SALMON — ON ROCKS. 



233 



and 1 atom of protoxide of iron. Fe Fe. An important and widely 

 distributed rock-constituent. 



1 1. Rock-salt. Na CI, but often mixed with many impurities. A most 



widely distributed mineral Haloid. 



12. Fluor, Fluor-spar. Ca F. A haloid mineral, more a vein- than a 



rock-constituent. 



13. Pyrtte, Iron-pyrites. Fe. 



/ /// 



14. Pyrrhotine, Magnetic pyrites. Fe^ Fe. Unimportant as a rock- 



mineral. 



B. Carbonates, Sulphates, and Borate. 



1.5. Calcite, Calc spar, Arrayonite. Ca 0'. This important mineral has 

 many varieties. The transparent one is Iceland-spar. Arragonite is 

 strictly a different mineral, although identical in chemical compo- 

 sition : it is not a rock-forming mineral. Haloid. 



16. Magnesite. IVTg 0. A haloid mineral. Unimportant. 



17. Dolomite, Bitter-spar. Ca C Mg C. Haloid mineral ; very important, 



forming the base of whole mountain-masses of IMagnesian Limestone. 



18. Chalabite, Siderite, Spai-ry-iron. J'e 0. An important ore of iron, 



but always mixed with some impurities. 



19. Anhydrite, Karstenite. Ca B". Haloid mineral. Occurs generally 



with gy|)sum. 



20. Gypsum. Ca S + IP. Haloid mineral ; very important ; the most 



abundant of the sulphates. The transparent variety is Selenite ; and 

 the white compact variety. Alabaster. 



21. Baryte, Heary-spar. Ba S. Haloid mineral ; more vein- than rock- 



forming. Not to be confounded with Witherite, Carbonate of 

 Baryta. 



22. BoRACiTE. ]\Ig^ An unimportant rock-mineral. Haloid. 



C. Silicates. 



23. OrtiiocTjASE, Potash-felspar ; Adularia ; Sanidine [.33^]. Is the most 



important felspar species, it Si^-f Al where K = 15 per cent. ; 

 but it always contains more or less Na. The transparent variety is 

 Adularia; Sanidine, or glassy felspar, is characteristic of the volcanic 

 rocks, and contains more Na than Orthoclase. This felspar de- 

 composes into Molin ; the alteration consists in the removal of the 

 alkalies together with a portion of the Si, while water is introduced, 

 producing a hydrated silicate of M. The gradual decomposition of 

 the original orthoclase is indicated by the increase of the O.Q., 

 which in Kaolin reaches .75. 



24. Leucite, Amphigene [.5]. Essentially K S'i + Al Si^ K (21) ; gene- 



rally contains some Na. This mineral is almost exclusively found in 



