428 



A JOURNEY IN BRAZIL. 



an instance of a fresh-water lake, which has now wholly 

 disappeared, formed in the same manner, and reduced suc- 

 cessively to lower and lower levels by the breaking down or 

 wearing away of the moraines which originally prevented 

 its waters from flowing out. Assuming then that, under 

 the low temperature of the ice-period, the climatic condi- 

 tions necessary for the formation of land-ice existed in the 

 valley of the Amazons, and that it was actually filled with 

 an immense glacier, it follows that, when these fields of ice 

 yielded to a gradual change of climate, and slowly melted 

 away, the whole basin, then closed against the sea by a 

 huge wall of debris, was transformed into a vast fresh- 

 water lake. The first effect of the thawing process must 

 have been to separate the glacier from its foundation, rais- 

 ing it from immediate contact with the valley bottom, and 

 thus giving room for the accumulation of a certain amount 

 of water beneath it ; while the valley as a whole would still 

 be occupied by the glacier. In this shallow sheet of water 

 under the ice, and protected by it from any violent disturb- 

 ance, 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 believe the first formation of 

 the Amazonian Yalley, with the coarse, pebbly sand beneath, 

 and the finely laminated clays above, to have been accu- 

 mulated. 



I shall perhaps be reminded here of my fossil leaves, 



