THE NATURAL HISTORY OF MARL 495 



it is frequently possible to recognize fragments of them with the 

 compound microscope. Finally the incrustation is distinctly 

 crystalline in ultimate form of the constituent particles, and 

 when it has disintegrated the crystals and their fragments are 

 found to constitute a large per cent, of the finer particles of the 

 resulting marl. On the growing tips of the younger branches 

 and the leaves of Chara, numbers of isolated crystals of calcium 

 carbonate may be seen. Farther back on stems and branches 

 the crystals become more numerous, then coalesce into a thin, 

 fragile covering, and finally on the lower part of the plant the 

 covering becomes dense and thick. It is evident therefore that 

 the decay of the younger parts of the plants would furnish a 

 mass of more or less free or loosely aggregated crystals of 

 microscopic size, which would retain their crystalline form, in 

 some degree at least, for an indefinite time and be recognizable, 

 hence the presence of microcrystals in marl may furnish addi- 

 tional evidence as to the origin of the deposits. 



The larger fragments of Chara incrustation as found in marl 

 are frequently much thicker and heavier than any which occur 

 among fragments of recent origin, /. e., those obtained from any 

 part of living, vigorously growing Chara from the beds of the 

 plant existing in ponds from which the marl may have been 

 obtained. While this subject needs further investigation, it is 

 probable that such thickened incrustations have originated in 

 several ways, the principal ones being, if the writer's notes have 

 any bearing on the subject, as follows : 



1. On short, stunted plants that grow for a long time on 

 unfavorable soil, such as sand or pure marl. Such plants have 

 relatively very short internodes and generally thick incrusta- 

 tions, much thicker than those on plants growing normally. 



2. :i^rom the growth of the lime-secreting, blue-green algae, 

 such as Schizothrix, Rivularia, etc., either upon living chara or 

 upon the fragments of broken incrustation, as a nucleus. .Such 

 a growth might produce considerable thickening of the Chara 

 incrustation. 



3. From the inclusion of the fragments within the nodules 



