Ch. II.] COMPOSITION OF GRANITE. 13 



noted when shorl characterises the rock, as in Cornwall ; or 

 when talc prevails, as in the Alps, in Corsica, and in some 

 parts of Norway. In each of these instances, the suite of rocks 

 of which the granitic range is composed is very different ; and 

 this variation is distinctly connected with the nature of the 

 characteristic mineral. 



This circumstance alone, therefore, independent of facts 

 to be hereafter adduced, is sufficient to show, that geologists 

 ought not to rest satisfied with stating that a country is com- 

 posed of granite : it is, certainly, in descriptions very con- 

 venient to be able to denote in general terms that a country 

 consists of granitic rocks ; but this ought not to obviate the 

 necessity of specifying the nature of the individuals of which 

 this group is constituted. 



These preliminary remarks may, perhaps, induce attention 

 to the contents of this Chapter, which otherwise might be 

 passed over as tedious details by those who have not interested 

 themselves in the examination of primary rocks ; but they 

 will be found a necessary introduction to the right under- 

 standing of the phenomena to be hereafter discussed, and may 

 enable the student to comprehend " those almost infinite vari- 

 ations in the composition of these rocks, which appear to set 

 classification at defiance ; but which, however perplexing at 

 first sight, will, it is presumed, ultimately lead to an accurate 

 knowledge of the nature of the granitic rocks." 



Granite, in the common and original acceptation of the 

 term, denotes a rock composed of felspar, quartz, and mica. It 

 oftentimes contains, in addition to these, some other minerals ; 

 but those just enumerated are considered, in the following 

 pages, as essential to true granite ; and if either of them is 

 wanting, the compound may then receive a distinct denomi- 

 nation. 



These component minerals of granite, both essential and 

 accidental, are united together by a confused crystallisation, 

 not only mutually penetrating and interfering with each other, 

 but sometimes the small crystals of one ape completely enve- 



