Hunt and Van Horn — Gerussite T%oins. 



47 



unusual growth of the brachyprisms r on either side of the 

 twinning plane. As all of the crystals showed the same gen- 

 eral type of distortion and distribution of forms, the accom- 

 panying drawing represents not the ideal form but the crystals 

 as they actually appeared looking into the re-entrant angle. 

 On five of the eight crystals the pyramids o and p and the 

 prism m were either wanting or scarcely perceptible, while the 

 brachydome zone was so striated and rounded that differentia- 

 tion of those forms would be highly speculative. However, 

 three crystals revealed without question all the forms repre- 

 sented in the drawing, as verified by the very close agreement 

 of the observed with the calculated angles in the following 

 table: 









Observed 



Calculated 



m 



: m = (110) 



: (110) = 



62° 



44' 





62° 



45' 50" 



r 



: r = (130) 



: (130) = 



57 



17 





57 



19 



r 



b = (130) 



(010) = 



28 



38 



30 



28 



39 30 



c 



: p — (001) 



(111) = 



54 



12 



30 



54 



14 



c 



: o — (001) 



(112) = 



34 



47 





34 



46 







: y = (001) 



(102) = 



30 



39 





30 



39 



c 



: x = (001) 



(012) = 



19 



51 





19 



52 30 



c 



: k = (001) . 



(011) = 



35 



53 





35 



52 



G 



: i = (001) ■ 



(021) = 



55 



20 





55 



20 



c 



: v = (001) 



(031) = 



65 



16 





65 



15 



r 



: r = (130) : 



(130) = 



65 



22 



30 



65 



20 



In the above computations the axial ratio as determined by 

 Kokscharow* was taken as a basis. 



In this brief note no attempt has been made to treat the 

 crystallography of cerussite exhaustively. References to some- 

 what similar and even more complicated twins have been 

 brought together by P. F. Hubrecht in his paper " Uber 

 Cerussitvierlinge von Sardinien," Zeitschr. f. Kryst., 1905, 

 xl, 147-188. 



Ann Arbor and Cleveland. 



* Loc. cit. — see also Dana, System of Min., 6th edition, p. 286. 



