possibly occur.
Russell, on the other hand, regards water not as the initial factor,
but as an occasional, though important, reinforcement. He suspects
that when the molten rock has risen to a considerable distance it
encounters that fluid, perhaps in a succession of pockets, and that
steam is then suddenly generated. The explosive effects which ensue
are of two kinds. By the expansion of the moisture which some of the
lava contains the latter is reduced to a state of powder, and thus
originate the enormous clouds of fine dust which are ejected. Shocks
of greater or less violence are also produced. The less severe ones no
doubt sound like the discharge of artillery and give rise to tremors
in the immediate vicinity. In extreme cases enough force is developed
to rend the walls of the volcano itself. Russell attributes the
blowing up of Krakatoa to steam. The culminating episode of the Pelee
eruption, though not resulting so disastrously to the mountain, would
seem to be due to the same immediate cause. To this particular
explosion, too, it seems safe to assign the upheaval which excited a
tidal wave.

The precise manner in which the plastic material inside of the
terrestrial shell gets access to the surface, is not entirely clear.
Nevertheless, it is possible to get some light on the matter. It is
now well known that in many places there are deep cracks, or "faults,"
in the earth's crust. Some of them in the remote past have been wide
and deep enough to admit molten material from below. The Palisades of
the Hudson are believed to have been formed by such an intrusion, the
adjacent rock on the eastern face having since been worn away by the
weather or other agents. It has been observed that many volcanoes are
distributed along similar faults.

The existence of a chain of volcanic islands in the West Indies
suggests the probability that it follows a crack of great antiquity,
though the issue of lava and ashes for several centuries may have been
limited to a few isolated points. Just how these vents have been
reopened is one of the most difficult questions still left for
investigation. Given a line of weakness in the rocks, though, and a
susceptibility to fresh fracture is afforded. Professor McGee suggests
that the overloading of the ocean bed by silt from the Mississippi
river or other sources may have been the immediately exciting cause of
the recent outbreaks. Other geologists have found a similar
explanation accept