Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/273

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263
Dr. Mac Culloch on the Vitrified Forts of Scotland.

quartz rock, and porphyry. A long mountain of trap rising at the borders of Loch Etive, skirts the edges of these hills for a considerable space, terminating on the plain of Connel by a trap breccia, that pudding-stone well known to tourists as occurring in various places from Connel to Oban. This breccia, where nearest the fort, is at least half a mile distant from it. As the geological site of the rock does not concern the present inquiry, I will limit myself to its mine description.

It consists of rounded pebbles of different magnitudes, cemented by a paste of a mixed white and brown colour. The pebbles are generally small, and are much more numerous under the size of an orange than above it. There are very few of considerable magnitude. They exhibit for the most part different varieties of trap, or greenstone, all of which have been rounded previously to their entanglement. Of these there are purple, red, gray, and dark blue specimens, varying as much in solidity of texture as they do in colour, and more or less homogeneous in their appearance. Many of them are of an amygdaloidal structure, containing imbedded grains of calcareous spar, zeolites, and green earth, and some are perfectly cavernous and scoriform. Besides these pebbles of trap, there are rounded pieces of quartz of different colours, white, gray, and red, cemented in the common ground. In the specimens which I examined I could not trace any other kind of rock. The paste by which the whole is cemented is of a peculiar quality. It is either dark purple, or brown, or mottled, or gray, or a dirty mixture of brown, white, and dull green. It may be scratched with the knife, has an earthy smell when breathed on, and effervesces with nitrous acid. Its fracture is not properly granular, but rather of the small splintery. Before the blow-pipe it is fused into a dark glass.

On a minute examination it appears to consist chiefly of frag-