70 SAND TUBES FORMED BY LIGHTNING, [chap. iii. 



every particular those from Drigg in Cumberland, described 

 in the * * Geological Transactions. " The sand-hillocks of 

 Maldonado, not being protected by vegetation, are con- 

 stantly changing their position. From this cause the 

 tubes projected above the surface ; and numerous frag- 

 ments lying near, showed that they had formerly been 

 buried to a greater depth. Four sets entered the sand 

 perpendicularly : by working with my hands I traced one 

 of them two feet deep ; and some fragments which evidently 

 had belonged to the same tube, when added to the other 

 part, measured five feet three inches. The diameter of 

 the whole tube was nearly equal, and therefore we must 

 suppose that originally it extended to a much greater depth. 

 These dimensions are, however, small, compared to those 

 of the tubes from Drigg, one of which was traced to a 

 depth of not less than thirty feet. 



The internal surface is completely vitrified, glossy, and 

 smooth. A small fragment examined under the microscope 

 appeared, from the number of minute entangled air or, 

 perhaps, steam bubbles, like an assay fused before the blow- 

 pipe. The sand is entirely, or in greater part, siliceous ; 

 but some points are of a black colour, and from their 

 glossy surface possess a metallic lustre. The thickness 

 of the wall of the tube varies from a thirtieth to a 

 twentieth of an inch, and occasionally even equals a 

 tenth. On the outside the grains of sand are rounded, 

 and have a slightly glazed appearance : I could not dis- 

 tinguish any signs of crystallisation. In a similar manner 

 to that described in the ^'Geological Transactions," the 

 tubes are generally compressed, and have deep longitudinal 

 furrows, so as closely to resemble a shrivelled vegetable 

 stalk, or the bark of the elm or cork tree. Their circum- 

 ference is about two inches, but in some fragments, which 

 are cylindrical and without any furrows, it is as much as 

 four inches. The compression from the surrounding loose 

 sand, acting while the tube was still softened from the . 

 effects of the intense heat, has evidently caused the creases 

 or furrows. Judging from the uncompressed fragments, 

 the measure or bore of the lightning (If such a term may 

 be vised), must have been about one inch and a quarter. 



* " Geologic.il Transactions," vol. ii., p. 528. In the "Philosophical 

 Transactions" (1790, p. 294) Dr. Priestley has described some imperfect siliceous 

 tubes and a melted pebble of quartz, found in digging: into the ground, under 

 a tree, where a man had been killed by lightning. 



