CHAPTER XV 

 THE CAUSES OF EARTHQUAKES 



NOW that we have before us some of the examples 

 of the changes which historic earthquakes have 

 brought about in the face of the country, it is 

 easy to see what an important effect they must have 

 exerted in geological history. But there are still weighty 

 questions to be answered about earthquakes. We have 

 seen that an earthquake can contort, upset, and twist 

 the surface strata of the earth as easily as we can 

 crumple a sheet of cardboard. We have yet to find 

 whether the crumpling of the strata is always produced 

 by earthquakes, or whether an earthquake is the cul- 

 minating symptom that some agency is at work crumpling 

 the strata. Let us try to imagine an example on a 

 small scale. Suppose we take the top of a pill-box, 

 and, holding it in the crook between our thumb and 

 forefinger, compress it very tightly on all sides. What 

 will happen? The lid of the pill-box, being subjected 

 to stress or strain on all sides, will presently buckle and 

 crack. We shall have produced an earthquake on a 

 small scale, and there will be an earthquake fracture 

 perhaps an earthquake fissure. If the whole pill-box 



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