Geology of the Amazon. 281 



all rapids, it is 1160 according to Humboldt ; and at the 

 junction of Araguaia with the Tocantins, it is 200 accord- 

 ing to Castelnau. These barometrical measurements repre- 

 sent the basin of the Amazon as a shallow trough lying 

 parallel to the equator, the southern side having double 

 the inclination of the northern, and the whole gently slop- 

 ing eastward. Farthermore, the channel of the great 

 river is not in the centre of the basin, but lies to the north 

 of it : thus, the hills of Almeyrim rise directly from the 

 river, while the first falls on the Tocantins, Xingu, and Ta- 

 pajos are nearly two hundred miles above their mouths ; the 

 rapids of San Gabriel, on the ISTegro, are one hundred and 

 seventy-five miles from the Amazon, while the first ob- 

 struction to the navigation of the Madeira is a hundred 

 miles farther from the great river. 



Of the creation of this valley we have already spoken. 

 No region on the face of the globe of equal extent has 

 such a monotonous geology. Around the rim of the basin 

 are the outcroppings of a cretaceous deposit ; this rests on 

 the hidden mezozoic and palaeozoic strata which form 

 the ribs of the Andes. Above it, covering the whole basin 

 from New Granada to the Argentine Republic,* are the f ol 

 lowing formations : first, a stratified accumulation of sand ; 

 second, a series of laminated clays, of divers colors, without 

 a pebble ; third, a fine, compact sandstone ; fourth, a coarse, 

 porous sandstone, so ferruginous as to resemble bog iron- 

 ore. This last was, originally, a thousand feet in thickness, 

 but was worn dio^n, jyerhaps, in some sudden escape of the 



* Messrs. Myers and Forbes found this red clay on the Negro, most abund- 

 antly near Barcellos ; also in small quantities on the Orinoco above Maipures. 

 The officers of the " Morona" assured us that the same formation was trace- 

 able far up the Ucayali and Hiiallaga. This clay from the Amazon, as ex- 

 amined microscopically by Prof. H. James Clark, contains fragments of gas- 

 teropod shells and bivalve casts. The red earth of the Pampas, according to 

 Ehrenberg, contains eight fresh-water to one salt-water animalcule. 



