THE WORK OF THE OCEAN. 387 



ice, on the other hand, is relatively more important in lakes than in the 

 sea. 



Changes taking place in lakes. — The processes in operation in lakes 

 are easily observed and readily understood. (1) The waves wear the 

 shores, and the material thus derived is transported, assorted, and 

 deposited as in the sea, and all the topographic forms resulting from 

 ^erosion or deposition along the seacoast are reproduced on their appro- 

 priate scale in lakes. (2) Streams bear their burden of gravel, sand, 

 and mud into lakes and leave it there. (3) The winds blow dust and 

 sand into the lakes, and in some places pile the sand up into dunes 

 along the shores. (4) Animals of 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 sub- 

 stance 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 be coherent, for the outflowing water is usually 

 clear, and therefore inefficient in corrasive work. These six proc- 

 esses 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 

 pecuUar to itself. 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 

 the sediment is deposited, a corresponding volume of water is dis- 

 placed, and, if there be outlets, forced out of the basins; the sixth 

 process is equally antagnostic to the lakes, while the seventh has Httle 

 influence on their permanence. Given time enough, these processes 

 must bring the history of any lake to an end. The lowering of the out- 

 let will alone accompHsh 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. The 

 antagonism of rivers and lakes long ago led to the epigram '^ Rivers 

 are the mortal enemies of lakes. " True as this statement is, it does 

 not follow that lakes will ever cease to exist, for the causes which pro- 

 duce new lakes may be in operation contemporaneously with those 

 which bring lakes now in existence to an end. 



