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.
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