214 
PHYSICAL HISTORY OF THE 
lying sandstone. To this period I refer the 
boulders of Errer6, sunk as they are in the 
clay of this final deposit. I suppose them to 
have been brought to their present position by 
floating ice at the close of the glacial period, 
when nothing remained of the ice-fields except 
such isolated masses, — ice-rafts, as it were; 
or perhaps by icebergs dropped into the basin 
from glaciers still remaining in the Andes and 
on the edges of the plateaus of Guiana and 
Brazil. From the general absence of stratifi¬ 
cation in this clay formation, it would seem 
that the comparatively shallow sheet of water 
in which it was deposited was very tranquil. 
Indeed, after the waters had sunk much below 
the level which they held during the deposition 
of the sandstone, and the currents which gave 
rise to the denudation of the latter had ceased, 
the whole sheet of water would naturally be¬ 
come much more placid. But the time came 
when the water broke through its boundaries 
again, perhaps owing to the further encroach¬ 
ment of the sea and consequent destruction of 
the moraine. In this second drainage, how¬ 
ever, the waters, carrying away a considerable 
part of the new deposit, furrowing it to its very 
