40 HENRY S. WASHINGTON 



with the above. The hypersthene he speaks of as present is 

 perhaps to be referred to the barkevikite just mentioned. 



The toscanite of Monte San Vito is structurally quite different 

 from that of Monte Calvario. The predominating dark brown- 

 ish black mass is highly vitreous, with a few irregular cavities 

 which are lined with a light blue gray opal. Through this are 

 scattered many quite large glassy tabular sanidines, in almost 

 every case twinned according to the Carlsbad law. They are 

 stained slightly yellow and many carry the bluish opal in the 

 crevices. Some irregular quartz grains are also visible, but 

 phenocrysts of ferro-magnesian minerals are wanting. 



The large sanidines are seldom met with in thin sections, but 

 of the smaller phenocrysts those of feldspar are the most abun- 

 dant. Those of plagioclase are in the majority over those of 

 orthoclase, and Michel-Levy's method shows this to be labra- 

 dorite of the composition Ab^Ang. The crystals, both of lab- 

 radorite and of orthoclase, are automorphic and sharp in outline, 

 showing the usual planes. They are clear and quite free from 

 even incipient alteration. Inclusions of pale brown glass are 

 common, often with a bubble and sometimes of the shape of the 

 host. Pale diopside, apatite and magnetite are also included in 

 the feldspars. These feldspars are often clustered together, but 

 no distinct relative order of crystallization could be made out 

 between the two. The diopside is in stout well-formed crystals, 

 usually with a pyramid largely developed. It is almost or quite 

 colorless, and inclusions are rare and almost wholly of brown 

 glass. A number of small thick tables of brown biotite are 

 present which invariably show a narrow border of fine-grained 

 augite-magnetite aggregate. A few large grains of magnetite 

 and some apatite needles complete the list. In my few speci- 

 mens I could detect none of the hypersthene mentioned by 

 Bucca as abundant. The groundmass is highly vitreous, con- 

 sisting of a largely predominating light brown glass base, in 

 which are sprinkled, with little evidence of flow structure, many 

 minute orthoclase and fewer prismatic diopside microlites. 

 Small magnetite grains are rare and are perhaps derived from 



