GENERAL CHEMICAL CONSTITUENTS OF ROCKS. 259 



silica, usually opaque, and variously coloured : the sub- 

 stance next in abundance is carbonate of lime ; then com- 

 binations of silicic acid with alumina, potash, and soda, 

 with lime, magnesia, and oxide of iron. The substances to 

 which we give the generic name of rocks are definite associa- 

 tions of a small number of minerals, to which some other mine- 

 rals attach themselves as it were parasitically, but always under 

 definite laws. These elements are not confined to particular 

 rocks : thus quartz (silicic acid), feldspar, and mica, are the 

 substances which, in their association, essentially constitute 

 granite ; but they are also found in many other rock forma- 

 tions, either singly or two of them combined. A single ex- 

 ample will suffice to shew how the proportions of these 

 elements may vary in different rocks, and how quan- 

 titative relations distinguish a feldspathic from a micaceous 

 rock. Mitscherlich has shewn, that if we add to feldspar 

 three times the quantity of alumina, and one-third of the 

 proportion of silex which previously belonged to it, we obtain 

 the composition of mica. Both these minerals contain 

 potassium; a substance of which the existence in many 

 kinds of rock was no doubt anterior to vegetation. 



The succession and the relative age of different forma- 

 tions are traced, partly by the order of superposition of sedi- 

 mentary strata, of metamorphic beds, and of conglomerates, 

 and partly by the nature of the formations which the erupted 

 rocks have reached or traversed, but most securely by the pre- 

 sence of organic remains and their diversities of structure. 

 The application of botanical and zoological evidence in deter- 

 mining the age of rocks, and in fixing points in the chro- 



logy of the crust of the globe, which the genius and 



