angular, masses of the greenish variety, being embedded in the white
variety;--in this latter case, the appearance was very much like that of a
sedimentary deposit, torn up and abraded during the deposition of a
subsequent stratum. Both these varieties are traversed by innumerable
tortuous veins (presently to be described), which are totally unlike
injected dikes, or indeed any other veins which I have ever seen. Both
varieties include a few scattered fragments, large and small, of dark-
coloured scoriaceous rocks, the cells of some of which are partially filled
with the white earthy stone; they likewise include some huge blocks of a
cellular porphyry. (The porphyry is dark coloured; it contains numerous,
often fractured, crystals of white opaque feldspar, also decomposing
crystals of oxide of iron; its vesicles include masses of delicate, hair-
like, crystals, apparently of analcime.) These fragments project from the
weathered surface, and perfectly resemble fragments embedded in a true
sedimentary tuff. But as it is known that extraneous fragments of cellular
rock are sometimes included in columnar trachyte, in phonolite (D'Aubuisson
"Traite de Geognosie" tome 2 page 548.), and in other compact lavas, this
circumstance is not any real argument for the sedimentary origin of the
white earthy stone. (Dr. Daubeny on Volcanoes, page 180 seems to have been
led to believe that certain trachytic formations of Ischia and of the Puy
de Dome, which closely resemble these of Ascension, were of sedimentary
origin, chiefly from the frequent presence in them "of scoriform portions,
different in colour from the matrix." Dr. Daubeny adds, that on the other
hand, Brocchi, and other eminent geologists, have considered these beds as
earthy varieties of trachyte; he considers the subject deserving of further
attention.) The insensible passage of the greenish variety into the white
one, and likewise the more abrupt passage by fragments of the former being
embedded in the latter, might result from slight differences in the
composition of the same mass of molten stone, and from the abrading action
of one such part still fluid on another part already solidified. The