THE TERTIARY VOLCANOES 
BOOK VIII 
182 
at least of volcanic origin. 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 had shown itself at any 
previous period in the geological history of the whole continent. The present 
active vents of Iceland and Jan Mayen are not improbably 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, fioe-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 the magnitude of the last episode in the 
extended volcanic history 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. This formation, as we have seen, was laid down between 
the great ridge of the Outer Hebrides and some other land to the east, of 
which a few of the higher mountains, once buried under the sandstone, are now 
being revealed by denudation between Loch Maree and Loch Broom, and also 
in Assynt. The conglomerates and volcanic rocks of Lome may represent 
the site of one of the older water-basins of this ancient hollow. The 
Carboniferous rocks, which run through the X or t 1 1 of Ireland, cross into 
Cantyre, and are found even as far north as the Sound of Mull, mark how, 
in later Palseozoic 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 J urassic strata to 
that depth filled it up. Before the Cretaceous period, underground move- 
ments 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 unconformahility on everything older 
than themselves, resting on many successive horizons of the J urassic 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 sub- 
sidences 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 basalt-plateaux. So far as we 
