ox ANOßTHITE FEOil MIYAKEJIMA. oU 



In convergent polarised light, the cleavage piece parallel to P 

 shoAVs one of the axial rings at the margin of the field, while that 

 parallel to J/ shows another, but in the latter the centre of the rino- 

 is much nearer to the maro-in of the field. These observations ao-ree 

 completely with Max Schuster's* on the Anorthite of Vesuvius and 

 Pesmeda. The position of the positive Bisectrix Avould hence be 

 nearly perpendicular to the face e ('2P'oc). The section cut as nearly 

 as possible parallel to this face, shows in a convergent polarised ray, 

 a biaxial interference-figure of a wide angle. Further optical charac- 

 ters can not at present be siven. 



The porphyritic crystals which are fou.nd within the basaltic 

 lava, show somewhat ditferent habitus, as compared with the crystals 

 already described. The f jlloAving observations ma}^ serve to indicate 

 the characters of the porphyritic crystals as seen under microscopic 

 slides of the lava. 



The section cut nearly j-arallel to P (oP) presents an almost 

 rectangular outline formed by the traces of the faces y and M. Fig. 10 

 represents a typical form (jf the section ; sometimes the corners being 

 differently cut oif as in fig. 9. Generally a few lamella3 of the Albite 

 type are found parallel to the cleavage-trace of i)7, the direction of ex- 

 tinction making 40° on both sides of the boundary. A noteworthy 

 feature which is often observed in this section, is the presence of fine 

 diagonal fissures (fig. I), 10), intersecting at angles of nearly 85° and 

 95°, the acuter angle being nearly bisected by the cleavage-trace of i)/. 

 Such fissures are also to be seen on the cleavage-face of P of separate 

 crystals with the effect of making them Aery brittle, but the larger 

 porphyritic crystals imbedded Avithin the lava, exhibit the structure 

 much more conspicuously. Such crystals Avhen taken from the 

 matrix fall into fine splinters along the fissures. Usually one of 



* Die optische Orientirung der Plagioklase.-Tschermak's. Min. ii. Petro. Mittli. Bel. 3. 

 1881. p. 213. 



