■294 Dr. Mac Culloch on the Geology of Glen Tilt, 



longs to the same order of rocks as mica slate, and clay slate, it 

 win only serve to point out their concurrence in another common 

 circumstance. It is perhaps almost superfluous to say that the 

 transition, which I am now describing, of quartz rock into granite, 

 is by no means a dubious one, and that it does not bear the least 

 resemblance to the common variety above referred to, which has so 

 often been mistaken for granite. It is perfectly crystallized through- 

 out, the felspar, the quartz, and the mica, having that aspect and 

 relation to each other which cannot for a moment be mistaken by 

 any one acquainted with the characters of rocks. The aspect of the 

 mica alone is so remarkable, and so different from its appearance when 

 found in quartz rock, that it is sufficient of itself to distinguish this 

 modification from every other. Before quitting this subject it is 

 necessary to remark, that one of the transitions between the granite 

 and quartz rock takes place at the junction of the latter with the 

 great body of granite. 



I have been thus particular in describing this singular rock, be- 

 cause it has often been confounded with gneiss, from which it Is 

 strongly distinguished in its mineral character, and because it is 

 important that a substance of so much consequence in the system of 

 rocks as this is should have its relations accurately ascertained, and 

 should not from laxity of definition be introduced into a series 

 with which it has no geological connection. Many of the West- 

 ern islands offer examples of It in its most genuine form. 



From the point last described, following still the course of the 

 stream, nothing is seen for the space of a mile and upwards, but a 

 continued succession of the same varieties of schist. These rocks are 

 also found occupying the hills on both sides of the glen, as far as 

 they are within sight of the river. 



If there are any other remarkable varieties, or any peculiar cir- 



