ITALIAN I'ETROLOGICAL SKETCHES 245 



scoriaceous. Most of these small cones were formed on the 

 outer flanks of the volcano, but three are found in the interior, 

 one being in the valley of Pratolungo between Mte. Sta. Croce 

 and Mte. Lattani. The tuffs belonging to this phase are few and 

 of small amount. That the eruption of the "basalts " was posterior 

 to those of the leucitites and trachytes is made evident by sev- 

 eral facts. The small cones formed by them are the best pre- 

 served of all, in many cases quite intact and covered with lapilli 

 and bombs and with their lava flows quite bare. At Sipicciano 

 on the northwest flank of Mte. Cortinelli a basaltic flow covers 

 the leucitites and leucite-tephrites ; while in the central eruption 

 at Pratolungo the flow of "basalt" is seen to be superposed on 

 the trachy-andesite of Mte. Lattani. 



Leucitite. — This is represented by only one specimen in my 

 collection, from a flow below Preta, on the outer northern side of 

 the volcano. This shows a very compact, fine grained, dark gray 

 groundmass, through which are scattered many small but quite 

 glassy leucites, with rare specks of augite. In thin sections these 

 large leucites are seen to be perfectly clear, and they show very 

 weak double refraction for such large fresh crystals. Over the 

 greater part of their area they carry almost no inclusions, but at 

 the edge of almost all is a very narrow peripheral line of small 

 augite microlites, which in turn is surrounded by a narrow mantle 

 of leucite. This shades into the groundmass, and is quite anal- 

 ogous to the alkali feldspar mantles so frequently noticed in the 

 preceding papers. Apart from the leucites only rare light green 

 augite phenocrysts are seen, which also carry no inclusions. 



The groundmass is fine grained and composed essentially of 

 small leucites with interestitial greenish gray augite prisms and 

 anhedra and a little magnetite. These small crystals are imbed- 

 ded in a colorless isotropic substance. It is possible that this is 

 o-lass, but, judging from the leucitic mantles around the leucite 

 phenocrysts and from the fact that there is no difference in 

 refractive index between this base and the small leucites, it seems 

 probable that it is leucite. It would thus correspond to the nephe- 

 line base of certain phonolites and other nepheline rocks, such 



