THE ORE DEPOSITS. 65 



of argentiferous galena with antimonial and arsenical combinations and 

 pyrite and the decomposition pi-oducts derived from these minerals. Com- 

 pounds of copper, zinc, and other metals occasionally accompany these 

 ores. In by far the greater number of instances the oxidation has been 

 tarried to a great depth, sometimes reaching or exceeding 1,000 feet. The 

 extent of this oxidation is in a great measure due to the absence of any 

 large quantity of water until considerable depth is reached. The charac- 

 teristic gangue of these ores is the hydrated oxide of iron with more or less 

 calcite. Quartz is rarely found in any great quantity, except where the 

 deposits occur in the form of contact lodes between limestone and por- 

 phyry, the quartz being probably derived from the decomposition of the 

 porphyry. An example of such lodes is offered by the 2 Gr. mine in Tybo 

 District, Nye County, Nevada. It is found to be more profitable to reduce 

 all these ores by smelting than by any other process. Among the princi- 

 pal districts where such ores are found may be mentioned Eureka, White 

 Pine, and Bristol, in Nevada; Cerro Gordo, in California, and Ophir, Big 

 Cottonwood, and Little Cottonwood, in Utah — all of which occur in Pal- 

 aeozoic rocks, though in some cases, at all events, the deposition of ore is 

 referable to a much more recent era. 



Deposits of the upper Mississippi. — The lead deposits of the Upper Mississippi" 

 occur in dolomites of the Lower Silurian. The ore is found in caves, in 

 openings between the strata, and in so-called gash veins. The stratification 

 of the country rock is flat, and it shows scarcely any signs of dynamic dis- 

 turbance or alteration through chemical causes even in the neighborhood of 

 the ore. The lead is found almost always in the form of galena, accompa- 

 nied by limonite and occasionally smithsonite (zinc carbonate) and blende, 

 rarely by pyrite. Calcite and barite occur, but quartz and combinations of 

 lead with arsenic are not met with. The galena contains but traces of sil- 

 ver and gold. There are no signs of fissure veins and the ore is not found 

 at any great depth below the surface. From the occurrence of the galena 

 in the forms of stalactites and stalagmites in the caves, and the absence of 

 decomposition in the country rock near the deposits, it is evident that the 

 openings were first formed and the ore was deposited in them from solu- 



"J. D. Whitney. Metallic wealth of the United States. 

 2654 L 5 



