Washington — Plauenal Monzonose (Syenite). 135 



measured mode as given above. It is clear, however, that the 

 mode is decidedly abnormative, the hornblende taking up 

 nearly all of the normative diopside and hypersthene, with a 

 very considerable amount of the anorthite, and a little of the 

 albite, magnetite and ilmenite. The greater part of the ilmen- 

 ite is used up in forming titanite, the silica and lime needed 

 coming from diopside and the ferrous oxide of the ilmenite 

 taking its equivalent of silica from the normative quartz to 

 enter the hornblende. On the other hand the estimated 

 amounts of quartz, orthoclase and albite as given above corre- 

 spond well with the figures for these in the norm calculated 

 from III. That the mode here given is close to the truth is 

 indicated by the specific gravity. That of my specimen was 

 found to be 2*73 at 23°, identical with Zirkel's, while that cal- 

 culated from the amounts of the several minerals shown by 

 Rosiwal's method was 2'697, a very satisfactorily close agree- 

 ment. 



Name. — In a general way therefore this rock may be spoken 

 of as hornblende grano-monzonose, but on account of its 

 importance we may regard it as a type, to which the name of 

 plauenal monzonose may be appropriately given. This name, 

 it will be seen, implies the modal and textural characters 

 involved in Rosenbusch's Plauen Typns of the hornblende- 

 syenites, but more strictly defined and with a very definite 

 indication of the chemical composition as well. 



Locust, N. J., June. 



