
Parr, Seer.i.§1.] LIGHTNING. 319 
loose sand or more compact rock, tubes termed fulgurites, which 
range up to 24 inches in diameter. ‘These descend vertically but 
sometimes obliquely from the surface, occasionally branch, and 
rapidly lessen in dimensions till they disappear. ‘They are formed 
by the actual fusion of the particles of the soil or rock surrounding 
-the pathway of the electric spark. They have been most frequently 
- found in loose sand. Abich has observed examples of such tubular 
perforations with vitreous walls in the porous reddish-white trachyte 
at the summit of Little Ararat. A piece of the rock about a foot 
long may be obtained perforated all over with irregular tubes 
having an average diameter of 3 centimetres. Each of these is 
lined with a blackish green glass, due to the fusion of the rock 
by the passage of the electric spark through it. As the whole 
summit of the mountain, owing to its frequent storms, is drilled in 
this manner, it is evident that the action of lightning may consider- 
ably modify the structure of the superficial portions of any mass 
of rock exposed on lofty eminences to frequent thunderstorms. 
Humboldt collected fulgurites from a trachyte peak in Mexico, and in 
two of his specimens the fused mass of the walls has actually over- 
flowed from the tubes on the surrounding surface.’ 
Effects of changes of temperature.—Of far wider 
geological importance are the effects that arise among rocks and 
soils from the alternate expansion and contraction caused by daily 
or seasonal changes of temperature. In countries with a great 
annual range of temperature considerable difficulty is sometimes 
experienced in selecting building materials liable to be little 
affected by rapid or extreme variations in temperature, which induce 
an alternate expansion and contraction that prevents the joints of 
masonry from remaining close and tight.” If the daily thermo- 
metric variations are large, the effects are frequently striking. In 
Western America, where the climate is remarkably dry and clear, 
the thermometer often gives a range of more than 80° in the twenty- 
four hours. Thus in the Yellowstone district, at a height of 9000 
feet above the sea, the author found the temperature of rocks ex- 
posed to the sun at noon to be more than 90° Fahr., and the thermo- 
meter at night to sink below 20°. In the Sahara and other African 
regions, as well as in Central Asia, the daily range is even greater. 
This rapid nocturnal contraction produces a strain so great as to 
disintegrate rocks into sand, or cause them to crack or peel off in 
skins or irregular pieces. Dr. Livingstone found in Africa (12° S. 
lat., 34° H. long.) that surfaces of rock which during the day were 

1 G. Rose, Z. Deutsch. Geol. Gesch. xxv. p. 112. 
2 In the United States, with an annual thermometric range of more than 90° Fahr., 
this difficulty led to some experiments on the amount of expansion and contraction in 
different kinds of building stones, caused by variations of temperature. It was found 
that in fine-grained granite the rate of expansion was 000004825 for every degree Fahr. 
of increment of heat; in white crystalline marble it was ‘000005668; and-in red sand- 
stone ‘000009532, or about twice as much as in granite. Tottenin Silliman Amer. Journ: 
xxii. p. 136. See ante, p. 284. : 
