OVERLYING ROCKS. 489 



most remarkable in the case of syenite ; since it is 

 only where the latter kind of felspar enters into 

 the composition, that the rock presents the 

 pseudo-granitic aspect : where it is formed of 

 compact felspar and hornblende, as in the rock 

 of Ailsa, it possesses no resemblance to granite. 



The glassy variety of felspar is also found, 

 though more rarely, entering into the compo- 

 sition of these rocks ; in some cases.it forms the 

 porphyritic imbedded crystal ; in others, but 

 more rarely, it occupies the place of common 

 felspar in the compounds of granitic structure. 

 In this latter way it occurs in the augit rock of 

 Rum and in the hypersthene rock of Sky. 



It is still necessary to enumerate a few mine- 

 rals which, though far less frequent ingredients 

 in the rocks of the trap family, and therefore less 

 essential, sometimes enter into their composition 

 as integrant parts ; often producing considerable 

 confusion in their characters. 



The first of these is mica. This occurs in 

 the greenstones and in the porphyries, but most 

 rarely in the former. It is a more important cir- 



