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. 60, representing a section at Wallsend, Newcastle, 



.sj 





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 

 ®f the mine. When the props have been reduced in size, they are pTessed 



