SALMON — ON ROOKS. 
333 
and 1 atom of protoxide of iron, f c Fc. An important and widely 
distributed rock -constituent. 
11. Rook-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. Pyeite, Iron-pyrites, t'c. 
I HI 
14. Pyrrhotine, Magnetic pyrites. Fe= Fe. Unimportant as a rock- 
mineral. 
B. Carbonates, Sulphates, and Borate. 
15. Calcite, Calc spar, Arrayonite. Ca C. 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. ]VIg C. A haloid mineral. Unimportant. 
17. TioiiOWiT'E, Bitter-spar. Ca C Mg C. Haloid mineral ; very important, 
fonning the base of whole mountain-masses of Magnesian Limestone. 
18. CnALABiTE, Siderite, Sparry-iron. Fe 0. An important ore of iron, 
but always mixed with some impurities. 
19. Anhydrite, Karstenite. Ca B. Haloid mineral. Occurs generally 
with gypsum. 
20. Gypsum. Ca 8 h- H^. Haloid mineral ; very important ; the most 
abundant of the sulphates. The transparent variety is Selenite ; and 
the white compact variety, Alabaster. 
21. Baryte, Eeaiy-spar. Ba S. Haloid mineral ; more vein- than rock- 
forming. Not to be confounded with Wither ite, Carbonate of 
Baryta. 
22. Boracite. Jig' B*. An unimportant rock-mineral. Haloid. 
C. Silicates. 
23. Oviii\ocijA.B^, Potash-felspar ; Adularia ; Sanidine [.333]. Is the most 
important felspar species. K Si'-f Al SP where = 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 kaolin ; 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 Al. The gradual decomposition of 
the original orthoclase is indicated by the increase of the O.Q., 
which in Kaolin reaches .76. 
24. Leuoite, Amphigene [.6]. Essentially K Si ■¥ Al gi^. K (21) ; gene- 
rally contains some Na. This mineral is almost exclusively found in 
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