F. ETJTLET ON BOTJTEILLENSTErN". 155 



more resemble those of flint cores from which flakes have been 

 artificially struck, or the surfaces of prisms of starch. They 

 depolarize strongly from strain, and the glass immediately sur- 

 rounding them is similarly afi'ected, so that each glass enclosure is 

 bordered by a nimbus which is traversed by dark brushes. The 

 similarity of these glass enclosures to the one figured from the 

 fulgurite of the Lorn du Goute is very striking. A great number 

 of capillary glass rods pass obliquely through the preparation and 

 are cut off" by the planes of section, so that from a mere examination 

 of the slide one can form only an imperfect estimate of the 

 enclosures. The section shows one doubly refracting microlith, 

 about j-jfo^h inch in length, with a dark zone. A few other 

 diminutive doubly refracting specks are also visible under a 

 tolerably high magnifying power. Some of the gas-pores are large, 

 and, through grinding, have become filled with emery mud. They 

 thus fail to show the bright central spot, and merely appear as 

 circular, black disks. What looks like a clear glass ball, containing 

 a nearly opaque brownish sphere about Jth of its diameter, and 

 numerous minute angular granules, chiefly of glass, occurs in one 

 part of the section. This ball is about -^th. inch in diameter, and 

 causes a marked deflection of the banding in the adjacent glass ; but 

 in this case there is no strain developed, both the ball and the 

 adjacent glass remaining dark during rotation between crossed 

 nicols. I have no doubt that this is merely a hemispherical cavity 

 (part of a gas-bubble) filled with Canada balsam and crumbs detached 

 from the section during mounting. At another spot an opaque 

 circular disk (part of a gas-bubble filled with emery) has also produced 

 a deflection in the banding *. Although bouteillenstein is certainly 

 a remarkably pure natural glass, the glass of the fulgurite from the 

 Dom du Goute is still purer, and while the former possesses a well- 

 marked banded structure and a few microliths, the latter is absolutely 

 structureless, so far as can be seen with an amplification of over 

 1000 linear. That bouteiUen stein is an obsidian is denied by Ma- 

 kowsky t ; but (apart from its banded structure) its glass enclosures 

 and numerous gas-bubbles and its almost perfect freedom from any 

 products of crystallization, render its comparison with fulgurite 

 not merely admissible but possibly instructive. The bibliography 

 of fulgurite consists, I believe, chiefly of notes by De Saussure and 

 Humboldt, and more recent papers by Abich, G. Eose J, Giimbel §, 

 Harting ||, Eomer %, "Wichmann**, and DiUerft- 



* That the strain in the bands results from a tension or stretching-out, and 

 not from a compression at right angles to the direction of the banding, cannot 

 be proTcd, since, in either case, one would expect an elongation of the gas-bubbles 

 b?/ compression. In this instance the pressure of the gas upon the wall of its 

 cavity has been greater than the pressure upon the surrounding glass. 



t Ueber die Eouteillensteine von Mahren u. Bohmen, in Min. Mittheil. vol. iv. 

 p. 43. 



I Zeitschr. d. d. G-eol. Ges. xxv. p. 112. § Ibid, rxxiv. p. 642. 



II Soc. Batav. Amsterdam, 1873, p. 13. f N. Jahrb. f. Min. 1876, p. 33. 

 ** Zeitschr. d. d. Geol. Gess. xxxv. p. 849. 



tt Amer. Journ. Sci. xxviii. p. 252 



