76 MALDONABO. 



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 Maldo- 

 nado, 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 diam- 

 eter of the whole tube was nearly equal, and there- 

 fore 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 



* 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. 



