XCU PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [vol. LxXV, 



From the character and relations of the deposits, it has heen 

 generally recognized that they were accumulated in an area of 

 differential subsidence, and Mr. Cosmo Johns. 1 in discussing this 

 matter, showed that the coal-seams can be used for measuring the 

 •direction and amount of the differential moYements by the method 

 which I have since used in the Jurassic rocks. 



The coalfields of the Midlands afford evidence of local and 

 irregular subsidences and re-elevations in the same great belt, but 

 ihe northern basin appears to have become shallow and broken 

 to the south, and the structures are disconnected and complex. On 

 the whole, however, they appear to indicate that the centre of 

 •depression in Carboniferous times lay farther west than after- 

 wards, so that the Carboniferous, Triassic, and Jurassic basins 

 ■of the Midlands, though in part confluent, had their greatest depths 

 ranging in order from west to east, like the present outcrops. 



Farther south-westward, the Carboniferous field of South Wales 

 is another example of a tilted syncline in which strong differential 

 movement during the period is implied by the arrangement of the 

 rocks ; and of this area it is remarked by Sir Aubrey Strahan ' that 

 there may be some connection in shape between the present Car- 

 boniferous basin and the area of subsidence in which the maximum 

 •development of the Carboniferous rocks took place/ 2 



Beyond the Carboniferous basins it is not safe to pursue the 

 argument, as we know so little about the concealed areas of the 

 older rocks, and to consider only the exposed tracts would be 

 manifestly unsound. But I cannot help suspecting, on general 

 considerations, that the great piles of Devonian, Silurian, and 

 Ordovician rocks of the Welsh borderland may mark the positions 

 -of sedimentary basins which have been filled and upheaved, and 

 that the very fact of the persistence of the piles is an indication 

 "that they represent the thicker parts of the original formations. 

 This brings me to a corollary arising from the study of the wedge- 

 structures, which I think may be of wide application. 



1 ' On Differential Earth-Movements during Carboniferous Times. & their 

 Significance as Factors in Determining the Limits of the Yorkshire. Derby- 

 shire, & Nottinghamshire Coalfield : Proc. Y r orks. Geol. Soc. vol. xv (1905) 

 pp. 372-79. 



- 'The Geology of the South Wales Coalfield: Part II — The Country 

 around Abergavenny ' Mem. Geol. Surv. 1900. p. 19 ; see also 'The Coals of 

 ■South Wales &c." ibid. (2nd ed.) 1915 ; pp. 81-83. 



