The basal strata of these hills, as well as some neighbouring, separate,
bare, rounded hillocks, consist of compact, fine-grained, non-crystalline
(or so slightly as scarcely to be perceptible), ferruginous, feldspathic
rocks, and generally in a state of semi-decomposition. Their fracture is
exceedingly irregular, and splintery; yet small fragments are often very
tough. They contain much ferruginous matter, either in the form of minute
grains with a metallic lustre, or of brown hair-like threads: the rock in
this latter case assuming a pseudo-brecciated structure. These rocks
sometimes contain mica and veins of agate. Their rusty brown or yellowish
colour is partly due to the oxides of iron, but chiefly to innumerable,
microscopically minute, black specks, which, when a fragment is heated, are
easily fused, and evidently are either hornblende or augite. These rocks,
therefore, although at first appearing like baked clay or some altered
sedimentary deposit, contain all the essential ingredients of trachyte;
from which they differ only in not being harsh, and in not containing
crystals of glassy feldspar. As is so often the case with trachytic
formation, no stratification is here apparent. A person would not readily
believe that these rocks could have flowed as lava; yet at St. Helena there
are well-characterised streams (as will be described in an ensuing chapter)
of nearly similar composition. Amidst the hillocks composed of these rocks,
I found in three places, smooth conical hills of phonolite, abounding with
fine crystals of glassy feldspar, and with needles of hornblende. These
cones of phonolite, I believe, bear the same relation to the surrounding
feldspathic strata which some masses of coarsely crystallised augitic rock,
in another part of the island, bear to the surrounding basalt, namely, that
both have been injected. The rocks of a feldspathic nature being anterior
in origin to the basaltic strata, which cap them, as well as to the
basaltic streams of the coast-plains, accords with the usual order of
succession of these two grand divisions of the volcanic series.