ITALIAN PETROLOGICAL SKETCHES 543 



also contains the complete set of Ricciardi's analyses, which were 

 published by him elsewhere.' 



In the same year Moderni^ published a short paper on the 

 trachyte and tuff of Rispampani near Toscanella, which lies to 

 the south of Lake Bolsena. He states that the trachyte forms a 

 dome resting on Eocene beds. The trachytic tuff is remarkable 

 for its radiating columnar structure, and resembles a lava in 

 external characters. 



In an important paper on the extinct volcanoes of the north- 

 ern Apennines de Stefani^ takes Lake Bolsena as the type of the 

 crater-lake eruptions and gives a detailed description on the 

 basis of his own observations. He differs from Verri in several 

 points, notably in regard to the genesis of the lake, which he 

 regards as a large central crater, at the same time recognizing 

 the existence of flank eruptions, and comparing the volcano to 

 those of the Hawaiian Islands. The interesting general conclu- 

 sions regarding the Italian volcanoes which he gives at the end 

 of his paper will be noticed elsewhere. 



The last writer to treat of the region is Bucca^* who gives 

 short petrographical descriptions of a number of the leucite rocks. 



Topography. — The center of the region is Lake Bolsena, which 

 lies northwest of Rome and southwest of Orvieto, in Lat. 42° 35' 

 and Long. 12° 20' E. Its shape is quite regularly elliptical, the 

 major axis running north and south. Its dimensions are 13 by 

 10 kilometers, with a surface area of about 1 12 square kilometers, 

 being thus the largest crater-lake in Italy, and one of the largest 

 in the world. The surface of the lake is 305 meters above sea 

 level, and the greatest observed depth is 140 meters. The two 

 small islands of Bisentina and Martana rise a few meters above 

 the surface in the southern and southwestern parts of the lake. 

 They are composed chiefly of tuff and seem to be the remains 

 of small volcanic cones. 



' RiCClARDi, Gazz. Chim. Ital., Palermo, 1888. 

 -MODERNi, Boll. Com. Geol. Ital., X, 19-25, 1889. 

 3 De Stefani, Boll. Soc. Geol. Ital., X, 499-537, 1891. 

 •* BuccA, Rivista Min. e Crist., 18-30, 1893. 



