76 
dirt parting. On the Blocking coal f swamp plants did not 
flourish so luxuriantly, and no submergence occurred, and so 
a coal bed, thinner than the Silkstone, and with no dirt part- 
ing, was the result. 
The breaking up of the seam as it approaches the margin of 
the swamp may have been caused in a way that will be under- 
stood by a reference to Fig. 4, Plate II. The barrier separating 
the two swamps may have consisted of a ridge of land, A B 
C, somewhat raised above the low ground on either side. On 
this barrier, for some reason or other, plants did not flourish 
so luxuriantly as over the adjoining swamps. While, there- 
fore, vegetable growth went on freely at a point D, some way 
out on the flat, it would decrease as we approached the ridge, 
and a layer of coal would be found thinning away towards A. 
Again, rain and other denuding agents would sweep the loose 
soil produced by the atmospheric disintegration of the ex- 
posed surface of the ridge down its slopes on to the flat, but 
when the running water reached the level surface it would 
soon come to rest and drop its burden of sand or mud. In 
this way the margin of the ridge would become fringed with 
accumulations of sediment, but these would not reach out far 
on to the flat, but would be wedge-shaped banks, such as 
E F A. In the meantime, the growth of plants out on the 
flat would add another layer of coal material above A D ; 
and away from the ridge this would be clear and pure, but on 
approaching the barrier would become mixed with sediment, 
and gradually thin nwaj as shown at H. On the top of 
Gr H, another band of sediment would accumulate in the 
neighbourhood of the ridge, and another layer of clean coal 
material out on the flat. By a continuation of such a process 
as this there would result a seam of coal, thick and free from 
partings on the left, but splitting up into numerous sub- 
divisions, and falling ofi" in quality and thickness as the 
point A was approached : in short, exactly such a seam as 
