took place during the Mesozoic era, we are impressed with the fact
that, all through those changes, a wide land-surface persisted in the
north and north-west of the European area, just as was the case in
Palæozoic times. 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 being
those of the Harz, the Riesen Gebirge, and other regions of Palæozoic
and Archæan rocks. The lower parts of England and the great plains of
central Europe were sometimes submerged in the waters of a more or
less continuous sea; but ever and anon elevation ensued, and the sea
was divided, as it were, into a series of great lakes. In the south of
Europe a Mediterranean Sea would appear to have endured all through the
Mesozoic era--a Mediterranean of considerably greater extent, however,
than the present. Thus we see the main features of our Continent were
already clearly outlined before the close of the Cretaceous period. The
continental area then, as now, consisted of a wide belt of high-ground
in the north, extending roughly from south-west to north-east; south of
this a vast stretch of low-grounds, sweeping from west to east up to
the foot of the Urals, and bounded on the south by an irregular zone of
elevated land having approximately the same trend; still further south,
the maritime tracts of the Mediterranean basin. During periods of
depression the low-grounds of central Europe were invaded by the sea,
and the Mediterranean at the same time extended north over many regions
which are now dry land. It is in these two low-lying tracts, therefore,
and the country immediately adjoining them, that the Mesozoic strata of
Europe are chiefly developed.
A general movement of upheaval[DH] supervened at the close of the
Cretaceous period, and the sea which, during that period, overflowed
so much of middle Europe had largely disappeared before the beginning
of Eocene times. The southern portions of the continent, however, were
still mostly under water, while great bays and arms of the sea extended
northwards now and again into central Europe. On to the close of the
Miocene period, indeed, southern and south-eastern Europe consisted
of a series of irregular straggling islands and peninsulas washed
by the waters of a genial sea. Towards the close of early Cainozoic
times, the Alps, which had hitherto been of small importance, were
greatly upheaved, as were also the Pyrenees and the Carpathians. The
floor of the Eocene sea in the Alpine region was ridged up for many
thousands of feet, its deposits being folded, twisted, inverted, and
metamorphosed. Another great elevation of the same area was effected
after the Miocene period, the accumulations of that period now forming
considerable mountains along the northern flanks of the Alpine
chain. Notwithstanding these gigantic elevations in south-central
Europe--perhaps in consequence of them--the low-lying tracts of what
is now southern Europe continued to be largely submerged, and even the
middle regions of the continent were now and again occupied by broad
lakes which sometimes communicated with the sea. In Miocene times, for
example, an arm of the Mediterranean extended up the Rhone valley, and
stretched across the north of Switzerland to the basin of the Danube.
After the elevation of the Miocene strata these inland stretches of
sea disappeared, but the Mediterranean still overflowed wider areas in
southern Europe than it does in our day. Eventually, however, in late
Pliocene times, the bed of that sea experienced considerable elevation,
newer Pliocene strata occurring in Sicily up to a height of 3000 feet
at least. It was probably at or about that period that the Black Sea
and the Sea of Asov retreated from the wide low-grounds of southern
Russia, and that the inland seas and lakes of Austria-Hungary finally
vanished.
[DH] I now doubt whether any vertical upheaval of a wide continental
area is possible. The so-called "continental uplifts" are probably in
most cases rather negative than positive elevations. In other words,
the land seems to rise simply because the sea retreats owing perhaps to
the sinking of the crust within the great oceanic basins. See on this
subject, Article XIII.
The Cainozoic era is distinguished in Europe for its volcanic
phenomena. The grandest eruptions were those of Oligocene times. To
that date belong the basalts of Antrim, Mull, Skye, the Faröe Islands,
and the older series of volcanic rocks in Iceland. These basalts speak
to us of prodigious fissure eruptions, when molten rock welled up along
the lines of great cracks in the earth's crust, flooding wide regions,
and building up enormous plateaux, of which we now behold the merest
fragments. The ancient volcanoes of central France, those of the Eifel
country and many other places in Germany, and the volcanic rocks of
Hungary, are all of Cainozoic age; while, in the south of Europe, Etna,
Vesuvius, and other Italian volcanoes date their origin to the later
stages of the same great era.
Thus before the beginning of Pleistocene times all the main features
of Europe had come into existence. Since the close of the Pliocene
period there have been many great revolutions of climate; several very
considerable oscillations of the sea-level have taken place, and the
land has been subjected to powerful and long-continued erosion. But
the greater contours of the surface which began to appear in Palæozoic
times, and which in Mesozoic times were more strongly pronounced,
had been fully evolved by the close of the Pliocene period. The most
remarkable geographical changes which have taken place since then 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.
The general conclusion, then, to which we are led by a review of the
greater geographical changes through which the European continent
has passed is simply this--that the substructure upon which all our
sedimentary strata repose is of primeval antiquity. Our dry lands are
built up of rocks which have been accumulated over the surface of a
great wrinkle of the earth's crust. There have been endless movements
of elevation and depression, causing minor deformations, as it were,
of that wrinkle, and inducing constant changes in the distribution
of land and water; but no part of the continental ridge has ever
been depressed to an abysmal depth. The ridge has endured through
all geological time. We can see also that the land has been evolved
according to a definite plan. Certain marked features begin to appear
very early in Palæozoic times, and become more and more pronounced
as the ages roll on. All the countless oscillations of level, all
the myriad changes in the distribution of land and water, all the
earthquake disturbances and volcanic eruptions--in a word, all the
complex mutations to which the geological record bears witness--have
had for their end the completion of one grand design.
A study of the geological structure of Europe--an examination of
the manner in which the highly folded and disturbed strata are
developed--throws no small light upon the origin of the larger or
dominant features of our Continent. The most highly convoluted rocks
are those of Archæan and Palæozoic age, and these are developed chiefly
in the north-western and western parts of the Continent. Highly
contorted strata likewise appear in all the mountain-chains of central
Europe--some of the rocks being of Palæozoic, while others are of
Mesozoic and of Cainozoic age. Leaving these mountains for the moment
out of account, we find that it is along the western and north-western
sea-board where we encounter the widest regions of highly-disturbed
rocks. The Highlands of Scandinavia and Britain are composed, for
the most part, of highly-flexed and convoluted rocks, which speak to
titanic movements of the crust; and similar much-crushed and tilted
rock-masses occur in north-west France, in Portugal, and in western
Spain. But when we follow the highly-folded Palæozoic strata of
Scandinavia into the low-grounds of the great plains, they gradually
flatten out, until in Russia they occur in undisturbed horizontal
positions. Over thousands of square miles in that country the Palæozoic
rocks are just as little altered and disturbed as strata pertaining to
Mesozoic and Cainozoic times.
These facts can have but one meaning. Could we smooth out all the
puckerings, creases, foldings, and flexures which characterise the
Archæan and Palæozoic rocks of western and north-western Europe, it
is certain that these strata would stretch for many miles out into
the Atlantic. Obviously they have been compressed and crumpled up by
some force acting upon them from the west. Now, if it be true that the
basin of the Atlantic is of primeval origin, then it is obvious that
the sinking down of the crust within that area would exert enormous
pressure upon the borders of our continental area. As cooling and
contracting of the nucleus continued, subsidence would go on under the
oceanic basin, depression taking place either slowly and gradually,
during protracted periods, or now and again more or less suddenly. But
whether gradually or suddenly effected, the result of the subsidence
would be the same upon the borders of our Continent; the strata along
the whole western and north-western margins of the European ridge would
necessarily be flexed and disturbed. Away to the east, however, the
strata, not being subject to the like pressure, would be left in their
original horizontal positions.
Now it can be shown that the mountains of Scandinavia and the British
Islands are much older than the Alps, the Pyrenees, and many other
conspicuous ranges in central and southern Europe. Our mountains and
those of Scandinavia are the mere wrecks of their former selves.
Originally they may have rivaled--they probably exceeded--the Alps
in height and extent. It is most likely, indeed, that the areas
of Palæozoic rocks in France, Portugal, and Spain also attained
mountainous elevations. But the principal upheaval of the western
margins of our Continent was practically completed before the close of
the Palæozoic period, and since that time those elevated regions have
been subjected to prodigious erosion, the later formations being in
large measure composed of their débris. I do not, of course, wish it
to be understood that there has been no upheaval affecting the west
of Europe since Palæozoic times. The tilted position of many of our
Mesozoic strata clearly proves the contrary. But undoubtedly the main
disturbances which produced the folding, fracturing, and contortion
of the Palæozoic strata of western Europe took place before the close
of the Palæozoic period. The mountains of Britain and Scandinavia are
amongst the oldest in Europe.
When we come to inquire into the origin of the mountains of central
Europe we have little difficulty in detecting the chief factors
in their formation. An examination of the Pyrenees, the Alps, and
other hill-ranges having the same general trend shows us that they
consist of flexed and convoluted rocks. They are, in short, mountains
of elevation, ridged up by tangential thrusts. Of this we need not
have the slightest doubt. If, for example, we approach the Alps from
the low-grounds of France, we observe the strata as we come towards
the Jura beginning to undulate--the undulations becoming more and
more marked, and passing into sharp folds and plications, until, in
the Alps, the beds become twisted, convoluted, and bent back upon
themselves in the wildest confusion. Now, speaking in general terms, we
may say that similar facts confront us in connection with every true
mountain-range in central Europe. Let it be noted, further, that all
those ranges have the same trend, which we may take to be approximately
east and west, or nearly at right angles to the trend of the Palæozoic
high-grounds of western and north-western Europe. Looked at broadly,
our continental ridge may be said to be traversed from west to east
by two wide depressions or troughs, separated by the intervening
belt of higher grounds just referred to. The former of these troughs
corresponds to the great central plain, which passes through the south
of England, north-east France, the Low Countries, and Denmark, whence
it sweeps east through Germany, and expands into the wide low-grounds
of Russia. The southern trough or depression embraces the maritime
tracts of the Mediterranean and the regions which that sea covers.
Such, then, are the dominant features of our Continent, to which all
others are of subordinate importance. Now it cannot be doubted that
the two great troughs are belts of subsidence in the continental ridge
itself. And their existence explains the origin of the mountain-ranges
which separate them. We know that the northern trough is of extreme
antiquity; it is older, at all events, than the Silurian period. Even
at that distant date its southern limits were marked out by ridges
of Archæan rocks, which seem to have formed islands in what is now
middle Germany, and probably also in Switzerland and central France.
The appearance of those Archæan rocks in central Europe was doubtless
due to a ridging up of the crust induced by those parallel movements
of subsidence which produced the northern and southern troughs. The
northern trough 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 seas which overflowed what are now the central plains
of Europe were of less considerable depth than that which occupied
the Mediterranean trough. As time rolled on, therefore, the northern
trough eventually became silted up; but so low even now is the level of
that trough that a relatively slight depression would cause the sea to
inundate most extensive regions in middle Europe.
In Cainozoic times, as we have seen, the last great elevation of the
Alps was effected--an elevation which can hardly have been due to any
other cause than the more or less abrupt depression of the earth's
crust under the Mediterranean basin. The area of that sea is now much
less considerable than it was in Tertiary times--a change due in part
to silting up, but chiefly perhaps to the sinking down of its bed to
profounder depths.
Thus we may conclude that from a very early period--a period
ante-dating the formation of our oldest fossiliferous strata--the
physical structure of our Continent had already been planned. The
dominant features of the primeval continental ridge are those
which have endured through all geological time. They are the
lines along which the beautiful lands in which we dwell have been
constructed. Tilted and convoluted, broken and crushed by myriad
earth-movements--scarred, furrowed, worn and degraded by the frosts,
the rains, the rivers, and the seas of countless ages--the rocks of our
Continent are yet eloquent of design. Where the ignorant sees nothing
save confusion and discord, the thoughtful student beholds everywhere
the evidence of a well-ordered evolution. Such is the conclusion to
which we are led by all geological research.
[Illustration:
SKETCH-MAPS ILLUSTRATING THE GEOGRAPHICAL EVOLUTION
OF CONTINENTAL AREAS
By PROFESSOR JAMES GEIKIE, LL.D., D.C.L., F.R.S.
PLATE V
+-----------------------------+------------------------------+
| MAP SHOWING THE | MAP SHOWING THE |
| AREA OF CONTINENTAL PLATEAU | AREA OF CONTINENTAL PLATEAU |
| OCCUPIED BY SEA IN | OCCUPIED BY SEA IN |
| PALÆOZOIC TIMES. | TERTIARY TIMES. |
+-----------------------------+------------------------------+
| MAP SHOWING THE | MAP SHOWING THE |
| AREA OF CONTINENTAL PLATEAU | AREAS OF DOMINANT DEPRESSION |
| OCCUPIED BY SEA IN | AND ELEVATION. |
| MESOZOIC TIMES. | (Below & Above the 1000 |
| | fathom Contour Line) |
+-----------------------------+------------------------------+
The Edinburgh Geographic Institute J. G. Bartholomew, F.R.G.S.
]
XII.
The Evolution of Climate.[DI]
[DI] Address delivered before the Royal Physical Society at the opening
of the Session 1889-90.
One of the most interesting questions with which geological science
has to deal is that of the evolution of climate. Although there is no
general agreement as to how former climatic fluctuations came about,
yet the prevalent opinion is that in the past, just as in the present,
the character of the climate must have depended mainly on latitude
and the relative position of the great land- and water-areas. This
was the doctrine taught by Lyell, and its cogency none will venture
to dispute. It is true he postulated a total redistribution of oceans
and continents--a view which the progress of science has shown to be
untenable. We can no longer speculate with him on the possibility of
all the great land-areas having been grouped at one time round the
equator, and at some other period about the poles. On the contrary, the
evidence goes to show that the continents have never changed places
with the ocean--that the dominant features of the earth's crust are
of primeval antiquity, and ante-date the oldest of the fossiliferous
formations. The whole question of climatic changes, therefore, must
be reconsidered from the point of view of the modern doctrine of the
permanency of continental and oceanic areas.
But before proceeding to this discussion, it may be well to glance
for a moment at the evidence from which it has been inferred that the
climate of the world has varied. Among the chief proofs of climatic
fluctuations are the character and the distribution of former floras
and faunas. It is true, fossils are, for the most part, relics of
extinct forms, and we cannot assert of any one of these that its
environment must have been the same as that of some analogous living
type. But, although we can base no argument on individual extinct
forms, it does not follow that we are precluded from judging of
the conditions under which a whole suite of extinct organisms may
have lived. Doubtless, we can only reason from the analogy of the
present; but, when we take into account all the forms met with in some
particular geological system, we seem justified in drawing certain
conclusions as to the conditions under which they flourished. Thus,
should we encounter in some great series of strata many reef-building
corals, associated with large cephalopods and the remains of tree-ferns
and cycads, which last from their perfect state of preservation could
not have drifted far before they became buried in sediment, we should
surely be entitled to conclude that the strata in question had been
deposited in the waters of a genial sea, and that the neighbouring
land likewise enjoyed a warm climate. Again should a certain system,
characterised by the presence of some particular and well-marked flora
and fauna, be encountered not only in sub-tropical and temperate
latitudes but also far within the Arctic Circle, we should infer that
such a flora and fauna lived under climatic conditions of a very
different kind from any that now exist. The very presence, in the
far north, of fossils having such a geographical distribution would
show that the temperature of polar seas and lands could not have been
less than temperate. When such broad methods of interpretation are
applied to the problems suggested by former floras and faunas, we
seem compelled to conclude that the conditions which determined the
distribution of life in bygone ages must have been, upon the whole,
more uniform and equable than they are now. It is unnecessary that I
should go into detailed proof; but I may refer, by way of illustration,
to what is known of the Silurian and Carboniferous fossils of the
arctic regions. Most of these occur also in the temperate latitudes
of Europe and North America, while many are recognised as distinctive
types of the same strata nearly all the world over. As showing how
strongly the former broad distribution of life-forms is contrasted
with their present restricted range, Professor Heilprin has cited the
Brachiopoda. Taking existing species and varieties as being 135 in
number, he remarks that "there is scarcely a single species which can
be said to be strictly cosmopolitan in its range, although not a few
are very widely distributed; and, if we except boreal and hyperboreal
forms, but a very limited number whose range embraces opposite sides
of the same ocean. On the other hand, if we accept the data furnished
by Richthofen concerning the Chinese Brachiopoda we find that out
of a total of thirteen Silurian and twenty-four Devonian species,
no less than ten of the former and sixteen of the latter recur in
the equivalent deposits of western Europe: and, further, that the
Devonian species furnish eleven, or nearly 50 per cent. of the entire
number, which are cosmopolitan or nearly so. Again, of the twenty-five
Carboniferous species, North America holds fully fifteen, or 60 per
cent., and a very nearly equal number are cosmopolitan." The same
palæontologist reminds us that by far the greater number of fossils
which occur in the Palæozoic strata of Australia are present also in
regions lying well within the limits of the north temperate zone. "In
fact," he continues, "the relationship between this southern fauna and
the faunas of Europe and North America is so great as to practically
amount to identity."
But, side by side with such evidence of broad distribution, we are
confronted with facts which go to show that, even at the dawn of
Palæozoic times, the oceanic areas at all events had their more or less
distinct life-provinces. While many of the old forms were cosmopolitan,
others were apparently restricted in their range. It would be strange,
indeed, had it been otherwise; for, however uniform the climatic
conditions may have been, still that uniformity was only comparative.
An absolutely uniform world-climate is well-nigh inconceivable. All we
can maintain is that the conditions during certain prolonged periods
were so equable as to allow of the general diffusion of species over
vastly greater areas than now; and that such conditions extended
from low latitudes up to polar regions. Now, among the chief factors
which in our day determine the limitation of faunas and floras, we
must reckon latitude and the geographical position of land and water.
What, then, it may be asked, were the causes which allowed of the much
broader distribution of species in former ages?
It is obvious that before a completely satisfactory answer to that
question can be given, our knowledge of past geographical conditions
must be considerably increased. If we could prepare approximately
correct maps and charts to indicate the position of land and sea
during the formation of the several fossiliferous systems, we should
be able to reason with some confidence on the subject of climate.
But, unfortunately, the preparation of such correct maps and charts
is impossible. The data for compilations of the kind required are
still inadequate, and it may well be doubted whether, in the case of
the older systems, we shall ever be able to arrive at any detailed
knowledge of their geographical conditions. Nevertheless, the
geological structure of the earth's crust has been so far unravelled as
to allow us to form certain general conceptions of the conditions that
must have attended the evolution of our continents. And it is with such
general conceptions only that I have at present to deal.
I said a little ago that the question of geological climates must now
be considered from the point of view of the permanency of the great
dominant features of the earth's crust. I need not recapitulate the
evidence upon which Dana and his followers have based this doctrine
of the primeval antiquity of our continental and oceanic areas. It is
enough if I remind you that by continental areas we simply mean certain
extensive regions in which elevation has, upon the whole, been in
excess of depression; by oceanic area, on the other hand, is meant that
vast region throughout which depression has exceeded elevation. Thus,
while the area of permanent or preponderating depression has, from
earliest geological times, been occupied by the ocean, the continental
areas have been again and again invaded by the sea--and even now
extensive portions are under water. It is not only the continental dry
land, therefore, but all the bordering belt of sea-floor which does not
exceed 1000 fathoms or so in depth, that must be included in the region
of dominant elevation. Were the whole of this region to be raised above
the level of the sea, the present continents would become connected so
as to form one vast land-mass, or continental plateau. (D, Plate IV.)