330 THEORIES OF OEE DISPOSITION HISTORICALLY CONJ^IDERED. 



through which these waters would circulate most freely, and which 

 hence were most favorable to ore deposition, were rock fractures pro- 

 duced by dynamic movements in the crust; faults or joints to which 

 Daubree had given the designation " lithoclases." In no case were 

 these fractures found to be contraction fissures, which Werner and 

 many subsequent writers assumed to be the typical vein fissure, disre- 

 garding the consideration that contraction fissures could not traverse 

 two distinct bodies of rock. To the joint-like fissures that are con- 

 fined to a single bed, Wiiitney bad already given the name "" gash " 

 veins. 



In the Comstock Lode report, Becker had discussed mathematically 

 the mechanics of faulting as applied to vein fissures, and had shown 

 that an important characteristic of faulting on a fissure in solid rock 

 is the tendenc}^ of the movement to separate the rock into sheets by 

 subordinate fissures parallel to the main one. From j)ractical obser- 

 vation Emmons had similarly concluded that the faulting movement 

 which produced vein fissures was often distributed on a number of 

 parallel fissures, thus producing a sheeting of the country rock. 

 Where these fissures were sufficiently close together, so that the inter- 

 mediate sheets of country rock were Aery thin and had been partially 

 replaced by vein material, a banding would result which might be 

 mistaken for that of the typical vein of incrustation. Where they 

 were farther apart and of approximately equal strength, the mineral 

 filling, instead of being confined to a single fissure, might be distrib- 

 uted on several, thus rendering frequent crosscutting advisable in 

 their exploitation. 



The idea that later-formed ore deposits are necessarily the filling of 

 considerable cavities or open spaces in the inclosing rocks has been 

 considerably modified by the important role that the process of meta- 

 somatic replacement or substitution has been shown to have played 

 in the formation of ore bodies. The idea of replacement had been 

 suggested in the conversion theories of the early speculators and more 

 distinctly expressed by Charpenticr. By th(» geologists of the second 

 period it was c()nlparati^■ely neglected, though iu a few cases admit- 

 ted as a subordinate factor, especially in the formation of deposits 

 in limestone. Even in Posepny's frequentl}' quoted studies of the 

 lead and zinc deposits of Raibl in Carinthia (1ST8). he aduiits this 

 mode of formation only for the oxidized ores, considering the sul- 

 phide ores to haA'e been deposited in open cavities. 



In America Pumpelly first ajiplied this process to the copper 

 deposits of the Lake Superior region (1871 ) , of which he says : '' Ln at 

 least very many instances, if not in all, the deposition of the copper is 

 the result of a process of displacement of preexisting minerals." 



Leadville and Eureka were the first large mining districts in which 

 it was proved that extensive ore deposits were entirely formed by 



