spm?:ri:lites and lithoimiys/E - 265 



five index is about 1.54. It is probable that these spherulites consist 

 chiefly of quartz intergro^^^l witli some alkali-feldspar. 



Not all of the spherulites in this specimen are entirely compact and 

 gray-black in color. Portions of many of them are porous and then 

 usually lighter in color and coarser grained. The appearance of such 

 spherulites leads one to infer that gas was evolved during their crystalli- 

 zation, and that the volatile components thus set free acted on the spheru- 

 litic material of the walls and caused its recrystallization. In other 

 words, each little spherulite, with its portion of volatile components, 

 which were liberated during its crystallization and were inclosed in the 

 thick viscous hot glass, may be likened to a chemical flask filled with 

 appropriate reagents and crystal compounds and heated to such a tem- 

 perature that certain chemical reactions take place. It is evident that 

 the physico-chemical equilibrium conditions during the partial crystalli- 

 zation of a melted magma, from which appreciable amounts of volatile 

 components are being liberated, are difl^erent from the equilibrium con- 

 ditions obtaining in a system of the same total composition, but at a 

 much lower temperature and containing the volatile components chiefly 

 as vapor phases and the other components as crystallized units. The 

 result of this shift in distribution of the elements from a homogeneous 

 solution (magma) crystallizing at high temperatures to a heterogeneous 

 system consisting of solid and vapor phases held at a lower temperature 

 is a redistribution of the constituents in the solid phases, such that the 

 mineral association which we encounter in the normally crystallizerl com- 

 ])act spherulites or in rhyolite or in granite is different from that of the 

 liollow spherulites or lithophyste. This difference will appear more clearly 

 ill the descriptions below, l)ut it is essential that the fundamental difPer- 

 t'licc ill behavior and in stability relations of the two cases be emphasized. 



[n certain bands of this specimen the aggregate volume of the gas 

 cavities is relatively large, but they are not elongated as in specimen 

 SS429 and lia\(' the ap[)earance rather of a spongy structure. The cavi- 

 ties here are associated with the spherulites and were evidently developed 

 in situ. 



In one larger cavity a white coating of clear, secondary hyalite was ob- 

 served. This, mineral is abundant in the more altered specimens of 

 obsidian and pumice, especially in the specimens gathered at the small 

 circular pond mentioned above. At this place highly siliceous solutions 

 were evidently active and not only deposited hyalite but also alunite, and 

 corroded the black obsidian glass in a remarkable manner, so that many 

 of the fragments resemble in outer forms the Bohemian moldavites, 



XX — lili.i.. <;i;n[,. Siir. A.M., Vol.. JC, 11114 



