486 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



vol. 59. 



given below is 811 for the Dana position and 927 for the Goldschmidt 

 position the difference in each instance favoring the former orien- 

 tation. In the majority of occurrences of crystallized datolite the 

 crystals are so developed that the zone adopted as prismatic by 

 Goldschmidt is much more prominent and recognizable than the 

 zone taken as prismatic in the Dana orientation, this constituting 

 the most practical argument in support of the Goldschmidt orien- 

 tation, as it is consequently much easier to adjust crystals in polar 

 position for measurement on the 2-circle goniometer in this orien- 

 tation than in that of Dana. In the present work the Dana position 

 is used, however, because it is more familiar to American mineralo- 

 gists and because of priority of its use for datolite, homolite, and 



Figs. 10-12.— 10, portion of crystal b7. clinographic projection showing new forms j (263), ^3 (192), 

 q (1.10.2), and t (1.20.15). 11, crystal bs. clinographic projection showing development of 

 the crystal and the new form ? (205). 12, crystal b8. clinographic projection of detail show- 

 ING Mi (231), 93 (3S2), <$ (192), O (1.10.2), g (380). 



gadolinite. The indices in general are not simpler in the Gold- 

 schmidt orientation, and the Dana orientation shows the crystallo- 

 graphic and optical relationships between the several members of 

 the group equally well. Perhaps the most convincing support for 

 this orientation is the symmetry relations existing between the 

 common forms These relations are brought out by the stereo- 

 graphic projections, figures 1 and 2, winch show the zonal relations 

 existing between the commonly occurring forms for datolite in the 

 two orientations. 



The use of two orientations for datolite has been a source of con- 

 siderable confusion since new forms have been described in either 



