Page:The Journal of geology (IA journalofgeology21894univers).pdf/243

Copper pyrites is often, and even usually, when found in nature, mechanically mixed with iron pyrites, and the relative amounts of copper pyrites and iron pyrites in a deposit may vary considerably. Different analyses of this mixture of the two minerals may, therefore, show varying proportions of copper and iron, but the composition of the copper pyrites itself is constant.

On page 23 the author speaks of the "pale yellow rust" of lead ores, and by "rust" he means doubtless the carbonate of lead formed by the action of surface agencies on the superficial parts of certain lead deposits. This product is often stained yellow or brown by the oxidation of iron pyrites, which is frequently associated with galena, the common ore of lead; but the normal color of the "rust," or carbonate of lead, is white. An oxide of lead of a yellow color may be formed when certain lead minerals are highly heated under suitable conditions, but this process would obviously be a very unusual one in nature, and the common product of the superficial alteration of galena ores is first the sulphate and then the carbonate of lead.

On page 132 the author, in speaking of iron ores, says: "The carbonate, siderite, may be considered to be a combination of iron and calcite in which the percentage of iron varies even to the point of complete replacement of the calcium." Siderite is a definite chemical compound containing iron protoxide and carbonic dioxide in fixed proportions, while calcite is also a definite chemical compound containing calcium oxide and carbonic dioxide in fixed proportions. Both siderite and calcite are isomorphous carbonates, and the two crystallize together in various proportions. The carbonate of iron, however, can in no way be called a "combination of iron and calcite."

The Lake Superior copper and iron districts, which, taken together, form one of the most important mining regions in the world, are discussed very briefly, but even the descriptions given are inaccurate. On page 210, in speaking of the Lake Superior copper ores, the author says that "in some of the mines, mineralized ores of copper are the source of the metal, but the most common ore is native copper frequently associated with native silver." The fact is that none of the copper produced in the Lake Superior region is derived from "mineralized"^[1] ores of copper, but all of it is obtained from native copper. The native copper is sometimes slightly stained green by the forma-