Dr. Mac Culloch on the Geology of Glen Tilt. 303 



rigiit ridge of Glen Tilt is part of this great mass, and it in fact con- 

 stitutes its termination in a southern direction. To ascertain its con- 

 nection w ith the quartz rock, it is necessary to compare numerous 

 observations made on the slopes of the hills, and in the channels of 

 the torrents where such connections are visible. I have already- 

 mentioned the very conspicuous junction visible at Glen Criny, and 

 it would be tedious to repeat the rest of the observations from 

 which the general conclusion -is drawn. That general conclusion is, 

 that the quartz rock is superimposed on the granite,* and that every 

 instance of apparent alternation in these rocks may be resolved into 

 this. The confusion which arises in the disposition of these rocks 

 depends on two causes, the discontinuous arrangement of the 

 \^[uartz rock, and the irregular protuberances, into which the surface 

 of the granite is formed. These latter are the cause of the 

 semblance of alternation before noticed, both on the sides of the 

 •right ridge, and in the bed of the Tilt. In no instance have I dis- 

 covered them proceeding to such a distance from the body of the 

 granite as to deserve the name of veins, and assurediy they are not 

 veins traversing the main body of granite, but on the contrary por- 

 tions of its solid mass. The junctions therefore of the limestone and 

 schist with the granite, which are visible in the bed of the Tilt, 

 are not to be viewed as consisting in the passage of granite veins 

 through those rocks, a phenomenon sufficiently common in other 

 j)laces, but as the points of junction, as I shall soon proceed to show, 

 between a great bed of stratified rocks and a central mass of granite. 



It is necessary for that purpose to enquire into the structure of the 

 left ridge or southern side of Glen Tilt. 



Beginning from Gow's bridge and ascending the hill at right an- 



* Vide PI. 20. %, 3. 



