TIIK CAIMIOMI-'KUOUS SYSTEM 305 



be a break at the top of the sandstones, for there ifl nothing 

 comparable to the Lower or to the Middle Coal-measures of the 

 Midlands, at any rate from a palaeontological point of view. 



In South Wales the so-called Millstone grit may be of any age, 

 for there appears to be a break at the base of it, and the overlying 

 beds which have hitherto been called " Lower Coal-measures " 

 contain the flora of the middle stage, and must be classed as 

 Middle Coal-measures. They everywhere rest on a sandstone 

 \vhich is known as the Farewell Rock, and though this is at 

 present grouped with the Millstone grit it may properly belong to 

 the overlying Coal-measure Group. 



In the following account the stages found in South Wales will 

 be described first, as the succession is there more complete. 



Millstone Grits. In South Wales the group is very variable. 

 At the eastern end of the basin north of Cardiff it is about 460 

 feet thick, consisting of a pebbly grit at the base, of shales and 

 sandstones in the middle, and a massive sandstone (the Farewell 

 Rock) at the top. Westward, however, the group expands to a 

 thickness of over 1500 feet, the greater part consisting of dark- 

 grey shale, but always having the Farewell Rock above, while the 

 basal grits are sometimes present and sometimes thin out, so that 

 in Gower, where it is absent, no line can be drawn between the 

 Bishopston cherts and shales and the overlying barren shales. 6 



The normal succession is well developed in the Ammanford 

 district on the northern outcrop, where the thicknesses are : 



Feet. 



Farewell Rock green and white sandstones . . about 200 

 Dark barren shales with bands of sandstone . . ,, 500 

 Quartzitic grits with some bands of shale . . ,, 600 



In the Bristol and Somerset Coalfield the fine-grained 

 sandstones which have been called Millstone Grit form a complete 

 ring round the Coal-measure basins, but are apparently an upward 

 continuation of the Avonian Series, and the relation of their 

 highest beds to the overlying shales has not yet been ascertained. 

 In most places they are about 1000 feet thick, but in the Mendip 

 district they have thinned to 500 feet. 



Middle Stage. According to the evidence of the plants this 

 only occurs in South Wales where it varies much in thickness, 

 and like other members of the system expands toward the west 

 and south-west. Thus at the eastern end of the basin it is only 

 from 625 to 850 feet thick, but in the centre it is about 1400 

 feet, and near the south-west margin upwards of 4000 feet This 

 expansion appears to be due not only to the increased thickness 

 of sandstones and shales, but also to the incoming of fresh 



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