IGNEOUS AGENCIES. 157 



thirty miles per minute, or even much more. Sometimes 

 the spread is equally rapid in all directions, and the spread- 

 ing wave is circular, or nearly so ; sometimes it is more 

 rapid in one direction than another, and the spreading 

 wave is elliptical. 



Cause of Earthquakes. The origin of earthquakes 

 being deep beneath the surface and hidden from obser- 

 vation, their cause is very obscure. Yet their association 

 with other forms of igneous agency suggests probable 

 causes : 



1. Volcanic eruptions, especially of the explosive type, 

 are always accompanied by slight and sometimes by seri- 

 ous earthquakes. This fact suggests the sudden formation 

 of gases or the sudden collapse of vapors as a possible 

 cause. On this view an earthquake would be like the 

 earth-jar produced by a mine-explosion, or by the explo- 

 sion of large quantities of gunpowder or nitro-glycerine. 



2. But great earthquakes are oftener associated with 

 bodily movements of extensive areas of the earth-crust. 

 Thus, for example, in 1835, after a severe earthquake on 

 the western coast of South America, it was found that the 

 whole coast-line of Chili and Patagonia was raised from 

 two to ten feet above sea-level. Again, in 1822, the same 

 phenomenon was observed in the same region after a great 

 earthquake. Again, in 1819, after a severe earthquake 

 which shook the delta of the Indus, a tract of land fifty 

 miles long and sixteen miles wide was raised ten feet, and 

 an adjacent area of 2,000 square miles was sunk, and 

 became a lagoon. In commemoration of the wonderful 

 event, the elevated tract was called Ullah bund, or, the 

 mound of God. Again, in 1811, a severe earthquake 

 perhaps the severest (except the Charleston earthquake of 

 August, 1886) ever felt in the United States shook the 

 valley of the Mississippi. Coincidently with the shock, 

 large areas of the river-swamp sank bodily, and have ever 

 since been covered with water. In commemoration of the 



