ON ANORTHITE FKOM MIYAKEJIMA. 41 



lialf, after the lattei* had experienced a sudden contraction, the magma 

 still retaining a certain degree of fluidity. 



The section parallel to Jl/(ooPco), is generally a parallelo- 

 g-ram with anHes of 98° and 82°, but sometimes becomes rhombic 

 in outline by the development of the faces o, p, combined with P, the 

 angles of the rhomb measuring 52° and 128° (compare fig. 7). Peric- 

 line-lamellas making an angle approaching to -16 with the trace of 

 I\ are often found. 



The section perpendicular to the zone of P : BI, presents a rec- 

 tangular form formed by the traces of P and M, the corners being 

 usually cut off by the faces e, u. Fig. 11. 12, show the usual forms 

 of this section ; fig. 11 representing the first type, fig. 12 the second 

 type, according to the classification of separate crystals already given. 

 In this section, broader l^erichne-lamellaî are inserted, which would 

 be parallel to the trace of P, if the section falls just parallel to the 

 jMacroaxis. Some of these lamellae thin out at the extremities. The 

 broad band arranged parallel to M is probably twinned according to 

 the Albite type (fig. 12). This band itself is divided into two halves 

 a, h in the manner of a penetrating twin, with different optical ori- 

 entation, (a/l) = 40°/25°), so that these halves may be twinned 

 according to the Carlsbad law. 



The sections parallel to M and perpendicular to the Brachyaxis are 

 traversed by irregular fissures, some of whicli riui nearly in the direction 

 of the Vertical axis. The substance of these felspars is always fresh, 

 and nearly homogeneous; some variation in chemical composition being 

 sometimes indicated by zonal structures as in fig. 12, wdien examined 

 under polarised light. The enclosures are Olivine-grains, glass, and 

 gas pores which are sometimes arranged parallel to a crystallographic 

 direction. Olivine-grains are generally found as enclosures within the 

 Anorthite, but occasionally we observe the inclusion of Anorthite 

 within the Olivine. 



