Paek. — Thermal Activity and Metalliferous Veins. 31 



formation, which in turn is capped by a flow of basalt. The 

 sandstones and slates are broken and fissured in such a way as 

 to form a breccia. The interspaces are filled partly with a still 

 soft or already indurated siliceous paste, containing finely 

 disseminated metallic sulphides, and partly with cinnabar, for 

 the most part in coherent crusts.* In the same mine the 

 basalt is reduced to a porous mass, and traversed by irregular 

 fissures filled with sulphur and cinnabar. ] Hot mineral 

 water and gases carrying H 2 S force their way through the 

 interstices of the deposit in the fissured sandstones and slates. 



The silica-deposits are found in all stages of consolidation, 

 from a gelatinous mass to chalcedony, and alternate with 

 layers of metallic sulphides, consisting of cinnabar and 

 pyrites. 



Unfortunately, no information is obtainable as to the 

 nature of the fresh-water formation lying between the Cre- 

 taceous sandstone and basalt. 



According to Becker, the hot water is rich in chlorides, 

 borax, and sodium-carbonate. The gases liberated from the 

 water cousisted of 893 parts of C0 2 , 2 parts of H 2 S, 79 parts 

 of marsh-gas (CH 4 ), and 25 parts of nitrogen, in 1,000 parts. 



According to Dr. Melville the marcasite associated with 

 the cinnabar contains traces of gold and copper ; and in the 

 efflorescence from the mine-workings Becker detected traces 

 of cobalt and nickel. 



In the upper zone only sulphur was found ; lower down 

 sulphur and cinnabar, and in depth cinnabar and pyrites 

 occurring upon or within deposits of silica. 



Steamboat Springs Cinnabar-deposits. 



The Steamboat Springs in Nevada also furnish important 

 evidence of vein-filling by thermal waters. They have been 

 fully described by Le Conte, } Becker, § and other writers. 



In a valley surrounded with eruptive rocks, and underlain 

 by altered sedimentaries believed to be of Archaean age, ther- 

 mal springs issue from several points from north-and-south 

 fissures. The floor of the valley is covered in places with a 

 sheet of calcareous sinter in which there are many fissures, 



*J. Le Conte, "On Mineral Veins now in Progress at Steamboat 

 Springs compared with the same at Sulphur Bank," Am. Jour, of 

 Science, vol. xxv, p. 404. 



t Prof. F. Posepny, "The Genesis of Ore-deposits," Trans. Amer. 

 Inst. Min. Eng., vol. xxiii, p. 197. 



\ J. Le Conte, " On Mineral Veins now in Progress at Steam- 

 boat Springs compared with the Same at Sulphur Bank," Am. Jour. 

 Sci., vol. xxv, p. 424. 



§ G. F. Becker, "Geologv of the Quicksilver deposits of the Pacific 

 Slope," U.S. Geol. Surv., Washington, 1888, p. 331. 



