the sound and shock should approximately coincide. The inference that the sound-vibrations heard before and after the shock come from the margins of the focus is also supported by the observations on the relative duration of the sound and shock. If we take only those records which are free from doubt, in 78 per cent. of the total number, the duration of the sound was greater than that of the shock; while, in Aberdeenshire, according to 93 per cent. of the observers, the durations of sound and shock were equal. We may imagine, then, that the slip within the seismic focus would be greatest in a central region, and that it would die outwards in all directions towards the edges. The friction arising from the slipping in the central region would produce chiefly the comparatively large oscillations that formed the perceptible shock; the evanescent creep within the marginal regions would produce the small and rapid vibrations that were sensible only as sound. ORIGIN OF THE EARTHQUAKE. While the seismic evidence enables us to determine the surface-position and the horizontal dimensions of the seismic focus, it unfortunately throws no light whatever on a point of some importance--namely, the direction of the movement which caused the earthquake. We cannot infer from it whether it was the rock on the south-east or north-west side of the fault that slipped or whether both sides slipped at once; nor, if that point had been settled, do we know if the movement of the displaced side was upward or downward. In the formation of the fault, however, it is clear that either the south-east side has been depressed or the north-west side elevated; and, as the bed of Loch Ness is below the level of the sea, that the former movement has predominated. If the displacements which gave rise to the earthquake were merely a continuation of the original series of movements--and this is, to say the least, a very probable view to take--then we may imagine that, for a distance of five or six miles, and at a depth of about a mile or less, there was a sudden sag downwards of the rock on the south-east side of the fault through a distance which perhaps in no part exceeded a fraction of an inch. Fig. 66 is an attempt to represent roughly the displacement which caused the principal earthquake. The diagram makes no pretence to accuracy, and the scale in the vertical direction is enormously greater, perhaps a hundred thousand times greater, than that in the horizontal direction. The straight line is supposed to represent a straight line drawn before the earthquake on the surface of the rock adjoining the fault on the south-east side and at a depth of about a mile, and the curve the form of the same line after the earthquake. [Illustration: FIG. 66.--Diagram to illustrate supposed fault-displacement causing Inverness earthquake.]