176 Transactions of the Royal Microscopical Society. 



III. — On so7ne Structures in Obsidian, Perlite, and Leucite. 



By Fbank Kutley, F.G.S. (H.M. Geological Survey). 



(Read before the Royal Microscopical Society, March 1, 1876.) 



Plates CXXXIII. and CXXXIV. 



In this paper I shall endeavour to demonstrate certain peculiar 

 structures in spherulitic obsidian and leucite which appear to be, 

 in some respects, analogous ; and an explanation will also be 

 attempted of the spheroidal structure which characterizes the 

 vitreous rocks known as Perlites. 



So far back as 1857 your President, Mr. Sorby, was at work 

 with his microscope upon the pitchstones of Arran. Since that 

 time Vogelsang, Zirkel, Allport, and others have made very con- 

 siderable additions to our knowledge of the minute structures 

 which rocks of this class contain. 



The section of obsidian which I am about to describe was cut 

 from a specimen collected by my friend, Mr. J. W. Judd, from a 

 lava flow in the Isle of Lipari. The mode of occurrence of this 

 lava stream is described in his " Contributions to the Study of 

 Volcanos," published in the 'Geological Magazine,' 1875. The 

 specimen is a grey glass, but when seen in moderately thick pieces 

 by reflected light it appears black. Through this glass run parallel 

 bands of white spherules, which are often elongated, and in most 

 cases have coalesced so as to form either continuous parallel-sided 

 bands (approximately cylindrical) or moniliform strings. Isolated 

 spherules also occur in the specimen, and sometimes adhere to the 

 sides of the spherulitic bands, as shown in Fig. 8, PI. CXXXIII. 

 The isolated spherules have a distinctly radiate structure, which in 

 the bands fonned of coalesced spherules is almost absent. Both 

 the isolated spherules and the spherulitic bands are surrounded by 

 a broad external crust, which by transmitted light appears of a 

 reddish-brown colour, and is but feebly translucent, while by 

 reflected light it looks almost snow-white. AVithin this lies 

 granular or crystalline, and in some cases radiately fibrous matter, 

 which in many instances exhibits a reddish, rusty colour, both by 

 reflected and by transmitted light ; while in others, although rust- 

 coloured by transmitted light, it appears white or greyish white 

 when viewed as an opaque object. At times this inner substance is 

 sharply separated from the cortical layer by a thin, trans j)arent, 

 vitreous band. In the spherulitic bands there are usually other 

 innermost bands which are more translucent and less rusty in appear- 

 ance than the intermediate ones, and along these inner cores there 

 frequently run lines of almost opaque bodies, giving, as a rule. 



