5 56 HENRY S. WASHINGTON 



It is true that Rosenbusch uses the term phonolite in a very 

 broad sense, covering the leucite as well as the nepheline rocks. 

 The advisability of such an extension of its meaning may well 

 be doubted. As to the term leucitophyr used by Rosenbusch, I 

 must agree with Zirkel in his objections to it, as a name which 

 does not convey any idea of the presence of nepheline, and con- 

 veying the erroneous idea that a porphyritic structure is charac- 

 teristic of it. Further discussion of this subject is uncalled for, 

 but the terms leucite-trachyte and leucite-phonolite will be used 

 in Zirkel's sense. 



Leucitite. — Of true leucitites there seem to be comparatively 

 few in this region compared with others of Italy, such as those 

 of Bracciano and Albano. Klein describes eight which belong 

 to this group, though they all carry a little plagioclase, and 

 Bucca also notices a few. They are basic, with SiOg from 48 to 

 50, and Ricciardi's analyses show less K3O than we would 

 expect to find. Megascopically they are basaltic looking, dark 

 gray and very fine-grained and compact, with only rare pheno- 

 crysts of leucite and augite. Their micro-structure is the char- 

 acteristic one, well known to most petrographers, of a ground- 

 mass made up of round spots of leucite with interstitial augite 

 needles. 



The most typical of my specimens is from a flow near the 

 lake shore, about half a kilometer north of Bolsena. Its color 

 is dark gray with a slightly greenish tinge. 



Under the microscope it shows the typical structure. Leu- 

 cite crystals are extremely abundant in the groundmass, few 

 being of phenocrystic dimensions. With them are some large 

 green augites, a few biotite crystals almost wholly "altered," and 

 one or two well shaped orthoclase phenocrysts. These lie in a 

 cement composed of colorless glass, with a felt of slightly green- 

 ish augite microlites, some magnetite grains and very few plagi- 

 oclase laths. 



The leucites, which show feeble double refraction, seldom 

 reach a diameter greater than o™™.25, and from this run down to 

 microlitic dimensions. These smallest leucites show only 



