296 THE JOURNAL OF GEOLOGY. 



Another physical effect of surface influences on ore deposits 

 is seen in certain forms of brecciation due to physical or chemi- 

 cal causes, such as expansion by hydration, etc. Such breccia- 

 tion, however, has usually occurred in the country rock before the 

 concentration of the ore deposit ; in fact its existence, by offer- 

 ing favorable conditions for deposition, has often been the cause 

 of the formation of the ore deposit in a given place. Though 

 brecciation, therefore, is very important as a factor in the con- 

 centration of ore deposits, it does not belong, to any large extent, 

 in a discussion of the surface alteration of ore deposits after their 

 formation, and, therefore, it will not be treated further in this paper. 



Depth of alteratio7i. — Having thus discussed briefly the means 



by which superficial alteration in ore deposits is produced, and 



the results of this alteration, the next feature to be taken up is 



the depth to which it extends. As already shown, the alteration 



is primarily one of oxidation and generally of hydration ; and, 



though either may occur without the other, they both very often 



occur together. When surface waters percolate into the rock, 



their influence is more active near the surface, because they carry 



large quantities of oxygen, and because the oxygen of the air 



itself also has some influence. As they sink deeper, the effect 



of the oxygen of the air becomes less active, and the oxygen 



dissolved in the water is consumed in oxidizing various materials 



which it meets on the way, until finally most of the oxygen is 



lost and active oxidation ceases. Theoretically, this oxidizing 



action may extend down as far as, and sometimes below, the level 



of the drainage of the surrounding country, which is called also 



the zone of permanent saturation. Above that level, there is a 



constant circulation of water from the surface downwards, thus 



affording means of active oxidation ; but when the water reaches 



that level, not only has most of the oxygen contained in solution 



generally been used up, but also the circulation of the water is 



much more sluggish, so that oxidation is less active.^ 



^ It is possible that the oxidation near the surface is due largely to free oxygen in 

 the waters, while, when this becomes exhausted at a depth, the oxidation may be due to 

 the abstraction by mineral matter of the oxygen in combination with materials in 

 solution. 



