Washington — Plauenal Monzonose (Syenite). 133 



If the ferrous oxide of I is divided up in about the ratio 

 shown by III, that is 3*89 Fe 2 3 and 3*50 FeO, the norm calcu- 

 lated on this basis is as follows : 



Quartz 7*08 



Orthoclase. -_ .. 38'92 



Albite 20-44 



Anorthite __ _ 15-57 



Diopside 5*19 



Hypersthene _ . 7'31 



Magnetite.. 5*57 



100-08 

 Water 1-29 



101-37 

 Extra oxygen 0*38 



100-99 



This gives as the systematic position ciminose (II. 5. 2. 2), a 

 rare subrang which is abundantly represented at the Italian 

 volcanoes, but which is unknown outside of these except as the 

 durbachite of the Schwarzwald, a rock with very abundant 

 biotite, which is quite absent from the Plauen syenite. 



The figures for the alkalies in the new analyses are remark- 

 ably constant, Na 2 Q varying only from 4*34 to 4'49, while the 

 range of K 2 is slightly greater, from 4'33 to 4*93 : the for- 

 mer being within the acceptable limits of analytical error and 

 the latter scarcely more than this.* Silica varies within a 

 range of 3'19 per cent, and the other constituents are some- 

 what higher in IY than in III, though not very much so. 

 Taken as a whole, when we remember that silica is the most 

 abundant constituent and the one in which a greater range of 

 variation is to be expected, and when the different times when, 

 and the different parts of the mass where, the specimens were 

 collected are considered, the results of my analyses imply a 

 remarkable uniformity in the mass of syenite. It is true that 

 highly feldspathic, as well as highly hornblendic, schlieren 

 occur here and there, but these are of very minor importance, 

 besides being complementary to each other. While the close 

 correspondence between I and II might argue a composition 

 similar to these, their earlier date and unsatisfactory character, 

 as well as the improbability of the systematic position to which 

 they lead, make one feel confident that the true composition of 

 the mass is shown by the new rather than by the older analy- 

 ses, and that it is best represented by the average of these, as 

 given in VII. 



*Dittrich, Neues Jahrb., 1903, ii, p. 81 ; Washington, Manual Chem. Anal. 

 "Rocks, 1904, p. 24. 



