147 
Geological Society. 
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 water, 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 54y .1 8 
2. The five-quarter 62 3 6 
3. The high main 73 6 3 
4. The Maudlin 83|- 5 0 
5. The Hutton 107 3 8 
