There is no clear line of demarkation between the Coal-
measures and the Millstone Grit, nor between the Millstone Grit and underlying
Yoredale rocks.
On comparing a series of vertical sections in a north-westerly direction from
Leicestershire and Warwickshire into North Lancashire, we find, says Mr. Hull,
within a distance of 120 miles an augmentation of the sedimentary materials to
the extent of 16,000 feet.
Leicestershire and Warwickshire: 2,600 feet.
North Staffordshire: 9,000 feet.
South Lancashire: 12,130 feet.
North Lancashire: 18,700 feet.
In central England, where the sedimentary beds are reduced to about 3000 feet in
all, the Carboniferous Limestone attains an enormous thickness, as much as 4000
feet at Ashbourne, near Derby, according to Mr. Hull's estimate. To a certain
extent, therefore, we may consider the calcareous member of the formation as
having originated simultaneously with the accumulation of the materials of grit,
sandstone, and shale, with seams of coal; just as strata of mud, sand, and
pebbles, several thousand feet thick, with layers of vegetable matter, are now
in the process of formation in the cypress swamps and delta of the Mississippi,
while coral reefs are forming on the coast of Florida and in the sea of the
Bermuda islands.
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