VOL. XIV.(1) | PRE-RHAETIC DENUDATION 49 
Silurian, the Old Red Sandstone, and the Carboniferous 
system, as exhibited in the West of England, may there- 
fore be taken at about 25,000 feet, or nearly five miles. 
The accumulation of these five miles of strata implies 
the subsidence of the earth’s crust to nearly the same 
extent. I say “nearly,” for in the epoch of the Coal 
Measures the sea must have become choked with sediment, 
and the land surfaces must have remained for ages at about 
sea-level, with slight oscillations above and below it. But 
the depression cannot have been much less than five miles, 
for the shallowness of the water throughout the period of 
accumulation is indicated as clearly by the corals of the 
Silurian and Carboniferous limestones as by the coarseness 
of the deposits of the Old Red Sandstone epoch. 
Towards, or at,’ the close of the Carboniferous epoch, 
this great series of sediments was affected by forces which 
bent the strata into folds, arch-like flexures alternating with 
curved hollows, the folds being sometimes shortened so 
as to resemble elongated domes or ellipsoidal basins. It 
was now that the Silurian strata west of the Malverns were 
contorted, the anticlinal domes of Woolhope and 
May Hill were ridged up, and the Coal Measures of 
Bristol and the Forest of Dean were bent int 
basin-like depressions. The earth-crumpling was 
not, indeed, confined to our area. Similar foldings 
affected Great Britain and large parts of Western 
Europe, while, thousands of miles to the west, vast 
masses of Paleozoic strata were bent into folds whose 
denuded stumps now form the great mountain system of 
the Appalachians. 
The contortions which affected the strata in pre-Permian 
times brought the Somerset and Gloucestershire area above 
the level of the waves. This mass of land probably trended 
1 The data for determining this question are at present insufficient. 
