66 C. Davison — Briiish Earthquakes. 



unreasonable to infer, then, that the Edinburgh earthquake was in 

 some way connected with this fault. 



If the inclination of the fault were known in the neighbourhood 

 of the epicentrum, this would enable us to determine appi'oximately 

 the depth of the seismic focus, assuming the inclination to be con- 

 stant to a depth of a few miles below the surface. The exact incli- 

 nation is, however, unknown ; but Prof. J. Geikie informs me that 

 it "cannot be less than 7O°-80° from the horizontal, say, about 15° 

 from the vertical." If this be the case, then, the depth of the centre 

 of the seismic focus is not much less than 6 miles, nor much greater 

 than 12| miles, and perhaps does not differ considerably from 8f 

 miles. 



The bearing of the observations made on the first direction of 

 motion of the principal vibration will now be obvious. The six 

 places (Edinburgh, Balerno, Trinity, Davidson's Mains, Linlithgow, 

 and Polmont) where the movement was first upward, lie on the 

 downthrow side of the fault, whilst Penicuik, where the movement 

 was downward first, is on the upthrow side. Now, if the mass of 

 rock on the downthrow side slipped slightly downwards, or if the 

 mass on the upthrow side slipped upwards, the particles on the 

 downthrow rock-face would be drawn upwards first, and those oa 

 the upthrow rock-face would be drawn downwards. The earth- 

 wave or the rock-masses on either side of the fault would thus start 

 in opposite phases of vibration, and the resulting effects at the seven 

 places on the surface would be those described above. 



I believe we may, then, with some probability, conclude : (1) 

 that the Edinburgh earthquake was caused by a slip of the fault 

 marked BB on the map, at a spot vertically below the position 

 indicated for the epicentrum, and therefore not far from the middle 

 of the fault, where, probably, the throw is a maximum and where 

 earthquake-action has been most frequent or most intense ; (2) that, 

 on account of the simple character and short duration of the dis- 

 turbance, the horizontal length of the faialt over which the slip took 

 place was very short, possibly less than a mile; (3) that the slip of 

 the downthrow side was downwards or that of the upthrow side 

 upwards, resulting, in either case, in an increase of the throw of the 

 fault in the neighbourhood of the seismic focus ; and (4) that, while 

 the region of maximum slip, the focus of the earthquake proper, was 

 probably at a depth of several (perhaps about 8) miles, the slip 

 extended uj)wards to within a short distance of the surface, this part 

 of the slip-area being the focus of the sound-vibrations. This latter 

 conclusion will be considered somewhat more fully in treating of 

 the Lancashire earthquake, and at greater length in a subsequent 

 paper. 



" When we reflect," says Dr. A. Geikie, " ujDon the extent of 

 depression produced by these faults, we see at once that the Car- 

 boniferous rocks must formerly have stretched across the area of the 

 Pentland Hills, and that it is to the agency of these dislocations, 



1 On the Existence of Undisturbed Spots in Earthquake-shaken Areas, Geol. 

 Mag. (April, 1886), Vol. III. pp. 157-159. 



