COAL 



25 



such a variety of local circumstances from the day they 

 were forming, that it is only possible to unravel the 

 causes when an individual case is closely studied. The 

 main sequence, however, is constant and is that illus- 

 trated in fig. 13. 



The second type of seam is that in which the under- 

 clay floor is not present, and is replaced either by shales 

 or by a special very hard rock of a finely granular nature 

 called "gannister". In the gannister floor it is usual 

 to find traces of rootlets and basal stumps of plants, 

 which seem to indicate that the grannister was the 





LIMESTONE 



SHALY ROOF 

 COAL 



GANNISTER 



Fig. 14. — Diagram of Coal Seam with Gannister Floor, in which are 

 traces of rootlets r, and of stumps of root-like organs s 



ground in which the plants forming the coal were 

 rooted. The coal itself is generally very pure plant 

 remains, though between its layers are often found 

 bands of shaly stone which are called "dirt bands". 

 These are particularly noticeable in thick seams, and 

 they may be looked on as corresponding to the roof 

 shales; as though, in fact, the roof had started to form 

 but had only reached a slight development when the 

 coal formation beg-an agfain. 



That the coal is strikingly different from the rocks in 

 which it lies is very obvious, but that alone is no indi- 

 cation of its origin. It is now so universally known and 

 accepted that coal is the remains of vegetables that no 

 proofs are usually offered for the statement. It is, how- 

 ever, of both interest and importance to marshal the 



