C. Davison — British Earthquahes. 65 



sound is generally said to have accompanied the shock, but this is a 

 vague expression. Probably it overlapped the shock slightly at 

 either end; at different places in and near Peel:)les, Dr. Gunn informs 

 me that it was said in one case to precede the shock, in two to follow 

 it, and in one to both precede and follow it. 



Position of the Seismic Focus and Geological Relations. — The faults 

 of the Edinburgh district "group themselves naturally into two 

 series, one more or less at right angles to the strike of the beds, that 

 is, east and west, or from south-east to north-west; the other usually 

 of greater magnitude, in long parallel north-east and south-west 

 lines." The latter faults " lie almost wholly along the axis of the 

 Pentlands. In place of cutting across the strike of the country as 

 the other faults do, they run parallel to it. Flanking each side of 

 the anticline, their effect has been to depress the Carboniferous strata 

 against the older rocks of the hills, so that on the west side their 

 downthrow is to the west, and on the east side to the east." Of 

 these main faults, which are four in number, three are shown on the 

 accompanying map of the Edinburgh earthquake. The first of these 

 (marked AA on the map) extends from the sea at Portobello to 

 be^'ond Cai'lops in Peeblesshire ; its downthrow is to the south-east, 

 and the amount of the throw in parts probably not less than 3000 feet. 

 The second fault (BB) reaches from the head of the Logan valley to 

 North Black Hill, where it seems to die out. The third fault (CC) 

 extends in a wavy line from Arthur's Seat, near Edinburgh, to the 

 neighbourhood of Bevelaw ; but in certain pai'ts its course is some- 

 what doubtful. The downthrow of the two latter faults is to the 

 north-west, but the amounts of their throw are unknown.' 



Now, the centre of the disturbed area, as indicated on the map, 

 lies about 3 miles W. 42° S. of Balerno ; and this, as before remarked, 

 may be regarded as the approximate position of the epiceutrura, 

 which is therefore to the north-west of all three of the faults. The 

 earthquake cannot, then, have any connexion with the first of these 

 faults. From the line of the third fault (CC), the perpendicular 

 distance of the epicentrum is about 1-| miles. The shock might ap- 

 parently have been caused by a slight extension of the fault towards 

 the south-west, or by a slip of the fault, supposing it to extend 

 underground far enough in this direction. But, unless we assume 

 that the earthquake originated at one fault, and the earthquake- 

 sounds at exactly the same moment at another, it does not seem 

 possible to connect the earthquake with this fault ; for the centre of 

 the sound-area lies about two or three miles to the south or south- 

 east of the epicentrum, and therefore to the south or south-east of 

 the line of fault at the surface. 



Turning next to the second fault (BB), we find the perpendicular 

 distance of the epicentrum from the line of fault at the surface is 

 about 2^ miles ; and this would admit of the sound-focus being on 

 the fault-plane, but probably close to the surface. It does not seem 



1 H. H. Howell and A. Geikie, Geology of the Neighbourhood of Edinburgh, 

 pp. 118-119. 



DECADE lEI. — VOL. VIII. MO. II. 5 



