1 66 THE ROMAN COMAGMATIC REGION. 
DISTRIBUTION OF MAGMAS. 
It is well known through recent researches that a number of igneous masses 
of large dimensions, as laccoliths, show a very pronounced change in chemical 
and other characters as one goes from the center to the periphery, the change being 
sometimes from more salic to more femic, and sometimes the reverse. These 
changes are usually held to be the results of local or laccolithic differentiation, and 
typical cases have been discussed by Brogger, Pirsson, and the writer. Similarly 
in a number of instances there have been observed such progressive changes 
extending over petrographic provinces or comagmatic regions of large area, the 
most detailed being that of central Montana, recently described by Pirsson. 
Although it may seem at first glance that there is no progressive change in the 
rocks of the Roman Region, but that the different districts show an irregular 
alternation of magmatic characters, study reveals strong evidence of a regular 
progression from each extremity to the center. In considering this matter some 
peculiarities of the Roman Region must be borne in mind. In the first place, it 
does not cover an area extended in all directions, as in the central Montana, Chris- 
tiania, Bohemia, and other well-known regions, but rather consists of a long, narrow 
line or band, with very few occurrences of igneous rocks on either side of the main 
line, and those very small. In the second place, the occurrences are entirely vol- 
canic, that is, effusive and superficial, the few dikes being in the masses of volcanic 
cones, and hence analogous rather to lava flows so far as their bearing on problems 
of differentiation go, deep-seated dikes penetrating the country rock and plutonic 
or deep-seated masses in general being wholly unknown. A third consideration 
is that the various centers are strung along the line somewhat irregularly and often 
with very notable spaces between them. 
When we study the descriptions of the different districts given above several 
points are brought out. 
The districts at the extremities, the Vulsinian and the Campanian, are the most 
complex vulcanologically, those inside of these, the Ciminian, Sabatinian, and 
Auruncan, somewhat simpler, though still complex, and the innermost, the Latian 
and Hernican, the smallest and the simplest of all. 
Concomitantly with this progressive change in vulcanological structure, though 
whether causally connected or not need not be here considered, is a more or less 
regular change from ends to center in the variety of the magmas represented. Thus 
the Vulsinian District shows a very great diversity of magmas, varying from per- 
salic to salfemic ones, and with orders running from perfelic to feldolenic, while, 
on the other hand, the rangs are consistently domalkalic and the subrangs are 
mostly dopotassic. At the other extremity we find at the Campanian District 
a similarly wide diversity of magmas from perfelic phlegrose to braccianose, though 
here the number is not as great, the Vesbian Volcano being almost wholly of 
braccianose, while the Phlegrean and Epomean Volcanoes are almost entirely of 
phlegrose, near the border of nordmarkose. The penultimate districts, the Cimin- 
