66 SILVEK-LEAD DEPOSITS OF EUREKA, NEVADA. 



tions or otherwise, and that substitution of minerals for country rock played 

 no important part. With regard to the age of the country rock and the 

 occurrence of limonite with the galena, these deposits resemble those of 

 Eureka District, but in other respects, such as structure and manner of ore 

 deposition, they differ widely. 



Deposits of Missouri. — The lead deposits of Missouri and Arkansas also occur 

 in the dolomites of the Lower Silurian. The ore is galena, with but traces 

 of silver and gold. It is often accompanied by calamine (zinc silicate) and 

 limonite. It is found in nests in clay, also in beds as fine impregnations in 

 the calciferous limestone, as well as in flat masses between the strata. In 

 the flat veins cerussite occurs. Galena is also found in impregnations in 

 dolomite and in cavities and caves mixed with gravel, zinc ore, clay, and 

 barite. Calamine and some blende, as well as barite, almost always accom- 

 pany the galena. Quartz crystals are also found. The occurrence of these 

 deposits is very similar to those of the Upper Mississippi, but very dissim- 

 ilar to those of Eureka. 



Deposits of Leadviiie. — The deposits of Leadville, Colorado, resemble those 

 of Eureka in a great many respects, as can be seen upon reference to an 

 "Abstract of a Report upon the Geology and Mining Industry of Lead- 

 ville, Colorado, by S. F. Emmons."" Mr. Emmons states that the investiga- 

 tions made in Leadville have proved the following facts: 



"As regards their origin — 



"I. That they have been derived from aqueous solutions. 



"II. That these solutions came from above. 



"III. That they derive their metallic contents from the neighboring 

 eruptive rocks. 



"IV. That in their original form they were deposited not later than the 

 Cretaceous period. 



"As regards their mode of formation — 



"I. That the metals were deposited from their solutions mainly as sul- 

 phides. 



"II. That the process of deposition of the vein-material was a chemi- 



° Second Annual Report of the Director of the U. S. Geological Survey, 1881. 



