98 AMAZONIA AND LA PLATA. 



plateau as tbe Tapnjoz, and. like it, is obstructed by a series of reef's and rapids, 

 inaccessible to boats. Although visited in the eighteenth century by the Jesuit, 

 Hundertpfund, and again in 1842 by Adalbert of Prussia, so little was known of 

 the Xingu till quite recently that its southern affluents figured on many maps as 

 tributaries of the Tapajoz. But the charts prepared from the surveys of Von 

 den Steinen in 1884 and 1887 may be regarded as fairly accurate. 



At its confluence the Xingu, already under the influence of the tides, expands 

 into a vast lake like that of the Tapajoz. The strip of land separating it from 

 the Amazons is divided by creeks into an archipelago of wooded islands, while a 

 labyrinth of other channels is developed above the confluence along the right 

 bank of the mainstream. 



Throughout its course of about 2,000 miles between Tabatinga and Macapa, 

 the Amazons maintains a somewhat uniform aspect, varying in breadth far less 

 than the E,io Negro, and nearly everywhere nairow enough for at least a fringe 

 of verdure to be visible on both banks from midstream. Below the Trombetas 

 confluence it even contracts, at the Obidos narrows, to 5,000 or 6,000 feet, during 

 the floods in June, with a mean depth of 250 feet, and a velocity of about 8,000 

 yards an hour. From these datait may be inferred that at this season the Amazons 

 discharges at least 3,500,000 cubic feet per second, before receiving the contri- 

 butions of the Tapajoz, Xingu, and some other affluents. During the great 

 inundations the overflow south of Obidos runs into the Lago Grande de Villa 

 Franca, a vast reservoir 34 miles long and from four to 10 miles wide. Many 

 billions of cubic feet are thus withdrawn from the sea at this point. In the same 

 place both Spix and Martins and Wallace have estimated the discharge in the 

 dry season at not more than 530,000 cubic feet per second. 



The annual rainfall of the whole basin cannot be calculated at less than 100 

 inches, which would supply a uniform discharge of at least 18,000,000 cubic feet. 

 But large quantities are lost by evaporation in the vast reservoirs lining both 

 banks of all the northern and southern affluents below the rapids. 



The Amazonian Mediterranean. 



Throughout the whole of the Amazonian basin, from the Andean foothills to 

 the shores of the Atlantic, there everywhere occur tabular or horizontal terraces 

 of sandstone and argillaceous rocks, ranging from 100 to about 1,000 feet in 

 height. In the central part of the depression the northern and southern terraces 

 recede to a distance of some 500 miles from each other ; but at Obidos and Monte 

 Alegre they approach much nearer to the fluvial banks. Between these two 

 towns, Santarem on the left side stands at the extremity of a fragment of the 

 same rocky formation, which extends to the shores and islands of the estuary, 

 including most of the large island of Marajo and the sea-coast stretching south- 

 eastwards in the direction of Piauhy and Ceara. 



Whatever be the origin of this vast system of sedimentary strata, whether 

 it is to be referred with Agassiz to glacial action, or with other geologists more 

 probably to the paleozoic and especially the carboniferous ages, there can be little 



