IRONSTONE FORMATION OF THE FOREST OF DEAN. 
267 
the most important, and 
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extends over between 15,000 and 
16,000 acres. The intervening 
beds of rock of this division are 
sandstones and grit-stones ; and 
the bed immediately under the 
Upper Trcnchard seam is coarse 
shale, containing a band of argil- 
laceous ironstone nodules — lo- 
cally called " cherkers," — and 
known as the " Blue BaU Vein." 
This band, although it has been 
only very little explored, is pro- 
bably of considerable economic 
importance. Below the Lower 
Trenchard Vein is a deposit, ten 
yards thick, of fire-clay, which 
is most extensively worked for 
the manufacture of fire-brick. 
The aggregate thickness of the 
coal measures is about 35,000 
feet, and the over-clays or " clod 
tops," of the coal seams, together 
with the argillaceous rocks and 
shales, contain a vast variety of 
fossil vegetable remains. 
One fact concerning the occur- 
rence of these remains is worthy 
of note. It is this : the over- 
clays alone have contained the 
fossils yet discovered, and the 
vegetable markings, often mere 
carbonaceous spots, extend up- 
wards into the rock which is 
superincumbent, but they are not 
found in that which underlies 
the coal-seam. If I may venture 
an explanation, I would suggest that the origin of the coal-seams — and the 
history of one seam is nearly the history of them all — was probably a peat- 
y 2 
