64 



GRANITE VEINS. 



erto been noticed. The junction of the granite rock and the schist 

 may be distinctly seen : they form together a sloping beach uncov- 

 ered by any fragments : the line of junction is waving from the coast 

 into the sea, as represented Plate 11. fig. 3., g. the Granite, s. the 

 Schist. 



It is truly worthy of notice, that the veins of granite may be dis- 

 tinctly seen penetrating both the schist and the granite; for the gran- 

 ite, in the veins, is finer-grained than the granite rock, and may as 

 easily be distinguished in the granite, as in the schist. The granite 

 rock itself is smaller-grained near the line of junction of the two 

 rocks, than it is a little distance from it, where it contains large white 

 crystals of felspar in a smaller-grained reddish granite. What is 

 further remarkable, the largest granite vein, in passing into the schist, 

 cuts through a vein of quartz thicker than itself ; and, a few yards 

 nearer the sea, a small quartz vein cuts through the same granite 

 vein : see Plate II. fig. 3. What is called the schist or killas in 

 Cornwall, in the places where I have observed it in immediate junc- 

 tion with granite, is highly indurated and of a dark colour, and ap- 

 pears to have been changed by the junction : it has no appearance 

 of slate ; — indeed the change, in the size of the grain of granite, as 

 the latter approaches the killas at Mousehole, would indicate that 

 the two rocks were passing into each other. Perhaps the best de- 

 signation of the killas rock on this situation is, that of a minutely 

 grained and highly indurated gneiss, that had lost its schistose char- 

 acter.* 



Granite veins, of large size, traverse rocks of small-grained gran- 

 ite and gneiss in the vicinity of Aberdeen : in these veins, both the 

 felspar and mica occur in crystalline plates and laminae of consider- 

 able magnitude, accompanied with tourmaline. At Glentilt in Scot- 

 land, a singular intermixture of granite, in veins and amorphous 

 masses, occurs with slate and limestone, and has been described by 

 Dr. MacCulloch in the Geological Transactions, vol. i. page 145. 

 It seems impossible to conceive how masses of granite could be in- 

 termixed with, or imbedded in limestone, without admitting that the 

 two substances have been both, in a fluid or semi-fluid state at the 

 same time ; and we are not acquainted with any cause except heat 

 combined with pressure, which could effect a simultaneous fusion of 

 both rocks. 



Some geologists describe the granite under gneiss and the granite 

 over gneiss as different formations ; but, as gneiss is itself a schistose 

 granite, it would be more correct to state, that the massive and schis- 

 tose granites sometimes occur, alternating with each other. When 

 the mica becomes abundant, the granite passes to the state of gneiss; 



* In the Phil. Mag. March, 1829, there is a full description of the granite veins 

 in killas, by two German geologists, but no new or important facts are communi- 

 cated. 



