102 THE ROMAN COMAGMATIC REGION. 
Orthoclase, Or 3 Ab : 10.5 
Labradorite, OrjAbj 26. i 
Leucite 29.0 
Nephelite 2.8 
Augite 21.6 
Barkevikite.. . 2.0 
Olivine 5.0 
Magnetite 2.0 
Apatite i . o 
In this mode the decidedly alferric character and the predominance of labra- 
dorite over orthoclase are marked. As compared with the viterbal and bagnoreal 
types of vicose the alkali-feldspar contains a much smaller proportion of the albite 
molecule, while the labradorite is distinctly more sodic, being on the border of 
andesine. In these respects it corresponds very closely with the mode of orvietal 
auruncose described above. 
As compared with the norm it is clear that the only differences of serious 
importance are, as usual, those involved in the formation of leucite, much of the 
normative orthoclase being used for this, and the nephelite, changed to albite, 
entering into the feldspars. The type may therefore be described as leucite interserti- 
aphyro- vicose. 
Occurrence. So far as known orvietal vicose occurs only in the Vulsinian Dis- 
trict. It is abundant near the town of Orvieto, especially to the south of this, as 
at Monte Cavallo and Porano, where it forms extensive flows. It also makes up 
the main part of the hill of Monte Bisenzo on the southwest shore of Lake Bolsena. 
Name. The derivation of the subrang name is discussed above. That of 
the type is based on the name of the town of Orvieto, near which it occurs in 
abundance. 
In the prevailing systems of classification the name of leucite-tephrite, which 
was applied to the type in a previous description, is the appropriate one, on account 
of the quantitative predominance of labradorite over orthoclase, as well as its greater 
conspicuousness in the thin section. The amount of olivine is so small that its 
presence may be disregarded, and the name of leucite-basanite is unwarranted. 
ORVIETAL VICOSE. II. 6. 2. 2. 
Megascopic characters. Medium gray, compact, aphyric, very fine-grained, phanero- 
crystalline. 
Microscopic characters. Hole-crystalline, microporphyritic, intersertal fabric. Micropheno- 
crysts: about 50 per cent, leucite, labradorite, orthoclase, augite. Microgroundmass : about 50 
per cent, augite, orthoclase, olivine, barkevikite, nephelite, magnetite, apatite. 
Orthoclase, OrjAb^ Microphenocrysts: about 3 per cent, o.i to 0.5 mm., subhedral, 
thin tabular, twinning uncommon, arrangement diverse. Microgroundmass : about 8 per cent, 
anhedral, as interstitial areas. 
Labradorite, AbjAn^ Microphenocrysts: about 25 per cent, o. i to 0.5 mm., subhedral 
thin, tabular, twinning common, arrangement diverse. 
Leucite. Microphenocrysts: about 30 per cent, o.i to i.omm., anhedral, equant, more 
often irregular, often incloses the feldspar tables, inclusions few. 
Nephelite. Microgroundmass: about 2 per cent, anhedral, formless, interstitial areas. 
Not always present. 
