380 CHANGE OF RIYER-COUKSES. 



two hemispheres of the same planet, we may conceive that 

 in the hemisphere most abundant in waters the different 

 systems of rivers required more time to separate themselves 

 from one another, and establish their complete indepen- 

 dence. The deposits of mud, which are formed wherever 

 the running waters lose somewhat of their swiftness, con- 

 tribute, no doubt, to raise the beds of the great confluent 

 streams, and augment their inundations ; but at length 

 these deposits entirely obstruct the branches of the rivers 

 and the narrow channels that connect the neighbouring 

 streams. The substances washed down by rain-waters form 

 by their accumulation new bars, isthmuses of deposited 

 earth, and points of division that did not before exist. It 

 hence results that these natural channels of communication 

 are by degrees divided into two tributary streams, and from 

 the effect of a transverse rising, acquire two opposite slopes ; 

 a part of their waters is turned back towards the principal 

 recipient, and a buttress rises between the two parallel 

 basins, which occasions all traces of their ancient communi- 

 cation to disappear. From this period the bifurcations no 

 longer connect different systems of rivers ; and, where they 

 continue to take place at the time of great inundations, we 

 see that the waters diverge from the principal recipient only 

 to enter it again after a longer or shorter circuit. The 

 limits, which at first appeared vague and uncertain, begin to 

 be fixed ; and in the lapse of ages, from the action of what- 

 ever is moveable on the surface of the globe, from that of 

 the waters, the deposits, and the sands, the basins of rivers 

 separate, as great lakes are subdivided, and as inland seas 

 lose their ancient communications.* 



The certainty acquired by geographers since the sixteenth 

 century, of the existence of several bifurcations, and the 

 mutual dependence of various systems of rivers in South 

 America, have led them to admit an intimate connection 



* The geological constitution of the soil seems to indicate that, not 

 withstanding the actual difference of level in their waters, the Black Sea, 

 the Caspian, and lake Aral, communicated with eadi other in an era 

 anterior to historic times. The overflowing of the Aral into the Caspian 

 Sea seems even to be partly of a more recent date, and independent of the 

 bifurcation of the Gihon (Oxus), on which one of the most learned geo- 

 graphers of our day, M. Ritter, has thrown new light. 



