198 THE GEOLOaiST. 



myself to those tubes which are hollow ; 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 fulgiirites, 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 cnly 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 hornst one-porphyry, 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 descriptions given of trees struck 

 by the electric fluid, when there have been found at the depth of from 



* Trans. Geol. Soc. vol. ii. + Annales de Chim. et Phy. torn. 37. 



