TAHITI.--Passage from wacke into trap; singular volcanic rock with the
vesicles half-filled with mesotype.

MAURITIUS.--Proofs of its recent elevation.--Structure of its more ancient
mountains; similarity with St. Jago.

ST. PAUL'S ROCKS.--Not of volcanic origin.--Their singular mineralogical
composition.


CHAPTER III.--ASCENSION.

Basaltic lavas.--Numerous craters truncated on the same side.--Singular
structure of volcanic bombs.--Aeriform explosions.--Ejected granite
fragments.--Trachytic rocks.--Singular veins.--Jasper, its manner of
formation.--Concretions in pumiceous tuff.--Calcareous deposits and
frondescent incrustations on the coast.--Remarkable laminated beds,
alternating with, and passing into obsidian.--Origin of obsidian.--
Lamination of volcanic rocks.


CHAPTER IV.--ST. HELENA.

Lavas of the feldspathic, basaltic, and submarine series.--Section of
Flagstaff Hill and of the Barn.--Dikes.--Turk's Cap and Prosperous Bays.--
Basaltic ring.--Central crateriform ridge, with an internal ledge and a
parapet.--Cones of phonolite.--Superficial beds of calcareous sandstone.--
Extinct land-shells.--Beds of detritus.--Elevation of the land.--
Denudation.--Craters of elevation.