194 Canadian Record of Science. 



The absorption is C > I* > a. 



It also occurs in the peculiar rock which was referred to 

 above as a gabbro and was found in a number of places be- 

 tween the true anorthosite and the gneiss. 



Biotite. — JBiotite never occurs in large amount but is 

 present rather frequently in very small amount as an acces- 

 sory constituent of the normal gabbro. It is usually found 

 with iron ores or with the hypersthene and shows the 

 characteristic brown color, strong pleochroism and parallel 

 extinction. 



Muscovite or Paragonite. — (See under " Plagioclase.") 



Chlorite. — Occasionally in small quantity as a decom- 

 position product of pyroxene or biotite. 



Quartz. — It is doubtful whether this mineral ever occurs 

 as a primary constituten t of the anorthosite. In a hand-spe- 

 cimen from the west side of the Achigan Eiver, near New 

 Glasgow, it was noticed in the form of rather small round 

 grains disseminated through the rock and looking like a 

 primary constituent. But the rock is much decomposed 

 and doubtless some secondary quartz is present as a pro- 

 duct of decomposition of pyroxene, so that the quartz which 

 appears to be primary at first sight may be in reality of 

 secondary origin. 



In the gabbro which occurs as above stated in many 

 places between the typical anorthosite and the gneiss, 

 quartz is quite frequent. But in this rock many facts point 

 to the secondary origin of the quartz. It occurs often for 

 example in more or less sharply defined veins made up of 

 large individuals. When it occurs in the form of separate 

 irregular grains these extinguish uniformly, although they 

 are often more or less fissured, but they are by no means so 

 much broken as one would expect, if they were primary 

 ingredients, in view of the extremely broken condition of 

 the feldspar and the other constituents of the rock. 



Ilmenite and Magnetite. — In nearly every section of 

 anorthosite some irregularly shaped grains of an opaque 

 black iron ore are seen. These are usually few in number. The 



