CHAPTER L

EFFECTS OF DENUDATION

Among the more impressive lessons which the basalt-plateaux of North-Western Europe teach the geologist, the enormous erosion of the surface of this part of the continental area since older Tertiary time takes a foremost place. He may be ready almost without question to accept the evidence adduced in favour of a vast amount of denudation among such soft and incoherent strata as those of the older Tertiary formations of the south-east of England or the north-west of France. But he is hardly prepared for the proofs which meet him among the north-western isles that such thick masses of solid volcanic rocks have been removed during the same geological interval.

To gain some idea of the amount of this waste we must, in the first place, picture to our minds the extent of ground over which the lavas were poured, and the depth to which they were piled upon it. Though we may never be able to ascertain whether the now isolated basalt-plateaux of Britain were once united into a continuous plain of lava, we can be quite certain that every one of these plateaux was formerly more extensive than it is now, for each of them presents, as its terminal edge, a line of wall formed by the truncated ends of horizontal basalt-sheets. And there seems no improbability in the assumption that the whole of the great hollow from the centre of Antrim up to the Minch was flooded with lavas which flowed from many vents between the hills of ancient crystalline rocks forming the line of the Outer Hebrides on the west, and those of the mainland of Scotland on the east.

It is certain that the depth to which some parts of this long hollow were overflowed with lava exceeded 3000 feet, for more than that depth of rock can be shown to have been in some places removed. The original inequalities of surface were buried under the volcanic materials which were spread out in a vast plain or series of plains, like those that have been deluged by modern eruptions in Iceland. Owing, however, to a general but unequal movement of subsidence, the lava-fields sank down here and there to, perhaps, an extent of several hundred feet, so that the old land-surface on which they began to be poured out now lies in those places below the level of the sea.

I have shown that even during the volcanic period, while the lavas were still flowing from time to time, erosion was in active progress over the surface of the volcanic plain. The records of river-action in Canna and Sanday, and the buried river-channel of the Scuir of Eigg, prove that, while eruptions still continued, rivers descending from the mountains of the Western Highlands carried the detritus of these uplands for many miles across the lava-fields, swept away the loose material of volcanic cones, and cut channels for themselves out of the black rugged floor of basalt.

The erosion thus early begun has probably been carried on continuously ever since. The present streams may be looked upon as practically the same as those which were flowing in the Tertiary period. There may have been slight changes of level, oscillations both upward and downward in the relative positions of land and sea, and shiftings of the water-courses to one side or other; but there seems no reason to doubt that the existing basalt-plateaux, which were built up as terrestrial areas, have remained land-surfaces with little intermission ever since, although their lower portions may have been in large measure submerged.

In the existing valleys, fjords and sea-straits by which these plateaux have been so deeply and abundantly trenched, we may recognize some of the drainage-lines traced out by the rivers which flowed across the volcanic plains. The results achieved by this prolonged denudation are of the most stupendous kind. The original lava-floor has been cut down into a fragmentary tableland. Hundreds of feet of solid rock have been removed from its general surface. Outliers of it may be seen scattered over the mountains of Morven, whence they look into the heart of the Highlands. Others cap the hills of Rum, where they face the open Atlantic. Several miles from the main body of the plateau in Skye, a solitary remnant, perched on the highest summit of Raasay, bears eloquent witness that the basaltic tableland once stretched far to the east of its present limits.

Two lines of observation and of argument may be followed in the effort to demonstrate how great the denudation has been since older Tertiary time. In the first place, there is the evidence of the level or nearly level sheets of basalt that form the plateaux, and, in the second place, there is the testimony of the dykes, sills and bosses by which these lavas have been disrupted.

1. The study of the denudation of the Tertiary volcanic rocks of North-Western Europe is most satisfactorily begun by an attempt to measure the minimum amount of waste which in certain places the basalt-plateaux can be proved to have undergone. For the purposes of this study, the stratification of the lavas and their nearly horizontal, or at least very slightly disturbed, position afford exceptional facilities. Amorphous rocks, such as granites and gabbros, or even foliated masses like the old gneisses and schists, may have been enormously denuded. Their mere presence at the existing surface may be taken as proof of such waste, yet they furnish in themselves no criterion by which the amount of removed material may be estimated.

But in the case of the basalt-plateaux, as in that of horizontal sedimentary formations, the successive lines of superposition of the component beds of the whole stratigraphical series supply admirable datum-lines which, on the one hand, vividly impress the imagination by the demonstration which they afford of the reality and magnitude of the denudation, and, on the other hand, furnish a measure by which the minimum amount of this denudation may be actually computed.

Availing ourselves of this kind of evidence it is easy to show that valleys many miles long, several miles broad, and from crest to bottom several thousand feet deep, have been excavated out of the basalt-plateaux since the close of the volcanic period. And if this conclusion can be demonstrated for these plateaux, it must obviously apply equally to the rest of the country. We thus obtain a most important contribution to the investigation of the origin and relative age of the present topographical features of the surface of the land.

Let me give a few illustrations of the nature of the investigation and of the results to which it leads. Throughout the Western and Faroe Islands the level bars of basalt present their truncated ends in the great escarpment-cliffs which wind mile after mile along their picturesque coasts. Where they front the open sea, it is obviously impossible to say how much further seaward they once extended. But where they retire in fjords or sea-lochs, and sweep inland into glens, it is easy to measure the distance from the bottom of the eroded hollow to its bounding watersheds, and to estimate the amount of material that has been worn out of it. The only uncertainty in this computation arises from our inability to determine to what extent movements of subsidence may have come into play to aid in the disappearance of the basalts. Where the bottom of the lavas can be seen at the same level on either side of an inlet, with no evidence of faulting, or where a definite horizon in the volcanic series can be traced round the head of a glen or sea-loch, the influence of underground movements may be eliminated. The evidence of vast denudation is always visible, the proofs of subsidence are much less frequently observable.

The island of Mull supplies many striking examples of the enormous waste of the basalt-plateau. The Sound of Mull, for instance, has been eroded out of the volcanic series for a distance of 20 miles, with a mean breadth of about two miles. From the deepest part of this fjord to the summit of the Mull plateau is a vertical height of 3600 feet. The whole of this vast excavation has taken place since older Tertiary time. On the opposite side of Mull the hollow of Loch Scridain has been eroded to a mean depth of at least 1200 feet below the average level of the surrounding plateau, with a breadth of rather more than a mile.

The scattered islands which lie to the west of Mull tell the same tale. They are all outliers of the same basalt-plateau, and have not only been greatly lowered by the removal of their upper lavas, but have been separated by the erosion of long and deep hollows between them. Thus from the summit of the Gribon cliffs in Mull to the deepest part of the sea-floor between that precipice and the Treshnish Isles a vertical depth of at least 2000 feet of rock has been removed since the basalts ceased to be erupted.

I have referred to the impressive evidence of denudation displayed on the west side of the island of Eigg. The vertical distance from the summit of the Eigg plateau to the bottom of the submarine valley between this island and Rum is about 1500 feet, but as that summit lies below the original surface of the lava-field, the depth of rock which has been removed must exceed 1500 feet. We thus learn that since the close of the volcanic period the hollow between the islands of Eigg and Rum has been eroded to this great depth.

Still more striking is the evidence of enormous waste presented by the Faroe Islands. The cliffs there are loftier and barer, and the fjords have been cut more deeply and precipitously out of the basalt-plateau. I shall never forget the first impression made on my mind when the dense curtain of mist within which I had approached the southern end of the archipelago rapidly cleared away, and the sunlit slopes and precipices of Suderö, the two Dimons, Skuö and Sandö, rose out of a deep blue sea. Each island showed its prolongation of the same long level lines of rock-terrace. The eye at once seized on these features as the dominant element in the geology and the topography, for they revealed at a glance the true structure of the islands, and gave a measure of the amount and irregularity of the erosion of the original basalt-plateau. And this first impression of stupendous degradation only deepened as one advanced further north into the more mountainous group of islands. Probably nowhere else in Europe is the potency of denudation as a factor in the evolution of topographical features so marvellously and instructively displayed as among the north-eastern members of the Faroe group.

Availing ourselves of the datum-lines supplied by the nearly level bars of basalt, we easily perceive that in many parts of the Faroe Isles the amount of volcanic material left behind, stupendous though it be, is less than the amount which has been removed. Thus the island of Kalsö is merely a long narrow ridge separating two broad valleys which are now occupied by fjords. The material carved out of these valleys would make several islands as large as Kalsö. Again, the lofty precipice of Myling Head, 2260 feet high, built up of bedded basalts from the summit to below sea-level, faces the north-western Atlantic, and the sea rapidly deepens in front of it to the surface of the submarine ridge 200 to 300 feet below. The truncated ends of the vast pile of basalt-sheets which form that loftiest sea-wall of Europe bears testimony to the colossal denudation which has swept away all of the volcanic plateau that once extended further towards the west.

Nevertheless, enormous as has been the waste of this plateau of the Faroe Islands, we may still trace some of its terrestrial features that date back probably to the volcanic period. Even more distinctly, perhaps, than among the Western Isles of Scotland, we may recognize the position of the original valleys, and trace some of the main drainage lines of the area when it formed a wide and continuous tract of land.

A line of watershed can be followed in a south-westerly direction from the east side of Viderö, across Borö to the centre of Osterö, and thence by the Sund across Stromö and Vaagö. From this line the fjords and valleys diverge towards the north-west and south-east. There can hardly be any doubt that on the whole this line corresponds with the general trend of the water-parting at the time when the Tertiary streams were flowing over the still continuous volcanic plain. Considerable depression of the whole region has since then sent the sea up the lower and wider valleys, converting them into fjords, and isolating their intervening ridges into islands.

The topography of the Faroe Islands seems to me eminently deserving of careful study in the light of its geological origin. There is assuredly no other region in Europe where the interesting problems presented by this subject could be studied so easily, where the geological structure is throughout so simple, where the combined influences of the atmosphere and of the sea could be so admirably worked out and distinguished, and where the imagination, kindled to enthusiasm by the contemplation of noble scenery, could be so constantly and imperiously controlled by the accurate observation of ascertainable fact.

2. Impressive and easily comprehended as are the proofs of denudation supplied by the basalts of the plateaux, they are perhaps to a geological eye less overwhelming than those furnished by the eruptive rocks which have been injected into these plateaux. In the case of at least the basic intrusions, we may reasonably infer that they assumed their present position under a greater or less depth of overlying rock which has since been removed. When, therefore, they are found at or above the summits of the plateaux, they demonstrate that a vast amount of material has been removed from these summits.

The argument from the position of the dykes has already been enforced. It is absolutely certain that valleys several thousand feet deep must have been excavated since these dykes were erupted, for had such valleys existed at the time when the dykes were injected across their site, the molten rock, instead of ascending to the tops of the surrounding mountains, would obviously have rushed forth over the valley-bottoms. I have shown that this reasoning applies not merely to the volcanic districts, but to the whole surface of the country within the region of dykes. Thus the uplands of Southern Scotland, and wide areas in the Southern and Western Highlands, can be proved to have had glens cut out of their mass to a depth of hundreds of feet since the Tertiary volcanic period.

Not less convincing is the evidence afforded by the great eruptive masses of gabbro. We have seen that these complex accumulations of sills, dykes, and bosses include rocks so coarse in grain as to show that they must have consolidated at some considerable depth, but that they now appear in hill-groups 2000 to 3000 feet in height, the whole of the original basaltic cover having been stripped off from them. But these gabbro hills have been in turn traversed up to the very crests by later basalt-dykes, which thus supply additional proof that the erosion here has been stupendous.

The granophyre bosses tell the same tale. Though, like the domite Puys of Auvergne, they may still retain, in their conical forms, indications of the original shapes which their component material assumed at the time of its protrusion, we may be confident that their existing surfaces have been reached after the removal of much rock which once lay above them. This inference is confirmed by the fact that these eruptive bosses have been invaded by a younger system of dykes. The black ribs of basalt which may be traced along their pale declivities, which cross the glens that have been eroded in them and which mount up to their very crests, prove that since the latest manifestations of volcanic energy in the West of Scotland, extensive changes in the topography of the land have been effected by the operation of the subærial agents of degradation.

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.

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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.

As in the modern eruptions of Iceland, new fissures are successively opened through the older lava-sheets, so in the Tertiary volcanic areas, renewed ruptures of the earth's crust allowed later dykes to be formed. The basalt-plateaux are traversed by such dykes, even up to their highest sheets. It is impossible to say how often the process of dyke-making may have been repeated. Not improbably it recurred again and again during the building of the basalt-plateaux, and we know that it was renewed even after the protrusion of the granophyre bosses which mark one of the latest phases of volcanism in the region.

For a protracted geological period, with long intervals of quiescence, various basic lavas (basalts, dolerites, etc.), with occasionally some of intermediate composition (andesites, trachytes), and perhaps in Antrim acid rhyolites, flowed out from fissures and vents until they had filled up the hollows of the great valley, which then stretched from the south of Antrim northwards between the west coast of Scotland and the chain of the Outer Hebrides. In some places the accumulated pile of these ejections even now exceeds 3000 feet in thickness, but we cannot tell how much material has been bared away from its top by denudation. The volcanic discharges consisted mostly of lava, fragmentary materials being comparatively insignificant in amount and local in origin, though layers of fine tuff and basalt-breccias occur in all the plateaux. None of the erupted materials thicken towards any centres that might be taken to mark volcanoes of the type of Vesuvius or Etna. On the contrary, the persistent flatness and uniformity of the volcanic series, and the thinning out of the separate beds in different directions, show that the lavas issued from many points all over the region. The positions of some of the actual vents can still be ascertained. They are now filled sometimes with dolerite, sometimes with coarse agglomerate.

The surface over which the lava flowed seems to have been mainly terrestrial. Here and there, between the successive sheets of basalt, the leaves, stems, and fruit of land-plants, sometimes in most perfect preservation, may be observed, together with the remains of insects and fresh-water fish. Distinct relics of old river-channels can be recognized which have been buried under streams of lava. Among the deposits left by these streams the uppermost layers are commonly dark with decayed vegetation, while layers of coal are found here and there between the basalts.

As the pile of erupted materials gradually thickened, and the subterranean energy possibly grew feebler, the ascending magma was forced between the layers of sedimentary strata underneath the basalts, or between these strata and the overlying volcanic series, or along any other plane of weakness in the terrestrial crust. In this way arose the multitudinous sills or intrusive sheets.

When the great volcanic plateaux had been built up to a thickness of several thousand feet, another remarkable episode in the history occurred. At certain points large bodies of coarsely crystalline basic rocks were pushed into and through the plateaux-basalts, upraising them in dome-shaped elevations, and ultimately solidifying as dolerites, gabbros, troctolites, picrites, etc. There is reason to believe that the points of extravasation of these materials were mainly determined by the positions of the larger or more closely clustered vents of the plateau-period, where points of weakness consequently existed in the terrestrial crust. Rising as huge bosses through such weak places, the gabbros and associated rocks raised up the overlying bedded basalts, and forced themselves between them, forming thus a fringe of finer-grained intrusive sills and veins around the central banded and amorphous masses of more coarsely crystalline material. Whether, in any of these vast domes of upheaval, the summit was disrupted, so as to allow the basic intrusion to flow out as lava at the surface, cannot now be told, owing to enormous subsequent denudation.

The next chapter in the chronicle shows us that probably long after the eruption of the gabbros, when possibly all outward symptom of volcanic action had ceased, a renewed outbreak of subterranean activity gave rise to the protrusion of another and wholly different class of materials. This time the rocks were of a markedly acid type. They included varieties that range from obsidians, pitchstones, flinty felsites and rhyolites, through porphyries and granophyres, into compounds which cannot be classed under any other name than granite. These masses likewise availed themselves of older vents in the plateaux, and broke through them. They now form huge conical hills, which, in their outer aspect, and even to some extent in their inner structure, recall the trachytic puys of Auvergne. But the granophyres not only ascended through the basalt-plateaux and the gabbro-bosses; they sent into these rocks a network of veins, pushed their way in huge sheets or sills between the strata below, and actually incorporated a considerable proportion of the basic materials into their own substance. Around the bosses of gabbro and granophyre, the bedded basalts have undergone considerable contact-metamorphism.

The gabbro and granophyre bosses of the Inner Hebrides demonstrate with singular force how unreliable petrographical characters are as a test of the relative age of rocks. No one, looking at hand-specimens of these rocks, or even studying them in the field, would at first suspect them to be of Tertiary date. They closely resemble rocks of similar kinds in Palæozoic and even Archæan formations. Yet, of their late appearance in geological time, there cannot be any possibility of doubt.

After the uprise of the granophyre, and the injection of the network of felsitic veins, there came once more a period of terrestrial convulsion, like that of the earliest basic dykes, but of less intensity. Again, the crust of the earth over the volcanic region was pushed upward and rent open by another system of parallel fissures. Again, from a reservoir or basin of basic lava underneath, molten rock was forced upwards into the rents, and thus another system of basic dykes was formed. These dykes are found crossing those of earlier date, and rising through the other volcanic rocks. They traverse the plateau-basalts from bottom to top; they climb to the summits of the gabbro mountains, and they even pursue their undeviating course over the huge domes of granophyre. No proof has yet been found that from any of these dykes there was a superficial outflow of lava. But so great has been the subsequent denudation of the areas, that such outflows might quite well have taken place, and have subsequently been destroyed.

Whether these basic dykes were the last manifestation of volcanic energy in our region cannot yet be decidedly affirmed. So far as the evidence at present goes, they are possibly older than another series of acid veins and dykes (pitchstone, felsite, and granophyre), which are found at many points from Antrim to the far end of the Inner Hebrides. These protrusions traverse every other member of the volcanic series, except some of the youngest basic dykes, and do not appear to be themselves cut by any.

Since the close of the volcanic period considerable disturbance of the basalt-plateaux has taken place. The whole volcanic region has subsided, some districts having sunk more than others. In Britain the most striking evidence of such depression is supplied by the basin of Lough Neagh. But throughout the Inner Hebrides much of the lower portion of the terrestrial lava-plateaux is now below sea-level. In the Faroe Islands and in Iceland the subsidence has been still more marked. Dislocations, also, sometimes amounting to more than a thousand feet of displacement, have occurred among the volcanic masses. The bedded basalts, originally on the whole nearly flat, have thus been broken up into large blocks of country wherein the sheets are now inclined in various directions.

One of the most important lessons taught by the Tertiary volcanic series of the north-west of Europe is the extent of the denudation of the land since the close of the volcanic period. The horizontal or gently inclined layers of bedding among the basalts afford datum-lines from which the minimum amount of material removed may be measured. As a reasonable estimate it may be inferred that in the case of the Mull plateau, for example, the average amount by which its surface has been lowered since the close of the volcanic period cannot be less than 2000 feet. If the rate of lowering of the land-surface in western Europe by subærial denudation be taken as 1/6000 of a foot in a year, then the lapse of time required for the degradation of the Mull plateau must amount to about twelve millions of years. Some such interval has therefore elapsed since the last Tertiary volcanoes became extinct.