Vol. 5] 



Louderback. — . 



Benitoite. 



371 



the benitoite-bearing veins are exceptional in the lower tempera- 

 ture and perhaps more moderate pressure under which they were 

 formed. The crystallization of complex and highly acid titano- 

 silicates at the comparatively low temperature suitable for the 

 production of natrolite demand the presence and activity of 

 crystallizing agents (agents mineralisateurs) whose nature is not 

 indicated by an analysis of the vein materials. 



KECENT PAPEKS. 



A. P. Rogers: Note on the Crystal Form of Benitoite. Science n.s., 

 28 (1908), p. 616. He gets an average of 40° 10' for the pole angle of the 

 unit pyramid. He discusses the possible symmetry and inclines to the 

 ditrigonal bi-pyramidal class. 



W. E. Ford: Neptunite Crystals from San Benito County, California. 

 Am. Jour. Set. (1) 27 (1909), pp. 235-240, 8 figs. Describes the crystal- 

 lography and optical properties of neptunite. Finds optic axial plane in 

 plane of symmetry; c A c = 24°; b r= (j ; O yellow, b red, c red. /3=1.7; 

 2F = 48°40'; optically +. Dispersion of optic axes v > p. 



The common habit described by Ford is practically the same as that 

 described by the writer; the less usual types are somewhat different. He 

 notes the form g as new, and the drawings for this paper were changed to 

 adopt this symbol, r was not reported. 



Received May 19, 1.909. 



SUPPLEMENTARY NOTES. 



To the proof of the above paper the writer is permitted to add 

 the following notes on contributions to the study of benitoite and 

 neptunite during the past summer. 



The symmetry of benitoite. The writer notes that no evidence 

 has been published unfavorable to the view of the strictly 

 trigonal 10 symmetry of benitoite presented by the writer in the 



is Not rhombohedral, as the writer is reported to have said, in the 

 Referat of his paper, Zeit. filr Kryst. u. Min., 46 (1909) pp. 386-387. In 

 the original paper (loc. cit. p. 150) he says "It crystallizes in the hexa- 

 gonal system, trigonal division. The observed forms are the basal plane, 

 the plus and minus trigonal pyramid and the corresponding trigonal 

 prisms. . . . The development of faces at one end of the principal 

 axis always corresponds so well with those at the other, that it gives the 

 impression that the horizontal plane of symmetry is present. ' ' Trigonal 

 bipyramids and corresponding trigonal prisms can only occur in the ditrigonal 

 bipyramidal and in the trigonal bipyramidal symmetry classes in which the 

 rhombohedra are not possible, and the Referent therefore misrepresented the 

 writer's view of the symmetry relations when he wrote: 



p. 386 line 4 from bottom, rhomboedrisch for trigonal 



Rhomboeder for trigonale Pyramiden 

 line 3 " " Rhomboeder for trigonale Pyramide. 



