1922] Whitman: Genesis of the Ores of the Cobalt District 299 



not have removed all there was, and have deposited it entirely in the 

 veins? My assistant, W. L. Whitehead, proposed a comparison of the 

 production of the mines with the calculated volume of diabase which 

 is presumed to have contributed to them. My figures for these com- 

 putations follow. In making the computations I assume that the aver- 

 age distance from the point of origin to the point of deposition of a 

 particle of ore was 500 feet, and that as much ore was deposited 

 along one margin of the sheet as along the other. This assumption 

 necessitated the estimation of the volume and weight of a sheet of 

 diabase 500 feet thick over or imder each group of veins, and sur- 

 rounding it for a radius of not more than 500 feet, upon the assumption 

 of a thickness of 1000 feet for the total diabase mass. Many errors are 

 undoubtedly represented by the resulting figures ; but I have tried 

 to throw all non-estimable errors onto the side of excessive richness in 

 silver oh the part of the diabase. 



The computations assume the weight of the diabase to be 187 

 pounds per cubic foot. The production of silver is taken as being 

 314,391,494 ounces from the years 1904 to 1919 inclusive, as indicated 

 by the government reports; and the volume of diabase involved is 

 estimated at 37,739,500,000 cubic feet, which would weigh 3,528,643,250 

 tons. This estimate would give a silver content of 0.089 ounces per 

 ton of diabase, or 0.0000037 parts, or 0.00037 per cent by weight, or 

 0.26 gms., of silver per cubic foot of diabase. 



Such a silver content is not unusual in igneous rocks. In ' ' The Data 

 of Geo-Chemistry, " 23 Clark quotes Hillebrand's analyses of Leadville 

 porphyries as indicating an original silver content of 0.000009 parts. 

 The same ratio was obtained from a volcanic ash from Tunguragua, 

 while an ash from Cotopaxi contained 0.0000119 parts of silver. J. B. 

 Harrison is also reported to have found in igneous rocks from British 

 Guiana a silver content of 0.0000016. Perhaps the reason why the 

 original silver content was found in these rocks is that they did not 

 contain also solvents for the silver, and whatever ground water may 

 have reached them was deficient in like manner. 



Deposition. — Split joints have already been discussed, and it has 

 been pointed out that those in which ore was deposited were the ones 

 the walls of which were the most free from lateral compression, being 

 under longitudinal compressive stress. Attention was also drawn to 

 a remarkable relationship existing between the mineralogical contents 

 of veins and their dips. The arrangement of the mineral constituents 



23 U. S. G. S., Bull. 148. 



