BLACK HILLS GEOLOGY. 307 



ores. That this is a general rule can only be demonstrated 

 when those districts from which the upper measures of the Cam- 

 brian have not been eroded have been more completely opened 

 up. 



It seems, however, reasonable, that such should be the case 

 when we consider that the lower horizon is the first replaceable 

 zone to be attacked by uprising solutions, and that unless por- 

 phyry intrusions have rendered it difficult of access, the solutions 

 would in general first expend their strength in its replacement ; 

 the mineralization of the upper contacts would then only be 

 performed by solutions of unusual strength and volume. 



Character of the Ores. — Of the character of the ores Profes- 

 sor Smith says •} 



" While some deposits (such as that of the Dividend Mine, on Green Mountain) 

 yield pay-ore of a gouge-like, decomposed character, in general the ores may be de- 

 scribed as thoroughly reorganized sandstones, showing, under the microscope, many 

 druses lined with innumerable quartz crystals, and containing calcite and fluorite. Of 

 these ores, those which have not suffered oxidation frequently show considerable fine 

 grained pyrite, and are locally called ' blue ' oi-es ; the oxidized ores, though fre- 

 quently showing only a small iron content, are usually stained with iron, and are 

 called ' red ' ores. Both kinds are usually exceedingly tough and difficult to break 

 or pulverize. 



" Analysis of typical samples yielded the following results : 



Red Ore. 



Silica 84-45% 



Alumina. . 407 



Ferric Oxide 7-28 



Calcium Oxide 0.85 



Magnesium Oxide 0.25 



Sulphur trioxide 3.71 



Tellurium 8.426 oz. per ton. 

 Gold 0.574 " " " 



Silver 2.875 " " " 



Total ' . . . 100.61 



1 Trans. American Inst. Mining Eng., XXVII, 415. 



2 See " Tellurium and Gold Ores," Trans. Am. Inst. Mining Eng., XXVI, 

 485, Sept., 1896. 



