764 C. B. BROWN ON THE ANCIENT 
They evidently at one time filled a great part of the valley of the 
Amazon, and can now be traced from Montealegré to Manaos, but 
are not found on the banks of the Upper Amazon, or Solimées, as it 
is usually termed. 
Both the elevated plateau of the ancient river-deposit and the 
plain are, as a general rule, thickly clothed with trees and vegetation 
of the most luxuriant description. With the exception of a narrow 
strip, in many places along the river’s edge, which is raised a foot or 
two above the water-level in time of floods, the whole of the allu- 
vial plain is at certain seasons covered with a sheet of water, pro- 
duced by the rising of the river’s surface above the level of its 
banks. On these occasions, which occur periodically, the inhabitants 
who cultivate the rich alluvial soil in the immediate vicinity of the 
river’s borders retire to the small towns, villages, and hazéndas 
built on the elevated river-deposit. 
The cliffs then seen from the river are the face-sections of an 
elevated tract of almost level country, spreading continuously along 
the course of the Amazon in both northerly and southerly directions 
for many miles, and terminating generally where the underlying 
rocky stratum, by its superior elevation, has been left untouched by 
fluviatile action. They afford the means of determining the compo- 
sition of the whole area occupied by the deposit in the vicinity of the 
Amazon ; whilst similar sections on the borders of most of the great 
tributaries give equal facilities for studying it in opposite directions. 
Their crests vary in height from levels of 10 to 160 feet* above the 
river's flood-mark, whilst their bases are generally exposed when the 
river is at a low ebb. 
From hasty examinations made in numerous places, I have, as a 
general rule, found the cliffs to be composed of layers of sand and 
clay, exhibiting every degree of false bedding, and varying greatly 
in structure and composition when traced in lateral directions. 
The sand beds are of white, red, yellow, or variegated colours, 
while the clay beds are chiefly bluish or variegated. No very coarse 
materials ever enter into their composition. Resting upon, and 
sometimes passing into, them is a homogeneous deposit of reddish © 
loam, capped by a yellow clay or a dark vegetable soil. 
In order to afford a clear idea of the structure of these cliffs, I 
here give some sections, taken in various places, the localities of 
which will: be seen by reference to the accompanying map (PI. 
XXXVIII.). The measurements of these are approximate only, as, 
owing to the often inaccessible nature of the cliffs and to the short 
time at. my disposal, exact measurements could not be taken. 
Beginning with the Lower Amazon, I give a sketch section of a 
portion of the Barreiras (cliffs) of Montealegré (fig. 2), the vertical 
scale of which is exaggerated in order to show their structure 
more clearly. 
* In my paper on the Tertiary deposits of the Amazon, in the Quarterly 
Journal of the Society, vol. xxxy. part 1, I said that the old river-deposit 
is seen in cliff-sections ‘‘ ranging from 10 to 800 feet in height.” Since that was 
written I have gone more thoroughly into the matter, and haye modified my 
views regarding its thickness. 
