LOWERING OF THE OUTLET. 251 



to me worthy of suggestion that the rhythmical action may be due to a 

 rhythmical factor inherent in the mode of degradation of the river valley 

 below the outlet. ^Yhenever the bed of a river is formed of material of 

 unequal i-esistance, it is scarcely possible for it to sink its channel by a 

 uniform downward cutting. If the bed is formed of horizontal strata, and 

 these have different degrees of resistance, it is almost inevitable that the 

 stream shall develop an alternation of levels and falls, expressing them- 

 selves by a succession of slack water and rapids, as is well known. Con- 

 sidered as a mode of excavation, the process is what miners would call 

 "stoping." When one of the stopes, in working up the river, reaches any 

 reservoir or lake that may lie in its course, it promptly lowers it to an 

 amount corresponding to the depth of the stope. If the bed of the river lies 

 upon drift, essentially the same mode is followed, but with some variation 

 in detail. The resistant factor in this case consists of transverse barriers, 

 such as may be formed by bowlder belts, heavy masses of till, aggregates 

 of coarse gravel, and similar inequalities of the deposit. These arrest the 

 down-cutting of the stream for a time and form rapids on their lower sides. 

 These rapids work away at the barrier in stope fashion. Meanwhile the 

 stretch of river above removes the less resisting material of its bottom 

 down to a gradation plane or to an actual base-level with reference to the 

 barrier. This brings about essentially the same condition of alternating 

 slack water and rapids that arises in connection with horizontal strata. 

 When the barrier is cut through, erosion works rapidly up stream through 

 the soft material, and if a lake lies in its course there will follow a sudden 

 lowering. It is not in the nature of the case that a lake drained by a long 

 river flowing over horizontal beds or over drift should be lowered uniformly. 

 It does not seem necessary, therefore, to appeal to a series of sudden accel- 

 erations and halts in a movement of the crust to account for the rliythmical 

 drainage of Lake Agassiz and of the many other lakes that exhibit like 

 phenomena, unless there is independent evidence of such movements. 



T. C. Chamberlin. 



