Vol. 63.] OCHIL EARTHQUAKES OF 1900 TO 1907. 373 



of the length of the seismic focus, it would appear that, in the 

 principal earthquake of September 21st, 1905, the length of the focus 

 was about 6 J miles ; in the shocks of the second class, the average 

 length was about 4 miles ; while, for the third class, we may infer, 

 from the brevity of the sound and shock, that the focus was 

 generally of small dimensions. 



IV. Origin of the Earthquakes. 



Prom the evidence of the earthquakes of September 17th and 

 22nd, 1900, it may be inferred that the mean direction of the origi- 

 nating fault in the neighbourhood of the epicentres is E. 11° N. and 

 W. 11° S. ; while that deduced from the earthquakes of July 23rd 

 and September 21st, 1905, is E. 27° N. and W. 27° S. Owing to 

 the paucity of observations in some parts of the disturbed areas, 

 especially towards the east and north-east, these estimates cannot 

 be regarded as very accurate, and it is uncertain whether one or 

 two faults were responsible for these earthquakes. 



The direction in which the fault hades is determined by one 

 earthquake only, namely, the principal earthquake of September 21st, 

 1905. As the distance between the isoseismals is greater on the north 

 than on the south side, it may be inferred that the fault hades 

 towards the north. In this case, then, the fault-line must lie a 

 short distance (about a mile or two) on the south side of the centres 

 of the isoseismal lines. 



On the maps of the earthquakes, the course of the great Ochil 

 Fault in the neighbourhood of the epicentres is shown. 1 As will 

 be seen, it passes through or near the Hillsfoot villages, at which 

 so many of the slighter shocks were felt. Its mean direction in 

 this district is E. 13° K and W. 13° S. In the portion between 

 Blairlogie and Alva, near which most of the epicentres lie, its hade 

 is unknown ; but, farther east, near Dollar, as Dr. J. Home, F.R.S., 

 kindly informs me, there is direct evidence that it hades towards 

 the south at an angle of about 20° from the vertical. Now, as in 

 all the stronger shocks (figs. 1-3 & PI. XXVI), the greater part of 

 the disturbed area lies on the north side of this fault, we must 

 conclude that none of these shocks was connected with the main 

 Ochil Fault, unless there is a change of hade, of which we have no 

 evidence, to the west of Tillicoultry. 



With regard to some, if not most, of the weaker shocks, however, 

 it seems clear that they must be attributed to slips along a fault 

 passing not far from the Hillsfoot villages, for (i) they were 

 observed as a rule at these villages only, and (ii) the intensity of 

 the shocks of December 28th and December 30th (4.15 p.m.), 1906, 

 was great at Menstrie, and died rapidly out from this place. 

 "Whether these alone should be referred to movements along the 

 Ochil Fault, or whether all the earthquakes originated in a fault 



1 Eeproduced from Sheet 39 of the 1-inch map issued by the Geological 

 Survey of Scotland. 



