12 HOLGATE : CARBONIFEROUS STRATA OF LEEDS. 
floated fronds of ferns and calamites are blue in colour, and not so 
dense or tough. 
We also realise that even if our coals were to run short, we 
should still have beds of immense thickness which, though not so 
valuable as pure coals, might in case of need be made to render good 
service (See Table Xo. 20, 50, etc.) 
The sharp line of demarcation between the coal and the over- 
lying stratum has most probably been produced by such stratum 
sliding over the coal during the elevations and depressions that have 
taken place since they were deposited, and by the different distances 
apart at which the force producing lateral pressure has caused the 
strata to joint or cleave. 
Sometimes we find that the trees actually took root under water 
and grew for some time whilst a considerable amount of mud was 
being deposited ; that the mud afterwards ceased to be brought down 
and that consequently the coals are pure. We thus have an impure 
coal immediately above the seat earth and below the pure coal. 
This is what has taken place in the Beeston Bed (Table No. 41). 
For about two feet between the seat earth and the coal at present 
worked is a stratum which is left unworked at the present time. 
Years ago this was sold at a low price to the very poor under 
the name of " Doggies." It is a very heavy and dirty coal. At 
Churwell it is known as the " Churwell thick," and possesses the 
same properties. Here the Beeston Bed actually divides into two 
distinct coal seams, the distance between the upper and the lower 
part increasing from nothing up to a distance of ten yards, with 
intervening measures of shale, showing that at Churwell the forest 
was completely overwhelmed by the mud which had been brought 
down by the water, but that at Hunslet it was left free from mud to 
grow with greater purity. This it did, making a coal seam 8ft. in 
thickness but with several mud partings. It is interesting to know 
that the latter continued growing, whilst that at Churwell was 
being covered up with mud to the height above named and 
that the forest again extended itself over the Churwell area, the 
upper part of the Beeston Bed making the Upper Churwell thin 
coal. There are few coal seams that did not vary in character during 
