54 SILVER-LEAD DEPOSITS OF EUREKA, NEVADA. 



and in yellow masses more or less mixed with sulphate. The occur- 

 rence of crystals is rare, but the "yellow carbonate" often contains consid- 

 erable quantities of this mineral. As the galena which has been found in 

 the mines of Ruby Hill rarely contains much arsenic, it is not likely that 

 mimetite was formed through the oxidation of galena alone, but that it re- 

 sulted from the simultaneous decomposition of that mineral and arseno- 

 pyrite. This is made probable by the fact that the "yellow carbonate," a 

 widely distributed ore, although it is sometimes composed of sulphate of 

 lead and hydrated oxide of iron alone, is usually a mixture of sulphate of 

 lead, chloro-arsenate of lead, and hydrated oxide of iron. If the "yellow 

 carbonate " resulted from the decomposition of arsenical galena alone it 

 would not contain the hydrated oxide of iron except as an admixture. That 

 the iron is not always an ingredient resulting from a subsequent mixture of 

 the products of oxidation is shown by fragments here and there in the mass 

 which retain the original structure of the minerals which composed them. 



This mimetite has been found in the form of stalactites, stalagmites, 

 and in columns in vuggs in some of the ore bodies. It occurs as minute 

 hexagonal crystals surrounding a core of some brown mineral, which is 

 probably limonite. The vugg in which the specimen belonging to the 

 collection was found occurred in the upper part of an ore body, which 

 was distinctly stratified, indicating that the material composing it had 

 been re-arranged since it was oxidized. The minerals in the interior of the 

 ■vugg had evidently been crystallized from solutions since the rearrange- 

 ment of the ore. The manner of formation of these stalactites, etc., seems 

 to be plain. The arsenopyrite, pyrite, and galena, which formed the orig- 

 inal ore, were oxidized, sulphate of iron being first formed. This sulphate 

 of iron trickled down, forming numerous columns, upon which the later 

 product of decomposition, mimetite, was afterwards deposited. In time the 

 sulphate of iron lost its sulphuric acid and became limonite, which remained 

 as a core. 



Wulfenite (molybdate of lead) is of frequent occurrence in the ores 

 of Ruby Hill. It is found as aggregates of fine tabular crystals coating 

 nodules of galena changed into sulphate and carbonate, and frequently 

 mixed with crystals of the latter as well as in minute crystals disseminated 



