34 THE ROMAN COMAGMATIC REGION. 
BOLSENAL VULSINOSE. I. 5. 2. 2. 
Megascopic characters. Light gray, sometimes mottled through darker schlieren, compact 
porphyritic. Feldspar phenocrysts rather abundant, i to 5 mm., stout prisms, clear, glassy. 
Augite phenocrysts few, 0.5 to i mm., prismatic, black. Biotite phenocrysts very few, 0.5 
to i mm., thin tabular, bronzy brown. Groundmass: light gray, aphanitic, rough fracture. 
Specific gravity, 2.534 at 25 C. 
Microscopic characters. Holocrystalline, megaporphyritic, dopatic, mediophyric. Pheno- 
crysts: about 20 per cent, orthoclase, anorthite, augite, biotite. Groundmass: about 80 per 
cent, trachytic fabric, orthoclase, labradorite, augite, magnetite, apatite, titanite. 
Soda-orthoclase, Or 9 Ab 4 . Phenocrysts: about 10 per cent, i.o to 5mm., subhedral, 
prismatic, Carlsbad twinning common, often surrounded by a mantle of later orthoclase sub- 
stance extending irregularly into the surrounding groundmass. Groundmass: about 60 per 
cent, 0.05 to 0.2 mm., anhedral, subprismatic and irregular, arrangement often subparallel. 
Anorthite. Phenocrysts: about 6 per cent, 0.5 to 3.0 mm., subhedral, stout prismatic, 
often equant, usually multiply twinned, often surrounded by later mantle of orthoclase like 
alkali-feldspar phenocrysts. Groundmass: none, except as below. 
Labradorite, AbjAn^ Phenocrysts none. Groundmass: about 12 percent, 0.05 to o.2omm., 
subhedral, prismatic to irregular, arrangement subparallel. In many specimens there is no 
difference in composition between the plagioclase of the phenocrysts and of the groundmass, 
both having about the composition AbjAnz. 
Augite. Phenocrysts : about 2 per cent, 0.2 to i.omm., subhedral, stout prismatic and 
fragmentary, colorless or pale greenish-yellow. Groundmass : about 5 per cent, o . 01 to o . 03 mm., 
anhedral, prismatic and equant, gray or pale yellowish, 
Biotite. Phenocrysts: about i per cent, 0.5 to i.omm., subhedral, thin tabular, brown, 
usually altered. Groundmass, none. 
Magnetite. Groundmass: about 4 per cent, o.oi to 0.2 mm., anhedral, equant. 
Apatite and titanite. Very rare accessories, in minute crystals of usual form. 
Chemical composition and norm as on p. 31. 
Type specimen from below castle, Bolsena, Lake Bolsena, Vulsinian District. 
I. 5. 2. 2. Viterbal Vulsinose [Leucite-Trachyte, Viterbo Type]. 
With this type of vulsinose we meet the first of the long and interesting series 
of leucitic rocks for which the Roman Region is so famous, and which give it such a 
strongly marked petrographic character. The rocks to be now described, further- 
more, exhibit a habit which is one of the best marked in the region and which rivets 
the attention of every geologist. The assumption of this leucitic mode by a per- 
salic magma is of some interest, as leucite rocks are, for the most part, of the dosalane 
or salfemane classes. 
Megascopic characters. The rocks, which would be called leucite-melaphyres 
in the field, are characterized especially by the abundance and size of the phenocrysts 
of leucite. These constitute about 40 per cent of the rock volume, and vary in diam- 
eter from 2 to 15 mm., with an average of about 8 mm. These leucites are highly 
euhedral trapezohedra, though sometimes fragmentary through breaking up during 
the flow of lava. They are lightly spotted with black inclusions of augite and glass, 
and carry as well some inclusions of labradorite, which are not distinguishable mega- 
