396 GEOLOGY. 



nitrogen, hydrogen, aqueous vapor, and carbonic acid, which make up 

 the mass of the present atmosphere, are all found in lavas and in their 

 cooled products. Probably all the rarer elements also occur in igneous 

 rocks. HeHum is known to be given forth by springs. 



Leading elements. — But although nearly or quite all the known 

 chemical elements enter into the igneous rocks, only a few of them are 

 abundant. These are regarded as normal or essential constituents, 

 while the rarer substances are regarded as incidental. By combining 

 a large number of the most trustworthy analyses of rocks of all sorts, 

 F. W. Clarke^ has estimated the relative amounts of the more abundant 

 elements in the crust of the earth with the following result : 



Element. Symbol. Percent, in the Solid Crust. 



Oxygen (O) 47 . 02 



Silicon (Si) 28.06 



Aluminum (Al) 8. 16 



Iron (Fe) 4.64 



Calcium (Ca) 3 . 50 



Magnesium (Mg) 2 . 62 



Sodium (Na) 2.63 



Potassium (K) 2.32 



Titanium (Ti) 41 



Hydrogen (H) 17 



Carbon (C) 12 



Phosphorus (P) 09 



Manganese (Mn) 07 



Sulphur (S) 07 



Barium (Ba) 05 



Strontium (Sr) 02 



Chromium (Cr) 01 



Nickel (Ni) 01 



Lithium (Li) 01 



Chlorine (CI) 01 



Fluorine (Fl) .> 01 



' ' 100.00 



It will be seen that only eight of the elements hold a high rank in 

 quantity. Many that are of the utmost importance in the history of 

 the earth and the affairs of men are low in the Hst, or do not even appear 

 in it at all, because their quantity is too small to be estimated in per- 

 centages. The precious metals, and even some of the more common 

 metals, as lead, zinc, and copper, are too scarce to form an appreciable 

 percentage. 



^Analyses of Rocks, Bull. 168, U. S. Geol. Surv., 1900, p. 15. 



