III.—The Sgùrr of Eigg: some Comments on Mr. Bailey's Paper

Extract

MR. Bailey has very kindly permitted me to see his manuscript, and also to examine a specimen and slice of the rock which he regards as a volcanic breccia underlying the pitchstone. Since he attaches some importance to this rock, I will consider it first.

The natural base of the thick pitchstone sheet is made by a band of more glassy aspect, almost like an obsidian. The microscope shows that this, apart from the usual enclosed crystals, is a light-brown glass, free from the crowds of crystallites found in the ordinary pitchstone. Another feature is that it encloses numerous little fragments of basalt. These indeed, with occasional larger pieces, pass up for some feet into the pitchstone, but they are most abundant near the lower surface. Such inclusions, picked up from the contiguous rocks, may occur both in lava-flows and intrusive sills, but, so far as my experience goes, are more frequent in the latter. The obsidian-like band is well seen, fresh and intact, at places on the northern side of the SgÙrr; but elsewhere it is much brecciated, and usually much decomposed in addition. The brecciation is of such a kind that pieces of the normal pitchstone have become mingled with the debris of the more glassy variety. Mr. Bailey has quoted part of my description of this brecciated basal band as it is exposed, in a decomposed state, on the southern side of the ridge. That it is a part of the pitchstone sheet, not a separate underlying deposit, is there clearly demonstrated: see fig. 6 on p. 60 of my paper.