300 RKCORDS OF THE AUSTRALIAN MUSRUM. 



fault-, or gliding-planes^ may have led to the distortion of the 

 morphological plane, which should be parallel to 010. The 

 parallelism of the optic axial plane, iu all the quadrants of the 

 crystal, proves that the crystal is optically continuous, and not 

 twinned. ' 



From the arrangement of the inclusions theBirabowrie mineral 

 may be described as a modified made pentarhombique. Instead 

 of having a solid dark rhomb in the centre, the core is usually of 

 light-coloured material, and is surrounded by four dark lines, 

 forming a rectangle or lozenge, according to the direction of cutting. 

 Moreover, the inclusions appear on the prism angles as roughly 

 triangular areas and not as rhombs. In some cases the central 

 rhomb is very small or absent, and, as a rule, the angular patches 

 of inclusions are well developed ; occasionally, as in Fig. 2, reaching 

 almost to the centre. 



A few typical specimens have been selected for particular 

 description ; some are the property of the Trustees, the others 

 belong to the Geological Survey collection. 



Fig. 1. — On the cross section this specimen measures 2|^ inches, 

 and is undoubtedly one of the finest chiastolites discovered. 

 On the polished surface the colour is brownish-red, which 

 unfortunately prevents the photograph from doing justice to the 

 mineral. The triangular areas are dark in colour, but the rest 

 of the inclusions are mostly reddish-brown, and are possibly 

 oxide of iron. The cleavages, crossing at approximately 90°, are 

 indicated by lines of inclusions, parallel to the prism faces. 



Fig. 2 is a photograph of one of the Geological Survey's specimens, 

 and represents a typical Bimbowriechiastolite. Its greatest breadth 

 is 1^ inches. The central rhomb and the dark patches are well 

 defined, the latter being differentiated into two areas, the inner 

 being darker in colour, while the outer is greyish. The latter 

 portion is very evidently clay-slate, and externally shews the 

 schistosity clearly. These included wedges of clay-slate can be 

 distinguished without reference to the cross section by the presence 

 of this schistosity, and by the absence of mica in the crust. 



Fig. 3 was originally five inches long, but was cut into seven 

 segments, in order to see how the inclusions varied in different 

 parts of the crystal. The result was not conclusive, but seemed 

 to shew that the central rhomb increased in size from the middle 

 towards either end. Before cutting, a depression was noticeable 

 running from end to end on either side. These depressions are 

 occupied by soft and friable clay-slate, and have evidently resulted 



I Rosenbush — Mior. Phys. of the Rock-making Minerals, trans. Iddings, 

 4th edit., 1900, p. 196, states that such gliding-planes exist in andalusite 

 parallel to 100, and give rise to mechanical deformation of the crystal. 

 Such gliding planes would account for the mechanical deformation of the 

 chiastolite crystals described in this paper. 



