THE UNSTABLE LAND. 105 



cleft open for a length of more than eighteen miles, and in 

 some places, the fissure was several yards in width. In 

 another place, a cleft occurred 131 feet deep, over a mile long 

 and 32 feet wide. Sometimes the disturbances of an earth- 

 quake leave the surface permanently elevated or permanently 

 depressed. In one remarkable instance in the country of 

 Cutch, the Great Runn sank down over an extent of some 

 thousands of square miles, so that, during a part of the year, 

 it remained inundated by the sea, and during another part 

 was a desert without water. 



A few other facts connected with earthquakes must be 

 mentioned, and then we will endeavor to ascertain how they 

 are caused. Through the monumental patience and industry 

 of M. Perrey, we have been placed in possession of thousands 

 of statistics of earthquakes between 1842 and 1850. Through 

 the similar labors of Mr. Robert Mallet and his son John W. 

 Mallet, we have been placed in possession of the facts respect- 

 ing other earthquakes between 1606 B. C. and 1842. From 

 both sources we have a record of six or seven thousand sepa- 

 rate earthquakes. The laborious discussion of this immense 

 catalogue shows very important results respecting the distri- 

 bution of earthquake occurences through the year. Earth- 

 quakes are found to occur most frequently at new and full 

 moon ; also, more frequently at perigee than at apogee ; also, 

 more frequently when the moon is on the meridian than when 

 in the horizon ; also, more frequently in winter than in sum- 

 mer; and finally, more frequently at night than during the 

 day. The results of this interesting analysis we will presently 

 apply to earthquake theory. 



It requires but little acquaintance with the phenomena 

 usually ascribed to earthquakes, to discern that they do not all 

 belong to one class. Most widely destructive earthquakes are' 

 characterized by vibrations of the earth, and these alone are 

 admitted by Mallet as true earthquakes. These may be de- 

 nominated earthquakes of vibration. Other movements of the 

 earth are translatory. The surface is bodily uplifted or 

 depressed, or both alternately. This may be denominated an 



