GRANITE. 229 



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so as to give a foliated appearance to the rock, 

 and, very frequently, communicating to it a ten- 

 dency to split in one direction. 



In certain cases, however, the distinction be- 

 tween granite and gneiss is evanescent, and the 

 two become mineralogically identical. The 

 cautious geologist will therefore beware of draw- 



'V ' I 



ing any general conclusions of a geological na- 

 ture, from the mere examination of specimens ; 

 but will examine the connections of th& rocks 

 themselves, and their general relations to the 

 surrounding strata. 



The minute structure of granite sometimes 

 approximates to that of porphyry ; or distinct 

 crystals of the felspar which belongs to its com- 

 position, are imbedded in the general granular 

 base. This variety has been termed porphyritic ; 

 and it is also proper here to remark, that the 

 porphyries sometimes lose their characteristic 

 structure in particular places, and assume that of 

 granite so nearly, as to be scarcely distinguish- 

 able when in hand specimens. In this case, as 

 in many others, such specimens can only be truly 

 referred to their place in this system, by examin- 

 ing their geological connections. 



