THE ORIGIN AND DESCENT OF ROCKS. 483 



tent, rather than diminishing it by deposition. But this is only general; 

 particular elements of the solution may be deposited. In ascending, 

 as the same water must later, it is predisposed to deposition from loss 

 of solvent power through reduction of pressure and temperature. The 

 theoretical balance is here clearly in favor of preponderant deposition 

 by the ascending portion of the current. So far as precipitation is 

 dependent on the mingling of differently mineralized waters, descend- 

 ing and ascending currents seem to be situated much ahke, in general, 

 for both are subject to accessions and mutual unions. 



The amount of water that circulates in the deeper horizons is much 

 less than that nearer the surface. Allowing a few hundred, or at most 

 one or two thousand feet for the special short-circuit zone next below 

 the water-level (it is known to reach 1000 to 1500 feet in some cases), 

 the water circulating through the next 1000 or 2000 feet is probably 

 several times greater than all that circulates at greater depths, and this 

 greater circulation above doubtless offsets, in greater or less measure, the 

 intensified action of the deeper circulation. Much of the upper and 

 more rapid circulation is lateral, being actuated by the sloping surface of 

 the ground-water, which in turn is determined by topography, precipita- 

 tion, and other surface conditions. Theoretical considerations, therefore, 

 favor the view that lateral flow is an important factor in the concentra- 

 tion of ore material. But as descending and lateral currents almost inevi- 

 tably meet and mingle with ascending currents, it is difficult to dis- 

 tinguish, in the ore-deposits, the special functions of each phase of action. 

 It is even more difficult to determine whether the different phases are 

 not alike essential to the mutual reactions on which the deposition 

 depends. It may be as necessary to have a precipitant as to have a 

 metallic constituent in solution to be precipitated, and what is more, 

 this precipitating agency may be a substance of no economic value in 

 itself and of no obvious relations to the substances that form the ores. 

 If the deposition is due solely to a physical state, as rehef of pressure 

 or lowering of temperature, these considerations do not hold. 



Summary. — The general results are probably these: In the deeper 

 circuits, more ore material is brought upward and deposited than is 

 carried downward and deposited, so that metallic values are shifted 

 toward accessible horizons. In the lateral currents, more metallic values 

 are shifted toward the trunk-lines of circulation — the great crevices and 

 other waterways — than are carried from these into the rock and dis- 



