REMARKS ON THEORIES ADVANCED FOR EXPLAINING ICE PERIODS.
On Nov. 12, 1891, Professor Geikie made his presidential address before
the Edinburgh Geological Society, the subject being “Supposed Causes of
the Glacial Period.”
Many of his views advanced in this lecture were so much in accordance
with my own that I am induced to repeat them. He said that the glacial
period was a general phenomenon due to some widely acting cause, and
that where we now have the greatest rain-fall the greatest snow-fall
took place, and that the Pleistocene period was characterized by great
oscillations of climate, extremely cold and very genial conditions
alternating. He also said that in glacial and post-glacial times
changes in the relative level of the land and sea had taken place, and
any suggested explanation which did not fully account for these various
climatic and geographical conditions could not be satisfactory. And,
while examining the earth-movement hypothesis, he pointed out that in
the first place there was not the least evidence of great continental
elevations and depressions in the northern hemisphere, such as the
hypothesis postulated. Next he showed that, even if the diserrated
earth-movements were admitted, they would not account for the phenomena.
Such changes, no doubt, would profoundly affect the maritime regions of
North America and Europe; but they would not bring about the conditions
that obtained at the climax of the ice age.
Another objection to the earth-movement hypothesis was this: it did not
account for interglacial conditions. The advocates of that hypothesis
imagined that these conditions would supervene when the highly elevated
northern regions were depressed to their present level. But these
were the conditions that obtained at the present time; and yet in
spite of them the climate was neither so equable nor so genial as that
which obtained in interglacial times and during the mild stage of the
necessary post-glacial period.
Therefore, he said that the earth-movement hypothesis should be
rejected, not only because it was highly improbable that such
wonderfully rhythmic elevations and depressions of northern lands could
have taken place, but chiefly because it did not explain the conditions
of the glacial periods and interglacial times.
Still, Professor Geikie says that in glacial and in post-glacial times
changes in the relative level of the land and sea had taken place; and
it is reasonable to suppose that such changes were obtained in the high
latitudes of both hemispheres during the breaking up of the last ice
age.
We have previously pointed out that much of the ice of the glacial
period in the southern hemisphere was melted away, and its waters
warmed sufficiently to assist the Gulf Stream and Japanese current to
bring about a mild period in the northern hemisphere; for without such
assistance they would be unable to disperse the vast ice-sheets of the
northern latitudes.
Still, the attraction of the southern ocean waters into the northern
seas must have commenced as soon as the growing ice-sheets of the large
continents and islands of the high northern latitudes surpassed the
growth and weight of the glaciers on the smaller lands of the southern
hemisphere.
Hence the attraction of the ocean waters northward overcomes the force
of the prevailing winds from moving an undue portion of the ocean’s
surface waters southward. Consequently, the movement of water from the
southern seas into the northern latitudes continued so long as the
vast northern ice-sheets increased in weight greater than the glaciers
of the southern hemisphere. Therefore, at the perfection of a frigid
age straits and channels situated so far southward as the Magellan and
Cape Horn channels were much diminished in width and depth or entirely
deprived of their waters. Through this cause such reduced channels were
readily filled with glaciers in a region of great snow-fall. The depth
of water on the submerged northern lands at the close of the glacial
period is not known.
According to Professor Dawson, in the township of Montague in Ontario
the skeleton of a whale was found in post-glacial deposits 440 feet
above tide-water, and marine shells are known to occur on Montreal
mountain at an elevation of 520 feet above the ocean; and it is said
that there are traces of submergence of over one thousand feet in the
higher latitudes, including the islands of Great Britain.
According to the researches of Dr. J. W. Spencer, one great sheet of
water covered most of the great lake region about the close of the
ice age; and the lower strands of these inland seas are known to be
connected with old marine shore lines. The probable reason why so few
sea-shells collected on the glacial drift during such times was because
of so much marine life having been exterminated in the high northern
latitudes during the frigid age. Therefore, the sea, in the short
period of northern submergence, left but few traces on the glacial
drift it once flowed.
Thus it will be seen that, if the ocean waters were attracted northward
through the preponderance of northern ice-sheets, they not only
assisted in melting the northern ice, but also served to greatly reduce
the waters in the Cape Horn channel, and so largely prevented the
independent circulation of the southern ocean, thus furthering a mild
climate in the southern hemisphere until the prevailing winds, after
the northern ice-sheets were melted, were able to move more of the
ocean waters southward than they could move northward, owing to the
ocean currents setting southward being less obstructed than the lesser
currents setting northward. This tendency of the ocean waters to move
southward I have before explained in the preceding pages.
But I will say in addition that, on further consideration, it seems
that one of the main causes of the waters of the augmented northern
oceans moving southward so soon after the melting of the ice from
the northern lands was on account of so much water being attracted
southward to the great low sea-level east of Cape Horn. This vast low
sea-level remained a great area of attraction for the northern seas
until so much northern water was moved into the southern ocean as to
reduce the seas of the northern hemisphere and augment the southern
ocean sufficiently to enlarge the Cape Horn channel, thus causing
the extinction of the vast low sea-level that furnished such great
attraction for the waters of the more northern latitudes.
If the earth-movement hypothesis, so wholly rejected by Professor
Geikie, fails to explain the cause or causes of a northern ice age, it
seems to be still more inadequate for explaining the occurrence of ice
periods extending over both hemispheres. For it is not probable that
portions of continents and large islands rose above the snow-line in
both temperate zones during the same period of time, and then again
obtained their present level with the occurrence of a mild era.
Those who maintain that the continents of North America and Europe
rose to great elevations during the ice age, in order to prove their
assertions, point to the fiords which indent the eastern and western
coasts of North America, and also to the fiords of Norway, as having
been eroded by streams of ice that flowed along the bottom of such
gorges when they were above the sea.
But it appears that such erosion could be performed by heavy glaciers
with the lands at their present level. A glacier three thousand feet
thick would fill and press heavily on the bottom of a gorge fifteen
hundred feet in depth. Therefore, should the bottom of a fiord sink
hundreds of feet below the sea-level, a glacier several thousand feet
thick flowing through and over it into a sea of much greater depth, the
erosion at the bottom of the sunken channel would be greater than on
the land above the sea, where the ice possessed less weight.
Therefore, it is not necessary that lands pierced by deep fiords
should have acquired a higher level during the ice age than they now
maintain. And it is probable that on the antarctic continent ice
erosion may be going on at much greater depths below the sea-level than
the deepest channels in the high northern latitudes. For it is likely
that the temperature of a glacier is so low in such frigid regions
that it holds firmly in its freezing grasp such bowlders as may become
detached from the rocks, thus giving it great erosive power.
But this great eroding ability could not be maintained by glaciers in
the lower latitudes, where a higher temperature would largely deprive
the ice of its abrading properties except on the steep slopes of
mountainous lands.
There are deposits of ice on the North American coast bordering the
arctic shores, and also on Northern Siberia, that are supposed to have
existed since the last frigid period, and are likely to be preserved
into a future cold age, which now appears to have made considerable
progress on Greenland and other ice-clad arctic shores on account of
the independent circulation of the Arctic Ocean waters, which largely
excludes the Gulf Stream from the polar seas; and it is for this reason
that the glaciers on the elevated lands of Iceland are being enlarged
and rapidly advancing. Yet, notwithstanding the gathering of ice
and increasing coldness of lands largely removed from the warm Gulf
currents, there are still mountain regions where glaciers may have been
preserved through post-glacial times, although directly to the leeward
and under the influence of the Gulf Stream and Japanese currents.
These glaciers are situated in the Alpine districts of Europe and on
the mountain ranges of Alaska. It would appear that, were the climate
growing gradually colder in the northern temperate zones, such glaciers
should be increasing in size.
Yet it is said that such is not always the case. This is probably
owing to their being subject to the genial influence of the tropical
currents. For, although the climate of Europe and Alaska may have been
slowly growing colder for centuries, still the slow shrinkage of these
once immense glaciers may still be going on, although at a much slower
rate than formerly, even if the tender plants of these latitudes,
because of the growing coldness, have gradually moved southward.
As to the Alpine glaciers, M. Forel reports from data he has collected
that there have been several enlargements and diminutions during the
last century. And since 1875 enlargements have taken place, their
shrinkage being caused by warm and dry weather, while their enlargement
was brought about during cold and rainy seasons. The glaciers of
Alaska cannot attain much extension until the waters of the great
Japanese stream acquire a lower temperature. There is at this date a
small current setting down through the eastern side of Bering Strait,
bearing field-ice in the spring season down to Anadyr Gulf. The Okhotsk
Sea in the spring season furnishes considerable field-ice to cool the
north Pacific waters, and the wintry winds which sweep down from the
high lands of Northern Asia also serve to chill the Pacific seas; but
all such sources of cold combined at this age have but little general
effect on the vast Japanese current, which still has warmth sufficient
to prevent the increase of glaciers on Alaska.
This great ocean stream in its impact against the shores of Oregon
causes a high sea-level, which is mostly turned southward by the
prevailing north-west winds. Still, a comparatively small stream
sets along the shore of the Alaska Gulf, and also through the island
passages toward a slight low sea-level, to the leeward of the Alaska
peninsula; and it is probable that this current which warms these
in-shore waters is favored by the difference of temperature and density
between the waters abreast Oregon and the Gulf of Alaska, and it may be
owing to the same cause that a small stream is sent along the eastern
shore of Bering Strait into the deep portions of the Arctic Ocean. Thus
because of the warm waters that proceed from the great Japanese current
the glaciers of Alaska are prevented from increasing their bulk.
The only way to furnish the Japanese stream with colder water, and
so cause glaciers to increase on the north-west coast of America, is
through the great Humboldt current, which has its rise in the southern
ocean west of Patagonia and the Cape Horn channel, where a moderate but
vast high sea-level is formed on account of the great drift current of
the southern ocean being somewhat obstructed on its passage through the
Cape Horn channel, which is about one-third the breadth of the westerly
wind-belt.
Therefore, the northern portion of the waters of the high sea-level
so caused are attracted northward to the low sea-level abreast Peru,
from whence they are moved by the south-east trade winds as a drift
current to the equatorial latitudes, thus meeting and mingling with the
returning Japanese current abreast Central America, and so giving head
to the great equatorial stream which moves westward over the Pacific
Ocean, partly impelled by the trade winds, and, on gaining the western
side of the ocean, sends off from a moderate high sea-level a large
stream to the low sea-level caused by the westerly winds abreast Japan,
from whence it is drifted by the same winds over to the north-west
coast of America, thus forming the great Japanese current.
Meanwhile the temperature of the Humboldt current, being governed by
the temperature of the southern ocean from which it takes its rise, is
cooling at a slow rate through the enlargement of ice-sheets in the
antarctic regions, while the increase of glaciers on Patagonia will
in time greatly add to its coolness, and so lower the temperature of
the equatorial current from which the Japanese current branches, the
latter current being made cooler through the increase of coldness of
the former streams. Therefore, the temperature of Alaska, which is
governed by the Japanese current, will slowly acquire a colder climate;
and, consequently, its glaciers will increase in size sufficient to
launch icebergs into the Pacific to be currented southward, and so
still further lower the temperature of the Eastern Pacific waters,
and consequently the equatorial current from which the Japanese stream
branches, and so eventually, under the above conditions, cause heavy
ice-sheets to spread widely over the north-west coast of North America.
It will be seen from the above explanations how an increase of cold
in the southern hemisphere is necessary to cause a wider spread of
ice-sheets on lands in the northern hemisphere.
Especially is this the case to promote the gathering of glaciers on the
west coast of North America. The great equatorial current while on its
way to the Indian Ocean not only sends off the Japanese stream, but
also the East Australian current, which is like the Japanese current,
having its temperature lowered in proportion as the equatorial stream
is cooled. Therefore, the southern ocean is slowly being deprived of
equatorial heat from this source.
I have explained how the increasing coldness of the superior oceans of
the southern hemisphere affects more or less the temperature of the
Gulf Stream, which meanwhile is only able to enter a small portion
of its waters into the Arctic Ocean after undergoing a long cooling
process as a drift current; and, while thus mingling with the arctic
waters, it is not able to prevent the gathering of ice-sheets on
Greenland, where glaciers are launching bergs to float southward as far
as the latitude of 40° north. Consequently, the northern seas are now
being cooled as well as the seas of the southern hemisphere.
Yet this cooling process is so slow there is a lack of data to
show that the temperature of the high latitudes is lowering. Our
thermometrical observations are of such recent date they cannot be used
to determine climatic changes which requires centuries to bring about.
Still, it is generally known that the climate of Northern Europe has
been accused of growing colder. The vine no longer flourishes on the
shores of Bristol Channel or in Flanders or Brittany; and vineyards
are no longer planted on the elevated shores of France where they
flourished three hundred years ago. Arago did not refuse to believe
that the laws regulating the temperature of Western Europe had notably
altered. This is proved, he said, by the general retrogradation of the
vineyards southward.
The recent deadly freezing of the orange groves of Florida makes it
uncertain whether the cultivation of the orange can again be successful
in the counties where during this generation it has been very
profitable.
Travellers visiting Iceland say that the old accounts of its prosperity
seem strange to those who now visit its shores; and it is narrated in
the Sagas that in early times sheep could shift for themselves during
winter, and that there were large forests and that corn ripened.
Several years ago a correspondent of the _Spectator_, writing from
Northern Russia where the Volga is locked with ice for six months in
the year, stated that “the people were beginning to show increased
resentment at the climate, and that there was reason to believe that
the northern government of Russia would be abandoned to the desert. The
people silently glide south by the tens of thousands every year, so the
life of Russia was concentrating in the south.”
It is now the opinion of travellers in arctic lands that the
inhabitants of the Esquimaux regions are decreasing, as are also the
inhabitants of Northern Siberia.
A writer in the North China _Herald_, of Shanghai, says that “the
climate of Asia is becoming colder than it formerly was, and its
tropical animals and plants are retreating southward at a slow rate.
In the time of Confucius elephants were in use on the Yangtse River.
A hundred and fifty years after this Mencius speaks of the tiger, the
leopard, the rhinoceros, and the elephant as being in many parts of
China.
“It is also said that the ferocious alligator, that formerly infested
the rivers of South China, has retreated southward.
“The flora of the country is also affected by the increasing coldness
of the climate. The bamboo is not found in the forests of North China,
where it grew naturally two thousand years ago, but is still grown in
Pekin, with the aid of good shelter, as a sort of garden plant only.”
A letter from Hong Kong, published in the London _Standard_, reports
that on the 15th of January, 1893, the temperature of Hong Kong, a
tropical seaport of China, was below freezing for three days, and was
colder than ever before known. The rocks and also vegetation were
covered with a coating of ice. The thermometer at times stood at 23°
and 26° Fahrenheit.
I have previously explained how the slow increasing coldness of the
northern temperate zone is also being carried out in the southern
hemisphere. The meteorological records for the lofty table lands of
Ecuador, although very incomplete, furnish strong evidence to show that
the mean temperature of that region is gradually lowering.
Observations made by Boussingault at Quito in 1831, compared with those
from 1878 to 1881, showed a decrease from 15.2° Centigrade to 13.27°
Centigrade.
Records made by Hall from 1825 to 1827 give averages of 16.1°
Centigrade, 15.52° Centigrade, and 15.6° Centigrade. This decrease
holds good for all points in the inter-Andean region where records have
been kept.
Yet we know that the falling temperature in the northern temperate
latitudes is not brought about by a yearly increase of cold, because,
when the arctic channels are somewhat obstructed with icebergs, the
movement of arctic waters through them is lessened; and, therefore,
during such times the Gulf Stream, meeting with less opposition from
arctic currents while flowing northward, is able to move a larger
volume of its waters into the arctic seas, thus warming their waters
sufficiently in a few seasons to clear the obstructed channels, and
also somewhat soften for several successive years the temperature of
such lands as border on the seas of that region.
And in this way we account for the mild seasons which at times follow
those of lower temperature in high northern latitudes.
But, when the detained icebergs are set adrift, and currented into the
temperate North Atlantic, the heat consumed while melting such numerous
bodies of ice is able to more than overcome the warmth gained during
the temporary detention of ice in the northern seas. Thus, under such
considerations, it appears that the conditions are favorable for the
growth of glaciers in the high northern latitudes.
I have pointed out the manner in which the superior oceans in the
southern hemisphere are obtaining a lower temperature, and how
they impart their coldness to the tropical currents, and in this
way slowly cool the waters of all oceans. Thus it appears that the
northern temperate zone, with all other parts of the earth, is slowly
approaching a cold epoch.
Several writers on climatic changes have expressed their views as to
the number of glacial and mild periods that have been perfected since
the conditions have been favorable for their appearance on the globe.
According to my views, while considering the reasons for the occurrence
of the great glacial periods which have left such extensive traces on
the land, it seems certain that two very cold epochs have possessed
the earth, separated by a warm period; and, possibly, other preceding
cold epochs of less intensity have possessed the high latitudes, with
intervening periods of mildness. But the earlier cold periods, if they
ever existed, were comparatively short, because the Cape Horn channel
during such times possessed less capacity than in the later periods,
and, therefore, was more easily and quickly obstructed by the natural
methods previously explained.
Consequently, the independent circulation of the southern ocean was
sooner arrested than during the later epoch, when the channel had
become enlarged by erosion from heavy glaciers and icebergs; and
meanwhile the same conditions may have governed the arctic channels
which give an independent circulation to the arctic waters which
surround Greenland, and thus, in connection with cold epochs in the
southern hemisphere, have caused periods of cold of small intensity to
occur in the high northern latitudes, and it may happen in the future
that more ice periods will be perfected than the one now progressing.
Still, it is well to bear in mind that the Cape Horn channel, which
is the real cause of glacial periods having occurred in both the
northern and southern hemispheres, in the manner previously explained,
is being made wider and deeper during each succeeding ice age. For
this reason the latest cold epoch will require a longer continuance
of cold to obstruct the channel than the cold period preceding.
Therefore, it appears that the time will come when there will be such
great accumulations of ice stored on the land and in the sea before
the enlarged Cape Horn channel can be closed that, when it is closed,
there will not be sufficient warmth remaining in the tropical seas to
unite with the sun’s rays to subdue the intense cold stored in the
immense gatherings of ice. And thus the earth, which began its career
with a warm temperature, and so continued for long ages, will finally
terminate in an endless glacial age.
The statements made by General Cowell in _Science_ of Nov. 25, 1892, in
reference to the alleged discovery of the second rotation of the earth
by Major-general Drayson, represents the discovery as affording a new
solution for the cause or causes of an ice age.
The second rotation as defined consists in the pole of the heavens
describing a circle around a point which is ascertained to be
situated six degrees distant from the pole of the ecliptic. And it
is asserted that by a knowledge of the second rotation it is proved
that a variation of twelve degrees in the extent of the arctic circle
and the tropics occurred not later than 13,500 B.C., “the tropics
varying in distance from the equator from the minimum of 23° 25′
47″ to the maximum of 35° 25′ 47″, thus extending the torrid zone
during its widest expansion from Cape Hatteras to the river Plate....
It is calculated that at this date we are about 403 years distant
from the time when the pole of the heavens in its revolution, the
pole of the ecliptic and that of the second rotation, will be in
the same colure,--that is, in the year 2,295 A.D.; and then the
least differences in temperature between summer and winter will be
experienced. From that time forward this difference will increase, and
about 6,000 years later, or about the year 8,300 A.D., the earth will
enter the next glacial period, and attain its greatest severity about
the year 18,136 of our era.” General Cowell does not state how the
widening of the tropical zone, as above set forth, would bring about
a glacial period. The winters of the temperate zones would evidently
be colder than now; but, on the other hand, the summers would be
proportionally warmer, while the westerly winds above the latitudes of
40° would prevail the same as now.
Therefore, their general effect on the surface waters of the ocean
in the high latitudes would not be changed with such an extension of
the tropical zone, neither would the trade winds change their general
direction with a wider torrid zone; yet the boundaries of the trade
winds and also the westerly winds would be more shifting according to
the declination of the sun, such winds being governed as now by the
position of the sun during the summer and winter solstice. Yet the
natural process for moving tropical water into the high latitudes, or
excluding it therefrom, would not be greatly changed.
Consequently, the expansion of the torrid zone to the latitudes named
by General Drayson would not affect the climate of the hemispheres
sufficiently to cause a frigid epoch. On the contrary, the summer
monsoons, which now blow from the north-east, along the shores of
Eastern Africa, and also along the coast of Southern Brazil, would be
much stronger with a vertical sun in midsummer as far south as river
Plate, thus forcing the surface waters of the tropical oceans into the
higher latitudes with greater facility than at this age.
Moreover, according to the statements of General Cowell, the present
period of mildness should be on the increase, and obtain perfection
in the year 2,295, or about 400 years hence; while, on the contrary,
according to the explanations we have given in the preceding pages,
there is much to show that an ice age is advancing, and has made
considerable progress in the high latitudes of both hemispheres.
Furthermore, if the second rotation, as claimed by General Cowell,
is able to perfect a glacial period at regular intervals of 31,600
years, it seems that traces of frigid epochs should not be confined to
late geological records, as there appear to be little or no traces of
glacial work prior to the Quaternary or Post-tertiary periods.
It appears that explanations so far given, which depend on the
astronomical theory to account for the ice age, are not in harmony with
well-known geographical facts. The explainers neglect the attention due
to the great prevailing winds which since the earlier geological ages
have, in connection with continents, moved the surface waters of the
ocean from torrid latitudes to colder zones, and from the colder zones
to the warmer latitudes.
This exchange of ocean waters between the zones is as old as the
continents which shape their courses. The important change wrought in
the ocean currents sufficient to have caused the glacial age which
ended the early warm epochs was brought about through the action of the
prevailing winds, which, in connection with the form of continents,
became able to move the ocean waters from the northern hemisphere into
the southern sufficient to submerge the low lands of the southern
hemisphere, causing a great diversion of the tropical currents from
the high southern latitudes, such as I have pointed out in preceding
chapters.
Those writers who believe that ocean currents have been the cause of
great climatic changes have suggested that the existence of an ancient
channel through the isthmus of Panama would have caused a frigid period
on lands bordering on the northern shores of the Atlantic by turning
the head-waters of the Gulf Stream into the Pacific Ocean.
Professor Agassiz thinks that such a channel existed during some remote
geological age, judging from the semblance of the fauna pertaining to
the Caribbean Sea and Pacific Ocean.
Yet it may be said that an open channel through Central America would
have connected two high sea-levels.
For this reason there would be little or no exchange of water between
the Caribbean Sea and the Pacific Ocean.
The high sea-level on the Pacific side is caused by the prevailing
north-west winds which blow down the North American coast past
California as far south as Central America; while, on the other hand,
the south-east trade winds impel the surface waters of the South
Pacific along the coast of Peru down to the equator, and so onward 5°
to 8° north latitude. Thus the space between the ending of the two
ocean winds obtains a high sea-level, corresponding to the high level
of the Caribbean Sea. This has been proved from levellings for the
Nicaragua ship canal.
Consequently, the Atlantic waters would not run into the Pacific Ocean,
even if a channel opened through Central America.
Therefore, the Gulf Stream has never been turned away from the North
Atlantic.
Writers, while seeking a cause for the mild climate of ages preceding
the glacial epochs, have thought that during such times channels
opening through Asia from the Indian Ocean by the way of the Persian
Gulf into the arctic seas would be the means of furnishing the Arctic
Ocean with warm water. But it is evident that such a movement of water
could not be brought about, because the winds would not be favorable
for it. For, when we reflect that the prevailing winds would blow in
the same direction as now, and that the seas of Eastern Europe and
Western Asia were enlarged during the warm epochs, it seems that they
would obtain high levels superior to the high level seas of the Indian
Ocean.
Besides, we should consider that there is a continuous range of high
land separating the Persian Gulf from the northern seas, which probably
existed anterior to the ice age. Still, during later periods, while
the ice-sheets were being melted from the northern hemisphere and also
on the ending of the last ice age, the Isthmus of Suez was submerged,
as were all other low lands in that latitude; but it is probable that
the waters of the high sea-level of the Indian Ocean abreast tropical
Africa did not flow largely into the Mediterranean Sea for the reason
that the enlarged European seas, being within the westerly wind-belt,
maintained a high sea-level, while at the same time the high level
tropical Indian Ocean waters were strongly attracted into the southern
oceans through the Mozambique and Agulhas currents in the manner I
have previously explained. Yet the waters of the high sea-level of the
southern European seas must have been strongly attracted to the low
sea-level abreast the Canary Islands.
While considering the causes which brought about the glacial periods,
it is well to reflect that the natural mode of action which could have
produced a frigid age was as extensive as the surface of the globe;
and, therefore, any geographical change that would affect only a
comparatively small portion of the earth cannot serve to account for
ages of warmth which extended over the globe, or for glacial epochs
which were separated by warm periods of time, which seem to have
affected all lands and seas.
And it appears from the geographical explanations given in preceding
pages of the general movements of the winds and currents of the sea how
impossible it is for heat to be conveyed to the antarctic latitudes
sufficient to prevent the growth of glaciers on their lands while the
Cape Horn channel is in possession of its present capacity.
For, as has been shown, this channel furnishes opportunity for the
westerly winds to impel the surface waters of the great southern ocean
constantly around the globe, and so largely turns away the tropical
currents from the high southern latitudes.
Consequently, there seems to be no method yet devised through nature’s
mode of action that can carry sufficient heat into the antarctic
latitudes to melt the ice-sheets from the southern continent, or even
arrest their growth, while the Cape Horn channel maintains its present
width and depth.
Therefore, the increase of glaciers and icebergs will slowly continue
until a glacial epoch is perfected.
And it seems that this arrangement for bringing about a frigid age
made slower progress in its early stage than at this date, owing to
there having been a lack of glacial ice in the polar regions to produce
icebergs for cooling the ocean waters. But the independent circulation
of the great southern ocean, after turning away the tropical currents
from the high southern latitudes for thousands of years, did at length
cause glaciers to form on the antarctic lands, which have been slowly,
but constantly increasing; and, consequently, the cooling of the ocean
has been accelerated proportionate to the increase of ice-sheets.
Therefore, with the cooling process so well advanced as it now appears
to be, it seems that more than half of the time required to bring a
frigid age to perfection has been expended since ice-sheets began to
gather on the antarctic shores. For, when we realise how the facilities
for making ice have advanced through the increase of glaciers in both
hemispheres, and how large a portion of the ocean waters have been
cooled below a temperate or tropical temperature even in the torrid
latitudes where the warm upper waters of the ocean have been reduced
to a comparatively thin stratum when compared to the vast bulk of the
cooled under-waters, it appears that the cold will increase at a faster
rate for the next thousand years than was the case during the last ten
centuries. Therefore, the climate will be less favorable for plants and
animals existing on lands in the high latitudes for the next thousand
years than during the ten centuries preceding; and, when we take into
consideration the accelerative growth of a frigid epoch, it seems that
the increasing cold will in a few thousand years drive the greater
portion of both plants and animals from the now temperate latitudes to
maintain an existence in the tropical zone, where a large part of the
existing species of such life must have taken refuge during the last
ice period.
And, from what can be learned from the relics of man’s prehistoric
life, it seems to point to the lands of the tropical latitudes as
having been his home during the frigid ages; and, because of his long
undisturbed residence in favored portions of the tropics, he there
attained his earliest civilization. For it appears that the tropical
zone was not only less burdened with ice in glacial times than the
higher latitudes of the globe, but was also more exempt from the
great flooding of lands which obtained in the more northern latitudes
through the shifting of the ocean waters, from causes set forth in the
preceding pages. Yet it may be said that the low lands of the tropical
zone south of the equator during cold epochs were much more extensive
than at this age, on account of the shrinkage of the sea, because of
the great amount of water evaporated from its surface, and stored in
ice-sheets on the great continents and islands. Hence the reefs and
shallows which surround such tropical islands as include the Seychelles
Archipelago, and also the extensive banks covered with shoal water
in that portion of the Indian Ocean, were during the glacial period
elevated above the surface of the sea, possessing a climate favorable
for vegetable and animal life. But, owing to the great rain-fall of
that region, it is probable that the highest lands were glaciated,
as it is reported that granite bowlders still rest on the mountain
slopes of the highest island. The numerous islands and shoals of the
south-western tropical Pacific must also have afforded wide land areas,
with a temperate climate, owing to their having been situated on one of
the warmest regions of the earth during the ice age.
Moreover, it is probable that these tropical lands afforded space for
numerous lagoons which had little connection with the surrounding
oceans, and consequently were able to maintain, in their secluded
shallow basins, a warmer temperature than obtained in the open seas;
and at the same time, owing to the great rainfall in such tropical
portions of the Indian and Pacific regions, the waters of the lagoons
were rendered less salt than the briny depths of the shrunken oceans
of a cold period. Hence because of such conditions the fauna of the
tropical seas were preserved from the destructive rigor which beset the
earth during the frigid epochs.
Transcriber’s Notes
Punctuation, hyphenation, and spelling were made consistent when a
predominant preference was found in the original book; otherwise they
were not changed.
Simple typographical errors were corrected; unbalanced quotation
marks were remedied when the change was obvious, and otherwise left
unbalanced.