22 THE ROMAN COMAGMATIC REGION. 
cent, trachytic fabric, soda-orthoclase, augite, lavenite, sodalite, magnetite. A very little glass 
base may be present. 
Soda-orthoclase. Phenocrysts: about 30 per cent, 10 to 20 mm., euhedral to subhedral, 
often fragmentary, mostly tabular parallel to b (oio) or stout prismatic parallel to axis a, 
microperthite and microcline structures generally wanting, Carlsbad twinning common, 
inclusions rare. Groundmass: about 60 per cent, 0.05 to 0.20 mm., subhedral, prismatic 
parallel to axis a, arrangement parallel or subparallel. 
Augite. Phenocrysts: about i per cent, o . 2 to i . o mm., subhedral to anhedral, prismoidal, 
very pale gray or slightly yellowish green, non-pleochroic, no inclusions. Groundmass: about 
2 per cent, 0.05 to o. 20 mm., anhedral, prismatic, colorless or very pale gray. 
Magnetite. Phenocrysts: about i per cent, 0.2 to 0.4 mm., anhedral, equant. Ground- 
mass: about 2 per cent, o.oi to 0.05 mm., subhedral, equant. 
Sodalite. A small amount, about 3 per cent, may be present, either as equant anhedra 
in the groundmass or in crevices of the rock, but is not essential to the type. 
Lavenite. Groundmass: about i per cent, o. i to 0.2 mm., subhedral prismatic or anhe- 
dral equant, pale to deep yellow, nonpleochroic, not essential to type. 
Chemical composition as in analysis I, p. 20. 
Type specimen from Marecocco, Ischia. 
I. 5. 1. 3. Cutnal Phlegrose [Phonolitic Trachyte, Cuma Type]. 
Megascopic characters. This type is closely similar to that just described, as 
far as the mode is concerned, but differs texturally in the much smaller number and 
size of the phenocrysts. The rocks are very light gray, in some cases slightly 
greenish, and with a somewhat greasy luster. There may be schlieric patches of 
darker material, which does not differ essentially from the lighter, except in color, 
but these are not regarded as characteristic of the type. 
The phenocrysts constitute only about 10 per cent of the mass. The majority 
of them are of alkali-feldspar, in small, subhedral stout prisms or thick tables. 
Very small phenocrysts of sodalite are also seen, but in much less amount than those 
of feldspar, and crystals of this mineral may be found in crevices. Their color is 
sometimes blue, but they are more often colorless. Still less abundant are the 
small prismatic phenocrysts of augite. 
In some specimens, as those of this type from the tuffs of the Vico Volcano, 
there are present rare, rounded white phenocrysts of leucite, about 5 mm. in diam- 
eter. Their number is very few, not over two or three being visible in a good- 
sized hand specimen. The light-gray groundmass is aphanitic, sometimes with a 
dull, sometimes with a slightly greasy, luster. 
Microscopic characters. In thin section these rocks are typically holocrystal- 
line, though very small amounts of glass may be present. The fabric is a trachytic 
one, though in this type most of the specimens do not show such a well-marked 
fluidal arrangement of the feldspar laths as in the preceding one. The rare pheno- 
crysts of soda-orthoclase offer nothing specially noteworthy, resembling those of the 
ischial type, except in their smaller size. The phenocrysts of augite are either 
euhedral or subhedral, are decidedly olive-green in color, markedly pleochroic, and 
with the axis of greatest elasticity a at an angle of 30 with the vertical axis. They 
are therefore of segirite-augite. The sodalites are either colorless, when they are 
