CONSTITUENT ELEMENTS OF ROCKS. 53 



Unlike alumina, magnesia unites with carbonic acid, forming car- 

 bonate of magnesia (MgO,C0 2 ). 



(5.) Calcium. — The oxyd of the metal calcium is common quick- 

 lime. Like magnesia, it enters into various silicates ; and it also 

 forms a carbonate, carbonate of lime (CaO,C0 2 ), and this carbonate is 

 the material of limestones. Moreover, with sulphuric acid and water 

 it forms sulphate of lime, or gypsum. 



The peculiar position of lime in the system of nature is that of 

 a medium between the organic and inorganic world. Carbonate of 

 lime is soluble in water which holds a little carbonic acid in solu- 

 tion ; and both this and the sulphate are found in river, marine, and 

 well waters. It is made into shells, corals, and partly into bone 

 by animals, and then turned over to the inorganic world to make 

 rocks. Lime is, therefore, the medium by which organic beings aid 

 in the inorganic progress of the globe, as above stated: far the 

 greater part of limestones have been made through the agency 

 of life, either vegetable or animal. 



Lime also unites with phosphoric acid, forming phosphate of lime, 

 the essential material of bone, and a constituent also of other 

 animal tissues. Like the carbonate, this phosphate is afterwards 

 contributed to the rock-material of the globe, and is one source 

 of mineral phosphates. 



(C.) (7.) Potassium and sodium. — Potassium is the metallic base 

 of potash, and sodium of soda. The alkalies potash and soda, 

 besides some other oxyds, form glass or fusible compounds with 

 silica; and this fact indicates one of their special functions in the 

 earth's structure. Silica, alumina, and the pure silicates of alu- 

 mina are quite infusible ; but by the addition of the alkalies, or 

 the oxyds of iron or lime, fusible compounds are formed. And, 

 as the earth's early history was one of universal fusion, the alkalies 

 performed an important part in the process, as they have since in 

 all igneous operations. Feldspars, which are found in all igneous 

 rocks, are silicates of alumina with potash, soda, or lime. A heated 

 solution of potash or soda will also dissolve silica, and so aid in 

 distributing quartz or making silicates. 



Sodium is likewise the basis of common salt in sea-water. 



(8.) Iron. — Iron combines with oxygen and forms two com- 

 pounds, a protoxyd FeO, and a sesquioxyd Fe 2 3 , and one or the 

 other occurs along with alumina, magnesia, or lime in many sili- 

 cates, which are mostly fusible. Silica and magnesia or lime with 

 protoxyd of iron make part of the very abundant mineral horn- 

 blende, found in syenite, hornblendic slate, etc. ; and also the 



