THE LESSER METALS, CONTINUED. 273 



TIN. 



2.15.13. Ores : Cassiterite, SnO 2 , Sn. 78.67, O. 21.33. The sul- 

 phide stannite is a rather rare mineral. 



Cassiterite occurs in small stringers and veins on the borders 

 of granite knobs or bosses, either in the granite itself or in the 

 adjacent rocks, in such relations that it is doubtless the result of 

 fumarole action consequent on the intrusion of the granite. It 

 appears that the tin oxide has probably been formed from the 

 fluoride. A favorite rock for the ore is the so-called greisen, a 

 mixture of quartz and muscovite or lithia mica, and probably an 



FIG. 67. Horizontal section of the Etta knob. After W. P. Blake, Min- 

 eral Resources, 1884, p. 602. 



original granite altered by fumarole action. Topaz, tourmaline, 

 and fluorite are found with the Cassiterite, indicating fluoric and 

 boracic fumaroles. Cassiterite seems also to crystallize out of a 

 granite magma with the other component minerals. Cassiterite, 

 being a very heavy mineral, accumulates in stream gravels, like 

 placer gold, affording thus the stream tin. When of concentric 

 character it is called wood tin. It is not yet demonstrated that 

 the United States have workable tin mines. 



2.15.14. Example 51. Black Hills. Knobs of granite rock 

 containing cassiterite, disseminated in a mass of albite and mica, 

 and associated with immense crystals of spodumene. Columbite, 

 tantalite, and beryl are also found. There are two granite knobs 

 which are best known, the Etta and the Ingersoll. The former is a 

 conical hill, 250 feet high by 150 feet by 200 feet, piercing mica and 

 garnetiferous slates. Tunnels show it to have a concentric struct- 

 ure first, a zone of mica ; second, a zone of great spodumene 

 crystals, with an albitic, so-called greisen and cassiterite in the 



