198 Mr. AiKiN on the Wrek'tn^ and on the 



yield can only be applied to the burning of lime : these are called 

 stinking coals. The first coal forms the ninth bed from the surface, at 

 the depth of one hundred and two feet, and is not more than four 

 inches thick : it is very sulphureous. In the space between this and 

 three hundred and ninety-six feet, lie nine other beds of stinking coal, 

 none of which exceeds the thickness of seven inches except the 

 lowest, which is a little more than a foot and a half thick. The 

 first bed that is worked is a five-foot coal at the depth of four hun- 

 dred and ninety feet, between which and the big-flint sandstone, 

 already mentioned, are a ten-inch and a yard coal. 



But the greatest deposit of coal is in the space (about one hun- 

 dred feet) between the big and little flints, consisting of nine beds, 

 the aggregate thickness of which is about sixteen feet. Beneath this, 

 and the lowest bed of the whole formation, is a sulphureous eight- 

 .inch coal. 



The depths and thicknesses mentioned above are such as present 

 themselves in the meadow pit in Madeley colliery, which from its 

 offering a greater number of strata than are to be found in any other 

 pit, deserves to be considered as the best authority for the whole 

 coal field, as far as regards the number and order of the beds 

 of which it is composed. But, by consulting the registers of the 

 other collieries, we learn, that some of the strata composing this 

 formation, especially the beds of coal and clay, are by no means 

 so regular, either in their extent or thickness, as is generally repre- 

 sented to be the case with stratiform floetz rocks. 



Thus, if we confine our attention only to those beds which lie 

 between the big and little flints, and which constitute by far the 

 most regular part of this coal field, we shall find that the pennystone 

 bed, which in the Madeley pit varies in thickness from six to eight 

 feet, is fifteen feet thick dX Lightmoor, about twenty feet atDa^^l^7, 



