The Monteregian Hills. 231 



magnesia and other constituents, form a sort of ground- 

 mass in the rock. 



The constituent minerals of the rock are biotite, horn- 

 blende, (pyroxene), soda-orthoclase, nepheline, sodalile, 

 apatite, magnetite, and sphene. The darker constituents 

 are identical in character with those occurring in the 

 essexite, and therefore do not require to be described 

 again. Not only are they as a class much less abundant 

 in this pulaskite, but the mica here preponderates, being 

 the prevailing iron-magnesia constituent, while the horn- 

 blende is much less abundant and the pyroxene is entirely 

 absent. It may be noted, however, that the hornblende 

 sometimes possesses the greenish tint referred to as 

 occasionally seen about the borders of the hornblende 

 individuals in the essexite, indicating probably that, the 

 pulaskite magma being richer in soda, the hornblende 

 crystallizing out of it has a tendency to take up this 

 element more abundantly. 



The feldspar in 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 microscope, probably owing to the incipient 

 alteration. The larger feldspars are what is commonly 

 described as soda-orthoclase. When examined under the 

 microscope they are seen to be composed 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 un- 

 twinned. 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 



