EARTHQUAKES 279 



also occur where there has been an overloading with sediment. The 

 great earthquake of the Mississippi Valley in 181 1 may have been 

 caused b)^ a readjustment of the crust as a result of the great weight 

 of sediment laid down there in recent geologic time. Boundaries 

 were thrown into such confusion as a result of these shocks that it was 

 necessary for the government to make a resurvey of 1,000,000 acres. 



Fig. 278. — Earthquake regions of the Eastern Hemisphere, shown in black. 

 (After Montessus de Ballore.) 



The earthquake " danger spots " of the United States are situated 

 on the Pacific coast, in the Great Basin (Utah), and in the lower 

 Mississippi Valley. New England has been remarkably free from 

 severe shocks, although many slight earthquakes have been recorded. 

 In the last-named region the danger is greatest where lines of fracture 

 intersect. Over the world as a whole two zones are recognized in 

 which earthquakes are most frequent : the Mediterranean belt, which 



