SALMON — ON KOCKS. 
55 
(1.) Carbon, (6.) //e?n<i<?/e (Sesqui-Oxide or Per- 
(2.) Sidphur, Oiide of Iron), 
h.) Water, (7.) i?ocZ,-AS'ci;^ (Chloride of Sodium), 
(4.) Conmcbnn (Alumina), (8.) Fluor (Fluoride of Calcuini), 
(5.) Silica Quartz (Silicic Acid), (9.) Pyrite (Bi-Sulphide of Iron), 
all the other rock-forming minerals are essentially either — 
I. Silicates, III. Sulphates, or 
II. Carbonates, IV. Borates ; 
that is, combinations of silicic acid, carbonic 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 them 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 sulphates 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 
( Ba S), 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 cai'bonates are those of Lime, Magnesia, and Iron. 
The first, in the form of Calcite (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. Magnesife (Mg C), a cai'bonate of magnesia, is 
neither an abundant nor important mineral in its pui-e 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, Chalybite (FeC), is not 
