MINERAL VEINS 247 



ground-water, according to the assumed average porosity of 2% per cent 

 or 5 per cent at the surface and .0 per cent at a depth of 6 miles. 34 They 

 estimate, however, that the average porosity is between 5 and 10 per cent 

 for the outer lithosphere. C. R. Van Hise, in 1904, estimated it as equiva- 

 lent to a layer over the continental land areas of 226 feet. 35 Finally, 

 M. L. Fuller, after long experience with the subject in the water-supply 

 work of the United States Geological Survey and on the basis of the best 

 obtainable data, gave as his estimate 96 feet if the underground water 

 were spread over the entire surface, and considered this amount as over 

 rather than under the truth. 35a 



Now, in the writer's contribution of 190 1 31 it was stated that, under 

 extremely arid regions, the ground-water might fail entirely ; that in deep 

 mines, if impounded, it ceased at depths from 500 to 2,000 feet, below 

 which pumping became unnecessary; but that cases such as the deep 

 workings of the Comstock Lode, in regions of expiring igneous activity, 

 must be ruled out, because the waters might be magmatic. 



If, with Chamberlin and Salisbury, we assume porosities of 5 and 10 

 per cent, we have in the one case 25 to 50 feet of water; in the other, 

 100 to 200 feet. If we average all four of these estimates, by an odd 

 coincidence we obtain 93.75 feet, checking remarkably with M. L. Fuller. 

 This number would be reduced somewhat by the experience in arid re- 

 gions, which have very deep ground-water level or none at all. If, again, 

 Ave average the 500 feet and 2,000 feet of mining experience at 1,250 

 feet, and take Chamberlin and Salisbury's other assumptions of porosity 

 at 2y 2 per cent and 5 per cent, we obtain, respectively, 31.25 feet and 

 62.5 feet. Obviously, there is much less meteoric ground-water than was 

 formerly believed to exist. 



MAGMATIC EMISSIONS AS PRODUCERS OF VEINS 



As the first decade of the present century passed into its second half, 

 we find in 1906 Waldemar Lindgren's address at the Mexican Interna- 

 tional Congress on "The relation of ore deposition to physical condi- 

 tions," 36 wherein the agents which I have been passing in review are very 

 much to the fore. In the same author's presidential address before the 

 Geological Society of Washington, in December, 190 7, 37 we observe quite 

 as pronounced a tendency. At the close of 1907 J. E. Spurr's valuable 

 contribution, entitled "A theory of ore deposition," 38 published in this 

 same month, there is a comprehensive and philosophical summary which 

 I quote almost entire. 



"This theory proposes that metalliferous fluids, from which most ore de- 

 posits are precipitated, are extreme differentiation phases of rock magmas; 



