ITALIAN PETROLOGICAL SKETCHES 375 



greater since there are indications of a division of the basic 

 leucitic subgroup into two divisions corresponding to the 

 ciminites and vulsinites. These would include on the one hand 

 the Icucitites and leucite-basanites, and on the other the leucite- 

 tephrites. There exist then along the Bolsena-Vesuvius line at 

 least two well-defined series with correlated members in each, 

 the Ciminite-Vulsinite-Toscanite Series and the Leucitite-Leucite- 

 tephrite-Leucite-trachyte Series. These two resemble each 

 other very closely, except for the presence of leucite in the 

 latter and the uniformly greater basicity of the correlated mem- 

 bers of this series as compared with those of the former. There 

 also occurs a typically trachytic group in the Vesuvian region, 

 while the phonolites of Viterbo mineralogically stand apart, but 

 chemically are related to the leucite-trachytes. These series 

 are, however, not gradual in Brogger's sense of the term,' but 

 form related but separated groups of rocks. 



Differentiation of the magma. — The question of the process or 

 processes of differentiation and the composition of the parent 

 magma is of great interest, but needs far more detailed and 

 extensive knowledge of the regions involved and of the rocks 

 than is yet available. The question is somewhat complicated by 

 the presence of leucite in such large amounts, and by the fact 

 already noted, that it is possibly derived from a potential biotite 

 molecule, or at least that some connection seems to exist 

 between the two minerals. It is pretty generally believed that 

 leucite is essentially an effusive mineral, though some cases of 

 intrusive leucitic rocks are known. Therefore, although the first 

 impulse is to consider the leucitic rocks as distinct differentia- 

 tion products from the trachydolerites, yet caution must be used, 

 since it seems possible that their differences are due rather to 

 differing conditions of extrusion than to differing secondary 

 magmas. This, of course, would not be true in all cases, since 

 leucite would be formed in a potash-rich magma poor in silica, 

 while orthoclase would take its place in one more acid. This is 

 seen clearly in the rocks with silica below 55 which are almost 

 'Brogger, Grorudite — Tinguait Serie, Kristiania, 1S95, 169. 



