accumulated over the surface of that great plateau, and consequently
are of comparatively shallow-water origin. They show us, in fact, that
at no time in geological history has that plateau ever been drowned in
depths at all comparable to those of the deeper portions of our oceanic
troughs. The stratified rocks teach us, moreover, that the present
land-areas have been gradually evolved, and that, notwithstanding
many oscillations of level, these areas have continued to increase
in extent--so that there is probably more land-surface now than at
any previous era in the history of our globe. To give even a meagre
outline of the evidence bearing upon this interesting subject is here
impossible. All that I can do is to indicate very briefly some of the
general results to which that evidence seems to lead.
The oldest rocks with which we are acquainted are the so-called Archæan
schists[DJ] But these have hitherto yielded no unequivocal traces
of organic life, and as their origin is still doubtful, it would
obviously be futile to speculate upon the geographical conditions of
the earth's surface at the time of their formation. Reliable geological
history only begins with the fossiliferous strata of the Palæozoic
era. From these we learn that in the European area the Archæan rocks
of Britain, Scandinavia, and Finland formed, at that time, the most
extensive tract of dry land in our part of the world. How far beyond
the present limits of Europe that ancient northern land extended we
cannot tell; but it probably occupied considerable regions which are
now submerged in the waters of the Arctic Ocean. Further south, the
continental plateau appears to have been, for the most part, overflowed
by a shallow sea, the surface of which was dotted by a few islands of
Archæan rocks, occupying the sites of what are now some of the hills
of middle Germany and the Archæan districts of France and the Iberian
Peninsula. Archæan rocks occur likewise in Corsica and Sardinia,
and again in Turkey: they also form the nuclei of most of the great
European mountain-chains, as the Pyrenees, the Alps, the Carpathians,
and the Urals. These areas of crystalline schists may not, it is true,
have existed as islands at the beginning of Palæozoic times, for they
were doubtless ridged up by successive elevations at later dates; but
their very presence as mountain-nuclei is sufficient to show that at
a very early geological period, the continental plateau could not
have been covered by any great depth of sea. We can go further than
this--for all the evidence points to the conclusion that, even so far
back as Cambrian times, the dominant features of the present European
continent had been, as it were, sketched out. Looked at broadly, that
part of the great continental plateau upon which our European lands
have been gradually built up may be said to be traversed from west to
east by two wide depressions, separated by an intervening elevated
tract. The former of these depressions corresponds to the great Central
Plain which passes through the south of England, north-east of France,
and the Low Countries, whence it sweeps through Germany, to expand into
the extensive low-grounds of central and northern Russia. The southern
depression embraces the maritime tracts of the Mediterranean, and the
regions which that sea covers. To these dominant features all the
others are of subordinate importance. The two great troughs are belts
of depression in the continental plateau itself. The northern one is of
extreme antiquity--it is older, at all events, than the Cambro-Silurian
period. Even at that distant date its southern limits were marked
out by ridges of Archæan rock, which, as I have said, seem to have
formed islands in what is now central Europe. It was probably always
the shallower depression of the two, for we have evidence to show that
again and again, in Mesozoic and later times, the sea that overflowed
what are now the central lowlands of Europe was of less considerable
depth than that which occupied the Mediterranean trough.
[DJ] I need hardly remind geologists that some of the so-called
"Archæan schists" may really be the highly altered accumulations of
later geological periods.
If we turn to North America, we find similar reason to conclude, with
Professor Dana, that the general topography of that region had likewise
been foreshadowed as far back as the beginning of the Palæozoic era.
Dana tells us that even then the formation of its chief mountain-chains
had been commenced, and its great intermediate basins were already
defined. The oldest lands of North America were built up, as in
Europe, of azoic rocks, and were grouped chiefly in the north. Archæan
masses extend over an enormous region, from the shores of the Arctic
Ocean down to the great lake country, and they are seen likewise in
Greenland and many of the Arctic islands. They appear also in the
long mountain-chains that run parallel with the coast-lines of the
Continent. In a word, the present distribution of the Archæan rocks,
and their relation to overlying strata, lead to the belief that in
North America, just as in Europe, they form the foundation-stones of
that continent, and stretch continuously throughout its whole extent.
We know comparatively little of the geology of the other great
land-masses of the globe, but from such evidence as we have there is
reason to believe that these in their general structure have much the
same story to tell as Europe and North America. In South America,
Archæan rocks extend over vast areas in the east and north-east, and
reappear in the lofty mountain-chains of the Pacific border. They have
been recognised also in various parts of Africa, alike in the north
and east, in the interior, and in the west and south. In Asia, again,
they occupy wide areas in the Indian Peninsula; they are well developed
in the Himalaya, while in China and the mountains and plateaux of
central Asia, azoic rocks, which are probably of Archæan age, are well
developed. The crystalline schists, which cover extensive tracts in
Australia and in the northern island of New Zealand, have also been
referred to the same age. Thus, all the world over, Archæan rocks seem
to form the surface of the ancient continental plateau upon which all
other sedimentary strata have been accumulated. And in every region
where Palæozoic rocks occur, we have evidence to prove that at the time
these last were formed vast areas of the old continental plateau were
under water.
The geological structure of the Palæozoic tracts of Europe and America
has shown us that, during the protracted period of their accumulation,
and notwithstanding many oscillations of level, the land-surface
continued to increase. The same growth of dry land characterised
Mesozoic and Cainozoic times--the primeval depressions that traverse
the continental plateau became more and more silted up, and the sea
eventually disappeared from extensive regions which it had overflowed
in Palæozoic ages. This land-growth, of course, was not everywhere
continuous. Again and again, throughout wide tracts, depression was
in excess of sedimentation and elevation. Even at the present time,
broad tracts of what was once dry land are submerged. But the simple
fact that the younger fossiliferous strata do not extend over such wide
areas as the older systems, is sufficient proof that our land-masses
have all along tended to grow, and to become more and more consolidated.
Reference has already been made to the remarkable fact that no abysmal
accumulations have yet been detected amongst the stratified rocks of
the earth's crust. Ordinary clastic rocks, such as shale, sandstone,
and conglomerate--altered or unaltered, as the case may be--form by
far the largest proportion of our aqueous strata, and speak to us only
of shallow waters. It is true that some of our limestones must have
accumulated in moderately deep clear seas, yet none of these limestones
is of abysmal origin. They prove that portions of the continental
plateau have now and again been submerged for several thousand feet,
but afford no evidence of depths comparable to those of the present
oceanic basins. The enormous thickness obtained by the sedimentary
strata can only be explained on the supposition that deposition took
place over a gradually sinking area. And thus it can be shown that,
within the continental plateau, movements of depression have been
carried on more or less continuously during vast periods of time--and
yet so gradually, that sedimentation was able to keep pace with them.
Take, for example, the Cambrian strata of Wales and Shropshire--all,
apparently, shallow-water deposits--which attain a thickness of 30,000
feet, or thereabout; or the Silurian strata of the same regions, which
are not much less than 20,000 feet thick; and similar great depths of
sedimentary rocks might be cited from North America. Passing on to
later periods, we find like evidence of long-continued depression in
the thick sediments of the younger Palæozoic systems. It is noteworthy,
however, that when we come down to still later ages, the movements of
depression, as measured by the depths of the strata, appear to have
become less and less extensive and profound. Each such movement of
depression was eventually brought to a close by one or more movements
of upheaval--slowly or more rapidly effected, as the case may have
been. Here, then, we are confronted with the striking fact that the
continental plateau has, from time to time, sunk down over wide areas
to depths exceeding those of existing oceans, and yet at so slow a
rate, that sedimentation prevented the depressed regions from becoming
abysmal. It is obvious, then, that such areas are now dry land simply
because, in the long-run, sedimentation and upheaval have been in
excess of depression.
And yet, notwithstanding the numerous upheavals which have taken place
over the continental plateau, these have succeeded in doing little
more than drain away the sea more or less completely from the great
primeval depressions by which that plateau is traversed. If it be true,
therefore, that the continental plateau owes its existence to the
sinking down of the earth's crust within the oceanic basins--if the
continents have been squeezed up by the tangential thrusts exerted by
the sinking areas that surround them--then it follows that while lands
have been gradually extending over the continental plateau, the bed of
the ocean has been sinking to greater and greater depths.
If this general conclusion holds good, it is obvious that the oceanic
troughs of early geological times could not have been so deep as they
are now. During the Palæozoic period, the most continuous areas of
dry land, as we have seen, were distributed over the northern parts
of our hemisphere, while, further south, groups of islands indicated
the continuation of the continental plateau. Doubtless South America,
Africa, Asia, and Australia were, at that distant date, represented by
similar detached areas of dry land. In a word, the primeval continental
plateau was still largely under water. Judging from the character and
broad distribution of the Palæozoic marine faunas the temperature of
the sea was wonderfully uniform. There is certainly nothing to indicate
the existence of such climatic zones as those of the present. We know
very little of the terrestrial life of early Palæozoic times--the
Cambro-Silurian strata are essentially marine. Land-plants, however,
become more numerous in the Old Red Sandstone, and, as every one knows,
they abound in the succeeding Carboniferous and Permian systems. And
the testimony of these floras points to the same conclusion as that
furnished by the marine faunas. The Carboniferous floras of the arctic
regions, and of temperate Europe and America, not only have the same
_facies_, but a considerable number of the species is common to both
areas; while many European species occur in the Carboniferous strata
of Australia and other distant lands. This common _facies_, and the
presence of numerous cosmopolitan forms, surely indicate the former
prevalence of remarkably uniform climatic conditions. The conditions,
of course, need not--indeed, could not--have been absolutely uniform.
At present the various climates which our globe experiences depend
upon the amount of heat received directly and indirectly from the
sun--oceanic and aërial currents everywhere modifying the results that
are due to latitude. It cannot have been otherwise in former times.
In all ages the tropics must have received more direct sun-heat than
temperate and polar regions; and however much the climatic conditions
of the Palæozoic era may have differed from the present--however
uniformly temperature may have been distributed--still, as I have
said, absolute uniformity was impossible. It was doubtless owing to
the fact that the dry lands of Palæozoic times were not only much less
extensive than now, but more interrupted, straggling, and insular,
that the climate of the globe was so equable. Under such geographical
conditions, great oceanic currents would have a much freer course than
is now possible, and warm water would find its way readily across
wide regions of the submerged continental plateau into the highest
latitudes. The winds blowing athwart the land would everywhere be moist
and warm, and no such marked differences of temperature, such as now
obtain, would distinguish the arctic seas from those of much lower
latitudes. At the same time, the comparatively shallow water overlying
the submerged areas of the continental plateau would favour the
distribution of species, and thus bring about that wide distribution of
cosmopolitan forms and general similarity of _facies_, which are such
marked features of the Palæozoic faunas. It is even quite possible that
migration may have taken place here and there across the great oceanic
depression itself; for it may well be doubted whether, at so early a
period, the depression had sunk down to its present depth below the
level of the continental plateau.
Yet, notwithstanding such facilities for migration, and the consequent
similarity of _facies_ I have referred to, the Palæozoic faunas of
different regions have usually certain distinctive characters. Even at
the very dawn of the era the marine faunas were already grouped into
provinces, sometimes widely separated from one another, at other times
closely adjacent, so that it is evident that barriers to migration
here and there existed. It could hardly have been otherwise; for local
and more widely-spread movements of elevation and depression took place
again and again during Palæozoic times.
While the younger Palæozoic systems were being accumulated, excess
of upheaval over depression resulted in the gradual increase of the
land.[DK] The continental plateau came more and more to the surface,
in spite of many oscillations of level. It is quite possible, nay,
even probable that this persistent growth of land, and consequent
modification of oceanic currents may have rendered the climatic
conditions of later Palæozoic times less uniform: but, if so, such
diminished uniformity has left no recognisable impress on either faunas
or floras; for fossils characteristic of the Devonian and Carboniferous
strata of temperate latitudes occur far within the Arctic Circle.
[DK] See footnote p. 341.
Descending to the Mesozoic era, we find that the character and
distribution of marine faunas are still indicative of uniformity.
There could have been little difference of temperature at that time
between arctic seas and those of our own latitude. Cosmopolitan species
abounded in the Jurassic waters, but were relatively less numerous
in those of the Cretaceous period. Professor Neumayr maintains that
already, in the Jurassic period, the climate had become differentiated
into zones. This, he thinks, is indicated by the fact that coral reefs
abound in the Jurassic strata of central Europe, while they are wanting
in the contemporaneous deposits of boreal regions. Dr. Heilprin, on
the other hand, is of opinion that this and certain other distinctive
features of separate Jurassic life-provinces may not have been due to
differences of temperature, but rather to varying physical conditions,
such as character of the sea-bottom, depth of water, and so forth.
Perhaps the safest conclusion we can come to, in the present state
of the evidence, is that the climatic conditions of the Mesozoic era
were, upon the whole, less obviously uniform than those of earlier
ages, but that marked zones of climate like the present had not as
yet been evolved. At the same time, when we consider how many great
geographical revolutions took place during the period in question, we
must be prepared to admit that these could hardly fail to influence the
climate, and thus to have induced modifications in the distribution of
faunas and floras. And probably evidence of such modifications will yet
be recognised, if indeed the phenomena referred to by Neumayr be not
a case in point. It may be noted, further, that while, according to
many botanists, the plants of the Palæozoic periods bespeak not only
uniform climatic conditions but the absence of marked seasonal changes,
those of late Mesozoic times are indicative of less uniformity. The
Cretaceous conifers, for example, show regular rings of growth, and
betoken the existence of seasons, which were less marked, however, than
is now the case.
The geographical changes of Mesozoic times were notable in many
respects. The dominant features of Europe, already foreshadowed in
early Palæozoic times, had become more clearly outlined before the
close of the Cretaceous period. Notwithstanding many movements of
depression, the chief land-areas continued to show themselves in the
north and north-west. The highest grounds were the Urals, and the
uplands of Scandinavia and Britain. In middle Europe the Pyrenees and
the Alps were as yet inconsiderable heights, the loftiest lands in that
region being those of the Harz, the Riesen Gebirge, and other tracts of
Archæan and Palæozoic rocks. The lower parts of England and the great
lowland plains of central Europe were sometimes submerged in the waters
of a wide, shallow sea, but ever and anon elevation ensued, new lands
appeared, and these waters became divided into a series of large inland
seas and lakes. In the south, a deep Mediterranean sea would appear
to have persisted all through the Mesozoic era--a sea of considerably
greater extent, however, than the present.
While in Europe the dominant features of the continental plateau run
approximately east and west, in North America they follow nearly the
opposite direction. In early Mesozoic times, vast tracts of dry land
extended across the northern and eastern sections of the latter area.
Over the Rocky Mountain region, low lands and saline lakes appear
to have stretched, while further west the area of the Great Plateau
and the Pacific slope were covered by the sea. Towards the end of
the Mesozoic era, the land in the far west became more continuous--a
broad belt extending in the direction of the Pacific coast-line from
Mexico up to high northern latitudes. In short, before the Cretaceous
period closed, the major portion of North America had been evolved. A
considerable tract of what is now the western margin of the continent,
however, was still under water, while from the Gulf of Mexico (then
much wider than now) a broad Mediterranean sea swept north and
north-west through Texas and the Rocky Mountain region to communicate
with the Arctic Ocean. All to the east of this inland sea was then, as
it is now, dry land. Thus, up to the close of the Cretaceous period,
in America and Europe alike, oceanic currents coming from the south
had ready access across the primeval continental plateau to the higher
latitudes. Southern Europe indeed, during Mesozoic times, was simply a
great archipelago, having free communication on the one hand across the
low-grounds of central and northern Russia with the arctic seas, and,
on the other, across vast regions in Asia with the Indian Ocean.
Of the other great land-masses of the globe our knowledge is too
limited to allow us to trace their geographical evolution with any
confidence. But from the very wide distribution of Mesozoic strata
in South America, Africa, Asia, and Australia, there can be no doubt
that, at the time of their accumulation, enormous tracts in those
regions were then under water. The land-masses, in short, were not so
continuous and compact as they are at present. And although we must
infer that considerable areas of Mesozoic land are now submerged, yet
these cannot but bear a very small proportion to the wide regions which
have been raised above the sea-level since Mesozoic times. In short,
from what we do know of the geological structure of the continents
in question, we can hardly doubt that they have passed through
geographical revolutions of a like kind with those of Europe and North
America. Everywhere over the great continental plateau elevation
appears, in the long-run, to have been in excess of depression, so
that, in spite of many subsidences, the tendency of the land throughout
the world has been to extend its margins, and to become more and
more consolidated. The Mesozoic lands were larger than those of the
preceding Palæozoic era, but they were still penetrated in many places
by the sea, and warm currents could make their way over wide tracts
that are now raised above the sea-level. Under such circumstances
approximately uniform conditions of climate could not but obtain.
Great geographical changes supervened upon the close of the Cretaceous
period. North America then acquired nearly its present outline.
Its Mediterranean sea had vanished, but the Gulf of Mexico still
overflowed a considerably wider region than now, while a narrow margin
of the Pacific border of the continent continued submerged. In Europe
elevation ensued, and the sea which had overspread so much of the
central and eastern portions of our Continent disappeared. Southern
Europe, however, was still largely under water, while bays and inlets
extended northwards into what are now the central regions of the
Continent. On to the close of the Miocene period, indeed, the southern
and south-eastern tracts of Europe were represented by straggling
islands. In middle Cainozoic times the Alps, which had hitherto been of
small importance, were considerably upheaved, as were also the Pyrenees
and the Carpathians; and a subsequent great elevation of the Alpine
area was effected after the Miocene period. Notwithstanding these
gigantic movements, the low-lying tracts of what is now southern Europe
continued to be largely submerged, and even the central regions of the
Continent were now and again occupied by broad lakes, which sometimes
communicated with the sea. After the elevation of the Miocene strata,
these inland seas disappeared, but the Mediterranean still overflowed
wider areas than it does to-day. Eventually, however, in late Pliocene
times, the bed of that sea experienced considerable elevation; and it
was probably at or about this stage that the Black Sea and the Sea of
Asov retreated from the broad low-grounds of southern Russia, and that
the inland seas and lakes of Austria-Hungary finally vanished.