MATERIALS OF THE EARTH 73 



zone are quite certainly more dilute than those below. The horizon 

 of greatest solution lies quite surely between the surface and a 

 level slightly below the ground-water surface; or, in other words, 

 in the zone where atmosphere and hydrosphere co-operate. 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, concentration by residual accumulation may take place, 

 as already noted, if the metallic compounds resist solution; other- 

 wise this zone is depleted of its ore material by solution, and prep- 

 aration 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. 



The influence of contacts. As ore-deposits depend on a dis- 

 solving state followed by a depositing state of the waters, and per- 

 haps on a complex succession of these alterations, it is obvious 

 that conditions which favor changes of state and the commingling 

 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 between formations of different character. The 

 contact of igneous rock with limestone is a rather notable instance. 

 It is not to be inferred 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 hence that on mingling at the contact, reactions are 

 specially liable to take place, and that when a valuable metallic 

 substance is present, it is liable to be involved and, by chance, to 



