The Comstock Lode

Abstract

THE great interest attaching to the mines of the Comstock lode has led to their being carefully and minutely studied by competent observers at different times. Prominent among these is the original investigation of Baron von Richtofen, published at San Francisco in 1866, who, bringing to the task an unusual knowledge of the class of volcanic rocks in which the lode is inclosed, was enabled to sketch out the broad features of the subject in so masterly a manner as to leave little more for later explorers than the filling in of details. These were supplied in very full measure in the magnificent volume of Clarence King and James D. Hague, forming part of the United States Survey of the 4oth parallel, copies of which, by the enlightened liberality of the United States Government were freely supplied to the geologists and engineers of other countries. The rapid increase of the mines in depth, from 500 to 2,000 feet and upwards during the last few years, has, however, to some extent superseded, or rather rendered a supplement necessary to the earlier accounts, and this is supplied by the volume under consideration. The Comstock lode was discovered in 1859 by some gold miner in a pit sunk for a water-hole, and “milling,” or reduction of the ore, commenced in the same year, but during the first twelve months the amount of precious metals produced did not exceed 20,000l. in value. Since then it has become the largest gold and silver producing locality in the world, the yield during the nineteen years of its history having been, according to different estimates, from 60,000,000l. to 70,000,000l. in gold and silver. The ore is of two kinds, poor or low grade, averaging in yield from 4l. to 7l. per ton, and rich, worth from 8l. to 22l. per ton. These richer ores occur in large bodies or “bonanzas” recurring at irregular intervals both along the course of the lode and in depth. One of the most remarkable, that of the Consolidated Virginia and California mine, discovered in 1873, at 1,300 feet below the surface, measuring 500 feet in depth, 700 feet in length, and 90 feet in thickness, yielded in six years over a million tons of ore, averaging 19l. per ton value. The metal or bullion produced is worth from 9s. to 10s. per ounce, representing a composition of about 94 per cent, silver and 6 per cent. gold. The author discusses the various conditions under which these great masses may have been introduced into the lode, distinguishing the periods of eruption of the different volcanic rocks forming the walls from the so-called “chemical periods” when the strata of diorite, ande-site, and propylite were attacked by water containing silica and dissolved or disintegrated, the hollows formed being filled up by masses of quartz without metallic minerals of value. Subsequently a great eruption of trachyte took place, accompanied by movements of the walls of the lode, opening fissures more steeply inclined than those of the first period. These in the “second chemical period” were filled by quartz in the same manner as before, but this time accompanied by gold and silver-bearing minerals, a trace of this action being still recognisable in the hot waters of the Steamboat Springs about twelve miles distant, which, as shown by Mr. J. A. Phillips and others, deposit a siliceous sinter containing at times cinnabar and metallic gold. On an extensive study of the various phenomena presented by the distribution of the ore bodies both in length and depth, the author, like a true miner, takes a hopeful view of the future, and considers that the prospect of finding a second and lower zone of ore-production within attainable depths is very good. The spirit with which the explorations are followed is best shown by the statement that some half-dozen new vertical shafts are now sinking to cut the lode at depths of 2,500 feet and upwards, one of them being expected to attain a perpendicular depth of 4,500 feet.

The Comstock Lode; its Formation and History.

By John A. Church. 4to. (New York: John Wiley and Sons, 1879.)