ON THE FILLING OF MINERAL VEINS. 29 



placement, which, however, is rather a method of precipitation 

 than of derivation. No one longer believes in contemporaneous for- 

 mation, and descension has an extremely limited, if, indeed, any 

 application. Ascension by sublimation as gas or with steam, or by 

 igneous injection, has few, if any, supporters. The discussion is 

 practically reduced to lateral secretion and to ascension by infiltra- 

 tion. 



1.04.03. By lateral secretion is understood the derivation of 

 the contents of a vein from the wall rock. The wall rock may 

 vary^ in character along the strike and in depth. Three interpreta- 

 tions may be made, two of which approach a common middle ground 

 with ascension by infiltration. It may first be supposed that the 

 vein has been filled by the waters near the surface which are known 

 to be soaking through all bodies of rock, even where no marked 

 waterway exists, and which seep from the walls of any opening 

 that may be afforded. Being at or within comparatively short 

 distances of the surface, the waters are not especially heated. 

 As they emerge to the oxidizing and evaporating influence of the 

 air in the cavity, their burden of minerals is deposited as layers on 

 the walls. The second interpretation supposes the walls to be 

 placed during the time of the filling at considerable depth below 

 the surface, so that the percolating waters are brought within the 

 regions of elevated temperature and pressure. Essentially the 

 same action takes place as in the first case. The third interpreta- 

 tion increases the extent of the rock leached. Thus if a mass of 

 granite incloses a vein and extends to vast depths, we may suppose 

 the waters to come from considerable distances, and to derive their 

 dissolved minerals from a great amount of rock of the same kind 

 as the walls. Portions of this may even be in the regions of high 

 temperature, while the place of precipitation is nearer the surface. 

 These last two interpretations have much in common with the 

 theory of ascension by infiltration, and on this common middle 

 ground lateral-secretionists and infiltration-ascensionists may be in 

 harmony. 



1.04.04. Ascension by Infiltration. The theory of infiltration 

 by ascension in solution from below considers that ore-bearing 

 solutions have come from the- heated zones of the earth, and that 

 they rise through cavities, and at diminished temperatures and 

 pressures deposit their burdens. No restriction is placed on the 

 source from which the mineral matter has been derived. Indeed, 

 beyond that it is " below," and yet within the limits, reached by 



