GIBU. — A CflAPTEU ON' FOSSIL LIGHTNING. 
I!»7 
compared to a sliri veiled vegetable stalk, or the bark of certaiu trees, 
particularly the elm, the birch, and 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, witli 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 and 
La Plata are of a light-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 az'e 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 
