56 MALDONADO. [CHAP. in. 



These vultures certainly may be called gregarious, for 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 wheeling 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 Laguua 

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

 miles from Maldonado, I found a group of those vitrified, siliceous 

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

 tub?s resemble in every particular those from Drigg in Cumber- 

 land, described in the Geological Transactions.* The sand- 

 hillocks of Maldonado, not being protected by vegetation, arc 

 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 are of a black colour, and 

 from their glossy surface possess a metallic lustre. The thickness 



* 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. 



