COAL MEASURES. I75 



fact has been found to be generally applicable in our coal- 

 fields. Stigmaria ficoides has been shown by Prof. W. C. 

 Williamson to be the root alike of Lepidodendron and 

 Stgillaria} In many places conspicuous traces of stumps 

 with roots have been observed, affording relics of the old 

 Coal-measure forests. 



The physical and palasontological evidence prove that the 

 Coal-measures were formed in an area undergoing slow and 

 gradual subsidence, during which pauses occurred which are 

 marked by the different beds of coal. These seams of coal 

 can in some cases be correlated over large areas, while in 

 other cases it is scarcely possible to indicate the extent of 

 any one particular seam. 



That the beds were mostly deposited in a freshw^ater and 

 fluvio-marine area is proved by the organic remains ; at the 

 same time in some localities, especially in the Lower Coal- 

 measures, there are indications of purely marine accumula- 

 tions. The conditions presented were probably those of an 

 inland sea, bordered by swamps, into which several rivers 

 brought and deposited sand and mud. The luxuriant growth 

 of vegetation is suggestive of a warm and equable climate, 

 while the atmosphere no doubt was moist. The coal-beds 

 were formed when either an increase of sediment or a slight 

 elevation produced a land area, or an extensive swampy plain. 

 Many remains of plants were, however, drifted, although not 

 very far. The Cypress Swamps of the Mississippi and the 

 Great Dismal Swamp of Virginia (described by Lyell) appear 

 to furnish the nearest analogues to the conditions prevailing 

 in Coal-measure times. 



Although an immense amount of carbonic acid gas is now 

 locked up in our Carboniferous rocks,^ Mr. Carruthers thinks 

 it not at all likely that the atmosphere was charged with 

 more of that gas than it is at the present day."^ 



It has been remarked by Prof. A. H. Green that wherever we 

 approach one of the old margins of the swamps in which the coal 

 was formed, it becomes impure from earthy admixture, and is 

 more and more split up by partings of shale and sandstone until 

 the coal is entirely replaced.* Jukes too observed that in the South 

 Staffordshire Coal-field there is every gradation from a mere 

 carbonaceous shale to a perfectly pure bright coal with very little 

 earthy matter.^ In other cases, where the coal is sharply cut out 



^ Geol. Mag. 1881, p. 520. 



^ Dr. T. Sterry Hunt, Chemical and Geol. Essays, 1879. 



^ Geol. Mag. 1869, p. 300 ; 1871, p. 497 ; see also J. D. Hooker, Mem. Geol. 

 Survey, ii. part 2, 387. 



* Geology of Yorkshire Coal-field, pp. 20-22. 



^ Geology of the South Staffordshire Coal-field, edit. 2, p. 17. 



