ORIGIN AND EVOLUTION OF LIFE 305
Species of Halimeda, however, calcified algfc belonging to the very dif- ferent class f'hlorophyceae, are important agents in reef -building and land -forming, yet are almost non-niagnesian.^'
The Grenville series at the base of the Paleozoic is essentially cal- careous, with a thickness of over J)4,000 feet, nearly eighteen miles, more than half of which is calcareous.'* Thus it appears probable that the surface of the primordial continental 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 their 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 algie are much less numerous."" Silica is also an earth- borne, though mineral, constituent of sea-water which forms the prin- cipal skeletal constituent of the shells of diatoms, minute floating plants especially characteristic of the cooler seas which form the siliceous ooze of the sea-bottoms.
(To be cont:n,ud)
srM. A. Howe, letter of February 24, 1916.
»» PiraEOD, Louis V., aad Sfhuebert, Charles, 1915, pp. 54.5, 546.
»«Op. cit., p. 104.
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