228 THE JOURNAL OF GEOLOGY. 



On page i6 the author says that "the group of silicates is 

 extremely large, including many of the important rock-forming mine- 

 rals, but as ores they are of little importance." He has evidently 

 overlooked the fact that calamine, a hydrous silicate of zinc, is an 

 important ore, and that garnierite, a hydrous silicate of nickel and mag- 

 nesia, is the source of a large part of the nickel of commerce. The latter 

 is the ore mined at New Caledonia, off the coast of Australia, one of the 

 two largest nickel producing regions in the world. It also occurs in 

 the United States. The author again overlooks this silicate when, on 

 page 26, in enumerating the ores of nickel, he says, "nickel is obtainen 

 from the two sulphides, millerite, niccoliferous pyrrhotite, and the arse- 

 nide niccolite." In other deposits also silicates form a minor but an 

 important part of the ore, as in the case of chrysocolla in the copper 

 ores of Arizona. 



■ On page 18 the author states that, "Sometimes, though not com- 

 monly, gold occurs in iron pyrites in invisible grains." It is almost 

 unnecessary to say that one of the most common modes of occurrence 

 of gold is in intimate association with iron pyrites, so that this state- 

 ment is extremely misleading. 



On page 20 the author says : " Gold occurs in the earth in only 

 two mineralogical forms, so far as known, one in association with tel- 

 lurium, the other native, the latter being its typical occurrence and the 

 one from which the gold in use is obtained." It is true that native 

 gold is the source of most of the gold in use, but the telluride ores, far 

 from producing no commercial gold, are in many mines an important 

 source of that metal. At Cripple Creek, in Colorado, the tellurides 

 form an important part of many of the ores, and this district produced 

 between $2,500,000 and $3,000,000 in gold in 1893. In Boulder county, 

 Colorado, tellurides are also of importance, and have been so for many 

 years past, while tellurides frequently occur in still other places. 



On page 22 the author, in speaking of copper, says: "Its most 

 common occurrence, however, is as the sulphide, chalcopyrite (CuFeS,,), 

 or copper pyrites, which is in reality a sulphide of iron and coppes 

 combined, the proportion varying from an exceedingly cupriferous 

 variety (chalcopyrite) to pure iron pyrites." The sulphide of copper 

 known as copper pyrites is a definite chemical compound, with pro- 

 portions of iron and copper in a definitely fixed ratio, so that the 

 mineral cannot vary from an exceedingly cupriferous variety to pure 

 iron pyrites. The same may also be said of other sulphides of copper. 



