1857.] GEIKIE SKYE. 15 



of Arran ; while at the other it in no way disturbed either their dip 

 or their texture, save along the immediate line of contact, but in- 

 sinuated itself between their planes, and conformed to every in- 

 equality of the floor over which it rolled. There is usually, more- 

 over, a marked difference between the mineralogical texture and even 

 the composition of the two rocks. The disrupting class are coarse- 

 grained, and of a brownish-yellow tint ; the overlying and intrusive 

 are of a finer texture and lighter shade, and generally considerably 

 more felspathic. There are indeed exceptions to this rule, as in 

 Beinn na Charn, where the rock approaches more closely to the 

 texture of the northern hills ; but the distinction is in most cases 

 sufficiently obvious. It seems impossible, therefore, to avoid the con- 

 clusion that the one series of outbursts must be older than the other. 



But there is another source of evidence, of a somewhat negative 

 kind, which not only corroborates this inference, but indicates, as 

 far as merely negative evidence can do, to which of the two classes 

 the higher antiquity should be assigned. For, as in no observed in- 

 stance do they intersect each other, it is obvious that they do not 

 of themselves furnish material for a determination of their relative 

 ages. Yet I believe an answer to the question may be gathered 

 from a survey of the other igneous rocks of the district ; and to 

 these I shall now refer. 



Trap-dykes. — Certainly one of the most remarkable features in 

 the geology of Strath is the almost incredible number and variety of 

 its trap- dykes. From their greater permanence they are visible at 

 a considerable distance, now coursing up the hill-sides like ruined 

 walls, now plunging deep amid the heather of the glens, now dam- 

 ming up the channel of some mountain-torrent that pours over them 

 its white cascades. They preserve a general north-westerly strike, 

 but frequently intersect each other or unite, being seldom continuous 

 for long distances. These features are well displayed along the 

 eastern shore of Loch Slapin, where for upwards of two miles the 

 waves have cut a vertical line of cliff. Along this cliff, and on the 

 beach below, the dykes may be seen running parallel, uniting, again 

 bifurcating, interlacing in labyrinthine confusion, now thickening, now 

 thinning, terminating abruptly and commencing again, entangling 

 masses of limestone, and sending out minor veins ; at one time roll- 

 ing in an undulating course, wholly irrespective of the dip or strike 

 of the beds, at another running along the line of the natural joints ; 

 now seeming to conform to the planes of bedding, now cutting- 

 through them like walls of masonry, and at length ending off in a 

 point sometimes well nigh as fine as that of a pen. In truth there 

 are few localities where the nature of trap-dykes could be better 

 studied than along this coast-line, for there is both an admirable 

 ground-plan of them along the beach, and, for a considerable part of 

 the shore, they are exposed in section along the cliffs. (PI. I. 

 fig. 9, and MS. Sketches Nos. 8 and 9, illustrate the trap-dykes on 

 the shore of Loch Slapin.) 



The eastern margin of the island also exhibits the trap-dykes in 



