56o HENRY S. WASHINGTON 



The groundmass is rather trachytic in character, as many 

 orthoclase laths — best seen with crossed nicols — are present, 

 showing decided flow structure. With them are numerous small 

 leucite crystals, with rounded outlines, containing a few sporadic 

 inclusions and an abundance of prismatic microlites of aegirine- 

 augite, with C/s (^=38°. Some magnetite is also present. All 

 these small crystals lie in a colorless base of low refractive index, 

 which in many places shows weak double refraction, and which 

 treatment with acid proves to be nepheline. 



The other specimen of leucite-phonolite is from the large 

 quarries immediately to the north of Bagnorca, east of the lake, 

 whence the slabs are exported for paving stones. The same 

 rock is described by Bucca,^ who does not mention nepheline, 

 though he speaks of the base as easily gelatinizable and rich in 

 soda. It is ash gray and fine-grained, with rough texture and 

 small cavities whose walls bear leucite but no nepheline crystals. 

 The specific gravity was found to be 2.648 at 27° C. 



Examined under the microscope the leucites and pleochroic 

 aegirine-augites show no specially noteworthy features. The few 

 well-shaped orthoclase phenocrysts often carry a mantle of later 

 alkali feldspar substance like that previously described. There 

 are also a few biotite phenocrysts which have been entirely 

 altered to augite and magnetite in the usual way. 



In the o-roundmass there are abundant small leucites almost 

 free from inclusions, stout aegirine needles and some mag- 

 netite grains. Orthoclase laths are larger and more abundant 

 than in the rock just described, and there are a few flakes of 

 brown biotite which are among the last products of crystalliza- 

 tion. These lie in a nepheline base similar to that of the last 

 rock. 



Klein describes a leucite-phonolite from Gradoli, northwest 

 of the lake. Its megascopic appearance is that of a phonolite. 

 In thin sections appear as phenocrysts a green pleochroic augite 

 (probably aegirine-augite) , some large leucites and a few sani- 

 dines. In the holocrystalline groundmass nepheline, leucite and 



^BuccA, op. cit., 20. Also Boll. Com. Geol. Ital., XIX, 58, 1888. 



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