200 KEMP'S ORE DEPOSITS. 



Example 39. Agglomerates of rounded, eruptive boulders, 

 bombs, etc., in abandoned volcanic necks or conduits, and coated 

 with ores. The mines of Ouster County, Colorado, are the only 

 examples of ore deposits of this kind yet identified. 



Example 40. Contact deposits between igneous and sedimen- 

 tary rocks. No subdivisions are made on the kind of rocks. Ore 

 and gangue as in Example 37. Compare also Example 20, " Ari- 

 zona Copper ;" Example 2 la, " Triassic Copper;" Example 30, 

 " Leadville ; " and Example 30^, " Horn Silver Mine." 



Example 41. Veins in sedimentary rocks, generally cutting the 

 bedding, but at times parallel with it. Lateral enlargements are 

 frequent. The ore body may be largely due to replacement. Ore 

 and gangue as in Example 37. 



Example 42. Veins cutting both sedimentary and igneous 

 rocks, and therefore due to disturbances after the intrusion of the 

 latter. Ore and gangue as in Example 37. 



No special examples are made for metamorphic rocks. 



2.09.03. Silver minerals. 



Ag. S. As. Sb. Cl. 



Native silver 100. 



Argenite (silver glance), Ag 2 S 87.1 12.9 



Prousite (light ruby silver), 3Ag 2 S.As 2 S 3 .. 65.5 19.4 15.1 



Pyrargerite (dark ruby silver), 3Ag 2 S.Sb 2 S 3 . 59.8 17.7 22.5 



Stephanite (brittle silver), 5 Ag 2 S.Sb 2 S 3 68 . 5 16 . 2 15.3 



Cerargerite (horn silver), AgCl 75.3 24.7 



Silver also occurs with galena (Cf. "Lead") and with tetrahe- 

 drite (Cf. " Copper "). Gold occurs combined with tellurium in a 

 few rare tellurides, mechanically mingled with pyrites, and as the 

 uncombined native metal. From a metallurgical point of view the 

 ores of the precious metals are divided into two classes. 1. Those 

 whose amount of precious metal amalgamates readily with mer- 

 cury, and is thus obtained with comparative ease the free milling 

 ores. 2. Those which require roasting or some previous treatment 

 before amalgamation, chlorination, or similar process, or which 

 must be smelted primarily for lead or copper, from which the 

 precious metals are afterward extracted the rebellious ores. In 

 the subsequent description the endeavor has been made to work 

 from the distinctively silver mines to those of gold where geo- 

 graphically possible. 1 



1 Ann. Reps. Directors of the Mint. Rec. W. P. Blake, " The Vari- 



