Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/144

 bright, and the texture so compact, that they can scarcely be distinguished from jasper, a rock which, I may remark, although hitherto but little examined in its geological relations, possesses a very near affinity to the family of porphyry, as well in the extensive independence of its position among the regular rocks, as in its appearance and composition.

The occasional minerals which are found in these rocks are hornblende, quartz, and epidote, all of them entering into the composition of some of the varieties, and the latter in particular forming a very conspicuous feature among them, being disposed either in the form of veins, or in amygdaloidal cavities, or else in occasional grains. Hard breccias, of which the structure can scarcely be detected except on the weathered surfaces, and exactly resembling those so conspicuous on Ben Nevis, also occur dispersedly among the more simple rocks, the fragments consisting only of different varieties of the same substance.

Before quitting these rocks it will not be uninteresting to mark the principal circumstances in which they differ from the analogous rocks which occur in Arran, in the Ochil hills, and in many other parts of Scotland. They all consist alike of claystone and compact felspar, simple or porphyritic. But they differ in situation, the hills of Glenco reposing on granite and the older schists, while the former lie above the red sandstone. They also differ in their general features, since the former assume a spiry shape, while the others present a succession of tame and rounded outlines. They appear equally to differ in durability, since although the hills of Glenco are destroyed by the effects of the mountain torrents, they are not like those of Arran subject to decompose by the ordinary action of the atmosphere. In the variety of composition there is also a conspicuous difference, the infinite number of hard, coloured, and compounded