470 J. Prestwich, Esq., on the 



clays and marlsj with the subordinate calcareous seams of the southern dis- 

 tricts, may have been derived from the old red sandstone and upper Silurian 

 rocks. 



We find the peroxide of iron distributed in a greater or less degree 

 throughout the whole of the coal-measures ; but it is in those beds of shale 

 only which appear to have been formed gradually and slowly, that we find it in 

 any great plenty. This proves the abundance of iron in the rocks from which 

 the silt was derived. Now we know that all trap rocks contain more or less 

 iron. Dr. Withering found in those of Staffordshire, 20 per cent.* ; Klaproth, 

 in those of Hazenberg, 20 per cent.f ; and out of numerous specimens of 

 basalt which he analysed, Bergman found, upon an average, as much as 25 

 per cent.|. As the ironstone itself contains only about 33 per cent, of iron, it 

 would require no very extensive degradation of trap rocks to produce a bulk 

 of ironstone equal to the entire volume of that existing in the coal-measures ; 

 whilst the residual silica and alumina, in the proportion of about 73 of the 

 former to 27 of the latter, give nearly the composition of the ironstone shales. 



As a further support of this hypothesis, I have found, in several of the con- 

 glomerates of the coal-measures, traces of the debris of igneous rocks. 



It has also been shown, that the carboniferous series frequently reposes upon 

 beds of trap and basalt. Prom the soft and friable texture and rapid disin- 

 tegration of many of these rocks, when exposed to the action of the atmo- 

 sphere, they would admit of great abrasion, and afford ample materials for 

 extensive beds of ironstone. The presence of titanium in the iron ore (see 

 table of minerals. Appendix B.) tends to corroborate its derivation from 

 these igneous rocks, as that metal is of very common occurrence in them. 

 According to Mohs, sphene is found " in beds of syenite, and other trap 

 rocks§." De la Beche states that "Basalt is supposed to be essentially com- 

 posed of felspar, augite, and titaniferous iron |1 ;" whilst Berthier says, " mais 

 c'est surtout dans les roches volcaniques, que le titane abonde; il i/ est ton- 

 jours combine avec lefer et le manganese^." 



It will readily be seen that the Coalbrook Dale coal-field differs greatly, both 

 in its lithological character and its organic remains, from most of the English 

 coal-fields. The latter are in general nearly destitute of animal reliquije, and 

 exhibit but few conglomerate strata, whereas the former abounds in both. 



* Conybeare's Outlines of Geology, p. 139. f Ure's Chemical Dictionary, 3rd edition. 



X Ibid. § Mohs' Mineralogy, translated by Haidinger, vol. ii. p. 376. 



II Geological Manual, 1st edition, p. 122. 

 ^ Essaies par la Voie Seche. Paris, 1834. Vol. ii. p. 112. 



