PETROGRAPHY. 
The groundmass is composed in great part of small prisms of feldspar, both of 
orthoclase and labradorite, the quantity of the former exceeding by several times 
that of the latter. The arrangement of these laths is subparallel, giving rise to a 
well-marked trachytic fabric. There are scattered through the groundmass small 
prismoidal anhedra of colorless augite and some small anhedral grains of magnetite. 
Between the feldspar laths is a little nephelite cement. This is difficult to detect, 
but one or two crystal forms were seen, and its presence was verified by treatment 
of the rock powder with very dilute acid which furnished some gelatinous silica. 
Chemical composition. The only analysis made of this type was published 
some years ago and is presented below in more complete form, determinations of 
TiO 3 and P 2 O S having been made recently. With it is given for comparison an 
analysis of the bolsenal vulsinose [vulsinite] from Bolsena. 
Chemical Composition of Arsal Vulsinose-ciminose [Vulsinite]. 
I. 
II. 
I. 
II. 
SiOj 
57. 32 O.CK6 
58.08 
H 2 O + . . . ) 
( O. Zd. 
ALO,.. 
10.07 .187 
10. II 
H 2 O-... \ 
o-57 
) 51 
f O. II 
Fe^O, 
2 21 .014 
3. ^t; 
CO 2 
none 
none 
FeO 
2.3< .033 
i .00 
TiOj 
0.61 0.008 
0.82 
MeO . 
1 . 60 . 040 
i. os 
P,O<. 
o. 17 .001 
o. 20 
CaO 
3 . 82 . 068 
z. 76 
MnO 
n.d. 
n.d. 
"NTo f~ 
n 9.A 
K 2 O 
3-22 .052 
g.15 .097 
8.86 
100.09 
99.92 
SD orr . 
2 . 611 at 11 
2 . S34. at 25 
I. Arsal vulsinose-ciminose [vulsinite]. Near Vetralla, Monte Vico, Ciminian District. 
ington, analyst. Jour. Geol., IV, 1896, p. 849. 
II. Bolsenal vulsinose [vulsinite]. Bolsena, Vulsinian District. 
Norm of I. 
Ratios 0} I. 
Sal 
Rest, 
99.61 
100.18 
! 5 .26 
5-68 
i^iass 
86.94 
Order 
Fern 
F 
7.86 
4-47 
12-67 Rang 
L 
K 2 0'+Na 2 0' 
o-34 
Subraner . . 
CaO' 
Wash- 
6.86 
3-92 
= 1.87 
From the figures given above it is clear that this type falls well within the perfelic 
order, the domalkalic rang, and the dopotassic subrang. On the other hand, it is 
almost exactly on the border line of the persalane class, where it was formerly* con- 
sidered to belong. The change in its position is due to the recent determination of 
TiO 2 and P 3 O S , which were not reported in the original analysis, and which just 
*H. S. Washington, Prof. Paper U. S. Geol. Surv. No. 14, 1903, pp. 108. 199. 
