Dr. Mac Gulloch on the Geology of Glen Tilt, 293 



veins in their course, and even crossing the layers at considerable 

 angles. A rock observable at Garviemore illustrates this accident 

 v^ell. Here the transit of the granite vein through the schist is at 

 right angles, and is attended with a flexure of the layers on each 

 side of the vein, in a curve directed from the thickest to the thinnest 

 part of it. At each side of the vein the granite passes from it be- 

 t.ween the iaminse of the schist, producing a partial effect similar to 

 the more extensive one above described, and which has doubtless 

 owed its origin to a similar cause. 



The other case which I reserved for consideration, is that where 

 similar beds of granite are found alternating with, and graduating 

 into quartz rock. This alternation is visible in the bed of the river 

 not far from the entrance of the Mark, But a much better 

 instance occurs in the hill. of Grianan on the right of Glen Tilt, 

 where the transition from the quartz rock to the granite is so per- 

 fect that the boundary cannot be defined. I have met with similar 

 appearances in the vicinity of Dalnacardoch. 



In a paper which I presented to the Society last year, 1 describe.d 

 among several varieties of quartz rock, that, perhaps the most common 

 of all, in which grains of felspar are found mixed with grains of 

 quartz, neither of them being crystallized ; and I then pointed out 

 the distinction between this rock and granite, attempting to shew 

 that it was a recomposed rock, and that it had been formed from 

 the wearing of more ancient granites. Among the several tran- 

 sitions of quartz rock into mica slate, clay slate, &c. I did not 

 notice this transition into real granite, as I had not then met with 

 this appearance : it will add one to the number of those transitions. 

 <It will at the same time be obvious, that it can in no respect affect 

 the account which I then gave of the nature and connections of 

 -quartz rock. On the contrary, as I have shewn that this. rock be- 



