ITALIAN PETROLOGICAL SKETCHES 43 



carry few inclusions. The augite is in stout prismatic crystals, 

 green in color and generally darker toward the center. Large 

 grains of magnetite abound, which are accompanied by orange 

 hematite flakes. Many lath-shaped crystals of a basic labrado- 

 rite, with a little orthoclase, are present with these, and the 

 interstitial colorless glass base contains only few augite micro- 

 lites and some minute opaque grains. An analysis is given in 

 Table I, No. 6. 



Lencite-pJionolite. — My only specimen of this was collected at 

 a rather thick flow on the northwest shore of the lake close to 

 the water's edge. The groundmass is compact and fine-grained, 

 of a very light gray color and with a slightly greasy luster. 

 Clear rather glassy leucites and minute black augites occur as 

 phenocrysts. Examined with the microscope the large leucites 

 show the twinned structure very finely, and the prismatic augites 

 are of an olive-green color, with the pleochroism and other 

 characters of aegirine-augite. 



In the holocrystalline groundmass are many small olive- 

 green aegirine-augite prisms, a few colorless haiiynes showing 

 the characteristic inclusions, and some magnetite grains. The 

 feldspars are represented by stout crystals and grains of alkaline 

 feldspar, which are occasionally twinned. There is also present a 

 residual base of colorless, feebly doubly refracting nepheline, 

 whose identity was established by treatment with acid and 

 fuchsine. Plagioclase is absent. A peculiar feature of the 

 groundmass is the presence of round spots of a clear substance, 

 whose outlines are defined by rings of minute augite microlites 

 with some magnetite grains. These seem at first sight to be 

 groundmass leucites. Close examination under crossed nicols, 

 however, reveals the fact that onl_y a small percentage of them 

 is really of this mineral. The majority are composed of nephe- 

 line, with some orthoclase grains and crystals, the latter occa- 

 sionally in fan-shaped aggregates. We have here, then, a case 

 of paramorphism of leucite into " pseudo-leucite " — a mixture of 

 nepheline and orthoclase, such as has been observed in Arkansas,' 



'J- F. Williams, Ark. Geol. Sur., 1890, II, 267. 



