502 CHARLES A. DAVIS 



seems to be the limit of depth of recorded occurrence of Chara 

 plants. The remains of the plants, then, would only accumu- 

 late, in place, above that depth, and the material reaching 

 greater depths would be that held in suspension in the water, 

 and hence be relatively very small in quantity and accumulate 

 slowly. A possible additional cause of slow accumulation is 

 that in the greater depths, i. e., over ten meters, the greater 

 abundance of dissolved carbon dioxid held in solution by pres- 

 sure dissolves the finer particles of marl which reach these 

 depths. 



From these investigations it seems (i) that marl, even of 

 the very white pulverulent type, is really made up of a mixture 

 of coarser and finer matter covered up and concealed by the 

 finer particles, which act as the binding material. (2) That the 

 coarser material is present in the proportion of from 50 to 95 

 per cent. (3) That this coarser material is easily recognizable 

 with the unaided eye and hand lens, as the incrustation pro- 

 duced on the algae, Schizothrix and Chara, principally the latter, 

 to particles less than one one-hundredth of an inch in diameter. 

 (4) That the finer matter is largely recognizable under the com- 

 pound microscope as crystalline in structure, and is derived 

 from the algal incrustation by the breaking up, through decay 

 of the plants, of the thinner and more fragile parts, or by dis- 

 integration of the younger parts not fully covered. (5) That 

 some of this finer matter is capable of remaining suspended in 

 water a sufificiently long time, after being shaken up with it, to 

 make it unnecessary to advance any other hypothesis to explain 

 the turbidity of the waters of some marl lakes, than that it is 

 caused by mechanical stirring up the marl by wave or other 

 agency. (6) That shells and shell remains are not important 

 factors in the production of the marl beds which are of largest 

 extent. (7) That there is in marl a small amount of a water 

 soluble calcium salt, readily soluble in distilled water, after 

 complete evaporation. 



^A. J. PiETERS : Plants of Lake St. Clair, Bull. Mich. Fish Commission, No. 2^ 

 p. 6. 



