lest resemblance to rocks cooled down from a state of
fusion. The exhalations of sulphuretted hydrogen and muriatic acid,
which are disengaged continually from the Solfatara, also produce
curious changes on the trachyte of that nearly extinct volcano: the rock
is bleached, and becomes porous, fissile, and honey-combed, till at
length it crumbles into a white siliceous powder.[545] Numerous globular
concretions, composed of concentric laminae, are also formed by the same
vapors in this decomposed rock.[546]

_Vesuvian minerals._--A great variety of minerals are found in the lavas
of Vesuvius and Somma; augite, leucite, felspar, mica, olivine, and
sulphur are most abundant. It is an extraordinary fact, that in an area
of three square miles round Vesuvius, a greater number of simple
minerals have been found than in any spot of the same dimensions on the
surface of the globe. Hauy enumerated only 380 species of simple
minerals as known to him; and no less than eighty-two had been found on
Vesuvius and in the tuffs on the flanks of Somma before the end of the
year 1828.[547] Many of these are peculiar to that locality. Some
mineralogists have conjectured that the greater part of these were not
of Vesuvian origin, but thrown up in fragments from some older
formation, through which the gaseous explosions burst. But none of the
older rocks in Italy, or elsewhere, contain such an assemblage of
mineral products; and the hypothesis seems to have been prompted by a
disinclination to admit that, in times so recent in the earth's history,
the laboratory of nature could have been so prolific in the creation of
new and rare compounds. Had Vesuvius been a volcano of high antiquity,
formed when nature

Wanton'd as in her prime, and play'd at will
Her virgin fancies,

it would have been readily admitted that these, or a much greater
variety of substances, had been sublimed in the crevices of lava, just
as several new earthy and metallic compounds are known to have been
produced by fumeroles, since the eruption of 1822.

_Mass enveloping Herculaneum and Pompeii._--In addition to the ejections
which fall on the cone, and that much greater mass which finds its way
gradually to the neighboring sea, there is a third portion, often of no
inconsiderable thickness, composed of alluviums, spread over the valleys
and plains at small distances from the volcano. Aqueous vapors are
evolved copiously from volcanic craters during eruptions,