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.[1]
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
- ↑ See Cordier on the internal temperature of the earth.
