376 AMAZONIA AND LA PLATA. 



equal to the average discharge of the Mississippi, and from thirty to forty times 

 greater than that of the Seine at Rouen. But the Uruguay shows great devia- 

 tions, being nearly equal to the Parana when in flood, whereas during the dry 

 season it is reduced to quite a secondary stream with not more than one sixtieth 

 part of its volume at high water.* Such oscillations in the volume of its waters 

 are explained by the uniform character of the climate in the region traversed by 

 the Upper and Middle Uruguay, and by the lack of lateral reservoirs, by which 

 the discharge might be regulated. 



To the volume of water contained in the beds of the Parana and Uruguay 

 must be added the subterranean channels, which flow with a sluggish current 

 below the surface, and which are also due to the rains that fall in the fluvial basin. 

 Below the upper layers there stretches a bed of saturated sands intermingled with 

 a sheet of fresh water, and containing small fluviatile shells. This underground 

 basin, which has been discovered by soundings in the sub-soil of Buenos Ayres 

 at a depth of at least 75 or 80 feet, is in direct communication with the section 

 of the Parana between Rosario and San Pedro. It is replenished not only by 

 direct contributions from the main stream, but also by continuous underground 

 infiltrations ; hence it may be regarded as practically inexhaustible. t 



The Platk Esiuary. 



The visible Parana-Uruguay delta is continued by a submarine delta, which 

 must gradually rise above the surface, if the relative levels of land and sea are 

 maintained in these waters. Sandbanks separated by deep troughs are advancing 

 beyond the alluvial islands, and slowly connecting Martin Garcia with the main- 

 land. The fluvial sediment, which has already absorbed so much of the old marine 

 inlet, is steadily encroaching on the Plate estuary. 



Both the Parana and the Uruguay are ceaselessl}^ washing down alluvial 

 matter, which is deposited in sandbanks, and which at low water transforms the 

 whole estuary to a labyrinth of channels, where the shipping has already to thread 

 its way cautiously. The mean depth of the estuary measured between Monte 

 Video and Punta de las Piedras, is only 13 or 14 feet, and it must have greatly 

 diminished even within a comparatively recent geological epoch, for large 

 whales were formerly stranded above Buenos Ayres, where the water is now too 

 shallow to give them access. Before the recent dredgings and other harbour 

 works, all vessels had to ride at anchor far from the shore, passengers and goods 

 being lauded in flat-bottomed barges, and during fine weather in carts drawn 



* Comparative table of the Parana and Uruguay, according to Aguirre, Revy, and Bateman : — 



Parana. Uruguay. 



Extent of catchment basin . . 1,150,000 sq. miles 155,000 sq. miles 



Extieme length .... 2,900 miles 920 miles 



Minimum discharge per second . 73,000 cubic feet 19,500 cubic feet 



Mean ,, „ . 393,000 ,, „ 141,000 „ ,, 



Maximum „ ,, . 1,650,000 ,, „ 495,000 „ „ 



Proportion of sediment in both rivers : — tôooTj- 



t Emilio Godoy, Bolctin del Instituto Gcogrnjico Argentino, Vol. V., 18S4. 



