called TRAVERTINE. The gas is sometimes withdrawn and the deposit
produced in large part by the action of algae and other humble
forms of plant life.

At the Mammoth Hot Springs in the valley of the Gardiner River,
Yellowstone National Park, beautiful terraces and basins of
travertine are now building, chiefly by means of algae which cover
the bottoms, rims, and sides of the basins and deposit lime
carbonate upon them in successive sheets. The rock, snow-white
where dry, is coated with red and orange gelatinous mats where the
algae thrive in the over-flowing waters.

Similar terraces of travertine are found to a height of fourteen
hundred feet up the valley side. We may infer that the springs
which formed these ancient deposits discharged near what was then
the bottom of the valley, and that as the valley has been deepened
by the river the ground water of the region has found lower and
lower points of issue.

In many parts of the country calcareous springs occur which coat
with lime carbonate mosses, twigs, and other objects over which
their waters flow. Such are popularly known as petrifying springs,
although they merely incrust the objects and do not convert them
into stone.

Silica is soluble in alkaline waters, especially when these are
hot. Hot springs rising through alkaline siliceous rocks, such as
lavas, often deposit silica in a white spongy formation known as
SILICEOUS SINTER, both by evaporation and by the action of algae
which secrete silica from the waters. It is in this way that the