if they were exclusively due to one or the other, and whoever accepted
the former must reject the latter, and _vice versa_. When geologists
have combined these now discordant elements, and consider these two
periods as consecutive,--part of the phenomena being due to the
glaciers, part to the icebergs and to freshets consequent on their
breaking up,--they will find they have covered the whole ground, and
that the two theories are perfectly consistent with each other. I think
the present disputes upon this subject will end somewhat like those
which divided the Neptunic and Plutonic schools of geologists in the
early part of this century; the former of whom would have it that all
the rocks were due to the action of water, the latter that they were
wholly due to the action of fire. The problem was solved, and harmony
restored, when it was found that both elements had been equally at work
in forming the solid crust of the globe. To the stranded icebergs
alluded to above, I have no doubt, is to be referred the origin of the
many lakes without outlet existing all over the sandy tract along our
coast of which Cape Cod forms a part. Not only the formation of these
lakes, but also that of our salt marshes and cranberry-fields, I believe
to be connected with the waning of the ice period.

I hope at some future time to publish in detail, with the appropriate
maps and illustrations, my observations on our coast changes, and upon
other phenomena connected with the close of the glacial epoch in the
United States. It is reversing the natural order of things to give
results without the investigations which have led to them; and I should
not have introduced the subject here except to show that the fresh-water
denudations and the oceanic encroachments which have formed the
Amazonian Valley, with its river system, are not isolated facts, but
that the process has been the same in both continents. The extraordinary
continuity and uniformity of the Amazonian deposits are due to the
immense size of the basin enclosed, and the identity of the materials
contained in it.

A glance at any geological map of the world will show the reader that
the Valley of the Amazons, so far as any attempt is made to explain its
structure, is represented as containing isolated tracts of Devonian,
Triassic, Jurassic, cretaceous, tertiary, and alluvial deposits. As is
shown by the above sketch, this is wholly inaccurate; and whatever may
be thought of my interpret