182 REACTION OF THE INTERIOR OF THE EARTH 



a definite degree of tension, which it was necessary that 

 the vapours should reach before they could burst through 

 the molten mass in the interior of 'the cone of cinders. 

 Just as in the case described, in which no shocks were 

 felt on the sides of the cinder-cone of Vesuvius, so in 

 a quite analogous phenomenon, though on a far grander 

 scale, on the cone of ashes of the volcano of Sangai, which 

 rises south east of the city of Quito to a height of 

 17,100 English feet, a very distinguished observer, Herr 

 Wisse, having (in December 1849) approached within a 

 thousand feet of the summit and crater, perceived no 

 quaking of the ground, ( 252 ) although 267 explosions 

 (eruptions of scoriae) were counted in the course of a 

 single hour. 



A second, immensely more important, class is the very 

 numerous one which consists of earthquakes usually 

 accompanying or preceding great eruptions, whether of 

 volcanoes which, like our European ones, pour forth 

 streams of lava ; or of volcanoes which, like Cotopaxi, 

 Pichincha, and Tunguragua in the Andes, send forth 

 only scoriae, ashes, and vapours. It is especially in 

 regard to this class that volcanoes may be viewed as 

 safety-valves, or vents, according to the early remark 

 of Strabo on the fissure from which lava flowed, near 

 Lelante, in the island of Eubcea. The earthquakes 

 cease when a considerable eruption has taken place. 



The most wide-spread devastations ( 253 ) are those occa- 

 sioned by earthquake-waves which traverse partly non- 

 trachytic and non-volcanic countries, and partly trachytic 

 and volcanic ones, as the Cordilleras of South America 

 and Mexico, without exercising any influence on the 

 neighbouring volcanoes. These form the third class or 



