DIFFERENTIATION OF KEWEENAWAN DIABASES IN 

 THE VICINITY OF LAKE NIPIGON 



E. S. MOORE 



The Pennsylvania State College 



In recent numbers of Economic Geology two papers have been 

 published describing the differentiation products of the quartz- 

 diabases of the Nipissing District, Ontario. 1 Since these diabases 

 have generally been regarded as of Keweenawan age, certain 

 differentiation products of the Keweenawan diabases in the vicin- 

 ity of Lake Nipigon are also of interest. 



On the north shore of Lake Superior and extending northward 

 beyond Lake Nipigon there are masses of diabase and gabbro 

 which intrude the older crystalline rocks in the form of batholiths, 

 dikes, and bosses and the sediments in the form of the "Logan 

 sills." Although there are differences of opinion regarding the 

 geological age of these rocks, the writer concurs with those who 

 regard, as closely related in origin, the great amygdaloidal basalt 

 flows of Keweenaw Point, the Duluth gabbro, the "Logan 

 sills," the Sudbury Nickel eruptive, and the Cobalt diabases, as 

 well as many other masses of diabase in intervening areas. The 

 great igneous activity of this region seems to have been the result 

 of extensive crustal adjustment centered around Lake Superior 

 and diminishing in intensity as a greater distance from the center 

 was reached. It is probable that on the northern side of the Lake 

 Superior basin the intrusive masses were being injected into sedi- 

 ments which had already been formed while the alternate deposits 

 of sediments and lava flows were being deposited on the south 

 side and that a close relationship exists between all portions of this 

 great series of sediments and extrusive and intrusive igneous rocks. 



While a general description of the petrography of these rocks 

 is given here, the object of this paper is to call attention to certain 



1 W. H. Collins, Econ. Geology, V, No. 6, p. 538; R. E. Hore, ibid., VI, No. i, 

 P- Si- 



429 



