ANORTHOSITES OF THE RAINY LAKE REGION 909 



twinning according to the albite and frequently also the peri- 

 cline law, the former ranging in angle of extinction from the twin 

 plane between 17° and 37°. The average extinction angle in 

 thin sections from Bad Vermilion Lake is about 24°, and from 

 the mouth of Seine River 32°. The former feldspar is therefore 

 bytownite and the latter anorthite, both more basic than that of 

 the typical anorthosite, which Dr. Adams finds to be labra- 

 dorite. 



In the freshest section studied (783, mouth of Seine River) 

 the large interlocking feldspar individuals often show a thin 

 band of fresh, clear feldspar where one joins the other; and this 

 clear feldspar strip is seen, when examined with a high power, 

 to form a secondary enlargement of the adjoining crystals, the 

 twin striations running out into it. The extinction angles of 

 these secondary feldspar rims vary from 8° to 14°, correspond- 

 ing to labradorite, so that the later feldspar is more acid than 

 the older. In one case a bytownite crystal has been broken, 

 the parts slightly shifted, and then cemented with labradorite, 

 most of the twin lamellae running across the strip of cement. 



An analysis of a specimen from the mouth of Seine River 

 was made by Mr. William Lawson in the laboratory of the 

 School of Practical Science, Toronto, the results being given in 

 column No. I. In No. II an analysis of anorthosite from 

 Rawdon, Que., made by Sterry Hunt and quoted by Dr. Adams 

 is given for comparison.' No. Ill gives the results of an 

 analysis of granite adjoining the Bad Vermilion anorthosite 

 area, and is the work of Mr. Lawson. 



SiOa 



AUO3 



FezOj - 



FeO - - - - 



MnO 



CaO - - - - 



MgO 



Xa.O - - - - 



' Ueber das Norian, p. 494. 



