92 PALEOZOIC TIME. [CHAP. VI. 



Professor Green elsewhere points out that the great 

 series of beds which compose the divisions of the Mill- 

 stone Grit, the Lower and the Middle Coal-measures, have 

 so many common characteristics " that it is impossible to 

 resist the conclusion that they were all three formed under 

 substantially the same conditions, and that these condi- 

 tions were altogether different from those which gave rise 

 to the Carboniferous Limestone and the Yoredale rocks." l 

 He thinks the change was caused by the blocking up of the 

 communications between the inner and the outer sea, so 

 that the former was converted into a large freshwater lake, 

 but though this might account for the succession in the 

 principal English and Scotch coalfields, it is far too local 

 an explanation to account for the Irish, French, Belgian, 

 and other European coal-measures. 



In the first place let us consider the formation of the 

 Millstone Grit, which consists principally of coarse sand- 

 stones, the grains of which are mostly angular and very 

 little worn, and are sometimes of very large size. The 

 sandstones are in fact coarser than those which occur in 

 the Yoredale Beds below or in the Coal-measures above, 

 and Prof essorGreen remarks that the Upper Carboniferous 

 series exhibits a gradational change in the texture of its 

 sandstones from the Millstone Grit upwards. The Mill- 

 stone Grits are not only coarse and massive, but are re- 

 markably persistent over large areas. " In the Lower Coal- 

 measures sandstones still play an important part ; but as a 

 rule they are neither so coarse nor so strong as those of the 

 Millstone Grit, and they vary much more from place to 

 place both in thickness and texture. Throughout a large 

 portion of the Middle Coal-measures thick sandstone beds 

 are conspicuous by their absence, and when such beds do 

 occur, they are, with very few exceptions, fine in grain ; 

 they are also markedly local, and can seldom be traced 

 1 " Coal, its History and Uses," 1878, p. 50. 



