THE ROMAN COMAGMATIC REGION. 
the border of the domalkalic rang that the type is really transitional and the 
magma a ciminose-auruncose. This is in harmony with the facts noted below 
that the orvietal auruncose would be termed leucite-tephrite in prevailing systems, 
on account of the more prominent labradorite, while the teanal type, in which the 
soda-lime feldspar is less important, would rather be called a leucite-trachyte. 
Mode. The mode of this type could be measured microscopically very satis- 
factorily, except that the groundmass feldspars had to be lumped together. The 
readjustments of the normative molecules to calculate the mode offered no points of 
difficulty or special interest, the measured amount of leu cite being assumed, and 
otherwise the calculations being carried out as in previous cases. 
CALCULATED. 
MEASURED. 
Orthoclase, Or 2 Abi . 
Labradorite, AbjAi 
Nephelite 
Leucite 
Augite 
Biotite 
Magnetite 
Apatite 
30.0 
24-5 
0.9 
iS-7 
18.3 
2-3 
5-3 
i.o 
IOO.O 
Vol. %. Sp. gr. \Vt. %. 
58.0 X 2.6 = 150.8 53.8 
17-3 X 2.5 = 43.3 15.4 
17-2 X 3.3 = 56.8 20.3 
4.1X2.9= 11.9 4.2 
3.4 X 5.2 = 17.7 6.3 
280.5 ioo.o 
The two agree very well, though the calculations show that a very small amount 
of nephelite is probably present, which might easily escape detection. The error 
due to overlapping of the colored minerals is noticeable, but not serious. The cal- 
culated amount of biotite is that derived from all the normative olivine, and it is 
notably less than the measured. The discrepancy is to be attributed to the general 
and profound alteration of the biotite tables to a fine-grained aggregate of augite 
and magnetite, which were measured with the unaltered portions as biotite. As 
compared with the norm, leucite is the critical mineral and biotite the varietal, 
and as the fabric of the groundmass is granular rather than trachytic, the type may 
be described as biotitic leucite-graniphyro-ciminose-auruncose. 
Occurrence. The only known occurrences of teanal auruncose are in the 
Auruncan District, where it seems to be fairly common. Specimens were obtained 
in the southeastern part near Teano, as at Tuoro, as well as below Orchi, on the 
northern slope of the volcano. 
Name. As mentioned above, the subrang name is derived from that of the 
district where it seems to be most abundant. That of the type comes from the 
town of Teano, near which the type is met with. 
In the prevailing classifications these rocks could be called either leucite-teph- 
rites or leucite-trachytes, but as the alkali-feldspar is the more prominent, the name 
of leucite-trachyte would seem to be the most appropriate, and was the one given 
the type in a former description. 
