No. 2385. CRYSTALLOGRAPBIC STUDY OF DATOLITE— SHANNON. 485 



The Dana orientation has naturally been preferred by most Amer- 

 ican authors and also by Ungemach and other European authors 

 who still use the 1-circle goniometer. The majority of German 

 mineralogists and most others who use the 2-circle goniometer and 

 Goldschmidt methods have used the Goldschmidt orientation. 

 Various advocates of each of these two methods have endeavored to 

 justify their preference by showing either that the orientation which 

 they favored gave simpler indices or that it better illustrated the 

 relationships existing between datolite and the other members of 

 the group which includes homilite, gadolinite, and euclase. 



Figs. 8-9.-8, orthographic and clinographic projections of crystal 28. An unusual habit ■with 

 the steep pyramidal form of type 2 but ■with an orthodome series including 2 (205) as in crys- 

 tals of type 4. 9, orthographic and clinographic projections of crystal b9 showing unusual 



PYRAMIDAL FORM WITH THE FORMS t) (768), £ (454), f h (361), AND Wlj, (12.25.1). 



The difference in relative simplicity of indices is not greatly in 

 favor of either orientation when all the known forms are considered, 

 although it is possible to select groups of forms or zones which yield 

 comparisons favoring either position. This is well illustrated by the 

 method of adding the indices of the same forms for the two orien- 

 tations. Thus, when the sums of the indices of the forms given in 

 Table 1 are obtained the indices for the Dana orientation total 1,101, 

 while the sum of the indices for the same forms in the Goldschmidt 

 orientation is 1,113, the difference being too small to be of any 

 importance. Similarly, the sum of the indices of the new forms 



