350 ARKAN. GEOLOGY. GRANITE. 



the existence of cavities in which both the felspar and 

 quartz are crystallized at liberty, although of but small 

 size : more rarely the mica is also found in similar circum- 

 stances. The felspar is in these cases whitish and ill 

 defined, and the quartz is generally brown and trans- 

 parent; presenting its most common regular shape, the 

 hexagonal prism with the terminating pyramid. I formerly 

 mentioned in treating of St. Kilda, that the syenite of 

 that island, which is decidedly associated with trap, and 

 therefore of a posterior origin to granite, is marked by 

 the same peculiarities ; affording an instance, among many 

 others, of the great affinity which is often found to exist 

 between certain rocks of the primary, and others of the 

 secondary division. Another variety of this granite is 

 remarkable for containing two kinds of felspar, the one 

 of an earthy and dry aspect, as if from the loss of its 

 water, the other of the glassy kind. This appearance, 

 as will hereafter be seen, is very remarkable in some 

 of the porphyries which constitute so peculiar a feature 

 in Arran. The last variety which I shall notice, contains 

 blueish felspar. It is unnecessary to describe the remain- 

 ing aspects which were observed, since the description 

 of such varieties is scarcely intelligible without the speci- 

 mens, and affords no instruction. 



With respect to the granite veins of this island, it may 

 be remarked that graphic granite is found in detached 

 pieces, but I know not that any one has observed it in 

 situ, as the public is not in possession of the observations 

 which have been made by the numerous geologists who 

 have visited [this spot. It is probable that here, as else- 

 where, it is the produce of veins ; and that these are 

 independent, and of a different nature from the root^like 

 veins to be seen at the junction of the granite and schist, 

 as well as from those which traverse the granite itself. 



These latter are frequent, and seem always to have 

 their origin and termination in the rock which contains 

 them. It is impossible to prove that this supposition 



