﻿1851.] 



DUKE OF ARGYLL ON ARDTUN LEAF-BEDS. 



99 



very different kind ; sheets of lava of great solidity and thickness 

 were now poured forth upon the ground, and if surfaces completely 

 vitrified, such as well-marked obsidian, be any indication of subaerial 

 exposure, such must have been the condition of this lava. The confi- 

 guration of the country no longer remained the same, and so complete 

 was the change effected by this and subsequent convulsions, that the 

 spot which had so long been the receptacle of calm stagnant waters 

 under the lee of some great forest, became as we now see it, cut into 

 the sea-cliff of a naked headland, so peculiarly exposed to the surf of 

 a stormy ocean, as well to deserve the description of its Gaelic name, 

 "the Point of Waves." 



It is true, that no evidence remains in the form of visible craters 

 to mark the site of volcanos to which the traps of the Hebrides may 

 owe their origin, and to prove their subaerial character. Such indeed 

 may not have been the character of all of them during the immense 

 periods of time in which their activities were exerted. But in the 

 particular case of the beds here described, with other evidences so 

 strongly marked, I cannot feel that the absence of this particular proof 

 stands much in the way. Of nothing, perhaps, does the whole 

 geology of the Hebrides present more conclusive demonstration than 

 of the enormous changes in the relative position of sea and land, 

 which have been effected since the latest period of volcanic activity of 

 which any evidence remains to us. The position of the leaf-beds in 

 the cliffs of Ardtun is one example. A great portion of the later 

 tertiary period, as well as the whole of that period of submergence 

 to which the Drift is referable, lie between existing times and that to 

 which the sealing up of these beds may be referred. The Ross of Mull, 

 like all the rest of Scotland, presents banks of sea-worn gravel far 

 above the level of the highest Ardtun basalt, and its rocks and 

 boulders are deeply marked by those remarkable abrasions, which, 

 whatever be the particular material which caused them, are apparently 

 due to the action of something impelled by powerful currents. There 

 is some evidence, that the sheet of basalt which caps the Ardtun beds 

 was by no means the last or highest which once occupied the same 

 area. From out of the mosses, now covering its surface, tablets 

 of similar material are seen elevated here and there, with broken 

 joints strewn about at the foot of their little escarpments. They 

 have all the appearance of having formed part of a sheet which over- 

 lay the other, and of which these isolated portions are the only rem- 

 nants (see Fig, 2, p. 92). Unless therefore the original craters of erup- 

 tion had been above the highest level accessible to such changes, it is 

 not surprising that no vestige should remain of cones of scoriae, or other 

 accumulations of loose materials. Nor, when they had been once 

 given up to the sea, could we reasonably expect much evidence of this 

 former existence. The ocean cannot often be successfully called to 

 account for such acquisitions ; it is, however, a curious fact, that on 

 the shore of the island of Tyree, opposite the basalts of Mull, at no 

 great distance above the reach of the present tides, balls of pumice 

 have been found in considerable abundance. These are of course sea- 

 borne, and, although there is no proof of their having been Hebridean 



