PETROGRAPHY. 85 
TEANAL C1MINOSE-AURUNCOSE. II. 5. 2-3. 2. 
Megascopic characters. Medium gray, compact, porphyritic. Feldspar and leucite pheno- 
crysts few, i to 5 mm., white, not very conspicuous. Augite phenocrysts few, i to 2 mm., pris- 
matic, black. Biotite phenocrysts few, i to 4 mm., thin tabular. Groundmass, gray, aphanitic. 
Microscopic characters. Holocrystalline, mediophyric, perpatic to dopatic. Phenocrysts: 
about 15 per cent, leucite, biotite, orthoclase, labradorite, augite. Groundmass: about 85 per 
cent, somewhat interstitial fabric, orthoclase, labradorite, leucite, augite, magnetite, apatite, 
nephelite. 
Orthoclase, Or 2 Abj. Phenocrysts: about 2 per cent, i to 2 mm., subhedral, stout pris- 
matic, untwinned. Groundmass: about 28 per cent, 0.05 to 0.20 mm., anhedral, irregular 
patches, sometimes interstitial. 
Labradorite, A^An^ Phenocrysts: about 2 per cent, i to 2 mm., subhedral, stout pris- 
matic, twinned. Groundmass: about 22 per cent, 0.02 to o. 10 mm., subhedral, thin prismatic, 
twinned, arrangement diverse. 
Leucite. Phenocrysts: about 4 per cent, i to 5 mm., subhedral, equant, inclusions few. 
Groundmass: about 12 per cent, 0.05 to o. 10 mm., anhedral, spheroidal, few inclusions. 
Nephelite. Groundmass : about i per cent, anhedral, interstitial, difficult to detect. 
Augite. Phenocrysts: about 5 per cent, 0.30 to 2.0 mm., subhedral to anhedral, stout 
prismatic to equant and irregular, pale greenish-gray, few inclusions. Groundmass: about 
13 per cent, 0.02 to o. 10 mm., subhedral, prismatic, very pale greenish-gray. 
Biotite. Phenocrysts: about 4 per cent, i to 4mm., subhedral, thin tabular, brown, 
usually much altered. Groundmass: none. 
Magnetite. Groundmass: about 5 per cent, o.oi to 0.02, anhedral, equant. 
Apatite. Groundmass: about i per cent, 0.02 to 0.05 mm., subhedral, prismatic. 
Chemical composition and norm as on p. 83. 
Type specimen from below Orchi, Auruncan District. 
II. 5. 3. 2. Orvietal Auruncose [Leucite-Tephrite, Orvieto Type]. 
Megascopic characters. These rocks are of a uniform ash-gray color, and differ 
from those yet described in being entirely devoid of phenocrysts. They are, how- 
ever, not aphanitic, since the mineral particles are distinguishable to the naked eye, 
on close inspection, as white and dark, the former predominating. 
Microscopic diameters. Study of the thin sections shows that micropheno- 
crysts are almost entirely wanting, only very few individuals of augite and an excep- 
tional one of feldspar attaining dimensions notably greater than others, and the 
length even of these is seldom more than 0.5 mm. The texture shows no marked 
hiatus in the sizes of the particles, and the extreme limits in size of the various 
minerals are approximately the same. The fabric is intersertal, through the diverse 
arrangement of the feldspar laths. 
The feldspar, which constitutes about 50 per cent of the rock, is in part a sodic 
orthoclase, in untwinned anhedra, up to 0.4 mm., in length. The borders are very 
irregular, and this mineral plays the r61e, to a large extent, of mesostasis for the 
other constituents. There is about an equal amount of labradorite, with the average 
composition Ab 2 An 3 , in small plates and laths, which exhibit the usual multiple 
twinning. Leucite is quite abundant, about 16 per cent, in equant anhedra from 
0.05 to 0.2 mm. in diameter, which often carry a few inclusions of colorless augite 
either centrally or zonally arranged. The augite, which forms about one-fifth of 
