374 



University of California Publications. [Geology 



The following consideration decided the writer in the choice 

 of the positive unit form and seems to him to indicate its more 

 fundamental character than the complementary pyramid of the 

 same parameters. In growth the positive pyramid produces more 

 perfect planes and more brilliant faces, the negative pyramid 

 showing most commonly uneven, curved, influenced and other- 

 wise less perfect forms even when it is areally about equal to the 

 positive form. In the attack of corrosive agents, the positive 

 pyramid is much more resistant than the negative. In concen- 

 trated hydrofluoric acid the negative faces immediately become 

 dull and are rapidly corroded, the positive planes remain bright 

 and show the production of small well-formed etch figures. The 

 positive faces must eventually be attacked over their whole sur- 

 face but this was not observed during the progress of the experi- 

 ment which lasted at least two hundred times and more as long 

 as it took to entirely destroy the original surface of the negative 

 pyramids. That this same relative resistance of the positive 

 planes exists under very varying conditions is shown by the facts 

 that it was observed (1) in the natural weathering process, (2) 

 in hydrofluoric acid, both hot and cold, concentrated and dilute, 

 and (3) in fused caustic potash. 



Groldschmidt and Wright 2 ' 1 in their work on that form-rich 

 mineral, calcite, found that the more fundamental planes give the 

 best etch figures, and suggest it as a possible general method for 

 their determination. In benitoite there is a very marked super- 

 iority in this respect of the form selected as positive by the writer 

 over the corresponding negative form. Altogether, then, the 

 different lines of evidence are consistent and definite and indicate 

 the writer's positive unit form as the more fundamental. 



Refractive index of benitoite. Hlawatsch has also determined 

 the refractive index of benitoite by the prism method (loc. cit., 

 p. 301) with results almost identical with the writer's. 



Hlawatsch. 



Louderbaek. 



1.756 

 1.802 



1.757 

 1.804 



Neues Jahrb. fur Mineral., etc. (1903), Beilage-Bancl 17, p. 365. 



