CHAP. VI.] CARBONIFEROUS PERIOD. 93 



continuously for more than a few miles" (op. cit., p. 

 60). 



Now the persistence of the coarse-grain sandstones and 

 the impersistence of the finer-grain beds is just the oppo- 

 site of what might be expected, and is indicative of some 

 special conditions during the formation of the Millstone 

 Grit. Such widely-distributed and coarse-grained sand- 

 stones can hardly have been deposited during a con- 

 tinuance of the subsidence which led to the formation of 

 the underlying shales and limestones ; moreover, the thick- 

 ness of the grits does not increase in the direction of the 

 mainland, but is greatest in the south-western part of 

 what may be called the central basin, where the Yoredale 

 group is also the thickest. 



The distribution of the grits, indeed, is such as to 

 suggest that the materials were derived from the detrition 

 of the land which formed the southern shore of this central 

 basin, and the position of which has already been indicated. 

 Thick lenticular masses of Millstone Grit occur both on 

 the north and south sides of this land tract, the centre of 

 one great mass being in Lancashire and that of another in 

 the Bristol coalfield. From the Burnley coalfield, where 

 the sandstones alone are over 2,000 feet thick, the grits 

 thin away to the north-east through Yorkshire and Dur- 

 ham, and to the south-east through Derbyshire, but less 

 rapidly to the south, being still thick and coarse in North 

 Staffordshire, within thirty miles of the shore-line. More- 

 over, in the extreme north-east (Northumberland) their 

 texture is so fine that they are actually less coarse than 

 some of the Lower Carboniferous sandstones of that area ; 

 it is clear, therefore, that the material did not come from 

 that direction. In the Bristol coalfield the Millstone G-rit 

 is 950 feet thick, and though the group thins in all direc- 

 tions, yet the coarse grits are conspicuous along the north 

 side of the South Wales coalfield and in the Forest of 



