sses of calcareous stone, scarcely if at all distinguishable
from the older rock. I was shown, in 1828, in the upper part of the
travertin, the hollow left by a cart-wheel, in which the outer circle
and the spokes had been decomposed, and the spaces which they filled
left void. It seemed to me at the time impossible to explain the
position of this mould without supposing that the wheel was imbedded
before the lake was drained; but Sir R. Murchison suggests that it may
have been washed down by a flood into the gorge in modern times, and
then incrusted with calcareous tufa in the same manner as the wooden
beam of the church of St. Lucia was swept down in 1826, and stuck fast
in the Grotto of the Syren, where it still remains, and will eventually
be quite imbedded in travertin.

I have already endeavored to explain (p. 241), when speaking of the
travertin of San Filippo, how the spheroidal masses represented in
figure 22 may have been formed.

_Sulphureous and gypseus springs._--The quantity of other mineral
ingredients wherewith springs in general are impregnated, is
insignificant in comparison to lime, and this earth is most frequently
combined with carbonic acid. But as sulphuric acid, and sulphuretted
hydrogen are very frequently supplied by springs, gypsum may, perhaps,
be deposited largely in certain seas and lakes. Among other gypseous
precipitates at present known on the land, I may mention those of Baden,
near Vienna, which feed the public bath. Some of these supply singly
from 600 to 1000 cubic feet of water per hour, and deposit a fine
powder, composed of a mixture of sulphate of lime with sulphur and
muriate of lime.[318] The thermal waters of Aix, in Savoy, in passing
through strata of Jurassic limestone, turn them into gypsum or sulphate
of lime. In the Andes, at the Puenta del Inca, Lieutenant Brand found a
thermal spring at the temperature of 91 degrees Fahr., containing a large
proportion of gypsum with carbonate of lime and other ingredients.[319]
Many of the mineral springs of Iceland, says Mr. R. Bunsen, deposit
gypsum.[320] and sulphureous acid gas escapes plentifully from them as
from the volcanoes of the same island. It may, indeed, be laid down as a
general rule, that the mineral substances dissolved in hot springs agree
very closely with those which are disengaged in a gaseous form from the
craters of active volcanoes.

_Siliceous springs.--Azores._--In order that water should hold a very
large