168 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



Let a. a. (fig. 11. Plate ix.j be a lode-fissure dipping south, 

 and passing through the killas, and down into the granite. 

 Now, assuming the rocks to be everywhere pervious as we know 

 in fact they are. and assuming that there is no deep-seated 

 spring issuing at a, then the waters percolating through the rock 

 from the surface down to the water level e.e., which in this case 

 is also the sea level, will make their escape at e ; so giving rise 

 to a phreatic spring on the sea-shore ; while in any wells sunk 

 through the rock, or in the shaft b the water will stand per- 

 mantly at that level, all the rock below remaining saturated 

 with practically stagnant water. If now, by sinking the shaft d 

 and pumping, the water be lowered to c that will be for the time 

 the artificial water level, water will circulate through the rock 

 above that level, and all below c will in this case be stagnant. 



The same figure 1 1 will serve to illustrate the more ancient 

 circulation which has often filled such fissures with ore and 

 veinstone. Suppose by the action of subterranean heat a 

 constantly issuing deep-seated upward current or spring at a. In 

 this case there will be a constant circulation through the mass 

 of the rock down as far the deep-seated source, wherever that 

 may be. The condition of the water below e.e. or c.c, is no 

 longer static but dynamic. The waters " seeping" through the 

 mass of the rock will be constantly making their way into the 

 fissure at all depths, and joining the outward current at nearly the 

 temperature of the rock itself at those various depths. The 

 water circulation may be considered as an infinite assemblage of 

 molecules moving in files, and their normal courses will be 

 indicated hereafter. 



The mean temperature of the issuing water will of course 

 be less than if it had all travelled to, and come from the greatest 

 depth, so that in the absence of strictly local sources of heat we 

 may fairly assume that thermal springs always come from 

 greater depths than would be indicated by their temperatures. 

 For it will be seen at once that different portions of the 

 very same circulating water will reach the ascending stream 

 after passing through rocks at different depths. Hence taking 

 any particular point in the vein, say x, the water which reaches 

 the vein above that point has only percolated through the rocks 



