56 
THE GEOLOGIST. 
an abundant rock-mineral, although 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 ; and even of 
these, five are unimportant in quantity : — 
(1.) Calcite (Carbonate of Lime). (5.) ^?i7«?/rZri^e (Sulphate of Lime). 
(2.) il/a(/«e«ife (Carbonate of Mag- (0.) ^'y^jswrn (Hydrated Sulphate 
nesia). of Lime). 
(3.) C/irt/j'/6«<e (Carbonate of Iron). (7.) 5ary<c (Sulphate of Baryta). 
(4.) Z>o^oww<e (Carbonate of Lime (8.) ^oraciVe (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 tJte other rock-formmj minerals are 
silicates. 
XVII, Silicates. — The principal constituents of all rocks, with the 
exception of Limestones, Magnesian Limestones, and Gypsums, are 
silicated minerals. Indeed, the crystalline rocks, and many sedi- 
mentary strata which constitute by far the greater part of the known 
surface 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 occur much less 
frequently. Of the eai-ths, we have simple silicates of Alumina, 
Lime, and Magnesia, but not of Baryta. The .inhydrous silicates of 
Alumina, such as Andalusite and Kyanite (Al Si), are by no means abun- 
dant minerals ; indeed they are less so than the simple hydrous 
silicates, such s)i& Kaolin (Al Si -|- H^), which are more important, inas- 
much as they form the base of all clays which' have resulted from 
the decomposition of ancient fclspathic rocks. The simple silicates 
Bischof, ii. 82. 
