270 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
The Last Dollar mine is 1,200 feet deep, the shaft having a surface elevation of 
10,278 feet. In a long crosscut on the 1,200-foot level, under the Modoc vein, a 
temperature of +16° C. ( + 60.8° F.) was measured at the breast. This figure is 
doubtless not far removed from the actual temperature of the surrounding rock. On 
the assumption that the mean surface temperature is 41° F., this will correspond to 
an increment of 1° F. per 58 feet. In the same mine and on the same level the 
temperature in the stopes 40 feet above the rail was found to be +17.5° C. (+ 63.5° 
F.), which is probably higher than the normal temperature. At the shaft on the 
same level the temperature was 4- 13° C. 
The next observations were taken in the Vindicator mine, at the breast of the 
Christmas crosscut, 800 feet below the surface, the elevation of the collar of the 
shaft being 10,209 feet. The temperature obtained at the breast of the crosscut 
was +12° C. ( + 53.6° F.). Again assuming that the mean surface temperature is 
41° F. and that the temperature of the crosscut does not differ materially from that 
of the surrounding rock, we obtain an increment of 1° F. per 63.5 feet. 
The Ophelia tunnel extends for over 7,000 feet westward from Cripple Creek 
into the heart of the district. At the time of visit work had been discontinued at 
the breast for several months. Connection with the surface was established through 
the Callie shaft, 1,800 feet back from the breast. Xo artificial ventilation was 
resorted to, and it may again be assumed that the temperature at the breast does not 
differ more than 1° or 2° at most from the temperature of the surrounding rocks. On 
March 18, 1904, the temperature at the breast was + 12° C. ( + 54° F.). The vertical 
distance to the surface being 700 feet, and a mean annual temperature of 41.3° F. 
being assumed for that surface elevation, an increment of 1° F. per 55 feet is obtained. 
At the Conundrum mine, in the workings on level 6, 750 feet below the surface, 
a temperature of +53° F. was measured on March 21, 1904. There was some venti¬ 
lation at the time, and it is possible that the figures may be somewhat too low to 
represent the actual rock temperature. If we assume that it is approximately 
correct, however, the increment is calculated to be 1° F. per 60 feet. 
This is in some measure confirmed by the observed temperature of the water 
where it issues from the El Paso drainage tunnel, the portal of which has an elevation 
of 8,783 feet. This temperature was found to be +14° C. ( + 57.2° F.). Assuming 
that this water is drained from an average level of 1,000 feet below the surface 
throughout the large area affected by the tunnel and further that the mean annual 
temperature is +41° F., we obtain an increment of 1° F. per 62 feet. 
Although these observations are not accurate, they may be depended on as 
showing that the average increment of temperature in the Cripple Creek district 
approximates 1° F. per 60 feet. This is very nearly the normal figure usually given 
for the increase of temperature in depth and indicates that there is no special source 
of heat present which would interfere with mining at depths much greater that those 
attained at present. At the depth of 2,400 feet a temperature of +81° F. might be 
expected if the rate is maintained. At 3,600 feet the temperature should rise to 
101° F. 
As has been explained in more detail in the paragraphs devoted to subterranean 
gases, a higher temperature obtains at places where mine gases are especially 
abundant. 
