104 
THE ROMAN COMAGMATIC REGION. 
often reaching i mm., but usually much under this. They are always multiply 
twinned, with extinction angles which vary somewhat, but which indicate an average 
composition of about Ab 2 An 3 . As stated above, their arrangement is divergent, 
so as to give rise to an approach to a true intersertal fabric. Alkali-feldspar seems 
to be quite absent, but some small leucite anhedra are found in the groundmass. 
Small olivine grains, usually quite anhedral, but occasionally with crystal planes, 
are scattered through the groundmass, but the total amount of this mineral is always 
small and quite negligible for classificatory purposes. A little residual nephelite 
base is present in most cases, its place being taken in some specimens by glass. 
Chemical composition. An analysis of a specimen of this type from the flow of 
1872 of Mount Vesuvius, collected below the Observatory, is given below in I, hav- 
ing been previously published, but without petrographical description. The posi- 
tion assigned the rock at that time, namely, in vesuvose, II. 8. 2. 2, was erroneous 
through inadvertence, not on account of error in the analysis or in calculating the 
norm. As was pointed out later,* the analysis should have been placed in the 
subrang II. 7. 2. 2. With it is also given an analysis of the lava of the eruption of 
1891-93, which probably belongs to the same type, though the description of Mat- 
teuccif leaves this somewhat in doubt. 
Chemical Composition of Vesbal Braccianose [Leucite-tephrite]. 
I. 
II. 
I. 
II. 
SiO 2 
47.6s O. 7OJ. 
48.00 
CO 2 
none 
A1 2 O 3 
iS.ii .178 
19.82 
TiO 2 
I 1 1 O OI4. 
Fe 2 O 3 
2 6l . OI7 
2 . ?O 
ZrO 2 
o 02 
FeO 
6.48 OQO 
526 
P 2 O 5 
O 11 
MgO . . 
2 82 
SO 3 
trace 
CaO 
9 . OI . l6l 
8.17 
MnO 
n.d. 
Na 2 O 
2 . 78 .04? 
7.17 
BaO 
O. 24 
Ko 
H 2 O + 
O.I}) 
100-47 
100.17 
H 2 O 
6 C 
O. 1 1 \ 
n.d. 
I. Vesbal braccianose [leucite-tephrite]. Lava of 1872, below Observatory, Mount Vesuvius. 
Cf. Washington, Prof. Paper U. S. G. S. No. 14, 1903, p. 307. 
II. Vesbal (?) braccianose [leucite-tephrite]. Lava of 1891-93. Atrio del Cavallo, Mount 
Vesuvius. Mrha, analyst. F. Becke, Tsch. Min. Pet. Mitth., XVIII, 1898, p. 94. 
Norm of I. 
Or 
14 . 46 
An 
14. 73 
Lc 
23 . 54 
Ne 
12 . 78 
Di 
21 . 80 
Ol 
Mt 
11 
2. 13 
Ap .. 
I . 1O 
Rest 
ioo. 27 
O. sO 
6-07 
65-51 
34-76 
Class 
Ratios o) I. 
Sal 
Fem 
Order 
Rang 
F 
'L 
K 2 O'+Na 2 O' 
CaO' 
Subrang. 
K 2 O' 
'Na 2 O' 
= 0.80 
36 
= 1.78 
100.77 
* Washington, Prof. Paper U. S. Geol. Surv. No. 28, 1004, p. 68. 
t R. V. Matteucci. Tscherm. Min. Pel. Mitih., XV, 1895, p. 342. 
