PETROLOGY. 161 
impossible, to translate these into the terms of the quantitative classification, on 
account of the lack of reliable analyses and the statement of the mode only in 
qualitative terms. 
The data available on which to base the distribution of types and the char- 
acterization of each district vary widely. In some cases, as the Ciminian, the 
Latian, and the Campanian districts, they are satisfactorily complete and accu- 
rate. In others, as the Vulsinian and the Auruncan, they are as yet far from being 
complete, and in such cases the descriptions must be regarded as provisional. 
VULSINIAN DISTRICT. 
This district is not only the largest of all, but is the most complex in its volcanic 
structure, as has been seen, and, furthermore, shows the greatest diversity in its 
rocks. Of the seventeen subrangs known to be represented in the Roman Region 
at least eleven are found in this district, and some are confined exclusively to it. 
Rocks of the persalane class are rather common and are represented by types 
of vulsinose (I. 5. 2. 2), pulaskose (I. 5. 2. 3), procenose (I. 6. 2. 3), and possibly 
beemerose (I. 6. i. 3). The types of vulsinose are non-leucitic, while those of 
the other subrangs mentioned are all leucitic, this mineral forming usually large 
phenocrysts. These persalic rocks are most common in the northern and north- 
western parts, as around the Latera Volcano and near Bolsena. It is perhaps 
worthy of note that the subrang phlegrose (I. 5. i. 3), which is elsewhere common, 
is apparently unrepresented in the Vulsinian District. 
The subrangs of dosalane are much more abundantly represented, and offer a 
greater variety of types. Of these subrangs ciminose (II. 5.2.2) is probably quite 
common in the northern portions. The leucitic bagnoreal type is found to the 
northeast of the lake, while non-leucitic types, as the fiescolal and arsal, seem to 
occur to the north. The alkalicalcic auruncose (II. 5. 3. 2) is rare and is confined 
to the southern portion, so far as known. The lenic subrangs, vicose (II. 6. 2. 2), 
braccianose (II. 7. 2. 2), and vesuvose (II. 8. 2. 2), are more abundant, and to 
them belong many of the flows to the east, south, and southwest of Lake Bolsena. 
The types of these are invariably leucitic, rarely with large leucite phenocrysts, as 
in the viterbal vicose near Orvieto, but much more commonly quite without pheno- 
crysts, or with only microscopic ones, as in the orvietal, galeral, and hernical types. 
Rocks belonging to salfemane are almost, if not quite, as abundant as those 
in dosalane, but they are more monotonous in type. Of these magmas albanose 
(III. 8. 2. 2) is the most common and most widely distributed, occurring at various 
localities all around the lake, though mostly to the south. In general it is of the 
romal or saccal type, but the melilitic boval albanose occurs in the neighborhood 
of Montefiascone, southeast of Lake Bolsena. The only other subrangs of salfe- 
mane which are represented, jugose (III. 7. 2. 2) and fiasconose (III. 7. 3. 2), 
are rare and seem to be confined to the southeast portion. 
