PETROGRAPHY. 91 
Microscopic characters. Holocrystalline to percrystalline, mediophyric ) "dopatic. Pheno- 
crysts: about 30 per cent, augite, biotite, labradorite, orthoclase. Groundmass, about 70 per 
cent, orthoclase, labradorite, augite, magnetite, apatite, and a colorless base, sometimes aniso- 
tropic, occasionally isotropic. Fabric trachytic. 
Orthoclase, OraAbj. Phenocrysts: about 4 per cent, 0.30 to i mm., euhedral to sub- 
hedral, stout prismatic, sometimes twinned, inclusions often zonally arranged, few. Ground- 
mass: about 30 per cent, 0.0310 o.i mm., subhedral to anhedral, slender prismatic and 
interstitial. 
Labradorite, Ab 2 An 3 . Phenocrysts: about 7 per cent, 0.50 to 2.0 mm., euhedral to sub- 
hedral, stout prismatic, twinned, inclusions as in orthoclase. Groundmass: about 25 per cent, 
0.03 to o.iomm., subhedral slender prismatic, twinned. 
Augite. Phenocrysts: about 8 per cent, 0.5 to 2.0 mm., euhedral to subhedral, stout 
prismatic, pale greenish. Groundmass: about 5 per cent, o.oi to 0.05 mm., subhedral, pris- 
matic, very pale greenish. 
Biotite. Phenocrysts: about 8 per cent, 0.5 to 2.0 mm., subhedral, stout tabular, brown 
or greenish brown, usually altered on edges. Groundmass: none. 
Magnetite. Groundmass: about 5 per cent, o.oi to o. 10 mm., anhedral, equant. 
Apatite. Groundmass: about i per cent, 0.02 to 0.05 mm., subhedral, slender prismatic. 
Base. Groundmass: about 10 per cent, colorless, usually indefinitely anisotropic or 
microfelsitic, sometimes isotropic, probably composed of quartz and orthoclase, does not gela- 
tinize with acids. 
Chemical composition and norm as on p. 88. 
Type specimen from Monte Santa Croce, Auruncan District. 
II. 6. 2. 2. Foglianal Vicose [Leucite-Tephrite, Viterbo Type]. 
Megascopic characters. This type in the hand specimen resembles the viterbal 
vulsinose and ciminose previously described so closely that it is quite impossible to 
distinguish them in the field. It shows the same characteristic viterboid habit, 
very abundant large phenocrysts of leucite in a light gray groundmass. The leu- 
cite phenocrysts are from 5 to 20 mm. in diameter as a rule, but may run up to 50 
mm., and are highly euhedral in well-formed trapezohedra, sometimes fragmentary, 
either white or grayish, and often showing small dark inclusions. With these are few 
small black prismatic phenocrysts of augite, with still more rare prismatic pheno- 
crysts of feldspar. The groundmass is of a rather light gray, and is either quite 
aphanitic or very fine-grained phanerocrystalline, only the distinction between the 
light and dark minerals being evident. 
Microscopic cliaracters. In the thin section the large leucite phenocrysts 
exhibit the double refraction and twinned structure very clearly, and the rather 
numerous inclusions are seen to be of augite, magnetite, labradorite, and glass, 
commonly clustered toward the center. The few feldspar megaphenocrysts visible 
are mostly of a labradorite of about A^Arjj, in stout subhedral prisms, much 
twinned, while the phenocrysts of augite are in stout, subhedral prismoids, from 
0.5 to i mm. long, often fragmentary, and of the usual pale-gray or slightly greenish 
color. 
The groundmass in which these lie is holocrystalline typically, though a little 
glass base may be present in some cases. The fabric varies somewhat, but is usually 
