have successively clothed and peopled the world, from Palæozoic down
to the close of Cainozoic times, the general climatic conditions
of the globe, prior to the Glacial period, would seem to have been
prevalently insular rather than continental as they are now. The
lands appear to have been formerly much less continuous, and ocean
currents from southern latitudes had consequently freer access to high
northern regions than is at present possible. In no other way can we
account for the facts connected with the geographical distribution
and extent of the fossiliferous formations. But are we to infer, from
the occurrence of similar assemblages of marine organic remains in
arctic, temperate, and tropical latitudes, that the shores of primeval
Greenland were washed by waters as warm as those of the tropics? Surely
not: an absence of very cold water in the far north is all that we
seem justified in assuming. And so, in like manner, the presence in
Greenland of fossil floras having the same general facies as those that
occur in the corresponding strata of more southern latitudes, does
not compel us to believe that conditions at all similar to what are
now met with in warm-temperate and sub-tropical lands ever obtained
in arctic regions. A relatively high winter temperature alone would
permit the range northward of many tribes of plants which are now
restricted to southern latitudes. Yet, under the most uniform insular
climatic conditions that we can conceive of, there must always have
been differences due to latitude--although such differences were never
apparently so marked as they are now.
In order to appreciate the character of the climate which must have
prevailed when the lands of the globe were much more interrupted
and insular than at present, we have only to consider how greatly
isothermal lines, even under existing continental conditions, are
deflected by ocean-currents. In the North Atlantic, for example, the
winter isotherm of 32° F. is deflected northward from the parallel
of New York to that of Hammerfest--a displacement of at least 30° of
latitude. The Arctic Sea now occupies a partially closed basin, into
which only one considerable current enters from the south. But in
earlier ages the case was otherwise, and there was often communication
across what are now our continental areas. Instead of being girdled,
as at present, by an almost continuous land-mass, the Arctic Sea seems
to have formed with the circumjacent ocean one great archipelago. Thus
freely open to the influx of southern currents, it is not difficult to
believe that the seas of the far north might never be frozen, and that
an "inland-ice" like that of Greenland would be impossible. The present
cold summers of that country, as the late Dr. Croll has insisted,
are due not so much to high latitude as to the presence of snow and
ice. Could these be removed, the summers would be as warm at least as
those of England. Now the occurrence in arctic regions of Palæozoic
and Mesozoic marine faunas is strongly suggestive of the former
presence there of genial waters having free communication with lower
latitudes; and it is to the presence of these warm currents, flowing
uninterruptedly through polar regions, that we would attribute the high
winter temperature and uniform climate to which the fossil floras and
faunas of Greenland bear testimony.
If these views be at all reasonable, it seems unnecessary to call to
our aid hypothetical changes in the position of the earth's axis. It
may be admitted, however, that the climate of the Arctic regions must
have been from time to time more or less affected by those cosmical
causes to which Croll has appealed. So long, however, as insular
conditions prevailed, the changes induced by a great increase in the
eccentricity of the earth's orbit would not necessarily be strongly
marked. Dr. Nansen objects to Croll's well-known theory that "it cannot
account for the recurrence of conditions so favourable as to explain
the existence in Greenland of a climate comparable to what we now find
in tropical regions." No doubt it cannot, but, as we maintain, there
is no good reason for supposing that tropical or sub-tropical climates
ever characterised any area within the Arctic Circle. The remarkable
association in Europe, during so recent a period as the Pleistocene,
of southern and temperate species of plants and animals, ought to warn
us against taking the present distribution of life-forms as an exact
type of the kind of distribution which characterised earlier ages. It
is safe to say that were our present continental areas to become broken
up into groups of larger and smaller islands, so as to allow of a much
less impeded oceanic circulation, the resulting climatic conditions
would offer the strongest contrast to the present. And as the lands
of the globe were apparently in former times more insular than they
are now, it is hazardous to compare the climates of the present with
those of the past. It is reasonable to infer, from the occurrence in
Greenland of fossil floras which find their nearest representatives in
southern Europe and north Africa, that the winters of the far north
were formerly mild and clement. But we cannot conclude, from the same
evidence, that the Arctic summers were ever as hot as those of our
present warm-temperate and sub-tropical zones.
But if the recent expedition has thrown no new light on the disputed
question as to the cause of the high temperature which formerly
prevailed in Greenland, it is needless to say that it has added
considerably to our knowledge of the present physical conditions of
that country. The view held by many that Greenland must be wrapped in
ice has been amply justified, and we can now no longer doubt that the
inland-ice covers the whole country from the 75th parallel southwards.
A section of Greenland in the latitude at which it was crossed by
Nansen and his comrades "gives an almost exact mathematical curve,
approximating very closely to the arc of a circle described with a
radius of about 6500 miles. The whole way across the surface coincides
tolerably accurately with this arc, though it falls away somewhat
abruptly at the coasts, and a little more abruptly on the east side
than the west." Taking the observations of other Arctic travellers
with his own, Nansen is led to the conclusion that "the surface of the
inland-ice forms part of a remarkably regular cylinder, the radius
of which nevertheless varies not a little at different latitudes,
increasing markedly from the south, and consequently making the arc of
the surface flatter and flatter as it advances northwards." He points
out that this remarkable configuration must to a certain extent be
independent of the form of the underlying land-surface, which, to judge
from the character of the wild and mountainous coast-lands, probably
resembles Norway in its general configuration--if, indeed it be not a
group of mountainous islands. The buried interior of Greenland must
in fact be a region of high mountains and deep valleys, all of which
have totally disappeared under the enveloping _mer de glace_. It is
obvious, as Dr. Nansen remarks, that the minor irregularities of the
land "have had no influence whatever upon the form of the upper surface
of the ice-sheet." That surface-form has simply been determined by
the force of pressure--the quasi-viscous mass attaining its maximum
thickness towards the central line of the country, where resistance to
the movement due to pressure must necessarily have been greatest. Thus
although the larger features of the ice-drowned land may have had some
influence in determining the position of the ice-shed, it is not by
any means certain that this central line coincides with the dominant
ridge or watershed of the land itself. For, as Nansen reminds us, the
ice-shed of the Scandinavian inland-ice of glacial times certainly lay
about 100 miles to the east of the main water-parting of Norway and
Sweden. Similar facts, we may add, have been noticed in connection with
the old ice-sheets of Scotland and Ireland.
The greatest elevation attained by the expedition was 9000 feet. How
deeply buried the dominating parts of the land-surface may be at that
elevation one cannot tell. It is obvious, however, that the _mer de
glace_ must be very unequal in thickness. According to Dr. Nansen the
average elevation of the valleys in the interior cannot much exceed
2300 or 3300 feet, so that the ice lying above such depressions must
have a thickness of 5700 to 6700 feet. It cannot, of course, lie so
deeply over mountain-ridges. The eroding power of such a glacier-mass
must be enormous, and Dr. Nansen does not doubt that the buried valleys
of Greenland are being widened and deepened by the grinding of the
great ice-streams that are ever advancing towards the sea.
The expedition met with no streams of surface-water on the inland-ice;
indeed, the amount of superficial melting in the interior was quite
insignificant. And yet, as is well known, many considerable streams
and rivers flow out from underneath the inland-ice all the year
round. It is obvious, therefore, that this water-supply does not come
from superficial sources, as, according to Dr. Nansen, it is usually
supposed to do. But surely it has long been recognised that such rivers
as the Mary Minturn must be derived from sub-glacial melting. And the
various causes to which our author attributes this melting have already
frequently been pointed out. Earth-heat--the influence of pressure in
lowering the melting-point of ice--and the friction induced by the
movement of the ice itself have all long ago been recognised as factors
tending to produce the sub-glacial water-drainage of an ice-sheet.
Dr. Nansen's speculations on the origin of the "drumlins" and "kames"
of formerly glaciated areas will interest geologists, but are not so
novel as he supposes. His description of what are known as "drumlins"
is not quite correct. These long lenticular banks cannot be said
to lie upon boulder-clay, but are merely a structural form of that
accumulation. And it is hardly the case that geologists have "performed
the most acrobatic feats" in trying to explain the origin of the
banks in question. The usual explanation is that they have been formed
underneath the ice as ground-moraine--the upper surface of which
varies in configuration--being sometimes approximately even, as in
broad mountain-valleys; at other times ridged and corrugated, as in
open lowlands. And these modifications of surface are supposed to
have resulted from the varying movement and pressure of the overlying
ice-sheet. The drumlins, in fact, would appear to be analogous to the
banks that accumulate in the beds of rivers. Many drumlins, indeed,
are composed partly of solid rock and partly of boulder-clay, which
would seem to have accumulated in the lee of the projecting rock, much
in the same way as gravel and sand gather behind any large boulder
in a stream-course. Dr. Nansen, apparently, to some extent confounds
drumlins with "kames" and "åsar," of which certainly many strange and
conflicting explanations have been hazarded. These, however, differ
essentially from drumlins, for they consist exclusively, or almost
exclusively, of water-worn and more or less water-assorted materials.
And one widely-accepted view of their origin is that they have
accumulated in tunnels underneath an ice-sheet. This is practically
the same view as Dr. Nansen's. He thinks that when an ice-sheet has
its under-surface furrowed by running water, the ground-moraine will
tend to be pressed up into the river-channels. The water will, in this
way, be compelled to hollow out the roof of its tunnel to a greater
degree, and as the stream continues to work upwards the moraine will
follow it, so as to partially fill the tunnel and form a ridge along
the back of which the sub-glacial stream will run. The material forming
the upper portion of the ridge will thus come to be composed mainly
of water-worn and stratified detritus, derived from the erosion of
the ground-moraine. This is an ingenious suggestion which may be of
good service in some cases, but it is certainly inapplicable to most
kames and åsar. If it were a complete explanation we ought to find
these ridges consisting of an upper water-assorted portion and a lower
unmodified morainic portion (boulder-clay). But this is not the case,
for most kames consist entirely, from top to bottom, of water-assorted
materials. They are found running across an even or gently-undulating
surface of boulder-clay, and sometimes they rest not on boulder-clay
but solid rock.
Dr. Nansen considers another geological question which has given rise
to much controversy, and is still far from being settled--namely,
whether the oscillations of level which have left such conspicuous
traces in northern regions are in any way connected with the appearance
and disappearance of great ice-sheets. Can a big ice-sheet push down
the earth's crust by its weight? and does the crust rise again as the
ice melts away? Could a thick ice-sheet exercise sufficient attraction
upon the sea to cause it to rise upon the land, and thus explain the
origin of some of the so-called raised beaches of this and other
formerly glaciated lands? Can the weight of a great ice-sheet shift
the earth's centre of gravity, and, if so, to what extent? Each of
these questions has been answered in the affirmative and the negative
by controversialists, and, until the geological evidence has been
completely sifted, each, doubtless, will continue to be alternately
affirmed and denied. All that need be pointed out here is that some of
the movements which occurred during the Pleistocene period were on much
too large a scale to be explicable by any of the hypotheses referred
to.
XIV.
The Geographical Development of Coast-lines.[DO]
[DO] Presidential Address to the Geographical Section of the British
Association, Edinburgh, 1892.
Amongst the many questions upon which of late years light has been
thrown by deep-sea exploration and geological research, not the least
interesting is that of the geographical development of coast-lines. How
is the existing distribution of land and water to be accounted for? Are
the revolutions in the relative position of land and sea, to which the
geological record bears witness, due to movements of the earth's crust
or of the hydrosphere? Why are coast-lines in some regions extremely
regular, while elsewhere they are much indented? About 150 years ago
the prevalent belief was that ancient sea-margins indicated a formerly
higher ocean-level. Such was the view held by Celsius, who, from an
examination of the coast-lands of Sweden, attributed the retreat of
the sea to a gradual drying up of the latter. But this desiccation
hypothesis was not accepted by Playfair, who thought it much more
likely that the land had risen. It was not, however, until after Von
Buch had visited Sweden (1806-1808), and published the results of his
observations, that Playfair's suggestion received much consideration.
Von Buch concluded that the apparent retreat of the sea was not due
to a general depression of the ocean-level, but to elevation of the
land--a conclusion which subsequently obtained the strong support of
Lyell. The authority of these celebrated men gained for the elevation
theory more or less complete assent, and for many years it has been the
orthodox belief of geologists that the ancient sea-margins of Sweden
and other lands have resulted from vertical movements of the crust.
It has long been admitted, however, that highly-flexed and disturbed
strata require some other explanation. Obviously such structures are
the result of lateral compression and crumpling. Hence geologists have
maintained that the mysterious subterranean forces have affected the
crust in different ways. Mountain-ranges, they conceive, are ridged up
by tangential thrusts and compression, while vast continental areas
slowly rise and fall, with little or no disturbance of the strata. From
this point of view it is the lithosphere that is unstable, all changes
in the relative level of land and sea being due to crustal movements.
Of late years, however, Trautschold and others have begun to doubt
whether this theory is wholly true, and to maintain that the sea-level
may have changed without reference to movements of the lithosphere.
Thus Hilber has suggested that sinking of the sea-level may be due, in
part at least, to absorption, while Schmick believes that the apparent
elevation and depression of continental areas are really the results
of grand secular movements of the ocean. The sea, according to him,
periodically attains a high level in each hemisphere alternately, the
waters being at present heaped up in the southern hemisphere. Professor
Suess, again, believing that in equatorial regions the sea is, on the
whole, gaining on the land, while in other latitudes the reverse would
appear to be the case, points out this is in harmony with his view
of a periodical flux and reflux of the ocean between the equator and
the poles. He thinks we have no evidence of any vertical elevation
affecting wide areas, and that the only movements of elevation that
take place are those by which mountains are upheaved. The broad
invasions and transgressions of the continental areas by the sea, which
we know have occurred again and again, are attributed by him to secular
movements of the hydrosphere itself.
Apart from all hypothesis and theory, we learn that the surface of
the sea is not exactly spheroidal. It reaches a higher level on the
borders of the continents than in mid-ocean, and it varies likewise
in height at different places on the same coast. The attraction of
the Himalaya, for example, suffices to cause a difference of 300
feet between the level of the sea at the delta of the Indus and on
the coast of Ceylon. The recognition of such facts has led Penck to
suggest that the submergence of the maritime regions of north-west
Europe and the opposite coasts of North America, which took place
at a recent geological date, and from which the lands in question
have only partially recovered, may have been brought about by the
attraction exerted by the vast ice-sheets of the Glacial period.
But, as Drygalski, Woodward, and others have shown, the heights at
which recent marine deposits occur in the regions referred to are
much too great to be accounted for by any possible distortion of the
hydrosphere. The late James Croll had previously endeavoured to show
that the accumulation of ice over northern lands during glacial times
would suffice to displace the earth's centre of gravity, and thus cause
the sea to rise upon the glaciated tracts. More recently other views
have been advanced to explain the apparently causal connection between
glaciation and submergence, but these need not be considered here.
Whatever degree of importance may attach to the various hypotheses of
secular movements of the sea, it is obvious that the general trends
of the world's coast-lines are determined in the first place by the
position of the dominant wrinkles of the lithosphere. Even if we
concede that all "raised beaches," so-called, are not necessarily
the result of earth-movements, and that the frequent transgressions
of the continental areas by oceanic waters in geological times may
possibly have been due to independent movements of the sea, still we
must admit that the solid crust of the globe has always been subject to
distortion. And this being so, we cannot doubt that the general trends
of the world's coast-lines must have been modified from time to time
by movements of the lithosphere.
As geographers we are not immediately concerned with the mode of origin
of those vast wrinkles, nor need we speculate on the causes which may
have determined their direction. It seems, however, to be the general
opinion that the configuration of the lithosphere is due simply to the
sinking-in and doubling-up of the crust on the cooling and contracting
nucleus. But it must be admitted that neither physicists nor geologists
are prepared with a satisfactory hypothesis to account for the
prominent trends of the great world-ridges and troughs. According to
the late Professor Alexander Winchell, these trends may have been
the result of primitive tidal action. He was of opinion that the
transmeridional progress of the tidal swell in early incrustive times
on our planet would give the forming crust structural characteristics
and aptitudes trending from north to south. The earliest wrinkles to
come into existence, therefore, would be meridional or submeridional,
and such, certainly, is the prevalent direction of the most conspicuous
earth-features. There are many terrestrial trends, however, as
Professor Winchell knew, which do not conform to the requirements of
his hypothesis; but such transmeridional features, he thought, could
generally be shown to be of later origin than the others. This is
the only speculation, so far as I know, which attempts, perhaps not
altogether unsuccessfully, to explain the origin of the main trends
of terrestrial features. According to other authorities, however, the
area of the earth's crust occupied by the ocean is denser than that
over which the continental regions are spread. The depressed denser
part balances the lighter elevated portion. But why these regions
of different densities should be so distributed no one has yet told
us. Neither does Le Conte's view, that the continental areas and the
oceanic depressions owe their origin to unequal radial contraction of
the earth in its secular cooling, help us to understand why the larger
features of the globe should be disposed as they are.
Geographers must for the present be content to take the world as
they find it. What we do know is that our lands are distributed
over the surface of a great continental plateau of irregular form,
the bounding slopes of which plunge down more or less steeply into
a vast oceanic depression. So far as geological research has gone,
there is reason to believe that these elevated and depressed areas
are of primeval antiquity--that they ante-date the very oldest of
the sedimentary formations. There is abundant evidence, however, to
show that the relatively elevated or continental area has been again
and again irregularly submerged under tolerably deep and wide seas.
But all historical geology seems to assure us that the continental
plateau and the oceanic hollows have never changed places, although
from time to time portions of the latter have been ridged up and added
to the margins of the former, while ever and anon marginal portions
of the plateau have sunk to very considerable depths. We may thus
speak of the great world-ridges as regions of dominant elevation, and
of the profound oceanic troughs as areas of more or less persistent
depression. From one point of view, it is true, no part of the earth's
surface can be looked upon as a region of dominant elevation. Our
globe is a cooling and contracting body, and depression must always
be the prevailing movement of the lithosphere. The elevation of the
continental plateau is thus only relative. Could we conceive the crust
throughout the deeper portions of the oceanic depression to subside to
still greater depths, while at the same time the continental plateau
remained stationary, or subsided more slowly, the sea would necessarily
retreat from the land, and the latter would then appear to rise. It is
improbable, however, that any extensive subsidence of the crust under
the ocean could take place without accompanying disturbance of the
continental plateau; and in this case the latter might experience in
places not only negative but positive elevation. During the evolution
of our continents, crustal movements have again and again disturbed the
relative level of land and sea; but since the general result has been
to increase the land-surface and to contract the area occupied by the
sea, it is convenient to speak of the former as the region of dominant
elevation, and of the latter as that of prevalent depression. Properly
speaking, both are sinking regions, the rate of subsidence within the
oceanic trough being in excess of that experienced over the continental
plateau. The question of the geographical development of coast-lines is
therefore only that of the dry lands themselves.