258 



BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



Other specimens show simple, and more frequently, twinned color- 

 less cerusite crystals up to 1 cm. in length, forming crusts on bleached 

 quartzite. These are moderately elongated on the a axis. The 

 simple crystals have the same habits as those making up the pene- 

 tration twins. One of the simplest of these is shown in the drawing, 

 Figure 66, and the angles measured on it are given, for normal posi- 

 tion, in the following table: 



Measurements of cerusite, Lookout mine, Figure 66, normal position 



A crystal from this lot of specimens similar to the last, but some- 

 what more modified, with the additional forms o(112), x(012), i(021), 

 2(041), andr(130), is shown in the next drawing, Figure 67. The 

 angles for this crystal are as given in the following table, the elonga- 

 tion (a axis) again being made vertical: 



A third crystal of this series similar in most respects to the last has 

 the additional forms iy(211) and s(121), as shown in the drawing, 

 Figure 68. This crystal contains the largest number of forms found 

 on any cerusite crystal measured. The habit of the crystals is thus 

 not marked by any great complexity and no unusual forms are 

 present. The agreement between the measured and calculated 

 angles is so close that there can be no reasonable question as to the 



