70 CHARLES DARWIN 



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 evolu- 

 tions. 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 dis- 

 tance 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 particu- 

 lar those from Drigg in Cumberland, described in the 

 Geological Transactions. 10 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 



"Geolog. Transact, vol. ii. p. 528. In the Philosoph. Transact (1700, 

 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. 



