by which the constituent parts of a volcanic rock could be thus acted on.
(Beudant "Voyage en Hongrie" tome 3 pages 502, 504 describes kidney-shaped
masses of jasper-opal, which either blend into the surrounding trachytic
conglomerate, or are embedded in it like chalk-flints; and he compares them
with the fragments of opalised wood, which are abundant in this same
formation. Beudant, however, appears to have viewed the process of their
formation rather as one of simple infiltration than of molecular exchange;
but the presence of a concretion, wholly different from the surrounding
matter, if not formed in a pre-existing hollow, clearly seems to me to
require, either a molecular or mechanical displacement of the atoms, which
occupied the space afterwards filled by it. The jasper-opal of Hungary
passes into chalcedony, and therefore in this case, as in that of
Ascension, jasper seems to be intimately related in origin with
chalcedony.) I was led to the careful examination of these rocks, and to
the conclusion here given, from having heard the Rev. Professor Henslow
express a similar opinion, regarding the origin in trap-rocks of many
chalcedonies and agates. Siliceous deposits seem to be very general, if not
of universal occurrence, in partially decomposed trachytic tuffs (Beudant
"Voyage Min." tome 3 page 507 enumerates cases in Hungary, Germany, Central
France, Italy, Greece, and Mexico.); and as these hills, according to the
view above given, consist of trachyte softened and altered in situ, the
presence of free silica in this case may be added as one more instance to
the list.

CONCRETIONS IN PUMICEOUS TUFF.

The hill, marked in Map 2 "Crater of an old volcano," has no claims to this
appellation, which I could discover, except in being surmounted by a
circular, very shallow, saucer-like summit, nearly half a mile in diameter.
This hollow has been nearly filled up with many successive sheets of ashes