258 f, e. wright obsidian from iceland 



'I'he Hrafntinnuhryggur Obsjutan 

 general description 



The obsidian specimens containing the litliopiiysae were collected by 

 the writer in 1909. Unfortunately lack of time and of transportation 

 facilities permitted neither adequate field study of the occurrence nor 

 the collection of a representative set of specimens. Only a few interest- 

 ing random specimens were gathered to illustrate, as well as possible, 

 the different types which occur. 



The obsidian of Hrafntinnuhryggur forms a well developed, long ridge 

 southeast of the volcano Krafla. It is not uniform in structure through- 

 out, but ranges from dense black glass to a rock approaching pumice. 

 Banding caused by an alternation of layers of the dense black glass with 

 bands of semi-pumiceous or spherulitic material is characteristic of cer- 

 tain of the specimens. Near the west end of the ridge a small circular 

 pond, resembling a shallow crater lake, occurs; and there the rock is 

 apparently a breccia consisting of fragments of black obsidian glass 

 (showing remarkable etched surfaces, which resemble those of the Bo- 

 hemian moldavites and of certain desert rocks with etched surfaces) and 

 of a white crypto-crystalline, siliceous substance. 



The obsidian proper is a dense, black, brittle glass, remarkably uniform 

 in character. Prismatic jointing was observed at several points and is 

 of the ordinar}- columnar type. The obsidian glass takes a good optical 

 polish, has a refractive index of about 1.500, and might possibly l)e 

 serviceable as a source of material for making large telescope reflectors; 

 its coefficient of expansion is probably low, in vieAv of the high silica con- 

 tent. The fracture is conchoidal; in tlie field a single sharp l)low of the 

 hammer on a large uniform block a meter in diameter ma}" spall off 

 ashlars or shell-like pieces, which show most beautiful conchoidal frac- 

 ture lines. The development of the two sets of lines — the one set con- 

 centric and emanating like wave-ripples from the point at which the 

 blow was struck, the second set radiating from it in lines normal to the 

 first — is so perfect and fascinating that the lack of transportation facili- 

 ties is keenly felt by the geologist. 



Under the microscope the obsidian glass is seen to l)e full of verv 

 minute crystallites of a colorless, prismatic mineral, not over 0.005 mm. 

 in length and less than 0.002 mm. in width. The optical properties 

 which could be determined on this mineral are : Eefractive index, notice- 

 ably higher than that of the glass; birefringence, medium; extinction, 

 apparently parallel to the elongation (=y'). These properties are un- 

 fortunately not sufficient to identify the substance, but other and more 



