ALG/E AS ROCK-BUILDING ORGANISMS. n 



At the present day we find such genera as Lithothaiu- 

 nion, Lithophyllum and others taking a more or less promi- 

 nent part in rock construction ; in some cases adding their 

 calcareous skeletons to the accumulating masses of coral 

 reefs ; under other conditions spreading over the ocean floor 

 in hiofh northern latitudes. 1 The thick masses of limestone 

 of various geological ages containing numberless specimens 

 of calcareous Florideae testify to the widespread occurrence 

 of the same and similar forms in the ancient seas. Other 

 strata are obviously built up very largely of calcareous 

 algae of a simpler type, and reveal the past history of 

 numerous genera which must be placed in the same group 

 with Cymopolia and other recent forms. Not only do 

 many rocks supply clear and undoubted proof of their par- 

 tial or complete development from the accumulation of 

 calcareous plants, but in many cases there are striking facts 

 to hand which point to a phytogenetic origin for strata with- 

 out any present traces of organic structure. 



OOLITIC ROCKS AND ALG/E. 



Oolitic structure characterises many rocks of Palaeozoic, 

 possibly also pre- Palaeozoic, Mesozoic and Tertiary age, 

 and is of common occurrence in recent calcareous deposits. 

 The best known examples of oolitic rocks are those which 

 form part of the Oolitic series in the Jurassic forma- 

 tion. A typical oolite consists of "rounded particles 

 varying in size from a pin's head to a pea," 2 with usually 

 some foreign body as a central nucleus round which the cal- 

 careous substance has been deposited. The microscopic 

 structure and mode of origin of oolitic sediments were made 

 the subject of examination by Dr. Sorby some years ago, 

 and it was generally agreed that the carbonate of lime of 

 each grain had been deposited layer by layer round a grain 

 of sand, shell fragment or other nucleus as it was carried 

 to and fro in an eddying current. At all events the forma- 

 tion of oolitic grains was referred to some essentially inor- 

 ganic process. 



1 Kjellman, p. 66. 2 Green, p. 281. 



