OF THE APPALACHIAN CHAIN. 521 



a similar rise and fall, and so others in succession, giving rise to 

 a progressive motion of the ground, very similar in appearance to 

 a wave of the sea."* 



To the suggestion of a propagated vibrating jar being the cause 

 of the rocldng of the surface, we will reply by simply referring 

 to all the authentic accounts of earthquakes, in which the reg- 

 ularly progressive march of the billowy undulation is so frequently 

 described by eye-witnesses, and likewise to the statements of 

 MicheU, who gives, from abundant data, the very rate of the pas- 

 sage of the great Lisbon earthquake, across an area exceeding 

 three thousand miles in breadth. As regards the other supposed 

 difficulty, that the radius of each superficial curvature must be 

 very small, we contend that this is by no means a necessary 

 inference from the phenomena, since if we take into consideration 

 the prodigiously high velocity with which earthquakes seem to 

 move, we find a reason at once, why tall objects, like trees, may 

 rock from side to side with a rapid oscillation, while the wave 

 which disturbs them may possess an enormous breadth. A low 

 and broad wave, if moving slowly, would only tilt the objects under 

 which it might pass, into attitudes perpendicular to its gentle 

 slopes, but the lowest and broadest billow, passing with the amaz- 

 ing speed of the Lisbon earthquake, might, by suddenly shoving 

 the foundations or pedestals of objects from beneath them, and 

 as suddenly pushing them in the opposite direction, cause them 

 to swing rapidly through arcs of almost any extent. 



While the evidence, therefore, seems ample, of the existence of 

 a wave-like motion of the earth's surface during earthquakes, facts 

 are not wanting which indicate the recent production, from this 

 cause, of permanent anticlinal axes. Thus we find, in Darwin's 

 Journal of Travels in South America, the following interesting 

 statement, Mr. GiU, an engineer, mentioned to that intelligent 

 traveller, that following up the bed of a stream, strewed with sand 

 and gravel, and showing in one place, a channel in the solid rock 

 about forty yards wide, and eight feet deep, he found himself 

 suddenly going down hill, the whole descent amounting to forty 



* Lyell's Principles, Boston edition, vol. 11. 



34 



