230 GRANITE. 



The magnitude of the parts in granite is ex- 

 tremely various ; each constituent mineral some- 

 times exceeding an inch in dimensions, and, at 

 others, being almost invisibly minute. Various 

 textures are also often united in a very limited 

 space, or the rock passes imperceptibly from 

 fine to coarse-grained. Occasionally also, irre- 

 gular patches, or veins, of a fine texture, are 

 seen imbedded in a coarser variety. In one rare 

 instance the parts affect a spheroidal arrange- 

 ment. 



Mica, felspar, and quartz, have, by some 

 mineralogists, been enumerated as the sole mi- 

 nerals essential to granite. This distinction is 

 too limited for practical purposes; and, in a 

 geological sense in particular, it is inadmissible. 

 Within a very narrow space, either the quartz or 

 the mica may disappear : in graphic granite, the 

 latter is always wanting. Moreover, horn- 

 blende is very often present in one part of the 

 same continuous mass which, in another, con- 

 tains only the three ingredients above named : 

 and it is also very frequently the substitute for 



inica ; the rock being then a ternary compound 



i 



of quartz, felspar, and hornblende. The term 



