conditions that obtained in the ages that immediately preceded the
Pleistocene period. The distribution of the Pliocene marine beds of
Britain entitles us to assume that at the time of their accumulation
our lands did not extend quite so far to the south and east as now.
The absence of similar deposits from the coast-lands of North America
is supposed to support the view of great continental elevation in
pre-glacial times. All it seems to prove, however, is that in Pliocene
times the North American continent was not less extensive than it is at
present. It is even quite possible that in glacial times pre-existing
Pliocene beds may have been ploughed out by the ice, just as seems to
have been the case in the north-east of Scotland. But without going so
far back as Pliocene times, we meet with evidence almost everywhere
throughout the maritime regions of the glaciated areas of Europe and
North America, to show that immediately before those tracts became
swathed in ice the geographical conditions were much the same as at
present. The shelly boulder-clays in various parts of our islands,
and the similar occurrence of marine and brackish-water shells in and
underneath the Diluvium of north Germany, etc., prove clearly enough
that just before the coming-on of glacial conditions neither Britain
nor the present maritime lands of the Continent were far removed
from the sea. It is true that the buried river-channels of Scotland
indicate a pre-glacial elevation of some 200 or 300 feet above the
existing sea-level, but it is quite certain that the Minch, St.
George's Channel, the Irish Sea, the North Sea, and the Baltic were all
in existence at the commencement of the Glacial period. And we are led
to similar conclusions with regard to the geographical conditions of
North America at that time, from the occurrence of marine shells in the
boulder-clays of Canada and New England. We note indeed that there is
abundant evidence of land-submergence during glacial times. Indeed, we
may say that the Pleistocene marine deposits of northern latitudes are
almost invariably indicative of colder conditions than now obtain.
If it be true that cold climatic conditions were contemporaneous in
our latitude with submergence, it is equally true that an extensive
land-surface in north-west Europe has, sometimes at least, co-existed
with markedly genial conditions. In Tertiary times, for example, as
the Oligocene deposits of Scotland, the Faröe Islands, Iceland, and
Greenland testify, a land-connection existed between Europe and the
North American continent. Again, it has been shown that during the
interglacial phase of the Pleistocene period Britain was continental,
and enjoyed at the time a peculiarly genial climate. And somewhat
similar geographical and climatic conditions again supervened in
post-glacial times. In other words, when the land was more elevated
and extensive than now, it enjoyed a warmer climate. Nor can we escape
the conclusion that the excavation of the fiord-valleys of northern
latitudes, which is a very old story (far older than the Pleistocene),
was the work not of glaciers but of running water, at a time when
north-western Europe and the corresponding regions of America were much
more elevated than they are now.
Thus there appears to be no evidence either direct or indirect in
favour of the view that glacial conditions were superinduced by
great continental elevation. But it may be argued that even although
no evidence can be cited in proof of such elevation, still, if
the glacial phenomena can be well explained by its means, we may
be justified in admitting it as a working hypothesis. Movements
of elevation and depression have frequently taken place--the
Pleistocene marine deposits themselves testify to oscillations of
the sea-level--and there can be no objection, therefore, to such
postulations as are made by the hypothesis under review. All this
is readily granted, but I deny that the conditions that obtained in
Pleistocene times can be accounted for by elevation and depression. Let
us see how the desiderated elevation of northern lands would work. Were
north-western Europe and the corresponding latitudes of North America
to be upheaved for 3000 feet, and a land-passage to obtain between the
two continents by way of the Faröe Islands, Iceland, and Greenland,
how would the climate be affected? It is obvious that under such
changed conditions the elevated lands in higher latitudes might well
be subjected to more or less extensive glaciation. Norway would become
uninhabitable and glaciers might well appear in the mountain-valleys
of Scotland. But it may be doubted whether the climate of France and
Spain, or the corresponding latitudes of North America, would be
much affected. For were a land-passage to appear between Britain and
Greenland no Arctic current would flow into the North Atlantic, while
no portion of the Gulf Stream would be lost in Arctic seas. The North
Atlantic would then form a great gulf round which a warm ocean-current
would circulate. The temperature of that sea, therefore, would be
raised and the prevailing westerly and south-westerly winds of Europe
would be warmer than now. However much such warm moist winds might
increase the snow-fall in North Britain and Scandinavia, we cannot
suppose they could have much influence in central and southern Europe,
and in North Africa; and still less could they affect the climate of
Asia Minor and the mountainous regions of the far east, in most of
which evidence of extensive glaciation occurs. And how, we may ask,
could the postulated geographical changes bring about the glaciation
of the mountainous tracts on the Pacific sea-board? In fine, we may
conclude that however much the geographical changes referred to might
affect north-western Europe and north-eastern America, they are wholly
insufficient to account for the glacial phenomena of other regions.
The continuous research of recent years has shown that the lowering of
temperature of glacial times was not limited to the lands which would
be affected by any such elevation as that we are considering. A marked
and general displacement of climatic zones took place over the whole
continent of Europe; and similar changes supervened in North America
and Asia. Are we then to suppose that all the lands within the Northern
Hemisphere were extensively and contemporaneously upheaved?
We may now consider another form of the "earth-movement hypothesis."
It has frequently been suggested that our glacial phenomena may have
been caused by the submergence of the Isthmus of Panama, and the
deflection of the Equatorial Current into the Pacific. But it may be
doubted whether a submergence of that isthmus, unless very extensive
indeed, would result in more than a partial escape of Atlantic water
into the Pacific basin. The Counter Current of the Pacific which now
strikes against the isthmus might even sweep into the Caribbean Sea,
and join the Equatorial on its way to the Gulf of Mexico. But putting
that consideration aside, what evidence have we that the Isthmus of
Panama was submerged during the glacial epoch? None whatsoever, it
may be replied. It is only a pious opinion. Considerable movements of
elevation and depression of the islands in the Caribbean Sea would seem
to have taken place at a comparatively recent date, but those movements
may quite well belong to Pliocene times. Whether they be of Pliocene
or Pleistocene age, however, no one has yet proved that the Isthmus of
Panama was sufficiently submerged, either at the one time or the other,
to permit the escape of the Atlantic Equatorial into the Pacific basin.
But let it be supposed that the isthmus has become so deeply submerged
that the Equatorial Current is wholly deflected, and that no Gulf
Stream issues through the Straits of Florida to temper the climate of
higher latitudes. What would result from such an unhappy change? Can
any one conversant with the geographical distribution of the glacial
phenomena imagine that the conditions of the Glacial period could be
thus reproduced? Norway might indeed become a second south Greenland,
and perennial snow and ice might appear in the mountainous tracts of
the British Islands. The climate of Hudson's Bay and the surrounding
lands might be experienced in the Baltic and its neighbourhood, and
what are now the temperate latitudes of Europe, north of the 50th
parallel, would possibly approach Siberia in character. But surely
these changes are not comparable to the conditions of the Glacial
period. The absence of a Gulf Stream would not sensibly affect the
climate of south-eastern Europe and Asia, and could not have the
smallest influence on that of the Pacific coast-lands of North America.
Yes, but if we conceive the submergence of the Isthmus of Panama
to coincide with great elevation of northern lands, would not such
geographical conditions bring about a glacial epoch comparable to
that of Pleistocene times? It is hard to see how they could. No
doubt the climate of all those regions that would be affected by
the withdrawal of the Gulf Stream alone would become still more
deteriorated if they stood some 3000 feet higher than now. A vast area
in the north-west of Europe would certainly be uninhabitable, but it
is for the advocates of the "earth-movement hypothesis" to explain
why those inhospitable regions should necessarily be covered with an
ice-sheet. For the production of great snow-fields and continental
ice-sheets, considerable precipitation, no less than a low temperature,
is requisite. Under the conditions we have been imagining, however,
precipitation would probably be much less than it is at present. But
to whatever extent north-west Europe might be glaciated, it is obvious
that the geographical revolutions referred to could have little
influence on the climate of south-eastern Europe, not to mention
central and eastern Asia. Nor could they possibly influence the climate
of the Pacific coast-lands of North America. And yet, as is well
known, the climate of all those regions was more or less profoundly
affected during the Glacial period. To account for the widespread
evidences of glaciation by means of elevation it would therefore seem
necessary to infer that all the affected areas were in Pleistocene
times uplifted _en masse_ into the Arctic zone that stretches above our
heads. Now it seems easier to believe that the snow-line was lowered
by several thousand feet than that the continents were elevated to the
same extent. Glaciation, as we have seen, was developed in the same
directions and over the same areas as we should expect it to be were
the snow-line to be generally depressed. To put it in another way,
were the snow-line by some means or other to be lowered over Europe,
Asia, and North America, then, with sufficient precipitation, great
ice-fields and glaciers would reappear in the very regions which they
visited during Pleistocene times. Neither elevation nor depression
of the land would be required to bring about such a result. Certain
advocates of the "earth-movement hypothesis," however, do not maintain
that all the glaciated areas were uplifted at one and the same time.
The glaciation of the Alps, they think, may have taken place earlier
or later than that of north-western Europe, while the ice-period of
the Rocky Mountains may not have coincided with that of eastern North
America. It is not impossible, they suppose, that the glaciation of the
Himalayas may have been caused by an uplifting of that great chain,
quite independent of similar earth-movements in other places. It can be
demonstrated, however, that the glaciation of the Alps and of northern
Europe were contemporaneous, and the facts go far to prove that the
glaciers of the Rocky Mountains and the inland-ice of north-east
America likewise co-existed. At all events all the old glacial
accumulations of our hemisphere are of Pleistocene age, and it is for
the advocates of the hypothesis under review to prove that they are
not contemporaneous. Their doubts on the subject probably arise from
the simple fact that they are well aware how highly improbable or even
impossible it is that all those glaciated lands could have been pushed
up within the snow-line at one and the same time.
Let me, however, advance to another objection. We know that the
Glacial period was interrupted by at least one interglacial epoch of
temperate and even genial conditions. Two glacial epochs with one
protracted interglacial epoch are now generally admitted. How do the
supporters of the "earth-movement hypothesis" explain this remarkable
succession of climatic changes? Their views as to the cause of glacial
conditions we have considered. If we can believe that the glacial
phenomena were due to elevation of the land, then we need have no
difficulty in understanding how glacial conditions would disappear
when the continents again subsided to a lower level. Not only did
North America and Europe lose all their early glacial elevation, but
by a lucky coincidence the Isthmus of Panama reappeared, and the Gulf
Stream resumed its beneficent course into the North Atlantic. This we
are to suppose was the cause of the interglacial epoch. But I would
point out that the geographical conditions which are thus inferred to
have brought about the disappearance of the glacial climate, and to
have ushered in the interglacial epoch, are precisely those that now
obtain--and, nevertheless, we are not yet in the enjoyment of a climate
like that of interglacial times. The strangely equable conditions that
permitted the development of the remarkable Pleistocene flora and fauna
are not experienced in the Europe of our day. And what about the second
glacial epoch? Are we to suppose that once more the lands were greatly
uplifted, and that convenient Isthmus of Panama was again depressed?
Did the Alps, the Pyrenees, and the plateau of central France--in all
of which we have distinct evidence of at least two glacial epochs--did
these heights, one may ask, rise up to bring about their earlier
glaciation, sink down again to induce interglacial conditions, and
once more become uplifted at the succeeding cold epoch, to subside
eventually in order to cause a final retreat of their glaciers?
But the climatic changes to be accounted for were in all probability
more numerous and complex than those just referred to. Competent
observers have adduced unmistakable evidence of three epochs of
glaciation in the Alpine Lands of Europe. And we are not without
distinct hints that similar changes have taken place in northern and
western Europe. Nor in this connection can we ignore the evidence of
several interglacial episodes which Mr. Chamberlin and others have
detected in the glaciated tracts of North America. Even this is not
all, for the upholders of the "earth-movement hypothesis" have still
further to account for the climatic oscillations of post-glacial times.
If it be hard enough to allow the possibility of one great movement
of elevation having affected so enormous an area of our hemisphere,
if we find it extremely difficult to believe either that one such
widespread movement, or that a multitude of local movements, each more
or less independent of the other, could have lifted the glaciated
regions successively within reach of the snow-line--we shall yet find
it impossible to admit that such remarkable upheavals could be repeated
again and again.
We seem driven to conclude, therefore, that the "earth-movement
hypothesis" fails to explain the phenomena of Pleistocene times. One
cannot deny, indeed, that glaciation might be induced locally by
elevation of the land. It is quite conceivable that mountains now
below the limits of perennial snow might come to be ridged up to such
an extent as to be capable of sustaining snow-fields and glaciers.
And such local movements may possibly have happened here and there
during the long-continued Pleistocene period. But the glacial phenomena
of that period are on much too grand a scale and far too widely
distributed to be accounted for in that way. And if the occurrence
of even one glacial epoch cannot be thus explained, we may leave the
supporters of the "earth-movement hypothesis" to show us what light
is thrown by their urim and thummim on the origin of succeeding
interglacial and glacial climates.
There is yet another physical condition of the Pleistocene and
post-glacial periods which any adequate explanation must embrace. I
refer to the oscillation of sea-level, of which so many proofs are
forthcoming. It is very remarkable that almost everywhere throughout
the maritime regions of formerly glaciated areas we find evidence of
submergence. So commonly is this the case, that geologists have long
suspected that the connection between glaciation and submergence might
be one of cause and effect. The possible influence of great ice-sheets
in disturbing the relative level of land and sea is a question,
therefore, of very great importance. It is one, however, which must be
solved by physicists. Croll and others have advocated the view that the
great accumulations of ice of the Glacial period may have displaced
the earth's centre of gravity, and thus caused the sea to rise upon
the glaciated hemisphere. The various results arrived at by physicists
are hardly comparable, because each has used different data, but it
seems probable that we have in this view a _vera causa_ of oscillations
of the sea-level. Another hypothesis would explain the rise of the
sea as due to the attractive influence of the great ice-masses, but
Dr. Drygalski's and Mr. Woodward's elaborate investigations would
seem to have demonstrated that this notion does not account for the
facts. Yet another speculation has been advanced. Mr. Jamieson has
suggested that the mere weight of the ice-sheets would suffice to
press down the earth's crust into a supposed liquid substratum, and
this explanation has met with much acceptance. Unfortunately our
knowledge of the condition of the earth's interior is so very limited
that we cannot be certain as to how the crust would be affected by
the weight of an ice-sheet. No doubt Mr. Jamieson's hypothesis gives
a specious explanation of certain geological phenomena, but if there
be no liquid substratum underlying a thin crust it cannot be true.
At present the prevalent view of physicists appears to be that the
earth is substantially solid. Professor George Darwin has shown
that the prominent inequalities of the earth's surface could not be
sustained unless the crust be as rigid as granite for a depth of 1000
miles. "If the earth be solid throughout," he remarks, "then at 1000
miles from the surface the material must be as strong as granite. If
it be fluid or gaseous inside, and the crust 1000 miles thick, that
crust must be stronger than granite, and if only 200 or 300 miles in
thickness, much stronger than granite." This conclusion is obviously
strongly confirmatory of Sir William Thomson's view, that the earth
is solid throughout. But many geologists find it hard to account for
the convolutions of strata and other structural phenomena on the
supposition that the earth is entirely solid, and they are inclined,
therefore, to adopt the hypothesis of a sub-crust layer of liquid
matter. Whether this be actually the condition or not physicists must
be left to determine. All that we need note is, that if there be any
force in Professor Darwin's argument, it is obvious that the crust is
possessed of great rigidity, and could not be readily deformed by the
mere weight of an ice-sheet. According to Dr. Drygalski, however, the
presence of an ice-sheet, by reducing the temperature of the underlying
crust, would bring about contraction, and in this way cause the surface
to sink. When the ice-sheet had disappeared, then free radiation of
earth-heat would be resumed, the depressed isogeotherms would rise,
and a general warming of the upper portion of the lithosphere would
take place. But the space occupied by the depressed section, owing to
the spheroidal form of the earth, would be smaller than that which it
occupied before sinking had commenced, and consequently when the ice
vanished expansion of the crust would follow, and the land-surface
would then rise again. The whole question is one for physicists to
decide upon, but I may point out that if Drygalski's explanation be
well founded, then it is obvious that it throws no light upon the
origin and subsequent disappearance of an ice-sheet. Somehow or other
this ice-sheet comes into existence, and the cooling and contracting
crust sinks below it; and that depressed condition of the glaciated
area must continue so long as the ice-sheet remains unmelted.
Re-elevation can only take place when, owing to some other cause or
causes, the climate changes and the ice-sheet vanishes.
Those who advocate the "earth-movement hypothesis" as an explanation of
the origin of extensive glaciation have welcomed Mr. Jamieson's view as
harmonising well with their conclusions. They contend, as we have seen,
that glacial conditions were induced by an extensive upheaval of the
crust in northern latitudes, accompanied by a depression of the Isthmus
of Panama. They then proceed to point out that the ice-sheets brought
about their own dissolution by pressing down the crust, and introducing
with submergence a disappearance of glacial conditions. See now how
much they take for granted. In the first place, they assume an amount
of pre-glacial or early glacial elevation of northern regions for which
not a scrap of evidence can be adduced, while they can give no proof of
contemporaneous depression of the Isthmus of Panama. Next, relying on
Mr. Jamieson's hypothesis, they take for granted that the ice-sheets,
called into existence by their postulated earth-movements, succeeded
in depressing the earth's surface even below its present level. That
is to say, the land, which, according to them, was in glacial times
some 3000 feet higher than now, sank down under the weight of its
glacial covering for, say, 3600 feet in north-western Europe. In North
America, in like manner, all the pre-glacial elevation was lost--the
land sinking below its present level for some 200 feet in New England,
for 520 feet at Montreal, for 1000 to 1500 feet in Labrador, and for
1000 to 2000 feet in the Arctic regions. Now, even if we concede the
reasonableness of Mr. Jamieson's hypothesis, and admit that a certain
degree of deformation may take place under the mere weight of an
ice-sheet, it is difficult to believe that the crust can be so readily
deformed as the supporters of the "earth-movement hypothesis" seem to
imply. If it could yield so readily to pressure, one is at a loss to
understand how a great ice-sheet could accumulate--the ice would simply
float off as the land subsided. Take the case of north-western Europe.
The ice-sheet that covered Scotland did not attain, on the average,
3000 feet in thickness, and yet we are to suppose that it was able to
depress the land for some 600 feet below its present level--that is to
say, for 3600 feet below its assumed pre-glacial elevation. Either the
ice depressed the crust to that remarkable extent, or the land upon
which the ice accumulated was not nearly so high as the advocates of
the "earth-movement hypothesis" have supposed. But the average I have
taken for the thickness of the Scottish ice-sheet is excessive, for it
was only in the low-grounds that the _mer de glace_ attained such a
depth. A large part of our country, however, is mountainous, and the
mountain-tops were, of course, not nearly so thickly mantled with ice
as the valleys. And the same to even a larger extent holds good for the
Scandinavian peninsula. If we take the thickness of the Scandinavian
ice-sheet that coalesced with that of Scotland as 4000 feet, we shall
be over the mark. Now, I ask, is it possible to believe that a sheet
of ice of that thickness actually pressed down the crust of the earth
for not less than 3600 feet? But if we accept the "earth-movement
hypothesis," as it has been recently advocated, that is what we must
believe. If we cannot do so, then we cannot accept the assumption of
great elevation of the land in pre-glacial and glacial times. Let me
put the case shortly: if the glacial marine beds and raised beaches
of the Atlantic borders of Europe and North America owe their origin
to depression induced by the weight of an ice-sheet, then it is quite
certain that at the advent of glacial conditions the land could not
have been so highly elevated as the advocates of the "earth-movement
hypothesis" suppose. But if we are to accept the notion of great
elevation of the land, then we must conclude that the submergence
to which the raised beaches testify cannot have been caused by the
pressure of ice-sheets.
It is hardly necessary to pursue this particular subject further, but
before leaving it, attention may be drawn for a moment to the curious
conclusion that the ice-sheets were self-destructive. One is left to
guess at what particular stage the sinking process began, but if the
earth's crust were as readily deformed as the extreme views I have
been examining would compel one to imply, then depression must have
commenced almost immediately with the accumulation of snow and ice. The
several ice-sheets must soon have attained their maximum thickness,
and their disappearance must have been correspondingly rapid. And
yet all the evidence goes to show that a glacial epoch endured for
a comparatively long time--for a time sufficient to account for a
prodigious amount of rock-erosion, and for the accumulation of vast
sheets of glacial débris and fluvio-glacial detritus.[BY]