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 of 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 



