194 ON THE ORIGIN, MATERIALS, COMPOSITION, 



pound, also enter into their structure. Such frag- 

 ments vary in size, from the most minute visible 

 particles to others of many pounds weight, or even 

 hundreds; and these rocks offer, in consequence, 

 numerous varieties which are fully treated of in the 

 author's Classification of Rocks. 



The earths which produce the minerals that form 

 the ordinary or essential ingredients of rocks, are 

 silica, alumina, lime, and magnesia. If the others are 

 occasionally found, it is rather in those minerals which 

 cannot be considered essential to the constitution of 

 rocks, but which are frequently imbedded in them. 

 To these earths must be added iron in different states 

 of oxydation, and, from some observations which I 

 have made, in that of a carbonat also. Potash and 

 Soda are, lastly, essential ingredients in some rocks; 

 and it remains to be proved whether Lithion may 

 not sometimes be present where one or other of these 

 has been suspected. As the earths, as well as the 

 alkalies, are now known to be oxydes, and as it is also 

 known that silica, at least, acts the part of an acid in 

 some mineral combinations, it is probable that we 

 have much yet to learn respecting the origin and for- 

 mation of many rocks: but whatever splendid pro- 

 balities may open on us from this new source of 

 knowledge, we are scarcely yet able to build any 

 rational conjectures on it. 



The simple minerals formed of these substances, 

 and which constitute the essential ingredients of all 

 rocks, are quartz, felspar, mica, hornblende, hyper- 

 sthene, diallage, augit, serpentine, compact felspar, 

 actinolite, chlorite, talc, and schorl. Some of these 

 are, however, far more abundant than others ; nor is 

 it easy to define the limit between them, and those 

 which mav be considered accidental, or which are 



