DIFFERENTIATION OF KEWEENAW AN DIABASES 433 



by A. N. Winchell for such textures, a poikilophitic rock. 1 In 

 sections of diabase from "Haystack Mountain" the augite is 

 frequently twinned with two members, and instead of the usual 

 stout crystal it occurs in long, narrow forms, somewhat lath 

 shaped, and in this respect resembling the feldspars. 



DIFFERENTIATION PRODUCTS; PEGMATITE DIKES 



The differentiation products of the Keweenawan rocks of the 

 Lake Superior region have been frequently mentioned. Clements 

 states that the gabbro in Minnesota shows undoubted evidence of 

 differentiation in the large masses of anorthosite and the patches 

 of magnetite and titaniferous iron ore. 2 W. S. Bayley describes 

 peridotites and pyroxenites as very basic phases of the gabbro in 

 his description of the Lake Superior region. 3 



From Lake Nipigon A. P. Coleman describes picrite and other 

 very basic phases of the diabase and also certain acid dikes which 

 are described as post-Keweenawan but closely related to the Kewee- 

 nawan basic rocks and perhaps differentiation products of them. 4 

 These rocks are described as having a pegmatitic or micropeg- 

 matitic texture and as having the composition of granite or grano- 

 diorite. 



In "Haystack Mountain" north of Lake Nipigon the writer 

 found similar dikes and from their relationships suggested that 

 they represented an acid phase of the diabase magma. 5 The later 

 observation of similar dikes in the Duluth gabbro near Duluth, 

 Minnesota, confirmed the belief that these rocks are differentiation 

 products of the diabases and gabbros. 



The rock at "Haystack Mountain" is a coarse diabase, rather 

 gabbro-like, and shows small, dark patches of titaniferous magnet- 

 ite and in places lighter blotches consisting largely of feldspar. 

 The magnetite is sufficiently abundant in part of the hill to influence 

 the compass so that prospectors were led to record mining claims 



1 "Use of 'Ophitic' and Related Terms in Petrography," Bull. Geol. Soc. Am., 

 XX (1910), 661-67. 



2 U.S. Geol. Survey, Monograph XLV, 397-424. 

 i Jour, of Geology, II (1894), 814-25. 



1 Bureau of Mines of Ontario, XVII (1908), 163-64. 

 s Ibid., XVIII (1907), 162. 



