So much for what can be demonstrated. But how much more may, with the highest probability, be inferred! The original limits of the plateaux are unknown. The waves of the wide Atlantic now roll over many a square league of the old lava-plains, and wide tracts of the islands and the mainland from which the basalt has been entirely stripped, or where it remains only in scattered outliers, were once deeply buried under piles of lava-sheets. It would probably be no exaggeration to affirm that over the British area, as well as over the Faroe Isles, the amount of Tertiary volcanic rock that now remains, large as it is, falls short in amount of what has been removed. The geologist who has made himself familiar with the effects of denudation in other Tertiary volcanic districts, such as Central France, Saxony and Bohemia, will be prepared for almost any conceivable amount of erosion among the far older volcanic series of the north-west of Europe.
To the student of the origin of the existing topography of the land there is a profound interest in the demonstration which these volcanic rocks supply of the vast changes which the terrestrial surface has undergone within a period geologically so recent as older Tertiary time. When, on the one hand, he finds himself more and more restricted in his demands for time by the confident assertions of the physicist that all the phenomena of geological history must have been comprised within a few millions of years, and when, on the other hand, he watches the seemingly feeble and tardy operations of the forces of denudation and sedimentation which have played the chief parts in that history, he may well be excused if sometimes he is apt to despair of ever reconciling the facts which he observes with the physical deductions that are somewhat dogmatically brought forward in opposition to his interpretation of them. He may feel sure that his facts cannot be gainsaid, and he may be unable to find any other way of comprehending them save by the admission that they necessitate a liberal allowance of time. Yet he may not feel himself to be in a position to offer any valid objections to the arguments from physical considerations that would so seriously abridge the length of time which geology requires.
In these circumstances it is some satisfaction to be provided with definite measurements of the amount of geological change which has been effected within a limited and relatively recent period of time. This change has resulted from the operation of the same agents by which it is still being carried on. No break in the history can be detected. There is not the least reason to suppose that the agents of denudation and sedimentation have, during the period in question, differed in their rate of working. Their activity at the present time is probably neither greater nor less than it was then. If, therefore, during so recent an interval such a stupendous amount of material has been worn away from the surface of the land and deposited on the sea-floor as the Tertiary volcanic rocks demonstrate, the geologist may surely contemplate without misgiving the lapse of time required for the completion of older geological revolutions. He may oppose to the arguments of the physicist the measurements and computations which he himself makes from data which are at least as reliable as the postulates whereon these arguments are based. The rate at which denudation and sedimentation are now taking place has been measured with tolerable accuracy, and a fair average for it has been obtained. Whatever may be maintained as to this rate in early geological ages, there can be no serious opposition to its being taken as fairly constant since older Tertiary time. We are thus provided with data for estimating the minimum amount of time that can have elapsed since the volcanic plateaux began to be denuded. But as no relic remains of the original upper surface of those plateaux, and as we are consequently ignorant of how much rock has been removed from their highest surviving outliers, it is obvious that such estimates are more likely to err in understating than overstating the amount of time required.
It would be beyond the scope of the present volume to enter fully into the measurements and calculations required for the adequate treatment of this subject. I will merely illustrate my argument by again taking a few data from the plateau of Mull. The original height of this plateau is shown by the outlier of Ben More to have been at least 3200 feet. If to this figure we add the portion of the basalt-group submerged under the sea the height will probably be increased by several hundred feet. But let us take 3000 feet as a moderate computation for the average thickness of the volcanic series here at the close of the plateau-period. Until a number of sections have been carefully plotted from the Ordnance Maps, in order to ascertain with approximate accuracy the average height of the present surface of the Mull basaltic plateau, making due allowance for the vast erosion of the Sound of Mull and the numerous glens and sea-lochs that traverse the island, any estimate which may be offered as to this average must be merely provisional. If, in the meantime, we suppose the present mean level of the plateau to be 1000 feet above the sea, the difference between this amount and the assumed original height will be 2000 feet. If, further, we take the present average rate of degradation of the Mull plateau to be 1/6000 of a foot in a year, which has been shown to be probably a fair estimate, then the time required for the lowering of the Mull plateau from its original to its present average level amounts to twelve millions of years. Yet this period, vast though it be, does not carry us back even as far as the beginning of Tertiary time.
In concluding this lengthened discussion of the Tertiary volcanic history of Britain, I may, perhaps, usefully add a brief summary of the leading features of the long record.
The region within which volcanic activity displayed itself during older Tertiary time in the British Isles, if our estimate of its area is restricted to those parts of the country where igneous rocks, probably of that age, now appear at the surface, embraces the North of England and of Ireland, the southern half and the west coast of Scotland—a total area of more than 40,000 square miles. Over that extensive region volcanic phenomena were displayed during an enormously protracted interval of geological time. The earliest beginnings of disturbance may possibly have started in the Eocene, and the final manifestations may not have ceased until the Miocene period. So prolonged was the duration of the eruptions, that enormous topographical changes from denudation, and probably also considerable variation in the fauna and flora, alike of land and sea, may have been effected.
Owing to some cause which has not yet in this relation been investigated, but which is probably referable to secular terrestrial contraction, the volcanic region underwent elevation, while, at the same time, a vast subterranean lake or sea of molten rock existed underneath it. Enormous horizontal tension thus arose, and at last the stretched terrestrial crust gave way. A system of approximately parallel fissures opened in it, having a general direction towards north-west. The rapid and simultaneous production of such a gigantic series of rents must have given rise to earthquakes of enormous magnitude and destructive force. The great majority of the fractures, doubtless, did not reach to the surface of the ground, though probably not a few did so. Such was the potency of this development of terrestrial energy, that the fissures ran through the most varied kinds of rocks and the most complicated geological structures, crossing even earlier lines of powerful dislocation, and yet retaining their direction and parallelism for sometimes 50 or 100 miles.
Into the fissures thus formed the molten magma from underneath was forced for many hundreds or even thousands of feet above the surface of the subterranean lava-reservoir. Solidifying between the fissure walls, it formed the crowd of basic dykes that stand out as the most widespread and distinctive feature of the volcanic region.
Where the fissures reached the surface or near to it, the molten rock would seek relief by egress in streams of lava. This probably occurred in many places from which subsequent denudation has removed all vestige of superficial volcanic manifestations. But, in the great range of basalt-plateaux, from Antrim northwards through the chain of the Inner Hebrides, there are still left abundant remains of the surface-outflows. Like the modern lavas of Iceland, the molten material probably flowed out sometimes from the open fissures, sometimes from vents formed along the chasms. After the convulsions ceased which produced the earliest dykes, the communication that had been established between the magma-reservoir underneath and the air above would be maintained, and repeated eruptions might take place, either from the original fissures and vents or from others afterwards opened by the volcanic energy.