METHOD OF LIME DEPOSITION 475 



for many fresh-water limestones, as long ago pointed out by Lyell. 

 Such a fresh-water limestone was described by him from a com- 

 paratively recent deposit in b'orfarshire. In the Eocenic of the Paris 

 Basin, Chara lyelli is rock-forming. Chara-like bodies have also 

 been discovered in abundance in Devonic limestones of Ohio, as 

 above noted. 



Travertine and Siliceous Sinter Formed by Alg.e 

 IN Hot Springs. 



In the waters of hot springs, the world over, algre of various 

 types have been observed to grow, in temperatures ranging from 

 below 100° to 200° F. (55). In the hot springs of the Yellow- 

 stone National Park they occur in waters between the temperature 

 of 90° and 185° I"., more having been found in warmer waters; 

 but in California they have been found in waters of a temperature 

 of 200° F. These algc'e through their physiological activities sep- 

 arate lime or silica from the waters of these hot springs, producing 

 extensive deposits of travertine, or of siliceous sinter, according 

 to the nature of the spring and its water. Not all the deposits of 

 travertine or siliceous sinter are due to this cause ; many are of 

 purely hydrogenic origin, being precipitated from the water by loss 

 of pressure, by dififusion or by abstraction of the solvent gases, by 

 evaporation, or by chemical reaction. In widely separated regions, 

 however, plants have been found active in the separation of these 

 mineral matters from the water — examples being the travertine de- 

 posits of the Carlsbad Sprudel in Bohemia, and the hot springs 

 of Yellowstone National Park, in the United States, and the geyser- 

 ite or siliceous sinter deposits of the latter region. The extensive 

 sinter deposits of Iceland and New Zealand are probably also in 

 large part due to algous growth. 



Method of Lime Deposition by Plants. 



We have seen above that lime carbonate is held in solution by 

 the excess of COo in the water and that any agent which causes 

 the abstraction of the COo also causes the precipitation of much of 

 the calcium carbonate. Plants obtain the carbon of which their 

 tissues are built by decomposing the carbon dioxide derived either 

 from the air or from the water, according to the mode of life of 

 the plant. It is thus obvious that where aquatic life is abundant, 

 much CO. will be abstracted from the water, and thus deposition 



