The geological work of Mosses and Algce. — Weed. 51 
which beautifies the hot spring howls, channels and overflow slope 
with its brilliant and varied tints, that we must credit the forma- 
tion of the unique and beautiful terraces of the Mammoth hot 
springs. Each peculiar type of algous growth forms its own 
characteristic form of travertine and we have in consequence lam- 
inated, fibrous, thatch-like and other forms of tufa. * 
Deposits of iron are also formed by plant-life. It is now gen- 
erall}- admitted that bog iron ores owe their origin to chemico- 
organic agencies, the iron being taken into solution by the waters 
through the action of the products of decaying vegetation. The 
final precipitation of the ore is easily accounted for by simple oxi- 
dation on the surface of the bogs or ponds and lakes, on the bot- 
tom of which the ore accumulates, but as a matter of fact it has 
been found that the ore sometimes consists very largely of the re- 
mains of diatoms, the iron encrusting the siliceous tests and in 
some cases at least forming part of the sheath itself. The oxygen 
given off by the living plants would be a sufficient cause of this 
encrustation of iron, since it would oxydize the iron to the insolu- 
ble sesquioxide. It is by this action that Bischof explains the 
presence of large amounts of iron found in the travertine of 
Nauheim. Not onl}" does the vegetable life of these waters cause 
the separation of carbonate of lime by their absorption of CO., but 
the oxygen which they give off ox}~dizes the protoxides of iron 
present in the water and causes the formation of a highly ferrugi- 
nous deposit. 
A very striking case of ferruginous deposit clearl}- due to plant 
life was observed by the writer last summer. A spring of cold, 
clear and quite acid water highly charged with iron and carbonic 
acid gas, issued from the summit of a mound of what appeared to 
be iron ochre. The bowl on the summit of this mound was filled 
to the brim with the sparkling water and surrounded by a growth 
of velvety dark green moss, a species of Hypnum. Upon exam- 
ining the deposit of iron forming the mound it was found to con- 
sist entirel} T of the stems of a moss that proved identical with that 
found about the edge of the bowl. Specimens were obtained 
showing a continuous passage of the living moss at the surface 
into the mineralized stalk below. The mound was several feet 
high and composed entirely of this deposit and an old mound 
*Ninth annual reporl <>f the Director U. S. Geol. Survey, p. 619. 
