THE LIFE HISTORIES OF LAKES. 93 



especially Sphannum or peat moss, grow al)oiit their shores, and extend- 

 ing outward, tonu a tliiek mat of intertwined roots and stems that iloat on 

 the surface. The finer waste from this sheet of lloating verdure falls to 

 the bottom and forms a peaty stratum. To this layer contributions are 

 made by other aquatic vegetation, as the lilies, reeds, rushes, and many 

 l)eautiful sulHiquatic plants. It also receives the trunks of trees falling 

 from the sliore. The small lakes of tlie prairie region especially, are fre- 

 quently transformed in this manner into beautiful fields of wild rice. In 

 the central part of moss-encireled lakes, practically no mechanical sedi- 

 ments are deposited, l)ut moUusks, crustaceans, and fishes may there find 

 a Avell sheltered home and thrive in such abundance that the bottom soon 

 becomes covered with their remains. Microscopic forms also inhabit the 

 water and their siliceous cases frequently accumulate so as to form thick 

 layers, known as diatomaceous earth. A continuation of this })rocess under 

 favorable conditions leads to the rapid extinction of small lakes. The open 

 waters are converted into bogs and swamps, on which forest trees encroach 

 and still farther assist in the transformation. When these deposits of 

 organic matter are di-ained, they frequently furnish rich garden lands. 

 The lakes exterminated by this organic process in the drift-covered por- 

 tion of North America, can only be estimated in tens of thousands, and 

 probably equal in number the lakes still remaining.' 



Lakes of Arid Keg-ions. — On every continent there are broad areas 

 where the skies are without a storm cloud for many months each year and 

 the air is dry and hot in all but the winter season. The lakes in these 

 desert regions have a different general history from their sisters whose 

 banks are fringed with green vegetation and overshadowed by forests. 

 AVhere the rainfall is small and evaporation active, the lives of lakes 

 depend on delicate adjustments of climatic conditions. As the barometer 

 rises and falls in harmony with changes in atmospheric i)ressure, so en- 

 closed lakes fluctuate in sympathy with changes in liumidity or in tem- 

 perature. The ephemeral lives of playa lakes have already l)een described, 

 but the larger lakes of arid regions, although sul)ject to many fluctuations, 

 may have a longer span of existence than lakes of corresponding size and 

 similar topograpliic environment in humid rt'gions. As enclosed lakes do 

 not overflow, there is no loss of area owing to the lowering of outlet. 

 Tributary streams bring in material both in solution and in suspension, all 

 of which is left as evaporation progresses, and tends to fill tlieir basins, 

 but tlie volume of their waters is not directly diminished l)y this proee.ss. 



