162 THE ROMAN COMAGMATIC REGION. 
The magmas of the Vulsinian District are hence highly varied and the number 
of types large. Furthermore, there is evident a progressive change in composition 
from northwest to southeast. Thus the rocks of the northern and northwestern 
portions are very largely either persalane or dosalane, the former apparently the 
more common; those of the Bolsena and Capodimonte volcanoes on the northeast 
and southwest of the lake are predominantly in the dosalane class; while to the 
southeast, around the Montefiascone Volcano, salfemane rocks are the largely 
predominant ones; and furthermore, it is only in this portion that we find olivine 
and melilite entering into the mode to a notable extent. While this progression 
is not without exceptions and irregularities, and although my knowledge of the 
northern and northwestern parts of the district is not yet satisfactorily complete, 
the observations and collections which I was able to make seem to be sufficiently 
ample and the facts observed sufficiently concordant to justify the assumption 
that such a progressive magmatic change does occur. Our belief in the validity of 
this supposition is confirmed by the observation of similar relations in other districts. 
Of the various subrangs, ciminose, vicose, and braccianose are probably the 
most abundant, with vulsinose and albanose less so, though still common. The 
other subrangs are of comparative unimportance, with the possible exception of 
vesuvose, pulaskose, and procenose. 
CIMINIAN DISTRICT. 
The volcanic structure of this district is simpler than that of the preceding 
one, only two volcanoes being found, and these clearly distinct. Petrographically, 
also, the character is less complex, the number of subrangs represented being only 
seven. 
Persalane rocks are abundant, though only two subrangs are represented. Of 
these cumal phlegrose (I. 5. i. 3) [phonolitic trachyte] is quite common in blocks 
about the Vico Volcano, though the total amount is probably small. On the other 
hand, vulsinose (I. 5. 2. 2) is very common, partly of the non-leucitic arsal type 
as flows and tuffs about the Cimino Volcano, and to a less extent around the Vico 
Volcano; partly of the pallanzanal type, known locally as "peperino," with small 
and not numerous feldspar and leucite phenocrysts at the Cimino Volcano; and 
very abundantly at the Vico Volcano of the viterbal type, with large leucite pheno- 
crysts, though most of these rocks are more or less transitional toward ciminose. 
The dosalane rocks vie with the persalanes in point of abundance. Heading 
the list systematically is harzose (II. 4. 3. 3), the sorianal type of which is so common 
at the Cimino Volcano. The occurrence of this subrang here is noteworthy as 
the only quaric subrang found along the whole line of volcanoes of the Roman 
Region, if we except the Amiata Volcano, where toscanose (I. 4. 2. 3) and amiatose 
(I. 4- 3- 3) occur. 
The subrang ciminose (II. 5. 2. 2) is, with vulsinose, the most abundant of 
the district. The magnophyric non-leucitic arsal type [vulsinite] forms extensive 
