most symmetrical prisms are exposed on the faees of the mountains,
and form the most elevated range at the sides of the inland
valleys, where they could never have been in contact with the
torrents which flow beneath, and which could not have existed
when the streams of basalt were first ejected from the crater.
The opinion, that the columnar structure natural to the basalt has
been developed by the continued action of streams of water, which
formerly washed, but now flow beneath it, from having in the
course of ages hollowed out a much deeper bed than that which
it occupied in the first instance; this opinion struck me as very
probable, until I remarked the position of the columnar basalt just
alluded to, and represented in the coloured sketches, Plate 3, A, C*
and of that exposed high up on the sides of mountains, remote
from valleys, where it must have always been, like the columnar
basalt crowning the cliffs at the sea-side, out of the reach of the
action of water, unless we take into account the heavy rains, and
the torrents poured out from the crater during eruptions, causes,
seemingly too transient, to be adeouate to the effect, which could
only be ascribed to a long and continued action on the part of the
waters. Could the above opinion have been established, we might
have estimated, by comparing the actual depth of these ravines, or
valleys, with the observed increase of that depth, in a known period
of time, the probable age of the basalt. The valleys or ravines of
Madeira have, of course, been deepened by the agency of water, but
I cannot consider that they have been entirely formed by i t : the
various and partial directions of the streams of basalt, as they descended
from the crater to the sea, and the pre-existing hills and
valleys (for we shall presently discover that the base of the island
is of a t r ansitio n , if notof a primitive formation), being no doubt
the primary causes.
The columnar basalt is, generally speaking, compact, of a broad
conchoidal fracture, splitting in horizontal laminae, or at right
angles to the prism, dark grey within, or on the surface of a fresh