STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 495 



yielded many trunks of trees, some of which are now in the Museum 

 at Swansea. Many years ago, de la Beche and Logan''*' saw two 

 erect stems near the head of Swansea Valley in Glamorgan ; the 

 shale underlying the sandstone was uncovered and found to contain 

 abundance of vegetable remains, proving it to be a vegetable soil, but 

 the statement does not indicate that roots were found attached to 

 the stems. The trees were Sigillarla; vertical stems of this type 

 occur frequently. 



Coals in this field decrease in volatile content downwardly ; the 

 Upper and Pennant coals are gas, while those of the Lower Series 

 are steam coals. But the variation is more marked in all seams as 

 they are followed westward ; gas coals become steam coals and the 

 steam coals become anthracitic until at last in the western portion 

 anthracite prevails. It should be noted here that the total thickness 

 of measures in the anthracitic area is very much less than was esti- 

 mated by the earlier surveyors. Strahan®'^ has given an elaborate 

 discussion of the conditions, which well deserves careful consid- 

 eration. 



The South Staffordshire Coalfield.^^ — This is chiefly in the 

 southern part of Stafford but extends into the adjoining counties of 

 Worcester, Shropshire, Warwick and Salop. The region has un- 

 dergone great disturbance and correlation is hardly possible in some 

 portions, but the relations are clear in the northern districts. The 

 Lower Carboniferous and the Millstone Grit are not reached but 

 the Permian and the Coal Measures are present. The boundary 

 between these formations had not been determined at the time when 

 Jukes wrote ; they appear to pass gradually one into the other. No 

 unconformity has been seen between Coal Measures and the deposits 

 taken to be Permian. The latter according to Jukes are from i,ooo 

 to 3,000 feet thick and are extremely variable. Observations by 

 geologists in recent years make more than probable that the lower 

 part of this Permian belongs to the Radstockian Series of Kidston. 



The succession below Permian, according to Jukes, is: (i) The 



96 H. T. de la Beche, " Geological Observer," Amer. ed., 1851, p. 482. 



" A. Strahan and W. Pollard, " The Coals of South Wales," etc., Mem. 

 Geol. Survey, 1908, pp. 65 et seq. 



98 J. B. Jukes, "The South Staffordshire Coal-Field," Mem. Geol. Survey, 

 26. ed., 1859. 



