PETROLOGY. 157 
Leucite, The abundance of this mineral is one of the most characteristic 
modal features of the region, and has made the volcanoes of the Bolsena-Vesuvius 
line famous in petrography. Apart from this it can not be said that the mineral 
offers any features specially peculiar to the region, as its characters here are those 
of many of its occurrences. In certain magmas, especially those belonging to 
vulsinose, ciminose, and vicose, it forms well-formed phenocrysts of very large 
size, up to 5 or i o cm. in diameter, which are characteristic of the viterboid habit. 
It may also occur in the groundmass as small crystals, sometimes well shaped 
and again mere spheroids, and rarely as anhedral patches. The larger individuals 
usually show the birefringence and twinned structure, which are commonly want- 
ing in the smaller groundmass leucites. The larger ones also are usually relatively 
free from inclusions, while the characteristic regular arrangement of small inclusions 
in the groundmass, leucite microphenocrysts, and the frequent skeletal forms of 
these, are well known. 
The occurrence of leucite in relation to the composition of the magma is an 
interesting topic, the discussion of which will be taken up on a later page. 
Nephelite. It is a somewhat striking fact that, while nephelite is found in the 
norms of nearly all the rocks, sometimes to a very large amount, it is seldom present 
modally in more than small quantities, not over 10 per cent, most usually from i 
to 5, and is often absent entirely, the soda entering the albite molecule to form a 
soda-lime feldspar. The typical occurrence of nephelite in the Roman Region is 
in the form of an interstitial cement, the last product of crystallization. Euhedral 
or even subhedral crystals are excessively rare, except in crevices in the rocks, 
where small, short prisms are fairly common in some types. 
Sodalite minerals. While these are met with in the rocks of the region, their 
occurrence can not be regarded as characteristic of it. For the most part their 
amount is very small, scarcely more than accessory, except in tavolatal appianose, 
where haiiyne is one of the main constituents. Both sodalite and hattyne are found, 
but the former seems to be the more common. 
Their occurrence is practically confined to the persalane rocks, and especially 
to the sodipotassic subrangs, the sodalites observed by De Lorenzo and Riva in 
the bolsenal vulsinoses [vulsinites] of the Astroni Volcano being very rare and 
sporadic, and corresponding to the occurrence of leucite in the blocks of cumal 
phlegrose [phonolitic trachyte] at the Vico Volcano or in the arsal monzonose [cim- 
inite] at L'Arso. 
Augite. This alferric mineral is the one which is most characteristic of the 
region, and from the point of view of the alferric and femic minerals the region is 
decidedly an augitic one, as it is eminently a leucitic one from the salic point. It 
is invariably present in greater or less amount in all the rocks, without exception, 
the amount, of course, varying with the character of the magma as to the relation 
of salic and femic components. 
