84 



Y. KIKTJCHI 



terminations of the crystal frequently end in fine filaments, which are 

 often curved and branching into filiform threads (fig. 13a) shown 

 magnified in fig. 14. Numerous longitudinal fissures as well as 

 transversal clefts are also characteristically developed. The crystals 

 are often found in irregular stellate aggregates, which are probably 

 twin-groups. In some cases, the directions of the vertical axes 

 differ by 60°. Sometimes a narrow median band is inserted, in which 

 the extinction is straight (fig. 13b), showing that the Rhombic 

 Pyroxene is in parallel-intergrowth with the Augite in the manner 

 already described. 



These rudimentary forms of the Augite-crystals are very 

 characteristic in the glassy rocks of Bonin Island, especially of 

 the rocks found near Omura and Kiyose in Chichijima. In the 

 glasses of Miyanoura, and of Tatsumiura already mentioned as contain- 

 ing the crystals of the Rhombic Pyroxene, the monoclinic Pyroxene 

 appears also in skeleton-forms, which, as may be seen from the accounts 

 given below, must have had a close relation to the crystals of the Rhom- 

 bic Pyroxene during their growth from the magma. A series of these 

 micro-crystals are illustrated in fig. 15 — 20.* The most, simple form is 

 a rod-like body bifurcated at both ends, or sometimes X shaped with 

 numerous delicate spiny proeesses proceeding from the branches (i\g. 

 20). These are very delicate bodies measuring nearly *025 mm. in 

 length. The fine processes usually become more elongated in larger 

 forms, which measure '07 — '00 mm. in length, and arc so arranged that 

 one set attached to one branch is always parallel to the other branches 

 (fig. 15 — 18), these two sets being disposed nearly at right angle to 



* These skeleton-crystals are strikingly analogous to those described by H. Vogelsang from 

 a certain slag. (Die Krystalliten. p. 40. Tar. VI). The form assumed by these microlites also 

 suggests an analogy with the so called " hour-glass shaped" structure found in some crystals 

 of Augite. This structure is probably due to the unequal accretion of crystal-molecules in 

 different directions as indicated by these rudimentary or so to say retarded crystals, combined 

 with a certain chemical change during the growth of the crystal. 



