majority of volcanic cones are situated in the vicinity
of the oceanic waters, or of inland seas, was pointed to in confirmation
of this theory. But the existence in Western America and other volcanic
countries of fissures of eruption along which molten lava has been
extruded without explosions of steam, shows that water is not an
essential factor in the production of volcanic phenomena; and, as
Professor Prestwich has clearly demonstrated, it is to be regarded as an
element in volcanic explosions, rather than as a prime cause of volcanic
action. The main difficulty he shows to be thermo-dynamical; and
calculating the rate of increase in the elastic force of steam on
descending to greater and greater depths and reaching strata of higher
and higher temperatures, as compared with the force of capillarity, he
comes to the conclusion that water cannot penetrate to depths of more
than seven or eight miles, and therefore cannot reach the seat of the
eruptive forces. Professor Prestwich also points out that if the
extrusion of lava were due to the elastic force of vapour of water there
should be a distinct relation between the discharge of the lava and of
the vapour; whereas the result of an examination of a number of
well-recorded eruptions shows that the two operations are not related,
and are, in fact, perfectly independent. Sometimes there has been a
large discharge of lava, and little or no escape of steam; at other
times there have been paroxysmal explosive eruptions with little
discharge of lava. Even in the case of Vesuvius, which is close to the
sea, there have been instances when the lava has welled out almost with
the tranquillity of a water-spring.

(_f._) _Access of Surface Water to Molten Lava during Eruptions._--The
existence of water during certain stages in eruptions is too frequent a
phenomena to be lost sight of; but its presence may be accounted for in
other ways, besides proximity to the sea or ocean. Certain volcanic
mountains, such as Etna and Vesuvius, are built upon water-bearing
strata, receiving their supplies from the rainfall of the surrounding
country, or perhaps partly from the sea. In addition to this the ashes
and scoriae of the mountain sides are highly porous, and rain or snow can
penetrate and settle downwards around the pipe or throat through which
molten lava wells up from beneath. In such cases it is easy to
understand how, at the commencement of a period of activity, molten lava
ascendi