28 DYNAMICAL GEOLOGY. 



deposited is also changing from day to day, and even 

 from hour to hour now coarser, now finer and a very 

 distinct, though often irregular, stratification is the result. 

 General Law. We may therefore state it as a general 

 law that all deposits in water, whether still water, as 

 lakes and seas, or running water, as rivers, are stratified, 

 and, conversely, that all stratified materials, wherever we 

 find them, whether near water or high up on the tops of 

 mountains, and in whatsoever condition we find them, 

 whether as sands and muds or as hard stone, if the strati- 

 fication be a true stratification, i. e., the result of sorted 

 material, have been deposited in wafer. Upon this very 

 simple law nearly the whole of geological reasoning is 

 based. It is important, therefore, that every one should 

 habitually observe the phenomena described above, not 

 only in lakes and rivers but in shower-rills and pools. 

 We are now in position to explain all the phenomena of 

 rivers. 



1. Final Effect of Erosion of Rain and Rivers. 



There is a certain slope of a river-bed, depending on 

 the amount of sediment carried, at which the river neither 

 cuts nor deposits. This is called its base-level of erosion. 

 Every river is seeking this level. If above it, it seeks to 

 reach it by cutting ; if below it, by building up by sedi- 

 mentation. As soon as the river reaches this level, it 

 rests, so far as down-cutting is concerned, but now begins 

 to sweep from side to side, widening its channel. Mean- 

 while the side streams and rain-wash continue to cut 

 down the divides. If this continues without interruD- 

 tion, the final result is a gently undulating land- Surface 

 with low divides and broad river channels. This is called 

 a Peneplain. It means that the land has remained 

 steady and the rivers have been working on it, a long 

 time. It is old topography. If the land be now elevated 

 by interior forces the rivers increase in velocity and begin 



