LIN R I SfsoMK ND ] RELATIONS OF ORE BODIES TO DEPTH. 29 



far the greater part of the underground prospecting is at less depths, 

 there being usually little inducement to go deeper, unless, as in the 

 case of the Gold Coin and Portland mines, lodes are discovered in 

 which the ore, beginning several hundred feet below the surface, 

 extends deeper than the pay shoot upon which the mine was origi- 

 nally opened. Thus deep prospecting is usually confined to the 

 vicinity of the larger and more persistent pay shoots which have been 

 followed down from near the surface. Underground water has also 

 proved a most serious obstacle to deep prospecting, few properties 

 being able to develop below the 1,000-foot zone unless there is abun- 

 dant and high-grade ore in sight. 



It may thus be concluded, without necessarily advocating promiscu- 

 ous exploration below the 1,000-foot zone, that any ore bodies exist- 

 ing below that depth are far less likely to be discovered than those 

 above, where from the surface to depths of several hundred feet the 

 rocks of the district are riddled with shafts, drifts, crosscuts, and 

 adits. It is exceedingly difficult, however, to determine, even ap- 

 proximately, the relative importance of this factor in the problem. 

 It is probably safe to assume that th.e chances of discovering a given 

 ore body within the 1,000-foot zone are at least ten times those of dis- 

 covering an ore body below that zone, and the ratio may be very much 

 greater. It is probably true that there was originally more ore within 

 the 1,000-foot zone than there is in a corresponding zone below, but 

 this disparity is not necessarily anything like so great as is indicated 

 by the vertical distribution of known pay shoots. 



Another important line of inquiry bearing upon the relations of the 

 ore bodies to depth is concerned with the question of the relative size 

 and abundance of the fissures near the surface and at greater depth. It 

 has been shown that all the ore bodies are intimately connected with 

 fissures. If such fissures are generally smaller and less abundant 

 below the 1,000-foot zone than they are within it, obviously there is 

 introduced a factor which diminishes the supposed importance of sec- 

 ondary enrichment by affording an anterior and physical explanation 

 for the decrease of ore with increase of depth. 



Detailed examination of practically all the accessible mines in the 

 Cripple Creek district has led to the conclusion that the fissures, which 

 ordinarily are narrow and often appear as mere cracks, do become less 

 abundant and less conspicuous as greater depth is attained. No mine 

 exhibits this feature better than the Stratton's Independence, in which 

 the very complex systems of productive fissures on the fifth and higher 

 levels contrast most strikingly with the few, insignificant, and unpro- 

 ductive fractures visible on the fourteenth level. In less degree the 

 same feature is shown in many others of the deep mines, but the rule 

 is not without some very marked exceptions. 



