TRANSACTIONS OF THK SECTIONS. 83 



to make in 1851 , of the leaf-beds of Ardtiui, it has become clearly ascertained that 

 these islr.mls are the remains of volcanoes of that geological age to which an ever- 

 increasing interest seems to attach — that middle age of the great Tertiary division 

 of geological time to which Lj-ell g.ave the name of Miocene. The mountains of 

 Mull, and of Eigg, and of Rona, and of Skj'e, with all their valleys and intricate 

 lines of coast, have unquestionably an origin later than the Miocene — how much 

 later, is the question of j)liysical geography which geologists are called upon to solve. 



It is possible, indeed, to suppose that the hills of the mainland might be of a very 

 different age from those of the adjacent islands ; and against this, until some two 

 yeai's ago, there would have been nothing to advance except the suspicious 

 similarity and adjustment between the two gToups, the coincidence of their out- 

 lines, and of the way in which they have been cut and carried. But the admii-able 

 researches of Mr. Judd, in 1(S74, have brought one little fact to light which speaks 

 volumes for the enormous changes which must have taken place since the volcanoes 

 of the Miocene over a portion at least of the Highland area, and which may, 

 therefore, have taken place over the whole of it. The land upon which the 

 Miocene vegetation flourished, and upon which the lava-streams of its volcanoes 

 were poured out, seems to have been for the most part a land consisting of Cre- 

 taceous and Secondary rocks. The fragments of that country which remain are 

 generally consistent with the supposition that they were deposited in a sea which 

 washed round the bases of the Highland mountains, but which never covered 

 them. Like the fragments of the Old Red Sandstone, the remains of the Secondary 

 rocks lie along the margins and fringes of the Sikn-ian hills. But Mr. Judd has 

 made the startling discovery of an outlier of the whole series of the Secondary 

 rocks, including representative beds of the Trias, Lias, Greensand, and Chalk, to- 

 gether with deposits, probably Lacustrine, all lying on the top of one of the moun- 

 tains of metamorphic gneiss which constitute the district of Morven. This fragment 

 has been preserved by having been covered by a sheet of lava from some great neigh- 

 bouring volcanic centre, the position of which is probably indicated by Ben More 

 in MuU. But the mass of volcanic trap which has covered up and preserved this 

 relic of the Cretaceous land is itself a fragment occupying the top of a mountain of 

 gneiss, separated from the remainder of the sheet of lava to which it belongs by 

 deep valleys, precisely similar to those which di\ ide the hills from each other 

 throughout the whole area of the Highlands. This position of an outlier of the 

 Cretaceous rocks on the summit of a mountain of gneiss is rendered still more 

 curious by the circumstance that in that position the beds are not tilted or in any 

 way apparently distm-bed. They are arranged horizontally, as if the ocean floor in 

 which they were deposited had occupied that level, or as if its deposits had been 

 lifted up over so large an area that any small section of that area could retain its 

 original horizontality. The Lower Silurian gneiss beds on which these Secondary 

 deposits have been laid are violently twisted and contorted ; and this structure 

 must have belonged to them when they constituted the floor of the Cretaceous 

 sea. The position of the Miocene basalts capping the Secondary deposits proves 

 that the whole mormtain, as a mountain, is of later date than the Miocene age — 

 how much later we cannot tell ; and thus that the causes of geological change 

 which have cut up the coimtry into its present form, though they doiibtless began 

 in very remote epochs, have at least been prolonged into a comparatively late age 

 in the history of the globe. 



It would, I think, be affectation to pretend that our science enables us to follow, 

 with any thing like distinctness of conception, the exact nature and sequence of 

 operations which through such a vast lapse of time have brought about the final 

 result. But I believe in something like the following general outline of events. 



Mrst. That subsequent not only to the consolidation, but probably also to the 

 metamorphism of the Lower Silurian deposits, the whole area of the Western- 

 Central Highlands became an area of that kind of disturbance which arose from 

 lateral pressm-e due to secular cooling and consequent conti-action and subsidence 

 of the crust of the earth. 



Second. That the crumpling, contortion, and tilting of the Silurian beds which 

 we now see arose fi'om that disturbance. 



Third. That then were determined those great general lines of strike running 



9* 



