94 SILVER-LEAD DEPOSITS OF EUREKA, NEVADA. 



from a considerable distance below, and it is highly probable that they did, 

 it would be reasonable to suppose that the deposition would continue as far 

 as it would be possible to follow it. An investigation of the phenomena 

 attending- the formation of these deposits will show, however, that the man- 

 ner in which the ore was deposited has a very important bearing upon the 

 probabilities of finding ore at any considerable distance below the water 

 level. 



Theory of the formation of caves. — The formation of caves in limestone is usually 

 attributable to the action of waters percolating from the surface and carry- 

 ing carbonic acid in solution. As is well known, even rain-water contains 

 carbonic acid in solution, though in small quantities corresponding to the 

 traces of carbonic anhydride always present in the atmosphere. The air 

 occupying the pores of the soil for a considerable distance from the surface 

 is much more highly charged with carbonic anhydride than the free atmos- 

 phere, a fact no doubt due to the oxidation of organic matter, and the per- 

 colating waters are correspondingly charged with carbonic acid. Below the 

 permanent water-level of a limestone country the water is nearly saturated 

 with calcium carbonate, and though there is a slow circulation of subter- 

 ranean currents beneath this level no strong local action can be expected. 

 To term 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 which results from the 

 corrosion of the rock. Caves cannot, therefore, form at an indefinite depth 

 from the surface of the limestone under any circumstances, for, after pass- 

 ing a certain distance through limestone, the percolating waters would be 

 nearly or quite saturated. Caves, too, can only be found in a country with 

 deep drainage, since otherwise the saturated solvent could not be removed. 



The rate of cave formation is dependent upon the quantity of water, 

 the amount of carbonic acid that it contains, and the velocity with which 

 it flows. Climatic changes and changes in the formation from dynamic 

 causes accelerate or retard the action of these waters as the case may be, 

 but a tendency to the formation of caves exists wherever water percolates 

 through limestone. The solution of limestone ordinarily appears to be 

 accompanied by the deposition of more or less calcium carbonate in the 



