422 General Notes. [May, 



But the arrangement of twins around a point has not been 

 known until recently. A simple example of this new kind of 

 twinning and of the method of detecting it may be given in the 

 case of Romeite. This mineral crystallizes in simple octahedrons, 

 and had therefore been supposed to be isometric. Bertrand has 

 endeavored to show that the octahedron of Romeite is in reality a 

 twinned arrangement of eight rhombic crystals grouped symmetri- 

 cally around a point. He found that if a section be cut between 

 the summit of the octahedron and the center of the crystal, parallel 

 to the cubic face, and this be examined in polarized light, it will 

 show, if parallel rays be used, four similar right-angled triangles 

 each having its hypothenuse either parallel or perpendicular 

 to the plane of polarization ; if, however, converging rays be used, 

 each triangle will show a cross and a series of rings, indicating 

 an optic axis for each, which is oblique to the section, but which 

 bisects the right-angle of each triangle. If now a section is cut 

 parallel to any octahedral face, and is examined in converging 

 rays, a central cross and series of rings appear, just as in a uni- 

 axial crystal, and indicate an optic axis normal to the octahedral 



imposed of e 



The investigations of Descloiseaux, Vom Rath, Bertrand, and 

 more especially Mallard, tend to the belief that quite a number of 

 apparently simple crystals of one system are in reality groups of 

 crystals of a higher system. 



Among the psaido-isometric crystals, are, as Mallard has shown, 

 topazolite, formed of 48 triclinic crystals, having for their bases 

 the faces of a hexoctahedron, and arranged in 12 different posi- 

 tions ; ouvarovite, formed of 12 orthorhombic crystals, corres- 

 ponding to each face of the dodecahedron and arranged in 6 dif- 

 ferent positions; boracite, formed of 12 orthorhombic crystals 

 whose summits are at the center of the crystal, as in ouvarovite ; 

 leucite, an assemblage of monoclinic crystals ; senarmontite, 

 whose octahedrons are composed of 48 orthorhombic crystals as 

 in topazolite (this being an interesting result when taken in con- 

 nection with the orthorhombic form of valentinite, hitherto sup- 

 posed to be a dimorphic form of oxide of antimony) ; analcite, 

 whose anomalous optical characters have long been known, now 

 shown to be formed of 24 orthorhombic crystals, corresponding to 

 the faces of a tetrahexahedron ; fluorite, probably composed of 

 interlacing rhombic crystals. To these, Bertrand has added 

 Ralstonite, and, very recently, Rhodizite, both of which are com- 

 posed of twinned biaxial crystals. Among pseudo-tetragonal crys- 

 tals may be mentioned apophyllite, idocrase and zircon, now 

 shown to be assemblages of monoclinic crystals; while as 

 regards rutile, octahedrite and brookite, generally supposed to 

 prove the trimorphic character of titanic acid, the interesting 



