SPECIMENS OF CHIASTOLITE FROM BIMBOWRIE — ANDERSON. 299 



weathered out. Mr. Howden states that embedded crystals occur 

 with the long axis parallel to the foliation. The crystals are 

 columnar in habit, elongated in the direction of the vertical axis. 

 Some specimens reach a length of five to six inches, and one, 

 (Pi. xlvii., fig. 1), has a diameter of about two inches. None shew 

 well defined crystal outlines, the edges being more or less rounded. 

 When complete the crystals taper to either end, the relation of 

 length to breadth being about 4:1. They have a brownish crust, 

 and the surface is spangled with flakes of mica. The two cleavages 

 parallel to the prism faces are usually followed by lines of inclusions. 

 The specimens are altered in a greater or less degree, hence 

 the physical characters vary considerably. The hardness is about 

 3 to 4, and the specific gravity, determined on the freshest material 

 obtainable, selected as free as possible from inclusions, was found 

 to be 2 '99. The fracture is uneven. The colour of a cut crystal 

 varies, grey predominating, but some are reddish brown, and some 

 have a pinkish tinge. Slightly translucent. Difficultly fusible on 

 the edges before the blowpipe. 



Professor T. W. E. David, of Sydney University, and Mr. H. 

 Stanley Jevons, the University Lecturer in Mineralogy, have made 

 a detailed examination of sections under the microscope, and have 

 obtained some interesting results, which Professor David has 

 kindly communicated to me. " With a view to ascertain whether 

 these chiastolites are normal as regards optical properties, and 

 whether they are really single crystals and not twinned forms, a 

 section was cut at an angle of 48°, measured from the vertical 

 axis, and in a direction parallel to the macro-diagonal axis, so 

 that the plane in which the section was cut made an angle of 42° 

 with the basal plane. It was found that this section cut one of 

 the optic axes at right angles, which is evidence in favour of the 

 acute bisectrix being parallel to the vertical axis of the crystal. 

 This fact having been established, a section of the crystal cut at 

 right angles to the vertical axis was examined, and the optic sign 

 was found to be negative. The optic axial plane was found to 

 lie in a plane about normal to that of the macropinacoid. The 

 optic axial plane, however, does not lie parallel to the dark plane, 

 crossing the crystal diagonally and shewing the morphological 

 direction for the plane of symmetry parallel to the brachypinacoid, 

 but makes an angle of about 12° with it. From this it is obvious 

 that this morphological plane, indicated by the black lamina, is 

 not normal to the macropinacoid, but, in the crystal examined, 

 makes angles of about 102° and 78° with it. This distortion of 

 the morphological plane, which should be the plane of symmetry 

 parallel to the brachypinacoid, is, however, not accompanied by a 

 similar distortion of the direction of the planes of prismatic 

 cleavage. The optic axial plane, being normal to the macropina- 

 coid, bisects the obtuse angles of the prismatic cleavage. Mr. 

 Jevons suggests that perhaps very minute, numerous, parallel, 



