In other regions deep fissures, occupied in part by dikes of igneous rock, have permitted of the ascent of water charged with mineral matter from far below the surface; such waters are heated, in part by the general heat of the earth's interior, or, if in association with dikes, by the heat of the once molten intruded rock. The ascending hot water is an active solvent, and as it rises becomes cooled, and for this and other reasons precipitates many mineral substances. Veins are thus formed, which are many times banded--that is, result from the filling of fissures by the successive deposition of minerals of various kinds on their walls, each different layer of minerals indicating a change in conditions. Fissures filled in this manner from below, as denudation progresses, become exposed at the surface and reconcentration through the influence of disintegration and decay, and of solution and redeposition by descending water takes place. Ore bodies of this character carrying gold, silver, mercury, etc., are of wide occurrence, especially in the Pacific mountains, but the process of concentration is independent of the nature of the country rock. Segregated and fissure veins occur in either igneous, sedimentary, or metamorphic terranes, but are more commonly of economic importance in the metamorphic rocks than elsewhere, and will be referred to again in that connection. _Economic Importance of the Metamorphic Terranes._--The great laboratory in which rocks undergo important changes in their physical condition and in mineralogical and chemical composition, is what has been termed on a previous page the zone of metamorphism. The depth of the upper limit of this zone is variable, dependent in part on the nature of the rocks and on movements within them, as is the case of mountain building. In fact, there is probably no well-defined limit to the zone either above or below, as in the former direction metamorphism merges by gradations into alteration produced by the descent of surface water, and in the latter direction as heat increases passes again, as we imagine, by insensible and irregular gradations into a region where the rocks are so highly heated that diffusion rather than concentration results. Whether the rocks below the zone of metamorphism are fused or not depends on pressure. They are probably solid, but in a potentially plastic condition, and become fused and may be forced upward through fissures in the condition of igneous magmas when pressure is relieved. The zone of metamorphism lies between a superior zone where alteration by descending water is dominant, and a lower region where alteration due mainly to heat is in control. In the zone of metamorphism the influence of heated percolating waters, combined with movements in the rocks, are the principal factors which lead to the concentration of mineral substances.