THE MINERALS OF IDAHO 



251 



set stope" were found butterfly contact twins of colorless trans- 

 parent cerusite up to 3 inches in greatest diameter. The illustration, 

 Figure 58, is based on measurements made on one of the smaller 

 crystals of this lot. Brilliant crusts of small twinned crystals rang- 

 ing from colorless to gray and black or amber were common. 



The Mammoth mine at Mace was reopened by a lease through the 

 No. 2 Mammoth tunnel in 1913 and some cerusite ore obtained 

 which contained some cerusite crystals as well as line specimens of 

 plattnerite and pyromorphite. These were aggregates of tabular 

 crystals twinned on m(110). One such group, which is typical of 

 much of the twinned cerusite of this and other mines, is shown in 

 Figure 62. The main crystal of this group gave the following angles: 



Measurements of cerusite from Mammoth mine 



No. Letter 



Symbol 



Gdt. Miller 



9 p. 







Oco 

 ooO 

 oo 

 003 



oy 2 



01 

 02 



1 



001 

 010 

 100 

 110 

 130 

 012 

 011 

 021 

 111 



Quality, description 



Very good 



do 



do _ 



Poor-narrow. 



Very good 



do 



Very poor 



Medium 



Very good 



Measured 



00 



90 03 



58 39 



28 30 



03 



03 



03 



58 37 



00 



90 00 



90 00 



90 00 



90 00 



19 43 



35 55 



55 30 



54 12 



Calculated 



00 



SO 00 



53 37 



28 39 



00 



00 



00 



58 37 



00 



90 00 



90 00 



90 00 



90 00 



19 52 



35 52 



55 20 



54 14 



This group illustrates the twinning which produces the large masses 

 of reticulated plates. Much of the galena ore at the base of the oxi- 

 dized zone contains cavities, from the removal of masses of siderite 

 gangue which have cerusite crystals, usually water clear and colorless, 

 implanted upon the unaltered sulphide. Some of these are simple 

 and are difficult to distinguish from anglesite crystals which occur in 

 the same manner. A relatively simple and untwinned crystal from 

 a specimen of galena from the Last Chance mine, which is illustrated 

 in the drawing, Figure GO, gave the following measurements: 



