IRONSTONE FORMATION OF THE FOREST OF DEAN. 



267 



the most important, and 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 Trenchard seam is coarse 

 shale, containing a band of argil- 

 laceous ironstone nodules — lo- 

 cally called cherkers," — and 

 known as the "Blue Ball 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 

 / /// Itn^f^ 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- 



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