ITALIAN PETROLOGICAL SKETCHES 39 



great amount of contact metamorphism is apparent. In the 

 gray groundmass are large tabular glass}- sanidines and some 

 rounded grains of quartz, but no ferromagnesian phenocrysts 

 are to be seen. In many of the vesicles are slender brownish 

 black needles of breislakite, and hexagonal scales of tridymite.' 

 These enclosures are much less decomposed than the trachyte 

 surrounding them, though the sanidines are stained slightly 

 yellow. 



Under the microscope the structure is strikingly like that 

 of a dolerite. Very numerous prisms of colorless diopside, 

 slightly brownish on the edges, and longplagioclase laths, whose 

 extinctions show them to be labradorite of the composition 

 Ab^An,, with fewer orthoclase laths, lie in a holocrystalline 

 mesostasis of orthoclase. Some comb-like skeleton forms of 

 labradorite are also seen. Magnetite is present though not 

 abundant. A few larger phenocrysts of violet augite are seen, 

 and the few sections of the large tabular sanidine phenocrysts 

 met with are clear and show only a few inclusions of glass 

 and apatite. A narrow orthoclase mantle surrounds them. 

 There were found a number of grains of the peculiar brown 

 barkevikite-like hornblende noticed in a leucite-tephrite of 

 Bolsena. The extinction angle of C on r was 17° and the 

 pleochroism was identical. No olivine is present, but apatite 

 needles are quite abundant. 



The vesicular structure of this rock shows that it is not a 

 segregation proper, but an inclosure of an earlier solidified lava 

 mass which had been erupted on the surface. The breislakite 

 and tridymite were formed prior to the eruption of the toscanite, 

 since they are present in cavities revealed by breaking open good 

 sized masses of the enclosures. The rock corresponds mineral- 

 ogically, and probably more or less closely chemically, with the 

 vulsinites, though the structure is quite different. It probably 

 represents one of the earliest outflows of the volcanic center, 

 carried up by the later toscanite. Bucca {op. cit. 377) gives a 

 very good description of these enclosures which agrees closely 



' Bi'CCA also notes hypersthene and angite crystals. 



