290 THE JOURNAL OF GEOLOGY. 



which, under surface influences, give rise to intricate chemical 

 changes. In discussing the subject of the superficial alteration 

 of ore deposits, therefore, we treat a similar, but much less 

 understood, subject than the superficial alteration of rocks. 



Techiical najnes of alteratio?i prodticts.—The altered surface out- 

 crop of ore deposits is known by various names in different 

 regions. Among the Cornish miners of England it is known as 

 gossan, a name which has also been adopted into American 

 mining nomenclature, though other special names are given in 

 special classes of deposits. In France it is known as chapeau de 

 fer; in Germany as eisener hut; among the Spanish Americans 

 as pacos or colorados. As almost all deposits contain more or 

 less iron minerals, the outcrops are usually stained brown from 

 their oxidation, and hence the reference to iron in the French 

 and German names. Sometimes, however, the outcrops are 

 stained black by the oxidation of manganese carbonate or sili- 

 cate, or green by copper minerals, or other colors by the forma- 

 tion of other compounds. 



Agejits of alter atio7i. — The superficial alteration of ore deposits, 

 as of any rock, results from a combination of mechanical and 

 chemical disintegration, brought about by the combined action 

 of the atmosphere, surface waters, changes in temperature, and 

 the various organic and inorganic materials contained in the air 

 and water. In nature, we never deal with perfectly pure water, 

 but different waters contain different ingredients derived from 

 the air and from the different materials with which they come in 

 contact. Among the most important of these ingredients are 

 oxygen, numerous organic acids like carbonic, oxalic, malic, 

 citric, formic, propionic, butyric, acetic acids, etc., certain inor- 

 ganic acids, such as sulphuric, nitric, hydrochloric, hydrobromic, 

 etc., etc. Some of the acids mentioned occasionally occur in the 

 free state, but most of them are generally combined with some 

 of the bases present, such as the alkalies, lime, magnesia, iron, 

 alumina, etc. These various ingredients, of course, are not all 

 contained in the same waters, but are found in various associa- 

 tions in different waters. The organic acids mentioned represent 



