268 Dr. Mac Culloch's Supplementary Observations 



with which we are acquainted, it has occasionally been in a flexible 

 state. Some beds are also to be found there towards the east, on 

 that shoulder of Cairn Gower which hangs towards Loch Lochs, 

 containing rounded pebbles, such as I before described as occurring 

 in it at Jura. The beds in which this variety is found consist indeed 

 entirely of a loose aggregation of large and small rounded gravel, 

 This is only the second instance in which I have noticed a coarse 

 conglomerate mass as forming one of the varieties of quartz rock. 

 The pebbles are of considerable magnitude, and bear just such marks 

 of attrition as do those which have been rolled on a sea shore : 

 if they are not mechanically rounded pebbles I know not where 

 such can be found. I have little doubt that this variety will oftener 

 occur when these rocks shall have been more extensively examined ; 

 but I trust this fact is no longer necessary to prove that quartz rock 

 bears the marks of a mixed mechanical origin, and that it thus serves 

 to determine in some measure a corresponding set of circumstances 

 in which the schistose rocks associated with it were formed. 



Near Blair it may be observed passing into a regular granite in a 

 very distinct manner. Many of the specimens found were trans- 

 ported stones, but in many other instances, which I have had occasion 

 to notice in a paper on Glen Tilt,* the transition from quartz rock 

 to granite is to be seen in situ* I have there also noticed that, which 

 ought for the sake of uniformity to be repeated here, that the beds 

 of quartz rock which are in immediate contact with the granite often 

 pass into it by insensible degrees. It is well known that there is a 

 gradual transition from mica slate into gneiss, and that this rock again 

 by degrees equally evanescent passes into granite. I have shown 

 that quartz rock is here in contact with granite, and it is not there- 

 fore surprizing that it should, like the mica slate with which it 



* Geo. Trans. Vol. HI. 



