ain its heat for half a century. The reader may be
reminded, that when we thus suppose the lava near the volcano to have
been, together with the ejected ashes, more than five hundred feet in
depth, we merely assign a thickness which the current of Skaptar Jokul
attained in some places in 1783.

_Hollow sound of the plain when struck._--Another argument adduced in
support of the theory of inflation from below, was, the hollow sound
made by the steps of a horse upon the plain; which, however, proves
nothing more than that the materials of which the convex mass is
composed are light and porous. The sound called "rimbombo" by the
Italians is very commonly returned by _made ground_ when struck sharply;
and has been observed not only on the sides of Vesuvius and other
volcanic cones where there is a cavity below, but in such regions as the
Campagna di Roma, composed in a great measure of tuff and porous
volcanic rocks. The reverberation, however, may perhaps be assisted by
grottoes and caverns, for these may be as numerous in the lavas of
Jorullo as in many of those of Etna; but their existence would lend no
countenance to the hypothesis of a great arched cavity, four square
miles in extent, and in the centre 550 feet high.[589]

_No recent eruptions of Jorullo._--In a former edition I stated that I
had been informed by Captain Vetch, that in 1819 a tower at Guadalaxara
was thrown down by an earthquake, and that ashes, supposed to have come
from Jorullo, fell at the same time at Guanaxuato, a town situated 140
English miles from the volcano. But Mr. Burkhardt, a German director of
mines, who examined Jorullo in 1827, ascertained that there had been no
eruption there since Humboldt's visit in 1803. He went to the bottom of
the crater, and observed a slight evolution of sulphurous acid vapors,
but the "hornitos" had entirely ceased to send forth steam. During the
twenty-four years intervening between his visit and that of Humboldt,
vegetation had made great progress on the flanks of the new hills; the
rich soil of the surrounding country was once more covered with
luxuriant crops of sugar-cane and indigo, and there was an abundant
growth of natural underwood on all the uncultivated tracts.[590]

_Galongoon, Java_, 1822.--The mountain of Galongoon (or Galung Gung) was
in 1822 covered by a dense forest, and situated in a fruitful and
thickly-peopled part of Java. There was a circular hollow at its summit,
but no tradition exist