EARTHQUAKES IN GEOLOGY 



crevice. The most remarkable revelation of the process 

 of lake draining during earthquake shocks was furnished, 

 however, according to Professor Hobbs, by the former 

 Lake Eulalie near New Madrid. After the shocks of 

 1812 the lake completely disappeared. On the lake 

 bottom thus exposed there was revealed a series of fissures 

 down the funnel-shaped openings which the waters had 

 disappeared. 



It will be seen from many of the foregoing instances 

 that whatever are the principal causes of earthquakes, 

 they must have played a great part in the shaping of 

 events in the geological past ; and the only limitation 

 which we can place in the importance of the part they 

 played will depend on whether we regard the earth- 

 quake as having been caused by a movement of the 

 underlying strata, or whether we believe that the same 

 cause which produces earthquakes may produce alterations 

 in the lie of the strata themselves. In the next chap- 

 ter we shall describe some of the effects produced on land 

 by earthquakes. But impressive as some of these effects 

 are, it is by no means certain that the greatest earth- 

 quakes take place on land at all. They may take place 

 at sea, deep underneath the ocean. Our opportunities for 

 observing such quakes, however, are much smaller than 

 are afforded by land earthquakes. The instruments 

 which have been devised for observing earth tremors will 

 measure the smallest of such disturbances, and will 

 record earthquakes the centres of which are thousands of 

 miles away. These delicate instruments often record 

 distant earthquakes, the exact locality of which is never 



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