SUMMARY. 189 



The formation of caves. — The formation of caves in limestone is usually attrib- 

 utable to the action of waters percolating from the surface and carrying car- 

 bonic acid in solution. To form a cave at a given spot, water containing 

 free carbonic acid must be supplied in sufficient quantities, and an escape • 

 must be provided for the more or less saturated solution of calcium carbonate. 

 Caves cannot, therefore, form at an indefinite depth from the surface, and 

 their practical limit is reached at the water level. The caves in Eureka are 

 of more frequent occurrence near the surface than in depth, and they are 

 not found at all below the water level. If the theory of a simple deposition 

 of minerals from solutions in pre-existing caves were correct, it is evident 

 that the limit of the ore would be reached at the point where cave formation 

 was no longer possible. 



The substitution theory.— In the Eureka deposits nothing has been observed 

 which would indicate that the ore had been crystallized from solutions in 

 pre-existing cavities. The banded and concentric structure, characteristic 

 of that manner of deposition, is nowhere visible, and although it is conceiv- 

 able that it might have been obliterated in the oxidized ore bodies, it is im- 

 possible that such should have been the case in the unchanged masses of 

 sulphurets. The masses of sulphurets on the other hand offer strong evidence 

 in favor of the theory of substitution. The minerals have replaced the lime- 

 stone in such a manner that they have often retained the structural form of 

 that rock. Rounded bowlders of limestone have also been found as the nu- 

 cleus of masses of ore. 



The sulphuret ore shrinks to some extent owing to the leaching which 

 follows oxidation, and this accounts for the apparent relative size of many of 

 the caves over ore bodies. These caves were no doubt subsequently con- 

 siderably enlarged by the waters bearing carbonic acid. If the deposition 

 of ore is correctly referred to the solfataric action consequent upon the 

 rhyolite eruption, it is likely that the precipitation of the sulphurets began 

 soon after the outburst of volcanic rock, and before there could have been 

 much cave formation. 



That the lead deposits of Raibl (see Chapter VIII.) and of other 

 places should not have been formed by substitution is not an argument which 

 would prove that the same was the case in Eureka. In the Leadville de- 



