360 



to parts of pre-existing western lands. All through Tertiary 

 time the mountains must have continued to rise, though our 

 knowledge of later geological history is still meagre. We know 

 that Tertiary marine deposits occur in the Orinoco valley, and 

 It is likely that a narrow marine channel still separated north- 

 western South America from the rest of the continent during 

 the earlier part of the Tertiary Era. The Amazon valley no 

 doubt was at that time a bay of the Pacific ; still, I am unaware 

 of any geological or zoogeographical evidence for Professor 

 Osborn's supposition that north and middle South America 

 were completely divided in Miocene times by a wide sea. That 

 the Orinoco and Amazon valleys were in communication with 

 one another for a long time is shown by the fact that one of 

 the species of manatees (Trichechus inunguis) and the fresh- 

 water turtle Podocnemis expansa are confined to the upper 

 portions of these two great rivers. A most surprising 

 confirmation of the theory that an ocean bay extended to the 

 neighbourhood of the Andes has been discovered near the small 

 town of Pebas, on the upper Maranon, more than twenty 

 degrees of longitude west of the mouth of the Amazon. Pro- 

 fessor Boettger described deposits from 'this locality contain- 

 ing typically brackish water mollusks which could only have 

 lived in the neighbourhood of the sea. He naturally came to 

 the conclusion that the Atlantic then had invaded the Amazon 

 valley so as to extend near to the foot of the Andes. But 

 Dr. Katzer's view, already alluded to, according to which the 

 Amazon drainage only changed eastward in later Tertiary 

 times, appears to me to agree better with the zoogeographical 

 features of eastern South America. Professor Boettger * 

 looked upon the Pebas beds as being of Oligocene, possibly 

 Eocene age. The fresh-water fish fauna of the Pacific slopes 

 of southern Ecuador still exhibits such affinity to that of the 

 Amazon that the Ecuador mountains could only have had a 

 slight elevation until comparatively recent geological times. 

 Hence we may assume that the Pacific extended to the neigh- 

 bourhood of Pebas when these brackish water beds were laid 

 down. 



An interesting zoogeographical demonstration of the 



* Boettger, 0., "Die Tertiarfauna von Pebas," p. 503. 



