PETROGRAPHY. 
The distribution of the soda was fixed by the composition of the plagioclase 
and the use of the usual equations. 
CALCULATED. 
MEASURED. 
Orthoclase, Or 2 Ab t .. . 
Labradorite, Ab 2 An 3 . . 
Nephelite 
Leucite 
Augite 
Olivine 
Biotite 
Magnetite 
Apatite 
26.0 
28.0 
i.o 
15-8 
16.8 
2.O 
3-4 
6.0 
i.o 
100. O 
Vol. %. Sp. gr. 
62.2 X 2.6 = 161.7 
17.4 X 2.5 = 43-5 
15-6 X 3-3 
Wt. %. 
58-9 
3.1 X 2.9 
i-7 X 5-2 
51-5 
9.0 
3-3 
3-2 
274.5 100. o 
The two modes resemble each other very closely, except that in the calculated 
one the amount of magnetite is considerably greater than in the measured. As 
compared with the norm, the only difference of importance is the modal presence 
of leucite, which replaces much of the orthoclase and changes most of the normative 
nephelite to albite. The readjustments involved by the presence of augite and 
biotite are small and negligible. The type may therefore be described as leucite- 
grani-auruncose. 
Occurrence. Orvietal auruncose is a rare rock, the only known example 
forming flows near Toscanella, in the Vulsinian District. It very probably occurs 
elsewhere in the same district, and possibly in the Auruncan, but no other rocks of 
similar type can, as yet, be definitely referred to this magma. 
Name. The name of the subrang is derived from the Auruncan District, 
where this magma seems to be common, though usually of another type. The 
type name is derived from the city of Orvieto in the Vulsinian District, near which 
a homologous type of the vicose magma is met with. 
In the prevailing systems of classification the type would be called a leucite- 
tephrite, as stress is laid especially on the soda-lime feldspar. 
ORVIETAL AURUNCOSE. II. 5. 3. 2. 
Megascopic characters. Light gray, compact, aphyric, very fine-grained, phanerocrystalline. 
Microscopic characters. Holocrystalline, serial heterometric. Orthoclase, labradorite, 
leucite, augite, biotite, olivine, ores, nephelite, apatite. Fabric subintersertal. 
Orthoclase, Or 2 Ab r . About 25 per cent, 0.05 to 0.40 mm., anhedral, irregular patches, 
largely intersertal, untwinned. 
Labradorite, Ab 2 An 3 . About 28 per cent, o.io to 0.40 mm., subhedral to anhedral, 
tabular, twinned, arrangement diverse. 
Lettcite. About 16 per cent, 0.05 to 0.20 mm., anhedral, equant, small inclusions of 
augite, zonally or centrally arranged. 
Nephelite. About i per cent, anhedral, interstitial cement. 
Augite. About 17 per cent, 0.05 to o. 20 mm., subhedral to anhedral, prismatic to equant, 
pale gray, no inclusions. 
