78 THE ROMAN COMAGMATIC REGION. 
shown by the Cavaliere rock, this carries the leucite and olivine molecules crystal- 
lized as these minerals, and not combined into biotite. 
It is clear that the modes of both specimens are essentially the same, as the 
amounts of biotite in the one and of leucite and olivine in the other are so small as 
to be quite negligible. As compared with the norm the differences are negligible 
also in both cases, and the type may be described, neglecting the small quantities of 
alferric phenocrysts, as normative salphyro-monzonose. 
Occurrence. This type is not abundant in the Roman Region, occurring rather 
sparingly in but two districts, the Ciminian and the Campanian. In the former it 
is best represented by a massive flow below the Poggio Cavaliere, on the south shore 
of Lake Vico, and it also forms blocks in the tuffs on the northwest slope of the Vico 
Volcano, especially near the Villa Balestra on the Via Aurelia, about 4 km. south- 
west of Viterbo. In the Campanian District it forms the flow of 1302 A. D., called 
L'Arso, on the island of Ischia, and a vesicular form occurs at Le Cremate, the 
point of eruption above the flow. 
The type of bolsenal monzonose, which is based provisionally on the descrip- 
tion and analysis of De Lorenzo and Riva, is met with as blocks at the Astroni Vol- 
cano in the Phlegrean Fields, but is not yet known outside the Campanian District, 
though it probably occurs elsewhere. 
Name. The subrang name is derived from that of Brogger's group of mon- 
zonites, many typical examples of which fall in this subrang. The type name is 
derived from the well-known locality of L'Arso. The possibility and the advisabil- 
ity of distinguishing two types, the arsal and the vetrallal, based on the conspicuous- 
ness of the feldspar phenocrysts and the presence of olivine or biotite, has already 
been discussed under the type of arsal vulsinose-ciminose (p. 61). 
The remarks on the position of the homologous types of vulsinose and ciminose 
in the prevailing systems are applicable here. In general these rocks would be called 
trachytes, but they are commonly recognized as rather abnormal. The Poggio 
Cavaliere rock would be a typical vulsinite, while that of L'Arso is distinguished by 
Rosenbusch from the true trachytes as the type of the Arso-typus, is often spoken 
of as an oh'vine-trachyte, and was formerly called a ciminite by me. It is note- 
worthy that the present measurements show the very small amount of olivine actually 
present, and emphasize again the prevalent tendency to exaggerate the importance 
of unusual constituents which may be present in negligible amounts. 
De Lorenzo and Riva call attention to the chemical divergence of the bolsenal 
monzonose from the similar types which fall in vulsinose, and make this clear by 
naming the vulsinose rocks vulsinite, while the monzonose type is referred to as a 
trachydolerite. 
ARSAL MONZONOSE. II. 5. 2. 3. 
Megascopic characters. Medium gray, usually compact, porphyritic. Feldspar pheno- 
crysts common, 3 to 10 mm. long, stout prismatic, white, rather conspicuous. Augite pheno- 
crysts rare, small, prismatic, black. Biotite phenocrysts not always present, few, small, tabular, 
