30 THE ROMAN COMAGMATIC REGION. 
I. 5. 2. 2. Bolsenal Vulsinose [Vulsinite, Bolsena Type]. 
As is stated elsewhere, this type is homologous texturally with the cumal type 
of phlegrose, differing from the arsal type just as the cumal differs from the 
ischial. Modally, on the other hand, the arsal and bolsenal vulsinose differ 
from ischial and cumal phlegrose by the presence of considerable anorthite or 
labradorite. 
Megascopic characters. These rocks are light gray, in specimens from one 
locality somewhat mottled with streaks of darker gray and yellowish. As the 
"schlieren" seem to be due rather to slight weathering than to essential differences 
in composition, they are not of sufficient importance to justify the separation of 
these specimens from the others as a distinct type. While the type is distinctly 
porphyritic, it is not conspicuously so, as in the preceding one. The phenocrysts, 
which are mostly of feldspar with fewer of augite and biotite, are small, from i to 5 
mm. long, and not very abundant, making up not more than one-fifth of the rock 
volume. The feldspar and biotite phenocrysts are tabular, as usual, while those of 
augite are stoutly prismatic. The light-gray groundmass is aphanitic. 
Microscopic characters. The tabular phenocrysts of feldspar are seen in thin 
section to be of both orthoclase and anorthite, the former more abundant than the 
latter. They are both subhedral, with some crystal planes, occasionally fragmen- 
tary. Carlsbad twinning is common in the former feldspar, while the anorthite is 
often quite untwinned, though usually with multiple lamellae. Zonal structure is 
not often seen, and inclusions are not very common, of augite, magnetite, apatite, 
and glass in both, and also of anorthite in the orthoclase. In many specimens the 
phenocrysts of both feldspars are surrounded by a border of alkali-feldspar sub- 
stance of later growth, which is oriented parallel crystallographically with the 
inclosed crystal, extinguishes simultaneously all around it, and extends a short dis- 
tance into the groundmass, without definite form. The augite phenocrysts call for 
little notice. They are mostly subhedral, in stout prismoids, of the usual pale-gray 
color so characteristic of the rocks of the region, and contain few inclusions of mag- 
netite and of glass. The biotite phenocrysts are tabular, with irregular outlines, of 
a pale brown, and are uniformly much altered in the usual way. A few phenocrysts 
of magnetite, in irregular grains, may generally be seen. 
The groundmass is holocrystalline and composed in great part of small pris- 
moids of soda-orthoclase, which frequently have a subparallel arrangement, giving 
rise to a trachytic fabric. With them are some very small prismoids of colorless 
augite and still fewer small anhedra of magnetite. A little titanite and apatite are 
seen in accessory amounts, and some specimens show rare flakes and irregular areas 
of a pale-brownish hornblende; but none of these minerals can be considered as 
essential to the type. 
