ARRAN. GEOLOGY. OVERLYING ROCKS. 397 



With a smoother fracture, either plain or conchoidal, 

 and a translucent fragment, this rock becomes compact 

 felspar, (the hornstone of some) which, however occa- 

 sionally found unmixed in Arran, is more generally por- 

 phyritic; containing crystals of felspar more or less 

 numerous and remarkable. 



As the various porphyries form a conspicuous feature in 

 the island, it is necessary to give a general sketch of their 

 appearances. They are still but varieties of the simple 

 rocks above described, since their geological connexions 

 are the same, and their actual continuity can in most 

 cases be traced. Each of the substances just enumerated 

 becomes occasionally the base of a porphyry, and many 

 varieties in colour as well as texture are thus produced ; 

 further increased by the more dispersed or condensed 

 numbers of the felspar crystals, as well as by their varying 

 magnitude. Among the most remarkable of these, I may 

 enumerate one consisting of a grey felspar base with large 

 white crystals, another of a pale blue with white crystals, 

 and a third, still more remarkable, well known as being 

 found at Drumadune. This porphyry, in a base of com- 

 pact felspar or clinkstone, contains large crystals of earthy, 

 of common, and of glassy felspar; together with trans- 

 parent grains of quartz, which have often the form of 

 crystals of which the angles have been rounded. The 

 base varies in colour, from greyish white to dark lead 

 blue ; and, in the latter case, cannot be distinguished from 

 many varieties of trap. But the base of the porphyries, 

 simple in the above cases, becomes sometimes compound, 

 by the intrusion of quartz into its composition ; and thus 

 there is formed a rock from which the imbedded crystals 

 of felspar again disappear, leaving a simple and granitic 

 mixture of felspar and quartz, often stained with spots 

 of oxide of iron, as if from the decomposition of pyrites, 

 hornblende, or some other mineral. This compound has 

 obtained no distinguishing name, nor perhaps is it neces- 

 sary, since it is easily arranged with the varieties of 



