Mineral Content 



53 



diatoms require silica to build their shells. When the 

 diatoms are dead their shells, relatively heavy though 

 extremely minute, slowly settle to the bottom, slowly 

 dissolving; and so, analyses of lake waters taken at 

 different depths usually show increase of silica toward 

 the bottom. 



Iron, common salt, 

 sulphur, etc., often 

 occur locally in great 

 abundance, notably in 

 springs flowing from 

 special deposits, and 

 when they occur they 

 possess a fauna and 

 flora of marked pecu- 

 liarities and very 

 limited extent. 



An idea of the rela- 

 tive abundance of the 

 commoner mineral 

 substances in lake 

 waters may be had 

 from the 



following 

 figures that are con- 



densed from Birge and 



Fig. ii. A beautiful green desmid, M icra- Judav's report of 74- 

 sterias that is common in bog waters. .. 



analvses. 



This is the bill of fare from which green water plants 

 may choose. Forel aptly compared the waters of a 



