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, «fec. 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; 
