78 



SCIENCE PROGRESS 



The diorites are characterised by oligoclase and andesine ; 

 the gabbros by labradorite, bytownite, and anorthite. 

 The syenites are subdivided as follows: 



Felsic minerals 

 Group Ai. [Prefelsic syenites] . 



i. Alkali-syenites 



> 1 

 Mafic minerals x 3 



Alkali-felspars 



Lime-soda felspars 



>-? 



Alkali-felspars 



2. Calcialkalic-syenites 



Group A2. [Mafelsic syenites] . 



Mafic minerals 



Lime-soda felspar 



< 



5 < f > 



J. 



3 



Felsic minerals / 5 



Monzonites have only been divided into 



Felsic minerals 



Group Bi. [Prefelsic monzonites] 

 Group B2. [Mafelsic monzonites] 



3 X 5 



5 



Mafic minerals 

 Felsic minerals 

 Mafic minerals 





Group C. — Diorites and Gabbros — is divided similarly. 



Methods of subdivision similar to the above are adopted 

 in all the main Divisions. 



It is obvious that this method of classification fulfils the 

 conditions laid down in this paper. It is modal and it is 

 quantitative. The scheme forms an excellent ground-work 

 on which to build up a stable quantitative system of classifica- 

 tion based on the mode. Several criticisms of Iddings' arrange- 

 ment can be made. The first is that his sixth division, that 

 characterised by mafic minerals, is really unnecessary. The 

 rocks included in this division are collected from most of 

 the other divisions, in increasing quantity from the first or 

 second onward, and form merely the domafic and permafic 

 members of these divisions. Division VI. is not co-ordinate 

 with the other divisions, but traverses them, as can be seen 

 from the diagram. 



