BRIEF OUTLINE OF RESULTS. 



From tho year 1S69 up to the present time (1883) Eureka District has produced about $60,000,000 gold and silver, and 

 about 225,000 tons of lead. Owing to the fact that the deposits of this district have been more completely developed than 

 any other of a similar character on the Pacific slope, they offer very complete opportunities for the scientific investigation of 

 the phenomena attending this class of deposits. In several respects they resemble those of Leadville, Colorado. 



The structure of Prospect Mountain and Ruby Hill, the principal mining localities in the district, is explained in 

 detail in Chapters III. and IV. of this report. The investigation here described has resulted in showing that the dominant 

 factor of the structure of Ruby Hill is an extensive fault which has determined the present relations of the formations, the 

 aptitude of the ground for ore deposition, the ingress of ore-bearing solutions, and the fissure system by which the ore 

 bodies are connected. The presence of this fault, which has been called the Ruby Hill fault, is marked by a fissure filled 

 in places with rhyolite. 



The sedimentary beds of the district are of the Cambrian, Silurian, Devonian, Carboniferous, and Quaternary periods. 

 Hitherto no deposits of value, with one exception in the quartzite of the Silurian, have been found outside of the limestones 

 of the Cambrian and Silurian, and they have been mostly confined to the limestones of the former age, though the deposition 

 of the ore took place without doubt in Tertiary or post-Tertiary times. The igneous rocks of the district occurring near 

 the mines are granite, quartz-porphyry, hornblende- and augite-andesites, rhyolite, and basalt. Granite-porphyry anddacite 

 occur, however, in this region. 



The ore above the water-level is principally composed of the minerals galena, anglesite, cerussite, mimetite, and 

 wulfenite, with very little quartz and calcite; the gaugue being foi the most part hydrated oxide of iron. The ore also 

 carries considerable gold and silver, and zinc is present probably as carbonate and silicate. Below the water-level the ore 

 is chiefly composed of pyrite, arseuopyrito, galena, blende, and a few other sulphides, aB well as silver and gold. 



The ore deposits themselves are very irregular in form, sometimes resembling lodes, sometimes "stocks," and 

 sometimes beds. Ore bodies of any size are always capped by caves or in some way connected with such openings in the 

 rock and with fissures. This connection of ore bodies with fissures is universal in the district. The caves were probably 

 formed since the deposition of the ore, partly by the action of water carrying carbonic acid, and partly by the shiinkage of 

 the ore caused by decomposition. Since this last action took place the ore has in many instances been redistributed bv the 

 flow of underground waters. The former presence of these waters is shown by the stratification of portions of the ore 

 bodies, and by traces of aqueous action exhibited by the surrounding limestone. 



It is likely that the constituents of the ore were derived from some massive rock by solution, the solutions bein " 

 due to the BOlfataric action incident to the eruption of large masses of rhyolite. They entered the limestone from below 

 through fissures, and the greater part, at least, of the ore was deposited by direct substitution for that rock. The lime- 

 stone was fissurod and crushed in many directions by the various faulting movements and gave free ingress to the ore- 

 bearing solutions, which followed the channels of least resistance and deposited the ore in masses of very irregular form. 



The assays of country rock show conclusively that the materials for the ore could not have been derived from any 

 of the sedimentary formations. The quartz-porphyry is the only igneous rock of the district in which anything but traces 

 of the precious metals has been found, and although it does not cover much ground on the surface it may be of much more 

 considerable extent, below. The results obtained from its examination point to it as the source of the ore in its neighbor- 

 hood, at least. The granite which probably underlies the formations of Prospect Mountain and Ruby Hill may also have 

 been a source of the ore, but if such is the case the extraction of the heavy metals from it has been very complete, as when 

 found on the surface it contains scarcely a trace of silver, gold, or lead. 



The process of determining tho presence of an ore body by means of exact assays of the surrounding limestone has 

 as yet led to no practical application, although the results obtained by this method of prospecting coincide in a remarkable 

 manner with tho electrical experiments made by Dr. Barns with a view to the same object. The methods used in assaying 

 are fully explained in the chapter on that subject. 



The chances of finding ore in the deeper workings of Ruby Hill are considered to be favorable, though the quality 

 and size of ore bodies cannot be predicted with certainty. 



(xiii) 



