THE CARBONIFEROUS GROUP. 
395 
3. Mountain or TA calcareous rock containing marine shells, corals, and 
‘ Carboniferous-N encrinites.; devoid of coal; thickness variable, some- 
Limestone_ ( times more than 1500 feet. 
If the reader will refer to the section Fig. 85, p. 130, he 
will see that the Upper and Lower Coal-measures of the coal¬ 
field near Bristol are divided by a micaceous flaggy sand¬ 
stone called the Pennant Rock. The Lower Coal-measures 
of the same section rest sometimes, especially in the north 
part of the basin, on a base of coarse grit called the Mill¬ 
stone Grit (No. 2 of the above table). 
In the South Welsh coal-field Millstone Grit occurs in like 
manner at the base of the productive coal. It*is called by 
the miners the “Farewell Rock,” as when they reach it they 
have no longer any hopes of obtaining coal at a greater 
depth in the same district. In the central and northern 
coal-fields of England this same grit,Tncluding quartz peb¬ 
bles, with some accompanying sandstones and shales con¬ 
taining coal plants, acquires a thickness of several thousand 
feet, lying beneath the productive coal-measures, which are 
nearly 10,000 feet thick. 
Below the Millstone Grit is a continuation of similar sand¬ 
stones and shales called by Professor Phillips the Yoredale 
series, from Yoredale, in Yorkshire, where they attain a 
thickness of from 800 to 1000 feet. At several intervals 
bands of limestone divide this part of the series, one of 
which, called the Main Limestone or Upper Scar Limestone, 
composed in great part of encrinites, is 70 feet thick. Thin 
seams of coal also occur in these lower Yoredale beds in 
Yorkshire, showing that in the same region there were great 
alternations in the state of the surface. For at successive 
periods in the same area there prevailed first terrestrial con¬ 
ditions favorable to the growth of pure coal, secondly, a sea 
of some depth suited to the formation of Carboniferous 
Limestone, and, thirdly, a supply of muddy sediment and 
sand, furnishing the materials for sandstone and shale. 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-west¬ 
erly 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 “ 
South Lancashire ............ .12,130 “ 
North Lancashire. 18,700 “ 
