The isoseismal 4 may be regarded as the boundary of the disturbed area of the earthquake, for, so far as known, the shock was not noticed at any point outside it. Towards the north, it was felt at Wick, Castletown, and other intermediate places; towards the west at Tobermory in the island of Mull; and, towards the south, at Skelmorlie (in Ayrshire), Paisley, Belsyde (near Linlithgow), Gullane (near North Berwick), and Dunbar. Along the east coast of Scotland, between Wick and Dunbar, there are few places of any size where the shock was not felt. The disturbed area of the earthquake is thus 215 miles long from north-east to south-west, 198 miles wide, and contains about 33,000 square miles. _Position of the Originating Fault._--The only isoseismals which are drawn accurately enough to determine the earthquake-fault are the two inner ones, those marked 8 and 7; but these are sufficient for the purpose. It is clear, from the direction of their longer axes, that the average direction of the fault must be N. 33° E. and S. 33° W. Again, the isoseismals are farther apart towards the south-east than towards the north-west, implying that the fault hades to the south-east. Lastly, as the intensity of the shock is greater on the side towards which the fault hades, it follows that the fault-line must lie a short distance (about a mile or so) on the north-west side of the centre of the isoseismal 8. Now, the great fault alluded to above occupies almost exactly the position indicated by the seismic evidence. Its mean direction from Tarbat Ness to Loch Linnhe is N. 35° E. and S. 35° W., it hades to the south-east, and the fault-line passes through a point about three-quarters of a mile to the north-west of the centre of the isoseismal 8 (Fig. 66). There can be little doubt, therefore, that the earthquake was caused by a slip of this fault; and the evidence of the after-shocks, as will be seen, offers additional support to this conclusion. The region in which the slip took place may be determined roughly from the position and form of the innermost isoseismal. Its centre must have been close to the point marked A in Fig. 66, which corresponds to a point about 1-1/2 mile east-north-east of Dochgarroch. In a horizontal direction, its length must have been at least five or six miles; otherwise, the isoseismal 8 would have been less elongated. It must therefore have reached from about half-a-mile north-east of Loch Ness to about half-a-mile south-west of Inverness. Its width, measured along the dip of the fault-surface is unknown; but the small distance between the centre of the isoseismal and the fault-line shows that the principal movement took place at a depth which was probably under, rather than over, one mile. NATURE OF THE SHOCK.