256 



BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



The first type which is well shown in the illustration, figure 

 63, is prismatic by elongation on the a axis, the usual elongation 

 for cerusite, and they are attached by one end of the elongation. 

 The crystals are minute and line cavities in massive impure cerusite 

 which contains some admixed iron oxides, and malachite. The crys- 

 tals are not twinned and they do not look like cerusite. In order to 

 give a clearer idea of the habit and appearance of the crystals they 

 are shown in the drawing, as they were measured, with the elongation 

 vertical. The angles measured are stated for this orientation in the 

 following table. 



Measurements of cerusite, Lookout mine, Figure 63, a axis vertical 



The second type of cerusite, illustrated in Figure 64, is relatively 

 simple in development and occurs as small smoky gray crystals 

 resting on leached quartz. These are in part simple and in part 

 rayed twins and attached to them in parallel position are later 

 brilliant colorless and more highly modified crystals. The simple 

 crystals all have the habit shown in Figure 64, this crystal, like the 

 last, being measured and drawn with the a axis vertical in order to 

 better show its development. The angles measured, stated in the 

 abnormal position as drawn, are given in the next table. 



Measurements of cerusite, Lookout mine, Figure 64, a axis vertical 



as 



The more highly modified colorless prismatic cr} 7 stals which appear 

 a later growth attached in parallel position to the preceding are 



