306 Alfred Harker — The Sgurr of Eigg — Cominenis. 



III. — The Sgtjer of Eigg : some Comhents on Me. Bailey's Papee. 

 By Alfred Harker, M.A., F.K.S. 



ME. 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 pitch- 

 stone. 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 pitch- 

 stone. 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 Sgvirr ; 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 decom- 

 posed 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. Mr. Bailey's specimen 

 comes, I do not doubt, from a part of the same band which is 

 brecciated but not decomposed. The small fragments of basalt in it 

 represent, in my view, the inclusions which always occur in the basal 

 pitchstone or obsidian. In the course of brecciation they have usually 

 become detached from their more brittle matrix ; but one or two, as 

 seen in the slice, seem to me to have relics of brown glass still 

 adherent to them, and one is completely enclosed in a shell of glass, 

 a fact which my friend is content to record without comment. For 

 ine, therefore, his rock is no volcanic breccia, but merely the brecciated 

 base of the pitchstone sheet. Such brecciation again affords no 

 criterion as between a lava-flow and a sill. 



Mr. Bailey next carries us to the west coast, and discusses the frag- 

 mental accumulation there exposed in the cliff. This Sir Archibald 

 Geikie regarded as a thick deposit of river-gravel, thrown down 

 (a circumstance requiring some explanation) just where the valley 

 narrowed to a gorge. To me its characters are not like those of 

 a river-gravel, and it is certainly very different from the accumulation 

 seen on the south side of the Sgurr, with which it was correlated. 

 My own interpretation has been set forth elsewhere. Concerning 

 the thick dolerite sheet seen just south of this spot, I have no 

 new observation to record. I have not visited the locality since 

 Dr. Peach and I saw, or thought that we saw, the dolerite turning 

 abruptly upward on encountering the agglomerate mass. The form 



