COAL-MEASURES. 
39V 
10,000 feet of the coal-measures had been caiTied away be¬ 
fore the deposition even of the lower Permian rocks which 
were thrown down upon the already disturbed truncated 
edges of the coal-strata.'*' The carboniferous strata most 
productive of workable coal have so often a basin-shaped ar¬ 
rangement that these troughs have sometimes been supposed 
to be connected with the original conformation of . the sur¬ 
face upon which the beds were deposited. But it is now ad¬ 
mitted that this structure has been owing to movements of 
the earth’s crust of upheaval and subsidence, and that the 
flexure and inclination of the beds has no connection with 
the original geographical configuration of the district. 
COAL-MEASUKES. 
I shall now treat more particularly of the productive coal- 
measures, and their mode of origin and organic remains. 
Coal formed on Land.—In South Wales, already alluded to, 
where the coal-measures*attain a thickness of 12,000 feet, the 
beds throughout appear to have been formed in water of 
moderate depth, during a slow, but perhaps intermittent, de¬ 
pression of the ground, in a region to which rivers were bring¬ 
ing a never-failing supply of muddy sediment and sand. The 
same area was sometimes covered with vast forests, such as 
we see in the deltas of great rivers in warm climates, which 
are liable to be submerged beneath fresh or salt water should 
the ground sink vertically a few feet. 
In one section near Swansea, in South Wales, where the 
total thickness of strata is 3246 feet, we learn from Sir H. De 
la Beche that there are ten principal masses of sandstone. 
One of these is 500 feet thick, and the whole of them make 
together a thickness of 2125 feet. They are separated by 
masses of shale, varying in thickness from 10 to 50 feet. The 
intercalated coal-beds, sixteen in number, are generally from 
one to five feet thick, one of them, which has two or three 
layers of clay interposed, attaining nine feet. At other points 
in the same coal-field the shales predominate over the sand¬ 
stones. Great as is the diversity in the horizontal extent of 
individual coal-seams, they all present one characteristic fea¬ 
ture, in having, each of them, what is called its underclay. 
These underclays, co-extensive with every layer of coal, con¬ 
sist of arenaceous shale, sometimes called fire-stone, because 
it can be made into bricks which stand the fire of a furnace. 
They vary in thickness from six inches to more than ten feet; 
and Sir William Logan first announced to the scientific world 
in 1841 that they were regarded by the colliers in South 
* Edward Hull, Quart. Geol. Journ., vol. xxiv., p. 327. 
