209 



tain compact felspar as a conspicuous or leading 

 ingredient; and this substance seldom occurs, 

 except in very minute proportions, and as an oc- 

 casional constituent, in the granites. A large 

 proportion of the more equal mixtures of horn- 

 blende with felspar, both common and compact, 

 belongs to this division ; but as similar mixtures 

 are also found among the granites, they are not 

 necessarily and exclusively members of this family. 

 Even the finer compounds, forming one of the 

 varieties of basalt, are found accompanying 

 granite. The student, in these cases of doubt, 

 must unavoidably have recourse to the geological 

 position; and this last example, in particular, 

 offers one of the strongest which geology affords, 

 of the hazard of determining the geological cha- 

 racters of these rocks from their mineral struc- 

 ture or composition. 



The porphyritic and the amygdaloidal rocks 

 are recognized by the peculiar structures already 

 described in the chapter on that subject; and 

 may always safely be referred to this family, as 

 they occur in no other department of nature. 



p 



