THE CAUSES OF EARTHQUAKES 



when examining the geological earth strata. One 

 stratum, instead of lying evenly with another where 

 a crack has occurred, rests a little above it or below 

 it. This inequality or unevenness geologists call a fault. 

 Now we can easily see that whenever, and by whatever 

 causes, a fault is produced, there is probably at the 

 same time an earthquake. The fault cannot be pro- 

 duced without a great and shaking disturbance. Mr. 

 John Milne, the most distinguished of British author- 

 ities on earthquakes, says that all large earthquakes 

 originate from the formation or extension of these 

 " faults " or great cracks in the strata. 



The occurrences of these "faults'" are most frequent 

 when the process of mountain building has been going 

 on over the earth. In other words, if we imagine a 

 period in which some great continental area of the 

 world's surface was slowly rising above the oceans, then 

 at that time we should expect that there would have 

 been great " faults " occurring in the strata, and great 

 earthquakes following them. To quote his words : 

 "If therefore we wish to know when earthquakes were 

 greatest and most frequent, we must turn to those 

 periods in geological history when mountain ranges 

 were built, when volcanic activity was pronounced, and 

 when great 'faults' were made. The first of these 

 periods would be coincident with the creation of the 

 Ural Mountains, the Grampians, and other mountain 

 ranges. This took place in the earliest geologic times. 

 Another period of mountain formation was when the 

 Himalayas and the Alps were slowly but intermit- 



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