ations. My conviction in this instance is
founded, first, on the materials in the Amazonian Valley, which
correspond exactly in their character to materials accumulated in
glacier bottoms; secondly, on the resemblance of the upper or third
Amazonian formation to the Rio drift,[C] of the glacial origin of which
there cannot, in my opinion, be any doubt; thirdly, on the fact that
this fresh-water basin must have been closed against the sea by some
powerful barrier, the removal of which would naturally give an outlet to
the waters, and cause the extraordinary denudations, the evidences of
which meet us everywhere throughout the valley.

On a smaller scale, phenomena of this kind have long been familiar to
us. In the present lakes of Northern Italy, in those of Switzerland,
Norway, and Sweden, as well as in those of New England, especially in
the State of Maine, the waters are held back in their basins by
moraines. In the ice period these depressions were filled with glaciers,
which, in the course of time, accumulated at their lower end a wall of
loose materials. These walls still remain, and serve as dams to prevent
the escape of the waters. But for their moraines, all these lakes would
be open valleys. In the Roads of Glen Roy, in Scotland, we have an
instance of a fresh-water lake, which has now wholly disappeared, formed
in the same manner, and reduced successively to lower and lower levels
by the breaking down or wearing away of the moraines which originally
prevented its waters from flowing out. Assuming then, that, under the
low temperature of the ice period, the climatic conditions necessary for
the formation of land-ice existed in the Valley of the Amazons, and that
it was actually filled with an immense glacier, it follows that, when
these fields of ice yielded to a gradual change of climate, and slowly
melted away, the whole basin, then closed against the sea by a huge
wall of _debris_, was transformed into a vast fresh-water lake. The
first effect of the thawing process must have been to separate the
glacier from its foundation, raising it from immediate contact with the
valley bottom, and thus giving room for the accumulation of a certain
amount of water beneath it; while the valley as a whole would still be
occupied by the glacier. In this shallow sheet of water under the ice,
and protected by it from any violent disturbance, those finer triturated
materials always found at a glacier bottom, and ground sometime