380 
CHANGE OF RIVER-COURSES. 
two hemispheres of the same planet, we may conceive that 
in the hemisphere most abundant in waters the diff erent 
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 divisiou 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 ol 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, f rom 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 each other in an er® 
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 th« 
bifurcation of the Gilion (Oxus), on which one of the most learned S e0 ‘ 
graphers of our day, M. Ritter, has thrown new light. 
