62 THE ROMAN COMAGMATIC REGION. 
generally in the trachytes, they are distinguished from the most typical of these by the 
abundance of labradorite and by their chemical composition, on account of which 
the name of vulsinite has been bestowed on rocks of this type. To distinguish 
them from the vulsinites which belong to the type of bolsenal vulsinose, with the 
much smaller and less abundant phenocrysts, the present rocks may be referred to 
the Arso type, while the others are of the Bolsena type. 
ARSAL VULSINOSE-CIMINOSE. I-II. 5.2.2. 
Megascopic characters. Light gray, compact, rough, porphyritic. Feldspar phenocrysts 
rather numerous, large, tabular, colorless. Augite phenocrysts rare, very small, prismatic, black. 
Biotite phenocrysts very rare, thin tabular, brown. Groundmass: very light gray, phanerocrys- 
taDine, very fine-grained. 
Specific gravity 2.611 at 11 C. 
Microscopic characters. Holocrystalline, megaphyric, dopatic. Phenocrysts : about 20 per 
cent, orthoclase, labradorite, augite, biotite. Groundmass: about 80 per cent, orthoclase, labra- 
dorite, augite, magnetite, nephelite. Fabric trachytic. 
Orthoclase. Phenocrysts: about 10 per cent, 5 to 10 mm. long, subhedral, tabular, Carls- 
bad twinning common, inclusions few, of augite and labradorite. Groundmass: about 50 per 
cent, 0.05 to o. 20 mm., anhedral, stout prismatic and irregular equant, arrangement subparallel. 
Labradorite, Ab r An 2 . Phenocrysts: about 5 per cent, 3 to 7 mm., subhedral, tabular, 
much twinned, inclusions few, of augite and magnetite. Groundmass: about 10 per cent, 
0.05 to o.2omm., anhedral, stout prismatic, arrangement subparallel. 
Augite. Phenocrysts: about 4 per cent, 0.5 to 2.0 mm., anhedral, stout prismatic and 
fragmentary, pale greenish-gray, non-pleochroic. Groundmass: about 5 per cent, 0.02 to 
0.06 mm., anhedral, prismatic and equant, colorless or very pale-greenish. 
Nephelite. Groundmass : about 5 per cent, rarely subhedral, mostly anhedral, inter- 
stitial between the groundmass feldspar laths. 
Biotite. Phenocrysts: about 3 per cent, 0.5 to 2.0 mm., subhedral, thin tabular, light 
brown, somewhat altered at the edges. 
Magnetite. Groundmass: about 3 per cent, o.oi to 0.05 mm., anhedral, equant. 
Apatite. Groundmass: about 0.5 per cent, subhedral, prismatic. 
Chemical composition, norm, and mode as above. 
Type specimen from near Vetralla, Ciminian District. 
II. 5. 2. 2. Piescolal Ciminose [Ciminite, Piescoli Type], 
Megascopic characters. This type is related texturally to the arsal ciminose, 
just as is the bolsenal vulsinose to arsal vulsinose, the difference consisting in the 
size and number of the feldspar phenocrysts. It differs also modally from the arsal 
type in the presence of olivine instead of biotite, though this is of comparative 
unimportance. 
The rocks are rather light to rather dark gray, and in the hand specimen show 
but few and small phenocrysts of feldspar, which are tabular and not very con- 
spicuous. There are also small phenocrysts of black augite and of yellow olivine, 
but these are not very numerous. The gray groundmass is quite aphanitic, and 
most of the specimens are compact, though a few are somewhat vesicular. 
Microscopic characters. The not very abundant phenocrysts of feldspar are 
seen in the thin section to be of both orthoclase and of a labradorite of about the 
composition A^An^ They are usually tabular, sometimes prismatic, and often 
