208 COSMOS. 



its independence of the nature of the mineral masses in which 

 it manifests itself. Earthquakes have even been felt in the 

 loose alluvial strata of Holland, as in the neighbourhood of 

 Middleburg and Vliessingen, on the 23d of February, 1828. 

 Granite and mica slate are shaken as well as limestone and 

 sandstone, or as trachyte and amygdaloid. It is not, there- 

 fore, the chemical nature of the constituents, but rather the 

 mechanical structure of the rocks, which modifies the propa- 

 gation of the motion, the wave of commotion. Where this 

 wave proceeds along a coast, or at the foot and in the direction 

 of a mountain-chain, interruptions at certain points have some- 

 times been remarked, which manifested themselves during the 

 course of many centuries. The undulation advances in the 

 depths below, but is never felt at the same points on the sur- 

 face. The Peruvians* say of these unmoved upper strata that 

 " they form a bridge." As the mountain-chains appear to 

 be raised on fissures, the walls of the cavities may perhaps 

 favour the direction of undulations parallel to them; occa- 

 sionally, however, the waves of commotion intersect several 

 chains almost perpendicularly. Thus we see them simultane- 

 ously breaking through the littoral chain of Venezuela, and the 

 Sierra Parime. In Asia, shocks of earthquakes have been por- 

 pagated from Lahore and from the foot of the Himalaya (22nd 

 of January, 1832) transversely across the chain of the Hindoo 

 Chou, to Badakschan, the upper Oxus, and even to Bokhara.! 

 The circles of commotion unfortunately expand occasionally 

 in consequence of a single and unusually violent earthquake. 

 It is only since the destruction of Cumana, on the 14th of De- 

 cember, 1797, that shocks on the southern coast have been felt 

 in the mica slate rocks of the peninsula of Maniquarez, situated 

 opposite to the chalk-hills of the main land. The advance from 



* In Spanish, they say, rocas que. hacen puente. With this pheno- 

 menon of non-propagation through superior strata is connected the re- 

 markable fact that in the beginning of this century shocks were felt in 

 the deep silver mines a c Marienberg, in the Saxony mining district, 

 while not the slightest trace was perceptible at the surface. The miners 

 ascended in a state oi alarm. Conversely the workmen in the mines of 

 Falun and Persberg felt nothing of the shocks which in November, 1823, 

 spread dismay amongst the inhabitants above-ground. 



f Sir Alex. Burnes, Travels in Bokhara, vol. i. p. 18 ; and Wathen, 

 Mem. on the Usbek State, in the Journal of the Asiatic Society of 

 Bengal, vol. iii. p. 337. 



