Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/478

From Wikisource
Jump to navigation Jump to search
This page has been validated.
Dr. Mac Culloch on Quartz Rock.

summits of Jura. There is, in fact, no essential difference between them, as they consist in both cases of highly compacted grains of quartz, with interspersed grains of felspar, often earthy, and never, I believe, so perfectly crystallized as they are in granites and porphyries, but having the aspect of fragments rather than of crystals. I did not meet with any specimens on Schihallien of a formation so decidedly mechanical as those which I have described as occurring in Jura, but must rest the proofs I am desirous to bring forward of the mechanical and therefore secondary structure of this rock, on other and more circumstantial evidence. Possibly future observers, bearing this doubt in their minds, may with more time and opportunity discover such positive proofs of this nature as I failed to find, since our acquisitions in specimens are generally in proportion to our previous knowledge.

It will doubtless be admitted that if of two rocks precisely similar in the generality of hand specimens, similar also in structure and position, and possessing the same characters in general aspect and disintegration, the one is proved to be a mechanical deposit by its containing rounded pebbles or beds of breccia, it affords a very strong presumption that the other is of the same nature. But we have yet to see what further evidence of this may be found in the neighbouring rocks, as the occurrence of a single bed of a recomposed rock in a situation so remarkable as the ridge of Schihallien, might excite rational doubts, unless we could discover a greater extent of similar rocks connected with it. It is here necessary to remark that the whole country to the north of this ridge consists for a considerable space of rocks of the class called primitive. The vale of Tumel and the hills which bound it are composed chiefly of mica slate, which continue still it meets the granite of the central Grampians. The case is different on the southern side of Schihallien.