36 HENRY S. WASHINGTON 



Surrounding it is a circle of hills, whose steep inner sides come 

 down close to the water's edge, leaving only a narrow shore mar- 

 gin. These hills are highest on the north, where they reach 

 their maximum elevation in Monte di Rocca Romana (602""), 

 and from this gradually diminish in height around the lake 

 toward the southern shore, where their height at Monti is only 

 336"" above sea level. From this circular crest the land here, as 

 at Bolsena and Vico, slopes gradually down on all sides at a low 

 angle, and presents much the same characteristic features. 



To the east of the lake are three maar-like craters described 

 by vom Rath. The largest of these is the dry circular Valle di 

 Baccano, about 3^" in diameter. Between this and Lake Brac- 

 ciano lie Lake Martignano (whose water level is 43™ above that 

 of Lake Bracciano), and the Stracciocappa Marsh. These are 

 all surrounded by ridges of tuff. 



The walls and sides of the Bracciano Volcano are built up of 

 leucitic lava flows and tuff beds, except on the west. On this 

 side we find two small non-leucitic centers. About 3""" due west 

 of Bracciano is the group of low hills which include Monti 

 Oliveto and San Vito. These are partially covered by leucitic 

 tuffs, so that they are older than some of these eruptions. To 

 the north of them is a small solfatara, whose floor is white through 

 decomposition of the rocks, and where sulphurous vapors are 

 abundantly given off. To the north of this again is Monte Cal- 

 vario,' a domal mass of eruptive rock. According to Tittoni 

 this rests on Pliocene beds. 



Petrography. — In this region my stay was of very short dura- 

 tion, so that the specimens collected and the observations made 

 were few. A study of the geological map and my own observa- 

 tions are, however, sufficient to show that we have to deal here, 

 as in the other centers, with two prominent groups of rocks, a 

 non-leucitic and a leucitic. The group of phonolites proper 

 seems to be lacking, or perhaps is present in only small amount. 



' It may be noted that BucCA speaks of this last eruptive center as Monte Virginio. 

 The hill of this name, which lies just north of Monte Calvario, is composed of tuffs 

 enclosing blocks of leucitite, as shown on the geological map. 



