EARTH-MOVEMENTS DURING CARBONIFEROUS TIMES. 375 
IV. — Significance of the Thinning of the Coal Measures 
Towards the Margins. 
That the Coal Measures thin, sometimes with rapidity, 
towards tlieir margins has long been known. The case of the 
famous Dudley seam formed by the measures between the coal 
seams of the South Staffordshire coalfield thinning out to nothing 
against the Silurian rocks is perhaps the most striking instance. 
In the Yorkshire coalfield the thinning is very marked towards 
its southern boundary. We have already seen that the central 
area was characterised by differential subsidence ; if now we 
consider the southern boundary to have been an axis of relative 
stability, we are enabled to explain tlie known facts. This 
explanation, too, would be in harmony with movements in other 
portions of the area under discussion, and is clearly brought out 
in Fig. 3. In other words, the downward movement was less 
rapid as the margin is approached, and might have been nothing 
at the axis itseff. It is difficult to discover how much further 
back we must date the limiting action of this ridge, but it is 
suggestive that in South Wales* it was hinted " that there 
may be some connection in shape between the present Carbon- 
iferous basin and the area of subsidence in which the maximum 
development of the Carboniferous rocks took place." This 
would imply that the present configuration of the basin was 
determined by pre-Carboniferous movements, v/hich we have 
already seen is rather clearly indicated in the Yorkshire coal- 
field. The significance of the thinning of the Coal Measures 
towards their margins lies in the evidence they afford that the 
present limits of the basins were determined, not by post- 
Carboniferous upheavals, but by a series of differential move- 
ments of inter-Carboniferous age. 
V. — Character of Faulting. 
In the Yorkshire coalfield tlie faults are normal ones, that 
is, they hade to the downthrow side. This of course involves 
extension of the strata affected, and indicates that there was 
*The Geology of the South Wales Coalfield, Part TI., p. 19. 
