320 Y. KIKUCHI. 



which* exhibit some difference in structure; and, secondly, the inclusion 

 of foreign matter in the boundary between the outer and the inner 

 portions, and between the twinning sutures in the outer por- 

 tion. 



The inner portion or (lie core is different in character from the 

 outer, as mav be easily revealed under the microscope by the use of 

 oolarized li^ht. The twin-structure is soon evident under crossed 

 Niçois, but the three systems of the twinned individuals often inter- 

 penetrate in a very complex and irregular manner. Figure 3. is a mag- 

 nified representation of the structure of the inner portion, in which 

 the different patches having an analogous optical orientation are re- 

 presented by the same shading. Tt will he observed that these differ- 

 ent patches are reducible to three systems which are continuous 

 with those of the outer portion. The directions of extinction, 

 therefore, in the outer and the inner portions, are always parallel and 

 at r'12'ht angles to the outer sides, and differ by nearly GO from each 

 other. 



tinder a convergent polarized ray the different sectors of the more 

 uniformly built outer port ion show the Axial-image (Interference-figure), 

 the position of which in each sector is represented in Figure 1. The 

 Optical-plane is situated at right angles to the outer side of the crystal, 

 and therefore it is in the Macropinacoidal face (100). as in this twin 

 the outer side corresponds to the Brachypinacoid (010). The Axial 

 image examined with mica plate reacts negative, with a weak disper- 

 sion, p-<v. The distance between the two rings of the image is not 

 very wide. 



Traces of cleavage-plane are often distinctly seen in the trans- 

 verse section, as line fissures always running parallel to the outer 

 sides, i.e.. parallel to the Brachypinacoid (010). In Figure 1, the fine 

 lines parallel to the sides are the traces of cleavage, while the thicker 



