CHAPTER IX. 



SILVER AND GOLD. INTRODUCTORY : EASTERN SILVER MINES 



AND THE ROCKY MOUNTAIN REGION OF NEW MEXICO 



AND COLORADO. 



2.09.01. The two " precious " metals are so generally associ- 

 ated that they cannot be separately treated. While endeavoring to 

 preserve the distinctive impression given by examples, it is practi- 

 cally impossible to set forth all the widely varying phenomena of 

 the silver-gold veins of the West in any other than an approxi- 

 mate way. Hence geographical considerations are placed first, and 

 where markedly similar ore bodies in different States are to be 

 grouped together cross references are given. The following gen- 

 eral examples have been made because their individual features 

 are based on those geological relations which are most vitally con- 

 cerned with questions of origin. 



2.09.0*2. Example 37. Veins containing the precious metals 

 usually with pyrite, galena, chalcopyrite, and less common sul 

 phides, sulpharsenides, sulphantimonides, etc., in igneous rocks. 

 No special subdivision is made on the character of the gangue, 

 which may be quartz, calcite, barite, fluorite, etc., one or all. The 

 first named is commonest. A great and well-defined original fis- 

 sure, is not necessarily assumed, but some crack, or joint, or crushed 

 strip must have directed the ore-bearing solutions, which may have 

 then replaced the walls in large measure. For other structural 

 features see the discussion of veins (1.05.01) ; compare also Ex- 

 ample 17, Butte, Mont. 



Example 37a. Replacements more or less complete of igneous 

 dikes, w^hich have usually been described as porphyry. Compare 

 Example l7a under '.' Copper " (Gilpin County, Colorado), and 

 Example 20cl (Santa Rita, N. M.). Ore and gangue (where the 

 matrix is not 'the dike rock) as in Example 37. 



Example 38. Contact deposits between two kinds of igneous 

 rock or between two different flows. Ore and gangue as in Ex- 

 ample 37. 



