366 Notes on the Earthquake of October, 1860. 



laneous occurrence of the shock throughout Canada, perhaps in- 

 dicates that the wave did not move horizontally but reached the 

 surface from a great depth and at a high angle as Perrey seems 

 to suppose the earthquakes of Eastern America have usually done. 

 It must however be observed that at the rate of propagation given 

 by Mallet for earthquake waves through hard rock, which is not 

 less than 10,000 feet per second, it is quite possible that even a 

 horizontal wave may appear to be felt at the same instant at great 

 distances.* 



All the observers agree that the sound preceded the shock and 

 continued after it, and that the first shock was the most violent ; 

 and it is also very generally noted that it was most severely felt on 

 low ground and least so on rocky eminences. This last character 

 which belongs to earthquakes generally, seems to arise from the 

 greater resistance opposed to the vibrations by loose materials as 

 compared with hard rocks. 



It appears from the published lists that the late eaithquake is 

 the last of at least twenty-nine that have visited Canada since its 

 discovery by Europeans, and we now proceed to give some account 

 of these previous instances, availing ourselves mainly of the facts 

 and conclusions stated by Mallet and Perry, the two most exten- 

 sive and laborious collectors of earthquake statistics. 



Mallet defines an earthquake as " the transit of a wave of elastic 

 impression in any direction from verticality upward to horizontality 

 in any azimuth through the crust of the earth, from any centre 

 of impulse, or from more than one, and which may be attended 

 with tidal and sound waves dependent upon the impulse and upon 

 the circumstances of position as to sea and land." Such " earth- 

 waves " travel outward from the centre of impulse with immense 

 velocity and appear as upward shocks or undulating rolls accord- 

 ing to the greater or less verticality of the motion. They may also 

 be complicated with indirect shocks arising from unequal or 

 circuitous transmission of the vibrations, and these complex 

 shocks usually occur in great and destructive earthquakes. 



The causes of these vibratory waves are too deep-seated to be 

 directly known to us, but they must occur when any part of the 

 crust of the earth is subjected to tension, and when this is sudden- 

 ly reliev^ed by fracture or otherwise, and again when any part of 

 the earth's crust is left unsupported and collapses under the force 



* See Mallet on the Dynamics of Earthquakes. — Transactions Royal 

 Irish Academy^ Vol. XXI. 



