﻿FAgenmann: South America West of the Maracaibo 11 



tains. It would appear that the west Cordillera are late quar- 

 ternary. There are no crystalline rocks. 



The mountains of Darien have a different trend and form the 

 present boundary between Panama and Colombia. 



If Hettner is right about the age of the coast Cordilleras, then 

 the streams flowing westward from the western Cordilleras, the 

 present Calima, Cucurrupi, Jujiado, Sipi, Tamana, Condoto, and 

 upper San Juan, as well as the Raspadura, upper Atrato, Certegui, 

 Yurri, and Sucio flowed into the Pacific at no very remote period. 

 Along with the formation of the coastal Cordilleras there was 

 formed the trough between them and the western Cordilleras in- 

 dicated at present by the Gulf of Uraba on the north, and the 

 Bay of Buenaventura on the south. 



The rivers Atrato and San Juan must have been later de- 

 velopments, the Atrato gathering the waters of the Raspadura 

 and the streams north of it flowing from the western Andes and 

 emptying them into the Caribbean, the San Juan gathering the 

 waters of the streams between the upper San Juan and the Rio 

 Calima flowing from the western Cordilleras and emptying them 

 into the Pacific. 



It would seem then, that the oldest of the present rivers of 

 western Colombia is the Cauca. The Magdalena, the largest 

 river, developed with the formation of the newer Cordillera of 

 Bogota. The youngest rivers are the San Juan and Atrato, de- 

 scribed above. A lowering of Colombia north of Buenaventura, 

 by as much as 200 feet, would convert the valleys of the San 

 Juan and the Atrato into two long bays or a strait and cause the 

 Magdalena, the Cauca, and the Cesar to empty independently 

 into a great bay or lake extending from Santa Mart a to a little 

 way above El Banco. 



Horizontal Distribution of the Fishes of Eastern Pana- 

 ma, Western Colombia, and the Pacific Slopes of Ecuador 

 and Peru south to Pacasmayo. The following list gives the 

 distribution of all of the fishes in the various rivers in the area 

 outlined above. It answers the first of the problems in geogra- 

 phic distribution: What fishes are found in the area under con- 

 sideration? 



