46 GROUND-WATER 



/ tween the surface and a level slightly below the ground-water sur- 

 face (p. 31); in other words, in the zone where atmosphere and 

 hydrosphere co-operate. Surface-waters are charged with atmos- 

 pheric and organic acids and other solvents, and their general 

 effect upon the rocks is markedly solvent down to and somewhat 

 below the permanent water-level. Concentration by residual 

 accumulation may take place in this zone, 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 prob- 

 ably to the full depth of water circulation; but in the deeper circuit, 

 precipitation also takes place, and with the waters taking up and 

 throwing down material at the same time, it is difficult to estimate 

 the balance of results. It is probable, however, that the result of 

 these processes is to promote the development of the higher ore 

 values at levels near enough the surface to be accessible, and along 

 the main lines of ground-water circulation. 



Influence of contacts. As many ore-deposits depend on a dis- 

 solving state of the waters followed by a depositing state, it is 

 obvious that conditions which favor changes of state and the com- 

 mingling of different kinds of water, are apt to be favorable to ore 

 production. At any rate it is observed that many important ore- 

 deposits occur at the contact of unlike formations, as for example 

 at the contact of igneous rock with limestone. It is not to be in- 

 ferred that such contacts are generally accompanied by workable 

 ore-deposits, but merely that a notable proportion of workable 

 ore-deposits occur at such junctions. It is rational to suppose 

 that where the chemical nature of the two formations is in contrast, 

 the waters that percolate through the one are likely to be mineralized 

 very differently from those that course through the other, and that 

 on mingling at the contact, reactions are liable to take place. When 

 a valuable metallic substance is present, it may be involved and, 

 by chance, suffer precipitation. Reactions are the more probable 

 because the contact plane of formations is, in some cases, a plane 

 of crustal movement, and hence more or less open and accom- 

 panied by fractures, zones of crushed rock, and other conditions 

 that facilitate circulation and offer suitable places for ore formation. 

 The effect of igneous intrusions. A special case of much im- 



