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 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.
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