COAL MEASTJKES IN THE SOUTH-EAST OF ENGLAND. 239 
fragments of the same great original trough, should not exist 
underground between Somerset and the north of France and 
Belgium. 
With respect to the possibility of denudation having removed 
the intervening Coal Measures, enormous as the extent of 
denudation must have been previously to, the deposition of the 
Permian strata, we cannot admit its exceptional action in this 
case. Denudation has removed from the crest of the Mendips 
a mass of strata possibly equal to two miles or more in height, 
and from that of the Ardennes as much as three or four miles ; / 
and it has also worn extensive channels between many of our 
coal-fields ; so that the power of such an agent cannot be 
denied. (See Sect. Plate LXXXY.). But it is a power of plan- 
ing down exposed surfaces rather than of excavating very deep 
troughs. Notwithstanding the extent of its action on the Men- 
dips and Ardennes, deep troughs of coal measures are left flank- 
ing their northern slopes. These troughs descend to more than 
a mile beneath the level of the sea; and I do not think it 
probable that the intermediate underground portions of the 
trough through South Eastern England, where the axis lies 
lower, have suffered more than those on the higher levels, except 
to the extent caused by the later denudation which preceded 
the Cretaceous period. But this would not affect the main bulk 
of the coal measures. The Belgian coal-field, which was exposed 
to the action of both these denudations, still retains vast pro- 
portions. 
At the same time the pre-cretaceous denudation was very 
irregular in its action, giving rise to hills and plains. At one 
place near Mons the chalk and tertiary strata are above 900 ft. 
thick ; whilst at another, on about the same level, and only a 
few miles distant, they are not 100 ft. thick — an old under- 
ground hill of highly inclined coal measures rising in the 
midst of the unconformable newer strata, and giving rise to 
this difference. This shows that in the English chalk area we 
may possibly find irregular old surfaces of this kind, so that 
the coal measures may exist at places nearer the surface than 
we have estimated. 
We have alluded before to the great length and small width 
of the Belgian coal-fields. That of Liege is forty-five miles 
long, with a mean width of less than four miles, whilst that of 
Hainaut and Valenciennes, with a width scarcely greater, is 
119 miles long. The presence of lower carboniferous rocks 
so far north as Harwich, and the extent of north range of the 
Bristol coal-field, render it possible that the coal trough in the 
intermediate area may have a greater expansion than in 
Belgium ; but we have nothing else to guide us, unless it be 
that the lateral pressure in the intermediate ground was less 
