198 
THE GEOLOGIST. 
myself to those tubes which are liollow ; but, as I have stated, occa- 
sionally this character is wanting, and many of them terminate in the 
solid state, it is only necessary then to mention that even the vertical 
penetration is characterised by the same peculiarity; thus of the 
fulgurites from Natal, very small specimens do not appear to be 
tubular, but resemble the horizontal cylinders from Dresden. 
That the formation of fulgurites, whether recent or fossil, is due to 
the vitrification of the siliceous matter through which the electric 
fluid passes, we have clear proofs by our knowledge not only of the 
meteoric phenomena of the districts in which they are found, but also 
by the results of various series of experiments which have been from 
time to time instituted to produce them artificially. In relation to 
the first, the neighbourhood of the Rio Plata, close to where Mr. 
Darwin found them, was known to be remarkably subject to electric 
phenomena; and the sand-hillocks of Drigg, in Cumberland, from 
presenting themselves as direct objects to clouds coming from the sea, 
the marshes of Irt only intervening, are favourably situated for pro- 
moting electrical discharges. The experiments made seem, too, very 
conclusive, and are full of interest. The Drigg-sand consists of quartz 
and hornstone-porjjhj'ry, and, when submitted to the action of a power- 
ful blow-pipe, formed a clear glass mingled with olive discolorations 
resembling the fulgurites * MM. Hachette, Savart, and Beudant 
feebly imitated these bodies by the shocks of one of the most power- 
ful galvanic batteries in Paris on powdered quartz, the result being 
small tubes of an inch in length. t I have experimented upon various 
kinds of sand with a powerful blowpipe, and have produced glass 
varying in colour according to the composition of the sand. The 
majority of light-coloured sands I find to assume a slight rusty colour, 
on being submitted to a considerable heat, before actual fusion occurs. 
All these, however, are but feeble evidences of the power of electricity 
prepared by ourselves, when contrasted with that which constitutes 
the lightning's flash, the intensity of which can be estimated by the 
descriptions given of these remarkable bodies. Were any further 
evidence required to prove these fulgurites to be the result of light- 
ning, it is only necessary to refer to descri^Jtions given of trees struck 
by the electric fluid, when there have been found at the depth of from 
* Trans. Geol. Soc. vol. ii. + Annalcs dc Chini. et Phy. torn. 37- 
