CLASSIFICATION OF THE PLUTONIC ROCKS 257 



II. Nordmarkite, Tonsenas, Christiania. Quoted by Brogger in Das Gang- 

 gefolge des Laurdalits, loc. cit. Christiania, 1898, p. 375. 



III. Soda-syenite (" Natron-syenit " of Brogger). Brogger's average of ten 



analyses of typical soda-syenites (akerite, laurvikite, augite-syenite, etc.). 

 Die Triadische Eruptionsfolge bei Predazzo, loc. cit. Christiania, 1895, P- 33- 



IV. Essexite, Salem Neck, Essex County, Massachusetts, U.S. M. Dittmar, 



vide Rosenbusch's Elemente der Gesteinslehre, 1898, p. 172. 



In the Soda Series, the Basic Group is represented by the 

 essexites, the Acid Group by the soda-granites (Brogger), and 

 the Intermediate Group by the soda-syenites and nordmarkites 

 (Brogger). Nordmarkite is thus a quartz-soda-syenite. 



Al 2 0j -171 



0*0 133 

 K t O'KfctO -121 



■fegSP 



UgO '052 

 Kj.0 -030 



Fig. 5. — Diagram of the Molecular Proportions of the Soda Series. 



The general characteristic of the series, as shown by the 

 Iddings diagram (fig. 5), is the high alkali-content, especially 

 in regard to the soda, which is present in all its members in 

 about double the molecular proportion in which potash is 

 present. Lime is fairly high in the essexites, but rapidly 

 diminishes with increasing acidity, while magnesia is remarkably 

 low even at the basic end. 



The alkalies reach their maximum in the nordmarkites, a 

 total of 127 per cent, being contained in the example chosen. 

 The proportion of alumina is constant at a fairly high level in 

 the essexite, soda-syenite, and nordmarkite types, but falls 

 rather sharply towards the acid end of the series. 



17 



