9] At Coca, on the Napo, the altitude
is 850 feet, according to our observations; at Tinga Maria. on the
Huallaga, it is 2200 according to Herndon; at the junction of the Negro
with the Cassiquiari, it is 400 according to Wallace; at the mouth of
the Marmore, it is 800 according to Gibbon; at the Pongo de Manseriche,
below all rapids, it is 1160 according to Humboldt; and at the junction
of Araguaia with the Tocantins, it is 200 according to Castelnau. These
barometrical measurements represent the basin of the Amazon as a shallow
trough lying parallel to the equator, the southern side having double
the inclination of the northern, and the whole gently sloping eastward.
Farthermore, the channel of the great river is not in the centre of the
basin, but lies to the north of it: thus, the hills of Almeyrim rise
directly from the river, while the first falls on the Tocantins, Xingu,
and Tapajos are nearly two hundred miles above their mouths; the rapids
of San Gabriel, on the Negro, are one hundred and seventy-five miles
from the Amazon, while the first obstruction to the navigation of the
Madeira is a hundred miles farther from the great river.

[Footnote 159: Professor Agassiz gives the average slope as hardly more
than a foot in ten miles, which is based on the farther assertion that
the distance from Tabatinga to the sea-shore is more than 2000 miles in
a straight line. The distance is not 1600, or exactly 1500, from
Para.--See _A Journey in Brazil_, p. 349.]

Of the creation of this valley we have already spoken. No region on the
face of the globe of equal extent has such a monotonous geology. Around
the rim of the basin are the outcroppings of a cretaceous deposit; this
rests on the hidden mezozoic and palaeozoic strata which form the ribs of
the Andes. Above it, covering the whole basin from New Granada to the
Argentine Republic,[160] are the following formations: first, a
stratified accumulation of sand; second, a series of laminated clays, of
divers colors, without a pebble; third, a fine, compact sandstone;
fourth, a coarse, porous sandstone, so ferruginous as to resemble bog
iron-ore. This last was, originally, a thousand feet in thickness, but
was worn down, _perhaps_, in some sudden escape of the pent-up waters
of the valley. The table-topped hills of Almeyrim are almost the sole
relics.[161] Finally, over the undulating surface of the denuded
sandstone an ochraceous, unstratified sandy clay was deposited.

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