128 THE ROMAN COMAGMATIC REGION. 
far as the norm goes, it is seen to be well within all the divisions, and this, as well as 
the normative mode of the type, its fresh and unaltered condition, caused its locality 
to be chosen for the name of the subrang rather than the locality root of the longer- 
known ouachitite. The chemical resemblance to the Arkansas rock is very close on 
the whole, though there are some differences of importance, notably in the silica, 
ferric oxide, lime, and magnesia. As this last rock is far from fresh, some of these 
differences may be ascribed to alteration, and its analysis is not very significant. 
The Montana rock, while closely similar to the Italian in silica, alumina, and iron 
oxides, differs notably in the figures for magnesia, lime, and the two alkalis, the lime 
being very considerably lower and the others higher than in I. The differences are 
expressed in its position, III. 7. 2. 3. It is of interest to note that of these closely 
related magmas, the fiordinal type, a lava flow, is almost strictly normative, the 
leucites shown by the norm appearing modally as such, while in the other two types, 
which occur as dikes, the mode is abnormative, the normative leucite and much of 
the normative olivine molecules* combining to form modal biotite, the rocks showing 
no leucite. 
Mode. Owing to the difficulty of determining the exact areas of the dark pheno- 
crysts against the dark groundmass in the hand specimen, the amount of these 
could be determined only approximately, and the sections were not large enough 
to allow a representative area to be measured under the microscope. For these 
reasons the estimation of the mode by Rosiwal's method was abandoned, though 
sufficiently exact data were obtained to serve as some check on the results of the cal- 
culation of the mode from the norm. This offered no difficulties, all the potash 
going into leucite, the normative olivine being accepted as the modal, and the augite 
being assumed to have the usual composition. The amount of silica left to com- 
bine with the soda, after formation of the other modal minerals, is barely more than 
is requisite to form nephelite, the amount of albite being thus negligible, which 
agrees with the character of the plagioclase determined by the extinction angles as 
an almost pure anorthite. The very small amount of biotite was neglected in the 
calculation. 
Anorthite *3 * 
Leucite 17.3 
Nephelite 3.4 
Augite 48 . o 
Olivine 15.9 
Ores 1.7 
Apatite 0.6 
100.0 
The mode is thus normative and the type would be described as a normative 
alferfemphyro-fiasconose. 
Occurrence. As far as known this type occurs only in the vicinity of Monte- 
fiascone, in the southeastern part of the Vulsinian District, especially near Fiordine. 
Some of the small valleys and dry stream beds here yield many well-formed, small 
crystals of both augite and olivine, derived from this rock. A somewhat similar type 
* The ouachital type is apparently vitreous. 
