considerable extent caused by, a high excentricity of the earth's orbit, we
are naturally led to expect that earlier glacial epochs would have occurred
whenever the excentricity was unusually large. Dr. Croll has published
tables showing the varying amounts of excentricity for three million years
back; and from these it appears that there have been many periods of high
excentricity, which has often been far greater than at the time of the last
glacial epoch.[10] The accompanying diagram has been drawn from these
tables, and it will be seen that the highest excentricity occurred 850,000
years ago, at which time the difference between the sun's distance at
_aphelion_ and _perihelion_ was thirteen and a half millions of miles,
whereas during the last glacial period the maximum difference was ten and a
half million miles.
[Illustration: DIAGRAM SHOWING THE CHANGES OF EXCENTRICITY DURING THE LAST
THREE MILLION YEARS.]
Now, judging by the amount of organic and physical change that occurred
during and since the glacial epoch, and that which has occurred since the
Miocene period, it is considered probable that this maximum of excentricity
coincided with some part of the latter period; and Dr. Croll maintains that
a glacial epoch must then have {60} occurred surpassing in severity that of
which we have such convincing proofs, and consisting like it of
alternations of cold and warm phases every 10,500 years. The diagram also
shows us another long-continued period of high excentricity from 1,750,000
to 1,950,000 years ago, and yet another almost equal to the maximum
2,500,000 years back. These may perhaps have occurred during the Eocene and
Cretaceous epochs respectively, or all may have been included within the
limits of the Tertiary period. As two of these high excentricities greatly
exceed that which caused our glacial epoch, while the third is almost equal
to it and of longer duration, they seem to afford us the means of testing
rival theories of the causes of glaciation. If, as Dr. Croll argues, high
excentricity is the great and dominating agency in bringing on glacial
epochs, geographical changes being subordinate, then there must have been
glacial epochs of great severity at all these three periods; while if he is
also correct in supposing that the alternate phases of precession would
inevitably produce glaciation in one hemisphere, and a proportionately mild
and equable climate in the opposite hemisphere, then we should have to look
for evidence of exceptionally warm and exceptionally cold periods,
occurring {61} alternately and with several repetitions, within a space of
time which, geologically speaking, is very short indeed.
Let us then inquire first into the character of the evidence we should
expect to find of such changes of climate, if they have occurred; we shall
then be in a better position to estimate at its proper value the evidence
that actually exists, and, after giving it due weight, to arrive at some
conclusion as to the theory that best explains and harmonises it.
_Effects of Denudation in Destroying the Evidence of Remote Glacial
Epochs._--It may be supposed, that if earlier glacial epochs than the last
did really occur, we ought to meet with some evidence of the fact
corresponding to that which has satisfied us of the extensive recent
glaciation of the northern hemisphere; but Dr. Croll and other writers have
ably argued that no such evidence is likely to be found. It is now
generally admitted that sub-aerial denudation is a much more powerful agent
in lowering and modifying the surface of a country than was formerly
supposed. It has in fact been proved to be so powerful that the difficulty
now felt is, not to account for the denudation which can be proved to have
occurred, but to explain the apparent persistence of superficial features
which ought long ago to have been destroyed.
A proof of the lowering and eating away of the land-surface which every one
can understand, is to be found in the quantity of solid matter carried down
to the sea and to low grounds by rivers. This is capable of pretty accurate
measurement, and it has been carefully measured for several rivers, large
and small, in different parts of the world. The details of these
measurements will be given in a future chapter, and it is only necessary
here to state that the average of them all gives us this result--that one
foot must, on an average, be taken off the entire surface of the land each
3,000 years in order to produce the amount of sediment and matter in
solution which is actually carried into the sea. To give an idea of the
limits of variation in different rivers it may be mentioned that the
Mississippi is one which denudes its valley at a slow rate, taking 6,000
{62} years to remove one foot; while the Po is the most rapid, taking only
729 years to do the same work in its valley. The cause of this difference
is very easy to understand. A large part of the area of the Mississippi
basin consists of the almost rainless prairie and desert regions of the
west, while its sources are in comparatively arid mountains with scanty
snow-fields, or in a low forest-clad plateau. The Po, on the other hand, is
wholly in a district of abundant rainfall, while its sources are spread
over a great amphitheatre of snowy Alps nearly 400 miles in extent, where
the denuding forces are at a maximum. As Scotland is a mountain region of
rather abundant rainfall, the denuding power of its rains and rivers is
probably rather above than under the average, but to avoid any possible
exaggeration we will take it at a foot in 4,000 years.
Now if the end of the glacial epoch be taken to coincide with the
termination of the last period of high excentricity, which occurred about
80,000 years ago (and no geologist will consider this too long for the
changes which have since taken place), it follows that the entire surface
of Scotland must have been since lowered an average amount of twenty feet.
But over large areas of alluvial plains, and wherever the rivers have
spread during floods, the ground will have been raised instead of lowered;
and on all nearly level ground and gentle slopes there will have been
comparatively little denudation; so that proportionally much more must have
been taken away from mountain sides and from the bottoms of valleys having
a considerable downward slope. One of the very highest authorities on the
subject of denudation, Mr. Archibald Geikie, estimates the area of these
more rapidly denuded portions as only one-tenth of the comparatively level
grounds, and he further estimates that the former will be denuded about ten
times as fast as the latter. It follows that the valleys will be deepened
and widened on the average about five feet in the 4,000 years instead of
one foot; and thus many valleys must have been deepened and widened 100
feet, and some even more, since the glacial epoch, while the more level
portions of the country will have been lowered on the average only about
two feet. {63}
Now Dr. Croll gives us the following account of the present aspect of the
surface of a large part of the country:--
"Go where one will in the lowlands of Scotland and he shall hardly find a
single acre whose upper surface bears the marks of being formed by the
denuding agents now in operation. He will observe everywhere mounds and
hollows which cannot be accounted for by the present agencies at work....
In regard to the general surface of the country the present agencies may be
said to be just beginning to carve a new line of features out of the old
glacially-formed surface. But so little progress has yet been made, that
the kames, gravel-mounds, knolls of boulder clay, &c., still retain in most
cases their original form."[11]
The facts here seem a little inconsistent, and we must suppose that Dr.
Croll has somewhat exaggerated the universality and complete preservation
of the glaciated surface. The amount of average denudation, however, is not
a matter of opinion but of measurement; and its consequences can in no way
be evaded. They are, moreover, strictly proportionate to the time elapsed;
and if so much of the old surface of the country has certainly been
remodelled or carried into the sea since the last glacial epoch, it becomes
evident that any surface-phenomena produced by still earlier glacial epochs
_must_ have long since entirely disappeared.
_Rise of the Sea-level Connected with Glacial Epochs, a Cause of Further
Denudation._--There is also another powerful agent that must have assisted
in the destruction of any such surface deposits or markings. During the
last glacial epoch itself there were several minor oscillations of the
land, without counting the great submergence of over 1,300 feet, supposed
to be indicated by patches of shelly clays and gravels in Wales and
Ireland, and also in a few localities in England and Scotland, since these
are otherwise explained by many geologists. Other subsidences have no doubt
occurred in the same areas during the Tertiary epoch, and some writers
connect these subsidences with the glacial {64} period itself, the unequal
amount of ice at the two poles causing the centre of gravity of the earth
to be displaced when, of course, the surface of the ocean will conform to
it and appear to rise in the one hemisphere and sink in the other. If this
is the case, subsidences of the land are natural concomitants of a glacial
period, and will powerfully aid in removing all evidence of its occurrence.
We have seen reason to believe, however, that during the height of the
glacial epoch the extreme cold persisted through the successive phases of
precession, and if so, both polar areas would probably be glaciated at
once. This would cause the abstraction of a large quantity of water from
the ocean, and a proportionate elevation of the land, which would react on
the accumulation of snow and ice, and thus add another to that wonderful
series of physical agents which act and react on each other so as to
intensify glacial epochs.
But whether or not these causes would produce any important fluctuations of
the sea-level is of comparatively little importance to our present inquiry,
because the wide extent of marine Tertiary deposits in the northern
hemisphere and their occurrence at considerable elevations above the
present sea-level, afford the most conclusive proofs that great changes of
sea and land have occurred throughout the entire Tertiary period; and these
repeated submergences and emergences of the land combined with sub-aerial
and marine denudation, would undoubtedly destroy all those superficial
evidences of ice-action on which we mainly depend for proofs of the
occurrence of the last glacial epoch.
_What Evidence of Early Glacial Epochs may be Expected._--Although we may
admit the force of the preceding argument as to the extreme improbability
of our finding any clear evidence of the superficial action of ice during
remote glacial epochs, there is nevertheless one kind of evidence that we
ought to find, because it is both wide-spread and practically
indestructible.
One of the most constant of all the phenomena of a glaciated country is the
abundance of icebergs produced by the breaking off of the ends of glaciers
which terminate {65} in arms of the sea, or of the terminal face of the
ice-sheet which passes beyond the land into the ocean. In both these cases
abundance of rocks and _debris_, such as form the terminal moraines of
glaciers on land, are carried out to sea and deposited over the sea-bottom
of the area occupied by icebergs. In the case of an ice-sheet it is almost
certain that much of the ground-moraine, consisting of mud and imbedded
stones, similar to that which forms the "till" when deposited on land, will
be carried out to sea with the ice and form a deposit of marine "till" near
the shore.
It has indeed been objected that when an ice-sheet covered an entire
country there would be no moraines, and that rocks or _debris_ are very
rarely seen on icebergs. But during every glacial epoch there will be a
southern limit to the glaciated area, and everywhere near this limit the
mountain-tops will rise far above the ice and deposit on it great masses of
_debris_; and as the ice-sheet spreads, and again as it passes away, this
moraine-forming area will successively occupy the whole country. But even
such an ice-clad country as Greenland is now known to have protruding peaks
and rocky masses which give rise to moraines on its surface;[12] and, as
rocks from Cumberland and Ireland were carried by the ice-sheet to the Isle
of Man, there must have been a very long period during which the ice-sheets
of Britain and Ireland terminated in the ocean and sent off abundance of
rock-laden bergs into the surrounding seas; and the same thing must have
occurred along all the coasts of Northern Europe and Eastern America.
We cannot therefore doubt that throughout the greater part of the duration
of a glacial epoch the seas adjacent to the glaciated countries would
receive continual deposits of large rocks, rock-fragments, and gravel,
similar to the material of modern and ancient moraines, and analogous to
the drift and the numerous travelled blocks which the ice has undoubtedly
scattered broadcast over every glaciated country; and these rocks and
boulders would be imbedded in whatever deposits were then forming, either
from the matter carried down by rivers or from the mud ground off {66} the
rocks and carried out to sea by the glaciers themselves. Moreover, as
icebergs float far beyond the limits of the countries which gave them
birth, these ice-borne materials would be largely imbedded in deposits
forming from the denudation of countries which had never been glaciated, or
from which the ice had already disappeared.
But if every period of high excentricity produced a glacial epoch of
greater or less extent and severity, then, on account of the frequent
occurrence of a high phase of excentricity during the three million years
for which we have the tables, these boulder and rock-strewn deposits would
be both numerous and extensive. Four hundred thousand years ago the
excentricity was almost exactly the same as it is now, and it continually
increased from that time up to the glacial epoch. Now if we take double the
present excentricity as being sufficient to produce some glaciation in the
temperate zone, we find (by drawing out the diagram at p. 171 on a larger
scale) that during 1,150,000 years out of the 2,400,000 years immediately
preceding the last glacial epoch, the excentricity reached or exceeded this
amount, consisting of sixteen separate epochs, divided from each other by
periods varying from 30,000 to 200,000 years. But if the last glacial epoch
was at its maximum 200,000 years ago, a space of three million years will
certainly include much, if not all, of the Tertiary period; and even if it
does not, we have no reason to suppose that the character of the
excentricity would suddenly change beyond the three million years.
It follows, therefore, that if periods of high excentricity, like that
which appears to have been synchronous with our last glacial epoch and is
generally admitted to have been one of its efficient causes, always
produced glacial epochs (with or without alternating warm periods), then
the whole of the Tertiary deposits in the north temperate and Arctic zones
should exhibit frequent alternations of boulder and rock-bearing beds, or
coarse rock-strewn gravels analogous to our existing glacial drift, and
with some corresponding change of organic remains. Let us then see what
evidence can be adduced of the existence of such deposits, and whether it
is adequate to support the {67} theory of repeated glacial epochs during
the Tertiary period.
_Evidences of Ice-action during the Tertiary Period._--The Tertiary fossils
both of Europe and North America indicate throughout warm or temperate
climates, except those of the more recent Pliocene deposits which merge
into the earlier glacial beds. The Miocene deposits of Central and Southern
Europe, for example, contain marine shells of some genera now only found
farther south, while the fossil plants often resemble those of Madeira and
the southern states of North America. Large reptiles, too, abounded, and
man-like apes lived in the south of France and in Germany. Yet in Northern
Italy, near Turin, there are beds of sandstone and conglomerate full of
characteristic Miocene shells, but containing in an intercalated deposit
angular blocks of serpentine and greenstone often of enormous size, one
being fourteen feet long, and another twenty-six feet. Some of the blocks
were observed by Sir Charles Lyell to be faintly striated and partly
polished on one side, and they are scattered through the beds for a
thickness of nearly 150 feet. It is interesting that the particular bed in
which the blocks occur yields no organic remains, though these are
plentiful both in the underlying and overlying beds, as if the cold of the
icebergs, combined with the turbidity produced by the glacial mud, had
driven away the organisms adapted to live only in a comparatively warm sea.
Rock similar in kind to these erratics occurs about twenty miles distant in
the Alps.
The Eocene period is even more characteristically tropical in its flora and
fauna, since palms and Cycadaceae, turtles, snakes, and crocodiles then
inhabited England. Yet on the north side of the Alps, extending from
Switzerland to Vienna, and also south of the Alps near Genoa, there is a
deposit of finely-stratified sandstone several thousand feet in thickness,
quite destitute of organic remains, but containing in several places in
Switzerland enormous blocks either angular or partly rounded, and composed
of oolitic limestone or of granite. Near the Lake of Thun some of the
granite blocks found in this deposit are of enormous size, one of them
being 105 feet long, ninety feet wide, {68} and forty-five feet thick! The
granite is red, and of a peculiar kind which cannot be matched anywhere in
the Alps, or indeed elsewhere. Similar erratics have also been found in
beds of the same age in the Carpathians and in the Apennines, indicating
probably an extensive inland European sea into which glaciers descended
from the surrounding mountains, depositing these erratics, and cooling the
water so as to destroy the mollusca and other organisms which had
previously inhabited it. It is to be observed that wherever these erratics
occur they are always in the vicinity of great mountain ranges; and
although these can be proved to have been in great part elevated during the
Tertiary period, we must also remember that they must have been since very
much lowered by denudation, of the amount of which, the enormously thick
Eocene and Miocene beds now forming portions of them is in some degree a
measure as well as a proof. It is not therefore at all improbable that
during some part of the Tertiary period these mountains may have been far
higher than they are now, and this we know might be sufficient for the
production of glaciers descending to the sea-level, even were the climate
of the lowlands somewhat warmer than at present.[13]
_The Weight of the Negative Evidence._--But when we proceed to examine the
Tertiary deposits of other parts of {69} Europe, and especially of our own
country, for evidence of this kind, not only is such evidence completely
wanting, but the facts are of so definite a character as to satisfy most
geologists that it can never have existed; and the same maybe said of
temperate North America and of the Arctic regions generally.
In his carefully written paper on "The Climate Controversy" the late Mr.
Searles V. Wood, Jun., remarks on this point as follows: "Now the Eocene
formation is complete in England, and is exposed in continuous section
along the north coast of the Isle of Wight from its base to its junction
with the Oligocene (or Lower Miocene according to some), and along the
northern coast of Kent from its base to the Lower Bagshot Sand. It has been
intersected by railway and other cuttings in all directions and at all
horizons, and pierced by wells innumerable; while from its strata in
England, France, and Belgium, the most extensive collections of organic
remains have been made of any formation yet explored, and from nearly all
its horizons, for at one place or another in these three countries nearly
every horizon may be said to have yielded fossils of some kind. These
fossils, however, whether they be the remains of a flora such as that of
Sheppey, or of a vertebrate fauna containing the crocodile and alligator,
such as is yielded by beds indicative of terrestrial conditions, or of a
molluscan assemblage such as is present in marine or fluvio-marine beds of
the formation, are of unmistakably tropical or sub-tropical character
throughout; and no trace whatever has appeared of the intercalation of a
glacial period, much less of successive intercalations indicative of more
than one period of 10,500 years' glaciation. Nor can it be urged that the
glacial epochs of the Eocene in England were intervals of dry land, and so
have left no evidence of their existence behind them, because a large part
of the continuous sequence of Eocene deposits in this country consists of
alternations of fluviatile, fluvio-marine, and purely marine strata; so
that it seems impossible that during the accumulation of the Eocene
formation in England a glacial period could have occurred without its
evidences being {70} abundantly apparent. The Oligocene of Northern Germany
and Belgium, and the Miocene of those countries and of France, have also
afforded a rich molluscan fauna, which, like that of the Eocene, has as yet
presented no indication of the intrusion of anything to interfere with its
uniformly sub-tropical character."[14]
This is sufficiently striking; but when we consider that this enormous
series of deposits, many thousand feet in thickness, consists wholly of
alternations of clays, sands, marls, shales, or limestones, with a few beds
of pebbles or conglomerate, not one of the whole series containing
irregular blocks of foreign material, boulders or gravel, such as we have
seen to be the essential characteristic of a glacial epoch; and when we
find that this same general character pervades all the extensive Tertiary
deposits of temperate North America, we shall, I think, be forced to the
conclusion that no general glacial epochs could have occurred during their
formation. It must be remembered that the "imperfection of the geological
record" will not help us here, because the series of Tertiary deposits is
unusually complete, and we must suppose some destructive agency to have
selected all the intercalated glacial beds and to have so completely made
away with them that not a fragment remains, while preserving all or almost
all the _interglacial_ beds; and to have acted thus capriciously, not in
one limited area only, but over the whole northern hemisphere, with the
local exceptions on the flanks of great mountain ranges already referred
to.
_Temperate Climates in the Arctic Regions._--As we have just seen, the
geological evidence of the persistence of sub-tropical or warm climates in
the north temperate zone during the greater part of the Tertiary period is
almost irresistible, and we have now to consider the still more
extraordinary series of observations which demonstrate that this
amelioration of climate extended into the Arctic zone, and into countries
now almost wholly buried in snow and ice. These warm Arctic climates have
been explained by Dr. Croll as due to periods of high excentricity with
winter in _perihelion_, a theory which implies alternating {71} epochs of
glaciation far exceeding what now prevails; and it is therefore necessary
to examine the evidence pretty closely in order to see if this view is more
tenable in the case of the north polar regions than we have found it to be
in that of the north temperate zone.
The most recent of these milder climates is perhaps indicated by the
abundant remains of large mammalia--such as the mammoth, woolly rhinoceros,
bison and horse, in the icy alluvial plains of Northern Siberia, and
especially in the Liakhov Islands in the same latitude as the North Cape of
Asia. These remains occur not in one or two spots only, as if collected by
eddies at the mouth of a river, but along the whole borders of the Arctic
Ocean; and it is generally admitted that the animals must have lived upon
the adjacent plains, and that a considerably milder climate than now
prevails could alone have enabled them to do so. How long ago this occurred
we do not know, but one of the last intercalated mild periods of the
glacial epoch itself seems to offer all the necessary conditions. Again,
Sir Edward Belcher discovered on the dreary shores of Wellington Channel in
75-1/2deg N. Lat. the trunk and root of a fir tree which had evidently
grown where it was found. It appeared to belong to the species _Abies
alba_, or white fir, which now reaches 68deg N. Lat. and is the most
northerly conifer known. Similar trees, one four feet in circumference and
thirty feet long, were found by Lieut. Mecham in Prince Patrick's Island in
Lat. 76deg 12' N., and other Arctic explorers have found remains of trees
in high latitudes.[15]
Similar indications of a recent milder climate are found in Spitzbergen.
Professor Nordenskjoeld says: "At various places on Spitzbergen, at the
bottom of Lomme Bay, at Cape Thordsen, in Blomstrand's strata in Advent
Bay, there are found large and well-developed shells of a bivalve, _Mytilus
edulis_, which is not now found living on the coast of Spitzbergen, though
on the west coast of Scandinavia it everywhere covers the rocks near the
sea-shore. These shells occur most plentifully in the bed of a river which
runs through Reindeer Valley at Cape Thordsen. They {72} are probably
washed out of a thin bed of sand at a height of about twenty or thirty feet
above the present sea-level, which is intersected by the river. The
geological age of this bed cannot be very great, and it has clearly been
formed since the present basin of the Ice Sound, or at least the greater
part of it, has been hollowed out by glacial action."[16]
_The Miocene Arctic Flora._--One of the most startling and important of the
scientific discoveries of the last forty years has been that of the relics
of a luxuriant Miocene flora in various parts of the Arctic regions. It is
a discovery that was totally unexpected, and is even now considered by many
men of science to be completely unintelligible; but it is so thoroughly
established, and it has such a direct and important bearing on the subjects
we are discussing in the present volume, that it is necessary to lay a
tolerably complete outline of the facts before our readers.
The Miocene flora of temperate Europe was very like that of Eastern Asia,
Japan, and the warmer part of Eastern North America of the present day. It
is very richly represented in Switzerland by well preserved fossil remains,
and after a close comparison with the flora of other countries Professor
Heer concludes that the Swiss Lower Miocene flora indicates a climate
corresponding to that of Louisiana, North Africa, and South China, while
the Upper Miocene climate of the same country would correspond to that of
the south of Spain, Southern Japan, and Georgia (U.S. of America). Of this
latter flora, found chiefly at Oeninghen in the northern extremity of
Switzerland, 465 species are known, of which 166 species are trees or
shrubs, half of them being evergreens. They comprise sequoias like the
Californian giant trees, camphor-trees, cinnamons, sassafras, bignonias,
cassias, gleditschias, tulip-trees, and many other American genera,
together with maples, ashes, planes, oaks, poplars, and other familiar
European trees represented by a variety of extinct species. If we now go to
the west coast of Greenland in 70deg N. Lat. we find abundant remains of a
flora of the same general {73} type as that of Oeninghen but of a more
northern character. We have a sequoia identical with one of the species
found at Oeninghen, a chestnut, salisburia, liquidambar, sassafras, and
even a magnolia. We have also seven species of oaks, two planes, two vines,
three beeches, four poplars, two willows, a walnut, a plum, and several
shrubs supposed to be evergreens; altogether 137 species, mostly well and
abundantly preserved!
But even further north, in Spitzbergen, in 78deg and 79deg N. Lat. and one
of the most barren and inhospitable regions on the globe, an almost equally
rich fossil flora has been discovered including several of the Greenland
species, and others peculiar, but mostly of the same genera. There seem to
be no evergreens here except coniferae, one of which is identical with the
swamp-cypress (_Taxodium distichum_) now found living in the Southern
United States! There are also eleven pines, two Libocedrus, two sequoias,
with oaks, poplars, birches, planes, limes, a hazel, an ash, and a walnut;
also water-lilies, pond-weeds, and an iris--altogether about a hundred
species of flowering plants. Even in Grinnell Land, within 8-1/4 degrees of
the pole, a similar flora existed, twenty-five species of fossil plants
having been collected by the last Arctic expedition, of which eighteen were
identical with the species from other Arctic localities. This flora
comprised poplars, birches, hazels, elms, viburnums, and eight species of
conifers including the swamp cypress and the Norway spruce (_Pinus abies_)
which last does not now extend beyond 69-1/2deg N.
Fossil plants closely resembling those just mentioned have been found at
many other Arctic localities, especially in Iceland, on the Mackenzie River
in 65deg N. Lat. and in Alaska. As an intermediate station we have, in the
neighbourhood of Dantzic in Lat. 55deg N., a similar flora, with the
swamp-cypress, sequoias, oaks, poplars, and some cinnamons, laurels, and
figs. A little further south, near Breslau, north of the Carpathians, a
rich flora has been found allied to that of Oeninghen, but wanting in some
of the more tropical forms. Again, in the Isle of Mull in Scotland, in
about 56-1/2deg N. Lat., a plant-bed has been discovered {74} containing a
hazel, a plane, and a sequoia, apparently identical with a Swiss Miocene
species.
We thus find one well-marked type of vegetation spread from Switzerland and
Vienna to North Germany, Scotland, Iceland, Greenland, Alaska, and
Spitzbergen, some few of the species even ranging over the extremes of
latitude between Oeninghen and Spitzbergen, but the great majority being
distinct, and exhibiting decided indications of a decrease of temperature
according to latitude, though much less in amount than now exists. Some
writers have thought that the great similarity of the floras of Greenland
and Oeninghen is a proof that they were not contemporaneous, but
successive; and that of Greenland has been supposed to be as old as the
Eocene. But the arguments yet adduced do not seem to prove such a
difference of age, because there is only that amount of specific and
generic diversity between the two which might be produced by distance and
difference of temperature, under the exceptionally equable climate of the
period. We have even now examples of an equally wide range of well-marked
types; as in temperate South America, where many of the genera and some of
the species range from the Straits of Magellan to Valparaiso--places
differing as much in latitude as Switzerland and West Greenland; and the
same may be said of North Australia and Tasmania, where, at a greater
latitudinal distance apart, closely allied forms of Eucalyptus, Acacia,
Casuarina, Stylidium, Goodenia, and many other genera would certainly form
a prominent feature in any fossil flora now being preserved.
_Mild Arctic Climates of the Cretaceous Period._--In the Upper Cretaceous
deposits of Greenland (in a locality not far from those of the Miocene age
last described) another remarkable flora has been discovered, agreeing
generally with that of Europe and North America of the same geological age.
Sixty-five species of plants have been identified, of which there are
fifteen ferns, two cycads, eleven coniferae, three monocotyledons, and
thirty-four dicotyledons. One of the ferns is a tree-fern with thick stems,
which has also been found in the Upper Greensand of England. Among the
conifers the giant sequoias are found, and among {75} the dicotyledons the
genera Populus, Myrica, Ficus, Sassafras, Andromeda, Diospyros, Myrsine,
Panax, as well as magnolias, myrtles, and leguminosae. Several of these
groups occur also in the much richer deposits of the same age in North
America and Central Europe; but all of them evidently afford such
fragmentary records of the actual flora of the period, that it is
impossible to say that any genus found in one locality was absent from the
other merely because it has not yet been found there. On the whole, there
seems to be less difference between the floras of Arctic and temperate
latitudes in Upper Cretaceous than in Miocene times.
In the same locality in Greenland (70deg 33' N. Lat. and 52deg W. Long.),
and also in Spitzbergen, a more ancient flora, of Lower Cretaceous age, has
been found; but it differs widely from the other in the great abundance of
cycads and conifers and the scarcity of exogens, which latter are
represented by a single poplar. Of the thirty-eight ferns, fifteen belong
to the genus Gleichenia now almost entirely tropical. There are four genera
of cycads, and three extinct genera of conifers, besides Glyptostrobus and
Torreya now found only in China and California, six species of true pines,
and five of the genus Sequoia, one of which occurs also in Spitzbergen. The
European deposits of the same age closely agree with these in their general
character, conifers, cycads, and ferns forming the mass of the vegetation,
while exogens are entirely absent, the above-named Greenland poplar being
the oldest known dicotyledonous plant.[17]
If we take these facts as really representing the flora of the period, we
shall be forced to conclude that, measured by the change effected in its
plants, the lapse of time between the Lower and Upper Cretaceous deposits
was far greater than between the Upper Cretaceous and the Miocene--a
conclusion quite opposed to the indications afforded by the mollusca and
the higher animals of the two periods. It seems probable, therefore, that
these Lower Cretaceous plants represent local peculiarities of {76}
vegetation such as now sometimes occur in tropical countries. On sandy or
coralline islands in the Malay Archipelago there will often be found a
vegetation consisting almost wholly of cycads, pandani, and palms, while a
few miles off, on moderately elevated land, not a single specimen of either
of these families may be seen, but a dense forest of dicotyledonous trees
covering the whole country. A lowland vegetation, such as that above
described, might be destroyed and its remains preserved by a slight
depression, allowing it to be covered up by the detritus of some adjacent
river, while not only would the subsidence of high land be a less frequent
occurrence, but when it did occur the steep banks would be undermined by
the waves, and the trees falling down would be floated away, and would
either be cast on some distant shore or slowly decay on the surface or in
the depths of the ocean.
From the remarkable series of facts now briefly summarized, we learn, that
whenever plant-remains have been discovered within the Arctic regions,
either in Tertiary or Cretaceous deposits, they show that the climate was
one capable of supporting a rich vegetation of trees, shrubs, and
herbaceous plants, similar in general character to that which prevailed in
the temperate zone at the same periods, but showing the influence of a less
congenial climate. These deposits belong to at least four distinct
geological horizons, and have been found widely scattered within the Arctic
circle, yet nowhere has any proof been obtained of intercalated cold
periods, such as would be indicated by the remains of a stunted vegetation,
or a molluscan fauna similar to that which now prevails there.
_Stratigraphical Evidence of Long-Continued Mild Arctic Conditions._--Let
us now turn to the stratigraphical evidence, which, as we have already
shown, offers a crucial test of the occurrence or non-occurrence of
glaciation during any extensive geological period; and here we have the
testimony of perhaps the greatest living authority on Arctic
geology--Professor Nordenskjoeld. In his lecture on "The Former Climate of
the Polar Regions," he says: "The character of the coasts in the Arctic
regions is especially favourable to geological investigations. While the
valleys are for the {77} most part filled with ice, the sides of the
mountains in summer, even in the 80th degree of latitude, and to a height
of 1,000 or 1,500 feet above the level of the sea, are almost wholly free
from snow. Nor are the rocks covered with any amount of vegetation worth
mentioning; and, moreover, the sides of the mountains on the shore itself
frequently present perpendicular sections, which everywhere expose their
bare surfaces to the investigator. The knowledge of a mountain's geognostic
character, at which one, in the more southerly countries, can only arrive
after long and laborious researches, removal of soil and the like, is here
gained almost at the first glance; and as we have never seen in Spitzbergen
nor in Greenland, in these sections often many miles in length, and
including one may say all formations from the Silurian to the Tertiary, any
boulders even as large as a child's head, there is not the smallest
probability that strata of any considerable extent, containing boulders,
are to be found in the polar tracts previous to the middle of the Tertiary
period. Since, then, both an examination of the geognostic condition, and
an investigation of the fossil flora and fauna of the polar lands, show no
signs of a glacial era having existed in those parts before the termination
of the Miocene period, we are fully justified in rejecting, on the evidence
of actual observation, the hypotheses founded on purely theoretical
speculations, which assume the many times repeated alternation of warm and
glacial climates between the present time and the earliest geological
ages."[18] And again, in his _Sketch of the Geology of Spitzbergen_, after
describing the various formations down to the Miocene, he says: "All the
fossils found in the foregoing strata show that Spitzbergen, during former
geological ages, enjoyed a magnificent climate, which indeed was somewhat
colder during the Miocene period, but was still favourable for an
extraordinarily abundant vegetation, much more luxuriant than that which
now occurs even in the southern part of Scandinavia: and I have in those
strata sought in vain for any sign, that, as some geologists have of late
endeavoured to render probable, these favourable climatic conditions have
been broken off {78} by intervals of ancient glacial periods. The profiles
I have had the opportunity to examine during my various Spitzbergen
expeditions would certainly, if laid down on a line, occupy an extent of _a
thousand English miles_; and if any former glacial period had existed in
this region, there ought to have been some trace to be observed of erratic
blocks, or other formations which distinguish glacial action. But this has
not been the case. In the strata, whose length I have reckoned alone, I
have not found a single fragment of a foreign rock so large as a child's
head."[19]
Now it is quite impossible to ignore or evade the force of this testimony
as to the continuous warm climates of the north temperate and polar zones
throughout Tertiary times. The evidence extends over a vast area, both in
space and time, it is derived from the work of the most competent living
geologists, and it is absolutely consistent in its general tendency. We
have in the Lower Cretaceous period an almost tropical climate in France
and England, a somewhat lower temperature in the United States, and a mild
insular climate in the Arctic regions. In each successive period the
climate becomes somewhat less tropical; but down to the Upper Miocene it
remains warm temperate in Central Europe, and cold temperate within the
polar area, with not a trace of any intervening periods of Arctic cold. It
then gradually cools down and merges through the Pliocene into the glacial
epoch in Europe, while in the Arctic zone there is a break in the record
between the Miocene and the recent glacial deposits.[20]
{79}
Accepting this as a substantially correct account of the general climatic
aspect of the Tertiary period in the northern hemisphere, let us see
whether the principles we have already laid down will enable us to give a
satisfactory explanation of its causes.
_The Causes of mild Arctic Climates._--In his remarkable series of papers
on "Ocean Currents," the late Dr. James Croll has proved, with a wealth of
argument and illustration whose cogency is irresistible, that the very
habitability of our globe is due to the equalizing climatic effects of the
waters of the ocean; and that it is to the same cause that we owe, either
directly or indirectly, almost all the chief diversities of climate between
places situated in the same latitude. Owing to the peculiar distribution of
land and sea upon the globe, more than its fair proportion of the warm
equatorial waters is directed towards the western shores of Europe, the
result being that the British Isles, Norway, and Spitzbergen, have all a
milder climate than any other parts of the globe in corresponding
latitudes. A very small portion of the Arctic regions, however, obtains
this benefit, and it thus remains, generally speaking, a land of snow and
ice, with too short a summer to nourish more than a very scanty and
fugitive vegetation. The only other opening than that between Iceland and
Britain by which warm water penetrates within the Arctic circle, is through
Behring's Straits; but this is both shallow and limited in width, and the
consequence is that the larger part of the warm currents of the Pacific
turns back along the shores of the Aleutian Islands and North-west America,
while a very small quantity enters the icy ocean.
But if there were other and wider openings into the Arctic Ocean, a vast
quantity of the heated water which is now turned backward would enter it,
and would produce an amelioration of the climate of which we can hardly
form a conception. A great amelioration of climate would also be caused by
the breaking up or the lowering of such {80} Arctic highlands as now favour
the accumulation of ice; while the interpenetration of the sea into any
part of the great continents in the tropical or temperate zones would again
tend to raise the winter temperature, and render any long continuance of
snow in their vicinity almost impossible.
Now geologists have proved, quite independently of any such questions as we
are here discussing, that changes of the very kinds above referred to have
occurred during the Tertiary period; and that there has been, speaking
broadly, a steady change from a comparatively fragmentary and insular
condition of the great north temperate lands in early Tertiary times, to
that more compact and continental condition which now prevails. It is, no
doubt, difficult and often impossible to determine how long any particular
geographical condition lasted, or whether the changes in one country were
exactly coincident with those in another; but it will be sufficient for our
purpose briefly to indicate those more important changes of land and sea
during the Tertiary period, which must have produced a decided effect on
the climate of the northern hemisphere.
_Geographical Changes Favouring Mild Northern Climates in Tertiary
Times._--The distribution of the Eocene and Miocene formations shows, that
during a considerable portion of the Tertiary period, an inland sea, more
or less occupied by an archipelago of islands, extended across Central
Europe between the Baltic and the Black and Caspian Seas, and thence by
narrower channels south-eastward to the valley of the Euphrates and the
Persian Gulf, thus opening a communication between the North Atlantic and
the Indian Oceans. From the Caspian also a wide arm of the sea extended
during some part of the Tertiary epoch northwards to the Arctic Ocean, and
there is nothing to show that this sea may not have been in existence
during the whole Tertiary period. Another channel probably existed over
Egypt[21] into the eastern {81} basin of the Mediterranean and the Black
Sea; while it is probable that there was a communication between the Baltic
and the White Sea, leaving Scandinavia as an extensive island. Turning to
India, we find that an arm of the sea of great width and depth extended
from the Bay of Bengal to the mouths of the Indus; while the enormous
depression indicated by the presence of marine fossils of Eocene age at a
height of 10,500 feet in Western Tibet, renders it not improbable that a
more direct channel across Afghanistan may have opened a communication
between the West Asiatic and Polar seas.
It may be said that the changes here indicated are not warranted by an
actual knowledge of continuous Tertiary deposits over the situations of the
alleged marine channels; but it is no less certain that the seas in which
any particular strata were deposited were _always_ more extensive than the
fragments of those strata now existing, and _often_ immensely more
extensive. The Eocene deposits of Europe, for example, have certainly
undergone enormous denudation both marine and subaerial, and may have once
covered areas where we now find older deposits (as the chalk once covered
the weald), while a portion of them may lie concealed under Miocene,
Pliocene, or recent beds. We find them widely scattered over Europe and
Asia, and often elevated into lofty mountain ranges; and we should
certainly err far more seriously in confining the Eocene seas to the exact
areas where we now find Eocene rocks, than in liberally extending them, so
as to connect the several detached portions of the formation whenever there
is no valid argument against our doing so. Considering then, that some one
or more of the sea-communications here indicated almost certainly existed
during Eocene and Miocene times, let us endeavour to estimate the probable
effect such communications would have upon the climate of the northern
hemisphere.
_The Indian Ocean as a Source of Heat in Tertiary Times._--If we compare
the Indian Ocean with the South Atlantic we shall see that the position and
outline of the former are very favourable for the accumulation of a large
body of warm water moving northwards. Its southern {82} opening between
South Africa and Australia is very wide, and the tendency of the
trade-winds would be to concentrate the currents towards its north-western
extremity, just where the two great channels above described formed an
outlet to the northern seas. As will be shown in our nineteenth chapter,
there was probably, during the earlier portion of the Tertiary period at
least, several large islands in the space between Madagascar and South
India; but these had wide and deep channels between them, and their
existence may have been favourable to the conveyance of heated water
northward, by concentrating the currents, and thus producing massive bodies
of moving water analogous to the Gulf Stream of the Atlantic.[22] Less heat
would thus be lost by evaporation and radiation in the tropical zone, and
an impulse would be acquired which would carry the warm water into the
north polar area. About the same period Australia was probably divided into
two islands, separated by a wide channel in a north and south direction
(see Chapter XXII.), and through this another current would almost
certainly set northwards, and be directed to the north-west by the southern
extension of Malayan Asia. The more insular condition at this period of
Australia, India, and North Africa, with the depression and probable
fertility of the Central Asiatic plateau, would lead to the Indian Ocean
being traversed by regular trade-winds instead of by variable monsoons, and
thus the constant _vis a tergo_, which is so efficient in the Atlantic,
would keep up a steady and powerful current towards the northern parts of
the Indian Ocean, and thence through the midst of the European archipelago
to the northern seas.
Now it is quite certain that such a condition as we have here sketched out
would produce a wonderful effect on the climate of Central Europe and
Western and Northern Asia. Owing to the warm currents being concentrated in
inland seas instead of being dispersed over a wide ocean like the {83}
North Atlantic, much more heat would be conveyed into the Arctic Ocean, and
this would altogether prevent the formation of ice on the northern shores
of Asia, which continent did not then extend nearly so far north and was
probably deeply inter-penetrated by the sea. This open ocean to the north,
and the warm currents along all the northern lands, would so equalise
temperature, that even the northern parts of Europe might then have enjoyed
a climate fully equal to that of the warmer parts of New Zealand at the
present day, and might have well supported the luxuriant vegetation of the
Miocene period, even without any help from similar changes in the western
hemisphere.[23]
_Condition of North America during the Tertiary Period._--But changes of a
somewhat similar character have also taken place in America and the
Pacific. An enormous area west of the Mississippi, extending over much of
the Rocky Mountains, consists of marine Cretaceous beds 10,000 feet thick,
indicating great and long-continued subsidence, and an insular condition of
Western America with a sea probably extending northwards to the Arctic
Ocean. As marine Tertiary deposits are found conformably overlying these
Cretaceous strata, Professor Dana is of opinion that the great elevation of
this part of America did not begin till early Tertiary times. Other
Tertiary beds in California, Alaska, Kamschatka, the Mackenzie River, the
Parry Islands, and Greenland, indicate partial submergence {84} of all
these lands with the possible influx of warm water from the Pacific; and
the considerable elevation of some of the Miocene beds in Greenland and
Spitzbergen renders it probable that these countries were then much less
elevated, in which case only their higher summits would be covered with
perpetual snow, and no glaciers would descend to the sea.
In the Pacific there was probably an elevation of land counterbalancing, to
some extent, the great depression of so much of the northern continents.
Our map in Chapter XV. shows the islands that would be produced by an
elevation of the great shoals under a thousand fathoms deep, and it is seen
that these all trend in a south-east and north-west direction, and would
thus facilitate the production of definite currents impelled by the
south-east trades towards the north-west Pacific, where they would gain
access to the polar seas through Behring's Straits, which were, perhaps,
sometimes both wider and deeper than at present.
_Effect of these Changes on the Climate of the Arctic Regions._--These
various changes of sea and land, all tending towards a transference of heat
from the equator to the north temperate zone, were not improbably still
further augmented by the existence of a great inland South American sea
occupying what are now the extensive valleys of the Amazon and Orinoco, and
forming an additional reservoir of super-heated water to add to the supply
poured into the North Atlantic.
It is not of course supposed that all the modifications here indicated
co-existed at the same time. We have good reason to believe, from the known
distribution of animals in the Tertiary period, that land-communications
have at times existed between Europe or Asia and North America, either by
way of Behring's Straits, or by Iceland, Greenland, and Labrador. But the
same evidence shows that these land-communications were the exception
rather than the rule, and that they occurred only at long intervals and for
short periods, so as at no time to bring about anything like a complete
interchange of the productions of the two continents.[24] We may therefore
admit that the {85} communication between the tropical and Arctic oceans
was occasionally interrupted in one or other direction; but if we look at a
globe instead of a Mercator's chart of the world, we shall see that the
disproportion between the extent of the polar and tropical seas is so
enormous that a single wide opening, with an adequate impulse to carry in a
considerable stream of warm water, would be amply sufficient for the
complete abolition of polar snow and ice, when aided by the absence of any
great areas of high land within the polar circle, such high land being, as
we have seen, essential to the production of perpetual snow even at the
present time.
Those who wish to understand the effect of oceanic currents in conveying
heat to the north temperate and polar regions, should study the papers of
Dr. Croll already referred to. But the same thing is equally well shown by
the facts of the actual distribution of heat due to the Gulf Stream. The
difference between the mean annual temperatures of the opposite coasts of
Europe and America is well known and has been already quoted, but the
difference of their mean _winter_ temperature is still more striking, and
it is this which concerns us as more especially affecting the distribution
of vegetable and animal life. Our mean winter temperature in the west of
England is the same as that of the Southern United States, as well as that
of Shanghai in China, both about twenty degrees of latitude further south;
and as we go northward the difference increases, so that the winter climate
of Nova Scotia in Lat. 45deg is found within the Arctic circle on the coast
of Norway; and if the latter country did not consist almost wholly of
precipitous snow-clad mountains, it would be capable of supporting most of
the vegetable products of the American coast in the latitude of
Bordeaux.[25]
{86}
With these astounding facts before us, due wholly to the transference of a
portion of the warm currents of the Atlantic to the shores of Europe, even
with all the disadvantages of an icy sea to the north-east and ice-covered
Greenland to the north-west, how can we doubt the enormously greater effect
of such a condition of things as has been shown to have existed during the
Tertiary epoch? Instead of _one_ great stream of warm water spreading
widely over the North Atlantic and thus losing the greater part of its
store of heat _before_ it reaches the Arctic seas, we should have _several_
streams conveying the heat of far more extensive tropical oceans by
comparatively narrow inland channels, thus being able to transfer a large
proportion of their heat _into_ the northern and Arctic seas. The heat that
they gave out during the passage, instead of being widely dispersed by
winds and much of it lost in the higher atmosphere, would directly
ameliorate the climate of the continents they passed through, and prevent
all accumulation of snow except on the loftiest mountains. The formation of
ice in the Arctic seas would then be impossible; and the mild winter
climate of the latitude of North {87} Carolina, which by the Gulf Stream is
transferred 20deg northwards to our islands, might certainly, under the
favourable conditions which prevailed during the Cretaceous, Eocene, and
Miocene periods, have been carried another 20deg north to Greenland and
Spitzbergen; and this would bring about exactly the climate indicated by
the fossil Arctic vegetation. For it must be remembered that the Arctic
summers are, even now, really hotter than ours, and if the winter's cold
were abolished and all ice-accumulation prevented, the high northern lands
would be able to support a far more luxuriant summer vegetation than is
possible in our unequal and cloudy climate.[26]
_Effect of High Excentricity on the Warm Polar Climates._--If the
explanation of the cause of the glacial epoch given in the last chapter is
a correct one, it will, I believe, follow that changes in the amount of
excentricity will produce no important alteration of the climates of the
temperate and Arctic zones so long as favourable geographical conditions,
such as have been now sketched out, render the accumulation of ice
impossible. The effect of a high excentricity in producing a glacial epoch
was shown to be due to the capacity of snow and ice for storing up cold,
and its singular power (when in large masses) of preserving itself unmelted
under a hot sun by itself causing the interposition of a protective
covering of cloud and vapour. But mobile currents of water have no such
power of {88} accumulating and storing up heat or cold from one year to
another, though they do in a pre-eminent degree possess the power of
equalising the temperature of winter and summer and of conveying the
superabundant heat of the tropics to ameliorate the rigour of the Arctic
winters. However great was the difference between the amount of heat
received from the sun in winter and summer in the Arctic zone during a
period of high excentricity and winter in _aphelion_, the inequality would
be greatly diminished by the free ingress of warm currents to the polar
area; and if this was sufficient to prevent any accumulation of ice, the
summers would be warmed to the full extent of the powers of the sun during
the long polar day, which is such as to give the pole at midsummer actually
more heat during the twenty-four hours than the equator receives during its
day of twelve hours. The only difference, then, that would be directly
produced by the changes of excentricity and precession would be, that the
summers would be at one period almost tropical, at the other of a more mild
and uniform temperate character; while the winters would be at one time
somewhat longer and colder, but never, probably, more severe than they are
now in the west of Scotland.
But though high excentricity would not directly modify the mild climates
produced by the state of the northern hemisphere which prevailed during
Cretaceous, Eocene, and Miocene times, it might indirectly affect it by
increasing the mass of Antarctic ice, and thus increasing the force of the
trade-winds and the resulting northward-flowing warm currents. Now there
are many peculiarities in the distribution of plants and of some groups of
animals in the southern hemisphere, which render it almost certain that
there has sometimes been a greater extension of the Antarctic lands during
Tertiary times; and it is therefore not improbable that a more or less
glaciated condition may have been a long persistent feature of the southern
hemisphere, due to the peculiar distribution of land and sea which favours
the production of ice-fields and glaciers. And as we have seen that during
the last three million years the excentricity has been almost always much
higher than {89} it is now, we should expect that the quantity of ice in
the southern hemisphere will usually have been greater, and will thus have
tended to increase the force of those oceanic currents which produce the
mild climates of the northern hemisphere.
_Evidences of Climate in the Secondary and Palaeozoic Epochs._--We have
already seen, that so far back as the Cretaceous period there is the most
conclusive evidence of the prevalence of a very mild climate not only in
temperate but also in Arctic lands, while there is no proof whatever, or
even any clear indication, of early glacial epochs at all comparable in
extent and severity with that which has so recently occurred; and we have
seen reason to connect this state of things with a distribution of land and
sea highly favourable to the transference of warm water from equatorial to
polar latitudes. So far as we can judge by the plant-remains of our own
country, the climate appears to have been almost tropical in the Lower
Eocene period; and as we go further back we find no clear indications of a
higher, but often of a lower temperature, though always warmer or more
equable than our present climate. The abundant corals and reptiles of the
Oolite and Lias indicate equally tropical conditions; but further back, in
the Trias, the flora and fauna, in the British area, become poorer, and
there is nothing incompatible with a climate no warmer than that of the
Upper Miocene. This poverty is still more marked in the Permian formation,
and it is here that some indications of ice-action are found in the Lower
Permian conglomerates of the west of England. These beds contain abundant
fragments of various rocks, often angular and sometimes weighing half a
ton, while others are partially rounded, and have polished and striated
surfaces, just like the stones of the "till." They lie confusedly bedded in
a red unstratified marl, and some of them can be traced to the Welsh hills
from twenty to fifty miles distant. This remarkable formation was first
pointed out as proving a remote glacial period, by Professor Ramsay; and
Sir Charles Lyell agreed that this is the only possible explanation that,
with our present knowledge, we can give of them.
Permian breccias are also found in Ireland, containing {90} blocks of
Silurian and Old Red sandstone rocks which Professor Hull believes could
only have been carried by floating ice. Similar breccias occur in the south
of Scotland, and these are stated to be "overlain by a deposit of glacial
age, so similar to the breccia below as to be with difficulty distinguished
from it."[27]
These numerous physical indications of ice-action over a considerable area
during the same geological period, coinciding with just such a poverty of
organic remains as might be produced by a very cold climate, are very
important, and seem clearly to indicate that at this remote period
geographical conditions were such as to bring about a glacial epoch, or
perhaps only local glaciation, in our part of the world.
Boulder-beds also occur in the Carboniferous formation, both in Scotland,
on the continent of Europe, and in North America; and Professor Dawson
considers that he has detected true glacial deposits of the same age in
Nova Scotia. Boulder-beds also occur in the Silurian rocks of Scotland and
North America, and according to Professor Dawson, even in the Huronian,
older than our Cambrian. None of these indications are however so
satisfactory as those of Permian age, where we have the very kind of
evidence we looked for in vain throughout the whole of the Tertiary and
Secondary periods. Its presence in several localities in such ancient rocks
as the Permian is not only most important as indicating a glacial epoch of
some kind in Palaeozoic times, but confirms us in the validity of our
conclusion, that the _total_ absence of any such evidence throughout the
Tertiary and Secondary epochs demonstrates the absence of recurring glacial
epochs in the northern hemisphere, notwithstanding the frequent recurrence
of periods of high excentricity.
_Warm Arctic Climates in Early Secondary and Palaeozoic Times._--The
evidence we have already adduced of the mild climates prevailing in the
Arctic regions throughout the Miocene, Eocene, and Cretaceous periods is
supplemented by a considerable body of facts relating to still earlier
epochs.
{91}
In the Jurassic period, for example, we have proofs of a mild Arctic
climate, in the abundant plant-remains of East Siberia and Amurland, with
less productive deposits in Spitzbergen, and at Ando in Norway just within
the Arctic circle. But even more remarkable are the marine remains found in
many places in high northern latitudes, among which we may especially
mention the numerous ammonites and the vertebrae of huge reptiles of the
genera Ichthyosaurus and Teleosaurus found in the Jurassic deposits of the
Parry Islands in 77deg N. Lat.
In the still earlier Triassic age, nautili and ammonites inhabited the seas
of Spitzbergen, where their fossil remains are now found.
In the Carboniferous formation we again meet with plant-remains and beds of
true coal in the Arctic regions. Lepidodendrons and Calamites, together
with large spreading ferns, are found at Spitzbergen, and at Bear Island in
the extreme north of Eastern Siberia; while marine deposits of the same age
contain abundance of large stony corals.
Lastly, the ancient Silurian limestones, which are widely spread in the
high Arctic regions, contain abundance of corals and cephalopodous mollusca
resembling those from the same deposits in more temperate lands.
_Conclusions as to the Climates of Tertiary and Secondary Periods._--If now
we look at the whole series of geological facts as to the animal and
vegetable productions of the Arctic regions in past ages, it is certainly
difficult to avoid the conclusion that they indicate a climate of a
uniformly temperate or warm character. Whether in Miocene, Upper or Lower
Cretaceous, Jurassic, Triassic, Carboniferous or Silurian times, and in all
the numerous localities extending over more than half the polar regions, we
find one uniform climatic aspect in the fossils. This is quite inconsistent
with the theory of alternate cold and mild epochs during phases of high
excentricity, and persistent cold epochs when the excentricity was as low
as it is now or lower, for that would imply that the duration of cold
conditions was _greater_ than that of warm. Why then should the fauna and
flora of the cold epochs _never_ be {92} preserved? Mollusca and many other
forms of life are abundant in the Arctic seas, and there is often a
luxuriant dwarf woody vegetation on the land, yet in no one case has a
single example of such a fauna or flora been discovered of a date anterior
to the last glacial epoch. And this argument is very much strengthened when
we remember that an exactly analogous series of facts is found over all the
temperate zones. Everywhere we have abundant floras and faunas indicating
warmer conditions than such as now prevail, but never in a single instance
one which as clearly indicates colder conditions. The fact that drift with
Arctic shells was deposited during the last glacial epoch, as well as
gravels and crag with the remains of arctic animals and plants, shows us
that there is nothing to prevent such deposits being formed in cold as well
as in warm periods; and it is quite impossible to believe that in every
place and at all epochs all records of the former have been destroyed,
while in a considerable number of instances those of the latter have been
preserved. When to this uniform testimony of the palaeontological evidence
we add the equally uniform absence of any indication of those ice-borne
rocks, boulders, and drift, which are the constant and necessary
accompaniment of every period of glaciation, and which must inevitably
pervade all the marine deposits formed over a wide area so long as the
state of glaciation continues, we are driven to the conclusion that the
last glacial epoch of the northern hemisphere was exceptional, and was not
preceded by numerous similar glacial epochs throughout Tertiary and
Secondary time.
But although glacial epochs (with the one or two exceptions already
referred to) were certainly absent, considerable changes of climate may
have frequently occurred, and these would lead to important changes in the
organic world. We can hardly doubt that some such change occurred between
the Lower and Upper Cretaceous periods, the floras of which exhibit such an
extraordinary contrast in general character. We have also the testimony of
Mr. J. S. Gardner, who has long worked at the fossil floras of the Tertiary
deposits, and who states, that {93} there is strong negative and some
positive evidence of alternating warmer and colder conditions, not glacial,
contained not only in English Eocene, but all Tertiary beds throughout the
world.[28] In the case of marine faunas it is more difficult to judge, but
the numerous changes in the fossil remains from bed to bed only a few feet
and sometimes a few inches apart, may be sometimes due to change of
climate; and when it is recognised that such changes have probably occurred
at all geological epochs and their effects are systematically searched for,
many peculiarities in the distribution of organisms through the different
members of one deposit may be traced to this cause.
_General View of Geological Climates as dependent on the Physical Features
of the Earth's Surface._--In the preceding chapters I have earnestly
endeavoured to arrive at an explanation of geological climates in the
temperate and Arctic zones, which should be in harmony with the great body
of geological facts now available for their elucidation. If my conclusions
as here set forth diverge considerably from those of Dr. Croll, it is not
from any want of appreciation of his facts and arguments, since for many
years I have upheld and enforced his views to the best of my ability. But a
careful re-examination of the whole question has now convinced me that an
error has been made in estimating the comparative effect of geographical
and astronomical causes on changes of climate, and that, while the latter
have undoubtedly played an important part in bringing about the glacial
epoch, it is to the former that the mild climates of the Arctic regions are
almost entirely due. If I have now succeeded in approaching to a true
solution of this difficult problem, I owe it mainly to the study of Dr.
Croll's writings, since my theory is entirely based on the facts and
principles so clearly set forth in his admirable papers on "Ocean Currents
in relation to the Distribution of Heat over the Globe." The main features
of this theory as distinct from that of Dr. Croll I will now endeavour to
summarise.
Looking at the subject broadly, we see that the climatic {94} condition of
the northern hemisphere is the result of the peculiar distribution of land
and water upon the globe; and the general permanence of the position of the
continental and oceanic areas--which we have shown to be proved by so many
distinct lines of evidence--is also implied by the general stability of
climate throughout long geological periods. The land surface of our earth
appears to have always consisted of three great masses in the north
temperate zone, narrowing southward, and terminating in three comparatively
narrow extremities represented by Southern America, South Africa, and
Australia. Towards the north these masses have approached each other, and
have sometimes become united; leaving beyond them a considerable area of
open polar sea. Towards the south they have never been much further
prolonged than at present, but far beyond their extremities an extensive
mass of land has occupied the south polar area.
This arrangement is such as would cause the northern hemisphere to be
always (as it is now) warmer than the southern, and this would lead to the
preponderance of northward winds and ocean currents, and would bring about
the concentration of the latter in three great streams carrying warmth to
the north-polar regions. These streams would, as Dr. Croll has so well
shown, be greatly increased in power by the glaciation of the south polar
land; and whenever any considerable portion of this land was elevated, such
a condition of glaciation would certainly be brought about, and would be
heightened whenever a high degree of excentricity prevailed.
It is now the general opinion of geologists that the great continents have
undergone a process of development from earlier to later times. Professor
Dana appears to have been the first who taught it explicitly in the case of
the North American continent, and he has continued the development of his
views from 1856, when he discussed the subject in the _American Journal_,
to the later editions of his _Manual of Geology_ in which the same views
are extended to all the great continents. He says:--
"The North American continent, which since early {95} time had been
gradually expanding in each direction from the northern Azoic, eastward,
westward, and southward, and which, after the Palaeozoic, was finished in
its rocky foundation, excepting on the borders of the Atlantic and Pacific
and the area of the Rocky Mountains, had reached its full expansion at the
close of the Tertiary period. The progress from the first was uniform and
systematic: the land was at all times simple in outline; and its
enlargement took place with almost the regularity of an exogenous
plant."[29]
A similar development undoubtedly took place in the European area, which
was apparently never so compact and so little interpenetrated by the sea as
it is now, while Europe and Asia have only become united into one unbroken
mass since late Tertiary times.
If, however, the greater continents have become more compact and massive
from age to age, and have received their chief extensions northward at a
comparatively recent period, while the Antarctic lands had a corresponding
but somewhat earlier development, we have all the conditions requisite to
explain the persistence, with slight fluctuations, of warm climates far
into the north-polar area throughout Palaeozoic, Mesozoic, and Tertiary
times. At length, during the latter part of the Tertiary epoch, a
considerable elevation took place, closing up several of the water passages
to the north, and raising up extensive areas in the Arctic regions to
become the receptacle of snow and ice-fields. This elevation is indicated
by the abundance of Miocene and the absence of Pliocene deposits in the
Arctic zone and the considerable altitude of many Miocene rocks in Europe
and North America; and the occurrence at this time of a long-continued
period of high excentricity necessarily brought on the glacial epoch in the
manner already described in our last chapter. A depression seems to have
occurred during the glacial period itself in North America as in Britain,
but this may have been due partly to the weight of the ice and partly to a
rise of the ocean {96} level caused by the earth's centre of gravity being
shifted towards the north.
We thus see that the last glacial epoch was the climax of a great process
of continental development which had been going on throughout long
geological ages; and that it was the direct consequence of the north
temperate and polar land having attained a great extension and a
considerable altitude just at the time when a phase of very high
excentricity was coming on. Throughout earlier Tertiary and Secondary times
an equally high excentricity often occurred, but it never produced a
glacial epoch, because the north temperate and polar areas had less high
land, and were more freely open to the influx of warm oceanic currents. But
wherever great plateaux with lofty mountains occurred in the temperate zone
a considerable _local_ glaciation might be produced, which would be
specially intense during periods of high excentricity; and it is to such
causes we must impute the indications of ice-action in the vicinity of the
Alps during the Tertiary period. The Permian glaciation appears to have
been more extensive, and it is quite possible that at this remote epoch a
sufficient mass of high land existed in our area and northwards towards the
pole, to have brought on a true glacial period comparable with that which
has so recently passed away.
_Estimate of the comparative effects of Geographical and Astronomical
Causes in producing Changes of Climate._--It appears then, that while
geographical and physical causes alone, by their influence on ocean
currents, have been the main agents in producing the mild climates which
for such long periods prevailed in the Arctic regions, the concurrence of
astronomical causes--high excentricity with winter in _aphelion_--was
necessary to the production of the great glacial epoch. If we reject this
latter agency, we shall be obliged to imagine a concurrence of geographical
changes at a very recent period of which we have no evidence. We must
suppose, for example, that a large part of the British Isles--Scotland,
Ireland, and Wales at all events--were simultaneously elevated so as to
bring extensive areas above the line of perpetual snow; that {97} about the
same time Scandinavia, the Alps, and the Pyrenees received a similar
increase of altitude; and that, almost simultaneously, Eastern North
America, the Sierra Nevada of California, the Caucasus, Lebanon, the
southern mountains of Spain, the Atlas range, and the Himalayas, were each
some thousands of feet higher than they are now; for all these mountains
present us with indications of a recent extension of their glaciers, in
superficial phenomena so similar to those which occur in our own country
and in Western Europe, that we cannot suppose them to belong to a different
epoch. Such a supposition is rendered more difficult by the general
concurrence of scientific testimony to a partial submergence during the
glacial epoch, not only in all parts of Britain, but in North America,
Scandinavia, and, as shown by the wide extension of the drift, in Northern
Europe; and when to this we add the difficulty of understanding how any
probable addition to the altitude of our islands could have brought about
the extreme amount of glaciation which they certainly underwent, and when,
further, we know that a phase of very high excentricity did occur at a
period which is generally admitted to agree well with physical evidence of
the time elapsed since the cold passed away, there seems no sufficient
reason why such an agency should be ignored.
No doubt a prejudice has been excited against it in the minds of many
geologists, by its being thought to lead _necessarily_ to frequently
recurring glacial epochs throughout all geological time. But I have here
endeavoured to show that this is _not_ a necessary consequence of the
theory, because a concurrence of favourable geographical conditions is
essential to the initiation of a glaciation, which when once initiated has
a tendency to maintain itself throughout the varying phases of precession
occurring during a period of high excentricity. When, however, geographical
conditions favour warm Arctic climates--as it has been shown they have done
throughout the larger portion of geological time--then changes of
excentricity, to however great an extent, have no tendency to bring about a
state of glaciation, because warm oceanic currents have a {98}
preponderating influence, and without very large areas of high northern
land to act as condensers, no perpetual snow is possible, and hence the
initial process of glaciation does not occur.
The theory as now set forth should commend itself to geologists, since it
shows the direct dependence of climate on physical processes, which are
guided and modified by those changes in the earth's surface which geology
alone can trace out. It is in perfect accord with the most recent teachings
of the science as to the gradual and progressive development of the earth's
crust from the rudimentary formations of the Azoic age, and it lends
support to the view that no inportant[**important] departure from the great
lines of elevation and depression originally marked out on the earth's
surface has ever taken place.
It also shows us how important an agent in the production of a habitable
globe with comparatively small extremes of climates over its whole area, is
the great disproportion between the extent of the land and the water
surfaces. For if these proportions had been reversed, large areas of land
would necessarily have been removed from the beneficial influence of
aqueous currents or moisture-laden winds; and slight geological changes
might easily have led to half the land surface becoming covered with
perpetual snow and ice, or being exposed to extremes of summer heat and
winter cold, of which our water-permeated globe at present affords no
example. We thus see that what are usually regarded as geographical
anomalies--the disproportion of land and water, the gathering of the land
mainly into one hemisphere, and the singular arrangement of the land in
three great southward-pointing masses--are really facts of the greatest
significance and importance, since it is to these very anomalies that the
universal spread of vegetation and the adaptability of so large a portion
of the earth's surface for human habitation is directly due.
* * * * *
{99}
CHAPTER X
THE EARTH'S AGE, AND THE RATE OF DEVELOPMENT OF ANIMALS AND PLANTS
Various Estimates of Geological Time--Denudation and Deposition of
Strata as a Measure of Time--How to Estimate the Thickness of the
Sedimentary Rocks--How to Estimate the Average Rate of Deposition of
the Sedimentary Rocks--The Rate of Geological Change Probably greater
in very Remote Times--Value of the Preceding Estimate of Geological
Time--Organic Modification Dependent on Change of
Conditions--Geographical Mutations as a Motive Power in bringing about
Organic Changes--Climatal Revolutions as an Agent in Producing Organic
Changes--Present Condition of the Earth one of Exceptional Stability as
Regards Climate--Date of last Glacial Epoch and its Bearing on the
Measurement of Geological Time--Concluding Remarks.
The subjects discussed in the last three chapters introduce us to a
difficulty which has hitherto been considered a very formidable one--that
the maximum age of the habitable earth, as deduced from physical
considerations, does not afford sufficient time either for the geological
or the organic changes of which we have evidence. Geologists continually
dwell on the slowness of the processes of upheaval and subsidence, of
denudation of the earth's surface, and of the formation of new strata;
while on the theory of development, as expounded by Mr. Darwin, the
variation and modification of organic forms is also a very slow process,
and has usually been considered to require an {100} even longer series of
ages than might satisfy the requirements of physical geology alone.
As an indication of the periods usually contemplated by geologists, we may
refer to Sir Charles Lyell's calculation in the tenth edition of his
_Principles of Geology_ (omitted in later editions), by which he arrived at
240 millions of years as having probably elapsed since the Cambrian
period--a very moderate estimate in the opinion of most geologists. This
calculation was founded on the rate of modification of the species of
mollusca; but much more recently Professor Haughton has arrived at nearly
similar figures from a consideration of the rate of formation of rocks and
their known maximum thickness, whence he deduces a maximum of 200 millions
of years for the whole duration of geological time, as indicated by the
series of stratified formations.[30] But in the opinion of all our first
naturalists and geologists, the period occupied in the formation of the
known stratified rocks only represents a portion, and perhaps a small
portion, of geological time. In the sixth edition of the _Origin of
Species_ (p. 286), Mr. Darwin says: "Consequently, if the theory be true,
it is indisputable that before the lowest Cambrian stratum was deposited
long periods elapsed, as long as, or probably far longer than, the whole
interval from the Cambrian age to the present day; and that during these
vast periods the world swarmed with living creatures." Professor Huxley, in
his anniversary address to the Geological Society in 1870, adduced a number
of special cases showing that, on the theory of development, almost all the
higher forms of life must have existed during the Palaeozoic period. Thus,
from the fact that almost the whole of the Tertiary period has been
required to convert the ancestral Orohippus into the true horse, he
believes that, in order to have time for the much greater change of the
ancestral Ungulata into the two great odd-toed and even-toed divisions (of
which change there is no trace even among the earliest Eocene mammals), we
should require a large portion, if not the whole, of the Mesozoic or
Secondary period. Another case is furnished by the bats and whales, both of
which strange modifications of the {101} mammalian type occur perfectly
developed in the Eocene formation. What countless ages back must we then go
for the origin of these groups, the whales from some ancestral carnivorous
animal, and the bats from the insectivora! And even then we have to seek
for the common origin of carnivora, insectivora, ungulata, and marsupials
at a far earlier period; so that, on the lowest estimate, we must place the
origin of the mammalia very far back in Palaeozoic times. Similar evidence
is afforded by reptiles, of which Professor Huxley says: "If the very small
differences which are observable between the crocodiles of the older
Secondary formations and those of the present day furnish any sort of an
approximation towards an estimate of the average rate of change among
reptiles, it is almost appalling to reflect how far back in Palaeozoic
times we must go before we can hope to arrive at that common stock from
which the crocodiles, lizards, _Ornithoscelida_, and _Plesiosauria_, which
had attained so great a development in the Triassic epoch, must have been
derived." Professor Ramsay has expressed similar views, derived from a
general study of the whole series of geological formations and their
contained fossils. He says, speaking of the abundant, varied, and
well-developed fauna of the Cambrian period: "In this earliest known
_varied_ life we find no evidence of its having lived near the beginning of
the zoological series. In a broad sense, compared with what must have gone
before, both biologically and physically, all the phenomena connected with
this old period seem, to my mind, to be of quite a recent description; and
the climates of seas and lands were of the very same kind as those the
world enjoys at the present day."[31]
These opinions, and the facts on which they are founded, are so weighty,
that we can hardly doubt that, if the time since the Cambrian epoch is
correctly estimated at 200 millions of years, the date of the commencement
of life on the earth cannot be much less than 500 millions; while it may
not improbably have been longer, because the reaction of {102} the organism
under changes of the environment is believed to have been less active in
low and simple, than in high and complex forms of life, and thus the
processes of organic development may for countless ages have been
excessively slow.
But according to the physicists, no such periods as are here contemplated
can be granted. From a consideration of the possible sources of the heat of
the sun, as well as from calculations of the period during which the earth
can have been cooling to bring about the present rate of increase of
temperature as we descend beneath the surface, Sir William Thomson
concludes that the crust of the earth cannot have been solidified much
longer than 100 million years (the maximum possible being 400 millions),
and this conclusion is held by Dr. Croll and other men of eminence to be
almost indisputable.[32] It will therefore be well to consider on what data
the calculations of geologists have been founded, and how far the views
here set forth, as to frequent changes of climate throughout all geological
time, may affect the rate of biological change.
_Denudation and Deposition of Strata as a Measure of Time._--The materials
of all the stratified rocks of the globe have been obtained from the dry
land. Every point of the surface is exposed to the destructive influences
of sun and wind, frost, snow, and rain, which break up and wear away the
hardest rocks as well as the softer deposits, and by means of rivers convey
the worn material to the sea. The existence of a considerable depth of soil
over the greater part of the earth's surface; of vast heaps of rocky
_debris_ at the foot of every inland cliff; of enormous deposits of gravel,
sand, and loam; as well as the shingle, pebbles, sand or mud, of every
sea-shore, alike attest the universality of this destructive agency. It is
no less clearly shown by the way in which almost every drop of running
water--whether in gutter, brooklet, stream or large river--becomes
discoloured after each heavy rainfall, since the matter which causes this
discolouration must be derived from the surface {103} of the country, must
always pass from a higher to a lower level, and must ultimately reach the
sea, unless it is first deposited in some lake, or by the overflowing of a
river goes to form an alluvial plain. The universality of this subaerial
denudation, both as regards space and time, renders it certain that its
cumulative effects must be very great; but no attempt seems to have been
made to determine the magnitude of these effects till Mr. Alfred Tylor, in
1853,[33] pointed out that by measuring the quantity of solid matter
brought down by rivers (which can be done with considerable accuracy), we
may obtain the amount of lowering of the land-area, and also the rise of
the ocean level, owing to the quantity of matter deposited on its floor. A
few years later Dr. Croll applied the same method in more detail to an
estimate of the amount by which the land is lowered in a given period; and
the validity of this method has been upheld by Sir A. Geikie, Sir Charles
Lyell, and all our best geologists, as affording a means of actually
determining with some approach to accuracy, the time occupied by one
important phase of geological change.
The quantity of matter carried away from the land by a river is greater
than at first sight appears, and is more likely to be under- than
over-estimated. By taking samples of water near the mouth of a river (but
above the influence of the tide) at a sufficient number of points in its
channel and at different depths, and repeating this daily or at other short
intervals throughout the year, it is easy to determine the quantity of
solid matter held in suspension and solution; and if corresponding
observations determine the quantity of water that is discharged, the total
amount of solid matter brought down annually may be calculated. But besides
this, a considerable quantity of sand or even gravel is carried along the
bottom or bed of the river, and this has rarely been estimated, so that the
figures hitherto obtained are usually under the real quantities. There is
also another source of error caused by the quantity of matter the river may
deposit in lakes or in flooded lands during its course, for this adds to
the amount of denudation performed by the river, although {104} the matter
so deposited does not come down to the sea. After a careful examination of
all the best records, Sir A. Geikie arrives at the following results, as to
the quantity of matter removed by seven rivers from their basins, estimated
by the number of years required to lower the whole surface an average of
one foot:
The Mississippi removes one foot in 6,000 years.
,, Ganges ,, ,, 2,358 ,,
,, Hoang Ho ,, ,, 1,464 ,,
,, Rhone ,, ,, 1,528 ,,
,, Danube ,, ,, 6,846 ,,
,, Po ,, ,, 729 ,,
,, Nith ,, ,, 4,723 ,,
Here we see an intelligible relation between the character of the river
basin and the amount of denudation. The Mississippi has a large portion of
its basin in an arid country, and its sources are either in forest-clad
plateaux or in mountains free from glaciers and with a scanty rainfall. The
Danube flows through Eastern Europe where the rainfall is considerably less
than in the west, while comparatively few of its tributaries rise among the
loftiest Alps. The proportionate amounts of denudation being then what we
might expect, and as all are probably under rather than over the truth, we
may safely take the average of them all as representing an amount of
denudation which, if not true for the whole land surface of the globe, will
certainly be so for a very considerable proportion of it. This average is
almost exactly one foot in three thousand years.[34] The mean altitude of
the several {105} continents has been recently estimated by Mr. John
Murray,[35] to be as follows: Europe 939 feet, Asia 3,189 feet, Africa 2020
feet, North America 1,888 feet, and South America 2,078 feet. At the rate
of denudation above given, it results that, were no other forces at work,
Europe would be planed down to the sea-level in about two million eight
hundred thousand years; while if we take a somewhat slower rate for North
America, that continent might last about four or five million years.[36]
This also implies that the mean height of these continents would have been
about double what it is now three million and five million years ago
respectively: and as we have no reason to suppose this to have been the
case, we are led to infer the constant action of that upheaving force which
the presence of sedimentary formations even on the highest mountains also
demonstrates.
We have already discussed the unequal rate of denudation on hills, valleys,
and lowlands, in connection with the evidence of remote glacial epochs (p.
173); what we have now to consider is, what becomes of all this denuded
matter, and how far the known rate of denudation affords us a measure of
the rate of deposition, and thus gives us some indication of the lapse of
geological time from a comparison of this rate with the observed thickness
of stratified rocks on the earth's surface.
{106}
_How to Estimate the Thickness of the Sedimentary Rocks._--The sedimentary
rocks of which the earth's crust is mainly composed consist, according to
Sir Charles Lyell's classification, of fourteen great formations, of which
the most ancient is the Laurentian, and the most recent the Post-Tertiary
or Pleistocene; with thirty important subdivisions, each of which again
consists of a more or less considerable number of distinct beds or strata.
Thus, the Silurian formation is divided into Upper and Lower Silurian, each
characterized by a distinct set of fossil remains, and the Upper Silurian
again consists of a large number of separate beds, such as the Wenlock
Limestone, the Upper Llandovery Sandstone the Lower Llandovery Slates, &c.,
each usually characterised by a difference of mineral composition or
mechanical structure, as well as by some peculiar fossils. These beds and
formations vary greatly in extent, both above and beneath the surface, and
are also of very various thicknesses in different localities. A thick bed
or series of beds often thins out in a given direction, and sometimes
disappears altogether, so that two beds which were respectively above and
beneath it may come into contact. As an example of this thinning out,
American geologists adduce the Palaeozoic formations of the Appalachian
Mountains, which have a total thickness of 42,000 feet, but as they are
traced westward thin out till they become only 4,000 feet in total
thickness. In like manner the Carboniferous grits and shales are 18,000
feet thick in Yorkshire and Lancashire, but they thin out southwards, so
that in Leicestershire they are only 3,000 feet thick; and similar
phenomena occur in all strata and in every part of the world. It must be
observed that this thinning out has nothing to do with denudation (which
acts upon the surface of a country so as to produce great irregularities of
contour), but is a regular attenuation of the layers of rock, due to a
deficiency of sediment in certain directions at the original formation of
the deposit. Owing to this thinning out of stratified rocks, they are on
the whole of far less extent than is usually supposed. When we see a
geological map showing successive formations following each other in long
irregular belts across the country (as is well {107} seen in the case of
the Secondary rocks of England), and a corresponding section showing each
bed dipping beneath its predecessor, we are apt to imagine that beneath the
uppermost bed we should find all the others following in succession like
the coats of an onion. But this is far from being the case, and a
remarkable proof of the narrow limitation of these formations has been
recently obtained by a boring at Ware through the Chalk and Gault Clay,
which latter immediately rests on the Upper Silurian Wenlock Limestone full
of characteristic fossils, at a depth of only 800 feet. Here we have an
enormous gap, showing that none of earlier Secondary or late Palaeozoic
formations extend to this part of England, unless indeed they had been all
once elevated and entirely swept away by denudation.[37]
But if we consider how such deposits are now forming, we shall find that
the thinning out of the beds of each formation, and their restriction to
irregular bands and patches, is exactly what we should expect. The enormous
quantity of sediment continually poured into the sea by rivers, gradually
subsides to the bottom as soon as the motion of the water is checked. All
the heavier material must be deposited near the shore or in those areas
over which it is first spread by the tides or currents of the ocean; while
only the very fine mud and clay is carried out to considerable distances.
Thus all stratified deposits {108} will form most quickly near the shores,
and will thin out rapidly at greater distances, little or none being formed
in the depths of the great oceans. This important fact was demonstrated by
the specimens of sea-bottom examined during the voyage of the _Challenger_,
all the "shore deposits" being usually confined within a distance of 100 or
150 miles from the coast; while the "deep-sea deposits" are either purely
organic, being formed of the calcareous or siliceous skeletons of
globigerinae, radiolarians, and diatomaceae, or are clays formed of
undissolved portions of these, together with the disintegrated or dissolved
materials of pumice and volcanic dust, which being very light are carried
by wind or by water over the widest oceans.
From the preceding considerations we shall be better able to appreciate the
calculations as to the thickness of stratified deposits made by geologists.
Professor Ramsay has calculated that the sedimentary rocks of Britain alone
have a total _maximum_ thickness of 72,600 feet; while Professor Haughton,
from a survey of the whole world, estimates the _maximum_ thickness of the
known stratified rocks at 177,200 feet. Now these _maximum_ thicknesses of
each deposit will have been produced only where the conditions were
exceptionally favourable, either in deep water near the mouths of great
rivers, or in inland seas, or in places to which the drainage of extensive
countries was conveyed by ocean currents; and this great thickness will
necessarily be accompanied by a corresponding thinness, or complete absence
of deposit, elsewhere. How far the series of rocks found in any extensive
area, as Europe or North America, represents the whole series of deposits
which have been made there we cannot tell; but there is no reason to think
that it is a very inadequate representation of their _maximum_ thickness,
though it undoubtedly is of their _extent_ and _bulk_. When we see in how
many distinct localities patches of the same formation occur, it seems
improbable that the whole of the deposits formed during any one period
should have been destroyed, even in such an area as Europe, while it is
still more improbable that they should be so destroyed over the whole
world; and {109} if any considerable portion of them is left, that portion
may give a fair idea of their average, or even of their maximum, thickness.
In his admirable paper on "The Mean Thickness of the Sedimentary
Rocks,"[38] Dr. James Croll has dwelt on the extent of denudation in
diminishing the mean thickness of the rocks that have been formed,
remarking, "Whatever the present mean thickness of all the sedimentary
rocks of our globe may be, it must be small in comparison to the mean
thickness of all the sedimentary rocks which have been formed. This is
obvious from the fact that the sedimentary rocks of one age are partly
formed from the destruction of the sedimentary rocks of former ages. From
the Laurentian age down to the present day the stratified rocks have been
undergoing constant denudation." This is perfectly true, and yet the mean
thickness of that portion of the sedimentary rocks which remains may not be
very different from that of the entire mass, because denudation acts only
on those rocks which are exposed on the surface of a country, and most
largely on those that are upheaved; while, except in the rare case of an
extensive formation being _quite horizontal_, and wholly exposed to the sea
or to the atmosphere, denudation can have no tendency to diminish the
thickness of any entire deposit.[39] Unless, therefore, a formation is
completely destroyed by denudation in every part of the world (a thing very
improbable), we may have in existing rocks a not very inadequate
representation of the _mean thickness_ of all that have been formed, and
even of the _maximum_ thickness of the larger portion. This will be the
more likely because it is almost certain that many rocks contemporaneously
formed are counted by geologists as distinct formations, whenever they
differ in lithological character or in organic remains. But we know that
limestones, sandstones, and shales, are always forming at the same time;
{110} while a great difference in organic remains may arise from
comparatively slight changes of geographical features, or from difference
in the depth or purity of the water in which the animals lived.[40]
_How to Estimate the Average Rate of Deposition of the Sedimentary
Rocks._--But if we take the estimate of Professor Haughton (177,200 feet),
which, as we have seen, is probably excessive, for the maximum thickness of
the sedimentary rocks of the globe of all known geological ages, can we
arrive at any estimate of the rate at which they were formed? Dr. Croll has
attempted to make such an estimate, but he has taken for his basis the
_mean_ thickness of the rocks, which we have no means whatever of arriving
at, and which he guesses, allowing for denudation, to be equal to the
_maximum_ thickness as measured by geologists. The land-area of the globe
is, according to Dr. Croll, 57,000,000[41] square miles, and he gives the
coast-line as 116,000 miles. This, however, is, for our purpose, rather too
much, as it allows for bays, inlets, and the smaller islands. An
approximate measurement on a globe shows that 100,000 miles will be nearer
the mark, and this has the advantage of being an easily remembered even
number. The distance from the coast, to which shore-deposits usually
extend, may be reckoned at about 100 or 150 miles, but by far the larger
portion of the matter brought down from the land will be deposited
comparatively close to the shore; that is, within twenty or thirty miles.
If we suppose the portion deposited beyond thirty miles to be added to the
deposits within that distance, and the whole reduced to a uniform thickness
in a direction at right angles to the coast, we should probably include all
areas where deposits of the maximum thickness {111} are forming at the
present time, along with a large but unknown proportion of surface where
the deposits were far below the maximum thickness. This follows, if we
consider that deposit must go on very unequally along different parts of a
coast, owing to the distance from each other of the mouths of great rivers
and the limitations of ocean currents; and because, compared with the areas
over which a thick deposit is forming annually, those where there is little
or none are probably at least twice as extensive. If, therefore, we take a
width of thirty miles along the whole coast-line of the globe as
representing the area over which deposits are forming, corresponding to the
maximum thickness as measured by geologists, we shall certainly over rather
than under-estimate the possible rate of deposit.[42]
Now a coast line of 100,000 miles with a width of 30 gives an area of
3,000,000 square miles, on which the denuded matter of the whole land-area
of 57,000,000 square {112} miles is deposited. As these two areas are as 1
to 19, it follows that deposition, as measured by _maximum_ thickness, goes
on at least nineteen times as fast as denudation--probably very much
faster. But the mean rate of denudation over the whole earth is about one
foot in three thousand years; therefore the rate of maximum deposition will
be at least 19 feet in the same time; and as the total maximum thickness of
all the stratified rocks of the globe is, according to Professor Haughton,
177,200 feet, the time required to produce this thickness of rock, at the
present rate of denudation and deposition, is only 28,000,000 years.[43]
_The Rate of Geological Change Probably Greater in very Remote Times._--The
opinion that denudation and deposition went on more rapidly in earlier
times owing to the frequent occurrence of vast convulsions and cataclysms
was strenuously opposed by Sir Charles Lyell, who so well showed that
causes of the very same nature as those now in action were sufficient to
account for all the phenomena presented by the rocks throughout the whole
series of geological formations. But while upholding the soundness of the
views of the "uniformitarians" as opposed to the "convulsionists," we must
yet admit that there is reason for believing in a gradually increasing
intensity of all telluric action as we go back into past time. This subject
has been well treated by Mr. W. J. Sollas,[44] who shows that, if, as all
physicists maintain, the sun gave out perceptibly more heat in past ages
than now, this alone would cause an increase in almost all the forces that
have brought about geological phenomena. With greater heat there would be a
more extensive aqueous atmosphere, and, perhaps, a greater difference
between equatorial and polar temperatures; hence more violent winds,
heavier rains and snows, {113} and more powerful oceanic currents, all
producing more rapid denudation. At the same time, the internal heat of the
earth being greater, it would be cooling more rapidly, and thus the forces
of contraction--which cause the upheaving of mountains, the eruption of
volcanoes, and the subsidence of extensive areas--would be more powerful
and would still further aid the process of denudation. Yet again, the
earth's rotation was certainly more rapid in very remote times, and this
would cause more impetuous tides and still further add to the denuding
power of the ocean. It thus appears that, as we go back into the past,
_all_ the forces tending to the continued destruction and renewal of the
earth's surface would be in more powerful action, and must therefore tend
to reduce the time required for the deposition and upheaval of the various
geological formations. It may be true, as many geologists assert, that the
changes here indicated are so slow that they would produce comparatively
little effect within the time occupied by the known sedimentary rocks, yet,
whatever effect they did produce would certainly be in the direction here
indicated, and as several causes are acting together, their combined
effects may have been by no means unimportant. It must also be remembered
that such an increase of the primary forces on which all geologic change
depends would act with great effect in still further intensifying those
alternations of cold and warm periods in each hemisphere, or, more
frequently, of excessive and equable seasons, which have been shown to be
the result of astronomical, combined with geographical, revolutions; and
this would again increase the rapidity of denudation and deposition, and
thus still further reduce the time required for the production of the known
sedimentary rocks. It is evident therefore that these various
considerations all combine to prove that, in supposing that the rate of
denudation has been on the average only what it is now, we are almost
certainly over-estimating the _time_ required to have produced the whole
series of formations from the Cambrian upwards.
_Value of the Preceding Estimate of Geological Time._--It is not of course
supposed that the calculation here given {114} makes any approach to
accuracy, but it is believed that it does indicate the _order_ of magnitude
of the time required. We have a certain number of data, which are not
guessed but the result of actual measurement; such are, the amount of solid
matter carried down by rivers, the width of the belt within which this
matter is mainly deposited, and the maximum thickness of the known
stratified rocks.[45] A considerable but unknown amount of denudation is
effected by the waves of the ocean eating away coast lines. This was once
thought to be of more importance than sub-aerial denudation, but it is now
believed to be comparatively slow in its action.[46] Whatever it may be,
however, it adds to the rate of formation of new strata, and its omission
from the calculation is again on the side of making the lapse of time
greater rather than less than the true amount. Even if a considerable
modification should be needed in some of the assumptions it has been
necessary to make, the result must still show that, so far as the time
required for the formation of the known stratified rocks, the hundred
million years allowed by physicists is not only ample, but will permit of
even more than an equal period anterior to the lowest Cambrian rocks, as
demanded by Mr. Darwin--a demand supported and enforced by the arguments,
taken from independent standpoints, of Professor Huxley and Professor
Ramsay.
_Organic Modification Dependent on Change of Conditions._--Having {115}
thus shown that the physical changes of the earth's surface may have gone
on much more rapidly and occupied much less time than has generally been
supposed, we have now to inquire whether there are any considerations which
lead to the conclusion that organic changes may have gone on with
corresponding rapidity.
There is no part of the theory of natural selection which is more clear and
satisfactory than that which connects changes of specific forms with
changes of external conditions or environment. If the external world
remains for a moderate period unchanged, the organic world soon reaches a
state of equilibrium through the struggle for existence; each species
occupies its place in nature, and there is then no inherent tendency to
change. But almost any change whatever in the external world disturbs this
equilibrium, and may set in motion a whole series of organic revolutions
before it is restored. A change of climate in any direction will be sure to
injure some and benefit other species. The one will consequently diminish,
the other increase in number; and the former may even become extinct. But
the extinction of a species will certainly affect other species which it
either preyed upon, or competed with, or served for food; while the
increase of any one animal may soon lead to the extinction of some other to
which it was inimical. These changes will in their turn bring other
changes; and before an equilibrium is again established, the proportions,
ranges, and numbers, of the species inhabiting the country may be
materially altered. The complex manner in which animals are related to each
other is well exhibited by the importance of insects, which in many parts
of the world limit the numbers or determine the very existence of some of
the higher animals. Mr. Darwin says:--"Perhaps Paraguay offers the most
curious instance of this; for here neither cattle, nor horses, nor dogs
have ever run wild, though they swarm southward and northward in a wild
state; and Azara and Rengger have shown that this is caused by the greater
number in Paraguay of a certain fly, which lays its eggs in the navels of
these animals when first born. The increase of these flies, numerous as
they are, must be {116} habitually checked by some means, probably by other
parasitic insects. Hence, if certain insectivorous birds were to decrease
in Paraguay, the parasitic insects would probably increase; and this would
lessen the number of navel-frequenting flies--then cattle and horses would
run wild; and this would certainly alter (as indeed I have observed in
parts of South America) the vegetation: this again would largely affect the
insects, and this, as we have seen in Staffordshire, the insectivorous
birds, and so onwards in ever increasing circles of complexity."
Geographical changes would be still more important, and it is almost
impossible to exaggerate the modifications of the organic world that might
result from them. A subsidence of land separating a large island from a
continent would affect the animals and plants in a variety of ways. It
would at once modify the climate, and so produce a series of changes from
this cause alone; but more important would be its effect by isolating small
groups of individuals of many species and thus altering their relations to
the rest of the organic world. Many of these would at once be exterminated,
while others, being relieved from competition, might flourish and become
modified into new species. Even more striking would be the effects when two
continents, or any two land areas which had been long separated, were
united by an upheaval of the strait which divided them. Numbers of animals
would now be brought into competition for the first time. New enemies and
new competitors would appear in every part of the country; and a struggle
would commence which, after many fluctuations, would certainly result in
the extinction of some species, the modification of others, and a
considerable alteration in the proportionate numbers and the geographical
distribution of almost all.
Any other changes which led to the intermingling of species whose ranges
were usually separate would produce corresponding results. Thus, increased
severity of winter or summer temperature, causing southward migrations and
the crowding together of the productions of distinct regions, must
inevitably produce a struggle for existence, which would lead to many
changes both in the characters and {117} the distribution of animals. Slow
elevations of the land would produce another set of changes, by affording
an extended area in which the more dominant species might increase their
numbers; and by a greater range and variety of alpine climates and mountain
stations, affording room for the development of new forms of life.
_Geographical Mutations as a Motive Power in Bringing about Organic
Changes._--Now, if we consider the various geographical changes which, as
we have seen, there is good reason to believe have ever been going on in
the world, we shall find that the motive power to initiate and urge on
organic changes has never been wanting. In the first place, every
continent, though permanent in a general sense, has been ever subject to
innumerable physical and geographical modifications. At one time the total
area has increased, and at another has diminished; great plateaus have
gradually risen up, and have been eaten out by denudation into mountain and
valley; volcanoes have burst forth, and, after accumulating vast masses of
eruptive matter, have sunk down beneath the ocean, to be covered up with
sedimentary rocks, and at a subsequent period again raised above the
surface; and the _loci_ of all these grand revolutions of the earth's
surface have changed their position age after age, so that each portion of
every continent has again and again been sunk under the ocean waves, formed
the bed of some inland sea, or risen high into plateaus and mountain
ranges. How great must have been the effects of such changes on every form
of organic life! And it is to such as these we may perhaps trace those
great changes of the animal world which have seemed to revolutionise it,
and have led us to class one geological period as the age of reptiles,
another as the age of fishes, and a third as the age of mammals.
But such changes as these must necessarily have led to repeated unions and
separations of the land masses of the globe, joining together continents
which were before divided, and breaking up others into great islands or
extensive archipelagoes. Such alterations of the means of transit would
probably affect the organic world even more profoundly than the changes of
area, of altitude, or {118} of climate, since they afforded the means, at
long intervals, of bringing the most diverse forms into competition, and of
spreading all the great animal and vegetable types widely over the globe.
But the isolation of considerable masses of land for long periods also
afforded the means of preservation to many of the lower types, which thus
had time to become modified into a variety of distinct forms, some of which
became so well adapted to special modes of life that they have continued to
exist to the present day, thus affording us examples of the life of early
ages which would probably long since have become extinct had they been
always subject to the competition of the more highly organised animals. As
examples of such excessively archaic forms, we may mention the mud-fishes
and the ganoids, confined to limited fresh-water areas; the frogs and
toads, which still maintain themselves vigorously in competition with
higher forms; and among mammals the Ornithorhynchus and Echidna of
Australia; the whole order of Marsupials--which, out of Australia, where
they are quite free from competition, only exist abundantly in South
America, which was certainly long isolated from the northern continents;
the Insectivora, which, though widely scattered, are generally nocturnal or
subterranean in their habits; and the Lemurs, which are most abundant in
Madagascar, where they have long been isolated, and almost removed from the
competition of higher forms.
_Climatal Revolutions as an Agent in Producing Organic Changes._--The
geographical and geological changes we have been considering are probably
those which have been most effective in bringing about the great features
of the distribution of animals, as well as the larger movements in the
development of organised beings; but it is to the alternations of warm and
cold, or of uniform and excessive climates--of almost perpetual spring in
arctic as well as in temperate lands, with occasional phases of cold
culminating at remote intervals in glacial epochs,--that we must impute
some of the more remarkable changes both in the specific characters and in
the distribution of organisms.[47] {119} Although the geological evidence
is opposed to the belief in early glacial epochs except at very remote and
distant intervals, there is nothing which contradicts the occurrence of
repeated changes of climate, which, though too small in amount to produce
any well-marked physical or organic change, would yet be amply sufficient
to keep the organic world in a constant state of movement, and which, by
subjecting the whole flora and fauna of a country at comparatively short
intervals to decided changes of physical conditions, would supply that
stimulus and motive power which, as we have seen, is all that is necessary
to keep the processes of "natural selection" in constant operation.
The frequent recurrence of periods of high and of low excentricity must
certainly have produced changes of climate of considerable importance to
the life of animals and plants. During periods of high excentricity with
summer in _perihelion_, that season would be certainly very much hotter,
while the winters would be longer and colder than at present; and although
geographical conditions might prevent any permanent increase of snow and
ice even in the extreme north, yet we cannot doubt that the whole northern
hemisphere would then have a very different climate than when the changing
phase of precession brought a very cool summer and a very mild winter--a
perpetual spring, in fact. Now, such a change of climate would certainly be
calculated to bring about a considerable change of _species_, both by
modification and migration, without any such decided change of _type_
either in the vegetation or the animals that we could say from their fossil
remains that any change of climate had taken place. Let us suppose, for
instance, that the climate of England and that of Canada were to be
mutually exchanged, and that the change took five or six thousand years to
bring about, it cannot be doubted that considerable modifications in the
fauna and flora of both countries would be the result, although it is
impossible to predict {120} what the precise changes would be. We can
safely say, however, that some species would stand the change better than
others, while it is highly probable that some would be actually benefited
by it, and that others would be injured. But the benefited would certainly
increase, and the injured decrease, in consequence, and thus a series of
changes would be initiated that might lead to most important results.
Again, we are sure that some species would become modified in adaptation to
the change of climate more readily than others, and these modified species
would therefore increase at the expense of others not so readily modified;
and hence would arise on the one hand extinction of species, and on the
other the production of new forms.
But this is the very least amount of change of climate that would certainly
occur every 10,500 years when there was a high excentricity, for it is
impossible to doubt that a varying distance of the sun in summer from 86 to
99 millions of miles (which is what occurred during--as supposed--the
Miocene period, 850,000 years ago) would produce an important difference in
the summer temperature and in the actinic influence of sunshine on
vegetation. For the intensity of the sun's rays would vary as the square of
the distance, or nearly as 74 to 98, so that the earth would be actually
receiving one-fourth less sun-heat during summer at one time than at the
other. An equally high excentricity occurred 2,500,000 years back, and no
doubt was often reached during still earlier epochs, while a lower but
still very high excentricity has frequently prevailed, and is probably near
its average value. Changes of climate, therefore, every 10,500 years, of
the character above indicated and of varying intensity, have been the rule
rather than the exception in past time; and these changes must have been
variously modified by changing geographical conditions so as to produce
climatic alterations in different directions, giving to the ancient lands
either dry or wet seasons, storms or calms, equable or excessive
temperatures, in a variety of combinations of which the earth perhaps
affords no example under the present low phase of {121} excentricity and
consequent slight inequality of sun-heat.
_Present Condition of the Earth One of Exceptional Stability as Regards
Climate._--It will be seen, by a reference to the diagram at page 171, that
during the last three million years the excentricity has been _less_ than
it is now on eight occasions, for short periods only, making up a total of
about 280,000 years; while it has been _more_ than it is now for many long
periods, of from 300,000 to 700,000 years each, making a total of 2,720,000
years; or nearly as 10 to 1. For nearly half the entire period, or
1,400,000 years, the excentricity has been nearly double what it is now,
and this is not far from its mean condition. We have no reason for
supposing that this long period of three million years, for which we have
tables, was in any way exceptional as regards the degree or variation of
excentricity; but, on the contrary, we may pretty safely assume that its
variations during this time fairly represent its average state of increase
and decrease during all known geological time. But when the glacial epoch
ended, 72,000 years ago, the excentricity was about double its present
amount; it then rapidly decreased till, at 60,000 years back, it was very
little greater than it is now, and since then it has been uniformly small.
It follows that, for about 60,000 years before our time, the mutations of
climate every 10,500 years have been comparatively unimportant, and that
the temperate zones have enjoyed _an exceptional stability of climate_.
During this time those powerful causes of organic change which depend on
considerable changes of climate and the consequent modifications,
migrations, and extinctions of species, will not have been at work; the
slight changes that did occur would probably be so slow and so little
marked that the various species would be able to adapt themselves to them
without much disturbance; and the result would be _an epoch of exceptional
stability of species_.
But it is from this very period of _exceptional stability_ that we obtain
our only _scale_ for measuring the rate of organic change. It includes not
only the historical period, {122} but that of the Swiss Lake dwellings, the
Danish shell-mounds, our peat-bogs, our sunken forests, and many of our
superficial alluvial deposits--the whole in fact, of the iron, bronze, and
neolithic ages. Even some portion of the palaeolithic age, and of the more
recent gravels and cave-earths may come into the same general period if
they were formed when the glacial epoch was passing away. Now throughout
all these ages we find no indication of change of species, and but little,
comparatively, of migration. We thus get an erroneous idea of _the
permanence and stability of specific forms_, due to the period immediately
antecedent to our own being a _period of exceptional permanence and
stability_ as regards climatic and geographical conditions.[48]
_Date of Last Glacial Epoch and its Bearing on the Measurement of
Geological Time._--Directly we go back from this stable period we come upon
changes both in the forms and in the distribution of species; and when we
pass beyond the last glacial epoch into the Pliocene period we find
ourselves in a comparatively new world, surrounded by a considerable number
of species altogether different from any which now exist, together with
many others which, though still living, now inhabit distant regions. It
seems not improbable that what is termed the Pliocene period, was really
the coming on of the glacial epoch, and this is the opinion of Professor
Jules Marcou.[49] According to our views, a considerable amount of
geographical change must have occurred at the change from the Miocene to
the Pliocene, favouring the refrigeration of the northern hemisphere, and
leading, in the way already pointed out, to the glacial epoch whenever a
high degree of excentricity {123} prevailed. As many reasons combine to
make us fix the height of the glacial epoch at the period of high
excentricity which occurred 200,000 years back, and as the Pliocene period
was probably not of long duration, we must suppose the next great phase of
very high excentricity (850,000 years ago) to fall within the Miocene
epoch. Dr. Croll believes that this must have produced a glacial period,
but we have shown strong reasons for believing that, in concurrence with
favourable geographical conditions, it led to uninterrupted warm climates
in the temperate and northern zones. This, however, did not prevent the
occurrence of local glaciation wherever other conditions led to its
initiation, and the most powerful of such conditions is a great extent of
high land. Now we know that the Alps acquired a considerable part of their
elevation during the latter part of the Miocene period, since Miocene rocks
occur at an elevation of over 6,000 feet, while Eocene beds occur at nearly
10,000 feet. But since that time there has been a vast amount of
denudation, so that these rocks may have been at first raised much higher
than we now find them, and thus a considerable portion of the Alps may have
been more elevated than they are now. This would certainly lead to an
enormous accumulation of snow, which would be increased when the
excentricity reached a maximum, as already fully explained, and may then
have caused glaciers to descend into the adjacent sea, carrying those
enormous masses of rock which are buried in the Upper Miocene of the
Superga in Northern Italy. An earlier epoch of great altitude in the Alps
coinciding with the very high excentricity 2,500,000 years ago, may have
caused the local glaciation of the Middle Eocene period when the enormous
erratics of the Flysch conglomerate were deposited in the inland seas of
Northern Switzerland, the Carpathians, and the Apennines. This is quite in
harmony with the indications of an uninterrupted warm climate and rich
vegetation during the very same period in the adjacent low countries, just
as we find at the present day in New Zealand a delightful climate and a
rich vegetation of Metrosideros, {124} fuchsias and tree-ferns on the very
borders of huge glaciers, descending to within 700 feet of the sea-level.
It is not pretended that these estimates of geological time have any more
value than probable guesses; but it is certainly a curious coincidence that
two remarkable periods of high excentricity should have occurred, at such
periods and at such intervals apart, as very well accord with the
comparative remoteness of the two deposits in which undoubted signs of
ice-action have been found, and that both these are localised in the
vicinity of mountains which are known to have acquired a considerable
elevation at about the same period of time.
In the tenth edition of the _Principles of Geology_, Sir Charles Lyell,
taking the amount of change in the species of mollusca as a guide,
estimated the time elapsed since the commencement of the Miocene as
one-third that of the whole Tertiary epoch, and the latter at one-fourth
that of geological time since the Cambrian period. Professor Dana, on the
other hand, estimates the Tertiary as only one-fifteenth of the Mesozoic
and Palaeozoic combined. On the estimate above given, founded on the dates
of phases of high excentricity, we shall arrive at about four million years
for the Tertiary epoch, and sixteen million years for the time elapsed
since the Cambrian, according to Lyell, or sixty millions according to
Dana. The estimate arrived at from the rate of denudation and deposition
(twenty-eight million years) is nearly midway between these, and it is, at
all events, satisfactory that the various measures result in figures of the
same order of magnitude, which is all one can expect when discussing so
difficult and exceedingly speculative a subject.
The only value of such estimates is to define our notions of geological
time, and to show that the enormous periods, of hundreds of millions of
years, which have sometimes been indicated by geologists, are neither
necessary nor warranted by the facts at our command; while the present
result places us more in harmony with the calculations of physicists, by
leaving a very wide margin between geological time as defined by the
fossiliferous rocks, and that {125} far more extensive period which
includes all possibility of life upon the earth.
_Concluding Remarks._--In the present chapter I have endeavoured to show
that, combining the measured rate of denudation with the estimated
thickness and probable extent of the known series of sedimentary rocks, we
may arrive at a rude estimate of the time occupied in the formation of
those rocks. From another point of departure--that of the probable date of
the Miocene period, as determined by the epoch of high excentricity
supposed to have aided in the production of the Alpine glaciation during
that period, and taking the estimate of geologists as to the proportionate
amount of change in the animal world since that epoch--we obtain another
estimate of the duration of geological time, which, though founded on far
less secure data, agrees pretty nearly with the former estimate. The time
thus arrived at is immensely less than the usual estimates of geologists,
and is so far within the limits of the duration of the earth as calculated
by Sir William Thomson, as to allow for the development of the lower
organisms an amount of time anterior to the Cambrian period several times
greater than has elapsed between that period and the present day. I have
further shown that, in the continued mutations of climate produced by high
excentricity and opposite phases of precession, even though these did not
lead to glacial epochs, we have a motive power well calculated to produce
far more rapid organic changes than have hitherto been thought possible;
while in the enormous amount of specific variation (as demonstrated in an
earlier chapter), we have ample material for that power to act upon, so as
to keep the organic world in a state of rapid change and development
proportioned to the comparatively rapid changes in the earth's surface.