OJisijfiaiilff's. 42'. 



Reducing the foregoing results to percentages, we obtain the 

 " Norm." or " standard mineral composition." 



Mineral. Norm. 



Quartz = ,725 x 60 = 43.5 



Orthoclase = .020 x 556 = 11.1 



Albite = .025 x 524 = 13.1 



Aiiorthite = .057 x 278 = 15.8 



Corundum = .027 x 102 = 2.8 



„ ^, ( .047 X 100 I ,, , 



Hypersthene = <j ^-^ x 132 f " ^^'^ 



Tlinenite - .009 x 152 = 1.4 



Magnetite = .003 x 232 = .7 



The "salic" minerals, i.e., the quartz, felspar and corundum 

 total 86.3, and the "femic" minerals, i.e., the ferro-magnesian 

 minerals and iron oxides total 13.5. In the same way the norms 

 of the other obsidianites may be calculated, the results being as 

 follows : — 



N0RM8. 



