J. E. Pogue — Cerussite Twin. 



91 



Forms. — The forms identified are listed in the following 

 table : 



Letter 



c 



b 



a 



??l 



W 



X 



2 



fc 



S 



Symbol Gdt. 

 Miller 





 001 



Ooo 

 010 



ocO 

 100 



00 



110 



oo3 

 130 



0* 



012 



of 



023 



01 

 Oil 



03 



032 



Letter 



: i 



V 



z 



n 



y 



7T 



P 







fU 



Symbol Gdt. 

 Miller 



: 02 

 : 021 



03 

 031 



04 

 041 



05 

 051 



102 



302 



1 

 111 



112 



21 

 211 



Fig. 2. 



Fig. 2. Gnomonic projection of upper portion of cerussite twin. 



Of these all are common, with the exception of q, S and tt, 

 which have been noted on less than 1 1/2 per cent of the sev- 

 eral hundred cerussite crystals thus far examined crystallo- 

 graphically. ir for the first time is found present on a twin after 

 the ?'-law. r appears only as the twinning plane uniting the 

 two individuals. The basal planes, as shown in fig. la, are 

 pitted by natural etching. The combination of forms is the 

 richest yet noted for a twin of this type. 



Twinning. — Cerussite exhibits two twinning laws : (1) the 

 m-law, in which the common prism m = qo J110J, or a plane at 



