DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 75 



extended over the whole of this strip of country, they covered a space of some 7000 or 

 8000 square miles. But they were not confined to the area of the British Islands. 

 Similar rocks rise into plateaux in the Faroe Islands, and it may reasonably be conjectured 

 that the remarkable submarine ridge which extends thence to the north-west of Scotland, 

 and separates the basin of the Atlantic from that of the Arctic Ocean, is partly at least 

 of volcanic origin. And still further north come the extensive Tertiary basaltic plateaux 

 of Iceland, while others of like aspect and age cover a vast area in southern Greenland. 

 Without contending that one continuous belt of lava-streams stretched from Ireland to 

 Iceland and Greenland, we can have no doubt that in older Tertiary time the north-west 

 of Europe was the scene of more widely- extended volcanic activity than showed itself at 

 any other period in the geological history of the whole continent. Possibly, as I have 

 already suggested, the present active vents of Iceland and Jan Mayen are the descendants 

 in uninterrupted succession of those that supplied the materials of the Tertiary basaltic 

 plateaux, the volcanic fires slowly dying out from south to north. But so continuous and 

 stupendous has been the work of denudation in these northern regions, where winds and 

 waves, rain and frost, floe-ice and glaciers reach their highest level of energy, that the 

 present extensive sheets of igneous rock can be regarded only as magnificent relics, the 

 grandeur of which furnishes some measure of that which characterised the last episode in 

 the extended volcanic records of Britain. 



The long and wide western valley in which the basalt-plateaux of this country were 

 accumulated seems, from a remote antiquity, to have been a theatre of considerable 

 geological activity. There are traces of some such valley or depression even back in the 

 period of the Torridon sandstone of the north-west which was laid down in it between the 

 great ridge of the Outer Hebrides and some other land to the east. The Lower Old Red 

 Sandstone of Lome may represent the site of one of its lakes. The Carboniferous rocks, 

 which run through the north of Ireland, cross into Cantyre, and are found even as far 

 north as the Sound of Mull, mark how, in later Palaeozoic time, the same strip of country 

 was a region of subsidence and sedimentation. During the Mesozoic ages, similar 

 operations were continued ; the hollow sank several thousand feet, and Jurassic strata to 

 that depth filled it up. Before the Cretaceous period, underground movements had 

 disrupted and irregularly upheaved the Jurassic deposits, and prolonged denudation had 

 worn them away, so that when the Cretaceous formations came to be laid down on the 

 once more subsiding depression, they were spread out with a strong unconformability on 

 everything older than themselves, resting on many successive horizons of the Jurassic 

 system, and passing from these over to the submerged hill-sides of the crystalline schists. 

 Yet again, after the accumulation of the Chalk, the sea-floor along the same line was 

 ridged up into land, and the Chalk, exposed to denudation, was deeply trenched by valleys, 

 and entirely removed from wide tracts which it once covered. 



It was in this long broad hollow, with its memorials of repeated subsidences and 

 upheavals, sedimentation and denudation, that the vigour of subterranean energy at last 

 showed itself in volcanic outbreaks, and in the gradual piling up of the materials of the 



