PHYSICAL HISTORY OF THE AMAZONS. 427 
glacial origin of which there cannot, in my opinion, be any 
doubt ; thirdly, on the fact that this fresh-water basin must 
have been closed against the sea by some powerful barrier, 
the removal of which would naturally give an outlet to 
the waters, and cause the extraordinary denudations, the 
evidences of which meet us everywhere throughout the 
valley. 
On a smaller scale, phenomena of this kind have long 
been familiar to us. In the present lakes of Northern 
Italy, in those of Switzerland, Norway, and Sweden, as 
well as in those of New England, especially in the State of 
Maine, the waters are held back in their basins by moraines. 
In the ice-period these depressions were filled with glaciers, 
which, in the course of time, accumulated at their lower 
end a wall of loose materials. These walls still remain, 
and serve as dams to prevent the escape of the waters. 
But for their moraines, all these lakes would be open 
valleys. In the Roads of Glen Roy, in Scotland, we have 
be so, it is evident, on comparing the two formations, that the ochraceous 
sandy clay of the valley of the Amazons has been deposited under different 
circumstances ; that, while it owes its resemblance to the Rio drift to the fact 
that its materials were originally ground by glaciers in the upper part of the 
valley, these materials have subsequently been spread throughout the whole 
basin and actually deposited under the agency of water. A survey of the 
more southern provinces of Brazil, extending to the temperate zone, where the 
combined effects of a tropical sun and of tropical rains must naturally be want- 
ing, will, I trust, remove all the difficulties still attending this explanation. 
The glacial phenomena, with all their characteristic features, are already known 
to cover the southernmost parts of South America. The intervening range, 
between 22 and 36 of south latitude, cannot fail to exhibit the transition 
from the drift of the cold and temperate zone to the formations of a kindred 
character described above from the tropical zone. The knowledge of these de- 
posits will definitely settle the question ; and either prove the correctness of my 
generalizations or show their absurdity. I feel no anxiety as to the result. I 
only long fur a speedy removal of all doubts. 
