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INDIA \ A UN IVEESTTY STUDIES 



The IMagdalv^na descends rapidly from its sources to Neiva (1442 

 feet) and Girardot (1086 feet). Except for a short stretch at 

 Honda it is navigable beloAV G-irardot. The upper part of the 

 Caiica, on the other hand, flows in an elevated valley, which, near 

 the parallel of Gii-ardot. at Cartago, has an elevation of over 3000 

 feet. It is navigable between Call and Cartago. Below Cartago it 

 rapidly descends 2500 feet to Boca de Nechi. and is not navigable 

 in this stretch. The conditions in the ^lagdalena and Cauca, in 

 the latitude of Girardot and Cartago, with a difference of about 

 2000 feet in elevation, are quite different. 



The central and eastern Cordilleras unite in southern Colombia 

 and. like the western chain, continue southward. In Ecua- 

 dor the western chain and the united eastern and central chains 

 are joined by cross ridoes ; in Peru they a.re again distinct, with an 

 intervening valley drained by the Maranon and Ucayale. The 

 western Cordilleras, though not as high as the central, form a more 

 or less effective barrier between the Magdalena-Cauca on the east: 

 and the Atrato-San Juan basins on the west. They are reduced 

 in size toward the north and reach the coast at Cartagena in ]\Iount 

 Poupa. In the north the coastal lowlands of the Rio Sucio, a 

 tributary of the Atrato, of the Sinn, and of the Dique of the Mag- 

 dalena, may permit itermigration of fishes or may have permitted 

 it recently. The western Cordilleras extend southward the entire 

 length of the continent. North of Istmina, they form the water- 

 shed between the Atrato and the Magdalena Rivers, both of which 

 empty into the Caribbean Sea. South of Istmina, they form the 

 watershed and barrier between the Atlantic slope and the Pacific 

 slope, except where the Patia River pierces them in southern Co- 

 lombia, and where, in southern Chili, some of the Pacific slope 

 streams liave pierced them and drain reservoirs to the east of them. 

 The coast Cordilleras are of particular interest. They begin some 

 distance north of Buenaventura and extend northward into Pan- 

 ama. They overlap the western chain only a short distance (about 

 300 miles', leaving a trough open to the Atlantic and the Pacific 

 between them and the western Cordilleras. Their watershed in 

 places approaclies to within a few hundred feet of the Pacific ocean. 

 In places, as at Culebra, their summits are but a few hundred feet 

 above the ocean level. In Central America, the north and south 

 coast Cordilleras give place to chains extending east and west. At 

 different points there seem to have been opportunities, at one time 

 or another, for Atlantic slope syjecies to cross over this chain to the 

 Pacific slope and possibly, vice versa. 



