26 GRANITIC ROCKS OF OTHER COUNTRIES [Ch. III. 



disposition in the felspar, has been called porphyritic. In 

 general the felspar is white, and of the common kind ; but, in 

 the minuter states of intermixture, it has often a greenish 

 hue, and so far loses its crystalline appearance as to resemble 

 the ordinary compact felspar, which is more common in 

 the greenstones of the trap family : this, however, is not easily 

 determined, but I am inclined to believe that it does occur. 

 When the mixture of the two minerals becomes minute, the 

 rock is no longer distinguishable from ordinary basalt : and 

 in some specimens, it even appears that the felspar is at length 

 excluded ; so that there remains nothing but that compact, 

 yet minutely granular aggregation of hornblende, which, 

 according to some mineralogists, constitutes the only genuine 

 basalt. And it must also be observed that, among rocks of 

 this character, some specimens cannot be distinguished from 

 the black clay-stones, in which the peculiar lustre of horn- 

 blende is absent ; and they are soft, with an earthy fracture." 



From this statement we learn, that hornblende acts as im- 

 portant a part as shorl, giving rise to as many varieties of 

 granitic rocks : and this is not the only analogy between the 

 granite of Scotland and Cornwall, for other kinds of primary 

 unstratified rocks are to be found in the former country. Of 

 these none are more extensively diffused than the different 

 species of porphyry, which are subject to the same mutations 

 as those of Cornwall already described, and appear to have 

 a similar composition, more especially their bases, which are 

 only varieties of compact felspar assuming different charac- 

 ters, as its siliceous element varies in proportion. 



The vicinity of Loch Etive also particularly abounds in 

 porphyry, according to Macculloch; and its nature is well 

 exemplified in the following extract from the same author : 



" The northern side of Cruachan * presents a range of 

 nearly perpendicular precipices, extending many hundred feet 

 down the mountain. This section shows that the mass of the 



* Geol. Trans., vol. iv. p. 121. et seq. 



