SOURCE OF THE ORE. 89 



of the rocks which may have covered the present surface contain any 

 heavy metals. 



The manner in which the ore entered the limestone. The evidence ill regard to the actual 



source of the ore is rather of a negative than of a positive character. The 

 theory of segregation is untenable; and other theories, such as that of a 

 deposition in beds simultaneously with the country rock or of an infiltra- 

 tion from above, are not to be thought of. The only reasonable explana- 

 tion which can be given of the source of the ore, and the only one which 

 is not contradicted by the observed facts, is that the ore bodies were formed 

 by infiltration from below. It has already been shown that the ore cham- 

 bers are intimately connected with fissures. Some of these — for instance, 

 the main fissure, as exposed on the twelfth level of the Eureka, in the cross- 

 cut to the Locan shaft — evidently served as channels for ore-bearing solu- 

 tions, and it is extremely probable that most of them at one time or other 

 have carried mineral solutions to the ore bodies. All fissures are more or 

 less connected with the two principal ones, and many of the ore bodies 

 are also connected together, and they have in general a downward trend. 

 All the facts point to an ascension of the solutions, and these solutions were 

 in all likelihood a result of the solfataric action consequent upon the erup- 

 tion of rhyolite. 



cause of the soifataric action. — It has already been stated that there is a strong 

 probability, if it is not absolutely certain, that the eruption of rhyolite pre- 

 ceded the deposition of ore. Extensive eruptions of this rock took place at no 

 great distance from the mines, and, as has been described, a dike of it follows 

 one of the chief fissures of the mineral zone. The decomposition of this 

 dike and of other rocks accompanying it, especially the quartz-porphyry, is 

 such as is characteristic of volcanic regions, and its occurrence must almost 

 inevitably be ascribed either to the rhyolite eruption or to the still more 

 recent outburst of basalt. There is no basalt, however, either in or near 

 the mines, and therefore nothing to indicate a connection between its ejec- 

 tion and the deposition of ore. The solfataric action traceable in the mines 

 is therefore most naturally referred to the rhyolite eruption. It is of course 

 no objection to this hypothesis that the rhyolite is itself decomposed, since 

 the decomposition of lavas within a few days of their ejection, by the gases 

 and solutions of the same eruption, has frequently been observed; while 



