912 THE GEOLOGY OF THE NANDEWAR MOUNTAINS, 



their calculated norms that we see how closely allied they are. 

 The calculation of the norm in terms of standard minerals is a 

 great boon for purposes of comparison, and having the analysis 

 we can also calculate it in terms of other mineral-combinations, 

 and thereby ascertain what other rock-species might arise from 

 the same magma. 



As for the classification based on the norms, it must be said 

 that it is no better than previous classifications. Whether we 

 call a certain rock syenite-porphyry, phlegrose, or nordmarkite, 

 we must first know the chemical and mineralogical composition 

 corresponding to these names; and the Rosenbusch and Brogger 

 names have the advantage over the magmatic names that they 

 define the mineralogical composition much more accurately, and 

 give us some idea of texture, fabric and facies as well. 



As Professor Marshall^ of Dunedin and many others have 

 shown, the quantitative system brings together, under a common 

 name, rocks which are widely different, and separates closely 

 allied ones. 



This is strikingly exemplified in my studies on the petrology 

 of the Warrumbn.ngle and Nandewar Mountains. Thus the 

 labradorite porphyry (N.12) is seen from field-evidence and 

 microscopic examination to be a differentiation-product of a 

 pulaskite magma. There are many analogies in the mode of 

 occurrence and the composition and structure of the groundmass 

 to bind it to the alkaline series. Yet the fact that it is chemi- 

 cally poorer in alkali and richer in lime and magnesia than the 

 other members of the series removes it so far as to place it in the 

 subrang "andose," whereby its alkaline affinities are completely 

 obscured. 



Now N.15, a quartz-monzonite or akerite, an olivine-free rock, 

 of light grey colour and even grain-size, has the same magmatic 

 name as the orthoclase-sodalite-basalt, W.67, from, the Warrum- 

 bungle Mountains (p. 607), although the last-mentioned is a black, 



* "Geology of Dunedin." Q.J.G.S. Yol.lxii. 



