270 ANNUAL OF SCIENTIFIC DISCOVERY. 



teristic features, there is not a trace of larger or smaller boulders 

 in them; while tlie superineuml)ent drift, consisting of similar 

 parts, does not show the slightest sign of the indistinct stratilica- 

 tion characteristic of the decomposed metamorphic rocks below 

 it, nor any of the decomposed veins, hut is full of various kinds of 

 boulders of dilVt rent dimensions. The boulders have not yet 

 been traced to their origin; the majority consist of a kind of 

 greenstone, composed of nearly equal amounts of a greenish black 

 hornblende and felspar. Tliis greenstone is said by mining engi- 

 neers to be found in the Eiitre Rios on llie Parahiba, where iron 

 mines are worked in a rock like these boulders. Thus far, evi- 

 dence has been furnished of the action of glaciers only in the 

 extensive accumulations of drift similar in its characteristics to 

 Northern drift; l)ut no trace has been found of glacial action, 

 proi)('rly speaking, such as polished surfaces, scratches, and 

 furrows. 



The decomposition of the surface rocks to the extent to which 

 it takes place here is very rcmarkal)le, and points to a geological 

 agency tiuis far noi fully discussiul in our geological theories. 

 It is obvious tiiat the warm rains, falling upon the heated soil, must 

 have a very powerful action in accelerating the decomposition of 

 rocks ; it is like torrents of hot water falling for ages in succession 

 upon hot stones; and, instead of wondering at the amount of de- 

 composed rocks, we sliould rather wonder that there are any rocks 

 left in their primitive condition. 



GEOLOGY OF THE VALLEY OF THE AMAZON. 



Tlie Basin of the Amazon. — Prof. Agassiz, in his lectures before 

 the Lowell Institute, in Boston, in October and November, 18G6, 

 made the following statemcnits in regard to the geology of the val- 

 ley of the river Amazon, from his personal observation. The forma- 

 tion of this basin is the same for a distance of three thousand miles. 

 It consists: 1. Of a horizontal bed of white thinly-laminated clay, 

 on which the river Hows, and exposed at low water, this water-level 

 showing that the basin has not changed its relative position with 

 the ocean since its formation. 2. Over this is a ferruginous sand- 

 stone, in horizontal stratified layers, forty to eighty feet thick, 

 later than the old red sandstone, and even tlian the trias, to which it 

 had been refeiTed respectively by Humboldt and Martius. 3. 

 Over this, on the slopes near the water, and on the top of the flat 

 mountains seen from time to time, is an ochre-colored, very rich 

 loam, constituting the fertile lands of the valley. These flat- 

 topped hills, once a continuous mountain range, have assumed 

 their shape of rounded hills by denudation from the weather, 

 heavy rains, etc., — the only instance known of mountains made 

 by simple denudation, as these are generally the result of up- 

 heaval or subsidence. These strata must have been deposited 

 perfectly horizontal, in a walled basin ; in this valley we have the 

 Andes on the west, the mountains of Bolivia and the table-land 

 of Brazil on the south, the high land of Guiana at the north ; but 

 nothing on the east, toward the ocean. 



