No. 2385. CRYSTALLOGRAPHIC STUDY OF DATOLITE— SHANNON. 531 



ZONES. 



The several prominent zones present on datolite have been dis- 

 cussed by Gorgey and Goldschmidt, using the Goldschmidt orien- 

 tation. It is of interest here to compare some of the more promi- 

 nent zones in the Dana orientation with the normal series according 

 to the law of complication of Goldschmidt with the especial purpose 

 of criticising the new forms here described for this mineral. The 

 method is useful indeed in checking up the indices assigned to new 

 forms. This is illustrated by the fact that, although preceding por- 

 tions of this paper had been completed previous to the analysis of 

 the zones, the indices of at least two forms listed as new were shown 

 by this method to be incorrect. These two have been corrected. 

 The zones discussed are those shown on the gnomonic projection, 

 plate 1 of this paper. The excellent work of Schaller 5 has been a 

 most useful guide in this interpretation. 



Zone No. 1. — Prism zone. 



Symbol jr' 



Form... 



Symbol 

 N 4 



100 



a Forms bracketed are known for the species but were not observed during the present study and are not 

 shown on the gnomonic projection. 



This zone is normal with one form extra and four missing. The 

 extra form is the one here listed as new, 380. In place of this the 

 normal series requires the form 2/5, or 250. The form 380 is con- 

 sidered to be well founded, however, by its clean-cut face and sharp 

 signal. That this is not 250 is shown by the following comparison 

 of angles: 



250^ = 32° 13' 1,, , 



380 v> = 30° 35' | Measured <p = 30° 05'. 



Zone No. 2. — Clinodome zone. 



Symbol 



Form !{ooi 



Symbol 



N4 



[0. 1. 10] 

 1/10 



V. 

 [01 »1 

 1/8 



014 



1/4 

 1/4 



t \g I A 



013 012 | 023 



1/3 i 1/2 2/3 



1/3 s/s l/2 3 /5 2/3 



m 



067 011 

 6/7 1 



3/4 1 



9 



043 

 4/3 



S 

 [021] 

 2 



2 0/3 3 4 



b 

 010 



This zone is normal, with eight forms missing and three extra. 

 Of the extra forms (1/10) and (1/8) were not observed by the writer, 

 although 018 is reported from this locality by Whitlock. (067) 



6 Schaller, W. T., Mercury Minerals of Terlingua, Texas. U. S. Geol. Survey Bull. 405, 



