DIATOMS. 17 



fer in their yellowish-brown color and their 

 very peculiar siliceous shells. When the 

 plant dies the shell remains and settles to 

 the bottom of the ocean or the lake. Ag- 

 gregations of these shells harden into rock. 

 Many durable flint rocks of dry lands are 

 found to be made up entirely of them, the 

 same as chalk is known to be composed of 

 the shells of minute animals. In some 

 places the floor of the ocean is now being 

 slowly covered with these remains, 

 which will gradually harden into im- 

 pervious rock. Of all plants, the 

 diatoms are the most widely distrib- 

 uted. They abound amid the ice in 

 the polar seas, in hot springs, at a 

 depth of two thousand feet in the 

 ocean, sometimes on mosses and other '^' 

 plants which grow in moist places, and every- 

 where on submerged sticks and stones, upon 

 which they often make a slimy covering. 

 In the ocean the diatoms are eaten by mol- 

 lusks, which in turn are eaten by fish, and 

 the fish are eaten by birds. The little shells 

 are often found intact in beds of guano. 

 Many interesting species have been discov- 

 ered in the stomachs of fish. 



One of the most marvelous of all plants is 



2 



