Page:Popular Science Monthly Volume 22.djvu/521

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THE DECREASE OF GOLD.

zas"; but, besides these bonanzas, crevices contain no gold, and only the hope of encountering them leads to their being worked.

The places in which gold is found may be divided into the following three classes—auriferous ore, auriferous veins, and auriferous alluvium:

The first group contains ore, rich in magnesium, interspersed with gold. It is frequently found in the Ural Mountains. As a transition to the second class may be regarded the auriferous minerals, in rock of volcanic origin, as it occurs on the west coast of South America and in many parts of Brazil. The granite of the Erz Gebirge, in Bohemia, which contains tin, is a similar formation. Interesting as this class is, considered from a geological stand-point, it is of little practical value to the gold miner.

The next class consists of the auriferous veins, which are fissures filled by hot springs, geysers, or volcanic eruptions. The gold is found here together with silver, as in the case of the Comstock lode, in Nevada, the gold deposits in Queensland and New Zealand, as well as the mines near Kremnitz, in the Carpathian range, in Hungary. The gold is sometimes found in them pure, sometimes mixed with silver, copper, or sulphur. In the older volcanic veins the gold is not mixed with silver, and bonanzas are never found in them. In many of these veins, also, granite veins are encountered, and, although remote from volcanic regions, it is presumed that the gold was carried up by the granite. The celebrated "Mother lode" of California is a sample of this kind.

The third class is the gold-bearing alluvium. This alluvium (earth washed down by rivers upon lower lands) has been produced by the decomposition of auriferous rock, and the gold is found in grains and lumps to the size of a hen's egg. It is a peculiar fact that the gold found in these deposits is purer than that contained in the veins from which it originated, nor is the formation of lumps and grains satisfactorily explained. The deposits of California, Australia, and Siberia pertain to this group, which may again be subdivided into—a. Deposits on the earth's surface, from which the gold is obtained by simple washing; b. Deposits which have been covered by subsequent inundations, and from which the gold has to be mined only by difficult work and great expense. These old deposits in California are frequently covered with basalt or lava, and are called "deep leads." They are worked by the hydraulic system: the water is conducted through pipes, and directed with full force against the soil, which is hereby converted into a fluid mud, and passes in this condition through a long line of sluices, set in zigzag, in which the particles of gold are deposited.

The most remarkable deposits of this kind are to be found at Ballarat, Australia, where they are covered with four hundred feet of ground and four layers of lava, which have come from a neighboring volcano. These deposits are the banks and bars of former streams,