484 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 11, NO. 20 



It is probable, however, that the Copan obsidian is much like the 

 general run of Central American and Mexican obsidians, if we may 

 judge from analogy with similar regions in the western United States 

 and along the Andes, a comagmatic zone to which the volcanic rocks 

 of Mexico and Central America belong. 



OBSIDIAN FROM CHICHEN ITZA 



Among a large accumulation of sacrificial offerings (mostly of jade) 

 found some years ago in a cenote, or natural well, at the ancient Maya 

 city of Chichen Itza in Yucatan,'^ were a few beads of obsidian. 



The beads are cylindrical, from 2 to 5 cm. long and about 5 mm, 

 thick, with a shallow groove running spirally around them from end 

 to end. The smooth but unpolished surface is brownish black, with 

 irregular streaks of scarlet, which appear to be painted or burnt into 

 the surface, as the color does not extend into the material below. They 

 are apparently intended to simulate similarly mottled obsidians, 

 which are fairly common, but discussion of this question of technique 

 will be taken up elsewhere. 



The obsidian is of a somewhat peculiar hair-brown color, quite 

 different from the more usual brown of the Copan obsidian, but is 

 colorless in thin section. It contains some bubbles, which vary in 

 length from 0.02 to 0. 10 mm. These are mostly spindle- or pear-shaped, 

 the former frequently with one end drawn out to a sharp point. The 

 bubbles are arranged in streaks, evidence of flow texture. Apart 

 from these bubbles, the glass is perfectly clear and contains no mi- 

 crolites or phenocrysts. 



Dr. Merwin determined the refractive index of this obsidian by 

 the immersion method, and I would express my thanks to him for 

 his kindness in making this and the other determinations on the Copan 

 obsidian. He found the value Wd = 1.489 ±0.001. This is notably 

 lower than the refractive index of the Copan obsidian, but is about 

 the same as the indices for the Lipari and Milos obsidians. It is 

 in accord with the rather high silica and alkalies and low lime and 

 alumina. 



The density was not determined, because the small amount of 

 material available and the presence of the bubbles would have made 

 the determination of little value. 



^ This collection, now in the Peabody Museum in Cambridge, Massachusetts, is to be 

 described under the editorship of Prof. A. M. TozzER, to whom I am much indebted for 

 the privilege of studying the stone objects and publishing the present description, which 

 deals only with the petrological aspects of the obsidian. 



