The Quarterly journal of the Geological Society of London (13365415485)

1850.. MURCHISON VENTS OF HOT VAPOUR IN TUSCANY. 375
and myself contend, that the amorphous, variohtie gabbro must have
been erupted in a molten state, whether we consider its composition
and unbedded condition, or the part it has played in protruding
through, overturning, breaking, and altering the pre-existing strata.
And although my deceased friend Pilla has to a certain extent pub-
lished this opinion, he has not sufficiently illustrated his views, and I
am therefore the more anxious to do him justice, and to adduce some
of the reasons he assigned when we visited the tract together. The
opinion of an attentive and lively observer of igneous action like Pilla,
a Neapolitan by birth, who during many years was occupied in exa-
mining Vesuvius, is surely entitled to much consideration in deter-
mining such a question ; even had not the physical and geological
relations of the phsenomenon seemed p. o
to me quite conclusive. Between °'
Castel Anselmo and Civita Castel-
lina 1 inspected natural sections, of
one of which I here give a sketch
(see fig. 3), where the gabbro had a, «. Alberese. b. Gabbro rosso.
not only penetrated the alberese limestone, but had thrown it off in
shreds, contorted fragments, and folds on the sides of the eruption.
Now, the red gabbro which had manifestly thus acted was entirely
an unbedded, amorphous, felspathic mass, for the most part made
up of spheroidal concretions having a variolitic structure, i. e, with
small pustular or globular surfaces in each of the folds or concentric
layers into which the large nodules exfoliate. This variolitic surface
was specially pointed out to me by Pilla as a proof of the rock having
been in complete fusion ; inasmuch as the same forms occur frequently
in ancient plutonic rocks and in the modern volcanic products of
Vesuvius. The rock is, besides, often cellular and amygdaloidal as
well as veined, like some of our earthy Scottish traps, occasionally
containing crystals of carbonate of lime, analcime, and also the pe-
culiar mineral caporcianite, a variety of stilbite. Chemically con-
sidered, this rock is little else than a variety of greenstone. In other
words, it is one of those products, accompanying greenstone and
serpentine, which has been much impregnated by iron, and which
under the blowpipe melts as easily as wax. This is the ^'gabbro rosso,"
which I consider to be a true eruptive rock, and which rises up into
Fig. 4.
W. Civita Castellina. E.
e c e II
a. Alberese. b. Alberese with mineral veins. n, e. Gabbro rosso.
d. Miocene ? with alabaster. e, e. Subapennine.
an amorphous mountain mass at Civita Castellina, where it performs,
as above mentioned, the part of an intrusive agent. It there throws
off on its eastern summit the alberese limestone in a highly fractured

and mineralized condition, as seen in fig. 4. From the natural section