SECONDARY MINERALS AND ETCHINGS 279 



etching medium and on the chai-after of the surface etched. The etching 

 process is not unlike tlie aln-asive action of sand-laden winds on exposed 

 rock surfaces in deserts.^'"' The attacking acid solution etches in the 

 direction of least resistance and the material is carried away in solution. 

 The solution currents form whirls and eddies, and thus favor unequal 

 attack even on a perfectly homogeneous surface. Furthermore, any ir- 

 regularities in the surface or material are emphasized by the solutions. 

 An examination of the surface of the obsidian of the most uniform 

 specimen, 88428, shows the presence of fine point irregularities, in the 

 shape usually of minute triangular-shaped areas, as though at such points 

 the cohesion of the obsidian was different from that of the surrounding 

 points ; this difference finds expression in the character of the surface of 

 fracture obtained on the splitting off of the glass chips. Such points of 

 unique cohesion are probahly the minute bubble cavities which are scat- 

 tered through the glass and are visible in thin obsidian splinters undei' 

 llic microscope. These points and cavities offer favorable points of attack 

 for the etching solutions, which, as the dissolving action proceeds, con- 

 tinue to enlarge the cavities, and thus possibly to produce lunar ci-ater 

 forms on some of the specimens. Another explanation of these forms is 

 that they are etched enlargements of original half-moon fracture cracks, 

 ])roduced by striking the glass fragment a sharp blow. The distinct 

 wavelike lines, both radial from and concentric to the point of impact 

 of a blow which fractures a piece of obsidian, are Jines which exert a 

 directive influence on the attacking solution, and thus give rise to certain 

 t\-pes of the remarkable etch forms which we observe. Still another kind 

 of crack requires mention, namely, the shrinkage rupture cracks, as shown 

 in specimen 88429, described above. Into these fissures the solutions 

 cnior and tend to enlarge them. In the case of strain in the gla.ss the 

 solutions probably etch most rapidly along the lines of maximum strain, 

 and this again tends toward irregularity of etching on the exposed 

 surface. 



In addition to tliese factors inherent in the etched material, any foreign 

 substance, as a precipitate, attached to the surface serves as an obstruc- 

 tion to the acid streams of the solvent and forces them to flow along cer- 

 tain paths. Attention has been called above to the effect of precipitated 

 hyalite in this direction. Observation proves that the pitted character 

 of some of the slightly etched surfaces is not due to a spongy layer of 

 original bubble cavities which have been exposed by subsequent frac- 

 turing. 



«V. Goldschmldt and F. E. Wright: Neues .Tahrb., Beilage Bd. xvii, 190."., pp. 355- 

 390; Beilage Bd. xvlli, 1904, pp. 335-376. 



