176 



DR' GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



They appear not so much at the actual margins of the bosses (though they occur there, 

 as has already been described), as in that tract of altered basalt, with intrusive sheets and 

 dykes of basalt, dolerite, and gabbro, which lies within the great ring of heights between 

 Loch-na-Keal and Loch Spelve. In some areas the amount of injected material appears 

 to equal the mass of basic rock into which it has been thrust. Pale grey and yellowish 

 porphyries and granophyres, varying from thick dykes down to the merest threads, 

 ramify in an intricate network through the dark rocks of the hills, as shown in the 

 accompanying illustration (fig. 60), which represents a portion of the hill-side between 

 Beinn Fhada and the Clachaig River. Such a profusion of veins probably indicates the 

 existence here of some large mass of granophyre, or of granite at no great depth beneath 

 the surface. 



There are two horizons on which, as I have already had occasion to point out, 

 protrusions of acid materials have been specially abundant. One of these is the base of 



Fig. 60. — Section of Intruded Veins of various Acid Roiks (bb), in Basalt, Dolerite, &c. (aa), above River Clachaig, Mull. 



the bedded basalts of the plateau ; the other is the bottom of the thick sheets of gabbro. 

 Dykes and veins of granophyre, quartz-porphyry, felsite, and other allied rocks are 

 sometimes crowded together along these two horizons, though they may be infrequent 

 above or below them. But examples also occur of solitary veins in the midst of the 

 unaltered plateau-basalts, at some distance from the nearest visible body of acid eruptive 

 rock. Some of the most remarkable instances of this kind are to be seen among the 

 basalts that form the terraced slopes on the north side of Loch Sligachan. Several thick 

 dykes of granophyre run up the declivity, cutting across hundreds of feet of the nearly 

 level basalt beds. Some of them can be seen on the shore passing under the sea. They 

 trend in a S.S.E. direction towards Glamaig, and they are not improbably apophyses 

 from that huge boss. Another example may be cited from the basalt-outlier of Strath- 

 aird, where two veins of felsite, one of them a pale flinty rock showing flow-structure 



