90 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



and, stretching across Loch Sunart, includes the western part of the peninsula of Ardna- 

 murchan. That it formerly extended far beyond its present limits is impressively 

 indicated by its margin of cliff's and fringe of scattered islands and outliers. It went 

 west, at least, as far as the Treshnish Isles, which are composed of basalt. On its eastern 

 border, a capping of basalt on the top of Ben Iadain (1873 feet) in Morven, and others 

 further north, prove that its volcanic sheets once spread far into the interior of Argyle- 

 shire (fig. 20). On the south, its fine range of lofty cliffs, with their horizontal bars of 

 basalt, bear witness to the diminution which it has undergone on that side ; while, on 

 the north, similar sea-walls tell the same tale. Not only has it suffered by waste along its 

 margin, it has also been deeply trenched by the excavation of glens and arms of the sea. 

 The Sound of Mull cuts it in two, and the mainland portion is further bisected by Loch 

 Sunart, and again by Loch Aline. The island of Mull is so penetrated by sea-lochs that 

 a comparatively slight depression would turn it into a group of islands. But, besides 

 its enormous denudation, the Mull plateau has been subjected to great disruption from 



Fig. 20. — Basalt-Capping on top of Ben Iadain, Morven. 



subterranean movements. In the southern portion of the island, it has been broken up 

 by the intrusion of large bosses and sheets of gabbro, and by masses as well as innumer- 

 able veins of various granitoid and felsitic rocks. In Ardnamurchan, it has suffered so 

 much disturbance from the sama cause that its original structure has been almost 

 obliterated over a considerable area. Moreover, it has been dislocated by many faults, 

 by which different portions have been greatly shifted in level. The most important of 

 these breaks is one noticed by Professor Judd, and visible to every tourist who sails up 

 the Sound of Mull. It traverses the cliffs on the Morven side, opposite Craignure, 

 bringing the basalts against the crystalline schists, and strikes thence inland, wheeling 

 round into the long valley in which Lochs Arienas and Teacus lie. On its western side, 

 the base of the basalt-series is almost at the sea-level ; on its eastern side, that platform 

 rises high into the outliers of Beinn na h-Uamha (1521 feet) and Beinn Iadain. The 

 amount of displacement here is probably not much less than 1500 feet. Many other 



