668 HENRY S. WASHINGTON 



composed of alkali feldspar, with nephelite (partly altered to 

 cancrinite), and rare grains of aegirine-augite. The position of 

 this, outside the zone of arkite, is where we would look for such 

 a differentiate of the foyaite magma. 



The analysis, made by myself, of this rock is shown in I 

 above, those of the primary foyaite and the covite in II and III, 

 the molecular ratios of 1 being given in la, for comparison with 

 those of the others on a preceding page. It will be seen that 

 the values obtained for the main Diamond Jo foyaite, which is 

 quite typical of this rock found around the border, are inter- 

 mediate in every case between those for the other two rocks. 

 Reference to Diagram 2 will also make evident the fact that 

 the positions of the various oxides would be, as regards the 

 normal curves, exactly the inverse of those of the covite. The 

 figures for the new analysis fall above or below their respective 

 curves, where those of covite are below or above. This is just 

 what would be expected in the case of secondary differentiates, 

 as has been explained above. 



The results of a mixture of eight parts of I and one part of 

 III are shown in IV. It approximates fairly well to the compo- 

 sition of the typical foyaite, especially in SiOg, MgO, and K^O, 

 though AlgOg, and Na^O are considerably higher, and iron 

 oxides and CaO lower. 



It may be of interest to give the calculated mineralogical 

 composition of I, which works out as follows : 



Orthoclase . . . . 52.3 



Nephehte - . - - 36.9 



Cancrinite ----- 2.8 

 Aegirite - . . . 2.2 



Diopside ----- 3.7 

 Extra alumina - - - i.o 



99.9 



The possibility of a secondar}^ differentiation taking place 

 in a primary differentiate is, it must be conceded, difificult to 

 reconcile with the hypothesis already advanced, since according 

 to this the differentiation takes place by a successive crystalliza- 



