Geology of Coalhrook Dale. 471 



In the Coalbrook Dale field the mountain limestone has but a trifling com- 

 parative thickness; the millstone grit also is entirely wanting below the coal- 

 measures; and as the organic remains present a far closer analogy to those 

 of the carboniferous limestone of Yorkshire and Northumberland than to the 

 coal-fields of those counties^ it appears very probable, that this coal-field is of 

 older date than most of those in the north and south of England. 



The coal-fields to which it bears the closest resemblance are those of North 

 Northumberland^ Durham, parts of Yorkshire and Scotland. Many of the 

 Conchifera and Crinoideae, the Conularia, the Megalichthys and Gyracanthus, 

 which characterise the Coalbrook field, are abundant in the subordinate beds 

 of limestone and ironstone in the Scotch coal-fields. 



Mr. Phillips, in his excellent Illustrations of the Geology of Yorkshire, di- 

 vides the carboniferous formation into three groups, viz. the Millstone grit, 

 the Yoredale series, and the Great Scar limestone, each characterised by 

 peculiar but variable hthological composition, and, during the prevalence of 

 like conditions, by organic remains of a generally similar character. 



Upon comparing the sections of Wensley Dale,Whernside, Penyghent, and 

 Coverdale* with that of Lilleshall (pp. 425, 427), the general similarity in Htho- 

 logical structure is very striking ; there are the same alternations of arenaceous 

 strata with subordinate limestones, characterised by the presence of numerous 

 Polyparia, some Brachiopoda and Cephalopoda, but with very few Crinoidea. 

 Again, the resemblance between the black limestone group of Yorkshire and 

 that of Steeraways, both characterised by the presence of Lithodendra and 

 large Leptaenae (Producta), induce me to consider the limestone strata of 

 Lilleshall and Steeraways as the equivalents of the Yoredale series, which 

 Mr. Phillips has shown to attenuate and become finer grained as it ranges 

 southward. 



Supposing, therefore, that these limestones represent the Yoredale series, 

 or a portion thereof, it is probable, that the measures of Coalbrook Dale, which 

 immediately succeed, are coeval or synchronous with the lower millstone 

 grit, and that the variable fine conglomerates and concretionary shales, form- 

 ing the upper portion of the coal-measures, and eventually throwing out the 

 coals and ironstones in the north of the field, may be the representatives of 

 the Upper Millstone or Brimham grit. The greater abundance than is 

 usual in the millstone grit, of many marine remains of the limestone groups, 

 might arise from the augmented vertical range, in this district, of those con- 

 ditions suited to the existence of Testacea, and which prevailed during the 



* See Phillips's Illustrations of the Geology of Yorkshire, Part II. pp. 37 — 57. 



