THE ROMAN COMAGMAT1C REGION. 
GENERAL TOPOGRAPHY AND GEOLOGY. 
Introduction. 
The volcanoes of the Italian peninsula may be referred to several distinct co- 
magma tic regions (petrographic provinces), the distinctions between them being 
based largely upon the petrographic and chemical characters of their rocks, though 
they are also separated by their geologic ages and their topographic and geologic 
relations. The most prominent of these may be enumerated as follows, and are 
shown in the map on the following page: 
The Venetian Region, comprising the volcanic complexes of the Euganean and 
Berican Hills. 
The Apulian Region, comprising the volcanic complex of Monte Vulture. 
The Tuscan Region, comprising the volcanoes of Monte Amiata, Montecatini, 
Orciatico, Campiglia and Roccastrada, with which may be placed the volcanoes 
of Tolfa and Cerveteri, and Monti Calvario and San Vito near Bracciano. 
The Roman Region, comprising the main line of volcanoes which extend from 
Lake Bolsena southeast to Vesuvius and the Phlegrean Fields. 
All of these vary widely in the characters of their eruptive rocks, andHiffer as 
well in regard to their geological ages. The only one which concerns us here is the 
last mentioned, the Roman Region, so named from the fact that it embraces the chief 
territory of the ancient Roman Republic, as well as the capital city of modern Italy. 
These volcanoes are all of Quaternary age, the eruptions of some of them extending 
into historic and even modern times. Extending in a narrow line between the Apen- 
nines and the Tyrrhenian Sea, these volcanoes occupy the sites of old embayments 
in the Mesozoic and Tertiary sediments which form the backbone of the Italian 
peninsula. 
It will be found convenient to take up the description of the region under the 
different districts into which it may be subdivided, which surround distinct centers 
of volcanic activity, and which are more or less clearly separated from one another. 
These districts are seven in number, and may be thus enumerated, the names be- 
stowed upon them following as far as possible the usages of the Italian geologists: 
(1) Vulsinian District, comprising the volcanic complex around Lake Bolsena. 
Named from the ancient tribe of the Vulsinii. 
(2) Ciminian District, comprising the volcanoes near Viterbo. Named from 
the Latin appellation of Colli Ciminii. 
(3) Sabatinian District, comprising the volcanic complex around Lake Brac- 
ciano. Named from the Roman name of the lake, Lacus Sabatinus. 
