104 



COAL BASINS. 



mines several times, under different beds of coal, with a perfect simi- 

 larity both in the succession and thickness of each. In some instan- 

 ces, a single bed of stone of vast thickness separates two beds of coal. 

 In other instances, only a very thin stratum of shale or clay lies be- 

 tween coal beds. 



Though numerous beds or seams of coal occur in one coal-field, 

 very rarely more than three of these are worked. The thickness 

 of the coal strata in the same coal-field, often varies from a few in- 

 ches to several yards ; but each stratum generally preserves nearly 

 the same thickness throughout its whole extent. Instances to the 

 contrary sometimes occur, in which the same bed will become nar- 

 rower or wider, and sometimes be divided by a stratum of incombus- 

 tible earthy matter, in different parts of its course. Few beds of 

 coal are worked at a great depth, which are less than two feet in 

 thickness. The stratum lying over a bed of coal is called its roof, 

 and the stratum under it the floor. The facility of getting coal de- 

 pends very much on the compactness of the stone which forms the 

 roof, not only on account of the security from falling, but for keep- 

 ing out the upper water, and preserving the pit in a dry state. The 

 great expense incurred in supporting the roof when it is loose, fre- 

 quently prevents a valuable bed of coal from being worked, or ab- 

 sorbs all the profit. In some situations, the roof is indurated clay, 

 impregnated with bitumen and pyrites. When this falls down, and 

 is intermixed with water and small^coal at the bottom, it lakes fire 

 spontaneously ; on which account the miners close up the space with 

 common clay, where the coal has been worked, to prevent the access 

 of air to the combustible matter. This kind of combustible clay is 

 called tow ; it is common in the Ashby-de-la-Zouch coal-field, and 

 in Staffordshire. The floor or stratum on which the coal lies, con- 

 sists of clay in various degrees of induration, and is almost always of 

 that kind which will resist the action of fire, called fire-clay, suited 

 for furnace bricks and crucibles. 



It has been before observed that coal strata are frequently bent 

 in concavities, resembling a trough or basin, dipping down on one 

 side of the field and rising on the other. In Plate IV. fig. 2. the 

 section of a coal-field is represented, in which the coal strata c c c, 

 D D are inclined in this manner, but partially dislocated by a fracture 

 or fauh at f. The extremities of the lowest stratum c c, are sever- 

 al miles distant in some coal-fields, in others not more than one mile. 



In the great coal-field in South Wales, which is rather a long 

 trough than a basin, the strata are arranged in this manner over an 

 extent of nearly a hundred miles in length, and a variable breadth of 

 from five to twenty miles. It is partly broken into by Caermarthen 

 Bay, but it forms an extent of surface exceeding twelve hundred 

 square miles. It contains twenty-three beds of workable coal, which 

 are said by Mr. Martin to make together ninety-five feet in thickness 

 of this valuable mineral ; this will yield sixty-four million tons of coal 



