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.
* * * * *
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.