28 RESURVEY OF CRIPPLE CREEK DISTRICT. [BULL. 254. 



of 1,200 to 1,400 feet from the surface. The developments of the 

 next year or two will probably give a safer basis for generalization. 



Roughly speaking, the above-mentioned distribution holds good for 

 any elevation within the district. In other words, the principal pro- 

 ductive zone everywhere occupies the space from the surface down 

 to about 1,000 feet below it, and its lower limit thus forms a curved 

 surface approximately parallel to the surface of the ground. 



It is probable that the minimum depth of rock removed from the 

 district by erosion amounts to 1,000 feet in the central part and to 

 400 or 500 feet about the periphery. The shape and number of the 

 ore bodies formerly existing in this eroded zone can be only conjec- 

 tured. It is probable that the veins were formed shortly after the 

 close of igneous activity, while the volcano yet possessed a much 

 greater height than at present. The absence of hot waters and the 

 depth of oxidation attained indicate that vein formation at Cripple 

 Creek is by no means a recent phenomenon. 



The general features of the vertical distribution of the known ore 

 bodies recorded above have of late years received more or less recog- 

 nition, and there has been a decided tendency to attribute them to a 

 process of secondary enrichment effected by waters moving generally 

 downward from the surface. It has been supposed a that such waters 

 have carried down a part of the auriferous contents of those portions 

 of the lodes now removed by erosion and have enriched originally lean 

 pyritic ores by the secondary deposition of gold and silver tellurides 

 and argentiferous tetrahedrite, with associated gangue minerals. 



It is clear that the hypothesis in question is suggested by the dis- 

 tribution of known pay shoots. The question arises, How far does 

 the distribution of known pay shoots represent the distribution of all 

 the pay shoots in the district? In other words, How far has explora- 

 tion been impartial in revealing ore bodies near the surface and at 

 depths greater than 1,000 feet? 



It requires but little examination to make clear the fact that ore 

 bodies within 1,000 feet of the surface are far more likely to be discov- 

 ered than those at greater depth. While shafts have been sunk for a 

 few hundred feet without any indication of ore and have ultimately 

 been developed into productive mines, such a procedure is considered 

 bold prospecting, and few well-informed mining men would seriously 

 contemplate sinking a shaft over 1,000 feet in depth solely on the 

 expectation of finding possible ore bodies below that depth. Most of 

 the large mines in the district have started upon some indication of ore 

 near the surface and have grown by the subsequent discovery of other 

 lodes and ore bodies in the course of their underground development. 

 As few individual ore bodies persist for more than 1,000 feet in depth, by 



a Bancroft, Geo. J., Eng. and Min. Jour., vol. 74, 1902, pp. 752-753, and vol. 75, 1903, pp. 111-112. 

 Finch, J. W., Proc. Colorado Sci. Soc., vol. 7, 1904, pp. 193-252. 



