here and there, wherever eddies gave rise to more rapid and irregular
currents, characterized by torrential stratification. Thus has been
consolidated in the course of ages that continuous sand formation
spreading over the whole Amazonian basin, and attaining a thickness of
eight hundred feet.

While these accumulations were taking place within this basin, it must
not be forgotten that the sea was beating against its outer
walls,--against that gigantic moraine which I suppose to have closed it
at its eastern end. It would seem that, either from this cause, or
perhaps in consequence of some turbulent action from within, a break was
made in this defence, and the waters rushed violently out. It is very
possible that the waters, gradually swollen at the close of this period
by the further melting of the ice, by the additions poured in from
lateral tributaries, by the rains, and also by the filling of the basin
with loose materials, would overflow, and thus contribute to destroy
the moraine. However this may be, it follows from my premises that, in
the end, these waters obtained a sudden release, and poured seaward with
a violence which cut and denuded the deposits already formed, wearing
them down to a much lower level, and leaving only a few remnants
standing out in their original thickness, where the strata were solid
enough to resist the action of the currents. Such are the hills of Monte
Alegre, of Obydos, Almeyrim, and Cupati, as well as the lower ridges of
Santarem. This escape of the waters did not, however, entirely empty the
whole basin; for the period of denudation was again followed by one of
quiet accumulation, during which was deposited the ochraceous sandy clay
resting upon the denudated surfaces of the underlying sandstone. To this
period I refer the boulders of Errere, sunk as they are in the clay of
this final deposit. I suppose them to have been brought to their present
position by floating ice at the close of the glacial period, when
nothing remained of the ice-fields except such isolated
masses,--ice-rafts as it were; or perhaps by icebergs dropped into the
basin from glaciers still remaining in the Andes and on the edges of the
plateaus of Guiana and Brazil. From the general absence of
stratification in this clay formation, it would seem that the
comparatively shallow sheet of water in which it was deposited was very
tranquil. Indeed, after the waters had sunk much below the level which
they held during