analysis from that given by mineralogists of this mineral: the author
attributed this difference to an error in the analysis of Labrador
feldspar, which is very old.); but in their interstices there is some white
granular feldspar, abundant scales of mica, a little altered hornblende,
and, as I believe, no quartz. I have described these fragments in detail,
because it is rare to find granitic rocks ejected from volcanoes with their
MINERALS UNCHANGED, as is the case with the first specimen, and partially
with the second. (Daubeny, in his work on Volcanoes page 386, remarks that
this is the case; and Humboldt, in his "Personal Narrative" volume 1 page
236, says "In general, the masses of known primitive rocks, I mean those
which perfectly resemble our granites, gneiss, and mica-slate, are very
rare in lavas: the substances we generally denote by the name of granite,
thrown out by Vesuvius, are mixtures of nepheline, mica, and pyroxene.")
One other large fragment, found in another spot, is deserving of notice; it
is a conglomerate, containing small fragments of granitic, cellular, and
jaspery rocks, and of hornstone porphyries, embedded in a base of wacke,
threaded by numerous thin layers of a concretionary pitchstone passing into
obsidian. These layers are parallel, slightly tortuous, and short; they
thin out at their ends, and resemble in form the layers of quartz in
gneiss. It is probable that these small embedded fragments were not
separately ejected, but were entangled in a fluid volcanic rock, allied to
obsidian; and we shall presently see that several varieties of this latter
series of rock assume a laminated structure.

TRACHYTIC SERIES OF ROCKS.

Those occupy the more elevated and central, and likewise the south-eastern,
parts of the island. The trachyte is generally of a pale brown colour,
stained with small darker patches; it contains broken and bent crystals of
glassy feldspar, grains of specular iron, and black microscopical points,