PHYSICAL HISTORY OF THE AMAZOXS. 431 
during which was deposited the ochraceous sandy clay 
resting upon the denudated surfaces of the underlying 
sandstone. To this period I refer the boulders of Erere, 
sunk as they are in the clay of this final deposit. I sup- 
pose 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 stratification 
in this clay formation, it would seem that the compar- 
atively 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 deposi- 
tion of the sandstone, and the currents which gave rise 
to the denudation of the latter had ceased, the whole sheet 
of water would naturally become much more placid. But 
the time arrived when the water broke through its boun- 
daries- again, perhaps owing to the further encroachment 
of the sea and consequent destruction of the moraine.* 
In this second drainage, however, the waters, carrying 
away a considerable part of the new deposit, furrowing 
it to its very foundation, and even cutting through it 
into the underlying sandstone, were, in the end, reduced 
to something like their present level, and confined within 
their present beds. This is shown by the fact that in 
this ochre-colored clay, and penetrating to a greater or 
less depth the sandstone below, are dug, not only the great 
* I would here remind the reader of the terraces of Glen Roy, which indicate 
successive reductions of the barrier encasing the lake, similar to those assumed 
to have taken place at the mouth of the Amazons. 
