Augite from Vesuvius and Etna. 21 



books of mineralogy. The faces present are: a(lOO), 

 h{010), m(llO), and 5(111). A small proportion are 

 twinned, forming the common contact twins, with twin- 

 ning plane a (100). They vary, in general, from 3 to 5 

 mm. in length (parallel to the vertical axis), thongh some 

 attain lengths of one centimeter or more. The thickness 

 is from one-half to two-thirds of the length, and there is 

 usually a slight tabular development parallel to a(lOO). 

 The faces are fairly smooth and bright, much brighter 

 than those of the*^ Stromboli augites described ^ in a 

 previous paper,^ and the edges are sharp. There is but 

 little scoria adherent to the loose crystals, but microsco- 

 pical examination of the pow^dered material revealed the 

 presence of small amounts of inclusions of glass and 

 crystalline matter (magnetite, leucite, and feldspar). 



Physical data. — Of a dozen crystals studied, four had 

 sets of faces that were sufficiently good for approximate 

 measurements (by Merwin). These indicate the proba- 

 bility that crystallographically diopside does not repre- 

 sent augite. The measurements were as follows : 



mAm (110) A (lT0)=92^0'-93°57' ; mean=: 



92° 56' 4 measurements. 

 5 A 5 ( 111 ) A ( 111 ) =61 ° 20'-59 ° 50' ; mean= 



60° 36' 8 measurements. 

 For diopside the values are : 



mAm=92°50^ 5A5=59°ir. 



Measurements of <^ and p for four pvramid faces, 5, gave 

 values of 24°23'-25°5' and 34°l'-33°i5', respectively, as 

 compared with 25° 7' and 33° 4' for diopside. 



Speaking of the pyroxenes as a group, Dana long ago 

 pointed out:^ ^'It is noteworthy that the angles vary but 

 little even for a wide variation in composition." AYhile 

 this is quite true, yet since that time, with more exact 

 means of measurements and of chemical analysis, a more 

 systematic study of the relations of the physical and 

 chemical properties, and for other reasons, we are 

 beginning to -appreciate more than we did thirty years ago 

 the probable importance of even small variations. 



An optical examination showed that the Vesuvius 

 augites studied are variable in composition. The general 

 color is a slightly yellowish gray, and there is little indi- 



^ Kozu and WasMngtoii, this Journal, 45, 463, 1918. 

 *Dana, System, 363, 1892. 



