ITALIAN PETROLOGICAL SKETCHES 845 



abundant tlian in tlic jM-cccding rocks, which led Mercalli to call 

 it a Icucite-tephrite. They all carry very narrow irregular 

 mantles of orientated alkali feldsj)ar. The pyroxene is quite 

 similar to those already described, but the green color is deeper 

 and the augite molecule evidently surpasses that of the diopside. 

 It frequently carries inclusions of apatite needles. 



The most prominent difference in these Monte Venere rocks 

 is the presence of quite numerous phenocrysts of brown biotite, 

 which show strong pleochroism. They are quite fresh over the 

 greater part of their area, but carry borders of large and loosely 

 coherent augite and magnetite grains. A few are entirely 

 altered and only represented by clusters of these grains showing 

 roughly the original form. The groundmass is largely made up 

 of small round leucite anhedra. Between these lie many laths 

 and flakes of alkali feldspar, fewer augite microlites, and many 

 magnetite grains, all of which are imbedded in a colorless glass 

 base with no apparent flow structure. There is no evidence of 

 the presence of nepheline. An analysis of this rock is given in 

 Table II, No. 7. 



The above descriptions embrace all the leucitic rocks collected 

 by myself, but a few of a different character are described by 

 other observers. Deecke mentions leucite-basanite as occurring 

 on the south shore of Lake Vico, as well as among the lapilli of 

 Monte Venere ; and Mercalli notes a similar rock from San 

 Martino, between Viterbo and Monte Fogliano. Bucca also 

 describes an olivine-bearing leucite-tephrite from Capo d'Acqua, 

 near Vetralla. Mercalli speaks of leucite-tephrites as occurring 

 in several places, but his brief descriptions leave it uncertain 

 whether Icucite-trachytes are not intended. He also mentions a 

 leucitic -])honolite as occurring in erratic blocks in the tuff which 

 carries the phonolites described later. Deecke describes a 

 number of leucite-phonolites (leucitophyrs) from Piazza, on the 

 southeast shore of Lake Vico, from below San Rocco, and from 

 Borghetto near Civita Castellana, east of the volcano. This last 

 I)robably belongs to a flank eruption. They closely resemble 

 the leucite-trachyte first described, except that they carry 



