494 CHARLES A. DAVIS 



These samples represent (i) central, (2) north central, and 

 (3) southern parts of the lower peninsula respectively, and may 

 be taken as typical of the marl deposits of Michigan. When it 

 is stated that, in general, it is easily possible to recognize with a 

 simple microscope the particles which are held by the one 

 hundred-mesh sieve, or even those which pass through it, if the 

 finer matter has been carefully separated by washing, as charac- 

 teristic Chara incrustation or Schizothrix concretions, it will be 

 seen that these results show conclusively that a large part of the 

 marl from these three samples is identifiable as of algal origin, 

 and studies of the marls from other localities give similar 

 results. 



The Coldwater sample (3) was exceedingly fine in texture, 

 and it was difficult to avoid loss in sorting and weighing, as 

 every current of air carried away some of the particles, and 

 some also adhered to sieves and weighing dishes in spite of all 

 precautions. Even this sample shows nearly 50 per cent, of 

 easily identified Chara incrustation. The fineness of the parti- 

 cles in a given marl bed varies much in different parts of the 

 bed, and the degree of fineness is probably chiefly dependent 

 upon the conditions of current and wave action under which the 

 bed was formed, that which was deposited where the wave or 

 current action was strong being coarser than that in stiller water 

 or that on the lee side of exposed banks. This fact was noted 

 at Littlefield Lake when samples of marl were collected along 

 exposed shores above the wave line, which were 95 per cent, 

 coarse fragments of Chara incrustation and Schizothrix nodules, 

 while in other parts of the shore line the marl was of such fine- 

 ness that it was like fine, white clay. The fragments of the 

 Chara incrustation are generally easily recognized even when of 

 minute size, because they preserve, usually very perfectly, the 

 peculiar form of the stem and branches of the plant. This 

 structure of the stem and branches is, in brief, a series of small 

 tubes, grouped about a larger central one, and is easily seen 

 with the unaided eye in larger fragments. Even when the tubes 

 have been crushed, as is the case with many of the thinner ones. 



