ON A BASIC ROCK DERIVED FROM GRANITE. 669 



its origin. The examination resulted in the finding of several 

 small patches, which, though considerably altered, still retain 

 enough of their original character to- show conclusively that they 

 are granite. The transition between the fairly fresh granite 

 and the serpentine-like rock is very gradual, the feldspar los- 

 ing its fresh appearance and passing over into a green aggregate, 

 while the quartz decreases in quantity, till in some specimens it 

 is wholly lacking. To the naked eye no other minerals are 

 visible, the original granite having been a coarse, pegmatitic 

 variety. 



The alteration of an acid granite into a nearly black, appar- 

 ently basic, rock is an exceptional phenomenon, and yet in this 

 instance the field evidence alone is of such conclusive nature 

 that there can be no doubt that such is the case. Furthermore, 

 the operation has taken place on a large scale, for there are hun- 

 dreds of tons of the altered rock in sight, and it is impossible to 

 tell how much is underground. 



Additional evidence that the rock is an alteration of granite 

 is afforded by the fact that at the bottom of the underground 

 workings the ore rests upon a granite which differs only in 

 freshness from the material of the least altered patches in the 

 serpentine. But even this granite shows the beginnings of the 

 alteration. 



Mcgascopical aspect of the alteration. — A specimen of granite 

 from the bottom of the shaft is a decidedly coarse-grained rock, 

 made up of quartz and feldspar, the latter predominating. 

 Besides these distinct minerals there is a limited amount of 

 greenish material which fills the interstices between the essential 

 minerals. The latter have very irregular outlines, the feldspar 

 occurring in decidedly larger individuals than the quartz. The 

 former mineral generally has bright cleavage faces ; less often 

 they are dull and earthy. The color is gray with a very faint 

 pink tinge. The quartz is colorless and clear, with the usual 

 vitreous lustre. 



The granite of the least altered patches in the "serpentine" 

 differs from such a specimen in having a larger amount of the 



