NATUKE OF ORES. 311 



unaltered sulphide. Assuming it to be correct that the ores were originally 

 deposited as galena and pyrites, it is most difficult to see how such vast accu- 

 mulations of these sulphides could have been formed in any other way than 

 in the preexisting caves and openings. Any theory with which we are 

 acquainted of chemical and physical replacement of the limestone or dolo- 

 mite seems wholly inadequate to meet the necessary conditions. Pseudo- 

 morphs of galena and pyrites after calcite have been described as minera- 

 logical curiosities and possibilities, but nowhere have they been found in large 

 quantities in any mine, and so far as the writer is aware they have never 

 been recognized at Eureka. On the other hand, underground drainage 

 channels probably existed before the deposition of the ore bodies, and with 

 the coming in of the ascending mineral currents it is most natural that they 

 should have followed these channels in their upward course. 



The Ores. After the deposition of the metallic sulphides came the period 

 of oxidation, which probably continued throughout the greater part of 

 Quaternary time and was due to atmospheric agencies, mainly percolating 

 surface waters. On Ruby Hill this oxidation may be said to be nearly 

 complete, unaltered galena and pyrites being exceptional occurrences above 

 water level. It has produced a great variety of secondary minerals, but 

 such as it might be anticipated would follow the complete alteration of an 

 admixture of argentiferous galena and auriferous pyrites accompanied by 

 compounds of arsenic and molybdenum. Mr. Curtis has devoted consider- 

 able time to an investigation of the miueralogical character of the ore and 

 has published a catalogue of the minerals known to occur on Ruby Hill. 

 The secondary products of oxidation include a long list of carbonates, 

 sulphates, arseniates, molybdates, and chlorides. The ore is exceptionally 

 rich in gold. Wulfenite occurs in brilliant transparent crystals, varying in 

 color from lemon-yellow to bright orange, and is found in large clusters 

 filling cavities or incrusting other minerals. Since the opening of the 

 mines the wulfenite of Eureka has been much sought after by mineral- 

 collectors both in this country and in Europe. It appears to be the only 

 species which Ruby Hill has developed that has any exceptional value from 

 a mineralogical point of view. 



The following analysis of a sample of all the ores smelted at the 



