628 MR. C. DAVISON ON THE INVERNESS 



heard like the report of a distant cannon, lasting between 2 and 3 

 seconds. 



Dec. 12, 23^ 20"\ Bunchrew. A slight shock. 



VI. Origin of the Earthquakes. 



One of the most important faults in the British Isles is that 

 which, following the line of the Great Glen, runs along the base of 

 the cliffs of the Black Isle and the east coast of Boss-shire to Tarbat 

 Ness. Its direction is about N. 35° E. and S. 35° W., its down- 

 throw to the south-east. 



Earthquakes are of frequent occurrence along the line of this 

 fault. At Inverness especially, along the course of the Ness valley 

 and at Dores, many have been felt during the present century. 

 Slight shocks are numerous at Invergarry, and in the neighbourhood 

 of Fort William they are not uncommon. The positions of their 

 epicentra and the directions of the longer axes of their disturbed 

 areas suggest that some connexion may exist between the recent 

 earthquake and this important fault. 



1. Principal SJiocJc of JS'^ov. 15. — Whatever may have 'been the 

 case vrith the subsequent shocks, I think, however, that it is im- 

 possible, in face of the evidence already recorded in this paper, to 

 admit such a connexion in the case of the principal earthquake of 

 Nov. 15 *. At the same time, I believe that the evidence at our 

 disposal is quite sufficient to enable ns to indicate, roughly no doubt, 

 the position of the fault, to slipping along which this earthquake 

 was most probably due. Had a fault in the position assigned been 

 known to geologists, we might have felt considerable confidence in 

 the accuracy of the result arrived at. As it is, we must wait for 

 further research to verify or correct it ; but I can interpret the evi- 

 dence in no other way. 



The relative position of the two isoseismal lines of intensities V. 

 and IV. has an important bearing on the problem. We have seen 

 that these lines are much closer together on the south-east than on 

 the north-west side ; and this is probably not accidental, for the 

 rocks are of fairly similar constitution for some distance on both 

 sides of the Great Glen. Now, if the earthquake originated in a 

 fault, the intensity of the shock would be greatest, and its duration 

 least, not far from the point where the perpendicular to the fault- 

 plane through the seismic focus meets the surface of the earth, i. e. 



* The reasons for rejecting the theory of a connexion between this earthquake 

 and the great fault are briefly : — 



1. Six out of the eight places where the intensity was greatest (VII.) lie on 

 the N.E. or upthrow side of the fault, and the other two places (Balnafettack 

 and Inverness) are close to the line of fault. 



2. The two isoseismals are farther apart on the N.W. than on the S.E. side, 

 which is inconsistent with a south-easterly hade. 



3. The places where the movement was upward first and then downward 

 occur on both sides of the line of fault. 



4. The distribution of the places where the sound followed, or accompanied 

 and followed, the shock favours a north-westerly hade. 



