PETROGRAPHY. 
2 3 
distinguishable only with difficulty, as at the Phlegrean Fields, or blue, with the 
characteristic dusty inclusions arranged in lines, as in the blocks from Monte Vico. 
The groundmass is composed in very large part of a soda-orthoclase in very 
small prismoids, usually with a subparallel arrangement. There are also a few 
minute anhedra of very pale greenish or grayish augite, occasionally small anhedra 
of sodalite, and usually a little nephelite interstitial between the feldspar laths, and 
often difficult to detect. Magnetite is usually present in very small amount, but 
not invariably, while lavenite is sometimes found. In some specimens there are 
small euhedral individuals of titanite, but this is a purely accessory constituent. 
Glass may be present in some cases, but its amount is always small, and it is difficult 
to detect on account of the character of the groundmass. 
Chemical composition. Analyses of three occurrences of cumal phlegrose 
were published several years ago, and are given below with the addition of deter- 
minations of some of the minor constituents. Those from the Phlegrean Fields 
(Monte di Cuma and Monte Nuovo) scarcely differ from each other, but the third, 
from Monte Vico, shows a very considerably greater amount of potash relatively 
to the soda, though the ratios of the alkalis in all three are so near unity as to 
cause all to fall in the same sodipotassic subrang. It is also seen that in the Monte 
Vico rock sulphur trioxide partially replaces the chlorine of the other two, though 
the amounts are so small in all cases as to be negligible for purposes of classifica- 
tion. The analysis by vom Rath (IV) is very probably of the same type as III, but 
it is almost certain that through some error the alkalis have been interchanged. 
Chemical Composition of Cumal Phlegrose [Phonolitic Trachyte.] 
I. 
II. 
III. 
IV. 
SiO, 
en. 70 o 
OO7 
60 33 
I 006 
en. 2/1 O 
987 
60. 18 
A1 2 O 3 
10. o? 
l87 
18 yj 
T7O 
18 63 
183 
18.70 
Fe,O, . 
2.OC. 
019 
2.84 
.018 
3. 30 
O2I 
FeO 
I. 08 
OIC 
i . 20 
.018 
1 . 20 
OI7 
3-44 
MgO 
o. 36 
OOQ 
o. 38 
.010 
O. 12 
OO3 
0.32 
CaO 
I . IO 
O2I 
i . ic, 
.021 
2.O6 
0^7 
2.8o 
Na a O 
6. 70 
no 
7. ie 
.ne 
4.87 
O70 
9.67 
K 2 O 
7. IO 
O76 
7- 3O 
.078 
9- 1 4 
O97 
4.18 
H 2 O+ 
o. 24 
o. c.6 
0.86 
CO 2 
none 
none 
none 
| o-33 
TiO a 
o. t;6 
OO7 
0.43 
.ooc. 
0.56 
007 
ZrO 2 
O. IO 
OOI 
PjOc . . . 
O. IO 
OOI 
o. 04 
o. ic 
OOI 
SO, . . . 
none 
none 
O. 17 
OO2 
o. 19 
Cl 
O.M 
014 
0.4? 
.012 
0.08 
OO2 
o. 14 
MnO 
n.d. 
n.d. 
n.d. 
BaO 
0.06 
O = Cl 
99-74 
O. 12 
100. 17 
O. IO 
100.54 
O.02 
99-95 
99.62 
ioo 07 
IOO.52 
So. Er. . 
2.509 
