482 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCFS VOL. 11, NO. 20 



others. The ends are not sharply pointed, but are broken cleanly 

 across. The cores are bounded laterally by 8 to 10 curved, narrow 

 faces, produced by the perfect conchoidal fracture of the material, 

 and nearly all these faces extend the whole length of the core. These 

 narrow faces are slightly concave across their width. 



The obsidian is of a uniform, jet-black color, perfectly vitreous, and 

 with perfect conchoidal fracture. There are no streaks, and no 

 phenocrysts are visible. In splinters the glass is brownish and very 

 transparent. The thin section shows a clear colorless glass, homo- 

 geneous and with little or no evidence of flow. There are no bubbles 

 but some very minute microlites are scattered sparsely through the 

 glass; the larger (0.01 mm.) being irregular crystal fragments of 

 pyroxene, and the smaller (0.001 mm.) opaque grains of magnetite. 

 Their total percentage cannot amount to more than about 0.01 of 

 the rock. 



-^ - -^-> -^^~ L 



rjy)>) 



) 



IfJJj 



pyfpTj^'J '■)'jV Jr I I ; 





Fig. 1. Obsidian Cores from Copan. 



Dr. H. B. Merwin very kindly determined the refractive indices, 

 these being measured on a specially ground prism. They are as 

 follows: «c = 1.4896, 71d = 1.4920, tip = 1.4976, and Wg'= 1.5022. 

 It will be seen that the value for yellow light (D) is slightly higher 

 than the indices (determined by Merwin) of the obsidians of I^ipari 

 (Rocche Rosse = 1.488-9, Forgia Vecchia = 1.490), Monte Arci 

 (1.487-9), and Milos (1.490).- The first three have about the same 

 percentage of silica as the Copan obsidian, but are slightly more 

 potassic and with less lime; while that of Monte Arci is higher in 

 silica by about 2 per cent, with a little less soda, but about the same 



2 H. S. Washington. Amer. Journ. Sci. 50: 462. 1920. 



