428 A JOURNEY IN BRAZIL. 
an instance of a fresh-water lake, which has now wholly 
disappeared, formed in the same manner, and reduced suc- 
cessively to lower and lower levels by the breaking down or 
wearing away of the moraines which originally prevented 
its waters from flowing out. Assuming then that, under 
the low temperature of the ice-period, the climatic condi- 
tions necessary for the formation of land-ice existed in the 
valley of the Amazons, and that it was actually filled with 
an immense glacier, it follows that, when these fields of ice 
yielded to a gradual change of climate, and slowly melted 
away, the whole basin, then closed against the sea by a 
huge wall of debris, was transformed into a vast fresh- 
water lake. The first effect of the thawing process must 
have been to separate the glacier from its foundation, rais- 
ing it from immediate contact with the valley bottom, and 
thus giving room for the accumulation of a certain amount 
of water beneath it ; while the valley as a whole would still 
be occupied by the glacier. In this shallow sheet of water 
under the ice, and protected by it from any violent disturb- 
ance, those finer triturated materials always found at a 
glacier bottom, and ground sometimes to powder by its 
action, would be deposited, and gradually transformed from 
an unstratified paste containing the finest sand and mud, 
together with coarse pebbles and gravel, into a regularly 
stratified formation. In this formation the coarse materials 
would of course fall to the bottom, while the most minute 
would settle above them. It is at this time and under 
such circumstances that I believe the first formation of 
the Amazonian Valley, with the coarse, pebbly sand beneath, 
and the finely laminated clays above, to have been accu- 
mulated. 
I shall perhaps be reminded here of my fossil leaves, 
