General Discussion

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Vol. 5]	Louderback.—Benitoite.	371

the benitoite-bearing veins are exceptional in the lower temperature 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 miner alisateurs) whose nature is not indicated by an analysis of the vein materials.

RECENT PAPERS. 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. Pord: Neptunite Crystals from San Benito County, California. Am. Jour. Sei. (4) 27 (1909), pp. 235-240, 8 ftgs. Describes the crystallography and optical properties of neptunite. Finds optic axial plane in plane of symmetry; c Λ c = 24°; b = t; a yellow, b red, c red. β = 1.7; 2V = 48°40'; optically +. Dispersion of optic axes υ > 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, 1909.

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 trigonal¹⁹ 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, für Kryst. u. Min., 46 (1909) pp. 386-387. In the original paper (loc. cit. p. 150) he says "It crystallizes in the hexagonal 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.

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