56 



THE GEOLOGIST. 



an abundant rock-mineral, althongli it is a considerable ingredient in 

 some rock formations. 



Summing up these minerals, and arranging them in their order of 

 importance, we find that all the carbonates, sulphates, and borates 

 only produce the eight following rock-forming minerals j and even of 

 these, five are unimportant in quantity: — 



(1.) Calcite (Carbonate of Lime). (5.) ^4«/iyc?Wfe (Sulphate of Lime). 

 (2.) J/(7_^»es/^(S (Carbonate of Mag- (6.) 6-'yj:>s?^??i (Hydrated Sulphate 



nesia). of Lime). 



(3.) (Carbonate of Iron). (7.) ^arz/^^' (Sulphate of Baryta). 



(4.) i)oZo??i2Y(? (Carbonate of Lime (8.) ^oraaVe (Borate of Magnesia). 



and Magnesia), 



Hence, with the exception of these eight minerals, and the nine 

 minerals already enumerated, formed of elements and binary com- 

 pounds, in all 17, the whole of tlie other rock-forming minerals are 

 silicates. 



XVI I. Silicates. — The principal constituents of all rocks, with the 

 exception of Limestones, Mag-nesian Limestones, and Gypsums, are 

 silicated minerals. Indeed, the crystalline rocks, and many sedi- 

 mentaiy strata which constitute by far the greater part of the known 

 sui'face of the earth, consist chiefly of silicates ; and therefore a 

 knowledge of their chemical character becomes of the gi'eatest 

 importance.* 



Silicates may be either simple silicates, or compound silicates. A 

 simple silicate is a combination of the acid with one single base, un- 

 combined with any other ; a compound silicate is a compound between 

 the acid and two or more bases combined. 



XVIII. Simple Silicates. — The simple silicates are of much less 

 geological importance than the compound, as they occiu' much less 

 ft'equently. Of the earths, we have simple silicates of Alumina, 

 Lime, and Magnesia, but not of Baryta. The anhydrous sihcates of 

 Alumina, such as A)idalusite and Kyanite (Al Si), are by no means abun- 

 dant minerals ; indeed they are less so than the simple hydrous 

 silicates, such as Kaolin (Al Si -j- ), which are more important, inas- 

 much as they form the base of all clays which have resulted from 

 the decomposition of ancient folspathic rocks. The simple silicates 



* Bischof, ii. S2. 



