212 



COSMOS. 



of the interior on the exterior. The frequency and general 

 prevalence of a phenomenoji which is probably dependent on 

 the raised temperature of the deepest molten strata explain 

 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 neighborhood ofMid- 

 dleburg andVliessingen on the 23d of February, 1828. Gran- 

 ite and mica slate are shaken as well as limestone and sand- 

 Btone, or as trachyte and amygdaloid. It is not, therefore, the 

 chemical nature of the constituents, but rather the mechanical 

 structure of the rocks, which modifies the propagation 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 sometimes been re- 

 marked, which manifested themselves during the course of 

 many centuries. The undulation advances in the depths be- 

 low, but is never felt at the same points on the surface. 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 favor the di- 

 rection of undulations parallel to them ; occasionally, however, 

 the waves of commotion intersect several chains almost per 

 pendicularly. Thus we see them simultaneously breaking 

 through the littoral chain of Venezuela and the Sierra Parime. 

 In Asia, shocks of earthquakes have been propagated from 

 Lahore and from the foot of the Himalaya (22d of January, 

 1832) transversely across the chain of the Hindoo Chou to 

 Badakschan, the upper Oxus, and even to Bokhara.f The 

 circles of commotion unfortunately expand occasionally in con- 

 sequence of a single and unusually violent earthquake. It is 

 only since the destruction of Cumana, on the 14th of Decem- 

 ber, 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 advancf^ 



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

 non of non-propagation through superior strata is connected the remark 

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

 deep silver mines at Marienberg, in the Saxony mining district, while 

 not the slightest trace was perceptible at the surface. The mmers 

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

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

 1823, spread dismay among the inhabitants above ground. 



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

 Mem. on the Usbek State, in the Journ -2 of the Asiatic Society of Bengal, 

 vol. iii., p. 337. 



