On the Norian or " Upper Laurentian " Formation. 185 



their irregular distribution. The following minerals have 

 been observed in the rock : 



Plagioclase Muscovite and Paragonite Epidote 



Augite Bastite Zoisite 



Hypersthene Chlorite Garnet 



Ilmenite Quartz Zircon 



Orthoclase Magnetite Spinel 



Hornblende Apatite 



Biotite Calcite 



Of these, plagioclase, augite, hypersthene and ilmenite are 

 by far the more important and may be considered as the 

 essential constituents of the rock, while the others are in 

 most cases either accessory constituents or decomposition 

 products. 



Plagioclase. — As above mentioned, Hunt gave the name 

 anorthosite to these rocks on account of the great pre- 

 valence in many varieties of plagioclase or anorthose. He 

 considered the type which contains only feldspar as the 

 true anorthosite and estimated that three fourths of the 

 anorthosites in the Dominion did not contain over 5% of 

 other minerals. 1 



Like the other constituents of the rock, the plagioclase is 

 quite fresh, showing but very rarely any traces of decompo 

 sition, and when it is not granulated (that is " cataclastic " 

 in structure) presents in hand-specimens, almost without ex- 

 ception, a dark violet but more rarely a reddish colour. 

 This colour is still more plainly visible in thin sections, al- 

 though naturally much fainter, and is seen to be caused by 

 the presence of an immense quantity of minute opaque 

 black rods and extremely small opaque dark points, which 

 give the mineral in thin sections a pecular hazy appear- 

 ance. The latter probably represent in part cross sections 

 of the rods, but are for the most part round or slightly 

 elongated individuals of the same substance as the rods and 

 occurring with them. Vogelsang 2 estimated, in connection 

 with his studies of the anorthosite of Labrador, that these 



1 T. Sterry Hunt, On Norite or Labradorite Rock. Am. Journ. Sc, Nov. 1869. 



2 Vogelsang, Archives N6erlandaise T. III. 1868. 



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