Phenomena in South America. 603 



shocks two points to westward and only half a point to eastward ; stood at N. when the motion 

 was less violent. Four minutes afterwards, a shock more violent than any of the preceding ones, 

 affecting the compass as before : another violent shock, and then the movements became gradually 

 less distinct, and eight minutes after the first commencement, they entirely ceased." 



I have quoted Mr. Douglas's statement with regard to the compass^ although 

 it is not clear how any movement could have forced it to oscillate towards one 

 side more than to another. I presume, however, if the needle with its card had 

 not been acted on by the magnetic force, it would have been thrown in the 

 trough (if such an expression may be usedj of the undulation, that is, in a 

 N.W. and S.E. line, and, therefore, that the recwrrence of this tendency, act- 

 ing against the polar attraction, caused the unequal oscillations, as described. 

 In my Journal of Researches*, 1 have endeavoured to show, that the vorti- 

 cose movement, which in several earthquakes appears to have affected the 

 stones in buildings, possibly may be explained on the same principle, namely, 

 that the stones are so shaken that they arrange themselves according to their 

 forms, in the line of vibration, as the compass would have done, had it not been 

 acted on by the magnetic force. That the movement of the surface was un- 

 dulatory, is shown by the fact, that at Concepcion the walls which had their 

 extremities directed towards the chief point of disturbance generally remained 

 erect, although much fractured ; whilst those extending at right angles to these 

 first lines, were hurled to the ground ; for in the latter case we must suppose, 

 that the whole wall was thrown at the same moment out of its perpendicular 

 by coinciding with an undulation. 



The fact mentioned by Mr. Douglas of the trees almost touching the 

 ground from the effects of the movement, though very extraordinary, has 

 been noticed by eye-witnesses of earthquakes in other parts of the worldf . 

 The circumstance (even supposing it somewhat exaggerated) is the more 

 remarkable, since at Valdivia, which is situated on the coast between this 

 island and the centre of the disturbance at Concepcion, the shock produced 

 no such eff'ects. I was seated in a thick wood there, during the earthquake, 

 and the trees were only slightly shaken. 



The range of the Cordillera opposite Chiloe, a narrow island ninety miles 

 in length, is not nearly so lofty as in Central Chile, and a few only of the 

 culminant peaks, which are all active volcanos, exceed 7000 feet in height. 

 Mr. Douglas has given me a detailed account of the effect produced on them 

 by the shock. 



* Journal of Researches during the Voyage of the Beagle, p. 376. 



f This is mentioned by Dolomeu as a well-known fact during the Calabrian earthquake of 

 1783. Lyell's Principles of Geology (5th edition), vol. ii. p. 217. 

 VOL. V. SECOND SERIES. 4 I 



