Analytical Abstracts of Current 

 Literature. 



SUMMARY OF CURRENT PRE-CAMBRIAN NORTH 

 AMERICAN LITERATURE.' 



Lawson^' gives a resume of the geology of Northeastern Minnesota adjac- 

 ent to Lake Superior. Surrounding the Lake there are four geological prov- 

 inces, 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 io°. 

 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 out- 

 flow of the Keweenian rocks, the strata have suffered comparatively little dis- 

 turbance, the prevalent lakeward dip being probably due to the attitude of the 

 slopes upon which the lavas flowed, rather than entirely to a differential move- 

 ment 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, car- 

 bonaceous 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 



'Continued from p. Ii8. 



^Sketch of the Coastal Topography of the North Side of Lake Superior with 

 Special- Reference to the Abandoned Strands of Lake Warren, by A. C. Lawson. In 

 20th Annual Rep. Geol. & Nat. Hist, Sur., Minn. pp. 181-289. 



444 



