accumulate a deep layer, are apt to be retained on the surface of the
country. A great deal of this powdered rock drifts away in the wind,
sometimes in great quantities, as in those cases where it darkened the
sky more than a thousand miles from the cone. Moreover, the water of
the steam which brought about the discharges and the other gases which
accompanied the vapour have left no traces of their presence, except
in the deep channels which the rain of the condensing steam have
formed on the hillsides. Nevertheless, after all these subtractions
are made, the quantity of volcanic matter remaining on the surface
about the Bay of Naples would, if evenly distributed, form a layer
several hundred feet in thickness--perhaps, indeed, a thousand feet in
depth--over the territory in which the vents occur. All this matter
has been taken in relatively recent times from the depths of the
earth. The surprising fact is that no considerable and, indeed, no
permanent subsidence of the surface has attended this excavation. We
can not believe that this withdrawal of material from the under-earth
has resulted in the formation of open underground spaces. We know full
well that any such, if it were of considerable size, would quickly be
crushed in by the weight of the overlying rocks. We have, indeed, to
suppose that these steam-impelled lavas, which are driven toward the
vent whence they are to go forth in the state of dust or fluid, come
underground from distances away, probably from beneath the floors of
the sea to the westward.

Although the shores of the Bay of Naples have remained in general with
unchanged elevation for about two thousand years, they have here and
there been subjected to slight oscillations which are most likely
connected with the movement of volcanic matter toward the vents where
it is to find escape. The most interesting evidence of this nature is
afforded by the studies which have been made on the ruins of the