I04 THE ORIGIN AND EVOLUTION OF LIFE 



gonkian time,^ together with calcareous bacteria, developed 

 the massive limestones of the Tetons. Clarke observes: "We 

 are now beginning to see where the magnesia of the limestones 

 comes from and the algae are probably the most important 

 contributors of that constituent." 



Thus representatives of the Rhodophyceae contribute as 

 high as 87 per cent of calcium carbonate and 25 per cent of 

 magnesium carbonate. Species of IJalimeda, however, calci- 

 fied algas belonging to the very different class Chlorophyceae, 

 are important agents in reef-building and land-forming, yet are 

 almost non-magnesian.- 



The Grenville series at the base of the Palaeozoic is essen- 

 tially calcareous, with a thickness of over 94,000 feet, nearly 

 eighteen miles, more than half of which is calcareous.^ Thus 

 it appears probable that the surface of the primordial conti- 

 nental seas swarmed with these minute algae, which served as 

 the chief food magazine for the floating Protozoa; but it is very 

 important to note that algal life is absolutely dependent upon 

 phosphorus and other earth-borne constituents of sea-water, as 

 well as upon nitrogen, also earth-borne, and due to bacterial 

 action; for where the denitrifying bacteria rob the sea-water 

 of its nitrogen content the alga? are much less numerous.^ 

 Silica is also an earth-borne, though mineral, constituent of 

 sea-water which forms the principal skeletal constituent of the 

 shells of diatoms, minute floating plants especially charac- 

 teristic of the cooler seas, which form the siliceous ooze of the 

 sea-bottoms. 



1 Walcott, Charles D., 1914. - M. A. Howe, letter of February 24, 1916. 



' Pirsson, Louis V., and Schuchert, Charles, 191 5, pp. 545, 546. 

 ^ Op. cit., p. 104. 



