222 Canadian Record of Science. 



and was also observed in the thin sections from the 

 essexite at one point on the east side of the mountain not 

 far from the summit. It is very pale-green in color and 

 occurs as little grains inclosed in the biotite and 

 pyroxene. 



Plagioclase. — The plagioclase in the rock has well- 

 developed, lath-like forms and is, almost without exception, 

 excellently twinned according to the albite law. Twin- 

 ning according to the carlsbad and pericline laws is also 

 very common, occurring in the same individuals which 

 show the albite twinning. The laths of plagioclase can in 

 a few cases be seen to be distinctly twisted, evidently 

 owing to pressure exerted upon them by other crystals 

 during the consolidation of the rock, since the rock was 

 submitted to no dymamic action subsequent to its 

 crystallization. 



As before mentioned, all the plagioclase individuals are 

 not of the same dimensions. There are larger laths 

 associated with the large hornblende crystals, and between 

 these are smaller laths. The two sets are not, however, 

 sufficiently well marked to cause the resulting structure 

 to be classed as porphyritic. The plagioclase in the rock 

 is not all of the same composition, but varies somewhat^ 

 even in the same hand specimen, ranging from an 

 extremely acid labradorite to an oligoclase. It, however, 

 is chiefly andesine. Its character was determined by a 

 large 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 determinations and separations by 

 means of Thoulet's solution. The larger plagioclase 

 individuals were found, in the case of the rock on the 

 north-east 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 



