AKT. 17 



MINERALS OF OBSIDIAN CLIFF — FOSHAG 



11 



feldspar. Their small size and light color makes them inconspicuous 

 objects which are easily overlooked. In habit they are flat plates 

 tabular to the macropinacoid (100) with the edges modified by nar- 

 row faces of (120), (111), (101), and (010) (figs. 3, 4). The indices 

 of refraction are high. The crystals are weakly pleochroic; Y is 

 light brown, X light grayish brown. The optical orientation is 

 X=b, Y=c, Z=a; the broad front pinacoid gives an obtuse 

 bisectrix figure with the plane of the optic axis parallel to the base. 

 The larger lithophysae of the glassy rock contain fayalite crystals 

 that are larger and stouter, and with a fine chestnut brown color. 

 Many of them lia^e a red and green iridescence on the surface, 

 giving a metallic sheen. These larger crystals, under the microscope, 



Fig. 4. — Cey.stal habit of fayalite from the lithophysab of Obsidian Cliff 



have the same optical properties as the smaller, except that the 

 pleochroism is fainter in shades of chestnut brown. Many of the 

 crystals show a coarse cross grating on the surface, the lines being 

 parallel to the faces (010) and (001), apparently due to cleavage. 



The fayalite crystals are exceptionally sharp and the faces bright. 

 The measured ancles are as follows : 



Measured angles of fayalite crystals 



