MICROSCOPIC PLANTS. 155 



constant, steady shower, and in the course of ages they 

 have formed deposits many miles in extent and many 

 fathoms thick. The thickest known deposit is that upon 

 which Berlin is built, which is eighty-four feet deep ; there 

 is another eighteen feet thick beneath Richmond (Virginia) ; 

 and one at St. Petersburg is thirty feet thick. 



The "polishing slate" of Bilin, " mountain-meal " of 

 Sweden and Tuscany, " Richmond earth," kieselguhr, or 

 "flint froth " of Germany, the "Tripoli stone" of Africa, 

 Italy, Bohemia, Germany, France, and the United States, 

 as well as the beds of white earth on the banks of the 

 Amazons, and the " Bath-brick " from the bed of the river 

 Parret, in Somersetshire, all consist chiefly or wholly of the 

 indestructible cases of diatoms and other minute organisms, 

 some deposited in fresh, others in salt water. * 



The " infusorial earth " of Nova Scotia so called, 

 though the vegetable nature of the diatoms has long been 

 established looks much like chalk, and is a very light, 

 white, friable earth, which, when examined by a bright 

 light, shows an infinite number of glistening specks of 

 pure silica, whose hardness and sharpness make them use- 

 ful for polishing. 



"Electro-silicon" or "magic brilliant," the white sub- 

 stance employed for cleaning jewellery, is also a diatom 

 earth found in Nevada. 



In Lapland a similar earth is mixed with bark and used 

 as food, and in America the farmers have been trying it as 



* Rottenstone has a different origin. Carbonated water has filtered 

 through beds of silicious limestone, carrying away the lime and leaving a 

 light porous residuum of flint. 



