264 F. E. WEIGHT OBSIDIAN FROM ICELAND 



ditions of cooling- of tlie different parts of the lava, together with tlio 

 amount and character of the vohitile components dissolved in it. I ii 

 order to show this clearly, six of the specimens collected at Hrat'nlin- 

 nuhrvo-yur will be brieflv described. 



Specimen 88428 is a black compact obsidian glass, free from s])hcru- 

 lites. but containing fine hairlike crystallites and minute bubbles 0.05 

 mm. in maximal length; also dark opaque grains scattered through it. 

 Part of this specimen served for chemical analysis I and is described in 

 sufficient detail above. 



Specimen 88429 shows clearly the passage of obsidian glass to pumice. 

 One end of the specimen is typical pumice, with silvery luster in the 

 direction of elongation of the vesicles; the opposite end is of massive 

 obsidian glass, with only occasional large cavities, which trend either 

 approximately i)arallel with or transverse to the general lines of flow. 

 The transverse cavities are much larger than the others and appear to 

 be of the nature of rupture fissures — rather than elongated gas cavities — 

 produced during the final stages of the cooling and flow of the lava as a 

 result of the tensional stresses thereby developed. 



Specimen 88430 presents another structural type whiih was developed 

 during the period of cooling of the lava. Dull gray-black litlioidal 

 bands and irregular masses alternate with bands and patches of black 

 obsidian glass. The lusterless parts consist of spherulites which have 

 crystallized from the glass. Under the microscope these sphendites are 

 seen to be of two different types : 



(1) Typical radial spherulites, with Hbers radiating from a central 

 point, or more commonly from a minute central bul^ble. The elonga- 

 tion of the fibers is a' : a distinct cross is visible between crossed nicols.' 

 The refractive indices are difficult to obtain accurately, l)ut they lie be- 

 tween 1.520 and 1.54(». The birefringence is medium to weak. Between 

 many of the fibers are minute irregular cavities which greatly decrease 

 the transparency of the spherulites. The determinations indicate that 

 these spherulites are chiefly albite, with possildy a little admixed ]wfash 

 feldsi")ar and also free quartz. 



(2) Adjoining the radial spherulites are usually patches of substances 

 of a deeper brown color and of slightly stronger birefringence and less 

 pronounced radial spherulitic development. The elongation of the fibers 

 in this material is not pronounced, but in those cases in which an elonga- 

 tion was apparent its character was y'. The development approaches 

 that of an aggregate of overlapping crystallites and grains too fine and 

 too intimately intergrown for satisfactory determination. The refrac- 



