abundantly present as sills lying parallel to the stratification, resembling contemporaneous beds, and as vertical dikes, some of which have been observed in continuity with the sills. Faulting is a common occurrence in the Animikie, many scarps being due primarily to this cause.

The Archean shares the coast line with the Animikie and Keweenian from the vicinity of Port Arthur to the eastern end of Nipigon Bay, and beyond this point to the outlet of the lake is the dominant series. This complex consists of two divisions: 1) a great volume of profoundly altered sedimentary and volcanic rocks, characteristically schistose or in the form of massive greenstones, which have suffered intense disturbance, and which correspond to what has been designated the Ontarian system, and 2) immense batholites of irruptive gneiss and granite, which have invaded the rocks of the Ontarian system from below in the most irregular fashion, corresponding to that division of the Archean which is commonly recognized as Laurentian. These Laurentian rocks exhibit only to a very subordinate extent those evidences of disturbances and deformation which are so abundantly apparent in the schists which they have invaded. The Laurentian gneisses and granites occupy much more of the shore than do the metamorphic and schistose rocks of the Ontarian. Both divisions of the formation are cut by basic dikes, which, as a rule, do not exceed 100 feet in width, and are vertical or nearly so. The Archean forms the basement upon which the Animikie rests in glaring unconformity, the actual superposition being observed at several points, with the Keweenian lying flat on the latter. Very frequently, however, the Keweenian reposes directly upon the Archean.

Van Hise^[1] gives an historical sketch of the Lake Superior region to Cambrian time. The five divisions of this region are the Basement Complex or Archean; The Lower Huronian, Upper Huronian and Keweenawan, the last three together constituting the Algonkian, and the Lake Superior Cambrian Sandstone. Each of these divisions are separated by unconformities.

The Basement Complex consists mainly of granites, gneissoid granites, and of finely foliated dark colored banded gneiss or schist. The relations which obtain between the two divisions are frequently those of intrusion, the granites and gneissoid granites being the later igneous rocks. There is no evidence that any of the dark colored schists are sedimentary, but it is certain, if a massive granular structure be proof of an igneous origin, that a part of them are eruptive, for between the two are gradations.