Page:South African Geology - Schwarz - 1912.djvu/126

 not in the tin-wolfram series. Minerals deposited in this way lie scattered throughout the rock in which they occur, and such a deposit is called a stockwork in contradistinction to an ore-vein, because the ore has to be won by working out the whole rock in chambers, one above each other, like the rooms of a house (Stockwerk, German for a story of a house).

Ore Formation. — There are two schools of mining geologists who hold opposite views as to ore formation. The one, the Ascensionist School, regards ores as having come up in crevices from the interior of the earth in solution, the water even being supposed to be original magmatic water held in the centre of the earth, and coming to the surface for the first time. The other school, the Lateral Secretionists, relying on the fact that all rocks, if analysed sufficiently carefully, show traces of practically every metal known, maintains that the water percolating downwards from the surface dissolves out certain of these metals when it arrives in zones of great pressure, and then, if there is a crevice, the water oozes in, the pressure becomes lessened, the solvent power of the water is reduced, and the substances in solution become precipitated. Both views hold much that is true, but the facts that are important to notice are 5 that ores are deposited in fissures in the earth's crust from solution in water, and that they can only form in any abundance in those portions of the crust which have been deeply buried, and have been exposed on the surface only after the lapse of enormous periods of time during which the overlying rocks have been removed by denudation. The pneumatolitically deposited ores are, of course, excepted from this category.

A crevice exists in the earth's crust, water containing