PETROGRAPHY. 95 
of possible application, on account of the olivine present. But the amounts of 
nephelite and olivine are so small as to be quite negligible in any system which 
rationally takes into account the relative amounts of the minerals, and the choice 
must depend on the relative amounts of, and the importance attributed to, the 
soda-orthoclase and the labradorite. It has been shown above that in two speci- 
mens the relations of these are inverse, and that it might be advisable to distinguish 
two types, which would find expression in the prevailing systems in the names leucite- 
trachyte and leucite-tephrite. But in view of the facts that the soda-lime feldspar 
is rather more prominent, even in the rock in which it is subordinate in amount, 
the usual greater importance given to this feldspar, and the peculiar composition 
of the alkali-feldspar, which approaches an albite in composition, the name of 
leucite-tephrite would seem to be the most appropriate for the rock. Were a new 
name desirable for this type, which so much resembles the viterbal vulsinose and 
ciminose, that of mcoite would be appropriate. 
FOGLIANAL VICOSB. II. 6. 2. 2. 
Megascopic Characters. Light gray, compact, highly porphyritic. Leucite phenocrysts 
very abundant, almost one-half of rock, 5 to 20 mm., or larger, euhedral trapezohedra, some- 
times fragmentary, white to very pale gray. Feldspar phenocrysts very few, 2 to 20 mm., stout 
prismatic, colorless. Augite phenocrysts very few, i to 2 mm., prismatic, black. Groundmass: 
light gray, aphanitic or rarely very fine-grained phanerocrystalline. 
Microscopic characters. Percrystalline, magnophyric, dopatic. Phenocrysts: 35 percent, 
leucite, labradorite, augite, sometimes orthoclase. Groundmass: 65 per cent, microporphy- 
ritic, granular, sometimes partly felted fabric, orthoclase, labradorite, leucite, augite, olivine, 
magnetite, apatite, sometimes nephelite or glass base. 
Soda-orthoclase, OrjAb! to Or r Ab 2 . Phenocrysts: about 2 per cent, not always present, 
5 to 20 mm., subhedral, stout prismoidal, Carlsbad twinning, few inclusions. Groundmass: 
15 to 30 per cent, 0.02 to o. 20 mm., subhedral to anhedral, in part tabular, in part as interstitial 
areas. 
Labradorite, AbjAnj. Megaphenocrysts: about 3 per cent, 5 to 20 mm., subhedral, stout 
prismoidal, twinned, inclusions few. Microphenocrysts : from 5 to 20 per cent, o. i to 0.5 mm., 
subhedral to anhedral, thick tabular, twinned. Microgroundmass : about 5 per cent, not 
always present, 0.02 to o.o. 10 mm., anhedral, tabular. 
Leucite. Megaphenocrysts: about 30 per cent, 5 to 20 mm. or more, euhedral, equant 
trapezohedra, often fragmentary, twinned, inclusions common of augite, labradorite, magnetite, 
and glass, usually centrally arranged. Microphenocrysts: about 10 per cent, 0.05 to 0.50 mm. 
subhedral to anhedral, equant spheroidal, sometimes irregular, faint twinning, inclusions few. 
Nephelite. Groundmass: 5 per cent to none, anhedral, formless, interstitial areas. 
Augite. Phenocrysts: about 2 per cent, 0.5 to 2.0 mm., subhedral, stout prismoidal, gray 
to pale greenish-yellow, non-pleochroic, inclusions rare. Groundmass: about 10 per cent, 0.05 
to 0.20 mm., subhedral to anhedral, prismoidal to equant and irregular, colorless or pale gray. 
Olivine. Groundmass: about 3 per cent, 0.02 to o.iomm., anhedral, equant, colorless. 
Magnetite. Groundmass: about 2 per cent, 0.02 to 0.05 mm., anhedral, equant. 
Apatite. Groundmass: about i percent, o.oi to 0.05 mm., subhedral, thin prismoidal. 
Glass. Usually none, sometimes up to 5 per cent, colorless, difficult to detect. 
Chemical composition and norm as on p. 92. 
Type specimens from Convento di Sant'Angelo, Monte Fogliano, Vico Volcano, Ciminian 
District, and from Monte San Antonio, Auruncan District. 
