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.
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