ITALIAN PEIROLOGICAL SKETCHES 35 



devoted to the sedimentary terranes, while the latter half is 

 taken up with brief megascopic descriptions of the eruptive 

 rocks and their tuffs, and with their distribution. He considers 

 that all of the eruptives are post-Pliocene. 



The rocks of the Agro Sabatino collected by Tittoni, as well 

 as others of the same region from other sources, were examined 

 by Bucca' who also later^ adds a note on the enclosures in the 

 eruptive rock of Monte Calvario. His papers are entirely petro- 

 graphical, and will be referred to later on in detail. Busattis in 

 The same year gives a description of a trachyte from Tolfa, to 

 which Lotti adds some remarks on the genetic relationships. 

 De Stefani,4 later, also gives an account of the regions of Tolfa 

 and Cerveteri.5 



It will be evident from the above that the literature of the 

 regions in question is of a very scanty description, the important 

 leucitic rocks of Lake Bracciano being practically untouched, 

 and the others not having been studied very fully. The regions 

 in question are shown on the Tolfa and Bracciano sheets (Foglii 

 142 and 143) of the Italian Geological Map (scale i : 100,000). 



THE BRACCIANO REGION. 



Topography.— The center of this region is the large Lake of 

 Bracciano. This is almost circular in shape and with a diameter 

 each way of 9'^'"^ The surface of the lake is 164" above sea 

 level. On the north at Trevignano its symmetry is broken by a 

 small bay which apparently represents a small explosive crater. 



The depth of the water is not stated, though it seems to be 

 much deeper than Lake Vico. There are no islands in the lake. 



'BUCCA, Boll. Com. Geol. Ital., 1886, 212-223. 



^BuccA, Loc. cit., 377-379- 



3BUSATTI, Proc. verb. Soc. Tosc. 4, Luglio. 1886. 



4De Stefani, Boll. Soc. Geol. Ital. X, 4^7-499, iSQI- 



s In a paper received just before going to press, P. Moderni (Boll. Com. Geol. Ital. 

 1896, Nos. I and 2), describes the Bracciano center. He regards the mass as the 

 product of eruptions from about 50 centers surrounding the lake, and this not as a 

 true crater-lake but as due to a sinking in of the surface. He does not touch on tue 

 petrography. 



