THE MINERALS OF IDAHO 



259 



correctness of the orientation and the identification of the forms. 

 Many of the crystals are not simple individuals, but are made up of 

 a large number of units in parallel position, but these are so exactly 



Figs. 67-68.— Cerusite. Untwinned colorless crystals. Lookout Mountain min 



parallel that all of the faces, reflecting simultaneously, give a single 

 perfectly sharp signal, permitting very exact measurements. The 

 angles of the crystal of Figure 68, in normal position, are as follows : 



Measurements of cerusite, Lookout mine, Figure 68, normal -position 



