Page:The Journal of geology (IA journalofgeology21894univers).pdf/314
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 oxidation practically ceases. Below the level of permanent saturation, the waters may sometimes gradually sink to very great depths, even deep enough to become intensely heated and possibly 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, however, 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 alteration 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 conditions.
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 influences, 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,