SALMON — OX ROCKS. 



55 



(1.) Carbon, (6.) Hematite (Sesqui-Oxide or Pcr- 

 (2.) Sulphur, Oxide of Iron), 



h.) Wafer, (7.) i.V^- .Sa'^ (Chloride of Sodium), 



(4.) Conindum (Alumina), (8.) Fluor (Fluoride of Calcuim), 



(5.) SUka Quartz (Silicic Acid), (9.) Pyrite (Bi-Sulphide of Iron), 



all the other rock-forming minerals are essentially either — 



I. Silicates, III. Sulphates, or 



11. Carbonates, IV. Borates ; 



that is, combinations of silicic acid, carhonic acid, sulphuric acid, or 

 boracic acid, with the earths, alcalies, and oxides given in the table in 

 XIII. Of these, as already stated, the silicates are by far the most 

 abundant ; the carbonates come next ; then the sulphates : the 

 borates are insignificant. In examining in detail into the minerals 

 formed by each of these acids, it will be convenient to reverse the 

 order given above, and first dispose of the least important. We shall, 

 consequently, take tliem in the following order : — borates, sulphates, 

 carbonates, silicates. 



XVI. Borates, Sulphates, Carbonates. — The only essential borate 

 which we find as a rock- forming mineral — and it only rarely — is 

 Boracite (Mg-^ B^), a borate of magnesia. But boracic acid also occurs 

 as an essential constituent of the important mineral Tourmaline, 

 averaging about 9 per cent. 



The only sulj)hates forming rock-minerals are those of Lime and 

 Baryta. Anhydrite (CaS) and Gypsum (Ca S -\- H^) are respectively 

 an anhydrous, and a hydrous sulphate of lime. Gypsum is a most 

 abundant rock, occurring in many sedimentary formations. Baryte 

 (BaS), or heavy-spar, is a sulphate of Baryta ; it is a rare ingredient 

 in rocks, and indeed is more a vein-forming than a rock-forming 

 mineral. 



The rock-forming carbonates are those of Lime, Magnesia, and Iron. 

 The first, in the form of Ccdcite (Ca C), carbonate of lime, is the base 

 of all limestones and chalks, and is the most abundant of any mineral 

 not being a silicate. 2Iagnesiie (Mg C), a carbonate of magnesia, is 

 neither an abundant nor important mineral in its pure state ; but in 

 combination with carbonate of lime, it forms the mineral Dolomite 

 (CaC, MgC), or bitter-spar, which is the base of the large formation 

 of magnesian limestone. Carbonate of iron, Chalyhite (FeC), is not 



