extended in width has not been ascertained by direct observation,
for we have not seen how they sink away to the northward, and towards
the south the denudation has been so complete that, except in the very
low range of hills in the neighborhood of Santarem, they do not rise
above the plain. But the fact that this formation once had a thickness
of more than eight hundred feet within the limits where we have had an
opportunity of observing it, leaves no doubt that it must have extended
to the edge of the basin, filling it to the same height throughout its
whole extent. The thickness of the deposits gives a measure for the
colossal scale of the denudations by which this immense accumulation was
reduced to its present level. Here then is a system of high hills,
having the prominence of mountains in the landscape, produced by causes
to whose agency inequalities on the earth's surface of this magnitude
have never yet been ascribed. We may fairly call them denudation
mountains.

At this stage of the inquiry we have to account for two remarkable
phenomena. First, the filling of the Amazonian bottom with coarse
arenaceous materials and finely laminated clays, immediately followed by
sandstones rising to a height of more than eight hundred feet above the
sea; the basin meanwhile having no rocky barrier towards the ocean on
its eastern side. Second, the wearing away and reduction of these
formations to their present level, by a denudation, more extensive than
any thus far recorded in the annals of geology, which has given rise to
all the most prominent hills and mountain chains along the northern bank
of the river. Before seeking an explanation of these facts, let us look
at the third and uppermost deposit.

This deposit, essentially the same as the Rio drift, has been minutely
described in my former article; but in the north, it presents itself
under a somewhat different aspect. As in Rio, it is a clayey deposit,
containing more or less sand, and reddish in color, though varying from
deep ochre to a brownish tint. It is not so absolutely destitute of
stratification here as in its more southern range, though the traces of
stratification are rare, and, when they do occur, are faint and
indistinct. The materials are also more completely comminuted, and, as I
have said above, contain hardly any large masses, though quartz pebbles
are sometimes scattered throughout the deposit, and occasionally a thin
seam of pebbles, exactly as in