disappear from such broad areas of the continental plateau, induced
many changes in the floras and faunas of the globe. A notable break in
the succession occurs between the Cretaceous and the Eocene, hardly one
species of higher grade than the protozoa passing from one system to
the other. In the Cainozoic deposits we are no longer confronted with
numerous cosmopolitan species--the range of marine forms has become
much more restricted. Nevertheless, the faunas and floras continue
to be indicative of much warmer climates for arctic and temperate
latitudes than now obtain. But, at the same time, differentiation
of climate into zones is distinctly marked. In the early Cainozoic
period, our present temperate latitudes supported a flora of decidedly
tropical affinities, while the fauna of the adjacent seas had a
similar character. Later on the climate of the same latitudes appears
to have passed successively through sub-tropical and temperate
stages. In short, a gradual lowering of the temperature is evinced
by the character and distribution both of floras and faunas. The
differentiation of the climate during one stage of the Cainozoic era is
well illustrated by the Miocene flora. Thus, at a time when Italy was
clothed with a tropical vegetation, in which palm-trees predominated,
middle Europe had its extensive forests of evergreens and conifers,
while in the region of the Baltic conifers and deciduous trees were the
prevalent forms.
When one takes into consideration the fact that, notwithstanding many
oscillations of level, the land during Cainozoic times was gradually
extending, and the sea disappearing from wide regions which it had
formerly covered, one can hardly doubt that the seemingly gradual
change from tropical to temperate conditions was due, in large measure,
to that persistent continental growth. I confess, however, that it is
difficult to account for the very genial climate which continued to
prevail over the arctic regions. So far as one can gather from the
evidence at present available, some of the marine approaches to those
latitudes had been cut off by the movements of elevation which brought
the Cainozoic era to a close, while the arctic lands were perhaps more
extensive than they are now. The Cretaceous Mediterranean Sea of North
America had vanished, and we cannot prove that the Tertiary Sea of
southern Europe communicated across the low-grounds of Russia with the
Arctic Ocean. We know, however, that the archipelago of southern Europe
was in direct connection with the Indian Ocean, and it is most probable
that a wide arm of the same sea stretched north from the Aralo-Caspain
area through Siberia. Indeed, much of what are now the lowlands of
western and northern Asia was probably sea in Tertiary times. It seems
likely, therefore, that, even at this late period, marine currents
continued to reach the Arctic Zone across the continental plateau. When
the warm waters of the Indian Ocean eventually ceased to invade Europe,
and the Mediterranean became much restricted in area, the climate of
the whole Continent could not fail to be profoundly affected.
There is yet another line of evidence to which brief reference may be
made. I have spoken of the remarkable uniformity of climatic conditions
which obtained in Palæozoic times, and of the gradual modification
of these conditions which subsequently supervened. Now, it is worthy
of note that in their lithological characters the oldest sedimentary
strata themselves likewise exhibit a prevalent uniformity which in
later systems becomes less and less conspicuous. The Cambro-Silurian
mechanical sediments, for example, maintain much the same character
all the world over; and the like is true, although in a less degree,
of the marine accumulations of the Devonian period. The corresponding
mechanical deposits of later Palæozoic ages continue to show more
and more diversity, but at the same time they preserve a similarity
of character over much more extensive areas than is found to be the
case with the analogous sediments of the Mesozoic era. Finally, these
last are more or less strongly contrasted with the marine mechanical
accumulations of Cainozoic times, which are altogether more local in
character. This increasing differentiation is quite in keeping with
what we know of the evolution of our land-areas. In early Palæozoic
ages, when insular conditions prevailed and the major portion of the
primeval continental plateau was covered by shallow seas, it is obvious
that mechanical sediments would be swept by tidal and other currents
over enormous areas, and that these sediments would necessarily assume
a more or less uniform character. Indeed, I suspect that much of the
sediment of those early seas may have been the result of tidal scour,
and that marine erosion was more generally effective then than it is
now. With the gradual growth of the land and the consequent deflection
and limitation of currents, marine mechanical sediments would tend
to become more and more local in character. Thus the increasing
differentiation which we observe in passing from the earlier to the
later geological systems is just what might have been expected.
Summing up, now, the results of this rapid review of the evidence, we
seem justified in coming to the following conclusions:--
(1.) In Palæozoic times, Europe and North America were represented
by considerable areas of dry land, massed chiefly in the higher
latitudes, while further south groups of smaller islands were scattered
over the submerged surface of the primeval continental plateau. The
other continents appear, in like manner, to have been represented
by islands--some of which may have reached continental dimensions.
A very remarkable uniformity of climate accompanied these peculiar
geographical conditions.
(2.) In Mesozoic times, the primeval continental plateau came more and
more to the surface, but the land-areas were still much interrupted, so
that currents from tropical regions continued to have ready access to
high latitudes. The climate of the whole globe, therefore, was still
uniform, but apparently not so markedly as in the preceding era.
(3.) In Cainozoic times, the land-masses continued to extend, and
the sea to retreat from hitherto submerged areas of the continental
plateau; and this persistent land-growth was accompanied by a gradual
lowering of the temperature of northern and temperate latitudes, and a
more and more marked differentiation of climate into zones.
Having thus very briefly sketched the geographical evolution of the
land during Palæozoic, Mesozoic, and Tertiary times, and come to the
general conclusion that climate has varied according to the relative
position of land and sea, I have next to consider the geographical
and climatic conditions of the Quaternary period. These, however,
are now so well known, that I need to no more than remind you that,
so far as the chief features of our lands are considered, all these
had come into existence before the dawn of the Ice Age. The greater
contours of the surface, which were foreshadowed in Palæozoic times,
and which in Mesozoic times were more clearly indicated, had been fully
evolved by the close of the Pliocene period. The connection between the
Mediterranean and the Indian Ocean probably ceased in late Pliocene
times. The most remarkable geographical changes which have taken place
since then within European regions have been successive elevations
and depressions, in consequence of which the area of our Continent
has been alternately increased and diminished. At a time well within
the human period, our own islands have been united to themselves and
the Continent, and the dry land has extended north-west and north, so
as to include Spitzbergen, the Faröe Islands, and perhaps Iceland. On
the other hand, our islands have been within a recent period largely
submerged. Similarly, in North America, we are furnished with many
proofs of like oscillations of level having taken place in Quaternary
times. Is it possible, then, to explain the climatic vicissitudes of
the Pleistocene period by means of such oscillations? Many geologists
have tried to do so, but all these attempts have failed. It is quite
true that a general elevation of the land in high latitudes would
greatly increase the ice-fields of arctic regions, and might even give
rise to perennial snow and glaciers in the mountain-districts of our
islands. But it is inconceivable that any such geographical change
could have brought about that general lowering of temperature over the
whole northern hemisphere which took place in Pleistocene times. For we
have to account not only for the excessive glaciation of northern and
north-western Europe, and of the northern parts of North America, but
for the appearance of snow-fields and glaciers in much more southern
latitudes, and in many parts of Asia where no perennial snow now
exists. Moreover, we have to remember that arctic conditions of climate
obtained in north-western Europe even when the land was relatively much
lower than it is at present. The arctic shell-beds of our own and other
temperate regions sufficiently prove that geographical conditions were
not the only factor concerned in bringing about the peculiar climate of
the Pleistocene period. Then, again, we must not forget that at certain
stages of the same period genial conditions of climate were coincident
with a much wider land-surface in north-western Europe than now exists.
The very fact that interglacial deposits occur in every glaciated
region is enough of itself to show that the arctic conditions of the
Pleistocene could not have resulted entirely from a mere elevation of
land in the northern parts of our hemisphere.
The only explanation of the peculiar climatic vicissitudes in question
which seems to meet the facts, so far as these have been ascertained,
is the well-known theory advanced by Dr. Croll. After carefully
considering all the objections which have been urged against that
theory, there is only one, as it seems to me, that is deserving of
serious attention. This objection is not based on any facts connected
with the Pleistocene deposits themselves, but on evidence of quite
another kind. It is admitted that were the Pleistocene deposits alone
considered, Croll's theory would fully account for the phenomena.
But, it is argued, we cannot take the Pleistocene by itself, for
if that theory be true, then climatic conditions similar to those
of the Pleistocene must have supervened again and again during the
past. Where, then, we are asked, is there any evidence in Palæozoic,
Mesozoic, or Cainozoic strata of former widespread glacial conditions?
If continental ice-sheets, comparable to those of the Pleistocene,
ever existed in the earlier ages, surely we ought to find more or
less unmistakable traces of them. Now, at first sight, this looks
a very plausible objection, but it has always seemed to me to be
based upon an assumption that is not warranted by our knowledge of
geographical evolution. Dr. Croll always admitted implicitly that high
eccentricity of the earth's orbit might have happened again and again
without inducing glacial conditions like those of the Pleistocene. The
objection takes no account of the fact that the excessive climate of
the Glacial period was only possible because of special geographical
conditions--conditions that do not appear to have been fully evolved
before Pliocene times. No one has seen this more clearly than Mr.
Wallace,[DL] with the general drift of whose argument I am quite at
one. In earlier ages, the warm water of the tropics overflowed wide
areas of our present continents--most of the dry land was more or
less insular, and the seas within the Arctic Circle were certainly
not cold as at present, but temperate and even genial. If we go back
to Cambro-Silurian times, we find only the nuclei, as it were, of our
existing continents appearing above the surface of widespread shallow
seas. It is quite impossible, therefore, that under such geographical
conditions, great continuous ice-sheets, like those of the Pleistocene,
could have existed--no matter how high the eccentricity of the earth's
orbit may have been. The most that could have happened during such
a period of eccentricity would be the accumulation of snow-fields on
mountains and plateaux of sufficient height, the formation here and
there of local glaciers, and the descent of these in some places to the
sea. And what evidence of such local glaciation might we now expect
to find? No old land-surface of that far-distant period has come down
to us: we look in vain for Cambro-Silurian _roches moutonnées_ and
boulder-clay or moraines. The only evidence we could expect is just
that which actually occurs, namely, erratics (some of them measuring
five feet and more in diameter) embedded in marine deposits. It may
be said that a few erratics are hardly sufficient to prove that a
true Glacial period supervened in Cambro-Silurian times, and I do not
insist that they are. But I certainly maintain that if any lowering
of the temperature were induced by high eccentricity of the earth's
orbit during Cambro-Silurian times, then ice-floated erratics are the
only evidence of refrigeration that we need ever hope to find. The
geographical conditions of early Palæozoic times forbade the formation
of enormous ice-sheets like those of the Pleistocene period. Extreme
climatic changes were then impossible, and periods of high eccentricity
might have come and gone without inducing any modifications of
flora and fauna which we could now recognise. We are ignorant of
the terrestrial life of the globe at that distant period, and our
knowledge of the marine fauna is not sufficient to enable us to deny
the possibility of moderate fluctuations in the temperature of the seas
of early Palæozoic times. Moreover, we must not forget there were then
no such barriers to migration as now exist. If the conditions became
temporarily unsuitable, marine organisms were free to migrate into
more genial waters, and to return to their former habitats when the
unfavourable conditions had passed away.
[DL] See _Island Life_.
The uniform climate so characteristic of the Cambro-Silurian period
appears to have prevailed likewise during the later stages of the
Palæozoic era. This we gather from a general consideration of the
floras and faunas, and their geographical distribution. The dry land,
as we have seen, continued to increase in extent; but vast areas of
the primeval continental plateau of the globe still continued under
water, and currents from southern latitudes flowed unrestricted into
polar regions. During the protracted lapse of time required for the
formation of the later Palæozoic systems several periods of high
eccentricity must have occurred. But, so far as one can judge, the
disposition of the larger land-areas was never such as to induce a
true Ice Age. Nevertheless we are not without evidence of ice-action
in Old Red Sandstone, Carboniferous, and Permian strata. And it seems
to me probable that the erratic accumulations referred to may really
indicate local action, of more or less intensity, brought about by such
lowering of the temperature as would supervene during a period of high
eccentricity. It is true we may explain the phenomena by inferring the
existence of mountains of sufficient elevation--and this, indeed, is
the usual explanation. But it is doubtful whether those who adopt that
view have fully considered what it involves. Take, for example, the
case of the breccias and conglomerates of the Lammermuir Hills, which
have all the appearance of being glacial and fluvio-glacial detritus.
These deposits overlie the highly-denuded Silurian greywackés of
Haddingtonshire in the north and of Berwickshire in the south, and have
evidently been derived from the intervening high-grounds--the width of
which between the Old Red Sandstone accumulations in question does not
exceed eight or nine miles. The breccias reach a height of 1300 feet,
while the dominating point of the intervening uplands is 1700 feet.
Under present geographical conditions it is doubtful whether perennial
snow and glaciers of any size at all could exist in the region of the
Lammermuirs at a less altitude than 7000 feet or more. But between
the breccias of Haddingtonshire and the equivalent deposits in
Berwickshire there is no space for any intermediate range of mountains
of circumdenudation of such a height. Moreover, we must remember that
under the extremely uniform conditions which obtained in Palæozoic
times the snow-line could not possibly have been attained even at that
elevation. When the Devonian coral-reefs described by Dupont were
growing in the sea that overflowed western Europe, to what height must
the southern uplands of Scotland have been elevated in order to reach
the snow-line! We may make what allowance we choose for the denudation
which the Silurian rocks of the Lammermuirs must have experienced since
the deposition of the Old Red Sandstone, but it is simply a physical
impossibility that mountains of circumdenudation of the desiderated
height could ever have existed in the Lammermuir region at the time the
coarse breccias were being accumulated.[DM] It seems to me, then, that
these breccias are in every way better accounted for by a lowering of
temperature due to increased eccentricity of the orbit. This view frees
us from the necessity of postulating excessive upheavals over very
restricted areas, and of creating Alps where no Alps could have existed.
[DM] It may be objected that the conglomerates were probably not
marine, but deposited in lakes, the beds of which may have been much
above sea-level. But from all that we know of the Old Red Sandstone of
Scotland it would appear that the lakes of the period now and again
communicated with the sea, and were probably never much above its level.
When we consider the enormous thickness of the strata that constitute
any of our larger coal-fields, we can hardly doubt that one or
more periods of high eccentricity must have occurred during their
accumulation. It does not follow, however, that we should be able
to detect in these strata any evidence of alternating cold and warm
epochs. So long as ocean-currents from the tropics found ready entrance
to polar regions across vast tracts of what is now dry land, extreme
and widespread glacial conditions were impossible. Any lowering of
temperature due to cosmical causes might indeed induce new snow-fields
and glaciers to appear, or existing ones to extend themselves in
northern regions and the most elevated lands of lower latitudes; but
such local glaciation need not have seriously affected any of the
areas in which coal-seams were being formed. For nothing appears more
certain than this--that our coal-seams as a rule were formed over
broad, low-lying alluvial lands, and in swamps and marshes, along
the margins of estuaries or shallow bays of the sea. Some seams, it
is true, are evidently formed of drifted vegetable débris, but the
majority point to growth _in situ_. The strata with which they are
associated are shallow-water sediments which could only have been
deposited at some considerable distance from any mountain-regions in
which glaciers were likely to exist. It is idle, therefore, to ask for
evidence of glacial action amongst strata formed under such conditions.
The only evidence of ice-work we are likely to get is that of erratics.
And these are not wanting, although it is probable that most of those
which are found embedded in coals have been transported by rafts of
vegetable matter or in the roots of trees. The same explanation,
however, will not account for the boulders which Sir William Dawson
has recorded from the coal-fields of Nova Scotia. He describes them as
occurring on the outside of a gigantic esker of Carboniferous age, and
thinks they were probably dropped there by floating-ice at a time when
coal-plants were flourishing in the swamps on the other side of the
gravel embankment.
If the disposition of the land-areas in Carboniferous times rendered
such an ice-age as that of the Pleistocene impossible--in other words,
if the effects flowing from high eccentricity of the orbit must to
a large extent have been neutralised--the flora and fauna of the
period can hardly be expected to yield any recognisable evidence of
fluctuating climatic conditions. When our winter happened in aphelion
new snow-fields might have appeared, or already existing glaciers
might have increased in size; while, with the winter in perihelion,
the temperature in northern latitudes would doubtless be raised. But
the general result would simply be an alternation of warm and somewhat
cooler conditions. And such fluctuations of climate might readily have
taken place without materially modifying; the life of the period.
The breccias of the Permian system have been described by Ramsay as
of glacial origin. Some geologists agree with him, while others do
not--and many have been the ingenious suggestions which these last
have advanced in explanation of the phenomena. Some have tried to
show how the stones and blocks in the breccias may have been striated
without having recourse to the agency of glacier-ice, but they cannot
explain away the fact that many of the stones (which vary in size
from a few inches to three or four feet in diameter) have travelled
distances of thirty or forty miles from the parent rocks. Similar
erratic accumulations, which may belong to the same system or to the
Carboniferous, occur in India and Australia. According to Dr. Blanford,
the Indian boulder-beds are clearly indicative of ice-action, and
he does not think that they can be explained by an assumed former
elevation of the Himalaya. On the contrary, he is of opinion that the
facts are best accounted for by a general lowering of the temperature,
due probably to the action of cosmical causes. Daintree, Wilkinson, R.
Oldham, and others who have studied the Australian erratic beds have
likewise stated their belief that these are of true glacial origin.