426 A JOURNEY IN BRAZIL. 
or reducing them to small pebbles, and it must have 
accumulated at its lower end a moraine of proportions 
as gigantic as its own ; thus building a colossal Sea-wall 
across the mouth of the valley. I shall be asked at once 
whether I have found here also the glacial inscriptions^ 
the furrows, stria3, and polished surfaces so character- 
istic of the ground over which glaciers have travelled. 
I answer, not a trace of them ; for the simple reason that 
there is not a natural rock-surface to be found through- 
out the whole Amazonian Valley. The rocks themselves 
are of so friable a nature, and the decomposition caused 
by the warm torrential rains and by exposure to the 
burning sun of the tropics so great and imeeasing, that 
it is hopeless to look for marks which in colder climates 
and on harder substances are preserved through ages un- 
changed. With the exception of the rounded surfaces 
so well known in Switzerland as the roches moutonnees 
heretofore alluded to, which may be seen in many locali- 
ties, and the boulders of Erere, the direct traces of gla- 
ciers as seen in other countries are wanting in Brazil. 
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 investigations. 
My conviction in this instance is founded, first, on the 
materials in the Amazonian 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 
* As I have stated in the beginning, I am satisfied that the unstratih'ed 
clay deposit of Rio and its vicinity is genuine glacial drift, resulting from the 
grinding of the loose materials interposed between the glacier and the solid 
rock in place, and retaining to this day the position in which it was left by the 
ice. Like all such accumulations, it is totally free from stratification. If this 
