258 



SCIENCE PROGRESS 



The Felspathoid Division of the Alkali Series 

 Table of Analyses of Rock-types 



I. Nepheline-syenite. Brbgger's average of thirteen analyses of typical 

 nepheline-syenites. Die Triadische Eruptionsfolge bei Predazzo, loc. cit. 

 Christiania, 1895, p. 27. 

 II. Covite, Magnet Cove, Arkansas, Washington. Journ. of Geology, vol. ix. 

 1901, p. 612. 

 III. Theralite (Covite), Gordon's Butte, Crazy Mts., Montana. W. F. Hille- 

 brande. Quoted in Washington's Chemical Analyses, p. 347. 



Since quartz and the felspathoids are antithetical, the Nepheline 

 Series has representatives only in the Basic and Intermediate 

 groups, namely the theralite, the covite, and the nepheline- 

 syenite types. The theralite of Gordon's Butte in the Crazy 

 Mountains of Montana, the analysis of which is quoted in the 

 above table, has generally been considered as the type rock ; but 

 this rock contains too much orthoclaseto tally with Rosenbusch's 

 definition of a theralite, namely, as a plagioclase-nepheline rock. 

 It appears rather to approximate, as regards mineral com- 

 position, to Washington's Magnet Cove type (covite) ; but 

 is somewhat more basic than this. 



It will be noticed in the diagram (fig. 6), that the alumina, 

 which is rather low in the most basic representative, reaches a 

 very high level in the nepheline-syenite (21 '3 per cent.) ; and the 

 same is true of the alkalies (total=i375 per cent.). The mole- 

 cular proportion of the soda is about double that of the potash 

 at both ends of the series. Lime and magnesia are high at the 

 basic end, but very low in the nepheline-syenites. 



