DURING THE TERTIARY PERIOD IN" THE BRITISH ISLES. 91 



minor faults in the same district show how much the crust of the earth has been fractured 

 here since older Tertiary time.* 



Nevertheless, in spite of the extent to which the Mull plateau has suffered from 

 denudation and subterranean disturbance, and indeed in consequence thereof, this 

 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 a higher series of lavas than 

 can now be seen in any other of the plateaux. 



It will be readily understood that, in the case of this plateau, the difficulties already 

 referred to in regard to that of Antrim, of tracing the probable form of ground on which 

 the volcanic eruptions began, are considerably increased. We can dimly perceive that 

 the depression in the crystalline rocks of the Highlands which had, from at least the older 

 part of the Jurassic period, stretched in a N.N. W. direction along what is now the western 

 margin of Argyleshire, lay beneath the sea in Jurassic time, and was then more or less 

 filled up with sedimentary deposits. The hollow appears thereafter to have become 

 a land-valley, whence much of the Jurassic strata was cleared out by denudation before 

 its subsequent submergence under the sea in which the upper Cretaceous deposits 

 accumulated. Professor Judd has shown how relics of these Cretaceous strata appear on 

 both sides of the plateau from under the protecting cover of basalt-sheets. But, before 

 the volcanic eruptions began, the area had once again been raised into land, and the 

 youngest Secondary formations had been extensively eroded. 



In their general aspect the basalts of Mull agree with those of Antrim, and the 

 circumstances under which they were erupted were no doubt essentially the same. But 

 considerable differences in detail are observable between the succession of rocks in the 

 two areas. The total depth of basalt-sheets in Mull is greater than in any other of the 

 plateaux. When I first visited the island in 1866, the only available maps, with any 

 pretensions to accuracy, were the Admiralty charts ; but, as these do not give the interior 

 except in a generalised way, it was difficult to plot sections from them, and to arrive at 

 satisfactory conclusions as to the thickness of different groups of rock. Accordingly, as 

 the successive nearly flat flows of basalt can be traced from the sea-level up to the top of 

 Ben More, I contented nryself with the fact that the total depth of lava-beds in Mull was 

 at least equal to the height of that mountain, or 3169 feet. The publication of the 

 Ordnance Survey Maps now enables us to make a nearer approximation to the truth. 

 From the western base of the magnificent headland of Gribon, the basalts in almost 

 horizontal beds rise in one vast sweep of precipice and terraced slope to a height of over 

 1600 feet, and then stretch eastwards to pass under the higher part of Ben More, at 

 a distance of some 8 miles. They have a slight easterly inclination, so that the basement 



* There are no fewer than three faults in the basalt-capping on the summit of Ben Iadain. By bringing the 

 basalts and schists into juxtaposition, they have given rise to topographical features that can be seen even from a 

 distance. See fig. 20. 



