'496 CHARLES A. DAVIS 



formed by the growth of the incrusting- blue-green algae, in shallow 

 water, and the subsequent destruction of the nodules by wave or 

 other disintegrating action. In this case a thickened fragment 

 may be left either free or attached to other material. Several 

 fragments may be cemented, together, and such aggregations 

 have been observed. 



4. By the deposition of calcium carbonate on fragments of 

 incrustation, the source of the salt deposited being soluble 

 calcium-organic compounds left free in the water by the decay 

 of dead Chara, the precipitation being caused by the reducing 

 action of chemical compounds derived from the decay of 

 organic matter or the growth of bacteria. It may be conceived 

 also that the calcium carbonate thus deposited might also act as 

 a cementing material to fasten the finer particles of marl to the 

 incrustation as a nucleus. 



5. By the deposition in more or less coarsely crystalline 

 form of calcium carbonate which is dissolved by water perco- 

 lating through the marl. This is probably considerable in 

 amount and takes place in a manner analogous to, if not identical 

 with, the formation of concretions in clays and shales. It is 

 probable that in this way the crystals may be formed, which rather 

 rarely are found filling the cavities in the Chara incrustations, 

 left by the large axial cells of the plant. The fact that in the 

 great majority of cases these cell cavities are entirely empty or 

 are simply mechanically filled with fine particles of marl, is a 

 most serious objection to considering that this form of chemical 

 precipitation is an important one in the history of marl, but that 

 it is occasionally operative is extremely probable. 



6. It is possible that the thick incrustations may have been 

 formed at some earlier period in the history of the lakes when 

 conditions for the development of the plant forms and their 

 activities were greater. This is not probable however, for the 

 thick incrustations are often found from the surface of the marl 

 beds throughout the entire deposits. 



In addition to the marl analyses given above a check analysis 

 was made of a specimen of material made up from the washings 



