Penfield and Forbes — Chrysolite- Fay edits Group, etc. 131 



low Xa, a = 1-8236, j9 = 1'8642, y = 1-8736, f - a = '060, the 

 orientation being a = c, b = a, e = b as in crysolite. 



The plane of the optic axis is the base and a is the acute 

 bisectrix. The double refraction is, therefore, negative. The 

 dispersion is p> v, 2H,Li = 57° 27'; 2H,Xa = 56° 32'; 2H, 

 Tl = 55° 2'. The index of refraction for a-rnono-bromnaph- 

 thalene being 1-6577 at 23° C. for Yellow, Y y = 21° 55' and 

 2E 7 = 103° 25'. 2E y was also measured and found to be 103° 

 4'. From the values of a, t 3 and y, according to the usual 

 formula, Y was found to be 61° 12 r , hence Y a = 25° 18', agree- 

 ing closely with 21° 55' as given above. The pleochroism is 

 distinct, in sections about 0'5 mm thick for rays vibrating parallel 

 to b orange yellow, parallel to a and c greenish yellow. 



To make sure of the orientation of the optical properties as 

 given above, where only cleavages were available, a comparison 

 was made with the excellent crystals from the obsidian of the 

 Yellowstone Park described by Iddings and Penfield.* For 

 this purpose, material was supplied by Mr. Arnold Hague of 

 the U. S. Geological Survey, to whom the authors desire to 

 express their thanks. On these transparent, but minute crys- 

 tals, the basal cleavage and the emergence of the obtuse bisec- 

 trix at right angles to the pinacoid 100 were distinctly seen, 

 the plane of the optic axis being 001. Xo indication of a cleav- 

 age parallel to the pinacoid 100 was observed on the fayalite 

 from Rockport, and the statement in many mineralogies of a 

 cleavage in that direction is probably erroneous. 



Hortonolite. 



Under this name Professor Brushf has described a mem- 

 ber of the chrysolite-fayalite group, found in an iron mine 

 at Monroe, Orange Co., X. Y., and characterized by its 

 dark color and high iron percentage. In appearance it 

 resembled the fayalite from Rockport. In order to ascertain 

 the chemical composition of the mineral upon which the opti- 

 cal properties were to be determined, a new analysis was made 

 upon material from a large mass, showing cleavage in two 

 directions, which served for the optical orientation, this cleav- 

 able material being better adapted for optical work than the 

 crystals at our disposal. 



Grains of magnetite were disseminated through the mineral 

 and the material for analysis was therefore purified as described 

 under fayalite. 



The results of the analysis are given below, together with 

 those obtained by Mixter and quoted by Brush : 



* This Journal, vol. xxx. p. 58, 1885. 

 f Ibid., vol. xlviii, p. 17. 1869. 



