THE ANORTHOSITES OF THE RAINY LAKE REGION. 



A NUMBER of eruptive masses rising through the Keewatin 

 (Huronian) schists and schist conglomerates of the Rainy Lake 

 region in western Ontario were mapped and described by Pro- 

 fessor A. C. Lawson in 1887, ^^^^ most interesting group of erup- 

 tives occurring along the southern shore of Seine Bay and 

 between Bad Vermilion and Shoal lakes, just to the east/ 

 Here very basic and very acid rocks are found associated. The 

 acid members of the group, quartzose granites containing much 

 plagioclase, have been studied somewhat carefully from the fact 

 that they contain important gold-bearing veins, but the barren 

 anorthosites have been neglected. The soda granites, which 

 often weather into the greenish sericite variety, protogine, and 

 have been sheared and metamorphosed into sericitie schists near 

 the quartz veins, have been described by Winchell and Grant"" 

 and the present writer, 3 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 anortho- 

 site,"* deserve some further mention. 



The larof-est area of anorthosite encloses the southern arms 



O 



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 propor- 

 tion of darker minerals occupying spaces between more or less 

 perfect phenocrysts of plagioclase which range in size from a 



' Geol. Sur. Can., Part F, 1887, pp. 56 and 99. 



' Winchell and Grant, Geol. Nat. Hist. Sur. Minn., 23d Ann. Rep., pp. 58-60. 

 3 Ontario Bureau of Mines, 1894, P- 89. 



t Geol. Nat. Hist. Sur. Minn., Bull. No. 8, 1893, 2d part, p. 7. 



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