126 * THALLOPHYTA. [CH. 



Ordovician, Silurian, Carboniferous, Jurassic and other systems 

 is a striking fact, and lends support to the view that oolitic 

 structure is in many cases intimately associated with the 

 presence of a simple tubular organism. Among recent algae 

 we find different genera, and representatives of different families, 

 growing in such a manner and under such circumstances as are 

 favourable to the formation of a ball-like mass of algal threads, 

 which may or may not be encrusted • with carbonate of lime. 

 Similarly as regards oolitic grains of various sizes, and the 

 occurrence in rocks of calcareous nodules, the tubular structure 

 is not always of precisely the same type, and cannot always be 

 included under the genus Girvanella. 



Several observers have recorded the occurrence of low forms 

 of plant- life in the waters of thermal springs. It has been 

 already mentioned that Cohn described the occurrence of 

 simple plants in the warm Carlsbad Springs, and fission-plants 

 of various types have been discovered in the thermal waters 

 of Iceland, the Azores^ New Zealand, the Yellowstone Park, 

 Japan, India, and numerous other places. 



A few years ago Mr Weed, of the geological survey of the 

 United States, published an interesting account of the forma- 

 tion of calcareous travertine and siliceous sinter in the Yellow- 

 stone Park districts This author emphasizes the important 

 role of certain forms of plants in the building up of the calca- 

 reous and siliceous material. Among other forms of frequent 

 occurrence, Galothrix gypsophila and a species Leptothrix are 

 mentioned, the former being a member of the Nostocaceae, allied 

 to Rivularia, and the latter a genus of Schizomycetes. In many 

 of the springs there are found masses of algal jelly like those 

 previously described by Cohn in the Carlsbad waters. Sections 

 of such dried jelly showed a number of interlaced filaments 

 with glassy silica between them. Weed refers to the occur- 

 rence of small gritty particles in this mucilaginous material. 

 These are calcareous oolitic granules which are eventually 

 cemented together into a compact and firm mass of travertine 

 by the continued deposition of carbonate of lime. The presence 

 of the plant filaments is often difficult to recognise in the 

 1 Moseley, H. N. (75), p. 321. ^ Weed (87-88), vide also Tilden (97). 



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