294 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 pro- 

 duced 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 

 pervious unaltered parts. Here they are precipitated by meet- 

 ing 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 caus- 

 ing an expansion sufficient to brecciate and fold the associated 

 rocks, ^ and amounting to about 33 per cent, of the original 

 material.^ 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 



^Elie de Beaumont, Explic. Carte g^ol. de France, Vol. II., p. 89. R. A. F. 

 Penrose, Jr., Arkansas Geol. Survey, 1890, Vol. I, pp. 535-538. 



= A. Geikie, Text Book of Geology, 3d Edition, p. 345. 



3T. Sterry Hunt, Mineral Physiology and Physiography, 1889, p. 262. 



