IN OBSIDIAN FROM CALIFORNIA. 425 



To sum up the history of these spherulites, I thiuk we may 

 assume that iti the first instance small spherulitic bodies (the 

 piimitive spherulites) were developed in the obsidian before it 

 assumed a condition of rigidity. Secondly, tiiat these small spheru- 

 lites floated towards certain points in the still viscid lava and 

 seij;regated in more or less spherical groups, although what deter- 

 mined their movements, whether mutual attraction or some other 

 cause, there is no evidence to show. Thirdly, that from a point or 

 points, situated at or near the centre of each group, crystallization 

 was set up, giving rise to a radiating fibrous or rod-like structure, 

 which gradually developed zone after zone ot:' divergent tufts or 

 fibres, until the entire mass of primitive spherulites was permeated 

 by this secondary structure, a structure engendering a molecular 

 rearrangement of the mass such as would obliterate any trace of 

 structure which the primitive spherulites might have originally 

 possessed. 



There are two or three small spherulites visible in the section, 

 which apparently arrived too late to be incorporated in the general 

 mass and which occupy conspicuous posilions on the margin of the 

 crowd, and an independent divergent crystalline structure has been 

 developed about them as shown in fig. 0, PI. XVIf. 



Ir was not until the foregoing notes were made that I had the 

 good fortune to see Mr. Iddings, whose work has already been 

 alluded to, and to converse with him upon the {piestion now under 

 consideration. 



Mr. Iddings has examined the specimen and section now laid 

 before the Society, and kindly given me his opinion upon them. 



He considers that the bodies which I have here ventured to term 

 primitive spherulites are of secondary origin, and that they probably 

 consist to a large extent of tridymite, although from his cursory 

 examination of the section he could not speak positively as to the 

 presence of this mineral. 



In this case he stated that he reasoned from analogy, and that he 

 had seen very similar phenomena in the spherulites and lithophysie 

 occurring in the obsidians ot the Yellowstone National Park. Prom 

 an examination of a vast amount of material collected in that 

 district, he had been able to trace out structures of this class ranging 

 from some very obscure or minute to others occurring on a sufh- 

 ciently large scale to admit of more accurate determination. 



In the present case he con?idered, 1st. that the spherulites or litho- 

 physie(he regarded them as the latter) originated in the developnu-nt 

 of a radial crj'stallization, in which felspar microlites were developed 

 in a succession of twins which gave rise to delicate ramifying 

 growths ; :2nd, that diminution of volume resulted from this action 

 of crystallization, and that vacuities were thus produced between 

 the twigs, if one may so term them, of this divergent crystalline 

 structuTe, so that the mass of the spherulife or lithophysa was 

 thoroughly cavernous or spongy in texture ; :h'd, that within these 

 spaces tridymite was subsecjuently developed in small spherical bodies 

 consisting of aggregates of minute scales or jjlates of tridymite, and 



