Hypogene Action

Hypogene Action, the collective term for all those geological agencies, several of which are most imperfectly known, which act from below the earth's surface. They may, perhaps, be grouped under five heads: - (1) volcanic, (2) seismic, (3) those producing slow secular or widespread movements, (4) those producing folding and faulting of strata, and (5) metamorphic. How little our certain knowledge of these agencies is, may be gathered from our being only able to name several of them by their effects; but one great cause probably underlying most of them is the heated condition of the interior of the earth. [Earth.] The action of volcanoes, of earthquakes (seismic action), and of metamorphism is dealt with under separate headings. Secular movements of upheaval or depression may occur in volcanic regions, and be merely a part of the volcanic phenomena, as would seem to be the case with the so-called Serapeum (q.v.) near Naples. If not, they may really be of local origin, as when beds of rock-salt or gypsum are dissolved, and produce subsidence, or when hydration of a mineral substance causes expansion. It has been suggested that elevation may be produced by denudation (q.v.) reducing the pressure on a heated interior; and depression by the weight of deposits on the ocean-floor acting upon a yielding internal region, such as a molten zone between the solid crust of the earth and a solid nucleus. Another view is that all such movements are but the expression of the shrinkage of the earth due to its cooling, the crust accommodating itself to the shrinking interior. Such shrinkage would certainly produce extreme tangential, lateral, or horizontal pressure, and this pressure would give rise to such molar changes as folding into anticlinals (q.v.) and synclinals, contortion, inversion, shearing (q.v.), and faults (q.v.), and to such molecular or textural changes as cleavage (q.v.) and foliation (q.v.). Speaking generally, while the action of epigene, or sub-aerial and marine, agencies is denudation (q.v.), lowering the general surface, hypogene agencies tend rather either to raise the surface, to pour out matter from below or to harden rocks (by crystallisation) against denuding forces.