262 FRANK D. ADAMS 



ever, is chiefly andesine. Its character was determined by a 

 larofe number of extinction measurements carried out on the 

 albite twins, as well as by Michel-Levy's method, which can 

 readily be applied owing to the frequency of carlsbad twinning 

 in association with albite twinning. These determinations were 

 extended and checked by a number of specific-gravity deter- 

 minations and separations by means of Thoulet's solution. The 

 larger plagioclase individuals were found, in the case of the rock 

 on the northeast side of the mountain 320 feet above the plain, 

 to be somewhat more basic than the smaller crystals, having the 

 composition of a basic andesine, while the latter ranged in char- 

 acter 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, Ab^ An^. 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 

 Abj An J 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 proportion of the smaller grains 

 are also untwinned, but most of these must be identical in char- 

 acter 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' an analysis of the feld- 

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

 This is as follows : 



'^Geology of Canada, p. 477. 



