26 THE ROMAN COMAGMATIC REGION. 
The wollastonite which was left over was placed in the aegirite-augite molecule 
though it is a trifle high. There not being enough SiO 2 left to form albite from all 
the Na 2 O present, nephelite was calculated by the same set of equations used to cal- 
culate the norm, exactly as was done in the preceding mode. The average alkali- 
feldspar, then, will be Or 97 Ab 4 8, or almost exactly O^Abj. 
Comparison with the measured mode shows the same relations as in the case 
of the Cuma rock, though here it is considerably greater, and is also to be attrib- 
uted to overlapping. The calculated and measured amounts of haiiyne are very 
concordant, which in both cases is to be attributed to the rather fair size of the 
crystals, equaling or excelling the thickness of the section, and their equant shape 
and consequent small liability to overlapping. 
The mode of the Monte Nuovo rock was not calculated or measured, as it was 
deemed to be superfluous. It may be assumed to correspond closely with that of 
the Cuma phlegrose. 
Taking all the above facts into consideration, we may conclude that the mode 
of cumal phlegrose is best represented by the two calculated modes given above, 
rather than by the measured ones. The variations from the norm are inconsiderable, 
and these rocks can readily be classified as far as rang by a microscopic examina- 
tion alone, and, if the highly sodic character of the alkali-feldspar were known, as 
far as subrang. They would therefore be described as normative trachiphyro- 
phlegrose. 
Occurrence. Cumal phlegrose is most typically and abundantly represented in 
the Phlegrean Fields, where it occurs at Monte di Cuma, Monte di Procida, and as 
blocks at Monte Nuovo, as well as other localities. The well-known "piperno" of 
Pianura might be considered to be of this type, though the entire absence of pheno- 
crysts and the peculiar eutaxitic texture shown in the "flames" of darker material 
would justify the establishment of a separate type for this, which might be called 
the pianural. The cumal type is also frequent on Ischia, as at Monte Tabor, 
Monte Rotaro, Punto di Castiglione, Scanella, Panza, and elsewhere. Outside of 
the Campanian District these rocks are comparatively rare. They are met with in 
the Auruncan District, where they form flows east of Casi near Teano, below Orchi, 
and possibly elsewhere, and in the Ciminian District, where the type occurs as 
blocks in yellow tuffs around Monte Vico and possibly as a few flows. 
Name. The type name is derived from the locality of Cuma in the Phlegrean 
Fields, the earliest Greek settlement of the neighborhood and a prominent locality 
for the type. 
In the prevailing classifications these rocks would be properly called trachyte, 
and would, for the most part, fall under the head of phonolitic trachytes of Rosen- 
busch, though he classes some of the occurrences as acmite-trachyte, and others as 
augite-trachyte. The rocks of this type from Monte Vico were formerly called 
phonolite by me, but this name lay undue stress on the presence of nephelite and 
haiiyne, the amounts of which are in reality very small. 
