With regard to depth, the lower extremity of hardly a single mineral bed or vein of any note has as yet been pointed out, though many have been worked to a considerable depth. Thus, one of the pits of St. Andreasberg, in the Harz, is 2,485 feet deep, though, on account of its high situation in the mountains, not much more than 280 feet below the surface of the sea, while, in the coal mines of Valenciennes and Liège, the deepest shafts are sunk from 1,300 to 1,600 feet below the level of the ocean. The shaft of the colliery of Sacrée Madame, near Charleroi, is 800 yards deep, and that of Dukinfield Colliery 2,050 feet.
The great difficulty of carrying on mining operations at great depths will, probably, for ever prevent most metalliferous veins from being followed to their origin in the bowels of the earth; for while on high mountains the rarefaction of the atmosphere prevents respiration, the increasing pressure or impurity of the air at a depth of four or five thousand feet below the level of the sea must necessarily hinder the free expansion of the lungs. The weakness of our organisation soon sets limits to our progress, whether we wish to rise into the air or to penetrate into the interior of the earth; and it is only on spiritual wings that we soar aloft to the stars, or wander through the mysterious depths of our planet.
How have the ores been collected or precipitated in the veins or strata where they are often found in such enormous quantities? Partly they ascended as vapour from unknown depths, and then were condensed in the crevices, mixing with the gangue or the stones which filled the volcanic chasms; and partly their solutions, of which the mineral springs of the present day afford us so many examples, permeated the porous rocks, and saturated them on cooling, or were forced to relinquish their valuable contents by some more powerful chemical affinity. Thus, numberless years before man appeared upon the stage of his future empire, the means were provided without which it would never have been possible for him to establish his dominion over the earth, and to make himself master of the treasures it bears on its surface.
When we consider the frequent upheavings and subsidences of the earth- rind and the denuding power of water, which, in the long series of ages, cuts deep ravines into the mountains and washes away whole strata, we cannot wonder that many metalliferous veins emerge or crop out on the surface of the earth, so that a fortunate chance sufficed for their discovery. Thus, a poor Indian looking for wood first found the rich deposits of silver that had so long been buried in obscurity under the sterile soil of Copiapo in Chili (1632). Partridges, in whose stomachs grains of gold were found, are said to have led to the discovery of the rich mines of Kremnitz and Schemnitz in Hungary; and Ramm, a huntsman of the emperor Otho the Great, having bound his horse to a tree in a forest near Goslar in the Harz Mountains, the fretful steed, stamping with impatience and tearing up the soil, pointed out the celebrated lode of the Rammelsberg, which is still worked to the present day.