Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/477

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467
Dr. Mac Culloch on Quartz Rock.

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 unnecessary 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 so 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 modifications 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

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