PETROGRAPHY. 
81 
This analysis is not specially noteworthy in itself. Although the magnesia 
may appear to be low for a rock which carries olivine, the norm shows that con- 
siderable of this mineral may be present, more, indeed, than exists modally. The 
norm is remarkable among the rocks of the region in showing no diopside, all the 
femic lime being taken up by apatite. The alkalicalcic subrang is in harmony 
with the predominantly calcic character of the feldspar. 
As regards its position in the quantitative classification the rock is an uncom- 
monly transitional one. It falls well within the perfelic order, notwithstanding 
the considerable amount of leucite, but in class is in dosalane close to the border of 
persalane, in rang is almost exactly on the border between the domalkalic and alka- 
licalcic rangs, and in subrang is dopotassic but very close to the sodipotassic border. 
The proper symbol for its position then would be I-II. 5. 2-3. 3-2, and the name 
of the magma corresponding to this would be extremely unwieldy. It seems 
best, therefore, to indicate in the magmatic name the two divisions in regard to 
which it is most transitional, so that the name of ciminose-auruncose has been 
selected. 
Mode. On account of the very fine grain and confused arrangement of the 
groundmass constituents only the phenocrysts could be estimated optically. Under 
these circumstances the calculation of the mode from the norm is unsatisfactory 
and must be based on several assumptions which may be only approximately cor- 
rect. These were that about 5 per cent of leucite is contained in the groundmass, 
that the composition of the soda-lime feldspar phenocrysts is different from the 
feldspar of the groundmass, and that the amount of olivine is about 2 per cent. 
Making these assumptions, and with due regard for the optically estimated relations, 
we obtain the following mode: 
Orthoclase, Or t Ab x 22.2 
Andesine, AbjAiii 29.0 
Labradorite, AbjAiij 6.6 
Leucite 25 . 5 
Augite 9.1 
Olivine 2.3 
Ores 4.0 
Apatite 1.3 
While this mode can not be regarded as more than a rough approximation, it is 
probably not very far from the actual facts. Compared with the norm, the mode 
differs mainly in the replacement of some of the orthoclase and all of the nephelite 
by leucite and in the existence of augite instead of olivine, very little of which is 
present modally. Taking these into consideration the type may be called an augitic 
leucite-salphyro-ciminose-auruncose. 
Occurrence. So far as known this type occurs only at one locality, a somewhat 
vesicular flow at the Croce di San Martino, on the northwest crest of the Vico crater 
ring, in the Ciminian District. 
Name. The derivation of the subrang name, auruncose, will be explained on 
a later page, while the type adjective is derived from Monte Fogliano, the west- 
ern summit of the Vico Volcano. 
