CHAPTER VII. 



THE SOUKCE OF THE ORE. 



Theories in regard to the formation of ore deposits. One of the lHOSt important inquiries 



connected with the geology of the Eureka District relates to the source of 

 the ore, for a successful solution of this problem would afford information 

 valuable in the search for further deposits, besides possessing great scientific 

 interest. A discussion of the various theories which have been held in regard 

 to the formation of ore deposits in general, some of which might be found 

 applicable to the Eureka deposits, cannot find a place in this report, but it 

 may be well to mention the only solutions of this problem which are in any 

 way warranted by the facts which have been observed. These are: First, a 

 deposition of the ore in small particles simultaneously with the limestone, 

 the ore being afterward segregated into nearly isolated bodies, either by 

 chemical or mechanical action; second, a segregation of the ore in the 

 limestone from the country rock on either side of it; and, third, a deposition 

 from solutions which came from below. 



Relative time at which the minerals were deposited. Although the periods at which tll6 



different minerals which compose the ore bodies were brought into their 

 pi'esent position may have been separate and distinct, it is highly improb- 

 able that such has been the case, and it is not likely that some of them 

 should have been segregated from the country rock, and others either have 

 been washed in from above or brought up in solutions from below. Evi- 

 dence against this last supposition is not plentiful in the oxidized ore masses, 

 where the original position of the ore has often been changed by the flow 

 of underground streams, although it can still be found in the least disturbed 

 portions of them. But in the undecomposed sulphuret ores there is clear 

 proof that the various minerals were deposited simultaneously. They occur 

 irregularly, but in about the same relative proportions throughout the mass; 

 they show no signs of concentric structure or of successive deposition, and 

 although this is not positive evidence that their sources were tbe same, yet 



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