VALLEY OF THE AMAZONS. 
157 
fact that the surface of the drift, when baked 
by exposure to the hot sun, often assumes the 
appearance of decomposed rock, so that great 
care is required for a correct interpretation of 
the facts. A little practice, however, trains 
the eye to read these appearances aright, and 
I may say that I have learned to recognize 
everywhere the limit between the two forma¬ 
tions. There is, indeed, one safe guide, name¬ 
ly, the undulating line, reminding one of roches 
moutonnees , and marking the irregular surface 
of the rock on which the drift was accumu¬ 
lated ; whatever modifications the one or the 
other may have undergone, this line seems 
never to disappear. Another deceptive fea¬ 
ture, 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 examination of their struc¬ 
ture, however, will at once show the geologist 
whether they belong where they are found, or 
have been brought from a distance to their 
present resting-place. 
While the features to which I have alluded 
