passing this end of the island at sea, I could not imagine what the
white patches were with which the whole plain was mottled; I now found
that they were seafowl, sleeping in such full confidence, that even in
midday a man could walk up and seize hold of them. These birds were
the only living creatures I saw during the whole day. On the beach a
great surf, although the breeze was light, came tumbling over the
broken lava rocks.
The geology of this island is in many respects interesting. In several
places I noticed volcanic bombs, that is, masses of lava which have
been shot through the air whilst fluid, and have consequently assumed a
spherical or pear-shape. Not only their external form, but, in several
cases, their internal structure shows in a very curious manner that
they have revolved in their aerial course. The internal structure of
one of these bombs, when broken, is represented very accurately in the
woodcut. The central part is coarsely cellular, the cells decreasing
in size towards the exterior; where there is a shell-like case about
the third of an inch in thickness, of compact stone, which again is
overlaid by the outside crust of finely cellular lava. I think there
can be little doubt, first that the external crust cooled rapidly in
the state in which we now see it; secondly, that the still fluid lava
within, was packed by the centrifugal force, generated by
[picture]
the revolving of the bomb, against the external cooled crust, and so
produced the solid shell of stone; and lastly, that the centrifugal
force, by relieving the pressure in the more central parts of the bomb,
allowed the heated vapours to expand their cells, thus forming the
coarse cellular mass of the centre.
A hill, formed of the older series of volcanic rocks, and which has
been incorrectly considered as the crater of a volcano, is remarkable
from its broad, slightly hollowed, and circular summit having been
filled up with many successive layers of ashes and fine scoriae. These
saucer-shaped layers crop out on the margin, forming perfect rings of
many different colours, giving to the summit a most fantastic
appearance; one of these rings is white and broad, and resembles a
course round which horses have been exercised; hence the hill has been
called the Devil's Riding School. I brought away specimens of one of
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