Ores sometimes occur, like coal, in layers or beds, running parallel with the strata of the inclosing rocks, or in prodigious irregular masses. Most commonly, however, they are found in veins traversing the rocks in every conceivable direction, and filling the crevices and chasms which former terrestrial revolutions have rent in the hard stone. From the wild and titanic powers that have here been at work, it may easily be imagined how irregular the direction of these veins must be, and under how many various forms they must appear. Here they are horizontal, there vertical; here they form thin layers, there they fill chasms several hundred feet thick. Sometimes they split into several minor branches, or make abrupt bends, and frequently they have been rent, torn, or displaced in every possible manner by subsequent revolutions.
Their length is as various as their thickness or their direction. Some are short, while others traverse the rocks to a distance of many miles. Thus the argentiferous veins in the neighbourhood of Clausthal, in the Harz, are three leagues long, and the famous Veta Madre, or chief lode of Guanaxuato in Mexico, has been traced for a length of eight miles.
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.
But if in these and similar instances the treasures of the subterranean world have revealed themselves spontaneously to man, in many other cases laborious and costly investigations have been found necessary for their discovery.
As civilisation advanced, and the value of the metals, of coal and salt, came to be more and more appreciated, nothing could be more natural than the desire of no longer relying upon the discoveries of chance or upon the mines bequeathed by former ages, but of sounding the mysterious recesses of the earth, and forcing her, by dint of patient exploration, to reveal the riches she conceals under her surface.
Thus, as far back as the eleventh century, the divining rod came into practice, and found full credence in a superstitious age. A forked branch of the hazel-tree, cut during a peculiar phase of the moon, was the means employed in Germany for the discovery of buried treasures, of veins of metal, of deposits of salt, or of subterranean sources. But the miraculous rod did not indiscriminately show its powers in every hand; it was necessary to have been born in certain months, and soft and warm—or, according to the modern expression, magnetic—fingers were indispensable for handling it with effect. The diviner possessing these necessary qualifications took hold of the rod by its branches so that the stem into which they united was directed upwards. On approaching the spot where the sought-for treasure lay concealed, the magical rod slowly turned towards it, until finally the stem had fully changed its position and pointed vertically downwards. To increase the solemnity of the scene, the wily conjurors generally traced magical circles that were not to be passed, burnt strong smelling herbs and spices, and uttered powerful charms to disarm the enmity of the evil spirits that were supposed to guard the hidden treasures.
At present, the divining-rod has lost its old reputation, and more rational means are employed for the discovery of mineral wealth. Relying on experience, tact, and geological knowledge, the investigator carefully examines the country where ores or coal are supposed to be concealed, and having fixed upon an appropriate spot, has recourse either to experimental digging or to boring for testing the truth of his opinion. These expensive explorations, though often unsuccessful, frequently prove highly remunerative, and many a saline spring, or coal seam, or metallic vein would, without their assistance, have remained unknown and unproductive.