sses the mineral species found about it may be
divided: it will help us but little to know Pliny's notions of how
Pompeii was overwhelmed, or to re-engrave pictures, assumed to give the
exact shape of the Vesuvian or other cone at different periods, or its
precise altitude, which are ever varying, above the sea. Even much more
time and labour may be spent upon analysing the vapours and gases of
fumaroles and salfatares than the results can now justify.

Nothing, perhaps, tends more to the effective progress of any branch of
observational and inductive science, than that we should endeavour to
discern clearly the scope and boundary of our subject.

To do so is but to accord with Bacon's maxim, "_Prudens questio dimidium
scientiae_." That once shaped, the roads or methods of approach become
clearer; and every foothold attained upon these direct paths enables us
to look back upon such collateral or subordinate questions as at first
perplexed us, and find them so illuminated that they are already
probably solved, and, by solution, again prove to us that we _are_ in
the right paths.

I believe, therefore, that I shall not do disservice to the grand
portion of cosmical physics to which volcanic phenomena belong, by
devoting the few pages accorded to me for this Introduction to sketching
what seems to me to be the present position of terrestrial
_Vulcanicity_, and tracing the outlines and relations of the two
branches of scientific investigation--_Vulcanology_ and _Seismology_--by
which its true nature and part in the Cosmos are chiefly to be
ascertained.

The general term, _Vulcanicity_, properly comprehends all that we see or
know of actions taking place upon and modifying the surface of our
globe, which are referable not to forces of origin above the surface,
and acting superficially, but to causes that have been or are in
operation beneath it. It embraces all that Humboldt has somewhat vaguely
called "the reactions of the interior of a planet upon its exterior."

These reactions show themselves principally and mainly in the marking
out and configuration of the great continents and ocean beds, in the
forcing up of mountain chains, and in the varied phenomena consequent
thereon, as seen in more or less adjacent formations.

These constitute the mechanism which has moulded and fashioned the
surface of our globe from the period when it first became superficially
solid, and prepared it as the theatre for the action of all