NOTES ON SOME IGNEOUS ROCKS OF JAPAN 635 



inclusions, it is rarely met with in the general mass, and may be 

 considered as an accessory constituent of the rock. 



The matrix exhibits different textures, according to different 

 conditions under which the lava consolidated. The crustal part 

 of the ejecta is hypohyaline, while its inner part usually shows 

 typical cellular structure, the glass base being colorless. The 

 specimens taken from the dome are more crystalline than those 

 just described, but there is still abundant residual glass. It is 

 moderately clouded with magnetite dust and pyroxene microliths. 

 The megascopically cryptocrystalline part appears holocrystalline 

 under the microscope and consists essentially of granular pyroxene 

 and feldspar, scattered microporphyritically with skeletal crystals 

 of hypersthene. 



Feldspar. — The feldspar phenocrysts are of two kinds. One of 

 them is well-formed anorthite of considerable size, 13 mm. in 

 average length. Zonal structure is nearly wanting, or is indis- 

 tinct. These occur in the ejecta and peripheral part of the lava, 

 or even as isolated crystals, suggesting that their crystallization 

 was prior to that of other minerals. The other variety is smaller 

 in size, with an average length of 2 mm., and is commomly sub- 

 hedral in shape, sometimes with a strongly curved outline invaded 

 by the glassy groundmass. This variety differs from the first in 

 possessing zonal structure due to variation in the chemical compo- 

 sition and to the arrangement of abundant inclusions. In average 

 composition, the second variety is slightly more sodic than the 

 first. The most abundant inclusions are light-brown or colorless 

 glass with air bubbles in many instances. Apatite and magnetite 

 are also present, commonly in small quantity. In some crystals 

 pyroxene appears as inclusions, but more commonly the feldspar 

 is abundantly inclosed in the hypersthene and augite phenocrysts 

 and shows a distinct automorphic relation toward the pyroxene. 

 The larger crystals will be more fully described in the second part 

 of this article. 



Pyroxene. — The hypersthene is easily distinguishable from the 

 augite by marked pleochroism, low double refraction, parallel 

 extinction, and crystal habit. It occurs in crystals of two periods 

 of crystallization. The largest phenocrysts are 2.5 mm. in length 



