336 MR ROBERT KIDSTON ON THE FOSSIL FLORA OF THE 



Its extreme length from north to south is 26 miles, and its width, from 

 east to west (if the outlying Nailsea basin be included), is 24 miles. If the 

 Nailsea basin be excluded, its width from Bath in the east to Bristol in the 

 west is reduced to 12 miles. 



The Coal Measures are mostly covered by Secondary rocks, Jurassic and 

 Triassic, which are unconformable to the underlying Palaeozoic strata. 



The Carboniferous Formation lies in a trough surrounded at intervals on 

 the north, west, and south by the Old Eed Sandstone. 



The general geological structure of this coal field will be most easily under- 

 stood by referring to Plate XVIII., which gives a reduced sketch of a section 

 prepared by Mr J. M'Murtrie, F.G.S., for the Royal Coal Commission in 

 1868. 



This section shows the Secondary Formations lying unconformably on the 

 upturned edges of the Palaeozoic rocks. The centre of the basin is occupied 

 by the Upper Division of the Coal Measures (Nos. in section 1, 2, 3). This 

 rests on the Pennant Hock (No. 4), immediately beneath which is the Lower 

 Division of the Coal Measures (5 and 6). Succeeding this is the Millstone 

 Grit (7) and Carboniferous Limestone (8), which latter rests on the Old Eed 

 Sandstone (9). 



It is necessary, however, to study in somewhat fuller detail the various 

 horizons of the Coal Measures, — that is, all the Carboniferous rocks above the 

 Millstone Grit. These, as already mentioned, resolve themselves into three 

 great divisions, the Upper and Lower Divisions of the Upper Coal Measures, 

 and the Pennant Rock. 



The Upper Division of the Coal Measures, attaining a thickness of about 

 2200 feet, is separated into the Upper or Radstoek Series (1), and the Lower or 

 Farrington Series (3), between which are interposed a characteristic series of 

 Red Shales (2). 



The Upper Division (including the Radstoek and Farrington Series) is 

 separated from the Lower Division by the Pennant Rock (4), which attains a 

 probable thickness of from 2500 to 3000 feet. 



The Lower Division, of a thickness of about 2800 feet, is also divisible into 

 two series, the upper of which is named the New Rock Series, and the lower the 

 Vobster Series. 



These two series are not so clearly separable from each other as those of 

 the Upper Division are by the intervention of the Red Shales, being separated 

 rather on account of the character of the veins and the nature of the strata 

 than by the occurrence between them of any unproductive characteristic 

 stratum of rock. 



This coal field is traversed by many faults, some of which, especially the 



