410 RECORDS OF T]IK AUSTRALIAN MUSEUM. 



form so common with Broken Hill cerussite is due to repeated 

 twinning' on r or twinning on r combined with twinning on tn or 

 on some other law. Suitable specimens for this purpose are not 

 easy to get, but, from a group consisting of part of the plate 

 forming one side of the rhomb-shaped net and two small attached 

 crystals with elongation apparent!}' parallel to the two remaining 

 directions, the following measurements were obtained, all the 

 reliable data being utilised in order to get results as accur- 

 ate as possible : 



&i A h., 57° r (Calculated for r twin 57° 18). 



bi A h , 58 35 



From these figures it is appai-ent that I and II are twinned on 

 r while III is independent, or exemplifies a third twinning law. 

 Miigge, who was the first to describe the cerussite of Broken Hill 

 sayb^: — " Neben Zwillingen kommen audi Drillinge vor, ind^ssen 

 wurden polysynthetic Bildungen nach (130) audi ia Diinn- 

 schliffen nicht beobachtet, wohl aber Verbindungen von Zwillingen 

 nach (130) mit gitterformigen Drillingen nach (110), welche 

 letztere auch durch tafeligen Habitus nach (010) sich von 

 Zwillingen nach (130) unterscheiden." If Miigge means by this 

 that the mesh-like form is the result of twinning on (110) com- 

 bined with twinning on (130) I can only say that so far as my 

 observations go I am not able to substantiate his conclusions. 

 Unfortunately he does not give the measurements on which his 

 inferences are based, and it would be absurd for me to question 

 their correctness, but a tabular extension on b is not a criterion 

 of distinction between twinning on (130) and twinning on (110) 

 as the habit is a common one with cerussite. 



Zeehan, Tasmania. 

 (Plate Ixxvii., fig. 4). 



One specimen in the Museum collection shows several small 

 but well developed crystals, simple and twinned, on a matrix 

 of galena with patches of friable limonite. A doublet on m 

 was measured and yielded the forms c (001), b (010), vi (110), 

 r (130), X (012;, k (Oil), ? (021), v (031), ,:; (041), 2) (HI). The 

 faces in the zone [010, 001] are striated and slightly inter- 

 oscillating. A group (PI. Ixxvii., fig. 4), is made up of four indi- 

 viduals of which I and II, also III and IV are twinned to each 

 -other on m, while I is twinned to III and II to IV on a possible 

 face (760) for which the calculated value of <^ is 62*^ 24'. This 

 form has not been recoi'ded for cerussite, and it is just possible 

 that we have here merely a case of accidental grouping, but the 



^ Miigge — Loc. cit., p. 79. 



