PETROLOGY. 175 
panian District with its two subdistricts, each of very uniform rocks, the one highly 
salic and perfelic, and the other more lenic and more femic than the average. 
We have also seen in the discussion of the space relations of the types and 
magmas that the average compositions of the outer districts, the Vulsinian, Ciminian, 
Auruncan, and Campanian, are probably very close to that of the average magma 
of the region as a whole. In the first three of these districts the relations are too 
complex and the number of types too great to allow of very satisfactory estimates. 
But in the comparatively simple Campanian District there seems to be a possibility 
of testing this statement by calculation. For this district the average has been 
arrived at by taking the average of the three analyses of Vesuvian lavas given in 
the table to find the composition of the Vesuvian magma. That of the Phlegrean 
and Ischian magma was found by taking the average of the analyses of the phleg- 
rose rocks of the district, and of the vulsinose and monzonose rocks of Astroni and 
L'Arso, and weighting these in the proportion of 5 to i, as the phlegrose rocks are 
much the most abundant. These two averages were then averaged by taking 
equal parts, with the results here given. 
77.29 
22.51 
The position of this magma is in borolanose (II. 6. 2. 3). The general resem- 
blance to the average magma of the whole region is very close, both chemically 
and normatively, the only difference of importance being in the relations of potash 
to soda, which is here almost unity, potash being only very slightly higher than the 
soda. This is, of course, due to the peculiar character of the phlegrose rocks of 
the district, which are so high in soda as to be almost in nordmarkose (I. 5. i. 4). 
Time Relations. 
GEOLOGIC AGE OF THE ERUPTIONS. 
It would lead us too far astray, and would be outside of the province of this 
paper, to discuss this topic fully and adequately, so a few words must suffice. Fur- 
thermore, there exists considerable divergence of opinion in regard to the exact 
age of the eruptions in some cases. 
While the volcanoes of the Tuscan Region date from the Eocene or Miocene 
Period, those of the Roman Region have begun either in the late Pliocene or in the 
A\ 
COMPOSITION. 
SiO 2 c^ 
jera 
.0 
.2 
6 
9 
7 
6 
3 
5 
8 
4 
ge Cam-j. 
0.900 
.178 
.016 
54 
.068 
. IOO 
.069 
.080 
.010 
.003 
lanian Magma. 
NORM. 
Or *J\.A?,\ 
A1 2 O 3 
!8 
Ab 
. .11.26 [ 
63-8oj 
13-49 J 
16.32 
5-23 
0.96 
Fe 2 O 3 
2 
An 
8.06) 
FeO 
. -1 
Ne 
. . I 7 . 4.Q 
MeO . . 
2 
Di . . . . 
13-66) 
CaO 
. C 
Ol 
2 . 66 } 
Na 2 O 
4. 
Mt 
3. 71 ) 
K a O 
. 7 
11 
* t 
1.52) 
TiO 2 
o 
Ap .. 
. 0.06 
2 5 
nn.8o 
