n8 
THE ROMAN COMAGMATIC REGION. 
though some irregular forms are noticed. They do not show very marked birefrin- 
gence, and inclusions are few and irregularly arranged. The microphenocrysts of 
augite are anhedral to subhedral, generally prismatic, and of the same color as the 
larger ones. 
The microgroundmass between these is composed of small tables of labradorite, 
with twinning and extinction angles which correspond to the composition Ab 2 An 3 , 
small irregular grains of augite and fewer of olivine, and the usual magnetite grains 
and apatite needles, neither of which are numerous. The arrangement of the feld- 
spar tables is diverse, inclosing the other constituents. Between these well-defined 
minerals is a colorless residual cement, which in most specimens is somewhat bire- 
fringent and to be regarded as nephelite, which treatment of the rock powder with 
weak acid proves to be present, and there is occasionally an isotropic glass of similar 
composition. 
Chemical composition. An analysis of this type was made from material ob- 
tained at the quarries in the great flow of 1631, at La Scala, near Torre del Greco, 
Mount Vesuvius. It has been already published, but in incomplete form and with- 
out any petrographic details. For comparison are also given two of the older analy- 
ses of the same rock. 
Chemical Composition of Scalal Vesuvose-braccianose [Leucite-tephrite]. 
I. 
II. 
III. 
SiO 2 
47.71 O 
70 e 
47.47 o 
701 
46 . 4 i o 
A1 2 O 3 
17. 6l 
173 
1 6. 60 
164 
10.67 
IQ3 
Fe 2 O 3 
2 .46 
QIC 
4. 20 
026 
6.88 
043 
FeO 
s.63 
O7Q 
^ -QO 
082 
4.17 
osS 
MeO . 
4.80 
I2O 
4- 34 
IOQ 
<;. 23 
131 
CaO 
0.42 
1 68 
O.o8 
1 70 
10.53 
1 88 
Na 2 O 
2. 7? 
O4s 
2.28 
O37 
2. O2 
032 
K 2 O 
7.64 
08 1 
7.4.6 
080 
4. oo 
OC3 
H 2 O + 
/f 
trace / 
H 2 O - 
n. d.* \ 
0.08 
O. II 
CO, 
none 
n. d. 
n. d. 
TiO a 
o. 37 
OOs 
O. 23 
OO3 
n. d. 
ZrOj 
0.06 
n. d. 
n. d. 
P 2 O, 
O. 77 
oo? 
o. iq 
OOI 
n. d. 
SO 3 
none 
n. d. 
n. d. 
Cl 
O . 4.O 
OI I 
O. 41 
Oil 
s 
none 
n d 
n. d. 
MnO 
n. d. 
II? O 
16 
n. d 
BaO 
o . 26 
n d 
n. d 
99-53 
100.69 
100.42 
Sp. gr. . 
2 . 7OO 
* Material dried at no 8 . 
I. Scalal vesuvose-braccianose [leucite-tephrite]. La Scala, Torre del Greco, Mount Vesu- 
vius. Washington, analyst. Cf. Washington, Prof. Paper U. S. G. S. No. 14, 
!93> P- 37- 
II. Scalal vesuvose-braccianose [leucite-tephrite]. Same locality. Haughton, analyst. Trans. 
R. Irish Acad., XXVI, 1876, p. 77. 
III. Scalal braccianose [leucite-tephrite]. Same locality. Fuchs, analyst. Neu. Jahrb., 1866, 
p. 678. 
