22 SCIENCE PROGRESS. 



ing currents the plants assume a filamentous form, in quieter 

 pools they are united in membrane-like sheets. Not in- 

 frequently the filaments are embedded in travertine with the 

 tips alone projecting. The withdrawal by the plants of 

 carbonic acid from the water causes a separation and precipi- 

 tation of carbonate of lime, which gradually gives rise to 

 fan-shaped or terraced masses of calcareous rock. About 

 many of the springs there occur masses of gelatinous algal 

 growths, consisting of successive membranous sheets ; in 

 these are embedded minute gritty particles which on further 

 development become aggregated together in the form of 

 small grains of carbonate of lime. These spherules afford 

 another example of algal oolite. In the older layers the 

 oolitic structure is masked by the cementing together of the 

 separate pellets, and thus the gelatinous masses gradually 

 pass into firm and more or less compact travertine. The 

 travertine of the Mammoth hot springs which has not been 

 the result of such organic agencies forms but a small part of 

 the deposits. In describing the more fibrous part of the 

 calcareous masses Weed refers to its obvious organic origin, 

 and speaks of the fine algal threads which have been instru- 

 mental in its formation, "as if a skein of silk floating in the 

 shifting currents of a stream were suddenly turned to snowy 

 travertine ". 



The following causes are mentioned as having been 

 concerned in the production of travertine deposits : Relief 

 of pressure, diffusion of the carbonic acid by exposure to the 

 atmosphere, evaporation, heating and the influence of plant 

 life. 



Turning to the siliceous springs of the Yellowstone 

 Park region the same observer finds that much of the sili- 

 ceous sinter has been formed by the growth of such plants 

 as algae and mosses. The brilliant tints of red, yellow 

 and green are all produced by such living plants as have the 

 power of separating the substance of the sinter from the 

 hot siliceous waters. 



In some places the algae occur in the form of tough 

 leathery sheets of gelatinous material, in others they ap- 

 pear as delicate skeins of white filaments. As in the 



