668 J. W. JTTDD ON THE SECONDARY ROCKS OF SCOTLAND. 



first instance in the midst of strata of Secondary age, and that its 

 lavas almost everywhere rest on deposits of the same age. The 

 manner in which the central subsidence, which has been so very 

 marked in the case of the volcano of Mull, has operated in the more 

 perfect preservation of its lava plateaux from denudation than is the 

 case in the contemporaneous volcanos we have described on a 

 former occasion ; and the underlying Mesozoic rocks have, of course, 

 also been saved from destruction with their protecting capping of 

 lavas, though at many points they have been by the same action 

 depressed beneath the sea-level and thus withdrawn from the ken 

 of the geologist. 



The volcanos of Rum and Ardnamurchan were of smaller dimen- 

 sions than those of Skye and Mull, and they have, in consequence, 

 suffered more from denudation. On the other hand, wherever the 

 pleateaux of protecting lavas have been removed, as is the case in 

 the greater part of the district of Sleat in Skye, and in the northern 

 part of Rum, there all traces of the Secondary rocks have disappeared 

 also. It is when viewed in this relation that such minute outliers 

 of the volcanic rocks, preserving between themselves and the under- 

 lying gneissic rocks slices of Secondary strata, are of such especial 

 interest. Such examples occur at Ru-Geur, in Skye, and in the 

 remarkable mountains of Morvern, to be described hereafter. 



From an examination of all the circumstances of the case, it 

 appears to me to be impossible to avoid the conviction that these 

 patches of Secondary strata, although now so minute in dimensions 

 and isolated in position, once formed portions of a great series of 

 connected deposits which covered the greater part of the vast area 

 we have indicated, and attained in places a thickness of from 4000 

 to 5000 feet. 



But an attentive consideration of the facts of the case will, I 

 believe, compel us to go a step further in the same direction. We 

 find, 80 miles to the southward of the exposures of the Secondary 

 strata in Mull, and exactly at the point where there exists a recur- 

 rence of those conditions which have determined the preservation of 

 the Mesozoic rocks in the Western Isles, an almost precise repetition 

 of the familiar phenomena presented by the Scotch deposits in ques- 

 tion. In the island of Rathlin and the north-east of Ireland we 

 meet — just as in the Hebrides and Western Highlands — with a series 

 of Carboniferous, Poikilitic, Jurassic, and Cretaceous strata inter- 

 posed between the Palaeozoic schists and the Tertiary basalts. And, 

 as we shall show hereafter, the relations between the Scotch and 

 Irish Secondary deposits are of the most intimate and unmistak- 

 able character. Can we then avoid the conclusion that the absence 

 of traces of the Secondary strata in the intervening area is not to 

 be accounted for by the inference that they were never deposited 

 in that district, but that it is to be set down entirely to the ab- 

 sence of those peculiar conditions by which alone, as we have seen, 

 any portions of the strata of that age could have escaped destruc- 

 tion during the great ordeal of denudation to which the district has 

 been subjected ? 



