Ch. V.] 



CREEPS IN COAL-MINES. 



51 



act amount of depression in these cases can only be accurately measured 

 where water accumulates on the surface, or a railway traverses a coal-field. 

 When a bed of coal is worked out, pillars . or rectangular masses of 

 coal are left at intervals as props to support the roof and protect the 

 colliers. Thus in fig. 66, representing a section at Wallsend, Newcastle, 



-o 



the galleries which have been excavated are represented by the white 

 spaces a b, while the adjoining dark portions are parts of the original 

 coal-seam left as props, beds of sandy clay or shale constituting the floor 

 of the mine. When the props have been reduced in size, thev are pressed 



