Professor Agassiz remarks that no formation--known to geologists--
resembling that of the Amazon exists on the face of the earth. Its
extent is stupendous. It stretches from the Atlantic shore through the
whole width of Brazil into Peru, to the very foot of the Andes--one vast
extent of red sandstone, capped by a yellow-ochred clay; not only along
the banks of the main river, but forming the sides of those of its
tributaries, to their far-off sources, probably over the whole basin of
the Paraguay and the Rio de la Plata. How are these vast deposits
formed? is the question. The easiest answer, he observes, and the one
which most readily suggests itself, is that of a submersion of the
continent at successive periods--to allow the accumulation of these
materials--and its subsequent elevation. This explanation is rejected,
for the simple reason that the deposits show no signs whatever of a
marine origin. No sea-shells, or remains of any marine animal, have as
yet been found throughout their whole extent--over a region several
thousands of miles in length, and from five to seven hundred miles in
width. It is evident, he considers, that this basin was a fresh-water
basin, these deposits fresh-water deposits. It is true that calcareous
layers thickly studded with shells have been found interspersed with the
clay; but though supposed to be marine fossils, he recognised them for
what they really are--fresh-water shells of the family of the Naiades.
As their resemblance is very remarkable, the mistake as to their true
zoological character is natural: indeed, many travellers have confounded
some fresh-water fishes from the Upper Amazon of the genus of
Pterophyllum with the marine genus Platax. He considers that the
immense glacier which probably existed at the same time that ice,
thousands of feet thick, covered the centre of Europe, must have been
formed in this valley, and then, ploughing its bottom over and over
again, and grinding all the materials beneath it into a fine powder,
must ultimately have forced its way through the colossal sea-wall which
it had built up eastward into the Atlantic.