Yol. 59.] TORRIDONIAN, ETC. OP THE ISLE OF RUM. 189 



17. The Overthrust Torridonian Rocks of the Isle of Rum, and 

 the Associated Gneisses. By Alfred Harker, Esq., M.A., 

 F.R.S., F.G.S., Fellow of St. John's College, Cambridge; 

 Geological Survey of Scotland. 1 (Read March 11th, 1903.) 



[Plate XIV— Map.] 



Contents. 



Page 



I. Introduction : The Relatively-Unmoved Tract 189 



II. The Monadh-Dubh Overthrust 193 



III. The Overthrust Belt of the Mountain -Border 196 



IV. The Geological Relations of the Gneisses 201 



V. The Composition and Origin of the Gneisses 207 



VI. Summary of Conclusions 213 



I. Introduction : The Relatively- Unmoved Tract. 



The geological literature of the Isle of Rum is not extensive. The 

 earliest important contribution is found in Macculloch's ' Western 

 Islands of Scotland,' and illustrates the close observation and acute 

 reasoning which characterize that remarkable work. 2 In his small 

 sketch-map Macculloeh shows the northern part of the island and a 

 strip along the eastern coast occupied by red sandstone, which he 

 correctly identifies with that of Skye [Torridonian]. The rest of 

 the island he divides among the igneous rocks, of which he distin- 

 guishes syenite [granite and granophyre], c augit-rock ' [peridotite, 

 gabbro, etc.], and basalt and amygdaloid. In spite of the extremely 

 crude topography of the map, the distribution of these rocks is roughly 

 indicated, and some of the leading facts concerning their relations 

 are set forth in the text. Since Macculloch's time no systematic 

 account of the geology of Rum has been published. The igneous 

 rocks, now recognized as of Tertiary age, have received some notice, 

 and a valuable description of the peridotites in particular has been 

 given by Prof. Judd. 3 Numerous references to Rum appear in the 

 writings of Sir Archibald Geikie ; and in ' The Ancient Volcanoes 

 of Great Britain ' he has given an account not only of the igneous 

 rocks, but also of some of the highly disturbed strata, with associated 

 gneisses, to be described below. 4 The present communication is the 

 outcome of the detailed mapping of the island carried out by the 

 writer for the Geological Survey of Scotland. 



1 Communicated by permission of the Director of H.M. Geological Survey. 



2 John Macculloeh, ' A Description of the Western Islands of Scotland ' . . . 

 vol. i (1819) pp. 473-506 ; also map facing p. 71 of Atlas, and section on pi. xix, 

 fig. 5. The direction of the section in that figure is incorrectly stated ; the 

 letters 8. and N. should be N.W. and S.E. 



3 Quart. Journ. Geol. Soc. vol. xli (1885) pp. 354-416. 



4 'Ancient Volcanoes of Great Britain' vol. ii (1897) pp. 349-55. 



