xin Action of Water on the Land 255 



bed of the sea or remodelled by the deposit of boulder-clay, 

 it necessarily forms a series of lakelets which overflow into 

 one another by streams. As the river system cuts its 

 channels more deeply the smaller hollows are either drained 

 or filled up and remain as meadows along its course. The 

 abundance of fresh lakes is a testimony to the comparative 

 newness of the land surface and to the early stage of evolu- 

 tion of its rivers. A river issuing from a lake cuts down the 

 lip it flows over very slowly, except when the barrier is soft 

 clay, as all the sediment which gives to running water the 

 properties of a file is dropped on entering the lake. Lakes 

 thus act as filters for rivers. The exquisite deep blue 

 colour for which the lakes of Northern Italy and Switzer- 

 land are famous is due to the scattering of light from the 

 fine flakes of mica brought in by glacier rivers and suspended 

 in the water. This deposit tends to gradually fill up 

 the lake. The fans of alluvial deposit laid down by 

 each inflowing stream grow into deltas; and flat meadows 

 encroach on the water so rapidly that lawsuits are oc- 

 casionally required to determine the ownership of the 

 new land. Lakes regulate the flow of rivers by keeping 

 up their supply in times of drought, and checking floods 

 during rain. For example, if a river -^ of a mile wide 

 passes through a lake of 100 square miles in area, 

 10 miles from the sea, and a flood takes place in the 

 upper stream which, if passed on directly, would raise the 

 level of the lower 10 miles by 25 feet, and so produce a 

 disastrous flood, the immediate effect is to raise the level of 

 the lake 3 inches, causing a very slight increase of the lower 

 stream. 



335. Salt Lakes. In arid regions, where evaporation 

 is in excess of rainfall, rivers flowing into hollows of the 

 Earth's crust may fail to fill them up to the brim, and lakes 

 will thus be formed with no outlet. These are necessarily 

 salt, on account of the evaporation of the river-water, and 

 the salts contained differ ( 221) from those of the sea. 

 Analysis of the water of salt lakes shows this to be usually 

 the case ; but the salts of the Caspian are very similar to 

 those of ocean water, indicating that it is part of the sea 



