CONSTITUENT ELEMENTS OF ROCKS. 51 



Lime is, therefore, the medium by which organic beings aid in the in- 

 organic 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 afterward contributed 

 to the rock -material of the globe, and is one source of mineral phos= 

 phates. 



(6.) (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 alumina are quite infusible ; 

 but, by the addition of the alkalies, or the oxyds of iron or lime, fusi- 

 ble 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 compounds, 

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

 along with alumina, magnesia, or lime, in many silicates, which are 

 mostly fusible. Silica and magnesia or lime with protoxyd of iron 

 make part of the very abundant mineral hornblende, found in syenyte, 

 hornblendic slate, etc. ; and also the equally common pyroxene, char- 

 acteristic of the heavy, dark-colored lavas. 



(9.) Carbon. — Carbon is well known in three different states, — 

 that of the diamond, the hardest of known substances, that of graphite 

 or black lead, and that of charcoal. Combined with oxygen, it forms 

 carbonic acid (CO 2 ) ; and carbonic acid combined with lime makes 

 carbonate of lime, or common limestone ; with magnesia, carbonate of 

 magnesia, or magnesite ; with protoxyd of iron, carbonate of iron or 

 siderite; etc. 



Carbonic acid exists in the atmosphere, constituting ordinarily about 

 one part in twenty -five hundred by weight. 



This acid is the only acid in the mineral kingdom, in addition to silica, which enters 

 very largely into the constitution of rocks ; and, while silica has alumina and other ses- 

 quioxyds wholly to itself, carbonic acid shares with it in the magnesia, lime, and alka- 

 lies, that is, in all the protoxyds. Carbon, we have said, performs as fundamental a 

 part in living nature as silicon in dead nature; and it is mainly through living beings 

 that it reaches the mineral kingdom and forms limestones and coal-beds. The deposits 



