60 
UNDERLYING FORMATIONS. 
In comparing the underlying rocks of Somersetshire with those: 
of the French and Belgian Coalfield, a great disparity is found to- 
exist. In Somersetshire the Mountain Limestone is underlaid by 
the Old Red Sandstone, which is the oldest rock met with on the 
Mendip Hills, and the same remark applies throughout the: 
extended elevation of the older rocks which forms the southern 
boundary of the Forest of Dean and South Wales Coalfields.. 
Those members of the Devonian rocks which appear far to the- 
west in Devonshire, are nowhere met with on the Mendips, and 
the Silurian, if it exists, lies far out of reach. 
On the N. side of the Bristol Coalfield, in the vicinity of 
Tortworth, which was visited by the Field Club many years ago, 
the section would appear to be more complete, for the 
Carboniferous Limestone is there unlerlaid by the Old Red. 
Sandstone, and afterwards by the Silurian. 
But in the underlying rocks of the French and Belgian Coalfield 
the succession of the Paleozoic rocks is much more extensive and 
complete, embracing in the Ardennes, according to Dr. Hovelacque, 
the following strata in descending order :— 
-Upper Devonian... ... Tamennian 
... Frasnian 
Middle Devonian Givetian 
.... Hifelian 
Lower Devonian { ia Coblentatan 
... Gedinuan 
Silurian... ae ... (Middle) 
Cambrian ... a wee 
Although the Devonians are all represented within the limits 
of the Coal basin, they are not all invariably present, for while to 
the S. of the crest of Condroz, in the Ardennes, in the Basin of 
Dinant, all three divisions of the Devonian are represented, to the N. 
of the same crest only the Upper and Middle Devonians are present. 
The Silurian and Cambrian appear there in certain lines of 
elevation which must have preceded the deposit of the Devonian 
and Carboniferous rocks, as these rest upon them unconformably. 
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