344 



GEOLOGY 



about 3.9 miles (6.25 km.) for the second set that goes through the 

 earth. 1 



Distribution of earthquakes. Over large portions of the globe, 

 severe earthquakes are rare, but in certain regions they are, un- 

 fortunately, frequent. In general, earthquakes are likely to be 

 rather frequent where geologic changes are in rapid progress, as 

 along belts of young mountains where the stresses are not yet 

 adjusted, or at the mouths of great streams where deltas are accu- 



\ 



Slegen* N x x 



\ 



wied 



Fig. 278. Coseismal lines for each minute, .Herzogenrath (Germany), earth- 

 quake of October 22, 1873. (Lasaulx.) 



mulating, or about volcanoes where temperatures and strains aro 

 changing, or on the great slopes, particularly the submarine slopes, 

 where adjustments to inequalities of stress are in progress. Not 

 a few, however, occur where the special occasion is not obvious. 



The Geologic Effects of Earthquakes 



Geologically, earthquakes are of less importance than many 



gentler movements and activities. Disastrous as they sometimes 



are to human affairs, they leave few distinct marks which are more 



than temporary. During the passage of notable earthquake 



1 The Physics of Earthquake Phenomena, C. G. Knott, 1908, p. 226. 



