lso identical in its geological
structure with the lateral valleys of the Amazons. The same is true of
the large island of Marajo, lying at the mouth of the Amazons. And yet I
believe that even this does not cover the whole ground, and that some
future writer may say of my estimate, as I have said of Humboldt's, that
it falls short of the truth; for, if my generalizations are correct, the
same formation will be found extending over the whole basin of the
Paraguay and the Rio de la Plata, and along their tributaries, to the
very heart of the Andes.

Such are the facts. The question now arises, How were these vast
deposits formed? The easiest answer, 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. I reject this explanation for the simple reason that the
deposits show no sign whatever of a marine origin. No seashells nor
remains of any marine animal have as yet been found throughout their
whole extent, over a region several thousand miles in length and from
five to seven hundred miles in width. It is contrary to all our
knowledge of geological deposits to suppose that an ocean basin of this
size, which must have been submerged during an immensely long period in
order to accumulate formations of such a thickness, should not contain
numerous remains of the animals formerly inhabiting it.[B] The only
fossil remains of any kind truly belonging to it, which I have found in
the formation, are the leaves mentioned above, taken from the lower
clays on the banks of the Solimoens at Tomantins; and these show a
vegetation similar in general character to that which prevails there
to-day. Evidently, then, this basin was a fresh-water basin; these
deposits are fresh-water deposits. But as the Valley of the Amazons
exists to-day, it is widely open to the ocean on the east, with a gentle
slope from the Andes to the Atlantic, determining a powerful seaward
current. When these vast accumulations took place, the basin must have
been closed; otherwise the loose materials would constantly have been
carried down to the ocean.

It is my belief that all these deposits belong to the ice period in its
earlier or later phases, and to this cosmic winter, which, judging from
all the phenomena connected with it, may have lasted for thousands of
centuries, we must look for the key to the geological history of the
A