HANDBOOK FOR THE DEPARTMENT OP GEOLOGY, 507 



Chlorine. Symbol CI; atomic weight 35.5: Chlorine occurs free in 

 nature only in limited amounts and in volcanic vents. Its most com- 

 mon form is in combination with hydrogen, forming hydrochloric acid 

 or with the metals to form chlorides. It combiues with sodium to form 

 sodium chloride or common salt (]STaCl), which is the most important 

 mineral ingredient in sea water and which can usually be detected in 

 rain and ordinary terrestrial waters. In this form also it forms exten- 

 sive beds of rock salt, which are mined for commercial purposes. 



Phosphorus. Symbol P ; atomic weight 31 : Phosphorus never occurs 

 in nature in a free state, but is found in great abundance in combina- 

 tion with oxygen or one of the metals. Combined with calcium and 

 oxygen it forms calcium phosphate, which is found in the bones of ani- 

 mals, the seeds of plants, and also the minerals phosphorite and apatite. 



Fluorine. Symbol F; atomic weight 19: Fluorine does not occur 

 free in nature and can be isolated by artificial chemical methods only 

 with great difficulty. It is the only element that does not combine 

 with oxygen. It occurs chiefly combined with calcium to form fluorspar, 

 but traces of it are found in sea water and in the bones, teeth, blood, 

 and milk of mammals. 



Aluminum. Symbol Al; atomic weight 27.4: Aluminum is next to 

 oxygen and silicon, probably the most abundant element of the earth's 

 crust, of which it is estimated to form about one-twelfth. It has never 

 been found in nature in the free state, but commonly occurs in combi- 

 nation with silicon and oxygen, in which form it is an abundant con- 

 stituent of feldspar, kaolin, clay-slate, and many other rocks and 

 minerals. In combination with oxygen it forms the minerals corun- 

 dum, ruby, and sapphire. 



Calcium. Symbol Ca ; atomic weight 40 : Calcium is one of the com- 

 monest and most important elements of the earth's crust, of which it 

 has been estimated to compose about one-sixteenth. It does not occur 

 free in nature, but its most common form is in combination with car- 

 bon dioxide, forming the mineral calcite (CaC0 3 ), or the rock limestone. 

 In this form it is slighty soluble in water containing carbonic acid, and 

 hence has become an almost universal ingredient of all natural waters, 

 whence it furnishes the lime necessary for the formation of shells and 

 skeletons of the various tribes of mollusca and corals. In combination 

 with sulphuric acid, calcium forms the rock gypsum. 



Magnesium. Symbol Mg ; atomic weight 24: Magnesium does not 

 occur free in nature, but is most commonly found in combination with 

 carbonic acid as carbonate of magnesia forming thus an essential part 

 of the rock dolomite. The bitter taste of sea water and some mineral 

 waters is due to the presence of salts of magnesia. In combination 

 with silica it forms an essential part of such rocks as serpentine, soap- 

 stone and talc. 



Potassium. Symbol K ; atomic weight 39.1 : Potassium does not 

 occur free in nature, but combined with silica is an important element 



