THE GEOGRAPHICAL DEVELOPMENT OF THE AMERICAS IN CENOZOIC TIMES
I. TERTIARY PERIOD
In the interior regions of western North America the transition from the
Mesozoic to the Cenozoic was so gradual that there is great difficulty
in drawing the line between them and therefore, as might be expected,
there is much difference of opinion as to just where that line should be
drawn. From one point of view, the matter is of no great consequence;
but from another, it is of the utmost importance, for, unless the events
in different continents can be approximately synchronized, it will often
prove a hopeless undertaking to trace the course of migration of the
various mammalian groups and determine their place of origin and primary
home. Until a definitive answer can be given to the question as to when
the Cenozoic era began, many significant points must be left in doubt,
and much remains to be done in the geology of the Far West before that
definitive solution can be reached.
1. _Paleocene Epoch_
So far as North America is concerned, the best available evidence points
to the conclusion that we should regard the Fort Union, Puerco and
Torrejon as the most ancient of the Cenozoic formations (see Table, p.
17), though retaining so many features of Mesozoic life that a separate
division of the Tertiary, the Paleocene epoch, is made for them. Such
a separation is not the common practice in this country, where it is
more usual to employ the terms “Lowest” or “Basal” Eocene. In my
judgment, however, the balance of advantage is in favour of giving to
this so-called Basal Eocene a rank equivalent to that of the four other
universally recognized and admitted epochs of the Tertiary period. No
marine rocks of Paleocene date have yet been found in North America,
which indicates that the continent was at least as extensive as it is
now. The very scanty development of deposits representing this epoch
in Europe renders the comparison with the fossils of the Old World
unsatisfactory and hence leads to uncertainty, when it is attempted to
determine the land-connections of the time. During the Mesozoic era the
shallow Bering Sea had repeatedly been elevated into a land joining North
America with Asia and had as often been depressed, so as to separate
the continents and allow the waters of the Arctic Ocean to mingle with
those of the Pacific. A like alternation of junction and separation went
on during the Tertiary and Quaternary periods and, by a comparison of
the fossil mammals of Europe and America for any particular division of
geological time, it is almost always feasible to say whether the two
continents were connected, or altogether separated. This statement does
not imply that the proportion of common elements in the two faunas during
epochs of continental connection was a constant one at all times, for
that was by no means true. Mere land-connections or separations are not
the only factors which limit the spread of terrestrial animals; if they
were, the community of forms between North and South America would be
much greater than it actually is. Climatic barriers are of almost equal
importance in determining animal distribution, and changes of climate may
greatly alter the conditions of migration between connected continents.
As the connections between North America and the Old World were probably
in high latitudes, where the seas are narrow, changes of climate produced
a greater effect upon migration than they could have done had the
land-bridges been in the tropical or warm temperate zones. That these
vicissitudes of climate really did occur and are not mere guesses to
bolster up a tottering hypothesis, there is abundant evidence to prove.
In the Paleocene, or most ancient epoch of the Tertiary period, the
geographical condition of North America was approximately as follows: The
continent had attained nearly its modern outlines and on the Atlantic and
Pacific coasts probably extended farther seaward than it does to-day.
Florida, however, and perhaps a narrow strip of the northern Gulf coast
were still submerged, the Gulf of Mexico opening broadly into the
Atlantic. It is very probable that the continent was connected with the
Old World by a land occupying the site of Bering Sea and perhaps also
by way of Greenland and the North Atlantic; and there is some evidence,
though not altogether convincing, that it was also joined to South
America. The great mountain ranges were largely what they now are, though
subsequent upheavals greatly modified the Rocky Mountains, Sierra Nevada
and the ranges of the Pacific coast, while the lofty St. Elias Alps
of Alaska were not in existence. The region of high plateaus, between
the Rockies and the Sierras, was much less elevated than it is now.
The Appalachians, which were of far more ancient date than the western
ranges, had been worn down by ages of weathering and stream-erosion into
a low-lying, almost featureless plain, with some scattered peaks rising
from it here and there, of which the mountains of western North Carolina
were the highest. In general, it may be said that while the _average_
height of the continent above the sea-level may have been as great or
greater than at present, yet the inequalities of surface appear to have
been less marked, and both along the Atlantic coast and in the interior
were vast stretches of plains.
The Paleocene formations of the western interior are of non-marine or
continental origin. In northwestern New Mexico is the typical area
of the _Puerco_ and _Torrejon_, a series of beds 800 to 1000 feet in
thickness and for the most part quite barren of fossils, but there are
two horizons, one near the top and the other near the bottom of the
series, which have yielded a very considerable number of fossil mammals,
and of these the lower is the Puerco, the upper the Torrejon. The _Fort
Union_ is quite different in character and is composed of great areas
of sandstone and clay rocks, with a maximum thickness of 2000 feet,
in eastern Wyoming, South Dakota, Montana and the adjoining parts of
Canada. The modes of formation of these beds have not yet been fully
determined; that they may have been partly laid down in shallow lakes is
indicated by the masses of fresh-water shells in certain localities. In
others are preserved multitudes of leaves, which have given a very full
conception of the plants of the time, and great swamps and bogs have left
the traces of their presence in beds of lignite, or imperfectly formed
coal. Deposits made on the flood-plains of rivers and wind accumulations
are probably also represented. “Vast stretches of subtropical and more
hardy trees were interspersed with swamps where the vegetation was rank
and accumulated rapidly enough to form great beds of lignite. Here were
bogs in which bog iron was formed. Amid the glades of these forests
there wandered swamp turtles, alligators, and large lizards of the
characteristic genus _Champsosaurus_” (Osborn, p. 100).
Fort Union mammals are relatively rare and most of those that have been
found are very fragmentary; they are amply sufficient, however, to
demonstrate the Paleocene date of the beds and to make it probable that
they include both the Puerco and the Torrejon faunas.
The climate, as shown by the plants, was much milder and more uniform
than that of the Recent epoch, though some indication of climatic zones
may already be noted. The vegetation was essentially modern in character;
nearly all our modern types of forest-trees, such as willows, poplars,
sycamores, oaks, elms, maples, walnuts and many others, were abundantly
represented in the vast forests which would seem to have covered nearly
the entire continent from ocean to ocean and extended north into Alaska
and Greenland, where no such vegetation is possible under present
conditions. Numerous conifers were mingled with the deciduous trees,
but we do not find exclusively coniferous forests. Palms, though not
extending into Greenland, flourished magnificently far to the north of
their present range. On the other hand, the Paleocene flora of England
points to a merely temperate climate, while that of the succeeding Eocene
was subtropical.
_South America._—Nothing is definitely known concerning the condition of
Central America and the West Indies and very little as to South America.
As no marine rocks of Paleocene date have been found in any of these
regions, it may be inferred that all the existing land areas were then
above the sea, and there is some evidence that South America was much
more extended in certain directions than now. From the character and
distribution of modern plants, fresh-water fishes, land and fresh-water
shells, there is strong reason to believe that in late Mesozoic times a
land-bridge connected Brazil with equatorial Africa and this connection
may have continued into the Paleocene, though it is only fair to observe
that some highly competent authorities deny the reality of this bridge.
There is also evidence, though incomplete, of a connection between South
America and Australia by way of the Antarctic continent, and it is
clear that that polar region could not have had the rigorous climate of
the present time. In the upper part of the Cretaceous, the last of the
Mesozoic periods, there was a possibility of migration, however indirect,
between every continent and every other, for the huge land reptiles
called Dinosaurs have been found in the non-marine Cretaceous rocks of
every continent, which could not have been the case, had any of the great
land areas been isolated. There is no known reason to assume that the
land-bridges were essentially different in the Paleocene.
2. _Eocene Epoch_
_North America._—The Eocene witnessed quite extensive geographical
changes, though but little is known of it in Central or South America, or
the West Indies. Along the Atlantic and Gulf coasts of the United States
there was an extensive submergence of the coastal plain, the sea covering
the southern half of New Jersey and extending thence to the southwestward
in an ever broadening band, through the South Atlantic and Gulf states.
Northern Florida was under water and the Gulf extended as a narrow sound,
known as the “Mississippi Embayment,” up the valley of that river to
southern Illinois and westward into Texas. The Embayment was present
in the Cretaceous and again in the Eocene, but it is not known whether
it persisted through the Paleocene; probably it did not, as the whole
Atlantic coast region appears to have stood at a higher level then than
now. While the condition of Mexico and Central America during the Eocene
is not known in any save the vaguest manner, it is evident that there
was then a broad communication between the Atlantic and the Pacific,
completely severing North and South America, though the place of this
transverse sea has not been fixed. On the Pacific side, a long, narrow
arm of the sea occupied what is now the great valley of California,
extending north into Oregon and Washington. It will be noted that in
North America the Eocene sea was almost confined to the neighbourhood of
the present coast-lines, nowhere penetrating very far inland, except in
the Mississippi Embayment, and thus differing widely from the condition
of Europe at that epoch, where much of what is now land was submerged.
The greatly expanded Mediterranean covered most of southern Europe, where
the great mountain ranges, the Pyrenees, Alps, etc., had not yet been
formed. Very important, from the point of view of American geography,
is the fact that Europe was completely separated from Asia by a narrow
strait or sea, which ran down the eastern side of the Ural Mountains
from the Arctic Ocean and joined the enlarged Mediterranean. During the
existence of this Ural Sea any land connection of North America with
Europe must necessarily have been by means of a North Atlantic bridge,
or by one across the Arctic Sea, since communication with Asia by way of
Alaska would not have reached eastern Europe.
[Illustration: FIG. 48.—Map of North America during the Eocene epoch.
The present limits of the continent are shown in outline; white areas =
land; horizontal lines = sea; dotted areas = non-marine deposits; black
circles with white dots = active volcanoes. (After Schuchert.)]
Any such general statement of geographical conditions during the Eocene
as the foregoing sketch, cannot but be to some extent misleading, because
it brings together, as contemporary, arrangements which were, in some
cases at least, separated by considerable intervals of time and which
were subject to continual change. Along nearly all coasts the position of
the sea was quite different in the latter part of the epoch from what it
had been in the earlier portion. On the north side of the Gulf of Mexico,
for example, the sea retreated from time to time, and the successive
divisions of the Eocene rocks are so arranged that the later ones are
farther to the south. Limitations of space, however, forbid the attempt
to follow out these minor changes.
[Illustration: FIG. 49.—Bad Lands of the lower Eocene. Wasatch stage.
Big Horn Basin, Wyo. (Photograph by Sinclair.)]
In the western interior are found extensive non-marine or continental
deposits of Eocene date, which must be considered more in detail, because
of the highly important bearing which they have upon mammalian history.
With the exception of a few small areas in Colorado, these deposits are
all situated in the plateau region west of the Rocky Mountains, and were
made of the débris of older rocks washed down by rain and rivers and
deposited in broad basins. Some of them are the sediments of shallow or
temporary lakes, and one series, at least, is made up of volcanic ash and
dust showered upon the land, or into water of no great depth. The oldest
of these Eocene stages, known as the _Wasatch_ (see Table, p. 17) covers
a very large region, though in a discontinuous manner; the principal area
begins in New Mexico, where it lies over the Torrejon, of the Paleocene,
and extends far to the north through western Colorado and eastern Utah
to the Uinta Mountains, around the eastern end of which it passes in a
narrow band and then expands again over southwestern Wyoming. A second
area is in the Big Horn Basin of northwestern Wyoming and southern
Montana, and probably two small areas in southern Colorado are of the
same date. The Wasatch beds are richly fossiliferous and have yielded a
most interesting and important series of mammals, which were far more
advanced than those of the Paleocene; and, at first sight, the student
is tempted to believe that they must be of very much later date. A more
critical examination shows that this appearance of a great lapse of time
between the Paleocene and the Wasatch is deceptive; the more advanced and
characteristic of the Wasatch mammals were obviously not the descendants
of ancestors in the North American Paleocene, but were altogether
newcomers to this continent, immigrants from some region which cannot
yet be identified. On the other hand, a considerable number of the old,
indigenous types still persisted, and these, when compared with their
Paleocene ancestors, are found not to have changed so much as to require
a very great length of time, geologically speaking, for the degree of
development involved. This is the earliest recorded one of the great
waves of mammalian migration which invaded North America down almost to
our own time.
The same wave of migration extended to Europe, and that there was a broad
and easy way of communication between that continent and North America
is plain, for the similarity between the Wasatch mammals and those of
the corresponding formation in France, the _Sparnacian_, is remarkably
close. At no subsequent time were the mammalian faunas of North America
and Europe so nearly identical as during the Wasatch-Sparnacian age,
which is especially remarkable when the discrepancy is noted between the
vast stretches of the Wasatch (150,000 square miles) and the very limited
areas in France.
If, as is probable, the Ural Sea was in existence at that time, the
land-connection with Europe must have been across the North Atlantic,
most likely from Greenland eastward. At the present time a land-bridge
in such high latitudes would be of little service in bringing about a
similarity of mammals in the two continents, for the severity of the
Arctic climate would be as effective a barrier against the intermigration
of all save the Arctic mammals as the ocean itself; but in the mild and
genial Eocene climate the latitude of the bridge was of small consequence.
The second of the Eocene stages, the _Wind River—Green River_, is found
in two very different phases. The Wind River phase occupies the basin
of that stream, north of the Wind River Mountains in central Wyoming,
and in the Big Horn Basin of the same state it very extensively overlies
the Wasatch, and in this phase the sediments are very like those of the
latter, flood-plain and wind accumulations. A widely distant area of
this stage occurs in the Huerfano Cañon in Colorado. The Wind River beds
contain numerous mammals which were clearly sequential to those of the
Wasatch, of which they were the more or less modified descendants. With
two possible exceptions, there were no new immigrants and the connection
with the Old World may have been already severed, as it assuredly was in
the succeeding age, the Bridger, though divergent development had not yet
had time to produce the very striking differences in the mammals of North
America from those of Europe, which characterized the Bridger.
The Green River phase is a thick body of finely laminated “paper shales,”
which seem to have been deposited in a very shallow lake and occupy
some 5000 square miles of the Green River valley in southern Wyoming
and northern Utah, where they overlie the Wasatch, just as do the Wind
River beds in the Big Horn Basin. These fine-grained and thinly laminated
shales have preserved, often in beautiful perfection, countless remains
of plants, insects and fishes, but no traces of mammals, other than
footprints, have been found.
The third of the Eocene stages of the interior is the _Bridger_ of
southern Wyoming and northeastern Utah, where it lies upon the Green
River shales, but overlaps these shales both eastward and westward,
extending out upon the Wasatch. The Bridger beds are largely made up
of volcanic ash and dust deposited partly upon the land and partly in
shallow or temporary lakes. The frequency with which the remains of
fishes, crocodiles and fresh-water shells are found indicates deposition
in water, and the large crystals of gypsum which are abundant in certain
localities show that the water became salt, at least occasionally. From
the immense mass of volcanic débris, it is evident that volcanic activity
broke out at this time on a much greater scale than had been known in
that region since the Cretaceous period. Two different horizons, or
substages, are distinguishable in the Bridger, lower and upper, each of
which has its distinct mammalian fauna, though the two are very closely
allied. Their difference from the contemporary mammals of Europe is very
great, hardly any genera being common to the two continents. So striking
a difference indubitably points to a severance of the land-connection,
a severance which, as was shown above, probably took place during the
Wind River stage, for its effects would not be immediately apparent; time
would be required for the operation of divergent evolution, the fauna of
each continent developing along its own lines, to make itself so strongly
felt. Had the connection never been renewed, North America, on the one
hand, and Eurasia on the other, would to-day be utterly different from
the zoölogical point of view, instead of containing, as they do, a great
many identical or closely similar animals of all classes, a likeness due
to subsequent migrations.
The fourth and last of the stages referred to the Eocene is the _Uinta_,
the geological position of which is the subject of much debate; almost
as good reasons can be brought forward for placing it in the Oligocene
as in the Eocene, so nearly is it on the boundary line between those two
epochs. The Uinta is found in the Green River valley of northeastern
Utah and northwestern Colorado, where it lies upon the upper Bridger
and is the latest of the important Tertiary formations to be found in
the plateau region west of the Rocky Mountains. It is probable that the
separation of North America from the Old World still continued, for, as a
whole, the Uinta fauna is totally different from that of the upper Eocene
of Europe. There were, however, a few doubtful forms, which may prove to
be the outposts of a renewed invasion.
The Eocene climate was decidedly warmer than the present one, and
subtropical conditions extended over the whole United States and perhaps
far into Canada. On the other hand, signs of increasing aridity in
the western part of the continent are not wanting, and that must have
resulted in a great shrinkage of the forests and increase of the open
plains. The vegetation was essentially the same as in the Paleocene, when
it had already attained a modern character, the differences from the
present being chiefly in regard to geographical distribution. Large palms
were then flourishing in Wyoming and Idaho, and another indication of a
warm climate is furnished by the large crocodiles which abounded in all
of the Eocene stages.
So far as North America was concerned, the Eocene epoch was brought to
a close by extensive movements of the earth’s crust, which more or less
affected the entire continent and were registered both on the sea-coasts
and in the mountain ranges of the interior. Upheaval added a narrow belt
of land along the Atlantic and Gulf coasts and the Mississippi Embayment
was nearly closed. On the Pacific side the sea withdrew from the great
valley of California and Oregon, and in the interior the plateau region
was elevated by a great disturbance, which also increased the height of
the western mountains.
Our knowledge of Eocene land-mammals in North America is almost wholly
derived from the formations of the western United States, but it may
be inferred from the uniform climatic conditions that there were no
very great geographical differences among the animals. This inference
is confirmed by the discovery of a Bridger genus, very fragmentary but
identifiable, in the marine Eocene of New Jersey.
_South America._—No Eocene rocks, marine or continental, are known in
the West Indies or Central America, but the latter region has been so
imperfectly explored that no great importance can be attached to this
fact. North and South America were separated completely, as is proved by
the entire dissimilarity of their mammalian faunas, but the position of
the transverse sea or strait cannot be determined. There is much reason
to believe that the Greater Antilles were connected into a single large
land, which has been called “Antillia” and may have been joined to the
mainland of Central America. Certain marine rocks in Patagonia and Chili
have been referred to the Eocene by South American geologists, but the
reference is almost certainly erroneous, the rocks in question being
much more probably Miocene. The Andes, probably throughout their length
and certainly in their southern half, stood at a much lower level than
they do now, and, no doubt, were rising, either slowly and steadily, or
periodically and more rapidly, throughout the whole Tertiary period. At
all events, their present height in the south is due to movements in
the Pliocene or later. Continental deposits of Eocene date have been
discovered only in northern Patagonia (Casa Mayor) where they occupy
depressions in the worn and eroded surfaces of the Cretaceous rocks; the
mode of their formation has not been carefully studied.
There is great uncertainty as to the status of the land-bridge which,
it is believed, in the Cretaceous period connected South America with
Africa. Some of the evidence goes to show that the connection persisted
throughout the Eocene epoch, but the testimony is that of fragmentary
and therefore imperfectly understood fossils and is far from being
unequivocal. The connection with Antarctica probably continued.
3. _Oligocene Epoch_
_North America._—The Oligocene, or third of the Tertiary epochs, was a
time of great significance in the history of the American mammals and
of great geographical changes in the West Indian and Central American
regions, but in North America proper the changes were not so widespread.
On the Atlantic coast the marine Oligocene is but scantily displayed
except in the Florida peninsula, where it is found in a thickness of some
2000 feet, but it is well developed along the north shore of the Gulf of
Mexico, where the coast-line followed that of the Eocene, only a little
farther to the south, marking the retreat of the sea at the end of the
Eocene. The Gulf Stream entered the Atlantic over the site of northern
Florida and flowed northward nearer the coast than it does to-day, in
consequence of which warm-water conditions extended far to the north and
West Indian shells flourished on the New Jersey coast. In the middle
Oligocene part of northern Florida was elevated into an island and the
water over much of the remainder of the peninsula became shallower, but
this did not greatly alter the course of the Gulf Stream. The Pacific
encroached upon the western shore of Oregon and British Columbia and very
extensively upon that of Alaska, where strata no less than 10,000 feet
thick are assigned to this epoch.
In the western interior Oligocene formations are among the most important
and widely spread of the continental Tertiaries and are divisible into
two principal stages and each of these again into three substages.
Of these, the older or _White River_ stage covers a vast region in
northeastern Colorado, western Nebraska, eastern Wyoming and southern
South Dakota, with separate areas in the Black Hills, North Dakota and
the Northwest Territory of Canada. The deposits are believed to be
chiefly of fluviatile origin, and many of the ancient stream-channels,
some of great size, may still be traced, filled with the consolidated
sands and gravels of the old rivers. The country was very flat and the
divides between the streams very low, so that in seasons of flood great
regions were converted into shallow, temporary lakes, in which were
deposited the finer silt and mud, but were dry for most of the year. The
volcanic activity which had gone on so impressively in the Bridger Eocene
was renewed in White River times, as is proved by thick beds of pure
volcanic ash, which must have been carried long distances by the wind,
for they occur far from any volcanic vent.
[Illustration: FIG. 50.—Map of North America in the upper Oligocene.
Explanation as in Fig. 48. (After Schuchert.)]
The White River fauna is more completely known than that of any other
Tertiary formation of this continent. The first discovery of these
fossils was made more than 70 years ago and since then oft-repeated
expeditions have brought to light an astonishing number and variety of
mammals. Not only are these beds remarkable for the immense quantity
of material which they have yielded, but also for its completeness and
beauty of preservation, a most unusual number of skeletons having been
obtained. The mammals demonstrate that the land-connection with the Old
World had been re-established, for many European genera, which could not
have been derived from an American ancestry, are found in the White River
beds. At the same time, there was no such proportion of forms common to
both continents as there had been in the Wasatch-Sparnacian stage of
the lower Eocene, each having many genera and even families which did
not extend their range into the other. The reason for this remarkable
and, at first sight, inexplicable difference between the lower Eocene
and the lower Oligocene is probably to be found in climatic changes, in
consequence of which relatively fewer genera were able to take advantage
of the reopened connection, which lay far to the north. The White River
mammals, like those of the Recent epoch, are thus divisible into two
groups or elements, one set indigenous and descended from ancestors
which are found in the American Eocene, and the other composed of late
immigrants from the Old World. Migrants from North America likewise made
their way to Europe.
The upper continental Oligocene of the interior has received the peculiar
appellation of the _John Day_, from the river of that name in eastern
Oregon, a large part of which was buried to a depth of 3000 or 4000 feet
in stratified volcanic ash and tuff. This great mass of finely divided
volcanic material was derived from the craters of the Cascade Mountains
to the westward; a long-continued series of eruptions would be needed
to form such thick accumulations at such a distance from the sources of
supply. The John Day evidently succeeded the White River very closely
in time, but is marked by the disappearance of almost all the European
migrants. This fact, together with the absence of any new immigrant
genera, is evidence that the connection had again been broken and it was
not renewed until after a considerable lapse of time.
There are many reasons for believing that the Oligocene climate marked
the beginning of the very long and gradual process of refrigeration
which culminated in the glacial conditions of the Pleistocene epoch, but
the change was slight and probably chiefly affected the far north. The
climate, however, remained notably warmer than the present one of the
same extra-tropical latitudes, as is abundantly proved by the fossils.
The Atlantic coast, as noted above, was bathed in warm waters, the plants
of the Alaskan Oligocene point to temperate conditions and the vegetation
of Europe was subtropical, palms growing in the north of Germany. The
change which was distinctly to be noted in the Great Plains region of
North America was probably due rather to the elevation and increased
altitude of the western interior than to general climatic alteration.
Crocodiles are very rare indeed in the White River beds and those that
have been found all belong to dwarf species, while none are known from
the John Day. Unfortunately, hardly anything has been ascertained
concerning the Oligocene vegetation of the region, but the reptiles
indicate diminished warmth.
_South America._—Marine Oligocene strata have great extent around the
Gulf of Mexico and the Caribbean Sea, and the distribution of these shows
that Antillia was broken up by great submergences, the islands of the
Greater Antilles being much smaller than they are to-day. The greater
part of Central America and the Isthmus were under water, a broad sea,
broken only by scattered islands, separating North and South America.
Very little is known of the Oligocene in the latter continent save a
non-marine formation in northern Patagonia, the _Deseado_ stage (or
Pyrotherium Beds), which, like the Eocene of the same region, occupies
depressions in the worn and irregular surface of the Cretaceous rocks.
The attribution of the Deseado to the Oligocene is open to some doubt,
because of the entire absence in its mammalian fauna of any elements
which are also found in the northern hemisphere. Hence, there are no
means of direct comparison.
4. _Miocene Epoch_
_North America._—The Atlantic and Gulf coasts, which had been raised
in the Oligocene, were again depressed, almost restoring the Eocene
coast-line, the chief differences being the presence of the Florida
islands and the nearly complete closing of the Mississippi Embayment.
There was a remarkable change in the marine fauna from that of Oligocene
times; a cool current flowed southward along the coast and entered the
Gulf of Mexico through the strait between the Florida island and the
mainland, bringing the northern animals with it and driving out the
tropical forms. This complete faunal change, which might fairly be called
a revolution, was the most sudden and striking in the Tertiary history of
the continent.
On the Pacific coast also there was a depression, which caused a renewed
transgression of the sea. The Coast Range formed a chain of reefs and
islands in the Miocene sea, which again filled the great valley of
California, except in the northern part of what is now the Sacramento
Valley, where there was an accumulation of continental deposits. The
immense thickness (5000 to 7000 feet) of the California Miocene is
largely made up of volcanic material, which testifies to the great
activity of the vents. In the Sierras, the height of which was increased
in the upper Miocene, there was also a great display of vulcanism,
recorded in the lava-flows and tuffs of the time. In the region of
Lower California and northwestern Mexico considerable changes of the
coast-line took place during the Miocene; in the earlier half of the
epoch the Gulf of California was much shorter and narrower than it is
to-day and the peninsula was broadly united with the mainland to the east
as well as to the north. A wide submergence marked the upper Miocene,
reducing the peninsula to a long, narrow island and enlarging the gulf
considerably beyond its present limits, flooding an extensive area in
northwestern Mexico and sending a small bay into southeastern California.
There were great disturbances in the course of the epoch, for in the
Santa Cruz Mountains near San Francisco the lower Miocene strata were
crumpled into folds, before those of the upper Miocene were deposited
upon them. British Columbia, Washington and Oregon were invaded by the
sea, which extended up the valley of the Columbia River and its southern
tributary, the Willamette, though here the beds are far thinner than
those of California. Much of Alaska, both on the north and west coasts
and in the valley of the Yukon, was submerged, and the land-connection
with Asia appears to have been broken. This is made probable not only
by the submergence of the Alaskan coast, but also by the fact that the
marine animals of the California coasts and shoal waters, which could
not migrate across the ocean, were quite unlike the contemporary forms
of the eastern Asiatic shore, which would hardly have been the case, had
a continuous coast-line united the two continents. On the other hand,
there was a renewed connection with Europe, as is shown by the appearance
of Old World land-mammals, beginning scantily in the lower and becoming
numerous in the middle Miocene. This connection, it will be remembered,
had been interrupted during the upper Oligocene. Many students of the
problem have maintained that the land-bridge was by way of the West
Indies and the Mediterranean lands, but such a bridge would not account
for the facts of mammalian distribution, which would seem to require its
location in the far north.
[Illustration: FIG. 51.—Map of North America in the upper Miocene.
Explanation as in Fig. 48. (Modified from Schuchert.)]
Several distinct lines of evidence go to prove that the junction of
the Americas dates from the Miocene, possibly from the beginning of
it. The absence of Atlantic species from the Pacific Miocene is an
indication that the passage from ocean to ocean had been closed, and
this is confirmed by the geology of the Central American and Isthmian
region. In the middle Miocene of Oregon and Nebraska have been found
remains, which are unfortunately too incomplete for altogether convincing
identification, but which can be interpreted only as belonging to the
extinct and most characteristically South American group of edentates,
the †ground-sloths or †Gravigrada; if this reference is correct, the fact
of the junction cannot be questioned.
Continental deposits of Miocene date, chiefly accumulations made by
rivers and the wind, cover vast areas of the western interior, though but
rarely to any considerable depth. These have been divided into several
stages and have received various names; the lower Miocene, known as the
_Arikaree_, _Harrison_ or _Rosebud_, overlies the White River in South
Dakota, western Nebraska and eastern Wyoming, with smaller areas in
Montana and Colorado. In the deposits of this stage there are no mammals
of indisputably Old World type, though a few which I consider to be such
are a probable indication of renewed connection with Europe. The middle
Miocene is found typically in central Montana, where it is called the
_Deep River_ (or _Smith River_) stage, but occurs also in numerous small,
scattered and widely separated areas in Oregon, Wyoming, Colorado and
Texas, with local names in these different states. It is most likely
that these middle Miocene formations are not strictly contemporaneous in
the geological sense, but rather form a closely connected and successive
series. The mammals of the Deep River stage leave no doubt that the way
of migration from the Old World was again open.
The _Loup Fork_, or upper Miocene, itself susceptible of further
subdivision, is by far the most extensive of the Miocene formations and
covers much of the Great Plains region, in separate areas, from South
Dakota far into Mexico. Perhaps also referable to the upper Miocene is
a small, but very interesting formation, the _Florissant_, which is in
the South Park of Colorado; it was made by very fine volcanic material
showered into a small and shallow lake. The finely laminated papery
shales of the Florissant have preserved countless plants and insects
and many fishes, and these throw very welcome light upon the vegetation
and climatic conditions of the epoch and afford an interesting contrast
to the fauna and flora of the Green River shales of the lower or middle
Eocene. That the Florissant shales are Miocene, no one questions, but
their isolated position and the fact that they have yielded no mammals
make it somewhat doubtful whether they belong in the middle or later part
of the epoch.
In the western portion of the continent vulcanism was displayed on a
grand scale during the Miocene. Mention has already been made of the
quantity of volcanic material in the marine Miocene of California and
also in the lavas and tuffs of the Sierras. The magnificent cones,
such as Mts. Hood and Tacoma, which are the glory of the Cascades,
are believed to date from this time. In Idaho and eastern Oregon and
Washington are the immense lava-fields of the Columbia River, which are,
partly at least, of Miocene date and were chiefly extruded through great
fissures, the lava flooding the valleys and plains in a fiery sea of
molten rock. In Oregon these lavas rest upon the upper Oligocene (John
Day stage) and middle Miocene beds are deposited upon them, which fixes
their date sufficiently. In the Yellowstone Park was piled up a huge mass
of volcanic products, lava-flows and beds of ash and tuff, to a thickness
of several thousand feet. The ash-beds have preserved the petrified
forests, with their tree-trunks still standing one above another; one
locality in the Park shows seven such forests, each one killed and buried
by a great discharge of ash and then a new forest established and growing
upon the surface of the accumulation. In the tuffs are leaf-impressions
which permit identification of the plants.
In the latter part of the Miocene and at its close there were important
crustal movements, which affected all the Pacific coast mountain ranges,
though this epoch was no such time of mountain making in America as it
was in the Old World. The principal elevation of the Coast Range in
California and Oregon was due to these movements, and the Sierras and the
plateaus of Utah and Arizona were increased in height. On the Atlantic
side the Florida island was joined to the mainland and thus the present
shape of the continent was almost exactly gained.
The Miocene climate of North America, as indicated by the plants of
Florissant, the Yellowstone Park and Oregon, was distinctly milder than
at present, a southern vegetation of warm-temperate character extending
to Montana and perhaps much farther north, but it was not so warm as it
had been in the Eocene, and palms are not found in any of the localities
mentioned, nor do crocodiles occur in any of the northern Miocene
formations. In Europe the climate of the early Miocene was considerably
warmer than in North America, the vegetation of central and western
Europe being very much like that of modern India. This difference between
the two sides of the Atlantic was probably due, in large part, to the
manner in which Europe was broken and intersected by arms and gulfs of
the warm southern sea. In the latter half of the epoch, however, the
climate became colder, the subtropical flora giving way to a distinctly
temperate one.
_South America._—In Central America, where marine Oligocene beds are
of great extent, no Miocene is known, and on the Isthmus Oligocene is
the latest marine formation, except a narrow fringe of Pleistocene
on the Caribbean coast. These facts and others already cited lead to
the conclusion that in the Miocene the connection of the Americas was
complete and that the Isthmus was considerably broader than at present,
extending nearly to Jamaica. The condition of the Greater Antilles is but
vaguely understood, but they were involved in the general elevation of
the Caribbean region and were at least as large as they are now and may
have been considerably larger, and Cuba was perhaps joined to Central
America, as Hayti probably was.
In South America proper nearly the whole of Patagonia was submerged
by the transgression of a shallow, epicontinental sea, in which were
accumulated the beds of the _Patagonian_ stage, containing an exceedingly
rich and varied assemblage of marine fossils, an assemblage which has
very little in common with the contemporary formations of the northern
hemisphere. It is this lack of elements common to the northern faunas
which has led to the long debate concerning the geological date of the
Patagonian formation, the South American geologists very generally
referring it to the Eocene. However, the occurrence of genera of
Cetaceans (whales and dolphins), which are also found in the Miocene
of Maryland and Virginia, is very strong evidence that the proper date
of the Patagonian is Miocene. A continuous coast-line, or at least an
unbroken continuity of shoal-water conditions, seems necessary to account
for the similarity of the Patagonian fossils with those of New Zealand
and Australia, and that this connection was by way of the Antarctic
continent is indicated by the occurrence of similar fossils in the South
Shetland Islands, an Antarctic group. On the Chilian coast the _Navidad_
formation, which is believed to be approximately contemporaneous with
the Patagonian, has so different a fauna as to point to some kind of a
barrier between the Atlantic and the Pacific, and this barrier, Dr. von
Ihering holds, was the land-extension from South America to Antarctica.
After some oscillations of retreat and advance, the sea withdrew from
Patagonia and the terrestrial accumulations of the _Santa Cruz_ stage
were formed. These beds are partly composed of river-deposits, but
chiefly of more or less consolidated volcanic ash or tuff, and have
yielded a surprising number of beautifully preserved mammals. No other
assemblage of South American Tertiary Mammalia is so well known and
understood as the Santa Cruz fauna, and the very large number of all but
complete skeletons which have been found strongly suggests that many of
the animals were buried alive in the showers of volcanic ash. The Santa
Cruz fauna is completely and radically different from any of the North
American assemblages, and at that time no immigrant from the north had
penetrated so far as Patagonia.
In the upper Miocene the Andes stood at a much lower level than they
do now; fossil plants, some of them collected at a great height in the
mountains, are the remains of a luxuriant and purely tropical flora
nearly identical with the vegetation of the modern forests of Bolivia
and Brazil. Such a vegetation could not exist at the altitudes where the
fossils occur and these demonstrate a great elevation of the mountains
since those leaves were embedded. The same mild climatic conditions which
prevailed in the northern hemisphere during the Miocene must also have
characterized Patagonia, subtropical shells extending far to the south of
their present range.
Whatever may have been true of the land-bridge connecting South America
with Africa during the early Tertiary epochs, it must have been
submerged in the Miocene, otherwise there would not have been the open
pathway for the Cetacea of Patagonia to reach the Atlantic coast of North
America and _vice versa_.
5. _Pliocene Epoch_
_North America._—The Pliocene of North America is not nearly so well
displayed or so satisfactorily known as the preceding Tertiary epochs,
and only of comparatively late years has it been recognized at all upon
the Atlantic coast. The Atlantic and Gulf shores had very nearly their
present outlines, but with some notable differences. It would seem that
the northeastern portion of the continent stood at a higher level than
it does now, north Greenland being joined with the islands of the Arctic
archipelago and Newfoundland with Labrador, the Gulf of St. Lawrence
then being land. From Nova Scotia to southern New Jersey the coast-line
was many miles to the east and south of its present position, but the
sea encroached here and there upon the shores of Virginia, the Carolinas
and Georgia, and southern Florida was mostly under water, as was also a
narrow strip of the Gulf coast from Florida to Texas and along the east
of Mexico. On the Pacific side of the continent the marine Pliocene is
far thicker and more important than on the east coast and in California
is largely made up of volcanic materials. Quite extensive disturbances
in this region had marked the close of the Miocene, the strata of
which in the Coast Range had been violently compressed and folded. An
elevation of the land had caused the sea to withdraw from the central
valley of California and had restored Lower California to its peninsular
conditions, reducing the gulf to the narrow limits which it had had in
the lower Miocene and extending southern Mexico to the west and south.
British Columbia and southeastern Alaska stood at higher than their
present levels and the countless islands of that region were part of
the mainland. Bering Strait was closed, for at least a great part of
the epoch, and, as a continuous shore-line was thus formed and a way of
migration opened, the marine fauna of California and Japan became closely
similar.
[Illustration: FIG. 52.—Map of North America during the Pliocene epoch,
Bering Strait open. Explanation as in Fig. 48. (Modified from Schuchert.)]
In the interior, the Pliocene continental formations and faunas followed
so gradually upon those of the Miocene, that there is great doubt as to
where the line between them should be drawn. These interior formations
are mostly of small extent and are very widely scattered, and much
remains to be learned regarding the mammals of the epoch. In northern
Kansas are the _Republican River_ beds, which are so doubtfully Pliocene,
that they may almost equally well be called uppermost Miocene. Other
lower Pliocene stages, representing various divisions of time, are the
_Alachua_ of northern Florida, the _Snake Creek_ of western Nebraska,
the _Thousand Creek_ and _Virgin Valley_ of northwestern Nevada and the
_Rattlesnake_ of Oregon. Probably middle Pliocene is the _Blanco_ of
northwestern Texas, a valley cut in the middle and lower Miocene rocks
and filled in with Pliocene deposits. Possibly upper Pliocene, or, it may
be, lowest Pleistocene, are the _Peace Creek_ of southwestern Florida and
the so-called “_Loup River_” (not Loup Fork) of western Nebraska.
The volcanic activity of the Rocky Mountain and Pacific coast regions,
which was so remarkable in the Miocene, continued into and perhaps
through the Pliocene. The great outflow of light-coloured lava which
built up the central plateau of the Yellowstone Park is referred to the
Pliocene, and some of the enormous fissure-eruptions which formed the
vast Columbia River fields of black basaltic lava were probably Pliocene,
as some were demonstrably Miocene. Both of these epochs were remarkable
for volcanic activity in the western part of the continent.
The Pliocene climate, as may be inferred from the plants and marine
shells, was colder than that of the Miocene, and refrigeration was
progressive, as is shown by the proportion of Arctic shells in the
Pliocene beds of the east coast of England, rising from 5 per cent in
the oldest to more than 60 per cent in the latest beds. In the Arctic
regions the cold must have been severe, at least during the latter half
of the epoch, for in the succeeding Pleistocene we find an Arctic fauna
already fully adapted to the extreme severity of present day polar
conditions and time was necessary for such an adaptation. In the western
interior the climate was not only colder, but also drier than it had
been in the Miocene, the desiccation which had begun in the latter epoch
becoming progressively more and more marked.
_South America._—The Greater Antilles were larger than at present and
Cuba was much extended, especially to the southeastward, and was probably
connected with the mainland, not as one would naturally expect, with
Yucatan, but with Central America; this island, it is most likely, was
cut off from Hayti. The Isthmian region was considerably broader than
it is now and afforded a more convenient highway of intermigration.
Costa Rica was invaded by a Pliocene gulf, but it is not yet clear
whether this persisted for the whole or only a part of the epoch. In the
Argentine province of Entrerios is a formation, the _Paraná_, which is
most probably Pliocene, though it may be upper Miocene. This formation
is largely marine and shows that the present Rio de la Plata was then a
gulf from the Atlantic. A few northern hemisphere mammals in the Paraná
beds show that the migration had advanced far into South America. A large
part of Patagonia was again submerged beneath the sea, which extended
to the Andes in places, but just how general the submergence was, it
is impossible to say, for the _Cape Fairweather_ formation has been
largely carried away by erosion and only fragments of it remain. Along
the foothills of the Andes these beds are upturned and raised several
thousand feet above the sea-level, a proof that the final upheaval of
the southern mountains took place at some time later than the early
Pliocene. Continental formations of Pliocene date are largely developed
in Argentina; the _Araucanian_ stage is in two substages, one in the
province of Catamarca, where the beds are much indurated and were
involved in the Andean uplift, the other, of unconsolidated materials, is
at Monte Hermoso near Bahia Blanca on the Atlantic coast. The very small
proportion of northern animals in the Araucanian beds is surprising, but
not more so than the almost complete absence of South American types in
the upper Miocene and lower Pliocene of the United States. Intermigration
between the two Americas would seem to have been a much slower and more
difficult process than between North America and the Old World, and the
reason for the difference is probably the greater climatic barriers
involved in a migration along the lines of longitude. Upper Pliocene is
found in the Tarija Valley of Bolivia and probably also in Ecuador, in
both of which areas the proportion of northern animals was very greatly
increased.
II. QUATERNARY PERIOD
The Quaternary period was a time of remarkable geographical and climatic
changes, which had the profoundest and most far-reaching effects,
partly by migration and partly by extinction, upon the distribution of
animals and plants, effects which are naturally more obvious than those
of earlier geological events, just because they were the latest. It is
customary to divide the period into two epochs, (1) the _Pleistocene_ or
_Glacial_, and (2) the _Recent_, which continues to the present day.
1. _Pleistocene Epoch_
When Louis Agassiz first suggested (1840) the idea of a time,
comparatively recent in the geological sense, when northern and central
Europe was buried under immense sheets of slowly moving ice, like
the “ice-cap” of modern Greenland, the conception was received with
incredulity. Nearly thirty years passed before this startling theory
gained the general acceptance of geologists, but now it is one of the
commonplaces of the science, for no other hypothesis so well explains
the complicated phenomena of Pleistocene geology. One great obstacle
to the acceptance of the glacial theory was the supposed fact that the
Pleistocene glaciation was something quite unique in the history of
the earth, a violent aberration in the development of climates. Now,
however, we have every reason to believe that at least three other and
very ancient periods had witnessed similar climatic changes and that
“ice-ages” were recurrent phenomena. This is not the place to discuss or
even to summarize the evidence which has convinced nearly all geologists
of the reality of Pleistocene glacial conditions on a vast scale in Asia,
Europe and, above all, in North America. The reader who may wish to
examine this evidence will find an admirable presentation of it in Vol.
III of the “Geology” of Professors Chamberlin and Salisbury.
_North America._—There has long been a difference of opinion among
students of the Pleistocene as to whether the glaciation was single,
or several times renewed. That there were many advances and retreats
of the ice, is not denied; the question is, whether there were truly
interglacial stages, when the ice altogether disappeared from the
continent and the climate was greatly ameliorated. The present tendency
among American and European geologists is decidedly in favour of
accepting several distinct glacial stages (Chamberlin and Salisbury
admit six of these) separated by interglacial stages, and for this there
are very strong reasons. While it is out of the question to present the
evidence for this conclusion here, one or two significant facts may be
noted. On the north shore of Lake Ontario, near Toronto, are certain
water-made deposits, which rest upon one sheet of glacial drift and are
overlaid by another. The fossils of the aqueous sediments are in two
series, upper and lower, of which the older and lower contains plants and
insects indicative of a climate considerably warmer than that of the same
region to-day and corresponding to the temperature of modern Virginia. In
the upper and newer beds the fossils show the return of cold conditions,
much like those of southern Labrador, and this was followed by the
reëstablishment of the ice, as recorded in the upper sheet of drift.
Even far to the north, on the Hudson’s Bay slope, an interglacial forest
is embedded between two glacial drift-sheets. In Iowa and South Dakota
numerous mammals of temperate character occur in interglacial beds.
At the time of their greatest extension, the glaciers covered North
America down to latitude 40° N., though the great terminal moraine,
which marks the ice-front and has been traced across the continent from
Nantucket to British Columbia, describes a very sinuous line. The ice
was not a homogeneous sheet, moving southward as a whole, but flowed in
all directions away from several, probably four, centres of accumulation
and dispersal. At the same time, the western mountain ranges had a far
greater snow-supply than at present, and great glaciers flowed down all
the valleys of the Rocky Mountains as far south as New Mexico and in the
Sierras to southern California, while the Wasatch, Uinta and Cascade
ranges and those of British Columbia and Alaska were heavily glaciated,
but, strange to say, the lowlands of Alaska were free from ice. During
the periods of greatest cold the rain-belt was displaced far to the south
of its normal position, bringing a heavy precipitation to regions which
are now extremely arid. In the Great Basin were formed two very large
lakes; on the east side, rising high upon the flanks of the Wasatch
Mountains, was Lake Bonneville, the shrunken and pygmy remnant of which
is the Salt Lake of Utah, and on the west side, in Nevada, was Lake
Lahontan. Lake Bonneville, which was nearly two-thirds the size of Lake
Superior, discharged northward into the Snake River, a tributary of the
Columbia, but Lahontan had no outlet. Each of these lakes had two periods
of expansion, with a time of complete desiccation between them.
Over the Great Plains the principal Pleistocene formation is that known
as the _Sheridan_, or, from the abundance of horse-remains which are
entombed in it, the _Equus Beds_. These beds extend as a mantle of
wind-drifted and compacted dust from South Dakota to Texas and in places
contain multitudes of fossil bones; they correspond to one of the early
interglacial stages and in South Dakota pass underneath a glacial moraine.
The upheaval which came at or near the end of the Pliocene had raised the
continent, or at least its northeastern portion, to a height considerably
greater than it has at present, and this must have facilitated the
gathering of great masses of snow; but before the end of the Pleistocene
a subsidence of the same region brought about important geographical
changes. The depression, which lowered the coast at the mouth of the
Hudson about 70 feet below its present level, increased northward to 600
feet or more in the St. Lawrence Valley and allowed the sea to invade
that valley and enter Lake Ontario. From this gulf ran two long, narrow
bays, one far up the valley of the Ottawa and the other into the basin
of Lake Champlain. The raised beaches, containing marine shells and the
bones of whales, seals and walruses, give eloquent testimony of those
vanished seas. The recovery from this depression and the rise of the
continent to its present level inaugurated the Recent epoch.
When the ice had finally disappeared, it left behind it great sheets
of drift, which completely changed the surface of the country and
revolutionized the systems of drainage by filling up the old valleys,
only the largest streams being able to regain their former courses.
Hundreds of buried valleys have been disclosed by the borings for oil and
gas in the Middle West, and these, when mapped, show a system of drainage
very different from that of modern times. Innumerable lakes, large and
small, were formed in depressions and rock-basins and behind morainic
dams, the contrast between the glaciated and non-glaciated regions in
regard to the number of lakes in each being very striking.
On the west coast events were quite different; marine Pleistocene beds
in two stages are found in southern California. The upheavals late in
the Pleistocene, or at its close, were far greater than on the Atlantic
side, 4000 feet in southeastern Alaska, 200 feet on the coast of Oregon
and rising again to 3000 feet in southern California; all the western
mountain ranges and plateaus were increased in height by these movements.
The volcanoes continued to be very active, as may be seen from the
lava-sheets and streams in Alaska, all the Pacific states, Arizona and
New Mexico.
_South America._—No such vast ice-sheets were formed in the southern
hemisphere as in the northern. Patagonia was the only part of South
America to be extensively covered with ice and there traces of three
glaciations have been observed, of which the first was the greatest and
reached to the Atlantic coast, and there were great ice-masses on the
coast of southern Chili. Mountain glaciers existed throughout the length
of the Andes across the Equator to 11° N. lat., the elevation increasing
northward to the tropics. The surface of the great Argentine plain of the
Pampas between 30° and 40° S. lat. is covered with a vast mantle, largely
of wind-accumulated dust, the _Pampean_, which is the sepulchre of an
astonishing number of great and strange beasts. The Pampean formation
corresponds in a general way to the Sheridan or Equus Beds of North
America, but involves a much greater lapse of time, beginning earlier,
possibly in the late Pliocene, and apparently lasting through the entire
Pleistocene. While largely of æolian origin, the Pampean seems to be in
part made of delta deposits laid down by rivers. One striking difference
between the Pampean, on the one hand, and the Sheridan and the loess of
the Mississippi Valley and of Europe, on the other, is that the former is
in many places much more consolidated and stony, which gives it a false
appearance of antiquity. Another and very rich source of Pleistocene
mammals is found in the limestone caves of eastern Brazil, which have
yielded an incredible quantity of such material, but not in such a
remarkably perfect state of preservation as the skeletons of the Pampean.
Very little is known of the Pleistocene in the West Indies, though
probably to this date should be assigned the notable oscillations of
level which are recorded in the raised sea-terraces of Cuba and other
islands. The Windward groups were joined, at least in part, to the
continent and large extinct rodents reached Antigua, which would not be
possible under present conditions. The Isthmus of Panama was 200 feet or
more higher than it is now and correspondingly wider, but was depressed
to a lower than the present level, and finally raised to the height it
now has. Marine beds, of presumably Pleistocene date and certainly not
older, extend from the Caribbean shore to Gatun, some seven miles, and
are nowhere more than a few feet above sea-level.
The question of Pleistocene climates is a very vexed one and is far
from having received a definitive answer. Limitations of space forbid a
discussion of the problem here and I shall therefore merely state the
conclusions which seem best supported by the evidence so far available.
Such immense accumulations of ice might be due either to greatly
increased snow-fall, or to a general lowering of the temperature. The
balance of testimony is in favour of the latter factor and no great
refrigeration is required. Professor Penck has calculated that a
reduction of 6° or 7° in the average yearly temperature would restore
glacial conditions in Europe. Even the tropics were affected by the
change, as is shown not only by the glaciation of the Andes, but also by
Mt. Kenya, which is almost on the Equator in eastern Africa and still
has glaciers. The presumably Pleistocene ice covered the whole mountain
like a cap, descending 5400 feet below the present glacier limit. It was
pointed out above that the interglacial stages had greatly ameliorated
climatic conditions and that, in some of them at least, the climate was
warmer than it is to-day in the same localities. The cause of these
astonishing fluctuations and of the climatic changes in general, to which
Geology bears witness, still remains an altogether insoluble mystery.