[ 165 ] 



XIII. — On Subsidences produced hy working Beds of Coal. In a Letter to 

 C. Lyell, Esq., F.G.S.,/rom John Buddle, Esq., F.G.S. 



[Read November 6th, 1839.] 



Subsidence of the surface invariably follows the working of the subjacent coal, 

 when sufficient pillars are not left to support the overlying mass. The extent of 

 such subsidence is governed by the following circumstances : 

 1st. The depth of the seams of coal below the surface. 

 2nd. The thickness of the seam or seams. 



3rd. The nature of the strata between the surface and the seams of coal. 

 4th. Whether the pillars of coal are wholly or partially worked. 



If the depth from the surface does not exceed twenty-five or thirty fathoms, 

 and sandstones form the predominant strata, then the subsidence is nearly, if not 

 fully, equal to the thickness of the seam of coal which has been taken out ; but 

 should metal-stone or shale form the predominant strata, the subsidence is less. 

 This rule I consider to hold good in all cases, let the depth from the surface be 

 what it may ; that is to say, the subsidence is always greater under a sandstone 

 than under a metal-stone cover*. 



Whenever a lower seam is worked after an upper one has been removed, and 

 the surface lowered in the manner above stated, a second set, or lowering, takes 

 place. I have known this to occur three successive times, as will afterwards be 

 shown ; and that it might not, under certain conditions of the stratification, take 

 place an indefinite number of times, I am not prepared to say. 



The thickness of the coal removed affects the degree of subsidence ; but I am 

 not enabled to state in what proportion it operates, not having had an opportunity 

 of making correct observations on this point. 



The degree of subsidence, however, does not depend so much on the thickness of 

 the coal as on the effectual removal of it, technically called " Clean Working. " If a 

 considerable portion of the coal should be left, although by no means adequate to 

 the support of the superincumbent strata, it yet retards the subsidence and prevents 

 the downward pressure of the mass from exerting that degree of force and momen- 



* Cover means the whole thickness of the strata between the seam and the surface. 



