THE ROMAN COMAGMATIC REGION. 
The above analysis (I) calls for no comment, except to point out its great gen- 
eral resemblance to those of the other types of braccianose. It may also be noted 
that, while the total amounts of the two oxides of iron are about the same in all of 
the types, their relative amounts are distinctly different in the galeral and hernical 
from what they are in the types at Vesuvius, where they are very uniform, ferrous 
oxide being here largely in excess over ferric. 
Mode. As the rock was relatively large-grained, compared with the ground- 
masses of the other tvpes of the same subrang, and as the size and equant shape of 
the crystals made it seem probable that the greater part of them would extend through 
the section, an attempt at measuring the mode under the microscope was made, with 
the result shown below. In this the leucite, feldspars, and nephelite and glass were 
necessarily measured together, as proper discrimination between them was difficult. 
The mode was also calculated in the usual* way from the norm. 
CALCULATED. 
MEASURED. 
Orthoclase, Or 3 Ab 2 . 
Labradorite, AbjAn 
Leucite 
Nephelite 
Augite 
Olivine 
Magnetite 
Apatite 
6-5 
15 o 
3 2 -5 
6.8 
32.0 
2-3 
2.6 
Vol. %. Sp.gr. Wt.%. 
61.0 X 2.6 = 158.0 54.2 
33-9 X 3-3 = m-9 38.4 
2-5 X 3-3 = 8-3 2.8 
2.6 X 5-2 = 13.5 4-6 
291.7 100.0 
Evidently the influence of overlapping has been quite serious, being especially 
marked relatively in the case of the small magnetite grains. Of the two the 
calculated mode is the more authoritative. 
Occurrence. This type is abundant in the Latian District, as near Grotta Fer- 
rata and Rocca di Papa, and in the Hernican District, two typical localities in this 
being the Torre di Pidocchia near Frosinone and blocks in tuff near the Casa Co- 
lumbella near Pofi. It was also found in the Vulsinian District, especially near 
Montefiascone, as at the railroad tunnel and at Madonna d'Oro. It also probably 
occurs in the Sabatinian District, though my specimens of braccianose rocks from 
here are to be referred mostly to the galeral type, in which the leucite is phenocrystic 
rather than the augite. 
Name. The type name is derived from that of the Hernican District, where it 
seems to be common. 
In the prevailing systems of classification these rocks would be called leucitites, 
like the preceding type, as the amount of feldspar is very small, though in some cases 
the name of leucite-tephrite might be applicable. 
