Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/75

 putting on a great variety of aspects varying with the colour of the base, the quality of the felspar which forms the crystals, their magnitude, and the density of their aggregation. Blaven offers a remarkable variety, in which solitary crystals of glassy felspar nearly two inches in length are sparingly disseminated through the ground. The beautiful variety consisting of pure white crystals in a ground of dark blue, which is found in Raasa, also occurs in Glamich. But it would be fruitless to describe these varieties. In the same hill it is found of an amygdaloidal texture, but, as far as I have examined, the cavities contain only crystallizations of epidote similar to those occurring in the greenstone which I have already mentioned.

Such are the rocks which as far as my observations extended form that mass, which, for want of a better general term, I have designated by the name of mountain trap. It is however evident that the rugged aspect and permanency of these hills cannot in any degree be attributed to the clinkstone which enters into their composition. We have examples sufficient in Mull and in Arran to show that hills of clinkstone always form in the progress of their decomposition a feeble and smooth outline. The same indeed occurs in these very hills, since the outline of Glamich, and of the other mixed hills in which clinkstone forms the dark part, is equally tame and rounded. From this knowledge we are led still more strongly to conclude that the hard and serrated summits, and even the main body of the hills which present them, are composed of some of the varieties of greenstone before described, and hence to conjecture that this substance forms their principal ingredient, and that the clinkstone is only found on those outskirts where the passage or change into syenite takes place. We have I fear no means even of conjecturing the causes of the difference either in the disposition or