Geologic a I Society. 147 



If the depth from the surface does not exceed 30 fathoms, and sand- 

 stones form the predominant strata, the subsidence is about equal to 

 the thickness of the seam of coal removed ; but if metal-stone con- 

 stitute the greater portion of the intervening mass, the amount of de- 

 pression in the surface is less. This rule is considered to hold good 

 at all depths. 



The degree of subsidence does not depend so much on the thick- 

 ness of the bed of coal, as on the entire removal of it ; but Mr. 

 Buddie states, that he has had no opportunities of making correct ob- 

 servations on the relative effect produced in the surface. If a con- 

 siderable portion of the coal be left, although quite inadequate to the 

 support of the superincumbent strata, the subsidence is retarded. 

 This is more particularly the case in the Newcastle system of work- 

 ing, where rectangular pillars are left in the first instance and after- 

 wards removed. In working these pillars, stooks or blocks of coal of 

 considerable strength are left as props to protect the colliers from 

 the exfoliation of the roof; and though a subsidence of the super- 

 incumbent strata invariably takes place, yet the extent in the first 

 instance is governed by the degree of resistance of the stooks ; and 

 it frequently happens, that a large tract of a coal mine remains for 

 several years only filled in part, and without any perceptible change 

 occurring. In course of time, however, from the exfoliation of the 

 stooks and the operation of the atmosphere, a further subsidence, 

 called a second creep, takes place, and generally continues until the 

 excavation is completely closed. 



In the Yorkshire system, by which all the coal is taken out in the 

 first instance, except small pillars, the roof being principally sup- 

 ported by wooden props and stone pillars, the subsidence of the 

 strata takes place immediately after the coal is removed, and there is 

 no second settlement. 



It is only where water accumulates on the surface or a railway 

 traverses a coal-field, that the amount of subsidence can be accurately 

 ascertained. 



In one instance, mentioned by Mr. Buddie, the excavation of a 

 bed of coal 6 feet thick, one-fourth having been left in " stooks," 

 the depth of the bed from the surface being 100 fathoms, and the 

 overlying strata principally sandstone, the amount of subsidence was 

 shown by the accumulation of a pond of M'^ater, to have been rather 

 more than 3 feet deep. 



In another instance, it was found necessary to restore the level of 

 a railway three times, in consequence of three distinct sinkings of the 

 surface having followed the successive excavating of three seams of 

 coal. The tract in question is of a quadrangular form and about 23 

 acres in area, and contains the following five seams of coal : 



Coal. Depth below the surface. Thickness. 



fath. ft. in. 



1. The three-quarter seam 54.i- .- 1 8 



2. The five-quarter 62 3 6 



3. The high main 73 6 3 



4. The Maudlin 83-J- 5 



5. The Hutton 107 , 3 8 



L 2 



