ITALIAN PETROLOGICAL SKETCHES 559 



been selected as tv[)ical of Ricciardi's closely agreeing analyses 

 of this groujx We may note here that they are low in silica, 

 high in iron, lime and magnesia, and surprisingly low in alkalies, 

 considering the amount of leucite present. 



Leiicite-plionolite. — Representing this group are specimens 

 from two localities, which differ somewhat from each other and 

 which may be described separately. 



Two, from above St. Trinita near Orvieto, are from different 

 parts of the same flow, one being compact while the other is 

 highly vesicular, though not enough so to be a true scoria. The 

 groundmass is light gray and in it lie many large, sharp leucite 

 crystals up to i'^"\5 in diameter, of a pale yellowish white color 

 and waxy luster. They are much cracked and contain large 

 inclusions of augite and magnetite, generally as a nucleus in the 

 center. Lining the sharp trapezohedral cavities left by leucites 

 which have fallen out is a thin white crust, which under a lens 

 is seen to be minutely mammillary. 



In thin section the large leucites, which have generally lost 

 much of their substance through cracking and falling out, show 

 quite strong double refraction. The edges of their sections are 

 sometimes corroded, and in these places one observes that they 

 are separated from the groundmass by a narrow border of clear 

 colorless substance, with a refractive index slightly higher than 

 that of the leucite. This is the white crust just mentioned. 

 Between crossed nicols it shows weak double refraction, and in 

 places a radiall}- fibrous, spherulitic structure is developed, the 

 fibers diverging toward the leucite. Definite determination 

 could not be made, but it is probable that this border consists 

 essentially of orthoclase, perhaps due to a solution of the 

 leucite in the magma.' 



The green asgirine-augite phenocrysts are somewhat fragmen- 

 tarv. They show distinct pleochroism; C slightly bluish green, 

 9 light vellowish green. A ver}- few orthoclase and one or two 

 labradorite phenocrysts are present. 



'It may be noted that in a leucite-phonolite from Latera Bucca (p. 27) observed 

 an apparent formation of leucite out of orthoclase, while in that from Acquapendente 

 he notes the reverse process. 



