THE BEGINNINGS OF PLANT LIFE 13 



ordinary silica, but is built up of the tiny and 

 beautiful silicious shields of diatoms. And as the 

 forms found in rotten-stone are characteristic of 

 fresh-water species, we conclude that where the 

 deposits are found lakes or marshes existed in 

 previous ages. 



In long- periods of time, by the slow percolation 

 of water, these diatomaceous deposits (a minute 

 speck of a diatom deposit composed almost 

 entirely of one species is shown in Fig. 5) are 

 slowly dissolved and then redeposited as a hard 

 opal-like rock. Thus these apparently insignificant 

 plant atoms, by their vast numbers and rapidity of 

 multiplication, play most important parts in the 

 formation of the earth. The city of Richmond, in 

 Virginia, is said to be built on a stratum of di- 

 atoms 1 8 feet thick. Estuaries and harbours are 

 frequently considerably shallowed by the accumu- 

 lation of these flinty deposits, and the surface and 

 ooze of many seas reveal diatoms in abundance. 



Fig. 6 shows diatoms dredged from the Atlantic 

 by the Challenger at a depth of 1990 fathoms, or 

 just over 2J- miles. 



The variety of forms assumed by the frustules, 

 or shields, are as varied as their surface markings, 

 and quite beyond description. The most familiar 

 species found in ponds assume the long oval 

 or boat-shaped forms, such as those shown in 



