knowledge of petrographic science, that branch of geology which
considers the principles of rock formation. They will therefore not be
further considered in this work.


VOLCANOES.

Of old it was believed that volcanoes represented the outpouring of
fluid rock which came forth from the central realm of the earth, a
region which was supposed still to retain the liquid state through
which the whole mass of our earth has doubtless passed. Recent
studies, however, have brought about a change in the views of
geologists which is represented by the fact that we shall treat
volcanic phenomena in connection with the history of rock water.

In endeavouring to understand the phenomena of volcanoes it is very
desirable that the student should understand what goes on in a normal
eruption. The writer may, therefore, be warranted in describing some
observations which he had an opportunity to make at an eruption of
Vesuvius in 1883, when it was possible to behold far more than can
ordinarily be discerned in such outbreaks--in fact, the opportunity of
a like nature has probably not been enjoyed by any other person
interested in volcanic action. In the winter of 1882-'83 Vesuvius was
subjected to a succession of slight outbreaks. At the time of the
observations about to be noted the crater had been reduced to a cup
about three hundred feet in diameter and about a hundred feet deep.
The vertical shaft at the bottom, through which the outbursts were
taking place, was about a hundred feet across. Taking advantage of a
heavy gale from the northwest, it was practicable, notwithstanding the
explosions, to climb to the edge of the crater wall. Looking down into