PETROGRAPHY. 35 
scopically. Owing to their white color, thtir size, and the generally rather dark 
shade of the groundmass, these phenocrysts stand out very prominently and give 
the rocks of this and homologous types a most characteristic appearance so much 
so that they have the popular name of " fish-eye" (occhio di pesce). Accompanying 
these leucites, but very inconspicuously, are a few much smaller phenocrysts of feld- 
spar (mostly labradorite) and augite. The aphanitic groundmass is a rather dark 
ash-gray. 
The habit of these and homologous types, characterized by abundant, large, 
euhedral leucite phenocrysts, with only very small amounts of small augite pheno- 
crysts, and a rather light to rather dark gray aphanitic groundmass, may be called 
"viterboid." Microscopically the groundmass may or may not show some leucite, 
as it does in the homologous type of vicose, the relative proportions of alkali- and 
soda-lime feldspars in the groundmass may vary considerably, the former being 
usually the more abundant, and the fabric may be more or less trachytic. But 
these differences are microscopic, and it would not seem advisable at present to 
regard them as of sufficient importance to determine different habits. 
Microscopic characters. In thin section the large leucites show the usual 
double refraction and twinning, which are very well marked unless alteration has 
set in. Inclusions of augite, labradorite, magnetite, and glass are not uncommon. 
The less abundant feldspar phenocrysts are mostly of orthoclase, with some of lab- 
radorite. They are in the form of stout subhedral prismoids, often with numerous 
glass inclusions, mostly clustered toward the center. The augite phenocrysts are 
mostly well formed crystallographically, and are of the usual very pale gray, with 
only a slight tinge of green and non-pleochroic. The groundmass is holocrystalline, 
composed in very large part of small orthoclase prismoids, whose arrangement is 
diverse, forming a felted fabric. With these are small quantities of minute augite 
prismoids, some biotite flakes interstitial between the feldspar laths, magnetite 
grains, and an occasional small apatite and titanite. A few of the specimens which 
are of this type contain small amounts (not more than 2 per cent) of leucite in 
the groundmass, as very small, clear, equant euhedra, in this respect being transi- 
tional toward the viterbal vicose to be described later. 
Chemical composition. Two analyses were made of viterbal vulsinose and 
are presented below, along with one of bolsenal vulsinose for comparison. The 
second of these was made and published some years ago, though additional deter- 
minations of minor constituents have been made since. It differs from the other 
in several respects, notably in lower alkalies and alumina, and higher oxides of iron, 
lime, and magnesia. As will be seen when the mode is under discussion, there is 
strong reason for the belief that the sample analyzed was not representative of the 
rock, but contained less of the leucite phenocrysts and more of the groundmass. 
For this reason the analysis of the Grignano rock is to be regarded as the most 
typical. 
