PETROGRAPHY. 99 
of alkali and soda-lime feldspars are almost identical, leading to the uncertainty as 
to the classification under prevailing systems, though the difference in habit and 
prominence of the two in the groundmass reduces this to some extent. 
As in so many of the preceding cases, the modal divergence from the norm is 
brought about largely by the increase in leucite and decrease in orthoclase and 
nephelite, making leucite the critical mineral. The type may therefore be described, 
as in the case of the two preceding, as leucite salphyro-vicose. 
Occurrence. This type seems to be most abundant in the Sabatinian District, 
typical localities being flows at Madonna del Riposo, below the town of Bracciano, 
and at a quarry along the railroad below Poggio Cotognola, southwest of Bracciano. 
Some flows at Monte Bisenzo, on the south shore of Lake Bolsena, in the Vulsinian 
District, may also be referred here, and undoubtedly belong to the subrang of 
vicose, though most of them are so poor in phenocrysts as to belong rather to the 
aphyric orvietal type to be described later. 
Name. Both the subrang and type names have already been discussed. In 
the prevailing classifications this type must be considered as a leucite-tephrite, in 
view of the mode and texture, since although orthoclase and labradorite are present 
in almost equal amounts, the latter is the more prominent in the sections and is 
usually given more weight in classification. 
BAQNOREAL VICOSE. II. 6. 2. 2. 
Megascopic characters. Medium gray, compact, slightly porphyritic. Leucite pheno- 
crysts few, 3 to iomm., grayish, not very conspicuous. Augite phenocrysts rare, 0.5 to 2 mm., 
prismatic, black. Groundmass: medium gray, fine-grained, phanerocrystalline. 
Microscopic characters. Holocrystalline, magnophyric, perpatic. Phenocrysts: 10 per cent 
or less; leucite, augite. Groundmass: 90 per cent or more, holocrystalline or percrystalline, 
intersertal fabric, leucite, orthoclase, labradorite, augite, magnetite, apatite, sometimes nephelite 
or glass. 
Orthoclase, OrjAbj. Phenocrysts: none. Groundmass: about 18 per cent, in part o. i to 
0.5 mm., subhedral, tabular, with diverse arrangement, in part as anhedral, formless patches 
of interstitial cement. 
Labradorite, AbjAn^. Phenocrysts: none. Groundmass: about 17 per cent, o.i to 0.5 
mm., thin tabular, twinned, arrangement diverse. 
Leucite. Phenocrysts: about 5 per cent, 3 to lomm., subhedral to anhedral, equant to 
irregular, twinned, inclusions rare. Groundmass: about 30 per cent, o. i to i.o mm., anhedral, 
equant and irregular, few inclusions. 
Nephelite. Groundmass: 5 per cent to none, anhedral, as interstitial cement. 
Augite. Phenocrysts: about 3 per cent, 0.5 to 2.0 mm., subhedral to anhedral, 
prismatic and fragmentary, pale gray or greenish, non-pleochroic, inclusions few. Groundmass: 
about 15 per cent, o.i to 0.5 mm., subhedral to anhedral, prismatic to equant, pale gray or 
very pale greenish. 
Magnetite. Groundmass: about 4 per cent, 0.02 to 0.05 mm., anhedral, equant to 
irregular. 
Apatite. Groundmass: about i per cent, 0.05 to o.iomm., subhedral, thin prismatic. 
Glass. Groundmass: 5 per cent to none, colorless, difficult to detect. 
