48 LITHOLOGICAL GEOLOGY. 



(2.) The material of mineral origin includes all that is not directly 

 of organic origin, — all the sand, clay, gravel, etc., derived from the 

 trituration or wear of other rocks ; the material from chemical de- 

 position, like some limestones, or from volcanic action, like lavas and 

 trap or basalt. 



But, whether organic or mineral in origin, the material, when in the 

 rock, though sometimes under the form of fossils, is almost solely in 

 the mineral condition. The topics for consideration in connection 

 with this subject are, then, the following : — 



1. The elements constituting rocks. 



2. The mineral material constituting rocks. 



3. The kinds of rocks. 



1. ELEMENTS CONSTITUTING KOCKS. 



General considerations. — In the foundation-structure of the globe, 

 firmness and durability are necessarily prime qualities, while in liv- 

 ing structures, instability and unceasing change are as marked charac- 

 teristics. 



These diverse qualities of the organic and inorganic world pro- 

 ceed partly from the intrinsic qualities of the elements concerned in 

 each. 



In the inorganic kingdom (which includes minerals and rocks), — 



(1.) The elements which combine with oxygen to become the es- 

 sential ingredients of rocks, are mainly hard and refractory substances : 

 as, for example, silicon, the basis of quartz ; aluminum, the basis of 

 clay ; magnesium, the basis of magnesia. 



(2.) Or, if unstable or combustible elements, they are put into stable 

 conditions by combination with oxygen. Thus, carbon, which we 

 handle and burn in charcoal, becomes burnt carbon (that is, carbon 

 combined with oxygen, forming carbonic acid) before it enters into the 

 constitution of rocks. So all minerals are made of burnt compounds, 

 — called burnt because ordinary combustion consists in union with 

 oxygen and the production of stable oxyds. They are therefore dead 

 or inert in ordinary circumstances, and hence fit for dead nature. 



In organic nature (or, plants and animals) 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 sub- 

 stances. Nitrogen is present only very sparingly in plant3. 



(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 



