VALLEY OF THE AMAZONS. 
207 
a natural rock surface to be found throughout 
the whole Amazonian Yalley. The rocks them¬ 
selves are of so friable a nature, and the decom¬ 
position caused by the warm torrential rains 
and by exposure to the burning sun of the 
tropics so great and unceasing, that it is hope¬ 
less to look for marks which in colder climates 
and on harder substances are preserved through 
ages unchanged. With the exception of the 
rounded surfaces so well known in Switzerland 
as the rodies moutonnees , heretofore alluded 
to, which may be seen in many localities, and 
the boulders of Errere, the direct traces of 
glaciers as seen in other countries are wanting 
here. I am, indeed, quite willing to admit 
that, from the nature of the circumstances, 
I have not here the positive evidence which 
has guided me in my previous glacial inves¬ 
tigations. My conviction in this instance is 
founded, first, on the materials in the Ama¬ 
zonian Valley, which correspond exactly in 
their character to materials accumulated in 
glacier bottoms ; secondly, on the resemblance 
of the upper or third Amazonian formation to 
the Rio drift,* of the glacial origin of which 
* As I have stated in the beginning, I am satisfied that the 
