50 The American Geologist. Jan. 1891 
species <>f mosses are also possessed of this power of separating 
carbonate of lime. The association of mosses and algse with de- 
posits of carbonate of Lime was noticed many years ago at several 
European localities, and in 18G2 Dr. Ferd. Cohn described in de- 
tail the manner in which travertine is formed by the mosses of 
Tivoli and the alga? of the hot springs of Carlsbad, and the ex- 
planation advanced by him is now generall}- accepted as the pro- 
cess by which the great deposits of travertine found at Tivoli and 
the beds of that substance that underlie the city of Carlsbad, have 
been formed. 
This process is explainable on strictly chemical grounds whether 
the lime forms an integral part of the plant structure or merely 
encrusts it. or as is sometimes the case, forms a deposit that 
shows no trace of organic structure. The process is a simple one, 
resulting from the well known avidity of plants for carbonic acid 
gas. Water plants thus tend to impoverish the water of its carbon 
dioxide, thereby depriving the carbonate of lime which the water 
holds in solution, of its solvent. It is not necessary to suppose 
that the plants always decompose the bicarbonate and take up 
the carbonic acid thus liberated, though the fact that the mosses 
growing in waters containing but 0.031 per cent, of the CaC0 3 
were found covered with a beautiful encrust of lime, while the 
water did not deposit that substance under exceptional^ favorable 
conditions of evaporation, proves that it really is the action of the 
plants that produces the deposition of the lime. But even in 
waters very highly charged with carbonic acid gas, plants ma}' 
produce a deposition of carbonate of lime since it is well known to 
chemists that the amount of bicarbonate of lime that is soluble in 
a water is dependent upon the volume of free carbonic acid gas it 
holds, which must be largely in excess of the amount necessary 
to form the bicarbonate, so that in such waters the withdrawal of 
any of the carbonic acid gas will produce supersaturation of the 
solution and consequent deposition of the calcic carbonate ; this is 
actually the case both in the cold waters of the Arno at Tivoli and 
the hot waters of the Carlsbad springs. Observations made upon 
tin- origin of the travertine and calc sinter deposits of the Yellow- 
>tone park, show that they have a similiar origin in part at least 
and it is to the algous vegetation of the hot lime-bearing waters 
R. Ludwig. Pogg. Ann. Vol. 87, p. 91. 
