VALLEY OF THE AMAZONS. 
the Amazonian clay rests exists nowhere 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 Andes 
did not exist, and the ocean flowed between 
them through an open strait. It would seem 
(and this is a curious result of modern geo¬ 
logical 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 Newfoundland almost to the 
present base of the Rocky Mountains. This 
tendency may be attributed to various causes, 
— to the rotation of the earth, the consequent 
depression of its poles, and the breaking of 
its crust along the lines of greatest tension 
thus produced. At a later period, the up- 
