382 



Taking a broad view of the matter we may contend that 

 all the carbonate of lime is carried to a lake by the feeding 

 streams and is either held in suspension or dissolved as bicar- 

 bonate. Matter held in suspension in a stream will, of course, 

 be almost totally deposited in a lake. With this mechanical 

 deposition my argument has nothing whatever to do. 



In the lake two different organic processes are supposed 

 to cause a deposition of dissolved lime. 



The first is the assimilation of plants by which carbonic 

 acid is withdrawn from the water and combined to form the 

 organic material of the plants, and through them the animals 

 that prey upon plants. About the equivalent weight of CaCO.^ 

 to the CO<i thus combined in organic compounds (2.3 grms. of 

 CaCO^ for each gr. of CO2) will be deposited, provided the 

 water of the lake be on the whole saturated with calcium- 

 bicarbonate at the tension existing. But nearly all, and for the 

 sake of argument I will presently suppose that all, this organic 

 material is in due course again decomposed, whereby the car- 

 bonic acid is completely recovered. If nothing else takes place 

 this carbonic acid must redissolve all the carbonate of lime 

 deposited. 



The second process is the formation of shells by mussels. 

 These animals absorb calcium-bicarbonate, probably through 

 the gills; they retain the monocarbonate, but the carbonic acid 

 forming bicarbonate is liberated and will increase the tension 

 of the water causing it to dissolve from the lime-deposits of 

 the bottom , from dead shells and , indeed, from whatever 

 source exactly the quantity of lime which the living mussels 

 have taken from it. 



In the long run, therefore, organisms are altogether incapable 

 of either adding to or diminishing the lime-deposits in a lake^), 



') In a lake where permanent organic deposits are formed these may give 

 rise to the deposition of 2.3 grms. of CaCO^ for each gr. of carbonic 

 acid deposited as organic material. The organic material that is not 



