100 SILVER-LEAD DEPOSITS OF EUREKA, NEVADA. 



solution and must have attacked the limestone. It is true that in many caves 

 the ore has the appearance of having been placed in the cup-shaped cavities 

 which are so common and which were evidently formed before the ore came 

 into them; but it must be remembered that much of the ore, especially in 

 the upper part of the caves, has been brought into its present position by 

 water. 



partially formed caves and ore chambers. — In a portion of the ground already de- 

 scribed, between the seventh and ninth levels of the Richmond mine, there 

 is a great deal of open country; there are no caves, however, and it does 

 not often show signs of the action of surface waters. The ground is shattered 

 and there are large fissures which are filled for the most part with bowlders 

 and fragments which have fallen into them. Around some of these bowlders 

 ferric oxide and ore are found, but these masses are not of any great 

 size. Some of the fragments are rounded off as if ore had been substituted 

 for their exterior parts, and the whole mass presents the appearance of an 

 ore chamber, the formation of which had been interrupted. From the 

 seventh level a distinct ore channel can be traced up to the west ore body 

 as well as downward to the ninth level. 



Effects of oxidation on the bulk of the ore bodies. The chemical reactions which took 



place when the ore was substituted for the limestone are unknown, but from 

 observations made in small masses of oxidized ore resulting from the decom- 

 position of sulphurets, that were evidently formed by substitution, it would 

 seem that the ore replaced the limestone very nearly bulk for bulk ; that is 

 to say, very nearly though not quite filled the space originally occupied by 

 the limestone. In the masses referred to there were no signs of the vigorous 

 action of water carrying carbonic acid, and the walls were compact; there- 

 fore it is not likely that these bodies had been disturbed since the ore was 

 deposited as sulphurets, and it is probable the slight shrinkage of the masses 

 was due to oxidation. In the large chambers, where there is a cave directly 

 over the ore body, the size of this cave has usually, but not invariably, 

 borne a direct relation to the size of the ore body, a large cave being fol- 

 lowed by a large mass of ore. This would indicate that the cave owed its 

 origin in a measure to the shrinkage of the ore. Whether the ore shrinks 

 or expands in oxidizing is a point which depends upon its composition. 



