GENERAL TOPOGRAPHY AND GEOLOGY. 7 
which destroyed much of the earliest crater wall, are true crater lakes, due to later 
eruptions, the positions of the vents having shifted. The tuffs from this volcano 
have covered nearly the whole of ancient Latium. 
The most important towns to be mentioned are Rocca di Papa near Monte 
Cavo, Grotta Ferrata, and Frascati on the northwest outer slope of the earliest 
cone, Rocca Priora on the northeast, and Velletri on the southeast, while Marino, 
Castel Gandolfo, Albano, and Genzano are all in the immediate vicinity of the 
small, late crater lakes on the west and southwest. 
Hernican District. 
This district, which lies in the valley of the Sacco, about 50 kilometers east- 
southeast of the Alban Hills, differs from those described above in two respects 
the geologic structure and the volume of the eruptive products. Instead of forming 
a single volcano or volcanic complex of large dimensions, the eruptions were of such 
small amount and the vents so widely separated that they formed only six or eight 
small and isolated cones. These may be called the Cones of Ticchiena (between 
Frosinone and Ferentino), Pofi, San Francesco, San Marco, Sant' Arcangelo, Callame 
(the last four around the town of Ceccano), Giuliano, Patrica, Morolo, and Selva 
dei Muli. Of these the one which best preserves any well-defined vestiges of its 
original form is that of Pofi. This village lies on a hill 295 meters above sea-level 
and 130 above the Sacco Valley bottom, which is formed for the most part of tuffs 
and lapilli, with a few small lava flows, the crater having disappeared. The other 
cones have suffered so extensively from atmospheric degradation that they are now 
represented by areas of volcanic tuffs, with small exposures of lava here and there, 
and with but little trace of the original form. The total area of the volcanic prod- 
ucts is very small less than 50 square kilometers in all. 
Auruncan District. 
This district is sityated about 70 kilometers southeast of the preceding one 
and about 65 north-northwest of Vesuvius, and its volcanic rocks cover an area of 
about 400 square kilometers, roughly estimated. It consists of but one volcano, 
which is called Rocca Monfina. This is of the simple type of strato- volcanoes, analo- 
gous to that of the Alban Hills and the Somma- Vesuvius mass, and has compara- 
tively few parasitic cones. It possesses an external ring, sloping regularly outward 
on all sides and eroded into radial valleys. The crest of the ring reaches its maxi- 
mum height of 926 meters above sea-level on the southwest side, which is the best 
preserved, the original crater walls having been much degraded, or being at least 
much lower, on the north and east. In the center of the plain inclosed by the large 
crater ring, which measures about 6 kilometers in diameter, rises the dome of Monte 
Santa Croce, the site of the final eruption of the volcano, with the smaller Monte 
Lattani adjoining it on the north. 
Of the towns around this volcano, the only ones which need be mentioned are 
Rocca Monfina, at the east foot of Monte Santa Croce ; Teano, near the southeastern 
