538 
PHILOSOPHICAL SOCIETY OP WASHINGTON. 
Yellowstone Park, but are more common than is generally supposed, 
and embrace a variety of mineral deposits. 
In point of magnitude and frequency of occurrence, deposits of car¬ 
bonate of lime undoubtedly take first rank. The travertine deposits just 
alluded to, the limestones on the coast of Florida, described by Agassiz, 
and a host of other cases too numerous to mention, make it unnecessary 
to dwell upon deposits of this character further than to state that they 
sometimes result from a direct secretion of carbonate of lime to form the 
cell-wall of the plants, and sometimes from the precipitation of the car¬ 
bonate of lime because of the withdrawal of carbonic-acid gas from the 
water by the living plants—a sufficient explanation of the chemistry of 
the process in either case. 
It is not so well known that deposits of oxide of iron, as bog iron ore 
or as a silicious ocher, are also formed by living plants. This is probably 
because a simple oxidation resulting from exposure to the atmosphere 
will explain the deposition of the ferric oxide; yet it is well known to 
botanists that the algse, Leptothrix ochrocea, secretes ferric oxide in the 
algae sheaths, and that several species of diatoms form their tests of 
both silica and oxide of iron, and microscopic examinations of many bog 
ores prove them to consist very largely of these organic structures. In 
the Yellowstone mounds of silicious iron ocher have been found formed 
of the remains of a Hypnum in situ, with the living moss at the surface, 
whose green stems were formed very largely of iron. 
Both algae and mosses secrete silica and form strata of silicious sinter, 
and of what may be called moss sinter. As in the case of the diatoms 
forming the well-known deposits of diatomaceous earth, the secretion of 
silica by the plants seems to be due to some physiological need of the 
plants being first secreted as a silicious jelly by the algae, and in sandy, 
gritty grains by the moss Hypnum. Both varieties of silicious deposits 
are common in the Yellowstone Park, where they are of considerable 
magnitude. 
The strangest case of all is the formation of gypsum by the vegetation 
of sulphur waters. The slimy, white masses of algae that live in sulphur 
springs have been found to secrete sulphur which they oxidize to sul¬ 
phuric acid, the latter immediately forming sulphate of lime with the 
carbonate of lime in the water. 
Examples of all these deposits due to plant life may be found in the 
Yellowstone Park, where they have been studied by the writer. 
Also Published in part under the title of The Formation of Sili¬ 
cious Sinter by the Vegetation of Thermal Springs. Am. Jour, 
of Science, 8vo, New Haven, May, 1889, vol. 37, No. 221, pp. 351- 
359 ; also in The Ninth Annual Report (1887-88) of the Director 
of the U. S. Geological Survey (4to, Washington, 1890, pp. 613- 
676), under the title The Formation of Travertine and of Silicious 
Sinter by the Vegetation of Hot Springs. 
