NA TURAL HISTORY OF MARL 493 



chemically, the silica has been found to be in the form of diatom 

 shells, and hence, because of the small size and great delicacy of 

 these structures, it is available as a source of silica for calcium 

 silicate in cement making. If such deposits as are made up 

 largely of diatom shells were adjacent to marl beds, it is pos- 

 sible they might be considered as clay and be used in cement 

 making. 



From the above considerations, it is evident that both because 

 of the quality and quantity of its works, Chara may be consid- 

 ered an important agent in marl production, and it only becomes 

 necessary to account for the chalky structure of the deposits to 

 make the chain of evidence complete. All algae are plants of very 

 simple structure, without tough or complicated tissues. Chara 

 stems and branches are made up of aggregations of thin-walled 

 cells, and when the plants die the cell walls must rapidly decay 

 and the residue of lime be left. In a laboratory experiment to 

 determine this factor, it was found that a mass of the broken-up 

 plants placed in the bottom of a tall glass vessel filled with water 

 became decomposed very quickly, giving the characteristic odor 

 of decaying vegetable matter, and after a few weeks all organic 

 matter had disappeared, leaving the incrustations in tubular, very 

 brittle, fragments. In studying the structure of marl, the writer 

 has found that near the top of the beds there is usually a " sandy," 

 or even a coarsely granular structure. This is noticeable, at 

 times, at all depths from which the samples are taken, i. e., in 

 some cases it extends through the bed. Close examination of 

 such marl shows that this coarseness is due to the remains of 

 the characteristic Chara incrustations, and that the "sand" and 

 other course material is made up of easily identifiable fragments 

 of the coatings of stems and branches of the plant. The pres- 

 ence of such coarse matter near the top of the beds may be con- 

 sidered due to sorting action of the waves, and such surface 

 currents as may be caused in ponds and small lakes, in shallow 

 water, by wind action. If these agents are effective in producing 

 the coarser parts of the deposits they may be also considered 

 so in connection with the finer parts as well, for the matter 



