It is supposed that when the surface of the lava cools rapidly owing
to enfeebled action below, the gases force their way upwards through
small vents, which then serve as "blow holes" for the imprisoned
fluid beneath. This, rapidly cooling as it is ejected, forms a ring
on the surface of the crust, which, growing upwards by accretion,
forms a chimney, eventually nearly or quite closed at the top, so as
to form a cone. In this case the cone is about eighty feet above
the present level of the lake, and fully one hundred yards distant
from its present verge.
The whole of the inside was red and molten, full of knobs, and great
fiery stalactites. Jets of lava at a white heat were thrown up
constantly, and frequently the rent in the side spat out lava in
clots, which cooled rapidly, and looked like drops of bottle green
glass. The glimpses I got of the interior were necessarily brief
and intermittent. The blast or roar which came up from below was
more than deafening; it was stunning: and accompanied with heavy
subterranean rumblings and detonations. The chimney, so far as I
could see, opened out gradually downwards to a great width, and
appeared to be about forty feet deep; and at its base there was an
abyss of lashing, tumbling, restless fire, emitting an ominous
surging sound, and breaking upwards with a fury which threatened to
blow the cone and the crust on which it stands, into the air.
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