CONSTITUENT ELEMENTS OE ROCKS. 49 



bathed or penetrated, and combine with it. This state of strong at- 

 traction for oxygen — for something not in the structure itself — is 

 the source of activity in the vital functions, and involves unceasing 

 change as the means of existence and growth, and a final dissolution 

 of the structure at the cessation of life. 



Hence, strength and durability belong to the basement-material of 

 the globe, and instability to living structures. 



But inorganic nature is still not without change. ' For there are 

 diversities of attraction among the elements and their compounds. 

 The changes are, however, slow, and not essential to the existence of 

 the compounds. The processes of solution, of oxidation and deoxid- 

 ation, and other chemical interactions, changes by heat, and other 

 molecular and mechanical influences, give a degree of activity even to 

 the world of rocks. But this topic belongs to the dynamics and 

 chemistry of geology. 



Characteristic elements. — The elements most important in rocks 

 are the following : — 



(1.) Oxygen. — Oxygen is a constituent of all rocks, and composes 

 about one-half by weight of the earth's crust. 



Sand is, by weight, more than half oxj'gen; quartz, the principal material of sand, is 

 about 53 percent, oxygen; common limestone, 48 per cent.; alumina, nearly 47 per 

 cent. ; feldspar, 46 to 50 per cent. ; common clay, 50 per cent. : and thus it is with the 

 various ordinary rocks. Besides, the atmosphere contains 23 per cent, of oxygen, and 

 water — the material of the oceans, lakes, and rivers — 89 per cent. 



(2.) Silicon. — After oxygen, silicon is the element next in abun- 

 dance, constituting at least a fourth of the earth's crust. It is un- 

 known in nature in the pure state ; but, combined with oxygen, and 

 thus forming silica, or quartz, it is common everywhere. This silica 

 is an acid, although tasteless ; and its combinations with alumina, 

 magnesia, lime, and other bases (called silicates), along with quartz, 

 are the principal constituents of all rocks except limestones. Silica 

 constitutes about 60 per cent, of these ingredients ; and, including the 

 limestones, 50 per cent, of all rocks. Silicon has therefore the same 

 prominent place in the mineral kingdom as carbon in the organic. 



Granyte and gneiss are nearly three-fourths silica, — half of it as pure quartz, and 

 the rest as silicates; mica schist and roofing-slate are about two-thirds silica; trap and 

 lavas are one-half; porphyry, two-thirds; sandstones are sometimes all silica, and usu- 

 ally at least four-fifths. 



Silica is especially adapted for this eminent place among the arch- 

 itectural materials of the globe by its great hardness, its insolubility 

 and resistance to chemical and atmospheric agents, and its infusibility. 

 As it withstands better than other common minerals the wear of the 

 waves or streams, besides being very abundant, it is the prevailing 

 4 



