This is the cause of the difference
between those erupted lavas, and our whin-stone, toad-stone, and the
Swedish trap, which may be termed subterraneous lava. The visible
effects of those different operations may now be mentioned.
In the erupted lavas, those substances which are subject to calcine and
vitrify in our fires, suffer similar changes, when delivered from a
compression which had rendered them fixed, though in an extremely heated
state. Thus, a lava in which there is much calcareous spar, when it
comes to be exposed to the atmosphere, or delivered from the compressing
force of its confinement, effervesces by the explosion of its fixed
air; the calcareous earth, at the same time, vitrifies with the other
substances. Hence such violent ebullition in volcanos, and hence the
emission of so much pumice-stone and ashes, which are of the same
nature.
In the body of our whin-stone, on the contrary, there is no mark of
calcination or vitrification. We frequently find in it much calcareous
spar, or the _terra calcarea aerata_, which had been in a melted state
by heat, and had been crystallized by congelation into a sparry form.
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