488 CHARLES A. DA VIS 



fitting to examine into the possibility of the plant and animal 

 organisms living in the waters of the lakes being the agents 

 which bring about the reduction of the soluble calcium bicarbon- 

 ate to the insoluble carbonate even in waters low in- the amount 

 of dissolved mineral matter, and containing considerable carbon 

 dioxide. That mollusks can do this is shown by the fact, which 

 has frequently come under the writer's notice, that the relatively 

 thick and heavy shells of species living in fresh water are often 

 partly dissolved and deeply etched by the action of carbonic 

 acid after the animals have, by their processes of selection, fixed 

 the calcium carbonate in their tissues, precipitating it from water 

 so strongly acid and so free from the salt that re-solution begins 

 almost immediately. No natural water seems so free from 

 calcium salts that some species of mollusks are not able to find 

 enough of the necessary mineral matter to build their character- 

 istic shells. 



While some limited and rather small deposits of marl are 

 possibly built up, or at least largely contributed to, by molluscan 

 and other invertebrate shells, the deposits which are proving 

 commercially valuable in the region under consideration, do not 

 contain recognizable shell fragments in any preponderance, 

 although numerous nearly entire fragile shells may be readily 

 washed or sifted from the marl. The conditions under which 

 marl is found are such that the grinding of shells into impalpable 

 powder, or fine mud, by strong wave action is improbable, if not 

 impossible, for exposed shores and shallow water of considerable 

 extent are necessary to secure such grinding action, and these 

 are not generally found in connection with marl. 



We are, then, reduced to the alternative of considering the 

 action of plants as precipitating agents for the calcium salts. It 

 has been shown already that plants generally become incrusted 

 with mineral matter in our marly lakes, and it is easy to demon- 

 strate that the greater part of the material in the incrustation is 

 calcium carbonate. It is also easy for a casual observer to see 

 that the deposit is not a true secretion of the plants, for it is 

 purely external, and is easily rubbed off the outside of the plants 



