PHYSICAL HISTORY OF THE AMAZONS. 407 



well-stratified deposits, resembling somewhat the Recife of 

 Bahia and Pernarnbuco ; whereas the unstratified drift of 

 the south rests immediately upon the undulating surface 

 of whatever rock happens to make the foundation of the 

 country, whether stratified or crystalline. The peculiar 

 sandstone on which the Amazonian clay rests exists no- 

 where else. Before proceeding, however, to describe the 

 Amazonian deposits in detail, I ought to say something 

 of the nature and origin of the valley itself. 



The valley of the Amazons was first sketched out by 

 the elevation of two tracts of land ; namely, the plateau 

 of Guiana on the north, and the central plateau of Brazil 

 on the south. It is probable that, at the time these 

 two table-lands were lifted above the sea-level, the An- 

 des did not exist, and the ocean flowed between them 

 through an open strait. It would seem (and this is a 

 curious result of modern geological investigations) that 

 the portions of the earth's surface earliest raised above 

 the ocean have trended from east to west. The first 

 tract of land lifted above the waters in North America 

 was also a long continental island, running from New- 

 foundland almost to the present base of the Rocky Moun- 

 tains. This tendency may be attributed to various causes, 



% 



to the rotation of the earth, the consequent depres- 

 sion of its poles, and the breaking of its crust along the 

 lines of greatest tension thus produced. At a later 

 period, the upheaval of the Andes took place, closing 

 the western side of this strait, and thus transforming it 

 into a gulf, open only toward the east. Little or nothing 

 is known of the earlier stratified deposits resting against 

 the crystalline masses first uplifted along the borders of the 

 Amazonian Valley. There is here no sequence, as in North 



