556 S. KOZU 



Microscopical characters. — The phenocrysts are of abundant 

 euhedral, or sometimes subhedral, feldspar. Almost all of them 

 are of anorthoclase, in which a small quantity of plagioclase is pres- 

 ent, either as nucleus of zoning which can be seen very rarely, as 

 perthite composing the faint perthitic structure, or as local patches 

 in the anorthoclase crystals. The groundmass is holocrystalline, 

 though small amounts of brown glass are locally present in the 

 vicinity of feldspar phenocrysts. It consists essentially of pris- 

 moids of alkaline feldspar, elongated toward the axis a. They 

 are arranged as in typical trachytic fabric. A smaller quantity 

 of thin and long prismoids of aegirine-augite, crystals or grains of 

 magnetite, and slender needles of apatite are disseminated through 

 the feldspathic groundmass. 



Feldspar, as phenocrysts, is soda-microcline, containing anorthite 

 molecules, that is, calcium-bearing anorthoclase. The shape gener- 

 ally shows euhedral form, principally bounded by crystallographic 

 faces (ooi), (oio), and (201), and is thick tabular parallel to (010), 

 or is very stout prismoid, or cuboidal. The characteristic habit 

 is derived from its appearing in a rectangular form on the face 

 (010), owing to the domination of the planes (001), (201), and 

 (010), as is the case with feldspar in " Rectangelporphyre," described 

 by Th. Kjerulf. The well-known rhombic form is entirely absent. 

 Parting parallel to (100) is so distinct that the cleavage pieces are 

 very difficult to get, the crystal easily breaking into pieces along 

 that face, as seen in Figs. 1 and 2. The twinning according to the 

 Carlsbad law is the most common, very rarely the Manebach type 

 is megascopically recognizable. Two other types (albite and peri- 

 cline) appear faintly between crossed nicols; sometimes they are 

 locally and irregularly distributed in the inner part of the pheno- 

 crysts. In some crystals microperthitic and microline structure 

 are visible. Zonal structure is not uncommon, but is not so distinct 

 as in the case of plagioclase. The outer zone usually shows a 

 slightly lower refraction than that of the inner part, but both are 

 lower than balsam. In one instance, plagioclase appears as a 

 nucleus in the alkaline feldspar. Aegirine-augite and greenish- 

 brown glass are comparatively abundant as inclusions; besides 

 these, apatite needles and magnetite grains are usually inclosed 



