SUPERFICIAL ALTERATION OF ORE DEPOSITS. 289 



while if they occur in igneous rocks, they may sometimes be the 

 result of concentration by differentiation from fused magmas.^ 

 More usually, however, ore deposits are a result of a concentra- 

 tion 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 enclos- 

 ing rocks or closely adjacent rocks, as in the case of many, if 

 not most, iron ore deposits; or it may be derived from more dis- 

 tant 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 alteratioji 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 min- 

 eralogical 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 num- 

 ber 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 



' This has been shown by J. H. L. Vogt (Zeitschrift fiir praktische Geologie, Jan- 

 uary, 1893) to be true of certain titaniferous iron ores and other deposits in the eruptive 

 rocks of Norway. It may also be true of certain titaniferous iron ores in the United 

 States. 



