334 STUDIES FOR STUDENTS 



them, the whole, from the geographic descriptions, constituting a 

 more or less perpetually flooded and impenetrable tropical forest 

 jungle. This great basin is largely shut in on the east by the uplands 

 of Guiana and Brazil. 



A similar physiographic condition is graphically described by Mr. 

 H. H. Smith as characterizing the headwaters of both the Madeira 

 and the Paraguay.'' These streams in their upper portions flow 

 through more or less separate and inclosed basins. The upper Para- 

 guay rises 30 feet annually. All the flat lands above the Fecho dos 

 Morros to Villa Maria — over 400 miles in a direct line — are subject 

 to river floods, and these are deepest toward the north. The width 

 of the flood-plain at the mouth of the Sao Lourenfo can hardly be 

 less than 150 miles from the rocky lands on the east to the base of the 

 Serra dos Dourados. The whole region is a labyrinth of lakes, ponds, 

 swamps, channels, and islands in a grassy plain, the only forest being 

 near the river. Even at low water one-fourth of it is flooded ; when 

 the river is at its highest, the whcle plain is a vast lake, covered with 

 floating grass and weeds. 



The South American instances illustrate most fully -the manner in 

 which large interior basins may be filled with river sediments. Usu- 

 ally the process of aggradation is not so striking and rapid. Where 

 the down-warping is cf minor importance, as in the central plain of 

 Hungary, fertile and habitable plains may occur. Where the down- 

 sinking of the crust has been deep and far more rapid than the infilling 

 by the rivers, large interior seas may 'result. 



Europe is largely surrounded, and separated from the other con- 

 tinents, by a series of such interior basins, but the dividing bridges 

 are so low that they let in the ocean waters, and on the southern side 

 all but the Caspian and Black Seas are united into the Mediterranean. 

 Of these basins the upper Adriatic, the ^Egean, and the Black Seas have 

 witnessed great changes since the Tertiary, and Suess^ regards the 

 formation of the ^gean and Black Seas as even post-glacial. Pre- 

 viously to this recent down-sinking, fresh- water deposits were formed 

 over the site of the ^Egean, and still remain on certain islands in the 



1 J. B. Hatcher, "Origin of the Oligocene and Miocene Deposits of the Great 

 Plains," Proceedings of the American Philosophical Society, Vol. XLI (1902), No. 169. 



2 Das Antlitz der Erde, Eng. trans., Vol. I, pp. 344, 345; also Plate V, opp. p. 463. 



