THE 
AMERICAN GEOLOGIST. 
Vol. XVI. NOVEMBEK, 1895. No. 5. 
[Crucial points in the geology of the Lake Superior region. No. 9.) 
THE LATEST ERUPTIVES OF THE LAKE 
SUPERIOR REGION. 
By N. H. ^^ INCHELL, Minneapolis:, Minn. 
Prof. Irving has called attention to the view of Whittlesey 
and of Foster and Whitney that the Lake Superior hasin is a 
great sjniclinal, which began early in its history, and has illus- 
trated the depression with fresh facts and a lucid mapping by 
contours.* Two principal formations are concerned in giving 
the leading topographic expression of this general truth, viz., 
the Animikie and the Keweenawan. These strata display their 
entire thickness in l)lutfs that face outward from the basin 
and constitute the rim which discloses in the main the salient 
steps in the physical history, as well as of the geology of the 
great synclinorium. Tlie line of breaking which caused these 
strata to rise thus and present a i-am]>art against the older 
terranes was very regular on the iu)rthern side of the lake 
from Diduth to Thunder Ijay, })ut on the south side of the lake 
it was broken and even tortuous eastward from the base of 
Keweenaw i)oint. F'rom the western extremity of the basin 
eastward to about the iiiidcUc of the same the tilting of these 
formations was regular and s^'iu'lironous, the southern line of 
the rim running out to the extremity of Keweenaw jioiiit, 
where it turned toward tiu' southeast. The northern line of 
the rim runs about parallel with the southern, but begins to 
*The Copper-V)earing rocks of lake Superior, pi. xxvrir. 
