338 



CEMENTS, LIMES, AND PLASTERS. 



majority of marl deposits. His studies * led him to believe that more 

 important effects were due to the action of certain aquatic plants of 

 low type, notably Chara (sonewort) and Zonotrichia, another alga. 

 Plants of higher type are also influential in marl deposition, but to a 

 less degree. Davis has summarized f his views as follows : 



"All green plants, whether aquatic or terrestrial, take in the gas 

 (carbon dioxide) through their leaves and stems and build the carbon 

 atoms and part of the oxygen atoms of which the gas is composed into 

 the new compounds of their own tissues, in the process releasing the 

 remainder of the oxygen atoms. Admitting these facts, we have two 

 possible general causes for the formation of the incrustation (of cal- 

 cium carbonate) upon all aquatic plants. If the calcium and other 

 salts are in excess in the water, and are held in solution by free carbon 

 dioxide, then the more or less complete abstraction of that gas from 

 the water in direct contact with plants causes precipitation of the (lime) 

 salts upon the parts abstracting the gas, namely, the stems and leaves. 

 But in water containing the salts, especially calcium bicarbonate, in 

 .amounts so small that they would not be precipitated if there were 

 no free carbon dioxide present in the water at all, the precipitation 

 may be considered a purely chemical problem, a solution of which may 

 be looked for in the action, upon the bicarbonates, of the oxygen set 

 free by the plants. Of these bicarbonates, calcium bicarbonate is the 

 most abundant, and the reaction upon it may be taken as typical and 

 expressed by the following chemical equation: 



CaH 2 (C0 3 ) 2 + = CaCO 3 + C0 2 +H 2 O+ 0, 



Calcium bicarbonate + oxygen = calcium carbonate + carbon dioxide + water + oxygen 



in which the calcium bicarbonate is converted into the normal (and very 

 slightly soluble) carbonate by the oxygen liberated by the plants, and 

 both carbon dioxide and oxygen are set free, the free oxygen possibly 

 acting still further to precipitate calcium monocarbonate. It is probable 

 that the plants actually do precipitate calcium carbonate in both these 

 ways (i.e., by abstracting carbon dioxide from the water and by freeing 

 oxygen), but in water containing relatively small amounts of calcium 

 bicarbonate the latter would seem to be the probable method." 



Professor Davis has further proven that Chara acts in still a third 

 way, abstracting lime salts directly from the water as part of its life 

 processes and depositing them in its tissues. 



* See papers by Davis, cited in list on pp. 346-347. 

 t Vol. 8, pt. 3, Reports Michigan Geol. Survey, p. 69. 



