PETROGRAPHY. 41 
dered it evident that at least the majority of the occurrences fall in vulsinose, so 
that, as a description is called for on account of their importance in the Ciminian 
District, it may be inserted here. 
The rocks are light gray, megaphyric, and dopatic to perpatic. The majority 
of the phenocrysts are of feldspar, mostly of orthoclase, but with about half as much 
labradorite. These are up to 20 or 30 mm. in length and euhedral to subhedral 
tabular. The feldspar phenocrysts constitute from 10 to 15 per cent of the rock. 
Phenocrysts of leucite are less numerous, making up not more than about 5 per cent 
of the rock. They vary from 5 to 10 mm. in diameter, are anhedral equant, and 
always a dull white through kaolinization. Very few phenocrysts of augite and 
biotite are also present, of about the size of the leucites. 
The groundmass is either very fine grained or aphanitic, and is quite often 
somewhat vesicular, though never scoriaceous. It is also holocrystalline and closely 
resembles that of the bolsenal type, composed in very large part of small laths of 
orthoclase, which have a subparallel arrangement. With these are a very few 
small (0.05 to o.oi mm.), round spots of colorless, isotropic substance, which is prob- 
ably to be referred to leucite. Their amount, however, is negligible. There are 
the usual small prismoids and irregular grains of colorless augite, and fewer of 
magnetite, but none of the interstitial biotite seen in the preceding phase. Here 
this mineral seems to have developed entirely as phenocrysts. 
The occurrence of these rocks is confined to the Ciminian District, where they 
are rather common, being known locally as "petrisco." The typical "petrisco" is 
best developed in the northeastern part of the district, where it forms extensive 
flows from the Vico Volcano. The largest of these originated apparently on the 
north flank of Vico, and passed northward around Monte Pallanzana, which 
divided it into two branches. From this flow characteristic specimens were obtained 
by me at the Villa Lante near Bagnaia and at Cavorcie on the north branch and 
at I Cappuccini on the south branch, as well as from other localities. Mercalli 
describes several flows of "petrisco" on the east and south of Monte Vico, between 
Capranica and Ronciglione, but whether they are strictly of this type or not is 
uncertain, as I was unable to visit them. 
As stated above, no analysis was made of this type, on account of the weathered 
condition of all the specimens, so a formal description is uncalled for. 
I. 6-5. 2. 3. Paglial Procenose-Pulaskose [Leucite-Trachyte, Paglia Type]. 
Megascopic characters. In the hand specimen the rocks of this type are much 
like those of the type of sabatinal beemerose to be described later, the main differ- 
ence being the somewhat darker groundmass. They are medium gray in color, 
very compact, and markedly porphyritic. Leucite phenocrysts are not very numer- 
ous, making up but about one-tenth of the rock, but they are conspicuous and quite 
large, from 5 to 20 mm. in diameter. Most of them show crystal planes, but 
all are more or less fragmentary, so that completely bounded trapezohedra are very 
