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National Geographic : 1898 Oct
Contents
THE GEOSPHERES temperature higher than at the surface. It is a well-known property of water to dissolve certain substances, and its efficiency in dissolving many rock-substances is greatly increased when the substances are subjected to pressure and heat; and, under these conditions, it also ionizes complex substances-i. e., sepa rates them into their simple components or ions. Accordingly when moist rocks are subjected to strong pressure at high tem perature, as is frequently the case deep in the earthcrust, the rock-matter is dissolved at the points and planes of greatest pressure and precipitated or redeposited at neighboring points and planes of less pressure; so that, for example, a crystalline cube of wet and hot rock-matter may be permanently distorted by long-continued pressure on opposite faces, the crystals grad ually yielding to the stress in the direction of pressure and elongating themselves in the orthogonal directions. Through its property as a dissolving and ionizing agent, that portion of the hydrosphere which penetrates and suffuses the lithosphere has determined the texture and structure of most of our rocks; it has transformed the muds and sands and slimes of original deposition into shales, sandstones, and limestones; in some instances it has reconverted or metamorphosed these rocks into schists, quartzites, and marbles; still more significantly it has aided in remetamorphosing deep-seated rocks into lavas and other crystallines. This extreme effect of water is peculiarly instructive in that it reveals something of the character of the centrosphere, whose dense materials are brought within reach of observation only by water as a solvent and sublimant in the form of lavas, vein-stones, and other rocks of hypogean origin. There is reason for regarding the atmosphere as a differentiating and dissipating factor, and the hydrosphere as a unifying and conserving factor, both interacting with the centrosphere in such manner as to develop the lithosphere and convert it into the terrestrial home of humanity; but this relation need not be pursued for the present. Yet it is worth while to note a curious relation between lithosphere and centrosphere which is appar ently controlled by the waters both of the surface and the depths: The two inner geospheres are in unstable equilibrium ; this is shown by the occasional escape of the deep-seated ma terials from the foundation of the lithosphere (if not from the centrosphere itself) in the form of extruded lavas and subli mated vein-stones; it is shown also by the interminable heav ing of the centrosphere manifested in continental oscillation
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