210 
PHYSICAL HISTORY OF THE 
it from immediate contact with the valley bot¬ 
tom, and thus giving room for the accumula¬ 
tion of a certain amount of water beneath it; 
while the valley as a whole would still be occu¬ 
pied by the glacier. In this shallow sheet of 
water under the ice, and protected by it from 
any violent disturbance, those finer triturated 
materials always found at a glacier bottom, 
and ground sometimes to powder by its action, 
would be deposited, and gradually transformed 
from an unstratified paste containing the finest 
sand and mud, together with coarse pebbles and 
gravel, into a regularly stratified formation. 
In this formation the coarse materials would 
of course fall to the bottom, while the most 
minute would settle above them. It is at this 
time and under such circumstances that I be¬ 
lieve the first formation of the Amazonian Val¬ 
ley, with the coarse, pebbly sand beneath, and 
the finely laminated clays above, to have been 
accumulated. 
I shall perhaps be reminded here of my fos¬ 
sil leaves, and asked how any vegetation would 
be possible under such circumstances. But it 
must be remembered, that, in considering all 
these periods, we must allow for immense 
