§ 6. NATURE OF COAL DEPOSITS. 667 



general idea of the strata of which a coal-basin con- 

 sists. But it is necessary to enter more particularly on 

 the nature and arrangement of the beds of coal, and their 

 associated deposits ; for though many accumulations of car- 

 boniferous rocks have manifestly been formed by different 

 and local agencies, the grand series of ancient coal-measures, 

 setting aside unimportant discrepancies, present a re- 

 markable uniformity of character, not only throughout 

 Great Britain and Europe, but also in most other parts of 

 the world.* 



"We have seen that the strata constituting a coal-field 

 are alternating layers of coal, clay, shale, and sand, of 

 variable thickness, based either on grit, or limestone 

 abounding in marine shells and corals. Now, a very 

 remarkable fact is the uniform presence of a thick bed of 

 earthy clay beneath every layer of coal, and a stratum of 

 slaty clay or shale above it. 



One of the series of which a coal-field consists presents, 

 therefore, the following characters : — 



(1.) Lowermost; — a stratum of clay, called from its 

 position, the under clay ; a tough argillaceous substance, 

 which upon drying becomes a grey friable earth. Occa- 

 sionally this clay is of a black colour from the presence of 

 carbonaceous matter. This bed almost invariably contains 

 an abundance of the fossil vegetables termed Stigmarice^ 

 which are generally of considerable length, and have their 

 rootlets or fibres attached, and extending in every direction 

 through the clay. These stems commonly lie parallel with 

 the planes of the stratum, and nearer to the top than to the 

 bottom. 



# * The various Memoirs on the British coal-fields in the Geological 

 Transactions, by some of our most eminent observers, and in the works 

 of Messrs. Bakewell, Conybeare, Phillips, Lyell, De la Beche, Buck- 

 land, Murchison, and others, will afford those who wish to pursue the 

 inquiry, information of the most important and interesting nature, 

 f See Medals of Creation, p. 139. 



