294 C. ANDERSON. 



observed by him and by Hubrecht is shown by cerussite 

 from other localities. In my former paper (loc. cit., p. 409) 

 I gave the result of measurement made on two groups of 

 four crystals twinned in pairs on r. "Denoting the four 

 segments by I, II, III, IV, we have I and II, likewise III 

 and IV twinned on r, but although the orientation of III and 

 IV relative to I and II is nearly the same in the two groups, 

 I have not been able to prove it due to twinning on any 

 known face. Appended are the angles obtained between 

 the b pinacoids of the four segments. 



(1) b 2 a b 2 = 57° 13' (calculated for r-twin 57° 18°) 

 6i A b 3 = 61 26 (calculated for m-twin 62° 46') 

 bi a b 4 = 4 4 



(2) b 2 A b 2 = 57 18 

 bx A b 3 = 61 54 

 b Ab = 4 38" 



At the time this was written I had not seen the papers 

 by Goldschmidt and Hubrecht and was not aware that these 

 cry s tall ographers had observed divergences of the same 

 order, and it is one of the objects of this paper, now that 

 better material is available, to extend the investigation in 

 order to see if possible whether any general rule covering 

 these anomalies applies to the cerussite of Broken Hill and 

 Muldiva, where similar polyets are found. It may be 

 remarked that a departure from the exact angle demanded 

 by the twin law has been observed in other minerals than 

 cerussite; thus Des Cloizeaux 1 found that in albite twins 

 faces theoretically parallel may be inclined to one another 

 at an angle varying from 40' to 1° 40', and Miers 2 observed 

 a similar variation in twins of proustite and pyrargyrite. 

 Description of Groups. 



Group I. (Plate LIV, figs. 5, 6). — This specimen from 

 Block 14, is a triplet on r, II and III being twinned to I ; 



1 Des Cloizeaux, Man. de Mineralogie, i, p. 520. 

 2 Miers, Min. Mag., vni, 1888, pp. 74—76. 



