PHYSICAL HISTORY OF THE AMAZONS. 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 Ererd, 

 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 \vliich 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. 



