EARTHQUAKES. 183 



krachytic, volcanic regions, without exerting any influence 

 upon the neighbouring volcanoes. This is a third group of 

 phenomena, and is that which most convincingly indicates 

 the existence of a general cause, lying in the thermic nature 

 of the interior of our planet. To this third group also be- 

 longs the phenomenon, sometimes, though rarely, met with 

 in non-volcanic lands, but little disturbed by earthquakes, 

 of a trembling of the soil, within the most narrow limits, 

 continued uninterruptedly for months together, so as to give 

 rise to apprehensions of an elevation and formation of an 

 active volcano. This was the case in the Piedmontese val- 

 leys of Pelis and Clusson, as well as in the vicinity of Pig- 

 nerol in April and May, 1805, and also in the spring of 1829 

 in Murcia, between Orihuela and the sea-shore, upon a space 

 of scarcely sixteen square miles. When the cultivated sur- 

 face of Jorullo upon the western declivity of the plateau of 

 Mechoacan in the interior of Mexico was shaken uninter- 

 ruptedly for 90 days, the volcano rose with many thousand 

 cones of 57 feet in height (los Jwrnitos) surrounding it. 

 and poured forth a short but vast stream of lava. In Pied- 

 mont and Spain, on the contrary, the concussions of the 

 oarth 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 

 connexion of the phenomena. Sometimes the volcanic 

 activity embraces at one time or within short periods sc 

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

 excited may be ascribed simultaneously to many causes re 

 lated to each other. The years 1796 and 1811 present par- 

 ticularly memorable examples 30 of such a grouping of the 

 phenomena, 



30 The great phenomena of 1796 and 1797, and 1811 and 1812, 

 occurred in the following order : 



27th of September, 1796. Eruption of the volcano of the island of 



Quadaloupe, in the Leeward Islands, after a repose of many years 

 November, 1796. The volcano on the plateau of Pasto, between 



the small rivers Guaytara and Juanambu, became ignited and 



began to smoke permanently ; 



