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XXIX. — On the Characters of the Beds of Clay immediately below the Coal- 

 Seams of South Wales, and on the occurrence of Boulders of Coal in the 

 Pennant Grit of that district. 



By WILLIAM EDMOND LOGAN, Esq., F.G.S. 



[Read February 26, 1840.] 



In South Wales, immediately below every regular seam of coal, and coextensive 

 with it, hes a bed of clay, which is commonly called underclay, underclift, under- 

 stone, bottomstone, or pouncing. It is of so peculiar a character, and the miners 

 are so well acquainted with it, that there is scarcely one who would not imme- 

 diately recognize a specimen of it, and state its position in relation to the coal. 



From several sections made with care in various places, it has been clearly ascer- 

 tained that the coal-measures of South Wales, from the mountain limestone up- 

 wards, attain in the deepest part the great geological thickness of 12,000 to 13,000 

 feet. They contain nearly 100 thick and thin seams of coal, about one half of 

 which, measuring a foot and upwards in thickness, have in various places been 

 more or less worked, and I am not aware of a single instance of a seam unac- 

 companied in any part by this subjacent bed ; indeed so thoroughly is the Welsh 

 miner persuaded that the two things are essentially conjoined, that he would as 

 soon expect to live in a house without a foundation as to work in a coal-seam which 

 did not rest upon underclay. It is very necessary that he should be familiar with 

 the material, as it occasionally serves a most important purpose in prosecuting the 

 work of a coal-mine ; for though there is never coal without a subjacent bed of 

 underclay, there is sometimes underclay without superincumbent coal. A seam of 

 coal which becomes thinner by degrees will occasionally vanish altogether ; but, after 

 a short distance, it will appear with increased thickness, and ultimately regain its 

 original dimensions. When a collier, meeting with an irregularity of this descrip- 

 tion, perceives the coal disappear, he might find it difficult to know whether he had 

 not suddenly come upon a fault completely dislocating the measures, if he had not 

 the underclay to guide his course ; and it is only when this fails, that he feels assured, 

 the direction of his search must be changed. 



This underclay, as its name imports, is generally more or less argillaceous, but 

 it is never without a considerable admixture of sand ; and it is always of a fine 



