6 DE. CHARLES DAVISON ON THE [Feb. I9OO,. 



the lower limit of audibility. All the common types of earthquake- 

 sound were referred to, but a rather unusual preference is given to 

 those of medium duration, 37 per cent, of the comparisons being to 

 thunder, 25 to passing traction-engines, waggons, or trains, 24 to 

 explosions or the firing of guns, 6 to wind, 2 to the fall of heavy 

 bodies, 2 to the fall of a load of stones, while 4 per cent, are of a 

 miscellaneous kind. 



The time-relations of the sound and shock were those which 

 are generally observed in a small earthquake. The beginning of the 

 sound either preceded or coincided with that of the shock; their 

 epochs of maximum intensity coincided ; and the end of the sound 

 either followed or coincided with that of the shock. Thus the 

 duration of the sound was in all parts either equal to or greater 

 than that of the shock ; whence it follows either that the movements 

 which gave rise to the sound-vibrations lasted longer than those 

 which produced the shock, or else that the area from which the sound- 

 vibrations proceeded was of greater linear dimensions than that from 

 which the more prominent vibrations came, and extended beyond it 

 at both ends. The latter alternative seems the more probable ; in 

 other words, the sounds heard before and after the shock came from 

 the lateral margins of the focus. 



Earthquake of April 2nd, about 3 p.m. 



This was the slightest shock of the series, the intensity being 

 apparently less than 4. I have only four undoubted records, coming 

 from Mane, Hawnan Smith, and Trewardrevah. A weak tremor 

 was felt at each place, accompanied by a noise like distant thunder. 

 So far as can be judged from the limited number of records, the 

 epicentre appears to have undergone a further easterly displacement 

 by about 2 miles. 



Origin of the Earthquakes. 



On the Geological Survey map of the disturbed area, very few 

 faults are marked, and there is none that will agree even approxi- 

 mately with the conditions implied by the seismic evidence. There 

 is, however, no contradiction between these conditions and the 

 known geological structure of the district. To the south, in the 

 neighbourhood of Mullion, the strike of the beds is parallel to 

 the axes of the isoseismal lines, and there is a thrust-plane in 

 the same direction which hades towards the south-east. 1 Near the 

 epicentre itself the general strike of the lodes is about west-south- 

 west, and several elvan-courses are parallel, or nearly so, to the iso- 

 seismal axes. One of these, copied from the Geological Survey map, 

 is shown on the earthquake-map (p. 2). In position it satisfies 

 the seismic conditions, and if the surface is faulted and hades to the 

 south-east it is quite possible that a series of slips along it may 

 have given rise to the earthquakes here considered. 



1 I am indebted to Prof. Lapworth for this information. 



