378 AMAZONIA AND LA PLATA. 



sudden intervals, greatly imperil the navigation ; even large vessels often drao- 

 their anchors, break from their moorings, and run aground on the sandbanks, or 

 on some of the islands in process of formation. 



The Closed Basins of Argentina. 



Between the Juramento-Salado and the E.io Colorado on the Patagonian 

 frontier, none of the rivers rising between the eastern slopes of the Andes and 

 the Sierra de Cordoba reach the Atlantic through the Plate estuary. The closed 

 basins, which are comprised within this north-western region of Argentina, and 

 which are nearly all disposed in the direction from north to south parallel with 

 the Cordillera itself, appear to be of glacial origin. Their beds are strewn with 

 angular boulders, which have not been transported to any great distances, but 

 which have been slightly worn by the action of winds and sands. 



Within a recent geological period all the running waters descending from 

 the Andes west and south of Aconquija heights must have reached the Atlantic 

 through the Colorado, whose basin was formerly far more extensive than at 

 present. The northernmost streams of this now half-dried basin are all now 

 reduced to small dimensions. Thus the Chaschuil, with its Fiambala branch and 

 other affluents, shrinks to half its size in a saline sandy depression of lacustrine 

 origin, beyond which it penetrates through a defile into the plains of the Eio 

 Kioja, where its current is completely exhausted. From this point to the junction 

 with the San Juan the distance in a straight line is about 280 miles from north 

 to south. 



The Rio Yermejo (de la E,ioju) and the Jaehal, which descend to the west of 

 the Famatina heights, also receive lateral contributions from the snowy Cordillera, 

 by which their current is maintained for a greater distance than that of the eastern 

 rivers. But owing to the irrigation canals derived from both banks they do not 

 always reach their confluence. Hence their lower course, the Rio Zanjon, 

 alternately increases and decreases with the seasons, without ever reaching 

 the marshy basin in which it might effect a junction with the San Juan. 



This latter watercourse, with its widely ramifying headwaters, issues as a 

 foaming torrent from the mountains, but is immediately captured by numerous 

 irrigation rills winding in all directions over the plains. Nevertheless, sufficient 

 water remains in the main channel to form a little navigable stream, which is 

 joined lower down by the drainage of the irrigated lands, and which discharges 

 into the marshy Iluanacache lagoons. The same depression also receives the Rio 

 de Mendoza descending from the Cumbre and fed by the snows of Aconcagua and 

 Tupungato. 



Lake Bekkdeko. — Rios Salado and Atuel. 



The overflow of these saline lagoons is carried off by the Desaguadero, which 

 is nearly dry for a part of the year, and which flows south-east, throwing off a 



