74 SILVER-LEAD DEPOSITS OF EUREKA, NEVADA. 



caves are by no means always accompanied by ore bodies. They resem- 

 ble all caves found in limestone, and have been produced, in part at any 

 rate, by the solvent action of water carrying' carbonic acid. These waters 

 passed through fissures and cracks, enlarging them, and dissolved the lime- 

 stone, especially where it was crushed and broken. The finely crushed 

 limestone was dissolved first, and the large fragments and bowlders settled 

 down and' were finally either completely dissolved or remained in the bot- 

 tom of the caves. This action was naturally most considerable at those 

 points where the best opportunity was afforded for the free circulation of 

 the water, and as the limestone was not uniformly shattered, and as the 

 different varieties of rock did not offer equal resistance, the openings formed 

 were of a very irregular character. The roof and sides of the caves are 

 sometimes entirely bare and only show the characteristic surface which re- 

 sults from the action of a solvent. Deposition of calcium carbonate, how- 

 ever, as well as its solution, has taken place on a large scale and is still 

 going on. The roof and sides of most of these caves are covered by arag- 

 onite crystals, and in some of them crystals of this mineral are still form- 

 ing. 



connection or ore bodies and fissures. — In the neighborhood of seams the limestone 

 is often crushed to a powder or is broken into fragments, which are occa- 

 sionally cemented together by calcite, forming a breccia. This fissured and 

 crushed country gave ingress to waters both from above and below. The 

 surface waters, owing to the carbonic acid which they contained, had a 

 solvent action upon the limestone, and those from below carried ores in so- 

 lution, which were, at least in part, substituted for the limestone. The 

 waters from both these sources removed limestone, which was again depos- 

 ited when the solutions became supersaturated. The question whether 

 the caves were partially formed before the deposition of the ore, during 

 its deposition, or after it, will be discussed hereafter; but it may be stated 

 here that a great portion of the ore in these ore bodies was directly substi- 

 tuted for the limestone. The irregularity of the course of the dissolving- 

 waters is everywhere perceptible in the ore bodies and caves. They have 

 every possible form and vary greatly in size, sometimes being but small 

 stringers and occasionally measuring upwards of a hundred feet in all di- 



