110 



ELEMENTARY BOTANY. 



by percolating waters, which convey them to points where 

 vast deposits accumulate. Another proof of an Archaean flora 

 is the occurrence of graphite. This is formed by metamor- 

 phism of coal, which in turn had its origin in the plants that 

 previously grew in that region. Carbonaceous shales and beds 

 of limestone also indicate the existence of aquatic life in 

 Archsean time. 



2. In the Cam.brian and Silurian Bras the lower animals 

 were very abundant ; this is conclusive proof that plants 

 must have existed at the time in great quantity ; probably, 

 too, in great variety. For animals depend on the vegetable 

 kingdom for their sustenance, each animal consuming many 

 times its own weight of vegetable food. That fossil animals 

 are in every age found more abundantly than fossil plants is 

 not to be taken as proof that there were fewer of the latter ; 

 it must be borne in mind that the hard parts of the aniinals 

 are much less destructible than most vegetable tissue, and 





Fig. 129. 



Fig. 130. 



are, therefore, oftener preserved in the fossil state. The plants 

 of the Cambrian and Silurian eras which have been found 

 and described up to the present time are a number of cellu- 

 lar, and a few vascular, cryptogams. Many of them were 

 marine plants, apparently related to the genus Fucus; a 



