PHYSICAL HISTORY OF THE AMAZONS. 417 
deposits already described, and to have exactly the same 
geological constitution. In short, the Serra of Monte 
Aldgre, and of course all those connected with it on the 
northern side of the river, lie in the prolongation of the 
lower beds forming the banks of the river, their greater 
* 
height being due simply to the fact that they have not 
been worn to the same low level. The opposite range 
of Santarem, which has the same general outline and 
character, shares, no doubt, the same geological struc- 
ture. In one word, all these hills were formerly part of 
a continuous formation, and owe their present outline and 
their isolated position to a colossal denudation. The sur- 
face of the once unbroken strata, which in their original 
condition must have formed an immense plain covered 
by water, has been cut into ravines or carried away over 
large tracts, to a greater or less depth, leaving only such 
portions standing as, from their hardness, could resist the 
floods which swept over it. The longitudinal trend of 
these hills is to be ascribed to the direction of the cur- 
rent which caused the denudation, while their level sum- 
mits are due to the regularity of the stratification. They 
are not all table-topped, however ; among them are many 
of smaller size, in which the sides have been gradually 
worn down, producing a gently rounded surface. Of 
course, under the heavy tropical rains this denudation is 
still going on, though in a greatly modified form. 
I cannot speak of this Serra without alluding to the great 
beauty and extraordinary extent of the view to be obtained 
from it. Indeed, it was here that for the first time the- 
geography of the country presented itself to my mind as 
a living reality in all its completeness. Insignificant as 
is its actual height, the Serra of Erere commands a 
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