120 THE ROMAN COMAGMATIC REGION. 
anorthite is just sufficient to yield the right amount of albite for the observed com- 
position of the labradorite, using the regular equations. 
Labradorite, Ab 3 An 3 13 . 8 
Leucite 35-6 
Nephelite 9.9 
Augite 31.8 
Olivine 6.1 
Ores i . o 
Apatite 1.8 
100.0 
This result is very similar to the mode of the vesbal type, and is in harmony 
with what we have noted of the rock characters. The amount of labradorite is lower 
and that of leucite higher in accord with its transitional magmatic position, and the 
slightly greater quantities of augite and olivine and less of ores agree with the micro- 
scopic observations. The presence of considerable nephelite was confirmed by 
treatment with very dilute acid, as in the previous case. 
Occurrence. Rocks of this type are only met with in the Campanian District, 
especially at Mount Vesuvius. Here they are most typically represented by the 
great flows of 1631, which reached the sea at various points between Portici and 
Torre Annunziata. These are quarried extensively for use in Naples, especially at 
Granatello near Portici and La Scala near Torre del Greco. Specimens from the 
following flows, which have been examined by me, also belong to this type: 1551, 
1555, 1806, 1810, 1822, 1855. The descriptions of Fuchs and others do not allow 
of discrimination between this type and another with more abundant and larger 
augite megaphenocrysts. 
Name. The type derives its name from the locality, La Scala, of the quarry 
whence the material analyzed was derived. In prevailing classifications it would be 
known as a leucite-tephrite, as in the preceding types. 
SCALAL VESUVOSE-BRACCIANOSE. II. 8-7. 2. 2. 
Megascopic characters. Light gray, compact, somewhat porphyritic. Augite pheno- 
crysts not abundant, 2 to 5 mm. long, prismatic, dark-green or black. Leucite phenocrysts very 
rare or none, i to 3 mm., round, pale-gray or white. Olivine phenocrysts very few or none> 
0.5 to 2 mm., equant, pale greenish-yellow. Groundmass, light gray, very fine-grained, phanero- 
crystalline. 
Microscopic characters. Holocrystalline, mediophyric, perpatic. Phenocrysts: about 10 
per cent, augite, sometimes also leucite and olivine. Groundmass, about 90 per cent, micro- 
porphyritic. Microphenocrysts : about 50 per cent, leucite, augite. Microgroundmass: about 40 
per cent, labradorite, augite, nephelite, magnetite, olivine, apatite. 
Labradorite, Ab 2 An 3 . Groundmass: about 14 per cent, 0.05 to 0.20 mm., subhedral, 
thin tabular or prismatic, twinned, arrangement diverse. 
Leucite. Phenocrysts: none to 2 per cent 0.5 to 2.0 mm., anhedral, equant to irregular. 
Microphenocrysts: about 35 percent, o. 1 100.3 mm., anhedral, equant to irregular, inclusions few. 
Nephelite. Groundmass: about 5 to 10 per cent, anhedral, as interstitial cement. 
Augite. Megaphenocrysts: about 10 per cent, 2 to 5 mm., subhedral, prismatic, some- 
times fragmentary, greenish-gray, few inclusions. Microphenocrysts: about 10 per cent, o. i to 
0.4 mm., subhedral to anhedral, prismatic to equant, pale greenish-gray. Microgroundmass: 
about 10 per cent, 0.02 to o.iomm., anhedral, equant. 
