■ALGONKIAN ROCKS OF LAKE SUPERIOR, REGION 223 



the middle portion, and the upper part of the series is free from vol- 

 canics.* 



Professor Irving divided the Keweenaw into an upper and a lower 

 division. The upper division consists mainly of sandstones, the esti- 

 mated thickness of which is about 15,000 feet in the middle portion of 

 the Lake Superior basin. On Montreal river the upper division is formed 

 of some 12,000 feet of red sandstone and shale, about 500 feet of black 

 shale alternating with hard, gray, nearly quartzless sandstone, both shale 

 and sandstone being detrital material, and about 1,200 feet of very coarse 

 boulder conglomerate.! 



The thickness of the lower division is very great, and may be placed, 

 according to Irving, in round numbers, at from 25,000 to 30,000 feet. 

 On the eastern side of Keweenaw point the maximum thickness of the 

 lower division at surface is some 25,000 feet, which does not go to the 

 base of the series. 



The lower division is made up chiefly of a succession of flows of basic 

 rocks. It also includes layers of conglomerate and sandstone nearly to 

 the base, and more or less of original acid rocks. Detrital beds, chiefly 

 porphyry-conglomerates and red sandstones, occur throughout the series, 

 having been seen all the way from the base to the summit, but they are 

 rare in the lower third of the series, and as a rule increase in thickness 

 and frequency toward the top, only one instance being known of a 

 heavy bed at a low horizon.']; 



Loioer Huronian. — In the Lake Superior region the Lower Huronian is 

 formed of closely folded semi-crystalline rocks consisting of limestones, 

 quartzites, mica-slates, mica-schists, schist-conglomerates, and ferruginous 

 and jaspery beds intersected by basic dikes, and in certain areas by acid 

 eruptives. It includes volcanic elastics, often agglomeratic, and a green, 

 chloritic, finely laminated schist. The thickness of this series has not 

 been worked out with accuracy, but at its maximum it is probably more 

 than 5,000 feet.§ 



Upper Huronian. — The Keweenaw series rests unconformably on the 

 Upper Huronian. The latter is formed of the Animikie and Upper Ver- 

 milion formations in northern Minnesota, and of the Penokee-Gogebic 

 rocks in Michigan and Wisconsin. On Pigeon river, according to Irving, 

 the formations consist of about 10,000 feet of dark gray to black, more 

 or less highly argillaceous or clay-slate-like layers alternating with others 

 that are more quartzitic. Peculiar cherty layers are met with at lower 



* Loc. cit., p. 161. 



t Monograph V, U. S. Geol. Survey, 18S3, p. 153. 



t Monograph V, U. S. Geol. Survey, 1883, pp. 152-lCO. 



I Bull. U. S. Geol. Survey, no. 86, 1892, p. 499. 



XXXIII— Bull. Geol. See. Am., Vol. 10, 1898 



