266 



F. E. WRIGHT OBSIDIAN FROM ICELAND 



whose origin is still in some doubt. The formation of these surface 



markings is discussed in a separate section below. 



Specimen 88432, as shown 

 in figure 1, is filled Avith 

 spherulites, ranging in size 

 from mere specks to kernels 

 half a centimeter in diam- 

 eter. The radial spheru- 

 lites are usuall}^ white, as a 

 result evidently of the ac- 

 tion of the circulating solu- 

 tions whicli deposited the 

 alunite and hyalite in the 

 adjoining gas cavities. The 

 central part of many of the 

 radial spherulites is still 

 dark gray and unaltered. 

 Under the microscope the 

 secondary alteration is seen 

 to be of the nature of a 

 bleaching effect rather than 

 of complete recrystalliza- 

 tion 



Figure 1. — Oisidiaii coutuininu rudUiJ S/tJieni- 



lites and buhble Cavities 



Specimen 88432. Two-thirds natural size 



although there is evi- 



dence of partial recrystallization. 

 Specimen 88433. — In this 

 specimen there are iiumerous cav- 

 ities (0.5 to S mm. in diameter) 

 partly filled witb crystal fibers 

 which radiate from the walls to- 

 ward the center. They vary in 

 size and often in shape ; but when 

 undisturbed by adjacent cavities 

 they are roughly spherical in out- 

 line. On breaking open the cav- 

 ities, one is impressed by the fact 

 that the crystallized material does 

 not completely fill them (figure 

 2) ; also that the crystals in the center of the cavity are coarser than those 

 at the margins. The radial fibers are usually water-clear, and are capped 

 and studded -with tridymite crystals in twinned groups measuring up to 



FiGfRK 2. — fjithophysw iritli flitted Tongue of 

 Obsidian projecting into hoUoic Caritji. 

 shown in Center of Photofjraijh 



Specimen 88433. Magnification, 15 X 



