] UNDERGROUND WATER. 31 



conclusion is warranted in an investigation as yet incomplete, it 

 appears that the unoxidized ore deposits of the Cripple Creek district 

 represent the product of one general period of mineralization and that 

 they have not been appreciably modified by secondary enrichment 

 during the subsequent erosion of the region. 



UNDERGROUND WATER. 



The conditions of underground waters are interesting and somewhat 

 unusual. A dry climate and a heavy percentage of run-off minimize 

 the annual additions to the underground supply. Nevertheless, the 

 ground-water level is not unusually deep, and large quantities of water 

 are encountered in all the mines below that level. The original water 

 surface of the district in the volcanic rocks stood at elevations of 9,400 

 to 9,700 feet, or 100 to 600 feet below the surface of the ground. At 

 first pumping was commenced by individual mines, but it was soon 

 found that the radius of drainage had unusual length that is, that one 

 mine would drain others situated at a distance. Drainage tunnels were 

 then undertaken, and the Chicago and Cripple Creek, the Ophelia, the 

 Standard, and lately the El Paso tunnels were driven, each of which 

 practically accomplished the drainage of a large part of the district 

 almost down to its own level, thus showing that the ground water is 

 limited in quantity and is more of the nature of a local reservoir than 

 a ' ' subterranean sea. " 



The plug of volcanic rocks which fills the throat of. the old volcano 

 is rudely circular, with a diameter of 3 miles. This mass is extremely 

 porous, and is, moreover, cut in many directions by partly filled fissures 

 and sheeted zones, so that water can circulate within it with compara- 

 tive freedom in several directions. It retains this character down to 

 the greatest depth yet reached. On the other hand, the surrounding 

 granite is relatively impermeable and is less traversed by open fissures. 

 No doubt it contains ground water down to a depth of 2,000 feet or 

 more, but in very much smaller quantity, and the circulation of this 

 water must be extremely slow. This is clearly shown by the fact that 

 the water in the breccia is not drained by Cripple Creek and Arequa 

 Gulch below the level of the points where they leave the volcanic area. 

 Thus the volcanic plug resembles a water-soaked sponge inserted in a 

 hole cut in an impermeable substance. The drainage of the mines is 

 thereby greatly facilitated, as it is not necessary to extend the tapping 

 tunnels to each mine. 



The El Paso tunnel, completed in the winter of 1903-4, has an ele- 

 vation of 8,790 feet at the portal. Within a short time it effectually 

 drained not only the Beacon Hill mines but also the Gold Hill mines, 

 and its influence extended even to the Last Dollar and the Elkton 

 mines. But the foregoing statement in relation to draining the dis- 



