58 THE ROMAN COMAGMATIC REGION. 
SORIANAL HARZOSE.* 
Megascopic characters. Dark gray, compact, highly porphyritic. Feldspar phenocrysts 
abundant, 2 to 5 mm., tabular, colorless or white. Phenocrysts of biotite common, small, glisten- 
ing, dark-brown tables. Phenocrysts of pyroxene rare, small prismatic, black. Groundmass 
dark gray, aphanitic, sometimes vitreous looking. 
Microscopic characters. Hyalocrystalline, megaporphyritic, mediophyric. Phenocrysts: 
about 40 per cent, orthoclase, labradorite, biotite, hypersthene, augite. Groundmass: about 
60 per cent, glass with microlites of hypersthene and feldspar. Flow structure visible. 
Orthoclase, Or 4 Ab 3 ?. Phenocrysts: about 20 per cent, 2 to 5 mm., tabular or stout pris- 
matic, subhedral to euhedral, Carlsbad twinning. Groundmass: small indeterminable amount* 
o.oi to 0.03 mm., prismatic, scattered in flow structure through the glass base. 
Labradorite, Ab An 2 . Phenocrysts: about 7 per cent. 2 to 5 mm., euhedral to subhedral, 
tabular or stout prismatic, multiply twinned. 
Biotite. Phenocrysts: about 10 per cent, 0.05 to 0.5 mm., subhedral, thin tabular, 
brown, much altered. 
Hypersthene. Phenocrysts : about 4 percent, o. i to i.o mm., subhedral to anhedral, pris- 
matic to irregular, often fragmentary, colorless. Groundmass: very small, indeterminable amount, 
o.oi to 0.03 mm., anhedral prismatic, scattered with flow structure through the glass base. 
Augite. Phenocrysts: about 2 per cent, o. i to i.omm., subhedral to anhedral, stout 
prismatic or fragmentary, colorless. Groundmass: augite may be present in very small amount, 
as minute prisms along with hypersthene in the glass base. 
Glass. Nearly 60 per cent, colorless, often dusty, contains the small microlites described 
above, flow structure common. 
Chemical composition and norm as on p. 56. Mode indeterminate. 
Type specimens from Monte Ciliano, Soriano Volcano, Ciminian District. Type speci- 
men of breccia form from La Cava, near Viterbo, Ciminian District. 
I-II. 5. 2. 2. Arsal Vulsinose-Ciminose [Vulsinite, Arso Type]. 
Megascopic characters. These rocks are light gray when fresh, but weather 
to reddish tones, especially in the groundmass. They are decidedly porphyritic, 
the phenocrysts being quite large and making up rather over one-eighth of the rock 
mass. The great majority of these, as well as the largest, are of feldspar, which are 
mostly tabular parallel to b (oio) and from 5 to 10 mm. long. There are also few 
small, black, prismatic phenocrysts of augite, and still more rare small tables of 
biotite. The light-gray groundmass is clearly phanerocrystalline and fine-grained. 
Microscopic characters. In thin section the phenocrysts of alkali feldspar are 
seen to be more common than those of the lime-soda feldspar. The former do not 
show usually the common characteristics of soda-orthoclase ; though the analysis 
of the rock indicates that they are quite sodic, while the latter give extinction angles 
corresponding to an average composition of Ab r An 2 . Carlsbad twinning is common 
among the former and the usual multiple twinning among the latter. The feld- 
spars carry few inclusions, mostly of augite and glass, and the orthoclase phenocrysts 
also inclose occasionally small inclusions of labradorite. The augite phenocrysts 
are subhedral to anhedral, in stout prisms or fragmentary, of the usual pale gray, 
sometimes with a slightly greenish tinge, though never pleochroic. The thin tables 
of biotite are light brown and for the most part altered at the edges to the com- 
mon aggregate. 
* This description applies to the lava form of the type, as the breccia form was not readily measured, and its 
brecciated condition also does not make a formal description necessary. 
