IX.] THE WORK OF RAIN AND RIVERS. 141 



such work as this, it is necessary to recognize the co- 

 operation of other forces, the effects of which will be dis- 

 cussed in subsequent chapters. 



If running water is thus wasting away the land, year after 

 year and age after age, what ultimately becomes of the great 

 quantity of matter which must be removed ? To this question 

 a partial answer has already been incidentally given. The 

 coarser detrital matter is pushed along the bottom of the 

 stream, and thus slowly moved towards its mouth ; whilst 

 the finer detritus, being held in suspension, is carried more 

 rapidly away by the flow of the running water. When the 

 flow is checked, the sand and mud settle down, the coarser 

 particles being naturally the first to subside. In the minia- 

 ture river-system, self-established in the muddy bank left by 

 the receding tide, a minute stream may often be seen enter- 

 ing a quiet pool of sea-water, and depositing its burden of 

 sand, particle by particle, upon the floor of the little pond. 

 Exactly the same kind of thing occurs, on a much larger 

 scale, at the mouth of every river. In some cases, a river 

 during its course opens out into a lake, and then the 

 resemblance to our model on the sea-shore is even more 

 striking. On entering the lake, the flow of the stream 

 becomes suddenly checked, and a part of the suspended 

 sediment falls to the bottom ; so that by the time the stream 

 emerges, its waters have become purified. The effect of 

 a sojourn in the lake is somewhat like that of allowing 

 muddy water to stand in a glass ; in either case, much of 

 the sediment slowly subsides. 



A striking example of the purifying effect of a lake is 

 seen in the Lake of Geneva, through which the Rhone 

 flows. The river enters the upper end of the lake as a 

 turbid stream, laden with detritus brought down from the 

 Alps ; but, at the lower end of the lake, it issues forth well 



