228 GRANITE, 



some cases they appear to be the consequence 

 of an original concretionary structure in the 

 rock: in others, it is equally certain that they 

 are produced by the action of the atmosphere , 

 as they occur equally in masses of artificial forms ; 

 in the shafts of columns, for example, and in 

 blocks squared by the tool. 



With these comprehensive geological features, 

 are united the following mineralogical characters. 



The texture rs, with one exception, always 

 crystalline and confused ; the several minerals of 

 which it is composed, interfering with each 

 other's forms. With the single exception of the 

 graphic variety, it is also granular ; but varying 

 much in the fineness of the texture, or in the 

 magnitude of the parts. 



As the mineral composition of granite re- 

 sembles that of gneiss, and as, in many in- 

 stances, these rocks approximate, even in texture, 

 it will be convenient to state here, the characters 

 by which these substances may be distinguished. 

 In granite, the crystals of mica, or of hornblende 

 if present, are placed irregularly in all directions ; 

 but, in gneiss, they preserve a general parallelism, 



