CLASSIFICATION OF IGNEOUS ROCKS 



265 



to form magnetite. Silica is now allotted first to the group 

 CaO . ALO3 to form anorthite, next to the remaining ferrous 

 oxide and the magnesia to form the orthosilicates (FeO) 2 Si0 2 and 

 (MgO) 2 Si0 2 , and then to the K 2 . A1 2 3 and Na 2 . Al 2 6 3 to form 

 orthoclase and albite. Sufficient silica now remains to raise 

 the orthosilicates of iron and magnesia to metasilicates 

 FeO . Si0 2 and MgO . Si0 2 , and finally an uncombined residue 

 is left over to form quartz. 



In the next table the first column gives the proportions of 

 the different mineral molecules, and the second column the 

 actual percentage weights of the minerals obtained by multi- 

 plying by the mineral-molecular weights. 



felspars 6271 J-salic 95*25 

 174 \ metasilicates 2*19^ 



oxides, etc. 1 "54 



femic 373 



The anorthite includes a certain amount of strontium and 

 barium, and the ferrous metasilicate some manganese metasili- 

 cate. These have been allowed for in the above calculations. 



The close approximation of the total of the minerals of the 

 norm (98-98) to the original total of the analysis (less combined 

 water and moisture), 98*971, is strong evidence that the operation 

 has been correctly carried out. The authors of the system only 

 calculate the number of molecules to the third place. This 

 saves trouble, but the final figures are then only approximate, 

 especially in the case of the felspars whose mineral-molecular 

 weights are very high : 556 in the case of orthoclase. 



If there is not enough alumina to satisfy both the alkalies 

 and the lime, the latter has to go short, and is subsequently 

 allotted silica to form either Wollastonite, CaO.Si0 2 , or Aker- 

 manite, (CaO) 4 (Si0 2 ) 3 , according to the amount of silica available. 



When the silicate is insufficient to convert all the alkali- 



