Ill TUBES FORMED BY LIGHTNING 6i 



they seem to have pleasure in society, and are not solely brought 

 together by the attraction of a common prey. On a fine day a 

 flock may often be observed at a great height, each bird wheel- 

 ing round and round without closing its wings, in the most 

 graceful evolutions. This is clearly performed for the mere 

 pleasure of the exercise, or perhaps is connected with their 

 matrimonial alliances. 



I have now mentioned all the carrion-feeders, excepting the 

 condor, an account of which will be more appropriately intro- 

 duced when we visit a country more congenial to its habits than 

 the plains of La Plata. 



In a broad band of sand-hillocks which separate the Laguna 

 del Potrero from the shores of the Plata, at the distance of a few 

 miles from Maldonado, I found a group of those vitrified, sili- 

 ceous tubes, which are formed by lightning entering loose sand. 

 These tubes resemble in every particular those from Drigg in 

 Cumberland, described in the Geological Transactions} The 

 sand-hillocks of Maldonado, not being protected by vegetation, 

 are constantly changing their position. From this cause the 

 tubes projected above the surface ; and numerous fragments 

 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 blowpipe. The 

 sand is entirely, or in greater part, siliceous ; but some points 



' Geolog. Transact, vol. ii. p. 528. In the Philosoph. Transact. (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. 



