Page:The Journal of geology (IA journalofgeology21894univers).pdf/460

Lawson^[2] gives a résumé of the geology of Northeastern Minnesota adjacent to Lake Superior. Surrounding the Lake there are four geological provinces, from the top downward, the Potsdam, Keweenian, Animikie, and Archean.

The Rocks of the Potsdam are flat-lying shaly sandstones, generally of a red color.

The Keweenian occupies the entire Minnesota coast from Duluth to Grand Portage. The series consists in this area of a well stratified series of volcanic flows, having a gentle lakeward dip, which does not generally exceed 10°. The sedimentary formations are represented in the series, but occupy less than one-half per cent of the coast line. The lavas are largely vesicular or amygdaloidal in character, and in those of acid composition in which the vesicular structure is not so well developed are numerous irregular joints. The series has been invaded by many later intrusive masses, which occur as nearly vertical dikes, or more commonly as injected sills which coincide with the planes of stratification of the bedded flows. Since the time of the outflow of the Keweenian rocks, the strata have suffered comparatively little disturbance, the prevalent lakeward dip being probably due to the attitude of the slopes upon which the lavas flowed, rather than entirely to a differential movement of once horizontal strata. The pre-Keweenian labradorite rocks exposed at a number of points were profoundly eroded before the Keweenian was deposited upon them, and they were presumably Archean.

The Animikie rocks occupy the shore of the Lake from Grand Portage to Port Arthur. The series is composed altogether of sedimentary strata, and consists mainly of fine-grained sandstones, which are locally quartzites, carbonaceous shales or slates, and in small part of cherts and jaspers, beds of carbonate of iron, hematite and magnetite, conglomerate, and occasional lenses of non-ferruginous carbonate in the slates. Except in local instances