PETROGRAPHY. 
21 
among the fine feldspar laths of the groundmass. As the figure for chlorine is con- 
cordant with that shown in other analyses of closely related rocks from the same 
district, it is justifiable to assume that the amount is correct. Furthermore, a 
study of the norm shows that a lenad must be present in the mode, since there is not 
enough silica to form albite with all the soda. The amount of nephelite shown in 
the norm is just enough to form sodalite with the normative halite (3.88 per cent), 
leaving only 1.70 of nephelite free. Consequently, in the calculated mode given 
in the first column, obtained by readjustment from the norm, the sodalite is stated 
to be present as such, while in the measured mode it is bracketed with the ortho- 
clase. In any case the amount of this mineral, as well as of lavenite, is quite 
negligible; so the mode is a normative one, and the rock may be described as 
normative trachiphyro-phlegrose. 
CALCULATED. 
MEASURED. 
Soda-orthoclase, Or s Ab 9 
% 
86.1 > 
S.i 
i 3-9 
4-4 
Vol. % Sp. gr. 
94.16 X 2.6 
2-25 X 3.3 
0.68 X 3-5 
2.91 X 5.2 
= 244.82 
- 7-43 
= 2.45 
- I5-I3 
Wt.% 
9 -73 
2-75 
0.91 
5-6i 
Sodalite 
Augite 
Lavenite 
Magnetite 
IOO.O 
IOO . OO 
269.83 
IOO.OO 
Occurrence. Ischial phlegrose is especially abundant on the island of Ischia, 
where it was found, among other localities, at Marecocco and Zale, probably the 
site of the eruption of about 470 B. c. ;* at Monte Rotaro, probably due to an erup- 
tion between 400 and 352 B. c. ; at Carbone and near Scanella. The flow at Monte 
Olibano in the Phlegrean Fields may also be referred to this type, though somewhat 
transitional toward the cumal type, to be described presently. Outside of the 
Campanian District rocks of this type were met with by me only at the Molino di 
Casa Fredda, in the Auruncan District. 
Name. The name of the subrang is, of course, derived from the Phlegrean 
Fields, where this magma is predominant, and where specially well-known types 
of it occur. The type adjective is derived from the island of Ischia, on which the 
type is abundant. 
In the prevailing systems of classification this rock is regarded as a typical 
augite-trachyte, of the Ponza typus of Rosenbusch, though with affinities toward 
his phonolitic trachyte, through the presence of sodalite and lavenite. 
ISCHIAL NORDMARKOSE-PHLEQROSB. I. 5. 1. 4-3. 
Megascopic characters. Very light gray, compact, highly porphyritic. Feldspar pheno- 
crysts abundant, 10 to 20 mm. long, tabular or stout prismatic, clear and colorless. Augite 
and magnetite phenocrysts very few, 0.5 to i mm. long; augite stout prismatic, magnetite equant. 
Groundmass: very light gray, fine grained, phanerocrystalline. 
Microscopic characters. Holocrystalline, megaporphyritic, dopatic, magnophyric. Phen- 
ocrysts: about 33 per cent, soda-orthoclase, augite, magnetite. Groundmass: about 67 per 
* C. W. C. Fuchs, 18, p. 237. 
