218 Kraus and Cook — Tody rite from Tonopah. 



In order to avoid all confusion we have divided the basal 

 pinacoid and the various bipyramids into upper and lower 

 forms, following the practice of the more recent texts on crystal- 

 lography. Certain letters are then assigned to the various 

 bipyramids and the basal pinacoid, which, when unprimed, 

 indicate upper forms, when primed, lower forms; thus, o J1011 J, 

 upper; VjlOHJ, lower. We believe that this system of 

 nomenclature offers decided advantages over what has been 

 heretofore in use for hemimorphic crystals, in that it does away 

 with all confusion. 



In accordance with the plan outlined in the preceding para- 

 graph we have prepared a general summary of all forms thus 

 far reported as occurring on either natural or artificial silver 

 iodide. 



As previously stated, p. 211, the axial ratio generally adopted 

 for iodyrite is the one which was established by Zepharovich* 

 for artificial crystals. Inasmuch as it is always highly desirable 

 to have ratios based upon reliable observations on the natural 

 compound, we have taken a very large number of readings 

 with the view of establishing a new axial ratio. The values 

 thus obtained differ but slightly from those given by Zepharo- 

 vich. In order to show the close agreement of the two deter- 

 minations we place them below and at the same time also add 

 the values obtained by Des Cloizeauxf on natural crystals in 

 1854. 



a : c 



Des Cloizeaux 1 : 0-81438 



Zepharovich _ 1 : 0-81960 



Kraus and Cook. _. ___ 1 : 0-82040. 



In all fifteen crystals were measured. The observed and 

 calculated angles based upon the axial ratio, given by us, show 

 in nearly all cases very close agreement. 



Observed Calculated 



c : "i — (0001) : (2021) 62° 10' 30"* 



m : m = (1010) : (0110) 59° 59' 60° 00' 



c : r = (0001) : (7074) 58° 47' 58° 50' 



c' : s' = (0001) : (15.0.15.8) 60° 32' 60° 47' 



c : t = (0001) : (7073) ' 65° 33' 65° 39' 



e : w = (0001) : (9092) 76° 46' 76° 48' 



c : x = (0001) : (7071) 81° 22' 81° 25' 



c : y = (0001) : (9091) 83° 13' 83° 19' 



c : z = (0001) : (33._0.33. 2) 86° 20' 86° 21' 



i : i = (2021) : (0221) 52° 30' 52° 29' 



m : i = (1010) : (0221) 63° 43' 63° 45' 



* Loc. cit. f Loc. cit. 



