Dr. Mac CuLLOcu on the Geology of Glen Tilt. 299 



alternated, and that quartz rock was, as it has too often been con- 

 sidered, a modification of granite. This supposition I have else- 

 where shewn to be unfounded, and that this rock is a member of 

 those schistose formations which are found reposing on granite. I 

 shall prove hereafter, that in this instance also the apparent alterna- 

 tion of the quartz rock is a deception, and that it is, like the lime- 

 stone and the schist with which it really does alternate, superimposed 

 on the granite. 



I have in former papers presented to the Society, shown that the 

 polarity of granite, or its power of affecting the magnet, is by no 

 means so rare an occurrence as it was supposed. I may add to 

 these observations, that almost all the granite found here possesses the 

 same properties. But as this is a question involving some important 

 considerations which would interrupt the chain of the present 

 investigation, I shall consider it in a separate paragraph at the end 

 of this paper. 



As I reserved the mineralogical description of the granite which 

 is found in the bed of the river in contact with the schistose rocks, 

 till I had described the larger masses with which it is connected, I 

 shall now describe it, as well as such other varieties as appear 

 worthy of particular notice. I may here also say, that if a term 

 be required for the purpose of distinguishing it from that granite 

 which does not contain hornblende, it may be called syenitic gra- 

 nite ; in treating however of its geological relations, I do not think 

 it necessary to adopt so long a term, which it will be sufficient to 

 use whenever it i& requisite to distinguish its peculiar character as a 

 compound rock. One advantage to be gained by the use of such 

 a term will be, that the term granite will thus become a sort of 

 generic term, including species, or varieties, which differ from the 

 compound of mica, quartz, and felspar, and of which there arc 



2 p2 



