50 LITHOLOGICAL GEOLOGY. 



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

 trituration or wear of other rocks, or the material from chemical 

 deposition, 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, as follow : — 



1. The elements constituting rocks. 



2. The mineral material constituting rocks. 



3. The kinds of rocks. 



1. ELEMENTS CONSTITUTING ROCKS. 



52. General considerations. — In the foundation-structure of the 

 globe firmness and durability are necessarily prime qualities, 

 while in living structures instability and unceasing change are 

 as marked characteristics. 



These diverse qualities of the organic and inorganic world proceed 

 partly from the intrinsic qualities of the elements concerned in each. 



In the inorganic kingdom, — 



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

 essential ingredients of rocks are mainly hard and refractory sub- 

 stances : as, for example, silicon, the basis of quartz ; aluminium, 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. The metals potassium and sodium burn if put in 

 contact with water, and become oxyds. They are made into these 

 stable oxyds, potash and soda, before entering as ingredients into 

 rocks. Calcium also becomes lime, — one of the most refractory of 

 substances ; and magnesium magnesia, — even more refractory than 

 lime. Silicon unites with its full allowance of oxygen in order to 

 form quartz, the most abundant compound in the mineral kingdom 

 and the least liable to change. Aluminium combines with a satu- 

 rating quantity of oxygen to form alumina, the constituent of sap- 

 phire and emery, the characterizing ingredient of clay, and hardly 

 less universal than quartz. 



