THE DEPTH OF AN EARTHQUAKE CENTRUM. 221 



terranean forces may exert upon the crust above them is 

 evidently the height to which volcanoes eject materials. 

 Cotopaxi is said to have hurled a 200-ton block of stone 

 nine miles. Sir W. Hamilton tells us that in 1779 Vesu- 

 vius shot up a column of ashes 10,000 feet in height ; 

 and Judd tells us that this same mountain in 1872 threw 

 up vapours and rock fragments to the enormous height 

 of 20,000 feet. This would indicate an initial velocity of 

 1,131 feet per second. 



Notwithstanding Mallet's calculation that thirty miles 

 is the limiting depth for the origin of an earthquake, the 

 origin of the Owen's Valley earthquake of March 1872 

 was estimated as being at least fifty miles. ^ 



ForTYi of the focal cavity, — Among the various 

 problems which are put before those who study the 

 physics of the interior of our earth it would at first sight 

 appear that there was none more difficult than the attempt 

 to determine the form of the cavity, if it be a cavity, from 

 which an earthquake originates. Almost all investigators 

 of seismology have recognised that the birthplace of an 

 earthquake is not a point, and have made suggestions 

 about its general nature. The ordinary supposition is 

 that the earthquake originates from a fissure, and if the 

 focus of a disturbance could be laid bare to us it would 

 have the appearance of a fault such as we so often see 

 exposed on the faces of cliffs. 



A strong argument, tending to demonstrate that some 

 of the shakings which are felt in Japan are due to the 

 production of such fissures, is the fact that the vibrations 

 which are recorded are transverse to a line joining the 

 point of observation and the district from which, by time 

 observations, we know the shock to have originated. The 

 most probable explanation of this phenomena appears to 

 1 See Am. Jour. Sci. 1872. 



