532 T\1R. F. RUTLEY OX A SPHERULITIC AND 



fissures are filled, and which also occurs in the obsidian itself, either 

 fringing the perlitic cracks or disseminated in small spherulitic bodies, 

 indicates this, while the numerous globulites which are present are 

 probably due to a like cause. Whether solfataric action gives rise 

 to the formation of globulites in vitreous rocks is a question which 

 has not, I think, as }ct, been demonstrated ; but I may mention that 

 a specimen of sphenditic obsidian, collected by Mr. G. F. Hodwell 

 some years ago in the crater of Yulcano at a point where a powerful 

 jet of steam issued and where sulphur was deposited, no longer pre- 

 sents the bright glassy lustre so characteristic of fresh obsidians, but 

 appears perfectly dull and has a stony instead of a glassy aspect. 



In thin section under the microscope the rock is seen to be tra- 

 versed by a network of line, irregular cracks, and its loss of lustre is 

 found to be due to the development of innumerable globulites, as 

 shown in fig. 6, PI. XVIII. The distinctness of the spherulitic 

 .structure has also suffered considerably, owing to the devitrification 

 which, in this case, appears very probably to have been engendered 

 by the action of steam. 



EXPLANATION OF PLATE XVIII. 



Fig. 1. Spherulitic and perlitic obsidian from Pilas, Jalisco, Mexico, showing 

 complex perlitic structure and delicate fluxion-banding. X 18 linear. 

 Ordinary transmitted light. 



2. Ditto, showing perlitic fissures filled with doubly-refracting siliceous 



matter (chalcedony). X 140 linear. Crossed nicols. 



3. Ditto, showing portion of a spherulite, in which the growth has been 



temporarily arrested along a fluxion-band, the latter having served as 

 a basis for subsequent growth of the spherulite. X 18 linear. 

 Crossed nicols. 



4. Ditto, showing portion of a spherulite, the development of which has 



been permanently arrested along a fluxion-band. This figure also 

 shows that the perlitic structure does not traverse the spherulite, but 

 that it accommodates itself to the contours of the latter. The fluxion- 

 banding passes through the spherulite. X 18 linear. Ordinary 

 transmitted light. 



5. Ditto, showing globulites, longulites, and pellets of chalcedony, occurring 



in the glass of the obsidian. The fluxion-banding passes diagonally 

 upwards from left to right in this figure. X 250 linear. Ordinary 

 transmitted light. 



6. Devitrified spherulitic obsidian from the crater of Vulcano, Lipari Is. 



Collected at a point from which a powerful jet of steam was issuing. 

 This obsidian is completely, or almost completely, devitrified by the 

 development of globulites. In many parts of the section the globulites 

 are much more densely crowded than in the portion here figured. 

 X 250 linear. Ordinary transmitted light. 



Discussion. 



Prof. JuDD asked whether the Author had any information as to the 

 locality of this interesting specimen, and especially as to its relation 

 with au}^ of the well-known varieties of Mexican obsidians. 



Mr. G. F. KuNz stated that the locality in Jalisco was about one 

 hundred miles west of Mexico city, and one hundred and fifty miles 

 north-west of Pachuca (" Navajas "), the Hill of Knives. 



