212 
PHYSICAL HISTORY OF THE 
of the main stream must have been rushing 
towards the axis of the valley, seeking its nat¬ 
ural level, but spreading over a more extensive 
surface than now, until, finally gathered up 
as separate rivers, it flowed in distinct beds. 
In its general movement toward the central 
and lower part of the valley, the broad stream 
would carry along all the materials small 
enough to be so transported, as well as those 
so minute as to remain suspended in the 
waters. It would gradually deposit them in 
the valley bottom in horizontal beds, more or 
less regular, or here and there, wherever ed¬ 
dies gave rise to more rapid and irregular 
currents, characterized by torrential stratifi¬ 
cation. Thus has been consolidated in the 
course of ages that continuous sand formation 
spreading over the whole Amazonian basin, 
and attaining a thickness of eight hundred 
feet. 
While these accumulations were taking place 
within this basin, it must not be forgotten that 
the sea was beating against its outer walls,— 
against that gigantic moraine which I suppose 
to have closed it at its eastern end. It would 
seem that, either from this cause, or perhaps 
