14 RESCTRVEY OF CRIPPLE CREEK DISTRICT. [BULL. 254. 



Big Bull Mountain and Gold Hill occupies a chasm of profound depth 

 in the fundamental rocks of the region. From the Conundrum mine 

 on the western slope of Gold Hill to Stratton's Independence mine on 

 the south slope of Battle Mountain the contact plunges steeply down, 

 with dips ranging in general from 70 to vertical. In some instances 

 the granite walls of this chasm actually overhang the breccia. It is 

 probable that this entire southwest contact represents a part of the 

 wall of the great pit formed by the volcanic explosions that produced 

 the breccia. It is further probable that an arm or branch of this 

 volcanic abyss, now filled with breccia and intrusive rocks, extends 

 northeastward past Bull Cliff and the town of Altman. 



ECONOMIC GEOLOGY. 



EARLIER WORK. 



To the excellent work of Mr. R. A. F. Penrose, jr., apply statements 

 similar to those made in the discussion of the purely geological branch 

 of the subject. Few shafts had then attained a depth of 400 feet, and 

 most of the exposures were masked by surface oxidation. It would 

 be surprising, in view of the facilities created by the later develop- 

 ment of hundreds of mines, if a subsequent investigation should not 

 bring out some slight modifications of earlier results. 



EXTENT OF PRODUCTIVE TERRITORY. 



There is nothing in the history of the district since 1894 warranting 

 any extension of the bounds of the productive territory as then known. 

 Now, as then, a circle of 3 miles radius described from the summit of 

 Gold Hill would include all deposits of known or prospective value, 

 while the really important mines would be embraced by a circle of about 

 half that radius, with its center near the summit of Raven Hill. That 

 scattered deposits of greater or less value may be found in outlying 

 portions of the district is by no means improbable. But the close 

 dependence of the typical Cripple Creek ores upon the main volcanic 

 center, and the consequent remarkable compactness of the gold-bearing 

 area, are features highly characteristic of the district and are likely 

 always to remain so. 



The greater part of the ore has undoubtedly come from the central 

 area of breccia, particularly from that part of this area in which the 

 breccia extends to great depth. Very productive ore bodies have 

 been found, however, in the granitic rocks, usually within 1,000 feet 

 of the steep contact limiting the breccia on the southwest. The im- 

 portant Beacon Hill mines, with ore bodies nearly three-quarters of a 

 mile from this contact, are exceptional, and are probably genetically 

 connected with the intrusive mass of phonolite forming the core of 

 the hill. 



