crater, perhaps several miles in circumference and several thousand feet
high, is blown away. Such an occurrence is recorded in the case of
the volcano Coseguina, Nicaragua, in 1835. Or, an entire mountain may
disappear, being reduced to lapilli and dust and blown into the air, as
in the case of Krakatoa, in the Straits of Sunda, in 1883.

"The essential feature of a volcano, as stated above, is a tube or
conduit, leading from the highly heated sub-crust portion of the earth
to the crater and through which molten rock is forced upward to the
surface. The most marked variations in the process depend on the
quantity of molten rock extruded, and on the freedom of escape of the
steam and gases contained in the lava.

"The cause of the rise of the molten rock in a volcano is still a matter
for discussion. Certain geologists contend that steam is the sole motive
power; while others consider that the lava is forced to the surface
owing to pressure on the reservoir from which it comes. The view perhaps
most favorably entertained at present, in reference to the general
nature of volcanic eruptions, is that the rigid outer portion of the
earth becomes fractured, owing principally to movements resulting from
the shrinking of the cooling inner mass, and that the intensely hot
material reached by the fissures, previously solid owing to pressure,
becomes liquid when pressure is relieved, and is forced to the surface.
As the molten material rises it invades the water-charged rocks near
the surface and acquires steam, or the gases resulting from the
decomposition of water, and a new force is added which produces the
most conspicuous and at times the most terrible phenomena accompanying
eruptions."

The active agency of water is strongly maintained by many geologists,
and certainly gains support from the vast clouds of steam given off by
volcanoes in eruption and the steady and quiet emission of steam from
many in a state of rest. The quantities of water in the liquid state,
to which is due the frequent enormous outflows of mud, leads to the
same conclusion. Many scientists, indeed, while admitting the agency of
water, look upon this as the aqueous material originally pent up
within the rocks. For instance Professor Shaler, dean of the Lawrence
Scientific School, says:

"Volcanic outbreaks are merely the explosion of steam under high
pressure, steam which is bound in rocks buried underneath the surface
of the earth and there subject