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 tho 

 geography of the country presented itself to my rnind as 

 a living reality in all its completeness. Insignificant as 

 is its actual height, the Serra of Ercru commands a 



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