Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/155

 Rh it is common grauwacke, with quartz, and at its junction with the granite, it is traversed by veins of this rock, similar to those I have already mentioned in the valley of the Erme in Dartmoor. The southern side of the mount is nearly precipitous, and is composed from top to bottom of a granite split into irregular masses; at the bottom is a heap of large blocks, among which, I thought I observed some indications of copper ore, on their surface. Sometimes the felspar and sometimes the quartz predominates in this granite; when it is the quartz, it gives the rock a vitreous appearance: it contains also black tourmaline, and pinite is also said to have been found in it.

Admitting however that the mass of granite of St. Michael's Mount was detached from the land, before the grauwacke was deposited upon it, and conformable with it, the grauwacke could only have rested on the northern face, or that which is the least abrupt, as the southern face is almost perpendicular, which is shewn by the great depth of the sea at the bottom of the mount on that side. But without having recource to this hypothesis, to explain so partial a fact, it would perhaps be more reasonable to admit, that the epoch of the separation and transportation of St. Michael's Mount has been posterior to the deposition of the grauwacke which has remained adhering to the detached mass of granite; and that in settling it has taken such a degree of inclination, that the strata of grauwacke on the south have been completely concealed, and only exposed to view on the northern side. I should not have dwelt so long upon.