Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/143

 of veins proceeding from the mass, rendered conspicuous by their projection and superior durability, but in examining the surrounding rocks where in contact with them, no difference of structure or composition is perceptible. I did not any where perceive a tendency to the columnar form.

Although the naked surfaces of these rocks might lead us to suppose they were inimical to vegetation, this effect must rather be attributed to the steepness of their declivities, which prevents the accumulation of soil. From whatever cause it may arise, they seem particularly subject to be destroyed by the action of the mountain torrents, whence the enormous piles of fragments which annually overwhelm the road and are fast raising the level of the valley.

The simple rock, which, as I have already said, prevails over the porphyritic varieties, is at one extreme a claystone, and at the other a compact felspar, varying through several intermediate stages of hardness. In the intermediate stages of transition to porphyry, a single crystal only of felspar will sometimes be found in a large fragment, the ultimate accumulation of which produces porphyries of an infinite variety of aspects. Every variety of this substance indeed, whether in colour or composition, which occurs in veins throughout Scotland, is here found mixed together in the mass, sometimes placed side by side with a sudden and decided transition, at others graduating into each other by imperceptible degrees. The colours graduate into each other in these cases just as do the different structures, and among these gradations the most striking are those where black passes into red. It would be an useless task to describe the varieties of colour which occur, but the different shades of grey, purple, and red, are the predominant ones. In some cases dark blueish specimens are found veined with red, producing very beautiful and remarkable varieties: in others, the red colour is so