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iide, fo that the fuperincumbent matter with the feam of coals 

 has been invariably thrown upwards by the convulfion, whilft 

 the remaining part of the bed has been left as it was before the 

 motion. 



Whitehaven collieries abound with what they there call 

 Dykes, that is, beds of ftone of a confiderable thicknefs, which 

 feparates one field of coal from another. The principal ones run 

 in a diredion nearly eaft and weft. They divide the feams of 

 coal into fields, as they are called, that is, feparatd trafls of coal 

 almoft like the fields or inclofures of a farm. Thefe dykes or 

 foparations are very ufeful, by reftraining the water or inflammable 

 air from flowing out of any adjoining field of coal, where 

 no works are going on, into another where men are working, until 

 it is found convenient to cut through or work a new field. 

 Without thefe dykes it would frequently be very difiicult to keep 

 the works from being overcharged with water, but it is fome- 

 times very troublefome and expenfive to cut through them, 

 being of a confiderable thicknefs. Where the covering of fuper- 

 incumbent matter is not of fo great a thicknefs, which is to- 

 wards the rife of the feam or field, there pillars of coal are left 

 from five to ten yards fquare, and the workings are from three 

 to four yards wide, fo that about one-half of the coal is taken 

 away, and the other half left to fupport the earth above. Where 

 the coals lie from one hundred and fifty to three hundred yards 

 deep, and efpecially where the coal is drawn from under the fea, 

 the pillars are left from fixteen to twenty yards fquare, fo that 



about 



