The Monteregian Hills. 223 



latter ranged in character from andesine to oligoclase. In 

 this case no feldspar having a specific gravity of over 2.65 

 was found to be present in the rock. Again, in the rock 

 of one of the quarries on the south side of the mountain, 

 the larger feldspars tested by 'Michel-Levy's method were 

 found to have the composition of a very acid labradorite, 

 Abj Anj. The results of a separation of the constituents 

 of the rock by Thoulet's solution showed that the feldspar 

 was almost all andesine, although it varied from Ab t Anj 

 to an oligoclase. A crystal examined by Mr. Wright in 

 Professor Rosenbusch's laboratory gave on P an extinction 

 of 5°-6° and on M about 11°, showing the feldspar to be 

 on the line between andesine and labradorite. A very 

 small amount of orthoclase was also present, forming a 

 subordinate accessory constituent. That there is a 

 variation in composition even in the same individual of 

 plagioclase is indicated in many cases by marked growth 

 rings with different extinctions in the different rings. 

 The smaller plagioclases, although twinned in the same 

 manner as the larger, usually have the twinning 

 developed in a less -striking manner. A certain propor- 

 tion of the smaller grains are also untwinned, but most of 

 these must be identical in character with the twinned 

 feldspar, since the separations show that while orthoclase 

 is often present it occurs in only extremely small amount. 

 Dr. Sterry Hunt gives 1 an analysis of the feldspar from 

 the essexite of Mount Johnson (called by him diorite). 

 This is as follows : 



Si0 2 ---.-. 62.05 

 Al 2 O s - - - - - -22.60 



Fe 2 3 .75 



CaO - - : - - - 3.96 



Na 2 - 7.95 



K 2 - - 1.80 



Volatile .80 



99.91 

 Sp. G. = 2.659. 



1 Geology of Canada, p. 477. 



