sedimentary deposits are being formed as the mean rate for all ages,
we probably under-estimate the total amount of rock formed, because
during the many glacial periods which must have occurred in past ages
the amount of materials ground off the rocky surface of the land in a
given period would be far greater than at present. But, in reply, it
must be remembered that although the destruction in ice-covered regions
would be greater during these periods than at present, yet the quantity
of materials carried down by rivers into the sea would be less. At
the present day the greater part of the materials carried down by our
rivers is not what is being removed off the rocky face of the country,
but the boulder clay, sand, and other materials which were ground off
during the glacial epoch. It is therefore possible, on this account,
that the rate of deposit may have been less during the glacial epoch
than at present.
When any particular formation is wanting in a given area, the inference
generally drawn is, that either the formation has been denuded off
the area, or the area was a land-surface during the period when that
formation was being deposited. From the foregoing it will be seen that
this inference is not legitimate; for, supposing that the area had been
under water, the chances that materials should have been deposited on
that area are far less than are the chances that there should not.
There are sixteen chances against one that no formation ever existed in
the area.
If the great depressions of the Atlantic, Pacific, and Indian Oceans
be, for example, as old as the beginning of the Laurentian period—and
they may be so for anything which geology can show to the contrary—then
under these oceans little or no stratified rocks may exist. The
supposition that the great ocean basins are of immense antiquity, and
that consequently only a small proportion of the sedimentary strata
can possibly occupy the deeper bed of the sea, acquires still more
probability when we consider the great extent and thickness of the
Old Red Sandstone, the Permian, and other deposits, which, according
to Professor Ramsay and others, have been accumulated in vast inland
lakes.
CHAPTER XXIII.
THE PHYSICAL CAUSE OF THE SUBMERGENCE AND EMERGENCE OF THE LAND
DURING THE GLACIAL EPOCH.
Displacement of the Earth’s Centre of Gravity by Polar
Ice-cap.—Simple Method of estimating Amount of
Displacement.—Note by Sir W. Thomson on foregoing
Method.—Difference between Continental Ice and
a Glacier.—Probable Thickness of the Antarctic
Ice-cap.—Probable Thickness of Greenland Ice-sheet.—The
Icebergs of the Southern Ocean.—Inadequate Conceptions
regarding the Magnitude of Continental Ice.
_Displacement of the Earth’s Centre of Gravity by Polar
Ice-cap._[206]—In order to represent the question in its most simple
elementary form, I shall assume an ice-cap of a given thickness at the
pole and gradually diminishing in thickness towards the equator in the
simple proportion of the sines of the latitudes, where at the equator
its thickness of course is zero. Let us assume, what is actually the
case, that the equatorial diameter of the globe is somewhat greater
than the polar, but that when the ice-cap is placed on one hemisphere
the whole forms a perfect sphere.
I shall begin with a period of glaciation on the southern hemisphere.
Let W N E S′ (Fig. 5) be the solid part of the earth, and _c_ its
centre of gravity. And let E S W be an ice-cap covering the southern
hemisphere. Let us in the first case assume the earth to be of the same
density as the cap. The earth with its cap forms now a perfect sphere
with its centre of gravity at _o_; for W N E S is a circle, and _o_
is its centre. Suppose now the whole to be covered with an ocean a few
miles deep, the ocean will assume the spherical form, and will be of
uniform depth. Let the southern winter solstice begin now to move round
from the aphelion. The ice-cap will also commence gradually to diminish
in thickness, and another cap will begin to make its appearance on
the northern hemisphere. As the northern cap may be supposed, for
simplicity of calculation, to increase at the same rate that the
southern will diminish, the spherical form of the earth will always be
maintained. By the time that the northern cap has reached a maximum,
the southern cap will have completely disappeared. The circle W N′ E S′
will now represent the earth with its cap on the northern hemisphere,
and _o′_ will be its centre of gravity; for _o′_ is the centre of the
circle W N′ E S′. And as the distance between the centres _o_ and
_o′_ is equal to N N′, the thickness of the cap at the pole N N′ will
therefore represent the extent to which the centre of gravity has been
displaced. It will also represent the extent to which the ocean has
risen at the north pole and sunk at the south. This is evident; for as
the sphere W N′ E S′ is the same in all respects as the sphere W N E
S, with the exception only that the cap is on the opposite side, the
surface of the ocean at the poles will now be at the same distance from
the centre _o′_ as it was from the centre _o_ when the cap covered
the southern hemisphere. Hence the distance between _o_ and _o′_ must
be equal to the extent of the submergence at the north pole and the
emergence at the south. Neglect the attraction of the altering water on
the water itself, which later on will come under our consideration.
[Illustration: Fig. 5.]
We shall now consider the result when the earth is taken at its actual
density, which is generally believed to be about 5·5. The density
of ice being ·92, the density of the cap to that of the earth will
therefore be as 1 to 6.
[Illustration: Fig. 6.]
Let Fig. 6 represent the earth with an ice-cap on the northern
hemisphere, whose thickness is, say, 6,000 feet at the pole. The centre
of gravity of the earth without the cap is at _c_. When the cap is on,
the centre of gravity is shifted to _o_, a point a little more than
500 feet to the north of _c_. Had the cap and the earth been of equal
density, the centre of gravity would have been shifted to _o′_ the
centre of the figure, a point situated, of course, 3,000 feet to the
north of _c_. Now it is very approximately true that the ocean will
tend to adjust itself as a sphere around the centre of gravity, _o_.
Thus it would of course sink at the south pole and rise to the same
extent at the north, in any opening or channel in the ice allowing the
water to enter.
Let the ice-cap be now transferred over to the southern hemisphere,
and the condition of things on the two hemispheres will in every
particular be reversed. The centre of gravity will then lie to
the south of _c_, or about 1,000 feet from its former position.
Consequently the transference of the cap from the one hemisphere to the
other will produce a total submergence of about 1,000 feet.
It is, of course, absurd to suppose that an ice-cap could ever actually
reach down to the equator. It is probable that the great ice-cap of the
glacial epoch nowhere reached even halfway to the equator. Our cap must
therefore terminate at a moderately high latitude. Let it terminate
somewhere about the latitude of the north of England, say at latitude
55°. All that we have to do now is simply to imagine our cap, up to
that latitude, becoming converted into the fluid state. This would
reduce the cap to less than one-half its former mass. But it would not
diminish the submergence to anything like that extent. For although the
cap would be reduced to less than one-half its former mass, yet its
influence in displacing the centre of gravity would not be diminished
to that extent. This is evident; for the cap now extending down to
only latitude 55°, has its centre of gravity much farther removed from
the earth’s centre of gravity than it had when it extended down to the
equator. Consequently it now possesses, in proportion to its mass, a
much greater power in displacing the earth’s centre of gravity.
There is another fact which must be taken into account. The common
centre of gravity of the earth and cap is not exactly the point
around which the ocean tends to adjust itself. It adjusts itself not
in relation to the centre of gravity of the solid mass alone, but in
relation to the common centre of gravity of the entire mass, solid and
liquid. Now the water which is pulled over from the one hemisphere to
the other by the attraction of the cap will also aid in displacing the
centre of gravity. It will co-operate with the cap and carry the true
centre of gravity to a point beyond that of the centre of gravity of
the earth and cap, and thus increase the effect.
It is of course perfectly true that when the ice-cap does not extend
down to the equator, as in the latter supposition, and is of less
density than the globe, the ocean will not adjust itself uniformly
around the centre of gravity; but the deviation from perfect uniformity
is so trifling, as will be seen from the appended note of Sir William
Thomson, that for all practical purposes it may be entirely left out of
account.
In the _Reader_ for January 13, 1866, I advanced an objection to the
submergence theory on the grounds that the lowering of the ocean-level
by the evaporation of the water to form the ice-cap, would exceed the
submergence resulting from the displacement of the earth’s centre of
gravity. But, after my letter had gone to press, I found that I had
overlooked some important considerations which seem to prove that the
objection had no real foundation. For during a glacial period, say
on the northern hemisphere, the entire mass of ice which presently
exists on the southern hemisphere would be transferred to the northern,
leaving the quantity of liquid water to a great extent unchanged.
_Note on the preceding by Sir William Thomson, F.R.S._
“Mr. Croll’s estimate of the influence of a cap of ice on the sea-level
is very remarkable in its relation to Laplace’s celebrated analysis,
as being founded on that law of thickness which leads to expressions
involving only the first term of the series of ‘Laplace’s functions,’
or ‘spherical harmonics.’ The equation of the level surface, as
altered by any given transference of solid matter, is expressed by
equating the altered potential function to a constant. This function,
when expanded in the series of spherical harmonics, has for its first
term the potential due to the whole mass supposed collected at its
altered centre of gravity. Hence a spherical surface round the altered
centre of gravity is the _first_ approximation in Laplace’s method of
solution for the altered level surface. Mr. Croll has with admirable
tact chosen, of all the arbitrary suppositions that may be made
foundations for rough estimates of the change of sea-level due to
variations in the polar ice-crusts, _the_ one which reduces to zero all
terms after the first in the harmonic series, and renders that first
approximation (which always expresses the _essence_ of the result) the
whole solution, undisturbed by terms irrelevant to the great physical
question.
“Mr. Croll, in the preceding paper, has alluded with remarkable
clearness to the effect of the change in the distribution of the
water in increasing, by its own attraction, the deviation of the
level surface above that which is due to the _given_ change in the
distribution of solid matter. The remark he makes, that it is round
the centre of gravity of the altered solid and altered liquid that
the altering liquid surface adjusts itself, expresses the essence of
Laplace’s celebrated demonstration of the stability of the ocean, and
suggests the proper elementary solution of the problem to find the
true alteration of sea-level produced by a given alteration of the
solid. As an assumption leading to a simple calculation, let us suppose
the solid earth to rise out of the water in a vast number of small
flat-topped islands, each bounded by a perpendicular cliff, and let the
proportion of water area to the whole be equal in all quarters. Let all
of these islands in one hemisphere be covered with ice, of thickness
according to the law assumed by Mr. Croll—that is, varying in simple
proportion of the sine of the latitude. Let this ice be removed from
the first hemisphere and similarly distributed over the islands of
the second. By working out according to Mr. Croll’s directions, it is
easily found that the change of sea-level which this will produce will
consist in a sinking in the first hemisphere and rising in the second,
through heights varying according to the same law (that is, simple
proportionality to sines of latitudes), and amounting at each pole to
(1 - ω)it
—————————,
1 - ωw
where _t_ denotes the thickness of the ice-crust at the pole; _i_ the
ratio of the density of ice, and _w_ that of sea-water to the earth’s
mean density; and ω the ratio of the area of ocean to the whole surface.
“Thus, for instance, if we suppose ω = ⅔, and _t_ = 6,000 feet, and
take ⅙ and 1/(5½) as the densities of ice and water respectively, we
find for the rise of sea-level at one pole, and depression at the other,
⅓ × ⅙ × 6000
————————————,
2 1
1 - — × —
3 5½
or approximately 380 feet.
“I shall now proceed to consider roughly what is the probable extent
of submergence which, during the glacial epoch, may have resulted from
the displacement of the earth’s centre of gravity by means of the
transferrence of the polar ice from the one hemisphere to the other.”
_Difference between Continental-ice and a Glacier._—An ordinary
glacier descends in virtue of the slope of its bed, and, as a general
rule, it is on this account thin at its commencement, and thickens
as it descends into the lower valleys, where the slope is less and
the resistance to motion greater. But in the case of continental ice
matters are entirely different. The slope of the ground exercises
little or no influence on the motion of the ice. In a continent of one
or two thousand miles across, the general slope of the ground may be
left out of account; for any slight elevation which the centre of such
a continent may have will not compensate for the resistance offered to
the flow of the ice by mountain ridges, hills, and other irregularities
of its surface. The ice can move off such a surface only in consequence
of pressure acting from the interior. In order to produce such a
pressure, there must be a piling up of the ice in the interior; or, in
other words, the ice-sheet must thicken from the edge inwards to the
centre. We are necessarily led to the same conclusion, though we should
not admit that the ice moves in consequence of pressure from behind,
but should hold, on the contrary, that each particle of ice moves by
gravity in virtue of its own weight; for in order to have such a motion
there must be a slope, and as the slope is not on the ground, it must
be on the ice itself: consequently we must conclude that the upper
surface of the ice slopes upwards from the edge to the interior. What,
then, is the least slope at which the ice will descend? Mr. Hopkins
found that ice barely moves on a slope of one degree. We have therefore
some data for arriving at least at a rough estimate of the probable
thickness of an ice-sheet covering a continent, such, for example, as
Greenland or the Antarctic Continent.
_Probable Thickness of the Antarctic Ice-cap._—The antarctic continent
is generally believed to extend, on an average, from the South Pole
down to about, at least, lat. 70°. In round numbers, we may take the
diameter of this continent at 2,800 miles. The distance from the
edge of this ice-cap to its centre, the South Pole, will, therefore,
be 1,400 miles. The whole of this continent, like Greenland, is
undoubtedly covered with one continuous sheet of ice gradually
thickening inwards from its edge to its centre. A slope of one degree
continued for 1,400 miles will give twenty-four miles as the thickness
of the ice at the pole. But suppose the slope of the upper surface
of the cap to be only one-half this amount, viz., a half degree,—and
we have no evidence that a slope so small would be sufficient to
discharge the ice,—still we have twelve miles as the thickness of the
cap at the pole. To those who have not been accustomed to reflect on
the physical conditions of the problem, this estimate may doubtless
be regarded as somewhat extravagant; but a slight consideration
will show that it would be even more extravagant to assume that a
slope of less than half a degree would be sufficient to produce the
necessary outflow of the ice. In estimating the thickness of a sheet of
continental ice of one or two thousand miles across, our imagination
is apt to deceive us. We can easily form a pretty accurate sensuous
representation of the thickness of the sheet; but we can form no
adequate representation of its superficial area. We can represent
to the mind with tolerable accuracy a thickness of a few miles, but
we cannot do this in reference to the area of a surface 2,800 miles
across. Consequently, in judging what proportion the thickness of the
sheet should bear to its superficial area, we are apt to fall into the
error of under-estimating the thickness. We have a striking example
of this in regard to the ocean. The thing which impresses us most
forcibly in regard to the ocean is its profound depth. A mean depth
of, say, three miles produces a striking impression; but if we could
represent to the mind the vast area of the ocean as correctly as we can
do its depth, _shallowness_ rather than _depth_ would be the impression
produced. A sheet of water 100 yards in diameter, and only one inch
deep, would not be called a _deep_ but a very _shallow_ pool or thin
layer of water. But such a layer would be a correct representation of
the ocean in miniature. Were we in like manner to represent to the eye
in miniature the antarctic ice-cap, we would call it a _thin crust of
ice_. Taking the mean thickness of the ice at four miles, the antarctic
ice-sheet would be represented by a carpet covering the floor of an
ordinary-sized dining-room. Were those who consider the above estimate
of the thickness of the antarctic ice-cap as extravagantly great called
upon to sketch on paper a section of what they should deem a cap of
moderate thickness, ninety-nine out of every hundred would draw one of
much greater thickness than twelve miles at the centre.
The diagram on following page (Fig. 7) represents a section across the
cap drawn to a natural scale; the upper surface of the sheet having
a slope of half a degree. No one on looking at the section would
pronounce it to be too thick at the centre, unless he were previously
made aware that it represented a thickness of twelve miles at that
place. It may be here mentioned that had the section been drawn upon
a much larger scale—had it, for instance, been made seven feet long,
instead of seven inches—it would have shown to the eye in a more
striking manner the thinness of the cap.
But to avoid all objections on the score of over-estimating the
thickness of the cap, I shall assume the angle of the upper surface to
be only a quarter of a degree, and the thickness of the sheet one-half
what it is represented in the section. The thickness at the pole will
then be only six miles instead of twelve, and the mean thickness of the
cap two instead of four miles.
[Illustration: Fig. 7.
Section across Antarctic Ice-cap, drawn to a natural scale.
Length represented by section = 2,800 miles. Thickness at centre
(South Pole) = 12 miles.
Slope of upper surface = half-degree.]
Is there any well-grounded reason for concluding the above to be an
over-estimate of the actual thickness of the antarctic ice? It is not
so much in consequence of any _à priori_ reason that can be urged
against the probability of such a thickness of ice, but rather because
it so far transcends our previous experience that we are reluctant to
admit such an estimate. If we never had any experience of ice thicker
than what is found in England, we should feel startled on learning for
the first time that in the valleys of Switzerland the ice lay from 200
to 300 feet in depth. Again, if we had never heard of glaciers thicker
than those of Switzerland, we could hardly credit the statement that
in Greenland they are actually from 2,000 to 3,000 feet thick. We, in
this country, have long been familiar with Greenland; but till very
lately no one ever entertained the idea that that continent was buried
under one continuous mass of ice, with scarcely a mountain top rising
above the icy mantle. And had it not been that the geological phenomena
of the glacial epoch have for so many years accustomed our minds to
such an extraordinary condition of things, Dr. Rink’s description of
the Greenland ice would probably have been regarded as the extravagant
picture of a wild imagination.
Let us now consider whether or not the facts of observation and
experience, so far as they go, bear out the conclusions to which
physical considerations lead us in reference to the magnitude of
continental ice; and more especially as regards the ice of the
antarctic regions.
_First._ In so far as the antarctic ice-sheet is concerned, observation
and experience to a great extent may be said to be a perfect blank. One
or two voyagers have seen the outer edge of the sheet at a few places,
and this is all. In fact, we judge of the present condition of the
interior of the antarctic continent in a great measure from what we
know of Greenland. But again, our experience of Greenland ice is almost
wholly confined to the outskirts.
Few have penetrated into the interior, and, with the exception of Dr.
Hayes and Professor Nordenskjöld, none, as far as I know, have passed
to any considerable distance over the inland ice. Dr. Robert Brown
in his interesting memoir on “Das Innere von Grönland,”[207] gives
an account of an excursion made in 1747 by a Danish officer of the
name of Dalager, from Fredrikshaab, near the southern extremity of
the continent, into the interior. After a journey of a day or two, he
reached an eminence from which he saw the inland ice stretching in an
unbroken mass as far as the eye could reach, but was unable to proceed
further. Dr. Brown gives an account also of an excursion made in the
beginning of March, 1830, by O. B. Kielsen, a Danish whale-fisher, from
Holsteinborg (lat. 67° N.). After a most fatiguing journey of several
days, he reached a high point from which he could see the ice of the
interior. Next morning he got up early, and towards midday reached
an extensive plain. From this the land sank inwards, and Kielsen now
saw fully in view before him the enormous ice-sheet of the interior.
He drove rapidly over all the little hills, lakes, and streams, till
he reached a pretty large lake at the edge of the ice-sheet. This was
the end of his journey, for after vainly attempting to climb up on the
ice-sheet, he was compelled to retrace his steps, and had a somewhat
difficult return. When he arrived at the fiord, he found the ice broken
up, so that he had to go round by the land way, by which he reached
the depôt on the 9th of March. The distance which he traversed in a
straight line from Holsteinborg into the interior measured eighty
English miles.
Dr. Hayes’s excursion was made, however, not upon the real inland
ice, but upon a smaller ice-field connected with it; while Professor
Nordenskjöld’s excursion was made at a place too far south to
afford an accurate idea of the actual condition of the interior of
North Greenland, even though he had penetrated much farther than he
actually did. However, the state of things as recorded by Hayes and by
Nordenskjöld affords us a glimpse into the condition of things in the
interior of the continent. They both found by observation, what follows
as a necessary result from physical considerations, that the upper
surface of the ice plain, under which hills and valleys are buried,
gradually _slopes upwards towards the interior of the continent_.
Professor Nordenskjöld states that when at the extreme point at which
he reached, thirty geographical miles from the coast, he had attained
an elevation of 2,200 feet, and that the inland ice _continued
constantly to rise_ towards the interior, so that the horizon towards
the east, north, and south, was terminated by an ice-border almost as
smooth as that of the ocean.”[208]
Dr. Hayes and his party penetrated inwards to the distance of about
seventy miles. On the first day they reached the foot of the great Mer
de Glace; the second day’s journey carried them to the upper surface
of the ice-sheet. On the third day they travelled 30 miles, and the
ascent, which had been about 6°, diminished gradually to about 2°. They
advanced on the fourth day about 25 miles; the temperature being 30°
below zero (Fah.). “Our station at the camp,” he says, “was sublime as
it was dangerous. We had attained an altitude of 5,000 feet above the
sea-level, and were 70 miles from the coast, in the midst of a vast
frozen Sahara immeasurable to the human eye. There was neither hill,
mountain, nor gorge, anywhere in view. We had completely sunk the
strip of land between the Mer de Glace and the sea, and no object met
the eye but our feeble tent, which bent to the storm. Fitful clouds
swept over the face of the full-orbed moon, which, descending towards
the horizon, glimmered through the drifting snow that scudded over the
icy plain—to the eye in undulating lines of downy softness, to the
flesh in showers of piercing darts.”[209]
Dr. Rink, referring to the inland ice, says that the elevation or
height above the sea of this icy plain at its junction with the
outskirts of the country, and where it begins to lower itself through
the valleys to the firths, is, in the ramifications of the Bay of
Omenak, found to be 2,000 feet, from which level _it gradually rises
towards the interior_.[210]
Dr. Robert Brown, who, along with Mr. Whymper in 1867, attempted a
journey to some distance over the inland ice, is of opinion that
Greenland is not traversed by any ranges of mountains or high land,
but that the entire continent, 1,200 miles in length and 400 miles in
breadth, is covered with one continuous unbroken field of ice, the
upper surface of which, he says, _rises by a gentle slope towards the
interior_.[211]
Suppose now the point reached by Hayes to be within 200 miles of
the centre of dispersion of the ice, and the mean slope from that
point to the centre, as in the case of the antarctic cap, to be only
half a degree; this would give 10,000 feet as the elevation of the
centre above the point reached. But the point reached was 5,000 feet
above sea-level, consequently the surface of the ice at the centre
of dispersion would be 15,000 feet above sea-level, which is about
one-fourth what I have concluded to be the elevation of the surface
of the antarctic ice-cap at its centre. And supposing we assume
the general surface of the ground to have in the central region an
elevation as great as 5,000 feet, which is not at all probable, still
this would give 10,000 feet for the thickness of the ice at the centre
of the Greenland continent. But if we admit this conclusion in
reference to the thickness of the Greenland ice, we must admit that
the antarctic ice is far thicker, because the thickness, other things
being equal, will depend upon the size, or, more properly, upon the
diameter of the continent; for the larger the surface the greater is
the thickness of ice required to produce the pressure requisite to make
the rate of discharge of the ice equal to the rate of increase. Now
the area of the antarctic continent must be at least a dozen of times
greater than that of Greenland.
_Second._ That the antarctic ice must be far thicker than the arctic
is further evident from the dimensions of the icebergs which have been
met with in the Southern Ocean. No icebergs over three hundred feet in
height have been found in the arctic regions, whereas in the antarctic
regions, as we shall see, icebergs of twice and even thrice that height
have been reported.
_Third._ We have no reason to believe that the thickness of the ice
at present covering the antarctic continent is less than that which
covered a continent of a similar area in temperate regions during the
glacial epoch. Take, for example, the North American continent, or,
more properly, that portion of it covered by ice during the glacial
epoch. Professor Dana has proved that during that period the thickness
of the ice on the American continent must in many places have been
considerably over a mile. He has shown that over the northern border of
New England the ice had a mean thickness of 6,500 feet, while its mean
thickness over the Canada watershed, between St. Lawrence and Hudson’s
Bay, was not less than 12,000 feet, or upwards of two miles and a
quarter (see _American Journal of Science and Art_ for March, 1873).
_Fourth._ Some may object to the foregoing estimate of the amount of
ice on the antarctic continent, on the grounds that the quantity of
snowfall in that region cannot be much. But it must be borne in mind
that, no matter however small the annual amount of snowfall may be, if
more falls than is melted, the ice must continue to accumulate year by
year till its thickness in the centre of the continent be sufficiently
great to produce motion. The opinion that the snowfall of the antarctic
regions is not great does not, however, appear to be borne out by the
observation and experience of those who have visited those regions.
Captain Wilkes, of the American Exploring Expedition, estimated it at
30 feet per annum; and Sir James Ross says, that during a whole month
they had only three days free from snow. The fact that perpetual snow
is found at the sea-level at lat. 64° S. proves that the snowfall
must be great. But there is another circumstance which must be taken
into account, viz., that the currents carrying moisture move in from
all directions towards the pole, consequently the area on which they
deposit their snow becomes less and less as the pole is reached, and
this must, to a corresponding extent, increase the quantity of snow
falling on a given area. Let us assume, for example, that the clouds
in passing from lat. 60° to lat. 80° deposit moisture sufficient to
produce, say, 30 feet of snow per annum, and that by the time they
reach lat. 80° they are in possession of only one-tenth part of their
original store of moisture. As the area between lat. 80° and the
pole is but one-eighth of that between lat. 60° and 80°, this would,
notwithstanding, give 24 feet as the annual amount of snowfall between
lat. 80° and the pole.[212]
_Fifth._ The enormous size and thickness of the icebergs which have
been met with in the Southern Ocean testify to the thickness of the
antarctic ice-cap.
We know from the size of some of the icebergs which have been met with
in the southern hemisphere that the ice at the edge of the cap where
the bergs break off must in some cases be considerably over a mile in
thickness, for icebergs of more than a mile in thickness have been
found in the southern hemisphere. The following are the dimensions of
a few of these enormous bergs taken from the Twelfth Number of the
Meteorological Papers published by the Board of Trade, and from the
excellent paper of Mr. Towson on the Icebergs of the Southern Ocean,
published also by the Board of Trade.[213] With one or two exceptions,
the heights of the bergs were accurately determined by angular
measurement:—
Sept. 10th, 1856.—The _Lightning_, when in lat. 55° 33′ S.,
long. 140° W., met with an iceberg 420 feet high.
Nov., 1839.—In lat. 41° S., long. 87° 30′ E., numerous icebergs
400 feet high were met with.
Sept., 1840.—In lat. 37° S., long. 15° E., an iceberg 1,000
feet long and 400 feet high was met with.
Feb., 1860.—Captain Clark, of the _Lightning_, when in lat. 55°
20′ S., long. 122° 45′ W., found an iceberg 500 feet high and 3
miles long.
Dec. 1st, 1859.—An iceberg, 580 feet high, and from two and a
half to three miles long, was seen by Captain Smithers, of the
_Edmond_, in lat. 50° 52′ S., long. 43° 58′ W. So strongly did
this iceberg resemble land, that Captain Smithers believed it
to be an island, and reported it as such, but there is little
or no doubt that it was in reality an iceberg. There were
pieces of drift-ice under its lee.
Nov., 1856.—Three large icebergs, 500 feet high, were found in
lat. 41° 0′ S., long. 42° 0′ E.
Jan., 1861.—Five icebergs, one 500 feet high, were met with in
lat. 55° 46′ S., long. 155° 56′ W.
Jan., 1861.—In lat. 56° 10′ S., long. 160° 0′ W., an iceberg
500 feet high and half a mile long was found.
Jan., 1867.—The barque _Scout_, from the West Coast of
America, on her way to Liverpool, passed some icebergs 600 feet
in height, and of great length.
April, 1864.—The _Royal Standard_ came in collision with an
iceberg 600 feet in height.
Dec., 1856.—Four large icebergs, one of them 700 feet high, and
another 500 feet, were met with in lat. 50° 14′ S., long. 42°
54′ E.
Dec. 25th, 1861.—The _Queen of Nations_ fell in with an iceberg
in lat. 53° 45′ S., long. 170° 0′ W., 720 feet high.
Dec., 1856.—Captain P. Wakem, ship _Ellen Radford_, found, in
lat. 52° 31′ S., long. 43° 43′ W., two large icebergs, one at
least 800 feet high.
Mr. Towson states that one of our most celebrated and talented
naval surveyors informed him that he had seen icebergs in the
southern regions 800 feet high.
March 23rd, 1855.—The _Agneta_ passed an iceberg in lat. 53°
14′ S., long. 14° 41′ E., 960 feet in height.
Aug. 16th, 1840.—The Dutch ship, _General Baron von Geen_,
passed an iceberg 1,000 feet high in lat. 37° 32′ S., long. 14°
10′ E.
May 15th, 1859.—The _Roseworth_ found in lat. 53° 40′ S., long.
123° 17′ W., an iceberg as large as “Tristan d’Acunha.”
In the regions where most of these icebergs were met with, the mean
density of the sea is about 1·0256. The density of ice is ·92. The
density of icebergs to that of the sea is therefore as 1 to 1·115;
consequently every foot of ice above water indicates 8·7 feet below
water. It therefore follows that those icebergs 400 feet high had 3,480
feet under water,—3,880 feet would consequently be the total thickness
of the ice. The icebergs which were 500 feet high would be 4,850 feet
thick, those 600 feet high would have a total thickness of 5,820 feet,
and those 700 feet high would be no less than 6,790 feet thick, which
is more than a mile and a quarter. The iceberg 960 feet high, sighted
by the _Agneta_, would be actually 9,312 feet thick, which is upwards
of a mile and three-quarters.
Although the mass of an iceberg below water compared to that above
may be taken to be about 8·7 to 1, yet it would not be always safe
to conclude that the thickness of the ice below water bears the same
proportion to its height above. If the berg, for example, be much
broader at its base than at its top, the thickness of the ice below
water would bear a less proportion to the height above water than
as 8·7 to 1. But a berg such as that recorded by Captain Clark, 500
feet high and three miles long, must have had only 1/8·7 of its total
thickness above water. The same remark applies also to the one seen by
Captain Smithers, which was 580 feet high, and so large that it was
taken for an island. This berg must have been 5,628 feet in thickness.
The enormous berg which came in collision with the _Royal Standard_
must have been 5,820 feet thick. It is not stated what length the
icebergs 730, 960, and 1,000 feet high respectively were; but supposing
that we make considerable allowance for the possibility that the
proportionate thickness of ice below water to that above may have been
less than as 8·7 to 1, still we can hardly avoid the conclusion that
the icebergs were considerably above a mile in thickness. But if there
are icebergs above a mile in thickness, then there must be land-ice
somewhere on the southern hemisphere of that thickness. In short, the
great antarctic ice-cap must in some places be over a mile in thickness
at its edge.
_Inadequate Conceptions regarding the Magnitude of Continental
Ice._—Few things have tended more to mislead geologists in the
interpretation of glacial phenomena than inadequate conceptions
regarding the magnitude of continental ice. Without the conception
of continental ice the known facts connected with glaciation would
be perfectly inexplicable. It was only when it was found that the
accumulated facts refused to be explained by any other conception,
that belief in the very existence of such a thing as continental ice
became common. But although most geologists now admit the existence of
continental ice, yet, nevertheless, adequate conceptions of its real
magnitude are by no means so common. Year by year, as the outstanding
facts connected with glaciation accumulate, we are compelled to extend
our conceptions of the magnitude of land-ice. Take the following as
an example. It was found that the transport of the Wastdale Crag
blocks, the direction of the striæ on the islands of the Baltic, on
Caithness and on the Orkney, Shetland, and Faroe, islands, the boulder
clay with broken shells in Caithness, Holderness, and other places,
were inexplicable on the theory of land-ice. But it was so only in
consequence of the inadequacy of our conceptions of the magnitude of
the ice; for a slight extension of our ideas of its thickness has
explained not only these phenomena,[214] but others of an equally
remarkable character, such as the striation of the Long Island and
the submerged rock-basins around our coasts described by Mr. James
Geikie. In like manner, if we admit the theory of the glacial epoch
propounded in former chapters, all that is really necessary to account
for the submergence of the land is a slight extension of our hitherto
preconceived estimate of the thickness of the ice on the antarctic
continent. If we simply admit a conclusion to which all physical
considerations, as we have seen, necessarily lead us, viz., that the
antarctic continent is covered with a mantle of ice at least two miles
in thickness, we have then a complete explanation of the cause of the
submergence of the land during the glacial epoch.
Although of no great importance to the question under consideration, it
may be remarked that, except during the severest part of the glacial
epoch, we have no reason to believe that the total quantity of ice
on the globe was much greater than at present, only it would then be
all on one hemisphere. Remove two miles of ice from the antarctic
continent, and place it on the northern hemisphere, and this, along
with the ice that now exists on this hemisphere, would equal, in all
probability, the quantity existing on our hemisphere during the glacial
epoch; at least, before it reached its maximum severity.
CHAPTER XXIV.
THE PHYSICAL CAUSE OF THE SUBMERGENCE AND EMERGENCE OF THE LAND
DURING THE GLACIAL EPOCH.—_Continued._
Extent of Submergence from Displacement of Earth’s Centre
of Gravity.—Circumstances which show that the Glacial
Submergence resulted from Displacement of the Earth’s
Centre of Gravity.—Agreement between Theory and observed
Facts.—Sir Charles Lyell on submerged Areas during
Tertiary Period.—Oscillations of Sea-level in Relation to
Distribution.—Extent of Submergence on the Hypothesis that
the Earth is fluid in the Interior.
_Extent of Submergence from Displacement of Earth’s Centre of
Gravity._—How much, then, would the transference of the two miles of
ice from the southern to the northern hemisphere raise the level of the
ocean on the latter hemisphere? This mass, be it observed, is equal to
only one-half that represented in our section. A considerable amount
of discussion has arisen in regard to the method of determining this
point. According to the method already detailed, which supposes the
rise at the pole to be equal to the extent of the displacement of the
earth’s centre of gravity, the rise at the North Pole would be about
380 feet, taking into account the effect produced by the displaced
water; and the rise in the latitude of Edinburgh would be 312 feet. The
fall of level on the southern hemisphere would, of course, be equal to
the rise of level on the northern. According to the method advanced
by Mr. D. D. Heath,[215] the rise of level at the North Pole would be
about 650 feet. Archdeacon Pratt’s method[216] makes the rise still
greater; while according to Rev. O. Fisher’s method[217] the rise would
be no less than 2,000 feet. There is, however, another circumstance
which must be taken into account, which will give an additional rise of
upwards of one hundred feet.
The greatest extent of the displacement of the earth’s centre of
gravity, and consequently the greatest rise of the ocean resulting
from that displacement, would of course occur at the time of maximum
glaciation, when the ice was all on one hemisphere. But owing to the
following circumstance, a still greater rise than that resulting from
the displacement of the earth’s centre of gravity alone might take
place at some considerable time, either before or after the period of
maximum glaciation.
It is not at all probable that the ice would melt on the warm
hemisphere at exactly the same rate as it would form on the cold
hemisphere. It is probable that the ice would melt more rapidly on the
warm hemisphere than it would form on the cold. Suppose that during
the glacial epoch, at a time when the cold was gradually increasing on
the northern and the warmth on the southern hemisphere, the ice should
melt more rapidly off the antarctic continent than it was being formed
on the arctic and subarctic regions; suppose also that, by the time
a quantity of ice, equal to one-half what exists at present on the
antarctic continent, had accumulated on the northern hemisphere, the
whole of the antarctic ice had been melted away, the sea would then be
fuller than at present by the amount of water resulting from the one
mile of melted ice. The height to which this would raise the general
level of the sea would be as follows:—
The antarctic ice-cap is equal in area to 1/23·46 of that covered by
the ocean. The density of ice to that of water being taken at ·92 to
1, it follows that 25 feet 6 inches of ice melted off the cap would
raise the general level of the ocean one foot, and the one mile of
ice melted off would raise the level 200 feet. This 200 feet of rise
resulting from the melted ice we must add to the rise resulting from
the displacement of the earth’s centre of gravity. The removal of the
two miles of ice from the antarctic continent would displace the
centre of gravity 190 feet, and the formation of a mass of ice equal
to the one-half of this on the arctic regions would carry the centre
of gravity 95 feet farther; giving in all a total displacement of 285
feet, thus producing a rise of sea-level at the North Pole of 285 feet,
and in the latitude of Edinburgh of 234 feet. Add to this the rise of
200 feet resulting from the melted ice, and we have then 485 feet of
submergence at the pole, and 434 feet in the latitude of Edinburgh. A
rise to a similar extent might probably take place after the period
of maximum glaciation, when the ice would be melting on the northern
hemisphere more rapidly than it would be forming on the southern.
If we assume the antarctic ice-cap to be as thick as is represented in
the diagram, the extent of the submergence would of course be double
the above, and we might have in this case a rise of sea-level in the
latitude of Edinburgh to the extent of from 800 to 1,000 feet. But be
this as it may, it is evident that the quantity of ice on the antarctic
continent is perfectly sufficient to account for the submergence of
the glacial epoch, for we have little evidence to conclude that the
_general_ submergence much exceeded 400 or 500 feet.[218] We have
evidence in England and other places of submergence to the extent of
from 1,000 to 2,000 feet, but these may be quite local, resulting
from subsidence of the land in those particular areas. Elevations and
depressions of the land have taken place in all ages, and no doubt
during the glacial epoch also.
_Circumstances which show that the Glacial Submergence resulted from
Displacement of the Earth’s Centre of Gravity._—In favour of this
view of the cause of the submergence of the glacial epoch, it is a
circumstance of some significance, that in every part of the globe
where glaciation has been found evidence of the submergence of the
land has also been found along with it. The invariable occurrence of
submergence along with glaciation points to some physical connection
between the two. It would seem to imply, either that the two were the
direct effects of a common cause, or that the one was the cause of the
other; that is, the submergence the cause of the glaciation, or the
glaciation the cause of the submergence. There is, I presume, no known
cause to which the two can be directly related as effects. Nor do I
think that there is any one who would suppose that the submergence of
the land could have been the cause of its glaciation, even although he
attributed all glacial effects to floating ice. The submergence of our
country would, of course, have allowed floating ice to pass over it had
there been any to pass over; but submergence would not have produced
the ice, neither would it have brought the ice from the arctic regions
where it already existed. But although submergence could not have been
the cause of the glacial epoch, yet we can, as we have just seen,
easily understand how the ice of the glacial epoch could have been the
cause of the submergence. If the glacial epoch was brought about by an
increase in the eccentricity of the earth’s orbit, then a submergence
of the land as the ice accumulated was a physical necessity.
There is another circumstance connected with glacial submergence which
it is difficult to reconcile with the idea that it resulted from a
subsidence of the land. It is well known that during the glacial
epoch the land was not once under water only, but several times; and,
besides, there were not merely several periods when the land stood
at a lower level in relation to the sea than at present, but there
were also several periods when it stood at a much higher level than
now. And this holds true, not merely of our own country, but of every
country on the northern hemisphere where glaciation has yet been found.
All this follows as a necessary consequence from the theory that the
oscillations of sea-level resulted from the transference of the ice
from the one hemisphere to the other; but it is wholly inconsistent
with the idea that they resulted from upheavals and subsidence of the
land during a very recent period.
But this is not all, there is more still to be accounted for. It has
been the prevailing opinion that at the time when the land was covered
with ice, it stood at a much greater elevation than at present. It
is, however, not maintained that the facts of geology establish such
a conclusion. The greater elevation of the land is simply assumed as
an hypothesis to account for the cold.[219] The facts of geology,
however, are fast establishing the opposite conclusion, viz., that
when the country was covered with ice, the land stood in relation to
the sea at a lower level than at present, and that the continental
periods or times when the land stood in relation to the sea at a higher
level than now were the warm inter-glacial periods, when the country
was free of snow and ice, and a mild and equable condition of climate
prevailed. This is the conclusion towards which we are being led by the
more recent revelations of surface geology, and also by certain facts
connected with the geographical distribution of plants and animals
during the glacial epoch.
The simple occurrence of a rise and fall of the land in relation to
the sea-level in one or in two countries during the glacial epoch,
would not necessarily imply any physical connection. The coincidence
of these movements with the glaciation of the land might have been
purely accidental; but when we find that a succession of such movements
occurred, not merely in one or in two countries, but in every glaciated
country where proper observations have been made, we are forced to the
conclusion that the connection between the two is not accidental, but
the result of some fixed cause.
If we admit that an increase in the eccentricity of the earth’s orbit
was the cause of the glacial epoch, then we must admit that all those
results followed as necessary consequences. For if the glacial epoch
lasted for upwards of one hundred thousand years or so, there would be
a succession of cold and warm periods, and consequently a succession
of elevations and depressions of sea-level. And the elevations of
the sea-level would take place during the cold periods, and the
depressions during the warm periods.
But the agreement between theory and observed facts does not terminate
here. It follows from theory that the greatest oscillations of
sea-level would take place during the severest part of the glacial
epoch, when the eccentricity of the earth’s orbit would be at its
highest value, and that the oscillations would gradually diminish
in extent as the eccentricity diminished and the climate gradually
became less severe. Now it is well known that this is actually what
took place; the great submergence, as well as the great elevation or
continental period, occurred during the earlier or more severe part of
the glacial epoch, and as the climate grew less severe these changes
became of less extent, till we find them terminating in our submerged
forests and 25-foot raised beach.
It follows, therefore, according to the theory advanced, that the mere
fact of an area having been under sea does not imply that there has
been any subsidence or elevation of the land, and that consequently the
inference which has been drawn from these submerged areas as to changes
in physical geography may be in many cases not well founded.
Sir Charles Lyell, in his “Principles,” publishes a map showing the
extent of surface in Europe which has been covered by the sea since
the earlier part of the Tertiary period. This map is intended to show
the extraordinary amount of subsidence and elevation of the land which
has taken place during that period. It is necessary for Sir Charles’s
theory of the cause of the glacial epoch that changes in the physical
geography of the globe to an enormous extent should have taken place
during a very recent period, in order to account for the great change
of climate which occurred at that epoch. But if the foregoing results
be anything like correct, it does not necessarily follow that there
must have been great changes in the physical geography of Europe,
simply because the sea covered those areas marked in the map, for this
may have been produced by oscillations of sea-level, and not by changes
in the land. In fact, the areas marked in Sir Charles’s map as having
been covered by the sea, are just those which would be covered were the
sea-level raised a few hundred feet. No doubt there were elevations and
subsidences in many of the areas marked in the map during the Tertiary
period, and to this cause a considerable amount of the submergence
might be due; but I have little doubt that by far the greater part
must be attributed to oscillations of sea-level. It is no objection
that the greater part of the shells and other organic remains found
in the marine deposits of those areas are not indicative of a cold
or glacial condition of climate, for, as we have seen, the greatest
submergence would probably have taken place either before the more
severe cold had set in or after it had to a great extent passed away.
That the submergence of those areas probably resulted from elevations
of sea-level rather than depressions of the land, is further evident
from the following considerations. If we suppose that the climate of
the glacial epoch was brought about mainly by changes in the physical
geography of the globe, we must assume that these great changes took
place, geologically speaking, at a very recent date. Then when we ask
what ground is there for assuming that any such change in the relations
of sea and land as is required actually took place, the submergence
of those areas is adduced as the proof. Did it follow as a physical
necessity that all submergence must be the result of subsidence of the
land, and not of elevations of the sea, there would be some force in
the reasons adduced. But such a conclusion by no means follows, and,
_à priori_, it is just as likely that the appearance of the ice was
the cause of the submergence as that the submergence was the cause
of the appearance of the ice. Again, a subsidence of the land to the
extent required would to a great extent have altered the configuration
of the country, and the main river systems of Europe; but there is no
evidence that any such change has taken place. All the main valleys
are well known to have existed prior to the glacial epoch, and our
rivers to have occupied the same channels then as they do now. In the
case of some of the smaller streams, it is true, a slight deviation
has resulted at some points from the filling up of their channels with
drift during the glacial epoch; but as a general rule all the principal
valleys and river systems are older than the glacial epoch. This, of
course, could not be the case if a subsidence of the land sufficiently
great to account for the submergence of the areas in question, or
changes in the physical geography of Europe necessary to produce a
glacial epoch, had actually taken place. The total absence of any
geological evidence for the existence of any change which could explain
either the submergence of the areas in question or the climate of the
glacial epoch, is strong evidence that the submergence of the glacial
epoch, as well as of the areas in question, was the result of a simple
oscillation of sea-level resulting from the displacement of the earth’s
centre of gravity by the transferrence of the ice-cap from the southern
to the northern hemisphere.
_Oscillations of Sea-level in relation to Distribution._—The
oscillations of sea-level resulting from the displacement of the
earth’s centre of gravity help to throw new light on some obscure
points connected with the subject of the geographical distribution
of plants and animals. At the time when the ice was on the southern
hemisphere during the glacial epoch, and the northern hemisphere was
enjoying a warm and equable climate, the sea-level would be several
hundred feet lower than at present, the North Sea would probably be
dry land, and Great Britain and Ireland joined to the continent, thus
opening up a pathway from the continent to our island. As has been
shown in former chapters, during the inter-glacial periods the climate
would be much warmer and more equable than now, so that animals from
the south, such as the hippopotamus, hyæna, lion, _Elephas antiquus_
and _Rhinoceros megarhinus_, would migrate into this country, where
at present they could not live in consequence of the cold. We have
therefore an explanation, as was suggested on a former occasion,[220]
of the fact that the bones of these animals are found mingled in the
same grave with those of the musk-ox, mammoth, reindeer, and other
animals which lived in this country during the cold periods of the
glacial epoch; the animals from the north would cross over into this
country upon the frozen sea during the cold periods, while those from
the south would find the English Channel dry land during the warm
periods.
The same reasoning will hold equally true in reference to the old
and new world. The depth of Behring Straits is under 30 fathoms;
consequently a lowering of the sea-level of less than 200 feet would
connect Asia with America, and thus allow plants and animals, as Mr.
Darwin believes, to pass from the one continent to the other.[221]
During this period, when Behring Straits would be dry land, Greenland
would be comparatively free from ice, and the arctic regions enjoying a
comparatively mild climate. In this case plants and animals belonging
to temperate regions could avail themselves of this passage, and thus
we can explain how plants belonging to temperate regions may have,
during the Miocene period, passed from the old to the new continent,
and _vice versâ_.
As has already been noticed, during the time of the greatest extension
of the ice, the quantity of ice on the southern hemisphere might be
considerably greater than what exists on the entire globe at present.
In that case there might, in addition to the lowering of the sea-level
resulting from the displacement of the earth’s centre of gravity, be a
considerable lowering resulting from the draining of the ocean to form
the additional ice. This decrease and increase in the total quantity
of ice which we have considered would affect the level of the ocean as
much at the equator as at the poles; consequently during the glacial
epoch there might have been at the equator elevations and depressions
of sea-level to the extent of a few hundred feet.
_Extent of Submergence on the Hypothesis that the Earth is fluid in
the Interior._—But we have been proceeding upon the supposition that
the earth is solid to its centre. If we assume, however, what is the
general opinion among geologists, that it consists of a fluid interior
surrounded by a thick and rigid crust or shell, then the extent of the
submergence resulting from the displacement of the centre of gravity
for a given thickness of ice must be much greater than I have estimated
it to be. This is evident, because, if the interior of the globe be in
a fluid state, it, in all probability, consists of materials differing
in density. The densest materials will be at the centre, and the least
dense at the outside or surface. Now the transferrence of an ice-cap
from the one pole to the other will not merely displace the ocean—the
fluid mass on the outside of the shell—but it will also displace the
heavier fluid materials in the interior of the shell. In other words,
the heavier materials will be attracted by the ice-cap more forcibly
than the lighter, consequently they will approach towards the cap to a
certain extent, sinking, as it were, into the lighter materials, and
displacing them towards the opposite pole. This displacement will of
course tend to shift the earth’s centre of gravity in the direction
of the ice-cap, because the heavier materials are shifted in this
direction, and the lighter materials in the opposite direction. This
process will perhaps be better understood from the following figures.
[Illustration: Fig. 8. Fig. 9.
O. The Ocean.
S. Solid Crust or Shell.
F, F^1, F^2, F^3. The various concentric layers of the fluid
interior. The layers increase in density towards the centre.
I. The Ice-cap.
C. Centre of gravity.
C^1. The displaced centre of gravity.]
In Fig. 8, where there is no ice-cap, the centre of gravity of the
earth coincides with the centre of the concentric layers of the fluid
interior. In Fig. 9, where there is an ice-cap placed on one pole, the
concentric layer F^1 being denser than layer F, is attracted towards
the cap more forcibly than F, and consequently sinks to a certain depth
in F. Again, F^2 being denser than F^1, it also sinks to a certain
extent in F^1. And again F^3, the mass at the centre, being denser than
F^2, it also sinks in F^2. All this being combined with the effects
of the ice-cap, and the displaced ocean outside the shell, the centre
of gravity of the entire globe will no longer be at C, but at C^1, a
considerable distance nearer to the side of the shell on which the
cap rests than C, and also a considerable distance nearer than it
would have been had the interior of the globe been solid. There are
here three causes tending to shift the centre of gravity, (1) the
ice-cap, (2) the displaced ocean, and (3) the displaced materials in
the interior. Two of the three causes mutually react on each other in
such a way as to increase each other’s effect. Thus the more the ocean
is drawn in the direction of the ice-cap, the more effect it has in
drawing the heavier materials in the interior in the same direction;
and in turn the more the heavier materials in the interior are drawn
towards the cap, the greater is the displacement of the earth’s centre
of gravity, and of course, as a consequence, the greater is the
displacement of the ocean. It may be observed also that, other things
being equal, the thinner the solid crust or shell is, and the greater
the difference in the density of the fluid materials in the interior,
the greater will be the extent of the displacement of the ocean,
because the greater will be the displacement of the centre of gravity.
It follows that if we knew (1) the extent of the general submergence of
the glacial epoch, and (2) the present amount of ice on the southern
hemisphere, we could determine whether or not the earth is fluid in the
interior.
CHAPTER XXV.
THE INFLUENCE OF THE OBLIQUITY OF THE ECLIPTIC ON CLIMATE AND ON
THE LEVEL OF THE SEA.
The direct Effect of Change of Obliquity on Climate.—Mr.
Stockwell on the maximum Change of Obliquity.—How Obliquity
affects the Distribution of Heat over the Globe.—Increase of
Obliquity diminishes the Heat at the Equator and increases
that at the Poles.—Influence of Change of Obliquity on the
Level of the Sea.—When the Obliquity was last at its superior
Limit.—Probable Date of the 25-foot raised Beach.—Probable
Extent of Rise of Sea-level resulting from Increase of
Obliquity.—Lieutenant-Colonel Drayson’s and Mr. Belt’s
Theories.—Sir Charles Lyell on Influence of Obliquity.
_The direct Effect of Change in the Obliquity of the Ecliptic on
Climate._—There is still another cause which, I feel convinced, must to
a very considerable extent have affected climate during past geological
ages. I refer to the change in the obliquity of the ecliptic. This
cause has long engaged the attention of geologists and physicists,
and the conclusion generally come to is that no great effect can be
attributed to it. After giving special attention to the matter, I have
been led to the very opposite conclusion. It is quite true, as has
been urged, that the changes in the obliquity of the ecliptic cannot
sensibly affect the climate of temperate regions; but it will produce
a slight change on the climate of tropical latitudes, and a very
considerable effect on that of the polar regions, especially at the
poles themselves. We shall now consider the matter briefly.
It was found by Laplace that the obliquity of the ecliptic will
oscillate to the extent of 1° 22′ 34″ on each side of 23° 28′, the
obliquity in the year 1801.[222] This point has lately been examined
by Mr. Stockwell, and the results at which he has arrived are almost
identical with those of Laplace. “The mean value of the obliquity,” he
says, “of both the apparent and fixed ecliptics to the equator is 23°
17′ 17″. The limits of the obliquity of the apparent ecliptic to the
equator are 24° 35′ 58″ and 21° 58′ 36″; whence it follows that the
greatest and least declinations of the sun at the solstices can never
differ from each other to any greater extent than 2° 37′ 22″.”[223]
This change will but slightly affect the climate of the temperate
regions, but it will exercise a very considerable influence on
the climate of the polar regions. According to Mr. Meech,[224] if
365·24 thermal days represent the present total annual quantity of
heat received at the equator from the sun, 151·59 thermal days will
represent the quantity received at the poles. Adopting his method of
calculation, it turns out that when the obliquity of the ecliptic is at
the maximum assigned by Laplace the quantity received at the equator
would be 363·51 thermal days, and at the poles 160·04 thermal days. The
equator would therefore receive 1·73 thermal days less heat, and the
poles 8·45 thermal days more heat than at present.
ANNUAL AMOUNT OF SUN’S HEAT.
+-------------------+-------------+-----------+
| Amount in 1801. | Amount at | |
| Obliquity 23° 28′.| maximum, |Difference.|
| | 24° 50′ 34“.| |
+---------+---------+-------------+-----------+
|Latitude.| Thermal | Thermal | Thermal |
| | days. | days. | days. |
| 0 | 365·24 | 363·51 | −1·73 |
| 40 | 288·55 | 288·32 | −0·23 |
| 70 | 173·04 | 179·14 | +6·10 |
| 80 | 156·63 | 164·63 | +8·00 |
| 90 | 151·59 | 160·04 | +8·45 |
+---------+---------+-------------+-----------+
When the obliquity was at a maximum, the poles would therefore be
receiving 19 rays for every 18 they are receiving at present. The
poles would then be receiving nearly as much heat as latitude 76° is
receiving at present.
The increase of obliquity would not sensibly affect the polar winter.
It is true that it would slightly increase the breadth of the
frigid zone, but the length of the winter at the poles would remain
unaffected. After the sun disappears below the horizon his rays are
completely cut off, so that a further descent of 1° 22′ 34″ would make
no material difference in the climate. In the temperate regions, the
sun’s altitude at the winter solstice would be 1° 22′ 34″ less than
at present. This would slightly increase the cold of winter in those
regions. But the increase in the amount of heat received by the polar
regions would materially affect the condition of the polar summer.
What, then, is the rise of temperature at the poles which would result
from the increase of 8·45 thermal days in the total amount received
from the sun?
An increase of 8·45 thermal days, or 1/18th of the total quantity
received from the sun, according to the mode of calculation adopted in
Chap. II. would produce, all other things being equal, a rise in the
mean annual temperature equal to 14° or 15°.
According to Professor Dove[225] there is a difference of 7°·6
between the mean annual temperature of latitude 76° and the pole;
the temperature of the former being 9°·8, and that of the latter
2°·2. Since it follows that when the obliquity of the ecliptic is
at a maximum the poles would receive about as much heat per annum
as latitude 76° receives at present, it may be supposed that the
temperature of the poles at that period ought to be no higher than
that of latitude 76° at the present time. A little consideration will,
however, show that this by no means follows. Professor Dove’s Tables
represent correctly the mean annual temperature corresponding to every
tenth degree of latitude from the equator to the pole. But it must be
observed that the rate at which the temperature diminishes from the
equator to the pole is not proportionate to the decrease in the total
quantity of heat received from the sun as we pass from the equator to
the pole. Were the mean annual temperature of the various latitudes
proportionate to the amount of direct heat received, the equator
would be much warmer than it actually is at present, and the poles
much colder. The reason of this, as has been shown in Chapter II., is
perfectly obvious. There is a constant transferrence of _heat_ from
the equator to the poles, and of _cold_ from the poles to the equator.
The warm water of the equator is constantly flowing towards the poles,
and the cold water at the poles is constantly flowing to the equator.
The same is the case in regard to the aërial currents. Consequently
a great portion of the direct heat of the sun goes, not to raise the
temperature of the equator, but to heat the poles. And, on the other
hand, the cold materials at the poles are transferred to the equator,
and thus lower the temperature of that part of the globe to a great
extent. The present difference of temperature between lat. 76° and the
pole, determined according to the rate at which the temperature is
found to diminish between the equator and the pole, amounts to only
about 7° or 8°. But were there no mutual transferrence of warm and
cold materials between the equatorial and polar regions, and were the
temperature of each latitude to depend solely upon the direct rays of
the sun, the difference would far exceed that amount.
Now, when the obliquity of the ecliptic was at its superior limit, and
the poles receiving about 1/18th more direct heat from the sun than
at present, the increase of temperature due to this increase of heat
would be far more than 7° or 8. It would probably be nearly double that
amount.
“We may, therefore, conclude that when the obliquity of the ecliptic
was at a maximum, and the poles were receiving 1/18th more heat than
at present, the temperature of the poles ought to have been about 14°
or 15° warmer than at the present day, _provided, of course, that
this extra heat was employed wholly in raising the temperature_. Were
the polar regions free from snow and ice, the greater portion of the
extra heat would go to raise the temperature. But as those regions
are covered with snow and ice, the extra heat would have no effect in
raising the temperature, but would simply melt the snow and ice. The
ice-covered surface upon which the rays fell could never rise above
32°. At the period under consideration, the total annual quantity of
ice melted at the poles would be 1/18th more than at present.
The general effect which the change in the obliquity of the ecliptic
would have upon the climate of the polar regions when combined with the
effects resulting from the eccentricity of the earth’s orbit, would be
this:—When the eccentricity was at a very high value, the hemisphere
whose winter occurred in the aphelion (for physical reasons, which have
already been discussed)[226] would be under a condition of glaciation,
while the other hemisphere, having its winter in perihelion, would be
enjoying a warm and equable climate. When the obliquity of the ecliptic
was at a maximum, and 1/18th more heat falling at the poles than at
present, the effect would be to modify to a great extent the rigour
of the glaciation in the polar zone of the hemisphere under a glacial
condition, and, on the other hand, to produce a more rapid melting
of the ice on the other hemisphere enjoying the equable climate. The
effects of eccentricity and obliquity thus combined would probably
completely remove the polar ice-cap from off the latter hemisphere,
and forest trees might then grow at the pole. Again, when the obliquity
was at its minimum condition and less heat reaching the poles than at
present, the glaciation of the former hemisphere would be increased and
the warmth of the latter diminished.
_The Influence of Change in the Obliquity of the Ecliptic on the
Level of the Sea._—One very remarkable effect which seems to result
indirectly from a variation of the obliquity under certain conditions,
is an influence on the level of the sea. As this probably may have had
something to do with those recent changes of sea-level with which the
history of the submarine forests and raised beaches have made us all so
familiar, it may be of interest to enter at some length into this part
of this subject.
It appears almost certain that at the time when the northern winter
solstice was in the aphelion last, a rise of the sea on the northern
hemisphere to a considerable number of feet must have taken place from
the combined effect of eccentricity and obliquity. About 11,700 years
ago, the northern winter solstice was in the aphelion. The eccentricity
at that time was ·0187, being somewhat greater than it is now; but the
winters occurring in aphelion instead of, as now, in perihelion, they
would on that account be probably 10° or 15° colder than they are at
the present day. It is probable, also, for reasons stated in a previous
chapter, that the Gulf-stream at that time would be considerably less
than now. This would tend to lower the temperature to a still greater
extent. As snow instead of rain must have fallen during winter to a
greater extent than at present, this no doubt must have produced a
slight increase in the quantity of ice on the northern hemisphere had
no other cause come into operation. But the condition of things, we
have every reason to believe, must have been affected by the greater
obliquity of the ecliptic at that period. We have no formula, except,
perhaps, that given by Mr. Stockwell, from which to determine with
perfect accuracy the extent of the obliquity at a period so remote as
the one under consideration. If we adopt the formula given by Struve
and Peters, which agrees pretty nearly with that obtained from Mr.
Stockwell’s formula, we have the obliquity at a maximum about the time
that the solstice-point was in the aphelion. The formula given by
Leverrier places the maximum somewhat later. At all events, we cannot
be far from the truth in assuming that at the time the northern winter
solstice was in the aphelion, the obliquity of the ecliptic would be
about a maximum, and that since then it has been gradually diminishing.
It is evident, then, that the annual amount of heat received by the
arctic regions, and especially about the pole, would be considerably
greater than at present. And as the heat received on those regions is
chiefly employed in melting the ice, it is probable that the extra
amount of ice which would then be melted in the arctic regions would
prevent that slight increase of ice which would otherwise have resulted
in consequence of the winter occurring in the aphelion. The winters at
that period would be colder than they are at present, but the total
quantity of ice on the northern hemisphere would not probably be
greater.
Let us now turn to the southern hemisphere. As the southern winter
would then occur in the perihelion, this would tend to produce a slight
decrease in the quantity of ice on the southern hemisphere. But on this
hemisphere the effects of eccentricity would not, as on the northern
hemisphere, be compensated by those of obliquity; for both causes would
here tend to produce the same effect; namely, a melting of the ice in
the antarctic regions.
It is probable that at this time the quantity of warm water flowing
from the equatorial regions into the Southern Ocean would be much
greater than at present. This would tend to raise the temperature of
the air of the antarctic regions, and thus assist in melting the ice.
These causes, combined with the great increase of heat resulting from
the change of obliquity, would tend to diminish to a considerable
extent the quantity of ice on the southern hemisphere. I think we may
assume that the slight increase of eccentricity at that period, the
occurrence of the southern winter in perihelion, and the extra quantity
of warm water flowing from the equatorial to the antarctic regions,
would produce an effect on the south polar ice-cap equal to that
produced by the increase in the obliquity of the ecliptic. It would,
therefore, follow that for every eighteen pounds of ice melted annually
at present at the south pole twenty pounds would then be melted.
Let us now consider the effect that this condition of things would
have upon the level of the sea. It would evidently tend to produce an
elevation of the sea-level on the northern hemisphere in two ways. 1st.
The addition to the sea occasioned by the melting of the ice from off
the antarctic land would tend to raise the general level of the sea.
2ndly. The removal of the ice would also tend to shift the earth’s
centre of gravity to the north of its present position—and as the sea
must shift along with the centre, a rise of the sea on the northern
hemisphere would necessarily take place.
The question naturally suggests itself, might not the last rise of the
sea, relative to the land, have resulted from this cause? We know that
during the period of the 25-foot beach, the time when the estuarine
mud, which now forms the rich soil of the Carses of the Forth and
Tay, was deposited, the sea, in relation to the land, stood at least
20 or 30 feet higher than at present. But immediately prior to this
period, we have the age of the submarine forests and peat-beds, when
the sea relative to the land stood lower than it does now. We know
also that these changes of level were not mere local affairs. There
seems every reason to believe that our Carse clay, as Mr. Fisher
states, is the equivalent of the marine mud, with _Scrobicularia_,
which covers the submarine forests of England.[227] And on the other
hand, those submarine forests are not confined to one locality. “They
may be traced,” says Mr. Jamieson, “round the whole of Britain and
Ireland, from Orkney to Cornwall, from Mayo to the shores of Fife, and
even, it would seem, along a great part of the western sea-board of
Europe, as if they bore witness to a period of widespread elevation,
when Ireland and Britain, with all its numerous islands, formed one
mass of dry land, united to the continent, and stretching out into the
Atlantic.”[228] “These submarine forests”“ remarks De la Beche, also,
“are to be found under the same general condition from the shores of
Scandinavia to those of Spain and Portugal, and around the British
islands.”[229] Those buried forests are not confined to Europe, but
are found in the valley of the Mississippi and in Nova Scotia, and
other parts of North America. And again, the strata which underlie
those forests and peat-beds bear witness to the fact of a previous
elevation of the sea-level. In short, we have evidence of a number of
oscillations of sea-level during post-tertiary times.[230]
Had there been only one rise of the land relative to the sea-level, or
one depression, it might quite reasonably, as already remarked, have
been attributed to an upheaval or a sinking of the ground, occasioned
by some volcanic, chemical, or other agency. But certainly those
repeated oscillations of sea-level, extending as they do over so wide
an area, look more like a rising and sinking of the sea than of the
land. But, be this as it may, since it is now established, I presume,
beyond controversy, that the old notion that the general level of the
sea remains permanent, and that the changes must be all attributed to
the land is wholly incorrect, and that the sea, as well as the land,
is subject to changes of level, it is certainly quite legitimate to
consider whether the last elevation of the sea-level relatively to the
land may not have resulted from the rising of the sea rather than from
the sinking of the land, in short, whether it may not be attributed
to the cause we are now considering. The fact that those raised
beaches and terraces are found at so many different heights, and also
so discontinuously along our coasts, might be urged as an objection
to the opinion that they were due to changes in the level of the sea
itself. Space will not permit me to enter upon the discussion of this
point at present; but it may be stated that this objection is more
apparent than real. It by no means follows that beaches of the same
age must be at the same level. This has been shown very clearly by Mr.
W. Pengelly in a paper on “Raised Beaches,” read before the British
Association at Nottingham, 1866.
We have, as I think, evidence amounting to almost absolute certainty
that 11,700 years ago the general sea-level on the northern hemisphere
must have been higher than at present. And in order to determine the
question of the 25-foot beach, we have merely to consider whether a
rise to something like this extent probably took place at the period in
question. We have at present no means of determining the exact extent
of the rise which must have taken place at that period, for we cannot
tell what quantity of ice was then melted off the antarctic regions.
But we have the means of making a very rough estimate, which, at least,
may enable us to determine whether a rise of some 20 or 30 feet may not
possibly have taken place.
If we assume that the southern ice-cap extends on an average down
to lat. 70°, we shall have an area equal to 1/33·163 of the entire
surface of the globe. The proportion of land to that of water, taking
into account the antarctic continent, is as 526 to 1272. The southern
ice-cap will therefore be equal to 1/23·46 of the area covered by
water. The density of ice to that of water being taken at ·92 to 1,
it follows that 25 feet 6 inches of ice melted from off the face of
the antarctic continent would raise the level of the ocean one foot.
If 470 feet were melted off—and this is by no means an extravagant
supposition, when we reflect that for every 18 pounds of ice presently
melted an additional pound or two pounds, or perhaps more, would then
be melted, and that for many ages in succession—the water thus produced
from the melted ice would raise the level of the sea 18 feet 5 inches.
The removal of the 470 feet of solid ice— which must be but a very
small fraction of the total quantity of ice lying upon the antarctic
continent—would shift the earth’s centre of gravity about 7 feet to the
north of its present position. The shifting of the centre of gravity
would cause the sea to sink on the southern hemisphere and rise on the
northern. And the quantity of water thus transferred from the southern
hemisphere to the northern would carry the centre of gravity about one
foot further, and thus give a total displacement of the centre to the
extent of about 8 feet. The sea would therefore rise about 8 feet at
the North Pole, and in the latitude of Edinburgh about 6 feet 7 inches.
This, added to the rise of 18 feet 5 inches, occasioned by the melting
of the ice, would give 25 feet as the total rise in the latitude of
Scotland 11,700 years ago.
Each square foot of surface at the poles 11,700 years ago would be
receiving 18,223,100 foot-pounds more of heat annually than at present.
If we deduct 22 per cent. as the amount absorbed in passing through the
atmosphere, we have 14,214,000 foot-pounds. This would be sufficient
to melt 2·26 feet of ice. But if 50, instead of 22, per cent. were cut
off, 1·45 cubic feet would be melted. In this case the 470 feet of ice
would be melted, independently of the effects of eccentricity, in about
320 years. And supposing that only one-fourth part of the extra heat
reached the ground, 470 feet of ice would be removed in about 640 years.
As to the exact time that the obliquity was at a maximum, previous
to that of 11,700 years ago, our uncertainty is still greater. If we
are permitted to assume that the ecliptic passes from its maximum to
its minimum state and back to its maximum again with anything like
uniformity, at the rate assigned by Leverrier and others, the obliquity
would not be far from a maximum about 60,000 years ago. Taking the
rate of precession at 50″·21129, and assuming it to be uniform—which
it probably is not—the winter solstice would be in the aphelion about
61,300 years ago.[231] In short, it seems not at all improbable that
at the time the solstice-point was in the aphelion, the obliquity of
the ecliptic would not be far from its maximum state. But at that time
the value of the eccentricity was 0·023, instead of 0·0187, its value
at the last period. Consequently the rise of the sea would probably
be somewhat greater than it was 11,700 years ago. Might not this be
the period of the 40-foot beach? In this case 11,000 or 12,000 years
would be the age of the 25-foot beach, and 60,000 years the age of the
40-foot beach.
About 22,000 years ago, the winter solstice was in the perihelion, and
as the eccentricity was then somewhat greater than it is at present,
the winters would be a little warmer and the climate more equable than
it is at the present day. This perhaps might be the period of the
submarine forests and lower peat-beds which underlie the Carse clays,
_Scrobicularia_ mud, and other deposits belonging to the age of the
25-foot beach. At any rate, it is perfectly certain that a condition
of climate at this period prevailed exceedingly favourable to the
growth of peat. It follows also that at this time, owing to a greater
accumulation of ice on the southern hemisphere, the sea-level would be
a few feet lower than at present, and that forests and peat may have
then grown on places which are now under the sea-level.
For a few thousand years before and after 11,700 years ago, when the
winter solstice was evidently not far from the aphelion, and the sea
standing considerably above its present level, would probably, as we
have already stated, be the time when the Carse clays and other recent
deposits lying above the present level of the river were formed.
And it is also a singular fact that the condition of things at that
period must have been exceedingly favourable to the formation of
such estuarine deposits; for at that time the winter temperature of
our island, as has been already shown, would be considerably lower
than at present, and, consequently, during that season, snow, to a
much larger extent than now, would fall instead of rain. The melting
of the winter’s accumulation of snow on the approach of summer would
necessarily produce great floods, similar to what occur in the northern
parts of Asia and America at the present day from this very same
cause. The loose upper soil would be carried down by those floods and
deposited in the estuaries of our rivers.
The foregoing is a rough and imperfect sketch of the history of the
climate and the physical conditions of our globe for the past 60,000
years, in so far as physical and cosmical considerations seem to afford
us information on the subject, and its striking agreement with that
derived from geological sources is an additional evidence in favour
of the opinion that geological and cosmical phenomena are physically
related by a bond of causation.
_Lieutenant-Colonel Drayson’s Theory of the Cause of the Glacial
Epoch._—In a paper read before the Geological Society by
Lieutenant-Colonel Drayson, R.A., on the 22nd February, 1871,[232] that
author states, that after calculating from the recorded positions of
the pole of the heavens during the last 2,000 years, he finds the pole
of the ecliptic is not the centre of the circle traced by the pole of
the heavens. The pole of the heavens, he considers, describes a circle
round a point 6° distant from the pole of the ecliptic and 29° 25′ 47″
from the pole of the heavens, and that about 13,700 years b.c. the
angular distance of the two poles was 35° 25′ 47″. This would bring
the Arctic Circle down to latitude 54° 34′ 13″ N. I fear that this is
a conclusion that will not be generally accepted by those familiar with
celestial mechanics. But, be this as it may, my present object is not
to discuss the astronomical part of Colonel Drayson’s theory, but to
consider whether the conclusions which he deduces from his theory in
regard to the cause of the glacial epoch be legitimate or not. Assuming
for argument’s sake that the obliquity of the ecliptic can possibly
reach to 35° or 36°, so as to bring the Arctic Circle down to the
centre of England, would this account for the glacial epoch? Colonel
Drayson concludes that the shifting of the Arctic Circle down to the
latitude of England would induce here a condition of climate similar
to that which obtains in arctic regions. This seems to be the radical
error of the theory. It is perfectly true that were the Arctic Circle
brought down to latitude 54° 35′ part of our island would be in the
arctic regions, but it does not on that account follow that our island
would be subjected to an arctic climate.
The polar regions owe their cold not to the obliquity of the ecliptic,
but to their distance from the equator. Indeed were it not for
obliquity those regions would be much colder than they really are,
and an increase of obliquity, instead of increasing their cold, would
really make them warmer. The general effect of obliquity, as we
have seen, is to diminish the amount of heat received in equatorial
and tropical regions, and to increase it in the polar and temperate
regions. The greater the obliquity, and, consequently, the farther
the sun recedes from the equator, the smaller is the quantity of heat
received by equatorial regions, and the greater the amount bestowed on
polar and temperate regions. If, for example, we represent the present
amount of heat received from the sun at the equator on a given surface
at 100 parts, 42·47 parts will then represent the amount received at
the poles on the same given surface. But were the obliquity increased
to 35° the amount received at the equator would be reduced to 94·93
parts, and that at the poles increased to 59·81; being an increase at
the poles of nearly one half. At latitude 60° the present quantity
is equal to 57 parts; but about 63 parts would be received were the
obliquity increased to 35°. It therefore follows that although the
Arctic Circle were brought down to the latitude of London so that the
British islands would become a part of the arctic regions, the mean
temperature of these islands would not be lowered, but the reverse.
The winters would no doubt be colder than they are at present, but the
cold of winter would be far more than compensated for by the heat of
summer. It is not a fair representation of the state of things, merely
to say that an increase of obliquity tends to make the winters colder
and the summers hotter, for it affects the summer heat far more than
it does the winter cold. And the greater the obliquity the more does
the increase of heat during summer exceed the decrease during winter.
This is obvious because the greater the obliquity the greater the total
annual amount of heat received.
If an increase of obliquity tended to produce an increase of ice in
temperate and polar regions, and thus to lead to a glacial epoch, then
the greater the obliquity the greater would be the tendency to produce
such an effect. Conceive, then, the obliquity to go on increasing until
it ultimately reached its absolute limit, 90°, and the earth’s axis to
coincide with the plane of the ecliptic. The Arctic Circle would then
extend to the equator. Would this produce a glacial epoch? Certainly
not. A square foot of surface at the poles would then be receiving
as much heat per annum as a square foot at the equator at present,
supposing the sun remained on the equator during the entire year. Less
heat, however, would be reaching the equatorial regions than now. At
present, as we have just seen, the annual quantity of heat received at
either pole is to that received at the equator as 42·47 to 100; but at
the period under consideration the poles would be actually obtaining
one-half more heat than the equator. The amount received per square
foot at the poles, to that received per square foot at the equator,
would be in the ratio of half the circumference of a circle to its
diameter, or as 1·5708 to 1. But merely to say that the poles would be
receiving more heat per annum than the equator is at present, does not
convey a correct idea of the excessive heat which the poles would then
have to endure; for it must be borne in mind that the heat reaching
the equator is spread over the whole year, whereas the poles would get
their total amount during the six months of their summer. Consequently,
for six months in the year the poles would be obtaining far more than
double the quantity of heat received at present by the equator during
the same length of time, and more than three times the quantity then
received by the equator. The amount reaching the pole during the six
months to that reaching the equator would be as 3·1416 to 1.
At the equator twelve hours’ darkness alternates with twelve hours’
sunshine, and this prevents the temperature from rising excessively
high; but at the poles it would be continuous sunshine for six months
without the ground having an opportunity of cooling for a single
hour. At the summer solstice, when the sun would be in the zenith of
the pole, the amount of heat received there every twenty-four hours
would actually be nearly three-and-a-quarter times greater than that
presently received at the equator. Now what holds true with regard to
the poles would hold equally true, though to a lesser extent, of polar
and temperate regions. We can form but a very inadequate idea of the
condition of things which would result from such an enormous increase
of heat. Nothing living on the face of the globe could exist in polar
regions under so fearful a temperature as would then prevail during
summer months. How absurd would it be to suppose that this condition
of things would tend to produce a glacial epoch! Not only would every
particle of ice in polar regions be dissipated, but the very seas
around the pole would be, for several months in the year, at the
boiling point.
If it could be shown from _physical principles_—which, to say the
least, is highly improbable—that the obliquity of the ecliptic could
ever have been as great as 35°, it would to a very considerable
extent account for the comparative absence of ice in Greenland and
other regions in high latitudes, such as we know was the case during
the Carboniferous, Miocene, and other periods. But although a great
increase of obliquity might cause a melting of the ice, yet it could
not produce that mild condition of climate which we know prevailed in
high latitudes during those periods; while no increase of obliquity,
however great, could in any way tend to produce a glacial epoch.
Colonel Drayson, however, seems to admit that this great increase of
obliquity would make our summers much warmer than they are at present.
How, then, according to his theory, is the glacial epoch accounted for?
The following is the author’s explanation as stated in his own words:—
“At the date 13,700 B.C. the same conditions appear to have prevailed
down to about 54° of latitude during winter as regards the sun being
only a few degrees above the horizon. We are, then, warranted in
concluding that the same climate prevailed down to 54° of latitude as
now exists in winter down to 67° of latitude.
“Thus in the greater part of England and Wales, and in the whole of
Scotland, icebergs of large size would be _formed each winter_; every
river and stream would be frozen and blocked with ice, the whole
country would be covered with a mantle of snow and ice, and those
creatures which could neither migrate nor endure the cold of an arctic
climate would be exterminated.”—“The Last Glacial Epoch,” p. 146.
“At the summer solstice the midday altitude of the sun for the latitude
54° would be about 71½°, an altitude equal to that which the sun
now attains in the south of Italy, the south of Spain, and in all
localities having a latitude of about 40°.”
“There would, however, be this singular difference from present
conditions, that in latitude 54° the sun at the period of the summer
solstice would remain the whole twenty-four hours above the horizon;
a fact which would give extreme heat to those very regions which, six
months previously, had been subjected to an arctic cold. Not only
would this greatly increased heat prevail in the latitude of 54°, but
the sun’s altitude would be 12° greater at midday in midsummer, and
also 12° greater at midnight in high northern latitudes, than it
ever attains now; consequently the heat would be far greater than at
present, and high northern regions, even around the pole itself, would
be subjected to a heat during summer far greater than any which now
ever exists in those localities. The natural consequence would be, that
the icebergs and ice which had during the severe winter accumulated in
high latitudes would be rapidly thawed by this heat” (p. 148).
“Each winter the whole northern and southern hemispheres would be one
mass of ice; each summer nearly the whole of the ice of each hemisphere
would be melted and dispersed” (p. 150).
According to this theory, not only is the whole country covered each
winter with a continuous mass of ice, but large icebergs are formed
during that short season, and when the summer heat sets in all is
melted away. Here we have a misapprehension not only as to the actual
condition of things during the glacial epoch, but even as to the way
in which icebergs and land-ice are formed. Icebergs are formed from
land-ice, but land-ice is not formed during a single winter, much
less a mass of sufficient thickness to produce icebergs. Land-ice of
this thickness requires the accumulated snows of centuries for its
production. All that we could really have, according to this theory,
would be a thick covering of snow during winter, which would entirely
disappear during summer, so that there could be no land-ice.
_Mr. Thomas Belt’s Theory._—The theory that the glacial epoch resulted
from a great increase in the obliquity of the ecliptic has recently
been advocated by Mr. Thomas Belt.[233] His conceptions on the subject,
however, appear to me to be even more irreconcilable with physics than
those we have been considering. Lieutenant-Colonel Drayson admits that
the increase of heat to polar regions resulting from the great increase
of obliquity would dissipate the ice there, but Mr. Belt does not even
admit that an increase of obliquity would bring with it an increase of
heat, far less that it would melt the polar ice. On the contrary, he
maintains that the tendency of obliquity is to increase the rigour of
polar climate, and that this is the reason “that now around the poles
some lands are being glaciated, for excepting for that obliquity snow
and ice would not accumulate, excepting on mountain chains.” “Thus,”
he says, “there exist glacial conditions at present around the poles,
due primarily to the obliquity of the ecliptic.” And he also maintains
that if there were no obliquity and the earth’s axis were perpendicular
to the plane of its orbit, an eternal “spring would reign around the
arctic circle,” and that “under such circumstances the piling up of
snow, or even its production at the sea-level, would be impossible,
excepting perhaps in the immediate neighbourhood of the poles, where
the rays of the sun would have but little heating power from its small
altitude.”
Mr. Belt has apparently been led to these strange conclusions by the
following singular misapprehension of the effects of obliquity on
the distribution of the sun’s heat over the globe. “The obliquity of
the ecliptic,” he remarks, “_does not affect the mean amount of heat
received at any one point from the sun_, but it causes the heat and the
cold to predominate at different seasons of the year.”
It is not necessary to dwell further on the absurdity of the
supposition that an increase of obliquity can possibly account for the
glacial epoch, but we may in a few words consider whether a decrease
of obliquity would mitigate the climate and remove the snow from
polar regions. Supposing obliquity to disappear and the earth’s axis
to become perpendicular to the plane of its orbit, it is perfectly
true that day and night would be equal all over the globe, but then
the quantity of heat received by the polar regions would be far less
than at present. It is well known that at present at the equinoxes,
when day and night are equal, snow and not rain prevails in the arctic
regions, and can we suppose it could be otherwise in the case under
consideration? How, we may well ask, could these regions, deprived of
their summer, get rid of their snow and ice?
But even supposing it could be shown that a change in the obliquity of
the ecliptic to the extent assumed by Mr. Belt and Lieutenant-Colonel
Drayson would produce a glacial epoch, still the assumption of such a
change is one which physical astronomy will not permit. Mr. Belt does
not appear to dispute the accuracy of the methods by which it is proved
that the variations of obliquity are confined within narrow limits; but
he maintains that physical astronomers, in making their calculations
have left out of account some circumstances which materially affect the
problem. These, according to Mr. Belt, are the following:—(1) Upheavals
and subsidences of the land which may have taken place in past ages.
(2) The unequal distribution of sea and land on the globe. (3) The fact
that the equatorial protuberance is not a regular one, “but approaches
in a general outline to an ellipse, of which the greater diameter is
two miles longer than the other.” (4) The heaping up of ice around the
poles during the glacial period.
We may briefly consider whether any or all of these can sensibly affect
the question at issue. In reference to the last-mentioned element, it
is no doubt true that if an immense quantity of water were removed
from the ocean and placed around the poles in the form of ice it would
affect the obliquity of the ecliptic; but this is an element of change
which is not available to Mr. Belt, because according to his theory
the piling up of the ice is an effect which results from the change of
obliquity.
In reference to the difference of two miles in the equatorial diameters
of the earth, the fact must be borne in mind that the longer diameter
passes through nearly the centre of the great depression of the Pacific
Ocean,[234] whereas the shorter diameter passes through the opposite
continents of Asia and America. Now, when we take into consideration
the fact that these continents are not only two-and-a-half times denser
than the ocean, but have a mean elevation of about 1,000 feet above
the sea-level, it becomes perfectly obvious that the earth’s mass must
be pretty evenly distributed around its axis of rotation, and that
therefore the difference in the equatorial diameters can exercise no
appreciable effect on the change of obliquity. It follows also that the
present arrangement of sea and land is the best that could be chosen to
prevent disturbance of motion.
That there ever were upheavals and depressions of the land of so
enormous a magnitude as to lead to a change of obliquity to the extent
assumed by Lieutenant-Colonel Drayson and Mr. Belt is what, I presume,
few geologists would be willing to admit. Suppose the great table-land
of Thibet, with the Himalaya Mountains, were to sink under the sea,
it would hardly produce any sensible effect on the obliquity of the
ecliptic. Nay more; supposing that all the land in the globe were sunk
under the sea-level, or the ocean beds converted into dry land, still
this would not materially affect obliquity. The reason is very obvious.
The equatorial bulge is so immense that those upheavals and depressions
would not to any great extent alter the oblate form of the earth. The
only cause which could produce any sensible effect on obliquity, as has
already been noticed, would be the removal of the water of the ocean
and the piling of it up in the form of ice around the poles; but this
is a cause which is not available to Mr. Belt.
_Sir Charles Lyell’s Theory._—I am also unable to agree with Sir
Charles Lyell’s conclusions in reference to the influence of the
obliquity of the ecliptic on climate. Sir Charles says, “It may be
remarked that if the obliquity of the ecliptic could ever be diminished
to the extent of four degrees below its present inclination, such a
deviation would be of geological interest, in so far as it would cause
the sun’s light to be disseminated over a broader zone inside of the
arctic and antarctic circles. Indeed, if the date of its occurrence in
past time could be ascertained, this greater spread of the solar rays,
implying a shortening of the polar night, might help in some slight
degree to account for a vegetation such as now characterizes lower
latitudes, having had in the Miocene and Carboniferous periods a much
wider range towards the pole.”[235]
The effects, as we have seen, would be directly the reverse of what is
here stated, viz., the more the obliquity was diminished the _less_
would the sun’s rays spread over the arctic and antarctic regions, and
conversely the more the obliquity was increased the _greater_ would
be the amount of heat spread over polar latitudes. The farther the
sun recedes from the equator, the greater becomes the amount of heat
diffused over the polar regions; and if the obliquity could possibly
attain its absolute limit (90°), it is obvious that the poles would
then be receiving more heat than the equator is now.
CHAPTER XXVI.
COAL AN INTER-GLACIAL FORMATION.
Climate of Coal Period Inter-glacial in Character.—Coal Plants
indicate an Equable, not a Tropical Climate.—Conditions
necessary for Preservation of Coal Plants.—Oscillations
of Sea-level necessarily implied.—Why our Coal-fields
contain more than One Coal-seam.—Time required to form a
Bed of Coal.—Why Coal Strata contain so little evidence of
Ice-action.—Land Flat during Coal Period.—Leading Idea of the
Theory.—Carboniferous Limestones.
_An Inter-glacial Climate the one best suited for the Growth of the
Coal Plants._—No assertion, perhaps, could appear more improbable,
or is more opposed to all hitherto received theories, than the one
that the plants which form our coal grew during a glacial epoch. But,
nevertheless, if the theory of secular changes of climate, discussed
in the foregoing chapters, be correct, we have in warm inter-glacial
periods (as was pointed out several years ago)[236] the very condition
of climate best suited for the growth of those kinds of trees and
vegetation of which our coal is composed. It is the generally received
opinion among both geologists and botanists that the flora of the Coal
period does not indicate the existence of a tropical, but a moist,
equable, and temperate climate. “It seems to have become,” says Sir
Charles Lyell, “a more and more received opinion that the coal plants
do not on the whole indicate a climate resembling that now enjoyed in
the equatorial zone. Tree-ferns range as far south as the southern
parts of New Zealand, and Araucanian pines occur in Norfolk Island.
A great preponderance of ferns and lycopodiums indicates moisture,
equability of temperature, and freedom from frost, rather than intense
heat.”[237]
Mr. Robert Brown, the eminent botanist, considers that the rapid and
great growth of many of the coal plants showed that they grew in swamps
and shallow water of equable and genial temperature.
“Generally speaking,” says Professor Page, “we find them resembling
equisetums, marsh-grasses, reeds, club-mosses, tree-ferns, and
coniferous trees; and these in existing nature attain their maximum
development in warm, temperate, and subtropical, rather than in
equatorial regions. The Wellingtonias of California and the pines of
Norfolk Island are more gigantic than the largest coniferous tree yet
discovered in the coal-measures.”[238]
The Coal period was not only characterized by a great preponderance
over the present in the quantity of ferns growing, but also in the
number of different species. Our island possesses only about 50
species, while no fewer than 140 species have been enumerated as having
inhabited those few isolated places in England over which the coal has
been worked. And Humboldt has shown that it is not in the hot, but in
the mountainous, humid, and shady parts of the equatorial regions that
the family of ferns produces the greatest number of species.
“Dr. Hooker thinks that a climate warmer than ours now is, would
probably be indicated by the presence of an increased number of
flowering plants, which would doubtless have been fossilized with
the ferns; whilst a lower temperature, _equal to the mean of the
seasons now prevailing_, would assimilate our climate to that of such
cooler countries as are characterized by a disproportionate amount of
ferns.”[239]
“The general opinion of the highest authorities,” says Professor Hull,
“appears to be that the climate did not resemble that of the equatorial
regions, but was one in which the temperature was free from extremes;
the atmosphere being warm and moist, somewhat resembling that of New
Zealand and the surrounding islands, which we endeavour to imitate
artificially in our hothouses.”[240]
The enormous quantity of the carboniferous vegetation shows also that
the climate under which it grew could not have been of a tropical
character, or it must have been decomposed by the heat. Peat, so
abundant in temperate regions, is not to be found in the tropics.
The condition most favourable to the preservation of vegetable remains,
at least under the form of peat, is a cool, moist, and equable climate,
such as prevails in the Falkland Islands at the present day. “In these
islands,” says Mr. Darwin, “almost every kind of plant, even the coarse
grass which covers the whole surface of the land, becomes converted
into this substance.”[241]
From the evidence of geology we may reasonably infer that were
the difference between our summer and winter temperature nearly
annihilated, and were we to enjoy an equable climate equal to, or
perhaps a little above, the present mean annual temperature of our
island, we should then have a climate similar to what prevailed during
the Carboniferous epoch.
But we have already seen that such must have been the character of our
climate at the time that the eccentricity of the earth’s orbit was at
a maximum, and winter occurred when the earth was in the perihelion of
its orbit. For, as we have already shown, the earth would in such a
case be 14,212,700 miles nearer to the sun in winter than in summer.
This enormous difference, along with other causes which have been
discussed, would almost extinguish the difference between summer and
winter temperature. The almost if not entire absence of ice and snow,
resulting from this condition of things, would, as has already been
shown, tend to raise the mean annual temperature of the climate higher
than it is at present.
_Conditions necessary for the Preservation of the Coal Plants._—But
in order to the formation of coal, it is not simply necessary to have
a condition of climate suitable for the growth, but also for the
preservation, of a luxuriant vegetation. The very existence of coal is
as much due to the latter circumstance as to the former; nay more, as
we shall yet see, the fact that a greater amount of coal belongs to the
Carboniferous period than to any other, was evidently due not so much
to a more extensive vegetable growth during that age, suited to form
coal, as to the fact that that flora has been better preserved. Now,
as will be presently shown, we have not merely in the warm periods of
a glacial epoch a condition of climate best suited for the growth of
coal plants, but we have also in the cold periods of such an epoch the
condition most favourable for the preservation of those plants.
One circumstance necessary for the preservation of plants is that they
should have been covered over by a thick deposit of sand, mud, or clay,
and for this end it is necessary that the area upon which the plants
grew should have become submerged. It is evident that unless the area
had become submerged, the plants could not have been covered over with
a thick deposit; and, even supposing they had been covered over, they
could not have escaped destruction from subaërial denudation unless
the whole had been under water. Another condition favourable, if not
essential, to the preservation of the plants, is that they should have
been submerged in a cold and not in a warm sea. Assuming that the
coal plants grew during a warm period of a glacial epoch, we have in
the cold period which succeeded all the above conditions necessarily
secured.
It is now generally admitted that the coal trees grew near broad
estuaries and on immense flat plains but little elevated above
sea-level. But that the _Lepidodendra_, _Sigillariæ_, and other trees,
of which our coal is almost wholly composed, grew on dry ground,
elevated above sea-level, and not in swamps and shallow water, as
was at one time supposed, has been conclusively established by the
researches of Principal Dawson and others. After the growth of many
generations of trees, the plain is eventually submerged under the sea,
and the whole, through course of time, becomes covered over with thick
deposits of sand, gravel, and other sediments carried down by streams
from the adjoining land. After this the submerged plain becomes again
elevated above the sea-level, and forms the site of a second forest,
which, after continuing to flourish for long centuries, is in turn
destroyed by submergence, and, like the former, becomes covered over
with deposits from the land. This alternate process of submergence
and emergence goes on till we have a succession of buried forests
one above another, with immense stratified deposits between. These
buried forests ultimately become converted into beds of coal. This,
I presume, is a fair representation of the generally admitted way in
which our coal-beds had their origin. It is also worthy of notice that
the stratified beds between the coal-seams are of marine and not of
lacustrine origin. On this point I may quote the opinion of Professor
Hull, a well-known authority on the subject: “Whilst admitting,” he
says, “the occasional presence of lacustrine strata associated with the
coal-measures, I think we may conclude that the whole formation has
been essentially of marine and estuarine origin.”[242]
_Coal-beds necessarily imply Oscillations of Sea-level._—It may also
be observed that each coal-seam indicates both an elevation and a
depression of the land. If, for example, there are six coal-seams,
one above the other, this proves that the land must have been, at
least, six times below and six times above sea-level. This repeated
oscillation of the land has been regarded as a somewhat puzzling and
singular circumstance. But if we assume coal to be an inter-glacial
formation, this difficulty not only disappears, but all the various
circumstances which we have been detailing are readily explained.
We have to begin with a warm inter-glacial period, with a climate
specially suited for the growth of the coal trees. During this period,
as has been shown in the chapter on Submergence, the sea would be
standing at a lower level than at present, laying bare large tracts
of sea-bottom, on which would flourish the coal vegetation. This
condition of things would continue for a period of 8,000 or 10,000
years, allowing the growth of many generations of trees. When the warm
period came to a close, and the cold and glacial condition set in, the
climate became unsuited for the growth of the coal plants. The sea
would begin to rise, and the old sea-bottoms on which, during so long
a period, the forests grew, would be submerged and become covered by
sedimentary deposits brought down from the land. These forests becoming
submerged in a cold sea, and buried under an immense mass of sediment,
were then now protected from destruction, and in a position to become
converted into coal. The cold continuing for a period of 10,000 years,
or thereby, would be succeeded by another warm period, during which the
submerged areas became again a land-surface, on which a second forest
flourished for another 10,000 years, which in turn became submerged
and buried under drift on the approach of the second cold period.
This alternate process of submergence and emergence of the land,
corresponding to the rise and fall of sea-level during the cold and
warm periods, would continue so long as the eccentricity of the earth’s
orbit remained at a high value, till we might have, perhaps, five or
six submerged forests, one above the other, and separated by great
thicknesses of stratified deposits, these submerged forests being the
coal-beds of the present day.
[Illustration: Fig. 10.]
It is probable that the forests of the Coal period would extend inland
over the country, but only such portions as were slightly elevated
above sea-level would be submerged and covered over by sediment and
thus be preserved, and ultimately become coal-seams. The process will
be better understood from the following diagram. Let A B represent the
surface of the ground prior to a glacial epoch, and to the formation
of the beds of coal and stratified deposits represented in the
diagram. Let S S′ be the normal sea-level. Suppose the eccentricity
of the earth’s orbit begins to increase, and the winter solstice
approaches the perihelion, we have then a moderately warm period. The
sea-level sinks to 1, and forests of sigillariæ and other coal trees
cover the country from the sea-shore at 1, stretching away inland in
the direction of B. In course of time the winter solstice moves round
to aphelion and a cold period follows. The sea begins to rise and
continues rising till it reaches 1′. Denudation and the severity of
the climate destroy every vestige of the forest from 1′ backwards into
the interior; but the portion 1 1′ being submerged and covered over
by sediment brought down from the land is preserved. The eccentricity
continuing to increase in extent, the second inter-glacial period is
more warm and equable than the first, and the sea this time sinks to 2.
A second forest now covers the country down to the sea-shore at 2. This
second warm period is followed by the second cold period, more severe
than the first, and the sea-level rises to 2′. Denudation and severity
of climate now destroy every remnant of the forest, from 2′ inland,
but of course the submerged portion of 2 2′, like the former portion 1
1′, is preserved. During the third warm period (the eccentricity being
still on the increase) the sea-level sinks to 3, and the country for
the third time is covered by forests, which extend down to 3. This
third warm period is followed by a cold glacial period more severe than
the preceding, and the sea-level rises to 3′, and the submerged portion
of the forest from 3 to 3′ becomes covered with drift,—the rest as
before being destroyed by denudation and the severity of the climate.
We shall assume that the eccentricity has now reached a maximum, and
that during the fourth inter-glacial period the sea-level sinks only to
4, the level to which it sank during the second inter-glacial period.
The country is now covered for the fourth time by forests. The cold
period which succeeds not being so severe as the last, the sea rises
only to 4′, which, of course, marks the limit of the fourth forest. The
eccentricity continuing to diminish, the fifth forest is only submerged
up to 5′, and the sixth and last one up to 6′. The epoch of cold and
warm periods being now at a close, the sea-level remains stationary at
its old normal position S S′. Here we have six buried forests, the one
above the other, which, through course of ages, become transformed into
coal-beds.
It does not, however, necessarily follow that each separate coal-seam
represents a warm period. It is quite possible that two or more seams
separated from each other by thin partings or a few feet of sedimentary
strata might have been formed during one warm period; for during a warm
period minor oscillations of sea-level sufficient to submerge the land
to some depth might quite readily have taken place from the melting of
polar ice, as was shown in the chapter on Submergence.
It may be noticed that in order to make the section more distinct, its
thickness has been greatly exaggerated. It will also be observed that
beds 4, 5, and 6 extend considerably to the left of what is represented
in the section.
But it is not to be supposed that the whole phenomena of the
coal-fields can be explained without supposing a subsidence of the
land. The great depth to which the coal-beds have been sunk, in many
cases, must be attributed to a subsidence of the level. A series of
beds formed during a glacial epoch, may, owing to a subsidence of the
land, be sunk to a great depth, and become covered over with thousands
of feet of sediment; and then on the occurrence of another glacial
epoch, a new series of coal-beds may be formed on the surface. Thus
the upper series may be separated from the lower by thousands of feet
of sedimentary rock. There is another consequence resulting from the
sinking of the land, which must be taken into account. Had there been
no sinking of the land during the Carboniferous age, the quantity of
coal-beds now remaining would be far less than it actually is, for it
is in a great measure owing to their being sunk to a great depth that
they have escaped destruction by the enormous amount of denudation
which has taken place since that remote age. It therefore follows that
only a very small fraction of the submerged forests of the Coal period
do actually now exist in the form of coal. Generally it would only be
those areas which happened to be sunk to a considerable depth, by a
subsidence of the land, that would escape destruction from denudation.
But no doubt the areas which would thus be preserved bear but a small
proportion to those destroyed.
_Length of Inter-glacial Period, as indicated by the Thickness of a
Bed of Coal._—A fact favourable to the idea that the coal-seams were
formed during inter-glacial periods is, that the length of those
periods agrees pretty closely with the length of time supposed to be
required to form a coal-seam of average thickness. Other things being
equal, the thickness of a coal-seam would depend upon the length
of the inter-glacial period. If the rate of precession and motion
of the perihelion were always uniform the periods would all be of
equal length. But although the rate of precession is not subject to
much variation, such is not the case in regard to the motion of the
perihelion, as will be seen from the tables of the longitude of the
perihelion given in Chapter XIX. Sometimes the motion of the perihelion
is rapid, at other times slow, while in some cases its motion is
retrograde. In consequence of this, an inter-glacial period may not be
more than some six or seven thousand years in length, while in other
cases its length may be as much as fifteen or sixteen thousand years.