These are particular varieties of mineral veins, the principal characters of which have been given in the preceding section. Metalliferous veins are no exception to the rule that subterranean activities are not well understood, and among students of the subject there are many and strong differences of opinion concerning the mode of formation of such veins. However, there is general agreement that the contents of veins, both gangue and ores, have been deposited from solution in thermal waters and vapours, just as certain existing hot springs are making similar deposits now (see p. 194). The first requisite for the formation of a lode is a water channel, because the metals are present in minute quantities, and immense quantities of water must pass before any considerable deposit can be accumulated. Hence, ore deposits are found in fissures, shattered rock-masses, in joints, in porous and soluble strata, where water may pass with comparative freedom, and further these waterways must, directly or indirectly, communicate with great depths, or with highly heated rocks, permitting supplies of hot water to reach them.

There is no general agreement as to the source of the waters that have filled the veins with gangue and ores. Perhaps the majority of geologists incline to the opinion that such waters are meteoric, i.e. of atmospheric origin, and that the waters descending through the rocks dissolve the metallic and other minerals and penetrate to great depths until they become highly heated and rise again through fissures. As the waters, thus charged with ore and gangue minerals in solution, ascend to the cooler layers nearer the surface, they are chilled and precipitate the greater part of the dissolved substances along the waterways. An alternative view, which seems to be better founded, is that the solvent hot waters are largely of magmatic origin; that is to say, that they are derived from the immense quantities of superheated steam which impregnate the igneous magmas. How vast is the amount of this water, is shown us by every great volcanic eruption, but the slowly solidifying plu-tonic bodies must give off their steam much more gradually.

With the highly heated ascending magmatic waters are doubtless mingled a greater or less proportion of meteoric waters, varying in amount according to local circumstances.

The views held concerning the origin of the ore substances themselves are similarly divergent. The hypothesis that the solvent waters are mainly of meteoric origin seems to involve the conclusion that the metallic minerals are dissolved out of the rocks through which the waters descend, while the magmatic hypothesis finds the source of the metals in the plutonic masses. For lack of space, it is impracticable to present here the evidence for and against these conflicting opinions; it must suffice to point out that the exceptional occurrence of the metalliferous veins and the nearly or quite universal association of igneous rocks with such veins, seem, in the present state of knowledge, to lend greater probability to the magmatic hypothesis. Obviously, however, no definitive conclusion is yet possible.