E. B. Bailey— The Sgitrr of Eigg. 305 



Thk Form of the Pitchstone. 

 Dr. Harker writes [3, p. 48] — 



"The elongated and curving trend of the ridge, with its projecting arms, 

 seems at first to accord with the supposition that the pitchstone occupies an 

 old valley with smaller tributary glens ; and it is also very noticeable that, at 

 the base of the escarpment, the lower surface of the pitchstone has in most 

 places an inward slope." 



He then proceeds to point out difficulties in the way of accepting 

 this hypothesis, which, of course, is the one proposed by Geikie. I have 

 not examined all the evidence Harker cites in this connexion, but 

 I did visit the locality to which apparently he attaches most weight. 

 Here a spur of pitchstone running N.N.E. and S.S. W. joins the main 

 ridge near its western termination. Harker's contention is that if 

 this spur is the cast of a tributary valley, then the latter can be 

 shown to have drained uphill at some parts of its course. All that is 

 clear, it seems to me, is that the edge of the pitchstone in this spur 

 does undulate quite sharply in places, just as a road running parallel 

 with a stream constantly rises and falls. For my own part, I am 

 unable to assert the existence of a regular thalweg buried beneath the 

 pitclistone here, but I find no difficulty in imagining that such exists. 

 At tlie end of the spur the base of the pitchstone, as seen in cross- 

 section, is considerably higher than at the termination of the main 

 ridge in the neighbouring sea-cliff. 



Conclusion. 



A careful re-examination of the evidence has led me to accept, 

 Geikie's interpretation of the Sgurr of Eigg pitchstone as a lava-stream 

 moulded upon an ancient river- valley. 



With Harker, liowever, I believe that the apparent stratification of 

 tlie mass is due to injection, and not, as Geikie thought, to the piling 

 of lava-flow upon lava-flow. Certain other criticisms advanced by 

 Harker I have been unable to adopt. 



BIBLIOGRAPHY. 



1. A. Geieie, Scenery of Scotland, 1st ed., 1865, pp. 278-82. For details 

 see [2]. 



2. A. Geikie, "On the Tertiary Volcanic Eocks of the British Islands" : 

 Quart. Journ. Geol. Soc, vol. xxvii, p. 279, 1871. 



3. A. Harkek, " The Geological Structure of the Sgurr of Eigg " : Quart. 

 Journ. Geol. Soc, vol. Ixii, p. 40, 1906. 



4. A. Harki<:r, in Sheet 60 of the Geol. Survey 1 in. Map of Scotland, and 

 in the accompanying memoir, Tlie Geology of the Small Isles of Inverness-shire , 

 1908. For details see [3]. 



5. Tempest Anderson, "The Volcano of Matavanu in Savii " : Quart. 

 Journ. Geol. Soc, vol. Ixvi, p. 621, 1910. 



EXPLANATION OF PLATE XXII. 



Fig. 1. Pale felsite or porphyry injections (F of Text-fig. 2, p. 300) cutting 

 darker pitchstone of Sgiirr of Eigg ridge. Photo by A. Stenhouse. 



Fig. 2. Basal breccia of Sgurr of Eigg pitchstone (a little above the top layer 

 lettered D in Text-fig. 2, p. 300). x 14-5 diam. Slice No. 17181. The 

 fragments with network structure are basalt, the rest pitchstone and felspar 

 derived from same. Microphoto by J. Khodes, jun. 



decade vi.— vol. i.— no. vii. 20 



