VOLCANIC EARTHQUAKES 122 
disturbed area of British earthquakes of the same intensity is 
24,500 square miles, it is evident that the earthquake originated 
at a very slight depth. The mean direction of the longer axes 
of the isoseismal lines is N.37°W. ‘That of the Irwell Valley 
fault near the epicenter (represented by the broken line on the 
map) is N.34° W. It would therefore seem probable that the 
@) Heywood 
Bolton 
NO 
Middleton 
° 
fe} 
S Prestwich 
N 
Pendlebury 9 ne 
(6) 
Failsworth 
Stockport 
° 
Fic. 9.—Map of Pendleton earth-shake of November 25, 1905 
earth-shake was due to a slip along this well-known fault, and at 
a depth so small that it may well have occurred within the portion 
of the crust penetrated by the pit-workings at Pendleton. If 
this be the case, the fault-slip must have been precipitated, not 
by the natural and gradual growth of the fault, but by the removal 
of the rock which, it is known, has been effected right up to the 
