PETROGRAPHY. 117 
Name. The name of the type is derived from the Atrio del Cavallo, the semi- 
circular valley between Vesuvius and Somma, into which so many of the small flows 
of the present cone run, and from the southern part of which the analyzed specimen 
was obtained. 
In the prevailing systems the name of leucite-tephrite-obsidian is undoubtedly 
the correct appellation. 
ATRIAL BRACCIANOSE. II. 7. 2. 2. 
Megascopic characters. Very dark gray or black, sprinkled with small white spots, compact 
to vesicular, highly porphyritic. Leucite phenocrysts abundant, i to 3 mm., round, white. 
Augite phenocrysts, very few, i to 2 mm., prismatic, black, inconspicuous. Feldspar and olivine 
phenocrysts very rare or none, small. Groundmass: black, vitreous. 
Microscopic characters. Hyalocrystalline to dohyaline, porphyritic, dopatic to sempatic. 
Phenocrysts 20 to 40 per cent, leucite, augite, labradorite, magnetite, rarely olivine. Ground- 
mass : 80 to 60 per cent, wholly glass, sometimes dusty with microlites. 
Labradorite, Ab 2 An 3 to AbiAn 3 . Phenocrysts: 5 to 10 per cent, o.i to 0.5 mm., euhedral, 
thin prismatic or tabular, much twinned, few inclusions. 
Leucite. Phenocrysts: 20 to 30 per cent, 0.2 to 2.0 mm., euhedral to subhedral, often 
clustered, sometimes skeleton forms, inclusions few. 
Augite. Phenocrysts: 5 to 10 per cent, 0.2 to i.omm., euhedral to subhedral, equant 
to prismatic, greenish-gray, few inclusions. 
Magnetite. Phenocrysts: about i per cent, 0.02 to 0.05 mm., anhedral or skeletal. 
Glass. Yellowish-brown, often clear, sometimes mottled, sometimes dusty with indeter- 
minable microlites. 
Chemical composition and norm as on p. 116. 
Type specimen from flow of 1903, Valle del Inferno, Mount Vesuvius, Campanian District. 
II. 8-7. 2. 2. Scalal Vesuvose-Braccianose [Leucite-Tephrite, Scafa Type]. 
Megascopic characters. This type is porphyritic, but not as much so as in those 
just described. The phenocrysts are mostly of augite, with rare ones of olivine, but 
few or none of leucite, and do not make up more than about 10 per cent of the rock 
volume. The augites are from 2 to 5 mm. long, and prismatic rather than equant, 
the color a dark green. The rare olivines are about the same size, equant, and pale 
greenish-yellow. The groundmass in this type is a rather light gray, not nearly as 
dark as the preceding rocks, and is phanerocrystalline and fine-grained, the dark 
and light mineral particles being easily distinguishable with a lens. In the field 
rocks of this type would be called augite-leucophyres. 
Microscopic characters. The augite phenocrysts show subhedral and often 
fragmentary forms, and are of the usual very pale-gray color and quite free from 
inclusions. The very rare olivines present no features of special interest, but are 
not well developed crystallographically. An occasional leucite phenocryst is seen 
here and there, but these are as rare as are phenocrysts of augite in the vesbal type. 
The groundmass is holocrystalline typically, though a trifling amount of glass 
may rarely be present. It is microporphyritic, small phenocrysts of leucite and 
augite lying in a microgroundmass which resembles the preceding in its intersertal 
fabric. The leucite microphenocrysts are very abundant, as is expected, since it 
does not occur as megaphenocrysts. They are anhedral and mostly spheroidal, 
