266 SCIENCE PROGRESS 



alumina groups to felspars, it is divided up among felspars and 

 felspathoidsaccording to definite ruleswhich have theirfoundation 

 in petrological experience. Other contingencies of a similar 

 nature may arise, but are provided for by the scheme, which has 

 been elaborated in detail with the greatest care and exactness. 



The operations in the calculation of the norm are both 

 interesting and instructive, but they are to a large extent unreal, 

 and cannot compare in value and scientific character with that 

 of calculating from the analysis the proportions of the minerals 

 which have been actually ascertained to be present by micro- 

 scopical examination, 1 and verifying the result by means of a 

 quantitative determination by the linear method. 



(3) The Classes 



The primary divisions into which igneous rocks are classified 

 in this system are dependent on the separation of the rock- 

 forming minerals which may be present in the norm into two 

 main groups, the salic and femic minerals. The former consist 

 of quartz, zircon, corundum, the felspars and the "lenads," 

 viz. felspathoids, while all the remainder are classed as femic. 

 They include the silicates of iron, manganese, magnesia, lime, 

 and the alkalies (but not of course the felspars or felspathoids), 

 also the metallic oxides and apatite, fluor, pyrite, and other 

 minerals of a similar character. 



1 The minerals present in the microgranite consisted of quartz, orthoclase 

 and microcline, plagioclase (partly microperthitic, partly independent), biotite, 

 fluor, apatite, and pyrite. The proportions in which these were present were 

 calculated from the analysis to be as follows : — 



Per cent. 

 Quartz 32 -90 



Orthoclase and microcline . . . . . . . . 27*58 



Albite ) 1 . , ,,. , ,, . f2Q'Q2 



Anorthite/ P^gioclase alhed to albite | * * fi 



Biotite . . . . . . . . . . 4-28 



Apatite 0*24 



Fluor ........... o - i3 



Magnetite 072 



Pyrite C07 



Excess alumina .......... 0*46 



Excess water .... ...... 1 '05 



IOO'II 



The excess of alumina and part of the water may represent aluminium hydrates 

 formed in the course of incipient lateritisation. These figures do not agree very 

 well with the results which I actually obtained by the linear method, for reasons 

 that it is needless to enter upon here. 



