THE CAUSE OF THE GLACIAL PERIOD.
In searching for the cause of the Glacial period, it is evident that we
must endeavor to find conditions which will secure over the centre of the
glaciated area either a great increase of snow-fall or a great decrease
in the mean annual temperature, or both of these conditions combined in
greater or less degree. As can be seen, both from the nature of the case
and from the unglaciated condition of Siberia and northern Alaska, a low
degree of temperature is not sufficient to produce permanent ice-fields.
If the snow-fall is excessively meagre, even the small amount of heat in
an arctic summer will be sufficient to melt it all away.
From the condition of Greenland, however, it appears that a moderate
amount of precipitation where it is chiefly in the form of snow may
produce enormous glaciers if at the same time the average temperature
is low. In southeastern Alaska, on the other hand, the glaciers are of
enormous size, though the mean annual temperature is by no means low,
for there the great amount of snow-fall amply compensates for the higher
temperature.
Snow stores the cold and keeps it in a definite place. If the air becomes
chilled, circulation at once sets in, and the cold air is transferred to
warmer regions; but if there is moisture in the air, so that snow forms,
the cold becomes locked up, as it were, and falls to the earth.
The amount of cold thus locked up in snow is enormous. To melt one
cubic foot of ice requires as much heat as would raise the temperature
of a cubic foot of water 176° Fahrenheit. To melt a "layer of ice only
one inch and a half thick would require as much heat as would raise a
stratum of air eight hundred feet thick from the freezing-point to the
tropical heat of 88° Fahrenheit." It is the slowness with which ice melts
which enables it to accumulate as it does, both in winter and upon high
mountains and in arctic regions. Captain Scoresby relates that when near
the north pole the sun would sometimes be so hot as to melt the pitch on
the south side of his vessel, while water was freezing on the north side,
in the shade, owing to the cooling effect of the masses of ice with which
he was surrounded.
Thus it will appear that a change in the direction of the moist winds
blowing from the equator towards the poles might produce a Glacial
epoch. If snow falls upon the ocean it cools the water, but through the
currents, everywhere visible in the sea, the temperature in the water in
the different parts soon becomes equalized. If, however, the snow falls
upon the land, it must be melted by the direct action of the sun and
wind upon the spot where it is. If the heat furnished by these agencies
is not sufficient to do it year by year, there will soon be such an
accumulation that glaciers will begin to form. It is clear, therefore,
that the conditions producing a Glacial period are likely to prove very
complicated, and we need not be surprised if the conclusions to which we
come are incapable of demonstration.
Theories respecting the cause of the Glacial period may be roughly
classified as astronomical and geological. Among the astronomical
theories, one which has sometimes been adduced is that the solar system
in its movement through space is subjected to different degrees of heat
at different times. According to this theory, the temperate climate
which characterised the polar regions during the Tertiary period, and
continued up to the beginning of the Glacial epoch, was produced by the
influence of the warmer stretches of space through which the whole solar
system was moving at that time; while the Glacial period resulted from
the influence upon the earth of the colder spaces through which the
system subsequently moved.
While it is impossible absolutely to disprove this hypothesis, it labors
under the difficulty of having little positive evidence in its favor,
and thus contravenes a fundamental law of scientific reasoning, that we
must have a real cause upon which to rest our theories. In endeavouring
to explain the unknown, we should have something known to start with.
But in this case we are not sure that there are any such variations in
the temperature of the space through which the solar system moves. This
theory, therefore, cannot come in for serious consideration until all
others have been absolutely disproved. As we shall also more fully see,
in the subsequent discussion, the distribution of the ice during the
Glacial period was not such as to indicate a gradual extension of it from
the north pole, but rather the accumulation upon centres many degrees to
the south.
Closely allied with the preceding theory is the supposition broached
by some astronomers that the sun is a variable star, dependent to some
extent for its heat upon the impact of meteorites, or to the varying
rapidity with which the contraction of its volume is proceeding.
It is well known that when two solid bodies clash together, heat is
produced proportionate to the momentum of the two bodies. In other
words, the motion which is arrested is transformed into heat. Mr. Croll,
in his last publication[DR] upon the subject, ingeniously attempted to
account for the gaseous condition of the nebulæ and the heat of the sun
and other fixed stars by supposing it to be simply transformed motion.
According to this theory, the original form of force imparted to the
universe was that exerted in setting in motion innumerable dark bodies,
which from time to time have collided with each other. The effects of
such collisions would be to transform a large amount of motion into
heat and its accompanying forms of molecular force. The violence of the
compact of two worlds would be so great as to break them up into the
original atoms of which they are composed, and the heat set free would
be sufficient to keep the masses in a gaseous condition and cause them
to swell out into enormous proportions. From that time on, as the heat
radiated into space, there would be the gradual contraction which we
suppose is going on in all the central suns, accompanied, of course, with
a gradual decline of the heat-energy in the system.
[Footnote DR: Stellar Evolution and its Relation to Geological Time.]
Now, it is well known that the earth and the solar system in their
onward progress pass through trains of meteorites. The tails of some of
the comets are indeed pretty clearly proved to be streams of ponderable
matter, through which, from time to time, the minor members of the solar
system plunge, and receive some accession to their bulk and weight. The
shooting-stars, which occasionally attract our attention in the sky,
mark the course of such meteorites as they pass through the earth's
atmosphere, and are heated to a glow by the friction with it. It has been
suggested, therefore, that the sun itself may at times have its amount
of heat sensibly affected by such showers of meteorites or asteroids.
Upon this theory the warm period of the Tertiary epoch, for instance,
may have been due to the heat temporarily added to the sun by impact
with minor astronomical bodies. When, afterwards, it gradually cooled
down, receiving through a long period no more accessions of heat from
that source, the way was prepared for the colder epoch of the Glacial
period, which, in turn, was dispelled by fresh showers of meteorites
upon the sun, sufficient to produce the amelioration of climate which we
experience at the present time.
As intimated, this theory is closely allied to the preceding, the
principal difference being that it limits the effects of the supposed
cause to the solar system, and looks to our sun as the varying source of
heat-supply. It has the advantage over that, however, of possessing a
more tangible _vera causa_. Meteorites, asteroids, and comets are known
to be within this system, and have occasional collisions with other
members of it. But the principal objection urged against the preceding
theory applies here, also, with equal force. The accumulations of ice
during the Glacial period were not determined by latitude. In North
America the centre of accumulation was south of the Arctic Circle--a fact
which points clearly enough to some other cause than that of a general
lowering of the temperature exterior to the earth.
The same objections would bear against the theory ably set forth by Mr.
Sereno E. Bishop, of Honolulu, which, in substance, is that there may
be considerable variability in the sun's emission of heat, owing to
fluctuations in the rate of the shrinkage of its diameter, brought about
by the unequal struggle between the diminishing amount of heat in the
interior and the increasing force of the gravitation of its particles,
and by the changes in the enveloping atmosphere of the sun, which, like
an enswathing blanket, arrests a large portion of the radiant heat from
the nucleus, and is itself evidently subject to violent movements, some
of which seem to carry it down to the sun's interior. Unknown electrical
forces, he thinks, may also combine to add an element of variability.
These supposed changes may be compared to those which take place upon
the surface of the earth when, at irregular intervals, immense sheets of
lava, like those upon the Pacific coast of North America, are exuded in a
comparatively brief time, to be succeeded by a long period of rest. The
heat thus brought to the surface of the earth would add perceptibly to
that radiated from it into space in ordinary times. Something similar to
this upon the sun, it is thought, might produce effects perceptible upon
the earth, and account for alternate periods of heat and cold.
A fourth astronomical theory is that there has been a shifting of the
earth's axis; that at the time of the Glacial period the north pole,
instead of being where it now is, was somewhere in the region of central
Greenland. This attractive theory has been thought worthy of attention
by President T. C. Chamberlin and by Professor G. C. Comstock,[DS] but
it likewise labours under a twofold difficulty: First, the shifting of
the poles observed (450 feet per year) is too slight to have produced the
changes within any reasonable time, and it is not likely to have been
continuous for a long period. But still more fatal to the theory is the
fact that the warm climate preceding the Glacial period seems to have
extended towards the present north pole upon every side; a temperate
flora having been found in the fossil plants of the Tertiary beds in
Greenland and northern British America, as well as upon Nova Zembla and
Spitzbergen.
[Footnote DS: See papers by these gentlemen read at the meeting of the
American Association for the Advancement of Science, in Washington, in
August, 1891. Professor Comstock's paper appeared in the American Journal
of Science for January, 1893.]
A fifth astronomical theory, and one which has of late years been
received with great favour, is that so ably advocated by the late Dr.
James Croll and by Professor James Geikie. Following the suggestions
of the astronomer Adhémar, these writers have attempted to show that
not only one Glacial epoch, but a succession of such epochs, has been
produced in the world by the effect of the changes which are known to
have taken place in the eccentricity of the earth's orbit when combined
with the precession of the equinoxes--another calculable astronomical
cause.
[Illustration: Fig. 99.--Diagram showing effect of precession: _A._
condition of things now; _B._ as it will be 10,500 years hence. The
eccentricity is of course greatly exaggerated.]
It is well known that the earth's orbit is elliptical; that is, it is
longer in one direction than in the other, so that the sun is one side
of the centre. During the winter of the northern hemisphere the earth
is now about three million miles nearer the sun than in the summer; but
the summer makes up for this distance by being about seven days longer
than the winter. Through the precession of the equinoxes this state of
things will be reversed in ten thousand five hundred years; at which time
we shall be nearer the sun during our northern summer, and farther away
in winter, our winter then being also longer than our summer. Besides,
through the unequal attraction of the planets the eccentricity of the
earth's orbit periodically increases and diminishes, so that there have
been periods when the earth was ten million five hundred thousand miles
farther from the sun in winter than in summer; at which times, also,
the winter was nearly twenty-eight days longer than the summer. Such an
extreme elongation of the earth's orbit occurred about two hundred and
fifty thousand years ago.
It is easy to assume that such a change in astronomical conditions would
produce great effects upon the earth's climate; and equally easy to
connect with those effects the vast extension of ice during the Glacial
period. Since, also, this period of extreme eccentricity terminated
only eighty thousand years ago, the close of the Glacial period would,
perhaps, upon Mr. Croll's theory, be comparatively a recent event; for
if the secular summer of the earth's eccentricity lags relatively as
far behind the secular movements as the annual summer does behind the
vernal equinox, we should, as Professor Charles H. Hitchcock suggests,
have to place the complete breaking up of the Ice period as late as forty
thousand years ago.[DT]
[Footnote DT: Geology of New Hampshire, vol. iii, p.327.]
We have no space to indicate, as it deserves, the comparative merits
and demerits of this ingenious theory. It would, however, be a great
calamity to have geologists accept it without scrutiny. It is, indeed,
a part of the business of geologists to doubt such theories until they
are verified by a thorough examination of all accessible _terrestrial_
evidence bearing upon the subject. There is no reason to question the
reality of the variations in the relative positions of the earth and the
sun assumed by Mr. Croll; though there may be serious doubt whether the
effects of those changes upon climate would be all that is surmised,
since equal amounts of heat would fall upon the earth during summer,
whether made longer or shorter by the cause referred to. During the short
summers the earth is so much nearer the sun that it receives each season
absolutely as much heat as it does during the longer summers, when it
is so much farther away from the sun. Thus the theory rests at last upon
the question what would become of the heat reaching the earth in these
differing conditions. It is plausibly urged by Mr. Croll that when a
hemisphere of the earth is passing through a period of long winters the
radiation of heat will be so excessive that the temperature would fall
much below what it would during the shorter winters; and so ice and snow
would accumulate far beyond the usual amount. It is also supposed that
the effect of the summer's sun in melting the ice during the short summer
would be diminished through natural increase of the amount of foggy and
cloudy weather.
Adhémar's theory is supposed by Sir Robert Ball, Royal Astronomer of
Ireland, to be considerably re-enforced by a discovery which he has made
concerning the distribution of heat upon the earth during the seasons
culminating in the summer and winter solstices. Croll had assumed,
on the authority of Herschel, that a hemisphere of the earth during
the longer winter in aphelion would receive the same actual amount of
heat which would fall upon it during the shorter summer in perihelion;
whereas, according to Dr. Ball's discovery, "of the total amount of heat
received from the sun on a hemisphere of the earth in the course of a
year, sixty-three per cent is received during the summer and thirty-seven
per cent during the winter."[DU] When, therefore, the summers occur in
perihelion the heat is more intense than Croll had supposed, and, at
the same time, the winters occurring in aphelion are more deficient in
heat than he had assumed. This discovery of Dr. Ball will not, however,
materially affect the discussion of Croll's theory upon its inherent
merits, since it is simply an intensification of the causes invoked by
him. We will therefore let it stand or fall in the light of the general
considerations hereafter to be adduced.
[Footnote DU: Cause of an Ice Age, p. 90.]
The aid of theoretical consequent changes in the volume of the Gulf
Stream, and in the area of the trade-winds, has also to be invoked by
Mr. Croll. The theory likewise receives supposed confirmation from facts
alleged concerning the present climate of the southern hemisphere which
is passing through the astronomical conditions thought to be favourable
to its glaciation. The antarctic continent is completely enveloped in
ice, even down to the sixty-seventh degree of latitude. A few degrees
nearer the pole Sir J. C. Boss describes the ice as rising from the water
in a precipitous wall one hundred and eighty feet high. In front of such
a wall, and nearly twenty degrees from the south pole, this navigator
sailed four hundred and fifty miles! Voyagers, in general, are said to
agree that the summers of the antarctic zone are much more foggy and cold
than they are in corresponding latitudes in the northern hemisphere; and
this, even though the sun is 3,000,000 miles nearer the earth during the
southern summer than it is during the northern.
Another direction from which evidence is invoked in confirmation of Mr.
Croll's theory is the geological indications of successive Glacial epochs
in times past. If there be a recurring astronomical cause sufficient of
itself to produce Glacial periods, such periods should recur as often as
the cause exists; but glaciation upon the scale of that which immediately
preceded the historic era could hardly have occurred in early geological
time without leaving marks which geologists would have discovered. Were
the "till" now covering the glaciated region to be converted into rock,
its character would be unmistakable, and the deposit is so extensive that
it could not escape notice.
In his inaugural address before the British Association in 1880,
Professor Ramsey, Director-General of the Geological Survey of Great
Britain, presented a formidable list of glacial observations in
connection with rocks of a remote age.[DV] Beginning at the earliest
date, he cites Professor Archibald Geikie, one of the most competent
judges, as confident that the rounded knobs and knolls of Laurentian
rocks exposed over a large region in northwestern Scotland, together
with vast beds of coarse, angular, unstratified conglomerates, are
unquestionable evidences of glacial action at that early period. Masses
of similar conglomerates, resembling consolidated glacial boulder-beds,
occur also in the Lower Silurian formation at Corswall, England. In
Dunbar, Scotland, Professor Forbes also found, in formations of but
little later age than the Coal period, "brecciated conglomerates,
consisting of pebbles and large blocks of stone, generally angular,
embedded in a marly paste, in which some of the pebbles are as well
scratched as those found in medial moraines." In formations of
corresponding antiquity the geologists of India have found similar
boulder-beds, in which some of the blocks are polished and striated.
[Footnote DV: Nature (August 26, 1880), vol. xxii, pp. 388, 389.]
Still, this evidence is less abundant than we should expect, if there had
been the repeated Glacial epochs supposed by Mr. Croll's astronomical
theory; and it is by no means impossible that the conglomerates of
scratched stones described by Professor Ramsey in Great Britain, and
by Messrs. Blandford and Medlicott in India, may have resulted from
local glaciers coming down from mountain-chains which have been since
removed by erosion or subsidence. We are not aware that any incontestable
evidence has been presented in America of any glaciation previous to that
of _the_ Glacial period.
Upon close consideration, also, it appears that Mr. Croll's theory has
not properly taken into account the anomalous distribution of heat which
we actually find to take place on the surface of the earth. He has done
good service in showing what an enormous transfer of heat there is
from the southern to the northern Atlantic by means of the Gulf Stream,
estimating that the heat conveyed by the Gulf Stream into the Atlantic
Ocean is equal to one fifth of all possessed by the waters of the North
Atlantic; or to the heat received from the sun upon a million and a half
square miles at the equator, or two million square miles in the temperate
zone. "The stoppage of the Gulf Stream would deprive the Atlantic of
77,479,650,000,000,000,000 foot-pounds of energy in the form of heat per
day."
Among the objections which bear against this ingenious theory is one
which will appear with great force when we come to discuss the date of
the Glacial period, when we shall show that even Professor Hitchcock's
supposition that the lingering effects of the last great eccentricity of
the earth's orbit, continued down to forty thousand years ago, is not
sufficient to account for the recentness of the close of the period as
shown by abundant geological evidence. It is certainly not more than ten
or fifteen thousand years ago that the ice finally melted off from the
Laurentian highlands; while on the Pacific coast the period of glaciation
was still more recent.
From inspection of the accompanying map the main point of Mr. Croll's
reasoning may be understood. It will be seen that the direction of the
currents in the central Atlantic is largely determined by the contour of
the northeastern coast of South America. From some cause the southeast
trade-winds are stronger than the northeast, and their force is felt in
pushing the superficial currents of warm water farther north than Cape
St. Roque, the eastern extremity of Brazil. As the direction of the South
American coast trends rapidly westward from this point to the Isthmus of
Panama, the resultant of the forces is a strong current northwestward
into the _cul-de-sac_ of the Gulf of Mexico, from which there is only the
one outlet between Cuba and the peninsula of Florida. Through this the
warm water is forced into the region where westerly winds prevail, and
spreads its genial influence far to the northward, modifying the climate
of the British Isles, and even of far-off Norway.
[Illustration: Fig. 100.--Map showing course of currents in the Atlantic
Ocean: _b_ and _b'_ are currents set in motion by opposite trade-winds;
meeting, they produce the equatorial current, which divides into _c_ and
_c'_, continuing on as _a_ and _a'_ and _e_.]
But why are the southeast trade-winds of the Atlantic stronger than
the northeast? The ultimate reason, of course, is to be found in the
fact that the northern hemisphere is warmer than the southern. The
atmosphere over the northern-central portion of the Atlantic region is
more thoroughly rarefied by the sun's heat than is that over the region
south of the equator. The strong southeast trades are simply the rush of
atmosphere from the South Atlantic to fill the vacuum caused by the heat
of the sun north of the equator.
But, again, why is this? Because, says Mr. Croll, we are now in that
stage of astronomical development favourable to the increased warmth
of the northern hemisphere. In the northern hemisphere the summers are
longer than the winters. Perihelion occurs in winter and aphelion in
summer. This is the reason why the North Atlantic is warmer than the
South Atlantic, and why the trade-winds of the south are drawn to the
north of the equator. Ten thousand five hundred years ago, however, the
conditions were reversed, and the greater rarefaction of the atmosphere
would have taken place south of the equator, thus drawing the trade-winds
in that direction.
By again inspecting the map, one will see how far-reaching the effect on
the climate of northern countries this change in the prevalences of the
trades would have been. Then, instead of having the northwest current
leading along the northeast coast of South America into the Gulf of
Mexico augmented by the warm currents circulating south of the equator,
the warm currents of the north would have been pushed down so far that
they would augment the current running to the southwest beyond Cape St.
Roque, along the southeast shore of South America; thus the northern
portion of the Atlantic, instead of robbing the southern portion of
heat, would itself be robbed of its warm currents to contribute to the
superfluous heat of the South Atlantic.
This theory is certainly very ingenious. There is a weak point in
it, however. Mr. Croll assumes that when the winters of the northern
hemisphere occur in aphelion, they must necessarily be colder than now.
But, evidently, this assertion implies a fuller knowledge than we possess
of the laws by which the heat received from the sun is distributed over
the earth.
For it appears from observation that the equator is by no means so
hot now as, theoretically, it ought to be, and that the arctic regions
are not so cold as, according to theory, they should be, and this in
places which could not be affected by oceanic currents. For example,
at Iquitos, on the Amazon, only three hundred feet above tide, three
degrees and a half south of the equator, and more than a thousand miles
from the Atlantic (so that ocean-currents cannot abstract the heat from
its vicinity), the mean yearly temperature is but 78° Fahr.; while at
Verkhojansk, in northeast Siberia, which is 67° north of the equator, and
is situated where it is out of the reach of ocean-currents, and where
the conditions for the radiation of heat are most favourable, and where,
indeed, the winter is the coldest on the globe (January averaging--56°
Fahr.), the mean yearly temperature is two degrees and a half above zero;
so that the difference between the temperature upon the equator and that
at the coldest point on the sixty-seventh parallel is only about 75°
Fahr.; whereas, if temperature were in proportion to heat received from
the sun, the difference ought to be 172°. Again, the difference between
the actual January temperature on the fiftieth parallel and that upon the
sixtieth is but 20° Fahr., whereas, the quantity of solar heat received
on the fiftieth parallel during the month of January is three times that
received upon the sixtieth, and the difference in temperature ought to be
about 170° Fahr. upon any known law in the case.
Woeikoff, a Russian meteorologist, and one of the ablest critics of Mr.
Croll's theory, and to whom we are indebted for these facts, ascribes
the greater present warmth of the northern Atlantic basin, not to the
astronomical cause invoked by Mr. Croll, but to the relatively small
extent of sea in the middle latitudes of the northern hemisphere. The
extent and depth of the oceans of the southern hemisphere would of
themselves give greater steadiness and force to its trade-winds, and
lead to a general lowering of the temperature; so that it is doubtful
if the astronomical causes introduced by Mr. Croll, even with Dr.
Ball's re-enforcement, would produce any appreciable effect while the
distribution of land and water remains substantially what it is at the
present time.
Still another variation in the astronomical theory has been set forth
and defended by Major-General A. W. Drayson, F. R. A. S., instructor
in the Royal Military School at Woolwich, England. He contends that
what has been called the precession of the equinoxes, and supposed to
be "a conical movement of the earth's axis in a circle around a point
as a centre, from which it continually decreases its distance,"[DW] is
really a second rotation of the earth about its centre. As a consequence
of this second rotation, he endeavours to show that the inclination of
the earth's axis varies as much as 12°; so that, whereas the Arctic and
Antarctic Circles and the tropics extend to only about 23° from the poles
and the equator, respectively, about thirteen thousand five hundred years
ago they extended more than 35°; thus bringing the frigid zones in both
cases 12° nearer the equator than now. This, he contends, would have
produced the Glacial period at the time now more generally assigned to it
by direct geological evidence.
[Footnote DW: Untrodden Ground in Astronomy and Geology, p. 26.]
The difficulty with this theory, even if the mathematical calculations
upon which it is based are correct, would be substantially the same as
those already urged against that of Mr. Croll. It is specially difficult
to see how General Drayson would account for the prolonged temperate
climate in high northern latitudes during the larger part of the Tertiary
epoch.
It will be best to turn again to the map to observe the possible effect
upon the Gulf Stream of a geological event of which we have some definite
evidence, and which is adduced by Mr. Upham and others as one of the
important probable causes of the Glacial period, namely, the subsidence
of the Isthmus of Panama and the adjacent narrow neck of land connecting
North with South America. It will be seen at a glance that a subsidence
sufficient to allow the northwest current of warm water, pushed by the
trade-winds along the northeast shore of South America, to pass into
the Pacific Ocean, instead of into the Gulf of Mexico, would be a cause
sufficient to produce the most far-reaching results; it would rob the
North Atlantic of the immense amount of heat and moisture now distributed
over it by the Gulf Stream, and would add an equal amount to the northern
Pacific Ocean, and modify to an unknown extent the distribution of heat
and moisture over the lands of the northern hemisphere.
The supposition that a subsidence of the Isthmus of Panama was among
the contributing causes of the Glacial period has been often made, but
without any positive proof of such subsidence. From evidence which
has recently come to light, however, it is certain that there has
actually been considerable subsidence there in late Tertiary if not in
post-Tertiary times. This evidence is furnished by Dr. G. A. Maack and
Mr. William M. Gabb in their report to the United States Government in
1874 upon the explorations for a ship-canal across the isthmus, and
consists of numerous fossils belonging to existing species which are
found at an elevation of 150 feet above tide. As the dividing ridge is
more than 700 feet above tide, this does not positively prove the point,
but so much demonstrated subsidence makes it easy to believe, in the
absence of contradictory evidence, that there was more, and that the
isthmus was sufficiently submerged to permit a considerable portion of
the warm equatorial current which now passes northward from the Caribbean
Sea and the Gulf of Mexico to pass into the Pacific Ocean.
[Illustration: Fig. 101.--Map showing how the land clusters about the
north pole.]
An obvious objection to the theory of a late Tertiary or post-Tertiary
subsidence of the Isthmus of Panama presents itself in the fact that
there is at present a complete diversity of species between the fish
inhabiting the waters upon the different sides of the isthmus. If
there had been such a subsidence, it seems natural to suppose that
Atlantic species would have migrated to the Pacific side and obtained
a permanent lodgment there, and that Pacific species would have found
a congenial home on the Atlantic side. It must be confessed that this
is a serious theoretical difficulty, but perhaps not insuperable. For
it is by no means certain that colonists from the heated waters of the
Caribbean Sea would become so permanently established upon the Pacific
side that they could maintain themselves there upon the re-establishment
of former conditions. On the contrary, it seems reasonable to suppose
that upon the re-elevation of the isthmus the northern currents, which
would then resume their course, would bring back with them conditions
unfavourable to the Atlantic species, and favourable to the competing
species which had only temporarily withdrawn from the field, and which
might now be better fitted than ever to renew the struggle with their
Atlantic competitors. It is by no means certain, therefore, that with
the re-establishment of the former conditions there would not also be a
re-establishment of the former equation of life upon the two sides of the
isthmus.
Mr. Upham's theory involves also extensive elevations of land in the
northern part of America; in this respect agreeing with the opinions
early expressed by Professors J. D. Dana and J. S. Newberry. Of the
positive indications of such northward elevations of land we have already
spoken when treating in a previous chapter of the fiords and submerged
channels which characterise northern Europe and both the eastern and
the western coasts of North America. But in working out the problem the
solution is only half reached when we have got the Gulf Stream into
the Pacific Ocean, and the land in the northern part of the continents
elevated to some distance above its present level. There is still the
difficulty of getting the moisture-laden currents from the Pacific Ocean
to carry their burdens over the crest of the Sierra Nevada and Rocky
Mountains and to deposit them in snow upon the Laurentian highlands.
An ingenious supplement to the theory, therefore, has been brought
forward by Professor Carpenter, who suggests that the immense Tertiary
and post-Tertiary lava-flows which cover so much of the area west of
the Rocky Mountains were the cause of the accumulations of snow which
formed the Laurentide Glacier. This statement, which at first seems so
paradoxical as to be absurd, appears less so upon close examination.
The extent of the outflows of lava west of the Rocky Mountains is almost
beyond comprehension. Literally, hundreds of thousands of square miles
have been covered by them to a depth in many places of thousands of feet.
These volcanic eruptions are mostly of late date, beginning in the middle
of the Tertiary and culminating probably about the time of the maximum
extent of the Laurentide Glacier. Indeed, so nearly contemporaneous was
the growth of the Laurentide Glacier with these outflows that Professor
Alexander Winchell had, with a good deal of plausibility, suggested that
the outflows of the eruptions of lava were caused by the accumulation
of ice over eastern British America. His theory was that the three
million cubic miles of ice which is proved to have been abstracted from
the ocean and piled up over that area was so serious a disturbance of
the equilibrium of the earth's crust that it caused great fissures to
be opened along the lines of weakness west of the Rocky Mountains, and
pressed the liquid lava out, as the juice is pressed out of an orange in
one place by pressing upon the rind in another.
Professor Carpenter's view is the exact reverse of Professor Winchell's.
Going back to those orographic changes which produced the lava-flows
and the elevation of the northern part of British America, he thinks
the problem of getting the moisture transferred from the Pacific Ocean
to the Canadian highlands is solved by the lava-flows west of the Rocky
Mountains. This immense exudation of molten matter was accompanied by an
enormous liberation of heat, which must have produced significant changes
in the meteorological conditions.
The moisture of the atmosphere is precipitated by means of the
condensation connected with a lowering of its temperature. Ordinarily,
therefore, when moist winds from an oceanic area pass directly over a
lofty mountain-chain, the precipitation takes place immediately, and
the water finds its way back by a short course to the sea. This is what
now actually occurs on the Pacific coast. The Sierra Nevada condense
nearly all the moisture; so that very little falls on the vast area
extending from their summits eastward to the Rocky Mountains. All that
region is now practically a desert land, where the evaporation exceeds
the precipitation. In Professor Carpenter's view the heat radiated from
the freshly exuded lava is supposed to have prevented the precipitation
near the coast-line, and to have helped the winds in carrying it farther
onward to the northeast, where it would be condensed upon the elevated
highlands, upon which the snows of the great Laurentide Glacier were
collected.
It is not necessary for us to attempt to measure the amount of truth in
this subsidiary hypothesis of Professor Carpenter, but it illustrates
how complicated are the conditions which have to be considered before we
rest securely upon any particular hypothesis. The unknown elements of the
problem are so numerous, and so far-reaching in their possible scope,
that a cautious attitude of agnosticism, with respect to the cause of
the Glacial period, is most scientific and becoming. Still, we are ready
to go so far as to say that Mr. Upham's theory comes nearest to giving
a satisfactory account of all the phenomena, and it is to this that
Professor Joseph Le Conte gives his cautious approval.
Summarily stated, this theory is, that the passage from the Tertiary
to the Quaternary or Glacial period was characterised by remarkable
oscillations of land-level, and by corresponding changes of climate, and
of ice-accumulation in northern regions; that the northern elevation
was connected with subsidence in the equatorial regions; that these
changes of land-level were both initiated and, in the main, continued
by the interior geological forces of the globe; but that the very
continental elevation which mainly brought on the Glacial period added
at length, in the weight of the ice which accumulated over the elevated
region, a new force to hasten and increase the subsidence, which would
have taken place in due time in the natural progress of the orographic
oscillations already begun. Professor Le Conte illustrates the subject by
the following diagram, which, for simplicity's sake, treats the Glacial
epoch as one; the horizontal line, A B, represents time from the later
Pliocene until now; but it also represents the present condition of
things both as to land-level and as to ice-accumulation. The full line, c
d e, represents the oscillations of land (and presumably of temperature)
above and below the present condition. The broken line represents the
rise, culmination, and decline of ice-accumulation. The dotted line
represents the crust-movement as it would have been if there had been no
ice-accumulation.
[Illustration: Fig. 102.]
_Succession of Epochs, Glacial and Fluvial Deposits, and_
Eastern Provinces and Middle and Southern
Epochs. New England. Atlantic States.
Recent or Rise of the land to its Continued subsidence of
Terrace. present height, or coast at New York and
(Mostly within somewhat higher, soon southward, and rise of
the period of after the departure of the mountainous belt, by
traditional the ice. Rivers eroding displacement along the
and written their glacial fall line of the rivers.
history.) flood-plains, leaving Much erosion of the
remnants as terraces. Columbia formation since
Warmer climate than now, culmination of second
probably due to greater Glacial epoch;
Gulf Stream, formerly sedimentation in bays,
permitted southern sounds, and estuaries.
mollusks to extend to
Gulf of St. Lawrence, now
represented by isolated
colonies.
Glacial Period of Ice Age. Pleistocene Period.
Champlain. Land depressed under Less subsidence in
ice-weight; glacial latitude of New York and
(Close of the recession; continued southward than at north;
second Glacial deposition of upper till lower Hudson Valley, and
epoch.) and deep flood-plains of part of its present
gravel, sand and clay submarine continuation,
(modified drift). above sea-level. Gravel
Terminal moraines marking and sand deposits from
pauses or readvance englacial drift in
during general retreat of Delaware and Susquehanna
ice. Marine submergence. Valleys, inclosing
150 to 230 feet on coast abundant human implements
of Maine, to 520 feet in at Trenton, N.J.
Gulf and valley of St.
Lawrence.
Second Glacial. Second great uplift of Renewal of great
the land, 3.000 to 4,000 continental elevation
feet higher than now; (3.000 feet in latitude
snow-fall again all the of New York and
year; ice probably two Philadelphia), of
miles thick on Laurentide excessive snow-fall and
highlands, and extending rains, and of wide-spread
somewhat farther south fluvial deposits, the
here than in first Columbia formation, on
glaciation. Lower till the coastal plain, during
(ground moraine), and early part of this epoch.
upper till (englacial Implements of man at
drift). Terminal Claymont, Del.
moraines, kames, osars,
valley drift.
Inter-glacial. Ice-sheet melted here; Depression, but generally
probably not more ice in not to the present level.
(Longest epoch arctic regions than now. Deep channels cut in the
of this era.) bed-rocks by the
Fluvial and lacustrine Delaware, Susquehanna,
deposits of this time, Potomac, and other
with those of the first rivers. The Appomattox
Glacial epoch, were deposits much eroded.
eroded by the second
glaciation. Relative length of this
epoch made known by McGee
from study of this
region.
First Glacial. Begun by high continental Continental elevation;
uplift, cool climate and erosion of Delaware and
snow-fall throughout the Chesapeake Bays, and of
year, producing Albemarle and Pamlico
ice-sheet. Much glacial Sounds. Plentiful
erosion and snow-fall on the southern
transportation; till and Appalachian Mountains;
stratified deposits. snows melted in summer,
Ended by depression of and heavy rains,
land; return of warm producing broad
climate, with rain; final river-floods, with
melting of the ice. deposition of the
Isthmus of Panama Appomattox formation.
probably submerged (Gulf
Stream smaller), and
again in second Glacial
epoch.
_Changes in Altitude and Climate, during the Quaternary Era._
Mississippi Basin and Cordilleran Region. Europe and Asia.
northward.
Terracing of river Including a stage of Erosion and terracing
valleys. Northward rise considerable uplift, of stratified drift in
of area of Lake Agassiz with return of humid river valleys. Land
nearly complete before conditions, Alpine passage of European
the ice was melted on glaciation (third flora to Greenland;
the country crossed by Glacial epoch), and succeeded by subsidence
Nelson River; but rise the second great rise there, admitting warm
about Hudson Bay is still of Lakes Bonneville currents to Arctic Sea.
going on; 7,000 to 8,000 and Lahontan. Very Minor climatic changes,
years since ice-melting recent subsidence including a warmer
uncovered Niagara and and change to present stage than now. Upper
falls of St. Anthony. aridity. and outer portions of
Indo-Gangetic alluvial
plain; extensive
deposits of Hwang Ho,
and destructive changes
of its course.
Abundant deposition of Depression probably Final departure of the
englacial drift. Stone almost to the present ice-sheets; glacial
implements in river level. Restoration of rivers forming eskers
gravels of Ohio, Ind., arid climate; nearly or and kames. Loess
and Minn. Laurentian quite complete deposited while the
lakes held at higher evaporation of Lakes region of the Alps was
levels, and Lake Bonneville and Lahontan.depressed lower than
Agassiz formed in Red Formation of the "adobe"now. Upper (englacial)
River basin, by continuing through the till, and asar, of
barrier of retreating second Glacial, Sweden. Marine
ice, with outlets over Champlain, and Recent submergence 500 to 600
lowest points of their epochs. feet in Scotland,
present southern Scandinavia, and
water-shed. Marine Spitzbergen.
submergence 300 to 500
feet on southwest
side of Hudson Bay.
Ice-sheet here less Probable uplift 3,0 Second elevation and
extensive than in the feet, shown by general glaciation of
first Glacial epoch, and submerged valleys near northwestern Europe;
not generally bordered Cape Mendocino. Second the ice-sheets of Great
as then by lakes in ice-sheet on British Britain probably more
valleys which now drain Columbia and Vancouver extensive than in first
southward. Island; local Glacial epoch.
glaciation of Rocky Oscillations of
Terminal moraines at Mountains, Cascade and ice-front; British
extreme limit of the Sierra range, Nevada, Lower and Upper
ice-advance, and at ten south to latitude 37°. the Chalky, Purple, and
or more stages of halt or First great rise of bowlder-clays, Hessle
readvance in its retreat. Lakes Bonneville bowlder-clays. Terminal
and Lahontan. moraines in Germany.
Depression nearly to Continental depression. Recession, or probably
present level southward; Arid climate. Long- complete departure, of
more northward, but continued denudation of the ice-sheets.
followed there, by the mountains:
differential uplift of resulting very thick Land connection between
800 or 1,000 feet. subaërial deposits of Europe and Africa,
Great erosion of loess the "adobe." permitting southern
and other modified animals to extend far
drift, and of "Orange Intermittent volcanic northward.
Sand." Valleys of this action in various parts
epoch, partly filled of this region, Erosion of the Somme
with later till, are throughout the Valley below its oldest
marked by chains of Quaternary era to very implement-bearing
lakes in southern recent times, and gravels.
Minnesota. liable to break forth
again.
Pliocene elevation of Latest rise (3.000 Uplift and glaciation
continent brought to feet) of the Colorado of northwestern Europe:
culmination at Cañon district. Sierra maximum elevation.
beginning of Nevada and other Great 2,500 feet or more
Quaternary era; this Basin mountain-ranges (depth of the Skager
whole basin probably formed by immense Rack); France and
then uplifted 3.000 uplifts, with faulting. Britain united with the
feet; excessive California river- Färöe Islands, Iceland,
snow-fall and rain; courses changed; human and Greenland. Uplifts
deposition of the bones and implements in of the Himalayas and
"Orange Sand." Ice- the old river gravels, other mountain-ranges
sheet south to lava-covered. Ice-sheet attendant on both
Cincinnati and St. on British Columbia; Glacial epochs.
Louis, at length local glaciers
depressing the earth's southward.
crust beneath it;
slackened river floods
and shallow lakes,
forming the loess.
It is seen from the diagram that the ice-accumulation culminated at a
time when the land, under the pressure of the ice-load, had already
commenced to subside; and that the subsidence was greatest at a time when
the pressure had already begun to diminish. But the fact that the land,
after the removal of the ice-load, did not return again to its former
height in the Pliocene, is proof positive that there were other and
more fundamental causes of crust-movement at work besides weighting and
lightening. The land did not again return to its former level because the
cycle of elevation, whatever its cause, which commenced in the Pliocene
and culminated in the early Quaternary, had exhausted itself. If it had
not been for the ice-load interfering with and modifying the natural
course of the crust-movement determined previously and primarily by other
and probably internal causes, the latter would probably have taken the
course represented by the dotted line. It would have risen higher and
culminated later, and its curve would have been of simpler form.
We append a carefully prepared table by Mr. Warren Upham, showing the
probable changes in altitude and climate during the Quaternary era.[DX]
[Footnote DX: On page 106 and sequel I have summarised the reasons which
lead me to discard the Inter-Glacial epoch, and to look upon the whole
Glacial period as constituting a grand unity with minor episodes. It
does not yet seem to me that the duality of the period is proved. On the
contrary, Mr. Kendall's chapter on the Glacial phenomena of Great Britain
strongly confirms my view.]
On the part of many the theory here provisionally adopted will be
regarded with disfavour by reason of a disinclination to supposing
any great recent changes of level in the continental areas. So firmly
established do the continents appear to be, that it seems like invoking
an inordinate display of power to have them exalted for the sake of
producing a Glacial period. Due reflection, however, will make it
evident that within certain limits the continents are exceedingly
unstable, and that they have displayed this instability to as great an
extent in recent geological times as they have done in any previous
geological periods. When one reflects, also, upon the size of the earth,
a continental elevation of 3,000 or 4,000 feet upon a globe whose
diameter is more than 40,000,000 feet is an insignificant trifle. On a
globe one foot in diameter it would be represented by a protuberance of
barely one thousandth of an inch. A corresponding wrinkle upon a large
apple would require a magnifying-glass for its detection. Moreover, the
activity of existing volcanoes, the immense outflows of lava which have
taken place in the later geological periods, together with the uniform
increase of heat as we penetrate to deeper strata in the crust of the
earth--all point to a condition of the earth's interior that would
make the elevations of land which we have invoked for the production
of the Glacial period easily credible. Physicists do not, indeed, now
hold to the entire fluidity of the earth's interior, but rather to a
solid centre, where gravity overcomes the expansive power of heat, and
maintains solidity even when the heat is intense. But between the cooling
crust of the earth's exterior and a central solid core there is now
believed to be a film where the influences of heat and of the pressure
of gravity are approximately balanced, and the space is occupied by a
half-melted or viscous magma, capable of yielding to a slow pressure, and
of moving in response to it from one portion of the enclosed space to
another where the pressure is for any cause relieved.
As a result of prolonged enquiries respecting the nature of the forces
at work both in the interior and upon the exterior of the earth, and
of a careful study of the successive changes marking the geological
period, we are led to believe that the continental elevations necessary
to produce the phenomena of the Glacial period are not only entirely
possible but easily credible, and in analogy with the natural progress
of geological history. In the first place, it is easy to see that
two causes are in operation to produce a contraction of the earth's
volume and a shortening of its diameter. Heat is constantly being
abstracted from the earth by conduction and radiation, but perhaps to
a greater extent through ceaseless volcanic eruptions which at times
are of enormous extent. It requires but a moment's thought to see that
contraction of the volume of the earth's interior means that the hardened
exterior crust must adjust itself by wrinkles and folds. For a long
period this adjustment might show itself principally in gentle swells,
lifting portions of the continents to a higher level, accompanied by
corresponding subsidence in other places. This gradually accumulating
strain would at length be relieved along some line of special weakness in
the crust by that folding process which has pushed up the great mountain
systems of the world.
Careful study of the principal mountain systems shows that all the
highest of them are of late geological origin. Indeed, the latter part
of the Tertiary period has been the great mountain-building epoch in the
earth's history. The principal part of the elevation of the Andes and
the Rocky Mountains has taken place since the middle of the Tertiary
period. In Europe there is indubitable evidence that the Pyrenees have
been elevated eleven thousand feet during the same period, and that the
western Alps have been elevated thirteen thousand feet in the same time.
The Carpathians, the western Caucasus, and the Himalayas likewise bear
explicit evidence to the fact that a very considerable portion of their
elevation, amounting to many thousand feet, has been effected since the
middle of the Tertiary period, while a considerable portion of this
elevation of the chiefest mountain systems of the world has occurred in
what would be called post-Tertiary time--that is, has been coincident
with a portion of the Glacial period.
The Glacial period, however, we suppose to have been brought about, not
by the specific plications in the earth's crust which have produced the
mountain-chains, but by the gentler swells of larger continental areas
whose strain was at last relieved by the folding and mashing together
of the strata along the lines of weakness now occupied by the mountain
systems. The formation of the mountains seems to have relieved the
accumulating strain connected with the continental elevations, and to
have brought about a subsequent subsidence.
Doubtless, also, correlated subsidences and elevations of the earth's
crust have been aided by the transfer of the sediment from continental
to oceanic areas, and, as already suggested, during the Glacial period
by the transfer of water evaporated from the surface of the ocean to
the ice-fields of the glaciated area. For example, present erosive
agencies are lowering the level of the whole Mississippi basin from
the Alleghanies to the Rocky Mountains at the rate of a foot in five
thousand years. All this sediment removed is being transferred to the
ocean-bed. Present agencies, therefore, if not counteracted, would
remove the whole continent of America (whose average elevation above the
sea is only 748 feet) in less than four million years; while the great
rivers which descend in all directions from the central plateau of Asia
are transferring sediment to the ocean from two to four times as fast
as the Mississippi is, and the Po is transferring it from the Alps to
the Adriatic fully seven times as fast as the Mississippi is from its
basin to the Gulf of Mexico. This rapid transfer of sediment from the
continents to the ocean is producing effects in disturbing the present
equilibrium of the earth's crust, which are too complicated for us fully
to calculate; but it is by no means improbable that when accumulating for
a considerable length of time, the ultimate results may be very marked
and perhaps sudden in their appearance.
The same may also be said of the accumulation of ice during the Glacial
period. The glaciated areas of North America and Europe combined
comprise about six million square miles. At a moderate estimate,
the ice was three-quarters of a mile deep. Here, therefore, there
would be between four and five million cubic miles of water, which
had first relieved the ocean-beds of the pressure of its weight, and
then concentrated its force over the elevated areas of the northern
hemisphere. This disturbance of the equilibrium, by the known transfer
of force from one part of the earth's crust to another, certainly gives
much plausibility to the theory of Jamieson, Winchell, Le Conte, and
Upham, that the Glacial period partly contained in itself its own cure,
and by the weight of its accumulated weight of ice helped to produce
that depression over the glaciated area which at length rendered the
accumulation of ice there impossible.
This general view of the known causes in operation during the Glacial
period will go far towards answering an objection that has probably
before this presented itself to the reader's mind. It seems clear
that the Glacial period in the southern hemisphere has been nearly
contemporaneous with that of the northern. The Glacial period proper of
the southern hemisphere is long since passed. The existing glaciers of
New Zealand, of the southern portion of the Andes Mountains, and of the
Himalaya Mountains are but remnants of those of former days. In the light
of the considerations just presented, it would not seem improbable that
the same causes should produce these similar effects in the northern and
the southern hemisphere contemporaneously. At any rate, it would not
seem altogether unlikely that the pressure of ice during the climax of
the Glacial period upon the northern hemisphere (which, as we have seen,
there is reason to believe aided in the depression of the continent to
below its present level in the latter part of the Glacial period) should
have contributed towards the elevation of mountains in other parts of the
world, and so to the temporary enlargement of the glaciers about their
summits.
Nor are we wholly without evidence that these readjustments of land-level
which have been carried on so Vigorously since the middle of the Tertiary
period are still going on with considerable though doubtless with
diminished rapidity. There has been a re-elevation of the land in North
America since the Glacial period amounting to 230 feet upon the coast of
Maine, 500 feet in the vicinity of Montreal, from 1,000 to 1,600 feet in
the extreme northern part of the continent, and in Scandinavia to the
extent of 600 feet. In portions of Scandinavia the land is now rising
at the rate of three feet in a century. Other indications of even the
present instability of the earth's surface occur in numbers too numerous
to mention.[DY]
[Footnote DY: For a convincing presentation of the views here outlined,
together with abundant references to literature, see Mr. Warren Upham's
Appendix to the author's Ice Age in North America.]
But, while we are increasingly confident that the main causes of the
Glacial period have been changes in the relative relation of land-levels
connected with diversion of oceanic currents, it is by no means
impossible, as Wallace[DZ] and others have suggested, that these were
combined with the astronomical causes urged by Drs. Croll and Geikie.
By some this combination is thought to be the more probable, because
of the extreme recentness of the close of the Glacial period, as shown
by the evidence which will be presented in the following chapter. The
continuance of glaciers in the highlands of Canada, down to within a few
thousand years of the present time, coincides in a remarkable manner with
the last occurrence of the conditions favourable to glaciation upon Mr.
Croll's theory, which took place about eleven thousand years ago.
[Footnote DZ: See Island Life, chapters viii and ix.]