GIBB — A CHAPTER ON FOSSIL LIGHTNING. 



201 



the surface, which were vitrified and glassy in their interior, and the 

 chalk itself affected for a short distance around them. They were 

 quite or nearly straight; in size, from half an inch to an inch in 

 diameter, and were exposed in a fissure of the chalk. About three 

 feet of these tubes were visible, and then they became lost. 



The finest examples of these lightning-tubes which have come under 

 my notice in the London museums are those from Drigg, in Cumber- 

 land, in the Museum of the Geological Society, and which have been 

 described in that Society's Transactions. They appear to be the most 

 perfect, although larger tubes have been found at Natal. In speaking 

 of these I shall combine a general description of the whole. 



It is in banks, hillocks, or mounds of loose sand, that they are gene- 

 rally met with, sometimes, however, in immense hollows, or occa- 

 sionally on declivities of mounds of sand. At Drigg they were 

 discovered in the middle of sand-banks forty feet high, between the 

 river Irt and the sea, but projecting above the surface from the drifting 

 away of the sand. These sand-banks are exceeded both in extent and 

 height by those at Eskmeols, in the same county. On the shores of 

 La Plata, in South America, Mr. Darwin found them in the sand- 

 hillocks of Moldonado, which were constantly changing their position 

 from not being protected by vegetation. This circumstance caused 

 them to be found projecting above the surface, as at Drigg, and Mr. 

 Darwin, inferred, from finding numerous fragments lying about, that 

 they had at one time been buried at a greater depth.* These 

 occurred in a level area of shifting sand, situated among some high 

 sand-hillocks, distant about half a mile from a chain of hills 400 and 

 500 feet high. In the Sonne, Westphalia, similar tubes were found 

 on the declivities of mounds of sand thirty feet high; but occasionally 

 some were noticed in cavities, described as hollowed out in the heath 

 in the form of great bowls, 200 feet in circumference, with a depth of 

 from twelve to fifteen feet. 



The position in which these tubes are found is one of some interest 

 and importance. As usually encountered, they run vertically into 

 the sand, at a varying depth. On the banks of La Plata four sets 

 entered the sand perpendicularly, and one was traced 5 J feet into the 

 * Darwin's Journal, vol. iii. 1839, p. 69. 



