270 FRANK D. ADAMS 



richer in soda, the hornblende crystallizing out of it has a ten- 

 dency to take up this element more abundantly. 



The feldspar \Vi the pulaskite, as has been mentioned, occurs 

 in part as stout prisms and in part as smaller laths. The latter 

 usually have a somewhat cloudy appearance under the micro- 

 scope, probably owing to incipient alteration. The larger feld- 

 spars are what is commonly described as sodaorthoclase. 

 When examined under the microscope they are seen to be com- 

 posed of very minute intergrowths of two, and in some cases 

 perhaps even of three, different feldspars — causing them to 

 present between crossed nicols a mottled appearance. These 

 several feldspars have somewhat different indices of refraction, 

 and frequently under a high power, where two are present, one 

 of them can be seen to possess a very minute polysynthetic 

 twinning, while the other is untwinned. The relative proportion 

 of the several feldspars present differs in different grains. The 

 individuals as a whole occasionally present the form of carlsbad 

 twins but usually have the appearance of simple crystals, and 

 Professor Rosenbusch, to whom sections of the work were sub- 

 mitted, considers the feldspars composing them to be micro- 

 cline, and in part microcline-microperthite, with probably some 

 anorthoclase. 



The specific gravity of these phenocrysts was determined in 

 the case of two hand specimens of the pulaskite from different 

 parts of the mountain. In the first of these three specimens of 

 the feldspar were found to have specific gravities of 2.62, 2.609, 

 and 2.603, respectively ; while in the second, five specimens of the 

 feldspar were selected and found to have specific gravities lying 

 between orthoclase and albite, which bears out the results of their 

 microscopic study. 



The smaller lath-shaped feldspars, although more frequently 

 composed of a single species, often show an intergrowth of two 

 feldspars, as described in the case of the phenocrysts. Separa- 

 tions of the constituents of several species of the rock by means 

 of Thoulet's solution show that these smaller feldspars have a 

 somewhat lower specific gravity than the phenocrysts. Thus, 

 while the specific gravity of the phenocrysts lies between 2.591, 



