THE EARTH'S CRUST 53 



and of phosphatic limestone show evidence of living organisms 

 having been connected with their origin, as in limestone shells 

 and bony skeletons. 



Apatite is crystallized calcium phosphate, containing small 

 amounts of calcium chlorid or calcium fluorid. This mineral is 

 largely found in masses, but traces of it are found in nearly all 

 other rocks, whether of igneous or aqueous formation. 



Oxids. Oxids of silicon, iron, and aluminum are more or less 

 abundant and distributed almost universally, in quartz (SiO 2 ) and 

 quartz sand, in the iron ore, hematite (FegOg) , and in the aluminum 

 ore, bauxite (A1 2 O 3 ) and (Fe 2 O 3 ). 



Other deposits. Various other deposits found naturally in the 

 earth, but constituting extremely small percentages of the earth's 

 crust, include common rock salt (NaCl) ; potassium salts, as carnal- 

 lite (KClMgCl 2 6 H 2 O) ; and kainit (K a SO 4 MgSO 4 MgCl 2 6 H 2 O), 

 from which potassium chlorid (KC1) and potassium sulfate (K 2 SO 4 ) , 

 respectively, are separated; saltpeter (KNO 3 ), and Chile saltpeter 

 or sodium nitrate (NaNO 3 ) ; also the extensive deposits of anthra- 

 cite and bituminous coal, the former consisting of nearly pure 

 carbon, while the latter contains considerable amounts of hydro- 

 carbon compounds in addition to the free carbon. 



