[219] Mr. G. T. Clark, Quart. Journ. Geol. Soc. xxv. (1869) p. 163. For remarks on the connection of dykes with superficial lavas, see postea, [p. 268].
Thousands of square miles in the Western States and Territories of the American Union have been similarly flooded with basic lavas. Denudation has not yet advanced far enough to lay bare much of the platform on which these lavas rest. But the dykes that traverse the rocks outside of the lava-deserts afford an example of the structure which will ultimately be revealed when the wide and continuous basalt-plains shall have been trenched by innumerable valleys and reduced to fragmentary plateaux with lofty escarpments ([p. 267]).
It is to the modern eruptions of Iceland, however, that we turn for the completest illustration of the phenomena connected with dykes and fissures. An account of these eruptions will therefore be given in Chapter xl. as an explanation of the history of the Tertiary basalt-plateaux of Britain.
CHAPTER XXXVI
THE PLATEAUX
Nature and Arrangement of the Rocks: 1. Lavas.—Basalts, Dolerites, Andesites—Structure of the Lavas in the Field—2. Fragmental Rocks.—Agglomerates, Conglomerates, and Breccias—Tuffs and their accompaniments.
We have now to consider the structure and history of those volcanic masses which, during Tertiary time, were ejected to the surface within the area of the British Islands, and now remain as extensive plateaux. Short though the interval has been in a geological sense since these rocks were erupted, it has been long enough to allow of very considerable movements of the ground and of enormous denudation, as will be more fully discussed in Chapters xlviii. and xlix. Hence the superficial records of Tertiary volcanic action have been reduced to a series of broken and isolated fragments. I have already stated that no evidence now remains to show to what extent there were actual superficial outbursts of volcanic material over much of the dyke-region of Britain. The subsequent waste of the surface has been so enormous that various lava-fields may quite possibly have stretched across parts of England and Scotland, whence they have since been wholly stripped off, leaving behind them only that wonderful system of dykes from which their molten materials were supplied.
There can be little doubt, however, that whether or not other Phlegrean fields extended over portions of the country whence they have since been worn away, the chief volcanic tract lay in a broad and long hollow that stretched from the south of Antrim to the Minch. From the southern to the northern limit of the fragmentary lava-fields that remain in this depression is a distance of some 250 miles, and the average breadth of ground within which these lava-fields are preserved may be taken to range from 20 to 50 miles. If, therefore, the sheets of basalt and layers of tuff 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 an extensive plateau 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. 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, 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 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 Lorne may represent the site of one of the older water-basins of this ancient hollow. 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 Palæozoic 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 hillsides 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 basalt-plateaux. So far as we know, these outbursts were subærial. At least no trace of any marine deposit has yet been found even at the base of the pile of volcanic rocks. Sheet after sheet of lava was poured out, until several thousand feet had accumulated, so as perhaps to fill up the whole depression, and once more to change entirely the aspect of the region. But the volcanic period, long and important as it was in the geological history of the country, came to an end. It, too, was merely an episode during which denudation still continued active, and since which subterranean disturbance and superficial erosion have again transformed the topography. In wandering over these ancient lava-fields, we see on every hand the most stupendous evidence of change. They have been dislocated by faults, sometimes with a displacement of hundreds of feet, and have been hollowed out into deep and wide valleys and arms of the sea. Their piles of solid rock, hundreds of feet thick, have been totally stripped off from wide tracts of ground which were once undoubtedly buried under them. Hence, late though the volcanic events are in the long history of the land, they are already separated from us by so vast an interval that there has been time for cutting down the wide plateaux of basalt into a series of mere scattered fragments. But the process of land-sculpture has been of the utmost service to geology, for, by laying bare the inner structure of these plateaux, it has provided materials of almost unequalled value and extent for the study of one type of volcanic action.