480 GEOLOGY, 



ently mineralized, as implied above. In these cases the metal-bearing 

 current may be scarcely more important than the precipitating current. 



Since the solvent action is a condition precedent to deposition, the 

 location of the greatest solvent action first invites attention. At pres- 

 ent it must be treated in general terms, for it is not known what solu- 

 tions must be formed beyond the fact that they must include the ore 

 material. Probably they must include much besides. Furthermore, 

 it is not known that deep-seated rocks carry more ore material than simi- 

 lar rocks at or near the surface or at any other horizon. Fantastic 

 conceptions of deep-seated metallic richness are to be shunned as quite 

 beyond practical consideration. The water circulation is probably 

 very sUght below a depth of two or three miles at most, and above that 

 depth there is little ground to suppose that the rocks of one horizon 

 are inherently more metalhferous than others of their kind. There is 

 no assignable reason why the igneous rocks at the surface are not as 

 rich in ore material as the igneous rocks two or three miles below, since 

 all are probably eruptive and of much the same nature on the whole, 

 being in many cases parts of the same eruptions. 



Location of greatest solvent action. — Solvent action is probably most 

 intense where the temperature and pressure are highest, that is, in the 

 deeper reaches of water circulation; but the amount oi water passing in 

 and out of the deeper zone is but a small fraction of that which courses 

 through the upper horizons, and the total solvent action is quite cer- 

 tainly much greater in the upper zone than in the lower. At the same 

 time the solutions in the upper zone are quite certainly more dilute 

 than those below. The horizon of greatest solution lies between the 

 surface and a level slightly below the ground-water surface, or, in other 

 words, in the zone where atmosphere and hydrosphere cooperate. Sur- 

 face-waters are charged with atmospheric and organic acids and other 

 solvents, and their general effect upon the rocks is markedly solvent 

 down to or often below the permanent water-level. In this zone con- 

 centration by residual accumulation may take place, as already noted, 

 if the metallic compounds resist solution; otherwise this zone is depleted 

 of its ore material by solution, and preparation is made for deposition 

 elsewhere. 



Solution also continues to take place varyingly as the water descends 

 below this zone of dominant solution, and extends probably to the full 

 depth of water circulation, but in the deeper circuit, precipitation also 



