48 MK. A. HAEKER ON THE GEOLOGICAL [Feb. 1906, 



exposition of the view which regards the pitehstone of the Sgurr of 

 Eigg as a lava-flow, or succession of flows, poured out over an eroded 

 laud-surface of the basalts. This may be taken as the generally 

 accepted interpretation for the last forty years. It has been 

 endorsed by Prof. Judd, 1 Prof. Heddle, 2 and others ; but it does not 

 appear to have been critically tested on the ground. 



In 1892 the late Prof. Heddle 3 made an interesting contrib- 

 ution to the subject, recognizing that a porphyritic pitehstone 

 identical with that of the Sgurr of Eigg makes the isolated cluster 

 of low rocky islets named Oigh-sgeir (often written Hyskeir) ; an 

 observation subsequently confirmed by Sir Archibald Geikie. 4 

 Oigh-sgeir, rising only 34 feet above sea-level, lies between 18 and 

 19 miles from Bidein Boidheach, the seaward termination of the 

 ridge of the Sgurr, and bears about W. 15° N., agreeing with the 

 general trend of the pitchstone-ridge of Eigg. If, as Heddle 

 supposed, Oigh-sgeir and the Sgurr of Eigg are relics of one and the 

 same lava-stream, we must infer that the two islands formed, at 

 the epoch of the pitehstone, parts of one extensive land-surface. 

 Assuming that the nearly submerged pitehstone of Oigh-sgeir has a 

 thickness comparable with that seen on Eigg, Sir Archibald calculates 

 that the base declines westward at the rate of about 35 feet per 

 mile. 



When, in the course of the geological survey of the Inner 

 Hebrides, the present writer came, in 1903, to map the southern 

 part of Eigg, the work was begun on the received hypothesis that 

 the pitehstone of the Sgurr represents the remains of a lava-current 

 filling an old Tertiary river-valley. The detailed examination of 

 the ground, however, discovered circumstances which were difficult 

 to reconcile with that hypothesis, and eventually led to a reconsid- 

 eration of the whole question. The results of my observations, 

 though not sufficient to decide some of the minor questions which 

 arise, go to support the view that the pitchstone-sheet is intrusive 

 in the basalt-group. I have twice revisited Eigg during the present 

 year (1905), and on one visit had the advantage of accompanying 

 Dr. Peach, who brought to light other facts tending to the same 

 conclusion. I am indebted to him for useful criticism, as well 

 as for positive additions to the evidence bearing on the question. 

 I desire also to express my thanks to Mr. R. L. Thomson, the 

 proprietor of the island, who has interested himself in the enquiry, 

 and whose kindness has made the work easy and pleasant. 



The first point relates to the general form of the pitchstone- 

 complex as a whole. The elongated and curving trend of the ridge, 



i Quart. Journ. Geol. Soc. vol. xxx (1874) p. 267. 



2 Proc. Roy. Soc. Edin. vol. xi (1882) pp. 632-33 ; also (Appendix C) 

 ' A Vertebrate Fauna of Argyll & the Inner Hebrides ' by J. A. Harvie-Brown 

 & T. E. Buckley, 1892, pp. 247-50. 



3 Op. supra cit. p. 249. 



4 Eep. Brit. Assoc, for 1894 (Oxford) pp. 652-73 ; and Quart. Journ. Geol. 

 Soc. vol. lii (1896) pp. 371-73. 



