1832-3.] TUBES FORMED BY LIGHTNING. 43 



there exists a tradition that these birds, at the time of the Conquest, 

 were not found near Monte Video, but that they subsequently followed 

 the inhabitants from more northern districts. At the present day they 

 are numerous in the valley of the Colorado, which is three hundred miles 

 due south of Monte Video. It seems probable that this additional 

 migration has happened since the time of Azara. The Gallinazo 

 generally prefers a humid climate, or rather the neighbourhood of 

 fresh water; hence it is extremely abundant in Brazil and La Plata, 

 while it is never found on the desert and arid plains of Northern 

 Patagonia, excepting near some stream. These birds frequent 

 the whole Pampas to the foot of the Cordillera, but I never saw or 

 heard of one in Chile : in Peru they are preserved as scavengers. 

 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 introduced 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, siliceous 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 are of a black colour, and from their 



* 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. 



