REVIEWS. 229 



Copper pyrites is often, and even usually, when found in nature, 

 mechanically mixed with iron pyrites, and the relative amounts of cop- 

 per pyrites and iron pyrites in a deposit may vary considerably. Dif- 

 ferent 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 pro- 

 portions, 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 dis- 

 cussed very briefly, but even the descriptions given are inaccu- 

 rate. 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 "mineral- 

 ized"' ores of copper, but all of it is obtained from native copper. 

 The native copper is sometimes slightly stained green by the forma- 



' By "mineralized ores" it is supposed that the author means the ores in wliich 

 the copper is combined with other elements, forming sulphides, carbonates, oxides, etc. 



