458 MISS E. ASTON AND PROF. BONNET ON [Aug. 1 896,, 



the lining of glass is perhaps a shade thinner, and the material is a 

 little different. 



As it seemed hopeless to obtain a thin slice of the rock so as to 

 retain a good section of the actual lining of a tube, 1 a little of this 

 crust was flaked off and examined, when powdered, under the 

 microscope. It was taken from the second of the above-named 

 specimens. The lighter-coloured material (from the inner part of 

 the tubes) consists of a very dark brown glass (only translucent in 

 the very thinnest chips or edges), in which flakelets of the serpen*, 

 tinous mineral, 2 common in the rock-mass, are not infrequently 

 embedded. The black material (external part) is similar, so far as 

 can be ascertained, but here any approach to translucency is rare,.. 

 and the above-named minerals indicate their presence only by 

 occasionally projecting from the edges or making almost the whole- 

 thickness of a chip. 



It then occurred to us to examine the glass which could be- 

 obtained by artificial fusion of this rock. A small flake was 

 detached from one end of the third of the above-described specimens. 

 This was not affected by the ordinary blowpipe-flame, but was 

 fused at the edges and on the adjacent surface by supplying oxygen 

 instead of air ; that is to say at a temperature which certainly 

 exceeded 1700° C, or was well above the melting-point of pure iron. 

 In this operation the thinner parts of the flake were fused and a 

 small detached pellet — seemingly of glass, — about -J-g- inch in diameter, 

 was obtained, but on breaking up these the former proved to be 

 only a pellicle of glass — rarely attaining -^ inch in thickness, and 

 the latter to consist of an unmelted core of rock, covered by a film 

 of glass varying from about jfa to ^ inch. This glass also was of 

 a very dark brown colour, and its surface was slightly pitted with 

 minute depressions ; in fact it was practically identical with that 

 which had been produced by the lightning. Samples from each 

 were crushed, mounted, 3 and examined under the microscope with 

 various magnifications up to about 400. The glass, so far as can 

 be seen, does not differ in any respect from that produced by the 

 lightning. Only the thinnest parts are translucent, and these are 

 of a deep umber-brown colour, like the darkest varieties of tachylyte. 

 As in the other case it includes flake-like minerals, between which 

 it can be seen, as it were, to penetrate, so that a transitional zone 

 separates that of complete fusion from the unfused rock, both of 

 these zones being very narrow. In the first-named many mineral 



1 One of the slices had accidentally intersected a tube, but nothing satisfactory 

 could be made out from the fragments of fused material that still adhered. It 

 does, however, indicate that the minerals immediately below the glass assume, 

 for a very short distance, a cloudy aspect, due, so far as can be seen, to the 

 development of extremely minute cavities, sometimes tubular. — T. Gr. B. 



a I contrived to measure the extinction in some ; generally it was straight, but 

 in one or two cases it appears to be slightly oblique. Both tints (mentioned 

 already) were noted. — T. Gr. B. 



3 I am indebted to Miss C. A. Baisin, B.Sc, for preparing the slides of glass 

 used in this paper, and for independently examining them so as to verify my 

 results.— T.G-.B. 



