348 



University of California Publications. 



[Geology 



Basal sections show a perfect uniaxial cross which gives a dis- 

 tinct positive reaction with the mica plate. 



Color, most commonly pale to deep blue, generally with a 

 slightly violet tint; transparent. Colorless crystals occur, but 

 are more rare. The variation in color frequently occurs on 

 the same crystal, and the writer has many in which part of the 

 crystal is blue and part colorless. The transition is sometimes 

 gradual and irregular; sometimes it is sharp, the zones being 

 separated by crystallographic planes. The writer has a slab 

 cut parallel to the vertical axis to exhibit the pleochroism in 

 which the line separating the blue from the colorless portion 

 is parallel to that crystallographic axis. On another crystal 

 a colorless layer 2 mm. thick lies at the top, and is separated 

 from the bulk of the crystal which is blue by a plane parallel 

 to its base. The physical properties of the colorless material 

 are, except for those dependent on color, the same as those of 

 the most highly colored ones. 



An effort has been made to determine the source of the blue 

 color of most of the material, but the results have so far been 

 negative. Some of the colorless material was carefully sepa- 

 rated and submitted to Professor Blasdale for chemical analysis, 

 but it shows but slight variation (if any) from that of the 

 blue, as may be seen by referring to the analyses given farther 

 on. He also made a careful qualitative examination of a two- 

 gram portion, but failed to detect any appreciable amounts of 

 any element that might be reasonably supposed to influence the 

 color of the minerals. That practically all of the titanium is 

 in the highest state of oxidation was also shown by dissolving 

 the material in hydrofluoric acid in an atmosphere of carbon 

 dioxide. A colorless solution was obtained which failed to re- 

 duce potassium permanganate. In the preliminary report the 

 writer suggested that the color might be due to a small amount 

 of titanium in the reduced condition in solid solution in the 

 benitoite molecule. A comparison of the analyses of colorless 

 and blue samples shows that the TiO, of the former is a half 

 per cent, or more less than in the latter, and if this can be 

 accepted as an essential difference it at least is in consonance 

 with that view. The violet-tinted blue of the extraordinary ray 



