2 Professor T. G. Bonney — Fulgurites from the Andes. 



including grains of felspar and of iron-oxide ; (c) biotite, only in 

 a very few ill-developed flakes; {d) one or two small grains of 

 augite. The felspar, plagioclase, is variable in size ; inclusions are 

 inconspicuous ; some of the larger grains are broken ; these in 

 extinction agree best with andesine, but possibly oligoclase may be 

 also present ; a small zircon is seen, and a few little vesicles. The 

 base is a glass crowded with microliths, probably felspar, and 

 tinted with ferrite. Thus the rock is a hornblende-andesite. 



In this block fulgurites are abundant, commonly perforating it, 

 but occasionally forming channels on the surface. The tubes not 

 seldom branch irregularly and vary in size. When fairly circular 

 in shape the diameter ranges from about one-fifth to one-third of an 

 inch, but the latter measurement is exceeded at a fork. The 

 irregularity of their course makes a precise statement impossible. 

 Where the tube shows on a broken surface we see that it is coated 

 at first with a film, hardly so thick as the finger-nail, of a reddish- 

 white or warm-grey tint, over which is a layer of glass of a greenish 

 colour, not unlike some of the lighter-coloured varieties of common 

 bottle glass, and occasionally presenting in this respect and in 

 fracture a superficial resemblance to some varieties of olivine. 

 This glass appears not to form a perfectly regular coating, for 

 sometimes it is almost absent, at others it occurs in patches, the 

 thickness of which is about one-eighth of an inch ; while at others 

 the tubes are almost choked by it, but in the last case it forms 

 an aggregate of separate clots rather than a solid mass ; the 

 interstices, in many instances at least, being due to bubbles. In 

 a section of one branching tube the aggregated glass extends for 

 about two inches inwards from the surface and for about one inch 

 in a transverse direction at a short thick offshoot. 



On careful examination of the material I came to the conclusion 

 that a thin slice could not be made, and that the attempt to obtain it 

 might seriously damage the specimen, so that I have studied the 

 glass by crushing some small fragments and examining the powder 

 under the microscope. The fragments vary in form, but flattish 

 chips are not uncommon, and occasionally they have a rather fibrous 

 structure ; curved surfaces are visible, probably indicating the 

 former presence of bubbles. The glass now appears to be of a very 

 pale-yellow tint ; clear, and containing but few enclosures. These 

 are either minute bubbles or very small flaky microliths of ferrite 

 and opacite. 



In the fulgurites on a specimen of a very similar rock (measuring 

 about 4x3x2 inches), which was " lying loose in quantities on 

 the summit," the tubes run slightly smaller than in the last 

 specimen ; the glass is similar in appearance, but just a shade darker 

 in colour. With a strong lens many minute vesicles, or their broken 

 surfaces, can be seen. Under the microscope the glass is almost the 

 same as in the other case, except that it seems slightly more strongly 

 coloured ; bubbles and definite flakes are perhaps rather less 

 numerous, but here and there are tiny brown patches. The white 

 film mentioned above occurs in specimens from both ; it appears to 



