he
substances below become more and more melted, and formed into lava. The
water, which is all the time filtering in through the crevices and
openings, in the rocks around the sides of the mountain, is forced down
under this molten mass by the immense pressure given to it by the height
of the mountain. There it is turned into steam. For a time it is kept
down by the vast weight of the lava which is over it, but after a time
the elastic force of it gets so great that a bubble of it bursts up, and
comes out at the top of the mountain in a great, thundering puff,
bringing up some portion of the melted lava with it, and throwing it
high into the air.

The lava thus thrown up falls down again, and when there is no wind it
falls down close around the opening. Some of it falls _into_ the
opening, where it is melted again. The rest falls on the sides, and in
process of time it begins to build up a small hill, as it were, all
around the opening, though the puffs and explosions of steam that are
continually coming out keep a mouth open at the top.

Things go on in this way for some time, until at length, for some
mysterious reason which nobody understands, the interior of the mountain
begins to moderate its heat, and finally to grow cool--not entirely
cool, but cooler than it has been. The puffs and explosions gradually
cease. The lava within the bowels of the mountain shrinks as it cools.
The sides of the mountain being firm and solid, do not collapse; but the
top, being still more or less soft, falls in, not suddenly, but by a
slow and gradual motion, corresponding with the progress of the cooling
below. So slow, indeed, is this progress, that sometimes the ground
continues sinking slowly in this way for several years before the crater
is fully formed.

All this time, although the puffs and explosions have in a great measure
ceased, the steam continues to blow out, more or less steadily, from a
great many small openings, some of them in the bottom of the crater, and
some, perhaps, in the sides. This steam is changed into visible vapor
when it comes out where the air is cool, and the several streams,
mingling together as they rise into the air, form a cloudy column, which
is often called _smoke_. Strictly speaking, however, it is not smoke.
It is almost entirely composed of steam.

After continuing in this state for some time, the interior of the
mountain begins to grow hot again. Then the steam and hot lava begin to
puff o