METALLIC BASES. 



27 



The mechanical rocks nearly all contain alumina. 



Potassium and sodium are pretty widely dissemi- 

 nated among rocks, though not in very large quanti- 

 ties. Potash is the most abundant of the two, and 

 is also contained in vegetables, which derive it, 

 however, from the decomposition of rocks contain- 

 ing it. Nearly all the inferior stratified rocks con- 

 tain potash, generally from five to ten per cent. 

 Granite contains about seven per cent., and green- 

 stone, and rocks of that class, about the same pro- 

 portion. Soda is chiefly disseminated in the ocean, 

 in the form of muriate of soda, and also in rock 

 salt, which, in Poland and other countries, consti- 

 tutes immense beds. It is also found in certain 

 feldspars, in schorl, basalt, pitchstone, hypersthene, 

 &c. 



Magnesium, the metallic base of magnesia, is pres- 

 ent in all the inferior stratified rocks, with the excep- 

 tion of quartz rock and a few feldspars. There are 

 very few limestones which do not contain it; such 

 are called magnesian limestones. In dolomite it 

 forms more than 40 per cent. Magnesia abounds 

 in the waters of the ocean, in the form of a muriate 

 of magnesia. 



Calcium, the metallic base of lime, is found in nu- 

 merous rocks besides limestone, such as granite, 

 gneiss, mica slate, chlorite slate, talcose slate, clay 

 slate, hornblende rock, &c. Lime also abounds 

 among the fossiliferous rocks in the form of a car- 

 bonate. It is also disseminated through sea-waters, 

 though in small quantities. 



Imponderable agents. — -These are heat, light, elec» 

 tricity, and galvanism. Of these, the first is of the 

 most importance in a geological view, as it is con- 

 tinually effecting changes on the surface of our 

 earth. By heat or caloric is generally understood 

 the effect of hot bodies on our senses. When this 

 sensation is excited to a high degree, it is called 

 burning. Thus the term heat has two meanings ; 



