little tuffs; and hence they have a hairy appearance. These spicula are of
the same diameter throughout their length; they are easily detached, so
that the object-glass of the microscope soon becomes scattered over with
them. Within the cells of many fragments of the scoria, the lime exhibits
this fibrous structure, but generally in a less perfect degree. These cells
do not appear to be connected with one another. There can be no doubt, as
will presently be shown, that the lime was erupted, mingled with the lava
in its fluid state, and therefore I have thought it worth while to describe
minutely this curious fibrous structure, of which I know nothing analogous.
From the earthy condition of the fibres, this structure does not appear to
be related to crystallisation.

Other fragments of the scoriaceous rock from this hill, when broken, are
often seen marked with short and irregular white streaks, which are owing
to a row of separate cells being partly, or quite, filled with white
calcareous powder. This structure immediately reminded me of the appearance
in badly kneaded dough, of balls and drawn-out streaks of flour, which have
remained unmixed with the paste; and I cannot doubt that small masses of
the lime, in the same manner remaining unmixed with the fluid lava, have
been drawn out when the whole was in motion. I carefully examined, by
trituration and solution in acids, pieces of the scoriae, taken from within
half-an-inch of those cells which were filled with the calcareous powder,
and they did not contain an atom of free lime. It is obvious that the lava
and lime have on a large scale been very imperfectly mingled; and where
small portions of the lime have been entangled within a piece of the viscid
lava, the cause of their now occupying, in the form of a powder or of a
fibrous reticulation, the vesicular cavities, is, I think, evidently due to
the confined gases having most readily expanded at the points where the
incoherent lime rendered the lava less adhesive.