PHYSICAL HISTORY OF THE AMAZOXS. 407 
well-stratified deposits, resembling somewhat the Recife of 
Bahia and Fernambuco ; 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 
