-Januart 5, 1906.] 



SCIENCE. 



27- 



It may be stated that the porphyritic 

 igneous rocks which have furnished nearly 

 all the samples for the above analyses, are 

 as a rule extremely dense, and that their 

 absorptive capacity is more nearly that of 

 the compact granites than the open tex- 

 tured ones. It is highly improbable that 

 -underground water circulates through 

 these rocks to any appreciable degree ex- 

 •eept along cracks which have been produced 

 in the mechanical way, either by contrac- 

 tion in cooling and crystallizing, or by 

 faulting and earth movements. The 

 cracks from faulting are very limited in 

 extent and in the greater number of our 

 mining districts affect but narrow belts, 

 small fractions of the total. Of the cracks 

 from cooling and crystallizing those of us 

 who have seen rock faces in cross-cuts and 

 drifts underground, where excavations 

 have been driven away from the veins 

 proper, can form some idea, if we eliminate 

 the shattering due to blasting. My own 

 impression is that in rocks a thousand feet 

 or so below the surface they are rather 

 widely spaced, and that, when checked in 

 a general way by the ratios just given, they 

 are decidedly unfavorable materials from 

 which the slowly moving meteoric ground- 

 waters (if such exist) may extract such 

 limited and finely distributed contents of 

 the metals. 



I have also endeavored to cheek the con- 

 clusions by the recorded experience in 

 cyaniding gold ores in which fine crushing 

 is so important, and I can not resist the con- 

 viction that we have been inclined to believe 

 the leaching of compact and subterranean 

 masses of rock a much easier and more 

 probable process than the attainable data 

 warrant. 



As soon, however, as we deal with the 

 open-textured fragmental sediments and 

 volcanic tuffs and breccias the permeability 

 is so enhanced as to make their leaching a 

 comparatively simple matter. Yet so far 



as the available data go, they are poor in 

 the metals or else are open to the suspicion 

 of secondary impregnation. They cer- 

 tainly have been seldom, if ever, selected 

 by students of mining -regions as the prob- 

 able source of the metals in the veins. 



Should the above objections to the effi- 

 ciency of the meteoric waters seem to be 

 well established, or at least to have weight, 

 it follows that the arena where they are 

 most, if not chiefly, effective is the vadose 

 region, between the surface and the level 

 of the ground-water. Undoubtedly from 

 this section they take the metals into solu- 

 tion and carry them down. But it is 

 equally triie that they lose a large part of 

 this burden, especially in the case of cop- 

 per, lead and zinc, at or near the level 

 of the ground-water and are particularly 

 efiicient in the secondary enrichment of 

 already formed but comparatively lean ore- 

 bodies. 



Let us now turn to the magmatic waters. 

 That the floods of lava which reach the sur- 

 face are heavily charged with them, there 

 is no doubt. So heavily charged are they, 

 that Professor Edouard Suess, of Vienna, 

 and our feUow member. Professor Robert 

 T. Hill, of New York, have seen reason for 

 the conclusion that even the oceanic waters 

 have in the earlier stages of the earth's, 

 history been derived from volcanoes rather 

 than, in accordance with the old belief, 

 volcanoes derive their steam from down- 

 ward percolating sea-water. From vents 

 like Mont Pelee which in periods of ex- 

 plosive outbreaks yield no molten lava, the 

 vapors rise in such volume that cubic miles 

 become our standards of measurement. 



There is no reason to believe that many 

 of the igneous rocks which do not reach 

 the surface are any less rich and when 

 they rise so near to the upper world that 

 their emission , may , attain the surfaqe, 

 we must assign to the emitted waters a. 



