PHYSICAL HISTORY OF THE AMAZONS. 401 



(Mating line, reminding one of roches 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 thefr 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- 

 twn&es. 



z 



