GIBB. — A CHAPTER ( )X FOSSIL LIGHTNING. 



197 



compared to a shrivelled vegetable stalk, or the bark of certain trees, 

 particularly the elra, the birch, aud the cork. These furrows and 

 creases are considered to be the result of the compression of the sur- 

 rounding loose sand, while the tube was still softened from the effects 

 of the intense heat. Perfectly cylindrical fragments, free from furrows, 

 have, however, been discovered, with a circumference of four inches. 

 By the aid of a small magnifying glass, the external surface of the 

 tube is seen to be covered with a crust of agglutinated sand or quartz, 

 each particle being visibly surrounded by vitreous matter, the grains 

 being also round in form, some having a slightly glazed appearance. 

 The internal surface of the tubes is quite smooth and glassy, and com- 

 pletely vitrified, u ith a corresponding irregularity of form to that of 

 the exterior. Extreme compression- acting on the tube in its soft 

 state, has in some cases caused the opposite sides of many of the 

 furrows to come into contact and be welded together ; in this way the 

 tube is occasionally so much flattened as to be obliterated. The in- 

 ternal glassy smoothness is brilliant and porcellaneous, and resembles 

 many mineral substances, particularly opal and hyalite. 



The colour of these tubes varies according to the nature of the sand 

 into which the lightning has penetrated. Those from Drigg aud 

 La Plata ai'e of a hght-drab colour, and, if minutely examined, nume- 

 rous black specks are seen, with what seem to be air-blebs, or 

 perhaps bubbles of steam. It is the white quartzose grains which 

 have become vitrified in all the specimens, imparting to the tube-mass 

 a sufficient amount of hardness to permit the scratching of glass, and 

 even the striking of fire with steel; they are nevertheless easily broken. 

 All the vitrified tubes from Natal are of a dark ferruginous brown 

 colour, owing to impregnation with iron; the largest of these are not 

 quite so dark as the smaller. In certain localities the tubes have 

 been found, firstly on passing through the soil almost black, then 

 yellowish-grey, still lower of greyish-white, and finally colourless; 

 these variations depending upon the purity of the sand from, or its 

 admixture with, other substances. The electric fusion is not solely 

 confined to the less refractory quartz, but in some instances at Drigg 

 has extended to pebbles of hornstone-porphyry, many of which were 

 partly vitrified. 



In the foregoing description of " recent lightning," I have confined 



VOL. II. Q 



