TIIK CAUBONlKKIiOl'S SYSTK.M 307 



measures, and more coal-seams toward the south-west Many of 

 the coal-seams are not persistent, and it is not easy to correlate the 

 seams which occur in different parts of the basin. 



Transition Stage. In South Wales this stage is represented 

 by the Pennant Grit Series, which consists of massive felspathic 

 sandstones or fine-grained grits, with some subordinate bands of 

 shale and seams of coal, and it forms the greater part of the high 

 central moorland plateau which is known as the Mynydd. Near 

 Pontypool on the east it is only about 800 feet thick and the coal- 

 seams are thin, but west of Taff Vale the group rapidly thickens 

 and workable coal-seams set in, so that near Swansea it is over 3000 

 feet thick, and contains fifteen good seams of coal. 



In the Bristol coalfield the Transition stage includes two 

 distinct groups, the Vobster Coal-measures below and the Pennant 

 Grit above. The Vobster Beds consist principally of shales with 

 some sandstones and many seams of coal, and many of the shale 

 bands contain ironstone nodules, so that these measures are rich in 

 coal and iron. The thickness is about 2000 feet, and in the 

 central part of the area (east of Bristol) there are twenty-six 

 coal-seams, but many of these are thin and disappear both to the 

 north and south. 



The Bristol measures include several bands which contain 

 marine shells, and from these Mr. H. Bolton has obtained no fewer 

 than forty-three species. The commonest are Nucula (equalis, 

 Posidoniella kevis, Schizodus antiquus, Lingula mytiloides, Glyphio- 

 ceras carbonarium, and Gastrioceras Listen. Pterinopecten papyraceus 

 also occurs, and by comparison with the list on p. 293 it will be 

 seen that the shells hardly support the evidence of the plants. 

 Mr. Bolton compares the fauna with that of the Gin Mine band in 

 North Staffordshire, which, according to Dr. Kidston, should be 

 placed in the Lower Coal-measures. 



The precise age of the Pennant Grits in the Bristol coalfield 

 IB not yet known ; they may belong to the Transition or to the 

 Upper Coal-measures. They form a sandstone group from 1000 

 to 1700 feet thick, the prevalent material being a hard grey 

 felspathic sandstone, which makes a durable building stone, and 

 lies in thick beds, so that it can be cut as a freestone. There are 

 some bands of shale and four workable seams of coaL 



Upper Coal-measures. These form a variable series of grey 

 shales, light-coloured sandstones, and coals, which appears to be an 

 expanded equivalent of the red Upper Coal-measures of more 

 northern coalfielda 



In South Wales, near Swansea, the group is very thick, having 

 been estimated at about 3000 feet (Pellengare Series). 



