200 THE PROBLEM OF THE METALLIFEROUS VEITSTS. 



understand how a process, which could only progress by the intro- 

 duction of material in very dilute solution should, by the agenc}^ of 

 crystallization, drive out the only means of its production. Some 

 residue of water must necessarily remain locked up in the partially 

 cemented rock. This residue we of course do not find where rocks 

 are dry and drifts are dusty. In many cases also, where deep cross 

 cuts have penetrated the fresh wall rock of mines, cementation, if 

 present, has been so slight as to escape detection. 



If we once admit that this conclusion is well based, it removes the 

 very foundation from beneath the conception of the meteoi'ic waters 

 and tumbles the whole structure in a heap of ruins. 



While I would not wish to positively make so sweeping a statement 

 as this about a question involving so many uncertainties, there is 

 nevertheless a growing conviction among a not inconsiderable groujD 

 of geologists that the rocky crust of the earth is much tighter and 

 less open to the passage of descending waters than has been generally 

 believed and that the phenomena of S2:)rings which have so much 

 influenced conclusions in the past affect only a comparatively shal- 

 low, overlying section. Such phenomena of cementation as we see 

 are probably in large part due to the action of water stored up by 

 the sediments when originally deposited and carried down by them 

 with burial. Under pressure a relatively small amount of water 

 may be an important vehicle for recrj^stallization. 



It has been assumed in the above presentation of the case of the 

 meteoric waters that they are able to leach out of the deep-seated 

 wall rocks the finely disseminated particles of the metallic minerals, 

 but the conviction has been growing in my own mind that we have 

 been inclined to overrate the probability of this action in our discus- 

 sions. In the first place, our knowledge of the presence of the metals 

 in the rocks themselves is based upon the assay of samples almost 

 always gathered from exposures in mining districts. The rock has been 

 sought in as fresh and unaltered a condition as possible and endeavors 

 have been made to guard against the possible introduction of the 

 metallic contents by those same waters which have filled the neigh- 

 boring veins. But if we admit or assume that the assay values are 

 original in the rock, and, in case the latter is igneous, if we believe 

 that the metallic minerals have crystallized out with the other bases 

 from the molten magma, we are yet confronted with the fact that 

 their very presence and detection in the rock shows that they have 

 escaped leaching, even though they occur in a district where under- 

 ground circulations have been especially active. From the results 

 which we have in hand it is quite as justifiable to argue that the 

 metals in the rocks are proof against the leaching action of under- 

 ground circulations as that they fall victims to it. These considera- 



