470 Dr. Mac Culloch on Quartz Rock. 



will appear fully determined by the other examples of this rock, 

 which are described in the supplement to this communication. But 

 this circumstance, already mentioned as being supposed to occur in 

 Jura, does really appear to receive further confimation here, since if 

 the central ridge of Schihallien is surrounded by mica slate, a fact of 

 which Mr. Playfair entertains no doubt, and if this ridge is connected 

 by any system of alternation with the sandstone of Glen Lyon, 

 mica slate will appear to be a rock formed posteriorly to, or alter- 

 nately with a rock of recomposed structure. Thus a ^'' primitive'^ 

 rock will be found to alternate with a " transition " one, an anomaly 

 which either renders this distinction as useless as it is artificial, or 

 compels us to modify the definition of transition rocks, or to form 

 that total change of arrangement which I have more than once 

 suggested with regard to the primitive and transition classes. It is, 

 I trust, quite superfluous to say that it can have no title to the name 

 of " granite, " with which it certainly does not possess any one 

 common feature. 



Supplementary Remarks on Qjiartz Rock. 



Having had an opportunity since I presented the foregoing 

 memoir last year, of examining some other parts of Scotland where 

 a similar rock occurs, I have thought it right to lay these supple- 

 mentary observations before the Society, partly for the purpose of 

 correcting the errors and supplying the defects of that communica- 

 tion, but principally for the sake of elucidating the history of a 

 substance as yet but imperfectly understood, and but ill arranged, 

 although it must already be obvious that it occupies a considerable 

 space among the older rocks, and is entitled to rank as a principal 



