12 University of California Publications in Geology. [Vol. 7 



that the alternations were not rigidly parallel in many cases, 

 and that even the interfacial angles were sometimes distorted. 

 One of the best crystals which gave sharp signals measured 

 (0001) : (2021) =58° 43' and (0001) : (2021) =64° 12', with 

 angles lying between these for the adjacent readings. Similar 

 variations were noticed on other crystals, and it seems probable 

 that the oscillations in the growth, combined with a slight sec- 

 tility of the mineral, have caused a deformation of the angles. 

 Some of the prisms have their negative ends terminated by good 

 faces of the steep pyramid and small base, but the lower end 

 is rather characteristically drawn out into a long, wobbly and 

 curved, tapering pyramid. Many of the crystals, especially the 

 larger ones, are mere shells or hollow prisms with irregular 

 cavities. 



The imperfect character of most of the crystals renders good 

 measurements impossible, and readings were obtained which 

 correspond to various new and improbable forms. These crystals 

 have been described by Kraus and Cook, 4 who give several new 

 forms. From the nature of the crystals these new forms would 

 need to be substantiated by further observations of them. As 

 shown above, considerable variation in the angle between the 

 base and pyramid may exist and their forms (7074), (7073), 

 and (15.0.15.8) seem to be striated gradations into, or imperfect 

 readings of, the form (2021). The angle (0001) : (2021) = 

 62° 10', and their angles for these forms show a variation of 

 — 3° 23' to -4- 3° 23', as seen from their measurements : 



(0001) : (7074) = 58°47' 

 (0001) : (7073) = 65 33 

 (00010) : (15.0.15.8) = 60 32 



"With good crystals there might be no question about the validity 

 of these new forms, but on the Tonopah crystals their existence 

 is very doubtful. Their new form (9092) may be the known 

 form (4041). The angle (0001) : (4041) is 75° 12' and the 

 writer obtained measurements varying from 74° 55' to 76° 40' 

 for a form which is probably (4041), although the latter angle 

 yields more closely the indices (9092). 



* Kraus and Cook, Iodyrite from Tonopah, Nevada, Amer. Journ. Sci., 

 vol. 27, p. 210, 1909. 



