("Essai Geognostique" pages 176, 326, 328.), and likewise the descriptions
given by several authors (P. Scrope "Geological Transactions" volume 2
second series page 195. Consult also Dolomieu "Voyage aux Isles Lipari" and
D'Aubuisson "Traite de Geogn." tome 2 page 534.) of the trachytic regions
in the Italian islands, agree with my observations at Ascension. Many
passages might have been transferred without alteration from the works of
the above authors, and would have been applicable to this island. They all
agree in the laminated and stratified character of the whole series; and
Humboldt speaks of some of the beds of obsidian being ribboned like jasper.
(In Mr. Stokes' fine collection of obsidians from Mexico, I observe that
the sphaerulites are generally much larger than those of Ascension; they
are generally white, opaque, and are united into distinct layers: there are
many singular varieties, different from any at Ascension. The obsidians are
finely zoned, in quite straight or curved lines, with exceedingly slight
differences of tint, of cellularity, and of more or less perfect degrees of
glassiness. Tracing some of the less perfectly glassy zones, they are seen
to become studded with minute white sphaerulites, which become more and
more numerous, until at last they unite and form a distinct layer: on the
other hand, at Ascension, only the brown sphaerulites unite and form
layers; the white ones always being irregularly disseminated. Some
specimens at the Geological Society, said to belong to an obsidian
formation from Mexico, have an earthy fracture, and are divided in the
finest parallel laminae, by specks of a black mineral, like the augitic or
hornblendic specks in the rocks at Ascension.) They all agree in the
nodular or concretionary character of the obsidian, and of the passage of
these nodules into layers. They all refer to the repeated alterations,
often in undulatory planes, of glassy, pearly, stony, and crystalline
layers: the crystalline layers, however, seem to be much more perfectly
developed at Ascension, than in the above-named countries. Humboldt
compares some of the stony beds, when viewed from a distance, to strata of