VALLEY OF THE AMAZONS. 
193 
worn to the same low level. The opposite 
range of Santarem, which has the same gen¬ 
eral outline and character, shares, no doubt, the 
same geological structure. 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 surface 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 current which 
caused the denudation, while their level sum¬ 
mits are due to the regularity of the stratifica¬ 
tion. They are not all table-topped, however; 
among them are many of smaller size, in which 
the sides have been gradually worn down, pro¬ 
ducing 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 leave this serra without alluding 
9 
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