LESSONS IN GEOLOGY. 327 



origin, when the irregularity was initiated, the pressures due to 

 unequal contraction would continue to intensify them. 



Sometimes under this pressure the crust yields suddenly, is 

 fractured for a great distance, the rocks on one side of the frac- 

 ture being elevated or depressed with reference to the other. Thus 

 an earthquake is caused, a fissure vein is opened and a fault is 

 formed. The fault may be only a few feet at first, but it may in- 

 crease to hundreds of feet as it slowly yields to the continued 

 pressure. Other forces may cause earthquakes, as explosions of 

 superheated water or of gas, possibly in connection with volcanic 

 eruptions. These shocks cause vibrations of the crust, which 

 are often transmitted for immense distances. These waves may 

 be severe enough to overthrow buildings, produce avalanches 

 and land slips, sometimes changing springs and streams of water. 

 Again they cause great waves of the sea, which rush upon the 

 shore with great violence; in fact, the greatest destruction of life 

 by earthquakes has been from the ocean waves which they caused. 

 A wave that destroyed Lisbon in 1755 was nearly sixty feet 

 high. In 1868 an earthquake devastated the coast of Peru. 

 And less than half an hour after the earth wave came several 

 water waves from 50 to 60 feet high, adding greatly to the 

 destruction already begun. These waves reached the Sandwich 

 Islands, 5,580 miles, in 12 hours, and Japan, 10,000 miles dis- 

 tant, the next day. Such waves have been known to move at 

 the rate of 450 miles per hour. An earthquake which occurred 

 near Naples in 1857, by careful examination was shown to have 

 originated in a fissure about nine miles long, through about 

 three miles of rock, the center being about six miles below the 

 surface. The velocity of the earth wave in this case was between 

 nine and ten miles per minute. 



No portion of the earth is free from earthquakes, but they are 

 more common in volcanic regions. 



Again, a fissure is formed as noted above, and from it is poured 

 out vast quantities of molten matter or lava. Such fissure erup- 

 tions were frequent during Tertiary times in the Rocky Mountain 

 region and other localities. At the present time a volcano ia 



