PETROLOGY. 
Introduction. 
Having obtained a detailed idea of the petrography of the region, it remains 
to discuss its petrology, the relations of the magmas and of the rock types to each 
other, and the conclusions which may be drawn from them. In this way we shall 
obtain a knowledge of the general petrological character of the region and shall be 
in a position to discuss the bearing of the facts on the theories of differentiation and 
to compare the region with others. 
These regional or clan characters are of various kinds, of which the most 
important are the chemical, the mineralogical, and the textural, their importance 
being in the above order, as the chemical characters are the most, and the textural 
the least, dependent on the composition of the magma. With them should also be 
discussed the geologic occurrence of the types, the space relations or geographic 
distribution, the time relations or order of succession, and the quantitative relations 
or relative masses, of the different magmas and types, as well as the average com- 
position or that of the parent magma, whose discussion may well be included in 
that of the closely connected quantitative relations. 
Geologic Occurrence. 
The geologic occurrence of the rocks has already been described in a previous 
chapter, and it only remains here to point out some of the general facts observed 
in the geologic structure of the volcanoes and their relations to the occurrence of 
the different magmas. 
In the Roman Region the occurrence of the rocks is overwhelmingly that of 
lava flows and beds of tuff, which have been ejected at the surface from volcanic 
vents. No deep-seated masses have been revealed, the time for erosion since the 
close of volcanic activity having been too short. It is also a striking feature that 
dikes are extremely rare, the most abundant being in the inner walls of Monte 
Somma, and the only other known occurrences being a few sporadic cases in the 
Ciminian District observed by some geologists. It must, however, be said that 
my own researches failed to reveal these, and, as far as my personal knowledge goes, 
there is not a single dike in the region outside the Vesbian Volcano. 
The main feature of interest in connection with the geologic occurrence is that 
the more complex volcanic structures are situated at or toward the extremities of 
the zone, and that they are at the same time much the most varied, in their eruptive 
rocks, both as to number of types and range of chemical composition, while the 
simpler volcanoes are toward the center of the zone and show very few types, with 
a narrow range in chemical composition. As far as the structures go this will be 
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