﻿Anorthosites of the Rainy Lake Region. 231 



by Winchell and Grant ^ and the present writer,^ and need 

 no detailed mention here. The basic rocks of the group,, 

 briefly described as saussuritic gabbro by Lawson, but 

 afterward identified by him as anorthosite,^ deserve some 

 further mention. 



The largest area of anorthosite encloses the southern 

 arms of Bad Vermilion Lake, and surrounds or is bordered 

 by three areas of eruptive granite. Two or three miles to 

 the west, on Seine Bay, a series of points and islands of 

 anorthosite extend, with some interruptions, westward 

 along the southern shore of the bay for about ten miles. 

 The rock is generally white, almost like crystalline 

 limestone, with only a very small proportion of darker 

 minerals occupying spaces between more or less perfect 

 phenocrysts of plagioclase, which range in size from a 

 quarter of an inch to a foot in longest diameter. Towards 

 the western end of Bad Vermilion, however, there are 

 points where the green constituent becomes more import- 

 ant, and the rock may be called a porphyritic gabbro. 



Frequently portions of chloritic or sericitic schist have 

 been enclosed by the anorthosite, showing its post- 

 Keewatin age; and occasionally a green massive rock, appar- 

 ently weathered diabase, is seen, probably portions of mas- 

 sive Keewatin rocks swept off by the molten anorthosite. 



The rock, though clearly an anorthosite, presents sotne 

 points of difference from the typical rocks of the name, so 

 well described by Dr. Adams from the province of Quebec, 

 the feldspars being always white, never purplish in color,, 

 and comparatively rarely showing the sheared and granu- 

 lated character so often found in eastern Canada."^ The 

 marked tendency toward idiom orphism in the feldspars is 

 apparently unusual in other regions. The loss of the 



1 Winchell and Grant, Geo). Nat. Hist. Snr. Minn., 23d Ann. Rep., pp. 58-60. 



2 Ontario Bureau of Mines, 1894, p. 89. 



3 Geol. Nat. Hist. Sur. Minn., Bull. No. 8, 1893, 2nd part, p. 7. 



4 Ueber das Noriaii, Separat Abdruck, Neues Jalirbuch fiir Min., Beilageband VIII. ;. 

 and Can. Rec. Science, Vol. VI., No. 4, p. 190. 



