Ford — Neptunite Crystals from San Benito Co., Cat. 239 



plane of the crystal. As the angle between them was small 

 it follows that the direction c, the normal to the section, is the 

 acute bisectrix and the mineral is positive. Rays vibrating 

 parallel to the o crystallographic axis were colored red, while 

 those vibrating in the symmetry plane were yellow. These 

 relations are shown in figure 8. 



The above results differ from those given in the original 

 description in the position assigned to the axial plane. Flink 

 describes the optical orientation of the Greenland neptunite as 

 follows :* " The plane of the optical axes is perpendicular to 

 the symmetry plane and the acute bisectrix forms an angle of 

 18° with the vertical axis in the obtuse angle /3." A critical 

 reading of his description would suggest that the discrepancy 

 noted above was probably brought about by an accidental turn- 

 ing of one of his sections so that the true orientation was 

 reversed. To translate further from his description : " In the 

 first section orientated parallel to the symmetry plane one extinc- 

 tion direction forms with the vertical axis an angle of 18° in 

 the obtuse angle (3. The rays vibrating in this direction show 

 the greatest absorption and the color is deep red brown. 

 The rays vibrating perpendicularly to this direction are less 

 absorbed and the color is yellow-red. The same section shows 

 also in convergent polarized light an axial figure with a large 

 angle between the optical axes." With the exception of the 

 last statement the description agrees with that given above for 

 the California mineral. Flink describes the second section 

 which he cut as follows : u In the second section, orientated 

 perpendicular to the symmetry plane, and making an angle of 

 72° with the orthopinacoid and 7J° with the base, the rays 

 vibrating parallel to the symmetry axis are the least absorbed 

 and the color is yellow-red. The rays vibrating perpendicu- 

 larly to the symmetry axis and in the symmetry plane are more 

 strongly absorbed and the color is dark red. This section 

 shows in convergent polarized light an axial figure with a 

 small angle between the optical axes." It will be noted that 

 the elasticity direction which is common to the two sections, 

 namely the one lying in the symmetry plane and making a 

 small angle with the a-axis, is said in the first case to show the 

 least absorption and to be colored yellow-red ; and in the 

 second case the greatest absorption with a dark red color. 

 This discrepancy could be explained and the optical orienta- 

 tion of the Greenland mineral be brought into conformity with 

 that from California, if Flink' s second section was considered 

 to have been accidentally turned 90° from its true orientation. 

 The statement that an axial figure with large optical angle was 

 *Zeitschr. Ki\, xxiii, 350, 1894. 



