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.
Vulcanicity—or Geology, if we choose to make it comprehend that—must commence its survey of our world as a nebula upon which, for unknown ages, thermic, gravitant and chemical forces were operative, and to the final play of which, the form, density and volume, as well as order of deposition of the different elements in the order of their chemical combination and deposition was due, when first our globe became a liquid or partly liquid spheroid, and which have equally determined the chemical nature of the materials of the outward rind of the earth that now is, and with these some of the primary conditions that have fixed the characters, nature and interdependence of the vegetables and animals that inhabit it. Physical Astronomy and Physical Geology, through Vulcanicity, thus overlap each other; the first does not end where the second begins; and in every sure attempt to bring Geology to that pinnacle which is the proper ideal of its completed design—namely, the interpretation of our world's machine, as part of the universal Cosmos (so far as that can ever become known to our limited observation and intelligence)—we must carry with us astronomic considerations, we must keep in view events anterior to the "status consistentior" of Leibnitz, nor lose sight of the fact that the chain of causation is one endless and unbroken; that forces first set moving, we know not when or how, the dim remoteness of which imagination tries to sound in shadowy thought, like those of the grand old Eastern poem, "When the morning stars first sang together," are, however changed in form, operative still. The light and fragile butterfly, whose glorious garb irradiates the summer zephyr in which it floats, has had its power of flight—which is its power to live—determined by results of that same chain of causes that lifted from the depths the mountain on whose sunny side he floats, that has determined the seasons and the colour of the flower whose nectar he sucks, and that discharges or dissipates the storm above, that may crush the insect and the blossom in which it basked. And thus, as has been said, it was not all a myth, that in older days affirmed that in some mysterious way the actions and the lives of men were linked to the stars in their courses.
Whatever may have been the manifestations of Vulcanicity at former and far remoter epochs of our planet, and to which I shall return, in the existing state of regimen of and upon our globe it shows itself chiefly in the phenomena of Volcanoes and of Earthquakes, which are the subjects of Vulcanology and of Seismology respectively, and in principal part, also, of this Introduction.
The phenomena of hot springs, geysers, etc., which might be included under the title of Thermopægology, have certain relations to both, but more immediately to Vulcanology.
Let us now glance at the history and progress of knowledge in these two chief domains of Vulcanicity, preparatory to a sketch of its existing stage as to both, and, by the way, attempt to extract a lesson as to the methods by which such success as has attended our labours has been achieved.
It will be most convenient to treat of Seismology first in order.
Aristotle—who devotes a larger space of his Fourth Book, Περἱ Κοσμου, to Earthquakes—Seneca, Pliny, Strabo, in the so-called classic days, and thence no end of writers down to about the end of the seventeenth century—amongst whom Fromondi (1527) and Travagini (1679) are, perhaps, the most important now—have filled volumes with records of facts, or what they took to be such, of Earthquakes, as handed down to or observed by themselves, and with plenty of hypotheses as to their nature and origin, but sterile of much real knowledge.