408 QnCKSILVEU DEPOSITS OF TOE PACIFIC SLOPE. 



such cases, if they exist, are exceptional. All observations and theory 

 point to the conclusion that most fissures are formed under a compressive 

 stress of greater or less violence or that a tendency to compress the rocks 

 into folds gives rise to fissures and faults when the applied force exceeds the 

 tenacitv of the rocks. It is also well known that in faulting the hanging 

 wall commonlv sinks relatively to the foot-wall. Fissures formed under 

 such conditions cnnnot vawn. The walls must come together at intervals 

 and the intervening spaces must be filled to a greater or less extent with 

 tlie fragments of wall rock. (Observation shows that veins very usually 

 answer to this description and that the space really occupied by ore corre- 

 sponds in great part to the interstices between fragments of wall rock which 

 looselv filled the fissure before the ore was deposited. This is observed 

 with particular frequency in large veins, while small ones are comparatively 

 free from rock fragments Irregular ore bodies connected with fissures still 

 more often represent masses of rock fragments rather than caverns. 



In some cases irregular chambers connected witli fissures are solidlj' 

 filled with ore and gangue minerals. Such bodies are found in limestones 

 under conditions which preclude the supposition that they represent pre- 

 existing caverns, and they are usually accompanied by evidences of sub- 

 stitution of ore for carbonate of lime. The deposits of Eureka and of 

 Leadville are of this type. In these cases broken rock seems originally to 

 have filled the spaces in question, much as stopes in mines are often filled 

 with rock by miners to prevent their collapse after the removal of the ore. 

 Solutions of ore finding access to these spaces thi-ough the main fissures 

 have come in contact with ver}- extensive surfaces of limestone. The lime- 

 stone has been dissolved and ore has replaced the rock molecule for mole- 

 cule. Whether similar substitution occurs in other rocks than limestone 

 and with other ores than those of lead has not been sufficiently investi- 

 gated. 



Fissures in the earth's mass would extend indefinitely both laterally 

 and vertically if the rocks possessed neither plasticity nor elasticity. No 

 rocks, however, are devoid of either of these properties. It is well known, 

 accordingly, that fissures are not of indefinite length. They sometimes 



