222 



placed, are believed to contain an enormous 

 quantity of sulphur, and other inflammable mat- 

 ter. We forget, that the rapidity with which 

 the undulations are propagated to great dis- 

 tances, even across the basin of the ocean, proves, 

 that the centre of action is very remote from the 

 surface of the Globe. From this same cause no 

 doubt earthquakes are not restrained to certain 

 species of rocks, as some naturalists pretend, 

 but all are fitted to propagate the movement. 

 In order to keep within the limits of my own ex- 

 perience, I shall here cite the granites of Lima 

 and Acapulco; the gneiss of Caraccas; the mica- 

 slate of the peninsula of A ray a ; the primitive 

 thonschiefer of Tepecuacuilco, in Mexico ; the 

 secondary limestones of the Apennines, Spain, 

 and New Andalusia ; and finally the trappean 

 porphyries of the provinces of Quito, and Popa- 

 yan *. In these different places the ground is 

 frequently agitated by the most violent shocks ; 

 but sometimes, in the same rock, the superior 

 strata form invincible obstacles to the propaga- 

 tion of the motion. Thus, in the mines of Sax- 

 ony-^, we have seen workmen hasten up af- 



* I might have added to the list of secondary rocks the 

 gypsum of the newest formation, for instance that of Mont- 

 martre placed on a marine calcareous rock, which is posterior 

 to the chalk. See the M6m. de l'Acad^mie, t. i, p. 341, on 

 the earthquake felt at Paris, and in it's environs, in 1681. 

 + At Marienburgh in the Erzgebuerge. 



