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THE JOURNAL OF GEOLOGY.

differently, and, therefore, there is a great difference in the classes of salts formed by the same surface waters on the ores of different metals. In the same deposit there may be formed an oxide of one metal, a carbonate of another, a chloride of another, etc. In fact, in some of the silver deposits of southern New Mexico, there can be found hydrous sesquioxide of iron formed from iron sulphide, carbonates of copper formed from copper sulphides, and chloride of silver formed probably from silver sulphides, and yet in all probability the same surface waters produced all these changes practically simultaneously.

As a result of these various changes, certain materials are sometimes leached from the upper parts of ore deposits, which have become porous by alteration, and carried down to the less previous unaltered parts. Here they are precipitated by meeting other solutions or in other ways, and hence the richest bodies of ore in a deposit often occur between the overlying altered part and the underlying unaltered part. This is not always the case, but it is true of some copper, silver, iron and other deposits.

Physical effects of alteration.—From a physical standpoint, the effect of superficial alteration is generally to make the deposit more open and porous, to cause it to shrink, and, in some cases, to convert it to a loose material of the consistency of sand and clay. In some cases, however, especially where considerable hydration goes on, an expansion may be caused. This is well seen in the formation of gypsum by the hydration of anhydrite, often causing an expansion sufficient to brecciate and fold the associated rocks,[1] and amounting to about 33 per cent. of the original material.[2] In the conversion of carbonate of iron to the hydrous sesquioxide of iron, or limonite, it has been found[3] that there is a contraction of 19.5 per cent., giving the deposit the loose porous structure characteristic of limonite and forming the familiar

  1. ↑ Elie de Beaumont, Explic. Carte géol. de France, Vol. II., p. 89.R. A. F. Penrose, Jr., Arkansas Geol. Survey, 1890, Vol. I, pp. 535-538.
  2. ↑ A. Geikie, Text Book of Geology, 3d Edition, p. 345.
  3. ↑ T. Sterry Hunt, Mineral Physiology and Physiography, 1889, p. 262.
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