as illustrative of Geologi^. 329 



oxides of metals and metallic salts, found chiefly in veins, which 

 have been and still continue to be produced, sometimes by the 

 immediate abstraction of oxygen from air or water, sometimes 

 by decomposition of combinations of the metals with the 

 metalloids. 



If we consider the rocks of which the larger masses of the 

 earth's crust, formed by the influence of heat, consists, we shall 

 find that, notwithstanding their variety, they are composed of 

 but hvr different substances. In this respect the following are 

 of the greatest importance : silica, alimiina, magnesia, lime, po- 

 tash, soda, iron, and manganese. As far as quantity is con- 

 cerned, silica is by far the most prevalent : to it succeeds alu- 

 mina, and compared with these the other component parts are 

 on the whole but inconsiderable. Hence it is proved in what 

 manner the mass in general was composed, out of which the 

 above mentioned substances were formed, by the grand process 

 of oxydation of the crust of the earth. If we compare the Plu- 

 tonic with the volcanic rocks, of which the former exhibit de- 

 cided proofs of earlier formation, we recognise in their compo- 

 sition a grand difference, which consists in this circumstance, 

 that in a large portion of the latter class there exists a much 

 greater quantity of oxide of iron, and a much smaller quantity 

 of silica, than in those which form the chief mass of the former. 

 In the Plutonic rocks, the great prevalence of siHca is displayed 

 not only in the very general appearance of quartz, but also in the 

 great extension of the higher kinds, the bisilicate and the trisilicate. 

 In the volcanic rocks, on the other hand, quartz seldom ap- 

 pears as an essential constituent part. Besides, the higher sili- 

 cates are found also simple ones, sometimes in considerable 

 quantity ; and not only does iron, in various states of oxida- 

 tion, enter in greater abundance into the combination of the 

 silicates, but it displays itself also far more generally diffused, 

 and in much larger quantities than it does in these, partly as 

 oxide-oxidal and oxide individually — partly in union with titanic 

 acid. If it may now be assumed that the process of oxidation 

 of the earth's crust advances on the whole from the surface to the 

 centre, and that, consequently, the rocks of a later creation have 

 arisen from the oxidation of a mass .which was originally further 

 removed from the actual surface than that mass out of which 

 those of an earlier cicalion have proceeded, the consequence is at 



