176 COSMOS. 



waves of commotion, which pass sometimes through com- 

 pletely non-trachytic, non-volcanic countries, and sometimes 

 through trachytic, volcanic regions, without exerting any 

 influence upon the neighboring volcanoes. This is a third 

 group of phenomena, and is that which most convincingly 

 indicates the existence of a general cause, lying in the ther- 

 mic nature of the interior of our planet. To this third 

 group also belongs the phenomenon sometimes, though rare- 

 ly, met with in non-volcanic lands, but little disturbed by 

 earthquakes, of a trembling of the soil within the most nar- 

 row limits, continued uninterruptedly for months together, 

 so as to give rise to apprehensions of an elevation and for- 

 mation of an active volcano. This was the case in the Pied- 

 montese valleys of Pelis and Clusson, as well as in the vi- 

 cinity of Pignerol, in April and May, 1805, and also in the 

 spring of 1829 in Murcia, between Orihueia and the sea- 

 shore, upon a space of scarcely sixteen square miles. When 

 the cultivated surface of Jorulfo, upon the western declivity 

 of the plateau of Mechoacan, in the interior of Mexico, was 

 shaken uninterruptedly for 90 days, the volcano rose with 

 many thousand cones of 5 7 feet in height (los hornitos) sur- 

 rounding it, and poured forth a short but vast stream of 

 lava. In Piedmont and Spain, on the contrary, the concus- 

 sions of the earth gradually ceased, without the production 

 of any other phenomenon. 



I have considered it expedient to enumerate the perfectly 

 distinct kinds of manifestation of the same volcanic activity 

 (the reaction of the interior of the earth upon its surface), in 

 order to guide the observer, and bring together materials 

 which may lead to fruitful results with regard to the causal 

 connection of the phenomena. Sometimes the volcanic ac- 

 tivity embraces at one time or within short periods so large 

 a portion of the earth, that the commotions of the soil excited 

 may be ascribed simultaneously to many causes related to 

 each other. The years 1796 and 1811 present particularly 

 memorable examples* of such a grouping of the phenomena. 



shocks followed in preference the direction of the chains of mountains, 

 and were principally felt in Alpine districts. The frequency of the 

 movements in the soil of the Andes, and the little coincidence ob- 

 served between these movements and volcanic eruptions, must neces- 

 sarily lead us to suppose that in most cases thev are occasioned by a 

 cause independent of volcanoes" (Boussingault, Annettes de Chimie et de 

 Physique, t. Iviii., 1835, p. 83). 



* The great phenomena of 1796 and 1797, and 1811 and 1812, oc- 

 curred in the following order: 



