n6 
THE ROMAN COMAGMATIC REGION. 
Chemical Composition of Atrial Braccianose [Leucite-tephrite]. 
I. 
I. 
SiOa 
48. 10 o 
802 
CO 2 
none 
A1 2 O 3 
17. c6 
172 
TiO 2 
018 
Fe-,0, 
2.48 
016 
ZrO 2 
trace 
FeO 
6. 10 
08? 
P 2 Or. . 
MeO . . 
4.27 
IO7 
SO, . . 
CaO 
8.16 
146 
s 
none 
Na 2 O 
2.65 
OAA 
CuO 
none 
K 2 O 
7.Q2 
084 
MnO 
n d 
H 2 O+ 
O 12 
BaO 
o 08 
H 2 O 
99.91 
I. Atrial braccianose [leucite-tephrite obsidian]. Lava of August 27, 1903, Valle dell' Inferno, 
Mount Vesuvius. Washington, analyst. Cf. G. Mercalli, Boll. Soc. Sism. Ital., 
X, 1904, p. 21. 
Norm. 
Ratios. 
Or 
25 . 86 
An . . . 
12 . 23 
Lc ... 
16. 35 
Ne 
12 . ^O 
Di 
I7-89 
01 
6. 26 
Mt 
i. 71 
11 
2. 74 
Ap 
2 . 31 
Rest 
99-85 
o. 24 
2.31 
66.94 
32.91 
Class 
Order.. 
Rang. 
Subrang . 
Sal 
'Fern 
F 
'L 
KQ' + Na 2 O' 
CaO' 
K 2 O' 
'Na 2 O y 
= 2.03 
= 1-32 
= 2.91 
= 1.91 
IOO.OQ 
Of this analysis it need only be remarked that it resembles very closely those of 
the two preceding types. 
Mode. As the type is very highly vitreous, and therefore indeterminate, any 
exact statement of the mode is uncalled for. It will be of interest to state the rela- 
tive amounts of the various minerals and glass, which are so easily estimated, the 
statement being based, of course, on the specimen analyzed. These figures may 
vary widely in other specimens. 
Vol.%. Sp.gr. Wt.%. 
Leucite 26.4 X 2.5 = 66 . o 24.6 
Labradorite 5.9X2.6= 15.3 5.7 
Augite 4-5X3.3= 14-9 5- 6 
Magnetite 0.6X5.2= 3.1 i.i 
Glass 62.6 X 2.7 = 169.0 63.0 
100. o 268.3 100. o 
Occurrence. As already stated, this type occurs only as flows of very small 
volume, as well as borders of larger flows and crusts of bombs at Mount Vesuvius, 
and also as narrow dikes and salbands of thick ones at Monte Somma. It has not 
been observed elsewhere in the Roman Region. 
