298 THE JOURNAL OF GEOLOGY. 



It will thus be seen that in going from the surface downwards, 

 we pass from a zone of active oxidation into a zone in which oxi- 

 dation practically ceases. Below the level of permanent satura- 

 tion, the waters may sometimes gradually sink to very great 

 depths, even deep enough to become intensely heated and possi- 

 bly dissociated. Such water may have a very important effect in 

 the formation of ore bodies, though in a manner quite different 

 from their action on the surface. The present discussion, how- 

 ever, relates not to this, but to only superficial influences. 



Though theoretically, therefore, alteration of one kind or 

 another may extend down to, and in some cases much below the 

 level of permanent saturation, and if given sufficient time would 

 actually go to such depths ; yet in many, if not most, cases it 

 has not yet reached that level. The actual depth to which alter- 

 ation does extend varies with the topographic conditions of the 

 region, the chemical nature and the porosity of the deposits 

 affected, the character of the climate, and other minor con- 

 ditions. 



The topography of a region affects the depth of alteration, 

 because it is one of the principal features in determining the 

 depth of permanent saturation. The chemical nature of the 

 deposit affects the depth of alteration because on this depends 

 the degree of resistance it will offer to the chemical effects of 

 percolating waters. The porosity of the deposit affects the depth 

 of alteration because, in deposits of similar kind but of different 

 porosity, the more porous will be more accessible to surface influ- 

 ences, and will, therefore, be more affected, in a given time, than 

 the less porous deposit. 



The climatic conditions, such as the amount and manner of 

 occurrence of rainfall and other forms of atmospheric moisture, 

 and the rate and degree of variation in temperature have a large 

 influence on superficial alteration. On the amount of rainfall 

 and other forms of atmospheric moisture depends the amount of 

 moisture available as an agent of alteration ; while on their mode 

 of precipitation depends, other things being equal, the amount of 

 water which would sink into the deposit, thus effecting alteration. 



