G. P. Merrill- — Augite from Little Deer Isle, Me. 489 



Olivine is the prevailing constituent and in nearly every 

 case examined has gone over completely into serpentine. The 

 augite, which is the only constituent to which particular atten- 

 tion need here be called, is of the normal type, of a faint 

 yellow or wine red color in the thin section, and gives 

 maximum extinction angles on clinopinacoidal sections of 40°. 

 The mineral occurs in the form of broad plates with deep, 

 rounded embayments and in long armlike forms reaching 

 out and enfolding the altered olivines, the peculiar habit of 

 the mineral in acting as a binding constituent being here dis- 

 played in its best development. On casual inspection by ordi- 

 nary light the mineral presents no features other than of the 

 ordinary type, the rounded forms of the altered olivine abut- 

 ting closely against the fresh augite, while the line of separa- 

 tion is perfectly sharp and distinct as I have attempted to show 

 in figs. 1 and 2. Here the portions marked (a) and bounded 



by the heavy wavy line represent in each figure a single augite 

 individual.* More careful inspection, however, shows that in 

 nearly every instance the augite is surrounded more or less 

 completely by a narrow and extremely irregular colorless 

 border which projects in the form of sharp teeth or tongue- 

 like prolongations for a considerable distance into the serpen- 

 tine (olivine) granules. This is shown by the portions marked 

 (b) in the figures and is very noticeable when the section is 

 viewed between crossed nicols. This irregular border I am 

 inclined to consider as a true secondary growth, formed since 

 the consolidation of the rock and analogous to the hornblendic, 

 feldspathic and quartzose enlargements described by F. Becke,f 

 Irving % and Yan Hise.§ 



* In fig. 1 the rock has been fractured and re-cemented by serpentine. The 

 portions in the upper left and lower right field forming originally one crystal. 

 t Min. u. Petr. Mittheil., vol. v, 1883. \ Bull. U. S. Geol. Survey, No. 8, 1884. 

 § This Journal, May, 1887. ■ 



