GENERAL TOPOGRAPHY AND GEOLOGY. 3 
(4) Latian District, comprising the volcanic complex of the Alban Hills near 
Rome. It is so called from its position in the ancient Latium. 
(5) Hernican District, comprising the volcanoes in the valley of the Sacco 
River, formerly inhabited by the Hernici, whence the name is derived. 
(6) Auruncan District, comprising the volcano of Rocca Monfina. The name 
is derived from the ancient tribe of the Aurunci. 
(7) Campanian District, comprising the volcanoes of Vesuvius, the Phlegrean 
Fields, and of Ischia. It is named from the Roman province of Campania, in 
which it lies. 
In addition there are a number of very small cones, or remains of cones, at some 
distance from the main line, as at Radicofani, San Venanzo, and Rieti. The rocks 
of these are possibly connected with those of the main line, but they will not be 
described, and their rocks will be only incidentally mentioned. 
The relations of the volcanoes of the Roman Region to the earlier ones of the 
Tuscan Region, which lie rather closely to the north and west, are problematical. 
There is some evidence that the rocks of the two regions are connected genetically, 
as will be pointed out later, but it seems advisable for the present to confine our 
attention to the Roman Region, leaving the discussion of the volcanoes and rocks 
of the Tuscan Region to the future. 
The brief descriptions which follow do not pretend to be complete. They 
serve only to give a general idea of the topography and geology and the vulcano- 
logical structure of the several districts, sufficient for a proper understanding of some 
of the petrological relations to be discussed in subsequent pages. They are based on 
the most modern literature available, which is due to the activity of the able Italian 
and German geologists who have studied the region, supplemented by the observa- 
tions which I was able to make. 
Vulsinian District. 
This district, the most northerly of the zone, extends from the Paglia River on 
the north to the Ciminian District on the south, the boundary between the two 
being irregular and in places not well denned. It embraces an area of about 2,280 
square kilometers, and extends in all directions around Lake Bolsena, which is 
the most prominent topographical feature. 
The lake is quite regularly elliptical in shape, the major axis running north and 
south. Its dimensions are 13 by n kilometers, with an area of 114.5 square kilo- 
meters. The lake surface lies 305 meters above sea-level, and the bottom, with 
some irregularities, slopes in general toward a point near the center, where the depth 
is the maximum one of 146 meters. Two small islands project above the water in 
the southern half. 
Lake Bolsena is surrounded by a girdle of hills which slope steeply down 
to the narrow fringing shore. These are the highest on the north, where they 
attain an altitude of 702 meters above sea-level, and among these are the small 
