1172 A TREATISE ON METAMORPHISM. 



upper parts of the veins is segregated. Segregation by this method is 

 especially important at and near the surface. It thus appears that the 

 production of the free gold of the upper parts of lodes is largely a chemical 

 process, but is partly a process of residual concentration The two together 

 continued for a sufficient length of time are believed to adequately explain 

 the rich free gold ores of the upper parts of lodes. 



Cripple Creek furnishes an excellent illustration of rich free gold at and 

 near the surface. Near the surface numerous deposits in this district were 

 exceptionally rich, containing a large amount of free gold above and near the 

 level of ground water. Usually much of this free gold was in spongy 

 pseudomorphs after tellurides, showing its formation in large measure by 

 the direct oxidation of the tellurium of the tellurides, the gold being- left 

 behind. For some years it has been well known that below the upper belt 

 of rich material in many of the veins the deposits were very much poorer. 

 Indeed, many of the deposits were found to be so poor below the upper 

 narrow belt of rich ores as to lead to their abandonment. 



In reference to this poor ground Mr. Moore says: 



"It is undoubtedly true that some mines have entered poor ground in 

 the veins at depths of 250 feet [75 meters] to 400 feet [120 meters] below 

 the surface, and have sunk down to depths of 400 feet [120 meters] to 500 

 feet [150 meters] deeper without finding payable ore bodies again." But 

 this was by no means true of all the deposits. Mr. Moore further says, "It 

 is a fact that in a greater number of mines the values have been carried 

 down to more than 1,200 feet [360 meters] below the surface continuously, 

 and in at least two mines the values have greatly increased from 1,100 feet 

 [330 meters] downward."" A most interesting feature in reference to the 

 Cripple Creek district has been developed recently. Deep in some of the 

 mines which showed a depleted horizon there has been discovered a lower 

 horizon of very rich ores, partly sulphides and tellurides, but also in larg-e 

 part free gold. 



According to Mr. Hills: "The original ground-water level at Cripple 

 Creek is about altitude 9,500 feet [2,850 meters] for the west side of the 

 district and considerably higher for the Bull Cliff reg'ion." 6 The upper part 



"The Daily News, Denver, Colo., Jan. 1, 1903, p. 3. 



& Ninth Ann. Kept., Portland Gold Mining Co., Victor, Colo.; Report of Victor G. Hills, Consult- 

 ing Engineer, 1903, p. 87. 



