Wright — Three Contact Minerals. 549 



plane of the optic axes and the fine twinning lamellae was 

 measured at 57° '5 in sodium light. On this thick plate crossed 

 dispersion was unusually clearly marked, the angle between the 

 plane of the optic axes for red lithium light being about 57°*6, 

 and for green thalium light 57°*1. These measurements indi- 

 cate a dispersion of the bisectrices tp : c gr in the plane of sym- 

 metry of about 0'15°. At the same time a slight dispersion of 

 the optic axes was noticeable with 2Ep > 2E W . 



The optic axial angle was measured on a number of different 

 sections by the use of the double screw micrometer ocular and 

 also of the universal stage, the average being 2V ~ 39 0, 5, ± 1°; 

 w r hence 2E = 70°. Owing to the strong birefringence the 

 interference figure is unusually well marked even in normal 

 thin sections. The refractive indices were measured in sodium 

 light on several different plates on an Abbe-Pulfrich total 

 refractometer with reducing attachment, the different refractive 

 index lines from the polished plates being clearly marked and 

 easy to follow : 



y Na = 1-679 ± 



•002 



y — a = -039 



P Na = 1-674 ± 



•002 



y — ft — '005 



a Na = 1-640 ± 



•002 



P — a = '034 



From these values the calculated optical axial angle is 

 2 V = 41° 12', which agrees fairly well with the measured value. 

 Optical character negative. 



The birefringence values were checked by direct measure- 

 ments on plates in the thin section and closely accordant results 

 obtained.* 



y — a = -040 

 P — a = -036 



In the thin section spurrite is recognized by its high bire- 

 fringence, imperfect cleavage and small optic axial angle with 

 negative optical character and in thick sections noticeable 

 crossed dispersion. 



Still further evidence on the crystal system of spurrite was 

 gathered from etch figures on the good cleavage face. 

 Cleavage flakes were immersed for 10 seconds in cold 5 per 

 cent hydrochloric acid and the etch figures of fig. 2 obtained. 



Many of these figures appear asymmetrical but the upper 

 terminal endings are so variable and influenced by adjacent 

 cleavage cracks to such an extent that the general symmetrical 

 aspect of the figure with respect to a vertical plane of sym- 

 metry may have thus been disturbed. It must be admitted, 

 however, that the etch figures may be actually asymmetric, in 



* For these direct measurements of birefringence in the thin section the 

 writer is indebted to Mr. E. S. Larsen, Jr., of the Geophysical Laboratory. 



