EAETHQUAKE-SHOCKS. BEIDGES. 



/ 



of Mount Cenis, at Fenestrelles and Pignerol, the phenomena 

 commenced in April, 1808. The liquid contained in full 

 glasses exhibited a constant agitation and trembling. In the 

 United States, at New Madrid, and Little Prairie, north of 

 Cincinnati, the trembling commenced in December, 1811, and 

 continued through the winter of 1812. In the Pachalik of 

 Aleppo the shocks continued during the months of August and 

 September, 1822. 



37. Since the perturbation of the crust of the earth is produced 

 by the agitation of the subjacent igneous ocean, it is no otherwise 

 dependent on the nature of the matter which forms that crust 

 than so far as such matter may be more or less susceptible of 

 receiving and transmitting the undulations. Accordingly we find 

 earthquakes occurring in every sort of soil, and strata from the 

 loose alluvial soil of Holland to superficial strata of granite. 



38. It is, however, certain that the mechanical structure of 

 certain strata are such as to arrest the undulation. Thus, when 

 an earthquake shock, or wave as we shall call it, is propagated 

 along a line of coast, or along the foot of a mountain chain, certain 

 points of interruption have been observed, over which the wave 

 passes without producing any disturbance, resuming its character 

 beyond their limits. Such tracts are well known, and have preserved 

 their immunity from the shocks which affect the country at either 

 side of them for centuries. It appears that the undulation of the 

 subterranean fluid passes under these without affecting them. The 

 Peruvians, who are of all people the most familiar with earth- 

 quakes, call these tracts bridges. 



39. It does not at all follow, however, that because the super- 

 ficial strata are not affected by the undulation, the inferior strata 

 are exempt from it. At the beginning of the present century 

 earthquake shocks were felt with such violence in the workings of 

 the deep silver mines of Marienberg, in Saxony, that the miners 

 took flight in alarm, and ascended by the shafts to the surface, 

 where, nevertheless, they found that no shocks or trembling had 

 been felt. 



40. On the other hand, the superficial strata are sometimes 

 affected by undulations, from which inferior strata are exempt, as 

 is proved by what took place at Fahlun and Persberg, a mining 

 district of Sweden, in November, 1823, during a violent shock of 

 an earthquake, which spread terror among the inhabitants, while 

 the miners employed in the deep workings experienced no dis- 

 turbance whatever. 



41. The undulations of earthquakes proceed so often in direc- 

 tions parallel to mountain chains, that it might be conjectured 

 that they are directed by some influence exerted by the walls of 



M 2 163 



