1180 A TREATISE ON METAMORPHISM. 



the same district. In general, the effect is likely to be deep seated in pro- 

 portion as the lode worked is on high ground. (See p. 1181.) 



In arid regions the level of ground water is far below the surface and 

 the process of denudation is slow, so that the downward-moving waters 

 have both a wide belt in which to work above the level of ground water 

 and a long time in which to work upon a given horizon. In such regions 

 the oxides and carbonates are likely to occupy a considerable horizon. 

 This is very well illustrated by the copper mines of Arizona and New 

 Mexico and by the "colorados" of the silver-gold deposits in various arid 

 regions. 



In humid regions, on the other hand, the level of ground water is 

 likely to be near the surface. If this be combined with marked relief so 

 that denudation is rapid, the processes of oxidation and carbonation may 

 be very incomplete above the level of ground water. Indeed, in many cases 

 erosion may be so rapid that the sulphurets do not have time for more than 

 veiy partial oxidation, and they may extend nearly or quite to the surface. 

 If the rainfall is abundant, descending- waters are likely to be effective below 

 the level of g-round water; and if there are large openings and strong relief, 

 the waters are effective to. a considerable depth. Consequently even where 

 there are no oxidized products above the level of ground water, the ores are 

 likely to have been enriched by descending waters in the belt of cementa- 

 tion. It is very seldom that a deposit is found in which no effect of 

 descending waters can be discovered. 



It has already been seen that the level of ground water may vary from 

 the surface to a depth of 300 meters or more. Hence it is certain that 

 from the surface to a depth of 300 meters the ground waters may be a 

 potent factor in the production of the rich ore deposits. The deposits in 

 this belt are particularly profitable, not only because of the accessibility of 

 the material, but because of the fact that there is no expense for pumping; 

 furthermore, the products are commonly easily reducible. This may be 

 illustrated by the gold and silver deposits. In the former, the native gold 

 is free from its entanglement of sulphurets and tellurides; in the latter the 

 silver is largely in the form of the readily extracted chloride, or in some 

 instances as native silver. Aspen Mountain furnishes an excellent illustration 

 of a broad belt of weathering. Here, at a depth of 250 meters, at least one 

 great body of lead carbonate has been developed. This carbonate contains 

 a few small crystals of galena, showing its derivation from the sulphide, and 



