PHYSICAL HISTORY OF THE AMAZONS. 401 
dulating line, reminding one of roclies moutonnees* and 
marking the irregular surface of the rock on which the 
drift was accumulated ; whatever modifications the one 
or the other may have undergone, this line seems never 
to disappear. Another deceptive feature, arising from the 
frequent disintegration of the rocks and from the brittle 
character of some of them, is the presence of loose frag- 
ments, which simulate erratic boulders, but are in fact only 
detached masses of the rock in place. A careful examina- 
tion of their structure, however, will at once show the geolo- 
gist whether they belong where they are found, or have been 
brought from a distance to their present resting-place. 
But, while the features to which I have alluded are 
unquestionably drift phenomena, they present in their 
wider extension, and especially in the northern part of 
Brazil, some phases of glacial action hitherto unobserved. 
Just as the investigation of the ice-period in the United 
States has shown us that ice-fields may move over open 
level plains, as well as along the slopes of mountain val- 
leys, so does a study of the same class of facts in South 
America reveal new and unlooked-for features in the his- 
tory of the ice-period. Some will say that the fact of 
the advance of ice-fields over an open country is by no 
means established, inasmuch as many geologists believe 
all the so-called glacial traces - - viz. striae, furrows, polish, 
etc., found in the United States to have been made by 
floating icebergs at a time when the continent was sub- 
* The name consecrated by De Saussure to designate certain rocks in Swit- 
zerland which have had their surfaces rounded under the action of the glaciers. 
Their gently swelling outlines are thought to resemble sheep resting on the 
ground, and for this reason the people in the Alps call them roches mou- 
tonntes. 
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