ITA L I A N PE TROL O GICA L SKE TCHES 829 



and masses of lava Ijlocks, tuff generally constituting the highest 

 jKirt of the ridge. The lavas of Monte Vico are predominantly 

 leucitic, a few phonolites also having been observed. 



The southern part of the interior of the crater is occupied 

 by a lake whose surface is 507 meters above sea level. Its 

 depth docs not seem to have been ascertained, but it is appar- 

 ently shallow. At the southeast corner an emissary has been 

 cut for drainage purposes. Eventually the lake will probably 

 give place to a plain, through drainage of its waters and filling 

 up by denudation of the surrounding easily eroded tuff walls, as 

 is the case at Agnano and elsewhere in Italy. 



In the northern part of the crater there rises from the allu- 

 vial plain left by contraction of the lake from its original limits 

 the so-called Monte Venere (Venus Mountain). This is a 

 rounded hill with steep sides, whose summit is 317 meters 

 above the lake level. The dense growth with which it is covered 

 makes proper study of it difficult, but as far as I could judge it 

 is a compact unstratified mass of eruptive rocks — a "dome" of 

 leucite-trachyte. Part of the southeast flank is covered with 

 pumiceous scorise, but elsewhere all such detrital material is 

 lacking. Stoppani and Ricciardi mention a lava stream as flow- 

 ing down its western side, which however I did not see, nor is 

 it spoken of by Verri and Deecke. 



From the ring wall of Vico the surface slopes gradually down 

 on all sides at a low angle, except to the north where the Monti 

 Cimini break the regularity. This surface is made up of yellow 

 and gray tuffs containing leucitic, pumiceous, and some phono- 

 litic blocks, as well as blocks of metamorphosed limestone, 

 etc., such as are described by Deecke and Mercalli. Deep 

 ravines — -so characteristic of the surface topography of these 

 regions — have been cut out by erosion, and diverge radially on 

 all sides. Flank eruptions, which are so common at Bolsena, 

 seem to be almost entirely w^anting. 



On the outskirts of the volcano the tuff and lava beds rest 

 on late Eocene, and also in places on Pliocene deposits. Imme- 

 diately to the south of the Vico crater lies the Bracciano reeion 



