slower cooling. Crystals of augite
have been met with in the scoriae of furnaces, but never those of
hornblende; and crystals of augite have been obtained by melting
hornblende in a platina crucible; but hornblende itself has not been
formed artificially.[621] Mica occurs plentifully in some recent
trachytes, but is rarely present where augite is in excess.

_Frequency of eruptions, and nature of subterranean igneous
rocks._--When we speak of the igneous rocks of our own times, we mean
that small portion which, in violent eruptions, is forced up by elastic
fluids to the surface of the earth,--the sand, scoriae, and lava, which
cool in the open air. But we cannot obtain access to that which is
congealed far beneath the surface under great pressure, equal to that of
many hundred, or many thousand atmospheres.

During the last century, about fifty eruptions are recorded of the five
European volcanic districts, of Vesuvius, Etna, Volcano, Santorin, and
Iceland; but many beneath the sea in the Grecian archipelago and near
Iceland may doubtless have passed unnoticed. If some of them produced no
lava, others, on the contrary, like that of Skaptar Jokul, in 1783,
poured out melted matter for five or six years consecutively; which
cases, being reckoned as single eruptions, will compensate for those of
inferior strength. Now, if we consider the active volcanoes of Europe to
constitute about a fortieth part of those already known on the globe,
and calculate that, one with another, they are about equal in activity
to the burning mountains in other districts, we may then compute that
there happen on the earth about 2000 eruptions in the course of a
century, or about twenty every year.

However inconsiderable, therefore, may be the superficial rocks which
the operations of fire produce on the surface, we must suppose the
subterranean changes now constantly in progress to be on the grandest
scale. The loftiest volcanic cones must be as insignificant, when
contrasted to the products of fire in the nether regions, as are the
deposits formed in shallow estuaries when compared to submarine
formations accumulating in the abysses of the ocean. In regard to the
characters of these volcanic rocks, formed in our own times in the
bowels of the earth, whether in rents and caverns, or by the cooling of
lakes of melted lava, we may safely infer that the rocks are heavier and
less porous than ordinary lavas, and more crystalline, although compose