Dr. C. Davison — Earth-shakes in Mining Districts. 223 



this seam has been worked, and that, under the area affected by the 

 «arth-shake, it has been exhausted for at least eight or nine years. 

 The Parkgate, Thorncliffe, and Silkstone seams, which are at depths 

 of about 260, 300, and 360 yards respectively, below the Barnsley 

 bed, have also been worked largely. 



I)odiUi-0-rtlx\ '\ 



Scale of jVT i.le s 



Fig. 2. — Area of the Barnsley Eartli-shake. 



The areas disturbed by these three earth-shakes are unusually 

 large for this class of disturbance. It would appear, then, that the 

 boundary of the disturbed area is approximately circular in form 

 when the area is small and the shock of slight intensity ; but, when 

 the shock is strong and the disturbed area comparatively large, the 

 form of the boundary is distinctly elliptical, and the longer axis of 

 the ellipse is parallel to the neighbouring lines of fault. 



The evidence of these shocks thus supports the view that earth- 

 shakes in mining districts are produced by small fault-slips pre- 

 cipitated by the removal of the coal from the immediate neighbourhood 

 of the fault, or partly, perhaps, as Mr. Eichardson has suggested to 

 me, by the lowering of the water in that region by pumping in 

 other parts of the mine. In either case — and this is the main 

 point — the earth-shakes would owe their origin to operations of 

 nature, aided very effectively by those of man. 



