The Soil Is Varied
And Productive; And Its Almost Insular Form Gives It Two
Grand Lines Of Communication By The Rivers Parana And
Uruguay.
I was delayed here five days, and employed myself in examining
the geology of the surrounding country, which was
very interesting.
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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