now turn to the second branch of our subject--viz.,
Vulcanology--upon which, as yet, we have secured less firm standing
ground than we have seen we possess in Seismology, for which reason we
took that first into consideration.

It is the part of Vulcanology to co-ordinate and explain all the
phenomena of past or present times visible on our globe which are
evidences of the existence and action, whether local or general, of
temperatures within our globe greatly in excess of those of the surface,
and which reach the fusing points of various mineral compounds as found
arriving, heated or fused, at the surface.

The stratigraphic geologist sees that such heated or fused masses have
come up from beneath, throughout every epoch that he can trace; but he
cannot fail to discern more or less a change in the order or character
of those outcomings, as he traces them from the lowest and oldest
formations to those of the present day. He sees immense outpourings of
granitoid or porphyrytic rocks that have welled up and overflowed the
oldest strata--huge dykes filling miles of fissures that had been
previously opened for the reception of the molten matter that has filled
them, and often passing through those masses of previously outpoured
rock; later he sees huge tables of basaltic rock poured forth over all.
One grand characteristic common to all these--commonly called plutonic
products--being that, whether they were poured forth over the surface or
injected into cavities in other rocks, the movements of the fused
material were, on the whole, hydrostatic and _not explosive_.

At the present day, whatever other evidences we have of high temperature
below our globe's surface, that which primarily fixes the eye of the
geologist is the Volcano, whose characteristic, as we see it in
activity, _is explosive_. But though there is this great characteristic
difference between the plutonic and the volcanic actions and their
products, the two, when looked at largely, are seen so to inosculate,
that it is impossible not to refer them to an agency common to both,
however changed the modes of its action have been between the earliest
epochs of which traces are presented to us and the present day.

To us little men, who, as Herschell has well said, in referring to the
methods of measuring the size of our globe, "can never see it all at
once, but must creep like mites about its surface," the Volcano, in the
stupendous grandeur of its effects, te