PETROLOGY. 167 
ian and the Auruncan, are almost as complex and great in range, especially the 
Auruncan, while the Ciminian is distinctly somewhat simpler and with a smaller 
range, the more lenic orders of dosalane and the salfemanes not being found here. 
The three central districts, the Sabatinian, the Latian, and the Hernican, are each 
much more simple magmatically than the external pairs, the Sabatinian being the 
more complex, while the Latian and Hernican both show only a very limited range 
of magmatic variation. It is thus seen that, while certain irregularities are evident, 
there is on the whole a marked increase in magmatic range and complexity from 
the center to the extremities of the line. 
Corresponding with this variation another makes itself manifest, though the 
data on which it is based are less readily and satisfactorily determinable. If we 
estimate the average composition of the magma of each district we find that here 
also there is a distinct and definite variation between center and ends. This esti- 
mation may be carried out in two ways, either by weighting the different chemical 
analyses of the rocks of each district according to the relative amounts and taking 
the mean, or more roughly, though more expeditiously, by expressing the average 
composition, regard being had to the relative abundance of the different magmas 
at each center, in terms of the magmatic position as expressed by the quantitative 
system of classification. In view of the lack of data and imperfect knowledge of 
many of the rocks in some districts, as well as of their relative amounts, the second 
method would seem to be adequate for present purposes. 
The estimates below are based largely on my own observations, as well as on 
the published descriptions of others, so far as they could be applied, though the 
details can not be gone into here, and while the results are admittedly but roughly 
approximate, they may yet be considered to be sufficiently accurate to enable us 
to judge the main features with confidence. Of these estimates those for the Vul- 
sinian, Sabatinian, and Auruncan districts are the least reliable, while those of the 
others are based on fairly complete and satisfactory data. 
Average Magmas of the Districts. 
Vulsinian: Vicose (II. 6. 2. 2) or ciminose (II. 5. 2. 2). 
Ciminian: Ciminose (II. 5. 2. 2) or vulsinose (I. 5. 2. 2). 
Sabatinian: Braccianose (II. 7. 2. 2). 
Latian: Braccianose (II. 7. 2. 2) or vesuvose (II. 8. 2. 2). 
Hernican: Braccianose (II. 7. 2. 2) or albanose (III. 8. 2. 2). 
Auruncan: Ciminose (II. 5. 2. 2) or monzonose (II. 5. 2. 3). 
Campanian: Borolanose (II. 6. 2. 3). 
Assuming that these are true or not far from the truth, and with the knowledge 
of the chemical composition and norms of the various types shown by the numerous 
analyses presented on a previous page, it is clear that the average magmas at the 
ends are more salic and less lenic and femic, that is, higher in silica than the central 
ones, which are very distinctly higher in femic constituents (especially magnesia 
and iron oxides) and lower in silica. 
