94 DR G'EIKIE ON THE HISTORY OF VOLCANIC ACTION 



altogether. Yet it is not less than about 800 feet thick, and consists of bedded lavas, 

 which alternate with and follow continuously and conformably upon the top of the 

 ordinary plateau-basalts. I have described these rocks as dull, finely crystalline or compact, 

 light-grey in colour, and weathering with a characteristic platy form, which has been 

 mistaken for the bedding of tuffs. The fissility, however, has none of the regularity or 

 parallelism of true bedding, and may be observed to run sometimes parallel with the bed- 

 ding of the sheets, sometimes obliquely or even at right angles to it. Even where this 

 structure is best developed, the truly crystalline nature of the rocks can readily be 

 detected. Some of them are porphyritic and amygdaloidal, the very topmost bed of the 

 mountain being a coarse amygdaloid. Intercalated with these curious rocks there are others 

 in which the ordinary characters of the dolerites and basalts of the plateaux can be 

 recognised. The amygdaloids are often full of delicate prisms of epidote. 



In Mull, as in the other areas of terraced basalts, we everywhere meet with gently 

 inclined sheets, which do not thicken out individually or collectively in any given direction, 

 except as the result of unequal denudation. So far as I have been able to discover, they 

 afford no evidence of any great volcanic cone from which they proceeded. Their present 

 inclinations are unquestionably due, as in Ireland, to movements subsequent to the 

 formation of the plateau. In Loch-na-keal they dip gently to the E.N.E. ; in Ulva and 

 the north-west coast to N.N.E. ; near Salen to W.S.W. on the one side, and N.W. on the 

 other. Round the southern and eastern margins of the mountainous tract of the island, 

 they dip generally inwards to the high grounds. 



The Mull plateau presents a striking contrast to that of Antrim, in the extraordinary 

 extent to which it has been disrupted by later protrusions of massive basic and acid 

 rocks over a rudely circular area, extending from the head of Loch Scridain to the Sound 

 of Mull, and from Loch-na-keal to Loch Buy. The bedded basalts have been invaded 

 by masses of dolerite, gabbro, and granophyre, with various allied kinds of rock. They 

 have not only been disturbed in their continuity, but have undergone considerable 

 metamorphism. 



Again, further to the north, in the promontory of Ardnamurchan, the plateau has 

 been disrupted in a similar way, and only a few recognisable fragments of it have been 

 left. These changes will be more appropriately discussed in connection with similar 

 phenomena in the other plateaux further north. 



3. Small Isles. — This plateau, the smallest and most discontinuous of the four, 

 includes the islands of Eigg, Rum, Canna, and Muck, which form the parish of Small 

 Isles. That the fragments of the bedded volcanic masses, preserved on each of these 

 islands, were once connected can hardly be doubted. Indeed, as already stated, they 

 were not improbably united with the plateau of Skye on the north, and with that of 

 Ardnamurchan and Mull on the south. Taking the whole space of land and sea within 

 which the basalt of Small Isles is now confined, we may compute it at not much less than 

 200 square miles. In Eigg, Muck, and Canna, the basalts retain their almost horizontal 

 position, and from underneath them emerge the Jurassic strata on which they lie. The 



