194 
ELEMENTARY GEOLOGY 
THE 
DESTRUCTION OF LAKES 
SEDIMENT 
DEPOsITS. 
Lakes filter the water of the streams. Deltas 
built. Sediment accumulated over the bottom. 
Lake waves add to this supply. 
ANIMAL 
DEPOsITs. 
Every animal that dies in the lake adds some- 
thing to the deposits. Sometimes animals are 
abundant enough to cause accumulations of marl 
and of infusorial earth (see p. 91). 
PLANT 
DEPposITs. 
Plants do the same. ‘They are usually the 
final agents of lake filling. Reeds, moss, etc., 
are important. Peat bogs are formed, and the 
lake finally becomes a swamp. 
CHEMICAL 
DEPOSITS. 
In lakes without outlets the mineral substances 
brought in solution by the river are sometimes 
precipitated, when by evaporation the water be- 
comes saturated. In this way beds of carbonate 
of lime, gypsum, and salt are deposited. 
