This, however, is not the case, and we may rest the argument here. The
septa reach not the circumference; the surface of the stone is solid and
uniform in every part; and there is not any appearance of the spar in
the argillaceous bed around the stone.
It, therefore, necessarily follows, that the contraction of the
iron-stone, in order to form septa, and the filling of these cavities
with spar, had proceeded _pari passu_; and that this operation must have
been brought about by means of fusion, or by congelation from a state of
simple fluidity and expansion.
It is only further to be observed, that all the arguments which have
been already employed, concerning mineral concretions from a simply
fluid state, or that of fusion, here take place. I have septaria of this
kind, in which, besides pyrites, iron-ore, calcareous spar, and another
that is ferruginous and compound, there is contained siliceous
crystals; a case which is not so common. I have them also attended with
circumstances of concretion and crystallization, which, besides being
extremely rare, are equally curious and interesting.
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