CONSTITUENT ELEMENTS OF ROCKS. 51 



In organic nature, on the contrary, — 



(1.) The essential elements are combustible substances, and mostly 

 gases, — oxygen combined with carbon and hydrogen forming plants, 

 and oxygen with carbon, hydrogen, and nitrogen forming animal 

 substances. 



(2.) The elements in living beings, moreover, are not saturated 

 with oxygen : they are therefore in an unstable and constrained 

 condition. Both from their nature and their peculiar condition, 

 they have a strong tendency to take oxygen from the atmosphere 

 with which they are bathed or penetrated, and combine with 

 it. This state of strong attraction 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 cessa- 

 tion 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 oxydation and 

 deoxydati'on, 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. 



53. 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 oxygen ; quartz, the principal material 

 of sand, is about 53 per cent, 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 atmo- 

 sphere 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 

 unknown 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 alu- 

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

 quartz, are the principal constituents of all rocks except the lime- 

 stones. Silica constitutes about 60 per cent, of these ingredients ; 



