VIl] OOLITIC STEUCTURE. 123 



the presence of the same forms in the interior of the calcareous 

 matrix, and it has been concluded on good evidence that the 

 algae are responsible for the deposition of the carbonate of lime 

 of the oolitic grains. By extracting the carbonic acid which 

 they require as a source of food, from the waters of the lake, 

 the solvent power of the water is decreased and carbonate of 

 lime is thrown down. In similar white grains from the Red 

 Sea^ there is a central nucleus in the form of a grain of sand, 

 and cells of Chroococcaceae occur in the surrounding carbonate 

 of lime as in the Salt Lake oolite. Prof. Cohn of Breslau in 

 1862 demonstrated the importance of low forms of plant life in 

 the deposition of the Carlsbad " Sprudelstein^" On the bottom 

 of Lough Belvedere, near Mullingar in Ireland^, there occur 

 numerous spherical calcareous pebbles, of all sizes up to that of 

 a filbert. From a pond in Michigan (U.S.A.)* similar bodies 

 have been obtained varying in diameter from one to three and 

 a-half inches. In the former pebbles a species of Schizothrix> 

 one of the Nostocaceae occurs in abundance, in the form of 

 chains of small cells enclosed in the characteristic and com- 

 paratively hard tubular sheath, and associated with Schizothiix 

 fasciculata there have been found Nostoc cells and the siliceous 

 frustules of Diatoms. In the Michigan nodules the same 

 Schizothrix occurs, associated with Stigonema and Dichothrix> 

 other genera of the Nostocaceae. One of the Michigan pebbles 

 is shown in section in fig. 32 B. 



The connection between the well-known oolitic structure, 

 characteristic of rocks of various ages in all parts of the world, 

 and the presence of algal cells is of the greatest interest from a 

 geological point of view. In recent years considerable attention 

 has been paid to the structure of oolitic rocks, and in many 

 instances there have been found in the calcareous grains 

 tubular structures suggestive of simple cylindrical plants, which 

 have probably been concerned in the deposition of the car- 

 bonate of lime of which the granules consist. In 1880 Messrs 

 Nicholson and Etheridge' recorded the occurrence of such a 



1 Walther (88). « Cohn (G2). 



» Murray, G. (952). 4 Thiselton-Dyer (91) p. 226. 



» Nicholson and Etheridge (80) p. 23, PI. ix. fig. 24. 



