Stnidures in Obsidian, PerJite, and. Leucite. By F. Butleij. 181 



of this class may be governed by laws or brought about by con- 

 ditions of which we are yet ignorant. It may be that Fig. 10 

 represents an originally spherical civity, around which granular 

 matter segregated, and that, from pressure, .the spherical cavity 

 has become elongated and has been protru'led through the gra- 

 nular envelope as a straight cylinder. Figs. 9, 13' and 14 are 

 straight filiform cylinders, which have this peculiarity, that they 

 appear invariably to terminate in little nodes of granular matter.* 

 In Fig. 9 this granular matter either lies within or incrusts the 

 cylinder. Are these nodes of granular matter discharges which 

 have taken place from the ends of the cylinders ? (if so, the 

 cylinders are hollow) — or are they merely segregations which have 

 formed on the ends of the rods ? I submit these questions to 

 the members of this Society, in the hope that they may be able to 

 solve them. 



Having now described these structures, it may be well to sum up 

 the conclusions to be derived from them. 



i. That the cylindrical processes extruded from the spherulitic 

 bands in the obsidian of Lipari are solid rods of glass. 



ii. That the spherules in this obsidian have resulted from the 

 development of imperfect crystallization around minute foreign 

 bodies, such as granules of magnetite &c., causing devitrification of 

 the otherwise clear glass of the obsidian. 



iii. That upon the contraction of this cryptocrystalline mass 

 pressure was exerted upon small lacunae of molten glass which were 

 enveloped in it, and that fissures, produced in Wie outer crust of the 

 spherulitic bands by the same contractile force, afforded channels 

 of egress for this still fluid matter which after extrusion rapidly 

 solidified. 



iv. That the main glassy magma of the obsidian solidified last, 

 but that the interval which elapsed between crystallization around 

 nuclei, extrusion of glass rods, and solidification of the magma, was 

 a very brief one. 



v. That in the crystal of leucite described, there are structures 

 which bear some analogy to those in the spherulitic obsidian. 



vi. That the zoned spheroids in perhte are never, so far as 

 I have ascertained, traversed by any fissures of even microscopic 

 importance, but that, on the contrary, the spheroids always lie 

 between these planes of fission. 



vii. That the foregoing fact indicates a close relationship be- 

 tween this structure and the spheroidal structure sometimes seen 

 in basalt, and that the two structures are probably due to the same 

 cause. 



* So miicli has already been done by Vom Rath, Wedding, Zirkel, Des Cloiseaux, 

 and others, in the examination of leucite, that it is. quite j^ossible that they may 

 have observed similar structures ; but of tliis I have seen no record. 



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