FRESH-WATER MARLS. 339 



A further possible mode of derivation, which is admittedly the way 

 in which part of all the marl deposits have originated, is through the 

 direct action of molluscs. These animals are especially frequent in limey 

 waters and have the power of abstracting lime salts from the water 

 and utilizing the resulting lime carbonate in the formation of their 

 shells. On the death of the animals their shells sink to the bottom 

 and form an essential portion of any deposit which is in process of forma- 

 tion. In some marl-beds shells amount to an important percentage of 

 the total, but in most cases they will probably constitute less than 5 per 

 cent of the entire mass. 



The facts so far stated may be summarized as follows: Spring or 

 stream water, carrying lime carbonate in solution, deposits it in lakes 

 in the form of marl, this deposition being caused by: 



(a) Escape of carbon dioxide, owing to decrease in pressure; 

 (6) Supersaturation, owing to rise in temperature; 



(c) Abstraction of carbon dioxide by plants; 



(d) Freeing of oxygen by plants, resulting in formation of carbon- 



ates from bicarbonates; 



(e) Direct abstraction and crystallization of lime salts by Chara* 

 (/) Abstraction of lime by molluscs and formation of shell deposits.. 



The formation of a given marl-bed may be due to the operation of 

 any one of these causes, or the cooperation of two or more of them. 



Geographic distribution of marl deposits. The geographic dis- 

 tribution of marl deposits is intimately related to the geologic history 

 of the region in which they occur. Marl-beds are, as indicated in the 

 preceding section, the result of the filling of old lake basins. Lakes 

 are not common except in those portions of the United States which 

 were affected by glacial action, since lakes are in general due to the 

 damming of streams by glacial material or to irregularities in deposition 

 of such material. Workable marl deposits, therefore, are almost ex- 

 clusively confined to those portions of the Unit:d States and Canada 

 lying north of the former southern limit of the glaciers. 



Marl-beds are found in the New England States, where, however, 

 they are seldom of important size, and in New York, large beds occurring 

 in the central and western portions of that State. Deposits are frequent 

 and important in Michigan, and in the northern portions of Ohio, In- 

 diana, and Illinois. Marl-beds occur in Iowa, Wisconsin, and Minnesota, 

 but have not been as yet exploited for cement-manufacture. Extensive 

 marl-beds also occur in Ontario, Quebec, and other Canadian prov- 

 inces. 



