218 SIR ARCHIBALD GEIKIE ON THE BASIC AND [May 1 894, 



Thus the important fact is established that the ridge of which 

 Druim an Eidhne forms part consists of a varied group of volcanic 

 rocks belonging, not to a single eruption, but to a succession of 

 eruptions. All these various constituents of the ridge successively 

 abut against the edge of the granophyre. Each of them in turn is 

 abruptly truncated along the line of contact with the acid material. 

 This truncation is made particularly distinct by the way in which 

 the parallel structure of the banded gabbros, which so much 

 resembles bedding, is sharply cut off. The ends of the beds and 

 laminae disappear as suddenly as the ends of a series of shales 

 traversed by a basalt-dyke (fig. 1, p. 222). 



It is inconceivable that this truncation could ever have taken 

 place had the gabbro been an intrusive mass erupted through the 

 granophyre. It can only be explained in one or other of two 

 ways : either the granophyre has been faulted against the complex 

 basic assemblage of the ridge, or has broken across it as an eruptive 

 protrusion. The supposition of a fault is at once negatived by the 

 most cursory examination of the ground, as will be seen from the 

 evidence which I now proceed to state. 



III. The Granophyre Bosses. 



There can be no doubt that though, on the whole, simpler in 

 structure and more uniform in petrography than the gabbros, the 

 acid bosses are more complex than their external conical forms 

 would lead us to suspect. Even within the limits of a single con- 

 tinuous area, like that of the lied Hills of Skye, indications may be 

 found of successive protrusions of material, not always of precisely 

 the same lithological character. But until the structure of the 

 ground has been traced upon large-scale maps no satisfactory account 

 of these details can be given. 



The granophyre mass which abuts against the great ridge of basic 

 rocks above described may be taken as fairly typical of the Tertiary 

 acid protrusions of the Western Isles. It rises northward into the 

 prominent height of Meall Dearg (the red rounded hill), which from 

 the north seems to close in the upper end of Glen Sligachan, and 

 owing to its bright reddish-yellow colour offers a striking contrast to 

 the dark gabbro crags that rise behind it and on either side. The 

 line of junction between the acid and basic rocks, so far as it can 

 be observed, is vertical. This is best seen in the ravine which 

 descends from the western shoulder of Meall Dearg into the mouth 

 of Harta Corry. To the south of Meall Dearg the basic rocks rise 

 as a low, rugged, sometimes mural, crag from the summit of the 

 long talus of broken-up granophyre, and seen from a distance 

 present the deceptive appearance of an overlying cake. 1 A little 

 careful search among the debris and projecting knobs of rock along 



1 It was, I have little doubt, such sections as this that gave the impression 

 that the granitoid rocks underlie and are older than the gabbros. But if, 

 instead of looking at the rough mountains from a distanee, a geologist will 

 climb their sides, he can easily satisfy himself as to the true nature of the 

 junction-line between the two rocks. 



