842 HENRY S. WASHINGTON 



phenocrysts are of a very pale green diopside and are highly 

 automorphic. Some show evidences of fracture and corrosion, 

 while a few have at the ends a late fringing growth of small 

 diopside needles arranged parallel to the vertical axis. Except 

 for a few large magnetite grains other phenocrysts are wanting. 



The groundmass is decidedly andesitic in structure. Small 

 stout needles of diopside, and of the brown -pleochroic 

 barkevikite-like hornblende seen in Bolsena leucite-tephrites,^ 

 many small alkali feldspars and a few plagioclase laths, and 

 many well-shaped magnetites lie in a colorless glass base with 

 absolutely no evidence of flow structure. Here and there are 

 small round spots of a colorless substance whose inclusions and 

 analosfies with similar forms in others of these Italian rocks 

 show them to be leucite, though their double refraction is 

 scarcely visible. Apatite needles are also present in all the 

 specimens. The colorless base is isotropic and seems to be 

 entirely of glass. No nepheline could be detected with certainty. 

 In one rock some alkali feldspar flakes are also present, and the 

 glass in many places is of a light brown color. 



Analyses of two of these rocks are given in Table II, Nos. 

 4 and 5. They resemble each other fairly well though 5 is 

 lower in SiOg and AlgOg and higher in iron oxides and lime. 

 The analysis of the groundmass of a leucite-trachyte from the 

 Vico Crater, by Ricciardi (No. 6), shows somewhat lower silica, 

 extremely high alumina, considerably more lime, and less iron 

 and alkalis. These are what we would expect to find, except 

 the very high alumina, and perhaps the lower iron. 



A few specimens of much decomposed petrisco from blocks 

 in the latest tuff also deserve mention. They are so friable as 

 to crumble readily between the fingers, rendering it difficult to 

 obtain good specimens. The groundmass is black or brownish- 

 black and very fine grained. This is thickly peppered with 

 small and large leucites, with a few small sanidines, which stand 

 out prominently against the dark background. The leucites in 

 general are perfect and well formed, though some fragments 



'Cf. this Journal, IV, 562, 1896. 



