PETROLOGY. 159 
2. 2) [ciminite] and of fiordinal fiasconose (III. 7. 3. 2) [leucite-basanite], in which 
latter type it is abundant as large phenocrysts. It is also a frequent accessory in 
the various types at the Vesuvius Volcano, but is not as important or constant a 
constituent here as is often supposed. Its features are the usual ones and call 
for no comment, as they are not characteristic of the region. 
Melilite. While this mineral is, as usual, very rare, and is present in notable 
amount only in the type boval albanose (III. 8. 2. 2) [cecilite], its occurrence in 
these rocks may be noted as rather peculiar to the region. It would seem to be 
more widespread than has been thought, but any description of it is needless. In 
this connection its occurrence as an abundant and essential component of the cellal 
venanzose (IV. i s . i 3 . 2) [venanzite] at the small cone of Pian di Celle, San 
Venanzo, Umbria, is worthy of attention, since this small cone is possibly connected 
with the main line of volcanoes. 
Absence of minerals. The most notable absentee is hornblende, which may 
virtually be said not to exist among the rocks of the region. Extremely small 
amounts of a barkevikitic hornblende are present in one or two types, and breis- 
lakite (which is usually referred to the hornblendes) occurs in the crevices of types 
of braccianose, vesuvose, and albanose in the Sabatinian, Latian, and Campanian 
Districts. But these occurrences are exceptional and do not invalidate the general 
statement that hornblende, as a true rock component, is absent from the region. 
In view of the general character of the magmas and the fact that silica is 
scarcely ever in excess, the absence of quartz is not surprising, though it might be 
looked for among the harzose rocks. The majority of these are vitreous and show 
no quartz, but it has been observed among these rocks at the Cimino Volcano by 
Mercalli, though it does not seem to be common. 
Among the more uncommon minerals, titanite is extremely rare, having been 
observed in only a few cases in very small amounts. Zircon is still rarer, and, 
indeed, no authentic occurrences are met with. The pyroxenic and hornblendic 
minerals which contain the oxides of the rare earths, such as lavenite, rosenbuschite, 
hiortdahlite, etc., are practically absent, lavenite occurring as a rare accessory in 
ischial phlegrose. This subrang is sodipotassic, and the general association of 
these minerals with magmas rich in soda is well known. 
Textural Characters. 
Although the textural characters are very largely determined by the conditions 
of solidification and are more remotely, if at all, dependent on the chemical com- 
position of the magmas, so that they can only rarely and in a subordinate degree 
be considered as characteristic of a comagmatic region, there are certain features 
which merit a brief discussion. 
As regards crystallinity the rocks of the Roman Region, which are wholly 
effusive flows, and their tuffs, are somewhat remarkable for the rarity of hyaline 
types. In the great majority of cases the rocks are holocrystalline, and this is 
