MANNBK OF DEPOSITION. 99 



morphs of galena after calcite have been observed at Andreasberg and in 

 other mines. Had they been found at Eureka, it is scarcely probable that 

 they would have remained recognizable, in view of the subsequent oxida- 

 tion of the ore. It is probable, however, that calcite crystals were not 

 formed until after the period of ore deposition, and it is, therefore, in no 

 way remarkable that the search for pseudomorphs after calcite was unsuc- 

 cessful. 



Relation of the rhyolite eruption and the caves to the formation of ore. The disturbance inci- 

 dent to the rhyolite eruption caused the faulting and crushing which pre- 

 pared the limestone for the circulation of the metal-bearing fluids. It is 

 not likely that the waters carrying carbonic acid had effected any material 

 dissolution of the limestone before an opportunity was given for their free 

 circulation by the shattered condition of the country. If the deposition of 

 ore is correctly referred to solfataric action consequent upon the rhyolite 

 eruption, the precipitation of the sulphurets may have begun immediately 

 after the outburst of volcanic rock and before a sufficient period had elapsed 

 to allow of the formation of caves. Besides the caves above ore bodies, 

 there are many cavernous openings in the limestone in which no ore occurs; 

 but some of these empty caves, as has been mentioned, are connected by 

 open fissures or pipes with ore bodies. Had these caves existed at the time 

 of the deposition of ore, it is difficult to see why they failed to receive a 

 share of the deposits. Although this is not absolute proof that these cave* 

 were made after the ore was deposited, it is entirely consistent with such a 

 theory. 



If the caves had been formed first, they certainly would have contained 

 gravel, bowlders, etc., washed in by water or representing insoluble residues, 

 and this material would have been found underlying the ore. This, how- 

 ever, is not the case; the ore is very free from admixtures of foreign sub- 

 stances, and wherever this detritus is found, it either overlies the ore or 

 occupies a position adjacent to it, consistent with the hypothesis of subse- 

 quent placement. If the caves were not formed after the deposition of the 

 ore, they must necessarily have been enlarged during the oxidation of the sul- 

 phurets, for this can have been sustained only by supplies of oxygen carried 

 by water from the surface. This water must have held carbonic acid in 



