Structures in Obsidian, Perlite, and Leucite. By F. Riitley. 179 



chronously. This appears to me to be the most plausible way in 

 which to account for the phenomena which occur in the section 

 which we have been considering. 



The followins; translation from Zirkel's ' Mikroskopische Bes- 

 chaffenheit der Mineralian u. Gesteine ' (p, 75) may here prove 

 interesting, as indicating the existence of structures somewhat similar 

 to those described, but due to a different cause : 



" Some of the more rare enclosures of glass which occur within 

 a glass mass are peculiar. They may be recognized from the fact 

 that a tolerably concentric circle usually surrounds a dark, mostly 

 spherical cavity, but lies at some distance around it, the intermediate 

 part also consisting of glass which is as a rule identical with the 

 surrounding glass; it is. however, at times clearer or darker. These 

 glass enclosures in glass are probably due to a gas-bubble, sur- 

 rounded by a thin envelope of molten matter, having broken loose 

 from some spot and after reaching an adjacent part of the magma 

 becoming fixed. That this is in fact the case, is proved by the 

 circumstance that at times the matter surrounding the bubble has 

 developed a finely fibrous structure to a certain extent around its 

 zone of attachment in a. somewhat radial manner." 



A few points will now be noticed in connection with the micro- 

 scopic structure so characteristic of Perlites : rocks which are closely 

 related to pitchstone and obsidian. In these rocks a very peculiar, 

 concentric, shaly structure exists, which has been described by Zirkel, 

 Eosenbiisch, and other observers. In some perlites, microliths, 

 trichites, crystals of magnesian mica and felspars, and also small 

 spherules, very similar to those just described, occur. Some of the 

 latter are surrounded by a clear girdle, like that shown in the 

 Lipari obsidian, Fig. 6. It is possible that the glassy girdles of 

 these isolated spherules may merely represent a structural difieren- 

 tiation, and that their union with the cylindrical processes may only 

 be an accidental circumstance ; their probably identical constitution 

 and synchronous development having favoured their coalescence, 

 so that no perceptible demarcation exists between them. Under 

 the microscope a distinct fluxion texture is seen in many of 

 the perlites, being rendered evident by granular matter and micro- 

 liths lying in more or less continuous streams. These streams, 

 which thus apparently represent fluxion planes, are traversed at 

 all angles by tine fissures, which often intersect one another, and in 

 thin sections I have not yet been able to find any instances in 

 which these fissures traverse the perlitic spheroids, although the 

 fluxion structure cuts across them in a very marked manner. On 

 the contrary, I find that the spheroids are packed between these 

 fissures, and there is even evidence that in places their forms have 

 been affected or modified by the fissures, since they sometimes 

 appear to be more or less compressed where they come in contact 



VOL. XV. 



