Page:The Journal of geology (IA journalofgeology21894univers).pdf/305

while if they occur in igneous rocks, they may sometimes be the result of concentration by differentiation from fused magmas.^[1] More usually, however, ore deposits are a result of a concentration after the formation of the enclosing rock, whether the latter be of sedimentary or of igneous origin. The mineral matter represented in this concentration may be derived from the enclosing rocks or closely adjacent rocks, as in the case of many, if not most, iron ore deposits; or it may be derived from more distant sources, often from greater or less depths, as in some of the precious metal deposits. Occasionally, both these sources may be drawn on for mineral matter in one deposit. In this subject of the original source of an ore, we enter a field concerning which there has been much dispute of late years between the advocates of the lateral secretion theory and those who favor the idea of a deep-seated source for many ore deposits. It is not, however, the purpose of the present paper to enter into this discussion, and the following remarks are confined to what happens in the superficial parts of ore deposits, and to a less extent of allied formations, after the materials forming them have been brought into their present, or approximately their present, positions.

Relation of alteration in ore deposits and in country rocks.—Ore deposits are generally more or less changed in their upper parts by atmospheric influences, so that very rarely do the same mineralogical and physical features that are found in these parts, continue to very great depths. In considering this superficial alteration, we discuss a subject analogous to the secular decay of rocks. The latter, however, involves usually but a limited number of common rock-forming minerals, while the secular decay of ore deposits involves a great variety of minerals, not only the oxides, carbonates and silicates common in most rocks, but also sulphides, arsenides, tellurides, selenides, antimonides, chlorides, bromides, iodides, fluorides, sulphates, phosphates, tungstates, molybdates, and numerous other classes of minerals, many of