EARTHQUAKES OP NOV. lo TO Ui;C. II, 1890. Ci'2^.} 



on the downthrow side ; and, on this side, the intensity will dimi- 

 nish less rapidly than on the upthrow side of the fault*. It follows, 

 therefore, that on the side towards which the fault hades two given 

 isoseismal lines must be farther apart than on the opposite side, 

 and, consequently, that the fault in (juestion must hade to the 

 north-west. 



Now, if the mass of rock on the north-west side were to slij) 

 slightly downward relatively to that on the other side, the particles 

 on the north-west rock-face would be drawn upward by the fric- 

 tion, and those on the south-east rock-face downward. Hence, on 

 the north-west side, wherever the vertical component of the motion 

 is appreciable, the movement should be first upward and then down- 

 ward ; on the south-east side it should be first downward and then 

 upward. Now, the movement, we have seen, was upward first at 

 Aldourie, Beauly, Bunchrubin, Cawdor, Dingwall, Inverness, and 

 Torbreck, and it was downward first at Dyke. The line in which 

 the fault meets the surface must therefore pass between Cawdor and 

 Dyke and south-east of Bunchrubin. Bearing in mind also the 

 direction of the longer axis of the disturbed area (about N. 50 E. 

 and S. oO^ W.) and the position of the epicentrum, it is evident that 

 the fault-line must pass through a point 6 miles S.E. of Inverness, 

 and, when produced, must meet the west shore-line of Loch Ness ai 

 a point 5 miles from its southern extremity. The suggested fault, 

 if it exist, would appear to be a branch of the great fault which 

 occupies the course of the Great Glen f. 



I have carefully examined all the evidence at my disposal, and I 

 know of nothing opposed to this view of the origin of the earth- 

 quake. On the other hand, it is supported by two other phenomena : 

 (1) the fact that all the eight places where the intensity was greatest 

 (YII.) lie on the north-west or downthrow side of the suggested 

 fault, and (2) that three places (Clunes station, Dores, and Torbreck) 

 at which the sound was heard after the shock cannot be far from the 

 point where the perpendicular to the fault-plane through the focus 

 meets the surface, and at this point the shock would be felt before 

 any sound was heard. Also, the angle between the lines of the 

 branch fault and of the great fault is about 15^, which is not far from 

 the usual angle in such cases ; further, the direction of the branch 

 fault is approximately parallel to the main line of folding in the 

 district, and this, again, is in accordance with a known geological 

 law. It is clear we cannot regard the existence of this branch 

 fault as actually proved, but possibly many a fault has been laid 

 down upon a geological map on less satisfactory evidence. 



* This statement is not true in all eases ; the limits within which it is appli- 

 cable will be considered in a subsequent paper. 



!' The line of the branch fault, as I have drawn it, is approximately parallel 

 to the southern boundary of the Old Red Sandstone in the neighbourhood 

 of Inverness, but about a mile to the south-east of it, as shown in Geikie's 

 Geological Map of Scotland. There can be no direct connexion between tho 

 two, for there can be no doubt whatever, Mr. Ilorne informs me, that in 

 this part, at any rate, 'the junction of the Old Red Sandstone is not a fault, 

 but an unconformability.' 



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