PHYSICAL HISTORY OF THE AMAZONS. 437 
the last half-century, little attention has been paid to 
the results connected with the breaking up of the geo- 
logical winter and the final disappearance of the ice. I 
believe that the true explanation of the presence of a 
large part of the superficial deposits lately ascribed to 
the agency of the sea, during temporary subsidences of 
the land, will be found in the melting of the ice-fields. 
To this cause I would refer all those deposits which I 
have designated as remodelled drift. When the sheet of 
ice, extending from the Arctic regions over a great part 
of North America and coming down to the sea, slowly 
melted away, the waters were not distributed over the 
face of the country as they now are. They rested upon 
the bottom deposits of the ice-fields, upon the glacial paste, 
consisting of clay, sand, pebbles, boulders, etc., underlying 
the ice. This bottom deposit did not, of course, present 
an even surface, but must have had extensive undulations 
and depressions. After the waters had been drained off 
from the more elevated ridges, these depressions would 
still remain full. In the lakes and pools thus formed, 
stratified deposits would be accumulated, consisting of the 
most minutely comminuted clay, deposited in thin lami- 
nated layers, or sometimes in considerable masses, without 
any sign of stratification ; such differences in the formation 
being determined by the state of the water, whether per- 
fectly stagnant or more or less agitated. Of such pool 
deposits overlying the drift there are many instances in 
the Northern United States. By the overflowing of some 
of these lakes, and by the emptying uf the higher ones 
into those on a lower level, channels would gradually be 
formed between the depressions. So began to be marked 
out our independent river-systems, the waters always 
