ON ITS EXTERIOR. EARTHQUAKES. 183 



group of phenomena, and it is that which points most 

 strongly to the existence of a general cause in the 

 thermic constitution of the interior of our planet. To 

 this third group belongs also a case of rare occurrence 

 in which, in countries non- volcanic and rarely visited by 

 earthquakes, the ground trembles uninterruptedly for 

 several months, on a very restricted space, seeming to 

 presage an upheaval, and the formation of an active vol- 

 cano. This took place in the beginning of the present 

 century in the Piedmontese valleys of Pelis and Clusson, 

 as well as at Pignerol in April and May 1808, and also 

 in the spring of 1829, in Murcia, between Orihuela and 

 the sea-coast, on a space rather less than a (German) 

 square mile. When in the interior of Mexico, on the 

 western slope of the high land of Mechoacan, the cul- 

 tivated flat of Jorullo was incessantly shaken for ninety 

 days, the volcano rose surrounded by many thousand 

 small cones, about five or seven feet high (los hornitos), 

 and poured forth a brief but powerful stream of lava. 

 On the other hand, in Piedmont and in Spain, the 

 shaking of the earth gradually ceased without any great 

 natural event ensuing. 



I have thought it useful to enumerate the wholly 

 different kinds of manifestation of the same volcanic 

 activity (or reaction of the interior of the earth on 

 its exterior), in order to guide the observer, and*aid in 

 providing materials which may lead to fruitful results 

 respecting the causal connection of the phenomena. 

 Sometimes the volcanic activity embraces at one time, 

 or at times little distant from each other, so large a 

 portion of the earth, that the shocks which are felt may 

 be attributed to several kindred causes at once. The 



