418 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



a chonolith. The northern limit of the chonolith is so completely covered by 

 glacial drift and soil that there the relations of the body could not be deciphered. 

 On that side the relatively coarse-grained porphyry seems to pass under a thick 

 cover of its own effusive phase, the vesicular biotite trachyte. Yet it is quite 

 possible that the two rocks pass into each other gradually, or, thirdly, that they 

 have been faulted into contact. The difficulty of deciding between these 

 alternatives is curiously paralleled in the problem above noted in connection 

 with the rhomb -porphyry chonolith on Rock creek, where, again on the north 

 side, it makes contact with thick vesicular lavas of the same chemical 

 composition. 



The pulaskite porphyry is very uniform in habit and composition. It 

 is a light pinkish-fawn rock, carrying abundant phenocrysts of pale, flesh-pink, 

 thick-tabular feldspars which reach 1 cm. or more in diameter. A few biotites, 

 generally under 3 mm. in diameter form the only other phenocrysts. Under 

 the microscope the fawn-coloured ground-mass is seen to be essentially a typical 

 trachytic mass of tabular feldspars, with which a few small biotites and rare 

 prisms of green hornblende are associated. A little interstitial quartz and small 

 amounts of titanite, apatite, and magnetite are accessory. The ground-mass is 

 highly miarolitic with actual cavities between the feldspars. The resjultins? 

 porosity goes far to explain the low specific gravity obtained for the fresh 

 analyzed specimen, namely, 2-497. 



The feldspars are all more or less cloudy with decomposition-products, 

 calcite and kaolin. The phenocrysts are chiefly socla-orthoclase with extinc- 

 tion on (010) of 12°+. This feldspar has a tendency to a perthitic struc- 

 ture. Carlsbad twins are common. A few andesines (Ab 4 An 3 ), generally 

 surrounded with thick shells of soda-orthoclase, occur in some thick sections. 

 The feldspar of the ground-mass is generally twinned on the same Carlsbad law 

 and seems also to be chiefly soda-orthoclase. A little plagioclase may also be 

 there present, though none was certainly identified under the microscope. 



Professor Dittrich has analyzed a typical specimen (No. 1010) from a sill 

 cutting the sandstones 400 yards northeast of the Kettle river bridge. The 

 result is as follows, (Table XXVII, Col. 1.) :— 



