538 THE POPULAR SCIENCE MONTHLY. 



Dr. Gilbert describes * the fulgurites found by Humboldt upon the 

 high peaks of the Nevada de Toluca, in Mexico. The rock on which 

 they were found is described as a trap porphyry taken from the summit 

 of the Pic del Fraile at a height of 2,364 " toisen " (about 4,621 me- 

 tres). Masses some two feet square are said to occur covered with a 

 thin layer (about one half of a millimetre) of a pistacio-green glass. 

 In one instance, where a feldspar crystal intervened, the glass was 

 white. The peculiar shimmer of this glass in the sun led Humboldt 

 to ascend this precipitous peak even at the risk of his life. In many 

 places the rock was completely pierced by small cylindrical tubes, 

 which were lined with the same greenish glass, and this was found 

 to resemble closely the glass from the inner walls of the tubes found 

 at the Senner Heide. 



The most interesting and, so far as I am aware, the only investiga- 

 tion of this kind undertaken in America is that of Mr. J. S. Diller on 

 fulgurite from Mount Thielson, in Oregon.f The summit of this 

 mountain is described as being composed of a hypersthene basalt, very 

 precipitous and difficult of ascent. The fulgurite occurs both as a super- 

 ficial coating on the rock, and in the form of tubes. " Although spread 

 over a considerable surface, it is not evenly distributed, but is arranged 

 in patches of drops and bubbles of glass in very much the same way 

 as paint which has been put upon a greasy surface." The glass is 

 described as translucent and of a greenish color. In places the tube 

 penetrated the rock to a depth of a few inches, having a diameter of 

 from 10*5 to 20 millimetres. These have a glassy lining of some two 

 millimetres in thickness. The tubes are not regarded as having been 

 produced by the lightning itself, but to be pre-existing tubes and cavi- 

 ties (the rock is naturally spongy), into which the fluid passed and 

 fused the walls. The chemical composition of this glass was found 

 to be silica, 55*04 per cent ; alumina and iron, 28*99 per cent ; lime, 

 7*86 per cent ; magnesia, 5*85 per cent ; potash and soda not deter- 

 mined ; loss by ignition, 1*11 per cent ; total, 98*85 per cent. This 

 was also found to be practically the composition of the groundmass, 

 or non-crystalline portion of the rock. 



The fulgurite found on the summit of Dom du Gonte, one of the 

 peaks forming a part of the chain of Mont Blanc, has been recently 

 described by Rutley.J The rock is represented as a hornblendic 

 gneiss. The fused material formed merely a superficial coating in the 

 form of attached globules or irregularly fused pellets and blotches of 

 brownish, black, or white glass. The white glass resulting from the 

 fusion of the feldspar, while the dark in like manner was of horn- 

 blendic origin. This occurrence of the two glasses, unmixed even 



* " Ann. dcr Physik.," B. 61, 1819, pp. 261 and 315. See also "Ann. de Chim. et de 

 Physiqne," pp. 281 and 299. 



f " Am. Jour. Sci.," vol. xxviii, October, 1884, p. 252. 

 % " Quar. Jour. Geol. Soc.," vol. xli, 1885, p. 152. 



