88 REVIEWS 



deposition of the lime carbonate ; or, (3) animal or vegetable agencies, or both, cause 

 the deposition of the marl. 



The writer disposes of the first two possibilities, stating that the marl is formed at 

 and below the present water level, even coating pebbles and living shells ; and that 

 the outlets of the marl lakes of Michigan are so large that little, if any, concentration 

 occurs in the lakes. 



Molluscan and other invertebrate shells, in fragments of recognizable size, do not 

 contribute largely to the deposits of commercial value ; and it is improbable that wave 

 action in these lakes has been sufficiently strong to reduce shells to powder or mud. 

 The sole remaining possible cause, then, is plant action; and this the author discusses 

 in detail. 



All green plants absorb carbon dioxide and release oxygen. If the calcium salts 

 are in excess in the water, and are held in solution by carbon dioxide, then the more 

 or less complete abstraction of the carbon dioxide from the water near the plant will 

 cause direct precipitation of lime carbonate upon the leaves and stems. If, however, 

 only minute percentages of lime salts are present in the water, the precipitation of 

 lime carbonate is accomplished, not by the abstraction of carbon dioxide, but through 

 the agency of the oxygen released by the plant, according to the following equation : 

 CaH2(C03)3 + = H30 + C02+0 + CaC03 . 



Not all plants are equally active agents. The algse seem to be of the greatest 

 importance in the formation of marl deposits, one group — the Characae or Stoneworts 

 — being of particular importance. Professor Davis collected five stems, each about 

 60"^™ long, of a species of Chara; these yielded 3. 6504 grains of solid matter, the 

 mineral portion of which carried 93.76 per cent, of lime carbonate, 2.93 per cent, of 

 magnesium carbonate; 2.40 per cent, of silica and undetermined, and 0.89 per cent, of 

 oxides of iron and aluminum. Chara fragilis seems so be the most active form in the 

 Michigan marl region, but it is probable that close examination will reveal a number 

 of allied species engaged in the same work. 



Pebble-like masses of marl, characteristically ellipsoidal in shape and showing 

 both radial and concentric structure, are attributed to the action of another alga, 

 determined to be a species of Zonotrichia. Similar, if not identical, material has been 

 described by Murray, who identified the alga &?, Sckizothrix fasciculaia ; but compari- 

 son of these materials was not possible. 



Davis, C. A. Second Cotttribtitiott to the Natural History of Marl. Journ. 



GeoL, Vol. IX, No. 6. September to October, iqoi, pp. 491-506. 



Continued investigation, along three distinct lines, confirms the writer's opinion 

 that close relationship exists between certain algae (especially Chara) and marl 

 deposition. 



A series of careful mechanical analyses of typical marls from various (Michigan) 

 localities was made. In general, it is easily possible to recognize with a simple micro- 

 scope the particles held by a hundred-mesh sieve as being Chara incrustations or 

 Schizothrix concretions ; and it is therefore evident that a large part of the samples 

 are of demonstrable algal origin. 



The milky appearance of certain lakes has been cited as evidence of the probable 

 presence of calcium carbonate, precipitated from the water by loss of carbon dioxide 

 or change of temperature on reaching the lakes. Experiments show, however, that 

 this turbidity is probably caused by the stirring up of marl deposits by wave action. 



