238 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
« 
open fissures between the Beacon Hill phonolite and the main volcanic neck. The 
relation between the flow from the El Paso tunnel and the fall of the water in the 
Elkton mine is shown in the diagram of fig. 22. 
CONDITION'S OF AVATER STORAGE. 
The records of the various drainage tunnels all suggest the same conclusion, 
namely, that the volcanic neck, filled with breccia and eruptive rocks, contains a 
body of water which is prevented from escaping into the streams of the region by the 
relatively impervious character of the older crystalline rocks of the prevolcanic 
plateau. The relation is suggestive of a water-soaked sponge set in a glass cup. 
That these granites, gneisses, and schists are not strictly impervious is shown by the 
evident connection between the Beacon Hill water and that in the main volcanic 
neck. This connection, however, is certainly due to one or more zones of persistent 
Assuring and not merely to passage through slightly jointed rocks. The fact that the 
tunnels are all comparatively dry until they penetrate the volcanic breccia or until 
they cut open fissures extending into the breccia and that they then receive a sudden 
influx of water followed by a lowering of the water level within the breccia shows that 
natural seepage through the prevolcanic rocks does not exceed the rate of supply to 
the central reservoir. The granite and related rocks undoubtedly contain water to a 
depth of over 2,000 feet, but this water must circulate very slowly, as shown by the 
absence of copious springs in the lower parts of the district, in spite of the consider¬ 
able head of water formerly stored at higher levels in the breccia. For all practical 
purposes, therefore, the underground water of the central part of the Cripple Creek 
district is to be regarded, not as an underground sea extending to indefinite distances 
from the Amlcanic center, but rather as an underground lake or pond bounded by the 
steep walls of the old \ T olcanic throat. In this particular pond, however, rock is far 
more abundant than water, and the simile should not be carried too far. 
SOURCE OF THE AVATER. 
The question of the source of this water is of considerable scientific and prac¬ 
tical importance. The statement is sometimes made that the water never rises after 
being lowered by pumping. This, of course, is not strictly true, otherwise such 
mines as the Gold Coin might long ago have stopped their pumps. It may be well 
doubted whether any wet mine has become dry by pumping alone, though after long- 
continued and heavy pumping, such as has been practiced in the Elkton, Stratton’s 
Independence, and Gold Coin mines, the water seldom rises again to its original level. 
The drainage tunnels have been far more potent in effecting a permanent reduction of 
water level. The facts that these tunnels run dry in a few years and that the springs 
of the district liaA r e never been more than insignificant trickles show that the supply 
of water to this underground reservoir must be small—certainly less than 2,000 
gallons a minute, the quantity which was being pumped from the mines before the 
completion of the El Paso tunnel. Assuming, however, the supply of water to the 
underground reservoir to be at the rate of 2,000 gallons a minute and conservatively 
estimating that the volcanic neck or reservoir has a diameter of 2 J miles, a rough cal¬ 
culation shows that this supply will be maintained if a sheet of water approximately 
