a schistose sandstone. Sphaerulites are described as occurring abundantly
in all cases; and they everywhere seem to mark the passage, from the
perfectly glassy to the stony and crystalline beds. Beudant's account
(Beudant "Voyage" tome 3 page 373.) of his "perlite lithoide globulaire" in
every, even the most trifling particular, might have been written for the
little brown sphaerulitic globules of the rocks of Ascension.

From the close similarity in so many respects, between the obsidian
formations of Hungary, Mexico, Peru, and of some of the Italian islands,
with that of Ascension, I can hardly doubt that in all these cases, the
obsidian and the sphaerulites owe their origin to a concretionary
aggregation of the silica, and of some of the other constituent elements,
taking place whilst the liquified mass cooled at a certain required rate.
It is, however, well-known, that in several places, obsidian has flowed in
streams like lava; for instance, at Teneriffe, at the Lipari Islands, and
at Iceland. (For Teneriffe see von Buch "Descript. des Isles Canaries"
pages 184 and 190; for the Lipari Islands see Dolomieu "Voyage" page 34;
for Iceland see Mackenzie "Travels" page 369.) In these cases, the
superficial parts are the most perfectly glassy, the obsidian passing at
the depth of a few feet into an opaque stone. In an analysis by Vauquelin
of a specimen of obsidian from Hecla, which probably flowed as lava, the
proportion of silica is nearly the same as in the nodular or concretionary
obsidian from Mexico. It would be interesting to ascertain, whether the
opaque interior portions and the superficial glassy coating contained the
same proportional constituent parts: we know from M. Dufrenoy ("Memoires
pour servir a une Descript. Geolog. de la France" tome 4 page 371.) that
the exterior and interior parts of the same stream of lava sometimes differ
considerably in their composition. Even should the whole body of the stream
of obsidian turn out to be similarly composed with nodular obsidian, it
would only be necessary, in accordance with the foregoing facts, to suppose