62 Mr. H. Woodward on the Tertiary Shells 



formation does not reach back to an epoch anterior to the 

 excavation of the valley itself. The total thickness of the 

 Amazonian drift does not exceed 300 metres (984 feet) ; it 

 covers the whole basin of the Amazons, from the Andes of 

 Peru and Bolivia to Cape Sao B-oque; or, in other words, it is 

 the most colossal drift formation kno■w^l. 



'' Professor Agassiz believes that the Beds I., II., III. IV., 

 or the coarse sands and clays, were deposited in a lake or 

 sheet of fresh water occupying the valley of the Amazons, and 

 sustaining on its suiface a glacier descending eastward from 

 the Andes, and furnished with a gigantic moraine in front 

 sti'etching across the mouth of the valley and converting it 

 into an inland freshwater lake. After the ice had broken up 

 and become more or less disintegrated, and the waters of the 

 lake had swollen, the sandstone formation V., VI., VII., VIII., 

 IX. was laid down; then the barrier was burst; the waters of 

 the lake, suddenly released, fmTOwed and wore down the sand- 

 stone beds, sweeping them entirely away over an immense 

 area, leaving only isolated hills, like those of Ererd, Obydos, 

 Cupati, Almeyi'im, &c., standing as remnants of the once uni- 

 versal sandstone sheet. After this period of turbulence and 

 denudation came on an epoch of quiet, and in the bottom of 

 the diminished lake the clays (No. X.) were deposited, while 

 ice-rafts floating on its smface di'opped here and there 

 boulders to be buried in the accumulating material. Then 

 the moraine was destroyed ; the drainage of the waters fui*- 

 rowed deeply those clays, and even cut through them into 

 the sandstone below, in which the various channels of the 

 system of the Amazons are excavated. Professor Agassiz 

 believes that the great barrier stretched across the Amazonian 

 valley far eastward of its present extremity ; and he has called 

 attention to the similarity between the formations found spread 

 over the coast of Maranhao and Piauhy and the Amazonian 

 formations here described, showing conclusively that these 

 deposits were once continuous. It is his belief that the Ama- 

 zonian formation formerly extended a hundred leagues out to 

 sea beyond the present mouth of the Amazons. There can be 

 no doubt that there is a rapid waste of land now going on 

 along the sea-shores of the mouth of the Amazons and of the 

 coast eastward for a long distance, a waste amounting to even 

 so much as two hundred yards in ten years in the Bay of 

 Braganza — or a mile in twenty, as on the coast near Vigia, 

 where an island a mile wide disappeared in that time. Since 

 the Tertiary period," saj's Professor Hartt, " at least, and, I 

 believe, for the greater part since the drift, the whole Eastern 

 Brazilian coast has suffered denudation by the sea to an im- 



