THE MINERALS OF IDAHO 



355 



one locality being given as in fractures in shale in the footwall of the 

 vein in the east crosscut on the 1,000-foot level. Several specimens 

 of the mineral from this locality were examined, two of them from 

 the Brush collection of Yale University, which 

 were kindly loaned by Prof. William E. Ford. 



The crystals are nearly colorless and trans- 

 parent. They occur singly and in groups and 

 reach a length of 3 or 4 mm. The smaller ones 

 are the more perfect, the larger, as usual with 

 this mineral, being made up of numerous smaller 

 units which diverge slightly. The broad face 

 has a pearly luster and many of them are doubly 

 terminated. The illustrations, Figures 115 and 

 117, show two crystals drawn with the elongation 

 upright, in orthorhombic position, the pearly 

 lustered face, parallel to which the best cleavage 

 is developed, being made the 6(010) pinacoid. 

 All of the crystals show the pyramidal faces con- 

 spicuously developed. The measurements ob- 

 tained from the faces are not precise as they 

 yield poor signals on the goniometer. The 

 average of the angles measured on three crystals 

 are given below. 



Fig. 117.— Stilbite. Min- 

 nie Moore mine. Crys- 

 tal SIMULATING TETRAG- 

 ONAL SYMMETRY. Orient- 

 ed ACCORDING TO GOLD- 

 SCHMIDT 



Measurements of stilbite, Minnie Moore mine 



In Figure 116 one of the crystals is drawn, as measured, in the 

 position adopted for stilbite by Dana. This habit Dana considers 

 as pseudo-orthorhoinbic by twinning as shown by the optical struc- 

 ture. The measured angles on one crystal are compared with the 

 angles of Dana as follows: 



Measured Dana 



/H(110)Am" ' (HO) 63° 19' Gl° 10' 



C (001)Aa (100) 51° 10' 50° 50' 



This agreement, the quality of the compound faces considered, is 

 as close as can be expected. 



