22 H. S. Washington and H. E. Merwin — Note on 



cation of zonal structure. There is little if any pleo- 

 chroism. The index (3 varies from 1.700 to 1.711. The 

 extinction angle is large, as usual, about 45°, but no exact 

 measurements were made, because of the variability in 

 composition and the consequent indeterminateness of the 

 data. 



The specific gravity of the crystal fragments used for 

 the analysis was found to be 3.242 referred to water at 

 23°. This low result is probably to be ascribed to the 

 presence of the glass inclusions. 



Chemical compositioyi. — For my analysis some of the 

 cleanest loose crystals were selected, and, in order to 

 obtain sufficient material, to these were added crystals 

 obtained from specimens of the scoria from about the 

 orifice. Great care was taken to free the crystal 

 fragments from adherent scoria, and the analysis was 

 made on what was probably as pure material as could be 

 obtained from this source. It was, however, not practi- 

 cable to obtain material entirely free from the small 

 inclusions of glass, etc., and the analysis, therefore, does 

 not represent an entirely pure augite substance. In 

 spite of this defect, however, it is thought best to publish 

 it, as a slight contribution to our knowledge of the Vesu- 

 vian augites, and as illustrative of the dangers, in the 

 form of included foreign matter, that may lurk in the 

 chemical study of many minerals. The presence of the 

 inclusions is also of interest in view of the very close 

 correspondence in chemical comiDosition between these 

 augites and a pyroxenite published by Lacroix, to be 

 mentioned later. 



TABLE I. 

 Analyses of Augites of Vesuvius. 



12 3 4 



SiO., 47.60 46.95 46.42 47.90 



ALO, 6.01 6.75 9.14 6.58 



FeoO 3.17 4.47 5.03 1.52 ^ 



Feb 4.59 4.09 4.87 3.33 



MgO 14.43 16.04 13.19 14.20 



CaO 21.52 19.02 20.86 22.51 



Na.0 0.70 n.d. n.d. 1.14 



K,6 0.76 n.d. n.d. 0.38 



H,0+ 0.08 n.d. n.d. 0.62 



TiO, 1.52 n.d. n.d. 1.49 



P2O-, n.d. n.d. n.d. 0.70 



MnO 0.13 n.d. 0.14 n.d. 



SrO none n.d. n.d. n.d. 



100.51 100.32 99.65 100.39* 



* Includino: F = 0.02 



