DEC. 4, 1921 



WASHINGTON : OBSIDIAN FROM COPAN 



483 



percentages of potash and lime. These observ^ations are in conformity 

 with the general rule, that silica and alkalies tend to lower, and lime 

 tends to raise, the refractive indices of obsidians. 



The specific gravity, determined with the balance on a piece weigh- 

 ing about 31 grams, is 2.372 at 23.2 °C., giving a density of 2.366. 



SiOo... 



AI2O3. . 



FeaOs. . 



FeO... 



MgO.. 



CaO... 



NaoO.. 



K2O... 



H2O+. 



H0O-. 



Ti02.. 



P2O3... 



MnO. . 



100.26 



100.11 



100.42 



100.29 



(1) Obsidian, Copan, Honduras. Washington analyst. 



(2) Obsidian, Corinto, Nicaragua. J. PETERSEN analyst. J. PETERSEN. Neues Jahrb. 

 1898: II, 157. 



(3) Obsidian, Cerro de los Navajos, Mexico. F. Baerwald analyst. C. A. TennE. 

 Zeitschr. deutsch. geol. Ges. 37: 616. 1885. 



(4) Obsidian, Cerro de los Navajos, Mexico. F. Baerwald analyst. C. A. Tenne. 

 Loc. cit. 



Chemical analysis gave the results shown in No. 1 of table 1, analyses 

 of obsidians from Nicaragua and Mexico being given for comparison. 

 In chemical composition the Copan obsidian closely resembles many 

 others from widely separated localities, as may be seen by reference 

 to U. S. Geological Sur\^ey, Professional Paper 99, pages 113 to 151. 

 Its norm is as follows, represented by the symbol 1.4. 1 (2). 3(4). 



Quartz 28.86 



Orthoclase 26 . 13 



Albite 38.25 



Anorthite 2.22 



Diopside 2.75 



Magnetite . 70 



Ilmenite 0.91 



Apatite 0.30 



The Corinto (Nicaragua) obsidian, which is the only other Central 

 American obsidian of which we have an analysis, is higher in silica 

 and with lower alkalies, the soda greatly dominating over potash. 

 One of the analyses of the Mexican obsidian is like that of Copan, 

 but none of the analyses cited can be regarded as quite satisfactory. 



