106 THE ROMAN COMAGMATIC REGION. 
while the descriptions of Fuchs suggest that flows of 1731, 1754, 1767, 1779, 1786, 
1802, 1810, 1855, 1858, and 1867 are also of this type. The type seems also to 
occur at Monte Somma, but to a more limited extent. Outside of the Campanian 
District these rocks seem to be rare, and the only specimen in my possession which 
may probably be referred here is the lava of Poggio Romolo, south of Lake Bolsena. 
Name. The name of the subrang is derived from the lake and town of Brac- 
ciano, the chief geographical features of the Sabatinian District, in the southern 
part of which this magma is very abundant. The type name is derived from 
Vesbius, an old Latin name of Vesuvius. 
In the prevailing classifications the type should be regarded as a leucite-teph- 
rite, since the amount of olivine is so small that the name of leucite-basanite often 
given it is not justified if regard is paid to quantitative relations. 
VESBAL BRACCIANOSE. II. 7. 2. 2. 
Megascopic characters. Dark gray, sprinkled with small white spots. Compact to vesic- 
ular, highly porphyritic. Leucite phenocrysts very abundant, i to 3 mm., round, white or pale 
gray, sometimes not very conspicuous. Augite phenocrysts very few, i to 3 mm. long, prismatic 
black or dark green. Olivine phenocrysts very rare, not essential, i to 2 mm., equant, yellow. 
Biotite phenocrysts very rare, not essential, i to 3 mm., tabular, brown. Groundmass, rather 
dark gray, aphanitic. 
Microscopic characters. Holocrystalline, mediophyric, sempatic. Phenocrysts: about 40 
per cent, leucite, augite, sometimes olivine and biotite. Groundmass: about 60 per cent, holo- 
crystalline, somewhat intersertal fabric, augite, labradorite, nephelite, leucite, olivine, magnetite, 
apatite. 
Labradorite, Ab 2 An 3 . Phenocrysts: 2 to o per cent, i to 2 mm., subhedral, stout prismatic, 
twinned, few inclusions. Groundmass: about 15 per cent, 0.05 to 0.50 mm., thin prismatic 
or tabular, much twinned, arrangement diverse. 
Leucite. Phenocrysts : about 35 per cent, 0.5 to 3 . o mm., subhedral to anhedral, equant to 
irregular, sometimes in clusters, inclusions few. Groundmass : about 2 per cent, o . 05 to o . 10 mm., 
anhedral, equant and irregular. 
Nephelite. Groundmass: 5 to 10 per cent, anhedral, as interstitial cement. 
Augite. Phenocrysts: about 5 per cent, often less, 0.5 to 3 . o mm., subhedral to anhedral, 
prismatic or fragmentary, pale yellowish-brown or yellowish-green, nonpleochroic, inclusions 
few. Groundmass: about 25 per cent, 0.05 to 0.20 mm., anhedral, prismatic, fusiform, 
equant, pale greenish- or yellowish-gray, arrangement generally diverse, sometimes tangential 
about the leucite phenocrysts, no inclusions. 
Biotite. Phenocrysts: 2 to o per cent, 0.5 to 3.0 mm., subhedral, tabular, brown, usually 
altered. 
Olivine. Phenocrysts: 2 to o per cent, 0.5 to 2.0 mm., subhedral to anhedral, equant 
to irregular, colorless. Groundmass: about 3 per cent, not always present, 0.05 to o.iomm., 
anhedral, equant, colorless. 
Magnetite. Groundmass: about 2 per cent, 0.02 to 0.05 mm., anhedral, equant, and 
irregular. 
Apatite. Groundmass: about i per cent, 0.05 to 0.20 mm., subhedral, prismatic. 
Chemical composition and norm as on p. 104. 
Type specimen from flow of 1872, below Observatory, Mount Vesuvius, Campanian 
District. 
