of the fact that in them we encounter evidence of ice-action of much
the same kind as that met with in Palæozoic strata. While, on the
one hand, the Mesozoic floras and faunas bespeak climatic conditions
similar to those of earlier ages, but probably not quite so uniform; on
the other, the occurrence of erratics in various marine accumulations
is sufficient to show that now and again ice floated across seas,
the floors of which were tenanted by reef-building corals. The
geographical conditions continued unfavourable to the formation of
extensive ice-sheets in temperate latitudes, no matter how high the
eccentricity of the orbit might have been. The erratics which occur
in certain Jurassic and Cretaceous deposits are admitted by most
geologists to have been ice-borne. Now, it is highly improbable that
the transporting agent could have been coast-ice, for it is hardly
possible to conceive of ice forming on the surface of a sea in which
flourished an abundant Mesozoic fauna. The erratics, therefore, seem
to imply the existence in Mesozoic times of local glaciers, which here
and there descended to the sea, as in the north-east of Scotland. The
erratics in the Scottish Jurassic are evidently of native origin, and
it is most improbable that those which have been met with in the Chalk
of England and France could have floated from any very great distance.
How, then, can we explain the appearance of local glaciers in these
latitudes during Mesozoic times? The geographical conditions of the
period could not have favoured the formation of perennial snow and ice
in our area, unless our lands were at that time much more elevated than
now. And this is the usual explanation. It is supposed that mountains
much higher than any we now possess probably existed in such regions
as the Scottish Highlands. It is easy to imagine the former existence
of such mountains. So long a time has elapsed since the Jurassic
period, that the Archæan and Palæozoic areas cannot but have suffered
prodigious denudation in the interval. But, when one considers how
very lofty, indeed, those mountains must have been, in order to reach
the snow-line of Jurassic times, one may be excused for expressing a
doubt as to whether the suggested explanation is reasonable. At all
events, the phenomena are, to say the least, as readily explicable on
the supposition that the snow-line was temporarily lowered by cosmical
causes. Even with eccentricity at a high value, no great ice-sheets,
indeed, could have existed, but local snow-fields and glaciers might
have appeared in such mountain-regions as were of sufficient height.
And this might have happened without producing any great difference
in the temperature of the sea, or any marked modification in the
distribution of life. In short, we should simply have, as before,
an alternation of warm and somewhat cooler climates, but nothing
approaching to the glacial and interglacial epochs of the Pleistocene.
These conclusions seem to me to be strongly supported by the evidence
of ice-action during Tertiary times. The gigantic erratics of the
Alpine Eocene do not appear to have been derived from the Alps, but
rather from the Archæan area of southern Bohemia. The strata in which
they occur are, for the most part, unfossiliferous; they contain only
fucoidal remains, and are presumably marine. How is it possible to
account for the appearance of these erratics in marine deposits in
central Europe at a time when, as evidenced by the Eocene flora and
fauna the climate was warm? Are we to infer the former existence of an
extremely lofty range of Bohemian Alps which has since vanished? Is
it not more probable that here, too, we have evidence of a lowering
of the snow-line, induced by cosmical causes, which brought about the
appearance of snow-fields and glaciers in a mountain-tract of much
less elevation than would have been required in the absence of high
eccentricity of the orbit? If it be objected that such cosmical causes
must have had some effect upon the distribution of life, I reply
that very probably they had, although not to any extreme extent. The
researches of Mr. Starkie Gardner have shown that the flora of the
English Eocene affords distinct evidence of climatic changes. But as
the geographical conditions of that period precluded the possibility of
extensive glaciation, and could only, at the most, have induced local
glaciers to appear in elevated mountain-regions, it seems idle to cite
the non-occurrence of erratics and morainic accumulations in the Eocene
of England and France as an argument against the application of Croll's
theory to the case of the erratics of the Flysch. I repeat, then, that
under the geographical conditions of the Eocene, all the more obvious
effects likely to have resulted from the passage of a period of high
eccentricity would be the appearance of a few local glaciers, the
existence of which could have had no more influence on the climate of
adjacent lowlands than is notable in similar circumstances in our own
day. It is absurd, therefore, to expect to find evidence in Eocene
strata of as strongly contrasted climates as those of the glacial and
interglacial deposits of the Pleistocene. There must, doubtless, have
been alternations of climate in our hemisphere; but these would consist
simply of passages from warm to somewhat cooler conditions--just such
changes, in fact, as are suggested by the plants of the English Eocene.
The evidence of ice-action in the Miocene strata is even more striking
than that of which I have just been speaking. The often-cited case
of the erratics of the Superga near Turin I need do little more
than mention. These erratics were undoubtedly carried by icebergs,
calved from Alpine glaciers at a time when northern Italy was largely
submerged. The erratic deposits are unfossiliferous, and are underlaid
and overlaid by fossiliferous strata, in none of which are any erratics
to be found. What is the meaning of these intercalated glacial
accumulations? Can we believe it possible that the Miocene glaciers
were enabled to reach the sea in consequence of a sudden movement
of elevation, which must have been confined to the Alps themselves?
Then, if this be so, we must go a step further, and suppose that,
after some little time, the Alps were again suddenly depressed, so
that the glaciers at once ceased to reach the sea-coast. For, as Dr.
Croll has remarked, "had the lowering of the Alps been effected by
the slow process of denudation, it must have taken a long course of
ages to have lowered them to the extent of bringing the glacial state
to a close." And we should, in such a case, find a succession of beds
indicating a more or less protracted continuance of glacial conditions,
and not one set of erratic accumulations intercalated amongst strata,
the organic remains in which are clearly suggestive of a warm climate.
The occurrence of erratics in the Miocene of Italy is all the more
interesting from the fact that in the Miocene of France and Spain
similar evidence of ice-action is forthcoming.
Opponents of Dr. Croll's theory have made much of Baron Nordenskiöld's
statement that he could find no trace of former glacial action in any
of the fossiliferous formations within the Arctic regions. He is
convinced that "an examination of the geognostic condition, and an
investigation of the fossil flora and fauna of the polar lands, show
no signs of a glacial era having existed in those parts before the
termination of the Miocene period." Well, as we have seen, there is no
reason to believe that the geographical conditions in our hemisphere,
at any time previous to the close of the Pliocene period, could have
induced glacial conditions comparable to those of the Pleistocene Ice
Age. The strata referred to by Nordenskiöld, are, for the most part,
of marine origin, and their faunas are sufficient to show us that the
Arctic seas were formerly temperate and genial. If any ice existed
then, it could only have been in the form of glaciers on elevated
lands. And it is quite possible that these, during periods of high
eccentricity, may have descended to the sea and calved their icebergs;
and, if so, erratics may yet be found embedded here and there in the
Arctic fossiliferous formations, although Nordenskiöld failed to see
them. One might sail all round the Palæozoic coast-lines of Scotland
without being able to observe erratics in the strata, and yet, as we
know, these have been encountered in the interior of the country.
The wholesale scattering of erratics at any time previous to the
Pleistocene, must have been exceptional even in arctic regions, and
consequently one is not surprised that they do not everywhere stare the
observer in the face.
The general conclusion, then, to which I think we may reasonably
come, is simply this:--That geological climate has been determined
chiefly by geographical conditions. So long as the lands of the
globe were discontinuous and of relatively small extent, warm
ocean-currents reaching polar regions produced a general uniformity
of temperature--the climate of the terrestrial areas being more or
less markedly insular in character. Under these conditions, the sea
would nowhere be frozen. But when the land-masses became more and more
consolidated, when owing to the growth of the continents the warm
ocean-currents found less ready access to arctic regions, then the
temperature of those regions was gradually lowered, until eventually
the seas became frost-bound, and the lands were covered with snow and
ice. But while the chief determining cause of climate has been the
relative distribution of land and water, it is impossible to doubt that
during periods of high eccentricity of the orbit, the climate must
have been modified to a greater or less extent. In our own day the
geographical conditions are such that, were eccentricity to attain a
high value, the climate of the Pleistocene would be reproduced, and our
hemisphere would experience a succession of alternating cold and genial
epochs.
But in earlier stages of the world's history, the geographical
conditions were not of a kind to favour the accumulation of vast
ice-fields. During a period of extreme eccentricity, there would
probably be fluctuations of temperature in high latitudes; but nothing
like the glacial and interglacial epochs of the Pleistocene could
have occurred. At most, there would be a general lowering of the
temperature, sufficient to render the climate of arctic seas and lands
somewhat cooler, and probably to induce the appearance in suitable
places of local glaciers; and, owing to precession of the equinox,
these cooler conditions would be followed by a general elevation of
the temperature above the normal for the geographical conditions of
the period. In Palæozoic and Mesozoic times, the effects of high
eccentricity of the orbit appear to have been, in a great measure,
neutralised by the geographical conditions, with a possible exception
in the Permian period. But in Tertiary times, when the land-masses had
become more continuous, the cosmical causes of change referred to must
have had greater influence. And I cannot help agreeing with Dr. Croll
that the warm climates of the Arctic regions during that era were, to
some extent, the result of high eccentricity.
In concluding this discussion, I readily admit that our knowledge
of geographical evolution is as yet in its infancy. We have still
very much to learn, and no one will venture to dogmatise upon the
subject. But I hope I have made it clear that the evidence, so far
as it goes, does not justify the confident assertions of Dr. Croll's
opponents, that his theory is contradicted by what we know of the
climatic conditions of Palæozoic, Mesozoic, and Cainozoic times. On
the contrary, it seems to me to gain additional support from the very
evidence to which Nordenskiöld and others have appealed.
Note.--The accompanying sketch-maps (Plate IV.) require a few words
of explanation. The geology of the world is still so imperfectly
known that any attempt at graphic representation of former
geographical conditions cannot but be unsatisfactory. The approximate
positions of the chief areas of predominant elevation and depression
during stated periods of the past may have been ascertained in a
general way; but when we try to indicate these upon a map, such
provisional reconstructions are apt to suggest a more precise and
definite knowledge than is at present attainable. For it must be
confessed that there is hardly a line upon the small maps (A, B,
C) which might not have been drawn differently. This, of course,
is more especially true of South America, Africa, Asia--of large
areas of which the geological structure is unknown. But although the
boundaries of the land-masses shown upon the maps referred to are
thus confessedly provisional, the maps nevertheless bring out the
main fact of a gradual growth and consolidation of the land-areas--a
passage from insular to continental conditions. I need hardly say
this is no novel idea. It was clearly set forth by Professor Dana
upwards of forty years ago (_Silliman's Journal_, 1846, p. 352; 1847,
pp. 176, 381), and it received some years later further illustration
from Professor Guyot, who insisted upon the insular character of
the climate during Palæozoic times (_The Earth and Man_, 1850). It
must be understood that the maps (A, B, C) are not meant to exhibit
the geographical conditions of the world at any one point of time.
In Map A, for example, the area coloured blue was not necessarily
covered by sea at any particular stage in the Palæozoic era. It
simply represents approximately the regions tended. But, as already
stated, numerous oscillations of level occurred in Palæozoic times,
so that many changes in the distribution of land and water must have
taken place down to the close of the Permian period. The land-areas
shown upon the map are simply those which appear to have been more
or less persistent through all the geographical changes referred to.
Similar remarks apply to the other maps representing the more or
less persistent land-areas of Mesozoic and Tertiary times. Thus, for
example, there are reasons for believing that Madagascar was joined
to the mainland of Africa at some stage of the Mesozoic era, but was
subsequently insulated before Tertiary times. Again, as Mesozoic
era a land-connection obtained between New Zealand and Australia.
The same naturalist also points out that a chain of islands, now
represented by numerous islets and shoals, served in Tertiary times
to link Madagascar to India. Map D shows the areas of predominant
elevation and depression. The area coloured brown represents the
great continental plateau, which extends downwards to 1000 fathoms or
so below the present sea-level. The area tinted blue is the oceanic
depression. From the present distribution of plants and animals, we
infer that considerable tracts which are now submerged have formerly
been dry land--some of these changes having taken place in very
recent geological times. And the same conclusions are frequently
suggested by geological evidence. There can be little doubt that
Europe in Tertiary times extended further into the Northern Ocean
than it does now. And it is quite possible that in the Mesozoic and
Palæozoic eras considerable land-areas may likewise have appeared
here and there in those northern regions which are at present under
water. There is, indeed, hardly any portion of the continental
plateau which is now submerged that may not have been land at some
time or other. But after making all allowance for such possibilities,
the geological evidence, as far as it goes, nevertheless leads to
the conclusion that upon the whole a wider expense of primeval
continental plateau has come to the surface since Tertiary times than
was ever exposed during any former period of the world's history.
[Mr. Marcou states (_American Geologist_, 1890, p. 229) that the idea
of a gradual growth of land-areas originated with Elie de Beaumont,
who was in the habit of showing such maps, and used them in his
lectures at Paris as early as 1836. Professor Beudant published three
of these same maps for the Jurassic, Cretaceous, and Tertiary seas in
his _Cours élémentaire de Géologie_ (1841); and Professor Carl Vogt
in his _Lehrbuch der Geologie und Petrefactenkunde_ (1845), which
was confessedly based on Elie de Beaumont's lectures during 1844-46,
gives four maps of the Carboniferous, Jurassic, Cretaceous, and
Tertiary seas.]
XIII.
The Scientific Results of Dr. Nansen's Expedition.[DN]
[DN] From _The Scottish Geographical Magazine_, 1891.
In the Appendix to his most interesting and instructive work, _The
First Crossing of Greenland_, Dr. Nansen treats of the scientific
results of his remarkable journey. The detailed enumeration of these
results, he tells us, would have been out of place in a general
account of his expedition, but will appear in due time elsewhere.
Hence he confines attention in his present work to such questions as
are of most obvious interest, such as the extent, outward form, and
elevation of the inland-ice of Greenland. By way of introduction his
readers are presented with some account of the geological history of
the country, which, although it contains nothing that was not already
familiar to geologists, will doubtless prove interesting to others.
After indicating that Greenland would appear to be composed almost
exclusively of Archæan schists and granitoid eruptive rocks, the author
glances at the evidence which the Mesozoic and Cainozoic strata of
the west coast have supplied as to the former prevalence of genial
climatic conditions. Heer is cited to show that during the formation
of the Cretaceous beds the mean temperature of north Greenland was
probably between 70° and 72° F., while in later Cainozoic times it
could not have been less than 55° F., in 70° N.L. These conclusions
are based on the character of the fossil floras. Now the mean annual
temperature on the west coast of Greenland, where the relics of these
old floras occur, is about 15° F., from which it is inferred that there
has been a decrease of 40° since Cainozoic times. In those times,
says Dr. Nansen, "the country must have rejoiced in a climate similar
to that of Naples, while in the earlier Cretaceous period it must
have resembled that of Egypt." He then refers to the well-known fact
that, long after the deposition of the Cainozoic beds of Greenland,
intensely arctic conditions supervened, when the inland-ice of that
country extended much beyond its present limits. This was the Glacial
period of geologists, during which all the northern regions of America
and Europe, down to what are now temperate latitudes were likewise
swathed in ice. Various hypotheses have been advanced in explanation of
these strange climatic vicissitudes, and some of them are very briefly
discussed by Dr. Nansen. None of the suggested solutions of the problem
quite satisfies him; but he appears to look with most favour on the
view that great climatic revolutions in what are now polar regions
may have resulted from movements of the earth's axis. He admits,
however, that there are certain strong objections to this hypothesis,
and concludes that we have not yet got any satisfactory explanation
to cover all the facts of the case. In discussing the question of a
possible wandering of the pole, the author cites certain astronomical
observations to show that the position of the axis is even now slowly
changing, the movement amounting to half a second in six months. This
is not much; but if the change, as he remarks, were to continue at the
same rate for 3600 years, the shift would amount to one degree. Thus
in a period of no more than 72,000 to 108,000 years Greenland might be
brought into the latitude required for the growth of such floras as
those of Cainozoic and Mesozoic times. Geologists will readily concede
these or longer periods if they be required, but they will have graver
doubts than Dr. Nansen as to whether any such great changes in the
axis are possible. The astronomical observations referred to, even if
they were fully confirmed, do not show that the movement is constant
in one direction. They indicated, as he mentions, a slight increase
of latitude during the first quarter of 1889, followed in the second
quarter of the same year by a decrease, which continued to January,
1890. Since the publication of Professor George Darwin's masterly paper
on the influence of geological changes on the earth's axis of rotation,
geologists have felt assured that the great climatic revolutions to
which the stratified rocks bear witness must be otherwise explained
than by a wandering of the pole. Indeed, the geological evidence alone
is enough to show that profound climatic changes have taken place
while the pole has occupied its present position. Thus, there is no
reasonable grounds for doubting that during the Glacial period the pole
was just where we find it to-day. For, under existing geographical
conditions, could a sufficient lowering of temperature be brought
about, snow-fields and ice-sheets would gather and increase over
the very same areas as we know were glaciated in Pleistocene times.
Still further, we have only to recall the fact that several extreme
revolutions of climate supervened during the so-called Glacial period,
to see how impossible it is to account for the phenomena by movements
of the earth's axis.
If it be true that the great climatic changes of the Pleistocene
period did not result from a wandering to and fro of the pole, then
it is not at all likely that the Mesozoic and Cainozoic climates of
Greenland were induced by any such movement. But does the geological
evidence justify us in believing that the climates in Greenland
during Cretaceous and Tertiary times really resembled those of
Egypt and southern Italy? It may be strongly doubted if it does.
Palæontologists, like other mortals, find it hard to escape the
influence of environment. They are apt to project the actual present
into the past, without, perhaps, fully considering how far they are
justified in doing so. Because there occur in Cretaceous and Tertiary
strata, within Arctic regions, certain assemblages of plants which find
their nearest representatives in southern Italy and Egypt, surely it is
rather rash to conclude that Greenland has experienced climates like
those now characteristic of Mediterranean lands. All that the evidence
really entitles us to assume is simply that the _winter temperature_ of
Greenland was formerly much higher than it is now. That great caution
is required in comparing past with present climatological conditions
may be seen by glancing for a moment at the character of the flora
which lived in Europe during the interglacial phase of the Pleistocene
period. The plants of that period are for the most part living species,
so that while dealing with these we are on safer ground than when we
are treating of the floras of periods so far removed from us as those
of Tertiary and Cretaceous times. Now, in the Pleistocene flora of
Europe we find a strange commingling of species, such as we nowhere
see to-day over any equally wide area of the earth's surface. During
Pleistocene times many plants which are still indigenous to southern
France flourished side by side in that area with species which are no
longer seen in the same region; some of these last having retreated
because unable to support the cold of winter, while others have retired
to the mountains to escape the dryness of the summer. Similar evidence
is forthcoming from the Pleistocene accumulations of Italy, northern
France, and Germany. In a word, clement winters and relatively cool and
humid summers permitted the wide diffusion and intimate association
of plants which have now a very different distribution, temperate and
southern species formerly flourishing together over vast areas of
southern and central Europe. And similarly we find that during the same
period the regions in question were tenanted by southern and temperate
forms of animal life--elephants, rhinoceroses, and hippopotamuses,
together with cervine, bovine, and other forms, not a few of which are
still indigenous to our Continent--that ranged from the shores of the
Mediterranean up to our own latitudes. We cannot doubt, indeed, that
the present geographical distribution of plants and animals differs
markedly from anything that has yet been disclosed by the researches
of geologists. The climatic conditions of our day are exceptional as
compared with those of earlier times, and the occurrence in Greenland
of southern types of plants, therefore, does not justify us in
concluding that climates like those of southern Italy and Egypt were
ever characteristic of arctic regions. It is a low winter temperature
rather than a want of great summer heat that restricts the range
northward of southern floras. If Greenland could be divested of its
inland-ice--if its winter temperature never fell below that of our own
island--it would doubtless become clothed in time with an abundant
temperate flora.