VALLEY OF THE AMAZONS. 
223 
winter and the final disappearance of the ice. 
I believe that the true explanation of the pres¬ 
ence of a large part of the superficial deposits 
lately ascribed to the agency of the sea dur¬ 
ing 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 in former publications as 
remodelled drift. When the sheet of ice, ex¬ 
tending 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 de¬ 
pressions. After the waters had been drained 
off from the more elevated ridges, these de¬ 
pressions would still remain full. In the lakes 
and pools thus formed, stratified deposits would 
be accumulated, consisting of the most mi¬ 
nutely comminuted clay, deposited in thin lam¬ 
inated layers, or sometimes in considerable 
