[63] The approximate circularity of the two outer isoseismals is due to the fact that the vibrations propagated to such great distances are those which start from the comparatively small central region of the focus.
[64] The above statement summarises the evidence of the majority of the observers in each portion of the disturbed area. In this, as in other similar cases, discrepancies in the observations are unavoidable; but it is important to notice that they are least frequent in the observations evidently made with the greatest care.
[65] Except in the case of Yorkshire, where the three Ridings are regarded as separate counties.
[66] The Derby earthquake of March 24th, 1903, was also a twin earthquake. The centres of the two foci were situated near Ashbourne and Wirksworth, above eight or nine miles apart, along a line running N. 33° E. and S. 33° W. The two parts of the shock coalesced along a rectilineal band about five miles wide running centrally across the lower isoseismals in a direction at right angles to their longer axes. The isacoustic lines are also elongated in the direction of this band. In this case, the impulses at the two foci must have taken place at the same instant. (Quart. Journ. Geo. Soc., vol. lx., 1904, pp. 215-232.)
[67] If the foci of the two impulses had been detached, there would, with so small an interval between the two parts, have been a variation in the nature of the shock like that observed during the Hereford earthquake.
[68] This part of Inverness-shire has not yet been mapped by the Geological Survey, but a fault is known to exist in the Findhorn valley near Drysachan Lodge, which lies about eleven miles down the valley from Dalarossie.