RECORDS LEFT BY RIVERS 



peculiar. The snails that live in lake waters are distinct 

 from the land snails of the adjoining shores. Their dead 

 shells gather at the bottom of some lakes in such numbers 

 as to form there a deposit of white crumbling marl, some- 

 times many yards in thickness. On the sites of lakes that 

 have been gradually filled up, or artificially drained, this 

 marl shows at once where the lake borders were, and, 

 roughly, the period of the lake. In some lakes also are 

 found concretions of iron-oxide, which are formed by the 

 chemical action of the water on some of the rocks by the 

 lake-side ; and in several Swedish lakes this ironstone 

 forms so fast that the lakes are regularly dredged for it. 



Thus among the rocks which form the dry lands of the 

 globe there occur masses of limestone, sand, marl, and 

 other materials which we know were deposited in lakes, 

 because they contain types of plants and animals like 

 those found in the lakes of our own time. From this 

 kind of evidence we can mark out the places of great 

 lakes that have long ago vanished from the face of 

 Europe and North America. 



There are also the so-called Salt Lakes to consider. 

 These are generally the lakes that have no outlet and 

 into which a small amount of water now flows, but never 

 enough to cause the lake now to overflow, whatever 

 it may have done in past times. The water that now 

 runs in escapes merely by evaporation. But just as the 

 bottom of a kettle in which hard water is constantly 

 boiled gradually becomes furred, so a lake bottom into 

 which water is continually pouring, bringing dissolved in 

 it all sorts of mineral salts, becomes coated with sediment. 



57 



