THE CAUSES OF EARTHQUAKES 



pressure of a screw was greater than it could bear, we 

 have considered cases where the pressure was slowly 

 applied. But a tap with a hammer or any sudden shock 

 would also produce a breakage and "faults," and an 

 earthquake on a small scale and it is possible that some 

 of the convulsions of nature and some of their permanent 

 effects are caused by sudden and violent causes. 



One such cause might be the violent and sudden 

 formation of steam by the contact of water with rock 

 at a very high temperature. Everybody knows what 

 happens to the kitchen boiler after a severe frost. The 

 frost clogs the pipes with ice, so that the kitchen boiler 

 becomes dry because no water is reaching it ; but it 

 continues to grow hotter and hotter till the iron plates 

 or iron lining become red-hot. Then the frost perhaps 

 gives way and a small amount of water finds its way into 

 the red-hot boiler. The water is converted instantly into 

 steam ; and, as a result, the boiler, if it is a weak one, is 

 blown out into the kitchen, causing grave personal in- 

 convenience to the cook. If the boiler is a strong one, 

 it may merely crack. Now, apply these considerations 

 to the instance of the earth, its oceans, its thin crust, 

 and its hot rocks situated at a depth of not more than 

 thirty miles, and perhaps at a good deal less depth than 

 that. What would happen if the ocean leaked through 

 into the strata of red-hot or molten rocks ? There would 

 be enormous quantities of steam formed; and if, owing 

 to the vast pressure of strata and water above, these 

 quantities of steam did not instantly produce violent 

 explosions, yet underneath there would be imprisoned, 



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