DISCUSSION OP THE PRISM ZONE. 55 



On all three of these crystals the form occurred in the zone c(001) : 

 1(2 10), and on crystal 19 it was also determined to lie in the zone of 

 and between 2(102) and J(112), so that its determination as (214) is 

 verified by its zonal relations. 



DISCUSSION OF THE PRISM ZONE. 



The prism zone contains numerous forms of somewhat complex 

 symbols and a discussion * of these forms is here given in order to 

 show how closely the forms approach the normal series. 



The entire prism zone from &(010) to a (100) shows, when plotted 

 in gnomonic projection (see fig. 35, p. 70), a partition into three parts, 

 the points of division being m(110) and d(310). The three zone 

 segments are then: 



(1) 6(010), r(120), 5(7.11.0), m(110). 



(2) w(110), Z(210), 72(15.7.0), C(940), .F(520), Q(830), d(310). 



(3) d(310), G(720), 3f(510), #(11.2.0), ;(610), (710), n(810), a(.100). 



In the following discussion, these three segments are taken up in 



turn : 



(1) Zone segment b ...... m. 



r S m 



'lOlO 120 7.11.0 110 

 Symbol= .......................................... \ ^ 1 



# 2 .................................................. \ 1 2 oo 



111 place of r or (7.11.0), the simple form 2 or (230) should occur. 



The form $(7.11.0) was not observed by me but was described by 

 Warren. The measured angle (7.11.0) : (7.11.0) is given as 74 51', 

 whereas the calculated angle (230) : (230) is 77 50', a difference too 

 great to allow of the form being referred to the symbol (230). Com- 

 pare also the remarks on page 69 on the indices of this form. 



(2) Zone segment m ...... d. 



1 For a description of the methods of the discussion, see Hillebrand, W. F., and Schaller, W. T., The 

 mercury minerals from Terlingua, Tex.: U. S. Geol. Survey Bull. 405, p. 13, 1909. 



