To do so is but to accord with Bacon's maxim, "_Prudens questio dimidium scientiæ_." 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 those superficial actions--such as those of tides, waves, rain, rivers, solar heat, frost, vitality, vegetable and animal (passing by many others less obvious)--which perpetually modify, alter or renew the surface of our world, and maintain the existing regimen of the great machine, and of its inhabitants. These last are the domain of Geology, properly so called. No geological system can be well founded, or can completely explain the working of the world's system as we now see it, that does not start from Vulcanicity as thus defined; and this is equally true, whether, as do most geologists, we include within the term Geology everything we can know about our world as a whole, exclusive of what Astronomy teaches as to it, dividing Geology in general into Physical Geology--the boundaries of which are very indistinct--and Stratigraphical Geology, whose limits are equally so. It has been often said that Geology in this widest sense begins where Astronomy or Cosmogony ends its information as to our globe, but this is scarcely true.