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28. 0;i a Sphekulitic and Perlitic Obsidian /ro7'rt Pilas, Jalisco. 

 Mexico. By Frank Huxley, Esq., F.G.S., Lecturer on Mine- 

 ralogy in the lloyal College of Science, London. (Read 



April 22, 1891.) 



[Plate XVIII.] 



The specimen upon which the following observations were made is 

 a greyish-green or leek- green obsidian with a waxy lustre, contain- 

 ing numerous deep brownish-red spherulitcs, ranging from the size 

 of peas to smaller dimensions. It was collected by Dr. A. E. Foote, 

 to whose kindness T am indebted for it. 



Under the microscope the thin section shows banded and perlitie 

 structures, the former being delicate and even, while the latter is 

 developed with a perfection which I have never seen equalled. Each 

 perlitic area, as a rule, includes several centres, sometimes five or 

 six, or more, around which perlitic structure has been developed 

 (PI. XVIII. fig. 1). A similar complex perlitic structure has already 

 been noted in certain dacites by Prof. Judd*. The glass of the obsi- 

 dian contains numerous globulites and longulites, which are densely 

 crowded together along the fluxion-banding. 



The perlitic fissures appear in all cases to have been filled up with 

 siliceous matter, showing, in some instances, minute crystalline rods 

 or fibres, passing from the walls towards the middle of the fissure, 

 where a distinct line of arrest parallel to the walls may be seen. 

 This secondary crystalline matter exhibits double refraction, so that 

 the perlitic structure appears brightly illuminated and in strong 

 relief against the glass of the obsidian, when viewed between crossed 

 nicols (PI. XYIII. fig. 2). 



It is only here and there, however, that these somewhat regular 

 growths from opposite walls of the cracks are to be seen. At times 

 the minute crystalline rods or fibres occur in little divergent groups, 

 and occasionally the growth has also passed from the walls of the 

 crack into the glassy substance of the obsidian, thus giving rise to 

 very minute pellets or spherulites of chalcedony which, when the 

 section is viewed between crossed nicols, cause the perlitic 

 fissures to appear broader than they do in ordinary light. These 

 divergent groupings or spherulitic bodies are very minute as a rule, 

 but in one or two instances, where they are sufiiciently large to 

 admit of the determination of their optical sign, they are found to 

 be positive. The longulites which follow the direction of the 

 fluxion-banding seem to pass through the material which has sealed 

 up the perlitic fissures ; but this appearance is, of course, deceptive, 

 and is due to the inclination of the cracks to the plane of section, 

 a longulite Avhen partially overlying or underlying an obliqiiel}- 

 inclined crack appearing to penetrate the transparent matter with 

 which the latter is sealed. 



* See ' The Volcanic Eocks of the North-east of Fife,' by J. Durham, with an 

 appendix by Prof. J, W. Judd, Quart. Journ. Geol. Soc. vol. xlii. (1886) p. 421). 



