28 
THE ROMAN COMAGMATIC REGION. 
Chemical composition and mode. Only one analysis was made of this type, and 
this of a rock of somewhat exceptional textural character. While, therefore, in the 
absence of analysis, and in view of the abundance of the glass base and the conse- 
quent indeterminability of the mode, the magmatic position can not be definitely 
established for all the specimens, there is little reason to doubt that all those 
obtained in the Campanian District belong to the subrang phlegrose, where the 
great majority of the holocrystalline, and hence determinable, rocks fall. There 
is all the more reason for this belief in the fact that no soda-lime feldspars were 
observed either among the phenocrysts or the microlites. In the rocks of the Phle- 
grean Fields which fall in vulsinose, and which carry modal labradorite, this mineral 
is invariably present as large phenocrysts. It is therefore justifiable to think that, 
were these rocks in question domalkalic, they would show phenocrysts of labradorite. 
No definite proof can be given to show that they are sodipotassic, except that of 
an analysis, and it is possible that some of them are dosodic, and hence in nord- 
markose. But when the general character of the district is considered, this does 
not seem very probable. 
Chemical Composition of Rotaral Phlegrose [ Trachyte Obsidian ]. 
I. 
II. 
I. 
II. 
SiO 2 
6l 62 I.O27 
60. 77 
Cl 
o. ic o 004 
A1,O* 
18. ii o. 177 
10.83 
MnO 
n.d. 
trace 
Vo r 
^e 2 u 3 
FeO 
2.36 .015 
1.28 .018 
4- J 4 
2.43 
100.67 
iQo-55 
MgO. . 
0.56 .014 
0.34 
CaO 
1.44 .026 
1.63 
Na a O 
i;. 77 .093 
4.90 
O = Cl 
0.03 
K 2 O . 
7.60 .081 
6.27 
, 
HO-4- 
o 78 
o. 46 
TiO 2 
087 .on 
n.d. 
Sp.gr. . 
2.44. 
P,Oc. 
O.I'? .OOI 
trace 
I. Rotaral phlegrose [trachyte-obsidian]. Monte Rotaro, Ischia. Washington, analyst. Cf 
Am. Jour. Sci., VIII, 1899, 289. 
II. Rotaral phlegrose [trachyte-obsidian]. Monte Rotaro, Ischia. Fuchs, analyst. Tsch. min. 
Mitth., 1872. p. 232. 
Rest ................ 0.78 
Class , 
Qi-45 
Order. 
Ratios of I. 
Sal 
' "Fern 
F 
L 
.40 Rang... 
Subrang . 
CaO' 
K 2 0' 
Na 2 O' 
=24.19 
34.80 
0.87 
100.63 
