early
that they were both deposited by the same water-system within the same
basin, but at different levels. Here and there the clay formation has so
pale and grayish a tint, that it may be confounded with the mud deposits
of the river. These latter, however, never rise so high as the
ochraceous clay, but are everywhere confined within the limits of high
and low water. The islands also in the main course of the Amazons
consist invariably of river-mud, while those arising from the
intersection and cutting off of portions of the land by diverging
branches of the main stream always consist of the well-known sandstones,
capped by the ochre-colored clay.

It may truly be said that there does not exist on the surface of the
earth a formation known to geologists resembling that of the Amazons.
Its extent is stupendous; it stretches from the Atlantic shore, through
the whole width of Brazil, into Peru, to the very foot of the Andes.
Humboldt speaks of it "in the vast plains of the Amazons, in the eastern
boundary of Jaen de Bracamoros," and says, "This prodigious extension of
red sandstone in the low grounds stretching along the east of the Andes
is one of the most striking phenomena I observed during my examination
of rocks in the equinoctial regions."[A] When the great natural
philosopher wrote these lines, he had no idea how much these deposits
extended beyond the field of his observations. Indeed, they are not
limited to the main bed of the Amazons; they have been followed along
the banks of its tributaries to the south and north as far as these have
been ascended. They occur on the margins of the Huallaga and the
Ucayall, on those of the Ica, the Jutahy, the Jurua, the Japura, and the
Purus. On the banks of the Japura, where Major Coutinho has traced them,
they are found as far as the Cataract of Cupati. I have followed them
along the Rio Negro to its junction with the Rio Branco; and Humboldt
not only describes them from a higher point on this same river, but also
from the valley of the Orinoco. Finally, they may be tracked along the
banks of the Madeira, the Tapajos, the Xingu, and the Tocantins, as well
as on the shores of the Guatuma, the Trombetas, and other northern
affluents of the Amazons. The observations of Martius, those of Gardner,
and the recent survey above alluded to, made by my assistant, Mr. St.
John, of the valley of the Rio Guruguea and that of the Rio Paranahyba,
show that the great basin of Piauhy is a