278 



F. E. WRIGHT OBSIDIAN FROM ICELAND 



the obvious connection between the deposition of hyalite (specimen 

 S8435) and the etch pits. In figure 12 a face of ol)sidian is shown from 

 which a crust of hyalite was broken off. The surface shows etched 

 grooves and markings like the lines on a turtle shell ; they Avere obviously 

 formed during the deposition of the hyalite. A careful study of the 

 entire specimen under a binocular microscope leads to the conclusion that 

 the solutions actually bored into the obsidian and continued to do so until 

 a protecting crust of liyalite Avas formed. The irregular distribution of 

 the etch channels seems to be, in part at least, the result of the irregular 

 precipitation of liyalite from an exceedingly mobile medium, probal)ly a 



I'lauuE 12. — Etched Surface of a small Ohsiilhin Frii<j"teiU 



The surface was protected in part by a coating of liyalite circulating solutions which 

 reached the obsidian along cracks in the hyalite mantle. Specimen 88435. Magniflca- 

 (ion, 10 X. 



hot solution with admixed vapors; in short, from liot volcanic emniuitious 

 which escaped from the intruded but still hot magma mass, in view of 

 the high silica content of the obsidian, 75 per cent, it is ])robable that 

 the etching solutions were alkaline and not acid. A glass bottle of the 

 composition of obsidian should be an excellent retainer for even hot acid 

 solutions. It is significant that the greater part of the hyalite was de- 

 posited before the alunite. This may indicate a gradual change in -the 

 composition of the volcanic emanations by the increased concentration 

 of sulphuric acid. 



(2) Experiments in etching both crystals and glasses have shown that 

 the nature of the etched surface produced is dependent on the kind of 



