Page:The Journal of geology (IA journalofgeology21894univers).pdf/241

ogy, and are the essential features of the volume. They, therefore, deserve consideration in some detail.

The most important feature of a book of this kind is the discussion of ore deposits, yet at the outset a faulty definition of the term "ore" is given. The author says, on page 15, "an ore may be defined as a mineral with a metallic base." Unless further qualified, this definition is, to say the least, vague, for though all ores have metallic bases, there are a number of important minerals with metallic bases which are not ores. Thus, oxide of iron, sulphide of lead, sulphide of copper and other materials have metallic bases, and under proper conditions are ores; but gypsum, calcite, baryta, mica, and many other minerals have metallic bases and are not ores. Moreover, though many ores are minerals, many others are not minerals at all, but are common rocks having some special metallic constituent as their only unusual feature. Thus the ore of the Calumet and Hecla copper mines is a cupriferous conglomerate, the Mansfeld copper deposits of Germany are cupriferous shales, and many other similar instances might be mentioned. The author adds that, "properly speaking, the metallic constituent should be a predominant constituent." Though in some ores the metallic constituent is a predominant one, yet in some of the most important ores the metal forms only a small, and often an insignificant, constituent. In most gold ores the metallic constituent forms but a fraction of one per cent. of the ore, and in most silver ores the silver forms but a slightly larger amount. In copper deposits, the copper rarely forms a large percentage of the ore, and in many other cases the metallic constituent is entirely subordinate.

The author states that "the miner considers an ore to be a mineral with a metallic base, occurring in sufficient abundance to be economically valuable; but from the scientific standpoint, a grain of magnetite in a granite rock is as much an ore as a bed of this mineral." The term ore is essentially a technical mining term, and has no scientific significance whatever. When a metal can be profitably extracted from a certain material, that material becomes an ore; but other materials may contain just as much of the same metal, and yet, on account of their mineralogical or other features, they may not be commercially profitable sources of the metal, and then they are not ores. Whether a material is an ore or not, is dependent on commercial conditions, which may vary from time to time; and this very fact prevents the term from having a scientific meaning.