646 SIR A. GEIKIE AND ME. J. J. H. TEALL ON THE [Nov. 1 894, 



remarkable arrangement of the component minerals in separate 

 layers, which in their alternation and occasional puckerings recall 

 in a striking way the characters of many ancient gneisses. 1 



We propose on the present occasion to offer a more detailed 

 account than has yet been given of this banded structure among the 

 Tertiary gabbros. The investigation appears to us to have a two- 

 fold interest. In the first place, the structure in question occurs 

 among the deep-seated bosses of the latest volcanic series in Britain. 

 These bosses have undergone no deformation or sensible disturbance 

 since their production. The structures which they present may 

 therefore be accepted as belonging to the original conditions of 

 igneous eruption, and hence a careful study of the nature of this 

 remarkable banded separation of their component minerals may 

 throw some light on the processes concerned in the protrusion and 

 consolidation of igneous magmas. 



In the second place, this structure is so closely parallel to that 

 found in some of our oldest gneisses that there can be little doubt 

 that if its nature and origin can be explained, a distinct step will be 

 gained in the interpretation of the history of these ancient rocks. 



We shall restrict our description to one locality in Skye- — the 

 rugged ridge which strikes from the southern side of Harta Corry 

 and extends between Strath na Creitheach and Coire Riabhach to 

 Loch an Athain. This ridge has no name on the Ordnance six-inch 

 map, and we have been unable to ascertain how it is distinguished 

 by the inhabitants of the district. But the south-eastern part, which 

 is crossed by the tourist footpath to Loch Coruisk, is marked ' Druim 

 an Eidhne ' on the map, and to avoid periphrasis we shall include 

 under this appellation the whole ridge up to the crest overlooking 

 Harta Corry. 



I. General Arrangement and External Characters 

 of the Rocks. 



The ridge which we now describe forms a part of the gabbro 

 mass of the Cuillin Hills. This great region of basic rocks, even if 

 we exclude Blaven and the eastern offshoots, covers an area of some 

 30 square miles. But as the gabbro can be traced continuously 

 into Blaven, save for the brief interruption of the alluvium of Strath 

 na Creitheach, the total area of this rock is probably not less than 

 40 square miles. It has been invaded by the granophyre of the 

 Bed Hills, which, sending a broad tongue into it as far south as 

 Loch an Athain, separates the Blaven hills from the rest of the 

 Cuillin group. Druim an Eidhne lies on the south-western side of this 

 invading tongue of acid rock. But although in one sense the rocks 

 of this ridge lie at the very edge of the gabbro mass, yet, if we 

 disregard the presence of the younger granophyre, we see that they 

 really belong to the central portion of the gabbro. 2 Hence, in 



1 A. Geikie, op. cit. p. 131 ; Eep. British Assoc. (Nottingham) 1893, pp. 754- 

 755. 



2 The evidence for the younger age of the granophyre has been fully 

 given from the same locality in this Journal. May 1894, vol. 1. pp. 212-231. 



