LAKES 333 



various sorts live in the lakes, and their shells and bones give rise 

 to deposits comparable to the animal deposits in the sea. (5) 

 Abundant plants grow in the shallow water about the borders of 

 many ponds and lakes, and as they die, their substance accumulates 

 on the bottom. (6) At the outlet, the water is constantly lowering 

 its channel. The lowering of the outlet is often slow, especially 

 if the rock is coherent, for the outflowing water is usually clear, 

 and therefore inefficient in corrasive work. These six processes 

 (except the last, which does not apply to lakes without outlets) 

 are essentially universal, and all conspire against the perpetuity of 

 the lakes. (7) In lakes where the temperature is low enough for 

 ice to be formed, it crowds on the shores and develops phenomena 

 peculiar to itself (Figs. 183-4). The ice of the sea may work in 

 similar ways, but its work is restricted to high latitudes. (8) In 

 lakes in arid regions, deposits are often made by precipitation from 

 solution. 



The first five and the last of these processes are filling the basins 

 of the lakes. As sediment is deposited in a lake, a corresponding 

 volume of water is displaced, and forced out of the basin if the lake 

 has an outlet. The sixth process is equally antagonistic to lakes, 

 while the seventh has little influence on their permanence. Given 

 time enough, these processes must bring the history of &ny lake 

 to an end. The lowering of the outlet alone will accomplish this 

 result if the bottom of the basin is above base-level. Many lakes 

 have already become extinct, either through the filling or draining 

 of their basins, or through both combined. It does not follow, 

 however, that lakes will ever cease to exist, for the causes which 

 produce them may be in operation contemporaneously with those 

 which bring lakes now in existence to an end. 



Lacustrine deposits. The beds of sediment deposited in lakes 

 are similar in kind, in structure, and in disposition, to beds of 

 sediment laid down in the sea; but river-borne sediment is more 

 commonly concentrated into deltas, since waves and shore-currents 

 are less effective in lakes than in the sea. Even the limestone 

 made in the sea has its correlative in limestone made in some lakes. 

 Some of it was made of the shells of fresh-water animals which 

 throve where the in-wash of terrigenous sediment was slight, some 



