We Here See At The Bottom Of The Cliffs,
Beds Containing Sharks' Teeth And Sea-Shells Of Extinct Species,
Passing Above Into An Indurated Marl, And From That
Into The Red Clayey Earth Of The Pampas, With Its Calcareous
Concretions And The Bones Of Terrestrial Quadrupeds.
This
vertical section clearly tells us of a large bay of pure salt-
water, gradually encroached on, and at last converted into
the bed of a muddy estuary, into which floating carcasses
were swept.
At Punta Gorda, in Banda Oriental, I found
an alternation of the Pampaean estuary deposit, with a
limestone containing some of the same extinct sea-shells; and
this shows either a change in the former currents, or more
probably an oscillation of level in the bottom of the ancient
estuary. Until lately, my reasons for considering the Pampaean
formation to be an estuary deposit were, its general
appearance, its position at the mouth of the existing great
river the Plata, and the presence of so many bones of
terrestrial quadrupeds: but now Professor Ehrenberg has had
the kindness to examine for me a little of the red earth,
taken from low down in the deposit, close to the skeletons
of the mastodon, and he finds in it many infusoria, partly
salt-water and partly fresh-water forms, with the latter
rather preponderating; and therefore, as he remarks, the
water must have been brackish. M. A. d'Orbigny found on
the banks of the Parana, at the height of a hundred feet,
great beds of an estuary shell, now living a hundred miles
lower down nearer the sea; and I found similar shells at a
less height on the banks of the Uruguay; this shows that
just before the Pampas was slowly elevated into dry land,
the water covering it was brackish. Below Buenos Ayres
there are upraised beds of sea-shells of existing species,
which also proves that the period of elevation of the Pampas
was within the recent period.
In the Pampaean deposit at the Bajada I found the osseous
armour of a gigantic armadillo-like animal, the inside
of which, when the earth was removed, was like a great
cauldron; I found also teeth of the Toxodon and Mastodon,
and one tooth of a Horse, in the same stained and decayed
state. This latter tooth greatly interested me, [3] and I took
scrupulous care in ascertaining that it had been embedded
contemporaneously with the other remains; for I was not
then aware that amongst the fossils from Bahia Blanca
there was a horse's tooth hidden in the matrix: nor was it
then known with certainty that the remains of horses are
common in North America. Mr. Lyell has lately brought
from the United States a tooth of a horse; and it is an
interesting fact, that Professor Owen could find in no species,
either fossil or recent, a slight but peculiar curvature
characterizing it, until he thought of comparing it with my
specimen found here: he has named this American horse Equus
curvidens. Certainly it is a marvellous fact in the history
of the Mammalia, that in South America a native horse
should have lived and disappeared, to be succeeded in after-
ages by the countless herds descended from the few introduced
with the Spanish colonists!
The existence in South America of a fossil horse, of the
mastodon, possibly of an elephant, [4] and of a hollow-horned
ruminant, discovered by MM. Lund and Clausen in the
caves of Brazil, are highly interesting facts with respect to
the geographical distribution of animals. At the present
time, if we divide America, not by the Isthmus of Panama,
but by the southern part of Mexico [5] in lat. 20 degs., where
the great table-land presents an obstacle to the migration of
species, by affecting the climate, and by forming, with the
exception of some valleys and of a fringe of low land on
the coast, a broad barrier; we shall then have the two
zoological provinces of North and South America strongly
contrasted with each other. Some few species alone have
passed the barrier, and may be considered as wanderers from
the south, such as the puma, opossum, kinkajou, and peccari.
South America is characterized by possessing many peculiar
gnawers, a family of monkeys, the llama, peccari, tapir,
opossums, and, especially, several genera of Edentata, the
order which includes the sloths, ant-eaters, and armadilloes.
North America, on the other hand, is characterized (putting
on one side a few wandering species) by numerous peculiar
gnawers, and by four genera (the ox, sheep, goat, and antelope)
of hollow-horned ruminants, of which great division
South America is not known to possess a single species.
Formerly, but within the period when most of the now existing
shells were living, North America possessed, besides
hollow-horned ruminants, the elephant, mastodon, horse, and
three genera of Edentata, namely, the Megatherium, Megalonyx,
and Mylodon. Within nearly this same period (as
proved by the shells at Bahia Blanca) South America possessed,
as we have just seen, a mastodon, horse, hollow-
horned ruminant, and the same three genera (as well as
several others) of the Edentata. Hence it is evident that
North and South America, in having within a late geological
period these several genera in common, were much
more closely related in the character of their terrestrial
inhabitants than they now are. The more I reflect on this
case, the more interesting it appears: I know of no other
instance where we can almost mark the period and manner
of the splitting up of one great region into two well-
characterized zoological provinces. The geologist, who is fully
impressed with the vast oscillations of level which have
affected the earth's crust within late periods, will not fear
to speculate on the recent elevation of the Mexican platform,
or, more probably, on the recent submergence of land
in the West Indian Archipelago, as the cause of the present
zoological separation of North and South America. The
South American character of the West Indian mammals [6]
seems to indicate that this archipelago was formerly united
to the southern continent, and that it has subsequently been
an area of subsidence.
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