THE AMAZON VALLEY. 295 



been going on, during the whole period of the elevation of the 

 Andes and the mountains of Brazil and Guiana, from the 

 ocean. At the commencement of this period, the greater 

 portion of the valleys of the Amazon, Orinooko, and La Plata 

 must have formed a part of the ocean, separating the groups of 

 islands (which those elevated lands formed on their first 

 appearance) from each other. The sediment carried down 

 into this sea by the rapid streams, running down the sides of 

 these mountains, would tend to fill up and level the deeper 

 and more irregular depressions, forming those large tracts of 

 alluvial deposits we now find in the midst of the granite 

 districts. At the same time volcanic forces were in operation, 

 as shown by the isolated granite peaks which in many places 

 rise out of the flat forest district, like islands from a sea of 

 verdure, because their lower slopes, and the valleys between 

 them, have been covered and filled up by the sedimentary 

 deposfts. This simultaneous action of the aqueous and 

 volcanic forces, of submarine earthquakes and marine currents, 

 shaking up, as it were, and levelling the mass of sedimentary 

 matter brought down from the now increasing surface of dry 

 land, is what has produced that marvellous regularity of surface, 

 that gradual and imperceptible slope, which exists over such an 

 immense area.* 



At the point where the mountains of Guiana approach 

 nearest to the chain of the Andes, the volcanic action appears 

 to have been continued in the interval between them, throwing 

 up the serras of Curicuriari, Tunuhy, and the numerous 

 smaller granite mountains of the Uaupes ; and it is here 

 probably that dry land first appeared, connecting Guiana and 

 New Granada, and forming that slightly elevated ridge which 

 is now the watershed between the basins of the Amazon and 

 Orinooko. The same thing occurs in the southern part of the 

 continent, for it is where the mountains of Brazil, and the 

 eastern range of the Bolivian Andes, stretch out to meet each 

 other, that the sedimentary deposits in that part appear to have 

 been first raised above the water, and thus to have determined 

 the limits of the basin of the Amazon on the south. The 

 Amazon valley would then have formed a great inland gulf or 



* The isolated granite domes and pillars show that the whole area has 

 been formerly covered with thick sedimentary rocks, which have been 

 removed by denudation. 



