i68 THE ROMAN COMAGMATIC REGION. 
Expressed mineralogically and in terms of the prevailing systems of classifi- 
cation, we find toward the ends of the line an abundance of non-leucitic rock types, 
trachytes, vulsinites and ciminites, with leucite-trachytes, leucite-phonolites, leucite- 
tephrites, and some leucitites; while at the central districts, the Sabatinian, Latian, 
and Hernican, we find no non-leucitic types, that leucite-trachytes and leucite- 
phonolites are rare, and leucite-tephrites of the more basic types and leucitites are 
largely predominant. 
A secondary progression, but in the same direction, is indicated by the local 
distributions observed at the Vulsinian, Ciminian, and Sabatinian districts, namely, 
that in each the southern portion is more femic and more lenic (less high in silica) 
than the northern. These have been already spoken of in preceding pages. It 
is needless here to go into details, but there is little doubt that as the volcanic 
foci shifted in these districts there was a progressive variation in the general 
magmatic characters of the erupted rocks. In the southern half of the region 
this is not as well marked, though we find here a magmatic division of the districts. 
Thus the more femic magmas seem to occupy the western and southwestern parts 
of the Auruncan District, while in the Campanian the more salic and more femic 
magmas are more sharply distinguished, the former constituting the western and 
the latter the eastern part. 
It would appear also that, while the central districts are very preponderatingly 
dopotassic, the exterior districts are less so and tend more to being sodipotassic ; 
that, in other words, there is a progressive increase of soda relatively to potash 
from the center outward. This is shown by the occurrence of rocks belonging 
to sodipotassic subrangs, as phlegrose (I. 5. i. 3), pulaskose (I. 5. 2. 3), procenose 
(I. 6. 2. 3), harzose (II. 4. 3. 3), monzonose (II. 5. 2. 3), and shoshonose (II. 5. 
3. 3) in the Vulsinian, Ciminian, Campanian, and Auruncan districts, while rocks 
belonging to sodipotassic subrangs are wholly wanting, or found only sporadically 
in the three central ones. The fact that the phlegrose rocks of the Campanian 
District are almost dosodic and near the border of nordmarkose, while those of 
the Vico Volcano are centrally sodipotassic, also points to similar relations. 
While it must be, and indeed freely is, admitted that the data on which the 
above conclusions rest are not complete, they are sufficiently so, in the judgment 
of the writer and in view of the descriptions and analyses found in the preceding 
pages, to warrant belief in a regular and symmetrical distribution of the magmas 
in the Roman Region, in which the central magmas are not only less varied and 
complex, but are more femic and lenic, lower in silica, and with potash relatively 
much higher than soda, than obtains in the districts toward both extremes of the line. 
Although anticipating a point which will be brought out later, it will be of 
interest to note here that the average composition of the regional magma as a whole 
is probably a ciminose-vicose (II. 5-6. 2. 2); that is to say, that the average exterior 
magmas are of about the composition of the original, or proximately original, magma, 
while the central ones show compositions approaching the more femic extremes 
of differentiation. 
