Geology and Mineralogy considered with reference to Natural Theology/Chapter 5
Volcanic Rocks, Basalt, and Trap.
In the state of tranquil equilibrium which our planet has attained in the region we inhabit, we are apt to regard the foundation of the solid earth, as an emblem of duration and stability. Very different are the feelings of those whose lot is cast near the foci of volcanic eruptions; to them the earth affords no stable resting place, but during the paroxysms of volcanic activity, reels to and fro, and vibrates beneath their feet; overthrowing cities, yawning with dreadful chasms, converting seas into dry lands, and dry lands into seas. (See Lyell's Geology, vol. i. passim.)
To the inhabitants of such districts we speak a language which they fully comprehend, when we describe the crust of the globe as floating on an internal nucleus of molten elements; they have seen these molten elements burst forth in liquid streams of lava; they have felt the earth beneath them quivering and rolling, as if upon the billows of a subterranean sea; they have seen mountains raised and valleys depressed, almost in an instant of time; they can duly appreciate, from sensible experience, the force of the terms in which geologists describe the tremulous throes, and convulsive agitations of the earth; during the passage of its strata from the bottom of the seas, in which they received their origin, to the plains and mountains in which they find their present place of rest.
We see that the streams of earthy matter, which issue in a state of fusion from active volcanos, are spread around their craters in sheets of many kinds of lava; some of these so much resemble beds of basalt, and various trap rocks, that occur in districts remote from any existing volcanic vent, as to render it probable that the latter also have been poured forth from the interior of the earth. We further find the rocks adjacent to volcanic craters, intersected by rents and fissures, which have been filled with injections of more recent lava, forming transverse walls or dikes. Similar dikes occur not only in districts occupied by basalt and trap rocks, at a distance from the site of any modern volcanic activity; but also in strata of every formation, from the most ancient primary, to the most recent tertiary (see Plate 1. section f 1—f 8. h 1—h 2. i 1—i 5): and as the mineral characters of these dikes present insensible gradations, from a state of compact lava, through infinite varieties of greenstone, serpentine, and porphyry to granite, we refer them all to a common igneous origin.
The sources from which the matter of these ejected rocks ascends are deeply seated beneath the granite; but it is not yet decided whether the immediate cause of an eruption be the access of water to local accumulations of the metalloid bases of the earths and alkalies; or whether lava be derived directly from that general mass of incandescent elements, which may probably exist at a depth of about one hundred miles beneath the surface of our planet.
Our section shows how closely the results of volcanic forces now in action are connected, both with the phenomena of basaltic formations, and also with the more ancient eruptions of greenstone, porphyry, syenite, and granite. The intrusion bath of dikes and irregular beds of unstratified crystalline matter, into rocks of every age and every formation, all proceeding upwards from an unknown depth, and often accumulated into vast masses overlying the surface of stratified rocks, are phenomena coextensive with the globe.
Throughout all these operations, however turbulent and apparently irregular, we see ultimate proofs of method and design, evinced by the uniformity of the laws of matter and motion, which have ever regulated the chemical and mechanical forces by which such grand effects have been produced. If we view their aggregate results, in causing the elevation of land from beneath the sea, we shall find that volcanic forces assume a place of the highest importance, among the second causes which have influenced the past, as well as the present condition of the globe; each individual movement has contributed its share towards the final object, of conducting the molten materials of an uninhabitable planet, through long successions of change and of convulsive movements, to a tranquil state of equilibrium; in which it has become the convenient and delightful habitation of man, and of the multitudes of terrestrial creatures that are his fellow tenants of its actual surface.
- See Cordier on the internal temperature of the earth.
- See further details respecting the effect| of volcanic forces in the description of Pl. I. Vol. ii.