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 is probably more than 1000 feet. Many other minor faults in the same district show how much the crust of the earth has been fractured here since older Tertiary time. [Illustration: Fig. 266.--Basalt-capping on top of Beinn Iadain, Morven. The hummocky ground to the right consists of the Highland schists against which the basalts are brought by lines of dislocation.[240]] [Footnote 240: There are no fewer than three faults in the basalt-capping on the summit of Beinn Iadain. By bringing the basalts and schists into juxtaposition, they have given rise to topographical features that can be seen even from a distance.] A little to the west of Mull, and belonging originally to the same plateau, lies the isle of Staffa, the famous columnar basalts of which first attracted the attention of travellers, and gave to the Tertiary volcanic rocks of Scotland their celebrity (Fig. 266_a_). [Illustration: Fig. 266_a_.--View of the south side of Staffa, showing the bedded and columnar structure of the basalt. The rock in which the cave to the left hand has been eroded is a conglomeratic tuff underlying the basalt; to the right is Fingal's Cave. These caverns bear witness to the enormous erosive power of the Atlantic breakers. ] In spite of the extent to which it has suffered from denudation and subterranean disturbance, and indeed in consequence thereof, the Mull plateau presents clear sections of many features in the history of the basalt-outflows and of the subsequent phases of Tertiary volcanic action which cannot be seen in the more regular and continuous tableland of Antrim. Moreover, it still possesses in its highest mountain, Ben More (3169 feet), a greater thickness, and probably a higher series, of lavas than can now be seen in any other of the plateaux.