38 HENRY S. WASHINGTON 



quartz grains are present. Biotite flakes are not rare, though 

 these and the augite phenocrysts have suffered much through 

 weathering, being represented by brown limonitic spots in some 

 of the specimens. Some large enclosures of a darker, fine- 

 grained, vesicular rock were seen, which will be described pres- 

 ently. 



In thin section the sanidine phenocrysts are seen to predomi- 

 nate over those of plagioclase. Examination of suitable sec- 

 tions of the latter shows it to be a labradorite of the approxi- 

 mate composition AbgAn^. The}^ are clear, but inclusions are 

 not uncommon, nearly always of brownish glass, and often in 

 the shape of the host. A few instances of parallel growth were 

 seen, a plagioclase core being surrounded by a border of alkali 

 feldspar. This, however, forms one crystal individual with the 

 plagioclase, and belongs to the same general period of crystal- 

 lization, being sharpl}^ outlined and distinct from the ground- 

 mass. It is thus of a different character from the alkali feldspar 

 mantles already noticed. A few rounded cr^'stals of quartz are 

 to be seen. The ferromagnesian minerals are represented by 

 brown biotite and fewer pale green diopside crystals, both of 

 which are much decomposed. The groundmass, which is almost 

 holocrystalline, is made up chiefly of alkali feldspar flakes, very 

 few laths being present, with some brown spots representing orig- 

 inal pyroxene microlites. A little quartz may be present, but no 

 plagioclase could be detected, though the rock is in such a con- 

 dition that a satisfactory study of it was impossible. 



This rock is described by Bucca as a quartz-trachyte. His 

 description agrees closel}^ with my own limited observations. 

 He mentions, however, the presence of hypersthene, whose exis- 

 tence in my specimens may have been concealed by its well- 

 known liability to decomposition. 



The enclosures in this trachyte are apparently fragments of an 

 early lava stream, brought up from probably no great distance 

 below. They are dark gray, fine-grained and compact, and quite 

 vesicular, though far from being scoriaceous. The line of junc- 

 tion between the enclosed and enclosing rock is sharp, and no 



