STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 501 



The Yorkshire Coalfield was described elaborately by Green^°^ 

 and his associates. It contains the whole column from Permian to 

 the upper part of the Lower Carboniferous. The succession is : 

 Permian, represented by the Magnesian Limestone, the Zechstein ; 

 Upper Coal Measures, perhaps 150 feet; Middle Coal Measures, 

 about 3,500 feet; Lower Coal Measures, about 1,600 feet; Millstone 

 Grit, perhaps 2,000 feet ; Yoredale Shales ; Carboniferous Limestone 

 is not reached. 



Permian and Upper Coal Measures deposits remain at very few 

 localities and for the most part the boundary is obscure, for the rela- 

 tions of the lower beds are in dispute. The Magnesian Limestone 

 rests unconformably upon the rocks in question. Near Pontyfract 

 is a great sandstone, averaging not less than 75 feet, resting on about 

 40 feet of purple shale and yellow sandstone. It seems to be con- 

 formable to the underlying beds but is distinctly unconformable to 

 the overlying Magnesian Limestone. This rock was referred by 

 Smith and by Sedgwick to the base of the Permian, their conclusion 

 being due in great measure to the red color, but Green asserts that 

 this cannot be taken as criterion, for red color characterizes many 

 deposits, which belong undeniably to the Coal Measures. Near 

 Conisborough, the Pontefract is wanting and the Magnesian Lime- 

 stone overlies 34 feet of very red beds. These rest conformably 

 upon the underlying beds and contain Coal Measures types of Nen- 

 ropteris, Sphenopteris and Stigmaria. The Red Rock of Rother- 

 ham, a great mass of sandstone and shale, occupies a trough eroded 

 in the Middle Coal Mieasures. Its age is in dispute and Green 

 declines to commit himself to either interpretation. The mass is 

 certainly unconformable to the Coal Measures but a distinct expo- 

 sure at one locality shows it distinctly unconformable to the un- 

 doubted Permian beds above. ^°^ 



Coal seams are the most nearly constant deposits, because formed 

 in swamps ; but swamps must end somewhere ; at their margins coal 

 becomes impure, is split by increasing number of clay or sand layers 

 until at length it is replaced with sandstone or shale. Evidence is 



102 A. H. Green, R. Russell and others, " The Geology of the Yorkshire 

 Coal-Fields," Mem. Geol. Survey, 1878, pp. xiii and 823. 



103 Green and Russell, pp. 481-486. 



PROC. AMER. PHIL. SOC, VOL. LiX, FF, DECEMBER 1 7, 192O. 



