36 ANNALS NEW YORK ACADEMY OF SCIENCES 



plausible, then, that the massive Amazon could have opened up its pres- 

 ent wide swampy valley and built up an extensive delta in a compara- 

 tively short time? 



That the Amazon has swampy margins is no more an objection to its 

 reversal than the original swampy margins of the Mississippi would be 

 to its reversal. The Amazon, like the Mississippi, if it was reversed, 

 has washed away the most of the old divide. 



Little or no exploration has been done far away from the forested 

 margins of the lower Amazon and its affluents. But an inland trip in 

 any of the region between Santerem and Obidos will usually reveal 

 sandy campos which are typical of the entire Piano Alto formations. 

 When this region is carefully explored, I feel sure that the old divide 

 will be definitely located. 



The view which I have expressed concerning the East Andean Sea 

 offers a ready explanation of the origin of the peculiar marine-like fauna 

 found in Lake Titicaca. Lake Titicaca was doubtlessly once connected 

 with the East Andean Sea by a stream. When the East Andean Sea 

 began to disappear, some of its fauna entered or was cut off in the Titi- 

 caca basin. When the Andes rose still higher, the amount of rainfall 

 •became more and more reduced until the exit of Lake Titicaca became 

 severed. 



Eigenmarm is states that Steinmann considers that the Titicaca basin 

 was a fresh-water basin whose southeastern exit was dammed by glaciers. 

 Glaciers may have assisted in closing the exit of Lake Titicaca, if they 

 were active at the time when the rainfall was so much reduced that the 

 precipitation scarcely exceeded evaporation; but it is evident that the 

 reduction of the rainfall due to the rise of the Andes was the more im- 

 portant factor. Otherwise, heavy rains would soon have filled up the 

 basin sufficiently to make a new exit- The Great Lakes of North America 

 were glaciated, and yet they made new exits. 



The absence of Manaius, Arapaima and Osieoglossum, above the Ma- 

 deira Falls and their presence above the falls of Tocantins, Tapajos, and 

 in some of the coastal streams north and south of the Amazon are ex- 

 actly what one would expect to find according to all of the facts which 

 have been considered. These three genera originally lived in the coastal 

 streams. When the Amazon became reversed, the Mamore, which had 

 previously flowed south west, changed its direction and suddenly formed 

 the Madeira Falls, Avhich have been barriers to the migration of these 

 three genera. 



Further zoological and topographical data could be given in support 



1S Princeton Patagonian Reports, Pt. Ill, p. 372. 1909. 



