OS COMPOSriE SPHERULITES IN OBSIDIAX FROM: CALIFORXIA. 423 



23. On Composite Spherulttes in Obsidian, from Hot-Springs, 

 near Little Lake, California. By Frank Rutley, Esq., 

 F.G.S., Lecturer oo Mineralogy in the Iloyal School of Mines. 

 (Read March 26, 1890.) 



[Plate XVII.] 



In a paper read before the Royal Society in 1885 *, allusion was 

 made to certain greyish or yellowish-white sphcrulites occurring in 

 a specimen of black obsidian which was given me by the late 

 Mr. John Arthur Phillips, F.R.S. 



These sphcrulites (portions of two occur in the specimen) arc 

 about an inch in diameter and are seen to consist of numerous 

 sphcrulites of very much smaller dimensions. Li the paper referred 

 to it was suggested " that the smaller spherulitic structure was set 

 up in the large spherule after its formation, the vestiges of a 

 radiating crystalline structure tending to confirm this view." This 

 opinion was based merely upon what could be seen on a fractured 

 surface by the help of a pocket-lens. The microscopic examination 

 of a section made through one of these bodies seems, however, to 

 show that the smaller sphcrulites were formed first, and that after 

 they had assembled together in spheroidal jiggregates, a radiating 

 crystallization was set up within the mass, travelling stage by stage 

 from the centre to the periphery. 



As I have not yet met with any account of precisely similar 

 spherulites, even in Mr. Iddings's monograph t, it seems to me that 

 the following notes may prove of some interest to potrogrnphers. 



In reflected light, but better when dark-ground illumination is 

 employed, the entire section of the spherulite is seen to be composed 

 of much smaller sj)herical or spheroidal bodies, so closely packed 

 that they are usually in contact, the matter occupyiug the interstices 

 appearing darker than the spherical or spheroidal bo.lies tliemselves, 

 which latter, by dark-ground illumination, appear brightly lighted, 

 fig. 4, PL XVII. In ordinary transmitted light these bodies are 

 scarcely to be recognized, except in a tew paits of the section, wiiere 

 they are somewhat darker than the remaining portions, although 

 they are more or less translucent (fig. 1, PI. XVII.). These are the 

 small pellets which, by the help of a lens or even by unassisted 

 vision, may be seen to com])osc the large spherulites in the hand- 

 specimen. In the following description these smaller bodies will be 

 termed primitive spherulites. 



The general aspect of the section, when viewed in ordinary trans- 



* " On the Microscopic Characters of some Specimens of Dovitrified Glass, 

 witli Notes on certain Analogous Structures in Rocks,*' by Douglas Hcnuan 

 and Frank Rutley. Comuuinicated by Prof. T. G. Ponney. J).Sc., F.K.8. 

 Proc. Roy. Soc. vol. xxxix. p. KKi. 



t "Obsidian Cliff, Yellowstone National Park," ISeventh Annual Report, 

 U.S. Geol. Surv. Washington, P*568. 



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