FRESH-WATER MARLS. 337 



percolating through beds of limestone or coarse limey clays, will, if 

 charged with carbon dioxide, dissolve and carry off lime carbonate, 

 the exact amount so dissolved being determined by the percentage 

 of CO 2 contained in the water, its temperature, etc. The tendency is 

 for every water to charge itself with its maximum possible amount 

 of calcium bicarbonate, and until it is so charged it will continue to 

 attack and dissolve limestones which it encounters in its course. 



When the water is almost or quite saturated with calcium bicar- 

 bonate, any increase in temperature or decrease in pressure will cause 

 the deposition of lime carbonate. Reactions of this type have been 

 appealed to as explanations of the formation of marl deposits through 

 the warming or loss of pressure which occurs when spring or stream 

 water enters a lake. A recent statement * of this theory is as follows : 



"This spring-water as it enters the lake is always colder than the 

 waters of the lake itself. The bicarbonate of lime is more soluble in 

 cold water than in warm and a part of the dissolved material is there- 

 fore precipitated in the form of a fine powder soon after the cold stream 

 enters the warmer, still water of the lake. Such precipitation of cal- 

 cium carbonate from cold water as it becomes warm is seen every day 

 in almost every household. The hard water heated in tea-kettles holds 

 while cold a large quantity of bicarbonate of lime in solution. As it 

 becomes warm, much, if not all, of this falls and forms a coating of 

 lime carbonate upon the bottom of the kettle. 



"Again, if there is a large amount of carbon dioxide in the perco- 

 lating waters the percentage of carbonate of lime held in solution will 

 be increased in proportion. As the spring-water enters the lake and 

 rises to the surface the pressure will be decreased and a part of the 

 carbon dioxide will escape and so cause a precipitation of another part 

 of the carbonate of lime according to the following formula: 



"CaH 2 (C0 3 )2-C0 2 = CaC0 3 + H 2 O." 



In support of his belief that the formation of most marl deposits 

 is due to the two causes above outlined, Blatchley urges that most, if 

 not all, of the marl lakes examined in Indiana are fed by subterranean 

 or subaqueous springs, even though they also have streams entering 

 and leaving them, and that "the larger deposits of marl in the lakes 

 are found in close proximity to these springs ". 



Davis, in studying the Michigan marls, came to the conclusion that 

 the causes above noted would not of themselves account for the 



* Blatchley, W. S. 25th Ann. Rep. Indiana Dept. Geology, p. 45. 



