372 HENRY S. WASHINGTON 



large number of transition forms toward the leucitites on the 

 one hand and the leucite-trachytes on the other. In general 

 they are characterized by a rather doleritic micro-structure and 

 the plagioclase is basic ; chemically they are very basic in char- 

 acter, their silica content scarcely running above 50 per cent. 

 Leucite-tephrites of this type abound at Bolsena, Rocca Mon- 

 fina, and Vesuvius, and typical analyses are represented by Nos. 

 6, 7, and 8. Other leucite-tephrites occur whose silica content 

 is higher. Examples are the rocks of Montalto (No. 10), in 

 which Klein reports the feldspar as anorthite, and that of Monte 

 Bisenzio (No. 9). Such acid leucite-tephrites, however, have 

 not come under my own observation, and the high silica is to be 

 explained by the presence of considerable orthoclase. Indeed 

 my observations and such analyses as I have been able to make 

 lead me to think that a typical Italian leucite-tephrite (which is 

 poor in orthoclase) has a silica percentage of 50 or less, and 

 that the plagioclase is generally, if not normally, a basic one. 



When we reach the leucite-trachytes and leucite-pho7wlites we 

 find a decidedly more acid group of rocks with silica ranging 

 from 55 to 59. These shade off into the leucite-tephrites to 

 some extent, but on the whole are distinctly separated from 

 them. Of leucite-trachyte we have few analyses, Nos. 14-18 

 being the only reliable ones known to me.' They show high 

 alumina, low magnesia and lime, and high alkalies — especially 

 potash, though this last feature is perhaps not as marked as 

 we might expect. The analyses of leucite-phonolite (Nos. 

 II, 12, 13) resemble these very closely, though soda is 

 somewhat higher, and are quite different from those of leucite- 

 phonolites from Germany,^ which are much more basic and with 

 soda much higher than potash. Indeed, so great is the resem- 

 blance to analyses of leucite-trachyte that these Italian leucite- 

 phonolites ought properly to be called nepheline-bearing leucite- 

 trachytes. They all carry orthoclase in large amount, especially 

 in the groundmass, and nepheline occurs as the last product of 

 crystallization. Only one case of a purely leucite-nepheline 



' ZiRKEL, Lehrbuch, II, 465, 1894. 



