NO. 2 PRE-CAM BRIAN ALGONKIAN ALGAL FLORA 85 



Recent calcareous deposits. — In discussing- with Dr. Charles 

 A. Davis of the United States Bureau of Mines the question of the 

 origin of calcareous deposits in fresh and brackish waters, he called 

 my attention to the Natural History of Marl in volume 8 of the 

 Geological Survey of Michigan.' 



Dr. Davis here disposes of the theory that the mineral salts are 

 deposited as the result of certain portions of the lake waters reach- 

 ing the saturation point by showing that the outflow of the lakes is 

 practically the same as the inflow and that the loss by evaporation is 

 too small a factor to be taken into account. He considers the pos- 

 sibility of the plant and animal organisms living in the waters of 

 the lakes being the agents which bring about the results of the 

 deposits of the soluble calcium bicarbonate as the insoluble car- 

 bonate. He shows that the deposits of marl that were largely con- 

 tributed to by Mollusca and other invertebrate shells are of minor 

 importance, and that the commercially valuable calcareous marl de- 

 posits do not contain recognizable shell fragments in any preponder- 

 ance, usually not to exceed 1.04 per cent. 



Next, considering the action of plants as precipitating agents for 

 calcium salts, he gives the following two possible general causes 

 for the formation of the lime incrustation upon all aquatic plants : ' 



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, which are easily demonstrated by any student 

 of plant physiology, we have two possible general causes for the formation 

 of the incrustation 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 the gas from the water in direct contact with plants causes precipitation 

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

 in water containing amounts of the salts, especially of the calcium bicarbon- 

 ate, 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 ox3-gen set free by the plants. Of these, calcium bi- 

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

 typical and expressed by the following chemical equation : 



CaH.(C03)3-f-0 =H.O -^ CaCOs + CO^ -f O 



calcium I + oxygen = water + ;<^^l<^i""^ 1 + / ^^'"bo" ] + oxygen 

 carbonate J 1 carbonate J ^ 1 dioxide J 



Geol. Survey IMichigan, Vol. 8, part 3, 1903, pp. 65-96. 

 Idem, p. 69. 



