292 



C. ANDERSON. 



the same direction and that in this case they endeavour 

 to take a middle position." Goldschmidt then, and follow- 

 ing him Hubrecht consider that twinning is to be explained 

 by the tendency of lines and planes of force in the two 

 individuals to place themselves parallel. In Text Fig. 1, 



8557 90° 



iswn 



f/9 





z&zg 



58W' *v: 



m i nv z 



Q0° 

 Fig. 1. 



we have a stereographic projection of a cerussite crystal 

 I in the conventional position, showing the poles of the pina- 

 coids and the m and r prisms, the corresponding normals 

 being drawn in full lines, with, on the right, the same poles 

 and normals (long-dashed lines) of a crystal II twinned to I 

 on m (110), and, on the left, the poles and normals (short- 

 dashed lines) of a crystal III twinned to I on r (130). The 



