492 STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 



is estimated by Jukes°^ at from i,ooo to 3,000 feet, but the lower 

 portion is almost beyond doubt of Coal Measures age. In other 

 fields it is much less. Coal rarely occurs ; Jukes notes a local seam, 

 10 inches thick and resting on fireclay. Numerous casts of Sigil- 

 laria were obtained from red sandstones lower in the section. 



The Carboniferous deposits have been grouped into : 



Upper Carboniferous. Upper Coal Measures ; Middle Coal 

 Measures or Pennant Series ; Lower Coal Measures or Canister 

 Series ; Millstone Grit. 



Lower Carboniferous. Yoredale Shales, equal in part to Pen- 

 dleside ; Carboniferous Limestone. 



Classification of the Coal Measures is perplexing and field work- 

 ers have employed designations in the local sense. Kidston has 

 offered a grouping based on an elaborate study of the plants : 



Radstockian Series ; Stafifordian Series ; Westphalian Series ; 

 Lanarkian Series. 



The first two are the Upper, the third is the Middle, while the 

 fourth includes the Lower Coal Measures and the Millstone Grit.^^ 



Happily, the matters involved in this study have, except in a few 

 cases, little to do with questions of classification, so that, in prepa- 

 ration of synopses of reports, it is sufficient usually to accept the 

 grouping employed by the authors. 



The South Wales Coalfield. — This, in the southern portion of 

 England and Wales, was restudied by A. Strahan, W. Gibson, R. H. 

 Tiddeman and T. C. Cantrill.^^ It extends from Monmouthshire at 

 the east to Pembrokeshire at the west. The Permian is not present 

 but the Carboniferous, Upper and Lower, is well marked. The 

 Coal Measures are readily divisible into Upper, Pennant or Middle, 

 and Lower, which are now regarded as equivalent to those of other 

 fields. The Millstone Grit is persistent and characteristic ; it and 

 the Coal Measures thicken greatly toward the east; but their total 



91 J. B. Jukes, "The South Stafifordshire Coal-Field," 2d. Ed., 1859, pp. 



12, 13- 



92 This classification was presented first in 1888, but the final statement 

 vv'ith explanations is given in R. Kidston, Trans. Roy. Soc. Edinb., Vol. I., 

 1914, pp. 74, 75. 



93 "The South Wales Coal-Field," Parts I.-X., Geo!. Snrv. Mem., 

 1899-1912. 



