Page:Popular Science Monthly Volume 33.djvu/657

Rh depth, the veins are prolonged indefinitely, and mining excavations, however deeply they may be carried, fail to reach their lower limit.

At first sight, metalliferous veins contrast in their mineral composition with the incasing rocks, to whatever category they may belong, even when they are welded to them. They are formed of very different minerals; and it is necessary to distinguish in them the useful substances or minerals, and the stony matter or gangue. The last very often occur in a decidedly predominating proportion, and much that is unforeseen in the returns of a mining operation results from their variations in quantity and richness. The various substances constituting veins sometimes assume a symmetrical disposition in respect to the two walls, showing that they result from deposits made successively one upon another, as happens in a crystallizing pan or in fountain pipes that are incrusted with stony substance.

Metalliferous veins are rarely isolated, but usually form systems or groups, connected by a bond of parallelism and by similarity of composition. They occur exclusively in regions that have suffered dislocations, of which they appear as if they were a consequence. The constitution of the soil of France well brings out this correlation. While veins are wanting in the districts in which the beds have nearly preserved their original horizontality, they are found by thousands, although of inferior richness, in the central plateau, the Vosges, the Pyrenees, and the Breton peninsula. They often border upon the eruptive rocks, with which they are visibly connected as if by a bond of relationship. Many countries famous for their metallic riches, like Cornwall, Hungary, and the State of Nevada, furnish striking examples of this last alliance.

Metalliferous veins betray their origin by their forms and the independent manner in which they cut rocks of every kind. Their formation is due to large vertical breaks, called faults, which have given an outlet to the substances and have ultimately been filled with them. The concomitance of the veins and of grand dislocations sufficiently testifies that metallic matters and their gangues have been brought from down up, that is, from the deeper regions of the globe toward the surface. From this fact, it was at first inferred that the ascension of the vein-filling minerals was accomplished by sublimation or at least by fusion, but this has been shown by many circumstances to be inexact. The specimens of the collections teach of themselves alone that their various minerals have been precipitated one after another, distributing themselves in an order quite different from their degree of fusibility and volatility. It should also be observed that the greater part of them are found outside of the veins, and in circumstances in