Ill 
TUBES FORMED BY LIGHTNING 
61 
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 
1 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. 
