Dr. Mac Culloch on Quartz Rock. 467 



the ridge bounding the southern side of Glenco takes its rise. This 

 ridge contains the other considerable elevation of Ferrogon, and 

 separates, not the " valley of the Tay" but that of the Lyon from 

 that of the Tumel. The reason for making this correction will 

 appear in the sequel. 



Mr. Playfair's accurate description of the spaces occupied by the 

 rocks which constitute the summit of Schihallien, renders it unneces- 

 sary for me to enter into any such detail. I may merely remark, 

 that by his report the whole of the mountain, except the central 

 ridge, consists of the various modifications of mica slate, hornblende 

 slate, and the usual associated limestone, which are so well known 

 as constituting the greatest tracts of the highland mountains. The 

 result of my own examination, which was however exceedingly 

 limited, coincides with his. All these strata are vertical, or very 

 nearly so, and the central ridge, or the " granular quartz, " appears 

 to be placed in a position absolutely perpendicular. 



This quartz rock is stratified, although vertical, in beds which, as 

 far as they can be seen, appear absolutely and nicely parallel, their 

 lines of separation being defined with a most mechanical exactness. 

 They have a tendency to break into rhombic and prismatic fragments, 

 by fissures perpendicular to their stratification, and being thus 

 broken, they have fallen on all sides around the summit, producing 

 the same appearances as those exhibited by the Paps of Jura, and 

 the mountains of Assynt, and, for the reasons which I have assigned 

 in my observations on these hills, contributing to form that elegant 

 conical outline for which the summit of Schihallien is so remarkable. 



An ample account having been given by Mr. Playfair of all the mo- 

 difications of this rock which he could observe, it will be superfluous 

 for me to enter into its history, particularly as I have already pointed 

 out its leading characters in describing the rock which forms the 



3 N 2 



