WEATHERING. :Most ore deposits have been changed superficially by weathering, the depth of this alteration varying from a few feet to several hundred; in the Rocky Mountains. 300 to 400 feet is nqt uncommon, and 1000 feet is extreme; in Chile a depth of 1500 has been recorded. Where the ore body contains iron-bearing minerals the oxidation of these may stain it heavily with limonite. and to this the name of yossait is often applied. At some localities the leaching out of the other minerals has naturally resulted in the concentration of the iron contents, so that the gossan can be worked as an ore of the latter min eral, while below the zone of weathering the de tions had brought the metals from below and followed the fissure upward. This is known as the theory of ascension. The views held by most geologists at the present day are an amplifica posit supplies a different typo of ore, namely.
that which originally predominated in it. The changes caused by weathering are both physical and chemical, or the same as would occur in all rocks, the difference being that the metallic minerals are more easily affected. The physical Changes ill1OlVe a disintegration of the mass. The chemical effects include processes of oxida tion. Bgcugation. and solution. Sulphides arc changed to sulphates and chloridot, oxides to carbonates and silicates. une result of this change is to convert many insoluble compounds into soluble taws; and these latter are taken up by percolating waters. carried to a lower depth and there deposited. in some cases producing a zone of howl enrichment at the water-line, that is, at the boundary of the gossan, or oxidized part, and the unaltered portion of the are holy.