564 HENRY S. WASHINGTON 



It resembles the preceding, though it is much finer grained, and 

 the leucite is seen to be quite subordinate in amount as com- 

 pared with the augite and plagioclase. This last is quite basic 

 and is a true bytownite.'' Quite noticeable in the sections are 

 many small flakes of brown biotite, which have given rise to 

 patches of a bright green, finely-granular viridite, probably 

 through atmospheric alteration. None of the other minerals 

 show signs of decomposition, except the magnetite, which is 

 sometimes accompanied by spots of limonite and bright orange 

 hematite. The augite is pale gray and none of the brown horn- 

 blende is present. 



Most of the leucite-tephrites described by Klein seem to 

 resemble the above and call for no special notice. Exception 

 must be made for those from Montalto, southwest of the lake, 

 which Verri regards as a distinct eruptive center. They 

 approach augite-andesites in character, but carry olivine pheno- 

 crysts, and small leucites only in the groundmass, so that Klein 

 calls them "leucite-tephrites with accessory olivine." In min- 

 eralogical composition they correspond to a leucite-basanite, 

 but their high content of silica allies them with the tephrites. 

 {Cf. anal. 11, page 565.) 



The phenocrysts are of green augite, a basic plagioclase, 

 magmatically altered biotite, olivine, magnetite and apatite. The 

 groundmass is andesitic in structure and consists largely of 

 olagioclase laths, augite crystals and magnetite grains, with 

 small colorless spots which were identified with leucite. All 

 these lie in a glass base. It is noteworthy that in one very 

 glassy specimen no leucite was to be found, while the other 

 minerals remained as before. 



Leucite-basanite . — None of my specimens belong here, so I 

 may briefly mention some described by Klein as "leucite rocks 

 of doleritic habit." All his specimens come from the southern 

 part of the region. They are distinguished from the true teph- 

 rites by their constant olivine content and structure, and Ric- 



' ROSENBUSCH (Mikr. Phys. II, 762, 1887) mentions the occurrence of acid 

 plagioclase in a leucite-tephrite from " Orvieto." 



