347 



sion. Where the wave proceeds regularly along a coast, or 

 by the foot and in the direction of a mountain chain, it is 

 occasionally observed that there is an interruption suffered at 

 certain points. This has been noticed for centuries. The 

 undulation advances along the depths, but at the points in 

 question it is never felt at the surface. The Peruvians say 

 of these unshaken superior strata ' that they form a bridge.' " 

 In the note No. 159 to the same work it is said, — " With 

 this phenomenon of non-transmission through superior strata 

 is connected the remarkable fact, that in the beginning of 

 the present century shocks of an earthquake were felt in the 

 deep silver mines of Marienberg, in the Saxon Erzgebirge, 

 which were not perceived at all on the surface. The miners 

 rushed up in alarm. Contrariwise, the people at work in the 

 mines of Falun and Persberg felt nothing of the smart shocks 

 (November, 1823,) which threw all the inhabitants above 

 ground into a state of great alarm." 



The celebrated traveller whom I have just quoted thus 

 bears unequivocal testimony to the fact that earthquakes 

 sometimes leave portions of ground unaffected in their 

 course ; but he states that the mechanical structure of 

 the mineral species modifies the propagation of the motion, 

 the wave of succussion." This assertion may probably be 

 correct, but it affords no explanation of the fact that similar 

 (and the same) formations of country and like rocks are, at 

 different times, affected by earthquakes in the most dissimilar 

 manner. Vast mountain ranges, composed of igneous and 

 primitive rocks, sometimes appear to escape all effects from 

 the wave of motion ; at other times the same mountains are 

 shaken and shattered to such an extent as to change the 

 whole face of the country. Many recent well-authenticated 

 instances of this have occurred, — as that related by the late 

 Sir Alexander Burnes, at Lahore, in February, 1832, when 

 several valleys were choked up by the masses of rock thrown 

 down. The shock of the 20th June, 1840, in the district of 



