PHYSICAL HISTORY OF THE VALLEY OF THE AMAZONS.
II.
MAJOR COUTINHO aand myself passed three days in the investigation of the Serra of Erreré. We found it to consist wholly of the sandstone deposits described in my previous article, and to have exactly the same geological constitution. In short, the Serra of Monte Alegre, 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 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 summits 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 leave 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 Erreré commands a wider prospect than is to be had from many a more imposing mountain; for the surrounding plain, covered with forests, and ploughed by countless rivers, stretches away for hundreds of leagues in every direction, without any object to obstruct the view. Standing on the brow of the Serra, with the numerous lakes intersecting the low lands at its base, you look across the Valley of the Amazons, as far as the eye can reach, and through its midst you follow for miles on either side the broad flood of the great river, carrying its yellow waters to the sea. As I stood there, panoramas from the Swiss mountains came up to my memory, and I fancied myself standing on the Alps, looking across the plain of Switzerland, instead of the bed of the Amazons, the distant line of the Santarem hills on the southern bank of the river, and lower than the northern chain, representing the Jura range. As if to complete the comparison, I found Alpine lichens growing among cactus and palms, and a crust of Arctic cryptogamous growth covered rocks, between which sprang tropical flowers. On the northern flank of this Serra I found the only genuine erratic boulders I have seen in the whole length of the Amazonian Valley, from Pará to the frontier of Peru, though there are many detached masses of rock, as, for instance, at Pedreira, near the junction of the Rio Negro and Rio Branco, which might be mistaken for them, but are due to the decomposition of the rocks in place. The boulders of Erreré are entirely distinct from the rock of the Serra, and consist of masses of compact hornblende.
It would seem that these two ranges
