122 THE ROMAN COMAGMATIC REGION. 
The rough figures given above must, however, be corrected for the small amount 
of nephelite, and some correction must also be made for the overlap, which we have 
seen to be a common source of error in all these rocks. The amount of nephelite 
was assumed to be 6 per cent, which is probably slightly below the true figure, if we 
can judge by the analogy of the calculated modes of galeral and hernical braccianose. 
To estimate the amount of error due to overlap, the calculated and measured modes 
of hernical braccianose (p. 114) were taken as the basis, as the two are similar in 
grain. On the assumption that the errors here are about the same, the observed 
amount of leucite was increased by one-tenth, that of magnetite reduced to one-half, 
and the balance referred to augite, after subtracting the 6 per cent of nephelite from 
this. The corrected mode would then be: 
Leucite 48. 2 
Nephelite 6.0 
Augite 42.8 
Magnetite 3.0 
100. o 
To calculate the chemical composition from this the leucite was assumed to 
have the composition of that of Mount Vesuvius as analyzed by Steiger,* the 
nephelite that of Vesuvius as analyzed by Rauff ,| and the augite that of Ticchiena 
as analyzed by myself.J The magnetite was assumed to be free from titanium. 
Using these data, the chemical composition of the rock and the norm are calculated 
to be: 
Chemical Composition. Norm. 
SiO 2 48.82 0.814 Or 7.23 
A1 *O 3 16.20 .159 An 6.12, --{.6270 
Fe 2 3 4-89 .031 Lc 40.55) 
FeO 3.32 .046 Ne 8.80 
MgO 5 .01 -125 Di 27.00 
CaO 8.65 .155 Wo 0.93 
Na 2 1.89 .031 Mt 7 .i 9 
K 2 O 9-99 -i6 II 2.28$ 
TiO 2 i . 24 . 015 
This analysis resembles very closely those of the braccianose types and, as 
will be seen, those of albanose rocks, the chief difference being the higher potash. 
Examination of the norm shows that while it is well within the limits of order 8, rang 
2, and sub rang 2, it is almost exactly on the line between dosalane (in which it 
falls) and salfemane, and is consequently an albanose-vesuvose. 
Occurrence. Rocks which may be provisionally referred to this type, though 
texturally some of them have a fabric which is saccal rather than romal, are found 
in the Vulsinian District at the Sassi Lanciati and in the Fossa Melona on the east 
shore of Lake Bolsena, in the Sabatinian at Oriolo and on the north side of Lagosello, 
and in the Auruncan below Preta. 
* Bull. U. S. Geol. Surv. No. 220, 1903, p. 30. 
t Zcits. Krysl., II, 1878, p. 454; Dana, Mineralogy, No. 6, p. 435- 
* Cf. p. 134- 
