1885.] Physical Geography of the Amazons Valley, 33 
by bluffs precisely like those along the Lower Tapajés and 
Xingu. It is evident, then, that these muddy rivers, in their 
lower courses, were once expanded into lake-like reaches, similar 
to those of the clear-water tributaries. 
‘If we now return to the northern side of the Amazons we 
shall find at least one clear-water tributary, the Trombetas, which 
is lake-like along its lower courses. But as the river here passes 
through low ¢erre-jirme, the bluffs are wanting; the borders are 
extremely irregular, and numerous small lakes open into the 
main one on either side ; islands or peninsulas of low, rocky land 
separate these lakes from the river, and smaller islands are cut off 
in the lakes or in the river itself} In fact the whole corresponds 
precisely to the irregular flood-plains of the other northern tribu- 
taries. The latter, being muddy, have filled up the lakes and 
channels with sediment, and they now wind about in broad allu- 
vial tracts, the borders of which seem to be inextricably mingled 
with the zerre-firme. 
I believe it can be shown that all the main tributaries of the 
Lower Amazons are, or have been, lake-like in their lower courses. 
_ The question then arises: Were these lakes produced by a dam- 
ming back of the tributaries by Amazonian silt, so that they filled 
up their valleys? I think not. No doubt the alluvial land, 
closing the mouths of the lakes, has tended to raise their waters; 
but the flood-plain of the Lower Amazons is everywhere so near 
the level of the sea that this uplift cannot have been very great, 
The tides, which are felt on the main Amazons as far as Obidos, 
are very apparent on the Lower Xingu and Tapajós; Bates no- 
ticed them on a secondary tributary of the latter river nearly six. 
hundred miles from the ocean. It is well, also, to note the simi- 
larity of the Tapajós and Xingu to the Tocantins, which opens. 
broadly into the sea and cannot owe its lake-like lower course to- 
any damming back of the waters. 
Having reached this point it requires but little imagination to 
apply the same reasoning to the Amazons itself; to look. upon 
the flood-plains as a filled-up sea or great bay, with many 
which now form the flood-plains or tidal-lakes of the tributaries. 
Let us go back in imagination to the period before this sea was 
filled up and map out the Amazonian system as it then was. 
Stretching eight hundred miles west-south-westward from the 
I owe these notes on the topography of the Trombetas to Professor Os A. Derby. 
VOL, XIX.—no. I, ' 3 
i 
