284 Transactions. — Geology. 



The time the shock took place at the centrum will be 

 between 4h. 4-85m. and 4h. 4-19m., or, say, 4h. 4m. 30s. 



But at stations like Ashburton and Christchurch, which 

 are at a considerable distance from the epiceutrum, the 

 depth of the centrum will affect the distance very little, and 

 therefore the velocity of propagation calculated from these 

 places will be almost independent of z. Assuming that the 

 time of shock at the centrum was lOh. 4-5m., and that the 

 depth of the centrum was 20 miles, the distance of Christ - 

 church from the centrum will be about 79 miles, and that of 

 Ashburtou about 104 miles. The time of shock at Christ- 

 church was 4h. 12m., and at Ashburton 4h. 13-5m. ; conse- 

 quently the velocity of propagation to Christchurch was 10-5 

 miles per mhuite, and to Ashburton 11-5 miles per minute, the 

 mean being 11 miles per minute, or 968ft. per second. This 

 indicates the depth of the centrum at 24 miles, and probably 

 about 20 miles is as near an approximation as the nature of 

 the data at our disposal will admit of. The size of the cen- 

 trum we have no means of estimating. 



From this it follows that the wave arrived at the epiceu- 

 trum at about 4h. 6m., and that the average velocity of propa- 

 gation along the surface was, from the epiceutrum to Boatman's, 

 1,584ft. per second, and from Boatman's to Westport 1,232ft. 

 per second. 



In attempting to locate the epicentrum from time-observa- 

 tions it is assumed that the rate of propagation was the same 

 in different directions ; and the result of that attempt agrees so 

 closely with the result arrived at by the methods of greatest 

 intensity and of direction of shock that we may conclude that, 

 to the places used fortius purpose, the rate of propagation was 

 approximately the same, and that it was about 12-3 miles per 

 minute, or 1,082ft. per second, along the surface. 



If, however, we take the distant stations, we find a much 

 faster rate : to Timaru, 28-4 miles per minute ; to Dunedin, 

 27-4 ; to Invercargill, 36-1 ; and to New Plymouth, 29-7 miles 

 per minute. As it is impossible to suppose that the earth- 

 quake travelled faster at a distance than it did nearits origin, 

 it looks at first as if there must be errors in the time. But 

 if we assume that it travelled at the rate of 12 miles per 

 minute all round, it should have arrived at Timaru at 4h. 18m. ; 

 at Dunedin at 4h. 26m. ; at Invercargill at 4h. 33m. ; and at 

 New Plymouth at 4h. 28m. This supposes errors in time of 

 from seven to eighteen minutes, which could not have been 

 the case. The only possible explanation that occurs to me is 

 that the shock was transmitted with great velocity along the 

 mountains in a south-west direction to Queenstown, and that 

 Invercargill and Dunedin received the shock from there. This 

 would agree with the direction of the shock given at Dunedin, 



