CERUSSITE CRYSTALS FROM BROKEN HILL AND MULDIVA. 307 



which shows that I is twinned to II and to III on m (calc. 

 angles 62° 46' and 117° 14). 



Group II (Plate LIII, fig. 2). Here the r law pre- 

 dominates, the group consisting of two arrow-head twins 

 united by their points ; measurement however reveals that 

 while I is twinned to II on r it is twinned to III on m, hence 

 also IV is twinned to II on m, so that here we have a fine 

 example of four individuals united by the two laws. The 

 faces in the prism zone are so strongly striated that their 

 readings barely suffice for identification, but the faces of p 

 and y give good readings, so that the orientation can be 

 relied upon to within a few minutes. The value of <£ for 

 I, II, III, IV respectively is 0° 0', 57° 9', 62° 30', 119° 42 

 and we have the following relations: — 



Segments. Meas. b a b 



III' IV 57 12 } Calc ' for r " twin 57 ° 18 



i' III 62 30 ) n , . , . AO o 4a , 



-q xy 62 qq J Calc. for m-twin 62 46 



This result does not throw much light on the general 

 question as the variations in the r-twins are away from 60° 

 and in the w-twins towards 60°; in any case the measure- 

 ments are too few. 



Group III (Plate LIII, fig. 3). — This group, which is 

 rather more complicated than the preceding two, has the 

 forms a (100), o (010), m (110), r (130), k (011), y (102), 

 p (111); the prism zone is striated vertically, b and r inter- 

 oscillating, while m is very narrow. 



Orientation. 



I II III IV V 



V o 102° 55' 160° 12' 165° 33' 228° 16' 225° 45' 



</> 57 17 62 38 125 21 122 50. 

 The twinning is therefore as follows: — 



