1 86 The American Geologist. ^'•<=^' i^^i. 
only at Placerville, where it is intended to work them for vanadium. 
Carnotite is associated with them in only trifling amount. Other sand- 
stones noticed owe their bright green color to chromium. In yet anoth- 
er case where the color was dull green, this was not due to either 
chromium or vanadium. c. h. w. 
A Contribution to the Natural History of Marl. By Ch.\rles A. 
Davis. (Jour. Gcol., 8, 485-497.) 
This valuable contribution to the chemistry of marl is based upon 
observations made in certain small lakes in Michigan ; but the prin- 
ciples which it embodies undoubtedly admit of general application. 
The marl is nearly pure carbonate of lime, the source of which is to 
be sought in the solution by meteoric waters of limestone or other cal- 
cerous strata, the drift of limestone districts and, more remotely, the 
carbonation of lime-bearing silicates. The real problem is the depo- 
sition of this dissolved carbonate of lime in the form of white chalky 
marl, which consists only to a small extent of shells or fragments of 
shells. After pointing out the inadequacy of animal life, evaporation 
and escape of carbon dioxide on relief of pressure, as causes of the 
precipitation of the carbonate of lime, the author discusses at length 
the only alternative hypothesis, namely, that the calcium salts are pre- 
cipitated through the agency of plant life. In the marly lakes aquatic 
plants of all kinds become incrusted with calcium carbonate, which is 
not a true secretion of the plants, for it is purely external, and the 
same species in other districts are not incrusted. The deposit is formed 
incidentally by chemical precipitation upon the surface of the plants, 
probably only upon the green parts, and in performance of normal and 
usual processes of the plant organism. All green plants inhale carbon 
dioxide and exhale oxygen ; and these are two possible causes of the 
calcareous incrustation. If the calcium carbonate is in excess in the 
water and is held in solution by carbon dioxide, then the absorption 
of the latter by the plants causes precipitation of the carbonate upon 
the parts (stems and leaves) abstracting the gas. But if the proportion 
of calcium corbonate in solution is so small that it would not be de- 
posited even if there were no carbon dioxide present, the precipitation 
is explained by the action of the oxygen set free by the living plants 
in converting calcium bicarbonate to monocarbonate : 
CaH„(C03)2+0=H,0+CaC03-fC02-fO. 
Plants varj- greatly in their power of precipitating the calcium car- 
bonate ; and the algae, and especially the Characes or stoneworts, are 
most efficient. Analyses are given showing that plants may precipi- 
tate mineral matter equal to several or many times the weight of their 
own dried tissues, that this mineral matter consists of CaO^, 93.76; 
MgCO^, 2.93 ; SiO,, 2.40 ; and iron and aluminum oxides 0.89 per cent. 
It is shown that the structure and distribution of the marl are entirely 
in harmony with the view that a species of Chara is an important 
agent in its formation ; but it is recognized that a species of Zonotrichia 
has always played an important part, explaining, especially, the more 
solid and nodular forms of the marl. w. 0. c. 
