Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/134

 appear to be in their places, turn out on examination to be only large rolled stones, and these invariably consist of granite and porphyry. It is not till we have attained about two thirds of the height of the mountain, that the natural rock makes its appearance in an unquestionable manner. From this part to both the summits there are abundant opportunities of examination, as immense faces of it are left uncovered even on the southern side where the acclivity is easiest. To the north it presents a range of nearly perpendicular precipices extending many hundred feet down the mountain. The mass of the mountain is easily seen from this point to consist of granite. This appears to be of uniform composition on both the summits, and to be formed of a very equal mixture of reddish felspar and white quartz with very little mica, nearly resembling the granite of Cairn Gorm. The porphyry veins which traverse it are here also as visible as they are below, but if there is any difference they appear of greater magnitude, and the red variety predominates. There are immense fissures on the northern side, which seem to have resulted from the wasting of these veins. It is on the lower of the two summits, as I before said, that the junction of the basaltic veins with the granite is visible.

I have remarked in a former paper that the magnet is much affected by the granite on the summit of Goattield. This is still more strikingly the case in Cruachan, its affections being indeed stronger here than they are even on many of the basaltic rocks of Canna. Both the porphyry veins and the granite affect it, but the former in the greatest degree. Having had occasion to observe these two instances of a fact but little noticed, I think it right to add, that in both cases I found this property confined to the masses which occupy the summits of Goatfield and Cruachan, and that I did not perceive it either in the rocks or detached masses at the foot of these