840 HENRY S. WASHINGTON 



of junction of the obsidian with the rest of the section. The 

 contact line is rather vague, and close to the obsidian there are 

 seen in the dusty groundmass some indications of a similar and 

 parallel flow structure. 



This patch might then be evidence in favor of the rock 

 being a much decomposed and devitrified, highly vitreous, ande- 

 sitic mica-trachyte, were it not for the wholly fragmentary con- 

 dition of the crystals and the general similarity of the rock 

 with other tuffs. The indications of flow structure seen in the 

 dusty groundmass might be explained by the idea that they are 

 due to the presence of part of the thin flake of obsidian extend- 

 ing beneath the tuff proper in the section. Such a flow struc- 

 ture is, however, clearly visible in such undoubted tuffs as those 

 of Monte Epomeo on Ischia, and Monte Barbaro in the Phleg- 

 raean Fields and the obsidian-like patch may be due to secondary 

 silicification. I may add that the view that the rock is a tuff 

 accords better with the lack of contact metamorphism in the 

 underlying Pliocene beds as commented on by Verri^ and 

 Deecke.^ 



Leiicitic rocks. — These rocks, as we have seen, are confined 

 apparently to the Vico center, though according to vom Rath 

 and Deecke the latest ejections of the Cimino volcano were 

 leucite-bearing. 



There seems to be much less variety among the leucitic 

 rocks of this region than was found among those of the Bolsena 

 region. All the specimens which I collected may be referred 

 to leucite-trachyte, and the same seems to be the case with 

 those described by others, with a few exceptions to be noted 

 presently. 



Leucite-trachyte? — This rock, which is known locally by the 

 name of ''petrisco," is very abundant in the region, forming flows 

 and blocks in tuff around Monte Vico, as well as the domal 

 Monte Venere. The flow occurrences are generally fresh, com- 



' Verri, op. cit., p. 26. = Deecke, op. cit., p. 228. 



3 In Zirkel's sense, leucite phonolite of Rosenbusch. Cf. this JOURNAL, IV, 555, 

 i8q6. 



