carried on probably to the sea.

_Spheroidal structure in travertin._--But what renders this recent
limestone of peculiar interest to the geologist, is the spheroidal form
which it assumes, analogous to that of the cascade of Tivoli, afterwards
to be described. (See fig. 22, p. 244.) The lamination of some of the
concentric masses is so minute that sixty may be counted in the
thickness of an inch, yet, notwithstanding these marks of gradual and
successive deposition, sections are sometimes exhibited of what might
seem to be perfect spheres. This tendency to a mammillary and globular
structure arises from the facility with which the calcareous matter is
precipitated in nearly equal quantities on all sides of any fragment of
shell or wood or any inequality of the surface over which the mineral
water flows, the form of the nucleus being readily transmitted through
any number of successive envelopes. But these masses can never be
perfect spheres, although they often appear such when a transverse
section is made in any line not in the direction of the point of
attachment. There are, indeed, occasionally seen small oolitic and
pisolitic grains, of which the form is globular; for the nucleus, having
been for a time in motion in the water, has received fresh accessions of
matter on all sides.

In the same manner I have seen, on the vertical walls of large
steam-boilers, the heads of nails or rivets covered by a series of
enveloping crusts of calcareous matter, usually sulphate of lime; so
that a concretionary nodule is formed, preserving a nearly globular
shape, when increased to a mass several inches in diameter. In these, as
in many travertins, there is often a combination of the concentric and
radiated structure.

_Campagna di Roma._--The country around Rome, like many parts of the
Tuscan States already referred to, has been at some former period the
site of numerous volcanic eruptions; and the springs are still copiously
impregnated with lime, carbonic acid, and sulphuretted hydrogen. A hot
spring was discovered about 1827, near Civita Vecchia, by Signor
Riccioli, which deposits alternate beds of a yellowish travertin, and a
white granular rock, not distinguishable, in hand specimens, either in
grain, color, or composition, from statuary marble. There is a passage
between this and ordinary travertin. The mass accumulated near the
spring is in some places about six feet thick.

_Lake of the Solfatara._--In the Campagna