The fragments of scoriae, embedded in the crystalline calcareous basis, are
of a jet black colour, with a glossy fracture like pitchstone. Their
surfaces, however, are coated with a layer of a reddish-orange, translucent
substance, which can easily be scratched with a knife; hence they appear as
if overlaid by a thin layer of rosin. Some of the smaller fragments are
partially changed throughout into this substance: a change which appears
quite different from ordinary decomposition. At the Galapagos Archipelago
(as will be described in a future chapter), great beds are formed of
volcanic ashes and particles of scoriae, which have undergone a closely
similar change.

THE EXTENT AND HORIZONTALITY OF THE CALCAREOUS STRATUM.

(FIGURE 2: SIGNAL POST HILL. (Section with A low and C high.)

A.--Ancient volcanic rocks.

B.--Calcareous stratum.

C.--Upper basaltic lava.)

The upper line of surface of the calcareous stratum, which is so
conspicuous from being quite white and so nearly horizontal, ranges for
miles along the coast, at the height of about sixty feet above the sea. The
sheet of basalt, by which it is capped, is on an average eighty feet in
thickness. Westward of Porto Praya beyond Red Hill, the white stratum with
the superincumbent basalt is covered up by more recent streams. Northward
of Signal Post Hill, I could follow it with my eye, trending away for
several miles along the sea cliffs. The distance thus observed is about
seven miles; but I cannot doubt from its regularity that it extends much