THE LIFE HISTORIES OF LAKES. 91 



than the lowering of their outlets, for the reason that while incoming 

 streams are frequently turbid and heav}' with sediment, the outgoing 

 watei-s are clear and therefore have Ijut little power to erode. The clear 

 outflowing waters deepen their chainiels by the slow process of chemical 

 solution, but when the rocks over which they pass are soft and inco- 

 herent, they may soon become recharged with sediment and make rapid 

 progress in deepening their channels and in (haining the basin above. The 

 lives of various lakes may differ in length and have minor variations 

 according to local conditions, but the main features in their histories will 

 conform to the same general outline. 



The filling of lake basins by sediment frequently progresses more 

 rapidly than at first might be supposed. In some instances its rate 

 may be observed from yyar to year, and attracts the attention of even the 

 casual observer. In countries that have been long inhabited, there is 

 sometimes historical evidence of the rate at which the boundaries of lakes 

 have contracted. At the head of Lake Geneva, Switzerland, for example, 

 the Rhone is bringing in large quantities of silt derived from the gla- 

 ciers on its head Avaters, and a low grade delta is being extended into the 

 lake. An stated b}^ Lyell,^ the town of Port Vallais (Portus Yalesie of 

 the Romans) once situated at the water's edge, is now more than a mile- 

 and-a-half inland, this extension of the shore having been made in 

 about eight centuries. 



The decrease in the capacity of lake basins, in ordinary cases, goes on 

 so much more ra})idly from filling than from the lowering of their outlets, 

 that it is the destiny of most lakes situated in liumid regions to become 

 exterminated mainly l)y sedimentation. B}' this process their basins are 

 transformed into alluvial plains, through whieli wander the streams that 

 were tributary to the antecedent lakes. These streams being no longer 

 robbed of the material they carry in suspension, are enabled to attack 

 tlieir channels below the former lakes with energy, and to dee])en and 

 broaden them. The grade of the streams through the alluvial jdain, 

 mai-king the former site of a lake, is iiicieased. and the reiini\al oV the 

 soft lakebe(Is progresses as tlie channel below is deej)ene(l. Streams How 

 through alluvial plains with slaekeneil sj)eed. and foiin winding channels, 

 and swing from side to side of their \alleys. thus reducing the general 

 level. The load previously deposited in the basin is again taken up and 

 the deferred task of trans[)orting it to the sea is resumed. Former lake- 



1 Principles of Geologj', lltli cilition. 187:1, vol. L p. 41;]. 



