,9o6.] SEE— THE CAUSE OF EARTHQUAKES. 387 



lava or molten rock take place in every important earthquake ; 

 confining walls and caverns of unequal pressure are reduced to one 

 common strain, and the resulting motion involves rapidly acting 

 and enormously powerful forces, which may shake the whole earth 

 and sometimes crack or derange the overlying strata of rocks many 

 kilometers deep. 



§ 52. Explanation of the New Madrid earthquake and some 

 other earthquakes often classed as tectonic. 



In his excellent work on " Earthquakes " Major Dutton remarks 

 that the New Madrid earthquake could not be said to have any real 

 depth, although he says it was strongly felt as far east as Boston, 

 and in fact nearly all over the United States. His reasoning is 

 based upon the supposition that it was due to the sinking of some 

 bottom land, which seems to be quite unjustifiable. The subsidences 

 happened, indeed, but we shall find the best reasons for holding 

 that they were produced by explosions within the earth, which 

 must therefore have been at least ten miles deep. Unless it had 

 been of about this depth this earthquake could not have been so 

 strongly felt throughout so wide an area of country. 



It seems probable that the water seeped down from the Mis- 

 sissippi River, which always overflowed badly here, and worse in 

 prehistoric times than now, because the country was then much 

 more heavily timbered; or the seepage was a survival effect of the 

 fractures of the Ozark Mountains, from the time when the Gulf of 

 Mexico extended far up the river, and thus was beginning to form 

 a sea valley about parallel to the general trend of the Ozark 

 Mountains which were left unfinished. 



Thus the cause producing this earthquake is apparently not dif- 

 ferent from those at Charleston, San Francisco and many other 

 places which have experienced so-called tectonic earthquakes. 



The most trustworthy accounts of the New Madrid earthquake 

 show that it was the most violent and destructive earthquake ever 

 felt in the United States. Lyell has given an excellent account of 

 this earthquake, based upon observations gathered during a per- 

 sonal visit in 1846, but the records here followed are of still earlier 

 date. 



Over a region of 300 miles in length, but especially from the 



