34 



PHYSICAL GEOLOGY 



condensed from the lower layers of the air, absorb and furnish to the 

 soil more gases and inorganic matter than rain. (5) Rain water is 



also effective in causing 

 certain rocks to swell. In 

 excavating the Panama 

 Canal it was found that 

 certain rocks which, when 

 first uncovered, had to 

 be blasted before they 

 could be removed, be- 

 came so soft after a few 

 months' exposure to the 

 tropical rains that they 

 could be excavated with 

 the steam shovels. The 

 slides which have oc- 

 curred in the Culebra 

 Cut were due both to the 

 softening of the rock in 

 this way and to gravity 

 which tends to cause the 

 rock to move toward the 

 excavation. The soften- 

 ing action is taken advantage of in extracting diamonds from the 

 inclosing rock in the South African mines. 



5. Wind. — The mechanical work of the wind carrying sand is 

 very effective in wearing away rock, especially in arid regions, 

 and will be discussed on another page (p. 45). 



6. Lightning. — When lightning strikes the earth it sometimes 

 fractures large masses of rock. When it strikes sand, drops or 

 bubbles of glass and irregular tubes or rods are sometimes formed by 

 the partial fusion of the soil. These fulgurites, as they are called, 

 are seldom more than a few inches long, but are sometimes several 

 feet in length and two and a half inches in diameter. The entire 

 summit of Little Ararat in western Asia, where electrical storms are 

 extremely common, is said to be drilled by lightning. " A piece of 

 rock about a foot long may be obtained, perforated all over with irreg- 

 ular tubes, having an average diameter of three centimeters. Each 

 of these is lined with a blackish-green glass." (A. Geikie.) This is, 

 however, unusual, and the total effect of lightning is inconsiderable. 



Fig. 9. — The work of rain water in sculpturing 

 a bank of earth containing bowlders, near Bogen, 

 Tirol. 



