ITALIAN PEIROLOGICAL SKETCHES 247 



flakes of brown biotite are also present. There are also numer- 

 ous grains of magnetite and a few apatite needles. 



Leucite is present in abundance, but shows an unusual habit. 

 While often in the form of small, rounded or trapezohedral crys- 

 tals, it generally occurs in irregularly shaped masses, occasionally 

 showing crystal planes here and there, or with small, definitely 

 shaped crystals attached to the larger mass. These leucites are 

 extremely clear, and show a remarkably faint double refraction. 

 Indeed so faint is the action of these leucites on polarized light, 

 that in most of the crystals recourse must be had to the quartz 

 wedge before any such action can be perceived, a proceeding not 

 often necessary in the Italian leucitic rocks. 



Later than the leucites, and often enclosing them micropoi- 

 kilitically,' are flakes and large patches of a plagioclase with 

 generally well developed twinning lamellae, whose high sym- 

 metrical extinction angles refer it to anorthite. There are 

 also some patches of a feldspar, which, though not showing any 

 signs of twinning, may also be referred to anorthite on the ground 

 of the equality of its refraction with that of the striated anorthite. 

 There is also considerable colorless base, which, while in general 

 isotropic, shows in places a very faint double refraction. Since 

 treatment of the powdered rock with acids produces abundant 

 gelatinous silica, this base may be regarded as nepheline, or 

 at least as a glass corresponding to nepheline in chemical com- 

 position. An analysis of this rock made for me by Dr. A. Roling 

 of Leipzig is here inserted : 



99.27 



' The marked difference in refractive indices between the leucite and anorthite 

 serve to bring the structure out very nicely in ordinary light, while the dark spots of 



