464 



GENERAL RELATIONS OF THE COAL-FIELD. 



while they enrich the stock of knowledge with details concerning a region so interesting 

 to men of science, from the complication and variety of its natural phenomena. 



General relations of the Coal-field. 



The surface of this field is much covered hy gravel, clay, and boulders. A few of the 

 latter, particularly in the Wolverhampton tract, have been derived from distant places 

 in the north ; but the chief masses of finer detritus have resulted from the abrasion or 

 breaking up of the Lower New Red Sandstone, mixed up with fragments of trap, parti- 

 cularly where the coal strata have been forced up through it. The debris of red sand- 

 stone constitutes, occasionally, mounds of great thickness, which might at first sight 

 be mistaken for undisturbed portions of the formation. These, as well as the other 

 loose superficial materials, will be considered in subsequent chapters. 



The youngest formation, in situ, is the Lower New Red Sandstone, by which, as 

 already shown, the coal-field is surrounded (p. 54 et seq.), and which passes down con- 

 formably hi to the coal-bearing strata. Notwithstanding this field contains the thickest 

 seams of coal in England, its associated strata are of very small dimensions compared 

 with the equivalent series in other parts of the kingdom. The millstone grit is scarcely 

 represented \ and the mountain limestone, the usual base of the system, as well as the 

 Old Red Sandstone, are entirely absent. "The absence of these formations," as Mr. 

 Conybeare has well observed, " and the immediate contiguity of the coal measures and 

 a transition rock (the Dudley limestone), constitute a remarkable and important cha- 

 racter in this coal-field 1 ." 



The real substrata of the coal deposits consist of various members of the Silurian 

 System. These ancient rocks, however, instead of appearing in the regular order in 

 which they have been described in other parts, rise up irregularly, like islands, through 

 the coal measures near Dudley and Sedgeley, and form the eastern boundary of the field 

 near Wallsall. Such relations will be understood by reference to the two general sec- 

 tions across the tract. (PL 37. figs. I and 2.) To convey, moreover, to the reader 

 some conception of the original condition of these deposits, both before and after their 

 elevation, I annex two wood-cuts. 



The first is an ideal representation of a carboniferous group of strata, one portion of 

 which (b) comprises overlying thick coal seams, the other (c) lower coal measures which 

 spread out beyond the thick coal ; the whole resting upon Silurian rocks (d) and covered 

 by the Lower New Red Sandstone (a). The second represents a generalized outline of 

 the condition of such strata after the sedimentary deposits were pierced and dislocated 

 by volcanic rocks (e) and thrown into their actual positions. 



1 Outlines of the Geology of England and Wales, p. 416. 



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