498 CHARLES A. DAVIS 



the total quantity of the marl. It is surprisingly small when all 

 things are taken into account. While it is probably true that not all 

 the minute shell fragments have been separated in any of these 

 analyses, it is also true that the weight of such overlooked 

 particles is more than counterbalanced by marl fragments which 

 are included within the cavities of the whole shells, and adhere 

 to both broken and whole shells in crevices and sculpturings in 

 such a way as to refuse to become separated in the processes of 

 washing out the marl. The whole shells are mainly small, fragile 

 forms, many of them immature, and it is evident that they would 

 be broken by any action that would crush the Chara incrusta- 

 tion. 



A second line of investigation took into consideration the 

 milky appearance of the waters of some marl lakes. This has 

 been considered by some investigators as possibly due to the 

 presence of calcium carbonate precipitated from the water 

 either by the liberation of dissolved carbon dioxid from the 

 water and hence from the calcium bicarbonate or by change of 

 temperature of the water after it has reached the lakes. 



The writer has not found among the marl lakes of the south- 

 ern peninsula of Michigan that those with turbid water were 

 common, even where marl banks were apparently forming with 

 considerable rapidity. 



" Merl" or Marl Lake in Montcalm county, situated on the 

 same stream as Cedar Lake and a mile or more below it, is, 

 however, one of the lakes in which the water is usually of 

 almost milky whiteness, and has sufficient suspended matter in 

 it to render it nearly opaque for depths of a meter or a little 

 more. The conditions in this lake are widely different from 

 those at Gedar Lake and other marl lakes in the vicinity and are 

 suggestive of the cause of the turbidity. At Cedar Lake there 

 is a border of grassy or sedgy marsh extending around the lake 

 on three sides and generally underlaid by marl, and the lake 

 bottom slopes sharply and abruptly from the edge of the marsh 

 to a depth of at least ten meters. In other words the lake is 

 simply a deep hole, with steep sides, and perhaps represents the 



