90S A. P. COLEMAN 



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 important, 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, apparently weathered diabase, 

 is seen, probably portions of massive Keewatin rocks swept off 

 by the molten anorthosite. 



The rock, though clearly an anorthosite, presents some 

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

 described by Dr. Adams from the province of Quebec, the feld- 

 spars being always white, never purplish in color, and compara- 

 tively rarely showing the sheared and granulated character so 

 often found in eastern Canada.^ The marked tendency toward 

 idiomorphism in the feldspars is apparently unusual in other 

 regions. The loss of the purplish color is no doubt the result 

 of weathering, which has generally progressed rather far, though 

 cleavage surfaces showing twin striations can be found gen- 

 erally. The freshest example studied comes from a hill at the 

 mouth of Seine River. 



In the numerous thin sections examined more than nine- 

 tenths of the rock is seen to consist of plagioclase, usually 

 sprinkled with zoisite particles or more or less completely 

 changed to a saussuritic mass. The darker portions lying here 

 and there in angles between the feldspars consist mainly of a 

 fibrous or scaly mineral with parallel or nearly parallel extinc- 

 tion and low double refraction, probably serpentine, but perhaps 

 a member of the chlorite group. Augite was found as a rem- 

 nant only once, and then was not of the diallage type. No 

 other primary minerals were observed, not even magnetite ; and 

 very few secondary ones require to be added to those men- 

 tioned, only epidote, probably some albite, and a very little cal- 

 cite. The feldspars, where fresh enough to study, show broad 



^ Ueber das Norian, Separat Abdruck, Neues Jahrbuch fiir Min., Beilageband VIII; 

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



