Dr. C. Davison—British Earthquakes. 19 
29. May 22, 1909: 5.1 p.m.—A shock, of intensity 4, felt at 
Airthrey ; accompanied by a noise like that of a slight explosion. 
30. May 22, 1909: 5.24 p.m.—A slight shock felt at Menstrie. 
31. May 22, 1909: 8.238 p.m.—A shock, of intensity 5, felt at 
Airthrey ; accompanied by a sound lke that of an explosion, the 
sound being louder than at 5.1 p.m. 
32. October 21, 1909: 8.37 a.m.—A concussion, of intensity 4, 
felt at Airthrey, and followed by a sound like that of an explosion. 
38. October 21, 1909: 9.53 a.m.—A slight shock felt at Menstrie. 
34. October 22, 1909: 6.55 a.m.—A slight shock felt at Menstrie. 
35. October 22, 1909: 7.57 a.m.—A concussion, of intensity 4, 
felt at Airthrey, preceded and followed by a sound like that of a heavy 
body falling. 
36. October 22, 1909: 9.8 p.m.—At Airthrey, three concussions, 
separated by intervals of two and five seconds, preceded and followed 
by sounds lke those of sharp explosions. 
Origin of the Ochil Earthquakes. 
From the only isoseismal line that can be drawn of the earthquake 
of October 20, 1908, it may be inferred that the direction of the 
originating fault is about E. 25° N., which agrees closely with that 
of KE. 27° N. obtained from the earthquakes of July 23 and September 21, 
1905. The corresponding direction given by the first earthquakes of 
the present series, those of September 17 and 22, 1900, is EK. 11° N. 
They originated, however, in a more westerly region, so that it is. 
uncertain whether two faults have been in action or a single fault. 
with varying direction. 
The Ochil fault passes through or near the Hillsfoot villages, at 
which alone the slighter shocks were felt, and has a mean direction 
there of about HE. 13° N. Moreover, at several of these places 
(Menstrie, Alva, and Tuillicoultry especially) the shocks attained an 
intensity which is out of all proportion to the areas disturbed. For 
instance, the earthquake of October 20, 1908, disturbed an area of 
about 1000 square miles, while the average area disturbed by a British 
earthquake of the same intensity is about 27,000 square miles. This 
points to an extremely shallow origin for the Ochil earthquakes, and 
therefore favours their connexion with the great fault of the district. 
With regard to the hade of the fault, the seismic and geological 
evidence are at first sight apparently in conflict. In the neighbour- 
hood of Dollar, which is about four miles east of Tillicoultry, it is 
known to hade to the south. In the district in which the earth- 
quakes originate, that between Airthrey and Tillicoultry, both the 
course and hade of the fault are unknown. ‘That the originating fault 
here hades to the north is, however, clear from the relative positions 
of the isoseismal lines of the earthquake of September 21, 1905, and 
from the fact that by far the larger portion of the disturbed areas of 
several of the slighter shocks! lies on the north side of the fault. 
Either, therefore, the great fault changes hade between Dollar or 
Tillicoultry, or there is another fault in its immediate neighbourhood 
1 Such as those of September 17 and 22, 1900; July 23, October 8, December 28 
and 30, 1905; and October 20 (4.13 p.m.), 1908. 
