ON ITS EXTERIOR. EARTHQUAKES. 199 



fears have been realised by the sudden appearance of volcanic 

 islands ; and a remarkable instance occurred in the eleva- 

 tion of the volcano of Jorullo, 1682 English feet above the 

 ancient level of its site and of the plain in which it now stands, 

 which took place the 29th of September, 1759, after eighty 

 days of earthquakes and subterranean thunder. 



If we could obtain daily intelligence of the condition of 

 the whole surface of the earth, we should very probably 

 arrive at the conviction that this surface is almost always 

 shaking at some one point ; and that it is incessantly affected 

 by the reaction of the interior against the exterior. The 

 frequency and universality of a phenomenon, which probably 

 owes its origin to the high temperature of the interior and 

 deep-seated molten strata, explain its independence of the 

 nature of the rocks in which it manifests itself. Earthquake 

 siiocks have been felt even in the loose alluvial soil of Hol- 

 land, Middelberg, and Flushing (23d Feb. 1828). Granite 

 and mica slate are shaken, as well as limestone and sand- 

 stone, trachyte and amygdaloid. It is not the chemical 

 nature of the constituent particles, but the mechanical struc- 

 ture of the rocks, which modifies the propagation of the 

 shock or of the wave which occasions it. Where such a 

 wave proceeds in a regular course along a coast, or at the 

 foot of and parallel to the direction of a mountain chain, 

 interruptions at certain points have sometimes been re- 

 marked, and continue for centuries ; the undulation passes 

 onward in the depth below, but it is never felt at those 

 points of the surface. The Peruvians say of these upper 

 strata which are never shaken, that they form a bridge. ( 189 ) 

 As the mountain chains themselves appear to have been 

 elevated over fissures., it may be that the walls of these 



